ABSORBENT STRUCTURE HAVING A LATEX SKIN

Abstract: An absorbent structure is provided which comprises a batt of lightly compacted cellulosic fibers which batt posses­ ses a thin skin of a film former on one surface thereof.

Description: ABSORBENT STRUCTURE HAVING A FILM-FORMING SKIN Background of the Invention The present invention relates to an absorbent structure containing loosely compacted cellulosic fibers, which structure is particularly suitable for use in absorbent products requiring a substantial liquid holding capacity. Absorbent structures such as disposable diapers, sanitary napkins, incontinence pads, and the like, are generally structured so as to have a facing sheet which is moisturepermeable, an absorbent batt which has a high liquid holding capacity, and a backing sheet which is moistureimpermeable. For any absorbent structure to be satisfactory, it is highly desirable for the structure to (1) readily accept liquid, (2) easily transport the liquid from one portion of the structure to another, (3) hold the liquid accepted, and (4) retain its original configuration even when wet. The facing sheet mentioned above must permit the liquid to penetrate the facing to reach the absorbent batt. The backing sheet keeps the liquid fron leaking and, therefore, must be moisture-impermeable. The present invention relates to an improvement in the absorbent batt used for these absorbent structures. Conventionally loosely compacted cellulosic fibrous batts are of substantially rectangular shape and are made by air-laying wood pulp fibers, cotton linters, or the like on a foraminous support. The batts may be laid in a prescribed shape or may be shaped subsequent to the airlaying of the fibers. These batts are incorporated by various techniques into products such as disposable diapers, sanitary napkins, incontinence pads, and the like. Various techniques have been developed or suggested for reinforcing an absorbent batt to improve the retention of structural integrity. For instance, it has been suggested that scrim be used intermediate in the absorbent batt to improve the structural integrity. Other attempts involve U.S. Patent 3,017,304 to Burgeni who teaches forming a paper-like densified skin on one surface of the absorbent batt. The skin assists in transporting the liquid and lends integrity to the batt. In another instance, U.S. Patent 3,938,522 to Repke suggests densifying portions of the batt to both assist in transporting of liquid and strengthening the absorbent batt. U.S. Patent 4,233,345 to Elias et al sprays a solution of a coagulating material for a resin on one side of the batt. The resin is then dispersed on the pretreated surface providing rapid coagulation of the resin. Still other suggestions have involved wrapping the absorbent batt with tissue or anchoring the absorbent batt to the facing and for the backing when the absorbent batt is used in a diaper-type product. Although these methods assisted in providing some integrity to absorbent structures, the absorbent structures still suffered from break up during shipping and use. The present invention provides an absorbent structure which readily accepts liquid, easily transports the liquid from one portion of the structure to another, holds the liquid accepted and substantially retains its original configuration. Summary of the Present Invention It has been discovered that a flexible absorbent batt having a high degree of integrity and contiguity can be made from loosely compacted cellulosic fibers, such as wood pulp fibers, which will readily accept and wick liquid while retaining a high liquid-holding capacity. The present invention provides an absorbent structure comprising an absorbent batt, substantially rectangular in shape, of loosely compacted cellulosic fibers having a thin film-forming skin on one surface of the batt. The new absorbent structure is made by air-laying cellulosic fibers to form a batt, applying a film-forming emulsion or suspension to at least a substantial portion of one surface of the batt and lightly pressing the batt. Suitable film forming compositions include cross-linking latexes which crosslink at room temperature, roomtemperature film formers, such as polyvinyl acetate, which form substantially water-insoluble films and the like. Brief Description of the Drawings Figure 1 is a perspective view of an absorbent structure of the present invention Figure 2 is a cross-sectional view of the absorbent structure of Figure 1 taken along line 2-2; Figure 3 is a top view illustrating a particular configuration of another embodiment of the present invention; Figure 4 is a top view illustrating still another particular configuration of an embodiment of the present invention; Figure 5 is a perspective view with certain portions broken away for clarity of illustration of one embodiment of a diaper of the present invention; Figure 6 is a perspective view of one embodiment of a sanitary protection product of the present invention; and Figure 7 is an enlarged cross-sectional view taken along line 7-7 of Figure 6. Detailed Description of the Present Invention Tensile strength and structural integrity have always been desired in cellulosic fibrous batts. Fibrous butts are commonly formed of loosely compacted, cellulosic fibers which are primarily held together by inter fiber bonds