At present, the most important modification method of kaolin is surface chemical modification. Commonly used surface modifiers mainly include silane coupling agent, silicone (oil) or silicone resin, surfactant and organic acid.
Silane coupling agent is the most commonly used and most effective surface modifier for kaolin fillers. Since R of silane coupling agent is an organophilic group, calcined kaolin can be compatible with organic substrates such as rubber and plastic after surface modification. . When modified kaolin is used as a filler in rubber, the R group will react with the rubber macromolecules during the vulcanization process, so that the kaolin molecules are completely dispersed and fused in the rubber matrix molecules.
The treatment process using silane coupling agent is relatively simple. Generally, kaolin powder and the configured silane coupling agent are added to the modifier for surface coating treatment. The process can be carried out continuously or in batches.
The factors that affect the final treatment effect are mainly the particle size, specific surface area and surface characteristics (surface functional groups and activity) of kaolin powder, the variety, dosage, usage of silane coupling agent, the performance of the modification equipment, and the time and temperature of the surface modification treatment. Wait.
In addition to silane coupling agents, kaolin used for wire and cable fillers (such as polyvinyl chloride) also often uses 1%-3% silicone oil for surface modification. The modification process and equipment are similar to those with silane coupling agents.
The calcined kaolin powder treated with silicone oil is used as a filler for wires and cables, which can not only improve the mechanical and physical properties of the cable, but also improve or enhance the electrical insulation and hydrophobic properties of the cable, as well as its electrical insulation in humid and cold environments. The improvement is significant.
The use of unsaturated organic acids, such as oxalic acid, sebacic acid, dicarboxylic acid, etc., can also be used for surface modification of aminated kaolin powder. This modified kaolin can be used as a filler for nylon 66 and the like.
For example, octadecylamine can also be used for surface modification of kaolin powder. Its polar groups interact with the surface of kaolin particles through chemical adsorption and physical adsorption. The surface hydrophobicity of kaolin modified by organic amine is enhanced.
Titanium dioxide, calcium carbonate, calcium sulfate, etc. can also be used for surface modification of calcined kaolin. The modification method is a surface precipitation reaction in an aqueous solution, and the modified product is washed, filtered and dried to obtain calcined kaolin coated with titanium dioxide on the surface.
The surface chemical modification of kaolin mainly adopts pretreatment method, and there are generally three kinds of process methods: wet method, semi-dry method and dry method.
(1) The wet process requires the pulping, dehydration and drying processes, and the process is complicated, especially the dehydration filtration. If the particle size of the mineral particles is less than 1250 mesh, it will be extremely difficult and complicated.
(2) The semi-dry modification process is to stir the powder in a mixer while adding an appropriate mixture of water, modifiers and additives, and heating to a certain temperature at the same time, the modification is completed after a certain time of reaction. Coupling effect of agent and mineral powder. The product after the reaction is in a very viscous state, and then a modified product can be obtained after a little drying. The semi-dry process eliminates the dehydration process, so the production efficiency is higher.
Due to the addition of a lot of water, coupling agent and auxiliary agent mixture in the wet and semi-dry process, and then through stirring and mixing, the powder particles can easily all contact with the coupling agent molecules, so that the modifier molecules It is easy to evenly coat the surface of the particles. Therefore, the rotation speed of the mixer required by these two processes is not required to be very high, and the equipment cost is relatively low.
(3) The dry modification process is to dilute the coupling agent and its additives with a small amount of diluent, and add them in the modifier while stirring, or use the spray method to add, and heat to a certain temperature at the same time After a certain period of reaction, the coupling effect is completed to obtain a modified product. The dry modification process requires relatively high technology and equipment. The process completely eliminates the dehydration and drying process, and the modification process is simple.
At present, the domestic equipment for surface modification of kaolin includes high-speed kneaders and continuous modifiers. High-frequency vibration mills, ball mills, etc. can also be used to modify the surface of kaolin.
High-speed kneader, also known as high-speed mixer, is currently the mainstream equipment for kaolin modification in China. A qualified high-speed kneader for kaolin modification needs to pay attention to the following aspects:
a) The number, position, radius and shape of the impeller, the gap between the impeller and the inner wall of the mixer;
b) Whether the stirring speed is adjustable;
c) Whether there is a suitable exhaust device;
d) Whether there is a constant temperature device.
These properties are important indicators that affect the modification effect of kaolin.
Because the high-speed kneader is a gap type modification equipment, it will cause uneven product quality and reduce production efficiency. In addition, the nano effect and small size effect of the kaolin powder during the modification process will cause the surface modifier and the powder particles to be inadequately contacted, coated or chemically reacted due to the high surface energy and the collision and friction of high-speed motion. Generate static electricity, and then condense into a "dump". This is the so-called "white spot" in composite materials.
