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PVC (Polyvinyl Chloride) is a polymer, by polymerization of vinyl chloride monomer (VCM) according to the free radical polymerization reaction mechanism, under the effect of peroxide, azoic compound or other initiators, or under the effect of light or heat. PVC is China’s first largest and the world’s second largest synthetic resin material for general purposes. Due to its excellent flame retardance, abrasion resistance, chemical corrosion resistance, comprehensive mechanical properties, product transparency, electrical insulating property and other features, at present, PVC has become one of the plastic varieties most widely applied in such fields as industry, building, agriculture, daily life, packaging, electricity and public utility; generally called Five Resins for general purposes together with polyethylene (PE), polypropylene (PP), polystyrene (PS) and ABS.
According to different applications, PVC can be classified into PVC resins for general purposes, PVC resins of high polymerization and cross-linking PVC resins. PVC resins for general purposes are polymerized by VCM under the effect of an initiator; PVC resins of high polymerization refer to resins polymerized after addition of a chain extender to the VCM system; cross-linking PVC resins refer to resins polymerized, after addition of diene- and polyene-containing cross-linking agents to the VCM system.
The VCM can be obtained by the calcium carbide method and the ethylene method.
According to the plasticizer content, PVC alkene-based plastics can be classified into: non-plasticized PVC, the plasticizer content of which is 0; hard PVC, the plasticizer content of which is less than 10%; semi-hard PVC, the plasticizer content of which is 10-30%; soft PVC, the plasticizer content of which is 30-70%; PVC paste resin, the plasticizer content of which is more than 80%.
PVC is prepared by the free radical addition polymerization method, mainly including suspension polymerization method, emulsion polymerization method, solution polymerization method and bulk polymerization method, where the suspension polymerization method has dominated, accounting for about 80% of PVC total output. Add pure water, liquefied VCM and dispersant to the reaction kettle, then add initiator and other aids. After increasing to a certain temperature, VCM will have free radical polymerization reaction and produce PVC particles. Persistent stirring allows the particle size to be uniform, and allows produced particles to suspend in water. In addition, the resin for production of PVC paste by micro-suspension method has good product performance and paste-forming properties.
Suspension polymerization method allows the monomer to suspend and disperse in the water phase in microdroplet shapes. The selected oil-soluble initiator is soluble in a monomer. The polymerization reaction occurs in these microdroplets. The polymerization reaction heat is absorbed by water in a timely manner. To ensure that these microdroplets can be dispersed in water in bead-like shapes, a suspension stabilizer should be added, e.g. polyvinyl alcohol and hydroxypropyl methyl cellulose. Initiator mostly adopts organic peroxide and azoic compounds. Polymerization occurs in the polymeric kettle with stirrer. After polymerization, the materials flow into the monomer recycling tank or the steam stripping tower to recycle the monomer, and then flows into the mixing kettle to obtain finished resin products after wash, centrifugal dewatering and drying. When polymerization, to ensure that the resins of specific molecular weight and molecular weight distribution range can be obtained, and that implosion can be avoided, the temperature and pressure during polymerization must be well controlled. The resin particle size and particle size distribution are controlled by the stirring speed and the selection and dosage of suspension dispersants. Resin quality is characterized by particle size and particle size distribution, molecular weight and molecular weight distribution, apparent density, porosity, fisheye, thermal stability, color, impurity content and powder free liquidity.
PVC is a flame-retardant material with good chemical stability, which only produces low internal tension crack force. Meanwhile, the material features also include high strength, high rigidity and high hardness; operating temperature range is from -15℃~60℃, adhesive and weldable. It has strong corrosion resistance, most suitable for anti-corrosive equipment in the chemical industry. It can be easily processed: cut, welded or bent. It has high strength, high rigidity and high hardness, good electric insulativity, good chemical stability, self-extinguishing, with low water absorption, easily adhesive, low in price, and has good printing adaptability and coating performance.
Sections and profiles have the largest consumption of PVC in China, mainly used in fabrication of doors, windows and energy-saving materials.
PVC pipes have the second largest consumption among the numerous PVC products. In China, PVC pipes were developed earlier than PE tubes and PP tubes, with numerous varieties, excellent performance and wide usage range, occupying an important position in the market.
PVC films have the third largest consumption of PVC. After mixing with additives and plasticization, PVC is manufactured into transparent or colored film with specified thickness by use of three-roll, four-roll calendering machine. The film processed by this method becomes calendered film. It can also be used in heat sealing and processing of packaging bags, raincoats, tablecloths, curtains and inflatable toys, by cutting. Wide transparent film can be used in greenhouses, plastic-covered tunnels and mulches. The film, after two-way stretching, can be used in shrink packaging due to the property of shrinkage, once heated.
PVC hardwoods and boards
After PVC is mixed with stabilizer, lubricant and filler, hard tubes, section tubes and corrugated tubes of various calibers can be extruded, via extruder, which can be used as sewer pipes, drinking water pipes, cable conduits or stair rails. The calendered slices can be overlapped and hot-pressed to fabricate hard boards of various thicknesses. Boards can be cut into the desired shapes, and then welded into storage tanks, air ducts and containers resistent to chemical corrosion with hot air, by use of a PVC welding rod.
PVC protective floors and walls
Due to the excellent abrasion resistance, elasticity, corrosion resistance, flame retardance and printing adaptability, novel and beautiful PVC floors are widely used in laboratories, hospitals, airport terminal grounds and other public places. Meanwhile, compared to floor tiles, PVC floors are easily constructed and maintained.
PVC protective walls are mainly used to substitute for aluminum protective walls.
PVC packaging materials
PVC is mainly used in containers, films and plates in packaging. PVC containers are mainly used in the PTP packaging of mineral water, drinks, cosmetics and drugs, as well as refined oils. Together with other polymers, PVC film can be co-extruded into laminated products of low production cost, and transparent goods with good barrier properties. PVC film can be also used in stretching or heat shrink packaging, used in the packaging of mattresses, clothes, toys and industrial commodities.
PVC general soft goods
PVC can be extruded as hoses, cables and wires via extruder, and can be manufactured into plastic sandals, soles, slippers, toys and automobile spare parts via injection molding machines in combination with various molds.
PVC goods for daily consumption
Luggage bags are a traditional product made from PVC. PVC is used to fabricate various leather cloaks. PVC fabric for clothing is generally absorptive fabric (not requiring coating), e.g. ponchos, baby pants, leatherette and rain boots. PVC is used in many sports and entertainment articles, e.g. toys, discs and sporting goods. At present, there is a significant increase in PVC toys. PVC toys and sporting goods have advantages due to low production cost, and easy molding.
Polymerization method, formula and technological conditions are the main factors affecting PVC structure morphology and cost.
Along with expansion of its application field and requirements, PVC resins for general purposes obtained by traditional technology cannot meet the requirements any more. PVC resin industry is developing towards the customization of resins for general purposes, i.e. production of PVC resins with more suitable performance and easier processability, through adjustment of the reaction system and technological conditions, e.g. PVC of high polymerization, PVC of ultralow polymerization, spherical resin, matting resin and mixing resin; and engineering of PVC resins, i.e. striving to fabricate PVC resins for special purposes with heat resistance, anti-oxidation resistance, impact resistance, weather resistance, aging resistance and enhancement to substitute some engineering plastics for general purposes.