requiring no added adhesive. However, when using the absorbent batt, integrity is important to prevent separation and breakage of the absorbent batt. One method is disclosed in U.S. Patent 3,017,304 wherein a "paperlike" skin is formed on one surface of an absorbent batt by applying moisture and then lightly compacting the cellulosic batt. Another method of densifying a selected area of fibers is found in U.S. Patent 3,938,522 wherein densified regions are provided. Still another method of forming the paper-like skin is found in U.S. Patent 4,233,345 wherein a coagulating agent is applied to one surface of the batt and, subsequently, a resin-forming substance is applied on top of the coagulating agent. An in situ reaction takes place providing a thin-skinned gel which, upon drying, provides a thin densified area lending some stability to the absorbent batt. The present invention, unlike the densified paper skin or thin gel skin, provides a thin layer or pattern of a substantially water-insoluble film on one surface of the absorbent batt. The film retains its continuity providing contiguity and structural integrity to the absorbent batt. It is surprising to observe that the film-former applied to one surface of the batt markedly improves the tensile strength without adversely affecting the liquid holding capacity or the wicking ability of the absorbent structure. Furthermore, the structure is still very flexible and has superior wet strength. Wet strength in an absorbent product like a disposable diaper, is highly desirable so that on the initial wetting the batt does not break apart. It is theorized that the three dimensional film-forming of the substantially waterinsoluble film may provide this high degree of tensile strength and wet strength without adversely affecting the other desirable products of the absorbent batt. The absorbent structure of the present invention is a fibrous batt formed of loosely compacted cellulosic fibers. When the film-former is a latex, preferably, it is a room temperature, crosslinking latex which is applied as a thin layer or in a continuous pattern as desired on one surface of the cellulosic fibrous batt. The batt is then subjected to slight compression so that the latex comes in contact with the fibers on the surface so treated. Although the density of the absorbent batt increases in the latex region, penetration of the latex is minimal so that only the immediate surface area is affected. The same is true for other suitable film formers. In Figure 1, an absorbent batt 10 is provided. The latex surface 14 represents a very small portion of the overall depth of the absorbent batt 10. The remainder of the batt 12 remains in loosely compacted form. Figure 2 is a cross-sectional view taken along lines 2-2 of Figure 1 providing an absorbent batt 20 having a fluffed, loosely compacted fibrous region 22 and a thin region of latex 24. Figure 3 illustrates a top view of a typical configuration of the absorbent structure of the present Invention. The absorbent batt 30 is reduced in width at the central portion 32 to provide a shaped, absorbent batt. To lend further to the integrity of the batt, embossed lines 34 are provided. The embossed lines 34 will assist in enhancing the wicking of liquid from one region to another of the absorbent batt 30. Figure 4 illustrates still another shaped absorbent batt 40 whvrvin the central portion 42 is diminished in size to provide an hour-glass shape. Figure 5 illustrates a typical disposable diaper structure 50 using the absorbent batt 54 of the present invention A moisture-Impermeable facing 56 with the absorbent batt 54 held in place between the backing and the facing. The absorbent batt has a fluffed loosely compacted portion 58 and a thin densified portion 60 having been treated with latex. Embossing lines 62 are provided to enhance the wicking in the diaper structure. The batt 54 is held in place and the facing 56 and backing 52 are laminated by glue lines 66. Tape tabs 64 are provided for securing the diaper about the waist of the infant. Figure 6 depicts a sanitary napkin 60 containing a thin absorbent batt of the present invention. The napkin 60 has a nonwoven fabric cover 62 and an adhesive strip which is made available by removing a release strip 64. A cross-sectional view of the napkin 6 is shown in Figure 7 wherein the napkin section 70 contains the cover 72, an adhesive strip 76 and a release strip 74. A liquidimpermeable barrier 75 is placed on the underside between the absorbent batt 71 and the cover 72. The absorbent batt 71 has a densified portion 73 at least a part of which is a polyvinyl acetate film. The absorbent batts of the present invention are prepared by forming a substantially uniform batt of air-laid loosely compacted cellulosic fibers. The fibers generally are wood pulp fibers which have been subjected to conventional chemical processing. Typically, chemically processed wood pulp fibers appear in ribbonw e form and, upon air-laying, become entangled to provide a degree of cohesiveness in fibrous web integrity. Non-delignified wood pulp fibers are also suitable and may have an additive present to enhance the cohesiveness of the fibers to form a reasonably stable absorbent batt. In accordance with the present invention, whether the batt is made from chemically processed