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Effect of Different Epoxies

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foam-n-fibre
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Effect of Different Epoxies

Postby foam-n-fibre » Tue Nov 28, 2017 6:47 pm

Hi, I know I have seen some talk about different epoxies in the past, but I'm wondering if anyone has done similar builds with different epoxies and has seen a real difference. I can imagine that a stiff epoxy when laid up as part of a thin skin might seem to be really stiff and dent resistant over a soft foam compared to another epoxy. BUT, when we make a mast/strut, and try to bend that, we have a lot of fibers in tension on one side and in compression on the other side on a relatively wide object. It would seem to me that particularly on the tension side that if the carbon fibers are pulling straight in tension the resin itself would not make much difference to flex. On the compression side I'm not sure if very small order buckling and compression would be possible, or whether this would just lead to full failure if there was any actual movement of fibers.

The more simple question is this: I used Resin Research epoxy for my first build. The strut has adequate stiffness, but I know some are stiffer. I think that my foam core and in retrospect no very well executed internal carbon stringer would matter more than some kind of different epoxy. Or, if I am wrong about this, is there a different epoxy that I should be using? Please don't reply and say "I used brand xxx and mine worked well". I would say that about what I am using too. I am looking for direct apples-to-apples comparisons, building the exact same thing with the same fibers but different epoxies. Has anyone done that?.

Thanks,
Peter

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Re: Effect of Different Epoxies

Postby BWD » Tue Nov 28, 2017 8:31 pm

I think if building a foil, it's probably worth it to go for an epoxy with a little more stiffness, and take the trouble to postcure it.
For a board to be 10% stiffer, it's easier and cheaper to add a stringer or make it a few mm thicker, but for a foil, you are probably not going to want it 17mm thick instead of 12-14mm!
So I would think it sensible to pay for a resin that might give you a little extra stiffness.
I started on boards with west system and system three, they are stiffer but very skin irritating, higher mix ratios (easy to get wrong), problematic blush, less UV resistance, expensive.
Switched to resin research, it's much easier to handle, 2:1 mix is easy, more flex, milder chemistry, less VOC's, good UV resistance, slightly cheaper.
The extra flex is a big advantage for building conventional boards. The longer "green" time and no blush allows multi layers and recoating without sanding.
But for a stiff mast or wing made in a mold, most of these issues disappear, except the stiffness.
I have yet to hear of a foil that was "too stiff"
I haven't built a foil yet, but I think I would try a "higher performance" (mainly higher elasticity modulus) epoxy for that, or any project that needed extra strength and stiffness.

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Re: Effect of Different Epoxies

Postby foam-n-fibre » Tue Nov 28, 2017 8:53 pm

So, are you saying to made a carbon foil with Resin Research epoxy and found it had more flex, or are you saying that in general, it is a more flexible epoxy? As I said in my first intro, I am wondering of the flex we feel in a thin skin laminate really translates into flex in a thicker object with a lot of carbon, where we are not just bending the thin layer of resin. If I pour a 0.5mm layer of pure epoxy and then bend it after it cures, does that mean much once we move up to a lot of carbon in a thick object?

Also, I didn't think the regular West System was a stiff resin, I thought it was pretty flexible, but I have not looked up specs.

Thx,
Peter

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Re: Effect of Different Epoxies

Postby rynhardt » Tue Nov 28, 2017 8:55 pm

Your resin's tensile modulus (around 3 GPa) is probably not going to matter much once you introduce glass (30 GPa) or carbon (150 GPa) reinforcement into the laminate.
Rather look at the resin's hardness, moisture absorbtion, shrinkage and strain to failure properties.

As a rule, elevated temp post-cure will always improve an epoxy's properties significantly. Glass transition temp especially is dramatically improved with post-cure.

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Re: Effect of Different Epoxies

Postby downunder » Wed Nov 29, 2017 3:21 am

Are you not using a prepreg?

If remember well from Levitaz vid, they are using only a prepreg carbon. Not the most stiffest mast, particularly with torsion from my brief experience holding it in hands.
As I understand the epoxy is a trade off, more 'hard' epoxies = better abrasion resistance, more 'soft' = better impact resistance. Would stick with the epoxy with long pot life for building a mast. 70mins for me was heaps better than 30-45mins life.

Duno how that relates to a prepreg tho.

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Re: Effect of Different Epoxies

Postby foam-n-fibre » Wed Nov 29, 2017 3:59 am

Pre-preg? No, I'm too old school for that. Considering that I order my epoxy and carbon from the US, I shudder to think what would be involved if I had to get stuff that had to stay refrigerated. My fast potlife was fine, but it might depend on how hot your shop is.

Rynhardt, I think you are describing what I was thinking too - the fibres are so stiff that resin details should not matter too much, but I thought I had heard someone here say otherwise.

Peter

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Re: Effect of Different Epoxies

Postby BWD » Wed Nov 29, 2017 4:42 am

To break, your carbon mast (or fuse or wing) has to bend. The resin helps keep it from bending, by keeping fibers from moving relative to each other. So with flexy resin you give up some strength and resistance to bending. A little too much bending and the carbon breaks. Stiffer resin keeps the part stiffer.

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Re: Effect of Different Epoxies

Postby rynhardt » Wed Nov 29, 2017 6:11 am

BWD wrote:
Wed Nov 29, 2017 4:42 am
To break, your carbon mast (or fuse or wing) has to bend. The resin helps keep it from bending, by keeping fibers from moving relative to each other. So with flexy resin you give up some strength and resistance to bending. A little too much bending and the carbon breaks. Stiffer resin keeps the part stiffer.
The layup should be designed such that the reinforcement fibres run in the direction of the expected strain. If your layup design does not cater for strain in a certain direction, then the resin's strength will come into play.
If you rely on the resin to provide stiffness then I think your design goals must be different.

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Re: Effect of Different Epoxies

Postby downunder » Wed Nov 29, 2017 9:13 am

foam-n-fibre wrote:
Wed Nov 29, 2017 3:59 am
Pre-preg? No, I'm too old school for that. Considering that I order my epoxy and carbon from the US, I shudder to think what would be involved if I had to get stuff that had to stay refrigerated. My fast potlife was fine, but it might depend on how hot your shop is.

Rynhardt, I think you are describing what I was thinking too - the fibres are so stiff that resin details should not matter too much, but I thought I had heard someone here say otherwise.

Peter
I would have thought it's much easier for you guys so close to the US. For me it's impossible. Not even the RR epoxy here.

It is hot, very hot. But as I've found it is not easy to work with a mast since I wrapped it all at once. The biggest problem is mast touching any surface while wrapping :nono:
That took significant time to fix and at the end binned all carbon, and scraped epoxy from the core.

This is why I've built a simple mast jig, not sure if you've seen it already. That made it heaps easier.

https://youtu.be/qaj0SMtY8CA

I can imagine it would be even more difficult to wrap a fuselage/mast at one go. Did wrap a fuselage only, with the mast attached and made to be removable.

D.

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Re: Effect of Different Epoxies

Postby BWD » Wed Nov 29, 2017 2:59 pm

If your layup design does not cater for strain in a certain direction, then the resin's strength will come into play.
the resin's strength is always in play.
imagine using sikaflex instead of epoxy.
That stuff sticks quite well, you would never get it off the fibers, but you couldn't make a mast of it, could you?
Granted, the difference with many epoxies might be 10% or less, but it is a difference, that could let your mast be 1mm thinner or a few hundred grams lighter. 8)


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