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MikeyDSlagle

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So I have been using US Composites 150 for a couple of years now. I always recommend it. Recently I got some of their 635 thin epoxy with 3:1 hardener (medium) to try with chopped fiberglass and possibly for sealing/strengthening/hardening MMTs and BTs and fins. My thinking is the stuff absorbs into the material just like CA does. I see it soak out around my internal fillets and harden the surrounding area.

So here is the problem. This 635 stuff will not cure. I laid some fillets with chopped fiber (right after I recommended it to another user) and after 6+ hours it was only a bit tacky so I rolled my fin can to do another set. By the time that set was tacky, another few hours, the first set had drooped. And it still had visible air bubbles. I scraped all that off and used the 150 stuff as normal and it set up with no issues. Thought maybe it was an issue with the thick fillets causing the epoxy to take time to set. So...

I went ahead used it to harden a MMT and a scrap piece of wood. This was last night, about 7pm. It is still tacky. I have had it under two lights, in the kitchen, for 3 hours now and it is STILL tacky. The website says 8 to 10 hours drying time for the 3:1 hardener. Shouldn't matter if it is thick or thin.

Is this normal for 635 epoxy?
Is it just super slow to fully cure?
I thought maybe the humidity, dew point, temp, pressure, something was affecting the cure, but like I say. They 150 cured just fine.

I am going to shoot them an email. Just wanted to see what you folks thought. And see if anyone has similar experiences.

Thanks,
Mikey D
 

REK

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So I have been using US Composites 150 for a couple of years now. I always recommend it. Recently I got some of their 635 thin epoxy with 3:1 hardener (medium) to try with chopped fiberglass and possibly for sealing/strengthening/hardening MMTs and BTs and fins. My thinking is the stuff absorbs into the material just like CA does. I see it soak out around my internal fillets and harden the surrounding area.

So here is the problem. This 635 stuff will not cure. I laid some fillets with chopped fiber (right after I recommended it to another user) and after 6+ hours it was only a bit tacky so I rolled my fin can to do another set. By the time that set was tacky, another few hours, the first set had drooped. And it still had visible air bubbles. I scraped all that off and used the 150 stuff as normal and it set up with no issues. Thought maybe it was an issue with the thick fillets causing the epoxy to take time to set. So...

I went ahead used it to harden a MMT and a scrap piece of wood. This was last night, about 7pm. It is still tacky. I have had it under two lights, in the kitchen, for 3 hours now and it is STILL tacky. The website says 8 to 10 hours drying time for the 3:1 hardener. Shouldn't matter if it is thick or thin.

Is this normal for 635 epoxy?
Is it just super slow to fully cure?
I thought maybe the humidity, dew point, temp, pressure, something was affecting the cure, but like I say. They 150 cured just fine.

I am going to shoot them an email. Just wanted to see what you folks thought. And see if anyone has similar experiences.

Thanks,
Mikey D
The thin epoxy is meant to wet out fabrics like fiberglass, carbon, kevlar, etc. It takes a much longer time to cure in order to provide complete wet out before it becomes rock solid.




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G_T

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Make sure the epoxy is weighed out accurately. Use a scale or it won't be. Mix well. When it looks well mixed, you are halfway there. Then, to speed the cure if you need to, increase temperature. Each 10 degree C increase in temperature cuts the cure time roughly in half. Don't over-do it. Once it looks fairly cured, you can ramp up the temperature higher to improve the mechanical properties. Again, don't over-do it. Look for the manufacturers recommendations on cure and post-cure temperatures.

Gerald
 

Handeman

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I've found that the surface can remain tacky even after it has hardened. A quick rub down/wash with some alcohol will usually take care of that.
 

Rob702Martinez

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I had this problem, similar anyway.

One of the epoxies I use, aeromarine 300/21, had an old batch, in a hurry mixed it by weight like I typically do, it never fully cured was that 3/4 tacky cured. Spoke to manf. and corrected to use volume instead. Turns out I forgot that this epoxy uses volume. Just as good as ever.

