3.1.1.1 Phthalic Anhydride

Phthalic anhydride is prepared by oxidizing o-xylene. The oxidation may be performed either in the gas phase with vanadium pentoxide as a catalyst or in the liquid phase with dissolved manganese, molybdenum or cobalt salts as catalysts:

At the present time, gas-phase oxidation is the favoured technique.

A third option involves oxidizing naphthalene, possibly with vanadium pentoxide as a catalyst.

Phthalic anhydride is a significant commercial product. Its main area of application is in synthetic resins and plasticizers. In 2014, the production volume worldwide was about 4.5 million tons.

3.1.1.2 Phthalonitrile

Phthalonitrile (also called ‘phthalodinitrile’) is obtained by oxidizing o-xylene with ammonia. To this end, o-xylene is treated with ammonia, either in the presence of an oxidation catalyst or with the aid of oxygen and a catalyst, at 330–340 °C:

3.1.2.1 Phthalonitrile Process

The first technical process involved heating phthalonitrile with copper bronze or copper(I)chloride at 200–240 °C in copper pans. Several variations of this technique were developed in Germany prior to the Second World War. The reaction was performed either without or in the presence of a solvent. A basic distinction is commonly made between the baking process and the solvent process; both may be carried out either by continuous or by batch technique.

3.1.2.1.2 Solvent Process

The solvent process involves treating phthalonitrile with any one of several copper salts in the presence of a solvent at 120–220 °C [11]. Copper(I)chloride is most important. The list of suitable solvents is headed by those with a boiling point above 180 °C, such as trichlorobenzene, nitrobenzene, naphthalene and kerosene. A metallic catalyst such as molybdenum oxide or ammonium molybdate may be added to enhance the yield, to shorten the reaction time and to reduce the necessary temperature. Other suitable catalysts are carbonyl compounds of molybdenum, titanium or iron. The process may be accelerated by adding ammonia, urea or tertiary organic bases such as pyridine or quinoline. As a result of improved temperature maintenance and better reaction control, the solvent method affords yields of 95% and more, even on a commercial scale. There is a certain disadvantage to the fact that the solvent reaction requires considerably more time than dry methods.

After completion of the reaction, the solvent is removed by filtration. Most often, however, the solvent is separated from the crude Copper Phthalocyanine Blue by distillation.

The solvent method may also be performed either by continuous (in cascades) or by batch operation. Continuous techniques in particular have gained considerable technical importance. A phthalonitrile/copper chloride solution is typically treated at 120–140 °C in a flow tube furnace and the temperature subsequently increased to 180–250 °C. The entire process requires approximately 1.5–2 h and affords the pigment in practically quantitative yield. The excellent purity of the product eliminates the need for additional purification with dilute acid or base prior to finishing, a procedure that plays a major role in the baking process. These are the advantages that make the baking process economically favourable, despite the problems connected with the separation and regeneration of the solvents.

The phthalonitrile process has the particular advantage over the phthalic anhydride process of forming ring-substituted chloro-copper phthalocyanines. Using copper(I)chloride produces so-called ‘semi-chloro’ Copper Phalocyanine Blue, a pigment that possesses a statistical average of 0.5 chlorine atoms per copper phthalocyanine molecule. Copper(II)chloride, on the other hand, affords a product that consists of an average of one chlorine atom per copper phthalocyanine molecule. A prerequisite for the formation of the chloro substituted compound, however, is the absence of ammonia or urea in the reaction mixture.

This simple one-step route leads to the starting material for the solvent-stabilized α-modification of Copper Phthalocyanine Blue.

The synthesis of the chlorine-free crude pigment by either the continuous or the batch version of the phthalonitrile process involves adding up to 20% urea or ammonia to the reaction mixture. The latter will provide an effective chlorine trap.

Moreover, the phthalonitrile process has the added advantage of being the more elegant of the two syntheses. This technique makes it possible to produce comparatively pure copper phthalocyanine without obtaining substantial amounts of side products, a phenomenon that is understandable in view of the fact that the phthalonitrile molecule provides the parent structure of the phthalocyanine ring. Formally, rearrangement of the bonds necessitates donation of two electrons to the system:

The advantages of the phthalonitrile process are that phthalonitrile is not only much more costly than phthalic anhydride but also less easily available. In view of the intermediates found so far, and in conjunction with a study of the thermal course of the reaction using differential scanning calorimetry, a reaction mechanism has been proposed for the phthalonitrile route that may be visualized as follows [12].

The reaction is initiated by attack of a nucleophile (Y⁻), usually the counterion associated with the Cu²⁺ ion, at one of the CN groups of the phthalonitrile, which is activated by its coordination with the Cu²⁺ ion. Subsequently, a cyclization reaction to an isoindoline derivative takes place. These steps are repeated three times by a series of similar reactions, finally resulting in a cyclization to a CuPc ring intermediate, whose formation is facilitated by the coordinating role of the copper ion. With a copper(II) salt as reactant it is suggested that Y⁺ is eliminated from the intermediate, for example the Cl⁺ ion in the case of CuCl₂. The monochloro derivate of CuPc is than formed by electrophilic attack of Cl⁺ on the CuPc initially formed:

3.1.2.2 Phthalic Anhydride/Urea Process

Likewise, this process [13] may also be carried out either as a solvent-free (baking) method or in the presence of solvents. Although initially performed as a solvent-free technique, it is the solvent version that currently dominates the field of copper phthalocyanine production from phthalic anhydride and urea. However, this trend is being reversed, such that solvent-free methods are attracting interest, especially for ecological reasons.

3.1.2.2.2 Solvent Process

There is somewhat less of a demand on the reaction equipment if phthalic anhydride and urea react in an organic solvent. In principle, the presence of a solvent makes almost no difference to the synthesis by baking process. Using suitable inert high-boiling solvents such as kerosene, trichlorobenzene or nitrobenzene obviates many of the difficulties connected with the reaction mixtures obtained in the baking process. Solvents make it much easier to adequately mix the components and to ensure even heat transfer. Long reaction times, however, remain somewhat of a problem, although suitable temperature control during condensation makes it possible to produce the complex in almost quantitative yield.

The solvent is removed from the solid product by filtration or centrifugation to afford a crude Copper Phthalocyanine Blue of a quality that makes intermediate purification unnecessary. In contrast to the product obtained by the baking process, this material is pure enough to be used directly for further pigment manufacture. Crude Copper Phthalocyanine Blue, on the other hand, which evolves as the solvent is removed by distillation, contains so many impurities that it must be boiled before being utilized further.

Phthalic anhydride and urea, together with copper(I) chloride and ammonium molybdate, are heated to 200 °C in trichlorobenzene. The ratios between the components are the same as in the baking process. Carbon dioxide and ammonia are released to yield Copper Phthalocyanine Blue. The reaction is complete after 2–3 h, producing a yield between 85% and >95%.

The main disadvantage of working in the presence of a solvent is the problem of regenerating this solvent.

Apart from nitrobenzene, trichlorobenzene in particular was the preferred solvent up until a few years ago. It has now been replaced by other solvents such as high-boiling hydrocarbons (kerosene, naphthalene) and also alcohols and glycols, because traces of polychlorinated biphenyls may be formed. These are not easily degradable. With hydrocarbons, however, the possibility of fire and explosion must be considered in designing suitable production units.

In the presence of a solvent, the crude pigment generally evolves in much purer form than if it is prepared by baking. Higher degrees of purity, that is, up to 98%, are achieved by additional alkaline and/or acidic treatment. Commercially available types of crude Copper Phthalocyanine Blue typically contain more than 90% pure pigment. This preliminary product is also commonly supplied to and finished by companies who do not manufacture crude pigment themselves.

Reaction between phthalic anhydride and urea always affords chlorine-free Copper Phthalocyanine Blue. Chlorinated derivatives are obtained only in the absence of bases (ammonia) or urea. The phase-stabilized α-modification is prepared by essentially the same but slightly modified route: it is derived from mixed condensation with chlorinated phthalic anhydrides (such as 4-chlorophthalic acid) (Section 3.1.2.4).

Despite the described disadvantages, but due to the high yield, economic production methods, and low-cost starting materials, the phthalic anhydride/urea method is presently the most significant industrial route to Copper Phthalocyanine Blue manufacture.

The gross reaction equation for the preparation of Copper Phthalocyanine Blue from phthalic anhydride and urea may be written as follows:

This overall reaction equation points to the fact that copper phthalocyanine formation formally requires two reduction equivalents.

Formally, copper phthalocyanine is formed by the following reaction pathway:

Phthalic anhydride initially reacts with ammonia, which in turn is liberated, for instance, by decomposition of urea. Diiminophthalimide is then produced via phthalimide and monoiminophthalimide. Subsequent self-condensation (as in the phthalonitrile process) under cleavage of ammonia affords polyisoindolenines, which form complexes with copper ions. Ring closure is achieved through further release of ammonia, and copper phthalocyanine is finally obtained by reduction.

The above-mentioned reaction mechanism is validated by proven intermediates.

Urea acts not only as an ammonia source but also forms decomposition products, such as biuret and higher condensation products. ¹⁴C labelling has indicated that the carbon atom of the urea molecule is not incorporated into the phthalocyanine structure. Employing a phthalic anhydride molecule bearing one radioactively labelled carbonyl function affords labelled copper phthalocyanine and phthalimide (as a side product), while the liberated carbon dioxide was found not to show any radioactivity. Labelled carbon dioxide, on the other hand, has been obtained in corresponding experiments using ¹⁴C labelled urea.

3.1.2.3 Manufacturing the Different Crystal Modifications

Unsubstituted Copper Phthalocyanine Blue is polymorphous. X-Ray diffraction diagrams point to eleven different crystal modifications (α, β, γ, δ, ɛ-, R, π, ρ-, σ, ζ, and a new eleventh one) [14, 15]. The five important ones are the α-, β-, γ-, δ- and ɛ-phases (Figure 3.1). Their relative thermodynamic stability decreases in the order: β > ɛ > δ > α ≈ γ [16–19]. The crystal structures are described in Section 3.1.3.

Amongst these modifications, it is particularly the phase-stabilized α-form and the β-form that are of high commercial interest.

Crude Copper Phthalocyanine Blue prepared by the phthalonitrile or urea process typically evolves as the β-modification with a coarse particle size.

The synthesis of the crystal modification is controlled primarily by the finishing technique of the crude pigment. There are basically two different methods to produce a finely dispersed pigment: treatment with acid to form copper phthalocyanine salts, followed by precipitation in water, on the one hand, and mechanical treatment (milling, kneading) on the other hand. The following methods are used:

• dissolving the crude pigment in concentrated sulfuric acid;
• swelling in 50–90% sulfuric acid;
• dry milling in the presence or absence of water soluble salts, as well as with or without small amounts of organic solvents;
• milling in organic solvents;
• kneading with salt in the presence of solvent.

Crude Copper Phthalocyanine Blue has become a commodity product, which is synthesized in large amounts. The finishing process is more demanding. Today, salt kneading and activation by ball milling with subsequent solvent finishing with additives play a major role. An alternative is acid pasting, that is, swelling in sulfuric acid followed by precipitation with water in the presence of amines or additives.

3.1.2.4 Phase- and Flocculation-Stabilized Copper Phthalocyanine Blue Pigments

There are various methods of ‘stabilizing’ a pigment to prevent conversion into a different phase (i.e. change of modification) and flocculation during pigment application. The two following techniques have been found to be most effective:

• minor chemical modification of the Copper Phthalocyanine Blue molecule, for instance by ‘partial chlorination’;
• admixing other agents to the Copper Phthalocyanine molecule; surface active additives may thus help to stabilize the surface.

Only a minor amount of chlorinated copper phthalocyanine, for instance, especially in the 4-position of the copper phthalocyanine molecule, prevents a change of modification from α into β. Approximately 3–4% chlorine is commonly used, which corresponds to the formula CuPc-Cl_0.5, also referred to as ‘semi-chloro-CuPc’. The phthalic anhydride/urea synthesis, for instance, affords a partially chlorinated product if 4-chlorophthalic anhydride is added to the reaction mixture. Copper chlorides in the phthalonitrile process have the same effect.

Partial sulfonation or sulfamide formation may stabilize a pigment toward flocculation. Combining partial sulfonation and chlorination will further improve the effect of each individual type of modification. Similar results are observed if carboxylic acid groups are introduced into the pigment molecule.

There is an interesting technique that makes it possible to introduce carboxylic acid groups into a copper phthalocyanine structure by an economic route. Carrying out the phthalic anhydride/urea process in the presence of a small amount of trimellitic acid or another benzene polycarboxylic acid will afford a carboxylated pigment.

Flocculation may likewise be prevented through partial introduction of dialkylaminomethylene groups into the aromatic ring system of the copper phthalocyanine molecule. The copper phthalocyanine structure tightly attaches the basic groups to the surface of the pigment particles:

a feature that imparts partial solubility on the parent molecule. Acidic constituents within the binder may interact with the amino groups.

There are other metal complexes, such as tin, aluminium, magnesium, iron, cobalt, titanium and vanadium complexes, which are similarly useful in stabilizing a particular phthalocyanine modification. Moreover, carboxy, carbonamido, sulfo or phosphono-copper phthalocyanine may be admixed during fine dispersion of the pigment.

The discussed additives, which stabilize a pigment with regard to change of modification and flocculation, are usually applied at a concentration of 3–10%.

A flocculation-resistant β-form is obtained by milling crude Copper Phthalocyanine Blue with salt in the presence of xylene or a similar hydrocarbon. In this case, as in general, impurities in the crude Copper Phthalocyanine Blue prevent flocculation.

Polyhalogenated green copper phthalocyanine pigments are not polymorphous and are thus exempt from change of modification.

3.1.2.5 Manufacture of Green Types

The early days of Copper Phthalocyanine Green synthesis were dominated by two competitive routes. One method was the synthesis of tetraphenyl copper phthalocyanine (Bayer), while the second method involved chlorination of copper phthalocyanine in carbon tetrachloride to form copper tetradeca to hexadecachloro phthalocyanine (BASF). It was on the grounds of economic considerations that manufacturers began to prefer the chlorination technique in industrial-scale production.

The oldest and still most prevalent large-scale synthesis for green copper phthalocyanine pigments proceeds via direct chlorination of copper phthalocyanine (1935). Chlorination is effected at 180–200 °C in a sodium chloride/aluminium chloride melt (eutectic blend) in the presence of a catalyst (metal chlorides such as iron(III)chloride). The temperature limits for chlorination are generally reported to be between 60 and 230 °C. There is also a battery of other media, such as chlorosulfonic acid, thionyl chloride, sulfuryl chloride and chlorinated hydrocarbons, possibly under pressure, which are equally suitable for chlorination. All these techniques afford mixtures of multiply chlorinated copper phthalocyanines. The structure is typically chlorinated in at least eight positions, but more often 14–15.5 times. The perchlorinated copper phthalocyanines produced either in AlCl₃/NaCl or in acid are then precipitated in water, filtered off, and washed, especially to remove aluminium salts. Since the pigment already evolves from the synthesis as an extremely fine powder, it may then be subjected directly to thermal aftertreatment in an aqueous/organic medium. This is attributed to the fact that perchlorinated copper phthalocyanine interacts with aluminium chloride as well as with strong acids, leading to the formation of salts, which are cleaved through hydrolysis.

Less widely used methods include acid pasting, that is, dissolving the material in a medium such as chlorosulfonic acid and precipitating it in water, or milling the crude pigment. These methods gain importance whenever phthalocyanine is to be chlorinated by a route other than with aluminium chloride or acids.

Commercially available Copper Phthalocyanine Green types typically contain approximately 15 chloro atoms per molecule, while yellowish green types bear not only chloro but also bromo substituents. Such derivatives are obtained through mixed halogenation. Chlorinated green copper phthalocyanines can also be prepared from already perchlorinated starting materials. Tetrachloro phthalonitrile, for instance, may be condensed in nitrobenzene with copper(II) chloride to form copper perchloro phthalocyanine. Costly starting materials, however, make this an industrially unattractive route. Tetrachloro-phthalonitrile may be synthesized from phthalonitrile by gas-phase chlorination, a quite demanding process.

The phthalic anhydride/urea process may also be employed to convert tetrachloro-phthalic anhydride into green copper hexadecachloro phthalocyanine by condensation. In this case, titanium or zirconium dioxides, particularly in the form of hydrated gels, are used instead of the molybdenum salts that are used in the phthalic anhydride process [25]. There is a certain disadvantage to the fact that the products lack brilliance and require additional purification.

The exact hue produced by a green copper phthalocyanine is defined only by the degree of halogenation and by the type of halogen used (chloro or bromo substitution) and not by different crystal phases. Only one crystal modification of copper polyhalophthalocyanine has been reported. A higher degree of chlorination is accompanied by a shift of colour from greenish blue to bluish green. A considerable difference is observed with the introduction of approximately the tenth chloro atom. Bromination affords yellowish shades of green.

Introduced in 1959 by GAF, a copper polybromochloro phthalocyanine was the first commercially used mixed halogenated copper phthalocyanine pigment. It was manufactured, as described above, in molten NaCl/AlCl₃ with CuCl₂ as a catalyst and bromine besides chlorine to afford a sequence of bromination followed by a chlorination reaction. The thus-prepared pigment contains approximately eleven bromine and three chlorine atoms per molecule. In the most yellowish pigments, the degree of halogenation is 11–12 bromo and 4–5 chloro substituents.

Copper Perbromo Phthalocyanine Green may also be obtained from tetrabromo phthalic anhydride by the phthalic anhydride/urea process in the presence of titanium or zirconium catalysts. This route has not yet been introduced on a commercial scale.

3.1.2.6 Metal-Free Phthalocyanine Blue

There are several pathways to metal-free Phthalocyanine Blue (P.B.16):

• by synthesizing suitable unstable ionic metal phthalocyanine salts, such as alkali or alkaline earth salts, with subsequent demetallization by alcohol or acid;
• by direct synthesis from phthalonitrile or amino-iminoisoindolenine;
• by fusing phthalic anhydride with urea.

The first route to preparing metal-free Phthalocyanine Blue involves treating phthalonitrile with the sodium salt of a higher-boiling alcohol, for instance with sodium amylate. The resulting phthalocyanine disodium salt is demetallized by stirring in cold methanol:

Phthalocyanine is likewise obtained by hydrolysing a corresponding calcium or magnesium salt in an acidic medium.

The route that proceeds via phthalonitrile has stimulated interest in connection with semiconductor technology for photoelectric copying, especially with respect to the pure Phthalocyanine Blue that is prepared by this path. An excellent pure product is obtained by heating phthalonitrile in 2-dimethylaminoethanol while passing ammonia through the reaction mixture. A second scheme involves heating 1,3-diiminoisoindolenine in the same solvent to 135 °C, a method that affords the product not only in equal purity but also in high yield (approx. 90%). A third route proceeds via phthalonitrile, which is heated with hydrogen under pressure in an inert solvent [26]. For dyeing, phthalocyanine as pigment can also be formed on cellulosic fibres itself [27].

The similarly blue metal-free phthalocyanine is chemically somewhat less stable than its copper complex [28]: it decomposes slowly in a sulfuric acid solution. On the other hand, it can be chlorinated to afford metal-free Phthalocyanine Green.

3.1.5.1 General

The α- and β-types of Copper Phthalocyanine Blue reign supreme amongst commercially available phthalocyanine pigments. There is also increasing interest in the phase-stabilized form of the α-crystal modification. Both modifications are also supplied as flocculation resistant types.

Because phthalocyanine blue pigments are utilized in a host of applications, the requirements for their use vary considerably. Numerous special-purpose types are therefore supplied that are custom designed to exhibit optimized properties in certain media to suit particular demands.

The ɛ-modification has attracted some interest. Commercial types of metal-free phthalocyanine blue are also available.

Copper phthalocyanine green pigments are primarily offered as partially chlorinated products containing 14–15 chlorine atoms per pigment molecule (Pigment Green 7). Various copper polybromochloro phthalocyanine derivatives featuring 4–9 bromine and 8–2 chlorine atoms per molecule (Pigment Green 36) play a role as yellowish green pigments.

Table 3.1 lists the types of phthalocyanine pigments that are currently supplied.

3.1.5.2 Pigment Blue 15

Under the name of Pigment Blue 15, the Colour Index lists types of α-Copper Phthalocyanine Blue that are not stabilized towards phase-transfer. Compared to other phthalocyanine blue pigments, Pigment Blue 15 types are reddish blue in shade, tinctorially strong and provide high colour yield and economy in use. Their impact on the market, however, is inferior to that of the corresponding stabilized types, by which they are increasingly being replaced. In many media, P.B.15 types are redder and frequently cleaner than stabilized types, an advantage that is often compromised by less tinctorial strength.

Non-stabilized α-Copper Phthalocyanine Blue is utilized in the printing industry, to a certain extent in oil-based binder systems, such as offset printing inks for packaging and metal deco printing. Under standard conditions, P.B.15 is stable to various organic solvents, such as alcohols, esters, ketones, aliphatic hydrocarbons, toluene, and plasticizers, such as dioctyl phthalate (Section 1.6.3). Ethylglycol or the standard DIN 16 524 solvent mixture, however, are slightly coloured. The prints are equally fast to acids and alkali and show perfect fastness to soap, butter and sterilization. P.B.15 is too reddish to be used in process colours, that is, to produce the standard cyan for three- and four-colour printing. A strong tendency to change its crystal modification in the presence of aromatics makes P.B.15 a doubtful candidate for packaging gravure inks. For the same reason, the pigment is only rarely found in paints.

The fact that unstable P.B.15 types tolerate less than 200 °C is a disadvantage in the colouration of plastics. With increasing temperature, the pigment frequently converts into the β-modification, a tendency that is especially pronounced in polymer materials that contain aromatic rings, such as polystyrene, ABS or PET.

As a result of their reddish shade, P.B.15 types are used to a certain extent to colour polyethylene, which is processed below 200 °C, to produce items such as films. Another suitable medium is PVC, which is commonly treated at moderate temperature. There is a certain disadvantage to the fact that the tinctorial strength of such systems is frequently inferior to that of stabilized Copper Phthalocyanine Blue varieties.

Incorporated in plasticized PVC, P.B.15, like other phthalocyanine pigments, is usually entirely fast to migration. Moreover, it provides excellent lightfastness. P.B.15 also finds use in various types of PUR foam materials as well as in rubber. Its redder and frequently cleaner shade compared to corresponding stabilized types makes it an equally useful pigment for other media. This applies especially for water-based systems. Textile printing, paper mass colouration, paper surface treatment and paper pulp are areas of application suitable for the use of P.B.15 in office articles, including coloured pencils, blackboard chalks for schools and watercolours.

Using P.B.15 in aqueous media may be problematic given that the highly non-polar character (Section 3.1.3) of the pigment may make it necessary to use a considerable amount of dispersants such as oxyethylated phenols, alcohols or alkylsulfonates. Adequate dispersion relies as much on sufficient wetting of the pigment particles as it does on vigorous mechanical shear. Thus, adiabatic heat is frequently developed during pigment processing. Unsubstituted types of α-Copper Phthalocyanine Blue may convert into the β-modification if they are treated in insufficiently cooled dispersion units. This process is associated with a colour change to a greener blue as well as with loss of cleanness and tinctorial strength. P.B.15 is thus supplied mainly in the form of highly pigmented aqueous pastes. Pigment manufacturers may either provide such products themselves or leave this task to specialized manufacturers of pigment preparations.

Introducing suitable substituents into the Copper Phthalocyanine Blue structure or covering the crystal surface with appropriate substances may specifically influence the hydrophilicity or the polarity of the pigment (Section 3.1.2.4). The ease of dispersion and wetting may thus be improved or optimized for certain applications, such as for aqueous media.

3.1.5.3 Pigment Blue 15:1

In the Colour Index, the designation P.B.15:1 refers to a phase-stabilized α-Copper Phthalocyanine Blue. This pigment has gained great commercial importance in almost all areas. Stabilization, however, for instance through partial chlorination, usually causes loss of tinctorial strength and cleanness as well as a colour shift towards a greener blue. Despite this disadvantage, however, stabilized P.B.15:1 types reign supreme amongst Copper Phthalocyanine Blue types as colourants for coatings and paints, packaging printing inks and plastics. They show good resistance to organic solvents. Excellent lightfastness and weatherfastness, superior migration fastness, and high heat stability, along with a reasonable price make these pigments attractive products. There is almost no limitation to pigment use in the above-mentioned areas. Moreover, P.B.15:1 types are often employed in combination with other pigments. Blends with Dioxazine Violet, for instance, provide shades of navy blue, while P.B.15:1 in conjunction with titanium dioxide produces good hiding power. In addition, P.B.15:1 blends are also utilized to brighten white paints, printing inks and other media.

