WO1998024974A1

WO1998024974A1 - Method for making wet strength paper

Info

Publication number: WO1998024974A1
Application number: PCT/US1997/021414
Authority: WO
Inventors: Tong Sun
Original assignee: Kimberly-Clark Worldwide, Inc.
Priority date: 1996-12-04
Filing date: 1997-11-25
Publication date: 1998-06-11
Also published as: KR20000069273A; CN1240010A; EP0943036A1; NZ336391A; CA2271384A1; AU5360698A; BR9713842A; ID22290A; JP2001505627A; AU729194B2

Abstract

The invention is a method for improving the efficiency of aqueous cationic wet strength additives by pretreating cellulose surfaces with reactive anionic compounds, thus providing the cellulose surface with additional anionic sites suitable for retaining a high proportion of said cationic wet strength additives on the cellulose. The wet strength additives on the cellulose surface are cured or reacted with the cellulose surface. The resulting fibrous material has unusually high wet strength with unusually low doses of cationic wet strength additive. The preferred reactive anionic compounds comprise compounds having a reactive group suitable for covalent bonding to hydroxyl groups on cellulose, and further having sulfonic or other anionic end groups capable of attracting cationic wet strength compounds in aqueous solution.

Description

METHOD FOR MAKING WET STRENGTH PAPER

Background of the Invention

In the art of papermaking, chemical materials exist for improving the strength of paper when wetted with water or aqueous solutions, including body fluids such as urine, blood, mucus, menses, lymph and other body exudates. These materials are known in the art as "wet strength agents" and are commercially available from a wide variety of sources. Any material that when added to a paper web or sheet results in providing the sheet with a wet geometric tensile strength:dry geometric tensile strength ratio in excess of 0.1 will, for purposes herein, be termed a wet strength agent. Typically these materials are termed either as permanent wet strength agents or as "temporary" wet strength agents. For the purposes of differentiating permanent from temporary wet strength, permanent will be defined as those resins which, when incorporated into paper or tissue products, will provide a product that retains 50% or more of its original wet strength after exposure to water for a period of at least five minutes. Temporary wet strength agents are those which show less than 50% of their original wet strength after exposure to water for five minutes. Both classes of material find application in the present invention.

The substantivity or effectiveness of many cationic wet strength agents is limited by low retention of the wet strength agent on the cellulose fiber. Much of the applied chemical may not be retained on the fiber, but remains in solution or is washed off after application, for there are relatively few anionic sites on the cellulose surface to attract the charged wet strength agent, and in some cases there may be a large number of anionic sites on colloidal particles or other particles in the fiber suspension which may adsorb a large portion of the wet strength agent, limiting its effectiveness in increasing wet strength. Cationic additives are sometimes used to help neutralize excess anionic sites on colloidal particles or "anionic trash" in the suspension, to allow more of a subsequently added cationic wet strength resin to attach to the fiber surface and not to be preferentially absorbed onto non-fiber components.

Further, the presence of anionic additives or agents in the pulp has a deleterious effect on the efficiency of cationic wet strength agents. This adverse effect can be reduced by adding "cationic promoters" to the stock, as is known in the art of papermaking. Cationic promoters typically include polyethyleneimine with a cationic charge of about 0.75 to 3.5 milliequivalents/gram, quaternized polyamines, such as polydiallyldimethylammonium chloride, or cationic starch. Commonly used cationic resins include polyquatemary amines and are available from Cytec Industries under the trade names CYPRO 514, 515, 516. Cationic promoters are added to the stock in advance of the wet strength resins to ensure adequate mixing and adequate contact with the fibers. When used, the cationic resins are generally used in an amount of about 1 to 10 pounds per ton or 0.05 to 0.5%. The cationic promoter can be used at 0 to 0.5 wt %, typically the resins are used in an amount of about 0.02 to 0.3 wt % and preferably 0.1 to 0.2 wt %. The manufacturer of the promoter will typically recommend a pH for its use. The Cypro resins are effective over a pH of about 4 to 9.

However, the use of cationic promoters does not increase the number of anionic sites on the fiber surface itself, and may decrease the number of such sites, such that the intrinsic potential of the cationic wet strength agent to increase wet strength is still limited by inadequate attachment sites on the cellulose surface. The extent of anionic sites on the cellulose can be measured in terms of the carboxyl group content of cellulose, which is typically measured to be about 2 to 5 milliequivalent per 100 grams of cellulose, or higher.