Try a mix by vol.
 

soopirV

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One last thing to consider, and as a heavy user of 635 for both lamination and for structural joints (when thickened with fillers), I have had good luck: 3:1 is a mix of the slow and the fast, and you need to really mix the hardener component well before use. The two components can settle out, leading to your situation perhaps.
That said, and I can't see where you are on my phone app, but this time of year I switch to their 4:1 fast set for 635. When it's hovering around 60 for me, even that compound takes over night to set up. Medium hardener around 6 could be 2-3 days if I remember their website correctly.

PS- 3:1 for me sets to leather stage in 2-3 hours in the summer when my garage is north of 115-120. I get a hard finish in ~6.
 
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Handeman

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I had this problem, similar anyway.

One of the epoxies I use, aeromarine 300/21, had an old batch, in a hurry mixed it by weight like I typically do, it never fully cured was that 3/4 tacky cured. Spoke to manf. and corrected to use volume instead. Turns out I forgot that this epoxy uses volume. Just as good as ever.

Try a mix by vol.
I hadn't thought of that, but I always mix my US Composites by volume and have never had any issues, except the surface tackiness. Even that goes away if after a few days.
 

Rob702Martinez

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I hadn't thought of that, but I always mix my US Composites by volume and have never had any issues, except the surface tackiness. Even that goes away if after a few days.
Test a small batch real quick by vol, see if it corrects that. Let us know.
 

soopirV

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I hadn't thought of that, but I always mix my US Composites by volume and have never had any issues, except the surface tackiness. Even that goes away if after a few days.
I only ever mix by weight, and like I said, nary a problem. My set is pretty old, too.
 

MikeyDSlagle

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Awesome.
Thanks for the responses guys.

The FAQ... that helps a bunch. Thanks mkadams001 The only FAQ I could find was about shipping, returns and general questions about US Composites. There are several scenarios there that apply to me: low air temp, resin applied too thin... I may not have stirred it quite enough. But I did shake the hardener for a 30 count.

soopirV, I am in Louisiana. Temperatures this time of year are anywhere between 40 and 80. Or below freezing. This morning we had ice on windshields and I was in shorts outside at dusk looking for tent stakes for my canopy for tomorrows launch. Didn't find em. So now I gotta stop by Wal-Mart on the way. Grrrr.

Volume vs weight. This was my first thought as US Composites says to mix by volume. I measured out the same amount by volume... and weighed them. Two separate cups of course. Honestly, I can't remember what the results were, but they were as such I moved forward mixing by weight. I have seen some epoxies that mix 3:1 by volume but 100:18 by weight, that makes a bit of difference. I will go back and weigh them again and use that as a basis for my future mixes.

I'll rub down with alcohol tomorrow after the launch and see if that takes care of the sticky.

Thanks folks

Mikey D
 

DGBrown

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The 635 is 100:32 by weight.
It is different for the different hardeners. I emailed them today and got the following.

Here are the ratios for mixing the 635 resin and hardeners by weight:

2:1 Slow Hardener: 100 parts 635 Resin: 46 Parts 2:1 Hardener

3:1 Medium Hardener: 100 parts 635 Resin: 32 Parts 3:1 Hardener

4:1 Fast Hardener: 100 parts 635 Resin: 24 Parts 4:1 Hardener
 

mikec

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FWIW, I use their pumps and have never had any problem with 635 with medium; I've never mixed the hardener in advance either. But I've never done anything much below 60F.
 

MikeyDSlagle

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It is different for the different hardeners. I emailed them today and got the following.

Here are the ratios for mixing the 635 resin and hardeners by weight:

2:1 Slow Hardener: 100 parts 635 Resin: 46 Parts 2:1 Hardener

3:1 Medium Hardener: 100 parts 635 Resin: 32 Parts 3:1 Hardener

4:1 Fast Hardener: 100 parts 635 Resin: 24 Parts 4:1 Hardener
Good info there, thanks.

A little update. I have used the 635 a bit more for some more fillets and this time it sat up as it should, or at least what I think is how it should. I still haven't got a technique for it yet. Smoothed the fillets just like I always do but they turned out awful, used Cabosil (fumed silica) as a filler. Not sure what I did wrong but I have since went over them with spot and glazing putty, just to smooth em out. Haven't been able to get back to it so don't know they turned out. Once I get me a technique down, I'm sure the stuff will be just fine. I obviously mixed the stuff wrong on my first attempt(s). I will try it again as a sealer and see what it does.

Mikey D
 
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