4.1.1.1 Chemistry, Manufacture

Alkali Blue type pigments are based on the following general structure:

Manipulation of the number of phenyl groups R¹ to R³ on the rosaniline nucleus and their substitution pattern has proven a useful tool in producing various commercially important derivatives. Currently, compounds with two (R¹, R² =C₆H₄CH₃/C₆H₅; R³ = H) and especially three phenyl and/or toluene moieties (R¹, R², R³ = C₆H₅/C₆H₄CH₃) are technically important. The CH₃ groups are primarily located at the meta-position relative to the secondary amino group.

Notably, all triarylcarbonium pigments are described by structures that are no more than a useful approximation of reality. These products in fact represent mixtures of various compounds that are obtained through an intricate reaction pattern, the respective written structure being the main component.

The substituted triphenylmethyl (trityl) system 121 may basically be synthesized by two different routes, both of which are in use.

Route A involves preparation of triaminotriarylmethane derivatives and their subsequent reaction with aromatic amines in the presence of acidic catalysts.

Route B proceeds via trihalogentriarylcarbonium compounds, which are reacted with aromatic amines.

The most important starting materials for process A are 4,4′,4″-triaminotriphenylmethane, pararosaniline (119a), and parafuchsin (119). Aniline and formaldehyde are treated at 170 °C to form, apart from some formaldehyde-aniline intermediates, 1,3,5-triphenylhexahydrotriazine as the main component. Subsequent treatment with an acidic catalyst, for instance with hydrochloric acid, in excess aniline as a solvent initially affords 4,4′-diaminodiphenylmethane, which is finally oxidized to yield parafuchsin (119). Iron(III)chloride and nitrobenzene, which in the past were employed as oxidants, are no longer used. The reaction is now performed by air oxidation in the presence of vanadium pentoxide as a catalyst:

Other intermediate products are [2]:

Pararosaniline, after being formed from parafuchsin with sodium hydroxide, is fused at 175 °C with aniline and/or m-toluidine, for instance in the presence of catalytic amounts of benzoic acid, and thus arylated:

The aniline or toluidine residue in the melt is removed by vacuum distillation at 150 °C. The melt is allowed to cool to room temperature and then broken.

Various degrees of arylation may be achieved, depending on the fusion time and on the amount of benzoic acid used. Longer fusion times afford more greenish Alkali Blue types, corresponding to a higher yield of phenylated triaryl product, while the amounts of the more reddish diaryl and monoaryl pararosanilines decrease with the time. Careful reaction control makes it possible to convert the materials into the corresponding water insoluble monosulfonates through sulfonation with concentrated sulfuric acid:

The crude pigment provides only limited tinctorial strength. This necessitates the following aftertreatment: dissolution as a sodium salt, precipitation with mineral acid, possibly in the presence of surface active agents, followed by drying or flushing to transfer the material from the aqueous to the oily phase (mineral oil, linseed oil).

Flushing involves kneading the aqueous pigment presscake with varnish until a complete exchange has taken place between the two liquid phases. The aqueous phase is then separated and residual water removed by vacuum distillation at elevated temperature. Flushing affords a product that can be incorporated directly into a printing ink (Section 1.6.5.8).

Route A is generally unacceptable, due to a poor parafuchsin yield (in the past 35%, now approx. 60%). The arylation step, on the other hand, proceeds with good yield. The formation of a comparatively large amount of by-products makes it necessary to purify the product extensively before it is processed further. Pathway B starts from p-chlorobenzotrichloride and chlorobenzene, which undergo a Friedel–Crafts reaction, forming the tetrachloroaluminum complex of trichlorotriphenylmethyl chloride (122). Compound 122 reacts with aromatic amines only if it is first condensed with at least one meta- or para-substituted aniline. Unsubstituted aniline, deactivating the aluminium complex, does not afford acylation products. Meta- or para-substituted anilines, however, produce arylated pararosanilines in almost theoretical yield.

Subsequent treatment with alkali provides a free base solution in excess amine, which can be separated from the aqueous/alkaline aluminate layer. The thus-prepared product is precipitated with acid, possibly converted into the free base with aqueous alkaline solution, and isolated. The dried base or its salt is then sulfonated with concentrated sulfuric acid to form the monosulfonic acid 121.

Route B of this process may be substantially improved in terms of yield and product quality (purity) of the resulting triarylaminoarylcarbonium pigment. To this end, the solution of the free dye base is treated with an excess of aqueous sulfuric acid (20–40%) in a solvent such as chlorobenzene or an aromatic amine. This method produces the sulfate of the basic dye, which is insoluble in this medium, together with the soluble sulfates of the primary aromatic amines, which can therefore easily be separated. The isolated sulfate of the basic dye is then washed and in dry or wet condition monosulfonated with 85–100% sulfuric acid. Based on the dye base sulfate, this step affords 96–98% yield, compared to only 83–89% achieved by the previously described method. The entire synthesis, including the intermediate isolation of the triarylaminoarylmethane sulfate, may also be performed by continuous process [3].

Route B has the added advantage over route A of making it possible to perform a stepwise reaction of trichlorophenylmethyl-tetrachloroaluminate 122 (i.e. the complex compound formed from tri(chlorophenyl)methyl chloride and AlCl₃) with various amines. It is thus possible to systematically synthesize triphenylmethane pigments with two or three differently substituted arylamino groups.

4.1.1.2 Properties and Application

As a result of the nonuniform reaction process, all commercial products within the Alkali Blue series represent mixtures of various products. The respective structure listed in the Colour Index only reflects the main component of a differently arylated mixture. Moreover, the aromatic moieties not only represent differently substituted compounds but also mixtures of various degrees of sulfonation.

Special-purpose triphenylmethane pigments are frequently even ‘tailor-made’, that is, designed to suit the individual needs of different areas of application.

Alkali Blue types cover a wide range of shades from the reddish blue to the violet portion of the spectrum. The shade becomes greener as the number of phenyl groups in the molecule increases. Both the tinctorial strength and the solubility of the pigment are controlled by manipulating the degree of sulfonation: one sulfonic acid function per molecule affords an optimum in tinctorial strength, while a higher degree of sulfonation adversely affects the strength but simultaneously improves the solubility of the pigment in water. This explains why compounds with two to three sulfonic acid groups (such as ‘Water Blue’ for paper mass colouration) are only infrequently used as pigments (by forming an insoluble aluminium lake on adding alum). Products featuring four to five sulfonic acid groups (such as ‘Ink Blue’ for inks) are completely unsuitable for use as pigments.

4.1.1.3 Commercially Available Alkali Blue Pigments

The list of significant Alkali Blue type triarylcarbonium pigments includes compounds derived from diarylated and triarylated rosanilines. Table 4.1 shows the pigments that are listed in the Colour Index.

