EP0031078B1 - Fine denier synthetic fibres and filaments and dry spinning process for their production - Google Patents

Abstract

Dry-spun synthetic fibers and filaments having an individual as-spun denier of 3 dtex are obtained by dry spinning a viscosity-stable spinning solution with a draft of at least 20.

Description

It is known to use the dry spinning process to produce acrylic fibers from spinning solutions with a solids content of 25 to 35% of thread-forming acrylonitrile polymers with at least 85 wt are cf. for example DE-A-25 02 195.

Recently, efforts have been made in the man-made fiber industry to produce synthetic fibers with particularly fine titers. Such fine-titer fibers, which generally have a final fiber titer between 0.4-0.8 dtex, have compared to conventional synthetic fibers, e.g. Acrylic fibers, which are in the titre range from 1.3 dtex, have a number of advantages such as: high gloss, appealing chandelier, elegance in the fabric, soft feel, high flexibility and pliability as well as high fiber strength, due to the high number of fine fibers in the yarn cross-section.

M. Okamato has summarized in Chemiefaser / Textilindustrie (1979), volume 1, pages 30 - 34 and volume 3, pages 175 - 178, all processes known to date. As can be seen from this overview, fine-titer synthetic fibers can mainly be changed by apparatus in the spinning process, e.g. by flash and blow spinning by shear, coagulation, impact or centrifugal force methods. With conventional spinning methods, only the spinning of mutually incompatible polymer mixtures to polymer blend fibers with a matrix / fibril structure has gained importance. Removal of the polymer matrix results in fine titre fibril fibers, which are mainly used as synthetic upper leather.

The object of the present invention was to produce fine-titer acrylic fibers by a dry spinning process.

In order to achieve very fine titre fibers using such a process, the spinning solution in the spinning shaft must be subjected to a high degree of warpage. The warping (V) during spinning is defined as the ratio of the take-off speed to the ejection speed

• F = Fördermenge in ccm/Min.
• Z = Anzahl der Düsenlöcher
• d² = Düsenlochdurchmesser in cm

The ejection speed (S) results in

• F = flow rate in ccm / min.
• Z = number of nozzle holes
• d ² = nozzle hole diameter in cm

In the conventional dry spinning process of, for example, acrylic threads, the spinning solutions are warped by about 10 to 20 times. Attempts to warp such spinning solutions higher under the usual spinning conditions used. thread breaks occur until the spinning pattern in the nozzle area finally breaks down. It is therefore not possible to obtain fine-titer threads and fibers by simply increasing the warpage in a dry spinning process.

It has now surprisingly been found that even with a dry spinning process, the high delays required to produce fine and finest titers can still be exerted if, on the one hand, spinning viscosity-stable spinning solutions and, on the other hand, choose mild thermal conditions in the spinning shaft which slow down the evaporation of the spinning solvent condition than usual in a conventional dry spinning process.

The present invention therefore relates to a process for the production of synthetic fibers and threads from spinning solutions with a solids content of 25 to 35% of thread-forming acrylonitrile polymers with at least 85% by weight of acrylonitrile units in polar organic solvents after a dry spinning process at a spinning shaft temperature of preferably between 30 ° C. and a value below the boiling point of the spinning solvent used and a spinning air temperature of at most 300 ° C and further processing of the spinning material in a manner known per se to produce fibers or threads, which is characterized in that thermally pretreated spinning solutions are used, the viscosity of which changes during the spinning time changes by a maximum of 5% that the spinning solution is spun with a delay of at least 20 and that spinning produces threads with individual spinning titers of 3 dtex and below.

The process according to the invention is in principle a dry spinning process which can be carried out with the same equipment as a process by which coarser titers are spun. So can for example, with the usual spinnerets with hole diameters of about 0.15 to 0.8 mm, preferably 0.2 to 0.4 mm, and in usual spinning shafts. With average K values of the polymers of about 80, the spinning solutions thus have viscosities of about 20 to 100 falling ball seconds at 80 ° C. (for the falling ball method, see K. Jost, Rheologica Acta (1958) Vol. 1, No. 2-3, page 303).

