Adhesion

Theories of adhesion

Adhesion intervenes in multiple fields, each one of them having given rise to a theory which is clean for him. There thus does not exist single theory of adhesion but a whole of complementary and sometimes contradictory models. Also we will describe each one of these theories briefly, in the case of a substrate covered with an unspecified coating:

the Mechanical theory

The theory of Mechanical anchoring , initiated by Mac Bath since 1926, is oldest of the theories of adhesion. This model considers that the origin of adhesion comes from an anchoring Physique of the coating in the asperities present at the Surface of the Substrat. It results from it a mechanical fixing between two surfaces. This mechanism is thus applicable only when there exists an intimate contact between two materials and that the coating (the adhesive, painting…) wet the surface of the substrate well in order to be able to penetrate anfractuosities of its surface. But adhesion on perfectly smooth surfaces supposes other theories.

the electric theory

The electric theory was developed by Deryagin. To the surface of layers of Oxide S in ionic matter, chemical heterogeneities and the defects of surface lead to the formation of electrostatic loads. The system coating/substrate is then compared to a plane condenser whose reinforcements would be the two electric layers formed in contact with two surfaces.

the theory of the Diffusion

The theory of the diffusion was proposed by Voyuskii. It applies to the compatible materials Polymère S (i.e. when at least a monomer is soluble in the other monomer), in particular with the autoadhésion, at the time of the contact of two materials of comparable nature, like rubber. In this theory, adhesion results from the interdiffusion of the molecules or the chains of one of the prépolymères in the other. The interface between two materials disappears with the profit from an interphase, whose physicochemical properties vary continuously, since the properties from first material until those from the second. This adhesion is controlled by the phenomena of diffusion, this is why it implies the compatibility of materials.

the Thermodynamic theory

This theory, so called theory of damping, was initiated by Sharpe and Schonhorn. According to this theory, adhesion is allotted to the intermolecular forces (chemical bonds of the type Van der Waals), existing with the interface. These intermolecular connections weak and are not directed. They have a sphere of activity about the intermolecular distances. So so that they are established, it is necessary to create a good contact between two surfaces.

the theory of the layers of weak Cohesion (weak boundary layers)

Bikerman noted the presence of layers of low cohesion during the analysis of the zones of rupture of the adhesive bonded joints, but this theory is often called theory of adhesion by abuse language. The interfacial forces are increasingly stronger than the force of cohesion of one of the many layers composing the assembly. It follows that the rupture will always take place in the layer whose force of cohesion is weakest, called layer of low cohesion. The composition of this layer can vary. The causes of this variation are numerous.

the Chemical theory

The chemical theory was proposed by Buchan and Rae. Adhesion is based here on the formation of iono-covalent chemical bonds. These connections are among strongest (up to 1000 kJ.mol^-1): they ensure the adhesive bonded joints a breaking strength important and a better durability, contrary to the connections type Van der Waals. The distance from interaction between atoms (0,15 to 0,24 Nm) thus imposes an intimate contact between two materials and thus, there still, a good damping.

Limit of adhesion

Stress concentration

According to the theory of Volkersen, there is a stress concentration at the edge of the joints of adhesives which increases with the length of the joint. This increase limits the transmissible force per unit of width of the adhesive bonded joint.

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