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KINETICS, CATALYSIS, AND REACTION ENGINEERING Mechanisms of Aldehyde-Containing Paper Wet-Strength Resins Nicole Chen, Shuwen Hu, and Robert Pelton* McMaster Centre for Pulp and Paper Research, Department of Chemical Engineering JHE-136, McMaster University, Hamilton, Ontario, Canada L8S 4L7
The ability of dextran (513 000 Da) with pendant acetaldehyde groups to increase the wet strength of filter paper was evaluated as a function of the aldehyde content, impregnation pH, and temperature. The impregnation of filter paper with dextran aldehyde increased the wet tensile strength. By contrast, neither dextran nor unhydrolyzed dextran acetal enhanced the wet paper strength. Drying of the impregnated sheets was a crucial step. Never-dried impregnated filter paper showed no improvement in tensile strength. This result contradicts the behavior of starch aldehyde, which can improve wet web strength. Impregnation under acidic conditions gave greater wet-strength improvements because of enhanced dextran-fiber bond formation compared with that occurring at neutral or alkaline pH. This result was explained by an equilibration process at low pH that allows dextran-dextran bonds to be converted to dextran-fiber bonds. Wet-strength improvements slowly disappeared with rewetting time as a result of the hydrolysis of acetal and hemiacetal dextran-fiber bonds. The hydrolysis rates were fastest at low pH. Introduction Most paper products become very weak when exposed to water because wood pulp fiber surfaces are very hydrophilic and swell in water to disrupt fiber-fiber bonds.1 Paper towels, tissues, and some paperboards are treated with water-soluble polymers, called wet-strength resins, to maintain strength. Although wet-strength resin technologies have been developed for many decades and have been the subject of book chapters2-4 and review articles,5,6 relatively little has been published about the detailed effects of wet-strength resin structure on wet-strength enhancement. The paper industry generally divides wet-strength resins into two groups, temporary and permanent. From a chemistry perspective, the permanent resins generally are able to form covalent bonds that are not readily hydrolyzed in water, whereas temporary resins are usually based on reactive aldehydes that form acetal or hemiacetal linkages with paper. However, a major difficulty is encountered in the use of aldehydefunctional polymers as wet-strength resins. Aldehydecontaining polymers are not stable upon storage because they can undergo cross-linking and oxidation. To overcome this storage problem, acetal-containing starch7,8 and polyacrylamide acetal have been developed.9 Acetals are stable functional groups in water at neutral pH; however, they can be hydrolyzed at low pH to generate active aldehydes (see Figure 1). Described in this paper are studies on the wetstrengthening mechanisms of dextran containing reactive aldehyde groups generated from the corresponding * Corresponding author. Phone: (905) 529 7070 ext. 27045. Fax: (905) 528 5114. E-mail:
[email protected].
Figure 1. Hydrolysis of dextran diethyl acetal to the corresponding aldehyde.
acetals. Dextran, a carbohydrate derived from bacteria, is not a commercial wet-strength resin. Instead, it is a well-defined carbohydrate that serves as a useful model for starch. Our work contributes to the mechanistic understanding of commercial starch acetal, dialdehyde starch, glyoxylated polyacrylamide, and polyacrylamide acetal wet-strength resins. Specifically, this work clarifies the effects of the degree of aldehyde substitution on dextran, the pH of treatment, and the drying temperature, providing information that can contribute to the design of superior resins. Some aspects of the literature are now summarized to put our work in context. The paper technology literature contains much discussion about the mechanism of wet-strength resins, and the following conclusions seem to be well founded: (a) Wet-strength resins function mainly by increasing the strength of wet fiber-fiber bonds. (b) Reactive resins, depending on the chemistry, form covalent bonds with the fiber surfaces (fiber coupling) or with themselves (cross-linking). (c) Aldehyde groups can react with carbohydrates to form hemiacetal or acetal linkages (Figure 2). Furthermore, these reactions are promoted by decreases in pH, the removal of water, and increases in temperature. (d) The maximum achievable wetstrength improvement upon polymer addition is usually
10.1021/ie020355m CCC: $22.00 © 2002 American Chemical Society Published on Web 09/26/2002
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5367 Table 1. Properties of Modified Dextrans
Figure 2. Hemiacetal and acetal formation between aldehyde groups and carbohydrates.
