Absorbent articles are typically used in contact with skin. Some absorbent articles such as disposable diapers, feminine pads, panty liners, incontinence pads and the like are held in contact with skin to absorb body liquids or exudates, while other absorbent materials such as paper towels, hand towels, and wipers may be held in the hands to absorb liquid on the skin or other surfaces. In virtually every case, it is desired that the absorbent article or material keep liquids off the skin to provide a clean, dry feel and to reduce skin health problems that arise from excess hydration or from contact with harmful biological or chemical materials in the liquid being absorbed.
While paper towels and wipers are often composed of a homogenous material, such as an entirely cellulosic web, absorbent articles intended to absorb body fluids typically have at least three layers of different materials. Next to the user""s skin is a topsheet layer, sometimes herein referred to as a liner, body-side liner or cover sheet. Beneath the topsheet is the absorbent core that is designed to retain liquid, and beneath the absorbent core is a fluid-impervious backsheet that prevents leakage and maintains the integrity of the product. The topsheet should feel soft and should have high liquid permeability to allow body fluid such as urine, menses, or runny bowel movement to be absorbed and transported away from the skin to reach the central absorbent core. Ideally, the topsheet provides a xe2x80x9cdry touchxe2x80x9d or xe2x80x9cdry feelxe2x80x9d by preventing liquid from flowing back to the skin. It is also desirable that the topsheets have high wet resiliency to maintain their bulk and shape when wet.
Traditional hydrophilic cover materials or topsheets in contact with the skin can serve effectively to transport body fluids into the absorbent core, but they cause a wet feel against the skin of the user and may adversely affect skin health. Further, they may wick liquid in the plane of the layer, allowing liquid to approach the edges of the absorbent article and possibly leak or seep out.
To achieve the goal of softness and a dry feel in topsheets of absorbent articles, many manufacturers have turned to nonwoven fabrics made of hydrophobic fibers for the body-contacting topsheet. While the use of hydrophobic nonwoven fabrics may have resulted in improved dry feel, the hydrophobic material hinders wicking into the absorbent core, offers little absorbent capacity and reduces liquid permeability. Further, the poor absorbency of most hydrophobic materials causes any liquid retained therein to be easily squeezed out by body motion of the wearer.
Others have sought to improve the poor wicking and absorbent properties of hydrophobic materials by applying a finish comprising surfactants on the surface of the hydrophobic fibers. This approach may offer some benefits when the article is first wetted, but the surfactants tend to be washed away, resulting in poorer performance upon further wetting.
In the case of absorbent pads for feminine care, two distinct approaches involving hydrophobic topsheets or covers are common. One approach is to use a soft, clothlike nonwoven hydrophobic material, which increases comfort but has the drawback of poor intake of menses. Another approach is to use an apertured plastic film of hydrophobic polymer or other materials. The hydrophobic cover material repels many body fluids, while the apertures allow wicking away from the cover into the absorbent material beneath.
In theory, the hydrophobic apertured material should allow the user""s skin to remain relatively dry while allowing wicking in the z-direction (normal to the plane of the cover) into the underlying absorbent core. In practice, hydrophobic apertured films present a number of problems. Apertured films have the drawback of being disliked by some users for their plastic feel and for their poor absorbency. Their hydrophobic nature resists transport through the material, possibly delaying wicking into the absorbent core. Likewise, pockets or pools of liquid may form between the film and the user""s skin. In the absence of hydraulic pressure or physical compression, menses in particular may pool on the hydrophobic surface and not penetrate into the apertures, especially if there is a significant interfacial gap between the cover and the underlying absorbent material.
Therefore there is a need for an improved topsheet material which provides the clean feel said to be characteristic of hydrophobic topsheet materials, while also providing for rapid z-direction (depthwise) transport of liquid through the topsheet into the underlying absorbent core, a characteristic more typical of hydrophilic materials. Preferably, these absorbent topsheets also have wet resiliency and absorbency properties which persist upon multiple insults of urine or other liquids.
The present invention pertains to composite, resilient materials that offer the once-thought mutually exclusive benefits of high absorbency and a clean, dry feel when used as skin-contacting layers that absorb body fluids or other liquids.
In copending U.S. application, Ser. No. 08/614,420, xe2x80x9cWet Resilient Webs and Disposable Articles Made Therewith,xe2x80x9d by F. -J. Chen et al., herein incorporated by reference, a novel wet-laid tissue web is taught having unusually high bulk, wet resiliency, in-plane permeability, and absorbency. The unusual properties of this material are achieved through a combination of high yield fibers, wet strength additives, and noncompressive drying of a molded, three-dimensional structure. The three-dimensional structure of this material does not collapse readily when wetted and thus reduces the contact area with the skin when wet, contributing to a relatively dry feel. It has been found that the inherently hydrophilic material of this previous invention and related materials can be made substantially more useful in personal care articles by the selective addition of hydrophobic material which can impart increased dry feel and, in some embodiments, improved softness. With hydrophobic material deposited on the uppermost, body-contacting regions of the three-dimensional hydrophilic web, the highest body-contacting regions are made substantially hydrophobic to increase the sensation of a clean, dry feel, while a plurality of hydrophilic regions in said web remain accessible to body fluids, allowing liquids to be wicked away from the body and into an absorbent medium. Thus, dry feel and high absorbency are achieved in a single unitary layer or in a single composite structure which may be a laminate of hydrophobic and hydrophilic materials. The hydrophobic material is bonded or integrally attached to the basesheet. Improved disposable absorbent articles comprising such materials include feminine pads and panty liners, incontinence products such as diapers and liners, bed pads, disposable diapers, pull-ups or disposable training pants, disposable menstrual pants, poultry pads, disposable sweat bands or pads, breast pads, odor absorbing pads for shoes, towels, moisturized wipes, wipers, medical pads, bandages and sterile pads for wounds, disposable garments, liners for helmets or other protective or athletic gear, pads for use in waxing automobiles and other surfaces, and so forth. A simple example of an absorbent article containing a topsheet, absorbent core and a backsheet is illustrated in U.S. Pat. No. 3,809,089 issued May 7, 1974 to Hedstrom et al., which is hereby incorporated by reference.
In general, it has been discovered that the addition of hydrophobic agents. or materials on relatively elevated portions of one surface of a three-dimensional, wet resilient fibrous web, said web predominantly comprising intrinsically hydrophilic fibers, can enhance the suitability of such webs for use in absorbent articles by reducing the amount of fluid that can remain in contact with the skin or flow back to the skin during use as an absorbent article, thus resulting in an improved dry feel. Certain hydrophobic materials such as short, fine synthetic fibers can provide a pleasant soft, fuzzy, and dry feel, while others such as hydrophobic resins, gels, emulsions, waxes, or liquids can increase the apparent smoothness or lubricity of the surface and improve the tactile properties.
Suitable basesheets can be prepared from aqueous slurries of papermaking fibers with known papermaking techniques. The fibers may be derived from wood or other sources of cellulose and preferably containing a portion of high yield or other wet resilient pulp fibers and an effective amount of wet strength agents. The basesheet can be textured by through-drying on a three-dimensional fabric or other means known in the art and preferably non-compressively dried to give a three-dimensional structure. The inherent stiffness of wet resilient pulp fibers may be reduced, if desired, by incorporation of a suitable plasticizer such as glycerol or by mechanical treatment such as microstraining, dry creping, or calendering.
Through-drying fabrics well suited for formation of three-dimensional webs are disclosed in U.S. Pat. No. 5,429,686, issued to Chiu et al., xe2x80x9cApparatus for Making Soft Tissue Products,xe2x80x9d issued Jul. 4, 1995, herein incorporated by reference. Other methods such as wet molding, forming on three-dimensional forming fabrics, drying on nonwoven substrates, rush transfer onto embossing fabrics, embossing, stamping, and so forth may be used to create useful three-dimensional structures. The basesheet may be formed as a unitary multilayer structure in which various plies are well bonded and intimately connected to each other. Unitary multilayer basesheets may be formed using layered or stratified headboxes in which two or more furnishes are provided into separate chambers of a headbox, or they may be formed using separate headboxes by couching the wet webs together prior to drying in order to allow extensive hydrogen bonding to develop between the plies during drying, or they may be formed during air-laying by varying the composition of the fibers and additives imparted to web. Multilayer sheets allow better control of physical properties by tailoring the material composition of each layer. For example, a unitary multilayer basesheet useful for the present invention would have an upper layer, corresponding to the upper surface of the basesheet, and at least one remaining layer below said upper layer and integrally attached thereto, preferably through hydrogen bonds formed between cellulosic fibers during drying, wherein said upper layer differs from at least one remaining layer of the basesheet in terms of material composition. The difference in material composition may be due to differences in fiber species (for example, percentage of hardwood versus softwood); fiber length; fiber yield; fiber treatment with processes which change fiber morphology or chemistry such as mechanical refining, fiber fractionation, dispersing to impart curl, steam explosion, enzymatic treatment, chemical crosslinking, ozonation, bleaching, lumen loading with fillers or other chemical agents, supercritical fluid treatment, including supercritical fluid extraction of agents in the fiber or supercritical fluid deposition of solutes on and into the cell wall, and the like. The difference in material composition between the upper layer and at least one other layer in the basesheet also may be due to differences in added chemicals, including the type, nature, or dosage of added chemicals. The chemicals added differentially to at least one layer of the web may include debonding agents, anti-bacterial agents, wet strength resins, starches, proteins, superabsorbent particles, fiber plasticizers such as glycols, colorants, opacifiers, surfactants, zinc oxide, baking soda, silicone compounds, zeolites, activated carbon, and the like. In a preferred embodiment, the basesheet structure has a wet resilient, noncompressively dried lower layer, preferably composed of softwood fibers, preferably including at least 10% of high yield fiber such as spruce BCTMP, and a soft upper layer containing a portion of finer fibers such as chemically pulped hardwoods. The multilayer basesheet structure is unitary, meaning that the two layers are intimately connected or bonded together. For example, a two-layer unitary basesheet could be formed with a layered headbox or by couching together two wet sheets prior to drying to form intimate contact and hydrogen bonding between the two layers.
