US2680682A

US2680682A - Paper of improved wet strength

Info

Publication number: US2680682A
Application number: US140746A
Authority: US
Inventors: William C Dearing
Original assignee: Allied Chemical and Dye Corp
Current assignee: Allied Corp
Priority date: 1950-01-26
Filing date: 1950-01-26
Publication date: 1954-06-08
Anticipated expiration: 1971-06-08

Description

paper.

Patented June 8, 1954 UNITED STAT PAPER F IMPROVED WET STRENGTH New York No Drawing. Application J anuary'26, 1950, Serial No. 140,746

4 Claims.

The invention relates to paper which contains a synthetic resin that imparts wet strength, and which is superior to the synthetic resin impregnated paper heretofore known.

When used for imparting wet strength to paper, a synthetic resin is usually incorporated at the wet end of the paper making process, for example, in the beater or at the head box. A synthetic resin that is incorporated at the wet end of the paper making process must be capable of dilution without recipitation of the resin, and must have an afiinity for the paper fibers so that a reasonably large proportion of the resin deposits on the paper fibers and so that an unreasonably large proportion of the resin is not lost in the waste water.

The principal object of the invention is to provide paper comprising a synthetic resin that has the special properties necessary to permit it to be incorporated at the wet end of the paper making process and also imparts substantially greater wet strength per unit of cost than the resins heretofore used for the wet-strengthening of More specific objects and advantages are apparent from the description, which illustrates and discloses but is not intended to limit the scope of the invention.

The present invention is based upon the discovery that paper has wet strength superior to that heretofore known if it contains a product of the reaction of formaldehyde, a substituted aliphatic carboxylic acid whose molecule has not more than eight carbon atoms and has not more than four carbon atoms per carboxy group, and a substance of the class consisting of urea and melamine, the monovalent substituents in said reaction product that are derived from said reactants consisting of NH2, OH and COOH groups.

The formaldehyde used in producing paper embodying the invention may be wholly or partially in the form of one of its polymers such as paraformaldehyde, but preferably is in the form of an aqueous solution (as hereinafter described).

The term resin former is used herein to refer to a substance of the class consisting of urea and melamine. The resin former may consist of either urea or melamine or a combination of both in any desired proportions. Urea is the preferred resin former, especially since it is less expensive to use than melamine.

The substituted carboxylic acid used in the practice of the present invention may be any sub- :stance whose molecule consists of a substituted straight or branched hydrocarbon chain having not more than eight carbon atoms, in which the substituents include at least one carboxy group and at least one hydroxy or amino group, each attached to a primary, secondary, or tertiary carbon atom, said molecule having no substituents other than carboxy, hydroxy or amino groups and having not more than four carbon atoms per carboxy group, i. e., having from one to three additional carbon atoms per carboxy group. Preferably, the substituents are attached to primary or secondary carbon atoms. Such substances include: glycolic acid, lactic acid, glycine, alanine, alpha-amino butyric acid, alpha-amino iscbutyric acid, aspartic acid, glutamic acid, bydroxyglutaric acid, hydroxyglutamic acid, hydroxybutyric acid, malic acid, serine, tartaric acid, and threonine. For the sake of brevity the term substituted carboxylic acid will be used hereinafter to designate such a substance.

The paper pulp which may be used in the practice of the invention may be of any type, such as bleached or unbleached K ra-ft, sulfite, or ground wood pulp. I

In the practice of the invention when the resin former is urea, not inore than about .2 mols of formaldehyde should be used per mol of urea, and it is preferable to use not more than about 2.15 mols per mol of urea. Not less than about 1.8 mols of formaldehyde should be used per mol of urea and it is preferable to use not less than about 1.9 mols per mol of urea. It is most desirable to use about 2.12 mols of formaldehyde per mol of urea. When the resin former is melamine at least 4 mols of formaldehyde should be used per mol of melamine, and not more than about '7 mols of formaldehyde per mol of melamine should be used. It is preferable to use at least about 6 mols of formaldehyde per mol of melamine and most desirable to use about 6% mols of formaldehyde per mol of melamine.

In the practice of the invention not less than .05 mol of substituted carboxylic acid should be used per mol of urea. It is preferable that the amount of substituted carboxylic acid used be not less than about .15 mol per mol of urea. It is preferable to use not more than about .25 mol of substituted carboxylic acid per mol of urea, and most desirable to use about .2 mol of substituted carboxylic acid per mol of urea. Increasing the amount of substituted carboxylic acid'to as much as .5 mol per mol of urea tends to decrease the degree of wet strength imparted to paper embodying the invention.

It is preferred that wet-strengthened paper embodying the invention comprise a product of the reaction of formaldehyde, a resin former and a substituted carboxylic acid in which carboxy radicals are neutralized with an alkali metal base (as hereinafter explained).

The term alkali metal base is used herein to mean any alkali metal compound that gives an alkaline solution, the preferred alkali metals being sodium and potassium. Such alkali metal bases include sodium hydroxide, potassium hydroxide, sodium alcoholates such as sodium methoxide, sodium ethoxide, and sodium beta-methoxy ethoxide (obtained simply by dissolving metallic sodium in methanol, ethanol or beta-methoxy ethanol, respectively), potassium carbonate, sodium tetraborate, potassium tetraborate, and sodium bicarbonate. The preferred alkali metal bases are sodium hydroxide and potassium hydroxide. Sodium hydroxide is more desirable from the standpoint of economy.

It is desirable that wet-strengthened paper embodying the invention comprise a product of the reaction of formaldehyde, a resin former, and a substituted carboxylic acid which has two carbon atoms (for example, glycolic acid, glycine, or serine), in which carboxy radicals are neutralized with an alkali metal base. In the practice of the invention the preferred substituted carboxylic acid having two carbon atoms is glycolic acid.

