US 3842151 A
Description (OCR text may contain errors)
Oct. 15, 1974 v.s'roY ETA METHOD FOR PREPARING FIBERS FROM POLYMER SOLUTIONS Filed Dec. 18, 1972 2 Sheets- -Sheet 1 FIG. I
Oct.15, 1974 v Y ETAL 3, 42,151
METHOD FOR PREPARING FIBERS FROM POLYMER SOLUTIONS Filed Dec. 18, 1972 z Sheets-Sheet u I I i I 2/ 8 I I 1 I 23 I 22 I W F163 l f F l I United States Patent US. Cl. 264-203 4 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for spinning polymer solutions into fibrous filaments wherein the solution is extruded through a spinneret and passed sequentially first through a gas medium, and then through a coagulating bath. Simultaneously the pressure of the gas medium is maintained less than the pressure on the liquid bath.
The present invention relates to a method and apparatus for treating polymer solutions to form fibers, strings, chords, tubings, films, etc. and particularly to a method and apparatus combining the advantages of dry and wet spinning.
Polymer solutions are usually spun to form fibers elther by the dry or by the wet method. In the following text all fiber-like articles having lateral dimension much smaller than the length are, for brevity sake, called fibers. The dry spinning consists in extruding the poly mer solution through a spinneret into a heated tubular shaft, where the solvent evaporates gradually, so that a substantially dry fiber exits from the bottom of the shaft. An additional step in which the fiber is washed is sometimes included, because the last traces of solvent are very diflicult to remove in the atmosphere of the shaft. The major advantage of this known method, compared with the wet method, lay in the ability to obtain 2 to 4 times higher spinning velocity, 1.5-2 times higher concentration of polymer in the starting solution (and, thus, resulting in higher output of the device and the need to recover a smaller amount of solvents). On the other hand, polymers which are not sufliciently thermo-stable or soluble in suitable volatile solvents, cannot be treated in this method. Further disadvantages are that considerable loss of solvent occurs, and that a device of considerable height is required if reasonable spinning velocity is to be achieved. Moreover, the solvent used must be properly volatile, since solvents with too high boiling points require a shaft that is enormously long and a temperature too high to effect evaporation. An additional washing is often necessary.
On the other hand, solvents that are too volatile, such as methylene chloride used for spinning cellulose triacetate, are recoverable only with great dilficulty. The fiber thus formed has poor mechanical properties, because the solvent vaporizes so quickly. Also the fiber cannot be oriented during subsequent drawing steps and microvoids often occur in its structure.
Wet spinning consists generally in extruding a polymer solution through a spinneret into a coagulation bath, where the polymer hardens and the solvent is washed off with a suitable liquid, e.g. with water. In this way thermal strain on the fiber does not occur, and the coagulation path is shorter than the atmospheric dry shaft. Moreover, solutions of polymers in non-volatile solvents can be treated by this method, as for example, polyacrylonitrile dissolved in concentrated aqueous zinc chloride or sodium rhodanide solution, proteins or cellulose xanthate in alkalineous solutions, etc.
On the other hand, the solvents used need to be recovered from more or less diluted solutions, and the concentration of the polymer in the spun solution and spinning velocity are rather low, so that the output of a wet device is usually several times smaller than that of a device for dry spinning, of the same size.
The above mentioned virtues and disadvantages will become obvious by comparing conditions of both methods of spinning:
In either case the extruded polymer solution leaves the spinneret forming a jet streaming through a surrounding fluid medium. The solvent penetrates by diffusion from the solution into the gaseous or liquid surrounding medium. If the diflz'usion into the liquid surrounding is very fast, a skin consisting of a coagulated, more or less plasticized polymer is formed on the surface of the polymer stream. The skin obstructs further diffusion and the entirety works as an osmotic cell due to the fact that the molecules of the solvent and those of the precipitant have a difierent size. The molecules of the solvent are usually more bulky, so that the precipitant (such as water) penetrates more rapidly into the spun stream of the polymer than the rate at which the solvent can diifuse outwardly. Thus microvoids occur in the structure of the resultant filament causing crimps, i.e. heterogeneous structure with poor mechanical properties. Coagulation must be carried out gently and slowly in order that the loss of solvent concentrated on the surface layer of the jet stream could occur only by diffusion from its inner parts. Thus the coagulation rate must be reduced, e.g. by reducing the temperature of the coagulation bath, or by coagulating the fiber in several successive baths with gradually increasing concentration of the precipitation agent.
