Patent Application
20050045296
This invention relates to fibrous web materials generally and particularly to stabilized absorbent composite materials and a method for making the materials.
Disposable absorbent products such as mortuary, veterinary and personal care absorbent articles such as diapers, training pants, disposable swimwear, incontinence garments and pads, sanitary napkins, wipes and the like often include one or more layers of fibrous web materials, especially liquid absorbent fibrous web materials, as an absorbent core material, and a backing layer or moisture barrier layer which is impervious to fluid. Personal care absorbent products typically also include a surface for contacting the body of the user, and the absorbent fibrous web material is generally disposed between the body-contacting surface and the moisture barrier layer so that body fluids are absorbed into the product and are contained by the moisture barrier.
Such absorbent fibrous web materials are frequently formed as nonwoven fibrous webs, such as for example, a pulp fluff/superabsorbent composite structure which may also contain longer synthetic staple fibers, such as thermoplastic binder fibers. In order to improve the structural integrity when utilized as, or as part of, an absorbent core material, it is beneficial to bond or otherwise stabilize the structure of the fibrous web material. Methods are known in the art for providing structural integrity to absorbent fibrous web material such as adhesive bonding or, where thermoplastic binder fibers are employed, by the application of heat such as by through-air bonding in an oven with heated air.
It is often highly desirable to have such absorbent composite materials produced in a highly densified state, for ease of storage and transportation when the materials are stored in roll goods form. Additionally, highly densified absorbent composite materials are easier to handle in product converting operations wherein the materials are incorporated into the product as the absorbent core and result in more aesthetically pleasing “thin” personal care absorbent products such as thinner diapers or thin feminine care pad products. However, conventional methods for forming densified absorbent composite materials include heat activation of thermoplastic binder followed by densification, or simultaneous heat activation and densification, both of which can damage the structure of the absorbent composite material and/or its components, particularly the structure of superabsorbent materials, which can deleteriously affect fluid absorption characteristics of the material. Thus, there remain continuing opportunities for improved stabilized absorbent composite materials and methods for making stabilized absorbent composite materials.
The present invention provides for a method of making a stabilized absorbent composite material including the steps of providing cellulosic fibers, thermoplastic bicomponent binder fibers and superabsorbent material, entraining in a stream of air as a mixture the cellulosic fibers, binder fibers and superabsorbent material, depositing the mixture on a moving forming surface to form an airlaid composite material, densifying the material, and then heating the airlaid composite to activate the binder fibers to stabilize the absorbent composite material. The superabsorbent material in the stabilized absorbent composite material may desirably be superabsorbent particles having a mass median particle size which is greater than about 80 percent, and more desirably at least about 90 percent, of the mass median particle size of those in a similar but non-densified material. In certain embodiments, the density of the stabilized absorbent composite material may be at least about 0.10 g/cm3, and for other embodiments the density may desirably be at least about 0.15 g/cm3 or at least about 0.20 g/cm3. The stabilized absorbent composite material may have a third insult fluid intake rate greater than about 3 ml/s, and desirably greater than about 4 ml/s. The method may further include the step of bonding one or more sheet layers to the stabilized absorbent composite material to form a laminate material, and the sheet layer or layers may desirably be such as a nonwoven sheet layer, a foam layer, a tissue layer or a thermoplastic film layer.
The invention further provides a densified stabilized absorbent composite material comprising cellulosic fibers, thermoplastic bicomponent binder fibers and superabsorbent particles, the stabilized absorbent composite material having a density of at least about 0.10 g/cm3, and wherein the superabsorbent particles have a particle size distribution defined by the weight percent of particles in each of a series of sized cuts. For the stabilized absorbent composite material, the weight percent of superabsorbent particles in the top two of the series of cuts is at least about 50 percent of the weight percent of superabsorbent particles in the top two cuts of a similar but non-densified material. In certain embodiments, the density of the stabilized absorbent composite material is desirably at least about 0.15 g/cm3. In another embodiment, the weight percent of superabsorbent particles in the top two of the series of cuts is at least about 70 percent of the weight percent of superabsorbent particles in the top two cuts of a similar but non-densified material. The stabilized absorbent composite material may further comprise one or more additional sheet layers bonded to it, and the sheet layer or layers may desirably be such as a nonwoven sheet layer, a foam layer, a tissue layer or a thermoplastic film layer. The stabilized absorbent composite material may desirably be used in or as a component of various personal care absorbent products.
