Elrik
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Conditioning hard black rubber?
Some of my older black rubber stoppers are turning rock hard on their surface. I'm not sure if its due to solvent exposure, acid exposure, or just age
and bad rubber.
Is there any trick to re-soften hard rubber to extend its useful lifespan?
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UC235
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Exposure to air crosslinks the elastic unsaturated chains in the rubber through a radical mechanism. It's not reversible.
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happyfooddance
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I have heard that methyl salicylate can improve old rubber somewhat. I had some stored in a syringe and the rubber plunger swelled up and got stuck,
so I know it does something. It is usually dissolved in isopropanol or something for the purpose of treating rubber.
[Edited on 12-5-2018 by happyfooddance]
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Abromination
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I have noticed the action of some more volatile oils on brittle rubber, although I can't remember exactly which ones. If you are relying on there
strength against more corrosive gasses like chlorine or HCl I would suggest buying some new ones.
List of materials made by ScienceMadness.org users:
https://docs.google.com/spreadsheets/d/1nmJ8uq-h4IkXPxD5svnT...
--------------------------------
Elements Collected: H, Li, B, C, N, O, Mg, Al, Si, P, S, Fe, Ni, Cu, Zn, Ag, I, Au, Pb, Bi, Am
Last Acquired: B
Next: Na
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Sulaiman
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I believe that the polymer strands cross-linking increases with time, sulphur and/or oxidants making the rubber more and more rigid as time goes bye.
Simultaneously the polymer chains are being broken by u.v. light , oxidants etc.
causing the exterior surface to perish/crumble.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Morgan
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I bought some black rubber stoppers and put the various sizes in ziplock bags and stored them in my garage. After some years they had started to exude
an oily substance and became hard. So they are as if someone had poured mineral oil all over them, they're that greasy.
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DavidJR
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Quote: Originally posted by Morgan  | | I bought some black rubber stoppers and put the various sizes in ziplock bags and stored them in my garage. After some years they had started to exude
an oily substance and became hard. So they are as if someone had poured mineral oil all over them, they're that greasy. |
I wouldn't be surprised if it was a phthalate ester of some kind.
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Morgan
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| Quote: Originally posted by Morgan | | I bought some black rubber stoppers and put the various sizes in ziplock bags and stored them in my garage. After some years they had started to exude
an oily substance and became hard. So they are as if someone had poured mineral oil all over them, they're that greasy. |
As an aside, I had two paintings hanging on the wall in the livingroom that hung there for probably a decade. They were about 33 inches in diameter
each and on the back of the frames they had 4 tiny off-white rubber tabs on the corners. One morning I woke up to find both paintings had dripped a
line of oily liquid on the wall maybe a foot and a half long, emanating from the little rubber tabs - one from each lower corner. The other tabs felt
squishy like a vitamin e gel tab but still uniform.
What struck me as odd is that the tabs decided to exude on precisely the same day, as if they were little clocks.
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Elrik
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Thanks for the tips 
The crosslinking and depolymerization route is what I had been taught in highschool, I just wasnt sure how much one could cheat against it to extend
lifespan. The propensity toward this deterioration seems to vary greatly, I have inherited rubber stoppers and a big rubber ended liebig condenser
from the second world war that are still good while stoppers I bought ten years ago are turning to rocks.
Now all my country has are the phthalate stoppers mentioned, I learned the joys of those when I turned my back on a warm flask and xylene vapors
pulled out goo that contaminated my reaction mix and produced a horrible sticky emulsion. This being why I dont just replace them all, I'd have to
import from other countries and still may get phthalates.
I'll try out the methyl salicylate. An automobile collector told me he uses brake fluid on old tires, I had assumed the polarity would mean it only
made the rubber look healthier, but if isopropanolic methyl salicylate isn't too polar perhaps isopropanolic brake fluid will be worth a try.
If I ever get truly desperate I suppose I could use bis(2-ethylhexyl) phthalate 
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Mr. Rogers
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Try rubbing them with Xylene.
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