About Coated Calcium Carbonate (Stearic Acid Treated)
Coated calcium carbonate is fine ground calcium carbonate whose particle surfaces have been treated with stearic acid. The acid reacts with the carbonate surface to form a thin layer of calcium stearate, turning a naturally hydrophilic mineral into a hydrophobic, organophilic one. As a result, the coated grade wets into polymers and plasticisers far more easily than untreated filler.
In rigid PVC pipes, fittings and window profiles, the coating improves dispersion, lowers melt viscosity and moisture pick-up, and helps maintain impact strength and surface finish at practical filler levels. Flexible PVC cable compounds, flooring and calendered sheet use it for similar reasons. Producers of polypropylene and polyethylene filler masterbatch rely on coated carbonate to reach high loadings without the agglomerates that cause film defects. In silicone, polyurethane and MS-polymer sealants, the treatment helps control rheology and storage stability.
Obsidian Minerals coats a fine GCC base under controlled temperature and dosage, then tests each batch for coating effectiveness, moisture and particle size. The coating level is matched to the surface area of the base so that little free stearic acid remains, since excess acid can migrate or contribute to plate-out. Coating level and base fineness can be adjusted for specific compounds on request.
Typical specifications
| Parameter | Typical Value | Test Method |
|---|---|---|
| CaCO₃ (base) | ≥ 98.0 % | ASTM C25 |
| Median particle size (D50) | 2–5 µm | Laser diffraction (ISO 13320) |
| Top cut (D97) | ≤ 15 µm | Laser diffraction (ISO 13320) |
| Coating agent | Stearic acid | — |
| Coating level | 0.8–1.5 % by mass | Internal method |
| Activation degree (hydrophobicity) | ≥ 95 % | Internal method (water flotation) |
| Moisture | ≤ 0.2 % | Internal method (105 °C) |
| Brightness (GE) | ≥ 93 | Internal method |
Typical values for guidance. Guaranteed coating level, particle size and moisture limits are stated on the Technical Data Sheet and batch CoA.
Key features & benefits
Hydrophobic surface
Stearic acid treatment converts the filler surface from water-loving to polymer-compatible, so coated calcium carbonate wets into PVC, polyolefins and plasticisers instead of forming agglomerates.
Dispersion at high loadings
Better wetting lets compounders raise filler content with fewer specks and gels, lower torque and more uniform melt flow than with uncoated calcium carbonate of similar size.
Low moisture pick-up
Moisture typically at or below 0.2 % and a water-repellent surface reduce the risk of voids, bubbles and surface streaks during extrusion and injection moulding.
Matched coating level
The stearic acid dose is matched to the surface area of the base, leaving little free acid that could migrate, cause plate-out or disturb the lubricant balance of the compound.
Processing aid effect
The calcium stearate layer adds mild lubrication in PVC processing, which can assist melt flow. Lubricant packages should be reviewed when switching from an uncoated filler.
Applications
Rigid PVC pipes and profiles
Used in pressure and non-pressure pipes, fittings, window profiles and sheet, where dispersion and surface finish matter and filler level is limited by the applicable product standard.
PVC cable and flexible compounds
A filler in cable insulation and sheathing, flooring and calendered sheet, where the hydrophobic surface supports electrical insulation properties and helps reduce plasticiser absorption by the filler.
Filler masterbatch
The main filler in polypropylene and polyethylene masterbatch for film, raffia, woven sacks and moulded goods, enabling very high loadings with good dispersion in the carrier resin.
Sealants and adhesives
Used in silicone, polyurethane and MS-polymer sealants, where the coated surface helps control rheology, reduces the moisture introduced into the system and supports cartridge storage stability.
Usage & handling
- Rigid PVC pipe compounds commonly use about 5–30 phr, subject to the product standard and pressure class.
- Filler masterbatch formulations can reach about 60–80 % by weight in PE or PP carriers.
- Review the lubricant balance when moving from uncoated filler, as the coating contributes lubrication.
- Store in a cool, dry place away from direct heat, and keep bags closed to preserve the low moisture level.
Packaging & supply
- 25 kg moisture-resistant HDPE or multi-wall paper bags with liner
- 1 MT and 1.2 MT jumbo bags for compounders and masterbatch plants
- Private-label bags for compound and masterbatch brands on request
Frequently asked questions
What is stearic acid coated calcium carbonate?
It is fine ground calcium carbonate treated with a controlled amount of stearic acid, which bonds to the particle surface as calcium stearate. The coating makes the powder hydrophobic and compatible with polymers. It is also called activated or surface-treated calcium carbonate and is widely used in PVC, polyolefin masterbatch and sealants.
Why use coated rather than uncoated calcium carbonate in PVC?
Uncoated calcium carbonate attracts moisture and tends to agglomerate in PVC, which can raise melt viscosity and reduce impact strength and surface quality. The coated grade disperses more evenly, adds less moisture and contributes some lubrication. This usually allows higher filler loading with more stable processing, subject to compound trials.
How much coated calcium carbonate can be used in a PVC pipe?
Typical levels in rigid PVC pipe compounds range from about 5 to 30 phr, depending on pipe type, pressure class and the product standard that applies in the destination market. Pressure pipes generally allow less filler than drainage or conduit pipes. Customers should confirm the permitted level against their standard and validate by testing.
How is coating quality checked?
Each batch is tested for activation degree, a flotation test that measures the proportion of powder that stays on the water surface rather than sinking, and for coating level, moisture and particle size. Results appear on the batch Certificate of Analysis, so compounders can confirm treatment quality before the material reaches the extruder.
Can you supply a different coating level or base size?
Yes. Coating level can be adjusted to suit a finer or coarser base, and the base particle size can be selected for a specific application such as pipe, cable or masterbatch. Share your current material data or target properties, and a trial lot can be prepared for evaluation on your line.