Effects of boron compounds impregnation on the physical and vibro-mechanical properties of spruce (Picea sp.)

Abstract

Abstract Boron compounds (BC) are eco-friendly and effective wood preservatives and have recently been found in the soundboards of Stradivari and Guarneri stringed musical instruments made in the 18th century. The effectiveness of BC protection against decay and insects has been validated extensively. However, the effects of BC on the vibro-mechanical properties of wood remain unclear. In this study, spruce wood was impregnated with the wood preservatives, disodium octaborate tetrahydrate (DOT), boric acid (BA), and sodium tetraborate pentahydrate (BX), and their effects on the physical and vibro-mechanical properties were investigated. The free-free flexural vibration method was sequentially applied to identical specimens before and after treatment at 23 °C and 30, 65, and 90% RH. The results indicated that the hygroscopicity increased and the dimensional stability decreased upon the three BC impregnation. Nevertheless, the acoustical properties were improved by the three BC treatments, particularly the acoustic conversion efficiency, which benefited from the decreased internal friction tan δ. FTIR spectroscopy combined with principal component analysis showed that an extensive boron-wood network was formed inside the wood, in contrast to the deionized water treatment. The effect of boron types on the acoustics and molecular structure overall depended on the solution pH and the network formed within the wood. One percent concentrations of the three BC-treated samples exhibited some acoustic enhancement and reduced humidity impacting performance compared to the higher concentration treatments. This study reveals the potential of BC treatment for improving the vibro-mechanical performance of spruce.