C.I. Name	C.I. Constitution No.	R	R′	R1	R2	R3
P.B.18	42 770:1	C6H5NH	H	H	H	Ha
P.B.19	42 750	NH2	CH3	H	H	H
P.B.56	42 800		H	CH3	CH3	H
P.B.61	42 765:1	C6H5NH	H	H	H	H
a The constitution which is listed in the Colour Index erroneously contains another SO3H group in the R2 and R3 carrying ring. In fact, however, P.B.18 and P.B.61 are chemically identical.

This chapter discusses relevant fastness properties and aspects of pigment performance as well as various facets of pigment application. The entire class is treated comprehensively instead of focussing on individual types. In Europe, Alkali Blue pigments are also referred to as Reflex Blue pigments, a trade name created by the local manufacturer.

The main area of application for these products is in printing inks, especially in offset and letterpress application and to a lesser extent also in aqueous flexographic inks. The pigments have less of an impact on the office articles sector. The printing ink industry employs triarylcarbonium pigments less as independent self colours but rather as shading pigments to tone black inks. Used at low concentrations, these pigments correct the brown touch of carbon black and provide a colouristically neutral, particularly deep black.

Alkali Blue types cover the reddish blue portion of the spectrum. Even the very greenish specimens are still considerably redder than α- or ɛ-Copper Phthalocyanine Blue. Alkali Blue pigments are uncommonly strong. At standardized conditions concerning printed film thickness, 1/3 SD letterpress proof prints are prepared from inks containing only approx. 3.5% pigment. More than twice as much pigment is needed to prepare corresponding α-Copper Phthalocyanine Blue prints.

As a consequence of their polarity, triarylcarbonium pigments show a considerable tendency to agglomerate. Compared to other organic pigments, they are very difficult to disperse. Normal powder pigments are therefore rarely used in actual practice – the pigments are supplied predominantly in predispersed form as flushed pastes (Section 1.6.5.8). Pigments may be flushed into very different types of binders, which makes it possible to optimize the products for specific printing purposes, such as heat set inks. The fact that a flushed paste consists of predispersed pigment makes it possible to add any amount of paste, as needed, to a ready-made dispersion of carbon black.

The rheological and colouristic differences between flushed pastes are partly attributed to the pigment concentration. Notably, however, tinctorial strength is a minor concern in a shading pigment: these products are selected primarily for their shading behaviour. This depends not only on the type of carbon black, for instance its colour cast, but it is also a function of the shade of Alkali Blue. Black prints made from inks containing Alkali Blue tend to bronze, a feature that equally contributes to the shading behaviour of the product. This effect, which is difficult to assess and measure, results from various factors [4]. Studies have indicated that, depending on the pigment concentration, practically all blue pigments bronze to a certain extent. The minimum extent of bronzing is given by the optical constants of the pigment. Stronger bronzing is attributed to floating of the blue pigment or to the penetration of a considerable amount of binder into the surface of the substrate, consequently increasing the pigment concentration on the surface of the dry printed film, a phenomenon that makes the reflection of light at the surface sensitive to the wavelength of the incident light.

Commercial flushed pastes commonly contain about 40% pigment; however, these products are standardized not in terms of pigment concentration but regarding their tinctorial strength. The ratio between carbon black and Alkali Blue pigment in a toned product may range between approximately 2 : 1 and 4 : 1, depending on the nature of the components and on the desired effect.

For several years, specialized Alkali Blue pigments, referred to as easily dispersible, have been available in powder form. Already during synthesis, these pigments are prepared with suitable resins or other agents to reduce the problem of pigment agglomeration during drying and milling. In addition, thus prepared pigments also have the advantage of facilitating the deagglomeration of agglomerated units within a printing ink. These types, however, do not disperse as readily as the easily dispersible members of other classes of organic pigments, for instance of the diarylide yellow pigment series.

Powder grades are commonly dispersed using agitated ball mills, often directly in combination with the carbon black. Theoretically, one should expect pigments that are used in offset and letterpress inks to dissolve in their medium or to more or less recrystallize. Such effects are normally expected in view of the considerable thermal demands placed upon pigments, especially carbon black, by modern dispersion technology. In practice, however, no colouristic evidence of dissolution or recrystallization has been found. The primary components in the respective oil-based binders, that is, offset and letterpress printing inks, are largely nonaromatic mineral oils as well as linseed oil and similar solvents. Alkali Blue pigments, tested for solvent fastness in pigment powders at standardized conditions (ambient temperature) (Section 1.6.2) prove to be completely resistant to these solvents, but not to alcohols and ketones. Insufficient fastness to alcohols is one of the main factors in precluding Alkali Blue from use in nitrocellulose-based printing inks. The printing ink often loses tinctorial strength and gloss, an effect that, due to pigment recrystallization, cannot be controlled.

Although Alkali Blue pigments are considerably faster to aromatic hydrocarbons than to alcohols and ketones, they are by no means entirely fast to these solvents. Alkali Blue is not applied in toluene publication gravure printing inks. Such black systems are typically shaded with Milori Blue (Iron[III]hexacyanoferrate), although sometimes in combination with Alkali Blue. Milori Blue exhibits a shading effect that is different from that of Alkali Blue types. At the concentrations needed to afford a satisfactory shading effect in carbon black, Milori Blue, incorporated in toluene-based publication gravure printing inks, unlike Alkali Blue, does not adversely affect the rheological properties of the black ink.

Prints containing Alkali Blue are not fast to the standard DIN 16524 solvent mixture, but they are fast to acid, paraffin, butter and other materials. Tested in accordance with normative testing standards (Section 1.6.2.2), the prints unexpectedly also show fastness to alkali. Notably, however, at higher alkali concentrations the tinctorial strength of the system declines and the shade becomes duller. This is because the pigment reacts with alkali.

Alkali Blue pigments are used to a considerable extent in aqueous flexographic inks, despite a slightly alkaline pH value. The resulting printing inks, however, may show a viscosity increase. Such inks are prepared from powder grades, because flushed pastes contain binders that are mostly incompatible with aqueous printing inks. Special-purpose pigment preparations have also recently been introduced to the market for this application.

Alkali Blue pigments may safely be exposed to 140–160 °C for 30 min. They are not fast to clear lacquer coatings and to sterilization. The pigment is therefore normally excluded from use in metal deco printing, although there are exceptions. Alkali Blue is employed, for instance, for blue cream containers that are not exposed to direct sunlight. Pigment blends of Copper Phthalocyanine Blue and Pigment Red 57:1 are frequently utilized as suitable shading pigments for black metal deco printing inks or for other black printing inks that are fast to overlacquering.

In terms of lightfastness, Alkali Blue performs moderately. 1/1 SD letterpress proof prints equal step 3 on the Blue Scale, while 1/3 to 1/25 SD formulations equal only step 2. However, as shading components their lightfastness is excellent and satisfies the requirements for all types of applications. This is because a large portion of the incident light is absorbed by carbon black.

