SORBENT SHEET PRODUCT

Abstract: A sorbent sheet product, particularly useful for surgical swabs and wound dressings. The sorbent sheet product comprises a coherent fibrous web that includes high sorbency, liquid sorbent fibers and liquid conductive transport fibers intermingled with thermoplastic binder fibers. The web may be intermittently thermally bonded.

Description: SORBENT SHEET PRODUCT Background of the Invention Field of the Invention The present invention relates to sorbent sheet products which provide rapid sorption, good wicking, and high liquid retention. Such products are particularly useful in wound dressings and surgical swabs» Description of the Prior Art A sorbent sheet product having high sorbency, high conformability, low density, and good wicking, and which is non-linting and economical has long been desired for use in wound dressings and surgical swabs. Gauzes are today the most common wound dressing, but gauzes are expensive to make and exhibit some linting which is undesirable in a wound. Many attempts have been made to find a substitute for gauzes, but until now, no fully satisfactory substitute has been found. One effort in sorbent products has involved incorporating sorbent additives to a fibrous web. U.S. Patent No. 3,670,731 discloses the addition of hydrocolloidal particles to a fibrous mass, such as wood pulp, by cascading the particles into the fibrous mass. However, mechanical action causes the particles to dust out. U.S. Patent No. 4,235,237 discloses hydro¬ colloidal particles adhered to a fibrous web. This is achieved by contacting a fibrous web with hydro¬ colloidal sorbent particles dispersed in a volatile liquid, then vaporizing the liquid. Disadvantages of this approach include the need for multiple processing steps, limitations on the amount of particles that can be added to the web, brittleness of the dried web, and the tendency for particles to concentrate at the web surface. U.S. Patent No. 4,429,001 discloses the use of high sorbency particles in melt blown fiber webs. The melt blown fibers hold the particles to a large extent, but some dusting out of the particles may occur with mechanical action on the web. European Patent Application No. 0,080,382 also discloses sorbent particles in melt blown fiber webs. U.S. Patent No. 4,426,417 discloses a nonwoven fiber matrix which includes melt blown fibers and a staple fiber mixture of synthetic and cotton fibers. However, this product sorbs only ' a limited amount of fluid and retention of fluid when the web is under pressure is low. British Patent Application No. 2,006,614 A discloses a melt blown fiber web treated with a wetting agent. This product also has disadvantages in that only a limited amount of fluid is absorbed and retention of fluid when the web is under pressure is limited. U.S. Patent No. 4,100,324 discloses the use of wood pulp in blown fiber webs. This product would only absorb a limited amount of fluid and retention under pressure would also be limited. U.S. Patents No. 3,950,219, No. 4,082,886, and No. 4,219,024 disclose absorbent material containing wood pulp, treated wood pulp, and/or synthetic wood pulp which are heat fused with or without the use of plastic material. Similarly, ~ ~ these products would only absorb a limited amount of liquid and retention under pressure would be limited. U.S. Patent No. 4,160,059 discloses a product of thermally bonded supporting fibers, heat fusible fibers, and an absorptive material. This product would be expected to lint significantly at low concentrations of heat-fusible fiber and lo bo undesirably stiff at high concentrations of heat fusible fiber. _,.S. Patent No. 4,275,105 discloses a rayon web having a thermoplastic web melted into the bottom thereof to a depth of about 10 to 40 percent. U.S. Patent No. 4,195,112 discloses point bonded molded articles containing at least 20% potentially adhesive, thermally bonded fibers. Sorbent products are not disclosed. Summary of the Invention The present invention provides a sorbeht sheet product which exhibits no dusting out of sorbent material, is highly conformable, i.e., has a flexible hand, and has high liquid sorbency with excellent liquid retention. The sorbent sheet product is also low in density, has good wicking properties, and is economical to manufacture. Briefly, this new sheet product which is suitable for use as a wound dressing or surgical swab comprises a coherent fibrous web that includes binder fibers, high sorbency, liquid sorbent staple fibers and liquid conductive transport fibers. The latter fibers are intermingled with the binder fibers and uniformly dispersed and physically held within the web, and the web is intermittently thermally bonded to increase sheet integrity. Suitable binder fibers include entangled blown fibers and thermally bondable staple fibers. The high-sorbency, liquid sorbent staple fibers swell upon sorption of liquid. The liquid conductive transport fibers are fibers capable of rapidly wicking liquid. Sorbent sheet products of the invention generally contain 25 to 70 weight percent, more preferably 52 to 63 weight percent, binder fiber, 10 to 25 weight percent, more preferably 12 to 18 weight percent, sorbent fiber, and 20 to 50 weight percent, more preferably 25 to 30 weight percent, transport fiber. Where fibers such as the binder fiber are particularly stiff, the binder fiber content is preferably proportionately reduced to as low as 20 to 25 weight percent of the sorbent sheet product. Additionally, the web may contain other constituents such as wetting agents and resin binder. When blown fibers are used as the binder fibers, they may be prepared by extruding liquid fiber-forming material into a high-velocity gaseous stream, where the extruded material is attenuated and drawn into fibers. A stream of fibers is formed, which is collected, e.g., on a screen disposed in the stream, as an entangled coherent mass. According to the invention, high-sorbency, liquid sorbent