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Aug 18, 2021, 08:33 AM
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truffaldino's Avatar
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Epoxy excess and strength


Are epoxy rich composites are much less strong than the ones made with minimum necessary amount of epoxy?

Obviously, some strength will be lost, but how much?

For instance, will be DIY unidirectional carbon composite strips loose a lot of strenght due to excess epoxy, say if I put 1.5 -2 times more epoxy than necessary (I do not have necessary equipment to remove all excess epoxy)?
Last edited by truffaldino; Aug 18, 2021 at 08:38 AM.
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Aug 18, 2021, 04:06 PM
San Antonio TX.
sensei's Avatar
Quote:
Originally Posted by truffaldino
Are epoxy rich composites are much less strong than the ones made with minimum necessary amount of epoxy?

Obviously, some strength will be lost, but how much?

For instance, will be DIY unidirectional carbon composite strips loose a lot of strenght due to excess epoxy, say if I put 1.5 -2 times more epoxy than necessary (I do not have necessary equipment to remove all excess epoxy)?
You are better off to remove the excess epoxy/weight.

The real strength comes from the cloth layer/layers, twice the epoxy means you most likely have a ply, or plies that have floated from the surface, or from one another.

Best policy is a layup that is not resin rich or too lean.
Just my opinion though…

Bob
Last edited by sensei; Aug 19, 2021 at 06:57 AM.
Aug 18, 2021, 11:56 PM
Diesel Danny
danny mz's Avatar
Bob is 100% correct.

The strength comes from the mechanical properties the cloth or carbon fibres. Epoxy just holds all the bits in place and any excess adds weight, not strength
Aug 19, 2021, 10:59 AM
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truffaldino's Avatar
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Quote:
Originally Posted by danny mz
Bob is 100% correct.

The strength comes from the mechanical properties the cloth or carbon fibres. Epoxy just holds all the bits in place and any excess adds weight, not strength
I have another question.

I saw on the web that people sometimes uses CA glue instead epoxy if small pieces of reinforcement are needed. In principle, CA glue also holds bits, but how strength of such "quick" composite is compared to the epoxy based one?
Aug 19, 2021, 11:24 AM
San Antonio TX.
sensei's Avatar
Quote:
Originally Posted by truffaldino
I have another question.

I saw on the web that people sometimes uses CA glue instead epoxy if small pieces of reinforcement are needed. In principle, CA glue also holds bits, but how strength of such "quick" composite is compared to the epoxy based one?
When making repairs on things like wheel pants and cowlings, medium ca works great once you get them cleaned very good with acetone and then soapy water!

Tape the OML damage site, pre cut your glass pieces, add medium ca, lay your first ply in place and using your finger tip rub the glass down into the wet ca.

Ad additional ca as needed to wet out.

Repeat plies as needed, generally two plies of 6oz. is more than adequate to get the job done.

A couple last things… Practice on a scrap first to get a feel of it because you will need to work fairly fast, and you will need to have paper towels and acetone handy.

Do not use kicker until you are completely done with your repairs, then you can light it up!

By the way, it works kick a$$, been doing it that way for over 20 years.
Aug 19, 2021, 04:38 PM
Entropy is happening!
Jim.Thompson's Avatar

Strength to weight ratio.


Quote:
Originally Posted by truffaldino View Post
Are epoxy rich composites are much less strong than the ones made with minimum necessary amount of epoxy?
Obviously, some strength will be lost, but how much?
...........................................
The replies above are informative and I agree with all of them.
However, they don't answer the question about loss of strength.
It is my understanding (mainly assumption and deduction) that what is lost is the strength to weight ratio. Not the strength.
I have not seen a scientific evaluation of this claim, but this in my mind, makes more engineering sense.

I welcome any evidence to refute this and put my claim to rest, as I have been wondering about it ever since beginning composite practice.

