Transversal compression and strain behaviour of mature wood strands pressed at high temperatures and pressures

Abstract

The properties of a wood strand in a strandboard are dependent on the conditions surrounding it and these, in turn, change throughout the hot pressing cycle and are therefore dependent on the location of the strand within the mattress. The behaviour of a mattress and its subsequent behaviour as a final product is an average of the strands that make up the product. Models to predict the distribution of strands in a mattress and its compression behaviour have been developed. What is needed is a method to observe the rheological behaviour of individual strands during the compression phase of hot pressing cycle in order to improve understanding of the hot pressing stage. Such a method is presented in this paper. Results for strain incurred by strands, 0·9 mm thick, subjected to a constant stress of 5 MPa at 12% moisture content and temperatures of 180, 150 and 105°C, to mimic mattress surface, intermediate and core layers respectively, showed the inclination of wet low density (300–340 kg m⁻³) radial strands (i.e. compressed in the radial direction) pressed above 105°C towards the formation of an M shaped density profile, while high density (460–500 kg m⁻³) strands tended to promote a flat density profile. Despite the density profile for strands compressed in the same direction not forming the expected density profile, average strains incurred by low density radial and high density tangential strands (i.e. compressed in the tangential direction) pressed at simulated mattress surface, intermediate and core layer conditions produced the expected M shaped density profile. This work demonstrated the possibility for the development of a mattress consolidation model which combines deformation of a mattress and strands during hot pressing.