Relationships between PVC plastisol rheology and partical size distribution for filled and unfilled systems
To control the rheological properties of PVC plastisols commercial manufacturers take great care to control the particle size (PS) and particle size distribution (PSD) of a polymer latex, as these exert a strong influence over the flow characteristics of an uncompounded plastisol. However, the fundamental mechanisms controlling viscosity are not clearly understood. Several techniques have been developed to measure the PSD (sedimentation analysis and laser diffraction) and viscosity over an extended shear rate range for plastisols prepared from carefully selected paste polymers. Care has been taken to establish the reliability of these techniques, as previous workers have experienced problems in obtaining reliable correlations from plastisol systems. Xany have failed to measure the PSD within the plastisols, incorrectly basing their observations on latex PSD. By careful selection of emulsion, seeded emulsion and microsuspension paste polymers the combined effects of polymerisation technique, emulsifier type, spray drying, milling and paste preparation on plastisol rheology have been established. For many applications plastisals contain mineral fillers, which have a similar PS to the paste polymers. Addition of these materials can drastically change plastisol rheology. Until now, it has been assumed that plastisol viscosity rises with filler content and depends on plasticiser absorbtion characteristics. To elucIdate the actual mechanism controlling the response of plastisols to fillers, calcium carbonate (coated and uncoated), titanium dioxide and Spheriglass filled systems have been studied. To establish a model which will allow paste polymer systems to be designed more accurately, the paste polymers used in the initial part of this study have been spray dried under widely differing conditions. Careful measurement of the rheology and PSD of filled and unfilled plastisols prepared from these materials has highlighted the effect of PS, PSD, paste polymer porosity and filler interaction on plastisol rheology. It has been clearly demonstrated that the plasticiser absorbtion characteristics of a filler are not the only factors controlling filled plastisol rheology, as previously thought. I have shown that the response of a plastisal to filler addition also depends on the interaction between filler PS and the paste polymer PSD.