Patent Publication Number: US-2009224437-A1

Title: Electrostatic spray apparatus and method of electrostatic spray

Description:
TECHNICAL FIELD 
     The present invention relates to an electrostatic spray apparatus and a method of electrostatic spray for electrostatically spraying a solution containing a polymeric substance. 
     BACKGROUND ART 
     Electrostatic spray (electrospinning) is carried out according to the following procedure in which a polymeric substance serving as a material desired to be used for coating is mixed with a solvent to make a solution, the solution is stored in a container having a pointed tip, such as the needle tip of a syringe or a thin glass pipe, and a high voltage is applied between the container and an object to which the polymeric substance is sprayed. In the electrostatic spray, charges are provided to the polymeric substance inside the container, the charged polymeric substance is emitted in an atomized form due to the repulsion forces acting between the charges from the tip of the container to the side referred to as a collector having a polarity different from that of the spray side (the tip side of the container) or to the ground side using the Coulomb forces and then collected and laminated on the collector side. Electrostatic coating is a technique in which electrostatic spray is used, and it is generally known that electrostatic coating is applied to vehicle bodies, such as automobile bodies. Electrostatic coating is not limited to vehicle body coating, but is applicable to the coating of various types of products. The present invention relates to an apparatus particularly adapted for electrostatically spraying an artificial polymeric substance as a material and more particularly relates to an apparatus capable of easily producing nonwoven fabrics formed of fibers thinner than 100 nanometers [nm] in diameter, referred to as nanofibers. Nonwoven fabrics produced in this way can be used in a wide range of applications including filters. 
     Filters can also be produced for example using the “fusion method” that is one of methods for producing conventional nonwoven fabrics. However, filters formed of fibers several ten micrometers in diameter are mainly produced using such a conventional production method, and filters formed of fibers of several hundred nanometers are the limit of the products that can be produced using the conventional method. On the other hand, the production method using electrostatic spray can produce nonwoven fabrics formed of fibers thinner than those in the case of the conventional production method such as the “fusion method” by one or two orders of magnitude as described above. When only one nozzle having a spray port is used for electrostatic spray, only small filters of several centimeter square can be produced. Hence, the configuration wherein only one nozzle having a spray port is used is not realistic in view of mass productivity and productivity. When filters having a width of 100 cm or an equivalent size are produced by textile manufacturers and original fabric manufacturers, it is necessary to use multiple nozzles as in the cases of nonwoven fabrics and films having been produced using the conventional production method. 
     However, when multiple nozzles are used, there is a problem that the operation state of electrostatic spray becomes unstable due to the effects of interference, repulsion, etc. of charges, and some or most of the multiple nozzles disposed cannot carry out electrostatic spray in some cases. Hence, carrying out stable electrostatic spray using multiple nozzles is a task that must be achieved to put an electrostatic spray apparatus having multiple nozzles into practical use and to mass-produce products. 
     To achieve the above-mentioned task, a mechanism for suppressing the interference and repulsion of charges, referred to as a charge distribution plate, has been proposed in Japanese Patent Application Laid-Open Publication No. 2002-201559. However, the charge distribution plate is difficult to handle since the position of the charge distribution plate must be changed depending on the voltage applied to carry out electrostatic spray. Furthermore, in the case that multiple nozzles are arranged in row and column directions, there is a problem that the ratio of the coating amount (lamination amount) to the spraying amount, the so-called collection efficiency, becomes lower when the charge distribution plate is used. 
     Furthermore, a system in which electrodes are disposed on the nozzles and the mounting face of an object to which a material is sprayed and a high voltage is applied therebetween has been disclosed in Japanese Patent Application Laid-Open Publication No. H 8-153669. In the system in which the electrodes are disposed on the nozzles and the installation face and electrostatic spray is carried out for the object to which a material is sprayed as described above, the electrostatically sprayed material is laminated so as to cover the mounting face, that is, the surface of the opposed electrode, and the capability of the electrostatic spray becomes weak gradually. Hence, in the production method disclosed in Japanese Patent Application Laid-Open Publication No. H 8-153669, it is difficult to carry out electrostatic spray with high accuracy for a long time, and the method is not suited for mass production. 
