PROCESS FOR PREPARING IMPROVED WEAK ACID RESINS AND POROUS WEAK ACID RESINS

Abstract: Weak acid type cation exchange resins are prepared by subjecting to caustic hydrolysis a copolymer comprising in polymerized form (1) a major amount of a monoethylenically unsaturated monomer which is hydrolyzable under caustic conditions; (2) a minor amount of a monoethylenically unsaturated monomer which has a less hydrolyzable character than the hydrolyzable monomer, and (3) an effective amount of a crosslinking monomer. For example, copolymers prepared from methyl acrylate, butyl acrylate and divinylbenzene. The resin beads so prepared exhibit increased resistance to osmotic shock.

Description: PROCESS FOR PREPARING IMPROVED WEAK ACID RESINS AND POROUS WEAK ACID RESINS The present invention relates to ion exchange resins, and in particular, to ion exchange resins which are acidic in character. Weak acid resins have been prepared by suspen- sion polymerizing and crosslinking unsaturated carboxylic acids such as acrylic acid. Unfortunately, the polymeri¬ zation of unsaturated carboxylic acids is highly exother¬ mic making reaction control very difficult. Thus, the physical properties of the final resin product are ~~ typically very poor in quality. Weak acid resins have been prepared by the post-hydrolysis of suspension polymerized, crosslinked acrylic acid esters. Unfortunately, such a process provides resin products which have undergone incomplete hydrolysis and exhibit poor physical strengths. Thus, such resins can have low exchange capacities and low resistance to osmotic shock. In view of the deficiencies of the prior art, it would be highly desirable to provide a weak acid ion exchange resin with a high operating capacity which can be prepared in a controlled manner and exhibits high resistance to osmotic shock during regeneration. The present invention is a process for pre¬ paring weak acid type cation exchange resins by sub¬ jecting to caustic hydrolysis a copolymer comprising in polymerized form (1) a major amount of a monoethylenic- ally unsaturated monomer which is hydrolyzable under caustic conditions; (2) a minor amount of a monoethylenic¬ ally unsaturated monomer which has a less hydrolyzable character than the hydrolyzable monomer; and (3) an effective amount of a crosslinking monomer; and option- ally a minor amount of a non-hydrolyzable monomer. As used herein the term "hydrolysis" refers to caustic hydrolysis. The term "caustic" is to be broadly construed to mean a strongly basic species. In another aspect, the present invention is the weak acid type cation exchange resin which is prepared using the process as described hereinbefore. The process of this invention provides the skilled artisan with a method for preparing weak acid type cation exchange resins exhibiting a high operating ion exchange capacity and improved osmotic shock proper¬ ties. Resin beads of this invention can have a porous character, and in particular, a macroporous character. Such resin beads can be subjected to an increased number of regeneration cycles without a substantial amount of cracking of said resin beads. Such beads are useful in a wide variety of applications which can include the demineralization of water for enhanced oil recovery, the removal of heavy metal ions from various aqueous streams, and the like. Polymers useful in the practice of this invention are crosslinked polymers formed by the addi¬ tion polymerization of at least one polymerizable hydrolyzable monoethylenically unsaturated monomer, at least one polymerizable monoethylenically unsaturated monomer which is less hydrolyzable in character than the hydrolyzable monomer, and at least one polymeriz¬ able unsaturated monomer capable of providing cross¬ linking to the polymer. Hydrolyzable monomers useful herein are monoethylenically unsaturated carboxylic acids such as alkyl acrylates. The choice of alkyl acrylate will depend upon the degree or ease of hydrolysis which is desired. In general, any alkyl ester of acrylic acid, itaconic acid, etc., which is readily hydrolyzable under caustic conditions can be employed. Preferred alkyl acrylates include ethylaer