Abstract:
A process for manufacturing a composite sheet by joining a second web made of thermoplastic synthetic fiber and capable of inelastic extension to at least one surface of a first web capable of elastic stretch and contraction in an intermittent manner. The process involves a step of extending the first web and a step of allowing the first web to retract, and a step of joining a second web to the second web while in a retracted state are included.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a process for manufacturing an elastically stretchable and contractible composite sheet comprising a web capable of elastic stretch and contraction and a fibrous web capable of inelastic extension. 
     Japanese Patent Publication No. 1996-504693A discloses a multi-layered elastic panel, as one example of this type of composite sheet, and a process of manufacturing the same. This manufacturing process of a multi-layered elastic panel involves arranging a rubber elastic layer and an inelastic fibrous layer, one over another, joining them intermittently, extending the combination up to the vicinity of a breaking extension limit of the inelastic fibrous layer and finally relaxing the combination from tension. 
     In the composite sheet (elastic panel) obtained via the above-specified known manufacturing process, the rubber elastic layer after removal of the tension is unable to return to its original dimension. The resulting difference in dimension sometimes produces a permanent strain in the composite sheet. Such a permanent strain is a first factor that makes the composite sheet larger in dimension along a direction of extension than before it is extended. Also, the inelastic fibrous layer when extended undergoes plastic deformation so that its dimension after extension is made larger than before extension. When the rubber layer is released from the tension, this dimensional difference causes the inelastic fibrous layer to increase its apparent bulk. The increased bulk then becomes a second factor that makes the composite sheet larger in dimension than before extension by restraining the rubber elastic layer from recovering, i.e., by restricting elastic contraction thereof. By these first and second factors, the composite sheet when again extended to the vicinity of a breaking extension limit of the inelastic fibrous layer exhibits a lower percentage extension than when initially extended to the vicinity of the breaking extension limit of the inelastic fibrous layer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process for manufacturing a composite sheet which can reduce the influence of the above-described first factor encountered in the known manufacturing process and thus widen the range that permits elastic stretch and contraction of the sheet. 
     To achieve this object, the present invention is directed to a process for manufacturing a composite sheet capable of elastic stretch and contract in one direction, which includes the steps of continuously feeding a first web capable of elastic stretch and contraction in the one direction and having a top surface and a bottom surface, continuously feeding a second web capable of inelastic extension and composed of thermoplastic synthetic fibers on at least one surface of the first web and joining the first and second webs in an intermittent manner along the one direction. 
     In the above-described manufacturing process of the composite sheet, the present involves the steps of: 
     (a) feeding the first web continuously in the one direction and extending the first web in the one direction within the range that permits elastic stretch and contraction of the first web; 
     (b) allowing the extended first web to retract by the action of an elastic contraction force of the web; and 
     (c) superimposing the second web on at least one surface of the first web after retraction and joining the first and second webs in an intermittent manner along the one direction. 
     In one embodiment of this invention, subsequent to the step (c), the following steps are further included: 
     (d) a secondary extension step wherein the joined first and second webs are extended in the one direction within the range that permits elastic stretch and contraction of the first web; and 
     (e) a secondary contraction step wherein the extended first and second webs are allowed to retract by the action of an elastic contraction force of the first web. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a composite sheet; and 
     FIG. 2 is a view showing an exemplary process for manufacturing the composite sheet. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The process for manufacturing an elastically stretchable and contractible composite sheet in accordance with the present invention is below described in detail with reference to the attached drawings. 
     FIG. 1 is a perspective view of an elastically stretchable and contractible composite sheet  1  manufactured by the practice of the method according to the present invention. 
     The composite sheet  1  is suitable for use as a liquid permeable or impermeable facing material of a disposable wearing article such as a disposable diaper, a sanitary napkin, a disposable medical gown or the like, and has an upper layer  2  and a lower layer  3  united together at bond areas  4  by fusion. The composite sheet  1  is elastically stretchable and contractible at least in the Y—Y direction, out of mutually-perpendicular double-headed arrows X—X and Y—Y, as shown by chain lines. 
     The upper layer  2  of the composite sheet  1  is capable of inelastic extension at least in the Y—Y direction, out of the X—X and Y—Y directions. Such the upper layer  2  comprises a mass of thermoplastic synthetic fibers extending continuously between bond areas  4  and  4 , preferably long fibers, more preferably a mass of continuous fibers  6 . In the preferred upper layer  2 , the fibers  6  are fused to each other at bond areas  4  but are individualized between bond areas  4  such that they are neither fused nor mechanically entangled tightly with each other. The length of a portion of the individual fiber  6  that extends between adjacent bond areas  4 , e.g., the length of a portion of the fiber  6   a  that extends between bond areas  4   a  and  4   a  is larger than a linear distance between the bond areas  4   a  and  4   a . That is, the fiber  6  extends over an upper surface of the lower layer  3  while describing the shown irregular curves. When the composite sheet is extended in the Y—Y direction, the fibers  6  change their orientations between the bond areas  4  and  4  to extend linearly along the Y—Y direction. As the composite sheet  1  retracts, the fibers  6  describe curves again. 
