Basic Principles of Adhesion
Create the best bond
The main purpose of bonding is to bind two materials together in the most appropriate way possible. In general, the interaction between two different materials along an interface can occur by physical or chemical bonding, depending on the properties of the materials.
Although various theories and practical approaches have been developed, the joining of similar or different substances by gluing still covers many issues that are not fully explained. According to the studies done so far and the proposed theories and the general result, the following materials properties are interpreted as important in determining the degree of adhesion:
- Surface features
- Chemical and physical condition of the adhesive to be used in the joining
- Surface relations between the materials to be joined and the adhesives to be used.
Today's studies on adhesion or bonding with adhesives can be examined under two main groups. One deals with the chemistry and physics of surfaces, while the other deals with the fracture mechanics of bonded joints. While the first group generally focusing on bond formation and determination of the magnitude of interaction along bonding line, the second group deals with the development and analysis of test methods to measure the resistance of bonded bodies across the interface.
Chemical bonding occurs as a result of direct interaction between the molecules of the adhesive and the bonded substance molecules with covalent, ionic, hydrogen, Van der Waals bonds between the surfaces and the glue layer. Physical bonding, on the other hand, may be the result of mechanical interlocking when the adhesive solidifies on a rough surface where physical absorption forces occur between the adhesive molecules and the bonded molecules or the diffusion of the adhesive molecules into the adhered material.
The bond energies of some chemical bonds that should be considered in joining with glues are briefly shown below as “Bond type/Bond energy [kJ/mol]”:
- Ionic bond / 600-1,100.
- Covalent bond / 160-700.
- Lewis acid-base interaction/ Up to 80.
- Bronsted acid-base interaction / Up to 1,000.
- Hydrogen bonds / 10-25.
- Dipole dipole interaction / 4-20.
- Dispersion forces 0.08-40.