GB1595169A - Ion exchange polymer beads and their preparation - Google Patents

Description

(54) ION EXCHANGE POLYMER BEADS AND THEIR PREPARATION (71) We, ROHM AND HAAS COMPANY, a Corporation organized under the laws of the State of Delaware, United States of America, of Independent Mall West, Philadelphia, Pennsylvania 19015, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This. invention is concerned with ion exchange polymer beads of enhanced physical characteristics and their preparation.

The techniques of preparing cross-linked vinyl copolymers in bead form (as precursors for ion exchange resins) by free-radical catalyzed polymerization of the monomer mixture in aqueous dispersion are well known: The term "cross-linked vinyl copolymer" and the like is used for the sake of brevity herein to signify copolymers of major proportion, i.e. 50 up to 99.5 mole percent, normally 80 to 99%, of mono-ethylenically unsaturated monomer, preferably, mono-ethylenically unsaturated aromatic monomer, e.g. styrene, vinyl toluene, vinyl naphthalene, ethyl vinyl benzene, vinyl chlorobenzene and chloromethyl styrene, with a minor proportion, i.e. from 0.5 to 50 mole percent, preferably 1 to 20%, of polyethylenically unsaturated monomer having at least two active ethylenically unsaturated groups polymerizable with the aforesaid mono-unsaturated monomer to form cross-linked, insoluble, infusible copolymer, for example, divinyl benzene, trimethylolpropane trimethacrylate, ethylene glycol dimethacrylate, divinyl toluene, trivinyl benzene, divinyl chlorobenzene, diallyl phthalate, divinylpyridine, divinyltoluene, divinylnaphthalene, ethylene glycol diacrylate, neopentyl glycol, dimethacrylate, diethylene glycol divinylether, bisphenol-A-dimethacrylate, pentaerythritol tetra- and tri-methacrylates, divinylxylene, divinylethylbenzene, divinyl sulfone, divinyl ketone, divinyl sulfide, allyl acrylate, diallyl maleate, diallyl fumarate, diallyl succinate, diallyl carbonate, diallyl malonate, diallyl oxalate, diallyl adipate, diallyl sebacate, divinyl sebacate, diallyl tartrate, diallyl silicate, triallyl tricarballylate, triallyl aconitate, triallyl citrate, triallyl phosphate, N,N'methylenediacrylamide, N,N'-methylene dimethacrylamide, N,N'ethylenediacrylamide, trivinyl naphthene, polyvinyl anthracenes and the polyallyl and polyvinyl ethers of glycol, glycerol, pentaerythritol, resorcinol and the monothio and dithio derivatives of glycols. The copolymers may also have incorporated therein up to 5 mole percent of polymerized units of non-aromatic ethylenically unsaturated vinyl monomer of a kind which does not affect the basic nature of the resin matrix, for example, acrylonitrile, methyl acrylate and butadiene.

Conventional conditions of polymerization used lead to cross-linked vinyl copolymers, which, when converted to ion exchange resins by attachment of functional groups thereto, have certain operating deficiencies resulting from physical weaknesses.

The present invention may be used to yield ion exchange resins in which the polymer beads have high mechanical strength and resistance to those swelling pressures which are produced within a bead during acid/base cycling (i.e. osmotic shock). Greater mechanical bead strength manifests itself in improved resistance to physical breakdown as a result of external forces such as weight of the resin column bed, high fluid flows and back-washing. Thus, the physically strong ion exchange resins of this invention are especially useful in treating fluid streams at high flow rates, for example in condensate polishing applications in which resins of lesser quality are prone to mechanical breakdown and short life spans.

It is current practice not to include oxygen during the preparation of crosslinked vinyl polymer used as base matrix copolymer for ion exchange resins since oxygen presents a safety hazard and has been generally regarded as detrimental to the properties of polymer obtained by free-radical polymerization.

In accordance with this invention, the vinyl monomer, cross-linking monomer, and other optional monomer are polymerized in an aqueous dispersion in the presence of a free-radical initiator and with oxygen dissolved in and/or being swept over the monomer mixture at least until the gel point is reached and at a reaction temperature of from 30 to 950 C., preferably 50 to 700C. Thus, in order to improve absorption of oxygen by the monomer mixture, it is generally preferred to employ polymerization temperatures somewhat below, e.g. 5 to 250C below those normally used in suspension polymerization processes for making similar products.

Accordingly, the free-radical initiator used in this invention is preferably one suitable for catalyzing polymerization at such temperatures, for example, such initiators as di(4-t-butycyclohexyl) peroxydicarbonate, dicyclohexyl peroxydicarbonate, di-(sec-butyl) peroxydicarbonate, di-(2-ethylhexyl) peroxydicarbonate, dibenzyl peroxydicarbonate, diisopropyl peroxydicarbonate, azobis(isobutyronitrile), azobis(2,4-dimethylvaleronitrile), t-butyl peroxypivalate, lauroyl peroxide, benzoyl peroxide, t-butyl peroctoate and t-butyl peroxyisobutyrate. The amount of initiator employed is normally from 0.1 to 2 percent, based on monomer weight, preferably 0.3 to 1%. It also may be advantageous when using catalysts which are active at relatively low temperatures, such as 30 to 600C, to employ a second, so-called "chaser", catalyst which is active at higher temperatures in order to achieve higher yields of cross-linked vinyl polymer, for example, from about 0.05 to 0.1%, based on monomer weight of such initiators as benxoyl peroxide, t-butyl peroctoate and t-butyl peroxyisobutyrate.

