Abstract:
A process for making paper to enhance the dry strength of the paper produced without substantially reducing its softness comprising adding to a bleached pulp furnish, separately or together, (1) an anionic polymer selected from the group consisting of carboxymethyl guar, carboxymethyl bean gum, carboxymethyl hydroxyethyl guar, and a carboxymethyl hydroxypropyl guar, with (2) a cationic polymer selected from the group consisting of a cationic guar, a cationic acrylamide copolymer, a cationic bean gum, a cationic amine-epichlorohydrin wet strength resin, and both a cationic wet strength resin and at least one of the other said cationic polymers, a composition for adding to a paper-making pulp slurry comprising the said polymers, a method for making the said composition, and a paper product containing the said composition, are disclosed.

Description:
This application is a division of application Ser. No. 07/929,554, filed Aug. 14, 1992, now U.S. Pat No. 5,318,699, which is a continuation-in-part of U.S. application Ser. No. 07/812,534, filed Dec. 23, 1991, now abandoned. 
    
    
     This invention relates to a process for making paper to enhance the dry strength of the paper produced without reducing its softness, that comprises adding to a pulp furnish a combination of cationic and anionic polymers. 
     BACKGROUND OF THE INVENTION 
     One of the major problems that tissue and towel manufacturers face is the unacceptable reduction of dry strength in paper products such as tissue and toweling in which a high degree of softness as well as dry strength is essential, resulting from the use of an increasing percentage of recycled or secondary pulp, chemithermomechanical pulp (CTMP) (CTMP) and groundwood, and the consequent reduction in average fiber length. Softness is a very important property in paper used for making high quality tissues and toweling, and procedure modifications or additives that achieve a compensating increase in paper strength normally decrease paper softness or increase stiffness. There is therefore a need for an effective additive that will enhance paper strength without adversely affecting the softness of the paper. 
     The tissue and towel manufacturers get the softness of their products evaluated by the perception of a human panel. Because it is a very subjective test, correlation of any single laboratory test evaluation with the perception test results may sometimes be difficult. However, extensive research efforts by workers in this area have shown that the results of a bending stiffness test by itself or in combination with tensile stiffness data correlate very reasonably with human perception evaluation. 
     European published Patent No. 0 362 770 A2 (Application No. 89118245.3) discloses a mixture of cationic and anionic polymers as a strengthening additive for papermaking processes, essentially for unbleached pulps containing black liquor. The mixture comprises a water-soluble, linear, cationic polymer having a reduced specific viscosity greater than 2 dl/g and a charge density of 0.2 to 4 meq/g, and a water-soluble, anionic polymer having a charge density of less than 5 meq/g that is reactable in the presence of water with cationic polymer to form a polyelectrolyte complex. Combinations of cationic guar (for example, guar &#34;derivatized&#34; with glycidyltrimethylammonium chloride) and cationic acrylamide copolymers, with anionic polymers in addition to those already contained in the black liquor (including sodium carboxymethyl guar) are disclosed. The preferred anionic polymer content is constituted by those polymers naturally present in unbleached pulps made by either chemical or mechanical pulping. 
     U.S. Pat. No. 3,058,873 discloses a combination of polyamide-epichlorohydrin resins with cellulose ethers or cationic starches for improving paper wet strength, specifically the use of CMC CT, a crude technical grade of carboxymethyl cellulose (CMC), and a polyamide-epichlorohydrin resin to improve wet strength in paper. 
     Published Japanese patent application JP 53106803 A, for which no counterpart exists, discloses paper production having improved wet strength and transparency by forming sheet from cellulosic material, carboxyalkyl cellulose and polyamide-epichlorohydrin resin, drying and treating with acid. Specifically, the method comprises (1) preparing a sheet from a mixture of (a) cellulosic material and (b) partially substituted carboxy-(lower alkyl)-cellulose of which the degrees of substitution (D.S.) is 0.10-0.50, followed by coating the resulting sheet with (c) polyamide-epichlorohydrin resin (or preparing a sheet from a mixture of (a)+(b)+(c), (2) drying the sheet, and (3) treating the dried sheet with a diluted acid solution. The partially lower-alkyl-carboxylated cellulose (typically CMC of D.S. 0.10-0.50) is firmly fixed on the cellulosic pulp when the polyamide-epichlorohydrin resin is present. 
     It would be desirable to provide a process for making paper from a bleached pulp furnish that uses a combination of cationic and anionic polymers to enhance the dry strength of the paper more efficiently than the known processes. 
     SUMMARY OF THE INVENTION 
     According to the invention, a process for making paper to enhance the dry strength of the paper produced without reducing its softness comprises adding to a bleached pulp furnish, separately or together, (1) an anionic polymer selected from the group consisting of carboxymethyl guar, carboxymethyl bean gum, carboxymethyl hydroxyethyl guar (such as the carboxymethyl guar available under the name Galaxy 707D from Aqualon and the name Jaguar 8707 from Hi-Tek), and a carboxymethyl hydroxypropyl guar (such as the carboxymethyl guar available under the name Jaguar 8600 from Hi-Tek), and (2) a cationic polymer selected from the group consisting of a cationic guar, a cationic acrylamide copolymer, a cationic bean gum, and a cationic wet strength resin that is an amine polymer-epichlorohydrin resin, such as polyamide-epichlorohydrin (PAE) resin, a polyalkylenepolyamine-epichlorohydrin (PAPAE) resin, or an amine polymer-epichlorohydrin (APE) resin, preferably the reaction product of a dibasic acid, more preferable adipic acid, with a polyalkylenepolyamine, more preferably a polyethylenepolyamine, in which amine polymer-epichlorohydrin resin amine groups have been alkylated and crosslinked with epichlorohydrin to produce a polyamide-epichlorohydrin resin that has azetidinium or epoxide functionality. 
