Calcium Carbonate

Like many materials we take for granted, calcium carbonate turns out to be more complex that one might imagine and to have more uses that expected. We will discuss some of those properties and applications for calcium carbonate as a functional mineral filler for plastics, paints, coatings, sealants and adhesives here.

Sources and Types

• Chalk is a soft, cretaceous-era limestone made of nanofossils (e.g., coccoliths), featuring loosely bonded 3 μm calcite crystals.
• Limestone is consolidated sedimentary rock formed from marine skeletons, sometimes altered by pressure or water.
• Marble is metamorphosed limestone, recrystallized under heat and pressure into a denser, coarser material.

Calcium carbonate is often mined from large deposits of coccoliths which are microscopic, calcareous (calcium carbonate) plates secreted by coccolithophores, a type of marine phytoplankton. These plates form a coccosphere, that covers the cell surface. The mined deposits can be millions of years old but the same process is in action today. One can harvest newly created aragonite calcium carbonate from the ocean – see the separate page on AquaFlex renewable calcium carbonate for more on that topic.

As well as mining, calcium carbonate can be and is made commercially using synthetic routes. So called precipitated calcium carbonate (PCC for short) is preferred when very fine particles are needed (e.g. 1 micron in diameter or less) and / or when very high purity is needed.

Processing

• High-purity deposits are mined (often dry) and ground.
• Chalk yields fine powders (“whitings”) around 3 μm average particle size.
• Marble grinding gives broader size distributions (3–5 μm mean), but purer color.
• Ground limestones have similar size distributions but can vary in color (white to grey).

Producing ultrafine calcium carbonate (e.g., 0.25 μm) demands much more energy, with wet grinding being more efficient for such fine products. Regardless of particle size, calcite crystals maintain a rhombohedral shape with an approximate aspect ratio of 3:1.

Surfaces of ground calcium carbonate may have minor layers of calcium hydroxide or oxide. In chalk, coccoliths are often coated with thin layers of aluminosilicates and organic materials, which can remain even after grinding.

Chemistry

Calcium carbonate is formed when calcium ions react with carbon dioxide in the air. The calcium carbonate formed is insoluble in water, for example in the ocean, and precipitates out, sinking because it has a much higher density than water. Thus, calcium carbonate is nature’s way of sequestering (trapping) carbon dioxide and removing it from the atmosphere.

Carbonates are relatively inert but calcium carbonate is attacked even by weak acids like citric acid or acetic acid (vinegar). So, if you own a marble countertop, there will be a warning to wipe it down if it comes into contact with such acids from food preparation. The reaction is visible to the naked eye because bubbles of carbon dioxide are released as the reaction proceeds.

Calcium carbonate is therefore useful for neutralizing acids making it useful for soil remediation. In PVC plastic, calcium carbonate is added to scavenge hydrochloric acid formed when PVC starts to degrade, for example at high temperature during melt processing. Without the carbonate there to remove the acid, it would catalyze further degradation leading to discoloration, loss of properties and impaired durability.

Effect on Mechanical Properties

Blending mineral particles with other materials generally leads to properties intermediate between those of the mineral and the host material. For example, minerals are harder than plastics, so adding them leads to a composite that is harder than the starting plastic was. The same is true for stiffness which is not surprising as hardness is merely the stiffness of the surface. Calcium carbonate is used to improve the hardness and stiffness of polymer-based formulations. They are improved whether or not there is good adhesion between the mineral and the surrounding material.

Heat distortion temperature and Vicat softening point may also be raised by adding calcium carbonate because of the increase in stiffness at any given temperature. However, the effect is modest with low aspect ratio fillers like calcium carbonate and so fibrous or platy type minerals are selected when high HDT is required. Furthermore, minerals only work well for increasing HDT / Vicat for semi-crystalline plastics like PE, PP and nylons but are not effective in non-crystalline (amorphous) plastics like PC, PS, ABS and so on. The reason is that when heated, amorphous polymers get so soft that it doesn’t matter that stiff mineral was added, the whole material sags anyway. Semi-crystalline plastics on the other hand do retain significant stiffness right up until the temperature where the crystals melt. When it comes to HDT, adhesion to between the mineral is important, so adhesion promoters such as silane coupling agents are used.

