Abstract:
A fiber-based carrier structure is made of fractions of industrially produced reinforcing fibrous materials, which contains endless fibers in a tangled arrangement as a first reinforcing fiber component and which contains endless fiber bundles as a second reinforcing fiber component. The fiber-based carrier structure further has a pore system. Accordingly, the endless individual fibers and the endless fiber bundles are used as a mixture preferably in an isotropic tangled arrangement to produce a fiber-based carrier structure, the pore system of which is an open-cell pore system and is openly accessible from outside. The fiber-based carrier structure enables the absorption of liquids and/or solid particles by the selection of a defined mixture ratio of endless fiber bundles and endless individual fibers. A capacity to be impregnated with and the capacity to absorb liquid and/or solid powdery materials can be set by the mixture ratio.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2014/053201, filed Feb. 19, 2014, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2013 002 861.2, filed Feb. 20, 2013; the prior applications are herewith incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a fiber-based carrier structure containing proportions of industrially produced reinforcing fiber materials, containing finite fibers in a random arrangement as a first reinforcing fiber component and containing finite fiber bundles as a second reinforcing fiber component, wherein the fiber-based carrier structure further has a pore system. 
         [0003]    International patent disclosure WO 2012/072 405 A1, corresponding to U.S. Pat. No. 8,840,988, describes a fiber preform having unidirectional slivers which is composed of two zones which are different from one another. This known fiber preform thus has an anisotropic structure. In a first zone there are arranged reinforcing fiber bundles which have a resin composition, while the second zone contains a unidirectionally oriented reinforcing yarn strand and likewise a resin composition. The purpose of this document is to provide a fiber preform which makes it possible to adapt to local stresses in the component. 
         [0004]    International patent disclosure WO 2010/139077 A1 describes a method for producing a composite material having the features of the generic type mentioned at the outset. It contains a core layer that contains at least 20 vol. % air voids and is made of a thermoplastic reinforced with randomly oriented fibers, and reinforcement strips made of continuous, parallel, unidirectional reinforcement fibers, which are embossed into the surface of the core layer on one side or on both sides. The reinforcement fibers of the outer layer are thermally fused onto the core layer by thermoplastic binders. It is not clear how the air void content in the core layer could purposively be adjusted, or whether the voids are accessible from the outside at all. There is high product anisotropy between the core and the outer layer. The reinforcement fiber strips embossed into the surface of the core layer are oriented so as to be parallel to one another. This document relates to producing a composite material having high strength and stiffness, which has good sound absorption. The voids have no significance for the absorption of liquids or solid particles, that is to say impregnability is not sought. 
         [0005]    Published, non-prosecuted German patent application DE 10 2007 012 608 A1, corresponding to U.S. Pat. No. 8,568,549, relates to a method for producing a preform for a fiber composite structure that is in accordance with the force flux, in which a flat fiber band obtained by spreading a fiber bundle is placed at a predefined position and then fixed by a binder material. Because in this case it is desired to arrange the fiber bands in accordance with the load, high product anisotropy is obtained. No defined mixtures of single fibers and fiber bundles are used. 
         [0006]    Published, non-prosecuted German patent application DE 10 2008 026 161 A1 describes a method for producing a fiber composite component, in which there are used continuous reinforcing fibers which are combined with a matrix material in the immediate vicinity of a shaping nozzle to form an impregnated fiber material. This document also relates to arranging reinforcing fibers in accordance with the load, which leads to high anisotropy in terms of fiber orientation, continuous reinforcing fibers being used. 
         [0007]    In the production of fiber layers in the conventional textile industry in the field of nonwovens or nonwoven-like fillers, the aim is always 100% complete separation, where possible, of the finite fiber materials that are used. Any residual fiber bundles that are still present, which are also referred to as cut bundles in the case of chemical fibers, are defined as defects and, where possible, should not occur. Consequently, according to the current prior art, nonwovens are referred to which are characterized in that they consist of fibers whose position can be described only by methods of statistics. Nonwovens are distinguished by the fiber material (for example the polymer in the case of chemical fibers), the bonding method, the type of fiber (staple or continuous fibers), the fineness of the fibers and the orientation of the fibers. The fibers can thereby be laid specifically in a preferential direction or can be oriented entirely at random, as in the case of a random-layer nonwoven or random nonwoven. A nonwoven having defined proportions of fiber bundles and single fibers as a carrier structure for substances and the use thereof in the field of fiber composites is hitherto unknown. The carrier structure can here range from a loose fiber filling to the consolidated 2D and 3D structure, for example nonwoven=2D. 
