Absorbent article
FIELD OF INDUSTRIAL APPLICATION 
     The present invention relates to an absorbent article such as a sanitary napkin, a disposable diaper and a pad for incontinence. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an absorbent article, and description will hereinafter be made as regards a sanitary napkin as a representative example of the absorbent article since the same idea can basically apply to other absorbent articles. 
     A sanitary napkin generally utilizes, as basic constituents, three components, namely a liquid-permeable surface sheet, an absorbent layer made of fluff pulp, absorbent tissues, a water-absorbent polymer or the like, and a liquid impermeable-leakproof sheet. Recently, a sanitary napkin comprising a rayon fiber assembly (hereinafter referred to briefly as &#34;cushioning wool&#34;) or the like provided under the surface sheet (on the absorbent layer) has been generally used. 
     An object of the present invention is to greatly improve the defects of the absorbent layer and cushioning wool which have been generally used in a conventional sanitary napkin or the like as described hereinbefore. 
     First, fluff pulp is economical, flexible and highly hydrophilic. Thus, it is widely used as the absorbent layer. 
     Since, however, water absorbent by a laminate of fluff pulp is nearly completed based on a retention of liquid between fibers (capillary) and the laminate has no interfiber bonding strength, it is easily deformed and capillaries formed between the fibers are broken so that liquid retained is easily released when it undergoes a compressional force. The laminate shows a little recovery from compression when it is dry, but no recovery when it contains absorbent water. This causes slippage and wilting of the absorbent layer, and hence constitutes the main cause of the occurence of leakage. The following techniques for eliminating the above-mentioned defects have been disclosed. 
     (1) As shown in FIG. 4, a core material 4 having a shape retention property, such as a foam, is provided in the absorbent layer 3 or on the side of the non-effective surface to prevent the deformation of the absorbent article (see Japanese Utility Model Laid-Open No. 6,099/1981) 
     (2) As shown in FIG. 6, the absorbent layer itself is replaced with an elastic porous material 5 (e.g., a urethane skeleton foam, a polyester fiber assembly having fibers bonded with a binder, or the like) (see Japanese Patent Laid-Open No. 22,354/1986). 
     In the case of (1), however, although the shape retention property of the construction as a whole can be maintained, when the absorbent layer itself is deformed slippage and wilting are caused as shown in FIG. 5. 
     In the case of (2), although the recovery from deformation of the absorbent article is excellent, the hydrophilic nature of the elastic porous material is so extremely poor as compared with the fluff pulp that the material is not sufficient in liquid uptake capacity and liquid retention. 
     Since a cushioning wool 6 as shown in FIG. 7 is used mainly for the purpose of effectively absorbing a liquid excreted on the surface sheet 1 in the inside and causing the liquid to migrate into the absorbent layer 3, it is generally made of rayon fiber (occasionally pulp). When the amount of excretion is relatively small, it is very effective. In contrast, when the amount of excretion is large, the cushioning wool wilts due to its highly hydrophilic nature just as in the case of the above mentioned fluff pulp. As a result, a layer having a large amount of a liquid absorbed therein is formed between the absorbent layer 3 and the surface sheet 1, and the liquid readily flows back to the surface sheet, thus leading a to lowering in dryness and causing a side leak. 
     Various improvements of these defects have been proposed. For example, a method has been proposed to use a synthetic fiber instead of a rayon fiber, and another method use a fiber assembly (tow). Since the synthetic fiber and the fiber assembly both have a low hydrophilic nature, however, uptake of a liquid from a surface sheet is insufficient, and the liquid inevitably diffuses between the surface sheet and the cushioning wool, thereby causing poor dryness and a side leak. 
     Since no sufficient improvement in the resistance to compression is attained as well, no improvement is made as regards prevention of the ready back flow of a liquid under a pressure in the direction of compression due to wearing. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention of the present ivention have made intensive investigations with a view to solving the above-mentioned problems and, as a result, have completed the present invention. 
     Specifically, the present invention provides an absorbent article comprising a liquid-permeable surface sheet, a liquid-impermeable leakproof sheet, and an absorbent layer provided between the two sheets, characterized in that at least part of a material constituting the absorbent layer is made of a nodular fiber and that the nodular fiber contains a fiber having a wet to dry tenacity ration of 80% or more. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS: 
     FIGS. 1 to 3 are crosssectional views of examples of the absorbent article of the present invention, 
     FIG. 1 is an example of a sanitary napkin comprising nodular fiber in the upper 
     FIGS. 1 to 3 are crosssectional views of examples of the absorbent article of the present invention, 
     FIG. 1 is an example of a sanitary napkin comprising nodular fiber in the upper portion of the absorbent layer, 
     FIG. 2 is an example of a sanitary napkin comprising a nodular fiber provided in the whole of the absorbent layer, and 
     FIG. 3 is an example of a sanitary napkin having an absorbent layer constituted only of a nodular fiber. 
     FIG. 4 is a crosssectional view of a conventional sanitary napkin having a piece for shape retention, 
     FIG. 5 is a crosssectional view of this sanitary napkin having the piece for shape retention when the absorbent layer is deformed, 
     FIG. 6 is a crosssectional view of a conventional sanitary napkin having an absorbent layer using an elastic porous material, 
     FIG. 7 is a crosssectional view of a conventional general sanitary napkin, 
     FIG. 8 is a perspective view of a movable female waist model, and 
     FIG. 9 is a perspective view showing the state of a test sample worn on the movable female waist model. 
    
