Rubber Stoppers

Stoppers are the thing that closes a vial. Often made of butyl rubber, you push a needle through the stopper to draw out your medicine before injecting it. When making vials, a cap is crimped on top of a stopper.

Two important points

1. Silicone stoppers are NOT compatible with benzyl alcohol (BA), an essential component of your homebrew. Benzyl alcohol is studied to be able to pass through, and evaporate out of, silicone. Benzyl alcohol is present in the solution to prevent the proliferation of bacteria, it is not optional. Taking risks surrounding benzyl alcohol is not worth it.

2. Butyl rubber stoppers are NOT compatible with oven sterilization. You cannot sterilize them in the oven at all, either before you assemble the vials or as part of a complete, sealed vial. Butyl rubber will begin to off-gas and therefore leech harmful chemicals into your medicine. Butyl rubber must be sterilized in the autoclave.

Off Gassing

In my own experiments, I have discovered that at any valid sterilization temperature, the butyl rubber stoppers begin to breakdown in the heat. This one time it released a horrific off-gassing into my space, where everything smelled of burning rubber. This was at the lowest, temperature, 140°C/284°F. Now, imagine that off-gassing is happening, but the rubber stoppers are fully capped and enclosed. That gas is leaving the stopper and coming into contact with the HRT. It’s impossible to know if leeching takes place. I’m certainly not going to inject something that underwent that process. I also suspect the butyl rubber that’s been overheated cores more easily, but I have no data to prove this.

Butyl rubber cannot be replaced with silicone due to the documented ability for benzyl alcohol to pass through and evaporate out of silicone tubing.¹

Notes and Sources

• BB + Silicone are a bad match 
• BA evaporates through silicone barriers 
• Silicone absorbs MCT oil  (no sources cited)