PSI - Issue 26

Chiara Bertolin et al. / Procedia Structural Integrity 26 (2020) 147–154 Bertolin et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 1: GAB parameters obtained for the six tested specimens: left, V m ads ; center, C GAB ; right k GAB.

The presence of sealing coatings on the wooden sample radial surface also affects C GAB values permitting to quantify the strength of chemical bonds formed between the water molecules of the first layer and those creating the following ones, in turn linked to the strength of bonds existing between the first monolayer and the sample surface. Observing Fig. 1, a decrease in C GAB values can be noted for all the treatments with respect to untreated pine wood, especially for E that in this case follow the same pattern of the other treatments. This means that for all the modified specimens the monolayer of water is bound weaker to primary sorption sites (Timmermann et al., 2001; Broda et a., 2018) and consequently less energy (heat) is necessary to break such weak bonds and remove water molecules. Modifications of k GAB due to the presence of the treatment are associated to the linkage between the water multilayer and water deposited on the top in its totally liquid phase and the difference of one order of magnitude observed for C GAB and k GAB is directly related to the sorption heat of the monolayer and of the multilayer (de Oliveira et al., 2018). When analyzing k GAB data (Fig. 1) two different behavior can be noticed: for CS+P40, P40 and E k GAB decreases, while for CS and R a higher value with respect to untreated pine occurs. The result obtained for specimens with k GAB lower than k GAB of clean sample means that the surface saturation is more difficulty reachable than in the “idealized” case at RH=100%; on the other hand, when an increase in the k GAB value is found, surface saturation is favored as in the case of CS and R. This has been related to an induced modification (increase) in the dewpoint temperature ( T DP ) which reduces the difference between T DP and the T during the experiment ( i . e . 24°C) in equilibrium with the sample thus facilitating condensation on its surface. 3.2 Equilibrium Moisture Content The equilibrium moisture content (EMC) of wood is the moisture content value at which a hygroscopic material is neither gaining nor losing moisture. It is strongly influenced by changes in p / p 0 ( i . e . RH) and T . Fig. 2a, b report the sorption and desorption branches of the six tested samples at 24°C . Generally, all treated samples display lower EMC than clean pine wood because the presence of sealing coatings limit the possibility of moisture diffusion inside the specimens. Information about the p / p 0 value for which the multilayer sorption starts to prevail on the formation of a simple water monolayer can come from the analysis of the minima of first derivative of adsorption and desorption branches. Fig. 2c, d show obtained results. For untreated pine wood as well as for E and CS+P40 the first derivative minima correspond to RH= 30%, while a value equivalent to 40% is found for R, CS and