The bonding mechanism of adhesive(Part Three)

Previously, we have explained the three most important theories, the Weak Boundary Theory, the Diffusion Theory and the Adsorption Theory through two articles. We will continue to explore the remaining two major theories, Electrostatic Theory, and Mechanical Force Theory.

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), two electric layers are formed on both sides of The interface area, thus generating electrostatic attraction.

When stripping adhesive layer from metal surface quickly in dry environment, the light and sound phenomena of discharge can be observed by instruments or naked eyes, which proves the existence of electrostatic effect. 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/square centimetre , the electrostatic attraction can have a significant influence on the bonding strength. The maximum density of the double layer charge migration is only 1019 electrons/square centimetre (some think it is only 1010-1011 electron/square centimetre). Therefore, although electrostatic force does exist in some special bonding systems, it is not a leading factor.

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 and smooth surfaces, this effect is not significant.

The adhesion mechanism of these adhesives can be used for reference by our engineers in solving the adhesion problem of coatings on different substrates.

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