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This entry is reviewed by the project of sorting out and applying scientific entries of "Popular Science China".
PVC (polyvinyl chloride) is the initiator of vinyl chloride monomer (VCM) in peroxides and azo compounds. Or a polymer polymerized under the action of light and heat according to a free radical polymerization mechanism. Vinyl chloride homopolymer and vinyl chloride polymer are collectively referred to as vinyl chloride resin.
PVC is a white powder with amorphous structure, and its branching degree is small, and its relative density is about 1.4. When the glass transition temperature is 77~90℃ and 65,438+070℃, it begins to decompose [65,438+0], and its stability to light and heat is poor. When exposed to sunlight above 100℃ or for a long time, it will decompose and produce hydrogen chloride.
The molecular weight of industrial PVC is generally in the range of 50000 ~ 1 10000, which has great polydispersity, and the molecular weight increases with the decrease of polymerization temperature. Without a fixed melting point, it begins to soften at 80 ~ 85℃, becomes viscoelastic at 130℃, and becomes viscous at 160 ~ 180℃. It has good mechanical properties, the tensile strength is about 60MPa, and the impact strength is 5 ~ 10 kJ/m2. Has excellent dielectric properties.
PVC used to be the largest general-purpose plastic in the world, and it was widely used. Widely used in building materials, industrial products, daily necessities, floor leather, floor tiles, artificial leather, pipes, wires and cables, packaging films, bottles, foaming materials, sealing materials, fibers and so on.
Chinese name
polyvinyl chloride
Foreign name
polyvinyl chloride
colour
Light yellow
Other attributes
Translucent and shiny
structure
(CH2-CHCl)n-
abbreviate
polyvinyl chloride
catalogue
1
2 Historical evolution
Three main categories
4 preparation method
Polymerization method
Modification method of polyvinyl chloride
Performance index of PVC modification
Colorability
liquidity
Weather capacity
stability
Electrical property
Development status of PVC
5 development prospects
Physical and chemical properties of PVC
7 molding conditions
Rigid polyvinyl chloride
Soft polyvinyl chloride
8 material processing
9 main uses
Study on 10 new material
1. PVC uses plastic instead of steel.
2. PVC uses plastic instead of wood.
1 1 identification method
12 main hazards
Composition structure
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Poly (vinyl chloride) (PVC) is a kind of high molecular material with a chlorine atom instead of a hydrogen atom in polyethylene, and it is an amorphous polymer with a few crystal structures. The structure of this material is as follows: [― CH2 ― chcl ――] n. PVC is a linear polymer, in which VCM monomers are mostly connected in a head-to-tail structure. Carbon atoms are arranged in a zigzag pattern, and all atoms are connected by σ bonds. All carbon atoms are sp3 hybrid.
Polyvinyl chloride structure
There is a short syndiotactic structure in PVC molecular chain. With the decrease of polymerization temperature, syndiotactic regularity increases. In the macromolecular structure of PVC, there are unstable structures such as head structure, branched chain, double bond, allyl chloride and tertiary chloride, which are difficult to resist thermal deformation and aging. After pretending to be crosslinked, this shortcoming can be eliminated.
Stereoregular structure of PVC
Crosslinking is divided into radiation crosslinking and chemical crosslinking.
1. radiation crosslinking. High-energy rays are mainly used, generally the rays produced by cobalt 60 radiation source or the electron rays produced by electron acceleration. Then add crosslinking assistant (monomer with two or more carbon-carbon double bonds) for crosslinking. However, it is difficult to operate and requires high equipment.
2. Chemical crosslinking. Triazole dithiocarbamate (FSH) is used for cross-linking, and the cross-linking mechanism is that amine and sulfhydryl combine to attack carbon-chlorine polar bond for substitution reaction. After crosslinking, the ultraviolet resistance, solvent resistance, temperature resistance and impact toughness of the product will be comprehensively improved.
The development of history
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polyvinyl chloride
As early as 1835, V. Regnaud of the United States discovered PVC. When it is irradiated by sunlight, it will produce a white solid, that is, PVC.
