Light calcium carbonate is made by chemical processing methods. Because its sedimentation volume (2.4-2.8mL/g) is larger than the sedimentation volume (1.1-1.9mL/g) of heavy calcium carbonate produced by mechanical methods. Its chemical formula is CaCO₃, which reacts with all strong acids to form and corresponding calcium salts (such as calcium chloride CaCl2), and at the same time emit carbon dioxide. At temperature (25℃), the concentration product of light calcium carbonate in water is 8.7/1029 and the solubility is 0.0014; the pH value of the light calcium carbonate aqueous solution is 9.5 to 10.2; the PH value of the air-saturated light calcium carbonate aqueous solution 8.0-8.6; Light calcium carbonate is non-toxic, odorless, non-irritating, usually white, with a relative density of 2.7-2.9; the sedimentation volume is above 2.5ml/g, and the specific surface area is about 5㎡/g.

Characteristics of calcium carbonate

White powder or colorless crystal, odorless, tasteless. It is decomposed into calcium oxide and carbon dioxide at 82.5℃. Soluble in dilute acid and emit carbon dioxide, insoluble in alcohol. There are two kinds of crystals, one is orthorhombic aragonite and the other is hexagonal rhombohedral calcite. Calcite is irritating.

a. The particles have regular shapes and can be regarded as monodisperse powders, but they can be in various shapes, such as spindle, cube, needle, chain, sphere, flake and quadrangular column. These different shapes of calcium carbonate can be prepared by controlling the reaction conditions.

b. Particle size distribution is narrow.

c. Particle size is small, the average particle size is generally 1-3μm. To determine the average particle size of light calcium carbonate, the short axis particle size in the triaxial particle size can be used as the representative particle size, and then the median particle size as the average particle size. In addition to the description hereinafter, the average particle size refers to the average minor axis particle size.

Light calcium carbonate has small particle size and high surface energy. Intermolecular forces, electrostatic interactions, hydrogen bonds, oxygen bridges, etc. cause calcium carbonate particles to easily agglomerate, or as a filler, it will affect the actual use effect; In addition, the surface of calcium carbonate is hydrophilic the strong -OH, which is alkaline, is a kind of hydrophilic powder, which is unevenly dispersed in high polymer. Therefore, its surface must be modified in application to reduce surface energy, increase surface active groups, and improve the wettability of the interface with the polymer and the interaction with the polymer.

The physical properties of the polymer are affected by the degree of activation, and the degree of activation is not only related to the modifier, but the key point is whether the calcium carbonate particles are truly dispersed. Therefore, the degree of dispersion of calcium carbonate and the quality of the modification effect directly affect its use value and application fields.

Brief introduction of calcium carbonate surface modification

The surface modification method of calcium carbonate is mainly chemical coating, supplemented by mechanochemistry; the surface modifiers used include stearic acid (salt), titanate coupling agent, aluminate coupling agent, zirconium aluminate acid salt coupling agent and atactic polypropylene, polyethylene wax, etc.

Continuous surface modification process of calcium carbonate

Surface modification should be carried out with the help of equipment. Commonly used surface modification equipment is SLG type continuous powder surface modification machine, high-speed heating mixer, vortex mill and fluidization modification machine.

The main factors affecting the surface modification effect of calcium carbonate are: the variety, dosage and usage of the surface modifier (surface modifier formula); the surface modification temperature and residence time (surface modification process); the surface modification of agents and the degree of dispersion of materials, etc. Among them, the degree of dispersion of surface modifiers and materials mainly depends on the surface modification mills.

1. Commonly used reagents and processes for wet modification

Wet activation is to add an activator to a solvent (such as water), stir the calcium carbonate in it to coat the surface, and finally dry it. This is generally done in light calcium carbonate or nano calcium carbonate manufacturers.

The surface energy of calcium carbonate particles is reduced after the wet modification treatment. Even if the secondary particles are formed after pressure filtration and drying, only soft agglomerates with weak binding force are formed, which effectively avoids the chemical bond oxygen bridges cause hard agglomeration in the dry modification. This method is a traditional calcium carbonate surface treatment method, which is suitable for water-soluble surfactants. The advantages of this method are uniform coating and high production quality. However, certain temperature and conditions need to be controlled for drying. Some surface treatment agents are insoluble in water or easily decomposed in water. The use of other organic agents has cost and safety issues.

(1) Stearic acid (salt) surfactant

Stearic acid (salt) surfactant is one of the commonly used surface treatment agents for calcium carbonate modification. It belongs to anionic surfactants. The structure of a long-chain alkyl group at one end of the molecule is similar to the structure of a polymer. It is a lipophilic group, so it is different from high molecular base material has good compatibility, and the other end is a water-soluble polar group, such as a carboxyl group, which can physically and chemically adsorb on the surface of inorganic fillers such as calcium carbonate.

