Polyvinyl chloride (PVC for short) is the third largest synthetic polymer plastic in the world (second only topolyethyleneandpolypropylene）The annual production of PVC is about 40 million tons.PVC is vinyl chloride monomer (VCM)peroxide、Azo compoundSuch initiator or polymer polymerized according to free radical polymerization mechanism under the action of light and heat.Vinyl chloride homopolymer and vinyl chloride copolymer are collectively referred to asVinyl chloride resin。
PVC used to be the most common plastic in the world, and it was widely used.PVC can be hard (sometimes abbreviated as RPVC) or soft.Rigid PVC is used for building pipes, doors and windows.It is also used to make plastic bottles, packaging, bank cards or membership cards.Adding plasticizer can make PVC softer and more elastic.It can be used in pipes, cable insulation, floors, signs, gramophone records, inflatable products and rubber substitutes.
On October 27, 2017, the list of carcinogens published by the International Agency for Research on Cancer of the World Health Organization (WHO) was preliminarily sorted out for reference, and PVC was included in the list of category 3 carcinogens[2]。
PVC is a white powder with amorphous structure. The degree of branching is small. The glass transition temperature is 77~90 ℃, and it begins to decompose at about 170 ℃[1]The stability to light and heat is poor. When it is above 100 ℃ or exposed to the sun for a long time, it will decompose to produce hydrogen chloride, which will further automatically catalyze the decomposition, causing discoloration, and the physical and mechanical properties will also decline rapidly. In practical applications, stabilizers must be added to improve the stability to heat and light.
The molecular weight of PVC produced in industry is generally in the range of 50000~110000, which has great polydispersity. The molecular weight increases with the decrease of polymerization temperature, without fixed melting point. It begins to soften at 80~85 ℃, becomes viscoelastic at 130 ℃, and becomes viscous fluid at 160~180 ℃;Good mechanical properties, tensile strength of about 60 MPa, impact strength of 5~10kJ/mtwo；Have excellentDielectric properties。PVC is insoluble in common solvents, but will expand in monomer and some chlorinated hydrocarbon solvents.
Composition
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Polyvinyl chloride is a kind of macromolecular material that uses a chlorine atom to replace a hydrogen atom in polyethylene. It is an amorphous polymer with a small amount of crystalline structure.The structure of this material is as follows: - (CH2-CHCl) n -.PVC is a linear polymer with VCM monomers mostly linked by head tail structure.The carbon atoms are arranged in zigzag shape, and all atoms are connected by σ bond.All carbon atoms are spthreeHybridization.
There is a short syndiotactic regular structure on the PVC molecular chain.The syndiotactic regularity increased with the decrease of polymerization temperature.In the macromolecular structure of PVC, there are unstable structures such as head structure, branch chain, double bond, allyl chloride, tertiary chloride, etc., which make it difficult to resist heat deformation and aging.Such defects can be eliminated after the artificial crosslinking.
Stereoscopic and regular structure of PVC
cross-linkingIt is divided into radiation crosslinking and chemical crosslinking.
1. Radiation cross-linking: high-energy radiation is used, generally the radiation generated by cobalt 60 radiation source or the electron radiation generated by electron acceleration, and the latter is mainly used.Then cross-linking aids (two or more monomers with carbon carbon double bond structure) are added for cross-linking.However, the operation is difficult and requires high equipment.
2. Chemical crosslinking: use triazole dimercapto amine salt (FSH) for crosslinking, and the crosslinking mechanism is amine andMercapto groupThe substitution reaction is carried out in combination with attacking carbon chlorine polar bonds.After crosslinking, the UV resistance, solvent resistance, temperature resistance, impact toughness and other properties of the product will be comprehensively improved.
Historical evolution
Polyvinyl chloride was discovered by V. Lenio of the United States as early as 1835. It was exposed to sunlightvinyl chlorideA white solid, namely polyvinyl chloride, is formed when.
PVC was discovered twice in the 19th century, once in 1835 by Henri Victor Regnault and again in 1872 by Eugen Baumann.On both occasions, this polymer appeared in the beaker of vinyl chloride placed in the sunlight and became a white solid.At the beginning of the 20th century, Russian chemist Ivan Ostromislensky and German chemist Fritz Klatte of Griesheim Elektron tried to use PVC for commercial purposes at the same time, but the difficulty was how to process this hard and sometimes brittle polymer.
In 1912, Fritz Klatte, a German, synthesized PVC and applied for a patent in Germany, but he was unable to develop suitable products before the patent expired.
In 1926, BF. Waldo Semon of Goodrich Company synthesized PVC and applied for a patent in the United States.Waldo Semon and BF. Goodrich Company developed the method of plasticizing PVC by adding various additives in 1926, making it more flexible and easy to process and quickly gaining widespread commercial application.
In 1914, it was found that the polymerization of vinyl chloride could be accelerated by using organic peroxides. In 1931, the German company adopted emulsion polymerization to realize the industrial production of PVC.1933 W50. Simon proposed to use high boiling point solvent and trimethyl phosphate to heat and mix with PVC to produce soft PVC products, which made a real breakthrough in the practicality of PVC.In 1936, the suspension polymerization of vinyl chloride and the processing and application of PVC were almost simultaneously developed by British Burnemen Chemical Industry Company, American Union Carbide Company and Gutrich Chemical Company.In order to simplify the production process and reduce energy consumption, Saint Gobain developed the bulk polymerization method in 1956.In 1983, the world's total consumption was about 11.1 Mt, and the total production capacity was about 17.6 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 Liaoning Jinxi Chemical Plant in 1956. In 1958, the 3 kt plant was officially industrialized, with an output of 530.9 kt in 1984.
PVC was industrialized in the early 1930s.Since the 1930s, for a long time, the output of PVC has always occupied the first place in the world plastic consumption.In the late 1960s, polyethylene replaced PVC.Although PVC plastics now rank second, its output still accounts for more than a quarter of the total plastic output.
