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Wet Strength Improvement of TEMPO-Oxidized Cellulose Sheets

Dec 29, 2006 - Cellulose fibers were oxidized with a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) in water, and these TEMPO...
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Ind. Eng. Chem. Res. 2007, 46, 773-780

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Wet Strength Improvement of TEMPO-Oxidized Cellulose Sheets Prepared with Cationic Polymers Tsuguyuki Saito and Akira Isogai* Department of Biomaterial Sciences, Graduate School of Agricultural and Life Sciences, The UniVersity of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

Cellulose fibers were oxidized with a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxy radical (TEMPO) in water, and these TEMPO-oxidized cellulose fibers were subjected to sheet making with cationic polymers such as cationic poly(acrylamide) (C-PAM) and poly(vinylamine) (PVAm). The wet strength of the sheets thus prepared was evaluated in terms of the cationic polymer used. It was found that wet tensile strength of the sheets was clearly improved by the cationic polymer addition to the cellulose slurries. Especially, the C-PAM addition was effective in wet strength improvement of the sheets, when the TEMPO-oxidized cellulose fibers were used. The results obtained using the TEMPO-oxidized and then NaBH4-reduced cellulose fibers showed that aldehyde groups present in the TEMPO-oxidized cellulose fibers have some interactions with the cationic polymers in the sheets, contributing to their wet strength improvements. Hemiacetals, N-acylcarbinolamines, carbinolamines, and Schiff bases are the possible structures formed at the interfaces between the TEMPO-oxidized cellulose fibers and cationic polymers in the sheets. Repulping behavior of the once-dried sheets was studied as well. Introduction The wet tensile strength of paper, i.e., the tensile strength of paper soaked in water, is one of the significant functionalities for tissue paper, paper towel, paperboard for packaging, printing and writing grades of paper, and others. Chemistry and applications of commercial wet strength resins as well as their mechanisms of wet strength developments have been already reviewed in detail.1,2 Poly(amideamine-epichlorohydrin) (PAE) resin has been widely used as a superior wet strength resin. However, commercial PAE solutions more or less contain lowmolecular-weight organic chlorine byproducts such as 1,3dichloro-2-propanol, which is suspected to have some mutagenic activities and contributes to adsorbable organic chlorine (AOX) emissions from paper mills.1,3-5 Therefore, more environmentally friendly wet strength additives have been required as alternatives to PAE for papermakers. Under these circumstances, poly(vinylamine) (PVAm),6-8 aldehyde-containing natural and synthetic polymers,9-10 poly(carboxylic acids),11 and chitosan12,13 have been studied as the candidates. The aldehydecontaining resins improve wet strength of paper by forming interfiber covalent bonds through hemiacetal and/or acetal linkages with hydroxyl groups of cellulose/hemicellulose in paper sheets. During our studies on TEMPO-mediated oxidation of native cellulose fibers,14-20 we have found that wet tensile strength of sheets prepared from TEMPO-oxidized cellulose fibers was remarkably improved.15,21,22 The TEMPO-mediated oxidation begins by the addition of NaClO to aqueous cellulose suspensions in the presence of catalytic amounts of TEMPO and NaBr at pH 10-11 and room temperature. The C6 primary hydroxyl groups of cellulose are converted to carboxylate groups via C6 aldehyde groups, and only inexpensive NaClO and NaOH are consumed as the TEMPO-mediated oxidation proceeds. The initial fibrous morphologies of native celluloses are mostly maintained even after the TEMPO-mediated oxidation, and * To whom correspondence should be addressed. Tel.: +81 3 5841 5538. Fax: +81 3 5841 5269. E-mail: [email protected].