The portion of the surface area treated with hydrophobic materials should be great enough to provide an effective improvement in comfort, which will in part depend on the specific product . Accordingly, the fraction of the basesheet surface covered by hydrophobic material can be about 5% or greater, more specifically about 10% or greater, more specifically about 20% or greater, more specifically about 30% or greater, and still more specifically from about 40% to about 75%. The portion of the surface area of the basesheet that remains essentially hydrophilic can be about 10% or greater, more specifically about 20% or greater, more specifically about 30% or greater, more specifically about 40% or greater, more specifically from about 20% to about 90%, and still more specifically from about 50% to about 90%. For effective fluid removal, the lateral width of the depressed hydrophilic regions should be about 0.1 mm or greater, more specifically about 0.5 mm or greater, and still more specifically about 1 mm or greater. The spacing between depressed hydrophilic regions can be about 0.4 mm or greater, more specifically about 0.8 mm or greater, and still more specifically about 1.5 mm or greater. The minimum width of the elevated regions can be about 0.5 mm or greater, more specifically about 1 mm or greater, and still more specifically from about 1 to about 3 mm.
In one preferred embodiment, the hydrophobic matter comprises a substantially contiguous network of hydrophobic fibers having a plurality of macroscopic openings such that a portion of the depressed regions of the basesheet are aligned with openings in the overlaying network of hydrophobic fibers to allow body exudates to pass through the macroscopic openings into the basesheet. A macroscopic opening is defined as an opening that is large relative to the intrinsic pore size of the material. In a typical spunbond or bonded carded web, for example, a macroscopic opening would appear to the eye to be a deliberately introduced hole or void in the web rather than a characteristic pore between adjacent fibers, and specifically could have a characteristic width of about 0.2 mm or greater, about 0.5 mm or greater, about 1 mm or greater, about 2 mm or greater, about 4 mm or greater, about 6 mm or greater, or from about 1 mm to about 5 mm. The characteristic width is defined as 4 times the area of the aperture divided by the perimeter.
The nonwoven web may be made from synthetic fibers, as is known in the art, and may be a spunbond web, a meltblown web, a bonded carded web, or other fibrous nonwoven structures known in the art. For example, a polyolefin nonwoven web such as a low basis weight spunbond material could be provided with apertures through pin aperturing; perf embossing and mechanical stretching of the web; die punching or stamping to provide apertures or holes in the web; hydroentangling to impart apertures by rearrangement of the fibers due to the interaction of water jets with the fibrous web as it resides on a patterned, textured or three-dimensional substrate that imparts a pattern to the web; water knives that cut out desired apertures or holes in the web; laser cutters that cut out portions of the web; patterned forming techniques, such as air laying of synthetic fibers on a patterned substrate to impart macroscopic openings; needle punching with sets of barbed needles to engage and displace fibers; and other methods known in the art. Preferably, the openings are provided in a regular pattern over at least a portion of the topsheet of the absorbent article.
Preferably, the openings in the network of hydrophobic fibers are spaced and registered with respect to the structure of the basesheet such that a predetermined fraction of the openings are largely superposed over depressed regions of the basesheet. An opening is said to be largely superposed over a depressed region if at least half of the area of the macroscopic opening resides over a depressed region of the basesheet. The predetermined fraction of the openings that are largely superposed over depressed regions can be about 0.25 of greater, 0.4 or greater, 0.5 or greater, 0.7 or greater, 0.8 or greater, or from about 0.4 to about 0.85. The contiguous network of hydrophobic matter is laminated to or otherwise physically joined with the underlying basesheet. Preferably, the network of hydrophobic fibers is attached to the basesheet by means of adhesives and related agents, including hot melts, latexes, glues, starch, waxes, and the like, which adhere or join the upper regions of the basesheet with adjacent portions of the overlaying network of hydrophobic fibers. Preferably, adhesives are applied only to the most elevated portions of the basesheet to effect the bonding between the hydrophilic basesheet and the network of hydrophobic fibers with macroscopic openings therein, leaving the depressed regions substantially free of adhesives. Adhesive application can be through meltblown application of hot melt glues and thermoplastic materials, spray or swirl nozzles of melted or dissolved adhesives, printing of adhesive material onto one or both surfaces before joining, and the like. If adhesives are applied directly to the basesheet by means of spray, mist, aerosol, or droplets in any form, prior to contact of the basesheet with the hydrophobic matter, then it is desirable to use a template or patterned shield to prevent application of adhesive to the depressed regions of the basesheet and to ensure that adhesives are preferentially applied to the elevated portions of the basesheet.
For improved comfort, the network of hydrophobic fibers use in the above-mentioned embodiment preferably is one that is perceived as soft and conformable when next to the skin.
For optimum efficiency in the embodiment comprising a nonwoven web, the apertures or openings in the web should be arrayed in a pattern corresponding to the array of depressed regions in the tissue basesheet, or should correspond to a subset of the depressed regions of the basesheet. Applicant have found a useful means for providing apertures in a nonwoven web in a pattern which corresponds geometrically to the depressed regions of a molded, three-dimensional basesheet wherein the basesheet was molded on a foraminous textured substrate such as a three-dimensional through-drying fabric. The method involves hydroentrangling, which is a well known principle involving the use of high pressure water jets to modify a fibrous surface. Basic principles of hydroentangling are disclosed by Evans in U.S. Pat. No. 3,485,706 issued in 1969, and in U.S. Pat. No. 3,494,821 issued in 1970, both of which are herein incorporated by. reference. Hydroentangling, as is known in the art, can be used to impart apertures to a nonwoven web. In one well known technique, the nonwoven web is carried on a textured, permeable carrier fabric. The action of water jets on the nonwoven web as it resides on the textured fabric causes fibers to be moved away from the elevated portions of the carrier fabric on which the nonwoven web reside, resulting in apertures where the carrier fabric was elevated. If a nonwoven web is placed on the same kind of throughdrying fabric that was used to mold a three-dimensional through-dried sheet, preferably an uncreped or only lightly creped sheet in order to preserve texture in the basesheet, then the high places on the carrier TAD fabric will become apertured regions in the nonwoven basesheet. The high portions of the TAD fabric will correspond to the depressed regions on the fabric side of the through-dried sheet. Alternatively, if the nonwoven web is hydroentangled against the backside of a three-dimensional TAD fabric, the elevated regions of the TAD fabric""s backside will generally correspond to the depressed in the air side of the sheet that is through dried on the TAD fabric. In either case, a nonwoven web can be created having apertures that align with the real physical structure of the TAD fabric, namely, with the depressed regions of a through-dried sheet. When the apertured nonwoven material is then attached to the through-dried basesheet, the apertures can be aligned with the depressed regions of the basesheet using techniques known in the art, such as photoelectric eyes or high speed CCD cameras which can view the position of apertures in the nonwoven web relative to the position of the through-dried fabric as the two are brought together, whereupon the position of one material can be adjusted both in the cross-direction and the machine direction (e.g., by controlling the speed of one layer or by machine direction motion of an unwind roll of one material) for proper placement of the two layers together.
In embodiments comprising contiguous nonwoven webs with spaced apart openings for fluid access to the hydrophilic basesheet, Applicants have found excellent fluid intake and absorbency results when the absorbent web is superposed on a separate layer of densified fluff pulp or an air laid cellulosic web, preferably an air laid web stabilized with thermosetting materials or crosslinking chemistry such as Kymene wet strength resin. With a densified cellulosic web beneath the basesheet and hydrophobic matter of the present invention, an insult of fluid that enters the hydrophilic basesheet can be pulled out of the hydrophilic basesheet by capillary suction provided that the local pore size of the underlying absorbent layer is small enough. Experiments with dyed water and also with an aqueous egg white mixture have shown that the combination of a hydrophobicly treated cellulosic basesheet resting on a densified airlaid web can result in greatly improved intake, with fluid being largely directed into the air laid material and not spreading significantly laterally in the basesheet.
It has also been discovered that highly calendered versions of such webs are suitable as hand towels. The hydrophobic, originally uppermost regions are made relatively flat, offering significant hydrophilic areas initially in contact with the wet skin for rapid intake of fluid, but also having the ability to expand after wetting to provide improved dry feel as the wet, hydrophilic areas retract from the skin relative to the more hydrophobic, elevated regions. Webs so treated can achieve the once mutually exclusive goals of having high density for economical dispensing and low density once wetted for high absorbency, while also having a dry feel in use.
Hence, in one aspect, the invention resides in an absorbent web having a dry feel when wet, comprising: (a) an inherently hydrophilic basesheet comprising papermaking fibers and having an upper surface and a lower surface, said upper surface having elevated and depressed regions; and (b) hydrophobic matter deposited preferentially on the elevated regions of the upper surface of said basesheet.
In another aspect, the invention resides in an absorbent dual-zoned web providing a dry feel in use, said web having an upper surface comprising a plurality of hydrophobically treated regions surrounded by inherently hydrophilic cellulosic regions, wherein upon wetting said web expands such that the hydrophobically treated regions are preferentially elevated relative to said hydrophilic regions.
In another aspect, the invention resides in an absorbent web having a Rewet value of about 1 g or less, comprising: (a) an inherently hydrophilic basesheet comprising papermaking fibers and having an upper surface and a lower surface, said upper surface having elevated and depressed regions with an Overall Surface Depth of 0.2 mm or greater in the uncalendered and uncreped state, said basesheet further having a Wet Compressed Bulk of at least 6 cc/g; and (b) hydrophobic matter deposited preferentially on the elevated regions of the upper surface of said basesheet.
In another aspect, the invention resides in an absorbent web having a dry feel when wet, comprising: (a) an inherently hydrophilic basesheet comprising papermaking fibers and having an upper surface and a lower surface, said upper surface having elevated and depressed regions with an Overall Surface Depth of about 0.2 mm or greater; and (b) a substantially contiguous network of hydrophobic fibers having a plurality of macroscopic openings attached to the upper surface of said basesheet such that a portion of the depressed regions of the basesheet are aligned with openings in the overlaying network of hydrophobic fibers to allow body exudates to pass through the macroscopic openings into the basesheet.
In another aspect, the invention resides in an absorbent web having a dry feel when wet, comprising: (a) an inherently hydrophilic basesheet comprising papermaking fibers and having an upper surface and a lower surface, said upper surface having elevated and depressed regions, said basesheet preferably having a wet:dry tensile ratio of at least 0.1; and (b) a contiguous network of hydrophobic matter deposited preferentially on the elevated regions of the upper surface of said basesheet.
