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Effects of Charge Ratios and Cationic Polymer Nature on Polyelectrolyte Complex Deposition onto Cellulose Martin A. Hubbe,* Stephanie M. Moore, and Sa Yong Lee Department of Wood and Paper Science, North Carolina State University, Box 8005, Raleigh, North Carolina 27695-8005

Sequential addition of poly(diallyldimethylammonium chloride), which is highly cationic, followed by anionic carboxymethylcellulose, has been found to promote inter-fiber bonding during the manufacture of paper, with potential benefits to the recycling of fibers. The present results help to confirm a hypothesis that observed strength gains, in cases where the amount of the first additive exceeded the adsorption capacity of the fibers, were due to the formation of polyelectrolyte complexes in the solution phase, followed by their deposition onto fiber surfaces. Complex formation and retention of complexes on fiber surfaces occurred efficiently over a wide range of polymer charge ratios, cationic polymer attributes, and other conditions, regardless of whether the fibers had been pretreated to reverse their net charge. Introduction Polyelectrolyte additives to the papermaking process are playing an increasingly important role in meeting the strength requirements of paper products. The importance of such “dry-strength additives” such as cationic starch1-9 and carboxymethylcellulose (CMC)10-14 is becoming more and more critical as the industry deals with the competing issues of customer expectations for higher strength and losses in the bonding ability of fibers due to increased levels of recycling.15-18 Recent progress in various aspects of dry-strength technology has been published.7-9,12-14,19-25 In previous publications it was shown that a superior strengthening effect could be achieved if a slurry of fibers, to be made into paper, was treated sequentially by a highly charged cationic polyelectrolyte, poly(diallyldimethylammonium chloride) (DADMAC) followed by the anionic polyelectrolyte CMC.12 In the case of paper formed from fibers reclaimed from recycled xerographic copy paper, preliminary tests showed surprisingly that the greatest strength gains were achieved when the amount of the first additive was many times greater than the adsorption capacity of the fibers. It is proposed that polyelectrolyte complexes (PECs)26-31 were formed between the excess cationic polymers in solution and the subsequently added anionic polymer and that the complexes were able to deposit onto the fiber surfaces, where they acted as a bonding agent when the paper was dried. Support for the hypothesis included turbidimetric results, confirming the existence of polyelectrolyte complexes and demonstrating their ability to adsorb at the fiber surfaces. An especially puzzling aspect of the preliminary work was the weakness of the dependency of PEC retention on cellulosic fibers, relative to the initial charge of fibers. As shown in Figure 1, pretreatment of the fibers with a sufficient amount of highly cationic polymer to saturate the surfaces, reversing the net charge to positive, appeared to slightly increase the retention efficiency of * To whom correspondence should be addressed. E-mail: [email protected]. Fax: (919)515-6302. Tel.: (919)513-3022.

Figure 1. Retention efficiency of polyelectrolyte complexes formed from DADMAC and CMC as a function of their ratio of macromolecular charge, the initial sign of charge of the fiber surfaces, and the state of agitation of the suspension.

all of the polyelectrolyte complexes, regardless of the ratio between the amounts of cationic and anionic groups on the respective macromolecules. In other words, even polyelectrolyte complexes having an excess of cationic groups were retained efficiently on fibers, the surface of which had been treated to make them cationic. The present study was motivated by a desire to find out whether the efficient retention of PECs on fibers was limited to a certain combination of polyelectrolytes or whether the phenomenon has a wider range of potential applications. Emphasis was placed on varying the nature of the cationic polymer involved in complex formation, including its molecular mass, branched versus linear structure, and chemical nature. Microelectrophoresis and sedimentation tests were carried out to quantify the electrokinetic and colloidal stability properties of the formed complexes, relative to their ability to adsorb onto fiber surfaces. Experimental Section Water and Fiber Suspension. The studies were carried out in deionized water, to which sufficient

