The physical and mechanical properties of the wood and composite materials which was surface-densified by roll-pressing were investigated.
The surface-densified wood(control）which was simply pressed at high temperatre and the PP-surface-densified wood which the surface layer was simultaneously reinforced with polypropylene film(PP) were prepared. Also, the densifying and simultaneously gluing between spruce and birch veneer carried out by roll-pressing using same PP as a hot -melt type adhesives. The pressing conditions were as follows :
Roll temperature ; 150 and 200℃, Ratio of compression in press to onginal thickness ; 10%, Feed speed ; 10cm/min(spruce) and 5cm/min(birch-spruce composite material) .
The results obtained are as follows :
1) By varying roll-pressing conditions, the specific gravity gradient developed in the thickness direction of panels, and the increase in specific gravity of the densified layers of 20% depth from each face side was 8-22% compared with that of untreated wood, The overall increase in specific gravity was only the ranges of 5.4 to 8.3%. The springwood cells around annual ring boundary were deformed, and the lumina of 2 to 3 cells from surface were filled entirely with PP during the pressing process.
2) The PP-surface-densification of spruce increased the Brinell hardness of about 30% and abrasion resistance in the ranges of 5.8 to 7.6 times, than untreated wood. The increases of the modulus of rupture and work to maximum breaking load in bending of PP-surface-densified spruce were about 21% and 80%, respectively, than untreated wood. Furthermore, the properties in impact bending were improved, though that of the wood modified chemically reduced generally with increase in the degree of treatment.
3) The PP-surface-densified wood had about 2% lower moisture content ot condition of 20℃ and 93% RH than untreated spruce. On the other hand, the swelling in thickness direction was slightly greater. The contact angle of water on PP-surface-densified wood was above 90゜.