The specific reaction mechanism of stearic acid (salt) modified calcium carbonate is that under alkaline conditions, ROOH- reacts with Ca2+ and other components to form fatty acid calcium precipitates, which are coated on the surface of calcium carbonate, so that the surface properties of the particles are changed from affinity Water becomes lipophilic.

Yue Linhai and his team reported using sodium stearate saponification solution as a medium to prepare composite calcium carbonate by co-precipitation. Jin Ruidi and his team studied the in-situ modification of calcium carbonate by sodium stearate. In the presence of a modifier, modified calcium carbonate was prepared from calcium hydroxide through carbonization, indicating that the hydrophobicity is due to the combination of sodium stearate in the form of ionic bonds. On the surface of calcium carbonate, insoluble calcium stearate is formed.

(2) Phosphate and condensed phosphoric acid surfactants

Phosphate and other fatty acids (esters) are used for the surface modification of calcium carbonate. After the surface modification of calcium carbonate is carried out by polyphosphate (ADDP) with a special structure, the surface of calcium carbonate particles is hydrophobic and lipophilic. The agglomerated particle size is reduced, and the modified calcium carbonate is filled in the PVC plastic system to significantly improve the processing and mechanical properties of the plastic. Mixed use of stearic acid and sodium dodecylbenzene sulfonate for surface treatment of light calcium carbonate can improve the effect of surface modification.

(3) Quaternary ammonium salt surfactants

The quaternary ammonium salt is a cationic surfactant. Its positively charged end is electrostatically adsorbed on the surface of calcium carbonate, and the other end can be cross-linked with polymers to modify the surface of calcium carbonate.

Zhang Zhihong and others used a new type of cationic surfactant Cetyl dimethyl allyl ammonium chloride (CDAAC) to organically modify calcium carbonate, and the modified product was used as a rubber filler and achieved good results.

2. Commonly used agents and processes for dry modification

The dry modification process is to put the calcium carbonate powder into the high-speed mixer, and then put in the surface modifier. With the help of the mixer and a certain temperature, the modifier can be uniformly adsorbed on the surface of the calcium carbonate particles to achieve the modification effect.

The key technical requirements of the dry modification process are: rapid mixing to facilitate the uniform coating of the coupling agent on the surface of the calcium carbonate particles, a suitable temperature to facilitate the reaction and adsorption, and the drying of the calcium carbonate without moisture to avoid the coupling agent React with water first, not with -OH on the surface of calcium carbonate, which will affect the modification effect.

The surface modifier is generally a coupling agent. The coupling agent modifies the surface of calcium carbonate. The group at one end of the coupling agent can react with the surface of calcium carbonate to form a strong chemical bond. The coupling agent the other end of the polymer can undergo a certain chemical reaction or mechanical entanglement with the organic polymer, thereby closely combining two materials with extremely different properties, calcium carbonate and organic polymer. At present, coupling agents on the market mainly include titanate coupling agents, aluminate coupling agents, borate coupling agents and phosphate coupling agents.

(1) Titanate coupling agent

Shown is the process flow of dry surface coating modification with titanate coupling agent. The modification equipment is a high-speed heating mixer.

In order to improve the uniformity of the interaction between the titanate coupling agent and calcium carbonate, inert solvents such as liquid paraffin (white oil), petroleum ether, transformer oil, absolute ethanol, etc. are generally used for dissolution and dilution.

The amount of titanate coupling agent depends on the particle size and specific surface area of calcium carbonate, generally 0.5%-3.0%. The drying temperature of calcium carbonate should be as low as possible below the flash point of the coupling agent, generally 100-120°C. The titanate coupling agent and the inert solvent are mixed and added to the high-speed mixer in the form of spray or dropwise addition, which can be better dispersed and mixed with the calcium carbonate particles for surface chemical coating.

If continuous surface modification equipment is used, such as SLG continuous powder surface modifier, it is not necessary to pre-dilute the titanate coupling agent with solvent.

The calcium carbonate treated with titanate coupling agent has good compatibility with polymer molecules. At the same time, because the titanate coupling agent can form a molecular bridge between calcium carbonate molecules and polymer molecules, it enhances the interaction between organic polymers or resins and calcium carbonate, and can significantly improve thermoplastic composite materials, etc. The mechanical properties, such as impact strength, tensile strength, bending strength and elongation.

Compared with untreated calcium carbonate filler or stearic acid (salt) treated calcium carbonate, the properties of the modified calcium carbonate coated with titanate coupling agent surface have been significantly improved.

(2) Aluminate coupling agent

Aluminate coupling agents have been widely used in the surface treatment of calcium carbonate and the processing of filled plastic products, such as PVC, PP, PE and filler masterbatch. Studies have shown that light calcium carbonate treated with aluminate can significantly reduce the viscosity of the calcium carbonate/liquid paraffin mixed system, indicating that the modified calcium carbonate has good dispersion in organic media.

