Background of the Invention Webs having a high strength when they become wet (known in the art as wet strength) are useful for many applications. One application for such webs is as premoistened tissues, often used by travelers for cleansing the body. Such webs or tissues must maintain sufficient strength when stored in wet conditions for an extended period of time to withstand wiping and rubbing actions. Other applications for high wet strength webs is in articles that need to maintain integrity when wetted with body fluids, such as urine, blood, mucus, menses and other body exudates.

In the art of papermaking, chemical materials exist for improving the wet strength of paper. These materials are known in the art as"wet strength agents" and are commercially available from a wide variety of sources. For example, a polyamide/polyamine/epichlorohydrin resin is often used to enhance the wet strength of paper. This cationic resin is typically added to the papermaking slurry whereupon it bonds to the anionically charged cellulose. During the papermaking process the resin crosslinks and eventually becomes insoluble in water. The agent thus acts as a"glue"to hold the paper fibers together and enhances the wet strength of the paper. However, one needs to use chlorine in order to remove the resin and recycle products containing this resin, which presents environmental problems.

Cationic resins have other disadvantages, such as reacting with other anionic additives which it may be advantageous to add to the paper and, in many cases, increasing the dry strength of the paper as well, resulting in a less

soft paper. Moreover, the effectiveness of cationic wet strength agents can be limited by low retention of the agent on the cellulose fiber.

The use of formaldehyde and various formaldehyde addition products to crosslink cellulosic fibers is known in the art. However, formaldehyde is an irritant and a known carcinogen. Crosslinking with compounds comprising formaldehyde at elevated temperatures can be particularly rapid relative to many other crosslinkers, requiring times as low as 1 to 10 seconds. However, for higher molecular weight compounds and for formaldehyde-free crosslinkers in general, much longer reaction times are found.

Other references disclose absorbent structures containing individualized, crosslinked fibers, wherein the crosslinking agent is selected from the group consisting of Cl to C8 dialdehydes, with glutaraldehyde being desired. The cost associated with producing fibers crosslinked with dialdehyde crosslinking agents such as glutaraldehyde may be too high to result in significant commercial success.

The use of monomeric polycarboxylic acids to impart wrinkle resistance to cotton fabrics is known. A cellulosic material was impregnated with a solution of the polycarboxylic acid and a catalyst, followed by drying the material and then curing the material in an oven at 150 °C to 240 °C for 5 seconds to 30 minutes.

The prior art also teaches a method of imparting wrinkle resistance to cellulosic textiles by crosslinking monomeric cyclic aliphatic hydrocarbons having multi carboxylic acid groups to the cellulose. Curing is said to be performed at about 150 °C to 240 °C for 5 seconds to 30 minutes.

The use of C2 to Cg monomeric polycarboxylic acids to make individualized, crosslinked cellulosic fibers having primarily intra-fiber crosslinking (crosslinks between cellulose units in a single fiber) and purportedly having increased absorbency has been taught.

Polyacrylic acid has been taught as a crosslinking agent, preferably as a copolymer with polymaleic acid. The fibers were fiberized prior to curing to make individualized, crosslinked cellulosic fibers having primarily intra-fiber

crosslinking. The fibers are purportedly useful in absorbents. The crosslinking was achieved using temperatures of about 120 °C to 160 °C.

Various resinous maleic anhydride compositions have been used in conjunction with paper products. For example, prior art discloses paper products coated with a composition including an amine salt of a low molecular weight C6 to C24 olefin/maleic anhydride copolymer in combination with a bisulfite. Such paper products exhibit release properties. Various amine salts of half esters of maleic anhydride/alpha-olefin copolymers have been disclosed as useful paper sizing or water holdout agents. Similarly, prior art discloses paper products impregnated with a sizing and wet strength agent of a reaction product of an alkyl tertiary amino alcohol and a copolymer of maleic anhydride/styrene or derivatives thereof. The use of an agent consisting of epoxide resins and maleic anhydride copolymers as an agent for imparting wet strength is known.

Polymeric treatment agents for adding wet strength to paper, which can be applied to a slurry or to a paper web, wherein curing times are said to range from 5 minutes to 3 hours, with a desired time range of 10 to 60 minutes, have been disclosed. The application of a polymeric polyacid, a phosphorous containing accelerator, and an active hydrogen compound to a paper web followed by curing at 120 °C to 400 °C for 3 seconds to 15 minutes has also been disclosed.

Accordingly, what is needed is a method of improving the wet performance of cellulosic based webs using non-formaldehyde crosslinking agents.

Summary of the Invention The present invention is directed to methods for improving the wet performance of cellulosic webs. The methods impart high wet resiliency, high wet strength, and high wet/dry strength ratio to wet-formed webs. The methods include applying a polymeric anionic reactive compound (PARC) solution onto a web, with subsequent curing. The PARC solution can be applied through one of a number of methods including coating, foam finishing, roller fluid feeding,

and spraying.

The polymeric anionic reactive compounds useful in the methods are compounds that will cause crosslinking between the cellulose fibers. In one embodiment, the polymeric anionic reactive compounds include monomeric units having two carboxylic acid groups on adjacent atoms so that the carboxylic acid groups are capable of forming cyclic anhydrides which, at elevated temperature or other initiating force, will form an ester bond with the hydroxyl groups of the cellulose. Polymers, including copolymers, terpolymers, block copolymers, and homopolymers, of maleic acid are especially desired.

Particularly desired are terpolymers of maleic acid, vinyl acetate, and ethyl acetate.

The method further includes treating the cellulosic web with a debonder, before, during, or after treatment with the PARC. The debonder lowers the dry strength of the web, making it softer. Because the dry strength is less, the wet strength to dry strength ratio of the resulting product is increased over a product made without the debonder.

