Nano-scale Surface Modification Of Calcium Carbonate By Wet Ultrafine Grinding

Ground calcium carbonate (GCC) has numerous applications in industrial fields including plastics, rubbers, paints and papers in order to reduce cost and improve properties. The flat surfaces and sharp edge angles formed during ultra fine grinding of GCC go against the interfacial stress transmission of calcium carbonate with matrix materials. An effective way to solve these problems is to modify the surface morphology of calcium carbonate in batch and scale.In this paper preparation process and mechanism of nano-scale surface modification of GCC using nano CaCO₃ as modifier by mechanochemical method were studied. This research provides technical supports for exerting excellent properties of calcium carbonate, improving its utilization value and solving problems existed in the application of calcium carbonate.Influences of different factors on the modification effect of composite GCC particles were investigated by single-factor experiments. Composite GCC particles with high coating rate and uniform coating were prepared under the following conditions: wet grinding time in stage I 60min, and during stage II ball-to-powder weight ratio 4:1, agitation speed 1400r/min, composite time 30min, concentration of slurry 60%, dosage of sodium polyacrylate 0.6%, and proportion of nano-CaCO3 20%.Results of SEM observation show that calcium carbonate particles had better surface morphology after surface modification, the smooth cleavage planes were coated with nanometer particles and the sharp edge angles were blunt by coating of nanometer particles. The composite GCC powders had larger specific surface area more than original mineral powder with substantial increase of 140%. The composite GCC powders also had fine granularity, better whiteness, higher oil absorption value, and better settling property. Ultrasonic disintegrator was used to check the bond strength between the nanostructure coating layer and GCC matrix. Results showed that there was a strong bond strength between them for the coating layer still existed without break after disintegrating by large power ultrasonic.Using sodium stearate as modifier, optimum process conditions of wet surface organic modification of composite GCC particles were obtained: reaction temperature 70℃, dosage of modifier 0.8%, reaction time 15min. The surface free energy of modified composite GCC particles decreased from 74.21mJ/m² to 11.62mJ/m², and its dispensability in kerosene was obviously improved, the water absorption was decreased, and activation index was approximately 100%. IR analyses indicated that C₁₇H₃₅COO^- was bonded on the surface of composite GCC particles by chemical absorption.The surface modification mechanism was discussed. SEM analyses indicated that the coating on the surface of GCC was carried out step by step. Because the interaction force between nano-CaCO₃ particles and GCC particles were repulsion force, contact and reaction between them can only be realized by mechanochemical effect generated during ultrafine grinding. And IR analyses indicated increased polymerization degree of calcium carbonate. The border fractal dimension of calcium carbonate particles increased from 1.0709 to 1.1135 after nano-scale surface modification, indicating significantly improved surface roughness.