MXPA00006630A - Method for the continuous production of thermoplastic moulding materials - Google Patents

Abstract

The present invention relates to a method for producing impact-resistant modified styrene polymers, wherein said method comprises the anionic polymerisation of styrene in the presence of a rubber in one or more serially-mounted polymerisation reactors. The rubber is produced in the shape of a solution during a process carried out immediately upstream. The rubber solvent, which has a boiling point lower than that of the styrene or ethyl benzene, is removed by distillation with vaporisation cooling from at least one of said polymerisation reactors using the heat generated during the polymerisation process, and is further fed back to the rubber production step.

Description

CONTINUOUS PREPARATION OF THERMOPLASTIC COMPOSITIONS FOR MOLDING The invention relates to a process that can be carried out continuously for the preparation of modified styrene polymers for impact (HIPS) which contains, as a dispersed phase, a polystyrene matrix, a styrene-butadiene block copolymer or a rubber particulate polybutadiene, compatibilized by the introduction of the graft with styrene, by anionic styrene polymerization in a polymerization reactor or in more than one polymerization reactor arranged in sequence, in the presence of a rubber prepared in an immediately preceding process, and preferably in the presence of a subordinate amount of an alkyl aromatic solvent other than styrene, in particular toluene, ethylbenzene or mixtures of toluene and ethylbenzene. Polystyrene modified for impact is usually prepared by mass free radical polymerization, where rubber, obtained in a separate process, is dissolved in styrene. In addition, usually a small amount of a solvent is used, such as ethylbenzene. Polymerization is carried out on an industrial scale in more than one reactor arranged in sequence, known as a cascade. Processes of this type have been reviewed, for example, by Echte in Handbuch der Technischen Polymerchemie, Weinheim, 1993; the specific arrangements are described, for example, in Patents US 2 727 884 and 3 903 202. It has also been proposed that modified polystyrene for shock can be prepared by anionic polymerization.

This can be done, for example, in tubular reactors, the heat of the polymerization being dissipated through the wall. For this purpose, static mixers are incorporated in the cross section of the tube, and are used for radial mixing of the polymer (Nguyen Khac Tien et al., Chem.

Eng. Technol. 13, 214-220 1990). However, no process of this kind has been well established. Rubbers, such as polybutadiene, are usually prepared in solution. Reference is made in this regard to the review in the Encyclopedia of Polymer Science and Technology Vol. 2, NY, 1985. Anionic polymerization, as used to prepare block rubbers (styrene-butadiene block copolymers), for example, always requires a solvent (EP-A-510 410; EP -A-554 142; US 5 286 457). The disadvantage of the known processes for the preparation of impact-modified or impact-resistant polystyrene is that if a rubber is to be used for impact modification this first has to be isolated from the reaction mixture (ie from a solution whose resistance is, for technical reasons, no greater than 15 to 20%). Attempts have already been made to find processes useful for the industry that avoid costly rubber insulation. For example, the application of European Patent 334 715 proposes a polymerization process of styrene initiated by free radicals in which the polybutadiene is first prepared by anionic polymerization in ethylbenzene in a stirred reactor. After the completion of the reaction, the preheated styrene will be added as a diluent, followed by the polymerization. However, even this process would not be an effective cost operation due to the amount of the solvent (high boiling point) which is necessary to prepare the rubber and which has to represent from about 40 to 100% by weight of the amount of styrene subsequently added. British Patent 1 013 205 therefore proposes that the butadiene be initially polymerized in a relatively low boiling solvent, specifically cyclohexane, and that the solution be mixed with styrene and fractionated by cyclohexane distillation together with any remaining butadiene monomer. The solution of the styrene rubber thus obtained is then used in the normal manner in the polymerization of styrene. This elegant process per se has not been introduced on an industrial scale, probably because, as indicated in the British Patent, the cyclohexane would have to be removed in an additional step to allow the polymerization to be carried out with a suitably high monomer concentration. These considerations also apply, of course, to anionic polymerization processes for preparing impact resistant polystyrenes. On the other hand, in continuous anionic polymerizations it would be very unlikely to use a rubber that has been isolated from its solution, due to the auxiliaries added during the isolation. This is probably one of the reasons why the modified polystyrene preparation for impact by anionic polymerization has not been introduced on an industrial scale, although it has the per se advantage of producing polystyrene almost free of monomers. European Patent 059 231 clearly describes the disadvantages of the process of British Patent 1 013 205. A process is then proposed in which styrene and butadiene are initially processed in anionic form in a stirred reactor, i.e. with backward mixing, to obtain styrene-butadiene block copolymer. The reaction is carried out in such a way that the finally obtained reaction mixture contains butadiene. The latent chains are treated with a terminating agent (for quenching), and the excess monomeric butadiene is then removed. Then styrene is added, followed by polymerization by free radicals. Even this process is not cost effective, since it requires the removal of the excess butadiene in a separate step, so as not to deteriorate the styrene polymerization.