     Patent document 1: Japanese Patent Application Laid-Open Publication No. 2002-201559 
     Patent document 2: Japanese Patent Application Laid-Open Publication No. H 8-153669 
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     Material particles having been charged and electrostatically sprayed are attracted to a collector serving as a collecting face (lamination face) having charges of a different polarity and then collected (laminated) on the collector. Since the material particles sprayed have the same polarity, they are repulsed from one another due to the Coulomb forces and dispersed, and then reach the collector in a uniform state. At the same time when the material particles are collected (laminated) on the collector, the charges thereon are discharged to the collector. Even after the charges are discharged, the collector is required to hold the amount of charges equivalent to the amount of charges on the material particles at all times, the polarity of the charges being different from that of the charges on the material particles, or the collector is required to be grounded. The reason why is that when the sprayed material particles are attracted and collected continuously on the collector, in the case that the collector holds the amount of charges equivalent to the amount of charges on the material particles, the polarity of the charges being different from that of the charges on the material particles, or the collector is grounded, no charges are accumulated between the spray ports of the nozzles and the collector. As a result, the material particles are smoothly attracted from the spray ports to the collecting face (lamination face), and electrostatic spray is carried out. However, as the amount of charges on the sprayed material particles increases, charges are accumulated between the spray ports and the collector. Since the polarity of the charges accumulated as described above is the same as that of the charges on the material particles to be sprayed, the charges are repulsed from one another according to Coulomb&#39;s law and suppressed from being sprayed from the spray ports, resulting in a phenomenon in which electrostatic spray cannot be carried out from the spray ports of the nozzles. This kind of phenomenon occurs similarly in a configuration in which one nozzle is disposed and in a configuration in which multiple nozzles are disposed. The inventors of the present invention conducted experiments using electrostatic spray apparatuses having a configuration in which one nozzle is disposed and a configuration in which multiple nozzles are disposed, and the inventors have found a phenomenon that electrostatic spray is stopped when the amount of charges on the polymeric material is increased by gradually raising the applied voltage. However, since the amount of charges on the polymeric material to be sprayed in the electrostatic spray apparatus having one nozzle is smaller than that in the electrostatic spray apparatus having multiple nozzles, it is easier to carry out electrostatic spray in the configuration of the electrostatic spray apparatus having one nozzle. 
     When multiple nozzles are used to raise productivity, the amount of charges increases inevitably, the charges are repulsed intensely from one another, and electrostatic spray is thus prevented from being carried out in many cases. Hence, it is necessary to reduce the amount of charges, more specifically, to lower the voltage to be applied. However, if the voltage to be applied is lowered in this way, there occurs a problem that the spray speed drops and the productivity lowers. Furthermore, the repulsion forces between the charges are required for spray from the spray ports at the tips of the nozzles. If the amount of charges is reduced by lowering the voltage to be applied, the repulsion forces required for spray become insufficient, and spray cannot be carried out in some cases. 
     Hence, when electrostatic spray is carried out in the configuration provided with multiple nozzles under the same conditions, for example, the same voltage and same spray distance, as those in which electrostatic spray can be carried out efficiently in the configuration provided with one nozzle, some or most of the multiple nozzles cannot carry out electrostatic spray in many cases. Therefore, to carry out electrostatic spray, the electrostatic spray apparatus provided with multiple nozzles is required to have a special configuration under the conditions in which electrostatic spray can be carried out efficiently in the configuration provided with one nozzle. 
     For example, in a conventional electrostatic spray apparatus, the collector is disposed in parallel with the spray ports of the multiple nozzles, the collector is made of a highly conductive material, such as aluminum foil, so that charges are not accumulated between the spray ports and the collector, and/or the area of the collector is made as large as possible. However, even in this kind of configuration, the effect is limited to some extent and not improved dramatically. Hence, the configuration does not provide a fundamental solution. 
     Furthermore, the conventional electrostatic spray apparatus is low in collection efficiency even when electrostatic spray is carried out and causes a problem that collection (lamination) at only the intended locations cannot be carried out and particles not collected are dispersed around the apparatus. The inventors have found this kind of problem of dispersion by carrying out electrostatic spray using the apparatus covered with an acrylic cover or the like. This kind of problem that the sprayed material particles cannot be collected (laminated) causes a loss of the material as a matter of course. In addition, since the large amount of fibers of several nanometers to several ten nanometers in diameter is dispersed, attention must be given to the health hazards of workers. 
     Accordingly, the present invention is intended to provide an electrostatic spray apparatus and a method of electrostatic spray capable of carrying out electrostatic spray stably using multiple spray means and capable of mass-producing products at high collection efficiency, excellent productivity, high accuracy and high safety. 
     Means for Solving Problems 
     The electrostatic spray apparatus and the method of electrostatic spray according to the present invention are configured as described below to solve various problems encountered in the above-mentioned conventional electrostatic spray apparatuses. 
     The electrostatic spray apparatus according to the present invention comprises: 
     multiple spinning units each having multiple nozzles arranged two-dimensionally to electrostatically spray a polymer solution containing a polymeric substance and formed into a liquid using a solvent and a first collector disposed so as to be opposed to the multiple nozzles via an insulating sheet, the polarity of the voltage applied to the first collector being different from that of the voltage applied to the nozzles or the first collector being grounded to the ground potential, 
     a first power supply for applying a predetermined high voltage to the nozzles, and 
     collecting sections provided with the spinning units and movably holding a collecting sheet, wherein 
     the spinning unit is provided with air stream forming means flowing in a direction substantially orthogonal to the spray direction from said nozzles to said first collector for forming an air stream toward the collecting sheet. The electrostatic spray apparatus according to the present invention configured as described above can carry out electrostatic spray stably and continuously at high collection efficiency. 
