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From: "Richard D Elder" <horizonstr@rmisp.com>
To: <lancair.list@olsusa.com>
Subject: STRUCTURAL BONDS
Date: Thu, 24 Jun 1999 16:28:24 -0600
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LML homepage:   http://www.olsusa.com/Users/Mkaye/maillist.html

In point of fact epoxy bonds are weaker when excess clamping pressure is
used to create too close a fit.  If you want to prove this to yourself, bond
two  pieces of oak which are sanded to 80 grit profile using the minimum
clamping pressure to insure a continuos glue bond.  You will find that the
glue line will always be stronger than the wood.
Now repeat the same test using planed surfaces sanded  to 600 grit profile
and clamped with a big "C" clamp every 2".  You will find the bond line is
now weaker than the wood.
Micro-balloons should never be used as fillers in structural bonds.  The
balloons are hollow glass spheres which substantially degrade the strength
of the epoxy/filler matrix.  If your use micro-balloons  in a highly loaded
joint, IT WILL FAIL.  The proper filler for increasing viscosity in
structural bonding is silica.
Richard D. Elder


[I recently used some Cab-O-Sil-thickened epoxy as a surface layer over a
mold... the leftover material was extremely tough and I really had to 
work at getting it out of the little plastic cup I used for mixing.  The
mixture is also much smoother than a similarly viscous flox mixture, but 
I don't think that it has the strength of a flox matrix.  

    <Marv>    ]

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