EP-A 0 595 121 describes a continuous process for the anionic polymetion of styrene in the presence of a solvent, for example, toluene, and a block rubber, in a reaction cascade with division of the monomer stream. US 4,153,647 in the same manner describes the anionic polymetion of styrene in the presence of a rubber in an aliphatic, cycloaliphatic or aromatic solvent. The solvent is distilled at the end of the polymetion. US 3,957,914 describes a cyclic process for the polymetion in anionic solution of, for example, styrene-butadiene block copolymers, and the hydrogenation thereof. After the hydrogenated polymer has been precipitated, using steam, the phase containing the solvent is fractionated by distillation and the purified solvent is passed again for use in the anionic polymetion. The proton donor used to terminate the block copolymer has a boiling point of at least 40 ° C above the boiling point of the solvent. In summary, it can be said that all the processes described so far for the preparation of modified styrene for impact have additional steps to remove residual solvents or monomers from rubber synthesis and depend on the previous isolation of the rubber. Above all, the purely anionic preparation of modified styrene for impact on an industrial scale is a problem that remains unresolved. An object of the present invention is to provide an efficient cost process for the purely anionic preparation of modified styrene for impact (HIPS) which avoids the disadvantages of known processes and does not require immediate isolation of the rubber from its solution. Another objective is to provide an anionic polymetion process which to a substantial degree can be used in plants that already exist for the preparation by free radicals of modified polystyrene for impact. We have found that this objective is achieved by means of a process that has at least one circulating solvent system and, expressed in a simple manner, the significant feature is that the required rubber is introduced to the styrene polymetion plant in the form of a solution in a third solvent that is neither styrene nor ethylbenzene and whose boiling point is lower than that of styrene, and that of ethylbenzene if present, and that at least some of this third solvent is reintroduced after being eliminated from when less one of the reactors of the polymetion by means of distillation. In this regard, it is advantageous that at least some of the heat of vapotion required as a result of the evaporative cooling is fed through the heat of the polymetion. A second circulation system of the solvent results from the devolatilization, in a manner known per se, of the polymer formed, that is to say, the polystyrene modified for impact, that is, the elimination of any residual monomeric residue and the associated solvent (so common ethylbenzene and toluene entrained, and also residues of the third solvent according to the invention) and the condensation of the vapor stream produced and, if it is worth its reintroduction into the plant, directly or after the appropriate recovery procedures. The invention mainly provides a process of the type mentioned at the beginning, in which, according to the invention, at least one solvent circulation system is provided where the rubber is used in the form of a solution in a third solvent, which it is neither styrene nor any of the alkyl aromatic solvents used and whose boiling point is lower than that of styrene and of any alkyl aromatic solvent used and, where in a (first) circulating solvent system, the third solvent is removed from when less one of the reactors of polymetion by distillation with evaporative cooling and using the heat of the polymetion and, if desired, after suitable recovery procedures, i.e. directly or indirectly, it is introduced to the rubber preparation process which is immediately upstream, and where in a manner known per se, the impact modified polystyrene formed is released from the residual styrene and, where appropriate, from minor components, and the vapor stream formed is condensed and, if desired, into a second circulating solvent system, again after appropriate recovery procedures if required, from the same way it is reintroduced. The alkyl aromatic solvent used is ethylbenzene. As is known, the rubber that can be used according to the invention must be that which is intrinsically compatible with polystyrene in the manner that is common to particulate grafted rubbers, since grafting does not occur in the anionic polymerization. This may be a rubber SB (styrene-butadiene rubber), a block rubber of styrene-butadiene or a mixture of butadiene without styrene or with a low content of styrene and a block rubber of styrene-butadiene prepared in batches or preferably in the same way prepared in a continuous process. The third solvent according to the invention can be an aliphatic, cycloaliphatic or aromatic hydrocarbon having from 4 to about 8 carbon atoms, or mixtures of these with a boiling point (p.b.?oi3hp) below 130 ° C. Examples of suitable solvents are pentane, hexane, heptane, cyclohexane, methylcyclohexane, benzene and toluene, it is important that this third solvent meets the normal conditions for use in anionic polymerization, in particular the freedom of protic substances and oxygen. It is convenient that this be distilled before use and drying on alumina or molecular sieve. The plant for preparing the rubber is also mentioned below as reaction zone 1, while the first reactor, generally a reactor with agitation, and if another reactor is desired, in the same way a reactor with agitation, are called reaction zone 2. Other reactor (s) known for free radical polymerization, for example, tower reactors with piston flow to complete the conversion, would represent a reaction zone 3. Reaction zone 4 is the part of the reaction zone. plant commonly present in styrene polymerization plants to devolatilize and obtain the polymer. The styrene was introduced into the first container of the reaction zone 2 and at least the amount necessary to achieve phase inversion when the polymerization is complete. The evaporation of the monomer / solvent mixture as a result of the heat of the polymerization released is introduced to a rectification column assigned to the vessel and having a reflux condenser. If, exceptionally in an individual case, the heat of the polymerization is sufficient to evaporate the desired amount of the solvent, it is possible to provide additional heat for the reactor and / or the feed streams. The solvent is extracted as a product at the top of the rectification column and is introduced into the rubber preparation, while the residual product is reintroduced into the reactor. It is possible, in cases where a second reactor is provided and it is in the same way a reactor with stirring, to separate a solvent at this point also and, if it is desired to introduce the resulting vapors to an individual column, and reintroduce the residual product into the first reactor or directly in the rubber preparation. The above references for styrene and butadiene are proposed to include the respective customary technical equivalents. Styrene can, for example, be represented by 2-, 3- or 4-methylstyrene, tert-butylstyrene, ethylstyrene or α-methylstyrene or other styrene substitution products with similar behavior in polymerization (vinylaromatic compounds). Suitable monomers for preparing the rubber (in addition to styrene and butadiene) are other dienes or combinations of dienes and vinylaromatic compounds. Examples of other dienes are isoprene, dimethylbutadiene, 1,3-pentadiene and 1,3-hexadiene. The novel process is conveniently carried out in such a way that, to prepare the rubber, use is made of at least one solvent having a boiling point of, in each case at atmospheric pressure, less than 130 ° C, preferably less than 120 ° C, and for the preparation of the HIPS, if desired, use is made of a solvent having a boiling point above, respectively, 120 or 130 ° C. The use of toluene to synthesize the rubber is particularly advantageous. In this case, it is convenient to remove only some of the toluene during the polymerization of styrene. The remaining residue is removed during devolatilization and, depending on the composition of the vapors, it can be reintroduced for the preparation of the rubber or even for the polymerization of styrene.