         [0008]    Conventional nonwovens, which can be bonded mechanically, thermally or chemically, have, depending on the fiber material, fiber geometry (thickness, length), fiber material mixture and production or consolidation, demonstrable properties for binding liquids or solid particles in the form of powders. In the case of liquids, a specific absorption capacity is referred to, which manifests itself in the absorption of more or less large amounts of liquids—which in most cases are aqueous, low-viscosity systems for applications in the field of cleaning or soaking up within the meaning of disposal—in a nonwoven into the inner layers thereof. Viscous liquids or melts can only be absorbed at the surface of the nonwoven without additional measures. As the viscosity of the liquid increases, it becomes increasingly difficult to transport it into the core of a nonwoven layer. Assistance is provided here by long impregnation times, with additional complex suctioning of the nonwoven, for example by vacuum or by pressure injection, with only limited success. 
         [0009]    It is similarly problematical to introduce solid particles in the form of powders or fine particles into such conventional nonwovens of single fibers. In most cases they remain on the surface of the nonwoven and penetrate only into layers that are close to the surface. The narrow-pore system of voids when single fibers are used prevents deeper penetration into the core of the nonwoven. This high capacity of conventional nonwovens to retain particles is used to advantage for example in dust or aerosol filtration but is extremely obstructive in other fields of application, for example in the production of fiber-reinforced plastics materials, where subsequently curing matrix materials in the form of viscous liquids or powders must be introduced as homogeneously as possible into reinforcing fiber nonwovens, loose fillings or mats. 
         [0010]    In the field of fiber-reinforced plastics materials, pulverulent binder substances and highly viscous liquid binders must uniformly penetrate reinforcing fiber layers that are as thick as possible in order to obtain an uniform fiber content over the product cross section in the subsequent fiber composite material. Assistance is provided in this case by the occasional use of mats of 100% cut fiber bundles. Although such what are known as cut roving mats, for example based on glass fibers, have good penetrability by highly viscous liquids and binding powders, they are not capable of absorbing and retaining such binder substances in a binding manner in large amounts within the meaning of a depot formation. 
       SUMMARY OF THE INVENTION 
       [0011]    The object of the present invention is to define a fiber-based carrier structure having the features mentioned at the outset, produced, for example, from loose fiber fillings of randomly arranged/oriented finite fiber materials, which, when shaped in a mat-like manner or three-dimensionally as a reinforcing fiber pre-product, exhibits a high and controllable impregnability with viscous liquids and powders and at the same time is able to retain large amounts of these substances in the inside in a uniformly distributed manner. 
         [0012]    This object is achieved by a fiber-based carrier structure of the generic type mentioned at the outset having the features of the main claim. 
         [0013]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0014]    Although the invention is described herein as embodied in a fiber-based carrier structure for liquids and solid particles and a method for producing the fiber-based carrier structure, it is nevertheless, not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0015]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The invention provides a fiber-based carrier structure for producing fiber-reinforced composite materials which, through the use of a defined mixture of finite randomly arranged fiber bundles and finite randomly arranged single fibers, is particularly suitable for absorbing liquids, melts and/or solid particles. The impregnability and the absorption capacity for liquid, also viscous, and/or solid pulverulent substances can be adjusted via the mixing ratio of fiber bundles:single fibers. The fiber carrier structure has a uniform structure in terms of length, width and thickness and is distinguished by an open-cell pore system that is openly accessible from the outside. 
         [0017]    The fiber carrier structure according to the invention does not consist of at least two different product zones of different compositions and orientations, as described in international patent disclosure WO 2012/072405 A1, or of a core and a cover layer of continuous fibers having a completely different structure, as described in international patent disclosure WO 2010/139077. 
