Key words

wood adhesives; tannin adhesives; lignin adhesives; biobased adhesives; wood welding

23.1 Introduction

Wood and fiber adhesives from renewable raw materials have been a topic of considerable interest for many years. This interest, already present since the 1940s, became more intense with the world’s first oil crisis in the early 1970s and subsided again as the cost of oil decreased. At the beginning of the twenty-first century this interest is becoming intense again for a number of reasons. The foreseen future scarcity of petrochemicals still appears to be reasonably far into the future. It is a contributing factor but, at this stage, it is not the main motivating force. The main impulse of today’s renewed interest in bio-based adhesives is the acute sensitivity of the general public to anything that has to do with the environment and its protection. It is not even this concern per se that motivates such an interest. There are rather very strict, for some synthetic adhesives almost crippling, government regulations which are just starting to be put into place to allay the environmental concerns of the public.

First of all, it is necessary to define what is meant by bio-based wood adhesives, or adhesives from renewable, natural, non-oil-derived raw materials. This is necessary because in its broadest meaning the term might be considered to include urea-formaldehyde resins, urea being a non-oil derived raw material. This, of course, is not the case. The term ‘bio-based adhesive’ has come to be used in a very well specified and narrow sense to only include those materials of natural, non-mineral, origin which can be used as such or after small modifications to reproduce the behaviour and performance of synthetic resins. Thus, only a limited number of materials can currently be included, at a stretch, in the narrowest sense of this definition. These are tannins, lignin, carbohydrates, unsaturated oils, proteins and protein hydrolysates, dissolved wood and wood welding by self-adhesion. To this in the future will surely be added proteins, blood and collagen that are already being used to some extent based on technology from the far past. The bio-based wood adhesives approach does not mean, however, any need to go back to the technology of natural product adhesives as they existed up to the 1920s and 1930s before they were supplanted by synthetic adhesives. The bio-based adhesives we are talking about here are indeed derived from natural adhesives, but using or requiring novel technologies, formulations and methods.

Of the classes of bio-based wood adhesives mentioned above, in the case of tannins and lignins their interest has been directed primarily at substituting phenol-formaldehyde (PF) resins, because of the phenolic nature of these two classes of compounds. In some cases, some formaldehyde is still used, and in the case of lignin some other additives. It is then necessary to distinguish between bio-based adhesives in which a limited amount of synthetic additives are still used, and bio-based wood adhesives where no synthetic additives are used.

Three types of thermosetting adhesives based on bio-materials have reached commercial exploitation, although their use is not yet widespread, namely tannin-based adhesives at a level of up to 50,000 tons solids per year, lignin adhesives coupled with synthetic resins, and soy protein hydrolysate adhesives.

23.2 Tannin adhesives

The word tannin has been used loosely to define two different classes of chemical compounds of mainly phenolic nature: hydrolysable tannins and condensed tannins. The former are mixtures of simple phenols such as pyrogallol and ellagic acid and of esters of a sugar, mainly glucose, with gallic and digallic acids [1]. The lack of macromolecular structure in their natural state, the low level of phenol substitution they allow, their low nucleophilicity, limited worldwide production, and higher price somewhat decrease their chemical and economic importance.

Condensed tannins, on the other hand, constituting more than 90% of the total world production of commercial tannins (200,000 tons per year), are both chemically and economically more important for the preparation of adhesives and resins. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees. These include various Acacia (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), and Rhus (sumach extract) species, from which commercial tannin extracts are manufactured, and various Pinus bark extract species.

23.2.1 Condensed (polyflavonoid) tannins

Condensed tannins are polyhydroxyphenols, polyflavonoids (see Fig. 23.1), which are soluble in water, alcohols and acetone and can coagulate proteins. They are mainly obtained commercially by water extraction from wood and bark. The other main components of the extracts, called non-tannins, are simple sugars and polymeric carbohydrates. The content of non-tannins can reduce the performance and water resistance of tannin-bonded joints. The polymeric carbohydrates also increase the viscosity of the extracts.

Tannin extracts are usually sold as spray-dried powders. No purification step is usually carried out in industrial-scale production. The modification of the extracts is especially aimed at decreasing the sometimes too high viscosity to achieve better handling and application, but also a longer pot life and a better crosslinking [1,2].

As tannins contain many ‘phenolic’-type subunits, one may be tempted to think that they will exhibit a similar reactive potential to that of phenol and therefore procedures used in standard PF production can be transferred to those containing tannin. This, however, is not the case because tannins are far more reactive than phenol due to the resorcinol and phloroglucinol nuclei present in the structure of condensed tannins. This increase in hydroxyl substitution on the two aromatic rings affords in relation to phenol an increase in reactivity towards formaldehyde of 10–50 times.

