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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Treatments can range from heating (Hakkou et al., 2005; Esteves and Pareira, 2009) to impregnation with monomers polymerizing inside the material, or functionalization (Kumar, 1994). Different approaches have been proposed depending on the property to be modified: for water repellency hydrophobic groups can be bound to wood functionalities; the improvement of dimensional stability can be obtained by bulking cell walls with bonded chemicals, or using cross-linking polymers limiting cell wall expansion, or bounding functional group that can increase hydrophobicity. Also flame retardance is achievable, by bonding retardants or flame suppressants functional groups to the cell walls; similarly, UV-resistance is possible by bonding UV blockers or absorbers to lignin (Rowell, 2013). When the main aim is to limit variations in the material moisture content, the mechanism of water absorption desorption must be analyzed in detail. Moisture sorption process in wood has been widely described (see Tshabalala et al., 2003) and one of the most recent approaches involves the use of the Guggenheim-Andersen-de Boer (GAB) sorption equation (Hailwood and Horrobin, 1946; Bratasz et al., 2012), able to describe the full shape of type II isotherm (Timmermann, 2003; Bratasz et al., 2012) and covering relatively wide activity range ( a 0 ) (Timmermann, 2003). For porous materials like wood it is related to the creation of bonds between water molecules and hydroxyl groups (Camuffo, 2019) and in consequence the theoretical basis for the GAB sorption isotherm fits with the vapor adsorption. When additional water layers deposit, interactions with the sorbent surface are assumed to be gradually weaker until reaching the energy level of liquid water (Vertucci and Leopold, 1984; Rizvi, 1986; McMinn, 1997). In addition, the GAB equation also introduced a new parameter named k , related to the difference of the standard chemical potential between molecules creating the additional adsorbed layers and those in liquid state (Timmermann, 2003). The mathematical model behind type II isotherm, besides describing in detail the sorption behaviour at RH<93%, is representative of microporous materials where unrestricted monolayer-multilayer adsorption occurs at the internal surface of a material. The state of the sorbate molecules in the second layer is identical to that one in superior multi layers but different from that of the monolayer below (directly bound to the wood hydroxyl groups) or of the liquid state molecules on the top of the multi-layers. Therefore, as stated by Andrade et al., 2011, C GAB coefficient represents the water primary layer binding strength (respect to the multilayers (Quirijns et al., 2005)). This work is focused (or focuses) on the application of GAB equation to sorption data obtained from pine wood samples which radial surfaces were coated with different sealing materials in order to investigate i) how the GAB parameters modify when moisture induced response of wood is modified by the surface treatments; ii) the behavior of the k parameter as a function of the applied coatings; iii) discrepancies existing in the estimation of the wood specific surface area when the GAB water vapor sorption model is applied instead of the water film thickness layer ( t ). 2. Materials and Methods Six pine ( Pinus Sylvestris L .) wood samples were radially cut from an air- dried log maintained in the Trøndelag area (Norway) over a 3 years period. Five samples were coated on the radial surface following the procedure reported by de Ferri et al., in press, and resumed in Table 1, while one sample not treated by any type of coating was used as a standard.

Table 1: Resume of the surface treatments Sample Treatment CS

Cellulose sand seal spray (Chestnut products)

P40 Paraloid B72: solution 40% (w/v) in acetone. Deposited by brush CS+P40 The Paraloid solution was deposited by brush over the CS sprayed layer R Microcrystalline wax (Reinassence). Deposited by spatula E Epoxy resin (Loctite). Deposited by spatula

Measurements of the water vapor sorption were carried out at 24°C and for a full range of water vapor relative pressures with the aid of the vacuum microbalance (CI Electronics Ltd.). A microsample of 0.1 g was weighted and outgassed prior to a measurement under a vacuum of a residual pressure of less than 0.1 Pa, in order to move the air