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Long term integrety of Epoxy

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speedracer

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Yes!
Pigments incorporating the oxides of magnesium (white) or aluminum (silver) have similar reflective properties.
The heat gain of different colors go like this: White, yellow and pink, light blue and aluminum (silver), purple and blue, light green, orange and tan, red and green, brown, black. I know of one silver Long EZ and one Berkut, but there's probably others. My Long EZ is yellow. So are the other two EZ's at the Hood River OR. airport.
 

Vigilant1

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Also, glossy absorbs less than flat finish. Finish can make a bigger difference than color.

As mentioned previously, some special pigments have differential emissivity.
They absorb thermal radiation like a lighter color but emit (radiate) longer wavelength more like a darker color. This keeps them cooler.
 

Mad MAC

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This may help......

View attachment 103559

Kevin
If we are resorting to that "temperature graph", the following from EASA CS-VLA is probably a reference with better standing

AMC VLA 613 (c)

Material Strength Properties and Design Values (Acceptable Means of Compliance)

Test Temperature –

a. For white painted surface and vertical sunlight: 54°C. If the test cannot be performed at this temperature an additional factor of 1·25 should be used.

b. For other coloured surfaces the curve below may be used to determine the test temperature.

Curve based on: NASA Conference Publication 2036

NASA Contractor Report 3290

Skin temperature of composites as a function of temperature.
1604032894098.png1604032894098.png
 

BrianW

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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
Only the IR frequencies are important in heating airframes: is this true?
Yes & no!
Here is the Solar Spectrum: high & low.
Notice that most energy per 'color' or delta F occurs in the visible. But notice there is a long tail into the infra-red whose area is somewhat larger than the area under the visible range - so the IR wins by a head! That smaller skirt into the UV also does a number on breaking bonds, where no amount of lower frequencies can.


Solar-Spectrum-at-the-Top-of-the-Atmosphere-and-at-Sea-Level.png
 

Vigilant1

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The incident wavelengths of solar radiation are one factor. But when deciding on a paint color to reduce heat absorption, we should consider that paint pigments are an example of subtractive color. A blue paint appears blue to our eyes because it is selectively absorbing the non-blue colors (red, green, etc) and what remains ( and is reflected to our eyes) is blue.
White pigment works well because light that appears white to our eyes is composed of all wavelength. That paint is reflecting all visible wavelengths and not preferentially absorbing any of them, and that results in a cooler surface.

For homework: Given:
1) A blue paint is preferentially reflecting short visible wavelengths and absorbing long visible wavelengths.
2) A yellow paint is doing the opposite.
3) Per the graph in Post 66, there's more energy in the shorter wavelength spectrum (even after atmospheric effects).

So, why do we observe that blue surfaces get hotter than yellow surfaces (post 65)?
 
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MadProfessor8138

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Ah hell...just to be safe... paint the thing with a pearl additive and block all the wavelengths.
Joking but not joking...because that brings up the question of whether or not pearl additives would be beneficial...the aluminum powder is..so why not pearl ?

Kevin
 

opcod

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There<s a guy who build up a cozy and it<s all painted in black. Color don't matter at all. It's all about the mission. Do you want to exposed and be the only one.., fly just local. All will be fine. But of course, if you stay outside, in about 3yrs lot of real test with c-t scan will be the best to do. But for 3hr flight per 2 week as the vast majority of people, it's ok.
 

Kyle Boatright

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There<s a guy who build up a cozy and it<s all painted in black. Color don't matter at all. It's all about the mission. Do you want to exposed and be the only one.., fly just local. All will be fine. But of course, if you stay outside, in about 3yrs lot of real test with c-t scan will be the best to do. But for 3hr flight per 2 week as the vast majority of people, it's ok.
I wouldn't try it in New Mexico in the summer...
 
