US3505445A - Solvent removal process for wet spun fibers - Google Patents

Description

SOLVENT REMOVAL PROCESS FOR WET SPUN FIBERS Filed NOV. 17, 1967 RICHARD L. LEONARD THOMAS a. TRuscoTT THOMAS M. vEAzEY TTORNE Y United States Patent O 3,505,445 SOLVENT REMOVAL PROCESS FOR WET SPUN FIBERS Richard L. Leonard, Raleigh, N.C., and Thonlas B. Truscott and Thomas M. Veazey, Decatur, Ala., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware Filed Nov. 17, 1967, Ser. No. 683,966 Int. Cl. B29b 11/22 U.S. Cl. 264-233 Claims ABSTRACT OF THE DISCLOSURE Substantially solvent free acrylic fibers are produced by washing with water and simultaneously stretching the newly formed fibers, in all instances at a maximum temperature of less than about 86 C. and, in some instances, at lower maximum temperatures depending upon the minimum orientation stretch ratio employed.

BACKGROUND OF THE INVENTION This invention relates to fibers, filaments, yarns and the like manufactured from acrylonitrile polymers and to an improved process of producing the same. More particularly this invention relates to improvements in the process of removing solvents used in the polymerization of acrylonitrile polymers and in the coagulating bath following the spinning step in the production of acrylonitrile fibers, to thereby obtain a finished fiber with a minimum of residual solvent therein.

Since acrylonitrile polymers do not melt, filaments of such polymers are formed by dissolving the polymers in a suitable solvent and then removing the solvent from a flowing stream of the solution to form filaments therefrom. Commercially filaments of acrylonitrile polymers are prepared either by the dry spinning process or by the wet spinning process as is well known.

Ordinarily, in the wet spinning operation, coagulation is accomplished by extruding the polymer solution into an aqueous bath sometimes containing a percentage of solvent or dissolved salt. As used herein an aqueous or water bath refers to a composition using water as one of its main components. When the solvent is extracted from the extruded stream of spinning solution in a coagulating bath during wet spinning, solidification of the polymer in filamentary form results. The coagulatng bath most frequently employed in the art is an aqueous bath employing a solvent, such as N,Ndimethylacetamide, N,Ndimethylformamide or the like, preferably the same solvent which is used to dissolve the polymer.

Instead of an aqueous coagulating bath, the art also teaches the use of a bath composed essentially of materials of such dissimilar character as butyl alcohol, kerosene and the polyalkylene glycols. Generally these baths contain less than 1% water, and with a tolerance of up to of the solvent in which the polymer is dissolved. These coagulation baths are referred to as nonaqueous baths.

Most solvents used in the preparation of acrylonitrile spinning clopes are toxic and may impart an objectionable odor to the fiber. Residual solvents in the fibers also affect physical properties and dyeing characteristics of the fibers. Moreover, it is economically advantageous to remove all solvents from fibers for recovery and reuse.

Experience has shown that the presence of residual solvents in spun fibers has been greater or lesser depending upon the type of solvent used and upon the type of coagulation employed. Where coagulation is accomplished by extruding the solution into an aqueous bath which contains a percentage of solvent or dissolved salt, the residual Fice solvent in the fibers can usually be reduced to very low levels by washing and stretching the coagulated fibers in water at a temperature near boiling. This temperature usually conforms closely with required stretch temperatures for desired stretch ratios of fibers extruded into aqueous solutions. Where nonaqueous coagulation baths are used, for reasons unknown, ditliculties in the removal of residual solvent become more pronounced, although stretching without breakage is more easily accomplished. Residual solvent after use of nonaqueous coagulation bath systems, following which the fibers are washed and stretched in the usual manner, in boiling or near boiling water, are 1% or higher.

SUMMARY OF THE INVENTION It is an object of this invention to provide a process for the removal of substantially all residual solvents from fibers which have been made by spinning a solution of an acrylonitrile polymer in a solvent into any coagulating bath.

Other objects will become apparent from the following description of the invention and claims.

It has been found that unexpectedly improved results in the elimination of residual solvents are obtained by washing fibers in a bath or cascade of water the maximum critical temperature of which varies in accordance with the amount of stretch applied to the fiber (stretch ratio of the fiber) while in the wash.

These maximum critical temperatures are believed to be determined by the second order transition temperature, or glass temperature (Tg) of the acrylonitrile copolymer. That is to say, the temperatures reflected in the following examples are lbelieved to be just below the second order transition temperature of the acrylonitrile copolymer in each instance. Measured Tg values vary with the means used to determine Tg, and with the state of molecular order of the fibers. It is probable that ber Tg depends upon fiber orientation and therefore upon degree of stretch or drawing. It is also probable that the presence of the solvent in the fibers at transition stages of molecular orientation lowers the Tg, and that the amount of the solvent wihin the fiber is reduced as the fiber is stretched. This would indicate that a more highly drawn fiber would have a higher Tg; and it would account for our discovery that a lower temperature is required of the Wash for solvent removal where the stretch ratio is lower than where fibers are stretched to a greater extent.