Before the 1960s, the production of vinyl chloride monomer was mainly calcium carbide acetylene. Because calcium carbide production required a lot of power and coke, the cost was high.In the early 1960s, after the industrialization of ethylene oxychlorination to produce vinyl chloride, countries turned to cheaper oil as raw material.In addition, due to the fact that most of the raw materials of PVC (about 57% by weight) are chlorine gas, which is an inevitable by-product of the alkali manufacturing industry, not only the source of raw materials is rich, but also one of the important products for developing the chlor alkali industry and balancing chlorine gas.Therefore, although the proportion of PVC in plastics has declined, 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 a certain extent.However, people have successfully reduced the residual VCM by adding spiral plate heat exchangers and other means[3]So that the VCM content in PVC resin is less than 10 ppm, meeting the requirements of sanitary grade resin and expanding the application range of PVC.It can even make the VCM content in the resin less than 5 ppm, and the residual VCM after processing is very little.It is basically harmless to human body and can be used as food and drug packaging and children's toys.
Main categories
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PVC can be divided into: Universal PVC resin, high degree of polymerization PVC resin, cross-linked PVC resin.Universal PVC resin is formed by polymerization of vinyl chloride monomer under the action of initiator;High degree of polymerization PVC resin refers to the resin polymerized by adding chain growth agent in the vinyl chloride monomer polymerization system;Crosslinked PVC resin is polymerized by adding crosslinking agent containing diene and polyene into vinyl chloride monomer polymerization system.
Distinguish according to the method of obtaining vinyl chloride monomerIt can be divided into carbide method, ethylene method and imported (EDC, VCM) monomer method (ethylene method and imported monomer method are commonly referred to as ethylene method).
According to aggregation methodPVC can be divided into four categories: suspension PVC, emulsion PVC, bulk PVC and solution PVC.Suspension PVC is one of the largest varieties, accounting for about 80% of the total output of PVC.Suspension PVC is divided into six models according to absolute viscosity: XS-1XS-2……XS-6；XJ-1、XJ-2……、XJ-6。Meaning of each letter in the model: X-suspension method;S-loose type;J-Tight type.
According to the content of plasticizer, PVC plastic olefin plastics are often divided into: non plasticized PVC, plasticizer content is 0;Rigid PVC with plasticizer content less than 10%;Semi rigid PVC with plasticizer content of 10-30%;Soft PVC, plasticizer content is 30-70%;PVC paste plastic with more than 80% plasticizer content.The property difference between rigid PVC and soft PVC is shown in the table[4]：
nature
Company
Rigid PVC
Soft PVC
density
g/cmthree
1.3–1.45
1.1–1.35
Thermal conductivity
W/(m·K)
0.14–0.28
0.14–0.17
yield strength
psi
4500–8700
1450–3600
MPa
31–60
10.0–24.8
Young's modulus
psi
four hundred and ninety thousand
—
GPa
three point four
—
Bending strength (yield)
psi
10,500
—
MPa
seventy-two
—
compressive strength
psi
9,500
—
MPa
sixty-six
—
Coefficient of thermal expansion (linear)
mm/(mm·°C)
5×10−5
—
Vicat softening temperature
°C
65–100
—
resistivitya
Ω·m
tensixteen
tentwelve–10fifteen
Surface resistivitya
Ω
tenthirteen–10fourteen
teneleven–10twelve
Note: a, at 60% relative humidity and room temperature
Preparation method
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PVC can be made by substitution reaction of ethylene, chlorine and catalyst.Because of its fire and heat resistance, PVC is widely used in various products in all walks of life: wire skins, optical fiber skins, shoes, handbags, bags, ornaments, signs and billboards, building decoration supplies, furniture, hanging decorations, rollers, pipes, toys, curtains, rolling doors, auxiliary medical supplies, gloves, some food fresh-keeping paper, some fashion, etc.
Aggregation method
PVC is prepared by free radical addition polymerization. Polymerization methods mainly include suspension polymerization, emulsion polymerization, bulk polymerization and micro suspension polymerization. Suspension polymerization is the main method, accounting for about 80% - 82% of the total output of PVC, followed by emulsion polymerization, accounting for about 10% - 12% of the total output of PVC, followed by bulk polymerization, accounting for about 8%.The particle structure obtained by suspension method and bulk method is similar, and the average particle size is 100~160 μ m.The particle size produced by emulsion method and micro suspension method is about 0.2 μ m and 1 μ m.Only a small amount of coating is prepared with polyethylene copolymer by solution method.Add pure water, liquefied VCM monomer and dispersant into the reaction kettle, and then add initiator and other additives. When the temperature rises 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 suspended in water.In addition, the micro suspension method is also used to produce PVC paste resin, which has good performance and paste forming property.
① Suspension polymerization[5]: The basic formula of vinyl chloride suspension polymerization is composed of vinyl chloride monomer, water, oil soluble initiator and dispersant, but actually pH regulator, molecular weight regulator (mainly used for low polymerization degree varieties), anti sticking agent, defoamer, etc. are also added.According to the different requirements of loose and compact polychloroenes, the ratio of water and monomer in the formula varies between (1.2~2): 1.The process of suspension polymerization of vinyl chloride is roughly as follows: water, dispersant, other additives and initiators are successively added into the polymerization kettle, the air is pumped straight, nitrogen is filled and oxygen is discharged, then monomer is added, and the polymerization temperature is raised to a predetermined temperature.Temperature and pressure remain constant during polymerization.In the later stage, the pressure drops 0.1~0.2MPa, which is equivalent to 80%~85% conversion, and polymerization is ended.If the pressure drops too much, the resin will be dense.After polymerization, the monomer is recycled, discharged, and the finished PVC resin is obtained through the post-treatment process.
When vinyl chloride is polymerized, the transfer to monomer is the main chain termination mode, so that the polymerization degree of PVC (600~1600) has nothing to do with the initiator concentration, and only depends on the temperature (45~65oC, temperature fluctuation should be controlled within 0.2~0.5oWithin C.The polymerization rate is mainly regulated by the amount of initiator.At present, the heat transfer performance of polymerization kettle is good, and highly active initiators such as carbamate peroxide are mostly used, with the dosage of 0.02%~0.05%.If high activity and low activity initiators are used together and properly combined, if the half-life is 2h, the reaction is expected to approach uniform speed.Uniform reaction is beneficial to heat transfer and temperature control.Polyvinyl chloride vinyl chloride is a partially miscible system, forming two phases: one is the rich phase of polyvinyl chloride that swells vinyl chloride (about 30%), becoming the main place for polymerization; the other is the monomer phase that dissolves trace amounts of polyvinyl chloride (<0.1%), close to pure monomer.When the conversion rate is more than 70%, the monomer phase disappears, the system pressure begins to be lower than the saturation pressure of pure vinyl chloride, and vinyl chloride in the rich phase of polychloroene continues to polymerize.Polymerize to 85% conversion and end the reaction so as not to affect the loose structure of resin particles.The property of dispersant is very important to the morphology of PVC particles.When gelatin is selected, the surface tension of its aqueous solution is large (68 mN · m at 25 ° C-1）, will form a compact resin.When preparing loose PVC, the surface tension of the individual substance is required to be 50 mN · m-1As follows, partially hydrolyzed polyvinyl alcohol (the surface tension of aqueous solution is 50~55 mN · m-1）And light propyl methyl cellulose (surface tension of aqueous solution is 45~50 mN · m-1）It is used in combination, and sometimes a third component is added.Although the combination of composite dispersants can be used as a partial reference for surface tension, it also has some components of experience and technology.
