STABILITY OF GROUND AND PRECIPITATED CACO3 SUSPENSIONS IN THE PRESENCE OF POLYETHYLENIMINE AND SALT

The stability of precipitated and ground CaCO3 pigment suspensions, us ed as fillers in papermaking, was investigated in the presence of salt and polyethylenimine. By monitoring the change of light transmittance and the change of size of the aggregates with time, the stability rat io W, i.e., the ratio of the measured rate of destabilization to the m aximum rate, was determined The stability ratio was correlated with th e electrophoretic mobility of the particles. The maximum rate of desta bilization was observed around the point of zero charge, which indicat es that the process is dominated by electrostatic interactions. With a n increasing rate of destabilization the size of the aggregates also i ncreases. There is a difference in behaviour between the commercial gr ound and precipitated CaCO3. The ground CaCO3 is negatively charged an d forms a stable suspension. By introducing an increasing amount of ca tionic polyethylenimine, it initially aggregates, but becomes stable a gain when its charge is reversed by the adsorbing polymer. The precipi tated CaCO3, which is unstable and uncharged, acquires a positive char ge and stability due to the adsorbed polymer.