IL29188A - Process for producing acrylonitrile containing polymers in an aqueous polymerization system - Google Patents

Description

A PROCESS FOR PRODUCED ACRYLOHITRILB CONTAINING- POLYMERS IN AH AQUOUS POLYMERIZATION SYSTEM This invention relates to a process of pro ducing acrylonitrile containing polymers. More specifically 3 this invention deals with a process for improving the addition polymerization of acrylonitrile containing monomers in an aqueous medium through the use of redox catalyst system.

Acrylonitrile containing polymers produced by the polymerization of acrylonitrile alone, or in conjunction with one or more other vinyl monomers copolymerizable therewith, are useful in the manufacture^ of films, fibers, and the like. Several techniques are known whereby the polymerization of these monomers may be accomplished.

This invention relates to those processes where in the monomers are polymerized in an aqueous media using a redox catalyst system and thereby includes broadly the methods commonly referred to in the art as suspension, emulsion and dispersion polymerization. The term aqueous polymerization as used herein is meant to include all the above-enumerated techniques.

One redox catalyst system which may be conveniently employed in aqueous polymerization systems is comprised of a persulfate catalyst such as sodium or potassium persulfat and a sulfoxy reducing agent activator such as sodium or potas sium bisulfite or sulfur dioxide, in combination with a catalytic amount of iron. The function of the redox catalyst system is to provide free radicals, notably HSO^, which initiate the chain reaction of the vinyl monomers. Trace amounts of iron are desirable to initiate the free radical formation.

Heretofore, the aqueous polymerization of acrylonitrile containing monomers was typically accomplished by feeding to a reactor separate streams consisting of (1) monomer, (2) an aqueous solution of potassium persul ate and sodium bicarbonate and (3) an aqueous solution of sulfur dioxide and iron. The sodium carbonate serves to control the pH of the polymerization reaction within the operational limits of about 2 to 4. The sulfur dioxide in aqueous solution forms the bisulfite ions required for the persulfate-bisulfi te redox catalyst system.

One problem which exists in such a persulfate-bisulfite catalysed system is the loss of a part of the acry-lonitrile by combination with bisulfite ions to form an acrylonitrile-bisulfite addition product , hereinafter referred to as AAP. This addition product is characterized by being water soluble and nonvolatile. It is carried with the water and the dissolved salts during the polymer filtration and washing stages, and is discharged to waste from the distillation columns used to recover the unreacted monomers dissolved in the aqueous filtrate.

The acrylonitrile lost due to formation of AAP during polymerization and later during monomer recovery has ag been found to befmuch as three percent of the total acrylonitrile feed, accounting for a major part of the total monomer losses from the system and representing a significant economic loss .

Accordingly the difficulties of the prior art are overcome by partially decomposing the redox catalyst prior to its introduction into the polymerization reaction.

It is therefore an object of this invention to provide a method for the aqueous polymerization of acrylonitrile containing monomers using a persulfate-bisulfite redox catalyst system wherein the formation of acrylonitrile-bisulfite addi- tion product is substantially reduced in the polymerization reactor .

Another object of this invention is to provide a process wherein the amount of acrylonitrile-bilsulfite addition product formed during polymer filtration is reduced.

Further, it is an object of this invention to provide a process wherein the amount of acrylonitrile-bisulfite addition product formed during the stream distillation of un-reacted monomers is reduced.

These and other objects and advantages will become more readily apparent when read in conjunction with the following detailed description.

Specifically, the aqueous solution containing the potassium persulfate catalyst and the sodium bicarbonate is mixed and reacted with the aqueous solution containing the sulfur dioxide activator and the iron, and the resulting mixture is fed with the monomer into the polymerization vessel. The predecomposition of the persulfate-bisulfite catalyst system drives the HSO^ free radical formation reaction toward completion, thereby reducing the bisulfite ion concentration and consequently reducing the amount of AAP formed.

