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24 hr epoxy cured in 1 hr

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Jyoder
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24 hr epoxy cured in 1 hr

Postby Jyoder » Thu Nov 08, 2018 11:54 pm

I have some US Composites 2 part medium 3:1 epoxy that I’ve used many times. It takes about 2 hours to set at normal temp and cures in 24 hours or so. This is epoxy, not poly resin.

Last night I needed some filler and so experimented with some Bondo auto body filler powder, figuring (maybe incorrectly) that the powder is just inert like sawdust.

I mixed the filler in after mixing the epoxy and hardener, as if adding sawdust or graphite.

Everything seemed normal, I was filling holes in an XPS blank, though I noticed it had a lot of air bubbles on the surface. I used a heat gun briefly to try to cook the air bubbles off and it only kind of worked, worse than usual.

Oh well, can sand it out. I left it sit.

Came back in an hour and guess what... hard as a rock! Not even tacky. Wtf happened?

I’ll have to experiment to see if it happens again and if the strength or bond is affected.
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BWD
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Re: 24 hr epoxy cured in 1 hr

Postby BWD » Fri Nov 09, 2018 12:21 am

Many fillers (milled glass or carbon fibers, microballoons, sawdust/wood flour, etc) are insulators.
So if you make a large mass of epoxy with any of these fillers, the cure can be accelerated.
Sometimes it can even get out of control and generate fumes and bubbles in the epoxy. Most will not start a fire, at least.
Adding more heat with a lamp, heat gun etc increases the risk.
Often large pours need to be done at extra low temperatures or in stages instead of all at once.
If there are bubbles visible in the epoxy after a hot/fasts cure, beware: the physical strength of the epoxy may be compromised!

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sabraxas
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Re: 24 hr epoxy cured in 1 hr

Postby sabraxas » Fri Nov 09, 2018 1:18 am

I agree with BWD.

The heat can accelerate the cure exponentially.
Large mass of epoxy generates heat. And this heat increases the chemical reaction, which generates more heat.

It happened to me once. I always prepare 100 gr of epoxy, not much more than that every time. And if I need more (I always need more) I prepare another 100 gr.

But once I prepared 400 gr. And it cured very fast, even faster than I could fiberglass. In twenty minutes some parts of the mixture were solid as a rock.

For larger mixtures, it is better to prepare then in flat large shallow metallic containers.

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Re: 24 hr epoxy cured in 1 hr

Postby fluidity » Wed Nov 14, 2018 5:57 am

I've had the same effect, especially with microsphere fillers. Of which you'd need a lot less resin than normal anyway, it bulks up the resin a lot.

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Re: 24 hr epoxy cured in 1 hr

Postby plummet » Wed Nov 14, 2018 6:34 am

Also, a more hardener than recommended can decrease cure time.

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Re: 24 hr epoxy cured in 1 hr

Postby rynhardt » Wed Nov 14, 2018 9:28 am

plummet wrote:
Wed Nov 14, 2018 6:34 am
Also, a more hardener than recommended can decrease cure time.
Not sure if that's recommended.
Epoxy cross-links, so excess hardener doesn't get used, which is why it's recommended to be very precise with the ratios.
Polyester systems however has an activator, and the amount of activator can speed up the process. Polyester is more tolerant of imprecise ratios.

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Re: 24 hr epoxy cured in 1 hr

Postby plummet » Wed Nov 14, 2018 11:58 am

rynhardt wrote:
Wed Nov 14, 2018 9:28 am
plummet wrote:
Wed Nov 14, 2018 6:34 am
Also, a more hardener than recommended can decrease cure time.
Not sure if that's recommended.
Epoxy cross-links, so excess hardener doesn't get used, which is why it's recommended to be very precise with the ratios.
Polyester systems however has an activator, and the amount of activator can speed up the process. Polyester is more tolerant of imprecise ratios.
Im not suggesting incorrect ratios. but it can change cure times.


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