Thus the most desirable finished article embodying the invention consists of wet-strengthened paper impregnated with a product of the reaction of formaldehyde, urea, glycolic acid and sodium hydroxide.

Wet-strengthened paper is extremely useful in all paper products which may come in contact with water or moisture during use, for example toweling, food product wrappers, bag and wrapping paper, and map and blue print paper.

A synthetic resin for imparting wet strength to paper is desirably incorporated at the wet end of the paper making process before the paper is made. This is known as wet end addition, the term referring not only to addition in the beater but also to addition in the machine chest, head box, fan pump or any other desired point at the wet end of the paper making process. This more convenient and less expensive method of applying the resin to the paper results in a more porous paper which is not coated with a sealing. Since in the production of Wet strength paper, the mixture at the point of resin addition ordinarily comprises a very dilute suspension of pulp in water (less than two percent) and a synthetic resin used for imparting wet strength is usually present in this suspension in a concentration of about one to two per cent of the pulp concentration, the resin must be capable of dilution without precipitation. Such a resin should be a thermosetting composition so that it can b added in its soluble stage to disperse and dissolve throughout the paper pulp suspension at the wet end of the paper making process before the paper is made, and then can be converted to a thermoset resin on the paper fibers by heating during drying or aging during storage.

Although theoretically ordinary urea-formaldehyde (and melamine-formaldehyde) resins are soluble in the concentration range required during wet end addition, in actual practice these thermosetting resins form curds when incorporated at the wet end under the slightly acid conditions used, before they have had time to be dispersed and dissolved in the water. The curds cannot be readily dissolved, and stick to the apparatus so that much of the resin is deposited on the walls of the apparatus or lost in the Waste water. (The formation of curds necessitates frequent cleaning of the apparatus and leads to serious dimculties, because under the acid conditions used the curds, after depositing on the equipment are converted to the insoluble state.) The small amount of resin that does cling to the paper fibers does not coat them uniformly. Such a resin makes the paper non-absorbent and is undesirable for use in imparting wet strength to paper.

Since urea-formaldehyde (and melamineformaldehyde) resins when used as paper treating resins to impart a high degree of wet strength must be sufficiently water soluble to dissolve so rapidly that curds do not form, it is necessary to use a modifying agent which gives a high degree of water solubility to such resins. It has been found that this is accomplished by the presence of an electrolytic group attached to the resin molecule, which gives it an electrical charge when in solution. The substituted carboxylic acid, or a salt thereof, used in the practice of the present invention becomes part of the resin through its amine or hydroxy substituent. The ionization in solution of the substituted carboxylic acid which is thus a part of the resin molecule imparts a negative electric charge to the molecule. Such a soluble anionic resin imparts wet strength to to paper when it is converted to a thermoset resin on the paper fibers by heating during drying or aging during storage. It is believed that the presence of the solubilizing groups in the resin molecules on the fiber also imparts greater absorbency to the paper since the soluble resin is less water repellent and hence facilitates passage of water through the thin layer of resin on the fiber. The known sodium :bisulfite modified resins do not have as good water solubility and do not impart as high a degree of wet strength to paper per pound of resin used as the synthetic resins used in the practice of the present invention (as is hereinafter demonstrated).

Sodium bisulfite may be used along with the substituted carboxylic acid in the practice of the invention. Such a modifying agent should not be present in a concentration greater than about 30 per cent of the substituted carboxylic acid, since its use in larger amounts gives a paper having lower wet strength than the paper produced by using only the substituted carboxylic acid. (The term per cent as used herein to refer to quantities of material means per cent by weight unless otherwise qualified.)

The reaction mixture in the preparation of a resin for use in the present invention usually contains formaldehyde in the form of an ordinary commercial formaldehyde solution. Such a solution is in itself highly acid so that the pH of the reaction mixture containing also the substituted carboxylic acid is quite low. Since the reaction of formaldehyde and a resin former tends to go too rapidly in a strongly acid solution so that the product coagulates, and since the reaction does not proceed satisfactorily if the solution is strongly alkaline, it is desirable that the pH of the reaction mixture be adjusted within the range 5.2 to 5.6. Varying the reaction pH between 5.2 and 5.6 and varying the reaction temperature has no substantial effect in general on the properties of resins reacted to the same degree of condensation. Since, however, undesirable by-products form within this pH range at temperatures below around degrees C. it is desirable to lower the pH to within the range from 5.2 to 5.6 only after the temperature of the reaction mixture reaches about 95 degrees C., to avoid turbidity in the final product. (Although formation of an insoluble by-product has no effect on the wet-strengthened paper made from filtered resin, the presence of a precipitate makes colorimetric pH control during the resin preparation very difficult.) It is, therefore, most desirable to raise the initial pH with a base within a range of about 6.5 to 7.0 and to maintain the pH of the mixture in approximately this range until the temperature is about 95 degrees C.

It is preferable to use a strong base such an alkali metal base (as hereinbefore defined). Such a base will also neutralize part or all of the substituted carboxylic acid. At the time of reaction it is preferred that from 20 to' 100 mol per cent of the substituted carboxylic acid taking part in the reaction be in the form of a metal salt that is water soluble and does not interfere in the reaction. The product of such a reaction may be used in the production of wet-strengthened paper embodying the invention because the salt is large- 1y ionized in the aqueous reaction medium, and some of the anions produced by such ionization combine with hydrogen ions to form molecules of the carboxylic acid so that some of the molecules taking part in the reaction are molecules of the free carboxylic acid. If the water present during the reaction is removed by drying the reaction product, and if the cations from the metal base are present in an amount sufficient to combine with all of the carboxy groups in the molecules of the reaction product, the ionization which tool; place when the alkali metal salt dissolved in the aqueous reaction medium is reversed during the drying of the reaction product and all the carboxy groups in the dried reaction product are neutralized by the metal base.

