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Old 03-19-2014, 01:49 AM   #1
FlyWheel
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Unhappy Am I doing something wrong?

I am been having trouble with epoxy recently (for the last year, actually). I mix the two parts 50/50, and stir it thoroughly. Yet although it says "20 minute cure" on the label, it's been taking more like 20-40 hours to harden. At least ten just to get to the point where I can touch it without it sticking to my finger! And yes, I know it's winter now and it's cold, that's why I have been bringing my projects inside the house where it's a constant 70-75 degrees F. And it was taking a long time to cure last summer as well.

At first I though I just had a bad batch, but this has been the case for the last three packages I've had over the last year, which is why I'm thinking it's me.

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Old 03-19-2014, 02:22 AM   #2
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It doesn't sound like you are doing anything wrong.

I don't think my epoxy, hardens as quick as the label says either. 5 minute epoxy seems to take 30 minutes.

Some glues like Gorilla Glue can absorb moisture from the air in the bottle, so I try to squeeze the air out of the Gorilla bottle before I put the cap on. I have had several partially used bottles harden inside the bottle without expanding.

I don't know for sure if the epoxy glues can absorb moisture and slow down, but maybe.

It is really strange that 3 batches acted the same, unless there is something wrong with the glue.

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Old 03-19-2014, 02:52 AM   #3
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Depends on the epoxy... Quick cure epoxy will have a hard surface in appx 1.5 to 3 times the label time. Slow cure will have a hard surface (usually) in 5 to 20 times the label time

The label time "5 minute" epoxy means 5 minutes AVERAGE working time if you get the proportions right and you are using it at the recommended temperature.

Read the instructions fully... you'll probably find that the "20 minute" epoxy you are using is a "finishing resin" which says something like 24 hours for full cure (again... average if the proportions are right and temperature is right)

You can heat the epoxy to make it flow better AND make it cure faster. Don't go above 120 F when using this trick. It will make the epoxy "kick off" FAST and can make 2 hr epoxy harden (too hard to work with) in under 10 minutes. Keeping the epoxy hot will speed the final surface cure also...
But the final product will not be as strong if you heat it.
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Old 03-19-2014, 03:03 AM   #4
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Originally Posted by FlyWheel View Post
I am been having trouble with epoxy recently (for the last year, actually). I mix the two parts 50/50, and stir it thoroughly. Yet although it says "20 minute cure" on the label, it's been taking more like 20-40 hours to harden. At least ten just to get to the point where I can touch it without it sticking to my finger! And yes, I know it's winter now and it's cold, that's why I have been bringing my projects inside the house where it's a constant 70-75 degrees F. And it was taking a long time to cure last summer as well.

At first I though I just had a bad batch, but this has been the case for the last three packages I've had over the last year, which is why I'm thinking it's me.
Yeah
I've got some epoxy that has our local hobby shops name on the bottle. For the 5 minute epoxy, you've got about 5 minutes before that stuff is starting to set up. For the 30 minute stuff, you've got 20-30 minutes. It takes a full day for that 30 minute stuff to fully cure. If you are working with saturating fiberglass, it really helps to SLIGHTLY warm the epoxy with a heat gun. You can actually watch the epoxy instantly wet the epoxy where ever the heat gun warms up everything. IMHO, much better than thinning the epoxy with alcohol.

But, don't do it with the 5 minute stuff though. It will set much faster with gentle heat application.

Word to the wise, if you're making a big batch of this stuff, the chemical reaction can heat it up, making it cure faster. Before retiring at work, they used to mix up 5 gallon buckets of special potting epoxy for transformers and stuff. That stuff would heat up to over 200 degrees F if it wasn't immediately poured into the mold used to pot the transformer core and its windings.

It's always a very good idea to use protective rubber gloves with this stuff, so you don't become allergic to it years from now. And, if possible, do it in a ventilated area.

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Old 03-19-2014, 03:29 AM   #5
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I would be suspicious of the epoxy and also whatever you use to mix it, as in cup or sticks may be introducing a contaminant. The 5 min I have, and rarely use, has to mixed and where it is supposed to go in less than five minutes at room temp or it gets hard to move it around. I prefer Bob Smith's 15 minutes for parts assembly when I use epoxy, and like the 5 min it has to be in place in 15 minutes. For laminating ply doublers to balsa and skinning foam cores with balsa I use WEST resin with 206 hardener, not fast but great glue. I also use WEST to lam thin fiberglass on wing joints etc, and mash it down with peel ply to avoid sanding.

All my epoxies go off as advertised, and if your's doesn't, change brands. Some say all epoxy is the same but I've been using the stuff for many decades and that is not true.
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Old 03-19-2014, 05:09 AM   #6
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Originally Posted by FlyWheel View Post
I am been having trouble with epoxy recently (for the last year, actually). I mix the two parts 50/50, and stir it thoroughly. Yet although it says "20 minute cure" on the label, it's been taking more like 20-40 hours to harden. At least ten just to get to the point where I can touch it without it sticking to my finger! And yes, I know it's winter now and it's cold, that's why I have been bringing my projects inside the house where it's a constant 70-75 degrees F. And it was taking a long time to cure last summer as well.

