Biobased Acetal Vinyl Ester Thermosets for Recovery of Fiber Reinforcements in Composite Materials

Conventional thermosets and polymer matrix composites (PMCs) are sourced from petroleum and contain irreversible covalent bonds between adjacent polymer chains. These characteristics restrict material reprocessing and sustainability. Moreover, the high-value fiber reinforcement used in PMC's is lost at the end of the part's life. In this work, methacrylate cross-linkers containing hydrolyzable cyclic acetal groups were synthesized from vanillin and glycerol to create renewable thermosets that can be effectively broken down after use. The synthesis of the cyclic acetal monomers was characterized using H-1 NMR and C-13 NMR spectroscopy. It was determined that 5-and 6-member acetal rings were produced at a 1:1.5 molar ratio. Neat and formulated methacrylate resins were cured by free radical polymerization, with the resultant thermosets exhibiting high modulus (1.4-3.8 GPa at 25 degrees C) and glass-transition temperatures (130-174 degrees C). Hydrolysis experiments using the monomers showed the dependence of degradation rate on acetal structure. Additionally, the resulting thermoset polymer was found to degrade by acid-catalyzed hydrolysis of the acetal linkages over a period of several days. As a proof of concept, acid-catalyzed hydrolysis of a small piece of carbon fiber-reinforced composite was conducted that resulted in the recovery of clean fiber weaves from the PMC.