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Lignin (Figure 1.11) is the relatively large‐volume renewable aromatic feedstock. Next to heteropolysaccharides, it is one of the most abundant biopolymer on Earth, which is found in most global plants [48, 49]. It is deposited in the cell walls and the middle lamella.

Lignin, whose concentration systematically decreases from the outer layer to the inner layer of the cell wall, is generally responsible for reinforcing the plant structure. It is described as a water sealant in the stems, playing an important role in controlling water transport throughout the cell wall. Additionally, lignin of outer layers acts as a binding agent, holding the adjoining cells together, whereas the lignin within the cell walls gives rigidity by the chemical bonding with hemicellulose and cellulose microfibrils [50]. Moreover, it protects plants against decay and biological attacks [51].

Figure 1.11 Simplified structure of softwood lignin (including three monolignols, the building blocks of lignin) [48, 49].

Lignin is a complex and amorphous, three‐dimensional network of hydroxylated phenylpropane units. Its contents vary with different types of plants, and overall, it is about 15–40% of the dry weight of lignocellulosic biomass [52]. Lignin is cross‐linked with cellulose and hemicellulose through covalent and hydrogen bonds [53]. Generally, because of the complex structure and variety of possible degrees of polymerization, lignin is called by the term “lignins,” which refers to the complex and diverse chemical composition and structure [54]. Mentioned properties, along with amorphous and hydrophobic nature, have an influence on difficult process of the isolation of lignin in unaltered form [55]. That is why, ball‐milled wood lignin (MWL), isolated from finely powdered wood via the application of mild, neutral solvents, is considered to be the closest to in vivo lignin. In general, lignins contain a variety of alkyl‐ or aryl‐ether interunit linkages (∼60–70%), carbon–carbon (∼25–35%), and small amounts of ester bonds, which include β‐O‐4, β‐5, β‐β, β‐1, β‐5, β‐6, α‐β, α‐O‐4, α‐O‐γ, γ‐O‐γ, 1‐O‐4, 4‐O‐5, 1‐5, 5‐5, and 6‐5 (Figure 1.11) [56, 57]. Respectively, β‐O‐4‐aryl ether (β‐O‐4), β‐O‐4‐aryl ether (β‐O‐4), 4‐O‐5‐diaryl ether (4‐O‐5), β‐5‐phenylcoumaran (β‐5), 5‐5‐biphenyl (5‐5), β‐1‐(1,2‐diarylpropane) (β‐1), and β‐β (resinol) are major linkages, which are present within lignin macromolecules.

The lignin content is usually higher in softwoods (27–33%) than in hardwoods (18–25%), and herbaceous plants such as grasses (17–24%) have the lowest lignin contents [51]. Moreover, lignin originated from softwood and hardwood has different contents of methoxyl groups. Softwood lignin is composed of guaiacyl units, resulted from a polymerization of coniferyl alcohol (one methoxyl group per phenylpropane unit), whereas hardwood lignin is a copolymer of coniferyl and sinapyl alcohols (two methoxyl groups per phenylpropane unit). Additionally, hardwood lignin, on the one hand, contains fewer free phenolic hydroxyl groups but, on the other hand, contains more free benzyl alcohol groups than softwood lignin (Table 1.2).

Based on the literature [58], there is also a third type of lignin, which is formed by the polymerization of p‐coumaryl alcohol. However, the resulting p‐hydroxyphenyl lignin, is usually found in the form of a copolymer with guaiacyl lignin only in certain trees and tissues.