The present invention also is directed to high wet performance webs produced according to the methods of the invention and to articles made with the webs.

Detailed Description of the Invention Definitions "Papermaking fibers,"as used herein, include all known cellulosic fibers or fiber mixes comprising cellulosic fibers. Fibers suitable for making the webs of this invention comprise any natural or synthetic cellulosic fibers including, but not limited to: nonwoody fibers, such as cotton lines and other cotton fibers or cotton derivatives, abaca, kenaf, sabai grass, flax, esparto grass, straw, jute hemp, bagasse, milkweed floss fibers, and pineapple leaf fibers; and woody fibers such as those obtained from deciduous and coniferous trees, including softwood fibers, such as northern and southern softwood kraft fibers, hardwood fibers, such as eucalyptus, maple, birch, aspen, or the like. Wood fibers may be

prepared in high-yield or low-yield forms and include kraft pulps, sulfite pulps, groundwood pulps, thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), pressure/pressure thermomechanical pulp (PTMP), and bleached chemithermomechanical pulp (BCTMP). High brightness pulps, including chemically bleached pulps, are especially desired for tissue making, but unbleached or semi-bleached pulps may also be used. Any known pulping and bleaching methods may be used.

Synthetic cellulose fiber types include rayon in all its varieties and other fibers derived from viscose or chemically modified cellulose. Chemically treated natural cellulosic fibers may be used such as mercerized pulps, chemically stiffened or crosslinked fibers, sulfonated fibers, and the like.

Suitable papermaking fibers may also include recycled fibers, virgin fibers, or mixes thereof.

As used herein, the term"cellulosic"or"cellulose"is meant to include any material having cellulose as a major constituent, and specifically, comprising at least 50 percent by weight cellulose or a cellulose derivative.

Thus, the term includes cotton, typical wood pulps, cellulose acetate, rayon, thermomechanical wood pulp, chemical wood pulp, debonded chemical wood pulp, milkweed floss, and the like.

As used herein,"high yield pulp fibers"are those papermaking fibers produced by pulping processes providing a yield of about 75 percent or greater.

Yield is the resulting amount of processed fiber expressed as a percentage of the initial wood mass. High yield fibers are well known for their stiffness (in both dry and wet states) relative to typical chemically pulped fibers. The cell wall of kraft and other low-yield fibers tends to be more flexible because lignin, the "mortar"or"glue"on and in part of the cell wall, has been largely removed.

Bleached kraft fibers and other bleached fibers tend to be low-yield, with yields sometimes on the order of 50% or less. Such low-yield fibers have more exposed cellulose area to form bonds with the polymeric reactive compound.

The terms"paper","textile","web","tissue", and"towel"are often used herein synonomously.

The present invention is directed to methods for making high wet performance webs. The webs produced by the methods have a high wet strength as compare to webs made according to other methods. The web desirably has a dry tensile strength similar to that of webs made without the addition of the PARC, and a wet tensile strength greater than that of such webs. Accordingly, the wet: dry tensile strength ratio is greater than such webs. Desirably, the wet tensile strength index (wet tensile strength normalized for basis weight) is at least twice that of the control webs, and is at least about 0.7 Nm/g, more desirably at least 0.8 Nm/g, more desirably still at least 1 Nm/g, more desirably still at least 1.5 Nm/g, and most desirably from about 0.8 Nm/g to about 1.8 Nm/g, although webs having a higher tensile index could likely be achieved and may be useful for some applications. The wet: dry ratio is desirably at least twice that of the control, and is at least about 20%, desirably at least about 30%, and most desirably at least about 40% or higher.

A high wet performance web of the invention is made by first applying an aqueous solution of a polymeric anionic reactive compound (PARC) to a cellulosic fibrous web. A catalyst can be included in the solution to initiate crosslinking of the PARC to the cellulose. Other ingredients that are commonly included in the preparation of wet performance webs can also be included. The treated and dried web is then cured so that the PARC forms crosslinks with the cellulosic hydroxyl groups.

I. Compositions A. Polymeric Anionic Reactive Compounds Useful polymeric anionic reactive compounds are compounds having repeating units containing two or more anionic functional groups that will covalently bond to hydroxyl groups of the cellulosic fibers. Such compounds will cause inter-fiber crosslinking between individual cellulose fibers. In one embodiment, the functional groups are carboxylic acids, anhydride groups, or the salts thereof.

In a most desired embodiment the repeating units include two carboxylic acid groups on adjacent atoms, particularly adjacent carbon atoms, wherein the

carboxylic acid groups are capable of forming cyclic anhydrides and specifically 5- member ring anhydrides. This cyclic anhydride, in the presence of a cellulosic hydroxyl group at elevated temperature, forms ester bonds with the hydroxyl groups of the cellulose.

Polymers, including copolymers, terpolymers, block copolymers, and homopolymers, of maleic acid are especially desired, including copolymers of acrylic acid and maleic acid. Polyacrylic acid can be useful for the present invention if a significant portion of the polymer comprises monomers that are joined head to head, rather than head to tail, to ensure that carboxylic acid groups are present on adjacent carbons.

Exemplary polymeric anionic reactive compounds include the ethylene/maleic anhydride copolymers described in U. S. Patent No. 4,210,489 to Markofsky. Vinyl/maleic anhydride copolymers and copolymers of epichlorohydrin and maleic anhydride or phthalic anhydride are other examples. Copolymers of maleic anhydride with olefins can also be considered, including poly (styrene/maleic anhydride), as disclosed in German Patent No.

