Calcium carbonate manufacturing plant surface modification processPosted: 2020-01-13

  • Calcium carbonate is currently the largest amount of inorganic filler in organic polymer-based materials. However, calcium carbonate without surface treatment has poor compatibility with the polymer, which can easily cause uneven dispersion in the polymer base material. Causes interface defects in composite materials and reduces the mechanical strength of the material. As the dosage increases, these disadvantages become more apparent.

    Therefore, in order to improve the application performance of the calcium carbonate filler, it must be surface modified to improve its compatibility or affinity with the polymer binder.

    1. Brief description 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 Salt coupling agent and random polypropylene, polyethylene wax, etc.

    Surface modification is carried out by means of equipment. Commonly used surface modification equipment are SLG continuous powder surface modification machine, high-speed heating mixer, vortex mill and fluidized modification machine.
    SLG continuous powder surface modification machine
    SLG calcium carbonate powder surface modification machine

    The main factors affecting the surface modification effect of calcium carbonate are: the type, amount and usage of surface modifier (the so-called surface modifier formula); surface modification temperature, residence time (that is, surface modification process); surface modification The degree of dispersion of agents and materials. Among them, the degree of dispersion of surface modifiers and materials mainly depends on the surface modifier.

    2. Fatty acid (salt) modified calcium carbonate


    Stearic acid (salt) is the most commonly used surface modifier for calcium carbonate. The modification process can be dry or wet. Generally, wet processes use stearates such as sodium stearate.

    1. Dry modified calcium carbonate with stearic acid

    When using continuous powder surface equipment such as SLG powder surface modification machine and vortex mill, the materials and surface modifier are continuously and simultaneously fed. Stearic acid can be added directly as a solid powder, and the amount depends on the powder. Depending on the particle size or specific surface area, it is generally 0.8% -1.2% of the calcium carbonate mass. When surface coating modification is performed in high-speed mixers, horizontal paddle mixers and other temperature-controllable mixers, it is generally For intermittent operation, firstly add the metered and prepared materials and stearic acid to the modification machine, stir and mix for 15-60min to discharge the package. The amount of stearic acid is about 0.8% -1.5% of the calcium carbonate mass. The reaction temperature is controlled at about 100 ° C.

    In order to make the stearic acid better disperse and work uniformly with the calcium carbonate particles, the stearic acid can also be diluted with a solvent (such as anhydrous ethanol) in advance. Other additives can also be added during modification.

    2. Wet modified stearic acid calcium carbonate

    Wet modification is a surface modification treatment of calcium carbonate in an aqueous solution. The general process is to saponify stearic acid and then add it to the calcium carbonate slurry. After a certain period of reaction, it is filtered and dried. Dispersion of calcium carbonate in the liquid phase is easier than dispersion in the gas phase.

    In addition, by adding a dispersant, the dispersing effect is better, and therefore, the action of the calcium carbonate particles and the surface modifier molecules in the liquid phase is more uniform. When the calcium carbonate particles adsorb the stearates, the surface energy decreases. Even if secondary particles are formed after pressure filtration and drying, the agglomeration and binding force of the calcium carbonate particles will be weakened, and no hard agglomeration will be formed. It's redispersed.

    Wet surface modification equipment is generally relatively simple. Most of them are containers with agitators and static mixers. Strong stirring can improve the modification activation efficiency and shorten the reaction time, but it requires higher equipment performance.

    Although wet surface modification can also be performed at normal temperature, but the reaction time is long, so the surface modification is generally carried out by heating, and the modification temperature is generally about 50-100 ° C.

    Wet surface modification is commonly used for the surface modification of light calcium carbonate and wet-ground ultra-fine heavy calcium carbonate.

    In addition to stearic acid (salts), other fatty acids (salts), such as phosphates and sulfonates, can also be used for the surface modification of calcium carbonate.

    Activated calcium carbonate modified with fatty acid (salt) is mainly used for filling polyvinyl chloride plastics, cable materials, adhesives, inks, coatings, etc.

    3. Coupling agent modified calcium carbonate


    The coupling agents used for the surface modification of calcium carbonate are mainly titanate and aluminate coupling agents, or composite coupling agents.

    1. Titanate coupling agent

    In order to improve the uniformity of the interaction between the titanate coupling agent and calcium carbonate, it is generally dissolved and diluted with an inert solvent such as liquid paraffin (white oil), petroleum ether, transformer oil, and absolute ethanol.

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

    If continuous surface modification equipment is used, such as the SLG continuous powder surface modification machine, the titanate coupling agent may not be diluted in advance with a solvent.

    The calcium carbonate treated with the titanate coupling agent has better compatibility with polymer molecules. At the same time, because the titanate coupling agent can form a molecular bridge between the calcium carbonate molecule and the polymer molecule, the interaction between the organic polymer or resin and the calcium carbonate is enhanced, and the thermoplastic composite material can be significantly improved. Mechanical properties, such as impact strength, tensile strength, flexural strength, and elongation.

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

    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 filled masterbatch. Studies have shown that the 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 dispersibility in organic media.

    In addition, the surface modified and activated calcium carbonate 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 a calcium carbonate coupling agent, combined with other surface treatment agents, cross-linking agents, and processing modifiers to comprehensively process the calcium carbonate surface.

    The coupling agents and various auxiliaries in the composite coupling system are described below:

    Titanate coupling agent.

