APR shares findings

APR shares findings on inclusion of PP in bales

By Editorial Staff, Resource Recycling

The recently concluded meeting of the Association of Postconsumer Plastic Recyclers offered an update on potential design guidelines covering how much uncompatibilized polypropylene is acceptable in HDPE bales.

Dave Cornell, the former technical director for APR, summarized what the organization has determined so far: Research has shown that some amount of PP in HDPE is actually beneficial, but high levels create problems. While PP occurs naturally in HDPE bales due to the use of PP in caps, labels and pouring spouts, PP containers are also often found in HDPE bales and these higher levels of additional PP can lower ductility, making the HDPE brittle and prone to cracking.

Several impact tests undertaken by Lyondell Basell show that new HDPE containers can incorporate about one-quarter PP resin before cracking occurs, as measured by dart and drop testing. However, environmental stress crack resistance testing showed that problems occurred at far lower levels of PP content, which may be related to the presence of grit and silicone rubber in the melt.

Taken as a whole, the research shows 20 percent inclusion of PP in copolymer HDPE bottles is acceptable, but not in long-life products, such as drainage pipe, where a 5-percent inclusion level is reasonable. In fact, some of the problems from PP may not be from the resin, but from additives, such as silicon rubber in closures.

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