Polyvinyl Chloride has been widely used in the production of cable materials due to its excellent corrosion resistance, electrical insulation, especially flame retardancy and self-extinguishing properties.
PVC cable compound is a pellet made by mixing, kneading and extruding polyvinyl chloride as the base resin, adding stabilizers, plasticizers, lubricants and inorganic fillers (e.g. calcium carbonate).
Selection of Polyvinyl Chloride Resin
The higher the molecular weight of PVC, the higher the tensile strength, impact strength and modulus of elasticity of the products, but the fluidity and plasticity of the melt decreases, the better the heat and cold resistance, and the higher the processing temperature.
Generally speaking, cable materials require high molecular weight and easy absorption of plasticizers. Usually use loose type, high purity, less impurity, less fish-eye suspension method of PVC resin. SG-1 or SG-2 is generally used, but at present, these two resins are less, so many cable materials are used SG-3. SG-1 resin should be used for high-grade electrical insulation materials, and SG-2 and SG-3 should be used for general electrical insulation materials. SG-1 should be used for cable materials with high requirements for heat resistance.
1. Selection of plasticizer
The main role of plasticizers is to reduce the melt temperature and melt viscosity of polymers, thereby reducing the processing temperature of polymers, so that polymer products have flexibility and low temperature resistance. However, the addition of plasticizers will reduce the insulation properties.
The order of volatility and heat resistance of plasticizers is as follows: totm > dTDP > DUP > DIDP > DINP > DOTP > DOP.
2. Selection of stabilizer
Stabilizers are basic lead salts. Generally, a variety of stabilizers are used in conjunction with each other to exert synergistic effects and improve thermal stability.
When tribasic lead sulfate and dibasic lead phosphite are used together, thermal stability and light stability can be considered. The main stabilizer for high-temperature-resistant cables is lead dibasic benzoate with good heat resistance. At present, compound lead stabilizers are also widely used in PVC cable materials, the addition amount is 4 ~ 6phr. Calcium-zinc compound stabilizers are commonly used in environmentally friendly cable materials.
3. Selection of flame retardant
Using different plasticizers, the flame retardancy and smoke emission of PVC cable material will be different. When the content of plasticizer increases, the oxygen index of PVC will decrease. For different plasticizers, the rate of decrease is basically the same. Therefore, a certain amount of flame retardant needs to be added to the formulation. It is found that the flame retardant properties of PVC cable materials are significantly improved with the increase of flame retardant dosage.
4. Selection of lubricants
Due to the large amount of plasticizers, the requirements for internal lubricants are not very high. The lubricant is mainly used to improve the surface brightness of the cable material.
Usually metal soap, stearic acid and paraffin wax are used, and the added amount is about 1phr.
5. Selection of filler
Adding fillers to cable materials can improve electrical insulation properties, heat resistance and reduce costs, but excessive addition will lead to molding and cable material performance decline.
In order to improve insulating properties, insulation grade cable materials can choose calcined clay (electrical grade) as filler; sheath (layer) cable can choose calcium carbonate as filler.
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