As mentioned above, the process of this invention involves ensuring that oxygen is absorbed by the monomer mixture at least until the polymerization reaches the gel point, i.e. the point at which an infinite polymeric network is formed (see, for example, Fundamental Principles of Polymerization by G. F.

D'Alelio, John Wiley & Sons, Inc., 1952, page 93). Known procedures for involving gaseous reactants in polymerization systems may be used to incorporate the oxygen in the monomer mix before and/or during the polymerisation reaction. For example, the head space above the reaction medium may be purged with an oxygen-nitrogen gaseous mixture (prior to initiation of reaction by raising the temperature) and then a gaseous sweep of the appropriate O2-N2 mixture may be passed through the head space during the reaction period. The gaseous mixture may contain as much as 20% by volume oxygen, however, for safety in avoiding explosion-prone conditions, lower levels may be required depending on the explosive range of the mixtures of the specific vinyl monomer or monomers with oxygen in the vapor phase, e.g. less than 9% by volume oxygen in the case of styrene and divinylbenzene mixture. Since the absorption of oxygen by the monomer droplets depends not only on temperature and the partial pressure of the oxygen in the gas in the head space, but also on the area of reaction medium exposed to the head space, the configuration of the kettle will determine whether it is advantageous to operate at atmospheric pressure or at increased pressure, for instance up to five or more atmospheres, inasmuch as increased pressure causes greater oxygen absorption. An alternative method of introducing oxygen into the monomer mixture is to sparge the gaseous mixture into the monomer mixture before and/or during polymerization.

The aqueous medium in which the polymerization is conducted in dispersion form will usually contain minor amounts of the conventional suspension additives, that is, dispersants such as xanthan gum (biosynthetic polysaccharide), poly (diallyl dimethyl ammonium chloride), polyacrylic acid (and salts), polyacrylamide, magnesium silicate, and hydrolyzed poly (styrene-maleic anhydride); protective colloids such carboxymethyl cellulose, hydroxyalkyl cellulose, methyl cellulose, polyvinyl alcohol, gelatin, and alginates; buffering aids such as phosphate and borate salts; and pH control chemicals such as sodium hydroxide and sodium carbonate.

The cross-linked, high-molecular weight copolymers may be recovered from the reactor as hard, discrete beads of particle size 0.02 to 2 mm, average particle size usually being 0.2 to I mm. These copolymers may be converted to ion exchange resins by functionalization according to known processes, such as functional groups including sulfonamide, trialkylamino, tetraalkyl ammonium, carboxyl, carboxylate, sulfonic, sulfonate, hydroxyalkyl ammonium, iminodiacetate, amine oxide, phosphonate, and others known in the art. Functionalizing reactions which may be performed on vinyl aromatic copolymers to produce ion exchange resins are exemplified by sulfonation with concentrated sulfuric acid, chlorosulfonation with chlorosulfonic acid followed by amination, reaction with sulfurylchloride or thionyl chloride~followed by amination, and chloromethylation followed by amination. Ion exchange resins may be further delineated by the types: strong acid cation, i.e. containing the groupings sulfonic (-SO3H) or sulfonate (-SO3M, where M is usually an alkali metal ion); weak acid cation, i.e. containing the carboxy (-CO2H) or carboxylate (-CO2M, where M is usually an alkali metal ion) groupings; strong base anion, i.e. containing the tetraalkyl ammonium group: -NR3X, where R is an alkyl or hydroxy alkyl group and X is usually chloride or hydroxide; and weak base anion, i.e. containing a trialkylamino group: -NR2, where R is an alkyl or hydroxyalkyl group.

The properties of the copolymers produced by this invention can be seen in their characteristics under thermal analysis and solvent swelling and when converted to ion exchange resins by the attachment of the aforesaid functional groups. The enhanced physical strength of these latter resins is apparent from their resistance to crushing which is conveniently measured on the Chatillon instrument, as well as by visual inspection before and after use in ion exchange applications.

For example, strongly acidic styrene-type resins frequently exhibit Chatillon values of 900 to 5000 gm force per bead, preferably 1200 to 5000, in contrast to resins derived from copolymers prepared by conventional polymerization methods which usually have Chatillon values of 50 to 550 gm/bead. Similarly, anion styrene-type resins of the invention can be made which exhibit Chatillon values of 500 to 2500 particularly 600 to 2500 and more particularly 900 to 1500 range in contrast with resins derived from copolymers prepared by prior art methods which typically have Chatillon values of 25 to 400.

Gel ion exchange resins of the invention, particularly the most common commercial resins produced from aromatic copolymers, can be easily distinguished from the prior art resins by one or more of various physical parameters including (1) perfect bead count (fewer cracked and fragmented beads), (2) resin friability (Chatillon test), (3) resistance to fracture upon repeated cycles of exhaustion/regeneration (Microcycling Test), and (4) the birefringence patterns of the beads. Test methods and observations of these distinguishing characteristics are given hereinafter.

In some instances, prior art resins have exhibited high physical stability by one or more of tests (1) to (3) above, but have failed to achieve excellence in all three criteria. In the cation resins, about 75% to 90% of the resins used commercially have intermediate levels of cross-linker, that is about 4 to 12% by weight usually 7 to 10% cross-linker (preferably divinylbenzene, DVB). The most common anion resins, from a commercial standpoint, are those containing relatively low levels of cross-linker, that is about 1 to 10% by weight usually 2 to 5% cross-linker. Products of the invention having the physical characteristics described may be made with all levels of cross-linker, although illustrated herein are the most common types. The differences between the birefringence patterns of the novel resins disclosed herein and similar resins of the prior art may be less pronounced at lower cross-linker levels where internal residual resin stress is a less significant factor. The improvement can nevertheless be ascertained when comparing the novel resins with the same type (and cross-linker content) resins of the prior art in either a relaxed or artificially stressed state (e.g. in swelling solvents).