Therefore, an object of the present invention is to increase the number of anionic sites on the surface of papermaking fibers by pretreating the fibers, thus increasing the substantivity of subsequently added cationic wet strength agents that form covalent bonds with the cellulose. A further object of the present invention is substantially increasing the wet strength of paper that can be achieved with a given dose of wet strength agent. In particular, an object of the present invention is to increase the wet strength that can be achieved with a given quantity of wet strength resin by a factor greater than 20%, preferably greater than 40%, more preferably greater than 50%, and most preferably greater than 70%. Another object of the invention is to provide a method for enhancing wet strength in paper capable of achieving wet tensile strength values in substantially unrefined paper of over 1500 g/in, preferably over 2000 g/in, and most preferably over 2300 g/in, based on a 60 gsm Tappi handsheet. A further object of the invention is to provide wetdry strength ratios greater than about 0.2, preferably greater than about 0.3, more preferably above about 0.4, still more preferably between 0.2 and 0.5, and most preferably greater than 0.5.

Summary of the Invention The present invention resides in a method in which cellulosic fibers are pretreated with a fiber reactive anionic compound to increase the substantivity of cationic wet strength agents. In this method, an aqueous slurry of papermaking fibers is treated with a substantially colorless fiber reactive anionic compound, wherein the fiber reactive anionic compound comprises a fiber reactive moiety suitable for forming a covalent bond (for example, an ether-type linkage) with hydroxyl or other groups on the cellulose surface. The pH of the slurry must be adjusted to ensure that it is sufficiently high to drive the reaction of the reactive anionic compound with the cellulose. The slurry can be at a high fiber consistency, preferably from about 5 to about 50%. High consistency is desirable because it reduces the amount of water affected by the chemical treatment process and allows more efficient use of the fiber reactive anionic compound. Subsequently, the slurry is diluted, if necessary, and mixed with a cationic wet strength agent. The treated fibers are formed into a paper web using processes well known in the art and dried under conditions suitable for curing the wet strength agent.

The present invention also resides in a chemical treatment process that increases the number of anionic attachment sites for cationic polymers on cellulose by means of reaction between a triazine or other reactive group on a colorless reactive anionic compound further comprising at least one sulfonic or carboxylic group suitable for establishing an ionic bond with a cationic wet strength agent, followed by addition of a cationic polymer and alkalization to drive reaction of the reactive anionic compound with cellulose.

The invention also resides in a method of preparing paper with relatively high wet strength and low dry strength by first increasing anionic sites on the cellulose fibers with said fiber reactive anionic compound, followed by addition of a chemical debonder agent and a cationic wet strength agent. The debonder agent may be applied to the fibers while the fibers are in solution, followed by addition of the cationic wet strength agent, whereafter the paper is formed, dewatered, and dried. Alternatively, the debonder agent may be applied to dried or partially dried paper web that has been prepared with a fiber reactive anionic compound and a cationic wet strength agent. In either case, the debonder agent interferes with hydrogen bond formation, reducing the dry strength of the paper, while having relatively little effect on covalent bond formation. The result is a paper with an increased wet:dry tensile strength ratio. Such paper can have reduced stiffness and improved softness due to the reduced extent of hydrogen bonding, while still having high wet strength.

Detailed Description of the Invention The first step in the method of this invention is providing an aqueous slurry of papermaking fibers. It is anticipated that wood pulp in all its varieties will normally comprise the papermaking fibers used in this invention. However, other cellulosic fibrous pulps, such as cotton liners, bagasse, rayon, kenaf, milkweed fibers, and the like can be used. Wood pulps useful herein include both sulphite and sulfate pulps as well as mechanical and thermomechanical pulps all well known to those skilled in the art, including chemithermomechanical pulp and bleached chemithermomechanical pulp. High brightness pulps, including chemically bleached pulps, are especially preferred for tissue making, but unbleached or semi-bleached pulps may also be used. Pulps derived from both deciduous and coniferous trees can be used. Recycled fibers are included within the scope of the present invention.

The second step of the present invention is chemical pretreatment of the fibers by adding an effective amount of a fiber reactive anionic compound to the fiber slurry. The preferred amount of fiber reactive anionic compound added to the fiber slurry is from about 0.01 to about 4 wt % based on the dry fiber weight, preferably from about 0.05 to about 2 wt %, more preferably from about 0.08 to about 1.5%, and most preferably from about 0.1 to about 1 wt%. (All weight percentages referred to herein are on a dry basis unless otherwise stated.) The pretreatment of papermaking fibers to increase substantivity toward cationic compounds is achieved with colorless fiber reactive "dyes" modified to be without chromophore groups and further modified, if necessary, to ensure the presence of at least one anionic moiety such as a sulfonic or carboxylic group. The anionic moiety serves as the site for ionic bonds with cationic groups of a cationic wet strength agent, helping to form a bridge between the fiber and the wet strength agent to hold the wet strength agent on the fiber, thus increasing the effectiveness of a given dose of wet strength agent in a papermaking furnish.

