THE SURFACE-PROPERTIES OF PURE AND MODIFIED PRECIPITATED CALCIUM-CARBONATE BY ADSORPTION OF NITROGEN AND WATER-VAPOR

Representative laboratory samples of pure and modified precipitated calcium carbonate were characterized by nitrogen and water vapor adsorption measurements. All the nitrogen isotherms were analysed by means of alpha(s) and Frenkel-Halsey-Hill (FHH) plots in order to evaluate the porosity and surface energetics of the samples. The water vapor sorption isotherms for the samples showed pronounced low pressure hystereses and those of pure unmodified samples showed anomalous negative hystereses which have been related to an ageing process. All the samples underwent a change in the Brunauer-Emmett-Teller (BET) nitrogen surface area after exposure to water vapor, the effect (loss of surface area) being more significant in the case of the pure unmodified sample. An ageing mechanism has been suggested in terms of the cementation of the particles. In contrast, the stearate-coated sample showed an increase in the BET nitrogen surface area after exposure to water vapor. It has been suggested that adsorbed water caused a substantial deaggregation of the particles. These effects have been explained in terms of the presence of surface cations which appear to be directly responsible for the ageing of the particles. In addition the interaction of calcium carbonate fillers with some polyolefin-based polymers has been discussed in terms of acid-base interactions.