Plastics are made by shaping synthetic polymers, often called synthetic resins. They fall into two groups according to how they behave when heated.
Thermoplastics, such as polyethylene, poly(vinyl chloride) and PET, consist of linear or branched chains. When they are heated, the chains slide past each other, so these plastics soften and can be moulded again and again.
Thermosetting plastics consist of chains joined by cross-links into a three-dimensional network. Once they have set, they do not soften on heating, and at high temperatures they char instead of melting, so they are used for things that must stand heat, such as pan handles and electrical fittings.
Phenolic resin, made from phenol and methanal (formaldehyde), was the first fully synthetic plastic. Leo Baekeland made it in 1907 and called it Bakelite.
Methanal first adds to the benzene rings of phenol, and the products then condense with other rings, giving off water. Repeated many times, these steps link the rings through \mathrm{{-}CH_2{-}} groups into a three-dimensional network.
Urea resins and melamine resins are made with methanal in a similar way. All of these are thermosetting plastics and are used for electrical fittings, tableware and adhesives.
Different plastics have different densities, and this difference helps to separate them for recycling.
The densities of polypropylene and polyethylene are about 0.90 to 0.96 grams per cubic centimetre, so they float on water, while polystyrene (about 1.05), poly(vinyl chloride) and PET (both about 1.4) sink.
In a concentrated salt solution, whose density is about 1.2, polystyrene floats as well, so it can be separated from poly(vinyl chloride) and PET.
Used plastics are dealt with in three main ways. In mechanical recycling, PET bottles are washed, shredded and melted, and the plastic is made into fibres, sheets or new bottles.
In chemical recycling, the polymer is broken down into its monomers or other simple compounds, which are used to make new PET or other chemicals.
Plastics that cannot be recycled are often burned, and the heat is used to generate electricity. This is called energy recovery; because the plastic is not turned back into a material, it is not counted as recycling.
Polyethylene and other polymers whose chains are made only of bonds between carbon atoms are hardly attacked by water or microorganisms, so they stay in the environment for a very long time.
Poly(lactic acid), a polyester made from 2-hydroxypropanoic acid (lactic acid), is a biodegradable plastic. Its ester bonds are hydrolysed, and the lactic acid formed is broken down by microorganisms into carbon dioxide and water.
Even poly(lactic acid) breaks down quickly only in industrial composting, where it is kept warm and moist. In soil or in the sea, it can last for years.