Dry-spun hollow polyacrylonitrile fibres and filaments and process for the production thereof
This application is a continuation of application Ser. No. 311,493, filed Oct. 15, 1981 now pending. 
    
    
     The production of hollow fibres by melt spinning or wet spinning has long been known. The processes mentioned in numerous patents are based essentially on three approaches. 
     In the first method, a molten polymer, for example a polyester, is spun from nozzles as adjacent arcuate segments. Synthetic hollow fibres are produced by swelling the molten polymer beneath the nozzle and allowing the edges of the arcuate segments to coalesce into a continuous form. In the second method, a hollow neddle positioned in the centre of the orifice is used, gaseous substances or fillers being pumped through the hollow needle. The polymer flows round the needle and the gas fills the central void and maintains the form until the polymer has cooled. Hollow viscose filaments, in particular, are produced in this way and castor oil, for example, may be used as lumen-filling medium. Lastly, in the third method, a solid pin is positioned in the nozzle orifice. This is generally a difficult spinning process as the polymer wishes to assume a closed form. The process is particularly suitable for cross-section modifications, but air has to be supplied to the end of the pin or a vacuum has to be applied to produce hollow fibres. 
     Hollow filaments and fibres have found many applications. Thus, for example, they are used for the desalination of sea water, for the purification of liquids and gases, in ion exchanger, for reverse osmosis, dialysis and ultrafiltration (artificial kidneys) and, because of the low weight and the high bulk thereof, for comfortable clothes. In particular, the purification of substances, for example industrial gases, has recently come to the fore. Comprehensive articles about the production and importance of synthetic hollow fibres may be found in the Encyclopedia of Polymer Science and Technology 15, (1971), Pages 258-272, in Acta Polymerica 30, (1979), Pages 343-347 and in Chemical Engineering, February 1980, Pages 54-55. 
     There have also been numerous attempts to produce hollow acrylic fibres from a spinning solution by a dry spinning process. However, owing to the problems encountered, no commercial process for the production of hollow acrylic fibres by this technique has previously been disclosed. 
     For the present purposes, the term &#34;hollow fibres&#34; refers to fibres having an internal, linear, continuous longitudinal channel. 
     Although acrylonitrile polymers may be converted to hollow fibres relatively simply by the wet spinning technique by one of the above-mentioned methods, this leads to considerable difficulties in a dry spinning process owing to a different filament formation mechanism. In a wet spinning process, filament formation is effected by coagulation of the spinning solution in an aqueous precipitating bath containing a solvent for polyacrylonitrile, the precipitating bath concentration, temperature and additional coagulating agent, such as aqueous salt solutions, may be varied within wide limits. Thus, for example, German Offenlegungsschrift No. 2,346,011 describes the production of hollow acrylic fibres by the second wet spinning method using aqueous DMF as precipitating bath and German Offenlegungsschrift No. 2,321,460 uses aqueous nitric acid, the filaments being spun from nozzles having annular orifices and a liquid being introduced into the centre of the annular orifice as an internal precipitant. 
     In attempting to apply the three methods to a dry spinning process, considerable difficulties are encountered as, when spinning from a spinning solution, only a proportion of the solvent has to evaporate after issuing from the nozzle in order for a thread to be formed and solidify. Owing to the high production costs and the difficult process control when producing hollow acrylic fibres by dry spinning from spinning solutions, the second and third methods were not pursued. 
     When attempting to produce hollow fibres according to the first method using profile nozzles having adjacent segmental arcuate orifices by the dry spinning process, only dumbbell-shaped or irregular random cross-sections are generally obtained which have uneven air inclusions. If the concentration of polymer solids is increased in order to obtain the predetermined cavity profile by increasing the structural viscosity, unexpected problems arise. The increase in the solids content is subject to limits owing to the gelation, flowability and management of the spinning solutions. Thus, for example, an acrylonitrile copolymer having a chemical composition of 93.6% of acrylonitrile, 5.7% of acrylic acid methyl ester and 0.7% of sodium methallyl sulphonate and a K-value of 81 may only be dissolved and spun into threads in a spinning solvent, such as dimethylformamide, to a maximum solids content of 32%, by weight. If an attempt is made to raise further the solids content, the spinning solutions gel during cooling at temperatures of from 50° to 80° C., rendering disturbance-free spinning impossible. 
     Owing to the numerous possible applications of these hollow fibres and filaments, an object of the present invention was to propose a dry spinning process of this type for the production of hollow acrylonitrile fibres. 
     It has now surprisingly been found that hollow polyacrylonitrile filaments may also be spun by a dry spinning process if spinning solutions having a viscosity exceeding a certain value are used, if nozzles having loop-shaped orifices of specific dimensions are used and if the spinning air is allowed to act on the filaments in a specific manner. 
     The present invention therefore relates to dry-spun hollow polyacrylonitrile filaments. Suitable acrylonitrile polymers for the production of these filaments and fibres obtainable therefrom include acrylonitrile homo- and co-polymers in which the copolymers contain at least 50%, by weight, preferably at least 85%, by weight, of polymerised acrylonitrile units. 
