Compression-resistant sanitary napkin

Abstract: A sanitary napkin for placement in a crotch portion of an undergarment of a wearer having a longitudinal axis and being further characterized by having at least one preferential bending zone extending obliquely in relation to the longitudinal axis. The preferential bending zone may be created by mechanically embossing the surface of the sanitary napkin to locally densify the absorbing materials of the article. The preferential bending zone causes the sanitary napkin to resist lateral compression forces exerted by the thighs of the wearer and thereby prevents bunching of the article on the undergarment The preferential bending zone also enhances fluid distribution by channeling the fluid away from the point of contact. The advantages of the present invention are increased efficiency of the sanitary napkin in collecting bodily fluids and greater comfort for the wearer of the napkin, particularly when the napkin is thin.

Description: FIELD OF THE INVENTION The present invention relates generally to the art of fluid absorption and more particularly to a disposable sanitary napkin that is thin yet resistant to compression by lateral forces. BACKGROUND OF THE INVENTION One element that contributes to the performance of a sanitary napkin is the way the napkin withstands deformation when in use. It has been observed that the thighs of an individual exert lateral forces on a sanitary napkin when the article is positioned for use on the undergarment. This results in bunching of the sanitary napkin, affecting the efficiency of the article's ability to collect bodily fluids since there is a concomitant reduction in the article's surface area. This observation is especially true for thin sanitary napkins that are highly flexible. One approach to minimizing this problem is to make the sanitary napkin stiffer in order that it may better withstand the effects of lateral compression. This approach may, however, have an adverse effect on the comfort potential of the sanitary napkin because the added stiffness affects the movement of the napkin in all directions, making it more difficult for the napkin to conform to the natural shape of the wearer's body. According to US 3,993,820 a sanitary napkin can be improved due to a densified layer on a highly porous, loosely compacted batt having thickened and unthickened portions to provide an increased flow of fluid within the densified thickened portions of the absorbent structure, thereby distributing a greater amount of fluid to the outer areas of the sanitary napkin. The densified thickened portions can be arranged in strips or lines or can have a diamond-like shape. These portions can also be continuous or non-continuous. In the latter case the flexibility in the sections being void of densified-thickened portions tends to be increased. GB 1,236,904 discloses a sanitary napkin having a channelled resin pattern in a diamond-shaped form that serves to strengthen the covering web and anchors the non-woven web to the moisture-permeable layer. It also provides channels to conduct the liquid discharge away from its area of deposition to improve the absorbency. Finally, the channelling should improve the flexibility of the article and provide a pleasing design. DE 36 20 077 A1 discloses a sanitary napkin having embossments to improve the stiffness of the napkin and also the fluid flow. WO 97/09015 discloses a sanitary napkin having preferential bending zones outside the stabilizing zone of a sanitary napkin and extending from one longitudinal side to the other. The bending zones may be due to a change in the flexibility of the napkin along its length or due to a seam, space or embossment. The sanitary napkin according to WO 97/09015 provides stiffness in the lateral direction but not in the longitudinal direction. Against this background, it can be seen that there exists a need to provide a sanitary napkin that is comfortable and yet reduces the likelihood of bunching when in use, leading to an overall increase in its efficiency for collecting bodily fluids. SUMMARY OF THE INVENTION It has been discovered that when a sanitary napkin is provided with a preferential bending zone that extends obliquely to the longitudinal axis of the napkin, the effect is to stiffen the napkin in a lateral direction and thus enhance the napkin's ability to resist bunching. This oblique bending zone extends from one longitudinal side area of the sanitary napkin to the opposite longitudinal side area, crossing the imaginary longitudinal axis of the sanitary napkin. One advantage proffered by this preferential bending zone is that it stiffens the napkin in the transverse direction and also in the longitudinal direction by virtue of it orientation with relation to the longitudinal axis. The result is an engineered rigidity that creates resistance in the sanitary napkin to lateral compression and at the same time does not make the napkin overly stiff. Advantageously, the sanitary napkin is able to twist along its longitudinal axis and thereby conform to the shape of the wearers body, making the sanitary napkin comfortable. The obliqueness requirement of the preferential bending zone is met as long as a segment of the preferential bending zone is oblique with relation to the longitudinal axis. Thus, even when a part of the preferential bending zone is not oblique with relation to the longitudinal axis, the obliqueness requirement is still met as long as another portion is oblique. Preferably, the segment of the