It’s one of the mysteries of life that has confounded many students throughout history. That pasty substance we all call glue…it sticks things together, but doesn’t stick to the inside of the bottle! Why not?
Is it magic? Nope! It’s science — and it’s all about water.
The regular white glue most students know so well is made up of a variety of chemicals called polymers. These polymers are long strands that are either sticky or stretchy. Glue manufacturers have to find just the right combination of sticky and stretchy polymers to make the best glue.
White glue also contains water. Water acts as a solvent that keeps the glue liquid until you want it to stick.
When you put glue on a piece of paper, the solvent — water — is exposed to air. The water eventually evaporates (changes from a liquid to a gas). As the water evaporates, the glue dries and hardens. All that’s left are the sticky polymers that hold things together. This process is called mechanical adhesion.
So why doesn’t white glue stick to the inside of the bottle? When white glue is inside a bottle, there’s not enough air inside the bottle to cause the water to evaporate to make the glue sticky. Basically, the bottle protects the glue from the air and keeps the glue runny. If you’ve ever left the top off a glue bottle for a while, you may have noticed that, unfortunately, the glue dried up!
What about super glue? You may have noticed that super glue tends to dry out in its container more quickly than white glue.
Super glue works differently than white glue. Instead of the polymers in white glue, super glue is made of a chemical called cyanoacrylate. This chemical bonds things together when it reacts with water vapor in the air. This process is called chemical adhesion.
No matter how dry the air may seem, there’s always some water vapor in the air. To keep super glue from drying out, its container must be kept tightly-sealed to prevent water vapor from seeping in and reacting with the glue.
So whether you’re talking about white glue or super glue, water is the key. With white glue, you need the bottle to keep the water in the glue from drying up and becoming sticky. Super glue containers, on the other hand, keep water out to prevent the chemical reaction that will make the glue harden.