Interphase epoxy-metal

The value of the epoxy resins lies in their reactivity with a variety of chemical groups. This enhanced reactivity also means that the surface chemistry of the reinforcement which the epoxies are cured against, can alter the local structure in the interphase regionJl). The most common reinforcement surfaces cured in contact with the epoxies are carbon/graphite fibers, glass fibers, aramid fibers and metal oxides. The surface chemistry of these reinforcement surfaces is quite diverse and in many cases can be the reason for alteration of the interphase epoxy structure as compared to the bulk. 

The molecular structure of epoxy/metal interphases in the presence of an amino coupling agent was studied by Boerio and co-workers [28] by IR and by XPS. The formation of amide and imide groups in the interphase provided evidence of chemical reaction between the silane primer and the curing agent for epoxy resin. 

The primer chosen for this investigation consisted of an equimolar mixture of phenyl- and amino-functional silanes, suggested as a potential superior primer for aluminum/epoxy adhesive joints [7], The amino-functional silane is known to be effective as an adhesion promoter for fiber-reinforced composite materials [1, 2] as well as for epoxy/metal adhesive joints [8, 9] and provides for strong chemical interaction between the adhesive and primer, while the phenyl functional silane should reduce the overall concentration of polar, hydrophilic functional groups in the interphase region and at the same time maintain or improve the ability of the resin and primer to interpenetrate due to its structural similarity to the adhesive resin. 

Properties of the Interphase Epoxy-Amine/Metal Influences from the Nature of the Amine and the Metal... 

As the results in the literature refer mostly to bulk samples, the question that has to be asked is how aging proceeds in the epoxy/metal interphase, because the joints usually fail in that region. 

Fig. 9. A transmitted electron micrograph of an ultramicrotomed section of an aluminum-epoxy interphase. The highly ordered structure in the center is a 3.3 micron thick aluminum oxide layer present on the base metal. The featureless area is the epoxy matrix. The light areas within the oxide are fractures caused by the microtoming. The epoxy has however penetrated to the bottom of all of the 50 nm pores in the oxide...

Epoxy-amine liquid prepolymers are extensively appHed to metallic substrates and cured to obtain painted materials or adhesively bonded stractures. Overall performances of such systems depend on the interphase created between the organic layer and the substrates. When epoxy-amine Hquid mixtures are appHed to a more or hydrated metaUic oxide layer (such as Al, Ti, Sn, Zn, Fe, Cr, Cu, Ag, Ni, Mg, or E-glass), amine chemical sorption concomitant with metaUic surface dissolution appear, leading to the organometaUic complex or chelate formation [1, 2]. Furthermore, when the solubility product is exceeded, organometaUic complexes may crystaUize. These crystals induce changes of mechanical properties (effective Young s modulus, residual stresses, practical adhesion, durability, etc.). 

When liquid epoxy-amine prepolymers were applied and cured on metallic substrates, interphases were created within the organic layer in the vicinity of the metal surface. 

When epoxy-amine prepolymers were applied on metallic substrates, interphases between the coating part, having the bulk properties, and the metallic surface were created. Amine chemisorption onto oxidic or hydroxidic metallic surfaces, concomitantly with partial dissolution of the surface oxide (and/or hydroxide) on the metal substrate, was observed, according to the basicity characteristics of the amine monomers (pfC,>10). Then it could be assumed that either ... 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

Bentadjine S., Roche A. A., Bouchet J., Epoxy-diamine adhesives on metals the interphase formation and characterization, in Adhesion Aspects of Thin Films (Ed. Mittal K.L.), Vol. 1, 2001, pp. 239-260. 

Possart W., Kruger J. K., Wehlack C., Muller U., Petersen C., Bactavatchalou R., Meiser A., Formation and Stmcture of Epoxy Network Interphases at the Contact to Native Metal Surfaces, C.R. Chimie 2005, in press. 

For the study of the interphase tension at the boundary between the oligomer and a metal, a model system was used in which liquid mercury was used for the metal. The mercury was thoroughly purified and distilled prior to use. The interphase tension was determined hy the sessile drop method [230]. The error was estimated as 3mN/m. The oligomers and adhesives based on them were ED-20 epoxy resin-hased adhesive with PEP A curing agent (12%) and PN 609-2IM polyester resin-based adhesive with MEKP(O). 

Given that the metal surface tension was the same in all csises, the thermodynamic work of adhesion depends on the interphase tension and on the adhesive surface tension. In this case (see Fig. 2.22) a correlation is seen between the thermodynamic work of adhesion and the adhesion strength. This correlation was determined earlier by Lipatov and Myshko by applying the modified equation of Dupre-Young [102]. Epoxy, polyester, pol30irethane, and polyaciylate adhesives were used the type of the adhesive did not have a significant effect on the correlation dependence. 

Glass fibers sized with polyurethane and polyvinyl acetate formed different interfaces. This was due to the differences in reactivity and miscibility. Polyurethane forms a stronger interface because it is reactive and miscible with epoxy resin. " Surface tension of glass surface in a molten state correlates with the interface formation with polymer. The diffusion at interface contributes to a complex structure controlling properties of the interphase. The analysis of the diffusion at the interphase has helped to develop an understanding of the formation of metal-polymer interfaces and plastic welding. 

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