In nappy fabrication, researchers need to understand a lot of principles. The principle of bonding is one of them, that is, the process of connecting the substrates together by bonding is called bonding, which mainly includes surface wetting, movement, diffusion and penetration of bonding molecules to the surface of the substrate. There are many factors that affect the bonding, such as the tension between the substrate and the colloid, the surface free energy, the nature of the functional group, the reaction between the interfaces, and the process conditions of the bonding process. The existing bonding theory is elaborated from one aspect and is not comprehensive.
According to the adsorption theory, the main source of adhesive force is the intermolecular force generated when two materials are in contact, mainly including van der Waals force and hydrogen bond. The adsorption process of binder molecules and substrate surface molecules includes diffusion and adsorption. The diffusion process is that the liquid adhesive molecules diffuse to the surface of the substrate through Brownian motion, so that the adhesive interface is close to the polar substrate or link; when the distance between the adhesive and the substrate molecules reaches 0.5~1nm, the interface molecules The force increases as the distance decreases until the adhesive is fully cured. The premise of the adsorption theory is that the two interfaces need to be in close contact for complete wetting, but if the adhesive is too polar, it can sometimes lead to incomplete wetting, reducing bond strength. Therefore, in order to obtain a good bonding effect, in addition to selecting a highly polar tackifying resin, it is also necessary to adjust the fluidity of the adhesive to achieve a better wetting effect.
If two different materials have functional groups that are capable of chemical reactions, a chemical reaction will occur when they are contacted under certain conditions. The chemical reaction theory holds that there are intermolecular forces and chemical bonds between the adhesive and the substrate. Due to the existence of chemical bonds, the bond strength is greatly improved. In addition, the chemical bond improves the environmental resistance and aging resistance of the bonded joint. However, it is often not possible to form chemical bonds at the point of contact between the adhesive and the substrate, so coupling agents are sometimes added to react with the adhesive and substrate surfaces to facilitate bond formation.
Diffusion theory states that it is not enough to rely solely on the mutual contact between the adhesive and the substrate. Interdiffusion is necessary to form a strong bonded joint. This diffusion is interwoven between the interface of the adhesive and the substrate. The result of diffusion leads to the disappearance of the interface and the generation of the transition zone, so that the two parts become one and form a good bond. Therefore, in the design of hot melt adhesive formula, we choose raw materials according to the solubility parameters of matrix resin and tackifying resin, so that they have better compatibility and better adhesion.
The electrostatic theory believes that the adhesive force mainly comes from double-layer static electricity. The formation of the bilayer is caused by the adsorption and orientation of the polar groups of the adhesive on the surface of the substrate, and is the result of free electron transfer between the adhesive and the surface of the substrate. The electrostatic effect does exist, but it is not a necessary condition for the generation of adhesive force. The electrostatic theory is not very comprehensive and not universal, and cannot explain the influence of temperature, humidity and other factors on the bonding performance.
The theory of mechanical interlocking believes that the adhesive can be embedded in the unevenness or gap on the surface of the substrate to generate the same engaging force as the hook between the interfaces, that is, mechanical inlay, the essence of which is friction. But this effect is not the root cause of the bond, but a way to increase the bond strength. The theory of mechanical interlocking is not universal. When bonding porous materials, fabrics and other substrates, the mechanical effect is obvious, but the effect on some solid and smooth surfaces is not obvious. This theory cannot explain the effect of surface electrochemical properties on bond strength.
Among the above five bonding theories, the main control of the bonding force of nappy hot melt adhesive is physical adsorption theory and mechanical interlocking theory. nappy hot melt adhesive does not have chemical reaction, diffusion and electrostatic effect during the gluing process, so when designing the formula, the properties of the material to be adhered, that is, the substrate should be considered first, and the two aspects of physical adsorption and mechanical interlocking should be designed to improve the adhesion. Connectivity.