In the preparation of a resin for use in the practice of the present invention the reaction medium usually consists only of water, although it may contain small amounts of a solvent such as methyl alcohol or ethyl alcohol. Usually the water is that present in the commercial aqueous formaldehyde solution ordinarily employed. The components may be mixed in any desired order. It is usually desirable to neutralize the carboxylic acid first and then to add the formaldehyde solution, urea and sufiicient base to adjust the pH of the mixture to 6.5-7.0. The mixture is then either heated for one hour at 60 degrees C. or allowed to stand overnight. However, resins imparting equally good wet strengthening properties may be obtained without this much preliminary reaction. It is necessary merely to allow enough time for the components to dissolve before reacting the mixture.

It is believed that during this preliminary reaction stage one molecule of formaldehyde combines with one molecule of water to form methylene glycol, and it is believed to be the methylene glycol that actually takes part in the reaction with a substance such as urea, so that a methylol group is formed by the condensation of a methylene glycol molecule with an Nl-lz group in the urea molecule; one molecule of water being eliminated in such condensation:

HCHO H2O canon OH H 0 on H o Thus in a molecule formed by the reaction of formaldehyde and urea the only monovalent substituents are hydroxy groups and amino groups.

Similarly, the only monovalent substituents in the molecules of the substittued carboxylic acids that are used in the practice of the invention consist of hydroxy or amino groups and carboxy groups. It is believed that a hydroxy or amino group in the substituted carboxylic acid condenses with a hydroxy group in the resin-molecule.

Another procedure comprises preliminarily reacting the neutralized substituted carboxylic acid with the formaldehyde solution for about two hours, prior to the addition of the resin former and sufiicient base to adjust the pH to the proper range. This procedure is essential when melamine is used as the resin former in order to obtain a resin which will develop wet strength since melamine reacts more rapidly than urea and time must be allowed for the substituted carboxylic acid to react with the formaldehyde.

Still another procedure for preparing a synthetic resin for use in the present invention comprises adjusting the mixture of urea and formaldehyde solution to within the proper pH range and reacting for about 45 minutes prior to the addition of the neutralized substituted carboxylic acid.

The degree of water solubility of resins used in the practice of the present invention is dependent upon the number of solubilizing groups" which become part of the resin molecule. Since increasing the amount of substituted carboxylic acid beyond the desirable limits hereinbefore described tends to decrease the degree of wet strength imparted to paper, it is not desirable to improve the water solubility by so increasing the amount of substituted carboxylic acid.

The synthetic resins ordinarily used in the present invention to prepare paper superior to the synthetic resin-impregnated paper heretofore known are reacted in the molar proportions hereinbefore described in an aqueous reaction medium derived from a commercial formaldehyde solution. An ordinary commercial formaldehyde solution usually consists of 37 per cent formaldehyde and 67 per cent water. The only other source of water in the reaction mixture is that present, if any, in the substituted carboxylic acid, which may contain about 30 per cent of water. Further dilution during the reaction is undesirable, since it results in resins of decreased stability. It has been discovered that less dilution during the reaction, i. e., reacting at a higher solids concentration than is achieved using an ordinary commercial formeldehyde solution, results in resins having a greater degree of water solubility, since there is then a tendency for more of the solubilizing groups to become part of the resin molecule. Such resins impart higher wet strength to paper and, when diluted after the reaction as hereinafter described, have increased stability. The solids concentration of the reaction mixture may be increased suiilciently to yield resins having improved water solubility by using the neutralized substituted carboxylic acid. in the form of its crystalline salt. For example, use of crystaline sodium glycolate results in a more soluble resin than is obtained with an equivalent amount of a solution of '70 per cent glycolic acid neutralized with flake caustic. Even greater improvement in the water solubility of resins used in the practice of the invention is obtained if the concentration of solids in the resin reaction mixture is increased by using a formaldehyde solution containing a greater concentration of formaldehyde than 3'7 per cent. It is desirable to use an aqueous solution containing a concentration of 45 to 50 per cent of formaldehyde, since such use of a concentrated formaldehyde solution gives substantially improved properties economically and efilciently. If desired sufilcient paraformaldehyde may be dissolved in a 37 per cent aqueous formaldehyde solution to raise the concentration of formaldehyde to 45 or 50 per cent.

In general, more highly condensed resins impart better wet-strength to paper. However, the increase in wet strength imparted by a given resin may be inappreciable beyond a certain degree of condensation (viscosity), and since increased condensation tends to decrease both the stability and the water solubility of the resin, the reaction should be terminated when that viscosity has been reached. The viscosity of resins reacted to the same degree of condensation will, of course, difier in accordance with the solids concentration of the reaction mixtures. Resins used in the practice of the present invention in which the proportions of resin reactants are within the limits hereinbefore given and in which the concentration of formaldehyde in aqueous solution is approximately (a) 3'7 per cent, (1)) 50 per cent or (c) 45 per cent are reacted to a desirable degree of condensation as follows:

(a) The pH of the reaction mixture is initially adjusted to 6.5-7.0, the mixture is heated to a temperature of about 95 degrees C. before lowering the pH to 5.2-5.6, and the reaction is continued at this temperature until the viscosity of the resin solution is about 2'? seconds (measured by Ford cup, inch opening, at 25 degrees C.). The reaction usually requires about eight to nine hours when the pH is about 5.6, or from two to six hours when the pH is within the range 5.3 to 5.45. The mixture is then cooled to 60 degrees C. and held at this temperature until the viscosity of the solution is 32 to 34 seconds (Ford cup), the pH being adjusted to 5.6 at the beginning of this period. This latter reaction stage usually requires from one to three hours.

