Strategies for improving the sustainability of cross-linked PMMA: product and process development

The objective of this dissertation is to improve the sustainability of cross-linked polymethyl methacrylate (PMMA) in polymer composites, with a dual purpose: evaluating the replacement of cross-linked PMMA with alternative recyclable polymeric materials and simultaneously investigating the possibility of its chemical recycling. The first part of this work focused on the search for alternative polymers to cross-linked PMMA that could maintain, or even enhance, both the mechanical properties and the glass transition temperature of the polymer composite but, at the same time, which can also be recycled in already existing industrial plants. Various processing parameters, such as reaction temperature and time, preheating of reagents, and the mold material, were studied to optimize the process. The resulting specimens were mechanically characterized through flexural and Izod impact tests and by dynamic mechanical analysis (DMA). The experimental results were used to identify an optimal polymer formulation as well as a suitable synthesis process to obtain a polymer that outperforms, both mechanically and thermally, the crosslinked PMMA The second part of the study addressed the development of a chemical recycling process of cross- linked PMMA through microwave-assisted depolymerization, since in the current scenario the crosslinked PMMA is sent to landfill or thermal recovery. With respect to the state of the art, the present project will develop a recycling process at quite low temperature (200°C instead of 400°C) in order to further increasing the sustainability of the process, by reducing the energy consumption. Starting from thermogravimetric analysis (TGA) tests to determine the most effective catalyst for the depolymerization process, preliminary experiments were carried out using a thermally heated system, and subsequently, the process was transferred to a microwave reactor in order to fully exploit the specific advantages of this technology. The reaction products were chemically characterized using FTIR spectroscopy and GC- MS to identify their chemical structure and gel permeation chromatography (GPC) to evaluate the molecular weight distribution. First trials showed that yield in pure monomer as high as 70 % can be obtained. Although such yield is lower than the current values reported in the literature, it has been shown that the process can be successfully carried out at quite low temperature (200 versus 400 °C). Further optimization in the process set-up as well as the option to include recycling of the unreacted reagents shall be investigated in a future work. In conclusion, the project demonstrates that the proposed strategies have strong potential for enabling more sustainable management of cross-linked PMMA, thereby reducing its environmental impact and aligning with the principles of the circular economy and environmental sustainability.