AbstractWhen high‐molecular weight polyacrylic acid (PAA) macromolecules are incorporated into phosphating solutions, they significantly increase the stiffness and ductility of zinc phosphate conversion crystal films deposited on metal substrate surfaces. The degree of improvement in these mechanical properties depends primarily on the thickness and fineness of the highly dense PAA‐complexed zinc phosphate crystals that are formed. The presence of this complexed PAA polymer at the outermost surface sites on the crystal layer plays an essential role in promoting adhesion with polymeric topcoats by the formation of polymer—polymer chemical bonds. The use of low‐molecular weight PAA results in a poor interfacial adhesion because of a high degree of alternation in chain conformation caused by charge‐transfer reactions between the proton‐donating COOH groups and the nucleophilic Zn2+ions in the acidic aqueous medium. In addition, the protective polymer topcoat overlayed on the complex precoat surfaces increases the flexibility of the crystalline precoat itself. The most important factors controlling the flexibility of the precoat were low elastic modulus, high tensile strength, and elongation of the topcoat systems. These characteristics are more important than the magnitude of interfacial adhesive bonds at topcoat/pre
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