(b) The pl-l of the reaction mixture is initially adjusted to 6.57.0, and the mixture is heated to a temperature of 95 degrees 0., two subsequent pH adjustments being made, one to 7 .5 at 50 de grees C. and the other to 7.2 at 75 degrees C'. to avoid precipitate formation. It is desirable that the temperature rise from 65 degrees C. to 95 degrees C. in less than 45 minutes. The mixture is held at 95 degrees C. for five minutes before lowering the pH to about 5.6, and the solution is held at this temperature and pH until the viscosity of the resin solution is J (measured by Gardner-Holdt Bubble Viscometer standard method, the Ford cup viscosity method becoming increasingly inaccurate at the much higher viscosity encountered using the more concentrated reaction mixture). The mixture is then rapidly cooled to 60 degrees C. (about 10 minutes cooling time) and held at this temperature until the viscosity of the solution is VW (Gardner- I-Ioldt) (c) The resin is prepared by the procedure described in (5) except that the viscosity of the solution after the 95 degree stage should be H-- I and the final viscosity after the 60 degree stage should be U-V (Gardner-Holdt).

Using a two-stage reaction (that is, heating first at 95 degrees C. to a certain viscosity and then reaching the final viscosity by reacting at 60 degrees C.) ordinarily makes the reaction more controllable and gives more reproducible results than a one-stage reaction carried to the same final viscosity at degrees C., although there is no essential difference in the properties of the final product. Usually, the total reaction time must be at least three to four hours in order to allow sufiicient time for the viscosity measurements and the temperature and pH adjustments that are necessary for safe control.

The wet strength of paper embodying the invention is increased when the paper is treated With a resin which has been aged at room temperature or even lower temperatures for as long a period of time as the resin remains stable. It is desirable to neutralize the liquid resin with a base such as sodium hydroxide to a pH of at least 7.0 and most desirable to adjust the pH to the range 7.8 to 8.0, for greater stability of the resin. The pH of the resin solution decreases on standing, the decrease being more rapid at higher temperatures. The decreasing pH of the resin solution is probably at least partially due to the formation of formic acid through air oxidation (and Cannizzaro type reaction) of the free formaldehyde usually present. Thereiore, a buffer such as sodium bicarbonate or borax (0.5 per cent based on the weight of solids in the resin) or sodium borate (0.2 to 0.3 per cent based on the weight of solids in the resin) is usually added to retard the rate of pH drop. Resins prepared using a more concentrated reaction mixture are most desirably adjusted to a pH of 8.0 and buffered with 2 per cent (based on the weight of solids in the resin) of a buffer mixture containing 65 per cent boric acid and 35 per cent borax.

Resins reacted according to the procedure described in (a) ordinarily contain about 48 to 50 per cent solids, and those reacted according to procedures (19) and (c) ordinarily contain about 51 to 55 per cent solids. The stability of resins used in the practice of the invention may be further improved by diluting the resin to a concentration of about 45 per cent solids. Resins prepared as described herein and spray-dried remain stable over periods even longer than one year.

In the production of wet-strengthened paper of the invention the quantity of resin used is within the range ordinarily used in the art of making wet-strengthened paper and varies with the degree of wet strength desired. For example, the minimum desirable concentration of resin is the smallest concentration that gives an appreciable increase in the wet strength of the paper (i. 6., about 0.1 per cent resin based on the weight of dry pulp). The maximum desirable concentration of resin (i. e., about 10 per cent based on the weight of dry pulp) is that above which the wet strength imparted to paper is not increased enough to make a larger concentration of resin economical for most applications. A concentration between about 0.25 and about 5 per cent resin ordinarily gives high wet strength economically and efiiciently. The resin may be added to the pulp at the wet end of the paper machine as prepared (i. e., as an approximately 50 per cent water solution) or, if desired, it may be diluted to about 20 per cent solids, preferably with water at a temperature above 50 degrees F. It is desirable to add a catalyst such as slum or aluminum chloride about five minutes before the resin addition. The resin and catalyst may stand on the pulp for a few hours before the mixture is used for making paper. Such contact time should not, however, approach as long a period as 24 hours since a portion of the wet strength is lost under such conditions. The pH of the mixture should be adjusted 9 within a range of 4.0 to 5.5, the pH being lowered to this range using, for example, dilute sulfuric acid. It is most desirable that the pH of the mixture be about 4.5

The wet strength of the resin-treated paper of the invention is affected when the pH of the pulp suspension prior to the addition of resin is varied. The hardness in the water used to dilute the pulp suspension is another factor which affects wet strength.

The magnitude of the improvement in Wet strength of paper embodying the invention over previously known wet-strengthened paper may be demonstrated by tests carried out as follows:

,A resin former (240 grams of urea), methanol-free formalin (713 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) and sodium glycolate containing onehalf molecule of water of crystallization (78.5 grams prepared by reacting 82 grams of purified 70 per cent hydroxy acetic acid with 28.75 grams of solid sodium hydroxide at a temperature ranging from 70 to 80 degrees C. to a pH of 7.0, filtering the hot mixture and precipitatin th sodium lycolate from dilute alcohol solution with molecule of water) are mixed together in a 1 liter three-necked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals. The DH of the reaction mixture is raised :to 7.2 by the addition of sodium hydroxide. The mixture allowed to .stand overnight and is then heated on a boiling water .bath to a temperature of about 95 degrees C. The pH is adjusted to 5.5 bytheadd tion of formic acid, and the solution is held for eight and three-quarters hours at .95 degrees C. The resin is allowed to stand overni ht and then held at 60 degrees C. for five hours, at a pH of 5.6 before cooling to room temperature and neutralizing to a pH of 7.0. This resin is hereinafter referred to as Resin K.

Anhydrous sodium 'bisulfite (167 grams) is dissolved in methanol-free formalin (1960 grams of .a solution consisting of 37 per cent formaldehyde and 63 per cent water) A resin former (660grams of urea) and sodium acetate (4.5 grams) are dissolved in this solution in a 3 liter three-necked flask fitted as described above. The solution is allowed to stand overnight at room temperature. The solution is then heated on a boiling water bath for about eight and one-half hours ata temperature ranging from 94.5 degrees C. to 97.5 degrees .C. (The buffer action of the sodium acetate keeps the pH at approximately 5.6 during the latter part of this heating period.) The mixture is allowed to stand overnight and is then held at a temperature of 60 degrees C. and at a pH of 5.6 for five hours before cooling to room temperature. This resin is used .as a control in the wetstrength tests and is hereinafter referred to as resin B.

