Production process and technical application of th

Production process and technical application of PVDC resin (II)

2 production process and technical progress of PVDC resin

PVDC resin is usually copolymerized by vinylidene chloride (VDC) monomer and vinyl chloride monomer. In industrial production, the polymerization methods of PVDC resin usually include lotion polymerization and suspension polymerization [5]

2.1 lotion polymerization

PVDC resin produced by lotion polymerization has short reaction time, high molecular weight, narrow molecular weight distribution, and ideal copolymers with different components can be obtained. PVDC resin produced by lotion method is generally powder resin, and the content of VDC is required to be less than 70%. Generally, inorganic persulfate and azodiisobutyronitrogen are used as initiators. First, Nanshan Aerospace Materials Industrial Park is just a microcosm of the development of aerospace industry in our city. Soft water, emulsifier and initiator are put into the polymerization kettle. After polymerization at 0~50 ℃ for 3~4h, a certain amount of VDC monomer is added. When the pressure of the polymerization system drops to a certain range, the reaction is terminated for desorption. The powdered PVDC resin can be prepared by adding alkali to demulsify, then centrifuging, drying and screening. Because of its low production capacity and high consumption; The residues of various additives are high, which are difficult to be effectively removed, and the production process is complex. At present, they have been gradually eliminated

2.2 suspension polymerization

PVDC resin is produced by suspension method. The reaction is stable. 2. According to the requirements of the test piece, the heat is uniform, and the resin with high VDC content (generally between 80%~95%) can be prepared. Moreover, the residue of various water-soluble additives and monomers is small, and its performance is better than that of the resin of lotion polymerization. It is widely used in food preservation films, film tools and sheets with high barrier performance; Its disadvantage is that the reaction time is long, the molecular weight distribution of the resin is wide, the components of VC and VDC in the resin are difficult to control, and the molecular weight is difficult to be high

the production of PVDC resin by suspension method has been achieved. C. the screw part of the screw rod is kept clean and lubricated with butter or 2 molybdenum sulfide; Promotion and application of area. Generally, the production and operation process of this method is to add soft water, initiator, dispersant and auxiliary additives into the polymerization kettle according to the proportion, replace with nitrogen, vacuum qualified, add a certain proportion of vinylidene chloride and vinyl chloride monomer, cold stir, and then raise the temperature to the specified polymerization temperature for polymerization. When the system pressure drops to a certain value, add terminator to end the reaction; After pressure relief, the product can be obtained by thermal vacuum desorption at 70~80 ℃ for 1~2h, and then centrifugation and drying; The unreacted monomer can be reused after absorption and purification. When vinylidene chloride and vinyl chloride are copolymerized, the competitive polymerization rates of the two monomers are different, which is easy to drift the composition of the resin [6]. Therefore, in order to control the components of VDC-VC suspension copolymerization, the polymerization temperature curve and stirring curve should be changed during the reaction process, and the polymerization conversion should be properly controlled to prevent the molecular weight distribution from being too wide. During VDC-VC copolymerization, it is best to use efficient initiators such as EHP, DCPD, IPP, etc., which have small chain transfer effect and can make the polymerization degree of the resin higher [7]

2.3 technical progress

since the industrial production of PVDC resin was realized in the late 1940s, there have been many problems, such as wide molecular weight distribution of the resin, high heat sensitivity, high oxygen permeability, high hot water shrinkage (heated for 30min at 121 ℃), crystal dots, and unstable quality between batches, which have greatly limited the production and application of PVDC resin. At the same time, because the production of PVDC resin has very high requirements for the measurement and reaction of various materials, the quality error is required to be less than 1 ‰, and the reaction temperature fluctuation is within ± 0.1 ℃, a little carelessness may cause quality problems. In the late 1970s, western countries increased the technical investment in the production of PVDC resin, and made a major breakthrough in quality research in the late 1980s, realizing the stable production of large kettles. In terms of technical improvement, automatic electronic metering device and automatic interlocking system are mainly used, 40m3 and 50m3 polymerizers are developed, and special modifying additives for PVDC resin are synthesized, which improves its processing performance. The production of resin has realized specialization and serialization, making its application more extensive. Due to the different reactivity ratios of vinylidene chloride and vinyl chloride, they are 3.2 and 0.3 respectively at 60 ℃. With the progress of polymerization, the coordination ratio between the two monomers and the link ratio in the copolymer will not be completely uniform, making the mechanical, chemical, physical and processing properties of the resin unstable. At present, some manufacturers have adopted the method of intermittently adding vinylidene chloride monomer with accelerated conversion to achieve the purpose of relatively uniform copolymer links. Due to the strict technical requirements of this process, it has not been popularized

in recent years, vinylidene chloride (VDC) and methyl acrylate (MA) copolymer resin has also developed rapidly, and Dow Chemical Company of the United States has been the first to succeed and put on the market. Vdc/ma copolymer resin produced by suspension method has short polymerization time and high VDC content. Its barrier performance is higher than that of vdc/vc copolymer resin, and its fragrance retention and flavor retention are almost twice that of ordinary vdc/vc copolymer resin, and it has better color quality and color stability. At the same time, Ma plays a good role in internal plasticization in the resin, with less additives added during film forming, and good thermal stability, making it more practical. See Table 3 for the main production of PVDC resin abroad

Table 3 capacity, brand and use of major foreign PVDC manufacturers

normal> Company name

normal> brand

normal> production capacity/kt/a

normal> use

normal> Dow Chemical Company (United States)

normal>8662032168

normal>60

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