Treatment of Fractionated Fibers with Various Cationic-Modified Poly

Oct 2, 2009 - Limerick Pulp & Paper Center, and Department of Chemical Engineering, Head Hall, 15 Dineen Drive, University of New Brunswick, Frederict...
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Ind. Eng. Chem. Res. 2009, 48, 10485–10490

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Treatment of Fractionated Fibers with Various Cationic-Modified Poly(vinyl alcohols) and Its Impact on Paper Properties Pedram Fatehi, Jeffery E. Ward, and Huining Xiao* Limerick Pulp & Paper Center, and Department of Chemical Engineering, Head Hall, 15 Dineen DriVe, UniVersity of New Brunswick, Fredericton, New Brunswick, Canada, E3B 5A3

In this work, the effects of various high molecular weight cationic poly(vinyl alcohols), CPVAs, on the fiber morphology and the properties of papers made from fractionated refined fibers were investigated. At first, bleached sulfite fibers were refined under various revolutions and then refined fibers were fractionated under various time intervals to obtain refined fibers with a similar fiber length and fines content. Then, 20 mg/g CPVAs with various charge densities was adsorbed on the fractionated refined fibers prior to making hand sheets. The results showed that the lower the charge density of CPVA, the higher the effectiveness of CPVA in improving the paper strength at any refining revolution. The reasons for such a phenomenon were perhaps the different configurations of CPVAs on the surface of the fibers and the different adhesion/repulsion forces developed between CPVA-modified fibers in hand sheets. Also, by reducing the freeness of the fractionated fibers (increasing the refining revolutions), the effect of CPVA in improving the paper strength was reduced. In this case, the CPVAs tended to cover the fibrils on the fiber surface, which was demonstrated by SEM observations, thus reducing the contact area of the fractionated refined fibers. 1. Introduction Dry strength additives are commonly used in papermaking. By applying a dry strength additive, extra hydrogen bonds are developed between fibers, which improves the paper strength.1 Dry strength additives are mainly applied to increase the strength of papers, to compensate for the decrease in paper strength that results from using fillers,2 or to use less pulps by decreasing the basis weight of papers.3 Several dry strength additives have been introduced to papermaking in the literature.4-7 However, their effects on the morphology of fibers before and after refining have not been well-characterized. The preparation of various cationic-modified poly(vinyl alcohols), CPVAs, as dry strength additives, and their adsorption characteristics on bleached sulfite pulp were investigated in our previous work.8 We also reported that the most effective CPVA in improving the strength of the papers made from bleached sulfite fibers was the high molecular weight CPVA, with a charge density of 0.4 mequiv/g.9 However, the effectiveness of CPVA in improving the paper strength was generally reduced by increasing the refining revolution.10 CPVA also increased the fines retention in the hand sheets.10 However, it is unclear how CPVA modifies the surface morphology of refined fibers. As is well-known, by increasing the refining revolution, more fibrils are created and branched off from the fiber surface. Such fibrils increase the contact area of fibers and hence improve the paper strength. Upon applying polymers on the refined fibers, polymers can be adsorbed on the unfibrillated area of fibers and on the fibrils. The adsorption of CPVAs on the fibrillated (refined) fibers can be in two different manners: (1) CPVAs may improve the bridging effectiveness of fibrils. In this case, the adsorbed CPVAs on fibrils further increase the contact area of fibers, thus improving the paper strength. (2) CPVAs can cover the fibrils and lay them on the fiber surface. In this case, the contact area of CPVA-modified fibers might be less than * To whom correspondence should be addressed. Tel.: (506) 4533532. Fax: (506) 453-3591. E-mail: [email protected].