     The present invention also relates to a process for the production of hollow polyacrylonitrile filaments and fibres, characterised in that the filament-forming synthetic polymers are spun from a solution through a nozzle having loop-shaped orifices by a dry spinning process wherein the solution has a viscosity equivalent to at least 120 falling ball seconds, measured at 80° C., or at least 75 falling ball seconds, measured at 100° C., wherein the area of the orifice is less than 0.2 mm 2 , the sides of the loop-shaped orifice are a maximum of 0.1 mm apart and the overlap of the two ends of the sides of the loop-shaped orifice forms an angle of from 10° to 30° measured from the centre of the nozzle and wherein the spinning air acts on the filaments in a transverse direction to the filament take-off and the air direction forms an angle of from 80° to 100° with a straight line passing through the opening in the sides. 
     The other conventional steps of the polyacrylonitrile dry spinning process follow the spinning operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view of a spiral or loop-shaped nozzle orifice for use in the present invention wherein the overlap angle of the two ends of the sides is about 20°. 
     FIG. 2 is a partial view of an annular nozzle for use in the present invention showing a plurality of nozzle orifices as depicted in FIG. 1 wherein the openings of the nozzle are oriented in a transverse direction to the air jet. 
     FIG. 3 is a partial view of a nozzle showing a plurality of nozzle orifices wherein the air enters the openings between the sides directly--the opening between the sides of the nozzle holes has a different position from the transverse position to the center of the spinning duct. 
     FIG. 4 is an elevational view of a spiral or loop-shaped nozzle orifice wherein the overlap angle of the two ends of the sides is 55°. 
     FIG. 5 is a partial view of a nozzle showing a plurality of nozzle orifices wherein the air flows at an angle of about 125°. 
     FIG. 6 is a partial view of a nozzle showing a plurality of nozzle orifices wherein the opening between the ends of the nozzle form an angle of about 35° to the direction of the air from the center of the spinning duct. 
    
    
     The viscosity in falling ball seconds, measured at 80° or 100° C., was determined by K. Jost&#39;s method, Reologica Acta, Volume 1 (1958), Page 303. The area of the nozzle orifice is preferably less than 0.1 mm 2  and the side has a width of between 0.02 and 0.06 mm. Merging of the cross-sectional shape is observed in the case of nozzle orifice areas exceeing 0.2 mm 2 . Indefinite nodular to formlessly deformed, random configurations are obtained. 
     Spinning solutions having the specified viscosity which also contain a higher concentration of the filament-forming polymer than normally used are obtained, according to German Offenlegungsschrift No. 2,706,032, by producing suitably concentrated suspensions of the filament-forming polymer, which may easily be conveyed, in the desired solvent and by converting these suspensions into spinning solutions which are viscosity stable by briefly heating them to temperatures just below the boiling point of the spinning solvents used. 
     The suspensions for the production of these spinning solutions are obtained by reacting the spinning solvent with a non-solvent for the polymer to be spun, if necessary, and then adding the polyer with stirring. 
     &#34;Non-solvents&#34; in the context of the present invention include all substances which are non-solvents for the polymer and which may be mixed with the spinning solvent within wide limits. 
     The boiling points of the non-solvents may lie below, as well as above the boiling point of the spinning solvent used. Substances of this type which may be solid or liquid include, for example, alcohols, esters or ketones, as well as singly- and multiply-substituted alkyl ethers and esters of polyhydric alcohols, inorganic or organic acids, salts and the like. As preferred non-solvents, there are used, on the one hand, water, owing to its simple management, simple removal in the spinning duct without the formation of a residue and simple recovery, and, on the other hand, glycerin, mono- and tetra-ethylene glycol, as well as sugar. 
     When using non-solvents having boiling points below the boiling point of the spinning solvent, hollow acrylic fibres are obtained which are distinguished from the known compact types by a considerably greater water-retention capacity. When using non-solvents whose boiling point lies above that of the spinning solvent, acrylic fibres having a high water-retention capacity are obtained, as described in German Offenlegungsschrift No. 2,554,124. These fibres are distinguished by particular wear properties. While the non-solvent is removed in the spinning duct in the first case, the non-solvent has to be washed from the solidified fibre in an additional stage of the process after the spinning process in the second case. 
     When using water as a non-solvent, hollow fibres may be obtained from the nozzles by the dry spinning process by using the acrylonitrile copolymer mentioned above having a K value of 81 and a solids content in the spinning solution of 36%, by weight. 
     The water content of these suspensions of polyacrylonitrile and dimethylformamide is between 2 and 10%, based on the total suspension. With a water addition of less than 2%, by weight, a flowable transportable suspension is not obtained, but rather a thick inert slurry. On the other hand, if the water content exceeds 10%, by weight, the filaments disintegrate beneath the nozzle during the spinning process owing to the high water vapour partial pressure as they issue from the nozzle orifices. The percentage of water in the spinning solution does not influence the profiling at the nozzle. The only decisive factor is the concentration of polymer solids. Water contents of from 2 to 3% have proven to be optimal with solids contents of up to 40% in order still to obtain flowable transportable suspensions at room temperature. If another non-solvent such as propanol or butanol, is used instead of water, the same results are obtained. More highly concentrated spinning solutions may obviously also be produced for acrylonitrile copolymers having K values below 81. Thus, for example, it is possible to produce from an acrylonitrile copolymer of 92% of acrylonitrile, 6% of acrylic acid methyl ester and 2% of sodium methallyl sulphonate having a K value of 60, a suspension comprising 45% of copolymer solids content, 4% of water and 51% of dimethylformamide having a viscosity equivalent to 142 falling ball seconds, measured at 80° C., which is still flowable at room temperature and may be converted into hollow fibres by dissolution and spinning from a particular profiled nozzle. On the other hand, hollow fibres may be obtained when using polymers having higher K values, even at a lower solids concentration than the specified 36% spinning solutions having a K value of 81 during dry spinning from certain profiled nozzles. The only decisive factor for the shaping at the profiled nozzle is the viscosity. 
