2. Crystallization
The crystallinity of PV C is generally around 5% and is considered to be an amorphous polymer. If a certain degree of crystallization can be produced in PVC products, the heat resistance of PVC will also be improved. The author's research group has conducted long-term and deep research on PVC materials. Firstly, the PVC is crystallized and modified to prepare a kind of PVC micropowder with the crystal area size in the nanometer level. Its crystal nucleus melting point is as high as 210 ℃. It does not melt during the molding process and is a "self-nucleating agent". The self-nucleating agent can crystallize PVC quickly and uniformly, and produce a large number of fine and uniformly dispersed crystallites in the system. These crystallites act as physical cross-linking points to make PVC strong, rigid, tough and heat resistant. Sexual improvement. The results of testing the heat resistance of PVC pipes and profiles added with this micropowder show that the Vicat softening point of the pipes is increased by 2--11 ℃, and the Vicat softening point of the profiles is increased by 2--10 ℃. The appearance of this result may be related to the nucleation of PVC micropowders containing nano-scale crystal regions during the melting and recrystallization process. Under this effect, the three-dimensional network structure of the crystal lattice in the system is established, and the role of PVC molecules is enhanced. Thermal stability can be improved.

3. Filled with inorganic particles
Inorganic fillers such as calcium carbonate, zeolite, marble, red mud, barium sulfate, iron white powder, etc. generally have a higher melting temperature, which can improve the thermal properties of polymer products when added to polymers. The surface of the red mud (red mud) is porous and filled into PVC. The interface between the red mud and the matrix resin is embedded to improve the mechanical properties of the material. At the same time, the free alkali and alkali metal oxides in the red mud can be neutralized Absorbs the Ha released from the thermal decomposition of PVC, which can inhibit further decomposition of PVC. In addition, there is research to fill hard polyvinyl chloride plastic with attapulgite. With the addition of attapulgite, the Vicat softening point has been significantly improved. When the addition amount of attapulgite exceeds 10 parts, the compactness of the material decreases and the Vicat softening point begins to decrease. Adding 10 parts of calcium carbonate to the formula of 120°C PVC cable compound can significantly improve the heat resistance of the cable. Adding calcium carbonate treated with ferric acid vinegar coupling agent to the PVC/CPE blending system can significantly improve the strength of the original system. When the filling amount of calcium carbonate reaches 15 parts and 30 parts, the Vicat softening point of the system It can reach 93 ℃ and 120 ℃ respectively. Under normal circumstances, the addition of inorganic fillers will cause the toughness of PVC to decrease, and excessive addition of inorganic fillers will cause "blooming". In order to meet the mechanical properties, it is impossible to add 30 parts of calcium carbonate to the PVC material, so it is not advisable to increase the content of inorganic particles in a large amount to improve the heat resistance.

