A research result on repeatable printing paper. I saw the popular science article by Professor Yin Yadong, the author of the paper, on Scientific American, and found it quite interesting.

Paper is an important information carrier, and up to now, the consumption is still very large, and the electronic display device has not completely replaced it. There is also a big problem in the use of paper: printing paper is disposable, and those that have already been printed must go through tedious steps to re-produce recycled paper, which has a lot of adverse effects on the environment and energy consumption.

so material scientists hope to develop paper that can be erased and reprinted in a relatively simple way. And the achievement mentioned here is one of them. You don't need ink to print words and patterns on this paper, just use ultraviolet light. It is also easy to remove those handwriting and reprint, as long as the paper is heated (heated to about 120 ℃). It is said that such a piece of paper can be reprinted more than 80 times.

this kind of paper is printed with text and patterns like this:

the comfortable dark blue ~ the picture an in the upper left corner is the original appearance of this kind of paper-yes, what is added in the printing process is not the blue line, but the "blank" part, and the background color of this kind of paper is dark blue.

what provides blue here is actually a very classic blue dye-Prussian blue, which is mainly composed of a dark blue complex of ferrous ferricyanide (which actually includes potassium), potassium ferricyanide and trivalent iron. It is also an important blue pigment in art history, such as Van Gogh's Starry Night and GE Beizhai's Kanagawa Surfing.

this paper is filled with Prussian blue and titanium dioxide nanoparticles. Under the action of ultraviolet light, Prussian blue is reduced to Prussian white (when the iron in the complex is bivalent), and heating can promote it to be oxidized again and restore blue.

Printing process:

Printing effect:

erasing process:

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returning to a completely blue state:

by the way, changing the composition of the complex can also achieve other colors:

reference and read more:

https://www.scientificamerican.com/article/reprintable-paper-becomes-a-reality/

http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.6b03909