Study On Acrylic Based Polyurethane/Nanosilica Composite Coatings

Acrylic based polyurethane was widely used as the high-quality auto-clear coatings. In recent years, much better properties of acrylic based polyurethane, such as better scratch resistance and weather ability, were desirable for the strong competative market. This thesis was mainly focused on the study on acrylic based polyurethane coating embedded by nanosilica, which was financially supported by the National "863" Foundation and the Shanghai Special Nano Foudation.The colloidal silica sol with different particle size was prepared via sol-gel process according to Stober method, then modified by methyltriethoxylsilane (MTES), vinyltriethoxysilane (VTES), methacryloxypropyltrimethoxylsilane (MAPTMS) and octyltriethoxylsilane (OTES), respectively, and then embedded into acrylic polyol resin via in-situ polymerization and blending methods, then cured by isocyanates to obtain polyurethane coats. The distillation, centrifigation/redispersion process and reduced distillation were adopted to remove the alcohol in colloidal silica sol, respectively. The effects of various factors, e.g., particle size, surface properties, silica content, preparation method and ethanol removal technique, on the dispersion of silica particles, the interfacial interaction and the properties of acrylic based polyurethane/silica composite coats were investigated. Some main conclusions could be drawn as follows:The dispersion of silica particlesFor normal presure distillation, SCA modified nanosilica particles could be evenly dispersed into acrylic resins and polyurethane coats, but some aggregates were observed for unmofified silica particles even SCA modified small silica particles (15nm). For centrifigation/redispersion process and reduced presure distillation, only long chain SCA molecules modified silica particles were evenly dispersed into polymer matrix, while the unmodified and short chain SCA modified silica particles aggregated.The interfacial interactionThose C=C containing SCA modified silica particles can be chemically bonded with acrylate monomers during in situ polymerization, and thus led to the high adsorption of acrylic polyol resin on their surface. Furthermore, the long chain SCA (MAPTMS) was more favorable for the chemical adsorption than the short chain SCA(VTES). Those unmodified or incompletely protected silica particles interact with acrylic polyol resin through hydrogen bond besides the chemical bonded (for VTES modified silica) or Van der Waals (for MTES modified silica). The amount of chemically bonded acrylic polyol molecule decreases as the particle size of MAPTMS modified silica increases when the silica particles were introduced into acrylic polyol resin through in-situ polymerization. At low content of silica (6wt%), the viscosity of acrylic polyol/silica composite resin increases with increasing silica particle size. In addition, the viscosity of composite resin increases with increasing silica content regardless of the silica particle size, the surface property of silica and introducing method, but sharply increases when silica content is more than 10wt% (for 80nm silica particle) because of the crosslinking role of MAPTMS modified silica particles.The mechanical and optical propertiesThe lower storage modulus but higher tensile strength of acrylic based polyurethane/silica composite coatings was observed for the samples prepared by in situ polymerization with smaller particle size and higher content of MAPTMS modified silica particle. However, the glass transition temperature (Tg) and hardness were not influenced by the silica particles maybe due to the high crosslinking density of acrylic based polyurethane. The abrasion resistance was improved by addition of colloidal silica particle and more obviously for SCA modified colloidal silica, and reached the best when 7.5wt% MAPTMS modified silica with particle size of 80nm is used. The transparency of composite coatings was not influenced by the nano colloidal silica particles, but reduces after the particles with size larger than 160nm or silica aggregation embedded. The gloss slightly reduces when colloidal silica particles incorporated. The scratch resistance is also improved with silica embedding, and even 50% improvement comparing with pure acrylic based polyurethane coatings is attained for the composite coating prepared by in-situ polymerization with 7.5-10wt% of MAPTMS modified silica.Acid-catalyzed sol-gel processThe silica sol was prepared by the pre-hydrolysis and condensation of TEOS and MTES, and then blended with as-synthesized acrylic polyol resin. The structure and morphology of silica network and hybrid coatings were investigated by the 29Si NMR> SAXS and SEM. The compact silica structures were formed in the coatings prepared with high molar ratio of TEOS/MTES and high silica content, which resulted...