Peel-strength Behavior of bilayer thermal-sprayed polymer coatings

We prepared various bilayer polymer coatings of ethylene methacrylic acid (EMAA) copolymer and ionomer by the thermal-spray process under a range of preheat temperatures (PTs) to investigate their ability to be repaired. The thermal properties, crystallinity, microstructure, and interface strength of the coatings were investigated with differential scanning calorimetry, X-ray diffraction, scanning electron microscopy, and mechanical testing. Processing parameters influenced the final morphological structure of the coatings. The crystallinity of the coatings increased with a higher final temperature, whereas the coating density decreased. The decrease in density was attributed to the appearance of bubbles, 250 mum in size, formed in the coatings during the spray process. For the monolayer coating of polymer on a metal substrate, a higher PT produced a greater contact area of the coating to the substrate. The adhesion of EMAA ionomer to steel was always lower than that of EMAA copolymer to steel. This may have been largely due to the interfacial adhesion between the polymer and steel being dominated by strong secondary bond interactions. Experimental results also indicate that the peel strength between polymers was at least twofold stronger than that between the polymer and the steel substrate for PTs greater than 100degreesC. The mixed bilayer coating of ionomer on copolymer produced the highest peel strength. The interface between the plastic layers was clearly visible under the scanning electron microscope at lower PTs, becoming more diffuse with-an increase in PT. On the basis of these observations, the adhesion mechanism between polymers was explained by the formation of welding points. (C) 2003 Wiley Periodicals, Inc.