The viscosity stability of the spinning solution is important so that the high warpage, which is preferably 30 to 500 but can also be higher, can be exerted by the process according to the invention. Preferred are spinning solutions whose viscosity (measured in falling ball seconds) changes during the spinning time, i.e. usually less than 1% over hours, but ideally not at all. Such solutions have proven to be particularly highly warpable, while spinning solutions, the viscosity of which is not constant, are more prone to thread breaks at high warpage (see Example 2). A viscosity-stable spinning solution can be prepared by keeping the solution at a certain minimum temperature for a certain time before spinning.

It is obvious that the preparation of such a viscosity-stable solution depends on the nature of the polymer used and that of the solvent selected. The known polar organic solvents are suitable as spinning solvents, in particular dimethylacetamide, dimethyl sulfoxide, ethylene carbonate, N-methylpyrrolidone, but preferably dimethylformamide. In the case of polymers made from 100% acrylonitrile and with usual K values of e.g. 80, the above-mentioned thermal pretreatment when using dimethylformamide (DMF) as solvent is at least about 4 minutes at at least about 140 ° C. Acrylonitrile polymers containing comonomers, as are common in this technology, can be pretreated at somewhat lower temperatures of approx. 125-130 ° C for the required period of time in order to achieve the desired viscosity stability of the solution. Depending on the choice of polymer and solvent, some preliminary tests to determine the optimal conditions for thermal pretreatment to achieve viscosity stability are recommended, if not necessary.

The above-mentioned dependency of the process products on the boundary conditions explained below is understood as follows: It has been shown that, following the process according to the invention, it is completely surprising that not only the dumbbell-shaped fiber cross sections usually obtained in dry spinning can be obtained, but also circular, round and bean-bis kidney-shaped, depending on how the thermal conditions in the spinning shaft are selected.

As far as the thermal conditions in the spinning shaft are concerned, it is very difficult to provide absolute information about this, as is obvious to the person skilled in the art, since these thermal conditions e.g. depend on the physical data of the chosen spinning solvent.

If, for example, dimethylformamide is used as a solvent, it can be said in general about these thermal conditions in the spinning shaft that the spinning solution should not have a temperature of above 150 ° C, the spinning shaft temperature should not exceed 200 ° C and the spinning air temperature at most about 400 ° C should be.

At low spinning solution temperatures, extremely high warpage can be achieved and thus very fine titers can be spun. In general, it can be said again that the lower the spinning solution temperature, the higher the delay can be selected. However, low spinning solution temperatures require viscosity-stable spinning solutions, since this is the only way to prevent cold spinning of the spinning solution. For example, a single spin titer of 0.2 dtex could be obtained from a viscosity-stable acrylic spinning solution of 35 ° C. with a delay of 457, which after 3.6 times stretching resulted in threads with a final titer of 0.07 dtex (Example 1).

What was found for the spinning solution temperature applies equally to the shaft and air temperature in the dry spinning of fine (st) titer fibers according to the invention. Low temperatures allow spinning with high warpage due to weak solvent evaporation (e.g. DMF) in the spinning shaft and thus the production of extremely fine titers. With increasing spinning titer from approx. 1 dtex, however, the spinning temperatures should be increased due to the increased polymer throughput in order to avoid sticking and thread breaks.

In particular, a non-dumbbell-shaped cross-sectional shape of the fine-titer fibers is always obtained by the process according to the invention if the spinning conditions are chosen to be as mild as possible and the work is carried out with high delays. For this purpose, the spinning solution, for example, is cooled to temperatures of about 20 ° C. to about 100 ° C. after the viscosity-stabilizing thermal treatment and before spinning, at the same time the spinning shaft temperature is set to a value between about 30 ° C. and preferably below the boiling point of the solvent used and with Spinning air worked up to about 300 ° C. In other words, care is taken to ensure that the solvent from the solution stream emerging from the nozzle is not suddenly or at least relatively quickly evaporated, but rather very gradually and as uniformly as possible over the entire length of the shaft. This results in circular to round cross-sectional shapes that are completely unusual for dry-spun threads and fibers. On the other hand, if the thermal spinning conditions are shifted to the above-mentioned upper ranges, i.e. one spins e.g. an acrylonitrile polymer / DMF spinning solution which has a temperature of about 90-150 ° C, at shaft temperatures of e.g. 150-200 ° C and air temperatures of 300 ° C and more, the solvent evaporates more quickly, with the result that the delay can not be chosen as high as in the previous case, so that the fiber cross-sections show the well-known dumbbell shape. If the spinning conditions are set to values that are essentially between those shown above, the fiber cross section also has an intermediate shape, e.g. a bean or kidney shape.

