Phenolic pistons are an alternative to traditional metal pistons in automotive brake systems. Phenolic is great for daily use, but they cannot take too much of a load. They are made with heat treated plastic. Metal pistons last longer, and are harder. They also provide better performance under more extreme conditions. They would never use Phenolic pistons in automotive racing.
Phenolic plastic is a thermosetting resin made by condensation of a phenol with an aldehyde and used especially for molding and insulating and in coatings and adhesives. The substance is used for phenolic brake pistons due to it being a heat resistant plastic and less expensive to produce when compared to metal pistons that are made from more expensive steel.