CA1124446A - Ion exchange resins - Google Patents

Abstract

IMPROVED ION EXCHANGE RESINS
Abstract of the Disclosure Hard, infusible, discrete beads of crosslinked copolymer are prepared by free-radical catalyzed polymeri-zation of a monomer mixture in aqueous dispersion wherein oxygen is incorporated in the monomer mixture. Ion exchange resins having improved mechanical strength are obtained by attaching functional groups to the copolymer.

Description

This invention concerns an improved process for the preparation of crosslinked vinyl copolymers as discrete copolymer beads in aqueous dispersions using oxygen in the polymerization in a novel way. The invention also concerns the ion exchange ; resins having improved physical characteristics obtained by ap-pending conventional ion exchange functional groups to said co-polymers.
The techniques of preparing crosslinked vinyl copolymers in bead form (as precursors for conversion into ion exchange re-sins) 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 a major proportion, e.g., from 50 upwards to about 99.5 mole percent, normally 80 to 99~, of a monovinyl monomer, preferably, monovinyl aromatic monomers, e.g., styrene, vinyl toluene, vinyl naphthalene, ethyl vinyl benzene, vinyl chlorobenzene, chloromethyl styrene, and the like, with a minor proportion, e.g., of from about 0.5 up to 50 mole percent, preferably 1 to 20~, of polyvinyl compounds having at least two active vinyl groups polymerizable with the aforesaid monovinyl monomer to form a cross-linked, insoluble, infusible copolymer, for example, divinyl benzene, trimethylolpropane tri-methacrylate, ethylene glycol dimethacrylate, divinyl toluene, trivinyl benzene, divinyl chlorobenzene, diallyl phthalate, di-vinylpyridine, divinyltoluene, divinylnaphthalene, ethylene glycol diacrylate, neopentyl glycol dimethacrylate, diethylene glycol divinylether, bisphenol-A-dimethacrylate, pentaerythritol tetra-and tri-methylacrylates~ divinylxylene, divinylethylbenzene, di-vinyl 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 txicaxballylate, triallyl aconitr~te! txiallyl citrate, triallyl phosphate, N~N~methylenediacrylamide, N,NI-methylene dimethacrylamide~ N,N'-ethylenediacrylamide, trivinyl naphthalene, polyvinyl anthracenes and the polyallyl and poly-vinyl ethers of glycol glycerol, pentaerythritol, resorcinol and the monothio and dithio derivatives of glycols. The copolymer may also have incorporated therein polymerized units of up to about 5 mole % of non-aromatic vinyl monomers which do not effect the basic nature of the resin matrix, for example, acrylonitrile, methyl acrylate, butadiene and others known in the art.
The conventional conditions of polymerization used heretofore lea~ to crosslinked vinyl copolymers, which, when con-verted to ion exchange resins by attachment of functional groups thereto, have certain operational deficiencies resulting from physical weaknesses.
The practice of the present invention yields ion ex-change resins in which the polymer beads have greater mechanical ; strength and increased resistance to swelling pressures which are produced within a bead during acid/base cycling (i.e., osmotic shock). The greater mechanical strength of the beads manifests itself in improved resistance to physical breakdown from external forces such as weight of the resin column bed, high fluid flows and backwashingr Thus, the physically stronger ion exchange resins embodied herein are especially useful in treating fluid streams of high flow rates, for example, condensate polishing ap-plications in which resins of lesser quality are prone to mechan-ical breakdown and short life spans.
In the past, it has been the practice not to include oxygen during the preparation of crosslinked vinyl polymers used as the base matrix copolymer for ion exchange resins since oxygen presents a safety hazard and has been generally regarded as detri-mental to the properties of said polymer obtained by free-radical polymerization.

