Scanning or desorption isotherms? Characterising sorption hysteresis of wood

Abstract

Sorption isotherms describe the relation between the equilibrium moisture content of a material and the ambient relative humidity. Most materials exhibits sorption hysteresis, that is, desorption give higher equilibrium moisture contents than absorption at equal ambient climate conditions. Sorption hysteresis is commonly evaluated by determination of an absorption isotherm followed by desorption starting from the highest relative humidity used in the absorption measurement (typically 95%). The latter is often interpreted as the desorption isotherm but is in fact a scanning isotherm, i.e. an isotherm obtained from neither dry nor water-saturated state. In the present study, we investigated the difference between desorption isotherms and scanning isotherms determined by desorption from different high relative humidity levels reached by absorption and how this difference influenced the evaluation of sorption hysteresis. The measurements were performed on Norway spruce (Picea abies (L.) Karst.) using automated sorption balances. Hysteresis evaluated from desorption isotherms gave linear absolute sorption hysteresis for the studied relative humidity range (0-96%), whereas hysteresis evaluated from scanning isotherms gave non-linear curves with a peak between 50 and 80% relative humidity. The position of this peak depended on the relative humidity from which desorption was initiated. Consequently, understanding and evaluation of sorption hysteresis might be challenging if scanning isotherms are used instead of desorption isotherms, hereby increasing the risk of misinterpreting the results.

The interaction of water with archeological birch bark and its effects on swelling, shrinking and deformations

“…Samples where the phloem is still attached to the outer bark are characterized by a high hygroscopicity, similar to wood [21] while samples where only the phellem is retained are characterized by a four times smaller moisture content that has little dependency on the speci c birch specie ( Figure 2). We report here only the adsorption isotherm as the authors reporting desorption results [19] did not let the samples saturate at 100% RH, measuring therefore scanning isotherms whose hysteresis cannot be univocally interpreted [24]. The difference in sorption behavior between samples with or without phloem can be explained by the fact that the secondary phloem is formed structurally by open cells, the sieves tubes, and chemically mostly by polysaccharides that are hygroscopic due to the large number of hydroxyl and carboxyl polar groups that can form hydrogen bonds with water molecules.…”

On Wood–Water Interactions in the Over-Hygroscopic Moisture Range—Mechanisms, Methods, and Influence of Wood Modification

“…According to mercury intrusion porosimetry (MIP) results from the same study, 37% of the pores were larger than 0.48 µm. Since MIP measures the pore entry sizes rather than the pore sizes themselves [34], there was a good Since the equilibrium moisture content depends not only on the ambient climate, but also on the previous moisture level, there are also intermediate curves, connecting the desorption and isotherm isotherms, called scanning isotherms [4,31,32]. These curves describe the equilibrium moisture content obtained when starting from another initial moisture state than a dry or water-saturated state.…”

Relative humidity versus moisture content relationship for several commercial wood species and its potential effect on flame spread

“…Since these samples were not fully saturated in vacuum prior to the desorption isotherm, this represents a so-called "scanning" desorption isotherm. 22 This choice of scanning isotherm (rather than desorption from full water saturation) was justified as the goal of the study was to examine the range of potential MCs for standard flame spread tests where it is unlikely that the samples start from a fully waterlogged state prior to testing. After equilibration at 90% RH, the samples were then placed above one of the saturated salt solutions ).…”

“…Typically, two sorption isotherms are presented for wood: the “absorption isotherm” where the wood is equilibrated at higher RH from an oven‐dry state and the “desorption isotherm” where the wood is equilibrated at lower RH from a wet initial state . Although desorption isotherms are commonly plotted starting at an RH of 90% or 95%, true desorption isotherms need to be collected from a fully water‐saturated state . The absorption and desorption isotherms represent boundary MCs; if the RH is first increased, then decreased and then increased again, the actual MC will fall somewhere in between the two boundary isotherms…”

“…[10][11][12][13][14][15][16][17][18][19][20][21] Although desorption isotherms are commonly plotted starting at an RH of 90% or 95%, true desorption isotherms need to be collected from a fully water-saturated state. 22 The absorption and desorption isotherms represent boundary MCs; if the RH is first increased, then decreased and then increased again, the actual MC will fall somewhere in between the two boundary isotherms. 6 The fact that the sorption isotherms are both species dependent and hysteretic has implications for flame spread testing.…”