Paint Colour

[Pblais]

heat absorption is a separate issue from fading. It would be a mistake to assume the two go together. The reaction of the pigments to UV causing fading are not so closely related to heat absorption but it might play a part. For example white pigment often based on Titanium is a very resilient pigment while green is very unresistant. Dark blue does far better than green. That is the only test I know of that crosses many paint types that also would factor in there as well. Green pigments seem to fade substantially faster than Navy. This all goes upside down when you talk Sunbrella pigments as the blue and greens stay brighter longer than the reds while paints are opposite.

[MidLandOne]

In response to Alan's post -

The carbon black addition to plastics to absorb UV as I posted is definitely correct - I have double checked that and there are plenty of references to it on the internet.

As I said, with paints I am not sure what goes on and in the interests of brevity I perhaps did not make that clear enough but did ask for explanations. So I will launch into why I am not sure and why I remain unclear. Sorry, it is lengthy but I cannot think of any other way to set out the background for what posted and why I asked the question I did.

At UV (ie wavelength less than 400nm, so below blue light) and for purely subtractive surfaces (reflection relied on eg red surface reflects red light) it is correct to say black reflects UV (as well as absorbs), but all colours reflect at UV and black reflects it less than any of the others. So, for example, green paint reflects green light and UV (but it also absorbs some UV too) - and as red and yellow is furtherest away from blue they reflect blue and UV more than other colours reflect blue. A perfect white reflects all visible wavelengths as well as UV.

That was the basis of my comment that black absorbs and white reflects UV.

Now moving onto paints which get their colour from pigments. One assumes that the design of paints from a colour perception point of view ignores UV (ie wavelengths shorter than 400nm) and infrared because they cannot be seen. But paints are often (always?) stabilised against UV from a servicibility point of view by the addition of stabilizers.

Carbon black and most (all?) black organic molecules absorb UV (not reflect) and both are used as UV stabilizers in various applications. Carbon black is also common pigment used in the production of black paint and so one would expect that such black paints will both absorb UV and be stabilised for UV by the carbon black (just like carbon black does in other plastics). I assume, but don't know, that other pigments are likely to be organic and so also absorb UV.

UV stabilizers may be transparant at visible light (an example of a material that transmits almost all visible light but prevents the passage of most UV is glass) so it may be that such stabilizers exist, and which give stability by reflecting UV, and are added to paints. I don't know but as far as I know all organic molecules absorb at UV and plastics are organic molecules so if the stabilizer is organic it will absorb, not reflect UV.

So, as I see it then, if carbon black is the pigment and its absorptive effect is not overtaken by another stabilizer that is reflective then such a black paint is likely to get its UV resistance from absorption of UV by the carbon black and not by reflection of UV. Also, if the stabilizer is an organic molecule then it would seem to me that the UV protection will be by absorption. But as I said, it may be that reflective stabilizers are widely used in paints but if so I am unsure.

Getting now to white paint and whether it reflects or absorbs UV. A common pigment in white paint is titanium dioxide which is, like carbon black, a strong absorber of UV and again like carbon black gives UV protection in paint by absorption rather than reflection. This may be why, as I alluded to in my last post, it is common to paint exterior plant pipework and tanks white - the titanium dioxide gives UV protection by absorption of UV and heat protection by reflection of infrared.

Note I am not saying (and did not say in my earlier post) which of reflection or absorption UV stabilisation in paints is based on - just laying out the reasons I am unsure. It also outlines (in a lengthy way ) the reason in my last post I ended by asking if anyone has any professional knowledge of the matter. It seems to me that the answer is not as straight forward as it might seem on first blush.

Hopefully, the answer adds something to the discussion as to the desirability of black paint on boats .

[Alan Wheeler]

Thanks John. great post, excellent reading.

