(I) Mechanical Theory

This theory holds that the adhesive penetrates into the pores on the surface of the adherend and forms a mechanical interlock after curing, thereby generating bonding force. Just like a nail driven into wood, the adhesive forms "anchors" in the pores on the adherend's surface to connect the two together. This theory is applicable to materials with rough surfaces, such as wood and paper.

(II) Adsorption Theory

Molecules of the adhesive and molecules on the surface of the adherend are mutually adsorbed through intermolecular forces (e.g., van der Waals forces, hydrogen bonds, etc.), thus producing bonding force. Although these intermolecular forces are relatively weak, the total bonding force is still considerable due to the large number of molecules involved. The adsorption theory can explain the bonding phenomenon of adhesives on various materials with smooth surfaces.

(III) Chemical Bond Theory

Chemical bonds (such as covalent bonds, ionic bonds, etc.) are formed between the adhesive and the surface of the adherend, resulting in a strong bonding force. The strength of chemical bonds is much higher than that of intermolecular forces; therefore, bonded joints formed by chemical bonds have high strength and stability. However, to form chemical bonds, the adhesive and the adherend surface need to have specific chemical structures and reactive activities.