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Material · Synthetic

POM

The plastic that replaced small metal parts — stiff, slippery, dimensionally stable, and the standard material for a moulded gear.

POM is the engineering plastic for small precision parts that move. It is highly crystalline, which gives it stiffness, a low and stable coefficient of friction, excellent fatigue resistance and — unusually among plastics — very low moisture absorption, so a moulded part holds its dimensions in humid air where nylon would swell.

That combination is why it replaced machined metal in an enormous range of small mechanisms: gears, cams, bearings, clips, zip teeth, fuel-system components. It runs against itself and against steel without lubrication.

Its weaknesses are specific and sharp. It has poor resistance to acids, it is difficult to bond adhesively because nothing wets its surface well, and it burns readily. Above about 250 °C it depolymerises to formaldehyde, which makes overheating during processing a real hazard rather than merely a quality problem.

Processing

Injection moulded and extruded into rod and sheet for machining. It machines better than any other common plastic — it cuts cleanly, holds a thread and does not gum a tool — which is why prototype and low-volume mechanical parts are so often acetal.

Moulding it needs care with residence time and barrel temperature. Held too hot for too long it depolymerises, and the product is formaldehyde gas.

Uses

Gears, bearings, bushings and cams in appliances, cars and office machinery; zip teeth; fasteners and clips; the mechanism inside a seatbelt buckle; conveyor components; fuel system parts; and the interior of almost any consumer product with small moving plastic parts.

It is the default answer when a designer wants a metal part gone and the part has to keep moving.

History

Formaldehyde polymerises spontaneously — it had been observed doing so since the nineteenth century — but the polymer degraded from its chain ends and was useless. DuPont's contribution in the 1950s was end-capping: chemically blocking the chain ends so the unzipping cannot start. Delrin was commercialised in 1960.

The copolymer route, introduced by Celanese shortly after, achieves the same stability differently and tolerates hot water and alkali better, at some cost in stiffness. Both are still made and the choice between them is a real engineering decision.

Environmental impact

A thermoplastic and mechanically recyclable in principle, and almost never recycled in practice: it appears as small components inside assemblies rather than as identifiable products, so there is no stream to collect.

Its thermal decomposition product is formaldehyde, which makes uncontrolled burning of acetal waste a genuine hazard rather than a nuisance.

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 · alternating with oxygen — the backbone is nothing but carbon and oxygen, which is unusual and is where the crystallinity comes from
  • Oxygen — element · every other atom of the chain; the same bond that makes it stiff makes it unzip to formaldehyde when overheated
  • Hydrogen — element · two per carbon, and no side groups at all

is produced by

  • Polymerisation — process · formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends

is an input to

  • Injection moulding — process · small precision parts — gears, clips and cams — where dimensional stability is the requirement

is an alternative to

  • Nylon — material · the two engineering plastics for small moving parts. Acetal holds its dimensions in humid air and nylon absorbs water and swells; nylon is tougher and wears better against abrasive contamination

belongs to the group

  • Plastic — material · the precision one — gears, clips and anything small that moves

Sources

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

Questions this page answers

Where it comes from, and what it becomes

Follow POM 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

  • POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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
  • POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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
  • POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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
  • POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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
  • POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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
  • POM → is produced by (formaldehyde polymerised and then end-capped — the capping is the invention, because an uncapped chain unzips from its ends) → 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

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

Downstream — what it becomes

  • POM → is an input to (small precision parts — gears, clips and cams — where dimensional stability is the requirement) → Injection moulding