Tannin crosslinking by formaldehyde via methylene or methylene ether bridges in a polycondensation reaction is the traditional way for tannins to function as exterior-grade weather-resistant wood adhesives (see Fig. 23.2). Tannin reactions are based on their phenolic character similar to those of phenol with formaldehyde. Formaldehyde reacts with tannin in an exothermic reaction forming methylene bridges. At neutral or even slightly acid pH, formaldehyde reacts rapidly with the tannin. This leads to the advantage that there is no high alkali content in tannin adhesives, contrary to synthetic phenolic resins. Thus, the neutral, hardened glue line which is obtained yields a noticeable improvement in water and weather resistance. One example that natural adhesive can present equal or even better performance of synthetic adhesives is illustrated in Fig. 23.3. Figure 23.3 shows the condition of two commercial, industrial particleboard panels after 15 years unprotected exposure to the weather at 1,500 m of altitude in a high UV radiation area. The remarkable performance of the tannin-bonded panel is easy to see in relation to a pure melamine-formaldehyde (MF) bonded panel. It must be stated clearly that in the 25 years since this picture was taken, melamine resin engineering has improved considerably to the point that similar performances to those of the tannin-bonded board can now be obtained too with top of the range synthetic melamine-urea-formaldehyde adhesives (with at least 45:55 M:U by weight).

Due to their high reactivity, tannin adhesives, in reality natural novolaks, need addition of paraformaldehyde hardener just before use. Furthermore, their aldehyde content is much lower than for synthetic adhesives as this is exclusively used as hardener, the polymer having been built naturally already in the tree. One alternative way used industrially to add the hardener, other than just mixing it in the glue-mix, is its separate addition by, for example, dosing the paraformaldehyde via a small screw conveyor directly to the particles in the blender. Also, for a liquid crosslinker, e.g. a urea-formaldehyde prepolymer concentrate (formurea), this can be mixed with the tannin solution in a static mixer shortly before the blender. The higher viscosity of the tannin solution at higher pHs, even without addition of the hardener, can be overcome by warming up to 30–35°C or by adding water. However, the viscosity is decreased well before application by simple acid-base pretreatments to hydrolyse the polymeric carbohydrates in the tannin extract. A higher moisture content of the glued particles is no disadvantage with these adhesives, but rather necessary to guarantee a proper flow of the tannin [1–3], this being in reality a considerable industrial advantage these adhesives have over synthetic adhesives.

The hardeners used are mainly paraformaldehyde fine powder [1,2], and methylolurea mixtures such as urea-formaldehyde precondensate or formurea [4]. Tannins can also be hardened by addition of hexamethylenetetramine (hexamine) [3,5–8], whereby these boards show a very low formaldehyde emission [1–3,5,9–12]. The autocatalytic hardening of tannins without addition of formaldehyde or another aldehyde as crosslinker is possible, if small traces of alkaline SiO₂ is present as catalyst at a high pH, or with certain tannins just by the catalytic action induced by the wood surface [13–22]. Tannin autocondensation ensures a totally natural, environmentally-friendly adhesive, but it is only suitable for interior-grade applications.

23.2.2 New technologies for industrial tannin adhesives

Extensive, up-to-date and in-depth reviews of the technology of tannin adhesives based on the classical technology of tannin-formaldehyde resins, as summarized above, already exist [1,13]. These technologies are commercial now for several years and used in a number of countries. It is sufficient to state here that tannin-formaldehyde adhesives of very low emission (E0), fast pressing times and using unmodified tannin extracts are well known, are used commercially and their technology is commercially and perfectly mastered [23,24]. The new technologies are those based either on no addition of aldehydes, or on the use of hardeners which are non-emitting or manifestly non-toxic.

The quest to decrease or completely eliminate formaldehyde emissions from wood panels bonded with adhesives, although not really necessary in tannin adhesives due to their very low emission (like most phenolic adhesives), has nonetheless promoted some research to further improve formaldehyde emissions. This has centered into two lines of investigation: (i) the use of hardeners not emitting at all simply, because either no aldehyde has been added to the tannin, or because the aldehyde cannot be liberated from the system, and (ii) tannin autocondensation. Methylolated nitroparaffins and in particular the simpler and least expensive exponent of their class, namely trishydroxymethyl nitromethane [25,26], belong to the first class. They function well as hardeners of a variety of tannin-based adhesives while affording considerable side advantages to the adhesive and to the bonded wood joint. In panel products such as particleboard, medium density fiberboard and plywood, the joint performance which is obtained is of the exterior/marine grade type, while a very advantageous and very considerable lengthening in glue-mix pot-life is obtained. Furthermore, the use of this hardener is coupled with such a marked reduction in formaldehyde emission from the bonded wood panel to reduce emission exclusively to the formaldehyde emitted by heating just the wood (and slightly less, thus functioning as a mild depressant of emissions from the wood itself). Furthermore, trishydroxymethyl nitromethane can be mixed in any proportion with traditional formaldehyde-based hardeners for tannin adhesives, its proportional substitution of such hardeners inducing a proportionally marked decrease in the formaldehyde emissions of the wood panel without affecting the exterior/marine grade performance of the panel. Medium density fiberboard (MDF) industrial plant trials confirmed all the properties reported above and the trial conditions and results have been reported [25,26]. A cheaper but equally effective alternative to hydroxymethylated nitroparaffins is the use of hexamine as a tannin hardener.

