Working Principle And Selection Method of Adhesive

How do adhesives work?

In modern manufacturing and daily life, adhesives play an indispensable role. Based on traditional connection methods such as screws and welding, they have more flexible usage scenarios, and with the development of technology, current adhesives can also cope with reliability environments such as high temperature and high humidity. But on what principle do adhesives bond two objects together? How to choose and use adhesives to ensure the best bonding effect?

There are different types of adhesives we commonly use: acrylic, epoxy, etc. Their bonding process involves multiple aspects, mainly including physical adsorption, chemical bonding, electrostatic action and mechanical interlocking, which are introduced one by one below.

Physical adsorption

Physical adsorption refers to the Van der Waals force between the adhesive and the surface of the adherend. It is a manifestation of intermolecular forces and is generally divided into three types: ① The interaction between the permanent dipole moments of polar molecules. ② One polar molecule polarizes another molecule, generating an induced dipole moment and attracting each other. ③ The movement of electrons in the molecule generates an instantaneous dipole moment, which instantaneously polarizes the neighboring molecule, which in turn enhances the instantaneous dipole moment of the original molecule; this mutual coupling generates electrostatic attraction. The contributions of these three forces are different, and usually the third effect contributes the most.

Chemical bonding

Chemical bonding refers to the formation of chemical bonds such as covalent bonds, ionic bonds or hydrogen bonds between the adhesive and the adherend. The strength of these chemical bonds is much higher than the physical adsorption force, which can significantly improve the bonding strength. For example, silane coupling agents can form stable chemical bonds between metals and polymers, thereby improving bonding properties.

Electrostatic effect

Electrostatic effect refers to the attraction between the adhesive and the adherend due to uneven charge distribution. This effect can provide additional adhesion under certain conditions, such as when the adhesive and the adherend have opposite charges.

Mechanical interlocking

Mechanical interlocking refers to the penetration of the adhesive into the microscopic concave and convex structure of the adherend surface to improve adhesion through mechanical interlocking. The mechanical interlocking of thermoplastic adhesives of polypropylene and wood (oak) has been demonstrated by scanning electron microscopy. The adhesive penetrates pores of 15 μm to a depth greater than 150 μm, and can penetrate to a depth of hundreds of μm in pores with larger diameters. However, it is worth emphasizing that although the treatment of increasing surface roughness such as sandblasting on metal surfaces increases adhesion in result, its mechanism of action is not as simple as increasing mechanical interlocking. Diffusion theory has a few polymers that can become compatible under specific interactions and eliminate the initial boundary.

How to choose right adhesives ?

Choosing the right adhesive is crucial to choosing the right adhesive according to the type of adherend and the environment in which it is used. For example, for applications that require high temperature resistance, adhesives with high glass transition temperatures (Tg points) should be selected; for applications that require high humidity conditions, adhesives that do not hydrolyze should be selected. And considering small deformations, glues with low CTE and hygroscopic expansion coefficients should also be selected. Before deciding on a glue, it is best to conduct a detailed analysis of the operating conditions, including the environment, load, etc., to determine the actual stress on the glue bonding interface, and then compare it with the allowable stress provided by the specification or glue supplier to determine whether there is sufficient safety margin.