The L217BKP1 is a black PP plastic 53-400 smooth skirt lid with pressure sensitive (PS) liner. 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. A seal is developed by fitting the lid, with liner installed, to the container, at room temperature. This lid is made of polypropylene (PP), which means it has a high heat tolerance level and an excellent resistance to fatigue. This closure fits a container with a 53-400 neck finish. The first number refers to the diameter of the container's opening (in mm). The second number refers to the thread/style/depth of the closure's skirt. Please read 5 seals and how they make perfect liners for your container for more information on liner types.