Patent Publication Number: US-2005136218-A1

Title: Bulky sheet and process for producing the same

Description:
TECHNICAL FIELD  
      The present invention relates generally to a bulky sheet with a large number of projections and depressions, more particularly, a bulky sheet suited for use as a cleaning sheet or a sanitary article, such as a mask or gauze, and a process for producing the same.  
     BACKGROUND ART  
      Japanese Patent Application Laid-Open No. 8-158226 discloses a bulky sheet having unevenness, which is produced by entangling non-heat-shrinkable fibers and highly heat-shrinkable fibers (crimping fibers) into a unitary nonwoven fabric and heating the nonwoven fabric to develop unevenness. However, the degree of unevenness of the bulky sheet is relatively small. Further, the layer made of the crimping fibers gets denser and stiffer on being heated only to provide a sheet with considerably reduced softness. Furthermore, the heat treatment for developing the unevenness raises the cost of production.  
      Applicant of the present invention previously proposed a method of producing a bulky sheet comprising strengthening nonwoven fabric formed by fiber entanglement with a network sheet and shrinking the network sheet by heat application to form unevenness (see Japanese Patent Applications Laid-Open No. 5-25763 and 5-192285). The sheet obtained may be characterized by its moderate unevenness, a soft feel, and comfortable to the touch, but the method needs a heating unit for shrinking the network sheet. Additionally the pattern of the unevenness (projections and depressions) is ruled by the mesh pattern of the network sheet because the nonwoven fabric follows the shrinkage of the openings of the net to create the unevenness. Therefore, the patterns of projections and depressions that could be given to the sheet are limited.  
      Japanese Patent Application Laid-Open No. 4-327255 discloses a process of producing nonwoven fabric having an uneven pattern which comprises carrying a fiber aggregate on a carrier belt having patterning projections and applying high-pressure water or hot air downward to the fiber aggregate to press the fiber aggregate onto the belt thereby forming an uneven pattern while entangling the fibers. According to this technique, although nonwoven fabric with arbitrary uneven patterns or designs can be obtained, the nonwoven fabric cannot be thicker than its own thickness and is not bulky.  
     DISCLOSURE OF THE INVENTION  
      An object of the present invention is to provide a bulky sheet having a large number of projections and depressions that feels soft and agreeable to the touch and, when used as a cleaning sheet, is capable of picking up dirt or debris from grooves of flooring or uneven surfaces of furniture, appliances, etc. that is difficult to remove with conventional cleaning sheets.  
      Another object of the present invention is to provide a preferred process of producing the bulky sheet.  
      The above objects of the present invention are accomplished by a bulky sheet comprising a fiber aggregate formed by water needling of a fiber web, said bulky sheet having a number of projections and depressions comprising said fiber aggregate, said projections and said depressions being formed both by rearrangement of the constituting fibers of said fiber aggregate by water needling of said fiber aggregate and by the multiple bending manner of said fiber aggregate along the thickness direction thereof, and said projections and said depressions retaining the shape thereof by themselves.  
      The objects are also accomplished by a preferred method for producing the bulky sheet comprising the steps of: 
          water needling a fiber web to entangle the constituting fibers of said fiber web with each other thereby forming a fiber aggregate;     transferring said fiber aggregate onto a patterning member having a number of depressions and projections or a number of perforations; and     projecting part of said fiber aggregate into said depressions or said perforations to form a number of projections corresponding to said depressions or said perforations,     said patterning member having a thickness of 5 to 25 mm, or having an air permeability of 800 to 3000 cm 3 /(cm 2 sec),     the energy Em and the energy Ef are applied to said fiber web and said fiber aggregate, respectively, in such a manner that the energy Em and the energy Ef satisfy at least one of the following formulae: 
 
200 (kJ/kg)&lt;Em+Ef&lt;1250 (kJ/kg) 
 
Em/10&lt;E &lt;2Em/3 
 
 wherein Em is an energy which is applied to said fiber web to form said fiber aggregate by said water needling, and Ef is an energy which is applied to said fiber aggregate to project part of said fiber aggregate on said patterning member. 
       

      The objects are also accomplished by a bulky sheet comprising a fiber aggregate formed by water needling of a fiber web and a network sheet, said bulky sheet having a number of projections and depressions comprising said fiber aggregate, the constituting fibers of said fiber aggregate which are entangled with each other by said water needling are further entangled with and/or thermally bonded to said network sheet thereby forming a unitary body, said projections and said depressions being formed both by rearrangement of the constituting fibers of said fiber aggregate by water needling of said fiber aggregate and by the multiple bending manner of said fiber aggregate along the thickness direction thereof, and said projections and said depressions retaining the shape thereof by themselves.  
      The objects are also accomplished by a preferred method for producing the bulky sheet comprising the steps of, 
          water needling a fiber web to entangle the constituting fibers of said fiber web with each other thereby forming a fiber aggregate;     superposing said fiber aggregate on one side or both sides of a network sheet and partially thermally bonding said constituting fibers to said network sheet thereby forming a unitary body;     transferring said fiber aggregate onto a patterning member having a number of depressions and projections or a number of perforations; and     projecting part of said fiber aggregate into said depressions or said perforations to form a number of projections corresponding to said depressions or said perforations,     said patterning member having a thickness of 5 to 25 mm, or having an air permeability of 800 to 3000 cm 3 /(cm 2 sec),     the energy Em and the energy Ef are applied to said fiber web and said fiber aggregate, respectively, in such a manner that the energy Em and the energy Ef satisfy at least one of the following formulae: 
 
200 (kJ/kg)&lt;Em+Ef&lt;1250 (kJ/kg) 
 
Em/10&lt;Ef&lt;2Em/3 
 
 wherein Em is an energy which is applied to said fiber web to form said fiber aggregate by said water needling, and Ef is an energy which is applied to said fiber aggregate to project part of said fiber aggregate on said patterning member.
       