When using evaporative cooling according to the invention, the temperature that develops in the reaction zone 2 corresponds to the boiling point of the monomer mixture and the low boiling solvent third. Of course, it is also possible to carry out the process at a pressure different from atmospheric pressure, for example, at reduced or superatmospheric pressure. The pressure range from 0.1 to 5 bar is, for example, suitable. The behavior of the relative boiling of the solvents, which is important in this case, is generally not affected by this to any degree. In the preparation of the HIPS, a small amount of an aromatic solvent is commonly present which is conveniently used for the polymerization of styrene and which is not completely removed during the preparation of the styrene. Therefore, it usually accumulates until it constitutes a certain proportion of the reaction mixture. These solvents are suitable boiling point alkylbenzenes, such as xylenes (p.b.?oi3hP «* 140 ° C), and in particular ethylbenzene (p.i.i.sup.33p: 136 ° C). To prevent these from reaching an undesired high concentration, it may be useful to extract a portion of these alkylbenzenes by continuously removing an aliquot of the solvent from the circulation system and introducing it into a recovery process. Small amounts of solvent waste of this type can also be introduced into a heating system or the like, if this proves to be cost effective. If the available styrene does not contain ethylbenzene, it is not necessary to add ethylbenzene, since it can be replaced by the third solvent according to the invention. Depending on the proposed conversion and the viscosity of the solution that can be handled, the unconverted styrene can form a part in the evaporative cooling according to the invention. The second reaction zone, in which at least partial conversion is carried out according to the invention, can be a back-mixing unit (a unit that behaves like a reaction vessel), and according to the invention , has a stirred reactor to which is assigned a column with a heat exchanger, in which the constituents evaporated in the reactor are fractionated, the upper boiling fraction being reintroduced into the second reaction zone and the fraction with a point of lower boiling introduced into the rubber preparation, that is, in the first reaction zone. According to the invention, the second reaction zone may also consist of more than one stirred vessel and columns assigned thereto. The product from the upper part can be introduced into the next upstream reactor in each case. According to the invention, therefore, the solvent (s) and also some of the non-converted styrene which is still present in each case, are distilled together in the second reaction zone, but are fractionated and reused, in each case at the relevant point in the course of the process. The weight of styrene added is generally at least three times the weight of the rubber, that is, of the block copolymer. The mixture is subjected to anionic polymerization in a manner known per se, if desired in the presence of additives that reduce the reaction rate, for example organo magnesium or organoaluminum compounds. According to the invention, the polymerization heat is substantially dissipated by evaporative cooling, in such a way that the polymerization temperature that develops corresponds to the composition of the volatile constituents of the reaction mixture and is, for example, from 70 to 180 ° C, depending on the prevailing pressure, which can, for example, be from 0.1 to 10 bar. A desirable conversion, depending on the proportion of the solvent and polymerization process, is generally from 70 to 100%. Preferably, the polymerization is carried out at a conversion of 95% or greater, and until almost complete conversion if this allows the device used. For this purpose, the reaction mixture obtained from the last stirred reactor is polymerized in at least one other reactor for the desired conversion. These reactors can be known tower reactors for the polymerization of styrene, which can be considered as reactors with some degree of piston flow. Other reactors which in the same way are suitable are simple tubular reactors or circulation reactors which are completely filled (ie, operate without a gas space). The polymerization temperature in this case is, for example, from 80 to 200 °. The monomers that are not yet converted at the end of the second reaction zone and the solvent that is still present can be separated in a circulating solvent system corresponding to the first circulation system. However, these usually accompany the product to the third reaction zone, where more styrene is consumed by the polymerization and only a small amount of solvent and / or the third solvent, and possibly high styrene, is finally removed from the polymer by devolatilization in the customary manner, and advantageously reintroduced to the second reaction zone. It may be useful to divide the vapors and introduce the constituents to different parts upstream of the plant, for example to introduce the solvent into the rubber preparation. The average residence time in the individual reactors is, for example, from 0.1 to three hours, conveniently from Q.2 to two hours, if an initiating system is used that operates sufficiently slowly. The residence time may also be significantly shorter for polymerization exclusively with lithium alkylene. The process is described in detail in the attached diagram. The first reaction zone is indicated only as a scheme, since according to the invention it is constructed practically as in the prior art. The second reaction zone consists mainly of a stirred reactor Rl and a rectifying column Kll with a downstream condenser (heat exchanger) Wll. In the second reaction zone, at least one styrene monomer is polymerized until the phase inversion in the presence of the rubber introduced from the first reaction zone, or its solution in the third solvent and, if desired, a small amount of a high-boiling aromatic solvent (for example ethylbenzene). This reaction zone, therefore, serves to establish the morphology of the desired particle for rubber (see Echte, loe. Cit.).