In another aspect, the invention resides in an absorbent article comprising a liquid impermeable backsheet, a cellulosic absorbent core in superposed relation with said backsheet, and a liquid permeable absorbent web, said absorbent web comprising an inherently hydrophilic basesheet comprising papermaking fibers and having a wet:dry tensile ratio of at least 0.1, said basesheet having an upper surface and a lower surface, said upper surface having elevated and depressed regions and hydrophobic matter deposited preferentially on the elevated regions, wherein the basesheet is superposed on the absorbent core with the lower surface of the basesheet facing the absorbent core.
In another aspect, the invention resides in an absorbent article comprising a liquid impermeable backsheet, a cellulosic absorbent core in superposed relation with said backsheet, and a liquid permeable absorbent web, said absorbent web comprising an inherently hydrophilic basesheet comprising papermaking fibers, said basesheet having an upper surface and a lower surface, said upper surface having elevated and depressed regions, further comprising an apertured contiguous web of hydrophobic nonwoven material attached to the upper surface of the basesheet such that a portion of said apertures overlay the depressed regions of the basesheet, wherein the basesheet is superposed on the absorbent core with the lower surface of the basesheet facing the absorbent core.
In another aspect, the invention resides in calendered, low density structures of previously three-dimensional resilient webs having hydrophobic matter on the once uppermost regions of one or both sides of the web. Without limitation, such articles may serve as suitable hand towels by providing high initial uptake of fluid by the plurality of hydrophilic regions in the plane of the flat paper during initial wicking, followed by an enhanced dry feel as the dry-feeling treated areas rise out of the plane of the sheet during wetting. The hydrophobic matter in such articles may also be used to increase the apparent softness or lubricity of the article and be applied in contiguous or discontiguous forms.
In another aspect, the invention resides in a method for producing an intake material for an absorbent article, comprising the steps of (a) forming an embryonic paper web from an aqueous slurry of papermaking fibers; (b) through-drying the embryonic paper web on a three-dimensional through-drying fabric having a pattern of elevated and depressed regions; (c) completing the drying of the web; (d) aperturing a nonwoven web by means of hydroentangling, wherein the nonwoven web overlays a carrier fabric having substantially the same pattern of elevated and depressed regions as the through-drying fabric of step (b); and (e) joining the apertured nonwoven web with the through-dried paper web such that the apertures of the nonwoven web are substantially aligned with the depressed regions of the through-dried paper web.
In stating that hydrophobic matter is preferentially deposited on elevated portions of the basesheet, the term xe2x80x9cpreferentiallyxe2x80x9d implies that more hydrophobic matter is deposited on the elevated regions rather than in the depressed regions, in terms of a mass per unit area basis, such that the depressed regions have a significantly lower amount of hydrophobic matter present than the elevated regions. It is preferred that the percentage of the hydrophobic material deposited on the elevated regions be at least about 60 percent, more specifically at least about 70 percent, and still more specifically at least about 80 percent of the total amount deposited. The hydrophobic matter can comprise fine fibers, powders, resins, gels, and other materials, preferably applied with an average superficial basis weight of less than 10 gsm, more specifically from about 1 to about 10 gsm. When used as the skin-contacting layer of absorbent articles, said absorbent web serves as an absorbent improvement over nonabsorbent, plastic apertured films or other inherently hydrophobic materials. The elevated regions of said basesheet preferably comprise between about 5 and about 300 protrusions per square inch having a height relative to the plane of the basesheet, as measured in the uncalendered state, of about 0.1 mm or greater, preferably about 0.2 mm or greater, more preferably about 0.3 mm or greater, and most preferably from about 0.25 to about 0.6 mm.
In describing the webs of this invention and their fluid-handling characteristics, a number of terms and tests are used which are described below.
As used herein, xe2x80x9chigh yield pulp fibersxe2x80x9d are those papermaking fibers of pulps produced by pulping processes providing a yield of about 65 percent or greater, more specifically about 75 percent or greater, and still more specifically from about 75 to about 95 percent. Yield is the resulting amount of processed fiber expressed as a percentage of the initial wood mass. High yield pulps include bleached chemithermomechanical pulp (BCTMP), chemithermomechanical pulp (CTMP) pressure/pressure thermomechanical pulp (PTMP), thermomechanical pulp (TMP), thermomechanical chemical pulp (TMCP), high yield sulfite pulps, and high yield Kraft pulps, all of which contain fibers having high levels of lignin. The preferred high yield pulp fibers can also be characterized by being comprised of comparatively whole, relatively undamaged fibers, having a freeness of 250 Canadian Standard Freeness (CSF) or greater, more specifically 350 CSF or greater, and still more specifically 400 CSF or greater, and low fines content (less than 25 percent, more specifically less than 20 percent, still more specifically less that 15 percent, and still more specifically less than 10 percent by the Britt jar test). In addition to common papermaking fibers listed above, high yield pulp fibers also include other natural fibers such as milkweed seed floss fibers, abaca, hemp, kenaf, bagasse, cotton and the like.
As used herein, xe2x80x9cwet resilient pulp fibersxe2x80x9d are papermaking fibers selected from the group comprising high-yield fibers, chemically stiffened fibers and cross-linked fibers. Examples of chemically stiffened fibers or cross-linked fibers include mercerized fibers, HBA fibers produced by Weyerhaeuser Corp., and those such as described in U.S. Pat. No. 3,224,926, xe2x80x9cMethod of Forming Cross-linked Cellulosic Fibers and Product Thereof,xe2x80x9d issued in 1965 to L. J. Bemardin, and U.S. Pat. No. 3,455,778, xe2x80x9cCreped Tissue Formed From Stiff Cross-linked Fibers and Refined Papermaking Fibers,xe2x80x9d issued in 1969 to L. J. Bemardin. Though any blend of wet resilient pulp fibers can be used, high-yield pulp fibers are the wet resilient fiber of choice for many embodiments of the present invention for their low cost and good fluid handling performance when used according to the principles described below.
The amount of high-yield or wet resilient pulp fibers in the basesheet can be at least about 10 dry weight percent or greater, more specifically about 15 dry weight percent or greater, more specifically about 30 dry weight percent or greater, still more specifically about 50 dry weight percent or greater, and still more specifically from about 20 to 100 percent. For layered basesheets, these same amounts can be applied to one or more of the individual layers. Because wet resilient pulp fibers are generally less soft than other papermaking fibers, in some applications it is advantageous to incorporate them into the middle of the final product, such as placing them in the center layer of a three-layered basesheet or, in the case of a two-ply product, placing them in the inwardly-facing layers of each of the two plies.
xe2x80x9cWater retention valuexe2x80x9d (WRV) is a measure that can be used to characterize some fibers useful for purposes of this invention. WRV is measured by dispersing 0.5 grams of fibers in deionized water, soaking at least 8 hours, then centrifuging the fibers in a 1.9 inch diameter tube with a 100 mesh screen at the bottom of the tube at 1000 G for 20 minutes. The samples are weighed, then dried at 105xc2x0 C. for two hours and then weighed again. WRV is (wet weightxe2x80x94dry weight)/dry weight. High yield pulp fibers can have a WRV of about 0.7 or greater and characteristically have a WRV of about 1 or greater and preferably from about 1 to about 2. Low-yield, cross-linked fibers typically have a Water Retention Value of less than about 1, specifically less than about 0.7 and more specifically still less than about 0.6.
xe2x80x9cRewetxe2x80x9d is a measure of the amount of liquid water which can be wicked out of a moistened web into an adjacent dry filter paper and is intended to estimate the tendency of a moistened web to wet the skin. The Rewet test is performed by cutting a sample of a tissue web to a rectangle of dimensions 4 inxc3x976 in. The test is performed in a Tappi conditioned room (50% RH, 73xc2x0 F.). The initial air dry weight of the conditioned sample is recorded, then deionized water is sprayed onto both sides of the tissue sample to uniformly wet it, bringing the total wet mass of the tissue to a value of 4 times the previously recorded initial air dry weight of the sample, thus bringing the xe2x80x9capparent moisture ratioxe2x80x9d of the sample to a value of 3.0 grams (xc2x10.15 g) of added water per gram of conditioned air dry fiber. The process of repeatedly spraying and weighing the sample until the proper mass has been reached should take no more than 2 minutes. Once the sample is wetted, a single dry Whatman #3 filter, whose mass has been measured and recorded, is placed on the center of the wet tissue sample and a load is immediately placed on the filter disk. The load is a cylindrical disk of aluminum having a diameter of 4.5 inches and a thickness of 1 inch for a mass of 723 g. The aluminum disk should be centered about the filter disk. The filter paper on the wet sample remains under load for 20 seconds, at which time the load and the filter paper are immediately removed. The filter paper is then weighed, and the additional mass relative to the initial air dry mass is reported in grams as the Rewet value.
xe2x80x9cNormalized Rewetxe2x80x9d is the Rewet value of a sample divided by the conditioned dry mass of the sample.
xe2x80x9cAbsorbency at 0.075 psixe2x80x9d is a measure of basesheet absorbent capacity under a load of 0.075 psi. The test requires two metal plates cut to a length of 6 inches and a width of 4 inches. A lower plate is 0.125-inches thick and the upper plate is xc2xe-inch thick aluminum having a mass of 813 g, which imparts a load of 0.075 psi when placed flat on a tissue sample. The center of the upper plate has a cylindrical hole 0.25-inches in diameter. To perform the test, 4-inxc3x976-in samples of dry tissue are cut, with the 6-in length being aligned with the machine direction. Multiple tissue plies are stacked to achieve a tissue stack weight as close to 2.8 grams as possible. The tissue stack is placed between the two horizontal plates, which lie flat in a larger tray. A titrating burette with 50 ml of deionized water is aligned directly above the hole in the upper plate. The burette is opened and water is allowed to slowly enter the hole in the upper plate such that the hole is filled with a column of water that is maintained as high as possible without rising above or spilling onto the upper surface of the plate. This is done until the sample is apparently saturated. Apparent saturation is the point at which water begins to leave any edge of the sample. The mass of water that has been removed from the burette is taken as the value forxe2x80x9cHorizontal Absorbency at 0.075 psi.xe2x80x9d At that point, the tray containing the plates is tilted at a 450 angle for 30 seconds to allow some of the liquid in the sample to drain. The mass of any liquid that drains out is subtracted from the previous xe2x80x9cHorizontal Absorbency at 0.075 psixe2x80x9d value to yield xe2x80x9cTilted Absorbency at 0.075 psi.xe2x80x9d For the basesheet, the horizontal absorbency at 0.075 psi can be about 5 g or greater, or alternatively 7 g or greater, 9 g or greater, 11 g or greater, or from about 6 g to about 10 g. The tilted absorbency at 0.075 psi may be about 4 g or greater, about 6 g or greater, about 8 g or greater, about 10 g or greater, or from about 6 to about 10 g. The tilted absorbency of the cover may be about 5 to 40% less than that off the basesheet alone, while the horizontal absorbency may be greater or lower than that off the basesheet.
