A plastic is made of long chains built from small monomers. Recycling gives those chains, or the monomers inside them, a second life.
Different polymers melt at different temperatures and behave differently, so recycling starts with sorting. The number in a small triangle on a plastic item is its resin code, and it names the polymer. The codes were set up in the United States in 1988 and are used around the world. Codes 1 to 5 name the thermoplastics you met earlier: PET, HDPE, PVC, LDPE and poly(propene). There are seven codes in all.
| Code | Polymer | Used for |
|---|---|---|
| 1 | PET, a polyester | drinks bottles, food trays |
| 2 | high-density poly(ethene), HDPE | milk bottles, shampoo bottles |
| 3 | poly(chloroethene), PVC | pipes, window frames |
| 4 | low-density poly(ethene), LDPE | carrier bags, food wrap |
| 5 | poly(propene), PP | food tubs, bottle tops |
| 6 | polystyrene, PS | yoghurt pots, foam packaging |
| 7 | everything else | polycarbonate, PLA, nylon, mixed layers |
Polystyrene, code 6, is another addition polymer. Its monomer is phenylethene, also called styrene: ethene with one hydrogen swapped for a benzene ring. Blown full of gas bubbles, it makes the light white foam used to pack fragile goods, and that foam is mostly air.
Melting and moulding again
Most recycling is mechanical. The sorted plastic is washed, shredded into flakes, melted and made into small pellets for new products. This works for thermoplastics, whose chains are separate and slide past each other when warm. Each melting breaks a few chains, so recycled plastic is often mixed with new plastic, or made into things such as fleece jackets, which are spun from fibres of recycled PET bottles.
Back to the monomers
Chemical recycling splits the polymer itself. PET is made of two monomers joined by ester links, and an ester link can be split, just as hot sodium hydroxide splits the ester links in a fat to make soap. Heat old PET with water and a base and its chains come apart into ethane-1,2-diol and benzene-1,4-dicarboxylic acid. Large plants often use methanol or ethane-1,2-diol to split the same links. The monomers are cleaned and joined into new PET as good as the first.
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Poly(ethene) and poly(propene) have only C–C bonds along their chains, with no ester link for water to split. Heated without air, they crack into a mix of smaller hydrocarbons, an oil that can be refined like crude oil.