     A polyamide-epichlorohydrin resin may more specifically be described as a poly(aminoamide)-epichlorohydrin resin, and is sometimes called a polyamide-polyamide-epichlorohydrin resin. If the cationic additive is a wet strength resin the benefits of the invention are achieved and the wet strength of the paper is additionally increased. 
     In a bleached pulp furnish, the combination of additives according to the invention is significantly more effective as a dry-strength additive than the same amounts of each of the cationic guar or the anionic guar, while maintaining substantially the same degree of softness (as measured by bending stiffness) as found in paper lacking a conventional dry-strength additive. A combination of an anionic guar and a cationic acrylamide copolymer is similarly more effective than the acrylamide copolymer alone as a dry strength additive at the same level of addition. These advantages are only achieved if bleached pulp is used in the process according to the invention. 
     Preferably, the wet strength resin is added to the anionic/cationic guar combination according to the invention. A combination of an anionic guar, a cationic guar (or acrylamide copolymer) and the wet strength resin is more effective than a combination of a cationic guar (or acrylamide copolymer) and the wet strength resin, all the other conditions being the same. It is at least as effective as a combination of an anionic guar and the wet strength resin. 
     When clear solutions of the individual components of the mixtures according to the invention are added together, aggregates of fine colloidal particles (which are usually called coacervates), apparently bound together by some physical or chemical force, are formed. This combination provides dry strength enhancement for paper that is higher than the strength enhancement provided by the same amount of either the cationic guar or the anionic guar, demonstrating a synergistic effect from the aggregates of fine colloidal particles that is substantially identical to the results obtained by adding the same anionic and cationic compounds individually to the pulp system. The cationic additive may also be a wet strength resin, when added with the anionic compound either individually to the pulp system or as coacervates. In either case, the presence of the anionic compound very significantly enhances the efficiency of a wet strength resin, and the use of the coacervates has the advantage of convenience. 
     The invention also comprises a composition for modifying a paper pulp slurry to enhance the dry strength of the paper produced without substantially reducing its softness comprising (1) an anionic polymeric component selected from the group of polymers consisting of carboxymethyl guar, carboxymethyl bean gum, carboxymethyl hydroxyethyl guar, and a carboxymethyl hydroxypropyl guar, and (2) a cationic polymeric component selected from the group of polymers consisting of a cationic guar, a cationic acrylamide copolymer, a cationic bean gum, a cationic wet strength resin, and both a cationic wet strength resin and at least one of the other of said cationic polymers, the wet strength resin being an amine polymer-epichlorohydrin resin selected from the group consisting of a polyamide-epichlorohydrin (PAE) resin, a polyalkylenepolyamine-epichlorohydrin (PAPAE) resin, and an amine polymer-epichlorohydrin (APE) resin, in which amine polymer-epichlorohydrin resin amine groups have been alkylated with epichlorohydrin to produce a polyamine-epichlorohydrin resin that has azetidinium or epoxide functionality. 
     Preferably in the said composition, the anionic polymeric component is a carboxymethyl guar and the cationic polymeric component is a cationic guar and a cationic wet strength resin that is produced by reacting a saturated aliphatic dicarboxylic acid containing three to ten carbon atoms with a polyalkylenepolyamine, containing from two to four ethylene groups, two primary amine groups, and one to three secondary amine groups (such as diethylenetriamine, triethylenetetramine and tetraethylenepentamine), to form a poly(aminoamide) having secondary amine groups that are alkylated with epichlorohydrin to form a PAE resin. Most preferably, the wet strength resin is Kymene 557H (available from Hercules Incorporated), in which adipic acid is reacted with diethylenetriamine to form a poly(aminoamide) that is alkylated and crosslinked with epichlorohydrin to form a PAE resin. The invention also comprises a paper product containing the said composition according to the invention, and a process for making the composition comprising adding a mixture of the anionic guar and the cationic guar, preferably as art aqueous suspension, to a paper pulp slurry and then adding the wet strength resin later to form the composition in the slurry. 
     Under the normal wet end conditions of papermaking, the combination of additives according to the invention enhances paper strength through ionic bonds. That enhancement is an important feature for toweling, toilet tissue, or arty other fine paper in which softness and dry strength or a combination of dry and wet strength, without compromising softness, are valued properties. 
     A cationic guar molecule with no anionic guar will have all its ionic groups available bond with the cellulose-fiber ionic groups of opposite charge. Thus, a cationic guar by itself is expected to offer a higher number of ionic bonds. When art anionic and a cationic guar are mixed together either in the presence of pulp or by themselves, art interaction takes place between them and this results in a lower number of ionic sites in the combination to bond with cellulose fibers. (A similar effect occurs when a wet strength resin, such as Kymene 557H, is in the additive system). Hence, the higher effectiveness of a combination as a dry strength is unexpected, particularly since it is not present if used with unbleached pulp containing black liquor. 
