Tehnici de polimerizare

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1 VI. ELEMENTS OF TECHNOLOGY OF CHAIN POLYMERIZATION VI.1. Generalities Chain polymerization is one of the most used methods for the industrial synthesis of polymers and copolymers used as thermoplastic materials, rubbers, synthetic fibres, adhesives, resins, etc. All the mechanisms (free-radical, cationic, anionic and coordinative) have practical applications, but the majority of the industrial processes are based on free-radical polymerization, since it requires less restrictive reaction conditions (especially in what concerns the purity of the reagents and the reaction medium). As a general rule, if a monomer can be polymerized by free radicals, this is the method chosen in industry, except for the cases when the microstructure required for the final product must be obtained by other mechanisms available. Such a case is the industrial synthesis of rubbers via anionic and coordinative polymerization or the Ziegler-Natta polymerization of olefins. Whatever the nature of the active centre, some features are common for all the additive polymerizations: existence of the characteristic stages of a chain reaction (initiation, propagation,

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Tehnici de polimerizare or polymerization techniques

Transcript of Tehnici de polimerizare

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VI. ELEMENTS OF TECHNOLOGYOF CHAIN POLYMERIZATION

VI.1. Generalities

Chain polymerization is one of the most used methods for the industrial synthesis of polymers and copolymers used as thermoplastic materials, rubbers, synthetic fibres, adhesives, resins, etc. All the mechanisms (free-radical, cationic, anionic and coordinative) have practical applications, but the majority of the industrial processes are based on free-radical polymerization, since it requires less restrictive reaction conditions (especially in what concerns the purity of the reagents and the reaction medium).

As a general rule, if a monomer can be polymerized by free radicals, this is the method chosen in industry, except for the cases when the microstructure required for the final product must be obtained by other mechanisms available. Such a case is the industrial synthesis of rubbers via anionic and coordinative polymerization or the Ziegler-Natta polymerization of olefins.

Whatever the nature of the active centre, some features are common for all the additive polymerizations: existence of the characteristic stages of a chain reaction (initiation, propagation, termination and transfer) and presence of polymer with high molecular mass in the system in the early stages of the polymerization. The polymerization degree does not depend on conversion (except for ome spacial cases of anionic polymerization) but is determined (in absence of autoacceleration or chain transfer) by the ratio between the concentrations of the monomer and the initiator.

Another important feature of chain polymerization is its exceptional exothermicity, that imposes to find out suitable methods to eliminate efficiently the reaction heat.

The structure of the polymer depends on the mechanism of polymerization: linear and stereospecific chains are obtained using more selective mechanisms (such as coordinative polymerization) while via free-radicals the reaction is less specific and branched (or even cross-linked) chains are obtained in many cases.

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From a practical point of view, there are several polymerization techniques, different by the composition of the reaction mass, the number of phases and the properties of the final polymer. The classical chain polymerization techniques are bulk polymerization, solution polymerization, suspension polymerization and emulsion polymerization. The latter two techniques are heterogeneous ones, while the first two may be either homogeneous or precipitant.

In the following, each technique will be detailed and some practical examples will be presented.

VI.2. Bulk Polymerization

VI.2.1. Main Features of Bulk Polymerization

Bulk polymerization (also called mass polymerization) is the simplest technique in what concerns the number of components and phases of the reaction system.

At the beginning of the process, the reaction mass consists only of monomer and initiator, or even only monomer (if initiation is achieved by physical methods). The chemical initiator must be monomer-soluble.

Depending on the solubility of the polymer in the monomer, there are two ways in which the reaction may progress. If the polymer is soluble in the monomer, then the polymerization system is homogeneous (homogeneous bulk polymerization). If the monomer is not a solvent for the polymer (case of some monomers as for instance, vinyl chloride or ethylene) then the polymer, once formed, separates as a second phase. This is called precipitant bulk polymerization and will be treated separately.

VI.2.1.1. Homogeneous Bulk Polymerization

In the homogeneous bulk polymerization, the systems remains homogeneous until the end of the process; at 100% conversion, the reaction mass will consist only of polymer (that incorporates the initiator at the end of the macromolecular chains), that forms a compact block, hence the name of the technique. Obviously, in an industrial system, the temperature must be raised high enough to keep the polymer flowing, in order to be able to ensure the system’s mixing. In some practical applications however (dental surgery for instance) the system is not stirred and the polymerization is photochemically initiated in a stationary layer. If

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no additives (pigments, fillers, etc.) are added, then the polymer does not contain any impurities, so it will be completely transparent.

Homogeneous mass polymerization has the advantage of the simplicity and of the high purity of the reaction product, but at the same time there are some significant problems raised when scaling up the process for industrial plants.

The most important problems that have to be solved (and that are difficult enough to make bulk polymerization a process less used as compared to the other techniques) are the following:

a. Viscosity of the reaction massViscosity of polymer solutions increases with both the molecular mass and

the concentration of the polymer dissolved. In the homogeneous bulk polymerization, the reaction mass consists of monomer that dissolves polymer with high molecular mass and the concentration of the polymer increases with the conversion.

Polymer solutions are extremely viscous (105 to 108 times more viscous than water). Moreover, during a high conversion polymerization, viscosity increases several orders of magnitude (starting from the monomer viscosity, around 10-3

Pas to the viscosity of the molten polymer, 102-105Pas). Since stirrers are constructively adapted to the viscosity of the mixed fluid, the same stirrer cannot be used for both the initial and the final stage of the process. Usually the reaction is performed in several reactors, each one with a stirrer adapted to the range of viscosities corresponding to a given interval of conversion: helical stirrers and anchor-type mixers for lower viscosities, planetary stirrers or rotating profiled cylindrical mixers for the late stages of the polymerization. In some cases, stationary mixing devices are used (elements that ensure – by their architecture - a spiral flow of the reaction mass that produces a certain degree of mixing). There is also a difference in what concerns the rotating speed of the stirrers, that diminishes significantly with the viscosity (while the power consumption increases).

