While you may love the look of pristine white plastic items, they will show their age over time. When white plastic gets old and dirty, it tends to take on a yellow hue. Sometimes plastic turns yellow because of excessive sun exposure, while other times the yellow colouring is because of age, heat exposure or dirt. Often, the yellowing is evidence of chemical changes that have occurred in the plastic. If this is the case, you will not be able to get the plastic completely white again. In most cases though, you will be able to lighten the yellowing and improve the item's appearance.
Place the plastic item in a container of warm, soapy water and allow it to soak for 10 minutes.
Use a nylon scouring brush to clean the plastic. Apply steady, even pressure as you clean.
Rinse the item under cool running water to remove the debris you loosened.
Pour hydrogen peroxide over the plastic and scrub with the brush. Hydrogen peroxide will help to lighten the yellowed areas significantly. You can also allow the item to soak in the peroxide for up to two hours.
Create a mixture of 4 tablespoons of bleach and 4 cups of warm water. Soak the plastic in the solution for half an hour. Rinse the item under cool water thoroughly after removing it.
Let the plastic air dry. If the plastic is still yellow, repeat soaking processes.
Some yellowing is irreversible. If you can't remove the yellow from the plastic, consider painting the item.
Wear gloves when handling bleach; it can irritate the skin.