Why doesn’t glue stick together in the bottle? When you put two pieces of whatever together and squeeze, there is no more air than there is in the bottle. It seems to me that the circumstances are the same and yet the glue sets when it’s out of the bottle. Also, the top layer of the glue in the bottle should set when it is exposed to the air, especially in those large wood glue bottles. I am puzzled. Is there something else going on rather than exposure to air?
I use polyurethane wood glue as well as other types. Polyurethane glues will cure in the bottle if not used fast enough. I buy the small bottles for that reason. These glues cure faster in the presence of moisture, so a common technique is to apply glue to one surface and dampen the other surface. Perhaps there is not enough moisture in the capped bottle to allow the porcess to occur.
Cyanoacrylate (sp?) glues like ZAP and Crazy Glue will also cure in the bottles after a time. When I was building model airplanes, I’d buy several bottle and keeep them in the freezer to make them last longer.
Perhaps a chemist will be along to add a better explaination.
Well I’m stuck! 
While we are at it, why don’t glues and adhesives “stick” to air? Obviously that would impair their practicality, but it leads to another question: are there other glues and adhesives that are created for use in vacuums?
Yes, there is. It’s the removal of water or other solvents that makes some glues (ala Elmer’s) set.
While in the bottle, there is a homeostatis of solvent in the glue and in the air also in the bottle. Therefore, the glue doesn’t set.
But once its put between two pieces of something to glue together, the solvent starts to evaporate and the glue sets. Two pieces of wood will set together fairly quickly, because the wood is fairly porous and the solvent can transport away quickly. Two piece of metal won’t glue together as well if the area is fairly air-tight, both because the solvent can’t evaporate away and because the smooth metal doesn’t provide as good of gripping sites for the hardened glue.
Now, there may be glues that set with exposure to oxygen. But I’d think these would be fairly exothermic, and an actual fire danger would result in using them.
Finally, there are epoxies that won’t set until the two chemicals are mixed together. They’re probably also exothermic, but not near as much as an O[sub]2[/sub] reaction.
Yes, there is. It’s the removal of water or other solvents that makes some glues (ala Elmer’s) set.
While in the bottle, there is a homeostatis of solvent in the glue and in the air also in the bottle. Therefore, the glue doesn’t set.
But once its put between two pieces of something to glue together, the solvent starts to evaporate and the glue sets. Two pieces of wood will set together fairly quickly, because the wood is fairly porous and the solvent can transport away quickly. Two piece of metal won’t glue together as well if the area is fairly air-tight, both because the solvent can’t evaporate away and because the smooth metal doesn’t provide as good of gripping sites for the hardened glue.
Now, there may be glues that set with exposure to oxygen. But I’d think these would be fairly exothermic, and an actual fire danger would result in using them.
Finally, there are epoxies that won’t set until the two chemicals are mixed together. They’re probably also exothermic, but not near as much as an O[sub]2[/sub] reaction.