staple fibers may be . introduced into the stream of melt blown fibers, e.g., in the manner taught in U.S. Patent No. 4,118,531, which is incorporated herein by reference, and the mixture of melt blown fibers and staple fibers is collected as an entangled coherent mass in which the high sorbency, liquid sorbent staple fibers are entrapped or otherwise physically held. A sheet product of the invention containing blown fibers, high sorbency, liquid sorbent staple fibers and liquid conductive transport fibers is integral and handleable both before and after immersion in liquid, because the collected blown fibers are extensively tangled or snarled and form a coherent web. Also, the web may be preferably intermittently thermally bonded to add additional strength and coherency to the web and to reduce surface fuzz. The high-sorbency, liquid sorbent staple fibers and the liquid conductive transport fibers are lastingly held and retained within this web. When thermally bondable staple fibers are used as the binder fibers, a fibrous web is formed from the thermally bondable staple fibers, the high sorbency,. liquid sorbent staple fibers and the liquid conductive transport fibers by carding or by a standard air layering process. The resulting coherent but unbonded web is a collection of fibers interlaced into a web wherein the fibers .cross over and under each other so as to be held together in three dimensions by mechanical and frictional forces. The web is then preferably intermittently thermally bonded to add additional strength and coherency to the web, forming another embodiment of the sorbent sheet product of the invention. A sheet product of the invention containing intermittently thermally bonded thermally bondable staple fibers, high sorbency, liquid sorbent staple fibers and liquid conductive transport fibers is -integral and handlea le both before and after immersion in liquid, because of the thermal bonds formed between the fibers. Large quantities of liquid can be sorbed at a rapid rate, with the amount dependent principally on the sorption capacity of the sorbent staple fibers and the quantity of sorbent staple fibers contained within the web. Liquid is rapidly sorbed by portions of the sorbent staple fibers located in even the inner parts of the web, due to the sorbent staple fibers being held apart by the web structure. Also, the liquid conductive transport fibers and portions of the sorbent staple fibers located at the exterior of the web conduct the liquid to portions of the sorbent staple fibers located in the interior portion of the web. The binder fibers of the web are preferably wet by the liquid being sorbed, e.g., as a result of use of a fiber-forming material that is wet by the liquid or by addition of a surfactant during the web-forming process, which further assists sorption. The sorbent staple fibers swell and expand in size during sorption, and although the binder fibers are extensively entangled and the web is intermittently thermally bonded, the web of fibers expands as the sorbent staple fibers expand and the sorbed liquid tends to be retained in the product even when the product is subjected to pressure. The sorbent staple fibers and the transport fibers also serve to separate the binder fibers, especially when in crimped form, producing a less dense web with greater potential for expansion on sorption of liquid. On sorption of liquid, the sorbent staple fibers and the transport fibers allow the binder fibers to slip and move in unbonded areas to a degree that the fibrous web is pushed apart by the swelling sorbent staple fibers while the web integrity is maintained. The sorbent sheet product of the invention is low in density, soft, and highly conformable.• The sorbent sheet product has excellent wicking properties due to the presence of the liquid conductive transport fibers and is non-linting because all fibers used in the product are at least of staple length. Brief Description of the Drawings FIG. 1 is a schematic diagram of the apparatus used in practicing the present invention when the binder fiber is melt blown fiber. FIG. 2 is a cross-sectional view of a sorbent sheet product of the invention. FIG. 3 is a cross-sectional view of a preferred embodiment of the sorbent sheet product of the invention. FIG. 4 is a top view of another embodiment of the sorbent sheet product of the invention. FIG. 5 is a cross-sectional view of another embodiment of the sorbent sheet product of the invention. FIG. 6 is a cross-sectional view of a wound dressing incorporating the sorbent sheet product of the invention. Detailed Description A representative apparatus useful for preparing sheet product of the invention containing blown fibers, high sorbency, liquid sorbent fibers and liquid conductive transport fibers is shown schematically in Figure 1. The apparatus is generally similar to that taught in U.S. Patent No. 4,118,531 for preparing a web of melt-blown fibers and crimped bulking fibers . This apparatus prepares webs with melt-blown fibers (prepared by extruding molten fiber-forming material and which are preferred in many webs of the invention), but solution-blown and other types of fibers may also be used. The fiber-blowing portion of the illustrated apparatus can be a conventional structure as taught, for example, in Wente, Van A. "Superfine Thermoplastic Fibers", in Industrial Engin¬ eering Chemistry, Vol. 48, pages 1342 et seq (19-56), or in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled "Manufacture of Super¬ fine Organic Fibers" by Wente, Van A.; Boone, C. D.; and Fluharty, E. L. Such a structure includes a die 10 which has an extrusion chamber 11 through which liquified fiber-forming material is advanced; die orifices 12 arranged i line across the forward end of the die and through which the fiber-forming material is extruded; and cooperating gas orifices 13 through which a gas, typically heated air, is forced at very high velocity. The high-velocity gaseous stream