Microwave depolymerisation of PMMA: Power density, char management, and reactor design for high-purity monomer recovery

Poly(methyl methacrylate) (PMMA) is a versatile polymer valued for its optical and mechanical properties, durability and biocompatibility. However, its production carries a high carbon footprint, primarily due to the energy-intensive synthesis of its monomer, methyl methacrylate (MMA), while recycling rates remain below 10 %, largely due to limitations in current conventionally heated technologies. Microwave depolymerisation offers a promising route to address these challenges by enabling greener, more energy-efficient recovery of the monomer. However, due to the unique microwave heating mechanisms, innovation in microwave reactor design is required to realise this at industrial scale. This study defines the key processing parameters required for successful scale-up of microwave depolymerisation of PMMA into high-purity MMA. A semi-continuous microwave reactor was developed to explore these parameters under controlled conditions. Electromagnetic simulations were employed to optimise reactor performance and provide insight into the observed process behaviours. For the first time, the roles of power density and char accumulation in determining product quality during microwave depolymerisation of PMMA are quantitatively established, highlighting the need for reactor designs that enable high power density and effective char removal. These findings define key design principles for microwave reactor scale-up and represent a critical step toward economically and environmentally viable PMMA recycling within circular plastic systems.