Tannin-hexamethylenetetramine (hexamine) adhesives

These adhesives are already commercial. Under many wood adhesive application conditions, contrary to what was thought for many years, hexamine used as a hardener of a fast reacting species is not at all a formaldehyde-yielding compound, yielding extremely low formaldehyde emissions in bonded wood joints [27]. ¹³C NMR evidence has confirmed [28–30] that the main decomposition (and recomposition) mechanism of hexamine under such conditions is not directly to formaldehyde. It rather proceeds through reactive intermediates, hence mainly through the formation of reactive imines and iminoaminomethylene bases (Fig. 23.4). ¹³C NMR evidence has also confirmed [28–30] that in the presence of chemical species with very reactive nucleophilic sites, such as melamine, resorcinol and condensed flavonoid tannins, hexamine does not decompose to formaldehyde and ammonia. Instead, the very reactive but unstable intermediate fragments react with the tannin, melamine, etc., to form aminomethylene bridges before any chance to yield formaldehyde. These are also stable for 1–5 hours at temperatures as high as 120°C. The intermediate fragments of the decomposition of hexamine pass first through the formation of imines followed by their decomposition to imino-methylene bases. The latter present only one positive charge as the second methylene group is stabilized by an imine-type bond [28–30] (Fig. 23.4). Any species with a strong real or nominal negative charge under alkaline conditions, be it a tannin, resorcinol or another highly reactive phenol, be it melamine or another highly reactive amine or amide, or be it an organic or inorganic anion, is capable of reacting with the intermediate species formed by decomposition (or recomposition) of hexamine far more readily than formaldehyde [28–30]. This explains the capability of wood adhesive formulations based on hexamine to give bonded panels of extremely low formaldehyde emissions. If no highly reactive species with strong real or nominal negative charge is present, then decomposition of hexamine proceeds rapidly to formaldehyde formation as reported in the previous literature [31].

On this basis, the use of hexamine as a hardener of a tannin, hence a tannin-hexamine adhesive, is a very environmentally friendly proposition. Formaldehyde emissions in a great chamber has been proved to be so low as to be limited exclusively to what is generated by the wood itself, hence truly E0 panels. The panels obtained with tannin-hexamine adhesives, according to under which conditions they are manufactured, can satisfy both interior and exterior grade standard specification requirements [32]. Steam injection presses recently have shown to be better suited to give better results for exterior grade boards using tannin-hexamine adhesives [33,34]. Comparable results are obtained with pine tannins or other procyanidins hardened with hexamine [35]. In the same reference, catalysis of the reaction in the presence of small amounts of accelerators such as a zinc salt allows even better results or faster press times.

Hardening by tannin autocondensation

The auto-condensation reactions characteristic of polyflavonoid tannins have only recently been used to prepare adhesive polycondensates hardening in the absence of aldehydes [36]. This auto-condensation reaction is based on the opening under either alkaline or acidic conditions [36] of the O1-C2 bond of the flavonoid repeating unit and the subsequent condensation of the reactive center formed at C2 with the free C6 or C8 sites of a flavonoid unit on another tannin chain [36–40]. Although this reaction may lead to considerable increases in viscosity, gelling does not generally occur. However gelling takes place when the reaction occurs in presence of small amounts of dissolved silica (silicic acid or silicates) catalyst and some other catalysts [36–40], and on a lignocellulosic surface [36].

As in the case of other formaldehyde-based resins, the interaction energies of tannins with cellulose obtained by molecular mechanics calculations [41] tend to confirm the effect of surface catalysis induced by cellulose also on the curing and hardening reaction of tannin adhesives. The considerable energies of interactions obtained can effectively explain weakening of the heterocyclic ether bond leading to accelerated and easier opening of the pyran ring in a flavonoid unit, as well as the facility with which hardening by auto-condensation can occur. In the case of the more reactive procyanidins and prodelphinidin-type tannins, such as pine tannin, cellulose catalysis is more than enough to cause hardening and to produce boards of strength satisfying the relevant standards for interior grade panels [36]. Figure 23.5 shows that the slower reacting tannins can yield an upgraded IB strength of the board when mixed with small amounts of faster reacting tannins. In Fig. 23.5, the effect of adding pecan tannin is shown as an example, but similar upgrades can be obtained by adding pine tannin too. In the case of the less reactive tannins, however, such as mimosa and quebracho, the presence of a dissolved silica or silicate catalyst of some type is the best manner to achieve panel strength as required by the relevant standards [13]. Auto-condensation reactions have been shown to contribute considerably to the dry strength of wood panels bonded with tannins, but to be relatively inconsequential in contributing to the bonded panels’ exterior grade properties which are rather determined by polycondensation reactions with aldehydes [41,43]. Combination of tannin auto-condensation and reactions with aldehydes, and combination of radicals with ionic reactions, have been used to decrease both the proportion of aldehyde hardener as well as to decrease considerably the already low formaldehyde emissions yielded by the use of exterior tannin adhesives [41–43]. A variation on the same theme of wood adhesives by tannin autocondensation is acid-catalysed oxidative condensation [44].

