Vial Closure Rubber Types: Butyl, Halobutyl, EPDM, Silicone, and How to Choose
Vial closures are moulded from a small set of elastomer families, and for injectables the field narrows quickly. Bromobutyl and chlorobutyl, the two halobutyls, account for most parenteral stoppers; plain butyl is their parent polymer and still used; EPDM appears where heat and steam resistance matter more than barrier; silicone is chosen for extreme temperature range or surface inertness despite its very high gas permeability; and natural rubber and polyisoprene are largely confined to specialist components. The choice turns on barrier, reseal, temperature range, and formulation compatibility.
Definitions for the individual terms are on elastomeric closure and butyl rubber stopper, and the two labels are separated in rubber stopper versus elastomer. This page is the comparative survey: what each family is, how it behaves, how the standards classify and test them, and how to match one to a product.
Key takeaways
- Halobutyl (bromobutyl and chlorobutyl) is the default for injectable vial stoppers: very low gas and moisture permeability plus a versatile cure chemistry.
- EPDM resists heat, steam, and ozone well but is a weaker barrier than butyl, so it is more usual in device and infusion components than in vial stoppers.
- Silicone has the widest usable temperature range and one of the highest gas permeabilities of any elastomer, making it a specialist rather than default choice.
- Natural rubber latex is not permitted under Ph. Eur. 3.2.9, although dry natural rubber may be used.
- The compound matters more than the family: two bromobutyl stoppers with different cure systems and fillers can behave and extract very differently.
- The revised USP <382> became official on 1 February 2026; fragmentation testing was removed from it and remains in USP <381>.
- The ISO 8871 series is most often overlooked: five parts covering extractables, identification, particles, biological requirements, and function.
The rubber families used for vial closures
Six families cover essentially all pharmaceutical closure applications, and they separate on two properties: permeability and temperature range.
| Family | Abbreviation | Gas and moisture barrier | Temperature behaviour | Usual parenteral role |
|---|---|---|---|---|
| Butyl | IIR | Very low | Elastic well below freezing | Parent polymer; still used for stoppers |
| Bromobutyl | BIIR | Very low | As butyl | The most common stopper base |
| Chlorobutyl | CIIR | Very low | As butyl | Widely used stopper base |
| EPDM | EPDM | Moderate; higher than butyl | Excellent heat, steam, ozone resistance | Device and infusion closures |
| Silicone | VMQ | High | Widest range; flexible when very cold | Specialist closures, plungers, tubing |
| Polyisoprene / natural rubber | IR / NR | High | Elastic, but ages less well | Plungers, needle shields |
Butyl rubber
Butyl is a copolymer of isobutylene with a small proportion of isoprene. The isobutylene backbone is densely packed with very little free volume, which gives butyl its defining property: gas and water-vapour molecules cannot diffuse through it easily. That is why butyl displaced natural rubber for injectable closures, and why a butyl-based stopper is the sensible starting point for an oxygen- or moisture-sensitive product. Its limitation is chemical: few unsaturated sites means a slow, inflexible cure and poor co-vulcanisation.
Bromobutyl and chlorobutyl
Halogenating the butyl backbone with bromine or chlorine solves that. The halogen introduces reactive sites, so the compound cures faster and with a wider choice of systems, co-vulcanises with other elastomers, and bonds better to laminate films. Barrier performance stays at butyl levels.
The two differ in processing. Bromobutyl offers the more flexible cure chemistry and better adhesion, which matters for laminated constructions; chlorobutyl is regarded as more scorch-resistant in the mould. Both are mainstream, and the choice usually follows the cure system, the drug’s compatibility profile, and existing qualification history rather than a decisive technical gap.
EPDM
EPDM is a terpolymer of ethylene, propylene, and a diene monomer. Its strengths are resistance to heat, steam, ozone, and polar fluids, and it holds up to repeated autoclaving. Its weakness here is barrier: EPDM is appreciably more permeable to oxygen and water vapour than butyl, so it protects a sensitive drug less well. It is therefore common in device seals and infusion sets and uncommon as a vial stopper. The head-to-head is in butyl vs EPDM stopper.
Silicone
Silicone rubber is a polysiloxane rather than a carbon-backbone elastomer, and that changes everything about it. It stays flexible across an exceptionally wide temperature range, well below where hydrocarbon elastomers stiffen and well above where they degrade, and it is very inert, with clean grades producing low levels of organic extractables.
Against that, silicone is one of the most gas-permeable elastomers in common use, orders of magnitude more permeable than butyl, with lower tear strength and higher cost. That puts it in niches rather than general vial stoppering: very low-temperature storage, plungers and tubing, and cases where inertness outweighs barrier.
Natural rubber and polyisoprene
Natural rubber was the original closure material and is now largely displaced: more permeable than butyl, less stable on ageing, and carrying the protein allergens associated with latex. Ph. Eur. 3.2.9 does not permit natural rubber latex in closures for aqueous parenteral preparations, although dry natural rubber may be used because the allergens are removed in processing.
