Advancing chemical recycling of acrylic plastic via microwave processing: Understanding the impact of grade variability on yield and purity

Microwave pyrolysis is emerging as a promising technology for recycling poly(methyl methacrylate) (PMMA), offering improved yields, purity, energy efficiency and reduced carbon footprint compared with existing routes. However, the influence of variability in PMMA waste on microwave pyrolysis performance is poorly understood. In this study, we report the first systematic examination of three grades of clear cast PMMA, probing the link between physicochemical properties of the polymer and the efficiency and product purity during microwave depolymerisation. Our results show that although all grades exhibited rapid microwave heating without the addition of microwave susceptors, distinct differences emerged in their microwave power absorption profiles, depolymerisation temperature, and the resulting product yields and purities. These differences were associated with variations in glass transition temperature, crosslink density, molecular weight distribution, dielectric properties, thermal stability, and thermal decomposition behaviour. Correlation of these results revealed clear trends indicating that higher crosslink density is linked to greater thermal stability and char formation, resulting in higher microwave absorption and depolymerisation temperatures. This, in turn, led to higher impurity levels with low monomer yields. Regardless, all grades achieved high liquid (96.1-97.5 wt%) and methyl methacrylate monomer (95.4-97.8 wt%) recovery, with low methyl isobutyrate impurity levels (0.06-0.16 wt%), confirming the robustness and efficiency of microwave pyrolysis for PMMA depolymerisation, even in the presence of grade variability.