Polyvinyl chloride
- Dec 22, 2018 -

The preparation methods


PVC can be made from ethylene, chlorine and catalyst by substitution reaction. Due to its fireproof heat effect, polyvinyl chloride (PVC) are widely used in all walks of life all kinds of products: fiber wire skin, skin, shoes, handbags, bags, decorations, signs and billboards, building decoration supplies, furniture, hang act the role ofing, roller, pipes, toys, such as the famous Italian "Rody" jump vault), door, door, auxiliary medical supplies, gloves, some food tin foil, and fashion, etc.



Folding polymerization method


PVC is prepared by free radical addition polymerization, which is mainly divided into suspension polymerization, emulsion polymerization and bulk polymerization, with suspension polymerization as the main method, accounting for about 80% of the total production of PVC. Pure water, liquefied VCM monomer and dispersant were added into the reactor, and initiator and other auxiliaries were added. When heated to a certain temperature, free radical polymerization of VCM monomer was conducted to generate PVC particles. Continuous agitation makes the particles uniform in size and suspends the resulting particles in water. In addition, there is a micro suspension method to produce PVC paste resin, product performance and paste is good.



(1) suspension polymerization to assume monomer droplets suspended in the water phase, selection of oil-soluble initiator is dissolved in monomer, polymerization reaction in these droplets, polymerization reaction heat is absorbed by water in time, in order to ensure that these droplets bead dispersed in the water, need to join the suspension stabilizer, such as gelatin, polyvinyl alcohol, methyl cellulose, hydroxyethyl cellulose, etc. Organic peroxide and azo compound are mostly used as initiators, such as diisopropyl peroxide, dicyclohexyl peroxide, diethyl hexyl peroxide, azo diisoheptanitrile and azo diisobutyronitrile. Polymerization is carried out in an polymerization kettle with a agitator. After polymerization, materials flow into the monomer recovery tank or stripper to recover monomer. Then into the mixing kettle, wash and centrifugal dehydration, drying resin products. Vinyl chloride monomer should be removed from the resin as far as possible. PVC for food packaging, free monomer content should be controlled under 1ppm. In order to obtain the specified molecular weight and molecular weight distribution range of resin and prevent detonation during polymerization, the temperature and pressure of polymerization process must be controlled. The particle size and distribution of resin are controlled by mixing speed, suspension stabilizer selection and dosage. The quality of resin was characterized by particle size and particle size distribution, molecular weight and molecular weight distribution, apparent density, porosity, fisheye, thermal stability, color, impurity content and free flow of powder. Polymerization reactor is the main equipment, by the steel kettle body lining stainless steel or enamel made, equipped with a agitator and control the temperature of heat transfer jacket, or internal cooling exhaust pipe, reflux condenser. In order to reduce production costs, the volume of the reactor has been from a few cubic meters, more than 10 cubic meters gradually to large-scale development, the largest has reached 200 cubic meters (kettle reactor). The polymerization kettle must be descaled after repeated use. PVC made from polyvinyl alcohol and cellulose ether as suspension stabilizer is generally loose, porous, large surface area, easy to absorb plasticizer and plasticizer.



(2) emulsion polymerization is one of the earliest industrial production of PVC. In emulsion polymerization, in addition to water and vinyl chloride monomer, but also add alkyl sodium sulfonate and other surfactants as emulsifiers, monomer dispersion in the water phase and into emulsion shape, with water-soluble potassium persulfate or ammonium persulfate as the initiator, can also be used in the "oxidation-reduction" initiator system, polymerization process and suspension method is different. Polyvinyl alcohol is also added as emulsion stabilizer, dodecyl mercaptan as regulator and sodium bicarbonate as buffer. There are three polymerization methods: intermittent, semi - continuous and continuous. Polymerization products are emulsion, emulsion particle size 0.05 ~ 2 microns, can be directly applied or spray drying to powder resin. The emulsion polymerization method has short polymerization period, easy to control, high molecular weight of the resin obtained, uniform degree of polymerization, suitable for PVC paste, artificial leather or impregnated products. The formula of emulsion polymerization is complex and the impurity content is high.



