Mechanical properties of blends containing HDPE and PP

The mechanical properties of binary blends of high‐density polyethylene and polypropylene are quite good compared to those for blends of some other immiscible pairs. The property relationships observed depend strongly on the process used to fabricate the blends as shown by comparisons of specimens made by injection and compression molding with widely varied cooling rates in the latter. Strength and modulus may show additive behavior or have positive or negative deviations, depending on the process conditions; however, measures of ductility like impact strength or elongation at break always show negative departures from additivity and exhibit minima in some cases. Addition of an appropriate ethylene–propylene elastomer greatly improves the ductility of these blends but with a corresponding decrease in strength and modulus. The presence of weld lines has a serious detrimental effect on mechanical properties of these blends.