Effect of kaolin content on structure and functional properties of water-based coatings

Abstract

The water-based coatings for paperboard were pigmented with a mixture of kaolin and ground calcium carbonate (GCC). The effects of kaolin content on the morphology and gloss of the coatings were determined. Kaolin particle orientation in the coatings was measured with X-ray diffraction (XRD) technique using peak ratio method, which determines the ratio of the integrated peak area intensities of the ( 3 ¯ 3 ¯ 1 ) and (0 0 1) reflections. The gloss values and the number of platy kaolin particles oriented parallel to coating surface increased with increasing kaolin content. Moreover, when measured in machine direction, gloss was higher and the orientation index lower compared to measurements in cross-direction. The laser profilometer measurements indicated that the differences in machine and cross-directions are related to the macroroughness of the surface. Novel method to study the structure of coating was utilized by exposing the reverse-side of the coating with cupriethylenediamine (CED) solution. Scanning electron microscopy (SEM) studies of the reverse-side of the coating illustrated decreasing coating porosity with increasing kaolin content. CED-dissolving was utilized also for preparation of transmission electron microscopy (TEM) samples. Concurrently the utilized characterization techniques complemented each other and provided information of the correlation between the functional properties and the structure of the coatings.