FR2500499A1 - PROCESS FOR IMPROVING TENSILE STRENGTH IN WET CONDITIONS OF PAPER - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR IMPROVING TENSILE STRENGTH IN WET PAPER CONDITIONS. ACCORDING TO THE INVENTION, THE PAPER PULP OF WHICH THE PAPER IS FORMED IS TREATED WITH AN AMINE SALT OF A POLYURETHANE PREPOLYMER HAVING A PH OF BETWEEN ABOUT 6.0 AND ABOUT 9.0. THE INVENTION APPLIES IN PARTICULAR TO THE PAPER INDUSTRY.

Description

The present invention relates to the field of

  improvements in paper products and, in particular,

  to improve the tensile strength

in wet conditions of the paper.

  In the present description, the term "paper"

  will be used to indicate products obtained from aqueous suspensions of cellulose fibers, wood pulp or synthetic fibers and mixtures thereof. The fibrous structure of the paper limits its resistance to the action of water and other liquids, making it unsuitable for many applications. Water releases the bonds that exist between the fibers forming the paper with the result that the mechanical strength of the paper, when it

  is wet, is greatly reduced.

  It is commonly known that the physical properties of a plain paper sheet are considerably diminished when the sheet is wet. Many additives have been used to improve the physical properties of a sheet of paper when it is

wet.

  Isgur et al. In U.S. Patent Application No. 116,287 filed January 28, 1980 and entitled "Polyurethane Polymer Amine Salt as a paper additive",

  reveal an additive that helps to improve certain properties

  physical forms of paper. Among these properties, there is the tensile strength in wet conditions, the

  dry tensile strength, burst resistance

  dry strength, crush strength, tear factor, crease endurance and peak resistance. A method has been discovered to further improve the wet tensile strength of paper by using the amine salt of a polyurethane polymer of the above mentioned application after adjusting the

  pH of this salt by adding a base.

  The present invention relates to a method for improving the wet tensile strength of paper by treating the pulp with an amine salt of a water-curable polyurethane polymer having a high pH level. at a level where no premature gelling or water separation of the amine salt of a polyurethane polymer will occur. The pH of the amine salt of a water-based polyurethane polymer is adjusted by adding a basic solution to raise the pH to about 6.0

and about 9.0.

  The amine salt of a water-based polyurethane polymer for use in the present invention is the subject of U.S. Patent Application No. 034,375 filed April 30, 1979 which is incorporated herein by reference.

reference.

  The amine salt of a polyurethane polymer is made in four basic steps. First, a polyol is reacted with a polyisocyanate to prepare an NCO-terminated prepolymer. The prepolymer is blocked with an oxime in the second step. Thirdly, the oxime-terminated, NCO-terminated prepolymer is reacted with one or more selected polyfunctional amines as will be described hereinafter. The reaction product of amine is then reacted with an acid. In order to obtain a product having suitable properties, it is necessary to use, in the first and / or third step, a reagent having a functionality greater than 2. Thus, the functionality of the NCO-terminated prepolymer plus the functionality of the amine polyfunctional will be

greater than 4.

  The reaction product of polyfunctional amines

  with the oxime-terminated NCO-terminated prepolymer tends to increase in viscosity over time until complete gelation / setting of the product occurs. The gel time and the viscosity of the dispersion of the water-based polyurethane polymer can be controlled and / or adjusted by addition of a secondary amine in the reaction product. The NC0 terminated prepolymer is prepared by reacting a polyoxyalkylene polyol with an excess of a polyisocyanate. A suitable polyisocyanate is toluene diisocyanate. The polyol should have a molecular weight of between about 200 and about 20,000

  and preferably between about 600 and about 6,000.

  The hydroxyl functionality of the polyol and the corresponding isocyanate functionality following the reaction are 2

  at about 8. If the isocyanate functionality of the pre-

  polymer is 2, the functionality of the reactive amine

  in step 3 must be greater than 2. When the function

  isocyanate of the prepolymer is greater than 2, the functionality of the reactive amine in step 3 can

to reach only 2.