The heat exchanger Wll and the column Kll mentioned serve to fractionate and condense the vapors. The Kll column is to be dimensioned and operated in such a way that it performs the condensation of most of the unreacted styrene and the high-boiling aromatic solvent that may be present, and these are reintroduced directly into this second reaction zone (specifically to the reactor Rl), while the low-boiling solvent and, if desired, possibly some of the styrene, remain in vapor form and are introduced into the heat exchanger Wll to be condensed. Column Kll is, therefore, a component that must have some fractionation action, usually achieved, for example, by means of internal structures, packaging or simply adequate length. The amount of styrene in the low-boiling solvent can be up to 70% by weight, preferably up to 50% by weight, based on the total amount of the rubber used. The low boiling point solvent is then substantially separated by Wll condensation and, as is convenient in many cases, then it is collected in a tank, where it is reintroduced to the rubber synthesis after proper retreatment (purification and drying). Small amounts of the low-boiling solvent can remain in the reaction mixture of the second reaction zone, that is, they can be tolerated in consideration of the subsequent devolatilization zone. The material discharged from the second reaction zone is generally polymerized to term in another third reaction zone, before it is finally released from the low molecular weight constituents (devolatilized) in a suitable surface evaporator (devolatilizer E). The third reaction zone consists of at least one reactor with mainly piston-type flow (TI in the diagram) in which it is possible to obtain almost complete styrene conversion. Examples of the possible reactors are stirred tower reactors or tubular reactors filled by hydraulic means. The advantage of the novel process is that it is possible to produce impact resistant molding compositions without the need for a release step separate from the required rubber from its solvent. Another advantage is the possibility, at a relatively low cost, of converting the existing plants in which the necessary rubber has hitherto had to be dissolved in styrene, that is, to introduce an upstream rubber production step and thus improve considerably the cost effectiveness of the plants. The impact resistant polystyrene preparation is described in more detail below using the diagram. The rubber is prepared continuously or in batches, using the methods of the prior art. The rubber solution (line K + LMi) from the upstream plant PB and the initiator solution (line I) are added in a stirred reactor Rl to styrene, which can, if desired, contain a certain amount of toluene and / or ethylbenzene (line S + LM2), and polymerize from 60 to 220 ° C, preferably from 70 to 180 ° C. If the temperature to be maintained is below the boiling point of the solvent (eg,? Oi3hp) it may be necessary to operate at subatmospheric pressure to obtain sufficient cooling (vacuum pump P4). Otherwise, a suitable pressure is the atmospheric or superatmospheric pressure, adjusted according to the boiling points of styrene and / or the solvent and the highest proposed polymerization temperature. The amount of styrene added must be at least three times the weight of the rubber. The conversion in the stirred reactor Rl is selected in such a way that the phase inversion takes place in this reaction zone; it is known that this gives rise to the formation of rubber particles dispersed in the polystyrene matrix. It is possible to establish the desired size and structure of the rubber particles by means of a suitable selection of the cutting conditions (see Echte, loc. Cit.).

