All plastic bottles have one great disadvantage compared to glass bottles: The sidewalls of any plastic container are open to permeation of gases and liquids whereas glass bottle walls are totally impenetrable. (Gases can still move in and out of a glass bottle but only through the closure.) This is not a large issue with water for example but poses significant challenges for carbonated drinks and all products that are sensitive to oxidation and moisture loss or ingress. The plastic industry has worked hard to overcome many of these issues and is constantly improving technologies to push plastic and especially PET into new applications where they are replacing glass bottles.There are other ways to improving barrier performance such as adding a thin layer of a high-barrier material into the PET structure itself, the so-called multi-layer process, using a high barrier material such as PEN or pre-mixing barrier material with virgin PET and using it in a mono-layer structure. All of these are not the subject of this article.