23.3 Lignin adhesives

23.4 Mixed tannin-lignin adhesives

Mixed interior wood panel tannin adhesive formulations were developed in which lignin is in considerable proportion, 50%, of the wood panel binder and in which no ‘fortification’ with synthetic resins, such as the isocyanates and phenol-formaldehyde resins as used in the past, was necessary to obtain results satisfying relevant standards. A low molecular mass organosolv lignin obtained industrially by formic acid/acetic acid pulping of wheat straw was used. Environmentally-friendly, non-toxic polymeric materials of natural origin constitute up to 94% of the total panel binder. The wood panel itself is constituted of 99.5% natural materials, the 0.5% balance being composed of glyoxal, a non-toxic and non-volatile aldehyde, for the pre-glyoxalation of lignin and of hexamine already accepted as a non-formaldehyde-yielding compound when in the presence of condensed tannin. Both particleboard and two types of plywood were shown to pass the relevant interior standards with such adhesive formulations [75–77,79].

It is of interest to understand the main mechanisms according to which this formulation works. As lignin is of much lower reactivity than a flavonoid tannin, a series of parallel reactions occur to yield co-reaction of tannin with lignin. Thus, tannin reacts very rapidly in the hot press with hexamine to form a network based on the known reactions of tannin with hexamine [5,6–8]. Lignin, much slower, is pre-reacted in a reactor with glyoxal [75,78]. Although some condensation of glyoxalated lignin occurs during the reaction, if the reaction is protracted for longer times unduly increasing the viscosity of the glyoxalated lignin, condensation needs to be minimized and addition of glyoxal on the lignin maximized. The glyoxalated lignin is then rich in methylol-type groups obtained by glyoxal addition, but the aromatic nuclei of lignin are still too slow for glyoxalated lignin alone to condense to a sufficiently hardened network in just the very brief period the board remains in the hot press. The methylol-type groups obtained by glyoxal addition to lignin are then forced to react with the much more reactive flavonoid tannin introducing, then, the lignin in the final copolymer network. A simplified scheme explaining in brief the reactions involved is shown in Fig. 23.5.

23.5 Protein adhesives

Intense research induced by the sponsorship of the US United Soybean Board has revised interest in protein adhesives, soya protein primarily, but also others. These technologies must not be confused with the age-old protein and bone glues used in carpentry, or blood used as an additive in plywood glue-mixes. Certainly, of the traditional technologies, one stands out head and shoulders above the others, namely casein adhesives. These are still produced, and still used industrially to very good effect in some special plywood and related products. They work as such, they are very environmentally friendly and their technology was completely mastered a long time ago. They are definitely strong candidates for future expansion, in their actual form or with some further technological improvement. What is new is some interesting work on the correlation between the bond strength obtained and the molecular architecture of the protein [80].

On the other hand, soya and even gluten adhesives are definitely new. Both addition to traditional synthetic wood adhesives, as well as their use as panels adhesives after partial hydrolysis and modifications have been reported, and with acceptable results [81–84]. These products are not widely used industrially/commercially as yet, but one industrial user has been reported in the US.

Here, too, several different technologies can be distinguished. First, technologies based on the pre-reaction of soy protein hydrolysate with formaldehyde, and this pre-formulated soy protein being mixed with a PF resin and with isocyanate (PMDI) [85,86], thus an identical technological approach to what has already presented for lignin adhesives in Section 23.3.3. Second, the evolution of this technology, again along similar lines as for lignin, in which pre-glyoxalated soy protein [87] or even soy flour, or glyoxalated gluten protein hydrolysate [88], glyoxal being a non-volatile non-toxic aldehyde, compose the glue-mix with either a PF resin or with a flavonoid tannin, the whole been added to 20–25% isocyanate (PMDI) [87,88]. Both these systems work well, but the more interesting system, and the one that has started in industrial exploitation is the third one. This is based on the pre-reaction of the soy protein hydrolysate with malei anhydride to form an adduct that is then reacted in the panel with polyethyleneimmine [82,89]. The system works well, as one company has started using it industrially in the US, but it suffers the drawback of being excessively expensive, the price quoted being considerably higher than those of isocyanates.

23.6 Carbohydrate adhesives

Carbohydrates in the form of polysaccharides, gums, oligomers and monomeric sugars have been employed in adhesive formulations for many years. Carbohydrates can be used as wood panel adhesives in three main ways: (i) as modifiers of existing PF and UF adhesives, (ii) by forming degradation compounds which then can be used as adhesives building blocks, and (iii) directly as wood adhesives. The second route above leads to furanic resins. Furanic resins, notwithstanding that their basic building blocks, furfuraldehyde and furfuryl alcohol, are derived from the acid treatment of the carbohydrates in waste vegetable material, are considered today as purely synthetic resins [90]. This opinion might need to change as in reality they are real natural-derived resins, and extensively used in foundry core binders. Appropriate reviews dedicated to just them do exist [90]. However, both compounds are relatively expensive, very dark-colored and furanic resins have made their industrial mark in fields where their high cost is not a disadvantage. They can be used very successfully for panel adhesives, they are used very successfully in other fields (as foundry core binders), but the relatively higher toxicity of furfuryl alcohol before it is reacted is a problem that will have to be taken into consideration if these resins are to be considered for wood products.