  The preferred NCO-terminated prepolymer consists of a mixture of: (1) an isocyanate capped hydrophilic polyoxyethylene diol having an ethylene oxide content of at least 40 mole percent; and (2) an isocyanate capped polyol having a hydroxy functionality of between 3 and 8 before being capped; the isocyanate-capped polyol must be present in an amount of between 2.9 and 50% by weight

of (1) and (2).

  Polyoxyethylene diol is the reaction product of alkylene oxides, at least 40 mol% of which are represented by ethylene oxide, with an initiator such as

  ethylene glycol, propylene glycol, tetra-

  methylene glycol, hexamethylene glycol or mixtures thereof. The molecular weight of the diol is preferably

between 400 and about 6,000.

  Examples of suitable polyols include

  (to capping polyisocyanates): (A) essential polyols;

  linearly formed, for example, by reaction

  of ethylene oxide with ethylene glycol as the initiator

  tor. Mixtures of ethylene oxide with other alkylene oxides may be employed as long as the molar percentage of the ethylene oxide is at least 40%. If the linear polyethers are mixtures of ethylene oxide with other alkylene oxides, such as propylene oxide, the polymer may be a random or block copolymer. A second class of the polyol (B) includes those having a hydroxy functionality of 3 or more. Such polyols are commonly formed by reaction of alkylene oxides with a polyfunctional initiator such as trimethylolpropane, pentaerythritol and the like. To form the polyol B, the alkylene oxide used may be ethylene oxide or ethylene oxide mixtures

  with other alkylene oxides as described above.

  above. Useful polyols are further represented by (C) linear or branched polyfunctional polyols as in A and B above with an initiator or crosslinking agent. A specific example of C is a mixture of polyethylene glycol (molecular weight of about 1,000) with trimethylolpropane, trimethylolethane or glycerol. This mixture can be reacted subsequently

  with excess polyisocyanate to produce a pre-

  polymer useful in the present invention. Alternatively, the linear or branched polyols can be reacted

  (like polyethylene glycol) separately with a poly-

  isocyanate in excess. The initiator such as trimethylolpropane can also be separately reacted with a polyisocyanate. Subsequently, both materials

  capped may be combined to form the prepolymer.

  The polyoxyalkylene polyol is terminated or capped by reaction with a polyisocyanate. The reaction can be carried out in an inert and moisture-free atmosphere, for example under a blanket of nitrogen, at atmospheric pressure with a temperature of from about 0 ° C to about 120 ° C for about 20 hours. The duration

  precise depends on the temperature and the degree of agitation.

  This reaction can also be carried out under atmospheric conditions provided that the product does not

  not exposed to excess moisture.

  The polyoxyalkylene polyol can be capped

  using stoichiometric amounts of the reagents.

  It is preferable to use an excess of isocyanate to ensure that the polyol will be fully capped. Thus, the ratio of isocyanate groups to hydroxyl groups used is from about 2 to about 4 isocyanate groups per hydroxyl group and preferably from about 2 to about 2.5 isocyanate groups per hydroxyl group. For maximum strength, solvent resistance, heat resistance, and the like, isocyanate-capped polyoxyalkylene polyol reaction products are formulated to provide a network.

crosslinked polymer.

  For the second step, all ketoxime is effective.

  Suitable ketoximes include acetone oxime, butanone oxime, cyclohexanone oxime and the like. An oxime based on a relatively volatile ketone is preferred. The preferred oxime is butanone oxime, also known more commonly as methyl ethyl ketoxime. It is also possible to use mixtures of oximes. The proportions of the oxime used can range from about 0.7 to about 1.2 equivalents of isocyanate groups present. A range

  preferred is between 1.05 and 1.15 equivalents.

