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Why doesn't epoxy stick to silicone? Is there a type of epoxy that does?

OpticalResonator
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Geremia
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  • silicone epoxy might (guess). Could also be surface prep – Pete W Mar 23 '21 at 01:09
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    How about modifying the question to "What class of materials will adhere to silicone surfaces?" , as this is an interesting question about surface energies and various classes of bonding (van der Waals, hydrogen, interstitial, etc) – Carl Witthoft Mar 23 '21 at 11:45
  • Someone should un-edit the question, the edited version does not match the existing answer. Feel free to open a new question. – Jason Goemaat Mar 23 '21 at 22:47
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    Please do not make changes to your question that invalidate existing answers and please also do not edit the answer to match your new question. You can ask a new question and refer to this one if it provides context. – OpticalResonator Mar 24 '21 at 09:15
  • Try flame treating the silicone. Coronal discharge is used on many substrates to enhance adhesion. The process alters the surface energy exposed to the adherant. Perhaps an intermediate adherant could be applied to the flamed silicone to which the epoxy adheres. – Jim Clark Mar 25 '21 at 14:27

2 Answers2

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There are two reasons why.

First, for any glue to stick to something, that something needs to be wettable by the glue. A cured silicone surface is bristling with uncrosslinked silicone units which have extremely low surface energy and hence are very difficult if not impossible to wet by glues like epoxy. This means the cured epoxy exhibits no adhesion to the silicone surface.

Second, the elasticity of silicone and cured epoxy are wildly different, which means if the joint between them is stressed, the silicone stretches and the epoxy does not- and so the silicone "unzips" from the epoxy glue line and the joint falls apart.

niels nielsen
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  • Is this also the reason that once silicone has been applied to something like fiberglass, that it is almost impossible to get anything else to adhere to that spot ever again? – boatcoder Mar 23 '21 at 16:37
  • @boatcoder - with fiberglass, sanding should fix that – Pete W Mar 23 '21 at 16:54
  • What do you mean by "whettable"? Electrically polar? – Geremia Mar 23 '21 at 17:04
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    wettability refers to the surface energies of the glue and the solid. If you put a glob of epoxy on a surface and it spreads into a pancake, that surface is *wetted* by the glue. If instead the glue pulls itself together into a spherical droplet on top of that surface, it is not wetted by the glue. – niels nielsen Mar 23 '21 at 21:08
  • This seems like a good resource on what 'wettable' means: https://www.masterbond.com/techtips/surface-wetting – AncientSwordRage Mar 24 '21 at 14:40
  • @nielsnielsen Your second point seems clear and as Geremia already asked, what does "wettable" mean here - not in common-or-garden, every-day English but here? The closest you got was mentioning surface energy and don't you think most who followed that, already knew it? – Robbie Goodwin Mar 24 '21 at 21:24
  • @robbie goodwin, I have no idea. I was a surface guy in a previous lifetime, now I am a recovering ex-engineer. the 12-step program is notoriously ineffective. – niels nielsen Mar 25 '21 at 00:54
  • @nielsnielsen If you have no idea, why did you Post that? Can you explain? Might you rather Delete? – Robbie Goodwin Mar 25 '21 at 20:54
  • @RobbieGoodwin, I posted to answer the question in the negative. I could explain but it would run to three pages. You are free to post your own answer to this question if you want. – niels nielsen Mar 25 '21 at 21:46
  • @nielsnielsen Good for you. Would you rather post an explanation simple enough for we laymen, or just Delete the Post about which you yourself state, you had no idea? If "I have no idea…" was a mistake, why not say so? – Robbie Goodwin Mar 25 '21 at 21:55
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Silicone is not a single substance, it is a very very wide range of different types of materials with widely varying properties! All materials that have several siloxane monomers in them can reasonably be called silicones. This can take almost any shape; foams, glues, coatings, rubbers, oils, powders, gels. 3D printed solids, even.

Some silicones will adhere effortlessly to epoxies. Some even have epoxides in their molecular structure. And some are made to be a copolymer, designed to be coexistant with an epoxy in the final formulation. Some are made to be cured together with epoxy. Others to be cured onto epoxy.

Other silicones will absolutely not adhere to epoxy, silicones are usually quite hydrophobic and can be substituted to be even worse.

I'd say it is more about the silicone, than the epoxy, whether they will have mutual wetting and adherence.

Stian
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