We describe the development of microporous paper sheet that meets the demands for print quality in different types of digital printing. The paper sheet contains a microporous layer that has a surface pore diameter of 0.5 to 30 μm and a density of 0.2 to 0.5 g/cm3. The microporous layer can be easily obtained by coating a sheet substrate with a stirred resin-containing liquid, which forms bubbles. Applications for the microporous paper sheet in digital printing include thermal wax transfer (including resin type), thermal dye diffusion transfer, direct thermal, solid inkjet and toner-based marking. In the first application, the microporous layer acts as an ink-receiving layer (see Fig. 1) due to the heat-insulating nature, the compressibility and the surface structure. The microporous paper sheet has a thermal conductivity of 0.25 W/(m.K) or less. The compression stress of the microporous sheet under a high compression of 10% by volume is controlled to 8 kg/cm2 or less. The molten ink permeates sufficiently into the region of the microporous surface layer due to the presence of a large number (3,000/mm2 or more) of fine pores distributed on the surface. Thus the microporous surface layer enhances the print quality for use in thermal wax transfer printing. Print qualities of the microporous paper sheets are quantitatively analyzed using automated image analysis systems with test targets that consist of basic image elements such as dots, lines and solid areas. The print qualities ncluding dot raggedness, dot reproduction, line blurriness, line raggedness, tone reproduction, mottle, graininess and optical density of the microporous paper sheets are consistently higher than those observed for a plain paper sheet. For example, by measurement with the ISO-13660 draft standard, the lead line blurriness and raggedness of a microporous paper sheet are 27 μm and 1.8 μm, respectively, whereas those of a plain paper sheet are 57 μm and 7.4 μm, respectively.
Shuichi Maeda, Toru Nakai, Akira Nakamura, Masakazu Hakomori, Masaru Kato, "Development of Paper Having Microporous Layer for Digital Printing" in Journal of Imaging Science and Technology, 2000, pp 410 - 417, https://doi.org/10.2352/J.ImagingSci.Technol.2000.44.5.art00006