Calcium Carbonate Knowledge Hub
Calcium Carbonate Particle Size Distribution
2026-09-04 16:46:31
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Calcium carbonate particle size distribution (PSD) describes the full range and proportion of particle sizes in a GCC or PCC product. It is more useful than a single mesh or micron label because it determines how calcium carbonate packs, disperses, flows, absorbs binder, and performs in plastics, PVC, paper, coatings, rubber, sealants, adhesives, and construction products.
The most common PSD values are D10, D50, and D97. D50 shows the median particle size, while D97 controls the coarse tail—the oversized particles most likely to create grit, rough surfaces, film defects, or poor dispersion. Laser diffraction is widely used to measure calcium carbonate PSD because it provides a volume-based distribution across fine and ultrafine particle sizes.
What Is Particle Size Distribution?
Particle size distribution is the curve that shows how much of a calcium carbonate powder lies at different particle sizes. It answers questions that a label such as “800 mesh” or “5 micron” cannot answer on its own:
How much of the powder is very fine.
Where the main particle population is concentrated.
How many coarse particles remain.
Whether the grade is narrow, broad, or multimodal.
Whether two products with the same nominal D50 will pack, flow, coat, and disperse similarly.
For example, two GCC products may both have D50 = 5 µm. One may have D97 = 12 µm and a narrow distribution; the other may have D97 = 30 µm and a broad coarse tail. The first is generally more suitable for applications demanding low grit and smooth surfaces, while the second may be acceptable for less demanding fillers at lower production cost.
D10, D50, and D97 Explained
| PSD value | Meaning | Why it matters for calcium carbonate |
|---|---|---|
| D10 | 10% of the measured particle volume is finer than this size | Shows the fine fraction and helps indicate surface-area-related behavior |
| D50 | 50% of the measured particle volume is finer than this size | Median size used to describe nominal product fineness |
| D90 | 90% of the measured particle volume is finer than this size | Useful upper-distribution indicator for many filler applications |
| D97 | 97% of the measured particle volume is finer than this size | Controls the coarse tail, grit, surface quality, and potential film or coating defects |
| D98 | 98% of the measured particle volume is finer than this size | Used where especially strict oversize control is needed |
Laser-diffraction reports typically provide volume-based size distributions and can calculate D97 as an upper-percentile particle-size value. The report should also identify whether measurements are volume-based, the dispersion medium, dispersant, ultrasonic treatment, and refractive-index assumptions, because these settings can affect results.
Simple example
Consider two calcium carbonate powders:
| Property | Grade A | Grade B |
|---|---|---|
| D10 | 1.0 µm | 0.6 µm |
| D50 | 4.0 µm | 4.0 µm |
| D97 | 10 µm | 22 µm |
| Distribution shape | Narrower | Broader with more coarse particles |
| Likely use direction | Fine coating, smooth PVC, paper coating, premium sealants | General filler applications that can tolerate a larger coarse tail |
Both powders have the same D50, but they are not equivalent. Grade B may produce more surface defects or grit in thin film, high-gloss coating, or smooth PVC because its D97 is much larger.
Why PSD Matters
Particle size distribution affects nearly every functional property of calcium carbonate. It changes the way particles pack together, interact with resins or binders, and respond during mixing, extrusion, coating, drying, or curing.
| Property or process | How PSD affects it |
|---|---|
| Surface smoothness | Large particles and broad coarse tails can create roughness, visible specks, or defects |
| Gloss | Fine, low-grit distributions generally support smoother coating and film surfaces |
| Dispersion | PSD influences the energy needed to distribute particles in polymers, rubber, paint, sealants, and adhesives |
| Rheology and viscosity | Finer particles and higher surface area may increase viscosity or binder demand |
| Oil absorption | Typically rises as particle surface area increases |
| Bulk density and packing | Broad or multimodal distributions may pack differently from narrow distributions |
| Coating-agent demand | Finer powder with more surface area generally requires more surface modifier for equivalent coverage |
| Paper optical properties | Particle size and distribution influence light scattering, pigment packing, opacity, and coating behavior |
| Film and cable quality | Oversized particles can create weak points, surface defects, or puncture-related risk in thin products |
In paper coatings, light scattering depends on pigment size, size distribution, and particle packing within the coated layer. This is why paper producers evaluate calcium carbonate PSD together with brightness, morphology, slurry rheology, and coating formulation—not as an isolated number.
PSD by Application
| Application | PSD priority | Why it matters |
|---|---|---|
| Wall putty, mortar, and dry-mix products | Economical fineness and controlled oversize | Influences texture, flow, workability, and surface finish |
| Rigid PVC pipe and profile | Stable D50, controlled D97, low coarse residue | Supports smooth surfaces, dispersion, extrusion stability, and consistent appearance |
| PE and PP masterbatch | Fine, controlled distribution with low agglomerates | Helps filler dispersion, melt flow, film quality, and controlled filler loading |
| Wire and cable compounds | Low coarse tail and stable PSD | Reduces defect risk and supports uniform compound processing |
| Paints and coatings | Fine PSD, low grit, controlled surface area | Affects gloss, viscosity, film formation, hiding-related performance, and smoothness |
| Paper filler and paper coating | Fine PSD matched to the paper grade and coating system | Influences brightness, opacity, light scattering, coating rheology, and print surface |
| Rubber | PSD matched to target loading, dispersion, and compound properties | Influences processing, hardness, reinforcement balance, and surface finish |
| Sealants and adhesives | Controlled distribution and surface area | Affects viscosity, extrusion force, density, sag resistance, and storage behavior |
For paper applications, industry guidance commonly distinguishes very fine coating grades from broader filler grades: premium top-coat products may use a D50 around 1–3 µm, while packaging paper fillers may use around 8–15 µm, depending on the paper type and target properties.