significant amounts of carboxylate and aldehyde groups are formed on the surfaces of microfibrils in cellulose fibers. Relationships between the TEMPO-mediated oxidation conditions and wet or dry tensile strength of the sheets prepared from the TEMPO-oxidized cellulose fibers were studied in detail on the basis of the functional group contents.21,22 It was found that aldehyde groups formed on the surfaces of cellulose fibers as intermediate structures by the TEMPO-mediated oxidation clearly contribute to the wet strength development of the sheets;22 wet strength improvement of cellulose sheets appear by forming covalent bonds at the interfiber bonds through hemiacetal linkages between hydroxyl and aldehyde groups of the TEMPO-oxidized cellulose fibers in the sheets. However, the levels of wet strength of the sheets prepared only from the TEMPO-oxidized cellulose fibers are not as high as those prepared from the normal cellulose fibers with PAE. The effects of additions of aluminum sulfate to the TEMPO-oxidized cellulose fibers21 or proteins to the TEMPO-oxidized cellulose sheets23 on wet strength development have been already reported. In this study, the use of the TEMPO-mediated cellulose fibers in combination with cationic polymers was studied to further improve the wet strength of the sheets and to make clear the mechanisms of wet strength developments of the sheets. Experimental Procedures Materials. A commercial hardwood bleached kraft pulp was used as the original cellulose fibers, whose R-cellulose, carboxyl, and aldehyde contents were 90%, 0.054 mmol/g, and 0.006 mmol/g, respectively.21,22 Cationic poly(acrylamide) (C-PAM, Seiko PMC Co., Japan), poly(vinylamine) (PVAm, BASF Japan), and poly(amideamine-epichlorohydrin) resin (PAE, Seiko) were commercial products of papermaking grade. CPAM is a copolymer prepared from acrylamide, diallyldimethylammonium chloride, and a small amount of a divinyl monomer, thus having partly branched structures. Cationic starch (C-starch) having quaternary substituents with a degree of substitution of 0.03 (Hi-Cat, Roquette Co., France)24 was commercially available. C-PAM and C-starch are dry strength

10.1021/ie0611608 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006

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Table 1. Cationic Polymers Used with TEMPO-Oxidized Cellulose Fibers in Handsheet Making polymer cationic poly(acrylamide)

(C-PAM)a

poly(vinylamine) (PVAm)a poly(amideamine-epichlorohydrin) (PAE)b poly(ethyleneimine) (PEI)a poly(diallyldimethylammonium chloride) (PDADMAC)a cationic starch (C-starch)a chitosanc a

wt av molecular mass

amt of amino groups (mmol/g)

comments

ca. 4 000 000

ca. 0.75

unknown ca. 1 140 000 ca. 25 000 ca. 450 000 unknown ca. 420 000

ca. 22.5 ca. 3.27 ca. 23.8 ca. 6.19 ca. 0.18 ca. 6.13

quaternary amine partly branched primary amine mainly quaternary amine all types of amines quaternary amine quaternary amine primary amine

Supplier’s data or calculated from chemical formulas. b Reference 3. c Determined in our laboratory.

