There are problems of interface bonding between polymer base material and different materials between polymers, between polymers and non-metals or metals, between metals and metals and between metals and non-metals. Adhesive bonding is a kind of technology with strong comprehensiveness and complicated influencing factors. The existing bonding theories all explain their principles from a certain aspect, so there is no comprehensive and unique theory so far.
Adsorption theory
People regard the adsorption of solids to adhesives as the main reason for bonding, which is called the adsorption theory of bonding. The theory believes that the main source of adhesive force is the molecular force of the adhesive system, that is, Van der’s gravitation and hydrogen bonding force. When the distance between the adhesive and the adherend molecules reaches 10-5Å, the interface molecules generate mutual attraction, which further shortens the distance between the molecules to the maximum stable state.
According to calculations, due to the van der Waals force , when the distance between two ideal planes is 10Å, the gravitational strength between them can reach 10-1000MPa; when the distance is 3-4Å, it can reach 100-1000MPa. This value far exceeds the strength that can be achieved by the best modern structural adhesives . Therefore, some people think that as long as the two objects are in good contact, that is, the adhesive fully wets the bonding interface, the calculated value is to assume that the two ideal planes are in close contact, and to ensure that the interaction between the pairs of molecules on the interface layer is destroyed at the same time At this time, it is impossible to ensure that the forces between the pairs of molecules occur at the same time.
The polarity of the adhesive is too high, and sometimes it will seriously hinder the wetting process and reduce the adhesion. The intermolecular force is a factor that provides adhesion, but it is not the only factor. In some special cases, other factors can also play a leading role.
Chemical bond theory
The chemical bond theory believes that in addition to the interaction force between the adhesive and the adherend molecules, there are sometimes chemical bonds , such as the bonding interface between vulcanized rubber and copper-plated metal, the effect of coupling agents on bonding, and the bonding of isocyanate to metal and rubber. Research on the interface, etc., has proved that there is the formation of chemical bonds. However, the formation of chemical bonds is not common. To form chemical bonds, certain quantization components must be met, so it is impossible to form chemical bonds at all contact points between the adhesive and the adherend. Moreover, the number of chemical bonds on the unit adhesion interface is much less than the number of intermolecular interactions, so the adhesion strength from the intermolecular forces cannot be ignored.
Weak boundary layer theory
When the liquid adhesive cannot well infiltrate the surface of the adherend, air bubbles remain in the gap and form a weak zone. For another example, when the impurities contained therein can be dissolved in the molten adhesive, but insoluble in the cured adhesive, another phase will be formed in the solidified adhesive, and a weak interface layer (WBL) will be formed between the adherend and the entire adhesive. In addition to the process factors, the non-uniformity of the boundary layer structure is caused by thermodynamic phenomena such as adhesive and surface adsorption during the forming process of polymer network or melt interaction. WBL will appear in the uneven interface layer. The stress relaxation and crack development of this kind of WBL will be different, which greatly affects the overall performance of materials and products.
Diffusion theory
On the premise that the two polymers are compatible, when they are in close contact with each other, the phenomenon of mutual diffusion occurs due to the Brownian motion of the molecule or the pendulum of the chain segment. This diffusion effect is interwoven across the interface between the adhesive and the adherend. The result of diffusion leads to the disappearance of the interface and the creation of a transition zone. With the aid of diffusion theory, the bonding system cannot explain the adhesion of polymer materials to metal, glass or other hard bodies, because it is difficult for polymers to diffuse into such materials.
Electrostatic theory
When the adhesive and adherend system is a combination of electron acceptor-donor, electrons will be transferred from the donor (such as metal) to the acceptor (such as polymer), forming double electrics on both sides of the interface zone Layer, which creates electrostatic attraction.
When the adhesive layer is quickly peeled off from the metal surface in a dry environment, the light and sound phenomena of the discharge can be observed with instruments or the naked eye, which confirms the existence of static electricity. But the electrostatic effect only exists in the bonding system that can form an electric double layer, so it is not universal. In addition, some scholars pointed out that the charge density in the electric double layer must reach 10 21 electrons/cm 2 before the electrostatic attraction can have a significant effect on the bonding strength. However, the maximum density of electric double layer migration charge generation is only 10 19 electrons/cm 2 (some think it is only 10 10 -10 11 electrons/cm 2 ). Therefore, although electrostatic force does exist in some special bonding systems, it is by no means a dominant factor.
Mechanical force
From a physical and chemical point of view, mechanical action is not a factor that produces adhesion, but a way to increase the adhesion effect. The adhesive penetrates into the gaps or bumps on the surface of the adherend, and after curing, produces a meshing force in the interface area. These situations are similar to the effect of nails and wood joints or roots implanted in soil. The essence of mechanical connection is friction. When bonding porous materials , paper, fabrics, etc., mechanical connection is very important, but for some solid and smooth surfaces, this effect is not significant.
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