A primer on plastics

All plastics are part of a group of compounds called polymers, according to the Society of the Plastics Industry Web site, chains of large molecules attached together in a repeating pattern.

The molecules used in the makeup of polymers and the way they are connected to each other dictate what qualities the plastic will have.

Closely-related groups of plastics use the same base, called a resin.

Because different groups of plastics use different resins, it is impossible to mix two different plastics together for recycling as the end product will be contaminated and will not have the same characteristics it is supposed to have.

In order to make plastic sorting easier, the SPI Resin Identification Code was created in 1988 to help consumers and commercial recyclers sort out different types of plastic quickly.

"The SPI code was developed to meet recyclers' needs while providing manufacturers a consistent, uniform system that could apply nationwide," SPI's Web site states. "Because municipal recycling programs traditionally have targeted packaging - primarily containers - the SPI coding system offered a means of identifying the resin content of bottles and containers commonly found in the residential waste stream."

To make finding the code simpler, most manufacturers mold or imprint the codes on the bottom of containers.

• No. 1: PETE - Polyethylene Terephthalate is used in soda bottles, water bottles and plastic jars.

l No. 2: HDPE - High- Density Polyethylene is used in plastic milk jugs, other non-food bottles and plastic car fuel tanks.

• No. 3: V - Polyvinyl chloride, or PVC, is often used for plastic plumbing pipes and plastic furniture. It also is used for laundry detergent jugs, flooring and shrink-wrapping.

• No. 4: LDPE - Low- Density Polyethylene is used for plastic bags and food storage containers.

• No. 5: PP - Polypropylene is used in dairy containers, rope, plastic hinges and car bumpers.

• No. 6: PS - Polystyrene is best known in its foamed variant as Styrofoam and is also used in plastic cutlery, CD cases and take-out food containers

• No. 7: OTHER - The plastics labeled as OTHER either do not share a common resin with any of the other groups or are made up of a combination of more than one kind of plastic.