Plastic chairs are manufactured in 3 types of plastic: High Density Polyethylene (HDPE), Polypropylene (PP), and Polystyrene (PS). The type of plastic used depends on the chair's weight, price, durability, and what you want it to be made out of.
The answer to this question is not as obvious as it seems. While some people may think that the quality of the chair is only determined by the material used in its construction, others may argue that there are other factors too, such as style or design. There are many reasons you should know what type of plastic is used in manufacturing your chair.