    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an enlarged perspective view of an embodiment of the bulky sheet according to the present invention.  
       FIG. 2  is an enlarged cross section of  FIG. 1 , taken along line A-A.  
       FIG. 3  is a schematic view of an apparatus which can be used to produce the bulky sheet of  FIG. 1 .  
       FIG. 4 ( a ) is an enlarged plan view of an example of a patterning member  27 ,  FIG. 4 ( b ) is a cross section of  FIG. 4 ( a ) taken along line b-b, and  FIG. 4 ( c ) is a cross section of  FIG. 4 ( a ) taken along line c-c.  
       FIG. 5 ( a ) is a perspective view showing another patterning member and  FIG. 5 ( b ) is a cross section of the patterning member shown in  FIG. 5 ( a ). 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The bulky sheet according to the present invention will be described with particular reference to its preferred embodiment by way of the accompanying drawings. As shown in  FIGS. 1 and 2 , the bulky sheet  10  of this particular embodiment is composed of a fiber aggregate  1  which is formed by water-needling a fiber web and a network sheet  4  which lies inside the fiber aggregate  1 . The fiber aggregate  1  and the network sheet  4  form a unitary body in which the fibers constituting the fiber web  1  (hereinafter “constituent fibers”) and the network sheet  4  are entangled with each other by water needling as described later in detail.  
      As shown in  FIGS. 1 and 2 , the bulky sheet  10  has a first surface  10   a  and a second surface  10   b  and is provided with a great number of projections  2 ,  2  . . . projecting from one side to the other. Among the projections  2 ,  2  . . . are formed depressions  3 ,  3  . . . making the whole sheet uneven.  
      As depicted in  FIG. 1 , the projections  2 ,  2  . . . are approximately of a size and provided in a regular pattern, each having the shape of a mountain which is long and narrow in its plan view. The distance between the projections  2  and  2  is preferably 1 to 10 mm, still preferably 1 to 7 mm, in the sheet width direction (direction X indicated in  FIG. 1  or CD in this embodiment) and 4 to 20 mm, still preferably 4 to 15 mm, in the longitudinal direction (direction Y indicated in  FIG. 1  or MD in this embodiment). The projections  2  may be partially continuous in the width and/or lengthwise directions of the sheet. The projections  2  spaced at such intervals not only make the sheet  10  agreeable to the touch but provide a cleaning sheet excellent in removing dust in grooves of flooring or uneven surfaces and catching and holding relatively large foreign matter such as crumbs.  
      When the bulky sheet  10  is used as a cleaning sheet, it is preferred for both sides thereof to have substantially the same configuration of the projections  2 , i.e., the shape, size, and spacing, to have equal performance. It is particularly preferred that the total area of the projections  2  on the second surface  10   b  be 20 to 100%, especially 35 to 100%, of that on the first surface  10   a.  In a preferred embodiment, a projection  2  on a first surface overlies a depression  3  on a second surface. In another preferred embodiment, shape of the projection  2  is substantially the inverse of the shape the depression.  
      The projections  2  and the depressions  3  comprise the fiber aggregate  1  and are formed only by the entanglement of the constituent fibers of the fiber aggregate  1 . Unlike those projections formed by partially applying heat and pressure to thermoplastic resin fibers by, for example, embossing to cause thermal bonding, the projections  2  and the depressions  3  feel agreeable to the touch. When used as a cleaning sheet, in particular floor use, for example, the bulky sheet  10  exhibits excellent performance in catching and holding dust, hair, lint, etc.  
      The configuration of the projections  2  can be designed freely as stated above. To the contrary, the configuration of the projections of the bulky sheet described in Japanese Patent Application Laid-Open No. 5-25763 supra, being formed on thermal shrinkage of a network sheet, is governed by the mesh pattern of the network sheet with limited freedom of design.  
      Since the projections  2  and the depressions  3  of the bulky sheet  10  are formed by rearrangement and re-entanglement of the constituent fibers by the action of water needling, they retain their shape by themselves. Accordingly, the projections  2  and the depressions  3  hardly collapse under load. Because of the presence of the projections  2  and the depressions  3 , the bulky sheet  10  has a larger apparent thickness than the thickness of the fiber aggregate  1  before formation of the projections  2  and the depressions  3 . On the other hand, the projections formed only by shrinkage of a network sheet as in the 5-25763 and 5-192285 have no shape retention. Assuming that the network sheet is removed from the bulky sheet of the 5-25763 and 5-192285, the projections fail to retain their shape and would readily collapse. When used as a cleaning sheet, the bulky sheet  10  with the projections  2  and the depressions  3  having high shape retention is excellent in cleaning grooves or uneven surfaces and catching and holding bread crumbs, etc. These advantages are particularly pronounced when the bulky sheet  10  is used as attached to a cleaning tool such as a mop. The shape retention is further improved by entangling and/or thermal-bonding the constituent fibers of the fiber aggregate  1  with the network sheet  4 .  
      The shape retention of the projections  2  can be evaluated by the difference between the initial apparent thickness of the sheet under a load of 15 gf/25 cm 2  (=59 Pa) and the apparent thickness under a load of 96 gf/25 cm 2  (=376 Pa), which is a simulated load applied in a cleaning operation (hereinafter “thickness change”). The shape retention is preferably such that the shape of the projections  2  and the depressions  3  is retained even under a load and that the above-identified thickness change is 1 mm or less, particularly 0.8 mm or less.  
      The language “formed by rearrangement and re-entanglement” as used herein means that a fiber aggregate with its constituent fibers lightly entangled by water needling is again subjected to water needling on a patterning member having a large number of projections and depressions or a large number of perforations to have the fibers rearranged or re-entangled along the projections/depressions or the perforations.  