In addition, the activated calcium carbonate after surface modification can significantly improve the mechanical properties of the CaCO3/PP (polypropylene) blend system, such as impact strength and toughness.

(3) Compound coupling modification

The calcium carbonate composite coupling system is based on the calcium carbonate coupling agent, combined with other surface treatment agents, crosslinking agents, and processing modifiers for comprehensive technical treatment of the calcium carbonate surface.

The coupling agent and various auxiliary agents in the composite coupling system are described as follows:

Titanate coupling agent.

Stearic acid. The effect of treating calcium carbonate with stearic acid alone is not satisfactory. Using coupling agent alone to treat calcium carbonate has a higher cost. Combining stearic acid and titanate coupling agent can receive a better synergistic effect. The addition of stearic acid basically does not affect the coupling effect of the coupling agent. At the same time, it can also reduce the amount of coupling agent and reduce production costs.

Crosslinking agent bismaleimide. In the composite coupling agent system, the use of cross-linking agent can make the inorganic filler and the matrix resin tightly combined through the cross-linking technology, and further improve the mechanical properties of the composite material. This is difficult to achieve with “Bai Yanhua” or simple titanate coupling agent surface treatment.

Processing modifier-80 resin, etc. Various processing modifiers are mainly polymer compounds. Processing modifiers can significantly improve the melt fluidity, thermal deformation properties and gloss of the product surface of the resin.

In order to coat the surface of all calcium carbonate particles with a layer of coupling agent molecules, the spraying or dripping method can be changed to emulsion dipping, and then filtered, dried, crushed and kneaded with crosslinking agent and other additives at high speed (Mixing), evenly dispersed.

In summary, the main components of the calcium carbonate composite coupling system are calcium carbonate and titanate coupling agent. The titanate coupling agent played a major role. On this basis, adding crosslinking agents, surfactants, processing modifiers, etc. can further enhance the surface activity of calcium carbonate fillers, increase the amount of fillers, and improve the performance of composite materials.

The calcium carbonate filler after compound coupling modification is a white powder with a density of 2.7-2.8g/cm3, a pH value of 7-8, and good hydrophobic properties.

Calcium carbonate treated with coupling agent (including light calcium carbonate and heavy calcium carbonate), in addition to being used as a rigid polyvinyl chloride functional filler, it is also widely used as fillers and pigments for adhesives, inks, coatings, etc.

4. Polymer modification

Surface modification of calcium carbonate with polymers can improve the stability of calcium carbonate in the organic or inorganic phase (system). These polymers include oligomers, high polymers and water-soluble polymers, such as polymethyl methacrylate (PMMA), polyethylene glycol, polyvinyl alcohol, polymaleic acid, polyacrylic acid, alkoxy styrene -Copolymers of styrene sulfonic acid, polypropylene, polyethylene, etc.

The process of coating modified calcium carbonate on the surface of polymer can be divided into two types. The polymer is dissolved in an appropriate solvent, and then the calcium carbonate is surface-modified. When the polymer is gradually adsorbed on the surface of the calcium carbonate particles, the solvent is removed to form a coating. These polymers are adsorbed on the surface of calcium carbonate particles to form a physical and chemical adsorption layer, which can prevent calcium carbonate particles from agglomerating, improve dispersibility, and make calcium carbonate have better dispersion stability in applications.

Master batch filler is a new type of plastic filler. The method is to mix the filler and the resin masterbatch in a certain proportion, add some surfactants, pass through high shear mixing, extrusion, and pelletizing to make the masterbatch filler. This kind of master batch filler has good dispersibility, strong bonding force with resin, uniform melting, high addition amount, low mechanical wear and convenient application. Therefore, it is widely used in straps, woven bags, polyethylene hollow products (pipes, containers, etc.), films, etc. According to the different matrix resins, the commonly used masterbatch fillers mainly include atactic polypropylene calcium carbonate masterbatch (APP masterbatch), polyethylene wax calcium carbonate masterbatch and polyethylene calcium carbonate masterbatch fillers.

APP masterbatch is made of calcium carbonate and random polypropylene as basic raw materials, formulated in a certain proportion, and produced through internal smelting, open refining, and granulation. Calcium carbonate must undergo surface activation treatment before compounding with random polypropylene. The ratio of atactic polypropylene and activated calcium carbonate is generally 1:3-1:10. In order to improve the processing and molding properties of atactic polypropylene, part of isotactic polypropylene or part of polyethylene is generally added during molding. The ratio of atactic polypropylene and activated calcium carbonate determines the surface coating level of calcium carbonate particles, which ultimately affects the product quality of APP masterbatch.