The paint industry employs P.B.15:1 for its excellent fastness properties to colour all types of paints, including automotive refinishes and original automotive finishes, in both solid and metallic shades.

The so-called flop effect is a primary concern in connection with metallic finishes. Products may exhibit a different colour and/or brightness, depending on the direction of incident light and the angle of observation. Flop is an important consideration in automotive finishes because different auto parts, although finished with the same product, may appear in a different colour, depending on the angle at which they are viewed. Differences are particularly visible in locations such as between hood and fender. This phenomenon is attributed to a considerable extent of scattering in the coloured pigment, thus sufficiently transparent pigments present something of advantage. More extensive chlorination of α-Copper Phthalocyanine Blue makes it possible to afford products that exhibit very little flop. Suitable special-purpose products are commercially available.

In practical application, most types of P.B.15:1 are completely fast to overcoating in oven drying systems. The pigments are suitable candidates for powder coatings, for instance for acrylate or polyurethane-based systems, in which they do not exhibit plate-out (Section 1.6.4.1).

The use of P.B.15:1 in various types of paints has for many years been hampered by unsatisfactory flow behaviour. Newer methods involving surface treatment of the pigment with suitable agents have made it possible to improve sufficiently the rheology in certain media to afford pigments that satisfy the use requirements.

Many P.B.15:1 type types demonstrate very good stability to change of modification, a feature that largely eliminates application problems. A sample may be tested for its stability in this regard by refluxing the pigment in toluene or xylene, filtering it, dispersing it in its target paint system, and evaluating the resulting product by colouristic comparison with an untreated but equally dispersed reference sample. The tendency of a product to convert into a different modification may similarly be assessed by storing the pigmented paint at elevated temperature, for instance at 50 °C, for two weeks. The thus prepared dispersion is then compared with a reference system to assess possible colour change.

Incorporated in printing inks, phase-stabilized α-Copper Phthalocyanine Blue, like non-stabilized types, is too reddish to be employed as a process colour for three- and four-colour printing. It is used, however, to a considerable extent in all types of printing inks for special and packaging purposes. The prints are stable to common organic solvents and exhibit perfect fastness properties in special application (Section 1.6.2.3). Metal deco prints withstand up to 200 °C for 10 min or 170–180 °C for 30 min. They may safely be sterilized.

Many types of P.B.15:1 are initially very strong in printing inks. To produce 1/1 SD letterpress proof prints at standard conditions, a pigment concentration of 16–19% will suffice, depending on the type. 1/3 SD prints are produced using 7.5–10% pigment. The resulting prints are entirely fast to light: they score as high as step 8 and step 7 on the Blue Scale, respectively.

The list of applications also includes decorative printing inks for melamine based laminated plastic sheets. Notably, however, P.B.15:1 may not be used in decorative printing inks for polyester-based sheets, because the pigment reacts with accelerators and hardening agents. Moreover, most types of P.B.15:1 are not entirely fast to styrene monomer, which is a frequently used solvent for polyesters.

Incorporated in aqueous systems, the main areas of application for stabilized α-Copper Phthalocyanine Blue are in emulsion paints, pastes for textile printing, and aqueous or water soluble gravure and flexographic printing inks for packaging and wallpaper. The textiles printing industry frequently prefers stabilized over non-stabilized α-types, especially if the resulting product must be fast to dry cleaning, that is, to exposure to halogenated hydrocarbons. Even stabilized Copper Phthalocyanine Blue is frequently worked into aqueous application media in the form of an aqueous paste. Such pastes are concentrated pigment formulations that contain predispersed pigment. In contrast to the handling of non-stabilized types (P.B.15), it is not necessary to exercise particular caution in dispersing P.B.15:1 in the presence of dispersion or wetting agents or particular solvents.

Likewise, P.B.15:1 also lends blue shades to plastics, in which it is incorporated either individually or in combination with inorganic or organic pigments. The stabilized type, like its non-stabilized counterpart, is a pigment with high tinctorial strength in polyolefins. 1/3 SD colourations (1% TiO₂) are formulated at 0.08–0.1% pigment, depending on the type. Most grades resist exposure to approximately 300 °C and satisfy almost all demands. Pigment dispersibility, however, is somewhat of a problem. Very few commercial products meet the stringent requirements particularly for use in thin films and similar products. Like other types of Copper Phthalocyanine Blue, P.B.15:1 varieties also affect the shrinkage of partially crystalline thermoplastics to an appreciable extent. They are therefore only marginally suited to application in nonrotation-symmetrical injection-moulded articles, such as bottle crates. Many grades of P.B.15:1 are more or less fast to bleeding in plasticized PVC, depending on the type of stabilization. In this medium, the pigment is also very lightfast and weatherfast. Incorporated in rigid or impact resistant PVC, stabilized α-types are less fast to long-term outdoor exposure than β-Copper Phthalocyanine Blue or Green.

P.B.15:1 is also applied in polystyrene, polyamide, polycarbonate (in which it is heat stable up to 340 °C), PUR foam materials and cast resins. Notably, however, the hardening of cast resins that are based on unsaturated polyesters is usually much retarded.

Problems may arise if P.B.15:1 is to be used in natural rubber, because the presence of free copper affects not only the vulcanization process, but also considerably affects the fastness of the product to ageing. P.B.15:1 is thus considered a rubber poison. The free copper content in a pigment should therefore never exceed 0.015%. Commercial types are available that have been tested accordingly.

P.B.15:1, like other types of Copper Phthalocyanine, finds extensive use in the spin dyeing of polypropylene, polyester, polyamide, secondary acetate, viscose rayon and spun rayon. In these, as in other media, P.B.15:1 is very lightfast, and its textile fastness properties are almost entirely if not entirely satisfactory.

3.1.5.4 Pigment Blue 15:2

Several α-Copper Phthalocyanine types that are stabilized towards flocculation and change of modification are registered in the Colour Index as P.B.15:2. Their main area of application is in paints. They are employed wherever P.B.15:1 types show too much of a tendency to flocculate in application or where economic considerations make P.B.15:2 appear more attractive. In accordance with its good fastness to flocculation, P.B.15:2 demonstrates good resistance to rub-out [40] in most common binder systems. Gradual improvement of the flocculation behaviour has been achieved by suitably adjusting the particle size or particle size distribution as well as the shape of the pigment particles. However, stability to flocculation is imparted on these types primarily through chemical modification or through adsorption of suitable agents. The chemically induced resistance to flocculation, however, is frequently associated with a certain loss of fastness to overcoating, especially in systems containing aromatics. For this reason, flocculation resistant types have been introduced which largely eliminate this problem.

Other fastness properties in application largely equal those of P.B.15:1. In printing inks, P.B.15:2 is employed mostly in special gravure and flexographic inks. This is an area in which lack of fastness to overcoating is frequently of no consequence. P.B.15:2, like other α-Copper Phthalocyanine Blue types, is too reddish to be used as a standard cyan for three- or four-colour printing.

3.1.5.5 Pigment Blue 15:3

P.B.15:3, the β-modification of Copper Phthalocyanine Blue, affords a clean shade of turquoise. Pigments of this type are used primarily in graphical printing as well as in finishes and paints, plastics and rubber, textile printing and other areas, such as office articles.

The commercial importance of P.B.15:3 is reflected in the wide variety of available grades, which cover a range of colouristic and application properties. Differences amongst application media are particularly noticeable in aspects such as dispersibility, tinctorial strength, shade and transparency.

The printing inks industry uses P.B.15:3 especially as a blue component on different colour scales for three- and four-colour printing. The pigment corresponds to the CIE 12-66 standard shade of cyan on the European Colour Scale for offset and letterpress prints (Section 1.8.1.1). Also the printed edition of this book is printed with P.B.15:3.

Resinated types are supplied for use in so-called oily binder systems for offset printing inks. However, these are much fewer in number than corresponding P.R.57:1 types, which are used to produce the standard shade of ruby, or hydrazone yellow pigments, which are used for standard yellow.

β-Copper Phthalocyanine Blue is one of the tinctorially strong pigments, although its strength is approximately 15–20% below that of α-Copper Phthalocyanine Blue. At standard conditions, 1/1 SD letterpress proof prints are formulated at 16% of a tinctorially strong type of P.B.15:1, but require approximately 21% β-Copper Phthalocyanine Blue. 1/3 SD prints contain, respectively, 7.5% and 9% pigment. To produce the standard shade of cyan, inks containing 14–15% of a tinctorially strong grade are used.

The prints exhibit excellent application properties. They are, for instance, entirely fast to organic solvents, soap, alkali and acids. They are also fast to sterilization. Metal deco prints demonstrate very good heat stability. The products withstand exposure to 200 °C for 10 min or to 180 °C for 30 min. Although not quite as fast to heat as halogenated types of Copper Phthalocyanine Green, P.B.15:3 is thus somewhat more heat stable than stabilized α-Copper Phthalocyanine Blue.

The fact that P.B.15:3 provides good dispersibility has met with considerable appreciation. This development results from using more economic but less effective dispersion units (such as agitated bead mills) to produce web offset inks today. Excellent solvent fastness renders P.B.15:3 largely insensitive to high processing temperatures, which affect neither the tinctorial strength nor the transparency of the system, the latter being of considerable concern. This is in contrast to the yellow pigments that are employed in three-colour printing and which are accordingly processed at lower temperature. On the other hand, there is some danger that flocculation may occur in systems that are rich in mineral oil, a phenomenon that resembles pigment recrystallization.

In Europe, Copper Phthalocyanine Blue is usually supplied as a powder or as a granulate. The granulated product is somewhat less dusty but also more difficult to disperse. In the USA, P.B.15:3 also continues to be offered in the form of flushed pastes to be incorporated into oil-based printing inks. These pastes offer improved pigment dispersion and frequently afford more glossy and transparent prints.

As in coatings and paints, β-Copper Phthalocyanine Blue tends to flocculate if employed in publication gravure printing inks that contain a high content of aromatics and very little resin. Therefore, nonflocculating P.B.15:4 grades are becoming increasingly important in this area. Sometimes, also used are predispersed, solvent-containing pastes (toluene) or concentrates, nonaqueous dispersions (NADs, Section 1.8.2.2). These systems confer better stability toward flocculation and tinctorial strength on the consequently improved pigment.

P.B.15:3 is usually also offered as a powder, to be used in packaging gravure printing inks. β-Copper Phthalocyanine Blue, like other pigments, is also supplied in the form of nitrocellulose chips or other preparations, for instance on vinyl chloride/vinyl acetate mixed polymer basis, ethyl cellulose or polyvinyl butyral. These pigment preparations are used to advantage where superior transparency, high cleanness of shade and optimized gloss are prime considerations.

Pigment preparations are more commonly used in aqueous/alcohol flexographic printing inks than in other types of inks. Aqueous/alcohol systems present somewhat of a problem, since hydrophobic phthalocyanine pigments are very difficult to adequately wet and disperse. Stabilized α-Copper Phthalocyanine Blue types, which provide a redder shade, dominate throughout the coatings and paints industry. Despite this fact, β-Copper Phthalocyanine Blue types are also useful, although often in flocculation resistant form (see P.B.15:4). They colour industrial paints, architectural paints and emulsion paints. The β-phase, like the stabilized α-crystal modification, is also excellently lightfast and weatherfast in these media.

P.B.15:3, the stable crystal modification of Copper Phthalocyanine Blue, demonstrates excellent heat stability. Most commercial types are therefore entirely suitable candidates for the pigmentation of plastics. β-Copper Phthalocyanine Blue, however, like the α-modification, often presents dispersion problems, especially in polyolefins. As a result of these difficulties, P.B.15:3 is usually employed in the form of pigment preparations that contain predispersed pigment. Moreover, β-Copper Phthalocyanine Blue types, like α-types, tend to nucleate in polyolefins, a problem that may lead to distortion and stress cracking in injection-moulded parts. Good stability to organic solvents, plasticizers and so on, makes β-Copper Phthalocyanine Blue also a useful pigment for plasticized PVC. The resulting pigmented systems are fast to bleeding, although somewhat less so than halogenated Copper Phthalocyanine Green types. The pigment has equally excellent lightfastness and weatherfastness in plastics. Incorporated in rigid PVC, for instance, β-Copper Phthalocyanine Blue is one of the most stable organic pigments known. It is only slightly less fast than halogenated Copper Phthalocyanine Green types. Besides, P.B.15:3 is also found in polystyrene, impact resistant PS types, ABS and similar polymers. Transparent P.B.15:3 systems, like those containing α-Copper Phthalocyanine Blue and Copper Phthalocyanine Green types, are thermally stable up to 300 °C. In very light white reductions (0.01% pigment/0.5% TiO₂), however, the samples only withstand up to 250 °C, while corresponding green types (P.Gr.7) are stable up to 300 °C.

β-Copper Phthalocyanine grades are also supplied as special-purpose types with a limited or defined free copper content, targeted for the colouration of rubber (see Section 3.1.5.3).

P.B.15:3, like stabilized α-Copper Phthalocyanine Blue, markedly affects the hardening of unsaturated polyester cast resins. The list of applications also includes PUR foam materials, office articles, such as coloured pencils, wax crayons and watercolours, as well as spin dyeing of polypropylene, polyacrylonitrile, secondary acetate, polyamide, polyester and viscose. Used in polyester spin dyeing, P.B.15:3 satisfies the thermal requirements of the condensation process (Section 1.8.3.8). 1/3 and 1/25 SD samples equal step 7–8 on the Blue Scale for lightfastness. Textile fastnesses, such as stability to wet and dry crocking, are perfect.

3.1.5.6 Pigment Blue 15:4

Under the designation P.B.15:4, the Colour Index lists β-Copper Phthalocyanine Blue types that are stabilized towards flocculation. These products show largely the same colouristic and fastness properties as P.B.15:3 types, but often exhibit much better rheology. As with stabilized α-Copper Phthalocyanine Blue types, stabilization through surface treatment has proven to decrease the solvent fastness of β-Copper Phthalocyanine Blue, sometimes considerably so, making the pigment more sensitive to aromatics, alcohols, ethylene glycol and ketones.

Stabilized types of β-Copper Phthalocyanine Blue are becoming increasingly important throughout the printing inks and paints field. Improved stability to flocculation and other rub-out phenomena [40] make P.B.15:4 an attractive choice, especially for oven drying paints. The pigments have also gained importance in gravure printing inks, particularly in toluene-based publication gravure printing inks and in various types of flexographic printing inks. They now dominate in these areas. Often, P.B.15:4 types are optimized for use in specific media, such as publication gravure printing inks. In these media the pigment exhibits high tinctorial strength and good flow. Other products have been developed for processing in particular dispersion units.

3.1.5.7 Pigment Blue 15:6

P.B.15:6, which is the ɛ-modification of copper phthalocyanine, affords the reddest blue shade provided by any copper phthalocyanine blue pigment. It has become possible to sufficiently stabilize these pigments towards change of modification to largely eliminate undesirable colouristic effects in application. Section 1.6.5 illustrates the different processes that simultaneously occur during pigment dispersion if an insufficiently stabilized grade of this modification is used. Oven drying paints containing aromatics clearly undergo a colour change towards greener shades as the dispersion time increases. This phenomenon is attributed to a conversion into β-Copper Phthalocyanine Blue. Recrystallization effects, which accompany this phase transition, markedly decrease the tinctorial strength of the system. Very interesting colouristically, the ɛ-modification is used in colour filters for liquid crystal displays (LCDs) [28]. Corresponding reddish shades of blue may also be produced by combining α-Copper Phthalocyanine Blue types with small amounts of Dioxazine Violet (P.V.23). The shade of ɛ-Copper Phthalocyanine Blue demonstrates noticeably higher cleanness than the equally reddish blue shades provided by P.B.60.

In terms of fastness properties, P.B.15:6 performs like other modifications. In baking enamels, stabilized types are completely fast to overcoating, but they are not entirely fast to bleeding in plasticized PVC.

3.1.5.8 Pigment Blue 16

To date, metal-free phthalocyanine blue pigment has little commercial importance. Used to an appreciable extent as a greenish blue pigment until the 1950s, it was later replaced largely by β-Copper Phthalocyanine Blue. Of the different crystal modifications, only the α-form continues to play a certain commercial role. The pigment is supplied as a non-stabilized type and also as a type that is stabilized towards change of modification. Regular grades, like the unstabilized α-Copper Phthalocyanine Blue P.B.15, lose tinctorial strength if they are exposed to certain solvents and undergo a colour shift towards a greener blue. In several systems these types may present problems due to lack of dispersibility and flocculation stability.

P.B.16 is used especially to produce metallic finishes. Incorporated in acrylate resin systems for this purpose, the pigment is more weatherfast than types of Copper Phthalocyanine Blue. P.B.16 also lends colour to plastics, although to a more limited extent. It is not as heat stable as stabilized α- or β-Copper Phthalocyanine Blue types. P.B.16 is also used for artists' paints. The polymorph X of P.B.16 can potentially be used as a photoconductor on drums for laser printers and photocopying machines.

3.1.5.9 Pigment Blue 75

The colouristic properties of this cobalt phthalocyanine blue pigment are comparable with those of Indanthrone Blue, P.B.60. It affords rather dull reddish blue shades, which are noticeably redder and duller than α-copper phthalocyanine blue.

3.1.5.10 Pigment Blue 79

This aluminium phthalocyanine pigment is a copper-free alternate to copper phthalocyanine. Its chemical composition is given as ClAlPc. The pigment affords a greenish blue shade. It is used for printing inks.

3.1.5.11 Pigment Green 7

P.Gr.7 type pigments provide a bluish green shade. The fact that P.Gr.7 only exists in one crystal modification eliminates the problems associated with the possibility of phase transition.

P.Gr.7, like copper phthalocyanine blue pigments, demonstrates good overall fastness properties. Its lightfastness and weatherfastness, heat stability and solvent stability are superior even to those of the corresponding blue types. Therefore P.Gr.7 is broad enough in scope to be found in all areas of pigment application. P.Gr.7 grades, together with the yellower chlorinated/brominated green types of P.Gr.36, dominate wherever green shades are required that cannot be produced by combining α- or especially β-Copper Phthalocyanine Blue with suitable organic yellow pigments. Introducing halogen substituents into the copper phthalocyanine molecule reduces the tinctorial strength of the product, a tendency that becomes more noticeable as the halogen content and thus the molecular weight of the pigment increases.

The main area of application for P.Gr.7 is in paints.

P.Gr.7 is lightfast and durable enough to satisfy almost all requirements. It is almost entirely fast to overcoating. Moreover, the pigment performs very well in terms of other properties that are of interest in this type of application. As with other copper phthalocyanine pigments, problems may occasionally occur in attempting to disperse the pigment in its medium or to stabilize the pigment dispersion to flocculation. Improved versions, however, have been introduced into the market. Untreated green pigments frequently show a greater tendency to flocculate than grades of Copper Phthalocyanine Blue that have not been stabilized to flocculation.

P.Gr.7 is used in all types of paints, including high grade original automotive finishes. Paints containing P.Gr.7 are completely suitable for exterior application. Incorporated in powder coatings, the pigment performs better than β-Copper Phthalocyanine Blue. Plate-out is not observed (Section 1.6.4.1).

The printing ink industry utilizes P.Gr.7 particularly for packaging printing inks. In this area, however, green shades are frequently also produced by combining the less expensive β-Copper Phthalocyanine Blue with suitable organic yellow pigments.

Prints made of P.Gr.7 are tinctorially weaker than those containing Copper Phthalocyanine Blue types. At standard conditions, for instance, 1/3 SD letterpress proof prints are made of inks formulated at approximately 17% pigment. Only approx. 8–9% β-Copper Phthalocyanine Blue is needed for the same purpose. Phthalocyanine Green demonstrates excellent overall fastness properties in application. Metal deco prints are thermally stable up to 220 °C for 10 min, they are fast to clear lacquer coating and may safely be sterilized. Incorporated in special gravure printing inks, some types of P.Gr.7 may flocculate, a tendency that is also observed in paints and which results in loss of tinctorial strength, reduced gloss and other problems. In contrast to the different types of Copper Phthalocyanine Blue, P.Gr.7 is suited to use in decorative printing for laminated plastic sheets based on polyester. Melamine resin systems, on the other hand, render the pigment phototropic in daylight, a phenomenon that is in contrast to the behaviour of Copper Phthalocyanine Blue. Chemical compounds that undergo a reversible shade change by exposure to light are referred to as being phototropic. The occurrence of phototropicity in a material is limited to a particular portion of the spectrum and may be reversed by irradiating the sample with a source that emits different radiation, usually in the more long-wave region. Irradiating a decorative P.Gr.7 melamine resin-based print with a xenon arc lamp rapidly destroys the pigment, resulting in a dull shade. The lightfastness of such samples equals step 5 on the Blue Scale.

In plastics, phthalocyanine green pigments are also tinctorially much weaker than corresponding blue pigments. More than 0.2% Pigment Green 7, for instance, is required to produce 1/3 SD HDPE samples (1% TiO₂). Less than 0.1% β-Copper Phthalocyanine Blue and approximately 0.08% α-Copper Phthalocyanine Blue are needed for the same purpose. The pigment withstands more than 300 °C. Some types of P.Gr.7 influence the shrinkage of HDPE and other partially crystalline thermoplastics at elevated processing temperature much less than do Copper Phthalocyanine Blue types or brominated derivatives. This is an asset in the pigmentation of larger injection-moulded parts.

P.Gr.7 is also difficult to disperse, especially in polyolefins. Easily dispersible products, however, are available. In plasticized PVC, as in other polymers, the pigment is completely fast to bleeding and exhibits excellent lightfastness and durability. P.Gr.7 is one of the most durable organic pigments in rigid PVC and may even be considered for long-term exposure. It is equally suitable for polystyrene, impact resistant polystyrene, and ABS. Its heat stability in polystyrene, even in white reduction, is up to 300 °C, while blue grades only withstand up to 240 °C. Incorporated in unsaturated polyester-based cast resins, P.Gr.7, in contrast to Copper Phthalocyanine Blue types, does not affect the hardening of the medium.

Grades with a limited free copper content are supplied for the pigmentation of rubber. These pigments do not disturb caoutchouc vulcanization and do not affect the resistance of rubber to ageing.

Utilized in spin dyeing, P.Gr.7 lends colour to all types of commercially important fibres. The products demonstrate excellent lightfastness and weatherfastness. Used in polyacrylonitrile, for instance, P.Gr.7 satisfies the stringent requirements for use in outdoor textiles such as canvasses. Its textile fastness properties are almost, if not completely, satisfactory. This textiles field is another area in which Copper Phthalocyanine Blue types are more than twice as strong as P.Gr.7.

Moreover, P.Gr.7 is also found in various applications outside the previously discussed areas, for instance in surface covering colourations for leather and in furniture stains.

3.1.5.12 Pigment Green 36

Pigments of this type provide very yellowish shades of green. They are much yellower than P.Gr.7 types. The colour of P.Gr.36 becomes yellower as more chlorine atoms are replaced by bromine. In the USA, these products are therefore classified according to the yellowness of shade and the bromine content. The bromine content may range from 25–30 wt% for less yellowish to 50–53 wt% for considerably yellower varieties. Again, only one crystal modification has been found.

All types of P.Gr.36 provide excellent lightfastness and weatherfastness, heat stability and solvent fastness. Their main area of application is in paints. The pigment covers the entire range of applications, including various types of high grade automotive finishes. P.Gr.36 types are weaker and more expensive than P.Gr.7 or Copper Phthalocyanine Blue. Yellowish shades of green are thus also produced by combining more inexpensive chlorinated copper phthalocyanine green pigments with suitable organic yellow pigments, although some of the excellent fastness properties are lost in the process.

The list of applications also includes printing inks, in which P.Gr.36 is tinctorially weaker than its copper phthalocyanine counterparts. For comparison, 1/3 SD letterpress proof prints containing P.Gr.36, printed at standardized film thickness, require approximately 26% pigment, while only 17% of P.Gr.7 suffice for the same purpose. The fastness properties exhibited by P.Gr.36 in prints, as in paints, closely resemble those of P.Gr.7, including the phototropic behaviour of decorative printing inks for melamine-based laminated plastics sheets.