The specific process is as follows:
The monomer is suspended and dispersed in the water phase in the form of micro drops, and the selected oil soluble initiator is dissolved in the monomer. The polymerization reaction is carried out in these micro drops. The heat of polymerization reaction is timely absorbed by water. In order to ensure that these micro drops are dispersed in the water as beads, suspension stabilizers such as gelatin, polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, etc. need to be added.Organic peroxides and azo compounds are mostly used as initiators, such as diisopropyl peroxydicarbonate, dicyclohexyl peroxydicarbonate, diethylhexyl peroxydicarbonate, azodiisoheptanitrile, azodiisobutyronitrile, etc.Polymerization is carried out in a polymerizer with an agitator.After polymerization, the material flows into monomer recovery tank or stripper to recover monomer.Then it flows into the mixing kettle, which is washed, centrifuged, dehydrated and dried to obtain the finished resin.Vinyl chloride monomer shall be removed from the resin as far as possible.For PVC used for food packaging, the content of free monomer should be controlled below 1 ppm.The temperature and pressure during polymerization must be controlled to ensure that the resin with specified molecular weight and molecular weight distribution range is obtained and prevent explosion polymerization.The particle size and particle size distribution of the resin are controlled by the stirring speed and the selection and amount 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, fish eye, thermal stability, color, impurity content and free flow of powder.
Polymerization reaction kettle is the main equipment, which is made of steel kettle lined with stainless steel or enamel, equipped with agitator and heat transfer jacket for temperature control, or internal cooling exhaust pipe, reflux condenser, etc.In order to reduce production costs, the volume of the reactor has gradually expanded from several cubic meters to more than ten cubic meters, and the maximum volume has reached 200 cubic meters (kettle reactor).The high heat transfer capacity of the polymerizer ensures the constant polymerization temperature. In addition to the help of mixing materials and heat transfer, agitation also has a significant impact on liquid dispersion and resin particle characteristics.Heat transfer and agitation are two major engineering problems in vinyl chloride polymerization.The polymerizer shall be descaled after repeated use.PVC prepared with polyvinyl alcohol and cellulose ether as suspension stabilizer is generally loose, with many pores, large surface area, and easy to absorb plasticizer and plasticize.
② Emulsion polymerization: the earliest method for industrial production of PVC.In emulsion polymerization, in addition to water and vinyl chloride monomer, surfactants such as sodium alkyl sulfonate are added as emulsifiers to disperse the monomer in the water phase to form emulsion. Water soluble potassium persulfate or ammonium persulfate are used as initiators, and "oxidation reduction" initiator system can also be used. The polymerization process and suspension method are different.Polyvinyl alcohol is also added 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 with particle size of 0.05~2 μ m, which can be directly applied or spray dried into powder resin.Emulsion polymerization has short polymerization cycle and is easy to control. The resin obtained has high molecular weight and uniform degree of polymerization. It is suitable for making PVC paste, artificial leather or impregnated products.The formula of emulsion polymerization is complex, and the content of impurities in the product is high.
③ Bulk polymerization: the polymerization unit is special, mainly composed of vertical prepolymerization kettle and horizontal polymerization kettle with frame agitator.The particle characteristics of bulk PVC are similar to those of suspension resin, which is loose, but without film and more crystalline.In addition to heat dissipation and anti sticking, bulk polymerization also needs to solve the problem of maintaining the loose structure of particles, which is usually ensured by two-stage polymerization.The first stage is prepolymerization, which is carried out in a vertical kettle.A small amount of vinyl chloride and a limited amount of highly active initiator (such as ethylene peroxide sulfonate) are added into the kettleoThe conversion rate of prepolymerization to 7%~11% at C is dead end polymerization to prevent the conversion rate from being too high.Stir quickly to form a loose particle skeleton.The conversion rate is estimated by the heat taken away by the jacket and condenser[6]。
The prepolymer, more monomers and another part of initiator are added to another low speed stirring (30r/min) kettle, and the monomers continue to polymerize on the preformed particle skeleton, so that the particles grow up and remain unchanged.To 70%~90% conversion, end polymerization.The residual monomer is discharged, and then crushed and sieved to obtain the finished product.Prepolymerization only takes 1~2 h, while polymerization takes 5~9 h. One prepolymerization kettle can be equipped with several polymerization kettles.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 monomer.This method is simple in production process, good in product quality and low in production cost.
PVC modification method
PVC resin is a polar amorphous polymer, density: 1.38 g/cmthree, glass transition temperature: 87 ℃, so the thermal stability is poor and it is not easy to process.It cannot be used directly. It can only be used after modification and blending, and adding relevant additives and fillers.The properties and requirements of the prepared PVC materials are different due to the different types and contents of the relevant additives and fillers added.We usually call it PVC formula. Strictly speaking, it is PVC modified formula. PVC can only be used after modification.This class is often classified as polymer modified materials.The modification of polymer materials mainly focuses on the high performance of general plastics, the transformation from single component materials to multi-component composites (alloys, blends, composites), the functionalization of materials, and the optimization of performance and price.The main modification methods are chemical modification, filling modification, reinforcement modification, blending modification and nanocomposite modification.The basic principle of modification is to endow materials with functions or improve some properties through additives[7]。Therefore, the level of PVC formulation technology determines the level of technology and production capacity of a factory.