preferential bending zone that is oblique represents at least 25% of the length of the preferential bending zone, more preferably 50% and most preferably 100% of the length of the preferential bending zone. Preferably, the segment that is oblique crosses the longitudinal axis. The preferential bending zone can be straight, arcuate, form a repeating pattern, such as a wavy pattern or a zigzag pattern or a combination of these shapes. When a repeating pattern is present in a segment of the preferential bending zone, the obliqueness of that segment is determined by considering am imaginary line that is contained within the repeating pattern and that either forms or at least approximates a line of symmetry for the pattern. In one possible form of implementation, the preferential bending zone crosses the longitudinal axis of the sanitary napkin in the central region of the sanitary napkin. The central region is the region of the sanitary napkin that registers with the vaginal opening of the user when the sanitary napkin is worn. In one aspect of the invention, the sanitary napkin has a plurality of oblique preferential bending zones that are disposed across the surface of the main body of the article and intersect each other. Under this example of implementation, the oblique preferential bending zones are arcuate. One possible method of creating the preferential bending zones is to mechanically emboss the surface of the main body of the sanitary napkin, resulting in a series of local densifications of the absorbent material. Another advantage afforded by the oblique preferential bending zones so created is that they contribute to a more uniform distribution of bodily fluids along the surface of the sanitary napkin by channeling the fluids away from the point of contact. The sanitary napkin is also more efficient in collecting bodily fluids, since a larger surface area is available for contact with the fluids than when bunching occurs. Advantageously, the sanitary napkin includes a fluid-permeable cover layer, an absorbent system comprised of a first absorbent layer and a second absorbent layer, and a fluid-impervious barrier layer. The sanitary napkin, in one specific form of implementation also includes a fastener for retaining the main body of the napkin to an undergarment. This fastener may comprise at least one flap projecting laterally from a longitudinal side of the main body of the sanitary napkin when the napkin is in a flattened state and being capable of being folded over a crotch portion of a wearers undergarment. Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a top elevational view of a sanitary napkin in accordance with the present invention, the cover layer of the sanitary napkin being partly removed to show the absorbent system; Figure 2 is a perspective view of the sanitary napkin of Figure 1, depicted in a position attained when the sanitary napkin is placed in the undergarment of a wearer; Figure 3 is a bottom plan view of the sanitary napkin shown in Figure 1; Figure 4 is a cross-sectional view taken along the longitudinal axis of the sanitary napkin shown in Figure 3; Figure 5 is a schematic illustration of means for air-laying absorbent material for making an example of an absorbent layer of the sanitary napkin according to the invention, using four air-laying heads followed by means for compacting the air-laid material; and Figures 6(a) and 6(b) show three and four stratum embodiments, respectively, of the absorbent layer that can be used in the sanitary napkin in accordance with the invention. DETAILED DESCRIPTION Referring to Figures 1 and 2, there is shown an embodiment of the present invention, a feminine sanitary napkin 20. The sanitary napkin 20 has a main body 22 with a first transverse side 26 defining a front portion thereof and a second transverse side 28 defining a rear portion thereof. Each of these sides is arcuate or of any other suitable shape. The main body also has two longitudinal sides, namely a longitudinal side 30 and a longitudinal side 32. In a specific example of implementation, the sanitary napkin 20 has a thickness not exceeding about 5 mm. Preferably, the thickness is less than 3.5 mm, more preferably less than 3 mm, and most preferably, it is of about 2.8 mm. It should be noted that the invention is not limited to products having a thickness of less than 5 mm and can be used on sanitary napkins that have a thickness exceeding 5 mm. The sanitary napkin 20 has a longitudinal centerline 34 that is an imaginary line bisecting the sanitary napkin 20 in two identical halves. The sanitary napkin 20 shown in the drawings has flaps 38, 40. The flaps 38, 40 project laterally outward from each of the longitudinal sides 30, 32. The flaps 38, 40 are in the shape of an isosceles trapezoid with the top adjoining the longitudinal side and the base at the distal end. This is an example only as other flap shapes can also be used without departing from the spirit of the invention. Furthermore, the present invention is not limited to a sanitary napkin with flaps as the present inventive concept can also be embodied in a sanitary napkin without flaps. The main body 22 also has an imaginary transverse centerline 36 perpendicular to the longitudinal centerline 34 and simultaneously bisecting the flaps 38, 40. As depicted in Figure 4, the main body 22 is of a laminate construction and preferably comprises a fluid-permeable cover layer 42, an absorbent system 44, and a fluid-impervious barrier layer 50. The absorbent system has preferably