    Stearic acid. Treatment of calcium carbonate with stearic acid alone is not ideal. Using a coupling agent alone to treat calcium carbonate is costly. When stearic acid is used in combination with a titanate coupling agent, a good synergistic effect can be obtained. The addition of stearic acid does not substantially 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.

    Cross-linking agent bismaleimide. In the composite coupling agent system, the use of a cross-linking agent enables the inorganic filler to be tightly combined with the matrix resin through a cross-linking technology, thereby further improving various mechanical and mechanical properties of the composite material. This is difficult to achieve with "Bai Yanhua" or a 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 product surfaces.

    In order to coat the surface of all calcium carbonate particles with a layer of coupling agent molecules, the method of spraying or dropping can be changed to the method of emulsion impregnation, and then filtered, dried, and pulverized with high-speed kneading agents such as crosslinking agents. (Mixed), evenly dispersed.

    In summary, the main components of the calcium carbonate composite coupling system are calcium carbonate and titanate coupling agents. Titanate coupling agents play a major role. On this basis, the addition of cross-linking agents, surfactants, and processing modifiers can further enhance the surface activity of calcium carbonate fillers, increase the amount of fillers, and improve the performance of composite materials.

    The composite coupling modified calcium carbonate filler 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 (including light calcium carbonate and heavy calcium carbonate) treated with a coupling agent is widely used as a filler and pigment for adhesives, inks, coatings, etc., in addition to functional fillers for rigid polyvinyl chloride. .

    4. Polymer modification


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

    The process of coating the surface of the polymer with modified calcium carbonate can be divided into two types. First, the polymer monomer is first adsorbed on the surface of the calcium carbonate, and then the polymerization is initiated to form a polymer coating layer on the surface; The polymer is dissolved in a suitable solvent, and then calcium carbonate is surface-modified. When the polymer is gradually adsorbed on the surface of the calcium carbonate particles, the solvent is excluded to form an envelope. These polymers are directionally adsorbed on the surface of calcium carbonate particles to form a physical and chemical adsorption layer, which can prevent the agglomeration of calcium carbonate particles, improve dispersibility, and make calcium carbonate have better dispersion stability in applications.

    Masterbatch filler is a new type of plastic filler. The method is to mix the filler and the resin master batch in a certain ratio, add some surfactants, extrude through high shear mixing, and cut into pellets to make a master batch filler. The master batch filler has good dispersibility, strong binding force with resin, uniform melting, high addition amount, small mechanical wear, and convenient application. Therefore, it is widely used in packing tapes, woven bags, polyethylene hollow products (pipes, containers, etc.), films, etc. According to different matrix resins, commonly used master batch fillers are mainly random polypropylene calcium carbonate master batch (APP master batch), polyethylene wax calcium carbonate master batch, and polyethylene calcium carbonate master batch filler.

    APP masterbatch is based on calcium carbonate and random polypropylene as basic raw materials, and is formulated in a certain proportion. It is produced through compaction, milling and granulation. Calcium carbonate must be surface-activated before being compounded with random polypropylene. Blending of random polypropylene and activated calcium carbonate

    The ratio is generally 1: 3-1: 10. In order to improve the processing and forming properties of atactic polypropylene, a part of isotactic polypropylene or a part of polyethylene is generally added during molding. The ratio of random polypropylene to activated calcium carbonate determines the surface coating level of calcium carbonate particles, which ultimately affects the product quality of the APP master batch.

    In the APP master batch system, the calcium carbonate particles are surrounded by random polypropylene, that is, the calcium carbonate particles are uniformly dispersed in the random polypropylene base material. Assuming that calcium carbonate particles are standard cubes or spherical particles with side lengths or diameters of 10 μm, 50 μm, and 100 μm, respectively, the surface coating of each calcium carbonate particle can be calculated randomly based on the mass ratio of random polypropylene and calcium carbonate The average imaginary thickness of propylene. Theoretically, 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.

    Polyethylene wax or polyethylene is used instead of random polypropylene as the base material and filled with activated calcium carbonate to prepare polyethylene wax calcium carbonate master batch filler and polyethylene calcium carbonate master batch filler.

    5. Plasma and radiation modification


    Low-temperature plasma modification of heavy calcium carbonate (1250 mesh) powder using an inductively coupled glow discharge plasma system and a mixture of hydrogen (Ar) and high-purity propylene (C3H6) as the plasma treatment gas shows that the Ar- C3H6 mixed gas treated calcium carbonate filler has good interface with polypropylene (PP). This is because a non-polar organic layer exists on the surface of the modified calcium carbonate particles, so the polarity of the surface of the calcium carbonate particles is reduced, and the compatibility and affinity with polypropylene (PP) are improved.

    6. Inorganic surface modification


    Surface modification of calcium carbonate powder with condensed phosphoric acid (ie metaphosphoric acid or pyrophosphoric acid) can overcome the shortcomings such as poor acid resistance and high surface pH of calcium carbonate powder. The pH value of the modified product is 5.0-8.0 (down 1.0-5.0 compared with that before surface treatment), it is difficult to dissolve in weak acids such as acetic acid, and has good acid resistance.

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

    7. Conclusion and outlook


    In the future, special and functional products such as ultra-fine calcium carbonate and activated calcium carbonate will be the main development trend of calcium carbonate. Selective purification, ultra-fine crushing, surface modification and other mineral fine processing technologies are necessary to increase the added value of calcium carbonate. Means will also be the key to the transformation and upgrading of calcium carbonate production enterprises.
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