      As shown in  FIG. 2 , the projections  2  and the depressions  3  are formed by multiple bending manner of the fiber aggregate  1  along the thickness direction thereof A large number of bends formed in the fiber aggregate  1  correspond to the projections  2  and the depressions  3 . While the projections  2  and the depressions  3  are formed by rearrangement of the constituent fibers of the fiber aggregate  1 , distribution of the constituent fibers caused by migrating the fibers in the projections  2  toward the depressions due to high pressure of water jet is suppressed at a very low level during the formation of projections  2  and the depressions  3 . It is noted that progress of the distribution of the constituent fibers results in unfavorable formation of holes in the projections  2 . Such a bulky sheet  10  exhibits enhanced bulky structure in spite of its low basis weight. In contrast, the nonwoven fabric disclosed in Japanese Patent Application Laid-Open No. 4-327255 has no bends in the context of the present invention since an uneven pattern is provided in the nonwoven fabric caused by the fiber distribution due to the application of high-pressure water followed by the change in amount of the fibers in the thickness direction thereof As a result, it is not possible for this nonwoven fabric to achieve high bulkiness. Multiple bending of the fiber aggregate  1  extend over its plane direction including the machine direction (MD) and/or the cross direction (CD). Multiple bending of the fiber aggregate  1  without fiber distribution can be achieved by controlling the energy applied by water needling, as described later.  
      The degree of bending can be evaluated by the bending ratio, the measurement of which will be described later. The bulky sheet  10  has such a high bending ratio as of 2 to 15%, in particular 3 to 15%.  
      The density of the projections  2  on one side of the bulky sheet  10  is preferably 50 to 850, in average, in a unit area of 10 cm by 10 cm on any position on that side. With this projection density, the projections  2  and the depressions  3  are disposed in better balance, and the bulky sheet  10  shows more excellent performance as a cleaning sheet in catching and holding both small and relatively large foreign matter such as crumbs.  
      It is preferred for the bulky sheet  10  to have an apparent specific volume of 23 to 100 cm 3 /g, particularly 25 to 90 cm 3 /g, especially 30 to 80 cm 3 /g. With an apparent specific volume less than 23 cm 3 /g, the bulky sheet  10 , when used as a cleaning sheet, may have difficulty in sufficiently conforming to grooves or uneven surfaces of an object to be cleaned to catch up dirt or foreign matter. If the apparent specific volume exceeds 100 cm 3 /g, the fiber-to-fiber distance becomes too large, or the thickness of the fiber aggregate becomes too small, which may cause difficulty in holding foreign matter. The apparent specific volume is defined to be a quotient obtained by dividing the apparent thickness (defined below) by the basis weight of the fiber aggregate (the basis weight of the network sheet, if used, is excluded).  
      The bulky sheet  10  preferably has an apparent specific volume of 18 cm 2 /g or more, particularly 20 cm 2 /g or more, under the simulated load in a cleaning operation. The upper limit of the apparent specific volume can be 100 cm 2 /g.  
      As shown in  FIG. 2 , the bulky sheet  10  is extremely bulky, having a larger apparent thickness T (the distance between the highest peak on the first surface  10   a  and the deepest bottom on the second surface  10   b ) than the fiber aggregate 1&#39;s own thickness t.  
      The apparent thickness T of the bulky sheet  10  is preferably 1 to 5 mm, still preferably 1.4 to 4 mm, to secure ample space in the sheet to make the sheet bulky and suited for use as a cleaning sheet. The thickness t of the fiber aggregate  1 , which is decided by the basis weight of the fiber aggregate  1  and processing conditions, is preferably 0.5 to 4 mm, still preferably 1 to 3 mm. The height h of the projection (see  FIG. 2 ) is preferably 0.2 to 4 mm, still preferably 0.5 to 4 mm. The thickness t of the fiber aggregate  1  is measured by observing a cut area of the bulky sheet under an optical microscope with a load of 15 gf/25 cm 2  (=59 Pa) applied on the bulky sheet.  
      The bulky sheet has an elongation of  5 % or less, in particular 4% or less in the machine direction (MD) thereof measured under the load of 5N at the width of the specimen of 30 mm, in view of the prevention of deformation of the projections  2  and depressions  3  caused by elongating the bulky sheet  10  in the course of production or during the use.  
      The elongation in the machine direction is measured as follows. A 30 mm wide specimen is cut out of a bulky sheet with the lengthwise direction of the specimen being perpendicular to the machine direction of the sheet. The specimen is clamped in the jaws of a tensile tester to expose a 100 mm gauge length and pulled in the machine direction at a speed of 300 mm/min. The amount of elongation at which the tensile strength shows 5N/30 mm is read. Then, the obtained value is divided by the initial length of the specimen (100 mm) followed by multiplying 100 to obtain the elongation.  
      Next the fiber aggregate  1  and the network sheet  4  of the bulky sheet  10  will be described. The fiber aggregate  1  which constitutes the bulky sheet  10  is nonwoven fabric formed by water-needling a fiber web to entangle the constituent fibers with each other. Being formed only by the entanglement of the constituent fibers, the fiber aggregate  1  allows the constituent fibers a higher degree of freedom as compared with a web formed by thermal bonding or adhesion of its constituent fibers. As a result, the fiber aggregate  1  has excellent properties of catching and holding foreign matter, such as hair, lint or dust, and feels soft to the touch.  
      The constituent fiber of the fiber aggregate  1  includes, for example, those described in Applicant&#39;s previous application, U.S. Pat. No. 5,525,397, column 4, lines 3-10, the related portions of which are herein incorporated by reference. The fiber aggregate  1  preferably contains fibers having a fineness of 5 dtex or less at an amount of 50% by weight or more, in particular 3.5 dtex or less at an amount of 70% by weight or more, in view of preventing unfavorable formation of holes in the fiber aggregate  1 , and enhancing and maintaining the bulky structure of the fiber aggregate  1 . In addition, the bulky sheet containing such an mount of such fibers advantageously catch up and hold foreign matters such as hair and dust when it is used as a cleaning sheet. The basis weight of the fiber aggregate  1  and the length of the constituent fibers are selected according to the use of the bulky sheet in relation to the processability, the cost and the like. For example, for cleaning use, the fiber aggregate  1  preferably has a basis weight of 30 to 100 g/m 2 , particularly 40 to 70 g/m 2 , and the constituent fiber preferably has a fiber length of 20 to 100 mm, particularly 30 to 65 mm, in view of the prevention of unfavorable formation of holes in the bulky sheet  10  in the course of production, enhancing the sufficient bulky structure, and maintaining the bulky structure. The fiber aggregate may be combined with a surface active agent or a lubricant which can improve the surface physical properties of the fiber aggregate and can absorb dust, or combined with a lubricant which imparts gloss to the surface to be cleaned.  
