Supercritical degradation of unsaturated polyester resin composites using alcohols

Degradation of glass fibre reinforced (GFR) unsaturated polyester (UP) resin composites has been studied in this paper. The degradation was carried out using supercritical ethanol and supercritical 1-propanol and with a reaction time of 30 min. Resin elimination reached between 20-40 % after this treatment, why a second treatment step was necessary to achieve complete degradation. The second treatment step was carried out using heated acetone without any reaction time. This resulted in 100 % resin elimination. The partly solubilized resin after step one was analyzed using GC-MS. From the GC-MS investigations, interesting results were obtained. The majority of the identified compounds were derivatives of UP resin monomers, primarily phthalic acid and maleic acid, clearly indicating a degradation of the UP resin. However, these monomers were not identified as carboxylic acids or anhydrides as expected, but as esters. It was found that depending on the supercritical solvent applied for the degradation, the length of the ester group varied. This esterification of the monomers could, potentially, decrease the value of UP resin degradation using alcohols as solvents, since the original monomers cannot be recovered. However, it was also found that the esterification reactions enhanced the value of a degradation product. 4-phenylbutyric acid propyl ester (4-PBPE) is a product of the esterification of 4-phenylbutyric acid with 1-propanol. Since 4-PBPE is significantly more valuable, in terms of cost, than 4-phenylbutyric acid, production of such compounds is also considered to potentially enhance the overall value of GFR UP composite recycling.