Influence of main indexes of chlorinated polyethylene (CPE) on modification of rigid PVC (1)

Polyvinyl chloride (PVC) is widely used in chemical industry, building materials and other fields because of its advantages of high strength, flame resistance and chemical corrosion resistance, and is the second largest general plastic in the world today. However, the toughness of PVC is low, so that its application is limited, so it must be toughened and modified. The commonly used impact modifiers for PVC profiles are chlorinated polyethylene (CPE) and acrylic ester (ACR) impact modifiers. Due to the high cost performance of CPE, more than 80% of domestic PVC-U products use CPE as impact modifier.

The effect of CPE toughening and modifying PVC is mainly related to the CPE raw material, chlorine content and preparation conditions, and the best toughening effect is when the dosage of CPE is 6 ~ 15 parts. CPE raw material HDPE high molecular weight, generally slow plasticizing, toughening effect is good; Low molecular weight, general plasticizing fast, suitable for high-speed extrusion. When used for toughening and modifying PVC-U products, CPE has superior comprehensive properties when the chlorine content is about 36%. The practice shows that the main performance indexes of CPE have an effect on the modification effect of PVC profile, especially the distribution of chlorine atoms on the molecular chain of CPE and the melting enthalpy indexes have a great effect on the physical properties and processing properties of CPE and PVC blends. Therefore, HG/T 2704-2010 "Chlorinated polyethylene" industry standard stipulates nine chlorinated polyethylene quality index requirements, and the following are discussed on its impact on PVC modification.

1.Effect of CPE with different heat of fusion on PVC modification

The difference in performance of CPE products with the same chlorine content produced from the same raw material is mainly caused by the difference in melting enthalpy (residual crystallinity) and the distribution of chlorine atoms. The distribution of chlorine atoms in the macromolecular chain is related to the chlorination process, chlorination conditions (temperature, pressure, chlorination reaction rate, etc.), and the distribution of chlorine atoms determines the enthalpy of melting of CPE. Chlorine distribution is greatly affected by raw materials and chlorination process conditions. In the production and preparation of CPE chlorination, the chlorination reaction temperature is as high as possible within the melting point of raw polyethylene and the appropriate amount of chlorination can make the chlorination reaction uniform, the distribution of chlorine is more dispersed, and the crystallization of raw HDPE disappears, so as to obtain a good rubber CPE. If a relatively low reaction temperature is selected for chlorination, the distribution of chlorine on the PE molecule can be uneven, and the partial crystallization state can be maintained, although the rubber property is reduced, the special purpose CPE can maintain the good electrical properties of PE.

Therefore, chlorinated polyethylene can be different according to the chlorination process, by changing the process conditions to control the distribution of chlorine, although the chlorine content is the same, but will get non-crystalline rubber elastomer and moderate crystallization of different properties of the product. Usually its crystallinity is greater than 15% for the crystalline type, such CPE has good strength and toughness, often used as a thermoplastic resin, (can also be used as a plastic modifier). When the crystallinity of CPE is less than 5%, such products are amorphous, with good elasticity and low temperature properties, and are generally used as impact modifiers and special synthetic rubber.

In order to make a good elastic CPE product, it is necessary to destroy most of the crystallization of HDPE to form an amorphous structure when chlorinated, but it is difficult to destroy all the crystallization of HDPE, can not destroy the crystallization called residual crystallization, the measurement index is the residual crystallinity or melting enthalpy, the detection method has dissolution method and DSC method. Generally, CPE impact modification products with residual crystallinity below 3 ~ 6% (DSC detection of crystallization melting enthalpy below 2 ~ 4J/g) have the best comprehensive performance [1]. In the newly revised chemical industry standard, the residual crystallinity of CPE is determined by DSC and expressed as the enthalpy of CPE melting. Melting enthalpy (DSC) is a commonly used method at present, and it is also an important test item in the current CPE standard, which is an important index affecting the modification performance of CPE.

In the range of 0.6 ~ 4.7J/g, the plasticization time decreases and the plasticization increases with the increase of the melting enthalpy, but the difference is not great. The minimum torque also increases with the increase of melting enthalpy value. The maximum torque and balance torque of the sample are basically the same, which may be related to the same HDPE raw materials and the same molecular weight.

Through the detection of notch impact strength, tensile impact strength and welding strength of PVC/CPE composites with CPE samples with different melting enthalpy, it can also be found that with the increase of the enthalpy of CPE melting, the properties of composite materials with different melting enthalpy of CPE show a downward trend, and when the enthalpy of CPE melting exceeds 2.3J/g, the properties of PVC/CPE composites with different melting enthalpy decrease. The tensile impact strength of the resulting composite is slightly less than 600 KJ/m2. This may be because the melting enthalpy of CPE is large, the PE chain segment is more, and the compatibility with PVC is poor. The effect of melting enthalpy on welding strength is relatively small, which may be because of fast plasticization and high plasticization degree is beneficial to welding strength.

The different melting enthalpy of CPE has an effect on the plasticizing time of PVC/CPE system and the properties of composites. CPE with chlorine content of about 36% and melting enthalpy (DSC) ≤ 1.4J /g is more suitable as an impact modifier for PVC. The enthalpy of melting (DSC), as an index to measure the performance of CPE, is of practical guiding significance for the processing of PVC plastics.

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