two components, namely a first absorbent layer 46 (commonly known as "transfer layer") and a second absorbent layer 48 (commonly known as "absorbent core"). Alternatively, a single layer, namely the second absorbent layer 48, can form the absorbent system 44. Each of these layers is described in hereinbelow. Main Body-Cover Layer The cover layer 42 may be a relatively low density, bulky, high-loft non-woven web material. The cover layer 42 may be composed of only one type of fiber, such as polyester or polypropylene or it may be composed of bi-component or conjugate fibers having a low melting point component and a high melting point component. The fibers may be selected from a variety of natural and synthetic materials such as nylon, polyester, rayon (in combination with other fibers), cotton, acrylic fiber and the like and combinations thereof. An example is the non-woven cover layer of sanitary napkins sold by Johnson & Johnson Inc. of Montreal, Canada under the trademark Stayfree Ultra-Thin Cottony Dry Cover. Bi-component fibers may be made up of a polyester core and a polyethylene sheath. The use of appropriate bi-component materials results in a fusible non-woven fabric. Examples of such fusible fabrics are described in U.S. Patent 4,555,446 issued November 50, 1985 to Mays. Using a fusible fabric increases the ease with which the cover layer may be mounted to the adjacent first absorbent layer and/or to the barrier layer. The cover layer 42 preferably has a relatively high degree of wettability, although the individual fibers comprising the cover may not be particularly hydrophilic. The cover material should also contain a great number of relatively large pores. This is because the cover layer 42 is intended to take-up body fluid rapidly and transport it away from the body and the point of deposition. Advantageously, the fibers which make up the cover layer 42 should not lose their physical properties when they are wetted, in other words they should not collapse or lose their resiliency when subjected to water or body fluid. The cover layer 42 may be treated to allow fluid to pass through it readily. The cover layer 42 also functions to transfer the fluid quickly to the other layers of the absorbent system 44. Thus, the cover layer 42 is advantageously wettable, hydrophilic and porous. When composed of synthetic hydrophobic fibers such as polypropylene or bi-component fibers, the cover layer 42 may be treated with a surfactant to impart the desired degree of wettability. Alternatively, the cover layer 42 can also be made of polymer film having large pores. Because of such high porosity, the film accomplishes the function of quickly transferring body fluid to the inner layers of the absorbent system. Apertured co-extruded films such as described in U.S. Patent 4,690,679 and available on sanitary napkins sold by Johnson & Johnson Inc. of Montreal, Canada could be useful as cover layers in the present invention. The cover layer 42 may be embossed to the remainder of the absorbent system 44 in order to aid in promoting fluid transport by fusing the cover to the next layer. Such fusion may be effected locally, at a plurality of sites or over the entire contact surface of cover layer 42 with absorbent system 44. Alternatively, the cover layer 42 may be attached to the absorbent system 44 by other means such as by adhesive. Main Body - Absorbent System -First Absorbent Layer Adjacent to the cover layer 42 on its inner side and bonded to the cover layer 42 is a first absorbent layer 46 that forms part of the absorbent system 44. The first absorbent layer 46 provides the means of receiving body fluid from the cover layer 42 and holding it until an underlying second absorbent layer has an opportunity to absorb the fluid. The first absorbent layer 46 is, preferably, more dense than and has a larger proportion of smaller pores than the cover layer 42. These attributes allow the first absorbent layer 46 to contain body fluid and hold it away from the outer side of the cover layer 42, thereby preventing the fluid from re-wetting the cover layer 42 and its surface. However, the first absorbent layer 46 is, preferably, not so dense as to prevent the passage of the fluid through the layer 46 into the underlying second absorbent layer 48. These types of absorbent layers are commonly known as fluid transfer layers or acquisition layers. The first absorbent layer 46 may be composed of fibrous materials, such as wood pulp, polyester, rayon, flexible foam, or the like, or combinations thereof. The first absorbent layer 46 may also comprise thermoplastic fibers for the purpose of stabilizing the layer and maintaining its structural integrity. The first absorbent layer 46 may be treated with surfactant on one or both sides in order to increase its wettability, although generally the first absorbent layer 46 is relatively hydrophilic and may not require treatment. The first absorbent layer 46 is preferably bonded on both sides to the adjacent layers, i.e. the cover layer 42 and an underlying second absorbent layer 48. An example of a suitable first absorbent layer is a through air bonded pulp sold by BUCKEYE of Memphis Tennessee under the designation VIZORB 3008. Main Body - Absorbent System-Second Absorbent Layer Immediately adjacent to and bonded to the first absorbent layer 46 is the second absorbent layer 48. In one embodiment, the first absorbent layer 46 has a central width that is at least about the same as the central width of the second absorbent layer 48. In a