PVC was discovered twice in19th century, once by Henry Victor Raignault in 1835 and once by Eugen Baumann in 1872. In both cases, the polymer appears in a vinyl chloride beaker placed in the sun and becomes a white solid. At the beginning of the 20th century, Russian chemist Ivan Ostromislensky and German chemist Fritz Klatte tried to use PVC for commercial purposes at the same time, but the difficulty was how to process this hard and sometimes fragile polymer.
19 12 years, German Fritz Klatte synthesized PVC and applied for a patent in Germany, but a suitable product could not be developed before the patent expired.
1926, Waldo Semon of American B.F. Goodrich Company synthesized PVC and applied for a patent in the United States. Waldo Semon and B.F. Goodrich company developed a method of plasticizing PVC by adding various additives in 1926, which made it a more flexible and easy-to-process material and soon got a wide range of commercial applications.
It is found in 19 14 that organic peroxide can accelerate the polymerization of vinyl chloride. 193 1 year, the company adopts emulsion polymerization to realize the industrial production of PVC. In 1933, W.L. Simon proposed that soft PVC products could be processed by heating and mixing high boiling point solvent, triphenyl phosphate and PVC, which made a real breakthrough in the practical application of PVC. The suspension polymerization of vinyl chloride and the processing and application of PVC were almost simultaneously developed by Bremen Chemical Industry Company, Union Carbide Company and Gutteridge Chemical Company in 1936. In order to simplify the production process and reduce energy consumption, the French company Saint-Gobain 1956 developed bulk polymerization. 1983, the world total consumption was about 1 1. 1Mt, and the total production capacity was about17.6 mt; ; It is the second largest plastic variety after polyethylene production, accounting for about 15% of the total plastic production. The PVC production plant designed by China was put into trial production in Jinxi Chemical Plant in Liaoning Province in 1956, and the 3kt plant was officially industrialized in 1958, with an output of 530.9kt in 1984.
PVC was industrialized in the early 1930s. Since 1930s, for a long time, the output of PVC has been ranked first in the world's plastic consumption. In the late 1960s, polyethylene replaced polyvinyl chloride. Although PVC plastics are now relegated to the second place, the output still accounts for more than a quarter of the total plastic production.
Before the 1960s, the production of vinyl chloride monomer was mainly based on calcium carbide and acetylene, which consumed a lot of electricity and coke and had high cost. After the industrialization of ethylene oxychlorination to produce vinyl chloride in the early 1960s, countries turned to cheaper oil as raw materials. In addition, because a large part of raw materials (about 57% by weight) of PVC is an inevitable by-product of alkali industry, it is not only rich in raw materials, but also one of the very important products for developing chlor-alkali industry and balancing chlorine. Therefore, although the proportion of PVC in plastics has decreased, it still maintains a high growth rate.
PVC plastic products are widely used, but in the mid-1970s, people realized that the residual monomer vinyl chloride (VCM) in PVC resin and products is a serious carcinogen, which will undoubtedly affect the development of PVC to some extent. However, people have successfully reduced the residual VCM by automobile and other means, so that the VCM content in PVC resin is less than 10ppm, which meets the requirements of sanitary resin and expands the application scope of PVC. Even the VCM content in the resin can be less than 5ppm, and there is very little VCM left after processing. It is basically harmless to human body and can be used as food and drug packaging and children's toys.
Main classification
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According to the different application scope, PVC can be divided into: universal PVC resin, high polymerization degree PVC resin and crosslinked PVC resin. Universal PVC resin is polymerized by vinyl chloride monomer under the action of initiator; High polymerization degree PVC resin refers to the resin polymerized by adding chain extender into vinyl chloride monomer polymerization system; Crosslinked PVC resin is a resin polymerized by adding a crosslinking agent containing diene and polyene into the polymerization system of vinyl chloride monomer.
According to the methods of obtaining vinyl chloride monomer, it can be divided into calcium carbide method, ethylene method and imported (EDC, VCM) monomer method (ethylene method and imported monomer method are collectively referred to as ethylene method).