In all of this, it is of course important to ensure that the threads at the shaft exit are sufficiently solidified.

These explanations show that it is possible according to the method according to the invention to vary the fineness and the cross-sectional shape of the threads obtained. Such a definition of the fiber cross section may be desirable for one or the other application of the fibers.

• nicht überschreiten, wenn noch nicht-hantelförmige Querschnittsformen erhalten werden sollen. Bei längeren Verweilzeiten im Spinnschacht, beispielsweise 2 Sekunden, muß die Verdampfungsgeschwindigkeit geringer und bei kürzeren Verweilzeiten entsprechend höher sein.

The DMF evaporation rate per capillary in (mg / sec) in connection with the dwell time of the threads in the spinning shaft have proven to be suitable variables for describing the cross-sectional shape formed. As emerged from numerous spinning tests, the DMF evaporation rate with a residence time of one second in the spinning shaft may be the value of

• do not exceed if non-dumbbell-shaped cross-sectional shapes are to be obtained. With longer dwell times in the spinning shaft, for example 2 seconds, the evaporation rate must be lower and with shorter dwell times correspondingly higher.

• als Ordinate gegen die Verweilzeit (in Sekunden) im Spinnschacht als Abszisse aufträgt. Sie ist annäherod eine Hyperbel, welche das Gebiet in hantel- und nichthantelförmige Faserquerschnittsstrukturen aufteilt. Unter nichthantelförmigen Faserquerschnittsprofilen werden dabei sowohl bohnen- als auch nierenförmige und runde Querschnittsformen sowie Übergänge zwischen den einzelnen profilen verstanden. Wie aus Abb. 1 hervorgeht, stellen die Werte der Ordinate in Form der DMF-Verdampfungsgeschwindigkeit ein Maß für die thermischen Spinnbedingungen wie Schacht-, Luft- und Spinnlösungstemperatur dar, während die Werte der Abszisse in Form der Verweilzeit der Fäden im Spinnschacht ein Maß für die mechanischen Spinnbedingungen, wie Abzugsgeschwindigkeit und Schachtlänge, bedeuten. Jeder punkt auf der Kurve der Abb. 1 stellt eine bestimmte DMF-Menge dar, wobei der DMF-Gehalt im Faden je nach Titer unterschiedlich sein kann. Das heißt mit anderen Worten, der Verlauf der Kurve ist vom Spinntiter unabhängig. Dem Kurvenverlauf ist fernar zu entnehmen, daß jeweils eine bestimmte DMF-Menge verdampft werden muß, um die Querschnittsstruktur zu ändern. Diese ist bei niedrigen Verweilzeiten bedeutend größer als bei längeren Verweilzeiten im Spinnschacht. Andererseits werden unterhalb einer bestimmten Verdampfungsgeschwindigkeit unabhängig von der Verweilzeit nie hantelförmige Querschnitte erreicht.

Fig. 1 shows the curve obtained when the DMF evaporation rate in

• plots as ordinate against the dwell time (in seconds) in the spinning shaft as the abscissa. It is almost a hyperbola, which divides the area into dumbbell and non-dumbbell-shaped fiber cross-sectional structures. Non-dumbbell-shaped fiber cross-sectional profiles mean bean-shaped as well as kidney-shaped and round cross-sectional shapes as well as transitions between the individual profiles. As can be seen in Fig. 1, the values of the ordinate in the form of the DMF evaporation rate represent a measure of the thermal spinning conditions such as shaft, air and spinning solution temperature, while the values of the abscissa in the form of the residence time of the threads in the spinning shaft are a measure of the mechanical spinning conditions, such as take-off speed and shaft length, mean. Each point on the curve in Fig. 1 represents a certain amount of DMF, whereby the DMF content in the thread can vary depending on the titer. In other words, the course of the curve is independent of the spin titer. The curve shows that a certain amount of DMF must be evaporated in order to change the cross-sectional structure. This is significantly greater with low dwell times than with longer dwell times in the spinning shaft. On the other hand, dumbbell-shaped cross sections are never reached below a certain evaporation rate, regardless of the residence time.

