Adhesive bonding between polyamide and steel

Fluorescence and IR absorption spectra are taken of coatings obtained by applying polyamide 6 powder on a steel substrate heated above the polymer melting point and subsequently cooling to room temperature. It follows from the coating spectra that the energy of a $\pi^*\to n$ transition in the C–O bonds of polyamide decreases. Simultaneously, the maximum of a band assigned to the deformation vibrations of N–H bonds shifts toward longer wavelengths. These effects are explained by the formation of coordination bonds between Fe$^{2+}$ ions having diffused from the steel into the polymer and nitrogen atoms entering into polyamide 6 molecules. As a result, a coordination-compound-saturated diffusion layer up to 100 $\mu$m thick arises near the steel surface. Coordination compounds squeeze the framework of the polyamide 6 molecule roughly by 0.06%. Eventually, a polyamide layer that is stronger than the surroundings appears at the polyamide 6–steel interface.