CA2250176C - Temporary wet strength polymers from oxidized reaction product of polyhydroxy polymer and 1,2-disubstituted carboxylic alkene - Google Patents

Abstract

A temporary wet strength polymer and compositions for paper products, e.g.
toilet tissue, is disclosed. The temporary wet strength polymer is the oxidation product of an esterified polyhydroxy polymer, more preferably of an esterified polysaccharide. The esterified polymer compound is formed by reacting the polyhydroxy polymer with a 1,2-disubstituted alkene compound that has at least one carboxylic acid group. The temporary wet strength polymer provides paper products having an initial wet strength that enables use of the product in the moistened condition, along with a suitable wet strength decay rate.

Description

TEMPORARY WET STRENGTH POLYMERS FROM OXIDIZED REACTION PRODUCT OF
POLYHYDROXY POLYMER AND 1,2-DISUBSTITUTED CARBOXYLIC ALKENE
Field of the Invention This invention relates to wet strength polymers and compositions which can impart temporary wet strength to paper products, and to paper products having temporary wet strength.
BACKGROUND OF THE INVENTION
Wet strength is a desirable attribute of many disposable paper products that come into contact with water in use, such as napkins, paper towels, household tissues, disposable hospital wear, etc. In particular, it is often desirable that such paper products have sufficient wet strength to enable their use in the moistened or wet condition. Thus, the product should resist tearing, ripping, disintegration and the like such that it substantially maintains its integrity during the intended use.
For example, moistened tissue or towel may be used for body or other cleaning.
Unfortunately, an untreated cellulose feber assemblage will typically lose 95% to 97% of its strength when saturated with water such that it cannot usually be used in the moistened or wet condition.
Paper products develop dry strength in part due to interfiber hydrogen bonding. When the paper product is wetted, water disrupts the hydrogen bonds and, as a consequence, lowers the strength of the paper product. Historically, wet strength of paper products has been increased primarily by two approaches. One approach is to prevent water from reaching and disrupting the hydrogen bonds, for example, by coating the paper product. Another approach is to incorporate additives in the paper product which conuibute toward the formation of interfiber bonds which are not broken or, for temporary wet strength, which resist being broken, by water. The second approach is commonly the technique of choice, especially for tissue products.
In this latter approach, a water soluble wet strength resin may be added to the pulp, generally before the paper product is formed (wet-end addition). The resin generally contains cationic funetionalities so that it can be easily retained by the cellulose fibers, which are naturally anionic.
A number of resins have been used or disclosed as being particularly useful for providing wet strength to paper products. Certain of these wet strength additives have resulted in paper products with permanent wet strength, i.e., paper which when placed in an aqueous medium retains a substantial portion of its initial wet strength over time. Exemplary resins of this type include urea-formaldehyde resins, melamine-formaldehyde resins and polyamide-epichlorohydrin resins. Such resins have limited wet strength decay.
Permanent wet strength in paper products is often an unnecessary and undesirable property.

2 Paper products such as toilet tissues, etc., are' generally disposed of after brief periods of use into septic systems and tile like. C.'.logging c>I'these systems ca.rr result if the paper product permanently retains its hydrolysis-resistant strength properties. Therefore, manufacturers have more recently added temporary wet strength additives to paper Lrroducts for which wet strength is sufficient for the intended use, but which tlrerx decays upon soaking in water.
Decay of the wet strength facilitates flaw of the paper product through septic; systems.
Numerous approaches for providing paper products claimed as having good Initial wet strength which decays significantly over tune have°, ber;n suggested.
For example, U.S. Pat. No. 3,1:?96,228, 1)av et al., issued .tiny 2, 1983, U.S. Pat. No.

3,556,932, Coscia et al., issued Jan. 1~), 1971; l.i.S. Pat. No. 3,740,391, Williams et al., issued June 19, 1973; tJ.S. Pat. No. 4,60 >,7U", C.3r.Yerro et al,,, issure~9 :~,r,igust 12, 198f~, and tJ.S. Pat.
No. 4,675,394, Solarek, et al., issued ,lane 23, 1987, suggest various approaches for achieving temporary wet strength with polymers or other compounds.
While the art has provided a variety of Iaaper products having temporary wet strength, none has provided paper products in the manner of late laresent invention. It is an object of an aspect of this invention to provide paper products, including; paper tissue products such as toilet tissue, that have an initial wet strength sufticient for use of the paper product in the moistened condition, hut which also exhibit vet strength decay (i.e., temporary wet strength) such that very low strength levels are attained sr.rbser~uent tn the period of intended use.
Another object of an aspect of the present invention is to provide paper products having a combination of an initial wet strength sufi~ciont forty cYSe ot'tlne paper product :lor body cleaning in the moistened condition, anti a rate cyf wet strcnngtlr c:lecay sufficient for a ftushable product.
It is a further object of an aspect of the present invention to provide tissue paper products having an initial total wet tensile strength of at feast about 8U glinct, ~rreforably at Least about 120 g/inch. Yet another object of an aspect of this invention is to prcavide tissue paper products having, in addition to these initial total wet strengths, a 30 minute total wet tensile strength of not more than about 4U g/inch, preferably rYOt. snore than about 2U g/inch. Another object of an aspect of the inverrtiort is to provide tisscre laaher products havin~; such initial wet strength and which also exhibit a wet stren~,~t.h decay rate ~~fter :30 minutes of soaking in neutral pH water of at feast about '?U%, preferably at toast about 80%.
"~.:.~Jllnnarv ~5~.~1~.2.~j!~.:u The present invention :relates tc.~ temporary wet strength polymers anc:t compositions for paper products, including toilet tissue. The temporary wet strength polymer is formed by oxidizing the reaction product ol'a polyhydroxy pcalyn -Yes and a 1,2-disubstituted carboxylic alkene to form covalent linkages. 'fl7e hydroxyl groups of'tkre polymer are reacted with the carboxylic functional group of the alkene tca form the covalent linkages. The alkene preferably has at least one other carboxylic functional group such that the alkene is capable of forming an anhydride. '1"he temporary wet strength E~olymer° cc>ntains aldehyde groups, which tend to impart temporary wet strength to paper formed with the polymer.
In one embodiment of the present invention. there is provided a temporary wet strength polymer containing aldehyde groups, w~l~ereiry tltc; poiyYrYer is prepared by oxidizing the reaction product of (i) a water-soluble p~olyhydrryxy pcalyrner, Karpd a 1,2-disubstituted alkene, ~l having at least one carboxylic functiotlal group capable of reacting with hydroxyl groups wherein in said reaction product, at least a portion of said hydroxyl groups are reacted with said at least one carboxylic fuoctionai group of said aikene to form ester linkages, said reaction product being axidiLed to form aldeltyde groups.

