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I've been building a decently strength-sensitive project and received conflicting advice regarding the final bond strength of epoxies.

I've heard some say that bond strength has nothing to do with the rated curing time and that 6-minute epoxy is just as strong as 15 or 30 minute after it has cured, while some have told me that longer cure time epoxies will have higher bond strength.

Is there any merit to the argument that epoxy's bond strength depends on its rated cure time?

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  • Most strength of epoxy components comes from the fiberglas in them. Feb 11 '20 at 16:10
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It depends. And this applies to a bunch of characteristics, not just bond strength. Let me back up a step.

"Epoxy" is a collection of chemistries. There are a number of different base resins, a number of different kinds of hardeners, and various other things that can accelerate the reaction. Turning the liquid into a hard plastic entails a chemical reaction between the resin and hardener (the hardener isn't a catalyst that produces a fixed result based on the resin). The specific chemistry of the chosen resin and hardener combination produces results with different characteristics.

The setting time varies, and the end result can vary in all kinds of ways: performance differences, hardness, color, thermal and chemical properties, etc. For a particular purpose, the chemistry is optimized for the characteristics that are important (curing speed is just one; it's a balancing of requirements).

The difference between a fast-setting and slow-setting epoxy might involve different hardeners that produce different end products (for a slow- and fast-setting version of a particular product, the results should be pretty similar, since the products are designed for the same general purpose, and could be similar enough that you wouldn't notice a difference for your application). Or, it might accomplish faster setting by adding accelerators to the hardener, which would produce essentially the same end product.

With some products, you can even mix slow and fast hardeners to make a medium-speed epoxy. With other products, the slow and fast hardeners aren't compatible chemistries, and you can't.

It's not the curing time, itself, that can affect performance. But the chemistry that the product uses to accomplish the different setting speeds might affect performance. The only practical way to know is to contact the product manufacturer and ask (or test both versions).

If you have some chemistry background, you might find this paper by a scientist at one of the epoxy manufacturers relevant (it's a PDF link).

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I've been told by the proprietor of the local hobby shop that long-cure epoxy adhesives will be stronger than short-cure. I was told at the time that it has to do with the lower heat generated by the long-cure, but my research today suggests nothing of the sort.

The few links I pursued indicated that long-cure epoxies remain in a semi-liquid state for a longer period (duh) which allows the cross-linked molecule strings to increase in length, when compared to short-cure epoxies.

I have noted that when using 5 minute cure time epoxy adhesive, it will sometimes fail under lower stress conditions than expected, while in a similar mechanical configuration, the 30 minute stuff holds up.

In all of these epoxy references, including those for fiberglass and carbon fiber applications, it is noted that full strength will require 24 hours or longer.

One of the forum posts I discovered (radio control model building) provided an anecdote in which the OP discovered the 15 minute epoxy adhesive would shrink and crack and become brittle under conditions that did not cause similar problems for the 30 minute formulation.

I was unable to locate any solid technical papers covering research. It appears anecdotal and also my personal experience.

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  • That's very interesting... I didn't actually think it could matter at all... Could you post your research?
    – ifconfig
    Feb 11 '20 at 0:35
  • I tried to find something concrete and came up with anecdotes and references to epoxy casting resin, not pertinent to this question. I don't like having that limitation, but there's little out there.
    – fred_dot_u
    Feb 11 '20 at 1:10
  • Thermal effects in quick-curing may well be an issue, especially if the thermal contraction is very different compared to the workpiece. some of the quick-cure stuff also seems to be very sensitive to proportions and mixing: you don't mic much so you use it quickly, but then the proportions are off, and you can over- or under-mix easily
    – Chris H
    Feb 13 '20 at 17:00

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