Process for preparing uniform size spheroidal polymer beads

Abstract: Spheroidal polymer beads having a uniform size are prepared by polymerizing uniformly sized monomer droplets formed by the vibratory excitation of a laminar jet of monomeric material flowing in a continuous liquid medium containing a suitable suspending agent. For example, a laminar jet of a monomer mixture comprising a monovinyl aromatic such as styrene, a polyvinyl aromatic such as divinylbenzene and a polymerization initiator are subjected to vibratory excitation and the resulting monomer droplets polymerized to yield copolymer beads having a narrow particle size distribution. These copolymer beads can be employed to prepare ion-exchange resins having superior properties, particularly in continuous ion-exchange opera­ tions.

Description: The present invention relates to the preparation of spheroidal polymer beads, particularly to a method for preparing spheroidal polymer beads having a relatively uniform particle size. A wide variety of polymers, including polymers of a monovinyl aromatic such as styrene, or esters of an unsaturated carboxylic acid such as acrylic or methacrylic acid esters, are commonly prepared in spheroidal bead form using suspension polymerization techniques. Normally, this consists of dispersing a monomer as droplets in a vessel equipped with an agitator and containing a continuous phase in which the monomers and resulting polymer are essentially insoluble. The monomer droplets are subsequently polymerized while maintaining continuous agitation. Unfortunately, the polymer beads prepared by these techniques have a relatively broad particle size distribution. This is generally undesirable because the spheroidal polymer beads employed in most applications require a generally uniform particle size. For example, the ion-exchange resins prepared from copolymer beads of a monovinyl aromatic and a polyvinyl aromatic should have a narrow particle size distribution to provide the desired flow characteristics to an ion-exchange resin bed containing these resins. Previously, various suspension stabilizers (suspending agents) have been employed to help improve the uniformity of the polymer beads prepared using conventional suspension polymerisation techniques. While the suspending agents effectively maintain the monomer droplets in suspension, they do not substantially reduce the broad particle size distribution obtained when the monomer is suspended in the continuous polymerization medium. Alternatively, various modifications in the suspension polymerization process have been proposed to prepare more uniformly sized polymer beads. For example, in the continuous suspension polymerization of vinyl compounds, the coalescence of the suspended monomer droplets can be reduced by introducing substantial amounts of the polymer being prepared to the monomer prior to its suspension in the polymerization medium. However, an undesirably broad particle size distribution is obtained in suspending the monomer as droplets in the polymerization medium, thereby causing the resulting polymer beads to possess a relatively broad particle size distribution. In another method for preparing uniformly sized beads, U.S. Patent No. 3,922,255, Koestler et al., issued November 25, 1975, proposes forming monomer droplets by jetting a mixture of a polymerizable monomer and a polymerization initiator through capillary openings into a column of a continuous aqueous phase containing a suspension stabilizer, and partially polymerizing the thus formed monomer droplets. The polymerization ofthe partially polymerized drop lets is subsequently completed in a reaction vessel which is specifically designed and equipped with an agitation means for the polymerization of the monomer droplets. While the uniformity of the suspended monomer droplets can be increased using the proposed method, the particle size distribution is often sufficiently broad that an undesirably large portion of the final beads are wasted or must be employed in less economically desired applications. In addition, the preparation of polymer beads by the disclosed method requires substantial expenditures of time, energy and capital. In view of the stated deficiencies of the prior art methods for preparing uniformly sized polymer beads, it remains highly desirable to economically and efficiently prepare spheroidal polymer beads having improved uniform particle size distribution. Accordingly, the present invention is a method for preparing uniform size spheroidal polymer beads characterized by: (a) forming a monomer jet, having laminar flow characteristics, defined by a Reynolds number from 120 to 1,200 of a polymerizable monomer phase by flowing the monomer phase through an opening into a continuous phase which is immiscible with the monomer phase and which contains a sufficient amount of a suspending agent to stabilize droplets of the monomer phase; (b) breaking the monomer jet into droplets of uniform size by vibratorily exciting the jet defined by a Strouhal number from 0.15 to 1.5; (c) subsequently allowing the monomer droplets which have a density lower than the density of the suspension medium to rise through the medium, or the monomer droplets which have a density greater than the density of the suspension medium to fall through the medium, into one batch polymerization reactor vessel such that essentially no polymerization of monomer occurs prior to the transfer of the monomer droplets to the reactor vessel, and achieving a change in monomer concentration such that the concentration of monomer droplets in the reactor vessel is greater than the concentration of the monomer droplets as formed and ranges from 0.01 to 20 volume percent based on total volume of the continuous and monomer phases at the point of formation and ranges from 30 to 60 volume percent based on the total volume of the continuous and monomer phases at the point of polymerization; (d) polymerizing until completion the suspended monomer droplets with agitation in the batch reactor vessel under conditions which do not cause significant coalescence or additional dispersion. The apparatus for preparing spheroidal polymer beads, is characterized by (1) a monomer reservoir containing a monomer phase comprising a polymerizable monomer, (2) a column containing a continuous phase of a suspending liquid immiscible with the polymerizable monomer or monomer phase and a stabilizing amount of a suspending agent, (3) a jet forming means in intimate contact with the monomer phase comprising the polymerizable monomer and the continuous liquid phase and having a passage connecting the monomer reservoir with the column containing the continuous phase such that the monomer phase is capable of flowing as a jet having laminar flow characteristics therethrough into the continuous phase, (4) a vibratory exciter capable of exciting the monomer jet such that the jet is broken into droplets, and (5) a means for polymerizing the monomer in suspension at conditions such that significant coalescence or additional dispersion of the droplets does not occur. The suspension polymerization product in the form of spheroidal polymer beads have a particle size prior to the mechanical separation thereof wherein at least 80 volume percent of the beads possess a particle diameter from 0.9 to 1.1 times the volume average particle diameter of said beads. Surprisingly, by the practice of the present invention, monomer droplets exhibiting exceptionally uniform size, e.g., where the monomer droplets have a coefficient of variance (i.e., twice the standard deviation divided by the arithmetic mean) of less than about 0.05, can be formed. Although the droplets are known to collide with one another immediately upon formation, the droplets unexpectedly exhibit minimum coalescence, and spheroidal polymer beads having an unexpectedly uniform particle size can be prepared. For example, spheroidal polymer beads can be prepared that have a sufficiently uniform particle size such that about 90 volume percent of the beads possess a particle diameter from 0.9 to 1.1 times the volume average particle diameter of the beads. Conversely, using conventional suspension polymerization techniques, less than 30-40 volume percent of beads are found to exhibit such uniform particle size. Moreover, the spheroidal polymer beads prepared by the method of this invention generally exhibit the other properties desired of polymers prepared in bead form, e.g., the beads employed in the subsequent preparation of ion-exchange resins are not excessively cracked or spalled. The method of the present invention are particularly useful in the preparation of uniform size beads of a polymer of a monovinyl aromatic and a cross-linking agent copolymerizable therewith, e.g., a polyvinyl aromatic such as a divinylbenzene, which beads are useful in the subsequent preparation of ion-exchange resins. The ion-exchange resins prepared therefrom exhibit superior properties, particularly in continuous ion-exchange operations. Of specific interest are the use of said ion-exchange resins in the separation of fructose from glucose, where the maximum efficiency of each ion-exchange bead is obtained, thereby causing unexpectedly superior results in said separation. In addition, the method of this invention is useful in the preparation of expandable polymer beads such as expandable polystyrene, and in the preparation of seeded beads having a uniform particle size where the seeded beads are prepared from seed beads manufactured by the disclosed method. A better understanding of this invention will be achieved by referring to the accompanying drawings, in which Figure 1 is a schematic representation, partially in cross-section, which illustrates an embodiment of the present invention. Figure 2 is a schematic representation, partially in cross-section, illustrating in detail a portion of Figure 1. Figure 3 is a graphical representation of the particle size distribution of spheroidal beads prepared by the method of the present invention as compared to the size distribution of beads prepared using conventional techniques. Referring now specifically to the drawings, Figures 1 and 2 depict a monomer reservoir 1 which contains a monomer phase 14 comprising a polymerizable monomer. The monomer reservoir communicates with a source of monomer (not shown) by means of monomer feed conduit 2. Column 3 contains a continuous phase 7 of a suspension medium comprising a suspending liquid immiscible with the monomer or monomer phase in reservoir 1 and a stabilizing amount of a suspending agent. Column 3 communicates with a source (not shown) of the suspension medium by means of suspending liquid supply conduit 4. A monomer jet forming means such as orifice plate 11 is in intimate contact with the monomer phase contained by reservoir 1 and the continuous liquid phase of column 3. Orifice plate 11 has a passage or, more typically as depicted by the Figures, a plurality of passages 15 connecting the monomer and continuous liquid phases. A means for vibratorily exciting the laminar flow jet comprises a piston 6 or similar means such as a diaphragm which is in intimate contact with the monomer phase in reservoir 1. As depicted in the Figures, piston 6 is mechanically connected by rod 13 to a variable-frequency vibrator or oscillator 5 in a manner such that piston 6 vibrates at the frequency generated by vibrator 5. An effluent conduit 8 is provided at or near the end of column 3 opposite the monomer reservoir 1 to transfer the monomer droplets and any excess liquid from column 3 to polymerization reactor 9 equipped with an agitation means such as agitator 10. The reactor 9 is fitted with polymer product removal line 12. In operation, the monomer phase 14 comprising a polymerizable monomer, including mixtures of one or more copolymerizable monomers and mixtures of one or more copolymerizable monomers with a non-polymerizable material, e.g., inert porogenic or pore-forming material or prepolymer, is introduced into monomer reservoir 1 through conduit monomer feed 2. In general, the monomer phase 14 includes a polymerization initiator suitable for initiating the polymerization of the monomer. The monomer phase is fed into reservoir 1 at a rate such that the monomer phase is forced through the passages 15 of orifice plate 11 into the continuous phase 7 contained by column 3 at a rate sufficient to form a jet having laminar flow characteristics. In this invention, a laminar flow jet has a flow rate defined by the dimensionless Reynolds number (Re) from 20 to 2,000 wherein Reynolds number is defined as the product of the density of the monomer phase (p), including the polymerization initiator and other additional materials, expressed in grams per cubic centimeter (g/cm 3 ), the mean jet velocity (v) of the monomer phase expressed in centimeters per second (cm/sec) and the diameter (d), expressed in centimeters (cm) of the opening through which the monomer is flowed divided by the viscosity (m) of the monomer phase expressed in poise (i.e., Re= pvd m ). As the thus-formed monomer jet flows into the continuous phase, the jet is vibratorily excited at a frequency which breaks the jet into droplets. In general, the monomer jet is excited at conditions such that generally uniform sized droplets are prepared. By the term "generally uniform" is meant that the droplets exhibit a particle size distribution having a coefficient of variance of less than 0.1. In the practice of this invention, droplets having such distribution are prepared by vibratorily exciting the monomer jet at constant vibrational characteristics defined by the dimensionless Strouhal number (St) from 0.5 to 5 wherein the Strouhal number is the product of 2π, the frequency (f) of the vibration expressed in hertz (hz) and the diameter (d), expressed in cm, of the opening through which the