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 x 20 x 10 mm(3) were thermally modified at 140 degrees C, 160 degrees C, 180 degrees C, 200 degrees C and 220 degrees 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 degrees C and 160 degrees C was applied. Differences between studied species were more significant at temperatures higher than 160 degrees C. With the highest tested temperature (220 degrees 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 (R-2 = 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 (R-2 = 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.

Automated summary

The mass loss kinetics of thermally modified wood species were analyzed based on time-temperature function, with hardwoods showing higher sensitivity to thermal degradation compared to softwoods. The three-dimensional analytical function was confirmed as valid for all studied wood species, allowing for prediction of mass loss values and necessary material properties for wooden product design.