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All I can think of is that a black cap won't degrade as quickly because of ultraviolet exposure. Most plastics (long polymers) have chemical bonds that can be b
by lutusp 13y ago
All I can think of is that a black cap won't degrade as quickly because of ultraviolet exposure. Most plastics (long polymers) have chemical bonds that can be broken by the energy level provided by ultraviolet photons, so over time they turn from polymers into quasicrystals and lose their strength -- disintegrate. But a black plastic, by excluding the ultraviolet light, protects itself and lasts much longer.
So a bottle that's composed of a clear plastic that naturally resists ultraviolet degradation (there are several) should be capped by a more rigid plastic (to maintain its shape). So the cap needs to be black (or at least a dark color).
A reflective cap would work just as well at first, but plastics are made reflective by being painted, and the paint soon falls off, because of ... wait for it ... ultraviolet exposure.
- raldi 13y agoI thought black objects absorb all wavelengths, and white objects reflect them all away.
- eru 13y agoReal world materials are rarely ideal black bodies (or ideal reflectors).
- raldi 13y agoSure, but to the extent they're imperfect radiation absorbers, they're also imperfectly black. It's like how we can discuss pi and circles even though the real world has no perfect circles.
- lutusp 13y ago> Real world materials are rarely ideal black bodies (or ideal reflectors). That's true, but it's also true that all objects eventually reach thermal equilibrium with their surroundings. Were this not the case, it would violate the law of energy conservation, and that's a law that's never broken. In that sense, most objects, solids and plasmas in particular, are reasonable black bodies once they achieve equilibrium. Some of them show spectral lines -- emission, absorption or both -- but in most cases those lines appear on top of a blackbody curve. The sun, for example -- perhaps surprisingly, it exhibits a classic blackbody spectrum appropriate to its surface temperature, with many spectral lines contributed primarily by its comparatively low-pressure atmosphere (http://commons.wikimedia.org/wiki/File:Solar_Spectrum.png http://commons.wikimedia.org/wiki/File:Solar_Spectrum.png). The state of matter least like a black body is a low-pressure molecular cloud in space, which often shows only emission and absorption lines, no classic blackbody curve. The lowest pressure gases show the narrowest spectral lines, which is how we can estimate the pressure of a remote cloud of gas illuminated by distant energy sources.
- mattgrice 13y agoWhite objects are not perfectly reflective and whiteness is not so much a function of reflecting them well, but reflecting them relatively uniformly in the visual range. Not every white object reflects UV. Black plastic lasts longer because 1) the UV is absorbed by the pigment, not the polymer and 2) the pigment limits UV penetration to the surface. Here is a good doc on UV weathering of plastics and materials (such as carbon black) to absorb UV. http://www.cabot-corp.com/wcm/download/en-us/sb/UV%20WEATHERING.pdf http://www.cabot-corp.com/wcm/download/en-us/sb/UV%20WEATHER...
- lutusp 13y ago> I thought black objects absorb all wavelengths, and white objects reflect them all away. The point about a black piece of plastic is that the energy is absorbed at the surface -- it can't penetrate the plastic to any significant depth. That means the majority of the plastic is protected from the ultraviolet wavelengths that would destroy it.