     The method of electrostatic spray according to the present invention comprises the steps of: 
     feeding a polymer solution containing a polymeric substance and formed into a liquid using a solvent to multiple nozzles arranged two-dimensionally, 
     applying a high voltage to the nozzles to electrostatically spray the polymer solution from the nozzles, 
     generating an air stream flowing in a direction substantially orthogonal to the direction of electrostatic spray in a spinning unit having the nozzles, and 
     applying the air stream to the polymeric substance electrostatically sprayed and charged so that the polymeric substance is laminated on a collecting sheet disposed substantially orthogonal to the direction of the air stream. The above-mentioned method of electrostatic spray according to the present invention can securely carry out electrostatic spray stably at high collection efficiency. 
     EFFECT OF THE INVENTION 
     The present invention can provide an electrostatic spray apparatus and a method of electrostatic spray capable of mass-producing products at high collection efficiency, excellent productivity and high accuracy using multiple nozzles arranged two-dimensionally and serving as spray means. 
     Furthermore, since an organic solvent evaporating during the electrostatic spraying process can be recovered securely, the present invention can provide an electrostatic spray apparatus and a method of electrostatic spray having high safety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing the schematic configuration of an electrostatic spray apparatus according to Embodiment 1 of the present invention; 
         FIG. 2  is a schematic sectional view showing multiple spinning units installed on the ducts of the electrostatic spray apparatus according to Embodiment 1; 
         FIG. 3  is a perspective view showing the spinning unit of the electrostatic spray apparatus according to Embodiment 1; 
         FIG. 4  is a view showing the installation face of the spinning unit of the electrostatic spray apparatus according to Embodiment 1; 
         FIG. 5  is a view showing the rear face of the spinning unit of the electrostatic spray apparatus according to Embodiment 1; 
         FIG. 6  is a sectional view showing a spray block and its surrounding area in the spinning unit of the electrostatic spray apparatus according to Embodiment 1; and 
         FIG. 7  is an exploded perspective view showing the spray block and its surrounding area in the spinning unit of the electrostatic spray apparatus according to Embodiment 1. 
     
    
    
     EXPLANATION OF NUMERALS 
     
         
         
           
               1  nozzle 
               2  conductive plate 
               3  charged particles 
               4  first collector 
               5  insulating sheet 
               6  air intake port 
               7  second collector 
               8  collecting sheet 
               9  insulating cover 
               10  housing 
               11  flange 
               12  installation hole 
               13  collecting section 
               14  material supply pipe 
               15  material holding tank 
               16  metal net 
               17  air stream 
               18  intermediate roller 
               19  recovery unit 
               20  feed roller 
               21  take-up roller 
               22  spinning unit 
               30  spray block 
               40  first power supply 
               50  second power supply 
           
         
       
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     A preferable embodiment of an electrostatic spray apparatus and a method of electrostatic spray according to the present invention will be described below in detail referring to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  is a side view showing the schematic configuration of an electrostatic spray apparatus according to Preferred Embodiment 1 of the present invention. The electrostatic spray apparatus according to Embodiment 1 sprays a solution containing a polymeric substance from nozzles by electrostatic spray to fabricate fibers thinner than 100 nanometers [nm], referred to as nanofibers, and to produce nanofiber nonwoven fabrics. 
     First, the outline of the fabrication of nanofiber nonwoven fabrics by the electrostatic spray of the electrostatic spray apparatus according to Embodiment 1 of the present invention will be described below referring to  FIG. 1 . A substrate serving as a base is required to produce nanofiber nonwoven fabrics by collecting and laminating nanofibers. A collecting sheet  8  formed of an insulating resin sheet made of polyethylene for example is used as the substrate. This collecting sheet  8  is fed from a feed roller  20  and taken up by a take-up roller  21  via multiple intermediate rollers  18 , the transfer speed of which can be controlled. The electrostatic spray apparatus according to Embodiment 1 is configured such that the transfer speed of the collecting sheet  8  is determined depending on the thickness of a nonwoven fabric, the diameter and material of nanofibers, etc. and so as to be able to meet a wide range of transfer speed from several meters to several hundred meters per minute. 
     The collecting sheet  8  serving as a substrate is transferred in the zone ranging from the feed roller  20  to the take-up roller  21  via the multiple intermediate rollers  18 . Multiple spinning units  22  are disposed along the transfer path of the collecting sheet  8 . The spinning units  22  are installed on the ducts of collecting sections  13  incorporating the collecting sheet  8  on which nanofibers are collected and laminated. As the collecting sheet  8  passes through the inside of the collecting sections  13 , the nanofibers from the spinning units  22  are laminated on the collecting sheet  8  to a thickness of several hundred nanometers to several hundred micrometers. 