The applicability of the second order transition theory to the practice of the instant invention is largely theoretical inasmuch as it is obviously impractical to measure glass temperatures of transition state fibers during the washing and stretching operation. Whatever the reason, it has been found that substantially all residual solvents can be removed from acrylic fibers, after the 'spinning of the solution of acrylic polymer in an organic solvent into a coagulating bath of any type, by simultaneously stretching the coagulated fiber and washing it with water at a temperature not exceeding a given maximum for a given stretch ratio, as follows:

Maximum Stretch ratio: temperature, C. 1.5 65

3 BRIEF DESCRIPTION OF THE DRAWING To further understand the invention reference will be made to the attached drawing that forms part of the present application.

In the drawing, the figure is a side elevation view, partly in section, showing schematically an apparatus arrangement of a type which can be used in carrying out the process of the instant invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT By acrylonitrile polymer is meant polyacrylonitrile, copolymers, including terpolymers and higher interpolymers of acrylonitrile, and blends of polyacrylonitrile and copolymers of acrylonitrile with other polmeric materials, such as polystyrene, polyvinylchloride or other acrylonitrile copolymers. In general, a polymer made from a monomeric mixture of which acrylonitrile is at least 70 percent by weight of the polymerizable content is applicable to the practice of the present invention. Block and graft copolymers of the same general type are within the purview of the invention. Suitable other monomers include vinyl acetate, and other vinyl esters of monocarboxylic acids, vinyl chloride and other vinyl halides, dimethyl fumarate and other dialkyl esters of fumarie acid, dimethyl maleate and other dialkyl esters of maleic acid, methyl acrylate and other alkyl esters of acrylic acid, styrene, and other vinyl-substituted aromatic hydrocarbons, methyl methacrylate and other alkyl esters of methacrylic acid, vinyl-substituted heterocyclic nitrogen ring compounds, such as the vinyl imidazoles, etc., the alkylsubstituted vinylpyridines, vinyl chloroacetate, allyl chloroacetate, methallyl chloroacetate, allyl glycidyl ether, methallyl glycidyl ether, allyl glycidyl phthalate, and the corresponding esters of other aliphatic and aromatic dicarboxylic acids, glycidyl acrylate, glycidyl methacrylate, and other mono-olefinic monomers copolymerizable with acrylonitrile.

Many of the more readily available monomers for polymerization with acrylonitrile form copolymers which are not reactive with some dyestufis and may therefore be impossible or diflicult to dye by conventional techniques. Accordingly, these non-dyeable fiber-forming copolymers may be blended with polymers or copolymers which are in themselves more dye-receptive by reason of their physical structure or by reason of the presence of functional groups chemically reactive with the dyestutf, whereby the dyestufi is permanently bonded to the poly- .mer in a manner which lends resistance to removal thereof by the usual laundering and dry cleaning procedures. Suitable blending polymers may be polyvinylpyridine, polymers of alkyl-substituted vinylpyridine, polymers of other vinyl-substituted N-heterocyclic compounds, the copolymers of the various vinyl-substituted N-heterocyclic compounds and other copolymerizable monomers, particularly acrylonitrile.

Of particular utility are the blends formed of polyacrylonitrile or a copolymer of more than 90 percent acrylonitrile and up to 10 percent vinyl acetate, and a copolymer of vinylpyridine or an alkyl-substituted vinylpyridine and acrylonitrile, the said aciylonitrile being present in substantial proportions to provde heat and solvent resistance, and a substantial proportion of the vinylpyridine or derivatives thereof to render the blend receptive to acid dyestuffs. Of particular utility are the blends of copolymers of 90 to 98 percent acrylonitrile and l to 2 percent vinyl acetate and sutiicient copolymer of l0 to 7() percent acrylonitrile and 90 to 30 percent vinylpyridine to produce a blended composition with a total of 2 to l0 weight percent vinylpyridine.

The polymers just described may be prepared by any conventional polymerization procedure. The spinning dopes may also be prepared using conventional solvents and the spinning carried out in equipment generally used in the art. Any conventional coagulation means may be used although the advantages of the present invention are most readily apparent and of greatest economic importance when the fibers are wet spun into a non-aqueous liquid coagulating medium.