     When using monoethylene glycol as non-solvent, using the acrylonitrile copolymer mentioned above, spinning solutions having solids contents of 36%, by weight, or higher could be produced, the viscosities of which were equivalent to at least 75 falling ball seconds, measured at 100° C. From these spinning solutions, hollow filaments and fibres were spun which were distinguished by the high water-retention capacity thereof after washing out the non-solvent and after the conventional subsequent treatment. The non-solvent content of these suspensions of polyacrylonitrile, dimethylformamide and monoethylene glycol must be at least 5%, by weight, based on solvent and solid, as indicated in German Offenlegungsschrift No. 2,554,124, so that the filaments and fibres have a water-retention capacity of at least 10%. As shown in Table II, the percentage content of non-solvent in the spinning solution does not influence the profiling at the nozzle. The fact that the spinning solution has a minimum viscosity is far more decisive. In the case of solids contents of up to 40%, by weight, non-solvent contents of from 5 to 10%, by weight, have proven to be preferred in order to obtain hollow acrylic fibres having a water-retention capacity exceeding 10%. The solid composition surrounding the internal, linear continuous channel in the fibre has a core sheath structure. The thickness of the fibre sheath may be varied within wide limits by the ratio of the polymer solid to the non-solvent content. In accordance with the statements concerning the use of water as non-solvent it is also found that, when using non-solvents whose boiling point exceeds the boiling point of the spinning solvent, acrylonitrile copolymers having K values below 81 produce the required minimum viscosity in the spinning solution in a higher concentration and acrylonitrile copolymers having K values exceeding 81 in a lower concentration. 
     The minimum viscosity may be determined at two different temperatures, namely at 80° C. and 100° C. This feature takes into account the fact that it is difficult to determine the viscosity in spinning solutions containing water as non-solvent owing to the vaporisation of the water at 100° C., while it may be problematic to determine the viscosity in other spinning solutions containing as non-solvent a substance whose boiling point exceeds that of the spinning solvent at 80° C. owing to the gelation tendency. However, the viscosity of water-containing spinning solutions may also be determined at 100° C. if the process is carried out in a closed system. 
     Providing that the spinning solution to be spun produces a finite falling ball second value, it is basically possible to produce hollow acrylic fibres from that spinning solution. However, spinning solutions having viscosities exceeding the equivalent of 300 falling ball seconds, measured at 80° C. or 100° C. cannot be processed without difficulty in conventional spinning apparatus for economic reasons, thus producing a natural upper limit for the viscosity range. 
     Suitable spinning solvents include, in addition to dimethylformamide, even higher boiling solvents, such as dimethylacetamide, dimethylsulphoxide, ethylene carbonate and N-methylpyrrolidone and the like. 
     The process control of the spinning air during filament formation, as well as the particular geometry, size and arrangement of the nozzle orifices in the spinnerettes suitable for the production of hollow acrylic fibres represent other important factors in the production of hollow acrylic fibres by a dry spinning process according to the present invention. It has been found that, to produce round hollow-fibres which are uniform in shape and have cavity portions which are equal to each other, a spiral or loop-shaped nozzle according to accompanying FIG. 1 is particularly advantageous, the overlap angle of the two ends of the sides of the spiral nozzle holes being from 10° to 30°, preferably 20°. If the end of the side of the spiral nozzle orifices is lengthened, the overlap angle of the two ends of the sides is 55° for example (cf. accompanying FIG. 4) or if the opening between the sides of the spiral nozzle holes has a different position from the transverse position to the centre of the spinning duct (cf. accompanying FIG. 3), then hollow fibres which are uniform in shape and cavity portion are not obtained. Depending on the spinnerette geometry and arrangement of the opening between the sides relative to the centre of the spinning duct, kidney-shaped and other undesirable cross-sectional shapes are formed. In addition to this particular nozzle orifice geometry and arrangement, the method of air supply to the profiled filaments plays an important part in the formation of hollow fibres. Uniform hollow fibres are obtained only by intentionally blowing spinning air from the centre of the spinning duct onto the filaments. If the air is applied to the filaments in a different manner, for example from the interior and exterior, indefinite random fibre cross-sections having varying cavity portions are obtained. It is obviously important for the spinning air not to impinge centrally upon the openings of the sides of the profiling nozzle, but to enter in a transverse direction at an angle of from 80° to 100°, preferably 90° (cf. accompanying FIG. 2). If the spinning air enters the openings between the sides directly (cf. accompanying FIG. 3) the filaments swell to a marked extent and then deflate under the influence of the drawing operation. Non-uniform cross-sectional shapes and variable cavity portions are obtained. 
     In addition to the particular process control of the spinning air during filament formation, as well as the particular geometry and arrangement of the nozzle orifices of the profiling nozzle to be used, the diameter of the nozzle orifice and the nozzle orifice area play an important part, as mentioned. It has been found that, in the case of certain geometrical configurations, filament cross-sections having sharp contours may only be spun up to a specific width of the sides depending on the total nozzle orifice area. The term &#34;width of the side of a profiling nozzle&#34; refers to the distance between the outer limit of the predetermined profile shape in mm, but not the distance to the centre of the nozzle orifice. 
