Enzyme-catalyzed polyurethane adhesive degradation

Abstract

Polyurethanes represent a class of highly versatile synthetic polymers, suitable for a wide range of applications. Their biological degradation is of great interest since it can allow the design of specific formulations by selecting suitable building blocks and it can contribute to the development of sustainable recycling processes. In the current study, a commercial hydrolytic enzyme (cutinase from Humicola insolens, HiC) was investigated for its ability to degrade various polyurethane adhesive formulations, by focusing first on macrodiols, then on specific polyurethanes. The aim was to identify solvent-based polyurethane formulations susceptible to enzymatic hydrolysis. First, a semi-quantitative assay, namely the emulsion turbidity test, was carried out on some macrodiols. Then, weight loss tests were carried out on specific solvent-based polyurethane formulations, and three promising formulations have shown 90, 60 and 40% degradation, after 96 h of incubation with HiC. A study of the enzymatic degradation mechanism of macrodiols and the most degradable polyurethanes was also carried out, through the characterization of the solid residues after the enzymatic degradation by infrared spectroscopy, calorimetric and thermogravimetric analysis, and the identification and/or quantification of the monomers released during the hydrolysis of macrodiols within the liquid fraction (by high-performance liquid chromatography). According to the results, a prevalent exo-type action mode for HiC against some macrodiols was found under the conditions tested, while, from a chemical point of view, the degradation seems to determine, on the polyurethane residues, a general increase in crosslinking.