The present invention also relates to paper products, e.g., cellulosic fibrous non-woven webs, e.g., tissue, containing the temporary wet strength polymer. The present invention tends to provide tissue having a high initial wet tensile strength (e.g., at least about 80 g/inch, preferably at least about 120 g/inch), and a suitable 30 minute wet tensile strength. For example, tissue containing the temporary wet strength polymer of the invention may have an initial wet tensile strength of over 120 gram/inch and a 30 minute wet tensile strength of less than 20 gram/inch.
Tissue having this initial wet tensile strength can be moistened for use in perianal cleaning without excessive deterioration of the paper tissue during use. The 30 minute wet tensile ensures that the tissue remains flushable with a low risk of clogging.
In a preferred embodiment, at least one component of the paper product has a positive charge to enhance inter-fiber bonding. In alternative embodiments, the positive charge is achieved by forming the temporary wet strength polymer from a polyhydroxy polymer containing cationic groups, or by including a cationic retention aid in the composition.
Detailed Description of the Preferred Embodiments The temporary wet strength polymer of the present invention can be formed by oxidizing the reaction product of a water-soluble polyhydroxy polymer and a 1,2-disubstituted alkene containing at least one carboxylic functional group capable of reacting with the hydroxyl groups of the polyhydroxy polymer to form covalent linkages (the 1,2-disubstituted alkene is alternatively referred to herein as "carboxylic alkene"). The polyhydroxy polymer that has been reacted with the carboxylic alkene, yet not oxidized, is hereinafter alternatively referred to as "intermediate polymer."
As used herein, "water soluble" includes the ability of a material to be dissolved, dispersed, swollen, hydrated or similarly admixed in water. Similarly, as used herein, reference to the phrase "substantially dissolved," "substantially dissolving" and the like refers to the dissolution, dispersion, swelling, hydration and the like admixture of a material in a liquid medium (e.g., water). The mixture typically forms a generally uniform fluid mixture having, to the naked eye, one physical phase.
The water-soluble, polyhydroxy polymer may be naturally occur: ing or synthetic. In a preferred embodiment, the polyhydroxy polymer consists essentially of a polysaccharide or a water soluble polysaccharide derivative. Non-limiting examples of suitable polysaccharides are water-soluble cellulosic polymers, including starch; galactomannan polymers, including guar gum and locust bean gum; and water-soluble derivatives thereof. Derivatives include anionic and cationic derivatives of such polysaccharides and ethers and esters of such polysaccharides.
Starches for use herein may suitably be derived from corn, potatoes, tapioca, rice, wheat and the like. Suitable starches may vary over a wide molecular weight range and include dextrins and maltodextrin. Preferred starches are those which do not have a substantial anionic charge, more preferably those which are electronically neutral or cationic, most preferably those which are cationic. The cationic starches typically contain cationic groups such as tetra-alkyl ammonium groups. T'he cationic groups present in the starch tend to decrease the charge repulsion between the final temporary wet strength polymer and cellulose fibers in order to enhance interfiber bond formation and thus to develop higher initial wet tensile strengths. Exemplary cationic starches include those commercially available from the National Starch and Chemical Corp., New York, NY, under the trade names Rediba:nd~~' 5327 and CatoTM 31.
In general, the initial ,rt~d 30 rt7irrute we.t. t ensile str~:rtgths of paper products including a temporary wet strength polymer of the invention tend to increase with an increase in the polymer molecular weight. hhe: molecular weight crf tyre temporary wet strength polymer is determined primarily by the nroleeular weight of the water-soluble polyhydroxy polymer.
Preferred polyhydroxyl polymers of the invention will have a number average molecular weight in the range of from about 3,000 gramsr'mole (glmolej to about 1,006,000 g/mole, more preferably in the range of fxom abcaut .3,000 gtmale to about 10,000 g/rrrole.
The polyhydroxy polymer- is reacted with a 1,2-distrbstituted alkene containing at least one carboxylic functional l,~roup that is capable af'rca4ting with the hydroxyl groups of the polyhydroxy polymer, to thereby tor~rn the intermediate polymer. The carboxylic functional group may be. for example, a carboxylic ;acid k>,rt:>up (- C'C>OH) or an acid amide group (- CONH2), and is preferably a i:ar-baxylic acid grauh. 'fhe carboxylic acid group reacts with a hydroxyl group of the polyhydroxy polyener~, and an amide group may also react with such hydroxyl group to. 'The acid amides are far less reactive than the Carboxylic: acids and are therefore less preferred.
By "1,2-disubstituted," it is mean that each ot'the doubly bonded carbons is singly bonded to one carbon atom other than the doubly bonded carbon atom, and to a hydrogen atom (-HC-----CH-). Without intending to lre bound by theory, it is believed that if each doubly bonded carbon atom is not banded to at least one carbon at<:rrn, formaldehyde undesirably tends to form during the oxidation of the intermediate polymer- On the other hand, if more than one carbon atom is banded to each doubly bonded carbon atom, kekoncs are undesirably formed when the intermediate polymer is axidi~ed. 'f he carboxylic alkene rrcay contain one or more carbon-carbon double bonds and may captain other multiple bonds. The alkene will typically contain one carbon-carbon doubles bond.
The 1,2-disubstituted carbon-carbon double band is preferably in a c;,yclie structure.
Cyclic alkenes tend to lose fewer aldehyde groups during oxidation of the intermediate polymer, relative to acyclic alk~ne;s. Vl~'itlrr:>ut intending to be bound by theory, it is believed that the number of aldehyde groups should be rnaximired :in order to maximi;~e the number of hemiacetal and/or N-acylhemiarninal groups iha polyacrylarnide is present, in the final paper product" and thus to .maximize ~lh~ initial wet strengtlu c:~f"the paper product containing the temporary wet strength polymer of the lrresent invention.
