Production technology for strand-particle board (SPB) II. Effect of initial pressure during hot pressing on thickness swelling

A strand-particle board (SPB) was manufactured using unoriented strands for the surface layers and unoriented particles for the core layer. Todomatsu (Abies sachalinensis) strands, recycled waste wood particles, and melamine urea formaldehyde as a resin adhesive were used. The effect of initial pressure on physical properties under conditions of c onstant surface moisture content (17%) and press temperature (200 degreesC) were investigated using laboratory boards. The initial pressure was set at three levels (3, 4, 5 MPa) by controlling the press closing speed. The results are summarised as follows: 1) When the initial pressure was 4 MPa or more, the effective pressing time (EPT) was significantly shortened and the thickness swelling was significa ntly reduced despite the high surface density of the manufactured board. Th e thickness swelling was lowest when the initial pressure was 5 MPa. 2) When the initial pressure was 4 MPa or more, irreversible swelling was s ignificantly reduced and the retention of surface density after the swellin g test was significantly higher. In the relation between initial pressure a nd reversible swelling or retention of core density, there were no signific ant differences. Based on these results, it was assumed that the decreases in thickness swelling at 4 MPa or more were due to restraint of irreversibl e swelling by the surface layers. 3) In addition to surface mat conditions (strand thickness, moisture conten t) and press temperature, selection of a suitable initial pressure to short en the EPT was effective in reducing thickness swelling.