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Long term integrety of Epoxy

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ToddK

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Even Texans, huh?
It would be easy to make a bit of fun of the inferiority complex often displayed by our neighbors to the north, but the truth is that he is absolutely right. When you see a white Long EZ from Texas you know there is a builder out there who supresssed his inner desire to put a giant Lone Star flag on there, or a A&M/UT color scheme in the interest of saftey.
 

BrianW

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I was told by a wood kit company owner, it’s not 140 inside the wing. The surface might be 140 but by the time the temperature gets to the first glue joint it’s only 100. Wood is good at insulating. If it was steel, I would be worried. Big heat sink then.
I think he has it the wrong way round!
 

TFF

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I don’t know, but he is a pretty well known kit plane company and high end aircraft restorer. I believe he is also a R&D engineer for marine engines.
 

fly2kads

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I'm of the type that likes data. (Sorry, not sorry. Count me in as one of those nerdy types.) I was curious if BJC's simple, one-sentence summation (T-88 @ 180 F > spruce) would hold up to more detailed scrutiny. Yes, it does! Once again, ANC-18 has the goods. Section 2.91 is a discussion of glued joints. It classes glued joints into essentially two types:

1. Solid wood members with grain parallel to one another. The allowable stress is simply equal to the shear strength parallel to the grain. If the glued members are of different species, use the weaker of the two.

2. Pretty much everything else. (Solid wood to plywood, plywood to plywood with grain perpendicular, solid to solid with grain perpendicular.) The allowable stress is 1/3 of the shear strength parallel to the grain, of the weaker species in the joint.

The value for shear strength parallel to the grain is to be taken from column 14 of table 2.3, which is the main table of mechanical properties of solid woods. For spruce, this is 850 psi, and 1/3 of that is 283 psi.

So T-88, even at the elevated temp of 180 F (1000 psi), exceeds the values for spruce. Just like BJC said! (Yes, back on page 1.)

ToddK, your answer would appear to be #1.
Unless I am not seeing something, the only reasonable options are 1.) even at reduced strength due to heat, the epoxy is still sufficient
(Sorry, sometimes I have to figure things out for myself. I get there, eventually.)
 
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MadProfessor8138

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Not to put a bullseye on my back,buuuut....I'm going to ask a dumb question anyways.

I understand that epoxy will lose strength as the temperature rises.
I also understand the FOS aspect.
And last but not least...I understand the temperature variations that you can get with different colors.

With that being said.....

Will the epoxy return to full strength once the temperature has decreased ?
And.........if it does....
How many heat cycles can it go through before it no longer returns to full strength with the temperature decrease ?

Asking for a friend......lol

Kevin
 

ToddK

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Not to put a bullseye on my back,buuuut....I'm going to ask a dumb question anyways.

I understand that epoxy will lose strength as the temperature rises.
I also understand the FOS aspect.
And last but not least...I understand the temperature variations that you can get with different colors.

With that being said.....

Will the epoxy return to full strength once the temperature has decreased ?
And.........if it does....
How many heat cycles can it go through before it no longer returns to full strength with the temperature decrease ?

Asking for a friend......lol

Kevin
Seems like it really doesn’t matter, as full strength was not necessary to begin with, only sufficient strength.
 

fly2kads

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Will the epoxy return to full strength once the temperature has decreased ?
And.........if it does....
How many heat cycles can it go through before it no longer returns to full strength with the temperature decrease ?
I had the same dumb question. From what I read, at the range of temps we're talking about here, it will return to full strength. Apparently, if you don't get it hot enough to actually break down the polymer matrix, it will retain its resiliency.
 

Yellowhammer

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I have often pondered this question.

My thoughts are that the temps will be higher when sitting out in the sun. However, in flight, wouldn't the cooling of the air help this situation?



My next thought is the "wetted" areas of the aircrafts surfaces in flight. I know there is a small amount of air that makes contact but there still would be a cooling factor or am I wrong?