      The bulky sheet  10  according to the present embodiment has a network sheet  4  inside the fiber aggregate  1  as described previously. The network sheet  4  is a resin net of square mesh as shown in  FIG. 1 . The network sheet  4  preferably has an air permeability of 0.1 to 1000 cm 3 /(cm 2  sec). Other kinds of network sheet than the resin net such as nonwoven fabric, paper and film can be used, provided that the air permeability satisfies the above range. The constituent fibers of the fiber aggregate  1  are not only entangled among themselves but entangled with and/or thermally bonded to the network sheet  4  so that the bulky sheet  10  as a whole has improved tensile strength. The network sheet  4  preferably has a thread diameter of 50 to 600 μm, particularly 100 to 400 μm, and a mesh size (distance between adjacent net threads) of 2 to 30 mm, particularly 4 to 20 mm. The network sheet  4  can be made of, for example, the materials described in U.S. Pat. No. 5,525,397, column 3, lines 39-46, the related portions of which are herein incorporated by reference. The material making up the network sheet  4  may be thermally shrinkable provided that the bulky sheet which contains such a network sheet meets the requirement of the present invention. The network sheet  4  made of a thermally shrinkable material could be heat treated in the course of producing a bulky sheet to provide a bulky sheet with an increased apparent thickness T and sharper projections. The bulky sheet  10 , however, has not been subjected to a heat shrinking of the network sheet. Alternatively, when the bulky sheet is subjected to heat treatment, the network sheet preferably has a heat shrinkage of 3% or less as measured under 140° C. for 3 minutes.  
      The bulky sheet  10  preferably has a basis weight of 30 to 110 g/m 2 , particularly 40 to 80 g/m 2 , to secure a feel of moderate thickness and improved processability. From the standpoint of strength enough for practical use, the bulky sheet  10  preferably has a breaking strength of at least 5 N, particularly 7 N or more at the width of the specimen of 30 mm. While it is sufficient for the bulky sheet  10  to have the breaking strength 5 N or more in any direction within the plane direction of the bulky sheet  10 , it is preferred that the bulky sheet  10  has the breaking strength 5 N or more in the cross direction (CD) thereof which direction is considered to exhibit the lowest strength within the plane direction of the bulky sheet  10 . The upper limit of the breaking strength can be 20 N from the standpoint of practical use.  
      The breaking strength is measured as follows. A 30 mm wide specimen is cut out of a bulky sheet with the lengthwise direction of the specimen being perpendicular to the fiber orientation direction of the sheet. The specimen is clamped in the jaws of a tensile tester to expose a 100 mm gauge length and pulled in the direction perpendicular to the fiber orientation direction at a speed of 300 mm/min. The load at which the sheet begins to break (the first peak value of a chart in continuous measurement) is read.  
      A preferred embodiment of the process of producing the bulky sheet according to the present invention is described below with particular reference to the production of the above-described bulky sheet  10 .  FIG. 3  and FIGS.  4 ( a ) to  4 ( c ) are referred to here. The process according to this embodiment comprises, in the order described, the steps of superposing an upper fiber web la and a lower fiber web  1   b  on the upper and the lower sides of a network sheet  4 , respectively, water-needling the fiber webs  1   a  and  1   b  to entangle the respective constituent fibers with each other thereby forming a fiber aggregate while further entangling the constituent fibers and the network sheet thereby forming a unitary laminate  6 , transferring the laminate  6  onto a patterning member having a large number of depressions and projections, and allowing part of the fiber aggregate to project into the depressions to form a large number of projections corresponding to the depressions.  
      In  FIG. 3  is shown an apparatus  20  which is preferably used to carry out the above process to produce the bulky sheet  10 . The apparatus  20  is divided into a superposition section  20 A, an entanglement section  20 B, and a patterning section  20 C.  
      The superposition section  20 A comprises carding machine  21 A and  21 B for making fiber webs  1   a  and  1   b,  respectively, a pair of feed rolls  22  and  22  for feeding the fiber webs  1   a  and  1   b,  respectively, and a feed roll  24  for feeding a network sheet  4 .  
      The entanglement section  20 B has a web supporting belt  25 , which is an endless belt, and first water jet nozzles  26 .  
      The patterning section  20 C has a patterning member  27 , which is an endless belt, and second water jet nozzles  28 . The patterning member  27  rotates in the direction indicated by arrows. A transfer belt  29  is provided in the rear of the patterning section  20 C. The patterning member  27  preferably has a thickness to some extent, in particular 5 to 25 mm, especially 5 to 15 mm, in view of giving sufficient bulky structure to the sheet  10 , and energy efficiency in giving the bulky structure; For the same reasons, The patterning member  27  preferably has an air permeability of 800 to 3000 cm 3 /(cm 2  sec), more preferably 800 to 2000 cm 3 /(cm 2  sec).  
      Details of the patterning member  27  are shown in FIGS.  4 ( a ) to  4 ( c ).  FIG. 4 ( a ) is an enlarged plan view of the patterning member  27 .  FIG. 4 ( b ) is a cross section of  FIG. 4 ( a ) taken along line b-b, and  FIG. 4 ( c ) is a cross section of  FIG. 4 ( a ) taken along line c-c.  
      As shown in FIGS.  4 ( a ) to  4(   c ), the patterning member  27  is made up of straight wires  27   a  and helical wires  27   b.  The straight wires  27   a  have, for example, a circular or elliptic section and are arranged in parallel to each other at a regular interval. The straight wires  27   a  are preferably arranged on the same plane. A single helical wire  27   b  helically curves around every adjacent two straight wires  27   a.  Every adjacent two helical wires  27   b  have the same winding direction and winding pitch. A single helical wire  27   b  is made of two wires whose sections are circular and have the same diameter which are closely bonded side by side to have an m-shaped cross section. Of course, a is single wire or three or more wires can be employed. The helical wire  27   b  is curved in such a manner that the line connecting the centers of the two constituent wires is always parallel to the straight wires  27   a.  Alternatively, the cross section of each wire making a single helical wire  27   b  may be either circular or elliptic. Both the wires  27   a  and  27   b  are made of metal or synthetic resins.  