draws, out and attenuates the extruded fiber-forming material, whereupon the fiber-forming material solidifies as fibers during travel to a collector 14. The collector 14 is typically a finely perforated screen, which in this case is in the form of a closed-loop belt, but which can take alternative forms, such as a flat screen or a drum or cylinder. Gas-withdrawal apparatus may be positioned behind the screen to assist in deposi¬ tion of. fibers and removal of gas. Alternatively, two dies may be used and arranged so that the streams of melt blown fibers issuing from them intersect to form one stream that continues to a collector 14. The apparatus shown in Fig. 1 also includes means for introducing the sorbent staple fibers and the transport fibers into the sheet product of the invention. The sorbent staple fibers and the transport fibers are introduced into the stream of melt blown fibers through the use of a lickerin roll 16. A web 17 of sorbent staple fibers and transport fibers, typically a loose, nonwoven web such as prepared on a garnet machine or "Rando-Webber" , is propelled along a table 18 under a drive roll 19 where the leading edge engages against the lickerin roll 16. The lickerin roll 16 turns in the direction of the arrow and picks the sorbent staple fibers and transport fibers from the leading edge of the web 17, dissociating the sorbent staple fibers and transport fibers from one another. The picked sorbent staple fibers and transport fibers are conveyed in an air stream to inclined duct 20 and into the stream of melt blown fibers where the sorbent staple fibers become mixed with the melt blown fibers. The air stream is generated inherently by rotation of the lickerin roll, or that air stream may be augmented by use of an auxilliary fan or blower operating through a duct 21 as is known in the art. The mixed stream of melt blown fibers, sorbent staple fibers, and transport fibers then continues to the collector 14 where a web 22 of randomly intermixed and intertangled melt blown fibers, sorbent staple fibers, and transport fibers is formed. Accordingly, web 22, shown in FIG. 2, contains blown fibers 31, sorbent staple fibers 32, and transport fibers 33. A spray jet 25 may be used to apply materials, such as wetting agents, to the mixed stream of blown fibers, sorbent staple fibers prior to collection at collector 14. Melt-blown fibers are greatly preferred blown fibers for sheet products of the invention, but solution-blown fibers in which the fiber-forming material is made liquid by inclusion of a volatile solvent can also be used. U.S. Patent No. 4,011,067 describes useful apparatus and procedures for preparing a web of such fibers; however, in preparing sheet products of this invention fiber-forming material is generally extruded through a plurality of adjacent orifices rather than the single orifice shown in the patent. The sorbent staple fibers and transport fibers are preferably introduced into the fiber stream at a point where the blown fibers have solidified sufficiently that the blown fibers will form only a point contact with the sorbent staple fibers and transport fibers (as taught in U.S. Patent No. 3,971,373). However, the sorbent staple fibers can be mixed with the melt blown fibers under conditions that will produce an area contact with the sorbent staple fibers and transport fibers . Once the sorbent staple fibers and transport fibers have been intercepted in the blown fiber stream, the process for making the sheet product of the invention is generally the same as the process for making other blown fiber webs; and the collectors, methods of col¬ lecting, and methods of handling collected webs are generally the same as those for making known blown fiber ' webs. The layer of melt blown fibers, sorbent staple fibers, and transport fibers -formed in any • one revolution of the collection screen, and a completed sheet product of the invention may vary widely in thickness. For most uses of sheet products of the invention, a thickness between about 0.1 and 1.0 centimeters is preferred. More preferably the thickness is between about 0.15 and 0.45 centimeters for wound dressings. For some applications, two or more separately formed sheet products of the invention may be assembled as one thicker sheet product. Also, sheet products of the invention may be prepared by depositing the stream of blown fibers, sorbent staple fibers, and transport fibers onto another sheet material such as a porous nonwoven web which is to form part of the eventual sheet product. Other structures, such as impermeable films, can be laminated to a sheet product of the invention through mechanical engagement, heat bonding, or adhesives. The blown fibers are preferably microfibers, averaging less than about 10 micrometers in diameter, since such fibers offer more points of contact with the sorbent staple fibers per unit volume of microfibers. Very small fibers, averaging less than 5 or even 1 micrometer in diameter, may be used. Solution-blown fibers have the advantage that they may be made in very fine diameters, including less than one micrometer. Larger fibers, e.g., averaging 25 micrometers or more in diameter, may also be prepared, especially by .the melt-blowing process. Blown fibrous webs are characterized by an extreme entanglement of the fibers, which provides coherency and strength to a web and also adapts the web to contain and retain the sorbent staple fibers and liquid transport fibers. The aspect ratio (ratio of length to diameter) of blown fibers approaches infinity, though the fibers have been reported to be discontinuous. The fibers are long and entangled sufficiently that it is generally impossible to remove one complete fiber from the mass of fibers or to trace one fiber from beginning to end. Despite such entanglement, a sheet product will expand greatly in size during sorption. The fibers may be formed from a wide variety of fiber-forming materials. Representative polymers for forming