Problem with "Oily" Silicone

Dear sir,

I'm afraid this oily substance is what is generally called "extender" and used on cheap silicones. This can be removed with ketonic solvent. One can easily find better ones on the market, which do not tend to sweat out, but maybe of higher vicosity. If viscosity doesn't matters, you can find some nice product at Rhodia, General Electric, Dow or Wacker, just to mention the most known. Hope this will help you Regards

Nicolas DELFAU

a écrit dans le message de news: snipped-for-privacy@m58g2000cwm.googlegroups.com...

Is it the "forms" that are sweating? If so, are the forms silicone or a casting material such as polyurethane. I just want to be clear.

Hi,

Thanks for your reply. Sorry if my vocabulary is inaccurate or confusing.

We pour silicone into molds to create various forms....that is to say "products" (mostly shapes that mimic parts of human anatomy: hands, fingers, etc). It is these objects that "sweat" oil.

My first guess would be to cut back on the ratio of oil we add when mixing the silicone to pour. But I'm told by the shop foreman that he has already tried that and the results are the same.

Curious if there is anything we can do.

Thanks again!

Well of course it's sweating oil, if you're mixing it in. Stop doing that and the problem is solved (duh...)

Andrew Werby

I'd suggest talking to the foreman and finding out just _exactly_ what was done when this was tried.

It would not surprise me if the experiment was flawed in some way. Informal experiments are often worse than no experiments, because they can lead you to believe you know something, when what you think you know is actually wrong.

If you don't do any experiments, you have the advantage of at least knowing one thing (that you know nothing), which is better than thinking you know something and being wrong about that.

Also useful would be to trace the history that got you to the current situation. How was the current recipe developed? Is this a recent problem, or has it always had this problem? If it is recent, what was changed? New equipment? New procedure? New batch of raw materials? (Note that there is a large batch-to-batch variation in silicone materials.)

If you want to try an experiment that might solve the problem with minimal changes to the existing procedure, I'd suggest a step-cure. But from the sound of it, you are nowhere close to embarking on such experiments. You need to know where you are, before planning the route to where you're going.

Apparently it has always been a problem....one that heretofore everyone just "accepted" as being "just the way things are". But from a little asking around, it seems that the problem is not consistent: sometimes its worse than others. That leads me to believe that there is some variability somewhere that, if controlled, could lead to reducing or eliminating the problem.

Quite true!!

Ultimately, I would want to perform some sort of DOE-type experiement to determine the best ratios for the deisred results. But, to get there

--as you pointed out -- we must first know where we are. On this respect, I am in a bit of a quandry. I am not a chemist...nor is foreman charged with producing the silicone for pouring. Thus far, I have not gotten any responses from our supplier as to the exact constituents of the silicone mixing compounds. So getting answers to even basic questions like "Is it peroxide cure or metal (Pt, Sn) cure?" is proving difficult.

The current mix ratio is the one recommended by the supplier.

Basically, I am turning to the internet to try and find out what I need to know I don't know....then trying to figure out how I can "know it"...so I can then take the next step and figure out what I need to know to change it...

Well . . . the first thing I'd ask to see is a data sheet for the material you are using. Then, I'd ask to see your list of requirements. If they match, that's a good start.

If you don't have a list of requirements, you're standing at the bottom of a rather deep hole. What hardness are you looking for? What % shrinkage on cure is acceptable? Are there any limits on cure schedule? Do you have any absolute requirements? (For example, must it cure at room temperature, or would a thermal cure be acceptable?) Do you care about outgassing? Swelling on exposure to solvents? Tear resistance? Crystallization temperature? Color? Transparency?

Once you make up a list of requirements, you'll be able to talk to a silicone vendor in a language he understands. Right away, he'll be able to dismiss many possible candidates on the basis of properties that you require. And, if you name certain properties as very important, that will suggest certain products. For example, if you need high tear resistance more than anything else (a property in which silicone tends to fall down, compared to other elastomers), that will suggest a silicone with long chain length and a free-radical curing mechanism.

Then, you should discuss your requirements with every silicone vendor you would consider using. Don't just stop at the first one you like. Talk to all of them.

BTW, is there any reason you're not using polyurethane? Silicone is mostly used for its high temperature resistance and chemical resistance. But if you can use polyurethane, that's always much cheaper.

Back in the day...( 70's and early 80') DOW's silicones were famous for sweating an oily residue.

Today, it is a problem I only see rarely with very cheap silicones.

If you need the best, dryest, and most inert form of silicone, you should be asking for a platinum cured system that is food grade. Be aware that food grade platinum silicone has a LOT of cure inhibition problems and requires a very good, clean, shop practice to get reliable results from... ( i.e. no water or moisture, no tin or zinc, etc...)

But the real question is why are you mixing in oil in the first place? Are you sure you are using silicone?

The only reason I would ever add silicone oil to silicone is as a diluent... to lower the viscosity so it runs faster when poured. If you are adding silicone oil as a diluent, then never exceed 5%... anything over 5% will sweat back out.

And use the best grade of silicone you can find.

christopher

Once again, thank you for your reply.

For the most part, your suggestions fall in line with things I've already been working on. Once I know the constituent materials, I will verify the recommended mix ratios. Unfortunately vendors are foreign (non-US) so data sheets are proving to be a bit of a challenge to obtain. Also, I want to verify our production process (looking for things like inaccuracies and variability in the measuring, etc)

We are using silicone for is pliability. We tried using various PU types but could not find anything that was suitably "soft" and flexible. There is still a search underway to try and find a supplier of PU who can supply material we can use. But, for now, silicone has become the material of choice.

Thanks again!

Thank you for your reply.....

Thank you for this "tip"....just the sort of advice I have been looking for! I will verify this with our production department.

There are so many varieties of both silicone and polyurethane available; I would first look for one that meets your requirements without alterations. Both materials are available in very soft gel like consistencies. As to the material you are currently using, does it sweat when no oil is added? If it does, quit using it and find a better quality. If it only sweats when oil is added, you should be able to find a suitable percentage.

Thanks for your response....

Billy Hiebert wrote:

I doubt they have tried it without adding any oil.....but that will be one of the "levels" if/when I start experiementing with it. Thus far, all I have been able to get out of the supplier is "You need the to add the oil or else it will be too hard."

Thanks again!