Kelly Wrote
>
> For those of you who don't know--the normal method to get the bubbles out
> is to put the mix in a bell jar, and pull a vacuum--with a real vacuum pump
> capable of going to at least 28" of Hg.  Problem is, the mix froths up to
> MANY times its original volume--real high surface tension keeps the bubbles
> from popping until they get huge.  Usually end up with silicone all over
> everything.  Much worse than putting a double recipe in the bread machine. :-p
>
> Haven't tried it with epoxies or urethanes, but don't see why it wouldn't
> work.  Used to buy pre-mixed years ago that came in dry-ice, and had to be
> stored well below freezing. Let me know if any of you try this, and the
> results.
>
 Yes indeed that is what happens, with a vacuum on Ureathanes,
Epoxies, and Silicones.
We made around 50 Hydrophones a few years ago to find the best way of
making cheap..ish (as in not US$10,000each) Hydrophones for our whale
research.
It was a toss up between  Epoxy and Ureathane depending on the
application BUT  we were able to manage the expansion by
letting air IN while the pump was sucking it OUT. The point was we
had good control over the tap going in but the pump was all ' brute
force and ignorance'.Allowing the vacuum to increase only as the
mixture frothed to an acceptable height  each time.
We also found each substance was 'de-Gassed' best (as in no unwanted
bubbles on setting) at certain temperatures.. e.g. the epoxy we were
using was best at 40 deg C before two part mixing.
The chemical manufacturers are very helpful with such info.
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

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