[Polyethylene wax] PVC pipe extrusion process

  • Categories:News Center
  • Time of issue:2020-05-26 19:36

[Polyethylene wax] PVC pipe extrusion process

  • Categories:News Center
  • Time of issue:2020-05-26 19:36

PVC pipe extrusion process Chlorinated polyvinyl chloride resin (hereinafter referred to as cpvc) was successfully developed by Bfgoodrich in 1958. It is a chlorinated product of polyvinyl chloride resin (pvc), which has been chlorinated PVC chlorine content can be increased from 57% to 64%-75%. As the chlorine content increases, the corresponding temperature resistance, rigidity, chemical resistance, flame retardancy, smoke suppression, etc. can be enhanced, but the cpvc resin melts Increased viscosity, increased brittleness, decreased impact strength, and decreased thermal stability bring great difficulty to the extrusion processing of cpvc. At the same time, if the process is not properly controlled, a large amount of hcl gas will be generated and the processing equipment and molds will be severely corroded. Therefore, the extrusion process of cpvc resin has always been the focus of discussion. In the processing of cpvc resin, "plasticization" is the key. Therefore, how to obtain a good plasticization quality from the formula, equipment and process technology is the key to the processing of cpvc resin.

One, the formula of chlorinated polyvinyl chloride (pvc-c) pipe

1. Heat stabilizer cpvc resin is easier to decompose than pvc during processing, so more heat stabilizer than pvc needs to be added. During trial production, it can be added at 1.5 to 2.5 times the amount of pvc, and then appropriate according to the trial production situation The general principle of increase or decrease is: the heat stabilizer must not be added too much, otherwise it will affect the plasticization of the cpvc resin. Basically, all heat stabilizers for processing PVC can be processed cpvc, such as lead salt stabilizers, calcium-zinc stabilizers, composite lead stabilizers, metal soap stabilizers and liquid organic tin stabilizers, among many heat stabilizers When the pipes have different special requirements, the selection is different. When producing sanitary cpvc pipes, calcium-zinc stabilizers or organotin stabilizers should be used; when producing pipes with requirements for temperature resistance, try their best Avoid using liquid organic tin stabilizers; do not add calcium stearate when producing high-impact pipes.

2. Lubricant The role of lubricant is as important as the role of heat stabilizer. Too little addition will cause processing difficulties and even cause considerable damage to the equipment. Too much addition will not only affect the quality of plasticization, but also It will lower the Vicat softening temperature of the pipe, so how to master the amount of lubricant added is the key, and it is also the key to the design of the cpvc pipe formula. It can even be said that the design of the cpvc pipe formula is actually the design of the lubrication system. The melt viscosity of cpvc resin is large, at least twice that of pvc resin [1], so the amount of lubricant added is slightly more than that of pvc resin. When using different stabilizers, the ratio of internal and external lubricants in the formula is different. Generally speaking, the use of lead salt stabilizers requires more internal lubricants and appropriate external lubricants; use organic The tin stabilizer needs to be equipped with more external lubricants and an appropriate amount of internal lubricant; when using a composite stabilizer with a lubricating system, the amount of internal and external lubricants should be appropriately reduced. The internal lubricant can be stearic acid (hst), and the external lubricant should be high temperature paraffin wax, pe wax or oxidized polyethylene wax. Generally, paraffin wax should not be used as the external lubricant of cpvc resin alone. In short, if the ratio of internal and external lubricants is coordinated, although the formula cannot be said to be a mature formula, at least there will be no problems in extrusion molding.