A beaten pulp suspension is prepared and treated with samples of resins K and B by the procedure hereinafter described. The results of wet strength tests .on sheets made from the resintreated beaten pulp suspensions are shown in Table 1, in which the headings in columns .2

through 8 indicate the time at which the sample of resin for treating the pulp suspension is withdrawn during the resin preparation.

Elllp (containin the equivalent of 360 grams of oven-dried pulp) is soaked in water (10 lite-rs) overnight. The soaked pulp is then agitated for 1.0 minutes with pa Lightnin mixer (a high-speed motor-driven stirrer). The agitated suspension is then placed in a Valley beater (a standard beater designed for laboratory use) and enough water is added to bring the total volume of water to 23 liters (measured at a temperature of 25 degrees 0.). The beater is run for five minutes (slush period) before a load (4500 grams) is placed on the lever arm which applies a force to the beater roll. Samp es are with w a ious intervals during the beating to measure the rate .at which water passes through the pulp (freeness) as Schopper freeness. The freeness of an 800 m1. sample after twenty minutes beating time is about 700 and that for an 800 ml. sample after about thirty minutes beating time is 550. The beating is terminated after thirty minutes.

The beaten pulp is diluted to such an extent that a volume of approximately 300 m1. gives a dry sheet weighing 2.0 grams. The pH is adjusted to 6.5 by the addition of sulfuric acid. Alum (a fresh solution of 17.75 grams of anhydrous aluminum sulfate in about cc. of water) is added with stirring to the beaten pulp suspension which is then ready for the addition of the resin for imparting wet strength. The beaten pulp suspension is allowed to stand for five minutes before a resin for imparting wet strength (an amount of a resin solution suiiicient to give 3 per cent based on the weight of the dry pulp) is added. A volume of stock large enough to give a sheet of the desired 2.0 gram weight 800 m1.) is placed in the sheet machine and diluted to a total volume of 10.7 liters, and the pH is adjusted to 4.5 by addition of sulfuric acid. The handsheet is made within five minutes after the addition of the resin and the Operation is repeated four times without delay, to make four more sheets.

The handsheets are made according to Institute of Paper Chemistry-Tentative Method 411- B-Valleyf The sheets are pressed separately between six blotters under a pressure of pounds for two minutes. Each sheet is placed on the drier while still in contact with one blotter (sheet against the metal) and dried for five minutes at 250 degrees F. The sheets are conditioned for a 1 minimum of eight hours in a room at a temperature of 75 degrees F., and at 7-8 per cent relative humidity before testing.

Wet strength measurements are made on the .handsheets with a Mullen tester which measures the bursting pressure, expressed as points (approximately pounds per square inch) for a stand.- ardized circular area. Bursting strength of the paper is given herein as a burst factor, that is, points per 100 pounds of basis weight (basis weight is the weight of 500 sheets of the paper, 25 inches by 40 inches). Mullen wet burst values are obtained on paper samples wet with water from a brush (equivalent to about a ten second soak. of the paper samples).

The great improvement in the wet strength of paper embodying the invention (paper containing resin K) over previously known synthetic resin impregnated paper (paper containing resin B) is readily apparent. Not only is the maximum Mullen wetburst achieved using resin K much higher 11 than that with the control resin, resin B, but it is also obtained after a shorter reaction period.

Tests made as follows demonstrate further improvement in the degree of wet strength imparted to paper in the practice of the present invention. Results of these tests are shown in Tables 2 through 5.

The reaction period for the preparation of a resin such as K may be shortened even more by lowering the pI-Iof the reaction mixture during the first stage of the heating (1. e., during the heating at 95 degrees 0.), the resulting resin imparting slightly better wet strength than the resin K reacted at a pH of 5.6. The preliminary stage in which the resin is allowed to stand overnight for the formation of dimethylol urea may be eliminated, and it is not necessary for the resin to stand overnight before the heating period at 69 degrees C.

Table 2 shows the results of varying the pH of the resin reaction mixture during the initial heating period at 95 degrees C. Resin K1 is reacted at a pH of 5.45 for six hours at 95 degrees C. (the pH being lowered by the addition of formic acid) and resin K2 is reacted for three and one-half hours at a pH of 5.3 (the pH being lowered by the addition of formic acid). Both resin K1 and resin K2 are then held at 60 degrees C. at a pH. of 5.6 in the same manner as resin K, samples being withdrawn at various time intervals and used to treat paper prepared and tested as hereinbefore described.

TABLE 2 Mullen Wet Burst-Hours held at 60 degrees C.

The good wet strength obtained by using resin K to treat paper is surpassed by using resin K1 or resin K2, 1. e., using a resin reacted at a pH of around 5.3 or 5.4 instead of 5.6 during the first reaction stage. Maximum Mullen burst is reached when the resins are heated during the second stage at 60 degrees C. for about two hours, the decreasing Mullen burst after the longer heating periods probably being due to the fact that the resin becomes slightly less soluble upon increased condensation.

Considerable development of wet strength occurs during the normal drying of the paper, due to the rapid rate of cure of resins used in the present invention. Complete development of the wet strength can be accomplished by additional drying of the paper or by storing the resin for a few days at Warm temperatures. The following table shows the effect of aging a resin such as K1, the Mullen wet burst being measured on paper prepared as previously described but dried for only one minute at 250 degrees F. and conditioned for eight hours at 120 degrees F. at low humidity. From the table may be seen. also the increase in wet strength imparted to paper when the catalyst for precipitating the resin is aluminum chloride in place of alum. The last value in columns two and three is obtained using resin treated paper dried for an additional four minutes at 250 degrees F. and conditioned for eight hours at 120 degrees F., instead of using the aged resin.

