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.

Isn't there some kind of material you can layer the surface with after the peroxide treatment that protects it from UV light? And if the discolouration is caused by factors other than light, it might actually just be better to paint it over. Takeshi357 (talk) 03:05, 30 August 2014 (EDT)

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

-It's possible that the use of H2O2 could cause crosslinking...would be interesting to compare the probabilities of the different reactions. I should go ask my chem engineer buddies... --Autobus Prime 13:31, 9 July 2010 (EDT)

Updates

Anyone willing to go through, examine and evaluate the more recent first-hand reports on the peroxide treatment?--Nevermore 09:06, 22 April 2011 (EDT)

Modern examples

My six inch Titanium Ultra Magnus has visible discolouration. Some spots even have visible lines between discoloured and non-yellowed plastic. Takeshi357 (talk) 03:05, 30 August 2014 (EDT)

A more recent example would be BotCon Gigatron, who some report has some yellowing parts. But I'm not sure we need additional examples. --ItsWalky (talk) 03:29, 30 August 2014 (EDT)

Possible causes

This may not be true of all cases, but there is a known cause for yellowing in some plastics. In particular, bromine is what made the SNES prone to yellowing. Some types of plastic yellow as a simple result of UV exposure. These tend to stay on the pale side and can look spotty depending on exposure. Brominated plastics, however, are a result of the fire retardant chemical added to them not being held en matrix properly and migrating to the surface of the plastic. In the case of brominated plastics, they can yellow even if they aren't exposed to UV light. They tend to be more of a yellowish brown. This is what happened with the SNES, in fact. --King Starscream (talk) 14:47, 22 December 2015 (EST)

Recent occurrences on the wrong page?

Does anyone else think we should move the unpaintable plastic's section on recent occurrences of photodegradation here, and vise versa? I think it's weird that the page covering the phenomenon in more detail is the one not primarily about said phenomenon. ShootingStar7X (talk) 21:25, 3 May 2023 (EDT)