[Cowboy Sailer]

This thread started with the question of paint color versus interior temperature. It drifted into fading and ultraviolet adsorption or absorption. Quick now, what is the difference between adsorption and absorption? (Answer at 10:00!) It turns out that this question is something I dealt with in the laboratory. Satellites in space have only one way to get rid of heat, by radiation, there is no conduction or convection in space. It turns out that color is not the determining factor. For temperatures near room temperature the determining factor is the ratio of absorptivity at .55 micron (the peak wavelength of the sun) to the emissivity at 8 to 10 microns (the peak emission wavelength of a body at near room temperature.)
I remember how hot my polished aluminum airplane was compared to a painted airplane of any color because polished aluminum has an extraordinarily low emissivity at 8-10 microns. Visually, polished aluminum has very low absorptivity at visible wavelength. It is highly reflective so it can’t be absorbing much. But the interior temperature is determined by the ratio of the skin’s absorbtivity at short wavelengths to it’s emissivity at long wave lengths. Unfortunately one cannot judge the emissivity of a surface by its visible color.
A practical example of this is the coating of solar collectors with black chrome. Black chrome has low emissivity at long wavelengths but high absorption at visible wavelengths. It is not even real black. Hold a bunch of needles in a bundle and look at them point on; they look black even though they are made of brightly polished metal. Surface smoothness at the microscopic level has a large effect on the interior temperature of a vessel exposed to visible light.
The airplanes used in atomic bomb testing were painted white and not left bare aluminum to lower their energy absorption. It really didn’t have to be white but it looked nice. All this to say that color matters a little but one can’t judge the interior temperature on the basis of the surface color only. Once again I told you way more than you wanted to know! Some people just don’t know when to shut up!

[Alan Wheeler]

CS, great post as well. But I am not sure if it answered or just opened more questions:-)
I am not going to ask though.

[GordMay]

Would MidLandOne and/or Cowboy Sailer further explain (pull together) some of the applications of Radiant Energy Transfer theory & practice?

Energy transferred from a warm body to a cooler body, by radiation, is described by Planck's Law of Black Body Radiation which describes radiant energy as a function of wave length.
Radiant energy is proportional to the fourth power of the absolute temperature of the source. The amount of radiation produced by a perfect radiator is expressed by the Stephan-Boltzman Law.
Wave length is inversely proportional to temperature of the source. The relationship of the wavelength of maximum intensity of a black body to its absolute temperature is expressed by Wein's law.
The Stephan-Boltzman Law gives the total energy emitted, but as the temperature rises, more of the energy is emitted at shorter wavelengths as given by Wein's Displacement Law, and shown by the Planck blackbody curves.

[MidLandOne]

[quote=Cowboy Sailer;165358]Quick now, what is the difference between adsorption and absorption? (Answer at 10:00!) [/quote]

Absorption is the noun naming the process of absorbing.

Adsorption is the holding of molecules in or on a surface.

[MidLandOne]

Gord May you set some tough tasks .

I'll cheat a little bit and say that Black Bodies and Black Body radiation is well described in the "Black Body" Wiki Black body - Wikipedia, the free encyclopedia and some examples of "black body radiation" (in quotes as the term is used loosely for radiation from non black bodies) are given at SPI / library / blackbody radiators.

The Wiki on "Emissivity" Emissivity - Wikipedia, the free encyclopedia is also useful as is the one on "Absorption (electromagnetic radiation)" Absorption (electromagnetic radiation) - Wikipedia, the free encyclopedia.

I will just emphasise that no black bodies are known to exist ie all known bodies are imperfect absorbers and imperfect emitters (defined by their absorptivity and emissivity) and that for a black body to exist it has to be in thermal equilibrium with its environment eg the environment has to be able to absorb the radiation it emits and provide the radiation it absorbs with no change.

Also the examples ignore convection and conduction - both important in the case of a boat. And also important in the boat case is that while the radiation falls on the outside of the hull, heating it, the hull will reradiate from both its inside surface and its outside surface (and there will be convection on both surfaces also and conduction at least to the sea).

