Patent Description:
Cellulosic fibers find utility in many applications, including absorbents. Indeed, cellulosic fibers are a basic component of many absorbent products such as diapers. The fibers form a liquid absorbent structure, a key element in an absorbent product.

Cellulosic fluff pulp, a form of cellulosic fibers, has been used for absorbent applications because the fluff pulp form provides a high void volume, or high bulk, liquid absorbent fiber structure. However, this structure tends to collapse upon wetting, which reduces the volume of liquid that can be retained in the wetted structure. Further, such collapse may inhibit transfer of liquid into unwetted portions of the cellulose fiber structure, leading to local saturation.

Whereas the ability of an absorbent product containing cellulosic fibers to initially acquire and distribute liquid (such as from an initial liquid insult) relates to the product's dry bulk and capillary structure, the ability of a wetted structure to acquire additional liquid (such as from subsequent and/or extended liquid insults) relates to the structure's wet bulk. Due to diminished acquisition and capacity properties related to loss of fiber bulk associated with liquid absorption, the potential capacity of a dry high bulk fiber structure such as cellulosic fluff pulp may not be fully realized, with the liquid holding capacity instead determined by the structure's wet bulk.

Intra-fiber crosslinked cellulose fibers and structures formed therefrom generally have enhanced wet bulk as compared to non-crosslinked fibers. The enhanced bulk is a consequence of the stiffness, twist, and curl imparted to fibers as a result of crosslinking. Accordingly, crosslinked fibers are incorporated into absorbent products to enhance their wet bulk and liquid acquisition rate.

In addition to wet bulk and liquid acquisition rate, a material's suitability -for use in absorbent products may be characterized in terms of other performance properties, such as liquid permeability. As noted above, performance properties tend to result from different fiber characteristics such as fiber length, fiber stiffness, and so forth. However, relationships between some performance properties indicate the existence of trade-off trends for many cellulose fiber (and other) materials. For example, liquid permeability tends to decrease as capillary pressure, expressed in terms of medium absorption pressure, increases. As explained in greater detail below, this particular relationship manifests in a manner that can be mathematically approximated as a power curve function of the two properties, which is characteristic for many if not all materials used in absorbent applications, including cellulose fiber materials, synthetic fiber materials, blends, and so forth. Of these materials, the "trade-off" curve for cellulose fiber products is the highest, but successful efforts to raise this curve higher - that is, to produce materials that exhibit better liquid permeability value at a given capillary pressure value (and vice versa) than as predicted by the power curve function described by cellulose fibers - have not yet been observed.

There are a number of methods for preparing crosslinked cellulose fibers; several are summarized in <CIT> to Westland, et al. Much effort has been spent improving crosslinking processes, such as to lower production and/or material costs, to modify absorbent and/or other fiber properties of the products, and so forth. In one example, polycarboxylic acids have been used to crosslink cellulosic fibers (such as in <CIT>, <CIT>, and <CIT>, all to Herron, et al. , and so forth), to produce absorbent structures containing cellulosic fibers crosslinked with a C2-C9 polycarboxylic acid. Despite advantages that polycarboxylic acid crosslinking agents provide, cellulosic fibers crosslinked with low molecular weight (monomeric) polycarboxylic acids, such as citric acid, have been found to undergo reversion to a non-crosslinked condition and thus have a useful shelf-life that is relatively short. Polymeric polycarboxylic acid crosslinked fibers, however, such as disclosed in <CIT>, <CIT>, and <CIT>, amongst others, resist such aging or reversion, due in part to the participation of the polymeric polycarboxylic acid molecule in the crosslinking reaction with an increased number of reactive carboxyl groups than is the case with monomeric polycarboxylic acids such as citric acid. In another example, <CIT>. , discloses the use of a comparatively low molecular weight polyacrylic acid having phosphorous (in the form of a phosphinate) incorporated into the polymer chain as a crosslinking agent to achieve crosslinked cellulose fibers having improved brightness and whiteness (as well as other properties) as compared to those prepared with higher molecular weight phosphinated agents or polyacrylic acid agents without phosphinates. <CIT> discloses monommer cross linked to the cellulose fiber.

Thus, there is a continuing need to produce crosslinked cellulose fibers and compositions and materials including such fibers suitable for use in absorbent and other applications.

Various embodiments of crosslinked cellulosic materials composed of a cellulosic substrate, such as cellulose fibers, having grafted polymer chains, with such cellulose fibers and/or grafted polymer chains being crosslinked, and various methods of producing such compositions, are disclosed herein. The fibrous materials produced according to the present disclosure are also referred to herein as "grafted crosslinked cellulose" and may be described as "compositions" as well as "materials. " In embodiments of the grafted crosslinked cellulose of the present disclosure, the polymer chains are composed of monoethylenically unsaturated acid group-containing monomers (non-limiting examples include acrylic acid, maleic acid, methacrylic acid, etc., and combinations thereof), which may be crosslinked in a variety of manners (non-limiting examples include ester intra-fiber crosslinks via crosslinking agents such as homopolymers, hyperbranched polymers, pentaerythritol, and so forth).

In some embodiments, acrylic acid is used as the monoethylenically unsaturated acid group-containing monomer. Some embodiments are characterized by a graft yield of <NUM>-<NUM> weight %, and more particularly <NUM>-<NUM> weight %. Some embodiments are characterized by a wet bulk at least <NUM>% greater, and up to at least <NUM>% greater, than untreated cellulose (referring to the cellulose substrate in an untreated, i.e., non-grafted and non-crosslinked, state). Some embodiments are characterized by a wet bulk of about <NUM>-<NUM><NUM>/g, and more particularly of about <NUM>-<NUM><NUM>/g. Some embodiments include mainly intra-fiber chain-to-chain crosslinks composed of a crosslinking agent such as pentaerythritol or a hyperbranched polymer. Some embodiments include intra-fiber chain-to-cellulose crosslinks.