  To prepare the blocked prepolymer, ketoxime

  and the prepolymer are simply mixed with temperatures

  between 50 and 70 C for about half a

  hour to half past one. A solvent is generally not necessary although solvents such as butyl cellosolve acetate may be employed. Other suitable solvents include materials that do not react with the oxime group or the urethane group. Based on the moles of the oxime and NCO reactive groups involved, the NOH / NCO molar ratio should range from about 0.7 to about 1.2, and preferably from about 1.05 to about 1.15. . In general, it is more efficient to use enough oxime for a complete reaction

with NCO groups.

  To prepare the blocked prepolymer, the ketoxime is selected to provide a product undergoing curing reactions in a reasonable time at a reasonable temperature. Many of the ketoximes and catalysts that are to be employed are described in: Petersen, Liebigs Ann. Chem., 562 (1949), page 215; Wicks, Progress In Organic Coatings, 3 (1975), pp. 73-99; and

  Hill et al., Journal of Paint Tech., 43 (1971), page 55.

  The oximes having the above release temperatures are liquid materials at temperatures of the order

  80 C, and the condensation products with the pre-

  Urethane polymers are either miscible with water or can be dispersed in water using surfactants. In general, oximes are aliphatic, cyclic, straight chain or branched materials

  containing 2 to 8 (preferably 3 to 6) carbon atoms.

  The oxime-blocked NCO terminated prepolymer is reacted with an amine capable of forcing the polymer to cure at a low temperature. Many amines suitable for this step are well known and are called polyfunctional amines. We can mention as

  specific examples of amines, without limitation, ethylene-

  diamine, 1,3-propanediamine, diethylenetriamine,

  triethylenetetramine, iminobispropylamine, tetra-

  ethylenepentamine, methyliminobispropylamine, 2 (2-aminoethylamine) ethanol and polyoxypropyleneamines manufactured by Jefferson Chemical Company, Inc. and sold under the trade names JEFFAMINE (registered trademark) D-400, D-2000 and T-403. These polyoxypropyleneamines are di- and trifunctional primary amines of polyethersaliphatic derivatives derived from adducts

  of propylene oxide of diols and triols.

  As can be observed by the indicated amines, some of the amines can be represented by the general formulas NH 2 -R 'and HO-R' -NH 2, where R 'is a

group C2 to C6.

  Polyfunctional amines having a function

  At least two primary amine terminal groups are preferred amines to ensure proper curing of the subsequently produced polymer. Some of the polyfunctional amines may be represented by the formula H 2 N- (C n H 2 n -N) Z-C n H 2 n -NH 2 where z is an integer of 1 to 4; n is an integer greater than 1; and R is hydrogen, a group

  alkyl of 1 to 4 carbon atoms or a hydroxy group

  alkyl of 1 to 4 carbon atoms.

  Polyoxypropyleneamines may be

  with the formula NH2CH (CH3) CH2 [OCH2CH (CH3)] xH2 ox is greater than 2, and by the formula CH2- [OCH2-CH- (CH3)] x - NH2 CH3CH2- -C H2- [oCH2 CH- ## STR2 ## The sum of x + y + z is approximately 5.3. The molecular weights of these polyoxypropyleneamines are included

  between about 200 and 2,000 or more, polyoxypropylene

  preferred amines having molecular weights of the order

from 400 to 2,000.

  The amount of the polyfunctional amine added to the oxime-terminated, NCO-terminated prepolymer should be in the range of 0.6 to 1.5 equivalents, the preferable range being 0.9 to 1.1 equivalents, based on on the total equivalents of all groups

  isocyanates present in the prepolymer terminated in NCO.

  If the isocyanate functionality of the NCO terminated prepolymer is 2, a polyfunctional amine having a functionality greater than 2 is required to obtain satisfactory crosslinked products. If the isocyanate functionality of the prepolymer-terminated in NCO

  is greater than 2, the functionality of the polyamine amine

  may only reach 2. It will be understood that in the same active system, the total functionality of the

  prepolymer terminated with NCO and amine or polyoxy

  propyleneamines will be greater than 4.