For the same reason the polymer cannot be in a concentration which is too high. The more bulky the molecule of the solvent is compared with that of the precipitation agent, the lower the polymer concentration of the solution and the slower coagulation, is required.
Another factor limiting the spinning velocity is the splashing of the coagulation liquid by the movement of the filament and the rollers (galettes) at high drawing-off velocities, and the hydrodynamic resistance of the coagulation bath to the stream of the polymer solution before it solidified, resulting in diminishing the diameter of the polymer stream and sometimes even in its interruption.
Still another reason for interruption of the jet stream of the polymer solution is its viscoelastic behavior espec1ally at high spinning velocities. Usually the said interruption takes place in the area, where the jet is still in liquid state. Since it is not possible to eliminate this area, or to shorten it in a substantial extent, by acceleratmg the coagulation, due to above mentioned osmotic effect, the formation of the heterogeneous fiber having poor mechanical properties, results.
These troubles do not occur in dry spinning, since the gaseous medium forming the atmosphere through which the stream passes, creates a negligible resistance to the fiber, and the osmotic effect does not occur. That is why it is possible to form a skin on the surface of the jet stream of a polymer solution consisting of very concentrated polymer, to thus provide mechanical strength to the jet immediately on leaving the spinneret. The skin remains strongly swelled with solvent, and thus it may be deformed by the tension of the drawing-off step. This tension causes stretching and orientation of this halfsolidified jet below the spinneret and thus the resulting fiber may gain an enhanced mechanical property. A disadvantage of this preliminary stretching is, however, that the fiber is more oriented on the surface than in its inner core and is thus laterally heterogeneous. This so called skin effect decreases the quality or fuel of the fiber.
Still another disadvantage of the dry spinning is the thermal strain on the fiber, the need for bigger spinning devices and the limited choice on the solvents regarding their boiling points and the low possibility of their recovery, as mentioned above.
It is the object of the present invention to combine the advantages of both above mentioned methods, and at the same time eliminate their shortcomings.
It is another object of the present invention to improve the spinning of fibers from polymer solutions of all types, which is more economical and provides improved fibers over those obtained from the prior art.
It is a further object of the present invention to provide apparatus for the spinning of fibers from polymer solutions to carry out its method and obtain its advantages.
Broadly the present invention comprises the steps of sequentially and in a continuous manner extrcding a polymer solution into a gaseous or dry atmosphere maintained at a pressure below ambient atmospheric pressure and thereafter passing the extrusion through at least one coagulating bath.
According to the present invention a polymer solution is extruded through a spinneret into a tube or shaft, which is on its upper end and sealed against gas fiow by a lid connected with the said spinnerett, and its lower end is placed below the level of a suitable coagulation bath, open to atmosphere. Pressure within the tube, between the spinneret and the level of the coagulation bath is maintained lower than the pressure outside the shaft, so that the level of the coagulation liquid is higher in the shaft than in the outer coagulation bath.
Full details of the present invention are set forth in the following description of the several embodiments for carrying out the method, and are seen in the accompanying drawings.
In the accompanying drawings:
FIG. 1 is a schematic side view of extruding apparatus and dry and wet coagulation apparatus according to the present invention;
FIG. 2 is a view similar to that of FIG. 1 showing the means for circulating the coagulating liquid, and for maintaining the necessary pressure level in the system;
FIG. 3 is a view similar to that of FIG. 2 showing the gas circulation system; and
FIG. 4 is a view showing an alternative embodiment for the apparatus of FIG. 1.
Before turning to the description it will be appreciated that reference to polymer solutions is intended to cover all polymers, copolymers, and mixtures, in solutions, suitable for the production of fibers, filaments, etc. by the coagulation techniques in dry or Wet system. Specific disclosures of all polymer solutions are not believed necessary since they do not per-se form a part of this invention.