Thanks for the clarification fellas!
 

Yellowhammer

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The kit I am building (Pulsar I) is a FRP and I really want to paint it silver. My thinking is that silver would have a fair amount of reflective properties to it and help with solar heating of the epoxy.

I really don't like the idea of being limited to a white only color.

Anyone think I could get away with Silver?

Also, I think it has a lot to do with how the materials were cured. Whether or not an enclave and post cured should make a difference.

All of these questions would be properly answered by Billski.
 

BrianW

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The kit I am building (Pulsar I) is a FRP and I really want to paint it silver. My thinking is that silver would have a fair amount of reflective properties to it and help with solar heating of the epoxy.
I really don't like the idea of being limited to a white only color.
Anyone think I could get away with Silver?
/snip/
Yes!
Pigments incorporating the oxides of magnesium (white) or aluminum (silver) have similar reflective properties.
 

BJC

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The kit I am building (Pulsar I) is a FRP and I really want to paint it silver.
Be sure to use a primer that reflects uv, to protect the epoxy.

Visible colors are not what is important for limiting temperature; IR absorption / reflectivity is. There are examples of glass airplanes painted yellow, red, shades of charcoal, etc. One pilot of a dark colored airplane rationalized “It cools off from the propeller blast before takeoff.” The epoxy that you use, and the cure temperature atr relevant, as you said. Bottom line: do research beyond what you read here, and make your d3cision. Remember, E-AB is about experimenting and learning.


BJC
 

Lendo

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To my knowledge Post Cure of Epoxy will complete the cross-matching of the molecules (resin and hardener) to give the highest strength and resistance to heat soak to a certain temperature. You need to check what that temperature is.
Without Post Cure the Epoxy remains under cured and under strength, I've heard of wings distorting when left out to heat soak in the open.
I would suggest different Epoxies have different values for Post Cure, so too does the foam in a foam sandwich, where some foams out-gas earlier than others, which can lead to delamination.
Although Epoxy falls victim to UV degradation it doesn't happen overnight, however Epoxy is the strongest and has the highest adhesion, compared to Polyester and Vinyl-ester.
Hope that helps.
George
 

ToddK

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To my knowledge Post Cure of Epoxy will complete the cross-matching of the molecules (resin and hardener) to give the highest strength and resistance to heat soak to a certain temperature. You need to check what that temperature is.
Without Post Cure the Epoxy remains under cured and under strength, I've heard of wings distorting when left out to heat soak in the open.
I would suggest different Epoxies have different values for Post Cure, so too does the foam in a foam sandwich, where some foams out-gas earlier than others, which can lead to delamination.
Although Epoxy falls victim to UV degradation it doesn't happen overnight, however Epoxy is the strongest and has the highest adhesion, compared to Polyester and Vinyl-ester.
Hope that helps.
George
I actually asked they guys at System Three if T-88 benefited from a post cure and they said it did not. Many laminating epoxies do, just not T-88.
 

JimCrawford

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The most used epoxy resin for glider construction and repair in Europe is Scheuffler L285. The data sheet shows comprehensive information regarding the epoxy properties and post curing. For structural repair schemes it is usual to find that the type certificate holder mandates both this resin and the post cure schedule.
 

Yellowhammer

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Great information being shared here!

My question is do I need to apply the UV protector as a primer?

Also, don't most paints come with built in UV protection these day or is that not enough?

Back in the day, folks used fine lamp black powder and added it to their primer coat. Is that still the best method of protection?

Thank you all for your excellent comments and shared knowledge.

Yellowhammer
 

Yellowhammer

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Yes!
Pigments incorporating the oxides of magnesium (white) or aluminum (silver) have similar reflective properties.

Thanks again for your comment! Lets me know I am on the right track. I will try to post a picture of the paint scheme I saw one Pulsar builder in Switzerland did in sliver. Looked really good.
 
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