The use of carbohydrates directly dissolved in strong alkali as wood panel adhesives is not a new concept, but is an interesting and a topical one today. This technology has been extensively reported [91]. All sorts of agricultural cellulosic materials have been successfully adapted to this technology and the technology and its application have been extensively reported in the past [91].

Research on the first route has centered particularly on the substitution of carbohydrates for parts of PF resins. It has been reported that at laboratory level, up to 50–55% of phenol in a PF resin can be substituted with a variety of carbohydrates, from glucose to polymeric, tree-derived hemicelluloses [92–96]. Apparently reducing sugars could not be used directly as they are degraded to saccharinic acids under the acid conditions required in the formulation of the resin. Reducing sugars can be used to successfully modify PF resins if they are reduced to the corresponding alditols or converted to glycosides. Some carbohydrates appeared to be incorporated into the resin network mainly through ether bridges [92]. Generally the resin is prepared by co-reacting phenol, the carbohydrate in high proportion, a lower amount of urea and formaldehyde. Extensive and rather successful industrial trials of these resins have also been reported [96].

Carbohydrate-based adhesives in which the formulation starts with the carbohydrate itself have also been reported, but the acid system used during formulation readily degrades the original carbohydrate to furan intermediates which then polymerize. An interesting concept that was advanced early on in carbohydrate adhesives research was the conversion of the carbohydrate to furanic products in situ, which then homopolymerize as well as react with the lignin in wood.

Several research groups [92,97,98] have recently described the use of liquefied products from cellulosic materials, literally liquefied wood, which showed good wood adhesive properties. Lignocellulosic and cellulosic materials were liquefied in the presence of sulfuric acid under normal pressure using either phenol or ethylene glycol. The cellulosic component in wood was found to lose its pyranose ring structure when liquefied. The liquefied product contains phenolic groups when phenol is used for liquefaction. In the case of ethylene glycol liquefaction, glucosides were observed at the initial stage of liquefaction and levulinates after complete liquefaction.

23.7 Unsaturated oil adhesives

Saturated and unsaturated vegetable oils are now widely available as a bulk commodity for a variety of purposes and at very acceptable prices. All resin research to date has focused on oils that contain at least one double bond. These oils are predominantly a mixture of triglycerides, hence esters, with a small quantity of free fatty acid, the small proportion of free fatty acid being dictated both by the plant species and the extraction conditions. As the number of unsaturations increases, so does the overall molecule reactivity and its potential for side reactions. The majority of these technologies are not applied to wood and wood composite adhesives, but they can be translated eventually to this field. An excellent and detailed review of formulations and technology on the subject already exists [99].

Until fairly recently, only two examples could be found in the literature where seed oil derivatives were being employed as wood adhesives. Linseed oil, for example, has been used to prepare a resin that can be used as an adhesive or surface coating material [100,101]. The chemistry of this resin centers on an epoxidation of the oil double bonds followed by crosslinking with a cyclic polycarboxylic acid anhydride to build up molecular weight. The reaction is started by the addition of a small amount of polycarboxilic acid.

When the epoxidized oil resin was evaluated as a wood adhesive in composite panels, it could be tightly controlled through the appropriate selection of triglycerides and polycarboxylic anhydrides. This apparently enables a wide range of materials with quite different features to be manufactured. The use as wood adhesives is one among the many uses, the focus of the development being more on plastic materials. The literature states that this plastic is well suited for use as a formaldehyde-free binder for wood fibers and wood particles, including fibers and chips from cereal residues, such as straw and fiber mats.

The literature on this resin [100] claims that crosslinking can be varied through the addition of specialized catalysts and several samples were prepared at a range of temperatures (120–180°C) that exhibited high water tolerance even at elevated temperatures, but no actual test data were included. Since the resin of reference [100], research in a number of other countries by imitators has produced very similar epoxidized oil resins. These are suitable for a number of applications, but the author has tested one or two of them finding that for wood adhesive applications, these resins have two major defects: (i) their hot-pressing time is far too slow to be of any interest in wood panel products, with the exception perhaps of plywood (for which they have not been tried), and (ii) they are relatively expensive. Unless the slow hot-pressing problem is overcome, and at a reasonable price, these resins will remain at the stage of potential interest. There is no doubt that these resins can be of interest in other fields, but it is symptomatic that no industrial use for wood panel adhesives has been reported as yet, or is known to have occurred.

Bioresins based on soy bean and other oils have been developed also by other groups, mainly for replacement of polyester resins [102]. These liquid resins were obtained from plant and animal triglycerides by suitably functionalizing the triglyceride with chemical groups (e.g., epoxy, carboxyl, hydroxyl, vinyl, amine, etc.) that render it polymerizable. The reference claims that excellent inexpensive composites were made using natural fibers such as hemp, straw, flax and wood in fiber, particle and flake form. That soy oil-based resins have a strong affinity for natural fibers and form a good fiber-matrix interface as determined by scanning electron microscopy of fractured composites. The reference also stated that these resins can be viewed as candidate replacements for phenol-formaldehyde, urethane and other petroleum-based binders in particleboard, MDF, OSB and other panel types. However, no actual test data have been supplied, and no industrial use in wood panel adhesives has actually been reported as yet.

