Factors Influencing the Crack Formation in Thermally Modified Wood

• Wood anatomical changes during a thermal modification process are likely to adverselyaffect the properties of the final product. Therefore, an anatomical investigation was performed on Scots pine and Norway spruce material coming from different steps of the regular production of ThermoWood. A macroscopic evaluation using an evaluation scale between 1 and 3 revealed typical drying defects in some of the kiln dried boards. Nevertheless, at least 50 % of the unmodified boards were defect free (CR = 1), while at least 90 % of the thermally modified boards were not defect free (CR = 2 and CR = 3), thus indicating that surface cracks become more frequent and severe after the modification process. However there was no evidence of an impact of the conditioning stage on the crack formation. Even though most of the surface cracks were removed by planing, microscopic analysis by means of light and scanning electron microscopy revealed that there might still be micro-defects within the ThermoWood. Such micro-defects were mainly associated with parenchyma or epithelial cells of the rays or the resin canals. Furthermore, longitudinal cracks within the tracheids were observed within the earlywood latewood interface or within the earlywood. During repeated drying and wetting cycles, the crack formation was higher for thermally modified wood than for unmodified, despite a lower water uptake and a better dimensional stability. After the third cycle, the average crack rating of thermally modified wood was at least twice as high as for the unmodified wood. It was concluded that this increase in the crack formation might be caused by the presence of micro-defects that potentially act as a starting point for more severe cracks during swelling and shrinkage.

Ämnesord

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Trävetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Wood Science (hsv//eng)

Nyckelord

cracks

drying-wetting cycles

microscopy

ThermoWood

wood anatomical changes

Forestry and Wood Technology

Skog och träteknik

Publikations- och innehållstyp

ref (ämneskategori)

kon (ämneskategori)