Modelling hygroexpansion of compression and opposite wood of conifer branches: bridging the gap between molecular and cell wall level

Abstract The use of branch wood as a raw material in refined wood products has been limited. Compared with normal wood, this is due to the difference in structure from molecular level to tissue, especially for compression wood found in conifers. These differences result in distinct material behaviour. To provide greater insight into the underlying mechanisms that govern the difference in moisture-induced swelling between compression wood and opposite wood, a finite element model of a simplified predominant S2 layer was developed. To estimate the swelling of the cell wall, the model included nanoscale information from previous and more recent findings on the composition and properties of the cell wall biopolymers. Parametric investigations were carried out to determine the sensitive areas of the three-phase composite, composed of cellulose, hemicellulose, and lignin. Within reasonable bounds, it was found that the microfibril angle plays a significantly larger role in the swelling behaviour of opposite and compression wood, than the relative composition of the cell-wall biopolymers and the hygroexpansion coefficient of lignin. The latter is known to be chemically different between compression and opposite wood.