Who is Pete and why is his name on the bottom of a plastic bottle? You've probably seen the little triangles and numbers on the bottoms of plastic bottles. But what do all these little numbers and triangles mean?

The numbers are all about recycling. Because many plastics can be made cheaply, they are often used to make disposable items. This kind of use creates lots of waste. Recycling plastic is one way to reduce the amount of waste.

Before plastics can be recycled, they must be sorted. Many different kinds of materials are used as plastics, and most cannot be mixed, in the same way that oil and water can't be mixed. To help separate one kind of plastic from another, many items have a triangle printed on them, with a number inside. The numbers on your bottles tell you from what kind of plastic the bottle is made.

Many kinds of polymers are used, because for different uses, plastics with different properties are needed. Sometimes we need stiff plastics, and sometimes we need flexible ones. Sometimes we need transparent plastics, and sometimes we need plastics that aren't transparent.

Each number on a plastic object corresponds to a different kind of polymer from which the object is made. Each polymer has different properties because it has a different molecular structure. Below you can see a diagram showing which number corresponds to which polymer, and the chemical structure of that polymer.

	poly(ethylene terephthalate) (PET) Invented by J.R. Whinfield and J.T. Dickson, 1940. Uses: clothing, plastic films, plastic bottles
	high-density polyethylene (HDPE) Invented by Robert L. Banks and J. Paul Hogan, 1951. Uses: plastics of all kinds, high-strength fibers
	poly(vinyl chloride) Invented by Waldo Semon, 1926. Uses: water pipes, LP records, vinyl car tops
	low-density polyethylene (LDPE) Invented by Eric Fawcett and Reginald Gibson, 1935. Uses: plastic films, bags
	polypropylene (PP) Invented by Robert L. Banks and J. Paul Hogan, 1951. Uses: fibers for rope, indoor-outdoor carpeting, plastics
	polystyrene (PS) Invented by Eduard Simon, 1839. Uses: rigid plastics of all kinds, polystyrene foams
	anything else, including items made from more than one kind of polymer

Just a note: some of you may have noticed that for two of our polymers, the abbreviation on the recycling code is different from the abbreviation underneath the structure. Scientists call poly(ethylene terephthalate) PET, but the recycling code is PETE. This is because "PET" is a trademark, a brand of milk to be exact. Also, poly(vinyl chloride) is often called "PVC", is also commonly known as "vinyl." So the recycling code just uses the letter "V."

Next: Polymers and Supercritical CO₂