Why doesn’t glue stick inside a tube?
One of the benefits of being pig ignorant of the physical laws that govern our universe is that the answer to mundane questions such as this can elicit a sense of wonderment. When I understand what is going on I get that feeling of contentment that the likes of Isaac Newton and Albert Einstein must have had when they had cracked a particularly thorny problem.
It is all to do with water and something called mechanical adhesion. Ordinary glue – we will come on to superglue in a minute – is made up of a variety of polymers, long strands which are either stretchy or sticky. The key to a good glue is finding the optimal combination of sticky and stretchy polymers.
Anyway, ordinary glue contains water and it is this that acts as a solvent that keeps a glue liquid until you want to use it. When you apply the glue to a piece of paper, for example, the water is exposed to air and eventually evaporates. The process of evaporation causes the glue to dry and harden, leaving only the sticky polymers to do their job – to adhere to the surface.
OK, that makes sense. But why doesn’t this process of mechanical adhesion occur when the glue is in the tube? Well it is all down to the amount of air that is present in the tube. In a tube with the top firmly in place there is insufficient air present to cause the water present in the glue to evaporate. So the glue remains in a liquid form and not so sticky as to stick to the insides of the tube. If, however, you forget to put the top back on or don’t screw it tightly enough, you will find that the contents will dry up.
Super glue is another kettle of fish, though, being made up of a chemical called cyanoacrylate instead of polymers and working through chemical adhesion. The reaction of the cyanoacrylate with the water vapour in the air causes the glue to bond. As there is always water vapour present in the air, no matter how arid it may seem to you, super glue reacts more quickly and is more prone to dry out in poorly sealed containers. This chemical adhesion explains why I always get into an unholy mess when I use superglue – it gets everywhere and sticks to everything before I even get it to where I intended to apply it – and why ordinary glue seems easier to apply.
So the answer is water but with ordinary glue you need to keep the H2O in the tube to stop it drying out and becoming sticky whereas with superglue you need to keep the water out to stop the substance from hardening.
Isn’t science wonderful?!
Whilst we are on the subject of tubes, what gets my goat is my inability to wring the last drops of whatever is in it out of its container. This inherent design flaw increases the manufacturer’s profit. As we still live in times of austerity here’s a handy tip that is guaranteed to solve the problem. When you have reached maximum frustration point with the tube, simply snip off the top or bottom (or both) of the tube and access the contents that way. When you’ve finished use a clothes peg or a food tie to seal it up again. Of course, for the reasons discussed above this may not be terribly effective with glue but it works a treat with toothpaste!
Every little helps!