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NaHCO3 was added to achieve 10-4 mol/L. Sufficient Na2SO4 was added to achieve an electrical conductivity of 1000 µS/cm at approximately 23 °C. The solutions as well as fiber suspensions prepared therefrom were adjusted to a pH range of approximately 8.0-8.2 at the start of each test. The fiber suspension was from EnviroCopy xerographic paper, having 35% recycled fiber content, obtained from Office Depot. To prepare the fibers, the copy paper was dispersed with a TAPPI disintegrator,32 with 30 000 revolutions. Fine particles were removed from the slurry using a Bauer-McNett apparatus33 fitted with a 200-mesh wire screen and run with a 200 mL/s flow rate for 10 min. The fibers were then thickened for storage by collecting them on a TAPPI handsheet mold.32 Chemicals. The default cationic polyelectrolyte used for the adsorption and strength experiments was medium molecular mass, linear poly(diallyldimethylammonium chloride) (poly-DADMAC), distributed under the brand name Alcofix 169 from Ciba Specialty Chemicals Co. Other poly-DADMAC products used in the research were Alcofix 109, having a higher mean molecular mass, and Alcofix 110, having a substantial degree of branching within the macromolecule. Solutions were prepared with deionized water. For calibration, charge titrations were carried out with polyDADMAC from Aldrich Chemical Co. of molecular mass 400-500 kDa (Catalogue No. 40,903-0). Further tests involved poly(ethylenimine) (PEI) products. Polymin P from BASF is PEI homopolymer having a very high cationic charge density and highly branched structure. Polymin SKA is a copolymer having a high molecular mass. More detailed descriptions of both polyDADMAC34,35 and PEI36,37 products have appeared elsewhere. Solutions were prepared by dissolving the dry-bead product in deionized water. For calibration, charge titrations were carried out with poly-DADMAC from Aldrich Chemical Co. of molecular mass 40050 kDa (Catalogue No. 40,903-0) and with 0.0025 N polyvinyl sulfate, potassium salt (PVSK) (Catalog No. S5434), from Nalco Co. The anionic polyelectrolyte used together with various cationic products to form complexes was carboxymethylcellulose 7 M (CMC) from Hercules, Inc. The nominal degree of substitution of this product is 0.7 carboxymethyl groups per sugar unit. The CMC was prepared by sifting 1% by mass of the powder into stirred deionized water, with continued stirring for 20 h. Turbidity. Nephelometric tests were carried out with a DRT-15CE turbidimeter. This type of measurement was selected for use in the present study due to its high sensitivity as well as the rapidity of testing, making it possible to minimize the time between adsorption steps and subsequent evaluation of the filtrate (see later). Increasing amounts of 0.1% CMC solution were added to poly-DADMAC (final concentration 1.63 g/L) with 50 mL of 10-4 M buffer solution, as described above, with manual swirling continuing for 30 s. The contents were transferred to a cuvette, and the turbidity was evaluated. Results shown are the averages of five measurements, swirling the cuvette between each reading. Microelectrophoresis. Information related to the apparent ζ-potentials of PECs was obtained by use of a Charge Analyzer 2 microelectrophoresis instrument from SKS Associates.38,39 For each observation, the

velocities of 10 individual PECs were evaluated, and the results were averaged. Adsorption of Polyelectrolyte Complexes onto Cellulose. The 0.5% solid suspensions of fibers were optionally treated with sufficient default poly-DADMAC to saturate the adsorption capacity of the fibers. In each test an amount polymer solution equivalent to 0.029 g dry mass of polymer/100 g of dry fibers was added to 500 mL of fiber suspension with continuous stirring for 30 s before subsequent steps. Tests were carried out to evaluate the efficiency of PEC deposition onto cellulose suspensions prepared in the same way. The first step was to prepare PECs according to the procedure described earlier under “Turbidity”. PECs were prepared over a wide range of ratios of the two polyelectrolytes, poly-DADMAC and CMC. Next, the PECs were added to 500 mL portions of 0.5% solids pulp slurry, as described earlier, in a Britt Jar apparatus40,41 with a constant impeller speed of 800 rpm. The turbidity of filtrate passing through a standard screen with 0.72 µm openings was determined after 30 s, after discarding the first 15 mL to pass through the screen. A second set of turbidity measurements was obtained in each case by discontinuing the stirring and allowing a fiber mat to form on the Britt Jar screen. Again, samples of filtrate were obtained after discarding the first 15 mL passing through the screen. Reference experiments, to determine the contribution of the PECs to turbidity, were carried out by the same Britt Jar procedures, except that no fibers were present. Blank experiments, with fibers but no added chemicals, were carried out to evaluate the contribution to turbidity of the small amount of fine materials remaining in the pulp after fractionation. The percent retention efficiency was calculated in each case by

