US2790700A - Controlled coagulation of salt-spun polyacrylonitrile - Google Patents

Description

G. W. STANTON ET AL April so, 1957 CONTROLLED COAGULATION OF' SALT-SPUN POLYACRYLONITRILE Filed Jan. 2'?, 1954 s sheets-sheet 1 INV E NTORS.

April 30, 1957 G. w. STANTON ETAL y 2,790,700

CONTROLLED COAGULATION OF SALT-SPUN POLYACRYLONITRILE Filed Jan. 27. 1954 n 3 Sheets-Sheet 2 l5 20 Z5 30 35 40 5y Way/Hf ZnC/Z /n Caag Pressure Droplo. s.

ATTORNEYS.

April 30, 1957 G. w. STANTON ET Al.

CONTROLLED COAGULATION OF SALT-SPUN POLYACRYLONITRILE Filed Jan. 27, 1954 INVENTORS George W. .S/an for) Theodore 5. Le/f'eAa//nk Thomas C. Spence' OTE t@ Conga/offen BY l ATTORNEYS.

United States Patent C firice CNTRLLED CAGULATION F SALT-SPUN POLYACRYLONITRILE George W. Stanton, Walnut Creek, and Theodore B.

Lefferdink and Thomas C. Spence, Concord, Calif., assi'gnors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Application January Z7, 1954, Serial No. 406,516

5 Claims. (Ci. 18-5l) This invention relates to the Wet .spinning of polyacrylonitrile, and of linear fiber-forming copolymers containing at least 85 percent of acrylon'itrile in the polymer molecule, from aqueous saline solutions thereof, and is concerned particularly with .a method for controlling the rate of coagulation of the spun product so as to assure the production of fibers of consistency high quality.`

After the discovery in Germany by Dr. Herbert Rein (See U. S. 2,140,921) that many high polymers, including polyacrylonitri'le, can be dissolved in and spun from concentrated aqueous solutions of certain highly hydrated metal salts, such as zinc chloride, it has been reported by .other investigators that such salt .solutions commonly yield poor libers when wet spun. The product 'has been described as brittle, opaque and full of voids. ln consequence, much attention has been given to the discovery of organic solvents .suitable for spinning polyacrylonitrile by either the wet or the dry spinning procedures.

The use of organic solvents, however, 'is expensive and results in a tiber having an irregular cross-section, whereas the :fibers wet `spun from saline solutions are cylindrical. This difference between .fibers spun .from organic solvents and those spun from aqueous inorganic salts is due to the different behaviors of the two types of solution when `brought into contact with a .setting medium. Dry spun organic solutions of the polymer merely lose solvent to the evaporative atmosphere, and the fiberforming material collapses on itself as fthe solvent is removed. Wet spun organic solutions .of the polymer are converted into fibers by extraction of 'the organic medium from the spun dope by the action of a nonsolvent in the coagulating bath. The material inthe coagul-ating bath which is a non-solvent for the polymer must be miscible, however, with the solvent lfor the polymer. In common practice the coagulant is water, and the polymer solvent is water-misci-ble. When spun from such organic solutions, the polymer Vremains vhydrophobic. Hence, the freshly coagulated fiber skin acts -as a semi-permeable membrane in transferring solvent from the fiber to the bath and allowing essentially no migration of water into the core of the liber, even when lthe bath contains a high concentration of solvent. ln consequence, the cores of the wet spun Jbers from organic solutions Ishrink and set due to the removal of solvent therefrom land to the resultant radial collapse Iof the .fiber to a -denser condition, forming a [fiber of irregular cross-section. The effect, then, of both wet and dry spinning of organic solutions of hydrophobic -polymers, is primarily one of removing .solvent until the yconcentration of the polymer exceeds the capacity of the lremaining solvent -to dissolve it. There is no formation of a stretchable gel, and the polymer can only be ren- Patented Apr. 30, 1957 dered plastic by heat in order to be stretched significantly. Because the organic solutions behave in this way, the freshly rspun bers cannot be subjected to any significant amount of cold stretching. Consequently, `to make ltine bers from organic solutions of the polymer, one needs spinnerets with very small orilices, and this necessitates high extrusion pressure to move the viscose solutions through the spinneret-s.

