Chemistry, Technology and Properties of Synthetic Rubber

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Ministry of Education and Science of Russian Federation Federal State Educational Establishment of higher education «Kazan National Research Technological University»

I.M. Davletbaeva, D.V. Beskrovniy, O.R. Gumerova

CHEMISTRY, TECHNOLOGY AND PROPERTIES OF SYNTHETIC RUBBER Tutorial

Kazan KNRTU Publishing 2013

Copyright ОАО «ЦКБ «БИБКОМ» & ООО «Aгентство Kнига-Cервис»

UDC 678(075.8) Davletbaeva I.M. Chemistry, technology and properties of synthetic rubber : tutorial / I.M. Davletbaeva, D.V. Beskrovniy, O.R. Gumerova; Ministry of Education and Science of Russian Federation, Kazan National Research Technological University. – Kazan : KNRTU Publishing, 2013. – 201 p. ISBN 978-5-7882-1416-0 The manual is intended for the 5th year students of the Polymer Faculty of 020015 "Chemical Technology" course. The manual reflects the current state of the elastomers science and production technologies. The focus is on technological methods, based on the scientific understanding of the chemistry in the synthesis of general-purpose and special rubbers, their properties and applications. The content of the manual corresponds to the program of the course "Technology of Elastomers". Published by the decision of the Editing and Publishing Board of Kazan National Research Technological University

Reviewers: Head of Organic Chemistry Department, KFU, I.S. Antipin, Dr. of Chem., corr. member of RAS Head of Materials Science and Technology Department, KNRTU, E.R. Galimov, Dr. of Eng., professor

ISBN 978-5-7882-1416-0

© Davletbaeva I.M., Beskrovniy D.V., Gumerova O.R., 2013 © Kazan National Research Technological University, 2013

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CONTENTS 1 INTRODUCTION 1.1 Elastomers Classification 1.2. Structural Characteristics of Elastomers and their Properties 1.3 Status of Synthetic Rubber Industry 1.4 Main Stages of Polymerization Processes 1.5 Methods of Polymerization 1.6 Heat Exchange in Polymerization 1.7 Rubber Separation 2 EXAMPLES OF TECHNOLOGICAL PROCESSES OF PRODUCTION OF SYNTHETIC RUBBER 2.1 Obtaining Solution Stereoregular Isoprene Rubber Using Anionic Coordination Catalysts 2.2 Drying of Solvent 2.3 Preparation of Catalyst Complex 2.4 Polymerization of Isoprene 2.5 Deactivation of Catalyst Complex 2.6 Degassing of Polymerizate 2.7 Rubber Separation, Drying and Packing 2.8 Secondary Operations 2.8.1 Preparation of Stabilizer Suspension and Stopper Solution 2.8.2 Preparation of Antiagglomerator 2.9 Obtaining Butadiene-Styrene (αMethylstyrene) Rubbers by Free Radical Initiation of Emulsion Polymerization 2.9.1 Initiation of Polymerization 2.9.2 Reactions of Polymer Chain Growth 2.9.3 Chain Termination and Transfer 2.9.4 Components of Emulsion Polymerization 2.9.5 Polymerization Rate 2.9.6 Process of Obtaining Styrene3

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Butadiene Rubbers 2.10 Technology of Butyl Rubber Obtaining with Cationic Initiation in Methyl Chloride 2.11 Batch Preparation, Copolymerization and Rubber Separation 2.12 Producing Ethylene-Propylene and Stereoblock Propylene Rubbers by Gas-Phase Polymerization with Metallocene Catalysts 2.12.1 Chemical Structure of Elastomeric Polyolefins 2.12.2 Metallocene Catalysts 2.12.3 Obtaining Ethylene-Propylene Rubber 2.12.4 Producing Stereoblock Polypropylene Rubber 3. MAIN TYPES OF RUBBERS, PROPERTIES, PRACTICAL APPLICATION 3.1 Natural Rubber 3.2 Polyisoprene (Synthetic Natural Rubber) 3.3 Polybutadiene 3.4 Styrene-Butadiene Rubber 3.5 Butyl Rubber 3.6 Ethylene-Propylene Rubber 3.7 Nitrile Rubber 3.8 Epichlorohydrin Polymer 3.9 Polychloroprene 3.10 Polynorbornene 3.11 Chlorinated Polyethylene 3.12 Chlorosulphonated Polyethylene 3.13 Ethylene-Vinyl Acetate Copolymer 3.14 Ethylene-Acrylic Rubber 3.15 Polyacrylate Rubber 3.16 Ebonite 3.17 Propylene Oxide-Allyl Glycidyl Ether Copolymer 3.18 Fluorocarbon Rubber 4

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3.19 Polysulphide Rubber 143 3.20 Silicone Rubber 145 3.21 Polyurethane Elastomers 152 3.22 Thermoplastic Elastomers - General 177 Description 3.23 Thermoplastic Urethane Elastomers 181 3.24 Styrenic Block Copolymers 185 3.25 Copolyether Ester 187 3.26 Polyester Amide 190 3.27 Thermoplastic Olefin Elastomer 190 3.28 Thermoplastic Vulcanizate 192 194 BIBLIOGRAPHY

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INTRODUCTION 1.1 Elastomers Classification Generally speaking, elastomers are the polymers in the highly elastic state under normal conditions. Narrowly, they are constructional materials based on highly elastic polymers which have an extremely large significance in the development of technology. The high flexibility of the polymer chains makes it possible to develop highly significant strain of elastomers under the influence of external mechanical forces, and the nature of strain depends on the structure of the polymer. In view of relaxation processes, the connection between stress, strain and time for the polymers is complicated. In general, the total strain ε is made up of three components: ε=εel+εhe+εpl, (1.1) where εel, εhe и εpl – elastic, highly elastic and plastic strain respectively. The elastic strain due to distortion of the bond lengths and bond angles (as in any solid), develops almost instantly and it is completely reversible. Highly elastic strain is associated with the changes in chain conformation and supramolecular structure of the polymer; it develops during time (relaxationally) and is reversible as well. Plastic strain is caused by the motion of macromolecules relative to each other, is irreversible (flow of the material), and its magnitude is proportional to the exposure time of external forces. The basic component out of the three components of the total strain in elastomers is highly elastic strain εhe, developing in time and completely reversible. However, in many cases the irreversible plastic deformation εpl can make a significant contribution. Naturally, such elastomers are not suitable for the common use as structural materials, since their products do not have the dimensional stability. It should be considered as true elastomer the kind of the polymer, which is able only to reversible strains with highly flexible chains, which are unable to relative motion for some reason. The motion of macromolecules can be prevented in two ways: the 6