In some of the aforementioned embodiments, the material is characterized by an IPRP value of about <NUM> to <NUM><NUM>/MPa·sec and a medium absorption pressure (MAP) of about <NUM> to <NUM> H<NUM>O. Some embodiments are further characterized by power curve function wherein for a given IPRP value y (in cm<NUM>/MPa·sec) from <NUM> to <NUM>, the MAP value of the material (in cm H<NUM>O) is within +/-<NUM>% of the value of x in the formula y = mxz; wherein m is from <NUM> to <NUM>, and wherein z is from - <NUM> to -<NUM>. Some embodiments are more particularly characterized in that z is from -<NUM> to - <NUM> and/or m is from <NUM> to <NUM>. In some embodiments, at a given IPRP value (in cm<NUM>/MPa·sec) from <NUM> to <NUM>, the material has a MAP value that is equal to or higher (e.g., <NUM>-<NUM>% higher) than the corresponding MAP value possessed by non-grafted, crosslinked cellulose fiber, and/or at a given MAP value (in cm H<NUM>O) from <NUM> to <NUM>, the material has an IPRP value that is equal to or higher (e.g., <NUM>-<NUM>% higher) than the corresponding IPRP value possessed by non-grafted, crosslinked cellulose fiber. Some embodiments have an IPRP value of <NUM><NUM>/MPa·sec or above.

Example methods of producing grafted crosslinked cellulose in accordance with the present disclosure include grafting polymer chains of at least one monoethylenically unsaturated acid group-containing monomer from a cellulosic substrate to produce a grafted cellulosic material, followed by crosslinking the grafted cellulosic material by treating the material with a crosslinking agent adapted to effect crosslinking of one or more of the cellulosic substrate or the polymer chains. In some methods, the grafting is performed in situ and may include reacting the monomer with the cellulosic substrate in the presence of a grafting initiator such as cerium(IV) sulfate. In some methods, acrylic acid is used as the monomer. Some methods include varying the amounts of the initiator and/or the ratio of cellulose to monomer to achieve a desired graft level.

Such methods may include any of a variety of crosslinking procedures. Some methods include establishing intra-fiber crosslinks via an esterification reaction via one or more crosslinking agents such as pentaerythritol, a polymeric crosslinking agent (for example, a homopolymer formed of the at least one monoethylenically unsaturated acid group-containing monomer), a hyperbranched polymer, and so forth. Some methods include establishing intra-fiber crosslinks via an ionic reaction via a multivalent inorganic compound (such as aluminum sulfate) as a crosslinking agent. Some methods include establishing intra-fiber crosslinks via a radical reaction via a suitable inorganic salt (such as ammonium persulfate) as a cross-linking agent. Some methods include at least partially neutralizing the grafted polymer side chains by treating the grafted cellulosic material with an alkaline solution.

The materials, concepts, features, and methods briefly described above are clarified with reference to the detailed description below.

The complete disclosures of the aforementioned references, and those of all of the other references cited herein, are incorporated in their entireties for all purposes.

There has been much research on grafting copolymers, including grafting copolymers from cellulosic materials, such as with grafting polymer chains or "arms" consisting of acrylic acid monomers, from holocellulose (see, e.g., <NPL>)). However, there has been no investigation of subjecting grafted cellulose structures to conditions suitable to effect crosslinking in non-grafted cellulose materials.

The inventors have discovered, however, that various absorbent and other properties of certain grafted cellulose structures are changed upon undergoing crosslinking by various reactions and mechanisms. In particular, absorbent properties of some grafted, crosslinked cellulose structures, such as wet bulk, absorbent capacity, permeability (e.g., in-plane radial permeability or IPRP), capillary pressure (e.g., as measured by medium absorption pressure or MAP), as described in greater detail below, are consistent with or improved relative to those achieved by crosslinked cellulose products produced by other methods, and are favorable when compared to cellulose fibers that are not crosslinked and/or not grafted.

As an example of a non-crosslinked, non-grafted cellulose fiber, a bleached kraft pulp product available from Weyerhaeuser NR Company under the designation CF416 has a wet bulk of <NUM><NUM>/g and an absorbent capacity of <NUM>/g. When grafted with polymer chains composed of monoethylenically unsaturated acid group-containing monomers (with a graft yield in the range of about <NUM>-<NUM> weight %) the resulting grafted CF416 exhibited lower wet bulk values (of about <NUM>-<NUM><NUM>/g, with acrylic acid used as the graft species) and absorbent capacity values (of <NUM>-<NUM>/g, with acrylic acid used as the graft species), but when subjected to subsequent crosslink treatment, the grafted, crosslinked cellulose structures produced in accordance with the present disclosure exhibited improved wet bulk values of about <NUM>-<NUM><NUM>/g, and/or absorbent capacity values of about <NUM>-<NUM>/g. A crosslinked (and non-grafted) fiber product available from Weyerhaeuser NR Company under the designation CMC530, useful as a control, has a wet bulk of approximately <NUM><NUM>/g and an absorbent capacity of <NUM>/g. Accordingly, the grafted, crosslinked cellulose fibers of the present disclosure may have suitability, for example, in absorbent applications similar to those for which non-grafted crosslinked cellulose fibers are used.

As another example of this suitability, the grafted cellulose structures produced in accordance with the present disclosure exhibit IPRP and MAP values consistent with or improved relative to non-grafted cellulose fibers. As explained in greater detail below, IPRP and MAP values indicate a trade-off relationship that approximates a power law function of the two properties. IPRP and MAP values relating to CMC530 and other non-grafted controls as compared to that of example embodiments of grafted cellulose structures indicate that the grafted, crosslinked structures exhibit (or are predicted to exhibit, according to formulae expressing best-fit curves generated by measured IPRP and MAP data) comparable or increased values, up to <NUM> or <NUM>%, or even greater.

