Why some plastics last

A fallen leaf rots away in a year or two. A plastic bag in the soil can last for decades. The difference is in the bonds along their chains.

Bacteria and fungi break down dead plants and animals with enzymes. Many of these enzymes cut one kind of link using a water molecule, the way amylase cuts the links in a starch chain and lipase splits the ester links in a fat. The polymers of living things are full of links like these.

A poly(ethene) chain has only C–⁠C and C–⁠H bonds. Carbon and hydrogen share their electrons almost evenly, so the chain carries no charged or partly charged spots for an enzyme to grip. Like oil, the chains keep water out. Microbes' enzymes have nothing easy to break, so poly(ethene) lasts a very long time.

A plastic made to break down

PLA, poly(lactic acid), is a polyester made from plants. Bacteria ferment sugar from maize starch or sugar cane into lactic acid, the same acid that makes yoghurt tangy. Each lactic acid molecule carries both an -⁠OH group and a -⁠COOH group, so lactic acid molecules can join to each other into a polyester. PLA makes cold drinks cups, food packaging and the filament for many 3D printers.

PLA is sold as compostable, and the word means something exact. In an industrial composting plant, heaps of food and garden waste stay hot, around 60 °C, and damp. There, water splits PLA's ester links into short pieces, and microbes digest the pieces to carbon dioxide and water. Items that meet the European standard for compostable packaging, EN 13432, turn at least 90% of their carbon into carbon dioxide within six months in those conditions. In a cool garden compost heap, in soil or in the sea, PLA breaks down very slowly, over years.

Microplastics

Sunlight and waves break plastic left outdoors into smaller and smaller pieces. Pieces smaller than 5 mm across are called microplastics. They also come from fibres shed by clothes made of polyester and nylon in the wash, and from dust worn off car tyres. Scientists have found microplastics in the deep sea, in Arctic snow, in drinking water and in the human body, and they are studying what the pieces do there.