The third solvent, which is fractionated in the superposed column (evaporative condenser) Kll and separated by condensation in the heat exchanger Wll, is reintroduced (line K) in the synthesis of the rubber. Purification and drying, for example, by passage over alumina or molecular sieve, can be interposed. The reaction mixture present in the stirred reactor Rl is extracted continuously (pump Pl) and polymerized in another reactor with stirring R2 to the desired conversion, giving the desired thermoplastic molding composition the form of a similar solution (concentrate) to a fade. If the viscosity is relatively high it is likely, and it may be useful, instead of the stirred reactor, to use a hydraulic-filled circulation reactor, a tower reactor or a tubular reactor with static internal structures: success can be obtained independently of itself the reactor R2 operates with backmixing or provides piston-type flow or has intermediate mixing characteristics between them. The heat of the reaction that arises in R2, if this is a reactor with agitation, can of course, in the same way be dissipated by evaporative cooling. This can be useful in particular if another decrease in the amount of low-boiling solvents is to be achieved. Suitable technical solutions are in general any of those that have been introduced into the industry for traditional styrene free radical polymerization (eg tower reactors with internal cooling elements protruding between the arms of the aqitator, cooling jackets). If R2 is a backmixing reactor, then, as already mentioned, if the full conversion is to be obtained there must be a subsequent reaction zone with piston-type flow to accomplish this. The ends of the latent chains of the strand-like molecules produced are finally destroyed in a known manner by adding a terminating agent in a downstream mixer. This polymer solution which is finally obtained is extracted (pump P2) and devolatilized in a conventional apparatus (extruder or reduced pressure apparatus E, as is commonly used to devolatize polymers) from 190 to 320 ° C. The extracted vapors (Br in the diagram) consist mainly of unconverted styrene and the aromatic solvent, preferably almost exclusively the solvent. It is convenient that the vapors are condensed and these can, for example, be introduced directly into the reactor Rl. For the purposes of the invention, the rubber is at least one styrene-butadiene block copolymer, having, for example, linear or star-shaped structure with random or ordered block structure and with transitions of the sharpened blocks or in the shape of a wedge. Examples of suitable polymers are block copolymers of types (AB) n, ABA, BAB or (AB) mX, (BA) mX, (ABA) mX or (BAB) mX, where X is the coupling agent radical multifunctional and A is a block of styrene and B a block made of a diene, n is an integer from 1 to 6 and m is an integer from 2 to 10. It is also possible to make concomitant use of pure polydienes, as long as these are produced in the same process together with the block copolymer. It is preferred to use blends of polydienes with low styrene content or without styrene and block copolymers. The ratio of the two rubbers has an effect on the particle size and particle structure obtained. Block A and / or block B may contain butadiene and styrene in random distribution. Preferred block copolymers are those prepared from styrene, α-methylstyrene, butadiene and isoprene. The glass transition temperatures of the segments containing dienes (blocks) of the rubbers produced should be below -20 ° C, in particular below -40 ° C. A mono or multifunctional alkali metal alkyl compound or a mono or multifunctional alkali metal aryl compound can be used to initiate the polymerization. For known reasons, use is made mainly of organo-lithium compounds. Examples which may be mentioned are ethyl lithium, propyllithium, isopropyllithium, n-butyllithium, sec-butyllithium, tert-butyllithium, phenyllithium, hexyl-diphenyllithium, hexamethylenedilithium, and also butadienyl lithium and isoprenyldilithium. The amount added depends on the desired molecular weight, but is generally in the range from 0.002 to 5 mol%, based on the monomers. Concomitant use of alkyl agnesium compounds or alkylaluminium compounds, or derivatives and mixtures thereof, may be made to reduce the reaction rate and / or to improve the thermal stability of the polymerization products. Suitable compounds for destroying the ends of the latent chains after the polymerization has been completed are protic substances or Lewis acids such as water, alcohols, aliphatic or aromatic carboxylic acids and also weak mineral acids, for example carbonic acid or boric acid. Suitable monomers other than styrene for the preparation of impact-resistant thermoplastic molding compositions are, as stated at the beginning, other vinylaromatic compounds. It is possible, for example, that a-methylstyrene, tert-butylstyrene, vinyltoluene, p-methylstyrene and also mixtures of the mentioned compounds are used in place of styrene. The auxiliaries that can be added, generally after completing the polymerization, are lubricants, stabilizers, regulators or mold releasing agents.