Another factor to take into consideration is that for viscous monomer-polymer mixtures, achieving turbulent flow is impossible; the values of the Reynolds criterion for concentrated polymer solutions are very low (100 to 102) while Prandtl values are 10 to 1000 times higher than for low-molecular compounds. This has a significant impact on the removal of the reaction heat (by convection) that becomes extremely difficult when the conversion exceeds a certain level.

The high viscosity has to be taken into account also when polymer solutions (reaction mass with a given conversion degree) have to be transported through pipelines. Not only the energy consumption for pumping is very high, but also the viscous solution may block the pipes, fittings and pumps. Classical centrifugal and volumetric pumps cannot be used; they are replaced with screw pumps (similar

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with the body of a screw extruder) or peristaltic devices (where the moving parts are outside the hose that contains the polymer); another solution is not to use pumps at all, but to take advantage of the gravitational flow when transporting the reaction mass from one reactor to another. Moreover, to avoid blocking, diameters of the pipes are larger than the usual for low-molecular compounds.

b. AutoaccelerationAutoacceleration is a phenomenon typical for the bulk polymerization, that

consists of the increase of the reaction rate without any outside intervention (any change in the reaction parameters). As previously detailed (see chapter V.3.10), autoacceleration leads to a characteristic S – shaped conversion time curve (figure V.1) and to a peak of the polymerization rate.

The autoacceleration is due to the massive increase of the viscosity that accompanies the advancement of the reaction; this leads to a lower diffusion rate that affects mainly the macromolecular species. Consequently, the termination rate decreases and the overall polymerization rate increases, until the monomer concentration becomes small enough to decrease the rate of the process. Another consequence of the autoacceleration is an increase of the average molecular weight, which, in turn, leads to a supplementary raise of the viscosity.

The practical consequence of the autoacceleration is that at a given moment the reaction becomes very difficult to control, since the reaction rate (and the corresponding reaction heat evolved) increase is of at least one order of magnitude. Any computation of a chemical reactor has to take this phenomenon into account, and estimation of the heat transfer area and stirring power must be done for the less favourable case, i.e. the autoacceleration peak.

c. Polymerization heatAs already stated, the thermal effect of chain polymerization is higher than

the usual value for reactions between organic compounds or step-growth processes (see table V.1.). Moreover, the heat is generated inside a reaction mass with a low thermal conductivity (3-4 times less than water) and with a very high viscosity, that makes efficient stirring impossible. Therefore, removal of the reaction heat by either convection or conduction mechanisms is impossible in homogeneous mass polymerization. Only approximately 10% of the reaction heat can be removed by circulating a cooling agent through the shell or coil of an industrial reactor, and this amount reduces to less than 1% for the autoacceleration peak. The extra heat remains in the reaction mass and would lead, for a homogeneous mass polymerization, to a huge increase of the temperature for high conversion. This temperature does exceed both the boiling point of the monomer and the destruction temperature of the polymer, therefore it would produce explosive decomposition of the reaction mass.

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Thermal control of the homogeneous mass polymerization is the most important problem that has to be solved in industrial plants, and several solutions are applied, that will be detailed below, tailored according the particular features of each case of polymerization.

d. Monomer conversionMost chain polymerization may be considered as irreversible, so 100%

conversion can be obtained – at least in theory. In practice a total conversion might not be useful in industrial plants, for several reasons:

the viscosity increase might limit the stirring efficiency; in a reactor with a deficient mixing there may develop temperature gradients that may affect the properties of the product (overheating may lead to partial decomposition affecting the colour and the mechanical properties of the polymer);

the thermal control is so difficult during last stages of the polymerization that limiting conversion to lower values might be the only solution for ensuring an efficient heat removal;

in systems where chain transfer reactions have a high intensity, cross-linked polymer is obtained at high conversion due to the transfer towards the polymer; to limit the amount of insoluble product, conversion must be limited at lower than the unit values.

some polymers have melting points higher than the decomposition temperature, so the fluidity of the reaction mass cannot be maintained at high concentrations of the polymer.

In all the above cases, the final product contains a given amount of unreacted monomer; this needs to be quasi-totally eliminated from the polymer, since the high toxicity of the monomers imposes a residual monomer content in the commercial polymers of the order of ppm. Consequently, a demonomerization stage must be included in all the flows where the conversion is not total. The removed monomer is then purified and recycled in the synthesis.

To solve the above described problems, several technological solutions have been adopted for the homogeneous bulk polymerization:

1. Low conversion polymerizationThis methods aims at limiting the heat evolved during the process by limiting

the conversion at a value low enough not to enter the autoacceleration range. This way only a fraction of the reaction heat is generated, and the reaction rate is low enough for the heat flow-rate to be controlled. Moreover, at low conversion the viscosity is relatively low, with the advantage of a better mixing. Part of the reaction heat is removed using a cooling agent, part remains in the system and it is used for heating the raw material to the reaction temperature. The reactor is

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partially autothermal (hence has a lower specific energy consumption) but the separation and recycling of the residual monomer increases both the investment and the operation costs.

The method is usually used for monomers that can be easy separated (gaseous monomers) and it applies mostly for precipitant systems (see below).

2. Two stages polymerizationThe method consists of two separate stages of the polymerization. In the first

one, that occurs at low conversion (bellow the autoacceleration limit, that may range between 10 and 40% depending on the nature of the monomer) the reaction is performed in classical stirred reactors, with a cooling shell or coil. The viscous monomer-polymer solution (sometimes called prepolymer, or polymer syrup due to its viscosity) is then poured in rectangular or cylindrical shaped recipients that have a very large specific surface and where polymerization continues until reaching total conversion. The large specific surface ensures that an efficient cooling may be achieved without stirring, by circulation of cooling water (or even air) outside the shaped polymerization forms. In such processes, conversion may reach 100% so demonomerization is not needed.

The best known examples are block polymerization of methyl-methacrylate to obtain sheets with a 3-6 mm thickness and surfaces up to several square metres or anionic polymerization of ε-caprolactam, to obtain polymer rods that are further processes via mechanical methods, similar with metals.

