Patent Publication Number: US-2005123720-A1

Title: Laminate sheet having reinforcement film and method of manufacturing the same

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a laminate sheet which has a base layer and a reticular reinforcement layer laminated to the base layer, and a method of manufacturing the same.  
      2. Description of the Related Art  
      Conventionally, a known wrapping material for use in wrapping an article includes a reinforcement laminated to one or both sides of a base made of paper, film or the like for supplementing the strength of the base. JP-2002-144451-A, for example, discloses a wrapping material which comprises a base made of paper, film or the like, and a reinforcement or a reticular reinforcement made of a thermoplastic resin, laminated to one side of the base by thermo-compression bonding. The reticular reinforcement comprises a pair of uniaxially stretched films, each of which is produced by stretching a film formed with a plurality of slits parallel with one another in a direction parallel with the slits, and expanding the film in a direction perpendicular to the stretching direction. The two films are superimposed on top of another with their stretched directions perpendicular to each other, and are fused together. Such a reticular reinforcement layer, used for reinforcement, provides a laminate sheet which excels in balance of strengths in the longitudinal and transverse directions.  
      However, since the foregoing conventional laminate sheet has the reticular reinforcement layer laminated to the base through thermo-compression bonding, the laminate sheet can suffer from curling caused by a difference in thermal contraction ratio of the two materials unless the base and reinforcement are made of appropriately selected materials. The curling is not favorable because the resulting laminate sheet becomes more difficult to handle. Also, since the reticular reinforcement layer comprises two uniaxially stretched films fused to each other, the uniaxially stretched film can be adhered to the base over the interface therebetween, whereas the two uniaxially stretched films are partially adhered to each other on the interface therebetween, possibly resulting in a lower adhesion strength depending on the open area ratio of the net.  
      The laminate sheet can be improved to prevent the curling by laminating the reticular reinforcement layers on both sides of the base. However, since the reticular reinforcement layer per se is in a two-layer structure, the fabrication of the improved laminate sheet results in a large increase in the amount of material used. This leads to an increased cost of the laminate sheet. Also, the increased number of laminated layers causes the resulting laminate sheet to be correspondingly thicker and harder, thereby making the laminate sheet itself unsuitable for applications which require softness. Moreover, even though the reticular reinforcement layers are laminated to both sides of the base, the problem still remains unsolved with respect to the strength with which the uniaxially stretched films are adhered to each other in the reticular reinforcement layer.  
      The foregoing problem is not limited to the laminate sheet used for a wrapping material, but similarly arises when the laminate sheet is used for house wrapping, agricultural materials, filters, automobile interior materials, industrial materials, and the like.  
     SUMMARY OF THE INVENTION  
      It is an object of the present invention to provide a laminate sheet which prevents the curling without increasing the amount of material used and improves the adhesion strength between respective layers, and a method of manufacturing the laminate sheet.  
      To achieve the above object, a laminate sheet includes a base layer, a first uniaxially stretched reticular film made of a thermoplastic resin and laminated to top surface of the base layer, and a second uniaxially stretched reticular film made of a thermoplastic material of the same type as the first uniaxially stretched reticular film, and laminated to the back surface of the base layer. In this structure, the first uniaxially stretched reticular film and the second uniaxially stretched reticular film are laminated to the base layer through thermo-compression bonding with their stretching directions oriented perpendicular to each other.  
      By thus laminating uniaxially stretched reticular films made of a thermoplastic resin of the same type on both sides of a base layer, thermal contraction is equal on the surface side and on the back surface of the base layer during the lamination. From this fact, the resulting laminate sheet excels in balance of strengths in two directions perpendicular to each other, and is free of curling. Moreover, the amount of material used will not be increased as compared with a conventional laminate sheet which has a base layer, and a reticular reinforcement layer composed of two laminated uniaxially stretched reticular films, laminated to the base layer. In addition, since the uniaxially stretched reticular films are not laminated together, there is no interface which suffers from a low adhesion strength due to partial adhesion.  
      The uniaxially stretched reticular films for use in the present invention is preferably a film formed with a plurality of slits parallel with one another, stretched in the direction in which the slits extend, and expanded in a direction perpendicular to the direction in which the film is stretched.  
