Polyvinyl chloride (PVC) conveyor belt as a new type of plastic products, with its light weight, long life, good flame retardancy, not easy to produce friction and other advantages, is widely used in coal mines, mines and all kinds of factories. However, because PVC is a good insulator with large surface resistance and poor electrical conductivity, it is easy to cause explosions or fires and other major accidents due to the accumulation of static electricity in coal mines, which are rich in gas and easy to generate static electricity. Therefore, it is necessary to improve the antistatic performance of PVC flame retardant conveyor belt through formula design.
Electrostatic generation mechanism and antistatic methods 1. 1 electrostatic generation process is made of plastic and rubber conveyor belt in the coal mine is running, can cause the belt body by friction on the static charges resulting from the positive and negative charge out of balance, electrostatic charges accumulated on the surface of conveyer belt is not easy to disappear, when reaches a certain value, can produce spark discharge ground objects, Cause a fire or gas explosion.
At present, the method of plastic antistatic is mainly to control the generation and accumulation of static electricity, control the generation of static electricity is mainly to reduce the generation of static electricity by reducing or preventing friction; The accumulation of static electricity is mainly controlled by grounding conduction, increasing the relative humidity of the environment and adding antistatic agents or conductive substances (such as conductive carbon black, metal powder, etc.) to accelerate the static electricity leakage, to avoid a large amount of static electricity accumulation.
Shandong Tongtai Rubber Co., Ltd. In order to make the plastic with good insulation performance has a certain electrical conductivity, the following methods are mainly used: ① Add antistatic agent to the plastic or use antistatic agent to coat the surface of the products. (2) Add conductive filler (such as conductive carbon black, metal powder, etc.) to the plastic. (3) change the structure of the polymer by graft copolymerization, so that it has more polar groups or ionic groups, increase the electrical conductivity. (4) with strong oxidant oxidation of the surface of plastic products, increase electrical conductivity.
In the actual production process of PVC flame retardant conveyor belt, the method of adding antistatic agent and conductive filler to the coating formula is generally adopted to reduce the surface resistance and volume resistance of the conveyor belt, so that it has a certain antistatic performance.
2 antistatic formula of exploration and discussion for coal mine use PVC flame retardant conveyor belt, formula of antistatic ability is a very important problem, in order to make the conveyor belt running process of electrostatic charge timely transmission and leak off by itself, we must from the craft condition, the production cost and other factors to consider, select the appropriate antistatic agent and conducting material, To reduce the surface resistance of the PVC flame retardant conveyor belt, to ensure that the surface resistance of the conveyor belt is not more than 3×10 8Ω.
2. 1 selection of antistatic agent In order to make the PVC flame retardant conveyor belt has good antistatic performance, the traditional method used in China is to add quaternary ammonium salt cationic antistatic agent in the formula; The practice has proved that quaternary ammonium salts cationic antistatic agents have strong adhesion and excellent antistatic properties, but there are many shortcomings in the actual use. First of all, quaternary ammonium antistatic agent itself is poor in heat resistance, easy to be decomposable by heat, can reduce the thermal stability of PVC resin, accelerate the aging process of PVC resin, this point in our use of single fat alkyl trimethyl ammonium halide antistatic agent production process has been fully confirmed; Secondly, quaternary ammonium salts cationic antistatic agent can react with lead stearate, barium stearate and other metal soap-based heat stabilizers in the formula of PVC flame retardant conveyor belt to form a stable compound, thus affecting the antistatic effect of conveyor belt and reducing the thermal stability of resin. Therefore, in the production process of PVC flame retardant conveyor belt with high heat resistance requirements, it is not suitable to use quaternary ammonium salts cationic antistatic agent, and non-ionic antistatic agent with good thermal stability and antistatic property should be used.