The method of the present invention may be best exemplified by reference to the basic arrangement as seen in FIG. 1. The polymer solution 1 (e.g. polyacrylonitrile dissolved in 65% nitric acid) is extruded by a piston through the spinneret 2 fixed transversely in a tubular shaft body 4. The solution forms a stream in a shape of the fiber 3 passing through the space 6 between the spinneret 2 and a coagulation liquid 5 is open to atmosphere. The space 6 is limited by the body of the shaft 4. The pressure of the gas in the space 6 is reduced by means of vacuum forming attachment 7 which sucks off the liquid in bath 5, so that the level of the coagulation liquid can be elevated on any desired height and kept there. The stream of the polymer solution 3 proceeds in the space 6 through a flow of a gaseous medium, which may, if desired, be heated, and which is preferably led counter-currentwise to the movement of the stream. The gas is led into the space 6 through an intake port 8, situated above the level of the liquid 5, and out through the outlet port 9, situated below the fixed spinneret. The spinneret seals the upper end of the space 6 in gas-tight condition. As the jet of the polymer solution 3 proceeds through the space 6, a certain portion of the solvent in the polymer solution evaporates at the elevated temperature and at the reduced pressure, so that the polymer solution meets the interface of the coagulation liquid 5 with higher viscosity and concentration of polymer than if the polymer solution were to be extruded directly into the coagulation liquid or even through an air gap, having normal atmospheric pressure and temperature. The system shown on one hand, reduces the amount of the solvent, which must be recovered from the coagulation bath, and in the other hand increases the spinning velocity due to the higher viscosity of the jet of the polymer solution meeting the interface of the coagulation liquid.
The evaporation in space 6 of the solvent from the surface layer causes the formation of a skin on the fiber filament, which retards the more rapid diffusion of the solvent. Thus, a homogeneous fiber may be formed even if the coagulation is rather swift. The result is similar to that of the spinning of a polymer solution having a higher viscosity. Of course, the viscosity of the solution is, in practice, limited since the hydrodynamic resistance in the spinneret places restraints on it. Moreover, if the viscosity is too high, undesirable viscoelastic effects occur just below the opening of the spinneret.
The reduced pressure below the spinneret helps in the extruding of the solution, so that the actual spinning pressure exerted by the piston may be lower. The skin formed by the rapid evaporation of at least a portion of the sol vent from the surface of the polymer solution jet stream provides mechanical strength, so that the stream cannot be broken or interrupted due to the viscoelastic effects. Compared with conventional dry spinning, the advantage of the method according to the present invention is that the solvent evaporates at a reduced pressure and thus the fiber undergoes less thermal strain.
The rate of the evaporation can be controlled not only by the supply of heat to the gas and by the countercurrent flow of the gaseous medium in the space 6, but also by the retention of the solvent vapors in this space. The saturation of gas with solvent can be thus controlled more readily than in usual open shaft, and polymer solutions even being very volatile solvents (such as methylene chloride), can be spun. The solutions are otherwise not suitable for the dry spinning due to their excessive volatility. Moreover, the solvent may be recovered from the gas in a completely closed circuit, reducing substantially the loss of solvent. Moreover, the spinning device itself is much shorter than the usual apparatus and the last traces of the solvent, which can not be easily removed by evaporation can be washed off in the coagulation bath rather than extending the shaft indefinitely.
The present invention utilizes the dry spinning at the first stage of the coagulation, and the wet method at the second stage, both in the conditions, where their virtues can be utilized most advantageously. Accordingly even a bicomponent fiber can be produced besides the common homogeneous fiber and such fibers can be permanently crimped by suitable aftertreatment, eg by the stretching and the stabilization. In forming bicomponent fibers, the apparatus of FIG. 1 is provided with a split spinneret, each part charged with a polymer solution of diiferent properties feeding into each part of the orifice. Basically, the method of forming crimped bicomponent fibers through a split spinneret is well known as such, but its combination with the method according to our invention provides numerous advantages.