TABLE 3' Mullen Mullen Wet Burst 5 Days Wet Burst (6% alumi- (6% alum) nunicfhlor- 10 5 min. drying of the paper at 250 degrees Table 4 shows the effect on wet strength of paper embodying the invention when treated with resins prepared using various percentages of sodium glycolate (based on the weight of urea) with two different molar ratios of formaldehyde to urea. All resins are prepared by a procedure which is the same as that described in the preparation of resin K except that resins (1) and (3) are reacted for eight and threequarters hours at 92-965 degrees C., pH 5.6 and for three additional hours at 60 degrees C., pH 5.6; resin (2) for three and one-half hours at 95 degrees C., pH 5.3, two hours at 60 degrees C.,

pI-l 5.6; (4) two hours at 95-99 degrees C., pH 5.3, one hour at 60 degrees C., pl-I 5.6; (5) two hours at 95-99 degrees C., pH 5.3, two hours at 60 degrees C., pH 5.6; (6) two and three-quarters hours at 94-98 degrees C., pH 5.4, one hour at 3 60 degrees C., pI-I 5.6; ('7) two and one-quarter hours at 95-975 degrees C., pH 5.4, one hour at 60 degrees C., pH 5.6. The paper is prepared and tested for Mullen burst as described hereinbefore except that the Mullen burst tests for (3) are made at a temperature of 72 degrees F., 7''! per cent relative humidity.

TABLE 4 Percent Ratio For- 40 Resin Sodium melriehyde Glycolate to Urea It is evident from Table 4 that the best wet strength using a formaldehyde to urea molar ratio of 2.2 to 1 is obtained by using about 30 per cent (based on the weight of urea) of sodium glycolate. The wet strength is increased by using the more desirable formaldehyde to urea molar ratio of 2.12 to 1.

As hereinbefore mentioned, use of excessive amounts of a modifying agent such as sodium glycolate in a urea-formaldehyde resin tends to decrease the degree of wet strength imparted to paper by the resin. The Mullen wet burst value obtained using 34.8 per cent of sodium glycolate is essentialy no higher than that obtained by using the resin prepared with 25.19 per cent of sodium glycolate. Similarly, al-

though a resin prepared with a formaldehyde to urea molar ratio of 2.12 to 1 using 52.5 per cent of sodium glycolate imparts Mullen burst as high as 3%.2, greater wet strength can be obtained with a much smaller percentage of sodium glycolate such as 33.5 per cent or 31.4 per cent.

Table 5 shows the efiect of varying the concentration of alum and resin in the preparation of wet strengthened paper. The paper treated is prepared and tested by the procedure hereinstis before described, the pH indicated being that of the pulp adjusted with sulfuric acid before the addition of alum and resin. It is seen from the table that it is most desirable to use about 3 per cent of alum and 3 per cent of resin (based on the weight of dry pulp) when the pulp is at a pH of about 6.5 to 7.0. (The resin is one prepared by a procedure similar to that described for resin K).

TABLE Mullen Percent Percent Mullen Burst Alum Rosin f f (pH 6.5-7.0)

5 5 8. 5 8. 5 1 5 9. 5 9. 2 3 5 10.1 9. 4 6 .5 9.6 8. 5 5 3 l3. 2 l3. 9 l 3 l8. 5 22. 0 3 3 25. 8 27. 7 6 3 25. 7 25. 2

In the practice of the invention resins prepared using urea as the resin former are preterred to resins in which melamine is used not only because urea resins impart higher wet strength to paper but also because they are cheaper to use than melamine resins. -t may be desirable, however, in some cases to replace some or all of the urea with melamine in order to obtain certain advantages characteristic of melamine resins. For example, sodium glycolate melamine resin is superior to sodium glycolate urea resin when it is desired to obtain wetstrengthened paper having high humidity endurance. This may be demonstrated by tests carried out as follows:

Glycolic acid (763 grams in 32.7 cc. of water) is mixed with flake caustic (45 grams). Methanol-free formalin (242 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent Water) is heated with this mixture in a 3 liter three-necked flask fitted with a ther- -mometer, reflux condenser, stirring rod and oil seals, over a boiling water bath for two hours at a temperature of 95 degrees C. The pH of the mixture is 6.0. At the end of this heating period melamine (63 grams) and sodium bicarbonate 1.68 grams) are added to the mixture, which is then held for twenty minutes at a temperature of 7.0 degrees C. The pH of the mixture is 7.5. Additional melamine (63 grams) and methanolfree formalin (283 grams of a solution consisting of 37 per cent formaldehyde and as per cent water) are then added, and the mixture is held for one and three-quarters hours at temperatures ranging from 90 to 95 degrees C.

A sodium glycolate-urea resin in which ten per cent of the urea by weight is replaced with melamine is prepared by the procedure described for resin K2, the melamine being added after about two hours of the heating at 95 degrees C.

Samples of these two resins and samples of resin K (prepared as hereinbefore described) are used to treat paper prepared and tested for Mullen wet burst as hereinbefore described and for wet tensile strength. Other resin-treated paper samples are conditioned for ten and twenty days in an oven at a temperature of 120 degrees F., and at 75 per cent relative humidity, and are tested for wet tensile strength. The results of these tests are shown in Table 6.' Wet tensile measurements, made on a standard pendulumtype tensile tester, are given in grams per mm. paper strip, tested after soaking for one hour in water at 23 degrees C.

TABLE 6 Days at 120 F. and 75 percent R. 11.