Alkali Blue pigments are used in large volume to colour office articles, especially ribbons for typewriters and computers, as well as blue copy-paper. Other areas of application, such as the plastics industry, do not employ Alkali Blue pigments because of their lack of fastness.

4.1.2.1 Chemistry, Manufacture and Crystal Structures

The list of dye cations used to prepare these complex salts includes primarily two types of triarylcarbonium compounds, namely:

• triphenylmethane or diphenylnaphthylmethane derivatives with the chemical structure 123;
• phenylxanthene derivatives with structure 124.

The most important group in this series consists of compounds with the structure 123 (A⁻ = acid anion):

with R = methyl or ethyl and Ar = phenyl, 4-dimethylaminophenyl, or 4-ethylaminonaphthyl.

It is the substitution pattern that defines whether a product provides violet, blue or green shades.

Phenylxanthene compounds with the structure 124:

with X = hydrogen or methyl and Y = hydrogen or ethyl primarily afford bluish red (‘pink’) colours.

Another member of this group is a yellowish green pigment, structurally derived from a benzothiazolium system (125):

Compound 125, which is supplied in the form of a complex salt with a heteropolyacid, is used in combination with Pigment Green 1.

Heading the list of possible acid anions, A^–, are complex phosphoric acids, particularly those with Mo₃O₁₀²⁻ or W₃O₁₀²⁻ ligands. Formally, these acids are derived from phosphoric acid H₃PO₄ by replacing all oxygen atoms by molybdate and/or tungstate groups. The resulting heteropolyacids may be written as follows:

The same structures may also be represented as:

Alternatively, it is possible to replace the phosphorus atom by silicon, which affords products amongst which silicomolybdic acids H₄[Si(Mo₃O₁₀)₄]·aq and H₄H₄[Si(Mo₂O₇)₆]·aq are commercially of some significance. The second kind of formula emphasizes the fact that only three (or four) out of seven (or eight) hydrogen atoms provide possible substitution sites.

The dye salts are therefore represented as follows (D = dye group):

However, notably, the stoichiometric aspect is idealized. The ratios between the different components can vary widely and are in actual fact controlled by the pH value and the precipitation temperature.

Reducing agents, such as zinc dust or sodium dithionite, convert heteropolyacids into deeply coloured blue compounds consisting of heteropolyacids with four hydrogen atoms to be substituted. This makes it possible to precipitate more dye with the same heteropolyacid to produce an insoluble pigment.

Copper ferrocyanide complex salts, which are occasionally used, are derived from copper(I)hexacyano-iron(II)-acid, HCu₃[Fe(CN)₆]. Three equivalents of copper are required for each unit of ferrocyanide, which furnishes the following general pigment structure:

4.1.2.1.1 Dye Synthesis

This section outlines the different synthetic routes to the most significant cationic dyes that these pigments are derived from.

Basically, compounds with a central C atom, which are considered electrophilic reaction centres, are treated with a nucleophilic aromatic compound. Subsequent oxidation affords the desired carbonium compound:

4.1.2.1.2 Triphenylmethane and Diphenylnaphthylmethane Derivatives

Methyl Violet consists of the basic structure of Pigment Violet 3 (126).

It is prepared by air oxidation of dimethylaniline in the presence of phenol and copper salts as well as sodium chloride. The reaction product consists of tetra- to hexamethylated pararosanilines (see 119, Scheme 4.1).

In contrast to Methyl Violet, Crystal Violet (127) is a uniform compound. This is the dye which constitutes Pigment Violet 39.

The compound is obtained from ‘Michler's ketone’ (128) and dimethylaniline in the presence of POCl₃. Michler's ketone in turn is synthesized from N,N′-dimethylaniline (DMA) and phosgene with zinc chloride:

Another route proceeds via bis(dimethylaminophenyl)methane, which is synthesized from dimethylaniline and formaldehyde. The thus prepared intermediate may be oxidized to form the hydrol and then reacted again with dimethylaniline to afford the leuco base, which is finally oxidized to yield 127:

A blue-shift is observed if the phenyl ring Ar in structure 123 is replaced by a naphthyl ring. The product is known as ‘Victoria Blue’ (129):

The product may be obtained from tetraethyldiaminebenzophenone and N-ethylnaphthylamine with phosphorus oxychloride or with phosphorus trichloride; the route parallels the synthesis of Crystal Violet. The resulting colourant is the basic dye for Pigment Blue 1.

Substituting the Ar moiety in structure 123 by a phenyl group without alkylamino groups affords green shades. A typical product is ‘Brilliant Green’ (130), which is the basis for Pigment Green 1. With the chemical substituent N(C₂H₅)₂, this compound is homologous to Malachite Green with N(CH₃)₂:

Compound 130 is manufactured by condensing one equivalent of benzaldehyde with two equivalents of diethylaniline in the presence of zinc chloride or hydrochloric acid. Proceeding via the carbinole, the reaction ultimately affords the leuco base, which is oxidized with agents such as PbO₂ to afford the dye.

4.1.2.1.3 Phenylxanthene Derivatives

As a group, these typically pink compounds are derived from the xanthene structure (131):

Condensation of a m-dialkylaminophenol with one equivalent of benzaldehyde in the presence of sulfuric acid or zinc chloride, followed by oxidation (for instance with FeCl₃), provides the bis(dialkylamino)-phenylxanthenium skeleton, which is the parent compound for dyes with structure 124 (see above):

The most significant parent structure for pigments of this group is obtained by a slightly modified route by simply using phthalic anhydride instead of aldehyde. Reaction with m-diethylaminophenol at 180 °C in the presence of sulfuric acid or zinc chloride and subsequent oxidation with iron(III)chloride thus affords a dye known as ‘Rhodamine B’ (132), the basis of Pigment Violet 1:

The ethyl ester of (132) is the dye component of Pigment Violet 2.

Starting from 3-ethylamino-4-methylphenol, it is possible to synthesize the cation (133) by using phthalic anhydride, as described above, and form the ethyl ester. The thus prepared product is the dye cation of Pigment Red 81.

4.1.2.1.4 Benzothiazolium derivatives

The benzothiazolium component with structure 125 is obtained through so-called primuline fusion: p-toluidine is heated with sulfur to 200–280 °C. Subsequent distillation affords not only the ‘primuline base’:

but also a product referred to as dehydrothio-p-toluidine, that is, 2-(4-aminophenyl)-5-methylbenzothiazole (134). Permethylation of the nitrogen atoms with agents such as methanol and hydrochloric acid and simultaneous quarternization affords the thiazolium salt 135.