Figure 3. Addition of acetal groups to dextran.
far below the tensile strength of the corresponding dry paper because delignified wet pulp fibers fail when loaded in the transverse direction.10,11 By contrast, small multifunctional reagents that can enter small fiber pores and cross-link fiber walls can give high wet tensile and compression strengths; however, the corresponding dry paper is very brittle.12 From a micromechanical perspective, the origin of the wet-strengthening mechanism is not clear. For example, it has been proposed that the resins function by preventing swelling of the fiber-fiber bond region so that indigenous fiber-fiber hydrogen bonds are not completely destroyed. This is often called the protection mechanism.5 Alternatively, it has been suggested that the resins act as covalently bonded tethers holding the fibers together, even though the fiber-fiber hydrogen bonds have been disrupted by swelling of the bonding regions. However, such details are difficult to clarify. Experimental Section Dextran (513 000 Da) was supplied by Sigma, and bromoacetaldehyde diethyl acetal, sodium hydride (60% dispersion in mineral oil), and filter agent (Celite 521) were purchased from Aldrich. The water content in dimethyl sulfoxide (DMSO) solvent was minimized by the addition of sodium hydroxide pellets. Dialysis tubing (14 000 MW cutoff) from Membra-Cell was washed several times with boiling water before use. Dextran acetal was prepared by reaction with bromoacetaldehyde diethyl acetal; see Figure 3. In a typical reaction, 15 g of dextran was dissolved in 300 mL of DMSO, and the mixture was heated to 80 °C in a 500mL round-bottom flask fitted with a mechanical stirrer and a heating mantle. After the dextran was completely dissolved, 4.74 g of sodium hydride solution and 17.52 g of bromoacetaldehyde diethyl acetal were added
namea
dextranb (g)
NaH (g)
BDAc (g)
mass yield (g)
reaction yield (%)
DSd
MDEX-1 MDEX-2 MDEX-3 MDEX-4 MDEX-5 MDEX-6 MDEX-7
10 10 15 15 10 12 10
1.65 2.28 4.74 4.74 3.76 4.01 3.76
2.37 5.48 17.52 17.52 12.78 13 12.78
6.8 6.3 11.6 10.8 6.4 7.7 6.4
46 73 64 69 66 78 86
0.035 0.11 0.20 0.22 0.24 0.25 0.30
a Samples MDEX 1-5 were used for paper testing, sample MDEX 6 for dynamic mechanical analysis and MDEX-7 for dried film studies. b Dextran used for the modification has molecular weight of 513 kDa. c Bromoacetaldehyde diethyl acetal. d DS is the degree of substitution which is the number of acetal groups per anhydroglucose unit.