xe2x80x9cFabric sidexe2x80x9d of a through-air dried paper web is the side of the web that was in contact with the through-air dryer fabric (TAD fabric) during through-drying. Typically the fabric side of a through-dried sheet offers the most pleasant tactile properties for contact with skin.
xe2x80x9cAir sidexe2x80x9d of a through-air dried paper web is the side of the web that was not in contact with the through-air dryer fabric (TAD fabric) during through-drying. Typically the air side of a through-dried sheet feels somewhat more gritty than the fabric side of the same sheet.
xe2x80x9cDensityxe2x80x9d can be determined by measuring the caliper of a single sheet using a TMI tester (Testing Machines, Inc., Amityville, N.Y.) with a load of 0.289 psi, e.g., using a TMI Model 49-70 with an enlarged platen. Density is calculated by dividing the caliper by the basis weight of the sheet. The basesheets useful for the purposes of this invention can have low, substantially uniform densities (high bulks), which is preferred for wet laid structures, or may have a distribution of zones of varying density, which is preferred in airlaid basesheets. Substantial density uniformity is attained, for example, by throughdrying to final dryness without differentially compressing the web. In general, the density of the basesheets of this invention can be about 0.3 gram per cubic centimeter (g/cc) or less, more specifically about 0.15 g/cc or less, still more specifically about 0.1 g/cc or less and can be from about 0.05 to 0.3 g/cc or from about 0.07 to 0.2 g/cc. It is desirable that the basesheet structure, once formed, be dried without substantially reducing the number of wet-resilient interfiber bonds. Throughdrying, which is a common method for drying tissues and towels, is a preferred method of preserving the structure. Basesheets made by wet laying followed by throughdrying typically have a density of about 0.1 gram per cubic centimeter, whereas airlaid basesheets normally used for diaper fluff typically have densities of about 0.05 gram per cubic centimeter. All of such basesheets are within the scope of this invention.
As used herein, xe2x80x9cdry bulkxe2x80x9d is measured with a thickness gauge having a circular platen 3 inches in diameter such that a pressure of 0.05 psi is applied to the sample, which should be conditioned at 50% relative humidity and at 73xc2x0 F. for 24 hours prior to measurement. The basesheet as well as the uncalendered web can have a dry bulk of 3 cc/g or greater, preferably 6 cc/g or greater, more preferably 9 cc/g or greater, more preferably still 11 cc/g or greater, and most preferably between 8 cc/g and 28 cc/g.
xe2x80x9cWet strength agentsxe2x80x9d are materials used to immobilize the bonds between the fibers in the wet state. Typically the means by which fibers are held together in paper and tissue products involve hydrogen bonds and sometimes combinations of hydrogen bonds and covalent and/or ionic bonds. In the present invention, it is desirable to provide a material that will allow bonding of fibers in such a way as to immobilize the fiber to fiber bond points and make them resistant to disruption in the wet state. In this instance the wet state usually will mean when the product is largely saturated with water or other aqueous solutions, but could also mean significant saturation with body fluids such as urine, blood, mucus, menses, runny bowel movement, lymph and other body exudates.
There are a number of materials commonly used in the paper industry to impart wet strength to paper and board that are applicable to this invention. These materials are known in the art as xe2x80x9cwet strength agentsxe2x80x9d and are commercially available from a wide variety of sources. Any material that when added to a paper web or sheet results in providing the sheet with a wet geometric tensile strength:dry geometric tensile strength ratio in excess of 0.1 will, for purposes of this invention, be termed a wet strength agent. Typically these materials are termed either as permanent wet strength agents or as xe2x80x9ctemporaryxe2x80x9d wet strength agents. For the purposes of differentiating permanent from temporary wet strength, permanent will be defined as those resins which, when incorporated into paper or tissue products, will provide a product that retains more than 50% of its original wet strength after exposure to water for a period of at least five minutes. Temporary wet strength agents are those which show less than 50% of their original wet strength after being saturated with water for five minutes. Both classes of material find application in the present invention. The amount of wet strength agent added to the pulp fibers can be at least about 0.1 dry weight percent, more specifically about 0.2 dry weight percent or greater, and still more specifically from about 0.1 to about 3 dry weight percent based on the dry weight of the fibers.
Permanent wet strength agents will provide a more or less long-term wet resilience to the structure. In contrast, the temporary wet strength agents would provide structures that had low density and high resilience, but would not provide a structure that had long-term resistance to exposure to water or body fluids. The mechanism by which the wet strength is generated has little influence on the products of this invention as long as the essential property of generating water-resistant bonding at the fiber/fiber bond points is obtained.
Suitable permanent wet strength agents are typically water soluble, cationic oligomeric or polymeric resins that are capable of either crosslinking with themselves (homocrosslinking) or with the cellulose or other constituent of the wood fiber. The most widely-used materials for this purpose are the class of polymer known as polyamide-polyamine-epichlorohydrin (PAE) type resins. These materials have been described in patents issued to Keim (U.S. Pat No. 3,700,623 and 3,772,076) and are sold by Hercules, Inc., Wilmington, Del. as KYMENE 557H. Related materials are marketed by Henkel Chemical Co., Charlotte, N.C. and Georgia-Pacific Resins, Inc., Atlanta, Ga.
Polyamide-epichlorohydrin resins are also useful as bonding resins in this invention. Materials developed by Monsanto and marketed under the SANTO RES label are base-activated polyamide-epichlorohydrin resins that can be used in the present invention. These materials are described in patents issued to Petrovich (U.S. Pat. No. 3,885,158; U.S. Pat. No. 3,899,388; U.S. Pat. No. 4,129,528 and U.S. Pat. No. 4,147,586) and van Eenam (U.S. Pat. No. 4,222,921). Although they are not as commonly used in consumer products, polyethylenimine resins are also suitable for immobilizing the bond points in the products of this invention. Another class of permanent-type wet strength agents are exemplified by the aminoplast resins obtained by reaction of formaldehyde with melamine or urea.
Suitable temporary wet strength resins include, but are not limited to, those resins that have been developed by American Cyanamid and are marketed under the name PAREZ 631 NC (now available from Cytec Industries, West Paterson, N.J. This and similar resins are described in U.S. Pat. Nos. 3,556,932 to Coscia et al. and 3,556,933 to Williams et al. Other temporary wet strength agents that should find application in this invention include modified starches such as those available from National Starch and marketed as CO-BOND 1000. It is believed that these and related starches are disclosed in U.S. Pat. No. 4,675,394 to Solarek et al. Derivatized dialdehyde starches, such as described in Japanese Kokai Tokkyo Koho JP 03,185,197, may also provide temporary wet strength. It is also expected that other temporary wet strength materials such as those described in U.S. Pat. No. 4,981,557; U.S. Pat. No. 5,008,344 and U.S. Pat. No. 5,085,736 to Bjorkquist would be of use in this invention. With respect to the classes and the types of wet strength resins listed, it should be understood that this listing is simply to provide examples and that this is neither meant to exclude other types of wet strength resins, nor is it meant to limit the scope of this invention.
Although wet strength agents as described above find particular advantage for use in connection with this invention, other types of bonding agents can also be used to provide the necessary wet resiliency. They can be applied at the wet end of the basesheet manufacturing process or applied by spraying or printing, etc. after the basesheet is formed or after it is dried.
xe2x80x9cNoncompressive dryingxe2x80x9d refers to drying methods for drying cellulosic webs that do not involve compressive nips or other steps causing significant densification or compression of a portion of the web during the drying process. Such methods include through-air drying; air jet impingement drying; non-contacting drying such as air flotation drying, as taught by E. V. Bowden, E. V., Appita J., 44(1): 41 (1991); through-flow or impingement of superheated steam; microwave drying and other radiofrequency or dielectric drying methods; water extraction by supercritical fluids; water extraction by nonaqueous, low surface tension fluids; infrared drying; drying by contact with a film of molten metal; and other methods. It is believed that the three-dimensional basesheets of the present invention could be dried with any of the above mentioned noncompressive drying means without causing significant web densification or a significant loss of their three-dimensional structure and their wet resiliency properties. Standard dry creping technology is viewed as a compressive drying method since the web must be mechanically pressed onto part of the drying surface, causing significant densification of the regions pressed onto the heated Yankee cylinder. Technology to noncompressively dewater and dry tissue webs with an air press and optionally with a Yankee dryer operated without creping is disclosed in the following commonly owned copending applications: U.S. patent application Ser. No. unknown, xe2x80x9cMethod of Producing Low Density Resilient Websxe2x80x9d by F. G. Druecke et al., Attorney Docket No. 13,504, filed Oct. 31, 1997; U.S. patent application Ser. No. unknown, xe2x80x9cLow Density Resilient Webs and Methods of Making Such Webxe2x80x9d by S. Chen et al., Attorney Docket No. 13,381, filed Oct. 31, 1997; U.S patent application Ser. No. 08/647,508 filed May 14, 1996 by M. A. Hermans et al. titled xe2x80x9cMethod and Apparatus for Making Soft Tissue;xe2x80x9d and U.S Patent Application Serial No. unknown filed Oct. 31, 1997 titled xe2x80x9cAir Press for Dewatering a Wet Webxe2x80x9d by F. Hada et al., all of which are herein incorporated by reference. Also of potential value for the tissue making operations useful in the present invention is the paper machine disclosed in U.S. Pat. No. 5,230,776 issued Jul. 27, 1993 to I. A. Andersson et al.; and the capillary dewatering techniques disclosed in U.S. Pat. Nos. 5,598,643 issued Feb. 4, 1997 and U.S. Pat. No. 4,556,450 issued Dec. 3, 1985, both to S. C. Chuang et al., all of which are incorporated herein by reference. The dewatering concepts disclosed by J. D. Lindsay in xe2x80x9cDisplacement Dewatering to Maintain Bulk,xe2x80x9d Paperija Puu, 74(3): 232-242 (1992) are also of potential value.