     According to the results obtained with various guar additives, this synergistic effect of art anionic and a cationic guar additive is independent of the chain-length of the compound as well as molecular weight as measured by solution viscosity. It is also independent of the charge density of the additives. However, the degree of effectiveness of the combinations depends on the molecular weight. There is evidence that the relatively higher molecular weight guars produce relatively higher paper strength. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Guar is a natural copolymer consisting of galactose and mannose, usually in the ratio of 1 to 2, in a linear chain of β-d-mamopyransyl with λ-D-galactopyranosyl units as side branches. An anionic guar is obtained by reacting a natural guar with caustic and subsequently with monochloroacetate. The resultant product is a carboxymethyl guar (CMG). Similarly, carboxymethylhydroxypropyl guar (CMHPG) is prepared by reacting natural guar with caustic and subsequently with monochloroacetate and propylene oxide. Examples of CMG are Galaxy 707D, (Aqualon), Jaguar 8707 (Hi-Tek) and those of CMHPG are WG-18 (Aqualon), Jaguar 8600 (Hi-Tek). Carboxymethyl hydroxyethyl guars are other examples of anionic guar additives. 
     A cationic guar is obtainable by reacting natural guar with caustic and subsequently with quaternary ammonium chloride, and is available from Dow as Dow Quart 188; such a cationic guar is available under the name Gendrive 162. 
     The structure of natural guar gum is as follows: ##STR1## 
     Polyacrylamide is prepared by polymerization of acrylamide with N,N&#39;-methylene bisacrylamide. A cationic polyacrylamide is usually prepared by making reacting polyacrylamide with acryloxytrimethyl ammonium chloride (ATMAC), methylacryloxytrimethyl ammonium chloride (MTMAC) or diallyldimethyl ammonium chloride (DADMAC). The anionic acrylamide compounds are usually copolymers of acrylamides and sodium acrylates. 
     The preferred wet strength resins are produced by reacting a saturated aliphatic dicarboxylic acid containing three to ten carbon atoms, preferable adipic acid, with a polyaikylenepolyamine, containing from two to four ethylene groups, two primary amine groups, and one to three secondary amine groups, such as diethylenetriamine, triethylenetetramine and tetraethylenepentamine, to form a poly(aminoamide) having secondary amine groups that are alkylated with epichlorohydrin to form tertiary aminochlorohydrin groups. These groups self-alkylate to form hydroxyazetidinium groups which are considered responsible for wet strength in paper. They are cationic in character. If tertiary amines are present in the aminopolyamide or polyamine backbones, quaternary epoxide groups are produced. The actual procedure for synthesizing these wet strength resins differ from product to product, but the objective to generate aminopolyamide-epichlorohydrin functionality remains the same. 
     Kymene® wet strength resins are most preferred. Some examples of wet strength resins available from Hercules Incorporated are Kymene 557H, Kymene 450, and Kymene 2064 (an APE resin based on methyldiallylamine monomer that is polymerized to an amine polymer precursor), as well as low absorbable organic halide (AOX) versions such as Kymene 557LX. Most preferred is Kymene 557H, in which adipic acid is reacted with diethylenetriamine (DETA) to form a poly(aminoamide) that is alkylated and crosslinked with epichlorohydrin to form a PAE resin, namely, adipic acid-DETA poly(aminoamide) epichlorohydrin. 
     The specific amount and the type of the additives will depend on, among other things, the type of pulp characteristics. The ratios of the anionic and the cationic additives may range from 1/20 to 10/1, preferably from 2/1 to 1/2, and most preferably about 1/1. The combination according to the invention is effective when added to the pulp stock in the amount of 0.05 to 5 percent, depending on the type of pulp. The preferable level of addition is 0.1 to 2 % based on the dry weight of pulp. 
     Since the additive combinations of the invention consist of two or more components, they can be added to the furnish in different ways that may affect the rate of production in the plant and the properties of the final product. The usual procedure is to add these additives individually in the wet end system in a predetermined sequence to achieve what experiment shows to be the most desirable product. Preferably, however, these additives are introduced into the wet end system by combining the anionic and cationic additives beforehand and adding the resulting coacervates. 
     In the following examples, handsheets were prepared from pulp which was refined in a Valley beater to 500 ±5 cc Canadian Standard Freeness. The 22.50% consistency pulp slurry was diluted to 266 % solids with normal tap water in a proportioner where the combination of additives according to the invention were added to the pulp while stirring. An aliquot of this pulp slurry was further diluted to about 0.023 % consistency in a Deckle box for molding handsheets. Both refining and papermaking were made at pH 7.5 to 8.0. Thus, the papermaking process consists of three main steps. They are (a) formation of an aqueous slurry of cellulose fibers, (b) addition of dry strength additives and (c) formation of sheet and drying to a desired moisture content (preferably 3 to 6 percent). 
     The step (b) may be carried out by adding the anionic component to the pulp first, followed by the cationic component, and the wet strength resin if used. Blends of anionic and cationic components may also be added to the pulp directly in the papermaking system. Whether individually or blended together, the additives are mixed into the wet end of the paper machine, preferably under shear. 
     Tensile strength, modulus, and elongation were measured in an Instron, according to a standard procedure, Tappi 494, as a guide. Drying was to a moisture content of 3 to 6 percent. By the same testing procedure, the tensile energy per unit volume that the fibers have assimilated up to the point of rupture was also determined. This is referred to as tensile energy absorption (TEA). The results of bending stiffness presented here have been measured in a Handle O&#39;Meter (Thwing-Albert Instrument Co.). 
     The same testing procedure measures the combined effect of sheet stiffness, surface friction, and thickness that affect the subjective perception of softness of paper products. (Holger Hollmark, TAPPI Journal, p 97, February 1983; Handbook of Physical and Mechanical Testing of Paper and Paperboard, Ed. Richard E. Mark, Ch. 11, p 511, 1983). 
    