additives in papermaking, while PVAm and PAE are wet strength additives. Poly(ethyleneimine) (PEI, Aldrich Co., USA), poly(diallyldimethylammonium chloride) (PDADMAC, Aldrich), and chitosan (Wako Pure Chemicals Co., Japan) were of reagent grades. Approximate weight average molecular mass values and the amounts of amino groups of the polymers, that are provided by suppliers or determined in our laboratory, are listed in Table 1. TEMPO, sodium bromide, and 9% sodium hypochlorite solution were of reagent grades (Wako) and were used as supplied. TEMPO-Mediated Oxidation of Cellulose. The cellulose fibers (10 g) were suspended in water (750 mL) containing TEMPO (0.025 g) and sodium bromide (0.25 g). The 9% NaClO solution of 2.48 g corresponding to 0.30 mmol of NaClO/(g of cellulose) was added to the cellulose slurry under continuous stirring. The slurry was maintained to be pH 10.5 at room temperature by continuous addition of 0.5 M NaOH using a pH stat until no NaOH consumption was observed. It took about 10 min. The TEMPO-oxidized cellulose fibers thus prepared were washed thoroughly with water on a filter paper set in a Bu¨chner funnel, and then the wet cellulose fibers of 10% consistency were stored at 4 °C without drying before use.21,22 Total and surface carboxylate contents of the TEMPO-oxidized cellulose fibers were 0.089 and 0.050 mmol/g, respectively, and the total and surface aldehyde contents were 0.195 and 0.019 mmol/g, respectively.22 Reduction of the TEMPO-Oxidized Cellulose. Selective reduction of aldehyde groups in the TEMPO-oxidized cellulose fibers were carried out with sodium borohydride at pH 8, as follows.22 The TEMPO-oxidized cellulose fibers (2 g) were suspended in water (100 mL), and sodium borohydride (1 g) was added to the slurry at about pH 8, which was adjusted by adding a dilute ammonium hydroxide solution. After stirring the slurry at room temperature for 48 h, the TEMPO-oxidized and then NaBH4-reduced cellulose fibers were washed thoroughly with water by filtration. These NaBH4-reduced cellulose fibers were stored at 10% consistency in water without drying for the successive sheet making. Preparation of Cellulose Sheets with Cationic Polymers. The original or TEMPO-oxidized cellulose fibers were suspended in tap water (pH 7.8) under continuous stirring at 500 rpm. A designed amount of 0.5 or 1% cationic polymer solution was added to the cellulose slurry, and then cellulose sheets with a basis weight of 60 g/m2 were prepared from the slurry with tap water according to TAPPI Test Method T 205 om-88 (2005). The wet-pressed sheets were dried at 100 °C for 2 min using a rotary drum-dryer. In some experiments, the wet-pressed sheets were dried at 23 °C for 1 day for comparison. The cellulose sheets thus obtained were conditioned at 23 °C and 50% relative humidity for more than 1 day. Disintegration of Cellulose Sheets. Repulping or disintegration behavior of the once-dried cellulose sheets in water was evaluated by the following method. The cellulose sheet (ca. 1.25

g) prepared by the aforementioned method was broken into small pieces with fingers and stirred in water (200 mL) at either room temperature or 80 °C for up to 12 h using a magnetic stirrer. In some cases, the water was adjusted to pH 2, 11, or 13 by adding a diluted HCl or NaOH solution. After stirring for a designed time, the pulp slurry was filtered using a Bu¨chner funnel. The fiber fraction on the filter paper was collected and subjected to sheet making according to the aforementioned method. Formation of the sheets was evaluated from their transmitted light images obtained using a scanner. The water-soluble filtrate fraction was evaporated, freeze-dried, and then subjected to 1HNMR analysis as described below. Measurements and Analyses. Retention ratios of cationic polymers in the cellulose sheets were calculated from their nitrogen contents, which were determined by means of an elemental analyzer (FLASH EA1112NSC; Thermo Finnigan Co., USA). Wet and dry tensile strengths of the cellulose sheets were evaluated in accordance with TAPPI Test Method T 456 om-87 and T 494 om-87, respectively (2005). The soaking time of the 15 mm width specimen strips in deionized water was set to 30 min for the wet strength measurement. 1H-NMR spectra of the water-soluble fractions freeze-dried from the filtrates, which were obtained from cellulose sheets by the above disintegration method, were recorded on a Bruker AC-300 using D2O (Wako) and 3-trimethylsilyl-2,2,3,3-d4-propionic acid sodium salt (Aldrich) as a solvent and an internal standard, respectively. Results and Discussion Wet Tensile Strength of TEMPO-Oxidized Cellulose Sheets Prepared with Cationic Polymers. The original and TEMPO-oxidized cellulose fibers were subjected to sheet making with cationic polymers by adding them into the cellulose slurries. C-PAM, PAE, and C-starch have quaternary amino groups, which provide cationic sites at any pH values in cellulose slurries. PEI has primary, secondary, tertiary, and quaternary amino groups, and PVAm has only primary amino groups. When the original cellulose fibers were used, the addition of PAE resulted in clear wet strength improvement of the sheets. PVAm showed wet strength improvement, only when the addition level was 0.6% on the dry weight of the cellulose. Nearly no wet strength improvement was observed for C-PAM. Thus, PAE provides wet strength improvement of sheets prepared from the original cellulose fibers quite efficiently. As shown in Figure 1, the TEMPO-mediated oxidation of cellulose fibers under optimum conditions led to clear wet strength improvement of the sheets even without any cationic polymers. The addition of C-PAM, PVAm, PAE, PEI, or C-starch to the TEMPO-oxidized cellulose slurries resulted in higher wet strengths of the sheets. Particularly, it is noticeable that C-PAM, which had nearly no effect on wet strength improvement for the sheets prepared from the original cellulose