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creation of a polymeric material with areas of intensive physical interactions (physical cross-linking) and transformation of a linear polymer into the cross-linked polymer (formation of system of intermolecular chemical bonds). In both cases, the flexibility of chains should remain. Materials of the first type are called thermoplastic elastomers (TPE), since they behave as cross-linked elastomers at ordinary temperatures, and as linear thermoplastic polymers (plastics) at elevated temperatures. The destruction-recovery processes of the network of the material are completely reversible, associated only with temperature changes. Therefore, thermoplastic elastomers can be recycled, allowing waste and disabled goods recovering. The advantages of TPE are the simplicity of processing, an exception of vulcanization stage of the process. However, the operating temperature range of TPE is relatively narrow, and they are used only for the manufacturing of products, operating at normal temperatures (eg, footwear). Much more common are cross-linked elastomers which polymer chains are connected by sufficiently strong chemical bonds. The main (traditional) way of obtaining such materials is vulcanizing of rubbers - linear, high molecular polymers in the highly elastic state. Initially, it was used exclusively natural rubber (NR) to obtain vulcanizates (often called rubbers). However, in the twentieth century, which was called as the age of the polymers by academician N.N. Semenov, synthetic rubbers have appeared (SR), and they gradually became the main raw material for the rubber industry. In the world consumption of rubber the ratio of SR:NR have been keeping at about 2: 1 for many years. Cross-linked elastomers can be obtained by curing of reactive oligomers with the terminal functional groups ("oligomeric" technology) as well. Owing to the low molecular weight and consequently low viscosity of the oligomers, such technology makes it possible to strongly reduce energy use on the mixing and product forming processes. However, reactive oligomers are relatively expensive, only some products can be obtained in this way; therefore 7

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oligomeric technology is used mainly in the industry of sealants, adhesives, coatings, etc. As an example, the reactive oligomers are urethane prepolymers, liquid Thiokol (polysulfides), liquid polydimethylsiloxanes. Service conditions of elastomeric products are extremely varied, and as the technology develops the constructional materials requirements increasingly toughen. These requirements include: high elasticity retention at lower temperatures, providing of sufficient strength under the long-term exposure of corrosive mediums or high temperatures (often both of these factors), the significant effects of excess pressure (or vacuum), radiation and electrical fields, etc. Nevertheless, a significant number of products run under the normal conditions, which mean the temperature range from -50 to 150°C, no contact with any corrosive liquids or gases, etc. Meanwhile, the main feature of rubbers is realized in elastomers high elasticity, and of course, such general properties as strength, durability, etc. play the important role. Elastomers used in such conditions (as well as rubbers, from which they are derived) are called general purpose elastomers (rubbers). They are primarily hydrocarbon rubbers: natural and synthetic isoprene, butadiene and butadiene-styrene (α-methylstyrene) rubbers. The main consumer of general purpose rubbers is tire industry, so just tire manufacturers who are the "trendsetters" with respect to this group of rubbers. If the operating conditions are different from ordinary the rubber must have certain specific properties to maintain working capacity for the required period of time (special purpose rubbers). The range of special purpose rubbers is much wider, as a set of specific properties is quite large - above all, resistance to high (or low) temperatures, to the various corrosive liquids (primarily fuels, oils, solvents, etc.), light and radiation, etc. However, in terms of production volume, they are much smaller than general-purpose rubbers. The special properties of elastomers are primarily associated with the chemical structure of polymer chains, so the carbon-chain special purpose rubbers for the most part are composed of some polar 8

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atoms or groups, and many of them are heterochain or elementcontaining. The special purpose rubbers are chloroprene, butadienenitrile, acrylic, polysulfide (Thiokol), siloxane, urethane, fluorine rubber and many others. Some rubbers can be referred to any group, depending on whether the specific property plays a significant role in provision with the service properties of the product or not. For example, ethylene-propylene rubbers (EPR) are typical general purpose rubbers which are widely used as such. However, their rubbers offers highly resistance to atmospheric factors and dielectric properties, which allows to consider EPR as special purposes, such as insulating coatings of electric cables. In addition, the properties of the final constructional material largely depend on the nature of intermolecular chemical bonds (method of vulcanization), formulation factors (type and amount of fillers, plasticizers, modifiers, etc.). When choosing the special rubber for a particular application one should rely on the well-known structure–properties relations. For example, it is important for heat-resistant elastomers to have chemical bonds in the macromolecules and in the network junctions as strong as possible. Therefore, saturated polymers are more heat resistant than unsaturated ones, and fluorine rubbers and polysiloxanes are more highly efficient at elevated temperatures:

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Type of rubber

Type of bonds in main chain

Peak temperature of continuous service, °С

Isoprene (peroxide cure) С – С, С = С 90 ÷ 100 Ethylene-propylene (peroxide cure) С–С 130 ÷ 150 Acrylate rubber С–С 140 ÷ 160 Fluorine rubber –F2C – CF2– 200 ÷ 250 Siloxane rubber –Si – O 200 ÷ 250 When selecting rubber for cold-resistant rubbers you should primarily draw attention to the thermodynamic flexibility of its macromolecules, which can be estimated quantitatively by a factor of hindered internal rotation σ. Floor temperature of Rubber σ continuous service, °С Styrene-butadiene rubber (SKS-30) ~1,8 –40 ÷ –50 1,4-cis-Isoprene (NR, SKI-3) 1,7 –50 ÷ –60 1,4-cis-Butadiene (SKD) 1,5 –90 ÷ –100 Syloxane 1,3 –50 ÷ –90 The behavior of elastomers in contact with organic liquids is determined by the affinity between them, which can be seen from the values of cohesive energy density (CED) and the solubility parameter δ=(CED)1/2. In most cases, similar values of these parameters (δ1 ≈ δ2) indicate a significant swelling of the elastomer in the liquid. The comparative data for the resistance of rubbers of different rubbers to some organic media (on a 5-point scale; rubbers and liquids are arranged in order of the polarity growth) is given below:

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NR

Rubbers BSK Butyl PH BNK rubber

Thiokol

Hydrocarbons Aliphatics 1 1 1 3 5 5 Hydrocarbons Aromatics 1 1 1 2 3 5 Fats, vegetable oils 2 2 5 4 5 Ketones, alcohols 3 3 1 1 1 Unfortunately, not all of the specific properties have the explicit link with the chemical structure of the elastomer, so you often have to rely on experimental data and find some empirical relations. Historically the main form of commercial rubbers are the briquettes. For the processing of rubber it is required heavy and energy-intensive equipment, which is the most likely to be of periodic action. Therefore, besides the economic, environmental, sanitation, etc. problems there are difficulties with ensuring the consistency of product. The plastic processing industry which occurred later had to deal immediately with the polymers in the form of granules and powders and it offered continuous processes, which are much easier to automate and robotize. The use of these advanced technologies applied to rubbers is complicated by the fact that under normal conditions, the polymer is in a highly elastic state. This can lead to aggregation of the particles during storage and transportation, which is unacceptable for the successful operation of process equipment. Therefore, production of rubber in the form of granules is limited, and it is suitable for the most hard rubbers - chloroprene, ethylenepropylene, thermoplastic elastomer (TPE), etc. Last years the interest in powdered mixtures of rubber with fillers (especially with carbon black) has grown again. Such stable free-flowing powders are suitable for the use in continuous 11

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processes, but they can be successfully applied when working on the standard stirrers. Getting a mixture of latexes (including natural), it simplifies the process of selection of rubber for unnecessary coagulation, mechanical dewatering, drying the polymer at high temperatures, pressing briquettes. The use of rubber-filler mixtures is the most appropriate way in large recycling processes, for example, in the tire industry, so this commodity form is promising for the general-purpose rubbers. Part of the rubber is produced in the form of commercial latexes, which find wide application for obtaining some of the products from elastomers (latex technology), as the basis of adhesives, paints and coatings, impregnation of the reinforcing parts, as well, etc. Mainly the synthetic rubber of the emulsion polymerization (synthetic latex) is produced in the form of latexes, but it is also possible to derive latexes from elastomers obtained by other methods (artificial latex). Natural rubber latex, its types, properties and applications are not considered in this book. A very small amount of rubbers can be produced in the form of solutions, in those cases where the further use of rubber requires its dissolution (eg, for subsequent modification, or as the basis for coatings, etc.). 1.2. Structural Characteristics of Elastomers and Their Properties When establishing the correlation between the molecular structure of elastomers and their properties it should be kept in mind that synthetic rubber is a raw material for the rubber industry (tires, various technical rubber products, shoes, etc.). During the processing the rubber is mixed with various ingredients to get the rubber compounds ("raw", ie, not vulcanized). The property package associated with the behavior of rubbers and raw rubber compounds at various stages of the processing cycle, is combined into concept of technological properties of rubbers. First of all, these include 12

