Water sorption in wood and modified wood at high values of relative humidity. Part I: Results for untreated, acetylated, and furfurylated Norway spruce

Abstract

Desorption isotherms at 208C for untreated, acetylated, and furfurylated Norway spruce wPicea abies (L.) Karst.x sapwood were established in the 91.9-99.9% relative humidity (RH) range. Three methods were employed to secure various constant RH levels: saturated salt solutions, climate chambers, and the pressure plate technique. The curve form for the untreated samples did not show an upward bend, except perhaps above 99.5% RH, indicating that -contrary to what has hitherto been assumed -capillary condensation does not play a significant role for water sorption in wood below fiber saturation. Three additional results corroborate this conclusion: (1) calculation of the theoretical contribution of capillary condensation to the moisture content (MC) in wood based on idealized microstructural geometries by means of the Kelvin and Laplace equations resulted in very small contributions to the equilibrium moisture content (EMC), i.e., below 0.35% moisture at 99.9% RH. (2) The ratio between the EMC of acetylated and untreated samples did not show an increasing trend for increasing RH, as would have been the case if capillary condensation had taken place in both untreated and acetylated wood. (3) Low field time domain nuclear magnetic resonance results showed that only the relaxation curves from the furfurylated samples were affected systematically by freezing, indicating that neither untreated nor acetylated wood contained significant amounts of capillary condensed water.

A critical discussion of the physics of wood–water interactions

“…These methods yield FSP values in the 38.5-42.5 % MC range for different softwood species. There is thus a difference of about 10 % moisture between the FSP around 30 % MC which is obtained if a change in how strongly a physical property of wood (electrical conductivity, shrinkage, strength properties) depends on the moisture content is used as basis for the definition (Stamm 1929(Stamm , 1971) and the FSP of about 40 % MC which is obtained if based on the amount of water present in the cell wall at equilibrium conditions close to 100 % RH (Hernández and Bizoň 1994;Hill et al 2005;Engelund et al 2010;Thygesen et al 2010;Hoffmeyer et al 2011). This discrepancy raises the question why the last 10 % MC within the cell wall (i.e., from 30 to 40 % MC) do not affect the physical properties of the cell wall as much as the first 30 % MC, while still apparently being present as bound water Thygesen et al 2010;Hoffmeyer et al 2011).…”

“…There is thus a difference of about 10 % moisture between the FSP around 30 % MC which is obtained if a change in how strongly a physical property of wood (electrical conductivity, shrinkage, strength properties) depends on the moisture content is used as basis for the definition (Stamm 1929(Stamm , 1971) and the FSP of about 40 % MC which is obtained if based on the amount of water present in the cell wall at equilibrium conditions close to 100 % RH (Hernández and Bizoň 1994;Hill et al 2005;Engelund et al 2010;Thygesen et al 2010;Hoffmeyer et al 2011). This discrepancy raises the question why the last 10 % MC within the cell wall (i.e., from 30 to 40 % MC) do not affect the physical properties of the cell wall as much as the first 30 % MC, while still apparently being present as bound water Thygesen et al 2010;Hoffmeyer et al 2011). Perhaps one should think of fibre saturation not as a state reached at a certain moisture content (i.e., a point) but as a gradual transition from the situation where new water molecules entering the cell wall result in breaking of intra-and intermolecular H-bonds in the wood cell polymers (i.e., up to about 30 % MC) to the situation (i.e., from about 30 to 40 % MC) where new water molecules are accommodated in the cell wall without breaking further cell wall polymer H-bonds.…”

Mode of action of brown rot decay resistance in modified wood: a review

“…Furthermore, it explains the delayed but eventual onset of degradation, because the EMC will increase with time due to moisture uptake from the surrounding air via mall cracks and voids in the wood cell wall (Welzbacher and Rapp 2004;Thygesen et al 2010). …”

“…The following modifications are considered: acetylated wood (Militz 1991;Larsson Brelid et al 2000), furfurylated wood (Schneider 1995;Westin 1996;Lande et al 2008;Bryne and Wålinder 2010;Thygesen et al 2010), dimethylol dihydroxyethyleneurea (DMDHEU)-treated wood (Militz 1993;Dieste et al 2009b), and thermally modified wood (Tjeerdsma et al 1998;Welzbacher 2007;Windeisen et al 2009;Pfriem et al 2010). …”

Critical moisture conditions for fungal decay of modified wood by basidiomycetes as detected by pile tests

“…Information is scarce in terms of the changed water uptake behavior and the critical MC for onset of fungal growth (Meyer et al 2012). Furthermore, it is still controversially discussed to which extent decay is influenced by the presence of liquid water in cell lumens (Thygesen et al 2010;Ringman et al 2014). …”