Plastic chair requires once in a month cleaning during summer time .Generally people use washing powder to clean it which is not quite effective. Here is a simple way to clean plastic chair.
Take half bucket of hot water and put 1/4th cup bleach into it and mix it properly. With the help of a mug pour the water on the chair and let it stay for five minutes. Now take a brush (brush which is used in washing clothes or any circular brush) and rinse the chair in a circular motion. Once rinsed clean it off with water and let it dry on its own or place it under the sun.
Your plastic chair will be absolutely clean.