3. Polymerization methods tailored for specific monomersBulk polymerization of styrene is a process that takes into account some

specific features of this monomer: its polymerization can be initiated by heating (purely thermal initiation), the polymerization rate is lower than for other vinyl monomers (due to the conjugation of the double bond with the aromatic ring in the monomers structure) and both the monomer and the polymer are practically immiscible with water.

Bulk polymerization of styrene is performed in continuous stirred tubular reactors, disposed horizontally, at temperatures above the flowing range of the polymer, to maintain the fluidity of the reaction mass. Initiation is purely thermal, which ensures a quite low reaction rate even at high temperature. The reaction heat is eliminated by injecting cooling water in the reactor and eliminating it as vapour. Removal of the heat reaction as latent evaporation heat allows maintaining the temperature of the polymer bellow the thermal destruction range. To ensure a good mixing of the viscous reaction mass, shaped cylindrical stirrers are used. The product thus obtained is very pure, so it had a high degree of transparency.

The bulk polymerization of vinyl chloride - which is not a homogeneous process but a precipitant one (see bellow) but for which the above mentioned

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practical problems apply as well - combines the solutions above described. The polymerization is separated in two stages (with adapted systems of stirring and heat removal) but the conversion is limited to 60-65% (the main reason being to prevent branching and cross linking). The first stage, up to 10% conversion, uses classical stirred tank reactors, while the second – critical in what concerns the viscosity, the reaction rate and the heat generation) proceeds in reactors with a special construction, similar with the internal mixers with helical stirrers, that allow homogenisation of the viscous mixture. The reaction heat is eliminated by circulating cooling water (5°C) in the reactor shell; the excess heat is used to heat the reaction mixture – this facilitates the demonomerization stage that occurs by vaporization of the monomer (that is liquefied inside the polymerization reactor, that works at high pressure).

VI.2.1.2. Precipitant Bulk Polymerization

In the homogeneous bulk polymerization, the polymer is dissolved by the monomer to give a viscous solution. However, there are a few monomers that do not dissolve the corresponding polymer: the best known example is the acrylonitrile.

In this case, bulk polymerization evolves differently from the homogeneous case. Initiation occurs in the monomer phase (that dissolves the initiator) but – after a few elementary propagation acts – the length of the chain becomes large enough for the macroradical to become insoluble; the polymer precipitates and propagation continues in the “solid” phase (that may be, in fact, either glassy or biphasic polymer). Therefore, a few moments after the beginning of the polymerization, the system becomes heterogeneous: the continuous, liquid phase is the unreacted monomer (with the dissolved initiator) and the dispersed phase consists of very fine polymer particles. The growth of the chains continues on the surface of the dispersed particles using the monomer from the liquid phase, with a corresponding increase in the size of the particles.

Termination may happen both in the liquid phase and in the particles. In the liquid phase possible reactions are between two chains short enough to be soluble, between soluble chains and primary radicals or by transfer towards the precipitated polymer particles. While possible, all these reactions have a low impact on the process, since most of the active centres are in the dispersed phase. In the polymer particles termination occurs by reaction between two precipitated growing chains. However, in the precipitated phase the mobility of the growing chains is lower; some of the active centres may become “trapped” inside the particles so the termination rate diminishes with the increase in conversion (when particles are

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larger than in the initial stage). This leads to a significant increase of the polymerization rate (autoacceleration).

Note: due to the above features, the kinetic equations valid for homogeneous polymerization are not anymore valid; several mathematical models for describing the polymerization rate are found in the literature.

The high reaction heat coupled with autoacceleration imposes a very good thermal control of the polymerization, since overheating can lead easily to explosive decomposition of the reaction mass.

However, as compared to the homogeneous bulk polymerization, the viscosity of the polymer-monomer suspension is much lower (comparable with the viscosity of the monomer, so while there is still enough monomer to maintain the fluidity of the reaction mass an efficient stirring can be achieved, thus allowing heat removal by convection (using a cooling agent circulating through the shell or coil of the reactor).

The stirring requirements limit the total conversion possible to be obtained in practice, since if all the monomer would be consumed the reaction mass would transform into polymer powder. Therefore, the polymerization must be stopped at lower than the unit conversion and the unreacted monomer must be separated. Demonomerization is however a simple process in this situation, since the monomer can be separated by filtration. The polymer powder is then dried to eliminate the last traces of residual monomer.

Other cases where the polymerization is precipitant is the one where monomers are gases under normal conditions (such as ethylene and vinyl chloride). Polymerization of gaseous monomers occurs at high pressure (to ensure a high enough monomer concentration and thus the productivity of the plant). The unreacted monomer is separated by decompressing the system; thus the monomer passes in the gas state.

VI.3. Solution Polymerization

VI.3.1. Main Features of Solution Polymerization

Solution polymerization is the technique in which the monomer and the initiator are dissolved in a suitable solvent before the reaction.

Depending on the solubility of the polymer in the monomer-solvent mixture, there are two types of solution polymerizations: the process in homogeneous solution, where the polymer is soluble in the reaction medium and the precipitant solution polymerization (also called solution-suspension technique) in which the insoluble polymer precipitates as a separate phase.

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VI.3.2. Homogeneous Solution Polymerization

In this process, the monomer, initiator and polymer are soluble in the solvent. If the reaction proceeds at total conversion, at the end the product will be a (concentrated) solution of the polymer.

The presence of the solvent has a dilution effect on the monomer and initiator, which leads to lower polymerization rates as compared to the homogeneous bulk process.

The viscosity of the reaction mass also decreases with the amount of the solvent. It still remains orders of magnitude higher than the one of a low-molecular compound and it increases with the polymer concentration (thus with the conversion) – this phenomenon usually requires to split the process in two or several separate stages, each performed in a reactor provided with a different system of mixing (more powerful stirrers for the final stages).