In accordance with this invention, the yinyl monomer~
cross-linking monomert and other optional monomer or monomers, are polymerized in an aqueous dispersion in the presence of a free-radical initiator and (1) in contact with an oxygen-contain-ing gaseous mixture, (2) using a pre-oxygenated monomer mixture or (3) both (1) and (2), advantageously within a range of reaction temperatures of from about 30 to about 95C., preferably 50 to 70C. Thus, in order to improve absorption of oxygen by the mono-mer mixture, it is generally preferred to employ polymerization temperatures somewhat below, e.g.,5-25~C., those normally used hereto-fore in suspension polymerization for similar products. Accor-dingly, the free-radical initiator used herein is one suitable for catalyzing polymerization at such temperatures, for example, such initiators as di(`4-t-butylcyclohexyl) peroxydicarbonate, di-cyclohexyl peroxydicarbonate, di-(sec-butyl) peroxydicarbonate, - di-(2-ethylhexyl) peroxydicarbonate, dibenzyl peroxydicarbonate, diisopropryl peroxydicarbonate, azobistisobutyronitrile), azobis-(2,4-dimethylvaleronitrile), t-butyl peroxypivalate, lauroyl peroxide, benzoyl peroxide, t-butyl peroctoate, t-butyl peroxy-isobutyrate, and the like. The amount of initiator employed is normally from about 0.1 to about 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 30-60C., to employ a second so-called "chaser catalyst"
which is active at higher temperatures in order to achieve higher yields of crosslinked vinyl polymer, for example, from about 0.05 to 0.1~, based on monomer weight of such initiators as benzoyl peroxide, t-butyl eeroctoate, t-butyl peroxyisobutyrate, and the like.
As mentioned above, the process of this invention in-volves contacting the monomer mixture with oxygen such that oxy-gen is absorbed by the monomer mixture at least until the poly-; - 4 -r . `'`~

~.~.2~ 6 merization reaches the gel point~ i~e., the point at which an infinite polymeric network occurs (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 are used to incorporate the oxygen in the monomer mix. For example, the head space above the reaction medium is 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 is passed thxough the head space during the reaction period. The gaseous mixture may contain as much as 20~ oxygen, however, for purposes of safety in the avoidance of 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% oxygen in the case of a styrene and divinylbenzene mixture. Since the absorption of oxygen by the monomer droplets depends not only on temperature and the partial pressure thereof in the gas in the head space, but also on the area of reaction medium exposed to said head space, the configuration of the kettle will determine whether it is advantageous to operate at atmospheric pressure or under in-creased pressures, 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 mix-ture is to sparge the gaseous mixture into the monomer mixture before and/or during polymerization.
The aqueous medium in which the polymerization is con-ducted in dispersion form will contain minor amounts of the con-ventional suspension additives, that is, dispersants such as xanthan gum (biosynthetic polysaccharide) r poly (dially dimethyl ammonium chloride), poly acrylic acid (and salts), poly acryl-amide, magnesium silicate, and hydrolyzed poly (styrene-maleic ~S7 .

anhydride); ~xotective colloids such as 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 crosslinked, high~molecular weight copolymers are recovered from the reactor as hard, discrete beads of particle size within the range of about 0.02 to 2 mm, average particle size being on the order of 0.2 to 1 mm. These copolymers are converted to ion exchange resins by functionalization according to known means, such functional groups ineluding sulfonamide, trialkylamino, tetraalkyl ammonium, earboxyl, earboxylate, sul-fonie, sulfonate, hydroxyalkyl ammonium, iminodiaeetate, amine oxide, phosphonate, and others known in the art. Funetionalizing reaetions whieh may be performed on vinyl aromatie eopolymers to produce ion exchange resins are exemplified by sulfonation with coneentrated sulfurie acid, chlorosulfonation with chlorosulfonic aeid followed by amination, reaetion with sulfuryl chloride or thionyl ehloride followed by amination, and ehloromethylation followed by amination. Ion exehange resins may be further de-lineated by the types: strong aeid eation, i.e., eontaining the ; groupings sulfonie (~S03H) or sulfonate (-SO3M, where M is usually an alkali metal ion); weak aeid eation, i.e., containing the car-boxyl (-CO2H) or carboxylate (-CO2M, where M is usually an alkali metal ion) groupings; strong base anion, i.e., containing the tetraalkyl ammonium grouping: -NR3X, where R is an alkyl or hy-droxy 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 uni~ue properties of the copolymers produced accor-ding to this invention are reflected in their different charac-teristics under thermal analysis and solvent swelling and when .

