Even the most durable and well cared for plastic can yellow over time. When this happens, you may think that your only option is to replace the item in question or accept the discolouration. However, you can combat yellowing plastic with a simple solution that is probably sitting in your medicine cabinet right now.
Wash the plastic item. Use a damp cleaning rag and three drops of washing-up liquid. This will remove dirt and grime that could interfere with the rest of the cleaning process.
Rinse off the plastic item. Make sure no soap residue remains.
Cover the item with hydrogen peroxide. If it will fit in the plastic tub, then place it in the tub and pour the hydrogen peroxide over it. If it will not fit in the tub, spray it with hydrogen peroxide.
Place the item in the sun. Leave it there for two to six hours. If it is in the tub, rotate it occasionally. If it is not submerged in hydrogen peroxide, refresh the coating of hydrogen peroxide every 45 to 90 minutes.
Rinse off the plastic item. It should be much whiter, brighter and cleaner than before. Make sure all the cleaning residue is gone before you use the item.
Lemon juice is a good, natural whitening additive to this process.