% retention ) (tpolymer - ttest - xtblank)/tpolymer (1) where tpolymer is the turbidity of the filtrate in the absence of fibers, ttest is the turbidity of filtrate from tests in which fibers were present, and tblank values were obtained by evaluating filtrate in the absence of chemical additives. Preliminary observations showed that a further correction was needed since some of the calculated values of retention efficiency were higher than 100%. The coefficient x was used as a fitting parameter to keep the values within a physically possible range. It was found that, by setting the value of x equal to 0.75 for the present series of experiments, the calculated results fell within the range of 0-100% retention efficiency. A physical explanation, to justify the correction term, is that PECs can be expected to act as a retention aid for residual fiber fines present in the fiber slurry. It is noted that the results of turbidity tests may depend on several factors, including particle size distribution and refractive index. In the present study, the time between formation of the PECs and their analysis was kept short and relatively constant in order to simplify interpretation of the results. A followup study, involving a different substrate, will employ thermogravimetric analysis (TGA) for determination of adsorbed amounts of PECs of the type considered in the present work. Results and Discussion PEC Formation and Turbidity. Turbidimetric tests revealed that the conditions under which the PECs

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Figure 2. Dependency of turbidity of polyelectrolyte mixtures on charge ratio. (A) Effect of background electrolyte type. (B) Effect of polyelectrolyte concentration.

Figure 3. Effect of time on the turbidity of mixtures of poly-DADMAC and CMC at different ratios of anionic to cationic macromolecular groups. (A) Continuously swirled mixtures. (B) Mixtures left unstirred except very briefly at the times of evaluation.

could be formed were relatively broad and that complex formation was not highly sensitive to changes in the nature of the background electrolyte. As shown in Figure 2A, the turbidity of mixtures containing different ratios of CMC charged groups added to the default polyDADMAC solution did not depend on whether the tests were carried out in deionized water or in the buffer solution of 1000 µS/cm conductivity, which was described earlier. These results are somewhat surprising in light of some previous work that showed a sharper dependency of PEC formation on a nearly equal number of positive and negative macromolecular charges at the limit of low ionic strength solutions.31 It is worth noting that the turbidity increased gradually up to the point of theoretical one-to-one pairing of macromolecular charges, consistent with the progressive formation of approximately stoichiometric complexes. The partial drop in turbidity values corresponding to yet higher relative amounts of CMC is tentatively attributed to restabilization of macromolecules in forms that were less efficient in the scattering of light.31,42 As shown in Figure 2B, very similar results were obtained at two different concentrations of the default cationic polyelectrolyte. The differences in turbidity between the two sets of results are consistent with the differing amounts of polymeric materials present, if it is assumed that essentially all of the macromolecules contribute to the observed turbidity in proportion to their mass. The fact that the shapes of the two curves were very similar helps to support the assertion that PEC formation was not highly sensitive to the conditions of testing. Colloidal Stability of Complexes. As shown in Figure 3A, the colloidal stability of PECs, as indicated by changes of turbidity with time, depended on the ratio

of CMC added to the default poly-DADMAC. It is worth noting that the least change with time was observed in the case of a mixture having the largest excess of CMC relative to the cationic polymer (see results corresponding to 120%). Similarly, the results corresponding to the lowest ratio (90%) showed less change with time as compared to mixtures that were closer to a 1:1 stoichiometry of charged groups. These results are consistent with a hypothesis that an excess of one or the other charged polymer is able to charge-stabilize PECs31 and reduce the likelihood of sticking collisions that could lead to growth of PECs, settling out of the suspension, or other changes affecting the measured turbidity. Figure 3B shows results of similar tests, except that the polyelectrolyte mixtures were not stirred after their initial mixing. Under these conditions the least change, with time, in the measured turbidities was observed when there was an excess of the cationic polymer (see results for 90%). The fact that the turbidity values otherwise tended to increase with time in the absence of stirring is attributed to the fact that the PECs were observed to settle into a coagulum at the bottom of the test vessel between the times that the turbidity was tested. Although the mixture was momentarily swirled, just before each measurement, to achieve representative values for the sample as a whole, this swirling did not reverse the effects of sedimentation the resulting contact between adjacent PECs. Effects of Cationic Polyelectrolyte Attributes. The independent variables considered included cationic polyelectrolyte molecular mass, linear versus branched nature, and chemical type. The efficiency of PEC retention on fiber surfaces is given in Figures 4 and 5 for two different chemical classes of cationic polymer, to which CMC was added with stirring. The relative

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Figure 4. Effect of charge ratio on the retention efficiency of PECs on fibers that had been optionally treated to make them cationic, with two different procedures of agitation. (A) PECs formed from the default poly-DADMAC and CMC. (B) PECs formed with higher mass poly-DADMAC and CMC.

