Wheaton w224216 pressure release aluminum seals, 20 mm, ptfe/gray butyl septa (20 mm headspace seal, alum, press release) these seals are special crimp closures with bridges and score lines in the aluminum seal. This design is an added safety feature, allowing the internal pressure to be released when 3.0 +/-0.5 bar has been exceeded. The seals come with ptfe faced grey molded butyl liners. They are offered in packs of 100 or can be purchased with vials in convenience packs. 20 mm crimp closure with liner for headspace vials score lines allow for internal pressure release purchase seals separately or together with vials in convenience packs. Warning: this product can expose you to chemicals including tetrafluoroethylene, which are known to the state of california to cause cancer. Prop 65 warning