1. The development history of carbon black acting on plastic conductivity
More than 95% of carbon black is used in rubber products. As a conductive material, carbon black, especially acetylene carbon black, is mainly used in dry batteries. It is a relatively new field to be used as a conductive functional filler in plastics. However, the production of acetylene carbon black causes serious environmental pollution, and the processability of acetylene carbon black in rubber and plastics is poor, and the mechanical properties of the finished products are also poor.
Carbon black, a by-product of synthetic ammonia and heavy oil gas production, can not be used as a conductive material because of its high manufacturing cost and its poor dispersion in plastics, which seriously damages the mechanical properties of plastic products. The use value and dosage are very limited.
There is no special conductive carbon black for plastics in the oil furnace carbon black variety, which was listed in ASTM D 1765-96a. N472 (XC-72 produced by American Cabot Company) in the standard of "carbon black for rubber" was introduced in the 1960s. A conductive carbon black for rubber was developed. Because of its large particle size, it has good dispersibility in plastics and is a good conductive carbon black for plastics. Afterwards, the German Degussa Company developed the Corax L series conductive carbon black, and the Japanese Asahi Carbon Company also developed a conductive furnace black for rubber in the mid-1960s, and so on.
The TD series of conductive furnace carbon black for rubber developed by the Carbon Black Industry Research and Design Institute in the past few years is based on reducing the particle size of carbon black particles and increasing the specific surface area of carbon black to obtain better conductivity. When in use, good dispersion can be obtained with the help of strong shearing force, and conductive rubber products with good conductivity and good mechanical properties can be obtained. However, when it is used in plastic products, it still has the defects of poor mechanical properties and poor use effect. This is also due to the fact that TD series carbon blacks cannot be well dispersed in plastics.
After carbon black is added to plastics, the conductive mechanism of carbon black in plastics is different due to the different filling amount and dispersion degree of carbon black. On the one hand, due to the developed structure of carbon black, the carbon black particles contact each other to form conductive channels; on the other hand, due to the insufficient number of carbon black particles, or after uniform dispersion, the carbon black particles cannot contact each other. There is a thin layer of resin between the particles to form a potential barrier, and the electrons cannot flow directly, but when there is a voltage, the electrons can conduct electricity through the tunnel effect, that is, the electrons will undergo electronic transitions at the potential barrier to form a tunnel conduction. In fact, in the conduction process of polymer composites, the two conduction mechanisms exist at the same time, but the conduction efficiency is different. Due to the different conditions of the conductive compound in use, such as AC, DC, high frequency, low frequency, electromagnetic wave shielding, etc., the role of carbon black in it is different.
2. Factors affecting the electrical conductivity of carbon black
1. Carbon black particle size
Theoretically, the smaller the particle size of carbon black, the more particles per unit volume, which is beneficial to improve the conductivity. This is normal in conductive products for rubber. However, when used in conductive plastic products, if the carbon black particles are too small, the dispersion will be poor due to the small shear force after the plastic is plasticized. The performance is reduced and the practical value is lost. Therefore, the particle size of carbon black must be controlled within a certain range to ensure that carbon black can not only be well dispersed in plastics, but also greatly increase the number of carbon black particles per unit volume in plastics, and improve the electrical conductivity of plastic products. At the same time, it does not destroy or less damages the original mechanical properties of the product.
2. Carbon black structure
The size of the DBP value represents the level of the carbon black aggregate structure. Generally speaking, when the DBP value is high, the carbon black has a chain branch structure and better conductivity. The DBP values of various by-products of heavy oil and gas-making carbon black are very high, and the electron microscope finds that they have an empty shell microstructure, which shows that their structure is not very high. Its high conductivity may be due to its larger volume per unit mass, and the shearing destroys part of the primary structure, resulting in a large number of new particles. In order to obtain good electrical conductivity in plastics, carbon black must have a larger particle size, and the structure should not be too high. It is best to make the structure of carbon black into a linear structure. On the one hand, it can promote the dispersion of carbon black in plastics, on the other hand, it is conducive to the formation of a conductive network, and the antistatic effect can be achieved with a small amount of carbon black.
3. Carbon black roughness
Since the conduction of carbon black requires a certain roughness, the carbon black is easy to form a conductive channel, so the difference between the nitrogen adsorption surface area and the CTAB surface area of carbon black is required to be large.
4. Surface volatile matter
The volatile matter on the surface of carbon black is mainly composed of some organic groups and the oil film that has not been completely cracked to form an insulating layer, which increases the potential barrier between carbon black particles and seriously affects the conductivity. The volatile matter must be controlled within a lower limit.
5. Ash and moisture
The high content of ash and moisture in carbon black actually reduces the content of carbon black, which also has an adverse effect on conductivity. In production, attention should be paid to controlling the content of ash and moisture in carbon black. In contrast, the carbon content of Ketjen BlackEC and acetylene carbon black is as high as about 9918%, while that of general carbon black is less than 98%. The moisture content should generally be controlled below 215%, otherwise a large number of air bubbles will be generated, which will affect the mechanical properties of the product.
3. Conduction principle of superconducting carbon black in plastics
When superconducting carbon black conducts electricity in plastics, it comes from the mutual contact between carbon black aggregates or agglomerates to form a conductive channel.