monomer is flowed divided by the mean jet velocity (v) of the monomer phase expressed in cm/sec (i.e., St= 2πfd v ). As depicted in the Figure, such vibrational excitation can be provided by the variable-frequency vibrator 5 and transmitted to the monomer phase by piston 6. The specific conditions at which the droplets are formed, i.e., the flow characteristics of the monomer jet and vibratory excitement thereof, which are most advantageously employed depend on a variety of factors, particularly the desired size and uniformity, i.e., particle size distribution, of the monomer droplets and the resulting spheroidal polymer beads. The monomer droplets are preferably prepared having a particle size distribution such that the coefficient of variance thereof is less than about 0.07, more preferably less than about 0.05. Most preferably, the coefficient of variance of the particle size of the monomer droplets is negligible, i.e., less than about 0.01. At the formation of monomer droplets having such uniformity, the subsequent polymerization of the monomer at conditions which do not cause significant coalescence or additional dispersion will give spheroidal polymer beads having a particle size such that at least about 50 volume percent of said beads have a particle diameter from 0.9 to 1.1 times the volume average particle diameter of the beads. Advantageously, at least 80, preferably at least 95, volume percent of the beads exhibit such particle size, wherein particle size is measured employing conventional techniques such as described in the examples. The Strouhal and Reynolds numbers which impart such uniformity to the monomer droplets and the subsequently prepared polymer beads will vary depending on the components of the monomer phase, including the types and amounts of monomers being employed, and the composition of the continuous suspending medium. Typically, the monomer jet exhibits flow characteristics defined by a Reynolds number preferably from 225 to 700. The excitation of such monomer jet is conducted at vibrational conditions defined by a Strouhal number preferably from 0.4 to 1.2. At such conditions, droplets of the monomer phase equal in volume to the amount of the divided by the vibrationary frequency are prepared. By properly selecting the frequency and monomer flow rates, discrete particles of a predetermined particle size can be prepared. Therefore, the particle size uniformity of the beads can be represented using this calculated, predetermined particle size by magnifying (at least 50×) a statistically significant sample of beads (e.g., from at least 100 to 1000 beads) and measuring the particle diameter of the magnified beads. At least about 70 percent of the beads in the statistical sample advantageously have a particle size which is from 0.95 to 1.05 times the predetermined size. At least 80, more preferably at least 90, percent by number of the beads exhibit such particle size. Such uniformity is expressed graphically in Figure 3 where curve C represents a typical particle size distribution of beads prepared using conventional suspension polymerization techniques, and curve 1 represents, the particle size distribution of beads prepared by the method of this present invention. Of the total amount of beads, at least 80 percent by number (and preferably on a corresponding weight basis) preferably have a particle size between 0.95 and 1.05, more preferably between 0.97 and 1.03 times the number average particle size of the beads comprising the 80 percent portion. At least 90, preferably at least 95, percent by number of the beads exhibit such a uniform particle size when compared to the number average particle size of the beads comprising the corresponding 90 or 95 percent portion. Alternatively, droplets having almost any other particle size distribution, can be prepared by varying the vibrational characteristics as defined by the Strouhal number, such as by varying the frequency of vibration or by employing an orifice plate having different size passages. The suspending liquid 7, which liquid is immiscible with the monomer or monomer phase comprising the continuous phase of column 3, is introduced by means of line 4. In general, sufficient amounts of suspending agent to stabilize the monomer droplets are added. Alternatively, the suspending agent can be introduced separately into column 3 at various positions throughout column 3. In those operations, such as depicted by the Figures where the monomer phase possesses a lower density than the density of the suspension medium, the monomer droplets are allowed to rise through the suspension medium for subsequent removal from the top of column 3 for transfer to the polymerization reactor 9 by transfer line 8. In such operations, the suspending liquid is advantageously introduced into column 3 in close proximity to the monomer reservoir 1,. flowed upwardly and transferred from column 3 to the polymerization reactor in a cocurrent type operation. Alternatively, but less preferably, the suspending liquid can be introduced at or near the end of a column such as column 3 opposite monomer reservoir 1 in a countercurrent type operation, or the monomer droplets can be prepared in a suspension medium having a zero flow motion, i.e., the continuous liquid phase 7 contained by column 3 is static. The monomer phase may possess a higher density than the suspension medium and the monomer droplets are allowed to drop through. Here, the suspending liquid may also be employed in cocurrent, countercurrent or zero flow motion, with cocurrent flow being most advantageous. Although the monomer can be partially polymerized as it moves through column 3, normally the conditions are maintained such that essentially no polymerization of monomer occurs prior to the transfer of the monomer droplets to the polymerization reactor 9. The monomer and continuous phases are maintained at temperatures from ambient temperatures (e.g., from 18°C to 25°C) to 70°C. To prevent substantial polymerization in the column and to most effectively employ the suspending agent, temperatures from 40°C to 50°C are most preferred. Both the monomer droplets and suspension medium are transferred to the polymerization reactor 9. The polymerization of the monomer is subsequently conducted employing conventional techniques. These generally comprise batchwise polymerization techniques consisting of heating the monomer under free radical forming conditions, with the suspending liquid serving as the polymerization medium. Additional amounts of the same or a different suspending agent are advantageously added to polymerization reactor 9 to help maintain the uniform particle size of the droplets during polymerization. Upon completion of polymerization, the uniformly sized polymer beads are withdrawn from the polymerization vessel for subsequent recovery using conventional dewatering techniques such as filtration, centrifugation, or decantation. With regard