      A method of manufacturing a laminate sheet according to the present invention includes the steps of providing a base layer, and a first and a second uniaxially stretched reticular film each made of a thermoplastic resin of the same type, and superposing the first uniaxially stretched reticular film, the base layer, and the second uniaxially stretched reticular film on one another in this order, while orienting the first uniaxially stretched reticular film and the second uniaxially stretched reticular film with their stretching directions perpendicular to each other, and bonding the first uniaxially stretched reticular film, the base layer, and the second uniaxially stretched reticular film together through thermo-compression bonding. This method requires only a single thermo-compression bonding process to complete the laminate sheet of the present invention described above.  
      Accordingly, the present invention provides a laminate sheet which is manufactured without increasing the amount of materials used, eliminates the curling, and has an increased adhesion strength of the uniaxially stretched reticular films, as compared with the conventional laminate sheet which employs a reticular reinforcement layer which is comprised of uniaxially stretched reticular films laminated to each other. Particularly, the method of manufacturing a laminate sheet according to the present invention can extremely simply manufacture the laminate sheet only through a single thermo-compression bonding process.  
      It should be noted that in the present invention, the “longitudinal direction” refers to a machine direction, i.e., a direction in which a laminate sheet, a film or the like is fed when it is manufactured, and the “transverse direction” refers to the direction perpendicular to the longitudinal direction, i.e., a transverse direction of the laminate sheet, film or the like.  
      The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view schematically illustrating a laminate sheet according to one embodiment of the present invention;  
       FIG. 2  is a diagram for describing an exemplary method of manufacturing the laminate sheet illustrated in  FIG. 1 ;  
       FIG. 3A  is a partial perspective view of a uniaxially stretched split-fiber film suitable for use as a reinforcement layer illustrated in  FIG. 1 ;  
       FIG. 3B  is an enlarged perspective view of an end face of the uniaxially stretched split-fiber film illustrated in  FIG. 3A ;  
       FIG. 4  is a partial perspective view of an original film for use in manufacturing the uniaxially stretched split-fiber film illustrated in  FIG. 3  when it is formed with slits;  
       FIG. 5  is a partial plan view of the laminate sheet which employs the uniaxially stretched split-fiber film illustrated in  FIG. 3  for a reinforcement layer;  
       FIG. 6A  is a partial perspective view of a uniaxially stretched slit film suitable for use as the reinforcement layer illustrated in  FIG. 1 ;  
       FIG. 6B  is an enlarged perspective view of an end face of the uniaxially stretched slit film illustrated in  FIG. 6A ;  
       FIG. 7  is a schematic diagram illustrating the configuration of an exemplary laminate sheet manufacturing apparatus according to the present invention; and  
       FIG. 8  is a schematic diagram illustrating a process for manufacturing a uniaxially stretched split-fiber film. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 1 , there is illustrated a schematic cross-sectional view of laminate sheet  1  according to one embodiment of the present invention which comprises base layer  2 , first reinforcement layer  3  laminated to the top or one surface of base layer  2 , and second reinforcement layer  4  laminated to the back or the other surface of base layer  2 . As is apparent from  FIG. 1 , base layer  2  is sandwiched by first reinforcement layer  3  and second reinforcement layer  4 . First reinforcement layer  3  and second reinforcement layer  4  are both made of the same type of thermoplastic resin, and are laminated to base layer  2  through thermo-compression bonding.  
      Base layer  2  can be made of any material, including, for example, paper, non-woven fabric, foamed material, metal foil, resin film, wood film, and the like. Which material to select for base layer  2  depends on the characteristics required for base layer  2  in accordance with a particular application of laminate sheet  1 . Base layer material  2  can be made of an air-permeable material if air permeability is required, or can be made of a metal foil, a resin film or the like if an air barrier property is required.  