The present inventors and cro-workers have disclosed in U. S. Patents 2,648,646 and 2, 648,647 (which were copending with the parent of the present appli-cation) that it is possible to obtain good fibers of high acrlonitrile polymers by wet spinning aqueous saline solutions thereof in which the saline constituents are mixed salts, one of which ifs capable at higher concentrations of dissolving the polymer and the other of which -is incapable of dissolving the polymer at yany concentration in water. It is shown in lthose patents that the minimum concentration of saline constituents in water to dissolve high acrylonitrile polymers is 55 percent.

It has been taught by Cresswell and his co-workers, in U. S. Patents 2,558,730-2,558,735, inclusive, and 2,558,781, that useful fibers of polyaciylonitrile .may be wet `spun from aqueous ysaline solutions thereof, provided that the coagulating bath is at a temperature not to eX- ceed -i- 10 C. and provided further that the bath be mainly water, with specific mention being made of water alone, aqueous ethanol (to prevent freezing the bath at temperatures down to -'l5 C), 0.24 normal hydrochloric acid, and dilute salt (sodium chloride) solutions. rIlhe so-.coagu'lated fiber is described as a swollen or gelled article having marked elasticity and toughness. It can -be washed free of salt and hot stretched to make strong libers. The same patents state that when the saline solutions ,of the polymer are coagulated at temperatures materially above .-ll0" C., the product 'is a non-transparent, weak gel -having little or no ductility. Because of the high viscosity of the saline solution of polymer, the solution is supplied under pressure to the spinneret and its mobility is increased by heating it before it passes lthrough .the small spinneret orifices into Athe cold coagu- -lating bath. The oriiices `mentioned 'in U. S. 2,558,730 ,are of 55 to 9.0 microns diameter, or from about 0.002 to 0.004 inch. A 40-iber tow, when given a standard after-treatment, had a denier of 97 (about 2.5 per fil) and a tenacity of 4.1 grams per denier, both being in a very useful range, though the 14 percent elongation indicates the toughness is not as great as may be desired .for many uses.

Each of the aforementioned patents relative to wet spinning of saline solutions of -polyacryl'om'trile is an `advance over the prior art, yet none 4of them 4teaches a spinning method which is free from certain serious limitations. Thus, as Ashown by the small orifices required to make ne bers, and the low temperatures lrequired for .coagulation in wat-er and dilute acids, .alcohols or salts, it is apparent that ylow coagulation 4temperature alone does not assure high rates of production at low extrusion pressures. lt has been found, as well, that when bers are spun from the aforementioned mixed salt brines into such an abrupt coagulant as water, there is Aobtained a physically non-uniform product. This is due, apparently, to the initial formation of a hard sheath about the still fluid core of the freshly spun ber, and any stretching to effect a reduction in diameter results in rupturing'the-tber.

It is accordingly among the objects of this invention to provide a method for wet spinning aqueous saline solutions of polyacrylonitrile, or of high acrylonitrile copolymers having similar characteristics, whether those solutions contain a single salt or a plurality of salts, whereby controlled coagulation may be obtained consistently: (a) to give clear gels at any desired operating temperature up to 30 C.; (b) to .permit the freshly extruded fiber to be cold stretched `to effect a reduction in diameter to a desired size smaller than the extrusion orifice; (c) to deliver the fibers in a uniformly gelled state to the washing and hot stretching zones which normally follow coagulation; and (d) to effect substantial economy in the recovery of the saline constituent of the spinning dope. A related object is to provide a method whereby there may be produced consistently fibers which, at any desired denier and tenacity, are capable of high elongation. Other objects may appear hereinafter.