Percent of Resin Mullen Original Burst Wet Tensile Strength Wet iilen- Strength 0 10 20 After 20 Resin K (100% urea) 29. 6 l, 270 430 320 25 90% Urea, 10% Melamin 28. l l, 200 590 355 30 100% Melamine .l 21. 3 915 495 360 39 As indicated by the results in Table 6, wetstrengthened paper containing a sodium glycolate-melamine resin has improved humidity resistance over wet-strengthened paper containing a sodium glycolate-urea resin, although the wet tensile strength of the former is initially poorer than the latter.

In summary, paper of the instant invention possesses greater wet strength than the resintreated paper heretofore known, because the resin used to treat paper of the invention is modified with an electrolytic group which imparts a high degree of water solubility to the resin. The hydroxy or amino group in the substituted carboxylic acid modifying agent from which the electrolytic group is derived condenses with a methylol group in the resin molecule so that the electrolytic group can become part of the resin molecule. Because of the high reactivity of an amino group with a methylol group, it would be expected that the reaction whereby the electrolytic group becomes part of the resin molecule would be facilitated and a greater improvement in wet-strengthening properties would result when an amino-substituted earboxylic acid is used as the modifying agent rather than a hydroxy-substituted carboxylic acid. However, it has been found on the contrary that better results are obtained in the instant invention when the carboxylic acid used is hydroxy-substituted. The specific reason why a hydroxy-suhstituted carboxylic acid gives better results in the instant invention is not known, but it is believed that there may be a tendency for free formaldehyde present to react with the amino group in an amino-substituted carboxylic acid, and that such a separate reaction may interfere with the reaction that is desired in the practice of the instant invention.

The following examples illustrate the practice of the invention:

Example 1 A substituted carboxylic acid (252 grams of glycolic acid in 108 cc. of water) is mixed with a caustic solution (100.3 grams of flake caustic in 131.7 cc. of water) and maintained at temperatures rangingrfrom 7-0 to degrees C. for 25 minutes. To this mixture in a 3 liter threenecked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals is added a resin former (690 grams of urea) and methanolfree formalin (1980 grams of a solution consisting of 3'7 per cent formaldehyde and 63 per cent water). The pH of the mixture is adjusted within the range 6.5 to 7.0 with additional iiake caustic, and the mixture is heated to a tenperature of about degrees C. The pH is then adjusted to 5.4 with glycolic acid and the heating is continued at approximately 95 degrees C. for two and one-half hours. The mixture is then cooled to 60 degrees C., (the pH being adjusted to 5.6 with sodium hydroxide during the cooling) and is held at this temperature for one hour. The resin is then cooled to room temperature, and the pH is adjusted to 7.0 with sodium hydroxide.

A beaten pulp suspension of any type of paper pulp, such as bleached or unbleached sulphite, kraft, or ground wood pulp, is prepared, in accordance with the procedure hereinbefore described, for the addition of a resin for imparting wet strength. A resin for imparting wet strength (the resin prepared as described in the preceding paragraph in an amount sufiicient to give 3 per cent based on the weight of dry pulp) is added to the beaten pulp suspension. Handsheets made as hereinbefore described from such a resin-treated beaten pulp suspension are superior in wet strength to resin-impregnated paper sheets heretofore known.

Example 2 Paper having superior. wet strength is prepared as described in Example 1 except that the resin solution added to the beaten pulp suspension is prepared by one of the following procedures:

(a) To a substituted carboxylic acid (85 grams of lactic acid in 15 cc. of Water) in a 1 liter threenecked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals, is added flake caustic (38 grams), and the pH of the mixture is adjusted to 6.8 by the addition of dilute sodium hydroxide. A resin former (230 grams of urea) and methanol-free formalin (660 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) are then added to the flask and the mixture is held for three hours at a temperature of about 95 degrees C., the pH being adjusted to approximately 5.5-5.6 with lactic acid when the temperature reaches 89 degrees C. and lowered to 5.4 with additional lactic acid during the last fifteen minutes of the heating. The mixture is then cooled to 60 degrees C. The pH is raised to 5.6 with dilute sodium hydroxide, and the solution is held at 60 degrees C. for one hour. The resin is cooled to room temperature and neutralized with dilute sodium hydroxide to a pH of 7.0.

(b) A substituted carboxylic acid (75 grams of tartaric acid in 10 cc. of water) is mixed with flake caustic grams), methanol-free formalin (344 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) and a resin former (120 grams of urea) by the procedure described in (a). The mixture is held for four hours at a temperature of 95 degrees C., the pH being adjusted to 5.4 with tartaric acid when the temperature reaches 90 degrees C. The mixture is then cooled to 60 degrees C. and held at this temperature for one hour, the pH being adjusted to 5.6 with sodium hydroxide at the beginnin of this stage of the reaction. The resin is then cooled to room temperature and neutralized with dilute sodium hydroxide to a pH of 7.0.

The procedure described in the preceding paragraph may be carried out using, as the substituted carboxylic acid, glutamic acid (75 grams) or glycine (38 grams).

(0) Glycoiic acid (105 grams in cc. of water), flake caustic (52.9 grams), methanol-free formalin (569 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) paraformaldehyde (44 grams), and urea (240 grams) are mixed and heated as described in (a). When the temperature approaches 90 degrees C. the pH is adjusted to 5.4 with glycolic acid. The heating is continued for one and one-half hours at temperatures ranging from 95 to 100 degrees C. The mixture is cooled to degrees C. and held at this temperature for one-half hour, the pH being raised to 5.6 at the beginning of this period with sodium hydroxide solution. The resin is then cooled to room temperature and neutralized with dilute sodium hydroxide to a pH of 7.0.

