Synthesis & Characterization of Molecularly Hybrid Bisphenols Derived from Lignin & Cashew Nutshell Liquid: Application in Thermosetting Resins

Vinyl ester (VE) resins are used to produce polymers that have a variety of commercial and military applications due to their favorable properties, including relatively high strength, moduli, thermal stability, and chemical resistance. Most of the commercially available VE resins include derivatives of bisphenol A (BPA). However, unfavorable qualities of BPA include its toxicity as a human endocrine disruptor and derivation from unsustainable petrochemical resources. Therefore, bio-based alternatives to BPA-based VE resins are being investigated for their sustainability aspect and possibility of reduced toxicity.

The polymer properties exhibited by BPA-based VE resins are largely due to their aromaticity. Therefore, bio-based alternative bisphenols must uphold the aromaticity of BPA to achieve comparable properties. Lignin, the second most abundant natural resource and a major byproduct of the pulp and paper industry, has a high aromatic content. Vanillin, derived from lignin, can be reduced to vanillyl alcohol, which has been shown to be a viable platform chemical for the production of bisphenols. Cashew nutshell liquid (CNSL), a renewable, non-edible byproduct of the cashew nut industry, contains a number of phenolic compounds (phenolic lipids), which makes it a promising feedstock to synthesize bio-based bisphenols. Cardanol, a main component of CNSL, has a long C15 alkyl chain that naturally can be saturated or unsaturated. Due to the phenolic lipid structure of cardanol, water and thermal resistant properties may be imparted to overall polymer networks.

In this study, lignin-derived vanillyl alcohol and CNSL-derived cardanol were coupled to create asymmetric bisphenols similar in structure to BPA. From these bisphenols, VE monomers were synthesized and subsequently blended with a reactive diluent and cured. Testing was performed on the cured bio-based VE resins to determine if the thermomechanical, thermogravimetric, and mechanical properties of the synthesized bisphenolic analogues were comparable to a standard industrial BPA-derived VE resin.