US2432448A - Bleaching polyacrylonitrile - Google Patents

Description

8-111 SR P15 8309 XE 2;?321443 ratemeu U85. 0, Lil-II BLEACHING POLYACRYLONITRILE John Cadwallader Richards, Kenmore, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application January 10, 1945, Serial No. 572,261

6 Claims.

This invention relates to the removal of color from shaped articles comprising a polymer of acrylonitrile and is particularly concerned with the bleaching of yarn composed of acrylonitrile polymer.

Polymers of acrylonitrile such as acrylonitrile polymers in which the polymers are composed entirely of acrylonitrile, and interpolymers and copolymers of acrylonitrile with other materials such as vinyl chloride, vinyl acetate, acrylic acid, its esters and homologues, styrene, butadiene, isobutylene and other polymerizable substances, especially those containing olefinic and diolefinic linkages, although potentially of great value on account of their highly desirable properties, are insoluble in ordinary organic solvents when the polymers contain at least 85% by Weight of acrylonitrile (vinyl cyanide) in the polymer molecule. Solutions of such polymers in organic solvents such as dimethyl formamide and other organic solvents, as well as their conversion by wet and dry or evaporative processes into various articles such as filaments, films, etc. are described and claimed in the copending applications of George Henry Latham, Serial No. 447,466, filed June 17, 1942, and Ray Clyde Houtz, Serial No. 447,446, filed June 17, 1942. This invention is broadly concerned with polymers of acrylonitrile containing at least 85% by weight of acrylonitrile in the polymer molecule although it is most particularly concerned with and will be specifically described in connection with polyacrylonitrile, i. e. the polymer which is composed substantially entirely of acrylonitrile in the polymer molecule.

One method which may be used to produce polyacrylonitrile yarn is the so-called wet spinning process which comprises extruding a solution of polyacrylonitrile through a perforated nozzle (spinneret) into a bath composed of a liquid which will readily leach out the solvent from the polymer solution and coagulate or precipitate the polymer from its solution, the yarn being carried through the bath for a period of time sufiicient to solidify the polymer to the desired extent and being subjected, if desired, to a stretching operation, preferably while it is in the gel state, in order to increase tenacity as well as .otherwise to improve physical properties by orienting the micelles of which the filaments are composed. Yarn produced by means of this wet spinning process has excellent physical properties, but does not possess that degree of freedom from color which is required by the exacting standards of the textile industry. Moreover, the

customary methods utilized in the textile art for.

the bleaching of color from filaments, fibers and the like do not efiect the desired elimination of color from polyacrylonitrile yarn. The yellowish or golden-brown color present in polyacrylonitrile articles occurs either during polymerization or processing of the polymer prior to or during shaping of the polymer to the desired form and may be caused by prolonged treatment of the polymer at high temperature, e. g. C. and above.

The invention will be described most particularly with reference to yarn composed of continuous filaments, but it will b 'understood that this is merely by way of illustration since the invention may also be applied to staple fibers, thin transparent films such as may be used as wrapping tissue, monofils, artificial straw, hollow pellicles such as tubing, and other industrialproducts composed of acrylonitrile polymer.

It is an object of this invention to provide a process for. removing color .from articles composed of acrylonitrile polymer. It is a particular object to produce color-free acrylonitrile polymer filaments, fibers and yarns without substantially impairing th physical properties of the products. Other objects will appear hereinafter.

The objects of the invention are accomplished in general by extruding a solution of polyacrylonitrile through a spinneret submerged in a liquid which will rapidly coagulate the polymer, passing the bundle of filaments through the bath of coagulating liquid for a distance sufficient to solidify or set the filaments, preferably collecting the yarn so formed on a bobbin or other suitable collecting device, maintaining it in the gel state, i. e. before the gel structure has collapsed, as for example while the yarn still possesses a large amount of absorbed coagulating bath liquid, e. g. 100% or more by weight of the acrylonitrile polymer and soaking the gel yarn with an aqueous solution of hydrogen peroxide or hypochlorous acid (which function to remove color by oxidation) in the critical concentrations set forth hereinafter at a temperature of 50 to 100 C. for a short period of time. Objectionable coloring present inthe gel yarn is removed by the bleaching treatment with no substantial impairment of the desired physical properties of the yarn.

