Patent Publication Number: US-2019194846-A1

Title: Method for strengthening a nonwoven fabric

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a method for strengthening a nonwoven fabric by means of a water jet treatment. 
     The strengthening of nonwoven fabrics by means of water jets, which is also referred to as “hydroentanglement” or “spunlacing”, is well known to a person skilled in the art. 
     In the production of nonwoven fabrics according to the water jet process, the strengthening of the presented carded fleece is achieved by enlacing and swirling the fibers. The presented fibers are encompassed by the water jets, set in motion and interlaced with each other three-dimensionally by a swirling motion. 
     Cotton is generally regarded as a particularly suitable fiber material for hydroentanglement, see, for example, the article “Aquajet Spunlace Verfahren—Technik für Baumwollfasern” by Alfred Watzl, Messrs. Fleissner. The low wet modulus of the cotton fibers as well as the fact that the fiber does not exhibit a round and smooth fiber cross-section are thereby regarded as beneficial. 
     Fibers having a high elastic modulus (hereinafter referred to as “e-modulus”) are suitable for obtaining high web strengths. These are essentially non-cellulosic fibers. 
     In order to achieve sufficient web strengths, high pressures are required in the course of hydroentanglement, as a result of which the method is energy-intensive. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a method for the hydroentanglement of nonwoven fabrics which is feasible with little energy input. 
     This object is achieved by a method for strengthening a nonwoven fabric by means of a water jet treatment which is characterized in that the nonwoven fabric contains flat fibers in the form of collapsed hollow viscose fibers with a ratio of width B to thickness D of B:D≥10:1. 
     Furthermore, the present invention relates to a hydroentangled nonwoven fabric containing flat fibers in the form of collapsed hollow viscose fibers with a ratio of width B to thickness D of B:D≥10:1. 
     Preferred embodiments are set forth in the subclaims. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Surprisingly, it has been shown that the use of collapsed hollow viscose fibers in a nonwoven fabric to be strengthened by water jets has the effect that, during the hydroentanglement with equal energy input (i.e., application of equally high treatment pressures), higher web strengths result than with a similar nonwoven material which does not contain any flat fibers. Likewise, a desired web strength can be achieved with an energy input which is smaller than in case of a nonwoven material which does not contain any cellulosic flat fibers. 
     In this manner, energy can be saved, and process costs can thus be reduced. In addition, the expenditure with regard to the equipment can be kept smaller due to the lower pressures. In addition, it is possible to perform a more gentle strengthening, i.e., at lower pressures. This is advantageous, for example, in case of mixtures with cellulose, wherein high pressures cause cellulose to be washed out, or also in case of mixtures with delicate fibers. In addition, the use of synthetic fibers (for achieving particularly high strengths) can be avoided or at least reduced, respectively. 
     In the method according to the invention, the flat fibers contained in the nonwoven fabric preferably have a ratio B:D ranging from 10:1 to 30:1, particularly preferably of 20:1. 
     Preferably, the flat fibers may have a titer ranging from 0.9 to 5 dtex, particularly preferably from 1.3 to 1.9 dtex. 
     Flat fibers and their manufacture are known. In contrast to the cross-section of fibers which commonly is essentially round, flat fibers have an essentially flat or, respectively, oblong cross-section. 
     On the one hand, cellulosic flat fibers can be produced by spinning a spinning dope containing cellulose or a cellulose derivative through slot-shaped spinnerets. In case of viscose fibers, flat fibers can alternatively be produced in the form of collapsed hollow fibers. In doing so, a gas, e.g. nitrogen, or a blowing agent, e.g., sodium carbonate, is admixed to the spinning viscose. During the spinning of the fibers through dies, which are per se conventional, hollow fibers are formed whose walls, however, are so thin when appropriate process conditions are chosen that the fibers will collapse and will then be provided in the form of flat fibers. 
     The manufacture of cellulosic flat fibers is known, for example, from GB 945,306 A, U.S. Pat. Nos. 3,156,605 A, 3,318,990, GB 1,063,217 A. Such fibers have been recommended especially for use in paper production, as is described in part in the above-mentioned documents. 
     The article by C. R. Woodings, A. J. Bartholomew; “ The manufacture properties and uses of inflated viscose rayon fibers ”; TAPPI Nonwovens Symposium; 1985; pp. 155-165. Source: http://www.nonwoven.co.uk/publications_cat4.php, describes different types of hollow fibers and their use. 
     WO 2006/134132 describes the use of viscose flat fibers in a fiber composite for the purpose of improving the dissolubility of the fiber composite in water. According to WO 2006/134132, the flat fibers used preferably have a crenelated (pinnacle-type) surface and, in contrast to collapsed hollow fibers, are thus produced by being spun through a slot die. The ribbed surface of such flat fibers reduces the fiber-fiber adhesion and hence the strength. On the other hand, with conventional flat fibers, the achievable thickness is limited by the geometry of the die. Final spinnings with dies having openings with heights of 25 μm generally lead to a fiber thickness of approx. 4-6 μm. In order to consistently produce a fiber thickness of approx. 2-3 μm such as with collapsed hollow fibers, a die opening with a height of approx. 12.5 μm would be required, which is economically feasible neither in the manufacture of dies, nor in the production of viscose fibers using conventional methods. 
     In contrast, the viscose flat fibers used according to the invention are collapsed hollow fibers which, as has been mentioned above, are producible by introducing gas or a blowing agent (in particular sodium carbonate) into the spinning viscose. The fiber may be completely collapsed or still slightly opened. However, the water retention of the fiber should preferably be 200% or less (measured according to DIN 53814). The fiber cross-section of the fibers should be predominantly flat and preferably not branched. 
     The amount of flat fibers in the nonwoven fabric is preferably 5% to 100%, in particular 20% or more, particularly preferably 50% or more. Hence, the nonwoven material can be made up entirely of the flat fibers or may also contain a mixture of the flat fibers with other fibers. All cellulosic and non-cellulosic fiber materials which are suitable for hydroentanglement are possible mixing partners. It is obvious to a person skilled in the art that the effect according to the invention (i.e., the increase in strength of the nonwoven material and the saving of energy, respectively) is the more pronounced, the higher the content of flat fibers in the nonwoven material. 
     The invention also relates to a hydroentangled nonwoven fabric containing flat fibers in the form of collapsed hollow viscose fibers with a ratio of width B to thickness D of B:D≥10:1. Regarding details as to the flat fibers as well as their proportion in the nonwoven fabric, see the subclaims and the above explanations, respectively. 
     Examples 
     For the production of hydroentangled nonwoven fabrics, the following fibers, each having a titer of 1.7 dtex, were used:
         a) standard viscose fiber (Type Danufil®)   b) viscose flat fiber from a hollow fiber process; fiber thickness approx. 2-3 μm; ratio width:thickness=approx. 20:1       

