Waste artificial marble pyrolysis and hydrolysis

Abstract

Artificial marble, a composite material consisting of 40 % (g g⁻¹) Poly Methyl Methacrylate (PMMA) and 60 % (g g⁻¹) aluminium hydroxide $\text{Al(OH)3}$, combines the durability and aesthetics of real marble with the lightweight and moldability of plastic. It is the most sought-after synthetic stone in the world, with a production volume of over 1 million t in 2021. However, due to a high level of cross-linking, mechanical recycling of the composite is impossible, while chemical recycling is achievable, yet unprofitable. The only economically viable recycling solution is to retain the value of both the organic and inorganic fraction of the composite. We investigated the pyrolysis and hydrolysis of post-consumer end-of-life artificial marble to identify possible valorization routes, examining the effects of temperature, water content, catalyst presence, and heating style. Temperature directly accelerates thermolysis, and indirectly hydrolysis. The water inherently present in $\text{Al(OH)3}$ drives initial hydrolysis, and temperature expedites inorganic fraction dehydration, increasing local water partial pressure near polymer ester sites. Above 350 °C, PMMAeq depolymerizes faster than it hydrolyzes, balancing the effects of temperature on water dehydration with the depletion of available ester sites for hydrolysis. Contrary to intuition, PMMA does not depolymerize to its monomer MMA and then hydrolyze its acid (methacrylic acid); instead, PMMA partially hydrolyzes to poly methacrylic acid (PMAA) while also depolymerizing to MMA. PMAA then dehydrates and degrades, releasing CO and CO₂. The optimal method involves a heating ramp that first releases water at 300 °C, minimizing hydrolysis, and then favors MMA production at 400 °C, achieving a 66 % (g g⁻¹) MMA yield. Regardless of the operative conditions, the inorganic fraction transforms from $\text{Al(OH)3}$ to a $\gamma$-alumina precursor, boehmite. Additionally, the remaining polymer in the residue, about 9 % (g g⁻¹), has the required heat capacity for an energy-self sufficient calcination to $\gamma$-alumina. This dual-phase process not only maximizes MMA recovery but also retains the value of the inorganic fraction, providing a sustainable and economically viable recycling method for artificial marble.