Direct Feeding of Calcium Carbonate During PVC Extrusion
Calcium carbonate is employed when extruding PVC to modify mechanical properties, improve the processing behaviour and reduce material costs. In actual practice, two factors limit the amount of calcium carbonate that can be added: segregation during pneumatic conveying of the dry blend and formation of deposits on the wall of the hot mixer. Both situations can be observed at loading levels exceeding 20–30 phr. Starting at this concentration, the calcium carbonate does not bond completely to the PVC particles during the hot mixing process and remains as separate particles. This leads to unstable behaviour during extrusion, to fluctuating mechanical properties and occasionally also to visible flaws in the extrudate.
To prevent these problems, the company Omya has conducted trials under near-production conditions where additional calcium carbonate was added to the dry blend directly at the extruder. The dry blend used was prepared conventionally on a hot/cold mixer and already contained 10 phr of calcium carbonate. With the aid of a cold mixer, 21 and 27 phr, respectively, of this calcium carbonate were added to the dry blend.
The extruded pipe was visually perfect; no agglomerates or flaws on the outer and inner surfaces were observed. Scanning electron microscope images of pipe cross-sections showed that no agglomerates were present inside. Testing of the mechanical properties confirmed these observations.
By installing an additional cold mixer, it is possible to increase the level of loading and avoid the segregation that occurs during pneumatic conveying of the dry blend. The potential savings resulting from use of calcium carbonate thus increase. A higher percentage of calcium carbonate leads to noticeably increased stiffness, which can be used to reduce the wall thickness. Incorporation of additional calcium carbonate also results in a considerable reduction in material costs even if the wall thickness remains unchanged.