Лако-красочные материалы — производство

Bascom [35] employed a variety of techniques in his study of the structure of silane films deposited on glass and metal substrates and concluded that vinyl-, amino-, and chloro — functional silane films were deposited as polysiloxanes, some of which could easily be removed from the surface by organic solvents or water. Contact-angle measurements on the remaining strongly retained material indicated it to be of an open polymeric structure since it was easily penetrated by the wetting liquids. That the critical surface tension of a silane film is not an important factor in adhesion promotion by silanes is indicated by many measured values below the minimum Yc value of about 35 dyn/cm for polyesters and 43 dyn/cm for epoxides for optimum wetting to occur [16]. It is suggested that on glass the performance of reactive silanes parallels reactivity rather than polarity (as described by the solubility parameters) of the organofunctional groups [36]. Plueddemann concludes that reactivity of the silane in copolymerization is much more significant than polarity or wettability [16].

Films deposited from nonpolar solvents are relatively thick (>1000 A) and resistant to desorption; films from polar solvents are generally thinner (<100 A) and easily disrupted by polar solvents. An adsorbed silane film can consist of different strata: a silane interface with covalent bonding [10], a relatively cross-linked intermediate layer, and a super­imposed layer of relatively un-cross-linked material. In practice, adsorbed films on both glass and metals are discontinuous and consist of discrete islands or agglomerates, called the button-down theory [37].

The molecular structure of silane films has been shown to depend to a great extent on the pH of the solution from which it was deposited. Using modified infrared spectroscopy to examine films of g-aminopropyltrimethoxysilane (APS) adsorbed on iron and alumi­num surfaces, Boerio and Williams [38] demonstrated that the nature of the film was highly pH dependent. When deposited from solutions of pH below 9.5, the films were of the structure indicated by the expected interaction with oxides with the amine func­tional groups uppermost and available for reaction with the polymer. When the solution pH was greater than 9.5, the film structure was reversed, suggesting a reaction between the amino groups and the surface. In this case the organofunctional groups were not available for reaction with the polymer. Further, this upside-down structure resulted in less hydro­lytically stable bonds.

In an investigation of epoxide joints on iron and titanium using g-APS as a primer, Boerio [39] concluded that although the film structures formed by g-APS adsorbed onto the two metals were very similar, the performance of the films as adhesion promoters was very different. He concluded that the performance was determined by the orientation of the APS molecules at the oxide surface rather than by the overall structure of the film. The orientation was determined by the isoelectric point of the oxide and the pH at which the films were adsorbed onto the oxide [39,40]. A comprehensive account of the structure of APS silane films is provided by Ishida and co-workers [41].