    
     EXAMPLE 1 
     This Example is a laboratory evaluation of strength properties and bending stiffness on handsheets prepared with 70/30 Northern Softwood/CTMP furnish. The results are shown in Table 1. The anionic additives were first added to the pulp followed by the cationic additive. The control used in this case is a handsheet prepared with the same pulp with no additive. Galaxy 707D is a carboxymethyl guar (DS 0.08), and Gendrive 162 is a quaternary ammonium chloride treated guar (DS 0.075). Jaguar 8600, available commercially as Hi-Tek, and WG-18, are hydroxypropylcarboxymethyl guars. Guar AQU-3129 and High DS cationic guar (404-48-3) are available from Aqualon, a Hercules Incorporated unit. WC100 and Hercofloc 1129 sodium acrylate-acrylamidecopolymer and sodium polyacrylate homopolymer, respectively. Reten® is a polyamide-epichlorohydrin polymeric material used as a retention. The &#34;Jaguar&#34; products are available from High-Tek. Co. 
     
                                           TABLE 1__________________________________________________________________________                                                    Bending                                   Enhancement, % Control                                                    StiffnessAnionic         Cationic                Tensile          % ofAdditive   Percent           Additive           Percent                                   Strength                                         TEA  Elongation                                                    Control__________________________________________________________________________None       --   Gendrive 162       1.0  6.5   --   --    --Galaxy 707D      0.5  Reten ® 200    0.4  8.3   --   --    --Galaxy 707D guar      0.5  Gendrive 162       0.5  33.9  --   --     94None       --   Jaguar CP-13       1.00 6.6   13.9 10.0  --Jaguar 8600      0.5  Jaguar CP-13       0.50 16.0  23.9 13.0  106None       --   High MW Cationic Guar (0083-40-3)                              1.00 4.0   12.7 3.1   104K0341A 2 (WG-18)      0.5  0083-40-3          0.5  12.0  23.1 13.0  105None       --   Percol 743         1.00 2.5   3.5  2.8   101WC-100     0.5  Percol 743         0.50 14.0  6.9  22.0  113Hercofloc 1129      0.5  Reten 157          0.50 15.4  3.2  8.9   101None       --   404-48-3           1.0  3.7AQU-D3129  0.5  404-48-3           0.5  14.3Jaguar 8707      0.5  Jaguar CP-13       0.5  21.3  48.0 19.4__________________________________________________________________________ 
    
     EXAMPLE 2 
     Results on laboratory evaluation of strength properties, tensile stiffness and bending stiffness on handsheets prepared as in Example 1 are presented in Table 2. Pulp system employed in Set No. 1 is 50/50 recycled/northern softwood bleached kraft pulp. In Set No. 2, the pulp is 100 percent bleached kraft. The process to prepare the guars is similar to what has been explained in Example 1, except that the anionic guar was a carboxymethyl guar (available from Aqualon under the designation AQU-D3 129) having a DS of 0.15 and the cationic guar (available from Aqualon under the designation 404-48-3) was a quaternary-modified guar having a DS of 0.10. 
     
