Why is calcium carbonate used in PVC?
Calcium carbonate is the most widely used filler in rigid PVC. It lowers formulation cost, improves stiffness and dimensional stability, helps fusion and gives a good surface. Fine, coated grades can even improve impact strength at moderate loadings. It is used in pipes, window profiles, fittings, cable ducts, sheets and foam boards.
How is calcium carbonate dosed in PVC?
PVC formulations are written in phr, parts per hundred resin. If a recipe has 100 kg PVC and 20 kg calcium carbonate, the loading is 20 phr.
| Product | Typical loading | Grade direction |
|---|---|---|
| Pressure pipes | Low (limited by pipe standards) | Fine, coated |
| Non-pressure and drainage pipes | Medium to high | Fine to medium, coated |
| Window and door profiles | Low to medium | Very fine, coated, high brightness |
| Fittings (injection moulded) | Low to medium | Fine, coated |
| Cable ducts and conduits | Medium to high | Medium, coated |
| Foam boards and sheets | Medium to high | Fine to medium, coated |
Which calcium carbonate properties matter in PVC?
How does calcium carbonate affect PVC processing?
Calcium carbonate changes heat transfer, fusion time and melt flow. Higher loadings usually need adjustment of lubricants and processing temperatures. Coated grades interact with the external lubricant package, so changing the coating level of the filler can shift the balance between fusion and slip in the extruder.
Calcium carbonate also reacts with hydrogen chloride released during degradation, which gives a modest support to thermal stability.
Common problems and their filler-related causes
| Problem | Possible filler cause |
|---|---|
| Low impact strength | Coarse grade, high top cut, poor dispersion, too high a loading |
| Surface roughness or matte patches | Agglomerates, coarse particles, high moisture |
| Yellowing or grey tint | Low brightness or iron/manganese impurities |
| Bubbles and voids | Moisture in the filler |
| Unstable fusion | Change in coating level or loading without lubricant adjustment |
| Die build-up | Excess free stearic acid |