For what its worth, pop bottles are made from PET. They have excellent puncture and burst properties. They must withstand C02 and drop tests. In the industry, they are tersted to high pressure and drop testing for impact resistance. They have much better resistance to bursting or breaking than the kevlar tanks used in jets. Kevlar is ideal for molding to conformal tanks that fit well inside certain airframes but will break and splinter in a crash. Too many people slam the use of pop bottles because they are inexpensive, but in reality they are more suited to the task.

As for shampoo bottles, there are two types, PET and PE. PE is the same material the dubro tanks are made from. The difference is one loos clear like a pop bottle (PET) or cloudy/milky color (PE).

As for the closures, they are usually tested up to 150 psi for blow off consideration.