Abstract:
The invention relates to a nanofiber web preparing apparatus and method via electro-blown spinning. The nanofiber web preparing method includes feeding a polymer solution, which is a polymer dissolved into a given solvent, toward a spinning nozzle, discharging the polymer solution via the spinning nozzle, which is charged with a high voltage, while injecting compressed air via the lower end of the spinning nozzle, and collecting fiber spun in the form of a web on a grounded suction collector under the spinning nozzle, in which both of thermoplastic and thermosetting resins are applicable, the solution does not need to be heated and electrical insulation is readily realized.

Description:
[0001]    The present invention relates to a nanofiber web preparing apparatus and method via electro-blown spinning, in particular, in which both of thermoplastic and thermosetting resins are applicable, such that the polymer solution does not need to be heated and electrical insulation is readily realized. Herein, “electro-blown” means injecting compressed air while applying a high voltage during spinning of nanofiber, and “electro-blown spinning” means spinning using an electro-blown method. 
         [0002]    In general, consumption of non-woven cloth is gradually increasing owing to various applications of non-woven cloth, and manufacturing processes of non-woven cloth are also variously developing. 
         [0003]    A variety of studies have been carried out in many countries including the USA for developing technologies for manufacturing non-woven cloth composed of ultra-fine nanofiber (hereinafter it will be referred to as ‘nanofiber web’) which is advanced for one stage over conventional super-fine fiber. Such technologies are still in their initial stage without any commercialization while conventional technologies remain in a stage in which super-fine fibers are prepared with a diameter of about several micrometer. Nanofiber having a diameter of about several nanometer to hundreds of nanometer cannot be prepared according to conventional super-fine fiber technologies. Nanofiber has a surface area per unit volume, which is incomparably larger than that of conventional super-fine fiber. Nanofiber having various surface characteristics, structures and combined components can be prepared so as to overcome the limitations of physical properties of articles made of conventional super-fine fiber while creating articles having new performance. 
         [0004]    It is well known that a nanofiber web using the above nanofiber preparing method can be used as an ultra precise filter, electric-electronic industrial material, medical biomaterial, high-performance composite, etc. 
         [0005]    The technologies in use for preparing ultra-fine fiber up to the present can be classified into three methods: flash spinning, electrostatic spinning and meltblown spinning. Such technologies are disclosed in Korean Laid-Open Patent Application Serial Nos. 10-2001-31586 and 10-2001-31587, entitled “Preparing Method of Ultra-Fine Single Fiber” previously filed by the assignee. 
         [0006]    Korean Laid-Open Patent Application Serial No. 10-2001-31586 discloses that nanofiber in nanometer scale can be mass-produced with high productivity and yield by systematically combining melt-blown spinning and electrostatic spinning.  FIG. 3  schematically shows a process for explaining this technology. Referring to  FIG. 3 , a thermoplastic polymer is fed via a hopper  10  into an extruder  12  where the thermoplastic polymer is melted into a liquid polymer. The molten liquid polymer is fed into a spinneret  14  and then spun via a spinning nozzle  16  together with hot air into an electric field. An electric field is generated between the spinning nozzle  16  charged with voltage and a collector  18 . Nanofibers spun onto the collector  18  are collected in the form of a web by a vacuum blower  20 . 
         [0007]    Korean Laid-Open Patent Application Serial No. 10-2001-31587 discloses that nanofiber in nanometer scale can be mass-produced with high productivity and yield by systematically combining flash spinning and electrostatic spinning.  FIG. 4  schematically shows a process for explaining this technology. Referring to  FIG. 4 , a polymer solution is fed from a storage tank  22  into a spinneret  26  with a compression pump  24 , and spun into an electric field via a decompressing orifice  28  and then via a spinning nozzle  30 . An electric field is generated between the spinning nozzle  30  charged with voltage and a collector  32 . Nanofibers spun onto the collector  32  are collected in the form of a web by a vacuum blower  34 . 
         [0008]    It can be understood that the nanofiber webs composed of nanofiber can be prepared according to the two technologies as above. 
         [0009]    However, the foregoing conventional technologies have many drawbacks in that electrical insulation is not readily realized, applicable resin is restricted and heating is needed. 
       SUMMARY OF INVENTION 
       [0010]    The present invention has been made to solve the foregoing problems and it is therefore an object of the present invention to provide a nanofiber web preparing method in which both of thermoplastic and thermosetting resins are applicable, such that a polymer solution does not need to be heated and electrical insulation is readily realized. 
