The only things that keep CO2 absorbed in carbonated drinks is the pressure of gas above the liquid surface and the temp. Compare how quickly two identical bottles go flat, one opened the other unopened. The unopened bottle is pressurised, the opened bottle is at atmospheric pressure. A squeezed PET bottle has more volume to fill with gas before the bottle returns to it's original shape and the gas pressure above the liquid reaches the equilibrium point where it supresses any more release of C02. A fizzy drink in a squeezed PET bottle will go flat quicker than a non-squeezed bottle.
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