Development and Evaluation of Nano-Silica Dispersed Polyurethane Based Coatings for Improved Anti-Graffiti and Scratch Resistance
Open Journal of Applied Sciences
Polyurethane based anti-graffiti coatings have been developed by incorporating varying amounts of OH-functional Silicone modified polyacrylate. Optimization of Silicone polyacrylate concentration in neat PU was done by testing its performance for graffiti resistance. Tests which include static contact angle measurements with water and spray paint, visual inspection of stain removal and color change measurements were performed. It was observed that 5 wt % of Silicone polyacrylate in the neat PU
... ate in the neat PU showed 95% stain removability. These coatings showed excellent graffiti resistance but poor mechanical properties. Hence modification of these coatings was done by nano-particle incorporation. Nano-silica particle concentration was optimized for surface mechanical properties such as surface hardness, elastic modulus and scratch resistance. FEG-TEM revealed that nano-silica particles were uniformly distributed over the surface. 5 wt % nano-silica particle additions in PU-5 wt % silicone polyacrylate base showed superior mechanical properties than other concentrations. Color change measurements showed that nano-silica particle addition had no effect on its graffiti resistance. Hence PU-5 wt % silicone polyacrylate, 5 wt % nano-silica showed excellent graffiti resistance and mechanical properties. Keywords Silicone Modified Polyacrylate, Nano-Silica Particles, Anti-Graffiti, Scratch Resistance P. Adapala et al. 809 causing unauthorized defacement of the surfaces which may include metallic structures, buildings, monuments etc. It is derived from Italian word Graffiare which means scratch. In recent times, graffiti in the form of spray paints is being considered as art. However, graffiti on ancient buildings and monuments is considered as vandalism. The impact of graffiti on artefacts of cultural heritage is very high in both social and economic terms because graffiti degrades the sprayed region and also the affected bodies are devaluated. Cleaning of graffiti also involves high costs. Although, the exact cost of the graffiti removal is not known, the estimates say that it is around 700 million dollars for Germany, 1.5 dollars for London, 12 billion dollars in the US annually . The general mechanism of these coatings is based on developing a low energy surface using the components such as fluoro, silicone and per-fluoro components that the graffiti solvent does not wet the surface. It enables easy cleaning thereby leading to the longevity of the coating. This also avoids usage of aggressive and powerful chemicals generally adopted in cleaning procedures  . Polysiloxanes possess the properties of hydrophobicity, thermal stability and biological resistance . Feng et al. investigated the surface properties of polyurethanes modified by polysiloxane . Mohammad Rabea et al. studied the surface properties of these coatings against UV exposure. They have tested for the effects of this additive on the surface chemistry and anti-graffiti properties . Nano-silica and Nano-alumina particles are generally used for improving mechanical properties of coatings. On Mohs scale of hardness, silica and alumina have hardness 9 and 7 respectively and hence they are the widely used nanoparticles for scratch and abrasion coatings  . Nano-silica particles are preferred to nano-alumina as they do not affect the transparency of the coating. Nano-silica particles possess low refractive index, high modulus and hardness  . In this work neat polyurethane was modified by OH-functional silicone polyacrylate for anti-graffiti property. Optimization of OH-functional silicone polyacrylate concentration was done based on their graffiti resistance. It was observed that they possessed inferior mechanical properties. Nanoparticles were incorporated into these coatings to enable improvement in the surface mechanical properties and scratch resistance. Nano-silica particles were added as their refractive index matches with the resin based coatings. Optimization of nano-silica particle concentration was done based on their mechanical behavior and scratch resistance. Mechanical, antigraffiti and surface properties of the final formulation were evaluated.