Are airplanes more in need of cooling or heating? The outside temperature at cruising altitude is around -50° (roughly where the Celsius and Fahrenheit scales meet, btw), is the insulation good enough to just keep the inside at acceptable temperatures for humans?
Whitest paint is now thin enough to coat cars and planes
41–50 of 73 posts
Re: Whitest paint is now thin enough to coat cars and planes
#42Couldn’t this be revolutionary for spacecraft? Also would it make a better solar sail?
Colour me stupid, but in a simple white vs. black comparison, wouldn't a VantaBlack-clad solar sail absorb more energy, so be more useful?
- ed I'm misunderestimating 'equal and opposite' photon reflections here, aren't I?
Re: Whitest paint is now thin enough to coat cars and planes
#43Earlier quoted context omitted.
Nope. Silver reflectivity is about 95%, aluminum reflectivity is about 90%. So the best metallic mirrors are worse than the best white paints, even if they can be better than the frequently used white paints based on titanium white (89.5% reflectivity). Total internal reflection might be closer to 100%, but that cannot be used for the light that comes from the air on the surface of a mirror.
You can have dielectric coatings on a mirror that increase the reflectivity to >99.9% (and over a wide range of wavelengths). You can of course get a lot more 9's if you only care about a narrow band. https://materion.com/products/precision-optics/application-f... https://en.wikipedia.org/wiki/Dielectric_mirror
99.9% for visible light does not mean 99.9% for all the solar light.
Most dielectric coatings absorb ultraviolet light so they reflect much less of the total solar light, most likely less than 90%.
Such a mirror with dielectric coatings would not be able to cool the object on which it is applied, unlike the paints which are discussed in the article, which are based on boron nitride, barium sulfate, calcium carbonate or fluorinated polymers, all of which do not absorb ultraviolet light.
The common white paints based on titanium white or zinc white also absorb ultraviolet light, so they cannot be used for cooling an object illuminated by the Sun.
Re: Whitest paint is now thin enough to coat cars and planes
#44I know white clothing often contains "brighteners" which fluoresce, converting received UV light into visible wavelengths, so the resulting white is brighter than what is reflected in the visible spectrum.
I'm just curious what role that ever plays in paint, if this 98% reflectivity is a big deal, or if we already have white paint with over 100% because of brighteners.
Re: Whitest paint is now thin enough to coat cars and planes
#45Are airplanes more in need of cooling or heating? The outside temperature at cruising altitude is around -50° (roughly where the Celsius and Fahrenheit scales meet, btw), is the insulation good enough to just keep the inside at acceptable temperatures for humans?
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Re: Whitest paint is now thin enough to coat cars and planes
#46If you mixed it with Vantablack, would you get the greyest possible paint? /s
You can't mix Vantablack. It actually isn't paint, it is a coating grown through vapour deposition.
Re: Whitest paint is now thin enough to coat cars and planes
#47Are airplanes more in need of cooling or heating? The outside temperature at cruising altitude is around -50° (roughly where the Celsius and Fahrenheit scales meet, btw), is the insulation good enough to just keep the inside at acceptable temperatures for humans?
It does not matter for airplanes. Both cooling and heating are in ample supply on an airplane.
Re: Whitest paint is now thin enough to coat cars and planes
#48Re >> "with an ability to reflect around 98% of incoming sunlight" Seems like a mirror would reflect even more light. Now, obviously we don't want to make the exteriors of our buildings and cars a mirror! . but it made me wonder: What's the difference between a really really really white paint that reflects 98%+ light, vs. a reflective surface? Is it that the white paint reflects the light, but in a "scrambled" form,…
Nope. Silver reflectivity is about 95%, aluminum reflectivity is about 90%. So the best metallic mirrors are worse than the best white paints, even if they can be better than the frequently used white paints based on titanium white (89.5% reflectivity). Total internal reflection might be closer to 100%, but that cannot be used for the light that comes from the air on the surface of a mirror.
What I’ve seen says silver and aluminium are both a bit better than that.
https://www.researchgate.net/figure/Reflectance-versus-wavel...
https://www.researchgate.net/figure/Reflectance-of-Ag-Au-and...
I think this better fits my observation that a clean mirror can easily be unnoticed in an environment similar in all directions, such as woodland or a messy room.
Re: Whitest paint is now thin enough to coat cars and planes
#49Couldn’t this be revolutionary for spacecraft? Also would it make a better solar sail?
Colour me stupid, but in a simple white vs. black comparison, wouldn't a VantaBlack-clad solar sail absorb more energy, so be more useful? - ed I'm misunderestimating 'equal and opposite' photon reflections here, aren't I?
For solar sail you don't want to absorb, you want to reflect.
When you absorb you only get half of the push (assuming reflective surface reflecting directly back at 100% efficiency vs absorbed heat that gets radiated omnidirectionally).
Re: Whitest paint is now thin enough to coat cars and planes
#50I know white clothing often contains "brighteners" which fluoresce, converting received UV light into visible wavelengths, so the resulting white is brighter than what is reflected in the visible spectrum. I'm just curious what role that ever plays in paint, if this 98% reflectivity is a big deal, or if we already have white paint with over 100% because of brighteners.
This 98% reflectivity refers to the total solar light, including infrared and ultraviolet light, because the purpose is to cool whatever is painted, despite being illuminated by the Sun.
With fluorescent paints, the energy of the emitted visible light is always less than the energy of the absorbed ultraviolet light. The difference is heat that warms the paint.
The paints discussed in the article also reflect the ultraviolet light. Moreover, they also emit infrared light in the frequency band where the air is transparent, and that infrared light leaves the Earth if the painted object is outdoors, which will be true for a building or for a car while it is used. By emitting more energy than it is absorbed from the solar light, the painted object is cooled (if it is outdoors).