The decomposition rate of the redox catalyst system is a function of time and temperature, and the degree of decomposition must be controlled to achieve the maximum benefit of this invention. If the decomposition is allowed to proceed too far before the catalyst is in\troduced into the polymerization reaction, the e ectiveness of the catalyst is diminished and results in a lower conversion of monomer to polymer. If the decomposition is allowed to proceed to only a very small degree the benefit of reducing the amount of acrylonitrile- bisulfite addition product formed in the reactor is reduced. In general 3 the optimum degree of decomposition will be the maximum possible without decreasing the monomer conversion below an acceptable level. Since the monomer conversion depends upon many polymerization variables, for example, the monomer to water ratio, monomer to catalyst ratio, persulfate to bisulfite ratio, and polymerization time and temperature, the optimum degree of decomposition will also depend upon these polymerization variables, and consequently a numerical value cannot be assigned. In other words, the amount of catalyst predecomposition optimum for any particular polymerization system will necessarily have to be determined experimentally for that system. This is easily accomplished by starting the polymerization reaction with a very low degree of catalyst decomposition, and then gradually increasing the amount of decomposition by increasing reaction time or temperature, or both, until the monomer conversion value begins to decrease, and thereafter decreasing the reaction time or temperature a small amount to again obtain maximum monomer conversion.

In order to maintain a given polymer specific viscosity, it may be necessary to increase the ratio of catalyst and activator to monomer when employing the predecomposition technique of this invention. For example, under the polymerization conditions specified in the examples included herein, it was found necessary to increase the ratio of the redox catalyst system to monomer by about 6 percent in order to maintain a polymer specific viscosity of 0.155· The specific viscosity (Nsp) of the polymer is determined by measuring the viscosity of a solution of 0.1 grams of polymer in 100 grams of dimethylformamide at 25°C. and cal- culated as follows : Polymer Nsp = Viscosity of polymer solution Viscosity of dimethylformamide -1 Utilizing the catalyst predecompositlon technique of this invention not only reduces the amount of acry-lonitrile-bisulfite' addition product formed in the polymerization reactor itself, but also reduces the amount formed during the following process steps of polymer filtration and steam distillation of unreacted monomer.

The amount of AAP present in an a$queous solu-tion may be determined by vapor chromatography. Although the acrylonitrile-bisulfite addition product is stable and nonvolatile at the conditions existing during the steam distillation of the unreacted monomers, the complex does break down to release the acrylonltrile at the more elevated temperatures employed in the vapor chromatograph . It is possible therefore to directly analyze an aqueous sample containing AAP for acrylonltrile content.

The practice ^and advantages of the instant in-vention may be more fully understood by reference to the following examples which are given by way of illustration only, ivith no limitation intended by the particular polymerization recipes or conditions cited. For example, it is contemplated that wide variations in the total amount of persulfate-sulfur dioxide redox system may be used as well as variations in the ratio of persulfate to sulfur dioxide, without departing from the scope of this invention. Likewise, other alkali metals or ammonia may be substituted for the potassium and sodium ions in the redox system, and a bisulfite or metabisulfite may be substituted for the sulfur . dioxide. fully understood, the following examples are given primarily by way of illustration. No details appearing therein should be construed as limitations on the present' invention, except as they appear in the appended claims.

EXAMPLE I To a continuous polymerization reactor equipped with baffles and an agitator, were charged separate feeds of (1) monomer (90 percent acrylonitrile and 10 percent vinyl acetate), (2) an aqueous solution containing potassium per-sulfate and sodium bicarbonate and (3) an aqueous solution containing sulfur dioxide and iron, the relative proportions of all the constituents being as follows on a parts by weight basis : Polymerization Recipe I (1) Fe as parts per million based on monomer The rate of addition of the three feed streams to the reactor was such that the polymerization dwell time was fifty minutes. In other words, the combined volume of the feeds added over a fifty minute period was equal to the volume of the polymerization reactor. The reactor was maintained at a temperature of ^5°C. After 150 minutes of continuous operation, at which time the reaction had attained equilibrium conditions, the reactor overflow was sampled and analyzed with the following results being obtained.