      First reinforcement layer  3  and second reinforcement layer  4 , which provide a required tensile strength to laminate sheet  1 , are made of a uniaxially stretched reticular film stretched in one direction, and are laminated to base layer  2  with their directions of stretching oriented perpendicular to each other. Since the uniaxially stretched reticular film is stretched in one direction and is in a reticular structure, the uniaxially stretched reticular film has a high tensile strength in the direction of stretching with a small amount of material. Thus, by laminating the two uniaxially stretched reticular films (first reinforcement layer  3  and second reinforcement layer  4 ) on base layer  2  with their directions of stretching oriented perpendicular to each other, resulting laminate sheet  1  excels in balance of strengths in the directions perpendicular to each other (for example, the longitudinal direction and transverse direction).  
      Also, first reinforcement layer  3  and second reinforcement layer  4  are made of the same type of thermoplastic resin, so that even if they are laminated to both sides of base layer  2  through thermo-compression bonding, the thermal contraction ratio on the top surface of base layer  2  is substantially equal to that on the back of the same. Consequently, resulting laminate sheet  1  is free of curling, and therefore will not compromise its own handling, including mechanical workability when it is processed, field workability when it is actually used, and the like.  
      The uniaxially stretched reticular film, which makes up first reinforcement layer  3  and second reinforcement layer  4 , may be made of a single film which forms part of the respective reticular reinforcement layers used in the laminate sheet described also in “DESCRIPTION OF RELATED ART.” As such, even if these reinforcement layers  3 ,  4  are laminated to both sides of base layer  2 , the amount of material used will not be increased, and laminate sheet  1  will not be increased in overall thickness, as compared with the conventional laminate sheet which has a reticular reinforcement sheet laminated to one side of a base layer.  
      Moreover, in laminate sheet  1  of this embodiment, first reinforcement layer  3  and second reinforcement layer  4 , each of which has a net structure, are not laminated to each other, in the conventional laminate sheet, but instead are laminated to base layer  2  which is not in net structure. As a result, reinforcements  3 ,  4  are entirely bonded to base layer  2 . The whole surfaces of reinforcements  3 ,  4  which face base layers  3 ,  4 , that is interfaces with base layers  3 ,  4 , are bonded to base layer  2 , thereby eliminating the interface on which the adhesion strength is poor due to partial adhesion, as found in the conventional laminate sheet. In conclusion, each reinforcement layer  3 ,  4  will not suffer from a lower adhesion strength. The “net structure” used herein refers to a structure which has at least a plurality of first components that are arranged along a certain direction in parallel with and spaced apart from one another, and a plurality of second components that are arranged along a direction, in which the second components intersect with the first components, in parallel with and spaced apart from one another, and is formed with apertures surrounded by the first components and second components. Therefore, while paper, for example, may be microscopically regarded as a net structure because it has apertures, the paper is not included in the “net structure” herein defined because of its random fiber directions.  
      As described above, laminate sheet  1  is highly advantageous over the conventional laminate sheet, and are suitable for use in a variety of applications, including wrapping materials, house wrapping, agricultural materials, filters, automobile interior materials, industrial materials, and the like.  
      Now, a general procedure of manufacturing laminate sheet  1  will be described with reference to  FIG. 2 .  
      As illustrated in  FIG. 2 , elongated base layer  2 , first reinforcement layer  3 , and second reinforcement layer  4  have been prepared in advance as wound in rolls. When first reinforcement layer material  3  is made of a uniaxially stretched reticular film stretched in a longitudinal direction, second reinforcement layer  4  is made of a uniaxially stretched reticular film stretched in a transverse direction. Alternatively, when first reinforcement layer  3  is made of a uniaxially stretched reticular film stretched in the transverse direction, second reinforcement layer  4  is made of a uniaxially stretched reticular film stretched in the longitudinal direction.  
      Then, these base layer  2 , first reinforcement layer  3 , and second reinforcement layer  4  are fed out from the rolls such that they are superimposed on one another in the order of first reinforcement layer  3 , base layer  2 , and second reinforcement layer  4 , i.e., base layer  2  is sandwiched between first reinforcement layer  3  and second reinforcement layer  4 , and are supplied between a pair of thermo-compression bonding rollers  10 . Thermo-compression bonding rollers  10  have been heated to a temperature equal to or higher than the melting point (or softening point) of the thermoplastic resin which makes up first reinforcement layer  3  and second reinforcement layer  4 . As base layer  2 , first reinforcement layer  3 , and second reinforcement layer  4 , which are superimposed on one another in the aforementioned order, are passed between thermo-compression bonding rollers  10 , first reinforcement layer  3  and second reinforcement layer  4  are thermally and compressively bonded onto both surfaces of base layer  2  to produce laminate sheet  1 .  
