Chemists at Nagoya Institute of Technology in Japan have invented a new and simple coating process that colors metals, strengthens their performance and saves energy. It is a chemical modification of non-ionic polymers involving nanotechnology.

llustration of the electrophoretic deposition (EPD) of sulfone-containing nano-latex shows negatively charged nano-particles in suspension migrating towards the positive electrode. Source: Nagoya Institute of Technology

The current industrial method for coating metal is called electrophoretic deposition. This is used particularly for rust prevention. This method is complex and expensive involving three steps. The costs for this process really add up. The team at Nagoya found that non-ionic polymers and electrophoretic deposition can be used to simplify the coating process to one step, reducing the energy and money needed for the process.

The process was actually discovered by accident when a specific chemical was added to the non-ionic polymer molecule.

"It was accidentally found in a project designing a new material for dental implants. When a non-ionic polymer had a sulfonyl group, it moved towards the anode in electrophoresis," explains Akinori Takasu, head of the research team at Nagoya.

When the new process was first discovered, the coating became very thick when electrophoretic disposition was applied at low voltages. Combining their findings, the team made it possible to skip multiple coatings on a metal. The new problem that the team is facing is in order to make the product commercially available, it must come in any color. Takasu and the team had to look at how color properties of non-ionic polymers reacted in water after it was applied as a coating.

"Our breakthrough was to include this non-ionic polymer into nano-particles. The new particles show structural color like opal stones, a.k.a. colorless color. The wavenumber of the particle should be controllable by changing the size of the particles used to coat the surface," said Takasu.

Takasu didn’t have a problem reacting the non-ionic polymers with the sulfonyl group. The difficult part was controlling the size of the particles. The team had to develop size control technology. They prepared the particles by soap-free emulsion copolymerization. This made nanoparticles 300 nanometers in size. The team oxidized the particles in water and generated the sulfonyl group. In the last step, electrophoretic deposition was applied to coat steel. Electron microscopic images confirmed the particles were uniformly covering the steel like a honeycomb.

The new technique helps overcome struggles of color fading or damage from UV radiation due to structural coloring. This will provide for wider application of electrophoretic dispersion.

This article, titled “Electrophoretic non-ionic nano-spheres (latexes) for structural coloring,” was published in Polymer.