PVC generally needs to be modified and pelletized first. After the particles are prepared by the screw extruder, the plasticization is more sufficient and the processing is easier, especially for products with injection molding technology.Screw extruder is one of the most important equipment for plastic molding. It carries out solid conveying, compaction, melting, shearing, mixing and extrusion molding of plastic through external power transmission and heat transfer of external heating elements.Screw extruder plays an important role both as a plasticizing and granulating machine and as a molding and processing machine. Strictly speaking, PVC products with special requirements and PVC modified formula are customized according to customer requirements.The other is copolymerization and derivation in the production process of PVC. Such modified varieties include vinyl chloride copolymer, PVC blend and chlorinated polyvinyl chloride[8]。
Colourability
PVC has poor thermal stability and light fastness.At 150 ℃, hydrogen chloride begins to decompose, and adverse reactions will occur with the amount of plasticizer.In addition, the influence of pigment on PVC is reflected in whether the pigment reacts with PVC and other components of PVC products, and the migration resistance and heat resistance of pigment itself.Some components in the colorant may promote the degradation of the resin.For example, iron ion and zinc ion are catalysts for degradation reaction 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 interact with degradation products of PVC resin.For example, ultramarine pigments have poor acid resistance, so they will interact with the hydrogen chloride generated from the decomposition of PVC and lose their proper color during the PVC coloring process.Therefore, in terms of PVC coloring, the characteristics of resins and related additives used and the characteristics of pigments should be considered.The following issues should be noted when selecting colorants[9]。
1. Some metal ions in the pigment will promote the thermal oxidative decomposition of PVC resin.
The determination method is the color change of polyethylene with pigment when heated to 180 ℃.The metal ions contained in the pigment accelerate the decomposition of PVC, resulting in color change.At the same time, it should be noted that the addition of lake red can make the color difference of PVC different. If calcium is contained, the color difference is small;The color difference of manganese is large, because manganese and other metals promote the dehydrochlorination of PVC.
Sulfide colorants (such as cadmium red) are used for coloring PVC, which may release hydrogen sulfide due to the decomposition of colorants.Such colorants should not be mixed with lead stabilizers to avoid producing black lead sulfide.
2. Influence of pigments on electrical insulation of PVC
As a cable material, PVC and polyethylene should consider the electrical properties after coloring.Especially PVC has a worse insulation than PE, so the influence of pigment is greater.It shows that the electrical insulation of PVC dyed with inorganic pigments is better than that of organic pigments (except furnace black and anatase titanium dioxide).
Mobility
Migration occurs only in plasticized PVC products and when dyes or organic pigments are used.The so-called migration refers to that some soluble dyes or organic pigments existing in the surrounding solvents penetrate to the surface of PVC products through plasticizers, and those dissolved dye (pigment) particles are also brought to the surface of products, thus leading to contact bleeding, solvent bleeding or frosting[10]。
Another problem is "scaling".It refers to that the colorant is free from the system and deposited on the surface of the processing equipment (such as the inner wall of the barrel of the extruder and the inner wall of the die hole) due to the poor compatibility or incompatibility of the colorant.
Weatherability
It refers to the ability of pigments to withstand various climates.These include visible and ultraviolet light, moisture, temperature, atmospheric chlorination, and chemicals encountered during the use of the product.The most important weather resistance, including color fastness, chalking resistance and durability of physical properties.Organic pigments have different advantages and disadvantages due to their different structures.In addition, in the formula containing white pigment, the weatherability of pigment will be seriously affected.
The fading, darkening or tone change of pigments is generally caused by the reaction genes of pigments.These reactive genes can interact with water in the atmosphere or chemical agents - acids and bases.For example, cadmium yellow will fade under the action of moisture and sunlight. Lisol red has good light resistance and is suitable for most indoor applications, while it will fade severely when used outdoors with acid and alkali components.
Dehydrochloride is determined according to JIS-K-6723 at 180 ℃.Based on the time of dechlorinated vinyl chloride of the unpigmented PVC compound, the extension or retardation time is calculated at intervals of 5% and 10%, and a negative value indicates accelerated decomposition.
stability
The softening point of PVC resin is low, about 75-80 ℃, and the embrittlement temperature is lower than - 50~- 60 ℃. The long-term use temperature of most products should not exceed 55 ℃, and the special formula can reach 90 ℃.If the PVC resin belongs to the linear structure of the head to sex interface and has no branched chains and unsaturated bonds inside, the stability of the PVC resin should also be relatively high, although the C-Cl bond energy is relatively small.However, even if the PVC resin with high purity is kept above 100 ℃ for a long time or exposed to ultraviolet radiation, hydrogen chloride gas will start to escape.It indicates that there are sharp groups or unstable structures in its molecular structure.The longer the time, the more the degradation, and the higher the temperature, the faster the degradation, and the faster the degradation in the presence of oxygen or air[11]。
Electrical performance
Polyvinyl chloride is a polar polymer with a high affinity for water and other conductive substances, so its resistance is smaller than that of non-polar polyolefins, but it still has a high volume of cathode and breakdown voltage.The polar group of PVC is directly attached to the main chain. Below the glass transition temperature, the dipole chain segment is restricted by the main chain atoms of the frozen structure and cannot move, so it does not produce dipole polarization, and can be used as high-frequency insulation materials at room temperature.When used for wire insulation, the electrical insulation of suspension resin is 10-100 times higher than that of floating resin.The presence of chloride ions generated by degradation will reduce the electrical insulation.
PVC development
PVC materials with special requirements generally need to be imported from abroad. The famous foreign companies are United Carbonization Company and Nordic Chemical Company. With the continuous research and development and technology accumulation of major research institutes and production units in China, the formula design and manufacturing of domestic PVC modified materials have reached the international advanced level, and have completely replaced the imported materials,Many products have been exported abroad[12]。
Physical and chemical properties
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PVC is yellowish and translucent with luster.Transparency is better than polyethylene and polypropylene, and worse than polystyrene. Depending on the amount of additives used, it can be divided into soft and hard PVC. Soft products are soft and tough, and feel sticky. The hardness of hard products is higher than that of low-density polyethylene, but lower than that of polypropylene. Whitening phenomenon will appear at the bend.stable;Not easy to be corroded by acid and alkali;It is relatively resistant to heat.
PVC has the advantages of flame retardant (flame retardant value is above 40), high chemical resistance (resistance to concentrated hydrochloric acid, 90% sulfuric acid, 60% nitric acid and 20% sodium hydroxide), good mechanical strength and electrical insulation.