(3) the body polymerization polymerization device is relatively special, mainly composed of vertical pre-polymerization kettle and horizontal polymerization kettle with frame agitator. The polymerization takes place in two stages. Monomer and initiator were prepolymerized in the pre-polymerization kettle for 1h to generate seed particles. At this time, the conversion rate was up to 8% ~ 10%, and then flowed into the second polymerization kettle. Monomer equivalent to the pre-polymerizer was added and continued to polymerize. When the conversion rate reaches 85% ~ 90%, the residual monomer is discharged, and then the finished product is obtained by crushing and sifting. The particle size and shape of the resin are controlled by agitation speed, and the reaction heat is carried out by monomer reflux condensation. This method has simple production process, good product quality and low production cost.



Folding modification method


PVC resin is a polar non-crystalline polymers, density: 1.380 g/cm3, glass transition temperature: 87 ℃, so the thermal stability is poor, not easy processing. Can not be used directly, must be modified mix, add relevant additives and filler can be used. Due to the addition of related additives and filler types and grades of different, which determines the PVC material properties and requirements are not the same. We usually call it PVC formula, strictly speaking, PVC modified formula, and 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 research of high performance of general plastics, the transformation of single-component materials into multi-component composite materials (alloy, blend, composite), the functionalization of materials, the optimization of performance and price, etc. The modification methods are mainly chemical modification, filling modification, reinforcing modification, blending modification and nano composite modification. The basic principle of modification is to give the material functions or improve some properties by adding materials. Therefore, PVC formula technology, determines the technical and production capacity of a factory.



PVC is generally modified granulation, with a screw extruder unit prepared into particles, plasticization is more complete, processing is easier, especially the process is the product of injection molding. Screw extruder is one of the most important equipment for plastic molding processing. It carries out plastic solid transportation, compaction, melting and shearing extrusion molding through external power transfer and heat transfer of external heating elements. Screw extruder as a plasticizing granulation machinery or molding machinery has an important position strictly speaking, has special requirements of PVC products, PVC modified formula, is tailored according to customer requirements. Still have copolymerization to derive in PVC production process namely, the variety that this kind of modification has PVC copolymer, polyvinyl chloride blend and chlorinated polyvinyl chloride to wait.



Index of folding modification


coloration



PVC has poor thermal stability and light resistance. At 150 ℃ starts to break down the hydrochloric acid as plasticizer content of adverse reactions. In addition, the impact of pigments on PVC is reflected in whether the pigments react with PVC and other components that make up PVC products, as well as the resistance of pigments to migration and heat resistance. Some of the ingredients in the colorant may contribute to the degradation of the resin. For example, iron ions and zinc ions are catalysts for PVC resin degradation. Therefore, the use of iron oxide (red, yellow, brown and black) pigments or white pigments such as zinc oxide, zinc sulfide and lide powder will reduce the thermal stability of PVC resin. Some colorants may interact with the degradation products of PVC resin. If the group of cyan pigment acid resistance is poor, so in the PVC coloring process, and the PVC decomposition of hydrogen chloride produced by the interaction and loss of due color. Therefore, in terms of PVC coloring, considering the characteristics of the resin used and related additives, combined with the characteristics of pigments. The following questions should be paid attention to when choosing colorants.



1. Some metal ions in the pigment will promote the thermal oxygen decomposition of polyvinyl chloride resin, as shown in figure 1.



Determination method for pigment with polyethylene of hue changes when heated to 180 ℃. Because the pigment contains metal ions to accelerate PVC decomposition, resulting in color changes. At the same time, it should be noted that the same addition of lake red can make PVC color difference, such as calcium, color difference is small; Manganese is a large color difference, this is due to manganese and other metals to promote PVC dehydrochlorination.