specific embodiment, this central width is greater than about 64mm. In another embodiment, the first absorbent layer 46 has a central width that exceeds the central width of the second absorbent layer 48. The term "central width" refers to a specific area of a layer, such as an absorbent layer determinable as follows. A reference point on the sample layer that is disposed beneath the center of the vaginal orifice, when worn, is located. A plane parallel to the transverse centerline 36 and 3.75 centimeters forward from the reference point in the direction of the wearer's mons pubis is located. Another plane parallel to the lateral centerline 36 and 5.0 centimeters rearward from the reference point in the direction of the wearers buttocks is also located. The greatest flat-out, uncompressed, unmanipulated, lateral width of the sample layer between the two planes is the absorbent width of the sample layer. The central width of the absorbent system, when the absorbent system includes a plurality of absorbent layers is the central width of the layer of the absorbent system that has the largest central width. In a specific example, the central width of the absorbent system exceeds 64 mm. In one embodiment, the second absorbent layer 48 is a blend or mixture of cellulosic fibers and superabsorbent disposed in and amongst fibers of that pulp. In a specific example, the second absorbent layer 48 is a material containing from about 40 weight percent to about 95 weight percent cellulosic fibers; and from about 5 weight percent to about 60 weight percent SAP (superabsorbent polymers). The material has a water content of less than about 10 weight percent As used herein, the phrase "weight percent" means weight of substance per weight of final material. By way of example, 10 weight percent SAP means 10 g/m 2 SAP per 100g/m2 basis weight of the material. Cellulosic fibers that can be used in the second absorbent layer 48 are well known in the art and include wood pulp, cotton, flax and peat moss. Wood pulp is preferred. Pulps can be obtained from mechanical or chemimechanical, sulfite, Kraft, pulping reject materials, organic solvent pulps, etc. Both softwood and hardwood species are useful. Softwood pulps are preferred. It is not necessary to treat cellulosic fibers with chemical debonding agents, cross-linking agents and the like for use in the present material. The second absorbent layer 48 can contain any superabsorbent polymer (SAP), which SAPs are well known in the art. For the purposes of the present invention, the term "superabsorbent polymer" (or "SAP") refers to materials which are capable of absorbing and retaining at least about 10 times their weight in body fluids under a 0.5 psi pressure. The superabsorbent polymer particles of the invention may be inorganic or organic crosslinked hydrophilic polymers, such as polyvinyl alcohols, polyethylene oxides, crosslinked starches, guar gum, xanthan gum, and the like. The particles may be in the form of a powder, grains, granules, or fibers. Preferred superabsorbent polymer particles for use in the present invention are crosslinked polyacrylates, such as the product offered by Sumitomo Seika Chemicals Co., Ltd. Of Osaka, Japan, under the designation of SA60N Type II*, and the product offered by Chemdal International, Inc. of Palatine, Illinois, under the designation of 2100A*. In a specific example the second absorbent layer 48 is a material containing from about 50 to about 95 weight percent cellulosic fibers and, more specifically from about 60 to about 80 weight percent cellulosic fibers. Such a material may contain from about 5 to about 60 weight percent SAP, preferably from about 20 to about 55 weight percent SAP, even more preferably from about 30 to about 45 weight percent SAP, and most preferably about 40 weight percent SAP. The second absorbent layer 48 can be manufactured by using air-laying means well known in the art (See Figure 5). In accordance with Figure 5, cellulosic fibers (e.g., pulp) are processed using a hammer mill to individualize the fibers. The individualized fibers are blended with SAP granules in a blending system 1 and pneumatically conveyed into a series of forming heads 2. The blending and distribution of fibers and SAP granules can be controlled separately for each forming head. Controlled air circulation and winged agitators in each chamber produce uniform mixture and distribution of pulp and SAP. The SAP can be thoroughly and homogeneously blended throughout the material or contained only in specific strata by distributing it to selected forming heads. Fibers (and SAP) from each forming chamber are deposited by vacuum onto a forming wire 3 thus forming a layered absorbent web. The web is subsequently compressed using calenders 4 to achieve desirable density. The densified web is wound into a roll 5 using conventional winding equipment. The forming wire 3 can be covered with tissue paper to reduce the loss of material. The tissue paper layer can be removed prior to calendering or incorporated into the formed material. In a possible variant, the first absorbent layer 46 can be formed integrally with the second absorbent layer 48 to provide a unitized absorbent system 44. This can be achieved by providing the apparatus depicted in Figure 5 with an additional forming head (not shown in the drawings) to deposit on the second absorbent layer 48, by air laying and prior to calendering, a layer of material to form the first absorbent layer 46. The second absorbent layer 48 of the present invention