What are the principles of adhesives?

Bonding (bonding, bonding, cementing, gluing) refers to the technology of bonding the surfaces of homogeneous or heterogeneous objects together with adhesives. It has the characteristics of continuous stress distribution, lightweight, or sealing, and low temperature in most processes. Bonding is especially suitable for the connection of different materials, different thicknesses, ultra-thin specifications, and complex components.
Any of a class of natural or synthetic, organic or inorganic substances that can make two or more parts or materials join together through the adhesion and cohesion of interfaces, and so on, are collectively referred to as adhesives, also called adhesives, or adhesives for short. In short, an adhesive is a substance that binds things together through adhesion. “Adhesive” is the general standard term.

Adsorption theory

The theory that the adsorption of solid to adhesive is the primary element of bonding is called the adsorption theory of bonding. According to the theory, the primary source of adhesion is the molecular force of the bonding system, namely the van der Polder attraction and hydrogen bonding force. Adhesive and adhesive surface adhesion and adsorption force have certain same properties. There are two processes of interaction between adhesive molecules and adhesive surface molecules: in the first stage, liquid adhesive molecules disperse to the surface of the adhesive by means of Brownian motion, so that the polar groups or chains at the two interfaces are close to each other. In this process, temperature rise, touch pressure, and viscosity reduction of adhesive are conducive to the strengthening of Brownian motion. The second stage is the onset of adsorption. When the interval between the adhesive and the adhesive molecules reaches 10-5, the interfacial molecules will attract each other, so that the interval between molecules is further shortened to the maximum stability.

The bonding strength differs greatly from the theoretical calculation, which is because the mechanical strength of the solid is a mechanical property, rather than a molecular property, and its size depends on the properties of each part of the data, rather than equal to the sum of molecular forces. The calculation value is assumed that the two planes are tightly touched, and ensure that the interface layer of the interaction between the molecules are damaged together, it is not possible to ensure that the interaction between the molecules at the same time. The polarity of adhesive is too high, sometimes it will severely hinder the wetting process and decrease the adhesion. Intermolecular force is the factor of adhesion supply, but not the only factor. In some exceptional circumstances, other factors can take the lead.

Theory of bond formation

According to the chemical bond theory, in addition to the force between the adhesive and the molecules of the adhesive, sometimes there are chemical bond attacks, and the strength of the chemical bond is much higher than the Vanderbilt force. Chemical bond structure can not only improve adhesion strength, but also overcome the disadvantages of desorption caused by adhesive joint damage. But the composition of chemical bond is not general, it is necessary to be satisfied with certain conditions to form a chemical bond, so it is impossible to make the adhesive and the adhesive touch points between the chemical bond. 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 force cannot be ignored.

Weak boundary layer theory

When the liquid adhesive can not very well moisten the external surface of the adhesive, the air bubble remains in the open space to form a weak zone. For example, the impurities on the side can be dissolved in the molten adhesive, but not dissolved in the cured adhesive, the adhesive will form another phase after solids, and the weak interface layer (WBL) will occur between the adhesive and the adhesive. Attack WBL in addition to the skill elements, in the formation process of polymer mesh or melt interaction, adhesives and appearance of adsorption, and another thermodynamic scene in the attack of the boundary layer structure of non-uniformity. The inhomogeneous interface layer will have a WBL presentation. The stress relaxation and crack development of this WBL will be different.

Dispersion theory

When the two polymers are in close touch with each other under compatible conditions, they are separated from each other by the Brownian motion of the molecules or by the pendulums of the chain segments. This dispersion is staggered through the interface of the adhesive and the adhesive. Dispersion results in the disappearance of the interface and the onset of transition zones. The theory of dispersion cannot explain the adhesion of polymer data to metal, glass, or other hard bodies, because the polymer is difficult to disperse to such data.

Electrostatic theory

When the adhesive and the adhesive system is an electron acceptor-donor combination, electrons will transfer from the donor (such as a metal) to the acceptor (such as a polymer), forming a double layer on both sides of the interface region, and then electrostatic attraction occurs. However, electrostatic force is not the dominant factor, although it does exist in some exceptional bonded systems.

Theory of mechanical forces

From the physicochemical point of view, mechanical action is not an element of adhesion, but a way to add adhesion. The adhesive is soaked into the gap or height of the surface of the adhesive, and the meshing force occurs in the interface area after curing. These conditions are similar to the joint of nails and wood or the effect of tree roots implanted in the soil. The essence of mechanical connection is friction. In bonding, the arrangement of connections is very important, but for some solid and lubricated appearances, this effect is not significant.

Extension: PP polypropylene material surface bonding base material and glue bonding solution.

Polypropylene PP material is a non-polar, surface-insensitive material, a relatively large inertia, and surface difficult to bond, often appears in the actual surface bonding process of the phenomenon of weak bonding degumming, generally in the bonding of polypropylene PP needs to be used together with Jiong shining primer PP adhesive primer to achieve a strong bonding effect. Halogen-free has passed the RoHS test and is widely used in the bonding and flocking process of PP automotive interior decoration.