Ping Lew Hodgett

I've been using glass & epoxy to effect some repairs. One surface was very irregular so the plan was to trowel on thickened epoxy, smooth on the glass squeezing out any thickened epoxy then add unthickened epoxy if necessary to wet out the glass.
I normally use Cab-o-Sil to thicken but this time I used micro balloons. After I finished there was maybe 1/4 - 1/2" of both thickened and unthickened in little pot pie type pans. The thick went off like gang busters...once it started, it got rock hard in minutes. It also expanded by about 3X. Meanwhile, the unthickened had barely gotten syrupy.
What's going on with that? The fast hardening and - especially - the expansion.
dadiOH
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"dadiOH" wrote:

------------------------------------------------------------------------- I'm clueless, but based on your description, sounds like thermal run away caused the expansion.
I like to use slow hardener with microballoons and limit thickness to 1/4" max per pass.
Using Cab-O-Sil to thicken produces a patch that is a BEAR to sand.
Can you talk to your supplier's tech service group?
Lew
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Could be, it was hot as a pistol. Way to hot to handle.

I had 3:1 hardener and 2:1, used the 3:1

Quite true. I try to limit its use to places where I don't need or intend to sand.

Naah, not that important.

Thanks, Lew, appreciated.
--

dadiOH
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Thanks. If I have occasion to do it again I'll keep it on ice.
--

dadiOH
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Yes, and warnings about exothermic runaway are found in every manufacturers technical notes. I've seen a pot of mixed epoxy catch fire whe something went wrong with the part fit so it didn't get used immediately as expected. The reaction rate typically doubles for every 10 deg K rise in temperature so if you are using a fast hardener and larger volumes of mixed resin its very easy to get into trouble when the self heating runs away. If you are using epoxy in warm weather you should always consider using a slow hardener.
Another problem is melting the mixing pot. I favour polyproplyene plastic pots because cured epoxy doesnt stick well to them so they can easily be reused, but for safety you need to put them in a tin in case you get a runaway batch and it is always a good idea to have something heatproof handy to grab them with and chuck them out the door or into a drum of water.
Just hope you never get an in-situ exothermic runaway - that rarely ends well!
Except for pure volume filling, I wouldn't use just microballoons. The cured epoxy tends to be too brittle. Adding microfibre (cellulose fibre) makes it a lot tougher but not significantly harder to sand.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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"Lew Hodgett" wrote:

----------------------------------------------------- "dadiOH" wrote:

--------------------------------------------------------- "Lew Hodgett" wrote:

------------------------------------------------------- "dadiOH" wrote:

--------------------------------------------------------- Break open the 1/2" piece. If it is foamed like an ice cream soda inside, two things.
1. You definitely had a run away exothermic reaction. 2. You have a structure that is totally useless.
Time to grind out and start over.
Lew
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2. You have a structure that *MAY* *BE* totally useless.
So far we understand you applied a skim coat of thickened epoxy to an irregular surface then glassclothed over it.
If you have any blistering or discolouration then Lew is right and you've had an exotherm in-situ. Grind out and start over, with slow hardener at a cooler time of day.
If you had any holes in the original surface bigger than a pea, you may have a problem. If they are near an edge or the part is under significant stress or it is going to be immersed, you should consider investigating further e.g. drill through the glass cloth with a 1/16" bit in a hand drill (not a power drill) then probe with a mounted pin to check the epoxy underneath isn't spongey. Make good with a dab of thickened epoxy.
However if it exothermed in the pot at least 5 minutes after you finished working on the surface, and the surface's cure rate appeared similar to the unthickened leftover pot, you've probably got away with it this time.
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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Yeah, the suppy batch was foamed. What I was working on was fine.
My garden tractor grill - plastic - was broken when a repairman pushed it too hard onto his trailer and bumped the front wall of the trailer. I would have preferred ro buy a new grill but they aren't available. There was one piece missing so the I covered some thin cardboard with celluose tape and used that taped to the inside to get a contour and to contain the thickened epoxy. Once set, I took out the cardboard and glassed over the repaired area. Looks terrible, but functions fine :)
--

dadiOH
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