Application of sericite to LWC coatings

Sericite is an indigenously produced mineral from the east coast of Taiwan. It is a layered inorganic that swells in liquid, and it possesses high negative charges on the plate surfaces and high aspect ratio that are useful as adjustor of water retention value and viscosity of coating colors. For the properties of LWCs, the platy structure and swelling delamination nature of the mineral could raise the surface strengths of the coated paper while facilitate ink drying time and improve opacity of the sheet. In this study, we aim to partially or totally replace delaminated clay use in the LWC coating formulation with the swelling sericite, and explore its effect on the coating properties and the optical and printing properties of the coated sheets produced. These will enable us to understand the role sericite plays in the formulation of coating colors and the properties of sericite containing coated paper. Observations on the coating colors included solid contents, pH value, low-shear viscosity, high-shear viscosity, and water retention value (WRV) etc. The coated papers, after passing through a supercalender stack, were measured for their optical properties: opacity, brightness, gloss and printed gloss, and printing density etc. Among the coating color properties, the WRV dropped with increasing amounts of sericite replacement, reflecting an increase in water retention of the coating color of about 30%. Except for the 100% SA (oxalic acid treated sericite) replacement, low-shear viscosities dropped with increasing amounts of replacement, but all were within the operating range of coating colors. Adding sericite tended to lower the high-shear viscosity of the coating color, but with increasing amounts of replacement, the viscosity often rebounded thus could effectively increase the elasticity of the coating color under high shear loading conditions. Among the coated paper properties, sericite addition could raise opacity of the sheet. At 25% replacement, opacity gained 1.5 to 2%, along with increasing amounts of replacement, opacity would show further gain but at a slower rate. Brightness, however, dropped with increasing sericite replacement, showing a linear correlation between the two. Print gloss too, showed decreasing trend with sericite addition in a linear fashion. Print density, on the other hand, was not affected. There were notable swelling and delamination effects during the process of coating color preparation using sericite. We propose that a 3-stage reaction mechanism was involved that affect coating color properties, viz., swelling of sericite, particles migration, and bridge structure formation that lend to improved WRV, adjustment of low-shear viscosity and high-shear viscosity, as well as strengthening of the coating layer etc.