melt-blown fibers include polypropylene, polyethylene, polyethylene terephthalate, and polyamides. Representative polymers for forming solution-blown fibers include polymers or copolymers of vinyl acetate, vinyl chloride, and vinylidene chloride. Inorganic materials also form useful fibers. Fibers of different fiber-forming materials may be used in the same sheet product in some embodiments of the invention, either in mixture in one layer or in different layers. Many of the fiber-forming materials form hydrophobic fibers, which can be undesirable in water sorbing sheet products. To improve the sheet product for such a use, a surfactant may be introduced into the sheet product, as by spraying liquids onto the web after it is formed. Useful surfactants, which typically comprise molecules having oleophilic and hydrophilic moieties, include sodium alkyl ester sulfonate, octylphonoxypolyethoxyethanol, and alkylaryl polyether alcohol. A small amount of the surfactant, such as 0.05 to 1 weight percent of the sheet product, will generally provide adequate hydrophilicity, but larger amounts can be used. Use of oleophilic blown fibers together with water sorbent staple fibers can have the advantage of dual absorption, in that the blown fiber portion of the fibrous web sorbs organic liquids, such as oils, while the sorbent staple fibers sorb water. For another embodiment of the invention, using a fiber mixture of thermally bondable staple fibers, high sorbency, liquid sorbent staple fibers, and liquid conductive staple fibers, any conventional carding or airlaying process, such as with a "Rando-Webber" sold by Rando Machine Corporation, Macedon, New York, may be used to form the web. Such a web 36, shown in FIG. 3, contains thermally bondable staple fibers 37, sorbent staple fibers 38, and transport fibers 39. Preferably, the thickness of the web is between about 0.1 and 1.0 centimeters, more preferably between about 0.15 and 0.45 centimeters. For some applications, two or more separately formed sheet products may be assembled as one thicker sheet product. Other structures such as films can be laminated to a sheet; product of the invention through mechanical engagement, heat bonding, or adhesives. The thermally bondable staple fibers can be bicomponent fibers or coated fibers having at least a portion that forms the outer surface of the fiber, i.e., the sheath or coating, of a lower softening point component and a core or base of a higher softening point component. The lower softening point component provides the fiber with thermal bondability and the higher softening point component adds strength to the fiber and the sorbent sheet product of the invention. The lower melting component of the bicomponent fiber may be selected from thermoplastic bondable polymers, such as polyolefins, polyamides and copolyamides, • polyesters and copolyesters, acrylics, and the like. The higher melting component of the bicomponent fiber may be selected from fiber-forming polymers,- such as polyolefins, polyamides, polyesters, acrylics, and the like. Normally, the thermally bondable fibers useful in the present invention have a denier in the range of 1 to 10, preferably 1 to 4, so that the frequency of the bonds is enhanced, and a length in the range of 2 to 15 cm, preferably less than about 7 cm, for ease of processability and web formation. Such fibers are well-known in the art and are described, for example, in U.S. Patent No. 3,511,747 (Davies). A particularly useful thermally bondable bicomponent fiber is an all polyester sheathed fiber. The fiber is believed to comprise a core of polyethylene-terephthalate and a sheath of polyester resin comprising a random copolyester composite of 68% terephthalic acid units and 32% isophthalic acid units polymerized with ethylene glycol, and is available as "Melty Fiber Type 4080" from Unitika, Ltd., Osaka, Japan. The fiber core (base fiber) has a melting temperature of approximately 245 C. The sheath has a sticking temperature of about 110 to 120 C. Another particularly useful thermally bondable bicomponent fiber is an all polyolefin core-sheath fiber. The fiber is believed to comprise a core of polypropylene and a sheath of polyolefin at a 1:1 ratio, the sheath having a sticking temperature of 110° to 120°C. Such fiber is available as "Diawa NBF Type H" from Diawabo Co. Ltd., Osaka, Japan. The selection of the thermally bondable binder fiber and amount of thermally bondable binder fiber used in the sorbent sheet product depends on the softness and conformability desired in the sheet product, as well as the amount of intermittent thermal bonding. To obtain soft, conformable sorbent sheet products, lesser amounts of stiffer thermally bondable binder fibers are used than when using softer thermally bondable binder fibers. As indicated above, the sorbent staple fibers used in the invention are super absorbent fibers, which rapidly absorb and retain under pressure large quantities of liquids. Tests useful in evaluating the sorbency of the super sorbent fibers and the sorbent sheet of the invention include the following: Demand Sorbency Test A 4.45 cm (1.75 inch) diameter test sample ' of sorbent sheet or, in the case where sorbent fibers are to be tested, an air-laid web of sorbent fibers prepared by using, for example, a "Rando Webber", is placed on a 25-50^*4 porous plate in a filter funnel. A pressure of 1.0 kPa is applied to the sample by a plunger which is freely movable in the barrel of the funnel. Test fluid at zero hydrostatic head is conducted from a reservoir through a siphon mechanism to the upper surface of the porous plate where the test sample sorbs the test fluid. The amount of test fluid withdrawn from the reservoir by the test sample is then measured to determine the amount of test fluid sorbed by the test sample. Centrifugal Retention Test A 1 g sample of fiber or sheet product is placed in a centrifuge basket and the basket with sample is submerged in test solution for one