sequentially. The reaction mixture was stirred at 80 °C for about 8 h under a nitrogen blanket. The product was dialyzed against water for 7-10 days to remove solvent and unwanted chemicals, such as NaBr. After dialysis, the product was filtered through a medium-porosity glass crucible supporting a thin layer of filter agent (Celite 521) and then vacuum-dried at -50 °C. A series of modified dextrans was prepared, and depending on the feed of bromoacetaldehyde diethyl acetal, the reaction yield ranged from 46 to 86%. The corresponding degree of substitution, DS, (i.e., the number of acetal groups per anhydroglucose unit) was between 0.035 and 0.3 as determined by NMR spectroscopy.13 The dextran modification conditions and properties are summarized in Table 1. The dextran acetal polymers were converted to the corresponding aldehyde derivatives by hydrolysis at pH 3 and 80 °C (see Figure 1). The normal hydrolysis time was 2 h; however, hydrolysis kinetic results are also presented. The hydrolyzed polymers were either freezedried for NMR analysis or used immediately for paper impregnation. The wet-strengthening capabilities of the modified dextrans were evaluated by impregnating filter paper with polymer, drying the paper, rewetting the paper, and measuring the wet tensile strength. The filter paper was Whatman #1 (47 × 56 cm, 100% cotton fiber) with a basis weight of 88 g/m2 and a thickness of 0.18 mm. Paper impregnation consisted of soaking dry filter paper for 30 min at room temperature using polymer solutions with dextran concentrations up to 1%. The polymer pickup was determined gravimetrically. Preliminary tests showed that both the pickup and wet strength were independent of impregnation time for times longer than a few minutes. Impregnated sheets were restraint-dried either at room temperature or at 120 °C for 15 min using a Speed Dryer, Labtech Instruments Inc. (St.-Laurent, Que´bec, Canada). The dried, impregnated papers were conditioned for at least 5 h at 23 ( 0.5 °C and 50 ( 1% and then were cut in the machine direction into strips of width 15 mm and length ∼160 mm using a model 2224 twin blade cutter from Testing Machines Inc. (Montre´al West, Que´bec, Canada). For rewetting, the test strips were soaked in water, usually for 30 min. Some results are reported as functions of rewetting time. The pH’s of the impregnation and rewetting solutions were the same and were either 3 or 8. Tensile measurements were performed using an Instron model 4411 universal testing system (Instron
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Figure 4. Influence of dextran acetal on the wet strength of impregnated filter paper. RT, room-temperature drying; HT, 120 °C drying.
Figure 5. Kinetics of dextran acetal hydrolysis to dextran aldehyde.
Corporation, Canton, MA). The load cells used for dry and wet specimens were 500 and 50 N, respectively. The elongation rate was 10 mm/min, and the initial gauge span between the jaws was fixed at 100 mm. In most cases, six samples were tested for each experimental condition. The mechanical properties of dry cross-linked polymer films were examined using a Dupont 983 dynamic mechanical analyzer (DMA). Testing samples were prepared by impregnation of an inert glass fiber braid with 10% aqueous polymer solutions. Measurements were made at a constant fixed frequency of 1 Hz and in a temperature range of 100-250 °C. Both the shear storage modulus and tan δ values are reported. Results This work is a study of the wet-strength properties of finished sheets of paper impregnated with dextran aldehyde. By contrast, commercial wet-strength resins are usually adsorbed onto the papermaking fibers in the “wet end” before the paper is formed. Wet-end addition was not used for this work because (1) cationic resins must be used to achieve adsorption; (2) the amount of adsorbed resin is limited to a monolayer; (3) the amount of adsorbed resin must be measured; and (4) the wetstrength resin can cause fiber flocculation, giving poorly formed paper with nonuniform mass distribution. Using impregnation, large amounts of resin can be accurately added, and paper formation is constant. Previous work has shown that, for dry strength, the two methods give similar results at similar polymer contents.15 A series of dextran acetals was prepared and characterized by NMR spectroscopy. The polymer properties are summarized in Table 1; characterization details and sample NMR spectra are provided in N. Chen’s thesis.13 The dextran polymers bearing acetals (structures in Figure 3) did not improve wet strength. For example, Figure 4 shows the wet tensile strength of filter paper as a function of the content of dextran acetal with a degree of substitution of 0.24. Indeed, the wet web strength was slightly decreased by impregnation with dextran acetal. The acid-catalyzed hydrolysis of the dextran acetals to the corresponding aldehydes is shown in Figure 1. The hydrolysis kinetics was followed by measuring the disappearance of the ethoxy methyl groups in the NMR spectra. Figure 5 shows the acetal concentration as a function of hydrolysis time and the corresponding degree
Figure 6. Influence of dextran concentration and degree of aldehyde substitution on the wet tensile strength of filter paper. The error bars show the standard deviation of the mean of six measurements.