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plasticity, viscosity (Mooney), hardness (Defoe), recoverability, as well as more specific properties - millability, calenderability, extrudability, cold flow, shrinkage, and tackiness. The most important technological properties of rubber are associated with its processability on rubber mixing and molding equipment. The cost of energy on mixing, processing temperature, and even the possibility of its implementation are due to the plastoelastic parameters of rubber. They are determined for raw rubbers, and the most important characteristic is the value of the effective viscosity for a particular strain mode and shear rate. The most common measurement is Mooney viscosity. Compare the values of shear rate (γ), typical for the different processing of rubber compounds, as well as realizable in the Mooney type viscometers when determining the viscosity of the material: Table 1. The values of shear rate (γ) for the different processing of rubber compounds Process γ, sec-1 Measurement of Mooney viscosity ~1,5 Press molding 1 ÷ 10 Calendering 10 ÷ 102 Extrusion 102 ÷ 103 Diecasting 103 ÷ 104 As it can be seen from data given above, Mooney viscosity may be used only as indicative or comparative characteristic, but it cannot be used in the calculation of processing of the elastomer. More information about the viscous properties of rubbers can be obtained from flow curves representing the dependence of the shear rate on stress (σF). For liquids obeying the Newton's viscosity, this dependence is linear. The liquids that flow faster than the Newtonian are called pseudo-plastic, and almost all rubbers and rubber compounds are belong to this group. 13

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In most cases the flow curve can be described by a law equation of Ostwald de Vila: σF = K⋅⋅γn , (1.2) Where K and n are rheological constants of material The constant n, called the flow index, in this equation characterizes the deviation of flow from Newtonian one, as for the last n = 1. For elastomers n is a few tenths and it depends on molecular weight, molecular weight distribution, chains branching, the concentration of filler in rubber, as well as on temperature. Despite the greater information value of the rheological constants their determination requires a fairly long experiment, and there are used mainly the Mooney viscosity to characterize the technological properties of rubbers. This index for linear polymers is related to their molecular mass characteristics: Mooney viscosity increases with increasing of average molecular weight and decreases with increasing of polydispersity index. The presence of branched macromolecules with long lateral branches also leads to an increase in Mooney viscosity in comparison with linear polymers of the same molecular weight and polydispersity. Thus, technological properties of the rubbers are etermined by their molecular structure, which, in turn, is associated with the specific synthesis of elastomers. The molecular weight of most of the rubbers is in the range from 105 to 106. The formation of natural rubber as a result of biochemical reactions does not allow to adjust the length of the macromolecules, so before processing NR is subjected to mastication to reduce the molecular weight and improve the technological properties. When obtaining the synthetic rubber the molecular weight is easy to adjust in the process of synthesis, and, as a rule, the same type of SR is produced in the form of various grades which vary in viscosity. All synthetic rubbers and even oligomers are polydisperse namely a mixture of macromolecules of different molecular weight. Polydispersity of polymers is quantitatively described by the polydispersity index (f) or a function of molecular weight distribution 14

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(MWD). Practically, it is more likely to use the values of the polydispersity, quantitatively equal to the ratio average and number average molecular weight of the polymer: f = Mw/Mn. For monodisperse substance f = 1, for polydisperse – f > 1, so the higher the value of this coefficient for the real rubber, the greater the polydispersity. The molecular weight distribution function is the dependence of the relative number or weight fraction of macromolecules with particular molecular weight of the value M. The molecular weight distribution function of the macromolecules depends on the conditions of synthesis of rubber, mainly on the nature of the used catalyst system or the initiator. The peculiarities of the structure of the active center in ionic polymerization determine the ratio of the rates of elementary reactions of the polymerization process - initiation, growth, chain termination, and the dependence of these rates on the length of the chain. Thus, in the anionic polymerization of diene monomers the active center is highly stable, which nearly rules out the possibility of such reactions as limiting the growth of macromolecular chains, deactivation or transfer of the kinetic chain to the monomer and polymer. Derived polydiene rubbers are characterized by a narrow molecular weight distribution and their polydispersity index verges to one. When using the Ziegler-Natta catalysts the reactions of chain transfer to monomer, to catalyst complex or to its components could take place that leads to extension of MWD function and increasing the values of f respectively. The processes of polymerization by a radical mechanism are even more complicated by numerous reactions of chain transfer and deactivation of active centers. With narrow MWD and in the absence of branching the rubbers are processed poorly, often exhibit a high cold flow, making them difficult to transport and long-term storage. Rubbers with wide MWD have the best technological characteristics. The presence of high fractions creates a "frame", which prevents from flowing at low shear stresses (and prevents cold flow). At the same time the low 15

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molecular weight fractions existing in rubber act as a plasticizer to facilitate the flow at high shear stresses (during the processing). A small branching also creates a "frame" and helps to reduce the cold flow of rubber, but also increases the tendency of polymers to mechanical degradation. Rubbers are characterized by statistical branching of macromolecules, which has a quantitative measure as the density of branching ρ, equal to the ratio of the number of nodes to the total number of branching units in the polymer chain. Due to the statistical nature of branching the average number of branch points in the macromolecules is proportional to their molecular weight. The main reason for the branching of the macromolecules during their formation is the transfer reaction of the active center to the polymer chain. Owing to occurrence of additional active centers in this macromolecule it is often observed the acceleration of its growth and the increase of the probability of further branching. As a consequence, the branching process leads to expansion of the molecular weight distribution, and the highest molecular fractions contain the highest number of branches. If the density of branching reaches some critical value ρcr there are supramolecular particles arising in the system, and then the macroscopic particles, which represent three-dimensional spatial structures. The formation of such structures is shown in the sharp increase in viscosity of the system during polymerization in bulk, and in the appearance of a gel in rubber solutions. Thus, the synthetic rubber is a polymer system with a complex molecular (topological) structure, which determines not only the technological properties of elastomers, but also the service properties of rubbers obtained from them. The ability of raw rubber mixtures to vulcanize describes the group of vulcanization characteristics, the most important among them are the cure rate and the maximum attainable spatial cross-link density. Vulcanization characteristics of mixtures are determined increasingly by the composition of vulcanizing group, i.e. mixtures 16