hour. The basket with sample is removed, drained for 2-3 minutes, and placed in a centrifuge tube. The tube is placed in a centrifuge and subjected to a centrifugal force of 180 G for 10 minutes. The samples are removed and the amount of test solution retained is measured. In the Demand Sorbency Tests and the Centrifugal Retention Tests where synthetic urine is used as the sorbed liquid, the synthetic urine has the following formulation: 0.06% calcium chloride 0.10% magnesium sulfate 0.83% sodium chloride 1.94% urea 97.07% deionized water The synthetic urine solution has a conductance of 15.7 mΛ . Fibers useful as sorbent staple fibers in the present invention are those having synthetic urine absorbency of at least 10 ml/g, preferably 30 ml/g, more preferably 70 ml/g, when tested according to the Demand Sorbency Test. The sorbent staple fibers preferably have a synthetic urine retention value of at least about 5 ml/g, more preferably at least 10 ml/g when tested according to the Centrifugal Retention Test. To achieve high liquid sorbency and good liquid retention under pressure, the sorbent staple fiber should have at least one outside portion of highly hydrophilic material. Examples of such highly hydrophilic fibers are those prepared by treating acrylonitrile fibers with an alkali-metal hydroxide to form a hydrophilic crosslinked polymer on the surface thereof as disclosed in U.S. Patent No. 4,366,206 and U.S. Patent No. 4,374,175. Also useful are fibers having a sorbent coating such as a crosslinked, saponified cόpolymer of methacrylic acid and ethacrylic acid or a homopolymer of acrylic acid. A particularly useful fiber is "Lanseal F", an acrylonitrile fiber having a hydrophilic crosslinked polymer on the surface thereof, available from Japan Exlan Co., Ltd., Osaka, Japan. The size of the sorbent staple fibers is preferably in the range of about 0.5 to 10 denier, more preferably about 1 to 6 denier, the fibers being slender elongated structures. The size of the sorbent staple fibers depends on the end use of the product with sorbent staple fibers of lower denier providing a softer hand. When using equipment such as a lickerin roll to dissociate the sorbent staple fibers during production of the product, the fibers should average between about 2 to 15 centimeters in length. Preferably, the sorbent staple fibers are less than about 7 centimeters in length. The sorbent staple fibers may be crimped to further enhance the anti-blocking effect provided by the fibers. Crimped sorbent staple fibers provide additional freedom of expansion to the product as the sorbent fibers swell during liquid sorption. Crimped sorbent staple fibers also provide bulk and resilience to the sorbent web, desirable characteristics for end uses such as surgical sponges and dressings for heavy drainage applications. The amount of sorbent staple fibers included in the sheet product of the invention will depend on the particular use to be made of the prodfct and the sorbency of the sorbent fibers. For most uses, at least 10 weight percent sorbent staple fibers based on total web weight will be used to provide sufficient sorbency of the sorbed liquid. Generally, the amount of sorbent staple fiber will not exceed about 25 weight percent sorbent staple fibers based on total web weight to maintain the strength and integrity of the web. Preferably, the sheet product contains about 12 to 18 weight percent sorbent staple fibers based on total web weight To further enhance the sorbency and wicking properties of the sorbent sheet products of the invention, transport fibers which wick or transport the liquid being sorbed to inner portions of the sheet product are included in the fibrous web. The transport fibers are generally absorbent staple fibers which rapidly absorb and wick the fluid being absorbed. Fibers useful as transport fibers are those having a water retention value of at least about 10%, preferably about 20% and more preferably about 25% when tested according to ASTM Test Method D2402 and a synthetic urine retention value of about at least 1 ml/g when tested according to the Centrifugal Retention Test. Fibers having such a water retention value have been found to provide a desired transport of -liquid into the interior of the web. Such fibers include rayon, cotton, wool and silk. A particular preferred fiber is "Absorbit" rayon fiber supplied by American Enka Company. The size of the transport fibers is preferably in the range of about 0.5 to 10 denier, more preferably about 1 to 6 denier. The size of the transport fibers depends on the end use of the product. Transport fibers of lower denier provide a softer hand. The transport fibers should average between about 2 to 15 centimeters in length. Preferably, the transport fibers are less than about 7 centimeters in length. The transport fibers may be crimped to further enhance the anti-blocking effect provided by the fibers and to increase the resilience of the sorbent sheet product. Crimped staple transport fibers provide additional freedom of expansion to the product as the sorbent staple fibers swell during liquid sorption. This additional freedom of expansion reduces the tendency for the web to limit expansion of the web and thereby limit the quantity of water sorbed by the sorbent staple fibers. Crimped transport fibers provide a mechanical release of the web which reduces the constrictive forces on the swelling sorbent staple fibers during liquid sorption. However, the amount of crimp in the fiber cannot be so great as to excessively separate the binder fibers and sorbent staple fibers to the extent that the interstitial movement of fluid though the web is reduced. The amount of liquid conductive transport fibers included in the sheet product of the invention will depend on the degree of wicking desired and the amount and type of sorbent staple fibers included in the sheet product. The transport fibers are preferably included in an amount to provide wicking of at least 10 