to the various components of the present invention, jet forming means 11 is suitably any means comprising one or more passages through which the monomer phase can be passed at conditions such that one or more jets of the monomer phase is formed having laminar flow characteristics. Although the jet forming means can consist of one or more capillary tubes, it is more advantageous to employ a plate or similar device having one or more passages, e.g., a perforated disc, where each passage is generally a cylindrical or slightly frustoconical passage of the same dimensions. The diamter of the passage openings can range from less than 0.05 mm to greater than 6 mm, where diameter refers to that cross-section of the passage having the smallest diameter. The diameter of each opening is primarily determined by the desired size of the monomer droplets. Typically, the desired droplet size will vary from 0.15 to 5 mm, most typically from 0.2 to 1 mm. The passage diameter which will produce this size droplet is dependent on a variety of factors including the physical properties, e.g., viscosity, density and surface tension of the monomer and continuous phases, and the conditions of the vibrational excitation. Typically, passage diameters from 0.06 to 0.6, more typically from 0.08 to 0.4, mm are employed. Using passages having such diameters, the length of each passage is advantageously less than about 10 times the diameter of the opening. Preferably, the length to diameter ratio (L/d) of the passages is from 0.2 to 2. The passages in the orifice plate are spaced a sufficient distance apart from each other such that the formation mechanism required to prepare the uniformly sized monomer droplets, and the stability of the resulting droplets, are not significantly affected by the laminar jet and droplet formation of an adjacent jet. Interactions between the droplet formation of adjacent jets are not significant when a passage is spaced at a distance of at least 20 times the diameter of each opening apart from the nearest passage thereto, when said distance is measured from the center of each passage. A minimum distance, center to center, of at least about 1 mm is preferred. In addition, modifications which minimize the effect of the adjacent monomer jets, such as providing a protective cavity which protects each of the monomer jets until broken into droplets, can be provided in the orifice plate. Although the orifice plate can be prepared from a variety of materials including metal, glass, plastic or rubber, a perforated metal plate is preferably employed. The vibration is suitably provided by any means which oscillates or vibrates at a frequency capable of exciting the monomer jet at vibrational characteristics such that the monomer jet is broken into droplets, preferably, droplets of a generally uniform size. Although transverse excitation can be employed, the monomer jet is preferably subjected to longitudinal excitation, Generally, frequencies from 70 to 7,000, preferably from 100 to 1,000 hertz, give the desired characteristics. Vibratory means such as mechanical, electroacoustic, hydroacoustic or electromagnetic vibrators and magnetoresistive transducers well-known in the art can be employed. Electromagnetic vibrators are generally preferred. The monomer phase comprises one or more polymerizable monomers which forms a discontinuous phase dispersed throughout the suspension medium upon the formation of droplets thereof. Polymerizable monomers advantageously employed in the practice of this invention are polymerizable monomers or mixtures of two or more copolymerizable monomers, which monomers or monomer mixtures are sufficiently insoluble in a liquid (which liquid may contain a surfactant), to form droplets upon the dispersion of the monomer therein. Monomers having only limited miscibility in water such as acrylonitrile can also be employed herein. Advantageously, the monomers are polymerizable using suspension polymerization techniques. Such monomers are well-known in the art and reference is made to Polymer Processes, edited by Calvin E. Schildknecht, published in 1956 by Interscience Publishers, Incorporated, New York, Chapter III, "Polymerization in Suspension" by E. Trommsdoff and C. E. Schildknecht, pp. 69-109. In Table II on pp 78-81 of Schildknecht are listed diverse kinds of monomers which can be employed. Of particular interest are the water-insoluble monomers including the monovinyl aromatics such as styrene, vinyl naphthalene, alkyl-substituted styrenes (particularly monoalkyl-substituted styrenes such as vinyltoluene and ethyl vinylbenzene) and halo-substituted styrenes such as bromo- or chlorostyrene; the polyvinyl aromatics such as divinyl benzene, divinyltoluene, divinylxylene, divinylnaphthalene, trivinylbenzene, divinyldiphenyl ether and divinyldiphenyl sulfone; halo olefins, particularly the vinyl halides such as vinyl chloride; esters of α,β-ethylenically unsaturated carboxylic acids, particularly those of acrylic or methacrylic acids, such as methyl methacrylate and ethyl acrylate; vinyl acetate and mixtures of one or more of said monomers. Of said monomers, the monovinyl aromatics, particularly styrene or a mixture of styrene with a monoalkyl-substituted styrene; the polyvinyl aromatics, particularly divinylbenzene; esters of α,β-ethylenically unsaturated carboxylic acid, particularly methyl methacrylate or combinations thereof, or a mixture of styrene and divinylbenzene or styrene, divinylbenzene and methyl methacrylate are preferably employed herein. Also included within the polymerizable monomers useful herein are those monomers which form a solution with a liquid, generally water, where the resulting solution is sufficiently insoluble in one or more other liquids. Such liquid is generally a water-immiscible oil such that the monomer solution forms droplets upon its dispersion in said other liquid. Representative of such monomers are water-soluble monomers which can be polymerized using conventional water-in-oil suspension (i.e., inverse suspension) polymerization techniques. Included are ethylenically unsaturated carboxamides such as acrylamide, methacrylamide, fumaramide and ethacrylamide; aminoalkyl esters of unsaturated carboxylic acids and anhydrides; and ethylenically unsaturated carboxylic acids, e.g., acrylic or methacrylic acid. Preferred for use herein are ethylenically unsaturated carboxamides, particularly acrylamiue, and the ethylenically unsaturated carboxylic acids, particularly acrylic or methacrylic acid. The monomer phase of such water-soluble monomers will generally contain sufficient amounts of water to solubilize the monomer. In such cases, the amounts of the monomer and water most advantageously employed will depend on many factors. including the specific polymer and desired end use application. The monomer generally constitutes less than 90 weight percent of the monomer phase. Preferably, these water soluble monomers