Tikalon Blog by Dev Gualtieri

Rewritable Paper

January 12, 2015

Writing existed before paper, with clay tablets being the preferred medium for three millennia, ending about 500 BC. After clay tablets, wax tablets, vellum, and parchment became the preferred media. Papyrus, an early paper material, coexisted with these other media, but it was less durable. Later, paper became the predominant medium for writing.

Paper, prepared from cellulose pulp from wood and other Natural sources, was first introduced about 150 AD in China. Papermaking diffused into Europe in the centuries thereafter. The Gutenberg Bible (c. 1455) was printed on very fine paper. Today, paper is ubiquitous.

In the early days of desktop computing it was thought that the ability to store documents electronically would lead to a "paperless workplace." Aside from adding cost, paper has a significant environmental impact. There's deforestation from wood harvesting, now largely mitigated through reforestation, and the fact that papermaking is an energy-intensive chemical process. I can still remember the sulfur odor of papermaking when my family drove through Ticonderoga, New York, on vacation in the 1960s.

Well, the paperless workplace hasn't arrived. In 1900, only two percent of harvested wood became paper. Presently, the proportion is about 25% with consumption still increasing (see figure).[1] Although tablet devices seem to be the optimum way to transition from paper, as e-books have demonstrated, paper will be with us for a while, so there's still an incentive to reuse and recycle paper.

US paper consumption, 1900-2000.

(Fig. 2 of ref 1,
simplified for clarity).

About 90% percent of business documents are on paper, much of which is discarded after being used just once. A team of chemists from the University of California (Riverside, California) has invented a rewritable paper using a photocatalytic reduction-oxidation (redox) reaction.[2-3] Rewritable paper is paper that can be written on and erased many times.[3] The paper uses the color switching property of redox dyes. After the paper is uniformly dyed, printing is done using ultraviolet light to photobleach the dye in a negative image, and erasing is done by simple heating.[2-3]

The laboratory version of the rewritable paper was prepared in the three primary colors, red, green and blue through use of the the commercial redox dyes, methylene blue, neutral red and acid green. The dyes incorporate titania nanocrystals that act as a catalyst, hydrogen cellulose as a thickening agent. This combination of materials gives high reversibility and repeatability, and the paper can be erased and rewritten at least twenty times. Presently, the print medium is glass or plastic, but the process can be used on paper.[2-3]

An example of the University of California, Riverside, rewritable paper.

(Yin Lab/University of California, Riverside, image.)[3]

The writing process uses ultraviolet light to reduce the redox dye to its colorless state in a negative image. Erasure is accomplished by re-oxidation of the reduced dye, which reverts to its original color. This happens slowly at room temperature, but complete erasure is accomplished by heating at 115 °C for about 10 minutes.[3] The image is legible at room temperature for about three days.[2] Says research team leader, Yadong Yin,

"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... 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."[3]

Erasure of a TiO₂/methylene blue/hydrogen cellulose rewritable paper by heating for up to eight minutes at 115 °C.

(Data from ref. 2, rendered using Inkscape.)

Yin's lab is working to increase the number of erasure cycles to a hundred and increase the room temperature legibility limit from its present three days. Work is also being done on a better catalyst. One barium-doped colloidal titania nanoparticle catalyst has given good results.[3]

Says 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."[3] This research was funded by the U.S. Department of Energy, and a patent has been filed on this invention.[3]

Rewritable Paper

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