More than 0.3% pigment is required to formulate 1/3 SD HDPE samples. P.Gr.36 is thus much weaker than P.Gr.7 types, although it withstands more than 300 °C and is thus equally heat stable. Considerable influence on the shrinkage of injection-moulded polyethylene parts necessitates some caution in using P.Gr.36 for this purpose. Many types are difficult to disperse in plastics, especially in polyolefins.

3.2.1.1 Thermal Ring Closure

The oldest known industrially used synthesis is carried out in a solvent and involves several intermediate steps. It is not necessary to isolate the intermediates, and the reaction can proceed in one vessel [44].

The starting material is succinic dialkyl ester^*, which is easily obtained from maleic anhydride. Cyclization is accomplished with sodium alcoholate in a high boiling solvent or solvent mixture (such as diphenyl ether/diphenyl) to afford the succinylosuccinic dialkyl ester 35:

A newer synthetic pathway, found by Lonza, involves chlorinating diketene to form the γ-chloroacetoacetic ethyl ester, followed by condensation to afford the succinylosuccinic diethyl ester [45].

The typical quinacridone synthesis may be exemplified by the manufacture of unsubstituted quinacridone.

Succinylosuccinic dialkyl ester is treated with two equivalents of aniline to yield the 2,5-dianilino-3,6-dihydroterephthalic dialkyl ester 36. Without separating the intermediate, the ring is closed thermally (at 250 °C) in the same reaction medium to afford the α-modification of dihydroquinacridone 37 [44, 46]:

According to the patent literature, this synthesis proceeds in one vessel. The yield is approximately 75% relative to the starting material succinic dialkyl ester.

The condensation reaction with aniline may also be accomplished in two separate steps. It is possible to separate succinylosuccinic ester, followed by condensation in boiling ethanol in the presence of an acid (acetic acid, hydrochloric acid, phosphoric acid).

Oxidation of dihydroquinacridone to quinacridone may be achieved, for instance, with the sodium salt of m-nitrobenzene sulfonic acid in aqueous ethanol in the presence of sodium hydroxide solution [47]. A distinction is made between heterogeneous and homogeneous oxidation. The reaction is referred to as a ‘solid state oxidation’ if the solvent contains approximately 2% sodium hydroxide solution. A content of approximately 30% sodium hydroxide solution relative to the solvent mixture, on the other hand, converts the reaction into a so-called ‘solution oxidation’. The type of ring closure defines the crystal modification of the resulting dihydroquinacridone, while the oxidation technique defines the crystal phase of the quinacridone pigment.

Solid state oxidation, both of the α- and the β-phase [48] of dihydroquinacridone, affords crude α-quinacridone. Subsequent milling with salt in the presence of dimethylformamide produces the γ-modification, while the β-form evolves in the presence of xylene. Solution oxidation of dihydroquinacridone, possibly performed as air oxidation in the presence of 2-chloroanthraquinone [49], forms crude β-quinacridone. Milling with xylene likewise affords β-quinacridone pigment.

Other patents describing variations of this method have been issued to Du Pont. However, no new insight has yet been gained from these publications of the described processes.

3.2.1.2 Acidic Ring Closure

Acidic ring closure resembles the Du Pont process in that the central benzene ring of the quinacridone structure is synthesized totally. The process bears resemblance to the Liebermann method [42] (see also References [41, 43]). Condensation of succinylosuccinic ester with two equivalents of arylamine affords 2,5-diarylamino-3,6-dihydroterephthalic diester. Subsequent oxidation with suitable agents yields 2,5-diarylaminoterephthalic diester (38). Hydrolysis and cyclization in polyphosphoric acid or other acidic condensation agents produces crude quinacridone [50]. This product already consists of very small particles.

Performing the last step (thermal aftertreatment) in an appropriate solvent provides a simple and convenient method of producing the particular target crystal modification:

The β-crystal modification is prepared by pretreating the crude quinacridone product with alkali base before finishing with the solvent. Immediate solvent treatment, on the other hand, produces the γ-modification of the quinacridone pigment.

The β- and γ-modifications can be directly obtained from the synthesis, if the hydrolysis of the polyphosphoric acid mixture is performed at 135–165 °C with a controlled amount of water or orthophosphoric acid [51, 52].

3.2.1.3 Dihalo-terephthalic Acid Process

Amongst syntheses that start from preformed aromatic systems, the Sandoz process, described here, in particular has stimulated interest. It is the only route that allows the manufacture of asymmetrically substituted quinacridones without obtaining a statistical mixture of symmetrical and asymmetrical molecules. 2,5-dibromo-1,4-xylene or its 2,5-dichloro derivative is obtained by bromination or, correspondingly, chlorination of 1,4-xylene. It is oxidized to form 2,5-dibromoterephthalic acid or its dichloro derivative 40. Subsequent reaction with arylamine, for instance in the presence of copper acetate, affords 2,5-diarylaminoterephthalic acid (41). It is also possible to replace the halogen atoms stepwise by arylamino moieties [53]. Cyclization to form linear trans-quinacridones, as in the above-mentioned method, is achieved by using acidic condensation agents:

Although the synthesis is completed in very few steps, oxidation of 1,4-xylene to the corresponding terephthalic acid does not afford a uniform product. Partial dihalogenation gives rise to side products. The condensation reaction requires two equivalents of arylamine per halogen atom. One equivalent is needed to neutralize the generated hydrohalogen acid, which is subsequently separated as arylamine-hydrohalide and recycled as hydrohalide and arylamine.

3.2.1.4 Hydroquinone Process

The hydroquinone process was developed by BASF [54]. Hydroquinone-2,5-dicarboxylic acid is prepared by a modified Kolbe–Schmidt synthesis from hydroquinone and carbon dioxide. Subsequent reaction with arylamine in an aqueous-methanolic suspension in the presence of an aqueous sodium chlorate solution and a vanadium salt affords the product in good yield:

Subsequent ring closure of the 2,5-diarylamino-1,4-benzoquinone-3,6-dicarboxylic acid (42) is performed in concentrated sulfuric acid (or with thionyl chloride/nitrobenzene) to afford the linear trans-quinacridone quinone 43:

Compound 43 is also a component in a solid solution (mixed-crystal) with quinacridones, types of which are commercially available (Section 3.2.4). Quinacridone quinone is reduced with zinc or aluminium powder in dilute sodium hydroxide solution, in an aluminium chloride/urea melt or in sulfuric/phosphoric acid under pressure to form quinacridone. There are certain disadvantages to this method in view of the fact that hydroquinone is relatively expensive, quinacridone is not easily reduced, and ecological problems prevail (wastewater pollution).

To summarize this discussion, the two latter methods have failed to gain the commercial recognition enjoyed by the first two processes (Sections 3.2.1.1 and 3.2.1.2).

3.2.1.5 Substituted Quinacridones

Certain disubstituted quinacridones are also used commercially. The two peripheral rings in such quinacridone systems (34) are partially chlorinated or methylated. In the sequence 38 → 39 → 34 (Section 3.2.1.2), possible substitution sites on the peripheral rings are indicated by dashes.

Succinylosuccinic diester is cyclized according to the common route by using chlorinated or methylated aniline. Starting from anilines, which are substituted in 2- or 4-position, the reaction affords the symmetrical 4,11- or 2,9-disubstitution products. Reaction with 3-substituted anilines, on the other hand, produces a mixture of both of the symmetrical 1,8- and 3,10-disubstitution products, as well as unsymmetrical 1,10-disubstituted compound. The following chart illustrates this point:

3.2.1.6 Quinacridone Quinone

Quinacridones are not the only industrially significant products. The list may be extended to include the derivative mentioned in Section 3.2.1.4, the linear transquinacridone quinone 43. There are two other synthetic pathways besides the hydroquinone method. The older method involves cyclization of the 2,5-bis-(2-carboxyanilino-)-1,4-benzoquinone (44) with concentrated sulfuric acid or polyphosphoric acid at 150–200 °C. The starting material 44 is obtained through condensation of 1,4-benzoquinone with anthranilic acid:

A more recent synthesis proceeds via 2,5-diarylamino-3,6-dicarbethoxy-1,4-hydroquinone (46), which is obtained by reducing 2,5-diarylamino-3,6-dicarbethoxy-1,4-benzoquinone (45) with sodium hydrogen sulfite or sodium in ethanol:

The hydroquinone 46 is cyclized in an organic solvent at 230–270 °C to yield 6,13-dihydroxy-quinacridone 47. Oxidation with suitable agents (such as nitrobenzene, chromic acid, nitric acid) provides the linear trans-quinacridone quinone (43):

3.2.1.7 Polymorphism

Most quinacridone pigments are polymorphic, that is, they exhibit multiple crystal modifications with different colours and different fastness and application properties. The individual polymorphs are identified by X-ray powder diffraction. The formation of the different polymorphs depends mainly on the procedures used for synthesis and finishing. This effect may be illustrated on unsubstituted quinacridone.

Most synthetic methods afford crude α-quinacridone [55]. This modification, however, has not been commercialized because of its lack of fastness properties. α-Quinacridone may be converted into the β- or γ-phase by organic solvents, especially at elevated temperature. Moreover, the α-phase is much less lightfast and weatherfast than its β- and γ-counterparts. It is by employing suitable solvents, therefore, that the β- and γ-phases are commonly obtained. The list of options includes milling the crude pigment with salt in the presence of a solvent, by first dissolving and then precipitating crude quinacridone, or by heating the crude product. Grinding with salt in a ball mill in the presence of xylene or o-dichlorobenzene, for instance, yields the β-modification [56]. The more stable γ-form [57] evolves in the presence of DMF [58].

The β-form is obtained by dissolving crude quinacridone in any one of various solvents (such as concentrated sulfuric acid/toluene or methylated sulfuric acid), followed by precipitation with water. The same end is achieved by dissolving the product in polyphosphoric acid, followed by rapid precipitation with ethanol at 45 °C. Depending on the process, the β-product, however, may contain some α-crystal modification as well.

The γ-modification is produced by heating crude quinacridone in ethanol (under pressure) or in dimethylformamide or dimethyl sulfoxide at 150 °C.

Non-polar solvents generally aid the formation of the β-phase, while polar solvents assist in producing the γ-modification.

In the patent literature many further phases of quinacridone have been described, including the phases B_I [59], γ′ [60], γ^I [61], γ^II [62], γ^III [63], γ^IV [64], δ [65], Δ [66], ɛ [67], another ɛ-phase [66] and at least one ζ-phase. All phases were characterized by X-ray powder diffraction. However, X-ray powder diagrams depend not only on the polymorphic form, but also on the crystal sizes, on the crystal quality and on impurities (which may cause additional lines, or distort the lattice and result in modified reflection positions and intensities). Additionally, the diffractometer plays a role. (Most older X-ray powder diagrams have been measured in a reflection mode, which frequently causes incorrect intensities due to preferred orientation effects.) Closer inspection [68] reveals that B_I and β actually describe the same phase (β-phase). Similarly, all γ-phases correspond crystallographically to the same polymorph (γ-phase). The δ-phases correspond either to the γ-phase or they consist of a mixture of γ with a trace of β-phase. Additionally the Δ-phase is the same polymorph as γ. The two ɛ-phases, which are claimed to be clearly different from each other, are actually both the γ-phase. The ζ-phase appears to be a mixture of at least three different phases (α, β and γ). One could come to the conclusion that quinacridone exhibits only the three polymorphic forms α, β, and γ, which were already described by Struve in 1958 [55, 56, 58]. However, quinacridone in fact exists in four polymorphs. From the X-ray powder data it is evident that the crude quinacridone, which is usually described as the ‘α-phase’, does not represent a uniform polymorph, but two different polymorphs (or a mixture of them), depending on the synthesis. These two polymorphs, called α^I and α^II, differ in their X-ray powder diagrams and in their colours (Figure 3.12). The α^I-phase corresponds to the phase described by Struve [55] and Labana & Labana [41]. This phase can be obtained by synthesis [55], by milling with NaCl or by sublimation at 140–170 °C [69]. The α^II-phase emerges from synthesis under different conditions, or from recrystallization in concentrated sulfuric acid followed by treatment in 3-nitrotoluene at 70–80 °C for 4 weeks [70]. According to Jaffe, the ‘α-phase’ (apparently meaning the α^I-phase) offers similar colour attributes as the γ-phase [71], whereas Schmidt describes the α^I-phase as a dull reddish-violet powder [68]. The α^II-phase is red (Figure 3.12).

The α^I- and α^II-phases are less stable than the β- and γ-phases, which impedes their use as pigments. Both α-phases are stable as powders at room temperature at day-light for at least 20 years.

3.2.1.8 Crystal Structures

The crystal structure of the β-phase of unsubstituted quinacridone was determined several times by different authors using single-crystal X-ray diffraction [71–74]. The same is valid for the γ-phase [68, 72, 73, 75–79].

For the α^I and α^II-phases, no single crystals could be grown. The crystal structure of α^I-quinacridone was determined from unindexed X-ray powder data by Leusen and Paulus in 1994 [68, 80, 81]. In the first step Leusen predicted the possible crystal structures of quinacridone by global lattice-energy minimizations in various space groups with free lattice parameters. Besides the structures of the β- and γ-phases they found a structure that had a simulated powder diagram similar to the experimental powder diagram of the α^I-phase. Subsequent Rietveld refinement by Paulus resulted in the crystal structure of the α^I-phase. This was probably the first instance in which an organic crystal structure has ever been solved by a full crystal structure prediction. The structure was independently confirmed by Ogawa using electron diffraction (Figure 3.13) [68, 69].

The crystral structure of α^II-quinacridone was determined in 2015 by T. Gorelik by means of electron diffraction, using crystals with a thickness of 10 nm only [70]. The diffraction pattern showed strong diffuse streaks through the reflections. Nevertheless Gorelik could solve and refine the structure from the electron diffraction data. The structure was confirmed by lattice-energy optimization with dispersion-corrected density-functional theory calculations (DFT-D).

In all structures of unsubstituted quinacridone (and most structures of substituted quinacridones) the molecules form four COHN hydrogen bonds (two as donor, two as acceptor). In the phases α^I, α^II and β the molecules are connected to two neighbouring molecules by two hydrogen bonds each, leading to chains (Figures 3.14a and 3.15a). These chains are stacked on top of each other leading to two-dimensional sheets. The chains themselves are not fully planar, but exhibit small steps between the molecules (Figure 3.15b). In the β-phase the steps have a height of 0.35 Å. Lattice-energy minimizations show that the formation of these steps allows a slightly denser packing of molecules than for exactly planar chains. In the α^I-phase all chains are parallel (Figure 3.14b). The structure of the α^II is disordered and exhibits a statistical sequence of herringbone arrangements and parallel arrangements of neighbouring chains, with the herringbone arrangement being more frequent (Figure 3.14c). This disorder causes the observed diffuse scattering. The disorder is also the reason, why the structure of the α^II-phase could not be found in the crystal structure prediction in 1994.

In the β-phase of quinacridone, chains of neighbouring sheets form an angle of 69° (Figure 3.15a,c). This feature is unique for the β-phase; it has not been observed for any other quinacridone pigment, and is rare for other chain structures as well. In the chain direction the molecules interact through hydrogen bonds, in the stacking direction through van der Waals and Coulomb forces, and in the third direction through weak van der Waals interactions between carbon and hydrogen atoms only.

In the γ-phase of unsubstituted quinacridone each molecule is connected to four neighbouring molecules (Figure 3.16a), which leads to a hunter's fence (criss-cross) pattern (Figure 3.16b).

The strong intermolecular hydrogen bonds between the CO and the NH-groups combined with the van der Waals and Coulomb forces lead to very stable crystal structures with high lattice energies. This all together causes the very good thermal stability and solvent resistance of these pigments. The solutions, in contrast to the pigment itself, are only poorly lightfast.

In diluted solutions, for example in DMSO (dimethyl sulfoxide) at 180 °C, quinacridone shows a pale yellow to pale orange colour. Quantum-mechanical calculations on an isolated molecule point to a yellow or orange shade as well. The red or violet colour of quinacridone pigments is a solid state effect. The shift from the yellow–orange colour of an isolated molecule to the red or violet shades of quinacridone in the solid state and the colour differences between the individual polymorphs are apparently caused by two effects:

1. The formation of the hydrogen bonds causes a weakening of the CO and the NH bonds and an increase of the conjugation between the aromatic rings. In the isolated quinacridone molecule the π-systems of the three benzene rings are only weakly conjugated by the NH and CO bridges; the resulting yellow colour resembles the colour of other substituted benzene compounds (e.g. nitrobenzene). In the solid state the improved conjugation leads to a strong bathochromic shift (Figure 3.17). The different hydrogen bond patterns probably result in different values of the bathochromic shift.
2. The colour is affected by exciton interactions (see Section 1.5.3.1) between an excited molecule and all surrounding molecules, including those not connected by hydrogen bonds. The two molecules above and below the excited molecule play a major role. Depending on the polymorphic form, the position of these molecules varies (Figure 3.18). Correspondingly the colours vary.

From Figures 3.14 and 3.15 it is evident that in the chain structures there is no space for any substitution in the positions 1, 4, 8 or 11, that is, in ortho-positions to the NH and CO groups. Any substituent (except hydrogen) in one of these positions would cause negative steric interaction with the neighbouring molecule and impede the formation of a chain structure. The hunter's fence structure of γ-quinacridone tolerates substituents (e.g. Cl) in the positions 1, 4, 8 or 11, if the substituent is not too large. With increasing size of the substituent (H < F < Cl < CH₃) the hydrogen bonds are distorted and weakened; correspondingly the lattice energy is reduced and the compounds become increasingly soluble and the lightfastness drops. This explains the orange–red colour and the reduced lightfastness of 4,11-dichloroquinacridone. Even larger substituents in the 4- and 11-positions, for example, tertiary butyl groups, impede the formation of hydrogen bonds even in a hunter's fence arrangement, and the compounds become soluble.

The same effect is observed with substituents on the NH group; for example, N,N′-dimethylquinacridone is the least lightfast and dissolves even in ethanol.

Molecules with substituents in the positions 2, 3, 9 and 10 could, in principle, either form chain structures such as α^I-quinacridone or β-quinacridone, or form a hunter's fence structure such as γ-quinacridone. Both cases are observed for 2,9-dimethylquinacridone: The commercial phase exhibits a chain structure such as α^I-quinacridone (Figure 3.19) [72, 82, 83], but in addition there exists a second phase of 2,9-dimethylquinacridone, which is described as isostructural to γ-quinacridone [84].

The thermodynamically stable phase of 2,9-dichloroquinacridone exhibits a chain structure (Figure 3.20) [72, 85]. The metastable black δ-phase, which is obtained only from sublimation, exhibits a strange structure without any hydrogen bonds [86].

Pure 3,10-dichloroquinacridone also crystallizes in a chain structure (Figure 3.21) [87]. The structure of the commercial product P.R.209, which contains a mixture of the 3,10-, 1,8- and 1,10-isomers, has not been determined yet.

Crystal structure data of quinacridone quinone pigments have not yet been published.

3.2.1.9 Solid Solutions

Quinacridones tend to form solid solutions with other quinacridone compounds or with quinacridone quinone. In a solid solution, also called ‘mixed crystal phase’, different molecules adopt a common crystal lattice; on each position within the lattice either a molecule of the one kind or of the other kind is situated, generally with a statistical distribution. Ternary solid solutions exist as well, for example, formed by 2,9-dimethyl-, 2-methyl- and unsubstituted quinacridones. The formation of solid solutions allows the stabilization of less stable polymorphic forms, or the tuning of the molecular packing and thereby of the properties (Section 3.2.4). Solid solutions are identified by their X-ray powder pattern, which is different from the powder pattern of a physical mixture of the individual compounds. Solid solutions can, like the pure pigments, be polymorphic.

3.2.4.1 General

Although quinacridone pigments were first prepared in 1935, their technical value initially went unrecognized. It was only when their polymorphism was detected that quinacridones gained importance. The systematic synthesis of different crystal modifications by appropriate aftertreatment of crude quinacridone made it possible to obtain various highly stable pigments, which rapidly gained commercial interest as a new class of pigments.

A considerable number of mono- to tetra-substituted quinacridones has been described so far. A publication that only includes the literature up to the year 1965 already lists more than 120 compounds, and many exhibit more than one crystal modification [88]. However, this diversity of compounds does not reflect commercial variety. Very few species have been offered, and even less command a major share of the pigments market.

Heading the list are two crystal modifications of Pigment Violet 19 (the parent compound of the quinacridone series). Both the red-violet β- and the red γ-modification are encountered in the market, while the pure α-forms are not lightfast and durable enough to have any commercial value. However, traces of the α-modifications are found in various types.

By addition of 0.1–10% of 2,9-dichloroquinacridone to β-quinacridone as well as to γ-quinacridone during the finishing process, 2,9-dichloroquinacridone acts as an inhibitor for the α-phase. Without α-quinacridone the colour of the β-quinacridone shifts to a bluer violet shade and the colour of the γ-quinacridone to a bluer red shade.

Optionally, 4,11-dichloroquinacridone can be added together with 2,9-dichloroquinacridone to the finishing processes of β- or γ-quinacridone. The incorporation of 4,11-dichloroquinacridone with the 2,9-dichloroquinacridone not only inhibits the formation of the α-phase, but the formed milled products show also increased heat stability properties [89].

Of the substituted quinacridone systems, it is primarily the 2,9-dimethyl derivative (Pigment Red 122) which enjoys appreciation for its excellent fastness properties and its pure bluish red (magenta) shade. In addition, the list also includes 2,9-dichloro-, 3,10-dichloro-, 4,11-dichloro- and 4,11-dimethyl-quinacridones. Some of these are available as solid solutions with other quinacridone derivatives [91].

Other solid solutions are made from quinacridone quinone with unsubstituted quinacridone or 4,11-dichloroquinacridone as a second component.

Quinacridone quinone itself (Section 3.2.1.6) is a tinctorially relatively weak yellow compound with poor lightfastness. Formation of solid solutions with other quinacridones (Section 3.2.1.9) as well as treatment with various metal salts [92] leads to an improvement of light- and weatherfastness. Oxidation of dihydroquinacridone with less than molar amounts of chromate affords a quinacridone/quinacridone quinone solid solution that offers an interesting shade of gold.

Table 3.2 lists representative commercial quinacridone pigments. Each specimen is offered in only one crystal modification. Unsubstituted quinacridone is somewhat of an exception in that two crystal phases are commercially available.

C.I. Name	C.I. Constitution Number	R2	R3	R4	R9	R10	R11	Range of shades	Total number of known modifications
Quinacridone pigments
P.V.19	73900	H	H	H	H	H	H	red-violet (β-phase), bluish red (γ-phase)	4
P.R.122	73915	CH3a)	H	H	CH3	H	H	bluish red (magenta)	4
P.R.192	—	CH3	H	H	H	H	H	bluish red	1 [51]
P.R.202	73907	Cl	H	H	Cl	H	H	bluish red to violet	1 (2,9-dichloro: 4)
		solution with unsubstituted and monochloroquinacridone
P.R.207	73906/	H	H	Cl	H	H	Cl	yellowish red	1 (4,11-dichloro:4)
	73900	solid solution with unsubstituted quinacridone
P.R.209	73905	H	Cl	H	H	Cl	H	red	1
		mixed with 1,8- and 1,10-dichloroquinacridone
P.R.282	—	solid solution of 2,9-dimethylquinacridone with 2-methylquinacridone and unsubstituted quinacridone						magenta	—
Quinacridone quinone pigments (Quinacridone quinone, C.I. 73920)
P.R.206	73900/	H	H	H	H	H	H	maroon (golden yellow)	1
	73920	solid solution with quinacridone quinone
—	—	H	H	Cl	H	H	Cl	scarlet	1 (4,11-dichloro:4)
		solid solution with quinacridone quinone
P.O.48	—	solid solution of unsubstituted quinacridone + quinacridone quinone						maroon (golden yellow)	1
P.O.49	—	Unsubstituted quinacridone						maroon (golden yellow)	1
		+ quinacridone quinone
P.V.42	—	—						maroon	1
P.V.55	—	solid solution of 2,9-dimethoxyquinacridone and 2,9-dichloroquinacridone						bluish violet	1
a Also available as a solid solution with unsubstituted quinacridone.