      In the embodiment shown in  FIG. 4 ( c ), the helical wire  27   b,  when seen in the helix axial direction, depicts an ellipse whose major axis is parallel with the plane on which the straight wires  27   a  are arrayed. Alternatively, the helical wire  27   b  may depict a circle or a triangle when seen in the axial direction.  
      As shown in  FIG. 4 ( b ), the helical wires  27   b  have a great number of peaks  27   d  and valleys  27   e  about a base plane (the plane  27   c  formed of the straight wires  27   a ). In short, the patterning member  27  has a large number of projections and depressions. The term “projection” denotes the peak  27   d  and its vicinity, which are inclusively represented by symbol  27   f,  while the term “depression” means the portion in the middle of two adjacent peaks  27   d  which is represented by symbol  27   g.    
      The patterning member runs in direction Y indicated in FIGS.  4 ( a ). The distance a p  between adjacent straight wires  27   a  determines the length of the projections  2  of the bulky sheet  10 . The winding pitch b p  of the helical wire  27   b  governs the pitch of the projections of the bulky sheet  10 . The width b d  of the cross section of the helical wire  27   b  decides the width of the projections. The minor axis b h  of the ellipse depicted by the helix of the helical wire  27   b  (the diameter in case where the helix has a circular cross section) regulates the apparent thickness of the resulting bulky sheet. The apparent thickness of the resulting bulky sheet also depends on the energy Em and Ef which will be described later.  
      The width a d  and the pitch a p  of the straight wires  27   a  are preferably 1 to 5 mm, still preferably 1 to 3 mm, and 4 to 20 mm, still preferably 4 to 15 mm, respectively. The width b d  and the winding pitch b p  of the helical wire  27   b  are preferably 1 to 10 mm, still preferably 1 to 6 mm, and 2 to 12 mm, still preferably 2 to 7 mm, respectively. The minor axis b h  of the helix of the helical wire  27   b  is preferably 3 to 18 mm, still preferably 5 to 15 mm. With the wires  27   a  and  27   b  satisfying these conditions, the patterning member  27  is capable of giving a sufficiently uneven surface to the fiber aggregate to provide a bulky sheet.  
      In the superposition section  20 A of the apparatus  20 , the fiber webs  1   a  and  1   b  are fed from the respective carding machine  21 A and  21 B via the respective feed rolls  22 . The network sheet  4  is unwound from a roll  23  disposed between the cards  21 A and  21 B, and the fiber webs  1   a  and  1   b  are superposed on the respective sides of the network sheet  4  by the feed rolls  22  to form a superposed body  5 .  
      In the entanglement section  20 B, the superposed body  5  is transferred onto the web supporting belt  25  and needled with high-pressure water jet streams spouted from the first water jet nozzles  26 , whereby the constituent fibers of the fiber webs  1   a  and  1   b  are entangled with each other to form a fiber aggregate and, at the same time, these constituent fibers are also entangled with the network sheet  4  to provide an unitary laminate  6 . It is preferred for the fibers constituting the fiber aggregate in the laminate  6  to have a low degree of entanglement. The degree of entanglement of the fiber aggregate is preferably such that an entanglement coefficient is in the range of from 0.05 to 2 N·m/g, particularly 0.2 to 1.2 N·m/g. By controlling the degree of entanglement of the fiber aggregate in the laminate so as to have the above-specified range of an entanglement coefficient, the laminate can be given a distinct uneven pattern in the subsequent patterning in the patterning section  20 C without making unfavorable holes to provide a bulky sheet which, when used as a cleaning sheet, for example, will catch and hold fibrous foreign matter, such as hair, satisfactorily.  
      The coefficient of entanglement as referred to above, which is a measure representing the degree of fiber entanglement, is represented by the initial slope of the stress-strain curve measured in the direction perpendicular to the fiber orientation direction. The smaller the coefficient, the weaker the entanglement. The “fiber orientation direction” is a direction in which the maximum load in a tensile test is the highest, the “stress” is the quotient of a tensile load divided by the width of a specimen clamped in the tensile tester and the basis weight of the fiber aggregate  1 , and the “strain” means an amount of elongation.  
      The laminate  6  is then transferred onto the patterning member  27  of the patterning section  20 C. While carried on the patterning member  27 , the laminate  6  is partially pressed by high-pressure water jet streams spouted from the second water jet nozzles  28 , whereby the parts of the laminate  6  which are positioned on the depressions  27   g  are pressed down and made to project downward into the depressions  27   g  to form depressions  3  (see  FIGS. 1 and 2 ). On the other hand, those parts of the laminate  6  which are positioned on the projections  27   f  are not allowed to project downward thereby becoming projections  2 . As a result, the laminate  6  is made uneven as a whole, having a large number of projections  2 ,  2  . . . and a large number of depressions  3 ,  3  . . . among the projections  2 . The configuration of the projections  2  is decided by the configuration of the patterning member  27  and the entangling energy applied to the fiber aggregate by the high-pressure water jet streams in the entanglement section  20 B and the patterning section  20 C. The entangling energy is controlled by such conditions as the shape of the water jet nozzles, the alignment of the nozzles (the pitch, the number of rows and the number of nozzles), the water pressure, the line speed, and so forth.  
      The laminate  6  thus patterned is then transferred from the patterning member  27  onto the transfer belt  29 , where it is dried to give the bulky sheet  10  shown in  FIGS. 1 and 2 . The resulting bulky sheet  10  of continuous length can be wound into a roll or cut to lengths. The entangle coefficient of the fiber aggregate  1  after it is preferably substantially same as that of the fiber aggregate  1  before it is patterned, i.e., the fiber aggregate  1  after it is patterned preferably has an entangle coefficient of 0.05 to 2 N·nM/g, particularly 0.2 to 1.2 N·m/g.  
      In the above production process, it is preferred that the energy Em and the energy Ef are applied to the fiber web and the fiber aggregate, respectively, in such a manner that the energy Em and the energy Ef satisfy at least one of the following formulae: 
 