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with different vulcanizing activity can be prepared on the base of the same rubber. However, the variation of the composition and the structure of macromolecules of rubber can also affect this group of properties. Thus, as the reactivity of double bonds and the mobility of hydrogen atoms in the polymer chains increases, the rate of sulfur vulcanization enhances. The ratio of links of different structure (1,2-, 3,4-, 1,4-cis, 1,4-trans) strongly influences the curing activity of chloroprene rubber. The incorporation of the carboxyl groups in macromolecules of rubber enables the metal oxides vulcanization. Halogenation of butyl rubber is one of the ways to increase its cure rate. However, the main methods of regulating the curing activity of rubber compounds are formula factors. The properties of rubber as a structural material are often called as technical (or service). Depending on the application of rubber priorities may be different, but in nearly all cases required properties are high strength, elasticity, time consistency. It is possible to obtain vulcanizates with the highest tensile strength only on the basis of rubber with sufficiently high molecular weight (about 250 ÷ 600 thousand). The molecular structure of rubber, which ensures good processing characteristics of mixtures and at the same time allows obtaining the rubber with high mechanical properties, demands various, often opposite requirements. Thus, the best physical and mechanical properties have rubber-based rubber with regular structure, high molecular weight and narrow molecular weight distribution. As a result of vulcanization of such rubbers there are less defective network structures, which are also characterized by a narrow distribution of molecular weights of network links. However, the rubbers of such structure have poor technological properties. Therefore, for each specific application of rubber there should be considered the required service characteristics and processing conditions. For example, in the case of butadiene rubber in the tire industry, the polymer with a molecular weight (3÷3,5)·105 and polydispersity index equal to 2,5÷3,0 satisfies the technological and 17

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physical-mechanical properties. At the same time, for the producing of industrial rubber products, butadiene rubber with such MWD is unacceptable, it requires more enhanced MWD (f = 4,0÷5,0). The main difference of elastomers from other types of macromolecular compounds is that their temperature range of high elasticity occurs in the region of temperatures, the most important for the practical use of the material. To show a high elasticity of the polymer it is needed that the macromolecules are long, continuously fluctuating statistical coiled chains. The polymer chain can take such conformations only if it has sufficient internal mobility, which is provided primarily by the rotation of the separate fragments of the chain around the single bonds (C-C in the case of carbon-chain rubber). The smaller size of this fragment, the higher conformational mobility (thermodynamic flexibility) of chain and, consequently, the higher the high elastic properties of the polymer. In the case of elastomers high thermodynamic flexibility of isolated macromolecules combines with relatively weak intermolecular interactions in the polymer. Quantitative criterion of these interactions is the cohesive energy density (CED), which has the physical meaning of energy required to separate molecules in a volume unit of substance (for low-molecular liquids, it coincides with the energy of evaporation). Typically, the value of CED is expressed in MJ/m3 or equivalent units - MPa. Equivalent intensity characteristic of interactions is the solubility parameter δ = (CED)0.5. The ability of polymers to dissolve or swell in liquids of different nature, to combine with each other and with the plasticizers, the properties associated with the cohesion of polymer chains (primarily mechanical properties, gas and water resistance, etc.) depend on the value of CED (or δ). The chemical structure of polymer chains uniformly influences the intra- and intermolecular interactions, i.e. with increasing polarity of the polymer both the potential barrier of rotation (which makes the conformational transformations in 18

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polymer chains complicated) and the cohesive energy density increase. Here are the values of cohesive energy density of some common polymers: Polymer Polyethylene Polypropylene Polyisoprene Polybutadiene Polychloroprene

CED, MPa 265 275 275 298 328

Polymer Polystyrene Polyvinyl chloride polyethylene terephtalate Polycaproamide Cellulose

CED, MPa 328 384 483 780 1025

Comparing these data with the mechanical properties of polymeric materials, we can conclude that the polymers with the lowest values of CED are the most low-modulus (elastomers). The higher the intensity of intermolecular interactions, the harder the polymer - the maximum values of CED are in fiber-forming polyamide and cellulose. It is also clear that the elastomers with low values of CED are not oil- and petrol-resistant, as the nonpolar hydrocarbons have similar values of the intensity of intermolecular interactions. Cohesive energy density determines the correlations between the rates of molecular rearrangements and the rates of cooling or heating the polymer samples. Molecular mobility and free volume associated with the cohesive interactions are responsible for the transition temperature of polymers to the glassy state (Tg). Thus, the increasing CED weakens the segmental motion and thus Tg increases. In addition, the glass transition temperature is significantly affected by the thermodynamic flexibility of the polymer chains, which depends on their chemical structure and composition. At Tg the elastomer loses its high elastic properties completely, so it is always below the lower usage temperature limit of elastomeric materials. Depending on the chemical nature and structure of the 19

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monomer units the glass transition temperatures of the different elastomers is between minus 130 and 0°C (Table 2). In the processing of rubber mixtures their cohesive strength has often a great importance, i.e. the ability of the unvulcanized blends to stand the relatively high strain without breaking (up to 1 MPa) under significant deformations (~400%). The problem of increasing the cohesive strength of the blends is particularly burning for general-purpose rubbers, as they have the lowest values of the CED, and the most large-tonnage products are produced on their base. The chemical structure of the elastomers specifies their supramolecular organization, which may show itself in three varieties. First, this is a certain kind of order and heterogeneity caused by morphology in the amorphous polymer. Secondly, the regularity of the macrochain structure can cause the formation of various crystalline structures. Finally, almost all the block copolymers, and in some cases in statistical copolymers, due to the incompatibility of chemically dissimilar fragments of the polymer chains, there are segregated areas of microscopic or submicroscopic size (domains). Table 2. The glass transition temperature of some rubbers Raw rubber Tg, °C Siloxane ACT 123 Trans-polipentenamer -105 ÷ -100 1,4 - cis-butadiene (SKD) -110 ÷ -105 Butadiene (SKDL) -90 ÷ -60 Polybutadiene emulsion № 80); 1,4 - cis-isoprene (NR, SKI-3) -70 ÷ -68 Isoprene lithium (SKIL) -69 ÷ -66 Styrene-butadiene emulsion SKS-30 -64 ÷ -59 Styrene-butadiene emulsion SKS-10 72 Styrene-butadiene solution DSSK -78 ÷ -75 20

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Butadiene-nitrile SKN-18 Butadiene-nitrile SKN-26 Butadiene-nitrile SKN-40 Butadiene-nitrile SKN-50 Chloroprene Ethylene-propylene (SKEP, SKEPT) Butyl rubber Fluorine rubber SKF-26 Fluorine rubber SKF-32 Acrylate (BAC, Bakhit-7) Urethane SKU-7 Polysulfide Polipropilenoxide (SKPO)

Continuation of Table 2 55% 42 25 -10 ÷ -7 -45 ÷ -40 -55 ÷ -50 -69 ÷ -67 -22 ÷ -20 -20 ÷ -18 -32 ÷ -35 30 -53 ÷ -43 75%

Despite the fact that the role of supramolecular structures in the properties of elastomers is relatively small, the processes of crystallization and microphase separation can affect the technological and operational properties of materials. With increasing of crystallinity the rigidity of the polymer increases. Therefore, the rubbers which are able to crystallization become more rigid during the storage, and it is necessary to melt the crystalline phase before the processing (additional technological operation). Crystallization in cross-linked polymers proceeds much slower than in linear ones, so it is often possible to obtain noncrystallized rubbers on the base of crystallizing rubbers. At the same time, tensile ability of the polymer to crystallize leads to the effect of self-empowerment, which enables the strength of materials. Thus, the distinction of the structure of elastomeric chains is that the polymer crystallization should occur only in tension.