In preferred embodiments, the carboxylic alkene is a polycarboxylic compound that contains at least one additional carboxylic t"urrctional group such that the compound is capable of ~~ _._.__._.. _,.~._..~..~......__._..._._._..._.__.,...._._.._..~.._~_r_~.-~~_._ -forming an anhydride. Such polycarboxylic compounds tend to more readily react with the hydroxyl groups of the polyhydroxy polymer to form the intermediate polymer such that yields of the temporary wet strength polymer of the present invention are higher than when the carboxylic alkene is not capable of forming an anhydride. As used herein, "anhydride" refers to chemical compounds 5 derived from an acid by the elimination of a molecule of water. The second carboxylic functional group may suitably be a carboxylic acid group or an acid amide group. Thus, the carboxylic alkene may be capable of forming a dicarboxylic acid anhydride or a cyclic imide. It is preferred that each of the carboxylic groups be a carboxylic acid group.
More preferably, the carbon atoms of the carboxylic groups of the polycarboxylic compound are separated by 2-3 carbon atoms in order to facilitate the formation of the anhydride (i.e., the carboxylic groups are positioned 1,2 or 1,3 relative to one another). Most preferably, the carbon atoms of the carboxyl group are separated by 2 carbon atoms since the 1,2 polycarboxylic compounds form anhydrides more readily at lower temperatures than the 1,3 potycarboxylic compounds.
The 1,2-disubstituted alkene group and the carboxylic functional groups) are preferably unconjugated. Without intending to be bound by theory, it is believed that Michael Addition (1,4) can occur to the alkene bond during the esterification reaction where the alkene group and the carboxylic groups) are conjugated. This addition reaction would destroy the alkene bond and thus negate aldehyde formation during oxidation of the intermediate polymer.
Preferred carboxylic alkenes are water soluble so as to enable a water-based process.
Suitable carboxylic alkenes include, but are not limited to, cis-1,2,3,6-tetrahydrophthalic acid and 1,2,3,6-tettahydrophthalamic acid. Derivatives of such compounds, e.g., substituted analogs thereof wherein any of the carbon atoms other than the doubly bonded carbon atoms are mono- or poly- substituted, are also suitable for use herein. A variety of substituent groups may be present. However, the substituent groups should not provide steric hindrance or electronic deactivation of the esterification step such that the rate of esterification is decreased. For reasons of availability and rapid reaction times, the carboxylic alkene is preferably cis-1,2,3,6-tetrahydrophthalic acid.
The intermediate polymer can be formed by a process including the steps of preparing a fluid mixture of the polyhydroxy polymer, the carboxylic alkene, and at least one suitable solvent, heating the mixture to a temperature sufficient to evaporate the solvent and to react the alkene with the polyhydroxy polymer.
The fluid mixture of the polyhydroxy polymer and carboxylic alkene is preferably prepared by mixing a solution of the polymer and a suitable solvent with a solution of the carboxylic alkene and a suitable solvent. The solutions of polymer and the carboxylic alkene are formed by at least substantially dissolving the respective compound in one or more suitable solvents.
Alternatively, a single solution can be prepared by substantially dissolving the polymer and f>
carboxylic alkene in one or more solvents. 'fhe. sole~ent or mixture thereof' is typically selected to provide maximum solubility of the solute(s). Suitable solvents include water, pyridine, or other aprotic solvents. Water is the preferred solvent for both solutes.
The solutions are prepared and combined such that the fluid mixture°
contains polyhydroxy polymer and carboxylic alkene in an amount to provide a degree of substitution on the polymer molecule of from about 0.25 to about 1.5, more preferably from about. O.S to about 1.0, most preferably about 1Ø It is typically preferred to maximize the concentration of the solutes in the fluid nrixturo in ordee- to reduce t:he tune a.nd energy required to evaporate the solvent. Heating can be employed to enhance solubility of the solute in the solvent. For example, the mixture of solute and solvent may be l~eateci to temperatures of'up to abirut 100°C, e.g., from about 70°C to about 99°C. E.g., at temperatures of at least about 90°C, mixtures of up to about 50 weight'% solute in water rare st.ritable.
The solutions) can be mixed together by any suitable method such as are known in the art. Mixing should 'be suf"licierrt to ~:n5ure substar~t~ally i.rniform reaction between the polysaccharide and the carboxylic alkene.
The fluid mixture is then heated to a temperature arnd for a time sufficient to substantially remove the solvent from the mixture arid to react the polymer hydroxyl groups with the carboxyl group of the alkene to form covalent linkages. Where the preferred carboxylic alkenes, the polycarboxylic compounds, are used, the fluid mixture is heated to a temperature and for a time sufficient to substantially remove the solvent from the mixture, to form the anhydride of the polycarlaoxylie compound" a:cnd tc~ react the anhydride with the hydroxyl groups of the polyhydroxy polymer to farm the intermediate polymer (generally by heating to at least 100°C). 'I"hey anlrydridt: and intermediate polymer are typically fornred by heating the mixture to a temperature in the range of fi-om 120°C' -130°C for a period of 1-2 hours. The steps of farming the anhydride and the irrtermecliate polymer are preferably performed by heating under vacuum in order to remove any residual solvent and to minimize the presence of oxygen.
In a preferred embodiment, the reaction betv4~een the polyhydroxy polymer and the carboxylic alkene is catalyzed ~by a suitable catalyst. rfl:re catalyst tends to result in a faster reaction rate, less decomposition of the polyhydroxy polymirr, and a higher yield of the intermediate polymer. Any catalyst such as are knoovn in the art of esterification may be used. A preferred catalyst is sodium hypophosphite ~NaHzPO~), which tends to provide higher yields and less decomposition of the polymer at higher reaction temperatures. 'the use of sodium hypophosphite as an esterifrcation catalyst has been described, for example, in U.S.
Patent 4,820,307, issued to C.W. Welclr. '~1"he catalyst is suitably included in the polyhydroxy polymer or carboxylic alkene solution().
Where the preferred polysaccharide polymers and 1,2-disubstituted alkenes are used, the resultant ester with a degree: of substitution of 'L.d:J~ has thi;
fcn'llowing structure:

