During the block polymerization of styrene, a solution of the synthesized polymer is formed in an unreacted monomer. With increasing depth of the process (degree of monomer conversion), the concentration solution and grows accordingly refractive index. By measuring the refractive index of the solution during polymerization, it is possible to obtain information about the kinetics of the process (in this case, the polymerization of styrene).
5 ml of styrene are placed in three test tubes with ground stoppers and weighed portions of the initiator - AIBN - taken on an analytical balance are added in quantities of about 10, 25 and 50 mg (the concentration of the solutions is, respectively, about 0.2, 0.5 and 1% wt). The test tubes are purged with an inert gas for 5 minutes and placed in a thermostat with a temperature of about 70 0 . In 10 minutes. After the start of thermostatting, a few drops of the solution are taken from each test tube onto a watch glass with a glass rod and the refractive index is determined. From each test tube take at least five samples, each time noting time from the beginning of polymerization.
The degree of monomer conversion is determined from the table below.
Dependence of the refractive index n D on the degree of conversion (p) of styrene
| p,% | n D | p, % | n D | p, % | n D |
| 1,5420 | 1,5475 | 1,5518 | |||
| 1,5429 | 1,5482 | 1,5519 | |||
| 1,5435 | 1,5488 | 1,5523 | |||
| 1,5441 | 1,5492 | 1,5525 | |||
| 1,5446 | 1,5495 | 1,5528 | |||
| 1,5451 | 1,5500 | 1,5531 | |||
| 1, 5455 | 1,5504 | 1,5534 | |||
| 1,5461 | 1,5508 | 1,5537 | |||
| 1,5465 | 1,5511 | 1,5540 | |||
| 1,5468 | 1,5515 | 1,5543 |
Initiator concentration(in mol/l) is found by the formula:
Where g is the weight of the initiator (in g)
V – volume of the polymerizing mixture (in this case – 5 ml)
M 1 – molecular weight of the initiator (for AIBN M 1 = 164)
The tangent of the angle of inclination of the resulting straight line is equal to order of reaction according to initiator.
CATIONIC POLYMERIZATION OF STYRENE
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Polymerization of styrene can occur in various ways, including the cationic mechanism. Inorganic Lewis acids are often used as catalysts for cationic polymerization—in this case, TiCl 4 . The use of this catalyst requires the reaction to be carried out under conditions that exclude the ingress of moisture - first of all, absolutely dry equipment.
Freshly distilled styrene 3.5 ml
Titanium tetrachloride distilled 1 ml
Dichloroethane dry 70 ml
70 ml of dry dichloroethane is placed in a three-neck flask equipped with a stirrer, thermometer and dropping funnel and purged with an inert gas for 3-5 minutes and cooled to 0 0 C in a bath with a cooling mixture.
Using a dry pipette, add 1 ml of TiCl 4 from a dropping funnel for 15-20 minutes. The monomer, styrene, is introduced drop by drop, making sure that the temperature does not exceed 0 0. After introducing the monomer, the mixture is stirred for another 30 minutes, and then 80 ml of alcohol is added (to decompose the reaction mixture). After a few minutes, carefully decant the solvent from the resulting oily reaction product, add another 10-15 ml of alcohol and rub with a stick until it hardens. The solid polymer is filtered, washed with alcohol and dried. The polymer yield and the degree of monomer conversion, as well as the catalyst consumption in g/g polymer are determined.
Expandable polystyrene (EPS), with surface treatment of particles, is produced by suspension polymerization of styrene in the presence of pentane and bulk polymerization. Polystyrene is produced in the form of spherical particles (beads), the surface of which is treated with various substances that improve the processability of the polymer during processing and give it new properties (for example, antistatic properties, non-flammability).
In the production of foaming polystyrene, the main methods are suspension polymerization and bulk polymerization. The most modern and effective is the second method of obtaining IPN.
Bulk polymerization of foaming polystyrene
The method of producing polystyrenes by bulk polymerization (block polystyrene) with incomplete conversion of monomers is currently one of the most common due to its high technical and economic indicators. Most modern industries operate precisely according to this scheme, as it is the most productive. This method has an optimal continuous process flow. The process is carried out in 2-3 devices connected in series with mixers; the final stage of the process is often carried out in a column-type apparatus.
