【Popular science stickers】Analysis of the processing technology of CPVC pipe

In the process of processing and shaping CPVC resin into finished or semi-finished products, "plasticization" is a key. Whether it is extrusion molding or injection molding, the quality of plasticization is directly related to the internal quality and surface quality of the product. Therefore, how to obtain good plasticization 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 plasticization during CPVC processing:

1. The molecular formula of CPVC

CPVC (PVC-C) is a chlorination product of PVC. It 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, and at the same time improve the weather resistance and flame resistance, and reduce the smoke density. This product is usually used to make products for occasions with special requirements;

2: Improve the solubility of materials. This product is commonly known as perchlorovinyl, and is mainly used for the production of adhesives, coatings and chlorinated fiber.

The chlorination method of CPVC is the aqueous suspension (slurry) chlorination method. Its molecular structure is: -CHCl-CHCl-CH2-CHCl-

The performance of CPVC is determined by two factors: the content of chlorine and the distribution of chlorine on the CPVC molecular chain. Therefore, CPVC with the same chlorine content will have a larger performance difference due to the different distribution of chlorine atoms.

2. 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 has caused difficulties in processing and molding and corrosion of equipment.

In the extrusion process, how to make CPVC, a high-viscosity material, achieve the ideal "plasticization" (plasticization in place) is the key to CPVC processing technology.

Therefore, it has special requirements for the formula:

Requirements for heat stabilizers

Due to the high processing temperature of CPVC, the amount of heat stabilizer in the formulation is much higher than that of PVC. Obviously, it is not suitable to use traditional three-salt and two-salt heat stabilizers. At present, the more mature heat stabilizer is the composite lead series stabilizer with lubricating system.

Requirements for lubricants

Due to the high melt viscosity of CPVC, especially melt fracture during injection molding, it is not appropriate to use traditional paraffin, stearic acid and metal soap lubricating systems.

In the extrusion process of CPVC, because CPVC has a tendency to adhere to the metal surface of thermal post-processing equipment (especially the head and die), in order to eliminate this adhesion, an external lubricant must be added to the formulation. The external lubricant and CPVC resin should be incompatible.

In the CPVC molding process (especially the injection process), under pressure, the friction between the CPVC resins generates frictional heat. The generation of frictional heat is unfavorable to the molding process and must be controlled. The internal lubricant can reduce the friction between the resins of CPVC during processing. The internal lubricant and CPVC resin should be compatible.

It should be pointed out that the balance of internal and external lubrication in the formulation is the key. Excessive internal lubrication will seriously affect "plasticization" and deteriorate the quality of products; excessive external lubrication will cause lubricant to precipitate, and even lead to screw slippage and other phenomena that seriously affect normal production. OP wax (montan ester wax) in saponified wax is an ideal internal and external compound lubricant.

The effect of processing aids on improving the quality of plasticization

In the extrusion and injection molding of CPVC, processing aids must be used, the purpose of which is to improve the quality of plasticization and increase the low-temperature impact resistance of CPVC materials and the toughness of products.

As far as CPVC is concerned, even at the temperature of viscous flow state (such as 195～205℃), its flow unit is still primary particles, and the interaction between the fine resin particles is poor, the heat transfer effect is poor, and the melt fracture is easy to occur. Poor chemical quality.

ACR is a processing aid that promotes plasticization. It can be dispersed in the CPVC melt into a network structure unit with a size of less than 0.01μm, and evenly distributed among the CPVC fine particles, increasing under the action of shearing force. The friction between the CPVC particles promotes the heat and mass transfer of the molten system, increases the torque, speeds up the plasticization process, and improves the quality of the plasticization.

The melt viscosity of CPVC resin is large, the processing torque is large, the plasticization is difficult, and it is easy to decompose. Therefore, the selection and ratio of internal and external lubricants is another key to the formulation design of CPVC pipes and fittings.

Using different heat stabilizers and applying different processing methods, the ratio of internal and external lubricants in the formulation is different.

Generally speaking, the use of lead stabilizers requires more internal lubricants and appropriate external lubricants; the use of organotin stabilizers requires more external lubricants and appropriate internal lubricants; extruded pipes More internal lubrication brakes are required than injection pipe fittings, and injection pipe fittings require more external lubricant than extrusion pipe fittings.

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 very low, it will also reduce the Vib softening temperature of the product.

It is worth noting that in the formula, in addition to organotin stabilizers, do not add any liquid lubricants, because this will significantly reduce the Vibr softening temperature and other physical and mechanical properties of the pipes and fittings.

An effective internal lubricant should have the characteristics of good compatibility with the melt, can significantly reduce the melt viscosity, and promote the plasticization of the melt; some high-temperature paraffins can significantly reduce the resistance of the melt to flow in the mold. It is an ideal external lubricant with the characteristics of plasticization which has little effect on melting and rest.

Three, CPVC requirements for special PVC resin

Because CPVC products are brittle at low temperature and poor impact resistance, only through the adjustment of formula and process can the toughness of CPVC products be increased and their impact resistance can be improved.

Requirements for the special PVC resin for the production of CPVC resin: CPVC resin is the product of PVC resin modified by chlorination. In the production process, in order to improve the comprehensive performance of CPVC resin, in addition to improving the chlorination conditions and the level of chlorination technology , It is also very important to use special PVC resin. According to the successful experience of BFGoodrich in the United States, the production of special PVC resin is to add special additives to the VC suspension polymerization formula to meet 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 and avoid the poor fluidity, high viscosity and processing difficulties caused by the high surface chlorine content.

3. The specific surface area and porosity of PVC special resin should have an appropriate ratio, which can be determined by analyzing BFGoodrich's products.

Performance parameters of special PVC resin for the production of CPVC resin (for reference)

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