mm/min, more preferably at least 25 mm/min in the sheet product when tested according to TAPPN-UM-451 using a 1% aqueous saline solution and a time interval of 60 'seconds. Generally, at least 20 weight percent transport fiber based on the weight of the web will be used to provide sufficient transport and wicking of the sorbed liquid to overcome any blocking effect resulting from the swollen sorbent fibers and to achieve the desired rapid sorbency. Generally, the amount of transport fiber will not exceed about 50 weight percent transport fiber based on the weight of the web to maintain the strength and integrity of the web, though greater weight amounts of transport fiber may be ' used when the denier of the fiber is higher. Preferably, the sheet product contains about 25 to 30 weight percent transport fiber based on total web weight. The web is preferably thermally bonded to provide additional sheet integrity and reduce fuzzing, particularly to adapt it for use as in wound dressings or surgical swabs. The thermal bonds are present in discrete areas located throughout the webs, but separated from each other by land areas in which thermal bonds are absent. Webs having such a distribution of thermal - bonds are frequently known as point-bonded webs. A sorbent sheet product of the invention 41, as shown in FIG. 4, has point-bonded areas 42 in land areas 43. The degree of thermal bonding should generally provide sufficient cohesion between the fibers for the specific end use desired. When the web is to be used as a surgical swab, a greater αegree of bonding is required than when the web is bonded to another layer s v... as a porous film, gauze, or adhesive layer for use as a wound dressing. Conversely, the degree of thermal bonding in the web must not be too great for the desired end use, e.g., to make the web unsuitably 5 stiff and reduced in absorbency. Preferably, the degree of bonding is such that the sorbent sheet product has a density of 1l«ess than 25 g/cm , more preferably The thermal bonds can be produced by any- 10 well known means including pin roll, engraved calendar, and sonic welding. To achieve a degree of point bonding which provides sufficient cohesion without undue stiffness, the bond area is preferably within the range of 10 to 60 percent, more preferably 15 to 25 15 percent, of the total surface area. The bond area can be of any shape, including square, circular, diamond, bar, and multi-patterned. Alternatively, the web may be lightly bonded over the surface by contact with a hot roll. 2 . 0 The absorbent sheet product of the invention can be provided with a surface treatment of binder resin to increase coherency of the sheet product as shown in FIG. 5. The binder resin 52 can be applied to only one face of the sheet 51, as shown in FIG. 25 5, or to both faces of the sheet. Preferably, the binder resin is applied only to the fibers near the surface of the web as such resin tends to reduce the absorbency. Suitable resins include acrylates, urethanes, styrene/butadiene, polyvinyl acetate, and styrene 30 acrylics. Examples of commercially available resins include Nacrylic 4260, available from National Starch and Chemical Co., Amsco-res 4402, available from Union Chemical Co., and Airflex 140, available from Air Products Co. Generally the resin can be applied at 35 a rate of 2 to 40, preferably 5 to 20, weight percent based on the weight of the web. The absorbent sheet product of the invention can be advantageously used for absorbent wound dressings 61 as described in U.S. Patent No. 3,285,245 (Eldredge et al. ) wherein a porous film 62 is adhered to the sorbent sheet product 63 for contact with the wound, as shown in FIG. 6. In such a wound dressing, the porous film is adhered to the absorbent sheet material by any well known means such as, for example, thermal bonding, adhesives, and sonic bonding. The advantages of the sorbent sheet product of the invention are illustrated in the following examples which are not to be construed as limiting its scope. In the examples, the following test methods were used to determine the various properties of the sorbent sheet products: Dry tensile strength at break: ASTM-D-1682-64 Test conditions: tested after conditioning at about 20°C and 50% r.h. . : sample width - 2.5 cm : jaw opening - 12.5 cm : extension rate - 12.5 cm/min Wet tensile strength at break: ASTM-D-1682-64 Test conditions: tested after 60 second immersion in 1% aqueous saline and drain for 90 seconds . : sample width - 2.5 cm : jaw opening - 12.5 cm : extension rate - 12.5 cm/min Absorbency: ASTM-D-1117-74 (5) Test conditions: sample size - 10 cm x 10 cm : test solution - 1% aqueous saline : time: 60 seconds : 10 minutes : 60 minutes Test results reported in weight percent solution absorbed as a percent of dry sample weight. Wicking: TAPPI-UM-451 Test conditions: time interval - 60 seconds : test solution - 1% aqueous saline Retention: Centrifugal Retention Test test results reported in percent of one hour absorbency value retained. Hand: Subjective determination of sorbent sheet softness Scale: very soft A : soft B : medium C : stiff D Examples 1 and 2 Webs were formed by an air laying process using Rando Webber equipment. The webs contained 4 denier, 51 mm in length "Melty Fiber Type 4080" as binder fiber, 7 denier, 51 mm in length "Lanseal F" sorbent fiber and 1.5 denier, 40 mm in length regular rayon fiber from Avtex Fibers as transport fiber in the amount shown in Table 1. The webs were then thermally bonded using a vertical calender having one smooth steel roll and an embossing roll at 138 C with a nip pressure of 3 kg/cm 2. The embossing roll provided point bonded sorbent sheet products of the invention having bond areas of 18 to 20 percent. The samples were tested for various properties with the results as shown in Table 1. Table ± Example 1 Example 2 Sheet composition Binder fiber (%) 25 40 Sorbent fiber (%) 25 20 Transport fiber (%) 50 40 2 Weight (g/m ) 80 80 Density (g/cc) 0.17-0. 