Common adhesion theories of adhesives

Mechanical anchoring theory:
Adhesive must penetrate into the gaps on the surface of the adhered object and eliminate the air adsorbed on its interface in order to produce adhesive effect.. The mechanical theory is very convincing in explaining the bonding of porous materials such as wood and materials with rough surfaces, and has been verified in practice. The effect of polishing dense materials with adhesive bonding surface is better than that of smooth dense materials, because after mechanical anchoring, a clean bonding surface is formed, generating a reactive surface, increasing the surface area, and improving the quality of bonding. However, mechanical theory has certain limitations as it cannot explain the bonding phenomenon of non porous materials such as glass, metal, and other objects, nor can it explain the influence of chemical changes on the bonding effect of material surfaces.

Adsorption theory:
Adsorption theory suggests that bonding is caused by molecular contact and interfacial forces between two materials.; Adhesive force mainly comes from intermolecular forces such as hydrogen bonding and van der Waals forces, while bonding is the result of the combined action of physical adsorption and chemical adsorption. Adhesive molecules move towards the surface of the adhesive through Brownian motion, bringing their polar molecular groups and segments closer together. When the distance between molecules is less than 0.5-1nm, intermolecular forces, known as van der Waals forces, are generated, forming adhesion. The process of continuous contact between adhesive and the adhered object is called wetting. In order to wet the solid surface, the surface tension of the adhesive should be less than the critical surface tension of the solid. The depressions and gaps on the solid surface where the adhesive penetrates will form good wetting, but if the depressions are elevated, it will reduce the actual contact area between the adhesive and the adhered object, and lower the bonding strength of the joint. The same adhesive can bond different materials, indicating the widespread use of adsorption. However, adsorption theory cannot explain the phenomenon of cohesive failure in bonding, nor can it explain the bonding of non-polar materials.

Diffusion theory:
Diffusion theory, also known as molecular permeation theory, holds that adhesion is generated by the diffusion of molecules at the interface between the adhesive and the adhered object. Large molecules intertwine or dissolve at the interface, resulting in the disappearance of the interface and the formation of transition zones, thus forming a strong bond after curing.. The diffusion theory is mainly used to explain the bonding between polymer materials of the same type or structure with similar properties. Self adhesion of polymers under the action of solvents or heat. The surface adhesion of polymers with similar solubility cannot explain the bonding process between polymer adhesives and inorganic substances such as metals, glass, ceramics, etc; Unable to explain the phenomenon where some adhesives have similar solubility parameters to the bonded material but are difficult to achieve good bonding.

Electrostatic theory:
Electrostatic theory, also known as double layer theory, believes that a double layer is formed at the interface between the adhesive and the adhered object, and adhesive force is generated due to the mutual attraction of static electricity.. When the adhesive peels off from the adhered object and there is a clear charge present, it is a strong confirmation of this theory. However, electrostatic theory cannot explain the bonding between polymers with similar or identical performance; Unable to explain the bonding process of conductive adhesives and adhesives using carbon black as fillers; Unable to explain the influence of temperature and other factors on the results of the peeling experiment.

Chemical bond theory:
This theory believes that the bonding action is the result of the chemical reaction between the adhesive molecule and the surface of the adherent to form a chemical bond. The chemical bond energy is 1 to 2 orders of magnitude higher than the intermolecular force. If the chemical bond can be formed, a high-strength, anti-aging bonding will be obtained. It is very instructive for the bonding of wood and other materials. Chemical bonding can be achieved by reacting the active groups between the adhesive and the bonded material under certain conditions to form chemical bonds. Coupling agents can be added or active groups can be generated through surface treatment. However, chemical bond theory cannot explain most bonding phenomena that do not undergo chemical reactions.

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