epoxy fun

[Sarah Fox]

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Hi John,

There's a somewhat fine line, short of which it takes forever for the =
stuff to cure, and beyond which the stuff goes thermonuclear.  When I'm =
using the stuff, I mix it in a small plastic cup (6 oz, I'd guess).  If =
the ambient temperature is a bit cooler, perhaps at or below 70F, I can =
mix up two or even three pumps without the mixture quickly "flashing."  =
However, for this to work, the cup needs ventillation.  Do NOT cover the =
cup, or it will flash quite rapidly.  (Covering it doesn't do anything =
useful anyway.)  If I'm working at a moderate to higher ambient =
temperature, only one or two pumps can be used.  If you're working with =
the stuff, and it's getting warm in the cup, it won't be long before it =
hardens.  To slow the process, cool the mixture.  I do this by spreading =
the mixture over the walls of the cup with my spatula.  With more =
surface area, the mixture will lose heat faster.  Finally, I *think* I'm =
correct that fillers hasten the cure, I suspect because they help to =
insulate the mixture.

If you want a longer pot life, put a fan by the cup.  If you want the =
ultimate pot life, put the cup in some cool water (not cold enough to =
create condensation on the inside of the cup).  Cool epoxy will take =
forever and a day to cure.  I know this from having done hull repair on =
a boat during cold weather.  :-(  If your workpiece is warm, it will =
speed the curing once the epoxy is applied.  To speed curing further =
once applied, cover it to protect it from drafts.  Be careful if you =
have too much of it in too well insulated a space, though.  I'm pretty =
sure the stuff could catch something on fire under extreme conditions.  =
If in doubt, use the 209 hardener, as Terry suggested.

Peace,
Sarah
  ----- Original Message -----=20
  From: John Voigt=20
  To: pianotech@ptg.org=20
  Sent: Tuesday, September 21, 2004 8:11 PM
  Subject: Re: epoxy fun


Just today I had the same thing happen to me, only my batch was only 1 =
pump of each and I was using a 6 oz. plastic cup.  I had made my repairs =
and then returned to the cup after 5-10 min. and the mixture was already =
hardened.  It was still warm and had caused the cup to wrinkle.  I was =
beginning to wonder if my materials had gotten beyond their shelf life.  =
I have used West Systems epoxy numerous times before (sometimes larger =
batches) without this happening and yet, recently, I have noticed that =
my mixing cups were wrinkled as if by heat when I returned, maybe days =
later.  The only difference I am aware of is the size of the cup.  10 =
0z. cup - no problem. 6 oz. cup - it gets hot.  Maybe there is another =
variable I am not aware of.

John Voigt
Assoc.
Avon, SD


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.








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