Carbon black is a plastic colorant, UV aging agent and antistatic agent. With the development of the plastics industry, the use of carbon black in the plastics industry is also growing, from ordinary daily-use plastics to high-grade engineering plastics are inseparable from carbon black. Compared with carbon black in the rubber industry, the amount of carbon black used in plastics is very small, but it has a great influence on the properties of plastics. Therefore, the performance requirements for carbon black for plastics are more stringent, and the performance of carbon blacks for different purposes There are different requirements, and an introduction to the application of carbon black in plastics is now available.
1 Carbon black basic properties Carbon black basic properties, primary particle size, structure, porosity and surface functional groups, the impact on plastic properties are as follows: 1.1 carbon black primary particle size (large specific surface area) (1) can improve carbon black (2) can improve the tinting strength of carbon black; (3) can enhance the ability of carbon black to resist ultraviolet rays; (4) can improve the conductivity of carbon black; (5) can increase the viscosity of plastic; ) reduce the dispersibility of carbon black; (7) increase the water absorption of carbon black and plastic. The water absorption of carbon black has a great relationship with the nitrogen adsorption specific surface area of ​​carbon black. When mixed with a resin to form a plastic, the primary particle size of the carbon black will be a major factor affecting the amount of water absorbed by the plastic.
1.2 high carbon black structure (1) will reduce the blackness and tinting strength of carbon black; (2) can improve the dispersion of carbon black; (3) can increase the viscosity of plastic; (4) improve the conductance of carbon black Rate; (5) In the use of conductive carbon black, the amount of carbon black used can be reduced. The carbon black structure is a representation of the degree of carbon black chain or the size of the carbon black aggregate. The carbon black structure is divided into a primary structure, or a primary structure and a secondary structure. The primary structure refers to the structure formed by the primary aggregates. The primary structure is relatively firm and not easily destroyed, so the primary structure is the structure actually existing in the plastic system. The secondary structure is an agglomerate formed by the aggregation of carbon black aggregates by van der Waals force. The secondary structure is not strong and is easily damaged during the processing of plastics. The pore volume of carbon black is directly related to the structure. The carbon black with high structure has a large pore volume; the carbon black with low structure has a small pore volume, so the pore volume is a measure of carbon black structure. Generally, the pore volume of carbon black is determined by a liquid absorption method and a compression specific volume method. The pore volume of carbon black affects the "weight" of carbon black of a certain particle size to some extent. Therefore, adding carbon black of the same weight but with a large pore volume can increase the volume of carbon black particles in plastic, and at the same time improve The electrical conductivity of plastic products.
1.3 Carbon black surface functional groups (high volatile matter, low pH) (1) reduce the conductivity of carbon black; (2) improve the wettability of carbon black; (3) increase the water absorption of carbon black. The surface functional groups of carbon black are natural oxygen-containing groups on the surface of carbon black ions, which affect many physical and chemical properties of carbon black. The surface functional groups of carbon black are mainly acidic surface groups, mainly including carboxyl groups, phenolic hydroxyl groups, mercapto groups and lactone groups. The amount of functional groups on the surface of carbon black is determined by measuring the reduction of carbon black after a certain period of time under certain temperature and environmental conditions. In addition, other factors affecting the properties of plastics are sulfur, ash, impurities, etc. in carbon black, as well as the physical form of carbon black, such as powder or granular.
2 Physical Properties of Carbon Black Particles The particle size, aggregate size and shape, and distribution of carbon black can be tested by transmission electron microscopy with automatic image analysis techniques. By applying transmission electron microscopy to multiple images of carbon black aggregates, the three-dimensional structure of the aggregates can be seen. By rotating the aggregate structure of the carbon black by an angle meter on a transmission electron microscope, it can be found that the appearance of the aggregates seen at different angles is different. From the aggregate images appearing at different angles, we can create a three-dimensional image of the aggregate. In summary, by rotation we can see that aggregates with high structural carbon black are larger and more complex than aggregates of low structure carbon black. At the same magnification, the image tested with a transmission electron microscope and a three-dimensional image simulated with a computer are shown in FIG. From the simulation image on the right, although the image is two-dimensional, we can clearly perceive the three-dimensional image of the aggregate; and by observation, we can understand the spatial relationship between the carbon black particles. In addition, such model images can be rotated as needed to observe and measure aggregates of carbon black. With the development of 3D image analysis technology, the future 3D analysis technology will explain how the aggregate structure affects the application performance.