     As shown in  FIG. 1 , the spinning units  22  are installed in a nearly horizontal direction on the side faces of the ducts of the collecting sections  13  extending in the vertical or horizontal direction so as to be orthogonal to the side faces, and are configured so as to be independent of one another. The number of the spinning units  22  to be installed is arbitrarily set depending on the production speed, specifications, etc. of a nanofiber fabric to be produced. The electrostatic spray apparatus according to Embodiment 1 is configured such that the number of the spinning units  22  to be installed can be arbitrarily selected from one to the maximum number of the spinning units  22  installable on the collecting sections  13 . Furthermore, since the spinning units  22  are configured so as to be able to operate independently of one another, only arbitrarily selected spinning units  22  can be operated while all the spinning units  22  are installed. For this reason, in the electrostatic spray apparatus according to Embodiment 1, specific spinning units  22  can be removed for repair or maintenance without stopping production. In addition, if an accident occurs suddenly in some spinning units  22  during production, the production can be continued by stopping only the spinning units  22  relevant to the accident, without stopping the entire apparatus. 
     Furthermore, in the electrostatic spray apparatus according to Embodiment 1, materials to be supplied to the spinning units  22  are not required to be the same. Since the spinning units  22  are each provided with a material holding tank  15  (see  FIG. 2 ), described later, for supplying a material to the nozzles, it is possible to supply different kinds of materials to the respective spinning unit  22  and to produce nonwoven fabrics formed of the different kinds of materials laminated. It is also possible to supply the same material to the respective spinning unit  22  and to hold the same material in the respective material holding tank  15 , and it is further possible to supply the same material to the respective spinning unit  22  at all times by connecting a common supply pipe to the respective spinning unit  22 , as a matter of course. 
     Next, the configuration and operation of the spinning units  22  in the electrostatic spray apparatus according to Embodiment 1 will be described below. 
       FIG. 2  is a side sectional view showing the configuration of the multiple spinning units  22  installed on the collecting section  13 .  FIGS. 3 to 5  are views showing the configuration of the spinning unit  22 ;  FIG. 3  is a perspective view showing the spinning unit  22 ,  FIG. 4  is a front view showing the front face serving as the installation face of the spinning unit  22 , and  FIG. 5  is a rear view showing the rear face of the spinning unit  22 . 
     As shown in  FIG. 2 , each spinning unit  22  is provided with the material holding tank  15  for storing a solution containing a polymeric substance, and the solution is supplied from the material holding tank  15  to multiple nozzles  1  provided inside the spinning unit  22  via a material supply pipe  14 . The multiple nozzles  1  are integrally molded with a conductive plate  2  made of a conductive material and serving as a metal block, and arranged two-dimensionally and linearly in row and column directions (in a lattice-like pattern). In the electrostatic spray apparatus according to Embodiment 1, the conductive plate  2  has a function of a solution reservoir for temporarily reserving the solution containing the polymeric substance that is used as the material, and the multiple nozzles  1  are disposed on the lower side of the solution reservoir. The multiple nozzles  1  and the conductive plate  2  described above constitute a spray block  30  that is used as a spray section. 
     A solution prepared by mixing a polymeric substance, such as polyurethane, serving as the material of nanofibers with a solvent, such as toluene, is supplied from each of the material holding tanks  15  independently disposed on the respective spinning unit  22  to the conductive plate  2  of the spray block  30  configured as described above via the material supply pipe  14 . 
     In the electrostatic spray apparatus according to Embodiment 1, although the spray block  30  having the multiple nozzles  1  and the conductive plate  2  is integrally molded from a metal block made of a material, such as aluminum or stainless steel, by press working, it may be made by cutting. The nozzle  1  has a spray port of a predetermined diameter and is formed into a conical shape so as to be pointed at the tip. In addition, the nozzle  1  is configured such that its spray port communicates with the solution reservoir of the conductive plate  2 . 
     The conductive plate  2  is configured such that a high voltage (for example, a high voltage between several kV to several ten kV) is applied thereto from a first power supply  40 , and the upper portion thereof on which the nozzles  1  are not disposed is covered with an insulating cover  9  made of an insulating resin. With this insulating cover  9 , the solution inside the solution reservoir of the conductive plate  2  is kept in a state of being prevented from being contaminated with dust, etc. Furthermore, in the spinning unit  22 , the spray block  30  is secured in a state of being insulated from the housing  10  of the spinning unit  22  using the insulating cover  9 . 
     Furthermore, on the entire lower face of the inside of the spinning unit  22 , an insulating sheet  5  and a first collector  4  formed of a thin metal plate are stacked from above in this order so as to be opposed to the nozzles  1 . In the electrostatic spray apparatus according to Embodiment 1, although the first collector  4  is grounded, it may be configured such that a voltage having a polarity different from that of the spray block  30  is applied thereto. 