constitute from 5 to 80, more preferably from 30 to 55, weight percent of the monomer phase. Although the monomers can be polymerized using free radical initiation such as UV light or heat, the monomer or monomer mixture normally contains a chemical free radical initiator. Free radical polymerization initiators used previously for preparing polymers from the monomers described herein are advantageously employed. For example, a monomer-soluble free radical initiator such as a peroxygen, e.g., benzoyl peroxide, or azobisisobutyronitrile can be used. Similarly, a water-soluble free radical initiator such as the persulfates, hydrogen peroxide or hydroperoxide can be used in conjunction with the water-soluble monomers such as acrylamide. Conventional polymerization aids, e.g., chain transfer agents and chelating agents, can also be included within the monomer phase. Pore-forming materials, i.e., those materials which impart a porous structure to the resulting polymer beads such as an organic material which forms a solution with the monomers, and which does not dissolve the resulting polymer (e.g., aliphatic hydrocarbons such as hexane and isooctane), can, also be included in the monomer phase. The continuous liquid phase contained by column 3 is a suspension medium comprising (1) a suspending liquid immiscible with the polymerizable monomer or monomer phase and the polymer prepared therefrom and (2) a stabilizing amount of a suspending agent. The suspension medium is selected such that the droplets of monomer can readily be prepared under the previously specified conditions. In general, the droplets are easily prepared when the suspension medium has a viscosity of less than about 10 -1 Pa·s. The droplet formation is more readily achieved when the viscosity of the suspension medium is of the same order of magnitude as the viscosity of the monomer phase when said viscosities are expressed at room temperature. Preferably, the suspension medium has a viscosity of less than 5.10 -2 Pa·s more preferably less than 10 -2 Pa·s which viscosity is from 0.1 to 10, preferably from 0.2 to 2, times the viscosity of the monomer phase. The suspension medium preferably has a sufficiently different density than the monomer phase to allow the droplets of monomer to rise or fall through the column. For example, when the droplets of monomer rise through the column to the polymerization reaction means, the density of the suspending medium is advantageously greater than the density of the monomer droplets, with the density of the suspension medium being more than 1.02, preferably more than 1.1, times the density of the monomer droplets. Alternatively, when the droplets of monomer are to descend through the suspension medium, the density of the suspension medium is less than the density of the monomer droplets, with the density of the suspension medium being less than 0.98, preferably less than 0.9, times the density of the monomer droplets. The suspending liquid is suitably any inert liquid which is immiscible with the polymerizable monomer or monomer phase, in which liquid the monomer or monomer phase can be dispersed as droplets. By "immiscible" is meant that less than 10 weight percent of the polymerizable monomer or monomer phase is miscible (or soluble) in the suspending liquid. Preferably, less than 1, more preferably less than 0.1, weight percent of the monomer or monomer phase is miscible in the suspending liquid. Normally, the suspending liquid will contain a suspending aid. Suitable suspending aids are those materials which enable the formation of the monomer phase into spheroidal droplets of a desired size, and which hinder the coalescence or dispersion of the thus-formed droplets before or during polymerization of the monomer. Typically, the suspending liquid employed with the suspension-polymerizable monomers is water, or mixtures of water with one or more water-miscible organic liquids such as the lower alkyl alcohols such as methanol or n-propyl or isopropyl alcohol. Preferably, water is employed as the suspending liquid. Alternatively, when the monomer phase comprises a water-soluble monomer, a water-immiscible oil is conventionally employed in the preparation of a water-in-oil suspension such as a halogenated hydrocarbon, e.g., methylene chloride, or a liquid hydrocarbon having from 4 to 15 carbon atoms. Included here are aromatic and aliphatic hydrocarbons, or mixtures thereof, such as heptane, benzene, xylene, cyclohexane, toluene, mineral oils and liquid paraffins, e.g., kerosene and naphtha. The suspending agents conveniently used here are dependent on the type and amount of monomer employed and the suspending medium. Suspending agents used in conventional suspension polymerizations are advantageously used here. Representative suspending agents are gelatin, polyvinyl alcohol, starch, polyacrylic acids, including salts thereof such as sodium oleate, water-insoluble inorganic compounds such as magnesium silicate, and cellulose ethers such as carboxymethyl methylcellulose and hydroxyethylcellulose. In the practice of the present invention, the suspending medium contains a stabilizing amount of the suspending agent. "Stabilizing amount" means an amount sufficient to maintain an essentially uniforms dispersion of the monomer droplets in the suspending liquid following droplet formation until subsequent polymerization, while preventing excessive coalescence or additional dispersion. Such amount of suspending agent will vary depending on the types and relative amounts of the monomer or monomers and the suspension medium and the specific suspending agent. In the preparation of uniformly sized monomer droplets from the water-insoluble monomers, the suspending medium contains from 0.05 to 5, advantageously from 0.075 to 0.75, percent by weight of the suspending agent based on the total weight of the suspending liquid. The amount of suspending agent used in the preparation of droplets of the more water-soluble polymers is typically from 0.1 to 10, preferably from 0.15 to 1.5, weight percent based on the weight of the suspending liquid. It is sometimes preferable to add additional amounts of suspending agent to the polymerization reactor to help maintain a uniform size during polymerization. The amounts will vary depending on the type of suspending agent and the type and amounts of the monomer droplets. Normally, an additional amount of suspending agent ranging from 0.05 to 5.0, preferably from 0.075 to 0.75, weight percent of the total suspending liquid is added to the polymerization reactor. Provided that uniform droplets of monomer can be prepared and the resulting monomers subsequently polymerized, the concentration of the continuous and monomer phases is not particularly critical to the practice of the present invention. The optimum amounts of the monomer and continuous phases used depend on a variety of factors, including the composition of the monomer and continuous