Jim.
Last edited by Jim.Thompson; Aug 19, 2021 at 11:24 PM.
Aug 19, 2021, 07:59 PM
Registered User
If we consider load sharing between adjacent strands of fiber, the epoxy in between is stressed in shear. I wouldn't be surprised if it turns out that a thick layer of epoxy has less shear resistance than a thin one. I'm too lazy to do the math or practical experiment to prove it, though.
Aug 19, 2021, 11:24 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by jkettu
If we consider load sharing between adjacent strands of fiber, the epoxy in between is stressed in shear. I wouldn't be surprised if it turns out that a thick layer of epoxy has less shear resistance than a thin one. I'm too lazy to do the math or practical experiment to prove it, though.
That's logical. Good thinking!
Aug 20, 2021, 07:07 AM
San Antonio TX.
sensei's Avatar
Quote:
Originally Posted by jkettu
If we consider load sharing between adjacent strands of fiber, the epoxy in between is stressed in shear. I wouldn't be surprised if it turns out that a thick layer of epoxy has less shear resistance than a thin one. I'm too lazy to do the math or practical experiment to prove it, though.
This is the exact reason why vacuum bagging processing, and pressure vessel processing is so important, it’s not just to remove weight, it’s to consolidate the fiber content while removing excess resin. I have been processing pre preg control flow systems now for over 30 years for this very reason.

38-42% resin to fiber content is used in many pre preg mil specifications.

Bob
Last edited by sensei; Aug 20, 2021 at 06:36 PM.
Aug 20, 2021, 11:39 PM
Registered User
Roguedog's Avatar
Quote:
Originally Posted by truffaldino View Post
Are epoxy rich composites are much less strong than the ones made with minimum necessary amount of epoxy?

Obviously, some strength will be lost, but how much?

For instance, will be DIY unidirectional carbon composite strips loose a lot of strength due to excess epoxy, say if I put 1.5 -2 times more epoxy than necessary (I do not have necessary equipment to remove all excess epoxy)?
If you're seeking optimum mechanical performance then adding more epoxy resin will just cause cracking as the strength comes from the fiberglass or carbon fiber cloth.

The optimum amount, as sensei has mentioned, is in the 38-42% area if resin to cloth.

If you want more strength then add another layer of carbon strips and keep the resin content below 40%.

If you want actual mathematical formulas seek out a structural engineer.

Here's link that describes the basics for stromg composite laminate - https://theepoxyexperts.com/fiberglass-layup-technique/
Aug 22, 2021, 03:26 PM
Registered User
In the video, they mention 22-27% for best strength. This seems very low. What would be a typical application for such a lean layup?

Lars

Quote:
Originally Posted by Roguedog

Here's link that describes the basics for stromg composite laminate - https://theepoxyexperts.com/fiberglass-layup-technique/
Aug 22, 2021, 04:58 PM
Scott
Pylonracr's Avatar
Quote:
Originally Posted by LN-JET
In the video, they mention 22-27% for best strength. This seems very low. What would be a typical application for such a lean layup?

Lars
The resin content varies greatly depending on the specific gravity of the resin and the weave of the fabric. I have seen Ideal resin ratios between 22% and 41%. There is no one answer. Any fabric / resin combination will achieve its maximum strength at the ideal resin content. The certification that comes with the fabric will give you the starting point for resin calculation.

Or you can cheat and oversaturate the fabric, use peel ply and a perforated bleeder with an absorbent, and bag it. Let the vacuum pull the excess resin out.

Scott
Aug 22, 2021, 05:09 PM
Registered User
Roguedog's Avatar
Good question! I'll see if i can get a hold of the engineer who made the video and ask him. It may take a few days though.

He also happens to be the owner of Polymer Composites, Inc, where I buy my epoxy resin.
Aug 22, 2021, 05:24 PM
Beater tester
gizzo's Avatar
Quote:
Originally Posted by Pylonracr

Or you can cheat and oversaturate the fabric, use peel ply and a perforated bleeder with an absorbent, and bag it. Let the vacuum pull the excess resin out.

Scott
Can that cause the layup to become too dry at all? Or will the resin stop getting pulled out of the layup when it reaches it's ideal saturation?

It's how I (and just about everyone else) do it and it seems to work but sometimes I wonder...

Cheers
Aug 22, 2021, 07:23 PM
San Antonio TX.
sensei's Avatar
You can use different size P release media’s directly on the back of the last ply, or on the back of the peel ply to control resin flow into the blotter, or blotter/breather ply inside the vacuum bag.

Bob


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