       FIG. 6  is a sectional view showing the spray section of the spinning unit  22  of the electrostatic spray apparatus according to Embodiment 1, the spray section comprising the spray block  30 , the insulating cover  9  for covering the spray block  30 , etc. In addition to the spray block  30  and the insulating cover  9 , the material supply pipe  14  and the material holding tank  15  are provided in the spray section.  FIG. 7  is an exploded perspective view of the spray section shown in  FIG. 6 . 
     The conductive plate  2  integrally molded with the multiple nozzles  1  is formed into a tray shape. In the bottom face section thereof, the multiple nozzles  1  each provided with a spray port having a conical shape and pointed at the tip are arranged two-dimensionally and linearly in row and column directions; in other words, the multiple nozzles  1  are disposed at the intersection positions of the lattice-like pattern at equal intervals with one another. The spray block  30  having the conductive plate  2  and the multiple nozzles  1  is integrally molded from a metal block by press working as described above. The solution containing the polymeric substance is stored in the space of the tray section, that is, the concave space in the conductive plate  2 , and the solution is supplied from the material holding tank  15  to the tray section via the material supply pipe  14  such that the amount of the solution inside the tray section is maintained constant. The multiple nozzles  1  disposed on the bottom face of the conductive plate  2  are arranged at equal intervals in the row and column directions. In Embodiment 1, 48 nozzles  1  are formed in 4 rows and 12 columns on the conductive plate  2  measuring 250 mm (width)×500 mm (length)×180 mm (height). The size of the conductive plate  2  and the number of the nozzles  1  described above are merely examples and set appropriately depending on the specifications, production speed, etc. of a fabric to be produced, the voltage applied to the nozzles  1  and the intensity of an air stream  17  flowing in a direction substantially orthogonal to the spray direction of the nozzles  1 . 
     Generally speaking, a material being easily processable and having conductivity, such as stainless steel, is preferable as the material of the spray block  30 . If the inside diameter of the spray port of the nozzle  1  is too small, the spray port may have a risk of being blocked; conversely, if the inside diameter is too large, liquid drooping may occur. Hence, the inside diameter is preferably in the range of 0.1 to 1.0 mm. The through hole serving as the spray port of the nozzle  1  is processed accurately by drilling. Furthermore, at the time of press working, the nozzle  1  is formed to have a height (the distance between the tip serving as the spray port and the root of the nozzle  1  having a conical shape) of at least 5 mm to take advantage of the property that charges are concentrated at pointed portions. The upper portion of the spray block  30  having the conductive plate  2  and the nozzles  1  formed as described above is covered with the insulating cover  9  made of an insulating resin, and the spray block  30  and the insulating cover  9  are fastened to each other. 
     The insulating cover  9  serves as a lid for covering the concave space formed in the conductive plate  2  and having a tray shape, thereby preventing the solution containing the polymeric substance injected from the material holding tank  15  via the material supply pipe  14  into the concave space in the conductive plate  2  from being contaminated with pollution substances, etc. The solution stored in the concave space in the conductive plate  2  flows into the nozzles  1  integrally provided at the bottom section of the conductive plate  2  and is sprayed. 
     In the electrostatic spray apparatus according to Embodiment 1, since an organic solvent, such as toluene, is used as a solvent for dissolving the polymeric substance, etc. to obtain a solution, it is preferable that the portions making contact with the solution, such as the insulating cover  9 , the material supply pipe  14  and the material holding tank  15 , are made of resin materials having corrosion resistance, such as fluororesin, PE (polyethylene resin) and PP (polypropylene resin). 
     The solution containing the polymeric substance to be supplied from the material holding tank  15  to the conductive plate  2  via the material supply pipe  14  is uniformly supplied to the respective nozzle  1  provided on the bottom face of the concave space in the conductive plate  2 . The level of the solution is detected at all times using a general-purpose level detector so that all the nozzles  1  are sufficiently filled with the solution, so that a predetermined level is maintained in the concave space in the conductive plate  2 , and so that the solution is supplied when it becomes insufficient. 
     As shown in  FIG. 7 , an installation hole  12  in which the material supply pipe  14  is installed and a concave section that is fitted with the solution reservoir of the spray block  30  to hermetically seal the solution reservoir are formed in the insulating cover  9 . The material supply pipe  14  and the spray block  30  are configured so as to be easily removable from the insulating cover  9 , thereby having structures being excellent in maintainability, such as cleaning. 