Compound 134 should first be removed from the primuline base by distillation:

4.1.2.2 Properties

As a class, these pigments are characterized by uncommonly clean and brilliant shades. They provide several shades, particularly red and violet ones, that can be duplicated by other organic pigments, but without their brilliance and cleanness. These pigments fail to satisfy more stringent fastness requirements. They are not stable to polar solvents, such as alcohols, ketones and ethylene glycol. Moreover, they also decompose if exposed to alkali.

For economic reasons these complex pigments are declining in use, despite their excellent colouristic properties. Many attempts have been made to replace more expensive types, such as those based on phosphotungstomolybdic acid, with less costly ones. As a consequence of these efforts, the properties of the pigments have improved greatly in recent decades, especially in terms of tinctorial strength and lightfastness. Complex pigments typically demonstrate average lightfastness. This property is a function both of the type of heteropolyacid and of the depth of shade. Copper ferrocyanide types, for instance, score an average of 2 steps lower on the Blue Scale than phosphomolybdic acid, phosphotungstomolybdic acid and silicomolybdic acid types.

4.1.2.3 Application

These complex pigments are utilized almost exclusively in printing inks, especially in packaging and special printing inks. Of the large number of pigments falling within this class, only the most significant are surveyed here. The exact chemical compositions of the individual species, especially the respective dye/heteropolyacid ratio, remain to be published.

4.1.2.4 Commercially Available Dye Salts with Complex Anions

4.1.2.4.1 General

The pigments are commonly referred to not only by their Colour Index name but also by an added abbreviation for the respective heteropolyacid. For instance:

1. PM (or PMA), phosphomolybdic acid,
2. PT (or PTA), phosphotungstic acid,
3. PTM (or PTMA) for phosphotungstomolybdic acid,
4. SM (or SMA) for silicomolybdic acid,
5. CF, copper ferrocyanide (copper(I)-hexacyano-iron(II)-acid).

Table 4.2 lists the predominant species among these pigments.

C.I. Name	C.I. Constitution Number	A−	Ar	R	Common name of the corresponding dye
P.V.3	42 535:2	PTM PM		CH3	Methyl Violet (Basic Violet 1)
P.V.3.1	42 534:4	SM		CH3	Methyl Violet (Basic Violet 1)
P.V.4	42 510:2			CH3	Methyl Violet(Basic Violet 1)
P.V.27	42 535:3	CF		CH3	Methyl Violet (Basic Violet 1)
P.V.39	42 555:2	PM PTM		CH3	Crystal Violet
P.B.1	42 595:2	PTM PM		C2H5	Victoria Pure Blue B (Basic Blue 26)
P.B.2	44 045:2	PTM PM		CH3	Victoria Blue 4R
P.B.9	42 025:1	PTM PM		CH3	—
P.B.10	44 040:2	PTM PM PT		CH3	—
P.B.14	42 600:1	PTM PM		CH3	Ethyl Violet
P.B.62	44 084	CF		C2H5	Victoria Blue R
P.Gr.1	42 040:1	PTM PM		C2H5	Diamond Green G
P.Gr.4	42 000:2	PTM PM		CH3	Malachite Green
P.Gr.45	—	CF	—	—	—

4.1.2.4.3 Phenylxanthene Derivates

The structures are given in Table 4.3.

C.I. Name	C.I. Constitution Number	A−	R	X	Y	Common name of the corresponding dye
P.R.80	45 160	PTM	H	CH3	C2H5	Rhodamine 6G (Basic Red 1)
P.R.81	45 160:1	PTM	H	CH3	C2H5	Rhodamine 6G (Basic Red 1)
P.R.81:1	45 160:3	STM	H	CH3	C2H5	Rhodamine 6G
P.R.81:2	45 161:1	SM	H	CH3	CH3	Basic Red 1:1
P.R.81:3	45 161:2	PM	H	CH3	CH3	Basic Red 1:1
P.R.81:4	45 161:5	PTM	H	CH3	CH3	Basic Red 1:1
P.R.81:5	45 161:4	SM	H	CH3	C2H5	Rhodamine 6G
P.R.81:6	45 161:7	PM	H	CH3	C2H5	Rhodamine 6G
P.R.169	45 160:2	CF	H	CH3	C2H5	Rhodamine 6G
PV.1	45 170:2	PTM	C2H5	H	H	Rhodamine B (Basic Violet 10)
P.V.1:1	—	PM	C2H5	H	H	Rhodamine B
P.V.1:2	45 170:4	M	C2H5	H	H	Rhodamine B
P.V.2	45 175:1	PTM	C2H5	H	C2H5	Rhodamine 3B ethylester

4.1.2.4.3.1 Pigment Red 81/81:1/81:2/81:3/81:4

P.R.81 affords a very clean bluish red shade, which matches the purple-red on the DIN 16508 colour scale for letterpress application and also on the DIN 16509 offset scale. The shade is not accessible by using other pigments (Section 1.8.1.1). Pigments, which are closely related by shade, such as P.R.147, provide shades which are not even remotely as clean. The standard magenta on the European Colour Scale for letterpress (DIN 16538) and offset printing (DIN 16539) is somewhat yellower. For such printing inks, certain high solvent fastness properties are required to safely overlacquer the prints. These conditions cannot be met with P.R.81. The pigment in particular lacks fastness to polar solvents, such as alcohols, ketones and esters, as well as to the DIN 16524 solvent mixture. On the other hand, P.R.81 prints are very fast to aliphatic and aromatic hydrocarbons, paraffin, butter and many other fats, although they are not entirely stable to sterilization. Like other members of its class, P.R.81 affords prints that are not fast to alkali and soap but largely fast to acid.

Tinctorially, P.R.81 is a strong pigment, which, as with other representatives, is especially true for the PTM types. The pigment commonly darkens noticeably as it is exposed to light. 1/3 SD letterpress proof prints equal step 4 on the Blue Scale for lightfastness. The lightfastness of P.R.81 systems may be adversely affected if the surface of the pigment particles is attacked by polar solvents, such as esters, ketones, low molecular weight alcohols or glycol ether. Alkali has a similar effect.

P.R.81 is used especially in three and four colour printing and lends itself to various printing processes; therefore, pigments of this type are referred to as ‘Process Red’ in the USA. Used as a colourant for NC-based printing inks, SM types of P.R.81 may present problems as they are dispersed with steel balls or stored in steel containers as well as at elevated temperature. Catalytic decomposition of the binder and damage to the pigment may induce a colour shift and increase the viscosity.

4.1.2.4.3.2 Pigment Red 169

P.R.169 parallels the P.R.81 types in terms of shade and fastness properties. Thus, P.R.169 types are equally suited to letterpress and offset printing inks, where they provide the standard purple red for three and four colour printing in accordance with the so-called DIN scale (Section 1.8.1.1). Moreover, P.R.169 is frequently used for toluene-based publication gravure printing inks, where it is sometimes applied in combination with P.R.57:1. Several grades, however, are not as fast to toluene as comparable P.R.81 varieties. One of the major fields of application for P.R.169 is in aqueous flexographic printing inks.

