Dark glasses go green

New light-sensitive glass can be recycled cleanly.

Researchers in Japan have developed recyclable light-sensitive glass. The new ’ecoglass’ does not contain the environmentally damaging halogen elements chlorine, bromine or iodine. These elements are essential to the photochromic glass that is currently used for car windscreens, sunglasses and visual display units.

Like photographic film, today’s photochromic glasses darken because they contain compounds of silver and halogens, such as silver iodide. Ultraviolet light in sunlight gives some of the electrons in the halogen ions enough energy to move around – these electrons combine with silver ions to make neutral atoms of metallic silver. The silver atoms then aggregate into tiny particles, scattering light and turning the glass dark.

This process is reversible – some glasses go light again after a few minutes away from ultraviolet radiation, whereas others must be heated. In each case, electrons come away from the silver atoms, causing the clusters to fall apart into silver ions once more.

Instead of halogens, which react with carbon-based molecules to form toxic and carcinogenic compounds, Tetsuo Yazawa and colleagues at the National Institute of Advanced Industrial Science and Technology in Osaka add silver ions in the form of silver nitrate to a fairly standard mixture of glass ingredients¹.

The silver-nitrate glass turns from clear to yellow under ultraviolet light. When the yellowed glass is heated to 500 oC for 15 minutes, it turns clear again. This colour change, from clear to yellow and back, can be repeated many times.

The researchers have not made glass in any other colours yet – a wider spectrum is needed for some applications. And the colour change has so far been produced only by several minutes of irradiation with an ultraviolet laser, rather than with natural sunlight. But the new material shows that halogens are not essential to the process.

Yazawa’s and his colleagues point out that their material might find more high-tech applications in ’optical memory’ devices that can be reversibly imprinted with information using lasers.

References

1. Chen, S., Akai, T., Kadono, K. & Yazawa, T. A silver-containing halogen-free inorganic photochromic glass. Chemical Communications, 2001, 2090 – 2091, (2001).