The basic theory of adhesives

Between the polymer, the polymer and the non-metal or the metal, the metal and the metal and the metal and the non-metal adhesive, etc., there is a problem that the interface between the polymer base material and the different materials are glued. Bonding is the result of post-contact interaction between different material interfaces. Therefore, the role of the interfacial layer is a fundamental issue in the study of adhesive science.

I. Adsorption theory

People consider the adsorption of solids to the adhesive as the main reason for the glue, which is called the glued adsorption theory. Adsorption theory believes that bonding is caused by the molecular contact between two materials and the generation of interfacial forces. 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 involved in adhesion and cohesion: (1) Ionic bonds (2) Covalent bonds (3) Metal bonds (4) Van der Waals forces.

The polarity of the adhesive is too high and can sometimes seriously interfere with the wetting process and reduce the adhesion. Intermolecular force is a factor that provides adhesion, but it is not the only factor. In some special circumstances, other factors can also play a leading role.

Second, chemical bond formation theory

Chemical bonding theory believes that in addition to the interaction between the adhesive and the adherent molecules, there are sometimes chemical bonds, such as the bonding interface between the vulcanized rubber and the copper plating metal, the effect of the coupling agent on the bonding, and the isocyanate to metal and rubber adhesives. Studies of the interface, etc., have all demonstrated the generation of chemical bonds. The strength of the chemical bonds is much higher than the van der Waals forces; the formation of chemical bonds can not only increase the adhesion strength, but also overcome the disadvantages of desorption and destruction of adhesive joints. However, the formation of chemical bonds is not common. To form chemical bonds must satisfy certain quantized parts, it is impossible to form chemical bonds at the contact points between the adhesive and the adherend. Moreover, the number of chemical bonds at the unit adhesion interface is much less than the number of interactions between molecules, so the adhesion strength from the intermolecular forces can not be ignored.

Third, the weak boundary theory

When the liquid adhesive does not wet the surface of the adherend very well, air bubbles remain in the void and form a weak zone. For another example, impurities contained in the molten adhesive can be dissolved in the molten adhesive, but not in the cured adhesive, the adhesive after solidification forms another phase, and a weak interface layer (WBL) is formed between the adherend and the adhesive as a whole. In addition to the process factors, WBL generates non-uniformity of the interfacial structure in thermodynamic phenomena such as adhesives and surface adsorption in the process of polymer web formation or melt interaction molding. Inhomogeneous interface layer will appear WBL. The stress relaxation of the WBL and the development of cracks will be different, thus greatly affecting the overall performance of materials and products.

Fourth, diffusion theory

Under the premise of compatibility of the two polymers, when they are in close contact with each other, mutual diffusion occurs due to the Brownian motion of the molecules or the pendulum of the segments. This diffusion is done through the interface between the adhesive and the adherend. Diffusion results in the disappearance of interfaces and the creation of transitional regions. The adhesion system cannot explain the adhesion of the polymer material to metal, glass or other hard materials by means of diffusion theory because it is difficult for the polymer to diffuse into such materials.

Fifth, electrostatic theory

When the adhesive and the adherend system is a combination of an electron acceptor-supply body, electrons will transfer from the donor (such as metal) to the acceptor (such as a polymer), forming double electrical charges on both sides of the interface area. Layers, which produce electrostatic attraction.
When the adhesive layer is quickly peeled off from the metal surface in a dry environment, the light and sound of the discharge can be observed with the instrument or the naked eye, and the existence of the electrostatic effect is confirmed. However, the electrostatic effect only exists in a bonding system capable of forming a double electric layer, and thus is not universal. In addition, some scholars have pointed out that when the charge density in the electric double layer must reach 1021 electrons/cm 2 , the electrostatic attraction force can be used to displace the adhesive.芏鹊淖畲笾 芏鹊淖畲笾 芏鹊淖畲笾 SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN SPAN. Therefore, although the electrostatic force does exist in some special bonding systems, it is by no means a dominant factor.

Six, mechanical force theory

From a physicochemical point of view, mechanical action is not a factor in the production of adhesion, but a means of increasing the bonding effect. The adhesive penetrates into the gaps or bumps on the surface of the adherend and, after curing, creates an interfacial force in the interfacial zone that resembles the bond of the nail to the wood or the effect of the root of the tree. The nature of the mechanical connection force is friction. Mechanical bonding is important when bonding porous materials, paper, fabrics, etc., but for certain solid and smooth surfaces, this effect is not significant.

The basic points considered by the glue theory mentioned above are all related to the molecular structure of the adhesive and the surface structure of the adherend and the interaction between them. Destruction experiments from glued joints show that there are four different conditions when glued joints are destroyed: 1. Interface failure: All the adhesive layers are separated from the surface of the adhesive body (complete separation of the adhesive interface); 2. Cohesive failure: Damage occurs in the adhesive or is Adhesive body itself, but not between the adhesive interface; 3. Mixed damage: the adherend and adhesive layer itself are partially damaged or only one of the two. These damages indicate that the bonding strength is not only related to the force between the adherend and the adherend but also related to the interaction between the molecules of the polymer adhesive. The chemical structure and the aggregation state of the polymer molecules strongly influence the bonding strength. It is important to study the molecular structure of the adhesive base material for designing, synthesizing, and selecting adhesives.

Source: Thermosetting resin network

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