Talk:Photodegradation

I asked a chemist...

...And he told me that the hydrogen peroxide basically destroys the sun-damaged top layer of the plastic, thereby revealing the correctly-colored layer below. Thus, this treatment isn't infinitely reproducable.--Nevermore 08:31, 20 April 2010 (EDT)

...without damaging the paint? --Detour 09:24, 20 April 2010 (EDT)

It basically destroys a layer of the plastic. If the same problem occurs again, and you repeat the process, another layer is destroyed. If you repeat that often enough, there will be no plastic left.--Nevermore 05:33, 21 April 2010 (EDT)

I'm not so sure about that explanation, Nevermore. It doesn't take much etching for its effects to become visible, and the gloss would certainly change, but the treated plastic I've seen in pictures remains glossy and unchanged except for color.

I got a different one from Dr. Rudy Deanin, remember:

(From the Allspark)

QUOTE(Dr. Rudy Deanin) In answer to your December 8 inquiry, the 3 monomer units of ABS all contain somewhat unstable atoms or groups which are activated by UV + atmospheric oxygen (photooxidation). They all contain double or triple bonds, which absorb UV. When one C=C bond absorbs light, it is only in the far UV, so we don't see it. It activates an adjacent group and forms another (conjugated) C=C group. This C=C-C=C (allylic) group is more active, so it forms a third, and so on. As this conjugated (-C=C-) chain grows longer, it absorbs longer and longer wavelengths. When it is about 6 C=C groups in a row, it reaches the wavelength of visible light, and we see the degradation. As it grows till longer, it absorbs mores and more visible light, and the color grows darker and darker. The reason peroxide and sunlight cause bleaching is that they destroy a C=C bond and break the conjugated chain.

Embrittlement is caused by crosslinking. When UV and/or oxygen attack an unstable -C:H bond, they split it into -C. and .H radicals. When 2 adjacent polymer molecules have -C. radicals, they pair and form a -C:C- crosslink.

That's my simple-minded explanation. If you read very far into the literature, they get much more complicated, sometimes even contradictory.

Rudy Deanin