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

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  • $\begingroup$ silicone epoxy might (guess). Could also be surface prep $\endgroup$ – Pete W Mar 23 at 1:09
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    $\begingroup$ 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) $\endgroup$ – Carl Witthoft Mar 23 at 11:45
  • $\begingroup$ Someone should un-edit the question, the edited version does not match the existing answer. Feel free to open a new question. $\endgroup$ – Jason Goemaat Mar 23 at 22:47
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    $\begingroup$ 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. $\endgroup$ – OpticalResonator Mar 24 at 9:15
  • $\begingroup$ 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. $\endgroup$ – Jim Clark Mar 25 at 14:27
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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.

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  • $\begingroup$ 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? $\endgroup$ – boatcoder Mar 23 at 16:37
  • $\begingroup$ @boatcoder - with fiberglass, sanding should fix that $\endgroup$ – Pete W Mar 23 at 16:54
  • $\begingroup$ What do you mean by "whettable"? Electrically polar? $\endgroup$ – Geremia Mar 23 at 17:04
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    $\begingroup$ 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. $\endgroup$ – niels nielsen Mar 23 at 21:08
  • $\begingroup$ This seems like a good resource on what 'wettable' means: masterbond.com/techtips/surface-wetting $\endgroup$ – Pureferret Mar 24 at 14:40
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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.

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