RUN A Percent Conversion 60 Percent Acrylonitrile lost as AAP 1.30 The above polymerization was repeated with the following results: RUN B Percent Conversion 60 Percent Acrylonitrile lost as AAP 1.24 Polymer samples from the above polymerization reactions were filtered from the reactor overflow, washed, and dried. The filtrate comprised of water, unreacted monomer, and dissolved salts was neutralized with sodium bicarbonate to a pH of 5 and steam distilled to recover the monomers. The aqueous bottoms from the steam distillation column were analyzed for AAP, and the total amount of acrylonitrile lost due to AAP formation during polymerization and monomer recovery was calculated as percent of original monomer feed.

Run A 2.63$ acrylonitrile lost as AAP Run B 2.51Ϊ acrylonitrile lost as AAP Average 2.57¾ The method encompassed in this example is contemplated in the prior art.

EXAMPLE II To the same polymerization reactor described in Example I were charged separate feeds of (1) monomer (90 percent acrylonitrile, 10 percent vinyl acetate) and (2) an aqueous solution comprising the redox catalyst system formed by mixing and reacting (a) an aqueous solution containing potassium persulfate and sodium bicarbonate with (b) an aqueous solution containing sulfur dioxide and iron for a period of ten minutes at 65°C The relative proportions of all the constituents were as followed on a parts by weight basis.

Polymerization Receipe II (1) Pe as parts per million based on monomer.

The rate of addition of the two feed streams to the polymerization reactor was such that the polymerization dwell time was 50 minutes. The temperature of the polymerization reaction was maintained at 45°C. After 150 minutes of continuous operation the reactor overflow was sampled and analyzed to obtain the following results: Run C Percent Conversion 64 Percent acrylonitrile lost as AAP 0.86 The above polymerization was repeated with following results : Run D Percent Conversion 66 Percent acrylonitrile lost as AAP 0.80 Polymer samples from the above polymerization reactions were filtered from the reactor overflow, and the filtrate was steam distilled to remove unreacted monomers and then analyzed for AAP as described in Example I. The amount of acrylonitrile monomer lost due to AAP formation calculated as percent of original acrylonitrile monomer fed to the reactor was as follows: Run C 1.28% acrylonitrile lost as AAP Run D 1.42$ acrylonitrile lost as AAP Average 1.35? acrylonitrile lost as AAP A comparison of the above results for acrylonitrile loss due to AAP formation with the results obtained when operating according to the method of the prior art as described in Example I illustrates the advantages of this invention.

Specifically, whereas 2.57 percent of the original acryloni-trile monomer fed to the polymerization reaction was lost as AAP in Example I, only 1.35 percent was lost in Example II, a significant reduction of approximately 48 percent.

EXAMPLE III A series of polymerization runs were made using the recipe and procedure described in Example II to determine the effect of various degrees of catalyst decomposition on percent monomer conversion and on polymer quality as indicated by its color stability. Color stability was determined by preparing a 25 percent solution of polymer in dimethyl-acetamide solvent, heating the solution for one hour at 100°C., and measuring color by comparison with a set of Gardner color standards. The results of the test are presented below. A comparison was made with the polymer produced under Run A, Example I as the standard control sample.

^Gardner color scale 1.0 to 10, light to dark.

The above data illustra s the increase in mono conversion resulting from catalyst predecomposition up an optimum (Runs E and P), followed by a sharp decrease in conversion as the optimum degree of decomposition is exceeded (Runs G and H). For the particular polymerization conditions and recipe employed for this series, the optimum decomposition occurred in 10 minutes at 65°C.

Increasing either the time or the temperature resulted in excessive decomposition and loss of catalyst activity as exemplified by the decrease in monomer conversion. It must be stressed hoxvever, that the optimum conditions of 10 minutes at 65°C. apply to this particular polymerization circumstance only, and that the optimum for other polymerization reactions must be determined experimentally for each operating condition.

The above data also illustrate^ a small but significant improvement in the heat stability of the polymer- solvent solution for Run No. F at the optimum level of catalyst decomposition. Run H shows that polymer heat stability decreases sharply if the catalyst decomposition is allowed to proceed to an excessive degree.

EXAMPLE IV The polymerization process of Example II was repeated with a modified polymerization recipe which employed a greater amount of sulfur dioxide and a lesser amount of potassium persulfate. The relative proportions of all the constituents on a parts per weight basis were as follows: (1) Fe as parts per million based on monomer.