O
)C~
'OC
O
vn n wherein R' is OH or NH2; and n is the degree of polymerization (i.e., DP) of the polysaccharide and is at least one ( 1 ), preferably 1-10,000.
As understood in the art, the DP is the inverse of the dextrose equivalent (i.e., DE) of a polysaccharide. Preferably, the DE (or DP) is such that the polysaccharide is soluble in boiling water.
In general, the initial wet tensile strength of the temporary wet strength polymers of the present invention increases with decreasing DE (or increasing DP), while the wet tensile decay rate increases with increasing DE (or decreasing DP). Polysaccharides having a DE
of 5 tend to provide a preferred initial wet tensile strength and wet tensile decay rate.
The resultant intermediate polymer is then oxidized to form the temporary wet strength polymer of the present invention. Oxidation is preferably accomplished by forming a fluid mixture of the intermediate polymer in a suitable solvent and introducing a suitable oxidizing agent into the mixture under conditions such that oxidation occurs to form a polymer having aldehyde groups.
The fluid mixture preferably comprises the intermediate polymer substantially dissolved in a suitable solvent. The solvent, temperature of the mixture, and the concentration of the intermediate polymer. are preferably selected such that the intermediate polymer and oxidation products thereof are substantially dissolved in the solvent during the oxidation step. Without intending to be limited by theory, it is believed that the oxidizing agent may not e~ciently access the intermediate polymer when it is present in solid form, with a resultant reduction in yield of the temporary wet strength polymer. Room temperature (20-25oC) is typically sufficient for dissolution.
Water is the preferred solvent for the oxidation reaction. Typically, the fluid mixture contains up to about 10 weight% of the intermediate polymer and at least about 90 weight% water, preferably about 10 weight% intermediate polymer and about 90 weight% water.
The intermediate polymer is preferably converted to salt form to maximize its solubility in water. The salt can be foamed by adding a suitable base to the mixture to neutralize the free carboxylic groups which are present in the intermediate polymer. Suitable bases include monovalent metal hydroxides, e.g., NaOH and KOH. Neutralization to a pH of from about 7-8 is preferred.

