Butyl Rubber (IIR) vs Polyisobutylene (PIB): What Actually Differs

Polyisobutylene (PIB) and butyl rubber (IIR) are among the most frequently confused pair of materials in the sealing industry, and the confusion is understandable. They share the same backbone chemistry, the same characteristic gas-tightness, and often appear in the same product datasheets. The difference between them comes down to a single ingredient present in one and absent in the other — and that one ingredient decides whether the material can ever be turned into a cured rubber.

• Polyisobutylene (PIB) is a homopolymer. Its chain is built from isobutylene units and nothing else. Every carbon-carbon bond along the backbone is saturated; there are no residual double bonds anywhere in the repeating structure. It is produced across a very wide molecular weight range, from low-molecular-weight tacky liquids used as tackifiers and plasticisers up to high-molecular-weight rubbery solids
• Butyl rubber (IIR) is a copolymer of isobutylene with a small amount of isoprene — typically only a few mole percent. Those isoprene units introduce a sparse population of double bonds distributed along an otherwise saturated chain. Chemically, IIR is PIB with a deliberate, minimal defect built into it

That "defect" is the entire point. A double bond is a reactive site. Without reactive sites, a polymer chain has nothing to react with when you try to bond chains to one another. This gives you the fundamental split:

1. PIB cannot be vulcanised. With no unsaturation in the chain, there is no site for sulphur or resin cure systems to attack. PIB remains a thermoplastic, viscoelastic material for its entire life — it can be melted, reprocessed, and reshaped, but never permanently networked
2. IIR can be vulcanised. The isoprene units provide just enough crosslinkable sites to build a permanent three-dimensional network. Cured butyl behaves as a true elastomer: it recovers its shape after deformation and resists permanent flow
3. Both share the isobutylene gas barrier. Because the barrier property comes from the densely packed isobutylene units, not from the isoprene, PIB and IIR are both exceptionally impermeable to gas and water vapour
4. Both are ozone- and weather-stable. The isoprene content in IIR is so low that the chain is still, in practical terms, a saturated one — which is why butyl inherits nearly all of PIB's ageing resistance

So the honest one-line summary is this: PIB is butyl without the crosslinking sites. Everything that follows — mechanical behaviour, application, product form — flows from that single structural fact.

The presence or absence of a crosslinked network is not a laboratory nicety. It determines how the material behaves under sustained load, which is precisely the condition every seal lives in.

An uncrosslinked polymer under continuous stress will slowly flow. Chains slide past one another, and the deformation never fully recovers. This is cold flow (or creep), and it is the defining engineering characteristic of PIB and of non-curing butyl compounds. A crosslinked network resists this: the chains are chemically tied together, so deformation is elastic and recoverable.

Characteristic	Polyisobutylene (PIB)	Butyl Rubber (IIR)
Polymer type	Homopolymer of isobutylene	Copolymer of isobutylene + small isoprene fraction
Unsaturation in chain	None	Low — a few mole percent
Can be vulcanised / crosslinked	No	Yes
Behaviour under sustained load	Viscoelastic flow (cold flow / creep)	Elastic recovery when cured; flow when uncured
Elastic recovery / compression set	Poor — deformation is largely permanent	Good when vulcanised
Gas & vapour impermeability	Outstanding	Outstanding
Ozone & weather resistance	Excellent (fully saturated)	Excellent (near-saturated)
Reprocessable / remeltable	Yes — thermoplastic throughout	Only before cure
Tack	High, especially at lower molecular weight	High when uncured; reduced once vulcanised
Typical role	Base polymer, tackifier, plasticiser, non-curing sealant	Cured rubber goods; non-curing sealing compounds and tapes

Two nuances matter for anyone specifying a real product:

• Not all butyl products are cured. This is the point most often missed. Butyl sealing compounds and tapes — including Garmy's HY-1, HY-2, CN-1, CN-FR, SD-1, and S-3 grades — are deliberately non-curing. They are formulated to stay permanently soft and tacky so the seal can wet the substrate, self-heal after minor movement, and never harden into a brittle bead. In this sense they behave much more like PIB than like a cured rubber part
• Cold flow can be a feature, not only a defect. In a compression joint, controlled flow is what lets a butyl bead conform to surface irregularities and maintain intimate contact under clamping load for decades. The design discipline is to confine the material — in a channel, under a flange, behind a plate — so that flow works for the seal rather than squeezing it out of the joint

Garmy's compound grades are engineered around exactly this balance: enough softness and tack for permanent wetting, with published peel strengths from 36.86 N/cm (S-3) up to 81.07 N/cm (HY-1 and CN-FR), and a service window of -40°C to +120°C on the membrane grades.

If your design needs a permanently soft, non-curing barrier seal rather than a moulded rubber part, Garmy's butyl compound range is built for that duty.

Once the structural difference is clear, the division of labour in industry becomes easy to read. PIB and butyl are not competitors so much as different tools drawn from the same chemistry.