     The fibers were presented as a carded nonwoven and strengthened on both sides in two passages. 
     Nonwoven fabrics with two weights per unit area and, in each case, two strengthening levels (less−higher strengthening) were produced from each fiber. 
     Weights per unit area: 50 g/m 2  and 80 g/m 2 , respectively 
     Strengthening levels: (strengthening pressure in each case indicated as sum of all pressures of all die bars in both passages) 
       Weight per unit area 50 g/m 2 −minor strengthening: 65 bar
 
       Weight per unit area 50 g/m 2 −higher strengthening: 95 bar
 
       Weight per unit area 80 g/m 2 −minor strengthening: 95 bar
 
       Weight per unit area 80 g/m 2 −higher strengthening: 145 bar
 
     The higher strengthening pressure is thus, in each case, approx. 50% above the low strengthening pressure. 
     Examination: 
     With standard test specimens of 5×25 cm, the following parameters were determined for all nonwoven fabrics:
         maximum tensile strength [N/5 cm] in the production direction (MD) and transversely to the production direction (CD), in each case wet and dry   maximum tensile strength elongation       

     and, respectively, the ratio MD/CD as well as the sum MD+CD, which have been derived therefrom 
     The results are summarized in the following tables: 
     
       
         
           
               
             
               
                 TABLE 1a 
               
             
            
               
                   
               
               
                 nonwoven fabric from a viscose flat fiber (according to the invention) 
               
            
           
           
               
               
            
               
                   
                 dry 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 maximum 
                   
                   
               
               
                   
                   
                 tensile 
               
               
                 weight 
                   
                 strength 
                 elongation 
               
               
                 per unit 
                 strengthening 
                 [N/5 cm] 
                 [%] 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 area 
                 level 
                 MD 
                 CD 
                 MD 
                 CD 
                 MD/CD 
               
               
                   
               
               
                 50 g/m 2   
                 minor 
                 40.1 
                 34.0 
                 17.0 
                 31.4 
                 1.18 
               
               
                 50 g/m 2   
                 high 
                 38.9 
                 33.3 
                 16.2 
                 29.1 
                 1.17 
               
               
                 80 g/m 2   
                 minor 
                 62.0 
                 60.9 
                 18.3 
                 34.5 
                 1.02 
               
               
                 80 g/m 2   
                 high 
                 59.8 
                 59.8 
                 14.7 
                 30.6 
                 1.00 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 1b 
               