Synthetic polyisoprene has the same base chemistry without the natural proteins and gives excellent elasticity and puncture recovery, which is why it appears in plungers and needle shields, but its permeability keeps it out of most vial-stopper work. Thermoplastic elastomers, moulded without vulcanisation, sit in the same category.
Why the compound matters more than the family
“Bromobutyl” names a base polymer, not a material. A finished stopper compound is the polymer plus a filler system (typically mineral, and the reason most stoppers are grey), a cure system, activators, and processing aids. Two stoppers from the same base polymer can differ substantially in hardness, compression set, penetration force, particle shedding, and extractables. So a specification written to a polymer family is not a specification: the compound reference is what has to be fixed and held under change control, and a change of compound is a change to the closure even though the family name has not moved.
Coatings and laminates
The drug-contact face of a stopper is frequently not bare rubber. Two surface treatments are widespread, and both are part of the closure specification and notifiable as changes.
Fluoropolymer films are laminated onto the drug-contact surface, and sometimes the full stopper, forming a barrier between the elastomer and the product. The film substantially cuts extractables reaching the drug and gives a harder, lower-friction surface that runs better through washing and filling equipment: a common choice for biologics.
Silicone treatment reduces friction so stoppers feed and insert reliably at line speed. Silicone is itself a particulate and interaction source, so some products specify silicone-free stoppers and rely on a laminate instead.
Matching the rubber to the product
The deciding property changes with the product, and the common error is optimising for barrier when reseal or temperature range is the constraint.
| If the product is | Deciding property | Usual direction |
|---|---|---|
| An oxygen- or moisture-sensitive small molecule | Permeability | Halobutyl, uncoated or laminated |
| A lyophilised product | Moisture vapour transmission, plus slotted geometry | Halobutyl slotted closure to ISO 8362-5 |
| A biologic or protein formulation | Extractables and particulate burden | Laminated halobutyl, often ready-to-use |
| A multiple-dose vial | Self-sealing after repeated penetration | Compound qualified against USP <382> |
| A deep-frozen product | Elasticity at the storage temperature | Cold-chain halobutyl, or silicone at the extreme |
| A terminally steam-sterilised product | Behaviour at 121 °C without added extractables | Halobutyl or EPDM, cure system chosen for it |
| A preservative-containing multi-dose product | Sorption of the preservative into the elastomer | Compound screened for preservative uptake |
Preservative sorption deserves emphasis as an often late-discovered failure mode: a preservative that partitions into the rubber loses its antimicrobial function while the assay for the active looks normal. It bites hardest on preserved multi-dose presentations such as insulin and hormone vials.
How the standards classify and test vial closure rubbers
Closure rubbers are qualified on four fronts: material chemistry, biological safety, function in use, and dimensions. No single document covers them all.
| Aspect | Reference |
|---|---|
| Physicochemical material requirements | USP <381> |
| Functional suitability | USP <382>, with guidance in USP <1382> |
| European requirements, Type I / Type II classification | Ph. Eur. 3.2.9 |
| Dimensions for injection vial closures | ISO 8362-2:2024 |
| Freeze-drying closures | ISO 8362-5:2016 |
| Extractables in aqueous autoclavates | ISO 8871-1:2003 |
| Identification and characterization | ISO 8871-2:2020 |
| Released-particle count | ISO 8871-3:2003 |
| Biological requirements and test methods | ISO 8871-4:2006 |
| Functional requirements and testing | ISO 8871-5:2025 |
Three points are worth drawing out.
The revised USP <382> became official on 1 February 2026. It covers needle and spike access functional suitability: penetration force, self-sealing capacity, spike retention, and sealability. Fragmentation testing was removed from the revised chapter after concerns about the proposed particle size limits, and remains in USP <381>. The self-sealing requirement was aligned with ISO practice at 1.0 times the maximum number of scheduled penetrations. What changed against the previous chapter is summarised in the revised USP <382> testing requirements.
Ph. Eur. 3.2.9 classifies closures as Type I or Type II. Type I meets the stricter requirement set; Type II has mechanical properties suited to special uses, may not meet every Type I requirement, and so has to be justified. The chapter was last revised through Supplement 11.1, applicable from 1 April 2023, and is carried in the 12th Edition. It is mandatory for aqueous parenterals because binding general monographs reference it.
The ISO 8871 series is the one most often missed. Its five parts sit alongside the pharmacopoeial chapters and are frequently cited in supplier specifications.
What the rubber choice means for the aluminium seal
Changing the stopper compound can change the crimp, even when the stopper’s nominal dimensions do not move. Hardness and compression set determine how far the stopper deforms under the capping head and how much force it keeps holding, so the same settings applied to a softer or harder compound give a different residual seal force. A stopper change is therefore a reason to re-verify the capping setup and, where significant, to re-confirm container closure integrity. The interaction is covered in pairing the rubber stopper and aluminium seal.
