The Screw Caps for internal thread tubes are made out of Polypropylene (PP). These Screw Caps have a Silicone O-ring added to the cap as well. The Screw Caps for external thread tubes are made out of PolyEthylene (PE). The TPE Push Caps are made from Thermo Plastic Elastomer (TPE). The EVA Push Caps are made from Ethyl Vinyl Acetate (EVA).