Cashew nut shell liquid, mainly composed of cardanol but containing also other compounds (Fig. 23.6), is an interesting candidate for wood-based resins. Its dual nature, phenolic nuclei + unsaturated fatty acid chain, makes a potential natural raw material for the synthesis of water-resistant resins and polymers. Cardanol resins are known from the past, but their use has not been very diffuse simply because the raw material itself was rather expensive. The price, however, appears to be more affordable now since the extensive cashew nut plantations in Mozambique are in production.

The phenol, often resorcinol, group and/or double bonds in the chain can be directly used to form hardened networks. Alternatively, more suitable functional groups such as aldehyde groups and others can be generated on the alkenyl chain. Generally, modifications of this kind take several reaction steps, rendering the process too expensive for commercial exploitation in wood adhesives. However, the Biocomposite Center in Wales [101] has developed a system of ozonolysis [101,103] in industrial methylated spirit [101,103] through which an aldehyde function is generated on the alkenyl chain of cardanol. The first reaction step yields as major product a cardanol hydroperoxide that following reduction by glucose or by zinc/acetic acid yields a high proportion of cardanolaldehyde groups. These crosslink with the aromatic groups of cardanol itself, thus a self-condensation of the system yielding hardened networks [101] (Fig. 23.6).

Exploratory laboratory particleboard and lap shear bonding yielded good results. Nonetheless, neither the press times used, nor other essential conditions that could help to evaluate the economical feasibility of these products were reported [101]. It remains to evaluate if the cost of the ozonolysis allows wood adhesives of a suitably low cost and, again, if the pressing times can match those of industrial resins. However, the resorcinolic structure of the cardanol phenol group would appear to indicate that the molecule should be able to achieve industrial pressing times.

More recently, alternative and very encouraging techniques involving unsaturated oils for wood and wood fiber adhesives have come to the fore [104]. Wheat straw particleboards were made using UF and acrylated epoxidized soy oil (AESO) resins with two resin content levels: 8% and 13%, and three pressing times: 8, 10 and 12 minutes. The boards’ physical and mechanical properties showed that AESO-bonded particleboards have higher physical and mechanical properties than UF-bonded boards, especially in internal bonding and thickness swelling [104]. All properties of AESO-bonded boards increase by increasing both resin content and pressing time. AESO bonded boards can compete with wood particle boards according to EN standards. The good properties of these particleboards are because of high compatibility between straw particles and AESO resin due to its oil-based structure; their good properties can be maintained even at low board densities. This work was based on a number of different technologies presented and discussed in a most appropriate monograph [99].

23.8 Wood welding without adhesives

Assembly techniques with mechanical connectors or with adhesives are common in joining solid wood in the furniture, civil engineering and wood joinery industries. Both kinds of connections show several problems. With mechanical metal connectors, rust stains may appear on the connectors and corrosion of the connectors can and does occur. With adhesively-bonded joints working with liquid adhesives, the costs are higher than for the use of mechanical connectors as regards manufacturing equipment maintenance. With adhesives the process is relatively longer unless high investments in adhesive materials and machinery (high-frequency or microwave systems) are made in order to speed up the hardening phase. Thermoplastic welding techniques which are widely used in the the plastic and car industries have recently been applied also to joining wood, by melting a thermoplastic polymer between the two wood surfaces to be joined. A variety of techniques such as ultrasound, mechanical friction and others have been used to melt the thermoplastic polymer in situ. In friction welding techniques the heat needed to melt the material is generated by pressing one of the samples to be joined against the other and to generate friction, hence heat, which increases the temperature of the weldline rapidly and considerably.

The same mechanically-induced friction welding techniques which are widely used in the plastic and car industries have recently been applied also to joining wood, without the use of any adhesive [105–111]. These work by melting some wood components and forming at the interface between the two wood surfaces to be joined a composite of entangled wood fibers drowned into a matrix of melted wood intercellular material, such as lignin and hemicelluloses [105–111] (see Figs. 23.7 and 23.8). Linear mechanical friction vibration has been used to yield wood joints satisfying the relevant requirements for structural applications by welding at a very rapid rate [105,107,108]. Crosslinking chemical reactions have also been shown to occur by CP-MAS ¹³C NMR. These reactions, however, are lesser contributors during the very short welding period proper [112,113]. They gain more, but still very limited, importance during the subsequent brief pressure holding period [105,112,113].

Also recently, high speed rotation-induced wood dowel welding, without any adhesive, has been shown to rapidly yield wood joints of considerable strength [105,110,111]. The mechanism of mechanically-induced high speed rotation wood welding is due to the temperature-induced softening and flowing of the intercellular material, mainly amorphous polymer material bonding the wood cells to each other in the structure of wood. This material is mainly composed of lignin and hemicelluloses. This flow of material induces high densification of the bonded interface [105,110,111]. Wood species, relative diameter differences between the dowel and the receiving hole, and press time were shown to be parameters yielding significant strength differences [105,110,111]. Other parameters were shown to have much lesser influence.