phases, particularly the monomer and suspending liquid used. The concentration of the monomer phase varies, with the formation of the monomer droplets being conducted at lower monomer concentrations than the subsequent polymerization. The formation of the monomer droplets can be conducted at a monomer phase concentration varying from that concentration provided by the monomer phase jetted through a single passage or nozzle into a relatively large amount of the suspension medium, to as much as 25 volume percent of the total volume of the monomer and continuous phases. Normally, the concentration of the monomer phase at the point of droplet formation ranges from 0.01 to 20, preferably from 1 to 15 volume percent, where said volume percent is based on the total volume of the continuous and monomer phases. Alternatively, in the subsequent polymerization of the monomer, the polymerization medium comprises, preferably from 40 to 55, volume percent monomer phase. Such change in the concentration of the monomer phase is readily achieved by varying the relative feed rates of the monomer and continuous phases to column 3. Normally, the monomer droplets are prepared at temperatures and conditions which minimize the polymerization reaction prior to or during droplet formation. For most monomers, temperatures from 18°C to 25°C are preferred for droplet formation. Normally, the monomer is polymerized at elevated temperatures under free radical forming conditions. Typically, temperatures from 50°C to 100°C are generally sufficient for polymerization. During polymerization, which may be continuous or batchwise, sufficient agitation is provided to maintain a dispersion of the uniformly sized droplets and to maintain a reasonably uniform temperature throughout the reaction medium. Upon completion of polymerization, the resulting suspension having beads comprising an aqueous solution of a water-soluble polymer dispersed throughout a continuous oil phase, is a liquid which is easily pumped or poured and which is readily used by adding it to an aqueous liquid which inverts the emulsion to an aqueous solution of the polymer. Normally, a water-soluble surface active agent such as an alkali metal, ammonium or amine soap of a fatty acid such as lauric, oleic or a comparable acid is used. Alternatively, after polymerization, the resulting polymer beads which comprise a normally solid polymer are recovered by conventional dewatering techniques such as filtration. The recovered beads can then be further processed. For example, the spheroidal polymer beads of a monovinyl aromatic and a polyvinyl aromatic can be converted to ion-exchange resins using techniques well-known in the art for converting cross-linked, aromatic polymers to such resins. Generally, anion resins are prepared by haloalkylating the cross-linked polymer and subsequently attaching anion active exchange groups to the haloalkylated polymer. Methods of haloalkylating the polymers and the haloalkylating agents employed in such methods are well-known in the art. A preferred haloalkylating agent is chloromethyl methyl ether. The anion resins, both weak and strong base resins, are prepared from the haloalkylated resins by techniques also well-known in the art. Typically, a weak base resin is prepared by contacting the haloalkylated polymer with ammonia or a primary amine such as methylamine or ethylamine, or a secondary amine such as dimethylamine at a temperature, generally from 25°C to 150°C, sufficient to react the aminating agent with the halogen atom attached to the carbon atom alpha to the aromatic nucleus. Strong base resins are prepared in a similar manner, using a tertiary amine such as trimethylamine or dimethylisopropanolamine as the aminating agent. Chelate resins are also easily prepared from the cross-linked polymers by techniques well-known in the art. For example, the haloalkylated polymer can be aminated and the aminated polymer subsequently reacted with a suitable carboxyl-containing compound, e.g., chloroacetic acid. Also, the haloalkylated polymer can be directly reacted with a suitable amino acid such as iminodiacetic acid or glycine, or an aminopyridine such as 2-picolylamine or N-methyl-2-picolylamine. Typically, strong acid cation-exchange resins are prepared by sulfonating the polymer, which polymer is swollen in a swelling agent such as a sulfonation-resistant chlorinated hydrocarbon, e.g., chlorobenzene or tetrachloroethylene, or an aliphatic or aromatic hydrocarbon, e.g., toluene or xylene, using sulfuric or chlorosulfuric acid or sulfur trioxide. Generally, the sulfonating agent is employed in amounts from 2 to 7 times the weight of the polymer and sulfonation conducted at temperatures from 50°C to 200°C. Alternatively, upon recovery, the spheroidal beads of various polymers, e.g., polystyrene, can be prepared as an expandable material. Although the blowing agent required to make such polymers expandable can be incorporated within the monomer phase prior to droplet formation, the beads are conveniently prepared without a blowing agent and the blowing agent subsequently incorporated. Following conventional techniques, the beads can be suspended in water in a pressure vessel and the blowing agent, e.g., fluorocarbon gas, introduced to the vessel using pressure and elevated temperatures, e.g., 80°C-100°C; the beads then absorb the gas. The normally solid polymeric beads can also be employed in the preparation of seeded beads having a uniform particle size. In preparing these beads, polymeric seed beads having uniform size, preferably having a particle size from 0.1 to 1.0 mm, are prepared by the method of this invention, i.e., the seed beads are prepared at the desired size by breaking a monomer jet by vibratorily exciting the jet and then polymerizing the resulting monomer droplets. Subsequently, a polymerizable monomer is imbibed within this seed bead and the imbibed monomer is then polymerized to form the uniform size seeded beads. If desired, these seeded beads can be chemically modified, such as by the attachment of active ion-exchange groups to form ion-exchange resins. In the preparation of the seeded beads useful in preparing ion-exchange resins, the seed bead comprises at least 0.15, preferably at least 0.2, and less than 2, preferably less than 1, weight percent of a polyethylenically unsaturated cross-linking agent, e.g., polyvinyl aromatic, based on the total weight of monomers employed in preparing the seed bead. Of the monomer imbibed by the seed bead, the amount of cross-linking agent employed can vary widely, with a polyethylenic monomer preferably constituting from 0.2 to 8, more preferably from 2 to 6, weight percent of the imbibed monomer. In general, the seed bead will constitute from 1 to 50, preferably from 10 to 30, weight percent of the total dry weight of the seeded bead. The remainder of the seeded bead is composed of the monomer imbibed and subsequently polymerized in