converted to ion exch~nge resins by the attachment of the afore-said 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 appli-cations. For exampler strongly acidic styrene-type resins fre-quently exhibit Chatillon values in the range of about 900 to about 5000 gm. force per bead, preerably 1200~5000, in contrast to resins derived from copolymers prepared by prior art polymeri-zation methods which have Chatillon values in the range of about 50 to 550 gm./head. Similarly, anion styrene-type resins of the invention exhibit Chatillon values of about 500 to 2500 prefer-ably 600-2500 and often in the 900-1500 range in contrast to resins derived from copolymers prepared by prior art methods which have typical Chatillon values of 25 to 400.
The improved gel ion exchange resins of the present invention~ particularly the most common commercial resins pro-duced from aromatic copolymers, can be easily distinguished from the prior art resins by one or more of various physical para-meters including (1) perfect bead count (fewer cracked and frag-mented beads~, (2) resin friability (Chatillon test), (3) resis-tance 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 follow.
IN THE DRAWINGS
.
Figure 1 illustrates the birefringence pattern of cation exchange resins made in accordance with the present invention, ~ 30 Figure 2 represents photomicrographs of birefringence ; pattern cation exchange resins made in accordance with several methods existing in the prior art;

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~ igure 3 represents photomicro~raphs of birefringence patterns of anion exchange resins prepared in accordance with the present invention except for ~igure 3D which is a comparison be-tween present resins and the prior art resins; and Figure 4 represents photomicrographs of anion exchange resins available from competitive prior art sources.
In some instances, prior art resins have exhibited high physical stability by one or more of tests (1)-(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 crosslinker, that is about 4-12%
usually 7-10% crosslinker (preferably divinylbenzene, DVB). The most common anion resins, from a commercial standpoint, are those containing relatively low levels of crosslinker, that is, about 1~10% usually 2-5% crosslinker. The improved products of the invention result with all levels of crosslinker, although illus-trated herein principally with the most common types. The dif-ferences between the birefringence patterns of the novel resins disclosed herein and similar resins of the prior art may be less pronounced at lower crosslinker levels where internal residual resin stress is a less significant factor. The improvement can ; nevertheless ~e ascertained when comparing the novel resins with the same type (and crosslinker 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 thR 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 this invention contain at least 90% or more ~' of perfect beads, typically 23~99% perfect beads, by visual ob-servation and count. Prior art resins typically contain about 40-99~ perfect beads. However, many grades of commercial resins typically have perfect bead counts of only 40-50~ (e.g., see~
Figure II, C and D described below).
ACID/BASE C~CLING (~MICROCYCLING) TEST
Microcycling is designed to simulate on an accelerated time scale the conditions under which the resin will be used.
These studies are conduct~d over a period of a few days rather than months or years typical of field conditions. Repeated ex-haustion-regeneration cycles are performed on the resin at pre-determined 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 a mono-layer of beads are observed and the average result for each of '~ the following is recorded:
(a) % perfect beads (b) % cracked beads 2Q (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 conver-sion of the resin during each step. The solutions used for ex-haustion 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 _ NaOH

Cation 0.5 _aOH 1.0 N HCL

_ g _ ~ ~Y ~ '3 6 ; 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, a deionized water rinse followed by mild vacuum, and dropwise addition of regenerant solution over 10 minutes followed by re-moval of bulk regenerant by mild vacuum and a water rinse; com-pletion of the aforementioned process represents an exhaustion-regeneration cycle and requires approximately 30 minutes. Com-plete 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.
The product of the invention generally show a reduction of perfect bead count of less than about 30%, normally not more than about 15% after 100 cycles by the Microcycling Test. The cation resins exhibit less reduction of perfect beads, not more than 10%r and usually 0-5%. Anion resins may show reductions of up to 30%t normally 0-15%. By comparison, prior art cation resins " are known to exhibit reductions of from 15-80% most typically 30-50%. Anion resins available heretofore show perfect bead re-ductions of 15-80% after 100 cycles with 15-50 being most typical.
CHATILLON TEST FOR RESIN FRIABILITY
The Chatillon test is named for an apparatus manufac-tured by John Chatillon and Sons, New York, N. Y. and designed to measure resin friability. This instrument (MODEL LTCM, Gauge DPP-2.5KG) measures the force (grams) required to crack or frac-ture 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 indi-vidual resin beads under actual use conditions.
Specifications for testing include converting the resin into the proper form (hydrogen or sodium for cation resins .~ .~,.

tested hexein and chloride form for anion resins tested hexein) 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 - 10 average (20 beads minimum, typically 30 beads), a standard devia-tion, 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 the same from prior art counterparts 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 law mganification (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 30~50 beads of the sample resin to be analyzed are then placed in the concave well of a deep-dish micro-scope 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 defi-nition 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 the same with patterns of contemporary commercial .~.

