Application and differentiation of heavy calcium, light calcium, active calcium carbonate, and nano calcium carbonate in rubber

Calcium carbonate is the largest inorganic filler used in the rubber industry. Its core function is to increase filling volume and reduce costs. Some grades can play a semi reinforcing role, adjusting the hardness, extrusion flowability, rolling shrinkage, surface texture of the rubber material, and affecting its tensile, tear, wear resistance, and aging properties. Many people are prone to confusion, and the essential distinction logic is: first, classify heavy calcium/light calcium according to the preparation process, then distinguish ordinary calcium and active calcium based on surface modification, and finally classify nano calcium based on the original particle size; Nano calcium can be either modified or unmodified, and active calcium does not necessarily have to be at the nanoscale. Below, we will explain the preparation, physical and chemical characteristics, identification methods, rubber application characteristics, and applicable scenarios one by one.

一、 Four basic definitions and essential differences of calcium carbonate

1. Ground calcium carbonate (GCC)

The raw materials are natural calcite, marble, and limestone ore, which are physically crushed and ground without any chemical reactions. Crystal morphology: retaining the natural rhombohedral crystal form of calcite, the particles are mostly irregular blocks with obvious edges and corners. Primary particle size range: ordinary heavy calcium 1-10 μ m; Ultra fine heavy calcium 0.5-2 μ m; Ultra fine heavy calcium can reach 0.2-0.5 μ m, but it is obtained by crushing and the particles are polycrystalline aggregates, not single crystal particles. Surface state: The surface is hydrophilic hydroxyl group, hydrophilic, poorly oleophobic, and easily aggregates when directly mixed in rubber.

2. Precipitated calcium carbonate (PCC, precipitated calcium carbonate)

Chemical production: Limestone is calcined to produce quicklime, digested into lime milk, and then carbonized and precipitated with carbon dioxide. Crystal morphology: spindle shaped (jujube nucleus shaped), with a significant aspect ratio, and ordinary light calcium particles with a diameter of 0.5-3 μ m. Features: Both are hydrophilic surfaces; Under the same filling amount, light calcium has a higher specific gravity and calcium oil absorption value, a lower bulk density, and a larger volume for the same weight.

3. Activated calcium carbonate (activated calcium)

It is not an independent mineral source, but a surface modified product of heavy calcium or light calcium powder. The most commonly used modifier is stearic acid (salt), as well as coupling agents (titanate, aluminate). Principle: One end of stearic acid is bound to the hydroxyl group on the surface of calcium carbonate, and the other end is a long-chain alkyl group. The surface of the powder changes from hydrophilic to oleophilic. Active calcium can be either active light calcium or active heavy calcium, and the vast majority of active calcium on the market is active light calcium. Simple indicators for determining activity: activation degree, commonly used in national standards for characterization of activation degree. The higher the activation degree, the better the hydrophobicity; Throwing the powder into water, active calcium floats on the surface, while ordinary heavy calcium and light calcium sink directly. Primary particle size: Ordinary active light calcium is generally 0.3-2 μ m and does not belong to the nanometer category.

4. Nano calcium carbonate (NPCC)

Belonging to precipitated calcium carbonate (chemical carbonization method), by controlling the carbonization temperature, concentration, and crystal form control agent, the primary single crystal particle size is reduced to within 100nm (≤ 0.1 μ m), and the mainstream commercial nano calcium primary particle size is 30-80nm. Crystal form: mostly cubic, there are also spindle shaped nano calcium; The particles are extremely small, with extremely high surface energy, and are highly prone to agglomeration. The vast majority of nano calcium on the market undergoes surface activation (stearic acid modification) before leaving the factory, which is called active nano calcium; There are also unactivated nano calcium, which are rarely used in the rubber industry and have severe agglomeration, making it difficult to disperse.

Key distinguishing point: The criterion for determining the core of nano calcium is the original single crystal particle size, not the apparent particle size of the powder surface; The test must use an electron microscope to observe the original particles, and cannot use a laser particle size analyzer to measure the particle size after aggregation. The ultrafine heavy calcium laser particle size analyzer measured 0.3 μ m, but the crushed agglomerates and primary particles are in the micrometer range and do not belong to nano calcium.