First, a spinneret with rather large orifices can be used. The diameter of the orifice may be much larger than that of the fiber formed, snice the latter can be diminished by stretching the polymer solution jet before it meets the interface of the coagulation liquid.
Second, the bicomponent fiber made by hitherto known methods often tends to split longitudinally into two parts, because both solutions meeting each other in the orifice do not have enough time to mutually penetrate into each other to an extent sufficient to form a lasting joint.
It has been found now that the adhesion of both parts of the bicomponent fiber is surprisingly good, if the fiber is produced according to the present method. Apparently, this results from at least a partial mixing of both solutions in the low pressure space 6 between the orifice of the spinneret 2 and the coagulation bath 5, supported also by common stretching of both parts of the viscous jet of the polymer solution under uniform and simultaneous conditions.
The enthalpy or pressure/heat conditions for evaporation of the solvent can be supplied either by heating the gaseous medium supplied to space 6, by recycling the gas below the spinneret, by radiant heating of the shaft (e.g. by means of heating elements in the wall of the shaft), or by means of a heated spinneret.
The advantage of the latter is that viscosity of the solution decreases due to the elevated temperature only in the smallest hydrodynamic cross-section, so that it is possible to use higher concentration of the polymer in the solution, or to raise the spinning velocity without resulting in the mentioned undesirable viscoelastic effects, or the need to apply a lower pressure by the piston before the spinneret. Importantly, the low pressure beneath the spinneret permits the use of less heat and a lower temperature in the space 6. Of course, it is often useful to combine more than one of the methods of heating. The amount of the applied enthalpy must not be too large, since the polymer solution is often not sufficiently thermo-stabile, and it may decompose if it were. Moreover, the solvent must not boil when the solution comes into the space 6 having the reduced pressure. The resultant bubbles of the boiling solvent vapor would cause the breaking and interruption of the filament. That is why it is most advantageous to supply a major part of the heat to the space 6 below the spinneret, and over a path as long as possible. This can be easily done by heating the gaseous medium itself before entry and/or by infrared irradiation on the length of the cylinder 4.
As the fiber pre-formed in space 6 mets the coagulation liquid 5, more of the solvent Washes off and the fiber further solidifies and hardens. The hardened fiber is drawn off via galette to a further washing, elongation, shape stabilization and reeling apparatus. As mentioned above, the coagulation rate must be necessarily limited to a reasonable value, for otherwise a heterogeneous fiber having bad mechanical properties may be formed. For this reason it is preferred to use a suitable technological measure to prevent rapid coagulation. For instance, the fiber may be led through an array of baths, each having a gradually increasing concentration of the precipitation agent and gradually decreasing concentration of the solvent.
It is another advantage of the present process that for mild and gradually proceeding coagulation it is not necessary to include any additional means such as the successive baths, but on the contrary, the desirable distribution of coagulating concentration may be established spontaneously, since it is possible to establish the highest concentration in the spinning shaft 4. This occurs because the vacuum causes a major part of the solvent to be transferred into the space 6. The difference of the concentrations in the shaft 4 and in the coagulation bath 5 itself equilibrates by diffusion and correction through the crosssection of the lower end of the shaft, where both spaces communicate with each other. Thus, higher concentration is set up in the shaft 4 than in the coagulation bath 5. The coagulation is further influenced by the skin, which is formed on the fiber due to the fact that some part of the solvent evaporates above the interface level of the coagulation liquid itself.
Another advantage of our process is the shape of the vessel holding coagulation bath 5 may be L-shaped. Such a shape does not take much room either in the vertical, or in the horizontal direction. Also the over-all volume of the bath may be smaller than that of those common in the wet spinning, because of the pre-formation of the filament and the previous evaporation of solvent.
To sum up, the method according to the present invention allows various advantages to be attained simultaneously:
1. Small overall size of the device;
2. The possibility of using higher spinning velocities;
3. The smaller amount of the solvent need be recovered;
from the coagulation bath;
4. The solvent evaporates at lower temperature;
5. The coagulation is more uniform due to the spontaneous establishment of a concentration gradient in the gas cylinder and coagulation bath and in the fiber itself;
6. Permits the use of a larger orifice of the spinneret and stretching the solution jet stream below the spinneret.
7. The spinneret is not in a contact with the coagulation liquid, and therefore its corrosion is prevented.
If the polymer is dissolved in a solvent mixture, it is preferred to establish the parameter to evaporate the most volatile solvent or an azeotropic mixture with minimum boiling point.