PVC has poor stability to light and heat.The softening point is 80 ℃, and the decomposition starts at 130 ℃.When the stabilizer is not heated, PVC starts to decompose at 100 ℃, and decomposes faster when the temperature is above 130 ℃.When heated, it decomposes into hydrogen chloride gas (hydrogen chloride gas is toxic gas), which changes color from white → light yellow → red → brown → black.The ultraviolet ray and oxygen in the sun will cause the photo oxidation decomposition of PVC, thus reducing the flexibility of PVC and finally making it brittle.This is why some PVC plastics become yellow and brittle after a long time.
Stable physical and chemical properties, insoluble in water, alcohol and gasoline, low gas and water vapor leakage;Under normal temperature, it can resist any concentration of hydrochloric acid, less than 90% sulfuric acid, 50-60% nitric acid and less than 20% caustic soda solution, and has certain chemical corrosion resistance;It is quite stable to salts, but can be dissolved in organic solvents such as ether, ketone, chlorinated aliphatic hydrocarbon and aromatic hydrocarbon.
The industrial PVC resin is mainly amorphous, but also contains some crystalline areas (about 5%), so the PVC has no obvious melting point. It begins to soften at about 80 ℃, and the thermal deformation temperature (1.82MPa load) is 70-71 ℃. It starts to flow at 150 ℃ under pressure, and slowly releases hydrogen chloride,It causes discoloration of PVC (from yellow to red, brown, or even black).
The weight average relative molecular weight of industrial PVC ranges from 48000 to 48000, and the corresponding number average relative molecular weight is 2-19500.The weight average relative molecular weight of most industrial resins is between 100000 and 200000, and the number average relative molecular weight is between 45500 and 64000.Rigid polyvinyl chloride (without plasticizer) has good mechanical strength, weather resistance and flame resistance, and can be used as structural materials alone, and used in chemical industry to manufacture pipes, plates and injection molding products.Rigid PVC can be reinforced.
Density: 1380 kg/mthree
Young's modulus of elasticity (E): 2900-3400 MPa
Tensile strength (σ t): 50-80 MPa
Elongation at break: 20-40%
Glass transition temperature: 87 ℃
Melting point: 212 ℃
Softening temperature: 85 ℃
Thermal conductivity (λ): 0.16 W/(m · K)
Thermal expansion coefficient (α): 8 × 10-5/K
Heat capacity (c): 0.9 kJ/(kg · K)
Water absorption (ASTM): 0.04-0.4
Refractive index: 1.52~1.55
The greatest feature of PVC is flame retardant, so it is widely used in fire prevention applications.However, PVC will release hydrogen chloride and other toxic gases, such as dioxin, during combustion.
The combustion of PVC is divided into two steps.First, hydrogen chloride gas and diene containing double bonds are decomposed by combustion at 240 ℃ - 340 ℃, and then carbon combustion occurs at 400-470 ℃.
It is one of the largest plastic products in the world, cheap and widely used. PVC resin is white or light yellow powder.Different additives can be added according to different uses, and PVC plastics can present different physical and mechanical properties.A variety of hard, soft and transparent products can be made by adding an appropriate amount of plasticizer into PVC resin.
The density of pure PVC is 1.4 g/cmthreeThe density of PVC plastic parts with plasticizer and filler is generally 1.15-2.00 g/cmthree。
Rigid PVC has good tensile, bending, compression and impact resistance, and can be used alone as structural materials.
The softness, elongation at break and cold resistance of soft PVC will increase, but the brittleness, hardness and tensile strength will decrease.
PVC has good electrical insulation performance, can be used as low-frequency insulation materials, and its chemical stability is also good.Due to the poor thermal stability of PVC, long time heating will lead to decomposition, release HCL gas, and discolor PVC, so its application range is narrow, and the use temperature is generally between - 15~55 ℃.
The conversion of hardness value Pa (Pa) and Shore hardness of PVC is as follows:
Pa
Shore hardness
forty-five
89±2，87~91A
fifty
86±2，84~88A
fifty-five
83±2，81~85A
sixty
80±2，78~82A
sixty-five
78±2，76~80A
seventy
75±2，73~77A
seventy-five
72±2，70~74A
eighty
69±2，67~71A
PVC hardness comparison table
Paasche hardness
thickness
Shore hardness test value
Average hardness value
30P
6mm
93-96 °
94.5 °
35P
6mm
87-93 °
90 °
38P
6mm
89-90 °
89.5 °
40P
6mm
88-90 °
89 °
45P
6mm
84-90 °
85 °
50P
6mm
82-83 °
82.5 °
55P
6mm
70-80 °
79 °
60P
6mm
74-76 °
75 °
65P
6mm
73-75 °
74 °
70P
6mm
72-74 °
73 °
75P
6mm
70-70.5 °
70 °
80P
6mm
67-68 °
67.5 °
85P
6mm
64-66 °
65 °
90P
6mm
63-64 °
63.5 °
95P
6mm
58-60 °
59 °
100P
6mm
57-59 °
58 °
110P
6mm
54-56 °
55 °
Another hardness comparison table
Pa
Shore hardness
thirty
95±2
forty-five
90±2
fifty
88±2
sixty
84±2
seventy
80±2
eighty
76±2
ninety
72±2
ninety-five
68±2
one hundred and five
64±2
Note: Shore hardness value is a 15 second reading
Forming conditions
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Rigid PVC
Tube temperature: 160-190 ℃
Mold temperature: 40-60 ℃
Drying temperature: 80 ℃ × 2 h
Injection pressure: 700-1500 kg/cmtwo
Density: 1.4 g/cm3
Molding shrinkage: 0.1-0.5%
Meat thickness: 2.0-50mm
Water absorption (24h): 0.1-0.4%
Melting degree softening point: 89 ℃
Hot deformation temperature: 70 ℃
Soft PVC
Tube temperature: 140-170 ℃
Mold temperature: 40-60 ℃
Drying temperature: 80 ℃ × 2 h
Injection pressure: 600-1500 kg/cmtwo
Density: 1.4 g/cmthree
Molding shrinkage: 0.1-0.5%
Meat thickness: 2.0-50mm
Water absorption (24h): 0.1-0.4%
Melting degree and softening point: 85 ℃
Hot deformation temperature: 55 ℃
material processing
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PVC plastics have different shapes and processing methods, which can be pressed, extruded, injected, coated, etc.The particle size, fish eye, bulk density, purity, foreign impurities and porosity of PVC resin have an impact on the processability;For paste resin, the viscosity and gelling property of paste should be considered.