Examples Polymerization plant (see diagram) Reaction area Rl is a stirred reactor with a capacity of 30 liters and equipped with an anchor stirrer and a superimposed column Kll and a descending condenser Wll, as shown in the diagram. The reaction mixture can be discharged from Rl using a mechanical pump Pl and transported to reactor R2. In Rl, a rubber solution is dosed, the solvent LM2, if desired the vapors that arise during the devolatilization (vapor line Br), the required amount of the monomer (St) and the initiator. As Rl, R2 is equipped with an anchor stirrer, and has a volume of 50 liters. If desired, R2 can be operated with evaporative cooling (using a column that is not shown in the drawing) or jacket cooling. Downstream there is a mechanical pump P2, which can discharge the reaction mixture in a normal way and can be introduced into a devolatilizing apparatus E. A tubular bundle heat exchanger serves for devolatilization and is located in a vessel that can be evacuated. If full styrene conversion is to be obtained, it can be used if desired of a TI tower reactor (not included in the drawing) which is downstream of R2 and has a capacity of 40 liters. Immediately after P2 or TI as appropriate, the terminating agent is homogenously incorporated using a mixing section (not included in the drawing) of length 540 mm and provided with a DN32 SMXL mixer (manufacturer: Sulzer, Winterthur, Sweden ). Another mechanical pump P3 is provided after E to discharge the product, followed by a nozzle plate and the granulator apparatus.