is of high density and in a specific example has a density of greater than about 0.25 g/cc. Specifically, the second absorbent layer 48 may have a density in the range of from about 0.30 g/cc to about 0.50 g/cc. More specifically, the density is from about 0.30 g/cc to about 0.45 g/cc and, even more specifically from about 0.35 g/cc to about 0.40 g/cc. Air-laid absorbents are typically produced with a low density. To achieve higher density levels, such as the examples of the second absorbent layer 48 given above, the air-laid material is compacted using calenders as shown in Figure 5. Compaction is accomplished using means well known in the art. Typically such compaction is carried out at a temperature of about 100 degrees C and a load of about 130 Newtons per millimeter. The upper compaction roll is typically made of steel while the lower compaction roll is a flexroll having a hardness of about 85 SH D. It is preferred that both the upper and lower compaction rolls be smooth, although the upper roll can be engraved. In one embodiment the second absorbent layer 48 has a ratio of Gurley stiffness, measured in milligrams (mg) to density, measured in grams per cubic centimeter (g/cc), of less than about 3700. In a specific example, that ratio of Gurley stiffness to density is less than about 3200 and, more specifically, less than about 3000. Gurley stiffness is one of many indices of softness. Gurley stiffness measures the bendability or flexibility of absorbent materials. The lower the Gurley stiffness value, the more flexible the material. The Gurley stiffness values are measured using a Gurley Stiffness Tester (Model No. 4171E), manufactured by Gurley Precision Instruments of Troy, N.Y. The instrument measures the externally applied moment required to produce a given deflection of a test strip of specific dimensions fixed at one end and having a concentrated load applied to the other end. The results are obtained in "Gurley Stiffness" values in units of milligrams. The second absorbent layer 48 is strong in light of its softness. Pad integrity is a well-known measurement of absorbent material strength. In a specific embodiment the second absorbent layer 48 demonstrates strength (high pad integrity) over a wide range of densities. In a specific example the second absorbent layer 48 has a pad integrity, measured in Newtons (N), to density (g/cc) ratio of greater than about 25.0. In a more specific example, that ratio is greater than about 30.0 and, could even be greater than about 35.0. The pad integrity is a test performed on an Instron Universal Testing Machine. Essentially, the test measures the load required to pierce through the test sample, as described in the PFI Method of 1981. A test sample having dimensions of 50 mm by 50 mm is clamped on the Instron with a suitable fastening device. A 20 mm diameter piston traveling at the rate of 50 mm/min punctures the stationary sample. The force required to puncture the sample is measured in Newtons (N). The second absorbent layer 48 can be prepared over a wide range of basis weights. The second absorbent layer 48 can have a basis weight in the range of from about 100 g/m2to about 700 g/m2. In a specific example, the basis weight ranges from about 150 g/m2 to about 350 g/m2. Preferably the basis weight ranges from about 200 g/m2 to about 300 g/m2 and, more preferably, to about 250 g/m2. The second absorbent layer 48 can be formed as three or four lamina or strata. Those strata include a bottom layer, one or two middle layers and a top layer. Specific examples of three and four layer material are set forth below. The SAP can be included in any or all of the layers. The concentration (weight percent) of SAP in each layer can vary as can the nature of the particular SAP. An interesting characteristic of the second absorbent layer 48 is its ability to retain SAP when subjected to mechanical stress. The second absorbent layer 48 retained over 85 percent by weight of its SAP content when subjected to 10 minutes of rigorous shaking. Specifically, a material of this invention retains over 90 percent, more specifically over 95 percent and, even more specifically over 99 percent of its SAP under these mechanical stresses. The percent of SAP retained was determined by shaking the material in a Ro-Tap Sieve Shaker manufactured by W. S. Tyler Co., Cleveland Ohio. More specifically the sample is placed in a 28-mesh (Tyler series) sieve. Additional sieves of 35-mesh and 150-mesh were attached to the first sieve forming a column of increasingly fine sieves. The column of sieves was capped on either end to prevent the loss of fiber and/or SAP. The sieve column was placed in the shaker and agitated for 10 minutes. The amount of SAP granules shaken loose from the sample, "free SAP", was determined by combining the residue contained in each of the sieves and separating the cellulosic fiber from the SAP. Even where prepared as from multiple layers, the final thickness of the formed second absorbent layer 48 is low. The thickness can vary from about 0.5 mm to about 2.5 mm. In a specific example, the thickness is from about 1.0 mm to about 2.0 mm and, even more specifically from about .25 mm to about 1.75 mm. One embodiment of the second absorbent layer 48 particularly well suited for use in the sanitary napkin 20 is depicted in Figure 6(a). Such second absorbent layer 48 has a basis weight of from about 200 g/m2 to about 350 g/m2 and a density between about 0.3 g/cc and 0.5 g/cc. In a specific example, the density is from about 0.3 