         [0011]    It is another object of the invention to provide a nanofiber web preparing apparatus for conducting the above preparing method. 
         [0012]    According to an aspect of the invention to obtain the above objects, it is provided a nanofiber web preparing method comprising the following steps of feeding a polymer solution, which is dissolved into a given solvent, to a spinning nozzle; discharging the polymer solution through the spinning nozzle, which is charged with a high voltage, while injecting compressed air via the lower end of the spinning nozzle; and collecting fiber spun in the form of a web on a grounded vacuum collector under the spinning nozzle. 
         [0013]    According to another aspect of the invention to obtain the above objects, it is provided a nanofiber web preparing apparatus comprising a storage tank for preparing a polymer solution; a spinning nozzle for discharging the polymer solution fed from the storage tank; an air nozzle disposed adjacent to the lower end of the spinning nozzle for injecting compressed air; high voltage charging means connected to the spinning nozzle; and a grounded collector for collecting spun fiber in the form of a web which is discharged from the spinning nozzle. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  shows a construction of a nanofiber web preparing apparatus of the invention; 
           [0015]      FIG. 2A  is a sectional view of a spinneret having an air nozzle on a knife edge; 
           [0016]      FIG. 2B  is a sectional view of another spinneret having a cylindrical air nozzle; 
           [0017]      FIG. 3  schematically shows a nanofiber preparing process via systematic combination of melt-blown spinning and electro-blown spinning; and 
           [0018]      FIG. 4  schematically shows a nanofiber preparing process via systematic combination of flash spinning and electrostatic spinning. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a construction of a nanofiber web preparing apparatus of the invention for illustrating a nanofiber web preparing process, and  FIGS. 2A and 2B  show nozzle constructions for illustrating spinning nozzles and air nozzles. The nanofiber web preparing process will be described in detail in reference to  FIGS. 1 to 2B . 
         [0020]    A storage tank  100  prepares a polymer solution via combination between polymer and solvent. Polymers available for the invention are not restricted to thermoplastic resins, but may utilize most synthetic resins, including thermosetting resins. Examples of the suitable polymers may include polyimide, nylon, polyaramide, polybenzimidazole, polyetherimide, polyacrylonitrile, PET (polyethylene terephthalate), polypropylene, polyaniline, polyethylene oxide, PEN (polyethylene naphthalate), PBT (polybutylene terephthalate), SBR (styrene butadiene rubber), polystyrene, PVC (polyvinyl chloride), polyvinyl alcohol, PVDF (polyvinylidene fluoride), polyvinyl butylene and copolymers or derivative compounds thereof. The polymer solution is prepared by selecting a solvent according to the above polymers. Although the apparatus shown in  FIG. 1  adopts a compression arrangement which forcibly blows compressed air or nitrogen gas into the storage tank  100  in order to feed the polymer solution from the storage tank  100 , any known means can be utilized without restricting feed of the polymer solution. The polymer solution can be mixed with additives including any resin compatible with an associated polymer, plasticizer, ultraviolet ray stabilizer, crosslink agent, curing agent, reaction initiator and etc. Although dissolving most of the polymers may not require any specific temperature ranges, heating may be needed for assisting the dissolution reaction. 
         [0021]    The polymer solution is discharged from the storage tank  100  through a spinning nozzle  104  of a spinneret  102  which is electrically insulated and charged with a high voltage. After heating in an air heater  108 , compressed air is injected through air nozzles  106  disposed on either side of the spinning nozzle  104 . 
         [0022]    Now reference will be made to  FIGS. 2A and 2B  each illustrating the construction of the spinning nozzle  104  and the air nozzle  106  in the spinneret  102 .  FIG. 2A  shows the same construction as in  FIG. 1  in which the air nozzle  106  is formed by a knife edge on both sides of the spinning nozzle  104 . In the spinning nozzle  104  shown in  FIG. 2A , the polymer solution flows into the spinning nozzle  104  through an upper portion thereof and is injected through a capillary tube in the lower end. Since a number of spinning nozzles  104  of the above construction are arranged in a line at given intervals, air nozzles  106  may be formed by knife edges at both sides of the spinning nozzles  104  parallel to the arrangement thereof, and nanofibers can be advantageously spun with the number of spinning nozzles  104 . Referring to preferred magnitudes of the components, the air nozzles  106  each have an air gap “a” which is suitably sized in the range of about 0.1 to 5 mm and preferably of about 0.5 to 2 mm, whereas the lower end capillary tube has a diameter “d” which is suitably sized with in the range of about 0.1 to 2.0 mm and preferably of about 0.2 to 0.5 mm. The lower end capillary tube of the air nozzle  106  has a suitable length-to-diameter ratio L/d, which is in the range of about 1 to 20 and preferably about 2 to 10. A nozzle projection “e” has a length corresponding to the difference between the lower end of air nozzle  106  and the lower end of spinning nozzle  104 , and functions to prevention fouling of the spinning nozzle  104 . The length of the nozzle projection “e” is preferably about −5 to 10 mm, and more particularly 0 to 10 mm. 
