Recent Developments in Advanced Lignin-Based Adhesives and Binders

Abstract

Recently several developments on the use of lignin and its derivatives as wood adhesive and for other binders have come to the fore in the literature. The novelty of these approaches has been dictated by the well-known low reactivity of lignin with aldehydes in its substitution of phenol in phenol-formaldehyde resins. A brief record of the more recent novelties having being published on the use of lignin in the more traditional field of lignin-phenol-formaldehyde (LPF) adhesive resins is reported. However, this review chapter is more focused on the types of more recent alternative approaches that have been used with encouraging results to go around the obstacle of the low lignin-aldehyde reactivity. Thus, approaches based on lignin demethylation coupled with specific oxidation, pre-glyoxalated lignin dialdehyde starch cross-linking by urea, lignin-based non isocyanate polyurethane (NIPU) adhesives and resins, lignin amine coatings, lignin-triethyl phosphate cross-linking for both wood surface coatings and biobinders for metals/Teflon assemblies, and finally direct wood bonding by lignin esterification by citric acid are described.

1. Introduction

It is for a very long time that researchers have been focusing on using lignin for wood adhesives and other binders. The phenolic oligomeric nature of lignin, its relatively low cost and its great abundance as a byproduct of the pulp and paper industry brings to mind its similarity with phenol and has focused on its use as phenolic resins. Thus, for many decades since the early 1960s the literature is extremely rich of articles describing how to substitute lignin fully or partially for phenol in phenol-formaldehyde (PF) resins for wood panel adhesives. Notwithstanding this strong focus on its use the industrial uses for these applications have been extremely rare. Thus in the 1970s and 1980s lignin has been used for up to 30% substitution of phenol in PF plywood adhesives by a major company in the USA and several plywood factories in Canada [1, 2]. This use was discontinued later as it was realized that while the cost of materials was lowered in Lignin-PF resins an equivalent lengthening of the press time resulted from the lignin substitution of phenol, due to the former lower reactivity with aldehydes. The slower press time allowed only the use of these LPF resins in plywood where swiftness in press time was less necessary, hence less limited by the lignin constraining reactivity. Thus, all was reduced to an economic equation: lower material costs but higher processing costs, with the latter being calculated to be predominant, and lignin substitution of phenol in industry was abandoned. Research work on lignin-phenol-aldehyde combinations did not stop however, without really being able to overcome the fundamental drawback outlined above.

More recently a Scandinavian company has been able to manufacture to good effect a plywood adhesive containing 50% lignin substitution of phenol [3] and to successfully market it. This has caused a concomitant increase of interest in other researcher and some literature increase on alternative methods to achieve such a result [4, 5, 6, 7]. The publication on these more recent and different approaches are already many, but this review will report briefly only some of them but not focus particularly on them, as they can be considered as an intermediate rather than a final or more advanced solution to the use of lignin in wood adhesives and other binders. This review will mainly focus instead on approaches where the lignin can be used alone or in a great and determinant majority to qualify the adhesive as a bioadhesive rather than to use the still interesting, but more conservative approach described above for LPF resins.

2. Lignin-phenol-formaldehyde adhesives

On this rather more traditional approach a few innovation of note have been proposed that will be very briefly described here. First the substitution of formaldehyde by a much less toxic aldehyde such as glyoxal, an approach still relatively conservative [4, 5].

Then some research work to determine the influence of lignin modification methods on lignin-phenol-formaldehyde (LPF) adhesive properties [6, 7]. Thus, lignin was modified with four different approaches, namely glyoxalation (G), phenolation (P), ionic liquid treatment (IL) (Figure 1), and maleic anhydride grafting (MA) (Figures 2 and 3). Phenol in PF resins was then substituted with the lignins so modified to a 50 wt% level to prepare modified LPF adhesives for bonding wood particleboard. Standard methods were used to determine the gel times, solids content, density, and viscosity of the these experimental LPF adhesives and the physical properties of the particleboard bonded with them such as internal bond (IB) strength, bending modulus, dimensional stability and even formaldehyde emission levels. Differential scanning calorimetry demonstrated that the MA-pre-reacted lignin presented the lowest curing temperature of all the other experimental LPF resins with modified lignin and non-modified lignin control. They also presented a faster gel time coupled with a higher solids content and viscosity than the resins differently otherwise modified. Moreover, the particleboards bonded with either maleic anhydride modified or ionic liquid modified lignin LPF resins presented lower formaldehyde emission and a much improved mechanical strength than all the others, control included. No significant difference was apparent between the panels bonded with all the experimental modified LPF adhesives.

Figure 1.

