"Plasticization" is a key in the process of processing […]
"Plasticization" is a key in the process of processing from CPVC resin into finished or semi-finished products. Whether it is extrusion molding or injection molding, the quality of plasticization is directly related to the intrinsic quality and surface quality of the product. Therefore, how to obtain good plasticizing quality from the process, equipment and technology is the key to CPVC processing.
The following points can help us understand how to improve the quality of plasticizing during CPVC processing:
First, the molecular formula of CPVC
CPVC (PVC-C) is a chlorination product of PVC, which is a chlorination modification of polyvinyl chloride. The modified PVC can achieve two different purposes:
1: Increase the thermal deformation temperature, increase the rigidity, improve the chemical stability, improve the weather resistance and flame retardancy, and reduce the smoke density. This product is usually used to make products for occasions with special requirements;
2: Improve the solubility of the material. This product, commonly known as perchloroethylene, is mainly used in the production of adhesives, coatings and polyvinyl chloride.
The chlorination process of CPVC is an aqueous phase suspension (slurry) chlorination process. Its molecular structure is: -CHCl-CHCl-CH2-CHCl-
The performance of CPVC is determined by two factors: the chlorine content and the distribution of chlorine on the CPVC molecular chain. Therefore, CPVC with the same chlorine content will have a large performance difference due to the different distribution of chlorine atoms.
Second, during the molding process
Relevant factors to improve the quality of plasticization
Since the melt viscosity of CPVC is at least twice that of PVC, the processing temperature is high, and the release of hcl due to thermal decomposition during processing causes difficulty in processing and corrosion of equipment.
In the extrusion process, how to achieve the ideal "plasticization" of CPVC, a high-viscosity material (plasticization in place) is the key to CPVC processing technology.
Therefore, it has special requirements for the formula:
Heat stabilizer requirements
Due to the high processing temperature of CPVC, the amount of heat stabilizer used in the formulation is much higher than that of PVC. Obviously, the use of the conventional three-salt, two-salt heat stabilizer is not suitable. At present, the more mature heat stabilizer is a composite lead series stabilizer with a lubricating system.
Since CPVC has a high melt viscosity, particularly melt fracture in injection molding, it is not suitable to use only conventional paraffin, stearic acid and metal soap lubricating systems.
In the extrusion processing of CPVC, since CPVC has a metal surface which tends to adhere to the thermal post-processing equipment (especially the head and the mold), in order to eliminate such adhesion, an external lubricant must be added to the formulation. The external lubricant should be incompatible with the CPVC resin.
In the CPVC molding process (especially injection processing), under the action of pressure, the mutual friction between the CPVC resins generates frictional heat, which is unfavorable for the molding process and must be controlled. The internal lubricant can reduce the friction between the resin during the processing of CPVC. The internal lubricant should be compatible with the CPVC resin.
It should be noted that the balance of internal and external lubrication in the formulation is critical. Excessive internal lubrication will seriously affect the “plasticization”, which will deteriorate the quality of the product; excessive external lubrication will cause the lubricant to precipitate, and even the screw slip will seriously affect the normal production. The OP wax (brown coal ester wax) in the saponified wax is an ideal internal and external composite lubricant.
The effect of processing aids on improving plasticizing quality
In the extrusion and injection molding processing of CPVC, processing aids must be used, the purpose of which is to improve the plasticizing quality and increase the low temperature impact resistance of the CPVC material and the toughness of the product.
In the case of CPVC, even at a viscous flow temperature (such as 195-205 ° C), its flow unit is still a primary particle, the interaction between the fine particles of the resin is poor, the heat transfer effect is poor, and melt fracture is easily generated. Poor quality.
ACR is a processing aid that promotes plasticization. It can be dispersed in CPVC melt into a network structure unit with a size of less than 0.01μm, and evenly distributed among CPVC fine particles, which is increased by shear force. The friction between CPVC particles promotes the heat and mass transfer of the molten system, increases the torque, accelerates the plasticization process, and improves the quality of plasticization.
CPVC resin has a large melt viscosity, large processing torque, difficulty in plasticizing, and easy decomposition. Therefore, the choice and ratio of internal and external lubricants is another key to the design of CPVC pipe and pipe fittings.
The use of different heat stabilizers, the application of different processing methods, the ratio of the internal and external lubricants in the formulation is different.
In general, the use of lead stabilizers requires the preparation of more internal lubricants and suitable external lubricants; the use of organotin stabilizers requires the preparation of more external lubricants and appropriate internal lubricants; extruded tubing The required internal lubrication brake is more than the injection tube, and the injection tube requires more external lubricant than the extruded tube.
The amount of lubricant added must be appropriate, not too much, too much lubricant will not only affect the plasticization of the melt, but also because the melting point of the lubricant is generally low, it will also reduce the Viby softening temperature of the product.
It is worth noting that in the formulation, besides the organotin stabilizer, it is forbidden to add any liquid lubricant, as this will significantly reduce the Weibo softening temperature and other physical and mechanical properties of the pipe and fittings.
The effective internal lubricant should have a good compatibility with the melt, can significantly reduce the viscosity of the melt, and can promote the plasticization of the melt; some high-temperature paraffin can significantly reduce the resistance of the melt flowing in the mold. It is an ideal external lubricant that has less influence on the plasticization characteristics of the fusion.
Third, CPVC requirements for special PVC resin
Because CPVC products have high temperature brittleness and poor impact resistance, only the adjustment of formula and process can increase the toughness of CPVC products and improve their impact resistance.
Requirements for special PVC resin for the production of CPVC resin: CPVC resin is a product of chlorination modification of PVC resin. In the production process, in order to improve the comprehensive performance of CPVC resin, in addition to improving chlorination conditions and improving the level of chlorination technology The use of special PVC resin is also crucial. According to the successful experience of B.F. Goodrich in the United States, the production of special PVC resin is to add special additives in the VC suspension polymerization formula to achieve the following basic requirements:
1. The structure of the special PVC resin should be as loose as possible to facilitate the chlorination reaction.
2. The film of the special PVC resin is as thin as possible to facilitate the deepening of chlorine, avoiding the poor fluidity caused by high surface chlorine content, large viscosity, and difficult processing.
3. The specific surface area and porosity of PVC special resin should have a suitable ratio, which can be determined by analyzing the product of B.F. Goodrich.