Why do worn slippers and used mobile phone cases turn pink?

All because of oxidation. One of the most critical factors that make the mobile phone case pink is that it contains sulfide. In the process of making mobile phone case, it usually contains sulfide, which is easily oxidized by air. The worn slippers change color due to oxidation. Any shoes made of rubber and plastic will be oxidized and accumulated gradually after contact with air, so they will turn yellow. The main material of slippers is usually rubber or plastic, so they will turn yellow, but this process is usually slow and takes a long time to turn yellow.