LIGHT-WEIGHT, LOW-DENSITY ABSORBENT STRUCTURE

Abstract: A light-weight, low-density absorbent structure suitable for use in disposable absorbent products which is formed from a fibrous web of cellulose and cellulose acetate fibers. The fibrous web is treated with a triacetin solvent and heat cured to bond the fibers. The absorbent structure has excellent structural integrity and an absorbent capacity and strength which is equal to or exceeds that of absorbent structures having twice the basis weight.

Description: LIGHT-WEIGHT. LOW-DENSITY ABSORBENT STRUCTURE FIELD OF THE INVENTION The present invention relates to fibrous webs which are suitable for use in products designed to absorb and retain body fluids, such as diapers, catamenials. and wound dressings. More specifically, the invention provides a light-weight, low-density absorbent structure formed from man-made fibers based on cellulose and cellulosic fibers. The fibers are treated with a chemical solvent and heat-cured to bond the fibers. The solvent treated absorbent structure of the present invention retains the absorbent capacity, fluid transport properties, and strength of known structures having at least twice the basis weight. BACKGROUND OF THE INVENTION A wide variety of absorbent structures for use in disposable absorbent articles are known in the art. Common commercial absorbent articles include diapers, adult incontinence products, catamenials and bandages. These products are provided with various functional components for receiving, absorbing and retaining fluids. As consumer demand for less expensive and less bulky disposable absorbent products increases, manufacturers continue to seek effective ways to reduce size and cost without sacrificing the quality of the fluid transport properties or structural integrity of the products during use. Efforts to improve disposable absorbent articles include changes in the barrier films and the geometric configuration of the products, as well as changes in the composition of the absorbent components of the products. Recently, ultrathin products have appeared on the market which incorporate acquisition layers formed from expensive cross-linked cellulose fibers or low density synthetic nonwovens to provide improved fluid transport properties. Nevertheless, due to a high absorption capacity and low cost, most manufacturers continue to favor natural cellulose fibers and super absorbing polymers as the principal absorptive components in the core of disposable absorbent products. As is well known in the an, cellulose fibers are typically utilized in the form of fibrous webs, which are manufactured by conventional wet-laid techniques. The fibrous webs then are used in air-laid systems to form absorbent structures used as a component of disposable absorbent products. The absorbent structures may vary considerably in weight, density, and composition, depending upon the properties required for a particular type of absorbent product. For example, superabsorbents may be added to enhance the absorbent capacity of the structure, and various synthetic or modified cellulose fibers may be combined with the cellulose fibers in order to achieve a desired characteristic. In an air-laid process, the fibers are suspended in a stream of air and conveyed to a forming screen where the fibers are condensed into a web. However, at this stage, the fibrous web lacks any significant structural integrity. Often, sheets of tissue are used on the top and bottom of the web to provided additional support to the web. Other means of stabilizing an air-laid web include thermal bonding by including specially treated synthetic fibers which melt upon heating and solidify upon cooling to bond with the cellulose fibers and promote retention of a desired shape. The use of latex binders within and on the surface of fibrous webs also has been proposed as a means to provide structural integrity to fibrous webs. While prior art methods may enhance the structural integrity of some absorbent products, most often these techniques result in a significant reduction in the absorption capacity, wicking rate, and other fluid transport properties which are critical to the effectiveness of an absorbent structure. Prior to the present invention, webs produced from cellulose fibers were somewhat bulky since the webs had to be of sufficient weight and density to provide minimally acceptable fluid transport properties. Moreover, absorbent structures formed from sufficiently dense cellulosic fibers tend to lose integrity and shape as the structure absorbs fluids. Accordingly, there remains a need for a low bulk absorbent structure made primarily from cellulose fibers which maintains its structural integrity upon saturation without loss of other significant fluid transport propeπies. Further, it is desirable to provide a method for producing an improved absorbent