Vol.:(0123456789) 1 3 European Journal of Wood and Wood Products https://doi.org/10.1007/s00107-020-01560-7 ORIGINAL ARTICLE Effect of heat treatment on some physical and mechanical properties of birch plywood Pavlo Bekhta 1 Received: 25 September 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract The plywood consumption in various applications is increasing, especially in construction. In service, plywood can be exposed to a range of outdoor conditions, sometimes at elevated temperatures. This requires an understanding of the struc- tural behavior and knowledge of the effects of the elevated temperatures on the properties of plywood. Therefore, the objec- tive of this research was to study the effect of exposure time on physical and mechanical properties of plywood at elevated temperatures. The plywood samples were tested after exposure to nine different temperatures (50 °C, 75 °C, 100 °C, 125 °C, 150 °C, 175 °C, 200 °C, 225 °C and 250 °C) and ten exposure time levels (0.5, 1, 1.5, 2, 2.5, 3, 6, 8, 10 and 12 h) at each temperature. Additionally, a set of control samples was tested at room temperature. The findings of this study indicated that exposure of plywood panels to elevated temperature caused significant degradation of their bending and bonding strengths, whereas the hydrophobicity of the panels improved. Three stages of plywood degradation could be distinguished. Below 175 °C, there is a slow decline in strength; the decisive change already occurs at 175 °C; but as temperature increases beyond 175 °C, a rapid decrease in strength is observed. Plywood samples lost 98.1% of their initial strength after 1.5 h of exposure at 250 °C, and lost 98.5% after 6 h of exposure at a temperature of 225 °C. Statistical regression-based models were also developed and evaluated for predicting the strength loss of plywood panels as a function of weight loss. As the weight losses of panels increased, the losses in bending and bonding strength increased too. The decrease in bending and bonding strength only becomes significant when the weight loss exceeds 5%. When the weight losses increased above 15%, plywood samples almost lost their strength. 1 Introduction Production and consumption of plywood is increasing. Global production and consumption of plywood reached 157 and 154 mln m 3 , respectively in 2017 (FAO 2018). Plywood is widely used in the construction side by side with solid wood. It is known more about the behavior of solid wood than plywood when exposed to elevated temperatures. With increasing consumption of plywood, it is important to evalu- ate its properties after exposure to elevated temperatures. There have been some studies on the use of heat treatment to improve the performance of wood or wood based materi- als. Several studies (Green et al. 1999; Bekhta and Niemz 2003; Kocaefe et al. 2008) have investigated the effect of heat treatment on mechanical properties of solid lumber. The authors observed that strength of lumber decreases with increasing temperature. Temperature is a major factor for the loss in strength of wood (Bekhta and Niemz 2003; Kocaefe et al. 2008). It is known that wood does not collapse at long exposure to 100 °C (Green et al. 1999). However, plywood is a layered composite containing wood veneer and glue. Even if the wood does not break down at 100 °C, the resins used in the plywood can be destroyed, which in turn causes a decrease in the strength of the plywood. Winandy et al. (1988, 1991) even observed room-temperature strength loss in plywood exposed to temperatures below 100 °C, but the degradation took much longer than 8 h (a few months). Several studies have reported on the influences of the post heat-treatment of wood-based composites, such as particleboard, flakeboard, waferboard, and oriented strand- board (OSB) (Suchsland and Enlow 1968; Hsu et al. 1989; Zhang et al. 1997; Bekhta et al. 2003; Ohlmeyer and Lukowsky 2004; Del Menezzi and Tomaselli 2006, Bekhta and Marutzky 2007; Okino et al. 2007). These studies often * Pavlo Bekhta bekhta@nltu.edu.ua 1 Department of Wood-Based Composites, Ukrainian National Forestry University, Gen. Chuprynky 103, Lviv 79057, Ukraine