Thermo-mechanical densification combined with thermal modification of Norway spruce (Picea abies Karst) in industrial scale -: Dimensional stability and durability aspects

Heat-treatments of wood to improve selected wood properties, e.g. durability and dimensional stability, are well established industrial processes. However, the main drawbacks of thermally modified timber are the reduced strength properties. In a previous study, thermo-mechanically densified wood with increased initial strength was successfully applied to an oil-heat treatment (OHT) in laboratory scale to overcome the problem of reduced strength properties. Consequently, the up-scaling of processes to industrial scale. was the objective of this study. Therefore, Norway spruce (Picea abies Karst.) was thermo-mechanically densified in laboratory scale at 140 degrees C, 160 degrees C, 180 degrees C, and 200 degrees C for 0.5 h, 1 h, 2 h, and 4 h and afterwards modified by a laboratory OHT-process at 180 degrees C, 200 degrees C, and 220 degrees C for 2 and 4 h. Swelling properties and biological properties were investigated on matched samples to identify suitable combinations of densification and OHT for use in outdoor application. Further on, the process-parameters assessed from laboratory scale were taken over for industrial scale production. The results show that compression-set recovery of densified and oil-heat treated spruce was almost completely eliminated by an OHT at temperatures above 200 degrees C, as demonstrated in laboratory tests and after 30 months natural weathering. Thus, with respect to the dimensional stability and improved durability, the industrially densified and oil-heat treated spruce timber appears to be suitable for weathered application.