  The reaction between the oxime-blocked prepolymer and the polyfunctional amine is controlled by adding an acid or a mixture of an acid and water before the end of the reaction. If the reaction of the blocked prepolymer with the oxime amine at the right time is not controlled, it may result in an amine reaction product which is too viscous. The appropriate portions of the blocked prepolymer and the amine

  polyfunctional elements are placed in a reaction vessel

  and is reacted under controlled conditions of heating and stirring. The state of the reaction is determined by observing the increase in viscosity. The reaction can

  be performed faster at high temperatures.

  Reaction times may be as low as about 3 minutes at about 95 ° C, 4 minutes at about 80 ° C, and so on. Preferred times of the reaction are from about one-half hour to about one hour with

  temperatures between about 40WC and 600C. One agitates enough

  acid or water-acid mixture in the amine reaction product to lower the pH value to

about 5 or less.

  The cationically stabilized water-based polyurethane polymers are prepared by dispersing the reaction product of amine in water in the presence of sufficient acid to obtain a pH of

  the order of 5 or less. In the preferred method of preparing

  of the waterborne polymers, a concentrated acid solution is added directly to the reaction product

  of amine, they are mixed and then diluted with water.

  However, it is also possible to first add the acid in water and then disperse the reaction product of amine in water. Other additives such as surfactants, ultraviolet absorbers, stabilizers, pigments and the like can be formulated into the original polyurethane polymers.

water according to what is required.

  It was found that if the pH was not controlled within the broad range above indicated, deposition or settling problems were posed and / or portions of the amine reaction product reacted with water to

  form a crust. The pH range is obtained by monitoring

  depositing the amine reaction product with a pH meter (Beckman, Model 96) equipped with a standard combination electrode (Fisher Scientific, 13-639-90) while adding the acid. The addition of the acid is stopped when the desired level of pH is reached. These water-based polymers have been found to be stable for periods of several months at ambient temperatures, i.e. 201C. They also have excellent

  resistance to freeze / thaw cycles.

  While any organic or inorganic acid will form the amine salt and perform the function of controlling the pH value, the acids that have actually been used include glacial acetic acid, acrylic acid, citric acid, ethylene-diaminetetraacetic acid (EDTA), formic acid, glycine (aminoacetic acid),

  hydrochloric acid, lactic acid (alpha-

  hydroxypropinic acid), orthophosphoric acid (H3PO4), phosphorous acid (H3P3), sulphamic acid, sulfuric acid, tartaric acid (dihydroxysuccinic acid),

  paratoluenesulfonic acid and mixtures thereof.

  Some of these acids, for example sulfuric acid, hydrochloric acid and acetic acid tend to discolour the final product. However, additives such as pigments and ultraviolet absorbers can be added in the water-based polyurethane,

  to reduce the tendency to fade by acids.

  A mixture of acetic and phosphoric acids seems

  discolour less than other acids or combinations of acids.

  Phosphoric acid alone gives good color stability. The paper pulp to be treated by this process is dispersed in water to a consistency such that a workpiece can be formed on a metal mesh screen. Suitable paper pulps include leavened and non-leached kraft paper pulp, sulfite from the upper leached and non-leached South Alfa, kraft paper

Hardwood and used newspaper.

  In the amine salt of the water-based polyurethane prepolymer, a sufficient amount of the basic solution is added to raise the pH to a value of

  between 6.0 and 9.0. The preferred range is between 7.0 and 8.3.

  However, if the pH is high above about 10.0, the water-based polyurethane can not improve the wet tensile strength of the

  paper product to obtain; it will actually hurt the resistance

  tensile strength in wet conditions of this paper.

  Suitable basic solutions include aqueous 10 to 30% ammonia solution and 50% aqueous sodium hydroxide solution. Other basic solutions that are also suitable include potassium hydroxide, calcium hydroxide, tetramethyl ammonium hydroxide, diethanolamine, diethylamine, potassium hydroxide, and the like.

  dibutylamine and diethylenetriamine.

  The selected pulp of paper is mixed with water to form a slurry. The concentration of the pulp in the slurry can be between 0.1% and 5% by

based on the weight of the porridge.

