Making brighter colours with air pockets

RESEARCHERS have found a way to produce purer structural red colours in coatings and paints by patterning a polymer with air pockets, resulting in a material which can be tuned to produce a full spectrum of matte colours.

In standard paint processing, silica nanoparticles are arranged in orderly crystals, producing bright structural colours, but making matte colours requires more unstructured arrangements. Matte blues can be produced the standard way, however, reds are more difficult. The space between particles in the material can be adjusted to reflect red light, but the nanoparticles can scatter blue light, meaning red structural colours can hold hints of blue.

A team from the Korea Advanced Institute of Science and Technology (KAIST) and Missouri University of Science & Technology (Missouri S&T) dispersed silica nanoparticles in a resin of polyethylene glycol dimethacrylate. They packed the particles closely in an unstructured arrangement, but controlled the particle concentration to ensure the silica particles did not touch.

The team created a solid resin by cross-linking the acrylate molecules. They treated the resin with acid to dissolve the silica, but left the polymer intact. This resulted in a polymer film with pockets of air approximately 200 nm in diameter, which produces matte red colours due to the low reflective index in each hole. The team also used 168 nm and 185 nm diameter silica particles to make blue and green films respectively.

Xiaodong Yang, a mechanical engineer at Missouri S&T said: “This method and the polymer used are [of] low cost and can likely be manufactured easily at large scales.”

The team hopes to expand the uses for these purer pigment materials for security materials, displays, optical barcodes, and sensors that detect toxic chemicals.

Shin-Hyun Kim, professor of chemical and biomolecular engineering at KAIST said the next stage for the team will be to produce films that incorporate multiple colours into a single sheet.

ACS Applied Materials & Interfaces, DOI: 10.1021/acsami.6b03217