This cap comes with a pressure sensitive (PS) liner. Pressure sensitive liners are made from foamed polystyrene. When the cap (with liner installed) is affixed to the bottle, the liner adheres to the bottle's landing/orifice. Before product can be dispensed, the lid must be removed and the PS liner peeled off and discarded. This lid is made of polypropylene (PP), which means it has a high heat tolerance level and an excellent resistance to fatigue.