The adhesive and the substrate are held together by lots of weak electrostatic forces. This means that for maximum adhesion the adhesive should be completely spread over the substrate to increase the contact surface area between the two substances. The wetting of an adhesive is its ability to form an interface with the solid surface. The degree of wetting, also known as the wettability, depends on both the surface tension and the viscosity of the adhesive. This can be determined by the contact angle between the substrate and the adhesive as shown in Figure 1.

Degree of Wetting of Substrate.

Figure 1: Degree of Wetting of Substrate.

The wettability of an adhesive is determined by a force balance between adhesive and cohesive forces. When the cohesive forces within the liquid are greater than the adhesive forces then the adhesive will retain a spherical shape. This is known as de-wetting. On the other hand, when the adhesive forces outweigh the cohesive forces, the adhesive will be attracted to the substrate surface. This leads to wetting which is the spreading of the liquid on the surface of the solid.

Substrate materials with low surface energy do not wet out easily as there will be a lower adhesive force between the liquid and surface. As a rule of thumb, the higher the surface energy the higher the strength of adhesion. The surface energies of some typical materials are shown in the table below.

Table 1: Surface energy of materials.
Material Surface energy (mN/m)
Silicone Rubber 26
PE/PP 30
Hydrocarbon Rubbers 30-40
PU 45
PA 45
Glass 100
Metals ~500

To ensure the wetting of the material, the surface tension of the adhesive must be less than the surface energy of the solid surface. The surface tension of selected liquids is shown in the following table.

Table 1: Surface tension of liquids.
Material Surface tension (mN/m)
Ethanol 22
MEK 24
Toluene 28
Water 70
Mercury 500

Another factor which determines the wetting of a substrate is the viscosity of the liquid adhesive. The viscosity of a fluid depends on the strength of its intermolecular forces between its molecules. The greater the attraction between the molecules the more viscous the liquid will be. High viscosity adhesives can be advantages as this can help to avoid the running of the adhesive at the edges of bonded joints.

The wetting of an adhesive increases the surface area between the substrate and adhesive materials. The surface area can also be increased by Mechanical Interlocking, illustrated in Figure 2. This occurs when the adhesive penetrates any pores or irregularities in the substrate surface. The interlocking also helps to provide a physical barrier which can limit crack propagation at the interface.

Mechanical interlocking between adhesive and substrate

Figure 1: Mechanical interlocking between adhesive and substrate.

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