structure which does not require substantial modifications of the equipment or processes currently employed by commercial manufacturers of disposable absorbent products. SUMMARY OF THE INVENTION The present invention satisfies these and other needs by providing a light-weight, low- density absorbent structure which is particularly suitable for use as a component in disposable absorbent products. Suφrisingly, it has been discovered that when the surface of a web formed from a combination of cellulosic and cellulose-based man-made fibers is treated with a chemical solvent and heat cured to bond the fibers, the resulting absorbent structure exhibits exceptional strength at half or less of the basis weight of untreated standard absorbent structures formed from only cellulose fibers, while retaining or enhancing the superior fluid transport properties which are characteristic of cellulose fiber webs. In addition, the method of the present invention can be easily incorporated into manufacturing systems currently utilized to produce a variety of disposable absorbent products without substantial modifications to the equipment or the process. Conventional wet-laid techniques may be used to combine the cellulose and cellulose- based man-made fibers to form standard webs or sheets. The sheets may then be fed through conventional air laying systems to form a fibrous layer having a desired weight and density. Alternately, the cellulose-based man-made fibers may be dry blended with the cellulose fibers to form a fluff which is men fed through conventional air laying systems to form the fibrous layer. At least one surface of the fibrous layer then is treated with a chemical solvent and subjected to a heat curing step to bond the cellulose-based man-made fibers to the cellulose fibers. The absorbent structure prepared by the method of the present invention exhibits exceptional strength and fluid transport ability at half or less of the basis weight of untreated standard absorbent structures containing only cellulose fibers. Preferably, the fibrous portion of the absorbent structure of the present invention is formed from between about 85% to about 95% by weight of cellulose fibers and between about 5% to 15% by weight of cellulose acetate fibers. In a preferred embodiment, the fibrous portion of the present absorbent structure is composed of between about 88% and about 92% by weight of cellulose fibers and between about 8% and about 12% by weight of cellulose acetate fibers. In a particularly preferred embodiment, the fibrous potion of the absorbent structure comprises about 90% cellulose fibers and about 10% cellulose acetate fibers by weight. Conventional air laying techniques are used to form an absorbent layer having a density of between about 0.03 g/cc and 0.15g/cc and a basis weight of between about 150 g/m 2 and about 1,000 g/m 2 . After forming an absorbent layer of the desired density and weight, a cellulose acetate solvent is applied to the layer. Most preferably, the solvent is 100% triacetin. The triacetin is applied in an amount of between about 2% and about 14% by weight of the total amount of bone dry fiber in the layer. The cellulose acetate solvent may be applied by spraying one or both surfaces of the absorbent layer, by pulling the solvent through tho layer in the form of a mist, or by any method which evenly distributes small amounts of the solvent throughout the absorbent layer. The solvent treated absorbent layer is then heat cured by passing the layer through a hot air oven or an air drier at a temperature sufficient to bond the cellulose acetate fibers. After curing, d e absorbent strucmre of the present invention has a drip capacity of at least 20 ml/g and a burst strength of at least 200g. The solvent treated absorbent structure of the present invention may be utilized as a single layer or combined with one or more additional solvent treated absorbent structures to form a layered component for use in a disposable absorbent product. Moreover, as is known in the art, superabsorbents or other additives may be combined with the cellulose and cellulose acetate fibers prior to solvent treatment, depending upon the desired characteristics of the final absorbent product. The solvent treated absorbent strucmre of the present invention also may be combined with other functional components to form a disposable absorbent product. For example, the solvent treated absorbent strucmre of the present invention may serve as the fluid storage layer in a disposable diaper, while a layer having a different strucmre and composition is used as the fluid acquisition and distribution layer. