Water Absorption, Swell, and Buckling

WPC materials will absorb variable amounts of moisture, some more, some less. Why so, it will be discussed later. When immersed into water, they absorb moisture typically between 0.7 and 3% by weight after 24 h of the immersion. This can be compared to water absorption by wood, such as pressure-treated lumber, which absorbs about 24% water by weight after 24 h of immersion. When immersed into water for much longer time, commercial WPC materials absorb up to 20-30% of water, wood more than 100% by weight. 

Water absorption by WPC materials may lead to a number of unpleasant events. One is board distortions, swelling, and buckling (Figs. 1.30 and 1.31). Another is mold propagation. Also, saturation of WPC boards with water sometimes decreases flexural modulus of the boards, hence, results in a higher deflection under load. Besides, water absorption leads to a faster board deterioration, oxidation (water is a catalyst of plastic oxidation), and other negative consequences. 

WPC materials absorb water due to their porosity. The base plastic material of WPC, such as neat HDPE, practically does not absorb water. However, after being filled with 

Water absorption accelerates mold growth because water is a necessary component for microbial life. Typically, materials that have moisture content of 19% or lower do not support the growth of mold. This amount of moisture can be retained in the very thin upper layer of WPC profiles in humid, moist areas, with inadequate deck ventilation, for an indefinitely long time. In cases where installation 

Most composite decking manufacturers utilize high-density polyethylene, (HDPE), polypropylene, or polyvinylchloride (PVC) as polymer matrix, and wood flour or rice hulls as the principal filler for their products. Some manufacturers also add mineral fillers, such as talc. These and other changes in compositions make composite materials to vary in their appearance, shape, strength, deflection, moisture absorption, fade resistance, microbial resistance, slip resistance, flammability, and other properties, which will be discussed later in this book. 

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