• Durchgesetze Menge an Polymerfeststoff in (g/min):
  
  

The DMF evaporation rate per capillary in (mg / sec) can be determined from the difference between the amount of spinning solvent per capillary (mg / sec) and the residual solvent amount per capillary (mg / sec). This should be shown on a model calculation for example 1. The following applies:

• Enforced amount of polymer solid in (g / min):
  
  

Amount of spinning solvent (g / min):

• Nach dem Spinnprozeß wurden 9,9 % an Restlösungsmittel DMF bezogen auf Feststoff gefunden.
• Es gilt:
• 5,76 g = 100 %
• x = 9,9 %
• x = 0,570 g DMF verbleiben im Spinngut.

Residual amount of solvent in the spinning material (g / min):

• After the spinning process, 9.9% of residual solvent DMF, based on solids, was found.
• The following applies:
• 5.76 g = 100%
• x = 9.9%
• x = 0.570 g DMF remain in the spinning material.

DMF evaporation rate (g / min) = 13.765-0.570

DMF evaporation rate per capillary (mg / sec)

wobei

• C_{1 DMF} = die eingesetzte Konzentration an Spinnlösungsmittel,
• C_{2 DMF} = 70,5 Gew.-% DMF und
• R₂ = die DMF-Verdampfungsgeschwindigkeit
  
• bedeuten. Den Wert für R₂ kann man direkt aus der Kurve der Abb. 1 für die entsprechende Verweilzeit im Spinnschacht (in Sek.) entnehmen. Dabei errechnet sich die Verweilzeit (in Sekunden) der Fäden im Spinnschacht aus der Beziehung
  
• Für Beispiel 6 errechnet sich demnach die DMF-Verdampfungsgeschwindigkeit R₁ für eine von 70,5 Gew.-% DMF verschiedene Spinnlösungskonzentration, bei der eine Änderung der Querschnittsform eintritt, wie folgt:
  
  
• Neben der veränderten Faserquerschnittsform feintitriger Fasern, die nach dem erfindungsgemäßen Verfahren hergestellt worden sind, weisen derartige Fasern mit nicht hantelförmigen Querschnittsprofilen noch einen außerordentlich hohen Glanz auf. Dies führt zu einer hohen Eleganz im Flächengebilde von Gebrauchsartikeln. Wie oberflächenmorphologische Untersuchungen mit dem Rasterelektronenmikroskop zeigen, besitzen die erfindungsgemäßen feintitrigen Fasern im Gegensatz zu herkömmlich trocken gesponnenen Acrylfasern keine borkige, fibrillierte Oberfläche mit Riefen begrenzter Länge unter wechselndem Winkel zur Faserachse. Die feintitrigen Fasern besitzen glatte Oberflächen und parallel zur Faserachse verlaufende Riefen und Streifungen, die nicht unterbrochen sind, so daß das Licht gerichtet reflektiert wird.
• Solche Acrylfasern und -fäden sind ein weiterer Gegenstand der Erfindung, bevorzugt mit runden bis bohnenförmigen Querschnitten.
• Infolge der größeren Garnfeinheit (Nm 100/1) zeigen feintitrige Fasern, z.B. bei Interlockware, aus 3-Zylindergarnen einen sehr weichen Griff gegenüber herkömmlicher Acrylware aus 1,6 dtex Fasern. Dies ist besonders für hautnah getragene Artikel von hohem Gebrauchswert.
• Im Falle der Nachbehandlung von feintitrigem Spinngut hat es sich als äußerst günstig erwiesen, das Spinngut vor dem Streckprozeß durch Hindurchleiten durch Wannen mit warmer Waschflüssigkeit, vorzugsweise Wasser, auf ca. 79-80°C aufzuwärmen, um eine gleichmäßigere Verstreckung zu erzielen. Das feintitrige Spinngut läßt sich auf übliche Weise durch Waschen-Strecken-Präparieren-Trocknen-Kräuseln-Schneiden zu fertigen Acrylfasern nachbehandeln. Wegen der großen Titerfeinheit der Fäden, besonders bei Spinntiter kleiner 1 dtex, ist es ferner vorteilhaft, die Verstreckung in Stufen vorzunehmen.
• Mit dem erfindungsgemäßen Verfahren ist es erstmals möglich, Fasern mit extrem feinen Endtitern von z.B. 0,1 dtex auch in größerem Tonnen-Maßstab herzustellen.
• Die Titerbestimmung nach der gravimetrischen Methode ist bei feinen Titern ( < 0,5 dtex) sehr ungenau. Die Titerbestimmung erfolgte deshalb nach der mikroskopischen Methode durch Ermittlung des Fadendurchmessers "d" mit dem Okularmikrometer nach DIN 53 811 gemäß der Formel:
  
