UV light prints without ink

A new method for rewritable printing on paper using light promises to cost less, and to cause less environmental damage, than ink-based printing.

Scientists have developed a nanoparticle coating that they claim is easy to apply to normal paper and changes colour when ultraviolet (UV) light shines on it. The colour change can be reversed when the coating is heated to 120 degrees Celsius, and allows for up to 80 rewrites.

It will not be replacing ink for any products needed for more than five days, as the print starts to fade following that.

Yadong Yin, chemistry professor, University of California, Riverside, says "The greatest significance of our work is the development of a new class of solid-state photoreversible colour-switching system to produce an ink-free light-printable rewritable paper that has the same feel and appearance as conventional paper, but can be printed and erased repeatedly without the need for additional ink. Our work is believed to have enormous economic and environmental merits to modern society."

"Our next step is to construct a laser printer to work with this rewritable paper to enable fast printing. Then we will work towards full-colour printing."

The researchers say the new coating consists of two types of nanoparticles: those made of Prussian blue, which is a common inexpensive, nontoxic blue pigment that turns colourless when it gains electrons; and titanium dioxide (TiO₂), a photocatalytic material, meaning it accelerates chemical reactions upon UV light exposure.

They explain that when the Prussian blue and TiO₂ nanoparticles are evenly mixed and coated onto paper, the plain unprinted paper appears solid blue. To print text or images, the paper is exposed to UV light, which photoexcites the TiO₂ nanoparticles. These nanoparticles then release electrons that are picked up by the adjacent Prussian blue nanoparticles, turning from blue to colourless.

Continuing, they say that since it is easier to read blue text on a colourless background than colourless text on a blue background, it is the background rather than the text that is typically printed by light, turning colourless. The paper can also be "reverse-printed" to show colourless text on a blue background. Different colours can be achieved by using Prussian blue analogues.

Once printed, the researchers say the paper retains its configuration for at least five days with high (5-µm) resolution, and then slowly fades back to solid blue. To erase the paper quickly, it can be heated for 10 minutes to return it to its solid blue state.

Yin says, "The light-printable paper is indeed cost-competitive with conventional paper. The coating materials are inexpensive, and the production cost is also expected to be low as the coating can be applied to the surface of conventional paper by simple processes such as soaking or spraying.

"The printing process is also more cost-effective than the conventional one as no inks are needed. Most importantly, the light-printable paper can be reused over 80 times, which significantly reduces the overall cost."

Source: NewZealand Printer