- resins produced by the various manufacturers 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 re-sins shall be descr~ed 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 10 Chemistry, Vol. 44, No. 8, August 1952, pp. 1796-1800). We have now further identified a number of characteristic patterns and empirically correlated such with the physical properties of the resin including residual internal bead stress so as to obtain a qualitative identification of resin origin and properties. Migh internal residual 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 appended hereto while some of the most - widely used resins presently available from various manufacturers are illustrated by patterns A-D in Figure II. The photographs showing the birefringence patterns were made at essentially the same film exposure and sample illumination.
In general, the patterns showing the highly stressed products (and therefore those more susceptable to fracture or breakage) can be identified by the brightness and sharpness of the pattern as well as the pattern type. Referring to the appen-ded Figures, the more highly stressed beads are found in Figure II
` which contains sharper, brighter individual bead patterns on an overall basis. Note that this observation and other observations described herein are often made on 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 :~ - 12 ~

some of the commercial samples observed consist of composites or physical mixtures of materials produced under different condi-tions and therefore the patterns may illustrate the sensitivity of the product quality to variations in the process of prepara-tion.
A Maltese cross, or some variation thereof, is typic-ally observed in resin strain birefringence patterns and is in-dicative of spherically symmetric stress or 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 the arms.
Applying the foregoing general considerations, the strain birefringence patterns in Figures I and II can be distin-guished. The low stress cation resin products illustrated in Figure Ir 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 ~hat 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 ran-domly 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 r A thru C containing a ~altese 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~) co-polymer backbone using oxygen moderated copolymerization tech-niques described in the examples contained herein. 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 thru 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 sometimes 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 sy$rene/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 con-; taining about 8~ DBV, and exhibiting many surface defects and poor physical siability by both the microcycling and Chatillon - 14 _ ~ ~L~

Tests described herein (sample obtained from m~nufacturerls regu-lar 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 illus-trated 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, one may easily distin~uish the products on an overall basis.
To illustrate, a similarity may be seen between some 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 res-ins of Figure II (even the best samples thereof) from ; a standpoint of the Chatillon test, perfect bead count and accel-erated use testing (Microcycling Test).
On the basis of the above, and other studies of strain birefringence patterns, it is postulated 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 it is not intended that the invention is 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 asso-ciated with the resins of Figure II, A-D, 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 follows that higher levels of stress would be expected to correspond to poorer physical quality. Bire-fringence patterns therefore offer a simple quantitative method of identifying and distinguishing the products of this invention.

Copolymer precursoxs for the Cation and ~nion resinsmay also be distinguished from the prior art copolymers on the basis of thermal analyses ana 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.

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ANIOr~ RESIriS
Anion exchange resins produced by the improved copolymerization techniques described herein may also be distinguished from prior art anion resins by strain birefrin-gence patterns which correlate with improved physical properties. In general it has been discovered that the anion resins are distinguished principally on the basis of differ-ences in the intensity of the birefringence patterns rather than differences in the shape or nature of the patterns themselves. Consequently, the experimental conditions must be standardized as much as possible and a sample used as an lntensity 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 lossesin the microscope, the positicn 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 anotner can be matched satisfactorily to allow direct comparison using photomicrographs.
The microscope and asgociated optics for obtaining birefringence patterns of anion resi~s were the same as had been used for the cation resins. However, the swelling solvent in which the anion resins were examined was ethanol rather than the water used for cation resins. Each anion resin was oven dried at 90-100C for ca. 4 hours in vacuo, equilibrated overnight under ambient conditions, and then lmmersed in ethanol until swelling equilibrium had been ,: IB

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achieved. All birefringence patterns of anion resins presented were obtained from samples in the chlorlde form - which had been swollen in ethanol 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 signifi-cantly more intense (brighter). Intensity differences inwater are more difficult to characterize.
Figure III, A-C illustrate the birefringence ; patterns of different samples of the improved resins at 34X
magnification. The uniformity of pattern intensity and configuration is typical of anion resins by this invention.
Figure III, D illustrates a composite sample of the new resin (top of photo/bright) together with beads produced by prior art methods (bottom of photo/dull) without oxygen in the copolymerization process.