     The housing constituting the outer face of the spinning unit  22  is made of a resin serving as an insulating material and having sufficient rigidity to support its own weight and is provided with a flange  11  so that the housing is installed on the duct of the collecting section  13 . The spinning unit  22  is configured such that the flange  11  of the housing  10  is screw-fastened to the duct by bolts to allow easy removal. An opening serving as an air intake port  6  and being used as means for forming an air stream is formed on the rear face side of the housing  10 , that is, on the side opposite to the flange  11 . A metal net  16  electrically insulated from the housing  10  is provided at the air intake port  6 . The metal net  16  is connected to a second power supply  50 . A voltage that is lower than that supplied from the above-mentioned first power supply  40  and is identical therewith in polarity is supplied from the second power supply  50 . The reference potential is the potential at the wall face of the duct constituting the collecting section  13 , and the reference potential of the first power supply  40  and the second power supply  50  is also the potential at the wall face of the duct. Furthermore, the reference potential is usually at the ground potential. 
     When a high voltage is applied from the first power supply  40  to the conductive plate  2 , charges are generated by an electrostatic field. Since the charges have the property of being concentrated at pointed portions, they are concentrated at the tips of the nozzles  1 . The high voltage to be applied to the conductive plate  2  is determined depending on the material and viscosity of the solution, the material and diameter of nanofibers to be fabricated, production environment (temperature and humidity), etc. and is in the range of several kV to several ten kV. Since the charges are concentrated at the tips of the nozzles  1  as described above, the surface tension portions of the liquids at the tips of the nozzles  1  become a charged state. Hence, minute charged particles  3  at the tips of the nozzles  1  are separated from the tips of the nozzles  1  and sprayed as charged liquid droplets by the strong electric field generated between the nozzles  1  and the first collector  4  disposed so as to be opposed to the nozzles  1  via the insulating sheet  5 . Since the sprayed charged liquid droplets have the same polarity, they are repulsed from one another due to the Coulomb forces exerted therebetween and dispersed uniformly and then attracted to the first collector  4 . 
     The distance from the tips of the nozzles  1  to the first collector  4  is preferably in the range of ten-odd cm to several ten cm. In Embodiment 1, the distance is set to 20 cm. The distance from the tips of the nozzles  1  to the first collector  4  is determined in consideration of the specifications of nanofibers to be fabricated, the value of the applied voltage, etc. Hence, the positions of the first collector  4 , the insulating sheet  5  and the nozzles  1  may be made changeable relatively. 
     Since the atomized liquid particles sprayed from the tips of the nozzles  1  are charged as described above and the charges are attracted to an electrode having the different polarity, the liquid particles are attracted toward the first collector  4 . While the charges are on the way toward the first collector  4 , the liquid particles are broken up repeatedly many times into minute particles by electrostatic repulsion because of the charges on the liquid particles. When the liquid particles are broken up into the minute particles in this way, the solvent used as a solvent for dissolving the material to obtain the solution evaporates, and only the material of nanofibers is left. The material is in the state of fibers referred to as nanofibers having a diameter of several nanometers to several ten nanometers. 
     Even when the solution containing the polymeric substance is transformed into the state of nanofibers as described above, that is, even when the solution is transformed into the solid state, the charges first given by the nozzles  1  remain preserved, and the nanofibers remain charged. 
     In the spinning unit  22  of the electrostatic spray apparatus according to Embodiment 1, the insulating sheet  5  is disposed between the nozzles  1  and the first collector  4  while having a distance of several mm to the first collector  4 . The distance between the first collector  4  and the insulating sheet  5  is set within a range from several mm to several ten mm in consideration of the material of the polymeric substance, the amount of charges, etc. The insulating sheet  5  is made of a dielectric material and is formed into a sheet shape or a thin plate shape. 
     The insulating sheet  5  has the property of a dielectric material; in other words, it has the property of inducing electric polarization by an external electric field, that is, the property of indicating induced polarization. Hence, when a high voltage is applied, the surface of the conductive plate  2  serving as a metal block is charged, and induced polarization occurs. In the induced polarization, the face of the insulating sheet  5  opposed to the spray ports of the nozzles  1  has the same polarity as that of the metal block. Hence, since the material that is sprayed from the spray ports of the nozzles  1  and is transformed into nanofibers has the same polarity as that of the insulating sheet  5 , the material is repulsive to the charged insulating sheet  5  by the Coulomb forces exerted such that “charges of the same polarity repel each other.” As a result, the material transformed into nanofibers is in the state of floating in the space of ten-odd cm to several ten cm between the spray ports of the nozzles  1  and the insulating sheet  5 . 
     The second power supply  50  is connected to the metal net  16  disposed on the rear face of the spinning unit  22  of the electrostatic spray apparatus according to Embodiment 1, and a voltage of several ten V to several hundred V is applied to the metal net  16 . The polarity of the voltage applied to the metal net  16  is the same as that of the voltage applied to the nozzles  1 . On the other hand, a voltage of several V is applied to a second collector  7  disposed inside the collecting section  13  so as to be opposed to the metal net  16 , and the polarity of the voltage is different from that of the metal net  16 . The voltage applied to the second collector  7  is set within a range of several V to several hundred V in consideration of the material of the polymeric substance, the amount of its charges, etc. or set to the ground potential serving as the reference potential. Therefore, the charged nanofibers in the state of floating in the space between the spray ports of the nozzles  1  and the insulating sheet  5  are repulsive to the metal net  16  having the same polarity as that of the charged nanofibers and attracted toward the second collector  7  having the different polarity by the Coulomb forces. 