g/cc to about 0.45 g/cc and, more specifically about 0.4 g/cc. The second absorbent layer 48 depicted in Figure 6(a) is air-laid as three strata: a bottom layer of pulp (without superabsorbent) with a basis weight of about 25 g/m2; a middle layer with a basis weight of about 150 g/m2 and which contains from about 10 to about 30 g/m2 superabsorbent and from about 120 g/m2 to about 140 g m2 pulp; and a top layer of pulp (without superabsorbent) with a basis weight of about 25 g/m2. Relative to the total basis weight of the second absorbent layer 48, the level of superabsorbent ranges from about 5 to about 15 weight percent (g/m2 of superabsorbent per g/m2 material). In a specific example, the level of superabsorbent is from about 7.5 weight percent to about 12.5 weight percent of the material. More specifically, the material contains about 10 weight percent of superabsorbent. Thus, the middle layer of the material could contain from about 15 g/m2 to about 25 g/m2 superabsorbent and from about 125 g/m2 to about 135 g/m2 pulp and, more specifically about 20 g/m2 superabsorbent and about 130 g/m2 pulp. The middle layer containing pulp and superabsorbent can be laid down as a homogeneous blend or as a heterogeneous blend wherein the level of superabsorbent varies with proximity to the bottom layer. In another embodiment depicted in Figure 6(b), the second absorbent layer 48 is air-laid as four strata. In this embodiment, the middle layer referred to above is replaced with two middle layers: a first middle layer adjacent the top layer and a second middle layer adjacent the bottom layer. Each of the first and second middle layers independently comprises from about 10 to about 30 g/m2 superabsorbent and from about 40 g m2 to about 65 g/ m2 pulp. When it is desired to keep absorbed fluid away from the cover layer 42 the amount of superabsorbent in the first and second middle layers is adjusted such that there is a higher level of superabsorbent in the second middle layer. The superabsorbent in the first and second middle layers can be the same or a different superabsorbent. In one embodiment, the cellulosic fiber for use in the second absorbent layer 48 is wood pulp. There are certain characteristics of wood pulp that make it particularly suitable for use. Cellulose in most wood pulps has a crystalline form known as Cellulose I which can be converted to a form known as Cellulose II. In the second absorbent layer 48, wood pulp with a substantial portion of the cellulose as Cellulose II could be used. Similarly, pulps having an increased fiber curl value are advantageous. Finally, pulps having reduced levels of hemicellulose are preferred. Means for treating pulps so as to optimize these characteristics are well known in the art. By way of example, treating wood pulp with liquid ammonia is known to convert cellulose to the Cellulose II structure and to increase the fiber curl value. Flash drying is known to increase the fiber curl value of pulp. Cold caustic treatment of pulp decreases hemicellulose content, increases fiber curl and converts cellulose to the Cellulose II form. Thus it could be advantageous that the cellulosic fibers used to produce the material of this invention contain at least a portion of cold caustic treated pulp. A description of the cold caustic extraction process can be found in U.S. patent application Ser. No. 08/370,571, filed on Jan. 18, 1995, pending which application is a continuation-in-part application of U.S. patent application Ser. No. 08/184,377, now abandoned filed on Jan. 21, 1994. The disclosures of both of these applications are incorporated in their entirety herein by reference. Briefly, a caustic treatment is typically carried out at a temperature less than about 60 degree C., but preferably at a temperature less than 50 degree C., and more preferably at a temperature between about 10 degree C. to 40 degree C. A preferred alkali metal salt solution is a sodium hydroxide solution newly made up or as a solution by-product in a pulp or paper mill operation, e.g., hemicaustic white liquor, oxidized white liquor and the like. Other alkali metal salts such as ammonium hydroxide and potassium hydroxide and the like can be employed. However, from a cost standpoint, the preferable salt is sodium hydroxide. The concentration of alkali metal salts is typically in a range from about 2 to about 25 weight percent of the solution, and preferably from about 6 to about 18 weight percent. Pulps for high rate, fast absorbing applications are preferably treated with alkali metal salt concentrations from about 10 to about 18 weight percent. For further details on the structure and the method of construction of the second absorbent layer 48 the reader is invited to refer to the US patent 5,866,242 granted on February 2, 1999 to Tan et al. The contents of this document are hereby incorporated by reference. Main Body-Barrier Layer Underlying the absorbent system 44 is a barrier layer 50 comprising liquid-impervious film material so as to prevent liquid that is entrapped in the absorbent system 44 from egressing the sanitary napkin and staining the wearer's undergarment. The barrier layer 50 is made preferably of polymeric film. The cover layer 42 and the barrier layer 50 are joined along their marginal portions so as to form an enclosure or flange seal that maintains the absorbent system 44 captive. The joint may be made by means of adhesives, heat-bonding, ultrasonic bonding, radio frequency sealing, mechanical crimping, and the like and combinations