         [0023]    The spinning nozzle  104  shown in  FIG. 2B  has a construction which is substantially equivalent to that shown in  FIG. 2A , while the air nozzle  106  has a cylindrical structure circularly surrounding the spinning nozzle  104 , in which compressed air is uniformly injected from the air nozzle  106  around nanofibers, which is spun through the spinning nozzle  104 , so as to have an advantageous orientation over the construction of  FIG. 2A , i.e. the air nozzles formed by the knife edge. Where a number of spinning nozzles  104  are necessary, spinning nozzles  104  and air nozzles  106  of the above construction are arranged within the spinneret. However, a manufacturing process of this arrangement is more difficult than that in  FIG. 2A . 
         [0024]    Now referring to  FIG. 1  again, the polymer solution discharged from the spinning nozzle  104  of the spinneret  102  is collected in the form of a web on a vacuum collector  110  under the spinning nozzle  104 . The collector  110  is grounded, and designed to draw air through an air collecting tube  114  so that air can be drawn through a high voltage region between the spinning nozzle  104  and the collector  110  and the suction side of a blower  112 . Air drawn in by the blower contains solvent and thus a Solvent Recovery System (SRS, not shown) is preferably designed to recover solvent while recycling air through the same. The SRS may adopt a well-known construction. 
         [0025]    In the above construction for the preparing process, portions to which voltage is applied or which are grounded are obviously divided from other portions so that electrical insulation is readily realized. 
         [0026]    The invention injects compressed air through the air nozzle  106  while drawing air through the collector  110  so that nozzle fouling can be minimized in an optimum embodiment of the invention. As not apparently described in the above, nozzle fouling acts as a severe obstructive factor in preparation processes via spinning except for melt-blown spinning. The invention can minimize nozzle fouling via compressed air injection and vacuum. The nozzle projection “e” more preferably functions to clean nozzle fouling since compressed air injected owing to adjustment of the nozzle projection “e” can clean the nozzles. 
         [0027]    Further, various substrates can be arranged on the collector to collect and combine a fiber web spun on the substrate so that the combined fiber web can be used as a high-performance filter, wiper and so on. Examples of the substrate may include various non-woven cloths such as melt-blown non-woven cloth, needle punched and spunlaced non-woven cloth, woven cloth, knitted cloth, paper and the like, and can be used without limitations so long as a nanofiber layer can be added on the substrate. 
         [0028]    The invention has the following process conditions. 
         [0029]    Voltage is applied to the spinneret  102  preferably in the range of about 1 to 300 kV and more preferably of about 10 to 100 kV with a conventional high voltage charging means. The polymer solution can be discharged in a pressure ranging from about 0.01 to 200 kg/cm 2  and in preferably about 0.1 to 20 kg/cm 2 . This allows the polymer solution to be discharged in large quantities adequate for mass production of nanofibers. The process of the invention can discharge the polymer solution with a high throughput rate of about 0.1 to 5 cc/min hole as compared with electrostatic spinning methods. 
         [0030]    Compressed air injected via the air nozzle  106  has a flow rate of about 10 to 10,000 m/min and preferably of about 100 to 3,000 m/min. Air temperature is preferably in the range of about room temperature to about 300° C. and more preferably between about 100° C. and room temperature. A Die to Collector Distance (DCD), i.e. the distance between the lower end of the spinning nozzle  104  and the vacuum collector  110 , is preferably about 1 to 200 cm and more preferably 10 to 50 cm. 
         [0031]    Hereinafter the present invention will be described in more detail in the following examples. 
         [0032]    A polymer solution having a concentration of 20 wt % was prepared using polyacrylonitrile (PAN) as a polymer and DMF as a solvent and then spun through a spinneret having knife edge air nozzles as shown in  FIG. 1 . The polymer solution was spun according to the following condition, in which a spinning nozzle had a diameter of about 0.25 mm, L/d of the nozzle was 10, DCD was 200 mm, a spinning pressure was 6 kg/cm 2  and an applied voltage was 50 kV DC. 
         [0033]    The spinneret on the knife edge constructed as in  FIG. 1  was used in the following examples. The diameter of the spinning nozzle was 0.25 mm, L/d of the nozzle was 10, and DCD was varied in examples 1 to 3 and set to 300 mm in examples 4 to 10. The number of the spinning nozzles was 500, the width of a die was 750 mm, the nozzle projection “e” was about 0 to 3 mm, and the flow rate of compressed air was maintained at 300 to 3,000 m/min through the air nozzle. 
         [0000]    
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Spinning 
                 App. 
               
