The present invention relates to ion exchange resin particles, and in particular, to those particles that are prepared using seeded polymerization technology-

Ion exchange resins are typically prepared by providing functional groups which have the capacity for ion exchange to crosslinked copolymer particles or beads. The copolymer beads provide a strong, insoluble and rigid substrate for carrying the ion exchange functional groups. Thus, the durability and hydraulic characteristics of the ion exchange resin are generally limited by those characteristics of the copolymer from which it is derived.

Typically, ion exchange copolymers are prepared using a batch process whereby monomer droplets are formed and suspended in an aqueous phase, and polymerized. Unfortunately, such a process can provide a wide distri¬ bution of bead particle sizes. Thus, it becomes neces¬ sary to mechanically screen the beads and/or ion exchange resin particles in order to obtain a desirable product

having a relatively uniform or narrow distribution of bead size.

In a batch seeded process, for example, a lightly crosslinked copolymer seed can be swelled in the presence of a monomer mix containing an initiator and a crosslinker. The imbibed monomer mixture is polymerized in situ by a standard suspension polymeri¬ zation process. Such particles have very high physical strengths. The copolymer particles are then function- alized by chemically treating the insoluble, crosslinked bead in order to attach an ion exchange group thereto.

Although the batch seeded process provides the skilled artisan with a means for preparing relatively highly uniformly sized particles, the particles so prepared can exhibit several disadvantages. For example, an ion exchange resin which is prepared using the batch seeded process may not provide sufficient ion exchange properties, or bed operation time before breakthrough may be very short. In addition, if such ion exchange resins are stirred in relatively pure water, a cloudy aqueous suspension may be observed to form. It is believed that insoluble organic materials which leach out of the resin particle provide this undesirable contamination of the water. Thus, the resin particles which have highly desirable physical characteristics are not acceptable for use in applications such as water treatment.

In view of the deficiencies of the prior art, it would be highly desirable to provide ion exchange resin particles having relatively uniform particle sizes, good physical characteristics, good ion exchange

properties, and a minimal amount of leachable organic species.

This invention is a seed process for preparing improved crosslinked copolymer particles capable of being functionalized to provide ion exchange copolymer particles having a high capability of withstanding osmotic shock during use, which process comprises:

(a) forming an agitated aqueous suspension of polymerized, lightly crosslinked seed particles, and

(b) contacting said suspended seed particles with a monomer or monomer mixture comprising a minor amount of the total crosslinking monomer which is employed in order to ultimately produce a seed swollen by imbibition of said monomer or monomer mixture and polyvinyl cross¬ linking monomer, and

(c) contacting the suspended swollen seeds with a suspending agent in an amount sufficient to provide suspension of said swollen seeds, prevent substantial agglomeration of swollen seeds, and also allow further imbibation of monomer, and (d) subjecting the suspended swollen seeds to polymerization conditions to the extent that at least substantial polymer¬ ization of imbibed monomers occurs, and (e) substantially stopping the polymerization reaction and contacting the partially polymerized particles with a second monomer or monomer mixture comprising a

ajor amount of the total crosslinking monomer which is employed, and (f) subjecting the further imbibed swollen seed to polymerization conditions in order to provide a copolymer bead.

In another aspect, this invention is a seed process for preparing improved crosslinked copolymer particles capable of being functionalized to provide ion exchange copolymer particles having a high capability of withstanding osmotic shock during use, which process comprises:

(a) forming an agitated aqueous suspension of polymerized, lightly crosslinked seed particles, and (b) contacting said suspended seed particles with a monomer or monomer mixture comprising a minor amount of the total polyvinyl crosslinking monomer which is employed in order to ultimately produce a seed swollen by imbibition of said monomer or monomer mixture and cross¬ linking monomer, and

(c) contacting the suspended swollen seeds with a suspending agent in an amount sufficient to provide suspension of said swollen seeds, prevent substantial agglomeration of swollen seeds, and also allow further imbibition of monomer, and

(d) subjecting the suspended swollen seeds to polymerization conditions to the extent that at least substantial polymer¬ ization of imbibed monomers occurs, and

(e) contacting the partially polymerized particles with a second monomer or monomer mixture comprising a major amount of the total crosslinking monomer which is employed, while controlling the rate of addition of the monomer mixture and polymerization conditions such that the monomer mixture can be imbibed into the swollen seed which is under polymer- ization conditions and polymerization continued to provide a compolymer bead.

