The interrelation of microfibril angle and the lignified S2 layer on hygrothermal recovery in juvenile and mature compression wood of Cryptomeria japonica and Chamaecyparis obtusa

Abstract

To release viscoelastically locked-in residual growth stress in wood rapidly, wood is treated by using boiling water. This process is called hygrothermal recovery (HTR). In this study, the mechanism of longitudinal HTR in Cryptomeria japonica (sugi) and Chamaecyparis obtusa (hinoki) treated at 120 °C was investigated. This study used juvenile and mature compression wood (CW) and normal wood (NW) and focused on the correlations of the HTR strain with the microfibril angle (MFA) and area ratio of lignified S2 (S2L) layer. The results revealed the following: In mature CW (M-CW) and juvenile CW (J-CW), both MFA and S2L area increased, and at the same time, large expansive HTR strain was observed. In M-CW, longitudinal HTR strain showed significant correlations with MFA and S2L layer. HTR strains ​​were more variable in J-CW than in M-CW. In mature NW (M-NW) and juvenile NW (J-NW), HTR strains were smaller than those of CW, and correlations between MFA and HTR strains were low. Based on these results, the following hypothesis is proposed: During the cell wall maturation, increased lignin and (1→4)-β-galactan concentrations in the S2L layer generate a large compressive growth stress along the fiber axis in CW, part of which remains as viscoelastically locked-in compressive stress. Hygrothermal treatment induces the softening of lignin and the water swelling of (1→4)-β-galactan in the matrix of S2L layer, which leads to a large expansion of the S2L layer, i.e., the release of the viscoelastically locked-in growth stress. Finally, the large MFA of the S2 layer and the large expansion of the matrix of the S2L layer generate a large expansive HTR strain in the longitudinal direction of the CW.