Лако-красочные материалы — производство

Most historical surveys treat the work of McBain and Hopkins in 1925 as the earliest application of modern scientific investigation to the study of adhesion [9]. McBain and Hopkins considered that there were two kinds of adhesion, specific and mechanical. Specific adhesion involved interaction between the surface and the adhesive: this might be ‘‘chemical or adsorption or mere wetting.’’ Specific adhesion has developed into the model we today describe in terms of the adsorption theory.

In contrast, mechanical adhesion was only considered possible with porous materi­als. It occurred ‘‘whenever any liquid material solidifies in situ to form a solid film in the pores.’’ They cite as examples adhesion to wood, unglazed porcelain, pumice, and char­coal. For McBain and Hopkins mechanical adhesion was very much a common sense concept, ‘‘It is obvious that a good joint must result when a strong continuous film of partially embedded adhesive is formed in situ.’’

Despite its ‘‘obvious’’ nature, the mechanical theory of adhesion fell out of favor, and was largely rejected by the 1950s and 1960s. This rejection was prompted by observa­tions that the roughening of surfaces in some instances lowered adhesion and by the tendency to rationalize examples of increased adhesion to rough surfaces in terms of the increased surface area available for ‘‘specific adhesion’’ to take place. In 1965 Wake summarized the position by stating that ‘‘theories that mechanical interlocking… added to the strength of a joint have been largely discredited’’ [10].

However, by the 1970s the mechanical theory was again being taken seriously. The extent of the change can be judged by again quoting a review by Wake, writing this time in 1976: ‘‘adhesive joints frequently possess an important mechanical component essential to the performance of the joint’’ [11].

This radical change resulted from new work from the 1960s cited by Wake, most of which falls into one of two categories. The first is associated with the electroless deposition
of metals onto plastics such as acrylonitrile-butadiene-styrene (ABS) copolymer and polypropylene. In the process the plastics must be etched in a way which produces pits on a micrometer scale. Such a topography had been shown to be a necessary, but not sufficient condition for adequate adhesion.

The second category was concerned with adhesion to microfibrous or porous sur­faces on metals, examples of which are shown Fig. 1. A range of polymers had been shown

to penetrate pores on anodized aluminum [12], dendritic electrodeposits on copper and nickel [13], and needlelike oxides on copper [14] and titanium [15].

Following these theoretical developments in the late 1960s and early 1970s, there was a burgeoning of interest in the relation between surface topography and adhesion [18]. This was facilitated by developments in electron microscopy (scanning electron microscopy and scanning transmission electron microscopy) and in electron spectroscopy (Auger and x-ray photoelectron spectroscopies) that enabled the physical structure and chemical composi­tion of surface layers to be established in detail previously impossible. Considerable work on pore-forming surface treatments for aluminum and titanium was stimulated by the increasing need of the aerospace industry for strong, consistent, and durable adhesive bonds [19-30]. Such work led to Boeing’s adopting a standard phosphoric acid anodizing pretreatment producing a porous surface for structural bonding of aluminum [31].

The broad consensus that comes from most of this work is that strong bonds, and more particularly bonds of high durability, tend to be associated with a highly porous surface oxide, providing, of course, that the values of viscosity and surface tenstion of the adhesive are such as to allow it to penetrate the pores [18]. The importance of porosity was brought out strongly in a 1984 review by Venables [32]. He concluded that for aluminum and titanium ‘‘certain etching or anodization pretreatment processes produce oxide films on the metal surfaces, which because of their porosity and microscopic roughness, mechanically interlock with the polymer forming much stronger bonds than if the surface were smooth.’’ This is as unequivocal a statement of the mechanical theory of adhesion as can be found in the original work of McBain and Hopkins.

Since this time, the acceptance of a ‘‘mechanical theory’’ has not been seriously challenged, and it now has a generally accepted place within the canon of adhesion theories [33-37]. The main features of the mechanical theory have been confirmed in a wide range of experimental situations. Plasma treatment of polymers [38] and of carbon [39] and polymer fibers [40] usually results in a roughening which has been seen as making a mechanical contribution to subsequent adhesion. In developing pretreatments for metals, interest has broadened to include techniques, such as plasma-sprayed coatings [41,42] and metal sintering [43], which produce roughness on a coarser scale. Here again mechanical effects have been postulated as adding significantly to the adhesion.

Thus the theory has proved a ‘‘useful’’ one in the sense that it has stimulated the development of new surface treatments for metals, polymers and fibers and has assisted in giving an understanding of their efficacy. There has perhaps been a tendency, now that the theory is again “respectable,” to invoke “mechanical effects’’ somewhat uncritically wher­ever an increase in surface roughness has been observed. A more detailed review of these developments may be found in references [18] and [44].

Given that the roughening of surfaces often has a beneficial effects on adhesion, how can it be explained? It might have been sufficient in 1925 for McBain and Hopkins [9] merely to assert that the mechanism of adhesion to a porous surface was ‘‘obvious,’’ but the wide range of experimental examples known today demands a more detailed discussion of the mechanisms involved. This, in turn, requires a critical examination of the common sense terms ‘‘surface’’ and ‘‘roughness.’’