Sulfide colorants (such as cadmium red, yellow, etc.) used for polyvinyl chloride coloring, may be due to the decomposition of the colorants released hydrogen sulfide. Such colorants should not be mixed with lead stabilizers to avoid the formation of black lead sulfide.



2. Influence of pigments on electrical insulation of polyvinyl chloride



As a cable material, polyvinyl chloride and polyethylene, should be considered after coloring electrical properties. Especially polyvinyl chloride because of its poor insulation than polyethylene, so the impact of pigment is greater. It is indicated that the choice of inorganic pigment to color PVC has better electrical insulation than organic pigment (except furnace black and anatase titanium dioxide).



mobility



Mobility occurs only in plasticized PVC products and when dyes or organic pigments are used. The so-called migration is in the presence of part of the surrounding solvents soluble dyes or organic pigments, through plasticizer penetration into the surface of PVC products, those dissolved dye (color) material particles are also brought to the surface of products, so as to lead to the solution of seepage color, solvent seepage or frost.



Another problem is scaling. Refers to the colorant in the coloring process, because the color of the solubility is poor or simply incompatible and free from the system, deposited on the surface of the processing equipment (such as the extruder barrel inner wall, the inner wall of the die hole).



Weather resistance



The ability of a pigment to withstand various climates. These include visible and ultraviolet light, water, temperature, atmospheric chlorination, and chemicals encountered during the use of the product. The most important weather resistance, including fade-fastness, pulverization resistance and durability of physical properties. Organic pigments, on the other hand, vary in their structure. In addition, in the formula containing white pigment, the weatherability of pigment will be seriously affected.



Discoloration, darkening, or tonal change of pigments, usually caused by the reactive genes of pigments. These reactive genes can interact with water in the atmosphere or with chemicals - acids and bases. For example, cadmium yellow will fade under the action of water and sunlight, and lisol red has better light resistance, which is suitable for most indoor applications, while it fades seriously when used outdoors with acid and alkali components.



Dehydrochlorination according to JIS - K - 6723, the determination methods of measuring temperature 180 ℃. With the uncolored polyvinyl chloride compound dechlorinated time as the benchmark, extended or retarded time with 5%, 10% interval, negative value indicates accelerated decomposition.



The stability of



Polyvinyl chloride (PVC) resin of low softening point, about 75-80 ℃, brittle temperature below - 50 ~ 60 ℃, most of the products use for a long time temperature should not be more than 55 ℃, special formulation of up to 90 ℃. If the polyvinyl chloride resin is purely head - sex interface afraid of linear structure, internal no branched chain and unsaturated bond, although the C-Cl bond energy is relatively small, the stability of polyvinyl chloride resin should be relatively high. But even if the high purity of PVC resin, long-term by ultraviolet (uv) radiation in 100 ℃ or above has hydrogen chloride gas to escape. It indicates 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 is in the presence of oxygen or air.



electrical



Polyvinyl chloride belongs to polar polymer, which has a greater affinity for water and other conductive substances. Therefore, its resistance is smaller than that of non-polar polyolefin, but it still has a higher volume negative current and breakdown voltage. The polar group of polyvinyl chloride is directly attached to the main chain. Under the vitrification temperature, the dipole chain segment is restricted by the main chain atoms of the frozen structure and cannot move. Therefore, it does not produce dipolarization and can be used as a high-frequency insulating material at room temperature. When used for wire insulation, the electrical insulation of suspension resin is 10-100 times higher than that of floating liquid resin. The presence of chloride ions from degradation reduces electrical insulation.



Folding development


Has a special request of PVC material, generally need to be imported from abroad, in foreign countries is famous with union carbide and the Nordic chemical companies in the United States, with China's major scientific research institutes and production unit of continuous research and development and technology accumulation, domestic formula design of PVC modified materials, manufacturing has reached the international advanced level, have sprung up in xuzhou han yunghsin material co., LTD. Company with independent intellectual property rights, such as has been completely replace the imported materials, there are a number of products have been exported abroad. [3]