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1.3 Status of Synthetic Rubber Industry Rubbers are natural or synthetic materials that are characterized by elasticity, water resistance and electrical insulating properties, of which the rubbers are received by special treatment. Natural rubber is a product produced by many plants (rubber-bearing plants or rubber gens). Rubber is produced in the form of colloidal dispersions in water, called milky sap or natural latex. Among the large number of rubber gens the industrial importance has only Brazilian Hevea (Hevea brasiliensis) - a tree native to the tropical zone of South America. Therefore, the birthplace of natural latex and natural rubber (NR) is considered to be Brazil. However, it was found that in Mexico, Honduras and Guatemala, where Mayan culture once flourished, there were widespread ball games made of flexible material, called "kaa-uchu," which translated from the Indian language as "tears of a tree." Their tribes made waterproof cloth, shoes, water tanks, and various religious figures from the same material. The Spaniards who arrived in South America with the famous Columbus expedition (1493-1496) have seen these items, but they were described much later. It took a long time, almost 2.5 centuries, until scientists learned to process rubbers into rubber and to apply these materials in technology. With the technical and economic point of view the discovery of NR occurred in Brazil. In Brazil wild Hevea occupies large areas in the Amazon basin. It is a powerful tree reaching 45 m in height and 2.5 m in diameter. There is a system of vessels in the bark of these trees filled with milky sap (latex), and the pressure in the vessels is up to 1 MPa. Therefore, when cutting of the bark (tapping) the latex begins to ooze out. There are various clones of Hevea, which productivity varies from 20 to 200 g of rubber per day. The content of the polymer in the latex ranges from 2 to 45% as well. Climatic conditions, land treatment, tree age, the way of machining a cut - all these factors play an important role. 22

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At first the transportation of the gathered latex over considerable distances was impossible because of its instability, so up to 1853 it was coagulated on the collecting ground and transported in the form of rubber. But after the discovery of the stabilization methods of the latex, it became possible to transport, that was not profitable due to the low concentration of polymer in the latex. At the beginning of the XIX century the first processing of NR appeared primarily for waterproof cloths and shoes. The discovery of the effect of mechanical mastication (T. Genkok, 1826) and curing (Ch. Goodyear, 1839), of natural rubber laid the foundations of the traditional technology of processing rubbers into rubber products, which have continued generally to this day. At the same time it was studied the structure of the NR. The studies by M. Faraday, J. Dumas, F. Himli, J. von Liebig, J. Dalton, G. Williams et al. (1826-1860) led to the conclusion that the structural element of natural rubber is a hydrocarbon C5H9 called isoprene. This conclusion was confirmed by G. Busharda who received the rubber-like material in the processing of isoprene with concentrated hydrochloric acid. The researches of Russian scientists in the field of obtaining and polymerization of unsaturated hydrocarbons had great importance for solving the problem of synthesis of rubber. The first was the work of A.M. Butlerov who established the possibility of zinc and sulfuric acid polymerization of butylenes, which initiated the study of the polymerization of unsaturated compounds. In 1878 A.A. Krakau in Russia obtained a polymer of styrene on metallic sodium. Later L.M. Kucherov reported receiving sodium-isoprene rubber (1908), but this information was published only in 1913. Serious series of studies of thermal polymerization of dienes was carried out by S.V. Lebedev, which in 1910 published his findings and pointed out that the butadiene and dimethylbutadiene form rubber-like substances along with oligomers upon heating. 23

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At the same time it became known about the work of K.D. Garries, received a rubber by thermal polymerization of isoprene, and in 1913 he got such rubber in the presence of metallic sodium. However, the quality of the rubber was poor, and therefore there was given more attention to dimetilbutadiene rubber. During the First World War, Germany was cut off from markets of NR that was the reason for organizing the production of synthetic dimethylbutadiene-based rubber. During the war, this rubber was released at the rate of 2350 tons. However, after the war that production was shut down, because the received rubber was too expensive, and the rubber on its basis was significantly worse than the rubber from the NR. In the first half of the twentieth century, the focus of scientists was aimed at developing methods of producing monomers that are suitable for rubber obtaining, as well as methods of their polymerization. In 1910 O.G. Filippov described the butadiene production process through the reduction of vapors of diethyl ether, and three years later I.I. Ostromyslensky and S.S. Kelbasinsky showed the possibility of synthesizing this monomer from ethanol and acetaldehyde. The method was of industrial interest to the United States and it has been used when organizing the production of butadiene from ethyl alcohol (1942-1944). The second method of obtaining butadiene proposed by I.I. Ostromyslensky was multistage. Initially, ethanol was converted to acetaldehyde, and the latter through the acetaldol and 1,3-butanediol - to butadiene. This method has been implemented in industry in Germany in 1936-1938, but acetaldehyde was obtained by the hydration of acetylene in the presence of sulfate salts of mercury using the method described by L.M. Kucherov in 1881, not from alcohol. Since 1913 B.V. Byzov conducted systematic research to find the opportunities to use oil as a raw material for monomers. In 1916 he proposed a synthesis method of diene hydrocarbons through the oil pyrolysis at normal and low pressure, or through the dilution of 24

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the initial hydrocarbons with an inert gas. Later in the USSR there was a pilot plant for the synthesis of butadiene according to this method (SR Plant, letter A). In that time it was impossible to overcome the serious difficulties connected with the organization of industrial production of butadiene by this method. After the Second World War, the work in this direction had been renewed. Owing to the works of N.D. Zelinsky, A.A. Balandin and other domestic scientists there were found the conditions and catalysts for the dehydrogenation of hydrocarbons allowing to nearly theoretical yield of butadiene from butylenes. In our country, the first industrial method of the butadiene obtaining was the catalytic decomposition of ethanol proposed by S.V. Lebedev. This method was developed in the laboratory of general chemistry of the Military Medical Academy (Leningrad) and in the laboratory of oil in Leningrad State University with the participation of his students and employees I.A. Volzhinsky, S.G. Kibarkshtis, V.P. Krauze, Ya.M. Slobodin, A.I. Yakubchik, A.V. Voronova and F.F. Voronov. Butadiene, obtained by this method, was converted into butadiene rubber through the polymerization on the metal sodium. This method was implemented in early 1931 on a pilot plant (SR pilot plant, letter B). The first synthetic rubber plant was started in Yaroslavl, June 15, 1932, the second plant of SR went into operation in October in Voronezh, the third the Efremov Plant of SR began to manufacture industrial products in July1933, the fourth the Kazan plant of SR was implemented in 1936. Thus a new industry of chemical manufacturing was born, which allowed the rubber industry of our country to develop independently from the supply of the natural rubber that played an important role in World War II. The polymerization technology of butadiene on the sodium catalyst has been developed under the leadership of G.G. Koblyansky. At Yaroslavl and Voronezh plants polymerization was carried out periodically in the liquid phase while at the Efremov and Kazan factories it was proceed in the gas phase. 25

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In 1938 in Germany polymerization of isobutylene was commercially ran at low temperature in the presence of boron trifluoride. This polymerization catalyst had been previously suggested by A.M. Butlerov. Later, since 1940, in the U.S.A. R.E. Tomas and V.J. Sparks had been published their works on the copolymerization of isobutylene and isoprene underlying the production of butyl rubber. This rubber was first released in1940 by "Standard Oil Jersey" company. Due to the work of G. Bayer, G.L. Fisher, M. Moskovits and J.K. Patrik, each of which stroke a blow for the theory and practice of polycondensation of dichlorderivatives and alkali metal polysulfides, the production of polysulfide rubbers (Thiokol) started in 1930. Soon, however, it was halted as the rubber had a strong odor and poor quality. Later, numerous studies had been conducted to improve the properties of these polymers, but the most interesting were liquid Thiokol (polysulfide oligomers), not rubber-like ones. In our country, the works on establishing of such oligomers industry were conducted under the direction of N.P. Apuhtina. In 1957 it was started the production of polysulfide rubbers and oligomers with different molecular weight. The studies of American scientists (J. Newland, etc.) on the synthesis of chloroprene from acetylene through vinyl acetylene and the discovery of its polymerization method allowed the U.S. to start production of chloroprene rubber called Dupre in 1932, and it was produced in amount of 250 tons that year. In the Soviet Union there were successful studies of the chloroprene synthesis and its rubber by A.L. Klebansky, I.M. Dolgopolsky, L.G Tsurih et al. As a result of these studies the pilot batches of polychloroprene were produced in 1934, and the industrial production of this rubber began in Yerevan in 1940. In 1922 in Germany (the company "IG Farbenindustrie") there was extensive research on the synthesis of rubber based on butadiene. G. Ebert developed two types of rubber (Buna 85 and Buna 115) for the industry, obtained by polymerization of butadiene 26