Cellulosic fibers useful for making the grafted crosslinked cellulose of the present disclosure are derived primarily from wood pulp. Although suitable wood pulp fibers may be obtained from chemical processes such as the kraft and sulfite processes, with or without subsequent and/or prior mercerization and/or bleaching, the pulp fibers may also be processed by thermomechanical or chemithermomechanical methods, or various combinations thereof. Ground wood fibers, recycled or secondary wood pulp fibers, and bleached and unbleached wood pulp fibers may be used. One example starting material is prepared from long-fiber coniferous wood species, such as southern pine, Douglas fir, spruce, hemlock, and so forth. Details of the production of wood pulp fibers are known to those skilled in the art. Suitable fibers are commercially available from a number of sources, including the Weyerhaeuser NR Company. For example, suitable cellulose fibers produced from southern pine that may be used as the cellulose substrate in the materials of the present disclosure are available from the Weyerhaeuser NR Company under the designations CF416, CF405, NF405, NB416, <CIT>, <CIT>, PW416, and PW405, amongst others.

The graft species suitable for grafting to the cellulosic fiber "backbone" to produce the grafted crosslinked cellulose materials of the present disclosure include those that may be described as monoethylenically unsaturated acid group-containing monomers, which include, for example, acrylic acid, methacrylic acid, crotonic acid, isocrotonic acid, maleic acid, fumaric acid, itaconic acid, vinylsulfonic acid, <NUM>-acrylamido-<NUM>-methyl-<NUM>-propane sulfonic acid, vinyl acetic acid, methallyl sulfonic acid, and so forth, as well as their alkali and/or ammonium salts, and various combinations of the aforementioned examples. The choice of suitable graft species is guided in part by the nature of the backbone from which the grafted arms are grown, achieving a suitable polymer architecture, the desired end result of the crosslink treatment, and so forth.

For example, grafting, in the context of polymer chemistry, refers in general to the synthesis of polymer chains attached to a substrate, and thus encompasses mechanisms such as "grafting to," which refers to a polymer chain adsorbing onto a substrate out of solution, as well as "grafting from," which refers to initiating and propagating a polymer chain (such as by step-growth addition of monomer units) at a grafting site on the substrate. The latter mechanism is generally considered to offer greater control over the resulting polymer architecture, density of grafting sites, polymer chain lengths and linearity, and so forth. Considering these factors, graft species that include one or more acid groups are chemically appropriate when considered against a goal of establishing intra-fiber crosslinks, such as chain-to-chain crosslinks between grafted arms of an individual cellulose fiber. Further, monoethylenenically unsaturated graft species are suitable for the grafted crosslinked cellulose materials herein because of their ability to graft to a cellulose substrate without creating additional branches or side chains (as opposed, for example, to species with more than one unsaturated group, the use of which is more difficult to control). Monomeric graft species are considered to be easier to control, in terms of reactivity, density of polymer chains, establishing desired chain lengths and polymer chain architectures (e.g., linear, unbranched polymers grafted at one end to the cellulose backbone), suppressing crosslink reactions from occurring in the grafting stage, and so forth.

"Grafting," as the term is used herein, refers collectively to the processes of initiation, growth, and termination of growth polymerization of the (monomeric) graft species from one or more grafting sites on the cellulosic substrate. Typically, grafting according to methods discussed herein is performed in situ, in which an initiator is used, such as to create active centers on the substrate and, usually to a lesser extent, initiate homopolymerization in the aqueous phase. As such, although the grafting processes described herein proceed mainly by way of the "grafting from" mechanism described above, the term does not exclusively refer to this mechanism. Rather, "grafting" also encompasses the "grafting to" mechanism, other mechanisms, and/or combinations thereof. Moreover, the terms "to" and "from," when used when referring to grafting, do not exclusively refer to the corresponding grafting mechanism, but instead may each encompass some degree of the other grafting mechanism (or mechanisms).

It was found that by controlling the amount of graft species used, various levels of grafting were obtained, characterized by graft yield %, defined below as the additional weight of a grafted sample attributable to the polymerized graft species. For example, acrylic acid readily grafted from cellulose up to approximately <NUM>% graft yield. Additionally, by varying the levels (e.g., weight %) of initiator, the number of graft sites available on the cellulosic substrate was altered. Variations in graft yield were also obtained by varying the ratio of cellulose to graft species.

In general, grafted cellulose fibers were produced in situ by dissolving a measured amount of initiator, such as cerium(IV) sulfate, in deionized water, and then dissipating a designated amount of graft species, such as <NUM> - <NUM> weight % acrylic acid (based on the oven-dry weight of the cellulose), in the solution. The levels of cerium(IV) sulfate initiator were varied between about <NUM> - <NUM> weight % (based on the oven-dry weight of the cellulose). The grafting solution was added to the cellulosic substrate, the treated cellulose was allowed to react, then washed and filtered to remove unreacted grafting solution and excess homopolymers of the graft species, dried, and weighed to determine graft yield %. In some examples, cellulose-graft-poly(acrylic) acid was then treated with dilute solutions of sodium hydroxide to at least partially neutralize the grafted arms on the cellulosic substrate.

A variety of cross-linking agents and reaction mechanisms were applied to various grafted cellulose materials, including ionic crosslinking reactions using multivalent inorganic compounds (such as aluminum sulfate, a trivalent salt, and titanium-based crosslinking agents), covalent ester crosslinking reactions using polymeric crosslinking agents (for example, a homopolymer of the graft species, such as poly(acrylic) acid or "PAA" when acrylic acid was used as the graft species), pentaerythritol, and various hyperbranched polymers ("HPB"), and a radical-based cross-linking mechanism using ammonium persulfate. Each was performed, at various levels, on various graft yield levels of fiberized, grafted cellulose and/or partially neutralized grafted cellulose, with various factors determining the conditions and amounts of reagents selected (e.g. solubility in water, ability to distribute evenly across the grafted cellulose, etc.).

In general, the grafted structures assumed a polymer architecture consisting mainly of linear, unbranched polymer chain arms attached to the cellulose backbone at one end. When subjected to the various cross-linking reactions, the resulting crosslinked, grafted structures generally exhibited intra-fiber crosslinks between grafted arms (such as via pentaerythritol, or a hyperbranched polymer), also referred to as chain-to-chain crosslinks. Additionally, in some cases, for example when treated with polymeric crosslinking agents, some of the chain arms attached to the cellulose backbone at more than one point, also referred to as chain-to-cellulose crosslinks, and some polymer bonded along the cellulose backbone. While not being bound by theory, it is believed that the stiffness and resiliency imparted by establishing intra-fiber chain-to-chain and/or chain-to-cellulose crosslinks strengthen the high void volume structure and provide the observed, improved absorbent properties as compared to non-crosslinked grafted cellulose fibers and non-crosslinked, non-grafted cellulose fibers.