However, the dilution by the solvent increases the mobility of the macroradicals, therefore the auto-acceleration effect attenuates with the amount of solvent added and, for a concentration of the solvent about a certain limit (that depends on the polymer’s nature and its molecular mass) it disappears completely. In these cases, the characteristic S-shaped conversion-time curve for autoaccelerated systems is replaced by a platformed one (see figure VI.3.). The reaction rate diminishes gradually in time, due to the consumption of the monomer.

Figure VI.3. Conversion-time curve for homogenous solution polymerization.

Technically there is no limitation of the conversion that can be reached in solution polymerization. However, some cases may require lower than the unit

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conversions, due to the particular features of the chemical process (excessive branching or cross-linking due to chain transfer) – in all these situations, a demonomerization stage is needed after polymerization.

Thermal control of the reaction in solution polymerization is easier as compared to the bulk technique, due to several factors: lack of autoacceleration, better mixing of a lower viscosity reaction mass and especially the possibility to eliminate the reaction heat as latent heat by solvent’s vaporization. In fact, the solution polymerizations almost always occur with vaporization of the solvent (or of the azeotropic mixture monomer-solvent); the vapour is passed through a heat exchanger to be condensed and then recycled in the reactor. This has the advantage that the cooling surface is not limited to the shell/coil area of the reactor but imposes an upper limit to the reaction temperature (boiling point of the solvent or of the monomer/solvent azeotropic mixture).

The presence of the solvent introduces some supplementary factors that may affect the reaction and the product, depending on the nature of the monomer, the solvent and the active centre. The solvent may participate in transfer reaction (for free-radical processes), with several consequences:

If the radical obtained by transfer towards the solvent is able to reinitiate polymerization, then the conversion may reach 100% but the average molecular mass of the polymer will decrease, proportional with the intensity of the chain transfer (and thus with the solvent concentration). In the polymerization of monomers that produce very active radicals in chain transfer, a compromise has to be reached between the need for dilution and the lowering of the polymerization degree. Usually, this problem may be solved by gradually adding the solvent (reaction starts in concentrated solution and subsequently, more solvent is added in the following stages, corresponding to higher conversions.

If the radical resulted by transfer is more stable than the monomeric active centre, the degradative chain transfer may limit conversion, together with a severe reduction of the polymerization degree. These factors narrow the choice in what concerns solvent as compared to the reactions in organic chemistry (not involving polyreactions).

In ionic polymerization the dielectric constant of the solvent strongly influences the reaction rate, through is solvation capacity (thus determining whether the polymerization occurs via free ions or ionic pairs).

Solvent selection The ideal solvent for a homogeneous solution polymerization should dissolve

the monomer, the initiator and the polymer, be inert in chain transfer reactions, and have a boiling point in the range of temperatures at which the polymerization rate is considered acceptable. High values of the vaporization latent heat and specific

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heat will ensure a better thermal control. The viscosity of the solution should be the minimum possible for a given polymer concentration (note that “good” solvents for the polymer produce more viscous solutions due to a more advanced disentanglement of the molecular coil as compared to the “poor” solvents).

Other characteristics for an acceptable solvent are the low cost, availability, absence of toxicity and flammability, low corrosion and facility of purification. The ideal solvent, according to the above criteria, is water; however, few polymers are hydrosoluble. Most organic solvents are, toxic and flammable, so the plants should be provided with efficient ventilation and the equipment should be sealed to prevent solvent loss in the atmosphere.

Polymer separationThe final product in a solution polymerization is a concentrated polymer

solution that may contain a variable amount of unreacted monomer. Separation of the residual monomer is done by distillation or stripping. Separation of the polymer from the solution is more difficult and can theoretically be achieved using two different approaches:

Solvent evaporation If the solvent has a high volatility, it can be eliminated by vaporization, either

under vacuum or using a flow of heated gas (usually nitrogen singe the mixtures of air and organic solvents have a high explosion risk). The solvent vapour should then be condensed, and the solvent will be recycled in the process after purification. The process has some major drawbacks: the need for perfectly sealed equipment (due to the toxicity of most of the solvents), explosion risks, and the difficulty of an advanced separation. The last is the most important problem, since the viscosity increases during evaporation. To reach a high degree of separation, the last stages have to occur in thin-layer equipment, that is expensive and supposes long operational times or high surfaces.

The process is used mostly when concentrated solutions of polymers are used for further processing: solution spinning of the synthetic fibres or manufacture of paints and adhesives that dry after application on the support. In both cases, the specific surface is large enough to allow the total removal of the solvent in an acceptable time range. Note also that if drying is made in atmospheric conditions (as for paints) the solvent is completely lost by evaporation, with a cost increase together with an environment and health risk.

Polymer precipitationThe method supposes separation of the polymer by adding a non-solvent to

the polymer-solvent mixture (the non-solvent has to be miscible with the solvent). It is used mainly for situations where the solvent has a high boiling point. In order to ensure a complete precipitation of the polymer, the volume of the non-solvent

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added to the solution must be 5 to 10 times larger than the one of the solvent. The polymer precipitates as a fine powder and it can be separated by filtration or centrifugation; afterwards it is dried under a current of heated inert gas and granulated (to reduce the manipulation losses).

The liquid phase is a mixture between the solvent and the non-solvent that must be separated – usually by column rectification – and recycled in the process.

The method has several disadvantages: using large volumes of solvent and non-solvent, the need for rectification of the liquid phase and the risks associated with working with organic solvents.

The problems listed above, linked to the separation of the polymer from the solvent at the end of the solution polymerization make this method extremely inefficient form the point of view of the costs involved. Therefore, the use of the method in industrial plants is limited to some particular cases:

Polymers that do not melt (that have the destruction temperature below the melting point or the flowing range). For these, the bulk methods cannot be used, since the last stages suppose a molten polymer.

