Moisture in modified wood and its relevance for fungal decay

Abstract

Water plays an essential role in fungal decay of wood, and limiting the cell wall moisture content by chemical modification can effectively improve the durability of the material. Investigating the wood-water relations of modified material under climatic conditions relevant for fungal decay are, however, experimentally challenging. Most studies in literature therefore focus on moisture sorption under conditions outside those of importance for fungal decay. This review discusses the validity of such data for characterising the wood-water relations at very humid climatic conditions, relevant for fungal decay. Moreover, the review attempts to cover the basics of fungal decay, the important role of water, and how controlling water content by modification can improve durability.

The Importance of Moisture for Brown Rot Degradation of Modified Wood: A Critical Discussion

“…This means that at a given RH, capillary condensation will form liquid water in pores of a particular size range. As described in Thybring et al [96], the minimum RH required for fungal growth is in the range of 92%-97% [120][121][122][123]. Using the Kelvin equation and assuming the surface tension between water and wood is 0.1 J/m 2 [124], capillary water should only form in nanopores of 34-96 nm diameter or smaller at the brown rot growth RH threshold.…”

“…Whether this is due to the fact that the 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. Thybring et al [96] suggest that reductions in cell wall MC prevent fungal decay by hindering the transport of fungal agents into the wood cell walls, presumably from a disruption of the continuous water network within cell walls. Zelinka et al [97] also discussed the possible mechanisms behind the decay resistance seen in modified wood: (i) inhibition of diffusion through an increase in the glass transition temperature of hemicelluloses (assumed to be the medium of transport); (ii) inhibition of diffusion through nanopore blocking; (iii) no inhibition of diffusion but instead a lower rate of diffusion and/or inhibition of chemical reactions leading up to the Fenton reaction through e.g., alteration of the pH level.…”

“…Unfortunately, this is difficult at the high RH levels relevant for the brown rot decay threshold. As discussed in Thybring et al [96], conditioning wood samples to near saturation is problematic using traditional methods for controlling RH, like sorption balances, saturated salt solutions and climate chambers. It is difficult to obtain stable saturated water vapor because of temperature differences between the vapor and the surrounding surfaces.…”

“…In the study of moisture relationships in modified wood and its effect on fungal decay, controlling the moisture conditions in the samples is of utmost importance. Performing decay tests under controlled moisture conditions is a challenge, as pointed out by Thybring et al [96]. As mentioned above, the threshold RH for brown rot decay is >95% and maintaining such a high RH is hard to achieve with conventional methods.…”

“…One way to do this is to use saturated salt solutions or pressure plates to create a steady, high RH. However, as mentioned previously, temperature fluctuations as small as 0.5 • C will affect the RH [96]. Additionally, the sealed vessels used in such analyses create another issue.…”