It has now been found that markedly improved fibers may be spun from aqueous salt solutions of polyacrylonitrile, or of high acrylonitrile linear copolymers, containing a single salt or a mixture of solvent and non-solvent salts, when the saline solution medium is extracted gradually and under controlled conditions in the coagulating bath. According to the invention, the saline solution of the polymer is spun first into an aqueous coagulant bath consisting of a solution in water of the same salt or salts as are in the spinning dope, at concentrations controlled within a narrow range but whose absolute values vary with variations in spinning rate, spinneret size, spinning dope concentration, and coagulation temperature. In all cases, the coagulating -bath has a concentration of the salt or salts such that the spun fiber can be taken away from the spinneret orifice, without break-ing, at a linear rate 4 or more times that at which the spinning dope enters the bath from -the spinneret. Unless the freshly spun fiber is `coagulated under conditions which would permit such a takeaway rate, no significant dimensional adjustment can be made in the coagulating bath,'and, for fine fibers, small orifices are required. Further, unless the fibers initially exhibit liquid-like flow, coagulation is non-uniform, and lany after treatment gives non-uniform products. In the case of zinc chloride solutions of polyacrylonitrile, to get this effect, the concentration in the coagulating bath must be at least percent and not to exceed -about 47 percent by weight, and for any specific set of operating conditions the optimum operative range of concentrations falls within a narrow bracket Within the stated range, as will be illustra-ted hereinafter. When the strongly saline solution of the polymer lis spun into the appropriate lower concentration of the same salt system, complete coagulation is not instantly effected and the fibers are not self-supporting for `a short distance, usually more than l inch, from the spinneret, as will be described more fully.

Between lthe orifice and the point at which the fibers are self-supporting or shape-retaining, is a zone in which liquid-like fiow of the fiuid fibers occurs. In this zone the superticially coagulated fiber has different properties than after it has gelled more completely, and the length of this liquid flow zone can usually be observed ocularly. When the coagulating conditions of the present invention are employed, the zone of liquid fiow extends at least 0.025 inch and not over ,0.2 inch from the spinneret for each foot per minute linear rate at which the fibers are withdrawn from the coagulating ba-th. When coagulating conditions are too abrupt, as when water is the coagulant, that zone extends less than 0.025 inch from the spinneret for each foot per minute rate of withdrawal of fibers from the .coagulating bath. A liquid zone length much over 8 times the stated minimum accompanies inadequate coagulation. These factors show that the fiber forming material must pass through the liquid flow zone in from 1A; to l second after leaving the orifice, and should be sufficiently gelled by that time to be self-supporting.

The accompanying drawings illustrate various features of the invention, as follows:`

Fig. 1 is a liow diagram of a typical spinning opera-l tion useful with the invention;

Fig. 2 is a typical curve showing the effect of spin-- neret size on spinning pressure;

Fig. 3 is a typical curve showing the maximum permissible stretch without rupture in coagulating baths of various concentrations;

Fig. 4 is a plot showing the effects of variations in temperature and concentration in the coagulating bath on the character of the fibers spun in such baths; and,

Fig. 5 is a typical curve showing the amount of water required to be evaporated when recovering the saline constituents from coagulating baths of various concentrations.

As may be seen from the drawings, all other factors being kept constant, the lower the concentration of thc coagulating bath, the more difiicult it is to make good fibers economically. Thus, when water or weak brines are used for coagulation, low temperatures must be used to get good fibers (Fig. 4); little cold stretch is possible (Fig. 3) and hence, small orifices must be used to make fine fibers; small orifices require high pressures (Fig. 2), and weak brines multiply the cost of salt recovery (Fig. 5). However, at salt concentrations much over 47 percent in the coagulating bath (in the case of zinc chloride systems) Athere is so little coagulating tendency that the spun dope tends to dissolve or disperse in the bath rather than to form a self-supporting fiber, and the theoretical economies of operating at such high bath concentrations are nullied. The most practical conditions as to spinneret size and coagulant concentration are those found below and to the right, on Figs. 2 and 5, of the points at which a line drawn tangent to the curves has a slope of 45, i. e., sp-inneret diameters of 0.015 inch or greater and zinc chloride concentrations in the bath of 25 to 47 percent.

l't has thus become apparent that the optimum conditions for making fibers of good quality are those in which the coagulation is effected slowly enough, and the first formed superficial gel is plastic enough, so that the fiber has time to be brought to the dimensions at which it will leave the coagulating bath before it has coagulated completely, or has formed a somewhat rigid sheath of coagulum. The desired slow coagulation is accomplished by reducing the rate of diffusion of the saline constituent from the fiber to the bath, through the use of a strong solution in water of the same saline material. This practice slows the initial rate of coagulation on the surface of the fiber, and permits the diffusion to proceed further into the interior before the surface layers become so low in salt concentration as to be unstretchable under the low takeaway tension. The practice also produces a softer and more plastic gel in the surface layers of the fiber, and these layers can undergo the necessary deformation duc to stretching or due to volume changes during leaching without becoming cracked, strained or disproportionately oriented.

