Rubber Stopper vs Elastomer: What the Terms Really Mean

A rubber stopper and an elastomer are not two competing things: a rubber stopper is an elastomeric closure, and “elastomer” is simply the material class that rubber stoppers are made from. “Elastomer” names the family of rubber-like materials; “rubber stopper” names the finished moulded part that seals a vial. So the honest answer to “rubber stopper vs elastomer” is that they describe the same component at different levels: the material (an elastomer) and the part made from it (a rubber stopper, which is an elastomeric closure). The real question buyers usually mean is which elastomer family a stopper is made from, and how the aluminium seal fits over it.

This is the terminology page. It settles what each word in a vial closure specification refers to, at which level, and why the standards use one term rather than another. It does not compare the materials against one another: the barrier, temperature and cure behaviour of butyl against bromobutyl, chlorobutyl, EPDM and silicone is the subject of vial closure rubber types, the material-by-material guide.

Key takeaways

A rubber stopper is an elastomeric closure; the two terms are not alternatives, they describe the same component.
“Elastomer” is the material class: rubber-like polymers that stretch and recover, used to mould the stopper.
“Rubber stopper” (or “elastomeric closure”) is the finished part that seats in the vial and forms the resealable barrier.
The useful comparison is between elastomer families, and that comparison is made material by material in vial closure rubber types, not here.
A stopper is qualified on its material under USP <381> and on its function under USP <382>.
The aluminium (or aluminium-plastic) seal crimps over the elastomeric stopper to hold it in place; the stopper is the elastomer, the seal is the metal cap.
Stopper, neck and seal share one nominal size: 13 mm takes 13 mm, 20 mm takes 20 mm, through 28, 32 and 34 mm.

What is the difference between a rubber stopper and an elastomer?

There is no functional difference to choose between: “elastomer” is the type of material, and a “rubber stopper” is the finished closure moulded from that material, so a rubber stopper is one kind of elastomeric closure. Calling something an elastomeric closure and calling it a rubber stopper are two ways of naming the same part.

An elastomer is a polymer with rubber-like elasticity: it stretches under load and returns to shape when released. That elastic recovery is exactly why elastomers are used for vial closures, because a stopper has to be compressed by the seal, be pierced by a needle, and recover to reseal. A rubber stopper is the moulded elastomeric part that does this in a vial. So when a specification says “elastomeric closure” and a drawing says “rubber stopper,” they are referring to the same component; “elastomeric closure” is the formal term and “rubber stopper” is the everyday one.

A rubber stopper is an elastomeric closure

In pharmaceutical packaging, “elastomeric closure” is the precise, standards-facing term for the rubber stopper that seals an injectable vial. The pharmacopoeias and ISO use the elastomer language because the requirements are written around the material class and its behaviour, not around a single trade name for rubber.

That is why the material and functional standards are named for elastomers: USP <381> covers elastomeric components in injectable packaging (the material), and USP <382> covers elastomeric component functional suitability (how the part performs, including penetrability and self-sealing). A rubber stopper is assessed under both because it is an elastomeric closure. The container closure system is completed when this elastomeric stopper is held in the vial by a crimped seal.

The closure vocabulary, term by term

Most of the confusion in this area is not about materials at all, it is about which of five words names which physical object. The table fixes each term to one object and one level.

Term	What it names	Level	Who supplies it
Elastomer	The rubber-like material class (butyl, halobutyl, EPDM, silicone)	Material	Compound supplier
Rubber stopper	The moulded closure made from an elastomer	Finished part	Stopper manufacturer
Elastomeric closure	The standards-facing name for that same closure	Finished part	Stopper manufacturer
Vial seal, or cap	The aluminium or aluminium-plastic component crimped over the stopper	Separate part	Seal manufacturer (Autofits)
Crimp	The action that rolls the seal skirt under the neck flange	Process step	The filler’s capping line
Container closure system	Vial plus stopper plus seal, assembled and qualified together	Assembly	The drug manufacturer

Two mistakes follow from getting this wrong. The first is asking a seal manufacturer for a quotation on “13 mm closures” and receiving aluminium seals when rubber stoppers were meant, or the reverse. The second is writing a specification against a polymer family, “bromobutyl”, as though that named a material: it names a base polymer, and a finished compound adds fillers, a cure system and processing aids that move hardness, penetration force and extractables. The compound reference, not the family name, is what belongs under change control.

Related words behave predictably once the table is fixed. A “flip-off cap” or “flip-off seal” is a vial seal with a plastic button that lifts, described in what is a flip-off seal. A “crimp cap” is the same object named after how it is applied. A “pilfer-proof” or ROPP cap is not in this family at all, because it threads onto a screw finish instead of crimping over a stopper. A “closure” on its own is ambiguous and worth avoiding in a purchase order.

Naming the elastomer families

The comparison that actually matters is between the elastomer families a stopper can be moulded from, and for injectable vials that is dominated by butyl and its halogenated grades. What follows is the naming, so that a specification can be read correctly. How the families behave against one another is the subject of the material-by-material guide.