The initial reaction temperature is 80-100°C, the final temperature is 200-220°C. Polymerization is interrupted when the degree of styrene conversion is 80-90%. The unreacted monomer is removed from the melt under vacuum and then with water steam until the styrene content in the polymer is 0.01-0.05%. Stabilizers, dyes, fire retardants and other additives are added to polystyrene and granulated. Polystyrene is characterized by high purity. This technology is the most economical (it does not involve the operations of washing, dehydrating and drying finely dispersed products) and is practically waste-free (unreacted styrene is returned for polymerization).
Carrying out the process until incomplete conversion of the monomer (80-90%) allows the use of high polymerization rates, control of temperature parameters, and ensure acceptable viscosities of the polymerized medium. When carrying out the process to deeper degrees of monomer conversion, it becomes difficult to remove heat from the highly viscous reaction mass, and it becomes impossible to carry out polymerization in an isothermal mode. This feature of the bulk polymerization process has led to increasing attention being paid to other production methods, and, first of all, to the suspension method.
Suspension polymerization
Suspension polymerization is a competing technological process based on the low solubility of vinyl monomers in water and the neutrality of the latter in radical polymerization reactions. The suspension production method is carried out in a reactor; it is a semi-continuous process, which is characterized by the presence of additional technological stages (creation of a reaction system, isolation of the resulting polymer) and periodic use of equipment at the polymerization stage. Styrene is suspended in demineralized water using emulsion stabilizers; The polymerization initiator (organic peroxides) is dissolved in monomer drops, where polymerization occurs. As a result, large granules are formed in a suspension of the polymer in water. Polymerization is carried out by gradually increasing the temperature from 40 to 130°C under pressure for 8-14 hours. The polymer is isolated from the resulting suspension by centrifugation, after which it is washed and dried. Then they are sorted by grade on vibrating screens. In this process, heat removal and mixing of system components are significantly facilitated.
Applicable:
- in the production of polystyrene foam blocks and slabs of various configurations of buildings and premises for any purpose (walls, roofs, floors, warehouses, pavilions, residential buildings, garages, basements, loggias);
- in the manufacture of packaging of complex shapes for various devices that require shock protection during storage and transportation;
- in the manufacture of automotive components;
- in the production of polystyrene concrete - lightweight concrete based on cement binder and foamed polystyrene filler, used in the manufacture of thermal insulation blocks and slabs, monolithic thermal insulation of attics, roofs, external walls, floors, etc.;
- for monolithic housing construction and shells for thermal insulation of pipelines.
- for the production of polystyrene foam gasified models used in metal casting.
In the production of finishing materials for the ceiling - tiles, baseboards, rosettes;
Copolymers of styrene with acrylonitrile SAN
The copolymer of styrene with acrylonitrile (SAN) usually contains 24% of the latter, which corresponds to the anisotropic composition of the mixture of monomers and makes it possible to obtain a product of constant composition. SAN is superior in heat resistance, tensile strength, impact strength and resistance to cracking in aggressive liquid environments, but inferior in dielectric properties and transparency. The cost of SAN is significantly higher than polystyrene. The ternary copolymer styrene-acrylonitrile-methyl methacrylate (SAM) has similar properties, but better transparency and resistance to UV irradiation; however, its cost is even higher than SAN.
SAN copolymers are usually produced by suspension or emulsion polymerization, similar to the production of PS.
SAN copolymers have higher chemical resistance and surface hardness than homopolymer. The starting material has a yellowish tint and has to be bluish. The weather resistance is good, which allows it to be used, for example, for cladding and in expensive household appliances instead of fragile and not frost-resistant general purpose polystyrene.
Copolymers of acrylonitrile, butadiene and styrene: ABS plastic
Such copolymers are called “ABS plastics”. There are several methods for producing a three-unit polymer (terpolymer), but their main principles are clear from the following examples: 1) styrene and acrylonitrile are added to a polybutadiene emulsion, mixed and heated to 50C; then a water-soluble initiator such as potassium persulfate is added and the mixture is polymerized; 2) butadiene acrylonitrile latex is added to styrene acrylonitrile latex, the mixture is coagulated and spray dried.
Properties vary widely depending on composition and production method. In general, however, ABS plastics have high impact strength, chemical resistance and ductility; not resistant to methyl ethyl ketone and esters.
ABS is very technologically advanced and can be easily processed by both injection molding and extrusion. Manufacturers produce grades of ABS plastic with different melt flow indices, with increased gloss and matte. Thin sheets are thermoformed into jars and trays. ABS plastics are widely used in the manufacture of household appliances, where high strength, high gloss, manufacturability in painting with masterbatches, environmental neutrality and heat resistance are in demand. Decorative coatings and designs are applied to products made from ABS plastic better than to polystyrene products.