Figure 5. Effect of charge ratio on PEC retention efficiency on fibers optionally treated with poly-DADMAC, with two different procedures of agitation. (A) PECs formed from the poly(ethylenimine) (PEI) and CMC. (B) PECs formed with higher mass copolymer of PEI and CMC.

standard deviations within 10-fold replication of individual turbidity observations ranged from near zero to about 50% (in the case of very low turbidities), with an average value of the relative standard deviation of 15.6%. Thus, differences in estimated retention of less than about 10% were not considered in interpretation of the results. As shown in Figure 4, PECs formed from the cationic polymer of higher mass with CMC were usually retained on fibers at significantly higher efficiency as compared to PECs that contained the same amount of the default poly-DADMAC. Within the range of charge ratios between 8:10 and 10:8 cationic to anionic groups on the macromolecules in the PECs, the results were generally insensitive to pretreatment of the fibers with sufficient default poly-DADMAC to saturate the fiber surfaces. These results are in contrast to those reported by Kekkonen et al.29 for adsorption of PECs onto anionic silica. In the cited study, essentially no adsorption was detected when the net charge of the PEC had the same sign as that of the substrate and that adsorption was maximized when the PEC was weakly charged, opposite in sign to the substrate. The difference between the two studies is tentatively attributed to the higher anionic charge density of the silica as compared to cellulose used in the present study. Generally similar results were obtained under two contrasting conditions of agitation. In some of the experiments, as noted in Figure 4, the suspension was

stirred continuously at 800 rpm, during collection of filtrate, preventing formation of a fiber mat. In other cases the agitation was stopped immediately before collection of filtrate. Although the two procedures yielded raw turbidity results that differed by as much as a factor of 2 from each other, it is notable that application of eq 1 yielded the results in Figure 4, showing similar ranges of PEC retention. It can be concluded, based on these results, that PEC-to-fiber attachments, once established, were not highly sensitive to shear forces. This is a significant result in light of the high levels of hydrodynamic shear present at different points in a typical paper machine process.43 Figure 5 shows related results obtained with two PEItype cationic polymers, following their interaction with the same anionic CMC solution. As shown in Figure 5A, complexes formed with the PEI homopolymer achieved high retention, based on the turbidimetric tests, under all conditions that were considered with the exception of the least cationic complex (8:10 ratio) under the conditions of continuous agitation during collection of the filtrate (800 rpm) and untreated (net negative) fiber surfaces. It makes logical sense that the weakest adhesion between PECs and fibers would be observed under conditions of net negative charge on both entities and that such attachments would be the most vulnerable to detachment in the presence of shear. By contrast, it is to be expected that the presence of a fiber mat during collection of filtrate would be able to act as a sieve44-48 and retain a higher level of PECs when the filtrate was collected in the absence of stirring. As shown in Figure 5B, the higher mass PEI-type polymer in combination with CMC showed a significantly lower efficiency of retention onto fibers in most cases considered. Thus, the nature of the cationic polymer clearly makes a difference relative to PEC retention. It is also worth noting, again, that generally similar results were obtained under the two contrasting hydrodynamic conditions. It is also worth noting that some of the charge relationships appear out of line with a simple model based on net charges. For example, the lowest calculated retention result (the calculated value coming out in the negative) corresponded to the most negatively charged PEC adsorbing onto net-negative fiber surfaces, with the filtrate collected from an unstirred suspension, allowing the formation of a fiber mat. To summarize, there did not seem to be a clear relationship between PEC net charge (based on added amounts) or fiber pretreatment and the retention results. A mechanism related to charged-patch coagulation49 can be used to explain the weakness of the observed dependencies of PEC deposition on charge relationships, including the PEC charge ratio and whether or not the fiber surfaces were pretreated with cationic polymer. It is tentatively proposed that the surfaces of the PECs contained a nonuniform distribution of charged segments. A nonuniform, patchy distribution of macromolecular segments of opposite charge can explain sticking collisions between a wide range of PEC compositions onto fibers of either positive or negative charge. A nonuniform distribution can be further justified by the irreversible nature of some interactions among high-mass polyelectrolytes of opposite charge, which results in trapped nonequilibrium structures.31,50 If this hypothesis is valid, then it can be expected that strategies based on PEC deposition can be used within