2,936,239. Copolymers and terpolymers of maleic anhydride that could be used are disclosed in U. S. Patent No. 4,242,408 to Evani et al. Desirably the copolymer or terpolymer includes a polymer having adjacent anionic groups and a polymer having a low glass transition temperature, such as vinyl acetate. The inclusion of the polymer having a low glass transition temperature gives the crosslinking copolymer or terpolymer more flexibility and thus the final product is more flexible.

Desired polymeric reactive compounds are terpolymers of maleic acid, vinyl acetate, and ethyl acetate known as BELCLENEO DP80 (Durable Press 80) and BELCLENEO DP60 (Durable Press 60), from FMC Corporation.

The polymeric anionic reactive compound desirably has a relatively low molecular weight and thus a low viscosity to permit effective spraying onto a tissue web. The polymeric anionic reactive compound desirably is a copolymer or terpolymer to improve flexibility of the molecule relative to the homopolymer alone. Improved flexibility of the molecule can be manifest by a

reduced glass transition temperature as measured by differential scanning calorimetry. Useful polymeric anionic reactive compounds according to the present invention can have a molecular weight less than about 5,000, with an exemplary range of from about 500 to 5,000, more specifically less than about 3,000, more specifically still from about 600 to about 2,500, and most specifically from about 800 to 2000. The polymeric anionic reactive compound BELCLENEO DP80 used in the Examples below is believed to have a molecular weight of from about 800 to about 1000. As used herein, molecular weight refers to number averaged molecular weight determined by gel permeation chromatography (GPC) or an equivalent method.

In aqueous solution, a low molecular weight compound such as BELCLENEOO DP80 will generally have a low viscosity, greatly simplifying the processing and application of the compound. In particular, low viscosity is especially desirable for spray application, whether the spray is to be applied uniformly or nonuniformly (e. g., through a template or mask) to the product. A saturated (50% by weight) solution of BELCLENE DP80, for example, has a room-temperature viscosity of about 9 centipoise, while the viscosity of a solution diluted to 2%, with 1% SHP catalyst, is approximately 1 centipoise (only marginally greater than that of pure water). In general, it is preferred that the polymeric anionic reactive compound to be applied to the paper web have a viscosity at 25 °C of about 50 centipoise or less, specifically about 10 centipoise or less, more specifically about 5 centipoise or less, and most specifically from about 1 centipoise to about 2 centipoise. The solution at the application temperature desirably should exhibit a viscosity less than 10 centipoise and more specifically less than 4 centipoise. When the pure polymeric anionic reactive compound is at a concentration of either 50% by weight in water or as high as can be dissolved in water, whichever is greater, the liquid viscosity desirably is less than 100 centipoise, more specifically about 50 centipoise or less; more specifically still about 15 centipoise or less, and most specifically from about 4 to about 10 centipoise.

As used herein, viscosity is measured with a Sofrasser SA Viscometer (Villemandeur, France) connected to a type MIVI-6001 measurement panel. The viscometer employs a vibrating rod which responds to the viscosity of the surrounding fluid. To make the measurement, a 30 ml glass tube (Corex II No.

8445) supplied with the viscometer is filled with 10.7 ml of fluid and the tube is placed over the vibrating rod to immerse the rod in fluid. A steel guide around the rod receives the glass tube and allows the tube to be completely inserted into the device to allow the liquid depth over the vibrating rod to be reproducible.

The tube is held in place for 30 seconds to allow the centipoise reading on the measurement panel to reach a stable value.

Another useful aspect of the polymeric anionic reactive compounds of the present invention is that relatively high pH values can be used when the catalyst is present, making the compound more suitable for neutral and alkaline papermaking processes and more suitable for a variety of processes, machines, and fiber types. In particular, polymeric anionic reactive compound solutions with added catalyst can have a pH above 3, more specifically above 3.5, more specifically still above 3.9, and most specifically of about 4 or greater, with an exemplary range of from 3.5 to 7 or from 4.0 to 6.5.

The polymeric anionic reactive compounds of the present invention can yield wet: dry tensile ratios much higher than traditional wet strength agents, with values reaching ranges as high as from 40% to 85%, for example.

The PARC need not be neutralized prior to treatment of the fibers. In particular, the PARC need not be neutralized with a fixed base. As used herein, a fixed base is a monovalent base that is substantially nonvolatile under the conditions of treatment, such as sodium hydroxide, potassium hydroxide, or sodium carbonate, and t-butylammonium hydroxide. However, it can be desirable to use co-catalysts, including volatile basic compounds such as imidazole or triethyl amine, with sodium hypophosphite or other catalysts.

B. Catalysts Suitable catalysts include any catalyst that increases the rate of bond formation between the PARC and cellulose fibers. Desired catalysts include

alkali metal salts of phosphorous containing acids such as alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphonates, alkali metal phosphates, and alkali metal sulfonates. Particularly desired catalysts include alkali metal polyphosphonates such as sodium hexametaphosphate, and alkali metal hypophosphites such as sodium hypophosphite. Several organic compounds are known to function effectively as catalysts as well, including imidazole (IMDZ) and triethyl amine (TEA). Inorganic compounds such as aluminum chloride and organic compounds such as hydroxyethane diphosphoric acid can also promote crosslinking.

Other specific examples of effective catalysts are disodium acid pyrophosphate, tetrasodium pyrophosphate, pentasodium tripolyphosphate, sodium trimetaphosphate, sodium tetrametaphosphate, lithium dihydrogen phosphate, sodium dihydrogen phosphate and potassium dihydrogen phosphate.

