3.1.1 Development of modification technology
Since 1838, French physicist and chemist Henri Victor and German Eugen Baumann accidentally discovered vinyl chloride in 1872, research began on PVC, but due to its hardness and brittleness, its use became a problem. The story is as strange as birth of vulcanization technology, when sulfur was added to natural rubber due to worker mismanagement. In 1926, a random experiment B. Softens in solvents with a high boiling point, easy to process and provides elasticity, this is a technology plastification of PVC. This unintentional discovery opens door to industrial production and application of PVC.
In connection with industrial production and use of PVC, as time requires, various technologies for polymerization of PVC have appeared. From initial production of acetylene from calcium carbide to production of vinyl chloride monomer by direct oxidative cracking of ethylene, from photopolymerization and hot press polymerization to initiation polymerization, solution polymerization, emulsion polymerization, continuous emulsion polymerization, suspension polymerization and special aggregation and many other methods . After many years of application and screening, commonly used polymerization methods are: suspension method, emulsion method, bulk method and solution method. Among them, suspension method is easy to control and large-scale production due to its simple production process and durable product. suitability is main production method. From a global point of view, production of PVC by slurry method accounts for about 80% of total; emulsion polymerization uses water as dispersion medium, and resulting particles are relatively fine, and thermal stability and electrical insulation are poor, so it is suitable for pasty resin. Therefore, it occupies a place in polymerization field. Its resin is mainly used to make artificial leather, impregnated gloves, screen windows, paddy shoes, tool handles, wallpaper, floor coils, battery separators and toys, etc. The pouring method does not use water and dispersants. in polymerization process, and production cost is relatively high. It belongs to eliminated process, and its production capacity is less than 10% of total. At present, in my country, only Tianyuan, Yibin, Sichuan use volumetric method for PVC production; solution polymerization process is maintained due to use of polymerized PVC in coatings and specialty products. In USA, share of resins produced using different polymerization methods is: suspension method 87.8%, emulsion and microsuspension method 6.4%, polymerization methodtions in mass of 4.4%, method of solutions 1.4%. In our country, more than 90% of PVC is produced by suspension method.
If PVC plasticizing technology, accidentally discovered by W. L. Simon (Waldo Semon) in 1926, became first milestone in development of PVC research, processing and application technology, then whether it is filling modification technology or reinforcement modification technology, increase amount modification technologies such as Strength Modification Technology are technical categories resulting from this research and application. Then in 1991, under leadership of Wu Qie, based on idea of non-elastomer reinforcement proposed in article "Energy Absorption in Polycarbonate Blends with ABS and SAN" published by T. Kurauchi and T. Ohita in 1984, first PVC not modified with an elastomer Research aimed at making PVC a reality should be seen as another milestone in PVC technology. The subsequent cladding and core hardening and hardening technologies, as well as nano-hardening and hardening technologies, are based on research of this technology.
3.1.2 Aggregation method
Four types of PVC polymerization are currently used, namely suspension polymerization, emulsion polymerization, bulk polymerization and solution polymerization. The complexity of polymerization is actually same as our production. Determination of technological route in our production process is relatively simple. The difficulty lies in rationality of formulation and processing technology. so node management is very important. For technical reasons, quality gap between domestic PVC and imported PVC is obvious.
Because everyone is a consumer of PVC, not a producer, this is only a brief introduction to PVC processing process.
A. Pause method
Suspension polymerization is main PVC production method due to its simple production process, easy operation and large-scale production, as well as high product suitability. From a global point of view, PVC production by suspension method accounts for about 80% of all. A free radical polymerization method in which a monomer containing an initiator is suspended in water as droplets is called a suspension polymerization method. In general, water is continuous phase and monomer is dispersed phase. Polymerization is carried out in each small drop, and reaction mechanism is same as that of bulk polymerization, which can be considered as low mass polymerization. Similarly, it can be divided into homogeneous and heterogeneous polymerization depending on solubility of polymer in monomer. If an aqueous solution of a water-soluble monomer is suspended in an oil continuous phase as a dispersed phase, and polymerizationcarried out under action of an initiator, then this method is called reverse-phase suspension polymerization.
A slurry polymerization system usually consists of four main components: monomer, initiator, water and dispersant. The slurry polymerization system is a thermodynamically unstable system, stability of which must be maintained by mixing and dispersion. Under action of agitation and shear, monomer dissolved in initiator is dispersed into fine droplets which are suspended in water to initiate polymerization. The water-insoluble monomer is crushed and dispersed into small droplets under strong agitation, which is unstable. As reaction proceeds, dispersed droplets may condense into blocks. A dispersant must be added to system to prevent sticking. The particle size of suspension polymerization product is typically 0.05-0.2 mm. Its shape and size depend on intensity of mixing and nature of dispersant.
Suspension polymerization process is shown in fig. 3.1.
Figure 3.1 Suspension polymerization flowchart
B, emulsion method
Water is used as a dispersion medium during emulsion polymerization, and resulting particles are smaller, with poor thermal stability and electrical insulation, which is suitable for production of paste resin. The output of PVC paste resin in my country is less than more than 4% of total PVC production.
Emulsion polymerization is a widely used synthetic method in polymer synthesis process, since it uses water as a solvent, which is very beneficial for environment. Under action of an emulsifier and mechanical agitation, monomer is dispersed into an emulsion in water, and polymerization reaction is initiated by an initiator.