Alkaline pH tends to destroy the aldehyde groups that are formed during the oxidation step.
Typically, one equivalent of base per free carboxylic group is added to the mixture.
Suitable oxidizing agents include, for example, ozone and potassium permanganate. Ozone is the preferred oxidation agent for reasons of simplicity, economics, environmental impact, safety, and reaction efficiency.
Ozone oxidation can be accomplished by introducing ozone into the fluid mixture of the intermediate polymer, e.g., by injecting the gas under pressure into the mixture. Although the flow rate and pressure of the ozone may vary over a wide range, exemplary conditions include a flow rate of about 8.0 liters/minute and a flow pressure of about 8 psig. The mixture is preferably cooled to a temperature as low as possible without freezing the mixture (e.g., to temperatures down to about 0°C) in order to maximize the solubility of the ozone in the mixture.
Antifoaming agents such as are known in the art may be added to the mixture to minimize foaming. The oxidation reaction is typically completed by introducing the ozone under the foregoing conditions for a period ranging from I S to 75 minutes.
The resultant oxidized ester comprises aldehyde groups that can be identified and quantified by known analytical techniques such as NMR. For example, where the preferred polysaccharide polymers and 1,2-disubstituted alkenes are used, the resultant temporary wet strength polymer with a degree of substitution of 1.0 has the following structure:
ECHO
COOH
-, vtl wherein R' and n are as defined above.
The temporary wet strength polymers of the present invention are useful for a wide variety of paper and paper products. As used herein, the terms "paper" and "paper products" include sheet-like masses and molded products containing fibrous cellulosic materials which may be derived from natural sources, such as wood pulp fibers, as well as other fibrous material characterized by having hydroxyl groups attached to the polymer backbone.
Cellulosic fibers of diverse natural origin are applicable to the invention.
Digested fibers from softwood (derived from coniferous trees), hardwood (derived from both deciduous trees) or cotton (inters are preferably utilized. Fibers from Esparto grass, bagasse, kemp, flax, and other lignaceous and cellulosic fiber sources may also be utilized as raw material in the invention. The i~
optimum eellulosic fiber source utilized ire conjunction with this invention will depend upon the particular end use ccmtemplated. Generally wc:~od pulps will be utilized.
Applicable wood pulps include chemical pulps, such as Kraft (i.o., sralfate) and sulfite pulps as well as mechanical pulps including, for example, groundwoad, thermomechanical pulp (i.e., TMP) and chemithermomecharrical pulp (i.e., C"l"MP). C:hrernical pulps, however, are preferred since they impart a superior tactile sense crf'soi'tness t~~ tiss~..re sheots made therefrom.
Completely bleached, partially bloached and unblc;ached tilaers are applicable. It may frequently be desired to utilize bleached pulp for its scrperiEjr brightness and consumer appeal.
For products such as paper tissue, paper towels and absorbent pads ibr diapers, sanitary napkins, catamenials, and other similar absorbent paper products, it is especially preferred to utilize fibers from nor~th~rn soFtwaod pulp due to its premium strength characteristics.
Also useful in the present invention are tit~c:rs derived from recycled paper, which can contain any or all of the above categories as well as other non-tibrous materials such as fillers and adhesives used to facilitate the original paper making.
The paper products may also coniair~ non-c;e llulosic fibrous polymeric material characterized by having hydrcr:xyl groups attached to tire polymer backbone, i'or example glass fibers and synthetic fibers modified with hydroxyl groups. Other fibrous material, e.g.., synthetic fibers, such as rayon"', polyethylene and polypropylene fibers, can also be utilized in combination with natural cellulosic fibers or other fibers containing hydroxyl groups.
Mixtures of any of the foregoin g fibers rnay be used. ~inue the strength of tl~~e paper product tends to increase with the number of hydroxyl groups rn the fibers, it will usually be preferred to employ primarily, more preferably wholly, fiber, having hydroxyl groups.
Cellulosrc fibers are economically preferred.
The temporary wet strength polymers of' the present invention are combined with the cellulosic f<bers in a manner which allows tho. polymer and iibars to fbrm a bonded fiber mass, generally in the form of as sheet containing the lihers. The banded fiber mass has a dry strength and an initial wet strength that is higher than a comparable fiber mass without the polymer.
The paper products are typicalhv fornmd by a vvet laid paper making process.
'Jet laid paper making processes tylaically include the steps of providing a slurry containing the cellulosic tibers (the slurry is alternatively reforrod to herein as a paper rrrakir~g furnish), depositing the slurry of fibers on a substrate such as a foraminaus forming wire (e.g., a Fourdrinier wire), and setting the fibers into a sheeted form while the fibers are in a substantially untlocculated condition. The step c>f setting this fibers into sheeted form may be perforrr~ed by allowing the fluid to drain and pressing the fibers against the foraminous wire (dewater-ing), for example, with a screened roll, such a~~ a cylindrical I7~andy Roll. Once set, the fibrous sheet may then be dried and optionally compacted as desired.
'Treatment of the paper ar paper products with the temporary wet strength polymer may involve spraying or ,printing the cellulc:~sic~ i'ibers that have 'been substantially sot in the preparation of the paper producr., o.g., by a we laid pr~a~;oss. The set fibers arc: preferably sprayed or printed with the temporary wet strength polymer in the form of a temporary wet strength composition which comprises a fluid mixture of the polymer substantially dissolved in a suitable solvent. Water is the preferred solvent. The fluid mixture typically contains from about I-10 weight % of the polymer and about 90-99 weight % of the solvent, for example, a mixture of about 5 weight % of the polymer and S about 95 weight % of the solvent, is suitable. In a preferred embodiment, treatment is accomplished by spraying the set fibers.
Alternatively, the temporary wet strength polymer is combined with the cellulosic fibers in the wet-end of a wet laid paper-making process. Thus, the temporary wet strength polymer may suitably be included in the paper-making furnish.
10 The amount of temporary wet strength polymer that is combined with the cellulosic fibers is generally selected to provide a balance of initial wet strength, wet tensile decay, and optionally other properties, including dry strength, consistent with the objects of the invention. In general, with increasing amounts of the polymer there is an increase in dry strength and initial wet tensile strength and a decrease in the rate of wet strength decay. The paper products will typically contain from about 0.5 to about 5 weight % of the polymer, based on the weight of the cellulosic fibers and optionally other fibers containing hydroxyl groups. Preferably, the paper products will contain from about 0.5 weight % to about 2 weight % of the polymer, based on the weight of such fibers.
