Aggregation of CaCO3 particles in PP composites: Effect of surface coating

The occurrence and effect of aggregation in PP composites containing seven different precipitated CaCO3 fillers coated with stearic acid are described in this study. The particle size and specific surface area of the filler varied in a relatively wide range, the latter changed between 2 and 12 m2/g. The fillers were characterized by various methods including surface area, particle size and bulk density. PP composites were prepared in an internal mixer in the composition range of 0–0.3 volume fraction filler content and their structure was studied by optical microscopy. The tensile and rheological properties of the composites were related to their structure. The results prove that the unambiguous detection of the presence of aggregation is difficult in particulate filled composites. The coating of CaCO3 fillers with a surfactant changes the behavior of the particles considerably, but does not eliminate aggregation completely. The association of filler particles depends on the relative magnitude of adhesion and separating forces. Although coating decreases the surface free energy of the filler significantly, gravitational forces are much smaller than adhesion between either uncoated or coated fillers thus powder particles always aggregate. Different forces act in suspensions used for the determination of the particle size distribution of the filler; the shape of the distribution may indicate the presence of aggregation. Coated fillers form much looser aggregates with more diffuse interphases, than uncoated particles. Composite stiffness is completely insensitive to changes in structure or interaction, but the direct evaluation of other tensile properties may also lead to erroneous conclusions. Model calculations, oscillatory viscometry, as well as the proper representation of the results allow the unambiguous detection of aggregation.