[LlyrLumen]

Hei community! Im new here and I've cross posted this from the chemistry Reddit to get more eyes on this, to see if any if you here can help. Thanks in advance for reading this.

Due to a heavy interest in/use of fabrication materials for the last 9 years (5 of which were spent working as a costume/prop maker and SFX artist in the film industry) and a sudden overwhelming interest in material science/engineering for polymers, I am currently studying chemistry from the ground up. Due to not having the time or being able to afford formal studies I am stuck with self study using books. In light of that information I would like to ask you guys/gals a question about this please because I am not at this level of understanding yet:

I received a call yesterday from a representative of a Silicone manufacturer to discuss an email I had sent regarding medical grade Silicone gels.

She warned me about an issue that various scientists/researchers (and the company she worked for I guess) had been struggling with and that the majority of Silicone manufacturers/engineers do not mention (she couldn't say whether this issue was solved for obvious reasons but just that it took several years)

The conversation was very technical so I might miss something out (we were on the phone for an hour and five minutes!)

Essentially she said to me that due to the layering system I am intending to use with different shore <0 hardness, RTV2 addition cure, medical grade Silicone gels that I would run into the same problem these people had using the same method (layering and curing different gels on top of each other inside a mold).

She mentioned the different molecular weights between the gels caused each layer to swell or shrink. So the top layer would shrink/wrinkle out of it's moulded shape because the layer below it had swelled, also out of it's shape.

She never stated how much swelling and shrinkage occurred but she sounded like it was a very serious issue to overcome. I remember mentioning that this sounded similar to osmosis but in a different system with no membrane, she wouldn't confirm or deny that.

From my limited knowledge of chemistry I've acquired so far, what this lady is describing sounds like osmosis but not. It sounds like, over time (days, weeks, months?) post cure the different Mwt* of the silicone gel layers in the casting are trying to achieve equilibrium between them, with the higher Mwt gel losing atoms to the lower Mwt gel causing the shape of the silicone cast to warp/deform. From what I can gather on Google, this happens when silicone is submerged in solvents. Apparently due to the two systems trying to equalise chemical, the solvent ions can transfer inbetween the cross-links of the silicone because there's so much space in-between the chains, causing the silicone to swell. But there's no mention of this happening between two silicone gels bonded/casted to each other. And at a cost of £12 /kg with an MOQ of 50-200 units...I'd like to find out why before I buy anything...Which I guess she was also trying to warn me about subtlety!

Does this sound close at all? Would anyone like to share an opinion on this and how to stop it happening? My first initial thought to ask her was, "WHY not just paint a thin barrier of something in-between the layers to stop this transfer happening?" It can't be that simple though? Plus there's the added problem of finding a similar material to the gels that will bond with them, is flexible/stretchy/soft but that isn't silicone itself.

Thank you for reading! Any help on this would be greatly appreciated as it's for a good cause! (I am trying to develop a more realistic mammary prosthesis prototype to help mastectomy survivors and the transgendered.)

*If Mwt=molecular weight=molar mass...Aren't weight and mass mutually exclusive?

[wildfyr]

Oof. A lot to sort out here. My first thought is about how two identical polymers of different molecular weights can be immiscible and phase separate.

The bit about the two layers shrinking and swelling, is this due to solvents or water penetrating them? If you have two polymers of different degrees of crosslinking, they will swell to different degrees, which will mean there is a mismatch at the interface and deformation will occur.

This bit about atoms being traded between layers makes very little sense to me. Do you mean residual solvent or monomers are flowing from one to the other? Where is the solvent coming from? In a problem this complicated correct use of jargon is very critical.

 I'm a polymer chemist by trade and I'm happy to help you, but some strange or incorrect use of jargon is making it very hard for me to understand the issue. Could you perhaps get this woman (a chemist? ) to write up what the concern is?

[LlyrLumen]

Hi, thanks for taking the time to reply. I really appreciate it. My apologies for the confusions, I'm still learning chemistry so I am making a lot of mistakes with terminology and mechanical processes. Which is why I thought to ask, so I can hopefully solve this and at the same time really learn about polymer science!

With regards to your questions, there is no solvent or water or anything in the system. Nothing. It's a straight up silicone gel to silicone gel bond.

