Application of Sericite to LWC Coating Formulation, 2006 TAPPI Advanced Coating Fundamentals Symposium

Sericite is a mica mineral locally produced from the east coast of Taiwan. It carries high negative charges on the plate surfaces and has a high aspect ratio that might be useful as an adjustor of water retention value and viscosity of coating colors. In this study, we partially or totally replaced delaminated clay in a LWC coating formulation with sericite, and examined the resulting effect on the color properties and the optical and printing properties of the coated sheets. The water retention value (WRV) of the color decreased with increasing amounts of sericite substitution, reflecting an increase in water retention of the color by about 30%. The low-shear viscosities decreased with increasing amounts of sericite substitution, while the high-shear viscosities showed a V shaped response, suggesting an increase in the elasticity of the coating color under high shear conditions. Among the coated paper properties, sericite addition raised the opacity of the sheet. Brightness of the coated sheets, however, decreased linearly with increasing sericite substitution. Printed gloss too, showed a linear decreasing trend with sericite addition. Print density was not affected. There were notable swelling and delamination effects during the preparation of coating color when sericite was applied. We propose that a 3-stage mechanism to explain the observed phenomena, entailing swelling of sericite, particles migration, and bridge structure formation that lend to improved WRV, adjustment of the low- and high-shear viscosities, as well as strengthening of the coated layer.