Formation, Characterization and Application of a Microporous Layer using a Bubble Coating Method

Abstract

There has been increasing interest in receiving sheets having porous layers for use in digital printing because of their porosity, low thermal conductivity and high compressibility. The simplest route to obtain paper sheet having a porous layer is to coat a sheet substrate with a coating mixture containing fine air bubbles. In our own research program we have focused on the preparation of the coating mixture using a bubble coating method of introducing and dispersing air bubbles in a resin-containing liquid. Recently we reported the way to disperse and stabilize fine bubbles as the key technology in obtaining successful microporous layer.In this paper we describe the formation, characterization and application of the microporous layer prepared by the method. The microporous layer has been analyzed in terms of its structure, thermal conductivity and compressibility. The microporous layer has an average surface pore diameter of 5 to 12μm and a density of 0.14 to 0.25g/cm3. The paper sheet having the microporous layer has a thermal conductivity of 0.25W/(m·k) or less. The compression stress of the sheet under a high compression of 10% by volume is measured to be 6.2kg/cm2 or less.A recording sheet for a thermal wax transfer printer is considered to be one of the most likely commercial applications for the microporous paper sheet. This is because the thermal wax transfer system requires not only low thermal conductivity and high compressibility but also a high affinity to wax ink. In this system a molten ink permeates into the microporous surface layer due to the capillary action of a large number of fine pores. The microporous layer acts as an ink-receiving layer in thermal wax transfer printing and enhances the print quality such as the optical density, tone reproduction, dot reproduction and color brightness.