The theory that regards the adsorption of solids by adhesives as the main cause of adhesion is called the adsorption theory of adhesion. This theory holds that the main source of adhesive force is the molecular force in the adhesive system, namely van der Waals force and hydrogen bonding force. The adhesive force between the adhesive and the surface of the adhered object has certain similarities with the adsorption force.
There are two processes in the interaction between adhesive molecules and the surface molecules of the adhered object: In the first stage, the liquid adhesive molecules diffuse towards the surface of the adhered object with the help of Brownian motion, bringing the polar groups or chain segments at the two interfaces closer to each other. During this process, increasing the temperature, applying contact pressure, and reducing the viscosity of the adhesive are all conducive to enhancing Brownian motion. The second stage is the generation of adsorption force. When the distance between the adhesive and the molecules of the adhered object reaches 5-10 Å, an intermolecular attractive force will be generated between the interface molecules, further shortening the intermolecular distance to the most stable state.
According to calculations, due to the action of van der Waals force, when the distance between two ideal planes is 10 Å, the gravitational strength between them can reach 10-1000 MPa; when the distance is 3-4 Å, it can reach 100-1000 MPa. This value far exceeds the strength that the best modern structural adhesives can achieve. Therefore, some people believe that as long as the two objects are in good contact, that is, when the adhesive fully wets the bonding interface and reaches an ideal state, the action of only the dispersion force is sufficient to produce a very high adhesive strength. However, the actual adhesive strength differs greatly from the theoretical calculation. This is because the mechanical strength of a solid is a mechanical property rather than a molecular property, and its magnitude depends on every local property of the material, rather than being equal to the sum of the molecular forces.
The calculated value is based on the assumption that two ideal planes are in close contact, and it is ensured that the interactions between each pair of molecules in the interface layer are simultaneously destroyed. In reality, it is impossible to ensure that the forces between each pair of molecules occur simultaneously. If the polarity of the adhesive is too high, it will sometimes seriously hinder the wetting process and reduce the adhesive force. The intermolecular force is a factor contributing to the adhesive force, but it is not the only factor. Under certain special circumstances, other factors can also play a dominant role.