thereof. The peripheral seal line is shown in Figure 1 by the reference numeral 52. Flaps The flaps 38 and 40 are preferably made as integral extensions of the cover layer 42 and the barrier layer 50. These integral extensions are joined to one another along their marginal seal portions by adhesives, heat-bonding, ultrasonic bonding, radio frequency sealing, mechanical crimping, and the like and combinations thereof. Most preferably, such joining is made at the same time the cover layer 42 and the barrier layer 50 are bonded to one another to enclose the absorbent system 44. Alternatively, the flaps may include absorbent material between the cover layer and the barrier layer extensions. Such absorbent material may be an extension of the first absorbent layer 46, the second absorbent layer 48 or both. Adhesive system Referring to Figures 2 and 3, in order to enhance the stability of the sanitary napkin, the garment facing surface of the barrier layer is provided with positioning adhesive material 58, typically hot-melt adhesive material capable of establishing a temporary bond with the undergarment material. A suitable material is the composition designated HL-1491 XZP commercially available from H.B. Fuller Canada, Toronto, Ontario, Canada. The positioning adhesive 58 may be applied to the garment-facing surface of the barrier layer 50 in various patterns, including complete adhesive coverage, parallel longitudinal lines, a line of adhesive following the perimeter of the structure, transverse lines of adhesive or the like. Standard release paper 82 (shown only in Figure 3) covers the positioning adhesive 58 before the napkin is used to prevent the unwanted adherence of the napkin to itself or foreign objects. The release paper is of conventional construction (e.g. silicone coated wet-laid Kraft wood pulp) and suitable papers are available from Tekkote Corporation (Leonia, New Jersey, USA), and bear the designation FRASER 30#/61629. Referring back to Figure 1, the main body of the sanitary napkin is provided with at least one preferential bending zone. In one possible form of implementation, the preferential bending zone crosses the longitudinal axis of the sanitary napkin in the central region of the sanitary napkin. It is not necessary that the preferential bending zone is oblique with relation to the longitudinal axis over its entire length. Preferably, the segment of the preferential bending zone that is oblique represents at least 25% of the length of the preferential bending zone, more preferably 50% and most preferably 100% of the length of the preferential bending zone. Preferably, the segment that is oblique crosses the longitudinal axis. In the specific example of implementation illustrated in the drawings, the sanitary napkin 20 has a plurality of oblique preferential bending zones that intersect each other to create a pattern extending over a major portion of the longitudinal dimension of the main body. The preferential bending zones are oblique over 100% of their length with relation to the longitudinal axis and they are arcuate. In a variant the oblique preferential bending zones could also be straight or form a repeating pattern, such as a wavy pattern or a zigzag pattern. The oblique preferential bending zones are created on the cover layer and absorbent system. Each preferential bending zone extends generally along an angle of 45 degrees with respect to the longitudinal axis or lateral axis of the main body of the sanitary napkin. The pattern is designed such that each preferential bending zone intersects at least two other preferential bending zones. Also, each preferential bending zone extends from one longitudinal side area of the sanitary napkin to the opposite longitudinal side area, crossing the imaginary longitudinal axis of the sanitary napkin. A longitudinal side area is defined as a portion of the sanitary napkin that extends inwardly from and includes a respective longitudinal side edge 30, 32, the side edge forming the outer boundary of the side area. Each side area has a width that is about 25% of the maximal transverse dimension of the sanitary napkin 20. This dimension is measured without taking into account the flaps. It is the maximal distance defined between the longitudinal side edges 30, 32 of the sanitary napkin 20. In a specific example, the spacing between parallel embossing lines is about 2 cm. In an alternative embodiment, the preferential bending zones are created on the absorbent system only, so that they are less visible on the sanitary napkin than when the preferential bending zones are made on the cover layer and absorbent system. Method of manufacture The above-described embodiment of the sanitary napkin 20 is fabricated in a conventional manner in accordance with conventional techniques. Specifically, a laminate structure, sometimes referred to in the art as a web, is created. This laminate structure comprises an expanse of the materials from which the napkin will be created. I.e. the laminate structure comprises the following layers of material in a top-to-bottom order: an expanse of cover layer material; an expanse of first absorbent layer material; an expanse of second absorbent layer material (manufactured as described above); and finally an expanse of barrier layer material. Some of the materials are necessarily not continuous within the laminate structure, and where such is the case, they are positioned precisely, one with respect to another, in the relationship they will occupy in the final products. The cover