What causes white plastic cases to yellow?

I've always assumed it was a surface oxidation process or from airbourne acids. It can often be cut back with oven cleaner but any legends etc are lost doing that. White paint goes yellow, paper goes brown in a reaction with airbourne acids.

perhaps try sci.chem

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

There are two or three causes. Firstly, UV light can cause 'color centers' to form, kind of like the purple tint that develops in desert glass. Secondly, polymers are developed from unstable (monomer) ingredients, and there are complex aging characteristics, which cause the plastics to shrink, embrittle, or change color.

Dirt, also, may bond to the surface; if normal household cleaners (soap, Windex, Formula 409 etc.) don't work, it might be useful to apply a mild abrasive (toothpaste or metal polish).

Unusual cleaners including waterless hand cleaner, denatured alcohol (for fingerprints on keyboards), dilute lye (for smokestains), vinegar (for some mineral dusts) have proven effective. Isopropyl alcohol is NOT recommended for translucent or transparent plastics.

this was discussed recently elsewhere, lots on the subject:

HTH

Dirt, also, may bond to the surface; if normal household cleaners (soap, Windex, Formula 409 etc.) don't work, it might be useful to apply a mild abrasive (toothpaste or metal polish).

Novus, too.