200 (kJ/kg)&lt; Em+Ef&lt; 1250(kJ/kg), in particular 400&lt; Em+Ef&lt; 1000 
 
 Em/ 10 &lt;Ef&lt; 2 Em/ 3, in particular  Em/ 4&lt; Ef&lt; 3 En/ 5 
 
 wherein Em is an energy which is applied to the fiber web to form the fiber aggregate by water needling, and Ef is an energy which is applied to the fiber aggregate to project part of the fiber aggregate transferred onto the patterning member  27 . Within the energy Em and Ef of the above range, the bulky sheet  10 , which has sufficient bulky structure and strength, and has no unfavorable holes and fiber falling-off which are likely to occur in the course of production, can be obtained. 
 
      The energy Em and Ef are calculated by the following formula,  
           Energy   ⁡     (     Em   ,   Ef     )       ⁢     (     kJ   /   kg     )       =         n   ⁢           ⁢   ρ   ⁢           ⁢     v   2     ⁢   Ca       2   ⁢   VB       ⁢         2   ⁢   P     ρ             
 
 wherein: 
          n represents number of holes provided in the nozzles per 1 m of the Width direction of the nozzles;     p represents the density of water (kg/m 3 );     it represents the flow rate of floater at the tip of the nozzle (m/sec);     C represents the flow coefficient due to energy loss (in case of water: 0.592˜0.68)     a represents the cross-sectional area of the nozzle at the tip thereof;     V represents the processing speed of the web (m/sec);     B represents the basis weight of the web (g/m 2 );     P represents the pressure of water (Pa) in the nozzle.        

