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