Suitable reactive anionic compounds are organic molecules comprising at least one anionic moiety such as a sulfonyl or carboxyl group and at least one fiber reactive group capable of forming a covalent bond such as an ether-type linkage to a hydroxyl group on cellulose, selected from the group consisting of monohalotriazine, dihalotriazine, trihalopyrimidine, dihalopyridazinone, dihaloquinoxaline, dihalophtalazine, halobenzothiazole, acrylamide, vinylsulfone, β-sulfatoethylsylfonamide, β- haloethylsulfone, and methylol, with dihalotriazine believed to be particularly advantageous because of an ability to allow reaction with the fiber to occur at lower temperatures than monohalotriazine and related compounds; and with chlorine as the preferred halogen. The reactive anionic compound further comprises a bridging group between the fiber reactive moiety and the anionic group, said bridging group comprising an aliphatic, an aromatic, an inertly or essentially inertly substituted aromatic, a cyclic, a heterocyclic, or an inertly or essentially inertly substituted heterocyclic radical, characterized by low absorption of visible light. In one embodiment, the bridging group is bonded to the fiber reactive moiety by means of an -NH-group or by a peptide bond involving the group

O

II

— C— NH— . In another preferred embodiment, the reactive anionic compound is substantially water soluble and has a molecular weight less than 5,000, preferably less than 3000, more preferably less than 1500, and preferably between 300 and 1000.

More specific examples of suitable reactive anionic compounds are given by the formula:

W— R— Y— X— B wherein W is sulfonyl or carboxyl or salts thereof; R is either an aliphatic, an aromatic, an inertly or essentially inertly substituted aromatic, a cyclic, a heterocyclic, or an inertly or essentially inertly substituted heterocyclic radical, characterized by low absorption of visible light, and preferably being resistant to attack or cleavage at 70°C over a pH range of 6 to 8, preferably 6 to 9, more preferably 5 to 9, and most preferably 4 to 10; Y signifies NH or

O

II — C— H— ;

X is a moiety suitable for forming a covalent bond such as an ether-type linkage to a hydroxyl group on cellulose, selected from the group consisting of monohalotriazine, dihalotriazine, trihalopyrimidine, dihalopyridazinone, dihaloquinoxaline, dihalophtalazine, halobenzothiazole, acrylamide, vinylsulfone, β-sulfatoethylsylfonamide, β-haloethylsulfone, and methylol, with dihalotriazine believed to be particularly advantageous because of an ability to allow reaction with the fiber to occur at lower temperatures than monohalotriazine and related compounds; and with chlorine as the preferred halogen; and

B is either hydrogen, a group of the formula Y-R, wherein Y and R are defined as above; or a group of the formula Y-R-W, wherein Y, R, and W are defined as above. A particular commercially available example of a suitable fiber reactive anionic compound, discovered to be useful for the present invention, is the nylon dye retardant Sandospace S produced by Clariant Corp., Charlotte, North Carolina. While the formula of Sandospace S is proprietary, chemical analysis and partial information from the supplier confirms that it has a chlorinated triazine group, aromatic structures, and sulfonic groups. The above formula provides one class of suitable structures. Related structures within the scope of this invention can have multiple sulfonyl or carbonyl groups attached to various locations of the molecule, including on segments of the bridging group or even directly attached to part of the fiber reactive group. Multiple fiber reactive groups may also be attached to one or more bridging groups, allowing the reactive anionic compound to attach to multiple adjoining sites on a cellulose surface.

Whereas treatment with fiber reactive dyes are typically carried out in dilute slurries, such as about 2% consistency, it has been surprisingly discovered that the reaction of the present invention can be successfully earned out with low amounts of liquid, including high consistency fiber slurries with fiber consistencies over 5%, preferably between 5 and 50%, preferably greater than 10%, more preferably greater than 15%, and most preferably greater than about 20%, and desirably between 10 to 30% consistency. The reduced use of water improves process efficiency and reduces water treatment burdens. For high consistency treatment, it is desirable to employ high consistency mixers such as those recently known in the art of papermaking and bleaching. Hobart batch mixers, for example, may be useful in preparing the slurry at high consistency. Useful continuous high consistency mixers are produced by Sunds Defibrator, Norcross, Georgia, and other vendors. For best results, mixing should be done with adequate shear to thoroughly and uniformly mix the reagents with the fiber slurry.

The third step is adjusting the pH and temperature of the slurry to effectively drive the reaction between the fiber reactive anionic compound and the fiber. Once applied to an aqueous fiber slurry, the reactive anionic compound added in the second step may not react significantly with the cellulose until the pH and the temperature are both sufficiently high. Alkalization is typically necessary to raise the pH above 6, preferably above 7, more preferably above 8, still more preferably between 8 and 11 , and most preferably between 8 and 10, in order to drive the reaction toward completion. Alkaline agents such as sodium hydroxide, trisodium phosphate, sodium bicarbonate, and sodium carbonate, either singly or in combination, are preferred for their low cost, their chemical effectiveness, their general compatibility with tissue making operations, and their ease of handling and processing, but other alkaline compounds may be selected as well, including but not limited to calcium oxide, potassium hydroxide, potassium carbonate, and related compounds.