Cases where the solution is used as such. There are some applications where polymer solutions are the final

(commercial products) or represent the feed for another processing line. Some examples are:

a. Paints – some paints are polymer solutions in an organic solvent. The solvent evaporates after application on the support. Some adhesives are used in a similar manner.

b. Polymer analogous reactions. These are reactions in which the polymer chain is not affected but the pending groups are transformed, using suitable reagents. All reactions on polymers must be performed in solution since the solvation of the macromolecular coils allows penetration of the reagents to all the pending groups. The best known example is the polymerization of vinyl acetate in methanol solution, that is used for polyvinyl alcohol synthesis.

c. Solution spinning – the method can use both evaporation of the solvent (dry spinning) and precipitation of the polymer (wet spinning) – in this case, both evaporation and precipitation are facilitated by the high specific surface of the filaments.

VI.3.3. Precipitant Solution Polymerization

Precipitant solution polymerization (also called solution-suspension polymerization) starts from a solution of the monomer and initiator in the solvent, but the polymer is not soluble in this mixture so it precipitates as a fine powder

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after the first moments of polymerization (initiation and the first elementary growth acts occur in solution but after reaching a certain length, the chains precipitate and the subsequent growth proceeds on the surface of the particles).

The technique is similar to the bulk precipitant polymerization; the difference is that the reaction can reach 100% conversion since the presence of the solvent will ensure the fluidity of the reaction mass even when all monomer is transformed into polymer.

Note that separation of the polymer as a different phase implies a significant reduction of the macroradicals’ mobility, the phenomenon that is the basis of autoacceleration. However, the reaction heat can be removed efficiently by evaporation of the solvent (as evaporation latent heat). The solvent is then condensed outside the reactor (with no geometrical limitation of the heat transfer area) and recycled in the reactor; this allows a good thermal control of the reaction.

At the end of the process the reaction mass is a suspension of the polymer in the solvent; the mixture is easy to separate by filtration or centrifugation followed by drying of the polymer particles in a current of heated inert gas. Note also that since the polymer is not soluble in the solvent, drying will be facilitated by the presence of the solvent only on the surface of the particles and not in their bulk.

If, for reasons linked to the chain transfer phenomena, the reaction is limited to a lower than the unit conversion, then the unreacted monomer has to be removed before polymer separation; this is made either by distillation or by steam stripping.

The precipitant solution polymerization is cost effective and it uses relatively simple installations, therefore it is preferred if there are solvents that do not dissolve the polymer. The main practical application is the synthesis of the acrylonitrile copolymers in aqueous solution.

VI.4. Suspension Polymerization

VI.4.1. Main Features of Suspension Polymerization

Suspension polymerization is a heterogeneous technique in which the monomer is dispersed in a continuous liquid phase. For a suspension polymerization, the condition is that the monomer must be immiscible with the dispersion medium (or to have a negligible solubility).

Usually the dispersion medium (the continuous phase) is water, since most monomers are organic substances insoluble in water. In a typical suspension polymerization, the discontinuous phase (the monomer) is also called organic phase while the continuous dispersion medium (water) is designated as aqueous phase.

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There are, however, some monomers with polar molecules that have a non-negligible water solubility. In these cases, the dispersion medium would be a non-polar organic solvent (linear or cyclic hydrocarbons); such systems are called inverse suspension systems. Except for the composition of the two phases, the polymerization occurs according to the same laws, and there is not any difference between a polymerization in aqueous suspension and a polymerization in an organic dispersion medium. Therefore, the following presents polymerization in aqueous suspension that represents the majority of the practical industrial and laboratory applications.

The initiator – soluble in the monomer – must be dissolved in the monomer phase previous to its dispersion (preparation of the suspension). The organic phase may contain also other components soluble in the monomer, such as chain transfer agents or additives (plasticizers, lubricants, stabilizers) that are inert in what concerns polymerization, do not decompose at the reaction temperature and reduce the energy used for mixing in the processing stage.

After reaching the polymerization temperature, the reaction proceeds in the monomer droplets similarly to the bulk polymerization (homogeneous or precipitant). In fact, each monomer droplet may be considered a mini-reactor for bulk polymerization, which gave to the technique the alternative name of micro-bulk polymerization. The advantage of the suspension technique is that the droplets have a high specific surface, therefore they can be efficiently cooled by contact with the aqueous phase. Due to this feature, suspension polymerization may be considered a bulk polymerization cooled with water.

The reaction is usually carried out to total conversion, and the final product consists of polymer particles (called beads) that are suspended in the continuous phase.

Since the beginning of the process (dispersion of the monomer in the aqueous phase) until the end, the reaction mass must be maintained under intense stirring, to ensure the dispersion of the organic phase in the aqueous one. In absence of mixing, the monomer droplets will form a continuous layer and the polymerization will continue in bulk, with a high risk of losing the thermal control of the reaction. This imposes the use of suitable stirrers (helical or impeller-type), to reach a turbulent regime of mixing and to prevent any accidental loss of power for the stirrer (usually the stirrers for the suspension reactors have an alternate power source – power generator – in case of accidental loss of the electrical power form the grid).

Even under intense mixing, there is a strong tendency towards coalescence of the monomer droplets (the driving force is the lower superficial tension of larger drops, that have a lower specific surface). This tendency increases when part of the monomer is transformed into polymer (if the polymerization inside the droplet is

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homogeneous) due to the corresponding raise of the viscosity. At a specific conversion the monomer-polymer particles become sticky, so to prevent agglomeration of the droplets, suspension (or protective) agents are added to the polymerization system. Suspension agents are substances soluble in the continuous phase that tend to cover the surface of the droplets forming a protective layer that reduces the tendency of the droplets towards agglomeration (prevent coalescence). Another effect of the suspension agents (especially when they have a macromolecular structure) is to increase the viscosity of the aqueous phase, thus reducing the frequency of the collisions between the droplets.

Substances used as suspension agents are:Natural macromolecular compounds: starch, gelatine, etc.Synthetic or artificial water-soluble polymers: poly(vinyl alcohol),

polyethylenoxide and its derivatives, copolymer vinyl acetate – maleic anhydride, water-soluble cellulose derivatives (such as caroxymethylcellulose), etc.