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graphic presentation of the effects of varying amounts of triacetin on the drip capacity of absorbent structures prepared according to the method of the present invention; and Figure 2 is a graphic presentation of the effects of varying amounts of triacetin on the strength ofthe absorbent structures prepared according to the method of die present invention. DETAILED DESCRIPTION OF THE INVENTION The absorbent strucmre of the present invention is formed by combining cellulose fibers with cellulose acetate fibers and compressing the fibers into a layer of desired density using standard airlaying techniques. The fibrous layer is treated with triacetin and heat cured to bond the cellulose and cellulose acetate fibers. The resulting light-weight low density strucmre has exceptional fluid transport properties, a high absoφtion capacity, and excellent strucmral integrity upon saturation with fluids. The cellulose fibers used in the present absorbent strucmre may be cotton linters, 4 SUBSTITUTE SHEET (RULE 2Q CTMP, northern or southern softwood fibers or hardwood blends. Chemically pulped wood fibers, such as southern softwood fibers, are especially suitable for use in the absorbent strucmre of the present invention. In a particularly prefeπed embodiment, the cellulose fibers are chemically pulped southern softwood fibers sold by Buckeye Cellulose Coφoration, 1001 Tillman Street. Memphis, Tennessee 38108, under the tradename FOLEY FLUFF. The cellulose acetate fibers used in the absorbent strucmre of the present invention may be produced from cellulose acetate tow. Preferably, the cellulose acetate fibers have a length of between about 1/8 and 1/4 inches and a denier of between about 1.7 and 2.7 dpf. Suitable cellulose acetate tow is available from Hoechst Celanese Coφoration, 2300 Archdale Drive, Charlotte. North Carolina 28210; Rhone-Poulenc, 25, Quai Paul Doumer, 92408 Courbevoie Cedex. Paris, France; Tennessee Eastman Division. Eastman Chemical Company, P.O. Box 511. Eastman Road, Kingsport, Tennessee 37662; Daicel Chemical Industries, Ltd. , 8-1 Kasumigaseki 3-Chome. Chiyoda-ku. Tokyo, 100 Japan; and Couπaulds pic, 50 George Street, London W1A2BB, England. Although any solvent which will bond the cellulose acetate fibers to the cellulose fibers may be used in me present method, the prefeπed solvent has a low toxicity and is effective at low concentrations. In a particularly prefeπed embodiment, 100% triacetin is used as the solvent for the cellulose acetate in the present absorbent strucmre. Typically, the solvent is applied to at least one surface of the absorbent strucmre by conventional spraying techniques. However, the solvent may be applied by any suitable alternate method may be utilized to apply the solvent to the fibrous layer. After application of the solvent, the absorbent strucmre is heat cured to bond the cellulose acetate to the cellulose fibers. Any conventional means for heat curing is suitable for use in the present method. However, in order to achieve acceptable bonding, it is desirable to heat the solvent treated absorbent strucmre to a temperamre of at least 74 °C. In order to demonstrate the superior fluid capacity and strucmral integrity of the present light-weight, low-density absorbent strucmre, comparative tests were conducted which measure the drip capacity and burst strength of absorbent structures prepared according to the present method and treated with varying amounts of triacetin. The experimental data show that the present solvent treated absorbent structures exhibit a drip capacity which is equal to or greater than the standard 100% cellulose control structures having a significantly higher density and basis weight. The data also shows that the present absorbent strucmre has strength at least equal to or greater than a standard 100% cellulose control strucmre having twice the basis weight. Preparation Of Standard Control Structures The cellulose fibers used in the standard control pads were provided as machine sheeted 100% cellulose pulp obtained from Buckeye Cellulose Coφoration. The dry, machine sheeted pulp was converted into a fluff form which did not contain a large quantity of fiber clumps by cutting me pulp into 1 " x 4" strips with the 4" dimension in the machine direction. The strips were individually fed into a fluffer at a consistent rate of about 1 strip every four seconds to produce a uniform fluff. A laboratory scale padmaker which duplicates the commercial padforming process was used to air lay the dry uniform fluff into pads in a conditioned environment. In order to overcome the effects of disintegrating the pulp in an unconditioned atmosphere and expose me pads to the conditioned environment, d e pads were allowed to remain in the padmaker for 4-5 minutes with the compressed air off and the vacuum on. Additionally, this procedure overcomes the possible effect of me compressed air used in padmaking not being at 50% relative humidity. A ply of tissue which measured 14-1/2" x 14-1/2" was placed on the forming screen of the padmaker. The