The temporary wet strength polymer is allowed to remain in contact with the cellulose fibers for a time and at a temperature sufficient to enable adsorption of the polymer by the fibers and bonding between the polymer and fibers such that significant wet strength is developed via the bond formation (inter-fiber bonds are formed). Bonding may involve ionic bonding and/or covalent bonding. The temporary wet strength polymer is typically readily absorbed by the cellulose fibers where the pH of the temporary wet strength polymer composition is within the range of about 3 to about 8. In general, for a given amount of wet strength polymer (% fiber basis), the initial total wet strength and the 30 minute total wet tensile strength decreases with an increase in pH. Where a cationic starch is used as the polyhydroxy polymer and the wet strength polymer is added at a level of about 1.5-2.0 % (fiber basis), a pH of about 8 tends to provide boat a relatively high initial total wet strength and a suitable wet strength decay rate over a 30 minute period.
The paper product that is being treated with the temporary wet strength polymer is subjected to a drying step to remove water and any other solvents so as to develop the wet strength. Drying may be accomplished by subjecting the paper product to elevated temperatures, e.g., in the range of from 85°C - 125°C, for a time sufficient to achieve the desired level of dryness. Typical conditions are a temperature of from 20oC to about 100oC and a contact time of from about 60 minutes to about 5 minutes. For example, a period of about 5 minutes at 50°C
provides a product having preferred initial and 30 minute wet tensile values.
Without intending to be bound or otherwise limited by theory, it is believed that the aldehyde groups of the temporary wet strength polymer bond to the cellulosic fibers by formation of hemiacetal and/or N-acylhemiaminal graups through reaction of at least a portion of the cellulosic hydroxyl groups and at least a portion of the aldehyde groups as the paper product dries. The resultant network tends to have a relatively high initial wet tensile strength. The hemiacetal and/or N-acylhemiarninal linkages are reversible in water, slowly reverting to the original temporary wet strength polymer. 'T'his revc::rsibility confers temporary wet strength to the paper product. ('f'he reversibility c>f the hemiaminal l;~roups is typically ,:lower than that of the hemiacetal groups. Therefore, for a maximum rate of wet tensile decay, preferred paper products are those which do not have: hen~arninal groups.) The paper product may further contain conventional paper-making additives such as are known in the art, e.g., retention aids and paper softeners. Ln a preferred embodiment of the invention, the paper product is treated with a cationic retention aid to decrease the charge repulsion between the temporary wet strength polymer arrcl the cellulose fibers. Fibers treated in this manner tend to have mere and stronger intertiber ponds, which serve to provide higher initial wet tensile strengths. rfhe retention aid can be added to the temporary wet strength composition to be applied to tike fibers (c:.g., as a spt°ay, E~rint mixture, or in t:lre furnish).
Suitable cationic retention aids and their use in paper making applications are well known in the art. Exemplary cationic retention aids include those commercially available as AccoTM 71.1 and CyproTM 514 (American C',yanamicl C,"c~r1>. of~ Wayne., N.J.), and RetenTM 201 (T-iercules Inc. of Wilmington, I:)el.). 'hhe ret:entiorr aid is typically used in an amount of 1 - 5 % based on the weight of the temporary wet strength polymer of this invention.
The present invention is particularly adapted for paper products which are to 'be disposed into sewer systems, s,ueh as toilet tissue. llawover, it is to be understood that the present invention is applicable to a variety of paper products including, but not limited to disposable absorbent paper products such as those used fir household, body, or other cleaning applications and those used for the absorption of body tluicls such as iu-ine and menses, Exemplary paper products thus include tissue paper including toilet tissue and facial tissue, paper towels, absorbent materials for diapers, feminine hygiene articles including sanitary napkins, pantiliners and tampons, adult. incontinent crrticlc.s and the like, and writing paper.
With regard to paper tissue, the temporary vret strength polymers of the present invention can be used in any type of tissue paper construction. For example.
tissue paper of the present invention can be homogeneous ar mufti-layered constructoon; anti tissue paper products made therefrom can lae of a single-ply or mufti-ply construction.
Tire tissue paper preferably has a basis weight of between about 10 gim' and about 65 g/mZ, and density of about 0.6 g/cm3 or less. More preferably, the basis weight vrill be about 40 g!m' or less and the density will be about 0.3 g~'cm' or less, l~Iost preferably, the density will be between about 0.04 g/cm' and about 0.2 g/cm3. See Column 13, lines 6 1-67, of U.S.
Patent 5,059,282 (Ampulski et al), issued t,7ctober 22, 1991, w'hi4h dcscribts how the density of tissue paper is measured. (Unless otherwise specified.. ail armount:~ <itad weights relative to the paper are on a dry basis.) The tissue paper may be conventionally pressed tissue paper, pattern densified tissue paper, and urtcompacted, nonpattern-densified tissue paper. These types of tissue paper and methods fi>r making such paper are well known in the art and are described, for example, iry iJ.S. Patent .5,:334,286, issued on August ;Z, 1994 in the names of Dean V. Phan and Paul D. 'hrokhan.
With respect tc~ paper° lxroducts that are tea be used in the moistened condition, and with particular reference to tissue paper products including toilet paper to be used in the moistened condition for body cleaning c>r other purposes, it is preferred that the product have an initial wet tensile strength that is high enough for it to withstand the stresses encountered in use. >:'referably, the paper product leas an initial wet tensile strength of at least about 80g/inch, more preferably at least about 120 g/inch.
Moreover, it is desirable for tissue paper products to exhibit a wet strength decay rate such that it can be flushed without a signi .fic.ant ri~;k of sewer system clogging. Preferred products have a total wet ten,;ile strength after 30 rrrinutes of soaking irz neutral pH water of less than about 40 g/in, preferably less than about 20 giinch. Flushable paper products may exhibit a wet strength decay rate after 3t) minutes of,9oaking in neutral pH
water of at least about 70°'°, preferably ~zt least about 80"~a.
In addition, with respect to tissue paper products, and with particular reference to products such as toilet paper, wherein high levels of softness are desired in addition to good initial wet tensile strength with wet strength decay a#tc;r the period of usage to low strength levels, it is highly preferred for the paper to have art .initial total wet tensile strength/total dry tensile strength of at least about 10°!°, preferably at least al:~out. 12%. Lower ratios are less desirable since they tend to by accompanied b~~ a har.;l~ tactile impression.
ii-iowever, paper softening agents may be used to provide greater softness as may be desired.
Paper tissue products formed with t'he teml>or~ary wet strength polymers of the present invention tend to have a high initial total wet tensile strength, a suitable initial total wet strength/dry strength ratio, and a wet strength decay rate suitable for flushability without a significant risk of sewer system clogging under normal use conditions. The aforementioned tensile properties may be determined ~zs described izr the fi~llowing c;xpE~rimr~rrtal section.
EXPERIMf;IW'AL
Strength Tests The paper products are aged prior to tensile testing a minimum of 24 hours in a conditioned room where the temperature is 73 °F ~ 4 °F (22.8 °C t 2.2 °C) and the relative humidity is 50°% ~ 10'i~.
1. Total Tensile Strength ('°CD'T") This test is performed on one inch by five inch (about 2.5 cm X 12.7 cm) strips of paper (including creped tissue paper, handsheets, as well as other paper sheets) in a conditioned room where the temperature is 73"F -r: .~"I~° (abcout:
28°C =~ 2.2°C') and the relative humidity is 50% t 1t)°/t.