         [0018]    The finite fiber bundles in the present invention preferably result from reinforcing fiber bundle pieces or multifilament yarn pieces which were originally continuous but have been reduced to finite lengths, in which the single fibers adhere to one another in parallel in a mechanically detachable manner over at least 50% of the length thereof by non-natural binding means. They can, however, also be fiber materials from recycling processes, if these are obtained in the form of bundles within the meaning of this invention. 
         [0019]    With this definition, the known fiber bundles of natural fibers such as flax, hemp, untreated cotton and kenaf are excluded. 
         [0020]    These long fibers in the fiber bundle which adhere to one another in parallel and in a mechanically detachable manner are substantially different from the fiber bundles mentioned in the literature, for example in the case of the intermeshing or needling of nonwovens, which are formed from single, isolated fibers over only a short length &lt;&lt;50% of the fiber lengths, for example when random fibers are brought together in an intermeshing hook or in the barbed hook in the case of needling. In contrast to the finite fiber bundles described in the present application, the fiber bundles described in the literature are held together only locally by external pressing forces or binding points. They form only during textile processing and are also referred to in the specialist literature as mechanical consolidation points, whereas the fiber bundles in the described invention are already present in the starting material, the fiber material, and are not formed purposively during the processing operation. 
         [0021]    The single fibers used in the mixture that is employed can consist of the same or a different polymer as the fiber bundles that are used. This special carrier structure formed of the two fiber systems, fiber bundles and single fibers, in defined uniform or different thickness and/or mass per unit area is stabilized mechanically, thermally and/or chemically and fixed in such a way that at least 10 and not more than 90% of the resulting, consolidated fiber carrier consists of fiber bundles having a minimum number of 10 single fibers that adhere to one another in parallel, and an open-cell pore system that is openly accessible over the entire structure is formed. The pore system contains a plurality of interconnected voids, which are interconnected by transport channels in order to be able to transport powders and/or liquids applied from the outside into the voids. 
         [0022]    It is provided according to the invention to produce the product from a defined mixture of single finite fibers and finite fiber bundles, to shape it to a surface or three-dimensionally and then to fix it. The proportion of fiber bundles used thereby determines to a significant degree the impregnability or the depth of penetration of highly viscous and pulverulent substances into the product layer. The proportion of single fibers thereby determines to a significant degree the penetrability of storable liquids or powders. The higher the proportion of fiber bundles in the structure, the higher the proportion of larger voids with macropores and partially continuous channels; the higher the proportion of single fibers in the structure, the higher the proportion of smaller voids. The voids are thereby interconnected by transport channels in order to be able to transport powders and/or liquids or melts applied from the outside into the voids. Surprisingly, it has been shown that the impregnability of such a structure with viscous liquids and powders changes in accordance with the proportions by mass of single fibers to fiber bundles. The higher the proportion of single fibers, the longer impregnation takes and the poorer the penetration of a powder into the structure. 
         [0023]    It is provided according to the invention that this effect is purposively developed and controlled via a purposively adjustable mixture of the two components. Therefore, in the case of the product according to the invention, defined proportions of finite fiber materials of the same or different types are processed in two forms, a bundle-like, non-unraveled form and a single-fiber form, in such a manner that, in a nonwoven-like mat or a three-dimensional molding, at least 10% but not more than 90% of the fibers used still remain in the form of non-unraveled bundles, and the further usability as a fiber pre-product for reinforcing plastics materials is thereby significantly determined. In a preferred embodiment, the fiber bundles and the single fibers are in a random arrangement without a defined orientation, as is achieved, for example, by a random loose filling material. 
