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Material · Synthetic

Butyl rubber

The rubber that gas will not pass through — the inner tube, the tyre liner, and the seal on a pharmaceutical vial.

Butyl rubber's defining property is gas impermeability, and it is not a marginal advantage: it is roughly ten times less permeable to air than natural rubber. The reason is the densely packed methyl groups along an otherwise saturated backbone, which leave very little free volume for a gas molecule to move through.

It also damps vibration unusually well — it is deliberately lossy, converting mechanical energy to heat rather than returning it — and resists weathering, ozone, acids and steam.

What it does not have is resilience. A butyl ball barely bounces, which is the same property that makes it a good vibration mount and a poor choice anywhere elastic energy return is wanted.

Processing

Polymerised at around −95 °C in methyl chloride with a Lewis acid catalyst — one of the very few industrial polymerisations run at deep cryogenic temperature, which is a substantial part of its cost.

Only about two per cent isoprene is included, and it is there solely to provide the double bonds vulcanisation needs. Halogenated grades — bromobutyl and chlorobutyl — cure faster and, importantly, bond to other rubbers, which is what makes a tyre's butyl inner liner possible.

Uses

The inner liner of every tubeless tyre, which is where most of it goes: a thin butyl layer bonded inside the carcass is why a modern tyre holds pressure for months. Inner tubes for bicycles and older vehicles.

Pharmaceutical vial stoppers and syringe plungers, where a drug must not lose sterility or gain oxygen through the closure. Vibration and engine mounts. Roofing and pond-liner membranes. Chemical protective clothing, including chemical-warfare protective suits, where its impermeability is the requirement. And chewing gum base, which is a genuine and slightly startling use of a synthetic elastomer.

History

Invented at Standard Oil in 1937 and, like SBR, scaled urgently during the Second World War when natural rubber supply was cut. Its wartime importance was specific: no other available synthetic could make an inner tube that held air.

The tubeless tyre, which followed in the 1950s, depended on it entirely and made the inner tube itself largely obsolete — the material's most important product replaced its own first product.

Environmental impact

Vulcanised and not remeltable; it shares the end-of-life route of other tyre rubbers. Its contribution is on the use side and is substantial: a tyre that holds its pressure is a tyre with lower rolling resistance, and under-inflation is a measurable and widespread cause of excess fuel consumption.

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.

is produced by

  • Polymerisation — process · cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold

is used as

  • Sealing and gasketing — application · pharmaceutical vial stoppers, where nothing may pass in either direction through the closure

is an alternative to

  • Natural rubber — material · for anything that must hold gas: butyl is roughly ten times less permeable, which is why the tubeless tyre has a butyl liner and not a natural rubber one

is associated with

belongs to the group

  • Rubber — material · the one gas will not pass through

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 Butyl rubber 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

  • Butyl rubber → is produced by (cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold) → 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
  • Butyl rubber → is produced by (cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold) → 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
  • Butyl rubber → is produced by (cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold) → 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
  • Butyl rubber → is produced by (cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold) → 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
  • Butyl rubber → is produced by (cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold) → 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
  • Butyl rubber → is produced by (cationic polymerisation at around −95 °C, one of very few industrial polymerisations run that cold) → 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. Butyl rubber can be traced through others besides.

Downstream — what it becomes