Study on thermal stability of water-based polyurethanes derived from hydroxypropyl terminated poly(dimethylsiloxane)

In this paper, thermal and thermo-oxidative stability of three water-based polyurethanes (PUs) derived from hydroxypropyl terminated poly(dimethylsiloxane) (HPT-PDMS) were studied. The structural properties of the synthesized samples were investigated by 1H NMR measurements, while characterization of their surface, performed by atomic force microscopy, revealed the existence of microphase-separated structures. According to the results obtained by thermogravimetric analysis (TGA), thermo-oxidative and thermal stability of the investigated water-based PUs decreases with decreasing HPT-PDMS content. Furthermore, obtained results also suggest that both, thermo-oxidative and thermal degradation reaction of water-based PUs occurred as a three-step process. In both atmospheres the first step of degradation is attributed to the degradation of weak urethane and urea bonds and the second step to the decomposition of polysiloxane chains. In order to undertake a detail investigation of thermal degradation process of water-based PUs, TG measurements in nitrogen atmosphere at four different heating rates (5, 10, 15 and 20 °C/min) were performed. Different iso-conversional model-free methods, such as Ozawa-Flynn-Wall, Kissinger-Akahira-Sunose and Friedman, were applied to determine apparent activation energy of thermal degradation reaction (Ea) of water-based PUs, at different conversions (α). Deviation in parallelism of the obtained fitted lines confirmed that thermal degradation reaction of water-based PUs occurred in three successive steps, followed by different degradation mechanism. Since linear fitting of the experimental data by Friedman method had the lowest correlation coefficient, Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose methods were considered to be the most reliable for Ea calculation. A comprehensive analysis of the obtained results revealed that, despite their similar structure, each water-based PU shows unique profile of Ea = f(α) plot.