The SPI resin identification coding system is a collection of symbols applied to plastic products to enable efficient separation of polymer types for the purpose of recycling. The symbols consist of a number between one and seven contained within three arrows arranged clockwise into a triangle. The numbers represent the type of polymer that composes the plastic.
The Society of the Plastics Industry developed the SPI resin identification coding system in 1988 to aid in the proper recycling of plastics. The numbers used do not represent how hard an item is to recycle, but rather they are a part of an arbitrarily agreed-upon system solely indicating the polymer used in manufacturing a product.
The number one represents polyethylene terephthalate. This polymer is common in soda bottles and water bottles, and manufacturers can recycle it into objects such as furniture and carpets. Two represents high-density polyethylene, which manufacturers commonly use in milk jugs and grocery bags. Polymer three is polyvinyl chloride, which curbside recycling programs do not commonly accept. Four corresponds to low-density polyethylene, a substance common in bread bags and plastic shopping bags. The number five designates polypropylene. Polystyrene, also known as styrofoam, is assigned the number six. Any polymers that do not fit into the previous six categories fall under the number seven.