Polystyrene production technology
In industry, polystyrene is produced by radical polymerization of styrene. Methods for producing polystyrenes differ in the work cycle, product removal per unit volume, and conditions for the polymerization process. The properties of the resulting polystyrene depend on the specific production method. There are 4 methods of polymerization of styrene: polymerization in the mass (block) of the monomer, polymerization of the monomer in an emulsion (mainly the production of ABS plastics), suspension polymerization (impact-resistant polystyrene and expanded polystyrene) and polymerization in solution (block copolymers of butadiene and styrene).
In the production of general purpose polystyrene, the main methods are suspension polymerization and bulk polymerization. Emulsion polymerization is used on a relatively small scale.
To obtain impact-resistant copolymers of styrene with rubber, the most widely used method is block-suspension polymerization, in which polymerization is first carried out in bulk (until a conversion of 20% - 40% is achieved), and then in an aqueous dispersion.
The general trend in the development of synthesis technology is to increase the power of individual units, both due to an increase in reaction volumes and due to the intensification of synthesis modes. Currently, the productivity of individual synthesis units reaches 15-30 thousand tons of polymer per year.
Bulk polymerization
The production method by bulk polymerization with incomplete conversion of monomers is currently one of the most common due to its high technical and economic indicators. In the domestic industry, the bulk polymerization method was chosen as the main one in the 70s, and currently about 60% of products are produced using this method. This method has an optimal process flow diagram. The process is carried out according to a continuous circuit in a system of 2-3 devices connected in series with mixers; the final stage of the process is often carried out in a column-type apparatus. The initial reaction temperature is 80-100°C, the final temperature is 200-220°C. Polymerization is interrupted when the degree of styrene conversion is 80% - 90%. The unreacted monomer is removed from the polystyrene melt under vacuum and then with water vapor until the styrene content in the polymer is 0.01% - 0.05%.
Stabilizers, dyes, fire retardants and other additives are added to polystyrene and granulated. Block polystyrene is characterized by high purity. This technology is the most economical (it does not involve the operations of washing, dehydrating and drying finely dispersed products) and is practically waste-free (unreacted styrene is returned for polymerization). Carrying out the process until incomplete conversion of the monomer (80% - 90%) makes it possible to use high polymerization rates, control temperature parameters, and ensure acceptable viscosities of the polymerized medium. When carrying out the process to deeper degrees of monomer conversion, it becomes difficult to remove heat from the highly viscous reaction mass, and it becomes impossible to carry out polymerization in an isothermal mode. This feature of the bulk polymerization process has led to increasing attention being paid to other production methods, and, first of all, to the suspension method.
Suspension polymerization
Suspension polymerization is a competing technological process that develops in parallel with bulk polymerization and is based on the low solubility of vinyl monomers in water and the neutrality of the latter in radical polymerization reactions. The process is used to produce special grades of polystyrene, mainly expanded polystyrene. The suspension production method is a semi-continuous process and is characterized by the presence of additional technological stages (creation of a reaction system, isolation of the resulting polymer) and periodic use of equipment at the polymerization stage.
The process is carried out in reactors with a volume of 10-50 m 3, equipped with a stirrer and a jacket. Styrene is suspended in demineralized water using emulsion stabilizers; The polymerization initiator (organic peroxides) is dissolved in monomer drops, where polymerization occurs. As a result, large granules are formed in a suspension of the polymer in water. Polymerization is carried out by gradually increasing the temperature from 40 to 130°C under pressure for 8-14 hours. The polymer is isolated from the resulting suspension by centrifugation, after which it is washed and dried. The laws of suspension polymerization are close to the laws of polymerization in the monomer mass, but the heat removal and mixing of the system components are significantly facilitated.
Emulsion polymerization
In the production of polystyrene, the emulsion method of polymerization has not received such development as polymerization in mass or suspension. This is due to the fact that emulsion polymerization produces a product of too high a molecular weight. Most often, for subsequent processing it is necessary to roll it or reduce its molecular weight by some other method. The main direction of its application is the production of an intermediate product for the subsequent production of expanded polystyrene using the extrusion method. The emulsion polymerization system contains styrene, water as a dispersion medium, a water-soluble initiator (potassium persulfate), an ionic emulsifier, and various additives, in particular those designed to regulate the pH of the environment.
Polymerization occurs in emulsifier micelles containing monomer. The resulting polymer is a highly dispersed suspension (latex), insoluble in water. The system as a whole is multicomponent, which makes it difficult to isolate the polymer in its pure form. Therefore, various methods of washing it are used. The use of the method is gradually being reduced, as it involves a large amount of wastewater.
