Sciencemadness Discussion Board

Silicone Sealant Chemical Resistance

Petit Homme - 1-9-2025 at 17:17



As I look for the Chemical Resistance of silicone it rates good versus glacial acetic acid. However, the Silicone Sealant I have seems to be silicone dissolved in... glacial acetic acid. How come exposure to glacial acetic acid wouldn't dissolve it?

Radiums Lab - 1-9-2025 at 19:03

Good question, the monomer is soluable in AA, but the polymer is not soluable in AA.

j_sum1 - 1-9-2025 at 20:21

I think this is quite a common phenomenon.
Consider your standard acrylic house paint. It is fully miscible with water when wet. But when it is fully dry, it has polymerised and is water resistant.
I recently asked a similar question about a tarry compound used for sealing concrete. Same deal.

Actually, I suspect there are a huge number of hardware store products that are designed with exactly this in mind -- some with water as the solvent and some with other compounds.

Petit Homme - 2-9-2025 at 00:36

Quote: Originally posted by Radiums Lab  
Good question, the monomer is soluable in AA, but the polymer is not soluable in AA.


Nice!

I now wonder "what makes in polymerize outside of solution and not in solution?" One guess is it must polymerize in the presence of (most likely) oxygen and that acetic acid is a bad solvent for O2?

Petit Homme - 2-9-2025 at 00:39

Quote: Originally posted by j_sum1  
I think this is quite a common phenomenon.
Consider your standard acrylic house paint. It is fully miscible with water when wet. But when it is fully dry, it has polymerised and is water resistant.
I recently asked a similar question about a tarry compound used for sealing concrete. Same deal.

Actually, I suspect there are a huge number of hardware store products that are designed with exactly this in mind -- some with water as the solvent and some with other compounds.


I never thought about that but you're right!

MrDoctor - 2-9-2025 at 03:30

silicone cures through a hydrolysis reaction with humidity in the air, now i dont know though if its the sort where each hydrolysis also liberates an additional water molecule causing a chain reaction, i assume it must otherwise it doesnt make sense that opened tubes would cure so far in if the water gets consumed upon reaction. methanol, ammonia and/or acetic acid are released though, either just squeezed out no longer solvated, or literally forming due to the hydrolysis, i can imagine they at least improve the mobility of water, but i dont think any of them actively pull moisture in.

bnull - 2-9-2025 at 03:57

Ah, @MrDoctor, there are so many types of silicone sealant and so many mechanisms of polymerization... It is amusing, even.

Tin salts are used as catalysts. Some sealants release acetic acid, some release water, some others release alcohols. Air humidity still helps to start polymerization.

F. de Buyl, Silicone sealants and structural adhesives (https://doi.org/10.1016/S0143-7496(01)00018-5).

Chemgineer - 2-9-2025 at 04:47

I used silicone sealant to attach a still head to a steel can when I was producing benzene from NaOH and sodium benzoate. It handled the 200 deg C and the benzene produced.

BromicAcid - 2-9-2025 at 05:19

I suppose my pet peeve is that if you start trying to search for things related to silicone you inevitably get people using the term silicon interchangeably. :(

I didn't think any of the silicones out there were solvated in acetic acid, it was my understanding that the polymerization of them on curing liberated acetic acid (i.e., that was the leaving group). There are different silicones out there using different mechanisms but that was the old stand-by.

Petit Homme - 2-9-2025 at 06:59

Quote: Originally posted by BromicAcid  


I didn't think any of the silicones out there were solvated in acetic acid, it was my understanding that the polymerization of them on curing liberated acetic acid (i.e., that was the leaving group). There are different silicones out there using different mechanisms but that was the old stand-by.



You might be right. I assumed it was the solvent because of the smell, but it may be the case that it is simply/only liberated in the polymerization process.

One could tell apart one case from the other by leaving a big amount in a vacuum chamber and immediately smelling upon re-pressurization to see if acetic acid has filled the chamber in high concentrations (in which case it would be present beforehand and be a solvent, in addition to perhaps also being released in the polymerization process, or if not and it only strongly smells when exposed to air, that only the later is the case as you write and may very much be correct).

Thanks for the contributions!

angrygiraffe - 8-9-2025 at 17:57

Fresh sealant isn’t the same as cured silicone. The acetic acid is just part of the curing process — that’s why it smells like vinegar. Once it’s fully cured, you’ve got cross-linked silicone rubber, and that stuff doesn’t dissolve in acetic acid anymore.

Lionel Spanner - 9-9-2025 at 13:18

Quote: Originally posted by Petit Homme  
You might be right. I assumed it was the solvent because of the smell, but it may be the case that it is simply/only liberated in the polymerization process.

That is indeed the case. Acetoxy silicones release some acetic acid as the cross-linking reaction occurs in air.
Quote: Originally posted by j_sum1  
I think this is quite a common phenomenon.
Consider your standard acrylic house paint. It is fully miscible with water when wet. But when it is fully dry, it has polymerised and is water resistant.
I recently asked a similar question about a tarry compound used for sealing concrete. Same deal.

In both cases, the product contains a resin which is either a water-based emulsion or a solution in a suitable solvent, which coagulates into a film as the solvent evaporates - the process is purely a physical one, with no chemical reaction taking place.

MrDoctor - 9-9-2025 at 15:19

what is the mechanism that allows silicone, regardless of what specific kind, to cure all the way through just because the tube was opened? ignoring descriptions of the process ive read, i still have always assumed a chain reaction must occur that is activated, but not neccesarily sustained, by air and/or moisture.
I have some uses for silicone as a single use gasket of sort, that would work really great if i could take a specific fancy roofing silicone sealant and split it into several small syringes. same with just being able to keep automotive gasket maker handy at all times, because if nothing else, no single reaction will ever really degrade that stuff so hard it disintegrates. maybe a few cycles of heat, oxidation, solvent stuff like pyrolysis, etc, but if its a non pressurized reaction and its just plugging a hole or sealing two pieces of connected glass, its the best thing ever then