     Furthermore, the electrostatic spray apparatus according to Embodiment 1 is configured such that dry air is passed through the metal net  16  of the spinning unit  22  installed on the collecting section  13  and fed toward the second collector  7  inside the duct of the collecting section  13 . The pressure inside the duct of the collecting section  13  is made lower than the ambient air pressure, that is, a negative pressure, so that the nanofibers can be collected securely. Furthermore, the humidity of the dry air fed into the spinning unit  22  is set to 40% or less so that the nanofibers floating inside the spinning unit  22  are transferred in the dry state. 
     As described above, the electrostatic spray apparatus according to Embodiment 1 is configured so as to be able to securely collect the nanofibers inside the collecting section  13  by setting the pressure inside the collecting section  13  to a negative pressure, by setting the polarity of the second collector  7  to the polarity different from that of the metal net  16  or to the ground potential, and by feeding dry air from the metal net  16  to the duct of the collecting section  13 . The electrostatic spray apparatus according to Embodiment 1 is configured such that a fan is provided for each spinning unit  22  to feed indoor dry air (humidity, 40% or less). However, the electrostatic spray apparatus may be configured such that a duct is provided to feed dry air having a desired humidity from the outside of the apparatus. 
     In Embodiment 1, the metal net  16  is used to allow dry air to be fed from the rear face of the spinning unit  22  so that the air stream  17  toward the collecting section  13  is generated securely. Other components having this kind of function may also be used. For example, it may be possible to use a conductive component having a structure obtained by drilling multiple through holes in a metal plate insulated from the housing  10 . 
     The spinning unit  22  of the electrostatic spray apparatus according to Embodiment 1 is provided with the air intake port  6  having a metal net structure as air stream forming means. However, the air intake port  6  according to the present invention may have a structure other than the metal net structure. For example, the air intake port  6  may have a structure wherein dry air, the pressure of which is adjusted, is allowed to be introduced from an air intake port formed at the end on the rear face side of the spray block  30 . In this case, the metal block in which the air intake port  6  is formed is connected to the first power supply  40 , and the voltage having the same polarity as that of the nozzles  1  is applied to the metal block. As a result, the air stream  17  from the air intake port  6  to the collecting section  13  is generated in the space between the nozzles  1  and the insulating sheet  5 , whereby the charged nanofibers are attracted from the air intake port  6  to the second collector  7 . 
     As described above, the nanofibers floating inside the spinning unit  22  are fed toward the collecting section  13  and attracted to and collected and laminated on the surface of the collecting sheet  8  that is transferred along the second collector  7 . As a result, a polymeric web formed of the nanofibers can be made. Hence, the lower the transfer speed of the collecting sheet  8 , the longer the time required for the collecting sheet  8  to pass through the spinning unit  22 , whereby the amount of the nanofibers to be collected and laminated per unit area increases. As a result, the polymeric web formed of the nanofibers and serving as a nonwoven fabric to be produced can be made thicker. In addition, the more the spinning units  22  to be installed on the collecting section  13 , the thicker the nonwoven fabric. This makes it possible to control the thickness of the nonwoven fabric of the polymeric web formed of the nanofibers by changing the number of the spinning units  22  to be installed and/or by changing the transfer speed of the collecting sheet  8 . 
     Since the nanofibers sprayed from the nozzles  1  are collected and laminated on the collecting sheet  8  that is transferred along the face of the second collector  7 , the charges on the minute charged particles  3  of the nanofibers are not accumulated in the space between the spray ports of the nozzles  1  and the insulating sheet  5  made of a dielectric material but transferred toward the second collector  7  along the air stream  17 . Hence, in the electrostatic spray apparatus according to Embodiment 1, the material is sprayed continuously from the nozzles  1 , and it is thus possible to produce a desired nonwoven fabric. 
     Hence, the problem encountered in the above-mentioned conventional apparatuses, wherein charges are accumulated between the spray ports and the collector, the accumulated charges are repulsed, and newly charged material particles cannot be sprayed from the nozzles, is solved by using the electrostatic spray apparatus according to Embodiment 1 as described above. 
     In the electrostatic spray apparatus according to Embodiment 1, dry air is fed through the air intake port  6  of the spinning unit  22  installed on the collecting section  13 , and the pressure inside the collecting section  13  is made lower than the ambient air pressure, that is, a negative pressure, to collect the nanofibers. The feeding speed of the dry air may be low, approximately several cm/sec, to the extent that the nanofibers are smoothly collected on the collecting sheet  8  and the air stream inside the spinning unit  22  is not disturbed. A high air speed of several ten cm/sec or more is not favorable since the nanofibers in the collected state are stirred by the pressure of the air and the generation of extra static electricity. 