3. Impact modifiers and processing aids. Because of the high chlorine content of cpvc resin, its impact resistance is worse than that of pvc resin. It is modified by adding impact resistance agent. Commonly used impact resistance agents are: cpe, mbs, abs And impact-resistant acr, etc., adding cpe in the processing of cpvc pipes has a very obvious toughening effect. The addition of 8-12 parts has a good toughening effect, but adding cpe will significantly reduce the Vicat softening temperature of the pipe; mbs and abs have little effect on Vicat softening temperature, but because their molecular chains contain unsaturated double bonds, they have poor weather resistance; impact-resistant acr is an ideal impact modifier, but the price is higher. Based on the above reasons, the author recommends using a mixture of cpe/mbs as an impact modifier, which can take into account the product's requirements for Vicat softening temperature, impact resistance, weather resistance, and price. The ratio of the two is 1.5 :1 is appropriate. At present, the processing aid used in China is mainly acr, which is a product of several acrylic compounds mixed together. It can increase the degree of plasticization of the resin, but the cpvc resin does not need to rely on the molecules due to its high melt viscosity. Other additives to increase frictional heat, it is easier to plasticize than pvc resin, so when extruding cpvc pipes, you should add less or no acr accelerator.

4. Other additives Other additives include fillers and colorants. Filler Adding an appropriate amount of filler can not only reduce costs, but also improve processing performance. It can reduce the processing viscosity of cpvc resin, improve the dispersion of the mixture, and reduce the adhesion to equipment and molds during extrusion processing. At the same time, as a filler for rigid particles, it can also improve the impact resistance of cpvc pipes. The commonly used filler is calcium carbonate. The dosage should not exceed 5 parts, otherwise it will affect the performance of the product. The coloring agent cpvc pipe is slightly darker than pvc pipe, and it is off-white, but you can add some colorants as needed. A small amount of rutile titanium dioxide should be added at the same time as the colorant to cover up the gray color of the cpvc pipe and reduce its impact on the colorant.

2. Processing technology of chlorinated polyvinyl chloride (pvc-c) pipe

1. Extruder Parallel or conical twin-screw extruder is usually used for extrusion processing of cpvc pipe. In view of the characteristics of cpvc that is easier to plasticize than pvc, it is easier to control the extrusion production of cpvc pipe by using parallel twin-screw extruder. If a lead salt stabilizer is used in the formulation, the extruder must have better plasticizing performance; if the formulation uses organotin as a stabilizer, the compression ratio of the extruder screw must not be too large.

2. The mixing process of processing materials. The mixing process of cpvc resin is the same as that of pvc resin. It requires two processes: high-speed mixing and low-speed cooling and stirring. Generally, the high-speed mixing temperature should be controlled at 115~125℃, not too high, otherwise it will be easy to mix yellow. The material causes decomposition of the material or the phenomenon of "over plasticization" during the extrusion process. The temperature of low-speed cooling and stirring should be controlled at 40~50℃, and it should not be too high. Otherwise, the mixed materials will absorb too much moisture in the air and affect the performance of the products due to the large difference between the materials and the room temperature. Extrusion temperature The focus of the extrusion process of cpvc pipe is the process temperature, which will directly affect the plasticization quality of the pipe. Generally, the process temperature will vary greatly due to the plasticizing performance of the extruder, and sometimes the difference will be 20-30°C. Theoretically, the processing temperature of cpvc material is higher than that of pvc, but in fact, according to our many years of processing experience, the processing temperature of cpvc is 5~8℃ lower than that of pvc. This is because the melt viscosity of cpvc is higher than that of pvc. There will be a lot of frictional heat between them. At this time, if the extruder provides too much heat to it, it will easily cause the decomposition of the material. In the process temperature setting, the curve should be kept as smooth as possible, which is conducive to the plasticization quality of cpvc resin. The curve up and down is not conducive to the plasticization of the pipe. Generally, the "u" shape of the curve is more suitable for the entire process temperature The control can be roughly divided into three parts: barrel, confluence core and mold. The temperature of the barrel decreases from the first zone. The temperature of the confluence core is slightly lower than the temperature of the barrel. In the mold temperature setting, it is worth noting The temperature of the die is the temperature of the die and the temperature of the core die. The temperature of the die should be about 10°C lower than the temperature of the heating section of the barrel, otherwise it will affect the longitudinal shrinkage rate of the pipe. Due to this limitation, the core mold heating can be disconnected after the pipe is normally pulled out, and the heat of the cpvc melt and the heat generated by the friction against the core mold can maintain the temperature of the core mold.

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