When an aqueous saline solution of a fiber-forming polymer or copolymer of acrylonitrile is spun into a co agulating bath, diffusion starts at once, and the interface between the fiber and the bath immediately comes to a concentration which does not vary greatly from the average of the two initial concentrations in the bath and in the spinning dope. In order to form a coherent fiber, it is necessary that this interface be at a low enough concentration so that a superficial gel is formed within a fraction of a second to give enough strength to the fiber so that the spun dope does not sink to the bottom of the bath, or dissolve in the bath, or develop a rough and irregular surface.

For each solution of any polymer or copolymer of acry- .lonitrile in any aqueous saline medium, there is a minimum concentration of the salt or salts below which the polymer will not remain dissolved at the prevailing temperature, and coagulation must occur in a bath which is at such a concentration that the salt concentration at the Spinning dope aconcentratonPrecipitation concentration=tPreoipitation concentration-.Upper limit .of coagulant concentration.

Thus, at C. the p-recipitation value for polyacryloni- -trile spun from aqueous Yzinc 'chloride -solution is 'about 5 3 percent zinc chloride by weight, and a concentration below this value must be attained immediately at the liber surliace when Aan 'aqueous :zinc `chloride solution of polyacrylonitrile lis Ispun into an aqueous coa'gulant at 15 C. The precipitation value is slightly flower at higher :temperatures, :as is shown by the line :EF in Fig. 4v4. At 15 C., then, with an aqueous zinc chloride system, the most concentrated coagulant which can be used is determined by the equation:

Spinning :dope concentration-53% =53 -upper limit of coagulant concentration.

Assigning the conventional symbol X to the unknown upper 'limit of coagulant concentration, the foregoing equation may be simpliiied, as follows;

Percentzsaline concentration tinspinnin'g dope -l- X;% 2

This maximum concentration 'of zinc chloride in the co- :agull'ant 'is shown, :for ra rrange of Y'ter'nperatures 'and for a spinning dope containing polyacryl-onitrile in -6.0 `percent aqueous zinc chloride, as curve ASB 'in Fig. '4. It is apparent that the coagulrant vconcentration must "be 'reduced proportionately whenever ythe concentration of zinc chloride is increased above 60 percent "in the spinning dope. It is considered undesirable vand irnpraot-ic'al to use much more salt 1in the spinning dope than is necessary to 'dissolve v'the polymer, and applicants prefer to use zinc chloride solutions -o'f 58 'to 6l percent concentration in the spinning dope.

`=It is `recognized that the line AfB of lFig. 4 ('or similar 'lines determined in `analogous manner for other systems) yd'etines a limi-ting bath :concentration tor vproducing quality iibers which, lafter standard iinishing operations have high toughness values and uniform orientation. For -ease in controlling the bath concentration, Tand to provide a safety-margin yforprotection against-the possible variations Yin concentration of the bath across 'a multiple larnent tow of simultaneously extruded fibers, it is considered desirable to use `a zcoa'gu-latin'g bath yconcentration to the left of curve AB (lFig. 4), since the ease of operation away from critical limiting lvalues more than offsets small losses in toughness. yThe coagulation baths of the :present invention should :be at concentrations to the rig'ht vof curve CD, however, as th-ose to the `left Aof `CD 'rigidity 'the ,'gel so rapidly that poor tiibers are obtained, 4and so little liquid ow is possible that the extruded product can be stretched not over 42.5 Vtimes in the bath, often much less than that, without breaking. For lreasons given before, it should be possible to vary the speed at which the .tiber is Vtaken rfrom :the coagulating bath 'from 'the linear speed 'at which it is vextrud'ed'into the lbath to 4 tor more times .tha-t speed Without breaking the tiber. it is not always necessary to withdraw a iber lat 4 or more times the linea-r rate of extrusion, but, to make goodibers consistently `the coagulation rate -should :be such yas to make possible the use of 4 to l, or greater rates of 'takew-ay.