Although the methods disclosed herein primarily establish intra-fiber crosslinks (such as chain-to-chain crosslinks between grafted arms of an individual cellulose fiber, chain-to-cellulose crosslinks between a grafted arm of a cellulose fiber to elsewhere on the cellulose fiber, and so forth), some materials also exhibited a minor degree of inter-fiber crosslinking, such as between separate cellulose fibers and/or grafted chains thereon. In general, inter-fiber crosslinking is thought to increase the number of "knots" in the resulting fibers. Although a higher knot content is generally not considered to be a desirable characteristic in cellulosic material used in absorbent applications (both for the sake of overall product appearance, as well as for the comparative ease of processing lower knot content fibers), a greater degree of inter-fiber crosslinking may be achieved by using the processes described herein, such as, for example, by applying and curing the crosslinking agent to cellulose in sheet form, rather than to fiberized cellulose. Other variations may include selecting graft species and/or crosslinking agent(s) appropriate for establishing such inter-fiber crosslinking, suitably varying process conditions, and so forth, to achieve a desired amount of inter-fiber crosslinking. All of such variations are considered to be within the scope of this disclosure.

Some performance properties of the grafted, crosslinked cellulosic compositions according to the present disclosure - specifically, absorbent properties of wet bulk, wick time, wick rate, and absorbent capacity - were determined using the Automatic Fiber Absorption Quality (AFAQ) Analyzer (Weyerhaeuser Co. , Federal Way, WA), according to the following procedure:.

A dry <NUM>-gram sample of the pulp composition is placed through a pinmill to open the pulp, and then airlaid into a tube. The tube is then placed in the AFAQ Analyzer. A plunger then descends on the airlaid fluff pad at a pressure of <NUM> kPa. The pad height is measured, and the pad bulk (or volume occupied by the sample) is determined from the pad height.

The weight is increased to achieve a pressure of <NUM> kPa and the bulk recalculated. The result is two bulk measurements on the dry fluff pulp at two different pressures.

While the dry fluff pulp is still compressed at the higher pressure, water is introduced into the bottom of the tube (to the bottom of the pad), and the time required for water to wick upward through the pad and reach the plunger (defined as wick time) is measured. The bulk of the wet pad at <NUM> kPa is also calculated. From distance measurements used to calculate the bulk, the wick rate is determined by dividing the wick time by the distance traveled by the water (e.g. the height of the wetted fluff pad). The plunger is then withdrawn from the tube and the wet pad is allowed to expand for <NUM> seconds. In general, the more resilient the sample, the more it will expand to reach its wet rest state. Once expanded, this resiliency is measured by reapplying the plunger to the wet pad at <NUM> kPa and determining the bulk. The final bulk of the wet pad at <NUM> kPa is considered to be the "wet bulk at <NUM> kPa" (in cm<NUM>/g, indicating volume occupied by the wet pad, per weight of the wet pad, under the <NUM> kPa plunger load) of the pulp composition. When the term "wet bulk" is used herein, it refers to "wet bulk at <NUM> kPa" as determined according to this procedure.

Absorbent capacity is calculated by weighing the wet pad after water is drained from the equipment and reported as grams water per gram dry pulp.

Permeability generally refers to the quality of a porous material that causes it to allow liquids or gases to pass through it and, as such, is generally determined from the mass flow rate of a given fluid through it. The permeability of an absorbent structure is related to the material's ability to quickly acquire and transport a liquid within the structure, both of which are key features of an absorbent article. Accordingly, measuring permeability is one metric by which a material's suitability for use in absorbent articles may be assessed.

The following test is suitable for measurement of the In-Plane Radial Permeability (IPRP) of a porous material. The quantity of a saline solution (<NUM>% NaCl) flowing radially through an annular sample of the material under constant pressure is measured as a function of time.

Testing is performed at <NUM> ± 2C° and a relative humidity <NUM>% ± <NUM>%. All samples are conditioned in this environment for twenty four (<NUM>) hours before testing.

The IPRP sample holder <NUM> is shown in <FIG> and comprises a cylindrical bottom plate <NUM>, top plate <NUM>, and cylindrical stainless steel weight <NUM>.

Top plate <NUM> comprises an annular base plate <NUM><NUM> thick with an outer diameter of <NUM> and a tube <NUM> of <NUM> length fixed at the center thereof. The tube <NUM> has an outer diameter of <NUM> and an inner diameter of <NUM>. The tube is adhesively fixed into a circular <NUM> hole in the center of the base plate <NUM> such that the lower edge of the tube is flush with the lower surface of the base plate, as depicted in <FIG>. The bottom plate <NUM> and top plate <NUM> are fabricated from Lexan® or equivalent. The stainless steel weight <NUM> has an outer diameter of <NUM> and an inner diameter of <NUM> so that the weight is a close sliding fit on tube <NUM>. The thickness of the stainless steel weight <NUM> is approximately <NUM> and is adjusted so that the total weight of the top plate <NUM> and the stainless steel weight <NUM> is <NUM> ± <NUM> to provide <NUM> kPa of confining pressure during the measurement.

