Ambient-curable Polysiloxane Coatings:Preparation, Structure And Properties

A variety of polysiloxane coatings have been developed by a sol-gel process of organofunctional alkoxysilanes since these coatings can provide corrosion protection, scratch resistance, and various surface wettabilities (i.e., hydrophobicity, superhydrophobicity) for the coated substrates. However, the traditional high-temperature-cured siloxane hybrids are usually brittle, although hard. They are generally limited to fabrication of thin films with thickness of several microns or less than one micron, and cannot act as the binder of pigmented coatings or thick clearcoats with thickness more than ten microns. Thus room-temperature (RT) curing process has to be developed for preparation of polysiloxane coatings.In this study, polysiloxane oligomers were first prepared by pre-hydrolysis and condensation of organoalkoxysilanes using deficient amount of water, and further mixing with aminopropyltriethoxysilane (APS) to fabricate polysiloxane coatings. The coatings can be quickly cured at an ambient temperature with the help of the moisture in the air and the dried films can reach around twenty micrometers in thickness. Four kinds of organoalkoxysilanes, namely,3-methacryloxypropylmethyldimethoxysilane (MPDS), methyltriethoxysilane (MTES), phenyltrimethoxysilane (PTMS), and dimethyldimethoxysilane (DMDS), were employed. With these precursors, three kinds of RT-curable polysiloxane coatings were prepared and their preparation-structure-properties relationships were carefully correlated. The detailed research contents and results are as follows:The first RT-curable polysiloxane coatings were prepared by hydrolysis and condensation of MPDS/MTES and subsequently mixing with APS. The structures of the as-obtained polysiloxane oligomers as well as the dried polysiloxane coatings prepared on tinplates were analyzed by FTIR and29Si-NMR. The mechanical properties of the coatings were thoroughly examined at both macro-level and micro-level using a pendulum hardness rocker, an impact tester, and a nanoindentation/nanoscratch instrument. Effects of the molar ratio of MPDS/MTES, the dosage of aqueous ammonia solution, and the catalytic condition on the structure of polysiloxane oligomers as well as the structure and mechanical properties of the polysiloxane coatings were investigated. The dried coatings with thickness of15-26μm are highly elastic. The hardness (Koenig hardness and microhardness), impact resistance and scratch resistance are mainly dependent on the condensation degree of polysiloxane coatings rather than on the organic component of the coatings. A proper pre-hydrolysis process or more APS is benefit for enhancing the mechanical strength of the polysiloxane coatings. Polysiloxane coatings with high hardness and excellent scratch resistance can be prepared preferentially at low molar ratio of MPDS/MTES.The second RT-curable polysiloxane coatings were prepared by pre-hydrolysis/condensation of PTMS/DMDS in the presence of ammonia solution and subsequently mixing with APS. The mechanical properties of coatings were thoroughly examined at both macro-and micro-level and the thermal stability and wettability of coatings was characterized by thermogravimetic analysis and water contact angle analyzer, both of which were correlated with coating composition and the hydrolysis/condensation degree of polysiloxane oligomer. It was found that pro-hydrolysis step is essential for fabrication of thick crack-free coatings (18-35μm). Higher PTMS/DMDS molar ratio, more APS dosage and lower hydrolysis/condensation degree of polysiloxane oligomer favor enhancing the hardness. Excellent impact resistance (50cm kg) of coatings was obtained at5%and10%APS dosage, despite of the type and structure of polysiloxane oligomer. Whatever, the best scratch resistance of coatings was attained using the polysiloxane oligomer, prepared at PTMS-to-DMDS molar ratio of2:8and water-to-precursor molar ratio of1:1, and5%APS dosage. The polysiloxane coatings exhibit high thermal stability and adjustable wettability, however, both of which strongly depend on the coating composition.The third RT-curable polysiloxane coatings were prepared by pre-hydrolysis/condensation of PTMS/MPDS/DMDS (molar ratio:6:4:4) in the presence of ammonia or acetic acid solution and subsequently mixing with APS. Karl-Fisher titration method, FTIR spectroscopy, gel permeation chromatography and29Si NMR were employed to examine the structure of the polysiloxane oligomers. The mechanical properties of coatings were thoroughly examined at both macro-and micro-level. The hardness (Koenig hardness and microhardness) and scratch resistance of polysiloxane coatings are strongly dependent on the content of the remained active groups (Si-OH and Si-OR) in the oligomers. Relatively lower pre-hydrolysis temperature and acetic acid catalytic condition favor acquiring of oligomer with more active groups and thus lead the polysiloxane coatings to possess better mechanical properties.