The bonding mechanism of adhesives (Part One)

In hot melt adhesive, we will have to face this words "Bonding".

Bonding is the result of post-contact interaction between different material interfaces. Therefore, the role of the interface layer is a fundamental problem in the study of gluing science.In the next few articles,  the iSuoChem group will be divided into different articles to briefly introduce the bonding mechanism in several aspects. It can be briefly summarized as the five bonding mechanisms of the adhesive. Weak  boundary theory, diffusion theory, adsorption theory, electrostatic theory, and mechanical force theory.

1.Weak Boundary Theory

Regarding the weak boundary theory, when the liquid adhesive does not well wet the surface of the adherend, the air bubbles remain in the voids to form a weak zone. For example, impurities can be dissolved  in the molten adhesive, and when insoluble in the cured adhesive, another phase will be formed in the solid adhesive layer, and a weak boundary layer (WBL) will be formed between the adhesive and the whole  adhesive. In addition to process factors, in the molding process of polymer web or melt interaction, the unevenness of the boundary layer structure caused by thermodynamic phenomena such as adhesive and  surface adsorption is also caused. WBL will appear in the uneven interface layer. The stress relaxation and crack development of this WBL will be different, thus greatly affecting the overall performance of  materials and products.

2.Diffusion Theory

If the two polymers are compatible, when they are in close contact with each other, the interdiffusion phenomenon occurs due to the Brownian motion of the molecules or the oscillation of the segments. The  effect of this diffusion is through the interfacial interweaving of the adhesive and the adherend. The result of the diffusion leads to the disappearance of the interface and the creation of a transition zone.  Bonding systems cannot explain the adhesion of polymeric materials to metals, glass or other hard materials by diffusion theory because polymers are difficult to diffuse into such materials.

In the following articles, we will elaborate on the remaining theories.

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