Perfect Bead Count Perfect bead count is determined microscopically after functionalization of the copolymer such as by sulfonation or chloromethylation and amination of the copolymer. Perfect beads are those which contain no visible flaws, that is, beads which are perfectly spherical with no cracks, fragments, pits or surface defects.

Products of the invention contain at least 90% or more of perfect beads, typically 93 to 99% perfect beads, by visual observation and count. Prior art resins typically contain about 40 to 99% perfect beads. However, many grades of commercial resins typically have perfect bead counts of only 40 to 50% (e.g. see, Figure II, C and D described below).

Acid/Base Cycling (Microcycling) Test Microcycling is designed to simulate on an accelerated time scale the conditions under which the resin will be used. These studies are conducted over a period of a few days rather than months or years typical of field conditions.

Repeated exhaustion-regeneration cycles are performed on the resin at predetermined intervals in a fully automated apparatus.

The resin to be tested is screened to a -20+30 U.S. mesh cut size and examined under a microscope for appearance before microcycling: four different fields of view of monolayer of beads are observed and the average result for each of the following is recorded: (a) % perfect beads.

(b) % cracked beads.

(c) % fragmented/broken beads.

A small portion of the screened resin (0.5 ml) is placed in a sintered glass filter tube such that a monolayer of beads is formed in the tube. This small quantity of resin beads assures good contact between solution and resin and total conversion of the resin during each step. The solutions used for exhaustion and regeneration are made up in advance and stored in 50 liter tanks. The solutions used for anion and cation resins are described below: Resin Type Exhaustion Solution Regeneration Solution Anion 0.25 N H2SO4 1.0 N NaOH Cation 0.5NaOH 1.0 N HCL During a typical experiment, approximately 200 ml of exhaustion solution is added dropwise to the resin sample over 10 minutes, followed by removal of bulk exhaustant by mild vacuum, deionized water rinse followed by mild vacuum, and dropwise addition of regenerant solution over 10 minutes followed by removal of bulk regenerant by mild vacuum and a water rinse; completion of this process represents an exhaustion-regeneration cycle and takes approximately 30 minutes.

Complete automation allows 100 cycles to be completed in about 48 hours. After completion of 100 cycles, the resin is recovered and inspected microscopically for appearance. The reduction in % perfect bead content is recorded as the breakdown.

Products of the invention may generally show a reduction of perfect bead count of less than 30%, normally not more than 15% after 100 cycles by the Microcycling Test. The cation resins generally exhibit less reduction of perfect beads, not more than 10%, and usually at most 5%. Anion resins may show reductions of up to 30%, normally at most 15%. By comparison, prior art cation resins are known to exhibit reductions of from 15 to 80% most typically 30 to 50%.

Many prior art anion resins show perfect bead reductions of 15 to 80% after 100 cycles with 15 to 50 being most typical.

Chatillon Test for Resin Friability The Chatillon test is named after an apparatus manufactured by John Chatillon and Sons, New York, N.Y. and designed to measure resin friability. The instrument (Model LTCM, Gauge DPP-2.5 KG) measures the force (grams) required to crack or fracture a resin bead when it is placed between two parallel plates. The plates are gradually brought together at a uniform rate until the resin "breakpoint" is reached. The purpose of this test is to simulate the frictional and pressure forces exerted on individual resin beads under actual use conditions.

Specifications for testing include converting the resin into the proper form (hydrogen or sodium for the cation resins tested herein and chloride form for the anion resins tested herein) by well known standard procedures. The converted resin is screened to a -20+30 U.S. mesh cut size and then allowed to fully hydrate in deionized water for at least 15 minutes prior to testing. Actual testing is done on a single resin bead (covered by a small drop of water) in the Chatillon instrument using the lowest practical speed of descent of the crushing plate. The individual fragmentation forces are recorded from the instrument in grams per bead and the results are presented as an average (20 beads minimum, typically 30 beads), a standard deviation, a 95% confidence interval, and the percentage of beads which meet a minimum friability standard.

Birefringence Test An analytical test which aids in identifying gel resins of the invention and generally distinguishing them from prior art counter parts is the birefringence test.

The technique for obtaining birefringence patterns involves the use of an optical microscope (e.g. Carl Zeiss Photomicroscope) set up for bright field illumination at low magnification (e.g. 34x). Polarized lenses are inserted above and below the microscope stage and oriented perpendicular to one another. A piece of frosted glass is mounted on the stage to provide diffuse illumination of the samples.

Approximately 30 to 50 beads of the sample resin to be analysed are then placed in the concave well of a deep-dish microscope slide. The well is filled with water and then covered with a large coverslip. The slide so prepared is placed on the frosted glass mounted on the stage, the focus adjusted to optimize definition of the outer edge of the beads, and a photomicrograph is made to illustrate the birefringence pattern.

Observations of a large number of birefringence patterns taken of ion exchange resin samples produced by the invention and comparison of them with patterns of contemporary commercial resins have revealed clear distinctions between the patterns. Both cation and anion resins are distinguishable from prior art counterpart resins, but on a somewhat different basis and therefore the cation and anion resins will be described separately.