Alkalization of the fibrous slurry can be done either before, during, or after addition of the reactive anionic compound to the fibers in the second step. Based on experimental results, alkalization after addition of the reactive anionic compound is preferred because it results in higher yield and efficiency (higher substantivity of the wet strength agent, manifest by higher wet strength of paper at a given dosage of wet strength agent). Without limitation, it is believed that alkalization too early in the process can cause some hydrolysis of the reactive group of the reactive anionic compound, resulting in lower yield.

In an especially preferred embodiment of the invention, slightly more of an alkaline compound is added to the slurry than would be needed to neutralize the acidic byproduct of reaction between the reactive anionic compound and a hydroxyl group of the cellulose. For example, when the reactive group is monochloro triazine, the acidic byproduct is hydrogen chloride. Adding sufficient sodium hydroxide in the post- alkalization treatment to more than neutralize the hydrogen chloride, assuming complete reaction, has proven to be effective in achieving the desired reaction and the desired wet strength properties. Thorough mixing of the slurry during alkalization is desirable.

Simultaneously or subsequent to the alkalization, a temperature of 20°C to 150°C is typically needed for practically rapid reaction rates, with a preferred temperature range of 20 to 120°C, more preferably 20°C to 100°C, more preferably still 40 to 85°C, and most preferably 50 to 80°C. Of course, the optimum temperature will depend on which fiber reactive anionic compound is used. If the slurry is below a suitable temperature range, temperature elevation may be achieved by contact heating through the use of a heat exchanger, heated vessel walls, steam injection, or any of the many means known in the art. For uniformity of reaction, good mixing of the slurry during heating is desirable. The adjustment of temperature need not be simultaneous with the addition of alkaline compounds or with the addition of fiber reactive anionic compound, but preferably will follow addition of the alkaline compound. The proper temperature should be maintained for a sufficient period of time to drive the reaction to a useful degree of completion.

The fourth step is adding an effective amount of cationic wet strength agents and water to said aqueous slurry, creating a papermaking furnish. Mixtures of compatible wet strength resins, including those described previously, can be used in the practice of this invention. Additional compounds and fillers or solid components may be added. This step may be done simultaneously with the second step, or could even precede the second step, if desired, although better efficiency is obtained by performing the addition of cationic wet strength agents after chemical pretreatment of the fibers. Any amount of wet strength agent may be added, but for efficient use and reasonable cost it is desirable that less than about 30 pounds per ton or 1.5 wt % on a dry fiber basis be added, preferably between about 0.02 to 1.5 wt %, more preferably between 0.02 to 1.0 wt %, and most preferably between about 0.05 to 0.8 wt %. Any cationic wet strength agent suitable for papermaking may be used. For high wet resiliency tissue, preferable agents should be capable of forming covalent bonds with cellulose. In the usual case, the wet strength resins are water-soluble, cationic materials. That is to say, the resins are water-soluble at the time they are added to the papermaking furnish. It is quite possible, and even to be expected, that subsequent events such as cross-linking will render the resins insoluble in water. Further, some resins are soluble only under specific conditions, such as over a limited pH range. Wet strength resins are generally believed to undergo a cross-linking or other curing reactions after they have been deposited on, within, or among the papermaking fibers. Cross-linking or curing does not normally occur so long as substantial amounts of water are present.

Particular permanent wet strength agents that are of utility in the present invention are typically water soluble, cationic oligomeric or polymeric resins that are capable of either crosslinking with themselves (homocrosslinking) or with the cellulose or other constituent of the wood fiber. Such compounds have long been known in the art of papermaking. See, for example, U.S. Pat. Nos. 2,345,543 (1944), 2,926,116 (1965) and 2,926,154 (1960), all herein incorporated by reference. One class of such agents include polyamine-epichlorohydrin, polyamide epichlorohydrin or polyamide-amine epichlorohydrin resins, collectively termed "PAE resins." These materials have been described in patents issued to Keim (U.S. Pat. Nos. 3,700,623 and 3,772,076 herein incorporated by reference) and are sold by Hercules, Inc., Wilmington, Delaware, as Kymene, e.g., Kymene 557H. Related wet strength agents are sold by Georgia Pacific under the name Amres, e.g., Amres 8855. Other suitable materials are marketed by Henkel Chemical Co., Charlotte, North Carolina. Materials developed by Monsanto and marketed under the Santo Res label are base-activated polyamide-epichlorohydrin resins that can be used in the present invention. These materials are described in patents issued to Petrovich (U.S. 3,885,158; U.S. 3,899,388; U.S. 4,129,528 and U.S. 4,147,586) and van Eenam (U.S. 4,222,921) all herein incorporated by reference.