that of the unmodified fibers in hand sheets. Therefore, the paper strength might be impaired by adsorbing CPVAs on the refined fibers. The charge density of polymers affects the configuration of polymers on the fiber surface and the electrostatic attraction/ repulsion force developed between neighboring fibers in hand sheets.11-16 The objective of the current study was to investigate how CPVAs with various charge densities modify the surface morphology of refined fibers and affect the properties of their resulting papers. As is well-known, by refining of pulps, the fiber length is reduced and the fines content is increased. To eliminate the effects of fiber length and fines content in our investigation, refined fibers should possess a similar fiber length and fines content. In this work, bleached sulfite fibers were first refined under various revolutions, and then the refined fibers were fractionated under various time intervals to obtain refined fibers with different degrees of fibrillation and flexibility, but with a similar fiber length and fines content. Then, approximately 20 mg/g CPVAs having various charge densities was adsorbed on fibers, and the resulting paper properties were evaluated. The results of this paper demonstrate how CPVAs modify the morphology of refined fibers and affect the properties of their resulting papers. 2. Materials and Methods 2.1. Raw Materials. Bleached sulfite softwood pulp was obtained from Fraser Papers Co., New Brunswick, Canada. The pulp was washed with 10 L of water prior to using. The moisture content of the pulp was measured in accordance with TAPPI T412. Poly(vinyl alcohol) (PVA) with a molecular weight (MW) of 124 000-186 000, 99% hydrolyzed, and glycidyl-trimethylammonium chloride (GTMAC), 75 wt % in water, were both obtained from Sigma-Aldrich Co. and applied as received. Anionicpoly(vinylsulfate)(PVSK)withaMWof100 000-200 000, 97.7% esterified, was obtained from Wako Pure Chem. Ltd. Japan. 2.2. CPVA Preparation. The cationic modification of PVA was carried out according to our previous work.9,10 At first, PVA

10.1021/ie900999n CCC: $40.75  2009 American Chemical Society Published on Web 10/02/2009

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Table 1. Reaction Conditions of GTMAC with PVA and Charge Density of CPVAs GTMAC to PVA ratio, mol %

temperature, °C

charge density, mequiv/g

0.375 0.500 0.375

80 80 95

0.4 0.7 1.3

Table 2. Properties of Refined Pulps refining load, revolutions

fiber length (LW), mm

fines content, %

CSF, mL

0 500 3000 8000

1.82 ( 0.08 1.80 ( 0.07 1.67 ( 0.06 1.45 ( 0.04

23.3 ( 2.0 24.1 ( 2.3 27.4 ( 2.3 37.8 ( 2.1

700 ( 10 690 ( 10 585 ( 5 385 ( 15

(7.25 g) was dissolved in water (60 mL) at 80 °C and stirred for 1 h. Then, 5 mL of NaOH (5 N) was added to the solution. Afterward, GTMAC was added to the solution and the mixture was stirred for 1 h. The reaction conditions of GTMAC with PVA to obtain various CPVAs are listed in Table 1. Finally, unreacted GTMAC was separated using a membrane dialysis tube with a MW cutoff of 1000, while the water was changed every 2 h for the first 6 h, and then once a day for 2 days. 2.3. Refining and Fractionating of Fibers. The pulp refining was carried out according to TAPPI T 248 by a PFI refiner, No. 158, Norway, under different revolutions. The pulp characteristics were analyzed using a fiber quality analyzer (FQA), Optest Equipment Inc., Ontario, Canada, as listed in Table 2. The fibers smaller than 200 µm are accounted as fines by FQA. The pulp freeness was measured by using a Canadian Standard Freeness (CSF) according to TAPPI T 227. A BauerMcNett classifier was employed to fractionate the unrefined and refined fibers according to TAPPI T 233. To obtain pulp with a similar fiber length and fines content, refined fibers were fractionated under various time intervals, as listed in Table 3. The fractionated refined fibers were collected from the bottom of the first tank that had an R14 mesh screen (opening 1.19 mm). 2.4. Adsorption Analysis and Fiber Treatment. The fractionated pulp fibers were dispersed in a 2 L three-neck glass flask at 3% consistency, neutral pH, and 30 °C for 1 h. Then, 20 mg/g CPVA was added to the fiber suspensions and stirred for 1 h. Afterward, fibers were washed twice with distilled water. Then, hand sheets were made from each pulp sample according to TAPPI T 205 and kept overnight in a conditioning room in accordance with TAPPI T 402. For the adsorption analysis, 20 mg/g CPVA was added to 125 mL Erlenmeyer flasks containing the suspension of fractionated unrefined fibers under the same conditions addressed above. Then, the samples were shaken in a water bath shaker, Innova 3100, Brunswick Scientific. Control samples without pulps were prepared under the same conditions. The titration was conducted with the PVSK solution (0.5 mN) using a particle charge detector, Mu¨tek PCD 03, Herrsching, Germany. Also, the charge density of CPVA was measured using the same PCD titrator. The CPVA modification of fibers and the adsorption analysis were conducted for various CPVAs having different charge densities, as addressed above. Three repetitions were performed to get an average value for each sample.

