Pentair Pentek RFC Series Cartridges are constructed with a 70-micron porous polyethylene outer shell and durable polypropylene endcaps. The 2-3/4" outer diameter cartridges have a polypropylene core and the 4-1/2" outer diameter cartridges incorporate a spun polypropylene core. Sandwiched between the outer shell and the core is a bed of granular activated carbon (GAC).