Although they are not as commonly used in consumer products, polyethylenimine resins are also suitable for immobilizing fiber-fiber bonds. Another class of permanent- type wet strength agents includes aminoplast resins (e.g., urea-formaldehyde and melamine-formaldehyde).

The permanent wet strength agent is typically added to the paper fiber in an amount up to about 20 pounds per ton or 1.0 wt %. The exact amount will depend on the nature of the fibers and the amount of wet strength required in the product. As in the case of the temporary wet strength agent, these resins are generally recommended for use within a specific pH range depending upon the nature of the resin. For example, the Amres resins are typically used at a pH of about 4.5 to 9.

Temporary wet strength agents are also useful in the method of this invention. Suitable cationic temporary wet strength agents can be selected from agents known in the art such as dialdehyde starch, polyethylene imine, mannogalactan gum, glyoxal, and dialdehyde mannogalactan. Also useful are cationic glyoxylated vinylamide wet strength resins as described in U.S. Pat. No. 3,556,932 issued to Coscia et al. on Jan. 19, 1971, and in U.S. Pat. No. 5,466,337, "Repulpable Wet Strength Paper," issued to William B. Darlington and William G. Lanier on Nov. 14, 1995. Useful water-soluble cation resins include polyacrylamide resins such as those sold under the Parez trademark, such as Parez 631 NC, by American Cyanamid Company of Stanford, Conn., generally described in the above-mentioned patent issued to Coscia et al. and in U.S. Pat. No. 3,556,933 issued to Williams et al. on Jan. 19, 1971. U.S. Pat. No. 4,605,702, Guerro et al., issued August 12, 1986, discloses temporary wet strength resin made by reacting a vinylamide polymer with glyoxal, and then subjecting the polymer to an aqueous base treatment. The product is said to provide tissue paper which loses a part of its wet strength when soaked in water at neutral pH. U.S. Pat. No. 4,603,176, Bjorkquist and Schmidt, issued July 29, 1986, discloses related temporary wet strength resins. Generally, the cationic temporary wet strength agent is provided by the manufacturer as an aqueous solution and is added to the pulp in an amount of about 0.05 to 0.4 wt % and more typically in an amount of about 0.1 to 0.2 wt %. Depending on the nature of the resin, the pH of the pulp is adjusted prior to adding the resin. The manufacturer of the resin will usually recommend a pH range for use with the resin. The Parez 631 NC resin can be used at a pH of about 4 to 8.

The fifth step is depositing said papermaking furnish on a foraminous surface to form an embryonic web. This step may further comprise dewatering and other operations known in the art prior to drying of the web.

The sixth and final step is drying the web. 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 heat supplied by air moving around, over, or through the web; by contact with a heated surface; by infrared radiation; by exposure to superheated steam, or by a combination of such methods. The exact point at which the wet strength agent begins to cure during the drying of the wet fibrous web is an indistinct one. What is required in the present invention is that the fibrous web be substantially dried and that the wet strength bonds of whatever nature as provided by the wet strength resin begin to form. The extent of formation of these bonds must have proceeded to such an extent that subsequent process steps will not appreciably interfere with their ultimate completion and the corresponding wet strength development. In general, though not necessarily in all cases, it is desired that the temperature of said web be sufficiently elevated to effectively cure the wet strength agent (i.e., drying may or may not require high temperature curing). The wetdry tensile strength ratio of the dried web should be at least 0.1 , preferably at least about 0.2, and more preferably at least about 0.3 when the process has been properly executed. As used herein, the "wetdry ratio" is the ratio of the geometric mean wet tensile strength divided by the geometric mean dry tensile strength. Geometric mean tensile strength (GMT) is the square root of the product of the machine direction tensile strength and the cross-machine direction tensile strength of the web. Tensile strengths are measured with standard Instron test devices having a 4-inch jaw span using 3-inch wide strips of tissue, conditioned at 50% relative humidity and 72°F for at least 24 hours, with the tensile test run at a crosshead speed of 10 in/min.

The final wet strength of the paper for a given dose of wet strength agent should be at least 10% higher than is achieved by the use of the wet strength agent without addition of the reactive anionic compound.