Action of ionic liquids on lignin units.

Figure 2.

Reaction of maleic anhydride on lignin units to form maleated lignin.

Figure 3.

Maleated lignin bridges with a PF resin.

These works had the merit to show the mechanism of ionic liquid activation on lignin repeating units (Figure 1) as well as the effect of maleic anhydride maleation of lignin increasing the number of reactive site of the material by grafting C〓O and C〓C linkages on the lignin structure (Figure 2) and the way how the maleated lignin linked to the phenol-formaldehyde resin so to form a LPF adhesive (Figure 3).

3. Predominant lignin proportions approaches to wood adhesives and binders

The difficulty experienced in developing LPF resins indicates that a different reaction approach has to be taken to avoid the reactivity drawbacks of the material described in the numerous LPF approaches.

3.1 Lignin demethylation coupled with specific oxidation

Lignin was successfully modified to prepare a bio-based wood adhesive by just two synthesis steps: demethylation and periodate oxidation [8]. These lignin resins have been based on the reaction of ∙OH carrying biocompounds such as sugars, protein, tannins etc. to produce vicinal aldehyde groups on the biostructure by cleavage of vicinal C‒OH bonds as shown in Figure 4.

Figure 4.

Schematic example of aldehyde generations by specific oxidation of vicinal C-OH groups.

Specific oxidation of carbohydrates through this route is known for a long time [9] but has recently been applied for all sorts of resins from biomaterials [8, 9, 10, 11, 12, 13, 14, 15]. The same is true for lignin. Thus, the basic reaction in the case of lignin develops according to a mechanism that is similar but not identical to that of some other biomaterials (Figure 5) [8].

Figure 5.

Lignin cleaving sites and cleaved products by specific oxidation with sodium periodate and other oxidants.

Where in the β-O-4 lignin bond between two lignin units is then cleaved in two sites as shown.

The two aldehyde groups formed still attached to sizable lignin oligomeric residues can than react with the more reactive demethylated groups of the residual lignin to form a hardened network. It must be noted that also other oxidizing agent such as KMnO₄ and sodium persulfate can also induce similar cleaving, but only this last has been found to be as specific as the periodate.

Fourier Transform Infrared Spectroscopy (FTIR), Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) Mass Spectrometry and Thermomechanical Analysis (TMA) were combined to characterize the bio-based adhesives obtained. Furthermore, the adhesion performance of the bio-based lignin adhesive was measured by testing the tensile shear strength of the bonded joints. The results showed that demethylated lignin gives to the wood joint acceptable dry shear strength, but no wet shear strength. Thus, sodium peroxide (NaIO₄) oxidant was used to improve the bonding performance of lignin-based adhesives. The optimal formulation for oxidized demethylated lignin adhesives was found to be by 20% addition of NaIO₄. The adhesive presented is a lignin-derived non-aldehyde addition, high biomass content, and its preparation is convenient, low-cost product and can be applied successfully to interior plywood bonding.

In the case of the wet shear strength of oxidized demethylated lignin-based adhesives bonded plywood, nevertheless, is inferior to the requirements of the China national standard (GB/T 9846-2015, ≥0.7 MPa). Therefore, some further modifications will need to be implemented in future research work to reach the relative standard requirements for this aspect.

A further new bio-sourced adhesive approach has been tested based on urea-crosslinked glyoxalated lignin by using dialdehyde starch obtained by hydrogen peroxide oxidation of starch. Urea appears to react with both the dialdehyde starch and the glyoxalated lignin to cross-link the resin to a hardened state [16]. The reaction as determined by instrumental analysis was a shown in Figure 6.

Figure 6.

Reaction of cross-linking by urea of dialdehyde starch and glyoxalated lignin.

The adhesives based on this approach were used to bond wood particleboards, and tested by both differential scanning calorimetry (DSC), and thermomechanical analysis (TMA). The panels internal bond strengths obtained with this approach were satisfying the strength requirements of the relevant European Norms. Cross-linking and bond strength were further improved by adding 5% on resin solids of a bio-sourced glycerol glycidyl ether.

3.2 Lignin-based non-isocyanate polyurethane bioadhesives (NIPU)

The formation of lignin-based NIPU bioadhesives and bioresins is based to a succession of two reactions. First the carbonation of any C‒OH carrying compound that can be obtained with a variety of reagents such as cyclic carbonates or even CO₂. However, the system that very recently has succeeds at a relative swift rate of reaction is by reacting C‒OH carrying biocompounds with dimethyl carbonate (DMC) to carbonate them. This is a good reagent for the first step to non-isocyanate polyurethanes [17, 18]. DMC is a liquid inexpensive, easily available, that, while flammable, has a pleasant smell similar to methanol and more importantly is classified as presenting no irritating or mutagenic effects, neither by contact nor by inhalation [18].