• Literatur: Chemiefasern (1975), Heft 7. Seite 593.
• Die folgenden Beispiele dienen der näheren Erläuterung der Erfindung. Teil- und Prozentangaben beziehen sich, wenn nicht anders vermerkt, auf das Gewicht.

When carrying out the process according to the invention, DMF spinning solutions with a polymer content of 29.5% by weight were generally used. At higher concentrations, as can be seen from example 6, a lower evaporation rate R is necessary in order to obtain non-dumbbell-shaped cross sections. The values follow the empirical formula:

in which

• C _{1 DMF} = the concentration of spinning solvent used,
• C _{2 DMF} = 70.5% by weight DMF and
• R ₂ = the DMF evaporation rate
  
• mean. The value for R ₂ can be taken directly from the curve in Fig. 1 for the corresponding dwell time in the spinning shaft (in seconds). The residence time (in seconds) of the threads in the spinning shaft is calculated from the relationship
  
• For example 6, the DMF evaporation rate R ₁ for a spinning solution concentration other than 70.5% by weight DMF, at which the cross-sectional shape changes, is calculated as follows:
  
  
• In addition to the changed fiber cross-sectional shape of fine-titer fibers which have been produced by the process according to the invention, such fibers with non-dumbbell-shaped cross-sectional profiles still have an extraordinarily high gloss. This leads to a high degree of elegance in the fabric of consumer goods. As shown by surface morphological studies with a scanning electron microscope, the fine-titer fibers according to the invention, in contrast to conventionally dry-spun acrylic fibers, have no barky, fibrillated surface with grooves of limited length at varying angles to the fiber axis. The fine-titer fibers have smooth surfaces and grooves and striations that run parallel to the fiber axis and are not interrupted, so that the light is reflected in a directed manner.
• Such acrylic fibers and threads are a further subject of the invention, preferably with round to bean-shaped cross sections.
• As a result of the greater yarn count (Nm 100/1), fine-titer fibers, for example in interlock fabrics, made from 3-cylinder yarns have a very soft feel compared to conventional acrylic fabrics made from 1.6 dtex fibers. This is particularly useful for articles worn close to the skin and of high utility value.
• In the case of the aftertreatment of fine-tinned spinning material, it has proven to be extremely advantageous to warm the spinning material to about 79-80 ° C. prior to the stretching process by passing it through troughs with warm washing liquid, preferably water, in order to achieve a more uniform stretching. The fine titre spun material can be post-treated in the usual way by washing-stretching-preparing-drying-crimping-cutting to give finished acrylic fibers. Because of the large titer fineness of the threads, especially in the case of spinning titer smaller than 1 dtex, it is also advantageous to carry out the drawing in stages.
• With the method according to the invention, it is possible for the first time to produce fibers with extremely fine end titles of, for example, 0.1 dtex, even on a larger ton scale.
• The titer determination according to the gravimetric method is very imprecise for fine titers (<0.5 dtex). The titer was therefore determined by the microscopic method by determining the thread diameter "d" with the eyepiece micrometer according to DIN 53 811 according to the formula:
  
• Literature: Chemical fibers (1975), No. 7, page 593.
• The following examples serve to explain the invention in more detail. Unless otherwise noted, parts and percentages relate to weight.