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on the metallic sodium. For a number of significant problems of this method of polymerization, even in 20-30 years "IG Farbenindustrie" company received numerous patents for carrying out the polymerization of dienes in emulsion. (the "Bayer" company was the first one which patented an emulsion polymerization in 1912). Simultaneously, the copolymerization of butadiene with other monomers was studied. As a result there had been developed emulsion styrene-butadiene and butadiene-nitrile rubbers (Buna S and Buna N, respectively), received at a pilot plant since1934. In the Soviet Union the work on emulsion polymerization was carried out under the direction of B.A. Dolgoplosk, who proposed the redox systems for initiating the polymerization of butadiene. Famous Soviet scientists as S.S. Medvedev, H.S. Bagdasaryan, A.D Abkin, A.I. Yurzhenko, P.M. Homikovsky, etc. made an invaluable contribution to developing ways to initiate radical polymerization and studying the mechanism of emulsion. During the Great Patriotic War the leading scientists of our country were keeping up developing the processes of petroleum monomers production for synthetic rubber. In 1941-1945 it was first established the possibility of copolymerization of α-methylstyrene with butadiene (butadiene-α-methylstyrene rubbers). Commercially synthesis of such rubbers had been implemented in the SR plant after the war. Center for the basic researches on the synthesis of elastomers was all-USSR S.V. Lebedev Scientific Research Institute of Synthetic Rubber (VNIISK) organized in 1945 in Leningrad. After the release of our country temporarily occupied by Germans, the Voronezh and Efremov Synthetic Rubber Plants were restored, and in 1948 the pre-war level of rubber production was reached. Simultaneously, retooling and modernization of the existing SR plants were conducted. Particularly, the Voronezh plant was renovated and enlarged, where for the first time in the country the emulsion styrene-butadiene rubbers were received. Due to the rapid development of economy and increasing demand for the synthetic 27

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rubber such new SR plants as Sterlitamak, Nizhnekamsk, Togliatti, Krasnoyarsk, Volzhsk and Omsk were put into operation in the late 50s and early 60s. During construction there were used the most advanced methods for monomers production (mostly petroleumbased monomers) and new processes of radical and ion-coordination polymerization. In 1953-54 a revolutionary event took place in the field of polymerization (including the synthesis of rubber) - there were catalytic systems, allowing to obtain stereoregular polymers. Such catalysts were discovered by German scientist K. Ziegler and studied in details by the Italian chemist G. Natta, so they are called by the names of two researchers. With the use of Ziegler-Natta catalysts it was possible to obtain the synthetic 1,4-cis-polyisoprene, identical in structure and approaching the properties of natural rubber. The development of research in the field of stereospecific polymerization and industrial development of isoprene, butadiene, ethylenepropylene and other rubbers became rapid. In our country, the research in this area was carried out under the direction of A.A. Korotkov (1948-1953), as well as B.A. Dolgoplosk and his numerous students and staff. In 1960 the "Shell" company in California started the first stereospecific polyisoprene production plant. Commercialization of the rubber was growing rapidly, and in the seventies of last century the United States had produced 140 tons already, Western Europe 150 tons, Japan - about 90 tons of stereoregular isoprene rubber. However, the largest volume of production of 1,4-cis-polyisoprene reached in the Soviet Union - about 800 tons per year. Such large production volumes of isoprene rubber in our country are related to the tendency to avoid the import of the NR, to preserve the economic independence of the country and fully provide the mechanical engineering with rubber products of necessary quality. Currently in the United States and Western Europe the tire rubber is composed of NR from 40 to 55%, while in the tire industry 28

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of Russia its part does not exceed 1% of the total volume of the used elastomers. In the postwar years, Malaysia had been long led in the production of NR, where the peak production was reached in 1988 (1.66 million tons), but it has been producing only about 20% of the world's NR as yet. In the 90's of Twentieth century, Thailand became the largest producer of NR (1.79 million tons in 1995, 2.24 million tons in 2000). Still a lot of natural rubber is produced in Indonesia; its production is rapidly growing in India, China, Vietnam and other countries. Amid the energy crisis and the oil reserve depletion the production of NR acquires special significance. It should be noted that the energy intensity of NR production is only 10% of styrene butadiene synthetic rubber energy use. The environmental aspects of the problem are also important: a small impact on the environment when receiving the NR and sustainability of vegetable raw materials. Table 3 shows the synthetic rubber production capacity in various regions of the world in 2004. Table 3. The SR production capacity in 2004 Regions and countries Th. tonnes Total production share, % Russia 1598 12.9. North America 2494 21 Western Europe 1854 15 Asia and Oceania 3835 32 Central Europe and the CIS 1787 15 China 1087 9 Latin America 658 6 Middle East and Africa 145 1 Modern, developing and increasingly complex technology requires various properties and high-quality rubbers, which would 29

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not dissolve in oils and gasoline, withstand high and low temperatures, show resistance to oxidants and various corrosive mediums. In this regard, the research works on the modification of rubbers and rubber based on them gain in contemporary importance. Rubber-based rubber tires are made for vehicles, aircraft, farm equipment, bicycles, various industrial, household and medical rubber products; rubbers are used for electrical insulation, as well as construction equipment. The main rubber consumers are the tire industry and the manufacturing of various industrial rubber products. Rubbers accounts for more than 50% of all the materials used for tires manufacturing. The range of industrial rubber products produced in our country exceeds 100 thousand items. To complete a modern car it is necessary to have 300-500 rubber products on the average (KamAZ has about 800 components). One plane contains 10 000-12 000, and the ship - up to 30 000 rubber products. The output characteristics of rubber and tires in the most depend on the nature and quality of rubbers used for their manufacturing. In the world rubber tire manufacture 30% of the total amount of used rubber accounts for natural rubber (NR). For the necessity of a significant import relief of NR Russia turned out the world's leading producer of synthetic 1,4-cis-isoprene rubber, the natural rubber counterpart. In terms of production volume synthetic polyisoprene occupies the first position in Russia, and its share now accounts for about 37% of the total production volume of SR. In the world the main type of producible synthetic rubber is general purpose butadiene-styrene rubber (BSR) which accounts for 58% of industrial capacities of SR. In our country, BSR production volumes occupy the second position and account for 25%, while butadiene rubber accounts for about 13% of the total production of SR, and butyl rubber - 11%. In 2005 total production volume of synthetic rubber in Russia total a little over 1.1 million tons. Table 4 presents some types of 30