Cation Resins The use of birefringence strain patterns to identify the stresses in cation resins is not new to the art of ion exchange (see, for example, Wheaten R. M., et al., Industrial and Engineering Chemistry, Vol. 44, No. 8, August 1952, pp. 1796- 1800). We have now further identified a number of characteristic patterns and empirically correlated them with the physical properties of the resin including residual internal bead stress so as to obtain a qualitative identification of resin origin and properties. High internal stress in the resin beads has been found to correspond directly with low physical stability. The cation, low stress resins of this invention are generally illustrated by patterns A-D in Figure I of the accompanying drawings while some of the most widely used resins presently available from various manufacturers are illustrated by patterns A-D in Figure II of the accompanying drawings. The photographs showing the birefringence patterns were made at substantially the same film exposure and sample illumination.

In general, the patterns showing the highly stresed products (and therefore those more susceptible to fracture or breakage) can be identified by the brightness and sharpness of the pattern as well as the pattern type. Referring to the drawings, the more highly stressed beads are found in Figure II which shows sharper, brighter individual bead patterns on an overall basis. It is important to realise that this observation and other observations herein are made an overall or gross appearance of a sample owing to differences between the individual beads in a single batch or sample. Further, it is postulated that some of the commercial samples observed may consist of composites or physical mixtures of materials produced under different conditions and therefore the patterns may illustrate the sensitivity of the product quality to variations in the process of preparation.

A Maltese cross, or some variation thereof, is typically observed in resin strain birefringence patterns and is indicative of spherically symetrical stress orientation.

Any application of physical stress on an ion exchange resin bead produces a strain pattern, typically a Maltese cross. This phenomenon may be observed when compressing a relatively unstressed bead between parallel planes and when inducing stress through osmotic pressure such as when swelling a bead in solvent.

The width and sharpness of the arms of the Maltese cross furnish a qualitative (sometimes quantitative) indication of strain. The sharper, narrower arms indicating higher stress, especially when accompanied by bright areas between arms.

Applying the foregoing general considerations, the strain birefringence patterns in Figures I and II can be distinguished from each other. The low stress cation resin products illustrated in Figure 1, A-D resins in hydrogen form have at least one of three identifying patterns, namely: (1) a broad Maltese cross enclosed by an extinction (dark) ring around the periphery of the bead (pattern predominating in Figure I, A and B), (2) a broad Maltese cross enclosed by an extinction ring that is distinctly inside and separated from the periphery of the bead (Figure I, A, B and C - perhaps best observed in lower half of C), and (3) an irregular pattern, sometimes resembling a randomly oriented chain and sometimes recognizable as a distorted version of (2), above (Figure I, patterns C and D, but best observed in D).

Each of the patterns of Figure I, A to C containing a Maltese cross are relatively dull, with broad, and somewhat blurred arms comprising the Maltese cross. While the pattern of Figure I, D is less distinct, it too is somewhat dull with more random stress patterns, probably indicative of random stress orientation. All of the patterns in Figure I are atypical of the prior art cation gel resins which. are illustrated in Figure II. All cation resins in Figure I were produced from a styrene/divinylbenzene (8%) copolymer backbone using the process of the invention as described in the Examples given later. The material of Figure I, C is a composite of three laboratory-prepared samples. All were sulfonated to produce strongly acidic resins.

Typical strain birefringence patterns for prior art gel resins are illustrated in Figure II, A to D, which patterns have at least one of three identifying criteria: (a) a square superimposed upon a Maltese cross (see Figure II, A), (b) a sharp Maltese cross having narrow arms and bright regions between the arms, with or without an outer extinction ring (see Figure II, C and some beads in B), and (c) an irregular pattern, sometimes resembling a distorted cross (or swastika) and somtimes a square superimposed upon a cross resembling (a), above (Figure II, D).

The pattern in Figure II, B represents a sample of relatively. high quality prior art styrene/DVB gel resin containing about 8%'DVB in the copolymer backbone (sample of manufacturers regular product line). The pattern in Figure II, C represents a sample of relatively poor quality prior art styrene/DVB gel resin containing about 8% DVB, and exhibiting many surface defects and poor physical stability by both the microcycling and Chatillon Tests described herein (sample obtained from manufacturer's regular product line). Another poor styrene/DVB resin containing surface defects and bubbles and having low physical stability is illustrated in Figure II, D (manufacturer's commercial product). The sample from which the pattern of Figure II, A was produced was a styrene/8% DVB gel resin of intermediate prior art quality (manufacturer's normal commercial product). All resins illustrated in Figure II were strongly acidic and in the sulfonic acid form.

While individual beads in a given pattern in Figure I may have strain patterns nearly the same as patterns in Figure II, the products are readily distinguished overall. For example, a similarity may be seen between some individual beads in Figure I, A or B and Figure II, B but a substantial number of beads are dissimilar.

The resins of Figure I are highly superior to the resins of Figure II (even the best samples thereof) in the Chatillon test, perfect bead count and accelerated use testing (Microcycling Test).

On the basis of the above, and other studies of strain birefringence patterns, it is postulated, although we do not wish to be bound by this theory, that the differences in patterns between the new resins of the invention and those of prior art resins reflect different levels of residual- stress within the resin beads. Although the invention is not dependent upon any theory expressed herein, the patterns associated with the new resins are believed to represent conditions of low internal stress, whereas those patterns associated with the resins of Figure II, AND, are believed to reflect higher levels of internal resin stress. Since the stress which is responsible for the birefringence pattern is believed to be the residual stress within the bead, it seems logical that higher levels of stress correspond to poorer physical quality. Birefringence patterns therefore offer a simple qualitative method of identifying and distinguishing the products of this invention.

Copolymer precursors for cation and anion resins may also be distinguished from prior art copolymers on the basis of thermal analyses and solvent swelling characteristics. Since these copolymers result in improved resins, it is clear that the copolymers are improved in composition over the prior art copolymers.

