Plastics are often blended or reinforced with other materials to obtain desired physical or mechanical properties. Generally speaking, the addition of rubber degrades the mechanical properties of recycled plastic. However, if rubber is used as an impact modifier, the properties of the product may be improved. The relative density of commercially available plastic lumber products typically varies from about .7 to .96, making them 1.5 to 2 times heavier than wood. Susceptibility to moisture for 100 percent recycled plastics and plastic-rubber blends tends to be slight.
Blends containing wood and paper products tend to have higher rates of water absorption.
Studies have shown that the strength of wood and paper products decreases with increasing moisture content. In addition, since wood fibers expand with moisture content, the entire product will tend to expand. Various additives can be used to reduce moisture uptake. Because the coefficient of thermal expansion of plastic products can be up to 12 times greater than wood or steel, they will expand and contract to a much greater extent for a given change in temperature. This fact should be considered in the design of connections and construction details.