IIt is noted that, at coagulation Itemperatures approaching C., the -AB curve and .the CD curve of Fig. 4 appreach one another. This means that, [in the zone near the .BC segment of the curve, operating conditions become more critical and there is less yfreed-om of choice of con- `dit-ions Liff good iibers are to be made. lIn 'the BC zone, diftusion 'rates and coagulation rates .are much higher than at Alower temperatures.

Unlike the coagulated wet-spun organic solutions of .acrylonitrile polymers, the coagulated salt-spun vfibers behave like permeable membranes, losing salt to the bath and absorbing wlater therefrom, and the concentraition of salt in the coagulatin-g bath-controls 'the rate of this interchange and the Iquality of the bers. When coagulation is controlled as here described, the libershave vthe vproperty tot liquid il'ow for a short but significant time, and Venough reduction may be made in Ithe diameter of the .freshly coagulated bers .to make it .possible to .defliver to the iinal wash baths and subsequent hot vstretching operations a fiber especially suited for the intended use.

The maximum cold stretch obtainable Without breaking the lgelled bers in the `coagulating bath is shown in Fig. 3 tor operations lat 27 C., with various concentrations of coagulating baths, using a spinneret 0.015 inch in diameter. vrl`he Aapproximate extent to which the curve is alected by changes in coagulation temperature is shown by ,dotted lines. The vfollowing table gives a relationship between the required amount of cold stretch to produce a 3-denier liber, assuming in each case the Washed y-gelled fiber will be 'given a 10-to-1 1ro-t stretch, when spinning a solution of polyacrylonitrile in 60 percent zinc chloride solution through various sizes of spinnerets into a coagulating Vbath consisting of 4l percent zinc chloride solution.

Orifice diameter: Required cold stretch 0.015 inch 10:1 0.0106 inch 5:1 0.0075 inch 2.5:1 0.00475 inch 1:1

The procedure of the invention provides a uniformly gelled liber by the time the ber has been drawn down to the desired diameter and Washed essentially free of salt. The liber is then subjected to such typical after-treatment as stretching in steam `or hot water to effect orientation and vfurther reduction in diameter. It is found that the properties of the so-'oriented bers, when dried, ,follow an orderly pattern dependent upon the concentration of the bath in which they were yfirst coagulated rand up'on the iinal hot stretch ratio. The term stretch ratio employed herein refers to the additional length to which a iiber is stretched for each unit length of the fiber entring fthe stretching zone. The following typical data illustrate the effects of coagulant concentration and inal hot stretch ratio on the ultimate properties or iibers of polyocrylonitr-ile produced under otherwise identical conditions, vThe spinning dope was an 11 percent by weight solution of polyacrylonitrile'in 60 percent aqueous zinc chloride solution. It 'had a viscosity of 2000 poises and was spun at the rate of 0.012 pound per orifice per hour through a 15'00-hole spinneret, :each orifice being 0.015 inch in diameter, into the stated concentration of aqueous zinc chloride coagulant at 20 C. All samples were given a uniform cold stretch in the coagulating bath of 8:1, and, after being washed free of salt Without further stretching, were stretched the indicated amount in wet steam. When dried, the so-formed iib'ers had the properties shown in the following table. rThe table shows a slight loss in tenacity with increasing coagulating bath concentration at a given steam stretch ratio. However, when higher concentrations are used, it is possible to steam stretch the fibers at a greater ratio, thus ultimately producing a superior product. Por example, when fibers were spun into a coagu-lating bath vconcentration of 42.5% ZnClz, .it was not possible to steam stretch without breaks vat over 12:1, but with .a coagulating bathconcentration of 45.5% ZnClz, a steam stretch ratio of 16:1 was achieved anda ber of 4.4 grams per denier tenacity and 29% elonga- .tion wasprodu-ced.

