Calcium carbonate fillers are ideal for many polymer applications, with global consumption reaching several million tons per year. Calcium carbonate is available in two forms: natural or ground calcium carbonate (GCC), with particle sizes of 0.5–100 µm, and synthetic or precipitated calcium carbonate (PCC), with particle sizes below 100 nm for applications requiring nanoscale particles, such as elastomers and PVC. Calcium carbonate fillers are widely used in thermoplastics, thermosets, and elastomers.
Calcium carbonate is abundant in nature and is extracted from various mineral sources. It is sometimes found with significant amounts of other materials, particularly magnesium carbonate (MgCO₃) and dolomite (MgCO₃·CaCO₃). It occurs in three crystalline forms—calcite, aragonite, and vaterite—as well as in an amorphous form. Among these, calcite is the most widely used as a filler. The specific gravity of calcite is 2.7, while that of aragonite is 2.8–2.9.
GCC is recognized as an ideal filler due to its ease of use and low cost. Its low price has led to its use as a partial replacement for polymers. However, the cost reduction is not as significant as it may initially appear. Due to its high specific gravity, 2.5 to 3 times the weight of the polymer being replaced must be added as calcium carbonate to produce the same unit volume of a product. The greater the required weight, the higher the cost. Therefore, this factor should be considered when using calcium carbonate for cost reduction.
Production of GCC
After mining, the main processes include crushing, grinding, impurity removal, and, when required, surface coating and drying. Grinding can be performed using either dry or wet methods. Dry grinding is more economical for coarse particle sizes, while wet grinding is more suitable for finer particles. Grinding aids may be used and can remain in the final product, making impurity removal necessary through methods such as froth flotation, magnetic separation, and other techniques.
Surface Modification of Calcium Carbonate
The incompatibility between the hydrophilic, high-energy surface of calcium carbonate and the hydrophobic, low-energy surface of polymers must be addressed before calcium carbonate can be effectively used as a filler. Therefore, surface modification of calcium carbonate particles is required to improve their compatibility with polymers.
Fatty acids and coupling agents are commonly used for surface modification. The most widely used fatty acid is stearic acid. Fatty acids are usually added during the final stage of calcium carbonate processing, although they can also be introduced during other stages, including grinding. In this case, stearic acid also acts as a grinding aid. The fatty acid reacts with the calcium carbonate surface and forms monoacid salt bonds.
Calcium Carbonate in Thermoplastics
More than 60% of global GCC consumption is in thermoplastics, particularly PVC and polyolefins. The largest share is used in flexible and plasticized PVC, mainly in the wire and cable industry, flooring, and parts of the carpet industry. Flexible PVC can accommodate large amounts of filler, even exceeding 400 parts per 100 parts of resin (phr). The amount of filler varies depending on the application; for example, filler content must be relatively low in wire and cable insulation.
Rigid PVC, or u-PVC, generally contains lower filler levels of around 20–40 phr. The main applications of u-PVC include pipe extrusion, cable management systems, windows, and other construction profiles. Processing u-PVC is challenging because of its thermal instability. During processing, polymer granules must fuse together rather than simply melt, and fillers have a significant effect on this fusion process. Smaller particles accelerate fusion, while fatty-acid-coated particles can also improve mechanical properties.
Based on these characteristics, PCC appears to have strong potential in the u-PVC industry. However, advances in grinding technology have resulted in GCC being used more widely than PCC in u-PVC, primarily because of its lower cost.
In polypropylene applications, GCC is the most important filler not only because of its economic advantages but also because it increases stiffness without significantly affecting other properties, particularly impact resistance. Fillers such as talc, which have a high aspect ratio, are highly effective in increasing stiffness but tend to affect other properties more significantly than calcium carbonate. Fatty acid coating and finer calcium carbonate particles contribute considerably to achieving better overall properties.
Another important application of calcium carbonate is in microporous or breathable films based on polyethylene and polypropylene. GCC particles create micropores by separating from the polymer matrix when the film is stretched during production. One of the major applications of these porous films is the production of disposable diapers.
Calcium Carbonate in Thermosets
GCC is used in bulk applications and bulk molding compounds in thermosets, particularly unsaturated polyester resin (UPR). In many of these applications, GCC and glass fibers are used together. The GCC should have very low moisture content, low cost, low oil absorption, and minimal impurities.
UPR contains numerous functional groups that interact effectively with the filler; therefore, surface modification with fatty acids is generally unnecessary.
Approximately 20% of global GCC consumption is in the thermoset industry.
Calcium Carbonate in Elastomers
The remaining 20% of global GCC consumption is used in elastomers, including carpet applications and carpet backing based on rubbers such as SBR. GCC used in elastomers must have a very fine particle size.
In elastomers, calcium carbonate is often used alongside reinforcing fillers such as glass fibers, carbon black, or silica. Both fatty-acid-modified and unmodified GCC can be used in elastomer applications.
Precipitated Calcium Carbonate
Although precipitated calcium carbonate is more expensive than natural calcium carbonate, it offers several advantages:
- Ability to produce specific crystalline structures
- High purity
- Very small particle sizes
These advantages have led to the widespread use of precipitated calcium carbonate in the paper industry. PCC is still used in polymers; however, advances in grinding technology have made it possible to produce very fine GCC grades, which has limited the use of PCC in the polymer industry.
