Chemists create clever rewritable paper
The paperless office – one of the promises of the new digital age – has not really eventuated. Instead, most organizations still print out large amounts of documents on paper that, according to the WWF, is being produced at around 400 million tons (362 million tonnes) a year and rising. So, wouldn’t it be great if we could just simply wipe off the print on paper and use it over and over again? Chemists at the University of California, Riverside (UCR) thought that this might be a good idea too, and have now created rewritable paper that can be printed on and erased more than 20 times before it needs to be discarded.
Many years ago, monks wrote on cured calfskin – vellum – to produce manuscripts. If they made a mistake, or simply wanted to reuse the vellum, they would scrape away the top layer to produce a fresh surface on which to work. This wasn’t ideal because some of the original writing could be faintly seen (giving rise to the term "palimpsest") which meant that even though it was recycled, it didn’t take too many scrapings and rewrites to make it unusable.
But even that sort of recycling was far more environmentally-friendly than today, where we not only print paper and then discard it soon after, we also manage to use up print cartridges in the process that generally get thrown away too.
That’s where the UCR prototype rewritable paper is different. Not only is it able to be printed upon, completely erased, and subsequently reprinted, it also takes advantage of the unique chemical properties of commercial inks known as redox dyes. These dyes are applied once and then, when a photomasked template is overlaid and the combination exposed to ultraviolet light, the uncovered portions are photo-bleached away, leaving behind the print. To reuse the sheet and erase the print it is then exposed to heat of around 115 °C (239 °F) and a blank sheet is once more created.
"Even for this kind of paper, heating to 115 °C poses no problem. In conventional laser printers, paper is already heated to 200 °C in order to get toner particles to bond to the paper surface," said Professor Yadong Yin, professor of chemistry at UCR. "This rewritable paper does not require additional inks for printing, making it both economically and environmentally viable. It represents an attractive alternative to regular paper in meeting the increasing global needs for sustainability and environmental conservation."
Not to be confused in any way with unpowered electronic rewritable displays, the UCR paper is essentially a rewritable media made of glass or plastic film on which prints can be repeatedly made, and able to retain the image applied to it for days until it is erased by heating.
Available so far in the primary colors of red and blue, and the secondary color green, the hues are created using the redox dye colors neutral red, methylene blue, and acid green. The dyes also contain titania nanocrystal catalysts and hydrogen cellulose as a thickening agent. According to the researchers, this amalgam of dye, catalysts and hydrogen cellulose provides high reversibility properties and continued printing repeatability to the paper.
"The printed letters remain legible with high resolution at ambient conditions for more than three days – long enough for practical applications such as reading newspapers, said Professor Yin. ”Better still, our rewritable paper is simple to make, has low production cost, low toxicity and low energy consumption."
Presently concentrating on a real paper (that is, one made from wood pulp) version, researchers at the UCR laboratory are also occupied with improving the number of cycles of printing and erasing that it can be made to perform so as to reduce costs and make the paper more commercially viable. Currently the goal is around 100 cycles. The team is also investigating methods to lengthen the time that the text remains legible to broaden the potential fields of use for the technology.
"One way is to develop new photocatalyst nanoparticles that become highly reductive when irradiated by ultraviolet light," said Professor Yin. "We are exploring, too, the possibility of multi-color printing. The design principle can be extended to various commercial redox dyes to produce rewritable paper capable of showing prints of different colors. All these efforts will help increase the practical applications of the technology."
Researchers in the study with Professor Yin included Wenshou Wang, Ning Xie, and Le He, and the research was funded by a grant from the US Department of Energy. A provisional patent has been filed for this technology.
The research was recently published in the journal Nature Communications.
The short video below shows the rewritable paper being erased and printed.