         [0024]    Such mixtures can be produced in a defined manner on the one hand by gravimetric weighing of one or more fiber components in bundle form with one or more single-fiber fiber components and subsequent mixing, for example by a textile mixing technique. On the other hand it is possible, starting from a starting material that is &gt;90% in fiber-bundle form, to produce a ratio by mass between fiber bundles and single fibers that is adjustable according to opening intensity by subsequent processing operations such as textile fiber opening by openers, treatment by means of a mill (see published, non-prosecuted German patent application DE102009023641) with a teaser, picker or units that operate in a similar manner. The opening technique, number of passes and parameters to be used in conjunction with the nature of the starting material in fiber-bundle form are to be adapted to one another in test series in such a way that the desired residual proportion of bundles in the product is obtained. The main influencing parameters in the starting material are the fiber bundle length and the intensity with which the single fibers adhere in the fiber bundles that are provided. The mechanically detachable parallel adhesion of the single fibers over a length &gt;50% of the single fiber length in the bundles is significantly determined by the nature and amount of substances having an adhesive action that are foreign to fiber polymers and are found on the fiber surface, such as sizes and finishes. Uncross-linked and/or uncured polymers can also be used as binders in the bundles. The important criterion is that it must be relatively easy to separate the bundles mechanically. 
         [0025]    In this manner, it is technically possible to produce constant proportions of from 10% to 90% fiber bundles and the remainder as single fibers in a fiber mixture which can subsequently be built up into a constant or varying fiber layer of defined thickness and mass per unit area by mechanical and/or pneumatic processing operations. The fiber bundles are characterized in such a way that they consist of at least 10 mutually adhering parallel single fibers which adhere to one another over at least 50% of the length thereof. The fiber bundles can be mechanically separated into smaller bundles or single fibers relatively easily and without damaging the fibers. 
         [0026]    This mixture of fiber bundles and single fibers having a defined proportion of fiber bundles and single fibers that remains constant over the production time leads to a defined thickness and mass per unit area profile, it being possible for both the thickness and the mass per unit area to be developed uniformly or purposively differently over time and area during the formation of the loose fiber filling. 
         [0027]    Conventional fiber-processing units of the textiles field, such as openers, mixing chambers, filling hoppers, airlay and fiber-blowing systems, can be used for the production and processing of this fiber mixture of bundle-like and single-fiber components, but such units must be modified technically and technologically in such a way that the desired mixing ratio of bundle-like components and individual fibers is ensured. Such modifications include reducing the number of fiber-opening passes, dispensing with carding and roller carding in the processing operation, reducing opening roller speeds, using coarser roller coverings during mixing, homogenization and metering, and increasing the distances between operating units having an unraveling action. In contrast to the prior art, all the measures have the purpose of not unraveling or only unraveling to a lesser degree the fiber bundle components that have an advantageous effect in the end product. Therefore, in a preferred embodiment, carding and roller carding is not used in the processing operation. The nature of the modifications is dependent on the system technology employed, the fiber material used, and the desired proportion of fiber bundles in the end product. All the influencing factors are therefore to be matched to one another in tests, and the necessary system and technology modifications are to be made. 
         [0028]    Suitable units for laying the loose fiber layer of bundle-like and unraveled components are in principle mechanically and/or pneumatically operating units, such as filling hoppers, airlay or fiber-blowing systems. Here too, systems and methods must in particular be configured, via the above-mentioned measures, in such a way that, on the one hand, they act to homogenize the mixture and, on the other hand, the fiber-opening effect brought about by their fiber-opening/unraveling intensity is defined and only such that the desired target range of fiber bundle proportion and single fiber proportion is reached. If fiber-opening units are used to a certain degree, the unraveling action of those units must be taken into account through a higher proportion of fiber bundles in the starting material that is provided. This mixture of fiber bundles and single fibers is laid randomly. 
         [0029]    During the operations of mixing and homogenizing and of laying the fiber layer, pulverulent substances, thermally binding components or liquid binders that are not originally a constituent of the fibers and/or fiber bundles used can be introduced at the same time. These binding components are used to fix the pore system and the carrier structure of single fibers and fiber bundles and/or as a binding component in the formation of the fiber-reinforced composite materials. After the loose fiber layer of homogeneously distributed fiber bundles and single fibers in defined proportions has been formed, it is necessary to fix this special structure and render it pressure and traction-stable to handling stresses. For this purpose, it is possible to use mechanical methods such as needling or intermeshing or binder consolidations of loose layers containing binders or thermoplastics. 