             
            
               
                   
               
               
                 nonwoven fabric from a viscose flat fiber (according to the invention) 
               
            
           
           
               
               
            
               
                   
                 wet 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 maximum 
                   
                   
               
               
                   
                   
                 tensile 
               
               
                 weight 
                   
                 strength 
                 elongation 
               
               
                 per unit 
                 strengthening 
                 [N/5 cm] 
                 [%] 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 area 
                 level 
                 MD 
                 CD 
                 MD 
                 CD 
                 MD/CD 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 50 g/m 2   
                 minor 
                 27.4 
                 22.6 
                 27.8 
                 34.1 
                 1.21 
               
               
                 50 g/m 2   
                 high 
                 29.9 
                 24.6 
                 27.9 
                 31.0 
                 1.22 
               
               
                 80 g/m 2   
                 minor 
                 44.3 
                 40.3 
                 28.6 
                 35.8 
                 1.10 
               
               
                 80 g/m 2   
                 high 
                 45.0 
                 37.8 
                 28.5 
                 31.7 
                 1.19 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2a 
               
             
            
               
                   
               
               
                 nonwoven fabric from a standard viscose fiber (comparison) 
               
            
           
           
               
               
            
               
                   
                 dry 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 maximum 
                   
                   
               
               
                   
                   
                 tensile 
               
               
                 weight 
                   
                 strength 
                 elongation 
               
               
                 per unit 
                 strengthening 
                 [N/5 cm] 
                 [%] 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 area 
                 level 
                 MD 
                 CD 
                 MD 
                 CD 
                 MD/CD 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 50 g/m 2   
                 minor 
                 21.0 
                 33.5 
                 26.1 
                 41.6 
                 0.62 
               
               
                 50 g/m 2   
                 high 
                 38.8 
                 42.5 
                 30.6 
                 37.8 
                 0.91 
               
               
                 80 g/m 2   
                 minor 
                 10.8 
                 51.3 
                 13.6 
                 42.4 
                 0.21 
               
               
                 80 g/m 2   
                 high 
                 29.7 
                 69.2 
                 19.5 
                 36.1 
                 0.43 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2b 
               
             
            
               
                   
               
               
                 nonwoven fabric from a standard viscose fiber (comparison) 
               
            
           
           
               
               
            
               
                   
                 wet 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 maximum 
                   
                   
               
               
                   
                   
                 tensile 
               
               
                 weight 
                   
                 strength 
                 elongation 
               
               
                 per unit 
                 strengthening 
                 [N/5 cm] 
                 [%] 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 area 
                 level 
                 MD 
                 CD 
                 MD 
                 CD 
                 MD/CD 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 50 g/m 2   
                 minor 
                 18.0 
                 19.8 
                 16.8 
                 29.9 
                 0.91 
               
               
                 50 g/m 2   
                 high 
                 21.3 
                 24.7 
                 25.1 
                 30.5 
                 0.86 
               
               
                 80 g/m 2   
                 minor 
                 5.9 
                 21.5 
                 16.8 
                 29.9 
                 0.27 
               
               
                 80 g/m 2   
                 high 
                 18.5 
                 41.2 
                 25.1 
                 30.5 
                 0.45 
               
               
                   
               
            
           
         
       
     
     From the above-indicated data, the following conclusions can be drawn: 
     Elongation: 
     In the dry nonwoven fabric, the maximum tensile strength elongation (with equal weight per unit area and equal strengthening) is clearly lower in the nonwoven fabrics produced from flat fibers, as opposed to a nonwoven fabric produced from standard viscose fibers. Presumably, this is due to the higher amount of fiber-fiber bonds. 
     MD/CD Ratio: 
     Under equal experimental settings, nonwoven fabrics comprising flat fibers exhibit a substantially higher MD/CD ratio than nonwoven fabrics made of standard viscose fibers. 
     Starting from a low MD/CD ratio with minor strengthenings, the MD/CD ratio increases as a result of a reorientation of the fibers in the strengthening process. The MD/CD ratio of the nonwoven fabrics made of flat fibers, which, under equal pressures, is substantially higher than that of nonwoven fabrics made of standard viscose fibers, indicates the considerably higher flexibility of the flat fiber, which significantly facilitates the strengthening process. 
     Strength: 
       
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 nonwoven fabric from a viscose flat  
               
               
                 fiber (according to the invention) 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 dry 
                 wet 
               
               
                   
                   
                   
                 maximum  
                 maximum  
               
               
                   
                 weight 
                   
                 tensile strength 
                 tensile strength 
               
               
                   
                 per unit 
                 strengthening 
                 [N/5 cm] 
                 [N/5 cm] 
               