example 1

70.5 kg of dimethylformamide (DMF) were mixed with 29.5 kg of an acrylonitrile copolymer composed of 93.6% acrylonitrile, 5.7% methyl acrylate and 0.7% sodium methallylsulfonate with a K value of 81 and stirred in a 60 cm long, double-walled Tube of 8 cm inner diameter heated with steam of 3.2 bar pressure. The temperature of the solution, which had a solids concentration of 29.5% by weight, was 135 ° C. at the tube outlet. There were several mixing combs in the tube to homogenize the spinning solution. The spinning solution was filtered after leaving the heating device and fed to the spinning shaft. The residence time from the heating device to the spinneret was 8 minutes. The spinning solution had a viscosity of 30 ball falling seconds measured at 80 ° C. This value remained unchanged after 1, 3 and 5 hours. The spinning solution was then cooled to 35 ° C. and dry spun from a 720 hole nozzle with nozzle hole diameters of 0.2 mm. The shaft temperature was 50 ° C, the air temperature 200 ° C and the air volume 40 m ³ / h. The take-off speed was 400 m / min. The dwell time of the threads in the spinning shaft was 0.87 seconds. 19.8 ccm / min were conveyed from the spinning pump. The total spin titer was 144 dtex and the residual solvent content of DMF in the spun material was 9.9% by weight, based on polymer solids. The DMF evaporation rate is then calculated to be 0.305

The single spin titer was 0.2 dtex. The warpage V was 457. The threads were wetted with oil-containing preparation at the shaft exit, wound on bobbins, folded into a cable, stretched 1: 3.6 times in boiling water and aftertreated in the usual way to give fibers with a final titre of 0.07 dtex.

For the microscopic assessment of the cross-sectional geometry, the fiber capillaries were embedded in methyl methacrylate and cross-cut. The light microscopic images produced in the differential interference contrast method showed that the sample cross sections are completely uniform and round. The titer value was calculated from the thread diameter d = 2.8 pm with the specified density = 1.17 g / cm ³ . The mean thread diameter was determined with the fiber measuring eyepiece. The fibers had an extremely high gloss. When examined in a scanning electron microscope, the fibers showed smooth surfaces with longitudinal stripes. The striations were completely parallel to the fiber axis and, unlike those with conventional acrylic fibers, were not interrupted.

Example 2 (comparison)

Part of the batch from Example 1 was dissolved in the heating device at 80 ° C. instead of 135 ° C. and the viscosity of the spinning solution after filtration was determined at 80 ° C. The spinning solution had a viscosity of 76 ball falling seconds. In reproduction measurements, the viscosity was 72 seconds after 1 hour, 67 after 3 hours and 64 seconds after 5 hours. The spinning solution thus had a decreasing viscosity. After the filtration, the spinning solution was cooled again to 35 ° C. and dry-spun into threads from a 720-hole nozzle, as described in Example 1. Thread breaks repeatedly occurred in the nozzle area. As light microscopic cross-sectional images showed, there were also numerous titre fluctuations.

Example 3

An acrylonitrile copolymer with the chemical composition of Example 1 was, as described there, dissolved in DMF, filtered and the spinning solution was cooled to 40 ° C. in front of the nozzle. Then, dry spinning was carried out from a 720-hole nozzle with a nozzle hole diameter of 0.2 mm. The shaft temperature was 50 ° C, the air temperature 200 ° C and the air volume 40 m ³ / h. The withdrawal speed was 250 m / min and the dwell time of the threads in the spinning shaft was 1.39 seconds. 52.8 ccm / min were conveyed from the spinning pump. The total spin titer was 648 dtex. The residual solvent content in the spun material was 10.8%. The DMF evaporation rate was 0.856

The single spin titer was 0.9 dtex.

The delay was 107. At the shaft exit, the threads were again wetted with oil-containing preparation, wound on spools, folded into a cable, stretched 1: 3.6 times in boiling water and aftertreated in the usual way to give fibers with a final titer of 0.3 dtex. The fiber cross sections were again completely uniform and circular. The fibers also had a very high gloss again and showed a smooth surface in the scanning electron microscope with striations that were longitudinally striped parallel to the fiber axis.