Referring now to the figure, a water coagulable solution comprising an acrylonitrile polymer dissolved in N, N-dimethylacetamide, N,Ndimethylformamide, dimethylsulfoxide, or the like is passed under pressure from a supply tank (not shown) through a conduit 10 and then through a candle lter 11 wherein undissolved particles and foreign materials in the solution are removed. Ordinarily, gear pumps are used to pump the solution through the filter 11 and to meter same to the spinnerette assemblyV 12. This assembly includes a spinnerette 13 and is suitably disposed below the upper surface of the coagulating liquid 14 contained in trough or bath 15. Fresh liquid 14 is supplied to trough 1S through pipe 18 and is withdrawn therefrom through pipe 20. Liquid 14 may be an aqueous solution of the same solvent as used in preparation of the spinning dope, or a non-aqueus liquid such as kerosene, butyl alcohol or a polyalkylene glycol containing a desirable quantity of the polymer solvent, typically about 5 to 20% of the total liquid. The coagulated filaments are withdraw by employment of a positively driven roller 21 or other thread advancing means, the peripheral speed of which preferably is synchronized with the extrusion speed so that the filaments during their travel may he attenuated. After passing around roller 21 and an idler roll 22, the laments are directed into a second trough 23 containing liquid 24. Liquid 24, in this preferred embodiment, is water at a maximum temperature of less than about 86. It may be circulated slowly, as a bath or rapidly, as a cascade. Fresh liquid is supplied to trough 23 through pipe 25 and is withdrawn through pipe 26. While it is quite possible to employ only a single bath, or alternatively, three or more, two have been illustrated to show a preferred embodiment including alternative subsequent treatments. The laments before emerging from the liquid in second trough 23 and being directed around a set of positively driven rolls identied by numerals 27 and 28 are passed under guides 30 and 31. The peripheral speed of rollers 27 and 28 are adjusted so that a predetermined orientation stretch will be imparted to the filaments 16 during their travel in second trough 23.

The washing operation can be accomplished in more than one stage of the process and by employment of other known washing means, However, in this invention the temperature of liquid 24 and the stretch imparted to fiber 16 between rollers 20 and 27, are critical. Minimum travel time and distance of fiber 1-6 in the critically controlled wash bath, for the accomplishment of the objective of this invention, will vary with the turbulence of the bath, the amount of stretch imparted to the fiber, as well as the number of, and distance between individual filaments. Additional and subsequent washing of the fiber may be at higher or lower temperatures.

To roller 27 additional washing liquid such as hot Water may be supplied from a spray or shower head 32, the liquid being collected in a container or tray 33. After leaving rollers 27 and 28, the filaments may be directed through a third trough 34 by being passed under guides 35 and 36. The liquid 37 in this trough is normally water, and, as with the water applied from spray or shower head 32, may or may not conform in temperature to that of bath 24. In addition to further washing of the fibers, bath 37 may serve a Variety of purposes. It may be used as a situs to relax and shrink the ber thereby imparting dimensional stability, in which case the filaments are withdrawn from trough 34 by means of driven roller 38 and associated idler roller 40 operated at a peripheral speed less than that. of thev peripheral speed of rollers 27 and 28. The bath may be used to impart a Vfurther stretch to the fiber resulting in a finer denier filament, in which c ase the fiber will` of course be withdrawn from the bath at a higher speed than the speed of entry. The bath may also be used to heat set the fiber under constant tension in boiling water in which case the fibers would be withdrawn from the bath at the same speed as the speed of entry. Each of the above additional uses of bath 37 would involve temperatures of the liquid in excess of the maximum critical temperature of liquid 24. Fresh water is supplied to trough 34 through an inlet pipe 41 and is withdrawn through outlet pipe 42. The filaments may then be passed through a finish bath liquid 43 contained in a vessel 44 and composed of a lubricant or like beneficial treating agent. The filaments after being withdrawn from liquid 43 are dried. As illustrated in FIGURE l the filaments are continuously directed around a pair of driven drying drums 45 and 46 heated internally with steam or the like. Thereafter the filaments are subjected to additional operations such as crimping and then are collected in the form of continuous filament yarn, tow, or they may be cut to staple fibers.

EXAMPLES Four polymeric compositions are selected as demonstrative of the breadth of the invention: A is a binary copolymer of acrylonitrile, B is a terpolymer, C is a blend of two acrylonitrile copolymers, and D is a blend of the same two copolymers with a polyvinylchloride hornopolymer. Spin dopes were prepared from each of these polymeric compositions by dissolving in an appropriate quantity of N,Ndimethylacetamide to provide solutions of satisfactory viscosity for spinning. Compositions and physical properties are given in Table I below.

then spun into laments under identical conditions except that the temperature of the water in the cascade was maintained at a maximum of 85 C. The results of solvent analysis on the dried filaments are given in Table 3.