3.2.4.2 Pigment Violet 19, β-Modification

The β-modification affords a reddish violet shade. These pigments are frequently used in combination with inorganic pigments, especially with iron oxide, or to a decreasing extent with Molybdate Red pigments, to colour industrial paints. Such pigment combinations provide comparatively dull shades of red to bordeaux. The excellent hiding power of the resulting systems is attributed to the content of highly scattering inorganic pigments with higher refractive indices. Combinations with opaque organic pigments, especially the P.O.36 type, are equally common. The products are used mostly in original automotive and automotive refinishes. The shade of the β-modification of P.V.19 is close to that of P.R.88, a tetrachlorothioindigo pigment. The tinctorial strength of the reddish violet quinacridone grades, however, is considerably higher in combination with these inorganic pigments, especially the Molybdate Red types. They also produce cleaner shades than thioindigo pigments. Quinacridone pigments, despite their higher price, have the advantage of being more economical in use. Moreover, the β-modification of P.V.19 is also more lightfast and weatherfast than its thioindigo counterparts. Thioindigo pigments and quinacridone pigments, however, demonstrate different metameric effects (also in combination with inorganic pigments) and may thus not be exchanged directly. The higher tinctorial strength of quinacridone pigments is less noticeable in white reductions, but the pigment is appreciably more lightfast and weatherfast. Full shades tend to darken. The β-modification of P.V.19, like other quinacridone pigments, shows good resistance to the common organic solvents. Incorporated in baked enamels, the pigment is correspondingly fast to overcoating. The systems show excellent fastness up to 140 °C, while some bleeding is observed at 160 °C. In this respect, the β-crystal modification of P.V.19 is inferior to the γ-form. Paints made of β-P.V.19 are fast to acid and alkali.

Like several other quinacridone pigments, P.V.19 types of the β-modification are prone to cause problems, especially through pigment flocculation, while being dispersed in paint systems.

The different grades exhibit a wide range of particle size distributions. As with other pigments, a decreasing average particle size is accompanied by enhanced transparency in full shade, increased tinctorial strength in white reduction and a general colour shift towards more intense reddish violet shades. There is no noticeable change in lightfastness and weatherfastness if the particle size is changed. The transparent types are also suited to application in metallic finishes. In addition, they lend colour to various other media throughout the paint industry wherever high lightfastness and weatherfastness, chemical fastness or heat stability are required. The list includes coil coating and various types of powder coatings, although it is recommended to adjust the pigment to the hardener before it is applied in a powder coating. β-P.V.19 does not show plate-out in most powder coatings. The β-modification of P.V.19 is also used as a colourant for plastics. In the colouration of PVC and PUR coatings, β-P.V.19 is also often used in combination with inorganic iron oxide and Molybdate Red pigments. The pigment is especially suitable for applications that require high hiding power as well as good lightfastness and weatherfastness, such as for automobile interiors, purses, and canvasses for tarps, awnings and so on. Importantly, β-P.V.19 is more lightfast and weatherfast in combination with Iron Oxide or Molybdate Red pigments than in combination with TiO₂.

The β-modification of P.V.19 is very fast to bleeding in plasticized PVC. Its lightfastness in this medium satisfies the requirements for long-term weathering, as long as the demands are not exceptionally high. The pigment is thermally stable in polyolefins, it withstands exposure up to 300 °C. Its tinctorial strength in polyolefins, like in other media, is average. Approximately 0.2% pigment is needed to formulate 1/3 SD HDPE samples containing 1% TiO₂, if a tinctorially strong type with a high specific surface area is used. However, the pigment noticeably affects the shrinkage of PE and other partially crystalline polymers. High heat stability makes β-P.V.19 a suitable colourant for polystyrene and other industrial synthetics. Likewise, β-P.V.19 is also used in spin dyeing, for instance of PP. The printing ink industry utilizes β-P.V.19 only where high lightfastness or other special fastness properties are required. It is found, for instance, in inks for printing on PVC and for posters.

Metal deco prints containing β-P.V.19 withstand up to 180 °C for 10 min and 170 °C for 30 min, respectively. The prints are fast to sterilization and calendering and to several special-purpose media, such as particular organic solvents.

3.2.4.3 Pigment Violet 19, γ-Modification

The γ-crystal modification of P.V.19 affords bluish red shades that are much yellower than those obtained by the β-modification. Commercial types of γ-P.V.19 show a wide range of particle size distributions. The specific surface areas vary accordingly; they range from approximately 30 to 70 m² g⁻¹.

The particle size of γ-P.V.19 has more of an effect on the lightfastness and weatherfastness of the resulting pigmented system than can be said of the β-modification. The grades of γ-P.V.19 with finer particle sizes, although less lightfast and weatherfast, have the advantage of being more transparent, tinctorially stronger and bluer than varieties with coarser particle sizes.

High lightfastness and durability makes γ-P.V.19 a useful product for various media throughout the coatings and paints industry, including high grade industrial paints and automotive finishes. γ-P.V.19 is used both in solid shades and as a shading pigment. Applied in combination with Molybdate Red, γ-P.V.19 affords comparatively brilliant, opaque shades, which are not accessible by using other quinacridone pigments. Moreover, γ-P.V.19 is also utilized in combination with other opaque pigments. Full shades and similarly deep shades darken slightly as they are exposed to light or weather. On the whole, γ-P.V.19 grades with fine grains are less weatherfast than the β-crystal modification and also various other types, such as P.R.122. The products of γ-P.V.19 with coarser particle sizes, however, frequently perform better than these quinacridone pigments. Excellent weatherfastness makes γ-P.V.19 a suitable candidate for exterior house paints. Its use in these areas, however, is somewhat limited by the fact that long-term exposure to alkali, for instance on freshly applied plaster, may considerably reduce the lightfastness and weatherfastness of the product as well as its tinctorial strength.

The stronger and more economical types with fine particle sizes are also found in transparent paints. If it is to be used in two-coat metallics, however, the application of the pigment must be combined with UV absorbants in topcoats to render it weatherfast enough to be applied in original automotive finishes.

The printing ink industry mostly prefers the type with finer particle sizes. These are incorporated into high grade printing inks for metal deco and poster printing. Combination with 2,9-dimethylquinacridone (P.R.122) affords a bluish shade of red, which closely approaches the standard magenta for three- and four-colour printing. Compared to other types of pigments covering the same portion of the spectrum, γ-P.V.19 types lack tinctorial strength. Inks formulated at almost 20% pigment, for instance, are needed to produce 1/3 SD letterpress proof prints at standard conditions. The γ-crystal modification is thus utilized primarily in areas where other pigments are not fast enough to satisfy the use requirements. In terms of lightfastness, 1/3 SD letterpress proof prints equal step 6–7 on the Blue Scale. The prints are thermally stable up to 190 °C for 10 min or to 170 °C for 30 min. They are fast to sterilization and calendering. Excellent fastness properties make γ-P.V.19 an equally suitable pigment for decorative printing inks for melamine and polyester based laminated plastic sheets. The samples equal step 8 on the Blue Scale for lightfastness.

γ-P.V.19 is frequently used for its high heat stability to replace more expensive bluish Cadmium Red pigments (cadmium selenide/cadmium sulfide mixed crystals) in lightfast thermoplastic systems; 0.2% pigment is required to formulate 1/3 SD HDPE samples containing 1% TiO₂. Some types exhibit a very moderate tendency to nucleate, which is why there is no limitation to pigment use in HDPE.

γ-P.V.19 as a colourant for plastics is available both as a powder and as a pigment preparation containing predispersed pigment. The list of applications includes PVC films, technical articles and toys made of PVC or PO, as well as spin dyeing products of PP fibres and PP monofilaments. Employed in polyester spin dyeing, the pigment is heat stable enough to be used in the condensation process at 240–290 °C for 5–6 h. A certain colour change at low concentrations is attributed to the fact that γ-P.V.19 dissolves in polyester, a feature that makes it unsuitable as a shading pigment. Like in other media, the pigment is often incorporated in polymers together with grades of Molybdate Red.

The γ-form of P.V.19 is also applicable in injection-moulded and extrusion-made polyamide. It satisfies not only the high thermal requirements in connection with these purposes but has the added advantage of being, like P.R.122 and 209, chemically inert to the slightly alkaline and reducing plastic melt.

Moreover, γ-P.V.19 is also found in various other media, such as powder coatings and cast resins. This includes systems based on unsaturated polyester resins whose hardening is not affected by the pigment. The list of application media includes plastics that are processed at very high temperature (such as polycarbonates), in which the pigment is thermally stable up to 320 °C. PUR foams and polyacetals should also be mentioned, although γ-P.V.19, like other quinacridone pigments, tends to migrate in polyacetals if it is used at concentrations below 0.1%. The pigment is also found in watercolours for artists.

3.2.4.4 Pigment Red 122

2,9-Dimethylquinacridone is more weatherfast than most other unsubstituted types. It possesses excellent fastness to migration and outstanding heat stability. P.R.122 offers a very clean bluish shade of red, which is usually referred to as pink or magenta. Its main areas of application are in high grade paints, printing inks and plastics, which is also true for the γ-modification of unsubstituted quinacridone.

P.R.122, which is more durable than unsubstituted types of quinacridone with fine particle sizes, may safely be used in automotive metallic finishes. Highly transparent types are available for this important purpose. However, such finishes occasionally present problems in terms of viscosity and flow. Its clean bluish shade makes P.R.122 an equally suitable pigment for use in combination with Molybdate Orange. The resulting colourations are particularly lightfast and weatherfast in automotive finishes. The thus-prepared shades are distinctly cleaner than combinations of Molybdate Red with β-P.V.19. Combinations of Molybdate Red with the γ-modification are less economical than those with P.R.122 and afford somewhat cleaner and more bluish shades. The pigment is used primarily as a shading pigment. Excellent stability to common organic solvents renders the pigment completely fast to overcoating in oven drying systems up to 160 °C. In this respect, P.R.122 performs better than the β-modification of unsubstituted quinacridone. Moreover, P.R.122 is recommended for use in architectural paints: its excellent weatherfastness is of interest for exterior application. P.R.122, like P.V.19, is also used in powder coatings.

2,9-Dimethylquinacridone has a slightly nucleating effect on partially crystalline polymers such as HDPE. The plastics industry therefore uses P.R.122, like the γ-modification of unsubstituted quinacridone, primarily in systems such as PVC films and coatings made from PVC or from PUR, in technical products or toys made from PVC or polyolefins, or in PO films. In addition, P.R.122 is also utilized in PP spin dyeing. In polyester spin dyeing, P.R.122 satisfies the high thermal requirements of the condensation process (exposure to 280 °C for 5–6 h). At low concentrations, however, the pigment dissolves to a certain extent in its medium and accordingly changes colour. Its lightfastness is also affected. Plastics are advantageously coloured with pigment powders or preparations containing predispersed pigment. P.R.122 grades are considerably stronger than γ-P.V.19, they match the β-modification in tinctorial strength: 0.21% pigment is required to produce a 1/3 SD HDPE sample containing 1% TiO₂. Comparative systems containing γ-P.V.19 are formulated at 0.26% pigment. P.R.122 demonstrates excellent heat stability, which makes it a suitable colourant for polystyrene, impact-resistant polystyrene, ABS and polycarbonate. In addition, it is also used in various other media wherever high lightfastness, good heat stability and other special fastnesses are a prime consideration. Examples are systems such as unsaturated polyester resins and PUR foams.

P.R.122, like other quinacridone pigments, shows excellent application properties in high grade printing inks. It is fast to sterilization and to calendering. However, P.R.122 is somewhat weaker in these media than P.V.19 types. At standard film thickness, 1/3 SD letterpress proof prints are made from printing inks that contain approximately 25% pigment. To compare values, notably, approximately 13% are needed of the β-modification of P.V.19 and about 19% of the γ-modification. The standard magenta for three- and four-colour printing on cans, posters, and packaging may be approached by combining P.R.122 with pigments of the γ-P.V.19 type. P.R.122 is also found in decorative printing inks for laminated plastic sheets. Complete fastness to the solvents that are commonly used in this area, such as styrene monomer and acetone, makes P.R.122 a useful colourant for polyester-based sheets, as well as for melamine resin systems. The lightfastness of such products equals step 8 on the Blue Scale.

3.2.4.5 Pigment Red 192

Production of P.R.192, 2-methyl-quinacridone, has been discontinued. The pigment never gained considerable commercial recognition. It provides a shade somewhere between the shades of the quinacridone pigments P.R.122 and γ-P.V.19. The commercial type was distinguished by good transparency, but inadequate flow behaviour. In some systems, the pigment was almost as durable as P.R.122. It was recommended for use in high grade industrial paints, especially for automotive finishes, plastics and special-purpose printing inks.

3.2.4.6 Pigment Red 202

P.R.202, a very lightfast and weatherfast solid-solution quinacridone pigment, provides more bluish and considerably duller shades than 2,9-dimethylquinacridone. (2,9-Dichloroquinacridone, which is listed in the Colour Index as Pigment Violet 30, has been discontinued.) P.R.202 is primarily applied in automotive finishes. Even highly transparent types with fine particle sizes provide two-coat metallic finishes that offer excellent rheological properties. P.R.202 is accordingly superior to 2,9-dimethylquinacridone (P.R.122). On the other hand, metallic finishes made from P.R.202 are duller and more bluish. P.R.202 types exhibit more apparent flop (Section 3.1.5.3) than grades of P.R.122. At similar transparency, the solid-solution pigment is more durable than the individual components, especially more so than unsubstituted quinacridone pigment. Its response to weathering corresponds to that of 2,9-dimethylquinacridone. Two-coat finishes made of P.R.202, however, are frequently less weatherfast than single-layer products. In terms of fastness to overcoating and other aspects of pigment performance and properties, P.R.202 largely behaves like P.R.122.

A version of P.R.202 in a platelet form is obtainable by recrystallizing of the crude pigment from a polar solvent such as DMF in the presence of a thiol compound and sodium methoxide. The new form affords, for instance in PVC, the appearance of a lustrous copper bronze tone [92].

3.2.4.7 Pigment Red 207

P.R.207 is a pigment made from a solid solution of unsubstituted and 4,11-substituted dichloroquinacridone in a ratio of about 2 : 1. Pure 4,11-dichloroquinacridone is not commercially available. P.R.207 affords a yellowish red shade, which is very yellowish compared to other non-oxidized quinacridone pigments. P.R.207 is still somewhat yellower, but also duller than P.R.209. The commercial type of P.R.207 is very opaque. In fact, it has better hiding power and is considerably weaker in white reductions than the commercially available version of 3,10-dichloroquinacridone. P.R.207 demonstrates excellent weatherfastness, and its response to weathering approximately equals that of P.R.209. This is also true for other aspects of pigment performance, including resistance to organic solvents, fastness to overcoating, tendency to flocculate and other properties and fastnesses. P.R.207 is used primarily in automotive finishes but is also utilized as a colourant for plastics and artists' colours.

3.2.4.8 Pigment Red 209

P.R.209 is a speciality product. It is used in much less volume than the β- and γ-modifications of unsubstituted quinacridone and also than 2,9-dimethylquinacridone. P.R.209 has stimulated a certain amount of interest, not only for its excellent lightfastness and weatherfastness, migration fastness and heat stability, which are comparable to those of P.R.122, but also for its very yellowish shade of red, which is unusual for a quinacridone pigment.

P.R.209 is applied in paints, especially in various types of automotive finishes. Exceptionally clean shades of red are furnished if P.R.209 is combined with Molybdate Orange. There is a certain disadvantage to the fact that such combinations are formulated at a relatively high P.R.209 content. The need for a high pigment content considerably compromises the economy of such systems in contrast to pigment combinations with other quinacridones. Moreover, P.R.209 is found in metallic automotive finishes, for instance in conjunction with transparent Iron Oxide pigments. Comparatively poor tinctorial strength, however, restricts the use of P.R.209 in this area to occasions where the desired shade is not accessible through pigments derived from 2,9-dimethylquinacridone or 2,9-dichloroquinacridone.

Other areas of application include several plastics that are processed at elevated temperature. P.R.209 lends colour to polyolefins (in which it only slightly affects the shrinkage), to ABS, to polyacetal resins and other industrial polymers.

3.2.4.9 Quinacridone Quinone Pigments

In contrast to most quinacridone pigments, commercial products based on quinacridone quinone afford relatively dull, slightly less weatherfast red and reddish to yellowish shades of orange. These pigments are used as special-purpose products to create metallic effects in paints. It is possible, particularly in combination with transparent Iron Oxide Yellow pigments, to produce interesting shades of gold for fashionable automotive finishes, but these products exhibit more of a flop than transparent grades of quinacridone pigments. These solid-solution pigments are used particularly throughout the USA.

3.2.4.10 Other Quinacridone Pigments

Several other members of this class are also available, although not all are listed in the Colour Index. Their exact chemical and physical properties remain to be published. The list includes solid-solution pigments made of unsubstituted quinacridone with dimethylquinacridone or other substituted quinacridones. Solid-solution pigments of two substituted quinacridones have been published, too [94]. These pigments are considered speciality products for automotive finishes, especially for metallic finishes.

3.4.1.1 Preparation of the Starting Materials

The primary starting material for the synthesis of perylene tetracarboxylic acid pigments is the dianhydride 48a. It is prepared by fusing 1,8-naphthalene dicarboxylic acid imide (naphthalic acid imide 51) with caustic alkali, for instance in sodium hydroxide/potassium hydroxide/sodium acetate at 190–220 °C, followed by air oxidation of the molten reaction mixture or of the aqueous hydrolysate. The reaction initially affords the bisimide (peryldiimide) 52, which is subsequently hydrolysed with concentrated sulfuric acid at 220 °C to form the dianhydride:

A new imaging method executed by scanning tunnelling microscopy has shown clearly the molecular structure of perylenetetracarboxylic acid anhydride (Figure 3.22) [95].

Naphthalic acid imide (50) is obtained through air oxidation of acenaphthene (53) with vanadium peroxide as a catalyst. The intermediate, naphthalic anhydride (54), is subsequently reacted with ammonia:

3.4.1.2 Chemistry, Manufacture

Perylene pigments are derivatives of the general structure 55:

In industrially interesting pigments, X usually stands for O or NR, and R represents H, CH₃ or possibly a substituted phenyl moiety. The phenyl ring may possess a methyl, a methoxy, an ethoxy, or a substituent:

Heating perylene tetracarboxylic dianhydride 52, as described in Section 3.4.1.1, to 150–250 °C with a primary aliphatic or aromatic amine in a high-boiling solvent affords the desired crude pigment. The reaction may be accelerated by adding agents such as sulfuric acid, phosphoric acid or zinc salts [96]. Pigment synthesis through condensation in aqueous medium has also been described.

A different route proceeds by alkali fusion of the correspondingly substituted naphthalic acid imide. This pathway parallels the synthesis of perylene tetracarboxylic acid diimide 51 (Section 3.4.1.1). The method is particularly suited to aliphatic amines.

Unsymmetrically substituted perylene pigments are a comparatively recent novelty. Selective protonation of the tetra sodium salt of perylene tetracarboxylic acid affords the monosodium salt of perylene tetracarboxylic monoanhydride in high yield. Stepwise reaction with amines produces unsymmetrically substituted perylene pigments [97].

There are various techniques of aftertreating the crude product which convert it into an industrially useful pigment. Important methods include reprecipitation from sulfuric acid, milling, and recrystallization from solvents. It is also customary to combine these methods in order to optimize the results [98].

The dianhydride of perylene tetracarboxylic acid is converted into the pigment form by preparing the corresponding alkali salt and then reprecipitating the compound with an acid. The dianhydride is formed after separating the acid by thermal aftertreatment at 100–200 °C, possibly under pressure, with an organic solvent. The list of suitable media includes alcohols, ketones, carboxylic acid esters, hydrocarbons and dipolar aprotic solvents.

Opaque perylene pigment varieties are treated in ball mills, either in the presence or absence of milling auxiliaries. The pigments are typically heated to 80–150 °C in solvents such as methyl ethyl ketone, isobutanol, diethylene glycol, and N-methylpyrrolidone, possibly in the presence of water.

Perylene tetracarboxylic acid dimethylimide may also be prepared by methylating the corresponding diimide.

3.4.1.3 Crystal Structures and Properties

Perylene pigments exist in a wide range of hues, they provide red, bordeaux, violet, brown and black shades. The pigments exhibit excellent solvent stability, good-to-very good migration stability in plastics, fastness to overcoating in paints, high chemical inertness and superior thermal stability. Only the basic compound, the dianhydride of perylene tetracarboxylic acid, is not fast to alkali.

As a group, perylene pigments offer high tinctorial strength. They are frequently found to be appreciably stronger than quinacridone pigments. Moreover, perylene pigments provide excellent lightfastness and weatherfastness. In this respect, they approximately equal the performance of quinacridone pigments.

Several perylene pigments are polymorphic, including P.R.224 and P.R.149. Crystal structures have been determined for a large series of commercial and non-commercial perylene pigments by single-crystal X-ray diffraction [99–105], by X-ray powder diffraction [106] and by electron diffraction [107]. The perylimide moiety is always almost planar. Alkyl and aryl substituents protrude from this plane. Phenyl groups attached to the imide nitrogen atoms are rotated by almost 90° from the perylene plane (Figure 3.23).

P.Bl.32, with p-methoxybenzyl substituents, adopts an S-shaped geometry (Figure 3.24).

In all perylene pigments, the molecules adopt a very dense packing in the crystals (Figures 3.25–3.33). The density reaches a value of 1.67 g·cm⁻³ for P.R.224, which is an extremely high density for an organic compound containing C, H, N and O atoms only. The mean atomic volume for organic crystals, which is generally around 18 ų per atom (H atoms not counted) [108], drops to 13.0 ų per atom for P.R.224 (Figures 3.25 and 3.26). The molecules are held together by strong van der Waals interactions, supported by some Coulomb interactions between the partially charged atoms. Hydrogen bonds are only present in P.V.29 (X = NH) (Figure 3.27) and in a few non-commercial compounds containing, for example, hydroxyalkyl substituents.

Interestingly, the colours of perylene pigments vary from orange (R = ethyl) via scarlet–red (R = p-ethoxyphenyl), red (R = xylyl) (Figures 3.36 and 3.37), bluish red (R = p-methoxyphenyl) to black (p-methoxybenzyl), although the substituent R does not take part in the chromophoric perylene system (Table 3.3). The reason for this has been sought in exciton interactions between neighbouring molecules [109]. Depending on the steric requirement of the substituent R, the crystal structure varies, causing variations in the relative position and orientation of the neighbouring molecules (Figures 3.34–3.39), leading to differences in the exciton interactions and thus modifying the absorption bands and, hence, changing the colour. In 1989 Klebe et al. found an empirical correlation between the relative position of neighbouring molecules and λ_max [99].

3.4.1.4 Application

Most perylene pigments, like quinacridone pigments, are used primarily in high grade industrial paints, especially in original automotive finishes and in automotive refinishes. Different types are available of some pigments, ranging from varieties with very fine particle sizes and high specific surface areas and correspondingly high transparency to versions with coarse particle sizes and low specific surface areas as well as high hiding power. Types with fine particle sizes are used especially in metallic and transparent paints, while opaque types provide full shades, frequently in combination with inorganic or other organic pigments. Some types are particularly suited to use in plastics and in spin dyeing products, in which they demonstrate excellent heat stability. However, perylene pigments are rarely used to colour HALS (hindered amine light stabilizer) stabilized polyolefins, that is, polyolefins that are stabilized by steric amines. At medium to high pigment concentrations, these stabilizers may be inactivated or even destroyed by exposure to light, rapidly converting the polyolefin system into a brittle material.

3.4.1.5 Commercially Available Perylene Pigments

3.4.1.5.1 General

Table 3.4 lists representative commercial perylene pigments.

CI. Name	C.I. Constitution Number	X	Shade
RR.123	71145		scarlet to red
P.R.149	71137		red
P.R.178	71155		red
P.R.179	71130	H3CN	red to maroon
P.R.190	71140		bluish red
P.R.224	71127	O	bluish red
P.V.29	71129	HN	red to bordeaux
P.Bl.31	71132		black
P.Bl.32	71133		black

3.4.2.1 Preparation of the Starting Materials

Perinone pigments are obtained from naphthalene-1,4,5,8-tetracarboxylic acid or its monoanhydride.

