Compared with other conductive materials, Conductive Carbon Black has a reinforcing effect on plastics and rubber. In addition, polymers with different conductivities can be obtained by controlling the amount of conductive carbon black added.
General research believes that conductive carbon black can be explained by conductive energy bands and tunnel effects. When conductive carbon black fills polymers, electrical conduction occurs along the contacting particles or is separated into small gaps. The average internal aggregate distance is affected by many parameters, including concentration, structure, aggregate size and morphology, size distribution, mixing effects, temperature, etc.
Conductive carbon black can be divided into several categories according to different preparation methods and raw materials. Only part of it is suitable for conductivity. They must have some basic characteristics, such as large specific surface area, few impurities for capturing electrons, and relatively good crystallinity.
The structural nature of conductive carbon black is expressed by the degree to which conductive carbon black particles are aggregated into chains or grapes. Conductive carbon black composed of aggregates composed of the size, shape and number of particles in each aggregate is called high-structure conductive carbon black.
At present, the oil absorption value is commonly used to indicate the structure. The greater the oil absorption value, the higher the structure of the conductive carbon black, which is easy to form spatial network channels and is not easy to be destroyed. High-structure conductive carbon black has fine particles, tightly packed network chains, large specific surface area, and many particles per unit mass, which is conducive to the formation of chain conductive structures in polymers.
Among many types of conductive carbon black, acetylene carbon black is the leading one. Scientists also found that beilum conductive carbon black particles with a wide particle size distribution can give polymers more conductivity than conductive carbon black particles with a narrow distribution, and used statistical methods to explain this phenomenon.
For carbon black with a wide particle size distribution, a small number of large-diameter particles require a huge number, which is compensated by particles with smaller diameters. Conductive carbon black with a wide particle size distribution has a greater total number of particles than carbon black with a narrow distribution for the same average particle size distribution.
If the surface of conductive carbon black contains a large number of oxygen functional groups, it will affect the migration of free electrons and thus affect the conductivity. For example, in addition to the poor structure of some conductive carbon blacks produced by thermal cracking and channel methods, the surface of channel conductive carbon black also has a large number of active groups, so the conductivity of these two types of conductive carbon black is very poor.
As the filling amount of conductive carbon black increases, the resistance value decreases. Generally, at the critical volume fraction, its resistance value decreases sharply. Most of the various studies at home and abroad that explore the dependence of the filling amount are geometric studies on the contact of conductive specialty carbon black particles. This theory holds that the greater the filling amount of Beilum conductive carbon, the greater the density of conductive carbon particles or aggregates of conductive carbon black particles in a dispersed state, the smaller the average distance between particles, and the higher the probability of mutual contact. Conductive specialty carbon black The more conductive paths formed by black particles or conductive pigment carbon black particle aggregates.
The greater the polarity of the mixed system of polymers and Beilum conductive carbon black with different polarities, the greater the critical volume fraction of conductive carbon black, which means that the conductivity of the system decreases because the surface of conductive carbon black contains strong polar groups. Cluster, the polarity of the matrix is large, and the effect is enhanced.
At this time, the strength is increased, but it hinders the agglomeration of the conductive particles themselves, resulting in poor conductivity. Polystyrene is more polar than polypropylene and polyethylene, and is more similar in polarity to conductive specialty carbon black, so more carbon black is needed to achieve the same conductivity.
However, in a mixed system composed of multi-component matrix resin and conductive carbon black, due to the different polarities of different matrices, filling conductive carbon black will cause segregation. At this time, the conductive performance depends on the concentration and distribution of carbon black particles in the segregation phase. The state also depends on the proportion of segregation phase polymer.
Treating the surface of conductive pigment carbon black with titanate coupling agents can not only improve antistatic properties, but also improve melt fluidity and material mechanical properties, while dispersants and non-surfactants can prevent the aggregation of carbon black particles. So that it can be evenly dispersed in the polymer. In addition, the modification effect can also be improved by using conductive carbon black with clay, talcum powder and other substances.
Adding low-molecular wax to conductive carbon black will reduce the viscosity of the system, causing the shear force on the system to decrease. At the same time, the low-molecular wax contains and infiltrates the conductive carbon black particles, causing the cohesion of the conductive carbon black particles to decrease. The magnitude of the decrease in these two forces varies with the amount of molecular wax. When the decrease in shear force is greater than the decrease in cohesion, the carbon black cannot be dispersed evenly and the conductive performance is poor.
The processing technology is also an important factor affecting the conductive performance, including: mixing conditions, polymer viscosity, molding method, heat treatment and storage aging, etc.
There are many studies on the impact of mixing conditions on electrical conductivity. Foreign scientists measured the corresponding resistance value, mechanical properties and dispersion by adjusting the mixing time. The results showed that the resistance value dropped rapidly as the mixing time increased. After reaching a certain value, it dropped again. Reverse rise.
This kind of mixing affects different Beilum conductive special carbon black types, contents and different matrices. Acetylene conductive specialty carbon black with a well-developed structure is easily damaged by long-term shearing, so the resistance value rises quickly, while ketene with an undeveloped structure EC conductive carbon is less affected by time.
In the case of low content of conductive carbon black, this effect is not obvious, but at the critical content, due to shearing serious damage to a small amount of conductive carbon linked to form a path, the effect is very sensitive at this time, and at high content, Since there will be a lot of conductive carbon black linked together, when chain damage occurs, a new chain structure will be formed, which will weaken again due to the influence of mixing conditions.
Different molding methods have a great impact on the conductivity of materials. Thermal conductive materials show different conductive characteristics when using different molding methods. The press molding process has little effect on the conductive characteristics of the material. The injection molding process adds a melting and mixing process to redistribute the conductive black particles longitudinally and transversely during the injection molding process, thereby increasing the resistance. During extrusion molding, conductive specialty carbon black particles are further dispersed under a certain shear force, and the conductivity of the material is improved. Therefore, many studies believe that the most suitable molding method is extrusion molding.
Molding temperature, pressure and elongation all have an impact on the resistance value. During the processing and molding of conductive plastics, in order to achieve the effect of dispersion into anisotropy and minimize the structural damage of conductive specialty carbon black caused by shear force, some Conductive plastics with elastomers and rubber as the matrix often need to be cross-linked or vulcanized. At this time, cross-linking, vulcanization conditions, maturation and storage time of the mixture will affect the conductivity of the material.
Through the heat treatment molding process, the conductive carbon particles that change in the chain structure during processing can be arranged in an orderly manner, forming an arrangement that is more conducive to conductivity. Heat treatment makes the crystallization of the matrix perfect, and the increase in crystallinity narrows the fusion resistance range of the matrix. The resistivity increases rapidly in an extremely narrow temperature range, and the crystallinity increases at the same time.
Development in the field of conductive plastics mainly focuses on the modification of conductive specialt carbon black fillers and the development of new conductive specialty carbon blacks. Modification of conductive carbon black usually involves high-temperature heat treatment to increase the specific surface area of conductive carbon black and improve surface chemical properties.
The research and development of new conductive carbon blacks is also eye-catching. Conductive specialty carbon blacks produced from petroleum and tar using high-temperature cracking method has a large specific surface area and high porosity. Filling it into low-density polyethylene can make composite materials The resistivity decreases, while the mechanical properties remain basically unchanged.
Although research on conductive carbon black-filled conductive plastics has made great progress, experimental data on some influencing factors are still lacking. With the development of nanotechnology, conductive carbon black-filled conductive materials have new characteristics, and the research on conductive carbon blacks-filled conductive materials will also be broader.
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