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When rubber (I don't know the specifics, but I assume it's butyl rubber) gets old, it turns into one of two states usually:

  • It becomes sticky and gummy.
  • It becomes hard and brittle.

I'd like to know if there's a way to change hardened or sticky rubber back into its original condition. The only thing I tried so far, is heating it up gently with a hot air gun to ~60°C which makes hard rubber more malleable.

I understand there are different kinds of synthetic rubbers, made of different polymers, and henceforth, there might be a range of chemicals that might be used to treat those various kinds. I couldn't find any information on such "pairings", for instance I was searching specifically to treat butyl rubber, but I couldn't find anything.

The air ducting in cars is made of some sort of hard rubber (I'm not sure what kind of polymer), and I've heard that Ballistol can be used to soften it, but it's hearsay, essentially, I'm not sure about that.

I'm not looking for a way to completely "rejuvenate" rubber parts like that, just to have them restored a bit, so they are easier to work with.

Same thing with sticky rubber. Is there a way to make it less sticky?

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The embrittlement and goop-forming reactions are fundamentally different, and different measures would be needed to deal with each.

Rubber embrittlement occurs when the rubber oxidizes, and stops being rubber anymore because the rubber molecule chains get cut and the chemical bonds between adjacent molecules get broken. This degradation process is made worse by exposure to heat, ozone, and ultraviolet. Once it happens, you cannot repair the rubber part simply because it's no longer rubber.

The goop reaction occurs because to make rubber soft and flexible, it has mineral oil or other oils called plasticizers milled into it during its manufacture. The oil molecules form tiny globs of oil within the rubber matrix and interfere with the bonding between adjacent rubber molecules, letting them slip past one another instead of getting caught and pulling back against each other. With the passage of time, the oil sometimes tends to diffuse out of the bulk and accumulate at the surface, which renders the surface sticky. Sometimes the oil globs slowly dissolve into the bulk rubber and disconnect most of the molecules from one another, and the once-solid rubber object turns into something like tar and dribbles apart.

There is a substance called ARMOR ALL which can be used to rejuvenate decaying rubber. It contains a water emulsion of silicone oil that soaks into the rubber surface, but the improvement is just cosmetic since it can't re-connect broken chemical bonds.

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  • $\begingroup$ Great, that's a lot of very useful information to me. Just to clarify, when old rubber gets hard, that's essentially a precursor to embrittlement, correct? And there's essentially no way to soften it? $\endgroup$ – polemon Jun 19 at 1:41
  • $\begingroup$ Correct. ArmorAll helps sometimes and is worth trying if you have some on hand. $\endgroup$ – niels nielsen Jun 19 at 5:57

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