Why do sealed plastic bottles of pop collapse as the contents get old?

Question

A number of bottles of pop (two varieties) in unopened plastic bottles have sat for a few years in a room-temperature environment. All of the bottles have now collapsed a certain amount (all to about the same degree).

They all started out under pressure from the carbonation, but now, not only is that gone, but the internal pressure has dropped below ambient. They all still have an "air" space above the liquid contents; not sure if it is or is not more than when originally purchased. If one of these bottles is opened, it draws in air and regains its original shape.

I have the following questions:

1. What happened to the carbonation? Did it leak out, or did it undergo some sort of chemical reaction, or perhaps both?

2. How could the pressure inside the bottle drop below ambient? What chemical or physical process is causing it, and what is driving the process? Why would it not simply reach equilibrium with ambient?

3. What would be the ultimate result if these bottles were left even longer?

Answer 1

1. It is most likely the CO2, H2O and air in the pop bottle headspace diffused (leaked) out of the pop bottle.

2. You are correct: the pressure has reached ambient (not dropped below ambient). Because the pressure in a freshly bottle pop bottle was originally higher than the pressure in the room (normally atmospheric pressure), and the bottle's plastic walls were smooth and strong, any decrease in pressure inside the pop bottle would cause the pop bottle walls to collapse.

3. If the bottles were left longer, the CO2 and water would continue to evaporate from the liquid phase until the headspace contained approximately the same pressure as the air in the room. If somehow all the water evaporated, you'd be left with sugar and a bunch of other solids (that were dissolved). I doubt full evaporation of water would occur.

Also, if the bottle is made from PETE (polyethylene terephthalate), prolonged contact with water (including pop) will result in many by-products. Two of them are:

a. UV exposure, air and water (together) promote peroxide formation on polymer end-groups (OH, COOH). Another way of saying this is that the bottle walls degrade.

b. The PETE plastic could leach some trace antimony (if that was used in the polymerization process) and/or phthalates, suspected to be endocrine disruptors.

IF water permeates through the bottle walls, it can allow for microoganisms to diffuse in/out of the bottle. You might get some mould in there!

Those are just some things that could happen.

Answer 2

The weakest point in a bottle with a lid is usually the lid - note that differences in temperature can see the lid loosen and gas escape (I'm going to guess there's no wax or similar over the lid to prevent this). Alternatively, some other form of slow leak exists. I'm also going to guess that there's some temperature variation in the room they're stored in. In this case, when it's warmer, more gas will escape and when it's cooler the pressure will be lower than external ambient . There is a famous experiment where you take an empty bottle or similar, and place it in hot water with no lid, then place it cold water with a lid on, whereupon it collapses, illustrating this last idea.

Answer 3

CO2 diffused through the plastic or possibly leaked out the closure. PET is the most common "plastic" bottle in the USA but others are used. However, when PET was developed many things were stored for extensive periods in bottles to satisfy FDA rules and provide technical information. As I remember ,carbonated beverages went over 2 years with no change ( this may be searchable under Amoco Chemicals after about 1970.) I suspect you had a different plastic ; For example , H2S totally diffuses through vinyl in 24 hours.