**3. Lignin-based epoxy resin synthesis**

Epoxy resins were first introduced in Europe by P. Schlack in 1939 [25]. They are a group of thermosetting resins (see **Figure 1**) that require curing to harden. The ratio of the curing agent, or the hardener, to the resin affects the overall performance of the polymer [26]. In general, epoxy possesses excellent thermal and mechanical properties. Hence, it is favorable in the fields of coating, electronic packaging, and thermal insulation [27], to name a few.

One of the most common epoxy resins is the diglycidyl ether of bisphenol A (DGEBA). DGEBA is the product of an epichlorohydrin (ECH) reaction with bisphenol A (BPA) in the presence of a basic catalyst [26]. Since BPA is a petroleum-based chemical, DGEBA is not biodegradable, thereby having a great potential to damage

*Perspective Chapter: Potential of Lignin Valorization with Emphasis on Bioepoxy Production DOI: http://dx.doi.org/10.5772/intechopen.108263*

**Figure 2.** *Different types of epoxy resins.*

the environment [28]. Being an environmental hazard has never been enough of an incentive to search for safer options. However, the recent volatility of oil and gas prices adds to the list of reasons that justify those options [29].

Lignin is a natural polymer whose structure is a complex network of phenylpropane units, namely *p*-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol [29, 30]. **Figure 2** shows the three building blocks of lignin. Lignin structure is very similar to that of BPA, which qualifies the former to be an excellent substitute for the latter [28]. Luckily, lignin is the second most abundant macromolecule after cellulose. Therefore, it is no surprise that researchers find lignin-based epoxy resins worthy of their attention. By far, there are three methods to integrate lignin into epoxy resin synthesis. They are a) physical blending of lignin and epoxy resin, b) epoxidation of lignin after pretreatment, and c) epoxidation of unmodified lignin [31, 32].

## **3.1 Physical blending of lignin and epoxy resin**

In this method, lignin is blended with petroleum-based epoxy resin to form a binary mixture, which is then cured at a proper temperature. *Simionescu et al.* [33] stated that epoxy resins with 25 ~ 50 wt% lignin exhibited good mechanical and dielectric properties after curing at elevated temperatures. In a different study, *Behin et al.* [34] reported that adding small amounts (< 2.5 wt%) of kraft lignin and Sal-A nanoparticles to uncured epoxy resins boosted their overall mechanical performance after cross-linking. They also mentioned that the additives had a positive impact on the curing reaction. In particular, both peak temperature and total heat of the curing reaction dropped significantly.

**Figure 3.** *Lignin's basic components: p-coumaryl alcohol (A), coniferyl alcohol (B), and sinapyl alcohol (C).*

*Pan et al.* [35] considered the addition of lignin as a cross-linker. They mixed aminated lignin (see **Figure 3**) with a regular curing agent (W93) at 80°C. Then, a proper amount of liquid epoxy was added to the blend after cooling to room temperature. The epoxy-hardener mixture was cured in a baking oven afterward. Results showed that the thermal behavior of epoxy gradually improved with increasing the amount of aminated lignin in the hardener system up to 50%. Further increase in lignin content caused a drastic deterioration in performance, owing to agglomeration.

In light of the abovementioned studies, it is clear that the simple blending approach is straightforward and versatile. However, owing to lignin heterogeneity (steric-hindrance effect), the complete substitution of petroleum-based materials is not feasible with this technique.