tissue completely covered the forming screen and curved up me sides. This tissue represents the bottom side of the airiaid felt pad. Fifty-two grams of the fluff sample was added to the padf ormer in four equal increments to form a uniform pad. After the fluff was added to the air-laid pad. the forming screen was removed with me pad on it and carefully transfened to a smooth, flat surface. A second covering tissue was marked to indicate the top side of the airiaid felt pad and placed on top of the pad, making sure that the machine direction of me second tissue ply is in the same direction as that of the first ply. A weight which measured 14" x 14" was placed on the pad in a manner which did not disturb the formation of the pad. The weight was allowed to remain on the pad for a minimum of 5 minutes and then carefully removed. The pad was cut into a 12-3/4" x 12-3/4" square by removing approximately the same from each edge with a standard paper cutter board. This pad was cut into nine square pads which measured 4-1/4" x 4-1/4" each. Each pad weighed about 4.75 grams, which represented 4.25 grams of fluff and 0.50 grams for me two cover sheets. The airiaid felt pads were then stored in an area maintained at 23 ± 1°C (73.4 ± 2°F) and 50 +. 5% relative humidity until needed for testing. The covering tissues on me 4-1/4" x 4-1/4" pad were carefully removed and the pad was placed on the bottom half of an aluminum press plate. The press plate is made from two blocks of aluminum measuring 6" x 6" x 1 ". One 6" x 6" face of each block was machined to a perfectly flat surface. Aligning pins are fixed near two corners of one plate. Coπesponding holes are formed in the other plate for receiving the pins. The top half of the press plate was placed over the pad to pressed and the entire press plate was placed on a Carver hydraulic press (Model No. 16600-224). Each pad was pressed at the appropriate pressure to produce the desired density. Since the size of the pad increased as a result of pressing, the pad was trimmed to measure 4" x 4" each and weighed. After waiting 120 seconds for delayed rebounding, the thickness of each pad was measured. The density of me pad was then calculated according to the following formula: Density in grams/cc of a 4" x 4" pad = 0.00379 x weight (gms) thickness (inches) Procedure For Drip Capacity Test In order to demonstrate the fluid transport capability of the absorbent strucmre of the present invention to retain fluids at a lower density and basis weight, airiaid experimental pads were prepared according to method of the present invention. Standard pads which contained 100% cellulose fibers were prepared according to the procedure described above for use as control pads. The fluid transport capability of each pad was measured by determining the drip capacity in milliliters of liquid per grams of cellulose in a pad without covering tissues. Synthetic urine was prepared by dissolving 108.4 g of a dry synthetic urine mixture in 20 liters of distilled water. The dry synthetic urine mixmre may be obtained from Endovations, Inc., Reading, Pennsylvania. A burette was filled with the synthetic urine solution and the flow rate of the pipette was adjusted to deliver 2 mis of urine per second. The delivery tip on the stopcock of me burette was positioned 1 " above and peφendicular to a cube made of 0.5 inch wire mesh. The cube was placed in a pan for receiving the excess fluid. The top face of the cube was maintained in a level position. Immediately after pressing to the desired density, the pad was placed on the cube so that the fluid impact point is at a crosswire position. Simultaneously, the stopcock on the burette was opened and the timer was started. The synmetic urine was allowed to drip at a controlled rate onto the center of me pad. The timer was stopped when the first drop of liquid was released by the pad and fell into the pan. The time required for the first drop of liquid to pass through tiie pad was recorded. The wet pad was removed from the cube and discarded. The cube was dried completely and returned to the pan. The above procedure was repeated on two more pads which were identical to the first in weight, density and composition. The weight, density, and time was recorded for each of the three individual pads. The drip capacity for each pad was calculated according to the following formula: Drip capacity in milliliters liquid/gram sample = Time in sec, x 2 Weight (grams) of 4"x 4" pad The average drip capacity of the three pads was then determined. Procedure For Burst Strength Test In order to demonstrate the burst strength of the present absorbent strucmre at a lower density and basis weight, airiaid experimental pads were prepared according to the method of the present invention. Standard pads which contained 100% cellulose fibers were prepared according to the procedure