               
                   
                   
                   
                   
                 DCD 
                 Pressure 
                 Voltage 
               
               
                 No. 
                 Polymer 
                 Solvent 
                 Conc. (%) 
                 (mm) 
                 (kgf/cm2) 
                 (kV) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ex. 1 
                 PAN 
                 DMF 
                 15 
                 350 
                 3 
                 30 
               
               
                 Ex. 2 
                 PAN 
                 DMF 
                 20 
                 160 
                 4 
                 40 
               
               
                 Ex. 3 
                 PAN 
                 DMF 
                 20 
                 200 
                 6 
                 50 
               
               
                 Comp. 
                 PAN 
                 DMF 
                 25 
               
               
                 Ex. 1 
               
               
                   
               
             
          
         
       
     
         [0034]    Example 1 was good in fluidity and spinning ability, but poor in formation of web. Examples 2 and 3 were good in fluidity, spinning ability and formation of web. Examination of SEM pictures showed fiber diameter distribution of about 500 nm to 2 μm. In particular, Example 3 demonstrated uniform fiber diameter distribution in the range of 500 nm to 1.2 μm. In Comparative Example 1, it was difficult to prepare a PAN 25% solution and thus no result was obtained. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 Spinning Pressure 
                 App. Voltage 
                 Diam. Distribution 
               
               
                 No. 
                 (kgf/cm 2 ) 
                 (kV) 
                 (nm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Ex. 4 
                 3 
                 21 
                 933.96-1470     
               
               
                 Ex. 5 
                 3 
                 30 
                 588.69-1000     
               
               
                 Ex. 6 
                 2.9 
                 40 
                 500.9-970.8 
               
               
                 Ex. 7 
                 3 
                 60 
                 397.97-520.85 
               
               
                 Ex. 8 
                 3.1 
                 80 
                 280.01-831.60 
               
               
                 Ex. 9 
                 3.5 
                 40 
                 588.69-933.77 
               
               
                 Ex. 10 
                 4 
                 40 
                 633.9-1510  
               
               
                   
               
             
          
         
       
     
         [0035]    Table 2 reports conditions and their results of Examples 4 to 10, which used nylon 6,6 for polymer and formic acid for solvent. The polymer solution concentrations were 25%. Fiber diameter distributions in Table 2 were determined by SEM picture examination, in which nanofibers having uniform diameters are irregularly arranged in the form of a web. 
         [0036]    As set forth above, the present invention forms webs of nanofibers with a fiber fineness ranging from about several nanometers to hundreds of nanometers. Also the preparing process of the invention has a higher throughput rate compared to conventional electrostatic spinning, thereby potentially mass producing nanofibers. Further, since a polymer solution is used, the invention has advantages in that the necessity of heating polymer is reduced and both thermoplastic and thermosetting resins can be used. 
         [0037]    Moreover, in the arrangement used for the electro-blown spinning, the spinneret can be readily electrically insulated while solvent can be recovered via vacuum.