DMC-induced hydroxyl groups carboxymethylation is in generally carried out at a temperatures around 90°C, this being close to the DMC boiling point.]It is a bimolecular nucleophilic substitution, acyl-cleaving, under basic catalysis [18]. Recently, some research on preparing polyurethanes without isocyanates from mono and disaccharides [19, 20], or hydrolysable or condensed tannins showed very good performances [17, 21, 22], for wood coating and wood adhesives applications [17, 19, 20, 21, 22].

The second reaction is to react the carbonated biomaterial with a diamine. Thus, the schematic sequence of reactions to form a non-isocyanate urethane linkage and NIPU adhesives is as shown in Figure 7.

Figure 7.

Sequence of reactions with dimethyl carbonate (DMC) and diamines or polyamines to obtain non-isocyanate polyurethanes (NIPU).

Lignin-based NIPU adhesives for wood particleboards have been developed [23] using organosolv lignin, giving good results. However, good results are obtained only by using hot press temperatures of 220–230°C. While such a hot press temperature is not unusual for modern continuous press wood particleboard factories it is less suitable for older plants. To overcome this drawback a 10–15% addition on total resin solids of an epoxy silane (Figure 8) allowed to achieve a very acceptable performance even at the lower temperature of 180°C.

Figure 8.

Silane coupling agent for upgrading lignin NIPU cross-linking [23].

The hardened adhesive crosslink density is increased by the epoxy end of the additive while adhesion to the substrate is rendered easier by the silane end of the additive. MALDI ToF analysis of the oligomers formed during the formation of lignin NIPU resins have shown the formation of compounds with two urethane linkages for a single lignin unit, as well as lignin units dimers linked by a urethane bridge.

The long series of the species formed and found by MALDI ToF leads to a series of linear oligomers that respond to the general structural formula shown in Figure 9.

Figure 9.

An example of species formed by reaction of all ∙OH groups of lignin, both aromatic and aliphatic, with dimethyl carbonate (DMC) and then with the diamine to form urethane linkages [23].

The question of how these NIPU adhesives cross-link to form a hardened tridimensional network should involve a mechanism inducing tridimensional networking, as in reality these NIPU resins cure by just being heated. MALDI analysis detected numerous species presenting tridimensional branching, thus explaining the NIPU resins capacity of cross-linking to a hardened network. The types of oligomers formed can be classed in two categories: (i) oligomers where branching has taken place just by reaction of DMC and the diamine, without reaction with further lignin units, thus branched but without cross-linking, and (ii) branched oligomers where branching has led to cross-linking with several other lignin units in the structure, explaining how these resins can cross-link and harden with heat. An example of this is the compound in Figure 10.

Figure 10.

Example of compounds detected in the formation of lignin NIPU resins showing the cross-linking and networking characteristics of this adhesive system [23].

Thus, a generalizable formula for the branched type of NIPU resins leading to cross-linking can rather be more correctly shown as in Figure 11.

Figure 11.

A schematic representation of a generalized network repeating structure for lignin-based NIPU resins and adhesives.

Parasite side reactions do also occur such as for example the reaction of DMC with only the diamine, hence linking two diamines through a DMC molecule, as well as others [23].

FTIR spectrometry analysis also confirmed that urethane bridges formed.

A further development of this approach consisted in considerably increasing the bio content of such resins by aminating demethylated lignin to transform any ‒OH groups present in ‒NH2 groups by reacting the lignin oligomers mix with ammonia. This reaction has already been used extensively and successfully for other biopolyphenols such as condensed tannins [24, 25]. This then allowed the elimination of the hexamethylene diamine, triethylene tetramine and polymamine used to remain with only the ‒C〓O of the DMC residue in the urethane bridges being thus the only one derived from a synthetic compound, but in reality also biosourced as derived from CO₂, thus with a NIPU biocontents of more than 95% to form a structure as shown in Figure 12.

Figure 12.

Structure of lignin NIPU almost 100% of bio green sources.

3.3 Lignin amine reactions

The limitations to form usable resins that are imposed by the low reactivity of the lignin aromatic rings with aldehydes needed to be overcome by the use of alternative reaction, for example of isocyanates with the lignin methylol groups of hydroxymethylated lignin [26, 27]. This approach, although not “green”, yielded the realization that the traditional lignin low reactivity and poor crosslinking can only be overcome for its resins application by using reactions that are not based on the classical phenols-aldehyde approach. While polymeric isocyanates initially served well such a purpose, as they are now much less acceptable for a “green” use due to their uncured toxicity, mainly helped to realize that alternative reaction routes were needed.