The polymerization reactor was sampled after a ISO minutes of continuous operation. Conversion of monomer to polymer was determined to be sixty percent. Polymer quality was good as evidenced by color stability of 1.5 on the Gardner Color Scale, measured as described in Example III.

The preceding examples illustrate the advantages which may be gained in the practice of this invention. Specifically, the examples show that by providing for predecomposi-tion of the redox catalyst-activator system in the aqueous polymerization of acrylonltrile containing monomers, the losses of acrylonitrile due to the formation of the acrylonitrile-blsulfite addition product may be substantially reduced, and that additional benefit may be realized by increasing the monomer conversion to polymer and by improving the polymer heat stability.

The invention is applicable to suspension, emulsion, dispersion, or other aqueous polymerization processes for the polymerization of acrylonitrile or mixtures of acrylonltrile and one or more other copolymerizable mono-olefinic monomers. Suitable copolymerizable monomers include acrylic, alphachloroacrylic and methacrylic acids, the acrylates, such as methylmethacrylate, ethymethacrylate , butylmethacrylate , chloroacrylic/ methoxymethyl methacrylate , beta 5SS-Ci-?^&5y3i^ acids ; vinyl chloride, vinyl fluoride, vinyl bromide, vinylidene chloride, 1-chloro-l-bromoethylene methacrylonitrile, acrylamide, and methaerylamide ; alpha-chloroacrylamide , or monoalkyl substiu-tion products thereof methyl vinyl ketone; vinyl carboxylates , such as vinyl acetate, vinyl chloroacetate , vinyl propionate, and vinyl stearate; N-vinyl-imides , such as N-vinylphthal-imldes, and N-vinylsuccinimide ; methylene malonic esters; itaconic acid and itaconic ester; N-vinyl carbozole; vinyl furan; alkyl vinyl esters; vinyl sulfonic acid; ethylene alpha, beta-dicarboxylic acids or their anhydrides or derivatives, such as diethyl citraconate, diethylmesaconate ; styrene; vinyl naphthalene; vinyl-substituted tertiary heterocyclic amines such as the vinylpyridines and the alkyl-substituted vinyl-pyridines for example, 2-vinylpyridine , iJ-vinylpyridine , 2-methyl-l-vinylpyridine, and the like; 1-vinylimidazole and alkyl-substituted 1-vinylimidazoles , such as 2-, **-, or 5-methyl-l-vinylimidazole, vinylpyrrolidone^ vinylpiperi^done , and other mono-ole inic copolymerizable monomeric materials.

While the preferred polymers produced by the practice of the instant invention are those containing at least 80 percent acrylonitrile , generally recognized as fiber forming acrylonitrile polymers, it will be understood that the invention is likewise applicable to polymers containing less than 80 percent acrylonitrile which are also useful in forming fibers, lacquers, coating compositions, and molded articles .

While the preferred redox catalyst system for use in the instant invention is that comprised of persulfate-sulfur dioxide, it will be understood that the invention may likewise be practiced using any catalyst-activator system containing both an oxidizing agent and a reducing agent, the reducing agent being a water-soluble oxidizable sulfoxy compound in which the valence of a sulfur atom does not exceed . In such systems, the catalyst may comprise perborates, perchlorates , the preferred persulfates, persulfuric acid, and perdlsulfates . The sulfoxy reducing agent may be sulfur dioxide, and alkali metal bisulfite, or a metabisulfite .

For example j sodium metabisulfite may be used in any of the examples as a replacement on a equivalent basis for the disclosed sulfur dioxide. However, the metabisulfite is strongly basic and Instead of using sodium bicarbonate as a pH adjuster to obtain the desired pH of from about o about 4, it will be necessary to use an acidic agent pH adjuster, preferably sulfuric acid, to reduce the pH to the preferred value. The polymerization results are the same as when using sulfur dioxide- and sodium bicarbonate.

The foregoing examples illustrate the essential features of the invention as well as some of the manners in which it may be practiced. Various changes and modifications may be made in practicing the invention without departing from the spirit and scope thereof, and therefore, the invention should not be limited except as defined in the appended claims.

