Physical properties of mPVC over time

It is known that plastics, in particular PVC and mPVC, experience changes in properties over time. The tensile rupture strength and elastic modulus are known to decrease with time. The design stresses and deflections of PVC pipe therefore need to account for the long-term properties. Laboratory tests indicate that strength does decrease over time if a load is applied continuously. However, short-term or temporary loads are resisted by higher strengths, i.e. the short-term strength. This means that water hammer and live loads can rely on a higher strength than the long-term rupture strength normally quoted in the literature. The elastic modulus also drops over time but tests conducted indicate that for short-term loads, such as water hammer or live loads, a higher elastic modulus nearer the short-term modulus is effective.