The copolymer particles prepared via the process of this invention can be functionalized to provide ion exchange resin beads, which resin beads have good osmotic shock and mechanical resistances to breaking. That is, for example, functionalized copolymer p rticles prepared via the process of this invention possess crush strengths (i.e., mechanical load required to break individual resin beads) of at least 500 g/bead crush strength and a resistance to osmotic shock such that when said particles are contacted with 10 cycles of alternating treatments with 8 molar sodium hydroxide and 8 molar hydrochloric acid, separated by backwashings with deionized water, fewer than about 15 percent by number of the particles are broken. One full cycle of said treatment comprises (a) immersing a quantity of beads into 8 M HC1 for one minute, (b) washing with deionized water until the wash water is neutral, (c) immersing the beads in 8 M NaOH for one minute and (d) washing the beads with deionized water until the ^' wash water is neutral. All references to alternating treatments with 8 M HCl and 8 M NaOH contained herein refer to repeating cycles of this test. The resistance to osmotic shock of the beads is measured by the number

of beads which remain unbroken after 10 cycles of the test. Typically, at least 85 percent of the function¬ alized beads of this invention will remain unbroken after 10 cycles of the osmotic shock test. Ion exchange resins can be either anionic or cationic in nature.

Such ion exchange resins are useful for a wide variety of applications known in the art. Of particular interest is the treatment of aqueous fluids in order to obtain highly pure water.

The monoethylenically unsaturated monomers useful herein are those commonly employed in the produc¬ tion of ion exchange resins. Examples of suitable mono¬ mers are disclosed in US 4,419,245. Reference is made to Polymer Processes, edited by Calvin E. Schildknecht, published in 1956 by Interscience Publishers, Inc., New York, Chapter III, "Polymerization in Suspension" by E. Trommsdoff and _^ C. E. Schildknecht, pp. 69-109 for purposes of illustration. In Table II on pp. 78-81 of Schildknecht are listed diverse kinds of monomers which can be employed in the practice of this invention. Styrene is the most preferred monomer.

Suitable crosslinking monomers are preferably those polyethylenically unsaturated monomers including those listed in U.S. Patent No. 4,419,245. The preferred polyethylenically unsaturated monomer is divinylbenzene.

The crosslinked seed particles useful in this invention are those which are lightly crosslinked in order to achieve the degree of swelling required to

produce the final copolymer products of the desired size. Preferably, the seed particles are spheroidal beads derived from polymerized monoethylenically unsat¬ urated monomer(s) and a crosslinking agent therefor. The crosslinking agent is preferably a polyethylenically unsaturated monomer. Generally, the amount of cross¬ linking agent in the seed can range from about 0.1 to about 3 weight percent, based on the weight of total monomer used in preparing the seed.

Typically, the amounts of each of the mono- and polyethylenically unsaturated monomers most advan¬ tageously employed in the preparation of the seed and seeded bead depend on a variety of factors including the type of each monomer employed and the desired size of the seed, seeded bead and resulting ion exchange bead. In addition, the amount and type of the mono- and polyethylenically unsaturated monomers employed.in preparing the seeded bead from the seed bead (i.e., those monomers imbibed by the seed bead) most advantageously employed herein will also vary depending on the size

(i.e., diameter) and composition (i.e., the amount and type of monomers) of the seed bead. In general, the seed and seeded beads are advantageously prepared using amounts of the mono- and polyethylenic monomers such that the seeded beads can be converted into ion exchange resin beads via techniques such as sulfonation, chloro- methylation, amination, and the like. The resulting functionalized beads when completely saturated with water preferably have a particle size of 0.3 mm to 1.0 mm and exhibit improved integrity (i.e., spheroidal character) and increased resistance to osmotic shock when compared to a conventionally prepared copolymer bead functionalized using similar conditions.

The size of the seed can vary. Typically, the size varies from 100 μ to 600 μm, preferably from about 200 μm to about 400 μm. The seed is generally swelled from about 1.5 to about 2.2 times its original diameter. It is imbibed with monomer to about 3 to about 10 times its original weight.

