It's all too easy to leave the plastic cap on the plastic bottle before tossing the container into a curbside recycling bin. But most people do not realize that move can affect the value of the plastic when it is being recycled. That is why researchers from RecycleWorlds Consulting Corp. and the University of Wisconsin-Madison are working to develop a cap that would be compatible in the recycling process.
Most plastic containers are made of PET or high density polyethylene. But most caps are made of polypropylene, which can decrease the quality of the recycled material when processed.
The difference can be 10-15 cents per pound for pelletized plastic, and up to 20 cents per pound for flake, said Peter Anderson, president of Madison-based RecycleWorlds.
``If you can enhance the value by that much, it can be an enormous payoff,'' he said.
Removing caps by hand can be labor-intensive and costly. Because PET is denser than water, reasonably priced water separation machines, averaging around $10,000, are used to separate PET from PP. But the mechanical separation equipment used for HDPE can cost $200,000. In that case, it makes sense to address the contamination issue on the front end, Anderson said.
The main question behind the study is whether the PP qualities adversely effect the performance of the plastic when it is remolded into a bottle, or if the problem is the plastics' viscosity.
``If it's viscosity, then it can be done without serious cost or loss of performance,'' Anderson said. ``If it's the polypropylene, then it can be a little more tricky.''
Researchers are blending various resins at contamination levels and conducting strength tests.
``We're clearly not going to argue performance,'' Anderson said. ``The idea is not to be pushy and say performance should be compromised. The idea is, with the technology available, to come up with solutions.''
The study is funded through a $6,000 grant from the Wisconsin Recycling Market Development Board.
``We're hearing from our waste collectors that they have a lot of caps in their bales,'' said Gail Miller Wray, the market development board's executive director.
``When the prices are good, it doesn't seem to bother the purchasers. But when the prices are bad, it knocks down the prices of the bales, so that's a concern we have in terms of the economics of recycling,'' she said.
Plastic caps also represent a lot of wasted material that is recovered but not recycled, she said.