Material · Synthetic

Polypropylene

The lightest common plastic, and the only one that survives being bent forever — the living hinge, the food tub, and half the world's rope.

Polypropylene is the second most produced plastic in the world and the one whose properties are least like the others. It is the lightest common polymer, floats in water, melts about fifty degrees higher than polyethylene, and resists fatigue in a way almost nothing else does.

That last property is why the living hinge exists. A thin section of polypropylene can be flexed hundreds of thousands of times without cracking, which is what makes the integral lid on a shampoo bottle or a toolbox possible — a hinge moulded as part of the part rather than assembled onto it.

Processing

It is made by polymerising propylene from oil or gas refining, and its properties depend on a subtlety of how the chain is assembled. Each unit has a methyl group hanging off it, and if those groups are arranged randomly the chains cannot pack and the material is a useless gum. Get them all on the same side — isotactic — and the chains crystallise, and the result is a structural plastic.

Giulio Natta and Karl Ziegler shared a Nobel Prize for the catalysts that made that control possible, and it is the clearest example in polymer chemistry of the arrangement mattering more than the composition.

Its weakness is ultraviolet light. Polypropylene degrades outdoors faster than most plastics and needs stabilising for any outdoor use, which is why cheap unstabilised rope goes brittle and powdery after a season.

Economic significance

Polypropylene took the food packaging market that polystyrene used to hold, largely on temperature: it survives a dishwasher and a microwave and polystyrene does not. It is also the fibre in most carpet, in the sacks that carry bulk goods, and in the non-woven fabric of surgical masks and wipes — a market that expanded abruptly in 2020.

It is one of the more recyclable plastics in principle and among the less recycled in practice, because it arrives in the waste stream as thousands of small, dirty, differently pigmented items.

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.

contains

Carbon — element · the backbone, and a methyl group on every second one — the arrangement of those groups is what decides whether the material is useful or a gum
Hydrogen — element · the rest of it; polypropylene is carbon and hydrogen and nothing else

is produced by

Polymerisation — process · 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

is used as

Packaging — application · food tubs and closures, which it took from polystyrene on temperature: it survives a dishwasher and a microwave
Textiles — application · carpet fibre, bulk sacks and the non-woven fabric of masks and wipes

is an alternative to

Polystyrene — material · in food containers, and it won on temperature: polystyrene deforms in a dishwasher and a microwave and polypropylene does not
Polyethylene — material · the two commodity polyolefins, chosen between on stiffness and temperature — polypropylene is harder and melts fifty degrees higher, polyethylene is tougher in the cold
Polytetrafluoroethylene — material · as a chemically resistant lining, where PTFE is the answer when polypropylene is not enough and cost stops being the deciding factor
Wool — material · in carpet, where wool holds a premium on resilience and on charring rather than melting
PLA — material · in disposables, and the substitution is not like for like: PLA softens near 60 °C, so it cannot hold a hot drink or survive a dishwasher

is sourced from

Propylene — compound · polymerised with a Ziegler-Natta or metallocene catalyst that lines the methyl groups up

is an input to

Injection moulding — process · closures, tubs and living hinges — polypropylene's fatigue resistance is why a moulded hinge can flex a million times
Mechanical recycling — process · improving, and held back by the variety of objects it appears in rather than by the polymer

belongs to the group

Plastic — material · the second, and the one that survives a dishwasher

Sources

Wikidata
Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
Material World
Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Polypropylene 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

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 the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
Polypropylene → is sourced from (polymerised with a Ziegler-Natta or metallocene catalyst that lines the methyl groups up) → Propylene → is produced by (a co-product from naphtha and barely a product at all from ethane, which is why dedicated propylene plants exist) → Steam cracking → takes as input (the European and Asian cracker feed; North American crackers mostly run on ethane from natural gas instead, and make a different product slate as a result) → Naphtha → is sourced from (the fraction boiling between about 30 and 200 °C, separated by distillation rather than made) → Crude oil
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 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
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 extracted from (by electrolysis of brine, which yields chlorine and sodium hydroxide together) → Halite
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 tetrafluoroethylene, which is where a substantial share of industrial fluorine chemistry ends up) → Fluorine → is extracted from (the only significant source; hydrofluoric acid is made from it and everything fluorinated follows) → Fluorite
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 (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation

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

Downstream — what it becomes

Polypropylene → is used as (food tubs and closures, which it took from polystyrene on temperature: it survives a dishwasher and a microwave) → Packaging
Polypropylene → is used as (carpet fibre, bulk sacks and the non-woven fabric of masks and wipes) → Textiles
Polypropylene → is an input to (closures, tubs and living hinges — polypropylene's fatigue resistance is why a moulded hinge can flex a million times) → Injection moulding
Polypropylene → is an input to (improving, and held back by the variety of objects it appears in rather than by the polymer) → Mechanical recycling