PVC is amorphous polymer with small shrinkage.The powder should be preheated before processing to remove moisture, enhance the plasticizing effect and prevent bubbles.Moreover, PVC is very easy to decompose, especially when contacting with steel and copper at high temperature (decomposition temperature 200 ℃).The forming temperature range is small, and the material temperature must be strictly controlled.When using screw injection machine and straight nozzle, the hole diameter should be large to prevent material stagnation in dead corners.The mold pouring system should be large, the gate section should be large, and the mold should be cooled, with mold temperature of 30-60 ℃ and material temperature of 160-190 ℃.
At glass transition temperature (Tg, below 80 ℃), PVC is in glass state;At Tg→ Viscous flow temperature (Tf, about 160 ℃) is in the form of highly elastic rubber and plastic;At Tf→ Thermal decomposition temperature (Td）It is viscous flow. The higher the temperature, the easier the flow.When the temperature exceeds TdPVC decomposes into a large amount of hydrogen chloride (HCl), and the material loses its chemical stability and physical properties, so TdIt is the upper limit temperature for processing and forming.Due to the large intermolecular force of polyethylene, TfIt is very high, even close to the decomposition temperature, so it is necessary to add plasticizer to reduce Tf.On the other hand, it is also necessary to add stabilizer to improve the TdCan be processed and formed.
Glass transition temperature (Tg）It is only related to the chain segment structure of the molecular chain and has little relationship with the molecular weight, while the viscosity flow temperature (Tf）It is the temperature at which macromolecules start to move, which is related to the size of molecular weight. The larger the molecular weight, the higher Tf.Therefore, for some processing molding (such as injection molding), it is necessary to appropriately reduce the molecular weight of the resin.According to the different molecular weight, domestic suspension PVC resins are divided into 1-7 grades. The larger the serial number, the smaller the molecular weight.XJ-4 (XS-4) to XJ-7 (XS-7fHigh, need to add a lot of plasticizer to make TfIt is often used to manufacture soft products.PVC with an average degree of polymerization below 1000 is called low degree of polymerization PVC, which has good processing properties. During processing, less plasticizers can be added, so that the products will not be accelerated aging due to the migration of plasticizers.Low polymerization degree PVC products have good transparency, and are widely used in building materials, food and drug packaging materials, as well as replacing plexiglass products.
PVC melt belongs to non Newtonian pseudofluid. The higher the shear speed is, the smaller the apparent viscosity is, and the change is quite sensitive.When the temperature is raised, the viscosity will not decrease much. Even if the plastic is below the decomposition temperature, it will heat up and degrade by oxidation due to being at a high temperature for a long time, which will affect its performance.Therefore, to improve the fluidity of PVC melt, the main consideration should be to increase the shear rate (increase the pressure).In fact, increasing the external force will help the movement of macromolecules, so that TfThe macromolecule can flow at a lower temperature.
Main purpose
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Vinyl chloride profile
Profile and profiled bar are the largest areas of PVC consumption in China, accounting for about 25% of the total consumption of PVC. They are mainly used for making doors and windows and energy-saving materials, and their application is still growing significantly nationwide.In developed countries, the market share of plastic doors and windows also ranks first, such as 50% in Germany, 56% in France and 45% in the United States.
PVC pipe
Among many PVC products, PVC pipes are the second largest consumption area, accounting for about 20% of its consumption.In China, PVC pipes have been developed earlier than PE pipes and PP pipes, with many varieties, excellent performance and wide application range, occupying an important position in the market.
PVC membrane
The consumption of PVC in the field of PVC film ranks third, accounting for about 10%.After PVC is mixed and plasticized with additives, a three roll or four roll calender is used to make a transparent or colored film of specified thickness. The film is processed in this way to become a calendered film.You can also cut and heat seal packaging bags, raincoats, tablecloths, curtains, inflatable toys, etc.The wide transparent film can be used for greenhouses, plastic greenhouses and mulching films.The biaxially stretched film can be used for shrink packaging due to its heat shrinking characteristics.
PVC hard materials and plates
PVC is mixed with stabilizers, lubricants and fillers. After mixing, hard pipes, profiled pipes and corrugated pipes of various diameters can be extruded with an extruder to be used as sewer pipes, drinking pipes, wire casings or stair handrails.The rolled sheets can be overlapped and hot pressed to make hard plates of various thicknesses.The plate can be cut into the required shape, and then welded into various chemical resistant storage tanks, air ducts and containers with hot air using PVC electrodes.
PVC general soft products
Extruder can be used to extrude hoses, cables, wires, etc;With injection molding machine and various molds, plastic sandals, soles, slippers, toys, auto parts, etc. can be made.
PVC packaging materials
PVC products are mainly used for packaging various containers, films and hard pieces.PVC containers are mainly used to produce mineral water, beverage, cosmetics bottles, as well as refined oil packaging.PVC film can be used to co extrude with other polymers to produce low cost laminated products and transparent products with good barrier properties.PVC film can also be used for stretch or heat shrink packaging, used to package mattresses, cloth, toys and industrial goods.
PVC dado and floor
PVC wall panels are mainly used to replace aluminum wall panels.In addition to a part of PVC resin, the other components of PVC floor tiles are recycled materials, adhesives, fillers and other components, which are mainly used in airport terminal floors and hard floors in other places.
PVC consumer goods
Luggage bag is a traditional product made of PVC. PVC is used to make all kinds of imitation leather for luggage bags and sports products, such as basketball, football and football.It can also be used to make belts for uniforms and special protective equipment.PVC fabrics for clothing are generally absorbent fabrics (without coating), such as ponchos, baby pants, imitation leather jackets and various rain boots.PVC is used in many sports and entertainment products, such as toys, records and sports goods. PVC toys and sports goods have increased significantly, and they have advantages due to their low production costs and easy molding.
PVC coated products
The synthetic leather with substrate is made by applying PVC paste on cloth or paper, and then plasticizing at above 100 ℃.It can also be formed by calendering PVC and additives into a film, and then pressing with the substrate.The substrate free artificial leather is directly rolled into a soft sheet of a certain thickness by a calender, and then pressed with patterns.Artificial leather can be used to make leather cases, bags, book covers, sofas and car cushions, as well as floor leather, which can be used as floor covering materials for buildings.