     In addition to the above-mentioned properties for dialysis and ultrafiltration purposes, the fibres according to the present invention are distinguished, in particular, by the high water-retention capacity thereof. Textile sheets made of these fibres have good comfort in wear, as mentioned in German Offenlegungsschrift No. 2,719,019. The water-retention capacity is at least 10% whenever there is a closed, uniform hollow fibre having a constant cavity portion. Varying values for the water-retention capacity are found in the case of non-uniform hollow fibre cross-sectional shapes, as well as partially open, partially closed shapes, depending on the cavity portion. The water-retention capacity is determined in accordance with the DIN regulation 53 814 (cf. Melliand Textilberichte 4, 1973, page 350). 
     The fibre samples are immersed for two hours in water containing 0.1% of wetting agent. The fibres are then centrifuged for ten minutes at an acceleration of 10,000 m/sec 2  and the quantity of water retained in and between the fibres is determined by gravimetric analysis. To determine the dry weight, the fibres are dried at 105° C. to constant weight. The water-retention capacity (WR) in percent, by weight, is: ##EQU1## m f  =weight of the moist fibre material m tr  =weight of the dry fibre material. 
     The cross-sections of such hollow fibres tend to deform under stress of high temperatures owing to the structure thereof. If, for example, a continuous hollow cable is dried at temperatures above 160° C., individual hollow capillaries break open, forming irregular, partially open fibre cross-sections and high proportions of short fibres. The following after-treatement procedure has been found to be the best for the subsequent treatment of the fibres according to the present invention: washing-drawing-preparation-crimping-cutting-drying to a maximum of 140° C. A drying temperature of from 110° to 130° C. is preferred. If the hollow acrylic fibres according to the present invention are subjected to an after-treatment, as just mentioned, closed, uniform hollow fibres having uniform cavity portions are obtained. 
     EXAMPLE 1 
     59 kg of dimethylformamide (DMF) are mixed with 3 kg of water in a heated chamber at room temperature with stirring. 38 kg of an acrylonitrile copolymer composed of 93.6% of acrylonitrile, 5.7% of acrylic acid methyl ester and 0.7% of sodium methallyl sulphonate having a K value of 81 are then added at room temperature with stirring. The suspension is pumped via a gear pump into a heated spinning chamber provided with a stirrer. The suspension, which has a solids content of 38%, by weight, and a water content of 3%, by weight, based on total solution, is then heated in a double-walled tube using steam at 4.0 bar. The residence time in the tube is seven minutes. The temperature of the solution at the tube outlet is 138° C. The tube contains several mixing chambers for the homogenisation of the spinning solution. The spinning solution, which has a viscosity equivalent to 176 falling ball seconds at 90° C., is filtered after leaving the heating apparatus without intermediate cooling and is supplied directly to the spinning duct. 
     The spinning solution is dry spun from a 36-orifice nozzle having spiral nozzle orifices (cf. accompanying FIG. 1). The nozzle orifices are arranged round an annular nozzle in such a way that the openings of the profiled nozzle are orientated transversely to the air jet (cf. accompanying FIG. 2). The nozzle orifices have an area of 0.08 mm 2  and the width of the sides is 0.06 mm. The duct is at a temperature of 160° C. and the air at a temperature of 150° C. The quantity of air passed through, which issues in the immediate vicinity of the spinnerette onto the filament bundle issuing from the spinnerette in a transverse direction to the filament take-off at one end from the centre of the spinnerette in all directions, is 30 m 3  /h. The take-off speed is 125 m/min. The spun material having a titre of 790 dtex is collected on bobbins and twisted into a tow having a total titre of 158 000 dtex. The fibre cable is then washed in water at 80°  C., drawn 1:4 in boiling water, provided with an antistatic preparation, crimped, cut into staple fibres having a length of 60 mm and subsequently dried on a perforated belt drier at 120° C. The hollow fibres which have a final titre of 6.7 dtex have a tensile strength of 2.7 cN/tex and a breaking elongation of 31%. The water-retention capacity is 37.6%. For microscopic examination of the cross-sectional geometry, the fibre capillaries were imbedded in methacrylic acid methyl ester and cut transversely. The light-microscopic photographs produced by the differential interference contrast method showed that the cross-sections of the samples had a complete, uniform round cavity structure. The cavity portion formed about 50% of the total cross-sectional area. 
     Table 1 below shows the limits to the process according to the present invention for the production of hollow acrylic fibres by the dry spinning process, with reference to further Examples. In all cases, an acrylonitrile copolymer having the chemical composition from Example 1 is again used and converted into a spinning solution in the manner described therein. The solids content, as well as the type and proportion of non-solvent for polyacrylonitrile were varied. A loop-shaped 36-orifice nozzle (cf. accompanying FIG. 1) with the orifice arrangement indicated in accompanying FIG. 2 was used for spinning. The spinning and after-treatment conditions correspond to the data given in Example 1. The viscosities were measured in falling ball seconds at 80° C. 