As used herein and in the claims, the term “comprising” is inclusive or open-ended and does not exclude additional unrecited elements, compositional components, or method steps.
Accordingly, the term “comprising” encompasses the more restrictive terms “consisting essentially of” and “consisting of”.
As used herein the term “polymer” generally includes but is not limited to, homopolymers, copolymers, such as for example, block, graft, random and alternating copolymers, terpolymers, etc. and blends and modifications thereof. Furthermore, unless otherwise specifically limited, the term “polymer” shall include all possible geometrical configurations of the material. These configurations include, but are not limited to isotactic, syndiotactic and random symmetries.
As used herein the term “monocomponent” fiber refers to a fiber formed from one or more extruders using only one polymer. This is not meant to exclude fibers formed from one polymer to which small amounts of additives have been added for color, anti-static properties, lubrication, hydrophilicity, etc. These additives, e.g. titanium dioxide for color, are generally present in an amount less than 5 weight percent and more typically about 2 weight percent.
As used herein the term “conjugate fibers” refers to fibers which have been formed from at least two polymers, or the same polymer with different properties, extruded from separate extruders but spun together to form one fiber. Conjugate fibers are also sometimes referred to as multicomponent or bicomponent fibers. The polymers are usually different from each other though conjugate fibers may be monocomponent fibers. The polymers are arranged in substantially constantly positioned distinct zones across the cross-section of the conjugate fibers and extend continuously along the length of the conjugate fibers. The configuration of such a conjugate fiber may be, for example, a sheath/core arrangement wherein one polymer is surrounded by another or may be a side by side arrangement or an “islands-in-the-sea” arrangement. Conjugate fibers are taught in U.S. Pat. No. 5,108,820 to Kaneko et al., U.S. Pat. No. 5,336,552 to Strack et al., and U.S. Pat. No. 5,382,400 to Pike et al. For two component fibers, the polymers may be present in ratios of 75125, 50/50, 25/75 or any other desired ratios.
As used herein the term “biconstituent fibers” refers to fibers which have been formed from at least two polymers extruded from the same extruder as a blend. Biconstituent fibers do not have the various polymer components arranged in relatively constantly positioned distinct zones across the cross-sectional area of the fiber and the various polymers are usually not continuous along the entire length of the fiber, instead usually forming fibrils or protofibrils which start and end at random. Biconstituent fibers are sometimes also referred to as multiconstituent fibers. Fibers of this general type are discussed in, for example, U.S. Pat. No. 5,108,827 to Gessner.
As used herein the term “nonwoven fibrous web” means a web having a structure of individual fibers or filaments which are interlaid, but not in an identifiable manner as in a knitted or woven fabric. Nonwoven fabrics or webs have been formed from many processes such as for example, meltblowing processes, spunbonding processes, and dry-forming techniques such as carding and air-laying processes. The basis weight of nonwoven fabrics is usually expressed in grams of material per square meter (gsm) or ounces of material per square yard (osy) and the fiber diameters useful are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.)
As used herein, an “airformed” or “airlaid” web is a fibrous web structure formed primarily by a process involving deposition of loose, air-entrained fibers onto a porous forming surface. Generally the web comprises cellulosic fibers having typical lengths ranging from about 1 to about 5 millimeters (mm) such as those from fluff pulp that have been separated from a mat or “board” of pulp fibers, such as by a hammermilling process, and then entrained in a moving stream of air and deposited or collected on the porous forming surface. Longer cellulosic fibers may be used as well. There may also be other longer fibers such as synthetic staple fibers or thermoplastic binder fibers present, and typically following collection of the fibers on the forming surface the web is bonded by such means as thermal bonding or adhesive bonding and/or densified. Further, an airlaid web to which binder material is subsequently added can be considered within the scope of the term “airlaid” according to the present invention. In addition, superabsorbent materials in particulate or fiber form may be included in airlaid webs where desired. Equipment for producing airlaid webs includes the Rando-Weber air-former machine available from Rando Machine Corporation of Macedon, N.Y. and the Dan-Web rotary screen air-former machine available from Dan-Web Forming International A/S of Risskov, Denmark.
As used herein, the term “cellulosic” is meant to include materials having cellulose as a major constituent, and specifically comprising at least about 50 percent by weight cellulose or a cellulose derivative. Therefore the term cellulosic includes, without limitation, cotton, typical wood pulps, non-woody cellulosic fibers, cellulose acetate, cellulose triacetate, rayon, thermomechanical wood pulp, chemical wood pulp, debonded chemical wood pulp, milkweed, or bacterial cellulose.
