Polymer networks are renowned for their excellent thermal and dimensional stability, mechanical strength, and chemical resistance. However, due to the irreversible nature of their network structures, such materials are not recyclable. Thermosetting systems used in polymer matrix composite (PMC) applications are often confronted with this problem. PMCs are lightweight materials which combine costly reinforcements, such as carbon or glass fibers, within a continuous thermoset phase. Conventional end-of-life processes for PMCs include mechanical grinding, pyrolysis, and landfilling. In all cases, the lack of recyclability of matrix materials results in value loss of reinforcements and increases the amount of petrol-derived plastic waste in the environment. Recyclable polymer networks have been designed by incorporating labile functionalities, such as esters, imines, or acetals, onto precursor molecules. The labile nature of these chemical groups induces degradable sites within a polymer network for eventual recycling at end-of-life. Resultant materials developed from previous studies are typically thermoplastics or elastomers which exhibit insufficient thermomechanical properties of many PMC applications.<br/>In this work, acetal linkages are introduced to the main structure of bio-derived dimethacrylate monomers to create readily recyclable thermosets from renewable feedstocks which exhibit high-performance characteristics. Acetal bonds are ubiquitous protecting groups used in organic synthesis known to readily hydrolyze under acidic conditions. By this method, recovery of natural carbon and mitigated loss of high-value materials from the current linearized economy of thermosets and PMCs can be realized. Acetal linkages were successfully formed by reacting the aldehyde on methacrylated vanillin with glycerol, a highly abundant, low-value, industrial waste product. Monomer structure was characterized by several spectroscopic techniques to confirm the reaction of aldehyde groups (90-95%) and formation of cyclic acetal units. Dynamic mechanical analysis showed that homopolymerized materials formed resins exhibited excellent storage modulus (2.9 GPa @ 35°C) and glass-transition temperature (183°C by tan delta). Finally, exposure of the polymer networks to 0.1 M HCl solution at 40°C resulted in complete hydrolysis, and degradation byproducts were analyzed for reuse.

Recyclable and renewable thermosetting polymers based on acetal linkages

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