In the APP master batch system, the calcium carbonate particles are covered by atactic polypropylene, that is, the calcium carbonate particles are evenly dispersed in the random polypropylene base material. Assuming that the calcium carbonate particles are standard cubic or spherical particles with side lengths or diameters of 10μm, 50μm, and 100μm, respectively, the mass ratio of random polypropylene and calcium carbonate can be used to calculate the surface of each calcium carbonate particle coated with random poly the average imaginary thickness of acrylic. In theory, the more calcium carbonate filled, the better, that is, the smaller the imaginary thickness, the better. But the actual thickness depends on the process equipment and operating conditions.

Using polyethylene wax or polyethylene instead of random polypropylene as the base material and active calcium carbonate filling compound can prepare polyethylene wax calcium carbonate master batch filler and polyethylene calcium carbonate master batch filler.

5. Plasma and radiation modification

Using an inductively coupled glow discharge plasma system and using a mixture of argon (Ar) and high-purity propylene (C3H6) as the plasma treatment gas to modify the heavy calcium carbonate (1250 mesh) powder by low-temperature plasma. The results show that the Ar- Calcium carbonate filler treated with C3H6 mixed gas has good interface adhesion with polypropylene (PP). This is because there is a non-polar organic layer on the surface of the modified calcium carbonate particles, which reduces the polarity of the surface of the calcium carbonate particles and improves the compatibility and affinity with polypropylene (PP).

6. Inorganic surface modification

Condensed phosphoric acid (metaphosphoric acid or pyrophosphoric acid) is used to modify the surface of calcium carbonate powder, which can overcome the disadvantages of poor acid resistance and high surface pH of calcium carbonate powder. The pH of the modified product is 5.0-8.0 (1.0-5.0 lower than before surface treatment), it is hardly soluble in weak acids such as acetic acid, and has better acid resistance.

In addition, zinc sulfate and water glass are added in the calcium carbonate carbonization process for surface modification. When the resulting product is applied to styrene butadiene rubber, its elongation and tear strength can be improved.

The dry modification process is simple, the investment in production equipment and production costs are low, and it can be packaged directly after discharging. However, compared with the wet method, the activation degree is not good, and it is difficult to uniformize the primary particle size of calcium carbonate particles. Therefore, the dry activation process is currently suitable for filler-grade calcium carbonate modification treatment, and it needs to be further improved for functional nano-calcium carbonate.
3. Evaluation of modification effect of calcium carbonate

The evaluation of the effect of modified calcium carbonate can be roughly divided into two categories: direct method and indirect method. The indirect method is to combine the modified calcium carbonate filler with the application system to determine the application performance of the application system. Direct method refers to the determination of surface physical and chemical properties of modified calcium carbonate, such as activation degree, specific surface area, oil absorption value, coating amount, surface structure, and morphology.

(1) Degree of activation

Inorganic fillers generally have a relatively high density and have a hydrophilic surface, which naturally settles in water, while the surface of inorganic fillers treated with surface modification changes from hydrophilic to hydrophobic. This kind of hydrophobic fine particles floats in water without sinking due to the huge surface tension. According to this phenomenon, the concept of activation degree is proposed, which is represented by ω.

ω=weight of floating part in sample (g)/total weight of sample (g)
The change process of ω from 0-100% reflects the degree of surface activation of modified calcium carbonate from small to large.

The test method is as follows, weigh about 5g sample, accurate to 0.01g, add 200ml of water to a 250ml separatory funnel, shake back and forth for 1 min at a speed of 120 times/min, gently place it on the funnel rack, and let it stand for 20 -30min, after the obvious stratification, put the sinking calcium carbonate in a glass sand crucible with a constant weight (accurate to 0.001g) at 105±5℃ in one time, suction and filter the water, and place it in a constant temperature drying box dry to constant weight at 105±5℃, accurate to 0.001g.

(2) Specific surface area

In addition to improving activity, the surface modification process can also effectively prevent secondary agglomeration. Unmodified nano calcium carbonate particles are prone to produce hard agglomerations, and the specific surface area is small. After surface modification, the agglomeration of calcium carbonate particles is greatly improved, and the specific surface area is significantly increased. The larger the specific surface area, the better the dispersion and degree of dispersion of the particles. This is because the surface of the modified nano calcium carbonate particles is coated with a layer of modifier, and the surface energy is reduced, making the particles in a stable state. Even if some particles are agglomerated together, their mutual agglomeration is a soft agglomeration, which is easier to open.

(3) Oil absorption value

The oil absorption value is related to the size, dispersion, degree of aggregation, specific surface area and surface properties of the calcium carbonate particles. Oil absorption value is an important property that affects the practical application of modified calcium carbonate, especially for coatings, plastics, and ink industries. If the oil absorption value is large, the viscosity will increase when used in the coating and ink industry, and the plasticizer consumption will be increased when used in the plastic industry, so the oil absorption value should be low.