As used herein, the term “hydrophilic” with regard to polymeric or cellulosic material means that the material has a surface free energy such that the material is wettable by an aqueous medium, i.e., a liquid medium of which water is a major component. The hydrophilicity of materials can be measured, for example, in accordance with the ASTM-D-724-89 contact angle testing procedure. For example, a hydrophilic polymeric material has an initial contact angle equal to or less than 90°. Depending on material application needs and degree of hydrophilicity desired, this term includes materials where the initial contact angle may desirably be equal to or less than about 75°, or even equal to or less than about 50°. The term “initial contact angle” as used herein indicates a contact angle measurement made within about 5 seconds of the application of water drops on a test film specimen. The term “hydrophobic” includes those materials that are not hydrophilic as defined. It will be recognized that hydrophobic materials may be treated internally or externally with surfactants and the like to render them hydrophilic, and that slightly or moderately hydrophilic materials may be treated to make them more hydrophilic.
The present invention is directed to stabilized absorbent composite materials and methods for making stabilized absorbent composite materials. The invention will be described with reference to certain embodiments and with reference to an illustrated process line in
The method of making the stabilized absorbent composite material comprises providing loose cellulosic fibers, thermoplastic binder fibers and superabsorbent materials and entraining these components into a moving airstream. The components are then collected on a moving forming surface to form a composite airlaid fibrous web material, which is densified and then undergoes heat activation of the binder to form the stabilized absorbent composite material. The fibrous web forming method may utilize conventional airlaying processes as are known in the art, such as is disclosed for example in U.S. Pat. No. 4,640,810 to Laursen et al., U.S. Pat. No. 4,494,278 to Kroyer et al., U.S. Pat. No. 5,527,171 to Soerensen and U.S. Pat. No. 4,375,448 to Appel et al., all incorporated herein by reference. The stabilized absorbent composite material comprises by weight from about 2 percent to about 20 percent thermoplastic binder fiber, from about 30 percent to about 80 percent superabsorbent material such as for example particulate superabsorbent material, and from about 18 percent to about 68 percent of cellulosic fibers. More particularly, the stabilized absorbent composite material comprises by weight from about 2 percent to about 10 percent thermoplastic binder fiber, from about 40 percent to about 80 percent superabsorbent particles, and from about 18 percent to about 58 percent of cellulosic fibers.
The thermoplastic binder fibers help to “stabilize” the material, i.e., help keep it from fragmenting or breaking apart under applied pressure, which is an especial hazard of pulp or cellulosic structures when wet. Suitable thermoplastic binder fibers include thermoplastic monocomponent and multicomponent fibers. Particularly suited are bicomponent thermoplastic binder fibers having one polymer component which melts at a temperature lower than the melting temperature of the second polymer component. Melting or partial melting of at least one polymer component of the bicomponent fiber allows the bicomponent fibers to form a tacky skeletal structure, which upon cooling, captures and binds or bonds to many of the pulp fibers. Additionally, the thermoplastic binder fibers may bond to each other and/or particles of superabsorbent material. Typically, the polymers of the bicomponent fibers are made up of different thermoplastic materials, such as for example, polyolefin/polyester (sheath/core) bicomponent fibers whereby the polyolefin, e.g. polyethylene sheath, melts at a temperature lower than the core, e.g., polyester. An example of a commercially available useful bicomponent thermoplastic binder fiber are the “CELBOND” fibers produced by KOSA Inc. (formerly Trevira Inc. and formerly Hoechst Celanese Corporation), Salisbury, N.C., under the designation T-255, which is a sheath-and-core fiber having a polyethylene sheath and a polyester core.
Typical thermoplastic and other polymers useful in fiber forming include polyolefins, e.g. polyethylene, polypropylene, polybutylene, and copolymers thereof, polyesters, e.g. polyethylene terephthalate, polyvinyl acetate, polyvinyl chloride acetate, polyvinyl butyral, acrylic resins, e.g. polyacrylate, and polymethylacrylate, polymethylmethacrylate, polyamides, namely nylon, polyvinyl chloride, polyvinylidene chloride, polystyrene, polyvinyl alcohol, polyurethanes, cellulosic resins, namely cellulosic nitrate, cellulosic acetate, cellulosic acetate butyrate, ethyl cellulose, etc., copolymers of any of the above materials, e.g. ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, styrene-butadiene block copolymers, and the like.
