Sub- and supercritical D-limonene technology as a green process to recover glass fibres from glass fibre-reinforced polyester composites

Abstract

Recycling of post-consumed composites based on thermosets reinforced with glass fibre (GF) uses different methods such as mechanical processing, high voltage fragmentation, thermal, and chemical treatments. Chemical treatments are a more interesting methodology because they allow the recovery of cleaner and polymeric matrices free glass fibres. However, depending on the solvent employed, limitations in chemical treatments such as a decrease in mechanical properties and low rate GF recovery are very common. In this paper, an original and green technology is proposed to overcome these limitations. Such technology uses D-limonene at sub- or supercritical conditions (ScD-limonene) in the absence of catalysts or auxiliary molecules, such as supercritical carbon dioxide (ScCO2), making the process more eco-friendly and so contributing to sustainability when compared to other processes that use catalysts or auxiliary molecules. Recovering almost 100% GF and preventing the deterioration or loss of properties, of the recovered GF (RGF) were achieved while the partially degraded resin can be separated and reused as a charge material in new composite formulations. Recycling glass fibre-reinforced polyester resin at composite/solvent (at sub-supercritical conditions) ratios up to 0.15 g mL−1 was possible due to the high solvation power of sub- and supercritical D-limonene. Scanning electron microscopy (SEM) images, X-ray photoelectron spectroscopy (XPS), and tensile strength analyses revealed that RGF exhibits morphological, chemical, and mechanical reinforcing capabilities comparable to virgin GF (VGF). Mass spectrometry of the resin residue (after sub- or supercritical D-limonene treatment) showed predominant fragmentation-patterns inferring the occurrence of partial degradation. The results found in this study, such as short reaction times of 45–60 min, and high recovery efficiencies ≈100%, are highly desirable for practical purposes. Furthermore, the preservation of the mechanical reinforcing properties of RGF at 64–85%, the ease of separation of the products, and solvent reuse, make this technology promising as compared to others. Based on the obtained results, ScD-limonene treatment can be employed as an efficient and eco-friendly strategy to recover GF from waste materials.