Rubber Stoppers & Plastic Caps

A solid rubber stopper is the right choice when you simply need to seal a flask, test tube, or bottle airtight with no need to pass anything through the seal. Measure the inside diameter of the vessel neck and choose a stopper size that fits within its tapered range — a correctly sized stopper should insert with light pressure and grip firmly without forcing. Solid stoppers are reusable and resistant to most common laboratory solvents.

Use a one-hole stopper when a single glass tube, thermometer, or delivery tube needs to pass through the seal, such as in gas collection or thermometer-monitored heating setups. Use a two-hole stopper when two separate tubes need to pass through simultaneously, such as in distillation or aeration apparatus where one tube carries vapour out and another carries liquid or air in. Check that the hole diameter matches your glass tubing snugly to maintain an airtight seal around the tube itself.

Polypropylene stoppers and screw caps resist a wider range of organic solvents and concentrated acids than natural or synthetic rubber, making them the better choice for storing aggressive reagents or samples that will sit sealed for extended periods. Polypropylene does not perish, crack, or absorb odours the way rubber can over time, which makes it a good option for long-term reagent or sample storage in pharmaceutical QC and industrial laboratories.

Cork stoppers allow slow airflow through the natural pores of the material, making them suitable for fermentation experiments and other applications where a small amount of gas exchange is needed without a full open vessel. Cork is also a traditional, low-cost option for general-purpose sealing in school laboratories where airtightness is not critical. Unlike rubber, cork does not form a perfect airtight seal, so it should not be used for gas collection or pressure-sensitive setups.

Rubber stoppers are sized using a numbered system where each size covers a small range of neck diameters, since the stopper is tapered. Measure the inside diameter of the vessel neck you want to seal and check it against the top and bottom diameter range of each stopper size. A correctly sized stopper inserts with light hand pressure and grips firmly without needing to be forced more than about a third of the way into the neck.

A one-hole stopper has a single bore through its centre for one piece of glass tubing, a thermometer, or a delivery tube. A two-hole stopper has two separate bores, allowing two tubes to pass through the same seal — commonly one for inflow and one for outflow, such as in a distillation or aeration setup. Solid stoppers have no holes at all and are used purely for sealing a vessel shut.

Standard rubber stoppers resist water, dilute acids, and dilute bases well, but can degrade or swell when exposed to concentrated organic solvents, oils, or strong oxidising agents over time. For applications involving aggressive solvents, a polypropylene stopper or cap is the more chemically resistant choice. Always check the chemical compatibility of the stopper material against the specific reagent before extended exposure.

Many synthetic rubber stoppers can withstand standard autoclave temperatures of 121 degrees Celsius for short cycles, but repeated autoclaving will eventually cause rubber to harden and lose its sealing flexibility. Check the specific material grade before autoclaving, and inspect stoppers for cracking or hardening after repeated sterilisation cycles. Silicone stoppers generally tolerate repeated autoclaving better than standard rubber compounds.

A tapered stopper fits any vessel neck within its size range without needing matching threads, making it more versatile across mismatched glassware. A screw cap requires the bottle and cap to share the same thread standard but provides a more secure, tamper-evident seal once tightened, which is preferable for sample transport and storage. Choose a stopper for general laboratory glassware with varying neck sizes, and a screw cap for dedicated sample bottles that need a secure travelling seal.