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Figure 6. Development of turbidity during addition of CMC solution to different solutions of cationic polymers. (A) Poly-DADMAC of moderate vs high molecular mass. (B) Comparison between linear vs branched poly-DADMAC.

relatively wide windows of polymer ratios and fiber treatment conditions, increasing the likelihood of practical success. To better understand the contrast between panels A and B of Figure 4, considering the molecular mass of poly-DADMAC and the retention of the resulting PECs, further tests were carried out in the absence of fibers but with the same background electrolyte as before. Figure 6A shows the evolution of turbidity with progressive addition of CMC solution to the cationic polymer solution. As shown, the general shapes of the two curves were similar, consistent with a similar charge dependency of interactions in each case. It is interesting that the higher mass cationic polymer yielded lower values of turbidity when combined with CMC. A macromolecular size dependency of PEC formation and stability can be rationalized based on the “guest-host” concept proposed by Kabanov and Zezin42 and, in the case of CMC, by Argu¨elles et al.51 According to this model, a relatively large macromolecule can act as a “host” for a smaller macromolecule of opposite charge, and such a combination can remain as a clear solution. The high mass poly-DADMAC has a molecular mass of about 2 million Da as compared to about 250-600 kDa in the case of the CMC. Figure 6B shows corresponding results for the case of a branched poly-DADMAC, in contrast with the default poly-DADMAC. It is interesting in this case that the maximum turbidity was achieved at a significantly lower ratio of CMC to cationic polymer in the case of the branched poly-DADMAC. It is tempting to attribute this finding to a lesser accessibility of at least some of the charged groups within the branched structure. Similar turbidity tests were carried out with the PEIrelated polyelectrolytes. In general the turbidities increased with addition of CMC to the mixtures, at least up to a theoretical 1:1 mixture of macromolecular charges. However, the measured turbidities were generally lower than those observed in the case of the polyDADMAC samples. In addition, it was not possible to observe the PECs under the microscope (see next section); this is further evidence that detailed chemical structures can affect the nature of PECs that are formed. Electrokinetics. Figure 7 shows the results of microelectrophoresis tests of mixtures formed with CMC solution added to poly-DADMAC solutions. The electrophoretic mobility (EM) rose as expected with increasing ratio of cationic to anionic macromolecular groups in the final mixture. Limit bars corresponding to the default polyelectrolyte show 95% confidence limits for the data.

Figure 7. Effect of charge ratio on the electrokinetic mobility of PECs formed from CMC being added to poly-DADMAC. Mean values for the default polyelectrolyte (shown with 95% confidence intervals) are compared to higher mass and branched polyDADMAC.

It is interesting that near-zero mobility was observed at a ratio of 8:10 cationic to anionic groups. By comparison with Figure 2, it is clear that the maximum in turbidity was achieved at a ratio of 10:10 cationic to anionic groups. These observations are similar to earlier findings that the sign of the observed electrokinetic potential of PECs does not necessarily follow from their net composition.31 In the cited study, for instance, it was found that results depended on the order of addition of the two polyelectrolytes, and the resulted PECs appeared to have an excess of the later-added polyelectrolyte (the titrant) on their outer portions at the end-point of charge titrations. As shown by the positions of the three curves in Figure 7, the relatively high mass cationic polymer in combination with CMC came the closest to matching expectations based on simple charge relationships. Thus, a near-neutral EM was observed at a 1:1 calculated ratio of macromolecular charges. By contrast, both the branched poly-DADMAC and the default polyDADMAC showed positively charged character of PECs having a 1:1 ratio of charges. Although the reasons for these differences are not known, it is reasonable to expect that tails or loops of the higher mass linear polyDADMAC extend farther from the PEC core and have a relatively greater effect on electrokinetic properties. It is interesting that in Figure 4B the highest retention was obtained in cases of PECs shown here to be either neutral or positive in electrokinetic potential, regardless of whether the fibers were negative or pretreated to be positive before addition of the PECs. Similarly, in Figure 4A, in which all of the PECs would