The following examples in which'parts, percentages and proportions are by weight unless otherwise specified and which are intended to be illustrative instead of limitative, set forth preferred forms of the invention.-

EXAMPLE A Yarn to be treated in accordance with the present invention may be prepared as follows:

A solution of 18 parts of polyacrylonitrlle possessing an average molecular weight of 120,000, as determined from viscosity measurements by the Staudinger formula, in 82 parts of dimethyl formamide is extruded at a temperature of 100 0. through a ten-hole spinneret into a spinning bath consisting of glycerol heated to"'a temperature of C., the solution being ex- 153 wa 1 i.

denier by means of a snubbing tension guide .5

comprising two stationary pins mounted at a distance of 16 inches from the spinneret face.

On leaving the bath, the yarn is passed without slippage about a positively driven feed :wheel possessing a peripheral speed of 3,600 inches per minute, the yarn being subsequently collected'on a rotating, perforated bobbin possessing thesame peripheral speed as the feed wheel. The bobbin and gel yarn wound thereon are then placed on a chuck and submitted to pressure washing with water until the glycerol is removed from the yarn. Where skeined yarn is desired, the yarn is wound into skeins from the collectingbobbin and washed in running water until free from glycerol. The yarns used in the following examples have been freed from glycerol in the manner described in this paragraph, the retained wash water maintaining the yarn in the swollen condition.

Example I Acrylonitrile polymer yarn, prepared by a process such as that described'in Example A and Wound on the perforated collecting bobbin which permits the passage of fluid through the bobbin and yarn, .is soaked, while still in the gel state, in water containing hydrogen peroxide at a temperature of 75 C. for ten minutes, is washed with distilled water and dried at 40 0.; the dried yarnis then'heat-treated while relaxed and free to shrink at 105 C. ,forfive minutes, to increase ya nelongation. As compared with completely white yarn in which the color is designated as zero, the yarn bleached in accordance with this example shows a color of 0.5 as contrasted with dried yarn (otherwise prepared and processed in thesame way except that the bleaching treatment was omitted), exhibiting a color of .4, and the dry tenacity of 5.2 grams per denier and the dry elongation of about 18% possessed by the dried, unbleached yarn are substantially unaf fected'by the bleaching treatment.

Example 11 Skeins of gel acrylonitrile polymer yarn prepared by aprocess similar to Example A are subjected "to soaking in a 10% aqueous solution of hydrogen peroxide for five minutes at 80 (3., followed by washing with distilled water, drying and heat-treating as in Example I, and show-a color of'0.5 on the same color scale used in Example I. The same degree of freedom from color isobtained by soaking the skeins of yarn-for one minute at 80 C. in the same aqueous hydrogen peroxide solution. The bleaching treatment effects no marked change in yarn tenacity and elongation.

Example III Yarn skeins identical with the'gel, unbleached yarn used in Example 11 are soaked, while still in the gel state, for ten minutes at 80 C. in 3% aqueous solution of hydrogen peroxide, are washedin distilled water, dried and heat-treated as in Example I, and show a color of 0.5 on the same color scale used in Example I, with no substantial decrease in tenacity and elongation. To show the effect of concentration of hydrogen peroxideon the removal of color, the same yarn, in the form of skeins, is subjected to 1% aqueous hydrogen peroxide solution for one minute, followed by washing and drying, and shows a a color in water.

4 of 1.0 -with.,little,if any, :change in tenacity and elongation.

Example IV Yarn skeins identical with the gel, unbleached .yarn-processedin accordance with Examples 11 and III are soaked, while still in the gel state,

fforgfive'minutes at a temperature of 75 C. in an aqueous-solution prepared by dissolving sodium hypochlorite inwater in an amount sufficient to provide a 0.5% concentration of available chlorine,concentratedhydrochloric acid being added 'in anamQunt suflicient to bring the pH of the solution -to 2.7. The yarn, after washing with distilled water and drying, but with no heattreatment subsequent to drying, shows a color of 0.0 on the same scale used in Examples'I to III, with no 'marked adverse effect on tenacity and elongation.

