Лако-красочные материалы — производство

Once interfacial contact between the adhesive and adherend has been established under favourable thermodynamic conditions, adhesive curing enables stress to be transmitted. There is some controversy regarding the basic nature of the intrinsic forces then acting across interfaces which prevent them from separating under an applied load. The four main theories of adhesion which have been proposed are: mechanical interlocking, adsorption, diffusion and electrostatic attraction. Detailed treatments of these attraction theories can be found in the literature(6-15). The adsorption mechanism is generally favoured, with mechanical keying also playing an important role.

Mechanical interlocking. This theory proposes that mechanical keying of the adhesive into the irregularities or pores of the substrate surface occurs, and it underlies the layman’s instinctive procedure of roughening surfaces to improve adhesion. Additionally, a rough surface will have a larger potential bonding area than a smooth one. As good adhesion to smooth surfaces such as glass is attainable, this theory seems limited to porous and fibrous materials such as textiles. Accordingly, penetration of adhesive into the micro­structure or porosity of high alloy metal oxide layers appears important. Thus micro-roughness, rather than macro-roughness, and consequent micro-mechanical inter-locking at a molecular scale is desirable with some metal surfaces, particularly in aiding the retention of adhesion under adverse conditions(2,3). In general, any improvement in joint strength from greater adherend rugosity may be ascribed to other factors such as the increased surface area, improved wetting, or enhanced energy dissipation of the adhesive during joint fracture(7,16).

Adsorption. This,’ the most generally accepted theory outside Russia, proposes that with sufficiently intimate contact, the adhesive macro-molecules are physically adsorbed on to the substrate surface because of the forces acting between the atoms in the two surfaces. In effect, the polar nature of the adhesive molecules acts like a weak magnet and they are attracted towards polar adherend surfaces. The most common interfacial forces are van der Waals’ forces, referred to as secondary bonds, although hydrogen bonding and primary chemical bonding (ionic or covalent) are involved in some cases. The terms primary and secondary imply the relative strength of the bonds (Table 3.2).

Primary bonding, although theoretically unnecessary to account for high joint strengths, may often increase measured joint strengths and is certainly of benefit in securing environmentally stable interfaces. Evidence of chemical bonding is generally limited to coupling agents, and Gettings and Kinloch(23,24) have provided strong evidence for chemical bonding at a particular silane primer/- steel interface.

The huge discrepancy between the magnitude of the attractive forces available for adhesion and measured joint strengths is attributed to insufficient interfacial contact, air voids, cracks, defects and stress concentrations.

Diffusion. This theory proposes that adhesive macromolecules diffuse into the substrate, thereby eliminating the interface, and so can only apply to compatible polymeric substrates. It requires that the chain segments of the polymers possess sufficient mobility and are mutually soluble. The solvent welding of thermoplastics such as PVC (polyvinyl chloride), softened with a chlorinated solvent, is an example of such conditions being met. Diffusion will also take place when two pieces of the same plastic are heat-sealed. The joining of plastic service pipes for carrying gas and water makes use of the diffusion mechanism.

Electrostatic. The electrostatic theory, like the diffusion theory, originated in Russia. It is postulated that adhesion is due to the balance of electrostatic forces arising from the transfer of electrons between adhesive and substrate, resulting in the formation of a double layer of electrical charge at the interface. The two layers thus formed can be likened to the plates of a capacitor and work is expended in separating the two charged capacitor plates. Supportive evidence for this theory includes the fact that the parts of a ruptured joint are sometimes charged, but of course the charge may have been produced during failure. If, as claimed, the work of peeling polymeric films from metal surfaces is much greater than can be accounted for by van der Waals’ forces, this may be due to the dissipation of energy through viscous and visco­elastic responses of the materials. The literature cites only a few special circumstances where the electrostatic mechanism may contribute to intrinsic adhesion.