PVC is the second largest yield universal plastics after PE. PVC has plasticity, corrosion resistance, flame retardant, insulation, transparent and other advantages, has a wide range of applications. PVC could be divided in to hard/rigid PVC, semi-rigid PVC, soft/flexible PVC. The mass fraction of chlorine is 56.8% and the oxygen index is 45%, so the hard products usually do not need to add flame retardant to achieve better flame retardant effect. Besides PVC material has high insulation performance. As a typical soft/flexible material, PVC compound adjusts its softness by adding different amount of plasticizer. But the amount of plasticizer shall decrease the mass fraction of chlorine and oxygen index. For example, when adding 60 hr plasticizer to 100 phr PVC, the mass fraction of chlorine shall be 36% and the oxygen index decrease to 22%. Cable as an important part of the power system and the stable operation of mechanical equipment, Once a fire breaks out, it will cause serious losses, and even directly endanger people’s life safety. So it is crucial to develop flame retardant cable compound.

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1. The mechanism and methods flame retardant and smoke suppression for cable compound

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During combustion, after the removal of HCl, PVC generated conjugated double bond unsaturated hydrocarbon, and then carbon chain break into low molecular products, free radicals, etc., molecular rearranged, cyclization to form benzene core, polymerized into thick ring aromatic structure resin, and finally become coke and graphite carbon particles. The final pyrolysis products of PVC include HCI, polyene, benzene, aromatic compounds, low molecular alkenes and alkanes, etc. There are sufficient amounts of intermediate products involved in pyrolysis, thus forming black smoke.

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In PVC cable, we usually add appropriate amount of flame retardant plasticizer and flame retardant to achieve smoke suppression and flame retardant effect. Common methods are as follows. First, the flame retardant plasticizer can be used to replace the conventional plasticizer.  The common used plasticizer for PVC compound including alkyl diaryl phosphate and triaryl phosphate ester. Secondly, by adding metal hydroxide, antimony compound, borate and other inorganic flame retardants in PVC cable material, could accelerate the formation of carbon layer in the combustion process, which plays a solid phase flame retardant role, and can effectively reduce the number of flue gas and soot formation, so that the cable compound can improve the cable’s flame retardant and smoke suppression performance. Finally, smoke suppressants such as ammonium octomolybdate and molybdenum trioxide can be added to promote the formation of carbon residue and the reduction of smoke density, mainly according to the mechanism of condensed phase and heterogeneous reaction.

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2. The processing technique of PVC cable compound

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Combined with the current production process of PVC cable compound in China, It can be divided into the following steps, first, you need to develop a reasonable proportion of relevant materials based on requirement, then high speed mixing all the material, so that each material can be fully mixed together. And then use screw extruder and cold mixing extruder to carry out the granulation work, the last is the product packaging processing. After the above process can finally get PVC cable compound. Although the process is relatively simple, it need strict process control and requirements.

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2.1 Kneading and cold mixing

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In the kneading process, after adding PVC resin, stabilizer, using the method of low speed mixing, stirring it for a few minutes, then adding plasticizer and color paste, followed by high speed mixing, in this process you need to gradually add other plasticizer materials. After the mixing temperature reaches up to 92℃, flame retardant, filler, lubricant and other substances are added and stirring all the materials until the temperature reaches 105℃. Under normal circumstances, according to the proportion of plasticizer added in the formula, the high speed mixing time is 8~15 min; Then it is necessary to place the material in the cooling machine for stirring, stirring time control in about 15 minutes, to ensure the final discharge temperature of PVC cable material in 30~40 ℃.  

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2.2 Plasticizing and granulating with an extruder

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In the plasticizing granulation stage, the two-step twin screw extrusion granulation unit is adopted to carry out the cable compound manufacturing. In the first stage of plasticizing granulation, an intermeshing co-rotating twin screw extruders can be used, and the equipment itself is also equipped with a forced feeder. In the second stage, single screw extruder with smaller L/D can be used to carry out related work. The cylinder molding temperature of these two machines are different, they can fully plasticized the cable compound. In the process of plasticizing granulation, we should pay attention to the control of feeding temperature, which is the key to ensure the dry mixed powder quality of cable compound.

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This article referenced to 《综合性能优良的阻燃PVC电缆料研究》 by Hu Ling. Feel free to contact Elena for more information.

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