Crosslinked PE can be recycled.

Crosslinked PE reclaim, whether industrial scrap or recovered post-consumer, doesn't have to end up on the scrap heap--not when a simple recycling procedure can turn it into a cost-effective filler. A paper given at the Association of Rotational Molders meeting last October in Oak Brook, Ill., detailed an inexpensive way to reuse the crosslinked material.

"Through a new process, we can now tell the world that crosslinked polyethylene can be recycled," says Elmer Good, the paper's author and president of Gyron Compounds in Mission Hills, Calif.

His recycling process simply involves pulverizing the reclaimed material to the right size and then compounding it as a filler with virgin resin. The resulting blend can then be used in rotational, injection or blow molding. Good recommends grinding or chipping to pellet size and then pulverizing to a 50-100 mesh. He says the cost should run around 15|cents~/lb.

Good has already produced a variety of rotational and injection molded parts containing 25% XLPE regrind level, including a 30-gal injection molded can. And recent tests conducted at Occidental Chemical Corp.'s lab in Houston have shown that parts containing the crosslinked filler exhibited comparable or better physical properties than virgin base resins.

REGRIND BONDS TO VIRGIN

The prevailing attitude has long been that crosslinked PE cannot be recycled. "This is all do to with the fact that crosslinked PE has a zero melt index," explains Good. But he notes that heating crosslinked PE to approximately 350 F causes the surface to become tacky, allowing it to bond to other materials. "To prove just how tacky it gets, you can spin weld a fitting onto a crosslinked PE part, and it will actually weld to the surface," he says.

To find out if bonding would occur between crosslinked PE and virgin material during molding, OxyChem made test parts with reclaim levels of 25%. Post-consumer bottles and milk jugs were the scrap source, having a TABULAR DATA OMITTED density of 0.960 and melt indexes ranging between 0.5 and 0.8. Before molding, the reclaim was extruded with a crosslinking agent, pulverized, and compounded with each of three virgin HDPE resins. (Table shows results from one virgin resin only.)

OxyChem then measured a number of physical characteristics, as shown in the table. "It was learned that not only did the bonding take place, but some of the physicals actually increased when 25% regrind was used," Good reports.

PARTICLE SIZE DOES MATTER

Similar trials with rotomolded samples additionally revealed that particle size makes a great deal of difference to the part's inside surface finish. Good found that reclaim ground to 35-mesh particles produced parts with a rough finish. Yet 100-mesh or finer particles made parts with a "much smoother" surface finish, Good says.

Other tests compared the molding behavior of the reclaim-filled resin with virgin crosslinked resins. After compounding 25% post-consumer reclaim (PCR) with virgin material, the resulting blend was crosslinked using Gyron's XL2000 process. Test parts made from the blend showed "no discernible difference over straight rotomolded crosslinked PE," Good says.

COMMINGLED PCR WORKS TOO

Another aspect of the tests showed the feasibility of putting commingled, heterogeneous PCR to work as a filler.

To each of the virgin HDPEs, Good also added commingled PE reclaim of different densities and melt indexes. He says the mix included garbage bags, butter tubs and bottles.

From these tests (also shown in the table), Good concludes that even mixed PCR can be turned into a filler for rotational, injection or blow molding. Just compound and crosslink the commingled reclaim and add it to a virgin base. (CIRCLE 1)