the seed bead. The following examples illustrate the invention. All percentages and parts are by weight unless otherwise indicated. Example 1 The preparation of generally uniform sized spheroidal polymer beads of a copolymer of styrene and divinylbenzene is achieved using a vertically mounted stainless steel column which is 2 meters high and has an inside diameter of about 15 cm at a jet forming means (an orifice plate in this Example), and a diameter of 3.8 cm at the point at which the monomer droplets are fed to a polymerization vessel. At the base of this column is an orifice plate having 9 openings of a diameter of 0.29 mm and a length of 0.1 mm (L/d of 0.34). The orifice plate is prepared such that the openings are situated to form a square having 3 openings per side, with each opening being at least 12 mm from the opening nearest thereto. Disposed beneath the orifice plate is a monomer reservoir connected to a monomer source. Disposed about 5 cm above the orifice plate is a piping connection communicating the column to a source of suspending medium. A piston, disposed within the monomer reservoir, is connected by a rod to a commercially available variable frequency acoustic exciter. The walls of the monomer reservoir serve as the cylinder walls, and the piston is employed to transmit the reciprocating motion produced by the exciter to the monomer mixture and laminar monomer jet. A suspending medium (density (p)=1.0 g/cc), comprising an aqueous solution of 0.1 weight percent of a carboxymethyl methylcellulose suspending agent, fills the column and the polymerization vessel. The suspending medium is at room temperature and is maintained at this temperature throughout the column. Throughout the droplet formation, the suspending medium has a zero flow motion. A monomer mixture (p=0.92 g/cc) comprising 84 parts styrene 8 parts divinylbenzene, 8 parts ethylvinylbenzene and 0.3 part of a peroxygen type free radical polymerization initiator (said weight percent being based on the total weight of styrene, divinylbenzene and polymerization initiator) is metered into the monomer reservoir at a constant rate of 45 grams per minute (g/min). This mixture is jetted through the openings in the orifice plate to form a jet having laminar flow characteristics defined by a Reynolds number of 375. At the feed ratio specified, the monomer phase comprises 0.01 volume percent of the volume of the monomer and continuous phases as it is broken into droplets. To break the monomer jets into uniformly sized droplets, the jets are vibratorily excited at a Strouhal number of 0.65, which corresponds to a frequency of vibration of a constant 520 cycles per second. The resulting uniformly sized droplets rise through the column due to their lower density than the dispersing medium. The average residence time of the monomer in the column is about 100 seconds. The monomer droplets flow from the upper end of the columr, into a polymerization reactor until a suspension comprising about 55 volume percent of the unpolymerized monomer droplets based on the volume of the monomer and continuous phase is obtained. Sufficient amounts of additional carboxymethyl methylcellulose are added to the polymerization reactor to make a total concentration of the suspending agent of 0.125 percent based on the weight of the suspending medium. The monomer is then polymerized by heating the reactor to 75°C for a period of 12 hours while agitating the suspension at conditions which minimize the coalescence or additional dispersion of the droplets. At the end of this period, the resulting polymer beads are recovered free of the suspending medium using conventional filtration techniques. The recovered beads (Sample No. 1) are screened using conventional dry screening techniques, using U.S. Standard Sieves designated No. 18, No. 20, No. 25, No. 30, No. 35, No. 40, No. 45, and No. 50. By such techniques, the beads are found to have the particle size distribution recorded in Table I. For purposes of comparison, the particle size distribution of spheroidal polymer beads (Sample No. C) prepared using conventional suspension polymerization techniques is also set forth in Table I. Particle size distribution, volume % Particle size (diameter), mm Sample No. C Sample No. 1 >1.0 1.8 - 0.84-1.0 2.8 2.4 0.71-0.83 3.6 1.8 0.59-0.70 13.4 2.7 0.50-0.58 40.0 90.9 0.42-0.49 27.0 1.6 0.35-0.41 7.0 0.3 <0.35 4.4 0.3 As evidenced by the particle size distribution recorded in Table I, the beads prepared by the present invention (Sample No.1) exhibit excellent uniformity, particularly when compared to conventionally prepared beads (Sample No. C). Example 2 In a similar manner, spheroidal polymer beads are prepared using 92 parts styrene, 0.3 part ethylvinylbenzene and 0.3 part divinylbenzene. The volume average particle size of these beads is found to be 0.67 mm and over 75 volume percent of the beads are found to have a particle size from 0.59 to 0.74. Example 3 Following the procedure employed in preparing Sample No. 1 of Example 1, a monomer mixture of 84 parts styrene, 8 parts of divinylbenzene, 8 parts of ethylvinylbenzene and 0.3 part of a peroxygen type free radical initiator are flowed through an orifice plate having 5 openings, each opening having a diameter of about 0.4 mm and a length of 0.1 mm (L/d-0.25) at a Reynolds number of 484, into a suspending medium of an aqueous solution of 0.05 weight percent carboxymethyl methylcellulose. The resulting laminar jets of the monomer mixture are vibrated at a Strouhal number of 0.83, which corresponds to a frequency of 440 cycles per second. The monomer droplets flow from the column into the polymerization vessel to form a suspension comprising 55 volume percent of the monomer droplets. The monomer is polymerized by heating the reactor for 12 hours at 75°C and an additional 9 hours at 95°C, and the resulting beads are filtered. As evidenced by Table II, the beads are found to have a particle size of exceptional uniformity. Particle size (diameter), mm Particle size distribution volume % 1 >1.68 2.3 1.41-1.68 0.8 1.19-1.40 1.0 1.00-1.18 5.0 0.84-0.99 28.5 0.71-0.83 61.1 0.59-0.70 0.6 <0.59 0.7 Example 4 In a manner similar to that employed in preparing Sample No. 1 of Example 1, uniformly sized monomer droplets are formed by flowing a monomer mixture of 54 parts styrene, 5.6 parts divinylbenzene, 40 parts isooctane and 0.4 parts of a peroxygen type free radical initiator through an orifice plate, similar in all respects to the plate employed in Example 1 at flow characteristics defined by a Reynolds number of 520, and vibratorily exciting the resulting monomer jet at the Strouhal number of 0.65. The monomer droplets flow from the column into a polymerization reactor until a suspension of 50 volume percent of the monomer mixture is prepared. The monomer is then polymerized for 24 hours at 75°C and an additional 8 hours at 95°C and the resulting porous beads recovered using conventional