Even though lignin is one of the most abundant natural polymers, its industrial applications are rather limited. That is why, recently, in the era of greater ecological awareness, as well as unstable and diminishing petrochemical resources, the intensive research is being performed on application of lignin and its valuable compounds. However, the strong chemical bonding of lignin with hemicellulose and cellulose microfibrils makes it hard to isolate for effective utilization. Hence, great effort is being put on the development of pretreatment methods for more effective separation of lignin. Generally, the isolation of lignin is performed by its extraction using different methods, such as Kraft, soda, lignosulfate, organosolv [59–61], hydrolysis, enzymatic, ionic liquids [62], and ultrafiltration by membrane technology. Because all the mentioned isolation methods require specific conditions such as pH, temperature, pressure, reagents, time, and variety of different solvents, the isolated lignin is characterized by diverse structural and chemical properties [63]. Utilization of lignin might be performed: (i) without its chemical modification (via the incorporation of lignin into matrix to give new or improved properties) and (ii) with the chemical modification to prepare a large number of smaller chemicals, which might be used to obtain other chemical compounds including polymers. The chemical modification of lignin (Figure 1.12) is performed by (i) fragmentation or lignin depolymerization to use lignin as a carbon source or to split the structure of lignin into aromatic macromers; (ii) creation of new chemical active sites, and (iii) chemical modification of hydroxyl groups.

Table 1.2 Proportions of interunit linkages in softwood and hardwood [50].

	Lignocellulosic material
Structure (%)	Softwood	Hardwood
Phenylpropane unit
Coumaryl	—	—
Coniferyl	90–95	50
Sinapyl	5–10	50
C9‐O‐C9
β‐O‐4	46	60
α‐O‐4	6–8	6–8
4‐O‐5	3.5–4	6.5
C9‐C9
β‐5	9–12	6
β‐1	7	7
β‐β	2	3
5‐5	9.5–11	4.5

Nearly 90% of epoxy polymers are obtained from bisphenol A (BPA). However, there is a growing demand to develop renewable aromatic compounds to replace the petroleum‐based BPA. Conducted studies are concentrated on maintenance in bio‐based materials the desirable thermomechanical properties, provided by aromatic rings of BPA‐based epoxy resins, which are linked to the rigidity provided by aromatic rings of BPA. That is why, efforts in the synthesis of novel epoxy resins are mainly directed toward renewable phenolic compounds derived from biomass feedstocks. Lignin is one of the most promising natural resource for replacement of bisphenol A because of the presence of aromatic structure with hydroxyl, carboxylic acid, and phenolic functional groups, which are able to react with epichlorohydrin to form bio‐based epoxy resins. The phenolic and alcohol hydroxyls, which are present within the lignin macromolecule, have found application in numerous research on incorporating that biopolymer into thermosetting resins, either as a component during the synthesis of epoxy monomers or as a reactive additive [64]. In general, the process of preparation of lignin‐based epoxy resins might be conducted by (i) direct blending of lignin with epoxy resin [65], (ii) modifying lignin derivatives by the glycidylation [66], or (iii) modifying lignin derivatives to improve their reactivity, followed by the glycidylation [67]. Recently, one of the most common approaches to obtain lignin‐derived polyols is the lignin depolymerization to lower molecular weight compounds, such as vanillin, vanillyl alcohol derivatives [68], phenol [69], isoeugenol [70], syringaresinol, and compounds based on propyl guaiacol and its demethylated product.

Figure 1.12 Summary of the main strategies for lignin conversion [49, 55].

Bio‐based epoxy resins are, for instance, synthesized from derivatives obtained on the course of lignin hydrogenolysis [71]. Lignin from pine wood is depolymerized by mild hydrogenolysis to give an oil product, containing aromatic polyols: dihydroconiferyl alcohol (DCA, 4‐(3‐hydroxypropyl)‐2‐methoxyphenol) and 4‐propyl guaiacol (PG, 4‐propyl‐2‐methoxyphenol), along with their dimers and oligomers. Then, the obtained oil product is dissolved in refluxing epichlorohydrin in the presence of solution of NaOH to give epoxy prepolymer (LHEP) (Figure 1.13), which is then blended with bisphenol A diglycidyl ether (DGEBA) in mass ratios of LHEP : DGEBA up to 3 : 1. Epoxy composition might be cross‐linked with diethylenetriamine (DETA).

Table 1.3 Thermal analysis data for epoxy resin blends containing DGEBA and cured with DETA [68].