3 carbon black dispersion equipment carbon black dispersion equipment commonly used in plastics is shown in Table 1.
| Table 1 | Various dispersing equipment for carbon black in plastics | ||
| Tab1 | Dispersing equipment for carbon black used in plastics applications | ||
| Dispersing equipment | application | Dispersion level | Premixing requirements |
| sanding machine | Plastic dispersion and pigment paste | Medium shear | need |
| Three roller machine | Plastic dispersion and pigment paste | High shear | need |
| Single screw extruder | Melted resin | Medium shear | No need |
| Twin screw extruder | Melted resin | Medium shear | No need |
| Continuous mixer | Melted resin | High shear | need |
| Closed plastic mixer | Melted resin | High shear | No need |
3.1 Sander In a sense, the sander is an extension of the ball mill. The grinding method of the sander is similar to that of a ball mill, and the shearing force required for dispersion is derived from the dispersion medium in the pigment. The sand mill relies on the centrifugal force generated by the high-speed rotating dispersing disc acting on the dispersing medium to provide the dispersive shearing force; while the ball mill relies on gravity to provide the dispersive shearing force. The advantages of the sand mill are fast dispersion time, high efficiency, continuous production, and dispersion of low viscosity systems. The disadvantage of sand mills is that they require premixing before dispersion. The dispersion medium used in the sand mill includes steel beads, glass beads, ceramic beads, and the like. When the sander is dispersed, the dispersion medium tends to be uniformly distributed. When the mass fraction of the dispersion medium in the system is reduced, the interval between the dispersion media will increase, which will reduce the dispersion efficiency of the carbon black. However, if too much dispersion medium is used, the wear between the dispersion medium is increased, and the particle size of the dispersion medium is reduced: sometimes the dispersion medium is broken and the quality of the product is affected. Generally, according to the production experience, the number and particle size of the dispersion medium are adjusted to maximize the dispersion efficiency of the carbon black. Sand mills are more suitable for processing low viscosity slurries and are not suitable for handling high viscosity slurries.
3.2 The main function of the three-roller three-roller machine is to overcome the cohesion of carbon black by pressure and shear force to achieve the purpose of crushing and dispersing carbon black. In order to achieve the best level of dispersion for the three-roller, the following conditions must be met: the spacing between the shearing surfaces (rollers) must be carefully adjusted and stabilized so that the pressure between the rolls reaches a constant state. The temperature of the roller should be strictly controlled to avoid the high and low, affecting the viscosity of the system and affecting the dispersion of carbon black. For the production of products with high gloss, high dispersion and deep blackness, the surface of the three-roller should be focused in advance. Cleaning; be sure to re-clean the shearing surface (roller) when producing different batches of the same product. The three-roller is more suitable for processing high viscosity slurries and is not suitable for handling low viscosity slurries.
3.3 Notes (1) In order to fully utilize the characteristics of carbon black, the following aspects should be completed before dispersion grinding: 1 mixing carbon black and resin uniformly; 2 replacing the air and moisture in the pores of carbon black with resin; The large agglomerates in the carbon black are ground and dispersed into small agglomerates or aggregates by the shear force provided by the dispersing device. If the carbon black is well handled in accordance with the above steps, carbon black will certainly achieve satisfactory blackness, hue, tinting strength, gloss, stability and activity. (2) Premixing As is apparent from Table 1, not all of the dispersing devices require premixing of carbon black and resin. However, in order to ensure the best effect of the plastic during extrusion, the necessary pre-mixing is still required to ensure that the resin is thoroughly mixed with the carbon black when it is melted, effectively reducing the grinding time to achieve the dispersion requirement.
4 The amount of carbon black is reduced to reduce the cost of the plastic. The amount of carbon black required is as high as possible. However, the performance and type of the grinding equipment will have an important influence on the dispersion of carbon black. Different plastics require different levels of dispersion. In many cases, the lack of plastic properties can be directly attributed to the lack of carbon black dispersion, such as cable shells and fiber optic products, which require a higher level of carbon black dispersion. For plastics with high performance requirements, the full dispersion of carbon black is very important. If the dispersion level of carbon black can be maximized, the color, hue, viscosity, gloss, stability and extrusion characteristics of the plastic can be better.
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