Colours to dye for: Spray on pigments

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  • Published: Jun 1, 2013
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Colours to dye for: Spray on pigments

Colour without colours

Japanese scientists have developed a new type of colouring agent that is not only environmentally friendly but does not fade like conventional textile dyes and pigments. The colourants consist of submicrometre-sized silicon dioxide particles and carbon black and can be sprayed on to the desired surface to generate bright colours that are permanent and highly fade resistant. Visible reflectance spectroscopy was used in this study published in the journal Angewandte Chemie

Japanese scientists have developed a new type of colouring agent that is not only environmentally friendly but does not fade like conventional textile dyes and pigments. The colourants consist of submicrometre-sized silicon dioxide particles and carbon black and can be sprayed on to the desired surface to generate bright colours that are permanent and highly fade resistant. Visible reflectance spectroscopy was used in this study published in the journal Angewandte Chemie.

Colours fade, it is the eternal lament of clothes designers and art historians alike. For the latter exposure to light and humidity are usually to blame for the degradation of paintings whereas the former might deliberately fade their textiles, it's sun exposure and the repeated pounding of the washing and drying that lead to fade. Now, researchers from Japan, explain how they have developed an entirely new type of colourant that is not only "fast" and so won't fade but is also more environmentally friendly than some other pigments. The colourants consist of submicrometre-sized silicon dioxide particles and carbon black, which can simply be sprayed on to the surface that needs a splash of colour.

Microscopic light

Commonly, organic dyes fade when exposed to ultraviolet light while inorganic pigments are resistant to fading but are often based on toxic heavy metals such as chromium. Now, Yukikazu Takeoka, Shinya Yoshioka and their colleagues at the Universities of Nagoya and Osaka have turned to the familiar substance of sandy grains, silicon dioxide, to build a new physical pigment system. Submicrometre particles of silica look white to the human eye, but as with many colourants in nature, physical colouration can arise because of the scale and structure of particles.

Conventional pigments, dyes, and paints absorb a portion of the visible light that impinges on them and reflect other wavelengths. Leaves are usually green because they reflect green light. However, the myriad shifting colours of avian plumage or the butterfly's wing often arise through another process, not absorption and reflection of light but through wavelength-dependent optical interference, refraction, and light scattering. The colours seen sometimes even depend on the physical shape and size of the particle but to some extent on light source and observer.

Indeed, structural colours are normally iridescent, the colours seen change depending on the angles of incident light and the angle from which they are seen. This phenomenon arises because of the high degree of order of the particles in the structural crystal lattice. To side-step this angle-dependent characteristic of structural colour, the researchers have attempted to preclude crystallization in their system, thus maintaining the sub-microscopic particles in a noncrystalline, amorphous arrangement.

Paint it black

The scientists overcome this difficult problem by dispersing silicon dioxide nanoparticles in methanol and spraying them on to the surface to be coloured. The methanol evaporates during the spraying process, so the silica lands on the surface as a dry powder, forming a thin, even membrane of amorphous particles; there is no opportunity for crystallization to take place. Furthermore, the team adds that they can stabilise this coating using a polyelectrolyte to fix the colloidal amorphous array. Depending on the particle size, the researchers obtained membranes that ranged in colour from a bluey-white (230 nm) to a pale pink (360 nm).

At the present time, these amorphous particles produce only very pale colours, which will please fans of pastels and Japanese watercolours. However, the team has also now developed a solution to this pale problem. They found that if they also added particles of carbon black to the spray, the resulting colour saturation was intensified markedly. They explain that the carbon black particles reduce light scattering over the entire visible spectrum and so effectively "turn up the contrast" on this structural colouration. They suggest that their colourants might be used in art, in architecture, for vehicle coatings and elsewhere in industry. Whether or not clothes designers will be spray their latest creations for the catwalk remains to be seen.

Related Links

Angew Chem Int Edn 2013, 52, online: "Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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