      During the thermo-compression bonding of base layer  2  with respective reinforcement layers  3 ,  4 , the adhesion strength is enhanced if their bonding surfaces have undergone such preprocessing as corona processing, ozone processing, or a frame processing. This method is effective particularly when layers made of different materials are laminated. It is believed that such preprocessing provides a polar group on the surfaces, which are to be bonded together, to enhance the adhesive force.  
      Resulting laminate sheet  1  is cut in predetermined dimensions, or wound up in rolls depending on its application.  
      As described above, laminate sheet  1  can be manufactured through a single thermo-compression bonding process by simultaneously supplying base layer  2 , first reinforcement layer  3 , and second reinforcement layer  4  between thermo-compression bonding rollers  10 . On the other hand, a conventional laminate sheet which uses a reticular reinforcement layer, involves the thermo-compression bonding process twice, one for making a reticular reinforcement layer from two uniaxially stretched reticular films, and one for adhering the reticular reinforcement layer to the base layer. In this embodiment, in turn, laminate sheet  1  can be more simply produced through a single thermo-compression bonding process at a lower cost because the thermo-compression bonding process is required a less number of times than the conventional laminate sheet.  
      Having described laminate sheet  1  above, detailed description will now be given on the uniaxially stretched reticular film suitable for use as respective reinforcement layer materials  3 ,  4  of laminate sheet  1 .  
      The uniaxially stretched reticular film is produced by forming a film made of a thermoplastic resin with a multiplicity of slits extending in parallel with one another, stretching the film in a direction in which the slits extend, and then expanding the film in a direction perpendicular to the direction in which the film has been stretched, or by stretching a film made of a thermoplastic resin in one direction, forming a multiplicity of slits in parallel with the direction in which the film has been stretched, and expanding the resulting film in a direction perpendicular to the stretching direction. Uniaxially stretched reticular films are roughly classified into a uniaxially stretched split-fiber film and a uniaxially stretched slit film. The following description will focus on the uniaxially stretched split-fiber film and uniaxially stretched slit film.  
      As illustrated in  FIG. 3B , uniaxially stretched split-fiber film  21  has a laminate structure composed of layer  21   a  made of a first thermoplastic resin, and layers  21   b  laminated to both sides of layer  21   a . Layers  21   b  are made of a second thermoplastic resin which has a lower melting point than the first thermoplastic resin. As illustrated in  FIG. 3A , uniaxially stretched split-fiber film  21  comprises a plurality of trunk fibers  23  extending in parallel with one another, and branch fibers  24  intersecting with truck fibers  23  to connect adjacent trunk fibers  23  to each other. Branch fibers  24  are thinner than trunk fibers  23 , so that the mechanical strength of uniaxially stretched split-fiber film  21  is mainly given by trunk fibers  23 .  
      Each layer  21   b  made of the second thermoplastic resin has a thickness smaller by 50% or less, and preferably 40% or less, than the overall thickness of uniaxially stretched split-fiber film  21 . For satisfying a variety of properties of uniaxially stretched split-fiber film  21  such as an adhesion strength and the like during the thermo-compression bonding, the thickness of layer  21   b  made of the second thermoplastic resin may be 5 μm or more, but is preferably selected from a range of 10 to 100 μm.  
      A method of manufacturing uniaxially stretched split-fiber film  21  may be carried out, for example, in the following manner.  
      First, original film  20  in a three-layer structure having layers  21   b , each made of the second thermoplastic resin, laminated to both sides of layer  21   a  made of the first thermoplastic resin is manufactured by extrusion molding such as a multilayer inflation method, a multilayer T-die method, or the like. Then, this original film  20  is stretched in a longitudinal direction (direction indicated by arrow L in  FIG. 4 ) and is formed with a multiplicity of cross-stitched parallel slits  20   a  in the longitudinal direction by a splitting process using a splitter or by a slitting process using hot blades. Then, original film  4  formed with slits  20   a  is widened in a direction perpendicular to the direction in which slits  20   a  extend. In this way, uniaxially stretched split-fiber film  21  is produced, as illustrated in  FIG. 3 , wherein trunk fibers  23  are aligned substantially in the longitudinal direction.  