      According to the above-described preferred process, a bulky sheet of the present invention can be produced at low cost because a dedicated heating unit is not required unlike the conventional techniques using latent crimping fibers or a thermally shrinkable network sheet. Since formation of projections does not rely on shrink of a shrinkable network sheet, any uneven pattern can be formed as desired irrespective of the mesh pattern of the network sheet simply by exchanging the patterning members  27 .  
      Another process for producing the bulky sheet  10  is then described only as to particulars different from the aforementioned process. Otherwise the description given above applies here. The difference from the aforementioned process resides in that entanglement of the fiber web by water needling to form a fiber aggregate having the above-specified entanglement coefficient precedes superposition on the network sheet and that the laminate of the fiber aggregate and the network sheet is made into a unitary body by partial thermal bonding by a prescribed means, which is then given an uneven pattern. The projections formed by this process also have high shape retention. When, in particular, the partial thermal bonding of the fiber aggregate formed by water-needling the fiber web and the network sheet is carried out by heat and pressure application by, for example, embossing, and the resulting unitary laminate is treated with high-pressure water jet streams on a patterning member having a large number of projections and depressions or a large number of perforations to allow part of the fiber aggregate to project into the depressions or the perforations, a large number of projections corresponding to the depressions or the perforations are formed and, at the same time, the fiber aggregate is entangled with the network sheet to form projections with improved shape retention. It is preferred that the energy Em and Ef applied by water needling is the same range as described above.  
      The fiber constituting the fiber aggregate used in this method is characterized by comprising a thermally bonding fiber. The thermally bonding fiber preferably includes core-sheath type conjugate fibers having a high-melting polymer as a core and a low-melting polymer whose melting point is lower than that of the high-melting polymer by at least 10° C. as a sheath and side-by-side type conjugate fibers having a high-melting polymer and a low-melting polymer joined together. Fiber made of a single low-melting polymer is also preferred. High-melting polymer/low-melting polymer combinations which constitute the conjugate fibers include polypropylene/polyethylene, polyethylene terephthalate/polyethylene, and high-melting polyester/low-melting polyester. The content of the thermally bonding fiber in the fiber aggregate is preferably 20 to 100% by weight, still preferably 40 to 80% by weight.  
      In the above production process, the patterning member shown in FIGS.  5 ( a ) and  5 ( b ) can be used instead of using the patterning member shown in FIGS.  4 ( a ) to  4 ( c ). The patterning member  30  shown in FIGS.  5 ( a ) and  5 ( b ) comprises a wire mesh having liquid permeability. The patterning member  30  comprises a base  31  and a number of projected portions  32  which project from the base  31 . The projected portions  32  are formed on one side of the base  31  by allowing part of the base  31  to project. The projected portions  32  are arranged in rows in constant intervals along the longitudinal direction of the patterning member  30 . Each rows are arranged in such a manner that the projected portions  32  may not adjoin each other in the width direction of the patterning member  30 . The distance between two adjacent projected portions  32  is preferably 3 to 15 mm, more preferably 4 to 10 mm, in view of proper arrangement of the projections  2  and depressions  3  which improves the catching up and holding of both fine dust and relatively large dust such as crumb when the bulky sheet is used as a cleaning sheet.  
      The projected portions  32  has a round contour in its plan view. Each projected portion  32  is the same size and shape, and has a mountain shape. The projected portions  32  are hollow. The projected portions  32  preferably have a height H (see  FIG. 5 ( b )) of 3 to 25 mm, more preferably 5 to 15 mm, in view of proper arrangement of the projections  2  and depressions  3  which enhances and maintains bulkiness of the bulky sheet. The projected portions  32  preferably has a diameter D as measured at the base  31  of 2 to 15 mm, more preferably 3 to 9 mm.  
      Further, the projected portions  32  preferably satisfy the relationship of P/9≦W≦P/3, in view of securing the distance between adjacent two projected portions  32  and enhancing sufficient bulky structure. In the relationship, W represents the two third-height width (see  FIG. 5 ( b )) of the projected portions  32  measured from the base  31 .  
      When the fiber aggregate is patterned by use of the patterning member  30 , the fiber aggregate is put on the patterning member  30  at the side where the projecting portions  32  are formed.  
      The present invention will now be illustrated in greater detail with reference to Examples. The basis weight of the resulting bulky sheets was measured. The apparent thickness (the initial thickness T and the thickness under load) and the initial apparent specific volume of the resulting bulky sheets were measured by the method previously described. The entanglement coefficient of the fiber aggregates prepared and the bending ratio of the bulky sheet were determined in accordance with the method described below. Further, the resulting bulky sheets were evaluated in terms of conformability to grooves, and property of catching large items of dust in accordance with the following methods. The results of measurement and evaluation are shown in Table 1.  
      1) Measurement of Entanglement Coefficient  
      A 100 mm long and 15 mm wide specimen was cut out of the fiber aggregate along the direction perpendicular to the fiber orientation direction. In case where the specimen contains a network sheet, it is removed to take out only a fiber aggregate. The specimen was clamped in the jaws of a tensile tester at a gauge distance of 50 mm and pulled in the direction perpendicular to the fiber orientation direction at a speed of 30 mm/min to measure the tensile load for the elongation. The value obtained by dividing the tensile load F (N) by the specimen width (m) and the basis weight W (g/m 2 ) of the fiber aggregate was taken as a stress S (N·m/g) to prepare a stress-strain curve. 
 