The acid, referred to as ‘tetra acid’, is prepared as follows: In a Friedel–Crafts reaction, acenaphthene 53 is reacted with malonic dinitrile and aluminium chloride. The resulting condensation product 56 is oxidized with sodium chlorate/hydrochloric acid to form the dichloroacenaphthindandione 57. Oxidation with sodium hypochlorite solution/sodium permanganate affords naphthalene tetracarboxylic acid 49, which mostly exists as the monoanhydride 49a. The dianhydride 49b, on the other hand, evolves only after drying at approx. 150 °C.

A different synthetic route involves halogenation (bromination, chlorination) of pyrene (58), which is thus converted into the tetrahalogen derivative. Oxidation with sulfuric acid to form a diperinaphthindandione with subsequent oxidation, once again in a sodium hydroxide solution [118], yields the tetra sodium salt of naphthalene tetracarboxylic acid 59:

3.4.2.2 Chemistry, Manufacture and Crystal Structures

The synthetic route to perinone pigments, as to perylene pigments, starts from an anhydride, in this case from the monoanhydride of naphthalene tetracarboxylic acid.

The perinone ring system is thus formed by condensation with aromatic o-diamines. Reaction of o-phenylenediamine with the monoanhydride is typically achieved in glacial acetic acid at 120 °C. A mixture of the cis- and trans-isomers evolves, which precipitates as a solid solution (mixed crystal):

By variation of the synthetic conditions, it is possible to enhance the yield of the economically interesting trans-isomer to about 60% [119].

The isomers are separated by taking advantage of the solubility characteristics of their respective salts. By heating the isomer mixture in ethanol/potassium hydroxide, the cis-isomer is dissolved, whereas the trans-isomer is precipitated as a sparingly soluble potassium salt. Satisfactory results are also achieved through fractionation with concentrated sulfuric acid. Either one of these techniques is followed by traditional methods of converting the product into a commercially useful pigment. The options include milling, acid treatment and solvent treatment at elevated temperature.

Other synthetic routes, which exclusively produce the desired trans-isomer, are possible, but have not been implemented industrially, for example [120]:

Figure 3.40 shows the crystal structure of the trans-isomer (P.O.43) [121, 122]. The molecule is situated on a crystallographic inversion centre. Molecules of neighbouring stacks form an interplanar angle of almost 90° (Figures 3.41 and 3.42).

The cis-isomer crystallizes with the same molecular packing as the trans-isomer [122, 123]. The molecule is situated on a crystallographic inversion centre. The molecule itself does not have inversion symmetry; it is disordered on two possible molecular orientations, each with a probability of 50% (Figures 3.43 and 3.44). The atoms of the naphthalene fragment can be superimposed in both orientations, but the positions of the N and O atoms and the positions of the atoms of the terminal phenylene moieties vary according to the molecular orientation. Lattice-energy calculations with dispersion-corrected density functional theory (DFT-D) methods show that the orientation of a given molecule is not fully random, but depends to some extent on the orientation of the neighbouring molecules [124].

The crystal structure of the solution containing cis- and trans-molecules was investigated by Erich Paulus by single-crystal structure analysis in 1996. However, the data did not allow to locate all atoms [124a]. The same crystal was remeasured 19 years later by Dieter Schollmeyer (Mainz) and Winfried Heyse (Frankfurt), using a more powerful diffractometer, which allowed to determine the crystal structure accurately [124]. In the solid solution, the molecules form the same molecular packing as in the pure cis- and trans-compounds. Each molecular position is either occupied by a trans-molecule, or by a cis-molecule. The cis-molecules are present in two orientations, the trans-molecules in only one, see Figure 3.45. Extensive lattice energy minimisations with DFT-D methods reveal that the distribution of cis- and trans-molecules and their mutual orientations are not fully random, but depend on their neighbours: In stacking direction, a trans-molecule is usually surrounded by two trans-molecules, whereas a cis-molecule prefers to be surrounded by two cis-molecules in the same orientation. The ordering length is calculated to be as large as 230 molecules for a trans-molecule, and 75 molecules for the cis-molecule. In the other spatial directions, cis- and trans-molecules tend to alternate, but the correlation is restricted to a length of 4 to 10 molecules [124].

The solid solution containing cis- and trans-molecules adopts the same molecular packing as the pure cis- and trans-compounds. Each molecular position in this crystal is either occupied by a trans-molecule, or by a cis-molecule (in either orientation). The distribution of molecules and their mutual orientations are not fully random, but depend on the type and orientation of the neighbouring molecules [124].

The colour of the orange trans-isomer and the red cis-isomer is not only caused by the molecular absorptions bands, but also by exciton interactions, as shown by quantum-mechanical calculations [125]. In a diluted DMSO solution, both compounds are much more yellowish.

3.4.2.3 Properties

Perinone pigments perform very much like perylene pigments. They exhibit shades in the range from orange to bordeaux. Only two members of this group, however, have gained commercial importance. Perinone pigments demonstrate high heat stability and are very lightfast and weatherfast.

3.4.2.4 Commercially Available Perinone Pigments and Their Application

At present, only the two isomers, which constitute the original cis/trans mixture (Vat Red 14), and, to a limited extent, also the mixture itself are commercially used as pigments (Table 3.5).

3.5.3.1 General

Commercially available DPP pigments are summarized in Table 3.6. Within the range of DPP pigments presently offered to the market, P.R.254 plays the most important role. The pigments differ in their substituents and position at the phenyl ring. The p-position is the preferred one; at present only one pigment with a substituent in the m-position is commercially available.

C.I. Name	Constitution Number	R3	R4	Shade
P.O.71	561200	CN	H	medium orange
P.O.73	561170	H	C(CH3)3	medium orange
P.O.81	—	—	—	reddish orange
P.R.254	56110	H	Cl	medium red
P.R.255	561050	H	H	yellowish red
P.R.264	561300	H	C6H5	bluish red
P.R.270	—	—	—	yellowish red
P.R.272	561150	H	CH3	medium red
P.R.283	—	—	—	yellowish red
DPP/quinacridone solid solutions	—	—	—	yellowish to bluish red

Dependent on the substituents the shade from orange to bluish red follows the order:

DPP/quinacridone and DPP/DPP mixed pigments (‘solid solutions’) are introduced or presently being offered to the market.

3.5.3.2 Pigment Orange 71

Some years ago the DPP pigment was introduced to the market. It produces medium orange shades and is recommended mainly for the use in plastics and printing inks. Special grades for these applications are on the market.

The plastics type is suitable amongst others for HDPE, PP and for PVC. 1/3 SD colourations of HDPE (1% TiO₂) require 0.35% pigment. These colourations are heat resistant up to 300 °C and equal step 8 on the Blue Scale for the lightfastness. The shrinkage of the polymer is only slightly affected at temperatures between 220 and 260 °C and somewhat affected at temperatures up to 300 °C. P.O.71 has a very good bleeding fastness in plasticized PVC. It is also of interest for the use in PP fibres.

The special grade for printing inks is recommended for all kinds of printing processes. The prints are very transparent and fast or almost fast to many solvents commonly used in printing inks. The lightfastness is also good.

3.5.3.3 Pigment Orange 73

P.O.73 affords clean shades of medium orange and is suitable to colour baking enamels based on organic solvent systems, powder coatings and coil coating. It is mainly recommended for high grade industrial finishes, especially for automotive finishes. The commercially available type produces opaque coatings, it is suitable for the replacement of lead-free pigment formulations. P.O.73 is less stable to some organic solvents, such as ethanol, methyl ethyl ketone and ethyl acetate, than P.R.254 or P.R.264. Therefore overcoating fastness is not perfect in certain systems.

The heat stability of P.O.73 is good, but deteriorates somewhat in white reductions. The pigment exhibits good weatherfastness, but performs poorer in this respect than P.R.254 and P.R.264.

3.5.3.4 Pigment Orange 81

P.O.81 is a DPP pigment, released some time ago to the market, whose exact chemical constitution has not been disclosed. It affords a very clean orange shade and shows good hiding power, high saturation to achieve deep shade areas, very good weatherfastness and excellent flow and gloss. The pigment can be used in automotive and industrial paints, plastics and industrial and powder coatings.

3.5.3.5 Pigment Red 254

P.R.254, which was introduced into the market as the first representative of DPP pigments, shows good colouristic and fastness properties and has within a short period of time developed into a widely used pigment for high industrial paints, especially in original automotive finishes and automotive refinishes. The commercially available type affords medium shades of red in full shades, while white reductions are somewhat bluish red. The pigment shows good hiding power, it is used primarily in automotive finishes wherever lead-free formulations are required. For economic reasons, P.R.254 is frequently used in combination with the somewhat bluer but less weatherfast opaque type of P.R.170. Combination, for instance, with quinacridone pigments affords opaque shades of bluish red.

P.R.254 shows very good fastness to organic solvents. It is therefore fast to blooming and bleeding in baking enamels. Full shades and similar deep shades are lightfast enough to reach step 8 on the Blue Scale. The pigment also shows very good weatherfastness – a reason for its primary use in original automotive finishes. Its fastness to flocculation can be improved by employing suitable additives.

P.R.254 is also used to colour plastics that are processed at high temperature. A special type has recently been introduced to the market that is used for this purpose. 1/3 SD HDPE colourations of this type are stable up to 300 °C for 5 min. The colourations exhibit high tinctorial strength; their shade is considerably yellower and cleaner than that of the type which is used in paints. 1/3 SD colourations (1% TiO₂) require 0.16% pigment. The pigment only affects the shrinkage of the plastic to a minor extent. The lightfastness of 1/3 SD colourations (1% TiO₂ and transparent) and of full shades equals step 7–8 on the Blue Scale.

In plasticized PVC, P.R.254 reaches step 8 on the Blue Scale for lightfastness. It shows high tinctorial strength and bleeding fastness.

3.5.3.6 Pigment Red 255

The pigment affords reddish shades of orange, actually scarlet shades, which are distinctly more yellowish than those of P.R.254. The commercially available grade of P.R.255 shows excellent hiding power due to its specific surface area of 15 m² g⁻¹. Lightfastness and weatherfastness of full shades are excellent, those of tints are very good-to-good, but not quite as good as those of P.R.254. Similar results are obtained for other fastness properties of the two DPP pigments. So, for instance, the stability of P.R.255 to some organic solvents, such as methyl ethyl ketone and ethyl acetate, commonly used in paint systems and finishes, is somewhat inferior compared with P.R.254. Baking enamels are fast to overcoating at common baking temperatures (140 °C, 30 min). Up to 200 °C heat resistance is very good, which is important for the use in powder coatings. Higher temperatures will cause a darkening of the shade.

P.R.255 is recommended for high grade lead-free industrial finishes, especially automotive OEM finishes. The commercially available grade tends to flocculate in many formulations. Additives to improve the fastness to flocculation are available on the market. The rheological behaviour of finishes is characterized by a distinct thixotropy. This can cause problems at the production of mill pastes. P.R.255 is also of interest for the use in tinting systems for decorative paints.

Within the plastics area it can be used in rigid and plasticized PVC. The list of applications includes packaging and decorative printing inks.

3.5.3.7 Pigment Red 264

P.R.264 provides bluish red shades, which are similar to those of P.R.177, an aminoanthraquinone pigment. The fastness properties are similar to the other types of the DPP pigment class. The commercially available grade is distinguished by a very high tinctorial strength. P.R.264 is broad in scope, its main application area is in paints. It is recommended for high grade industrial paints including automotive finishes of different kinds. In metallic and effect finishes it is suitable for very clean red shades. The weatherfastness of the coating is excellent. The coatings are completely fast to overcoating. The flow properties of the commercially available grade are moderate; the pigment exhibits high viscosity in high and medium solids. Special additives increase the tinctorial strength.

The heat stability is very good, which is a prerequisite for the use of P.R.264 in powder coatings and in coil coating. In the plastics industry P.R.264 can be used to colour PVC, and because of its good heat stability also PP, ABS, PA and PET. The tinctorial strength in plastics is very high. The pigment concentration required for 1/3 SD HDPE colourations (1% TiO₂) is only 0.09%. The colourations are heat stable up to 300 °C and the corresponding lightfastness equals step 8 on the Blue Scale. The shrinkage of such partially crystalline polymers is only slightly affected. In suitable polymers the weatherfastness is said to be good. The pigment is also recommended for PP dyeing.

In the printing ink area P.R.264 can be used for high grade packaging printing inks. The prints are transparent and show good lightfastness.

3.5.3.8 Pigment Red 270

The pigment was later introduced to the market. The chemical structure has not yet been disclosed.

P.R.270 exhibits a yellowish red shade and is recommended for the colouration of plastics and printing inks.

3.5.3.9 Pigment Red 272

This pigment has been published in the Colour Index. According to the data released by the producer, it is a medium red shade pigment and exhibits high colour strength. P.R.272 is said to be designed for use in PE, PP and PVC plastics applications, such as injection moulding, sheet and cable extrusion and calendering as well as PP fibres application. The heat stability in these plastic materials reaches 300 °C in the mass tone and 260 °C in white reductions with a very low influence on shrinkage in HDPE. The pigment demonstrates good lightfastness and weatherfastness, but mass tone colourations tend to darken.

Owing to its good opacity, the pigment is recommended in general industrial paint applications for lead- and cadmium-free formulations.

3.5.3.10 Pigment Red 283

P.R.283 is a DPP Pigment of which the chemical structure has not yet been disclosed. It presents a strong yellowish red shade, outstanding thermostability and very high transparency and is preferably applicable for the colouration of plastics. The pigment shows no nucleation in plastics and therefore no distortion in moulded plastics, such as HDPE.

3.5.3.11 DPP/Quinacridone Mixed Crystal Phase (‘Solid Solutions’) Pigments

Some of these pigment types which are claimed to be solid solutions, have been released to the market. The commercially available grades afford red shades, which are much bluer than the DPP pigments and much yellower than the quinacridone pigments. The ratio of the two components within the pigment crystal and the physical properties, such as particle size distribution and crystallinity, determine the colouristic and fastness properties. The commercially available grades are recommended for high grade industrial finishes, especially automotive finishes. They are transparent, which is a prerequisite for metallic and effect automotive OEM finishes. In such systems they produce very brilliant red shades with special flop effects (see Section 3.1.5.1.2.4). Pigmented water-reducible base coat systems form stable dispersions. The fastness of these mixed crystal phase pigments to some organic solvents is not perfect. So, for instance, they are somewhat soluble in butyl-glycol; the resulting solutions are fluorescing more or less. Connected with this behaviour is a decrease of the overcoating fastness of the two basic pigments, the DPP pigment and the quinacridone pigment.

The plastics industry uses these pigments mainly for polyolefins. The tinctorial strength is comparatively moderate. 1/3 SD HDPE colourations (1% TiO₂), for instance, require between 0.22% and 0.7% of these pigment grades. Such colourations are heat resistant up to 300 °C. Transparent colourations in 1/3 SD are stable up to 250 °C.

3.6.1.1 Crystal Structure

In indigo, the NH group donates a bifurcated intra-/inter-molecular hydrogen bond to the keto group of the same molecule and to a keto group of a neighbouring molecule (Figure 3.56). Each molecule is connected to four different neighbouring molecules, resulting in a criss-cross pattern as in the γ-phase of P.V.19 (quinacridone).

3.6.1.2 Properties and Application

Unsubstituted indigo is marketed as Pigment Blue 66, 73000. It is a suitable pigment for spin dyeing of synthetic viscose rayon and spun rayon, to which it lends navy blue shades. The pigment provides good lightfastness: the samples match step 5–6 to step 7 on the Blue Scale, depending on the standard depth of shade. However, Pigment Blue 66 performs poorly regarding several textile fastnesses, especially in terms of bleaching with chlorite, and vat resistance.

3.6.2.1 Chemistry, Manufacture and Crystal Structures

Thioindigo pigments have the following general structure:

In commercially important products, R typically represents a chlorine atom or a methyl group, and n stands for 2 or 3. The compounds assume the trans structure.

The synthesis is performed in two steps [143, 144]:

• attaching the five-membered ring to the benzene ring;
• oxidatively linking two molecules of the resulting thionaphthen-3-one.

The most important route involves cyclization of appropriately substituted phenylthioglycolic acids:

Suitable agents are chlorosulfonic acid monohydrate, or concentrated sulfuric acid. Other methods include cyclization of phenylthioglycolic acid chlorides, which is afforded by Friedel–Crafts reaction with aluminium chloride, possibly in the presence of sulfuric acid/sodium chloride. The acid chloride is obtained from phenylthioglycolic acid with thionyl chloride or phosphorus trichloride.

There are more routes to cyclization. Phenylmercaptoacetic acid may be replaced by its o-carboxy or o-amino derivative to form the thionaphthen-3-one. The first reaction is performed in molten alkali, the second proceeds via diazotization, Sandmeyer reaction with sodium cyanide/copper cyanide, alkaline hydrolysis and acid treatment.

Oxidation may be achieved in the presence of oxygen or air. Other suitable oxidants include sulfur, sodium polysulfide, iron(III)chloride, potassium ferrocyanide(III) or potassium dichromate, peroxydisulfate or salts of aromatic nitrosulfonic acids. An aqueous/alkaline medium is used in the presence of a high boiling organic solvent that is not miscible with water or which is almost immiscible with water. Cyclization with chlorosulfonic acid can be followed directly by oxidation with bromine to afford the thioindigo system, without separation of the intermediate.

The synthetic route to tetrachlorothioindigo may serve as an example for an industrial-scale synthesis. Starting material is 2,5-dichlorothiophenol, which is obtained by reduction of 2,5-dichlorobenzene sulfochloride or by reaction of the 2,5-dichlorobenzene diazonium salt with potassium-o-ethyldithiocarbonate and hydrolysis or with disodium sulfide and subsequent reduction:

2,5-Dichlorothiophenol is treated with chloroacetic acid to form 2,5-dichlorophenylmercaptoacetic acid. Cyclization followed by oxidation with chlorosulfonic acid at 35 °C affords 4,7-dichlorothionaphthen-3-one. Direct addition of bromine in chlorosulfonic acid without separating the intermediate yields the 4,4′,7,7′-tetrachlorothioindigo derivative through oxidative dimerization. It is also possible to separate the monoheterocycle and to oxidize it with oxygen in an alkaline medium:

3.6.2.1.2 Crystal Structures

Thioindigo pigments cannot form hydrogen bonds in the solid state. Figure 3.57 shows the molecular packing in P.R.88.

3.6.2.2 Properties and Application

Thioindigo pigments provide a range of hues from red violet and maroon to brown shades. The type and position of the substituents affects the shade.

Electron-donating substituents (methyl groups) at the 5-position cause the most drastic bathochromic shift, which decreases in the order: 4->7->6-position. The reverse is also true in the case of electron acceptors (such as chlorine): they cause more of a bathochromic shift in the 6-position than in the 5-position.

As a class, tetrachlorothioindigo pigments exhibit good-to-excellent lightfastness and weatherfastness as well as solvent stability and migration fastness. 4,4′,7,7′-Substituted derivatives demonstrate highest fastness to solvents. Chlorine substitution has a better effect in this respect than methyl substitution. Shifting only one of the substituents on 4,4′,7,7′-tetrachlorothioindigo, for instance, will adversely affect the migration resistance of the parent compound (fastness to bleeding).

3.6.2.3 Commercially Available Thioindigo Pigments and Their Application

3.6.2.3.1 General

Of the two remaining thioindigo pigments, only Pigment Red 88, the 4,4′,7,7′-tetrachlorothioindigo, has been able to maintain interest throughout the pigments industry. It is broad in scope. The other type, Pigment Red 181, is a special-purpose type for a limited number of applications. Table 3.7 lists the chemical constitutions of both pigments.

C.I. Name	C.I. Constitution Number	R4	R5	R6	R7	Shade
P.V.38	73395	CH3	Cl	H	CH3	reddish purple
P.R.88	73312	Cl	H	H	Cl	red-violet
P.R.181	73360	CH3	H	Cl	H	bluish red

Unsubstituted thioindigo, which is marketed as Vat Red 41, 73300, is used in rigid PVC, polystyrene and several other plastics. Dissolved in its medium, the pigment affords fluorescent bluish red shades.

Unsubstituted indigo is marketed as Pigment Blue 66, C.I. 73000. It is a suitable pigment for spin dyeing of synthetic viscose rayon, to which it lends navy blue shades. The pigment provides good lightfastness: the samples match step 5–6 to step 7 on the Blue Scale, depending on the standard depth of shade. However, pigment Blue 66 performs poorly regarding several textile fastnesses, especially in terms of bleaching with chlorite, and vat resistance.

3.6.3.1 Chemistry, Manufacture and Crystal Structures

Thiazine indigo pigments (THI pigments) have the general formula:

Thiazine indigo pigments are produced by condensation of o-thioanilines with dihalogenated maleic acid anhydrate. The synthesis yields thiazine-indigo derivatives in their cis-isomeric form, which are thermally transformed into the thermodynamically more stable trans-isomer:

In the solid state, THI compounds form chain structures via double hydrogen bonds [151] (Figure 3.58). Different arrangements of chains are possible, for example, parallel, antiparallel or with a herringbone pattern. Such molecular chains built by double hydrogen bonds are also found in DPP pigments and in several quinacridone phases. The arrangement of chains can be similar, too. For example P.O.80 (unsubstituted THI) has the same herringbone arrangement of molecular chains as P.R.254 (4,4′-dichloro-DPP), whereas P.R.279 (7,7′-dichloro-THI) has the same layered arrangement of chains as P.R.255 (unsubstituted DPP) and P.R.122 (2,9-dimethylquinacridone).

3.6.3.2 Properties

Thiazine indigo pigments offer yellow, orange, red or brown shades, depending on the substitution patterns and on the polymorphic form. For example, substitution with Cl atoms at positions 7 and 7′ results in a red pigment (P.R.279), whereas the compound with CF₃ groups at positions 6 and 6′ is yellow. According to preliminary investigations, thiazine indigo pigments have a high refractive index (strongly anisotropic, between η = 1.55 and about 2.1, on average η = 1.81) [151], which explains their outstanding hiding power. The fastness properties of thiazine indigo pigments are not as high as those of for example DPP pigments.

3.6.3.3 Commercially Available Thiazine Indigo Pigments and Their Application

In the Colour Index two thiazine indigo derivatives are registered: P.O.80 and P.R.279 (Table 3.8).

C.I. Name	C.I. Constitution Number	R7,R7′	Shade
P.O.80	35714	H	reddish orange
P.R.279	—	Cl	yellowish red

3.7.1.1 Synthesis of 1-Aminoanthraquinone

The traditionally most important route to 1-aminoanthraquinone [153] proceeds via nucleophilic exchange of anthraquinone-1-sulfonic acid or 1-chloroanthraquinone with ammonia. Replacing ammonia by amines affords the corresponding alkyl or arylaminoanthraquinones.

Sulfonation of anthraquinone to form the 1-sulfonic acid is achieved at approximately 120 °C with 20% oleum in the presence of mercury or a mercury salt as a catalyst [154]. Without this catalyst, the reaction produces the 2-sulfonic acid. Exchange with aqueous ammonia (30%) at about 175 °C under pressure converts the potassium salt of 1-sulfonic acid into 1-aminoanthraquinone in 70–80% yield. To avoid sulfite formation, the reaction is performed in the presence of an oxidant, such as m-nitrobenzosulfonic acid, which destroys sulfite.

A less important pathway for 1-aminoanthraquinone involves replacing the chlorine atom of 1-chloroanthraquinone by an amino group. The displacement is achieved with an excess amount of aqueous ammonia at about 200–250 °C in the presence of acid-binding agents.

Nitration of anthraquinone with a stoichiometric amount of nitric acid in sulfuric acid affords 1-nitroanthraquinone.

This reaction, which has been known for more than 100 years, has been reinvestigated extensively in an attempt to furnish a purer reaction product and to improve the yield. Interrupting the nitration at approximately 80% completion by removing 1-nitroanthraquinone by distillation makes it possible to separate the side products, which remain in the distillation residue.

The subsequent reaction of the pure 1-nitroanthraquinone to 1-aminoanthraquinone used to be carried out mainly by reaction with sodium sulfides in an alkaline medium and is now performed by nucleophilic replacement of the nitro group with ammonia in organic solvents, affording up to 98% yield. The overall reaction affords approximately 70% yield, in comparison to the roughly 50% yielded by the ‘classical method’ that proceeds via the 1-sulfonic acid. Moreover, the newer method is also superior to the old one because recyclization of the solvent makes it ecologically more attractive.

