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.

Wood modification processes based on citric acid in combination with various copolymers have gained in importance in recent years. These processes also include modification with citric acid (CA) and sorbitol, a sugar alcohol, which is currently in the industrial realisation phase in Germany (SorCA) and Norway (CIOL). The modification of wood with sorbitol and citric acid (SorCA) can significantly improve the dimensional stability and durability of the wood. It is hypothesised that the addition of a copolymer may improve the fixation of the chemicals within as well as the flexibility of the modified wood matrix. In this study, the macroscopic changes, the fixation of the chemicals and the impact bending properties of wood modified with CA and SorCA are compared. Both modifications cause a permanent increase in mass (WPG) and volume (CWB). The fixation of the chemicals was comparable for both modifications; at high chemical concentrations, the SorCA polyester fixed slightly better within the wood matrix. Both modifications led to embrittlement of the wood, measured by a decrease in the impact bending strength (IBS). This embrittlement was clearly concentration-dependent and slightly more pronounced for the modification with CA.

In Germany, Pinus sylvestris is a dominant tree species, and many trees with large diameters are not utilised due to difficulties with processing. However, older pines have larger volumes of sapwood, and boards with a high sapwood content can be produced. The durability of boards from large-diameter (>50 cm) P. sylvestris trees, treated with furfurylation, acetylation, DMDHEU (1.3-dimethylol-4.5-dihydroxyethyleneurea), and SorCA (Sorbitol/Citric Acid), was assessed. The samples were taken from different sections along the longitudinal axis and the cross-section. The durability was tested against Coniophora puteana, Rhodonia placenta, and Trametes versicolor, according to the EN 113-2 standard. All treatments had a median mass loss < 5%, so classed as “highly durable” (Durability Class 1) against all fungi. DMDHEU had a large deviation in mass loss against Coniophora puteana and could potentially be classified as “moderately durable” (Durability Class 3), if based on the mean mass loss. The inner part of the end section had a higher mass loss, indicating that there was poorer retention of the treatment at this location. Overall, chemical modifications on large-diameter pine trees were effective at increasing durability. Utilising large-diameter pine trees can help to make use of regional resources and potentially reduce reliance on imported timber. With favourable mechanical properties and easy-to-treat sapwood, large-diameter P. sylvestris trees could be used for commercial treatments.

Modification with citric acid and sorbitol (SorCA) or with pure citric acid (CA) both improve wood durability, yet the contribution of sorbitol remains unclear. Scots pine sapwood was treated with SorCA or CA at equal chemical loadings and exposed to Trametes versicolor , Coniophora puteana , and Rhodonia placenta in an EN 113-2 (2021) decay test. Mass loss caused by T. versicolor was low for all treatments. For C. puteana and R. placenta , mass loss decreased strongly with increasing treatment level. However, CA treated samples showed lower mass losses than SorCA treated samples at comparable weight percent gain (WPG) and comparable calculated CA-content, suggesting differences in reaction pathways within the wood matrix. Moisture contents determined after exposure differed markedly from values obtained in standardised water uptake tests. Across fungi and treatments, absolute water content increased with treatment level and was negatively associated with mass loss. It remains unclear whether this relationship indicates a causal link between moisture and decay, or is confounded by other mechanisms that increase decay resistance.