Effect of polyethylene and polypropylene cross-contamination on slow crack growth resistance

The alteration in lifetime relevant material characteristics, such as slow crack growth (SCG) resistance, of a virgin PE-HD pipe grade (PE80) with successively increasing amounts of a virgin PP block copolymer (PP-B) was studied in frame of this research. For this purpose, Cyclic Crack Round Bar (CRB) tests were performed, demonstrating the high negative impact of an already small amount of polymeric impurity, which leads to a significant reduction of the SCG resistance. Already 5% mixing content evokes a dramatic decrease of SCG resistances of up to 40 and 70 % for PE-HD and PP-B, respectively. As SCG is generally accepted to be a critical failure mechanism for plastics used in long-term applications, such as pipes, this effect must be carefully considered in all discussions about the use of nonvirgin polymers.