According to the actual processing conditions of the PVC conveyor belt, and considering the interaction between the antistatic agent and other additives in the formula, the use of non-ionic ester antistatic agent to reduce the surface resistance of the PVC flame retardant conveyor belt. Non-ionic antistatic agent has good heat resistance, good compatibility with other auxiliaries, no ionic antistatic agent is easy to cause the shortcomings of plastic aging, and no adverse effects on the physical and mechanical properties of products in the process of processing and use. The action mechanism of this kind of antistatic agent is mainly as follows: (1) the hydrophilic group of antistatic agent increases the hygroscopicity of the surface of the product, forming a monomolecular conductive film; ② the formation of antistatic agent molecular layer on the surface of plastic products, increase the smoothness of the surface of products, thereby reducing friction, reduce the generation of static charge. After repeated experiments show that the dosage of plasticizer more than 30 of the flame retardant conveyor belt cover formulated non-ionic antistatic agent KJ - 210 and flame retardants (beta - ethyl chloride) phosphate (TCEP), after mixing them evenly dispersed in PVC plastic was nonconductor, forming after it can gradually migrate to the surface of a conveyor belt, The formation of antistatic molecular layer, so as to reduce the surface resistance of the conveyor belt, PVC flame retardant conveyor belt has good antistatic and flame retardant performance.
2. 2 Selection of conductive materials Because carbon black is a natural semiconductor material with long-lasting and stable electrical conductivity, and can greatly adjust the resistivity of polymer materials, coupled with a wide range of sources and low price, carbon black is selected as a conductive filler. In order to reduce the production cost and other factors, the formulation of acetylene carbon black as a conductive material.
Shandong Tongtai Rubber Co., Ltd. uses carbon black as a conductive material, which can reduce the volume resistivity of PVC plastic and make it have a certain electrical conductivity. It is generally believed that carbon black is dispersed in the form of particles in PVC resin, with the increase of the amount of carbon black added, the particle spacing is shrinking, when the particles close to or contact, it will form a large number of conductive network channels, PVC plastic conductive property is greatly improved. However, the decline of the resistivity of PVC plastic products is not in a linear relationship with the amount of carbon black. When a certain amount of carbon black is added, the electrical conductivity change is no longer significant even if the carbon black is increased again. According to relevant data, after more than 7 sets for the amount of carbon black, carbon black conductive network has formed, adding more carbon black surface resistance of PVC plastic products, instead of increasing as the amount of carbon black, PVC plastic tensile strength, impact strength and elongation have fallen sharply, hardness, and in PVC plastics combustion would produce red shines, Reduced flame retardant effect. Therefore, to solve the anti-static problem of PVC flame retardant conveyor belt only by adding more carbon black method is not desirable, must choose accurate usage.
Now the suitable antistatic formula for PVC flame retardant conveyor belt is: PVC, 100; KJ-210, 7; Acetylene black, 5; TCEP, 9; Stabilizer, 6; Lubricants, processing AIDS, appropriate amount.
3. 1 Process and Requirements The trial production process of PVC flame retardant and antistatic conveyor belt is as follows: PVC resin and various auxiliary agents stirring and kneading, plasticizing, rolling plate forming, sample kneading temperature control: 105 ~ 115℃. Temperature control of two-roller mixer: front roller, 150 ~ 155℃; Rear roll, 145 ~ 150℃. Temperature control of three-roll calender: upper roll, 140 ~ 145℃; Middle roller, 150 ~ 155℃; Lower roller, 90 ~ 100℃. Plate forming temperature control: the high temperature does not exceed 165℃.
3. 2 Test results The PVC flame retardant antistatic conveyor belt made of the above antistatic formula has been tested by the National Coal Mining Machinery Quality Supervision and Inspection Center, and its conductivity and flame retardant properties are in line with the MT914-2002 "Coal Mine Fabric Whole Core Flame Retardant Conveyor Belt" standard requirements. The performance test results are shown as follows.
4 Conclusion In the production process of PVC flame retardant conveyor belt, adding non-ionic antistatic agent KJ-210 and conductive material acetylene carbon black can greatly improve the anti-static and flame retardant properties of PVC conveyor belt while reducing the production cost, and make it have good physical and mechanical properties.