According to the polymerization method, PVC can be divided into four categories: suspension PVC, emulsion PVC, bulk PVC and solution PVC. Suspension PVC is the largest variety, accounting for about 80% of the total PVC production. Suspended PVC can be divided into six types according to absolute viscosity: XS- 1, xs-2 ... xs-6; XJ- 1, XJ-2, XJ-6. The meaning of each letter in the model: X- suspension method; S- loose type; J- compact.
According to the content of plasticizer, PVC plastics are often divided into: unplasticized PVC with plasticizer content of 0; Rigid PVC with plasticizer content less than 10%; Semi-rigid PVC with plasticizer content of 10-30% and soft PVCPVC paste with plasticizer content of 30-70% have plasticizer content above 80%.
preparation method
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polyvinyl chloride
Polyvinyl chloride can be prepared from ethylene, chlorine and catalyst by substitution reaction. Because of its fire and heat resistance, PVC is widely used in all kinds of products in all walks of life: wire sheath, optical fiber sheath, shoes, handbags, bags, ornaments, signboards and billboards, architectural decorative products, furniture, ornaments, rollers, pipes, toys (such as the famous Italian "Roddy" vault), door curtains, rolling doors, auxiliary medical supplies, gloves and some foods.
Polymerization method
Polyvinyl chloride is prepared by free radical addition polymerization. Polymerization methods are mainly divided into suspension polymerization, emulsion polymerization and bulk polymerization, with suspension polymerization as the main method, accounting for about 80% of the total PVC production. Adding pure water, liquefied VCM monomer and dispersant into the reaction kettle, and then adding initiator and other additives. After heating to a certain temperature, VCM monomer undergoes free radical polymerization to generate PVC particles. Continuous stirring makes the particle size uniform, and the generated particles are suspended in water. In addition, there is a kind of resin for PVC paste produced by micro-suspension method, and the product performance and paste-forming performance are good.
① Suspension polymerization: The monomer is suspended and dispersed in the water phase in the form of droplets, and the selected oil-soluble initiator is dissolved in the monomer. Polymerization takes place in these droplets, and the heat of polymerization is absorbed by water in time. In order to ensure that these droplets are dispersed in water in the form of beads, suspension stabilizers such as gelatin, polyvinyl alcohol, methyl cellulose and hydroxyethyl cellulose need to be added. The initiators are mainly organic peroxides and azo compounds, such as diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, diethyl hexyl peroxydicarbonate, azodiisoheptylnitrile and azodiisobutyronitrile. The polymerization is carried out in a polymerization kettle with a stirrer. After polymerization, the materials flow into the monomer recovery tank or stripping tower to recover the monomer. Then flowing into a mixing kettle, washing with water, centrifuging, dehydrating and drying to obtain a finished resin product. Vinyl chloride monomer should be removed from the resin as much as possible. The content of free monomer in PVC for food packaging should be controlled below 65438±0 ppm. In order to obtain resin with specific molecular weight and molecular weight distribution range and prevent explosive polymerization, it is necessary to control the temperature and pressure during polymerization. The particle size and particle size distribution of resin are controlled by the stirring speed and the selection and dosage of suspension stabilizer. The quality of resin 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 fluidity. polyreaction
PVC waterproof membrane
The reaction kettle is the main equipment, which is lined with stainless steel or enamel in the steel kettle, equipped with stirrer and heat transfer jacket for controlling temperature, or internally cooled exhaust pipe and reflux condenser. In order to reduce the production cost, the volume of the reaction kettle has gradually developed from several cubic meters to more than ten cubic meters, and the largest volume has reached 200 cubic meters (kettle reactor). The polymerizer should be descaled after repeated use. PVC prepared by using polyvinyl alcohol and cellulose ether as suspension stabilizers is usually loose, porous and has a large surface area, and easily absorbs plasticizers and plasticizers.