3.7.1.2 Anthraquinone-Hydrazone Pigments

1-Aminoanthraquinone has only little importance as a diazo component for corresponding hydrazone pigments. Diazotization of 1-aminoanthraquinone with subsequent coupling onto different coupling components, which, especially by heterocyclic groups with additional carbonamide groups, leads to highly insoluble, migration fast pigments, has been described in several patents. This synthesis produces yellow to red monohydrazone pigments, which are recommended primarily for use in plastics.

1-Aminoanthraquinone is diazotized advantageously with nitrosylsulfuric acid after being dissolved in sulfuric acid. Coupling the diazo compound onto barbituric acid, for instance, affords a yellow pigment [155] with the structure 61:

Coupling 1-aminoanthraquinone with certain Naphthol AS derivatives produces red pigments [156] with the structure 62, which are suitable colourants for plastics:

The compounds are obtained by coupling diazotized 1-aminoanthraquinone onto 2-hydroxy-3-naphthoic acid, followed by separation, drying and conversion into the hydrazone dye acid chloride. Condensation with amines (structure 63) is achieved in an aprotic organic solvent:

Coupling 1-aminoanthraquinone onto pyrazolo[5,1-b]quinazolone with the general structure:

primarily affords red pigments with very good fastness properties. R represents a methyl or phenyl group and R′ stands for various substituents, but primarily for hydrogen, chlorine or bromine. This point is illustrated by Pigment Red 251, C.I. 12925 [157] as follows:

Other anthraquinone-hydrazone pigments are obtained by coupling 1-aminoanthraquinone derivatives onto bis-quinazolinone methanes with the following chemical structure (64):

Compound 64 is prepared through condensation of, possibly substituted, anthranilic acid amides with malonic diethyl esters in the presence of pyridine. Diazotization of 1-amino-5-benzoylaminoanthraquinone, which is then coupled onto 64, for instance, affords an orange pigment with the structure 65 [158]:

3.7.1.3 Other Aminoanthraquinone Pigments

As a class, these pigments include 1-aminoanthraquinone compounds that bear free amino groups, and also derivatives in which the primary amino group is substituted by an aryl or a heteroaryl moiety. Very few representatives are industrially important.

Formation of CC links in the anthraquinone molecule, especially substitutions with aryl moieties, proceed via copper-catalysed nucleophilic exchange of halogenated anthraquinone compounds. These methods include dimerization of 1-aminoanthraquinone.

Pigment Red 177, a red pigment that is listed in the Colour Index under Constitution No. 65300, has the chemical structure of 4,4′-diamino-1,1′-dianthraquinonyl (66) [159]:

Simultaneously with Hansa Yellow G, compound 66 was first described as early as 1909 by Farbenfabriken Bayer. Its preparation starts from 1-amino-4-bromoanthraquinone-2-sulfonic acid (bromamine acid) (67). Dimerization is achieved through the Ullmann reaction, that is, treatment with fine-grain copper powder in dilute sulfuric acid at 75 °C. The separated intermediate, the disodium salt of 4,4′-diamino-1,1′-dianthraquinonyl-3,3′-disulfonic acid (68), is heated to 135–140 °C in the presence of 80% sulfuric acid to cleave the sulfonic acid groups [160]:

The result is a bluish red pigment, the only one amongst the commercial products that possesses free amino groups.

X-Ray diffraction analysis of P.R.177 disclosed a twisting of the two anthraquinone units by 75° relative to each other [161, 162]. The amino group forms one intramolecular and one intermolecular hydrogen bond (Figures 3.59 and 3.60).

Pigment Yellow 199, Constitution Number 653200, can be considered as a dicarbonamide of P.R.177 with both amino groups substituted by lauric acid (C₁₁H₂₃COOH):

Acylation of 1-aminoanthraquinone affords 1-acylaminoanthraquinones. The carbonamide function renders these compounds very fast to migration. The migration fastness may be improved further by introducing substituents that carry additional carbonamide groups, or by molecular dimerization using aromatic dichlorides.

The most important route to 1-acylaminoanthraquinones involves reaction of 1-aminoanthraquinone with acid chlorides in an organic solvent. Reaction of 1-aminoanthraquinone with benzoyl chloride in nitrobenzene at 100–150 °C affords 1-benzoylaminoanthraquinone, a yellow pigment that is registered as Colour Index Constitution No. 60515, but has no C.I. name. The reaction may also be performed in the presence of a tertiary amine, which acts as a proton acceptor:

Benzoylation of 1-amino-4-hydroxyanthraquinone affords a bluish red pigment, which is known as Pigment Red 89, C.I. 60745:

1-Amino-4-hydroxyanthraquinone may be obtained from 1-nitro-4-hydroxyanthraquinone by reduction with sodium sulfide.

Condensation of 1-aminoanthraquinone with phthalic acid chloride in o-dichlorobenzene at 145 °C also yields a yellow pigment, registered as Pigment Yellow 123, 65049 [163]:

Its meta-isomer is Pigment Yellow 202, Constitution number 65410:

It exhibits a dull greenish yellow shade and is recommended to be used for the colouration of PVC.

The para-isomer, Pigment Yellow 193, C.I. 65412, provides a reddish yellow shade:

The four pigments P.R.89, P.Y.123, P.Y.202 and P.Y.193, however, currently lack significant commercial use.

Amongst heterocyclic substituted 1-aminoanthraquinone derivatives, the 2 : 1 reaction product with 1-phenyl-2,4,6-triazine, also referred to as Pigment Yellow 147, 60645, should be mentioned as an example [164]. It is a reddish yellow pigment with the chemical structure 69:

This pigment is prepared by condensation of two equivalents of 1-aminoanthraquinone with 2,4-dichloro-6-phenyl-1,3,5-triazine in the presence of a base in an organic medium. It is also possible and even preferable to treat one equivalent of phenylguanamine with two equivalents of 1-halogenanthraquinone in the presence of an additional tertiary base with copper(I)iodide as a catalyst. In contrast to copper iodide itself, the corresponding addition compounds have the advantage of dissolving easily in organic solvents, which facilitates their separation:

Phenylguanamine (2,4-diamino-6-phenyl-1,3,5-triazine) is synthesized from benzonitrile and dicyanamide:

3.7.1.4 Commercially Available Aminoanthraquinone Pigments

3.7.2.1 Commercially Available Hydroxyanthraquinone Pigments

3.7.3.1 Anthrapyrimidine Pigments

1,9-Anthrapyrimidine (72), which is not used commercially, provides the parent structure for an important yellow pigment:

Anthrapyrimidine and its substituted derivatives are obtained by condensation of 1-aminoanthraquinone (or its derivatives) with formamide or aqueous formaldehyde/ammonia in the presence of an oxidant, such as ammonium vanadate or m-nitrobenzosulfonic acid. A newly developed, simpler route proceeds via formamidinium chloride, which is prepared from 1-aminoanthraquinone with dimethylformamide and thionyl chloride or phosphorus oxychloride. Cyclization in a solvent in the presence of ammonium acetate affords the desired product:

Vat Yellow 20, a product long known and patented as early as 1935, has the following chemical constitution, derived from anthrapyrimidine:

The compound has gained commercial recognition primarily as an organic pigment (Pigment Yellow 108, C.I. 68420).

P.Y.108 is obtained under condensation from 1-aminoanthraquinone, 1,9-anthrapyrimidine-2-carboxylic acid and a chlorinating agent, or directly with 1,9-anthrapyrimidine-2-carboxylic acid chloride in an organic solvent in the presence of an acid trap.

Thus, P.Y.108 is commercially produced by heating 1,9-anthrapyrimidine-2-carboxylic acid with 1-aminoanthraquinone and thionyl chloride in a high-boiling solvent, such as o-dichlorobenzene or nitrobenzene, to 140–160 °C. The product is separated, washed with methanol, and residual solvent removed by steam distillation. The aqueous suspension is then boiled down with sodium hypochlorite solution.

Finer particle sizes are obtained if 1,9-anthrapyrimidine-2-carboxylic acid chloride is condensed with 1-aminoanthraquinone in a dipolar aprotic solvent (such as N-methylpyrrolidone) at a temperature between 70 and 110 °C. The reaction may be accelerated by using a proton acceptor such as triethylamine or tert-butanol, which reacts with hydrochloric acid. A particularly useful route proceeds via the acid chloride, which is prepared by reacting anthrapyrimidine carboxylic acid with thionyl chloride at 40 °C. A condensation reaction follows without separation of the intermediate [167].

3.7.3.2 Indanthrone and Flavanthrone Pigments

In 1901, R. Bohn synthesized indanthrone and flavanthrone. Both compounds are thus amongst the oldest synthetic vat dyes known.

3.7.3.2.1 Indanthrone

Vat dyes of the highest quality have derived their name from indanthrone, a blue compound, which was originally known as Indanthrene Blue. The compound has long been used as a pigment and is registered in the Colour Index as Pigment Blue 60, C.I. 69800 (73):

The primary synthetic route proceeds via oxidative dimerization of 2-aminoanthraquinone in the presence of an alkali hydroxide. 2-Aminoanthraquinone, for instance, is fused with potassium hydroxide/sodium hydroxide at 220–225 °C in the presence of sodium nitrate as an oxidant. New techniques involve air oxidation of 1-aminoanthraquinone at 210–220 °C in a potassium phenolate/sodium acetate melt or in the presence of small amounts of dimethyl sulfoxide. A certain amount of water that is formed during the reaction may be removed by distillation to improve both efficiency and yield.

One out of several possible options for the synthesis of indanthrone from 2-aminoanthraquinone is illustrated in the following scheme:

The reactions involve various preliminary equilibria, which open up several different pathways. Therefore, the reaction conditions during indanthrone synthesis must be followed exactly [168, 169].

The pigment form [170] is obtained from the leuco form, which in turn is prepared by oxidation with alkali fusion, followed by treatment with sodium hydrogen sulfite solution or sodium dithionite (vatting).

Heating the leuco form in an aqueous alkaline medium in the presence of air, possibly also in the presence of a surfactant, affords the pigment form. Oxidation may also be achieved with sodium m-nitrobenzenesulfonate. Another alternative is to precipitate the vat acid from the salt of the leuco form and to subsequently oxidize the product. It is also possible to mill the suspension of the leuco form together with aqueous sodium hydroxide/sodium dithionite in air, using a pearl mill.

Other methods start from crude indanthrone, which is dissolved in sulfuric acid or oleum (sometimes mixed with organic solvents). The product may be precipitated with water. It is more advantageous to separate the precipitated crude pigment, to reslurry it in water, and to heat the resulting aqueous suspension in the presence of a cationic surfactant. Subsequent treatment of the sulfuric acid solution with nitric acid, manganese dioxide or chromium trioxide is followed by transfer into a solution containing sodium sulfite or iron(II) sulfate.

Kneading or milling the crude indanthrone in the presence of finishing agents, such as polyols, or milling it with salt also affords a product that provides useful pigment properties.

In the past, not only unsubstituted indanthrone but also some chloro derivatives were commercially available. 3,3′-Dichloroindanthrone, also known as Pigment Blue 64, C.I. 69825, has had a certain impact on the market:

No current manufacturers are known of this and other indanthrone derivatives that are preferably chlorinated in α-position (4,4′-, 5,5′- or 8,8′-) and used as pigments.

3.7.3.2.2 Flavanthrone

Flavanthrone has also long been used as a vat dye. It gained recognition as a pigment when increasingly lightfast and durable paints were required.

The yellow pigment is registered in the Colour Index as Pigment Yellow 24, C.I. 70600. It has the structure 76:

The synthetic route involves treating 1-chloro-2-aminoanthraquinone with phthalic anhydride (PA), which initially affords 1-chloro-2-phthalimidoanthraquinone (77). Subsequent Ullmann reaction with copper powder in refluxing trichlorobenzene yields 2,2′-diphthalimido-1,1′-dianthraquinonyl (78) through dimerization. This product is cyclized with 5% aqueous sodium hydroxide solution at 100 °C, cleaving phthalic acid units, to yield 76:

This three-step synthesis may be performed in one step by heating 1-halogeno-2-aminoanthraquinone with copper in a highly polar aprotic solvent [171].

An older, equally interesting industrial route involves condensing 2-aminoanthraquinone in nitrobenzene in the presence of antimony pentachloride or titanium tetrachloride. Complex 79 prevents any undesirable formation of anthrimide (80):

The comparatively low yield, together with the high price of antimony pentachloride, make this process economically unattractive.

A more recent method starts from 2,2′-diacetylamino-1,1′-dianthraquinonyl, which is cyclized in a phase-transfer reaction in chlorobenzene using tetrabutylammonium bromide and a 30% aqueous sodium hydroxide solution [172]:

Flavanthrone, like indanthrone, must be extremely pure in order to develop useful pigment properties. Subsequent finishing converts the thus-prepared material into an appropriate product for use in paints or plastics.

The crude product may be purified by one of the following methods:

• dissolving the material in concentrated sulfuric acid at 50–70 °C and hydrolysing the thus prepared sulfate with water;
• converting the crude product into its leuco form, separating the intermediate, and reoxidizing the compound to form pure flavanthrone;
• extracting the crude product with a dipolar aprotic solvent, such as dimethylformamide or dimethyl sulfoxide.

Various milling techniques are available to develop the desired pigment properties, performing the finishing process either in an aqueous suspension or in the presence of organic solvents or milling agents. A pigment prepared by this route typically evolves in an opaque, reddish yellow form.

A product that provides the same shade but higher transparency is obtained by converting crude flavanthrone into its leuco form (for instance with sodium dithionite/aqueous sodium hydroxide). The resulting intermediate material is separated and resuspended in water. It undergoes facile reoxidation through shearing forces and/or in the presence of surfactants, affording the pigment [173].

The crude pigment may also be treated with an aromatic sulfonic acid (such as toluene sulfonic acid, xylene sulfonic acids, m-nitrobenzene sulfonic acid) in sulfuric acid or with nitric acid at 80 °C to yield a somewhat redder yellow transparent modification of flavanthrone [174].

3.7.3.2.3 Polymorphism and Crystal Structures of Indanthrone and Flavanthrone

Indanthrone exists in four polymorphic forms [175]. From the investigations of the British Intelligence Office and the corresponding US authorities in Germany after the Second World War it is known that these polymorphic forms were investigated by X-ray powder diffraction at the BASF company as early as in 1934 [176, 177], that is, only 18 years after the diffraction of X-rays on powders was discovered by Debye and Scherrer in 1916.

The α- and the β-modification afford greenish and reddish blue shades, respectively, while the γ-form provides reddish hues. The δ-modification is a reddish blue pigment of high transparency, strong weather resistance and superior dispersibility [178]. Being the most stable thermodynamically, the α-form is most suitable for use as a pigment. It affords reddish and greenish blue grades. The more greenish type is prepared by precipitating a crude indanthrone solution in sulfuric acid, or it is obtained from the leuco form with a surfactant by air oxidation. The only commercially available reddish form is also obtained through air oxidation, although it is necessary to simultaneously apply shearing forces [179, 180].

The indanthrone and flavanthrone molecules have a similar molecular shape. The crystal structures of the α-phase of indanthrone and of flavanthrone are similar as well (Figure 3.61) [181, 182]. The NH groups of indanthrone forms intramolecular hydrogen bonds. In both compounds the molecules are connected by van der Waals and Coulomb interactions only. The molecules crystallize in stacks, as it is frequently observed for organic pigments. Molecules from neighbouring stacks arrange in a herringbone pattern (Figures 3.61 and 3.62).

The density of indanthrone and flavanthrone is quite high (1.58 g cm⁻³), which results in favourable lattice energies and good fastness properties. Figure 3.63 shows the space-filling, dense packing of molecules in flavanthrone.

3.7.4.1 Pyranthrone Pigments

Pyranthrone pigments are related to the pyranthrone structure (81), which is formally derived from flavanthrone (Section 3.7.3.2) in that the nitrogen atoms are replaced by CH groups:

Manufacture of the unsubstituted compound 81 (Pigment Orange 40, C.I. 59700) has been discontinued. Other commercially available pyranthrone pigments include bromo, chloro or bromo/chloro derivatives of the parent structure.

Pyranthrone is commonly prepared by Ullmann reaction of 1-chloro-2-methylanthraquinone (82), followed by double ring closure.

1-Chloro-2-methylanthraquinone is treated with copper powder, pyridine and dry sodium carbonate in o-dichlorobenzene at 150–180 °C to form 2,2′-dimethyl-1,1′-dianthraquinonyl (83). Subsequent condensation is achieved after several hours of boiling in a sodium hydroxide/isobutanol solution at 105 °C, which affords the leuco form. The product is then oxidized to form pyranthrone by blowing air through the reaction mixture:

Heating 83 with aqueous sodium hydroxide in diethylene glycol monomethylether or with alkali acetate, a reaction that may also be performed in other polar organic solvents, such as dimethylformamide, N-methylpyrrolidone, or dimethylacetamide at 150–210 °C, also provides pyranthrone. As an alternative, 83 may be cyclized to form pyranthrone in a two-phase reaction, carried out in the presence of quaternary ammonium salts in a phase-transfer system consisting an aqueous and an organic phase [171].

Halogenated pyranthrones may be obtained by two different routes. One route proceeds via ring closure of halogenated 2,2′-dialkyl-1,1′-dianthraquinonyl, in accordance with the synthesis of pyranthrone. The same end may be accomplished by halogenation of ready-made unsubstituted pyranthrone.

Compound 83, for instance, may be chlorinated in o-dichlorobenzene, followed by ring closure through phase-transfer reaction with aqueous sodium hydroxide/water and dichlorobenzene. 3,3′-Dichloro-2,2′-dimethyl-1,1′-dianthraquinonyl is also accessible through ring closure by traditional alkaline condensation or by adding polar solvents in the presence of sodium acetate at 110–150 °C [183].

3,3′-Dichloro-2,2′-dimethyl-1,1′-dianthraquinonyl can be synthesized by Friedel–Crafts reaction from phthalic anhydride and 2,6-dichlorotoluene. Subsequent cyclization of the resulting substituted benzoylbenzoic acid 84 in sulfuric acid affords the corresponding anthraquinone derivative 85, which is dimerized by an Ullmann reaction:

Pyranthrone may be halogenated, for instance, in chlorosulfonic acid in the presence of small amounts of sulfur, iodine or antimony as a catalyst. This procedure necessitates intermediate separation and purification of pyranthrone after manufacture, because the products, unless purified, fail to furnish the solvent fastness that is characteristic of a typical pigment.

While 3,3′-dichloro-2,2′-dimethyl-1,1′-dianthraquinonyl is easily converted into well-defined 6,14-dichloropyranthrone, direct halogenation affords changing amounts of chloro and bromo derivatives with undefined substitution patterns on the pyranthrone ring. Depending on the amount of added halogen, an average of two to four halogen atoms per molecule is expected.

Bromination of 6,14-dichloropyranthrone in chlorosulfonic acid in the presence of a catalyst provides 0.1–2.2 bromine atoms per molecule, depending on the reaction conditions. The exact outcome is controlled not only by the choice of solvent but also by the type of catalyst and the amount of bromine, as well as by the reaction time and reaction temperature.

The halogenated pyranthrones dissolved in chlorosulfonic acid may be isolated by precipitation into ice water. Residual acid is removed by washing. The thus-prepared products, such as 6,14-dichloropyranthrone, do not require further treatment before bromination. An opaque version of 6,14-dichloropyranthrone as well as transparent types of chlorobromopyranthrones are accessible by initially dissolving the crude pigment in sulfuric acid, reprecipitating it with ice/water, and separating the product. The aqueous presscake is then treated with C₄–C₁₀-alkanols or alkanones, or with C₆–C₈-cycloalkanols or alkanones [184].

An opaque form of pyranthrone may also be prepared by treating the corresponding pigment in a polar organic solvent (such as isobutanol) at elevated temperature in the presence of some (0.5–10%) halogenated anthraquinone or another anthraquinone derivative.

While 7,15- and 3,11-dichloropyranthrone demonstrate such poor solvent fastness as to be unsuitable for use as pigments, 6,14-dichloropyranthrone possesses excellent solvent resistance.

In addition to 6,14-dibromopyranthrone, some mixed halogenated pyranthrone derivatives exhibit equally useful pigment properties.

3.7.4.2 Anthanthrone Pigments

Anthanthrone pigments are characterized by the basic structure 86:

The unsubstituted ring system, although exhibiting an orange shade, apart from other deficiencies is tinctorially not strong enough to stimulate interest. Only halogenated derivatives have gained some interest throughout the pigment industry.

Anthanthrone is synthesized from naphthostyril (87), which is saponified to form 1-aminonaphthalene-8-carboxylic acid (88). Naphthostyril itself is prepared from 1-naphthylamine with phosgene in the presence of dry aluminium chloride.

Diazotizing 88 and boiling the solution in the presence of copper powder affords 1,1′- dinaphthyl-8,8′-dicarboxylic acid (89), which is cyclized with aluminium chloride, but preferably with concentrated sulfuric acid at 30–40 °C, to produce anthanthrone (86):

The Friedel–Crafts reaction, which proceeds via electrophilic aromatic substitution, as illustrated in the following scheme, is unique to the manufacture of anthanthrone pigments. Most other polycyclic anthraquinone pigments are synthesized via nucleophilic ring closure.

4,10-Dibromoanthanthrone (90) is registered as Pigment Red 168, C.I. 59300. First synthesized as early as 1913 as a vat dye, this compound is the commercially most interesting halogenated anthanthrone derivative:

Compound 90 may be prepared directly from 86 without intermediate isolation of 86 by treating the dicarboxylic acid 89 with monohydrate or with concentrated sulfuric acid at 35 °C, followed by bromination in the presence of iodine as a catalyst.

P.R.168 exists in two polymorphic forms [185]. The metastable β-polymorph is formed upon synthesis. This orange product is mainly used as a vat dye. Finishing, for example by recrystallization from sulfuric acid, leads to the thermodynamically stable scarlet α-phase, which is used as a pigment.

In the α-phase of P.R.168 the molecules adopt a ‘brick’ packing, as it is also found in the β-phase of P.R.224 (Figure 3.64). While the molecules arrange in a herringbone arrangement in P.R.168, they form slightly wavy layers in P.R.224 (Figure 3.65). Similarly as in P.R.224, the molecular packing is extremely efficient and dense (Figure 3.66).

In contrast to the reddish orange colour of P.R.168, which is referred to as scarlet, the yellower 4,10-dichloroanthanthrone is of no commercial interest as a pigment and is therefore no longer produced.

3.7.4.2.1 Commercially Available Anthanthrone Pigments

3.7.4.2.1.1 Pigment Red 168

Pigment Red 168 dibromoanthanthrone, is a vat type pigment that demonstrates excellent fastness properties, a feature that is an asset in high grade paints. It provides a clean yellowish shade of scarlet, somewhere between that of P.O.43, a naphthalene tetracarboxylic acid derivative, and those of yellowish perylene tetracarboxylic acid type pigments.

P.R.168 is entirely or almost entirely resistant to most of the organic solvents commonly found in typical binder systems. P.R.168, like other vat type pigments, is not completely fast to overcoating in oven drying systems that are baked at 120–160 °C. The degree of bleeding depends on the individual system. The pigment is thermally stable up to 180 °C. It is one of the most lightfast and weatherfast organic pigments known. Excellent performance standards make it a suitable candidate for all types of coatings and paints, even at very low pigment concentrations. P.R.168 is used both as a shading pigment and in mixed systems. It exhibits comparatively low tinctorial strength. The commercial products, like those of other vat type pigments, are more or less transparent, which makes them useful products for metallic finishes. P.R.168 is used, for instance, in one- and two-coat automotive finishes. Moreover, it is weatherfast enough to be used in low concentrations. In such systems, P.R.168 is also employed in combination with highly fast, typically reddish yellow pigments to produce shades of bronze and copper. The pigment is equally useful in conjunction with more reddish organic pigments, such as with perylenetetracarboxylic acid pigments. High weatherfastness makes it a suitable candidate for use in architectural paints and in emulsion paints, including those targeted for outdoor use. The fact that P.R.168 is only used in very light tint allows its comparatively high price in this market. Specialized pigment preparations are also available for these purposes. P.R.168 is fast to alkali and plaster. The pigment is also recommended for use in coil coating, including PVC coatings (Section 1.8.2.2).