2.5. Analysis of Paper Properties. The light scattering coefficient and the brightness of hand sheets were tested according to TAPPI T 425 and T 452, respectively, by using a Technibrite Micro TB-1C, Indiana. The tensile and tear strengths were measured according to TAPPI T 494 and T 414, respectively, using Lorentzen and Wettre (L&W) tensile and tear testers, Sweden. The burst strength of papers was also measured according to TAPPI T 403, using a Burst-o-Matic, Lorentzen and Wettre (L&W), Sweden, burst tester. 2.6. Atomic Force Microscopic (AFM) Analysis. To investigate the interaction between CPVA and the fiber surface, thermally oxidized silicon wafers were obtained from Universitywafer and employed as substrates. The silicon wafers were first immersed in 98% H2SO4 for 6-8 h and rinsed with deionized distilled water prior to using. The silicon probes (tips) with a spring constant of 0.32 N/m, NP-S20, Veeco Instruments, and a resonance frequency of 273 kHz were employed to conduct the force analysis. The above-mentioned silicon probes were coated with CPVAs according to the following procedure: first, the probes were immersed in CPVA solutions (2 wt %) overnight. Then, the CPVA-coated probes were washed with deionized distilled water to remove unadsorbed CPVAs from the probes. In the literature17,18 this method has been applied to coat AFM probes. The adhesion/repulsion force developed between the CPVA-coated probes and silicon wafers was measured using an atomic force microscope (AFM), Nanoscope IIIa, Veeco Instruments Inc., Santa Barbara, CA, in Picoforce mode with a vendor-supplied fluid cell at the temperature of 25 °C and pH 7. For each set of measurements, 50 different spots and on each spot 5 times, on an area of 1 × 1 µm2, were scanned at a constant tip velocity of 0.5 µm/s. The force-distance curves were calculated from cantilever deflection and displacement of the piezo. This method has been widely applied in the literature18-20 to investigate the interaction between polymers and surfaces. 2.7. SEM Analysis. Samples were collected from the hand sheets made from unmodified and CPVA-modified fractionated refined fibers. The samples were dried and coated with carbon and gold to obtain high-resolution images. The images were then taken by a scanning electron microscope (SEM) JEOL, JSM-6400, Japan. 3. Results 3.1. Fiber Analysis. Usually fiber fragments smaller than 75 µm are classified as fines. Due to the fact that a broader range was used to identify fines by FQA in our analysis (see section 2.3), the fines content of pulp seemed to be high. The properties of refined fibers are listed in Table 2. As expected, by increasing the refining revolutions to 8000, the fiber length and CSF were reduced by 20% and 45%, respectively. Also, the fines content was increased by 62%. The properties of fractionated refined fibers are listed in Table 3. As seen, by fractionating the refined fibers under various time intervals, a similar fiber length was obtained. Also, the fines content of refined fractionated fibers was negligible, whereas the freeness of pulp was varied by 6%.

