Effect of temperature, humidity and aminoalkoxysilane additive on the low temperature curing of polyorganosilazane coatings studied by IR spectroscopy, gravimetric and evolved gas analysis

In this work the moisture curing of commercially available pure (Durazane 1033) and aminopropyltriethoxysilane (APTES) modified polyorganosilazane (HTA1500rc) coatings was studied by the use of gravimetric analysis and IR analysis of the coatings and evolved gases. These coatings were cured isothermally at temperatures between 22-40 degrees C and humidity levels between 3-90% RH (relative humidity) for several hours and subsequently heat treated at 300 degrees C. It was found that a criterion of reactivity/volatility ratio of both, Silicon containing polymer fragments (SiCPF) and water molecules, determines the final mass yield and curing kinetics of HTA1500rc. Thus, under non-reactive/evaporative conditions of low humidity and high temperature a large fraction of the SiCPF evaporates before it is able to incorporate in the coatings. Under reactive/non-evaporative conditions at low temperatures and high humidity the 'reactive' fraction of SiCPF reacts before it is able to evaporate but nevertheless, a fraction of 'non-reactive' SiCPF, which is traced back to the raw material, is lost. It is demonstrated that fully formulated HTA1500rc coatings containing ceramic particles cured under conditions of high reactivity/volatility ratio are smooth and exhibit non-stick properties, whereas those cured at low ratio have rough, porous surfaces with compromised non-stick properties. Evolved gas analysis (EGA) shows that the composition of the SiCPF evolved has a relatively high ratio of organosilazane to APTES derivatives below 40 degrees C and a low one at elevated temperatures. Furthermore, the evolved SiCPF are significantly richer in Si-H and Si-OEt groups compared to the raw material. As a consequence, evaporation of SiCPF not only reduces the mass yield but also reduces the Si-O-Si linkage per mass of polymer. Caution should be exercised in the interpretation of FTIR data of these coatings since the intensity of reactive groups, such as Si-H or Si-OEt, is reduced as a result of two different factors: hydrolysis reactions and evaporative loss of SiCPF with different chemical composition than the coating. Thus, it was found that the combination of characterization methods used here is essential in the interpretation of curing behavior of such systems. (C) 2016 Elsevier B.V. All rights reserved.