二、Performance characteristics and applicable scenarios in rubber systems

Basic premise: Calcium carbonate as a whole belongs to non reinforcing/semi reinforcing fillers, and its reinforcing ability is much lower than that of carbon black and white carbon black; Only nano calcium carbonate has a significant semi reinforcing effect, while ordinary heavy calcium and light calcium mainly fill in increments.

1. Ground calcium carbonate (GCC)

Performance characteristics: low oil absorption and less gum consumption; Low mixing viscosity, good extrusion and rolling flowability; The increase in hardness of the rubber material is small; Poor reinforcement, weak improvement in tensile and tear strength; Large filling volume and lowest cost; Disadvantages: The particles have sharp edges, and excessive filling can significantly reduce wear resistance and tear resistance; The elongation rate of the rubber material remains acceptable. Ordinary heavy calcium is easy to disperse and does not require a high plasticizing time; The performance of ultrafine heavy calcium is between that of ordinary heavy calcium and light calcium.

Rubber application scenarios focus on incremental cost reduction, suitable for products with low mechanical requirements:

Ordinary rubber sealing strips, inexpensive rubber pads, rubber flooring substrates, rubber floor tiles; Inner non pressure bearing layer of rubber hose, ordinary rubber miscellaneous parts; Recycled rubber products (recycled rubber shoe soles, recycled rubber pads); Rolled rubber sheets require good fluidity and stable shrinkage; Limitations: Cannot be used for high tear, wear-resistant, and dynamically stressed products; The wear resistance significantly decreases under high filling.

2. Precipitated calcium carbonate (PCC, ordinary light calcium, not activated)

Performance characteristics: high oil absorption value and large amount of glue consumption; The viscosity of the rubber material increases significantly during mixing; Strengthening is slightly better than regular heavy calcium, but much lower than active calcium; Hydrophilic surfaces have poor compatibility with rubber substrates, are prone to agglomeration, and are prone to particle formation and pitting on extruded surfaces; The tear strength of vulcanized rubber is average, and there is a large lag loss. Advantages: lower price than active calcium, high whiteness.

Nowadays, the rubber industry uses less and less ordinary unactivated light calcium directly, and most of them are replaced with activated light calcium; Only a small amount of low-end recycled rubber, low-end adhesive slurry, and low requirement rubber roller outer layer; ⚠️ Not recommended for extrusion, precision molded products, poor dispersion, and multiple appearance defects in finished products.

3. Active calcium carbonate (mainly active light calcium)

Performance characteristics: After modification with stearic acid, the interface between the powder and rubber is improved, and the dispersibility is significantly better than that of ordinary light calcium; The oil absorption value is slightly lower than that of unactivated light calcium with the same particle size; The stretching, tearing, and elongation of vulcanized rubber are better than those of ordinary light calcium; The extruded surface of the rubber material is smooth; Lower mixing energy consumption; Reinforcement belongs to the category of medium to low and still belongs to incremental fillers, not strong reinforcement fillers. Disadvantages: stearic acid modifier can bring certain plasticizing effects, and when added in large amounts, it can reduce the crosslinking density of the rubber material, resulting in poor heat resistance and compression permanent deformation; Some brands have too much stearic acid, which can easily cause frosting. Rubber application scenarios (calcium carbonate category with the largest usage in the rubber industry)

4. Nano calcium carbonate (active nano calcium)

Performance characteristics: Nano size brings significant semi reinforcement effect, with the best reinforcement among the four types of calcium: improving tensile strength, tear strength, and tensile stress; Can partially replace lightweight carbon black; High whiteness, does not affect colored rubber; The crystal structure is controllable, and the internal stress of cubic nano calcium is low. Shortcomings: ① Large specific surface area, high oil absorption value, and high filling amount. The viscosity of the adhesive material sharply increases, making processing difficult; ② High surface energy makes it difficult to mix and disperse, and poor dispersion can form agglomerated particles, which become stress concentration points and actually reduce strength; ③ The price is much higher than that of active light calcium and heavy calcium; ④ The modified nano calcium with stearic acid also contains residual stearic acid, and caution should be exercised when formulating formulations that require low compression permanent deformation under high temperature conditions.