Table 3. Properties of Fractionated Refined Fibers and Strength Properties of Their Resulting Papers refining revolutions time, min fiber length (LW), mm fines content, % CSF, mL tensile index, N · m/g burst index, kPa · m2/g tear index, N · m2/kg 0 500 3000 8000

15 11 9 9

2.82 ( 0.05 2.81 ( 0.06 2.81 ( 0.07 2.80 ( 0.04

1.7 ( 1.4 1.6 ( 1.2 1.5 ( 0.9 1.5 ( 1.0

770 ( 15 760 ( 10 740 ( 10 725 ( 5

25.8 ( 2.3 46.5 ( 2.6 72.4 ( 2.2 85.2 ( 3.1

1.8 ( 1.8 3.5 ( 1.1 5.8 ( 1.9 6.8 ( 1.2

25.2 ( 3.2 28.1 ( 4.1 18.4 ( 3.6 17.5 ( 3.7

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Figure 1. Variations in tensile, burst, and tear indices of papers made from fractionated refined pulps, modified with CPVAs having various charge densities (mequiv/g), versus freeness of fractionated pulps.

3.2. Paper Analysis. The adsorption analysis showed that almost all of the CPVAs added to the pulps were adsorbed on the fiber surface. Therefore, the adsorption of various CPVAs was approximately 20 mg/g on fibers. The strength properties of the papers made from the fractionated refined pulps are also listed in Table 3. As seen, the tensile and burst indices of the papers made from the fibers, refined under 8000 revolutions, were 3.3 and 3.8 times as much as those of unrefined fibers, respectively. To investigate the effect of CPVAs on fractionated refined pulps, the variations in the tensile, burst, and tear indices of papers, upon CPVA application, were plotted as a function of the freeness of fractionated refined pulps in Figure 1. As seen, the low-charged CPVA (0.4 mequiv/g) increased the tensile and burst indices of papers by 28% at the CSF of 725 mL. It also reduced the tear index of paper as much as 8%. The tear index is affected by several factors, e.g., fiber wall and bonding breakages.9,21 The decrease in the tear index of papers, while the tensile index is increased, has been attributed to the increase in fiber breakage, which requires less force than the pulling-out of fiber in a tear test.21-23 Generally, by decreasing the freeness of pulps (increasing the refining revolutions), the tensile and burst indices were less improved by CPVAs, while the tear index was more significantly improved (Figure 1). Also, the high-charged CPVA (1.3 mequiv/g) did not alter the paper strength at the CSF of 725 mL significantly. However, by decreasing the freeness of fractionated refined pulp to 725 mL (increasing the refining revolutions), the high-charged

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CPVA decreased the tensile and burst indices of papers as much as 25%, and increased the tear index by 15%. The optical and structural properties of the papers made from the fractionated refined pulps, unmodified or modified with CPVAs (20 mg/g), are listed in Table 4. As expected, by increasing the refining revolutions, the light scattering coefficient and the brightness of papers were reduced, whereas the apparent density was increased, regardless of the CPVA application. On fractionated unrefined fibers, the low-charged CPVA (0.4 mequiv/g) reduced the light scattering coefficient and increased the apparent density of papers to some extent. However, the high-charged CPVA (1.3 mequiv/g) marginally changed the light scattering coefficient and apparent density of papers. These results are in agreement with our results previously reported in the literature.24 Also, by increasing the refining revolution, the effect of the low-charged CPVA (0.4 mequiv/g) on the light scattering coefficient and apparent density of papers was reduced. However, by increasing the refining revolution, the high-charged CPVA (1.3 mequiv/g) tended to increase the light scattering coefficient and decrease the apparent density of papers to some extent. Additionally, the higher the charge density of CPVAs, the lower the brightness of papers at any refining revolution. 3.3. Force Analysis. The AFM analysis showed that there was no observable adhesion force developed between the uncoated AFM probe and the silicon wafer in water. Figure 2 shows the adhesion force developed between the AFM probe, coated with various CPVAs, and the silicon wafer in water. Interestingly, the adhesion force developed between the AFM probe coated with high-charged CPVA (1.3 mequiv/g) and the silicon wafer (-1.5 mN/m) was higher than that developed between the AFM probe coated with low-charged CPVA (0.4 mequiv/g) and the silicon wafer (-0.2 mN/m). A similar adhesion force (-1 mN/m) has been reported in the literature18 between the AFM probe coated with cationic starch having a charge density of 0.5 mequiv/g and the silicon wafer. 3.4. Image Analysis. Figure 3 shows the morphologies of fractionated refined fibers (8000 revolutions), unmodified or modified with the high-charged CPVA (1.3 mequiv/g), in hand sheets, revealed by SEM. As seen in Figure 3a, the surface of fractionated refined fibers was well fibrillated. Such fibrils increased the contact area of fibers and contributed to the development of fiber bonding. However, as seen in Figure 3b, the surface of the fractionated fibers, refined under 8000 revolutions and modified with the CPVA, was fairly smooth and fibrils could no longer be observed. Similar results were reported by applying carboxymethyl cellulose, CMC, to fibers.25 These results imply that the CPVA covered the fibrils located on the fiber surface and reduced the contact area of fibers in the hand sheets. A similar phenomenon was observed on other hand sheets made from fractionated refined fibers, modified with various CVPAs. 4. Discussion 4.1. Refining and Fractionating. As is well-known, the refining of pulps decreases the fiber length and increases the fines content, fibrillation, and flexibility of fibers. Decreasing the fiber length increases the freeness of pulps, while increasing the fines content, flexibility, and fibrillation of fibers decreases the freeness of pulps. By fractionating the refined pulps, the fiber length and fines content of pulps were kept approximately constant. However, the fractionated refined fibers possessed different degrees of flexibility and fibrillation. The results in Tables 2 and 3 show that, by increasing the

