The bonding of adhesives is a complex process of interaction between the interfaces of different materials, and the study of the interface layer is at the core of adhesive science. Numerous factors, such as the interfacial tension between the adherend and the adhesive, surface free energy, the nature of functional groups, and interfacial reactions, jointly influence the bonding effect. Bonding technology is highly comprehensive and has complex influencing factors. So far, there is no comprehensive and unified theory. The following are several mainstream bonding mechanism theories, which are of great reference value for engineers to solve the problem of the adhesion of coating layers on different substrates.

1. Adsorption Theory
The adsorption theory regards the adsorption of the adhesive by the solid as the main cause of bonding. This theory points out that the adhesive force mainly originates from the molecular forces in the bonding system, namely Van der Waals forces and hydrogen bonds. The adhesive force between the adhesive and the surface of the adherend is similar in nature to the adsorption force. The interaction between adhesive molecules and the surface molecules of the adherend occurs in two stages: In the first stage, the liquid adhesive molecules diffuse towards the surface of the adherend through Brownian motion, prompting the polar groups or chain segments at the two interfaces to approach each other. Measures such as increasing the temperature, applying contact pressure, and reducing the viscosity of the adhesive can all enhance the Brownian motion. In the second stage, when the distance between the adhesive molecules and the adherend molecules reaches 5 - 10 Å, mutual attractive forces are generated between the interface molecules, and the molecular distance is further shortened to the most stable state. According to calculations, under the action of Van der Waals forces, when the distance between two ideal planes is 10 Å, the gravitational strength can reach 10 - 1000 MPa; when the distance is 3 - 4 Å, it can reach 100 - 1000 MPa, which is far higher than the strength achievable by modern high-quality structural adhesives. However, there is a significant gap between the actual bonding strength and the theoretical calculation because the mechanical strength of a solid is a mechanical property, depending on the local properties of the material rather than simply the sum of molecular forces. In reality, it is difficult to ensure a perfect contact between two objects, and the interactions between each pair of molecules in the interface layer cannot be guaranteed to be broken simultaneously. In addition, if the polarity of the adhesive is too high, it may hinder the wetting process and reduce the adhesive force. Therefore, although molecular forces are an important factor in adhesive force, they are not the only determining factor.

2. Chemical Bond Formation Theory
The chemical bond theory holds that in addition to the mutual forces between the adhesive and the adherend molecules, chemical bonds may sometimes be formed. Studies on the bonding interfaces such as that between vulcanized rubber and copper-plated metal, the effect of coupling agents on bonding, and the bonding interface between isocyanates and metals and rubbers have all confirmed the generation of chemical bonds. The strength of chemical bonds is much higher than that of Van der Waals forces. The formation of chemical bonds can not only improve the adhesive strength but also overcome the problem of the destruction of the bonded joint caused by desorption. However, the formation of chemical bonds requires specific conditions, and it is not possible for chemical bonds to be formed at all the contact points between the adhesive and the adherend. Moreover, the number of chemical bonds per unit adhesion interface is much smaller than the number of intermolecular interactions. Therefore, the contribution of intermolecular forces to the adhesive strength cannot be ignored.