The least regarded consumable in a lab is a stopper. It takes no part in the reaction and never appears in a chromatogram, yet it can leave an unexplained peak in a blank and write off a whole bottle of standard.
In recent years trace analysis and repeated sterilisation have pushed labs from rubber to silicone stoppers. Silicone is not universal either: the wrong solvent swells it visibly within a week. This note covers why the switch happened, what to watch afterwards, and how to specify pierced stoppers and sterilisation.
1. Three reasons rubber left the bottle
- Curing residue: natural and SBR rubber are crosslinked with sulphur and accelerators, and residues migrate to the bottle mouth and the reagent. In trace organic analysis or cell culture the blank never comes down, and the stopper is often where it ends.
- Ageing and shedding: antioxidants and plasticisers are consumed by light, ozone and time; the surface goes tacky, sheds, sticks to the ground joint and drops particles into the bottle.
- Distortion after repeated sterilisation: after several 121 C cycles rubber stiffens, recovery drops and sealing force falls; some grades also leave a ring at the mouth that makes the next seal worse.
None of the three is fatal for short-term storage of ordinary reagents. In standards, trace analysis and cell culture, any one of them can ruin a dataset.
2. Four things silicone does well
- Inertness: platinum-cured silicone uses neither sulphur nor accelerators and contains no plasticiser, so volatiles and extractables are low and aqueous solutions, buffers and media are largely undisturbed.
- Temperature: commonly -50 to 200 C, so 121 C autoclaving can be repeated; a freezer does not stiffen it the way rubber stiffens.
- Elastic sealing: it recovers after compression and keeps sealing through repeated opening and closing - which is why it beats cork and rubber for daily use.
- Processability: it can be pierced for glass tubing, vented, made porous, flanged to stop it falling in, and made translucent or coloured to sort reagents.
3. Why a tapered stopper tightens as it goes in
A stopper seals by taper, not by tack. As it is pressed in, it is compressed radially and the rebound presses the surface against the ground joint; the further it goes, the higher the compression, friction and sealing force, so it locks itself.
Three values need control: the taper (selected to the bottle standard, commonly around 1:10), the interference between stopper and mouth, and how far the stopper enters. Too much interference and it is hard to pull and easy to tear; too little and pressure changes during autoclaving push it out. For pierced stoppers, wall thickness around the hole matters - thin walls crack at the hole first.
4. Where silicone fails: three media that swell it
Silicone dislikes non-polar solvents and does not block gases. The table below covers the common laboratory cases; check it before ordering.
| Application or medium | Silicone stopper | Safer choice |
|---|---|---|
| Aqueous solutions, buffers, media | Yes, the default | Silicone |
| Repeated 121 C autoclaving | Yes, replace on cycle count | Silicone |
| Toluene, hexane, dichloromethane and other non-polar solvents | Noticeable swelling, seal fails, reagent may be contaminated | Butyl rubber, FKM, or a PTFE-faced silicone stopper |
| Concentrated acids and alkalis, long contact | Attacked | PTFE-faced or all-PTFE stopper |
| Trace volatile organic analysis | Adsorption and permeation risk | PTFE-faced septum or a dedicated cap system |
| Dry heat above 160 C for long periods | Accelerated ageing and embrittlement | Glass stopper or metal cap |
One rule of thumb: water and heat go to silicone; non-polar solvents and trace volatiles need a different material. Swelling is not cosmetic - once the stopper grows, the sealing face distorts and autoclave pressure is more likely to push it out.
5. Other silicone in the same lab
Besides bottle stoppers: gaskets for Petri dishes and anaerobic jars, liners in centrifuge caps, peristaltic and perfusion tubing, glovebox and cleanroom seals, pipette seals and cryovial O-rings. They share the same requirements - repeated sterilisation, lasting rebound, no interference with the system - and the same selection logic.
Conclusion
An order for a stopper should state four things: the medium it contacts, the sterilisation method and cycle count, whether it must be pierced or vented, and the bottle size and taper. Write only "silicone stopper" and the supplier can only ship the most generic size - and if something goes wrong, nobody can say whose problem it was.
Related: properties library, R&D and testing for sterilisation, ageing and extraction work, and the series notes on tamping mats and quick-release plate pads.