Whitening of polyvinyl alcohol used as restoration material for Shohekiga

Polyvinyl alcohol (PVA) has been used to prevent flaking of paint pigments on traditional Japanese Shohekiga painted screens and panels. However, over time these PVA coatings whiten. To date, the mechanism of this whitening has not been studied from the point of view of polymer degradation. In this study, a piece of whitened PVA was obtained from a painted wooden panel. The PVA whitening mechanism was studied in this piece using scanning electron microscopy (SEM) and functional group analysis by Fourier transform-infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). SEM showed that the surface of the whitened PVA thin film was irregular and rugged, and many micrometer-sized cracks were observed. This suggests that efficient scattering of all visible light from the cracks causes whitening. FT-IR and XPS measurements revealed that inter- and intra-chain dehydration and chemical crosslinking (C–O ether linkage) occur on the surface of the whitened PVA. Polyvinyl alcohol (PVA), which has been used to prevent flaking of paint pigments on traditional Japanese Shohekiga , became white and we investigated its phenomenon. The surface of whitened PVA was irregular and rugged, and many micrometer size cracks were observed. This suggests that efficient scattering of all visible region light from the cracks causes whitening. Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy measurements revealed that interchain dehydration and chemical crosslinking (C–O ether linkage) occurred on the surface of whitened PVA.