Timber manufacturers are being challenged by the increasing quantities of fast-grown plantation softwoods that are being supplied for processing. Compared to traditional plantation softwood timber, this timber has increased warping characteristics and lower mechanical properties, which reduces the utility and value of the final timber products. The aim of this research was to design a thermomechanical densification procedure that could be used as a complete processing method to season structural or semi-structural timber in a time and energy efficient manner and to produce minimum cost final timber products with improved dimensional stability characteristics and mechanical properties.
The optimal thermomechanical densification procedure allows timber cells to redistribute under conditions of high plasticity with minimal internal timber stresses being established. To determine the design parameters for the thermomechanical densification procedure, existing processing techniques such as Compreg, Staypak and press drying were investigated in conjunction with theory on the chemistry, cellular structure, viscoelastic properties and drying responses of timber. The knowledge established in these areas provided the basis for timber temperature response and moisture content variation experiments which provided clear output on the design parameters required for the thermomechanical densification procedure.
The thermomechanical densification procedure was used to process timber samples at 40 mm and 45 mm initial thicknesses to a final nominal thickness of 35 mm, representing 13 % and 22 % densification levels respectively. Dimensional stability and mechanical properties of the densified timber samples were then compared to the kiln dried control sample timber.
Observation of the timber samples during densification indicated that high moisture content conditions were present during densification due to the large amount of steam and liquid water released by the timber samples during this process. Final densified timber samples did reveal minor discontinuous cracking along the length of the timber.
The final densified timber products were seasoned to within the target moisture content range of 8 to 16 % and achieved residual compression levels after conditioning that were approximately equal to the compression applied to the timber samples, nominally 13 % and 22 %, indicating the effectiveness of the pressing procedure that included a 2 mm over-press.
The mean MOE results were similar between the timber samples densified to 13 %, the timber samples densified to 22 % and the kiln dried control samples, for both the facewise and edgewise testing orientations. Evaluation of results also indicated that timber samples tested facewise produced lower average results compared to samples tested edgewise. This result occurred partially due to the higher number of defects included in the timber samples tested facewise.
Mean MOR results of timber samples tested facewise indicate that the timber densified to the 13 % level is slightly stronger than the kiln dried control samples, whereas the timber densified to the 22 % level is slightly weaker than the kiln dried control samples.
Hardness results show that there is a mean improvement of 23 % and 31 % over the kiln dried control samples for the timber samples densified to the 13 % and 22 % levels respectively.
The thermomechanical densification procedure designed in this research was successful. This was evident through observation of the timber samples being densified and by inspection of the final timber products. Timber samples were seasoned to within the target final moisture content range and achieved a high degree of dimensional stability. There was no significant improvement in MOE and MOR in the densified timber samples compared to the kiln dried control samples, however timber hardness was improved at both timber densification levels.
Thermomechanical densification technologies provide potential solutions to timber manufacturers processing increasing quantities of fast-grown plantation softwoods and that are looking to produce final timber products with improved utility and value.