Example 4

An acrylonitrile copolymer with the chemical composition from Example 1 was dissolved in DMF as described there. The spinning solution was then filtered, cooled to 90 ° C. and dry-spun from a 720-hole nozzle with a nozzle hole diameter of 0.2 mm. The shaft temperature was 150 ° C, the air temperature 200 ° C and the air volume 40 m ³ / h. The take-off speed was 180 m / min. It was spun on a shorter-sized spinning shaft, so that a residence time of 1.66 sec. revealed. The spinning pump became 82.8 ccm / min. promoted. The total spin titer was 1304 dtex. The residual solvent content in the spinning material was 13.5%. The DMF evaporation rate was 1.225

The single spin titer was 1.8 dtex. The warpage was 48. The threads were post-treated with 1: 4, 0-fold stretching to fibers with a final titer of 0.6 dtex. The fibers had a round to slightly bean-shaped cross-sectional profile. Her shine was again extraordinarily high. In the scanning electron microscope, striations and striations running on the surface parallel to the fiber axis were again observed, which showed no interruptions.

• demonstriert. Mit steigendem Spinntiter müssen die Energieverhältnisse im Spinnschacht angehoben werden, da mit steigendem Lösungsdurchsatz mehr Spinnlösungsmittel verdampfen muß, um eine Fadenverfestigung zu erhalten. Das Spinngut wurde jeweils 1:3,6-fach in kochendem Wasser verstreckt und wie üblich nachbehandelt. Die Einzelspinn- und Einzelendtiter wurden wiederum nach der lichtmikroskopischen Methode ermittelt und die Querschnittsformen anhand lichtmikroskopischer Aufnahmen nach dem differentiellen Interferenzkontrastverfahren bestimmt. Die unterschiedlichen Verweilzeiten im Spinnschacht wurden neben unterschiedlichen Abzugsgeschwindigkeiten auch durch andere Schachtlängen erzielt. Wie man der Tabelle entnehmen kann, entstehen von der Hantelform abweichende Querschnittsformen vornehmlich bei Spinntitern kleiner 3 dtex. Wie die Beispiele 12 und 17 zeigen, lassen sich jedoch auch bei Spinntitern ab 3,0 dtex und feiner hantelförmige Faserquerschnitte herstellen, wenn man nur die DMF-Verdampfungsgeschwindigkeit in
  
• hoch genug wählt. Man hat daher mit dieser Meßgröße, wie bereits erwähnt, einen geeigneten Parameter in der Hand, die Querschnittsform festzulegen.
  
  

The following table shows the dependence of the cross-sectional shape on the DMF evaporation rate in

• demonstrated. With increasing spinning titer, the energy ratios in the spinning shaft have to be increased, since with increasing solution throughput, more spinning solvent has to evaporate in order to achieve thread strengthening. The spinning material was stretched 1: 3.6 times in boiling water and post-treated as usual. The single-spin and single-end titers were again determined using the light microscopic method and the cross-sectional shapes were determined using light microscopic images using the differential interference contrast method. The different dwell times in the spinning shaft were achieved in addition to different take-off speeds by other shaft lengths. As can be seen in the table, cross-sectional shapes deviating from the dumbbell shape arise primarily with spin titers less than 3 dtex. As examples 12 and 17 show, however, even with spinning titers from 3.0 dtex and finer dumbbell-shaped fiber cross sections, only the DMF evaporation rate in
  
• chooses high enough. Therefore, as already mentioned, this measured variable has a suitable parameter in hand to determine the cross-sectional shape.
  
  

Example 5

a) An acrylonitrile copolymer with the chemical composition of Example 1 was dissolved in DMF as described there, filtered and the spinning solution was kept at 112 ° C. in front of the nozzle. Then, dry spinning was carried out from a 1050 hole nozzle with a hole diameter of 0.25 mm. The shaft temperature was 150 ° C, the air temperature 260 ° C and the air volume 40 m ³ / h. The take-off speed was 300 m / min and the dwell time of the threads in the spinning shaft was 1.76 seconds. 193.2 ccm / min were conveyed from the spinning pump. The total spin titer was 1903 dtex. The residual solvent content in the spun material was 8.3%. The DMF evaporation rate was 2.090

The single spin titer was 1.81 dtex.

