About GCC Fine Grade Calcium Carbonate (D50 < 5 µm)
GCC fine grade is ground calcium carbonate milled and air-classified to a median particle size below 5 µm, with a tight top cut that keeps oversize particles out of the product. Reducing particle size changes how calcium carbonate behaves. Surface finish becomes smoother, gloss retention improves, and the filler distributes more evenly through a paint film or polymer matrix.
In architectural and industrial paints, the fine grade works as a functional extender in satin, eggshell and semi-gloss finishes, where a coarser filler would dull the surface. It helps space titanium dioxide particles, which can support opacity at a given TiO₂ level, subject to formulation trials. In plastics such as polypropylene and polyethylene compounds and thermoset moulding compounds, the finer size has less effect on impact strength and surface appearance than standard grades. Sealants, adhesives and industrial primers also benefit from its smooth rheology and low grit content, which help both application and appearance.
Obsidian Minerals checks particle size on each batch by laser diffraction, alongside brightness and residue, so D50 and D97 stay consistent between deliveries. The grade is supplied uncoated. Where a hydrophobic surface is needed for polymer compounding at high loadings, the stearic-coated grade in the range is the recommended alternative.
Typical specifications
| Parameter | Typical Value | Test Method |
|---|---|---|
| CaCO₃ | ≥ 98.0 % | ASTM C25 |
| Brightness (GE) | ≥ 94 | Internal method |
| Median particle size (D50) | < 5 µm (typically 2–5 µm) | Laser diffraction (ISO 13320) |
| Top cut (D97) | ≤ 15 µm | Laser diffraction (ISO 13320) |
| Residue on 45 µm | ≤ 0.05 % | ASTM D185 |
| Oil absorption | 18–26 g/100 g | ASTM D281 |
| Moisture | ≤ 0.3 % | Internal method (105 °C) |
| Fe₂O₃ | ≤ 0.05 % | ASTM C25 |
Typical values for guidance. Guaranteed particle size and other limits are stated on the Technical Data Sheet and batch CoA.
Key features & benefits
Median size below 5 µm
A D50 below 5 µm gives fine GCC a smooth feel in paint films and compounds, supporting higher sheen levels and a more uniform surface than medium-grade calcium carbonate.
Narrow top cut
A D97 of about 15 µm or less and near-zero residue on 45 µm reduce the risk of bits, seeds and surface defects in thin films, coatings and extruded parts.
High brightness
Typical GE brightness of 94 or higher and very low iron content help maintain clean whites and bright pastel shades in decorative and industrial paints.
Supports TiO₂ efficiency
Fine particles help space titanium dioxide in the dry film, which can maintain opacity when part of the TiO₂ is reduced. The achievable reduction depends on the formulation and should be confirmed by trial.
Lower impact on mechanical properties
In thermoplastics and thermosets, the smaller particle size causes fewer stress concentration points than coarse filler, helping retain impact strength and surface quality at moderate loadings.
Applications
Satin and semi-gloss paints
A functional extender in satin, eggshell and semi-gloss emulsions and alkyds, where it adds solids and helps opacity without the sheen loss caused by coarser calcium carbonate.
Thermoplastic and thermoset compounds
Used in polypropylene and polyethylene compounds and in SMC and BMC moulding compounds, where fine size helps keep surface quality; the coated grade is preferred at high loadings.
Sealants and adhesives
Provides smooth, grit-free rheology in acrylic, polyurethane and hybrid sealants and in adhesive formulations, contributing body, tooling smoothness and controlled flow from the cartridge or applicator.
Industrial coatings and primers
Used in industrial primers, wood coatings and general metal coatings where a fine, bright extender supports smooth application, good sanding properties and consistent film build from coat to coat.
Usage & handling
- Paint loadings typically range from about 5–25 % by weight, often replacing part of a coarser extender where finish quality matters.
- Disperse under high shear in the grind stage and allow sufficient dispersant for the finer particle size.
- For polymer compounding at high filler levels, consider the stearic-coated grade, and pre-dry if the process is moisture-sensitive.
- Keep bags sealed in a dry store; fine powders pick up surface moisture more readily than coarse grades.
Packaging & supply
- 25 kg multi-wall paper or HDPE bags suited to paint and compounding plants
- 1 MT and 1.2 MT jumbo bags for bulk handling
- Private-label packaging on request
Frequently asked questions
What does D50 below 5 µm mean?
D50 is the median particle size: half of the particles by volume are smaller than this value and half are larger. A D50 below 5 µm means the fine GCC grade is considerably finer than standard fillers, which typically sit in the 8–15 µm range. It is measured by laser diffraction to ISO 13320 on each batch.
Why use fine GCC instead of standard GCC in paint?
Fine GCC gives a smoother film with better sheen and gloss retention, so it suits satin, eggshell and semi-gloss finishes. It also packs more efficiently around titanium dioxide, which can help opacity. The trade-off is slightly higher binder and dispersant demand, so the choice depends on the finish quality required and the formulation budget.
Can fine GCC partly replace titanium dioxide?
Fine GCC does not hide like titanium dioxide, but it can space TiO₂ particles more effectively, which in many formulations allows part of the TiO₂ to be reduced while keeping acceptable opacity. The achievable reduction varies with pigment volume concentration and binder. A ladder trial against your current formula is the reliable way to confirm it.
Is the fine grade coated?
No, the GCC fine grade is supplied uncoated, which suits water-borne paints, sealants and many adhesives. For PVC, filler masterbatch and other polymer applications that need a hydrophobic surface, the stearic-acid-coated grade is recommended. Customers unsure which version suits their process can request samples of both for comparison.
How is particle size tested?
Particle size distribution is measured by laser diffraction to ISO 13320, reporting D50 and D97, and coarse residue is checked by wet sieving on 45 µm to ASTM D185. Results are recorded on the batch Certificate of Analysis, so incoming QC teams can compare each delivery with the agreed specification.