As used herein, the xe2x80x9cwet:dry ratioxe2x80x9d is the ratio of the geometric mean wet tensile strength divided by the geometric mean dry tensile strength. Geometric mean tensile strength (GMT) is the square root of the product of the machine direction tensile strength and the cross-machine direction tensile strength of the web. Unless otherwise indicated, the term xe2x80x9ctensile strengthxe2x80x9d means xe2x80x9cgeometric mean tensile strength.xe2x80x9d The basesheets of this invention preferably have a wet:dry ratio of about 0.1 or greater, more specifically about 0.15 or greater, more specifically about 0.2 or greater, still more specifically about 0.3 or greater, and still more specifically about 0.4 or greater, and still more specifically from about 0.2 to about 0.6. Tensile strengths can be measured using an Instron tensile tester using a 3-inch jaw width, a jaw span of 4 inches, and a crosshead speed of 10 inches per minute after maintaining the sample under TAPPI conditions for 4 hours before testing. For enhanced wet resiliency and integrity, the basesheets of this invention also preferably have a minimum absolute ratio of dry tensile strength to basis weight of about 1 gram/gsm, preferably from about 2 grams/gsm, more preferably about 5 grams/gsm, more preferably about 10 grams/gsm and still more preferably about 20 grams/gsm and preferably from about 15 to 50 grams/gsm.
xe2x80x9cOverall Surface Depthxe2x80x9d. A three-dimensional basesheet or web is a sheet with significant variation in surface elevation due to the intrinsic structure of the sheet itself. As used herein, this elevation difference is expressed as the xe2x80x9cOverall Surface Depth.xe2x80x9d The basesheets useful for this invention possess three-dimensionality and have an Overall Surface Depth of about 0.1 mm. or greater, more specifically about 0.3 mm. or greater, still more specifically about 0.4 mm. or greater, still more specifically about 0.5 mm. or greater, and still more specifically from about 0.4 to about 0.8 mm.
The three-dimensional structure of a largely planar sheet can be described in terms of its surface topography. Rather than presenting a nearly flat surface, as is typical of conventional paper, the molded sheets useful in producing the present invention have significant topographical structures that, in one embodiment, may derive in part from the use of sculptured through-drying fabrics such as those taught by Chiu et al. in U.S. Pat. No. 5,429,686, previously incorporated by reference. The resulting basesheet surface topography typically comprises a regular repeating unit cell that is typically a parallelogram with sides between 2 and 20 mm in length. For wetlaid materials, it is preferred that these three-dimensional basesheet structures be created by molding the moist sheet or be created prior to drying, rather than by creping or embossing or other operations after the sheet has been dried. In this manner, the three-dimensional basesheet structure is more likely to be well-retained upon wetting, helping to provide high wet resiliency and to promote good in-plane permeability. For air-laid basesheets, the structure may be imparted by thermal embossing of a fibrous mat with binder fibers that are activated by heat. For example, an air-laid fibrous mat containing thermoplastic or hotmelt binder fibers may be heated and then embossed before the structure cools to permanently give the sheet a three-dimensional structure.
In addition to the regular geometrical structure imparted by the sculptured fabrics and other fabrics used in creating a basesheet, additional fine structure, with an in-plane length scale less than about 1 mm, can be present in the basesheet. Such a fine structure can stem from microfolds created during differential velocity transfer of the web from one fabric or wire to another prior to drying. Some of the materials of the present invention, for example, appear to have fine structure with a fine surface depth of 0.1 mm or greater, and sometimes 0.2 mm or greater, when height profiles are measured using a commercial moirxc3xa9 interferometer system. These fine peaks have a typical half-width less than 1 mm. The fine structure from differential velocity transfer and other treatments may be useful in providing additional softness, flexibility, and bulk. Measurement of the surface structures is described below.
An especially suitable method for measurement of Overall Surface Depth is moirxc3xa9 interferometry, which permits accurate measurement without deformation of the surface. For reference to the materials of the present invention, surface topography should be measured using a computer-controlled white-light field-shifted moirxc3xa9 interferometer with about a 38 mm field of view. The principles of a useful implementation of such a system are described in Bieman et al. (L. Bieman, K. Harding, and A. Boehnlein, xe2x80x9cAbsolute Measurement Using Field-Shifted Moirxc3xa9,xe2x80x9d SPIE Optical Conference Proceedings, Vol. 1614, pp. 259-264, 1991). A suitable commercial instrument for moirxc3xa9 interferometry is the CADEYES(copyright) interferometer produced by Medar, Inc. (Farmington Hills, Michigan), constructed for a 38-mm field-of-view (a field of view within the range of 37 to 39.5 mm is adequate). The CADEYES(copyright) system uses white light which is projected through a grid to project fine black lines onto the sample surface. The surface is viewed through a similar grid, creating moirxc3xa9 fringes that are viewed by a CCD camera. Suitable lenses and a stepper motor adjust the optical configuration for field shifting (a technique described below). A video processor sends captured fringe images to a PC computer for processing, allowing details of surface height to be back-calculated from the fringe patterns viewed by the video camera.
In the CADEYES moirxc3xa9 interferometry system, each pixel in the CCD video image is said to belong to a moirxc3xa9 fringe that is associated with a particular height range. The method of field-shifting, as described by Bieman et al. (L. Bieman, K. Harding, and A. Boehnlein, xe2x80x9cAbsolute Measurement Using Field-Shifted Moirxc3xa9,xe2x80x9d SPIE Optical Conference Proceedings, Vol. 1614, pp. 259-264, 1991) and as originally patented by Boehnlein (U.S. Pat. No. 5,069,548, herein incorporated by reference), is used to identify the fringe number for each point in the video image (indicating which fringe a point belongs to). The fringe number is needed to determine the absolute height at the measurement point relative to a reference plane. A field-shifting technique (sometimes termed phase-shifting in the art) is also used for sub-fringe analysis (accurate determination of the height of the measurement point within the height range occupied by its fringe). These field-shifting methods coupled with a camera-based interferometry approach allows accurate and rapid absolute height measurement, permitting measurement to be made in spite of possible height discontinuities in the surface. The technique allows absolute height of each of the roughly 250,000 discrete points (pixels) on the sample surface to be obtained, if suitable optics, video hardware, data acquisition equipment, and software are used that incorporates the principles of moirxc3xa9 interferometry with field-shifting. Each point measured has a resolution of approximately 1.5 microns in its height measurement.
The computerized interferometer system is used to acquire topographical data and then to generate a grayscale image of the topographical data, said image to be hereinafter called xe2x80x9cthe height map.xe2x80x9d The height map is displayed on a computer monitor, typically in 256 shades of gray and is quantitatively based on the topographical data obtained for the sample being measured. The resulting height map for the 38-mm square measurement area should contain approximately 250,000 data points corresponding to approximately 500 pixels in both the horizontal and vertical directions of the displayed height map. The pixel dimensions of the height map are based on a 512xc3x97512 CCD camera which provides images of moirxc3xa9 patterns on the sample which can be analyzed by computer software. Each pixel in the height map represents a height measurement at the corresponding x- and y-location on the sample. In the recommended system, each pixel has a width of approximately 70 microns, i.e. represents a region on the sample surface about 70 microns long in both orthogonal in-plane directions). This level of resolution prevents single fibers projecting above the surface from having a significant effect on the surface height measurement. The z-direction height measurement must have a nominal accuracy of less than 2 microns and a z-direction range of at least 1.5 mm. (For further background on the measurement method, see the CADEYES Product Guide, Medar, Inc., Farmington Hills, Mich., 1994, or other CADEYES manuals and publications of Medar, Inc.)
The CADEYES system can measure up to 8 moirxc3xa9 fringes, with each fringe being divided into 256 depth counts (sub-fringe height increments, the smallest resolvable height difference). There will be 2048 height counts over the measurement range. This determines the total z-direction range, which is approximately 3 mm in the 38-mm field-of-view instrument. If the height variation in the field of view covers more than eight fringes, a wrap-around effect occurs, in which the ninth fringe is labeled as if it were the first fringe and the tenth fringe is labeled as the second, etc. In other words, the measured height will be shifted by 2048 depth counts. Accurate measurement is limited to the main field of 8 fringes.
The moirxc3xa9 interferometer system, once installed and factory calibrated to provide the accuracy and z-direction range stated above, can provide accurate topographical data for materials such as paper towels. (Those skilled in the art may confirm the accuracy of factory calibration by performing measurements on surfaces with known dimensions.) Tests are performed in a room under Tappi conditions (73xc2x0 F., 50% relative humidity). The sample must be placed flat on a surface lying aligned or nearly aligned with the measurement plane of the instrument and should be at such a height that both the lowest and highest regions of interest are within the measurement region of the instrument.
Once properly placed, data acquisition is initiated using Medar""s PC software and a height map of 250,000 data points is acquired and displayed, typically within 30 seconds from the time data acquisition was initiated. (Using the CADEYES(copyright) system, the xe2x80x9ccontrast threshold levelxe2x80x9d for noise rejection is set to 1, providing some noise rejection without excessive rejection of data points.) Data reduction and display are achieved using CADEYES(copyright) software for PCs, which incorporates a customizable interface based on Microsoft Visual Basic Professional for Windows (version 3.0). The Visual Basic interface allows users to add custom analysis tools.