There are various systems in which the salt character of these pigments adversely affects the ease of processing in an ink or even the finished print. Most of these problems are attributed to the copper ferrocyanide content, which is used as a complex acid in P.R.169 and comparable types of other basic dyes. In offset inks and other oxidatively drying printing inks, catalytic acceleration of the drying process may occur, resulting in a thickening of the ink, film formation on the surface of the ink and yellowing of the prints. Incorporation in polyamide vehicles has the disadvantage that P.R.169 catalytically decomposes the resin and thus causes the prints to stick and develop an odour. An unsuitable or shifting pH value in an aqueous ink may thicken the vehicle and affect the colouristic properties of the product. Highly acidic binder components may chemically react with the pigment or with the dye component and thus adversely affect the colouristic properties, an effect that is often accompanied by considerable flocculation. Deficiencies also tend to appear in NC-based printing inks. These are a consequence of dye nitrosation through formation of nitrous gases. As a result, gas develops, the shade demonstrates an often considerable shift, and the printing ink loses some of its strength. Pigment manufacturers supply their products with detailed information on how to obviate these difficulties.

4.1.2.4.3.3 Pigment Violet 1

P.V.1 is broad in scope; it is closely related to P.V.2, both in terms of colouristics and fastness properties. Although the shades are very similar, P.V.1 usually provides not only higher tinctorial strength and greater brilliance but also higher cleanness of shade. In terms of lightfastness, P.V.1 scores about 1/2 step on the Blue Scale less than P.V.2 types. Moreover, P.V.1 prints are also less fast to organic solvents and to agents such as soap and butter than prints made from P.V.2. The areas of application are the same as for the other violet type.

4.1.2.4.3.4 Pigment Violet 2

One of the predominant features of P.V.2 varieties is the fact that they are comparatively lightfast. At standardized conditions regarding printed film thickness and substrate, 1/3 SD letterpress proof prints equal step 5 on the Blue Scale. Like other representatives of this class of pigments, P.V.2 loses much of its lightfastness through exposure to polar solvents, such as low molecular weight alcohols, esters and ketones. Alkali has a similar effect. Moreover, such solvents (and also alkali) enhance the tendency of the pigment to migrate and reduce its fastness to overlacquering. PTM types are noticeably less fast to polar solvents than pigments based on other complex acids. P.V.2 is acid fast in accordance with standard test conditions (Section 1.6.2.2).

The pigment provides a very bluish shade of red, referred to as violet. It furnishes a very clean shade. P.V.2 systems show a brilliance that is not reproduced by using chemically different pigments. Tinctorially, P.V.2 is an especially strong product.

The printing ink industry utilizes P.V.2 particularly for special printing inks and as a shading pigment. It is much too blue to be used in process colours. As is the case with other PTM types and with types containing phosphomolybdic acid or silicomolybdic acid, there is some difficulty in dispersing P.V.2 in certain binders. Both the processing of the ink and the printed products may be affected. Inks containing polyamide, for instance, if printed onto polyolefin films may exhibit dye migration through the film. This phenomenon is attributed to the presence of small amounts of dye within the pigmented system that have not been converted into salts. NC-based printing inks containing P.V.2 are affected through contact with steel. This is likely to occur, for instance, as printing inks are prepared using steel balls or as they are transported in steel containers. Consequences include catalytic degradation of nitrocellulose and damage to the pigment, visibly resulting in increased viscosity and colouristic changes involving transparency, tinctorial strength and shade. Elevated temperature has a similar effect or may even enhance the action of steel. Pigments decompose in strongly basic aqueous systems and suffer colouristic and rheological changes. Interaction between pigment and polar solvents also frequently affects the shade and the rheology of the system, thickening the material or flocculating the pigment. Finally, the different components of a blend of this type of pigment, incorporated in a liquid printing ink, may interact with one another, not only forming a sediment on the bottom of the container but also affecting the colouristic properties of the ink. Pigment manufacturers provide their products with information on how to meet these problems.

4.1.2.4.4 Benzothiazolium Derivatives

4.1.2.4.4.1 Pigment Green 2

P.Gr.2 is a mixture of P.Gr.1 (see Table 4.2) and

P.Gr.2 provides yellowish shades of green that are considerably more yellowish than those furnished by P.Gr.1. P.Gr.2, like P.Gr.1, lacks brilliance, a deficiency that within this class of pigments is unique to these two species. The shade produced by P.Gr.2 is also obtained with blends consisting of other organic pigments, such as phthalocyanine green pigments and monohydrazone yellow pigments or diarylide yellow pigments. A prominent characteristic of P.Gr.2 is its high tinctorial strength. In recent years, P.Gr.2 has lost most of its commercial importance and continues to be used only in the USA. Both PTM and PM types are available. The pigment parallels other representatives of its series in terms of fastness properties. It shows good lightfastness, but darkens slightly upon exposure.

4.1.3.1 Pigment Red 172

P.R.172, C.I.45430:1 is the aluminium salt of tetraiodofluorescein (136):

Fluorescein (137) is prepared by the same route as Pigment Red 90 (Section 4.3.8). The compound is iodized with iodine and potassium iodate in an acidic medium. The iodic acid reoxidizes the resulting hydrogen iodide back to iodine:

Aluminium chloride is used to convert tetraiodofluorescein, also known as erythrosine, into the P.R.172 lake.

Provided it meets certain purity criteria, the pigment may be used in pharmaceuticals and cosmetics, but less in foodstuff. It is registered as E 127 throughout the EC and as FD&C Red No.3 in the USA. The bluish red pigment P.R.172 provides little tinctorial strength. The pigment generally shows poor application and fastness properties, including fastness to organic solvents, alkali and acids, as well as heat stability and lightfastness. Consequently, there are limits to its technical applicability.

4.1.3.2 Pigment Blue 24:x, Pigment Blue 24:1 and Pigment Blue 78

The parent structure is a trisulfonated triphenylmethane dye (138):

It is obtained by condensing benzaldehyde-o-sulfonic acid with 2 equivalents of N-ethylbenzylaniline, followed by sulfonation, oxidation and conversion into the ammonium salt. The dissolved dye is then immersed in a dispersion of aluminium hydroxide and converted into the corresponding salt using aluminium chloride or barium chloride solution:

Both types of lakes are available.

4.1.3.3 Pigment Blue 24:x

P.B.24:x, the Al salt of 138, a greenish blue compound, is registered in the USA as FD&C Blue 1 for use in foodstuffs, pharmaceuticals and cosmetics, provided certain purity standards are met. The pigment possesses poor lightfastness. It largely parallels P.B.24:1, also in terms of tinctorial strength and stability to organic solvents.

