Calcium Carbonate Filler and What It Does to Your Colour

Calcium carbonate is a white pigment that happens to be sold as a filler. That is the single most useful way to think about it, and it explains almost every colour surprise associated with filled compounds.

Filler is added to reduce cost, increase stiffness, improve thermal conductivity and modify surface properties. It also changes shade, changes opacity, changes the dosage of colourant needed, and – when its loading moves between lots – moves your colour with it.

What it actually is

Calcium carbonate for plastics comes in two forms.

Ground calcium carbonate (GCC) is mined limestone, marble or chalk, crushed and classified. Particle size distribution is relatively broad, particles are irregular, and cost is low. This is the bulk of what is used in Indian plastics.

Precipitated calcium carbonate (PCC) is chemically produced, giving controlled particle size, narrow distribution and controllable crystal morphology. Finer, more consistent, more expensive.

Both are commonly supplied surface-coated, usually with stearic acid. The coating matters more than it sounds:

– It makes a hydrophilic mineral compatible with a hydrophobic polymer – It improves dispersion and reduces agglomeration – It reduces melt viscosity at a given loading – It reduces moisture pickup

Uncoated filler in a polyolefin disperses poorly, raises viscosity more, and brings moisture with it. If a filled compound is showing dispersion problems and the filler is uncoated, that is where to look before examining the colourant.

Why it changes your colour

Calcium carbonate has a refractive index around 1.6, against roughly 1.5 for polyolefins. That difference is enough to scatter light, which is the definition of an opacifying pigment.

It is far weaker than titanium dioxide, whose refractive index is around 2.7, but it is present at 10 to 50% loading rather than at 1 to 5%, so the total effect is substantial.

Four practical consequences:

1. Filled compounds are lighter and less saturated. The white mineral dilutes and lightens everything. A colour formulated for unfilled PP will look pale and washed out in 30% filled PP.

2. You need more colourant. Reaching the same shade over a lighter, more opaque base takes higher pigment loading. This partly offsets the cost saving from the filler, and it is routinely left out of the cost calculation.

3. Your titanium dioxide requirement changes. For white and light shades, the filler is already providing opacity. A white masterbatch dosage optimised for unfilled material is often more TiO2 than a filled compound needs. There is real money in re-optimising this – see titanium dioxide in plastics and our white masterbatches.

4. Deep and saturated shades become harder. Black is the clearest case: filler lightens toward grey, so a deep jet black in a heavily filled compound needs considerably more carbon black, and beyond a point the depth simply cannot be reached. Rich, saturated colours have the same limit.

The variability problem

This is the part that causes disputes.

Filler loading is the most frequently adjusted variable in an Indian plastics plant, because it is the most direct cost lever available. It gets adjusted for raw material price movements, for stiffness requirements, for whatever filler grade is available this month.

And every adjustment moves the colour.

The pattern is familiar: filler percentage is raised by a few points to hold a cost target, the compound goes lighter, and a colour complaint is raised against the masterbatch supplier – whose material has not changed at all.

Two further sources of the same effect:

Filler grade changes. Different sources have different whiteness, different particle size and different impurity content. A GCC from one quarry is not optically identical to a GCC from another, even at the same nominal specification.

Filler quality varies. Iron and other mineral impurities tint filler grey or yellow, and low-grade material varies lot to lot.

Before raising a colour complaint against a masterbatch supplier, check whether the filler loading, filler grade or filler source has changed. In filled compounds this is the single most common cause, and it is checkable in an afternoon.

This is a specific instance of the broader diagnostic in why colour drifts between batches.

Effects beyond colour

Mechanical properties. Filler raises stiffness and modulus, and reduces impact strength and elongation. Fine, well-coated filler at moderate loading can improve impact in some polypropylene systems; coarse or poorly dispersed filler always reduces it.

Processing. Filler raises melt viscosity, increases screw wear and abrasion of tooling, and improves thermal conductivity – which actually shortens cooling time and can shorten cycle.