Figure IV, A-D illustrate prior art products pro-duced by four different manufacturers. All of the photo-micrographs of Figure III and IV were made at the same exposure under conditions controlled, as explained hereto-fore, to allow comparisons of the pattern intensities. Eventhe low-intensity patterns of Figure IV generally exhibit broad Maltese crosses, with a few beads showing strong stress patterns characteristic of the prior art cation resins.
The dominating characteristic of anion resins does 3 not appear to be the residual internal stress in the resin ' . , .

.
beads as is the case with cation resins. However, the intensity of the broad r~altese cross birefringence patterns correlates directly and consistently with physical stability of the resin, the brighter patterns serving as "fingerprints"
of the most physically stable resins. Whereas the dominating feature of cation resins was postulated to be the internal bead stress, applicant believes the greater swelling pressure that sustains in superior anion resins is evidence of a greater elastic component associated with the crosslinked gel network, which makes it possible for the network to better acoo~date an externally applied stress without craze or crack formation. The more intense patterns of the new resins swollen to equilibrium in ethanol is indicative of a higher swelling pressure for the new resins.
It is recognized, of course, that certain selected samples representing the extremes in pattern intensity for new and prior art resins may be difficult to distinguish.
Also, a given bead in a pattern may deviate substantially from the overall pattern of a prior art sample. However, based upon data for a substantial number of products exam-ined, ambiguities in determining the quality of a particular product can be resolved by multiple analyses, preferably of different lots of the same product. In some cases, both for cation and anion resins of the p~ior ar~, the overall patterns showing wide divergence in patterns and/or intensity - are indicative of the quality of the overall product rather than the presence of both good and poor quality beads within a single sample. To illustrate,the birefringence pattern~ in Figure IV, B and D, should be interpreted as "fingerprinting"
a typical prior art product which overall correlates with ':
B -20_ : .

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physical stability inferior to the products of this inven-tion, 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 principal]y to compositional factors such as crosslinking uniformity and the level of primary crosslinking 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 vs. 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 copolyrners prepared by the novel oxygen copolymerization method. 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

4 ~

characterized with cation resins whose main "fingerprints"
are differences in pattern types.
Reactlon rate kinetic studies have indicated some moderation of the crosslinker (DVB) reaction rate when using oxygen-copolymerization compared to conventional me~thods leading to a premise that the copolymer matrlx may be more homogeneously crosslinked by the method of the inven-tion, and explaining the improved elasticity of the resins.
All of the resins represented by Figures III and IV contained low crosslinker levels typical of the most widely used styrene/divinyl benzene type anion exchange resins, that is, between about 2% and ~bout 5~ crosslinker (DVB).
In the cation resins (Figures I and II, above), about 75% to 90% of the resins used car ~ cially have inter-; mediate levels of crosslinker, that is 7-10% crosslinker (usually divinylbenzene, DVB). The most common anion resins, from a commercial standpoint, are those containing relatively low levels of crosslinker, that is, about 2-5~ crosslinker.
The improved products of the invention resuit with all levels ; of crosslinker, although illustrated herein principally with the most common types. The differences between 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 as-certained when comparing the novel resins with the same type (and crosslinker content) resins of the prior art in either a relaxed or artificially stressed state (e.g., in swelling solvents).

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The process of the inventiPn is clarified by the following illustrative examples which are not to be construed as limitative thereof.
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 concen-tration in t;he gas stream is monitored by gas-liquid chro-matographic (GLC) techniques, and in the monomer mix is checked by a Beckman oxygen analyzer).
The monomer phase containing initiator is charged to the reactor, and the head space is swept with an appro-priate gas mixture (e.g., 2% oxygen in nitrogen) until equilibrium is reached at 25C.~ Then the aqueous phase is charged and the stirrer is set at about 210 rpm to produce the droplets of monomer in aqueous dispersion, while the gas sweep is maintained. The following is representative poly-merization reaction materials charge in grams.
Styrene 489.4 Divinylbenzene (54.7% conc.) 85.3 ~ Methyl acrylate 8.8 ; "Percadox-16" initiator ~ 2.04 (di(4-t-butycyclohexyl) peroxydicarbonate) Water 510.3 **
"Padmac A" dispersant20.1 poly(diallyl dimethyl 3 a.nmonium chloride) .~ ***
"Pharmagel" protective1.6 c~lloid (gelatin) Boric acid o.88 .
. * 'rradOEk ** Trademark *** Trademark -23-.~ .