Anion Resins Anion exchange resins produced by the improved copolymerization techniques described herein may also be distinguished from prior art anion resins by strain birefringence patterns which correlate with improved physical properties.

In general, it has been discovered that the anion resins are distinguished principally on the basis of differences in the intensity of the birefringence patterns rather than the differences in shape or nature of the patterns themselves. Consequently, the experimental conditions must be standardized as much as possible and a sample used as an intensity reference, in order to allow direct comparison of birefringence patterns from one day to the next. Normally, it is preferable to focus the microscope on the outer edge of the beads. Factors such as the intensity of the light source, radiation losses in the microscope, the position of.condensing lenses, the sensitivity of the film, and the exposure time greatly influence the overall intensity of the recorded image. However, for a given microscope, all of these factors are adequately reproduced and given a sample as an intensity reference, conditions from one day to another can be matched satisfactorily to allow direct comparison using photomicrographs.

The microscope and associated optics for obtaining birefringence patterns of anion resins were the same as those used for the cation resins. However, the swelling solvent in which the anion resins were examined was ethanol rather than water used for cation resins. Each anion resin was oven dried at 90--1000C for ca.

4 hours in vacuo, equilibrated overnight under ambient conditions, and then immersed in ethanol until swelling equilibrium had been achieved. All birefringence patterns of anion resins presented were obtained from samples in the chloride form which had been swollen in ethanol for at least 7 days.

All of the anion resins, including those produced by the novel oxygen moderated process described herein, exhibit patterns which may be described qualitatively as a broad Maltese cross having little or no extinction ring at the periphery of the bead. However, when compared to the prior art resins, the resins of this invention, swollen to equilibrium in ethanol, exhibit patterns that are significantly more intense (brighter). Intensity differences in water are more difficult to characterise.

In the accompanying drawings, Figure III, A-C illustrate the birefringence patterns of different samples of anion exchange resins of this invention at 34x magnification. The uniformity of pattern intensity and configuration is typical of anion resins of prior art product which overall correlates with physical stability inferior to the products of this invention, rather than as comprising both good (high brightness) and bad (low intensity) beads. The bright patterns for a few beads in those samples are indicative of high internal stress and are to be excluded from a comparison of the patterns.

The need for different interpretations for the birefringence patterns between cation and anion resins is thought to be a consequence of different inherent properties between the cation and anion resins under study, owing principally to compositional factors such as cross-linking uniformity and the level of primary cross-linking, which lead to differences in the relative contributions of swelling pressure and residual stress to the overall level of stress in the cation resins swollen in water as against the anion resins swollen in ethanol. The importance of backbone elasticity in the anion resins has, at least in part, been substantiated by thermal mechanical analysis (TMA) above the copolymer glass transition point where a secondary yield point has been detected for both cation and anion copolymers prepared by the novel copolymerization method of the invention. Residual stresses appear to be substantially less important in the anion resins since the broad Maltese cross patterns that typify both the resins of the improved technology and those of the prior art are not suggestive of highly stressed beads.

Although it may be theorized that elasticity plays an important part in the physical stability of both cation and anion resin stability it has not been independently characterized with cation resins whose main birefringence characterising features are differences in pattern types.

Reaction rate kinetic studies have indicated some moderation of the crosslinker (DVB) reaction rate when copolymerizing in the presence of oxygen compared to conventional methods leading to a presumption that the copolymer matrix may be more homogeneously cross-linked by the method of the invention and explaining the improved elasticity of the resins.

All of the resins represented by Figures III and IV contained low cross-linker levels typical of the most widely used styrene/divinyl benzene type anion exchange resins, that is, between about 2% and about 5% cross-linker (DVB).

In the cation resins (Figures I and II, above), about 75% to 90% of the resins used commercially have intermediate levels of cross-linker, that is 7 to 10% crosslinker (usually divinylbenzene, DVB). The most common anion resins, from a commercial standpoint, are those containing relatively low levels of cross-linker, that is, about 2 to 5% cross-linker. The improved products of the invention result with all levels of cross-linker, although they are principally illustrated herein with the most common amounts. The differences between the novel cationic resins disclosed herein and similar resins of the prior art may be less pronounced at lower cross-linker levels where internal residual resin stress is a less significant factor.

The improvement can nevertheless be ascertained when comparing the novel resins with the same type (and cross-linker content) resins of the prior art in either a relaxed or artificially stressed state (e.g. in swelling solvents).

The following - Examples illustrate some preferred embodiments of the invention.

Example I Polymerization Procedure The polymerization reactor is a two-liter, three neck, round bottom flask equipped with a two-blade paddle stirrer, thermometer, condenser, heating mantle with temperature controller, and provision for sweeping in a blanket of a blend of oxygen and nitrogen. (Oxygen concentration in the gas stream is monitored by gasliquid chromatographic (GLC) techniques, and in the monomer mix is checked by a "Beckman" (Registered Trade Mark) oxygen analyser).

A monomer phase containing initiator is charged to the reactor, and the head space is swept with an appropriate gas mixture (e.g. 2% by volume oxygen in nitrogen) until equilibrium is reached at 25"C. Then the aqueous phase is charged and the stirrer is set at about 210 rpm to produce droplets of monomer in aqueous dispersion, while the gas sweep is maintained. The following is a representative polymerization reaction material charge with amounts given in grams.

Styrene 489.4 Divinylbenzene (54.7% conc.) 85.3 Methylacrylate 8.8 "Percadox-16" initiator (di(4-t-butycyclohexyl) peroxydicarbonate) 2.04 Water 510.3 "Padmac A" dispersant poly(diallyl dimethyl ammonium chloride) 20.1 "Pharmagel" protective colloid (gelatin) 1.6 Boric acid 0.88 Sodium nitrate 0.59 Sodium hydroxide solution (50% conc.) added to pH ll0.5 The oxygen-nitrogen gas sweep is passed at 140 cc/min over the dispersion as it is heated from 25"C to 570C in 45 minutes, then maintained at 57+20C for 7 hours.