When a catalyst is used to promote bond formation, the catalyst is typically present in an amount in the range from about 5 to about 100 weight percent of the PARC. Desirably, the catalyst is present in an amount of about 25 to 75 percent by weight of the polycarboxylic acid, most desirably about 50% by weight of the PARC.

In one embodiment, a non-phosphorous co-catalyst is used along with the phosphorous containing catalyst, such as SHP. Preferred non-phosphorous catalysts include TEA and IMDZ. The use of a non-phosphorous co-catalyst allows the pH of the PARC solution to be above the pH of 2-3 when SHP is used alone. For example, a solution of 2% DP80 and 1% SHP has a pH of about 2.0. A solution of 2% DP80,1% SHP, and 0.5% TEA has a pH about 3.0. A solution of 2% DP80, 1% SHP, and 0.5% IMDZ has a pH about 4.0. The use of a higher pH is beneficial in some cases because it speeds up the curing process. The co-catalyst is preferably added at an amount of about 0.1 to 2%.

C Debonders The addition of a debonder to the cellulosic web is especially desired.

Debonders can be especially useful in providing high wet: dry tensile strength

ratios by lowering the dry strength without a correspondingly large decrease in the wet strength. Very high wet: dry tensile ratios can thus be achieved by combining treatment with chemical debonding agents with the treatment with a PARC. In one embodiment, debonder can be added to the web in the furnish or otherwise prior to application of the polymeric anionic reactive compound and subsequent crosslinking. However, debonder may also be added to the web after application of PARC solution and even after crosslinking of the PARC. In another embodiment, the debonder is present in the PARC solution and thus is applied to the web as the same time as the PARC, provided that adverse reactions between the PARC and the debonder are avoided by suitable selection of temperatures, pH values, contact time, and the like.

Debonders such as dialkyl dimethyl quaternary ammonium compounds, imidazoline diquaternary ammonium compounds, and diamidoamine based quaternaries are preferred. However, any debonding agent (or softener) known in the art may be utilized. Examples of useful agents are tertiary amines and derivatives thereof, amine oxides, quaternary amines, silicone-based compounds, saturated and unsaturated fatty acids and fatty acid salts, alkenyl succinic anhydrides, alkenyl succinic acids and corresponding alkenyl succinate salts, sorbitan mono-, di-and tri-esters, including but not limited to stearate, palmitate, oleate, myristate, and behenate sorbitan esters, and particulate debonders such as clay and silicate fillers. Useful debonding agents are described in, for example, U. S. Patent Nos. 3,395,708,3,554,862, and 3,554,863 to Hervey et al., U. S. Patent No. 3,775,220 to Freimark et al., U. S. Patent No. 3,844,880 to Meisel et al., U. S. Patent No. 3,916,058 to Vossos et al., U. S. Patent No. 4,028,172 to Mazzarella et al., U. S. Patent No. 4,069,159 to Hayek, U. S. Patent No. 4,144,122 to Emanuelsson et al., U. S. Patent No. 4,158,594 to Becker et al., U. S. Patent No.

4,255,294 to Rudy et al., U. S. Patent No. 4,314,001, U. S. Patent No. 4,377,543 to Strolibeen et al., U. S. Patent No. 4,432,833 to Breese et al., U. S. Patent No.

4,776,965 to Nuesslein et al., and U. S. Patent No. 4,795,530 to Soerens et al.

Preferred debonding agents for use herein are cationic materials such as quaternary ammonium compounds, imidazolinium compounds, and other

such compounds with aliphatic, saturated or unsaturated carbon chains. The carbon chains may be unsubstituted or one or more of the chains may be substituted, e. g. with hydroxyl groups. Non-limiting examples of quaternary ammonium debonding agents useful herein include hexamethonium bromide, tetraethylammonium bromide, lauryl trimethylammonium chloride, and dihydrogenated tallow dimethylammoniurn methyl sulfate. Other preferred debonding agents for use herein to improve fibrous structure flexibility are alkenyl succinic acids, and their corresponding alkenyl succinate salts. Non- limiting examples of alkenyl succinic acid compounds are n-octadecenylsuccinic acid and n-dodecenylsuccinic acid and their corresponding succinate salts.

The debonding agent will desirably be added at a level of at least about 0.1%, desirably at least about 0.2%, more desirably at least about 0.3%, on a dry fiber basis. Typically, the debonding agent will be added at a level of from about 0.1 to about 6%, more typically from about 0.2 to about 3%, active matter on dry fiber basis. The percentages given for the amount of debonding agent are given as an amount added to the fibers, not as an amount actually retained by the fibers.

D. Other Ingredients A wide variety of other compounds known in the art of papermaking and tissue production can be included in the webs of the present invention.

Softening compounds, emollients, silicones, lotions, waxes, and oils can also have similar benefits as debonders in reducing dry strength, while providing improved tactile properties such as a soft, lubricious feel. Fillers, fluorescent whitening agents, antimicrobials, ion-exchange compounds, odor-absorbers, dyes, and the like can also be added. Hydrophobic matter added to selected regions of the web, especially the uppermost portions of a textured web, can be valuable in providing improved dry feel in articles intended for absorbency and removal of liquids next to the skin, as disclosed in the commonly owned copending U. S. application Serial No. 08/997,287, filed Dec. 22,1997.

The above additives can be added before, during, or after the application of the PARC and/or drying step.

Other chemical treatments of the web can be considered, desirably after curing the PARC, including the inclusion of superabsorbent particles, incorporation of odor-control substances such as cyclodextrins, baking soda, or chelating agents, the topical application of waxes and emollients, and the application of hydrophobic material over portions of the web, including the patterned, topical application of hydrophobic matter to a textured web, as described in commonly owned copending US application,"Dual-zoned Absorbent Webs", Serial No. 08/997,287, filed December 22,1997.