Elastomer family	What the name refers to	Where it appears in a specification
Butyl rubber (IIR)	Isobutylene-isoprene copolymer	The parent polymer; still specified for stoppers
Halobutyl: bromobutyl (BIIR), chlorobutyl (CIIR)	Butyl with bromine or chlorine on the backbone	The most common stopper bases; the halogen is named, not the performance
Natural rubber (NR) and polyisoprene (IR)	Polyisoprene, from latex or synthesised	Largely confined to plungers and needle shields for parenterals
EPDM	Ethylene propylene diene monomer	Named where heat, steam or ozone resistance governs
Silicone (VMQ)	Polysiloxane, not a carbon-backbone rubber	Named for plungers, tubing and wide-temperature use

All five are elastomers, so choosing among them is choosing an elastomer family, not choosing “rubber stopper versus elastomer.” For how they actually differ on barrier, temperature range, cure chemistry, coatings and standards, read vial closure rubber types: that page is the material-by-material guide and covers each family in turn. The narrower head-to-head between the two most often confused is in butyl vs EPDM stoppers.

Where the aluminium seal fits

The elastomeric stopper and the aluminium seal are two different components that work together: the stopper is the elastomer that seals and reseals, and the seal is the metal (or metal-plus-plastic) cap crimped over it to hold it in compression. Confusing “rubber stopper” with “seal” is a common source of the terminology mix-up, so it is worth separating them.

The stopper seats in the vial mouth and forms the resealable barrier a needle passes through. The aluminium skirt of the seal is then crimped under the vial neck flange, compressing the stopper against the glass to maintain container closure integrity. On an aluminium-plastic flip-off seal, a plastic button flips up to expose the stopper while the crimp stays in place. The pairing of the elastomeric stopper with the aluminium seal is covered in detail in pairing the rubber stopper and aluminium seal. In short: the elastomer does the sealing, the aluminium seal does the holding.

Rubber stopper vs elastomer: the summary

Read the two terms as material and part, not as options to choose between; the choice a buyer actually makes is which elastomer family the stopper uses.

Term	What it refers to	Level
Elastomer	The rubber-like material class (butyl, halobutyl, EPDM, natural rubber)	Material
Rubber stopper	The moulded closure made from an elastomer	Finished part
Elastomeric closure	The standards-facing name for that same closure	Finished part
Aluminium seal	The metal cap crimped over the stopper to hold it	Separate component

So “rubber stopper vs elastomer” resolves to: the stopper is an elastomeric closure; the meaningful decision is the elastomer family and how the seal is matched to it.

Specifying the seal that goes over the stopper

Once the terms are straight, the specification is straightforward: the seal is ordered against the vial neck finish, one to one with the stopper’s nominal size, and the elastomer family does not enter into it. A 13 mm stopper on a 13 mm ISO 8362-1 neck takes a 13 mm seal, and the relationship holds through 20, 28, 32 and 34 mm.

Stopper and neck size	Inner Ø aluminium seal (D1)	Outer Ø plastic disc (D2)	Total seal height	Aluminium thickness	Bridges	Max opening force	Size page
13 mm	13.36 mm (min)	14.99 mm (max)	7.62–8.38 mm	0.16–0.20 mm	6 taper	30 N	13 mm seals
20 mm	20.22 mm (min)	23.14 mm (max)	9.02–9.91 mm	0.16–0.20 mm	6 taper	35 N	20 mm seals
28 mm	27.60–28.20 mm	30.10–31.10 mm	11.06–12.06 mm	0.17–0.23 mm	8	35 N	28 mm seals
32 mm	32.10–32.90 mm	35.50–36.50 mm	14.30–15.50 mm	0.21–0.25 mm	12 taper	65 N	32 mm seals
34 mm	33.60–34.00 mm	35.50–36.50 mm	15.00–16.00 mm	0.20–0.25 mm	8 taper	Available on request	34 mm seals

The nominal figure is a neck finish designation shared by all three parts, which is why a 20 mm seal measures 20.22 mm minimum on the inside rather than exactly 20 mm: the skirt has to clear the seated stopper flange before it is rolled in under the neck. Opening force is what the end user applies to break the bridges and lift the disc, and it rises with diameter and bridge count. Top designs across the FlipTop Optima range are Bridge, Button (intrinsic tamper evidence from a 12-stem flower under the disc) and Flower, in glossy, matte, embossed or printed finishes and a wide range of colours customised to requirement.

Forms of supply, shelf life and packing

Autofits supplies the seal in three forms. Regular is the standard non-sterile presentation. Ready-to-Use is sterilized by gamma irradiation or by ETO, washed to control particles, assembled in an ISO Class 8 cleanroom from double-lacquered aluminium and supplied double-bagged for transfer into an aseptic filling area. Ready-to-Sterilize is supplied clean and bagged for the customer’s own cycle. Tyvek bagging is available in place of the standard bag.

