An environmentally safe and nondestructive process for bleaching birch veneer with peracetic acid

Abstract

The industrial production of dyed wood veneers is increasing significantly. The bleaching process prior to dyeing is a key step. An effective and nondestructive process was developed in this work for bleaching the birch veneer by peracetic acid. The effects of bleaching parameters, such as whiteness increase, weight lose rate, dimensional stability and surface wettability were investigated, as well as a comparison of different bleaching processes with hydrogen peroxide and sodium hypochlorite as bleaching agents. Fourier transform infrared spectrum and Scanning electron microscope were used to characterize the bleached veneers. The obtained bleaching conditions are as follows: peracetic acid 10 g/L, sodium pyrophosphate 0.4 g/L, pH 6.5, material to liquor ratio 1:20, at 65 °C for 1 h. The whiteness increase from peracetic acid bleaching is a little lower than that of hydrogen peroxide bleaching, but far higher than that of sodium hypochlorite bleaching. The weight lose rate of veneers bleached with peracetic acid is only 1.74%, far lower than those with hydrogen peroxide and sodium hypochlorite. And the dimensional change sequence is peracetic acid <hydrogen peroxide <sodium hypochlorite. The initial contact angle of unbleached veneer is 82°. After bleached by peracetic acid, hydrogen peroxide and sodium hypochlorite, the initial contact angle changed to 81°, 65° and 56°, respectively. Fourier transform infrared spectrum and Scanning electron microscope results show that weakly acidic peracetic acid bleaching had no obvious impact on wood structure, however, the alkaline hydrogen peroxide and sodium hypochlorite bleaching indeed removed some lignin from the surface of the veneer. Peracetic acid bleaching is advantageous for wood and is a promise method for commercial production. The present work can help to make technical innovations in wood bleaching process.