                                           TABLE 2__________________________________________________________________________Anionic      Cationic               Enhancement, % of Control                                 Tensile Stiffness                                         Bending StiffnessNone    Guar   % Guar % Tensile Strength                        TEA                           Elongation                                 % of control                                         % of Control__________________________________________________________________________1   AQU-D3129      0.5        404-48-3             0.5               21.5     37.77                           13.3  108     952   AQU-D3129      0.5        404-48-3             0.5               15.2     31.22                           16.1  99      99__________________________________________________________________________ 
    
     EXAMPLE 3 
     Laboratory evaluation results of strength properties and bending stiffness on handsheets prepared with 70 30 northern softwood/CTMP (Nos. 1 and 2) and recycled pulps (Nos.3 to 5) are shown in Table 3. The anionic additive is added to the pulp prior to adding the cationic additive. The guar additives Galaxy 707D and Gendrive 162 are the same as those used in Example 1. Kymene®557H is the reaction product of an polyamide and epichlorohydrin conventionally used as a wet strength resin in paper and available from Hercules Incorporated. KN9-56CMG is a carboxymethyl guar. The combinations show minimal adverse effects on paper softness caused by the presence of the wet strength agent, as indicated by the stiffness data. 
     
                                           TABLE 3__________________________________________________________________________                                          Bending                     Enhancement, % of Control                                          Stiffness  Anionic     Cationic    Kymene 557H                             Dry          % ofNo  Additive    Percent         Additive                Percent                     Percent Tensile                                 Elongation                                       TEA                                          Control__________________________________________________________________________1 None   --   None   --   1.0     10.8                                 10.0  --  952 KM9-56CMG    0.15 Gendrive 162                0.15 0.75    18.3                                 13.9     1083 None   --   None   --   1.0     11.9                                 22.2  20.6                                          --4 None   --   Gendrive 162                0.5  0.5     19.6                                 25.9  34.4                                          --5 Galaxy 707D    0.25 Gendrive 162                0.25 0.5     34.5                                 33.2  77.6                                          104__________________________________________________________________________ 
    
     COMPARATIVE EXAMPLE 4 
     Results of the evaluation of strength on handsheets prepared with unbleached kraft containing about 2 percent black liquor are shown in Table 4. The results show that a combination of an anionic and a cationic guar additive is not more effective than the cationic guar additive alone when added at the same total level. The guar additives, Galaxy 707D and Gendrive 162, are the same as used in Example 1. 
     
                                           TABLE 4__________________________________________________________________________Anionic     Cationic    Enhancement, % of ControlAdditive  Percent       Additive              Percent                   Tensile Strength                            TEA                               Elongation__________________________________________________________________________Galaxy 707D  0.50 Gendrive 162              0.5  14.2     21.1                               12.5None   --   Gendrive 162              1.0  16.9     25.3                               12.5__________________________________________________________________________ 
    
     COMPARATIVE EXAMPLE 5 
     Results of strength properties evaluation on handsheets prepared with partially unbleached kraft incorporated externally with 0.9% black liquor are presented in Table 5. The results show that a combination of an anionic and a cationic guar, when added to this unbleached kraft-black liquor system, is in fact less effective than the cationic guar alone at the same total addition level. The guar additives are the same as those used in Example 1. 
     
                                           TABLE 5__________________________________________________________________________Anionic     Cationic    Tensile Strength                            TEA   ElongationAdditive  Percent       Additive              Percent                   (PSI)    (ft. lb/ft.sup.2)                                  (%)__________________________________________________________________________Control  --   --     --   5877     5.29  2.2Galaxy 707D  0.5  Gendrive 162              0.5  7644     7.58  2.6None   --   Gendrive 162              1.0  8684     10.62 3.0__________________________________________________________________________ 
    
     EXAMPLE 6 
     This series of tests examines the strength properties and bending stiffness of paper prepared in a small-scale pilot plant version of a conventional paper machine, located at Kalamazoo, Mich. and referred to herein as the Laboratory Former. The pulps used in the numbered tests were: Nos. 1 and 2, 50/50 NSW/NHW kraft; Nos. 3, 4, 7 and 8, 70/30 long fiber/sawdust; and Nos. 1 and 2, 70/30 virgin fiber/broke. In each case, a combination of an anionic and a cationic additive (guar or acrylamide copolymer) was incorporated in the pulp followed by the same amount of wet strength resin Kymene 557H. The anionic components used were all carboxymethyl guars. Among the cationic additives, Percol 743 is a polyacrylamide copolymer, the rest are guars. These results, recorded after 2 weeks natural curing and presented in Table 6, represent the enhancements of properties over what are obtained with 1 percent Kymene 557H alone. They demonstrate the fact that these combinations of an anionic and a cationic component provide synergistic effects on wet strength as well as dry strength of paper. These effects are significantly greater when one of the components is a conventional wet strength resin, such as Kymene 557H. 
     