late, butylacrylate, hexylacrylate, and the like. More preferred alkyl acrylates are those wherein the alkyl moiety has from 1 to 3 carbon atoms, with methyl crylate being especially preferred. Non-hydrolyzable monomers which are option¬ ally useful herein include, for example, acr lonitrile, styrene, ethylbenzene, vinyl toluene, methylstyrene, vinylbenzyl chloride, and halogenated styrene, hetero- cyclic aromatics such as vinylpyridine and substituted vinylpyridines, vinyl acetate, vinyl chloride, vinyli- dene chloride, N-vinylpyrirolidone, and methacrylates which are not highly hydrolyzable under conditions which the hydrolyzable monomers are converted to carboxylic acid moieties. Monomers which are less hydrolyzable than the aforementioned hydrolyzable monomers include those alkylacrylates which contain an alkyl functionality which provides a less hydrolyzable character to the monomer than said hydrolyzable monomer. In particular, the less hydrolyzable monomer does not undergo hydroly- sis to any substantial degree under conditions which the hydrolyzable monomer undergoes hydrolysis. Prefer¬ red less hydrolyzable monomers typically have alkyl functionalities which have higher amounts of carbon atoms than those hydrolyzable monomers. For example, if methylacrylate is the hydrolyzable monomer, then ethylacrylate or butylacrylate can be employed as the monomer which is less hydrolyzable in character. Similarly, if ethylacrylate is employed as the hydrolyz¬ able monomer, then butylacrylate or hexylacrylate can be employed as the monomer which is less hydrolyzable in character. Crosslinking monomers are those monomers which can introduce crosslinking to the resulting polymer. Examples of such monomers are those useful in preparing ion exchange resins and are those polyvinyl crosslinking monomers such as divinylbenzene, divinyl- toluene, divinylxylene, and divinylnapthalene; ethylene glycol dimethacrylate; trimethylol propane triacrylate; divinylsuccinate; and the like. See, also, those disclosed in U.S. Patent No. 4,419,245 and U.S. Patent No. 4,444,961. The monomer composition which is employed can vary depending upon factors such as the density of the bead desired, the amount of physical stability which is desired, the ion exchange capacity which is desired, and the like. Typically, the amount of hydrolyzable monomer can range from 70 to 90 weight percent; the amount of less hydrolyzable and optional non-hydrolyz- able monomer can range from 1 to 20 weight percent; and the amount of crosslinking monomer can range from 4 to 12 weight percent based on all monomers. If desired, the hydrolyzable monomer portion can be polymerized in conjunction with a monomer such as acrylic acid. The process of this invention is preferably performed by suspension polymerizing the aforementioned monomers under conditions such that polymerization occurs. Suitable—solvents, surfactants, diluents, activators and catalysts are known in the art. See, for example, U.S. Patent No. 4,224,415. The resulting polymer product is isolated and subjected to hydrolysis conditions. For example, the polymer can be contacted with a hydroxide solution. The resin can be washed with an acid solution to yield the H-form resin. Beads can be obtained in a gel form or in a porous form. While beads having a wide range of pore sizes can be employed, beads having pore sizes greater than o 100 A (0.01 microns) in diameter are preferred. That is, porous beads of this invention include the macro- porous beads. Macroporous beads are those macroreticu- lar types of beads as are defined in U.S. Patent No. 4,224,415. For example, macroporous beads are prepared by those techniques described in U.S. Patent No. 4,382,124. The resin beads of this invention are prefer¬ ably spherical beads having particle sizes ranging from 180 to 2,000 microns, preferably from 200 to 1,000 microns. The distribution of the particle size can be narrowed by employing the appropriate stabilizer. Narrow distributions of particle sizes can be obtained by using the method taught in U.S. Patent No. 4,444,961. The resins of this invention exhibit good osmotic shock resistance as well as high ion exchange capacity. It