I don't have an accordion bottle any more. Based on what I recall from how it felt like in my hand, I think it's made of PP, HDPE or something similar, and it was not PET or PVC. If my guess on PP or HDPE are indeed correct, these materials are good acid-resistant and moisture barrier but not a good gas barrier therefore not suited for storing developers. My experience with HDPE bottle and ascorbic acid formulae is like the bottle wall is as if nonexistent when it comes to the aerial oxidation killing my chicken soup. I'd never store my wine in HDPE bottles.(posted 8118 days ago)One point I made in that page is to make sure that the bottle is made from PET (PETE) before storing developer in it. For storing fixer, stop bath, toning bath, and almost everything else, you can use either PET or HDPE bottles. For concentrated acid stop baths, use HDPE.