NPG Asia Materials featured highlight | doi:

Transparent paper: Clearly different

We cannot see through ordinary paper because it’s constituent micrometer sized cellulose fibers and large cavities scatter light, which makes paper opaque. Now, Nogi and colleagues¹ have developed transparent paper. Using cellulose—as in normal paper—and downsizing the fibers using a simple processing technique, they produced ‘transparent paper’ which, unlike many transparent plastics, does not expand significantly on heating. This paper is ideal as an alternative substrate for electronics, which could even be used in roll-to-roll processing.

The transparent paper was made using wood flour, in which cellulose nanofibers are usually bundled together to make larger, 30 μm-wide fibers. The researchers started by swelling the bundled cellulose fibres in water and then mechanically grinding them just once. This broke them down into single nanofibres.

To form optically transparent sheets of paper, the fibres must be squashed together to prevent large gaps forming between them—large spaces would scatter light and render the material opaque. Nogi and colleagues filtered the suspension to bring the fibers together and then sandwiched the resulting layers between wire mesh and filter paper and dried it for three days. Once the fibers are compacted together in this way, hydrogen bonds hold them in this configuration even after the pressure is removed.

This process produced translucent sheets, but which still had surface roughness sufficient to scatter light so the paper at this stage was not completely transparent. The researchers polished the sheets using emery paper, which resulted in truly transparent films with 71.6% light transmittance at 600nm (Fig.1). The sheets were foldable like normal paper and had high strength and a thermal expansion coefficient comparable to that of glass.

“Everyone believes cellulosic paper is white and its mechanical properties are poor,” says Nogi. “But, 21st-century paper using cellulose nanofibers has high transparency like glass and plastics, high thermal stability like glass, and then high foldability like traditional paper—it’s not only flexible, but it’s foldable.” This flexibility makes it perfect for roll-to-roll processing, which will be vital for making future bendable electronic devices.

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