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Table 4. Structural and Optical Properties of Papers Made from Fractionated Unmodified or CPVA-Modified Fibers refining revolutions

unmodified modified with 0.4 mequiv/g CPVA modified with 0.8 mequiv/g CPVA modified with 1.3 mequiv/g CPVA

light scattering coefficient, apparent density, kg/m3 brightness, % ISO light scattering coefficient, apparent density, kg/m3 brightness, % ISO light scattering coefficient, apparent density, kg/m3 brightness, % ISO light scattering coefficient, apparent density, kg/m3 brightness, % ISO

m2/kg m2/kg m2/kg m2/kg

refining revolution, only 45 mL out of 315 mL in CSF reduction (14%) was attributed to the increases in the fibrillation and flexibility of fibers. 4.2. Effect of CPVA Charge Density on Paper Properties. The polydispersity of PVA was 1.47, which implies that the molecular weight distribution of PVA was relatively broad. However, the cationic modification of PVA was conducted using the same PVA polymers. Therefore, the influence of polydispersity of PVA in our investigation should be minimal. It is implied from Figure 1 that, by increasing the charge density of CPVA, the effectiveness of CPVA in improving the strength of the papers made from the fractionated fibers was decreased, regardless of the refining revolutions. It seems that the configuration and charge density of CPVA affect the development of fiber bonding and paper strength. In the literature, it has been claimed that low-charged polymers tended to create a tail-andloop configuration on fibers, whereas high-charged polymers tended to develop a flattened configuration.11-16 Therefore, the surface morphology of fibers is modified differently upon adsorbing polymers having various charge densities. One reason for such a phenomenon is the electrostatic adhesion/repulsion force developed between the charges associated with the adsorbing polymers and the fiber surface. As observed in Figure 2, the higher the charge density of CPVA, the higher the adhesion force developed between the CPVA and the silicon wafer. Such higher adhesion force perhaps contributed to the flattened configuration of polymers on the fiber surface. If polymers lie on the fiber surface (flattened configuration), they probably develop a thinner layer of polymers on the fiber surface. Such a phenomenon has been comprehensively demonstrated in the literature.26,27 Pelton hypothesized that the fiber bonding develops at the scale of 10 nm.28 Therefore, polymers having a tail-and-loop configuration on the fiber surface have higher effectiveness than those having a flattened configuration to bridge the fibers. In other words, the low-charged CPVA

Figure 2. Adhesion/repulsion force developed between the AFM probe coated with CPVAs having various charge densities (mequiv/g) and the silicon wafer in retrace mode in water.