Polymerization initiators useful herein include those initiators useful in the preparation of the seed bead. Preferably, the aqueous suspension of seed particles is contacted with the initiator along with the first monomer mixture comprising the minor -amount of polyvinyl crosslinking monomer. Preferably, the initiator is a conventional chemical initiator useful as a free radical generator in the polymerization of ethylenically unsaturated monomers. Representative of such initiators are UV radiation and chemical initiators including azo compounds such as azobisiso- butyronitrile; peroxygen compounds such as benzoyl peroxide, t-butyl peroctoate, t-butyl perbenzoate and isopropylpercarbonate; and the like. Several catalysts are disclosed in U.S. Patent Nos. 4,192,921; 4,246,386; and 4,283,499. The initiator is employed in an amount sufficient to cause the copolymerization of the monomeric components in the monomer mixture. Such amount will generally vary depending on a variety of factors including the type of initiator employed, reaction temperature, the composition of the seed bead and the type and proportion of monomers in the monomer mixture imbibed thereby. Generally, the initiator is employed in amounts from 0.02 to 1, preferably from 0.05 to 0.5, weight percent based on the total weight of the monomer mixture.

The seed beads are advantageously suspended, using relatively high agitation rates, in a suitable suspending medium such as water or other aqueous liquid. Suspending agents are most preferably added after the first monomer mixture has been allowed to imbibe into the seed. Suspending agents useful herein are those materials which assist in maintaining a more uniform dispersion of the swollen seed beads in the aqueous liquid. Although the suspending agents most advan- tageously employed herein are dependent on the type and amount of monomers employed in preparing the swollen seed bead, in general, suspending agents conventionally employed hereto in the suspension polymerization of mono- and polyethylenically unsaturated monomers are advantageously employed. Representative of such suspending agents are gelatin, polyvinyl alcohol, sodium, dodecyl sulfonate, sodium methacrylate, magnesium silicate, sodium cellulose glycolate, hydroxyethylcellulose, methylcelluloses and the like. Suitable suspending agents are disclosed in U.S. Patent No. 4,419,245. The amount of the suspending agent employed is dependent on a variety of factors and is advantageously that amount which prevents agglomeration of the swollen seed beads but does not prevent further imbibition of monomers. Typically, from 0.05 to 1.0 weight percent, of the suspending agent, based on the weight of the aqueous phase is advantageously employed.

While the amount of the suspending medium advantageously employed herein, will vary depending on the type and amount of the suspending agent and swollen bead, in general, the suspending medium is employed in amounts from 30 to 70, preferably from 40 to 60, weight

percent based on the weight of the swollen seed beads, i.e., the weight of the seed bead and monomer mixture.

The process of this invention involves two critical stages in the preparation of the copolymer bead. The first stage involves suspending the seed under conditions such that imbibition of monomers which are contacted with the seed can occur. That is, upon contacting the seed bead with the monomer mixture, the seed bead swells, which swelling is believed to be due generally to the absorption of monomer mixture by the seed bead. The monomer can be added continuously or batchwise. The initiator can be added to the medium or added along with the monomer. The seed bead can be suspended during imbibition using suitable agitation conditions.

It is preferable in the first stage of the preparation that for reasons of mechanical strength of the final resin bead, the monomer mixture includes a minor amount of the crosslinking agent which is employed. That is, 1 to 15, preferably 1 to 10, most preferably 1 to 5, weight percent of the total amount of cross¬ linking agent which is employed is added in the first stage.

The temperature employed to polymerize the imbibed monomers in the first stage can vary depending upon the choice of initiator. Polymerization is generally conducted at temperatures between 50°C and 100°C, pre¬ ferably between 60°C and 90°C, most preferably 80°C.

In one aspect of this invention, before the second stage of process is commenced, it is necessary to remove the reaction mixture from a state of polymer¬ ization conditions. In addition, it is highly desirable that polymerization conditions be ceased before total polymerization has occured. If desired, polymerization of imbibed monomer can be substantially completed. This typically occurs when the seeded, swollen bead reaches its gel point. Advantageously, polymerization in the first stage should continue until 40 to 80 per¬ cent monomer conversion to polymer. This typically means lowering the temperature of the reaction mixture such that the second mixture of monomers can be con¬ tacted and imbibed into the swollen seeds without significant latex formation. If desired, the second stage monomer mixture can include a suitable amount of initiator.

The second stage of the polymerization process is commenced after the second monomer mixture has been contacted with the partially polymerized bead. This is believed necessary in order to allow the second monomer mixture to be imbibed into said bead. The second stage monomer mixture includes a major amount of the cross¬ linking agent which is employed. That is, 85 to 99, preferably 90 to 99, most preferably 95 to 99, weight percent of the total amount of crosslinking agent which is employed is added in the second stage. The reaction mixture is subjected to polymerization conditions until essentially total polymerization of the monomer has been achieved. The polymerization reaction is then finished, for example, by raising the reactor temperature.