The batch is then heated to 750C over a 30 minute period and held at 750C for one hour. The copolymer beads are washed and excess water is removed by vacuum filtration on a Buchner funnel.

Sulfonation of Copolymer A portion of the wet polymer beads prepared as set out above (110 gms) is added to 600 grams of 95% H2SO4 in a one liter flask equipped with stirrer, condenser, dropping funnel, thermometer, caustic scrubber and heating means.

Thirty grams of ethylene dichloride (bead swelling agent) are added, and the suspension is heated from 30"C to 120"C over a 3 hour period. This is followed by a hydration procedure in which water is added to quench the product. The polymer beads are transferred to a backwash tower and backwashed to remove residual acid. The resulting ion exchange resin product is characterized by the following properties: Whole beads 99% Cracked beads 2% Fragmented beads 1% Perfect beads 97% Friability: Chatillon value, g/bead 2139 Solids, H+ form 44.7% Solids, Na+ form 51.5% Salt Splitting Cation Capacity, meq./g dry 5.21 Examples 2 to 4 and Comparative Tests A and B Further cross-linked styrene copolymers are prepared as above with variations in the oxygen concentration in the reactor head space, then sulfonated as above to yield ion exchange resins, the properties of which are compared to commercial sulfonated resins made from copolymers prepared without oxygen addition during polymerization. In Table 1, the resins of this invention are designated as A, B and C.

TABLE 1 Oxygen Chatillon, Microcycling Stability* Example Resin Level % g/bead Before** After 2 A 8 2150 97/3/0 96/4/0 3 B 8 2360 98.5/1.5/0 98.5/1.5/0 4 C 4 2300 100./0/0 98/2/0 Comparative Commercial Test A Resin A - 300 72/26/2 49/46/5 Comparative Commercial Test B Resin B - 510 98.5/1.5/0 55/42/3 Notes *100 cycles with 1 N HCI and 0.5 N NaOH solutions.

**Perfect/Cracked/Fragmented.

Examples 5 to 7 and Comparative Tests C and D Further cross-linked styrene copolymers are prepared in accordance with Example 1 but instead of sulfonation are chloromethylated and aminated in conventional manner to form strong base anion exchange resins, the properties of which are compared to commercial resins having the same functional groups and made from copolymers prepared without oxygen during polymerization. In Table 2, the resins of this invention are designated as D, E and F.

TABLE 2 Anion Exchange Appearance* Capacity, Initial Chatillon, after Example Resin meq/gm appearance* % Solids gm/bead Microcycling 5 D 4.32 97/1/2 47.8 698 - 6 E 4.34 98/1/1 48.9 768 - 7 F 4.44 99/1/0 46.7 1022 - Comparative Commercial Test C Resin C 4.20 93/7/0 46.0 140 5/90/5 Comparative Commercial Test D Resin D 4.40 94/5/1 42.5 400 79/21/0 *Appearance - Bead Count: Perfect/Cracked/Fragmented.

Examples 8 to 21 Using polymerization procedure similar to that of Example 1, further copolymers were prepared and they were subsequently functionalized to produce strong acid cation and strong base anion resins. Using the same reactor set-up as described in Example 1, a monomer phase (represented by "A" below) containing an initiator was charged to the reactor and either the monomer was previously saturated with oxygen or the reactor bead space was swept with an oxygencontaining gas, e.g. 8% by volume O2 in nitrogen, until equilibrium was reached at 25"C (typically 30 minutes). The aqueous phase ("B" below) was then charged (monomer phase:aqueous phase weight ratio=1.1:1.0) and the stirrer was set at about 210 rpm to produce droplets of monomer in aqueous dispersion. When an oxygen-nitrogen gas sweep was used it was passed over the dispersion at 140 cc/min for the remainder of the reaction. Alternatively a pressure of about 5 to 15 psig of the gas was used.

The following were representative reaction material charges in parts by weight per hundred of each solution.

Solution A (Monomer Phase) (a) Styrene 83.6 (b) Divinylbenzene (54.7% conc.) 14.6 (8.0 active) (c) Methylacrylate 1.5 (d) Di(4-t-butylcyclohexyl)-peroxydicarbonate: Percadox-16 (initiator) 0.35 (e) t-Butyl peroctoate (chaser) Solution B (Aqueous Phase) (a) Water 95.3 (br Poly(tetraalkyl ammonium chloride) (dispersant) 3.75 (c) Gelatin (protective colloid) 0.30 (d) Boric acid 0.16 (e) Sodium nitrate 0.11 (f) Sodium hydroxide solution (50% conc.) added to pH 10.5-11.0 0.2-0.4 The reaction mixture was heated from 25"C to 570C in 45 minutes and maintained at 57+20C for 7 hours. The batch was then heated to 750C over a 30 minute period and held at 750C for one hour (chase step); if a coinitiator was used as a chaser, e.g. t-butyl peroctoate (tBP), the batch was then heated to 950C over a 30 minute period and held at 95"C for one hour (final chase step). The batch was then cooled and the copolymer beads were washed and excess water was removed by vacuum filtration on a Buchner funnel. The specific reaction conditions and final product properties of the resulting styrene/DVB resins are summarized in Table 3, where cross-linker content is expressed as the percent 'active' cross-linker ingredient, and other monomer components of the commercial grade cross-linker (principally ethyl vinyl benzene) are calculated as part of the monovinyl monomer.

