How does carbon black fill the polymer to achieve the conductive function? Knowing this can give us a further grasp of the theoretical basis when manufacturing conductive materials, so that we can try to consider different conductive requirements in the molding process and take corresponding measures To get the required conductive material. Generally, there are two theories about the conductive mechanism of carbon black in polymers, namely chain-type conductive pathway and tunnel effect, but the final conclusions of both support the statement that the conductivity depends on the type and amount of carbon black .
Chain-Lock Conductive Paths
The mechanism of the chain-type conductive pathway believes that the carbon black particles must be within a distance of several A (1A=0.1nm) close (as shown in Figure 11-5), so that a voltage difference can be generated to make the ∏ electrons of the carbon black particles Relying on the chain locks to pass the movement through the current. The dispersion state of carbon black particles in the polymer is shown in Figure 11-6. From this equivalent circuit model, it can be understood that a certain amount of carbon black must be used to form a chain lock in order to have a strong electrical conductivity, thus dominating the electrical conductivity of polymer materials. The most important reason is the amount of carbon black. This is the most classic explanation.
tunnel effect
The chain-lock conductive path is proposed on the premise that carbon black must form a chain. However, recently, the rubber in the stretched state observed with an electron microscope does not have carbon black chain locks, but still conducts electricity, which is the tunnel effect. The main argument is that conductivity is determined by the tunneling of carbon black particles. At the same time, it has been proved by experiments that as the distance between carbon black particles increases, the volume resistance also increases.
There is also an electric field radiation conduction mechanism, because when studying the voltage and current characteristics of carbon black-filled polymer materials, it is found that the results do not conform to Ohm's law. It is believed that this is due to the high electric field intensity generated between carbon black particles. An electric current occurs resulting in electric field emission. In summary, no matter what kind of conductive mechanism is understood, it is believed that the type and amount of carbon black are the main factors that govern the final conductivity of the material.
Conductivity of carbon black filled materials.
The conductivity of carbon black-filled polymer materials is mainly determined by the type, dosage and compounding technology of carbon black. The variety of carbon black was mentioned earlier, and this section focuses on the relationship between the amount of carbon black, the composite technology and the conductivity of the material.
①Amount of carbon black
It can be seen from the conduction mechanism that when the performance is guaranteed to meet the requirements, the amount of carbon black should be increased in order to improve the conductivity. However, the relationship between the amount and the conductivity is not linear, but changes according to an exponential law, which can be expressed as follows
R=exp(a/W)p
In the formula, R-material volume resistance; W-mass fraction of carbon black
a, p-constants determined by the rubber type of carbon black
Figure 11-7 and Figure 11-8 are the volume resistance changes of different mass fractions of acetylene carbon black after adding neoprene and natural rubber respectively
Although the mass fraction of carbon black corresponding to the best conductivity is shown in Figure 11-7 and Figure 11-8, the amount of carbon black used in the actual process should also consider the impact on other properties including process performance. Table 11-3~Table 11-5 respectively show the effect of the amount of acetylene and carbon black on the electrical conductivity and physical and mechanical properties.
② Composite technology
If the structure and amount of carbon black are regarded as the main factors to realize the electrical conductivity of the material, then the composite technology is the objective condition to realize the electrical conductivity of the material. The relationship between the two is like an egg must be at the right temperature to hatch a chick.
Composite technology mainly has the following aspects.
Surface treatment of carbon black In order to improve the dispersibility of carbon black and the affinity with resin, it is necessary to use appropriate additives for surface treatment.
kneading
When the selected polymer and carbon black and their amount are determined, the conductivity of the material depends on the dispersion state of carbon black and the formation of chains. The structure that is most likely to be destroyed during kneading affects conductivity. This requires the selection of appropriate processing equipment and means.
The purpose of mixing, in addition to ensuring the smooth progress of subsequent processing, should also ensure that carbon black is fully dispersed in the polymer from the perspective of electrical conductivity. General mixing is carried out by internal mixer, and in order to achieve full dispersion, it is often easy to extend the mixing time and speed at will. Therefore, the influence of mixing time and degree of dispersion on conductivity should be recognized, and an optimal mixing time should be obtained to ensure good dispersion and thus good conductivity.
Figure 11-9 shows the relationship between the mixing time and the dispersion and conductivity of styrene-butadiene rubber after adding conductive carbon black.
However, for materials with poor plasticity, it is difficult to achieve uniform dispersion after short-term mixing, so carbon black can be mixed in a solution state (wet method), which can reduce the damage of carbon black structure caused by mixing. phenomenon, which significantly increases the conductivity. Figures 11-10 show that different mixing methods result in significantly different changes in conductivity.
mature
The semi-finished products after kneading are generally not formed into products immediately, but can only be formed after a certain period of storage or high temperature treatment. This post-kneading treatment process is called aging. Figures 11-11 show that different curing conditions have significantly different effects on conductivity. That is to say, the volume resistance increases after aging, and this increase increases with time, and the influence of temperature is not too great.
Molding time Molding time is not only an important process factor that determines the physical properties of conductive polymer materials, but also a factor that determines its electrical conductivity. Figures 11-12 show the experimental results of the increase in conductivity of chloroprene rubber filled with acetylene carbon black as the vulcanization time increases. This can be understood in this way. When under certain high temperature conditions, carbon black is subjected to high temperature, its impurities and moisture are continuously removed, and its structure is relatively gradually developed. As time goes on, the effect is better, and finally the conductivity becomes better. good. When the time is extended to a certain value, this effect is weakened so that there is no change in conductivity.
Molding temperature
In the molding process of polymer materials, the molding temperature is often considered together with the molding time. Generally, increasing the temperature will shorten the time accordingly, while decreasing the temperature should prolong the time. Then when the time is constant, as the temperature increases, the conductivity will naturally become better, and the reason is consistent with the explanation of time extension. Figures 11-13 show the relationship between forming temperature and electrical conductivity.