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Effect of Different Epoxies

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rynhardt
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Re: Effect of Different Epoxies

Postby rynhardt » Wed Nov 29, 2017 3:38 pm

BWD wrote:
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)
For sure the epoxy's strength is important. An epoxy resin is probably more suitable for the OP's application than for example a polyester resin, or an acrylic adhesive.
But if you look at the different epoxy resins that are suitable for wet layup applications, i.e. low viscosity, long pot life, their modulus should not be the deciding factor.
The OP's question was whether the epoxy's modulus will have a significant impact on the final laminate's stiffness.
The answer is still that with a proper layup schedule the reinforcement modulus will have the most effect.
The resin may contribute maybe 1%.

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Re: Effect of Different Epoxies

Postby tegirinenashi » Wed Nov 29, 2017 6:07 pm

rynhardt wrote:
Wed Nov 29, 2017 3:38 pm
...The resin may contribute maybe 1%.
You earlier mentioned that resin's tensile modulus is 3 GPa compared to carbon 150 GPa. That would be 2% :thumb: This number should be further adjusted up because the final product is never 100:1 or 1:100 proportion of resin and fiber. For that matter, what is the typical ratio of resin and fiber in the outer layers by weight?

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Re: Effect of Different Epoxies

Postby rynhardt » Wed Nov 29, 2017 6:26 pm

tegirinenashi wrote:
Wed Nov 29, 2017 6:07 pm
rynhardt wrote:
Wed Nov 29, 2017 3:38 pm
...The resin may contribute maybe 1%.
You earlier mentioned that resin's tensile modulus is 3 GPa compared to carbon 150 GPa. That would be 2% :thumb: This number should be further adjusted up because the final product is never 100:1 or 1:100 proportion of resin and fiber. For that matter, what is the typical ratio of resin and fiber in the outer layers by weight?
:wink:
Rough estimate, given a 50/50 resin to fibre ratio.
And my estimates for tensile modulus are indicative, of course. A more complete answer will need to take into account the fabric weave, crimp ratio, layup process (wet layup, infusion, compression mould), etc.
:cool2:

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Re: Effect of Different Epoxies

Postby BWD » Wed Nov 29, 2017 10:26 pm

And what's the safety factor in your engineering value for the carbon?
I wouldn't design to 150 GPa....
And how much safety factor do you need for the resin (less, since it depends on micro to molecular scale interactions, instead of meter long fibers and many process steps)?
If you mix the resin well and saturate the fibers, it pretty much does as advertised.
The carbon requires you to do a more things just right to get performance close to its potential, i think....

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Re: Effect of Different Epoxies

Postby tegirinenashi » Wed Nov 29, 2017 10:37 pm

What is the argument here? That different epoxy would give you some additional strength? You must be kidding, try eating with plastic fork at fast food restaurant without breaking it. The purpose of epoxy is to provide "a stable matrix [for CF fibres] to reside in and maintain their shape", that's it.

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Re: Effect of Different Epoxies

Postby foam-n-fibre » Thu Nov 30, 2017 1:57 am

Thanks for some interesting replies here. So far I am not shopping around for a new brand of epoxy, but I'm still interested to hear if anyone has some real head-to-head comparison results.

Peter

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Re: Effect of Different Epoxies

Postby BWD » Thu Nov 30, 2017 3:09 am

Guys, I’m out.
Do some learning.
You’ll be glad you did.

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Re: Effect of Different Epoxies

Postby tahoedirk » Thu Nov 30, 2017 8:18 pm

I think epoxy stiffness matters and varies greatly. Brittle, stiff and breakable or pliable and rubbery...hmmm

I have built entire foils out of RR, US composites and West. Foil profiles and subtle geometry differences seem to matter more than strut and wing stiffness. There is a brand from Soller composites which I would like to try for stiffness , especially with nanotubes. I still am using RR for all purposes, boards and foils. It sucks for hot coats but it is pretty good all around. USC is softish when warm or hot and takes weeks to cure. West is stiff but no UV? MGS from Germany is by far the best I've used but too expensive. My masts have always been solid carbon/epoxy though

Since I am more into design than production I choose a budget minded Epoxy for everything . It really does matter! but not really if you are smiling more than the other guy, even with a stiffer thingy.

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Re: Effect of Different Epoxies

Postby foam-n-fibre » Thu Nov 30, 2017 11:09 pm

With all due respect BWD, I'm here to learn, and asking for people with real world experience. I have 2 mechanical engineering degrees so have some understanding of materials. For that matter when I look at properties of RR vs West I don't see a big difference, and certainly don't see why West would be the stiffer option. The specs for RR are as shown below, with the numbers for West in brackets after them (from fiberglass supply website):
Weight 3.7 (West 3.1)
Tensile Modulus 405,000 psi (West 408,000)
Tensile Strength 9,800 psi (West 7,846)
Elongation 3.8% (West 3.4%)
Flexural Modulus 510,000psi (West 461,000)
Flexural Strength 14,800 psi (West 14,112)
Compression Strength 15,400psi (West 11,418)

I do see a larger elongation for the RR at 3.8%, but if that is 3.8% at failure, I also note that in tension it is stronger, so the actual elongation at the same force would probably be lower, if I am understanding these specs right.

So, the question remains, in something like a mast/strut, does a slightly softer resin hold fibers in place better to make a noticeable difference? And if so, what is the property that we need in the resin? For that matter, how well a resin works and wets out might also affect the finished ratio, and have a very big effect. There's a lot more to this than some basic calculations, which is why I'm hoping for comparisons from the finished product.

edit: found some MGS data too. Flex modulus 456,000, tensile 10,878, flex strength 16,679. Numbers might appear better, but how does that translate in the final product?

Thanks,
Peter

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Re: Effect of Different Epoxies

Postby downunder » Fri Dec 01, 2017 2:43 am

Pete,

I recon post curing does almost nothing for WS since designed for room curing.

Ie Tg for WS:
https://www.westsystem.com/products/com ... roperties/

Tg for Entropy:
https://entropyresins.com/product/super ... oxy-resin/

Tg for RR:
http://resinresearch.net/id9.html

Tg:
http://www.epotek.com/site/files/Techti ... /tip23.pdf

We need a Tg property to be high, plus post curing. I might be wrong but if post curing is not used and epoxy is designed for it, than we are not getting the best properties from resin.
Hence, if higher Tg in resin, and lower everything else, it will be still better than the one with higher everything but lower Tg. When post cured.

This is exactly why a prepreg is used in ie sail masts. Since that mast is huge, epoxy would go jelly with all the heat received from the Sun. However, it doesn't. Because epoxy used has huge Tg backed in autoclave. The trade off is super high Tg = high viscosity.

Hope this helps.

D.


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