FIBER PROPERTIES vs. COAGULATING BATH CONCENTRATION AWD HOT STRETCH RATIOS Elongation, Percent Coagulant Conccn- Steam Stretch Ratio Tenacity, grams per denier Steam Stretch Ratio Denier Steam Stretch Ratio tration, Percent ZnGla 'iigol -ift" 'ftl' 'ai-ifi "sli Average. 4. 1 3. 7 3. 2. 9

Notruf-In any single run, the variation in denier was of the order of 5 percent.

l Noria-This concentration of coagulnnt not; usable under these conditions, as thc bers break in thc eoagulating bath.

The following examples illustrate the practice of the invention:

Example l It was desired to malte 3-denier iibers, using a spinneret 0.003 inch in diameter. The spinning dope was a 10.5 percent solution by weight of polyacrylonitrile in an aqueous brine consisting of 35.35 percent zince chloride, 25.95 percent calcium chloride and 38.7 percent water. The coagulant was at 5 C. and consisted of 22 percent zinc chloride, 16 percent calcium chloride and 62 percent water. The zone of liquid iiow outside the orifice was about l inch long. The coagulated iilaments rem'ained in the coagulating bath for l seconds and were withdrawn from the bath at a linear rate 'of cabout 18 feet per minute, or about 4.5 times the linear extrusion rate. After further washing in more dilute ZnClz-CaClz solution and in fresh water, they were stretched 2-fold in wet steam, to about 10 times their original length. After a iinal water wash they were dried in an -air oven and had a denier of 3.0 and a tenacity of 2.43 grams per denier, and were capable of 28 percent elongation.

By way of contrast, some 'of the same dope was spun through the same spinneret into water at C. The zone of liquid iiow was less than 0.1 inch long. The so-coagulated fibers were removed from the bath at |about feet per minute, which was the maximum rate possible without rupture, and was `only 2.5 times the linear rate of extrusion. After further washing in water, they were essentially salt free, and were stretched as much as possible in wet steam, which was 1.7 times, to a total of about 4.25 times their original length. When washed Iand dried, rthese bers had a denier of 5.5, a tenacity of 1.60 grams per denier and were capable of 21 percent elongation.

Example 2 An l1 percent solution of p'olyaciylonitrile in 60 per cent zinc chloride was spun through a 150G-hole spinneret having 0.015 inch oriices at the rate of 2 pounds of polymer per hour. ln order to draw down the iibers enough inthe coagul'ating bath to make possible the production of l to 3 denier fibers after a normal 8-to-l hot stretching operation, it was found that the coagulating bath at C. must be at least 4l percent zinc chloride and not over 45.5 percent zinc chloride solution in water. At lower concentrations of zinc chloride in the coafgulant, only coarse fibers could be made, and at higher concentrations fibers could not be produced.

Example 3 A series of runs was made in which polyacrylonitrile solution in aqueous saline media were spun at each of several temperatures into each of several concentrations of the same salt o1' salts in water. The so-coagulated fibers were given as nearly as possible the same aftertreatments, including the maximum permissible cold and hot stretching treatments without rupture. The dried fibers were tested to determine their tenacities, in grams per denier, and their percent elongation under stress to the breaking point. A conventional, but arbitrary, toughness factor was determined for each fiber by means of the equation: Toughness=Vz tenacity x percent elongation. The results are tabulated below.

RELATIVE TOUGHNESS FACTOR vs. COAGULANT CONCENTRATION AND TEMPERATURE Temperature of Coagulunt, C. Saline Content. of

lt is observed that, while many of the tested conditions have been shown herein not to be ideal, the fibers coagulated in fairly strong solutions of the same salt system as that from which they are spun have a superior combination of tenacity and elongation values. Thus, fibers spun into a 43 percent solution in Water of the salt have at least as great a toughness factor when coagulated at 25 C. as those coagulated in water at 5 C., and, at any given temperature of coagulation the Order of preference is always the same.

The invention has been illustrated with respectto polyacrylonitrile, but is equally applicable to the liber forming copolymers of acrylonitrile, and is especially advantageous with those copolymers containing percent or more of acrylonitrile in the polymer molecule, as these most closely resemble the homopolymer in their behavior.

