Theories of Adhesion

Подпись: Mechanical Figure 10.6 Four methods Theories of Adhesion

It is now generally recognised that adhesives stick to surfaces by one of (or a combination of) four different methods (Figure 10.6) and these methods are discussed briefly here but more detail can be found in references [2] and [4].

• Mechanical keying

Perhaps the most widely used of all the adhesion theories is that the adhesive fills any interstitial spaces between the two surfaces and mechanically keys into the surface. The adhesive must not only wet the substrate, but also have the right rheological properties to penetrate any pores and irregularities before the adhesive starts to cure. Slightly rougher surface finishes are nearly always beneficial in any
adhesive bonding application and can often improve the strength and durability of the bonded joint.

• Diffusion

Some adhesives will ‘diffuse’ into the surface. The polymer chains of the adhesive interact locally with the polymer chains of the substrate and the two ‘fuse’ together. Solvent-based adhesives are probably the best example of this adhesion theory. Cyclohexanone and similar solvents are widely used in the medical industry for bonding tubular poly(vinyl chloride) (and other plastic) medical devices. The cyclohexanone is usually applied to the male part by dipping the tube into a container. The solvent evaporates quite quickly but there is sufficient time to assemble the components and as the solvent evaporates, it bonds the two parts together to give a very effective bond.

• Adsorption

The adsorption theory [4] essentially attributes adhesive strength to the forces which act between molecules and atoms in the structure of matter and are known as ‘valence forces’. These can include physical and chemical bonds. The chemical bonds include covalent and ionic bonding between atoms and electrons and the physical bonds include the presence of dipole (or polar) interactions (also known as van der Waals forces).

• Electrostatic

This theory is based on the presence of an electrical double layer between the adhesive and the substrate. It has been found difficult to quantify the electrostatic forces [4] but they can undoubtedly contribute to the total adhesive strength.

There are three modes of failure of an adhesive joint:

• Cohesive,

• Adhesive, and

• Substrate.

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