On Mon, 24 Mar 2008 19:27:09 -0700, The Last Mimsy
wrote:

On Tue, 25 Mar 2008 01:40:00 GMT, Rich Grise wrote:

On Thu, 20 Mar 2008 16:19:07 -0700, The Last Mimsy wrote:
On Thu, 20 Mar 2008 21:09:14 GMT, Esther & Fester Bestertester

See no n.g. for adhesives & epoxies, so if there's a better forum for this,
just point me to it.

I need an epoxy that is strong to the point of brittle. I want no flex; it
has to transmit vibration as close to 100% as possible.

Am I looking for a high Shore Hardness value? That's what Devcon uses in its
data sheets to specify hardness.

Is this something that I can achieve by changing the mix of the 2 parts? If
so, what do I lose if I use more hardener?

Any personal experiences with epoxies that you found to be harder than
others?

With the way you worded this, it would seem that a technical response
would go right over your head.

Epoxy mix ratios are NEVER meant to be altered. You need to find an
epoxy that matches your needs. The only time I ever saw mix ratio ranges
to alter behavior, it was with an epoxy branded as "stycast". Which,
would oddly meat your needs.

It will not likely be cheap, however.

Just FYI, when I was a kid, my Dad (RIP) got a two-bottle set of one of
the first epoxies, and its instructions gave one ratio for a rigid bond,
and another for "flexible".

I haven't seen anything like that lately, probably because so few
people would have the right measuring stuff. :-)

Thanks,
Rich


There are some epoxies that give different characteristics with a
different mix ratio, but not many, and NONE of those that do not should
be manipulated in such a way.

Curious, I have been using epoxies for more than 40 years and have
read many articles on epoxies in for instance Adhesives and Sealants
magazine, and I have never once seen or heard of any epoxy whose
properties do not depend on mix ratio; although in many cases exact
mix ratio is not critical.

The "stycast" I mentioned has about four different hardening schedules,
and uses about three different hardening medias. All yield differing
results for heat resistance, high temp softening after cure, etc.

When it is several grams of base media, and a few drops of "hardener",
I call it barely qualified to be called epoxy.

When the mix is closer to 50 50, one will find it is more stringently
adhered to as both constituents are critical to the finished medium.

Some mixes have to be accurate to within 0.1 grams or 0.1%.

"Epoxy" is not an adhesive, it is a family of adhesives that includes
many different specific chemistries. All high strength epoxies are
multi-functional, that is, they are a blend of two or more of the
basic epoxy chemistries. In any critical application it is foolish
not to discuss your specific requirements with one or more epoxy
formulators to obtain a blend closely optimized to your needs, and
then to qualify the materials and application process with testing.
Master Bond is one supplier of specialty epoxies with good tech
support.

On the other hand, for hobby applications, you can get good variation
in hardness with mix ratio with any nominally 5:1 ratio marine epoxy.
These epoxies are normally available with slow or fast hardeners in
order to allow reasonable working time in cool (use fast hardener) to
hot (use slow hardener) conditions. More hardener results in more
flexibility with lower strength and hardness. I mostly use "The West
System" with pump dispensers for easy measurement - one stroke of each
is 5:1, for small amounts I count drops. With the addition of fumed
silica (available from the same marine suppliers as the epoxy) it can
be thickened into a non-running paste very easily, or with the
addition of fine wood dust as a filler you can make epoxy foam.

BTW, by far the most common reason for lower than normal strength
epoxy is inadequate mixing, especially in the filled, pasty
formulations. It is best to consider the point where the epoxy
appears to be completely mixed to be the half way point in mixing. If
you actually measure the strength or hardness of the cured epoxy the
additional mixing will almost always result in better and more
consistent properties.