4.1.3.4 Pigment Blue 24:1

P.B.24:1, 42 090:1, the Ba-salt of 138, provides brilliant greenish shades of blue. Although a pigment with very good tinctorial strength, P.B.24:1 has largely been displaced by Copper Phthalocyanine Blue. In the past, P.B.24:1 used to be applied in large volume as a process colour blue for three and four colour printing. The pigment lacks fastness to acid, alkali and soap, and its lightfastness is poor. P.B.24:1 is found in inexpensive letterpress printing inks as well as in office articles such as coloured pencils and less costly water colours.

4.1.3.5 Pigment Blue 78

P.B.78 is listed under Constitution Number 42 090:2. The structure is identical with the aluminium salt of 138. The pigment is used for food, pharmceutical and cosmetic products and toys.

4.2.1.1 Azo Metal Complexes

Azo metal complexes may also be seen as ‘hydrazone metal complexes’, since some of them are synthesized from hydrazone pigments (see below). However, the difference between the azo form and the hydrazone form vanishes, when the proton is removed during complexation with the metal. Therefore, in this book the name ‘azo metal complexes’ is retained.

The above-mentioned Pigment Green 10 is the nickel complex of the hydrazone pigment obtained from p-chloroaniline and 2,4-dihydroxyquinoline (140):

Its synthesis follows the usual pathway of coupling diazotized 4-chloroaniline onto 2,4-dihydroxyquinoline and subsequently treating the product with a nickel(II) salt.

The fact that protons are released through complexation makes it possible sometimes to enhance the reaction and to improve the yield by adding a base (such as sodium acetate).

An interesting product in the range of azo metal complexes is P.Y.150, which is the 1 : 1 nickel complex of azo barbituric acid (142) [8]:

This molecule is believed to assume the structure of a 1 : 1 sandwich complex [9].

A reaction known as diazo group transfer produces diazo barbituric acid from barbituric acid and p-toluenesulfonyl azide. Additional barbituric acid affords azo barbituric acid [10]. Subsequent complexation with a nickel(II) salt yields a greenish yellow pigment.

4.2.1.2 Azomethine Metal Complexes

Copper, nickel and cobalt complexes reign supreme amongst industrially interesting products within this class. The complexes are manufactured by condensing aromatic o-hydroxyaldehydes with aromatic o-hydroxyamines, frequently o-aminophenols, in an aqueous medium or in organic solvents at 60 to 120 °C. The intermediate products, usually orange to red azomethines, are either separated or immediately reacted further by adding a Cu, Ni or Co(II) salt. The conversion is carried out at similarly elevated temperature, although the prime concern is the presence of a solvent to afford the largely insoluble azomethine metal complexes. As above, the presence of a base may sometimes be useful.

There is another structure which, although it does not represent a typical azomethine compound, is synthesized from nickel complexes of the anilide of diimino butyric acid (143).

The synthesis involves nitrosating the corresponding substituted acetoacetic anilide with sodium nitrite in acetic acid and subsequently, by adding hydroxylamine to the same reaction vessel, converting the resultant compound into the oxime. Finally, complexation is achieved by means of a Ni(II) salt.

Yet another structural principle is represented by metal complex pigments based on isoindolinones. Condensation of amino-iminoisoindolinones (iminophthalimide) with 2-aminobenzimidazole in a high boiling solvent affords an azomethine (144). This compound reacts with salts of divalent metals, such as Co, Cu, Ni, to yield yellow azomethine metal complex pigments [13, 14]:

The cobalt complex is commercially available as P.Y.179, C.I. 48 125.

Pigment Yellow 177, C.I. 48 120 is synthesized by using 2-cyanomethylene-benzimidazole instead of 2-aminobenzimidazole to react with iminophthalimide, followed by complexation with Co²⁺.

For more information about cobalt complexes, refer to Reference [15].

Crystal structures have been determined for P.Y.177 [13], P.O.59 (α- and β-phases) [16], P.O.65 [17], and for pyridine solvates of P.Y.129 [18, 19].

In P.O.65 the Ni²⁺ ion is coordinated in a square-planar geometry by the pigment dianion (Figure 4.1). The molecules form pairs that are held together by van der Waals and Coulomb interactions (Figure 4.2). The pairs arrange in a herringbone packing (Figure 4.3).

4.2.4.1 General

A small number of azo metal complexes and a large number of azomethine metal complexes are commercially available (Table 4.4).

C.I. Name	C.I. Constitution Number	Structure	Shade	Reference
Azo series
P.Gr.8	10 006		Green
P.Gr.10	12 775		Very greenish yellow
P.Y.150	12 764		Greenish yellow	[8, 20]
P.Br.2	12 071		Brown
Azomethine series
P.Y.117	48 043		Greenish yellow	[20, 21]
P.Y.129	48 042		Very greenish yellow
P.Y.153	48 54548		Greenish yellow	[20, 22, 23]
		R2, R4, R5 = H
P.O.59	5455	structure see P.Y. 153 R2 = OCH3, R4, R5 = H	orange
—	—	structure see P.Y. 153 R2, R4 = CH3, R5 = H	red
P.Y.177	48 120		Dull yellow
P.Y.179	48 125		Reddish yellow
P.O.65	48 053		Dull reddish orange
P.O.68	486 150		Orange	[24]
P.R.257	562 700		Red–violet	[24]
P.R.271	487 100		Red

4.2.4.2 Pigment Green 8

P.Gr.8, also referred to as Pigment Green B, provides dull yellowish shades of green or, respectively, greenish shades of yellow up to the olive hues. Over the last few decades, P.Gr.8 has largely given way to the cleaner phthalocyanine green pigments.

P.Gr.8 exhibits good fastness to organic solvents, but performs poorly towards esters, such as ethyl acetate, ketones, such as methyl ethyl ketone, and ethyl glycol (ethylene glycol monoethylether). In contrast to possessing excellent fastness to alkali and lime, P.Gr.8 is not stable to acids. The pigment exhibits good lightfastness, but it is not as lightfast as its copper phthalocyanine counterparts. 1/3 SD P.Gr.8 emulsion paint samples, for instance, equal step 7 on the Blue Scale for lightfastness, while more reduced specimens up to 1:200 equal step 6–7. Corresponding phthalocyanine green pigments match step 8 on the Blue Scale.

With its moderate price, P.Gr.8 is used in emulsion paints and also in concrete mass colouration. Like P.Gr.7, it is one of the few organic pigments suitable for a large variety of concrete formulations. P.Gr.8 in general is not lightfast and weatherfast enough to be used in outdoor application. Its dull shade is of no consequence to indoors application, the shade is cleaner than that of Chrome Oxide Green. As always in this type of application, preliminary testing is recommended.

