Colourless Coating Gives Superior Protection Against Rust

The vast majority of metal objects will end up on the scrapheap in a rusty shell of their formers selves – we can’t alter the fact that they will rust, but we can delay it and prevent it as best we can.

Electrodeposition, or electrophoretic deposition (EPD) is the standard industrial method for rustproofing metal objects; colloidal particles suspended in a liquid medium travel under the influence of an electric field and are deposited onto an electrode.

It allows for a uniform film to be applied even on a complex surface. However, it is an expensive and time consuming three-step process requiring lots of energy.

Polymer chemists from the Nagoya Institute of Technology in Japan believe they have invented a novel and simple process to coat metals that is superior to current approaches. They report in the journal Polymer that their method uses non-ionic polymers incorporated into nanoparticles to coat metals in a single step, using significantly less energy. Furthermore, the coating is essentially colourless, meaning it has a wide range of potential applications.

Professor Akinori Takasu and his colleagues suggest the key to their discovery was the addition of a specific chemical group to the non-ionic polymer molecule, a non-ionic poly(ester-sulfone) material which showed anode selective electrophoretic behaviour.

"It was accidentally found in a project designing a new material for dental implants,” explains Takasu, Professor in the Department of Life Science and Applied Chemistry. “When a non-ionic polymer had a sulfonyl group, it moved towards the anode in electrophoresis."

Previous work showed that the resulting coating becomes incredibly thick when electrophoretic deposition is applied at low voltages. However, by combining this with other findings, the team found they could skip the multiple coating processes in favour of a single step.

In order for it to be commercially viable, the coating needed to be available in any desired colour. This led to Takasu and his colleagues studying how the colour properties of the non-ionic polymers behaved in water after it was applied as a coating.

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

This means that particle size could be used to determine the colour emitted. But while reacting the non-ionic polymers with the sulfonyl group was straightforward, controlling particle size proved difficult. Takasu and his team developed a size control technology and prepared the particles by soap-free emulsion copolymerisation, which consistently gave spherical nanoparticles 300 nm in diameter.

They then oxidised the particles in water to generate the sulfonyl group. Finally, electrophoretic deposition was applied to coat steel. The electrophoretic nanoparticles gave a stable dispersion, uniform surface, and structural colouring based on a specific light diffraction by deposited nan-particle layers.

After EPD, a structural colour was observed, indicating that the coating had attached to the surface and exhibited specific light diffraction, in which the wavenumber is dependent on the incident angle. Scanning electron microscopic images confirmed that the particles 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 fibres," Takasu said.

He adds that this technique overcomes problems such as colour fading and damage from UV radiation because of structural colouring, thus will be provided for wider application of electrophoretic dispersion.