described above for use as control pads. The burst strength of each pad was determined by measuring the force required for the ball penetrator of a conventional tensile testing apparams to reach the point of no resistance in a pad without covering tissues. A Thwing Albert Intelect II tensile tester was used to measure the burst strength of me pads. The tensile tester includes a clamp platform and clamp plate for securing a test pad in a horizontal position between the platform and the plate. The platform and clamp plate are provided wim coπesponding holes for receiving a ball penetrator which is positioned directly above the holes. The tensile tester was set up in compression mode and attached to a gram cell which monitors any resistance encountered by me ball penetrator. The ball penetrator had a diameter of 1.5 cm. Immediately after pressing to the desired density, the pad was placed over the hole on the clamp platform, and the clamp plate was securely clamped over the pad to hold the pad in place. The Intelect was started, with me crosshead set to travel downward at 1.0 in/min or 2.54 cm/min. As the ball penetrator moves down and contacts the pad, an ever increasing force measurement shows continuously on me momtor. The penetrator continues to move completely dirough the pad until reaching me point of no resistance, which is typically when the pad breaks. At this point, me crosshead automatically rebounds upward to the starting position. The maximum force value on the momtor of me Intelect was recorded. This process was repeated two times with new pads. Three pad values were averaged and the maximum force value was reported in grams. Experimental Example 1 Experimental pads according to me present invention were prepared by wet blending cellulose fibers with cellulose acetate fibers in a disintegrator available from the Techmcal Association of the Pulp and Paper Industry ("TAPPI"). The cellulose fibers used in the experimental pads were provided as machine sheeted pulp obtained from Buckeye Cellulose Coφoration. The cellulose acetate tow fibers used in the experimental pads had a length of 1/4" and a denier of 2.7. The fibers were combined in a slurry in a ratio of 90% cellulose to 10% cellulose acetate tow. The fiber slurry was sheeted using a Williams handsheet mold, and men dried on steam drum cans. The dried handsheet was cut into 1 " x 4" strips which were defiberized in a d ree stage fluffer to produce a comminuted uniform airfelt. An airfelt pad measuring 14" x 14" and weighing about 75 g/m 2 was formed by airlaying 13 grams of the fibers in a laboratory scale padformer. This pad was men cut into experimental pads measuring 4-1/4" x 4-1/4". The surface of one experimental pad was sprayed with 100% triacetin in the amount of 7.6% by weight of the amount of bone dry fiber in the experimental pad. A second experimental pad without triacetin was placed over the triacetin coated surface of the first experimental pad so that the triacetin was layered in the center of the experimental pad. The total pad was heat cured by oven drying at a temperamre of 125 °C for about 1 hour. The experimental pad had a basis weight of 160 g/m : , which is about 50% less d an me basis weight 325 g/m 2 of a standard 100% cellulose fiber pad. The sandwiched pad was lightly pressed to a target density of 0.03 g/cc and trimmed to precisely 4" x 4" using the procedures described above. The experimental pad was evaluated by me drip capacity and airfelt burst strength tests described above. As shown in Figs. 1 and 2, the experimental pad prepared according to the method of me present invention had a drip capacity of about 28.2 ml/g and a burst strength of about 123g. A standard 100% cellulose fiber pad prepared according to me procedure described above has a basis weight of 325 g/m 2 and a density of 0.10 g/cc, and exhibits a drip capacity of 12 ml/g with a burst strengm of 185g. Experimental Example 2 Experimental pads were prepared as in Example No. 1 above, but sprayed with triacetin in me amount of 3.4 % by weight of me amount of bone dry fiber in me experimental pad. Each experimental pad was tested for drip capacity and burst strength as described in Example 1. As shown in Figs. 1 and 2, the experimental pads prepared according to die mediod of me present invention had a drip capacity of 14 ml/g and a burst strength of 122g. Experimental Example 3 Experimental pads were prepared as in Example No. 1 above, but sprayed with triacetin in the amount of 14% by weight of me amount of bone dry fiber in the experimental pad. Each experimental pad was tested for drip capacity and burst strength as described in Example 1. As shown in Figs. 1 and 2. me experimental pad prepared according to the method of me present invention had a drip capacity of 23.6 ml/g and a burst strength of 223g. Experimental Example 4 Experimental pads were prepared as in Example No. 1, however, NO triacetin was applied to the pads. The experimental pads were tested for drip capacity and burst strengm as described in Example 1. As shown in Figs. 1 and 2, diese experimental pads had a drip capacity of only 9.7 ml/g and a burst strengm of only 28.9g. Although the invention is described with respect to a prefeπed embodiment, it is expected mat various modifications may be made mereto without departing from the spirit and scope of me invention. Therefore, me scope of the invention is to be determined by reference to the claims which follow. 12 SUBSTITUTE Sr ΞT (RULE 26)

