What Makes Glue Stick?

Glue is a liquid mixture of substances used to join two surfaces together, generally permanently. There are many sorts or varieties of glue. A complete enumeration of each is beyond the scope of this article; however, a few varieties deserve mention here. One variety of glue includes the epoxies, made from two or more component substances, typically a component A and a component B. More common yet is ordinary household glue, consisting of a single component and may be found in grocery stores - water-based casein glue. For the hobbyist, there is model cement, an adhesive containing the rapidly evaporating solvent toluene (sometimes called toluol). Yet a third commonly used adhesive is the near-immediately responsive, cyanoacrylate variety. For each of these, however, the real question is, What enables the adhesive to join two surfaces together? What makes glue stick? Adhesion is predominantly a surface phenomenon.

Surfaces to be Glued

To the naked eye, surfaces to be glued may be flat and smooth, yet in most situations, an appropriate adhesive or glue can be found that will enable them to be joined together. While in some instances, there may be pores on the surface - as, say, when gluing two pottery shards - such isn't always the case. Even without pores, an adhesive can usually be found to accomplish the task. What is the chemistry or physics - the mechanism - that enables adhesion?

Mechanical Attachment

Mere mechanical adhesion can occur if the adhesive enters cracks, pores, or other openings and, after solidification, locks the glued object in place. This is the least exotic of the adhesion mechanisms. One special feature of mechanical attachment is that, unlike the contributions of chemical bond formation or van der Waals forces, complete contact of glue to surfaces at the molecular level is not necessarily essential to accomplish the task.

Attachment via Chemical Bond Formation

Another adhesion mechanism is chemical adhesion, through the formation of a covalent¹ chemical bond. Curiously even the slippery Teflon® can be glued using this mechanism, although with difficulty. The amazingly smooth polymeric coating consists of fluorine atoms attached along a backbone of carbon. Through the use of an exceptionally reactive organometallic agent, aromatic hydrocarbon rings can be attached via chemical bond formation. It is a relatively simple matter, afterward, to bond to those rings using glue.

Van Der Waals Contribution

Van der Waals forces are relatively weak chemical forces between atoms that are not united by means of a formal chemical bond. Glue sandwiched between two surfaces of significant area and allowed to cure provides sufficient strength to join them. To illustrate: consider the water molecule. Water, or H₂O, consists of two hydrogen atoms (very small) and an oxygen atom (larger). The two hydrogen atoms, which are not on opposite sides of the oxygen, but are on the same side, bear a charge of plus one (+1) each. The oxygen atom carries a negative two (-2) charge.

Although this gives an overall - or net charge - of zero, one side of the molecule constitutes a positive pole, the other side a negative pole. The cumulative effect of the immense number of these miniscule molecular charges serves to join surfaces together.

¹ less frequently via ionic or metallic bond