Can All Polymers Be Recycled?

Plastics pose challenges for recyling.

Recycling polymers, or plastics, is a growing concern in Western societies. Although not all commercial plastics can be recycled, and many problems exist with plastics recycling, solutions are being developed.

Problem

Plastic consumption has grown at an enormous rate in recent decades in the "consumer" societies of Europe and the Americas. Plastics are made using a non-renewable resource, petroleum, and are usually non-biodegradable. In addition, plastic products often have a life cycle of less than one year, after which they are discarded, according to the Practical Action document "Recycling Plastics." The slow rate at which plastics degrade in landfills may be less of a concern than their additives, however, suggests Ahmad Lofti, Ph.D., in the article "Plastic Recycling" on his website, Lofti.net. Researchers do not know whether plastics' additives contribute significantly to products leached from municipal landfills, Lofti writes.

Composition

Polymers are long chains of molecules. "Polymers" and "plastics" are often used interchangeably. In fact, however, many types of molecules, both biological and inorganic, are polymeric. All plastics are polymers, but not all polymers are plastic. Polymers are usually modified or compounded with additives, such as colors, to form the useful materials called "plastics," Lofti explains.

Types

Thermoplastic polymers and thermoset polymers are two basic types of polymers. Thermoplastic polymers can be repeatedly heated and formed. The linear or slightly branched shape of their polymer molecules means that they flow under pressure when heated above their melting point. When thermoset polymers are heated, however, they undergo a chemical change that creates a 3-D network, and they cannot be re-heated and re-formed. A simpler way to put it is that the linear and branched thermplastic polymers soften when heated while the cross-linked thermoset polymers harden when heated.

Uses

Thermoplastics make up 80 percent of polymers produced, according to the Practical Action document. Thermoplastics include high-density polyethyline (HDPE), which is used for items such as piping, bottles and toys; low-density polyethylene (LDPE), used in plastic bags and cling film; polyethylene terephthalate (PET), used in bottles and battery cases; polystyrene (PS), used in dairy product containers, and cups and plates; and polyvinyl chloride (PVC), used in window frames, flooring, credit cards and medical products. Thermosets include polyurethane (PU), used in coatings, finishes, gears, cushions and mattresses; epoxy, used in sports, electrical and automotive equipment; and phenolics, used in ovens, cutlery handles and circuit boards.

Significance

Because thermoset polymers are hardened by curing and cannot be re-melted or re-molded, they are difficult to recycle. The four types of plastics commonly recycled are polyethylene, polypropylene, polystyrene and polyvinyl chloride. Recovery of plastics can be made difficult by the fact that they are often composite, with fiber of some kind added for strength, or made of more than one kind of polymer, according to Practical Action. PVC recycling is highly problematic because of its high proportion of additives and high chlorine content of raw PVC, according to Lofti.

Additional Challenges

Plastics recycling usually involves only a single re-use, Lofti writes. It is technically possible to make a 100 percent recycled bottle, but the cost of production is prohibitive and the environmental impact is high in terms of energy use and hazardous byproducts. As of 2009, only about 3 1/2 percent of plastics are recycled, which is considerably less than paper, glass or metals.

Solution

Solutions are under development, such as the production of truly biodegradable polymers. The Bristol polymer group has been developing polyhydroxyalkonates, naturally occurring polyesters produced by many bacteria. Other efforts include developing systems of automated plastics sorting, a necessary step in recycling. Sorting plastics is a major concern because one stray PVC bottle in a melt of 10, 000 PET bottles can ruin the entire batch, according to Lofti.

References

• Practical Action: Recycling Plastics
• Lofti.net: Plastic Recyling; Ahmad Lofti, Ph.D.