  The amine salt of the water-based polyurethane prepolymer whose pH is adjusted is added to the slurry and is thoroughly mixed. The concentration of the amine salt of the polyurethane prepolymer whose pH is adjusted relative to the paper pulp can be between 0.02 and 30 parts by weight. The preferred concentration is 1 part by weight of the amine salt per parts by weight of the paper pulp. The pH of the

  slurry should be between 4.5 and 8.0.

  Sheets are then prepared for testing from the paper-processed pulp in a conventional manner. To make the sheets, a Williams standard sheet molding machine can be used. To demonstrate the effectiveness of the new process, an amine salt of a prepolymer of

typical polyurethane.

  The following process is used to prepare HYPOL (Trade Mark) WB4000, water-based polyurethane prepolymer amine salt manufactured by W. R. Grace

  & Co., Cambridge, Massachusetts, United States of America.

  A preferred isocyanate-terminated polyol prepolymer is prepared by mixing a hydrophilic polyoxyethylene diol having an ethylene oxide content of at least mole% with a polyol having a hydrophilic functionality of between 3 and 8. The concentration of the polyol in the mixture must be between 1% and 20% by weight. A quantity of diisocyanate equal to 1.8-1.9 equivalents of NCO / OH equivalent is reacted with the mixture at a temperature of 0 to 1200 ° C for a time sufficient to

  substantially capping all the hydroxyl groups of the mixture.

  Additional diisocyanate is added to obtain 0.1-0.3 equivalents of NCO per initial equivalent of OH in excess of the theoretical amount necessary to react with

hydroxyl groups.

  To 300 parts of the NCO-terminated polyol prepolymer at 24 ° C. in a stainless steel vessel was added 54.6 parts of butanone oxime with stirring. The reaction of

  the oxime with isocyanate is exothermic and the temperature

  ture rises to 60WC. A hot water bath is used to control the temperature at 801C-901C for about

minutes.

  After about 20 minutes, 29.4 parts of diethylenetriamine are added with stirring. The reaction with the amine is exothermic, thus accelerating the extension

of cat.

  The viscosity continues to increase. When the viscosity has reached 10,000 to 12,000 cps, a solution of portions of glacial acetic acid and 50 parts of ophosphoric acid dissolved in 150 parts of deionized water are added slowly to control the viscosity. After adding the acid solution, the

product is cooled.

  The typical physical properties of the amine salt of the polyurethane prepolymer are shown in the following table:

TABLE A

  Percentage of non-volatile products 53.8 pH 4.3 Viscosity (Brookfield LVF-spindle N 2 at 30 rpm at 26 C) 1980 cps Aspect liquid amber

  The following examples illustrate the invention.

EXAMPLE I

  HYPOL (Trade Mark) WB-4000, manufactured by W. R. Grace & Co, Cambridge, Mass., United States of America, is the amine salt of a prepolymer of

  water-based polyurethane for use in prepa-

  improved paper ration. HYPOL WB-4000 contains 53.8% non-volatile products. Water is added to WB-4000 salt to reduce non-volatile content

  about 35%. A sufficient amount of ammo-

  concentrated in the amine salt of a water-based polyurethane prepolymer (35% non-volatile) until the pH of the salt reaches 6.1. Salt with a pH adjusted is diluted with water to a

  10% level of non-volatile products.

  50 g of non-leached kraft paper pulp is added to 4,000 g of water to form a slurry. 5 cc of the amine salt solution of a pH-adjusted polyurethane prepolymer (10% non-volatile) was added to the slurry of the paper pulp, and the resulting solution was mixed for about 5 minutes. Hand sheets are prepared from the paper pulp treated with a Williams standard sheet molding machine. The leaves are then subjected to 103.4 bars for about 1 minute with a Williams 30.48 cm by 30.48 cm two-post manual hydraulic press. The leaves are then dried for minutes on each side using a Williams standard leaf dryer. The leaves are finally hardened during