Cross-linker content given elsewhere herein and in the claims is also calculated on an 'active' basis. Further, all ion exchange resin test values given herein and in the claims, are related to fully functionalized copolymers, that is to resins of high capacity and reasonable commercial quality.

TABLE 3 Final Product Properties (H+Form) Initiator Reaction Reaction Oxygen Addition Cross- Other Microcycling Resin Percadox- Hold Time Monmomer Reaction linker Monomers Appearance Chatillon Test (after Designa16(%) Temp( C) Hours Sparge Sweep DVB (%) An*% Perf/Cr/Fr (G/bead) Example tor 100 cycles) Cation Resins: 8 G 0.45 63 5 Air 8% O2 8.0 0 100/0/0 2038 99.5/0.5/0 9 H 0.35 63 6 8% O2 8% O2 8.0 0 100/0/0 1480 99/1/0 10 I 0.45 63 5 8% O2 N2 8.0 0 98/2/0 1368 98/2/0 11 J 0.35 62 6.5 Air 8% O2 8.5 0 98/2/0 1757 96.5/3.5/0 12 K 0.40 62 6 Air 8% O2 8.5 0 99/1/0 2019 98.5/1.5/0 13 L 0.45 63 4 Air 8% O2 10.0 100/0/0 2100 98.5/1/0.5 14 M 0.35 63 5.5 8% O2 10.0 0 99/1/0 2646 99/1/0 15 M' 0.35 63 6.0 8% O2 8% O2 8.0 - 100/0/0 3100 15 psig Anion Resins Cl Form 16 N 0.45 55 11 8% O2 4% O2 4.25 0.0 99/1/0 973 15 psig 17 O 0.45 58 8.5 8% O2 N2 3.8 2.0 99/1/0 740 94/6/0 18 P 0.45 58 8.5 8% O2 8% O2 3.8 2.0 100/0/0 771 94/6/0 15 psig 19 O 0.45 58 8.5 8% O2 8% O2 3.5 2.0 99/1/0 780 20 R 0.45 58 8.5 8% O2 N2 3.5 2.0 95/4/1 611 83/16/1 3.8 2.0 98/2/0 1168 21 S 0.45 58 9.0 8% O2 8% O2 15 psig *AN=acrylonitrile Additional Procedure (Macroporous Resins) In a manner similar to the process described above in the "polymerization procedure", macroreticular copolymers were prepared and functionalized to produce strong acid cation exchange resins. The polymerization process used, apart from the oxygenation in accordance with this invention, was according to known techniques.

Example 22 The monomer phase consisted of styrene 387.0 g. divinylbenzene 97.4 g. methylisobutylcarbinol 215.6 g. percadox- 16 2.18 g. t-butyl peroctoate 0.49 g.

The aqueous phase consisted of water 77.0 g. gelatin 3.0 g. boric acid 3.17 g. padmac A 27.9 g. sodium chloride 23.1 g.

Example 23 The monomer phase consisted of styrene 369.0 g. divinylbenzene 92.9 g. methylisobutylcarbinol 238.0 g. percadox-16 2.08 g. t-butylperoctoate 0.51 g.

The aqueous phase consisted of water 734.0 g. gelatin 2.24 g. boric acid 3.17 g. padmac A 40.0 g.

Upon completion of the copolymerizations using the aforementioned conditions, the copolymerization mixtures were heated slowly to 1000C to remove methylisobutylcarbinol. The copolymer beads were then washed and dried prior to sulfonation.

Sulfonation Procedure In a manner similar to that described in the "Sulfonation of Copolymer", the two macroreticular copolymers prepared above were also sulfonated; dried copolymer (100 g) is added to 98 percent H2SO4 (615 g), followed by ethylene dichloride (35 g). The stirred mixture is heated to 1220C in 65 minutes and held at 122"C for one hour. This is followed by hydration procedure in which water is added to quench the product. The quenched product is treated with 80 g of 50 percent NaOH to convert to the sodium salt.

Claims (33)

1. Macroreticular Strong Acid Cation Exchange Resins Examples Chatillon Percent cation exchange (11% DVB) (G/bead) solids capacity (meq/gm) control 365 "12 5055 4.44.5 (typical values)