The height map of the topographical data can then be used by those skilled in the art to identify characteristic unit cell structures (in the case of structures created by fabric patterns; these are typically parallelograms arranged like tiles to cover a larger two-dimensional area) and to measure the typical peak to valley depth of such structures. A simple method of doing this is to extract two-dimensional height profiles from lines drawn on the topographical height map which pass through the highest and lowest areas of the unit cells. These height profiles can then be analyzed for the peak to valley distance, if the profiles are taken from a sheet or portion of the sheet that was lying relatively flat when measured. To eliminate the effect of occasional optical noise and possible outliers, the highest 10% and the lowest 10% of the profile should be excluded, and the height range of the remaining points is taken as the surface depth. Technically, the procedure requires calculating the variable which we term xe2x80x9cP10,xe2x80x9d defined at the height difference between the 10% and 90% material lines, with the concept of material lines being well known in the art, as explained by L. Mummery, in Surface Texture Analysis: The Handbook, Hommelwerke GmbH, Mxc3xchlhausen, Germany, 1990. In this approach, which will be illustrated with respect to FIG. 7, the surface 31 is viewed as a transition from air 32 to material 33. For a given profile 30, taken from a flat-lying sheet, the greatest height at which the surface beginsxe2x80x94the height of the highest peakxe2x80x94is the elevation of the xe2x80x9c0% reference linexe2x80x9d 34 or the xe2x80x9c0% material line,xe2x80x9d meaning that 0% of the length of the horizontal line at that height is occupied by material. Along the horizontal line passing through the lowest point of the profile, 100% of the line is occupied by material, making that line the xe2x80x9c100% material linexe2x80x9d 35. In between the 0% and 100% material lines (between the maximum and minimum points of the profile), the fraction of horizontal line length occupied by material will increase monotonically as the line elevation is decreased. The material ratio curve 36 gives the relationship between material fraction along a horizontal line passing through the profile and the height of the line. The material ratio curve is also the cumulative height distribution of a profile. (A more accurate term might be xe2x80x9cmaterial fraction curve.xe2x80x9d)
Once the material ratio curve is established, one can use it to define a characteristic peak height of the profile. The P10 xe2x80x9ctypical peak-to-valley heightxe2x80x9d parameter is defined as the difference 37 between the heights of the 10% material line 38 and the 90% material line 39. This parameter is relatively robust in that outliers or unusual excursions from the typical profile structure have little influence on the P10 height. The units of P10 are mm. The Overall Surface Depth of a material is reported as the P10 surface depth value for profile lines encompassing the height extremes of the typical unit cell of that surface. xe2x80x9cFine surface depthxe2x80x9d is the P10 value for a profile taken along a plateau region of the surface which is relatively uniform in height relative to profiles encompassing a maxima and minima of the unit cells. Measurements are reported for the most textured side of the basesheets of the present invention, which is typically the side that was in contact with the through-drying fabric when air flow is toward the through-dryer. FIG. 8 represents a profile of Example 13 of the present invention, discussed below, having an Overall Surface Depth of about 0.5.
Overall Surface Depth is intended to examine the topography produced in the basesheet, especially those features created in the sheet prior to and during drying processes, but is intended to exclude xe2x80x9cartificiallyxe2x80x9d created large-scale topography from dry converting operations such as embossing, perforating, pleating, etc. Therefore, the profiles examined should be taken from unembossed regions if the basesheet has been embossed, or should be measured on an unembossed basesheet. Overall Surface Depth measurements should exclude large-scale structures such as pleats or folds which do not reflect the three-dimensional nature of the original basesheet itself. It is recognized that sheet topography may be reduced by calendering and other operations which affect the entire basesheet. Overall Surface Depth measurement can be appropriately performed on a calendered basesheet.
The xe2x80x9cWet Wrinkle Recovery Testxe2x80x9d is a slight modification of AATCC Test Method 66-1990 taken from the Technical Manual of the American Association of Textile Chemists and Colorists (1992), page 99. The modification is to first wet the samples before carrying out the method. This is done by soaking the samples in water containing 0.01 percent TRITON X-100 wetting agent (Rohm and Haas) for five minutes before testing. Sample preparation is carried out at 73xc2x0 F. and 50 percent relative humidity. The sample is gently removed from the water with a tweezers, drained by pressing between two pieces of blotter paper with 325 grams of weight, and placed in the sample holder to be tested as with the dry wrinkle recovery test method. The test measures the highest recovery angle of the sample being tested (in any direction, including the machine direction and the cross-machine direction), with 180xc2x0 representing total recovery. The Wet Wrinkle Recovery, expressed as a percent recovery, is the measured recovery angle divided by 180xc2x0, multiplied by 100. Basesheets of this invention can exhibit a Wet Wrinkle Recovery of about 60 percent or greater, more specifically about 70 percent or greater, and still more specifically about 80 percent or greater.
xe2x80x9cWet compressive resiliencyxe2x80x9d of the basesheets is defined by several parameters and can be demonstrated using a materials property procedure that encompasses both wet and dry characteristics. A programmable strength measurement device is used in compression mode to impart a specified series of compression cycles to an initially dry, conditioned sample, after which the sample is carefully moistened in a specified manner and subjected to the same sequence of compression cycles. While the comparison of wet and dry properties is of general interest, the most important information from this test concerns the wet properties. The initial testing of the dry sample can be viewed as a conditioning step. The test sequence begins with compression of the dry sample to 0.025 psi to obtain an initial thickness (cycle A), then two repetitions of loading up to 2 psi followed by unloading (cycles B and C). Finally, the sample is again compressed to 0.025 psi to obtain a final thickness (cycle D). (Details of the procedure, including compression speeds, are given below). Following the treatment of the dry sample, moisture is applied uniformly to the sample using a fine mist of deionized water to bring the moisture ratio (g water/g dry fiber) to approximately 1.1. This is done by applying 95-110% added moisture, based on the conditioned sample mass. This puts typical cellulosic materials in a moisture range where physical properties are relatively insensitive to moisture content (e.g., the sensitivity is much less than it is for moisture ratios less than 70%). The moistened sample is then placed in the test device and the compression cycles are repeated.
Three measures of wet resiliency are considered which are relatively insensitive to the number of sample layers used in the stack. The first measure is the bulk of the wet sample at 2 psi. This is referred to as the xe2x80x9cWet Compressed Bulkxe2x80x9d (WCB). The second measure is termed xe2x80x9cWet Springback Ratioxe2x80x9d (WS), which is the ratio of the moist sample thickness at 0.025 psi at the end of the compression test (cycle D) to the thickness of the moist sample at 0.025 psi measured at the beginning of the test (cycle A). The third measure is the xe2x80x9cLoadinq Energy Ratioxe2x80x9d (LER), which is the ratio of loading energy in the second compression to 2 psi (cycle C) to that of the first compression to 2 psi (cycle B) during the sequence described above, for a wetted sample. The final wet bulk measured at the end of the test (at 0.025 psi) is termed the xe2x80x9cfinal bulkxe2x80x9d or xe2x80x9cFBxe2x80x9d value. When load is plotted as a function of thickness, loading energy is the area under the curve as the sample goes from an unloaded state to the peak load of that cycle. For a purely elastic material, the springback and loading energy ratio would be unity. Applicants have found that the three measures described here are relatively independent of the number of layers in the stack and serve as useful measures of wet resiliency. Also referred to herein is the xe2x80x9cCompression Ratioxe2x80x9d, which is defined as the ratio of moistened sample thickness at peak load in the first compression cycle to 2 psi to the initial moistened thickness at 0.025 psi.
In carrying out the foregoing measurements of the wet compressive resiliency, samples should be conditioned for at least 24 hours under TAPPI conditions (50% RH, 73xc2x0 F.). Specimens are die cut to 2.5xe2x80x9cxc3x972.5xe2x80x9d squares. Conditioned sample weight should be near 0.4 g, if possible, and within the range of 0.25 to 0.6 g for meaningful comparisons. The target mass of 0.4 g is achieved by using a stack of 2 or more sheets if the sheet basis weight is less than 65 gsm. For example, for nominal 30 gsm sheets, a stack of 3 sheets will generally be near 0.4 g total mass.
Compression measurements are performed using an Instron 4502 Universal Testing Machine interfaced with a 286 PC computer running Instron Series XII software (1989 issue) and Version 2 firmware. The standard xe2x80x9c286 computerxe2x80x9d referred to has an 80286 processor with a 12 MHz clock speed. The particular computer used was a Compaq DeskPro 286e with an 80287 math coprocessor and a VGA video adapter. A 1 kN load cell is used with 2.25xe2x80x3 diameter circular platens for sample compression. The lower platen has a ball bearing assembly to allow exact alignment of the platens. The lower platen is locked in place while under load (30-100 lbf) by the upper platen to ensure parallel surfaces. The upper platen must also be locked in place with the standard ring nut to eliminate play in the upper platen as load is applied.
Following at-least one hour of warm-up after start-up, the instrument control panel is used to set the extensionometer to zero distance while the platens are in contact (at a load of 10-30 lb). With the upper platen freely suspended, the calibrated load cell is balanced to give a zero reading. The extensionometer and load cell should be periodically checked to prevent baseline. drift (shirting of the zero points). Measurements must be performed in a controlled humidity and temperature environment, according to TAPPI specifications (50%xc2x12% RH and 73xc2x0 F.). The upper platen is then raised to a height of 0.2 in. and control of the Instron is transferred to the computer.
Using the Instron Series XII Cyclic Test software with a 286 computer, an instrument sequence is established with 7 markers (discrete events) composed of 3 cyclic blocks (instructions sets) in the following order:
Marker 1: Block 1
Marker 2: Block 2
Marker 3: Block 3
Marker 4: Block 2
Marker 5: Block 3
Marker 6: Block 1
Marker 7: Block 3.
Block 1 instructs the crosshead to descend at 1.5 in./min. until a load of 0.1 lb. is applied (the Instron setting is xe2x88x920.1 lb., since compression is defined as negative force).
Control is by displacement. When the targeted load is reached, the applied load is reduced to zero.
Block 2 directs that the crosshead range from an applied load of 0.05 lb. to a peak of 8 lb. then back to 0.05 lb. at a speed of 0.4 in./min. Using the Instron software, the control mode is displacement, the limit type is load, the first level is xe2x88x920.05 lb., the second level is xe2x88x928 lb., the dwell time is 0 sec., and the number of transitions is 2 (compression, then relaxation); xe2x80x9cno actionxe2x80x9d is specified for the end of the block.
