The Effect of Temperature, Density and Chemical Composition upon the Limit of Hygroscopicity of Wood

Abstract

There are several factors influencing a fundamental property of a wood species, the limit of hygroscopicity of wood (LH) or fiber saturation point (FSP). The effect of temperature upon the limit of hygroscopicity has been mentioned in literature, but has not been precisely quantified. In this paper we have correlated the LHs with temperature of thermal pretreatment, taking also into account the properties of particular wood species, i.e. density and chemical composition. For our investigation we have selected the most important domestic wood species for industrial processing: oak, beech, poplar, fir and spruce. We have determined the limits of hygroscopicity by method of compression strength parallel to grain, for the following temperature ranges of thermal treatments, in dependence of wood species: for hardwoods from 20° to 80 °C and for softwoods from 20° to 90 °C. Our results demonstrate the general decrease of the limit of hygroscopicity with the increase of temperature of the thermal pretreatment of the examined wood species. Mathematical correlations have been evaluated on the basis of experimental data by polynomial regression statistical method. Wood density is another factor examined as influential upon the limit of hygroscopicity. We have determined the decrease of the LH with the increase of wood density, i.e. denser wood species have lower capacity for water adsorption. The differences obtained for the LHs between the examined wood species have been explained in relation to their chemical compositions. Our results indicate that cellulose is the major wood component determining its capacity for water adsorption.

Linear viscoelasticity of hot-pressed hybrid poplar relates to densification and to the in situ molecular parameters of cellulose

“…Hot pressing clearly decreases wood hygroscopicity, and the effect is more pronounced with the most severe hot-pressing conditions. These results corroborate previous observations for heat-treated wood (Kolin and Janezic 1996;Tabarsa and Chui 1997;Navi and Girardet 2000;Repellin and Guyonnet 2005). While at the lowest thermal treatment temperatures, the reduction of hygroscopicity is practically small, especially for oven-dry wood, it is again very marked when wood is hot-pressed at 250°C.…”

“…Figure 3 presents the sorption isotherms conducted at 20°C for control and hot-pressed samples. The maximum EMC at 20°C and 90 % RH reaches 18 %, which is the range of that reported for hardwoods (Kolin and Janezic 1996;Navi and Girardet 2000). Hot pressing clearly decreases wood hygroscopicity, and the effect is more pronounced with the most severe hot-pressing conditions.…”

Comparison of the hygroscopic behaviour of 205-year-old and recently cut juvenile wood fromPinus sylvestrisL.

“…Therefore, the changes that wood undergoes through natural ageing are not necessarily similar to ageing through artificial processes [13]. Although cellulose is the determining component of the cell wall for water sorption [19], other variations in the amorphous components of the wood (hemicelluloses and lignin) and volatile components may have contributed both to the lower crystallisation coefficient of the old wood and to an increase in its hygroscopicity. In terms of the volatile components such as polyphenols, a high concentration of these contributes to low wood hygroscopicity [36], while the influence of the alcohol-benzene extractives has an uncertain influence on the hygroscopicity [40].…”

Cork oak (Quercus suber L.) wood hygroscopic properties and dimensional stability

“…The shrinkage factor showed more constancy between 50-30% of relative humidity for both temperatures. Cork oak wood FSP (27%) is similar to other oak species, such as Q. robur (Kolin and Janezic, 1996), Q. pyrenaica, Q. rotundifolia, Q. rubra, Q. faginea (Carvalho, 1997), and red oak (Wang and Wang, 1999), and to the general average wood FSP of about 30% moisture content (Simpson and TenWolde, 1999). However the precise determination of the fibre saturation point is difficult and rather a FSP region corresponding to a range of values should be considered (Tsoumis, 1991).…”

“…To avoid under estimations, Almeida and Hernández (2006) used only volumetric shrinkage values obtained between 33% and 76% of relative humidity to estimate FSP by the intersection method. It should also be pointed out that the obtained results are only representative of in-use conditions and the situation may be different under kiln-drying since the FSP decreases with increasing temperature (from 20°C to 80°C) according to Kolin and Janezic (1996).…”