Лако-красочные материалы — производство

A. Mechanical Interlocking

The mechanical interlocking model, proposed by MacBain and Hopkins in 1925 [1], conceives of mechanical keying, or interlocking, of the adhesive into the cavities, pores, and asperities of the solid surface to be the major factor in determining adhesive strength. One of the most consistent examples illustrating the contribution of mechanical anchoring was given many years ago by Borroff and Wake [2], who have measured the adhesion between rubber and textile fabrics. These authors have clearly proved that penetration of the protruding fiber ends into the rubber was the most important parameter in such adhesive joints. However, the possibility of establishing good adhesion between smooth surfaces leads to the conclusion that the theory of mechanical keying cannot be considered to be universal. To overcome this difficulty, following the approach suggested primarily by Gent and Schultz [3,4], Wake [5] has proposed that the effects of both mechanical inter­locking and thermodynamic interfacial interactions could be taken into account as multi­plying factors for estimating the joint strength G:

G = (constant) x (mechanical keying component) x (interfacial interactions component)

Therefore, according to the foregoing equation, a high level of adhesion should be achieved by improving both the surface morphology and physicochemical surface proper­ties of substrate and adhesive. However, in most cases, the enhancement of adhesion by mechanical keying can be attributed simply to the increase in interfacial area due to sur­face roughness, insofar as the wetting conditions are fulfilled to permit penetration of the adhesive into pores and cavities.

Work by Packham and co-workers [6-9] has further stressed the notable role played by the surface texture of substrates in determining the magnitude of the adhesive strength. In particular, they have found [6] that high values of peel strength of polyethylene on metallic substrates were measured when a rough and fibrous type of oxide surface was formed on the substrate. More recently, Ward et al. [10-12] have emphasized the improve­ment in adhesion, measured by means of a pull-out test, between plasma-treated poly­ethylene fibers and epoxy resin. In that case, long-time plasma treatments create a pronounced pitted structure on the polyethylene surface, which can easily be filled by the epoxy resin by means of good wetting.

One of the most important criticisms of the mechanical interlocking theory, as suggested in different studies [9,13,14], is that improved adhesion does not necessarily result from a mechanical keying mechanism but that the surface roughness can increase the energy dissipated viscoelastically or plastically around the crack tip and in the bulk of the materials during joint failure. Effectively, it is now well known that this energy loss is often the major component of adhesive strength.