What does stearic acid coating do?
Stearic acid reacts with the calcium carbonate surface to form a thin layer of calcium stearate. The particle surface changes from water-loving (hydrophilic) to water-repelling (hydrophobic). This lets the mineral disperse evenly in PP, PE and PVC, absorb less moisture and flow better in feeders and extruders.
How much stearic acid is needed?
The right amount depends on the particle surface area, not on a fixed percentage. Finer powders have far more surface per kilogram, so they need more acid to form a complete layer. The target is roughly a single molecular layer: enough to cover the surface, with little free acid left over.
| Grade (CarboCore) | D97 (µm) | Stearic acid (kg per tonne) | Approx. % |
|---|---|---|---|
| CC4T | 3.8 | 13–14 | 1.3–1.4 |
| CC6T | 5.5 | 12.5 | 1.25 |
| CC8T | 7.8 | 12 | 1.2 |
| CC10T | 10 | 10 | 1.0 |
| CC15T | 15 | 8 | 0.8 |
| CC20T | 20 | 6 | 0.6 |
| CC25T | 25 | 6 | 0.6 |
What happens with too little coating?
Uncoated patches stay hydrophilic. The powder picks up moisture, agglomerates and disperses poorly. In film and raffia this shows as specks, holes, tape breaks and gels; in masterbatch it shows as screen-pack pressure build-up and unstable melt flow.
What happens with too much coating?
Excess (free) stearic acid does not bond to the surface. It can migrate, cause plate-out and die build-up, create smoke and odour at processing temperature, and reduce printability or adhesion of the final product. More coating is not better; correct coating is better.
How is coating quality tested?
A sample is placed on water and stirred. Well-coated powder floats; the proportion that sinks shows the uncoated fraction.
The floating fraction expressed as a percentage, often targeted at 95% or higher.
Measures the organic content, which indicates the total coating level.
A solvent removes unreacted stearic acid to show how much is not bonded.
Coated powder exposed to humidity should gain much less weight than uncoated.
Wet vs dry coating: what is the difference?
Most GCC for plastics is coated dry, in a high-speed mixer or during grinding, where heat and shear melt the acid onto the particles. Wet coating, done in slurry, is used more for precipitated calcium carbonate. Dry coating suits high volumes and gives consistent results when temperature, residence time and dosing are controlled.
Which grade of stearic acid is used?
Industrial stearic acid is usually a blend of stearic (C18) and palmitic (C16) acids. Blend ratio, iodine value and acid value affect reactivity and colour, so the acid itself should be specified and checked on arrival.