I have no idea what this thing is called, so I’ll describe how to find one (so you know what I’m talking about):
Find a plastic soda bottle
Take the lid off the bottle, and look inside it
There is a blue plastic thing in the bottom of the cap
What is this blue plastic thing?
4b) (You can remove it with a pencil or a screwdriver - or fingernails, if they’re long enough, I suppose.)
My best guess is that it serves as some sort of barrier against mold/mildew/etc. since a lot of moisture builds up in the bottle. (Since the moisture would rise from the soda and cling to the underside of the cap - maybe it’s harder for mold to thrive on this blue thing, rather than on the bare cap.)
Its the pressure seal. The softer plastic “liner” acts as a membrane which , coupled with the pressure from the lid, acts as a gas proof barrier, keeping your soda fresh and bubbly
I think it is just to make a better seal. Take the blue piece out of the cap from a fresh bottle, put the cap back on, tighten to a normal bottle-cap torque, and shake it up. I bet you end up with a mess.
As an example, clamp your jaw closed. You may notice that your teeth, having rather uneven surfaces, touch in a number of places (what dentists call an occlusion) but don’t form a tight seal. You can quite easily breathe through your clenched teeth. Now imagine fitting a piece of cardboard into your mouth and biting down on it. While your teeth are hard and do not compress under pressure, the cardboard is softer, and does, conforming to the surfaces of your teeth and forming a seal. Attempting to breathe through your clenched teeth now is much more difficult. When sealing a soda bottle, a fairly rigid plastic cap is screwed onto a fairly rigid plastic bottleneck and neither surface is inclined to mold to the other. Placing a compressible gasket between then, i.e. a disk made of softer plastic or rubber, allows a tight seal around all points of contact, covering any imperfections in the cap or bottleneck that might allow the CO[sub]2[/sub] to escape and let the soda go flat as it is shipped.
If you look under the cap of a microbrew, you find a seal that also acts as an oxygen-scrubber, absorbing the oxygen in the headspace so it doesn’t affect the beer.