     The lower layer  3  of the composite sheet  1  is elastically stretchable and contractible in the Y—Y direction, preferably in both the X—X and Y—Y directions. The lower layer  3  comprises a mass of short, long or continuous fibers made of elastic materials such as thermoplastic elastomers, or alternatively, comprises a film or the like made of such elastic materials. In the case of fibers, the lower layer takes the form of a non-woven or woven fabric, preferably integrated via mechanical entanglement or fusion bond of fibers. The lower layer  3  operates such that it extends elastically as the composite sheet  1  is extended in the Y—Y direction by an external force and causes the composite sheet  1  to retract as the composite sheet  1  is freed from the force. 
     FIG. 2 is a diagram illustrating a manufacturing process of the composite sheet  1  shown in FIG.  1 . On the left side of the drawing, a first endless belt  31  and a second endless belt  32  which both run toward the right are juxtaposed to interpose a first extension step  71  and a first contraction step  72  therebetween. A first extruder  33  and a second extruder  34  are disposed over the endless belts  31  and  32 , respectively. The extruder  33 ,  34  has a plurality of nozzles  37 ,  38  arranged in a row and transverse direction to the endless belt  31 ,  32 . A suction duct  31   a ,  32   a  is disposed right under the extruder  33 ,  34  through the endless belt  31 ,  32 . The first extension step  71  involves a pair of first rolls  73  and a pair of second rolls  74 . The second roll  74  rotates at a faster speed than the first roll  73 . The first contraction step  72  involves a plurality of third rolls  76  arranged substantially in a machine direction. The foremost third roll  76  rotates at a peripheral speed close to that of the second roll  74 , the following third rolls  76  rotate at peripheral speeds slowed in the sequence toward the rear, and the rearmost third roll is controlled to coincide in peripheral speed with the first roll  73 . A guide roll  75  is provided in both front and rear sides of the first contraction step  72 . 
     Plural streams of first continuous fibers  41 , made of a thermoplastic elastomer and capable of elastic stretch and contraction, are discharged from the nozzles  37  of the first extruder  33  and directed onto the first endless belt  31  under the suction action of the duct  31   a . The first continuous fibers  41  are preferably fused to each other over the first endless belt  31  and rendered into a first web  41   a  having the form of a non-woven fabric. The first web  41   a  is conveyed in a machine direction and then extended in the machine direction at a specific extension ratio as required. The first web  41   a  advances to the first contraction step  72  where it is released from the tension and allowed to retract while passing through the third rolls  76  arranged in a sequence of decreasing peripheral speed. The first web  41   a  subsequent to retraction advances toward the second endless belt  32 . Plural streams of second continuous fibers  52 , made of a thermoplastic synthetic resin and capable of inelastic extension, are discharged from the nozzles  38  of the second extruder  34  and directed onto the first web  41   a  under the suction action of the duct  32   a  to form a second web  52   a.    
     The superimposed first and second webs  41   a  and  52   a  are brought between a pair of hot emboss rolls  47  and united together by fusion at bond areas  4  (see FIG. 1) arranged at intervals in the machine direction to form a composite sheet  1  shown in FIG.  1 . 
     The composite sheet  1  can be controlled to further advance in the machine direction and treated into a second composite sheet  1   a . A next step that follows in the machine direction is a second extension step  82 , involving a pair of fourth rolls  84  and a pair of fifth rolls  85 , in which the composite sheet  1  is extended in the machine direction at a specific extension ratio as required. In the second extension step  82 , the fifth roll  85  rotates with a faster peripheral speed than the fourth roll  84 . After passage between the fifth rolls  85 , the composite sheet  1  advances to enter between a pair of carrying rolls  57  which rotate with almost the same peripheral speed as the first rolls  73 . The composite sheet  1  extended in the second extension steps  82  is released from tension in the second contraction step  82  involving the fifth rolls  85  and the carrying rolls  57 , allowed to retracts by the action of an elastic recovery force of the first web  41   a , and then wound round a roll as the second composite sheet  1   a . The composite sheet  1  shown in FIG. 1, when subjected to a single cycle of extension and contraction, results in the second composite sheet  1   a  which is applicable for the similar uses as the composite sheet  1 . 