(a) Mechanism of emulsion polymerization
The distribution of each component of emulsified system in each phase.
Depending on polymerization reaction rate and change in number of monomer droplets, latex particles and micelles in system, emulsion polymerization can be divided into three stages.
The first stage is called latex particle formation period, or nucleation period, acceleration period until micelles disappear.
The second stage is called period of constant speed.
The third stage is called deceleration period.
Emulsion polymerization was first developed in Germany. During World War II, United States used this technology to produce styrene-butadiene rubber, and then successively produced nitrile rubber and neoprene, polyacrylate emulsion paint, polyvinyl acetate latex (commonly known as white glue), and PVC. Unlike suspension polymerization, emulsion system is relatively stable, and there are batch, semi-batch and continuous production in industry, and it will not mix or delaminate during transportation or pipeline storage. In production of "seed polymerization" (ie latex containing active chains), addition of monomers or regulators can be used to control polymerization rate, molecular weight and particle size of colloidal particles. High concentration latex can also be produced directly.
(b) New development in emulsion polymerization
In 1980s, there were many new developments in field of emulsion polymerization. For example, to eliminate contamination of emulsifiers with polymers and difficulty of washing, emulsion polymerization without emulsifiers appeared; ,Coordination emulsion polymerization system for 4-butadiene polymerization, reverse emulsion polymerization, in which acrylic acid or acrylamide is dispersed in oil and emulsified with a water-in-oil emulsifier, polymer particle size is between particle size of suspension polymerization and emulsion polymerization products. SoFurther. Dispersion polymerization is used in industry for production of coatings. The block diagram of emulsion polymerization is shown in fig. 3.2.
Figure 3.2 Flowchart of emulsion polymerization
The block method does not use water and dispersant, post-polymerization processing is simple, and purity of product is high, but there are problems with mixing and heat transfer in polymerization process, and production cost is high. refers to excluded process, and its production capacity is less than 10% of total.%, in my country, only Tianyuan, Yibin, Sichuan use mass method to produce PVC in China.
Bulk polymerization is a polymerization reaction in which monomers (or low molecular weight materials) are polymerized by action of an initiator or by themselves under action of light, heat and radiation without addition of a solvent or other dispersant. Sometimes a small amount of coloring agent, plasticizer, molecular weight regulator, etc. may also be added. Bulk polymerization can be carried out for liquid, gaseous and solid monomers.
There are two types of homogeneous polymerization and heterogeneous polymerization. If polymers formed can be dissolved in corresponding monomers, then it is a homogeneous polymerization, since resulting block polymers, also known as block polymerization, such as styrene, methyl methacrylate, etc.; compounds are insoluble in their monomers, and they are continuously precipitated during polymerization process, which is a heterogeneous polymerization, also called precipitation polymerization, such as ethylene, vinyl chloride, etc. The bulk polymerization initiator is an oil-soluble initiator. The oil-soluble initiator mainly includes an azo initiator and peroxyinitiator. The azoinitiator includes azobisisobutyronitrile, azobisisoheptanonitrile, azodiisovaleronitrile, azobiscyclohexylcarbonitrile, dimethylazobisisobutyrate initiator, and like. Compared to peroxyinitiators, reaction of azoinitiators is more stable.
Bulk polymerization characteristics: product is pure, has good electrical properties, and can be cast directly; utilization rate of production equipment is high, operation is simple, no complicated separation and purification operations are required.
The bulk polymerization process is shown in fig. 3.3.
Fig. 3.3. Ontology Aggregation Flowchart
D, solution method
The monomer and initiator are dissolved in a suitable solvent, and polymerization reaction is carried out in dissolved state. Solution polymerization is an important synthetic method in polymer synthesis process.
Solution polymerization is a process in which monomers and initiators (catalysts) are dissolved in appropriate polymerization solvents. The solvent is usually an organic solvent, and polymerization reaction may also be a homogeneous reaction, which is a typical solution polymerization; if resulting polymer is insoluble in solvent, polymerization reaction is a heterogeneous reaction called precipitation polymerization or suspension polymerization. polymerization.
The viscosity of polymerization system is lower than that of bulk polymerization, mixing and heat dissipation is easier, production process and temperature are easy to control, and solvent evaporation can also be used to dissipate heat. polymerization. In case of free radical polymerization, auto-acceleration effect may not appear at a low monomer concentration, which makes it possible to avoid implosion and simplify design of polymerization reactor. The disadvantage is that in free radical polymerization, yield is often low and degree of polymerization is lower than other methods. A large amount of expensive, flammable and even toxic solvents are used and recovered, which not only increases production costs and investment in equipment, reduces production capacity equipment, but also cause environmental pollution. If solid polymers are to be obtained, separation equipment must be set up and processes such as washing, solvent recovery and purification must be added. Therefore, solution polymerization is used in industry only when it is difficult to use other polymerization methods or when a polymer solution is used directly.
The polymerization process is shown in Figure 3.4.
Figure 3.4 Flowchart of solution polymerization
E. Characteristics and main applications of PVC produced by four polymerization methods
The advantages and disadvantages of PVC obtained by four polymerization methods are compared in Table 3.1.
Table 3.1 Advantages and disadvantages of PVC produced by four polymerization methods
Source: "Practical Guide to Rubber and Plastic Technology"
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