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

摘要

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 °C, 160 °C, 180 °C, and 200 °C for 0.5 h, 1 h, 2 h, and 4 h and afterwards modified by a laboratory OHT-process at 180 °C, 200 °C, and 220 °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 °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.