is believed that the copolymerization of the hydrolyzable monomer with the non-hydrolyzable monomer or less hydrolyzable monomer causes the cross¬ linking in the bead to be less centralized. The more even spread of crosslinking in the bead is believed to provide a stronger product which exhibits less potential to crack or break during repeated shrinking and~swelling. The following examples are presented to further illustrate but not limit the scope of this invention. All parts and percentages are by weight, unless otherwise noted. Example 1 Into a jacketed stainless steel reactor was charged an aqueous solution comprising 1,500 grams (g) distilled water, 2.4 g carboxymethyl methylcellulose and 2.7 g sodium dichromate. To this solution was charged an organic mixture containing 763 g methyl- acrylate, 105 g n-butylacrylate, 200 g divinylbenzene, 100 g isooctane diluent, 0.5 g tertiary butylperoctoate and 0.9 g tertiary butyl perbenzoate. The reactor was purged with nitrogen for 15 minutes. The mixture was then agitated and the polymerization was conducted at 75°C for 5 hours, followed by 110°C for 3 hours. The reaction mixture was cooled. The spherical polymer product was- filtered and washed with distilled water. The diluent was removed by steam distillation. The polymer product was mixed with a 20 percent active aqueous sodium hydroxide solution at 100°C for 5-6 hours. The polymer product was filtered and washed with 1 normal hydrochloric acid. The final resin exhibits a bulk density in the acid form of 0.83 g/ml. The wet volume of the resin is 4.5 meq/ml. The water retention capacity of the resin is 51.6 percent. The osmotic shock resistance of the sample is high. After 500 cycles of treatment of the resin beads with alternating 5 percent hydrochloric acid and 5 percent sodium hydroxide solutions, 100 percent of the beads are left whole. The operating capacity of the resin is 47 Kg/ft 3 of resm. Other properties of resms which were similarly prepared (designated as Sample No;- 1-) are presented in Table I. Table I Wet Volume Water Osmotic Capacity Retention Shock (% Sample (meq/ml) Capacity (% ) ' Expansion whole bead) 1 4.2 48.7 64 100 C-l* 3.32 51.2 90 70 C-2* 4.1 44 71 80 Table I (cont. ) * Not an example of the invention. C-1 and C-2 are commercially available macroporous resins. C-1 is a carboxylated macroporous resin comprising substantial amounts of polymerized methacrylate and is sold commercially as Amberlite DP-1 by Rohm and Haas Company. C-2 is a carboxylated macroporous resin comprising substantial amounts of polymerized methyl acrylate and is sold commercially as Dowex CCR-2E by The Dow Chemical Company. Expansion is percent expansion based on volume from sodium form of resin to acid form. The data in Table I indicate that the sample of this invention (Sample No. 1) exhibits increased osmotic shock resistance as compared to commercially available macroporous resins. In addition, it is noted that Sample No. 1 has undergone 500 cycles of the treatment as described hereinbefore, while the compa a- tive samples (i.e. Sample Nos. C-1 and C-2) have under¬ gone only 100 cycles of treatment. Example 2 Into a reactor was charged an aqueous solution comprising 1496 g deionized water, 2.4 g carboxymethyl methylcellulose and 1.5 g sodium dichromate. To this solution was charged an organic mixture containing 150 g ethyl acrylate, 750 g methacrylate, 150 g of a 55 percent active divinylbenzene formulation, 340 g iso- octane, 2.5 g tertiary butylperoctoate and 2.5 g tertiary butyl perbenzoate. The mixture was treated and reacted as in Example 1, hydrolyzed with a 20 percent active aqueous sodium hydroxide solution as described in Example 1, and filtered and washed with 1 normal hydrochloric acid. The sample was designated as Sample No. 2. Into a reactor was charged an aqueous solution comprising 1496 g deionized water, 2.4.g carboxymethyl methylcellulose and 1.5 g sodium dichromate. To this solution was charged an organic mixture containing 105 g butyl acrylate, 663 g ethyl acrylate, 200 g of a 55 percent active divinylbenzene formulation, 100 g isooctane, 0.5 g tertiary butylperoctoate and 0.9 g tertiary butyl