      A stretching magnification (orientation magnification) of film  21  is preferably 1.1 to 15 times, and more preferably 3 to 10 times. When the stretching magnification is less than 1.1 times, the mechanical strength of the film would be insufficient. On the other hand, when the stretching magnification exceeds 15 times, it becomes difficult to stretch the film by an ordinary method, and such a problem arises in which an expensive machine for the stretching operation is required. The stretching is preferably performed at multiple stages in order to prevent an unevenly stretched film.  
      Uniaxially stretched split-fiber films  12  fabricated in the foregoing manner are bonded on both sides of base layer  2  (see  FIG. 1 ) through thermo-compression bonding such that trunk fibers  23  of one film are perpendicular to trunk fibers  23  of the other film. The thermo-compression bonding should be performed at a temperature equal to or lower than the melting point of the first thermoplastic resin and equal to or higher than the melting point of the second thermoplastic resin so as not to lose the stretching effect of layer  21   a  made of the first thermoplastic resin.  FIG. 5  illustrates a plan view of laminate sheet  1  using uniaxially stretched split-fiber film  21 .  FIG. 5  illustrates laminate sheet  1  through base layer  22  in order to show the reticular structure of two uniaxially stretched split-fiber films  21 . Actually, base layer  22  is sandwiched between two uniaxially stretched split-fiber films  21 .  
      When uniaxially stretched split-fiber films  21  illustrated in  FIG. 3A  is used for reinforcement layers  3 ,  4  illustrated in  FIG. 1 , one of uniaxially stretched split-fiber films  21  can be supplied as it is on base layer  22 , whereas the other one must be cut into tile-shaped pieces, each of which has a length equal to the width of laminate sheet  1  to be manufactured, such that the tile-shaped film pieces are supplied intermittently in a direction perpendicular to the direction in which base layer  22  is supplied, in order that trunk fibers  23  of one uniaxially stretched split-fiber films  21  are made perpendicular to trunk fibers  23  of the other uniaxially stretched split-fiber film  21 . For this reason, laminate sheet  1  illustrated in  FIG. 5  includes joints of tile-shaped uniaxially stretched split-fiber film  21  at regular intervals.  
      Next described will be the uniaxially stretched slit film.  
       FIG. 6A  illustrates uniaxially stretched slit film  31 . Uniaxially stretched slit film  31  can be made from an original film which has the same structure as that used for manufacturing uniaxially stretched split-fiber film  21  illustrated in  FIG. 3A . Specifically, as illustrated in  FIG. 6B , uniaxially stretched slit film  31  comprises layer  31   a  made of a first thermoplastic resin, and layers  31   b  laminated to both sides of layer  31   a . Each layer  31   b  is made of a second thermoplastic resin which has a melting point lower than the first thermoplastic resin. Then, the original film is formed with cross-stitched slits in the transverse direction (a direction indicated by arrow T in  FIG. 6A ) in a slitting process, and the resulting film is stretched in the transverse direction such that cross-stitched slits are opened in the longitudinal direction. This results in reticular uniaxially stretched slit film  31 . Though joints are formed as is the case with uniaxially stretched split-fiber film  21 , uniaxially stretched slit films  31  illustrated in  FIG. 6A  may be placed to sandwich a base layer such that their stretching directions are perpendicular to each other to produce a laminate sheet.  
      When a laminate sheet formed with joints is not desirable, uniaxially stretched split-fiber film  21  illustrated in  FIG. 3A  may be combined with uniaxially stretched slit film  31  illustrated in  FIG. 6A  to form a structure in which a base layer is sandwiched by uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31 . In this way, uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  are continuously supplied, while they are placed on the top and back surfaces of the base layer, respectively, and the three components are bonded together through thermo-compression bonding to provide joint-less laminate sheet  1  ( FIG. 1 ).  
      Next, an exemplary method of manufacturing a laminate sheet will be described with reference to  FIG. 7  when uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  are used for the reinforcement layers, respectively, on one and the other sides of the base layer.  