Bottom plate <NUM> is approximately <NUM> thick and has two registration grooves <NUM> cut into the lower surface of the plate such that each groove spans the diameter of the bottom plate and the grooves are perpendicular to each other. Each groove is <NUM> wide and <NUM> deep. Bottom plate <NUM> has a horizontal hole <NUM> which spans the diameter of the plate. The horizontal hole <NUM> has a diameter of <NUM> and its central axis is <NUM> below the upper surface of bottom plate <NUM>. Bottom plate <NUM> also has a central vertical hole <NUM> which has a diameter of <NUM> and is <NUM> deep. The central hole <NUM> connects to the horizontal hole <NUM> to form a T-shaped cavity in the bottom plate <NUM>. The outer portions of the horizontal hole <NUM> are threaded to accommodate pipe elbows <NUM> which are attached to the bottom plate <NUM> in a watertight fashion. One elbow is connected to a vertical transparent tube <NUM> with a total height of <NUM> measured from the bottom of bottom plate <NUM> (including elbow <NUM>) and an internal diameter of <NUM>. The tube <NUM> is scribed with a suitable mark <NUM> at a height of <NUM> above the upper surface of the bottom plate <NUM>. This is the reference for the fluid level to be maintained during the measurement. The other elbow <NUM> is connected to the fluid delivery reservoir <NUM> (described below) via a flexible tube.

A suitable fluid delivery reservoir <NUM> is shown in <FIG>. Reservoir <NUM> is situated on a suitable laboratory jack <NUM> and has an air-tight stoppered opening <NUM> to facilitate filling of the reservoir with fluid. An open-ended glass tube <NUM> having an inner diameter of <NUM> extends through a port <NUM> in the top of the reservoir such that there is an airtight seal between the outside of the tube and the reservoir. Reservoir <NUM> is provided with an L-shaped delivery tube <NUM> having an inlet <NUM> that is below the surface of the fluid in the reservoir, a stopcock <NUM>, and an outlet <NUM>. The outlet <NUM> is connected to elbow <NUM> via flexible plastic tubing <NUM> (e.g. Tygon®). The internal diameter of the delivery tube <NUM>, stopcock <NUM>, and flexible plastic tubing <NUM> enable fluid delivery to the IPRP sample holder <NUM> at a high enough flow rate to maintain the level of fluid in tube <NUM> at the scribed mark <NUM> at all times during the measurement. The reservoir <NUM> has a capacity of approximately <NUM> liters, although larger reservoirs may be required depending on the sample thickness and permeability. Other fluid delivery systems may be employed provided that they are able to deliver the fluid to the sample holder <NUM> and maintain the level of fluid in tube <NUM> at the scribed mark <NUM> for the duration of the measurement.

The IPRP catchment funnel <NUM> is shown in <FIG> and comprises an outer housing <NUM> with an internal diameter at the upper edge of the funnel of approximately <NUM>. Funnel <NUM> is constructed such that liquid falling into the funnel drains rapidly and freely from spout <NUM>. A stand with horizontal flange <NUM> around the funnel <NUM> facilitates mounting the funnel in a horizontal position. Two integral vertical internal ribs <NUM> span the internal diameter of the funnel and are perpendicular to each other. Each rib <NUM> is <NUM> wide and the top surfaces of the ribs lie in a horizontal plane. The funnel housing <NUM> and ribs <NUM> are fabricated from a suitably rigid material such as Lexan® or equivalent in order to support sample holder <NUM>. To facilitate loading of the sample it is advantageous for the height of the ribs to be sufficient to allow the upper surface of the bottom plate <NUM> to lie above the funnel flange <NUM> when the bottom plate <NUM> is located on ribs <NUM>. A bridge <NUM> is attached to flange <NUM> in order to mount two digital calipers <NUM> to measure the relative height of the stainless steel weight <NUM>. The digital calipers <NUM> have a resolution of ± <NUM> over a range of <NUM>. A suitable digital caliper is a Mitutoyo model <NUM>-492B or equivalent. Each caliper is interfaced with a computer to allow height readings to be recorded periodically and stored electronically on the computer. Bridge <NUM> has a circular hole <NUM> in diameter to accommodate tube <NUM> without the tube touching the bridge.

Funnel <NUM> is mounted over an electronic balance <NUM>, as shown in <FIG>. The balance has a resolution of ± <NUM> and a capacity of at least <NUM>. The balance <NUM> is also interfaced with a computer to allow the balance reading to be recorded periodically and stored electronically on the computer. A suitable balance is Mettler-Toledo model MS6002S or equivalent. A collection container <NUM> is situated on the balance pan so that liquid draining from the funnel spout <NUM> falls directly into the container <NUM>.

The funnel <NUM> is mounted so that the upper surfaces of ribs <NUM> lie in a horizontal plane. Balance <NUM> and container <NUM> are positioned under the funnel <NUM> so that liquid draining from the funnel spout <NUM> falls directly into the container <NUM>. The IPRP sample holder <NUM> is situated centrally in the funnel <NUM> with the ribs <NUM> located in grooves <NUM>. The upper surface of the bottom plate <NUM> must be perfectly flat and level. The top plate <NUM> is aligned with and rests on the bottom plate <NUM>. The stainless steel weight <NUM> surrounds the tube <NUM> and rests on the top plate <NUM>. Tube <NUM> extends vertically through the central hole in the bridge <NUM>. Both calipers <NUM> are mounted firmly to the bridge <NUM> with the foot resting on a point on the upper surface of the stainless steel weight <NUM>. The calipers are set to zero in this state. The reservoir <NUM> is filled with <NUM>% saline solution and re-sealed. The outlet <NUM> is connected to elbow <NUM> via flexible plastic tubing <NUM>.

An annular sample <NUM> of the material to be tested is cut by suitable means. The sample has an outer diameter of <NUM> and an inner hole diameter of <NUM>. One suitable means of cutting the sample is to use a die cutter with sharp concentric blades.

The top plate <NUM> is lifted enough to insert the sample <NUM> between the top plate and the bottom plate <NUM> with the sample centered on the bottom plate and the plates aligned. The stopcock <NUM> is opened and the level of fluid in tube <NUM> is set to the scribed mark <NUM> by adjusting the height of the reservoir <NUM> using the jack <NUM> and by adjusting the position of the tube <NUM> in the reservoir. When the fluid level in the tube <NUM> is stable at the scribed mark <NUM> initiate recording data from the balance and calipers by the computer. Balance readings and time elapsed are recorded every <NUM> seconds for five minutes. The average sample thickness B is calculated from all caliper reading between <NUM> seconds and <NUM> seconds and expressed in cm. The flow rate in grams per second is the slope calculated by linear least squares regression fit of the balance reading (dependent variable) at different times (independent variable) considering only the readings between <NUM> seconds and <NUM> seconds.

