Compared with other conductive materials, conductive carbon black has a reinforcing effect on plastics and rubber. In addition, polymers with different conductivity can be obtained by controlling the amount of conductive carbon black added.
It is generally believed that conductive carbon black can be explained by conductive energy band and tunnel effect. When conductive carbon black is filled with polymers, electrical conduction is carried out along the contacting particles or separated into small gaps. The average distance within aggregates is influenced by many parameters, including concentration, structure, size and morphology of aggregates, size distribution, mixing effects, temperature, etc. Conductive carbon black can be divided into several categories according to different preparation methods and raw materials. Only a part of them are suitable for conducting electricity. They must have some basic characteristics, such as large specific surface area, less impurities for capturing electrons, and better crystallinity.
The structure of conductive carbon black is represented by the extent to which the conductive carbon black particles gather into chains or grapes. Conductive carbon black composed of aggregates composed of the size, shape and number of particles of each aggregate is called high-structure conductive carbon black. At present, the oil absorption value is commonly used to indicate the structure. The larger the oil absorption value, the higher the structure of the conductive carbon black, and it is easy to form a space network channel, and it is not easy to damage. High-structure conductive carbon black has fine particles, tightly packed network chains, large specific surface area, and more particles per unit mass, which is conducive to the formation of chain-type conductive structures in polymers. Among many conductive carbon black varieties, acetylene carbon black is the leading one. The scientists also found that conductive carbon black particles with a wide particle size distribution imparted electrical conductivity to the polymer better than conductive carbon black particles with a narrow particle size distribution, and explained this phenomenon statistically. For carbon black with a wide particle size distribution, a small number of large-diameter particles need to be huge, and particles with smaller diameters are compensated. Conductive carbon black with the same average particle size distribution has more total particles than carbon black with a narrow distribution.
If the surface of conductive carbon black contains a large number of oxygen functional groups, it will affect the migration of free electrons, thereby affecting the conductivity. For example, some conductive carbon blacks produced by thermal cracking method and channel method have poor structure, and the surface of channel method conductive carbon black also has a large number of active groups, so the conductivity of these two conductive carbon blacks is very poor.
With the increase of conductive carbon black loading, the resistance value decreased. Generally, at the critical volume fraction, the resistance value drops sharply. Most of the various studies on the dependence of filling amount at home and abroad are studies on the geometry of the contact of conductive carbon black particles. According to the theory, the greater the filling amount of conductive carbon black, the greater the density of conductive carbon black 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. The more conductive pathways are formed by the black particles or aggregates of conductive carbon black particles.
The greater the polarity of the mixed system composed of polymers with different polarities and conductive carbon black, 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. Group, the polarity of the matrix is large, and the effect is enhanced. At this time, the strength increases, but it hinders the agglomeration of the conductive particles themselves, resulting in poor conductivity. Polystyrene is more polar than polypropylene and polyethylene, and more similar to conductive carbon black, so more conductive carbon black is needed to achieve the same conductivity.
However, in the mixed system composed of multi-component matrix resin and conductive carbon black, due to the different polarities of different matrices, segregation will occur when filling conductive carbon black. At this time, the conductivity depends on the concentration and distribution of conductive carbon black particles in the segregated phase. The state also depends on the proportion of polymers in the segregation phase.
Treating the surface of conductive carbon black with titanate coupling agent can not only improve the antistatic performance, but also improve the melt fluidity and mechanical properties of the material, while the dispersant and non-surfactant can prevent the aggregation of conductive carbon black particles, Make it evenly dispersed in the polymer. In addition, the combination of conductive carbon black and clay, talcum powder and other substances can also improve the modification effect.
Adding low-molecular-weight wax to the conductive carbon black reduces the viscosity of the system, which reduces the shear force on the system. At the same time, the low-molecular wax contains and infiltrates the conductive carbon black particles, reducing the cohesion of the conductive carbon black particles. The magnitudes of these two force reductions vary with the amount of molecular wax used. When the decrease of the shear force is greater than that of the cohesive force, the conductive carbon black cannot be uniformly dispersed, and the conductivity is poor.
The processing technology is also an important factor affecting the electrical conductivity, mainly including: mixing conditions, polymer viscosity, molding method, heat treatment and storage aging, etc.
There are many studies on the influence of mixing conditions on conductivity. Foreign scientists measured the corresponding resistance value, mechanical properties and dispersion by adjusting the mixing time. The results showed that the resistance value decreased rapidly with the increase of mixing time. reverse rise.
This kind of mixing affects different conductive carbon black types, contents and different substrates. The acetylene conductive carbon black with a well-developed structure is easily damaged by long-term shearing, so the resistance value rises faster, while the ketene with an underdeveloped structure EC conductive carbon black 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 the severe damage to a small amount of conductive carbon black linked into the path due to shearing, the impact is very sensitive at this time, and at high content, There will be a lot of conductive carbon black due to the link effect, and when it suffers chain damage, it will form a new chain structure, and at this time it will be weakened again due to the influence of mixing conditions.
Different molding methods have a great influence on the conductivity of the material. Thermally sensitive conductive materials show different conductive characteristics when different molding methods are used, and the compression molding process has little effect on the conductive characteristics of the material. The injection molding process adds a repairing and mixing process, so that the conductive carbon black particles are redistributed longitudinally and laterally during the injection molding process, and the resistance is increased. During extrusion molding, under the action of a certain shear force, the conductive carbon black particles are further dispersed, and the conductivity of the material is improved. Therefore, many studies have considered that the most suitable molding method is extrusion molding.
Molding temperature, pressure and elongation all have an influence on the resistance value. In order to achieve the effect of dispersing into anisotropy and minimize the structural damage of conductive carbon black caused by shear force during the processing and molding of conductive plastics, some Conductive plastics with elastomer and rubber as the matrix often need to be cross-linked or vulcanized. At this time, cross-linking, vulcanization conditions, aging and storage time of the mixture will all affect the conductivity of the material.
Through the heat treatment molding process, the conductive carbon black particles that change in the processing chain structure can be arranged in an orderly manner, forming an arrangement that is more conducive to conductivity. Heat treatment makes the crystallization of the matrix tend to be perfect, and the increase of the crystallinity narrows the range of the melting resistance of the matrix, and the resistivity increases rapidly in a very narrow temperature range, and the crystallinity increases at the same time.
The development in the field of conductive plastics mainly focuses on the modification of conductive carbon black fillers and the development of new conductive carbon black. Conductive carbon black modification usually carries out high-temperature heat treatment to increase the specific surface area of conductive carbon black and improve the surface chemical properties. The research and development of new conductive carbon black is also eye-catching. The conductive carbon black prepared from petroleum and tar by pyrolysis has a large specific surface area and high porosity. Filling it into low-density polyethylene can make composite materials The electrical resistivity decreases, while the mechanical properties remain basically unchanged.
Although the research on conductive carbon black filled conductive plastics has made great progress, the experimental data of some influencing factors are still lacking. With the development of nanotechnology, new properties of conductive carbon black-filled conductive materials appear, and the research on conductive carbon black-filled conductive materials will also be broader.