Polyvinyl chloride
The abbreviation code of polyvinyl chloride is PVC, according to the output, polyvinyl chloride occupies the second place of the five general plastics in the world and our country.
1, process characteristics:
Polyvinyl chloride has the following moulding process characteristics: 1, poor thermal stability. In order to avoid overheating decomposition of the material, should try to avoid all unnecessary heat phenomenon, strict control of the moulding temperature, to avoid the material to stay in the cylinder for too long (especially when the production start and shift handover), and should try to reduce the friction heat in the process of plasticisation. Polyvinyl chloride melt viscosity is high, melt processing process should try to avoid the use of molecular weight is too high grade, the ingredients should be added to the appropriate lubricant to increase the fluidity of the material, the stabiliser should be used to higher efficiency of the organotin class, such as dibutyltin maleate, dibutyltin di-n-octyl dithiocarbamate, and so on. Injection moulding should not use plunger type injection moulding machine.2, PVC melt viscosity is high, need high moulding pressure, in order to avoid melt rupture, injection, extrusion is recommended to use medium and low speed, avoid high speed.3, PVC thermal decomposition of hydrogen chloride, corrosive to the equipment, processing of metal equipment should be taken to plating protective measures or the use of corrosion-resistant steel.4, PVC melt cools fast ( Specific heat capacity is only 836-1170kJ/(kg-K), and there is no heat of phase change), short moulding cycle.
2, processing technology
PVC can be moulded by injection moulding, extrusion, blow moulding, calendering, plastic lining, foaming and other moulding processes. For plasticized PVC products, it is necessary to add plasticizer and other additives to the PVC powder or granule material for pre-mixing and plasticising before moulding and processing. The plasticised ingredients are prepared into shapes suitable for processing, for example, when using injection moulding and extrusion processes, extrusion granulation is required; when used in calendering processes, it needs to be pre-compressed into a soft plate. Injection moulding is mainly used for rigid PVC. The process conditions for injection moulding of hard PVC are listed in Table 4-1. PVC can be extruded into a variety of profiles and blown film. Table 4-2 and Table 4-3 are polyvinyl chloride pipe and polyvinyl chloride film extrusion moulding process conditions.
3, the application of polyvinyl chloride:
The application of polyvinyl chloride is mainly concentrated in the preparation of the following aspects of products: 1, film and artificial leather, film is mainly for agricultural use. 2, oil resistance, corrosion resistance, aging resistance, non-combustible wire and cable wrapping, insulation layer. 3, kinds of profiles such as pipes, rods, profiles, door and window frames; corrugated boards and building materials, indoor flooring and decorative materials, all kinds of panels. 4, furniture, toys, sports equipment, medical fittings, packaging coatings, etc..
4、Modification of polychloroprene:
The disadvantages of polychloroethylene are low softening point, poor heat and cold resistance, toughness is also poor, especially poor thermal stability, in addition to the melt viscosity is also higher, more difficult to process. In order to overcome these shortcomings, a variety of modified polyvinyl chloride. First, chlorinated polyvinyl chloride: chlorinated polyvinyl chloride, also known as perchloroethylene, chlorinated polyvinyl chloride resin by the chlorination of the proceeds. 1, chlorination method: the production of chlorinated polyvinyl chloride is mainly used in the suspension chlorination method. Polyvinyl chloride resin powder suspended in the concentration of about 20% of hydrochloric acid (or water), with chloroform or dichloroethane for the expansion agent, to form a stable suspension. Chlorination is carried out with peroxide, initiator or ultraviolet light irradiation, and chlorine is passed under atmospheric pressure and a temperature of 60-65°C. After chlorination, the polyvinyl chloride contains chlorine. The chlorine content of chlorinated PVC can reach 66%～67%.2. Performance improvement: Chlorinated PVC has obvious performance improvement compared with PVC, mainly in heat resistance and cold resistance. The maximum continuous use temperature of unchlorinated polyvinyl chloride is only 65～80C, and the chlorinated polyvinyl chloride can be increased to 100℃. Unchlorinated polyvinyl chloride embrittlement temperature is only one 20 ℃, chlorinated polyvinyl chloride embrittlement temperature can reach one 45 ℃. The tensile and flexural strength of chlorinated polyvinyl chloride has been improved compared with that of polyvinyl chloride, and the corrosion and aging resistance has been further improved, and the flame retardancy has been further improved, and the limited oxygen index has been improved from the original 47 to about 60. Chlorinated polyvinyl chloride density is greater than polyvinyl chloride, and the greater the chlorine content, the greater the density, when the chlorine content of 65%, the density of 1.52 g / cm. Second, copolymerisation modification: vinyl chloride and some other monomers copolymerisation, can improve some of the properties of polyvinyl chloride. 1. vinyl chloride copolymerisation with vinyl acetate. Peroxide initiator to make the two monomers for suspension or solution copolymerisation, can be obtained with vinyl acetate 10% 25% of the copolymer. Vinyl chloride – L acid vinyl copolymer molecular chain, containing side group chlorine atoms and acetyl, reducing the molecular chain order, so vinyl acetate into the copolymer molecular chain actually played a role in the plasticising effect, so that the copolymer melt mobility increased; toughness and cold resistance has been improved, but also so that the material is resistant to chemicals.
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