               
                   
                 area 
                 level 
                 MD + CD 
                 MD + CD 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 50 g/m 2   
                 minor 
                 74.1 
                 50.1 
               
               
                   
                 50 g/m 2   
                 high 
                 72.3 
                 54.5 
               
               
                   
                 80 g/m 2   
                 minor 
                 122.9 
                 84.7 
               
               
                   
                 80 g/m 2   
                 high 
                 119.6 
                 82.8 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 nonwoven fabric from a standard  
               
               
                 viscose fiber (comparison) 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 dry 
                 wet 
               
               
                   
                   
                   
                 maximum 
                 maximum  
               
               
                   
                 weight 
                   
                 tensile strength 
                 tensile strength 
               
               
                   
                 per unit 
                 strengthening 
                 [N/5 cm] 
                 [N/5 cm] 
               
               
                   
                 area 
                 level 
                 MD + CD 
                 MD + CD 
               
               
                   
                   
               
               
                   
                 50 g/m 2   
                 minor 
                 54.5 
                 37.8 
               
               
                   
                 50 g/m 2   
                 high 
                 81.3 
                 46.0 
               
               
                   
                 80 g/m 2   
                 minor 
                 62.1 
                 27.4 
               
               
                   
                 80 g/m 2   
                 high 
                 98.8 
                 59.8 
               
               
                   
                   
               
            
           
         
       
     
     For the sake of ease of examining, the sum of the tearing forces MD+CD is to be used in each case for assessing the strength: 
     As to the nonwoven fabrics according to the invention made of flat fibers, it is apparent that a higher strength is not achieved by increasing the strengthening pressure from “minor” to “high”. This means that, obviously, the nonwoven material was already strengthened to the maximum extent always at the low strengthening level. 
     With equal strengthening, the strength of a nonwoven fabric normally correlates with the weight per unit area. 
     In this case, the ratio of the weights per unit area is 80 g/m 2  to 50 g/m 2 =1.6. 
     Based, for example, on a measured strength of approx. 72 N/5 cm of the highly strengthened nonwoven fabric having a weight per unit area of 50 g/m 2 , a strength of 72*1.6=115 N/5 cm might thus be expected for the nonwoven fabric with a weight per unit area of 80 g/m 2  which has been strengthened equally high, which complies well with the actually measured value of approx. 120. 
     This means that, in all four configurations, the nonwoven material has already been strengthened to a maximum extent. 
     With the nonwoven fabrics made of standard viscose fibers, a different picture emerges: 
     In a haptic assessment, the two nonwoven materials of the minor strengthening level have been strengthened only insufficiently. 
     Upon an increase of the strengthening pressure from “minor” to “high” (always by approx. 50%), a clear increase in web strength can, in each case, be determined. Thus, a further pressure increase might in this case obviously result in still a further strengthening, which means that, for maximum strengthening, an even higher pressure would be required. 
     At the higher strengthening level of the 50 g/m 2  nonwoven fabric, the strength of the dry nonwoven fabric is even slightly above the strength of the nonwoven fabric made of flat fibers. This is probably caused by the higher single-fiber strength of the fibers used in the experiment (standard viscose fiber: 22 cN/tex; viscose flat fiber: 16 cN/tex). However, it may be assumed that the nonwoven material has thereby been strengthened to a large extent. 
     Thus, according to the above calculation, a strength of at least 81.3×1.6=130 N/5 cm might be expected for a completely strengthened nonwoven material with a weight per unit area of 80 g/m 2 . However, only about 99 N/5 cm were measured. Thus, even with the application of a higher strengthening pressure, the 80 g/m 2  nonwoven fabric is still far from completely strengthened. 
     At said strengthening level, the nonwoven material has achieved only about 75% of its strengthening potential. 
     Comparison: 
     The example of the 80 g/m 2  nonwoven fabrics clearly shows the advantages of the use according to the invention of flat fibers in hydroentanglement. 
     Nonwoven fabric from a viscose flat fiber—at a strengthening pressure of 95 bar: 
       Strength dry ( MD+CD )=120N/5 cm; 
       Strength wet ( MD+CD )=83N/5 cm 
     Nonwoven fabric from a standard viscose fiber—at a strengthening pressure of 145 bar: 
       Strength dry ( MD+CD )=99N/5 cm; 
       Strength wet ( MD+CD )=60N/5 cm 
     Hence, by using viscose flat fibers in the nonwoven material, strengths which are higher by 20% (dry) and, respectively, higher by 40% (wet) than with standard viscose fibers can be achieved at 50% lower pressures.