The wettability and bonding performance of densified VTC beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) bonded with phenol–formaldehyde adhesive and liquefied wood

The influence of viscoelastic thermal compression (VTC) on surface wettability and bonding performance of wood was evaluated. Low quality beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) were densified with the VTC process to different degrees of densification. Control and densified strips were bonded with phenol–formaldehyde (PF) adhesive and liquefied wood (LW). Shear strength of bonded assemblies was determined after 1 week of conditioning at 20 °C and relative humidity of 65 %. Wettability was determined on the basis of the contact angle of water, PF adhesive, and LW using the Wilhelmy method. Results showed that densification of beech and spruce wood did not significantly affect the shear strength of specimens bonded with PF adhesive. In beech assemblies bonded with LW shear strength decreased significantly with increased density, whereas in bonded spruce specimens decrease of shear strength was not significant. It was found that degree of densification and bonding process used in the study were not appropriately chosen for spruce wood specimens, since major deformations after the bonding process occurred. Wettability changed significantly after densification. Contact angle of water and LW increased after densification, whereas contact angle of PF showed inverse trend and decreased after VTC process. Furthermore, the degree of densification had a minor effect on the wettability.     

Die Xylemleitquerschnitte von Fichten (Picea abies (L.) Karst.) unterschiedlicher Vitalitätsgrade and Altersklassen

In mountaineous regions of Germany Norway Spruce (Picea abies Karst.) the most common and the most important tree species is severely affected by forest decline. At Göttingen University an integrated research project is conducted to analyse qualitative and quantitative relations between site quality, pollution (immision) and physiology and vitality of the affected trees. Within that context it is of particular interest to know if the conductive structure of the tree’s xylem can become a limitation for transport of water between the roots (as uptaking parts) and the crown/needles (as evaporating parts) of the hydraulic system. A total of 108 trees out of 5 young stands (appr. 40 yrs.) and 2 old stands (appr. 120 yrs.) situated in the Harz mountains and nearby have been intensively analysed. Two of the stands are severely affected by forest decline, and showed heavy needle-loss in the crowns, the other trees were obviously in normal, vital conditions. On cross cuts of 4 different levels (heights) of the trees, the following variables had been measured Absolute sapwood-area, density (ovendry), relative moisture content, water content of the xylem and cross-sectional area of conductive tissue in the sapwood. The results show clearly, that

• - the sapwood-area in the affected trees is lower than in the vital trees. The relative sapwood area increased from the bottom to the top of all trees, independent from their vitality.
• - the density raised from the pit to the bark of the stem, subvital trees showed higher densities.
• - the moisture content of the sapwood of the vital trees was higher than that of the subvital trees.
• - there is a clear distinction in moisture content between heartwood, inner sapwood and outer sapwood. This supports the hypothesis, that the latter is primarily responsible for the water conduction.
• - there was an inverse correlation between the area of the lumina of tracheids (“pores”) and the density, the subvital trees showed a lower area of pores compared with the vital trees.
• - the sapwood area showed high correlation with the ovendry weight of the total needles of the tree, this correlation can be even improved if the proportion of pores within the sapwood area and its water content are calculated as additional variables.

     

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 °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.