The thing I mentioned about molecular transfer is better explained by a reply on my Reddit post which I think is somewhat correct:

With respect to swelling, it sounds like a problem with migration of low Mw siloxane from one layer to the other. LSR/RTV isn't my area, but silicone gels are generally a lightly crosslinked network of polymers swollen with loose polymer and cyclics. The small molecules can swell the polymer just like a solvent. If there is a difference in gel fraction between the layers, the migration of small molecules may cause one to swell one one to shrink.

I couldn't phone the rep again sadly as that would be asking to much I think.

[wildfyr]

I poked into the reddit thread and I agree with that analysis of monomers and liquid lower MW materials migrating. This is obviously a difficult technical problem. The obvious answer is a thin layer with reactive groups sandwiched between the two layers. The groups should react with both layers.

Heres an idea: one silicone layer, exposure to ozone to make peroxide groups, then add a nonsilicone impermeable polymer, heat it up to get a covalent bond, then ozone again, add the other silicone layer, and heat it up again. UV could work in place of heat, but it's harder to control.

[LlyrLumen]

Thanks for the reply again. I'm glad it's somewhat more clear for you now.

Your suggestion echos my very first thought, which was, "why not just put a barrier in-between the layers?" This is the first thing we learn as fabricators, barrier coats to prevent these issues.

But, as you said, the solution isn't that simple. With your method could a thin layer of brushed on polyurethane plastic or rubber do the trick? I often use PU plastic to encapsulate silicone gels and no migration/leaching happens...But, the issue with PU plastic is it's not stretchy or soft and the issue with PU rubber is it won't bond to silicone. Even a thin layer would defeat the whole multi layered systems intent, which is movement. I know this as fact having examined  PU encapsulated silicone gel mammary prosthesis...You can see the gel wants to do it's wibble/wobble but is restricted by the PU encapsulation (even though the encapsulation is very thin). As far as my research has gone, there isn't a stretchy, soft PU plastic or PU rubber that would bind to AD cure silicone...Could you suggest another material at all? Flexible acrylate or something?

With regards to exposing layers to air, could this be done post cure? I couldn't expose the layers to air during the cast because it will be an injection process with a closed fibreglass mould (jacket and various depth cores for each layer). And removing the core before full cure would warp the shape and delaminate the silicone. Also regarding the baking, how long would you bake the gel and at what temperature to form a covalent bond?

I actually understand the covalent bond you mention (yay!), do you think you could perhaps suggest some areas of polymer science for me to Google research so I can figure out the mechanics behind the actions you have suggested? I am looking for a solution but I would like to understand along the way without straight up asking for explanations...Defeating the purpose of me learning properly.

Thanks again for this, I would love to study material science formally and get to your level but that's impossible sadly. A NuSil rep has just  been in contact with me now and is also lending support and I have mentioned this issue to her as well. Every little bit of help is appreciated greatly.

[wildfyr]

Peroxides on the silicon surface stand a good chance of reacting with almost any other polymer. They will probably work bond best with polymers containing double bonds.

I did a little research, and ozone is not the right answerway to activate the surface. Plasma activation is. This, unfortunately, y requires low pressure conditions to generate the plasma which will give you surface peroxides.

For baking... I don't know. Would probably need to be empirically established what the best combo of time/temp is. 5 min at 70C seems kinda reasonable just as a shot in the dark.

As for giving you materials, I'm sorry I don't have anything except to suggest getting a textbook on polymers such as "introduction to polymers" by young and lovell. However, a good understanding of organic chemistry is also critical, and that can only be attained the hard way.

Let me give some more thought to a better way to activate the silicone surface, plasma is not a scalable answer in my book. Perhaps there is an aqueous solution based method that would work.

[LlyrLumen]

I know I keep saying it but thank you. I have a few library books that I'm reading and some apps for basic to advanced chemistry, foundation stuff. I just bought introduction to polymer science and a few others. It'll be hard as you say but I'm learning/retaining information faster, having a real problem to solve and apply this knowledge to, instead of just reading theory. All the technical keywords you mention I am reading up on in the mean time.

I had a thought today, would neoprene rubber be a suitable polymer for the barrier coat?