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I've been reading a lot of conflicting articles/threads pertaining to which lubricant is compatible with rubber/plastics and I'm stumped.

From what I've come across, the consensus is that silicone is safe to use on rubber and that petroleum based eats at it over time. But then I heard the opposite.

What I'm wondering is, if rubber is composed of some petroleum then why would a petroleum lubricant or grease have a negative effect on it? Also what about plastics and vinyl, do they get damaged as well?

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    $\begingroup$ You need to know what sort of rubber. "Natural" is made from plant extracts not petroleum and is severely degraded by many petroleum-based solvents. But there are artificial "rubbers" of many varieties that are totally different and react very differently with solvents. And there are a lot of different types of solvents. So: no easy way to know the general answer. Bottom line research the material you are working with specifically or risk problems. $\endgroup$
    – matt_black
    Apr 12, 2016 at 15:47

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The simple version:

Petroleum-based rubbers can be dissolved by petroleum jelly eating away at the mechanical integrity.

The slightly longer version:

Petroleum jelly incorporates its way into the rubber, changing the plasticity (ie how rubbery or stiff) the rubber is. Changes in plasticity change the mechanical strength and toughness such that the rubber is then outside the material property specifications of the product it is with/on/in, and hence fails.

Even longer version:

Most rubbers have small molecules inside them that help the molecular chain is the rubber move. These molecules are called plasticizers (and occasionally tackifiers). They are small molecules usually 200-500 MW which is less than the several hundred thousand of the rubber itself. Petrleum jelly is basically hydrocarbon of roughly the size of the tackifiers. The petroleum jelly molecules can 'wiggle' their way into the rubber, moving and adjusting how the existing plasticizers interact with the rubber. In some cases the plasticizers may be extracted out, also changing the interaction, fundamentally changing the energies involves in intermolecular motion which is then related to toughness and strength (many steps in that derivation omitted). In extreme cases in poor quality rubbers, some of the rubber molecules themselves can be dissolved by the petroleum jelly. Changes in toughness, plasticity and mechanical integrity are common when petroleum jelly interacts with rubbers, and hence it should be avoided as a lubricant.

As you have pointed out silicone oil is a costly replacement, but not unbearable more expensive for most jobs.

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  • $\begingroup$ I see. I figured if a rubber had a petroleum base then greases with petroleum would be safe but in fact it causes the composition to change since it's made of the same base which allows it to change the characteristics? $\endgroup$
    – ohmmy
    Apr 12, 2016 at 4:01
  • $\begingroup$ The same 'base' allows it to solveate/dissolve, a chemical phenomena. The change in characteristics noticed is normally a physical phenomona, based on breakdown in mechanical qualities (see my more extended answer posted after your comment) $\endgroup$
    – Lighthart
    Apr 12, 2016 at 4:05
  • $\begingroup$ Thanks for that. So now I'm curious about the rubber boots used in some automotive parts that hold in petroleum based greases like chassis grease. Are these rubbers made without a petroleum base? $\endgroup$
    – ohmmy
    Apr 12, 2016 at 4:13
  • $\begingroup$ Probably. But I can't know for sure without a pair in my hands. $\endgroup$
    – Lighthart
    Apr 12, 2016 at 4:26
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    $\begingroup$ I am sorry I do not feel comfortable validating every product to you, particularly when I cannot speak to their contents A wikipedia search for Armor All indicates its likely composition. .You'll have to study some basic organic chemistry or hire a consultant. $\endgroup$
    – Lighthart
    Apr 12, 2016 at 4:33

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