         [0030]    The action of contact or radiation heat or the passage of hot air has a melting action or dry liquid binder components. The application of binders for fixing the pore system and the carrier structure of single fibers and fiber bundles and/or as a binding component in the formation of the fiber-reinforced composite materials after the formation of the loose layer in the form of powder application or spraying is likewise technically possible and is determined by the intended use of the consolidated layer. Here too, consolidation is generally carried out by a heat treatment that dries the binder or that effects melting or the start of melting. By these processes, the pore system purposively produced in the nonwoven pre-product is fixed. The open pore system of the nonwoven-like pre-product consists of small voids, which form in particular as gaps between the random, intersecting single fibers of very small diameter, and larger voids, which form as gaps between the random intersecting fiber bundles of substantially larger diameter. Depending on the proportion of fiber bundles in the single fibers, a correspondingly finer or coarser open-pore void system or pore system with differing impregnability and substance storage capacity is formed. In the subsequent further processing, these pores or voids, which are purposively adjustable according to the proportions of single fibers and fiber bundles, perform the function of binder transport, or binder infiltration of the nonwoven pre-product and of fixing the binder in the nonwoven pre-product. The coarser, open pore system forms transport channels for thick, viscous binding resins and powders, which channels extend into the center of the nonwoven layer. It is thus possible to substantially assist with a desired complete, continuous impregnation with viscous liquids and powders, which simplifies impregnation technology in terms of costs and technology, shortens impregnation times and makes it possible to use thicker reinforcing fiber pre-products. The finer pores based on the single fibers in the product ensure that the binder components that have penetrated are retained and incorporated in the product in the manner of a sponge. 
         [0031]    The proportion of fine and coarse pores is determined by the respective proportions of coarse fiber bundles and fine single fibers in the nonwoven. Depending on the thickness of the nonwoven, the impregnating medium to be used to form the consolidated carrier structure or the fiber-reinforced molding and the infiltration technology, the proportions of fiber bundles and single fibers are to be tested and specified in preliminary tests for the fiber material to be used in each particular case. By mechanical needling or a similar process, vertical puncture channels can be formed in addition to the existing pore structure, which channels assist with the transport of binder into the nonwoven layer and influence the function of the impregnability. By compressing the loose fiber layer, the penetrability of the nonwoven is generally reduced and the depot action is reduced. In the interplay of pressing processes for reducing the thickness of the nonwoven and related nonwoven consolidation, the impregnability that is produced is again purposively influenced and fixed in terms of the final quality and quantity thereof. 
         [0032]    The use of these fiber pre-products having defined fiber-bundle-like proportions is concentrated in the field of fiber composite materials. Accordingly, the fiber materials used reside in the field of conventional reinforcing fiber materials. They can be organic fiber materials such as para-aramids as well as glass and carbon fiber materials including fiber materials of this type from various recycling processes. 
       Examples 
     Practical Example 1 
       [0033]    Starting from a starting material based on mechanically prepared carbon fiber non-crimped fabrics having a high proportion of fiber bundles, loose fiber fillings having different proportions of fiber bundles were produced by different fiber opening intensities on a roller opener (material 1) and a carding machine 2 (material 2) and were built up to a uniform mass per unit area of 500 g/m 2  with a constant loose thickness. The carbon fibers used, which were from a mechanical recycling process, had a mean length of the fiber bundles of 45 mm. 
         [0034]    For both fiber materials, the proportion of fiber bundles, based on mass, was determined by manual screening, the air through-flow resistance was determined by an air through-flow method as a measure of the open-pore nature and accessibility to particles and liquids, and the drop sink-in time was determined by means of a drop test using a more highly viscous liquid in accordance with the TEGE-WA drop test as a measure of the impregnability. 
         [0035]    The following results were obtained: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Parameter 
                 Unit 
                 Material 1 
                 Material 2 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Proportion of fiber bundles* 
                 [%] 
                 80.5 
                 51.9 
               