In another aspect of this invention, the second stage of the process involves a continuous feed of the second monomer mixture to the suspended swollen, partially polymerized seeds. It is not necessary to remove the reaction mixture from a state of polymerization conditions. That is, by controlling the feed rate of the addition of the second monomer mixture, the suspension conditions, the rate of reaction, and the like, it is possible to continuously imbibe the second stage monomer mixture into the swollen bead, which is subjected to polymerization conditions where the imbibed monomers continue to undergo polymerization.

The amount of various monomeric components in the first stage monomer mix can range from 0.5 to 5 weight percent crosslinking agents, and from 95 to 99.5 weight percent monethylenically unsaturated monomer(s). The amount of various monomeric components in the second stage monomer mix can range from 5 to 20 weight percent crosslinking agent, and from 80 to 95 weight percent monoethylenically unsaturated monomer(s). The amount of second stage monomer mixture relative to the first stage monomer mixture can range from 50 to 80 weight percent based on the total weight of the monomer mixtures, subject to the limitation that the amount of crosslinking agent employed in the second stage mixture is a major amount based on the total amount of crosslinking agent which is employed. That is, the amount of crosslinking monomer employed in preparing the final resin is 2 to 18, pre¬ ferably 5 to 15 percent based on the total amount of organic material (i.e., seed plus polymerizable monomer).

The time periods over which each of the two (i.e., first or second) polymerization stages occur can vary as long as the desired species are obtained in each case. For example, the first stage monomer mixture can be added to the seed suspension either batchwise or continuously and subjected to polymerization conditions over a period ranging from 2 to 8 hours. The second stage monomer mixture is added either batchwise or con¬ tinuously, preferably over a period ranging from 3 to 5 hours. If batch addition is employed, the imbibition is preferably carried out at 20°C to 50°C and is allowed to occur, preferably with suitable mixing, for a period of 1 to 3 hours. The polymerization conditions are maintained in the second stage polymerization step for 8 to 14 hours. Finishing the reaction is performed for 1 to 3 hours, preferably at temperatures from 90°C to 120°C.

Following polymerization, the resulting seeded beads are recovered from the reaction media using con¬ ventional techniques, such as filtration, and the recovered beads are washed and dried. Functional groups are provided to the beads using known techniques. That is, copolymer beads are converted to anion and cation exchange beads. Beads can be fractionated into various size ranges using techniques such as screening.

The following examples are presented to further illustrate but not limit the scope of this invention.

Example 1 Into a 1-gallon (3.79 x 10 -3 m3) stainless steel reactor equipped with an agitator were loaded 875 grams (g) deionized water, 130 g of 0.3 percent cross¬ linked styrene/divinylbenzene copolymer seed of -50+70

mesh (from 210 μm to 297 μm) particle size with agitation, and 300 g of monomer I, comprising styrene and 1.3 percent active divinylbenzene, 0.05 percent active t-butylper- octoate ^' (TBPO) and 0.16 active percent t-butylperbenzoate (TBPB) based on monomer I. After 30 to 60 minutes of swelling time, 325 g of an aqueous solution of 6 g gelatin and 0.3 g sodium lauryl sulfate stabilizers, and 3.8 g sodium dichromate latex inhibitor was added. The reactor temperature was raised to and maintained at 80°C for 4 to 6 hours. The reactor mass was cooled to 40°C, and 860 g of monomer II containing styrene and 14.5 percent active divinylbenzene was added to reactor and after 60 to 120 minutes of swelling time, the reactor mass was heated to 85°C, 95°C, and 110°C for 12, 1.5, and 1.5 hours each, successively.

The copolymer, thus, made after appropriate screening was checked for crosslink density and then converted into a strong acid cation exchange resin. Fifty grams of the copolymer thus obtained was sulfonated to a cation exchange resin. The resin after appropriate washing was tested for capacity, crush strength, osmotic shock resistance, and water quality. A mixture of 50 ml each of resin and water was vigorously stirred magnetically for 30 minutes and the aqueous layer examined for clarity.

