Introduction to Surface Modification of Heavy Calcium Carbonate Fillers

Calcium carbonate is currently the most widely used category of inorganic filler. Due to its high whiteness, low cost, and non-toxic, non-irritating properties, it is extensively utilized in industries such as plastics, rubber, adhesives, inks, and coatings. Based on their respective manufacturing processes, calcium carbonate is classified into two categories: heavy calcium carbonate (heavy calcium) and light calcium carbonate (light calcium). Heavy calcium is produced from natural stones—such as calcite, marble, and limestone—through mechanical crushing and classification; it features irregular particle shapes and a relatively broad particle size distribution. Light calcium, conversely, is prepared via chemical methods, allowing for control over crystal morphology and resulting in finer particle sizes, albeit at a higher production cost.

Precipitated calcium carbonate exhibits a hydrophilic and oleophobic surface with strong polarity, resulting in poor affinity and compatibility with organic polymers (such as plastics, rubber, and resins). When used as a direct filler, the particles tend to agglomerate, making it difficult to achieve uniform dispersion within the organic matrix; furthermore, the weak interfacial bonding forces readily lead to interfacial defects, thereby compromising the mechanical properties of the resulting composite materials. As the particle size of the calcium carbonate decreases, its surface energy increases, rendering the agglomeration phenomenon even more severe.

Therefore, surface treatment is required prior to use, primarily to improve:

Reduce particle surface energy to prevent particle aggregation.

The surface of heavy calcium carbonate is transformed from hydrophilic to hydrophobic, thereby enhancing its compatibility with organic matrices.

Currently, the primary methods for modifying heavy calcium carbonate are the dry method, the wet method, and the mechanochemical method. Among these, the dry method is currently the dominant industrial process.
In the dry method, dry calcium carbonate powder is mixed with a modifying agent in a high-speed kneader to achieve surface coating at a specific temperature. This process is simple to operate—yielding the final product directly upon discharge—and is particularly suitable for oil-soluble modifying agents as well as for large-scale, continuous production. The wet method results in a more uniform coating; however, the process is more complex and requires a subsequent dehydration step. The mechanochemical method involves the simultaneous addition of the modifying agent during the ultrafine grinding process, thereby integrating both grinding and modification into a single operation.

The selection of the modifying agent directly affects the modification results; commonly used modifying agents include:

Stearic Acid (Salts):

One end of the molecule features a carboxyl group, which can chemically bond with calcium ions on the surface of calcium carbonate; the other end consists of a long-chain alkyl group, structurally similar to a polymer. This mechanism constitutes a single-point anchoring system.
The typical dosage ranges from 1.2% to 1.5%, achieving an activation degree of over 95%. Its advantages include low cost and wide availability; however, its disadvantages lie in a relatively low degree of anchoring and only moderate stability of the surface coating. The primary components utilized are stearic acid (octadecanoic acid) and its sodium salt.

Titanate Coupling Agents:

By forming chemical bonds with the surface of heavy calcium carbonate, these agents enhance the flowability and mechanical properties of the composite system; however, they do have some impact on the environment and human health. The primary examples include isopropyl tri(dioctyl pyrophosphate) titanate and isopropyl tri(isostearoyl) titanate.
Experimental data indicate that when the titanate coupling agent is added at a concentration of 2.0%, the activation degree reaches 97.70% and the oil absorption value decreases to 10.6 g/100g; furthermore, when incorporated into PVC, both the elongation at break and the impact strength show significant improvement.

Aluminate Coupling Agents:

Characterized by its light color, non-toxicity, and excellent thermal stability, it is capable of forming irreversible chemical bonds with the surface of calcium carbonate. It is suitable for applications with stringent requirements regarding product appearance and health safety, and primarily comprises isopropyl aluminum bis(ethyl acetoacetate).

Silane Coupling Agents:

There are numerous types of silanes, all of which are capable of reducing the polarity of heavy calcium carbonate and enhancing its hydrophobic and lipophilic properties; however, silanes exhibit distinct drawbacks in the modification of calcium carbonate, specifically their susceptibility to hydrolysis and self-condensation under alkaline conditions, as well as the relative complexity of the associated processing techniques.
Commonly employed silanes include γ-aminopropyltriethoxysilane (KH-550), γ-glycidoxypropyltrimethoxysilane (KH-560), and γ-methacryloxypropyltrimethoxysilane (KH-570), among others.

Composite Modifier:

Using a combination of two or more modifiers can yield synergistic effects. For instance, combining stearic acid with aluminates or titanates results in an activation effect superior to that of a single modifier used alone.

Precipitated calcium carbonate exhibits an alkaline surface (the pH of its aqueous suspension typically ranges from 9 to 10). Under alkaline conditions, silane coupling agents are prone to hydrolysis and self-condensation reactions, making it difficult for them to form a uniform, robust chemically bonded layer on the calcium carbonate surface.
However, this does not preclude the use of silane coupling agents; indeed, in certain applications, silanes may be the more suitable choice. Typically, they are used in conjunction with additives such as stearates. By utilizing stearates to mitigate the surface alkalinity of the calcium carbonate—or to form a composite coating layer—one can then apply the silane coupling agent, thereby leveraging its inherent advantage of strong affinity for specific resins.