PET is the one plastic which has the correct balance of properties ideally suited for packaging carbonated (fizzy) drinks like colas and lemonades. All plastics are porous to some degree and will allow gasses to pass through the structure given sufficient time. PET will retain sufficient carbon dioxide in a beverage bottle to keep the drink fresh within the normal shelf life of the product, which is not true for other widely available plastics. The same property also keeps oxygen outside the container and prevents spoilage from oxidation.
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