PVC foam products
When soft PVC is mixed, a proper amount of foaming agent is added to make a sheet, which is foamed into foam plastic, and can be used as foam slippers, sandals, insoles, and shock proof and cushioning packaging materials.It can also be used to form low foaming rigid PVC sheet and profile based on extruder, which can replace wood and is a new type of building material.
PVC transparent sheet
PVC is mixed with impact modifier and organotin stabilizer, and becomes transparent sheet after mixing, plasticizing and calendering.It can be made into thin transparent containers or used for vacuum blister packaging by thermoforming. It is an excellent packaging material and decorative material.
other
Doors and windows are assembled with hard profiled materials.In some countries, it has occupied the market of doors and windows together with wooden doors, windows and aluminum windows;Wood like materials, substitute steel building materials (north, seaside);Hollow container.
Virtual circuit is one of the services provided by the packet switching network (the other is datagram service). In short, it is to establish a virtual logical connection between user hosts through the internal control mechanism of the network, and ensure the correctness and orderliness of the packets transmitted on it. Virtual circuit shall be established and removed before and after communication.The permanent virtual circuit is a virtual circuit established during network initialization, and the virtual circuit is always maintained.10. Both the network and B-ISDN provide PVC services.PVC plastic bags are generally prohibited.
Research on New Materials
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At present, the total annual demand for modified plastics in China is about 5 million tons, accounting for about 10% of the total plastic consumption, which is far below the world average.There is still a big gap between China's per capita plastic consumption and that of developed countries in the world.In order to realize the rapid and stable development of China's modified plastics industry, innovative technology is the key point of future development.
Chemical industry analysts believe that the overall development level of China's modified plastics industry is not very high at present, the production scale of enterprises in the industry is generally small, there are many primary products in the product market, the quality of intermediate products is not stable enough, and the lack of advanced products is far from meeting the needs of China's current social and economic development.As an important part in the field of new chemical materials, modified plastics have been listed as one of the key scientific and technological fields of development by the state.The successive introduction of various domestic policies will further promote the development of the modified plastics industry.The automobile and home appliance industries are hot spots in the development of modified plastics, accounting for more than 50%.
The application of plastics in the automotive industry has a history of more than 50 years.With the development of automobiles towards light weight and energy saving, higher requirements are put forward for materials.Since 1 kg plastic can replace 2-3 kg steel and other heavier materials, the fuel consumption can be reduced by 6% - 8% for every 10% reduction in the dead weight of the vehicle.Therefore, increasing the amount of modified plastics in the vehicle can reduce the cost and weight of the whole vehicle, and achieve energy-saving effect.The amount of plastic used in passenger cars and commercial vehicles varies.The demand for modified PP, PC alloy and modified ABS was about 114.63, 1513 and 149700 tons respectively last year.The market of modified plastics for household appliances in China is mainly occupied by foreign enterprises, while domestic modified plastics enterprises occupy less than 1/3 of the market share.Because most of the products of domestic enterprises are limited to the level of low technology content and low standards, they are obviously lack of the ability to develop areas with high performance requirements.
According to the 2009-2012 Research Report on Market Analysis and Investment Value of China's Modified Plastics Industry, with the substantial improvement of people's living standards and the improvement of technical means, "plastic instead of steel" and "plastic instead of wood" will become a trend in China.The modified plastics industry, as one of the fastest growing and potential sub industries in the plastics processing industry, is expected to maintain a growth rate of more than 10% in the total market demand in the next five years.
PVC replaces steel material with plastic
Through the research of PVC modification technology, the advanced internal plasticizing process and additive formula abroad are used to ensure the mechanical and electrical properties of PVC plastic steel, improve the flame retardancy, and make the product have the advantages of high strength, corrosion resistance, flame resistance, good insulation, light weight, convenient construction, etc. It can completely replace steel pipes in the electrical wiring system.
PVC replaces wood with plastic
PVC wood plastic composite material is a new type of composite material, which is made of waste wood fiber and plastic as main raw materials, supplemented by appropriate processing aids, through hot pressing process.Its products fully reflect the recycling concept of renewable resources and petroleum products, which is of great significance to alleviate the current shortage of wood and petroleum resources, serious environmental pollution and other problems[13]。
Home building materials products with PVC as the main raw material have become the second pillar of China's plastic industry, with an average annual growth rate of more than 15%.In the next 10 years, it is estimated that 30 billion square meters of new housing construction area will be added nationwide. If these buildings achieve 50% energy conservation on the existing basis, the market demand for energy-saving building materials can reach several trillion yuan, which has created a huge space for the development of indoor energy-saving decorative materials.For a long time, the building materials industry has been characterized by high energy consumption and high pollution.In order to meet the requirements of low-carbon economy, home building materials enterprises have developed a batch of PVC high simulation building materials with plastic instead of wood after years of research and development, which have become a perfect combination of low-carbon and practical home products.
Experts in the industry pointed out that the PVC building materials with plastic instead of wood not only save costs, but also can be recycled, which is in line with the general trend of environmental sustainable development and circular economy.
the way to distinguish
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The conventional identification methods of PVC are generally summarized into the following three categories, namely:
Combustion identification
Softening or melting temperature range: 75~90oC；
Combustion: self extinguishing after ignition;
Combustion flame status: yellow at the top and green at the bottom with smoke;After the fire: off fire;Smell: pungent sour taste.
This method is the most simple and direct, and is generally preferred.
Add the suspected PVC plastic into the above solvents and observe the dissolution of the plastic to determine whether it is PVC.After the solvent is heated, the dissolution effect will be more obvious.
Specific gravity method
The specific gravity of PVC is 1.35~1.45, generally about 1.38.Polyvinyl chloride and other plastics can be distinguished by specific gravity difference or by measuring specific gravity.However, because PVC can add plasticizers, modifiers and fillers, the proportion of PVC varies greatly, and the hardness and softness of PVC vary greatly. At the same time, many properties of PVC plastic will change due to the addition of some components, resulting in the effect of our common identification methods is not obvious, and even the phenomenon changes, making it impossible to make accurate judgments.For example, the density of plasticized PVC (about 40% plasticizer) is 1.19~1.35;However, PVC hard products are increased to 1.38~1.50.For high filled PVC products, the density sometimes exceeds 2.
