You can clean plastic chairs using ammonia-free dish-washing soap, laundry detergent, or vinegar. Each method requires warm water and a scrubbing tool such as a brush or sponge.
To clean plastic chairs using dish soap, mix two tablespoons of soap per gallon of warm water. Scrub the chair manually with the soapy mixture until clean. For white chairs, mix in three-quarters of a cup of bleach. To clean with laundry detergent, mix one tablespoon of laundry detergent per gallon of warm water. Scrub the chairs until clean, then leave the soapy mixture on the chair for 15 minutes before rinsing. To clean plastic chairs with vinegar, mix one-quarter of a cup of vinegar per gallon of warm water. Rinse the chair thoroughly, then scrub with the vinegar mixture.