We are creating forms out of poured silicone.
We are having a problem in that a few hours after the forms come out of
their mold, they begin to "sweat" an oily substance.
We are following the manufacturers recommended ratio of
Before I start playing around with these ratios, I was wondering if
anyone else has experience this problem or has any advice on how best
to attack it.
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
Hope this will help you
Thanks for your reply. Sorry if my vocabulary is inaccurate or
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.
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.
Thank you for your response. All very good suggestions and right on
Mark Thorson wrote:
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.
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?
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.
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.
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.
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."
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.