I found an article today telling why plastic cases discolor with age. It explains why your old computer or game system turned yellow.
Executive summary: It’s the nature of the material it’s made of.
I’ve noticed that plastics turn brittle with age, but some happen more quickly than others. When I was a kid and built plastic model kits, some kits turned brittle within just a few years–quickly enough that I noticed it before I was in the seventh grade.
I also have some relics from Dad’s childhood–the Plasticville buildings he used on his Lionel train layout. While most of these didn’t discolor too badly, they do seem more brittle than something made of the same material (styrene) made last year. They don’t have as much flex to them and would break more easily than something made with the same molds this year.
The article explains that exposure to air causes plastics to break down over time. But in the end, it’s oxidation. Think rust.
Exposure to heat and certain kinds of light greatly accelerates the process. This is why monitors and Macintosh computers tend to yellow really badly–they generate more heat than a video game system does, so they turned yellow much faster.
The color of the plastic also makes a difference. Darker colors hide the yellowing better than lighter colors. So a Nintendo 64 probably won’t change color over time very much, but a Sony Playstation has that possibility.
Once the plastic has turned yellow, you can reverse it, at least to a degree, without chemicals. It’s gentler than using peroxide, which is the most popular method.