Optical Characteristic and Tensile Strength of Silk Textiles Dyed with Natural Dyes Coated with PVA and PVAc Exposed to UV Light

Poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) polymers have been used in the conservation of textiles for over 50 years. This study examines the physical and mechanical behavior of PVA- and PVAc- (Jade 403N) coated silk fabrics dyed with five type of natural dyes Japanese madder (Rubia akane L.), shikon (Lithospermum erythrorhizon L.), kihada (Philodendron amurense Rupr.), enjyu (Sophora japonica L.), and kariysu (Miscanthus tinctorius L). An ozone-producing UV lamp (8W X 2.GL8ZH. Sankyo Denki Co.) with an emission at 254 nm was used as the light source. The distance between the lamp and the samples was 28 cm, the duration of the exposure was 102 h. The fabric surfaces were photographed using a digital microscope (VHX-6000 Series Keyence, Japan) (×200) and an SEM (Hitachi SU-1500) (x 140, x 100). The surface color of the samples was measured with a spectrophotometer (CM-700d and CM-600dJ, Konika Minolta, Japan). The color difference was calculated according to CIELAB∆1E*. The tensile strength and elongation of the samples were measured using a universal material tester (Shimadzu universal tester S-500, Japan). The polymers are affected earlier than the silk fabric with exposure to light. The PVAc (Jade 403N) film showed the largest and the PVA film showed the least color difference among all the samples after UV irradiation. The color change pf the dyes has an effect on the optical appearance and that the dyes may disguise the overall discoloration caused by the polymer. Application of PVA and PVAc showed a decrease in tensile strength, which further decreased with UV irradiation. The results of this study showed that contrary to the intention, coating silk fabrics dyed with natural dyes with PVA and PVAc polymers does not exhibit stabilization as a conservation treatment, and when exposed to excessive light, the physical and chemical properties decrease further compared with that of the untreated silk fabric and the color change is influenced by the type to dyes. The common polymer treatment actually degrades the mechanical properties of the fabric. Historic textiles treated with PVA and PVAc need to be monitored and displayed under low levels of light and short duration. Careful consideration is necessary when applying PVA and PVAc to historic textiles as conservation treatments.