Stress  S ( N· m/g)=( F/ 0.015)/ W  
 
      The stress-strain curve of a fiber aggregate formed only by entanglement of the constituent fibers depicts a straight line in the initial stage. The slope of the straight line is an entanglement coefficient E (N·m/g).  
      2) Measurement of the Bending Ratio  
      A 100 mm long and 15 mm wide specimen was cut out of the bulky sheet along the direction perpendicular to the fiber orientation direction. The length of the specimen La (mm) in the longitudinal direction thereof was measured. Then, a weight weighing 500 g was put on the entire area of the specimen. After 5 minutes later, the weight was removed and the length of the specimen Lb (mm) in the longitudinal direction thereof was again measured. In case where the specimen contains a network sheet, the weight was put on the specimen after the network sheet was cut, and then the measurement of the length Lb was carried out  
      3) Conformability to Grooves  
      A catching ratio of soil present in grooves was measured in accordance with the procedure described below, and the conformability to grooves was evaluated based on the measured ratio. An acrylic resin plate having six grooves at a 3 cm interval was used as an object to be cleaned. Each groove has an inverted triangle-shaped cross section with an opening width of 3.0 mm and a depth of 1.5 mm. A mixture of seven kinds of soil for testing was spread in the grooves over a length of 20 cm each in an amount of 0.01 g per groove (total amount of soil=0.06 g). A sample sheet was attached to a mop-like cleaning tool, Quickie Wiper available from Kao Corp., and the acrylic plate was given two double strokes with the sheet. The weight of the soil caught up in the sheet was measured, and a soil catching ratio was calculated as a percentage of the weight of the caught soil to the weight of the soil spread in the grooves. The conformability of the sheet was rated A (satisfactory), B (not satisfactory). or C (substantial failure to catch up dust) based on the soil catching ratio.  
      4) Property of Catching Large Foreign Matter  
      A crumbs catching ratio was measured in accordance with the procedure described below, and the property of catching large foreign matter was evaluated based on the measured ratio. Crumbs having a particle size of 1.0 to 1.4 mm and weighing 0.3 g were scattered on a floor within an area of 1 m by 1 m. That area of the floor was wiped with a sheet attached to Quickle Wiper (Kao Corp.), and the weight of the crumbs caught up in the sheet was measured, from which the crumbs catching ratio was calculated. The property of catching large foreign matter was rated A (satisfactory), B (not satisfactory) or C (substantial failure to catch up) based on the crumbs catching ratio thus obtained.  
     EXAMPLE 1  
      Mixed fiber consisting of polyester fiber having a fineness of 0.8 denier (0.9 dtex) and a length of 38 mm and polyester fiber having a fineness of 1.45 denier (1.6 dtex) and a length of 51 mm at a mixing ratio of 70:30 by weight was carded in a usual manner to make a fiber web having a basis weight of 29 g/m 2 . The resulting fiber web was superposed on each side of a polypropylene net of square mesh (mesh size: 8 mm×8 mm; net thread diameter: 300 μm), and the superposed body was needled with water jet streams having a pressure of 1 to 5 MPa spouted from nozzles to form a unitary laminate having a fiber aggregate having an entanglement coefficient of 0.5 N·m/g. The energy Em applied was 295 kJ/kg. The laminate was transferred onto a patterning member having the structure shown in FIGS.  4 ( a ) to  4 ( c ), and water jet streams spouted from nozzles under a water pressure of 1 to 5 MPa were applied thereon to form an uneven pattern. The patterned laminate was dried in hot air to obtain a bulky sheet having projections and depressions shown in  FIGS. 1 and 2 . The energy Ef applied was 175 kJ/kg. A lubricant consisting of 90 wt % of liquid paraffin and 10 wt % of a nonionic surface active agent (polyoxyethylene alkyl ether) was applied to the bulky sheet in an amount of 5 wt % based on the sheet.  
     EXAMPLE 2  
      A unitary laminate having a fiber aggregate having an entanglement coefficient of 0.79 N·m/g was prepared in the same manner as in Example 1, except for changing the basis weight of the fiber web to 24 g/m 2  and changing the energy Em applied to 427 kJ/kg. The resulting laminate was transferred onto a patterning member having the structure shown in  FIG. 4  but different in configuration from that of Example 1, and water jet streams spouted from nozzles under a water pressure of 1 to 5 MPa were applied thereon to from an uneven pattern. The patterned laminate was dried in hot air to obtain a bulky sheet having projections and depressions shown in  FIGS. 1 and 2 , which was applied with a lubricant in the same manner as in Example 1. The energy Ef applied was 252 kJ/kg.  
     COMPARATIVE EXAMPLE 1  
      Fiber webs (0.9 dtex×38 mm/1.7 dtex×51 mm=70 wt %/30 wt %, basis weight 24 g/m 2 ) was overlaid on the upper and lower surfaces of a network sheet, respectively, which network sheet was the same as used in Example 1, and then a fiber aggregate was made from the fiber webs by water needling to obtain a laminate. The entanglement coefficient of the fiber aggregate was 0.8 N·m/g. A lubricant was applied to the resulting laminate in the same manner as in Example 1. It is noted that this laminate was not patterned.  
     COMPARATIVE EXAMPLE 2  
      A commercially available disposable cleaning sheet with a network pattern, Swiffer supplied by Procter and Gamble (Lot. 9  345  CA12 21 0402 06) was used.  
                           TABLE 1                                          Comparative           Example   Example                                     1   2   1   2                                             Basis Weight of Fiber Aggregate   58.0   48.0   48.0   52.4       (g/m 2 )       Apparent Thickness T (mm)   3.2   1.5   0.9   1.1       Apparent Specific Volume (cm 3 /g)   55.2   31.3   18.8   21.0       Apparent Thickness under Load   2.5   1.1   0.7   0.8       (mm)       Bending Ratio   8.5   3.7   1.2   1.0       Conformability to Grooves   A   A   C   C       Soil Catching Ratio (%)   70   65   10   23       Large Foreign Matter Catching   A   A   B   B       Performance       Crumbs Catching Ratio (%)   95   90   70   70                  
 
      As is apparent from Table 1, the bulky sheets according to the present invention are highly conformable to grooves and capable of satisfactorily catching up large foreign matter such as crumbs. The bulky sheets of Comparative Examples exhibit poor conformability to grooves, and have poor ability to catch up large foreign matter.  
      The bulky sheet according to the present invention is not limited in the shape, density and arrangement of the projections. While in the above-described processes, the fiber web is superposed on both sides of the network sheet, it may be superposed on only one side of the network sheet.  
      For some uses, the bulky sheet of the present invention does not always need the network sheet  4  as used in the aforementioned bulky sheet  10 .  
      The configuration of the network sheet is not particularly limited to that of the network sheet  4  of square mesh used in the embodiments described above, and any network sheet serving as a carrier with which a fiber web can be entangled to form a unitary body can be used.  
      While the fiber web is subjected to water needling at one side thereof in the above embodiment, both side of the fiber web may be subjected to water needling. In this case, the energy Em is the summation of the energy applied to each side.  
      The patterning member which can be used in the process of the present invention can have straight wires woven to form square openings instead of the straight wires  27   a  arranged in parallel. The straight wires  27   a  arranged in parallel can be displaced with a plate having a large number of square openings, around which the wires  27   b  are helically wound. Alternatively, perforated plate having a large number of circular openings can be used as the patterning member. The spacing between adjacent openings in the width and lengthwise directions of the plate corresponds to the spacing between adjacent projections of the resulting bulky sheet in the width and the lengthwise directions of the sheet.  
      Industrial Applicability  
      The bulky sheet according to the present invention has a large number of projections and feels soft and agreeable to the touch and is therefore suited for use as a cleaning sheet and a sanitary article, such as a mask and gauze. The process according to the present invention provides the bulky sheet of the invention at low cost.  
      When used as a cleaning sheet, in particular, the bulky sheet of the present invention is extremely superior in dust cleaning performance, being capable of catching and holding dirt such as dust or other small foreign matter on a surface to be cleaned in the interstices among the constituent fibers and also capable of collecting and holding relatively large foreign matter that is difficult to catch among fibers, such as crumbs, in the depressions on its surface. Further, the cleaning sheet conforms to grooves of flooring or uneven surfaces of furniture, appliances, etc. to remove dirt or debris in the uneven surfaces. Having a moderate thickness, the cleaning sheet is convenient to use in wiping operation.