       FIG. 7  illustrates an exemplary laminate sheet manufacturing apparatus according to the present invention. In the example illustrated in  FIG. 7 , base layer  2  and uniaxially stretched split-fiber film  21  have been in advance provided in rolls, and they are supplied to a manufacturing line for manufacturing uniaxially stretched slit film  31  to manufacture laminate sheet  1 .  
      Multilayer film manufacturing machine  42  has multilayer extruder  42   a  and die  42   b  for manufacturing original film  31 ′ in three-layer structure, as illustrated in  FIG. 6B . Die  42   b  used herein may be either for the multilayer inflation method or for the multilayer T-die method. Original film  31 ′ manufactured by multilayer film manufacturing machine  42  is formed with transverse slits by force-cutting slit forming roller  43 , while it is conveyed by rollers, and is further stretched in the transverse direction by transverse stretcher  44  to provide uniaxially stretched slit film  31 . Uniaxially stretched slit film  31  thus manufactured is supplied to thermo-compression bonding rollers  41 .  
      Uniaxially stretched split-fiber film  21 , after formed with longitudinal slits, is wound in a roll form before it is widened in the transverse direction. Uniaxially stretched split-fiber film  21 , before being widened, is fed out from the roll, and widened by spreader  45  in the transverse direction. Widened uniaxially stretched split-fiber film  21  is superimposed on uniaxially stretched slit film  31 , and they are supplied together to thermo-compression bonding rollers  41 .  
      When uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  are supplied to thermo-compression bonding roller  41 , base layer  2  is supplied between uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31 . Therefore, uniaxially stretched split-fiber film  21 , base layer  2 , and uniaxially stretched slit film  31  are superimposed on one another in this order before they are supplied between thermo-compression bonding rollers  41 . These components are bonded together by thermo-compression bonding rollers  41  through thermo-compression bonding. As a result, uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  are entirely bonded to base layer  2 , thereby fabricating laminate sheet  1 . Resulting laminate sheet  1  can be wound in a roll form.  
      In the exemplary method shown in  FIG. 7 , uniaxially stretched split-fiber film  21  has already been formed with slits before it is provided. Alternatively, uniaxially stretched split-fiber film  21  may be started with the manufacturing of a film, in a manner similar to uniaxially stretched slit film  31 . In this event, uniaxially stretched split-fiber film  21  wound in a roll form, shown in  FIG. 7 , is replaced with a process illustrated in  FIG. 8 . Specifically, the aforementioned original film  20  in three-layer structure is formed by multilayer film manufacturing machine  46  having multilayer extruder  46   a  and die  46   b . Next, original film  20  is stretched in the longitudinal direction by longitudinal stretcher  47 . Stretched original film  20  is formed with longitudinal slits by splitter  48 , and subsequently is widened in the transverse direction by spreader  45  shown in  FIG. 7 , and supplied to thermo-compression bonding rollers  41 .  
      When uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  are used for first reinforcement layer  3  and second reinforcement layer  4 , respectively, as mentioned above, the manufacturing of uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  can be directly followed by in-line manufacturing of laminate sheet  1 . In other words, laminate sheet  1  can be manufactured through a process continuous to the manufacturing of respective reinforcement layers  3 ,  4 . This can further simplify the manufacturing process of laminate sheet  1 , and as a result, provide laminate sheet  1  at a lower cost.  
      The resin used for making uniaxially stretched split-fiber film  21  and uniaxially stretched slit film  31  may be, for example, one of substances including polyolefin such as polyethylene and polypropylene, copolymer of these substances, polyester such as polyethyleneterephthalate and polybutyleneterephthalate, copolymer of these substances, polyamide such as nylon 6 and nylon 66, copolymer of these substances, polyvinyl chloride, methacrylic acid or polymer and copolymer of the derivative of methacrylic acid, polystyrene, polysulfone, polytetrachloroethylenepolycarbonate, and polyurethane. Among others, polyolefin, copolymer thereof, polyester, and copolymer thereof are preferred because they are easily subjected to the split-fiber processing.  
      Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made without departing from the spirit or scope of the appended claims.