The insertion of dowels into solid wood, for joinery and furniture, without any wood-to-wood welding, has been used for centuries. This simple technology has also been applied to join particleboard [114]. However, nowhere in the relevant literature has wood-to-wood welding ever been achieved or even mentioned. Only welding by fusion of interposed thermoplastic materials has been mentioned in previous literature.

The relative diameter difference between dowel and substrate was the most important parameter determining joint strength performance [105,110,111]. The real determining parameter, however, is how fast the lignin/hemicelluloses melting temperature is reached. The greater the relative difference between the diameters of the dowel and of the substrate hole, the greater is the friction, hence the more rapidly the lignin melting temperature is reached and a better welding is achieved.

23.8.1 Systems of frictional wood welding

Two wood welding systems exist today that give strength results of the joint which are higher than required by the relevant standards [105,106]. A third system has also been thoroughly tested [115], but with much poorer results. These systems are not limiting like other welding systems such as ultrasound, microwave and/or radiofrequency heating, and high rotation or high vibration spindle welding, laser welding as well as others are all likely to afford some level of wood welding. Experiments with some of them have also been carried out, and will be reported briefly later. The material flow and melting induced by the elevated temperatures reached lead to high densification of the interface between the two profiles and interfacial loss of the cellular wood structure in the joint, hence to increased strength of the interface [105–107].

Linear vibration welding

The wood samples to be joined together are first brought into contact, with a pressure between 1.3 and 2 MPa, to enable the joint areas to be rubbed together with a linear reciprocating motion (Fig. 23.9). The samples are vibrated with a displacement amplitude of about 3 mm and a vibration frequency of 100 Hz in the plane of the joint. The specimens welded up to now are of a length of up to 1.0–1.8 m [116], as the capacity of the existing machines is not greater than this. The time of welding is roughly 1.5–5 sec [105,108,109] and the holding time, still under pressure, after vibration has stopped is of 5 sec [105,108,109]. The results obtained satisfy the strength requirements for structural application. It must be pointed out, however, that the parameters used for wood-to-wood welding give widely different results as regards water resistance according to the technology used. Older linear welding technologies yield joints which are not water resistant to any great extent, and which then can only be used for strictly interior applications [105]. Newer linear welding technologies instead give joints of much greater resistance to water [108,109,113], while the geometry of the joint itself in rotational dowel welding yields joints of almost exterior grade level [118,119].

Wood grain orientation differences in the two surfaces to be bonded yield bondlines of different strength in no-adhesives wood-to-wood welding. Longitudinal wood grain bonding of tangential and radial wood sections yield approximately 10% difference in strength results of the joint. Cross-grain (± 90°) bonding yields instead much lower strength results, roughly half than observed for pieces bonded with the grain parallel to each other, although these results still satisfy the relevant wood-joining standards [117–119].

Of particular interest are studies on wood cross-grain welding [117–120], where the principles of the anisotropy of composites rule the type of strength results that can be obtained in welding. Of particular interest is the case of wood endgrain welding [120] where the formation of very strong butt joints by this technique does open up the possibility of eliminating wood adhesive bonded fingerjoints. An example of this is shown in Fig. 23.10 for oak wood joints that have been endgrain-welded.

High speed rotation dowel welding

Traditionally, wooden dowels of a given diameter (most commonly 10 mm) are forced into a pre-drilled hole of lower diameter by applying pressure. Alternatively, similar dowels can be inserted into holes of the same diameter after application of an adhesive, in general PVAc. In high speed rotation welding, fluted rib beech dowels (or even smooth dowels of other woods) are inserted at high rotation speed and insertion rate within a predrilled hole of smaller diameter [106] (Fig. 23.11).

In dowel welding, generally, cylindrical beech fluted dowels of 10 mm in diameter are used. They are placed into a drill, in the place of the drill bit, and inserted within pieces of wood having pre-drilled holes of 8 mm. For best results the drill rotation rate must be higher than 1,200 revolution per minute (rpm), and if possible equal to 1500–1600 rpm. When fusion and bonding are achieved, generally between 1 and 3 sec, the rotation of the dowel is stopped and the pressure may be briefly maintained [106,110,111].

Dowel welding by high speed rotation has been used to join two and even three wood block (Fig. 23.12). This is the ultimate aim of dowel welding. Strong joints were obtained [111,121]. An extremely important finding in dowel welding is that its water resistance is far superior to that obtained in linear welding [121]. This is due mainly to the geometry of the joint that allows approximately 80–90% of the dry strength to be conserved once it is wet, cold water soaked and up to 15% of the original dry strength once redried after 24 hours cold water soaking [121].

High-speed dowel rotation welding has been shown to be capable of holding together structures such as a suspended wood floor of size 4 m × 4 m × 0.2 m without using any adhesive, any nails or any other binding system other than welded dowels [122–124]. When this technique applies to solid wood, several physical, chemical and mechanical processes occur. The rheological behavior of wood that is compressed and heated simultaneously while a rapid vibrating shift is applied changes, the moisture content at the interface is reduced, and chemical modifications of the wood structure occur during heating and the solidification of the melted joint interface.

