Лако-красочные материалы — производство

The electronic theory of adhesion was proposed primarily by Deryaguin and co-workers [15-19] in 1948. These authors have suggested that an electron transfer mechanism between the substrate and the adhesive, having different electronic band structures, can occur to equalize the Fermi levels. This phenomenon could induce the formation of a double electrical layer at the interface, and Deryaguin et al. have proposed that the resulting electrostatic forces can contribute significantly to the adhesive strength. Therefore, the adhesive-substrate junction can be analyzed as a capacitor. During inter­facial failure of this system, separation of the two plates of the capacitor leads to an increasing potential difference until a discharge occurs. Consequently, it is considered that adhesive strength results from the attractive electrostatic forces across the electrical double layer. The energy of separation of the interface Ge is therefore related to the discharge potential Ve as follows:

where h is the discharge distance and ed the dielectric constant. Moreover, according to such an approach, adhesion could vary with the pressure of the gas in which the measure­ment is performed. Hence Deryaguin et al. have measured, by means of a peel test, the work of adhesion at various polymer-substrate interfaces, such as poly(vinyl chloride)- glass and natural rubber-glass or steel systems, in argon and air environments at various gas pressures. A significant variation in peel energy versus gas pressure was indeed evi­denced and very good agreement between the theoretical values, calculated from Eq. (1), and the measured values of Ge was obtained whatever the nature of the gas used. However, several other analyses [5,20] have not confirmed these results and seem to indicate that the good agreement obtained previously was rather causal. According to Deryaguin’s approach, the adhesion depends on the magnitude of the potential barrier at the sub­strate-adhesive interface. Although this potential barrier does exist in many cases (see, e. g., [21,22]), no clear correlation between electronic interfacial parameters and work of adhesion is usually found. Moreover, for systems constituted of glass substrate coated with a vacuum-deposited layer of gold, silver, or copper, von Harrach and Chapman [23]
have shown that the electrostatic contribution to peel strength, estimated from the mea­surement of charge densities, can always be considered as negligible. Furthermore, as already mentioned, the energy dissipated viscoelastically or plastically during fracture experiments plays a major role on the measured adhesive strength, but it is not included conceptually in the electronic theory of adhesion. Finally, it could be concluded that the electrical phenomena often observed during failure processes are the consequence rather than the cause of high bond strength.