fibers or nondelignified wood pulp fibers, the batt is provided with one surface area effectively treated with a thin layer of a film former. The layer may provide a continuous film or may be applied in a predetermined continuous pattern. At least 35 percent of the surface is treated with the thin layer of film-former and preferably the entire surface area is so treated. The layer is generally located on the planar surface away from the point of liquid entry when the product is in use. The air-laid, loosely compacted absorbent batt is subjected to a small degree of compaction so that the film-former polymer emulsion applied to the surface penetrates the surface slightly. Suitable latexes include the latexes which crosslink at room temperature. Typical of such latexes are those mentioned in U.S. Patent 3,678,098 to Lewis et al. Other suitable film formers include polyvinyl acetate and film formers which when applied as a suspension, dispersion or emulsion form a substantially water-insoluble film at room temperature. The film-former is applied after the loosely compacted cellulosic fibrous batt is formed. However, the final shape of the batt need not be determined at this point as further cutting or shaping of the batt may be made at a later time. The film-former at room temperature is applied to one surface of the batt in either a continuous form or in a continuous predetermined pattern. The filmformer should cover at least about 35 percent of the surface, but preferably 90 percent or more of the surface. After it is applied, the film forms at room temperature and the film increases in molecular weight building toward water insolubility and the thin, stabilized area of the batt is formed. The absorbent batts of the present invention are particularly useful in disposable products such as disposable diapers, sanitary napkins, incontinence pads and the like. Each of the disposable products usually contains a moisture-impermeable backing sheet, an absorbent batt in superposed position and a moisture-permeable facing or cover in superposed position on the side of the batt opposite the backing sheet. The disposable product, when ideally constructed, permits a void of liquid onto the facing to be rapidly drawn through the facing into the absorbent batt for storage. Leakage, when the batt becomes substantially wetted, is prevented by the backing sheet. However, if the absorbent batt has lost its integrity and in fact has separations present or breaks, there is a loss of ability to transport liquid to the extremities of the batt, thus liquid will have a tendency to leak out the side edges of the disposable product. Therefore, the provision of the batt with high tensile strength in accordance with the present invention, provides a suitable absorbent structure to place in such a disposable product. One advantage is that when the absorbent structure becomes wet, the film still retains the contiguity of the overall structure and prevents break up. Also, because the product retains its shape, sagging and destruction of the product's aesthetics is avoided. The following examples illustrat specific embodiments of the present invention and are not limiting in any way. Example I A batt of chemically delignified wood pulp fibers is formed by using a hammermill and light compression. The batt is rectangular, about 1/2 inch thick and weighs about 52 grams. A paper-like densified skin is formed on one side (see U.S. Patent 3,017,304) and five embossed lines running most of the length of the batt are placed substantially equidistant from one another. The batt just described is identified as "The Control Batt. Sample A is a batt having the characteristics of The Control Batt except that on the surface possessing the paper-like densified skin, a latex emulsion is applied in the amount of 1.75 gm by spraying over the batt surface. The sprayed emulsion is a six percent latex emulsion solution identified as E-1179N produced and sold by Rohm and Haas Corporation. The batts are tested for dry density, liquid-holding capacity, wet and dry t strength and batt stability. Results of the test appear in Table I below: Table I EMI10.1 <tb> <SEP> Liquid <SEP> Tensile <SEP> Batt <tb> <SEP> Dry <SEP> Holding <SEP> Str <SEP> ngth <SEP> Stability <tb> <SEP> Density <SEP> Capacity <SEP> Dry <SEP> Wet <SEP> Exposed <SEP> Weight <tb> Sample <SEP> gm/cc <SEP> ml/gm <SEP> <SEP> gm/in <SEP> gm/in <SEP> Film(%) <SEP> <SEP> Lossegm) <SEP> <tb> Control <SEP> .102 <SEP> 5.9 <SEP> 128 <SEP> 32 <SEP> 14.0 <SEP> 18.0 <tb> Sample <SEP> A <SEP> .103 <SEP> 5.9 <SEP> 321 <SEP> 57 <SEP> 0.3 <SEP> 2.5 <tb> It should be noted that the liquid holding capacity and the density of the batts are substantially equal. However, the tensile strength, both wet and dry, are substantially improved when a thin-coating resin is used on the absorbent batt described above. Thus, the objects of the invention are accomplished without loss of the liquid-holding capacity or markedly affectng the product density. Example I T Thin sanitary napkins are prepared. Pads having a length of about 7.5 inches, a width of about 2.5 inches and a thickness of about 0.25 inch are enveloped in a nonwoven cover and provided with an impervious barrier consisting of a sheet of polyethylene having a thickness of l mil. The impervious barrier is between the