PET is hygroscopic: Water diffuses out of a Sprite bottle

Polyethylene terephthalate (PET) is a plastic commonly used to make beverage bottles. PET is hygroscopic [1, 2]. Therefore, if you have a bottle of Sprite, you would expect the water to be absorbed by the plastic, diffuse through the bottle, then evaporate outside the bottle.

And that’s exactly what happens.

In the picture below are two bottles of Sprite in PET bottles. On the left, a partially-consumed 591 mL bottle, unopened for the last 9 years (2009). On the right, a fresh unopened 710 mL bottle (2018). As can be seen, the old bottle has been squished, as water has escaped but no air has entered to replace the lost water. How much water was lost? I’m going to find out…

(Spoiler: 28% water loss in 9 years.)

Methodology

If a substantial amount of water (but not sugar) has left the bottle, then the sugar concentration in the bottle should have increased. If I assume that the Sprite formula is the same for both bottles, then I can calculate the amount of water that left the bottle by comparing the sugar concentrations between the old and new bottles.

The methodology is fairly simple: Take a sample of Sprite and weight it, bake it until mostly dry, then weigh the amount of unevaporated solids (sugar) remaining. The old Sprite should have a higher fraction of solids because water was lost.

The Sprite was baked at around 150°C until nearly dry, then at around 90°C for an hour more. The intent is to dry the sugar without too much caramelization.

Sprite formula appears to be unchanged

2009
2018

Comparing the ingredients and nutrition facts from the labels of the 2009 and 2018 Sprite bottles suggests that my assumption that the formula has not significantly changed is likely true.

Results

Sprite samples before (top) and after (bottom) baking. Left: 2009 Sprite. Right: 2018 Sprite.
2009 2018
Sample weight (grams) 14.63 14.52
Solids remaining after bake (grams) 2.04 1.51
Solids fraction 13.9% 10.4%
Water : solids ratio 6.17 8.62
Fraction of water lost 28.3%
Table 1: Sprite sample weights

Table 1 above shows the weights of the Sprite samples before and after baking. The old Sprite has a higher concentration of solids remaining after baking. If we assume that the sugar concentration at manufacture was the same for both samples, then about 28% of the water (or about 100 mL) was lost in 9 years.

Also interesting is that my measured solids concentration for the fresh bottle (10.4% by mass) agrees quite well with the Nutrition Facts label (38g carbohydrates in 355 mL of Sprite with a density of around 1.04 g/mL [3] results in a sugar concentration of about 10.3%)

Conclusions

In a PET bottle of Sprite, about 28% (~100 mL) of the water diffused through the bottle over 9 years.

References

[1] J. Bozzelli, Injection Molding: You Must Dry Hygroscopic Resins, Oct., 2010. [Online]. Available: https://www.ptonline.com/columns/you-must-dry-hygroscopic-resins
[2] CWC Best Practices in PET Recycling: Drying Methods and Requirements, 1997. [Online]. Available: http://infohouse.p2ric.org/ref/13/12488.pdf
[3] L. Grace, 36.34 – Diet vs. regular Coke. [Online]. Available: http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/36.34.html

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