Development of Sustainable and Degradable Thermosets and Elastomers
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The development of renewable energy and material sources is of utmost importance due to the finite supply of petroleum and environmental implications of petroleum processing. In this study, epoxy thermosets and thiol-ene elastomers were synthesized from bio-derived materials – phenolic acids and soybean oil. Elastomeric polymer films derived from thiol-ene chemistry are attracting an increasing commercial interest, due to their ease of preparation and superior physical properties. Plant-sourced phenolic acids offer the presence of multiple hydroxyl and carboxyl groups leads to ease of functionalization. In this study, thiol-ene networks were synthesized through the photoinitiated reaction between allylated plant-based phenolic acids (salicylic acid, 3-hydroxybenzoic acid, 4-hydroxybenzoic acid, gentisic acid and gallic acid) and a multifunctional thiol. Allylation of the phenolic acids proceeded to high conversion and yield, and high conversion of both thiol and allyl functional groups was observed in the thiol-ene network formation. The phenolic acids produced networks with high degrees of homogeneity and few defects, as evidenced by narrow glass transitions and consistency of their tensile behavior with the ideal elastomer model at low-to-moderate strains. This work developed fundamental relationships between the molecular structure of the phenolic acids and the physical properties of the resulting networks. Epoxy thermosets are widely applied in coatings, adhesives, automotive components, wind turbine blades and other applications, due to their superior chemical, electrical and heat resistance, adhesion and mechanical properties. However, there are crosslinked polymers within which all molecules are connected by covalent bonds, making it difficult to recycle them after their useful lifetime. In this study, epoxidized soybean oil and epoxidized phenolic acid were employed to develop epoxy resins with degradability and high strength. Epoxy thermosets were synthesized containing epoxidized soybean oil and traditional petroleum-derived epoxy resin components (diglycidyl ether of bisphenol A [DGEBA] and methylene dianiline). Significant differences were observed in the hydrolytic degradation characteristics of the polymers in basic solutions. However, the flexible aliphatic chain of soybean oils decreased the glass transition temperature of the epoxy resins. As biorenewable monomers, plant-derived phenolic acids contain rigid aromatic rings which were expected to provide mechanical strength to the epoxy resins. Epoxidized phenolic acid were utilized to synthesize epoxy thermosets with an anhydride curing agent. The resulting epoxy resins exhibited thermal and mechanical behavior to traditional DGEBA-based epoxy resins.