pad and the cover on the underside of the product. A hot melt adhesive is applied in a single longitudinal hand on the garment facing surface of the product. The overall length of the product is 9 inches. The absorbent material of the pad consists of 3.8 grams of loosely compacted wood pulp fibers. Additional napkins are prepared in substantially the same manner as above except that the pad is treated on one side with an emulsion of a film former. An atomizer nozzle delivers a water/ polyvinyl acetate emulsion to one side of the pad to form a film on the pad. The pad is then subjected to a compression force of about 20 psi to form a thin densified layer on the side where the film is forming. In Table II below, different amounts of polyvinyl acetate (PVA) are delivered to the surface of the pad. In each instance an aqueous emulsion containing 50 percent PVA solids by weight is used in the increments shown with an amount of water added so as to keep the overall moisture content substantially even. For instance, the grams of water used in each sample equal 0.54. The theoretical moisture is calculated and does not vary significantly. The tensile strength is obtained on each sample. The cover and liquid barrier are removed and the pad is gripped at each end by an Instron instrument. The amount of force necessary to separate the pad into two pieces is expressed in grams per inch of the pad length tested. The compression which the napkin will withstand is expressed in pounds per square inch. In this test a sample of the product has the protective strip removed and is rolled into a cylinder by overlapping the transverse ends of the product to form a hollow cylinder. The product is maintained in this position by adhering the overlapped ends using the pressure sensitive adhesive element. The cylinder is then compressed in an Instron Tester at a crosshead speed of 10 inches per minute. The Tester is provided with a recorder adapted to provide a stress/strain curve. Crush resistance is the force measurement recorded at the first inflection point of the stress/strain curve. The results are set forth in Table II below: Table II PVA 508 Tensile Compres Sample Solids Water Theoretical Strength sion No. g/pad g/pad Moisture % g/in psi B 1.00 0.00 8.8 692 3.63 C 0.80 0.14 9.3 667 4.37 D 0.72 0.18 9.3 641 3.86 E 0.60 0.24 9.4 632 3*96 F 0.50 0.29 9.4 670 4.53 G 0;42 0.33 9.5 614 4¯03 H 0.36 0.36 9.5 396 2.99 I 0.24 0.42 9.6 440 2.96 J 0.18 0.45 9.6 277 2.51 K 0.00 0.54 9.7 44 2.08 It is apparent from the above data that the tensile strength is markedly improved with the addition of the polyvinyl acetate. The improvement escalates rapidly up to the addition of 0.42 grams of the 50 percent PVA emulsion. Additional amounts of PVA do not seem to add substantially to the tensile strength. Substantially, the same phenomenon is observed about the compression value. One might expect the addition of a film former to the surface of an absorbent batt would tend to stiffen and/or render the surface harder. Surprisingly, the pads treated with PVA feel softer than the control sample K. It will be understood by those skilled in the art that variations and modifications of the specific embodirnents described above may be employed without departing from the scope of the invention as defined in the appended claims.

Claims: Claims: 1. An absorbent structure comprising an absorbent batt substantially of rectangular shape, said batt being comprised of loosely-compacted cellulosic fibers having a thin skin formed on one surface of the batt, said skin being a film former which forms a film at room temperature and is substantially water-insoluble. 2. The absorbent structure of Claim l wherein said skin is continuous and covers at least about 90 percent of one surface of the batt. 3. The absorbent structure of Claim 1 wherein said skin is a continuous predetermined pattern and covers at least about 35 percent of one surface of the batt. 4. The absorbent structure of any one of Claims 1 to 3 wherein Ue dry tensile stretgth is at least about 225 gmfln. 5. The absorbent structure of any one of Claims 1 to 4 wherein the wet tensile strength of the absorbent structure is at least about 50 gm/in. 6. The absorbent structure of any one of Claims 1 to 5 Wherein said-filni former is a crosslinking latex which crosslinks at room temperature. 7. The absorbent structure of of any one of Claims lt to 6 wherein said film former is polyvinyl acetate. 8. A disposable diaper having a moisture-permeable facing sheet defining a diaper inside surface for direction toward the wearer, a moisture-impermeable sheet substantially coextensive with said facing sheet and defining a diaper outside surface, and an absorbent batt according to any one of claims 1 to 7 positioned between said facing sheet and said backing sheet with the thin skin in contact with said backing sheet. 9. A sanitary napkin having a moisture-permeable cover, a moisture-impermeable liquid barrier and an absorbent batt according to any one of claims 1 to 7 positioned with the thin skin in contact with said liquid barrier, said liquid barrier being between said cover and said batt on at least one side of said batt.

Inventors: HOLTMAN, DENNIS C.
Applicants: PERSONAL PRODUCTS COMPANY
Application year: 1984
Main classification: A61L15
IPC codes: A61F13, B32B27