    
     The following reference numerals are employed in the Figures: 
     1: liquid-permeable surface sheet 
     2: liquid-permeable leakproof sheet 
     3: absorbent layer 
     4: piece for shape retention 
     5: elastic porous material 
     6: rayon cushioning wool 
     7: nodular fiber 
     8: hydrophilic fiber 
     9: movable female waist model 
     10: liquid injection tube 
     11: test sample 
     The wet to dry tenacity ratio as used in the present invention is a value obtained by the formula: wet/dry tenacity ratio=(SW/SD)×100, wherein SD is a tensile strength in a standard state and SW is a tensile strength in a wet state. The tensile test is made as regards a single filament in accordance with the method as stipulated in the &#34;Tensile Strength Tests of Fibers&#34; of JIS L 1069. 
     The content of the fiber having a wet to dry tenacity ratio of 80% or more in nodular fiber according to the present invention is not particularly limited, and may be chosen with due consideration given to required characteristics. However, it is preferably substantially 20 wt.% or more, and more preferably 50 wt.% or more. When the content of the fiber having a wet to dry tenacity ratio of 80% is less than 20 wt.%, no sufficient compression resistance is obtained. 
     Hydrophilic fiber materials can be mentioned as a fiber material contained in the nodular fiber other than the fiber material having a wet to dry tenacity ratio of 80% or more. Use of nodular fiber comprised of only a fiber material having a wet to dry tenacity ratio of 80% or more (hereinafter referred to briefly as a &#34;skeletal fiber&#34;), or a skeletal fiber and a hydrophilic fiber material can provide an absorbent layer which does not undergo reduction in the liquid absorption, namely hydrophilic nature, and can effectively cause a liquid to migrate into the absorbent layer, and which shows such a compression resistance that a liquid does not flow back to the surface even when a pressure is applied thereto, that is a reduced wilting tendency. 
     Examples of the sketetal fiber having a wet to dry tenacity ratio of 80% or more to be used in the present invention include polyester, polypropylene, polyethylene, polyvinyl chloride, polyvinylidiene chloride, nylon, acrylic, cotton, and flax fibers. Conjugate fibers composed of two components selected from among polyester, polypropylene, etc. may also be used alone or in mixture. 
     Among others, polyester, polyester/polypropylene conjugate and polyester/polyethylene conjugate fibers are preferred from the viewpoint of elastic recovery from compression. The fiber size is not particularly limited and may be chosen according to the required characteristics. Generally, it is preferably about 1.5 to 6 deniers. When it is less than 1.5 deniers, no sufficient compression resistance is obtained. When it is more than 6 deniers, stiffness is felt in an aspect of hand unfavorably. 
     Surface modification such as a treatment of the fiber surface with a lubricant or a plasma may be done without limitation in so far as the compression resistance is not affected. 
     Examples of the hydrophilic fiber material include rayon, cuprammonium rayon, and fluff pulp. They may be used alone or in mixture. There are no limitations in fiber lenght and fiber size. 
     The process for preparing a nodular fiber may comprise cutting a fiber assembly to have adequate length and width (or adequate weight as the case may be), making the cut fibers nodular, and partially fusing the skeletal fiber by heating if it is fusable, or partially bonding the fiber with a binder which can be neither swollen nor dissolved in water. 
     Although the size of the nodular fiber according to the present invention is not particularly limited, it usually is preferably 1 mm to 60 mm. 
     The methods of using such a nodular fiber as a constituent in the absorbent article of the present invention include (1) one in which it is used under the surface sheet, (2) one in which it is mixed with other hydrophilic fiber and the mixture is used in the whole of the absorbent layer, and (3) one in which the absorbent layer is constituted only of the nodular fiber. In the cases of (1) and (2), an absorbent layer using 3 to 80 wt.% of a nodular fiber based on the absorbent layer is usually suitable used. Needles to say, however, the present invention is not limited to this. 
     EXAMPLES 
     The following Examples will illustrate the present invention in more detail but they should not be construed as limiting the scope of the invention. 
     EXAMPLE 1 
     In this embodiment a nodular fiber was used under the surface sheet on the upper side of the absorbent layer. This embodiment is shown in FIG. 1. As a test sample a sanitary napkin is comprised of a surface sheet 1, an absorbent layer 3, a leakproof sheet 2, and cushioning wool 6 provided under the surface sheet as shown in FIG. 7. 
     The surface sheet used herein was a heat-bonded non-woven fabric comprised of 65% of a polyester fiber and 35% of a polyethylene/polypropylene conjugate fiber and having a base weight of 20g/m 2 . The absorbent layer was comprised of 2 g of fluff pulp, 1.5 g of absorbent tissues, and 0.3 g of a super-absorbent polymer. 
     The leakproof sheet used herein was a waterproof paper (25 g/m 2 ) having a 10μ-thick polyethylene film laminated thereon. 
     A napkin having cushioning wool 6 made of a rayon fiber was used as a comparative sample. This wool was replaced with various nodular fibers according to the present invention as shown in Table 1 to examine the effects thereof. The rayon wool 6 and the nodular fibers 7 of the present invention were each used in an amount of 0.3 g. In order to confirm the effect of the present invention, the amount of liquid back flow was measured. The results are shown in Table 2. 
     Measurement of amount of liquid back flow 
     10 g of artifical blood was injected into a test sample. The liquid, which flowed back when a pressure of 50 g/cm 2  was applied to the test sample, was soaked up with the fiber paper. The weight of the liquid soaked up was defined as the amount of liquid back flow. 
     