When it comes to strength, calcium carbonate has little influence one way or the other. It is not a reinforcing filler because the particles of calcite have low aspect ratio, meaning they are round or blocky in shape. In order to attain reinforcement, high aspect ratio mineral particles are selected instead.

The above-mentioned properties change in relatively predictable ways and the volumetric linear rule of mixtures is a useful tool for estimating the behavior of such systems. In contrast, some properties do not depend on the average properties of the blend but rather on local flaws. One example is impact resistance. Large particles of carbonate can act as flaws where a crack initiates upon impact, then spreads rapidly such that the whole material fails. It is not the average particle size that matters but the size of the largest particle or agglomerate of particles. If high impact resistance is desired, then small, well-dispersed particles with coarse particles removed is the recipe for success. For brittles plastics like polypropylene homopolymer or uPVC, addition of calcium carbonate can actually increase impact resistance compared to the unfilled plastic.

Elongation to break is a term that refers to the maximum amount one can stretch the material before it breaks. This property mirrors impact resistance in that it depends on flaws. So, small, well dispersed particles are key when one wishes to retain elongation to break, for example in a flexible seal or a formulation for cables.

Effect on Thermal Properties

Minerals have much lower thermal expansion coefficients than organic materials do, so adding calcium carbonate to plastics, paints, sealants or adhesives is effective for reducing and tailoring CLTE (see separate page on modification of CLTE).

Specific heat capacity is another of the thermal properties and it is one that is much misunderstood. Even text books get the facts wrong in many cases. Apart from rare exceptions, minerals have the same ability to store heat as other solids do, meaning that a given volume of almost any solid takes the same amount of energy to heat up releases the same amount upon cooling. So, calcium carbonate is neither beneficial nor detrimental.

Density

Calcium carbonate is used to add weight to polymer-based materials. For example, additional heft may help increase the perception of quality. Sometimes the added density is needed to ensure that the part sinks in water. Added density also helps with sound deadening, so calcium carbonate is used as a cost-effective filler in that application.

Optical

Calcium carbonate has a higher refractive index than many common polymer-based materials, so it acts as a weak white pigment. It can also be used as a spacer to help optimize the performance of titanium dioxide, which is a much stronger white pigment. However, because CaCO3 has more than one refractive index in the crystal, one cannot match its refractive index to that of the surrounding polymer in order to create a transparent material. Calcium carbonate filler will always give some degree of opacity, which may be useful for example in light diffusers. In applications where colour is not critical, it is possible to purchase grades of calcium carbonate with lower brightness at a discount.

Abrasion

Calcium carbonate has low hardness and can be used as a mild abrasive or polish, for example in toothpaste. It is too soft to cause significant abrasion of polymer processing equipment, so if abrasion is observed, it is usually due not to the carbonate but to trace impurities, in particular quartz which has very high hardness and abrasivity. If abrasion is a problem, then changing the carbonate to a purer and / or finer grade is advisable. To test for abrasive behavior, the Einlehner test is useful whereby a slurry of mineral in water is passed through a brass mesh and the loss in weight of the mesh recorded. Similarly, one can extrude filled polymer through a die of softer metal like brass and record the weight lost due to abrasion.

Surface Treatment

Calcium carbonate is receptive to surface treatment and many such commercial products are available. Stearic acid is an effective and inexpensive dispersant and is therefore by far the most commonly used, although more effective alternatives are available as well. For ground calcium carbonate (GCC), stearic acid can be added during milling where it can improve milling efficiency. It can also be added post milling if needed. PCC tends to be surface treated with stearic acid anyway because it is added to help dewater the particles to save time and energy. By making the particles hydrophobic, water removal from the PCC filter cake is far easier. Silane coupling agents do not adhere to calcium carbonate, so acidic types like maleated polymers are used instead. More details about dispersants and coupling agents can be found here in this free training video.