Reactive extrusion recycling of polymethyl methacrylate to methyl methacrylate and methacrylic acid

The global production of polymethyl methacrylate (PMMA) reached 4 million tonnes annually, yet only 10% of PMMA, primarily post-industrial scraps, is recycled. Recycling end-of-life PMMA, which often contains additives or composite components, poses challenges in achieving crude methyl methacrylate (MMA) monomer with comparable purity to post-industrial scraps. Hydrolyzing end-of-life PMMA presents a viable alternative to produce methacrylic acid (MAA) and simplifies the purification process of crude MMA. However, current PMMA hydrolysis is limited to lab-scale and batch operations in fluidized/fixed beds and stirred tank reactors. In this study, we demonstrate a pilot-scale, two-stage reactive hydrolysis extrusion system for the continuous conversion of injection- and extrusion-grade PMMA scraps into MMA and/or MAA at 330 ◦C to 370 ◦C. Residence time distribution (RTD) tests characterized the hydrodynamics of the screw configuration for PMMA extrusion, revealing that lower screw speed and feeding rate increase reaction time. A Plackett-Burman design identified temperature and catalyst type as significant factors for MMA hydrolysis. Under optimized conditions, hydrolysis extrusion without any catalysts achieved the highest MMA yield of 89% and 96% PMMA conversion. Hydrolysis extrusion with 10% H-type zeolite Y with an SiO2/Al2O3 ratio of 80 at 370 ◦C resulted in a 5.3% MAA yield, a 67% MMA yield, and near-complete PMMA conversion. Liquid acid catalysts directly hydrolyzed PMMA to poly(MMA-co-MAA) copolymer and/or PMAA, followed by dehydration of two adjacent acid groups to form six-member glutaric anhydride. KOH solution hydrolyzed PMMA to poly(MMA-co-MAA) and/or PMAA potassium salt.