The welding or bonding that occurs at the interface is probably mainly explained by the melting and solidification of the hemicelluloses, mainly xylans [118,121], and intercellular middle lamella’s lignin and protolignin [105,118,121], and partly by the physical entanglement of the fibers interconnected between them as a result of the friction (Fig. 23.13). The direct welding of the cells is explained by the known properties of wood intercellular material: markedly thermoplastic, rigid and concentrated in the compound middle lamella of the wooden cells. Chemical phenomena occur too [105,112,113,118], mainly in the brief pressure holding phase immediately after welding, the main one of which is the formation and self-condensation of furfural [105,112]. The reactions involved are both of ionic type [105] as well as radical reactions [125].

23.9 Conclusion and future trends

Still many challenges confront bio-based adhesives for wood and for fibers. These can be divided into four broad classes:

This latter is both from the psychological point of view of operators, used for decades to using the same type of synthetic adhesives, thus their natural resistance to change, as well as the lobbying by chemical companies who already control the market with synthetic products, and whose interest is often not in using alternative materials but to maximize profits with the minimal possible effort: after all why change if they are already selling something and prices are increasing?

Research and development in adhesives and resins for wood and fiber composites are driven mainly by requirements of the bonding and production process and the properties of the fiber- and wood-based composites themselves, the main topics being:

The necessity to achieve shorter press times is omnipresent within the woodworking industry, based on a permanent and immanent pressure on costs and prices. An increased production rate is still one of the best ways to reduce production costs, as long as the market takes up the Surplus product. Shorter press times within a given production line and for a certain type of wood-based panel can be achieved, among others, by:

There is quite good industrial experience on how to improve the ability of a bondline to resist against moisture and water, especially at higher temperatures. However, it is also known that, for example, in outdoors applications all adhesive systems are working at the upper limit of their performance. Hence fiber and wood-based composites with optimized properties and performance, especially for new applications in construction purposes like facades, require new bonding ideas. Cheaper raw materials are another way to reduce production costs. In this respect, bio-based adhesives are in outright competition with improvements in synthetic adhesives, and these latter can be considerable enough to deny market entrance to bio-based adhesives. It is evident that for many applications bio-based adhesives can deliver the same performance, but while they are cheaper than certain adhesives such as synthetic phenol-formaldehyde resins, isocyanates and the melamine resins, they cannot compete either in present supply quantity or in price with urea-formaldehyde adhesives. Special legislation and the public pressure for environmentally friendly products do drive this trend at present. The increase in oil costs and other cost factors are also starting to favor interest in natural or partially natural adhesives or even in synthetic adhesives totally biosourced, these being traditional adhesives obtained by synthesis but for which the reagents used are obtained from transformed raw materials of natural origin.

One particular challenge, and possibly the greater one, is the supply and availability of the correct raw materials used for bio-based adhesives. One glaring example is tannin adhesives, now a mature technology that has been in industrial use in the southern hemisphere for 40 years. These adhesives give excellent PF resin substitutes but the amount produced yearly, in the order of a few hundred thousand tons, is literally a ‘drop in the ocean’ if one wants seriously to substitute them for phenolic resins in many of their applications. The availability of raw material is not the problem; after all the tree barks from which tannins are extracted are a worldwide resource and can yield millions of tons of usable tannin for adhesives and other resins. The problem is their present low installed extraction capacity. Thus, two alternatives exist for the future: (1) build new tannin extraction factories to increase production, an approach that seems to have been tentatively taken by a few corporations around the world, or (2) to dilute the tannin with an alternative bio-based material to increase the amount of adhesive available, the developments in this line too, namely mixed tannin/lignin and tannin/protein adhesives, having been developed and being of interest and being pursued by industry. Equally, widely available biomaterials such as lignin from the paper industry still need systems to upgrade further their reactivity, this being at present obviated by modifying them and mixing/co-reacting them with more reactive synthetic adhesives.

The wood and fibers themselves, especially the wood and fiber surface including the interface to the bondline, also play a crucial role for the quality of bonding and hence for the quality of the fiber- and wood-based composites. Low or even no bonding strength can be caused by unfavorable properties of the substrate surface, e.g. due to low wettability. Adhesives and resins are one of the important and major raw materials of fiber- and wood-based composites. Thus, each question concerning life cycle assessment and their recycling also is a question for the adhesives and resins used. This includes, for example, the impact of the resins on various environmental topics like wastewater and effluents, emissions during the production and from the finished composite or the energetic reuse of panels. Also for several material recycling processes the type of resin has a crucial influence on feasibility and efficiency.

The emissions of gases, especially from wood-based panels during and after their production, can be caused by wood-inherent chemicals, like terpenes or free acids, as well as by volatile compounds and residual monomers of the adhesive. The emission of formaldehyde is especially a matter of concern, but also possible emissions of free phenols or other monomers. The problem of the subsequent formaldehyde emission fortunately can be regarded as more or less solved, even in the case of bio-based adhesives due to stringent regulations having been implemented in many countries, and successful long-term R&D in industry.

However, this scenario cuts further into the competitiveness of some bio-based adhesives, as the upgrading caused by developments in synthetic adhesives to solve such problems diminishes the advantages in this area which are inherent to bio-based resins.