Example 2

Into a reactor as described in Example 1 was charged 850 g deionized water, 215 g of 0.3 percent crosslinked styrene/divinylbenzene copolymer seed of -50+60 mesh (from 210 μm to 297 μm) particle size with agitation, and 215 g of monomer I containing styrene and 1.7 percent active divinylbenzene, 0.036 active percent TBPO and 0.05 percent active TBPB. After 30 to 60 minutes

of swelling time, 325 g of an aqueous solution of the previously described stabilizers and latex inhibitor was added. The reactor was sealed, purged with nitrogen and the temperature of the mixture was raised to 80°C, 95°C and 110°C for 8, 1.5 and 1.5 hours, respectively. The reaction mixture was cooled to 40°C, and 995 g of monomer II containing styrene and 13.9 percent divinylbenzene, 0.015 percent TBPO and 0.05 percent TBPB was added to the reactor. The mixture was allowed 60 to 120 minutes swelling time. The reaction mixture was heated at 85°C, 95°C and 110°C for 10, 1.5 and 1.5 hours, respectively. The copolymer beads were isolated and converted to strong acid cation exchange resins.

Example 3 Into a 20 gallon (75.7 x 10 m ) stainless steel reactor equipped with an agitator were charged 70 lbs. (31.75 kg) of deionized water and with stirring, 13.3 lbs. (6.03 kg) of 0.3 percent divinylbenzene cross¬ linked polystyrene seed of -50+60 mesh (from 210 μm to 297 μm). Also charged was 20 lbs. (9.07 kg) of monomer I containing styrene, 1.44 percent active divinylbenzene, 0.26 percent active TBPO and 0.22 percent TBPB. After 30 to 60 minutes of imbibing time, 20 lbs. (9.07 kg) of an aqueous solution of 228 g gelatin and 20 g sodium lauryl sulfate suspending agent, and 11.5 g sodium dichromate latex inhibitor was added to the mixture. The reactor was purged with nitrogen and the reaction mixture was heated to 80°C. After 90 to 120 minutes at 80°C to the reaction mixture was added 66.7 lbs. (30.25 kg) of monomer II containing styrene and 15 percent active divinyl¬ benzene, over a 5 hour period, while polymerization conditions were continued. The reactor was maintained at 80°C for an additional 5 to 7 hours. At that time the

reactor mixture temperature was raised to 95°C and 110°C for 1.5 and 1.5 hours, respectively. The copolymer beads were isolated and converted to strong acid cation exchange resins.

Example 4

Copolymer particles were prepared employing the procedures generally described in Example Nos. 1-3. These samples were compared to beads prepared using a conventional methods of preparation as follows:

-3 3 Into a 1-gallon (3.79 x 10 m ) stainless steel reactor equipped with an agitator were loaded 875 g of water and 375 g of 0.3 percent crosslinked styrene/- divinylbenzene copolymer seed of -40+45 mesh (from 354 μm to 420 μm) particle size with agitation. To the suspended seed particles were added 875 g of monomer mix containing 15.6 percent active divinylbenzene, 0.036 percent active TBPO, and 0.05 percent active TBPB. The monomer was allowed to imbibe for 30 to 60 minutes and then 375 g of an aqueous solution containing a suspending agent and an aqueous phase inhibitor was added. The reaction mixture was then heated to and maintained at 80°C for 8 hours, and the temperature raised to 95°C and 110°C for 90 minutes each, respectively. The copolymer beads were isolated and converted to strong acid cation exchange resins.

Properties of the various sulfonated samples of this invention exhibit excellent osmotic shock properties (i.e., greater than 95 unbroken beads after 10 cycles of alternate treatments with 8 M HC1 and 8 M NaOH, separated by backwashings with deionized water). In addition, when 50 ml of beads are placed in 50 ml

deionized water and agitated for 30 minutes, beads of this invention yield clear solutions, whereas such a test performed using the beads prepared using the previously described conventional procedure yield cloudy solutions.

Example 5 Into a 1 gallon (3.79 x 10 -3 m3) stainless steel reactor, equipped with an agitator, were loaded 750 g deionized water, 215 g of 0.3 percent crosslinked styrene/divinylbenzene copolymer seed of -50+60 mesh (from 210 μm to 297 μm) particle size with agitation and 215 g of monomer I, containing styrene and 1.7 percent active divinylbenzene, 0.075 percent active TBPO and 0.25 percent active TBPB. After 30-60 minutes of swelling time, 325 g of an aqueous solution of stabilizer and latex inhibitor was added. The reactor temperature was raised to and maintained at 80°C for 6 hours. The reactor mass was cooled to 40°C and 640 g of monomer II containing styrene and 8.8 percent active divinylbenzene was added to the reactor. After 60 to 90 minutes of swelling time, the reactor mass was heated to 85°, 95° and 110°C for 10, 1.5 and 1.5 hours, each successively. The copolymers thus made, after appropriate washing and drying, were checked for toluene swell crosslink density and converted to both anion and cation resins, respectively.