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