An important application of P.Gr.8 is in the colouration of rubber. The pigment, however, is not suitable for use in blends that contain large amounts of basic fillers. It is somewhat sensitive to cold vulcanization. The coloured articles usually perform well in general application but are not entirely fast to aromatic hydrocarbons and to some fats, and they are sensitive to acid and sulfur dioxide. P.Gr.8 also colours some plastics, especially LDPE and polystyrene. Heat stable up to 220 °C, P.Gr.8 grades equal step 2–3 on the Blue Scale for lightfastness. Other areas of application include wallpaper and artist's colours.

4.2.4.3 Pigment Green 10

P.Gr.10, a nickel azo complex known since 1947, affords a dull, very greenish yellow shade that, as in the Colour Index, may also be considered a very yellowish green (in white reduction and in full shade, respectively). It is a comparatively weak product and is used primarily in high grade industrial paints, especially as a shading pigment for blue and green colours. The commercial types are highly transparent, which makes them suitable candidates for metallic, transparent and hammer tone finishes. P.Gr.10, however, fails to satisfy the requirements for use in automotive finishes. The colourations remain very lightfast even in highly reduced white reductions: reductions up to 1:200 SD equal step 8 on the Blue Scale for lightfastness. The weatherfastness level is equally very good. In this respect, P.Gr.10 performs similarly to P.Y.117, an azomethine copper complex pigment.

Incorporated in finishes, P.Gr.8 is heat stable up to about 180 °C. It is not entirely fast to overcoating. Most systems containing P.Gr.8 will fail to maintain their colour value if they are exposed to acids: there is a shift towards yellower hues, a phenomenon attributed to demetallation of the pigment.

4.2.4.4 Pigment Yellow 117

The marketing of P.Y.117, an azomethine copper complex, has recently been discontinued. It provides very greenish shades of yellow that resemble those of P.Y.129. Combination with TiO₂, for instance at a ratio of 1 : 50, considerably reduces the cleanness of the system, resulting in very dull olive green shades. As P.Y.117 exhibits high transparency, it was recommended for use in metallic finishes, especially throughout the automotive industry. It was also recommended for olive shades in automotive finishes. Blends with copper phthalocyanine blue and green pigments were equally recommended for automotive finishes.

Compared to other pigments covering the greenish yellow range of the spectrum, P.Y.117 exhibits good tinctorial strength. It shows good stability to organic solvents and under standardized conditions exhibits complete fastness to mineral spirits. Its fastness to toluene, alcohol or esters, such as ethyl acetate, equals step 4 on the 5 step scale; and the pigment equals step 3 in terms of stability to ketones, such as methyl ethyl ketone, or ethyl glycol.

P.Y.117 demonstrates good fastness to alkali and acids. It is completely fast to overcoating up to 160 °C and heat stable up to 180 °C. For a system consisting of P.Y.117 incorporated in a silicone resin paint, the manufacturer listed a temperature stability of up to 300 °C for 1 h exposure.

P.Y.117 is very lightfast and weatherfast. 1/3 to 1/25 SD formulations in an alkyd-melamine resin equal step 8 on the Blue Scale for lightfastness, while 1:200 SD samples still reach step 7. The weatherfastness of the pigment in this range of standard depths and in this binder system equals approximately that of P.Y.153, but is not quite as high as that of P.Y.129. In addition, P.Y.117 was also recommended for use in emulsion paints.

P.Y.117, like P.Y.129, undergoes recomplexation if it is incorporated in plasticized PVC in conjunction with tin stabilizers, such as dibutyltin thioglycolate. The colour shifts towards reddish shades, and the pigment begins to exhibit an appreciable tendency to migrate in the polymer. If suitable stabilizers are added, P.Y.117 is very weatherfast in PVC and shows high tinctorial strength. Some 0.65% pigment is required to produce 1/3 SD samples containing 5% TiO₂.

4.2.4.5 Pigment Yellow 129

P.Y.129 exhibits a clean, very greenish yellow shade. Its only field of application is in paints, especially in automotive and industrial finishes. High transparency makes it a useful pigment to produce interesting metallic finishes. Much of the cleanness is lost as white pigment, such as TiO₂, is added, resulting in olive shades. The coatings are very lightfast and weatherfast. Up to 140 °C, P.Y.129 is fast to overcoating. Reduced 1 : 50 with TiO₂, the systems are heat stable up to 150 °C. P.Y.129 is also recommended for use in architectural paints.

P.Y.129 is not employed in plastics. The type of metal in the stabilizer defines how much of a colour change is observed as the chelated metal in the pigment molecule is exchanged. Tin stabilizers produce red complexes, which, like corresponding lead or zinc chelates, respond very poorly to light and weather (see also Section 1.6.7).

4.2.4.6 Pigment Yellow 150

This pigment, an azo/nickel complex, affords dull, medium shades of yellow. The pigment is recommended for use in paints and printing inks. P.Y.150 produces very lightfast paints: samples up to 1/25 SD equal step 8 on the Blue Scale. The weatherfastness of these specimens, however, deteriorates as more white pigment, such as TiO₂, is added. Systems up to 1/3 SD are heat stable up to 180 °C. P.Y.150 is used as a colourant for general industrial and architectural paints wherever the durability requirements are not too high. It exhibits good resistance to organic solvents but is not entirely fast to overcoating.

P.Y.150 is also very lightfast in printing inks. The prints, however, are neither acid nor alkali resistant. Moreover, they are somewhat sensitive to several organic solvents, including the DIN 16524/1 standard solvent mixture, and soap. The systems are heat stable up to 140 °C. Prints made from P.Y.150 are almost completely fast to sterilization. A pigment with high tinctorial strength, P.Y.150 is particularly suitable for use in decoration printing inks for laminates.

A new special-purpose type of P.Y.150 was introduced to the market a few years ago. It is recommended for use in spin dyeing polypropylene and polyamide fibres. In this type of application, the pigment exhibits good heat stability and also good lightfastness and weatherfastness. Under common processing conditions in injection moudling, P.Y.150 is likely to react with the zinc sulfide that is often found in polyamide, a tendency that precludes its use in polyamide injection moulding.

4.2.4.7 Pigment Yellow 153

P.Y.153 is a nickel complex that was introduced to the market in the late 1960s. It produces slightly dull reddish shades of yellow. Although not fast to acids, the pigment may safely be exposed to alkali. It is fast to mineral spirits and alcohols, but only moderately so to aromatic solvents, such as xylene, and to esters, such as ethyl acetate.

Medium to light shades of this relatively weak pigment are used to colour high grade industrial paints, especially automotive solid shades, as well as metallic finishes. P.Y.153 withstands exposure to up to 160 °C for 30 min. Incorporated in an alkyd-melamine resin, full shades (10%) and 1/3 SD samples equal step 8 on the Blue Scale for lightfastness, while 1/25 SD specimens match step 7–8. In terms of lightfastness, P.Y.153 performs approximately like P.Y.138, a quinophthalone pigment. P.Y.153 is fast to overcoating up to 120 °C, but bleeds at higher temperatures.