How this works in practice at Autofits
Autofits manufactures the aluminium and aluminium-plastic seals that crimp over the stopper, not the stopper itself, so the compound is a customer decision Autofits designs around. The relevance is dimensional: the seal’s skirt length and diameter have to suit the seated height of whichever stopper the customer has qualified, and that height changes with compound hardness. The range covers FlipTop aluminium-plastic seals, tear-off and tear-down aluminium seals, and aluminium pilfer-proof (ROPP) caps in 13, 20, and 32 mm sizes, on the products page. Production runs under an ISO 15378:2017 quality system, with ISO 9001:2015 and ISO 14001:2015 certification and a Drug Master File, in a 75,000 sq ft Nashik facility with an ISO Class 8 cleanroom.
Frequently asked questions
What rubber types are used for vial closures?
The main families are butyl, bromobutyl and chlorobutyl (the halobutyls), EPDM, silicone, and polyisoprene or dry natural rubber. Halobutyl accounts for most injectable vial stoppers because it combines very low gas and moisture permeability with a versatile cure chemistry. EPDM suits cases where steam and ozone resistance matter more than barrier, and silicone where a very wide temperature range or surface inertness is needed.
What is the difference between bromobutyl and chlorobutyl stoppers?
Both are butyl rubber with a halogen added to the backbone, and both retain butyl’s very low permeability. Bromobutyl generally offers the more flexible cure chemistry and better adhesion, which suits laminated constructions; chlorobutyl is regarded as more scorch-resistant during processing. Neither has a decisive barrier advantage, so the choice follows the cure system, the drug’s compatibility data, and existing qualification history.
Can EPDM be used for injectable vial stoppers?
EPDM can be used, but it is uncommon for injectable vials because it is a weaker moisture and gas barrier than butyl, which matters over a two-year shelf life. Its strengths, resistance to heat, steam, ozone, and polar fluids, suit device seals and infusion components. Where the drug is sensitive to oxygen or moisture, halobutyl is the standard choice.
Why is silicone rubber not the default for vial stoppers?
Because silicone is one of the most gas-permeable elastomers in common use, orders of magnitude more permeable than butyl, so it protects oxygen- and moisture-sensitive drugs poorly. It also has lower tear strength and costs more. Its advantages, a wide temperature range and high inertness, suit specific applications such as very low-temperature storage, plungers, and tubing.
Is natural rubber allowed in vial closures?
Ph. Eur. 3.2.9 does not permit natural rubber latex in closures for containers for aqueous parenteral preparations, because of the protein allergens it carries. Dry natural rubber may be used, since those allergens are removed during processing. In practice butyl and halobutyl have replaced it, with synthetic polyisoprene where the elasticity is wanted without the latex proteins.
Which standards cover the rubber used in vial closures?
Material requirements are in USP <381> and, in Europe, Ph. Eur. 3.2.9, which also classifies closures as Type I or Type II. Functional suitability is in USP <382>, whose revised chapter became official on 1 February 2026, with guidance in USP <1382>. Dimensions are in ISO 8362-2:2024 and ISO 8362-5:2016. The five-part ISO 8871 series covers extractables, identification, released particles, biological requirements, and function.
Related reading
- Butyl rubber stopper: definition and properties
- Elastomeric closure: definition and role
- Butyl vs EPDM stopper
- Pairing the rubber stopper and aluminium seal
Sources
- USP-NF: General Chapters <381> Elastomeric Components in Injectable Pharmaceutical Product Packaging/Delivery Systems and <382> Elastomeric Component Functional Suitability in Parenteral Product Packaging/Delivery Systems, revised chapter official 1 February 2026 (https://www.usp.org/)
- USP-NF: notice on General Chapters <381> and <382>, including removal of the fragmentation section from <382> (https://www.uspnf.com/notices/381-382-nitr-20250211)
- EDQM: European Pharmacopoeia general chapter 3.2.9, Rubber closures for containers for aqueous parenteral preparations, for powders and for freeze-dried powders (https://www.edqm.eu/)
- ISO: ISO 8362-2:2024, Closures for injection vials (https://www.iso.org/standard/87251.html)
- ISO: ISO 8362-5:2016, Freeze drying closures for injection vials (https://www.iso.org/standard/65448.html)
- ISO: ISO 8871-2:2020, Elastomeric parts for parenterals, Part 2: Identification and characterization (https://www.iso.org/standard/76110.html)
- ISO: ISO 8871-5:2025, Elastomeric parts for parenterals, Part 5: Functional requirements and testing (https://www.iso.org/standard/85863.html)
*Last updated: 2026-07-31. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and requirements with the issuing bodies.*
