06 May 2015, 01:01
Some more details about the process, for those who are interested. Sorry if I get too technical in the end xD .
ABS is basically a "mixture" of three polymers, those of acrylonitrile, butadiene, and styrene. Of the three, it's the polybutadiene phase that's most vulnerable. Exposure to UV leads to a discolouration of the ABS as well as a considerable loss of impact strength. Once this degradation has started, it can be reversed as well as combated, but will over time always return.
When exposed to UV radiation, the methylene bond in the trans-butadiene structure is photolysed, which yields an allylic radical. This species can then react in several ways, either forming more radicals, or reacting with molecular oxygen to form oxygenated species, such as hydroperoxides, esters, and unsaturated carbonyls. The peroxide species are susceptible to decomposition under the influence of UV to form more radicals, accelerating the process. The carbonyl materials are responsible for the yellowing, and are also capable of engaging in cross-linking reactions with the rubber component of the material, which results in destruction of the elastomeric properties of the material and a concomitant embrittlement and loss of impact strength.
Sauces:
-Piton et al., Polym. Degrad. Stab. 55, 1997
-David et al., Polym. Degrad. Stab. 84, 2004
-Pickett et al., Polym. Degrad. Stab. 85, 2004
-Rajan et al., J. Appl. Polym. Sci. 124, 2012
and others
Once the material has been degraded like this, it has become susceptible to attack by oxygen as well as UV. Hence, even in dark rooms, materials will form which cause yellowing, as well as generate more radicals. If your board is yellowed, you can temporarily make it look nicer, but it will never be fully stable anymore.
(in other words, forget the bromine rubbish :p)
