1, mechanical theory The mechanical theory believes that the adhesive must penetrate into the gap on the surface of the adherend and remove the air adsorbed on the interface to produce the adhesive effect. Mechanical tacking is an important factor in bonding porous adherends such as foam. Adhesive bonding Surface-polished dense materials are better than dense surfaces with smooth materials because (1) mechanical inlays; (2) clean surfaces; (3) reactive surfaces; (4) increased surface area. As the surface roughness of the surface is roughened, it can be considered that the physical and chemical properties of the surface layer have been changed, thereby increasing the bonding strength. 2. Adsorption theory The adsorption theory suggests that bonding is caused by the contact between molecules and the interfacial forces between two materials. The main source of adhesive force is intermolecular forces including hydrogen bonding forces and van der Waals forces. The process of continuous contact between the adhesive and the adherend is called wetting. To make the adhesive wet the solid surface, the surface tension of the adhesive should be less than the critical surface tension of the solid, and the depression and voids of the adhesive immersed in the solid surface form a good wetting. If the adhesive is emptied in the recess of the surface, the actual contact area of ​​the adhesive with the adherend is reduced, thereby reducing the adhesive strength of the joint. Many synthetic adhesives readily wet metal adherends, and most solid adherends have surface tensions less than the surface tension of the adhesive. In fact, the conditions for good wetting are that the surface tension of the adhesive is lower than that of the adherend, which is why the epoxy adhesive is excellent for metal bonding, and for untreated polymers such as polyethylene, polypropylene and Fluoroplastics are difficult to bond. Through wetting, the adhesive and the adherend are in close contact, and the permanent bond is mainly produced by the intermolecular forces. There are four types of chemical bonds contained in adhesion and cohesion: (1) Ionic bond (2) Covalent bond (3) Metal keys (4) Van der Waals force 3. Diffusion theory The theory of diffusion believes that bonding occurs through the diffusion of molecules at the interface between the adhesive and the adherend. Diffusion theory is basically applicable when the adhesive and the adherend are both long-chain macromolecular polymers that can move. Solvent bonding and thermal welding of thermoplastics can be considered as the result of molecular diffusion. 4, electrostatic theory due to the formation of double electric layer at the interface of the adhesive and the adherend has generated electrostatic attraction, that is, the resistance to separation. The existence of a significant charge when the adhesive is peeled from the adherend is a strong confirmation of this theory. 5. Weak boundary layer theory The theory of weak boundary layer states that when the bond failure is considered to be interface failure, it is actually a cohesive failure or a weak boundary layer failure. The weak boundary layer comes from the adhesive, the adherend, the environment, or any combination of the three. If impurities are concentrated in the vicinity of the bonding interface and loosely bonded to the adherend, a weak boundary layer may appear inside the adhesive and the adherend. When damage occurs, although the majority occurs at the interface between the adhesive and the adherend, it is actually the destruction of the weak boundary layer. The adhesion of polyethylene to metal oxides is an example of a weak boundary layer effect. Polyethylene contains low-intensity oxygen-containing impurities or low-molecular-weight substances, and its interface has weak boundary layers with little damage stress. If surface treatment methods are used to remove low molecular weight or oxygen-containing impurities, the bond strength is greatly improved. In fact, it has been demonstrated that there is a weak boundary layer at the interface, resulting in a decrease in the bonding strength.
Source: 21st Century Fine Chemicals Network