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Sodium nitrite 0.59 Sodium hydroxide solution (50 conc.) added to pH lO - 10.5 The oxygen-nitrogen gas sweep is passed at 140 cc/
min. over the dispersion as it is heated from 25C to 57C.
in 45 minutes, then maintained at 57 + 2C. for 7 hours. The batch is then heated to 75C. over a 30 minute period and held at 75C. 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 above (llO gms.) is added to 600 grams of 95~ H2S04 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 30C to 120C 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 ` 20 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 ~i Solids, H+ form44.7~
Solids, Na+ form51.5%
;~ Salt Splitting Cation Capacity, meq./g dry 5.21 At ~ _ ~6 Additional crosslinked sytrene copolymers are prepared as above with variations in the oxygen concentra-tion in the reactor head spaceS 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 the following table, the resins of this invention are desig-nated as A, B and C.
Oxygen Chatillon, Microcycling Stability*
Resin Level % g/bead Before** After 8 2150 97/3/0 96/4/o B 8 2360 98.5/1.5/0 98.5/1.5/0 C 4 2300 100/0/0 98/2/o Commercial Resin A - 300 72/26/2 49/46/5 15 Commercial Resin B - 510 98.5/1.5/0 55/42/3 Notes:
*100 cycles with lN HCl and 0.5N NaOH solutions.
**Perfect/Cracked/Fragmented.
Other crosslinked styrene copolymers are prepared in accordance with this invention using oxygen incorporation, then chloromethylated and aminated in a 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 addition during polymerization. In the following table, the resins of this invention are designted as D, E
and F.

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Additional Specific Examples In a manner similar to the process described above in the "Polymerization Procedure" additional copolymers were prepared and functionalized to produce strong acid cation and strong base anion resins. Using the same reactor set-up as described above 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 swept with an oxygen-containing gas, e.g., 8% 2 in nitrogen, until equilibrium was reached at 25C. (typically 30 minutes). The aqueous phase ("B" below) was then charged (monomer : aqueous ratio = 1.1 : 1.0) and the stirrer was set at about 210 rpm to produce droplets of monomer in aqueous dispersion. The oxygen-nitrogen gas ` 15 sweep, if any, was passed over the dispersion at 140 cc/min ;~ for the remainder of the reaction (alternatively a pressure - of about 5-15 psig was used).
The following were representative reaction material charges in parts per hundred of each solution.
Solution A (Monomer Phase) (a) Styrene 83.6 (b) Divinylbenzene (54.7% conc.) 14.6 (8.o active) (c) Methylacrylate 1.5 (d) Di(4-t-butylcyclohexyl)-peroxydicarbonate:Percadox-16 (initiator) 0.35 (e) t-But;yl peroctoate (chaser) ~ .
Solution B (Aqueou`s Phase) (a) Water 95.3 3o (b) Poly~tetraall~yl ammonium chloride)(dispersant) 3.75 (c) Gelatin (protective colloid) 0.30 r~L~

(d) Boric acid 0.16 - (e) Sodium nitrite 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 25C to 57C in 45 minutes and maintained at 57 + 2C for 7 hours.
The batch was then heated to 75C over a 30 minute period and held at 75C 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 95C over a 30 minute period and held at 95C 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 styrene/DVB resins are summarized in the following table, where crcsslinker content is expressed as the percent "active" crosslinker ingredient, and other ~; monomer components of the commercial grade crosslinker (principally ethyl vinyl benzene) are calculated as part of the monovlnyl monomer. Crosslinker content given else-where 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, relate to fully . 25 functionalized copolymers, that is to resins of high capa-city and reasonable commercial quality.