An electronic tensile tester (Model 1122, Tnstrc~n (:'or°p., Canton.
Mass.) is used and operated at a crosshead speed of 2.0 irGChes per minute (;about 1.3 can per min.) and a gauge length of

4.0 inches (about 10.2 cm). Keference to a machine direction means that the sample being tested is prepared such that the 5" dirnensiov corresponds to that direction.
Thus, fox a machine direction (MD) TD'l'. the strips arc cut such that the 5" dimension is parallel to the machine direction of manufacture of the paper product. For a cross machine direction (CD) TDT, the strips are cut such that the 5" dirnensiorr is parallel to the cross-machine direction of manufacture of the paper product. Machine-directiow~ and cross-machine directions cof manufacture are well known terms in the art ofpuper-making.
T'he MD and CD tensile strengths are determined using the above equipment and calculations in the conventional manner. 'l'he reported value is the arithmetic average of at least eight strips tested for each directional strength. The TDT is the arithrr~etic total of the MD and CD tensile strengths.
2. Wet Tensile An electronic tensile tester (Model 1122, Inst.ron C"orp.) is used and operated at a crosshead speed of 0.5 inch (srbout 1.3 cm) per minute and a gauge length of 1.0 inch (about 2.5 enr), using the same size strips as for 'I'I_) f. The two ends of the strip are placed in the ,jaws of the machine such and the center of the strip is placed around a stainless steel peg. The strip is soaked in distilled water at about 20°C' for the:
desii°ed soak time, and then measured for tensile strength. As in the case of the 'TD~T, reference to a machine direction means that the sample being tested is prepared such that the: 5" dimension corresponds to that direction.
The MD and CD wet tensile strengths are determined using the above equipment and calculations in the conventional manner. The reported value is the arithmetic average of at least eight strips tested for each directional strength. fhe total wet tensile strength for a given soak time is the arithmetic total of"thu MI:> arid t'I'> tertsil~ stx~eragths for that soak time. Initial total wet tensile strength ("I'fVlr''f ") is measured whoa the paper has been saturated for 5 t 0.5 seconds. 3U minute total wet tensile ("30 M'fW'r") is measured when the paper has been saturated l:or 30 ~ 0.5 minutes.
3. Wet tensile strength decay rate is defined according to the following;
equation:
% Decay = [ (ITWT - 30MTWT of paper including the temporary wet strength polymer of theinvention)X 10U]
divided by:
(ITWT- 3C)MTVhT of comparable paper without any strength additive) The following non-linuting examples are provided to illustrate the present invention.
The scope of the inventic:>n is to be determined by the claims which follow.
EXAMPLE 1 - preparation of temporary wet strength polymer of the present invention 1. preparation of ozone oxidized cis-1,2,3,6-tetrahydrophthalic acid ester of maltodextrin M040 available fi~om Drain ProcessiralC;'orpor-ation of Musctine, Iowa (DE = 5).
A 500 grn quantity of MaltrinTM M040 (Grain 1'rocessinl; C'op.), 500 gin quantity ofcis-1,2,3,6-tetrahydrophthalic acid(THPA), and 30grn quantity of sodium hypophosphite are stirred and heated in 1.0 liter of distilled water until a homogeneous solution is obtained. The reaction mixture is then placed in a container suitalUs; fcrr efficient evaporation of wager from the solution, e.g., PyrexT"' glass pans. 'fhe container is then placed in a Despatch Model LAC1 -67-4 forced air c:rven at 1'?S° 1:.", ;and the wat:er° is evaporated, e.g., by leaving in the oven for about 12-~l6 hcaurs. 'The resultant mixture i:placed in a vacuum oven at 80" - 85° C
for two hours to remove any residual water, then heated at 12>° -130° (.'. for f"our hours while the esterification proceeds. At the end of this period beating is terminated and the product is allowed to cool to room temperature under vacuum for about t2-16 hours. The resultant product is crushed into a powder, e.g., with mortar arvd pestle, and suspended with stirring for 30 minutes in 2.0 liters of cold water. 'l'he resultant lar~oduct is separated .from the aqueous phase, pressed into a container suitable tier efficient waporation of water from the product, e.g., a Pyrex glass pan, and <lric:d for about 12-1 (i hours in vacuum oven at ,~5° C. The resultant maltodextrin ester is powdered in a blender.
A 100gm batch of the starch ester i5 prepared by suspending the unpurif'red powder in 950 ml water and mixing with 20 gm Na::CO3. 'This sc:~lution is oxidized for 1.5 hours at 8.0 1!min ozone flow, 115 volts, gauge pressure 8 prig using a Polymetrics Model T816 ozone generator with oxygen feed. I-lexanc>1 is added as needed to control foaming.
2. preparation of ozone oxidized cis-I ,2,3,6-tc~trahydrophthalic acid (THPA) ester of cationic starch (Redibond 5327 cationic starch from National Starch & Chemical ('orp.) is esterified and the resultant ester oxidized as described for the naaltodextrin. 30 grams cis-1,2,3,6-tetrahydrophthalic acid, 120 ~rams R.cdibond 5327, i .8 grams sodium llypol>hosphite and 210 grams boiling distilled water are mixed to dissolve the solids in the water.
'L'he solution is evaporated to dryness by heating at about 10S°C for about 12 hours. The resultant white solid is placed in a vacuum oven at 12S°C.', for ,'.? llcnrrs. 'fh~
resr.sltant THPAlcatic>nic starch ester, a yellow solid, is washed with water, filtered and oven dried.
The starch ester is oxidized as fellows. 8 grams starch ester, 72 ml water, and 20 ml IN NaOH are mixed until the ester is dissolved in the liquids. The solution has a pH of 7.18.
Hexanol is added to the solution 1o central foaming. '1'he solution is chilled to about 5°C, then ozone is bubbled into the salutiorr unti'1 the color crf tkre solution is bleached (ozone flow rate 2 1/min., oxidation time '?5 min.) A small amount of white suspension is seen in the solution. NMR analysis shows the preaence of-al~ehydc pc°aks and very small alkene peaks.
EXAMPLE 11 - preparation of paper treated with various wet strength compositions a) creped tissue paper preparation Creped tissue paper treated is made according to tire teachings of Sanford and Sisson, U.S. Pat. No. 3,301,746, issued Jan. 31, 1967, and U.S. Pat. No. 3,994,771, Morgan and Rich, issued Nov. 30, 1976. ~1'I1e paper is treated with various wet strength compositions.