(d) A resin former (720 grams of urea) and methanol-free formalin (2064 grams of a solution consisting of 37 per cent formaldehyde and 63 per cent water) are mixed in a three liter three-necked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals. The pH of the mixture is adjusted with sodium hydroxide within the range 6.5 to 7.0. Crystalline H20 (370 grams) is added and the mixture is heated to a temperature of 95 degrees C. The pH is adjusted to approximately 5. L with glycolic acid and the mixture is reacted at this temperature and pH to a viscosity of 48 seconds (Ford cup viscosity after about two and three-quarters hours heating). The mixture is then cooled to a temperature of 60 degrees C., the pH is adjusted to 5.6 with sodium hydroxide, and the reaction is continued until the viscosity of the solution is 66.5 seconds (Ford cup viscosity after about one and one-half hours heating at 60 degrees C.). The resin is cooled to room temperature and the pH is adjusted to 7.0 with sodium hydroxide.

(e) A substituted carboxylic acid (215 grams of an aqueous solution consisting of per cent glycolic acid and 30 per cent water) is mixed with flake sodium hydroxide (85 grams) in a 2 liter three-necked flask fitted with a thermometer, reflux condenser, stirring rod and oil seals,

and the mixture is maintained with cooling for about eight minutes at temperatures from to degrees C. Methanol-free formalin (874 grams of an aqueous solution consisting of 50 per cent formaledhyde and 50 per cent water, prepared by vacuum distillation of commercial 37 per cent formalin) is added, and the pI-I of the mixture is adjusted within the range 6.5 to 7.0 by addition of sodium hydroxide. A resin former (413 grams of urea) is dissolved in this mixture, the pH being adjusted to 7.5 when the temperature, which drops sharply and then starts rising slowly, reaches 50 degrees C. (held with cooling during the adjustment). The temperature of the mixture is raised to degrees C. by heating over a period of about forty minutes, the pH being adjusted to 7.2 with sodium hydroxide when the temperature reaches 75 degrees C. The heating is continued. at 95 degrees C. for five minutes before the pH is lowered to 5.6 with glycolic acid. The solution is reacted to a viscosity of K-L (Gardner-Holdt) which is attained about one and one-quarter hours after the mixture reaches 95 degrees C. The mixture is cooled to 60 degrees C. (over a period of about ten minutes) and held at this temperature and at a pH of 5.6 until the viscosity of the resin is V-W (approximately two and one-quarter hours of heating at 60 degrees 0.). The resin is cooled to room temperature, the pH is adjusted to 8.0 with sodium hydroxide, and water (167 grams), and an aqueous bufier solution (12.7 grams of a mixture comprising 65 per cent boric acid and 35 per cent borax in 114.3 cc. of water) are added.

What is claimed is:

1. Paper of improved wet strength containing 0.1-10 per cent of its dry pulp weight of a condensation product of (w) formaldehyde (b) a substance of the class consisting of urea and melamine and (c) 0.05-0.25 mol per mol of (b) of a carboxylic acid derivative of the class consisting of substituted aliphatic carboxylicacids and alkali metal salts thereof, whose molecule has not more than eight carbon atoms and has from two to four carbon atoms per carboxy group; said condensation product containing no monovalent substituents derived from (w), (b) and other than NH2, OH, -CO'OH and COOM groups, wherein M is an alkali metal, the molar ratio of (a) to (1)) being from 1.8:1 to 22:1 when (b) is urea and the molar ratio of (a) to (1)) being from 4:1 to 7:1 when (b) is melamine.

2. Paper of improved wet strength containing 0.1-10 per cent of its dry pulp weight of a condensation product of (a) 1.8-2.2 mols of formaldehyde (b) 1 mol of urea and (c) 0.15-0.25 mol of a carboxylic acid derivative of the class consisting of substituted aliphatic carboxylic acids and alkali metal salts thereof, whose molecule has not more than eight carbon atoms and has from two to four carbon atoms per carboxy group; said condensation product containing no monovalent substituents derived from (a), (b) and (0) other than NH2, -OH, COOI-I and COOM groups, wherein M is an alkali metal.

3. Paper of improved Wet strength containing a condensation product as claimed in claim 2 wherein carboxy groups are neutralized with an alkali metal base.

4. Paper of improved wet strength as claimed in claim 3 wherein the carboxylic acid derivative has twocarbon atoms.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,922,690 Lougovoy Aug. 15, 1933 2,322,887 I Schwartz et a1 June 29, 1943 2,325,302 Britt July 27, 1943 2,338,602 Schur Jan. 4, 1944 2,389,415 DAlelio Nov. 20, 1945 2,443,368 Alexander et al. June 15, 1948 2,446,991 Alexander et al Aug. 17, 1948 2,524,111 La Piana et a1 Oct. 3, 1950 2,524,112 La Piana et al. Oct. 3, 1950 2,601,666 Niles June 24, 1952

Claims

1. PAPER OF IMPROVED WET STRENGTH CONTAINING 0.1-10 PER CENT OF ITS DRY PULP WEIGHT OF CONDENSATION PRODUCT OF (A) FORMALDEHYDE (B) A SUBSTANCE OF THE CLASS CONSISTING OF UREA AND MELAMINE AND (C) 0.05-0.25 MOL PER MOL OF (B) OF A CARBOXYLIC ACID DERIVATIVE OF THE CLASS CONSISTING OF SUBSTITUTED ALIPHATIC CARBOXYLIC ACIDS AND ALKALI METAL SALTS THEREOF, WHOSE MOLECULE HAS NOT MORE THAN EIGHT CARBON ATOMS AND HAS FROM TWO TO FOUR CARBON ATOMS PER CARBOXY GROUP; SAID CONDENSATION PRODUCT CONTAINING NO MONOVALENT SUBSTITUTENTS DERIVED FROM (A), (B) AND (C) OTHER THAN -NH2, -OH, -COOH AND COOM GROUPS, WHEREIN M IS AN ALKALI METAL, THE MOLAR RATIO OF (A) TO (B) BEING FROM 1.8:1 TO 2.2:1 WHEN (B) IS UREA AND THE MOLAR RATIO OF (A) TO (B) BEING FROM 4:1 TO 7:1 WHEN (B) IS MELAMINE.