     
                                           TABLE I                                 
__________________________________________________________________________
          Visc.                                                           
               Chemical                                                   
          (falling                                                        
               composition % of                                           
Non-solvent                                                               
          ball sec)                                                       
               spinning solution                                          
                            Fibre cross-                                  
                                        Contour                           
No.                                                                       
   for PAN                                                                
          at 80° C.                                                
               PAN                                                        
                  non-solvent                                             
                        DMF section     sharpness                         
__________________________________________________________________________
1  Water  41   34 3     63  Kidney-shape                                  
                                        No hollow                         
                                        fibre                             
2  Water  73   35 3     62  Hollow fibre +                                
                                        No hollow                         
                            kidney-shape                                  
                                        fibre                             
3  Water  120  36 3     61  Hollow fibre                                  
                                        in order                          
4  Water  176  38 3     59  Hollow fibre                                  
                                        in order                          
5  Water  243  40 3     57  Hollow fibre                                  
                                        in order                          
6  Water  75   35 4     61  Hollow fibre +                                
                                        No hollow                         
                            kidney-shape                                  
                                        fibre                             
7  Water  79   35 5     60  Hollow fibre +                                
                                        No hollow                         
                            kidney-shape                                  
                                        fibre                             
8  Water  124  36 4     60  Hollow fibre                                  
                                        in order                          
9  Water  105  30 10    60  No spinning possible -                        
                                        --                                
                            breaking of the                               
                            filaments                                     
10 Butanol                                                                
          106  35 4     61  Hollow fibres +                               
                                        No hollow                         
                            kidney-shape                                  
                                        fibre                             
11 Butanol                                                                
          127  36 4     60  Hollow fibre                                  
                                        in order                          
12 Butanol                                                                
          233  38 4     58  Hollow fibre                                  
                                        in order                          
__________________________________________________________________________
 
    
     EXAMPLE 2 
     (a) A proportion of the spinning solution from Example 1 is dry spun in the manner described therein from a 36-orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIGS. 1 and 2) under spinning conditions which are identical, except that the spinning air passed through at 30 m 3  /h may act on the filament bundle issuing from the spinnerette in the direction of the filament take-off in the immediate vicinity of the spinnerette from the outside, as well as from the inside. The spun material is collected on bobbins and, as described in Example 1, is twisted into a tow having a total titre of 158 000 dtex and is subsequently treated to form fibres having a final titre of 6.7 dtex. The cross-sections of the fibre sample do not have a uniform shape and have varying cavity portions. About 50% of the fibre cross sections are completely compact. 
     (b) A further proportion of the spinning solution from Example 1 is dry spun in the manner described therein from a 36-orifice nozzle having loop-shaped nozzle orifices according to accompanying FIGS. 1 and 2, under spinning conditions which are identical except that the spinning air passed through at 30 m 3  /h may act on the issuing filament bundle in the immediate vicinity of the spinnerette in the transverse direction from the outside instead of from the inside. Spun material is again collected as described in Example 1, twisted and subsequently treated to form fibres having a final titre of 6.7 dtex. The cross-sections of the fibre sample again do not have a uniform shape and have varying cavity portions. About 60% of the fibre cross-sections were completely compact. 
     EXAMPLE 3 
     A proportion of the twisted hollow fibre cable from Example 1 having a total titre of 158 000 dtex was washed in water at 80° C., drawn 1:4 in boiling water, provided with an antistatic preparation and dried under tension at 160° C. in a drum drier. The filaments were then crimped and cut into staple fibres having a length of 60 mm. The hollow fibres which have a final titre of 6.7 dtex, have a water-retention capacity of 14.1%. The cross-sections of the fibre samples comprise, in addition to about 30% of round hollow fibres which are uniform in shape, about 70% of fibres which are deflated in shape having varying cavity portions, for example half-moon-shaped to sickle-shaped configurations, as well as hollow fibres having several breakages in cross-section. A super-pressure is obviously formed in the air enclosed in the cavity when drying hollow fibre cables at high temperatures, so that the hollow fibres break open with collapse of the cross-sectional structure. The breaking of the hollow fibres is demonstrated in the drier by grating noises. 
     EXAMPLE 4 
     An acrylonitrile copolymer having the chemical composition from Example 1, was dissolved, filtered and dry spun from a 36 orifice nozzle having spiral nozzle orifices (cf. accompanying FIG. 3) in the manner described therein. In contrast to Example 1, however, the nozzle orifices are arranged in such a way that the opening between the sides is orientated exactly toward the centre of the spinning duct so that the spinning air may enter the spinning orifices centrally from the centre of the spinning duct (air jet angle=0°). The overlap between the ends of the sides of the nozzle orifices is again 20°, the nozzle orifoice area 0.08 mm 2  and the width of the sides 0.06 mm. The other spinning and after-treatment data correspond to the particulars in Example 1. The hollow fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 16.4%. The cross-sections of the fibre samples reveal irregularly deformed tubular to loop-shaped collapsed hollow fibres having varying cavity portions, as well as some completely compact structures. 
     EXAMPLE 5 
     An acrylonitrile copolymer having the chemical composition from Example 1 was dissolved, filtered and dry spun from a 36-orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 4) in the manner described therein. One end of the sides of the loop-shaped nozzle orifices is lengthened in comparison with the profiling nozzle from Example 1 in such a way that the overlap angle of the ends of the sides is 55°, so that the air no longer flows transversely to the openings between the sides of the profiling nozzle, but at an angle of 125° (cf. accompanying FIG. 5). The nozzle orifices have an area of 0.095 mm 2  and the width of the sides is 0.06 mm. The other spinning and after-treatment conditions correspond to the particulars in Example 1. The fibres, which have a final titre of 6.7 dtex, have a water retention capacity of 10.7%. The cross-sections of the fibre sample do not exhibit a closed cavity shape but have half-moon-shaped to curved configurations. 