' Example V 'Acrylonitrile polymer yarn similar to thatproducedin accordance with Example A and wound on a perforated bobbin, is soaked, while still'in the gel state, on the bobbin, in 2.5 liters of the hypochlorite solution of Examples IVby heating the yarn and solution in a beaker for twenty minutes until a temperature of 68 C. was reached. This temperature is maintained :2 C. for ten minutes, after which the yarn :is thoroughly washed with tap water, twisted, skeined, dried and subjected, while relaxed, to heat-treatment at a temperature'of 114 'C. for twelve .minutes. Yarn tenacity and elongation are not materially affected by the bleaching treatment and the-bleached yarn is almost completely white ascompared with-the light goldenbrown color of the same yarn which had not been subjected to the bleaching treatment.

Thebleaching treatments of Examples .I-to V are all carried out at atmospheric pressure.

The acrylonitrile polymer for use with this invention is preferably prepared by the ammonium persulfate catalyzed polymerization of monomeric acrylonitrile dissolved or emulsified It can, however, .be prepared by any other suitable type of polymerization reaction, such as *for example the emulsion type reaction disclosed by U. S. Patent No. 2,160,054 to Bauer et al. The polymer preferably possesses a molecular weight within the range of 15,000 to 250,000 'oreven higher, as calculated from viscosity measurements :by the Staudinger equation:

. I 81 Molecular we ght Kmc wherein:

and

C'=concentration of the solution expressed as :the

U iii weight (as calculated by the above equation) of approximately 60,000. Increasing or decreasing the amount of the catalyst, while maintaining the other conditions constant, decreases or increases the molecular weight of the polymer.

Although the examples illustrate the use of glycerol as the coagulating medium in the wet spinning process for forming polyacrylonitrile yarn, other coagulating liquids, for example water, organic solvents such as alcohol, ether, etc., or aqueous solutions of salts, alkalies or acids may be used. The tension spinning of acrylonitrile polymers from solution, involving the use of coagulating baths which may, for example, comprise glycerol is described and claimed in the copending application of William W. Watkins, Serial No. 496,476, filed July 23, 1943.

The heat treatment imposed on the dried yarn as described in Examples I to III and V is used to increase elongation, but yarns which are not heat treated may, of course, be bleached in accordance with the invention as illustrated in Example IV. Reference is made to the copending application of Shirleigh Silverman, Serial No. 496,396, filed July 28, 1943, which describes and claims the heat treatment of acrylonitrile polymers to increase elongation.

In order to effect the bleaching action characterized by the present invention, the acrylonitrile polymer yarn must be wet spun and must be maintained in the gel state up to and through the bleaching treatment; the coagulating bath liquid, for example glycerol, may, if desired, be replaced before the bleaching treatment by another swelling liquid, for example water and other liquids which can be used as coagulating bath liquids for acrylonitrile polymers, but the yarn must not be allowed to lose its swollen condition prior to bleaching. The examples illustrate the removal of glycerolby washing with water, this procedure representing an inexpensive and preferred expedient to remove from the yarn relatively non-volatile materials present in the coagulating bath.

The bleaching solutions used in accordance with the present invention must be maintained within certain concentrations. Thus, if an aqueous solution of hydrogen peroxide is used as the bleaching agent, the hydrogen peroxide must be within the range of 1% to based on the weight of the total aqueous solution and is preferably present in a concentration of 3% to-10%. If aqueous solutions of hypochlorous acid are used, they must contain 0.05% up to 0.5% of available chlorine, with the preferred concentration being in the neighborhood of 0.25%; hypochlorous acid solution may readily be prepared by dissolving a material such as sodium hypochlorite, or calcium hypochlorite, or other soluble hypochlorite, in water, acidifying with a mineral acid such as hydrochloric or sulfuric acid to a pH of less than 7.0 and preferably 5.0 or less, the term available chlorine signifying the chlorine present as hypochlorous acid. An excess of mineral acid added to the hypochlorite solution does no harm and the pH can be as low as 2.0 or even close to 0.0, if desired.

