CPVC pipe extrusion processCPVC pipe extrusion process
Chlorinated polyvinyl chloride resin (hereinafter referred to as CPVC) was successfully developed by BF Goodrich in 1958. It is a chlorinated product of polyvinyl chloride resin (PVC). The chlorine content of chlorinated PVC can be 57% to 64% to 75%, with the increase of chlorine content, the corresponding temperature resistance, rigidity, chemical resistance, flame retardancy, smoke suppression, etc. can be enhanced, but the melt viscosity of CPVC resin increases, brittleness Increased, reduced impact strength, and reduced thermal stability have brought 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 mold will be severely corroded. The issue of extrusion processing has always been the focus of discussion. In the processing of CPVC resin, "plasticization" is the key. Therefore, how to get good plasticization quality from the formula, equipment and process technology is the key to the processing of CPVC resin.
1. Formula of chlorinated polyvinyl chloride (PVC-C) pipe
1. Heat stabilizer
CPVC resin is easier to decompose than PVC during processing, so it needs to add more heat stabilizer than PVC. During trial production, it can be added according to 1.5 to 2.5 times the amount of PVC added, and then increase or decrease according to the trial production situation. The total The principle is: Do not add too much heat stabilizer, otherwise it will affect the plasticization of CPVC resin. Basically, heat stabilizers for processing PVC can process CPVC, such as lead salt stabilizers, calcium zinc stabilizers, compound lead stabilizers, metal soap stabilizers and liquid organic tin stabilizers, among many heat stabilizers. , When the pipe has different special requirements, there is a difference in the selection. When producing hygienic grade CPVC pipe, non-toxic calcium zinc stabilizer or organic tin stabilizer should be used; when producing pipes that require temperature resistance , Should try to avoid the use of liquid organic tin stabilizers; when producing high impact pipes, do not add calcium stearate.
The role of the lubricant is as important as the role of the heat stabilizer. If the amount is too small, it will cause processing difficulties and even cause considerable damage to the equipment. If the amount is too much, it will not only affect the quality of plasticization, but also reduce the pipe Vicat softening temperature, so how to master the amount of lubricant is the key to the design of CPVC pipe formulations. It can even be said that the design of CPVC pipe formulations is actually the design of the lubrication system. The melt viscosity of CPVC resin is large, at least twice that of PVC resin , so the amount of lubricant added is slightly more than PVC resin. When using different stabilizers, the ratio of internal and external lubricants in the formulation is different. Generally speaking, the use of lead salt stabilizers requires the configuration of more internal lubricants and an appropriate amount of external lubricants; use organic Tin stabilizers need to be equipped with more external lubricants and appropriate amounts of internal lubricants; when using compound stabilizers with a lubrication 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, PE wax or oxidized polyethylene wax. Generally, paraffin wax should not be used alone as the external lubricant of CPVC resin. In short, the coordination between the internal and external lubricants is well coordinated, so although the formula can not be said to be a mature formula, at least there will be no problems in extrusion molding.
3. Impact modifier and processing aid
Due to the high chlorine content of CPVC resin, its impact resistance is worse than that of PVC resin. It must be modified by adding an impact agent. Commonly used impact agents are: CPE, MBS, ABS and impact ACR, etc., in CPVC pipe When CPE is added in the processing, its toughening effect is very obvious, and the addition amount is between 8 and 12 parts. The toughening effect is the best, but the addition of CPE will significantly reduce the Vicat softening temperature of the pipe; MBS and ABS affect the Vicat softening temperature Smaller, but because their molecular chains contain unsaturated double bonds, weather resistance is poor; impact ACR is an ideal impact modifier, but the price is higher. Based on the above reasons, the author recommends the use of CPE / MBS mixture 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. It is a product of several acrylate compounds mixed together. It can improve the degree of plasticization of the resin. However, due to the large melt viscosity of CPVC resin, there is no need to resort to between molecules. Other additives to increase frictional heat, it is easier to plasticize than PVC resin, so when extruding CPVC pipes, ACR type accelerator should be added with little or no addition.
4. Other additives
Other additives include fillers and colorants.
Adding the right amount of filler can not only reduce the cost, but also improve the processing performance. It can reduce the processing viscosity of CPVC resin, improve the dispersion of the mixture, reduce the adhesion to the equipment and mold during extrusion processing, and at the same time, as a filler of rigid particles, it can also improve the impact resistance of CPVC pipes. The most commonly used filler is calcium carbonate, the amount added should not exceed 5 parts, otherwise it will affect the performance of the product.
CPVC pipes are slightly darker than PVC pipes, and are grayish white, but some colorants can be added as needed. A small amount of rutile titanium dioxide should be added at the same time as the colorants are added to cover the gray of CPVC pipes and reduce their colorants. Impact.
2. Processing of chlorinated polyvinyl chloride (PVC-C) pipes
Extrusion processing of CPVC pipes usually uses parallel or conical twin screw extruders. In view of the fact that CPVC is easier to plasticize than PVC, it is easier to control the extrusion production of CPVC pipes by using parallel twin screw extruders. If a lead salt stabilizer is used in the formula, the extruder is required to have better plasticization performance; if the formula uses organotin as the stabilizer, the compression ratio of the extruder screw is not too large.
2. Processing technology
2.1 Mixing of materials
The mixing process of CPVC resin is the same as that of PVC resin, which requires two processes of 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 materials during the extrusion process. Cause the decomposition of the material or the phenomenon of "overplasticization". The temperature of low-speed cooling and stirring is controlled at 40 ～ 50 ℃, and it should not be too high, otherwise the mixed materials will have too much relationship with room temperature, so that the materials absorb too much moisture in the air and affect the performance of the product.
2.2 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. The general process temperature will be very different due to the different plasticizing properties of the extruder, and sometimes this difference will be 20 ~ 30 ℃. Theoretically, the processing temperature of the CPVC material is higher than that of PVC, but in fact, according to our years of processing experience, the processing temperature of CPVC is 5-8 ° C lower than that of PVC. This is because the melting viscosity of CPVC is higher than that of PVC, and the molten molecules A large amount of frictional heat will be generated between them. At this time, if the extruder provides it with too much heat, it is easy to 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, and the curve is not conducive to the plasticization of the pipe. The general curve is "U" shape is more suitable. The control of the entire process temperature can be roughly divided into three parts: the barrel, the confluence core and the mold. The barrel temperature gradually decreases from one zone, and the temperature of the confluence core is slightly lower than the barrel temperature. In the temperature setting of the mold It is worth noting the temperature of the die and the temperature of the core die. The temperature of the die should be about 10 ℃ lower than the temperature of the heating section of the barrel, otherwise it will affect the longitudinal shrinkage rate of the pipe, and there is no requirement for the longitudinal shrinkage rate. Pipes are not subject to this limitation. The heating of the core mold can be disconnected after the pipe is pulled out normally. The heat of the CPVC melt and the heat generated by friction against the core mold can fully maintain the temperature of the core mold.