The two main types of polymer insulator available today are Silicone rubber and Ethylene Propylene Diene Monomer (EPDM.) The insulator pictured is a EPDM rubber type, which is molded right to the fiberglass core of the unit. An easy way to tell if a polymer insulator is a Silicone or EPDM rubber is from the color. Generally, EPDM is always black or a very dark grayish color while Silicone rubber insulators can vary from shades of light blue all the way to light gray and white.