This application is a continuation-impart of our c0- pending 'application Serial No. 228,755, tiled May 28, 1951 now abandoned.

We claim:

1. The method which comprises spinning an aqueous inorganic saline solution of a fiber-forming polymer containing at least 85 percent of acrylonitrile in the polymer molecule through a spinneret into a coagulating bath which is at a temperature not in excess of 30 C. and which consists essentially of a solution in water of thc same saline material as is in the solution being spun, the saline concentration of the coagulating bath being in the range from 25 percent to a higher concentration of X percent and having such a value in said range that the freshly extruded fiber exhibits a zone of liquid iiow extending from the spinneret orilice a distance equal to from 0.025 to 0.2 inch for each foot per minute linear rate at which the liber is withdrawn continuously from the coagulating bath, thereby permitting the rate of withdrawal of fiber from the bath without breaking to be at least 4 times the linear rate at which the polymer solution is spun into the coagulating bath; Washing the so-coagnlated fiber to free it of salt, and hot stretching it; the limiting concentration X percent being determined from the equation:

Percent saline concentration of spinning dopc-l-X] the length of said zone of liquid flow, and the permissible rate of withdrawal of liber from the coagulating bath, both varying in the same direction as variations 9 10 in the coagulating bath concentration in said range from mer is polyacrylonitrile, the saline constituent of the 25 percent to X percent. solution being spun is zinc chloride, and the coagulating 2. The method claimed in claim 1, wherein the polybath is a solution of zinc chloride in water. mer is polyacrylonitrile. Y

3. The method claimed in claim 1, wherein the saline 5 References Cited in the me 0f this pant constituent of both the spinning dope and the coagulat- UNITED STATES PATENTS ing bath is predominantly zinc chloride.

4. The method claimed in claim 1, wherein the saline 2,025,730 Dickie et al Dec. 31, 1935 constituent in the spinning dope is essentially zinc chlo- 2,140,921 Rein Dec. 20, 1938 ride and the coagulating bath is a solution of zinc 10 2,558,630 Cresswell July 3, 1951 chloride in water. 2,577,763 Hoxie Dec. 11, 1951 5. The method claimed in claim 1, wherein the poly-

Claims (1)

1. THE METHOD WHICH COMPRISES SPINNING AN AQUEOUS INORGANIC SALINE SOLUTION OF A FIBER-FORMING POLYMER CONTAINING AT LEAST 85 PERCENT OF ACRYLONITRILE IN THE POLYMER MOLECULE THROUGH A SPINNERET INTO A COAGULATING BATH WHICH IS AT A TEMPERATURE NOT IN EXCESS OF 30*C. AND WHICH CONSISTS ESSENTIALLY OF A SOLUTION IN WATER OF THE SAME SALINE MATERIAL AS IS IN TH ESOLUTION BEING SPUN, THE SALINE CONCENTRATION OF THE COAGULATING BATH BEING IN THE RANGE FROM 25 PERCENT TO A HIGHER CONCENTRATION OF "X PERCENT" AND HAVING SUCH A VALUE IN SAID RANGE THAT THE FRESHLY EXTRUDED FIBER EXHIBITS A ZONE OF LIQUID FLOW EXTENDING FROM THE SPINNERET ORIFICE A DISTANCE EQUAL TO FROM 0.025 TO 0.2 INCH FOR EACH FOOT PER MINUTE LINERAR RATE AT WHICH THE FIBER IS WITHDRAWN CONTINUOUSLY FROM THE COAGULATING BATH, THEREBY PERMITTING THE RATE OF WITHDRAWAL OF FIBER FROM THE BATH WITHOUT BREAKING TO BE AT LEAST 4 TIMES THE LINEAR RATE AT WHICH THE POLYMER SOLUTION IS SPUM INTO THE COAGULATING BATH; WASHING THE SO-COAGULATED FIBER TO FREE IT OF SALT, AND HOT STRETCHING IT; THE LIMITING CONCENTRATION "X PERCENT" BEING DETERMINED FROM THE EQUATION:

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