How PSD Is Measured
Laser diffraction is the most common method for measuring fine and ultrafine calcium carbonate. It passes a laser through dispersed powder or slurry and calculates particle-size distribution from the scattering pattern. The method can provide D10, D50, D90, D97, D98, and the full distribution curve.
Typical measurement workflow
Take a representative powder sample from the production lot.
Disperse it in a suitable dry or wet medium using an agreed dispersant and, if needed, controlled ultrasonication.
Set the optical model, refractive index, obscuration or concentration range, and measurement conditions.
Run the measurement and review the full PSD curve, not only D50.
Compare results with the product specification and historical control data.
Laser-diffraction testing has been used to compare D10, D50, and D90 values among different GCC samples and evaluate sizing-process stability. For coated GCC, sample preparation is particularly important because poorly dispersed soft agglomerates can appear as false oversize particles.
Other methods
| Method | Best suited for | Main limitation |
|---|---|---|
| Laser diffraction | Fine and ultrafine GCC or PCC; routine product QC | Results depend on dispersion protocol and optical-model settings |
| Sieve analysis | Coarse powders such as 200, 325, or 400 mesh grades | Less suitable for low-micron powders; screens can blind and agglomerates distort results |
| Air classification cut test | Plant process control and classifier performance evaluation | Does not replace laboratory PSD measurement |
| Microscopy or image analysis | Particle shape, contamination, agglomerates, and qualitative morphology review | May not represent the full bulk distribution without careful sampling |
| Sedimentation methods | Selected fine-powder or slurry measurements | Sensitive to particle shape, density, dispersion, and settling assumptions |
How PSD Is Controlled
In GCC production, PSD is controlled through the combined action of raw-material preparation, grinding, classification, airflow, recirculation, and product handling. The classifier is usually the main device that determines the coarse tail, while the mill and feed conditions influence the overall curve.
| Process variable | PSD effect |
|---|---|
| Feed size and feed-rate stability | Unstable feed can cause fluctuations in mill load, product fineness, and coarse residue |
| Feed moisture | High moisture can cause agglomeration, reduce grinding efficiency, and disrupt air classification |
| Mill energy and residence time | Too little energy leaves coarse particles; too much can create excessive ultrafines |
| Classifier rotor speed | Higher separation intensity generally produces a finer cut but may reduce yield |
| Airflow and pressure balance | Controls particle transport and classification sharpness; leaks can destabilize PSD |
| Circulating load | Determines how much coarse material returns to the mill for further grinding |
| Grinding media or wear condition | Changes milling efficiency, contamination risk, and the final particle-size curve |
| Surface treatment and handling | Can create or break soft agglomerates, changing apparent PSD and customer performance |
A closed-circuit mill and classifier system separates qualified fine particles from coarse material that returns for further grinding. This avoids both high coarse residue and unnecessary over-grinding. Process guidance identifies D97 as especially important because a single oversized particle can create a film tear or pinhole in sensitive plastic-film applications.
How to Specify PSD
A useful calcium carbonate purchase specification should define the full distribution and how it will be measured. Avoid specifications that only state “800 mesh,” “5 micron,” or “D50 = 3 µm.”
| Specification item | Why it should be included |
|---|---|
| D10, D50, D90, D97, or D98 | Defines the fine fraction, median, upper distribution, and coarse tail |
| Target value and tolerance | Prevents disputes over normal batch variation |
| Measurement method | Defines laser diffraction, sieve analysis, sedimentation, or another agreed test |
| Sample-dispersion procedure | Critical for fine and coated powders that can form soft agglomerates |
| Dry or wet measurement medium | Can influence result comparability |
| Coarse-residue limit | Provides additional control of grit and oversized particles |
| Specific surface area | Supports interpretation of coating demand, oil absorption, viscosity, and formulation behavior |
| Application trial requirement | Verifies that PSD translates into the desired result in the actual product |
Key Takeaway
Calcium carbonate particle size distribution is the full size profile of a GCC or PCC powder. D10, D50, and D97 are the most useful routine values: D50 describes the median size, while D97 shows the coarse tail that often controls grit, gloss, surface smoothness, and dispersion.
For PVC, masterbatch, paper, paint, rubber, sealants, adhesives, and construction products, the best calcium carbonate is not simply the finest grade. It is the grade with the PSD, surface area, purity, whiteness, moisture, and surface treatment that match the formulation. Always specify the complete PSD curve and test method rather than relying on mesh or a single micron number.