22 0.17-0.22 Dry tensile strength (g/2. ,5cm) 540 1700 Wet tensile strength (g/2. ,5cm) 400 1550 Absorbency (%) 60 sec 700 470 10 min ' 750 550 60 min 640 540 Wicking (mm/min) 55 50 Retention (%) - 71 Hand B B As shown by the data in Table 1, the sheet product of Example 1 hud yood dry and wcL Lensile t-lrongth, excellent absorbency and wicking, and a soft hand, while the sheet product of Example 2 which contained a greater . quantity of binder fiber had greater dry and wet tensile strength, good absorbency, excellent wicking and retention, and a soft hand. Examples 3 and 4 Samples were prepared as in Examples 1 and 2, except that the fibers used were 3 denier, 51 mm in length "Diawa H" binder fiber, 7 denier, 51 mm "Lanseal F" sorbent fiber, and 1.5 denier, 40 mm in length rayon transport fiber. The samples were tested as in Examples 1 and 2. The amounts of sample constituents and test results are shown in Table 2. . Table 2 Example 3 Example 4 Sheet composition Binder fiber (%) 65 50 Sorbent fiber (%) 15 20 Transport fiber (%) 20 30 2 Weight (g/m ) 40 65 Density (g/cc) 0.14-0. 19 0.14-0.19 Dry tensile strength (g/2.5cm) 950 1010 Wet tensile strength (g/2.5cm) 900 920 Absorbency (%) 60 sec 1010 1350 10 min 1080 1620 60 min 1080 1700 Wicking (mm/min) 32 46 Retention (%) - >70 Hand A A As shown in Table 2, the sheet products of Examples 3 and 4 had excellent dry and wet tensile strength, absorbency, wicking and retention. The hand of these sheet products was softer than the hand of the sheet products of Examples 1 and 2 due to the use of a softer binder fiber. Examples 5 and 6 Samples were prepared as in Examples 3 and 4 using varying amounts of "Diawa" binder fiber, "Lanseal" sorbent fiber, and rayon transport fiber. The samples 5 were tested as in Examples 1 and 2. The amounts of sample constituents and results are shown in Table 3. Table 3 Example 5 Example 6 10 Sheet composition Binder fiber (%) 40 50 Sorbent fiber (%) 20 15 Transport fiber (%) 1 40 35 Weight (g/m 2 ) 70 70 15 Density (g/cc) 0.14-0. 19 0.14-0.19 Dry tensile strength (g/ '22..55ccmm)) 840 940 Wet tensile strength (g/ '22..55ccmm)) 760 870 Absorbency (%) 60 sec 1420 1240 ' 20 10 min 1610 1300 60 min 1730 1330 Wicking (mm/min) 55 49 Retention (%) 70 >70 Hand A A 25 As shown in Table 3, the sheet products of Examples 5 and 6 had excellent dry and wet tensile strength, absorbency, wicking and retention and had very soft hand. Examples 7 and Comparative Example 1 30 Samples were prepared as described in Examples 1 and 2 using "Diawa" binder fiber, "Lanseal" sorbent fiber, and rayon transport fiber in Example 7 and "Melty" binder fiber, "Lanseal" sorbent fiber, and rayon transport fiber in Comparative Example 1. The samples were tested 35 as in Examples 1 and 2. The constituents of the samples and the test results are shown in Table 4. Table 4 Comparative Example 7 Example 1 Sheet composition Binder fiber (%) 65 65 Sorbent fiber (%) 15 15 Transport fiber (%) 20 20 Weight (g/m 2 ) 60 60 Density (g/cc) 0.14-0.19 0.17-0.22 Dry tensile strength (g/2.5cm) 900 4500 Wet tensile strength (g/2.5cm.) 810 4000 Absorbency (%) 60 sec 1170 610 10 min 1230 680 60 min 1190 600 Wicking (mm/min) 38 17 Retention (%) 69 66 Hand D In comparing the data of Example 7 with Comparative Example 1, it can be seen that ,in using equal amounts of "Diawa" binder fiber and "Melty" binder fiber much different properties are obtained due to the 4 denier "Melty" fiber being larger, stronger, and stiffer than the 3 denier "Diawa" fiber. In Comparative Example 1, the wet and dry tensile strength is much greater, but the absorbency and wicking are lower and the hand is unacceptably stiff for such end uses as wound dressings where conformability is important. Examples 9 and 10 Samples were prepared as in-Examples 1 and 2 using 3 denier, 51 mm in length "Diawa" binder fiber, 5 denier, 51 mm in length "Lanseal" sorbent fiber, and 1.5 denier, 40 mm in length regular rayon transport fiber in Example 9 and 2 denier, 51 mm in length "Melty" binder fiber, 7 denier, 51 mm in length "Lanseal" sorbent fiber, and 1.5 denier, 40 mm in length regular rayon transport fiber in Example 10. The sheet products were tested as in Examples 1 and 2. The amounts of constituents and the test results are shown in Table 5. Table 5 Example 9 Example 10 Sheet composition Binder fiber (%) 50 50 Sorbent fiber (%) 20 20 Transport fiber (%) 30 30 . Weight (g/m ) 60 60 Density (g/cc) 0.14-0. 19 0.17-0.22 Dry tensile strength (g/2.5cm) 1270 1940 Wet tensile strength (g/2.5cm) 1110 1690 Absorbency (%) 60 sec 1310 950 10 min 1560 1110 60 min 1620 1200 Wicking (mm/min) 45 42 Retention (%) 70 70 Hand A B As shown by the data in Table 5, sheet product containing 3 denier "Diawa" binder fiber is more absorbent and softer than sheet product containing 2 denier "Melty" fiber. However, the sheet product containing the "Melty" fiber had greater wet and dry tensile strength. Comparative Examples 2 and 3 Samples were made as in Examples 1 and 2 using 4 denier, 51 mm ' in length "Melty" binder fiber and 7 denier, 51 mm in length "Lanseal" sorbent fiber. No transport fiber was used. The sheet products produced were tested for various properties. The amounts of each constituent and test results are reported in Table 6. Table 6 Comparative Comparative Example 2 Example 3 Sheet composition Binder fiber (%) 75 60 Sorbent fiber (%) 25 40 • Transport fiber (%) 2 Weight (g/m ) 60 60 Density (g/cc) 0.17-0. 