Polymer chemists at Nagoya Institute of Technology in Japan have invented a simple coating process for coloring metals, which could save energy and lead to higher performance. The process involves a combination of chemically modifying non-ionic polymers and nanotechnology.
Electrophoretic deposition is a standard industrial method for coating materials, in which an electric current is used to deposit charged particles onto a conducting surface, and is especially used for rust prevention. Currently, however, electrophoretic deposition is complex and expensive, often requiring three separate coating steps.
In a paper in Polymer, Akinori Takasu and his team at Nagoya Institute of Technology now report that conducting electrophoretic deposition with novel non-ionic polymers can simplify the coating process to just one step and significantly reduce energy demands. The key to their advance was adding a specific chemical group to the non-ionic polymer molecule. "It was accidentally found in a project designing a new material for dental implant," explains Takasu. "When a non-ionic polymer had a sulfonyl group, it moved towards the anode in electrophoresis."
Previously, the research team had shown that conducting electrophoretic disposition at low voltages produces an incredibly thick coating. Using this approach with non-ionic polymers allowed them to skip multiple coating processes on a metal for rust resistance. For commercial purposes, however, it is important that the coat come in any desired color. Takasu and his colleagues therefore looked at how the color properties of non-ionic polymers behaved in water after being applied as a coating. "Our breakthrough was to include this non-ionic polymer into nanoparticles," Takasu says. "The new particles show structural color like opal stones, aka colorless color. The wavenumber of the particle should be controllable by changing the size of the particles used to coat the surface." Thus, by using nanoparticles of different sizes, Takasu and his team can produce coatings of different colors.
While Takasu could easily react the non-ionic polymers with sulfonyl groups, it proved more difficult to control the size of the nanoparticles. In this study, he and his team prepared the particles by soap-free emulsion copolymerization, which consistently produced nanoparticles of a specific size. They then oxidized the polymer nanoparticles in water to generate the sulfonyl group, before conducting electrophoretic deposition to coat them onto steel. Electron microscopic images confirmed that the nanoparticles uniformly covered the steel in a honeycomb pattern.
"I expect our study will lead to a new type of electrophoretic painting that can be applied to any coating technologies like cars and fibers," says Takasu. Because the color of the nanoparticles depends on their size, this technique also overcomes problems such as color fading and damage from UV radiation.