Synthesis of lignin-based resin using epoxidized cross-linkers from bio-sources.

Epoxy resins are an important class of thermosetting polymers, representing 70% of the total polymeric resin market. One of the main fossil-based aromatic monomers used for their synthesis is bis(4-hydroxyphenylene)-2, 2-propane, also known as bisphenol A (BPA). Still, this compound is toxic to human health and the environment. For replacing BPA, a promising natural solution is lignin due to its aromatic structure, making it an attractive natural replacement for aromatic monomers. Lignin is the most abundant aromatic biopolymer present in nature and is a highly available byproduct from pulp and paper industries. Extracted lignin has a variety of chemical structures that depend on the extraction method and the plant source, strongly impacting the mechanical properties, solubility, thermal stability, and reactivity. There are some methods, such as lignin fractionation or chemical modifications (i.e., epoxidation), which help to increase lignin reactivity and obtain a less dispersed structure with more homogeneous and reproducible properties but reduce the potential of lignin in sustainable bio-based materials because of the use organic solvents and the increase of the processing steps. Furthermore, fractionated lignin is not commercially available on an industrial scale. More sustainable approaches are based on reactive melt processing of technical lignin without any purification steps before or after the chemical modification [6]. Moreover, most lignin-based resins use non-renewable cross-linkers, increasing the negative impacts on the environment. In the literature, some studies used bio-based cross-linkers in lignin-based resins, but the lignin must be modified to increase the epoxy content. This work proposes a new strategy to produce fully bio-based epoxy resins that improve the sustainability of the final product by valorizing unmodified technical lignin and exploiting its natural advantages in combination with non-toxic bio-based epoxidized cross-linkers.