of hydrolysis. The kinetic data were fit to the first-order kinetics equation
-
d[A] ) k[A] dt
(1)
where [A] is the molar concentration of acetal groups. The rate constant, k, was determined to be 2.26 × 10-4 s-1. From a practical perspective, the hydrolysis reaction at 80 °C and pH 3 was slow; about 1 h was required to hydrolyze one-half of the groups. Dextran aldehyde was an effective wet-strength resin. Figure 6 shows that the wet-strengthening effect increased with the dextran content and also with the degree of substitution. Most of the improvements were observed over the polymer content range 0-1%; beyond this range, higher levels of dextran aldehyde impregnation gave decreasing benefits. The effects of pH and drying temperature are illustrated in Figure 7. Note that corresponding impregnation and rewetting steps were conducted with solutions of the same pH. The ability of dextran aldehyde to improve wet strength was not very dependent on the drying temperature. On the other hand, never-dried sheets showed absolutely no improvement in wet tensile strength with polymer treatment. Thus, drying is a critical step for wet-strength development, but the drying temperature is not very important. Filter paper impregnated and rewetted at pH 3 gave nearly twice the wet tensile strength of filter paper
Ind. Eng. Chem. Res., Vol. 41, No. 22, 2002 5369 Table 2. Water Solubility of Cast Dextran Aldehyde Films
pHa
drying temp (°C)
3
23
8
23
3
100
8
100
a
dextran dissolved instantly dissolved instantly dissolved instantly dissolved instantly
MDEX-7 (0.3 DS) nonhydrolyzed hydrolyzed dissolved instantly dissolved instantly dissolved instantly dissolved instantly
gel dissolved in about 8 h gel dissolved in about 13 h gel did not dissolve in 1 week gel did not dissolve in 1 week
pH of water used to dissolve the films.
Figure 7. Influence of pH (both impregnation and rewetting) and drying temperature on wet strength. RT, room-temperature drying; HT, 120 °C drying.
Figure 9. Dynamic mechanical analysis for dextran and MDEX (0.25 DS) polymer film. The storage modulus G′ and tan δ were recorded from 100 to 250 °C at a constant frequency of 1 Hz.
Figure 8. Influence of pH and rewetting time on wet tensile strength. The filter paper was impregnated with 2% polymer, and the error bars show the standard deviation of the mean tensile strength based on six samples.
a glass transition, whereas the aldehyde-containing dextran gave broad curves typical of a cross-linked polymer. Discussion
treated at pH 8 (see Figure 7). Thus, pH is an important factor for aldehyde-containing wet-strength agents. The rate of wet-strength loss with rewetting time was also sensitive to pH. Figure 8 shows the wet tensile strength as a function of time at pH 3 and 8. The initial wet tensile strengths at pH 3 and 8 were equal because the impregnation steps for both data sets were conducted at pH 3. The wet tensile strength decreased with rewetting time from ∼8 to ∼4 N‚m/g. The decrease in strength occurred more quickly at pH 3 than at pH 8. The wet tensile did not decay to 1 Nm/g, the value obtained without dextran (see Figure 6). Note that a 30-min rewetting time was used for the other results reported herein. To test water solubility, 3-mL portions of 1% dextran aldehyde solutions were dried in 1.5-in. aluminum weighing dishes to yield thin films. Table 2 summarizes the results as a function of drying temperature and pH. In all cases, the films formed from unhydrolyzed dextran acetal were water-soluble. By contrast, the dextran aldehyde solutions dried to form water-insoluble films. Extended soaking at pH 3 solubilized films dried at room temperature, whereas films dried at 100 °C did not dissolve with extensive soaking at pH 3 or 8. Perhaps more hydrolysis-resistant acetal groups were formed at high temperature. The dynamic mechanical properties of dextran and dried dextran aldehyde are presented in Figure 9. tan δ for dextran showed a peak around 210 °C indicating