(2) Emulsion polymerization is the earliest method for industrial production of PVC. In emulsion polymerization, besides water and vinyl chloride monomer, surfactant such as sodium alkyl sulfonate should be added as emulsifier to disperse the monomer in the water phase to form emulsion. Water-soluble potassium persulfate or ammonium persulfate can be used as initiator, and oxidation-reduction initiation system can also be used. Polymerization process is different from suspension method. There are also polyvinyl alcohol as emulsion stabilizer, dodecyl mercaptan as regulator and sodium bicarbonate as buffer. There are three polymerization methods: batch method, semi-continuous method and continuous method. The polymerization product is emulsion, and the particle size of emulsion is 0.05 ~ 2 microns, which can be directly applied or spray-dried into powder resin. The emulsion polymerization method has the advantages of short polymerization period, easy control, high molecular weight and uniform polymerization degree, and is suitable for manufacturing PVC paste, artificial leather or impregnated products. Emulsion polymerization has complex formula and high impurity content.
(3) Bulk polymerization polymerization device is special, mainly composed of vertical prepolymerization kettle and horizontal polymerization kettle with frame agitator. The polymerization is carried out in two steps. The monomer and initiator are pre-polymerized 1h in the pre-polymerization kettle to generate seed particles, at this time, the conversion rate reaches 8% ~ 10%, and then they flow into the second-stage polymerization kettle, and the monomer equivalent to the prepolymer is added to continue polymerization. When the conversion rate reaches 85% ~ 90%, the residual monomer is discharged, and then crushed and screened to obtain the finished product. The particle size and shape of the resin are controlled by the stirring speed, and the reaction heat is brought out by the reflux condensation of the monomer. The method has the advantages of simple production process, good product quality and low production cost.
Modification method of polyvinyl chloride
PVC resin is a polar amorphous polymer with a density of 1.380 g/cm3 and a glass transition temperature of 87℃, so it has poor thermal stability and is difficult to process. Can not be used directly, it must be modified and mixed, and can be used only after adding relevant additives and fillers. However, due to the different types and fractions of additives and fillers, the properties and requirements of PVC materials are also different. We usually call it PVC formula, strictly speaking, it is PVC modified formula, and PVC can only be used after modification. This kind of material is usually classified as polymer modified material. The modification of polymer materials mainly focuses on the high performance of general plastics, the transformation from single-component materials to multi-component materials (alloys, blends and composites), the functionalization of materials, and the optimization of properties and prices. Modification methods mainly include chemical modification, filling modification, reinforcement modification, * * mixed modification and nano-composite modification. The basic principle of modification is to endow materials with functions or improve some properties through additives. [2] So the technology of PVC formula determines the technology and production capacity of a factory.
Generally, PVC should be modified and granulated first. After being prepared into granules by screw extruder, it is more fully plasticized and easier to process, especially for products using injection molding technology. Screw extruder is one of the most important equipment for plastic molding. It transports, compacts, melts, shears, mixes and extrudes plastic solids through external power transmission and heat transfer of external heating elements. Screw extruder plays an important role in plasticizing granulation machinery and molding processing machinery. Strictly speaking, PVC products with special requirements and PVC modified formulas are tailored according to customer requirements. There are also * * * poly derivatives in PVC production. Such modified varieties include vinyl chloride polymer, polyvinyl chloride mixture and chlorinated polyvinyl chloride [3].
Performance index of PVC modification
Colorability
PVC has poor thermal stability and light resistance. Hydrogen chloride began to decompose at 150℃, and had an adverse reaction with plasticizer content. In addition, the influence of pigment on PVC is also reflected in whether the pigment reacts with PVC and other components that make up PVC products, as well as the migration resistance and heat resistance of the pigment itself. Some components in the colorant may promote the degradation of the resin. For example, iron ions and zinc ions are catalysts for the degradation of PVC resin. Therefore, the use of iron oxide (red, yellow, brown and black) pigments or white pigments such as zinc oxide, zinc sulfide and lithopone will reduce the thermal stability of PVC resin. Some colorants may react with degradation products of PVC resin. For example, ultramarine pigment has poor acid resistance, so it will interact with hydrogen chloride produced by PVC decomposition during the coloring process and lose its proper color. Therefore, in the aspect of PVC coloring, considering the characteristics of the used resin and related additives, combined with the characteristics of pigments. Attention should be paid to the following issues when selecting colorants. [4]
1, some metal ions in the pigment will promote the thermal oxidation decomposition of PVC resin as shown in figure 1.