I will also just claim that dark paints rise to a higher temperature when exposed to sunlight than lighter coloured paints do - that because all the books say so and while the temperature will depend on the difference in their emissivity and absorption (ie they are not perfect black bodies) as described by Cowboy Sailor this difference tends to more absorptive for darker colours all else being equal (eg reflectivity).

So a dark coloured hull will, in general, rise to a higher temperature than a lighter coloured one - it is simple physics to understand that this will result in heat transfer to the interior of the boat through radiation, conduction and convection from the inside hull surface. Unless ventilated or artificially cooled that will continue until the interior reaches temperature equilibrium with the hull.

I get the feeling I have cheated more than a little bit .

[Cowboy Sailer]

Yeah, color matters. Surface roughness down in the scale of the wavelengths of interest also matters. Most paints are similar in surface roughness so comparing paint color on identical surfaces is valid. Gord, you are exactly right, but painted surfaces are not perfect black bodies at room temperature. There is a law that a good adsorber is a good emmitter. What is usually forgoten is that what is important is that we are interested in absorption at one wavelength but emission at a much longer wavelength. The fact that there is such a disparity of wavelengths means that what is intuitive is sometimes wrong. The emission at long wavelenghts can't be judged by is color at visible wavelengths. In the case of airplanes it really doesn't make much difference in which glossy color the aluminum is painted, just that it is painted. Flat black may not give higher temperatures than glossy black all else being equal. They are both black but have differing emissive properties and it is the ratio that is important.
All this to say that I bet a black hull can be made that is as cool as a white one. I don't know how, just that it is theoretically possible. So let the man paint his hull whatever he wants! He just has to invent a new paint/surface preparation technique. LOL

[Cowboy Sailer]

And MidLandOne is the man!

Vacuum bottles have the glass walls silvered not to "reflect the heat" but because the shiney metal (actually Aluminum) has such low emissivity. So few molecules to transfer heat by convection and conduction and low radiant transfer equals low heat loss or gain. Your coffee stays hot and your cold drink stays cold (even if you paint the outside black!)

[GordMay]

Originally Posted by MidLandOne

Gord May you set some tough tasks ...

Originally Posted by Cowboy Sailer

... Gord, you are exactly right, but ...

Which is why I ask you two, who know more than I.
My only experience is with Radiant Comfort Heating (electric & hydronic) applications.

[Stevens 47]

My understanding is that epoxy and heat are a bad combination. I like the look of dark top sides. But have gone away from them because of the heat inside the boat and the possibility of damage to the epoxy in my laminate.

[Zach]

This thread is a great read! (Finally, the explanation for why chrome shift knobs are dangerous...)

Questions:

If something that is highly polished radiates slower than something with a dull finish, does a rough surface has a higher emmisivity? Once the energy is absorbed, does emmisivity go both ways, like radiating to the deck it is painted on?

Does this extend to the visible spectrums too? Some white paints are capable of turning retinas to dust... do flattening agents reduce emmisivity? Or just reflectivity?

Going between layers for a moment... If emmisivity works on both sides of the paint, can I spray a really low emmisivity product to act as insulator between the paint and the fiberglass? (Hollow ceramic spheres to insulate, or a sheet of aluminum foil to reflect... perhaps some other space age goodies...)

Or... do these paints emmisivity ratings only work when they are the outer coat?

If they need an air gap... hmm. Maybe I won't pursue gluing all that aluminum foil to the hull afterall. (grin) Gotta find the bubble wrap stuff... (Bigger grin.)

Thanks for your thoughts! The reflection off some white decks give me a headache without a black brimmed hat and sunglasses...

Zach

[sundown]

If you plan to be about anywhere in the night a Black Hull makes about as much sense as a white snow suit on a ski hill. You may look good but you are damned hard to see, especially if you just have a single anchor light on.

[Zach]

Anyone used either of these products?

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