Not all plastics are equal. For 25 years, that's been evident to New York City residents, as many puzzled over which of their plastic containers — soda bottles? yogurt tubs? — would be accepted for recycling, perhaps without completely understanding why they had to choose.
The answer is polymers. Polymers, or molecular structures, determine a plastic's properties. Some plastics will melt at just 325 degrees, while others must reach up to 720 degrees to liquefy. Some thermoset plastics, like desktop computer bodies, do not melt at all. Other plastics refuse to mix with different types of polymers when melted, limiting their application for reuse.
Since the purpose of recycling plastic is to reform the material into a new product, recycling processors must take all of these characteristics into account when deciding which plastics they will accept.
"Processors need to be able to balance their recipe and follow a formula," said Lois Levitan, program leader of the Recycling Agricultural Plastics Project at Cornell University.
Logistics also come into play. While polystyrene (commonly known as Styrofoam) can be recycled, most cities do not accept this lightweight, air-filled material because transportation costs outweigh the economic benefits of recycling it.
"Many more plastics are technically possible to recycle than makes sense to do given the value of things," Dr. Levitan said.
Polymer types are indicated with resin codes, the arrow-encircled numbers stamped on products. Nos. 1 to 6 refer to a specific polymer type, while No. 7 serves as a catchall for about 2,000 other kinds of plastics.
New York City officials announced in April that recycling had expanded to include hard plastics, an easier answer to the yogurt-vs.-soda question.
This article has been revised to reflect the following correction:
Correction: July 3, 2013
An article on Tuesday about polymers and their variable recyclability referred incorrectly to a type of polymer material that does not melt. Such materials, used in products like desktop computer bodies, are known as thermoset plastics, not "thermostat plastics."
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