TABLE 3 Percent solvent in dried fiber TABLE I Dope Polymer Polymer solids, Composition sp Tg, C. percent Polymer: A

A 94%aerylomtrile, 6% vinyl acetate 0.165 87 24 B 89.8% acrylonitrile, 7.5% vinyl acetate, 2.7% vinyl bromide 0 155 87 25 f 93% acrylonitrile, 7% vinyl acetate (88%)1 0.267 0 255 86 18 C 50% acrylonitrile, 50% methylvinyl pyridine (12%) 0.155 D 87% Blend C, 13% polyvinylchloride 0. 24 86 19 As a demonstration of the conventional prior art, the several dopes were then individually spun using conventional equipment into a nonaqueous coagulating bath maintained at 95 C. and consisting essentially of a polyethylene glycol of average molecular weight 1000 and approximately of N,Ndimethylacetamide. The filaments so formed were withdrawn from the coagulating bath after a travel therein of and thereafter directed through a boiling water cascade twenty feet in length and -wherein the fibers were stretched to 4.0 times their initial length. The filaments were then dried on rotating drums maintained at a temperature of 125 C. Samples of the dried filaments were analyzed for solvent content, the re- In each of the above examples, the residual solvent in the filaments is too high for commercial acceptability. In demonstration of the present invention, additional quantities of the same four polymeric compositions were peratures, as demonstrated with polymer B, are given in Table 4.

The critical nature of the above maximum temperatures and minimum stretch ratios is further demonstrated by the following experiments conducted with polymer A under conditions identical to the previous experiments except for the indicated variations in cascade stretch and water temperature. As shown in Table 5, Examples 20 and 23, temperatures lower than the critical maximum at a 'given stretch ratio, or stretch ratios higher than the critical minimum at a given temperature do not adversely affect the washing performance. However, it is preferable for reasons such as ease of fiber stretching and economic efficiency in the subsequent drying process to operate near the permissible maximum in both stretch ratio and water temperature.

Any departure from the description herein that conforms to the present invention is intended to be included within the scope of the claims.

We claim:

1. Ina process of producing a iilament from an acrylonitrile polymer wherein the said polymer is dissolved in a solvent and a stream of the resulting solution is eX- truded through a spinnerette and thereafter directed into a coagulating bath, and wherein the resulting solventladen filament is withdrawn from the coagulating bath and stretched in a water bath to orient the polymer molecules thereof after which the filament is dried and collected, the improvement of maintaining the water bath at just lbelow the glass transition temperature of the filament at the imparted stretch ratio, whereby the filament is rendered substantially solvent-free.

2. The process improvement of clai-m 1 wherein the filament is stretched at least about 4 times.

3. The process improvement of claim 1 wherein the temperature of the water bath is less than about 81 C. and the filament is stretched at least about 3 times.

4. The process improvement of claim 1 wherein the temperature of the Water bath is less than about 76 C.

and the filament is stretched at least about 2 times.

5. The process improvement of claim 1 wherein the temperature of the water bath is less than about 66 C. and the lament is stretched at least about 1.5 times.

6. In a process of producing a filament from an acrylonitrile polymer wherein the polymer is dissolved in 8 a solvent, and a stream of the resulting solution is extruded through a spinnerette and thereafter directed into a coagulating bath, and wherein the resulting filament is withdrawn from the coagulating bath, stretched for orientation in the presence of a liquid medium, washed, and dried, the improvement comprising combining the draw step with the wash step by using as a draw bath, a water bath at just below the glass transition temperature of the filament at the imparted stretch ratio, whereby the solvent-laden filament is simultaneously stretched and rendered substantially solvent-free.

7. The process improvement of claim 6 wherein the temperature of the water bath is below about 86 C., and the filament is stretched at least about 4X.

'8. The process improvement of claim 6 wherein the temperature of the water bath is less than about 81 C. and the filament is stretched at least about 3 X.

9. The Aprocess improvement Yof claim 6 wherein the temperature of the Water bath is less than about 76 C. and the filament is stretched at least about 2X.

10. The process improvement of claim 6 wherein the temperature of the water bath is less than about 66 C. and the filament is stretched at least about 1.5

y References Cited UNITED STATES PATENTS 2,716,586 8/1955 Terpay 264--182 2,723,900 11/1955 Hooper 264-182 3,088,188 5/ 1963 Knudsen 2.64-182 X FOREIGN PATENTS l100,414 11/1966 Japan.

38/23,264 1l/1963 Japan.

37/3,920 6/1962 Japan.

39/ 22,042 10/ 1964 Japan. lli/1,825 2/ 1965 `apan.

JULIUS FROME, Primary Examiner I. H. WOO, Assistant Examiner U.S. C1. X.R.

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