     EXAMPLE 6 
     An acrylonitrile copolymer having the chemical composition from Example 1, was dissolved, filtered and dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 3) in the manner described therein. One end of the sides of the loop-shaped nozzle orifices is lengthened in the manner described in Example 5 in such a way that the overlap angle of the ends of the sides is 55°. In contrast to Example 5, however, the nozzle orifices are arranged in such a way that the openings between the ends of the sides of the profiling nozzle form an angle of 35° to the direction of the spinning air from the centre of the spinning duct so that the spinning air may only flow obliquely into the nozzle orifices from the inside (cf. accompanying FIG. 6). The area of the nozzle orifices is 0.095 mm 2  and the width of the sides 0.06 mm. The other spinning and after-treatment conditions correspond to the particulars in Example 1. The hollow fibres, which have a final titre of 6.7 dtex, have a water retention capacity of 20.5%. The cross-sections of the sample fibres exhibit predominantly closed tubular to loop-shaped configurations which are, however, irregularly deformed. 
     EXAMPLE 7 
     (a) An acrylonitrile copolymer having the chemical composition from Example 1 was dissolved, filtered and dry spun from a 36-orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 1) in the manner described therein. The nozzle orifice arrangement and the overlap angle between the two ends of the sides correspond to the particulars in Example 1 so that the air flow angle between the centre of the spinning duct and the profiling nozzle opening is again 90°. In contrast to Example 1, the width between the sides of the profiling nozzle is 0.10 mm instead of 0.06 mm and the nozzle orifice area is 1.33 mm 2 . The other spinning and after-treatment conditions correspond to the particulars in Example 1. The hollow fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 35.3%. The cross-sections of the sample fibres are completely uniform and round and the cavity portion again forms about 50% of the total cross-sectional area. 
     (b) A proportion of the spinning solution from Example 7 is dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 1) as described in Example 1. The nozzle orifice arrangement, overlap angle of the ends of the sides and the air flow angle again correspond to the particulars in Example 1. The width of the sides of the profiling nozzle is 0.12 mm and the nozzle orifice area 0.16 mm 2 . The spinning and after-treatment conditions correspond to the data in Example 1. However, hollow fibres which are not uniform in shape are formed. In addition to completely round hollow fibres, loop-shaped forms and collapsed cross-sectional shapes having a tubular smaller volume cavity are also obtained. The water-retention capacity is 23.1%. 
     (c) A further proportion of the spinning solution from Example 7 is dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 1) as described in Example 1. The arrangement of nozzle orifices, overlap angle and air flow angle correspond to the particulars from Example 1. The width of the sides of the profiling nozzle is 0.15 mm and the nozzle orifice area 0.20 mm 2 . The spinning and after-treatment conditions correspond to the data in Example 1. Hollow fibres are no longer obtained. The profile shape merges, forming compact, irregular oval or irregular cross-sectional structures. The water-retention capacity is 6.3%. 
     EXAMPLE 8 
     51 kg of DMF are mixed with 4 kg of water in a heated chamber with stirring. 45 kg of an acrylonitrile copolymer containing 92% of acrylonitrile, 6% acrylic acid methyl ester and 2% of sodium methallyl sulphonate having a K value of 60 are then added at room temperature with stirring. The suspension, which has a solids content of 45%, is dissolved, filtered and dry spun from a loop-shaped profiling nozzle having 36 orifices according to accompanying FIGS. 1 and 2 in the manner described in Example 1. The viscosity of the spinning solution is equivalent to 142 falling ball seconds at 80° C. The other spinning and after-treatment conditions correspond to the statements in Example 1. The cross-sections of the sample hollow fibres, which have a final titre of 8.0 dtex, exhibit a completely uniform round profile having a cavity portion of about 50%. The water-retention capacity is 39%. 
     EXAMPLE 9 
     57 kg of dimethylformamide (DMF) are mixed with 6 kg of monoethylene glycol in a heated chamber at room temperature with stirring. 37 kg of an acrylonitrile copolymer containing 93.6% of acrylonitrile, 5.7% of acrylic acid methyl ester and 0.7% of sodium methallyl sulphonate having a K value of 81 are then added at room temperature with stirring. The suspension is pumped via a gear pump into a heated spinning chamber provided with a stirrer. The suspension, which has a solids content of 37%, by weight, is then heated in a double-walled tube using steam at 4.0 bar. The residence time in the tube is 7 minutes. The temperature of the solution at the tube outlet is 138° C. The tube contains several mixing chambers for the homogenisation of the spinning solution. The spinning solution, which has a viscosity equivalent to 186 falling ball seconds at 100° C., is filtered after leaving the heating apparatus without intermediate cooling and is supplied directly to the spinning duct. 
     The spinning solution is dry spun from a 36 orifice nozzle having spiral nozzle orifices (cf. accompanying FIG. 1). The nozzle orifices are arranged over an annular nozzle in such a way that the openings of the profiling nozzles are orientated transversely to the air flow (see accompanying FIG. 2). The nozzle orifice area is 0.08 mm 2  and the width of the sides 0.06 mm. The duct temperature is 160° C. and the air temperature 150° C. The quantity of air passed through, which issues in the immediate vicinity of the spinnerette onto the filament bundle issuing from the spinnerette in a transverse direction to the filament take-off at one end from the centre of the spinning duct in all directions, is 30 m 3  /h. The take-off speed is 125 m/min. The spun material having a titre of 790 dtex is collected on bobbins and twisted into a tow having a total titre of 158 000 dtex. The fibre cable is then washed in water at 80° C., drawn 1:4 in boiling water, provided with an antistatic preparation, crimped, cut into staple fibres having a length of 60 mm and subsequently dried on a perforated belt drier at 120° C. The hollow fibres, which have a final titre of 6.7 dtex, have a tensile strength of 2.3 cN/tex and a breaking elongation of 37%. The water-retention capacity is 50.3%. The cross-sections of the samples have a complete, uniform round cavity structure. The cavity portion amounts to about 50% of the total cross-sectional area. The solid composition surrounding the cavity consists of a porous core/sheath structure. 