  one hour at 1041C in a forced air oven. Eight samples

  lons are cut from each leaf, four from each direction. The samples are 1.25 x 11.43 cm. The cut samples are soaked for 18-24 hours in a 10% solution of AEROSOL OT (sodium dioctyl sulfosuccinate from American Cyanamid Co.). Tensile strengths under wet conditions are performed on an Instron test machine and on an average of eight samples. The process is repeated with the following modifications: (1) the pH of the amine salt of the polyurethane prepolymer is raised to 7.4 with concentrated NH 4 OH; (2) the pH of the amine salt of the polyurethane prepolymer is raised to 7.5 with NaOH; (3) the pH of the amine salt of the polyurethane prepolymer is raised to 8.3 with concentrated NH 4 OH; (4) The pH of the amine salt of the polyurethane prepolymer is raised to 10.0 with concentrated NH 4 OH Table I gives the results obtained when the pH of the amine salt of the polyurethane prepolymer is changed.

TABLE I

  pH of the amine salt of the prepolymer Polyurethane base None

NH40H conc.

NH40H conc.

NaOH

NH40H conc.

NH40H conc.

  , 3 (unadjusted, base not added) 6.1 7.4 7.5 8.3, 0

Resistance to trac-

  under wet conditions (N / cm) 21.9, 4 28.55 28.38 34.0 14.7

EXAMPLE II

  The process of Example I was repeated except

  following: the amine salt of the polyurethane prepolymer was allowed to stand for 3 weeks before adjusting

its pH.

  Table II gives the results obtained when the aging time of the amine salt is varied.

  polyurethane prepolymer before pH adjustment.

TABLE II

  pH of amine salt Age of amine salt Prepolymer resistance of polyurethane pre-polyurethane traction prepolymer before under conditions pH adjustment wet (N / cm) None 5.3 3 weeks 28.38 NH40H 6.1 3 weeks 38.9 None 5.3 1 day 21.9 NH40H 6.2 1 day 26.63

EXAMPLE III

  The procedure of Example I was repeated except for the following: the amine salt of the prepolymer of

  pH-adjusted polyurethane remained at room temperature

  room temperature for 15 days before the sheets of paper are prepared. Waterborne salt

  contained 10% non-volatile products.

  Table III gives the results obtained when aging the amine salt of the prepolymer of

  pH-adjusted polyurethane (10% non-volatile products).

TABLE III

Age of Amine Salt Resistance

  pH of the prepolymer salt of the trac-

  of amine of the polyurethane polymer of pH adjusted (10% of non-volatile polyurethane conditions) wetted Base before formation (N / cm) of a sheet None 5.3 15 days 32.23 NH4OH 6.1 15 days 35 , 91 NH4OH 7.4 15 days 38.54

EXAMPLE IV

  The procedure of Example I was repeated except for the following: the pH of the amine salt of the prepolymer

  polyurethane was adjusted to 8.3 and 10.0.

  Table IV gives the results obtained when the pH of the polyurethane additive is raised to a certain level.

extremely high.

TABLE IV pH of the amine salt Tensile strength of the prepolymer under conditions

  Wet Polyurethane Base (N / cm)

_

None 5.3 38.19 1)

NH40H 8.3 33.99 2)

NH40H 10.0 14.71 2)

  1) the sample containing% non-volatile products was allowed to stand at 221C for 18 days before forming the sheets; 2) the samples were maintained at 400C

before the pH is high.

  On reading the preceding tables, it can be seen that the method described makes it possible to improve the resistance

  traction under wet conditions of the paper.

  Table I shows that simply raising the pH of the salt of the polyurethane prepolymer as an additive improves the tensile strength under wet conditions of the pulp paper that has been treated with the additive. Table II shows that even if the amine salt of the polyurethane prepolymer is allowed to age before its pH is adjusted, there is an improvement in the wet tensile strength of the polyurethane prepolymer.

  paper formed of the pulp treated with the additive.

  Table III shows that even if the amine salt of the pH-adjusted polyurethane prepolymer is allowed to age, there is an improvement in the tensile strength under wet conditions of the paper formed from the

pulp treated with the additive.