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Figure 1. Tensile index of the rewetted sheets prepared from slurries of the original or TEMPO-oxidized cellulose fibers with cationic polymers.

Figure 3. Dry tensile index of the sheets prepared from slurries of the original or TEMPO-oxidized cellulose fibers with cationic polymers.

Figure 4. Effect of drying temperature of the wet webs prepared from slurries of the original or TEMPO-oxidized cellulose fibers with 0.3% (on dry weight of cellulose) cationic polymers on the wet tensile index of the dried sheets. Figure 2. Elongation at break of the rewetted sheets prepared from slurries of the original or TEMPO-oxidized cellulose fibers with cationic polymers.

fibers, showed clear wet strength improvement of the sheets, when the TEMPO-oxidized cellulose fibers were used. PAE and PVAm were also effective in wet strength improvement of the TEMPO-oxidized cellulose sheets in manners similar to that for C-PAM. C-starch and PEI were in the next rank. The addition of PDADMAC or chitosan had nearly no wet strength improvement of the sheets (data not shown). Fluctuations of each plot in Figure 1 were within (12%. Figure 2 shows elongation at the breaking point obtained from the strain-stress curves of the wet tensile test in Figure 1. Static Young’s moduli of the sheets were quite similar to each other, indicating that stiffness values of the sheets were unchanged even by the addition of the cationic polymers to the slurries (data not shown). In contrast, elongation values varied, depending on the cationic polymer added as well as the addition level. The elongation values in Figure 2 roughly corresponded to those of the wet strength of the sheets in Figure 1, and the improvement of plastic behavior of the wet sheets by the addition of cationic polymers, therefore, brought about the wet strength improvement. Dry Tensile Strength of TEMPO-Oxidized Cellulose Sheets Prepared with Cationic Polymers. Figure 3 shows the

dry tensile strength of the cellulose sheets prepared from the original or TEMPO-oxidized cellulose fibers with cationic polymers. The TEMPO-mediated oxidation of cellulose fibers resulted in a clear increase in the dry tensile strength of the sheets prepared without any cationic polymers. In both the original and TEMPO-oxidized cellulose fibers, dry tensile strength slightly increased with increasing the addition level of cationic polymers, although no significant differences in dry tensile strength were observed between the TEMPO-oxidized cellulose sheets prepared with various cationic polymers. Thus, the wet strength improvement of the TEMPO-oxidized cellulose sheets varied, depending on the cationic polymer used (Figure 1), more clearly than the dry strength improvement (Figure 3). Effect of Drying Temperatures of TEMPO-Oxidized Cellulose Sheets on Wet Strength Improvement. It is wellknown that the wet tensile strength of paper sheets prepared with PAE is clearly improved by thermal drying of wet webs and curing treatment of the once-dried paper sheets, resulting from enhancement of covalent bond formation between azetidinium rings of PAE and carboxyl groups of pulp fibers in sheets by heating.1-3 On the other hand, conditions of thermal drying of wet webs have less impact on the wet tensile strength of PVAm-treated sheets.7 Figure 4 shows the effect of drying temperature of wet cellulose webs on the wet tensile strength of the sheets. In all cationic polymers examined, thermal drying

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Figure 5. Retention ratios of cationic polymers in the sheets prepared from slurries of the original or TEMPO-oxidized cellulose fibers with 0.3% (on dry weight of cellulose) cationic polymers.