The warping was 80. At the shaft exit, the threads were again wetted with oil-containing preparation, collected on bobbins, folded into a cable, stretched 1: 4.0 times in boiling water and post-treated into fibers in the usual way. The final fiber titer was 0.56 dtex. The fibers show the typical dumbbell shape.

b) After the dissolving and filtration process, part of the batch from Example 5a was cooled to 40 ° C. in front of the nozzle and dry-spun from a 1050-hole nozzle with a nozzle hole diameter of 0.25 mm. The shaft temperature was 190 ° C, the air temperature 380 ° C and the air volume 40 m ³ / h. The take-off speed was 250 m / min and the dwell time of the threads in the spinning shaft was 2.11 seconds. 161 ccm / min were conveyed from the spinning pump. The total spin titer was 1891 dtex. The residual solvent content in the spinning material was 8.8%. The DMF evaporation rate was 1.727

The single spin titer was 1.80 dtex. The warpage was 80. The threads were post-treated as described in Example 5a. The final fiber titer was 0.58 dtex. The fibers in turn show the typical dumbbell shape.

• c) Part of the batch from Example 5 was dissolved in the heating device at 80 ° C. instead of 135 ° C., filtered and the spinning solution in front of the nozzle was again kept at 112 ° C. Then spinning was carried out as described in Example 5a. The threads could not be put on. There were constant tears below the nozzle.
• d) Another part of the batch was dissolved in the heating device at 80 ° C. instead of 135 ° C., filtered and the spinning solution was cooled to 40 ° C. The solution had a viscosity of 235 ball falling seconds at 50 ° C. At 40 ° C the viscosity increased to 356 falling ball seconds and the solution became cloudy. When trying to spin such a solution as described in Example 5a, no threads could be obtained. There were constant tears below the nozzle.

Example 6

35 kg of an acrylonitrile copolymer with the chemical composition from Example 1 were dissolved in 65 kg of DMF as described there. The spinning solution was then filtered, cooled to 35 ° C. and dry-spun from a 360-hole nozzle with a nozzle hole diameter of 0.3 mm. The shaft temperature was 50 ° C, the air temperature 200 ° C and the air volume 40 m ³ / h. The withdrawal speed was 300 m / min. The dwell time in the spinning shaft was 1.16 seconds. 126.8 ccm / min were conveyed from the spinning pump. The total titer was 1391 dtex. The residual solvent content in the spinning material was 35.5%. The DMF evaporation rate was 2.902

• zu erwarten ist, erfolgt somit der Übergang der Querschnittsform von rund nach hantelförmig bei einer 65 %igen Spinnlösungskonzentration gemäß
  
  bereits viel früher.

The single spin titer was 3.86 dtex. The warpage was 60. The threads were post-treated with 1: 4.0 times stretching to fibers with a final titer of 1.2 dtex. The fibers have a dumbbell-shaped cross-sectional profile. At 70.5% concentration of the spinning solution, the transition of the cross-sectional shape from round to dumbbell shape with a dwell time of 1.16 seconds in the spinning shaft according to Fig. 1 only at an evaporation rate of 3.05

• is expected, the cross-sectional shape changes from round to dumbbell-shaped at a 65% concentration of spinning solution
  
  much earlier.

Claims (5)

1. Process for the production of acrylic fires and filaments from spinning solutions having a solids content of 25 to 35% of filament-forming acrylonitrile polymers containing at least 85% by weight of acrylonitrile units in polar organic solvents by a dry-spinning process at a spinning chamber temperature of preferably between 30° C and a value below the boiling point of the spinning solvent used and a spinning air temperature of at most 300°C and with further treatment of the spinning material in a manner known per se to form finished fibres or filaments, characterised in that thermally pre-treated spinning solutions are used, the viscosity of which changes by at most 5% during the spinning time, in that the spinning solution is spun with a draft or at least 20 and in that filaments having individual spinning titres of 3 dtex and below are produced during spinning.

2. Process according to Claim 1, characterised in that dimethylformamide is used as the spinning solvent.

3. Process according to Claim 1, characterised in that the spinning solution is spun with a draft of 30-500.

4. Acrylic fibres and filaments of acrylonitrile polymers containing at least 85% by weight of acrylonitrile units, characterised in that they are smooth in the longitudinal direction, in that their surface has longitudinal striae and furrows parallel to the fibre axis and in that they have an individual spinning titre of at most 3 dtex.

5. Filaments and fibres according to Claim 4, characterised in that they have round to bean-shaped cross- sections.

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