PET is polyethylene terephthalate, HDPE is high density polyethylene. The former is made by reacting ethylene glycol and terephthalic acid, while the latter is made from ethylene.HDPE is cheaper that PET so it is always preferred, but it does allow for quite a bit of the interior moisture to pass through it and evaporate. With milk, this is not a problem as milk has a short self life, less than 3 weeks. For bottled water however, the shelf life is much longer (a year or two) and the amount of water in the bottle could eventually become less than what the bottle is labeled as containing, a violation of the law. The water loss is slower in PET than in HDPE. It's the same problem with the carbonation in soft drinks. New soda bottles will be rock hard while old ones will be softer.As for the environmental issues, a whole book would be needed, and then in the end it would still come down to a value judgement. How do you rank air pollution vs. water pollution vs. solid waste, keeping in mind that there are differing types and amounts and location of air/water/solid pollution in the two cases? Let me add one last thought: the connection between oil and polymers is pretty weak. Yes, they are all made from oil byproducts and if we didn't have oil, we would have a lot less polymers, but 90% of oil is used for making gasoline and similar fuels, only 10% is used by the chemical industry. People that see the oil leak in the Gulf and want to give up on plastics are 90% wrong in their approach.
Thanks John! I had been looking for a concise explanation to link to, but hadn't found one - thanks for providing it for me! I didn't know about water passing through the HDPE and evaporating - fascinating stuff!