Effect of manufacturing parameters on the linear expansion of densified wood composite panels I. Effect of element size and press temperature

Densified wood composite panels were fabricated in the laboratory with five different element sizes ranging from fibers to strands. Linear expansion ( LE) was measured in two ways: after reaching saturation at three wetting st eps from 0% to 95% relative humidity (RH) at 20 degrees C, and during sorpt ion from 40 to 80% RH at 20 degrees C. The effects of element size on the r elationships between moisture content (mt) and LE, and between sorption rat es and LE behavior were investigated from the aspects of thickness swelling (TS), density profiles and bending properties. The results obtained for mc -LE relationships are as follows: 1) The larger the element size, the small er was the LE except for medium density fiberboard. 2) The linearity of tho se relationships disappeared at the middle and high me regions, the effect being especially pronounced for panels with larger element sizes. 3) All pa nels tested exhibited a slight hysteresis in LE, this being related to an o pposite hysteresis loop in TS. With respect to sorption rate and LE behavio r: 1) The smaller the element size, the larger the sorption rate became. 2) The rates of increase were in the order LE>Moisture gain>TS. Some panels w ere fabricated with different press temperatures from 160 to 200 degrees C. No remarkable improvement of LE and TS was observed because the changes in panel hygroscopicity were insignificant.