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MMD > Archives > April 1996 > 1996.04.11 > 02Prev  Next

Re: Sealing Pouches: Migration of Silicone
By Craig Brougher

Answering Doug Rhodes (and others) regarding the migration of silicone:

There are tens of thousands of silicones, Doug. To say that silicone
does this and that is like saying that alcohol has this and that
property, or that all benzines do thus and so. You cannot start making
assumptions based upon generic qualities for elementally derived
compounds. One good example is cholesterol. It is closer to an alcohol
than anything else, so is classified as an alcohol. (The body turns
every pound of sugar it processes into one lb.  of LDL cholesterol
before it is able to do anything else with it, but doctors talk about
is as though it were a fat. ) There are silicones that we walk on at
the beach, and there are so-called "silicones" with the viscosity of
water, crystalline form that makes transistors, and everything
in-between. Just because they call it "silicone" doesn't mean you can
classify it as a material that can migrate through wood. H2O2 isn't
something you want to shower in either, but except for one extra
hydrogen ion, "it's the same as water!" Unless you tell a person WHO
makes it, and WHAT the product name is, and HOW it is used, and WHERE
to use it, and WHEN it should be used and when it should not be used,
then you don't have the full story.

    111 Dow Corning Silicone grease, used properly on pouches, will not
migrate anywhere else. It will stay in the pouch, as long as that pouch
is glued down with hot hide glue and not something that might be
dissolved by the thinner in the grease, it cannot develop a vapor
pressure. It is physically impossible, Doug.

    This same kind of grease is used in high tor vacuum experiments in
which any kind of contamination, like a silicone molecule released from
a sealing ring would prevent perfect vapor deposition. When you were in
high school, the physics prof may have evacuated a "bell jar" in order
to do some vacuum experiments, too. The simplest method was to place
the jar on a metal plate, sealed with--you guessed it-- 111 Dow Corning
Silicone Grease.

    As far as the inability to reglue over it is concerned, if you will
confine your sealing to each individual pouch, and you do not just dip
the entire stack into a cauldron of it, you nor the next guy will have
any problems. I've had to do it many times, myself, when changing
something or correcting something.

    Regarding the Schultz pouch pneumatics, They seal perfectly this
way, but as Bill Kavouras suggested, much pouch leather is porous and
has visible pinholes when held up to a strong light. Leather like this
must be marked and culled out of the lot before pouch pneumatics are
covered. And while sucking rubber cement into leather may seal it
better, it also stiffens it more, because I used  rubber cement when I
first started, too. And when you blow back through it, you get teeny
tiny little bubbles on top that pop, which means you just put the leak
back into the pouch. Yep, how well I remember.

    Rubber cement is easy to come by, fun to apply, makes a cartridge
tight seal instantly, and works just fine -- for awhile. And for those
who want longer-lasting rubber cement that will not turn to a power in
fifteen years, I suggest the cement sold by leather companies and shoe
companies, and folks, that's a far cry from Carter's rubber cement. The
problem with response to faint air signals is still the same with
either one. And you can thin it with MEK or whatever-- but to the
degree you have sealed the leather, it stiffens the pouch more, and as
time goes on, more and more. But things like this make just one of the
little detail differences between rebuilds. You can tell the difference
instantly in tests, and you can test the degradation as time goes on.
One changes, one doesn't. One preserves leather by keeping out
moisture.  The other is hygroscopic-- that means, it DRAWS moisture to
itself. One gets stiffer as time progresses. The other never changes.
You decide which is best for you. And remember, I also suggest talcum
power with any sealant.

    I have had to discuss the value of hot hide glue over Tightbond,
also. I do my best to convince them with basic principles, but have
found that I can't really tell anybody anything that they don't want to
hear when they are defending themselves.

    My grandpa used to say, "Don't tell me what you're going to do.
Tell me what you did." I guess the proof is in the instrument, down the
road. I have seen every kind of sealant used on pouches in every
condition imaginable, and will probably find a set sealed with
spaghetti sauce someday, too. ("The secret of course is the anchovie
paste," I'm told).

Also a note to David Wasson: Thanks for including me in your article
for MBSI.  Yes, you are welcome to anything I have on the stuff. As a
matter of fact, I have smeared a little on this note and will post it
shortly, so if you see another paper clip, that's the grease I have
associated with this file.  There's just enough to do one pouch,

Craig B.

(Message sent Thu 11 Apr 1996, 12:58:56 GMT, from time zone GMT.)

Key Words in Subject:  Migration, Pouches, Sealing, Silicone

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