When the amount of carbon black in the plastic is low, the aggregates of superconducting carbon black are dispersed in the polymer, and the distance between the aggregates is about 10nm or more. Dominators are non-conductive. When the compounding amount increases, the average aggregate distribution between the aggregates is uniform in the batch material, then the resistivity of the batch material will remain unchanged until the carbon black compounding amount can be increased until the aggregates can contact each other, and then increase Small amounts can drop quickly. However, carbon black will actually be dispersed into aggregates or agglomerates ranging in size from 1-10um. In the area where the agglomerates are locked or inside, the compounding amount of carbon black will be higher than that in the polymer. The average value of the amount, so its resistivity will be low, and secondly, the aggregates and agglomerates are not uniformly distributed in the batch, so the resistivity change of the batch will be relatively gentle rather than very steep.
The effect of agglomerate size on resistivity can be observed indirectly from the effect of mixing time on resistivity, because the longer the mixing time, the number of agglomerates will increase and the size will decrease. The amount of conductive carbon black added in different proportions in different carriers
Batches with superconducting carbon black are usually used at temperatures below the softening point of the polymer. When the temperature rises, the resistivity of the batch with superconducting carbon black will increase. This phenomenon is called The compounding amount has a text coefficient PTC, which is because the thermal expansion coefficient of most polymers is larger than that of carbon black, and the dimension increases, and the gap between the contact points of carbon black aggregates increases. This phenomenon can be applied to switching circuits, which can be used to switch on and off by changing the silver dimension of the circuit, or to limit the power in electric furnaces. There are two key points in this application. One is that the use of concentrated carbon black in combination is more sensitive to the change of resistivity when the dimension changes than that of a single superconducting carbon black. Another point is that the resistivity of the batch material will change after experiencing several periodic changes in temperature. Selecting the appropriate carbon black quality and dispersion of Hebei Moyu conductive carbon black can reduce the hysteresis of this temperature.
4. Application of special carbon black for plastic conduction in PE and PVC
When carbon black is used as a conductive material in a polymer, since it can be filled into the polymer in different proportions, the electrical properties of the polymer composite can be selected between 101 - 1014Ω·cm, such as the resistivity of antistatic polymers 105 - 106Ω·cm; the resistivity of conductive polymers is 103 - 105Ω·cm; the resistivity of highly conductive polymers is 102 - 103Ω·cm; the resistivity of superconducting polymers is 1 - 103Ω·cm.
Compared with foreign series of special carbon black for plastic conduction, there is no domestically produced special carbon black for plastic conduction in my country. When many plastic factories need this product, they have to import it at a high price, or use conductive carbon black for rubber and carbon black for rubber. replace. Judging from the technical indicators of foreign plastic conductive carbon black, the varieties suitable for plastics are mainly high-structure carbon blacks with relatively large particle sizes, which is a challenge for us technically. The top priority of the black industry.
Special conductive carbon black for plastics is widely used in the plastic industry as semi-conductive shielding materials for high-voltage cross-linked cables and other places where electrostatic protection is required. Since plastic is a highly insulating material, the electrostatic charge accumulated on the surface is not easy to discharge, and it will form a high electrostatic voltage of up to tens of thousands of volts. Therefore, plastic films used for packaging electronic devices or electrical products must be conductive, otherwise they will Static electricity on the surface will damage the electronic products it packs.
The conductive plastic products that urgently need to use special carbon black for plastic conduction mainly include semi-conductive shielding materials for high-voltage cables above 100,000 kV. According to the survey results, there are currently more than 60 production lines for inner and outer semi-conductive shielded cables (30 imported from abroad and more than 20 domestically manufactured) for high-voltage cross-linked cables, with a production capacity of about 50 Mm/a, about 25 kt of semi-conductive shielding material is required.
In fact , each production line is underutilized , but even if calculated on the basis of 10 kt / a , the filling amount of special carbon black for plastic conduction is generally 30 % , and the amount of special carbon black for plastic conduction in the semi - conductive shielding material can also reach 3 kt/a or so. Due to the high price of imported special conductive carbon black for plastics, it is difficult for domestic plastic manufacturers to afford it. 20% of the high-voltage cross-linked semi-conductive materials used in cable factories need to be imported. Even for the production of semi-conductive shielding materials for 35 kV cables, there are not many qualified manufacturers. Improving the conductivity of carbon black and promoting the localization of semi-conductive shielding materials can greatly reduce the cost of wires and cables and save a lot of foreign exchange.
With the continuous improvement of our country's industrial level, more and more plastics are used in production and life. At the same time, the problems of antistatic and flame retardant are becoming more and more urgent. Many industries, such as petroleum, chemical industry, textile, electronics, printing, etc. have put forward clear requirements for this, especially the antistatic and flame retardant problems of coal mine pipes are very prominent.
In addition, there are many electronic equipment, conductive coatings, paints, anti-static work clothes, etc. that require the use of conductive carbon black with various special properties. Although carbon black for plastic conduction is mainly used in conductive plastic products, it is also widely used in rubber products. Using special carbon black for plastic conduction with large particle size can produce highly elastic conductive rubber products in rubber, which can be used for various antistatic conveyor belts, plates, pipes, and antistatic and flame retardant air duct cloth for underground, etc. .