II. Methods of Making the High Wet Performance Webs The methods include applying a solution of the PARC onto a web with subsequent drying and curing. The PARC solution can be applied through any of a number of methods including coating, printing, and spraying.

A. Preparation of the Web The fibrous web is generally a random plurality of papermaking fibers that can, optionally, be joined together with a binder. Any papermaking fibers, as previously defined, or mixtures thereof may be used. Bleached fibers from a kraft or sulfite chemical pulping process are especially desired. Recycled fibers can also be used, as can cotton linters or papermaking fibers comprising cotton.

Both high-yield and low-yield fibers can be used, though low-yield fibers are generally desired for best results. Because of commercial availability, softwood and hardwood fibers are especially desired. To achieve good softness and opacity, it is desirable that the tissue web comprise substantial amounts of hardwood.

For good strength, substantial amounts of softwood are desired. In one embodiment, the fibers may be predominantly hardwood, such as at least 50% hardwood or about 60% hardwood or greater or about 80% hardwood or greater or substantially 100% hardwood. Higher hardwood contents are desired for high opacity and softness, whereas higher softwood content is desirable for strength.

In another embodiment, the fibers may be predominantly softwood, such as at least 50% softwood or about 60% softwood or greater or about 80% softwood or greater or substantially 100% softwood.

For many tissue applications, high brightness is desired. Thus the papermaking fibers or the resulting paper of the present invention can have an ISO brightness of about 60 percent or greater, more specifically about 80 percent or greater, more specifically about 85 percent or greater, more specifically from about 75 percent to about 90 percent, more specifically from about 80 percent to about 90 percent, and more specifically still from about 83 percent to about 88 percent.

The fibrous web of the present invention may be formed from a single layer or multiple layers. Both strength and softness are often achieved through layered tissues, such as those produced from stratified headboxes wherein at least one layer delivered by the headbox comprises softwood fibers while another layer comprises hardwood or other fiber types. Layered tissue structures produced by any means known in the art are within the scope of the present invention, including those disclosed by Edwards et al. in U. S. Patent No.

5,494,554. In the case of multiple layers, the layers are generally positioned in a juxtaposed or surface-to-surface relationship and all or a portion of the layers may be bound to adjacent layers. The paper web may also be formed from a plurality of separate paper webs wherein the separate paper webs may be formed from single or multiple layers. In those instances where the paper web includes multiple layers, the entire thickness of the paper web may be subjected to application of the PARC or each individual layer may be separately subjected to application of the PARC and then combined with other layers in a juxtaposed relationship to form the finished paper web.

In one embodiment, the PARC is predominantly applied to one layer in a multilayer web. Alternatively, at least one layer is treated with significantly less polymeric anionic reactive compound than other layers. For example, an inner layer can serve as the wet strength layer.

Suitable paper webs include tissue webs that have been creped or are intended for creping, and wet-pressed or through-dried webs in general, such as those of U. S. Patent No. 5,637,194 to Ampulski et al., U. S. Patent No. 4,529,480 to Trokhan, and U. S. Patent No. 4, 440,597 to Wells et al. Other suitable webs

include those that are uncreped, such as those of U. S. Patent No. 5,772,845 to Farrington, Jr. et al.

The web can be formed with normal papermaking techniques, wherein a dilute aqueous fiber slurry is disposed on a moving wire to filter out the fibers and form an embryonic web which is subsequently dewatered by combinations of units including suction boxes, wet presses, through drying units, Yankee dryers, and the like. Examples of known dewatering and other operations are given in U. S. Patent No. 5,656,132 to Farrington et al.

Dry airlaid webs can also be treated with polymeric anionic reactive compound solution to provide increased stability and wet strength, according to the present invention. Airlaid webs can be formed by any method known in the art, and generally comprise entraining fiberized or comminuted cellulosic fibers in an air stream and depositing the fibers to form a mat. The mat may then be calendered or compressed, before or after treatment with the polymeric anionic reactive compound, using known techniques, including those of U. S. Patent No.

5,948,507 to Chen et al. Following curing of the polymeric anionic reactive compound, the airlaid web may be used as a wipe, incorporated into an absorbent article such as a diaper, or used in other products known in the art.

Any of the techniques known to those skilled in the papermaking art for drying wet fibrous webs can be used. Typically, the web is dried by applying a heated gas around, over, or through the web, by contacting the web with a heated surface, by applying infrared radiation, by exposure to superheated steam, by microwave or radiofrequency radiation, or by a combination of such methods. Through drying and contact with a heated drum are desired methods of drying. Desirably the web is dried to about 60-100%, more desirably 70-96%, and most desirably 80-95% before application of the PARC solution.

The web desirably is substantially free of latex and substantially free of film-forming compounds. Desirably, the applied solution or slurry comprising the polymeric reactive compound is free of latex and its derivatives. The applied solution or slurry also is desirably free of formaldehyde or of cross-linking

agents that evolve formaldehyde. Most desirably, the PARC does not comprise formaldehyde nor require formaldehyde for crosslinking.

B. Application of the PARC The PARC desirably is applied in an aqueous solution to an existing papermaking web. The solution can be applied either as an online step in a continuous papermaking process along a section of a papermaking machine or as an offline or converting step following formation, drying, and reeling of a paper web.

The solution can be applied by spray, by coating technologies (e. g. blade coating such as with a commercial short-dwell coater, metered size presses, flooded nip coating, curtain coating, wire-wound rod coating, slot coating, and the like), printing technologies (gravure printing, ink jet printing, flexographic printing, offset printing, and the like), and including foam finishing and roller fluid feeding. These methods will generally homogenously distribute the PARC to the web. However, it may be advantageous to distribute the PARC heterogenously and these methods can be adapted to do so.