Resin	Tga) (°C)	T5%b) (°C)	Tsc) (°C)
DGEBA	117	328	169
LHEP/DGEBA 1 : 1	80	289	161
LHEP/DGEBA 2 : 1	70	270	156
LHEP/DGEBA 3 : 1	68	258	151
LHEP	53	236	144

a) T_g – the glass transition temperature.

b) T_5% – the initial decomposition temperature.

c) T_s – the statistic heat‐resistant index temperature.

Cured epoxy material LHEP/DGEBA is less thermally stable than the DGEBA resin (Table 1.3) because of the presence of methoxy groups on the aromatic ring.

The initial decomposition temperature (T_5%) and the statistic heat‐resistant index temperature (T_s) are the lowest for samples containing the highest proportion of LHEP. On the other hand, the presence of methoxy groups in the lignin hydrogenolysis products is likely to contribute to the superior mechanical properties of the cured LHEP/BADGE blends. Values of flexural modulus and strength of bio‐based materials are 52% and 28%, respectively, greater than DGEBA alone. Additionally, it is worth to mention here the research on the influence of the presence of lignin on the thermal performance and thermal decomposition kinetics of lignin‐based epoxy resins [72]. The presence of lignin‐based epoxy resin (depolymerized Kraft lignin, DKL‐epoxy resin, and depolymerized organosolv lignin, DOL‐epoxy resin, respectively) in epoxy composites, prepared by mixing the DGEBA and a desired amount (25, 50, and 75 wt%) of DKL‐epoxy resin and DOL‐epoxy resin at 80 °C, then cured with 4,4′‐diaminodiphenylmethane (DDM), results in a significant effect on the activation energy of the decomposition process, in particular, at the early and the final stage of decomposition (Table 1.4).

The increase in the percentage value of lignin‐based epoxy resins in the composites reduces the initial activation energy of the system. Additionally, the obtained bio‐based materials exhibit higher limiting oxygen index (LOI) than that of the conventional BPA‐based epoxy resin, which might indicate that the lignin‐based epoxy composites are more effective fire retardants than the conventional BPA‐based epoxy resin.

An interesting example of a novel approach to finding new epoxy application for bio‐based derivatives from lignin is a conversion of lignin to epoxy compounds throughout the reaction of epichlorohydrin with partially depolymerized lignin (PDL) in the presence of benzyltriethylammonium chloride and dimethyl sulfoxide (Figure 1.14) [67].

Figure 1.13 Cured epoxy resins from lignin hydrogenolysis products.

Table 1.4 Thermal decomposition of BPA‐based epoxy resin and the DGEBA/lignin‐based epoxy resin [72].

Sample	IDT (°C)	Tmax (°C)	Char800 (%)	LOI
		Shoulder peak	Main peak
DGEBA‐DDM	370	—	405	12.5	22.5
25% DKL‐DDM	359	—	397	18	24.7
50% DKL‐DDM	330	—	396	25	27.5
75% DKL‐DDM	300	335	407	32	30.3
100% DKL‐DDM	290	325	416	38	32.7
25% DOL‐DDM	383	—	404	17	24.3
50% DOL‐DDM	352	—	399	21	25.9
75% DOL‐DDM	338	—	398	24	26.7
100% DOL‐DDM	335	—	397	29	29.1

The lignin‐based epoxy material is characterized by comparable thermal and mechanical properties to those of BPA‐based epoxy resin (DER332) cured with the same bio‐based curing agent (the Diels–Alder adduct of methyl esters of eleostearic acid, a major tung oil fatty acid, and maleic anhydride [MMY]). The obtained bio‐based epoxy product might be applied as a modifier for asphalt applications in the same manner as petroleum‐based (and mostly BPA‐based) epoxy resins, which are currently used for asphalt modification to improve its temperature performance. The PDL epoxy asphalt, in the same way as DER332‐asphalt, exhibits significant improvement on the viscoelastic properties, especially at elevated temperatures.

