Modification and evaluation of Egyptian kaolinite as pigment for paper coating

The modification of low quality Egyptian kaolinite to be acceptable as a pigment for paper coating was reported in this work. Sedimentation, chemical bleaching, preparation of organo kaolinite and preparation of nano-kaolinite were the modification methods used in this study. The modified pigments were characterized using X-ray fluorescence, an X-ray diffractometer, Fourier-transform infrared spectroscopy and scanning electron microscopy. The original and modified kaolinites were applied in paper coating mixture. The results of XRF analysis showed that Fe2O3 impurities reduced by using sodium dithionite from 0.41 to 0.25 (wt %). XRD data revealed shifting of basal space of kaolinite from 7.14 to 11.8 Å with an intercalation ratio of 80% upon grinding with urea for 5 h. Meanwhile, IR spectra showed that NH–CO molecule (refer to urea molecule) exists in the intercalated kaolinite. The characteristic peak of kaolinite completely disappeared during dispersion of kaolinite–urea in paper coating suspension which showed the exfoliation of kaolinite–urea layers through the binder (nano-kaolinite). The SEM images of intercalated kaolinite–urea and exfoliated kaolinite/binder revealed thin flacks with particle size ranging from 500 nm and 300 nm, respectively. Nano-kaolinite caused a significant decrease in coated paper roughness and an increase in optical properties compared with the original and commercial kaolinite. Air permeability of coated paper increased and was better than commercial kaolinite. Although, nano-kaolinite reduced the mechanical properties of coated paper, it produced coated paper having print density and print gloss higher than the ones containing the original and commercial kaolinite.