Permeability k is then calculated by the following equation: <MAT>
Where:.

In-plane radial permeability is dependent on the fluid being used, so the IPRP value (in cm<NUM>/MPa·sec) may be defined and calculated as follows: <MAT>
Where:.

Capillary pressure can be considered representative of a material's ability to wick fluid by capillary action and is expressed in the context of the present disclosure in terms of Medium Absorption Pressure (MAP), as explained below.

Capillary pressure measurements are made on a TRI/Autoporosimeter (TRI/Princeton Inc. of Princeton, N. The TRI/Autoporosimeter is an automated computer-controlled instrument for measuring capillary pressure in porous materials, which can be schematically represented in <FIG>. Complimentary Automated Instrument Software, Release Version <NUM>. 6WD, is used to capture the data. More information on the TRI/Autoporosimeter, its operation and data treatments can be found in <NPL>, incorporated here by reference.

As used herein, determining capillary pressure hysteresis curve of a material as function of saturation, involves recording the increment of liquid that enters a porous material as the surrounding air pressure changes. A sample in the test chamber is exposed to precisely controlled changes in air pressure which at equilibrium (no more liquid uptake/release) corresponds to the capillary pressure.

The equipment operates by changing the test chamber air pressure in user-specified increments, either by decreasing pressure (increasing pore size) to absorb liquid, or increasing pressure (decreasing pore size) to drain liquid. The liquid volume absorbed (or drained) is measured with a balance at each pressure increment. The saturation is automatically calculated from the cumulative volume.

All testing is performed at <NUM> ± 2C° and a relative humidity of <NUM>% ± <NUM>%. A saline solution of <NUM>% weight to volume in deionized water is used. The surface tension (mN/m), contact angle (°), and density (g/cc) for all solutions are determined by any method known in the art. Alternatively (as done for measuring the Examples below), reference values for these parameters may be provided to the TRI/ Autoporosimeter's software.

Surface tension (mN/m), contact angle (°), and density (g/cm<NUM>) is provided to the instrument's software. Reference values used for the tests described herein were as follows: surface tension of <NUM> mN/m; contact angle of <NUM>°; and liquid density of <NUM>/cm<NUM>. The balance is leveled at <NUM> and equilibration rate set to <NUM>/min. The pore radius protocol (corresponding to capillary pressure steps) scans capillary pressures according to the following equation: <MAT>
Where:.

Tests are performed with the sample compressed with an applied load of approximately <NUM> psi. The weight applied to the sample is <NUM> and is <NUM> in diameter.

The pressure sequence in Table <NUM>, below, is applied to the measurement cell in the standard test protocol which corresponds to an individual pore radius as indicated.

The sample is cut into a circle with <NUM> diameter and then conditioned at <NUM> ± 2C° and a relative humidity <NUM>% ± <NUM>% for at least <NUM> hours before testing. The sample weight (to ± <NUM>) is measured. The empty sample chamber is closed. After the instrument has applied the appropriate air pressure to the cell, the liquid valve is closed and the chamber is opened. The specimen and confining weight are placed into the chamber and the chamber is closed. After the instrument has applied the appropriate air pressure to the cell, the liquid valve is opened to allow free movement of liquid to the balance and the test under the radius protocol is started. The instrument proceeds through one absorption/desorption cycle (also called a hysteresis loop). A blank (without specimen) is run in like fashion.

For calculations and reporting, the mass uptake from a blank run is directly subtracted from the uptake of the sample. Medium Absorption Pressure (MAP) is the pressure at which <NUM>% of the liquid uptake has been achieved - or, in other words, the pressure that corresponds to <NUM>% of the total liquid absorbed on the absorption branch of the hysteresis loop generated by the autoporosimeter.

The following examples summarize representative methods of treating cellulose fibers in accordance with the methods and concepts discussed above, and are illustrative in nature. The reagent amounts, times, conditions, and other process conditions may be varied from those disclosed in the specific representative procedures disclosed in the following examples without departing from the scope of the present disclosure.

Representative procedure: cellulose pulpsheets (CF416, <NUM>% solids, Columbus Mill, from Weyerhaeuser NR Company, Federal Way, WA) were cut into rectangles (of <NUM>" x <NUM>"), weighed, and placed into re-sealable plastic bags in pairs. A Ce<NUM>+ catalyst solution was produced by stirring and dissolving a measured quantity of ammonium cerium(IV) sulfate (<NUM>%, from Sigma Aldrich) in <NUM> deionized water. Acrylic acid (<NUM>%, with <NUM>-200ppm MEHQ inhibitor, from Sigma Aldrich) in a measured volume was then added to the Ce<NUM>+ solution and stirred for <NUM> minutes. The resulting solution was slowly poured over the cellulose pulpsheets, on both sides, in the bag, which was then sealed and allowed to equilibrate at room temperature overnight.

The sealed bag was then cured in a ventilated oven at <NUM> for <NUM> hours, followed by cooling to room temperature. The treated cellulose (cellulose-graft-poly(acrylic) acid) was then washed with <NUM> deionized water in a Waring Blendor at low speed. Unreacted grafting solution, excess homopolymers of poly(acrylic) acid, and other impurities, were removed via vacuum filtration with Buchner funnel and filter paper, washed and vacuum filtered again, then oven-dried overnight at <NUM>.

Graft yield was calculated using the following formula: <MAT> Where W<NUM> = weight of starting cellulose material, and W<NUM> = grafted product weight.

Representative data indicating weights and volumes used in a number of runs performed according to Example <NUM>, and graft yields achieved, are shown in Table <NUM>.

A variety of cross-linking agents and reaction mechanisms were then applied to cellulose-graft-poly(acrylic) acid materials prepared in accordance with the procedure in Example <NUM>, as described in Examples <NUM>-<NUM> infra.