0

500

3000

8000

28.7 ( 0.3 615 ( 10 88.9 ( 0.3 26.4 ( 0.5 627 ( 9 87.8 ( 0.6 27.8 ( 0.5 620 ( 8 87.4 ( 0.5 29.5 ( 0.3 600 ( 10 86.2 ( 0.3

26.1 ( 0.2 691 ( 7 88.5 ( 0.4 25.3 ( 0.4 696 ( 9 87.3 ( 0.4 26.4 ( 0.5 663 ( 11 87.0 ( 0.3 26.8 ( 0.6 640 ( 9 85.4 ( 0.7

19.6 ( 0.4 739 ( 12 86.4 ( 0.6 19.5 ( 0.4 735 ( 10 85.1 ( 0.4 20.1 ( 0.6 721 ( 11 84.3 ( 0.4 20.9 ( 0.5 701 ( 11 83.3 ( 0.6

17.3 ( 0.3 741 ( 6 85.0 ( 0.7 17.9 ( 0.2 739 ( 7 82.1 ( 0.5 18.4 ( 0.4 727 ( 8 81.3 ( 0.2 18.8 ( 0.3 715 ( 8 80.4 ( 0.6

performed a similar task to fibrils; i.e., it increased the contact area of fibers and developed extra hydrogen bonds between fibers, which was observed as a reduction in the light scattering coefficient in Table 4. However, by increasing the charge density of CPVA, the repulsion force, developed between the adsorbed

Figure 3. (a) Surface morphology of fractionated unmodified fibers, refined under 8000 revolutions, revealed by SEM. (b) Surface morphology of fractionated fibers, refined under 8000 revolutions and modified with the high-charged CPVA (1.3 mequiv/g), revealed by SEM.

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CPVAs on neighboring fibers, was perhaps increased. Such a repulsion force might prevent fiber closure and impair the development of fiber bonding. The flattened configuration and the higher repulsion force developed between the neighboring fibers, modified with high-charged CPVA (1.3 mequiv/g), were possibly the reasons for the low effectiveness of high-charged CPVA (1.3 mequiv/g) in improving the paper strength. It was reported that, by applying low molecular weight polymers on fibers, some polymers might diffuse into the pores of fibers.11,14 The diffusion characteristics of CPVA into the pores of kraft fibers were evaluated in our previous work.29 It was also reported that the high-charged polymers diffuse less than low-charged ones into the fiber pores.30,31 Therefore, the high-charged CPVA (1.3 mequiv/g) might have diffused less than low-charged CPVAs into the pores, implying that the outer surface of fibers was more significantly affected by high-charged CPVA (1.3 mequiv/g) than by other CPVAs. Considering the charges of CPVAs, one can also conclude that greater cationic charges were introduced on the outer surface of fibers upon applying high-charged CPVA (1.3 mequiv/g), which affected the development of fiber bonding. 4.3. Effect of CPVA on Refined Fibers and Their Resulting Papers. It is also implied from the results of Figure 1 that, by decreasing the freeness of pulps (increasing the refining revolutions), the effectiveness of CPVA in improving the paper strength was reduced. Generally, by increasing the refining revolution, the available surface area of fibers is increased, due to the increase in the fines content and fibrillation, and to the decrease in the fiber length. However, the adsorption dosage of CPVA was kept at 20 mg/g on fibers. Therefore, one reason for such a reduction was the decrease in the surface coverage of fibers by CPVA.10 However, as addressed earlier, the application of CPVAs, especially the high-charged one, on fractionated refined fibers could even reduce the paper strength. It has been reported in the literature that the refining of bleached fibers created fibrils with an average size of 18-20 nm on the fiber surface.32 We previously reported that the hydrodynamic size of CPVAs was approximately 12.5 nm.24 Since fibrils and CPVAs had a fairly comparable size, the adsorbed CPVA partly covered the fibrils located on the fractionated refined fibers and flattened the fibrils on the fiber surface (see Figure 3). In other words, the fibrils, which were supposed to increase the contact area of fibers and to contribute to fiber bonding, were covered by CPVA. In this case, it seems that there is a compromise between the covering of fibrils by CPVAs and developing extra hydrogen bonds between fibers by applying CPVAs. In our previous work, we reported that paper strength was improved more by refining than by applying the lowcharged CPVA (0.4 mequiv/g).10 The CPVAs increased the shear strength of fiber bonding on one hand, and reduced the contact area of refined fibers by covering the fibrils on the other hand. It seems that the increase in the hydrogen bonding, as a result of CPVA application, could not compensate for the decrease in the contact area of fibers due to the coverage of fibrils. This hypothesis is confirmed by the increase in the light scattering coefficient and the decrease in the apparent density of papers in Table 4. Similar results were reported by Jokinen et al. on applying CMC on fibers.25 5. Conclusions By refining the bleached sulfite softwood pulp up to 8000 revolutions, approximately 14% of the reduction in CSF was owing to the increase in the fibrillation and flexibility of fibers. By increasing the charge density of CPVA, the adhesion force