Tests: Impact resistance with notch according to DIN 53453 in normal notched samples. Elastic limit according to DIN 53 455 in samples according to ISO 3167.

Example 1 The following raw materials were prepared: - Technical styrene monomer, anhydrous, freshly distilled with 800 ppm ethylbenzene content; - Sec-butyllithium, 1.6 molar in n-hexane / cyclohexane (volume ratio 1: 9) diluted in a weight ratio of 1: 4 with toluene. - Triisobutylaluminum, 6% by weight in toluene. - Mixture of 10% by weight of methanol and 90% by weight of toluene to complete the polymerization (elimination of the ends of the latent chains). The quantities per hour dosed continuously to the stirred reactor Rl were 7.4 kg of styrene, 4.8 kg of a solution preheated to 90 ° C and composed of 0.8 kg of styrene-butadiene block copolymer of molar mass 220,000 g / mol and having a proportion of 25% by weight of styrene, 4 kg of toluene and also initiator. The initiator used was 95 g / h of sec-butyllithium solution and 125 g / h of triisobutylaluminum. The pressure in the stirred reactor was established in such a way that the temperature of the mixture was 105 ° C. 2.2 kg per hour of a mixture of 91% by weight of toluene and 9% by weight of styrene were extracted by Kll. The reaction mixture in the steady state had a solids content of 30% by weight; this was transferred to the reactor with agitation R2 and polymerized at 110 ° C to a solids content of 55% by weight, and then to complete the conversion at 150 ° C in IT. Through the mixer, 200 g / h of the methanol-toluene mixture were added to the discharged product, followed by heating to 250 ° C in the heat exchanger W21 and, to remove the volatile constituents, transferred with pressure release in a vessel evacuated 5% by weight of the condensed vapors were removed from the circulating system to prevent an increase in the level of ethylbenzene carried along with these. The melt was discharged using a mechanical pump, and granulated. The notched impact strength of 6.1 kJ / m and an elastic limit of 32 MPa were measured.

Example 2 The quantities per hour dosed continuously to a stirred reactor Rl were 7.4 kg of styrene, 4.8 kg of a solution heated to 60 ° C consisting of 0.8 kg of styrene-butadiene block copolymer having a molar mass of 228,000 g / mol and a proportion of 25% by weight of styrene, 4 kg of cyclohexane, all recirculated vapors (2.2 kg / h of ethylbenzene, in addition to small amounts of other hydrocarbons) and also initiator. The initiator was 95 g / h of sec-butyllithium solution and 125 g / h of triisobutylaluminum solution of Example 1. The reactor size and the resulting residence time provided a steady state solids content of 31% by weight . The pressure in the stirred reactor was adjusted in such a way that the set temperature was 105 ° C, with boiling. 4.1 kg / h of the solvent vapors were extracted from the column as a product in the upper part and introduced for the preparation of the rubber. In addition to cyclohexane, small amounts of toluene were also present in the vapor. The reaction mixture was discharged to the reactor with stirring R2, where the polymerization took place at 110 ° C to a solids content of 55% by weight. The polymerization was carried out until complete conversion at 150 ° C in the tower reactor. 200 g / h of the methanol-ethylbenzene mixture were added to the discharged product, followed by heating to 250 ° C and pressure release. 5% by weight of the condensed vapors were removed from the system. The melt was discharged by a mechanical pump and granulated. The impact resistance with notch was 6.8 kJ / m and the elastic limit was 31 MPa.

