Synthesis of lignin-based epoxy resins using epoxidized cross-linkers from bio-sources

The growing demand for non-toxic and sustainable alternatives has led to increased interest in bio-sourced epoxy resins. Epoxy resins are an important class of thermosetting polymers widely used in adhesives, coatings, and as matrices in composites. Most conventional epoxy resins are composed of bis(4-hydroxyphenyl)- 2,2-propane, also known as bisphenol A (BPA), a petroleum-based compound with environmental and health concerns. Lignin, the most abundant aromatic biopolymer in nature and a readily available byproduct from the pulp and paper industry, is a promising natural alternative to BPA due to its aromatic structure. However, its chemical composition varies depending on the extraction method and plant source, significantly affecting its mechanical properties, solubility, thermal stability, and reactivity. In the literature, lignin fractionation and chemical modification (e.g., epoxidation, acetylation, and allylation, etc.) are considered effective methods for enhancing lignin reactivity and lowering its polydispersity to achieve more uniform and reproducible lignin properties. However, these methods can diminish lignin's potential in sustainable bio-based materials, as they rely on multiple chemical reagents and introduce additional processing steps, which increase both the complexity and environmental burden of the epoxy resin production process. More sustainable approaches focus on the reactive melt processing of technical lignin without purification steps before or after chemical modification. Furthermore, most lignin-based resins utilize non-renewable cross-linkers. While some studies have employed bio-based cross-linkers in lignin-based resins, lignin is typically modified to increase the epoxy content. This work proposes a new strategy for producing bio-based epoxy resins by valorizing unmodified technical lignin to leverage its natural advantages and employing non-toxic, bio-sourced epoxidized cross-linkers such as epoxidized plant oils. These can effectively react with lignin's native hydroxyl groups, enabling a simple and greener route to high-performance thermoset materials.