How-To: Restore the color of old Lego bricks
Turns out the yellowing of old ABS plastic is due to degradation of bromine-containing fire retardants which are added to the plastic during manufacture, which release elemental bromine, causing the yellow color. Shining UV light on the gel accelerates the decomposition of the fragile oxygen-oxygen bond in the peroxides it contains, generating reactive hydroxyl radicals which scavenge the free or loosely-bound bromine in the plastic that causes discoloration.