of wet cellulose webs at 100 °C for 2 min using a drum-dryer gave a higher wet tensile strength of the TEMPO-oxidized cellulose sheets than those dried at 23 °C. Thus, some interactions between the TEMPO-oxidized cellulose fibers and cationic polymers, which cause the wet strength improvement of the sheets, must be enhanced by thermal drying. Retention of Cationic Polymers in TEMPO-Oxidized Cellulose Sheets. The wet strength of cellulose sheets is influenced by not only the properties of cationic polymers used as additives in sheet making but also their retention ratios. Electrostatic interactions between anionic sites of cellulose fibers and cationic sites of polymers are the predominant factors influencing adsorption behavior of cationic polymers on cellulose fibers in water, and the anionic charge densities of cellulose fiber surfaces affects the resultant retention ratios of the added cationic polymers for the sheets. Figure 5 shows retention ratios of cationic polymers for the sheets at the 0.3% addition level. In all cases examined, the TEMPO-oxidized cellulose fibers showed 13-30% higher retention ratios of cationic polymers than those for the original cellulose fibers. The TEMPO-oxidized cellulose fibers had surface and total carboxyl contents of 0.050 and 0.089 mmol/g, respectively, while those of the original cellulose fibers had 0.029 and 0.054 mmol/g, respectively. Thus, especially the higher surface carboxyl content for the TEMPO-oxidized cellulose fibers is likely to have brought about the higher retention rations of the cationic polymers by more electrostatic interactions.15,16,21 Because PVAm has primary amino groups, only a part of which are dissociated and have cationic charges in water around pH 7, its retention ratios are lower than those of the other cationic polymers. The data of wet tensile strength of the sheets obtained in Figure 1 were then replotted to cationic polymer contents in the cellulose sheets (Figure 6). In the case of the original cellulose fibers, the efficiency of improving wet tensile strength of the sheets was in the order of PAE > PVAm > C-PAM at the same cationic polymer content in the sheets. When the TEMPO-oxidized cellulose fibers were used, the efficiency was in the order of PVAm > PAE ≈ C-PAM > PEI. Thus, PVAm gave the highest efficiency to improve the wet tensile strength of the sheets at the same cationic polymer content, when the TEMPO-oxidized cellulose fibers were used. Effect of Aldehyde Groups in TEMPO-Oxidized Cellulose Fibers on Wet Strength Improvement. We reported in the previous paper that aldehyde groups formed on cellulose fiber surfaces by the TEMPO-mediated oxidation bring about the wet strength improvement through the formation of interfiber

Figure 6. Relationships between cationic polymer contents in the sheets and their wet tensile indices shown in Figure 1.

Figure 7. Wet tensile index of the sheets prepared from slurries of the original, TEMPO-oxidized, or TEMPO-oxidized and then NaBH4-reduced cellulose fibers with 0.6% cationic polymers.

hemiacetal linkages with hydroxyl groups of adjacent fiber surfaces in the cellulose sheets.21,22 Also in the case of cationic polymer additions carried out in this study, aldehyde groups of cellulose fibers formed by the TEMPO-mediated oxidation may participate in the wet strength improvement of the cellulose sheets. Figure 7 shows the wet tensile strength of cellulose sheets prepared from the original, TEMPO-oxidized, and TEMPOoxidized and then NaBH4-reduced cellulose fibers with or without 0.6% cationic polymers. The use of the TEMPOoxidized cellulose fibers in combination with C-PAM, PEI, or PVAm resulted in remarkable wet strength improvement of the sheets, as shown in Figure 1. On the other hand, wet tensile strengths of the sheets prepared from the TEMPO-oxidized and then NaBH4-reduced cellulose fibers with the cationic polymers mostly returned to those of the sheets prepared from the original cellulose fibers with the cationic polymers. Because aldehyde groups present in the TEMPO-oxidized cellulose fibers can be mostly converted to alcohol groups by the NaBH4 treatment,22 the aldehyde groups formed clearly contribute to the wet strength improvement of the sheets by some interactions with C-PAM, PEI, and PVAm retained in the sheets. One of the mechanisms hypothesized for wet strength improvement of paper by PVAm is due to Schiff base formation between primary amino groups of PVAm and aldehyde groups present in bleached kraft pulp fibers.25 However, as shown in Figure 7, the wet strength value of the sheet prepared from the TEMPO-oxidized and then NaBH4-reduced cellulose fibers with 0.6% PVAm was almost equal to that prepared from the original

