Abstract
The effect of wood modification on wood-water interactions in modified wood is poorly understood, even though water is a critical factor in fungal wood degradation. A previous review suggested that decay resistance in modified wood is caused by a reduced wood moisture content (MC) that inhibits the diffusion of oxidative fungal metabolites. It has been reported that a MC below 23%–25% will protect wood from decay, which correlates with the weight percent gain (WPG) level seen to inhibit decay in modified wood for several different kinds of wood modifications. In this review, the focus is on the role of water in brown rot decay of chemically and thermally modified wood. The study synthesizes recent advances in the inhibition of decay and the effects of wood modification on the MC and moisture relationships in modified wood. We discuss three potential mechanisms for diffusion inhibition in modified wood: (i) nanopore blocking; (ii) capillary condensation in nanopores; and (iii) plasticization of hemicelluloses. The nanopore blocking theory works well with cell wall bulking and crosslinking modifications, but it seems less applicable to thermal modification, which may increase nanoporosity. Preventing the formation of capillary water in nanopores also explains cell wall bulking modification well. However, the possibility of increased nanoporosity in thermally modified wood and increased wood-water surface tension for 1.3-dimethylol-4.5-dihydroxyethyleneurea (DMDHEU) modification complicate the interpretation of this theory for these modifications. Inhibition of hemicellulose plasticization fits well with diffusion prevention in acetylated, DMDHEU and thermally modified wood, but plasticity in furfurylated wood may be increased. We also point out that the different mechanisms are not mutually exclusive, and it may be the case that they all play some role to varying degrees for each modification. Furthermore, we highlight recent work which shows that brown rot fungi will eventually degrade modified wood materials, even at high treatment levels. The herein reviewed literature suggests that the modification itself may initially be degraded, followed by an increase in wood cell wall MC to a level where chemical transport is possible.
Highlights
When wood is exposed to humid conditions, it becomes highly vulnerable to fungal attack [1,2].It is estimated that 10% of the lumber harvested each year is used to replace timber decayed by fungi [3]
Together with the in vitro tests above, these results indicate that the fungus was not able to induce OH radical formation in these materials. Whether this is due to the fact that the chelator-mediated Fenton (CMF) metabolites cannot be transported through the modified wood cell wall or that the Fenton reaction somehow is inhibited cannot be concluded from the current literature
Even though the RH thresholds may be different for CMF metabolites, this study clearly shows that acetylation of wood has a negative effect on diffusion through the wood cell wall
Summary
When wood is exposed to humid conditions, it becomes highly vulnerable to fungal attack [1,2]. Higher temperatures and humidity due to climate change will likely improve conditions for fungal decay and further shorten the service life of wood products in exterior applications [4]. Forests 2019, 10, 522 brown rot fungi are the most common and most destructive organisms involved in the degradation of softwood products [1,5,6]. Brown rot fungi preferentially attack and rapidly depolymerize the structural carbohydrates in the cell wall, cellulose and hemicelluloses, leaving behind highly modified lignin residues [7,8,9]. Brown rot fungi are relevant in the built environment where they can rapidly compromise the strength of structural wood products
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