   22 508 55.4 4.47

   23 651 48.9 4.50 WHAT WE CLAIM IS: 1. Ion exchange resin beads comprising functionalized cross-linked gel copolymer beads containing less than 50% by weight units of polyethylenically unsaturated cross-linking monomer, 0 to 5% by weight units of non-aromatic monomer and at least 50% by weight units of monoethylenically unsaturated monomer, said beads having a birefringence pattern substantially as depicted in any one of Figure 1A, B, C or D when the beads are cationic and swollen in water and substantially as depicted in any one of Figure III A, B or C when the beads are anionic and swollen to equilibrium in ethanol.
2. 2. Cation exchange resin beads as claimed in Claim 1, wherein the copolymer contains units of styrene and divinyl benzene, containing 4 to 12% by weight divinyl benzene units, and which beads exhibit strain birefringence patterns substantially as described by one or more of the following patterns: (a) a broad-armed Maltese cross enclosed by an extinction ring around the periphery of the bead, (b) a broad-armed Maltese cross enclosed by an extinction ring that is distinctly inside and separate from the periphery of the bead, and (c) an irregular pattern defining a randomly oriented chain or recogniseable as a distorted version of (b).
3. 3. Cation exchange resin beads as claimed in Claim 2 which exhibit a strain birefringence pattern which appears substantially as a broad-armed Maltese cross enclosed by an extinction ring around the periphery of the bead.
4. 4. Cation exchange resin beads as claimed in Claim 2 which exhibit a strain birefringence pattern which appears substantially as a broad-armed Maltese cross enclosed by an extinction ring that is distinctly inside and separated from the periphery of the bead.
5. 5. Cation exchange resin beads as claimed in Claim 2 which exhibit a strain birefringence pattern which appears substantially as an irregular pattern defining a randomly oriented chain or recognizable as a distorted broad-armed Maltese cross enclosed by an extinction ring that is distinctly inside and separated from the periphery of the bead.
6. 6. Cation exchange resin beads as claimed in Claim 2 containing divinyl benzene units in an amount of 7 to 10% by weight of the copolymer.
7. 7. Cation exchange resin beads as claimed in any preceding claim (1) of which at least 90% are perfect beads, by count, (2) which have a Chatillon value of at least 900 gm/bead and (3) which exhibit a loss of perfect bead count of not more than 10% upon 100 cycles by the Microcycling Test.
8. 8. Cation exchange resin beads as claimed in Claim 7 which exhibit a loss of perfect bead count of not more than about 5% after 100 cycles by the Microcycling Test.
9. 9. Cation exchange resin beads as claimed in any preceding claim which have a Chatillon value of at least 1200 gm/bead.
10. 10. Cation exchange resin beads with sulfonic acid functionality comprising cross-linked resin beads (1) of which at least 95% are perfect beads, by count, (2) which have a Chatillon value of at least 900 gm/bead and (3) which exhibit a loss of perfect bead count of not more than 5% upon 100 cycles by the Microcycling Test.
11. 11 Cation exchange resin beads as claimed in Claim 10 which have a Chatillon value of at least 1200 gm/bead.
12. 12. Cation exchange resin beads as claimed in Claim 10 in respect of which the perfect bead count is at least 98% and which exhibit a loss of perfect bead count of not more than 2% upon 100 cycles by the Microcycling Test.
13. 13. Anion exchange resin beads as claimed in Claim 1, wherein the copolymer contains units of styrene and divinyl benzene, containing 1 to 10% by weight of divinyl benzene units and resin beads exhibit a strain birefringence pattern which appears substantially as a broad Maltese cross of relatively high intensity when the beads are swollen to equilibrium in ethanol.
14. 14. Anion exchange resin beads as claimed in Claim 13 containing 2 to 5% by weight of divinyl benzene units.
15. 15. Anion exchange resin beads as claimed in Claim 13 or 14, wherein (1) of which at least 90% are perfect beads, by count, (2) which have a Chatillon value of at least 600 gm/bead and (3) which exhibit a loss of perfect bead count of not more than 30% upon 100 cycles by the Microcycling Test.
16. 16. Anion exchange resin beads as claimed in Claim 15 of which at least 93% are perfect beads by count.
17. 17. Anion exchange resin beads as claimed in Claim 15 or 16 which have a Chatillon value greater than 1100 gm/bead.
18. 18. Anion exchange resin beads as claimed in Claim 15 which exhibit a loss of perfect bead count of not more than 10% upon 100 cycles by the Microcycling Test.
19. 19. Anion exchange resin beads as claimed in Claim 15 which exhibit a loss of perfect bead count of not more than 5% upon 100 cycles by the Microcycling Test.
20. 20. Anion exchange resin beads with quaternary'ammonium functionality (I) of which at least 90% are perfect beads, by count, (2) which have a Chatillon value of at least 600 gm/bead and (3) which exhibit a loss of perfect bead count of not more than 30% upon 100 cycles by the Microcycling Test.
21. 21. Anion exchange resin beads as claimed in Claim 20 of which at least 98% are perfect beads by count.
22. 22. Anion exchange resin beads as claimed in Claim 20 having a Chatillon value of greater than 100 gm/bead.
23. 23. Anion exchange resin beads as claimed in Claim 20 which exhibit a loss of perfect bead count of not more than 3% upon 100 cycles by the Microcycling Test.
24. 24. Ion exchange resin beads as claimed in Claim I substantially as described in any of the foregoing Examples.
25. 25. A process of preparing hard, cross-linked, discrete copolymer beads by free-radical polymerization in an aqueous dispersion of a monomer mixture comprising at least 50% by weight of monoethylenically unsaturated monomer and less than 50% by weight of polyethylenically unsaturated cross-linking monomer having at least two active ethylenically unsaturated groups, wherein the polymerization, at least until the gel point is reached, is conducted at a temperature of 30 to 950C with oxygen dissolved in, and/or being swept over, the monomer mixture.
26. 26. A process as claimed in Claim 25, wherein the reaction temperature is from 50 to 70 C.
27. 27. A process as claimed in Claim 26, wherein the mono-ethylenically unsaturated monomer is mono-ethylenically unsaturated aromatic monomer.
28. 28. A process as claimed in Claim 27, wherein the aromatic monomer is styrene and the cross-linking monomer is divinyl benzene.
29. 29. A process for producing ion exchange resin beads which comprises adding to copolymer produced by a process as claimed in any one of Claims 25 to 28 anion or cation exchange functional groups.
30. 30. A process as claimed in Claim 25 or 26, wherein the monomer mixture is saturated with air before the polymerization is begun.
31. 31. A process as claimed in Claim 30, wherein an oxygen-containing gas is swept over or through the monomer mixture during polymerization and the gas contains less than 9% oxygen by volume.
32. 32. A process as claimed in any one of Claims 29 to 31 as applied to the production of ion exchange resin beads as claimed in any one of Claims 1 to 24.
33. 33. Ion exchange resin beads whenever prepared by a process as claimed in Claim 32.