     The structure of feeding the dry air according to Embodiment 1 is used not only to take measures against the dispersion of the nanofibers but also to make it possible to collect the organic solvent contained in the material solution. Since the material, a polymeric substance, used in Embodiment 1 is used in the state of solution as described above, an organic solvent or the like is used as a solvent, and a large amount of the organic solvent is consumed when the production is carried out. In the conventional apparatuses, collecting the organic solvent evaporating during the spraying operation is not considered. Hence, health hazards, fires, etc. due to the evaporated organic solvent as well as health hazards to human beings due to the dispersion of the nanofibers have been serious issues to be concerned about. In the electrostatic spray apparatus according to Embodiment 1, a recovery unit  19  is disposed at the top of the collecting section  13  so as to recover the dry air containing the organic solvent or the like and passing from the inside of the spinning units  22  through the collecting section  13 . This recovery unit  19  is configured such that the dry air is exhausted using a fan provided in a recovery device (not shown) installed outside the electrostatic spray apparatus. 
     The operation of collecting the nanofibers in the electrostatic spray apparatus according to Embodiment 1 will be further described below. 
     Dry air having a humidity of 40% or less is fed into each spinning unit  22  via the air intake port  6  using transfer passage forming means, such as a pipe and a duct, and the pressure inside the collecting section  13  communicating with the spinning unit  22  is set to a negative pressure using the recovery unit  19 . With this configuration, the organic solvent evaporated inside the spinning unit  22 , etc. is not exhausted to the outside of the apparatus but fed into the collecting section  13  and recovered using the recovery unit  19 . As described above, the electrostatic spray apparatus according to Embodiment 1 has a structure capable of securely recovering the evaporated organic solvent without leaking the solvent outside the apparatus. An exhaust fan is incorporated inside the recovery unit  19  according to Embodiment 1 as in the case of a general-purpose draft chamber. The speed of the air flowing inside the collecting section  13  toward the recovery unit  19  is within the range of approximately several cm/sec to 10 cm/sec, and the pressure inside the collecting section  13  is set to a weak negative pressure that is lower than the pressure inside the spinning unit  22  by approximately 0.02 kPa. 
     As described above, the electrostatic spray apparatus according to Embodiment 1 is configured such that the pressure inside the recovery unit  19  is set to a negative pressure so that the pressure inside the collecting section  13  is maintained at a negative pressure, such that the generated nanofibers are laminated on the collecting sheet  8  inside the collecting section  13 , and such that the organic solvent evaporated in the spinning unit  22 , etc. is recovered via the duct of the collecting section  13 . 
     In the electrostatic spray apparatus according to Embodiment 1, the structure in which the dry air is fed from each spinning unit  22  to the duct of the collecting section  13  is important. However, the structure should only be made such that the air stream is stably generated inside the spinning unit  22  mainly from the air intake port  6  to the collecting sheet  8 . 
     The electrostatic spray apparatus according to Embodiment 1 is provided with a controller capable of independently driving the respective spinning unit  22 , thereby being configured so as to be able to select the spinning units  22  required depending on a product to be produced and to drive only the selected spinning units  22 . 
     As described above, the electrostatic spray apparatus according to the present invention can carry out stable electrostatic spraying operation using the multiple nozzles and can mass-produce desired products. 
     Furthermore, as described above, the electrostatic spray apparatus according to the present invention makes it possible to mass-produce nonwoven fabrics and filters formed of nanofibers of several nanometers [nm] to several ten nanometers [nm] in diameter. Hence, the filters produced using the electrostatic spray apparatus according to the present invention can fulfill the requirements for the conventional filters as a matter of course and can eliminate dust and bacteria, such as anthrax, that could not be eliminated using the conventional filters. 
     In addition, the filters produced using the electrostatic spray apparatus according to the present invention has an excellent effect not only in view of “elimination” but also in view of “selection.” The fact that particles of several nanometers in diameter are captured makes it possible to not only eliminate undesired substances but also take out nanoparticles. For example, in the case of diamond abrasive grains and the like, if only the abrasive grains of several ten nanometers [nm] in diameter can be selected, the conventional grinding accuracy is improved by two or more orders of magnitude. Furthermore, the filters produced using the electrostatic spray apparatus according to the present invention can also be used for drug delivery. As described above, the electrostatic spray apparatus and the method of electrostatic spray according to the present invention have excellent features for “selection” in nano-level. 
     Moreover, the products produced using the electrostatic spray apparatus according to the present invention can also be used for regenerative medicine for generating “artificial biological membranes” and the like although the medicine is still in the stage of research at present. The present invention is thus expected to be also useful in this kind of special field. 
     INDUSTRIAL APPLICABILITY 
     The electrostatic spray apparatus according to the present invention carries out electrostatic spray stably using multiple nozzles, thereby being useful for apparatuses intended for mass production.