First, the role of conductive carbon black in the development of plastic conductive history
Carbon black more than 95% for rubber products, carbon black as a conductive material, especially acetylene carbon black, mainly used in dry cell batteries, used in plastics as a conductive functional filler is a relatively new field. However, the production of acetylene carbon black caused by environmental pollution is more serious, and acetylene carbon black in rubber, plastic processing is poor, the mechanical properties of the finished product is also poor.
Synthesis of ammonia and heavy oil gas by-production of carbon black, due to its manufacturing cost is too high, and it is very poor dispersion in plastics, so that the loss of the mechanical properties of plastic products is serious, and therefore can not be used as a conductive material, its value in the use of plastics, the dosage is very limited.
Oil furnace method of carbon black varieties are not specialized in plastics with conductive carbon black, was included in the ASTM D 1765 -96a.
When carbon black is added to plastics, due to the different filling amount and dispersion degree of carbon black, the conductive mechanism of carbon black in plastics is not the same.
On the one hand, due to the developed structure of carbon black, so that the carbon black particles contact each other to form a conductive channel;
on the other hand, due to the insufficient number of carbon black particles, or dispersion of uniformity, carbon black particles can not be in contact with each other, there is a layer of carbon black particles between the formation of a thin layer of resin barriers, electrons can not directly circulate, but when there is a voltage, the electrons can rely on the tunneling effect of conductive, that is, the electrons should be in the barriers to the electronic leap, the formation of tunneling conductivity.
Formation of tunneling conductive. In fact, in the polymer complex conductive process, the two conductive mechanism exists at the same time, just conductive efficiency is different. As a result of conductive compounds in the use of different conditions, such as AC, DC, high-frequency, low-frequency, electromagnetic wave shielding, etc., the role of carbon black in which there are differences.
Second, the factors affecting the conductive properties of conductive carbon black
1. Carbon black particle size
Theoretically, the smaller the particle size of carbon black, the more particles per unit volume, is conducive to improving electrical conductivity. This is normal in rubber conductive products. However, when used in conductive plastic products, if the carbon black particles are too small, because the plastic shear after plasticization is small, so the dispersion is poor, carbon black to a large number of small clumps exist in the base material, so that the mechanical properties of conductive plastic products to reduce the loss of practical value.
Therefore, to control the particle size of carbon black in a certain range, in order to ensure that the carbon black can be good dispersion in the plastic, but also greatly increase the number of carbon black particles per unit volume of plastic, improve the electrical conductivity of plastic products, while not destroying or less destructive of the original mechanical properties of the products.
2. Carbon black structure
DBP value of the size of the carbon black aggregates on behalf of the structure of the high and low, in general, DBP value is high when the carbon black is chain dendritic structure, conductivity is better. The DBP values of various heavy oil gas by-products of carbon black are very high, and from the electron microscope found that they are the microstructure of the empty shell form, indicating that its structure is not very high. Its high electrical conductivity may be due to the result of its larger volume per unit mass, and shear destruction of part of the primary structure, resulting in a large number of new particles.
To obtain good electrical conductivity in plastics, carbon black must be a larger particle size, 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 conductive network, with a small amount of carbon black can achieve anti-static effect.
3. Carbon black roughness
As the conductivity of carbon black needs to have a certain degree of roughness, so that carbon black is easy to form a conductive channel, so the nitrogen adsorption surface area of carbon black and CTAB surface area of the difference between the larger.
4. Surface volatile matter
The volatile fraction of carbon black surface is mainly composed of some organic groups and not completely cleaved oil film formation, forming a layer of insulation, increasing the potential barrier between the carbon black particles, seriously affecting the conductivity, the volatile fraction must be controlled at a lower limit.
5. Ash and moisture
Carbon black in the ash and moisture content is high, actually reduces the content of conductive carbon black, the same conductivity has a negative impact, in the production of carbon black to pay attention to the control of ash and moisture content.
Third, superconducting carbon black in the plastic conductive principle
When the superconducting carbon black in plastics conductive role from the carbon black aggregates or aggregates between the mutual contact of the conductive channel.
When the amount of carbon black in plastics with a low, superconductive carbon black aggregates in the polymer dispersion, aggregates and aggregates are about 10nm or more away from each other, which is the resistance of the matching material by the polymer phase dominated by the non-conductive. When the amount of coordination increases, the average aggregates between the aggregates are uniformly distributed in the compounds, then the resistivity of the compounds in the amount of carbon black can be increased to the aggregates can be in contact with each other will remain unchanged, waiting for a small increase in the amount of can be rapidly decreased.