The measurement method is the hue change of polyethylene with pigment when it is heated to 180℃. Because the pigment contains metal ions, it accelerates the decomposition of PVC and leads to the change of hue. At the same time, we should also pay attention to the fact that adding lake red can make the color difference of PVC different, such as containing calcium, and the color difference is small; When containing manganese, the color difference is great, which is due to the promoting effect of manganese and other metals on dehydrochlorination of PVC.
Sulfide colorants (such as cadmium red, yellow, etc. ) used for PVC coloring, hydrogen sulfide may be released due to the decomposition of colorant. This colorant should not be mixed with lead stabilizer to avoid producing black lead sulfide.
2. Influence of pigments on electrical insulation of PVC
As a cable material, PVC, like polyethylene, should consider the electrical properties after coloring. In particular, the insulation of PVC is worse than that of polyethylene, so the influence of pigments is greater. It shows that the electrical insulation of PVC colored with inorganic pigments is better than that of organic pigments (except furnace black and anatase titanium dioxide).
liquidity
Migration only occurs in plasticized PVC products, and when dyes or organic pigments are used. The so-called migration means that some soluble dyes or organic pigments in the surrounding solvents penetrate into the surface of PVC products through plasticizers, and those dissolved dye (pigment) particles are also brought to the surface of the products, leading to color bleeding, solvent color bleeding or frosting. [5]
Another problem is "scaling". It means that the colorant is released from the system due to poor compatibility or incompatibility with the colored object during the coloring process and deposited on the surface of processing equipment (such as the inner wall of extruder barrel and the inner wall of die hole).
Weather capacity
Refers to the ability of pigments to withstand various climates. These factors include visible light and ultraviolet light, humidity, temperature, atmospheric chlorination and chemicals encountered during product use. The most important weatherability includes fastness, pulverization resistance and durability of physical properties. But organic pigments are good and bad because of their different structures. In addition, in the formula containing white pigment, the weather resistance of pigment will be seriously affected.
The fading, darkening or discoloration of pigment is generally caused by the reaction gene of pigment. These active genes can interact with moisture or chemical reagents (acids and bases) in the atmosphere. For example, cadmium yellow will fade under the action of water and sunlight, and Lisu red has good light resistance and is suitable for most indoor use, but it will fade seriously when used outdoors with acid and alkali components.
The determination method of dehydrochlorination refers to JIS-K-6723, and the determination temperature is 180℃. Based on the dechlorination time of uncolored PVC composites, the extension or delay time is in the range of 5% and 10%, and negative values indicate accelerated decomposition.
stability
The softening point of PVC resin is low, about 75-80℃, and the brittleness temperature is lower than -50~-60℃. The long-term use temperature of most products does not exceed 55℃, and the special formula can reach 90℃. If PVC resin is a pure linear structure with head-to-head interface, and there are no branched chains and unsaturated bonds inside, the stability of PVC resin should be relatively high even if the bond energy of C-Cl is relatively small. However, even if the high-purity PVC resin is exposed to ultraviolet rays above 100℃ or for a long time, hydrogen chloride gas will begin to escape. It shows that there are sharp groups or unstable structures in its molecular structure. The longer the time, the more degradation, the higher the temperature, the faster the degradation rate, and the faster the degradation rate in the presence of oxygen or air. [6]
Electrical property
PVC is a polar polymer, which has great affinity for water and other conductive substances, so its resistance is lower than that of nonpolar polyolefin, but it still has a higher volume cathode and breakdown voltage. The polar group of PVC is directly connected to the main chain. Below the glass transition temperature, the dipole segment is restricted by the atoms in the main chain of the frozen structure and cannot move, so it does not produce dipole polarization, and can be used as a high-frequency insulating material at room temperature. When used for wire insulation, the electrical insulation ratio of suspended resin