Effect of untreated silica on matt finished of polyacrylic/polyurethane waterborne topcoat

Abstract

Matt-finished coating is one of trendy aesthetics characters that regarded to a reduction in the amount of a specular reflected light as a result of the surface roughness. Transparency is a challenge to produce a clear matt-finished topcoat. Incorporating matting agent into the system will affect the transparency of the coating. Waterborne polyacrylic/polyurethane (PA/PU) topcoats at three different PA/PU ratios (100:0, 80:20, 60:40) were blended with untreated silica as a matting agent at different weight percentages, namely, 0.5 wt%, 1.0 wt% and 1.5 wt%. Liquid paint properties and dried film were analysed and characterized. From a Fourier transform infrared analysis, the absorption peaks of PU were detected at 1720 – 1730 cm-1 for –C=O vibration and 3360 – 3380 cm-1 for –NH vibration. Meanwhile, the characteristic groups of PA were observed at 1440 – 1450 cm-1, 1720 – 1730 cm-1 and 1150 – 1160 cm-1 corresponding to C-H bonding, -C=O vibration and –C-O-C vibration, respectively. For fumed silica, the absorption peaks were detected at 801, 975 and 1066 cm-1, corresponded to the silanol group. All these peaks confirmed the successful of incorporation of silica into PA/PU blend. For the liquid paint properties, non-volatile content of the waterborne paints showed increment when adding the untreated silica and PU as both contribute 100% and 40% respectively of solid content. Meanwhile, the gloss values of dried PA/PU film decreased with increasing silica amount due to the increase of surface roughness. Silica particles that embedded on the surface of film reflected the incident light at a certain specular angle (600). As more silica was added into the formulations, more specular lights were scattered, hence imparted more matt-finished look (dull and flat). This was supported by the images from scanning electron microscopy that showed the formation of rough surface with the addition of silica. However, a sample that had PU in the formulation showed a bit smoother surface compared with the sample with 100% PA. This was thought due to the elastomeric part of PU had buried the silica particles deep into the PU matrix. Hence, less scattered specular lights were projected to the surface. The arguments were supported by the opacity analysis where sample with 20 wt% PU at 1wt% silica had a lower contrast ratio of 6.26 (less matte) compared to the sample with 100% PA at 1 wt% silica which had 6.32 (more matte). This implied that PA/PU ratio had also influenced the opacity of the sample. Based on the study, incorporation of untreated silica will affect the transparency and will reduce down the gloss of the film coating. The higher the dosage is, the less transparent the film coating will be, thus more matt the finishing will be. On the other hand, it was failed to prove that the untreated silica can be the associative thickener in the system due to the incorporation of the PU thickener.