1. The mechanical theory ：mechanical theory holds that the adhesive must infiltrate into the void in the surface of the adhesive and remove the adsorption air from the interface, so as to produce adhesion. Mechanical inlaying is an important factor in the adhesion of porous plastic foam. The effect of adhesive bonding through surface grinding is better than that of dense material with smooth surface, because (1) mechanical setting; (2) to form a clean surface; (3) generating reactive surfaces; (4) increased surface area. Due to the roughness of the surface, the physical and chemical properties of the surface layer can be considered to change, thus improving the bonding strength.
2. Adsorption theory ： adsorption theory holds that adhesion is caused by molecular contact and interface force between two materials. The main source of adhesion is the intermolecular force including hydrogen bond force and van der Waals force. Adhesive and glued continuous contact with the process of wetting, to make the adhesive wetting solid surface, the surface tension of the adhesive should be less than the critical surface tension of solid, adhesive in solid surface depression and gap to form good wetting. If the adhesive is suspended in the concave of the surface, the actual contact area of the adhesive and the adhesive is reduced, thus reducing the bonding strength of the joint.
Many synthetic adhesives are easy to moisten the metal and the surface tension of most solids is less than the surface tension of the adhesive. Actually good wetting condition is adhesive is lower than the surface tension of the glued objects, this is epoxy resin adhesive to metal bonding very good reason, for the untreated polymer, such as polyethylene, polypropylene and fluorine plastic hard to glue.
Through wetting, the adhesive is closely related to the adhesive, mainly by the force of the intermolecular force to produce permanent adhesion. There are four types of chemical bonds in adhesion and cohesion:
(1) ionic bonds
(2) covalent bonds
(3) metal key
(4) van der Waals force
3. Diffusion theory：diffusion theory holds that the adhesion is generated by the diffusion of molecules through the adhesive and the surface of the adhesive. Diffusion theory is generally applicable when adhesives and adhesives are long chain large polymer molecules that can move. The solvent bonding and thermal welding of thermoplastic plastics can be considered as the result of molecular diffusion.
4. The electrostatic force is generated by the formation of a double layer on the interface between the adhesive and the viscous interface, which is the resistance to the separation. When the adhesive is removed from the viscous material, the obvious charge exists, which is a strong confirmation of the theory.
5. The weak boundary layer theory：weak boundary layer theory holds that when the bonding damage is considered to be an interface failure, it is usually the cohesive failure or weak boundary layer destruction. The weak boundary layer comes from adhesive, adhesive, environment, or any combination of the three. If the impurities are concentrated near the adhesion interface and are not securely bonded with the adhesive, the weak boundary layer can be found in the adhesives and the adherents. When damage occurs, although most occur in adhesives and adhesion interfaces, it is actually the destruction of the weak boundary layer.
Polyethylene and metal oxide adhesive is an instance of weak boundary layer effect, strength of polyethylene with low oxygen impurities or low molecular material, make its interface has weak boundary layer on the failure stress very little. If the surface treatment method to remove low molecular impurity or oxygen, the bonding strength greatly improved, the fact has proved that interface, the existence of weak boundary layer and the bonding strength is reduced.