Hygroscopic thickness swelling rate of compression molded wood fiberboard and wood fiber/polymer composites

The objective of this study was to investigate the effect of ambient environment, temperature and relative humidity, on the hygroscopic thickness swelling rate of wood fiberboard and wood fiber/polymer composites. A swelling model describing the thickness swelling process of composites exposed to water vapor conditions was developed, from which the parameter, KSR, can be used to quantify the swelling rate. All the composites examined in this study were manufactured using a compression molding method. The adhesive used to make the composites was polymeric diphenylmethane diisocyanate (pMDI) resin. Polymers used in the wood fiber/polymer composites were processed from reclaimed automobile polymer mixtures, called polymer fluff. A polymer content of 30% was used to process the wood fiber/polymer composites. Four different ambient temperatures (17, 40, 60, and 80 °C) and four relative humidities (66%, 75%, 93%, and 100%) were examined in the experiment. The results of this study indicated that the swelling model developed was a good predictor of the hygroscopic swelling process of wood fiberboard and wood fiber/polymer composites. The lower the composite swelling rate, the better the swelling prediction was obtained. The standard errors of the swelling model were related to the temperature and relative humidity. The higher the temperature, or the lower the relative humidity, the greater the prediction error was obtained. Temperature had a significant effect on the hygroscopic thickness swelling rate. The swelling rate increased as the temperature increased. From the activation energy values calculated from Arrhenius plots, the temperature had less effect on the swelling rate for wood fiber/polymer composites compared to the wood fiberboard. A poor relationship was found between the relative humidity and the swelling rate.