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ADDITIONAL PROCEDURE (I~ACROPOROUS RESINS) - In a manner similar to the process described above in the "Polymerization" procedure, macroreticular copolymers were prepared and functionalized to produce strong acidation exchange resins.
Example I
, . The monomer phase consisted of styrene (387 g.), divinyl benzene (97.4 g.), methylisobutylcarbinol (215.6 g), Percadox-16 (2.18 g.) and t-butyl peroctoate (0.49 g). The ; 10 aqueous phase eonsisted of water (770 g.), gelatin (3.0 g.), borie acid (3.17 g.), Padmac A (27.9 g.), and sodium ehloride (23.1 g.).
,. .
. EXAMPLE II
The monomer phase consisted of styrene (369 g.), , divinylbenzene (92.9 g.), methylisobutylearbinol (238 g.), 'r Percadox-l6 (2.08 g.), and t-butylperoetoate (0.51 g.).

Upon eompletion of the copolymerizations using the aforementioned conditions, the copolymerization mixtures were heated slowly to 100C. to remove methyl isobutyl carbinol, and the eopolymer beads were then washed and dried prior to sulfonation.
SULFONATION PROCEDURE
In a manner similar to that deseribed in the "Sulfonation of Copolymer" the two macroreticular copolymers prepared above were also sulfonated. Dried eopolymer (100 g.) is added to 98~ H2SO4 (615 g.) followed by ethylene diehloride (35 g.). The stirred mixture is heated to 122C. in 65 minutes and held at 122C. for one hour. This is followed by a hydration proeedure in whieh water is added to queneh the produet and the quenched product is treated with 80 g. of 50%
NaOH to convert to the sodium salt.

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MACRORETICULAR STRONG ACID CATION EXCHANGE RESINS
:, EXAMPLES CHATILLON CATION EXCHANGE
(llPCT. DVB) (G/BEAD PCT. SOLIDS CAPACITY (MEQ/GM) Control (Typical 36S-412 50-55 4.4 - 4.5 values) I 508 55.4 4.47 ~ II 651 48.9 4.50 .` 10 `:

Claims (34)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Ion exchange resin beads having improved mechanical strength and resistance to swelling pressures comprising functionalized crosslinked gel copolymer beads of a minor amount of a polyvinyl monomer, up to 5 mole %
of a non-aromatic monomer and the remainder a monovinyl aromatic monomer, said beads having a birefringence pattern corresponding predominantly to one of Figure I A, B, C or D
of the drawings when the beads are cationic and swollen in water and corresponding predominantly to one of Figure III A, B or C of the drawings when the beads are anionic and swollen to equilibrium in ethanol.

2. The improved ion exchange resin beads of claim 1 wherein the copolymer is a styrene/divinyl benzene gel-type containing about 4 - 12% by weight divinyl benzene and the functionalized resin derived therefrom is cationic and exhibits a strain birefringence pattern which predomi-nantly consists of at least one pattern selected from:
(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 separated from the periphery of the bead, and (c) an irregular pattern defining a randomly oriented chain or recognizable as a dis-torted version of (b).

3. The improved cation exchange resin of claim 2 wherein the resin exhibits a strain birefringence pattern which predominantly consists of a broad-armed Maltese cross enclosed by an extinction ring around the periphery of the bead.

4. The improved cation exchange resin of claim 2 wherein the resin exhibits a strain birefringence pattern which predominantly consists of a broad-armed Maltese cross enclosed by an extinction ring that is distinctly lnside and separated from the periphery of the bead.

5. The improved cation exchange resin of claim 2 wherein the resin exhibits a strain birefringence pattern which predominantly consists of 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. The improved cation exchange resin of claim 2 wherein the divinyl benzene component of the copolymer is 7 - 10% by weight of the copolymer.

7. The improved cation exchange resin of claim 6 wherein the resin has (1) at least 90% perfect beads, by count (2) a Chatillon value of at least 900 gm/bead and (3) a loss of perfect bead count of not more than 10% upon 100 cycles by the Microcycling Test.

8. The improved cation exchange resin of claim 2 wherein the resin has (1) at least 90% perfect beads, by count (2) a Chatillon value of at least 900 gm/bead and (3) a loss of perfect bead count of not more than 10% upon 100 cycles by the Microcycling Test.

9. The improved cation exchange resin of claim 6 wherein the resin exhibits a loss of perfect bead count of not more than about 5% after 100 cycles by the Microcycling Test.