     The limits of the process according to the present invention for the production of hollow acrylic fibres by the dry spinning process are indicated in Table II below with reference to further Examples. In all cases, an acrylonitrile copolymer having the chemical composition in Example 9 is again used and is converted into a spinning solution in the manner described therein. The solids concentration, as well as the type and proportion of non-solvent for polyacrylonitrile were varied. A 36 orifice nozzle having a loop-shape (cf. accompanying FIG. 1) with the orifice arrangement indicated in accompanying FIG. 2 was used for spinning. The spinning and after-treatment conditions correspond to the particulars from Example 9. The viscosities were measured in falling ball seconds at 100° C. in the manner described above. 
     
                                           TABLE II                                
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                           Viscosity                                      
           Chemical composition of                                        
                           (falling                                       
Non-solvent for                                                           
           the spinning solution %                                        
                        WR ball sec.)                                     
No.                                                                       
   PAN     PAN                                                            
              non-solvent                                                 
                    DMF %  at 100° C.                              
                                 Fibre cross-section                      
__________________________________________________________________________
1  Tetraethylene                                                          
           38 7     55  35.3                                              
                           152   Round hollow shape                       
   glycol                        with core/sheath                         
                                 structure                                
2  Tetraethylene                                                          
           36 7     57  42.1                                              
                           100   Round hollow shape                       
   glycol                        with core/sheath                         
                                 structure                                
3  Tetraethylene                                                          
           35 7     58  27.4                                              
                           72    oval, 80% hollow                         
   glycol                        fibre; 20% compact                       
4  Tetraethylene                                                          
           34 7     59  19.3                                              
                           58    oval, 30% hollow                         
   glycol                        fibre; 70% compact                       
5  Tetraethylene                                                          
           38 5     57  22.7                                              
                           134   round hollow shape                       
   glycol                        with core/sheath                         
                                 structure                                
6  Tetraethylene                                                          
           36 5     59  26.8                                              
                           87    round hollow shape                       
   glycol                        with core/sheath                         
                                 structure                                
7  Tetraethylene                                                          
           36 4     60  17.6                                              
                           78    round hollow shape,                      
   glycol                        indefinite core/                         
                                 sheath structure                         
8  Tetraethylene                                                          
           35 3     62  11.9                                              
                           55    oval, 40% hollow                         
   glycol                        fibre; 60% compact                       
9  Tetraethylene                                                          
           36 10    54  55.2                                              
                           184   round hollow shape                       
   glycol                        with core/sheath                         
                                 structure                                
10 Tetraethylene                                                          
           34 4     62  13.9                                              
                           48    irregular to oval,                       
   glycol                        30% hollow fibre,                        
                                 70% compact                              
11 Tetraethylene                                                          
           34 5     61  17.2                                              
                           50    irregular to oval,                       
   glycol                        30% hollow fibre,                        
                                 70% compact                              
12 Tetraethylene                                                          
           34 6     60  15.4                                              
                           61    irregular to oval                        
   glycol                        30% hollow fibre                         
                                 70% compact                              
13 Monoethylene                                                           
           34 5     61  16.6                                              
                           70    irregular to oval                        
   glycol                        30% hollow fibre                         
                                 70% compact                              
14 Monoethylene                                                           
           36 8     56  44.4                                              
                           156   round hollow fibre                       
   glycol                        with core/sheath                         
                                 structure                                
15 Glycerin                                                               
           36 8     56  39.3                                              
                           168   round hollow fibre                       
                                 with core/sheath                         
                                 structure                                
__________________________________________________________________________
 
    
     EXAMPLE 10 
     (a) A proportion of the spinning solution from Example 9 is dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIGS. 1 and 2) in the manner described therein, under identical spinning conditions, except that the spinning air passed through at 30 m 3  /h may act on the filament bundle issuing from the spinnerette in the direction of the filament take-off in the immediate vicinity of the spinnerette both from the outside and the inside. The spun material is collected on bobbins and twisted into a tow having a total titre of 158 000 dtex in the manner described in Example 9 and is subsequently treated to form fibres having a final titre of 6.7 dtex. The cross-sections of the sample fibres do not exhibit a uniform shape and have varying cavity portions. About 50% of the fibre cross-sections are completely compact. 
     (b) A further proportion of the spinnng solution from Example 9 is dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices according to accompanying FIGS. 1 and 2 in the manner described therein, under identical spinning conditions, except that the spinning air passed through at 30 m 3  /h may act on the issuing filament bundle in the immediate vicinity of the spinnerette in a transverse direction from the outside instead of from the inside. The spun material is again collected, twisted and subsequently treated to form fibres having a final titre of 6.7 dtex as described in Example 9. The cross-sections of the sample fibres again do not exhibit a uniform shape and have varying cavity portions. About 60% of the fibre cross-sections are completely compact. 