The present invention offers multiple advantages over prior art techniques for enhancing wet strength. The present invention requires no coloration or dying of the fibers, and requires no bleaching or discharging of chromophores to maintain a white sheet. The present invention requires no addition of NaCI or other chlorides to drive the reaction of the reactive anionic compound with the fiber. Further, the present invention does not require highly dilute fiber slurries in the fiber pretreatment step but has been demonstrated successfully at fiber consistencies as high as 30%. Further, the present invention does not rely on ionic bonds to enhance strength, but takes advantage of reactive wet strength agents that form covalent bonds with the cellulose surface, though ionic bonds do provide the initial attachment of the cationic polymer with the sulfonic groups of the reactive anionic compound.

The novel use of fiber reactive anionic compounds in the present invention can also be coupled with chemical debonder agents to make paper with relatively high wet strength and low dry strength. One or more fiber reactive anionic compounds are used with cationic wet strength resins to establish water-resistant covalent bonds, while chemical debonders are used to reduce the number of hydrogen bonds between fibers, thus reducing the dry strength of the paper. This is best done by first increasing anionic sites on the cellulose fibers with said fiber reactive anionic compound, according to steps one through three as previously described, followed by addition of a chemical debonder agent and a cationic wet strength agent. The debonder agent may be applied to the fibers after step three while the fibers are in solution, followed by addition of the cationic wet strength agent as in step four, whereafter the paper is formed, dewatered, and dried according to steps five and six above. In this case, wherein the debonder agent is added to the fibers while they are in slurry form, it is desirable that the cationic wet strength resin be added after the debonder agent has been added to the slurry. Otherwise, the cationic wet strength agent may occupy most anionic sites on the fibers and interfere with retention of the chemical debonder agent. Chemical debonder agents typically have a single cationic site, such as a quaternary ammonium salt, with fatty acid chains.

Alternatively, the debonder agent may be applied to the dried or partially dried paper web during step six through known means such as spraying, printing, coating, and the like. Preferably, the web has been dried enough to begin formation of covalent bonds in the web. The web should then be at a solids level of preferably at least 40%, more preferably at least 60%, more preferably still at least 70%, most preferably at least 80%, and desirably between 60 and 90%. The debonder maybe applied at other times, but for best results it should be either between steps 3 and 4 or during step 6 of the process described above.

When properly applied, the debonder agent interferes with hydrogen bond formation between the fibers, thus reducing the dry strength of the paper, while having relatively little effect on covalent bond formation. The result is a paper with an increased wet:dry tensile strength ratio. Such paper can have reduced stiffness and improved softness due to the reduced extent of hydrogen bonding, while still having high wet strength.

Desirable chemical debonder agents have less than five cationic sites per molecule and preferably no more than one cationic site which can bond with the anionic sites on the cellulose fiber surface. Large numbers of cationic sites could interfere with the anionic sites provided by the fiber reactive anionic compound if the debonder is applied to the fibers before covalent bonds have formed. Examples of useful chemical debonder agents include fatty chain quaternary ammonium salts (QAS) such as Berocell 584, an ethoxylated QAS made by Eka Nobel, Inc. (Marietta, Georgia), or compounds made by Witco Corp., Melrose Park, Illinois, including C-6027, an imidazoline QAS, Adogen 444, a cethyl trimethyl QAS, Varisoft 3690PG, an imidazoline QAS, or Arosurf PA 801 , a blended QAS. Agents known as softeners in the art of tissue making are also likely to be suitable as chemical debonder agents. Relative to the dry mass of the fibers, debonder may be added at a level in the range of 0.1% to 2%, preferably 0.2% to 1.5%, and more preferably 0.5% to 1%.

Examples Example 1.

100 gm of a dried bleached virgin northern softwood kraft pulp (Kimberly-Clark LL-19 pulp) was saturated with 1200 ml of water and dispersed into a slurry through agitation in a Hobart mixer. The slurry was dewatered to a fiber consistency of about 25%. This was repeated several times to obtain multiple batches of high consistency pulp. For each batch of pulp, between 1 and 4 grams of Sandospace S (Clariant Corp., Charlotte, NC) was prepared and diluted with 5 parts of water per part of reagent (thus, the amount of dilution water ranged from 5 to 20 grams of water). Each batch of fiber slurry, comprising 100 gm of fiber per batch, was then reloaded into the Hobart mixer and a Sandospace S solution, containing between 1 and 4 gm of Sandospace S was added during agitation of the pulp. The mixture was thoroughly blended at 25°C for 25 minutes. Then NaHCO₃ was added to each batch at a dose of 0.5 gm of NaHCO₃ per gm of Sandospace S (for a range of 0.5 to 2 gm of NaHCO₃), with the NaHCO₃ having been first dispersed in 5-10 ml of water prior to addition to the mixture of fiber, water, and Sandospace S. Following addition of NaHCO₃, the mixture was further blended in the Hobart mixer for 20 min at 25°C. Thereafter, the mixture was heated to 100°C in an oven and maintained at said temperature for 2 hours without mixing. After cooling the slurry to 25°C, without post-washing of the slurry, the slurry was formed into 60 gsm handsheets using standard Tappi procedures. Kymene 557LX wet strength agent was added to the diluted handsheet slurry at a level of 1 % Kymene on a dry fiber basis. The properties of these handsheets are shown in Figures 2-5. Sheet wet strength is shown to have increased substantially as the level of Sandospace S was increased, even though the amount of wet strength agent was constant. This demonstrates the ability of the fiber reactive anionic compound to improve the efficiency and substantivity of the Kymene, which is a cationic wet strength agent.