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BrianW

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/snip/ paint pigments are an example of subtractive color. A blue paint appears blue to our eyes because it is selectively absorbing the non-blue colors (red, green, etc) and what remains ( and is reflected to our eyes) is blue.
White pigment works well because light that appears white to our eyes is composed of all wavelength. That paint is reflecting all visible wavelengths and not preferentially absorbing any of them, and that results in a cooler surface.

For homework: Given:
1) A blue paint is preferentially reflecting short visible wavelengths and absorbing long visible wavelengths.
2) A yellow paint is doing the opposite.
3) Per the graph in Post 66, there's more energy in the shorter wavelength spectrum (even after atmospheric effects).

So, why do we observe that blue surfaces get hotter than yellow surfaces (post 65)?

I looked over your homework. Couple of problems; assessing energy absorption on a painted surface by the color response of the eye is not a great idea.
Pigments which provide a particular color response may absorb frequencies either side to a varying degree.
Here's an assignment you may enjoy.
Locate a non contact thermometer.
Locate a Walmart parking lot on a sunny payday.
Note the temperature of each car roof in three categories: silver, white, blue.
When you have at least 16 values in each column average each column and draw your conclusion as to the difference in solar absorption between silver and white, and between either of those and blue.
 

Vigilant1

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Locate a non contact thermometer.
Locate a Walmart parking lot on a sunny payday.
Note the temperature of each car roof in three categories: silver, white, blue.
When you have at least 16 values in each column average each column and draw your conclusion as to the difference in solar absorption between silver and white, and between either of those and blue.
Interesting, but we'd need to adjust for some things.
1) A non-contact IR thermometer won't tell us the temperature of the roofs. They work by measuring IR radiation and making an assumption about the emissivity of the surface, then telling is how hot such a surface would have to be to radiate this amount of energy. If we have a white surface and a black surface of identical temperature, the IR thermometer will read higher for the black one. There are IR thermometers that overcome this by allowing a custom calibration for the actual surfaces being measured, but you need to know their true temp to perform the calibration.
2) Brightness vs hue: I suspect this is the biggest confounding factor. Saying a surface is blue or yellow (its hue) doesn't tell us much about its reflectivity. More at this link concerning hue, brightness, and saturation. A very light blue roof can be brighter, (i.e. more reflective in the visual spectrum) than a dark yellow roof.
As it happens, most blue cars are fairly low in brightness (doesn't have to be that way, but darker blues are more popular), and most silver cars are fairly high in brightness. So even if we got actual temps of car roofs and averaged them by color, we might find a stronger correlation if we instead grouped them by brightness.
 

autoreply

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What seems overlooked in this thread is that there are many different epoxies on the market. Plenty with a Tg of 180C (356F) that could endure operational temps just below that forever.

Room-temp epoxies without a post-cure... you could have permanent sagging/deformation if you heat it to 50C (122F).

Bottom line:
*Post-cure epoxy such that Tg is high enough. What high enough is depends on skin color, external (exhaust!) heating etc.
*Pick an epoxy that can achieve a sufficiently high Tg. Most quality epoxies can at least get to 80C (176F).

Post-cure for small parts is pretty simple. Some foam and an electric heater. Make sure ramp rate is low; if it's too high, dump some thermal mass (steel) in there.

For bigger parts/molds it's still not that hard.
 

BrianW

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Interesting, but we'd need to adjust for some things.
1) A non-contact IR thermometer won't tell us the temperature of the roofs. They work by measuring IR radiation and making an assumption about the emissivity of the surface, /snip/
2) Brightness vs hue: I suspect this is the biggest confounding factor. Saying a surface is blue or yellow (its hue) doesn't tell us much about its reflectivity.
#1 is a rational point - resolved with a thermocouple thermometer.
#2 cites brightness, which harks back to your preoccupation with visual cues: here's a definition:
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to luminance. This is a subjective attribute/property of an object being observed.....

But enough of this amusing discussion: if we say with a broad brush, that white and silver finishes tend to reflect well, it is safe to suppose that their substrate will be cooler than surfaces of other colors, even in Texas.
 
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