i read this awhile back, hope it helps.
http://www.vintagecomputing.com/index.php/archives/189
> The plastics most commonly used to make the structural cases for electronic equipment are polypropylene, impact styrene, and ABS, replied Deanin. These all tend to discolor and embrittle gradually when exposed to UV and/or heat. They become oxidized and develop conjugated unsaturation, which produces color. They crosslink or degrade, which causes brittleness.
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> From looking at a stamp on the Super Nintendos plastic case, I learned that the case is composed of ABS, which is a rugged, durable plastic that is sadly more susceptible to discoloration and degradation from both UV and heat than the alternatives.
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> There are other plastics which would be more stable, Deanin continued, but manufacturers avoid them because they are more expensive and/or more difficult to process.
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> Instead of using more expensive plastics, manufactures put additives known as stabilizers, absorbers, or blockers into the plastic mixture to reduce the effects of degradation. They also get creative with their use of pigmentation.
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> Since most discoloration is toward yellowing, some manufacturers add a little blue to neutralize the yellow, Deanin said. This gives a temporary reprieve, but eventually the yellow keeps growing and overpowers it anyway.