Hydrophilic inorganic powders such as: Mg(OH)2, BaSO4, CaCl2, Na3PO4,

etc.The suspension agents do not play any role in the chemistry of the process

(they are chemically inert) so they can be separated at the end of the polymerization by washing the beads with a suitable detergent (emulsifier). Some traces of the suspension agents may be found however in the final product, affecting its properties (mainly its transparency, but also – in a lower degree – the processing parameters and the mechanical properties).

The mechanism of the polymerization is a typical chain mechanism, that evolves – inside the droplets – similar with any bulk polymerization, with all the features previously described. However, the presence of the water as a dispersion medium leads to several practical consequences:

Viscosity of the reaction massThe reaction mass is heterogeneous and its overall viscosity (hs) depends on two

parameters: the viscosity of the continuous phase (water in which the suspension agent is dissolved), hl, and the volume concentration of the suspension, j:

ηs=ηl (1+2. 5ϕ ) ϕ<0 .3 (6.1)

ηs=0.59ηl

(0. 77−ϕ )2ϕ>0 .3

(6.2)Therefore, the viscosity of the system will be close to the water viscosity.

Even if the dissolved suspension agent is a polymer, its concentration (of the order of magnitude of 0.5-1.5%) is low enough not to register a significant increase of the viscosity, that remains close to the values typical for low molecular

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compounds, orders of magnitude lower than the one registered in homogeneous bulk or solution polymerization. Moreover, the viscosity remains constant during all the reaction (opposite to the aforementioned techniques, in which it increases with the concentration of the polymer, hence with the conversion).

A first consequence of this feature is that the polymerization can be carried out from 0 to 100% conversion within the same reactor, without the need to adapt the stirrer to the raise of the viscosity.

Another consequence is the intensive mixing is possible to be achieved, with turbulent flow. This fact has direct consequences on the intensity of the heat transfer phenomena. Typical rotational speeds for the helical or impeller type stirrers used for suspension polymerization are between 3 and 5 rpm.

Specific heat and thermal conductivityWater has high specific heat (double than the one of the organic phase) and

thermal conductivity (around 3 times higher than the polymer); the values for the overall reaction mass will be computed as averages, depending also on the concentration of the suspension (that usually ranges between 30 and 50%). As a result, heat transfer will be facilitated, allowing the removal of the reaction heat by circulating a cooling agent in the shell or coil of the reactor.

Particle size and specific surfaceThe average size of the droplets in the suspension polymerization ranges

usually between 0.1 and 2 mm; the droplets are not equal in size, there is a distribution of the diameters that follows a Gaussian curve, with values between 100 nm and 5 mm. The polymer beads have a lower volume due to the 15-20% shrinkage typical for any polymerization (polymer densities are lower than the one for the corresponding monomers). This leads to a very high specific surface. Suppose 1 m3 of monomer, as a single sphere; this means that the radius will be 0.43 m (according to the relation V=4R3) and the corresponding surface (according to the relation S=4R2) of 2.32 m2. If this volume of monomer is divided in droplets with the diameter of 1 mm, then the volume of a droplet will be 1.5710-9 m3 and there will be 6.4108 droplets. Since the surface of a droplet is 3.1410-6 m2 the total surface will be of about 2000 m2, an increase of 3 orders of magnitude (860 times). This means that the reaction heat, generated inside the droplets, will be easily transferred to the aqueous phase even if the process is always autoaccelerated. From the water, due to the high thermal conductivity and specific heat and to the turbulent regime of mixing the heat can be transferred to the cooling agent in the reactor’s jacket.

The above features make the suspension polymerization very efficient in what concerns the thermal control; this allows the reaction to be carried out in relatively large reactors, even giant ones for some monomers, like styrene, where the volume of a batch can reach 100 m3 of suspension.

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Note: usually the volume of the chemical reactors is limited by the ratio surface/volume, since the reaction mass must be heated/cooled through the shell. An increase in the specific surface of heat transfer can be achieved by replacing the shell with an internal coil, but always there is an upper limit of the volume, determined by the need of an efficient heat transfer.

Densities of the two phasesUsually the monomers are lighter than water while polymers are heavier (the

exception is polyethylene, that has a density slightly lower than the water). Therefore, with the advance of the reaction and the raise of the conversion, the density of the droplets (computed as average value between the polymer and the monomer) increases. For each monomer there is a given critical concentration at which the density of the monomer-polymer particles equals the one of the dispersion medium. At this point (immersion point) the tendency of the droplets to migrate (in absence of stirring) towards the surface of the reaction mass will be reversed (tendency towards sedimentation). This is the critical point in what concerns mixing, since the risk of coalescence is maximum. Excessive agglomeration of the particle will result in a lower specific surface. The consequence would be that the reaction heat will not be completely eliminated so temperature will increase, leading to the raise of the reaction rate and correspondingly of the generated heat flow. If the phenomenon continues, it may lead to overheating of the reactor, with the accompanying risks (degradation of the polymer).

Another consequence of the presence of agglomerated particles is a higher erosion of the installation (especially the pallets of the centrifugal pumps) and the risk of sedimentation inside the pipes and fittings.

Polymer separationPolymer separation in suspension polymerization is easy to be carried out,

since the beads are not soluble in the continuous phase. A first operation is the removal (by washing) of the suspension agent. The beads tend to sediment, so they can be easy separated from the aqueous phase by filtration or centrifugation, followed by drying in a current of heated gas (air or nitrogen). Note that the drying agent temperature must not exceed the glass transition temperature (for the amorphous polymer) or the melting point of the crystalline phase (for biphasic polymers) because if the beads loose rigidity there is the risk of agglomeration inside the dryer. The moisture must be completely eliminated (except for polymers that absorb a certain amount of water vapour) since the presence of volatile compounds may lead to problems in processing (bubbles or other superficial defaults).

