PVC has been widely used in the production of cable materials because of its excellent corrosion resistance, electrical insulation, especially its flame resistance and self extinguishing.
PVC cable material is pellet prepared by mixing, kneading and extruding PVC as the base resin, adding stabilizers, plasticizers, lubricants and inorganic fillers such as calcium carbonate.
Selection of PVC resin
The higher the molecular weight of PVC, the higher the tensile strength, impact strength and elastic modulus of the product, but the fluidity and plasticity of the melt decrease, the better the heat and cold resistance, and the higher the processing temperature.
Generally, cable materials are required to have high molecular weight and easy to absorb plasticizers. PVC resins of loose type, high purity, less impurities and less fish eyes by suspension method are often selected. SG-1 or SG-2 is generally selected, but at present, these two resins are relatively few, so many cable materials use SG-3. SG-1 resin shall be selected as high-grade electrical insulation materials, and SG-2 and SG-3 shall be selected as general electrical insulation materials. SG-1 cable materials with high requirements for heat resistance shall be selected.
Selection of plasticizer
The main function of plasticizer is to reduce the melting temperature and melt viscosity of polymer, thereby reducing the processing temperature of polymer and giving polymer products flexibility and low temperature resistance. However, the addition of plasticizer will reduce the insulation performance.
The order of volatility and heat resistance of plasticizers is as follows: totm > dTDP > DUP > DIDP > DINP > DOTP > DOP.
- The content of plasticizer in PVC cable is generally 50 ~ 60phr. Generally, varieties with better heat resistance and electrical insulation, such as DOP, are selected. It has excellent comprehensive properties, high plasticizing efficiency, low volatility, good cold resistance and electrical properties, and is an ideal main plasticizer. However, DOP alone cannot meet the requirements of electrical insulation performance of insulating materials. The properties of DOTP and DOP are similar, but the cold resistance, thermal stability and electrical insulation are better than DOP (about 18 times higher than DOP). Therefore, DOP and DOTP plasticizer need to be used together to obtain insulation materials with required properties.
- For cable materials with high electrical insulation, phosphate ester can be selected as the main plasticizer, and phthalate ester can be selected as the main plasticizer for general grade. Chlorinated paraffin can improve electrical insulation. Fatty acid esters and epoxy plasticizers can improve the low temperature resistance of cable compounds, and the latter also has good weather resistance.
- For cable materials with temperature resistance of 70 ℃, plasticizers such as diisopropyl phthalate (DIDP) or diisopropyl phthalate (DINP) can be used; For cable materials with temperature resistance of 90 ℃, double undecyl phthalate and double tridecyl phthalate shall be used; For cable materials with high temperature resistance of 105 ℃, plasticizers with higher heat resistance shall be selected, such as trioctyl trimellitic acid (TOTM).
- The acid value of the plasticizer has an impact on the electrical insulation and heat resistance of the cable material, and the plasticizer with smaller acid value should be selected.
- The molecular weight and flash point of the plasticizer have an impact on the heating loss of the cable material. The plasticizer with higher flash point and larger molecular weight should be selected, such as dibutyl phthalate compared with dioctyl phthalate and dioctyl adipate compared with dioctyl sebacate. The former has smaller molecular weight and lower flash point, so the heating loss is also larger.
- Considering the plasticizing efficiency, the plasticizer with high plasticizing efficiency should be selected, which can reduce the amount of plasticizer in the formula. The amount of plasticizer is related to the insulation performance. Reducing the amount of plasticizer is beneficial to improve the insulation performance.
Selection of stabilizer
The stabilizer is the base lead salt. Generally, a variety of stabilizers are used together to play a synergistic role to improve the thermal stability.
When tribasic lead sulfate and dibasic lead phosphite are used together, thermal and light stability can be considered. The main stabilizer of high temperature resistant cable is dibasic lead benzoate with good heat resistance. At present, the composite lead stabilizer is also widely used in PVC cable materials, with an addition of 4 ~ 6phr. Calcium/zinc composite stabilizer is often used in environmental protection cable materials.
Selection of flame retardant
The flame retardancy and smoke emission of PVC cable materials will be different with different plasticizers. When the content of plasticizer increases, the oxygen index of PVC decreases. For different plasticizers, the speed of decline is basically the same. Therefore, a certain amount of flame retardant needs to be added to the formula. It is found that the flame retardancy of PVC cable material will be improved obviously with the increase of the amount of flame retardant.
- When talc and clay are added as fillers, the oxygen index increases by 1 to 2 times, but the combustion speed of the cable material also doubles, and the maximum light reduction coefficient remains unchanged.
- When Al (OH) 3 is added as a filler, the combustion speed increases by 50%, the oxygen index increases, and the maximum light reduction coefficient decreases, so it has a better effect on suppressing smoke emission.
- With the addition of CaCO3 filler, the oxygen index of PVC cable material decreases with the increase of the amount, and the maximum light reduction coefficient also decreases. Therefore, the amount of CaCO3 added should not be too large, otherwise the flame retardant and smoke suppression effect of the cable material will be affected. However, CaCO3 can be used as a trapping agent for HCl gas, which has a very significant effect.
Selection of lubricants
Since there is a large amount of plasticizer, the requirements for internal lubricant are not very high. Lubricant is mainly used to improve the surface brightness of cable material.
Metal soaps, stearic acid and paraffin are often used, and the addition amount is about 1phr.
Selection of filler
Adding filler into cable material can improve electrical insulation performance, heat resistance and reduce cost, but excessive amount will lead to deterioration of formability and cable material performance.
In order to improve insulation, calcined clay (Electrical Grade) can be selected as filler in insulation grade cable materials; Calcium carbonate can be selected as filler for sheath (layer) cable.