In addition, it can also be determined by determining whether the material contains chlorine, but because vinyl chloride copolymer, neoprene, polyvinylidene chloride, chlorinated polyvinyl chloride, etc. all contain a high proportion of chlorine, they must be identified by pyridine color reaction.Note that before the test, the test sample must be extracted with ether to remove the plasticizer. Test method: dissolve the sample taken with ether benzene in tetrahydrofuran, filter out the insoluble components, add methanol to precipitate them, and dry them below the first 75 degrees after extraction.A small amount of dried sample does not need 1 mL pyridine to react with it. After a few minutes, add 2 to 3 drops of 5% sodium hydroxide methanol solution (1 g sodium hydroxide is dissolved in 20 mL methanol), immediately observe the color, and observe again after 5 minutes and 1 hour.Different chlorine containing plastics can be identified according to their colors.
In daily life, PVC and PE plastic films (bags) need to be distinguished if they are exposed more frequently. Simple methods:
1. Touch method
There is a sense of lubrication when you touch it with your hand, and the surface is like coated with a layer of wax (chemically called wax sense). This is a non-toxic polyethylene film bag, while the PVC film is somewhat sticky when you touch it.
2. Dithering method
When shaking by hand, the sound is crisp, and the polyethylene film bag is easy to float.The PVC film bag with low voice when shaking by hand.
3. Combustion method
Flammable in case of fire, the flame is yellow, paraffin oil drops when burning, and there is gas when burning candles. It is a non-toxic polyethylene film bag.If it is not easy to burn, it will be extinguished immediately after leaving the fire. The flame is green and it is a PVC film bag.
4. Immersion method
Dip the plastic bag into the water, press it into the water by hand, and polyethylene will emerge from the water surface, while polyvinyl chloride will sink into the water bottom (the density of polyethylene is less than that of water, and the density of polyvinyl chloride is greater than that of water; at normal temperature, it is about 0.92 g/cm respectivelythreeAnd 1.4 g/cmthree）。
A copper wire can also be taken and burned red in the fire. Then, contact the copper wire with the plastic film for test to produce chemical changes, and put the copper wire dipped with the plastic composition back into the flame.At this time, careful observation is required. If there is a colorful and dazzling green flame, it means that this plastic material contains chlorine, which belongs to PVC.
Health and Safety
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degradation
Aspergillus fumigatusDegradable plasticized polyvinyl chloride[14]。In mineral saltagarPhanerochaete chrysosporium grows on PVC in.Ascomycetes flavescens, Aspergillus tigrisAspergillus nigerAspergillus occidentalis can effectively degrade PVC.
Plasticizer
Phthalates are often added to plastics as plasticizers.Phthalates are not designed to covalently bind to the polymer matrix, so they are easily leached.Phthalates are high in plastics.For example, by weight, the content of phthalate in intravenous medical bags can reach 40%, and in medical tubing can reach 80%[15]。Vinyl products are ubiquitous, including toys, car interiors, shower curtains and floors, which initially release chemical gases into the air.
Hazards of lead
Previously, people often added lead in PVC to improve its processability and stability.It has been proved that lead can seep into drinking water from PVC pipes.
Lead stearate, an antioxidant of PVC, is toxic.Lead will be released from polyvinyl chloride (PVC) products containing lead salt antioxidant in contact with ethanol, ether and other solvents.PVC containing lead salt is used as food packaging. When it meets with fried dough sticks, fried cakes, fried fish, cooked meat products, cakes and snacks, the lead molecules will diffuse into the oil. Therefore, PVC plastic bags cannot be used to contain food, especially oily food.In addition, PVC plastic products will slowly decompose into hydrogen chloride gas at a relatively high temperature, such as about 50 ℃, which is harmful to human health. Therefore, PVC products should not be used as food packaging.
Vinyl chloride monomer
In the early 1970s, the carcinogenicity of vinyl chloride (usually called vinyl chloride monomer or VCM) was related to the cancer of PVC industry workers.In particular, workers in the polymerization department of a B.F. Goodrich factory near Louisville, Kentucky, were diagnosed with hepatic angiosarcoma, also known as angiosarcoma, which is a rare disease.Since then, studies on PVC workers in Australia, Italy, Germany and the United Kingdom have all shown that some types of occupational cancer are related to exposure to vinyl chloride, and it has been accepted that vinyl chloride monomer is a carcinogen[16]。
Dioxin (TCDD)
Dioxin (TCDD), the most lethal substance in the dioxin family, is a well-known carcinogen, hormone decomposer and toxic compound, which is very harmful to humans and animals.HCl produced during combustion of PVC is almost proportional to its chlorine content.Some studies have shown that the chlorine in discharged dioxins does not come from HCl in flue gas.On the contrary, most dioxins are generated in the solidification phase through the reaction of inorganic chlorides with the graphite structure in the soot containing particles.Copper is the catalyst for these reactions.
The research on household waste incineration shows that with the increase of PVC concentration, the production of dioxin is also increasing.Landfill fires may be a larger source of dioxins in the environment.An international research survey has consistently shown that the concentration of dioxin in the area affected by open-air waste incineration is high. A study has investigated the homologous model and found that the sample with the highest concentration of dioxin is "typical PVC pyrolysis"[17]。
The second largest source of dioxins is medical and municipal waste incinerators.Various studies that have been carried out have yielded contradictory results.For example, a study of commercial scale incinerators showed that there was no relationship between the content of PVC in waste and dioxin emissions.Other studies have shown that there is a clear correlation between the formation of dioxins and the content of chlorides, and that PVC is an important factor in the formation of dioxins and polychlorinated biphenyls in incinerators.
other
Because disposable medical devices are mostly made of medical grade polyvinyl chloride (PVC) or polycarbonate (PC), and the thermal decomposition products in the processing of PVC have strong corrosivity to steel, while PC has high hardness and viscosity, the materials of parts in the plasticizing part must be corrosion resistant, abrasion resistant and have high polishing performance.Most medical injection molding machines adopt the method of hard chromium plating on barrel screws or use stainless steel as the material to make simple screws to meet the above special requirements.In addition, in order to prevent gas generated from thermal decomposition during PVC processing, it is required to aluminize the surface of the movable and fixed formwork, and aluminize the outer wall plate gold or use stainless steel plate to make plate gold, and use non-toxic silica gel to seal the plate metal seam,In order to prevent the gas generated during plastic processing from running out (the gas generated during plastic processing can be collected by special equipment and then discharged into the atmosphere after purification).
Recycling
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Polyvinyl chloride (PVC) is recyclable, and its resin identification code is "3"