It is on the basis of this background that the recent development to crosslink other polyphenols, such as tannins, by alternate polycondensation reactions [27, 28, 29] has led to check the possibility of applying these same reactions to obtain new, hardened, cross-linked polycondensation resins based on lignin. One of these approaches was by reacting a commercial, desulfurized kraft lignin with amines [30, 31, 32], diamines and with polyamines, to yield hardened resins [33]. In this work (a) covalent bonds were obtained by reacting the amine groups to substitute the phenolic ‒OH groups on the aromatic rings of lignin. (b) Equally, covalent bonds are also formed by reaction between the amine groups and the aliphatic side chain of lignin units by substituting the lignin side chain alcoholic ‒OH groups. (c) A proportion of ionic bonds were nonetheless still formed and subsisted between the lignin phenolic and side chain aliphatic ‒OH groups and the amine at both 180°C and 100°C. (d) Lignin yields hardened resins when reacted with 1,6-hexanediamine at 180°C, and also but to a lesser extent at 100°C.

Precedents to this reaction with amines were also known through reaction of aromatic hydroxyl groups with ammonia [24, 25].

3.4 Lignin triethyl phosphate for wood surface coatings and biobinders for metals/Teflon assembly

A recent research work aimed at obtaining new thermosetting resins by reacting lignin and triethyl phosphate (TEP) for developing through a different set of reactions a range of novel bio-based and heat-resistant paints, lacquers, coatings, resins, and binders for a number of different applications and surfaces [28, 34]. Polycondensation of the two materials appears to occur by reaction of TEP with the phenolic hydroxyl groups of lignin as well as with the aliphatic hydroxyls groups of its side chain. The reaction is favored by higher temperatures and by the presence of ammonia. Initially the reactions was codified by using simple model compounds, namely guaiacol and glycerol for the aromatic and aliphatic parts of lignin. The findings with the two models was then confirmed by reaction of TEP with a desulphurised softwood kraft lignin, namely Biochoice kraft lignin.

The resins prepared by reacting lignin and TEP at 180°C and 220°C produced acetone insoluble rigid, hard, dark solids [34]. Glycerol reacting with TEP at 180°C yielded a liquid transparent resin, but guaiacol reacted with TEP, either with or without NH₃, was so burned that no resin solids were recoverable after being heat treated. Finally, a flexible black semisolid was obtained when the lignin and TEP resin was heated at 90°C, soluble in acetone but not in water [34]. Meanwhile, the TEP heated at 90°C with glycerol, remained a transparent liquid before and after heating it. The resins were evaluated by MALDI ToF, CP-MAS 13C NMR and FTIR spectrometries. The NMR analysis showed, together with some unreacted lignin units, lignin units where TEP had reacted with the lignin phenolic ‒OH groups, such as shown in Figure 13.

Figure 13.

Mode of linkage of triethyl phosphate with phenolic hydroxyls.

The aliphatic ‒CH2OH of the lignin unit side chain was also shown to react with TEP [34].

The combination of MALDI and 13C NMR brought to the fore a great variety of structures being formed, such as those shown in Figures 14 and 15.

Figure 14.

An example of the variety of structures detected from the reaction of lignin with triethyl phosphate.

Figure 15.

Example of one of the type of structures formed in the reaction of lignin with triethyl phosphate in which a phenolic hydroxyl has been substituted by nitrogen.

The possibility that the ammonia used in some of the cases has substituted the phenolic hydroxyl groups of two lignin units also exists (Figure 15). This type of reaction is facile and has already been reported for other phenolic compounds [24, 25].

This would yield structures of the following type, several of which have effectively been found [34].

For surface coating applications the effectiveness of such a surface coating on wood were determined by a sessile water drop test on the wood surface coated with the lignin and TEP reaction product. The results shown in Figure 16 do attest the effectiveness of the lignin-TEP coat as a surface finish.

Figure 16.

(left) Water contact angle variation as a function of time of the lignin-TEP-based resin coated beech wood surface and of the untreated beech sample control, and (right) water drop shape after 60 seconds on: (a) an untreated beech wood (control), and (b) a beech wood surface coated with a lignin-TEP-based resin [34].

Figure 16 shows clearly that not only the initial water contact angle is much higher for the surface treated with the lignin-TEP-based resin coating than for the untreated wood surface, but that, moreover, the water contact angle values for the untreated surface decreases rapidly during the testing period, while they remain almost constant for the lignin-TEP resin coated wood surface.

