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What’s the composition of the (slighlty sticky) substance we find on foam earplugs that allow them to be easily moldable?

This substance slows down the expansion of the foam, so it makes the foam a little bit more rigid and help the foam to stick into the ear canal. (It might also have some insulation property in combination with the foam.) This substance get out when you wash the earplugs with soap, it's probably sprayed on the foam, or impregnated on the foam.

Any idea ?

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    $\begingroup$ AFAIK they were always just made out of one material. Have you found this with a particular brand? Have you noticed it with more than one brand? $\endgroup$ – CL22 May 12 '16 at 13:17
  • $\begingroup$ I always thought they were just some formulation of polyurethane closed-cell foam with a compacted surface layer. $\endgroup$ – wwarriner May 13 '16 at 0:54
  • $\begingroup$ I have seen it on 3 brands after trying to wash them. Try to use one, then wash and dry it and put it back again you might find a difference (if you are use to it). The PU is foam it quite "dry" by itself and would not be so easely moldable. $\endgroup$ – Lisa May 13 '16 at 3:34
  • $\begingroup$ I've used plugs and other objects which don't have a smooth surface, and their expansion remains slow. I strongly suspect it's a function of the relaxation time of the base material, even if a "restraining" skin slows the process a bit more. $\endgroup$ – Carl Witthoft May 13 '16 at 11:26
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That sticky oily substance found in the ear plug foam is the plasticizer leaching out. The plasticizer is unlikely to be added on after the foam is made, I think it would be mixed in with the source polymer before it gets foamed.

Usually the more plasticizer there is, the softer the foam gets, and the slower the foam recovers from being compressed.

What is likely happening when you wash them, is that the soap is removing the oil based plasticizer from the foam. A short wash should only remove plasticizer from the surface of the material, but since it is a foam, internally it has a high surface area, making more plasticizer available to be washed away.

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  • $\begingroup$ So this plasticizer would just be added to slow down the foam expansion when compressed? I can imagine the “mechanical property” of an oily “wet effect” to slow it down, but I don’t understand the “plasticizer” thing. Since it doesn’t polymerized with the foam (because it’s washable) so is it a kind of lubricant, just a liquid that would change a bit the foam structure, or what else? $\endgroup$ – Lisa May 16 '16 at 18:26
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    $\begingroup$ The plasticizer is used to make the foam polymer soft and compressible, not just to slow its recovery rate. If you wash out the plasticizer it wont just make the foam recover slower, it will actually make the foam harder. Plasticizers behave somewhat like a solvent for polymers, the more you add, the softer it gets. Add enough plasticizer to something like hard PVC and you can turn it into a sticky gel. Plasticizers also tend to migrate to the surface too, for example stuff like artificial rubber getting a sticky coating after a while. $\endgroup$ – Netduke May 16 '16 at 20:23
  • $\begingroup$ Thanks for your answer, that "solvent effect" explains a lot of things! Ok but why washing (several time) the earplugs (with soap) doesn't make them harder? $\endgroup$ – Lisa May 17 '16 at 8:37
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Mouldable earplugs are often made from PU foam which can be self skinning ie the foam structure is created in a mould (as opposed to being cut from bulk material). I n this case the smooth skin is just a change in the density of the material, created as it is forced against the surface of the mould rather than a separate coating. again in this case the 'slow recovery' is a material property of the foam rather than any additional coating.

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  • $\begingroup$ That's interesting, but why washing it changes does change its property? (Its not sticky anymore, and the foam expansion is faster) $\endgroup$ – Lisa May 16 '16 at 18:27
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It might not be an answer, but it's an interesting explanation

Washing of foam earplugs does not have a major effect on strength or attenuation, but... foam earplugs do retain moisture. You can never "squeeze" it all out. And that has two negative after-effects:

First, the moisture causes the foam to swell in size a bit (affecting fit and expansion time).

A way to test that hypothesis would be to wash a new earplug in water, without soap to see if it affects its structure. If it does that would confirm the moisture hypothesis which could increase the size of bubbles inside which could affects its compression and soundproofing properties.

So the following question would be: how can we efficiently dry the foam?

Any idea?

source: http://www.hearforever.org/blogs/01/24/2011/qa-washing-foam-earplugs-good-or-bad-idea

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