We believe that this work is the first published systematic study of the influence of aldehyde concentration on the performance of wet-strength-enhancing polymers for paper. Although aldehyde-containing polymers have been known for decades to increase the wet strength of paper, the detailed relationships between polymer structure and paper strength have been proprietary to companies involved in these technologies. The importance of aldehyde groups is illustrated by the fact that dextran and acetal-containing dextran do not increase wet tensile strength, whereas dextran aldehyde is an effective wet-strength-enhancing polymer. The pendant aldehyde groups on dextran can react with hydroxyls on the same dextran molecule (a process called cyclization), with hydroxyl groups on other dextran molecules (cross-linking), and with hydroxyls on fiber surface carbohydrates (fiber coupling). For all three cases, aldehydes react with hydroxyls to form both hemiacetal and acetal groups in sequential, reversible reactions. Because all of these reactions involve carbohydrates, the reaction rate constants should be approximately the same for cyclization, cross-linking, and fiber coupling. On the other hand, topochemical effects will be operative. For example, only dextran next to the fiber surface will be capable of coupling with fiber. By contrast, cyclization will be favored by reaction in dilute solution at early stages of water removal. Hemiacetal groups will hydrolyze in water under acidic and neutral pH conditions, whereas acetal functional groups will hydrolyze in water only at low pH.5
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Figure 7 shows that paper impregnated at pH 3 is stronger than that impregnated at pH 8. When paper impregnated at pH 3 is dried, it seems reasonable to propose that hemiacetal bonds will form and hydrolyze in an equilibration process, thus providing the opportunity to replace dextran-dextran bonds with dextran-fiber bonds. By contrast, at pH 7, hemiacetal bond formation is effectively irreversible, so dextran-dextran bonds cannot transform into the more desirable dextran-fiber bonds. A related observation (see Figure 8) is that the acid-catalyzed hydrolysis of hemiacetal and acetal groups leads to a much more rapid rate of wetstrength loss compared with soaking in water under neutral pH conditions. The conventional view of wet strengthening is that the wet-strength resin increases the strength of fiberfiber bonds. Although the details are open to debate, it seems clear that an effective wet-strengthening polymer must be located in the fiber-fiber bonding region or at the bond periphery. Dextran is a neutral water-soluble polymer that does not adsorb onto fiber surfaces.14 Thus, we postulate that the dextran aldehyde concentrates in the aqueous phase as the water evaporates from an impregnated paper sheet. Once air starts to penetrate the drying sheet, it seems reasonable to propose that capillary forces will concentrate the dextran solution at the edges of fiber junctions. In previous work, this effect was illustrated by photographing a drop of fluorescently labeled dextran solution as it dried on the junction of crossed rayon fibers. With drying, the polymer concentrated in the crevice formed by the fiber junction.15 Consider a sheet of paper with a specific surface area accessible to dextran of 2 m2/g. If that sheet is impregnated with 1% dextran, a uniform coverage would give a dextran film 50 nm thick. If, as proposed above, capillary forces concentrate the dextran at fiber-fiber junctions, then the thickness of the dextran domains will be greater than 50 nm, which, in turn, implies many layers of dextran molecules. To give strength when wet, the dextran domains must be cross-linked and must also be coupled to the fibers. The influence of DS on crosslinking is now considered. The range of DS values (i.e., the number of acetal groups per anhydroglucose unit) of 0.035-0.24 corresponds to 29-4 anhydroglucose units in the dextran chain per potential cross-link. The network properties of the cross-linked dextran depend on the degree of aldehyde substitution and the relative contributions of cyclization, cross-linking, and fiber coupling. Cyclization (reaction within one dextran chain) will not prevent solubilization of dextran in water, whereas both fiber coupling and cross-linking contribute to network building and thus prevent solubilization. The relative contributions of the three reactions are determined by the kinetics of the condensation reactions. In the absence of detailed information about the dextran network structure, the influence of the aldehyde DS on dextran swelling was estimated from simple Flory-Huggins theory. The following equation was used to predict the degree of gel swelling as a function of DS16
(
V1υ φ21/3 -
)
φ2 ) -[ln(1 - φ2) + φ2 + χφ22] 2
(2)
where V1 is the molar volume of water, φ2 is the volume fraction of polymer in the swollen gel, υ is the number of cross-links per unit volume, and χ is the Flory-
Figure 10. Wet tensile strength as a function of the total aldehyde content of the impregnated handsheets. The data correspond to the results in Figure 6. The two points within the circle have about the same aldehyde content; however, the polymer loadings for 0.11 DS and 0.24 DS were 3 and 1%, respectively.
