Figures 5 and 6 show the effects of soaking time on the flexural modulus and the flexural strength of foamed composites with various mAC contents of 0.5, 1.0, 1.5, and 2.0 wt%.
After soaking for 90 days, both the flexural strength and flexural modulus increased only minutely due to the water uptake; the influences of water were mainly on the porous structure of the foam and the interfacial adhesion between PVC and rice hull. Maximum increments of the flexural modulus and the flexural strength around 4.6 and 5.0 % were found in all the PVC/rice hull composites foamed with 2.0 wt% mAC. As illustrated in Fig. 2, the PVC matrix was found to possess the greater cell population density and hence the maximum number of pores in the PVC/rice hull foamed with 2.0 wt% mAC; it is believed to have more of the dioctyl phthalate (DOP) leached out, consequently making the foamed composites with 2.0 wt% mAC more rigid.
In comparison, when the WPC absorbed water molecules, the wood particles was found to have swollen and warped. Both the swelling and the warpage inevitably affected the bonding between the polymer matrix and rice hull filler, thus decreasing the flexural modulus and the strength of the unfoamed composite drastically [16, 17].
On the contrary, all the foamed PVC/rice hull composites did not encounter the problem of swelling and warpage as there were numerous cell pores to accommodate the water molecules