The continuous modifier has the advantages of stable product quality and high production efficiency, and is relatively mature in use abroad. At present, it is mainly used in the modification of talc, mica, calcium carbonate and other powders in China.
Due to the high cohesiveness of kaolin, it is easy to cause blockage during screw conveying, and the dispersion effect is not as good as using high-speed kneaders. Therefore, the use of continuous modifiers to modify kaolin in China is subject to certain restrictions and needs to be further improved.
The grinder relies on a vibration exciter to cause the medium in the cylinder to vibrate with high frequency and small amplitude. Because the vibration acceleration is much larger than the gravitational acceleration, the medium in the mill tube can produce high-strength impact and rotational movement, and the kaolin can quickly heat up, so that the material can be quickly and effectively dispersed and mixed under strong vibration and a certain temperature.
This equipment can disperse and modify the calcined kaolin at one time. However, due to the strong impact of mechanical force, some of the coated particles will re-impact new sections, which will affect the final modification effect.
Wet modification requires relatively low equipment. The most commonly used equipment is a ball mill. The main method is to de-sand the kaolin into a slurry with a certain concentration, mix it with the modifier under heating, and then put it in the ball mill. After modification for a certain period of time, the liquid is centrifuged, the sediment is dried and then crushed. Studies have found that ultra-fine pulverization of coal calcined kaolin particles by wet method can cause lattice distortion and amorphization of the crystal structure, which will reduce the density of the particles and increase the whiteness.
The main factors affecting the surface modification effect of kaolin are particle size, specific surface area and surface characteristics (surface functional groups and activity), the variety, dosage, usage, performance of the modification equipment, and the time and temperature of the surface modification treatment.
The physical and chemical properties of kaolin are the basis for surface modification, especially the surface functional groups, which determine the adsorption characteristics, chemical reactivity, electrical properties and wettability of kaolinite under certain conditions.
Kaolin is a layered silicate mineral. The surface of kaolin particles after crushing process has hydroxyl groups and oxygen-containing groups, which is acidic. The pH value is between 6-7. The Si-O and Al-O bonds formed in the ultrafine grinding and calcination process become the main surface functional groups, especially the Al-O bonds formed after the Si-O-Al bond is broken, which will play a role in the modification reaction. Important role. At this time, since the hydroxyl group is removed after calcination, the pH value measured by a potentiometer is generally 5.6-6.1, and the acidity is further enhanced.
Therefore, the surface of coal-based kaolin is weakly acidic whether it is calcined or not, and it is most suitable for surface modification with silane surface modifiers.
In addition, the structural defects formed on the surface of the kaolin by calcination will also become the main active points of the modification chemical reaction. Especially after calcination of kaolin, in addition to the changes in surface functional groups, the internal structure also changes. Especially when the calcination temperature exceeds 600°C, the calcined kaolin is already in a disordered amorphous phase. The disordered structure of kaolin will inevitably affect its physical and chemical properties, and further affect the surface modification process, mechanism and effect of kaolin.
In addition to the fineness requirements of the particle size of kaolin after deep processing, the range and distribution characteristics (uniformity) of its particle size distribution have a great impact on the application of kaolin in the paper, coating, rubber and plastic industries. At the same time, the particle size distribution has a great influence on the modification of kaolin, which determines the amount of coupling agent and the determination of reaction conditions.
The specific surface area and surface energy of kaolin are closely related to its particle size composition. The finer the particle size, the higher the specific surface area and surface energy, and the stronger the binding ability with organic compounds. In addition, after calcined, coal-based kaolin exhibits great surface activity due to the loss of hydroxyl groups and the existence of a large number of broken bonds on the surface, and the surface energy also increases accordingly.
However, when the specific surface area and surface energy of kaolin are too high, agglomeration is likely to occur, and it cannot be uniformly dispersed when filled with organic polymer materials such as rubber and plastic, which is not conducive to the improvement of product performance. From this point of view, the coal-series kaolin must be surface modified to reduce its surface energy.
The type, amount and method of use of surface modifiers directly affect the effect of surface modification. If we only consider the interaction between the surface modifier molecules and the surface of the inorganic powder, of course the stronger the interaction between the two, the better, but in actual operation, the cost and cost of the modified product must be considered comprehensively. Factors such as application purpose.
The use method of surface modifier is also very important. Proper use method can not only improve the surface modification effect, but also reduce the dosage and reduce the production cost. When modifying the surface of the powder, you can add an appropriate amount of solvents, diluents, emulsification, spraying and other methods to ensure that the modifier is uniformly dispersed on the surface of the material and improve the dispersion of the modifier in the powder.