                       TABLE 1                                                     
______________________________________                                    
Materials Used in Test                                                    
                                        wet/dry                           
                  Fiber    Fiber        tenacity                          
                  fineness length                                         
                                 Extent of                                
                                        ratio (1)                         
No.  Material     (denier) (mm)  crimping                                 
                                        (%)                               
______________________________________                                    
a    rayon        3        15    small  60-65                             
b    rayon        6        15    small  60-65                             
c    rayon        3        38    small  60-65                             
d    cottony pulp --       1-2   --     --                                
e    dewaxed cotton                                                       
                  --       --    --     --                                
f    polyester    1.5      38    large  100                               
g    polyester    3        38    medium 100                               
h    polyester    6        51    medium 100                               
i    polypropylene                                                        
                  1.5      38    medium 100                               
j    polypropylene                                                        
                  3        38    medium 100                               
k    polypropylene                                                        
                  6        51    large  100                               
l    polyethylene/                                                        
                  3        38    medium 100                               
     polypropylene                                                        
     conjugate fiber                                                      
m    polyethylene/                                                        
                  3        38    medium 100                               
     polyester                                                            
     conjugate fiber                                                      
______________________________________                                    
 Note:                                                                    
 (1) A tensile strength test of a single filament was conducted in        
 accordance with the method as stipulated in the &#34;Tensile Strength Tests o
 Fibers&#34; of JIS L 1069. The wet to dry tenacity ratio was obtained by the 
 following equation: wet/dry tenacity ratio (%) =  (SW/SD) × 100    
 wherein SD is a tensile strength in a standard state and SW is a tensile 
 strength in a wet state.   as stipulated in the &#34;Tensile Strength Tests o
 Fibers&#34; of JIS L 1069. The wet to dry tenacity ratio was obtained by the
 following equation: wet/dry tenacity ratio (%)=(SW/SD)×100 wherein
 SD is a tensile strength in a standard state and SW is a tensile strength
 in a wet state.
 
    
     
                       TABLE 2                                                     
______________________________________                                    
                              Amount of                                   
               Blending       liquid                                      
Sample         weight         back flow                                   
No.   Material ratio    Shape (g)     Remarks                             
______________________________________                                    
1-1   f        100      sphere                                            
                              1.4     present                             
1-2   g        &#34;        &#34;     1.2     invention                           
1-3   h        &#34;        &#34;     1.1                                         
1-4   i        &#34;        &#34;     1.7                                         
1-5   j        &#34;        &#34;     1.5                                         
1-6   k        &#34;        &#34;     1.4                                         
1-7   l        &#34;        &#34;     1.6                                         
1-8   m        &#34;        &#34;     1.4                                         
1-9   f/a      60/40    &#34;     1.1                                         
1-10  l/e      80/20    &#34;     0.9                                         
1-11  m/d      70/30    &#34;     0.9                                         
1-12  a        100      staple                                            
                              2.3     comparative                         
1-13  c        &#34;        &#34;     2.5                                         
1-14  e        &#34;        &#34;     2.2                                         
______________________________________                                    
 
    
     EXAMPLE 2 
     In this embodiment a nodular fiber was used throughout the whole of the absorbent layer. This embodiment is shown in FIG. 2. 
     The surface sheet 1 and the leakproof sheet used herein were the same as those used in Example 1. The absorbent layer was a mixture of 0.3 g of a water-absorbent polymer and 3.5 g of a mixture of a nodluar fiber 7 and a hydropilic fiber 8 such as fluff pulp, rayon or cotton. Samples thus obtained are shown in Table 3. 
     