The research on utilization of lignin derivatives toward the synthesis of the epoxy system is ongoing for several years; thus, there are numerous methods described in the literature. Among them, it is worth to mention the synthesis of epoxies by (i) direct epoxidation of the phenolic hydroxyl group in the technical lignin with epichlorohydrin and (ii) obtaining bisguaiacyl structure via the reaction using ketone compound and then the epoxidation (Figure 1.15a) [66].

The other route of lignin utilization toward the synthesis of epoxy system is the cleavage of lignin intermolecular bond and creating the phenolic hydroxyl group in the molecule (Figure 1.15c). The process is usually done by treating the Kraft lignin with acid (hydrochloric or sulfuric acid) and phenol derivatives. The obtained phenolic hydroxyl group is epoxidized with epichlorohydrin, resulting in the lignin‐based epoxy resin, which in the next step is cross‐linked using DETA or phthalic anhydride. The phenol derivative within the lignin structure might also be obtained on the course of the lignin phenolization with bisphenol A in the presence of hydrochloric acid and BF₃‐ethyl etherate as catalysts (Figure 1.15b) [73]. The obtained product is soluble in organic solvent such as acetone because of the contribution of bisphenol A.

Figure 1.14 Synthesis of lignin‐based epoxy and epoxy asphalt.

Figure 1.15 Schematic routes of lignin modification and crosslinking: (a) epoxidation with bisguaiacyl structure stage, (b) lignin' phenolization with bisphenol A and (c) direct epoxidation of the phenolic hydroxyl group in the technical lignin with epichlorohydrin.

It is worth noting that a substantial amount of lignin decomposing aromatics is characterized by the structure of phenol substituted by inert methoxy and alkyl groups (structures such as guaiacol or creosol), making polycondensation or radical polymerization especially difficult. Thus, there are numerous studies on (i) introducing the reactive groups, which are promoting further polymerization reactions [74], (ii) utilization of the reactive ortho‐ and para‐sites of phenol for hydroxymethylation or obtaining novolac or resol‐type resin using formaldehyde chemistry [75] otherwise, (iii) connecting lignin‐derived compounds to make oligomers with additional functional groups [76]. Bimetallic Zn/Pd/C catalytic method for converting lignin via the selective hydrodeoxygenation (T = 150 °C and 20 bar H₂, using methanol as a solvent) directly into two methoxyphenol products has been reported [77]. The compound characterized by the increased content of hydroxyl groups might be obtained using the above method, via the reaction of o‐demethylation of 2‐methoxy‐4‐propylphenol and aqueous HBr. In the next step, propylcatechol is glycidylated to epoxy monomer (Figure 1.16).

Other techniques described in the literature involve the ozone oxidation of Kraft lignin toward splitting its aromatic rings and generation of the muconic acid derivatives. The ozonized lignin (Figure 1.17a) might then be dissolved in an alkali water solution and cross‐linked with the water‐soluble epoxy resin (glycerol polyglycidylether).

Another interesting synthesis described in the literature begins from the dissolution of alcoholysis lignin or lignin sulfuric acid in ethylene glycol and/or glycerin (Figure 1.17b) [78]. Next, the hydroxyl group in the lignin molecule is reacted with succinic acid to convert the lignin into multiple carboxylic acid derivatives. In the last step, the resulting products react with epoxy compound (ethylene glycol diglycidyl ether [EGDGE]) in the presence of dimethylbenzyl amine as a catalyst to provide the cross‐linked epoxidized lignin resin. In the obtained curried epoxy material, lignin acts as a hard segment (increasing value of T_g with increasing lignin derivatives). Additionally, a slight decrease of T_d with increasing content of biocomponent in epoxy resin suggests that the thermal stability of obtained epoxy system is not affected by the presence of lignin derivatives.

Based on numerous studies, one can conclude that lignin is a very promising natural resource for replacement of bisphenol A in the synthesis of epoxy resins, as it has aromatic structure with hydroxyl, carboxylic acid, and phenolic functional groups, which can react with epichlorohydrin to form bio‐based epoxy resins. One of the biggest problems for commercial application of lignin's derivatives, because of its complex and multifunctional nature, is isolation and the synthesis of monomers.