Suitable superabsorbent materials useful in the present invention include superabsorbent particles as are known in the art. Generally, superabsorbent materials include water-swellable, generally water insoluble materials capable of absorbing at least about 10 times their weight in water. More specifically, superabsorbent materials are capable of absorbing as much as 20 times their weight in water, and more specifically 50 times, 100 times, or even up to 300 times or more their weight in water (or other dispersion medium). Superabsorbent materials may be formed from organic material which may include natural materials such as agar, pectin, and guar gum, as well as synthetic materials such as synthetic hydrogel polymers. Synthetic hydrogel polymers include, for example, carboxymethylcellulose, alkali metal salts of polyacrylic acid and its copolymers, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropylcellulose, hydroxypropyl acrylate, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine, and the like. Other suitable polymers include hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, and isobutylene maleic anhydride copolymers and mixtures thereof. The hydrogel polymers are suitably lightly crosslinked to render the material substantially water insoluble. Crosslinking may, for example, be by irradiation or by covalent, ionic, Van der Waals, or hydrogen bonding. Suitable materials are available from various commercial vendors, such as the Dow Chemical Company (Midland, Mich.), Stockhausen Inc. (Greensboro, N.C.), and BASF (Charlotte, N. C.). The noncellulosic, synthetic hydrogel polymers are suitable for use in the present invention. Exemplary commercially available superabsorbent materials include the synthetic hydrogel polymer superabsorbent particles such as are available from Stockhausen Incorporated (Greensboro, N.C.) under the name “FAVOR” and designated “SXM-9543”.
Suitable cellulosic fibers for use in the invention include conventional pulp fibers as are well known in the industry such as thermomechanical pulp fibers, chemithermomechanical pulp fibers, chemimechanical pulp fibers, refiner mechanical pulp (RMP) fibers, stone groundwood (SGW) pulp fibers, and peroxide mechanical pulp (PMP) fibers. Suitable cellulosic pulp fibers commercially available include those designated “NB-416” available from the Weyerhaeuser Corporation (Federal Way, Wash.) and “Caressa 1300” available from Buckeye Technologies, Inc. (Memphis, Tenn.).
As shown in the exemplary process line 10 shown in side view in
Following web formation on forming surface 60, the composite airlaid fibrous web material is compacted or densified at nip 70 defined by rollers 72 and 74. Desirably, the bulk density of the composite airlaid fibrous web material following densification is at least about 0.10 g/cm3 and more desirably at least about 0.15 g/cm3. In certain embodiments, the bulk density is desirably even higher, as high as about 0.20 g/cm3 or 0.30 g/cm3, or higher. Rollers 72 and 74 defining nip 70 may be any rollers useful for compacting or densifying webs as are known in the art, such as for example steel rollers, rubber or plastic covered steel rollers, rubber rollers, or combinations thereof. Rollers 72 and 74 defining nip 70 may be heated or unheated but are desirably operated unheated or at ambient temperature. Where rollers 72 and 74 are heated they should be operated at a temperature less than the softening point of the thermoplastic binder fibers.
While the specific means for compacting or densifying the composite airlaid fibrous web material is not crucial, it is important that the material be compacted to or substantially to its final desired density prior to heat activation of the thermoplastic binder, and that the material not undergo further compaction or densification after heat activation. As is discussed below with reference to the Examples and Comparative materials, densification of the stabilized absorbent composite material following heat activation of the thermoplastic binder material produces deleterious effects on the final stabilized absorbent composite material's components, such as by grinding or crushing the SAM particles, and negatively affects the material's fluid handling ability such as the material's ability to rapidly intake fluid insults. Returning to
The stabilized absorbent composite material may desirably range in basis weight from about 50 grams per square meter (gsm) or less up to about 1000 gsm, or higher. More particularly, the composite airlaid fibrous web material may desirably range in basis weight from about 200 gsm to about 800 gsm. In order to produce materials with basis weights in the heavier ends of these ranges, it may be useful or desirable to employ an airlaying process substantially as described with respect to
It should be noted that although
Depending on the desired end use for the nonwoven material, it may be beneficial to add other treatments or additives to the web such as for example treatments to impart or increase hydrophilicity, or colorants or pigments. Such additives or treatments may be incorporated in the stabilized absorbent composite material by introducing them into the airlaying process at some point prior to the collection of the composite airlaid fibrous web upon the forming surface, or may be added later in the process by spraying or otherwise depositing them on the web material. Other alternatives are also possible, and various additional potential processing and/or finishing steps known in the art such as aperturing, slitting, treating, or lamination of the fibrous web material with films, foams, nonwoven web layers or tissue layers, may be performed without departing from the spirit and scope of the invention.