Adequate bleaching can be obtained in accordance with the present invention only by maintaining a temperature during the bleaching period of between 50 and 100 C. The higher the temperature, the shorter the necessary period of contact between the yarn and the bleaching solution. Normally, adequate bleaching is accomplished in a few minutes at the higher temperature and in less than an hour at the lower end of the temperature range. The preferred temperature is about 70 to C. and the preferred time of bleaching is about one to thirty minutes.

Hydrogen peroxide and sodium hypochlorite have been used in very dilute concentrations for the treatment of textile fibers such as cotton and rayon, but they have no useful effect, in the customary concentrations andat temperatures below those used in the practice of thepresent invention, as far as acrylonitrile polymer yarn is concerned. Alternatively, the use of the relatively concentrated solutions of bleaching agent used in accordance with this invention would so de-v grade the said textile materials as to make them practically useless. V

The wet spun, gel yarn of polyacrylonitrile may be bleached in a substantially continuous manner after the coagulation thereof and before the yarn is collected on a bobbin, bucket or other collecting device. Alternatively, the gel yarn may be collected, for example on a bobbin in a bobbin spinning process, or in a bucket in a bucket spinning process, and the resultant gel yarn cake subjected to treatment with the bleaching solution. A preferred method of bleaching yarn in accordance with this invention comprises skeining the yarn with the skeining reel dipping in water or even submerged under water to insure the retention of the yarn in its swollen, gel state. The skein's of gel yarn are then dipped into the bleaching solution,

By the process of the present invention polyacrylonitrile yarn, filaments, fibers, film, monofilaments, artificial straw, hollow tubing and other structures can .be bleached to a satisfactory colorless or white character Without deleterious action on the structure. The bleached polyacrylonitrile structures of the present invention, except for improved color, exhibit substantially the same physical characteristics as the unbleached structure.

Since it is obvious that many changes and modifications can be made in the above described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to the details described herein except as set forth in the appended claims.

' I claim:

1. In the process of removing color from an article composed of a polymer of acrylonitrile containing at least by weight of acrylonitrile in the polymer molecule, the step which comprises subjecting said article in the gel state, at a temperature of 50 to C. for a time sufiicient for bleaching and less than one hour to treatment with an aqueous solution taken from the class consisting of hydrogen peroxide in 1% to 10% concentration by weight and hypochlorous acid in a, concentration corresponding to 0.05% to 0.5% by weight available chlorine.

2. In the process of removing color from yarn composed of a polymer of acrylonitrile containing at least 85% by weight of acrylonitrile in the polymer molecule, the step which comprises subjecting Said yarn in the gel state, at a temperature of 50 to 100 C. and for a period of about one to thirty minutes, to treatment with an aqueous solution taken from the class consisting of hydrogen peroxide in 1% to 10% concentration by weight and hypochlorous. acid in a concentration corresponding to 0.05% to 0.5% by weight available chlorine,

3. In the process of removing color from yarn composedv of polyacrylonitrile, the step which comprises subjecting said yarn in the gel state, at a temperature of 50 to 100 C. for a time sufiicient for bleaching and less than one hour, to treatment with an aqueous solution of hydrogen peroxide in 1% to 10% concentration by weight.

4. In the process of removing color from yarn composed of polyacrylonitrile, the step which comprises subjecting said yarn in the gel state, at a; temperature of 50 to 100 C. for a time sufiicient for bleaching and less than one hour, to treatment with an aqueous solution of hypochlorous acid in a concentration corresponding to 0.05% to 0.5% by Weight available chlorine.

5. In the process of removing color from yarn composed of polyacrylonitrile, the step which comprises subjecting said yarn in the gel state, at a temperature-of 70 to 80 C. for a time sufficient for bleaching and less than one hourfto treatment with an aqueous solution of hydrogen peroxide in 3% to 10% concentration by weight.

6. In the process of removing color from yarn composed of polyacrylonitrile, the step which 8 comprises subjecting said yarn in the gel state, at atemperature of 70 to 80 C. for a time sufiicient for bleaching and less than one hour, to treatment with an aqueous solution of hypochlorous