Epoxy Fun

[William R. Monroe]

List,

I had an interesting experience with epoxy the other day, so I thought I'd
post my situation as well as the response from West System.  It helped me
and may help others who are not as familiar with the use of epoxy.

Just started working on some finger worn keys, following the method posted
by Ed Foote some time back.  Basically it involves using West System A group
epoxy, 105 Resin and 206A hardener as well as 410 microlite fairing.

I mixed up some epoxy, four pumps each into a 2oz. plastic cup, added a bit
of fairing to thicken, and about 10 minutes into ladeling the mixture onto
the keys, the mixture began to smoke in the cup it was in.  It had been
slowly gaining heat the whole time.  I set it down, and the whole thing just
solidified within about 3-5 minutes.  I mixed a fresh batch, same result.
Then I mixed another a bit thinner (less fairing), [and in retrospect, only
two pumps] and didn't seem to have any trouble this time.  What's going on?

Response from West System:
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 The more mass that is mixed, the faster the product cures.  Also, the
longer the product is contained the faster it cures.  Ambient heat also
accelerates the cure (for every 20F increase in temperature, pot time
halves).  The cure mechanism/energy produces heat - exothermic.  4 pumps
of each is a lot of material and the time it takes to blend in #410
Microlight to a thick consistency in a container with little surface
area, plus the ambient heat from your hand holding the pot is sufficient
to produce a run-away exothermic condition.  Suggest mixing less - 2
pumps each and after blending resin/hardener together spread out into a
shallower pot when blending #410 - increased surface area extends
working time.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Ed also pondered the idea of a cold bath/wrap to extend the working time,
and this seems to support that idea.

Hope this helps some folks.

William R. Monroe
Madison, WI
Assoc.