Thickness Swelling of Oriented Strandboard Under Long-Term Cyclic Humidity Exposure Condition

Thickness swelling (TS) measurements for oriented strandboard (OSB) were carried out under cyclic relative humidity (RH) conditions at 25°C. Measurements were made by placing test materials in a climate-controlled conditioning chamber until specimens reached their steady-state equilibrium moisture content (EMC) at each given RH.Thickness swelling hysteresis or residual TS developed in all panels as a result of cyclic humidity exposure. The largest hysteresis occurred during the first adsorption cycle. Subsequent adsorption processes led to significantly smaller increases in the hysteresis. TS rate from the first adsorption cycle increased with increase in panel MC level and density, and decreased with increase in resin content. Flake alignment level and flake weight ratio for the three-layer boards played a less significant role in controlling the total TS and the swelling rate. The mean swelling rate was the largest from the first adsorption cycle. The rate decreased significantly during subsequent adsorption cycles.A procedure was developed to predict TS and TS distribution for panels with a density gradient based on measured layer TS rate and density. The predicted total TS matched experimental data well. The predicted TS distribution across panel thickness followed the distribution of EMC change, rather than the vertical density profile. For a given RH exposure condition, TS was generally smaller in the high-density surface region compared to the low-density core because of smaller EMC changes in the face