Synthesis of protective coatings on steel by surface spontaneous polymerization

Abstract

We have previously reported a new method for applying protective coatings on metal surfaces, the surface spontaneous polymerization process. Polymerization occurs spontaneously on the substrate upon immersion into a monomer solution, forming a layer of uniform coating in situ. A redox mechanism was proposed to explain the spontaneous initiation [1]. In this paper, we report studies of the propagation mechanism involving several monomer systems. It was found that propagation via both a charge transfer complex and addition of free monomers is possible, and also that both mechanisms can be operating at the same time. This is the case for the 4-carboxyphenyl maleimide (4CPMI)/styrene/acrylonitrile system, in which the donor monomer, styrene, forms a charge transfer complex with the strong acceptor monomer, 4CPMI, and copolymerizes alternatingly while the other acceptor monomer, acrylonitrile, is incorporated randomly. The surface spontaneous polymerization process can therefore be used to copolymerize a wide variety of monomers to yield coatings with desired properties.

Introduction

Coatings are widely used on metal surfaces for decorative and/or protective purposes. The protective coating industry is currently under tremendous pressure to develop new coating methods and materials to remain competitive and to meet increasingly stringent environmental regulations. Coating methods that can provide high-quality coatings economically, while causing less pollution during manufacturing are very desirable in industrial applications. In a previous paper [1], we reported the discovery of a new method of forming protective coatings on metal surfaces, the surface spontaneous polymerization process (S-Poly), in which the polymerization occurs spontaneously on a cleaned metal substrate upon immersion into a monomer solution, forming a uniform layer of coating with thickness ranging from 1 to 50 μm. This process is applicable to a variety of metals, such as aluminum 2, 3, copper, zinc, tin, and silver. Polymerization, deposition, and crosslinking, if necessary, can be achieved in a single step. A variety of monomers can be copolymerized, such that the coating properties can therefore be easily tailored to specific application needs by adjusting the monomer feed composition.

The phenomenon of spontaneous polymerization of two monomers has been studied by several researchers in the literature 4, 5, 6, 7, 8. However, these studies were limited to organic solvent and the polymerization occurred in the solution. The S-Poly process is different in that initiation occurs at the metal surface instead of in the bulk of the monomer solution and the process is conducted in a partially to completely aqueous medium. A redox mechanism has been proposed [1]to explain the initiation of polymerization. The active metal substrates are oxidized and the monomer reduced into a free radical form in an aqueous reaction environment; the radical then initiates polymerization. The process is thermodynamically favorable. Studies on a 4-carboxyphenyl maleimide (4CPMI)/styrene system were previously reported [1]. In this article, we report our studies on the polymerization propagation behavior of several other monomer systems.