Material · Synthetic

Polyethylene

The most-produced plastic in the world — a chain of carbon and hydrogen, whose properties depend almost entirely on how the chains are branched.

Insulator

Polyethylene is chemically the simplest useful polymer: a long chain of carbon atoms with hydrogens attached, and nothing else. Its enormous range of behaviour comes not from chemistry but from architecture — how long the chains are and how much they branch.

Heavily branched chains cannot pack closely, giving a low-density material that is soft and flexible: carrier bags and squeeze bottles. Unbranched chains pack into crystalline regions, giving a high-density material that is stiff and tough: pipes, fuel tanks, chopping boards. Same formula, entirely different materials.

Processing

Ethylene, obtained by cracking petroleum fractions or natural gas liquids, is polymerised under conditions chosen to control branching. High-pressure free-radical processes give the branched low-density grades; catalytic processes at lower pressure give the linear high-density ones, and the development of those catalysts is what made the distinction controllable.

The polymer is then melted and shaped — extruded into film, pipe and sheet, or moulded — which it tolerates repeatedly, since it is a thermoplastic that softens on heating rather than degrading.

Uses

Packaging dominates: film, bags, bottles and containers. Beyond that, water and gas distribution pipe, where polyethylene's resistance to corrosion and its ability to be joined by fusion welding have displaced metal in much of the network.

It also insulates cable, lines landfills and ponds as geomembrane, and, as ultra-high-molecular-weight polyethylene, serves as the bearing surface in artificial hip and knee joints and as the fibre in cut-resistant gloves and body armour.

Environmental impact

Polyethylene's chemical inertness is exactly what makes it durable in use and a problem afterwards. Nothing in the environment metabolises it readily, so discarded material fragments mechanically and persists, and it is a major component of marine plastic debris.

It is straightforward to recycle in principle — it remelts cleanly — and the obstacles are practical: mixed and contaminated waste streams, and the low cost of new material. Its carbon footprint per unit is modest compared with alternatives such as glass or metal, which complicates substitution arguments that treat plastic as uniformly worse.

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

is used as

Electrical conduction — application · as insulation around the conductor, not as the conductor
Packaging — application · film, bags and the majority of packaging by tonnage — the cheapest barrier there is against water

is an alternative to

Polyvinyl chloride — material · in pipe and packaging: polyethylene is tougher and easier to weld and contains no chlorine, where PVC is stiffer and cheaper per unit of pressure rating
Polypropylene — 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
Paper — material · in packaging, where paper is winning back ground on recyclability and losing it on barrier — a paper wrapper that keeps water out generally has a polymer film on the inside, which is what makes it hard to recycle

is produced by

Polymerisation — process · from ethylene; chain length and branching decide whether it is a milk bottle or a fibre

is sourced from

Ethylene — compound · the monomer the polymer is named for, and essentially the only input

is an input to

Extrusion — process · blown film for bags and sheet, and pipe — the extruded forms are the majority of polyethylene's use
Mechanical recycling — process · rigid HDPE from bottles recycles well; thin film largely does not, because it is contaminated and clogs the equipment

is used in

Joint replacement — object · the cup, as cross-linked UHMWPE — the deliberately sacrificial surface, and the source of the debris that ends the device
Solar panel — object · as the ethylene-vinyl-acetate encapsulant sealing the cells, which is also why a panel cannot be taken apart at end of life

is associated with

The wartime materials programmes — event · its first significant use was radar cable insulation, and its existence was classified

belongs to the group

Plastic — material · the highest-tonnage plastic there is, and the cheapest barrier against water

Sources

Material World
Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Polyethylene 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

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 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
Polyethylene → is sourced from (the monomer the polymer is named for, and essentially the only input) → Ethylene → is produced by (the principal product, and the largest-tonnage organic chemical made anywhere) → 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
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
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 extracted from (by electrolysis of brine, which yields chlorine and sodium hydroxide together) → Halite
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 extracted from (the only significant source; hydrofluoric acid is made from it and everything fluorinated follows) → Fluorite
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 (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. Polyethylene can be traced through others besides.

Downstream — what it becomes

Polyethylene → is associated with (its first significant use was radar cable insulation, and its existence was classified) → The wartime materials programmes complete chain
Polyethylene → is used in (the cup, as cross-linked UHMWPE — the deliberately sacrificial surface, and the source of the debris that ends the device) → Joint replacement → is used in (around two million hip and knee replacements a year) → Medical devices
Polyethylene → is used in (as the ethylene-vinyl-acetate encapsulant sealing the cells, which is also why a panel cannot be taken apart at end of life) → Solar panel → is used as (and the price fell roughly ninety-nine per cent in thirty years on thinner wafers, finer saws and rising cell efficiency) → Photovoltaics
Polyethylene → is used as (as insulation around the conductor, not as the conductor) → Electrical conduction
Polyethylene → is used as (film, bags and the majority of packaging by tonnage — the cheapest barrier there is against water) → Packaging
Polyethylene → is an input to (blown film for bags and sheet, and pipe — the extruded forms are the majority of polyethylene's use) → Extrusion

These are the most distinct paths onward. Polyethylene ends up in others besides.