Folks:Warning! Science.Warning 2! Amateur.
(I'm trained as Wheeljack, not Quickmix.)
You have been warned. Now prepare for the geekstorm.WHOA.
This is awesome to the max. UV-catalyzed peroxide bleaching! Now we need to figure out what's happening.
I think I've got a partial theory.
Photooxidation creates chains of alternating single-double carbon bonds in the hydrocarbon molecules - the conjugated unsaturation mentioned in the NES article below. This forms chromophores in the molecules, color-causing regions, turning the molecules into dyes. Chromophores formed by conjugated systems...are yellow!
The hydrogen peroxide bleaches these by breaking double bonds and forming an epoxide group - an oxygen atom bonded to two adjacent carbons, which remain bonded to each other by a single bond. This turns the chromophores colorless.
Now, here's what I'm wondering -
-where in the ABS structure is the chromophore forming? The polybutadiene chain or the cross-chains?
-brittleness (due to UV-induced cross-linking) and yellowing seem to be separate modes of deterioration. What effect might this relatively fast UV-catalyzed oxidation have on cross-linking?
-how does the UV light catalyze this bleaching, if indeed it is doing that?
There's a LOT of papers on photooxidation and photodegradation of polymers! I have been browsing abstracts, because I don't want to buy the actual papers. If anybody has access to polymer science journals, it would be well worth a look. I think there is a neat puzzle here for the chemists here to figure out! I know you're out there.
One thing I can suggest from an engineering POV is a test for brittleness. Take some identical parts and treat them, then do a tension test to see how easily they break, and examine the broken ends for signs of embrittlement. Compare yellowed parts, bleached yellowed parts, and unbleached, unyellowed parts. I think a good source would be a scrap computer case; they're flat, so you could get good test specimens, and they're usually ABS.
Nerdy excitement! Let it spill into the streets.
Edited by Autobus Prime, 08 December 2008 - 12:58 PM.