The attachment for maintaining the interface between liquid and gas in the shaft at a constant chosen predefined elevation is seen in FIG. 2. The space 6 between the spinneret and the surface level of the bath is evacuated by means of a vacuum pump not shown but indicated by the arrow A through the pipe 7. The mouth 11 of the pipe is placed at the desired height of the level to be obtained and valve 12 is incorporated between the mouth 11 and the vacuum pump. The valve comprises a seat 13 placed at its upper end in which a plug 14a formed at the end of float 14 is adapted to seat. The float 14 is less dense than the coagulation liquid so that when the level in the shaft reaches the mouth 11, the valve 12 closes. On the contrary, if vacuum in the shaft decreases (and, consequently, the level of the bath 5 in the shaft falls), the valve 12 will open and the bath fills the vessel until it reaches to the mouth 11 again.
The conduit pipe 7 leads from the valve 12 to a separator device 15 into which the coagulation liquid flows. The separator is equipped with three-way cock valve 16 at its bottom which is opened either towards a receiving vessel 17a or 17b. If either of the two vessels is full, the cock 16 is turned to the opposite position. Vacuum is eliminated in the full receiving vessel 17a or 17b by opening a cock valve 18:! or 18b, respectively, and the liquid is lead or discharged through the cock valve 19a or 19b back into the coagulation bath, at a rate to maintain the level in the system at the desired height.
Of course, it is possible to use many other systems for keeping the level at the desired height, but the described one proved itself to be very simple and reliable.
Air, nitrogen, argon, carbon dioxide, etc. may be used as the gaseous medium flowing in space 6 below the spinneret. The purpose of the circulation of the gas below the spinneret is to remove vapors of the solvent from this space to prevent saturation or the accumulation of an equilibrium concentration to be established there. Such saturation would prevent evaporation of the solvent from the filament stream.
In some other cases it is nevertheless desirable to control the evaporation in the cylinder 4. For instance, if polyacrylonitrile is spun from a zinc chloride solution only water would evaporate in the gaseous atmosphere. Such evaporation would be useless, and even undesirable. There is no advantage of solvent recovery in such cases. The other advantages, however, hold (e.g. advantageous establishment of concentration, shape of the device, etc.).
Apparatus for the circulation of the gaseous medium without influence of vacuum or pressure in the shaft is shown in FIG. 3. The gas together with the desired amount of solvent vapor is sucked off through the outlet 9 by means of the pump 20. The gas is cooled in the heat exchanger 21 where some part of the vapors condenses. The condensed part of the solvent is separated from the gas in the separator 22 which is itself preferably cooled by a cooling jacket such as a Water or refrigerant system. The gas is then led through the heater 23, the inlet 8 back into the space 6 between the spinneret 2 and the level of the bath 5. The condensed solvent is let out of the separator 22 through three-way cock valve 16' alternately to the reservoirs 17'a and 17b. If either of the two reservoirs 17'a or 17'!) is full, it is disconnected from the separator 22 by turning the cock valve 16 to the opposite position, air or vacuum in the reservoirs is eliminated by turning on the valve 18a or 18'b, respectively, and the recovered solvent is led through the cock valves 19a and 19b to apparatus for further treatment. Various other arrangements can be used to obtain the circulation of solvent vapors without destroying the vacuum in the cylinder between the spinneret and the level of the coagulation liquid.
The solvent may be recovered from the gaseous medium by condensation of the solvent vapors in a condenser, by absorption in a suitable liquid with a correspondingly high boiling point, or by adsorption on a solid surface (e.g. on charcoal). The choice of the solvent recovery method will depend mainly on the type of the solvent and on economical considerations.
The outlet for sucking-off the solvent vapors is advantageously placed as near to the spinneret as possible, and it is particularly advantageous to place the outlet in the body of the spinneret, itself offset from the spinneret nozzle.