The paper machine u;~es a fixed roof' former type of headbox. The fiber furnish comprises 80 weight% eucalyptus and 20 weight°ro I~ortl'uerrr Softwood Kraia fbers fcarmed homogeneously. The h~:adbox dilution water is natural water which is aciditied with sulfuric acid to an approximate pH of from about 5.0 to 5.9,

5 T'he sheets are formed on a polyester 84M lbrming wire. This wire is an "84M"; that is the weave was 84 X 7li filaments per inch wire woven irn a five-shed pattern to form an embryonic web. The embryonic paper web is transi°er-red tc:> a 3fi !~
32 five-shed fabric. These patterns and their use are described in'frcikhan, Ll.S. l'at. l~io. 4,191,609, anti Trokhan, L1.S.
Pat, No. 4,239,065. The embryonic paper sheet is fnrst driEd with hot air in a flow-through 10 dryer to a moisture level of about 50% by weight of the sheet. Such a hot air dryer is well lrnown to those skilled in the ari. 'fhe final drying is accomplished on the surface of a Yankee dryer (to which the web has been adhered with poly inyl alcohol). The paper is dried to approximately 3'% moisture, and then i:reped from the Yankee with a doctor blade and reeled to provide an ultimate residual crepe of' about 2(7°4a.
15 The following sc>lutior~s are spray applied onto different samples of the above described creped paper at a le~~el of 2 weight '% solutic:7n on a fiber basis.
Spraying is executed with two air atomized nozzles spraying at 30 ml/minute ( l S ml/nozzle). Paper consistency at the spray point is 90% and is lowered to 45-4'7°,~o after spraying.
"The paper is then redried to 90% consistency. Solutions A-C. arc representative of the present invention.
(A) Aqueous solution of the none oxidi;~ed cis-1,~,~i,f>-tetc°ahydrophthali~ acid ester of maltodextrin M040 made according to I~xa.mple 1, at: a solids level of 1.~) weight %. The solution pH is 5.86.
(B) A sample of Solution (A) is tittated with CaC:I,, until cloudiness is observed. 25 weight°,% of the quantity of Ca<:~ 1 ~, that produced cloudiness is added and mixed into the above 1.9% solids aqueous Solution (A) on a proportionately scaled yp basis.
The solution pH is 5.91.
(C) A sample of Solution (A) is titrated with ~.'ypro 514 (a cationic retention aid available from the American Cyanamid Corp. of Wayne, N.J.) until cloudiness is observed.
25 weight%
of the quantity of Cypro 514 that produced cloudiness is added and mixed into the above 1.9°!° solids aqueous Solution (A) on a proportionately scaled up basis. "The solution pH is 5.90.
(D) (Comparative) CoBond~r"~ 100U (available from ;fsfational Stanch ~c.
(~hemical Corp. of NY, NY) is cooked in standard preparatory manner by heating in water at pH 2.5 until a solution is attained and diluted with water to a 1.9 weightA"° solids concentration. The solution pH is 2.83.
T'he resultant paper products have tensile properties such as reported in Table 1.
f"ABLE 1 Wet strength InitialInitialITWT 30 minute30 minute30 MTWT
additive - wet wet tensile,wet tensile, wet tensile, MD CD
tensile,CD
MD

D (Comparative)314 140 454 > 187 135 >322 i aore i snows that each of the samples have a high initial total wet tensile strength. Each of the Examples A-C according to the present invention demonstrate significant wet tensile decay. The Comparative Example D has a significantly higher 30 minute total wet tensile strength that is indicative of permanent wet strength. ' In an alternative embodiment, paper products are treated in the manner described in reference to Example II (A), but with an aqueous solution of the ozone oxidized cis-1,2,3,6-tetrahydrophthalic acid ester of cationic starch made according to Example I.
The oxidized solution of the THPA/cationic starch ester is spray applied onto different samples of the above described creped paper at a level of 1, 1.5, or 2 weight % solution on a fiber basis. The total solution sprayed in each case is 0.55 grams. The pH of the solution is adjusted prior to spraying. The paper is then air dried to constant weight, cured at 105°C for S minutes, and creped 5 times on a twin roll press.
The resultant paper products have tensile properties such as reported in Table 2.
~renr >; ~
solutionpolymerInitial InitialITWT 30 minute30 minute30 MTWT
pH % fiberwet wet wet tensile,wet tensile, basis tensile,tensile, MD CD
MD CD

4 2.0 167 80 247 102 45 147

6 2.0 154 64 218 66 35 101 8 2.0 136 64 200 36 25 61 4 l.0 81 39 120 39 20 59 6 1.0 54 30 84 - _ 8 1.0 42 22 64 - _ -8 1.5 71 34 105 17 8 25 For a given pH, the initial total wet strength and 30 minute total wet strength tends to increase with an increase in the polymer % fiber basis. For a given polymer %
fiber basis, the initial total wet strength and 30 minute total wet strength tends to decrease with an increase in pH of the 1'7 solution being applied. I~or a polymer % fiber basis of about 1.S-2.0, a pH of about 8 tends to provide an initial total weight strength and a 3U minute total wet strength which are preferred for flushable tissue products.
While particular embodiments of the present unvention have been illustrated and described it would be obvious to those ski fled in the: art that various other changes and modifications can be made without departing from thc~ spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modi:bcations that are within the scope of this invention.

Claims (15)

WHAT IS CLAIMED IS:

1. A temporary wet strength polymer containing aldehyde groups, wherein said polymer is prepared by oxidizing the reaction product of (i) a water-soluble polyhydroxy polymer, and a 1,2-disubstituted alkene having at least one carboxylic functional group capable of reacting with hydroxyl groups of said polyhydroxy polymer, wherein in said reaction product, at least a portion of said hydroxyl groups are reacted with said at least one carboxylic functional group of said alkene to form ester linkages, said reaction product being oxidized to form aldehyde groups.

2. The temporary wet strength polymer of claim 1 wherein the water-soluble polyhydroxy polymer is a polysaccharide or a water-soluble polysaccharide derivative.

3. The temporary wet strength polymer of claim 1 or 2 wherein the water-soluble polyhydroxy polymer is starch or a water-soluble derivative of starch.

4. The temporary wet strength polymer of any one of claims 1 to 3 wherein the alkene is a cyclic alkene.

5. The temporary wet strength polymer of any one of claims 1 to 4 wherein the reaction product is oxidised with ozone.

6. The temporary wet strength polymer of any one of claims 1 to 5 wherein said polyhydroxy polymer comprises cationic groups.

7. The temporary wet strength polymer of any one of claims 1 to 6 wherein said alkene has at least two carboxylic functional groups, said at least two carboxylic functional groups being capable of forming an anhydride.

8. The temporary wet strength polymer of claim 7, wherein the at least two carboxylic functional groups are positioned 1,2 or 1,3 relative to one another.

9. The temporary wet strength polymer of claim 7 or claim 8, wherein the 1,2-disubstituted alkene group and the at least two carboxylic functional groups are unconjugated.

10. The temporary wet strength polymer of any one of claims 7 to 9 wherein said at least two carboxylic functional groups are independently selected from the group consisting of carboxylic acid groups and acid amide groups.

11. The temporary wet strength polymer of claim 10, wherein the at least two carboxylic functional groups are carboxylic acid groups.

12. The temporary wet strength polymer of claim 11, wherein said alkene is selected from the group consisting of cis 1,2,3,6 tetrahydrophthalic acid, cis 1,2,3,6-tetrahydrophthalamic acid, and mixtures thereof.

13. A temporary wet strength resin composition comprising the temporary wet strength polymer of any one of the claims 1 to 12.

14. A paper product comprising cellulosic fibers combined with the temporary wet strength resin polymer of any one of claims 1 to 12.

15. A paper product comprising cellulosic fibers combined with the composition of claim 13.