22 0.17-0.22 Dry tensile strength (g/2.5cm) 1300 1530 Wet tensile strength (g/2.5cm) 1470 1400 Absorbency (%) 60 sec 350 540 10 min 520 630 60 min 350 450 Wicking (mm/min) 1.5 3.5 Retention (%) 62 - Hand C C • As can be seen from the test results, although the comparative samples had excellent dry and wet tensile strength and retention and good absorbency, wicking was poor and the hand was of medium stiffness . Examples 11 and 12 Samples were prepared as in Examples 3 and 4 using the amounts Of constituents shown in Table 6. The sheet products were further treated with "Nacrylic" 2 4260 resin binder at a rate of 5 to 7 g/m by foam application. The samples were tested as in Examples 1 and 2, the results being reported in Table 7. Table 7 Example 11 Example 12 Sheet composition Binder fiber (%) 60 60 Sorbent fiber (%) 10 10 Transport fiber (%) 30 30 Weight (g/m 2 ) 40 70 Density (g/cc) Dry tensile strength (g/2.5cm) 1120 950 Wet tensile strength (g/2.5cm) 1030 900 Absorbency (%) 60 sec 1090 990 10 min 1250 1030 60 min 1270 1050 Wicking (mm/min) 44 45 Retention (%) - 72 Hand B B As can be seen from the test results in Table 7 the sheet product treated with resin binder had excellent dry and wet tensile strength, absorbency, wicking, and retention as well as very soft hand. Examples 13 and 14 Sheet products of Example 7 were adhered to porous releasable film, as described in U.S. Patent No. 3,285,245 (Eldredge et al . ) which is incorporated herein by reference, using, as the film in Example 13, 2 XXSP#1050 (lOg/m ) available from Kendall Co., and, 2 as the film in Example 14, 0.02 mm (24 g/m ) low density polyethylene film. The samples were tested for absorbency, wicking, retention, and hand. The results are shown in Table 8. Table 8 Abscjrbency(%) Wicking Retention Hand 60 10 Example • Sec min (mm/min) (%) 13 620 850 31 0.62 A 14 460 780 30 0.65 A Comparative Examples 4 and 5 Eight-ply gauze, available from Johnson and Johnson, (Comparative Example 4) and "Soft Wick", a multi-ply "spun-laced" rayon nonwoven dressing, also available from Johnson and Johnson, (Comparative Example 5) were tested as in Examples 1 and 2. The test results are shown in Table 9. Table 9 Comparative Comparative Example 4 Example 5 Weight (g/m 2 ) 250 280 Density (g/cc) 0.25-0. 33 0.25-0.33 Dry tensile strength (g/2.5cm) 1840 2020 Wet tensile strength (g/2.5cm) 1300 900 Absorbency (%) 60 sec 320 1200 10 min 420 1500 60 min 300 1400 Wicking (mm/min) 12 18 Retention (%) 42 46 Hand A C Example 15 and Comparative Example 6 In Example 15, sample was prepared using polypropylene microfibers as binder fiber, 7 denier, 51 mm in length, "Lanseal" sorbent fibers, and 1.5 denier, 40 mm in length, rayon transport fibers as described in U.S. Patent No. 4,118,531. In Comparative Example 6, sample was prepared using polypropylene microfibers as binder fiber and 7 denier, 51 mm in length "Lanseal" „- sorbent fibers. The webs were then thermally bonded and tested for various properties as in Examples 1 and 2. The amounts of constituents and test results are shown in Table 10. Table 10 Comparative Example 15 Example 6 Sheet composition Binder fiber (%) 60 75 Sorbent fiber (%) 20 25 Transport fiber (%) 20 2 Weight (g/m ) 60 60 Density (g/cc) 0.18-0. 21 0.17-0.20 Dry tensile strength (g/2 ..55ccmm)) 1210 1030 Wet tensile strength (g/2 ..55ccmm)) 1030 950 Absorbency (%) 60 sec 650 400 10 min 710 420 Wicking (mm/min) 20 7 Retention (%) 69 73 Hand B C As can be seen from the test results, the sample of Example 15 had excellent dry and wet tensile strength, absorbency, wicking, and retention and a soft hand. The sample of Comparative Example 6 had poor wicking and was stiffer, having a medium hand. Various modifications and " alterations of this invention will become apparent to those skilled in the art without departing from the scope of the invention set forth in the claims.

Claims: WHAT IS CLAIMED IS: 1. A sorbent sheet material useful in wound dressings and surgical swabs comprising a coherent fibrous web of high sorbency, liquid-sorbent fibers and liquid conductive transport fibers intermingled with thermoplastic binder fibers. 2. The sorbent sheet material of claim 1 wherein said web contains about 10 to 25 weight percent of said high sorbency liquid sorbent fibers, about 20 to 50 weight percent liquid conductive transport fibers, and about 25 to 70 weight percent of said thermoplastic binder fibers. 3. The sorbent sheet material of claim 1 wherein said thermoplastic binder fibers are thermally bondable fibers and said web is intermittently thermally bonded. 4. The sorbent sheet material of claim 1 wherein said thermoplastic binder fibers are blown polymeric fibers and said web is intermittently ' thermally bonded. 5. The sorbent sheet material of claim 4 wherein the blown polymeric fibers are microfibers having an average diameter of less than about 10 micrometers. 6. The sorbent sheet material of claim 1 wherein said transport fibers have a retention value of at least 5.0 ml/g for synthetic urine. 7. The sorbent sheet product of claim 6 wherein said transport fibers are selected from rayon, cotton, wool, and silk. 8. The sorbent sheet product of claim 7 wherein said transport fibers are rayon. 9. The sorbent sheet product of claim 1 wherein said bond area is 10 to 60 percent of the total surface area. 10. The sorbent sheet product of claim 9 wherein said bond area is 15 to 25 percent of the total surface area. 11. The sorbent sheet product of claim 1 wherein said sheet product has a retention value of at least about 1.5 ml/g for 1% by weight saline solution. 12. The sorbent sheet product of claim 1 wherein said sheet product has a wicking rate of at least 10 mm/min for a 1% aqueous saline solution. 13. The sorbent sheet product of claim 1 further comprising a thin, highly porous thermoplastic facing adapted to releasably contact a wound surface. 14. The sorbent sheet product of claim 13 wherein said facing is intermittently thermally sealed to said web. 15. The sorbent sheet product of claim 1 further comprising resin binder on at least one surface of said web. 16. An absorbent wound dressing comprising the sorbent sheet product of claim 1. 17. An absorbent surgical swab comprising the sorbent sheet product of claim 1.

Inventors: INSLEY THOMAS I, NELSON DAVID L, RIEDEL JOHN E, INSLEY, THOMAS, I., NELSON, DAVID, L., RIEDEL, JOHN, E.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1985
Main classification: A61F13
IPC codes: A61F5, A61F13, A61L15, A61M35, D04H1