Sulaiman - 18-6-2018 at 09:33
There are a lot of 'standard' red rubber stoppers available, the type that I am familiar with,
and some silicone rubber ones that I've not tried,
and this type of solid white rubber https://www.ebay.co.uk/itm/SOLID-RUBBER-STOPPER-BUNG-LABORAT...
I have one such #10 stopper on order and one #8 in use,
the one that I've used is resistant to steam,
slightly softened at the edge by hot ethanol/water vapour,
and discoloured but not severely corroded by conc. sulphuric acid.
The 'texture' is similar to the red bungs but more 'crumbly'/granular.
(I carved a hole in it with a knife to fit a male 24/29 glass joint when I was bed-ridden)
Does anyone know what rubber they are made from ?
(I suspect whatever is available from recycling, they are cheaper than the red rubber types)
and more importantly (but related)
- anyone know their chemical resistance/compatibility chart ?
So far this white rubber seems as good as the red.
(except that dirty white is uglier than dirty red 
[Edited on 18-6-2018 by Sulaiman]
Dr.Bob - 18-6-2018 at 12:26
Those are just butyl rubber, or natural rubber, likely with many additives. The red rubber is basically white rubber with red pigment in it.
Black stoppers are sometimes viton, or some chlorinated rubber, but not always.
Translucent white ones are sometimes silicone rubber, which does well with many solvents, but not so well with halogenated ones.
zed - 18-6-2018 at 18:11
Have not seen red ones around here.
Got black, which is low-Sulfur rubber ( I haven't yet looked for Viton offerings). Green, which is neoprene, without Sulfur. Silicone is also
available, and might have superior heat resistance.
I used to use the black ones for hydrogenations, without a thought, and without apparent problems. Though recently, I have seen cautions by a few
authors, suggesting the black rubber stoppers might contain enough Sulfur to poison some catalysts.
So.... The green ones might be superior for some applications.
https://www.usplastic.com/catalog/item.aspx?itemid=124340
https://www.usplastic.com/search/default.aspx?it=item&ke...
http://www.usplastic.com/search/default.aspx?keyword=silicon...
I haven't done business with these folks, but their prices look decent. Gotta buy a lot though. Priced by the pound!
You can actually make your own Silicone stoppers, if need be.
Silicone is able to stand up to low-melting metals type heat. Makes decent molds for Pewter, Zinc, Lead, Etc..
https://www.youtube.com/watch?v=dVd4ePHhhmM
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Magpie - 19-6-2018 at 07:44
I recommend checking out the Widget company. They have most kinds of rubber stoppers. You can buy them by the unit.
Leafs - 19-6-2018 at 18:54
I wrap my rubber stoppers with ptfe plumber tape when I'm worried about potential degdradation/reactivity.
weilawei - 19-6-2018 at 19:14
I have purchased from US Plastic (Tygon tubing). It was a smooth, painless, utterly uneventful transaction. Shipping was quick enough, as I recall.