A Natural Adhesive: Lignin

Introduction

As is known, lignin is the most abundant natural polymer on earth after cellulose. The primary function of lignin in plant cells can be summarized as holding other chemical compounds that form cell wall substances together within a matrix structure and providing resistance. Lignin is currently obtained most commonly from lignocellulosic material through chemical approaches from cellulose production (delignification). Chemical delignification processes are applied according to process variables where lignin is removed at the highest rate and most effectively while cellulose and carbohydrates sustain minimum damage. Following chemical pulp production, millions of tons of lignin are released, regarded as waste, and mostly burned to meet the energy needs of facilities. However, depolymerized lignin fractions cause significant environmental problems due to toxic chemical structures in their composition. In major pulp-producing countries, as awareness of environmental and wildlife protection has developed, paper production has been subjected to strict regulation, prompting intensive research into more rational use of wastes resulting from delignification and their conversion into high-value-added alternative products. Despite being released in very large quantities worldwide (>70 million tonnes/year) and desired to be evaluated as high-value-added products, the fact that chemical properties vary depending on the plant source from which it is obtained and the delignification approach used, and its lack of homogeneous structure like other raw material sources, represent the greatest difficulties in utilizing lignin (Matsushita, 2015; Stewart, 2008). With advances in chemistry and technology, certain additional purification/derivatization processes such as etherification, esterification, sulfonation, oxidation, etc., have demonstrated the potential for lignin to be used, albeit limitedly, in producing certain alternative products such as adhesives, dispersants, carbon fibres, plastics, and bioactive materials. However, these products are generally more expensive than petrochemical equivalent materials and require lengthy processing, creating commercialization difficulties. In this work, general chemical properties of lignin and certain studies on its use as adhesive are briefly described. More detailed information on lignin's chemical structure, reaction properties, and utilization can be obtained from other sources (Fengel and Wegener 1984; Sjostrom, 1993; Young, 2008).

1- Chemical Properties of Lignin

Lignin is generally understood to be formed from phenylpropane (C9) units through C-C and C-O-C (ether) bonds and produced as a result of oxidative coupling reactions of p-hydroxy sinapyl alcohols. For this reason, it has been explained that lignin structure is formed by three alcohol groups with phenylpropane moieties bonding in various ways, but the proportion of these building blocks varies depending on plant source. In this case, it has been explained that softwood lignin consists of 90% coniferyl alcohol, with the remainder comprising lower proportions of p-coumaryl alcohol. Hardwood and non-wood plant material lignin, by contrast, consists of nearly equal proportions (1:1 by weight) of coniferyl alcohol and sinapyl alcohol structures (Fengel and Wegener 1984; Stenius, 2000). Additionally, certain functional groups such as methoxyphenolic hydroxyl, aldehyde, ether, etc., are present within the polymeric structure. A fraction of an exemplary lignin polymer composed of the alcohol building blocks of lignin and their various bonding patterns is shown in Figure 1. Figure 1. Chemical structure of lignin (A: Building blocks forming lignin, B: Radical reactions in lignin structure, C: Example softwood lignin fraction) (Fengel and Wegener 1984; Sjostrom, 1993; Stenius, 2000; Young, 2008). As can be understood from Figure 1, lignin is a highly complex, three-dimensional amorphous polymeric substance formed by phenylpropane units bonding in various ways. Although extensive research has been conducted on lignin's building block, repeating units, and general chemical properties, and many important results have been reported, unlike cellulose and other polysaccharides with which it occurs, a single chemical formula representing all plant material has not yet been established. Only generalizations exist as hypotheses on chemical formula structures through general classification such as softwood lignin, hardwood lignin, and non-wood herbaceous plant lignin (Fengel and Wegener 1984; Sjostrom, 1993; Stenius, 2000; Young, 2008).

2. Utilizing Lignin as Adhesive

The fact that lignin serves as an adhesive in the cell wall and that its chemical structure comprises phenolic compounds has prompted investigation of its potential use as a natural adhesive in industry. Particularly in the wood-based composite material industry, the cost of adhesive used is higher than all other costs (raw materials, energy, and operations, etc.). For this reason, intensive research has been conducted to utilize lignin, already available in large quantities as waste in the cellulose industry, as an adhesive for the forest products industry. Especially during the last 10-15 year period, as a result of research on more effective use of lignin, certain products have been developed and commercialization attempts have been made. This is because lignin's polyphenolic structure is expected to demonstrate properties similar to phenol-based (phenol-formaldehyde) adhesive. While this may be true for pure lignin with hydrophobic properties as it occurs in the cell wall, following delignification its complex polymeric structure is fragmented into smaller molecules and oligomers; additionally, acidic sulfon groups (–SO3H) are added to its structure (derivatization) and new functional groups are created, rendering it water-soluble. This particular situation, as the adhesive does not remain insoluble during curing, limits its application as an adhesive.

Lignin's function as adhesive and its ability to form cross-links result fundamentally from condensation and/or radical coupling reactions.