Block 3 uses displacement control and limit type to simply raise the crosshead to 0.2 in. at a speed of 4 in./min., with 0 dwell time. Other Instron software settings are 0 in first level, 0.2 in second level, 1 transition, and xe2x80x9cno actionxe2x80x9d at the end of the block.
When executed in the order given above (Markers 1-7), the Instron sequence compresses the sample to 0.025 psi (0.1 lbf), relaxes, then compresses to 2 psi (8 lbs.), followed by decompression and a crosshead rise to 0.2 in., then compress the sample again to 2 psi, relaxes, lifts the crosshead to 0.2 in., compresses again to 0.025 psi (0.1 lbf), and then raises the crosshead. Data logging should be performed at intervals no greater than every 0.02xe2x80x3 or 0.4 lb. (whichever comes first) for Block 2 and for intervals no greater than 0.01 lb. for Block 1. Preferably, data logging is performed every 0.004 lb. in Block 1 and every 0.05 lb. or 0.005 in. (whichever comes first) in Block 2.
The results output of the Series XII software is set to provide extension (thickness) at peak loads for Markers 1, 2, 4 and 6 (at each 0.025 and 2.0 psi peak load), the loading energy for Markers 2 and 4 (the two compressions to 2.0 psi previously termed cycles B and C, respectively), the ratio of the two loading energies (second cycle/first cycle), and the ratio of final thickness to initial thickness (ratio of thickness at last to first 0.025 psi compression). Load versus thickness results are plotted on the screen during execution of Blocks 1 and 2.
In performing a measurement, the dry, conditioned sample is centered on the lower platen and the test is initiated. Following completion of the sequence, the sample is immediately removed and moisture (deionized water at 72-73xc2x0 F.) is applied. Moisture is applied uniformly with a fine mist to reach a moist sample mass of approximately 2.0 times the initial sample mass (95-110% added moisture is applied, preferably 100% added moisture, based on conditioned sample mass; this level of moisture should yield an absolute moisture ratio of about 1.1 g. water/g. oven dry fiberxe2x80x94with oven dry referring to drying for at least 30 minutes in an oven at 105xc2x0 C.). (For the uncreped throughdried materials of this invention, the moisture ratio could be within the range of 1.05 to 1.7 without significantly affecting the results). The mist should be applied uniformly to separated sheets (for stacks of more than 1 sheet), with spray applied to both front and back of each sheet to ensure uniform moisture application. This can be achieved using a conventional plastic spray bottle, with a container or other barrier blocking most of the spray, allowing only about the upper 10-20% of the spray envelopexe2x80x94a fine mistxe2x80x94to approach the sample. The spray source should be at least 10xe2x80x3 away from the sample during spray application. In general, care must be applied to ensure that the sample is uniformly moistened by a fine spray. The sample must be weighed several times during the process of applying moisture to reach the targeted moisture content. No more than three minutes should elapse between the completion of the compression test on the dry sample and the completion of moisture application. Allow 45-60 seconds from the final application of spray to the beginning of the subsequent compression test to provide time for internal wicking and absorption of the spray. Between three and four minutes will elapse between the completion of the dry compression sequence and initiation of the wet compression sequence.
Once the desired mass range has been reached, as indicated by a digital balance, the sample is centered on the lower Instron platen and the test sequence is initiated. Following the measurement, the sample is placed in a 105xc2x0 C. oven for drying, and the oven dry weight will be recorded later (sample should be allowed to dry for 30-60 minutes, after which the dry weight is measured).
Note that creep recovery can occur between the two compression cycles to 2 psi, so the time between the cycles may be important. For the instrument settings used in these Instron tests, there is a 30 second period (xc2x14 sec.) between the beginning of compression during the two cycles to 2 psi. The beginning of compression is defined as the point at which the load cell reading exceeds 0.03 lb. Likewise, there is a 5-8 second interval between the beginning of compression in the first thickness measurement (ramp to 0.025 psi) and the beginning of the subsequent compression cycle to 2 psi. The interval between the beginning of the second compression cycle to 2 psi and the beginning of compression for the final thickness measurement is approximately 20 seconds.
The utility of a web or absorbent structure having a high Wet Compressed Bulk (WCB) value is obvious, for a wet material which can maintain high bulk under compression can maintain higher fluid capacity and is less likely to allow fluid to be squeezed out when it is compressed.
High Wet Springback Ratio values are especially desirable because a wet material that springs back after compression can maintain high pore volume for effective intake and distribution of subsequent insults of fluid, and such a material can regain fluid during its expansion which may have been expelled during compression. In diapers, for example, a wet region may be momentarily compressed by body motion or changes in body position. If the material is unable to regain its bulk when the compressive force is released, its effectiveness for handling fluid is reduced.
High Loading Energy Ratio values in a material are also useful, for such a material continues to resist compression (LER is based on a measure of the energy required to compress a sample) at loads less than the peak load of 2 psi, even after it has been heavily compressed once. Maintaining such wet elastic properties is believed to contribute to the feel of the material when used in absorbent articles, and may help maintain the fit of the absorbent article against the wearer""s body, in addition to the general advantages accrued when a structure can maintain its pore volume when wet.
The hydrophobically-treated absorbent webs of this invention and the untreated, inherently hydrophilic basesheets useful in producing this invention can exhibit one or more of the foregoing properties. More specifically, said absorbent webs and basesheets can have a Wet Compressed Bulk of about 6 cubic centimeters per gram or greater, more specifically about 7 cubic centimeters per gram or greater, more specifically about 8 cubic centimeters per gram or greater, and still more specifically from about 8 to about 13 cubic centimeters per gram. The Compression Ratio can be about 0.7 or less, more specifically about 0.6 or less, still more specifically about 0.5 or less, and still more specifically from 0.4 to about 0.7. Also, they can have a Wet Springback Ratio of about 0.6 or greater, more specifically about 0.7 or greater, more specifically about 0.85, and still more specifically from about 0.8 to about 0.93. The Loading Energy Ratio can be about 0.6 or greater, more specifically 0.7 or greater, more specifically still about 0.8 or greater, and most specifically from about 0.75 to about 0.9. Final bulk can be about 8 cubic centimeters per gram or greater or preferably about 12 centimeters per gram or greater.
xe2x80x9cIn-Plane Permeabilityxe2x80x9d. An important property of porous media, particularly for absorbent products, is the permeability to liquid flow. The complex, interconnected pathways between the solid particles and boundaries of a porous media provide routes for fluid flow which may offer significant flow resistance due to the narrowness of the channels and the tortuosity of the pathways.
For paper, permeability is commonly expressed in terms of gas flow rates through a sheet. This practice is useful for comparing similar sheets, but does not truly characterize the interaction of flowing fluid with the porous structure and provides no direct information about flow in a wet sheet. The standard engineering definition of permeability provides a more useful parameter, though one less easily measured. The standard definition is based on Darcy""s law (see F. A. L. Dullien, Porous Media: Fluid Transport and Pore Structure, Academic Press, New York, 1979), which, for one-dimensional flow, states that the velocity of fluid flow through a saturated porous medium is directly proportional to the pressure gradient:                     V        =                              K            μ                    ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢              P                        L                                              (        1        )            
where V is the superficial velocity (flow rate divided by area), K is the permeability, xcexc is the fluid viscosity, and xcex94P is the pressure drop in the flow direction across a distance L. The units of K are m2. In Equation (1), the permeability is an empirical proportionality parameter linking fluid velocity to pressure drop and viscosity. For a homogeneous medium, K is not a function of xcex94P, sample length, or viscosity, but is an intrinsic parameter describing the flow resistance of the medium. In a compressible medium, permeability will be a function of the degree of compression. Darcian permeability is a fundamental parameter for processes involving fluid flow in fibrous webs.
Darcian permeability has units of area (m2) and for simple uniform cylindrical pores is proportional to the cross sectional area of a single pore. However, the permeability of most real materials cannot be predicted from an optical assessment of pore size. Permeability is determined not only by pore size, but also pore orientation, tortuosity, and interconnectedness. Large pores in the body of an object may be inaccessible to fluid flow or accessible only through minute pores offering high flow resistance. Even with a full three-dimensional description of the pore space of a material from x-ray tomography or other imaging techniques, it is difficult to predict or calculate the permeability. Permeability and pore size determinations are related but distinct pieces of information about a material. For example, a sheet of metal with discreet, nonoverlapping holes punched in it may have very large pores (the holes), while still having negligible In-Plane Permeability. Swiss cheese has many large pores, but typically has negligible permeability in any direction unless sliced so thin that individual holes can extend from one face to the other of the cheese sample.
Most studies of permeability in paper have focused on flow in the z-direction (normal to the plane of the sheet), which is of practical importance in wet pressing and other unit operations. However, paper is an anisotropic material (for example, see E. L. Back, xe2x80x9cThe Pore Anisotropy of Paper Products and Fibre Building Boards,xe2x80x9d Svensk Papperstidning, 69: 219 (1966)), meaning that fluid flow properties are a function of direction. In this case, different flow directions will appear to have different apparent permeabilities. The many possibilities of flow direction and pressure gradients in such a medium can be encompassed with a multidimensional form of Darcy""s law,                                           v            _                    =                                                    -                                                      K                    _                                    _                                            ·                              ∇                P                                      μ                          ,                            (        2        )            
where {overscore (v)} is the superficial velocity vector (volumetric flow rate divided by cross-sectional area of the flow), xcexc is the viscosity of the fluid, {double overscore (K)} is a second-order tensor and ∇P is the pressure gradient. If a Cartesian coordinate system is chosen to correspond with the principal flow directions of the porous medium, then the permeability tensor becomes a diagonal matrix (see Jacob Bear, xe2x80x9cDynamics of Fluids in Porous Media.,xe2x80x9d American Elsevier, New York, N.Y., 1972, pp. 136-151):                                                         K              _                        _                    =                      [                                                                                K                    x                                                                    0                                                  0                                                                              0                                                                      K                    y                                                                    0                                                                              0                                                  0                                                                      K                    z                                                                        ]                          ,                            (        3        )            
where Kx, Ky, and Kz are the principal permeability components in the x-, y-, and z-directions, respectively. In paper, these directions will generally correspond to the cross-direction (taken here as y) and the machine-direction (taken as x, the direction of maximum In-Plane Permeability) in the plane, and the transverse or thickness direction (z). Thus, the anisotropic permeability of typical machine-made paper can be characterized with three permeability parameters, one for the machine-direction, one for the cross-direction, and one for the z-direction. (In some cases, as when there are unbalanced flows in the headbox of the paper machine, the direction of maximum permeability may be slightly off from the machine direction; the direction of maximum In-Plane Permeability and the direction orthogonal to that should be used for the x- and y-directions, respectively, in that case.) In handsheets, there may be no preferential direction of orientation for fibers lying in the plane, so the x- and y-direction permeability values should be equal (in other words, such a sheet is isotropic in the plane).