Representative procedure: using cellulose-graft-poly(acrylic) acid prepared according to the procedure in Example <NUM>, <NUM>" x <NUM>" British handsheets were prepared using all the material from one run (there was some loss during process). The handsheets were equilibrated to <NUM>% solids in a humidity-controlled room. Each handsheet was cut into strips (of <NUM>" x <NUM>").

Polyacrylic acid ("PAA") crosslinking agent (Aquaset™ <NUM> available from The Dow Chemical Company; other suitable examples of suitable crosslinking agents are listed in <CIT>) was applied, in some cases in the presence of a sodium hypophosphite ("SHP") catalyst, to the handsheet strips. The treated strips were allowed to equilibrate, then air-dried, fiberized with a Kamas hammermill, and cured.

Representative data indicating weights and volumes used are shown in Table <NUM>.

Representative procedure: using cellulose-graft-poly(acrylic) acid prepared according to the procedure in Example <NUM>, an aqueous slurry was prepared using all the material from one run, which was then vacuum-filtered using a Buchner funnel to produce a pad. The pad was then oven-dried at <NUM> to constant weight.

A solution of <NUM> pentaerythritol (from Sigma Aldrich) and <NUM> sodium hypophosphite in <NUM> deionized water was prepared at <NUM>, which was then cooled to room temperature and evenly applied to both sides of the pad via transfer pipette, and the treated pad was placed in a sealed plastic bag and allowed to equilibrate at room temperature overnight.

The pad was then fiberized in a Waring Blendor, and cured in an oven at <NUM> for <NUM> minutes.

Representative procedure: using cellulose-graft-poly(acrylic) acid prepared according to the procedure in Example <NUM>, an aqueous slurry was prepared using all the material from one run, which was then vacuum-filtered using a Buchner funnel to produce a pad. The pad was then air-dried to <NUM>% solids content.

An aluminum sulfate solution was prepared by dissolving <NUM> of aluminum sulfate octodecahydrate (from Sigma Aldrich) in <NUM> deionized water. The solution was evenly applied to both sides of the air-dried pad via transfer pipette, and the treated pad was placed in a sealed plastic bag and allowed to equilibrate at room temperature overnight.

The treated pad was then vacuum filtered in a Buchner funnel and gently rinsed, once, with <NUM> deionized water, then air-dried at room temperature until constant weight was achieved.

Representative procedure: using cellulose-graft-poly(acrylic) acid prepared according to the procedure in Example <NUM>, an aqueous slurry was prepared using all the material from one run, which was then vacuum-filtered using a Buchner funnel to produce a pad. The pad was then oven-dried to constant weight.

A solution was prepared by dissolving <NUM> of Tyzor® LA (lactic acid titanate chelate, from DuPont) in <NUM> deionized water at room temperature. The solution was evenly applied to both sides of the air-dried pad via transfer pipette, and the treated pad was placed in a sealed plastic bag and allowed to equilibrate at room temperature overnight.

Representative procedure: using cellulose-graft-poly(acrylic) acid prepared according to the procedure in Example <NUM>, <NUM>" x <NUM>" British handsheets were prepared using <NUM> of the material. The handsheets were equilibrated to <NUM>% solids in a humidity-controlled room. Each handsheet was cut into strips (of <NUM>" x <NUM>").

A solution of <NUM> of ammonium persulfate (from Sigma Aldrich) in <NUM> deionized water was prepared and evenly applied across the handsheet strips via transfer pipette, and the treated strips were placed in a sealed plastic bag and allowed to equilibrate at room temperature overnight.

The treated strips were then air-dried to about <NUM>% solids and then fiberized with a Kamas hammermill. The material was then gently and evenly sprayed with <NUM> deionized water, placed in a foil pouch that was perforated to allow evaporation, and oven-cured at <NUM>°F (<NUM>) for <NUM> minutes. After removal from the oven, the pouch was cooled to room temperature, and then the material was removed and allowed to air-dry at room temperature until constant weight was achieved.

A solution of a measured quantity of a hyperbranched polymer (e.g., <NUM> Lutensit® Z96 or <NUM> Lutensol® FP620, both from BASF) along with <NUM> sodium hypophosphite in <NUM> deionized water was prepared and evenly applied across the handsheet strips via transfer pipette, and the treated strips were placed in a sealed plastic bag and allowed to equilibrate at room temperature overnight.

The treated strips were then air-dried to about <NUM>% solids, fiberized with a Kamas hammermill, air-dried overnight, and cured at <NUM>°F (<NUM>) for <NUM> minutes in a large dispatch oven.

AFAQ analysis was performed on the cellulose-graft-poly(acrylic) acid materials prepared in accordance the representative procedure of Example <NUM>, as well as on various crosslinked cellulose-graft-poly(acrylic) acid materials prepared in accordance with the representative procedures of Examples <NUM>-<NUM>. The representative values presented in Table <NUM> (below) are averages from multiple runs of the indicated crosslink method on the indicated graft yield % level of cellulose-graft-poly(acrylic) acid.

Table <NUM> shows, for example, that cellulose-graft-poly(acrylic) acid produced from CF416 cellulose pulp fiber tends to exhibit lower wet bulk and absorbent capacity values as compared with untreated (i.e. non-grafted, non-crosslinked) CF416, across a range of graft yield levels tested. However, when subjected to subsequent crosslink treatment, the grafted, crosslinked cellulose structures produced thereby generally exhibited improved wet bulk values and/or absorbent capacity values. For example, ester cross-linking using PAA or a hyperbranched polymer as the cross-linking agent yielded improvements in wet bulk and absorbent capacity values, from about <NUM>% to over <NUM>%, as compared to untreated cellulose. Indeed, the improved values of these properties were found to be comparable with those exhibited by non-grafted, crosslinked cellulose samples (e.g., CMC530). Some crosslinking treatments, however, such as ionic trivalent salt and titanium-based reactions, did not increase, and in some cases further decreased, wet bulk and absorbent capacity values as compared to untreated cellulose. Also, in general, cellulose-graft-poly(acrylic) acid materials produced from CF416 exhibited lower wick rates than non-grafted, crosslinked cellulose samples, but when subjected to subsequent crosslink treatment, wick rates were seen to further decrease with some PAA/SHP and radical treatment processes, but increase with others (such as with ester cross-linking with HPB and pentaerythritol, and ionic processes). Thus, various absorbent properties may be modified through a selection of graft species, cross-linking reaction, and other process conditions.