However, this requires the use of additional mineral acid and elevated temperature (polycondensation reactions), and the low proportion of free phenolic hydroxyl groups also causes lignin molecules' bonding strength to be lower than other synthetic adhesives (phenol-formaldehyde). Research has been conducted on whether these adverse conditions arising from lignin use as an adhesive can be regulated through the addition of certain chemicals and the effect of temperature. It has been explained that this way lignin molecules can achieve the level of cross-linking (bonding structure) necessary for adhesion (Mansouri, et al., 2007; Matsushita, 2015). Lignin's bonding reactions as adhesive are generally divided into two categories: condensation and oxidation reactions. Condensation Reactions: From lignosulfonates, in a strong mineral acid and elevated temperature environment (>180oC), diphenylmethane and sulfon groups form. The degree of these reactions is closely related to the cation in its structure. Generally, calcium-based lignosulfonates have the lowest, ammonia-based ones the highest, and sodium and magnesium-based lignosulfonates have intermediate levels of reactivity. Hydrobenzyl alcohol and sulfon groups in lignosulfonates undergo random reactions with phenylpropane groups to form diphenylmethane in strong mineral acid environment. Since this structure resembles the reaction given by phenol-formaldehyde adhesive, it has been explained that lignin reacted with formaldehyde can demonstrate thermoset adhesive bonding properties with cross-linking characteristics (Pizzi, 2006; Pizzi and Salvadó 2007; Pizzi and Mittal 2017). The major disadvantage of condensation reactions is that they require the use of strong mineral acid along with elevated temperature. This causes damage to the delicate structure of wood or other lignocellulosic plants during bonding, and when used together with formaldehyde, requires longer pressing times. To improve this adverse situation in lignin condensation reactions and to achieve hardening/cross-linking of lignosulfonates as adhesive without requiring mineral acid and elevated temperature, oxidative radical combination/coupling reactions have been developed.

Oxidative Radical Combination/Coupling

Reactions: In sulfite lignin (lignosulfonate), typically each building block (C9) contains 0.4 free phenolic hydroxyl groups, allowing cross-linking in the lignosulfonate structure. For this environment, hydrogen peroxide can be used oxidatively, and sulfur dioxide or potassium ferrocyanide (K4[Fe(CN)6] •3H₂O) can be used as catalyst. In sulfite black liquor at 50% solids content, with water, strong exothermic reactions result from redox reactions and lignin yields exceeding 70% can be obtained. However, certain impurities and carbohydrates may be present in this solution. The advantage of oxidative reactions over condensation reactions is that they do not require mineral acid and elevated temperature. Thus, since the reactivity of radicals and the system's activation energy are low, homogeneous temperature is created during pressing and cross-linking (adhesion) can be achieved under more favorable conditions. However, in this reaction environment, caution must be exercised as highly active peroxides are used that have effects damaging wood components. One of the fundamental aims in developing lignin-based thermoset adhesive has focused on developing formaldehyde-based adhesives as an alternative to those known to cause environmental and health problems. In this research, certain proportions of lignin were added to traditional formaldehyde-based adhesive structures (urea-, phenol-, or melamine-formaldehyde) to attempt to reduce the effect of formaldehyde at minimum, or new-type adhesives were developed without using formaldehyde at all. In this regard, the work of French researcher Antonio Pizzi and his group stands out (Pizzi, 2006; Pizzi and Salvadó 2007; Pizzi and Mittal 2017). Figure 2 shows a typical phenol-lignin reaction.

Figure 2. Phenol-lignin reaction

  In other alternative new-type adhesive development studies, the combined use of epoxy resin with lignin has been investigated. By adding depolymerized lignin in an epiclorohydrin environment to an amine-epoxy mixture, lignin functioning as a cross-linked (covalent) reinforcing element within the epoxy matrix structure can produce a new-type resin with lower curing temperature, it has been explained. The resin formed in the lignin-epoxy mixture has low solubility in organic solvents. Those produced from the epoxy-lignin mixture have been shown to provide higher resistance and technological properties compared to those produced from epoxy alone. Additionally, this mixture enables not only use as adhesive but also production of molded biomass-polymer composite materials (Li et al., 2018; Sun et al., 2016; Yin et al., 2012). It has been reported that Kraft lignin recovered from black liquor, used together with glycerin (biodiesel), can produce water-resistant adhesive that hardens rapidly and provides good adhesion resistance with properties similar to formaldehyde-based thermoset adhesives.

Figure 3. Lignin-epoxy reaction (Li et al., 2018; Sun et al., 2016; Yin et al., 2012).

The hardening of this adhesive has been noted to result from covalent (ether) bonds between lignin's hydroxyl groups and glycerin-derived epoxy (Li et al., 2018; Sun et al., 2016; Yin et al., 2012). One of the advantages of lignin-epoxy resin over other formaldehyde-free adhesive formulations is the non-use of caustic alkali, which has been identified as having carcinogenic effects. Below in Figure 3, the hardening mechanism (cross-linking) formed between lignin and epoxy is shown in summary.

3. Conclusions and Recommendations

Millions of tons of lignin are released worldwide in paper production and cannot be utilized in high-value-added product manufacturing. Principal reasons for this include: its heteropolymer chemical structure, varying chemical properties depending on plant source, different chemical groups (derivatized) depending on the method used in its production, and difficulties in its purification. Although its chemical composition contains abundant phenolic hydroxyl groups, most of these groups are in bound/derivatized form through chemical reactions (delignification), leaving very few phenolic hydroxyl groups available in free form for adhesion. However, with advancing technology, it is possible to partially or completely eliminate these disadvantages of lignin and expand its limited use as a heteropolymer, thereby opening avenues for its use in new areas of the chemical industry. Since utilizing or disposing of residual lignin from paper industry today is important for environmental protection, examination from this perspective can provide economic benefits from the process of purifying lignin and converting it into high-value-added new products. New or sub-sectors of the adhesive industry in particular may be created. Prof. Dr. Halil Turgut Şahin / Isparta University of Applied Sciences / Faculty of Forestry / Forest Industry Engineering Department

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