http://galileo.phys.virginia.edu/out...Newton3Frm.htm

"Note: The industrial specifications for the pop bottles maintain that the bottles can withstand 180 psi. For maximum safety, do not exceed 90 psi."


This was in reference to 2 liter bottles...not sure how reliable the info is. I would have to assume that sun exposure and general aging would diminish the max over time. Remember that the PSI will increase with temp.