Claims (9)

HA.VIKG NOW particularly described and ascertained tlie nature * of our said invention and in what manner the sarae io to be erformed, we declare that wha we claim is ί

1. A process of producing acrylonitrlle containing polymers in an aqueous polymerization system partially decomposin i characterized by| 'yc-py^ftyittg a bisulfite free-radical producing redox catalyst system which is capable of being used in the production of acrylonitrile containing polymers and feeding the catalyst system into a continuous polymerization reactor which is being fed with monomers.

2. The process of Claim ls characterized in that the catalyst system and the monomers are separately fed into the reactor.

3. The process of Claim 1 or 2 s characterized in that the monomers are selected from the group consisting of acrylonitrile monomers 3 and acrylonitrile monomers and one or more mono -olefinic monomers copolymerizable therewith.

4. The process of any of Claims 1-3 s characterized in that the redox catalyst system is formed by reacting: (a) an aqueous solution containing a persul- fate catalyst selected from the group con- sisting of sodium persulfate, potassum persulfate, alkali metal persulfates , and ammonium persulfates- and (b) an aqueous solution containing a sulfoxy reducing agent activator selected from the group consisting of sodium bisulfite, potassium bisulfite 3 sulfur dioxide s and other soluble bisulfites and metabisulfites in an aqueous solution containing t c/ (c) a catalyst amount of iron; and (d) a sufficient amount of a pH adjuster to control the pH of the polymerization reaction within functional limits of from about 2 to about 4.

5. The process of Claim 1, characterized in that the redox catalyst system is formed by reacting: (a) an aqueous solution containing a persul- fate catalyst selected from the group consisting of sodium persulfate, potassium persulfate, alkali metal persulfates, and ammonium persulfates; and (b) An aqueous solution containing a sulfoxy reducing agent activator selected from the group consisting of sodium bisulfite , potassium bisulfite, sulfur dioxide, and other soluble bisulfites and metabisulfites .

6. The process of Claim 1, characterized in that the redox catalyst system is formed by reacting: (a) an aqueous solution containing a persulfate catalyst selected from the group consisting of sodium persulfate , potassium persulfate, alkali metal persulfates, and ammonium persulfates; and (b) an aqueous solution containing a sulfoxy reducing agent activator selected from the group consisting of sodium bisulfite , potassium bisulfite, sulfur dioxide, and other soluble bisulfites and metabisulfites : in an aqueous solution containing (c) a sufficient amount of a pH adjuster to control the pH of the polymerization reac- -h6- tion within functional limits of from about 2 to about .

7. The process of Claim 1, characterized in that the redox catalyst system formed by reacting: (a) a catalyst selected from the group consisting of perborates, perchlorates , persul- fates, pe sulfuric acid, and perdisulfates ; and (b) an aqueous solution containing a sulfoxy reducing agent activator selected from the group consisting of sodium bisulfite , potassium bisulfite, sulfur dioxide,, and other soluble bisulfites and metabisul ites .

8. The process of Claim 1, characterized in that the redox catalyst system is formed by reacting: (a) a catalyst selected from the group consisting of perborates, perchlorates, per- sulfates, persulfuric acid, and perdisul- fate; and (b) a sulfoxy reducing agent activator selected from the group consisting of sodium bisulfite, potassium bisulfite , sulfur dioxide and other soluble bisulfites and metabisulfites ; in an aqueous solution containing (c) a sufficient amount of a pH adjuster to control the pH of the polymerization reaction within functional limits of from about 2 to about 4.

9. The process of any of Claims 1-3S charac- -i6- terized in that the redox catalyst system is formed by reacting (a) a catalyst selected from the group consisting of perborates, perchloratess, persul- fates, persulfuric acid, and perdisulfates : and (b) a sulfoxy reducing agent activator selected from the group consisting of sodium bisulfite, potassium bisulfite, sulfur dioxide, and other soluble bisulfites and meta- bisulfites in an aqueous solution containing (c) a catalytic amount of iron; and (d) a sufficient amount of a pH adjuster to control the pH of the polymerization reaction within functional limits of from about 2 to about 4. Dated this Twenty-first day of Decenfber 1967 Agent