The method according to this invention is suitable for manufacturing all fibers spinnable from a solution. Of course, it is more useful for spinning polymer solutions from which at least a part of solvents can be recovered by evaporation, as it is obvious from the previous description. Examples of such solutions are those of polyacrylonitrile in ethylene carbonate, dimethyl formamide, nitric acid or concentrated aqueous zinc chloride or sodium etc. rhodanide solutions, those of polyvinylchloride in cyclohexanone, those of chlorinated polyvinylchloride in acetone-alcohol mixtures, those of cellulose triacetate in methylene chloride, those of cellulose acetobutyrate in acetone, those of cellulose diacetate in acetone-alcohol mixtures. These examples should not be taken as limiting the use of the present invention.
The cylinder 4 in FIG. 1 may have any suitable crosssection, e.g. circular, squared, elliptical, etc. It may also be divided into two or more zones 25, 26, and 27 having different temperatures, and equipped with one or more heating or cooling jackets as seen in detail in FIG. 4. For instance, it is sometimes useful to keep different temperatures in adjacent portions of the spaces 6 in FIG. 4 and in the portion of the coagulation liquid in the cylinder 4. Also the temperatures of the coagulation liquid in the cylinder and in the coagulation bath need not be the same. It is especially advantageous, if the space between the spinneret and the level of the liquid zone 26 is heated, as well as the liquid, in the coagulation bath 5, while the liquid in the cylinder, zone 27, is cooled. This measure helps to diminish the evaporation of the solvent from the surface of the coagulation liquid into the space 6 below the spinneret and, consequently, permits the rate of evaporation of the solvent from the jet stream of the polymer solution to rise. Moreover, it helps to enhance the conditions necessary for formation of a homogeneous fiber in the critical first stage (dry coagulation) of the coagulation. It also helps to equilibrate concentrations of the liquid in the shaft and in the coagulation bath by convection and maintains a uniform gradient. The elevated temperature of the coagulation bath is useful for accelerating the washing off of the solvent in the area, where the coagulation rate is no longer critical for the quality of the fiber.
Various modifications and embodiments will of course be evident to those skilled in this art. Accordingly, the foregoing disclosure is to be taken as illustrative only, and not limiting of the scope of the present invention.
What is claimed is:
'1. A method of spinning a solution containing polymer dissolved in a solvent therefor, into fibrous filaments from a spinneret, comprising the steps of extruding said polymer solutionfrom said spinneret, passing said extruded polymer solution through a field of gaseous medium held at subatmospheric pressure, into which a portion of said solvent is removed from said polymer solution, continuously supplying gaseous medium to said field and simultaneouly exhausting gaseous medium carrying said solvent therefrom and thereafter passing said remaining polymer solution into a liquid coagulating bath and coagulating said remaining solution to form filaments, said field of gaseous medium being maintained at a pressure less than that of said liquid coagulating bath and while at least part of the surface of said liquid coagulating bath is in direct contact with said field of gaseous medium, the level of that part of the surface of said coagulating bath being dependent on the pressure of said gaseous medium thereon.
2. The method according to claim 1 wherein the temperatures of the gaseous medium and the coagulating bath are different.
3. The method according to claim 1 including maintaining the surface level of the liquid coagulating bath substantially constant while reducing the rate of exhaustion of the gaseous medium and increasing the level of said bath with respect to the gaseous medium.
4. The method according to claim 1 including recycling the gaseous medium after removing the part of the evap orated solvent therefrom.
References Cited UNITED STATES PATENTS 1,549,364 8/1925 Kempf et a1 425-67 2,246,990 6/ 1941 Wuppermann 425-68 2,318,679 5/ 1943 Dreyfus 264-l78 F 2,323,383 7/ 1943 Dreyfus 264178 F 1,619,768 3/1927 Schubert 264179 2,581,559 1/1952 Ryan 264-180 2,987,768 6/1961 Given 264179 FOREIGN PATENTS 289,233 4/ 1928 Great Britain.
27,765 1/1970 Japan.
JAY H. WOO, Primary Examiner U.S. Cl. X.R. 264-40, 184, 205