P.R.168 is found to a lesser extent in printing inks and plastics. The printing ink industry utilizes P.R.168 to produce special-purpose printing inks, which may be applied to substrates such as posters or metal deco prints. The pigment demonstrates equally excellent fastness in these materials. 1/1 SD systems equal step 8 on the Blue Scale for lightfastness, while 1/3 to 1/25 SD formulations match step 7. The prints are resistant to common organic solvents and chemicals. The pigment is thermally stable up to 220 °C for 10 min, and its prints may safely be sterilized.

3.7.4.2.1.2 Pigment Orange 77

The pigment is listed in the Colour Index under Constitution Number 59105:

It is described as bright yellowish orange and can be used for aqueous flexo inks and in the plastics application in dibutyl phthalate paste as plasticiser for PVC. Dyeing of technical leathers is as well described.

3.7.4.3 Isoviolanthrone Pigments

Isoviolanthrone (91) is an highly anellated polycyclic quinone system. It is derived from the chemical structure of isodibenzanthrone, which may be visualized as being obtained by unsymmetrical condensation of two benzanthrone (92) molecules. The compound isoviolanthrone itself affords an intense blue shade.

Isoviolanthrone is prepared from benzanthrone, which is synthesized from anthraquinone. Anthraquinone is reacted with glycerine and sulfuric acid in the presence of a reducing agent such as iron. In this reaction, anthraquinone is initially reduced to anthrone (93), which is condensed with acrolein. Acrolein, on the other hand, is an intermediate of the reaction between glycerine and sulfuric acid. The synthesis proceeds by oxidative cyclization with sulfuric acid. The resulting benzanthrone (92) is halogenated in sulfuric acid or in chlorosulfonic acid in the presence of a catalyst (sulfur, iodine, or iron) to afford 3-chloro or 3-bromobenzanthrone (93a). Subsequent treatment with sodium disulfide at 135 to 150°C under pressure leads to 3,3′-dibenzanthronyl sulfide (93b). Finally, treating boiling 93b with potassium hydroxide in alcohol (ethanol or isobutanol) leads to isoviolanthrone (91).

In contrast to earlier methods which involved an alcoholic potassium hydroxide fusion of 3-chloro-benzanthrone, this route affords pure isoviolanthrone, free of isomers.

Isoviolanthrone was first synthesized in 1907 and has long been known as a vat dye. However, in contrast to its halogenated derivatives, unsubstituted isoviolanthrone has failed to gain technical recognition as a pigment.

Pigment Violet 31, C.I. 60010, a dichloroisoviolanthrone (94), is commercially available. A monobromo derivative is registered as P.V.33.

Dichloroisoviolanthrone is prepared by chlorination of isoviolanthrone with sulfuryl chloride in nitrobenzene. Chlorine substitution occurs at positions 6 and 15:

Compounds containing 12–17% bromine (and up to 1% chlorine) are referred to as bromoisoviolanthrone. The product has been discontinued in the USA. It is obtained by bromination of isoviolanthrone at 80 °C in chlorosulfonic acid, using iodine as a catalyst. Two crystal modifications are obtained: the α-modification is produced by treating crude isoviolanthrone in 90% sulfuric acid and discharging it into an aqueous dispersion solution. The β-modification evolves if the aqueous α-paste is heated in the presence of N-methylpyrrolidone and a surfactant, or generally by stirring the α-modification with organic solvents [186].

A higher quality α-modification with enhanced tinctorial strength and transparency is prepared from the leuco form of bromoisoviolanthrone. This intermediate in turn is manufactured by vatting crude bromoisoviolanthrone with sodium dithionite/aqueous sodium hydroxide. The product is separated and oxidized in an aqueous alkaline medium in the presence of surfactants. Application of shearing forces, preferably by means of a sand or pearl mill, and maintaining a temperature of 50 °C produces improved pigment quality [187].

3.7.4.4 Violanthrone Pigments

Violanthrone, 95, also known as dibenzanthrone, is an isomer of isoviolanthrone (91). In contrast to isoviolanthrone, which has the molecular symmetry C_2h and possesses a molecular inversion centre, the violanthrone molecule has C_2v symmetry and no molecular inversion centre. Violanthrone is registered as Pigment Blue 65, and just as isoviolanthrone, has an intense blue colour.

Violanthrone is traditionally produced by the potash fusion of two molecules of benzanthrone (92) [189]. However, this process yields a crude product, which contains impurities such as isoviolanthrone and 4-hydroxybenzanthrone. A product of better purity is obtained when the condensation of the two benzanthrone molecules is carried out in two steps. In the first step, benzanthrone (92) is dimerized in an alcoholic alkali melt to form 4,4′-bibenzanthronyl (95a), which is then followed by alkaline or acidic ring closure, preferably in the presence of an oxidizing agent [189a, 189b, 189c].

The single-crystal structure determination [189d] confirms that the violanthrone molecule is planar and has C_2v symmetry. In the crystal the molecules are stacked on top of each other, as frequently found in pigments. As usual, the molecules are slightly inclined against the stacking direction, see Figure 3.67. The stacks are arranged in a way so that the space is perfectly filled, see Figure 3.68.

P.B.65 serves as a pigment as well as a vat dye and a precursor to other vat dyes. The compound, a violet–black powder, is used as a redish-blue polycyclic colourant. It is resistant against alkali and acids, insoluble in ethanol and exhibits a lightfastness of 8 on the Blue Scale. The pigment is reported to be used for printing inks, colouration of plastics and coatings.

Derivatives of violanthrone with chloro, bromo, nitro or methoxy substituents are produced as vat dyes.

3.8.2.1 Crystal Structures

Pigment Violet 23 forms a criss-cross packing (Figures 3.69 and 3.70) [73]. The molecule is planar; only the ethyl groups protrude from the molecular plane.

In Pigment Blue 80 the benzimidazolone moieties of neighbouring molecules are connected via an eight-membered HNCOHNCO ring (Figure 3.71), as is found for many other benzimidazolone compounds. The molecules form chains that arrange in layers (Figure 3.72). As for P.V.23, the molecules are planar, except for the ethyl groups. The packing is quite efficient; the density of P.B.80 is as high as 1.676 g m⁻³. The above-mentioned solid solution of methyl and ethyl derivatives (P.V.57 (Figure 3.73)) is isostructural to P.B.80 and even has a density of 1.754 g m⁻³, which is extraordinarily high for an organic compound. The strong intermolecular interactions, the efficient packing and the high density explain the insolubility and the good fastness properties of these compounds [194].

3.8.4.1 General

At present, this class produces only two representatives that continue to be used industrially: Pigment Violet 23, 51319 and Pigment Violet 37, 51345:

While P.V.23 is used in considerable volume, P.V.37 is a speciality product. Pigments of the P.V.34 and 35 type [198–200] have been discontinued. These pigments, which deviate from P.V.37 in their substitution patterns, are much inferior in terms of tinctorial strength and sometimes also in their migration fastness, especially in plastics.

At the beginning of the twenty-first century, P.B.80 and P.V.57 were introduced into the market [201]. Despite their extraordinary high colour strengths and their excellent fastness properties, their production was suspended a few years later.

3.8.4.2 Pigment Violet 23

P.V.23, also referred to as Carbazole Violet, is a universally useful product. Its colour, a bluish violet shade, is not accessible with other Pigments. P.V.23 is used in almost all media that are typically coloured with pigments. The list of suitable systems ranges from coatings and paints to plastics, printing inks, and other special-purpose media. P.V.23 is entirely fast to many organic solvents. At standardized conditions (Section 1.6.2.1), it is fast to alcohols, esters and aliphatic hydrocarbons as well as to plasticizers such as dibutyl and dioctyl phthalate. Other solvents, such as ketones, are coloured slightly (step 4).

Tinctorially, P.V.23 is an uncommonly strong pigment in almost all media, which qualifies it, even in very small amounts, for use as a shading pigment. Used to a considerable extent as a shading component for paints, P.V.23 adds a reddish tinge to the shade of copper phthalocyanine blue pigments. Although P.V.23 is not quite as lightfast and weatherfast as phthalocyanine blue pigments, it does satisfy most requirements, even very stringent ones. Moreover, P.V.23 is also a useful toning pigment for white enamels. The pigment is particularly important in systems based on TiO₂/rutile with their yellowish undertone. Trace amounts of P.V.23 are used to tone the system: only 0.0005–0.05 parts of P.V.23 are required per 100 parts of TiO₂. White enamels may also be toned by mixtures of P.V.23 and α-Copper Phthalocyanine Blue. P.V.23 is both completely lightfast and weatherfast, even in very light shades (for instance at 1:3000 reduction with TiO₂). Apart from this excellent fastness, P.V.23 has the added advantage of being entirely fast to overcoating. It is therefore used in all types of media throughout the paint industry. The list ranges from air drying house and trade sales paints, that is, decorative (architectural) paints, to general and high grade industrial finishes, such as original automotive (O.E.M) and automotive refinishes. P.V.23 is used in uniformly coloured finishes as well as in metallic finishes. Baking enamels containing P.V.23 are thermally stable up to 160 °C.

Most grades exhibit a high specific surface area, sometimes above 100 m² g⁻¹. For good dispersion and to avoid flocculation, it is therefore necessary to maintain an adequate pigment/binder ratio. Notably, the required amount of binder is higher in this case than for most other organic pigments.

P.V.23 is a favourite shading pigment for use in emulsion paints, where it lends a reddish tinge to Phthalocyanine Blue shades. Excellent weatherfastness makes it a suitable candidate for exterior application in media based on synthetic resin dispersions. The systems are fast to alkali and plaster.

Plate-out is observed with various hardeners if P.V.23 is incorporated in most types of powder coatings, for instance in epoxy systems. This phenomenon, however, is of no consequence for the applicability of the pigment: only minor amounts of P.V.23 are required in these media to lend a blue shade to white enamels.

The list of suitable applications also includes various special-purpose systems, especially where high lightfastness and durability, but also high heat stability or other perfect fastness properties, are a prime concern. This is true, for instance, for coatings to be applied onto aluminium window blinds. Bilaterally coated, usually in pastel colours, these aluminium strips are baked at 250 °C or less and subsequently subjected to considerable mechanical stress.

P.V.23 is also frequently encountered in plastics. Although plasticized PVC systems containing P.V.23 are not entirely migration resistant, the pigment exhibits unusually high tinctorial strength. Less than 0.3% pigment is needed, for instance, to produce 1/3 SD systems (5% TiO₂). The systems demonstrate excellent lightfastness: 1/3 SD specimens equal step 8 on the Blue Scale, while 1/25 SD samples match step 7–8. Moreover, P.V.23 provides excellent weatherfastness and is suited to long-term exposure. It is an important colourant for PVC and PUR plastisols.

In terms of heat stability, 1/3 SD polyolefin systems containing P.V.23 withstand exposure to 280 °C. In lighter tints, this value is appreciably lower: 1/25 SD samples only tolerate up to 200 °C. Dissolution effects shift the shade considerably towards the reddish region, while the pigment maintains its shade at higher temperatures. P.V.23 is an equally tinctorially strong product in these media. 1/3 SD HDPE samples (1% TiO₂) are formulated at less than 0.07% pigment. P.V.23 is a typical polycyclic pigment in that it affects the shrinkage of injection-moulded articles made of HDPE and other partially crystalline polymers. This is a tendency that somewhat restricts its use in such media. The pigment concentration in a transparent HDPE system should not be less than 0.05%. P.V.23 systems rapidly lose lightfastness as more white pigment is added. 1/3 SD samples equal step 8 on the Blue Scale, while 1/25 SD specimens only score as high as step 2.

Carbazole Violet is a suitable pigment for transparent polystyrene products. A certain degree of dissolution in this medium at elevated temperatures makes it necessary to limit the use to processes up to approximately 220 °C. P.V.23 is also used in polyester that is processed at very high temperature. The pigment is equally suitable for use in polyester spin dyeing, it is stable to the harsh conditions of the condensation process, during which it is exposed to 240–280 °C for 5–6 h. At low pigment concentrations there is also a colour shift towards redder shades, due to pigment dissolution. P.V.23 is highly lightfast. Although it generally offers high textile fastness properties, the pigment is not entirely fast to dry and wet crocking. P.V.23 also satisfies the thermal and other requirements for use in spin dyed polyacrylonitrile. In this context, the pigment is completely fast to dry and wet crocking. In PP spin dyeing, P.V.23 should not be applied at low concentrations in order to prevent dissolution effects. It provides excellent lightfastness in medium and deep shades. P.V.23 is also recommended for use in spin dyed viscose, where it exhibits good lightfastness and generally excellent fastness properties in application.

Worked into cast resins based on methacrylate or unsaturated polyester, P.V.23 has the advantage of being fast to peroxides, which act as catalysts in these media. The lightfastness of such systems is between step 7 and step 8 on the Blue Scale, both for transparent and opaque colourations.

P.V.23 has stimulated interest throughout the printing ink industry. It is frequently utilized in combination with copper phthalocyanine blue pigments to produce reddish shades of blue. P.V.23 is equally strong in these media. At standardized film thickness, 1/1 SD letterpress proof prints are prepared from inks formulated at only 8.7% pigment, while 1/3 SD specimens require inks containing 5.6%. In terms of lightfastness, these prints equal step 6 to step 7 on the Blue Scale. The prints are very fast to organic solvents and also exhibit excellent application properties, such as fastness to soap, alkali and fat. The prints may be exposed to 220 °C for 10 min or to 200 °C for 30 min and may safely be calendered or sterilized. P.V.23 is also used in textile printing. It is very lightfast and weatherfast in this application and also performs well in terms of other important fastness properties.

Several other media are also pigmented with P.V.23. The list includes office articles and artists' colours, such as drawing inks and fibre-tip pen inks, wax crayons, oil paints, and high quality watercolours, water- or solvent-based pigmented wood stains, cleaning agents and mass coloured paper.

3.8.4.3 Pigment Violet 37

P.V.37 provides a much more reddish shade than P.V.23 but is equally fast to organic solvents. P.V.37, although weaker in some media than P.V.23, is broad in scope. Printing inks based on nitrocellulose are unique in that P.V.37 provides high tinctorial strength and excellent gloss in these media, although the products are somewhat opaque. Despite the fact that, incorporated in oil-based printing inks for purposes such as offset printing, it is somewhat weaker than P.V.23, P.V.37 exhibits high gloss and good flow behaviour. Its main field of application within the printing ink industry is in metal deco printing. Its fastness properties in application equal those of P.V.23.

In paints, P.V.37 also exhibits a considerably more reddish shade and reduced tinctorial strength. Similar observations apply to its colouristic properties in polymers. In plasticized PVC, P.V.37 has the advantage of being considerably more migration resistant than P.V.23; in fact, it is almost entirely fast to migration. Since P.V.37, like P.V.23, dissolves to some extent in polymers, it loses much of its heat stability as the white pigment concentration in a system is increased. Similar observations apply to polyolefin systems: the shade provided by P.V.37 is considerably more reddish and the tinctorial strength much inferior to that of P.V.23: 0.09% P.V.37 is required to afford 1/3 SD in HDPE systems (1% TiO₂). Such systems are heat stable up to 290 °C but, notably, the heat stability declines rapidly in certain concentration ranges, a tendency that is also observed with P.V.23. Like P.V.23, P.V.37 also considerably affects the shrinkage of HDPE and other partially crystalline injection-moulded plastics. P.V.37 is also recommended for use in PP spin dyeing. As with P.V.23, systems formulated at low concentrations tend to undergo a colour shift due to pigment dissolution.

3.8.4.4 Pigment Violet 57

P.V.57 exhibits a bright reddish violet shade, and a high colour strength. The fastness properties in paints are as high as for P.B.80, which makes the pigment suitable for automotive coatings and paints.

3.8.4.5 Pigment Blue 80

P.B.80 is a reddish blue pigment, shadewise between P.V.23 and P.B.60 (Indanthrene Blue) [201]. The pigment shows outstanding colour strength, which even surpasses P.V.23 in this respect. In PVC the colour strength exceeds that of P.V.23 by about 30%. In paints, the compound can be mixed with 30 parts of TiO₂ to get 1/3 standard depth, and with 300 parts of TiO₂ to get a shade of 1/25 standard depth. Apparently P.B.80 has the highest colour strength of all commercial organic pigments. Lightfastness and weatherfastness are excellent, both in solid and metallic applications. The light fastness is at the highest grade (grade 8), even in 1/25 standard depth. The pigment passes the three-year Florida exposure test even better than P.B.60. The high transparency makes the pigment suitable for all kinds of effect shade applications. Because of its weather fastness, P.B.80 can even be used for shading silver-metallic coatings.

The pigment is completely resistant to the major solvents used in paint applications and the fastness to overpainting is excellent even at baking temperatures of 180 °C. The heat stability is outstanding and, thus, P.B.80 can be used in coil coating applications without any restriction. In principle, the heat stability for powder coating applications is also excellent, but the surface treatment of the pigment may cause some incompatibility in certain powder coating systems. P.B.80 is recommended for all kinds of high-performance applications, such as OEM, refinish and coil coating.

3.9.3.1 Pigment Yellow 138

P.Y.138 (Scheme 3.28) affords exceedingly lightfast and weatherfast greenish yellow shades with good heat stability. Its main fields of application are in paints and in plastics.

Most commercial P.Y.138 types have low specific surface areas of approximately 25 m² g⁻¹ and consequently provide good hiding power, a feature that qualifies them for use in opaque systems. In this field, P.Y.138 competes with opaque yellow pigments of other classes, which, although tinctorially weaker, sometimes offer better hiding power and cleaner shades. Paints formulated at higher pigment concentrations show good flow behaviour. P.Y.138 is entirely or at least largely resistant to a large number of organic solvents, such as alcohols, esters, ketones and aliphatic as well as aromatic hydrocarbons. Oven drying systems containing P.Y.138 are fast to overcoating at common baking temperatures; the pigment is thermostable up to 200 °C.

Full shades and similar deep shades exhibit excellent weatherfastness, but rapidly decrease in tints made by adding TiO₂. The cleanness of full shades increases noticeably by exposure to weather. There are certain binders in which P.Y.138 tends to chalk if employed at high concentration.

P.Y.138 is primarily applied in industrial finishes for commercial vehicles, automotive refinishes and original automotive finishes. Incorporated in these systems, P.Y.138 is acid and alkali fast. However, exterior house paints that are applied onto a basic substrate show some sensitivity to alkali.

Types that feature slightly higher specific surface areas are more transparent and demonstrate somewhat different colouristic properties. Such types are not only tinctorially stronger and more greenish but also somewhat less fast to light and weather than opaque varieties, and their flow behaviour is less favourable.

Another main field of application for P.Y.138 is in plastics, in which it demonstrates average to good tinctorial strength. 1/3 HDPE samples (1% TiO₂) are formulated at approx. 0.2% pigment. Such systems are heat stable up to 290 °C, as are full shades. 1/25 SD specimens only withstand 250 °C. The pigment nucleates in this polymer, as in other partially crystalline plastics, that is, it affects the shrinkage of injection-moulded articles. This effect becomes markedly less noticeable as the processing temperature rises. In terms of lightfastness, full shades equal step 7–8 on the Blue Scale, while white reductions down to 1/25 SD match step 6–7. P.Y.138 is an equally suitable colourant for polystyrene, ABS and various other plastics, such as polyurethane foam. A wide variety of pigment preparations are supplied for various areas of application within the paint and plastics industry. These preparations have the advantage of containing already dispersed pigment.

3.10.1.1 Isoindolinone Pigments

The important group of tetrachloroisoindolinone pigments are azomethine derivatives. They were introduced in the mid-1960s.

Formally, these pigments are disazomethine types, obtained by condensation of primary aromatic diamines with two equivalents of tetrachloroisoindoline-1-one:

In fact, two tetrachlorophthalimide molecules are linked via a disazomethine bridge:

Patents concerning isoindolinone type dyes and pigments were first published by ICI in 1946. Routes to other such pigments followed in 1952 and 1953 (see Reference [210]). All of these routes started either from the unsubstituted phthalimide or from a phthalimide backbone carrying only one or two substituents (105):

Only very few of these yellow and orange pigments have stimulated interest in terms of technical application, for example P.Y.173 (106) [211]:

This is a yellow pigment, obtained by condensing 2,5-dichloro-1,4-diaminobenzene dihydrochloride with 3-iminoisoindolinone in chlorobenzene at 140 °C:

A decisive step towards the development of isoindolinone pigments is credited to Geigy. Patents published in the late 1950s describe pigments with the basic structure 105, using tetrachlorophthalimide as starting material.

The technique of ‘perchlorinating’ the two aromatic rings surprisingly enhances both the tinctorial strength and the fastness properties of the product considerably. This quality increase is notably attributed to factors that are described in detail in Section 3.10.2.

3.10.1.2 Isoindoline Pigments

The methine type consists of isoindoline derivatives. One or usually two equivalents of a compound containing an activated methylene moiety are attached to one equivalent of isoindoline. The list of compounds featuring activated methylene groups includes cyanacetamide or heterocycles such as barbituric acid or tetrahydroquinolinedione.

Notably, none of these pigments feature chlorine substitution on the aromatic system.

3.10.2.1 Isoindolinone Pigments

Commercially used tetrachloroisoindolinone pigments have the general chemical structure 107 [210]:

The synthetic route generally involves condensing two equivalents of 4,5,6,7-tetrachloroisoindoline-1-one derivatives with one equivalent of an aromatic diamine in an organic solvent. Suitable tetrachloroisoindoline-1-one derivatives are substituted in 3-position, which is occupied either by two monovalent groups (A) or one divalent group (B). A may represent a chloro atom or a CH₃O group, while B usually represents a NH moiety:

Primary starting materials for this condensation reaction are 3,3,4,5,6,7-hexachloroisoindoline-1-one (108) and 2-cyano-3,4,5,6-tetrachlorobenzoic acid methyl ester (109):

Compound 108 directly reacts with a diamine to afford the desired pigment. Two pathways have been found effective for the synthesis of 108. Tetrachlorophthalimide may be chlorinated either:

• with one equivalent of phosphorus pentachloride in phosphorus oxychloride;
• with two equivalents of phosphorus pentachloride, affording 1,3,3,4,5,6,7-heptachloroisoindolenine (110) as an intermediate, which is to be converted into 108 with one equivalent of water or alcohol:

Compound 109 reacts with ammonia or alkali alcoholate to form the corresponding activated compounds 111 and 112:

Compound 109 is synthesized from 3,3,4,5,6,7-hexachlorophthalic anhydride, which is treated with ammonia to yield the ammonium salt of 2-cyanotetrachlorobenzoic acid. After being converted into the sodium salt, the thus-prepared intermediate is reacted with methyl halogenides or dimethyl sulfate to form the methyl ester 109.

The synthetic route may be exemplified by the preparation of a tetrachloroisoindolinone pigment. A mixture of one mole of 1,4-diaminobenzene in o-dichlorobenzene with a solution of two moles of 3,3,4,5,6,7-hexachloroisoindoline-1-one in o-dichlorobenzene is heated to 160–170 °C for 3 h. Closed filtration equipment is used to filter the hot product, which is then washed with o-dichlorobenzene and alcohol, dried and milled. The resulting product is a reddish yellow pigment with the structure 113, which is registered as P.Y.110:

o-Dichlorobenzene can be substituted advantageously by 1,2,3-trimethylbenzene [212].

The following published examples illustrate some developments in the field of tetrachloroisoindolinone pigments.

In 1973, a patent describing a new synthetic method was issued to Dainippon Ink [213]. According to this publication, the bisacylated compound 114 is prepared from easily accessible tetrachlorophthalic anhydride by reacting with an aromatic diamine at a 2 : 1 molecular ratio in the presence of ammonia and phosphorus pentachloride. Additional phosphorus pentachloride in a high-boiling solvent, such as trichlorobenzene, effects ring closure of 114 to form the tetrachloroisoindolinone pigment:

By finishing, different polymorphic forms can be obtained. A β-modification of P.Y.110 is formed by heating the known α-modification to temperatures between 200 and 350 °C in one of various high-boiling solvents [214]. A γ-modification of P.Y.110 is apparently accessible by slowly hydrolysing the corresponding potassium salt, which is obtained by the usual methods, with water [215].

Other publications describe mono and disazomethine pigments featuring heterocyclic ring systems [216]. There are also patents concerning bisiminoisoindolenine pigments [217].