Shrinkage falls, because the mineral does not shrink. This helps dimensional stability and is a genuine benefit in large mouldings – see plastic shrinkage and dimensional stability.

Density rises, which is the point most often missed commercially. Calcium carbonate is roughly 2.7 g/cm³ against about 0.9 for polypropylene. Filler reduces cost per kilogram and increases parts per kilogram far less than expected – because you are buying heavier material. On a part sold by piece, the saving is real but smaller than the price-per-kilo comparison suggests. On a part sold by weight, it can vanish entirely.

Dispersion. Poorly dispersed filler agglomerates are stress concentrations and surface defects, exactly like pigment agglomerates.

Surface finish. High filler loading reduces gloss and can give a chalky appearance – sometimes desirable for a matt finish, usually not.

Weathering. Filler particles at the surface can be washed out over time, leaving a rough chalky surface.

Getting the dosage right

Because filler changes the substrate, colourant dosage has to be established against the actual filled compound, not against virgin polymer.

– Match colour on a plaque of the production compound, at production filler loading and production thickness – Requalify colour if filler loading changes by any meaningful amount – Re-optimise white dosage for filled compounds, since the filler is already contributing opacity – State the filler loading on the masterbatch enquiry – a colourant quoted against unfilled PP is quoted against the wrong substrate – Control filler as a specification, not as a cost lever, if colour consistency matters to your customer

Our let-down ratio calculator covers the arithmetic of dosage; the point here is that the target itself moves with filler.

Talk to us

Life Color Pigments & Masterbatches formulates colour, white and black masterbatches against the compound you actually run, under ISO 9001:2015.

If you run filled compounds, tell us the filler type, loading and grade along with your polymer and shade target. And if your colour has drifted recently, check the filler before anything else – it is the most common answer and the fastest to rule out.

Related: household, furniture, packaging, masterbatch vs compound vs dry pigment.

FAQs

Does calcium carbonate filler change the colour of plastic? Yes, substantially. Calcium carbonate has a refractive index higher than polyolefins, so it scatters light and acts as a white opacifying pigment. At 10 to 50 percent loading it lightens and desaturates every colour in the compound.

Why do I need more masterbatch in a filled compound? Because the filler lightens and opacifies the base, so reaching the same shade requires higher pigment loading. This partly offsets the cost saving from the filler and is frequently left out of the cost calculation.

My colour changed but my masterbatch supplier says nothing changed. What should I check? Filler loading, filler grade and filler source, in that order. Filler percentage is the most frequently adjusted variable in a plastics plant and every adjustment moves the colour. Different quarries and grades also differ in whiteness and impurity content.

What is the difference between coated and uncoated calcium carbonate? Coated grades, usually treated with stearic acid, are compatible with hydrophobic polymers, disperse better, raise viscosity less and pick up less moisture. Uncoated filler in a polyolefin disperses poorly and brings moisture with it.

Does filler reduce my material cost as much as the price suggests? Usually less. Calcium carbonate has a density around 2.7 against about 0.9 for polypropylene, so filled compound is considerably heavier. You get fewer parts per kilogram than the price comparison implies, and on parts sold by weight the saving can disappear.

Can I use less titanium dioxide in a filled compound? Often yes, because the filler is already providing opacity. A white masterbatch dosage optimised for unfilled material is frequently more titanium dioxide than a filled compound requires, and re-optimising it is a real cost saving.

Does calcium carbonate affect shrinkage? Yes, it reduces it, because the mineral itself does not shrink. This improves dimensional stability and is a genuine benefit in large mouldings, alongside the increased stiffness and improved thermal conductivity that filler provides.

Why is deep black harder in a filled compound? Because the white filler lightens the compound toward grey, so achieving a deep jet black requires considerably more carbon black. Beyond a certain filler loading the depth simply cannot be reached, and the same limit applies to rich saturated colours.