                                           TABLE 6__________________________________________________________________________                       Total Enhancement over Control                                                 Bending Stiffness                  Kymene                       Additive                             1% Kymene 557H      % of 1%   Anionic   Cationic  557H Level Dry            Wet  KymeneNo Additive     %  Additive               %  %    %     Tensile                                  Elongation                                        TEA Tensile                                                 Control__________________________________________________________________________1  WG-18  0.30        Percol 743               0.20                  0.50 1.00  20.3 19.5  42.5                                            17.8 992  Galaxy 707D     0.30        Gendrive 162               0.20                  0.50 1.00  18.0 24.2  45.3                                            10.7 963  0087-08-2     0.30        Gendrive 162               0.20                  0.50 1.00  26.2 28.2  62.2                                            31.9 1094  0087-08-2     0.30        Percol 743               0.20                  0.50 1.00  26.3 32.2  63.1                                            40.5 995  0087-08-2     0.30        Percol 743               0.20                  0.50 1.00  20.5 26.8  57.1                                            17.5 996  WG-18  0.30        0083-40-3               0.20                  0.50 1.00  15.4 18.5  36.8                                            11.3 1077  D3129  0.30        Percol 743               0.20                  0.50 1.00  25.0 22.0  52.0                                            36.8 1048  WG-18  0.30        0083-40-3               0.20                  0.50 1.00  19.3 27.3  62.0                                            29.2 98__________________________________________________________________________ 
    
     EXAMPLE 7 
     This series of tests examines the strength properties and bending stiffness on handsheets prepared in the Laboratory Former. The pulps used in the numbered tests were: Nos. 1 to 6,55/30/15 northern softwood/CTMP/recycled pulp, and No. 7,50/50 northern softwood/hardwood furnish. The results in No 8 were produced from handsheets using 70/30 northern softwood/CTMP pulp. All the cationic additives were modified polyacrylamides. Percol 743 is a copolymer of acrylamide and 10 mole % MTMAC (Methylacryloxytrimethyl ammonium chloride), Reten 157 contains 10 mole % ATMAC (acryloxytrimethyl ammonium chloride) and Hercofloc 1154 contains 6 mole % DADMAC (dialcryloxydimethyl ammonium chloride). All the anionic additives are guar products available from Aqualon. The results are recorded in Table 7. 
     
                                           TABLE 7__________________________________________________________________________POLYACRYLAMIDE COPOLYMER - GUAR COMBINATIONS                         Enhancement, % of Control                                        Bending Stiffness  Anionic        Cationic     Tensile        % ofNo  Additive  Percent            Additive                    Percent                         Strength                              Elongation                                    TEA Control__________________________________________________________________________1 None      --   Percol 743                    1.00 6.4  15.0  22.6                                        --2 K0341 A2 WG-18)       0.50 Percol 743                    0.50 17.8 12.9  36.3                                        --3 None      --   Reten 157                    1.0  5.3  11.7  14.6                                        --4 AQU-D3129 0.50 Reten 157                    0.5  12.7 19.5  35.3                                        --5 None      --   Hercofloc 1154                    1.0  11.7 10.0  21.1                                        --6 AQU-D3129 0.50 Hercofloc 1154                    0.50 16.9 22.8  44.2                                        --7 AQU-D3129 0.50 Percol 743                    0.50 37.5 45.5  101 1068 Galaxy 707D       0.50 Hercofloc 1154                    0.50 16.3 8.0   26.4                                         92__________________________________________________________________________ 
    
     EXAMPLE 8 
     This series of tests examines the strength properties and bending stiffness of paper prepared in the Kalamazoo Laboratory Former with 70/30 northern softwood/CTMP furnish. The anionic component was added first followed by the cationic component, and the wet strength resin (Kymene 557H) was added last. The results, recorded in Table 8, show that the combination of a wet strength resin and an anionic and a cationic guar, and even the combination of a an anionic guar and a wet strength resin enhances not only the dry strength but also the wet strength very significantly over the corresponding properties obtained by the same amount of the wet strength resin alone. The bending stiffness of the paper samples is not adversely affected by the presence of these additive combinations. The additives AQU-D3129, Galaxy 707D and 0.1 DSCMG are anionic carboxymethyl guars. 404-48-3,404-48-1 and Gendrive 162 are Aqualon cationic guars of which the first two are developmental. The respective controls used were made with the same furnish, but with no additive. 
     