In spite of the past focus on z-direction permeability in paper, In-Plane Permeability (both Kx and Ky are in-plane factors) is important in a variety of applications, especially in absorbent articles. Body fluids or other liquids flowing into the absorbent article usually enter the article in a narrow, localized region. Efficient use of the absorbent medium requires that the incoming fluid be distributed laterally through in-plane flow in the absorbent article, otherwise the local capacity of the article to handle the incoming liquid may be overwhelmed resulting in leakage and poor utilization of the absorbent core. The ability of fluid to flow in the plane of the article is a function of the driving force for fluid flow, which can be a combination of capillary wicking and hydraulic pressure from fluid source, and of the ability of the porous medium to conduct flow, which is described in large part by the Darcian permeability of the material. Two-phase flow and non-Newtonian liquids or suspensions complicate the physics, but the in-plane permeability of the porous medium is a critical factor for rapid in-plane distribution of liquid insults. Especially in the case of urine management, where liquid flow rates may occur far in excess of the ability of capillary forces, high In-Plane Permeability is needed in the intake layer to allow the fluid to be distributed laterally rather than to leak.
While many past studies of liquid permeability in paper focused exclusively on measuring Kz for z-direction flow, more recently, methods have been taught for measuring permeability in the plane of a paper sheet. J. D. Lindsay and P. H. Brady teach methods for in-plane and z-direction permeability measurements of saturated paper in xe2x80x9cStudies of Anisotropic Permeability with Applications to Water Removal in Fibrous Webs: Part I,xe2x80x9d Tappi J., 76(9): 119-127 (1993) and xe2x80x9cStudies of Anisotropic Permeability with Applications to Water Removal in Fibrous Webs: Part II,xe2x80x9d Tappi J., 76(11): 167-174 (1993). Related methods have been published by K. L. Adams, B. Miller, and L. Rebenfeld in xe2x80x9cForced In-Plane Flow of an Epoxy Resin in Fibrous Networks,xe2x80x9d Polymer Engineering and Science, 26(20): 1434-1441 (1986); J. D. Lindsay in xe2x80x9cRelative Flow Porosity in Fibrous Media: Measurements and Analysis, Including Dispersion Effects,xe2x80x9d Tappi J., 77(6): 225-239 (June 1994); J. D. Lindsay and J. R. Wallin, xe2x80x9cCharacterization of In-Plane Flow in Paper,xe2x80x9d AlChE 1989 and 1990 Forest Products Symposium, Tappi Press, Atlanta, Ga. (1992), p. 121; and D. H. Horstmann, J. D. Lindsay, and R. A. Stratton, xe2x80x9cUsing Edge-Flow Tests to Examine the In-Plane Anisotropic Permeability of Paper,xe2x80x9d Tappi J., 74(4): 241 (1991).
The basic method used in most of these publications is injection of fluid into the center of a paper disk that is constrained between two flat surfaces to force the fluid flow to be in the radial direction, proceeding from the injection point at the center of the disk to the outer edge of the disk. This is illustrated in FIG. 9, which depicts a sheet 41 in which a central hole 42 has been punched and into which fluid is injected by means of an injection port of the same size as the punched hole. Fluid is forced to flow to the outer radial edge 43. For a liquid-saturated sheet of constant thickness subject to steady radial fluid flow in the manner described in the work of Lindsay and others, the equation relating average In-Plane Permeability to fluid flow is:                                                         K              r                        ≡                                                            K                  x                                +                                  K                  y                                            2                                =                                                    Q                μ                            ⁢                              ln                ⁡                                  (                                                            R                      o                                        /                                          R                      i                                                        )                                                                    2              ⁢              π              ⁢                              xe2x80x83                            ⁢                              L                p                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢              P                                      ,                            (        4        )            
where Ro is the radius of the paper disk 41, Ri is the radius of the central hole 42 in the sample into which fluid is injected through an injection port; Lp is the thickness of the paper; xcex94P is the constant pressure above atmospheric pressure at which fluid is injected into the disk (the gauge pressure at the injection pore); Q is the volumetric flow rate of liquid, and Kr is the In-Plane Permeability, technically the average radial permeability, defined as the average of the two in-plane permeability components. The disk diameter is 5 inches. The central inlet hole 42 was consistently 0.375 inches (xe2x85x9c inch) and was created using a paper punch tool. The test apparatus for In-Plane Permeability measurements is depicted in FIG. 10 and FIG. 11, which is similar in principle to the apparatus taught by Lindsay and Brady, previously cited. Tubing 45 connects water from a water reservoir to an injection port drilled into a 1-inch thick Plexiglas support plate 45. (The support plate is transparent to permit viewing of the wetted sample, especially in cases when an aqueous dye solution is injected into the sample. A mirror at a 45 degree angle below the support plate facilitates viewing and photography.) The water reservoir 51 provides a nearly constant hydraulic head 49 for fluid injection during the test. The volumetric flow rate is obtained by noting the change in water reservoir mass as a function of time, and converting the water mass flow rate to a volumetric flow rate. Vacuum-deaerated deionized water at room temperature is used.
In using the apparatus, a paper disk 41, cut to be 5-inches in diameter and having a central hole diameter of 0.375-inches, is placed on the support plate 46 over the injection port 44 (0.375 inches diameter also) and is then saturated with water. The fluid injection line 45 and the injection port 44 should be filled with water and efforts should be taken to avoid air bubbles being trapped in the sheet or in the injection area. To help eliminate air pockets, the sample 41 should be bent gently in the center as it is placed on the wet support plate to initiate liquid contact in the center of the sample; the edges can then be lowered gradually to create a wedge-like motion of the liquid meniscus to sweep air bubbles out from under the sheet. Multi-ply stacks of sheets can be handled in the same way, although preliminary sample wetting may be needed to remove interply air bubbles. The goal in removing air bubbles is to reduce the flow blockage that trapped air bubbles can cause.
Once the wetted sample is in place, a cylindrical metal platen 47, 5-inches in diameter, is gently lowered on top of the sample to provide a constant compressive load and to provide a reference surface on its top for thickness measurement with displacement gauges 48. Three displacement gauges 48 are used, spaced approximately evenly around the edge of the top of the metal cylinder 47, in order to measure the average thickness of the sheet 41. The sample thickness is taken as the average of the three displacement values relative to a zero point when no sample is present. A suitable thickness gauge is the Mitutoyo Digimatic Indicator, Model 543-525-1, with a 2-inch stroke (traveling distance of the contacting spindle) and a precision of 1 micrometer. The thickness gauges are rigidly mounted relative to the support plate. The contacting spindles of the thickness gauges can be raised and lowered (without changing the position of the body of the gauge) by use of a cable to provide clearance for moving the metal platen onto the sample. The small force applied by the thickness gauges 48 should be added to the weight of the metal platen 47 to obtain the total force applied to the sample 41; this force, when divided by the cross sectional area of the sample and platen, should be 0.8 psi.
A hydraulic head of 13 inches is used to drive the liquid flow. The head is the vertical distance 49 between the water line 50 of the supply reservoir 51 and the plane of the sample 41. This head is achieved by placement of a water bottle 51, filled to a specified level 50, on a mass balance 52 at a fixed height relative to the support plate 46 on which the sample rests. As the sample is being placed on the support plate, the water reservoir is at such a height that the water level 50 in the reservoir is nearly the same as (or slightly greater than) the support plate 46 on which the sample rests. When the sample has been moistened and placed under the compressive load of the metal platen, the water reservoir is then raised and placed on a mass balance 52 such that the water level is 13 inches above the support platen. A timer is activated and the water reservoir mass is recorded at 20 seconds or 30 seconds intervals for a least 90 seconds. The thickness readings of the three gauges is also recorded regularly during the test. To reduce creep, the saturated sample should be allowed to equilibrate under the compressive load for at least 30 seconds before the water bottle is raised and forced flow through the sample begins.
The change in water reservoir mass as a function of time gives the mass flow rate, which can easily be converted to a volumetric flow rate for use in Equation 4. Normal engineering principles should be used to ensure that the proper units (preferably SI units) are used in applying Equation 4.
In performing In-Plane Permeability measurements, it is important that the sample be uniformly compressed against the restraining surfaces to prevent large channels or openings that would provide paths of least resistance for substantial liquid flow that could bypass much of the sample itself. Ideally, the liquid will flow uniformly through the sample, and this can be ascertained by injecting dyed fluid into the sample and observing the shape of the dyed region through the transparent support plate. Injected dye should spread out uniformly from the injection point. In isotropic samples, the shape of the moving dye region should be nearly circular. In materials with in-plane anisotropy due to fiber orientation or small-scale structural orientation, the shape of the dye region should be oval or elliptical, and nearly symmetric about the injection point. A suitable dye for such tests is Versatint Purple II made by Milliken Chemical Corp. (Inman, S.C.). This is a fugitive dye that does not absorb onto cellulose, allowing for easy visualization of liquid flow through the fibrous medium.
As will be illustrated in the Examples, the webs and basesheets of this invention possess very high In-Plane Permeability. The In-Plane Permeability can be about 0.1xc3x9710xe2x88x9210 square meters or greater, more specifically about 0.3xc3x9710xe2x88x9210 square meters or greater, more specifically about 0.5xc3x9710xe2x88x9210 square meters or greater, still more specifically from about 0.5xc3x971010 to about 8xc3x9710xe2x88x9210 square meters, and still more specifically from about 0.8xc3x9710xe2x88x9210 to about 5xc3x9710xe2x88x9210 square meters.