As a few examples of the foregoing, the stabilized absorbent composite material may be combined into a laminate material that contains at least one layer of the stabilized absorbent composite material and at least one additional layer such as a woven or nonwoven fabric, a tissue, a film or a foam. The additional layer for the laminate may be selected to impart additional and/or complementary properties, such as additional liquid handling characteristics, liquid barrier and/or microbe barrier properties. The layers of the laminate can be bonded to form a unitary structure by a bonding process known in the art to be suitable for laminate structures, such as thermal, ultrasonic or adhesive bonding processes.
For example, the stabilized absorbent composite material may be laminated with one or more additional nonwoven sheet layers such as spunbond or meltblown layers, or combinations thereof such as the exemplary laminate structure disclosed in U.S. Pat. No. 4,041,203 to Brock et al., incorporated herein in its entirety by reference, which discloses a pattern bonded laminate of at least one fibrous nonwoven web, e.g., spunbond fiber web, and at least one microfiber nonwoven web, e.g., meltblown web. Alternatively, a breathable film can be laminated to the stabilized absorbent composite material to provide a breathable barrier laminate material useful as, for example, a backsheet material for personal care absorbent products. As yet another alternative, the stabilized absorbent composite material can be laminated to a non-breathable film to provide a high liquid barrier, or laminated to one or more foam type materials to provide resiliency and/or additional absorbency.
The stabilized absorbent composite material and/or laminates of the stabilized absorbent composite material with other layers is highly suitable for use as an absorbent layer or absorbent core layer in a personal care absorbent product. Personal care absorbent articles include such as diapers, training pants, disposable swimwear, incontinence garments and pads, sanitary napkins, wipes and the like.
Turning to
Disposed between liner 220 and outer cover 210 is an absorbent core 230 which comprises a layer of the stabilized absorbent composite material of the invention. Diaper 200 may further include optional containment flaps 240 made from or attached to body side liner 220. Suitable constructions and arrangements for such containment flaps are described, for example, in U.S. Pat. No. 4,704,116 to Enloe, incorporated herein by reference in its entirety. Still further, the diaper 200 can optionally include additional elements known to those skilled in the art, including but not limited to, elasticized leg cuffs, elastic waist band, and so forth.
To secure the diaper 200 about the wearer, the diaper will have some type of fastening means attached thereto. As shown in
The Example stabilized absorbent composite material was produced substantially in accordance with the process described herein and Comparative stabilized absorbent composite material was produced for purposes of comparison. The Example and Comparative materials were produced using a 2-head airlaid forming line using Dan-Web forming heads to airlay the component fibers and materials. The SAM particles were added to the forming heads using two “Christy Coat-O-Matic Model 20 inch-DI-S” particle feeders available from the Christy Machine Company of Freemont, Ohio which were placed above each Dan-Web forming head substantially in accordance with the teachings of co-assigned U.S. Pat. App. No. 10/036854 to Chambers et al. (U.S. Application Publication No. 20030116890, Jun. 26, 2003).
Both the Example and Comparative materials contained by weight percentage approximately 46 percent Buckeye Technologies Caressa 1300 pulp, 50 percent Stockhausen SXM-9543 superabsorbent particles and 4 percent KoSA T-255 binder fibers, and were produced at an undensified or pre-compacted density of approximately 0.09 g/cm3 and a basis weight of approximately 660 gsm. Following deposition, the Example material was densified using two sets of unheated steel-on-rubber compaction rolls located just downstream of the second forming head. The compaction nip pressure was approximately 350 pounds per linear inch or PLI (about 63 kg/linear cm) and the Example material had a final density of about 0.18 g/cm3. The Example material was then passed through an air-impingement oven (air temperature approximately 350° F. (178° C.) and air face velocity about 400 ft/min (about 2 m/s)) to activate the binder fibers. The Comparative material was passed through the oven in its undensified state to activate the binder fibers (air temperature again approximately 350° F. (178° C.) and air face velocity about 400 ft/min (about 2 m/s)), and was then densified to a final density of about 0.18 g/cm3. To densify the Comparative material a set of steel-on-steel compaction rolls downstream of the oven was used. In order to densify the Comparative material to the same density as the Example material it was found that a pressure of approximately 830 PLI (about 148 kg/linear cm) was required. While not wishing to be bound by theory, we believe the increased pressure required to density the Comparative materials may be the result of a certain amount of resiliency added to the composite due to prior heat activation of the binder fibers.
