Densification of New Zealand-grown Eucalyptus species: Effect of grain orientation and densification process on wood properties
Abstract
Eucalyptus fastigata and Eucalyptus nitens were densified using a thermo-hydro-mechanical (THM) densification process. The THM treatment was applied either as surface densification of one wood surface or as bulk densification of the entire wood thickness. To understand the effect of grain orientation on final wood properties, both quarter-sawn and flat-sawn boards were densified. The Eucalyptus nitens boards were able to be compressed to a greater degree without being damaged compared to Eucalyptus fastigata boards. This led to substantial increases in surface hardness and surface density in Eucalyptus nitens. Additionally, levels of set-recovery (irreversible swelling from contact with water) for bulk densified Eucalyptus nitens were substantially lower than Eucalyptus fastigata and lower than literature values for other species with a similar density. The reason for this unusually low set-recovery is not known, but it is of potential interest for the commercial application of densified wood, where set-recovery is unacceptable and would need to be eliminated. Density profiles showed that the peak density was generally at, or very close to, the wood surface, giving the maximum increase in surface hardness for a given degree of densification. The properties following densification were not substantially different between the quarter-sawn and flat-sawn boards suggesting that densification was effective irrespective of grain orientation.
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