Polymer quality

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The polymer synthesised by the suspension process has properties similar to the ones of the product obtained in bulk: high molecular mass (limited only by chain transfer reactions towards the monomer and the polymer) and purity. However, due to the traces of suspension agent that may be still present on the surface of the beads, the product has a lower transparency (it is translucent).

Conversion of the polymerizationDue to the biphasic nature of the reaction medium, the unreacted monomer is

difficult to be removed from the beads. Therefore, suspension polymerization is a technique used for monomers that can be polymerized at total conversion (without the risk of cross-linking). To ensure the complete consumption of the monomer (that cannot be found in the final products in amounts higher than the order of ppm) the usual procedure consists in raising the temperature, at the end of the process, above the glass transition temperature of the polymer, and maintaining the heating for 15-30 min. This increases the mobility of both macroradicals and monomer molecules and favours the complete consumption of the monomer.

Suspension polymerization in aqueous media has many advantages, such as high productivity, facility of thermal control, purity of the product, low cost of polymer separation.

Installations are always discontinuous; a continuous operation would suppose the uninterrupted flow of the suspension through the pipes and fittings, that can’t be accepted due to the risk of blocking the transport lines with sediment particles. Usually a discontinuous plant has a lower productivity than a continuous one, but for the suspension polymerization this limitation is balanced by the possibility of performing the reaction in high-volume reactors. Moreover, batch polymerization adds the advantage of flexibility: the same installation can be used for polymerizing various monomers or using different synthesis formulations (such as adding a volatile inert liquid to the organic phase to obtain particles that can be expanded in a further stage, manufacturing pre-plasticized products by adding the plasticizers in the organic phase previous to the synthesis, adding pigments for coloured products, etc.). Note that mixing the additives with the monomer before obtaining the macromolecular compound ensures a better mixing and reduces the energy consumption in the processing stage.

The drawbacks of the suspension polymerization are relatively few: a higher investment cost (volumes of the equipment have to consider a suspension, hence the water also) and the fact that not all monomers can be polymerized in aqueous solution: either due to their water solubility of because the reaction must be stopped before reaching 100%, to prevent excessive branching or cross-linking due to the chain transfer.

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Polymerization in inverse suspension is less cost-effective, since it replaces the water with organic solvents. These have lower specific heat, thermal conductivity and boiling points, so the reaction has to be carried out at lower temperature both to ensure the removal of the reaction heat and to prevent evaporation of the solvent. This leads to a lower polymerization rate, larger batch times and lower productivity. Moreover, using an organic solvent as dispersion medium supposes supplementary precautions in what concerns manipulation, ventilation, sealing of the equipment and higher costs for polymer separation (purification and recycling of the solvent separated in the filtration stage, condensation and recycling of the solvent evaporated during drying. Drying has to be performed using exclusively inert gases, to prevent the risk of explosion of mixtures air-vapour of organic solvents. Therefore, while suspension polymerization in water is a method widely used in industrial plants, polymerization in inverse suspension remains mostly a laboratory method.

VI.5. Emulsion Polymerization

VI.5.1. Main Features of Emulsion Polymerization

The name of the technique is given by the initial state of the reaction system, an emulsion of the monomer in water.

The size of the initial droplets of monomer is between 1 and 10 μ, while the final polymer particles, at the end of the process, are much finer (0.05-1.5 μ). The stability of the final emulsion (polymer latex) is also better than the one of the initial reaction mixture.

The main components of an emulsion polymerization system are:The monomer should exhibit a very low water solubility; it represents the

initial dispersed phase.The water is the reaction medium, the continuous phase. Usually, the

monomer/water ratio is between 30/70 and 60/40 (weight).The initiator that must be, in the emulsion technique, a water soluble one

(opposite to the suspension polymerization, where the initiator was soluble in the organic phase). The most used initiators are hydrosoluble peroxide compounds or redox systems.

The emulsifier. Emulsifiers (surfactants) are substances with molecules containing both a hydrophilic and a lyophilic (hydrophobic) group.

Due to the hydrophilic/lyophilic character, the water solubility of emulsifiers is very low (less than 0.25% ). The solubility limit is also called CMC – critical

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micelle concentration, since if this concentration is exceeded, the emulsifier molecules are not dissolved anymore but form aggregates (of 50-100 molecules) called micelles. In the micelles, molecules are oriented with their lyophilic groups toward each other and with the hydrophilic ones towards the water, forming either parallel layers or even spherical structures.

According to their chemical compositions, emulsifiers can be:Anionic emulsifiers: metallic salts of fatty acids (soaps), salts of alkyl-aryl-

sulphonic acids, alkyl sulphates, etc.Cationic emulsifiers: ammonium quaternary salts.Non-ionic emulsifiers: ethhylenoxide oligomers esterified with hydrophobic

substances (fatty acids), polyvinyl alcohol, ethoxylated alkyl-phenols, etc.

Mechanism of the emulsion polymerizationThe initial reaction system (see figure VI.8) consists of monomer droplets

with a diameter between 1 and 10 μ. A certain (small) amount of monomer (depending on its water solubility) is also physically dissolved in water.

If the emulsifier concentration in water is superior to the CMC, then the emulsifier molecules will be partially soluble and partially dispersed, as follows:

An amount equal to the CMC will be physically dissolved in water; since CMC is very low, the solubilised emulsifier molecules will be surrounded only by water (not forming aggregates in solution).

Part of the emulsifier molecules will be disposed at the interface between the monomer droplets and the water, with their hydrophobic parts oriented towards the organic phase, thus forming a protective layer that prevents the coalescence of the monomer droplets.

The rest of the emulsifier will form micelles; these micelles may contain monomer molecules, that is thus colloidally dispersed in water. Due to the much smaller size of the micelles as compared to the monomer droplets, the number of micelles will be much higher than the number of monomer droplets.