Shelf life is 3.5 years before capping for non-irradiated seals, plus a further 5 years after capping, and 2 years before capping for Ready-to-Use. Standard packing per box is 15,000 at 13 mm (5,000 in each of three bags), 6,000 at 20 mm (2,000 × 2), 3,000 at 28 mm (1,500 × 2) and 1,400 at 32 mm (700 × 2), in a 420 × 270 × 320 mm shipper. Packing at 34 mm is available on request.

Requests are easiest to price when the terminology is used precisely, so quote the nominal neck finish, the seal family and top design, the finish and colour, and the form of supply. Send it through the contact page, or browse the range on the products page first. Minimum order quantities and lead times are available on request. Autofits does not manufacture rubber stoppers, so name the stopper you have already qualified rather than asking for a stopper quotation.

How this works in practice at Autofits

Autofits does not manufacture the elastomeric stopper; it manufactures the aluminium and aluminium-plastic seals that crimp over whichever rubber stopper a filler selects. That means the terminology matters in practice: the stopper (the elastomeric closure) is chosen for its material and function, and the Autofits seal is matched to it and to the vial. The range includes FlipTop Optima flip-off seals in 13, 20, 28, 32 and 34 mm, FlipTop Pull Ring seals, tear-off and tear-down all-aluminium seals, and aluminium pilfer-proof (ROPP) caps. Autofits produces these seals under an ISO 15378:2017 quality system in a 75,000 sq ft Nashik facility with an ISO Class 8 cleanroom and 100% high-speed camera inspection, so the metal component that pairs with the elastomeric stopper is made under GMP-aligned controls. The aluminium is AA8011 alloy, epoxy lacquered to 21 CFR 175.300, with polypropylene discs to 21 CFR 177.1520.

Which elastomer family you end up with is decided by the fill. Biologic products push the choice toward the lowest extractables and leachables profile that will still seal, lyophilised presentations toward the best moisture barrier, and insulin and hormone vials toward reseal performance across many punctures. The study design behind the first of those is covered in extractables and leachables testing for vial closures.

Frequently asked questions

Is a rubber stopper an elastomer?

A rubber stopper is made from an elastomer, and the finished part is an elastomeric closure. “Elastomer” is the material class, the rubber-like polymers that stretch and recover; “rubber stopper” is the moulded part made from that material. So a rubber stopper is not something different from an elastomer, it is an elastomeric closure.

What is the difference between a rubber stopper and an elastomeric closure?

There is no difference in the object: “elastomeric closure” is the precise, standards-facing term used by the pharmacopoeias and ISO, and “rubber stopper” is the everyday name for the same component. Both refer to the moulded rubber part that seals a vial and is pierced by a needle. Standards such as USP <381> and USP <382> use the elastomer language because their requirements are written around the material class.

Which elastomer is used for vial stoppers?

Butyl rubber and its halogenated grades (bromobutyl and chlorobutyl) are the mainstream elastomers for injectable vial stoppers, chosen for their very low gas and moisture permeability and their self-sealing behaviour. Natural rubber has largely been superseded for parenterals, and EPDM is used more where chemical or heat resistance dominates than as a general parenteral closure.

Is the aluminium seal the same as the rubber stopper?

No. They are two separate components. The rubber stopper is the elastomeric part that seats in the vial and forms the resealable barrier. The aluminium (or aluminium-plastic) seal is the metal cap crimped over the stopper to hold it compressed against the glass and to provide tamper-evident opening. The stopper does the sealing; the seal does the holding.

Why do standards say “elastomeric” instead of “rubber”?

Standards use “elastomeric” because the requirements apply to the whole class of rubber-like materials and to the closure’s behaviour, not to a single named rubber. It is a more precise, material-neutral term that covers butyl, halobutyl, and other elastomers, and it describes the finished closure by its defining property, elasticity, rather than by a trade name.

Does the elastomer family change which aluminium seal I order?

No. The seal is ordered against the nominal neck finish of the vial, so a 20 mm stopper takes a 20 mm seal whether it is butyl, halobutyl, EPDM or silicone. What the compound can change is the seated height of the stopper and how much load the crimp leaves behind, because hardness and compression set differ between compounds. That is a reason to re-verify capping after a stopper change, not a reason to change the seal size.

Sources

USP-NF: General Chapter <381>, Elastomeric Components in Injectable Pharmaceutical Product Packaging/Delivery Systems (https://www.usp.org/)
USP-NF: General Chapter <382>, Elastomeric Component Functional Suitability in Parenteral Product Packaging/Delivery Systems (https://www.usp.org/)
ISO: ISO 8362-1, Injection containers and accessories, Part 1: Injection vials made of glass tubing (https://www.iso.org/)
ISO: ISO 8362-2, Injection containers and accessories, Part 2: Closures for injection vials (https://www.iso.org/)
Autofits FlipTop Seals dossier and engineering drawings (working/spec-sheets/)

*Last updated: 2026-08-18. This article is general technical information, not regulatory or compliance advice; confirm current standard editions and material specifications with the issuing bodies and your supplier.*