layer material and the barrier layer material are then bonded together by applying pressure in the appropriate positions, and what will become the peripheral seal is created. The seal may also be made by means of heat-bonding, ultrasonic bonding, radio frequency sealing, mechanical crimping, and the like and combinations thereof. The sealed structure is then severed by conventional means (i.e. die-cutting, fluid-jet cutting, or by laser) from the web to create a discrete article. The preferential bending zones are created preferably by embossing. The choice of embossing is not critical since the same result may be obtained by other methods also such as slitting, perforating or other techniques known to those skilled in the art. If the embossing operation is chosen to create the preferential bending zones, the sanitary napkin is passed between a pair of rolls, one of the rolls including projections according to the pattern of embossing desired. The projections locally compress the material of the sanitary napkin, which can be either a combination of the cover layer and the absorbent system or the absorbent system alone, thereby compacting it. The degree of pressure applied during the embossing operation can vary depending upon the type of material embossed and the physical integrity of the material embossed, among others. It is within the reach of a person skilled in the art to find the optimal process conditions in accordance with the specific application. In general, the embossing pressure should be selected to sufficiently densify the material locally so as to create the preferential bending zones and at the same time not to be too high so as to sever the material. Heating the embossing rolls has been found beneficial. Also, ultrasonic embossing may be used for forming the preferential bending zones. It is preferred to emboss the entire sanitary napkin as the embossing also holds the various layers of the sanitary napkin together and reduces the likelihood of the cover layer or the barrier layer gapping or coming loose when the sanitary napkin is bent. The positioning adhesive material is then applied to the barrier layer in the appropriate positions, and release paper is applied to cover the positioning adhesive. Alternatively, the positioning adhesive, or the positioning adhesive and the release paper may be applied to the web before the individual articles are severed therefrom. As indicated earlier, the sanitary napkin 20 has preferably a thickness of about 5 mm or less. The apparatus required to measure the thickness of the sanitary napkin is a footed dial (thickness) gauge, available from Ames, with foot 1 1/8" diameter with stand, 2 oz. deadweight accurate to 0.001". A digital type apparatus is preferred. If the sanitary napkin sample is individually folded and wrapped, the sample is unwrapped and carefully flattened by hand. The release paper is removed from the sample and it is repositioned back gently across the positioning adhesive lines so as not to compress the sample, ensuring that the release paper lies flat across the sample. Flaps (if any) are folded back under the sample, prior to taking the thickness reading in the center of the sample. The foot of the gauge is raised and the sample is placed on the anvil such that the foot of the gauge is approximately centered to the sample (or in the location of interest on the sample of interest). When lowering the foot, care is taken to avoid allowing the foot to "drop" or that undue force is not applied. The read out is allowed to stabilize for approximately 5 seconds. The thickness reading is then taken. Applications of the product and methods of the present invention for sanitary and other health-care uses can be accomplished by any sanitary protection, incontinence, medical and absorbent methods and techniques as are presently or prospectively known to those skilled in the art. Thus, it is intended that the present application cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.

Claims: A sanitary napkin for placement in a crotch portion of an undergarment of a wearer, said sanitary napkin having a main body with a longitudinal axis, two opposite longitudinal side areas and a central region intermediate said two opposite side areas, said sanitary napkin comprising a preferential bending zone on said main body extending from one longitudinal side area of the sanitary napkin to an opposite longitudinal side area, said sanitary napkin being characterised in that said preferential bending zone includes a segment crossing the longitudinal axis of the sanitary napkin, said segment crossing the longitudinal axis obliquely in relation to said longitudinal axis. A sanitary napkin as defined in claim 1, wherein said sanitary napkin having a thickness not exceeding about 5 mm. A sanitary napkin according to claim 1, comprising a plurality of preferential bending zones spaced apart from one another. A sanitary napkin according to claim 3, wherein said preferential bending zones intersect each other. A sanitary napkin according to claim 3, wherein said preferential bending zones are operative to create resistance to lateral compression when said sanitary napkin is in use. A sanitary napkin according to claim 1, wherein said preferential bending zone is arcuate.

Inventors: FELDMAN REBECCA, LARIVIERE CHRISTIANE, WEITZNER LORI, FELDMAN, REBECCA, LARIVIERE, CHRISTIANE, WEITZNER, LORI
Applicants: JOHNSON & JOHNSON INC, JOHNSON & JOHNSON INC.
Application year: 2000
Main classification: A61F13
IPC codes: A61F13