PET moisture barrier explained

Q: If PET is not a good moisture barrier, then why are hygroscopic foods and drugs are packed in PET bottles? Why then are Coke/Pepsi packed in PET bottles?

A: As I recall, the moisture barrier of a typical 25-micron PET film is about 77.5 gm/m2/day. So for many products it will allow the product to stale (gain or lose moisture) faster than an OPP film with a moisture barrier of perhaps 7.75 gm/m2/day. I would point out that the level of moisture barrier that is acceptable or necessary depends on what is being packaged and what the effect of the moisture permeating through it is on the product and what the shelf life is designed to be.

I.e.: How does the product fail with a moisture gain or loss and what is the shelf life needed. Knowing these answers along with the distribution cycle allows the determination of what a “good” barrier level is. So my use of “not good” in describing the moisture barrier of PET is a little sloppy as clearly it is “better” than other films but not as “good” as other films. These are all subjective descriptions as “good” and “bad” only have meaning relative to the packaging demands.

I would suspect that the products you are mentioning (hygroscopic foods & drugs) don’t have a noticeable quality impact with some moisture pick up or loss, and the drugs will likely need oxygen barrier more so than moisture. But this can only be a guess without evaluating the specific products and the desired shelf life.

Coke and Pepsi are packaged in PET bottles because the bottles are lighter weight than glass and have a carbon-dioxide barrier to keep the carbonation in. The lower weight of the bottles decreases the transportation costs and avoids the high energy use to make and use the glass bottles that the PET bottles replace. Because the Coke and Pepsi are essentially greater than 95% water, the loss of a small amount of water from the bottle does not affect the quality of the Coke or Pepsi, while the loss of the carbon dioxide will make the Coke and Pepsi or “flat.”