Experimental investigation of chemical recycling routes for crosslinked PMMA

TThe rapid growth in plastic production, 430 Mt in 2024, represents one of the major global environmental challenges due to the devastating effect produced by plastics waste on the environment. This has made the transition toward a more sustainable economy more urgent, a kind of economy in which plastic materials are kept in circulation for as long as possible. Poly methyl methacrylate (PMMA) is a high value plastic material, used in many different sectors, mainly in its thermoplastic form. Of the total PMMA production only 10%, and only in its thermoplastic version, is currently recycled, through mechanical and chemical recycling. The present work investigates chemical recycling routes capable of recovering high value monomer from end-of-life cross-linked poly methyl methacrylate. Indeed, this kind of PMMA is currently totally landfilled or incinerated since there are no available processes for its recycling. The aim of the process that has been developed in this work is to recover methyl methacrylate at lower temperature than the one usually employed for pyrolysis of thermoplastic PMMA (500-600°C) to further increase the sustainability of the process by reducing energy consumption. The experimental investigation was developed in two main phases. The first step was done to verify the feasibility of a purely thermal depolymerization, which was developed using two different heating systems, a conventional conduction heating reactor and a microwave multimode reactor, which both operated in a temperature range between 200-240°C in air atmosphere. This first phase of the work was carried out exploring the two different heating mechanisms to assess if they cause any noticeable differences in the reaction products. The operating conditions were identified through a preliminary thermogravimetric analysis (TGA) from which, due to the cross-linked nature of the polymer, a Head-Head (H-H) scission initiation mechanism emerged as the most promising alternative to perform a low energy thermal process. The effects of temperature and reaction time were also analysed to verify their contribution on the main process parameters (conversion, selectivity and yield) with the aim of maximizing the quantity and quality of recovered monomer. The second phase of the investigation explored the possibility of adopting a catalyst to further increase the efficiency on the purely thermal reaction, and it was developed through a preliminary screening with TGA and subsequently tested in full scale experiment to find the most promising candidates. In both cases, the characterization of the reaction products was performed with Fourier Transform Infrared Spectroscopy (FT-IR), Gas Chromatography coupled with Mass Spectrometry (GC-MS), Gel Permeation Chromatography (GPC) to identify the chemical species produced and assess their purity. In general, the final products are recovered as a biphasic system: one light liquid phase, composed of MMA, and a heavier liquid phase composed of side-products like aldehydes, water and glycols. For each experiment conversion, selectivity and yield were evaluated, and MMA yields as high as 80% were reached in the optimized conditions. Overall, this study demonstrated that low energy chemical recycling of cross-linked PMMA is technically feasible, yielding high purity MMA monomer operating at more energy efficient conditions than usual, offering a viable pathway towards a more sustainable material management.

The rapid growth in plastic production, 430 Mt in 2024, represents one of the major global environmental challenges due to the devastating effect produced by plastics waste on the environment. This has made the transition toward a more sustainable economy more urgent, a kind of economy in which plastic materials are kept in circulation for as long as possible. Poly methyl methacrylate (PMMA) is a high value plastic material, used in many different sectors, mainly in its thermoplastic form. Of the total PMMA production only 10%, and only in its thermoplastic version, is currently recycled, through mechanical and chemical recycling. The present work investigates chemical recycling routes capable of recovering high value monomer from end-of-life cross-linked poly methyl methacrylate. Indeed, this kind of PMMA is currently totally landfilled or incinerated since there are no available processes for its recycling. The aim of the process that has been developed in this work is to recover methyl methacrylate at lower temperature than the one usually employed for pyrolysis of thermoplastic PMMA (500-600°C) to further increase the sustainability of the process by reducing energy consumption. The experimental investigation was developed in two main phases. The first step was done to verify the feasibility of a purely thermal depolymerization, which was developed using two different heating systems, a conventional conduction heating reactor and a microwave multimode reactor, which both operated in a temperature range between 200-240°C in air atmosphere. This first phase of the work was carried out exploring the two different heating mechanisms to assess if they cause any noticeable differences in the reaction products. The operating conditions were identified through a preliminary thermogravimetric analysis (TGA) from which, due to the cross-linked nature of the polymer, a Head-Head (H-H) scission initiation mechanism emerged as the most promising alternative to perform a low energy thermal process. The effects of temperature and reaction time were also analysed to verify their contribution on the main process parameters (conversion, selectivity and yield) with the aim of maximizing the quantity and quality of recovered monomer. The second phase of the investigation explored the possibility of adopting a catalyst to further increase the efficiency on the purely thermal reaction, and it was developed through a preliminary screening with TGA and subsequently tested in full scale experiment to find the most promising candidates. In both cases, the characterization of the reaction products was performed with Fourier Transform Infrared Spectroscopy (FT-IR), Gas Chromatography coupled with Mass Spectrometry (GC-MS), Gel Permeation Chromatography (GPC) to identify the chemical species produced and assess their purity. In general, the final products are recovered as a biphasic system: one light liquid phase, composed of MMA, and a heavier liquid phase composed of side-products like aldehydes, water and glycols. For each experiment conversion, selectivity and yield were evaluated, and MMA yields as high as 80% were reached in the optimized conditions. Overall, this study demonstrated that low energy chemical recycling of cross-linked PMMA is technically feasible, yielding high purity MMA monomer operating at more energy efficient conditions than usual, offering a viable pathway towards a more sustainable material management.