However, the carbon black will actually be dispersed into aggregates or agglomerates ranging in size from 1-10 um, and the amount of carbon black in the area where the agglomerates are locked in or within the area will be higher than the average of the amount of conductive carbon black in the polymer, and therefore the resistivity will be lower, and secondly, in the uniform distribution of the aggregates and the agglomerates in the composite, the resistivity change of the composite will be more gradual and not very steep.
The effect of the size of the agglomerates on the resistivity can be indirectly observed from the effect of mixing time on the resistivity, because the longer the mixing time, the number of agglomerates will increase, while the size will decrease. Conductive carbon black additions in different proportions in different carriers
Compound with superconducting carbon black, usually used at temperatures below the softening point of the polymer, when the temperature rises, the resistivity of the compound with superconducting carbon black will rise, this phenomenon is known as the amount of coordination with a positive coefficient of PTC, this is because most of the polymer’s coefficient of thermal expansion is greater than that of the carbon black, the dimensionality of the rise is the carbon black aggregates between the contact points of the gap in the increase. This phenomenon can be applied to switching circuits, so that the circuit silver dimensional changes in the goods off, or used to limit the amount of power in the furnace.
In this application there are two key points, one is and use a concentrated conductive carbon black than a single superconducting carbon black in the dimensional changes in the change in resistivity is more sensitive. Another point is that the resistivity of the matching material in the experience of a number of cyclical changes in the temperature will change, the choice of Beilum Carbon conductive carbon black with the appropriate amount of carbon black quality and dispersion of conductive carbon black quality can reduce the lag of this temperature.
Fourth, the plastic conductive carbon black in PE, PVC applications
Carbon black in the polymer as a conductive material, because it can be filled into the polymer in different proportions, so that the electrical properties of polymer compounds can be selected between 101 – 1014Ω-cm, such as anti-static polymer resistivity in 105 – 106Ω-cm; conductive polymer resistivity in 103 – 105Ω-cm; highly conductive polymer resistivity in 102 – 103Ω-cm. Super conductive polymer resistivity in 1 – 103Ω-cm.
And foreign plastics conductive carbon black varieties compared to the series, China now has no domestic plastics conductive carbon black, many plastics factories in need of the product, only to import at high prices, or the use of rubber with conductive carbon black and rubber carbon black to replace. From the point of view of foreign technical indicators of conductive carbon black for plastics, suitable for plastics varieties are mainly some of the larger particle size of the high structure of the carbon black, which is technically a challenge to us, the development of conductive carbon black for plastics is the most urgent need of China’s conductive carbon black industry.
Plastic conductive carbon black in the plastics industry is widely used in high-voltage cross-linked cable semi-conductive shielding material and other needs for electrostatic protection. As plastic is a highly insulating material, the surface of the electrostatic charge is not easy to exclude the formation of up to tens of thousands of volts of electrostatic high pressure, so as used for packaging electronic devices or electrical products, plastic film, need to have a conductive, otherwise the surface of the electrostatic charge will be packaged in the electronic products will be damaged.
Now there is an urgent need to use plastic conductive special carbon black conductive plastic products are mainly 100 000 kV above the high-voltage cable semi-conductive shielding material. From the results of the survey, currently used for high-voltage cross-linked cables within and outside the semi-conductive shielding cable production line has more than 60 (30 introduced from abroad, more than 20 domestic manufacturing), the production capacity of about 50 Mm / a, about the need for semi-conductive shielding material 25 kt.
In fact, each production line is not enough work, but even if the 10 kt / a count, which is generally filled with plastic conductive carbon black 30%, then the semi-conductive shielding material in the plastic conductive carbon black dosage can be up to 3 kt / a or so. As the price of imported plastic conductive carbon black is too high, domestic plastics manufacturers can hardly afford. Cable plant’s high-voltage crosslinking semi-conductive material is also 20% need to be imported.
Even the production of 35 kV cable with semi-conductive shielding material, qualified manufacturers are not many. Improve the conductive properties of conductive carbon black, promote the localization of semi-conductive shielding material, can greatly reduce the cost of wires and cables, and save a lot of foreign exchange.
With the continuous improvement of China’s industrial level, plastics in production and life more and more applications, at the same time, anti-static and flame retardant issues are becoming more and more urgent. Many industries, such as petroleum, chemical, textile, electronics, printing, etc. has put forward clear requirements, especially the coal mine with the pipe anti-static, flame retardant problem is more prominent.
In addition, there are many electronic equipment, conductive coatings, paints, anti-static clothing, etc. need to use a variety of special properties of conductive carbon black. Although the plastic conductive carbon is mainly used for conductive plastic products, but also has a wide range of uses in rubber products. With a large particle size of plastic conductive carbon black, in the rubber can produce highly elastic conductive rubber products, for a variety of anti-static transportation belts, boards, tubes, and underground with anti-static flame retardant duct tape and so on.
