A drop of glue holds two pieces of wood so firmly that the wood often breaks before the joint does. The grip comes from the same small pulls between molecules that hold water together.
Pulls between molecules work only at very short range: the molecules must be less than about a nanometre apart, a few atoms' widths. Even a surface that feels smooth is a landscape of hills and dips at that scale. Press two solids together and they touch only at the tops of the highest hills, a tiny part of their area, so they fall apart again.
A glue goes on as a liquid, so it flows into every dip and spreads over the surface. Chemists call this spreading wetting. Once the glue wets a surface, its molecules sit right against the surface's molecules everywhere, and millions of small pulls add up to a strong grip.
Hydrogen bonds at the surface
The strongest of these pulls are hydrogen bonds, the pull of a δ+ hydrogen on an O–H group towards a lone pair on a δ− oxygen or nitrogen. Wood and paper are made largely of cellulose, chains of glucose covered in -OH groups. White wood glue is PVA, short for poly(vinyl acetate), a polymer whose chains carry ester groups. The δ− oxygen of each ester's C=O reaches a δ+ hydrogen of the wood.
A glue has two jobs. It has to hold on to each surface, which chemists call adhesion, and it has to hold itself together, called cohesion. Hydrogen bonds at the surface do the first job. For the second, the liquid glue turns solid, its long chains tangled and pulling on each other as in any plastic. Look at a broken joint: glue left on both pieces means the glue itself gave way, and a clean piece means the grip on that surface gave way.
| How it sets | Example |
|---|---|
| water leaves | PVA wood glue: the water soaks into the wood and dries off |
| it cools | a glue gun melts a stick of polymer, which hardens as it cools |
| molecules join | superglue and epoxy: a reaction builds long chains or a network |