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have been neutral or positive according to Figure 7B, approximately equal retention was found in each case, especially if one pools results for different conditions of hydrodynamic shear. To explain these observations, one possibility is that those PECs that had a net negative electrophoretic mobility were less adhesive with surfaces in general, including the surfaces of PECs, and therefore less efficient in being retained on fiber surfaces. Such a suggestion, although not proven, is qualitatively consistent with the greater stability of turbidimetric results, with time, shown for the most highly anionic PEC considered in Figure 3A. Conclusions (1) Polyelectrolyte complexes could be formed by addition of CMC solutions to various different cationic polymer solutions, usually with increasing complex formation up to a 1:1 ratio of macromolecular charges. Results showed a tolerance for different charge ratios, background electrolyte, and net concentration in the final mixture. Although the turbidity of the resulting complexes tended to reach a maximum when the added amounts corresponded to a stoichiometric match of macromolecular charges, complexes having a net excess of one or the other charge often showed less change in turbidity with the passage of time. (2) Complex formation was sensitive to the nature of the cationic polymer, to which CMC solution was added. A higher mass poly-DADMAC resulted in lower turbidity values as compared to the default linear cationic polymer. By contrast, a branched poly-DADMAC affected the ratio of charges at which the maximum in turbidity was achieved, consistent with a lesser accessibility of some of the cationic groups, in terms of complexation reactions with the anionic polymer. (3) The observed dependencies on charge relationships of PEC retention onto fibers were weak and not consistent with a simple attraction between net oppositely charged entities. Rather, the results suggest an irregular, patch-like charge distribution on PEC surfaces, allowing relatively efficient adsorption onto fibers over a fairly wide range of PEC charge ratio as well as differently charged fiber surfaces. This view was confirmed by electrokinetic tests, revealing the sign of ζ-potential of the PECs at various charge ratios for various different poly-DADMAC solutions in combination with CMC. Acknowledgment The authors are grateful for support from the National Science Foundation, as part of the Green Processing Undergraduate Research Program (REU), EEC9912339, which supported the work of S.M.M. Literature Cited (1) Moeller, H. W. Cationic starch as a wet-end strength additive. Tappi J. 1966, 49 (5), 211. (2) Marton, J.; Marton, T. Wet end starch: adsorption of starch on cellulosic fibers. Tappi J. 1976, 59 (12), 121. (3) Hofreiter, B. T. Natural products for wet-end addition. In Pulp and Paper Chemistry and Chemical Technology, 3rd ed.; Casey, J. P., Ed.; Wiley-Interscience: New York, 1980; Vol. III. (4) Lindstro¨m, T.; Flore´n, T. The effects of cationic starch wet end addition on the properties of clay filled papers. Svensk Papperstidn. 1984, 87, R99.