                       TABLE 3                                                     
______________________________________                                    
         Material of    Material of                                       
Sample   nodular fiber  hydrophilic fiber                                 
No.      (wt. %)        (wt. %)                                           
______________________________________                                    
2-1      g         70       d     30                                      
2-2      h         60       c     40                                      
2-3      l         60       e     40                                      
2-4      m         50       d     50                                      
2-5      1/c(60-40)                                                       
                   60       d     40                                      
______________________________________                                    
 
    
     EXAMPLE 3 
     In this embodiment the absorbent layer constituted only a nodular fiber. This embodiment is shown in FIG. 3. 
     The surface sheet 1 and the leakproof sheet 2 used herein were the same as those used in Example 1. The absorbent layer was a mixtue of 3.5 g of a nodular fiber 7 and 0.3 g of a water-absorbent polymer. A mixture or a lamination of nodular fibers constituted of different materials were also made. 
     
                                           TABLE 4                                 
__________________________________________________________________________
Sample                                                                    
    Nodular fiber                                                         
No. (1)   wt %                                                            
              (2)   wt %                                                  
                        (3)  wt % Remarks                                 
__________________________________________________________________________
3-1 1/c(60/40)                                                            
          100 --        --        Only a single                           
                                  component                               
3-2 m/b(80/20)                                                            
          30  1/c(50/50)                                                  
                    70  --        mixture of two                          
                                  components                              
3-3 h/b(80/20)                                                            
          30  m/c(50/50)                                                  
                    70  --        laminate of                             
                                  (1) and (2),                            
                                  (1) being pro-                          
                                  vided on the                            
                                  side of surface                         
                                  sheet                                   
3-4 m/b(90/10)                                                            
          10  1/c(70/30)                                                  
                    30  1/c(50/50)                                        
                              60  laminate of                             
                                  (1), (2) and                            
                                  (3) in this                             
                                  order, (1) being                        
                                  provided on the                         
                                  side of surface                         
                                  sheet                                   
__________________________________________________________________________
 
    
     TEST EXAMPLE 1 
     In order to confirm the effects of these samples, the amount of liquid back flow and the amount of dynamic maxium absorption were measured. 
     The results are shown in Table 5. 
     Measurement of amount of liquid back flow 
     The same method as in Example 1 was used. 
     Measurement of amount of dynamic maximum absorption 
     A test sample 11 was put on a movable female waist model 9 as shown in FIG. 8 in such a way as that shown in FIG. 9. After shorts were put on, the model was set in walking movement at a rate of 50 m/min while artificial blood was injected into the test sample through a dropping tube 10. The amount of the artificial blood absorbed until side leak occurred was measured. 
     
                       TABLE 5                                                     
______________________________________                                    
      Amount of    Maximum amount                                         
      liquid       of dynamic                                             
Sample                                                                    
      back flow    absorption                                             
No.   (g)          (g)          Remarks                                   
______________________________________                                    
1-1   1.4          6.1          present                                   
                                invention                                 
1-2   1.2          6.8          present                                   
                                invention                                 
2-1   1.5          8.6          present                                   
                                invention                                 
2-2   1.4          9.2          present                                   
                                invention                                 
2-3   1.2          9.4          present                                   
                                invention                                 
2-4   1.5          7.4          present                                   
                                invention                                 
2-5   1.3          8.0          present                                   
                                invention                                 
3-1   1.8          6.2          present                                   
                                invention                                 
3-2   1.6          7.8          present                                   
                                invention                                 
3-3   1.5          8.3          present                                   
                                invention                                 
3-4   1.2          9.4          present                                   
                                invention                                 
1-13  2.5          4.9          comparative                               
______________________________________                                    
 
    
     It is apparent from the above that, since the absorbent article of the present invention is excellent in compression resistance and recovery from deformation, liquid back flow to the surface (surface dryness) and the amount of dynamic absorption (leakproofness) are greatly improved as compared with that of the conventional article. 
     FUNCTION AND EFFECTS 
     The absorbent article of the present invention comprises a nodular fiber containing a skeletal fiber having a wet to dry tenacity ratio of 80% or more in at least part of the absorbent layer. Thus, a liquid can be effectively caused to migrate into the absorbent layer without reduction in the liquid absorption, namely the hydrophilic nature, of the absorbent layer. Since the absorbent layer is excellent in compression resistance and recovery from deformation even when a liquid is absorbed, the absorbent layer neither slips nor wilts when the absorbent layer is deformed, whereby liquid back flow the surface and side leakage which are mainly caused by slippage and wilting can be remarkably reduced.