There is a large number of studies concerning the synthesis of new chemical species based on the isoindolinone system. The most notable publications are those that refer to modifications of the bridging diamine between the two isoindolinone systems. The following diamine derivatives have been mentioned:

[218–221]

Moreover, there are patents claiming monohydrazone and dihydrazone pigments, starting from compounds such as

acting as diazo components [222], or, after appropriate acetoacetylation with diketene, as coupling components [223].

The crystal structure of the commercial α-phase of P.Y.110 has been determined from X-ray powder data by a combination of molecular quantum-mechanical calculations, lattice-energy calculations and intuition [224]. The central phenylene group is rotated out of the plane formed by the isoindolinone groups by 52°. The molecules are arranged in chains connected by double hydrogen bonds, see Figures 3.76 and 3.77.

Treatment of the α-phase of P.Y.110 with solvents at elevated temperatures leads to the formation of the β-phase. The crystal structure of the β-phase was also determined by X-ray powder diffraction [224a]. The angle between the isoindolinone groups and the central phenylene unit increased to 62°. The molecules are arranged in chains similar to the α-phase, but the mutual arrangement of the chains is different (Figure 3.78).

3.10.2.2 Isoindoline Pigments

Several publications describe various isoindoline pigments with different chemical structures. Such species consist of an isoindoline ring attached to two methine bridges.

These pigments have the following general chemical structure:

R¹ through R⁴ represent CN, CONH-alkyl, or CONH-aryl. R¹ and R² on the one hand and R³ and R⁴ on the other hand can be also members of a heterocyclic ring system.

Examples, such as 115 type pigments, have been described by BASF and Ciba-Geigy [225, 226]:

in which R⁵ stands especially for a methyl group or possibly for a substituted phenyl moiety, or for the almost symmetrically substituted compound 116 (P.O.66) [227]. Pigments with this structure afford yellow shades.

Likewise, suitable starting materials are iminoisoindolines, especially diiminoisoindoline (1-amino-3-iminoisoindolenine), which reacts with a cyanoacetamide NCCH₂CONHR₅ to afford a mono-condensation product 117. Further reaction with a compound containing an activated methylene group (such as cyanoacetamide derivatives, barbituric acid) yields the desired pigment 115:

It is also possible to perform a simple one-step synthesis by adjusting the pH (initially pH 8–11, then pH 1–3).

The manufacture of P.Y.139 may serve as an example for the synthesis of a methine pigment. P.Y.139 is the condensation product of diiminoisoindoline with 2 mol of barbituric acid [228]. Gaseous ammonia is introduced into o-phthalodinitrile suspended in ethylene glycol, forming diiminoisoindoline. This compound is transferred into a mixture of an aqueous solution of barbituric acid, formic acid and a surfactant. After boiling at reflux for 4 h, the hot reaction mixture is filtered, washed to neutrality and free of auxiliaries, and dried. The resulting pigment is a greenish yellow compound, P.Y.139:

It is also possible to synthesize P.Y.139 from one equivalent of 1-amino-3-cyaniminoisoindolenine:

and two equivalents of barbituric acid in aqueous solution [229].

In another process [230] phthalonitrile, dissolved in an alcohol, is reacted with a base and, without isolating, condensed with barbituric acid in a mixture of formic acid and water.

A one-step synthetic route to methine pigments, starting from o-phthalonitrile, has been described by BASF [231].

An asymmetric isoindoline pigment preparation has been claimed, starting from 1,2-dicyanobenzene, which is reacted in alcohol and a base, followed by addition of a cyanomethylene compound and subsequently a barbituric acid derivative, whereby all reactions are accomplished without isolation of the corresponding intermediates [232].

Ciba-Geigy [233] has investigated other methine pigments, including bismethine compounds with one or two non-halogenated isoindolinone ring systems.

The crystal structure of P.Y.139 was determined from X-ray powder diffraction data by P. Erk [224, 224a, 234] using a combined refinement against the X-ray powder data and the lattice energy. The compound crystallizes in the rare space group Cmce with eight molecules per unit cell. The molecules are situated on a mirror plane going vertically through the isoindoline moiety. The molecule is not planar. Because of steric hindrance between the isoindoline and the neighbouring CO groups, the barbituric acid groups are rotated out of the isoindoline plane by 22° (Figure 3.79). A strong two-dimensional hydrogen bond network connects the molecules to their neighbours (Figure 3.80), which explains the good fastness properties of this comparatively small molecule. The network is not planar, but slightly wavy.

The crystal structures of P.Y.185 and P.R.260 were determined from single-crystal data [234]. In P.Y.185 the molecules are entirely planar. Packing effects obviously induce the concurrence of the plane of the barbituric acid group and the isoindoline plane. The hydrogen bond pattern of the barbituric acid fragments is the same as in P.Y.139. On the cyanomethylene side of the molecule a single hydrogen bond is formed between the NH group and the CN group of a neighbouring molecule. The molecules are arranged in layers, which are (in contrast to P.Y.139) completely planar.

In P.R.260 (118) the barbituric acid moiety is twisted against the isoindoline moiety, as in P.Y.139. In contrast, the cyanomethylenequinazolone fragment, which is sterically less demanding, is coplanar with the isoindoline. The quinazolone group is connected to the barbituric acid group of a neighbouring molecule via double hydrogen bonds, which form an eight-membered ring as for example in P.Y.139, P.Y.185 and P.Y.213. The barbituric acid group forms two of these hydrogen bonded rings, one with the quinazolone fragment, and the second one with a barbituric acid group of another neighbouring molecule. Thus the molecules form non-planar layers.

3.10.3.1 Isoindolinone Pigments

Tetrachloroisoindolinone pigments are available in shades from yellow to orange to red and brown. Commercially important pigments cover the spectral range from greenish to reddish yellow. Interestingly, the corresponding non-chlorinated compounds produce yellow shades, which undergo a bathochromic shift as chlorine atoms are introduced. In terms of explanation, two arguments should be considered. Steric problems, on the one hand, prevent the pigment molecules from assuming a coplanar arrangement but, on the other hand, absorption in the visible portion of the spectrum is possible only in the presence of electron conjugation. It is therefore reasonable to assume that the typical pigment molecule represents a donor–acceptor complex, with the amine functioning as a donor and the tetrachlorinated nucleus acting as an acceptor. The central part, that is the diamine moiety, contributes to the colour insofar as higher conjugation and a large number of π-electrons cause a noticeable bathochromic shift in diamines.

The pigments are only sparingly soluble in most solvents, which makes for good migration fastness. Stability to acids, bases and oxidizers, as well as to reducing agents is very good. The pigments exhibit good heat stability and melt around 400 °C. They are also excellently fast to light and weather, especially in white reductions.

3.10.3.2 Isoindoline Pigments

Isoindoline pigments afford yellow, orange, red and brown shades. They match the above-mentioned tetrachloroisoindolinone pigments in terms of solvent and migration fastness, heat stability and chemical inertness. Likewise, isoindoline pigments demonstrate good lightfastness and weatherfastness. Several azomethine pigments have also been found to be polymorphous.

3.10.5.1 General

Heading the list of commercially available pigments are yellow representatives, such as Pigment Yellow 109, 110, 139, 173 and 185, as well as Pigment Orange 61, 66 and 69, and Pigment Red 260. Another available type is Pigment Brown 38. Pigments providing other shades have failed to gain much commercial impact.

Table 3.9 lists examples of tetrachloroisoindolinone and isoindoline pigments (for general literature, see also Reference [235]). P.Y.173 is also available as a mixture of 30% with 70% of [236]:

C.I. Name	C.I. Constitution Number	Structure	Shade	Reference
a) Examples of the azo methine type
P.Y.109	56284		greenish yellow	[237, 238]
P.Y.110	56280		reddish yellow	[237, 238]
P.Y.173a)	561600		greenish yellow	[235]
P.O.61	11265		orange
–	–		red	[235]
b) Examples of the methine type
P.Y.139	56298		reddish yellow	[238, 239, 240]
P.Y.185	56280		greenish yellow
P.O.66	48210		yellowish orange	[227]
P.O.69	56292		yellowish orange	[241]
P.R.260	56295		yellowish red
P.Br.38	561660		yellowish brown	[242,243]
a) Also available as mixture (30%) with 70% of the monochloro compound. b) In the original literature [242] no information is given, which of the three possible isomers shown below is formed in the syntheses. A later publication written by scientists from the manufacturer (BASF) [243] shows the E,E-isomer.

3.10.5.2 Pigment Yellow 109

P.Y.109 affords clean, greenish shades of yellow. It is recommended for use in paints, plastics and printing inks. The paint industry uses P.Y.109 primarily to colour various high grade industrial finishes. The list includes, for instance, original automotive and automotive refinishes. P.Y.109 imparts very deep colours on automobile (OEM) finishes, particularly in combination with inorganic pigments. Green shades are produced in conjunction with blue compounds, such as phthalocyanine pigments. Full shades and related shades, reduced up to 1 : 1 with TiO₂, darken upon exposure to light. Depending on the type, the lightfastness in full shades (5%) equals step 6–7 to step 7 on the Blue Scale. Samples that are reduced with TiO₂ at a ratio of 1 : 3 to 1 : 25 match step 7–8. Both lightfastness and weatherfastness deteriorate rapidly with further reduction. P.Y.109 is fast to overcoating, its heat stability satisfies almost all requirements. Incorporated in an alkyd-melamine system, for instance, 1 : 50 reduction ratios with TiO₂ still afford products that are stable up to almost 200 °C. P.Y.109 is used not only in industrial finishes but also in architectural and emulsion paints.

Its main field of application within the plastics industry is in polyolefin colouration. 1/3 SD samples containing 1% TiO₂ withstand up to 300 °C, while 1/25 SD specimens are heat stable up to 250 °C. The shade, however, is less clean at this temperature, it becomes duller. In injection moulding, P.Y.109 considerably affects the shrinkage of the plastic. It is a tinctorially weak pigment: 0.4% pigment is required to produce 1/3 SD HDPE samples containing 1% TiO₂.

P.Y.109 is fast to migration in plasticized PVC and shows good heat stability. In white reductions (1/25 SD), it is fast up to 160 °C. A noticeable colour change is observed after a 10 min exposure to 180 °C, while full and related shades change colour only at 200 °C. In terms of lightfastness, P.Y.109 equals step 6–7 on the Blue Scale in white reductions (1/3 to 1/25 SD with 5% TiO₂). This is slightly inferior to the lightfastness of the redder dihydrazone condensation pigments P.Y.128, 94 and 93. P.Y.109 exhibits good weatherfastness in rigid PVC, but does not satisfy the requirements of long-term exposure. This is especially true for white reductions with a higher TiO₂ content. Moreover, P.Y.109 is used to colour polystyrene and other plastics, to rubber, polyurethane, aminoplastic resins and unsaturated polyesters. It is also recommended for use in polypropylene spin dyeing.

P.Y.109 colours high grade products throughout the printing ink industry, but it is in direct competition with a large number of similarly coloured products of various classes. Prints containing P.Y.109 exhibit good fastness to several organic solvents. They are not, however, entirely fast to the DIN 16 524/1 standard solvent mixture. The prints show good heat stability, are fast to overcoating and may safely be sterilized. This makes P.Y.109 a suitable candidate also for metal deco printing. Its lightfastness is good.

3.10.5.3 Pigment Yellow 110

P.Y.110 affords very reddish shades of yellow. Good fastness properties make it a widely used pigment.

The paint industry uses the relatively weak P.Y.110 frequently as a colourant for industrial finishes, especially for high grade finishes. The pigment is very lightfast and weatherfast, which also makes it a suitable product for automotive finishes, for instance original automotive finishes. High transparency is an asset in metallic shades. Deep colours, for instance at 1 : 1 TiO₂ reduction, initially brighten as they are exposed to light and weather, but further exposure only has a minor effect on the intensity of the colour. P.Y.110 turns redder as it is combined with TiO₂. It is completely fast to overcoating and heat stable up to 200 °C for 30 min. The pigment is also applied in emulsion paints and in architectural paints.

Incorporated in plastics, P.Y.110 is highly heat stable and possesses excellent lightfastness and weatherfastness. Full shades and related shades up to 1/3 SD in plasticized PVC withstand exposure to 200 °C for 30 min. Colourations up to 1/25 SD containing 5% TiO₂ equal step 7–8 on the Blue Scale for lightfastness. P.Y.110 also shows excellent durability in rigid PVC and impact resistant PVC types, as well as in plastisols for coil coating. It satisfies the requirements for long-term exposure. P.Y.110 is one of the most lightfast and weatherfast organic yellow pigments known. It shows average tinctorial strength. Between 1.4 and 1.9% pigment is needed to formulate 1/3 SD colourations with 5% TiO₂, depending on the type. Comparative values are listed for several other pigments covering the same range of shades.

P.Y.110 is fast to bleeding. Its high heat stability is used to advantage in polyolefins. 1/3 SD HDPE samples maintain their colour up to 290 °C, while 1/25 SD specimens tolerate up to 270 °C for 5 min. The colour turns duller, indicating pigment decomposition. In HDPE melt extrusion, P.Y.110 has considerable effect on the shrinkage of this partially crystalline polymer at processing temperatures between 220 and 280 °C. The pigment is also very lightfast in polyolefins.

P.Y.110 lends colour to polystyrene and styrene containing plastics. It is a suitable candidate for unsaturated polyester and other cast resins, as well as for polyurethane. P.Y.110 is used to an appreciable extent in polypropylene spin dyeing, it is very lightfast in this medium. It is utilized in polyacrylonitrile spin dyeing and sometimes also in polyamide. Its fastness properties, however, especially its lightfastness, do not meet special application conditions (Section 1.8.3.8).

The printing ink industry applies P.Y.110 in all types of printing, provided the pigment satisfies the demands. The prints are resistant to many organic solvents, including the DIN 165224/1 standardized solvent mixture. Prints made from P.Y.110 are fast to clear lacquer coatings, sterilization and are very heat stable. 1/1 to 1/25 SD letterpress proof prints equal step 7 on the Blue Scale for lightfastness.

Likewise, P.Y.110 is found in various other media, such as solvent-based wood stains.

3.10.5.4 Pigment Yellow 139

P.Y.139 is a reddish yellow pigment, used in plastics, paints, and printing inks. The commercial types exhibit a wide variety of particle size distributions and accordingly demonstrate very different colouristic properties, which is especially true for the hiding power. The opaque version is considerably redder. Incorporated in a paint, it is less viscous, which makes it possible to increase the pigment concentration without affecting the gloss of the product.

P.Y.139 is sometimes used in conjunction with inorganic pigments for paints, especially to replace Chrome Yellow pigments. The systems are fast to overpainting (up to 160 °C for 30 min), but they are not entirely fast to acids. The pigment performs very poorly in contact with alkali, therefore it is not suitable for use in amine hardening systems or in emulsion paints that are to be applied on alkaline substrates.

The paint industry uses P.Y.139 to colour general and high grade industrial finishes, including automotive finishes, up to a 1 : 5 TiO₂ reduction ratio. The pigment is very lightfast and weatherfast, but its full shade and related shades darken noticeably upon exposure to light and weather. The opaque version performs better than the transparent one. The difference between the two, depending on the depth of shade, is 1/2 to 1 step on the Blue Scale. Incorporated in an alkyd-melamine resin, for instance, 1/3 SD samples of the opaque type equal step 7–8 on the Blue Scale, while corresponding transparent specimens match step 6–7.

P.Y.139 shows average tinctorial strength in plastics. Approximately 1% pigment is required to produce 1/3 SD samples in plasticized PVC containing 5% TiO₂. 1/3 SD HDPE specimens containing 1% TiO₂ are made from 0.2% pigment. Comparative values are listed for various other pigments. P.Y.139 is bleed resistant in plasticized PVC. It is often used in combination with inorganic and organic pigments, both in plasticized PVC and in PE. HDPE systems containing P.Y.139 in its transparent form (0.1%) and in white reductions (0.1% Pigment + 0.5% TiO₂) equal step 7–8 on the Blue Scale for lightfastness. Full shades darken considerably. P.Y.139 is not durable enough to satisfy more stringent requirements. 1/3 SD HDPE samples withstand exposure to 250 °C, while 1/25 SD specimens are heat stable up to 260 °C. The colour becomes duller at higher temperatures, which is a result of pigment decomposition. P.Y.139 is an equally suitable candidate for PP spin dyeing. It is also found in PUR and in unsaturated polyester.

The printing ink industry utilizes P.Y.139 to colour high grade printing products. 1/3 SD letterpress proof prints equal step 7 on the Blue Scale for lightfastness.

3.10.5.5 Pigment Yellow 173

P.Y.173, an isoindolinone pigment, affords somewhat dull, greenish yellow shades. It shows average fastness to organic solvents, especially to alcohols (ethanol), esters (ethyl acetate) and ketones (including methyl ethyl ketone and cyclohexanone). Its solvent resistance equals step 3 on the 5 step scale. P.Y.173 is almost completely fast to mineral spirits and xylene.

P.Y.173 is weatherfast enough to be recommended for high grade industrial finishes, including automotive finishes, and for architectural paints. Notably, P.Y.173 is used advantageously to produce metallic effects in coatings. It is specifically recommended as a shading pigment, sometimes in conjunction with transparent iron oxide, to adjust flops of hue (Section 3.1.5.3). It is a colourant with high tinctorial strength compared to other pigments covering the same range of shades.

Alkyd-melamine resin systems may safely be overcoated; they withstand exposure up to 160 °C for 30 min. 1/3 SD specimens are heat stable up to a maximum of 180 °C. P.Y.173 shows excellent lightfastness and weatherfastness in white reductions, even as larger amounts of TiO₂ are added.

The plastics industry uses P.Y.173 to colour various polymers. Incorporated in plasticized PVC, P.Y.173 is one of the tinctorially weaker pigments of its class: 1.8% pigment is required to produce 1/3 SD specimens containing 5% TiO₂. The systems are almost completely fast to overcoating. P.Y.173 demonstrates excellent lightfastness and durability in rigid PVC and in PVC plastisols intended for coil coating.

Tinctorially, P.Y.173 is equally weak in PE. It exhibits good heat stability. 1/25 SD samples reduced with TiO₂ retain their colour up to 300 °C. 1/3 SD specimens containing 1% TiO₂ increase in strength as the temperature exceeds approximately 260 °C. At 300 °C, for instance, as more of the pigment dissolves, such colourations not only exhibit higher cleanness but are also approximately 35% stronger than at 260 °C.

In printing inks P.Y.173 it is a very green shade opaque yellow with excellent light, weather and solvent fastness properties, but low colour strength. It is recommended for paste and liquid inks where high fastness properties are required as well as for water based decorative inks.

3.10.5.6 Pigment Yellow 185

P.Y.185, a comparatively recent product, is an isoindoline pigment and provides clean greenish shades of yellow. Its main field of application is in packaging printing inks. P.Y.185 confers good tinctorial strength and high gloss on prints made from NC-based inks. Its tinctorial strength in print exceeds that of P.Y.17, a pigment of the diarylide yellow pigment series, which provides the same hue. In terms of tinctorial strength, P.Y.185 approaches the equally strong but redder P.Y.74. Likewise, the pigment exhibits high tinctorial strength in offset application. Prints made from P.Y.185 perform like those made from monohydrazone yellow pigments in terms of resistance to organic solvents, including fastness to the standardized DIN 16 524 solvent mixture. The same is true for the fastness to packaged goods, such as butter. However, P.Y.185 performs unsatisfactorily towards alkali (step 2). The prints are heat stable and show good lightfastness. 1/3 SD letterpress proof prints, for instance, equal step 5–6 on the Blue Scale for lightfastness. The commercial type provides good transparency.

Recently, special types for the paint industry and for application media apart from the areas of printing inks, paints and plastics were released to the market. The paint grade is tinctorially very strong. Its tinctorial strength in an alkyd melamine resin baking enamel is more than double as high as the distinctly greener P.Y.194, a benzimidazolone pigment. The fastness to overcoating in this system is perfect. The weatherfastness of these coatings is poor, therefore this pigment grade is mainly used for indoor applications, especially in good value powder coatings, if high lightfastness and weatherfastness are not required.

3.10.5.7 Pigment Orange 61

P.O.61 affords yellowish shades of orange. It is noticeably less fast to organic solvents than comparative yellow types within the same class of pigments.

The paint industry uses P.O.61, like other isoindolinone pigments, to colour high grade industrial finishes, including automotive (OEM) finishes, especially metallic shades. Other areas of application include general industrial and architectural paints and emulsion paints. The pigment is highly lightfast and weatherfast, but does not perform as well as the yellow types within the same class. Incorporated in an alkyd-melamine resin lacquer, the pigment may safely be overcoated up to 140 °C.

P.O.61 is migration resistant in plasticized PVC. Some 1.4% pigment is needed to formulate 1/3 SD colourations containing 5% TiO₂. P.O.61 is thus one of the weaker pigments within its range of shades. Its lightfastness is very good. At depths of shade up to 1/25 SD in plasticized PVC, the pigment equals step 7–8 on the Blue Scale. In terms of durability, P.O.61 performs equally well if it is incorporated in PVC plastisols intended for coil coating. This is especially true for transparent samples and those that contain small amounts of TiO₂. P.O.61 PVC plastisol systems are heat stable up to 200 °C. Particularly high heat stability is observed in polyolefins. 1/3 SD HDPE samples containing 1% TiO₂, for instance, withstand exposure up to 300 °C. P.O.61 produces a shade that is very similar to that of P.O.13. P.O.13 is, on the whole, less fast, but twice as strong tinctorially as P.O.61. P.O.61 causes an uncommon degree of shrinkage in HDPE, which is of great interest to injection moulding. The pigment is recommended as a colourant for polystyrene, polyurethane, unsaturated polyesters and elastomers. High heat stability and good lightfastness make it a suitable candidate for spin dyeing polypropylene and polyacrylonitrile.

The printing ink industry employs P.O.61 mainly in products that are to be printed on PVC films.

3.10.5.8 Pigment Orange 66

P.O.66, a member of the methine series, was introduced some years ago, but has recently already been withdrawn from the market. It was recommended especially for paints. Its colour is a yellowish orange. P.O.66 is a pigment with high tinctorial strength. Excellent lightfastness and weatherfastness qualified the pigment for use in high grade industrial finishes, especially in original automotive and refinishes. The available grade was used in metallic finishes for its high transparency, but was also utilized as a shading pigment. At baking temperatures of 140 °C and higher, P.O.66 is not entirely fast to overcoating. It shows excellent lightfastness in plasticized PVC. P.O.66 dissolves in polyolefins if the temperature exceeds 240 °C.

3.10.5.9 Pigment Orange 69

P.O.69 was introduced to the market some years ago, but meanwhile the marketing has been discontinued. P.O.69 was primarily applied in various types of industrial paints, and the pigment was also recommended for original automotive finishes. The commercial grade, which featured a specific surface area of 30 m² g⁻¹, has a comparatively coarse particle size. It provides good hiding power. The shade is a yellowish orange. The type was used especially in conjunction with other opaque organic pigments to produce lead-free colourations in the red and brown range. P.O.69 is fast to overcoating up to 140 °C. Both lightfastness and weatherfastness are good, but fail to meet more stringent use requirements. Full shades bleach noticeably and lose much of their gloss.

3.10.5.10 Pigment Red 260

This isoindolinone pigment is already being withdrawn from the market. The commercially available grade had a coarse particle size and thus provided good hiding power. The pigment was recommended for use in various types of industrial paints, including automotive finishes.

P.R.260 may safely be overpainted up to 140 °C. It provides somewhat dull shades in the yellowish red region of the spectrum. P.R.260 is comparatively strong. Full shades and related shades exhibit a slight haze, which is probably due to pigment flocculation. This problem may be met by using suitable additives. Both lightfastness and weatherfastness are good, although full shades darken slightly upon exposure and lose some of their gloss. P.R.260 bleaches considerably in white reductions.

3.10.5.11 Pigment Brown 38

P.Br.38 is already being withdrawn from the market. The commercial type was recommended as a colourant for plastics, especially for PVC and LDPE. The pigment provides yellowish shades of brown and is comparatively strong: 0.64% pigment is needed, for instance, to produce 1/3 SD colourations in plasticized PVC. P.Br.38 shows good migration fastness in plasticized PVC, but is not entirely fast to migration. Its lightfastness in rigid PVC is excellent. Full shade and related shades up to a 1 : 10 TiO₂ reduction ratio equal step 8 on the Blue Scale. P.Br.38 is heat stable up to 240 °C, which made it a suitable pigment, especially for LDPE.