Huggins interaction parameter. According to this theory, the water content of a swollen cross-linked dextran gel based on 0.24 DS dextran is 90%, and the lowest-DS dextran, 0.035, should swell to have 97% water. Thus, it seems clear that cross-linked dextran does not give wet strength by forming a water-repellent protective coating via the “protection mechanism”.5 Thus, wet strengthening must involve covalent fiber-to-fiber tethers via a cross-linked dextran network. Note, that in addition to all of the assumptions behind eq 2, we assumed that the aldehydes were randomly distributed and that they completely reacted to form cross-links, thus permitting υ in eq 2 to be calculated from the DS. Because water is a good solvent for dextran, we assumed that χ equaled -0.5. It is clear from Figure 6 that wet strength is a function of both the amount of impregnated dextran and the degree of substitution. The total quantity of reactive aldehyde groups added by impregnation was the product of the mass of impregnated polymer and the specific aldehyde content. Figure 10 shows the wet tensile strength data from Figure 6 plotted as a function of the corresponding total aldehyde content. The results from the three series of data in Figure 4 collapsed to a single master curve (Figure 10). For example, the two points inside the circle have the same aldehyde contents, whereas the mass fractions of 0.11 DS and 0.24 DS were 3 and 1%, respectively. The implication from Figure 10 is that the total aldehyde content is a better predictor of wet-strength improvement than the degree of substitution. In other words, within limits, one obtains the same wet-strength improvement by adding a small mass of highly substituted dextran as by adding a large mass of lightly substituted dextran. The master curve in Figure 10 must break down in the extremes. For example, a crosslinked network cannot form if the number of effective aldehyde groups per dextran chain is less than 2. Finally, Laleg and Pikulik7 showed that treatment of bleached kraft pulp mixtures with starch aldehyde increased the wet web strength, which contradicts our observation that never-dried impregnated filter paper was no stronger than untreated wet sheets (see Figure 7). One difference is that our work involved impregnation, whereas Laleg and Pikulik’s experiments employed wet-end fiber treatment, which gives a thin, uniform
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coverage of polymer, including the entire fiber-fiber bonding region. Perhaps this explains the difference. In closing, it should be emphasized that this is an impregnation study, whereas many commercial resins are adsorbed onto the papermaking fibers in the wet end before the paper is formed. The impregnated polymers will be able to penetrate into preformed fiberfiber bonds, so the conclusions of this study might not be entirely applicable to wet-end-treated paper. On the other hand, wet-end addition has two serious limitations in that the amount of resin that can be added is restricted and flocculation can result in poorly formed paper with a nonuniform mass distribution. In contrast, impregnation completely avoids the uncertainties caused by variations in mass distribution. Conclusions The conclusions from this work are as follows: 1. Impregnation of filter paper with dextran aldehyde increases the wet tensile strength. By contrast, neither dextran nor nonhydrolyzed dextran acetal enhances wet paper strength. 2. Drying of impregnated sheets is a crucial step. Never-dried impregnated filter paper were found to exhibit no improvement in tensile strength. This contradicts the behavior of starch aldehyde, which can improve wet web strength.7 3. Wet-strength improvements can be almost completely explained by the total aldehyde content added to the sheets. Thus, adding a small amount of dextran bearing many aldehyde groups provides the same advantage as adding a large amount of lightly substituted dextran. However, this observation is not consistent with the notion that wet strength is improved by the presence of a cross-linked dextran network. 4. Impregnation under acidic conditions provides greater wet-strength improvements because of enhanced dextran-fiber bond formation compared with that occurring at neutral or alkaline pH. This result can be explained by an equilibration process at low pH that allows dextran-dextran bonds to be converted to dextran-fiber bonds. 5. Wet-strength improvements slowly disappear with rewetting time as a result of the hydrolysis of acetal and hemiacetal dextran-fiber bonds. The hydrolysis rates are fastest at low pH.