perbenzoate. The mixture was treated and reacted, hydrolyzed and washed as described here¬ inbefore. The sample was designated as Sample No. 3. For comparison purposes was prepared a cross-- linked ethyl acrylate copolymer which was hydrolyzed and which was designated as Sample No. C-3. Into a reactor was charged 1500 g of an aqueous solution as was described in Example 1. To this solution was charged an organic mixture containing 763 g ethyl acrylate, 200 g of a 55 percent active divinylbenzene formulation, 160 g isooctane, 0.5 tertiary butyl per¬ benzoate and 0.9 g tertiary butyl peroctoate. The mixture was treated and reacted, hydrolyzed and washed as described in Example 1. For comparison purposes was prepared a cross- linked butylacrylate copolymer which was hydrolyzed and which was designated as Sample No. C-4. Into a reactor was charged 1500 g of an aqueous solution as was described in Example 1. To this solution was charged an organic mixture containing 763 g butyl acrylate, 200 g of a 55 percent active divinylbenzene formulation, 100 g isooctane, 0.5 g tertiary butyl peroctoate and 0.9 g tertiary butyl perbenzoate. The mixture was treated and reacted, hydrolyzed and washed as described in Example 1. For comparison purposes was prepared a cross- linked methyl acrylate copolymer which was hydrolyzed and which was designated as Sample No. C-5. Into a reactor was charged 1,500 g of an aqueous solution as described in Example 1. To this was charged an organic mixture containing 763 g methyl acrylate, 126 g of a 55 percent active divinylbenzene formulation, 100 g isooctane and catalyst. The mixture was treated and reacted hydrolyzed and washed as described in Example 1. Data for the Samples of this Example are presented in Table II. TABLE II Water Wet Volume Retention Osmotic Capacity Capacity Shock Percent Sample (meq/ml) (Percent) Whole Bead 2 3.0 42 97 3 * 3.2 44 98 C-3* 3.5 42 69 0 * 2.0 26 0 C-5 4.0 50 <20 Not an example of the invention. The data in Table II indicate that the samples of this invention (i.e., Sample Nos. 2 and 3) exhibit increased osmotic shock resistance as compared to the comparative samples.

Claims: 1. A process for preparing weak acid type cation exchange resins by subjecting to caustic hydroly¬ sis a copolymer comprising in polymerized form (1) a major amount of a monoethylenically unsaturated monomer which is hydrolyzable under caustic conditions, (2) a minor amount of a monoethylenically unsaturated monomer which has a less hydrolyzable character than the hydrolyz¬ able monomer, and (3) an effective amount of a cross¬ linking monomer. 2. A process of Claim 1 wherein said monomer which is hydrolyzable and said monomer which has a less hydrolyzable character than the hydrolyzable monomer are each alkyl acrylate monomers. 3. A process of Claim 2 wherein said alkyl acrylate monomer, which is hydrolyzable, is an alkyl ester of acrylic acid. 4. A process of Claim 3 wherein said alkyl acrylate monomer having a less hydrolyzable character is an alkyl ester of acrylic acid. 5. A process of Claim 4 wherein the alkyl acrylate monomer, which is hydrolyzable, is an alkyl ester of acrylic acid which contains an alkyl function¬ ality which contains less carbon atoms than the acrylate monomer having a less hydrolyzable character. 6. A process of Claim 1 wherein said alkyl acrylate monomer, which is hydrolyzable, is an alkyl acrylate, wherein the alkyl moiety less from 1 to 3 carbon atoms. 7. A process of Claim 1 wherein said mono- ethylencially unsaturated monomer which is the major amount of monomer is methyl acrylate, ethyl acrylate or butyl acrylate and said monoethylenically unsaturated monomer which is the minor amount of monomer is ethyl acrylate or butyl acrylate. 8. A process of Claim 1 wherein said cross¬ linking monomer is a polyvinyl crosslinking monomer. 9. A process of Claim 1 wherein said copoly¬ mer is hydrolyzed in the presence of sodium hydroxide solution. 10. A weak acid type cation exchange resin prepared by a process as claimed in any one of Claims 1 to 9.

Inventors: CHONDE YOHANNES, CHONDE, YOHANNES
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C08F8
IPC codes: B01J39, C08F8