This weekend I came across Biota brand spring water, which is the normal expensive kind of spring water (the kind where they make a point of saying where in the earth it came from rather than obfuscating the "municipal source" text). It was pretty good - maybe even one of the better waters I've had.
I'm not here to talk water, though. What's unique about this is the bottle. Most water bottles are PET, a polyester of terephthalic acid and ethylene glycol. These bottles are made of PLA, or polylactic acid.
Esters are pretty labile, and will break down pretty easily into their components. The nice thing about making a polymer out of a monomer like lactic acid is that there are a lot of biochemical pathways into which it can enter - in our bodies, it can be oxidized back to pyruvate, which goes all sorts of places (in your body, you can break it down to carbon dioxide in the citric acid/Krebs cycle, or store it as sugars). I'm not sure what the principal method of decomposition by bacteria is.
My only complaint was that the plastic felt a little sticky (sort of like saran wrap). Tomorrow, we'll take a look at an even weirder renewable/decomposable plastic.
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So would it be safe to say the polycarbonates have now been replaced by PET, PLA? I was having a discussion some time back on the physiological effects of Bisphenol A (used to cross link carbonates) and that it was used extensively in plastics, canning, etc (we were talking about the leachate problems with BPA). It would seem that this is no longer an issue with water bottles (at least) as it appears if it is used it is in the minority. Just curious.
Dunno, not a polymer guy. Anecdotally I've noticed almost all PET bottles and some polypropylene in my food and drink containers, but I just figured it was because polycarbonate was usually hard. I don't think there are many things that combine so many desirable properties (Strength, transparency) in the way polycarbonate does (nalgene-type drinking beakers, for example, are all polycarb as far as I've seen), so I think we're stuck with it for a lot of things.
So, dunno :) Hopefully a real polymer chemist will weigh in.
The single biggest reason for using PP or PET in packaging is expense. Either of those resins is about 1/2 the cost per pound as PC. PC has superb clarity and impact performance.
PLA is expensive as well, but since it's "biodegradable" -- IIRC, it actually requires composting to decompose on any reasonable timescale -- it can get away with an upcharge.
The biggest deficit of PLA is that it has a very low heat distortion temperature. Do not leave that water bottle in a hot car during the summer! It will start to warp and shrink at elevated temperatures. So unlike PET or PP there's no way to pasteurize foods in the package.