Screw caps and pouring rings for use with GL threaded bottles. For steam autoclaving, we recommend PBT (polybutylene theraphthalate) caps with a PTFE liner, -45 to 180 degrees C rated. Polypropylene is only rated to 140 degrees C. 316L stainless steel cap is for use with stainless steel laboratory bottles. Autoclavable to 121 Degrees C, Max use temperature 500 Degrees C.