Effect of Moisture on Physical and Creep Properties of Particleboard

Nine urea-bonded particleboards were made in the laboratory at specific gravity levels of 0.6, 0.7, and 0.8; and resin contents of 4 and 6 percent in the core and surface, respectively, 6 and 9 percent in core and surface, and 10 percent throughout. Low density, low resin, and medium density, high resin phenolic boards were also made,with and without steam post-treatment. Specimens were equilibrated to 14 humidity conditions ranging from 13 to 97 Percent relative humidity. Creep tests were made for most of the board types. Above 90 percent R.H. small changes in R.H. have a pronounced effect on particleboard—e.g., the effects of a change from 90 to 95 percent R.H. are similar, to those of a 50 to 90 percent R.H. change. When urea bonded specimens were taken from equilibrium at 30 percent R.H. to 97 percent R.H., moisture content increased 10 to 14 percent, thickness swelled 14 to 33 percent and springback ranged from 6 to 21 percent. Equilibrium moisture content varied between boards, and density was the main cause of variation below 90 percent R.H. Resin content contributed to variations above 90 percent R.H. Thickness swelling was higher for higher-density boards, while the lower EMC of high-resin high-density boards had an opposing effect. Relative creep varied somewhat between boards under steady moisture conditions, but much larger differences were observed with increasing moisture. Exposure to 97 percent R.H. after conditioning at 30 percent R.H. gave relative creep from 5 to 8 after loading for 4 weeks. With steady moisture content, creep was 1.3 to 1.4 after 1 week at 30 percent R.H., and 2.2 to 2.8 after 1 week at 97 percent R.H. Creep during adsorption related with moisture content increase and specimen thickness swelling. Phenolic-bonded particleboard had about the same EMC as urea boards, but the former swelled less at high humidity. Steam post-treatment of a low-density low-resin board reduced thickness swelling to half and springback to about one-quarter the values for untreated phenolic boards. However, the effect was less on a medium-density high-resin board; it is likely that treatment was not severe enough. Steam treatment reduced relative creep from 7 to 4.5 for the low-density board (after 2 weeks loading at 97 percent R.H. after conditioning at 30 percent R.H.). Creep of the treated and untreated medium-density board was similar.

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