Paper production and printing processes have a negative impact on the environment. Moreover, paper production is an important source of industrial pollution. Approximately 40% of landfills consist of discarded paper. In addition, paper manufacturing is the main activity responsible for deforestation: in the United States, about one third of all harvested trees are used for paper and cardboard production.
In order to reduce this excessive use of paper, researchers have invented a new way to print and reuse it, using light and heat . The discovery is based on the color change chemistry of nanoparticles, which can be applied in a thin coating on a variety of surfaces including plain paper.
The nanoparticle coating changes color when ultraviolet (UV) light hits the paper, and returns to its original color when the coating is heated to 120 degrees Celsius. A team of researchers from China and the United States say this process allows them to reuse paper up to 80 times, which is much more cost-effective and eco-friendly than ink-based printing.
The coating is made of two types of nanoparticles:
- Prussian blue, a semi-transparent blue pigment used in radioactive decontamination and in paint, which becomes colorless when acquiring electrons.
- Titanium dioxide (TiO2), a photocatalytic material that accelerates chemical reactions when exposed to UV light.
Mixing both substances creates a solid blue coating. With a little UV light, the TiO2 particles release electrons, rendering the Prussian blue pigment colorless.
Since it is easier to read blue text on a colorless background than the opposite, it is the background rather than the text that is usually printed by light—although the reverse can also be done for special displays of a colorless text on a blue background. Different colors, in addition to blue, can also be obtained using Prussian blue analogues of different colors.
Light-printable paper is convenient and perfect when printed information is only needed for a short time. It can be used for newspapers, magazines, posters, and labels indicating the shelf life of products, for example. Once printed, the paper retains its configuration at high resolution (5 μm) for at least five days, then slowly goes back to solid blue by oxidation under ambient conditions (see image below). To erase the print more quickly, the paper can be heated for about 10 minutes, bringing it back to its solid blue state.
Since the coating can also be applied to normal paper, the researchers expect the technology to be fairly affordable when brought to market. This invention will have a direct effect on deforestation and the waste of inkjet cartridges.
Anouer Kebir is currently working toward the PhD degree in electrical engineering at ÉTS. His research interests include real-time optimization and control of solar energy and bioenergy.
Program : Electrical Engineering
Research laboratories : GREPCI – Power Electronics and Industrial Control Research Group