Density of the materials was calculated by measuring the thickness or caliper of the material and the length, width and weight of the sample and calculating the density in units of grams per cubic centimeter. Thickness of a 4 inch by 4 inch (10.16 by 10.16 cm) sample of material was measured under a 3 inch (7.62 cm) diameter circular platen under a 0.2 PSI (1.38 kpa) load.
The Example and Comparative materials were tested for SAM particle size distribution and fluid intake rate as is described below. In addition, a similar but non-densified absorbent composite material was tested for SAM particle distribution. By the term “similar” what is meant is an absorbent composite material made under essentially the same process conditions and having essentially the same components except for the conditions or components mentioned. Here, the similar but non-densified absorbent composite material is the same material as either the Example or Comparative material at the point in the production process where material is deposited on the forming surface, but the similar but non-densified absorbent composite material does not undergo any densification step, either before or after heat activation of the binder material.
SAM Particle Size Distribution:
The SAM particle size distribution was measured for the Example and Comparative absorbent composite materials. The SAM particle size distribution was also measured for similar but non-densified absorbent composite material (i.e., after being airlaid into the composite material and deposited on the forming surface and undergoing heat activation of the binder material in the same way as the Example and Comparative materials but otherwise in a non-densified state) as a way to determine the initial or as-deposited particle size distribution. The particle size distributions were measured in a series of sized cuts using a stack of sieves over a pan with each lower sieve having smaller sized openings than the preceding sieve, and then measuring the weight of SAM particles retained in each sieve and on the pan. The sieves used had 850, 600, 300, 90 and 45 micron openings. The particle size distribution percent by weight was then calculated using the weight retained by each sieve and the pan divided by the initial total weight of SAM particles tested.
To perform the test, a 25 gram sample of the SAM particles is obtained from each of the materials by carefully pulling the material apart and separating out the SAM particles. The 25 gram sample of SAM is placed into the sieve stack by placing it into the sieve with the largest openings (850 microns). The sieve stack is then shaken for 10 minutes with a Ro-Tap mechanical Sieve Shaker, Model B available from W.S. Tyler, Inc. (Mentor, Ohio), or other similar shaking device. After shaking, the weight of SAM particles retained in each cut (retained by each sieve and by the bottom pan) is measured and recorded. The principle is that a particle retained in the top cut, i.e., retained by the sieve having 850 micron openings, is considered to have a particle size greater than 850 microns. A particle passing through that sieve but being retained by the sieve having 600 micron openings is considered to have a particle size greater than 600 microns but less than or equal to 850 microns. Similarly, particles passing through the sieve having 600 micron openings but being retained by the sieve having 300 micron openings are considered to have a particle size greater than 300 microns but less than or equal to 600 microns, etc. Finally, particles passing through all of the 850, 600, 300, 90 and 45 micron openings sieves to be retained on the pan are considered to have a particle size of 45 microns or less.
TABLE 1 lists the results of particle size distribution testing by listing the weight percentage of particles in each cut for each of the non-densified, Example and Comparative materials. Sample size for each particle size distribution test was 25 grams of SAM particles.
As can be seen from TABLE 1, the weight percentage of SAM particles greater than 600 microns is about 21 percent for the similar but non-densified material. This percentage represents the combined weight percent of particles in the top two cuts, where the top two cuts are the 850 and 600 micron cuts. After densification and heat activation/stabilization of the Example material, the combined weight percent of particles in the top two cuts is still about 18 percent. Stated another way, the Example stabilized absorbent composite material still had over 85 percent of the top two cuts SAM particle weight compared to the similar but non-densified material. However, after heat activation and then densification for the Comparative material, the combined weight percent of particles in the top two cuts drops to less than 9 percent. Therefore, the Comparative stabilized absorbent composite material only had about 42 percent of the top two cuts SAM particle weight compared to the similar but non-densified material. Furthermore, the weight percentage of particles in the 300 microns and smaller category (the bottom three cuts) increases from about 29 percent by weight in the similar but non-densified material to about 44 percent by weight for the Comparative material. However, for the Example material the bottom three cuts are still only about 28 percent by weight of the SAM particles.