High durability makes P.Y.153 a preferred product in emulsion paints, especially for exterior house paints. It is a relatively important pigment for these purposes.

4.2.4.8 Pigment Yellow 177

P.Y.177 is no longer listed as a commercial product. It was a special-purpose pigment for polypropylene and polyamide spin dyeing. As a colourant for these media, P.Y.177 has the added advantage of enhancing the stability of the fibres.

Baking enamels containing P.Y.177 are very sensitive to overcoating. Their shade depends largely on the depth of shade. Full shades are typically reddish brown, similar to iron oxide systems, while white reductions are greenish yellow.

4.2.4.9 Pigment Yellow 179

The production of P.Y.179, an isoindolinone/cobalt complex pigment, has been discontinued. It was recommended for use in paints, especially in automotive finishes. The pigment produces a reddish yellow shade. High lightfastness and excellent weatherfastness are an asset in pastel colours. Besides, good transparency made P.Y.179 a suitable product for metallic finishes. Yet, it is not quite as weatherfast as the equally reddish yellow P.Y.24, a flavanthrone pigment.

4.2.4.10 Pigment Orange 59

This pigment has been withdrawn from the market.

P.O.59 is a nickel complex that affords very dull yellowish shades of orange. The pigment was used to match deep to medium shades in the yellow and orange part of the spectrum for high grade industrial finishes, as well as for architectural paints.

P.O.59 exhibits good lightfastness and weatherfastness. Incorporated in a baking enamel based on alkyd-melamine resin, for instance, 10% full shades equal step 8 on the Blue Scale for lightfastness, while 1/3 to 1/25 SD samples correspond to step 7, and 1:200 SD specimens (with TiO₂) match step 6 on the Blue Scale. The systems rapidly decrease in weatherfastness as more TiO₂ is added. Up to 180 °C, the pigment is heat stable and fast to overcoating, as long as the baking temperature does not exceed 140 °C. P.O.59 is not alkali and acid resistant; pigment demetallation fades or lightens the shade.

4.2.4.11 Pigment Orange 65

P.O.65, was withdrawn from the market a few years after the production started. It is an azomethine/nickel complex pigment and produces dull, reddish shades of orange, but very brilliant shades of copper in metallic finishes. P.O.65 was therefore considered a speciality product for such purposes. The pigment is not entirely fast to overcoating. Its weatherfastness in metallic finishes is very good and satisfies the requirements for use in original automotive finishes. This is despite the fact that it fails to reach the durability standards of the reddish P.R.168, a dibromanthanthrone pigment.

4.2.4.12 Pigment Orange 68

P.O.68 lends itself for use in industrial paints, including automotive finishes, as well as for plastics. It provides dull, reddish shades of orange. Two varieties are commercially available: a version with a coarse particle size and a type with a fine particle size.

The pigment with a fine particle size provides good transparency at high tinctorial strength. It is recommended primarily for use in metallic finishes to produce shades of copper, gold and brown. Transparent P.O.68 is not completely fast to overcoating. This type exhibits good lightfastness and durability, but tends to darken. This is especially true for deep solid shades and colour intense metallic finishes.

The variety with a coarse particle size is a pigment with equally high tinctorial strength and provides good hiding power. It is bluer and somewhat duller than the version with a fine particle size. The opaque type, like the transparent variety, is not completely fast to overcoating. Lightfastness and durability equal those of the pigment with a fine particle size. Opaque P.O.68 is used in combination with quinacridone pigments, magenta or violet pigments to produce dull shades of red and maroon.

P.O.68 is also used in plastics for its high heat stability. 1/3 SD colourations in HDPE (high density polyethylene) containing 1% TiO₂ withstand temperatures up to 200 °C. The shade is a dull orange. Some 0.15% pigment is required to formulate 1/3 SD samples containing 1% TiO₂. Such specimens match step 7 on the Blue Scale for lightfastness. In some plastics (polycarbonate), which are processed at high temperature, the pigment withstands as much as 320 °C. This makes P.O.68 one of the most heat stable organic pigments known. The list of recommendations also includes polyamide colouration.

4.2.4.13 Pigment Red 257

P.R.257, a heterocyclic nickel complex, covers the range of reddish violet shades; its hue is distinctly yellower than that of the P.R.88 type thioindigo pigments and of the β-modification of P.V.19, an unsubstituted quinacridone pigment. In paints, P.R.257 is tinctorially weaker than these pigments. In combination with Molybdate Orange or opaque organic red pigments it is also less intense. The commercial grade demonstrates good hiding power, good rheology and good resistance to flocculation. Both lightfastness and weatherfastness are equally good and, both in full shades and in white reductions, comparable to the performance of P.R.88. In baking enamels, P.R.257 is fast to overcoating. Its full shade and similarly deep shades are used in original automotive and industrial finishes.

4.2.4.14 Pigment Red 271

This isoindoline nickel complex pigment is mainly recommended for the colouration of metallics and effect coatings, especially for water based systems. P.R.271 affords yellowish to medium red shades, providing a bright flop in metallics. Alkyd/melamine resin systems may safely be overcoated and withstand exposure of 140 °C for 30 min. The flow properties of P.R.271 in these systems and in polyestercellulose acetobutyrate base coat systems are good, but there is a certain tendency to flocculate.

4.3.5.1 Pigment Yellow 218

The chemical structure of P.Y.218 is listed in the Colour Index under Constitution Number 561 805. It is a 3-fluoroquinolonoquinolone:

P.Y.218 has a bright yellow shade and is a high performance pigment for inkjet, plastics and paints. Lightfastness and thermostability are outstanding. It can also be used as a bright and strong yellow inkjet pigment.

4.3.5.2 Pigment Yellow 220

P.Y.220 is a 2-fluoroquinolonoquinolone, that is, an isomer of P.Y.218. In the Colour Index, the structure is listed under Constitution Number 561 806:

P.Y.220 has a bright and neutral yellow hue. It is a strong inkjet pigment with outstanding fastness properties.

4.3.5.3 Pigment Yellow 221

The chemical structure of P.Y.218 is listed in the Colour Index under C.I. Constitution Number 561 801:

P.Y.221 is a mixture of 80–90% 3-chloroquinolonoquinolone (C.I. 561802, R³ = Cl, R⁹ = H), quinolonoquinolone (C.I. 561800, R³ = R⁹ = H) and 3,9-dichloroquinolonoquinolone (C.I. 561812, R³ = R⁹ = Cl).

P.Y.221 is a yellow pigment that can be used for plastics and paints. Lightfastness and thermostability are outstanding. It can also be used as a bright and strong yellow inkjet pigment.