Bopping molecules keep glue runny
![[elmers.com] A solvent keeps capped glue from sticking.](2002-03-20-glue.jpg) Q: This possibly has something to do with volume of air exposure, but I would like to know why glue
doesn't stick to the inside of the bottle. A little girl asked this question and I have no idea. I have ever
since wondered. --Mike M.
A: Glue and a solvent reside in a bottle of glue. The solvent keeps the glue from sticking inside the bottle.
[elmers.com] A solvent keeps capped glue from sticking.
When you squirt the glue out of the bottle onto a piece of paper or a broken china cup, the solvent gradually
evaporates, allowing the glue to get sticky.
The solvent molecules bounce and jiggle at the surface of the glue-solvent mixture and some get enough energy (when neighbors slam into
them) to escape. That's evaporation. Outside of the bottle, few solvent molecules remain near the spread glue to hit it and renter the
mixture. So, gradually enough solvent molecules leave the mixture that the glue gets sticky.
It's different in the capped bottle. The solvent molecules evaporate inside the bottle-just as they do outside the bottle when you spread
the glue. However, once airborne they don't have far to go. They're trapped in the bottle. Some of the solvent molecules near the surface
of the glue bop into and reenter the glue. That's condensation. Gradually--as more and more solvent molecules crowd into the airspace
above the glue-solvent mixture--more and more solvent molecules reenter the mixture. Until, eventually, as many solvent molecules leave
as reenter and the mixture reaches equilibrium.
Glue manufacturers make sure the glue mixture stays runny by choosing a sufficiently small air space when they fill the bottle. You may
have noticed that old glue in an almost empty bottle gets viscous or even solid. It's a good idea to buy only enough glue for use in the near
future.
As far as sticking to the bottle is concerned, here's the poop from Elmer's glue: "If it were to dry out or harden, it would form a very
weak bond with the special nonporous plastic used to make the bottle and could be de-bonded or removed easily," says Robert Thompson.
(Answered by April Holladay, science correspondent, Mar. 20, 2002)
Further Surfing:
Elmers: glue projects
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