The chemical bond theory holds that, in addition to the intermolecular forces, chemical bonds may sometimes be formed between the adhesive molecules and the adherend molecules. For example, studies on the bonding interface between vulcanized rubber and copper-plated metal, the effect of coupling agents on bonding, and the bonding interface between isocyanates and metal-rubber combinations have all proven the formation 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 adhesion strength but also overcome the drawback of adhesive joint failure caused by desorption. However, the formation of chemical bonds is not common. To form chemical bonds, certain quantization conditions must be met, so it is impossible to ensure that chemical bonds are 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 adhesive strength derived from intermolecular forces cannot be ignored.
When the liquid adhesive fails to wet the surface of the adherend properly, air bubbles remain in the gaps, creating weak areas. For another example, when impurities in the adhesive can dissolve in the molten adhesive but not in the cured adhesive, another phase will form in the cured adhesive, creating a weak boundary layer (WBL) between the adherend and the overall adhesive. In addition to process factors, during the molding process of polymer networking or the interaction between melts, as well as in thermodynamic phenomena such as the adsorption of the adhesive on the surface, the inhomogeneity of the boundary layer structure will occur. When there is inhomogeneity in the interface layer, a WBL will appear. The stress relaxation and crack development of this WBL will vary, thus greatly affecting the overall performance of the materials and products.
On the premise that two polymers are compatible, when they are in close contact with each other, a mutual diffusion phenomenon occurs due to the Brownian motion of the molecules or the swinging of the chain segments. This diffusion process occurs interwoven across the interface between the adhesive and the adherend. The result of the diffusion leads to the disappearance of the interface and the formation of a transition zone. The diffusion theory of the bonding system cannot explain the bonding between polymer materials and metals, glass, or other hard materials, because it is difficult for polymers to diffuse into such materials.
When the adhesive and adherend system is in the form of an electron acceptor-donor combination, 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 area, thereby generating electrostatic attraction.
In a dry environment, when the adhesive layer is quickly peeled off from the metal surface, the phenomena of light and sound from the 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, so it is not universal. In addition, some scholars have pointed out that the charge density in the double electric layer must reach 10²¹ electrons/cm² for the electrostatic attraction to have a significant impact on the bonding strength. However, 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 by no means a dominant factor.
From the perspective of physical chemistry, the mechanical action is not a factor that generates the adhesive force, but rather a method to enhance the bonding effect. The adhesive penetrates into the gaps or uneven surfaces of the adherend, and after curing, an interlocking force is generated in the interface area. These situations are similar to the connection between a nail and wood or the way tree roots are embedded in the soil. The essence of the mechanical connection force is friction. When bonding porous materials, paper, fabrics, etc., the mechanical connection force is very important. However, for some solid and smooth surfaces, this effect is not significant.
The basic points considered in the above bonding theories are all related to the molecular structure of the adhesive, the surface structure of the adherend, and the interaction between them. Experiments on the failure of the bonding system show that there are also four different situations when the bonding fails:
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Interface failure: The entire adhesive layer separates from the surface of the adherend (the adhesive interface is completely detached);
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Cohesive failure: The failure occurs within the adhesive or the adherend itself, rather than at the adhesive interface;
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Mixed failure: Both the adherend and the adhesive layer itself are partially damaged, or only one of the two is damaged. These failures indicate that the bonding strength is not only related to the force between the adhesive and the adherend, but also to the force between the molecules of the polymer adhesive. The chemical structure and the aggregation state of the polymer molecules strongly affect the bonding strength. Studying the molecular structure of the adhesive base material is of great importance for the design, synthesis, and selection of adhesives.