Claims (1)

1. CLAIMS A continuous process for preparing impact-resistant styrene polymers (HIPS) consisting of, as a dispersed phase in a polystyrene matrix, a styrene-butadiene block copolymer or a particulate polybutadiene rubber compatibilized by grafting with styrene by anionic polymerization of styrene in a polymerization reactor, or in more than one polymerization reactor arranged in sequence, in the presence of a rubber prepared in an immediately preceding process, and preferably in the presence of a subordinate amount of ethylbenzene, which consists of: a) introducing the rubber in the form of a solution in a third solvent that is neither styrene nor, if present, ethylbenzene and having a boiling point lower than that of styrene and, if present, ethylbenzene, and providing at least one solvent circulation system using the heat of polymerization to remove the third solvent from at least one of the reactants of polymerization by distillation with evaporative cooling and, directly or indirectly, reintroducing it in the process of preparing the rubber, and b) in a manner known per se, releasing the impact-resistant polystyrene resulting from any remaining styrene residues, and of the associated ethylbenzene and other minor components, by thermal treatment under reduced pressure, and condensing the resulting vapor stream and, if desired, reintroducing it into another solvent circulation system. The process as claimed in claim 2, wherein the third solvent used to prepare the rubber is an aliphatic, cycloaliphatic or aromatic hydrocarbon having from 4 to 8 carbon atoms or a mixture of these hydrocarbons having a boiling point (bp? oi3h) below 130 ° C. The process as claimed in claim 2, wherein the solvent used for the rubber preparation has a boiling point (p.b.? Oi3hp) below 120 ° C. The process as claimed in claim 1, which is carried out at a pressure different from atmospheric pressure. The process as claimed in claim 1, wherein no other solvent other than styrene is used in the polymerization of styrene and has a boiling point above 130 ° C at atmospheric pressure. The process as claimed in claim 2, wherein the second circulating stream (the vapors) is also, at least to some degree, reintroduced into the upstream process to prepare the rubber. The process as claimed in claim 1, wherein the rubber used is a block rubber SB or a styrene-butadiene rubber prepared in continuous or batch form. The process as claimed in claim 1, wherein the first reactor is a stirred reactor and the monomer / solvent mixture that evaporates as a result of the heat of the polymerization is introduced into a rectification column assigned to the reactor and having a reflux condenser. The process as claimed in claim 8, wherein the rectification column is placed under the reactor or placed downstream. The process as claimed in claim 8, wherein the solvent is extracted from the rectification column as a product in the upper part and is introduced into the rubber preparation, and the residual or lower product is reintroduced to the reactor. The process as claimed in claim 8, wherein a second reactor is disposed which in the same manner is a stirred reactor and to which a distillation equipment is assigned. The process as claimed in claim 11, wherein in the second non-converted styrene reactor together with, if desired, the third solvent, it is distilled, fractionated and, if desired, used again at the respective relevant point in the course of the process. The process as claimed in claim 8, wherein the reactor and / or the feed streams are further heated. The process as claimed in claim 1, wherein, in place of styrene and / or butadiene, use is made in each case of the normal technical equivalents, such as 2-, 3- or 4-methylstyrene, tert-butylstyrene. , ethylstyrene or α-methylstyrene, and isoprene, dimethylbutadiene, 1,3-pentadiene or 1,3-hexadiene.