4. Copolymerization
Copolymerization refers to the addition of monomers with high glass transition temperature, high polarity or large steric hindrance during the polymerization of vinyl chloride monomer (VCM).
With vinyl chloride, methyl methacrylate, acrylonitrile, N-substituted maleimide, etc., the softening temperature of the resulting copolymer will be significantly increased. Maleimide (MI) homopolymer and its derivatives are a kind of polymer with good effect in improving the heat resistance of PVC by blending method, but it is difficult to process, so it is often copolymerized with other monomers for heat resistance. Modifier [1,210 vinyl chloride and N-substituted maleimide are free-radically copolymerized, and the resulting random copolymer has excellent heat resistance and processability. With the increase in the amount of N-substituted maleimide (5%-15%), the glass transition temperature and heat distortion temperature of the copolymerized polyvinyl chloride resin show a linear upward trend, the melt viscosity of the resin decreases, and the processing performance When it becomes better, the impact strength is also improved to varying degrees. For example, a two-step suspension polymerization method is used to synthesize a copolymerized modified polyethylene resin (ChMI content of about 10%) and polyvinyl chloride resin are mixed, and the Vicat softening point of the product is 115 ℃. Mixing a small amount of heat-resistant acrylic resin into the N-substituted maleimide copolymer modified polyvinyl chloride resin can further improve the heat-resistant performance and processing performance of the polyvinyl chloride resin. For example, if 70 parts of ChMI-PVC copolymer resin is mixed with 30 parts of MMA-ChMI copolymer and 5.5 parts of additives, the Vicat softening point of the resulting blend is 142℃. If no MMA-ChMI copolymer is added, the Vicat softening point It is 110°C.
Graft copolymerization of N-substituted maleimide monomer on polyvinyl chloride resin or graft copolymerization of N-substituted maleimide polymer on polyvinyl chloride monomer. The copolymer has excellent heat resistance, Processability and processing fluidity. In order to further improve the heat resistance, impact resistance and processing performance of N-substituted maleimide resin modified polyvinyl chloride resin, a small amount of rubber, such as ethylene propylene diene rubber (EPDM), can be introduced into the resin. Graft copolymerization is carried out in the presence of rubber. For example, Japanese patent adopts suspension polymerization method to prepare EPDM-K-VC-ChMI (10/80/10) ternary graft copolymer, and add 3.5 parts of additives to 100 parts of the above resin , The Vicat softening point of the prepared product is as high as 123℃.
Due to the transformation of VCM polymerization equipment and process conditions, the feasibility is small when the current VCM polymerization process is very mature, and there are few studies and reports in this area. The blending method is to add a resin with a higher glass transition temperature to the PVC powder, and through the blending of the two polymers, the heat resistance of the PVC is improved. The process is simple, the implementation is convenient, and the cost is saved.

5. Chlorination
As far as the polymer structure is concerned, the introduction of polar or rigid groups and macromolecules in the main chain and side groups of the polymer can increase the heat distortion temperature, and at the same time regularize and chlorinate the chain structure. Etc. can improve the heat resistance of the product. The Vicat softening point of a CPVC system with a chlorine content of 64%-72% is greater than 110℃. When oxidized polyvinyl chloride is added to PVC, the Vicat softening point of the PVC material increases with the addition of CPVC content. The reason why it can improve the heat resistance of the material is because a large amount of Cl- is introduced in the main chain. On the one hand, it can increase the polarity of the polymer main chain and increase the intermolecular force; The movement of the polymer main chain is hindered, and the kinetic energy required for the movement and slippage of the molecular chain after the polymer material is heated is greatly increased, thereby improving the heat resistance of the material. However, if the chlorine content is too high (above 67.5%), CP VC and PVC will not be compatible, and the poor processing performance of CPVC, poor thermal stability, serious corrosion to equipment and other defects make the chlorination method unable to be an improved PVC material Excellent method of heat resistance.

6. Increase molecular mass
The above methods for improving the heat resistance of polyvinyl chloride are aimed at the polymer matrix of the same degree of polymerization, that is, the molecular mass and distribution of the polyvinyl chloride used are basically the same. However, the degree of polymerization of polyvinyl chloride has a great influence on the heat resistance of the product. The greater the degree of polymerization of polyvinyl chloride, the better the heat resistance and the worse the processability. Ultra-high molecular weight PVC can greatly improve heat resistance. A US company has developed a new type of PVC polymer whose molecular weight is 2 to 5 times that of conventional PVC. The relative molecular mass of conventional PVC is in the range of 31,000 to 75,000, while the relative molecular mass of the new brand is as high as Qiao million. In the mid-1990s, the company developed the first-generation brand and is currently developing ingredients for ultra-high polymer quality PVC. Applications include transparent hoses for medical peristaltic pumps, automotive polishing sealants, boots for cleaning hazardous environments, housings for manual resuscitators used in sub-zero temperature environments, outdoor power lines, and other wires and cables. This kind of ultra-high molecular weight PVC also has flame retardancy and prevents chemical penetration.


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