9mm Screw Neck PP Caps. Polypropylene Screw Cap with Septa – Polypropylene screw cap with precision-fit septa provide an evaporation-proof seal. Bi-layer include silicone and PTFE. Silicone septa for multiple injections. Rated 4.5 /5 based on 215 customer reviews. cap with center hole pp cap screw cap. Free Sample Get Price.