I. Key Properties of Nano Calcium Carbonate
With its unique nano-sized particle structure, nano calcium carbonate possesses surface effect, small-size effect and macroscopic quantum tunneling effect that conventional calcium carbonate does not have. It exhibits excellent physicochemical and application performances, and serves as a refined inorganic powder material under intensive research and development in the field of new materials.
In terms of performance, nano calcium carbonate shows outstanding advantages in magnetic properties, catalytic performance, photothermal barrier property and melting point characteristics, and delivers remarkable enabling effects across a wide range of industries. In rubber and plastic industries, it acts as a high-quality white reinforcing filler, which can effectively improve the surface gloss, tensile strength, bending resistance and crack resistance of finished products, and optimize the overall quality of products. In coatings and high-end ink industries, it functions as a functional filler. It not only reduces raw material costs, but also endows products with thickening and anti-settling properties to stabilize product performance. In feed processing, it can be used as an efficient calcium supplement additive to increase the available calcium content of feed. In the cosmetics industry, it can replace high-priced titanium dioxide. Relying on excellent purity, whiteness and fineness, it meets the raw material quality requirements of high-end cosmetics and boasts remarkable cost performance.
Nano calcium carbonate is a general refined powder material. Its end-use performance is mainly determined by core parameters including crystal form, microscopic morphology, particle size and particle size distribution. These parameters can be precisely optimized by adjusting preparation processes and adding crystal form control agents. Therefore, relying on the regulation of process parameters and compounding of functional additives to prepare nano calcium carbonate products with different morphologies and specific functions, further improving its industry adaptability and exploring new application scenarios, constitutes the core research direction in the field of nano calcium carbonate.
II. Analysis of Main Technical Indicators of Nano Calcium Carbonate
(I) Main Content Characteristics
Nano calcium carbonate finished products generally feature relatively low main content. The core reason lies in the surface modification required during production. Crystal form directing agents, surfactants, dispersants and other additives added in the process form single or multi-layer coating on the surface of calcium carbonate particles, which directly reduces the effective main content of calcium carbonate. Meanwhile, the smaller the particle size and the larger the specific surface area of the product, the more additives can be coated on the particle surface, and the lower the main content of calcium carbonate.
The main content of different types of calcium carbonate presents an obvious gradient, following the order: ordinary light calcium carbonate > activated light calcium carbonate > special nano calcium carbonate. The main content of high-end imported nano calcium carbonate is even lower than 95%, which is an important indicator distinguishing nano calcium carbonate from ordinary calcium carbonate.
(II) Correlation between Crystal Morphology and Particle Size
The crystal morphology of nano calcium carbonate is highly correlated with particle size, and the particle size range directly determines the crystal form. The corresponding relationships are as follows: when the particle size is greater than 200 nm, the crystals are mostly asymmetric structures such as spindle shape and rod shape; when the particle size ranges from 50 nm to 120 nm, the crystals are dominated by symmetric and regular structures such as cubic and spherical shapes; when the particle size is less than 30 nm, most crystals are cubic granular, and the particles tend to adhere to each other to form chain-like agglomerates.
Carbonization temperature, initial reaction rate and crystal form directing agent are three core process parameters for regulating the particle size and crystal form of calcium carbonate, and play a decisive role in the consistency of product microscopic structure.
First, carbonization temperature is a key factor for particle size control. Low-temperature carbonization at 0～30 ℃ enables ultra-fine particle formation. As the temperature rises, the crystal particle size gradually increases, the particle size distribution broadens, and the product uniformity declines.
Second, the reaction rate exerts a significant influence on crystal particle size. Increasing the reaction rate at the initial stage of carbonization accelerates nucleation and promotes particle ultra-fining, which is the core principle for preparing ultra-fine nano calcium carbonate via low-temperature carbonization.
Third, crystal form directing agents can precisely regulate crystal morphology, particle size and structural uniformity. Commonly used additives include soluble inorganic salts such as Al³⁺, Zn²⁺, Mg²⁺ and Na⁺, strong inorganic acids, as well as organic dispersants such as lower fatty acids and resin acids.
(III) Characteristics of Sedimentation Volume
Sedimentation volume is a core index to measure the fineness and quality of calcium carbonate powder. It is defined as the volume occupied by a unit mass of calcium carbonate sample after being fully shaken in 100 mL of water and standing for 3 hours. Sedimentation volume has a strong correlation with particle size and bulk density of the product. A larger sedimentation volume indicates smaller powder particle size, lower bulk density and higher product grade and quality.
There are obvious differences in sedimentation volume among various calcium carbonate types with clear quality gradients: the sedimentation volume of ground calcium carbonate is 1.1～1.4 mL/g, that of ordinary light calcium carbonate is 2.4～2.8 mL/g, while nano calcium carbonate can reach 3.0～4.0 mL/g, significantly superior to traditional calcium carbonate products. Besides particle size, crystal morphology is also an important factor affecting sedimentation volume.
(IV) Correlation between Oil Absorption Value and Application Performance
The oil absorption value of calcium carbonate is jointly determined by particle voids, surface properties and specific surface area, and directly affects the adaptability of products in various industries. Powder with uniform particle size distribution and smooth regular surface has smaller specific surface area and lower oil absorption value. On the contrary, products with uneven particle size, complex or defective crystal structure and larger specific surface area have higher oil absorption value.
Surface modification has a great impact on oil absorption value. Activated modified calcium carbonate has much lower oil absorption value than ordinary calcium carbonate, and the oil absorption value decreases with the increase of active agent coating amount on the particle surface. Unmodified nano calcium carbonate is prone to electrostatic effect and powder agglomeration, difficult to disperse during application. Its high oil absorption value will seriously restrict the processing performance and end-use effect of products.
Different application fields have significantly different requirements on the oil absorption value of nano calcium carbonate. For plastics, coatings and inks, the oil absorption value should be controlled as low as possible to facilitate product molding and performance stability. For rubber and papermaking industries, the oil absorption value can be appropriately increased on the premise of ensuring powder dispersibility to adapt to industrial production processes and product performance requirements.
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