Epoxy xtrength?

Longer set time usually means "stronger" epoxy. Naturally, the chemistry of the epoxy comes into play, BUT Devcon has pretty good datasheets available here :

the longer the set time the greater the shear strength seems to hold true.

Jim

On Wed, 27 Sep 2006 15:51:25 GMT, "Earl Scherzinger" wrote in :

All things being equal, I would expect the 30-minute to show some greater strength, if you know how to test it (I don't).

The longer curing time suggests to me that there will be a more thorough polymerization of the compound. Longer molecular chains = more strength (as a general rule).

Marty

I don't know for sure, but in my experience, 30-minute epoxy seems tough, while 5-minute epoxy seems more brittle.

I would also expect that the longer curing time allows deeper peneatration and more strength.

All theory; no test data.

I only use 5-minute if I NEED a quick cure.

Good flying, desmobob

If you are gluing hardwood together, 30 minute is the way to go. 6 minute does not allow enough time for the glue to soak into the wood.

30- minute in general

If 6 minute Epoxy were stronger than 30 minute, there would be no reason to have 30 minute would there?

Red S. "

just that little thing called "pot life"

reminds me of the time I tried to glue the deck back on a Lil swamp buggy with 5 min epoxy. NOt only did I not make it, I had to cut off all the epoxy and try again! mk

30 min epoxy CAN BE stronger than 6 min...

The longer pot life allows the stuff to form longer molecular chains.

But... in most cases, the 6 min is stronger than what you are gluin with it, so the extra strength of the 30 min is often moot.

2hr epoxy soaks into the wood better, because its typically thinner an has more time to do it... that adds up to a significantly stronger join even with the same material strength.

It all makes sense when you think about it

-- fhhuber50677

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| If 6 minute Epoxy were stronger than 30 minute, there would be no reason to | have 30 minute would there?

Now, that's hardly true.

What if you need more than six minutes to get everything all set up and adjusted? And in my experience, if it says six minutes, that generally means I've got about three minutes to get my act together once I mix it all up before it starts getting sticky enough that it's hard to work with.

Mostly because of that, I generally prefer the two hour epoxy -- I get more time to make things `just right', and it may be stronger than the

30 minute epoxy. I generally save the 5-6 minute epoxy for field repairs.

Agree, except there is 45 minute and longer epoxy for those needing time to fiddle and adjust. :-)

Curing at higher temperatur helps too, eg 50 deg C Oven etc. As does exposure to water!

Marc

It is not a hard and fast rule that quicker cure times mean less strength.. but that is only true when speaking of the adhesive itself, not it's interaction with the substrate. When working with porous substrates, it is true that longer cure time can improve wetting of the substrate, which ultimately determines how many square inches of bond area you achieve per square inch of projected area - which as you can imagine is a major factor in the quality of the bond. And also, please remember that there are several curative chemistries, it's not as if there is a generic 5, 30, 60 minute and

24 hour curative for epoxies and that's it.

One of the major issues IMHO with 5 minute type epoxies is that the average user, in the haste to get it mixed and applied, does only a mediocre job of blending the two materials. Mind you, this is likely true of some people with slower cure systems, but let's leave that for the moment.

Epoxies cure through an addition reaction - meaning that ideally the correct stoichiometric quanitities of the resin and curative are bleneded so that each reactive group on the resin side can find and react with a reactive group on the curative side. Some deviation left or right is fine, within a few percentage points, without serious impact on the properties of the bond. And, some curative/resin systems are more tolerant than others due to the fact that in some cases a limited amount of homopolymerization can occur after the A/B reactions, further increasing the integrity of the cured system.

Now to practical matters - when using short cure epoxies, plan ahead and have everything ready. Measure out the two components, then immediately start mixing vigorously - scrape the bottoms, sides etc., and do a really good job blending the two as absolutely thoroughly as possible. BUT, time yourself - most of you have mixed this or that brand of 5 minute epoxy and have an idea that in the summer it cures faster (heat always does that!) than in winter, etc. So use your judgement. As a minimum, for the typical small qty's of 5 minute that we normally use, mix rapidly, thoroughly and effectively for 30 seconds. With slower cure systems, change that to one minute plus. You may be amazed at the difference in the quality of the bond that occurs by proper mixing technique! I certainly have, and some years ago I did some quantitative analysis of the benefits - yes, it helps, let's put it that way.

A useful tip with 5 minute systems is phenolic microballoons. They tend to promote thorough curing without intolerable catalysis [translated - curing way too fast], and I find in practice that the integrity of bonds I have made with 5 minute epoxies blended with a moderate qty of phenolic microballoons is noticably improved. Also, they act as crack terminators and add a bit of flexibility a.k.a. toughness to the finished system. But keep them dry.

Mike D.

Whose? I thought Hobby Poxy went away via EPA

The 30 minute is stronger. It hardens better. I have a You Can Do 3D.46 ARf

Are you just letting us know you have one or is it for sale ?

Ken

Both are those are about the same.

24hr is far stronger over time. Also make sure you do not mix small quantities on a porous surface. Mix the largest amount of 24hr needed on a non porous surface for maximum strength and longevity.

Nigel has a great point here.... We've all used business cards or the like to mix epoxy or JB Weld...

It's not a good practice. Some of the hardener will be absorbed into the paper, and it's effect is diminished.

It's far better to mix these products on a plascic coffee can cover or something similar. Let the excess cure and flex the plastic to pop it off and ready the lid for another use...

Cheers,

Bill

This is Earl, the guy who asked the original question of epoxy strength.

Thanks for all your advice.

To Bill's answer, using a plastic coffee can lid.... I would like to add what I use to mix epoxy.

I tape a small piece of wax paper (app. 6 x 6) to a cutting board and mix the epoxy on that. I also use round tooth picks or an OLD table knife to mix. When you're finished, carefully fold the wax paper and throw in the trash. Be sure to clean the knife with denatured alcohol before the glue gets too hard.

P.S. I'm NOT the one that said I have a U-Can-Do. ________________________________ Earl Scherzinger, AMA #40329