               
                 Air through-flow resistance as a 
                 [mm 
                 377 
                 430 
               
               
                 measure of porosity** 
                 isopropanol] 
                   
                   
               
               
                 Sink-in time in the drop test*** 
                 [s] 
                 120-150 
                 280-310 
               
               
                   
               
             
          
         
       
     
         [0036]    The proportion of fiber bundles was determined by manual screening of a fiber sample of 1 g, weighing the bundle components from at least 10 single fibers and calculating the proportion by mass in percent. 
         [0037]    ** The air through-flow resistance was determined on the basis of a publication from 1964 by Geitel, K.: “Zur Theorie der Luftströmung durch Faserpropfen” [The Theory of the Flow of Air Through Fiber Pads], in “Faserforschung and Textiltechnik” [Fiber Research and Textile Engineering] 15 (1964) Volume 1, p. 21-29. The theory of the flow of air through fiber pads is described here. According to that publication, the pressure drop over an amount of fibers through which a medium flows is dependent on 
         [0000]    the amount of air that flows per unit time,
 
the dimensions of the measuring chamber (diameter, height),
 
the viscosity of the flowing medium,
 
the porosity of the fibers, and
 
the fiber surface area.
 
         [0038]    By means of a type 4/15/1 wool fineness tester from Medimpex (Hungary), the porosity of a fiber pad was determined by this air through-flow method. The fiber pad is in this case the test specimen, formed from the carrier structure produced according to the example. All the parameters apart from the fiber material to be tested were kept constant. The air resistance generated by the fiber pad is read off from the measuring instrument in [mm] of an isopropanol liquid column. The column height in [mm] is directly proportional to the air through-flow resistance that is established and thus indirectly proportional to the porosity. The tests were in each case carried out on 1.4 g of fiber material at an air through-flow speed of 400 l/min. 
         [0039]    In the modified drop test, a CMC solution having a viscosity (25° ° C.) at a shear gradient of 2/s of 1.7 Pas was used as the test liquid. The mass of the test drop was 0.5 g in all cases. 
       Practical Example 2 
       [0040]    Starting from a starting material based on mechanically prepared carbon fiber non-crimped fabrics and having a high proportion of fiber bundles, the starting material was intimately mixed with 7% of a thermally softening binding fiber GRILON MS 6.7 dtex/Varioschnitt via a mixing bed and 1 mixing pass by means of a coarse opener using a mixing/opening pin roller and supply of the material via a roller pair. Half of this constant starting fiber mixture of recycled carbon fibers having a high proportion of fiber bundles and thermally softening binding fibers in a mixing ratio of 93/7 was then laid via an FBK 536 feeder from Tru{umlaut over (t)}zschler at 2 m/min to form a loose filling of 370 g/m 2 . The other half of the starting material was processed as comparative material by carding twice with a roller card using three worker/stripper pairs at 10 m/min to form card web and placed in loose layers one above the other by means of crossplaiters so that a mass per unit area of 370 g/m 2  was obtained. 
         [0041]    The two loose fiber layers, feeder layer and carded and laid layer, were then partially consolidated to a mat by means of a Thermofix from Schott &amp; Meissner at a throughput speed of 2 m/min, a heating temperature of 190° C. and with a gap of 1.5 mm, in which the two fiber layers are guided in succession through the thermal consolidation system between an upper and a lower transport belt, and the void and pore structures formed were fixed. 
         [0042]    The water absorption according to DIN 53923 was then carried out on both nonwoven mats as a measure of the storage capacity for liquids. Before thermal consolidation, the proportion by mass of fiber bundles in the two loose fiber layers was determined. 
         [0043]    The following results were obtained: 
         [0000]    
       
         
               
               
             
               
               
               
             
               
             
               
               
               
             
           
               
                   
               
               
                   
                 Type of nonwoven mat 
               
             
          
           
               
                 Sample 
                 Laid by feeder 
                 Laid by carding machine 
               
             
          
           
               
                 Water absorption according to DIN 53923 
               
               
                   
               
             
          
           
               
                 1 
                 1202% 
                 929%  
               
               
                 2 
                 1069% 
                 930%  
               
               
                 3 
                 1097% 
                 866%  
               
               
                 4 
                 1065% 
                 846%  
               
               
                 5 
                 1097% 
                 902%  
               
               
                 Mean 
                 1106% 
                 895%  
               
               
                 Proportion of  
                  85% 
                 6.6% 
               
               
                 fiber bundles 
               
               
                   
               
             
          
         
       
     
         [0044]    Whereas a high proportion of fiber bundles was used in the first fiber-based carrier structure according to the invention (ratio proportion of fiber bundles to single fibers approximately 5.66:1), the proportion of single fibers is comparatively high in the case of the nonwoven mat laid by carding machine that was used as comparative material (on the right in the above table) (ratio proportion of single fibers to fiber bundles approximately 14.15:1). The above table shows that the water absorption is substantially higher in the case of the carrier material according to the invention than in the case of the comparative material.