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Figure 8. Possible formations of covalent bonds at the interfaces between aldehyde groups of the TEMPO-oxidized cellulose fibers and polymers in the sheets.

Figure 10. Wet tensile index of the sheets prepared by repulping of the once-dried sheets under various conditions. The original sheets were prepared from slurries of the TEMPO-oxidized cellulose fibers with or without 0.6% C-PAM. Figure 9. Reinforcement of the wet strength of the sheets by additional formation of covalent bonds at the interfaces between aldehyde groups of the TEMPO-oxidized cellulose fibers and the retained cationic polymers in the sheets.

cellulose fibers with 0.6% PVAm. Wet tensile strength of the sheets prepared from the TEMPO-oxidized and then NaBH4reduced cellulose fibers was still higher than those of the sheets prepared from the original cellulose fibers with either 0.6% C-PAM or 0.6% PEI, even though almost no aldehyde groups remained in the NaBH4-reduced cellulose fibers. Thus, the Schiff base formation does not seem to be the candidate for the mechanism of wet strength improvement of the sheets prepared from the original cellulose fibers with 0.6% PVAm. Mechanisms of Wet Strength Improvement of TEMPOOxidized Cellulose Sheets with Cationic Polymers. The results obtained in the previous sections indicate that aldehyde groups formed in the TEMPO-oxidized cellulose fibers more or less contribute to the wet strength improvement of the sheets prepared with C-PAM, PEI, PVAm, and cationic starch. It is known that NH2 groups of amides and amines form Nacylcarbinolamine and carbinolamine structures, respectively, with aldehyde groups, which are similar to hemiacetals formed

between hydroxyl and aldehyde groups.26 C-PAM can form the N-acylcarbinolamine structures with aldehyde groups in the TEMPO-oxidized cellulose sheets, while primary amino groups of PVAm and PEI can form carbinolamine structures followed by the formation of Schiff base structures with aldehyde groups in the TEMPO-oxidized cellulose fibers in the sheets by heating (Figure 8). Formation of a number of these covalent bonds at the TEMPO-oxidized cellulose fiber surfaces with either C-PAM, PEI, PVAm, or C-starch in the sheets is likely to cause their wet strength improvements, reinforcing the interfiber hemiacetal linkages formed between aldehyde and hydroxyl groups of the TEMPO-oxidized cellulose fibers in the sheets (Figure 9). This is the reason why C-PAM and C-starch is ineffective in wet strength improvement of the sheets prepared from the original cellulose fibers but effective for the TEMPO-oxidized cellulose fibers. The heating effect of the wet cellulose webs on wet strength development of the sheets observed in Figure 4 may correlate with the interfiber covalent bond formation mechanisms postulated in Figure 9. Probably, the heating treatment increases the number of interfiber covalent bonds by activation of aldehyde groups or their hydrated structures on the cellulose fiber surfaces, although further studies are needed to prove this

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Figure 11. Transmitted light images of the sheets prepared by repulping of the once-dried sheets under various conditions. The original sheets were prepared from slurries of the TEMPO-oxidized cellulose fibers and 0.6% C-PAM.

