Bio-Based Epoxy Polymers, Blends, and Composites. Группа авторов

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.

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