Claims: WHAT IS CLAIMED IS: 1. An absorbent strucmre for absorbing, transporting and retaining fluids which comprises: cellulose fibers in an amount of between about 85 and about 95 percent of me fibrous basis weight of the strucmre; cellulose acetate fibers in an amount of between about 5 and about 15 percent of the fibrous basis weight of the strucmre; and an organic solvent in an amount sufficient to solubilize and bond me cellulose acetate fibers to the cellulose fibers. 2. The absorbent strucmre of claim 1 wherein me organic solvent is a 100% solution of triacetin. 3. The absorbent strucmre of claim 2 wherein the triacetin is added in an amount of between about 2% and about 14% by weight of the total amount of bone dry fiber in the absorbent strucmre. 4. The absorbent strucmre of claim 1 wherein said cellulose acetate fibers are present in an amount of about 10 percent of the fibrous basis weight of the structure. 5. The absorbent strucmre of claim 1 which further comprises a predetermined amount of superabsorbent material. 6. An absorbent strucmre which comprises a dry laid fibrous web formed from a mixmre 13 SUBSTΓTUTE SHEET (RULE 26) of cellulosic fibers and cellulose acetate fibers: and a triacetin solvent, said strucmre having (i) a basis weight of about 150 to 1000 grams per square meter, (ii) a density of about .03 to .15 grams per cubic centimeter, (iii) a drip capacity of not less than 20 milliliters per gram of structure, and (iv) a burst strength of not less than 200 grams. 7. The absorbent strucmre of claim 6 wherein said mixmre of cellulose and cellulose acetate fibers is prepared by a dry blending technique. 8. The absorbent strucmre of claim 6 wherein said mixmre of cellulose and cellulose acetate fibers is prepared by a wet blending technique. 9. A disposable diaper containing at least one absorbent strucmre which comprises a dry laid fibrous web formed from a mixmre of cellulosic fibers and cellulose acetate fibers; and triacetin, said strucmre having (i) a basis weight of about 150 to 1000 grams per square meter, (ii) a density of about .03 to .15 grams per cubic centimeter, (iii) a drip capacity of not less than 20 milliliters per gram of strucmre, and (iv) a burst strengm of not less than 200 grams. 10. A sanitary napkin containing at least one absorbent strucmre which comprises a dry laid fibrous web formed from a mixmre of cellulosic fibers and cellulose acetate fibers; and triacetin, said strucmre having (i) a basis weight of about 150 to 1000 grams per square meter, (ii) a density of about .03 to .15 grams per cubic centimeter, (iii) a drip capacity of not less than 20 milliliters per gram of strucmre, and (iv) a burst strengm of not less than 200 grams. 11. A method of making a light-weight, low-density abosorbent strucmre which comprises the steps of: providing cellulose fibers in an amount of between about 85 and about 95 percent of the fibrous basis weight of the structure; adding cellulose acetate fibers to the cellulose fibers in an amount of between about 5 and about 15 percent of die fibrous basis weight of die structure; forming the cellulose and the cellulose acetate fibers into a fibrous web; adding triacetin to said fibrous web; and curing me fibrous web at a temperamre sufficient to bond the cellulose acetate fibers. 12. A method according to claim 11 wherein said fibrous web is formed by an air laying method. 13. A method according to claim 11 wherein said triacetin is added in an amount of between about 7 and about 10 percent of the total amount of bone dry fiber in the structure. 14. A method according to claim 11 wherein triacetin is added by spraying die triacetin on at least one surface of said fibrous web. 15. A method according to claim 11 which further comprises: combining said cellulose fibers and said cellulose acetate fibers by a wet laid method to form a fibrous sheet; and disintegrating said fibrous sheet to separate the fibers prior to forming said fibers into a fibrous web.

Inventors: BOST CHARLES E, BROUGHTON DENISE H, SCHOGGEN HOWARD L, BOST, CHARLES, E., BROUGHTON, DENISE, H., SCHOGGEN, HOWARD, L.
Applicants: BUCKEYE CELLULOSE CORP, BUCKEYE CELLULOSE CORPORATION
Application year: 1996
Main classification: A61F13
IPC codes: A61F13