Epoxy resin systems

On July 27 Mark Vine asked about Gasil 23, a fumed silica used as a
thickening agent with epoxy resins.  He mentioned that someone not
on the list is having difficulties with mixing and application, as
was seeking articles on the same.

Fumed colloidal silica is interesting stuff, with a wide variety of
applications.  Small amounts are added to cake mix to prevent
unsightly clumping of the powder when the mix is poured.  It is also
used as a matting agent in varnishes like Soluvar Mat (from
Liquitex).  The use of fumed silica with epoxy resin systems is
often misdescribed as adding a "thickening agent."  If a sufficient
amount is added, fumed silica will not "thicken" the resin
significantly, but will transform fluid epoxy to a gel through the
formation a weak three-dimensional network within the mixture.  When
stirred, this gel form exhibits no significant increase in
resistance to the tool due to its thixotropic character.  Start with
what appears to be equal "volumes" of resin and fumed silica, mixing
with a dental spatula on an appropriate palette.  When the resin has
thoroughly wet the powder and the mix stops flowing this gel state
has been achieved.

Bulking agents like microballoons and chalk provide both thickening
for resins (with increased resistance to stirring) and extension of
the mixture by occupying a significant amount of space.  If you want
to fill a large hole with a minimum of costly resin, thickening may
be the answer.  If you want epoxy resin to stay where you put it
without holding the adherends farther apart, fumed silica will work
better.

You may be able to find a fuller explanation of the applications for
fumed silica in the product literature.

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                  Conservation DistList Instance 10:19
                 Distributed: Thursday, August 15, 1996
                       Message Id: cdl-10-19-001
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