(1) Selection of flame retardant system
The main material used for PVC covering rubber is polyvinyl chloride (PVC) polymer material, which has a chlorine content of 57% and an oxygen index of 45%. It is a kind of flame retardant material that is self-extinguishing from fire, but due to the process of producing PVC covering rubber. A large amount of plasticizer is added to improve the processing property, so that the chlorine content in PVC is greatly reduced, the oxygen index is reduced, and the flame retardant property is lost to become a flammable material. Therefore, it is guaranteed to increase the chlorine content of the PVC cover rubber. An important prerequisite for flame retardant PVC conveyor belts.
In the original PVC cover rubber formula, we use the method of adding antimony trioxide (Sb2O3), composite flame retardant and trichloroethyl phosphate (TCEP) to achieve the flame retardant purpose. Through analysis, it was found that the flame retardant was repeatedly used in the original PVC cover rubber formulation, and the synergistic effect between various flame retardants was not fully exerted, resulting in waste of raw materials and high production costs. Therefore, the flame retardant system used in the original formulation
Theoretical analysis and optimization were carried out. It was decided to use halogenated flame retardant 52% chlorinated paraffin instead of TCEP flame retardant in the original formula, and used in combination with Sb2O3 flame retardant to form a halogen-ruthenium synergistic flame retardant system.
Its flame retardant mechanism is:
(1) using the endothermic decomposition of halogen-cerium oxide to reduce the combustion temperature and decomposition rate of the PVC polymer;
2 bismuth halide vapor coating on the surface of PVC polymer to form a "blank" effect of heat insulation and oxygen barrier;
3 Promote the char formation reaction, slow down the decomposition rate of the PVC polymer to form a combustible gas, and the generated carbon layer prevents the combustible gas from escaping and entering the combustion zone;
4 Capture the active radicals in the gas phase of the combustion zone to maintain the combustion chain reaction, change the reaction mode in the gas phase, reduce the heat of reaction to quench the flame, and achieve a good flame retardant effect.
(2) Selection of antistatic agent
Because PVC is a good insulator, the surface resistance is large, and the electrical conductivity is poor. The flame-retardant conveyor belt made of PVC material will generate static charge due to the imbalance of positive and negative charges on the belt due to friction during use. A gas-rich site produces a spark that causes a fire or gas explosion. Therefore, we have selected suitable antistatic agents and conductive materials to reduce the surface resistance of PVC flame retardant cover rubber, to ensure that the surface resistance of the conveyor belt is not more than 3 × 108Ω. The quaternary ammonium salt cationic antistatic agent CN-3 in the original PVC cover rubber formulation has poor heat resistance and is easily decomposed by heat, thereby reducing the disadvantage of PVC thermal stability. We reduce the amount thereof and add conductive conductive carbon black. Methods
The PVC conveyor belt has good antistatic properties. Through the reasonable combination of the two, the hydrophilic group of the antistatic agent CN-3 can be used to increase the hygroscopicity of the surface of the PVC conveyor belt, and a single-molecule conductive film is formed to improve the surface conductivity of the PVC conveyor belt, and at the same time, Under the action of shearing force, the special conductive carbon black particles become contact particles in the cover rubber to form a large number of conductive networks, and the conductive energy is generated by the movement of л electrons in the mesh chain and the electronic transition to reduce PVC transport. The volume resistance of the belt.
(3) Selection of filler
In the production cost of PVC conveyor belt, the cost of raw and auxiliary materials accounts for more than 70% of the total production cost. Therefore, the cost of raw and auxiliary materials is directly related to the level of profit. The addition of inorganic fillers is the best way to reduce material costs. To this end, on the basis of ensuring the physical properties and safety performance of the cover rubber, increase the amount of light calcium carbonate. Light calcium carbonate is the most commonly used inorganic powder filler. It is a non-toxic, odorless white powder that is not easily decomposed at high temperatures. Moreover, the light calcium carbonate texture is soft, and the wear on the kneader and the molding machine is small; at the same time, the relative density is small, and in addition to the incremental effect in the PVC cover rubber, the processing property of the PVC cover rubber can be improved. Improve the impact strength of PVC flame retardant conveyor belts; in addition, its low price and wide range of sources. Therefore, light calcium carbonate is used as a filler in the PVC cover rubber to reduce the material cost of the PVC cover rubber.
(4) Adjustment of the stability system
Since the molecular structure of PVC contains double bonds, branching points and initiator residues, it is degraded by dehydrochlorination (HCl) reaction when heated to 100 ° C, and degradation reaction at processing temperature (170 ° C or higher). Accelerating, eventually leading to PVC losing its chemical and physical properties.
Therefore, it is common to add a heat stabilizer with excellent performance in the processing of PVC to make it have a synergistic thermal stability system with PVC resin to improve the heat resistance of PVC. The original PVC cover rubber formula uses lead dibasic phosphite, lead stearate and barium stearate as heat stabilizers. In view of the large amount of lead stearate, it is easy to form a frosting on the surface of the product and the stearic acid tends to cause stickiness. Roller and other shortcomings, therefore need to make appropriate adjustments to the original stable system, select lead salt stabilizers with excellent heat resistance, especially suitable for high temperature processing, lead dibasic phosphite and lead sulfite lead as PVC cover rubber Heat stabilizers.
(5) The optimum amount of each component
Experiments have shown that the performance of the various components in the formulation is not proportional to its amount, and each component has a limit. A large number of experiments on halogen-fluorene flame retardant system, antistatic system and filler dosage found that Sb2O3 has the effect of prolonging the residual ash, the amount should not exceed 10 parts, and the flame retarding effect is 7:1 when the X/Sb ratio is 7:1. The best; at the same time, it is found that the conductive material carbon black is added in an amount greater than 7 parts, and then adding more amount has little effect on the surface resistance of the PVC covering rubber, but the tensile strength of the PVC covering rubber increases with the increasing amount of the coating. The impact strength and elongation are greatly reduced, the hardness is increased, and red enamel is generated when the PVC film is burned, thereby reducing the flame retardant effect. In addition, since both the plasticizer and the chlorinated paraffin can adsorb on the surface of the light calcium carbonate, the surface of the calcium carbonate is infiltrated, thereby improving the dispersibility of the light calcium carbonate in the PVC resin. However, since the light calcium carbonate is not subjected to surface treatment, its surface activity is not strong, so as the amount of calcium carbonate increases, the tensile strength and elongation of the PVC cover rubber are greatly reduced, and the hardness is increased. Therefore, it is necessary to add light calcium carbonate to the PVC cover rubber.
Based on the above analysis of the PVC cover rubber formula, the best formula for determining the PVC full core flame retardant conveyor belt cover adhesive is:
Liquid flame retardant 15
Solid flame retardant 4
Antistatic agent 2
Conductive filler 5
The improved PVC cover rubber new formula can fully meet the actual production needs of the workshop, and can produce qualified products with safety performance indicators meeting and exceeding national standards.