Why Does Glue Hold Wood Together?
Glue is one of the substances which, when heated, change their form gradually from the solid to the liquid, and not instantly, as ice melting to water. It is what men of science call viscous; that is to say, it is, for a time, only imperfectly fluid, and does not flow very quickly.
When we smear the surface of a piece of wood with glue in the semi-liquid state, and place another piece of wood on top, the glue is forced into the hollow air spaces of the wood, and as soon as the glue cools and hardens it is firmly anchored in each piece of wood, and the adhesion of the molecules of glue holds the whole structure together. The glue does not penetrate the cell walls of the wood, but merely enters the exposed openings of the cell cavities.
When a section is cut through two pieces of wood glued together, and examined under the microscope, it will be seen that the glue has filled the exposed pores of both pieces of timber. The more cavities in a piece of wood, and the larger the cavities are, the more easily it can be glued. Thus basswood can be glued more easily than oak. Woods having a fine texture and a large proportion of cells with very small cavities are the most difficult to glue.
The force that attracts like molecules is called cohesion. The force that attracts unlike molecules (molecules of different materials) is adhesion. Usually cohesion is a stronger force than adhesion. But in the case of glue when it is spread upon some other materials, such as wood, adhesion is stronger than cohesion. The glue adheres (sticks) to the wood.
