Surface Modification of Superfine Calcium Carbonate Powder By Coating Machine

1 Why should ultrafine calcium carbonate powder be activated and modified?

 (1) The smaller the particle size of calcium carbonate, the higher the surface energy, the stronger the absorption, and the easier it is to agglomerate with each other. So it is not easy to disperse in the compound matrix.
(2) Calcium carbonate is an inorganic filler, the surface of the particles is hydrophilic and oleophobic, and the bonding force with the polymer interface is weak. When impacted by an external force, it is easy to cause interface defects, resulting in material performance degradation.
 In order to give full play to the nano-effect of ultrafine calcium carbonate, improve its dispersibility in composite materials, It is necessary to adopt effective modification technology and surface modification methods to broaden its application fields.

2 The principle of surface coating modification of ultrafine calcium carbonate

 Surface organic coating modification is currently the most commonly used calcium carbonate surface modification method. Superfine calcium carbonate is surface-coated and modified, and the lipophilic group in the modifier can be firmly polymerized with the polymer. The hydrophilic group in the modifier binds to the surface of the calcium carbonate particles. This can reduce the interfacial energy between calcium carbonate and the matrix material, so that the two poorly compatible materials of calcium carbonate and polymer are tightly bound together by the molecular bridge of the surface coating modifier, which is beneficial to greatly improve the composite materials overall performance.

3 Several points that should be paid attention to in ultrafine calcium carbonate powder surface coating modification:

(1) Ultrafine calcium carbonate powder surface modification process
 Currently, the ultrafine calcium carbonate powder surface modification processes used in industry mainly include dry process, wet process, and composite process. Dry modification is to put calcium carbonate powder into a high-speed mixer, rotate it to a certain temperature, and add surface treatment agent for mixing. This method is simple and easy to implement, and is suitable for the surface treatment of various coupling agents. At present, the SLG type continuous surface modifier/coating machine is widely used in industry.
For wet modification, it needs to add activator into the solvent, or directly add it to the calcium carbonate in water.In this method, the surface treatment agent interacts with the calcium carbonate particles, and the modification is uniform and the effect is better.  
(2) Type and dosage of ultrafine calcium carbonate powder surface modifier
  The surface coating modifier of ultrafine calcium carbonate generally adopts organic compounds with low molecular weight and amphiphilic structure, such as stearic acid. If the ultrafine calcium carbonate surface modification process is chemical coating, the combination of surface modifier and Ca2+ should be given priority to generate various calcium salt precipitation problems, such as calcium stearate, calcium phosphate, calcium titanate, calcium aluminate, etc. . If the surface modification process is physical coating, the electrostatic absorption of surface modifiers and Ca2+ is given priority, such as anionic surface modifier and nonionic surface modifier, rather than cationic surface modifier. The contact angle of stearic acid modified calcium carbonate generally increases as the amount of modifier increases, and the ultrafine calcium carbonate powder modified gradually increases, and the modification effect gradually increases, so that calcium carbonate is transformed from a hydrophilic surface to a lipophilic surface.
When the dosage reaches a certain value, the surface of calcium carbonate is just coated with a layer of organic matter, and the modification effect is the best, so that the sedimentation volume is the smallest. When the amount of modifier exceeds this value, as the amount of modifier increases, the interaction between the modifiers will affect the modification effect, and the sedimentation volume will increase instead. Therefore, in actual production, the amount of modifier should not be too much, otherwise it will not bring good effect.
 (3) Ultrafine calcium carbonate powder modification time and temperature
 To achieve uniform coating modification of ultrafine calcium carbonate, it is necessary to appropriately extend the modification time and increase the modification temperature. However, it should not be too high, and the modification effect and energy consumption should be considered comprehensively. If stearic acid is used to modify light calcium carbonate, the best process: modification temperature is 85℃, the amount of fatty acid is 2.5% of dry calcium carbonate, and the amount of sodium hydroxide to saponify stearic acid is 10% of stearic acid, the modification reaction time is 55-60min, the activation rate is over 99%, and the oil absorption is less than 45mL/100g active calcium carbonate.
 In short, the activation modification of ultrafine calcium carbonate is actually to select a specific surfactant to modify ultrafine calcium carbonate for a functional filling material, The types of surface modifier and the modification process directly affect the surface modification effect. For ordinary filler grade calcium carbonate, dry or wet surface modification can generally be used. For ultra-fine, nano-level and special calcium carbonate, it is necessary to use wet modification process.

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