Untreated LL19 fiber handsheets with 1% Kymene had a wet strength of 1411 grams/in and a wetdry tensile strength ratio of 24.6%. With pretreatment by the Sandospace S fiber reactive anionic compound, the same level of Kymene resulted in a wet strength of 2374 g/in and a wetdry tensile strength ratio of 30.1% when 1% of the Sandospace S was applied. Results from tensile testing are shown in Table 1. Up to a 68% increase in wet strength was possible with fiber reactive anionic compound relative to the use of 1 % Kymene alone.

Table 1 : Results from Example 1 (post-alkalization)

Example 2

All steps were conducted as in Example 1 except that the NaHC0₃ solution was added prior to the addition of the Sandospace S solution, resulting in pre-alkalization rather than post alkalkization. Up to a 46% increase in wet strength with fiber reactive anionic compound was possible relative to paper made with the Kymene alone. Note that at 1% RAC (reactive anionic compound), a wet strength of 1606 g was achieved with pre- alkalization compared to 2374 g with post-alkalization.

Table 2: Results from Example 2 (pre-alkalization)

Example 3

45 kg of a bleached northern softwood kraft pulp was pulped at 25°C for 20 minutes in a high consistency pulper at a consistency of 8%. 3.6 kg (8% relative to the fiber mass) of Sandospace S paste, as received from Clariant Corp., was added to the slurry in the pulper and mixed for an additional 20 minutes. 0.9 kg of sodium carbonate powder was added to the slurry in the pulper and mixed for another 20 minutes. The slurry was then heated to 60°C and maintained at that temperature for 2 hours and then dewatered with a centrifuge to 35% consistency. The fibers were then ready for use in papermaking without any washing.

The 35% consistency fibers were then diluted with water to make handsheets according to Tappi procedures for handsheet making. Then Berocell 584 liquid (Eka Nobel Corp., Marietta, Georgia) was added to the dilute slurry at a dose of 1 gram of Berocell liquid per 100 grams of fiber (1% Berocell on a dry fiber basis) and stirred for 20 minutes. Thereafter, 1 % Kymene 557LX on a dry fiber basis was also added to the slurry and stirred for 20 minutes. Then 60 gsm handsheets were formed according to Tappi procedures and tested for dry and wet tensile strength properties.

The 60 gsm handsheets had a mean wet strength of 2160 g/inch and a mean dry strength of 4929 g/inch. The wetdry tensile strength ratio for the handsheets of this example was 43.8%, in contrast to typical values of 30-35% for sheets with Kymene but without debonder, as in Example 1. A handsheet made according to this Example but without any added debonder had a wetdry tensile strength ratio of 35.1%.

Example 4

Handsheets were prepared as described in Example 3, except that no debonder was added to the fibrous slurry. A 1% by weight aqueous solution of Berocell liquid was prepared and sprayed onto the dried handsheets using a common household hand sprayer. Spray was applied evenly to both sides of the handsheets until the added liquid mass was approximately 100% of the dry handsheet mass, resulting in a total application of 1% pure Berocell to the fibers on a dry fiber basis (1 gram of added Berocell per 100 grams of fiber). Then the handsheets were dried at 105°C for 20 minutes and then cooled, conditioned, and tested for tensile strength. The mean wet strength was 2897 g/inch and the dry strength was 6551 g/inch, yielding a wetdry tensile ratio of 44.3%.

It will be appreciated that the foregoing examples, given for purposes of illustration, are not to be construed as limiting the scope of this invention, which is defined by the following claims and all equivalents thereto.

