Process · Changes the form, not the material

Mechanical recycling

Sort, wash, shred, melt, re-pelletise. It works, it is the only route operating at scale, and it degrades the polymer a little every time.

Waste plastic is collected, sorted by polymer, washed, shredded into flake, melted and extruded into new pellets. Nothing is chemically changed — which is the process's strength, because it needs a fraction of the energy of making the polymer new, and its limitation, because every contaminant travels with it.

The chains shorten a little each cycle, through heat and shear and any residual moisture. Recycled material therefore tends downward in demand: a bottle becomes fibre or sheet rather than another bottle, unless the input stream is clean enough to justify the extra steps that keep it bottle-grade.

Uses

It reaches thermoplastics and only thermoplastics. PET and HDPE recycle best because they are collected in reasonably pure streams from identifiable objects; polypropylene is improving; mixed and multilayer packaging largely does not, because a laminate of three polymers and a foil layer cannot be separated by any economic means.

A thermoset cannot be melted at all. Its chains are chemically crosslinked, so a tyre or a cured epoxy can be ground into filler or burned for energy, and cannot be reprocessed into what it was.

History

Industrial scrap has been reground and remelted since the beginning of the plastics industry, because clean single-polymer offcuts of known history are the ideal feedstock and throwing them away was never sensible.

Post-consumer recycling is a much later and much harder problem, dating from kerbside collection programmes in the 1980s and 1990s, and it is harder for a reason that has not changed: the material arrives mixed, dirty, and of unknown provenance.

Economic significance

It competes with virgin polymer priced off oil, which means its economics move with a commodity it does not control. Cheap oil undercuts recyclate, and recyclers fail in the years when they are most needed.

Regulation is what has changed the arithmetic — recycled-content mandates and packaging taxes create demand that price alone did not — and food-contact approval is the bottleneck that decides whether recyclate reaches the highest-value use or a lower one.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

takes as input

Polyethylene terephthalate — material · the polymer that recycles best, because it arrives in a reasonably pure stream of identifiable objects — and even so a bottle usually becomes fibre rather than a bottle
Polyethylene — material · rigid HDPE from bottles recycles well; thin film largely does not, because it is contaminated and clogs the equipment
Polypropylene — material · improving, and held back by the variety of objects it appears in rather than by the polymer
ABS — material · recovered from electronics where separation is economic, and otherwise lost inside assemblies
Plastic — material · and it reaches the thermoplastics only, because a thermoset cannot be melted

Sources

Material World
Our own writing

Where it comes from, and what it becomes

Follow Mechanical recycling back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

Mechanical recycling → takes as input (rigid HDPE from bottles recycles well; thin film largely does not, because it is contaminated and clogs the equipment) → Polyethylene → is produced by (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Mechanical recycling → takes as input (the polymer that recycles best, because it arrives in a reasonably pure stream of identifiable objects — and even so a bottle usually becomes fibre rather than a bottle) → Polyethylene terephthalate → is produced by (a condensation polymerisation rather than an addition one — the units join and release water, which is why the reaction runs in reverse when the polymer is processed wet) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Mechanical recycling → takes as input (improving, and held back by the variety of objects it appears in rather than by the polymer) → Polypropylene → is produced by (polymerised from propylene with a Ziegler-Natta catalyst, which is what controls the arrangement of the side groups and so whether the product is a structural plastic or a gum) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Mechanical recycling → takes as input (recovered from electronics where separation is economic, and otherwise lost inside assemblies) → ABS → is produced by (styrene and acrylonitrile polymerised in the presence of polybutadiene rubber, so the rubber phase is grafted in rather than blended) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Mechanical recycling → takes as input (and it reaches the thermoplastics only, because a thermoset cannot be melted) → Plastic → is produced by (the reaction the whole class is defined by — long chains built from small repeating units) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
Mechanical recycling → takes as input (rigid HDPE from bottles recycles well; thin film largely does not, because it is contaminated and clogs the equipment) → Polyethylene → is produced by (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polymerisation → takes as input (as tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up) → Fluorine → is produced by (electrolysis of potassium bifluoride, which is molten and conducts — there is no chemical oxidant strong enough to displace fluorine from a compound, so electricity is the only route and always has been) → Molten salt electrolysis → takes as input (after concentration, as one of the two routes to lithium chloride) → Lepidolite

These are the most distinct paths back. Mechanical recycling can be traced through others besides.