Effect of lignin on veneer densification and set-recovery

Wood densification, aimed at enhancing mechanical properties through compression, faces challenges due to set-recovery, where densified wood regains its original shape upon re-moistening. However, understanding the role of lignin in set-recovery remains elusive. In this study, acidified sodium chlorite was applied to achieve approximately 50 % delignification without removing other wood components from 3.2 mm veneers of birch, aspen, and grey alder. The veneers were then densified at 100°C and 6 MPa, with wood moisture contents (MC) of 10 % and 16 %. The morphology, physical properties, and dimensional stability of densified wood are studied in relation to delignification and MC at densification. Delignification with subsequent densification at 16 % MC was better than at 10 % MC, increasing the resultant wood density by 89 %, 148 %, and 146 % for birch, aspen, and grey alder, respectively. In general, delignification and higher moisture content during compression resulted in better compression ratios and lower set recovery. This delignification-densification approach reduced set-recovery across all wood species to a range of 71–86 %, suggesting that under these conditions, lignin has only a marginal impact on shape memory and unrestrained set-recovery in wood. Therefore, cellulose microfibrils are likely playing a dominant role. • Partial lignin removal followed by densification increases density and compression ratio of wood. • Delignified samples exhibit lower set-recovery than undelignified. • Delignification improves compression efficiency by enabling polymer chain slippage and hydrogen bonding. • Lignin plays a role in set-recovery, although its contribution is limited.