At first I though I just had a bad batch, but this has been the case for the last three packages I've had over the last year, which is why I'm thinking it's me.
Its the Epoxy that your using, some are better than others, I like the Epoxy from Harbor frieght tools, I use their 5 min epoxy, it stays clear and does not yellow like some other epoxys do, swich epoxy brand, I belive thats your problem. Also, mix the epoxy for at least 1 min, it needs to be mixed very thoroughly.

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Old 03-19-2014, 01:06 PM   #7
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I buy "named brands" epoxy and stay away from generics. Before I use the epoxy, I keep the 2 bottles in a pan of warm water for about 20 minutes. If the epoxy is warm, it flows better and aids when mixing. The warm epoxy tends to set quicker as well - so you can go flying sooner.
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Old 03-19-2014, 01:12 PM   #8
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Originally Posted by gerrynj View Post
I buy "named brands" epoxy and stay away from generics. Before I use the epoxy, I keep the 2 bottles in a pan of warm water for about 20 minutes. If the epoxy is warm, it flows better and aids when mixing. The warm epoxy tends to set quicker as well - so you can go flying sooner.
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I do the same. I also measure epoxy by weight, rather than eyeball it.

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Old 03-19-2014, 09:24 PM   #9
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BroncoSquid beat me to it; I was going to say that the proportions are important, and that you have to avoid using too much resin. Too much catalyst will make it harden fast and be somewhat brittle. Too much resin retards its curing. As to how you determine which is which if it doesn't say on the tubes I can't say; I do it by smell.

You'll also notice that the resin is a little thicker, and while you may think you're squirting out equal amounts, the difference in viscosity can fool you.

And last, thorough mixing is necessary. I use a dowel of about 1/8" diameter. I'll put the 2 blobs of glue on a piece of whatever plastic is handy (try to avoid cardboard, the different viscosities also soak into cardboard at different rates), and mix with a circular motion. The epoxy spreads out as you mix, and periodically while I mix it I roll the stick to move the epoxy back into a compact little puddle before mixing it again.

I've always thought epoxy was better for resin casting than gluing things together.

Tom
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Old 03-19-2014, 09:51 PM   #10
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What are you mixing the epoxy in? If it is a waxed paper cup or anything with a wax coating, the epoxy will not cure properly in many cases. Just as a matter of information, all epoxy's, even 5 minute, do not fully cure (reach full strength) for about 24 hours under typical temperatures. Mild heat (less than 120 degrees F) will not cause a weaker product but excessive heat will. What will weaken the final cure is thinning with alcohol, acetone, lacquer thinner etc.. That will also make it more water permeable. Check out westsystem.com/ss/thinning-west-system-epoxy/ for some good info on how to use epoxy.
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Old 03-20-2014, 01:14 AM   #11
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Thanks peeps. Yeah, that seems about right, 20 minutes is about when I start seeing a serious change in the consistency. I guess I'm used to glues that advertise the actual drying/curing time, and I'm just misinterpreting the 'instructions'. I am using the little plastic cups 'designed' for mixing things, so wax isn't the problem.

Anyway, I am currently using it (thinned with denatured alcohol) AS a finishing resin for a sign, so in that application, the longer curing time actually works to my advantage, as it allows more time for the coating to level out. For aircraft use though, I think mixing a while before use may be in order.

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Old 03-20-2014, 01:49 AM   #12
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Originally Posted by Rodneh View Post
What are you mixing the epoxy in? If it is a waxed paper cup or anything with a wax coating, the epoxy will not cure properly in many cases. Just as a matter of information, all epoxy's, even 5 minute, do not fully cure (reach full strength) for about 24 hours under typical temperatures. Mild heat (less than 120 degrees F) will not cause a weaker product but excessive heat will. What will weaken the final cure is thinning with alcohol, acetone, lacquer thinner etc.. That will also make it more water permeable. Check out westsystem.com/ss/thinning-west-system-epoxy/ for some good info on how to use epoxy.
Yeah
Years ago we were using epoxy coating on our circuit boards used in our high voltage circuit breakers controls. The shop had an "Epoxy Thinner" solution used to dilute that epoxy before coating those boards.

That thinner was really some nasty stuff, nothing to mess around with. Or get on your hands or breath in. It would dissolve stuff that nothing else could.

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Old 03-20-2014, 03:33 PM   #13
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Probably acetone, I know that's used as a solvent for epoxy resins (any many other polymers).

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Old 03-20-2014, 04:29 PM   #14
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Originally Posted by FlyWheel View Post
Probably acetone, I know that's used as a solvent for epoxy resins (any many other polymers).
Or Chloro-Tri-Ethylene (sp?) stuff that will actually dissolve cured epoxy. Super nasty stuff. Had a quart in my garage stores and a visiting chemical engineer friend saw it and almost passed out from fright.
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