     In the above-described manufacturing process of the composite sheet  1 , SEPS or the like may be used, for example, for the thermoplastic elastomer which is raw material of the first continuous fibers  41 . The use of first continuous fibers  41  having a fiber diameter of 18 μm results in the first web  41   a  having a basis weight of 31.9 g/m 2 . This first web  41   a  has a machine-direction breaking strength of 2.35 N per width of 50 mm and a breaking extension of 447%. In the first extension steps  71 , the extension ratio is within the range that permits elastic stretch and contraction of the first web  41   a  and within a breaking extension of the second web  52   a . For example, a 100 mm length of the first web  41   a  can be extended in the first extension step  71  by 120% of its original length, i.e., to a length of 220 mm. This is allowed to retract in the first contraction step  72  to a length of 113.5 mm. That is, when the first web  41   a  is extended by 120% of its original length, a permanent strain of 13.5 mm (13.5%) is produced per 100 mm of the first web  41   a . This permanent strain is conceivably attributed to the rearrangement of the first continuous fibers  41  in the first web  41   a . Such a permanent strain is generally reduced if the first web  41   a  is in the form of an elastic film. 
     Examples of thermoplastic synthetic resins for use as raw material of the second continuous fibers  52  include polypropylene; a 60:40 (by weight) mixture of polypropylene and a terpolymer of propylene, ethylene and butene; polyester; polyethylene and the like. As an example, the aforementioned mixture of polypropylene and terpolymer can be used to form the second continuous fibers  52  having a diameter of 17.5 μm and a percentage extension of 311% and then form the second web  52   a  having a basis weight of 15.0 g/m 2  from such fibers  52 . 
     To illustrate the case where this second web  52   a  is joined to the aforementioned first web  41   a  obtained via extension by 120% and subsequent contraction and measuring a length of 113.5 mm to provide the composite sheet  1 , extension of the composite sheet  1  by 100% in the second extension step  82  and subsequent contraction in the second contraction step  77  results in the second composite sheet  1   a  which shows an elastic recovery of 93%, out of 100% extension, and a remaining permanent strain of 7%. 
     For the purposes of comparison to this composite sheet  1 , a prior art elastic panel was obtained by joining the 113.5 mm long first web  41   a  before subjected to the first extension to the 113.5 mm long second web  52   b . The elastic panel was extended by 100% and then allowed to retract. The elastic recovery was 80% and 20% remained as a permanent strain. The increased permanent strain remained in the elastic panel than in the composite sheet  1 . As clear from the comparison, the composite sheet  1  exhibits the superior elastic recovery, higher elastic extension and wider range of elastic extension and contraction, compared to the prior art elastic panel. 
     In the manufacturing process in accordance with this invention, the second web  52   a  capable of inelastic extension is used having a breaking extension of 40% or higher, preferably 70% or higher, more preferably 100% or higher, at least in the machine direction, out of the machine and cross directions. The first web  41   a  capable of elastic stretch and contraction preferably has a breaking extension higher than that of the second web  52   a . More preferably, the first web  41   a  sustains its ability to elastically stretch and contract even at the breaking extension of the second web  52   a . The composite sheet  1 , if constructed from such first and second webs  41   a  and  52   a , can be extended up to the vicinity of the breaking extension limit of the second web  52   a . In the case where the continuous fibers  52  forming the second web  52   a  are engaged with each other by mechanical entanglement or fusion bond thereof, it is preferred that fibers  52  are largely freed from the engagement and thereby individualized in the second extension step  76 . This increases a bulk of the second web  52   a  as it retracts, so that the composite sheet  1  provides a softer skin contact. 
     In this invention, the composite sheet  1  can be rendered into a three-layer structure by placing the second web  52   a  on top and bottom surfaces of the first web  41   a . In such a case, the second webs  52   a ,  52   a  joined to the top and bottom surfaces of the first web  41   a  may be of the same properties or made different from each other in any of the following properties; basis weight, density, type of the thermoplastic synthetic resin used to form the continuous fibers  52 , fiber diameter and fiber length. The illustrated first and second continuous fibers  41 ,  52  may be altered to short fibers with a length of 50 mm or less, or to long fibers having a length in the approximate range of 50-300 mm. 
     In the manufacturing process of an elastically stretchable and contractible composite sheet in accordance with this invention, an elastically stretchable and contractible web obtained via a sequence of extension and contraction effective to remove a major proportion of permanent strain and a web capable of inelastic extension are superimposed on each other and then united together. Accordingly, the resulting composite sheet exhibits a wider range of elastic extension and contraction, compared to an elastic panel resulting from a conventional manufacturing process wherein a web capable of elastic extension and contraction and a web capable of inelastic extension are superimposed and then united together.