In addition, a mass median particle size can be calculated from the particle size distribution data in TABLE 1. The mass median particle size is determined by plotting the cumulative particle size distribution curve (i.e., the total mass percentage of material not passing through a particular sieve vs. size of that sieve opening) and by interpolating the sieve opening which corresponds to 50 percent of the cumulative distribution. That is, the particle size where an equal amount of the total mass of particles is contained in particle sizes higher and lower than the mass median particle size. A linear interpolation can be done by identifying the two cuts that bracket 50 percent of the cumulative distribution and identifying the particle size that would correspond to 50 percent. For example, in the similar but non-densified material 20.87 percent of the total mass of particles is above 600 microns while 70.57 percent of the total mass of particles is above 300 microns. Therefore, the mass median particle size (MMPS) for the similar but non-densified material is calculated as:
MMPS (non-densified)=600−(600−300)*(50−20.87)/(70.57−20.87)=424 microns.
The MMPS is calculated similarly for the Example and Comparative materials:
MMPS (Example)=600−(600−300)*(50−17.82)/(72.04−17.82)=422 microns.
MMPS (Comparative)=600−(600−300)*(50−8.84)/(55.79−8.84)=337 microns.
TABLE 2 shows the cumulative particle size distributions for each of the similar but non-densified, Example and Comparative Materials. TABLE 2 also lists the MMPS for each material as was calculated above.
As can be seen from TABLE 2, the mass median particle size for the Example material (422 microns) is over 99 percent of the mass median particle size for the similar but non-densified material (424 microns). However, for the Comparative material the mass median particle size drops to 337 microns, which is less than 80 percent of the mass median particle size for the similar but non-densified material. Therefore, it appears that the heat activate-then-densify process for the Comparative material is unduly destructive of the SAM particles, fracturing and breaking the SAM particles up into smaller fragments. While not wishing to be bound by theory, we believe this SAM particle destruction is caused by either the higher pressures required to densify the material after binder heat activation, or by the interaction of drying of the SAM particles (during binder heat activation) followed by the densification pressure, or both, which have a grinding and damaging effect on the SAM particles and may crack and/or break individual particles into smaller sized particles. This destructive grinding can have deleterious effects on the fluid handling abilities of the stabilized absorbent composite material, especially in the case of multiple or serial fluid insults, as for example when the stabilized absorbent composite material is used in or as part of an absorbent core material in a personal care absorbent product and is expected to take up and retain bodily fluid exudates.
Therefore it is beneficial to minimize SAM particle damage during processing and the stabilized absorbent composite material should have at least about 50 percent of the top two cuts SAM particle weight compared to the similar but non-densified material, and desirably this ratio may be 60 percent, 70 percent, 80 percent or even higher. It is also highly desirable that the mass median particle size or MMPS of the SAM particles in the stabilized absorbent composite material remain as close as possible to the MMPS of a similar but non-densified material. Desirably, the MMPS for the densified stabilized absorbent composite material will be higher than 80 percent of the MMPS for a similar but non-densified material, and more desirably will be 90 percent or more of the MMPS for a similar but non-densified material.
Fluid Intake Testing:
To simulate real-life fluid handling requirements and estimate potential leakage of the absorbent composite materials in-use in a personal care absorbent product, the rate of fluid intake upon a third insult was measured for the Example and Comparative materials. The third insult fluid intake was measured substantially in accordance with the fluid intake measurement portion of the “Fluid Intake and Flowback Evaluation” or “FIFE” test described in U.S. Pat. No. 5,147,343 to Kellenberger, incorporated herein by reference, with the modifications as described below. Generally, for the fluid intake portion of the test, the sample to be tested is placed flat and smooth under a 360 gram cylinder plate assembly such as is shown in
It will be apparent to those skilled in the art that other alterations or modifications than those discussed herein may be made without departing from the spirit and scope of the present invention. It is therefore intended that all such modifications, alterations and other changes be encompassed by the claims. Numerous other patents have been referred to in the specification and to the extent there is any conflict or discrepancy between the teachings incorporated by reference and that of the present specification, the present specification shall control.