The PARC solution is desirably added at about 50 to 200% add-on, most desirably 100% add-on, where add-on is the percent by weight of PARC solution to the dry weight of the web. In other words, 100% add-on is a 1: 1 weight ratio of PARC solution to dry web. The final percent by weight PARC to the web is desirably from about 0.3 to 8%, more desirably from about 0.7% to 2%. The concentration of the PARC solution can be adjusted to ensure that the desired amount of PARC is added to the web.

The catalyst is present in the PARC solution at an amount in the range from about 5 to about 100 weight percent of the PARC. Desirably, the catalyst is present in an amount of about 25 to 75 percent by weight of the polycarboxylic acid, most desirably about 50% by weight of the PARC.

The PARC may be added to any layer independent from other layers in a tissue or paper web, but in one embodiment it is added to the predominantly softwood component of a tissue web to enhance the physical properties of the strength layer. However, excellent results in physical property improvement

have also been observed in predominantly hardwood fiber structures (bleached kraft hardwood, for example), particularly a dramatic increase in tensile energy absorbed in the dry state during tensile tests, suggesting that layered tissue production with PARC in predominantly hardwood layers of a tissue could offer improvements in physical properties.

C Drying and Curing the Web Generally, the applied polymeric reactive compound is in a solution that must be dried while on the web and then cured. Drying and curing can be achieved in two separate steps or can be done in one process wherein evaporative water removal is followed by elevating the sheet to a temperature sufficient for curing.

The web, after treatment with the PARC and catalyst solution, can be dried and cured with a variety of methods. Desirably, the web is first dried at a temperature less than 150 °C, desirably less than 120 °C, more desirably less than 110 °C until the web has a dryness level of desirably about 90% or higher, more desirably about 94% or higher, and most desirably about 98% or higher.

Additional energy is then applied to the web to heat the web to a suitable curing temperature. The treated web should be cured at a temperature sufficient to cause the PARC to crosslink with the cellulose fibers. This will generally be at a temperature within the range of about 150 °C to 190 °C, for a period of time preferably less than 15 minutes, more specifically ranging from about 1 minute to 10 minutes, desirably from about 2 to 7 minutes. The time required to properly cure the material will depend upon several factors, including the temperature, the nature of the PARC, the nature and amount of catalyst, and the add-on amount of the PARC.

Suitable drying methods include any known in the art, including contact with a Yankee dryer, contact with other heated drums such as steam-filled cylinders, through air drying, impingement drying, superheated steam drying, infrared drying, and the like. Useful drying methods include through air drying in which a hot gas (preferably air) passes through the web, infrared drying, and

drying by conduction from a heated surface such as a Yankee dryer or an internally heated roll having combustion gases, electric elements, or induction heaters to heat the surface of the roll. Through air drying can be accomplished with a non-oxidative gas, but air is preferred for economic reasons. The drying apparatus can also combine both convective heating from hot air and radiative heat transfer, as disclosed in U. S. Patent No. 4,336,279 to Metzger.

Suitable heating methods for the curing step include contact with heated surfaces such as gas-fired cylinders or other heated drums, infrared heating, radiofrequency heating, microwave heating if suitable dipolar compounds are present in the web to respond to microwave radiation to produce heat, and impingement heating or through-air drying with sufficiently hot air or with other heated gases such as carbon dioxide or nitrogen, which offer the advantage of reduced oxidative damage to the web. The gas should be heated to a temperature sufficient for it to raise the surface of the web to the desired curing temperature.

During many methods of curing, the web should be supported on a porous surface capable of withstanding high temperatures. Open metal wires or other metal supports are especially desired.

Curing of the polymeric reactive compound can also be achieved by radio frequency drying if the polymer comprises abundant dipoles or if other materials are included that respond to radio-frequency radiation. For example, a variety of polymers such as copolyester binder fibers known in the nonwovens industry can be radiofrequency bonded. One example is the amorphous copolyester material CoPET-A which is used in Eastman's KODEL0410 binder fiber, according to W. Haile et al. in the article,"Copolyester Polymer for Binder Fibers,"Nonwovens World, April-May 1999, pp. 120-124. This fiber requires a minimum temperature of about 132 °C for good bonding.

The webs produced by the methods have a high wet strength as compared to webs made according to other methods. The web desirably has a dry tensile strength similar to that of webs made without the addition of the PARC, and a wet tensile strength greater than that of such webs. Accordingly, the wet: dry

tensile strength ratio is greater than such webs. Desirably, the wet tensile strength is at least twice that of the untreated web and, for a web having a basis weight of between about 40 to 60, is at least about 400 g/3 inches, more desirably at least about 600 g/3 inches, and most desirably at least about 800 g/3 inches.

Desirably, the wet tensile index (the wet tensile strength normalized for basis weight) is at least twice that of the control webs, and is at least about 0.7 Nm/g, desirably at least about 0.8 Nm/g, more desirably still at least about 1 Nm/g, more desirably still at least about 1.5 Nm/g, and most desirably from about 0.8 Nm/g to about 1.8 Nm/g. The wet: dry ratio is desirably at least twice that of the control, and is at least about 20%, desirably at least about 30%, and most desirably at least about 40% or higher. Unless otherwise specified, the dry and wet tensile properties of machine-made webs are taken in the machine direction of the web.

Desirably, the webs of the present invention have a wet tensile index that is similar taken in either the machine direction or cross direction.