10. The improved cation exchange resin of claim 6 wherein the resin has a Chatillon value of at least about 1200 gm/bead.

11. Improved cation exchange resin beads with sulfonic acid functionality having higher mechanical strength and resistance to swelling pressures comprising crosslinked resin beads with (1) at least 95% perfect beads, by count, (2) a Chatillon value of at least 900 gm/bead and (3) a loss of perfect bead count of not more than 5% upon 100 cycles by the Microcycling Test.

12. The improved cation exchange resin beads of claim 11 wherein the crosslinked resin beads have (1) at least 95% perfect beads, by count, (2) a Chatilion value of at least 1200 gm/bead and (3) the loss of perfect bead count is not more than 5,0 upon 100 cycles by the Microcyclin, Test.

13. The improved cation exchange resin beads of claim 11 wherein the (1) perfect bead count is at least 98%
(2) the Chatillon value is at least 1200 gm/bead and (3) the loss of perfect bead count is not more than 2% upon 100 cycles by the Microcycling Test.

14. The improved ion exchange resin beads of claim 1 wherein the resin is an anlon resin.

15. The improved anion exchange resin beads of claim 14 wherein the copolymer is a styrene/divinyl benzene-type containing about 1-10% by weight of divinyl benzene and the functionalized resin derived therefrom exhibits a strain birefringence pattern which predominantly is a broad Maltese cross of relatively high intensity when the beads are swollen to equilibrium in ethanol.

16. The improved anion exchange resin beads of claim 14 wherein the resin is a styrene/divinyl benzene-type containing about 2-5% by weight of divinyl benzene.

17. The improved anion exchange resin of claim 16 wherein the resin additionally has (1) at least 90% perfec beads, by count, (2) a Chatillon value of at least 600 gm/
bead and (3) a loss of perfect bead count of not more than 30% upon 100 cycles by the Microcycling Test.

18. The improved anion exchange resin of claim 17 whereln the resin contains at least 93% perfect beads.

19. The improved anion exchange resin of claim 17 wherein the resin has a Chatillon value greater than 1100 gm/bead.

20. The improved anion exchange resin of claim 17 wherein the loss of perfect bead count is not more than 10 upon 100 cycles by the Microcycling Test.

21. The improved anion exchange resin of claim 17 wherein the loss of perfect bead count is not, more than 5%
upon 100 cycles by the Microcycling Test.

22. Improved anion exchange resin beads with quaternary ammonium functionality having higher mechanical strength and resistance to swelling pressures comprising resin beads with (1) at least 90% perfect beads, by count, (2) a Chatillon value of at least 600 gm/bead and (3) a loss of perfect bead count of not more than 30% upon 100 cycles by the Microcycling Test.

23. The improved anion exchange resin beads of claim 22 wherein the resin contains at least 98% perfect beads.

24. The improved anion exchange resin beads of claim 22 wherein the resin has a Chatillon value of greater than 100 gm/bead.

25. The improved anion exchange resin beads of claim 22 wherein the loss of perfect bead count is not more than 3% upon 100 cycles by the Microcycling Test.

26. In the process of preparing hard, crosslinked, discrete copolymer beads by the free-radical polymerization in an aqueous dispersion of a monomer mixture comprised of a major proportion of monovinyl monomer and a minor propor-tion of a crosslinking monomer having at least two active vinyl groups, the improvement which comprises conducting the polymerization reaction at a temperature within the range of about 30 to about 95°C. with oxygen in contact with or dissolved in the monomer mixture, or a combination of both.

27. The process of claim 26 wherein the reaction temperature is from 50 to 70°C.

28. The process of claim 26 wherein the monovinyl monomer is a monovinyl aromatic monomer.

29. The process of claim 28 wherein the monovinyl aromatic monomer is styrene and the crosslinking monomer is divinyl benzene.

30. A process for producing an ion exchange resin having improved physical strength which comprises function-alizing the copolymer product produced according to the process of claim 26.

31. The process of claim 26 wherein the monomer mixture is saturated with air before polymerization.

32. The process of claim 31 wherein the monomer mixture is in contract with an oxygen-containing gas mixture comprising about 8% by volume of oxygen during polymerization.

33. The copolymer produced according to claim 26.

34. The copolymer produced according to claim 32.