Six basic theories of adhesives




Network Declaration: Invasion of deletion

Bonding is the result of post-contact interaction between different material interfaces. The role of the interface layer is the basic problem of research in the science of adhesion. Interfacial tension such as adherend and binder, surface free energy, functional group properties, interfacial reaction, etc. all affect the bonding. Gluing is a kind of technology with strong comprehensiveness and complicated influencing factors, and the existing bonding theory is based on a certain aspect to explain its principle, so the only comprehensive theory so far is not available. Between polymers, between polymers and non-metals or metals, between metals and metals, and between metals and non-metals, there is a problem of interfacial bonding between polymer binders and different materials.

     The basic points considered in the following bonding theory are related to the molecular structure of the binder and the surface structure of the adherend and the interaction between them. The bond strength is not only related to the force between the adherent and the adherend, but also to the force between the molecules of the polymer binder. The chemical structure of the polymer molecules, as well as the aggregation state, strongly influence the bonding strength. It is important to study the molecular structure of the adhesive binder for the design, synthesis and selection of adhesives.

1. Adsorption theory:

      The adsorption of solids on adhesives is seen as the main reason for the bonding, called the adsorption theory of bonding. The theory holds that the main source of adhesion is the molecular force of the bonding system, namely van der Waals gravitational force and hydrogen bonding force. Adhesion and adhesion to the surface of the adherend have some of the same properties as the adsorption force.

       The polarity of the adhesive is too high, sometimes severely hampering the wetting process and reducing the adhesion. Intermolecular forces are factors that provide adhesion, but not the only factor. In some special cases, other factors can also play a leading role.

      There are two processes in the action of the adhesive molecules on the surface of the adherend: the first stage is that the liquid adhesive molecules diffuse toward the surface of the adherend by means of Brownian motion, so that the polar groups or links of the two interfaces are brought closer to each other. During this process, warming, application of contact pressure, and reduction of viscosity of the adhesive all contribute to the enhancement of Brownian motion. The second stage is the generation of adsorption force. When the distance between the adhesive and the molecules of the adherend reaches 10-5 Å, mutual attraction between the interface molecules occurs, and the distance between the molecules is further shortened to the maximum stable state.

     According to the calculation, due to the effect of van der Waals force, when the two ideal planes are 10Å apart, 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 modern best structural adhesives can achieve. Therefore, it has been considered that as long as the two objects are in good contact, that is, the adhesive is sufficiently wetted to the bonding interface to achieve the desired state, only the effect of the dispersing force is sufficient to produce a high bonding strength. However, the actual bond strength differs greatly from the theoretical calculation because the mechanical strength of the solid is a mechanical property, not a molecular property, and its size depends on each local property of the material and is not equal to the sum of the molecular forces. The calculated value assumes that the two ideal planes are in close contact and that the interaction between the pairs of molecules on the interface layer is simultaneously destroyed, and it is impossible to ensure that the forces between the pairs of molecules occur simultaneously.

2. The theory of chemical bond formation:

     Chemical bond theory believes that in addition to the interaction 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 agent on bonding, and the bonding of isocyanate to metal and rubber. Studies such as interfaces have demonstrated the formation of chemical bonds. The strength of chemical bonds is much higher than that of van der Waals; the formation of chemical bonds not only improves the adhesion strength, but also overcomes the disadvantages of debonding to break the bonded joint. However, the formation of chemical bonds is not common, and it is necessary to form a chemical bond to form a certain quantum member, so that it is impossible to form a chemical bond between the contact point between the adhesive and the adherend. Moreover, the number of chemical bonds per unit adhesion interface is much less than the number of intermolecular interactions, so the adhesion strength from the interaction between molecules is not negligible.

 3. Weak boundary theory

      When the liquid adhesive does not wet the surface of the adherend well, the air bubbles remain in the voids to form a weak zone. For example, when the impurities are soluble in the molten adhesive and are not soluble in the cured adhesive, the solidified adhesive 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 produces a non-uniformity of the boundary layer structure in the thermodynamic phenomenon such as adhesive and surface adsorption during the molding process of polymer forming or melt interaction. WBL will appear in the unevenness interface layer. The stress relaxation and crack development of this WBL will be different, thus greatly affecting the overall performance of materials and products.

4. Diffusion theory

     The two polymers, when compatible, when they are in close contact with each other, cause interdiffusion due to Brownian motion of the molecules or pendulum of the segments. This diffusion is carried out by interlacing the interface of the adhesive and the adherend. The result of the diffusion leads to the disappearance of the interface and the creation of a transition zone. Bonding systems cannot explain the adhesion of polymeric materials to metals, glass or other hard materials by diffusion theory because polymers are difficult to diffuse into such materials.

 5, electrostatic theory

     When the adhesive and adherend system are a combination of an electron acceptor-supplier, electrons are transferred from the donor (eg, metal) to the acceptor (eg, polymer), forming a double charge on both sides of the interface region. The 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 by the instrument or the naked eye, confirming the existence of electrostatic action. However, the electrostatic effect only exists in a bonding system capable of forming an electric double 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 can have a significant influence on the bonding strength. The maximum density of the double-layered habitat charge generation is only 1019 electrons/cm 2 (some think it is only 1010-1011 electrons/cm 2 ). Therefore, although electrostatic force does exist in some special bonding systems, it is by no means a leading factor.

 6, mechanical force theory

     From a physicochemical point of view, mechanical action is not a factor that produces adhesion, but a way to increase the bonding effect. The adhesive penetrates into the gaps or irregularities on the surface of the adherend, and after solidification, an engaging force is generated in the interface region, which is similar to the joint of the nail with the wood or the root of the tree. The essence of mechanical connection is friction. The mechanical connection force is important when bonding porous materials, paper, fabrics, etc., but for some solid, smooth surfaces, this effect is not significant.