Thermosets are a class of polymers with excellent thermal and mechanical properties, which has prompted their use in a wide range of applications and industries. However, thermosets are inherently infusible and insoluble and consequently their recycling is challenging. To address this an acid cleavable, methacrylate cross-linker bearing two acetal groups was synthesised. and used as a cleavable cross-linker in thermosets. 1H NMR spectroscopy showed the hydrolysis products were HEMA, tri(ethylene glycol) and acetaldehyde and the extent of hydrolysis increased with lower acid pKa.
The cross-linker synthesis was then optimised and scaled up for consistency in future thermoset syntheses and mechanical testing, while also improving the sustainability of the process. Acetal-containing methyl methacrylate thermosets formed homogeneous solutions in acidic solvent/H2O mixtures. The degradation products were found to be p(methyl methacrylate-co-2-hydroxyethyl methacrylate), tri(ethylene glycol) and acetaldehyde, consistent with acetal hydrolysis. Comparison of the rate of acetaldehyde evolution and gravimetric loss of p(MMA-co-A1) showed the rate of degradation related to a complex pathway involving swelling, hydrolysis, and solvation of p(MMA-co-A1). These results showed that acetal cross-linkers may enable chemical recycling of thermosets.
Acid generators were then investigated as internal acid sources for catalysis of acetal hydrolysis and exchange reactions in covalently adaptable networks. Novel polymeric sulfonate thermal acid generators were synthesised and their thermolysis properties comprehensively characterised via thermogravimetric analysis and 1H NMR spectroscopy. However, upon thermolysis they underwent a cross-linking reaction and consequently are unsuitable for use in degradable thermosets.
Lastly, the cross-linker was used in covalently adaptable networks and exhibited re-processable adhesive behaviour.
