Structure (%)
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) Tg – the glass transition temperature.
b) T5% – the initial decomposition temperature.
c) Ts – 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 (T5%) and the statistic heat‐resistant index temperature (Ts) 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
