Performance of Sericite as a Pigment for Coating Paper

Sericite is one of the rare naturally swelling layered minerals that expands its volume when immersed in polar liquids. It possesses a high degree of negative charges on its platy surfaces and a high aspect ratio. These traits could make the mineral a suitable water retention agent and viscosity modifier for coating colors. Because of its platy structure and the swelling and delaminating characteristics, coated paper containing the pigment has greater surface strength, faster ink drying times, and improved opacity. The purpose of the study was to investigate the effects of substituting sericite for a portion of the delaminated clay in an art paper coating color formulation on the optical and printing properties of the coated paper, so as to understand the role it plays in coating color formulation and the characteristics of sericite-containing coated paper. The experimental design simulated a typical art paper coating color formulation, with 2 variables of sericite types (oxalic acid-treated, SA; and plain water-washed sericite, SB) and various replacement levels of sericite for delaminated clay in the formula. Parameters of coating color such as the solids content, pH, low- and high-shear viscosities, and water retention values (WRVs) were examined. The optical properties of opacity, brightness, gloss, and printed gloss, and printing properties such as IGT printability and print density of the coated and supercalendered paper were examined. As to the coating color properties, incorporating sericite increased water retention by 20%; whereas the low-shear viscosity of the colors increased 10.7% and 3.6%, while the high-shear viscosity increased by 5.6% and 14.0% for the optimal levels of SA and SB substitution for delaminated clay, respectively. These traits can be used in the future to adjust dosages of water retention aids and rheological modifiers to provide optimal coating color formulations. The opacity of the coated paper increased by 0.31% and 0.67% and the brightness of the coated paper decreased by 1.5% and 1.91% ISO when SA and SB were respectively substituted for part of the delaminated clay. IGT printability effectively increased to 300 cm/s or better, and print density gained 3.0% to 6.4%. In the process of the coating color preparation, swelling and delamination of the sericite mineral were quite notable, hence, we propose a 3-stage reaction mechanism to explain the effects of sericite on coating color: swelling, particle migration/intercalation, and the formation of bridge structures. These lead to retention of water, modified low- and high-shear viscosities, and a strengthened coat layer. Partial substitution of sericite for delaminated clay in an art paper coating formulation has been proven to improve water retention, raise the opacity of the coated paper, and increase IGT printability and print density, but at the cost of lowering brightness and glosses.