PVC can be made by substitution reaction of ethylene, chlorine and catalyst. Because of its fire and heat resistance, PVC is widely used in all kinds of products in all walks of life: wire sheaths, optical fiber sheaths, shoes, handbags, bags, ornaments, signboards and billboards, building decoration products, furniture, ornaments, rollers, pipes, toys (such as the famous Italian "Rody" jumping horse), door curtains, rolling doors, auxiliary medical supplies, gloves, and the preservation of certain foods. PVC is prepared by free radical addition polymerization. The polymerization methods are mainly divided into suspension polymerization, emulsion polymerization and bulk polymerization, with suspension polymerization as the main method, accounting for about 8% of the total PVC production. Pure water, liquefied VCM monomer and dispersant are added into the reaction kettle, and then initiator and other additives are added. After heating to a certain temperature, the VCM monomer undergoes free radical polymerization to generate PVC particles. Continuous stirring makes the particle size uniform and suspends the generated particles in water. In addition, there is also a resin for PVC paste produced by micro-suspension method, and the product performance and paste-forming property are good.
① The monomer is suspended and dispersed in the water phase in the form of droplets by suspension polymerization, and the selected oil-soluble initiator is dissolved in the monomer. The polymerization reaction is carried out in these droplets, and the heat of polymerization reaction 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 mostly 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 resin finished product. Vinyl chloride monomer should be removed from the resin as much as possible. The content of free monomer in PVC used for food packaging should be controlled below 1ppm. In order to obtain the resin with specified molecular weight and molecular weight distribution range and prevent explosion polymerization, it is necessary to control the temperature and pressure in the polymerization process. The particle size and particle size distribution of the 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. Polymerization reactor 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 the production cost, the volume of the reaction kettle has gradually developed from several cubic meters to more than ten cubic meters, and the maximum has reached 2 cubic meters (kettle reactor). The polymerizer should be descaled after repeated use. PVC prepared by using polyvinyl alcohol and cellulose ethers as suspension stabilizers is generally loose, porous, large in surface area, and easy to absorb plasticizers and plasticize.
② emulsion polymerization is 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 should be added as emulsifier to disperse the monomer in the water phase to form emulsion, and water-soluble potassium persulfate or ammonium persulfate can be used as initiator, and "oxidation-reduction" initiation system can also be used. The 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 .5 ~ 2 μ m, 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 making PVC paste, artificial leather or impregnated products. The formula of emulsion polymerization is complex, and the impurity content of the product is high.
③ The bulk polymerization polymerization device is special, which is mainly composed of a vertical prepolymerization kettle and a horizontal polymerization kettle with a frame agitator. The polymerization is carried out in two stages. The monomer and initiator were prepolymerized in the prepolymerization kettle for 1 hour to generate seed particles, at which time the conversion rate reached 8% ~ 1%, and then flowed into the second polymerization kettle, and the monomer equivalent to the prepolymer was added to continue polymerization. When the conversion rate reaches 85% ~ 9%, the residual monomer is discharged, and then the finished product is obtained by crushing and sieving. 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. This method has the advantages of simple production process, good product quality and low production cost. PVC resin is a polar amorphous polymer with density of 1.38 g/cm3 and glass transition temperature of 87℃, so it has poor thermal stability and is not easy to process. Can not be used directly, it must be modified and mixed, and relevant additives and fillers can be added before it can be used. However, due to the different types and fractions of additives and fillers, the properties and requirements of PVC materials are different. We usually call it PVC formula, strictly speaking, it is PVC modified formula, and PVC can only be used after modification. This kind 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 materials (alloys, blends and composites), the functionalization of materials, and the optimization of performance and price. The modification methods are mainly 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. Therefore, 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 a screw extruder, it is more fully plasticized and easier to process, especially for products with injection molding technology. Screw extruder is one of the most important equipment for plastic molding. It carries out plastic solid transportation, compaction, melting, shearing, mixing and extrusion molding through external power transmission and heat transfer of external heating elements. Screw extruder plays an important role in both plasticizing granulation machinery and molding processing machinery. Strictly speaking, PVC products with special requirements and PVC modified formula are tailored according to customer requirements. There is also * * * poly-derivation in the production of PVC. Such modified varieties include vinyl chloride * * * polymer, polyvinyl chloride * * mixture and chlorinated polyvinyl chloride. PVC has poor thermal stability and light resistance. Hydrogen chloride began to decompose at 15℃, and adverse reactions occurred with the content of plasticizer. In addition, the influence of pigment on PVC is 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 pigments have poor acid resistance, so during the coloring process of PVC, they will interact with hydrogen chloride produced by PVC decomposition and lose their proper color. Therefore, in terms of PVC coloring, considering the characteristics of the resin and related additives used, combined with the characteristics of pigments. The following problems should be paid attention to when choosing colorants.
1. Some metal ions in the pigment will promote the thermal oxidative decomposition of PVC resin, as shown in Figure 1.
The measurement method is the hue change of polyethylene with pigment when heated to 18℃. Because the pigment contains metal ions, the decomposition of PVC is accelerated, resulting in hue change. At the same time, it should also be noted that the same addition of lake red can make the color difference of PVC different, such as containing calcium, the color difference is small; The color difference is great when manganese is contained, which is due to the promotion of dehydrochlorination of PVC by metals such as manganese.
sulfide colorants (such as cadmium red, yellow, etc.) are used for coloring PVC, which may release hydrogen sulfide due to the decomposition of colorants. This kind of 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, PVC has a worse insulation than polyethylene, so the influence of pigments is even 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). Migration only occurs in plasticized PVC products and when dyes or organic pigments are used. The so-called migration is 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, which leads to color bleeding, solvent bleeding or frosting.
another problem is "scaling". It refers to that the colorant is released from the system during coloring processing because of poor compatibility or incompatibility of the colored object, and deposited on the surface of processing equipment (such as the inner wall of the barrel of extruder and the inner wall of die hole). Refers to the ability of pigments to withstand various climates. These include visible light and ultraviolet light, moisture, temperature, atmospheric chlorination and chemicals encountered during the use of products. The most important weatherability includes fastness, pulverization resistance and durability of physical properties. However, organic pigments are good or 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 color change of pigments are generally caused by the reaction genes of pigments. These reactive genes can interact with moisture in the atmosphere or chemical agents-acid and alkali. For example, cadmium yellow will fade under the action of water and sunlight, and Lissol red has good light resistance and is suitable for most indoor applications, but it will fade seriously when it is used outdoors with acid and alkali components.
the determination method of dehydrochlorination is in accordance with JIS-K-6723, and the determination temperature is 18℃. Based on the dechlorination time of uncolored PVC composite, the extension or retardation time is calculated at intervals of 5% and 1%, and negative values indicate accelerated decomposition. Generally, PVC materials with special requirements need to be imported from abroad. Among them, United Carbonization Company of the United States and Nordic Chemical Company are famous. With the continuous research and development and technology accumulation of major scientific research institutes and production units in China, the formula design and manufacturing of domestic PVC modified materials have reached the international advanced level, and companies with independent intellectual property rights such as Xuzhou Hanyongxin Materials Co., Ltd. have emerged, which have completely replaced imported materials and many products have been exported abroad.