Epoxy strength

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Epoxy strength

Postby firecat1981 » Sun Aug 14, 2011 6:39 am

Is there a major strength difference between fast curing epoxy (1:1) and slower curing epoxy (2:1)?

I've been using 2:1 for my last few builds, but I'd like to get some things done a little faster, like when doing non structural items like the console or bonding caps.....

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Re: Epoxy strength

Postby Oyster » Sun Aug 14, 2011 6:48 am

The key element is to make sure that you get proper wetout and saturation of the joining parts before the chemical process takes place. I use fast set a fair amount for smaller jobs. But i aso wet out the matting areas with uncured resin before joining the piece together with any thickened epoxy, if applicable.

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Re: Epoxy strength

Postby BruceDow » Sun Aug 14, 2011 6:58 am

you can always to a test.

Do two identical glue-ups... one with fast, and one with slow.

Then see if either joint fails before the wood breaks.

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Re: Epoxy strength

Postby galamb » Sun Aug 14, 2011 7:05 am

West System has done about a zillion tests to answer that question in thousands of different applications.

The main jist of their research has been that epoxy is upwards of ten times stronger (in shear strength) than the wood it's bonding regardless of the hardener used (in their case it's either 5:1 or 3:1).

Once cured, in general, 1:1 epoxy is more flexable than 5:1, with the others falling in between. So the arguement could be made to use a 1:1 for coating operations (covering a hull with cloth etc) since it will be less prone to fatigue over the long term, than a "stiffer" coat which may be more prone to cracking.

However, from what I have read (it gets kinda technical, so I really only got the jist of what they were saying) is that with average use, it takes somewhere between 800 and 1000 hours on the water before the epoxy loses 1/2 it's original integrity - so at that point it's only maybe 4-5 times stronger than the wood it's bonding/bonded to.

The guys at West consider the length of time to build as a key factor (since more boats built/repaired means more money/less man hours of work etc) and they suggest that you use the fastest hardener you can given the circumstances you are working under. And while your motivation may not be the same, these are, after all, the guys who practically invented marine epoxy and probably wouldn't use a practice that produced inferior results.

I used fast hardener for the majority of my bonding, only switching to slow or extra slow when it got so hot and humid that the fast was setting up in the pot before I could get it used up.

Once cured I can detect no difference between the three different hardeners as far as bonding strength -however, I haven't tried to rip apart a frame or batten to test it out :)

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Re: Epoxy strength

Postby firecat1981 » Sun Aug 14, 2011 9:40 am

Thanks guys, that's what I figured. Anything big or important I'l still stick with the 2:1, dries in about 6 hours or so. For smaller things or a quick fix I'll use the 1:1, which dries in about 2-3.

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