Poly(methyl methacrylate) (PMMA) is one of the most used thermoplastics of its market therefore PMMA is of a relatively high value and warrants the significance of reclaiming the original monomer,1 in a process known as depolymerisation.2,3 Amongst current recycling technologies,2 chemical recycling offers the only route for PMMA recycling with monomer recoveries > 95 %, however conventional depolymerisation techniques suffer challenges due to poor heat transfer properties of the feedstock. Microwave processing on the other hand has the potential to overcome these challenges and present a low carbon route to PMMA recycling.
PMMA undergoes depolymerisation via a series of radical directed reactions with % MMA recovery dependent on the degradation temperature and mechanistic pathway.2 The polar group in PMMA can locally move under the influence of microwaves facilitating dielectric heating4 to thermally degrade PMMA volumetrically, which provides better product purity than conventional heating.
A stepwise pyrolysis approach highlighted an overall increased impurity concentration with conversion, attributed to changes in the depolymerisation kinetics. Furthermore, characterisation of pyrolytic liquid crude from a microwave processing system showed fewer by-products were produced compared to an analogous conventional pyrolysis system, indicative of changes to the depolymerisation pathway.
The influence of pyrolysis processing parameters: temperature and residence time; were also explored for their impact to the kinetics and thermodynamics of depolymerisation to understand factors leading to formation of impurities.