Coated vs Uncoated Calcium Carbonate: When to Use Which?

In short: Coated means the calcium carbonate particle surface is treated with stearic acid so it becomes organophilic — easier to disperse in PP/PE, lower moisture pickup, less agglomeration. Uncoated is cheaper and adequate for low loadings or non-critical applications. The rule of thumb: the higher the loading, the thinner the product, and the finer the grade, the more coating pays off. For low loadings in thick products, uncoated is usually fine.

Calcium carbonate's surface is naturally hydrophilic (water-loving) — while polyolefin plastics like PP and PE are organophilic (oil-loving). This "mismatch" makes uncoated CaCO₃ particles hard for the resin to wet out evenly: they tend to clump and pick up moisture.

Coating resolves the mismatch. The most common treatment is stearic acid, which forms a thin layer (calcium stearate) on the particle surface and turns it organophilic. The effects:

• Better dispersion — particles wet out and spread evenly through the resin melt rather than clumping.
• Lower moisture pickup — the organophilic surface repels water, reducing the risk of moisture-related defects in processing. (Coating reduces, but does not eliminate, moisture pickup — storage in a humid warehouse and pre-drying before processing still matter.)
• Less agglomeration, and smoother melt flow during extrusion (the stearate layer provides internal lubrication).

The coating level is usually about 1% by weight for standard fine GCC, rising to 2–3% for ultrafine grades or PCC. The dose follows the particle's surface area — not a fixed number: the finer the particle, the more surface to cover, the more coating needed. Coating beyond a single layer adds no benefit and can show up on the line as over-lubrication — e.g. screw slip, plate-out at the die, or print/seal adhesion problems.

Uncoated is a reasonable — and more economical — choice when the technical need does not require coating:

• Low loading. At small loadings, defects from imperfect dispersion are rarer and more easily tolerated by the matrix.
• Thick / non-critical products. Some injection-molded goods or thick sheet tolerate less-than-perfect dispersion without losing function.
• Coarser grades. Coarse particles have less surface area and are less agglomeration-prone, so coating delivers less benefit.
• Plants with their own dispersion process. If your compounding line wets out and distributes particles well (e.g. via a properly run twin-screw), cheaper uncoated can be adequate — as a practical tell, if you already run an uncoated grade without dispersion problems at your current loading, your line is probably capable.

Coated generally earns its price premium when one or more of these apply:

• High loading. Calpet-into-resin loadings for sacks and film typically run from the teens of percent up to about 40% (that is the masterbatch percentage into resin, not the CaCO₃ content). The higher the loading, the more it matters that every particle disperses without clumping. (For thin film, coating generally helps even at low loading, because particle wet-out remains critical.)
• Thin film. At thin gauge, a single clump or undispersed particle can cause a film tear or speck. Coating reduces this risk.
• Surfaces that demand smoothness — printed or display products where surface specks are unacceptable.
• Very fine grades (D50 below ~2 µm). Fine grades are precisely the ones that most need coating (see next section).
• Moisture-sensitive processes. Lower moisture pickup reduces defects and drying needs.

There is a common misconception: "the finer the particle, the better the dispersion." That is only true if the grade is properly coated.

Very fine CaCO₃ particles have large surface area and strong inter-particle attraction, so they tend to stick together (agglomerate). Without coating, these clumps are hard for the resin to break apart during mixing — so uncoated fine grades can disperse worse than coarse grades. The remaining clumps act like large particles: stress-concentration points that cause film tears and can reduce strength. In raffia tape for woven sacks, the same clumps can break the tape during stretching (orientation) and raise pressure on the screen pack.

Stearic-acid coating lowers the particles' surface energy so the resin wets and separates them more easily. In other words: fineness and coating are two separate levers. Choosing a finer grade without making sure it is coated can make dispersion worse, not better. For guidance on choosing fineness (mesh/D50) by application, see our filler masterbatch / calpet guide.

When comparing offers, check the spec sheet (TDS):

• Coating status — whether the grade is described as "coated," "activated," or "surface-treated" (vs "uncoated"/"natural").
• Stearic-acid level (%) (or other surface activator) — usually listed; around 1% for standard fine GCC is typical, higher for ultrafine grades.
• Oil absorption and moisture (%) — indirect indicators; coated grades generally show lower moisture pickup.
• Remember: you usually buy calpet (filler masterbatch), not raw CaCO₃ powder. Beyond coating status, check the carrier (PP or PE) so it matches your resin, and the CaCO₃ content. See our filler masterbatch / calpet guide.

A simple line trial remains the best confirmation: run one lot at your target loading and check dispersion, surface smoothness, and defects before committing to volume.

What is the difference between coated and uncoated calcium carbonate?

Coated means the CaCO₃ particle surface is treated with an activator — most commonly stearic acid — so it changes from hydrophilic (water-loving) to organophilic (oil/plastic-loving). The result: better dispersion in polyolefins (PP/PE), lower moisture pickup, and less agglomeration. Uncoated is untreated — cheaper, and adequate for low loadings or applications that do not demand a smooth surface. For thin film and high loadings, coated generally performs better.

When should you use coated calcium carbonate?

Choose coated when: (1) you load at high levels (and for film, even at low loadings — particle wet-out still matters), (2) the product is thin film or demands a smooth surface, (3) the grade is very fine (D50 below ~2 µm — fine grades are the most prone to agglomeration without coating), or (4) the process is moisture-sensitive. Coating lets particles disperse evenly and reduces defects such as film tears and specks.

When is uncoated good enough?

Uncoated is adequate for low loadings, thick/non-critical applications (some injection-molded goods or thick sheet), coarser grades, and when your plant has its own dispersion process capable of wetting out the particles. Because it is cheaper, uncoated makes sense when the technical need does not require coating.

How much stearic acid is on coated calcium carbonate?

Typically about 1% by weight for standard fine GCC, rising to 2–3% for ultrafine grades or PCC. The dose follows particle surface area — the finer the particle, the more coating is needed — so 1% is not a fixed number for all grades. Coating beyond a single layer adds no benefit and can act as an over-lubricant.

Does coating reduce agglomeration?

Yes. Very fine CaCO₃ particles tend to stick together due to surface forces. A stearic-acid layer lowers the surface energy so they wet out more easily in the resin and resist clumping. That is why uncoated fine grades can disperse worse than coarse grades.

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Need help choosing between coated and uncoated grades for your application? Tell us — product type, application, target loading, and required fineness — and our team will respond with a matching grade, the right documentation, and advice on running a one-lot line trial.

We supply premium GCC/filler — coated and uncoated, high-whiteness (approaching 98%) — sourced directly from Vietnam and ready to ship to Indonesia. Tell us the whiteness and mesh you need.

See also: Filler Masterbatch (Calpet) & Calcium Carbonate for Plastics in Indonesia: A Buyer's Guide.