ars technica did some digging earlier this week on plastic oxidation and found a cool article on why this happens to some SNES consoles, and not others.
Since two different batches of plastics had two different aging results (as illustrated in the top and bottom halves of my SNES, or the old and new SNES units), then there must have been a difference of additives between them. Perhaps in one of the production runs of plastic, they didn’t get the catalyst or flame retardant mixture quite right and more residues were left over in the top half’s plastic batch, thus causing it to degrade more rapidly over time. And by the time Nintendo produced the later runs of Super Nintendos, they had perfected the manufacturing process of their plastic, meaning that those later models aren’t as susceptible to oxidation as the earlier models are.
[Original article from Vintage Computing and Gaming]