How does polystyrene recycling work?

Polystyrene is easily recycled, but does the cost outweigh the benefit? Check out these green science pictures!

Polystyrene is one of the most common forms of plastic. You see it in take-out coffee cups and egg cartons; it's the packing material used to cushion goods for shipping. Many call it Styrofoam, though that term is actually the brand name of a rigid blue insulation made by Dow Chemical Company. Polystyrene is a very versatile material, but recycling it isn't always easy.

The term polystyrene means that the plastic is derived from styrene, a liquid hydrocarbon. When heated, monomer styrene molecules link together into long chains, creating a polymer material that's solid when it cools to room temperature. That clear, hard, brittle plastic was developed on a commercial basis in Germany in the 1930s. The material is used today for CD and DVD jewel cases and plastic forks. In 1941, Dow scientist Ray McIntyre invented extruded polystyrene foam (Styrofoam), a light, waterproof material that was first used for making life rafts. Expanded polystyrene (EPS) is another, similar foam material that has found even more uses.

Because it's about 95 percent air, EPS is an excellent insulator. That's why it's used in beer coolers and home insulation, and why the hot coffee in a polystyrene cup doesn't burn your fingers. Because it's light, EPS is ideal for creating buoyancy in life vests and rafts. Its lightness and malleability make it a good packing material, adding cushioning but little weight. Also, EPS doesn't react with other materials and is resistant to heat, so it has found wide use in the food industry in things like meat and poultry trays and the boxes that fast-food hamburgers are served in.

However, some of the same qualities that make polystyrene useful can also work against it when it comes to recycling. Its lightness means that it's hard to collect from curbside containers -- it often blows away, becoming litter. Because it's bulky, it's difficult and expensive to transport. Many municipal recycling programs do not accept it (a few, like Los Angeles and Toronto, do).

One of the problems of all plastics recycling in general is that you have to gather the same types of materials together and sort them by their material container code -- a number usually found on the bottom of the container that makes it easy to identify the type of plastic in the object. Some other plastics -- like 1 (used for soda and water bottles), 2 (laundry detergent and other containers), and 4 (plastic bags) -- are easier to isolate. Polystyrene, which is number 6, presents more problems. While water and soda bottles are relatively clean when discarded, polystyrene used for food is often mixed with paper, food scraps and other types of plastic, like the straw that's thrown away with an EPS cup.

Polystyrene usually can't be recycled locally but has to be transported to a centralized plant, increasing costs to the recycler and reducing the incentive to recycle. Also, recycled polystyrene cannot in most cases be used for products that contact food because of health concerns, even though the material is usually sterilized by the recycling process. Recycled EPS might be used instead to create packaging or other materials, but new EPS is always needed for coffee cups and plates.

So what's the best way to recycle polystyrene and why might bacteria someday play a role in the process? Read on to find out.