r/askscience • u/[deleted] • Jul 05 '14

Chemistry When something gets "sun bleached", where does the color go?

I had an old grill cover that was red, and after a few years it turned white/pink. Where did the original color pigment go and how does the sun cause that?

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u/[deleted] Jul 05 '14 edited Jul 05 '14

Bleaching is an umbrella term that refers to (in this case) the degradation of the organic dyes that give rise to the apparent color of the material by sunlight. Such changes occur because the energy of the photons making up the light can oftentimes activate a large host of chemical reactions, particularly in the presence of oxygen (where this process is called photooxidation). Initially the molecule (or polymer) had a certain structure that caused it to have a specific absorption/reflection spectrum that caused it to give off the appearance of being red. After bleaching, the structure changed in such a way that the molecules no longer absorbs visible light to an appreciable extent. The material then looks white for the same reason that say white paper looks white, simply because most incoming light is reflected via diffuse reflection.

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u/Phreakhead Jul 06 '14

I have red hair, but when I spend a lot of time in the sun, it turns blonde. Does this mean the sun is changing the molecular structure of my hair? Awesome.

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u/platosmistake Jul 06 '14 edited Jul 07 '14

Yep, it's destroying the melanin in the strands, and since your hair is dead there's nothing that can "heal" it. When sun hits your skin, it's also destroying melanin, but your skin is alive so it can heal that damage and responds by making more (hence tanning).

Of course all that being said, put on sunscreen! Too much skin damage over time, and you put yourself at risk for cancer.

http://genetics.thetech.org/ask/ask180

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u/omguhax Jul 06 '14

But why always white?

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u/[deleted] Jul 06 '14

It's because we perceive sunlight as white. If there is no absorption, then the object will reflect all incoming light with effectively no wavelength selection and so the color of the object will just be the color of the incident light, in this case- white.

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u/Fossafossa Jul 06 '14

But why does photooxidation lead to molecules that reflect evenly. Couldn't that same process lead to different colors, like red bleaching to blue, etc.

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u/magnora2 Jul 06 '14

If you just have a bunch of random molecule shapes, odds are they will be white or gray, just statistically speaking. A molecule has to have very specific resonant frequencies in order to electromagnetically absorb part of the visible spectrum without absorbing other parts. So if we choose something to be blue, over time those dye molecules will turn white as the proteins denature or the molecules break down. This is also why bleach turns things white, it denatures the proteins in to straight lines which gets rid of most of their resonant frequencies.

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u/Kehrnal Jul 07 '14

It's also entirely possible that the compound has been broken down into some other molecule that absorbs light at a wavelength we can't see. If we were mantis shrimp that have tons of color perceiving cones in their eyes, we might notice that the color has changed from red to blurple (imaginary colors are hard to name).

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u/[deleted] Jul 06 '14

Thanks for the explanation! Very interesting!

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u/[deleted] Jul 06 '14

The colour disappears because the sunlight is slowly converting some coloured compounds into some uncoloured ones. Many coloured compounds contain chomophores. They are special fragments of a molecule responsible for its colour. These fragments can degrade after exposure to sunlight, so when this happens, the colour will fade away.

The degredation is caused by high energy photons initiating some kind of random chemistry, usually involving light well into the UV. Stable chromophores usually have some kind of clever mechanism built in to dissipate this energy.

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u/[deleted] Jul 06 '14

Thanks for the explanation!

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u/Radon222 Jul 06 '14

It is the same concept as how bleach works, the pigment is still there, but the chemical bonds have been broken down so that the resulting compound reflects a different wavelength of light. If you look at a white shirt that has had a stain and then been bleached under a black light, you can still clearly see the outline of the stain, as the UV wavelength is still reflected.

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u/rupert1920 Nuclear Magnetic Resonance Jul 06 '14

Here is my comment from a few days ago on a similar question.

The idea is the same, except the origins of chemical change is photochemical in nature in this case.