Especially after using the same bottle of glue repeatedly
In: 22
It does, eventually. If you leave an old glue bottle in a drawer for years and come back later it will be a solid chunk inside.
Since the bottle has a small hole at the top, not much air moves in or out of the bottle. The glue needs a fresh supply of air to harden, so the hardening process takes a very long time. Also, the glue at the top of the bottle hardens first, since it’s in direct contact with the air. This helps protect the rest of the glue underneath it, but not entirely. Eventually, it will all harden. It just takes time.
Many types of liquid glue generally don’t dry up because the glue is exposed to air.
There is a solvent inside glue (typically water or ethyl acetate), and once this evaporates the glue becomes hard. It can only do so when there is a lot of air for the solvent to evaporate into, much more than what ends up inside the bottle.
P.S: Note that this applies to the glue regular people come into contact with. Industrial glues function in many different ways, and for example many industrial glues do chemically harden (for example if they come into contact with water which sets off a curing process).
How most consumer based glues work is they have a solvent that Carries the sticky material such as water or alcohol and this solvent has a tendency to slowly evaporate as it’s exposed to air, which leaves behind a dried up version of the sticky stuff, forming your bond. Well the air that accepts this evaporated solvent only has so much room for the solvent between the air particles, so for the glue to dry in a usable matter, it needs to be exposed to a large volume of air ie it needs to be squirted out of the bottle and now the glue can be in contact with all the air in the room your using it in (this is why many glues will recommend leaving outside in well ventilated area to dry, as the more air the glue can touch, the more solvent can evaporate at once).
in contrast the bottle only can hold a small amount of air, and therefore can only allow a small amount of solvent to evaporate (this is why sometimes you need to shake up a bottle of glue before reusing it as the surface level of glue might be a bit harder as it was exposed to air. Obviously as you use the glue, air will be pushed out of the bottle, and new air sucked in, which is why over a long period of time, glue in a bottle can harden and form a solid rubbery chunk, especially if the lid isn’t sealed properly (that’s why they tell u to close when storing it).
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