hypothesis. Moreover, not only the covalent bond formation mechanisms shown in Figures 8 and 9 but also characteristics of the cationic polymers such as molecular mass, molecular mass distributions, and others (Table 1) probably influence the resultant wet strength behavior of the sheets. Different wet strength improvement behavior between the cationic polymers observed in Figure 1 might be related to their characteristics. As shown in Figure 1, PAE is effective in wet strength improvement of the sheets prepared from not only the original cellulose fibers but also the TEMPO-oxidized cellulose fibers. However, because PAE molecules have hydroxyl, amide, and various amino groups as well as reactive azetidinium rings,3 interactions between PAE and the TEMPO-oxidized cellulose fibers in the sheets must be more complicated. Thus, discussions about these interactions are skipped in this paper, which focuses primarily on the effect of C-PAM, PEI, and PVAm on the wet strength improvement of the sheets prepared from the TEMPOoxidized cellulose fibers. Repulping Behavior of TEMPO-Oxidized Cellulose Sheets. One of the problems for paper sheets having high wet strength is that once-dried sheets are difficult to be repulped to single fibers in water because of high repulping resistance. Therefore, recycling of these sheets, brokes, and waste stuff formed in papermaking is difficult, and incineration is often the favored method of these waste disposals. Therefore, if some switching functions to improve repulping abilities can be added to once-

dried paper sheets having wet strength, it must be favorable for recycling or reuse. Figure 10 depicts wet tensile strength of the sheets prepared from the TEMPO-oxidized cellulose fibers before and after some repulping treatments. Detailed procedures to evaluate the repulping behavior of the sheets are described in Experimental Procedures. The sheets prepared from the TEMPO-oxidized cellulose fibers without any cationic polymers were completely repulped or converted to single fibers by stirring of the oncedried sheets in water at room temperature for 24 h, and the sheets prepared from the repulped fibers had still high wet tensile strength (no. 1 in Figure 10). Thus, aldehyde groups of the TEMPO-oxidized cellulose fibers contributing to the wet strength improvement of the sheets have survived after this repulping treatment. When the once-dried sheets prepared from the TEMPO-oxidized cellulose fibers without any cationic polymers were stirred in water at 80 °C for 2 h, they were completely repulped or converted to single fibers. However, the sheets prepared from these repulped fibers had no longer sufficient wet strength (no. 2 in Figure 10). The sheets prepared from the TEMPO-oxidized cellulose fibers with 0.6% C-PAM had high wet strength. In this case, stirring the once-dried sheets in water at pH 11 and 80 °C for 1 h was needed for complete repulping or conversion to single fibers. However, also in this case, the sheets prepared from the repulped fibers had low wet strength (no. 3 in Figure 10).

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the sheets prepared from the repulped fibers, although complete repulping was achieved under the adopted conditions (no. 3 in Figure 10). Thus, the use of the TEMPO-oxidized cellulose fibers in combination with C-PAM developed in this study may be one of the candidates of alternatives to produce paper sheets having high wet tensile strength by environmentally friendly procedures as well as switching functions of repulping ability. Conclusions

Figure 12. 1H-NMR spectra of water-soluble fractions in the filtrates extracted by stirring of the sheets prepared from the original or TEMPOoxidized cellulose fibers in water under various conditions.

Figure 11 shows transmitted light images of the sheets prepared from repulped fibers obtained from the once-dried sheets, which were prepared from the TEMPO-oxidized cellulose fibers with 0.6% C-PAM. In this case, stirring of the once-dried sheets in water at pH 11 and 80 °C was required for complete repulping, stirring under other milder conditions was insufficient for repulping, and insufficiently repulped sheet pieces partly remained in the sheets. Thus, although the sheets prepared from the TEMPO-oxidized cellulose fibers with 0.6% C-PAM have quite high wet strength, they can be repulped under some conditions and are recyclable. However, these repulped fibers have no longer the capability to have high wet strength, when sheeted. The filtrate fractions, which were obtained by some repulping treatments of the once-dried sheets prepared from the TEMPOoxidized cellulose fibers without any cationic polymers, were collected and subjected to 1H-NMR analysis. As shown in Figure 12, some sugar and fatty acid components were extracted with water at 80 °C even from the sheets prepared only from the original cellulose fibers. When the sheets prepared from the TEMPO-oxidized cellulose fibers were stirred in water at 80 °C for 2 h, some aldehyde group-containing compounds were extracted together with some sugar and fatty acid components. This is the reason why the sheets prepared from these repulped fibers had no longer the high wet strength (no. 2 in Figure 10). On the other hand, when the sheets prepared from the TEMPOoxidized cellulose fibers without any cationic polymers were stirred in water at room temperature for 24 h, no such compounds having aldehyde groups were extracted. In this case, because aldehyde groups still remained in the repulped fibers, the sheets prepared from these repulped fibers still had high wet strength (no. 1 in Figure 10). When the sheets prepared from the TEMPO-oxidized cellulose fibers with 0.6% C-PAM were stirred in water at pH 11 and 80 °C for 1 h, aldehyde group-containing compounds were more or less extracted from the sheets and the original wet strength no longer appeared on