• PIB as a primary seal in insulating glass. The best-known dedicated PIB application is the primary seal in insulating glass units, where an extremely low gas transmission rate is the single governing requirement. PIB is applied as a thin, non-curing bead between the glass and the spacer, and the structural load is carried by a separate secondary seal. This is a textbook case of using a non-crosslinked polymer where barrier performance matters and load-bearing does not
• PIB as a formulation ingredient. Low-molecular-weight PIB is widely used as a tackifier and plasticiser inside other sealant systems, including butyl compounds. It contributes tack and flow without introducing unsaturation that could compromise ageing resistance
• Butyl compound as the workhorse sealing material. Where the seal must combine barrier performance with adhesion, thickness, formability, and a defined mechanical specification, a formulated butyl compound is the correct product. Garmy's compound is the raw material behind self-adhesive waterproofing membranes (HY-1, HY-2, CN-1), flame-retardant membranes (CN-FR), and butyl waterproof tapes (SD-1, S-3)
• Vulcanised butyl for moulded goods. Where genuine elastic recovery is required — tyre inner liners, moulded seals, diaphragms — the isoprene sites are used and the butyl is cured. This is the branch of the family that PIB simply cannot enter

A practical way to decide which branch your application belongs to:

1. Does the part need to spring back after deformation? If yes, you need a crosslinked elastomer — PIB and non-curing compounds are both out
2. Is the primary requirement a gas or moisture barrier in a confined joint? Then a non-curing isobutylene-based material is ideal, and the choice becomes which formulated product suits the geometry
3. Does the joint need adhesion to the substrate? A formulated butyl compound or tape gives you a specified peel strength; raw PIB does not come with that engineering package
4. Is the joint confined or open? Confine the material wherever possible so cold flow becomes conformability rather than extrusion
5. Are there fire, temperature, or documentation requirements? A formulated grade answers these — CN-FR carries a UL94 V-0 classification, all compound grades are rated -40°C to +120°C, and every lot ships with a Certificate of Analysis

The summary that serves procurement best: PIB is a polymer, butyl compound is a product. If you are buying a base polymer for your own formulation, PIB and butyl are alternatives. If you are buying a seal, what you actually need is a formulated compound with specified adhesion, temperature range, and lot traceability.

Garmy has formulated butyl compounds since 1999 and can advise which grade fits your joint geometry and barrier requirement.

Q: Is polyisobutylene the same thing as butyl rubber?

A: No, though they are close relatives. Polyisobutylene is a homopolymer made only from isobutylene. Butyl rubber is a copolymer of isobutylene with a small fraction of isoprene — typically only a few mole percent. That isoprene fraction supplies the double bonds needed for crosslinking, which PIB does not have. The simplest accurate statement is that PIB is butyl without the crosslinking sites.

Q: Why can't polyisobutylene be vulcanised?

A: Vulcanisation works by reacting at unsaturated sites — double bonds — along the polymer chain to tie chains together into a permanent network. PIB's backbone is fully saturated, so there is nothing for a sulphur or resin cure system to react with. It stays a thermoplastic, viscoelastic material throughout its life, which is why it can be remelted and reprocessed but never converted into a cured rubber.

Q: Do PIB and butyl have the same gas barrier performance?

A: Both are exceptionally impermeable, because the barrier property comes from the densely packed isobutylene units that dominate both chains rather than from the isoprene. That is why both materials appear in applications where stopping gas and water vapour is the governing requirement — PIB in insulating glass primary seals, butyl compounds in headlamp housings, electrical enclosures, and building envelope joints.

Q: What is cold flow, and is it a problem?

A: Cold flow, or creep, is the slow permanent deformation of an uncrosslinked polymer under sustained load. Both PIB and non-curing butyl compounds exhibit it. Whether it is a problem depends entirely on the joint design. In a confined compression joint it is beneficial: the material conforms to surface irregularities and maintains intimate contact. In an unconfined joint under high load it can squeeze the sealant out. The design rule is to confine the bead — in a channel, under a flange, or behind a plate.

Q: Are Garmy's butyl compounds cured or non-curing?

A: Our sealing compound and tape grades are non-curing by design. They are formulated to remain permanently soft and tacky so the seal wets the substrate, self-heals after minor joint movement, and never hardens into a brittle bead. Published peel strengths run from 36.86 N/cm on the S-3 tape grade to 81.07 N/cm on HY-1 and CN-FR, with the membrane grades rated from -40°C to +120°C.

Q: Should I buy PIB or a butyl compound for my application?

A: If you are formulating your own sealant and need a base polymer or tackifier, PIB is a raw material choice. If you need a finished sealing material with a specified peel strength, temperature range, colour, flame rating where required, and lot-level documentation, you need a formulated compound. Garmy has manufactured butyl compounds since 1999 at a 4,200 m² plant in Eumseong, Chungbuk, Korea, producing over 3,400 tonnes annually under IATF 16949, ISO 9001, and ISO 14001 certification, and exports to six countries.

Talk to a Garmy specialist about the right isobutylene-based material for your seal.