Esterification of wood with citric acid and sorbitol: effect of the copolymer on the properties of the modified wood. Part 2: swelling and shrinking, sorption behaviour and liquid water uptake

Formaldehyde-free, bio-based wood modification processes are gaining importance. One well-developed method is modification with citric acid and sorbitol as a copolymer (SorCA). However, some studies report that citric acid (CA) alone can be used for modification. In our previous paper, we showed that although the addition of sorbitol leads to a higher weight percent gain (WPG), the cell wall bulking (CWB), chemical fixation and impact bending strength were only marginally different compared to modification with pure citric acid. This study compares water-related properties, namely (1) dimensional stability and (2) swelling and shrinking behaviour, (3) water vapour uptake, (4) liquid water absorption and release and (5) capillary water uptake (CWU). Modification with pure citric acid resulted in higher dimensional stability compared to modification with SorCA. Dynamic vapour sorption (DVS) measurements showed that the modification with CA leads to a higher reduction in water vapour uptake. Liquid water uptake and release tests on the other hand revealed that, after leaching, SorCA-modified samples exhibited significantly lower water absorption. In summary, this study identified that while the addition of a copolymer has little effect on dimensional stability and water vapour sorption, it exerts a clear positive influence on liquid water absorption.