Effect of hygroscopic treatments and load applications on engineering properties of flakeboards

The study of mechanical properties of hardwood structural flakeboards (white oak, red oak, and sweetgum) as affected by hygroscopic treatments and load applications, individually or collectively, is reported. The shear moduli and moduli of elasticity determined by stress waves (Esw) are drastically reduced by cyclic conditions of 65/95/65 percent relative humidity (RH). These reductions were quite large after the first cycle. Only 51 percent and 49 percent of the original bending and internal bond (IB) properties were retained when the RH was increased from 65 to 95 percent, respectively. The engineering performance of sweetgum and red oak specimens was better than those of white oaks, and moderate improvement resulted in all three species groups when resin content was increased from 5 to 7 percent. No significant difference in bending modulus of elasticity (MOE) and modulus of rupture (MOR) was observed between the groups subjected to the long-term load applications and those not subjected to long-term loads under constant RH conditions of 65 and 95 percent. Reduction of the residual bending properties, MOR and MOE, in the specimens subjected to long-term loadings under fast cycles of 65/95/65 percent RH was the same as those loaded under constant 95 percent RH, but greater than those loaded under the identical RH cycles at a slowly changing rate. Improved engineering performance of the boards under severe environmental conditions was achieved by increasing the resin content. The residual bending properties of the boards were further reduced when the long-term loading level was increased. The hygroscopic treatments imposed a greater reduction effect on the residual mechanical properties of the boards than did the load applications. However, bending properties were further reduced when cyclic RH treatments were applied during the load application.

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