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Thermoanalytical study of siloxane-polyurethane thermosets: Kinetic deconvolution of overlapping heterogeneous curing reactions

The curing behaviour and kinetics of siloxane-polyurethane coatings based on difunctional hydroxyl-terminated polydimethylsiloxane (PDMS-OH), polycaprolactone triol (PCL-triol) and isophorone diisocyanate (IPDI) were studied. The curing kinetics were elucidated by dynamic differential scanning calorimetry (DSC) using the model-free Friedman technique. The curing kinetics showed a constant value of 96.3 ± 1.5 kJ/mol over the range of 0.05 < α < 0.72. The y(α) and z(α) functions showed that the kinetic process does not follow a single-step curing reaction and that a combination of reactions of carbon-linked and silicone-linked hydroxyls with isocyanate control the kinetics during curing due to the different chemical activities of the hydroxyl species. The detailed kinetics of the curing reactions were analyzed by applying an empirical kinetic deconvolution method. The results showed that the thermal behaviour of these systems is characterized by the reaction of PCL-triol with isocyanate, followed by another overlapping exothermic reaction of PDMS-OH with isocyanate at a higher thermal range, resulting in a reaction-induced phase-separated system. The activation energy of the two separated reactions was calculated using a model-fitting method, and optimization was performed to minimize the sum of the squared errors. Moreover, the surface topography of the cured systems was observed using an atomic force microscope (AFM) in the tapping mode, and the different isocyanate curing reactions were confirmed using Fourier transform infrared (FT-IR) spectroscopy.

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Volume 112, November 2017, Pages 234–243

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