Transparent Paper Expected to Become OLED Substrate
Hiroyuki Yano, professor at Kyoto University, developed a technology to make transparent paper by adding resin, etc to paper or its material (pulp).
This was announced at the New Technology Presentation Meetings, which the university organized Aug 24, 2012, in Tokyo. Yano expects that the new technologies will be applied to transparent OLED substrates, substrates of organic thin-film photovoltaic (PV) cells, electronic paper, materials for car windows, packaging containers, etc.
Cellulose nanofibers, which constitute pulp, have a width of 10-20nm and a length of 1μm or longer. It is seven to eight times stronger than steel but as light as 1.5g/cm3, Yano said. So, he considered that a transparent sheet having a low expansion coefficient can be made by adding resin to cellulose nanofibers.
Specifically, an improvement was made for preventing cellulose nanofibers from clumping together in the process of dehydrating pulp. And the fibers were impregnated with acrylate resin to make the pulp transparent.
The transparent sheet has a thickness of 100μm, a light transmittance of 87.8% and a coefficient of linear thermal expansion of 8.30ppm/K with a temperature range of 20-150°C. The coefficient of linear thermal expansion is about 1/3 that of a PET sheet and close to glass'.
The coefficient of linear thermal expansion of a transparent sheet that was made by adding cellulose nanofibers obtained by decomposing pulp to resin was 12.1ppm/K, Yano said. The new paper's coefficient of linear thermal expansion is smaller because pulp is made transparent without being decomposed and the orientations of its fibers are more uniform, he said.
Dehydrated pulp was impregnated with acrylate resin because the refraction index of acrylate resin is 1.52-1.60, which is close to the refraction index of cellulose (1.57). Materials other than acrylate resin can be used as long as their refraction indexes are close to that of cellulose, Yano said.
In general, the refraction indexes of resins change depending on temperature. But the light transmission of the new paper does not change much even at a temperature of 90°C because the pulp prevents the thermal expansion of the resin, Yano said.
In November 2011, Yano announced that he developed a technology to make a transparent sheet by using a material made by making a crab shell transparent.