(5) Roberts, J. C.; Au, C. O.; Clay, G. A.; Lough, C. The effect of C14-labelled cationic and native starches on dry strength and formation. Tappi J. 1986, 69 (10), 88. (6) Howard, R. C.; Jowsey, C. J. The effect of cationic starch on the tensile strength of paper. J. Pulp Pap. Sci. 1989, 15, J225. (7) Formento, J. C.; Maximino, M. G.; Mina, L. R.; Srayh, M. I.; Martinez, M. J. Cationic starch in the wet end: its contribution to interfiber bonding. Appita J. 1994, 47, 305. (8) Beaudoin, R.; Gratton, R.; Turcotte, R. Performance of wetend cationic starches in maintaining good sizing at high conductivity levels in alkaline fine paper. J. Pulp Pap. Sci. 1995, 21, J238. (9) Tanaka, A.; Hiltunen, E.; Kettunen, H.; Niskanen, K. Interfiber bonding effects of beating, starch, and filler. Nord. Pulp Pap. Res. J. 2001, 16, 306. (10) Horsey, E. F. Sodium carboxymethylcellulose for papermaking. Tech. Assoc. Pap. 1947, 30, 294. (11) Beghello, L.; Long, L. Y.; Eklund, D. Laboratory study on carboxymethylcellulose as a wet-end additive in paperboard making. Paperi Puu 1997, 79, 55. (12) Hubbe, M. A.; Jackson, T. J.; Zhang, M. Fiber surface saturation as a strategy to optimize dual-polymer dry strength treatment. Tappi J. 2003, 2 (11), 7. (13) Watanabe, M.; Gondo, T.; Kitao, O. Advanced wet-end system with carboxymethyl-cellulose. Tappi J. 2004, 3 (5), 15. (14) Ekevåg, P.; Lindstro¨m, T.; Gellerstedt, G.; Lindstro¨m, M. E. Addition of carboxymethylcellulose to the kraft cook. Nord. Pulp Pap. Res. J. 2004, 19, 200. (15) Howard, R. C.; Bichard, W. Basic effect of recycling on pulp properties. J. Pulp Pap. Sci. 1993, 19, J57. (16) Nazhad, M. M.; Pazner, L. Fundamentals of strength loss in recycled paper. Tappi J. 1994, 77 (9), 171. (17) Horn, R. A. What are the effects of recycling on fiber and paper properties? Prog. Pap. Recycl. 1995, 4, 76. (18) Hubbe, M. A.; Venditti, R. A.; Barbour, R. L.; Zhang, M. Changes to unbleached kraft fibers due to drying and recycling. Prog. Pap. Recycl. 2003, 12, 11. (19) Park, S.-B.; Tanaka, H. Effects of charge densities of cationic polyacrylamides on strength properties of handsheets. Mokusai Gakkaishi 1998, 44, 199. (20) Pelton, R.; Zhang, J.; Wågberg, L.; Rundlo¨f, M. The role of surface polymer compatibility in the formation of fiber/fiber bonds in paper. Nord. Pulp Pap. Res. J. 2000, 15, 400. (21) Zhang, J.; Pelton, R.; Wågberg, L.; Rundlo¨f, M. The effect of charge density and hydrophobic modification on dextran-based paper strength enhancing polymers. Nord. Pulp Pap. Res. J. 2000, 15, 440. (22) Zhang, J.; Pelton, R.; Wågberg, L.; Rundlo¨f, M. The effect of molecular weight on the performance of paper strengthenhancing polymers. J. Pulp Pap. Sci. 2001, 27, 145. (23) Kitaoka, Y.; Tanaka, H. Novel paper strength additive containing cellulose-bonding domain of cellulase. J. Wood Sci. 2001, 47, 322. (24) Yamauchi, T.; Hatanaka, T. Mechanism of paper strength development by the addition of dry strength resin. Appita J. 2002, 55, 240. (25) Wågberg, L.; Forsberg, S.; Johansson, A.; Juntti, P. Engineering of fiber surface properties by application of the polyelectrolyte multilayer concept. Part 1: Modification of paper strength. J. Pulp Pap. Sci. 2002, 28, 222. (26) Michaels, A. S. Polyelectrolyte complexes. Ind. Eng. Chem. 1965, 57 (10), 32. (27) Philipp, B.; Dautzenberg, H.; Linow, K.-J.; Ko¨tz, J.; Dawydoff, W. Polyelectrolyte complexessrecent developments and open problems. Prog. Polym. Sci. 1989, 14, 91. (28) Buchhammer, H.-M.; Kramer, G.; Lunkwitz, K. Interaction of colloidal dispersions of non-stoichiometric polyelectrolyte complexes and silica particles. Colloids Surf. A 1994, 95, 299. (29) Kekkonen, J.; Lattu, H.; Stenius, P. Adsorption kinetics of complexes formed by oppositely charged polyelectrolytes. J. Colloid Interface Sci. 2001, 234, 384. (30) Mende, M.; Petzold, G.; Buchhammer, H.-M. Polyelectrolyte complex formation between poly(diallyldimethyl-ammounium chloride) and copolymers of acrylamide and sodium acrylate. Colloid Polym. Sci. 2002, 280, 342. (31) Chen, J.; Heitmann, J. A.; Hubbe, M. A. Dependency of polyelectrolyte complex stoichiometry on the order of addition. 1. Effect of salt concentration during streaming current titrations with strong poly-acid and poly-base. Colloids Surf. 2003, 223, 215. (32) TAPPI Test Methods T205; TAPPI Press: Atlanta, 1996.

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Received for review November 13, 2004 Revised manuscript received February 4, 2005 Accepted February 14, 2005 IE048902M