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rubbers production data in the USSR and Russia in the period from 1988 to 2004 inclusive. Table 4. The production of some types of SR in the USSR and Russia Russia The USSR Type of rubber 1988 1998 2001 2004 Isoprene 986.8 221.5 351.6 406.1 Butadiene 368.2 84.6 95.8 129.6 Styrene-butadiene 545 143 238.1 307.1 (α -methylstyrene) Butyl rubber 56.4 84.6 95.8 129.6 Other 203.2 85 168.2 141.2 Total 2159.6 618.7 919.5 1113.6 Half of the produced domestic rubber is exported. At the same time, the share of isoprene rubber exports is 28%, butadiene 40%, BSR - 50%, while the export of butyl rubber accounts for 95% of its production volume. In 2005, four companies attained to more than 80% of the total production volume of rubber and latex in our country. For example, OAO "Nizhnekamskneftekhim" produces 27% of the total SR, LLC "Togliattikauchuk" - 23%, "Voronezhsintezkauchuk" 19%, JSC "Kauchuk" (Sterlitamak) - 15%. The share of OJSC "Omsky Kautchuk" accounts for 8% of rubber produced in Russia, JSC "Efremov plant of SR" - 5%, JSC "Krasnoyarsk plant SR" - 3%. The structure of tires production in Russia has significantly changed and come up to the world structure in recent years. The share of passenger car tires in the production volume considerably increased and in 2004 was 63%. For example, in Western Europe and the United States this rate is from 72 to 90%. A change in the structure of tire production has caused changings in tire requirements and their production processes. Among the most priority characteristics for tire consumers were grip 31

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on different road carpets (asphalt, wet asphalt, snow, ice), rolling loss, tires load-bearing and geometric inhomogeneity and materials consumption. In this regard, the trend of production of solution polymerized butadiene-styrene rubber has appeared. Solution styrene-butadiene rubber ensures higher wet grip and lower rolling losses with the same wear resistance, as compared with the emulsion rubber. Significant increase in tire grip, the vehicle stability and roadability are also achieved with the introduction of 8.5 wt. fraction of polyisoprene in a rubber compound with a predominant content of 3,4-units. The general trend in world production of SR is the growth of the role of thermoplastic elastomers (TPE), the main advantages of which are nearly complete wasteless recycling, reuse and elimination of vulcanization stage. Over the last decade owing to the opening of new generation of metallocene catalysts the manufacturing of previously unknown stereoblock propylene rubbers and copolymers of ethylene with higher α-olefins became possible. World demand for elastomers (NR and SR) in the next 30 years could double from 18 million tons in 2005 to 36 million tons in 2035, while maintaining the typical ratio SR: NR = 60: 40 for the present. 1.4 Main Stages of Polymerization Processes In the vast majority of cases, in the industrial production of synthetic rubber there are used the polymerization processes, which, in spite of the variety, can be represented by a single flow scheme: basic components preparation of polymerization process of extraction and treatment of the polymer. A few processes in which the polymer is formed by the polycondensation are also described by the same sequence of operations (except that the second stage is the polycondensation). 32

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Technical implementation of all stages of the process must ensure the maximum yield of product per reactor volume unit at the least cost to raw materials, capital investment, energy, etc. Besides this approach which is common to the whole chemical engineering for obtaining rubbers and other polymers, the set of properties of the product, defined by chemical structure and the polymer structure, is crucial. Therefore, in the technology of rubbers synthesis kinetic peculiarities of processes that affect the productivity of the equipment cannot be considered out of the reached molecular weight characteristics and microstructure of the polymer. The solution of these two related problems is implementation and the right selection of the conditions applied at each stage of the processes. The first stage of the process – the preparation of the source components – typically involves the preparation of solutions (rarely emulsions or suspensions) of certain concentrations and their dosing into the polymerization apparatus. The process of purification of monomers, solvents and other materials of the polymerization system is as a rule a separate technological cycle and is not included in the preparation of components stage. Processes of dosing, dilution, filtration, heating (or cooling), etc. occurring at this stage usually do not have any distinguishing features and can be equipped with a standard chemical equipment. In the second stage polymerization reaction proceeds, and the high molecular weight compound – rubber is formed. Conditions typical for chemical engineering are relatively rare in the process, and typical equipment is suitable only in certain manifactures. The feature of the polymerization step is quite significant heat. When joining of the molecule of unsaturated monomer to the growing chain one double bond breaks and two single bonds form, so the theoretical value of reaction heat is easy to calculate. If we take the average values of bond energies EC=C and EC-C, we will obtain: QR = 2 EC-C - EC=C ≈ 95 kJ/mol. (1.3) Actually, this value is slightly smaller, and the greater the size or number of substituents in the monomer molecule, the lower the 33

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heat of polymerization. Obviously, this is due to the fact that part of the energy is spent on overcoming the steric hindrances while growing of the chain. Polymerization with ring opening is also an exothermic reaction, meanwhile, the more strained the ring, the higher the heat. For example, ethylene oxide (a very strained three-membered ring) is polymerized with the thermal effect of 104.6 kJ/mol (2374 kJ/kg), whereas for tetrahydrofuran (low strained five-membered ring) QR = 21 kJ/mol (292 kJ/kg). Other features of polymerization stage are a significant viscosity of the reaction medium, the possibility of the polymer sticking to the walls of the equipment, etc., which makes the occurring processes of mass and heat transfer complicated, and requires special types of equipment. Microkinetics that studies chemical reactions, complicated by processes of mass and heat transfer, underlies the theory of calculating the polymerization apparatus. Since the temperature of the process greatly and variously affects the speed of the various reactions occurring during the polymer formation, the vessel design should ensure strict set temperature and the working capacity of equipment in a wide range of modes. During the conversion of monomer into the rubber, there are manifold and often multi-step processes of mass and heat transfer, such as diffusion of the monomer to the active centers, rearrangement of resulting macromolecules, the local heat release (by means of the viscous friction energy dissipation when stirring) and its removal from the system, etc. Therefore, analyzing the polymerization processes and their modeling it is important to combine theoretical and practical generalization of the both transport processes at the molecular level and the processes of convective mass and heat exchange for the reactor (or a group of reactors) as a whole.

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1.5 Methods of Polymerization The polymerization processes of synthetic rubber are carried out in different conditions, according to the method of initiation, the nature of the monomer, the desired microstructure of the polymer, etc. The processes of bulk polymerization of the monomer seem to be the most simple, as they do not require any solvents, thinners and others additional components. Meanwhile, the monomer could be liquid or gaseous. The process of liquid-phase polymerization could occur in two ways. 1) The polymer and the polymerization initiator are soluble in the monomer, the process begins and continues in the solution the viscosity of which greatly increases as the monomer exhausts. Therefore, in conventional reactors only the initial stage of the process can be conducted with intensive stirring, when the reaction mass remains sufficiently mobile. To complete the reaction the solution of the polymer in the monomer must be moved into small molds where the process takes place without stirring up to high conversion degrees. 2) The polymer is not soluble in the monomer; the polymerization process begins in a homogeneous system, and then continues in the particles of polymer, swollen in the monomer. In this case the chain termination reactions are complicated, and the polymer has high molecular weight values. Another way of polymerization in the bulk of the monomer is gas-phase process in which the monomer is used in the form of gas. The formation of the polymer begins and develops on the surface of the catalyst, resulting in a two-phase system throughout the process. At that rate, the conditions of removal of polymerization heat are substantially improving (with the help of remote heat exchangers, where the circulating monomer is cooled). The obtaining the butadiene rubber (SRB) on the metallic sodium in such a way is associated with serious problems, which are its periodicity, complexity of the catalyst deactivation and rubber perfection procedures, a low level of process mechanization and automation. 35