  Table IV shows that raising the pH of the amine salt additive of the polyurethane prepolymer

  at too high a value decreases the resistance to trac-

  in wet conditions paper made from pulp

  which has been treated with the additive.

Claims (11)

CLAIMS V E N D I C A T IO N S   1. A process for improving the tensile strength under wet conditions of a paper, characterized in that it comprises the step of treating the paper pulp of which said paper is formed, with an amine salt of a polyurethane prepolymer having a pH of between about 6.0 and about 9.0.   2. A process according to claim 1, characterized in that the amine salt of a polyurethane prepolymer mentioned above is an amine salt of a polyurethane prepolymer waterborne and hardenable.   3. A process according to claim 2, characterized in that the above-mentioned water-curable polyurethane has been prepared in:   (a) reacting an isocyanate prepolymer   cyanate consisting of a mixture of: (1) about 2.9 to about 50% by weight of an isocyanate capped polyol having a hydroxyl functionality of 3 to 8 before being capped, and (2) about 97, 1 to about 50% by weight of an isocyanate-capped hydrophilic polyoxyethylene diol, said diol having an ethylene oxide content of at least mol%, with (b) of the order of 0.8 to 1, 2 equivalents of a ketoxime to form an oxime-blocked prepolymer, (c) reacting said oxime-blocked prepolymer with a polyfunctional amine containing at least two functional groups and capable of forcing the   polyurethane polymers to be cured at low temperatures   to form an amine reaction product, (d) reacting said amine product with a   acid to form an amine salt of a poly-   low temperature, infinitely dilutable urethane urethane in water, and (e) adjusting the pH of said amine salt of a polyurethane prepolymer to about 6.0 to about 9.0 with a base.   4. Process according to claim 3, characterized   in that the aforementioned base is NH40H.   5. A process according to claim 3, characterized in that the aforementioned base is selected from the group consisting of sodium hydroxide, calcium hydroxide, tetramethyl ammonium hydroxide, diethanolamine, diethylamine,   dibutylamine and diethylenetriamine.   6. A method for improving the tensile strength under wet conditions of paper, characterized in that it comprises the steps of: (1) preparing a slurry of paper pulp (2) treating said pulp of paper with a salt of paper amine of a polyurethane prepolymer having a pH of from about 6.0 to about 9.0, and   (3) form paper of the treated pulp.   7. A process according to claim 6, characterized in that the amine salt of the polyurethane prepolymer   aforementioned is a curable polyurethane waterborne.   8. A process according to claim 7, characterized in that the amine salt of the curable water-based polyurethane prepolymer is prepared by: (a) reacting an isocyanate-capped prepolymer consisting of a mixture of: 1) about 2.9 to 50% by weight of an isocyanate-capped polyol having a hydroxyl functionality of 3 to 8 before being capped, and (2) about 97.1 to about 50% by weight of an isocyanate capped hydrophilic polyoxyethylene diol, said diol having an ethylene oxide content of at least 40 mole%, with (b) 0.8 to 1.2 equivalents of a ketoxime to form a prepolymer blocked at oxime, (c) reacting said oxime-blocked prepolymer with a polyfunctional amine containing at least two functional groups and capable of forcing the polyurethane polymers to cure at low temperatures to form an amine reaction product, (d) ) reacting said amine product with a   acid to form an amine salt of a poly-   low temperature, infinitely dilutable urethane urethane in water, and (e) adjusting the pH of said amine salt of a polyurethane prepolymer to about 6.0 to about 9.0 with a base.   9. A process according to claim 6, characterized in that the amine salt of a polyurethane prepolymer mentioned above has been pretreated with a base to raise its pH. at about 6.0 to about 9.0.   10. The process according to claim 9,   characterized in that the aforesaid base is NH40H.   11. A process according to claim 9, characterized in that the abovementioned base is chosen from the group consisting of sodium hydroxide, calcium hydroxide, tetramethyl ammonium hydroxide, diethanolamine,   diethylamine, dibutylamine and diethylenetriamine.