                                           TABLE 8__________________________________________________________________________                      Total                           Enhancement, % of Control                                            Bending                 Kymene                      Additive          Wet Stiffness  Anionic   Cationic  557H Level                           Dry          Tensile                                            % ofNo  Additive    %  Additive              %  %    %    Tensile                               Elongation                                     TEA                                        (P/I)                                            Control__________________________________________________________________________1 None   -- None   -- 1.00 1.00 9.1 3.4   13.0                                        5.65                                            1132 None   -- None   -- 1.50 1.50 --  9.9   15.5                                        5.85                                            1303 None   -- None   -- 2.00 2.00 11.9                               13.0  37.0                                        5.93                                            --4 AQU-D3129    0.13       404-48-3              0.13                 1.00 1.26 17.5                               24.0  52.3                                        6.44                                            885 Galaxy 707D    0.25       404-48-1              0.25                 1.00 1.50 16.3                               20.0  37.0                                        6.58                                            916 0.1 DSCMG    0.19       404-48-1              0.19                 1.50 1.88 28.2                               4.3   29.8                                        7.14                                            --7 0.1 DSCMG    0.50       Gendrive 162              0.50                 1.00 2.00 33.4                               8.9   38.9                                        7.15                                            898 0.1 DSCMG    1.00       None   -- 1.00 2.00 32.6                               9.8   30.9                                        7.67                                            --__________________________________________________________________________ 
    
     EXAMPLE 9 
     This series of tests examines the strength properties and bending stiffness of handsheets prepared from the following pulps: Nos. 1 to 4, 50/50 softwood/hardwood kraft (SWK/HWK); (Nos. 5 and 6), 70/30 northern softwood kraft/CTMP (NSK/CTMP). AQU-D3129 and Galaxy 707D are anionic carboxymethyl guars previously referred to, and Gendrive 162 is a cationic guar previously referred to, while 404-48-3 is a developmental cationic guar. The results, which are recorded in Table 9, show that the paper properties obtained by adding to the pulp coacervates formed by premixing the anionic and cationic components are about the same as those obtained by adding the additives individually to the pulp. They were significantly more convenient to use. 
     
                                           TABLE 9__________________________________________________________________________                              Enhancement over Control                              Containing    Bending                              1% Kymene 557H                                            Stiffness  Anionic   Cationic Modes of Addition                              Tensile       % ofNo  Additive    %  Additive              % to Pulp System                              Strength                                   Elongation                                         TEA                                            Control__________________________________________________________________________1 AQU-D3129    0.5       404-48-3              0.5                Added individually in pulp                              12.2 12.5  11.6                                            1022 AQU-D3129    0.5       404-48-3              0.5                Premixed to form                              12.8 8.3   21.7                                            107                coacervate before adding to                pulp3 AQU-D3129    1.0       404-48-3              1.0                Added individually                              28.5 25.0  45.0                                            1084 AQU-D3129    1.0       404-48-3              1.0                Premixed to form                              25.6 29.2  49.8                                            110                coacervate5 Galaxy 707D    0.5       Gendrive 162              0.5                Added individually to pulp                              17.8 6.5   28.5                                            916 Galaxy 707D    0.5       Gendrive 162              0.5                Premixed to form                              17.4 9.7   29.3                                            95                coacervate before adding to                pulp__________________________________________________________________________ 
    
     EXAMPLE 10 
     Dry strength properties and bending stiffness of paper prepared in the Kalamazoo Laboratory Former (KLF) with 55/30/15 NSK/CTMP/secondary furnish are presented in Table 10. The data recorded in K-17803 and K-17822 represent enhancement of dry strength properties over what was obtained with the control with no additive. The anionic additive employed are CMG (AQU-D3129, Galaxy 707D) and CMHPG (WG-18) while the cationic components are cationic guars (Jaguar CP-13-HiTek, and 0083-40-3) and acrylamide copolymer (Percol 743). The results show that in most cases, at the same level of addition, dry strength with a combination of an anionic and a cationic guar (or a cationic polyacrylamide) is significantly higher than what is obtained with a combination of an anionic guar and the wet strength resin Kymene 557H, with less adverse effect of paper softness, as indicated by the bending stiffness results. 
     