The reaction appears to be dependent on the temperature. Thus, it is favored certainly from 180°C, yielding insoluble hardened resins, as well as when ammonia is present, due to additional cross-linking reactions.

However, the reaction of lignin (and other bio polyphenols, namely condensed flavonoid tannins) with TEP was initially developed for another application, namely as a binder of Teflon on steel or aluminum for non-stick frying pans [28, 34] (Figure 17).

Figure 17.

Frying pan metal base with the polyphenolic-TEP binder applied to it (left) and finished pan with Teflon applied on the binder according to a proprietary process (right).

The biopolyphenols-TEP binder was developed to substitute phenol-formaldehyde resins. The needs were that it had to bind well both the metal base (steel and aluminum) of the pan and at the same time bind well the surface Teflon coating used for non-stick pans. Figure 17 shows the case in which the polyphenols-TEP resin was bonded to the metal base without any Teflon (left) and the finished pan with the Teflon applied (right), according to a proprietary process. The test need that was satisfied by this “bio” assembly was that it should be able to repeatedly resist a temperature of more than 400–430°C for more than 11 minutes each time the test was repeated, and it was repeated many times, a test that the ‘bio” assembly passed well. Such an application is of course patented for the tannin/TEP system [35], but it is not patented for the equally good lignin/TEP system.

3.5 Citric acid and wood lignin bonding

Citric acid has been promoted as a direct binder for wood [36, 37, 38] and also as an additive improving wood binding by traditional or new adhesives [39, 40, 41, 42]. While in direct bonding of wood substantial esterification of the polymeric carbohydrate constituents of wood does occur leading to cross-linking between contact surfaces [38], the reaction although less immediately noticed occurs extensively also with lignin [38]. Reaction with model compounds of lignin has led to the identification of citric acid esterified compounds such as shown in Figure 18.

Figure 18.

Example of one of the species detected by reaction of citric acid with a lignin repeating unit [38].

As well as esterification of model compounds of carbohydrate constituents of wood such as shown in Figure 19.

Figure 19.

Example of some of the species detected by reaction of citric acid with glucose as a model compound of wood cellulose and hemicelluloses [38].

Indications are that cross-linking occurs also between polymeric carbohydrates and lignin reinforcing the density of cross-linking of the wood surface network, hence increasing joint strength in plywood and LVL bonding [38]. Some internal rearrangements of lignin also accompany its esterification reaction by citric acid. The bonding results of LVL and plywood panels, thus of flat wood veneers, gave excellent bond strength notwithstanding that the hot pressing conditions used were harsher than what is usual for such an application (180°C for 10 minutes). The bond line interface examined by X-ray densitometry showed that the interface reached peaks up to 1015 kg/m³ density, this being up to 3 times higher than the density of the veneers between the interfaces (Figure 20). Conversely, the density of the veneers zones between the panels bonded interfaces was almost as low than that of veneers not pressed. Such an alternation of high and low densities throughout the panel thickness profiles is similar to what observed for wood welded joints [43].

Figure 20.

X-ray density profile of the 5-layer citric acid-bonded LVL [38].

SEM micrographies showed that the bond line between the veneers even at 3000× magnification (30 μm) was not identifiable, confirming undue densification of the veneers surfaces at the bonded interface [38]. It appeared that the veneers boundary melted and densified under the action of citric acid and that possibly materials from the two veneers surfaces flowed into each other. While the hot-pressing conditions also appeared to also have a marked influence on this effect by contributing to maintain a close contact between the veneers, they were definitely not the major cause of this. This indicated that the reaction between the untreated surface and the citric acid treated surface induced a certain level of some wood partial melting causing its flowing, compaction and thus increasing interfacial density.

It is interesting to note that even in wood friction welding the addition of a citric acid solution on the veneers surfaces caused not only increased interfacial melting of the wood substrate but improved water resistance of the welded bond line, indicating the importance of citric acid grafted lignin as regards the improved water resistance of the joint [42].

4. Conclusion

This brief review indicates some alternative routes for the use of lignin in wood adhesives and other binders. It stresses that alternative reactions need to be used if such an abundant natural polymer such as lignin is to take its rightful place in the development of new, more advanced lignin-based “green” materials of all kinds, without the need to reiterate old approaches and ideas on its use. It is also an encouragement for the growing number of applications which will most likely be developed in the future based on lignin and in which lignin will be eventually used. What presented here is only the beginning of what promises to be an even brighter future for lignin applications.

Conflict of interest

The author declares no conflict of interest.