III. Methods of Using the High Wet Performance Paper Webs The treated web may be provided with a number of mechanical, chemical, and physical treatments before or after treatment with the PARC. For example, the web may be creped, apertured, slit, embossed, calendered, converted to a multi-ply web, further treated with softening agents or lotions, printed with graphics, and the like.

Creped or throughdried tissue webs made according to the present invention can be particularly useful as disposable consumer products and industrial or commercial products. Examples include premoistened tissues, paper towels, bath tissue, facial tissue, wet wipes, absorbent pads, intake webs in absorbent articles such as diapers, bed pads, meat and poultry pads, feminine care pads, and the like. Uncreped through-air dried webs having high wet strength and desirably having a basis weight from about 10 grams per square meter (gsm) to about 80 gsm, alternatively from about 20 to about 40 gsm, may be particularly useful as wet resilient, high bulk materials for absorbent articles

and other uses, as illustrated by way of example in commonly owned copending U. S. application, Serial Number 08/614,420,"Wet Resilient Webs and Disposable Articles Made Therewith,"by F. J. Chen et al.

The invention is further illustrated by the following examples, which are not to be construed in any way as imposing limitations upon the scope thereof.

On the contrary, it is to be clearly understood that resort may be had to various other embodiments, modifications, and equivalents thereof, which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the present invention.

Examples Unless otherwise specified, tensile strengths are measured according to Tappi Test Method T 494 om-88 for tissue, modified in that an MTS SINTECHO 1/G tensile tester (or equivalent) is used having a 3-inch jaw width, a jaw span of 4 inches, and a crosshead speed of 10 inches per minute. Wet strength is measured in the same manner as dry strength except that the tissue sample is folded without creasing about the midline of the sample, held at the ends, and dipped in deionized water for about 0.5 seconds in water to a depth of about 0.5 cm to wet the central portion of the sample, whereupon the wetted region is touched for about 1 second against an absorbent towel to remove excess drops of fluid, and the sample is unfolded and set into the tensile tester jaws and immediately tested. The sample is conditioned under TAPPI conditions (50% RH, 22.7°C) before testing. Generally 3 samples are combined for wet tensile testing to ensure that the load cell reading is in an accurate range.

Tensile index is a measure of tensile strength normalized for basis weight of the web. Tensile strength can be converted to tensile index by converting tensile strength determined in units of grams of force per 3 inches to units of Newtons per meter and dividing the result by the basis weight in grams per square meter of the tissue, to give the tensile index in Newton-meters per gram (Nm/g).

Example 1

Standard TAPPI handsheets (60 gsm) were prepared from a bleached virgin northern softwood kraft pulp (Kimberly-Clark LL-19 pulp). The PARC solution was hand sprayed onto the sheet with a spray bottle, adding an aqueous solution having a weight equal to the dry weight of the sheet. In other words, the pick up was 100%. The aqueous solution included BELCLENEO DP80 (Durable Press 80), a terpolymer of maleic acid, vinyl acetate, and ethyl acetate or BELCLENE DP60 (Durable Press 60), also a terpolymer of maleic acid, vinyl acetate, and ethyl acetate, or polymaleic acid (PMA). The solution also included sodium hypophosphite (SHP) as a catalyst, with one part of SHP for each two parts of polymeric reactive compound. The solutions included the polymer at an appropriate concentration to result in the amount of polymer added as indicated in Table 1. For example, with 100% pickup, a solution of 2% by weight polymer yields a sheet containing 2% by weight of polymer when the sheet is dry.

Sheets were uniformly sprayed with the solution. After spraying, the sheet was dried at 80 °C for 6 hours to substantially dry the sheet but not cure the polymeric reactive compound. The dried, treated sheet can be stored before the polymeric reactive compound is cured.

The sheets were cured in a Pro-Tronix@ forced air convection oven (an oven with air flow in the chamber for good mass transfer) at a temperature of 180 °C for 3 minutes.

Tensile measurements of the handsheets were measured using a 5-inch gauge length (jaw span) in 1-inch wide strips cut from the center of the handsheets to have a length of about 6 inches. Physical properties of the treated and cured sheets are listed in Table 1. The control was a sheet without polymer treatement, cured at 105 °C for 20 minutes. The sheets treated with polymeric reactive compound were cured in the oven at 180°C for 3 minutes. The effect of drying the test sheets at 180 °C, rather than 105 °C as for the control was negligible because both conditions used are sufficient to dry the sheet. TEA is total energy absorbed, reported as g-cm per square cm.

Table 1. The Effect of Polymeric Reactive Compounds on Physical Properties of Handsheets Polymer to Dry Wet Wet : Dry Wet TEA PARC Fiber Weight Strength Strength (%) (g-cm/cm2) Ratio (%) (g/in) (g/in) None 0 4772 283 5. 9 1. 25 DP60 g 5573 4628 83 15. 88 DP60 5808 4165 71. 7 14. 98 DP60 2 5401 3846 71. 2 16. 29 DP60 0. 5 5190 2771 53. 4 8. 95 DP80 4 5480 4612 84 17. 18 PMA 4 4944 3355 68 14. 69

Example 2 An uncreped, through air dried tissue web with a basis weight of 37.8 grams per square meter was prepared on a pilot paper machine. A layered headbox was used to produce a three-layered web. Each of the two outer layers was 25% of the basis weight of the sheet and were made from a furnish of NB-88 bleached northern softwood kraft fibers, produced by Repap New Brunswick, a division of Miramichi, Inc. The center layer comprises the remaining 50% of the basis weight of the sheet and was prepared from a 50: 50 blend of NB88 northern softwood fibers and bleached chemithermomechanical (BCTMP) spruce pulp.