Khimich Irina
Block polystyrene is produced by bulk polymerization. Polymerization of styrene in mass (block) is currently widespread. It can be carried out in the presence or absence of the initiator.
Initiators of polymerization usually benzoyl peroxide, dinitrile azobiisobutyric acid, etc. The decomposition products of initiators are part of polystyrene macromolecules, as a result of which it is not possible to obtain polystyrene with high dielectric properties using this method.
In industry, to obtain high-purity polystyrene, polymerization is carried out without an initiator (thermal polymerization).
The kinetics of radical polymerization of styrene to deep conversions has been studied much more fully than the kinetics of polymerization of other monomers. This makes it possible to very accurately calculate the temperature regime of polymerization to obtain polystyrene with specified properties.
Thermal polymerization of styrene until complete conversion monomer in a continuous way in column-type devices without stirring (the principle of “ideal” displacement) is currently not used, since this process has a number of serious disadvantages. The main disadvantages of the technological process of polymerization of styrene in bulk with complete conversion of the monomer are its long duration and the need to carry out the process at high temperatures (200-230 °C) at the final stages to achieve high conversion (99%), as well as obtaining a polymer with a low molecular weight (Figure 1) and a wide molecular weight distribution. In addition, with the depth of conversion the viscosity of the reaction mass increases greatly, reaching by the end of the process 1 10 3 – 1 10 4 Pa s. Carrying out thermal polymerization of styrene to incomplete monomer conversion (80-95%) in a cascade of apparatus with stirring (the principle of “ideal” mixing) and removal of residual monomer allows the reaction to be carried out at lower temperatures (140-160 °C) and obtain polystyrene from narrower molecular weight distribution. This ensures significant intensification of the process and production of higher quality polystyrene.
Industrial processes of styrene polymerization to incomplete monomer conversion were developed using mathematical modeling methods.
The first stage of process modeling is a mathematical description (model) of the thermal polymerization reaction of styrene. To calculate industrial processes, not the full kinetic model can be used, but the dependence of the gross reaction rate on conversion.
For polystyrene in the operating range temperatures 110-150 °C the molecular weight of the polymer depends only on temperature and does not depend on the degree of monomer conversion: 
The second stage of process modeling consists of a mathematical description of the reactors for carrying out polymerization processes. It contains a description of the properties of the reaction medium and heat exchange conditions in the reactor.
The properties of the reaction medium include:
- viscosity,
- thermal conductivity,
- heat capacity,
- vapor pressure above the polymer solution.
A feature of styrene polymerization is high viscosity of the reaction medium, which fluctuates in reactors from 1 before 1·10 3 Pa·s.
To ensure a given heat exchange in reactors, mixers of a certain type are used and the power consumption for mixing is calculated. When converting to 40% and viscosity of the reaction medium up to 10 Pa s apply sheet mixers(in the first reactor), at higher viscosities they become advantageous spiral (belt) mixers.
One of the main issues during polymerization in an isothermal reactor is heat removal. High intensity of the styrene polymerization process can be achieved by heat removal by evaporation and return of the monomer for polymerization. In addition, partial heat removal is carried out through the jacket of the device. The required temperature difference between the reaction mass and the coolant in the reactor jacket is determined from the heat balance equation
Q E + Q N - Q BX -Q X = 0
Where Q e- heat of exothermic reaction; Q n- heat generated during operation of the mixer; Q BX- heat spent on heating the input flow of the reaction medium; Qx- heat removal through the reactor wall.
To ensure stable operation in the reactor, the following condition must be met: the change in heat removal depending on temperature must occur faster than the change in heat release.
After determining the conditions for stable operation of the reactors, the question of the possibility of controlling them and the selection of appropriate means of automatic control are decided.
Currently block polymerization of styrene until incomplete conversion of the monomer into polymer is carried out in a cascade of stirred reactors in two ways:
- in the absence of solvents;
- using solvents.
Production general purpose block polystyrene carried out in the presence of ethylbenzene (15-20%), the presence of which in the process facilitates heat removal, operation of equipment, especially pumps, due to a decrease in the viscosity of the reaction mass, as well as control of the process as a whole.
Below are descriptions of the technological processes for producing general purpose block polystyrene.
Production of general purpose block polystyrene up to incomplete monomer conversion in a cascade of stirred reactors
The most widely used technological scheme for the production of block general-purpose polystyrene in a cascade of two stirred reactors. The process includes stages:
- preparation of the starting styrene,
- polymerization of styrene in reactors of the 1st and 2nd stages,
- removal and rectificationunreacted monomer
- polystyrene melt dyeing,
- polystyrene granulation,
- packaging and packaging of polystyrene granules.
