Mechano-sorptive creep of heat treated and innate beechwood

Wood heat treatment (HT) is promising technique for improvement of dimensional stability and durability of wood, specifically needed for outdoor applications. Wood can be exposed in these cases to various loads and to continuously changing environmental and climatic conditions. Among many positive impacts the heat treatment also reduces the wood density and most of the mechanical properties, as well as changes rheological characteristics of wood (Gorišek and Straže, 2011). Characteristics of HT wood highly depend on the properties of the innate wood specie and used process parameters. Industrial heat treatment under steam atmosphere was performed on beechwood (Fagus sylvatica L.) with pre-drying, heating at 230 °C for 6 hours, followed by conditioning and cooling. Oriented micro-tensile wood specimens (n = 20), 1.5 mm (T) by 3.0 mm (R) by 50.0 mm (L), were made from innate- (C) and heat treated wood (HT). Firstly, short-term micro-tensile mechanical tests were carried out at half of the conditioned wood specimens in dry climate (T = 23 ± 0.1 °C ; RH = 33 ± 1%). Micro-tensile mechano-sorptive tests followed afterwards in specially designed apparatus (Navi et al., 2002), limited to the area of linear viscoelasticity (σ = 0.2×σmax) for 48 h, where climatic conditions were cyclically changed between relative humidities of 33 % and 75%, at temperature of 23 ± 0.1 °C. The mechano-sorptive test was combined of phase of free swelling and shrinkage (a), phase of transient moisture effect and loading on the mechanical response (b) and a phase of transient moisture effect on the strain recovery (c). Heat treatment significantly reduced density of beechwood in average for 21%, from 760 kg/m3 to 575 kg/m3. We confirmed also 15% decrease of modulus of elasticity (MOE) from 18.7 GPa to 15.7 GPa, and the loss of mean tensile strength, from 170 MPa (C) to 84 MPa (HT). The viscoelastic creep response of HT wood was slightly reduced, where the creep rate was decreased also (Fig. 1 ; Fig. 2) Differences were found out also at mechano-sorptive response between innate and HT beechwood, where the latter had almost no mechano sorptive deformation. The ratio of final mechano sorptive strain (εms) to initial elastic strain (ε0) reaches at HT beechwood less than half of the value of innate wood. The mechano-sorptive strain was the greatest in the first relative humidity cycle (Fig. 1 ; Fig. 2). Lower mechanical properties, less visco-elastic creep and mechano-sorptive deformations can be generally ascribed to chemical changes of HT wood, to reduced amount of water in the cell wall structure, to rearrangement of molecular structure as well as to less established and loosen bands between cellulosic microfibrils, hemicelluloses and matrix of lignin (Olsson et al., 2005). Micro deformations of the cell wall structure can not either be neglected.