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Hereupon, the gas-phase polymerization in this way is out of dated and finds limited use. The newly developed gas-phase polymerization processes in a fluidized bed of powdered catalyst overcame the majority of mentioned shortcomings, but these processes have not become widespread so far. In recent decades, the polymerization in solution has become the main method of producing synthetic rubber in our country. It should be noted that when manufacturing the SR all the processes of solution polymerization are ionic. In most commercial systems there are solvents in which the source monomer (or mixture of monomers) and the resulting polymer are highly soluble. The reaction mass preserves the homogeneity during the whole process and as the monomer converses to polymer its viscosity increases significantly. The resulting product (the polymerizate) is a solution of rubber and unpolymerized monomer, in addition, it contains the remains of a catalyst, so the rubber separation is related to the deactivation of the catalyst and distillation of monomer and solvent. The features of the process are the high viscosity of the reaction medium, making it difficult to eliminate the polymerization heat, and the possible sticking of polymer to the reactor walls. The version of the solution polymerization is less commonly used, when the polymer is not soluble in the solvent, and polymerizate is a suspension of the swollen polymer. At the same polymer concentration the viscosity of the dispersion is always lower than the viscosity of the solution, which facilitates heat elimination and allows higher concentrations of monomer in the source solution. This reduces energy consumption during the subsequent separation of the polymer and solvent recovery. The emulsion polymerization is widespread in the global SR industry and is always radical. During the polymerization, the source aqueous emulsion of the monomers turns into a colloidal dispersion of polymer (latex) with particle sizes of 30 to 300 nm. The low viscosity of the reaction medium makes it easy to take away the 36

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polymerization heat, but the unpolymerized monomers distillation and rubber separation from latex is rather complicated and energyintensive processes. However, these costs are much lower than when separating rubber from the solution and solvent recovering. Therefore, generally, emulsion rubbers have a lower cost than solvent rubbers of the same type. The resulting rubber is usually contaminated with the remains of an emulsifier and inorganic salts; besides, radical polymerization mechanism does not allow to obtain stereoregular rubbers. Large scale solution and emulsion polymerization are usually continuous processes in the cascade of reactors operating in the mode close to the ideal mixing, whereas the work of the whole cascade close to the mode of plugflow reactors. The higher the reactor volume the lower cost per unit of product, and the higher the level of automation, and the closer to the optimum process conditions, the higher the single aggregate economic capacity. But with the growth the reactor volume the problems of mixing and especially reaction heat removal become more complicated, and now it is commonly used the polymerizers of 16-20 m3. 1.6 Heat Exchange in Polymerization One of the most intensive methods of heat removal from the reaction volume is the evaporation of part of the monomer or solvent with their condensation in a separate condenser and return to the apparatus. However, the evaporation out of highly viscous mediums is accompanied by considerable foaming, which makes it hard to implement this interesting method in industrial scale. If to avoid foaming (eg, as receiving the ethylene-propylene rubber in liquid propylene), or to suppress it by antifoamers, the heat removal by evaporation will be very promising. When the convective heat removal in a continuous process of polymerization the heat balance for each unit of the cascade can be written as: 37

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QP + QN = QM + QF, (1.4) where: QP - the heat of polymerization; QN - the viscous flow energy dissipation; QM - the heat consumed in heating the incoming products to the temperature in the reactor; QF - the heat revealed through the heat exchange surface. Qp = Gp × [M] × ∆X × rp, (1.5) QM = Gp × cp × (tp - tin), (1.6) QF = F × α × (tr - tw), (1.7) where: Gp, cp - the amount and heat capacity of the reaction mass supplied to the apparatus; [M] - monomer concentration; ∆X - the increment of the degree of conversion (conversion) of monomer in the apparatus; rr - the monomer polymerization heat; α - the coefficient of heat transfer from the reaction mass to the wall; F - the heat exchange surface; tp, tin, tw - the temperature in the reactor, of the incoming reaction mass and wall of the apparatus, relatively. Therefore, the efficiency of heat dissipation is influenced by technological factors (temperature, rate of polymerization, monomer concentration, etc.) and structural characteristics of the equipment (F, α, QN). .Knowing these parameters of the process, it is possible to define the role of each member of the equation and draw some conclusions. Quantity of heat released in the device due to the polymerization reaction depends primarily on the values of ∆X and [M], since the amount of fed products is usually specified by the volume of the reactors of the cascade, and the value of rp for the system is constant. From the standpoint of simplifying the degassing and reducing costs of the monomer regeneration it is advisable to carry out the processes to the highest possible conversion, limited by the quality of the resulting polymer or the kinetics of the polymerization process. As a rule, the increment of conversion ∆X decreases from the first apparatus to the last in the cascade of polymerizers mainly due to lowering of monomer concentrations. For the more uniform distribution of heat load over the reactors of 38

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cascade it is reasonable sometimes to carry out the fractional feed of the monomer. To create highly productive polymerization processes it is necessary to ensure the highest possible output concentration of polymer in the reaction mass. However, during the solution polymerization, the viscosity of the system substantially increases with the growth of concentration, which is especially characteristic for solutions of flexible chain polymers, namely rubbers. Maximum allowable viscosity is achieved at concentrations of polymer in solution 11-13%. In consideration of the high conversion of monomers, the initial concentration of monomer [M] in the solution fed to the polymerization does not exceed 15% wt. In emulsions, the concentration of monomers in the initial reaction mass is much higher (30-50% wt.). When heat removing through the heat exchange surface the temperature difference (tr-tw) is very important which is often limited by the terms of the polymerization process, the ability of the polymer deposition on cooled surfaces, or economic considerations. Depending on the polymerization temperature tp the different refrigerants are used: Temperature of Refrigerant polymerization, °C 50 ÷ 80 industrial water 10 ÷ 50 chilled water, brine 0 ÷ 10 brine, ammonia, propane -80 ÷ -100 Ethylene The polymerization heat removal by means of heating supplied cooled reaction mass is important for the fast processes of polymerization. In principle, with a sufficiently large difference (tptin) the process can be transferred to the autothermal mode, i.e. the entire heat released in the apparatus is spent on heating the incoming products, and the cooling through the heat exchange surface is unnecessary. At relatively low temperatures of polymerization, the 39

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difference (tp-tin) cannot be large, and this mode is possible only at very low values of [M] or ∆X. In the synthetic rubber industry, both heat removal methods are used in combination, moreover the largest value of (tp-tin) is characteristic for the first machine of the cascade, where the increment of the monomer conversion is the largest and, therefore, the heat release is also the highest in the cascade. It is important to consider the viscous flow heat QN, which depends on the properties of the reaction mixture, and mixing intensity. With the increase in the rotary speed n of mixer the number of heat released in the apparatus increases, and at the same time (but much less) the heat transfer through the cooling surface enhances: QN ≈ A × na; QF ≈ B × nb, (1.8) where A and B - coefficients of proportionality, a ≈ 2 ÷ 3; b ≈ 0,35 ÷ 0,65. In this regard, the work of polimerizer is not reasonable under any conditions. Figure 1.1 schematically shows the change in the ratio of the heat balance equation summands for different rates of stirring (for two values of Qp). It is clear that the process will proceed without raising the temperature, provided that: ∆Q = (QF + QM) - (Qp + QN) ≥ 0. (1.9) Autothermal mode in these systems cannot be realized as QM