The scheme for producing block polystyrene in a cascade of stirred reactors is shown in Figure 1.
From capacity 1 styrene is continuously supplied by a dosing pump to 1st stage reactor, which is a vertical cylindrical apparatus with a conical bottom with a capacity of 16 m 3. The reactor is equipped with a sheet stirrer with a rotation speed 30-90 rpm. Polymerization in reactor 1st stage 2 occurs at temperature 110-130 °C before conversions 32-45% depending on the brand of the product received. Excess heat of reaction is removed due to the evaporation of part of the styrene from the reaction mass.
Reactor 2nd stage 3 similar in design and dimensions to the 1st stage reactor, but equipped with a belt mixer with a rotation speed 2- 8 rpm. This ensures effective mixing of highly viscous reaction media. Polymerization in the 2nd stage reactor proceeds until 75-88% degree of conversion at temperature 135-160 °C depending on the brand of the resulting polymer.
A solution of polystyrene in styrene from the 2nd stage reactor unloading pump 5 served in vacuum chamber 6 through a pipe heated by steam at a pressure of at least 2.25 MPa. This happens post-polymerization styrene up to 90% conversion rate.
The polystyrene melt enters vacuum chamber 6 with temperature 180-200 °C. In the superheater tube of the vacuum chamber, the polystyrene melt is heated up to 240 °C and enters a hollow chamber with a volume of 10 m 3 with a residual pressure of 2.0-2.6 kN/m 2. In this case, styrene evaporates from the melt and the content of residual monomer is reduced to 0.1-0.3%. Styrene vapor is supplied for regeneration and then returned to capacity 1.
Melt polystyrene from vacuum chambers 6 enters extruder 7 and for granulation.
When receiving general purpose polystyrene in the presence of ethylbenzene, the latter is in a closed cycle mixed with styrene. The amount of excess heat of reaction in the apparatus is carried out by evaporation under vacuum of part of the styrene and ethylbenzene. The evaporated mixture condenses and returns to the reaction zone. To maintain normal operation of the mixers in the polymerizers, the viscosity of the reaction mass is continuously monitored. The specified viscosity is maintained automatically by changing the supply of a mixture of styrene and ethylbenzene.
Both polymerizers operate under vacuum, the process temperature fluctuates at 115-135 °C And 140-160 °C respectively. Polymer content in 1st stage reactor reaches 30-40% , V 2nd stage reactor - 65-70%. The solution contains 15-20% ethylbenzene. From the 2nd stage reactor, the polymer solution enters the evaporator, in which a vacuum is maintained (residual pressure about 2.6 kPa). Vapors of styrene and ethylbenzene are removed, and the polymer melt is collected in the lower part of the evaporator, from where the temperature 200-230 °C sent for staining and granulation.
Vapors of styrene and ethylbenzene from the evaporator enter the scrubber for cleaning, then condense and return to the original container of styrene and ethylbenzene.
Thus, the technological scheme for producing block general-purpose polystyrene using ethylbenzene in the process differs from the technological scheme shown in Figure 1, only presence of a scrubber And styrene and ethylbenzene vapor condenser.
Comparative assessment of methods for block polymerization of styrene with complete and incomplete monomer conversion
The method of block polymerization of styrene with incomplete conversion of the monomer has a number of advantages over the method of block polymerization with complete conversion of styrene:
1) the productivity of the polymerization unit increases by more than 2 times due to the reduction in the duration of polymerization, which leads to a reduction in capital investments and energy costs;
2) the hardware design makes it possible to regulate the technological parameters of the process and obtain products of varying quality depending on the consumer’s requirements;
3) polystyrene leaving the vacuum chamber contains less residual monomer (up to 0.2%) than the product leaving the column with complete monomer conversion (0.5%).
However, when carrying out a process with incomplete conversion of the monomer, waste is inevitable - stripping condensates of styrene. When implementing large-scale production, there is a need to use stripping condensates. With a total production capacity of 100-120 thousand tons/year of polystyrene, about 10-12 thousand tons/year of stripping condensates are obtained.
Utilization of stripping condensates is carried out in two directions:
1) purification of stripping condensates to obtain styrene of standard purity (rectification);
2) polymerization of stripping condensates to produce polystyrene of somewhat worse quality, but which can be used for the production of less critical products. Both directions are developing in industry.
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