     EXAMPLE 11 
     A proportion of the twisted hollow fibre cable from Example 9 having a total titre of 158 000 dtex was washed in water at 80° C., drawn 1:4 in boiling water, provided with an antistatic preparation and dried under tension at 160° C. in a drum drier. The filaments were then crimped and cut to staple fibres having a length of 60 mm. The hollow fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 17.1%. The cross-sections of the sample fibres exhibit, in addition to about 30% of round hollow fibres which are uniform in shape, about 70% of collapsed fibres having varying cavity portions, some half-moon-shaped to sickle-shaped configurations, as well as hollow fibres with several breakages in cross-section. A super pressure is obviously formed in the air enclosed in the cavity when drying this hollow fibre cable at high temperatures, so that the hollow fibres break open and the cross-sectional structure collapses. The breaking open of the hollow fibres is demonstrated in the drier by grating noises. The core-sheath structure is also substantially lost. There are now only compact hollow fibres without a pore system. 
     EXAMPLE 12 
     An acrylonitrile copolymer having the chemical composition from Example 9 was dissolved, filtered and dry spun from a 36 orifice nozzle having spiral nozzle orifices (cf. accompanying FIG. 1) in the manner described therein. In contrast to Example 9, however, the nozzle orifices are arranged in such a way that the opening between the sides is orientated exactly toward the centre of the spinning duct (cf. accompanying FIG. 3) so that the spinning air may flow into the nozzle openings centrally from the centre of the spinning duct (air flow angle equals 0°). The overlap between the ends of the sides of the nozzle orifices is again 20°, the nozzle orifice area 0.08 mm 2  and the width of the sides 0.06 mm. The other spinning and after-treatment data correspond to the particulars in Example 9. The hollow fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 22.4%. The cross-sections of the sample fibres exhibit irregularly deformed tubular to loop-shaped collapsed hollow fibres having varying cavity portions, as well as some completely compact cross-sectional structures. 
     EXAMPLE 13 
     An acrylonitrile copolymer having the chemical composition from Example 9 was dissolved, filtered and dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 4) in the manner described therein. One end of the sides of the loop-shaped nozzle orifices is lengthened in comparison with the profiling nozzle from Example 1 in such a way that the overlap angle between the ends of the sides is 55° so that the air no longer flows transversely to the openings between the sides of the profiling nozzle, but at an angle of 125° C. (cf accompanying FIG. 5). The area of the nozzle orifices is 0.095 mm 2  and the width of the sides 0.06 mm. The other spinning and after-treatment conditions correspond to the particulars from Example 9. The fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 13.7%. The cross-sections of the sample fibres do not exhibit a closed cavity shape, but rather a half-moon-shaped to curved configuration. 
     EXAMPLE 14 
     An acrylonitrile copolymer having the chemical composition from Example 9 was dissolved, filtered and dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 4) in the manner described therein. One end of the sides of the loop-shaped nozzle orifices is lengthened in the manner described in Example 13 so that the overlap angle of the ends of the sides is 55°. In contrast to Example 13, however, the nozzle orifices are arranged in such a way that the openings between the ends of the sides of the profiling nozzle form an angle of 35° to the direction of the spinning air from the centre of the spinning duct (cf. accompanying FIG. 6), so that the spinning air may also flow obliquely into the nozzle openings from the inside. The area of the nozzle orifices is 0.095 mm 2  and the width of the sides 0.06 mm. The other spinning and after-treatment conditions correspond to the particulars in Example 9. The hollow fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 24.5%. The cross-sections of the sample fibres exhibit predominantly closed tubular to loop-shaped configurations which are, however, irregularly deformed in structure and have core/sheath structures. 
     EXAMPLE 15 
     (a) An acrylonitrile copolymer having the chemical composition from Example 9 was dissolved, filtered and dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 1) in the manner described therein. The nozzle orifice arrangement and the overlap angle of the two ends of the sides correspond to the particulars from Example 9 so that the air flow angle between the centre of the spinning duct and the profiling nozzle opening is again 90° (cf. accompanying FIG. 2). In contrast to Example 9, the width of the sides of the profiling nozzle is 0.10 mm instead of 0.06 mm and the area of the nozzle orifices is 1.33 mm 2 . The other spinning and after-treatment conditions correspond to the particulars in Example 9. The porous hollow fibres, which have a final titre of 6.7 dtex, have a water-retention capacity of 45.3%. The cross-sections of the sample fibres are completely uniform and round, the cavity portion is again 50% of the total cross-sectional area. 
     (b) A proportion of the spinning solution from Example 15 is dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 1) in the manner described in Example 9. The nozzle orifice arrangement, overlap angle of the ends of the sides and air flow angle again correspond to the particulars from Example 9. The width of the sides of the profiling nozzle is 0.12 mm and the area of the nozzle orifice is 0.16 mm 2 . The spinning and after-treatment conditions correspond to the data from Example 9. Hollow fibres are formed, but they are not uniform in shape. In addition to completely round porous hollow fibres, loop-shaped cross-sectional shapes and collapsed cross-sectional shapes in the manner of tubes having smaller cavity volumes are obtained. The water-retention capacity is 25.1%. 
     (c) A further proportion of the spinnng solution from Example 15 is dry spun from a 36 orifice nozzle having loop-shaped nozzle orifices (cf. accompanying FIG. 1) in the manner described in Example 9. The nozzle orifice arrangement, overlap angle and air flow angle correspond to the particulars from Example 9. The width of the sides of the profiling nozzle is 0.15 mm and the area of the nozzle orifices is 0.20 mm 2 . The spinning and after-treatment conditions correspond to the data from Example 9. Hollow fibres are no longer obtained. The profiled shape merges and forms compact, irregular oval to irregular cross-sectional structures. The water-retention capacity is 8.3%.