Re: ATM re failing silicone

["barb" <jdubois@essex1.com>]

The filler that is used in silicones to strengthen them is fumed silica.  It
is made by burning silicone monomer (like trichlorosilane) and collecting
the "white soot".  A big pillow size bag weighs only a few pounds.  A
quality silicone with fumed silica in it is much stronger than clear and
just as adhesive.  Fumed silica filled RTV has a translucent cast to it.
All high quality silicone rubbers are filled with fume silica.  When talc or
calcium carbonate extenders are added to silicone, the silicone is greatly
weakened, but costs less. Guess what you usually get in the $2.00 caulk tube
at the store.

The BIG problem with silicone rubber is its very poor tear resistance, and
the fumed silica greatly helps tear strength.  Put a thick section of clear
RTV about 2x2x1/4 inch and let it cure over night.  Take that 2x2 sample and
bend it over on itself. If it doesn't crack through at the 180 degree bend
after folding, slit the curved portion under tension just a little with your
fingernail and watch it tear apart.  That problem of poor tear resistance
has been a big problem for silicone rubber from the beginning.

Jule J. DuBois at 42N 89.6W


----- Original Message -----
From: Peter D. Brooks <pdbrooks@pacbell.net>
To: Peter John Smith <pjifl@bigpond.com.au>
Cc: <atm@shore.net>
Sent: Monday, June 25, 2001 9:06 PM
Subject: Re: ATM re failing silicone


>
>
>
> Peter John Smith wrote:
>  We talk of a caulking compound - never a caulk.  Interesting.
>
> In my experience, "caulking compound" was a crumbly stuff that (barely)
held
> its own when shoved into a crack.  That was before the silicone based
compounds
> came out (butyl used to be OK stuff, but I wouldn't hold a mirror with
it....)
>
> > > I don't agree with that reasoning. It assumes that the pigments must
> > degrade
> > > the properties. There are plenty of examples where filled adhesives
have
> > > much higher strengths. One common example is using glass fibers in
> > epoxies.
>
> I think it depends on the filler.  Modelers use glass microballoons to
fill
> with epoxy.  This is sandable, where the parent epoxy isn't, and I've
found
> it's *much* weaker than plain epoxy.  OTOH, fiberglas, when applied
properly,
> is a very high percentage of glass and a small amount of plastic.
>
> > > Polyurethanes are available in even a broader range than the silicone
> > > caulks. Polyurethane refers to the linkage between monomer segments.
The
> > > chemical nature are what is in between greatly affects the properties.
Do
> > > you have a specific recommendation of manufacturer and urethane type?
>
> I've used a polyurethane for a sealer (3M marine.  Don't have the number
handy,
> but if someone really wants to know, I'll look it up).  This ends up as a
very
> strong adhesive, but it is *not* flexible, at least orders of magnitude
from
> RTV.  On the other hand, I have a windshield that was sealed in place with
an
> infernal black goo that is still quite flexible, but also leaves black
streaks
> on anything that touches it.  Somewhere in the middle, and I'd be
interested...
>
> Pete Brooks
>
>