Claims

We claim:

1. A method for making wet strength paper comprising the steps of:

(a) providing an aqueous slurry of cellulosic papermaking fibers;

(b) adding a substantially colorless reactive anionic compound to said aqueous slurry, said reactive anionic compound having the formula:

W-R-Y-X-B wherein:

W is sulfonyl or carboxyl or salts thereof;

R is an aliphatic, an aromatic, an inertly or essentially inertly substituted aromatic, a cyclic, a heterocyclic, or an inertly or essentially inertly substituted heterocyclic radical, characterized by low absorption of visible light;

0 Y signifies NH or — C^_NH^— ;

X is a moiety suitable for forming a covalent bond to a hydroxyl group on cellulose, selected from the group consisting of monohalotriazine, dihalotriazine, trihalopyrimidine, dihalopyridazinone, dihaloquinoxaline, dihalophtalazine, halobenzothiazole, acrylamide, vinylsulfone, β- sulfatoethylsylfonamide, β-chloroethylsulfone, and methylol; B is hydrogen, a group of the formula Y-R, wherein Y and R are defined as above; or a group of the formula Y-R-W, wherein Y, R, and W are defined as above;

(c) adjusting the pH and temperature of said aqueous slurry to promote reaction of the reactive anionic compound with the cellulosic fibers;

(d) adding a cationic wet strength agent and water to said aqueous slurry to create a papermaking furnish;

(e) depositing said papermaking furnish on a foraminous surface to form an embryonic web; and (f) drying the web.

2. The method of Claim 1 , wherein the amount of the reactive anionic compound is from about 0.01 to about 4 dry weight percent of the dry fiber mass of the web.

3. The method of Claim 1 , wherein the amount of the cationic wet strength agent is from about 0.02 to about 1.5 dry weight percent of the dry fiber mass of said web.

4. The method of Claim 1 , wherein the consistency of fiber in said aqueous slurry is about 5% or greater during the step of adding the reactive anionic compound.

5. The method of Claim 1 , wherein the consistency of fiber in said aqueous slurry is about 20% or greater during the step of adding the reactive anionic compound.

6. The method of Claim 1, wherein group X of the reactive anionic compound is a moiety selected from the group consisting of dichlorotriazine, trichloropyrimidine, and dichloropyridazinone.

7. The method of Claim 1, wherein the amount of sodium chloride present in the aqueous slurry of step (c) is less than 0.01 g per gram of fiber.

8. The method of Claim 1 , wherein the step of adjusting the pH of said slurry is achieved through the addition of an alkaline agent selected from the group consisting of NaHCO₃, Na₂CO₃, Na₃PO₄ and NaOH.

9. The method of Claim 1 , wherein the cationic wet strength agent is a crosslinkable agent.

10. The method of Claim 1 , wherein the cationic wet strength agent is a permanent wet strength agent.

11. The method of Claim 1 , wherein the cationic wet strength agent is a permanent wet strength agent.

12. The method of Claim 1 , wherein the wet strength of the dried web is greater than 2000 grams per inch based on a 60 gsm Tappi handsheet.

13. The method of Claim 1 , wherein the wet strength of the dried web is at least 20% greater than the wet strength of an otherwise identical web made without the addition of the reactive anionic compound.

14. The method of Claim 1 , wherein the wetdry strength ratio of the dried web is 0.2 or greater.

15. The method of Claim 1, wherein the wetdry strength ratio of the dried web is 0.4 or greater.

16. The method of Claim 1 , wherein the pH in step (c) is adjusted to be in the range of from about 8 to about 11.

17. The dried web made according to the method of any one of Claims 1-12 having a wetdry strength ratio of about 0.2 or greater.

18. The method of Claim 1 , further comprising the steps of adding a chemical debonder agent to said aqueous slurry prior to the step of adding a cationic wet strength agent.

19. The method of Claim 1 , further comprising the step of adding a chemical debonder agent to said aqueous slurry after the step of adding a cationic wet strength agent.

20. The method of Claim 19, wherein said chemical debonder agent is applied to said web during the step of drying the web, such that the web is at least partially dried prior to application of said chemical debonder agent.

21. The dried web made by the method of Claim 18 or 19 having a wetdry strength ratio of 0.3 or greater.

22. A paper web comprising:

(a) cellulosic papermaking fibers;

(b) from 0.02 to about 1.5 dry weight percent, based on dry fiber, of a cationic wet strength additive; and (c) from 0.01 to about 4 dry weight percent, based on dry fiber, of a substantially coloriess reactive anionic compound, said reactive anionic compound having the formula:

W-R-Y-X-B wherein:

W is sulfonyl or carboxyl or salts thereof;

O

II Y signifies — H— or— C— NH— ;

X is a moiety suitable for forming a covalent bond to a hydroxyl group on cellulose, selected from the group consisting of monohalotriazine, dihalotriazine, trihalopyrimidine, dihalopyridazinone, dihaloquinoxaline, dihalophtalazine, halobenzothiazole, acrylamide, vinylsulfone, β- sulfatoethylsylfonamide, β-chloroethylsulfone, and methylol; B is hydrogen, a group of the formula Y-R, wherein Y and R are defined as above; or a group of the formula Y-R-W, wherein Y, R, and W are defined as above.

23. The paper web of Claim 22, further comprising 0.1% to 2.0% of a chemical debonder agent.