Effects of Urushi and Sumi ink on the mechanical properties and water repellency of Washi paper

The development of environmentally friendly natural fiber composite materials is desirable. In this study, Urushi (oriental lacquer), which is a natural paint, and Sumi ink (black ink), which is a natural ink, were applied to Washi (Japanese paper), and their effect on the functional properties (appearance, mechanical properties, and water repellency) of the coated Washi were evaluated. Coating the Washi specimens with Urushi significantly improved their water repellency and tensile strength compared to those of untreated Washi. This was because Urushi filled the gaps between the fibers; however, it also caused brittle fracture. In addition, the elongation at break and water repellency were improved when Washi was coated with Sumi ink compared with those of untreated Washi. Furthermore, Washi specimens coated with Sumi ink followed by Urushi exhibited filled gaps between the fibers, a smoother surface, and lower water repellency compared to Washi directly coated with Urushi. The brittle fracture occurred from the Sumi-Washi fiber interface.