How exactly does Ethanol Alcohol “expire” acting as a sanitizer?
Boring answer, but I believe it’s mostly because alcohol is extremely volatile, meaning it evaporates quickly.
This even a well designed container of hand sanitizer will have the alcohol evaporate over time and eventually you just have a container of the gel goop and scents they use to make it a nice product.
Also boring, but they probably only test/model the effectiveness of the sanitizer for a specific time period. They aren’t going to run an expensive analysis of “will the sanitizer” be effected 50 years from now? because that’s just a waste. So they can only print a confirmed expiration date for as far as they know it’ll be good for.
It “expires” as it evaporates when exposed to the air. There is some water content in ethanol, and when you open the container, some of the alcohol evaporates while the water is left behind. This causes the product to become more diluted over time. Furthermore, ethanol reacts (slowly) with oxygen, creating acetaldehyde, and this impurity increases as time passes. If the level of this impurity increases enough, the product may fail to meet its purity requirements for food or medical uses.
If you have a container of ethanol that is sealed and has never been opened, it is likely still good even well after its expiration date.
The ethanol reacts with oxygen to form vinegar. That too is a sanitizer but a different kind which might cause acid burns when applied to skin and stain objects it is applied to. There are also other things in regular ethanol sanitizer to help it gel and moisturize skin, these ingredients may also turn bad over time.