Theoretical basis of bonding




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In general, the bonding process is surface treatment, gluing, lamination, curing, post-treatment, etc. It is a complex physical and chemical process.


1. Mechanical theory


According to mechanical theory, the adhesive must penetrate into the voids on the surface of the adherend and remove the air adsorbed at the interface to produce a bonding effect. Mechanical embedding is an important factor in bonding porous adherends such as foam. Adhesive bonding Surface-polished dense materials are better than dense, smooth materials because (1) mechanical inlays; (2) clean surfaces are formed; (3) reactive surfaces are formed; and (4) surface area is increased. Since the grinding does make the surface rough, it can be considered that the physical and chemical properties of the surface layer are changed, thereby improving the bonding strength.


2, adsorption theory


Adsorption theory believes that bonding is caused by molecular contact between two materials and the generation of interfacial forces. The main source of adhesion is the 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. The depressions and voids of the adhesive immersed in the solid surface form good wetting. If the adhesive is erected in the recess of the surface, the actual contact area of ​​the adhesive with the adherend is reduced, thereby reducing the bond strength of the joint.


Many synthetic adhesives tend to wet metal adherends, while most solid adherends have a surface tension that is less than the surface tension of the adhesive. In fact, the condition for good wetting is that the adhesive has a lower surface tension than the adherend, which is why epoxy adhesives are excellent for metal bonding, and for untreated polymers such as polyethylene, polypropylene and Fluoroplastics are difficult to bond.


The adhesive is brought into intimate contact with the adherend by wetting, mainly by * intermolecular forces to produce a permanent bond. There are four types of chemical bonds contained in adhesion and cohesion:


(1) Ionic bond


(2) Covalent bond


(3) Metal bond


(4) Van der Waals force


3. Diffusion theory


Diffusion theory believes that bonding is produced by molecular diffusion at the interface between the adhesive and the adherend. The diffusion theory is basically applicable when both the adhesive and the adherend are capable of moving long-chain macromolecular polymers. Solvent bonding and thermal soldering of thermoplastics can be considered as a result of molecular diffusion.


4, electrostatic theory


Electrostatic attraction, that is, resistance to separation from each other, is caused by the formation of an electric double layer at the interface between the adhesive and the adherend. The presence of significant charge when the adhesive is peeled off from the adherend is a strong demonstration of this theory.


5. Weak boundary layer theory


The weak boundary layer theory holds that when bond failure is considered to be interface failure, it is often a cohesive failure or a weak boundary layer failure. The weak boundary layer comes from adhesives, adherends, the environment, or any combination of the three. If the impurities are concentrated near the bonding interface and are not firmly bonded to the adherend, a weak boundary layer may appear inside the adhesive and the adherend. When damage occurs, although most occur at the interface of 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 substances, and the weak boundary layer at the interface has little damage stress. If a surface treatment method is used to remove low molecular substances or oxygen-containing impurities, the bonding strength is greatly improved, and it has been confirmed that a weak boundary layer exists at the interface, resulting in a decrease in bonding strength.


6, the general process of bonding


Before bonding, the surface to be adhered is firstly treated, and then the prepared adhesive is evenly applied to the surface of the adherend, followed by wetting, rheology, diffusion, penetration, and superposition of the adhesive. After that, make it in close contact. When the distance between the macromolecule of the adhesive and the surface of the adherend is less than 0.5 nm, it will attract each other, generate van der Waals force or form hydrogen bonds, coordination bonds, covalent bonds, ionic bonds, metal bonds, etc., plus infiltration into the pores. The adhesive, after curing, produces a myriad of small "glue hooks" to complete the bonding process, thus obtaining a strong bond.