i have several large water bottles that i can use and throw away. i am just not sure if they will or will not be impermeable to oxygen. is there an easy way to tell
Do they have any markings to indicate what kind of plastic they are?
i am sure they are marked, i can look when i get home tonight
They should have “PP, LDPE or HDPE” on the bottom. I would use HDPE for fermentation.
My guess is that they are “Polypropylene”
thanks hopefully they can be useful
Even if not useful as fermenters (unfortunately most likely with water bottles), you can still use them for other things. I have two that I use to hold my filtered/campdened water prior to brewing.
I have one and I use it for Star San.
checked out the plastic carboys. only marking on the bottom is the recycle triangle with a number 7 in the middle.
thanks for the other ideas
Resin identification code:
http://en.wikipedia.org/wiki/Resin_identification_code
Looks like it may be safe and recycled. ??? I’d go for more identifiable plastic for fermentation. Just me though. 
from what i can find out this is a catchall number. some seem to have the potential to leach chemicals similar to the old nalgene bottles. probably not worth using for that reason but can find nothing about oxygen.
Why would the plastic from a water bottle potentially leach chemicals into yoyr beer when fermenting and not leach into water that is stored in them??
I have a couple that say “3” on the bottom According to the link it is PVC.
The chemicals in the plastic are said to potentially leak into the water as well, but evidently heat will speed up the process… here’s one article http://www.sciencedaily.com/releases/2008/01/080130092108.htm and there’s plenty more online. As for the number 7, here’s some info http://trusted.md/blog/vreni_gurd/2007/03/29/plastic_water_bottles#axzz0nxs2qIvq
my original concern was actually oxygen diffusion. i have a few left over from an old water cooler at work, and the things are relatively cheap to get. thought it would be worthwhile to try turning them into conicals. i am less worried about the chemicals leaching than i am about oxidation. i may still try them and see if i get much oxidation
I use the rectangular-shaped water jugs we occasionally get from BJ’s Wholesale Club as carboys. $5 for the bottle deposit. They are 5.28 gallons, and because of the rectangular shape they have a smaller footprint and just fit in a compact 'fridge I have set up for temperature-controlled fermentation. Because there isn’t a huge amount of head space I use a blow-off tube.
On the bottom of these bottles is a “7” in the recycling logo. They also have the name Greif molded into the plastic on the bottom. I ran that down to a plastic bottle mfr., found their web site, and sent an email to their Sales & Marketing Manager who responded that the plastic is a polycarbonate resin, FDA approved for water, and he supposed they could be used for fermentation as well.
beer contains alcohol which is a solvent to many plastics.
I would be more worried about chemicals that oxygen. The fermentation process is only going to last a few weeks and then it is going in to bottles and/or kegs.
Oxygen molecules traveling through plastic is probably measured in molecules per year. 
The alcohol in the beer acting as a solvent is more likely to leach out death. You will get more oxygen in the beer by transferring to a container than through the walls of the plastic.
Use only HDPE.
This is science as Tubercle understands it. :