Bromobutyl And Chlorobutyl Stoppers

Butyl rubber is roughly 98% isobutylene and 2% isoprene by mole. The isobutylene backbone is saturated and densely packed, which is the origin of the low permeability that makes the material useful as a closure. The small isoprene fraction supplies the double bonds needed for crosslinking, and halogenation converts those sites into allylic halides that are far more reactive.

Chlorobutyl carries roughly 1.1 to 1.3% chlorine by weight, and bromobutyl roughly 1.8 to 2.1% bromine by weight, as determined by elemental analysis or X-ray fluorescence on the raw polymer. The allylic bromide is the more reactive of the two, and that single fact drives most of the practical differences.

Neither name describes a finished stopper. A stopper is a compound: polymer plus filler, plus a crosslinking system, plus pigment and processing aids. Two stoppers of the same base polymer from different formulations can behave quite differently, which is why component specifications are written against a formulation code rather than a polymer family.

How the two compare

The zinc line is the one that most often decides a selection. Where a formulation must keep extractable metals low, a bromobutyl compound crosslinked without zinc oxide is the straightforward route, because the bromide is reactive enough to crosslink through alternative chemistry at practical process times. A chlorobutyl compound generally needs the zinc oxide system to reach the same crosslink density.

What the pharmacopoeial tests actually check

European Pharmacopoeia general chapter 3.2.9 sets the framework for rubber closures used with aqueous preparations, powders and freeze-dried powders. It divides into physical tests on the closure's mechanical behaviour and chemical tests on an aqueous extract of the rubber.

• Penetrability: the force required to pierce the closure, measured on a defined apparatus, with a limit set so that the closure is neither excessively hard nor unresisting.
• Fragmentation: closures are pierced under specified conditions and shed rubber particles are counted and filtered, with a limit of not more than 5 fragments across 100 closures pierced for aqueous preparations.
• Self-sealing: repeated piercing of one closure followed by a check for leakage, applicable to multiple-piercing designs.
• Ultraviolet absorbance of the aqueous extract, read across roughly 220 to 360 nm against a limit set in the chapter, which catches organic extractables broadly rather than identifying them.
• Acidity or alkalinity, reducing substances, residue on evaporation and extractable zinc: a set of bulk chemical checks on the same extract.

Fragmentation is the test most closely connected to what happens when a closure is pierced in ordinary use, and it is worth reading alongside the separate question of coring.

The failure mode: water carried in by the stopper itself

Halobutyl rubber absorbs water during steam sterilisation and holds a fraction of it, commonly in the range of 0.2 to 0.5% by weight, measured as gravimetric loss on drying. A stopper seated on a freeze-dried cake while still carrying that water becomes a slow internal moisture source. Over weeks to months the water redistributes from the elastomer into the headspace and then into the solid, and the residual moisture of the cake rises with no external ingress at all.

It shows up on stability as a moisture figure climbing across timepoints while container closure integrity results stay clean, which is the pattern that distinguishes it from a leaking seal. Where the solid has a low glass transition temperature, the same rising moisture shows up visually first, as a cake that has slumped or lost its original shape.

The established mitigation is a post-sterilisation drying step, typically several hours in a vacuum or forced-air chamber at around 100 to 105 °C, with the stoppers held dry until seating. It is a process control on the component, not something inspectable in the finished vial.

Where the published record stops

Elastomer formulation, extractables and leachables profiles, and container closure integrity results are not among the specifications Aurum publishes. What is published rests on two records: material is independently assayed for purity by reverse-phase HPLC, and identity is assayed by mass spectrometry. Neither of those speaks to the closure at all.

Sterility, endotoxin and pyrogen testing are not performed and are not claimed. That statement is worth repeating in a discussion of closures specifically, because closure integrity and sterility are frequently read as the same subject and they are not. A closure can hold pressure perfectly and carry no sterility claim whatsoever.

Common questions

Can the polymer be identified from the finished stopper?

Not by appearance. Colour and shape are formulation choices. Identification is by infrared spectroscopy or by halogen determination on the compound, and in practice by the component specification and lot documentation from the closure manufacturer.

Is bromobutyl the better material?

It is the more flexible one to formulate, particularly where low extractable metals are required. Chlorobutyl remains widely used and well characterised. The comparison is between finished compounds, not between the two base polymers in the abstract.

Why is butyl rubber used rather than natural rubber?

Permeability and oxidative stability. The saturated isobutylene backbone gives roughly an order of magnitude lower gas permeability than natural rubber and far fewer sites available for oxidation, and it carries none of natural rubber's protein content.

Does the crimp seal or the stopper make the seal?

The stopper makes the seal, by compression against the glass sealing surface. The aluminium crimp holds the compression constant for the life of the container.

Does the halogen leach out?

The halogen is bonded into the polymer and consumed in crosslinking rather than present as free halide. What extracts from a closure is dominated by the non-polymer parts of the compound, which is why the pharmacopoeial tests target zinc, reducing substances and total organic extractables rather than halogen.