filtration techniques. The particle size distribution of the resulting beads is determined using conventional dry screening techniques. The beads are found to have excellent uniformity, with 82.5 volume percent having a particle size between 0.5 and 0.71. These beads are suitably employed in the preparation of uniformly sized, macroporous (macroreticular) ion-exchange resin. Example 5 In a manner similar to Example 1, generally uniform sized monomer droplets are formed by flowing a monomer mixture of 28 parts styrene, 4 parts divinylbenzene, 68 parts methyl methacrylate and 0.2 part of a peroxygen type free radical initiator through the orifice plate employed in Example 1 at flow characteristics having a Reynolds number of 340, and vibratorily exciting the resulting monomer jet at a Strouhal number of 0.71. The monomer droplets flow from the column into a polymerization reactor until a suspension of 55 volume percent of the monomer mixture is prepared. The monomer is then polymerized for 24 hours at 75°C, and an additional 8 hours at 95°C and the resulting beads recovered using conventional filtration techniques. Using conventional wet screening techniques, the beads are found to have a narrow particle size distribution with 70 volume percent of the beads having a particle size between 0.50 and 0.70 mm. Example 6 Using a column similar in all respects to the column employed in Example 1, a monomer mixture comprising 99.56 parts styrene, 0.02 part divinylbenzene, 0.02 part ethylvinylbenzene and 0.4 part of a free radical polymerization initiator is metered into a monomer reservoir at a constant rate of 5.5 g/min. This mixture is jetted through the openings in an orifice plate, which plate has 14 openings having a diameter of 0.29 mm and a length of 0.1 mm (L/d of 0.34), to form a jet having laminar flow characteristics defined by a Reynolds number of 409. To break the monomer jets into uniformly sized droplets, the jets are vibratorily excited at a Strouhal number of 0.43, which corresponds to a frequency of vibration of a constant 520 cycles per second. The resulting uniformly sized droplets rise through the column which comprises an aqueous solution of 0.05 weight percent of a carboxymethyl methylcellulose suspending agent. Following an average residence time of about 100 seconds, the monomer droplets flow from the upper end of the column into a polymerization reaction with continuous agitation, until a suspension comprising 60 volume percent of the unpolymerized monomer droplets based on the volume of the monomer and continuous phases is obtained. Sufficient amounts of additional carboxymethyl methylcellulose are added to the polymerization reactor to make the total concentration of suspending agent 0.3 percent based on the weight of the suspending medium. The monomer is then polymerized by heating the reactor to 85°C for a period of 12 hours, followed by heating the reactor for an additional 6 hours at 95°C while agitating the suspension at conditions which minimize the coalescence or additional dispersion of the droplets. At the end of this period, the resulting polymer beads are recovered free of the suspending medium using conventional filtration techniques and are subsequently dried. The dried beads are screened using conventional dry screening techniques and are found to have a particle size such that 82 percent by volume of the beads have a particle size from 0.7 to 0.84 mm. The dry beads are resuspended in an aqueous solution of a carboxymethyl methylcellulose suspending agent contained by a pressure vessel. Fifteen parts of a commercial blowing agent per 100 parts of polymer beads are subsequently added to the pressure vessel. The vessel is then heated to 95°C for 2 hours, followed by an additional 2 hours at 110°C. Following this period, the reactor is cooled to ambient temperatures, and the beads washed, recovered by conventional filtration techniques and dried. The beads are bound to expand by 58 volumes, with the expanded beads generally exhibiting the excellent uniformity of the polymer beads from which the expandable beads were prepared.

Claims: A method for preparing uniform size spheroidal polymer beads characterized by: (a) forming a monomer jet, having laminar flow characteristics, defined by a Reynolds number from 120 to 1,200 of a polymerizable monomer phase by flowing the monomer phase through an opening into a continuous phase which is immiscible with the monomer phase and which contains a sufficient amount of a suspending agent to stabilize droplets of the monomer phase; (b) breaking the monomer jet into droplets of uniform size by vibratorily exciting the jet defined by a Strouhal number from 0.15 to 1.5; (c) subsequently allowing the monomer droplets which have a density lower than the density of the suspension medium to rise through the medium, or the monomer droplets which have a density greater than the density of the suspension medium to fall through the medium, into one batch polymerization reactor vessel such that essentially no polymerization of monomer occurs prior to the transfer of the monomer droplets to the reactor vessel, and achieving a change in monomer concentration such that the concentration of monomer droplets in the reactor vessel is greater than the concentration of the monomer droplets as formed and ranges from 0.01 to 20 volume percent based on total volume of the continuous and monomer phases at the point of formation and ranges from 30 to 60 volume percent based on the total volume of the continuous and monomer phases at the point of polymerization; (d) polymerizing until completion the suspended monomer droplets with agitation in the batch reactor vessel under conditions which do not cause significant coalescence or additional dispersion. The method of Claim 1 wherein the monomer droplets have a particle size distribution such that the coefficient of variance of their particle size is less than 0.1. The method of Claim 1 and further characterized in that the viscosity of the continuous phase is less than 5.10 -2 Pa·s at room temperature and from 0.1 to 10 times the viscosity of the monomer phase. The method of Claim 1 and further characterized in that the monomer droplets possess a lower density than the continuous phase and the monomer droplets rise through the continuous phase to a single batch polymerization reactor said continuous phase being flowed in the direction of the flow of the monomer droplets in a cocurrent operation. The method of Claim 1 and further characterized in that the polymerizable monomer is a suspension polymerizable, water-insoluble monomer. The method of Claim 1 and further characterized in that a polymerizable monomer is imbibed within the spheroidal polymer beads and the imbibed monomer subsequently polymerized.

Inventors: TIMM EDWARD EARL, TIMM, EDWARD EARL
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1981
Main classification: C08F2
IPC codes: C08F212, C08F2