Acknowledgment The Mechanical Wood-Pulps Network of Centres of Excellence is acknowledged for funding this work. Literature Cited (1) Niskanen, K. Paper Physics. In Papermaking Science and Technology; Neimo, L., Ed.; Finnish Paper Engineers’ Association and Tappi: Helsinki, Finland, 1999. (2) Linhart, F. The Applications of Wet-End Paper Chemistry. In Applied Wet-End Paper Chemistry; Au, C. O., Thorn, I. Eds.; Blackie Academic and Professional: London, 1995. (3) Bates, R.; Beijer, P.; Podd, B. Papermaking Chemistry. In Papermaking Science and Technology; Neimo, L., Ed.; Finnish Paper Engineers’ Association and Tappi: Helsinki, Finland, 1999. (4) Dunlop-Jones, N. Wet-Strength Chemistry. In Paper Chemistry; Roberts, J. C., Ed.; Chapman and Hall: New York, 1994 (5) Espy, H. H. The Mechanism of Wet-Strength Development in Paper: A Review. Tappi J. 1995, 78 (4), 90. (6) Neal, C. W. Wet and Dry Strength Short Course; Tappi Press: Atlanta, GA, 1988. (7) Laleg, M.; Pikulik, I. Modified Starches for Increasing Paper Strength. J. Pulp Pap. Sci. 1993, 19 (6), J248. (8) Solarek, D. B.; Jobe, P. E.; Tessler, M. M. Polysaccharides Containing Acetal Groups, Their Preparation from the Corresponding Acetals and Use as Paper Additives. U.S. Patent 4,675,394, 1987. (9) Jansma, R. H.; Begala, A. J.; Furman, G. S. Strength resins for paper. U.S. Patent 5,401,810, 1995. (10) Gurnagul, N.; Page, D. H. The difference between dry and rewetted zero span tensile strength of paper. Tappi J. 1989, 72 (12), 164. (11) Taylor, D. L. The Mechanism of Wet Tensile Failure. Tappi J. 1968, 51 (9), 410. (12) Yang, C. Q.; Xu, Y. Paper wet performance and ester crosslinking of wood pulp cellulose by poly carboxylic acids. J. Appl. Polym. Sci. 1998, 67, 649. (13) Chen, N. The influence of dextran aldehyde on wet paper strength. M. Eng. Dissertation, McMaster University, Hamilton, Canada, 2002. (14) Stone, J. E.; Scallan, A. M. A Structural Model for the Cell Wall of Water Swollen Wood Pulp Fibres Based on Their Accessibility To Macromolecules. Cellul. Chem. Technol. 1968, 2, 343. (15) Pelton, R. H.; Zhang, J.; Chen, N.; Moghaddamzadeh, A. The Influence of Dextran Molecular Weight on the Dry Strength of Dextran Impregnated Paper. Tappi J., in press. (16) Vold, R. D.; Vold, M. J. Colloid and Interface Chemistry; Addison-Wesley: New York, 1983.
Received for review May 15, 2002 Revised manuscript received July 26, 2002 Accepted August 19, 2002 IE020355M