(1) Wet tensile strength of the sheets prepared from the TEMPO-oxidized cellulose fibers is further improved by the addition of C-PAM, PVAm, PEI, PAE, or C-starch to the cellulose slurries. Especially, the C-PAM addition to the TEMPO-oxidized cellulose fiber slurries is effective in wet strength improvement of the sheets, while that is ineffective for the original cellulose fibers. (2) The wet strength improvement of the sheets prepared from the TEMPO-oxidized cellulose fibers with the above cationic polymers are reflected by the improved elongation at the breaking points rather than Young’s modulus of the wet sheets. (3) Thermal drying of wet webs enhances the wet tensile strength of the sheets prepared from the TEMPO-oxidized cellulose fibers with the cationic polymers. (4) Retention values of the cationic polymers in the sheets are clearly increased, when the TEMPO-oxidized cellulose fibers are used probably because of higher carboxyl contents in these fibers. (5) The results obtained using the TEMPO-oxidized and then NaBH4-reduced cellulose fibers showed that aldehyde groups present in the TEMPO-oxidized cellulose fibers have some interactions with the cationic polymers in the sheets, resulting in their wet strength improvements. Hemiacetals, N-acylcarbinolamines, carbinolamines, and Schiff bases are the possible structures formed at the interfaces between the TEMPO-oxidized cellulose fibers and cationic polymers in the sheets. (6) The once-dried sheets prepared from the TEMPO-oxidized cellulose fibers with or without 0.6% C-PAM can be repulped or converted to single fibers by stirring in water under the optimum conditions. However, in most cases, wet tensile strength no longer appears on the sheets prepared from the repulped fibers, because aldehyde group-containing compounds formed by the TEMPO-mediated oxidation of cellulose fibers are extracted to water-soluble fractions during the repulping treatments. Acknowledgment This research has been supported by Grants-in-Aid for Scientific Research (Grant Nos. 18380102 and 18-10902) from the Japan Society for the Promotion of Science (JSPS). Literature Cited (1) Dunlop-Jones, N. Wet-Strength Chemistry. In Paper Chemistry, 2nd ed.; Roberts, J. C., Ed.; Chapman & Hall: London, 1996; p 89. (2) Scott, W. E. Wet Strength Additives. Principles of Wet End Chemistry; Tappi Press: Atlanta, GA, 1996; p 61. (3) Obokata, T.; Isogai, A. Characterization of Polyamideamine-Epichlorohydrin (PAE) Resin: Roles of Azetidinium Groups and Molecular Mass of PAE in Wet Strength Improvement of Paper Prepared with PAE. J. Appl. Polym. Sci. 2005, 97, 2249. (4) Podd, B. D. The Third Generation of Neutrally Hardening PolyamideEpichlorohydrin Wet Strength Agents. Wochenblatt fu¨r Papierfabrikation 1999, 127, 1388. (5) Obokata, T. Paper Strength-Enhancing Agents. Fine Chem. 2004, 33 (3), 26.

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ReceiVed for reView September 2, 2006 ReVised manuscript receiVed November 19, 2006 Accepted November 27, 2006 IE0611608