1. Weak Boundary Layer Theory

When the liquid adhesive cannot fully wet the surface of the adherend, air bubbles will remain in the voids, forming weak areas. In addition, if the impurities contained in the adhesive are soluble in the molten adhesive but insoluble in the cured adhesive, another phase will be formed in the cured adhesive layer, creating a weak boundary layer (WBL) between the adherend and the overall adhesive. In addition to process factors, the inhomogeneity of the boundary layer structure caused by thermodynamic phenomena such as the interaction between the polymer network formation or the melt and the surface adsorption of the adhesive during the molding process can also lead to the occurrence of the WBL. In such an inhomogeneous interface layer, the stress relaxation and crack development of the WBL will vary, which has a significant impact on the overall performance of materials and products.
2. Diffusion Theory
When two polymers are compatible and come into close contact with each other, mutual diffusion occurs due to the Brownian motion of molecules or the swinging of chain segments. This diffusion occurs interwovenly at the interface between the adhesive and the adherend, ultimately leading to the disappearance of the interface and the formation of a transition zone. However, this theory cannot explain the bonding between polymer materials and hard bodies such as metals and glasses because polymers are difficult to diffuse into such materials.
3. Electrostatic Theory
When the adhesive and the adherend system form a combination of an electron acceptor - donor, electrons will transfer from the donor (such as metal) to the acceptor (such as polymer), forming a double electric layer on both sides of the interface region, thus generating electrostatic attraction. In a dry environment, when the adhesive layer on the metal surface is quickly peeled off, the light and sound phenomena of discharge can be observed with instruments or the naked eye, confirming the existence of electrostatic effects. However, the electrostatic effect only exists in bonding systems that can form a double electric layer and is not universal. Moreover, some scholars have pointed out that the charge density in the double electric layer needs to reach 10²¹ electrons/cm² for the electrostatic attraction to have a significant impact on the bonding strength. In reality, the maximum value of the charge density generated by the migration of charges in the double electric layer is only 10¹⁹ electrons/cm² (some believe it is only 10¹⁰ - 10¹¹ electrons/cm²). Therefore, although the electrostatic force does exist in some special bonding systems, it is not the dominant factor.
4. Mechanical Force Theory
From a physical and chemical perspective, the mechanical effect is not the fundamental factor generating the adhesive force but a means to enhance the bonding effect. The adhesive penetrates into the gaps or unevenness on the surface of the adherend, and after curing, a meshing force is generated in the interface region, similar to the connection between a nail and wood or the rooting of a tree in the soil. Its essence is frictional force. When bonding porous materials, paper, fabrics, etc., the mechanical connection force plays a significant role. However, for some solid and smooth surfaces, this effect is not obvious.