Another performance metric by which the grafted, crosslinked cellulose materials produced in accordance with the present disclosure may be characterized and/or compared is by means of liquid permeability and capillary pressure, two properties important for absorbent products. Liquid permeability may be measured by in-plane radial permeability (IPRP) and capillary pressure may be measured by medium absorption pressure (MAP), according to the tests described above.

As noted above, there is a trade-off between IPRP and MAP with known absorbent materials, including cellulose materials, synthetic fibers, blends, and so forth, in which IPRP tends to decrease as MAP increases. This trade-off is illustrated, for example, in <FIG>, in the form of a dashed line following the least-squares best-fit curve that corresponds to IPRP and MAP values exhibited by example cellulose fiber controls, including crosslinked cellulose products such as CMC530 (used as a control in the Examples and subsequent AFAQ analysis described above) as well as non-crosslinked cellulose products such as NB416, CF416, and so forth. The IPRP and MAP values are shown in Table <NUM>, below. The curve can be described mathematically as a power law function y = mxz, with IPRP value as the abscissa and MAP as the ordinate. For the Table <NUM> data, the best-fit curve for the non-grafted controls can be expressed by the formula y = <NUM>x-<NUM>, with R<NUM> = <NUM>.

As exemplified, for example, in <FIG> (and Table <NUM>), cellulose fibers have been observed to be bounded by a maximum IPRP value of about <NUM><NUM>/MPa·sec and a maximum MAP value of about <NUM> H<NUM>O. Higher IPRP values have been achieved, but only with blends of cellulose with synthetic fibers (e.g. polyethylene, polypropylene and/or polyester fibers) or synthetic nonwovens produced from, for example, polyethylene, polypropylene and/or polyester fibers or filaments.

Focusing in particular on crosslinked cellulose fibers, such products have been observed to be bounded by a maximum MAP value of about <NUM> H<NUM>O.

The grafted, crosslinked cellulose materials prepared in accordance with the present disclosure, however, exhibit IPRP values as high as <NUM><NUM>/MPa·sec and MAP values up to <NUM> H<NUM>O.

In some examples, IPRP and MAP values for grafted, crosslinked cellulose materials approximate a trade-off curve that is slightly shifted (i.e. raised) and also elongated (i.e. spans a broader IPRP range), with respect to that exhibited by the non-grafted cellulose controls, as shown in <FIG>. The trade-off for the grafted materials is shown as a solid line following the best-fit curve for the example data presented in Table <NUM>, below.

The best-fit curve generated by the example data set in Table <NUM>, corresponding to grafted cellulose materials, can be expressed by the formula y = <NUM>. 93x-<NUM> (with R<NUM> = <NUM>). Best-fit curves for example materials prepared in accordance with the present disclosure can be characterized by the same general power law function represented by the formula y = mxz, with m values ranging from about <NUM> to about <NUM> (and more particularly from about <NUM> to about <NUM>), and z values ranging from about -<NUM> to -<NUM> (and more particularly from about -<NUM> to about -<NUM>). These best-fit curve models for the grafted materials of the present disclosure correspond to or predict the IPRP value for a given MAP value (and vice versa) within about +/- <NUM>% of the value of x (or y) in the respective formula, particularly at IPRP values y (in cm<NUM>/MPa·sec) ranging from about <NUM> to about <NUM>.

Comparing the best-fit curves for the example non-grafted controls (dashed line) and the example grafted materials (solid line), the "shift" visible in <FIG> (and shown by the data in Tables <NUM> and <NUM>) illustrates that, in the range of IPRP values exhibited by non-grafted cellulose fiber control materials (that is, a range of from about <NUM> to about <NUM><NUM>/MPa·sec), the example grafted materials exhibit (or are predicted to exhibit) MAP values equal to or higher than the corresponding MAP values possessed by non-grafted cellulose materials. Also, for a given MAP value (in cm H<NUM>O) in a range of from about <NUM> to about <NUM>, the grafted materials exhibit (or are predicted to exhibit) IPRP values equal to or higher than the corresponding IPRP values possessed by non-grafted cellulose fiber. Focusing specifically on the data corresponding to crosslinked materials, the difference in IPRP values is generally up to about <NUM>% higher for the example grafted materials as compared to the example non-grafted controls over an MAP range of about <NUM> to <NUM> (although the differences are even greater in some instances, for example with IPRP values exhibited at MAP values between about <NUM> and about <NUM>), and the difference in MAP values is generally up to about <NUM>% higher for grafted materials over a range of IPRP values of about <NUM> to <NUM><NUM>/MPa·sec.

Comparing the best-fit curves for the example non-grafted controls and the example grafted materials, the "elongation" visible in <FIG> (and shown by the data in Tables <NUM> and <NUM>) illustrates that IPRP values greater than those achieved with non-grafted cellulose products (e.g., IPRP values greater than about <NUM><NUM>/MPa·sec) are exhibited by the grafted, crosslinked cellulose materials of the present disclosure.

Claim 1:
A cellulosic material comprising a cellulose fiber having grafted polymer chains composed of at least one monoethylenically unsaturated acid group-containing monomer, wherein one or more of said cellulose fiber having said grafted polymer chains are intra-fiber crosslinked, wherein the grafted polymer chains are grafted to the cellulose backbone, and
wherein for a given in-plane radial permeability ("IPRP") value y (in cm<NUM>/MPa sec) (as described in the description) from <NUM> to <NUM>, the medium absorption pressure ("MAP") value (as described in the description) of the material (in cm H<NUM>O) is within ± <NUM>% of the value of x in the formula y = mxz; wherein m is from <NUM> to <NUM>, and wherein z is from -<NUM> to -<NUM> wherein the parameters are measured as defined in the description.