                                           TABLE 10__________________________________________________________________________                                          Bending                   Kymene                        Total                             Enhancement, StiffnessAnionic     Cationic    557H Additive                             % of Control % ofAdditive  Percent       Additive              Percent                   Percent                        Level %                             Tensile                                 TEA                                    Elongation                                          Control__________________________________________________________________________AQU-D3129  0.50 Jaguar CP-13              0.50 None 1.00 43.3                                 76.9                                    29.0  87WG-18  0.50 Percol 743              0.50 None 1.00 32.9                                 63.2                                    27.8  89WG-18  0.50 0083-40-3              0.50 None 1.00 26.9                                 47.0                                    19.5  108AQU-D3129  0.50 None   --   0.50 1.00 23.5                                 54.2                                    23.9  112WG-18  0.50 None   --   0.50 1.00 9.3 28.0                                    18.0  110Galaxy 707D  0.50 None   --   0.50 1.00 25.7                                 57.0                                    23.9  118__________________________________________________________________________ 
    
     EXAMPLE 11 
     Dry strength properties and bending stiffness of paper prepared in the KLF using 70/30 NSK/CTMP furnish are recorded in Table 11. The data demonstrated enhancement of dry strength properties over what was obtained with the control containing no additive. The anionic additives are CMG and cationic components are either cationic guar or Kymene 557H, a wet strength resin. The results show that in most cases, at the same level of addition, a combination of an anionic and a cationic guar provides significantly higher dry strength than what is obtained with the combination of an anionic guar and Kymene, with less adverse effect on paper softness. 
     
                                           TABLE 11__________________________________________________________________________                                          Bending                   Kymene                        Total                             Enhancement, StiffnessAnionic     Cationic    557H Additive                             % of Control % ofAdditive  Percent       Additive              Percent                   Percent                        Level %                             Tensile                                 TEA                                    Elongation                                          Control__________________________________________________________________________AQU-D3129  0.50 404-48-1              0.50 None 1.00 34.8                                 53.7                                    25.7  95AQU-D3129  0.60 404-48-3              0.40 None 1.00 25.6                                 45.9                                    21.0  92AQU-D3129  0.50 404-48-3              0.50 None 1.00 25.9                                 96.6                                    51.2  96AQU-D3129  0.50 None   --   0.50 1.00 18.4                                 26.7                                    9.8   110Galaxy 707D  0.50 None   --   1.0  1.50 26.5                                 51.5                                    22.8  1160.1 DS CMG  1.0  None   --   1.0  2.00 22.5                                 30.9                                    9.2   --__________________________________________________________________________ 
    
     COMPARATIVE EXAMPLE 12 
     Strength properties and bending stiffness of paper prepared at the Kalamazoo Laboratory Former with 70/30 NSW/CTMP are presented in Table 12. The results demonstrate enhancement of dry strength properties over what was obtained with the control with no additive while the wet strength tensile is the enhancement over what was obtained with 0.5 % Kymene® alone. To demonstrate the advantages achieved by the combinations of anionic and cationic components according to the invention over the prior art combinations described in U.S. Pat. No. 3,058,873, the anionic additives used according to the invention were CMG and CMHPG, while CMC-6CTL is a technical grade carboxymethyl cellulose such as that disclosed in the Patent. Gendrive 162 is a cationic guar and Reten®157 is an acrylamide copolymer. A sharp drop in dry strength accompanied by an increase in bending stiffness was noted when the carboxymethyl cellulose was used. 
     
                                           TABLE 12__________________________________________________________________________                       Kymene                            Enhancement, Wet Strength                                                 Bending StiffnessRun   Anionic     Cationic     557H % of Control Enhancement                                                 % ofNo.   Additive     Percent          Additive                  Percent                       Percent                            Tensile                                TEA                                   Elongation                                         % 0.5 Kymene                                                 Control__________________________________________________________________________1  Galaxy 707D     0.30 Gendrive 162                  0.20 0.50 31.0                                102                                   33.6  24.5    --2  Galaxy 707D     0.50 None    --   0.50 27.6                                68 30.6  27.0    --3  WG-18  0.20 Reten 157                  0.30 0.50 30.8                                76 --    --      964  WG-18  0.50 None    --   0.50 22.4                                60 37.3  25.7    94 5*   CMC-6CTL     0.50 None    --   0.50 13.5                                21 13.1  12.0    99__________________________________________________________________________ *See U.S. Pat. No. 3,058,873 described above. 
    
     TESTS OF ADDITIVES FOR EXAMPLES 
     Results of viscosity and relative specific viscosity (RSV) for 0.25 % aqueous solutions of the guar additives are shown in Table 13. The results indicate the range of relative molecular weights of typical additives employed in the examples. Since these data do not lead to the absolute molecular weights of the additives, no comparison can be made with similar data for materials of different molecular shapes. Charge densities of typical additives employed in the examples are shown in Table 14. 
     
                       TABLE 13______________________________________Additives       Viscosity (CP)                       RSV (dl/g)______________________________________Guar Gendrive 162           31.1        121.5Guar Galaxy 707D           9.0         32.4Guar Jaguar CP-13           66.5        223.8______________________________________ 
    
     
                       TABLE 14______________________________________        Charge Density                    Viscosity (cp)Product      (meq/g)     2% Solution______________________________________AQU-D3129    -1.34        2,300404-48-3     0.86         4,200Jaguar 8707  -0.012      12,000Jaguar LP-13 0.23        23,000______________________________________