Mass loss kinetics of thermally modified wood species as a time–temperature function

The mass loss kinetics of thermally modified wood species was analyzed as a time–temperature function. European beech (Fagus sylvatica L.), English oak (Quercus robur L.), Norwegian spruce (Picea abies L. Karst.) and Scots pine (Pinus sylvestris L.) wood specimens of dimensions 20 × 20 × 10 mm³ were thermally modified at 140 °C, 160 °C, 180 °C, 200 °C and 220 °C for 1–6 h using atmospheric pressure and superheated steam environment. The process intensity was determined by mass loss (M_L), based on oven-dry mass before and after the thermal modification. Furthermore, the equilibrium moisture content (EMC) was determined before and after thermal modification to analyze the effect of mass loss on the sorption properties. Measured mass loss data were compared with the three-dimensional analytical function and its applicability to mass loss prediction was verified. For the studied wood species, the M_L was found to be less than 1–1.5% when temperature of 140 °C and 160 °C was applied. Differences between studied species were more significant at temperatures higher than 160 °C. With the highest tested temperature (220 °C), mass loss reached 13.5% (beech), 18.8% (oak), 6.7% (spruce) and 13.5% (pine). According to the results, hardwoods have been shown to be more sensitive to the thermal degradation than softwoods as demonstrated by the higher mass loss recorded for the same modification time and temperature. The three-dimensional analytical function was confirmed as valid for all the species studied (R² = 0.89–0.99) and relevant for the mass loss prediction using fitted parameters. The EMC was reduced after thermal modification within the range of 4–48%, 0.4–47%, 1–32% and 0.7–40% for beech, oak, spruce and pine, respectively. Further, the EMC correlates exponentially (R² = 0.91–0.95) with the decrease in the specimens’ mass depending on the wood species used and modification temperature applied. However, the EMC seems to be almost stabilized beyond a limit value of approximately 10–12% of mass loss. The results provide a better insight into the mass loss and EMC kinetics of thermally modified wood species and can be used as a tool for prediction of mass loss values and required material properties (EMC) for designed wooden products.