Synthesis, characterization, and evaluation of cycloaliphatic substituted silane monomers and polysiloxanes for photo-curable aerospace coatings

A protective silicone/siloxane hybrid coating was prepared using a polysiloxane binder and nanophase metal-oxo-clusters. These clusters were formed in situ from sol-gel precursors during the UV-curing process, which was accomplished through a cationic curing mechanism at ambient temperatures. The binder of the coating was a substituted silicone oligomer and the metal-oxo-phase was prepared from pre-hydrolyzed tertraethoxysilane oligomers, zirconium(IV) propoxide, and titanium(IV) iso-propoxide. In addition, zinc alkoxides were also incorporated into the formulation and, with the aid of the other metal-oxo-clusters, will protect the underlying composite from UV induced degradation. Alkoxy silane groups were also grafted onto the silicone oligomer to couple with the metal-oxo-clusters.; A synthetic scheme was developed to prepare cationically polymerizable methyl, cyclopentyl, and cyclohexyl substituted polysiloxanes. Monomers, oligomers, and polymers were characterized by 1H and 29Si nuclear magnetic resonance (NMR), fourier-transform infrared spectroscopy (FT-IR), and electrospray ionization mass spectroscopy. The photo-induced curing kinetics and activation energies were investigated using photo-differential scanning calorimetry. Differential scanning calorimetry and dynamic mechanical thermal analysis was used in order to observe any physical changes in the films that are brought about due to the variation of the pendant groups. The cycloaliphatic substituents raised the glass transition temperature and affected the curing kinetics and permeability when compared to a methyl substituted polysiloxane.; The cured polysiloxanes were analyzed using scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, 1H NMR, traditional/solid state 29Si NMR, and solid state 13C NMR to elucidate the structure. Phase modulated FT-IR was also used to investigate the internal structure of the cured polysiloxane and indicated that the metal-oxo-clusters were comprised of mixed species of sol-gel precursors resulting in hetero-bonded (Si-O-Metal) species. Results also indicate that the cycloaliphatic groups inhibit the complete hydrolysis and condensation of the pendant triethoxysilane group with the incorporated sol-gel precursors. Cured films were subjected to a low Earth orbit simulator to gauge the films' performance in a simulated space environment and were found to reveal no micro-cracks when compared to an industrial standard. The release potential, mechanical properties, surface tension, and contact angle of the crosslinked polymer were also investigated under various curing sources.