The web was formed in a twin wire former. The pilot paper machine was configured substantially according to the description given by Farrington et al. in U. S. Patent No. 5,656,132 and operated substantially as described therein. The fibrous slurry used to make the wet web also included a debonding agent, Witco C 6027 (an imidazoline quaternary ammonium salt produced by Witco Corp.

(Melrose Park, Ill.), which was introduced into the recirculation loop for the water of the paper machine at a dose sufficient to apply 3 kg of debonder per metric ton of dry tissue.

After being formed on a forming fabric, the wet web with a consistency of about 12% was transferred to a carrier fabric, design T216-3 of Lindsay Wire, a division of Appleton Mills, Appleton, Wisconsin. A speed differential of 1.15% existed between the forming fabric and the transfer fabric, with the transfer fabric moving more slowly than the carrier fabric. The transfer was made over a vacuum shoe. The web was then transferred from the carrier fabric across a vacuum shoe to a Lindsay Wire T-1205-1 through drying fabric. The web was then through dried completely and reeled. A soft, high-bulk tissue was produced in this manner. The web had a geometric mean tensile strength of 1208 grams per 3 inches of tissue, a caliper of 32.7 mils, and a wet: dry ratio of 0.6%.

After having been dried and reeled, sections of the tissue were then sprayed by hand to apply a uniform treatment of polymeric reactive compound.

The applied polymeric reactive compound was 1% by weight of the dry weight of the web, and the catalyst (SHP) was 0.5% by of the dry weight of the web. After air drying, the treated sections of web were then cured in an oven at one of several temperatures, with longer cure times selected for cooler temperatures, according to Table 2. Dry strength, wet strength, and wet: dry ratio values were obtained from tensile testing (4-inch gauge length, 3-inch wide jaws, at 10 inches/min crosshead speed) and are reported in Table 2. MD is machine direction, CD is cross direction.

Table 2. Effect of Curing Time and Temperature on Wet Strength Development in PARC-Treated Webs. Curing MD Dry MD Wet MD CD Dry CD Wet CD Time & Strength Strength Wet: Dry Strength Strength Wet: Dry Temp. (%) (%) 4'tes 1540 1024.9 66.5 1520.6 765.4 50. 3 4 minutes 190 °C, 1461 971.4 66.4 1399 754.3 54 2 minutes 200 °C, 1457.2 871.4 59.8 1458.8 614.8 42.1 1 minute 210 °C, 1290.6 608.6 47.1 1198.3 504 42 32 seconds

While wet: dry ratio decreased as curing time decreased, even when the temperature was substantially increased to compensate for the time, the results nevertheless show that the PARC approach can successfully increase wet strength and wet: dry ratios to very high values (well over 30% and even over 40%, with some values above 60%).

Example 3 A 50/50 Mob/RF non-layered CHF-UCTAD (uncreped, through air dried) tissue web prepared on a pilot paper machine, having a basis weight of 42.5 grams per square meter was used. 0.2% Witco C6001 debonder (an imidazoline quaternary ammonium salt produced by Witco Corp. (Melrose Park, Ill.)) was applied to the tissue in the furnish. The same general process as desribed in Example 2 was used.

After having been dried and reeled, section of the tissue were then sprayed by hand to apply a uniform treatment of polymeric reactive compound.

The applied polymeric reactive compound (DP80) was 2% by weight of the dry weight of the web, and the catalyst (SHP) was 0.5% by of the dry weight of the web. After air drying, the treated sections of web were then cured in an oven at 180 °C for 3 minutes. Dry strength, wet strength, and wet: dry ratio were obtained as described above.

Table 3. Effect of Debonder on Dry Strength and Wet Strength in PARC- Treated Webs. Debonder (%) MD Dry Strength MD Wet Strength MD Wet:Dry (%) none 5956 1669 28 0.2 3558 1593 45 Wet end treatment with debonder lowered the dry strength substantially (by 40%) and marginally lowered the wet strength (4.5%). The wet: dry ratio was higher by 60.7% for the debonder treated tissue.

Example 4 A 50/50 Mob/RF CHF-UCTAD non-layered CHF-UCTAD (uncreped, through air dried) tissue web prepared on a pilot paper machine, having a basis weight of 42.5 grams per square meter was used. The same general process as desribed in Example 2 was used.

After having been dried and reeled, section of the tissue were then sprayed by hand to apply a uniform treatment of polymeric reactive compound.

The applied polymeric reactive compound (DP80) was 2% by weight of the dry weight of the web, and the catalyst (SHP) was 1% or 0.5% by of the dry weight of the web. 0.2% Berocell 584 debonder (an ethoxylated quaternary ammonium salt made by Eka Nobel, Inc. (Marietta, Ga.)) was included in the PARC solution.

After air drying, the treated sections of web were then cured in an oven at 180 °C for 2 minutes. Dry strength, wet strength, and wet : dry ratio were obtained as described above.

Table 4. Effect of Debonder on Dry Strength and Wet Strength in PARC- Treated Webs. SHP (%) Debonder (%) MD Dry MD Wet MD Wet : Dry (%) Strength Strength none 4921 2395 48. 7 1 0. 2 3044 1971 64. 8 0. 5 none 5239 2053 39. 2 0. 5 0. 2 3145 1543 49. 1 Inclusion of debonder decreased both dry strength and wet strength but wet: dry ratio was still increased over webs not treated with debonder.

The above description is intended to be illustrative and not restrictive.

Many embodiments will be apparent to those of skill in the art upon reading the above description. The scope of the invention should, therefore, be determined not with reference to the above description, but should instead be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. The disclosures of all articles and references, including patents, patent applications, and publications, are incorporated herein by reference.