Recycling of glass fibre reinforced composites using subcritical hydrolysis: Reaction mechanisms and kinetics, influence of the chemical structure of the resin

Hydrolysis in batch conditions was used to recycle composite materials reinforced with long glass fibres and made of unsaturated polyester resin cross-linked with styrene. Subcritical conditions of water (200 °C < temperature < 374 °C and pressure < 221 bars) were chosen regarding the involved chemistry for the case of simple esters. Experiments performed in light and heavy water confirmed the specific acid catalysis of the hydrolysis of ester bonds (kH2O/kD2O<1), indicating that it can be described by the Aac2 mechanism identified for simple esters. The hydrolysis kinetics was thus defined for three widespread commercial resins and showed a good fit to the experimental data. Secondary reactions like decarboxylation of carboxylic acid and dehydration of glycol were also described in terms of mechanisms and kinetics. They also showed a specific acid catalysis.The obtained results enabled the definition of smooth conditions for the recovery of valuable products. They also showed that the chemical structure of the resin has an influence on the kinetics of hydrolysis but also on the kinetics of the main secondary reactions. However iso- and ortho-phthalic polyesters could be treated together.