Chains, and Why They Last

A polymer is a molecule made by repeating a small unit many thousands of times. That is nearly the whole definition, and almost every property of the resulting material comes from two things: how long the chains are, and what holds one chain to the next.

Length first, because it is the more surprising. Two materials can share an identical repeating unit and behave entirely differently. Ultra-high-molecular-weight polyethylene is used in body armour and artificial hip joints. Ordinary polyethylene at shorter chain lengths makes a carrier bag. The chemistry is the same; the chains in the first entangle with each other far more, and entanglement is what resists being pulled apart. Much of polymer engineering is the control of chain length and of the distribution of lengths in a batch.

The second question (what connects the chains) divides plastics into two families with completely different fates. In a thermoplastic the chains are separate molecules in a tangle, so heating gives them enough motion to slide past each other and flow. The material melts, can be moulded, and melts again. In a thermoset the chains have been chemically cross-linked during curing into a single enormous network. There is nothing to slide relative to anything else, so heating does not soften it; eventually it just breaks bonds and chars. Epoxy, vulcanised rubber and the resin in a circuit board are all in this family, and none of them can be recycled by melting, because melting is not something they do.

Even for the thermoplastics, recycling is a spiral rather than a loop. Heating and shearing break chains, so each pass shortens them and degrades the mechanical properties, and contamination from mixed streams compounds it. A bottle typically becomes fibre or lower-grade packaging rather than another bottle. There is a finite number of turns, which is why chemical recycling back to monomer attracts so much interest despite its energy cost.

Then the persistence, which is the property everyone knows about and few can explain. Enzymes are shaped to specific bonds in specific molecular contexts, and evolution produced them for the molecules that existed. A polyethylene chain is a long, featureless, uniform hydrocarbon backbone presenting nothing any enzyme evolved to recognise. Organisms with some capacity to degrade certain plastics have since been found, and they work slowly under conditions that do not describe a hedgerow or the open ocean.

So the objects do not degrade chemically. They break up physically, under ultraviolet light and abrasion, into progressively smaller pieces of the same indigestible material. That is the origin of most microplastics, alongside two sources that get far less attention than cosmetic beads ever did: tyres, which shed continuously as they wear, and synthetic textiles, which shed fibres in every wash.

One consumer-facing conclusion is worth stating plainly. Biodegradable plastics generally meet their certification only under industrial composting: sustained temperature, controlled moisture, a particular microbial community. In a landfill, a hedge or the sea, most behave a great deal like the material they replaced, while reliably encouraging more careless disposal.