ABSTRACT:The curing and structure of an epoxy system containing dicyandiamide (DICY) as hardener were studied as a function of temperature and the presence or absence of copper with the use of differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy in photoacoustic mode (FTIR-PAS). Spectroscopic analysis of specimens taken from the DSC helped to clarify the reaction mechanism in terms of the different schemes that have been proposed. The initial stages, corresponding to the first peak in the DSC exotherm, involve the usual epoxide-amine reactions closely followed by a reaction between DICY nitrile groups and hydroxyl groups to form structures containing iminoether and urea groups. These reactions are slightly retarded in the presence of copper. At higher temperatures, corresponding to the second peak in the exotherm, these structures are transformed into others believed to contain urethane ester groups. This reaction, which may be considered to constitute a form of degradation, is significantly accelerated in the presence of copper. The effect is particularly large around 180°C, a temperature commonly used to cure such systems, so the results have important practical implications, for example, in the lamination of circuit boards.

Adhesion at copper-polyamide 11-copper and at aluminum-polyamide 11-aluminum laminate interfaces was studied. Metal-polymer-metal laminates were prepared by compression molding using processing conditions similar to the normal melt processing of polyamide 11. The results show that the time of contact at the molding temperature required to reach a constant level of adhesion is significant. Mild oxidation of the metal prior to molding improves the adhesion of polyamide 11 to aluminum; with copper, a monotonic slow decrease in adhesion with the oxidation time is observed. The presence of a metal surface affects the crystallization behavior of polyamide. With a cooling rate of 40-50°C/min, an approximately 15 µm transcrystalline polymer layer is formed with a degree of crystallinity that is almost 10% higher than the material away from the interface. The metal substrate surface oxidation prior to molding does not change the crystallinity profile of the polymer in the bulk. The polymer surface crystallinity is also a function of the time of contact with the metal substrate. The cooling rate and the metal substrate structure and its nucleating activity are responsible for the surface/bulk crystallinity ratio. Although the highly-crystalline polymer surface layer improves the adhesion to some extent, the formation of active species on the polymer surface which are able to react with the metal surface is mostly responsible for the increase of adhesion with time and its ultimate strength.

Adhesion at polyamide 11-copper and polyamide 11-aluminum interfaces was studied. Metal-polymer-metal laminates were prepared by compression molding using processing conditions similar to the normal melt processing parameters of polyamide 11. The results show that the time of contact at the molding temperature required to reach contact level of adhesion is significant (several minutes). The state of the metal surface affects adhesion. Mild oxidation (prior to molding) improves adhesion with aluminum; with copper, a monotonous slow decrease of adhesion with oxidation time is observed. The presence of the metal surface affects the crystallization behavior of polyamide. Upon a relatively slow cooling rate (40-50'C/min), a transcrystalline layer about 15 1Lm thick is formed having a degree of crystallinity almost 10% higher than the material away from the interface. In the absence of the transcrystalline layer, the level of adhesion is significantly lower.