The initiator is soluble in water. When the reaction temperature is reached, radicals are obtained that will migrate towards the organic phase. Since there are many more micelles than droplets, and they have also a much higher specific surface, there is a high probability for the radicals to penetrate the micelles (and not the monomer droplets). The addition of the primary radicals to the monomer molecules and the propagation will thus occur inside the micelles (with the monomer that was colloidally dispersed). As a result, the number of monomer molecules in the micelles will diminish, until it will be below the limit of colloidal solubility. At this point, the monomer molecules that are solubilized in water will

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migrate towards the micelles and, by the mechanism of solubility, part of the monomer in the droplets will pass in the aqueous phase.

Since the monomer is continuously consumed in propagation, this phenomenon will continue, with an increase of the size of the micelles at the same time with a consumption of the monomer in the droplets.

A similar equilibrium is established in what concerns the emulsifier. Since the size of the micelles increases, more emulsifier molecules that are dissolved in water will migrate towards the micelles, disposing themselves at the interface between the organic phase (monomer-polymer) and the water, while the emulsifier molecules on the surface of the monomer droplets (that gradually shrink) will be dissolved in water and thus transported towards the surface of the micelles. Another source for emulsifier molecules (to cover the surface of the polymer particle) is the dismantling of the micelles that have not been penetrated by any radicals.

Figure VI.8.Topochemistry of the emulsion polymerization.

The mechanism described above therefore supposes the existence of three different stages (see figure VI.8.):

I - Formation of the monomer-polymer particles by penetration of the radicals in the micelles, followed by growth of particles and absorption of both monomer and emulsifier molecules from the “empty” micelles that have not been penetrated by radicals. The first stage ends when all the “empty” (not penetrated by radicals) micelles have disappeared.

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II - Stationary stage when the increase of the monomer-polymer particles occurs by absorption of monomer from the droplets, intermediated by the solubility equilibrium in water. During this stage the number of monomer-polymer particles remains constant; this stage corresponds to conversions between 10 and 80% and its end is marked by the complete consumption of the monomer droplets. The constant polymerization rate is due that the process is diffusion cntrolled during this stage.

III – Final stage, where polymerization continues inside the monomer-polymer particles until all the monomer is consumed.

Figure VI.8.Conversion-time curve typical for an emulsion polymerization.

Polymer separationWhen the reaction is completes, a disperse system (called latex) is obtained. It

is composed of very fine polymer particles that are covered by an emulsifier layer. The emulsifier does not only physically cover the particle, but may be also chemically bonded (grafting reactions that occurs as a result of the chain transfer). Therefore, the removal of the emulsifier by washing is not possible, it will remain mixed with the polymer (and it will play a secondary role, as a plasticizer; however, the emulsifier concentration is low enough not to affect significantly the properties of the final product).

time

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Due to the dual (hydrophile - hydrophobe) nature of the emulsifier, all the polymer particles will have the same superficial charge (or polarity, for non-ionic emulsifiers). If ionic emulsifiers have been used, the latex particles will be surrounded by a double electrically charged layers: the ionic groups of the emulsifier and the corresponding counter-ions.

The fact that particles have the same electrical charge (or polarity) makes the emulsion very stable; even in absence of stirring the latex will not separate immediately (particle will not sediment, as it happens for suspension polymerization). The system is in fact a colloidal solution, it will separate spontaneously but in a long time interval.

Therefore, for separating the system, at the laboratory or industrial scale, two methods are used:

Latex coagulation consisting of neutralising the double electrical layer by adding a strong electrolyte to the polymer emulsion, thus compensating the electrical charges and eliminating the repulsion forces that keep the particles from sedimenting. The coagulated latex is then filtrated or separated by centrifugation. Note that the separation process is more difficult to be achieved as compared to the suspension polymerization due to the much smaller size of the particles, that may clog the filters.

Water removal (drying of the latex); this method supposes heating the latex at a temperature high enough to allow rapid evaporation of the aqueous phase without any thermal destruction of the polymer. The rate of evaporation may be raised by increasing the specific surface; the process is called atomization and consists in dispersing the latex in very fine drops (of both water and polymer particles), by contact with a mechanical system (method used in the emulsion polymerization of vinyl chloride).

Emulsion polymerization is one of the most used polymerization techniques, due to its advantages:

Using water as a reaction medium leads to the same advantages as outlined for suspension polymerization: low viscosity (as compared with the bulk and solution techniques), higher specific heat and thermal conductivity as compared to organic substances (approximately double values), easy removal of reaction heat (due to the possibility to achieve turbulent stirring regimes and to transfer the heat from the polymer particles to the water and from here to the cooling agent in the shell or coil of the reactor). Moreover, the thermal control of the reactor is easier since the reaction rate is constant for a large interval of conversion and there is no autoacceleration peak (as for suspension or bulk polymerization).

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The polymerization advances at high reaction rate even at low temperatures (using water soluble redox initiator systems that have a low activation energy for the radical generation reaction); this is especially convenient when monomers are gases under normal conditions and are liquefied previously to polymerization to increase concentration and hence the reaction rate.

The molecular mass of the polymer is high and its characteristics can be controlled through the nature and concentration of the components of the reaction system.

If – for various reasons – the final conversion does not reach 100%, the unreacted monomer can be easy removed, due to the high specific surface of the latex particles.

There are a number of applications (paints, adhesives, impregnation of tissues, etc.) where the latex is used as such, with no need for separation of the polymer.

The main drawback of the process is the impurification of the polymer with the emulsifier, that is grafted onto the surface of the latex particles. For usual processing applications this does not influence significantly the polymer properties (due to the low concentration of the emulsifier) and may also act as an advantage since the emulsifier acts like a plasticizer for the final polymer. However, the presence of other substances affects transparency, polymers obtained in emulsion are either translucent or white and cannot be used for optical applications.

Another limitation of emulsion polymerization is the fact that water can’t always be used as a continuous reaction medium. Some monomers, with water solubility, can be polymerized using the emulsion technique only if the dispersion medium is an organic solvent (inverted emulsion); in these cases, some of the advantages of the water are lost (organic solvents have lower heat transfer properties) and there are supplementary costs linked to the manipulation of organic substances (toxicity and flammability risks, need for recycling and purification).