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developed between the CPVA-coated AFM probe and the silicon wafer was increased. The lower the charge density of CPVA, the higher effectiveness of CPVA in improving the paper strength, regardless of the refining revolution. Also, by increasing the charge density of CPVA, the apparent density and brightness of papers were reduced, whereas the light scattering coefficient was increased. The different configurations of CPVAs on the fiber surface and the different adhesion/repulsion force developed between the CPVA-modified fibers were suggested to be the main reasons. If the fiber surface was not fibrillated, the low-charged CPVA (0.4 mequiv/g) could perform similarly to fibrils in that it increased the fiber bonding. However, if the fiber surface was well fibrillated, the low-charged CPVA (0.4 mequiv/g) covered the fibrils and impaired the contribution of fibrils on developing fiber bonding. Acknowledgment The authors would like to acknowledge Fraser Papers Co. for providing the pulp sample and NSERC, Canada, for funding this research. Literature Cited (1) Bobu, E.; Benea, G. H.; Bacaran, M. Performance and limits of starch as a papermaking additive. Cellul. Chem. Technol. 1997, 31, 499. (2) Gaiolas, C.; Mendes, P.; Silva, M. S.; Costa, A. P.; Belgacem, M. N. The role of cationic starch in carbonate filled papers. Appita J. 2005, 58 (4), 282. (3) Retulainen, E.; Kaarina, N. Fiber properties as control variables in papermaking? Part 2. Strengthening interfiber bonds and reducing grammage. Pap. Timber 1996, 78 (5), 305. (4) Baba, Y. New paper strength agent from grafted starch. Jpn. Tappi J. 2001, 55 (9), 11. (5) Mendes, P.; Sansana, P.; Silvy, J.; Costa, C. A. V.; Belgacem, M. N. Cationic starch as a dry strength additive for bleached Eucalyptus globulus kraft pulps. Appita J. 2001, 54 (3), 281. (6) Sanjay, N.; Soni, P. L.; Singh, S. V.; Kapoor, S. K. Application natural and modified guar and cassia tora gum as wet end additive vis-a`vis flocculant. Ippta J. 2001, 13, 97. (7) Gaiolas, C.; Costa, A. P.; Silva, M. S.; Belgacem, M. N. Influence of cationic starch addition on printing grade paper formation. Cellul. Chem. Technol. 2006, 40 (9-10), 783. (8) Fatehi, P.; Xiao, H. Adsorption characteristics of cationic-modified poly (vinyl alcohol) on cellulose fiberssA qualitative analysis. Colloids Surf., A: Physicochem. Eng. Aspects 2008, 327 (103), 127. (9) Fatehi, P.; Xiao, H. The influence of charge density and molecular weight of cationic poly (vinyl alcohol) on paper properties. Nord. Pulp Pap. Res. J. 2008, 23 (3), 285. (10) Fatehi, P.; Ates, S.; Ward, J. E.; Xiao, H. Impact of cationic poly (vinyl alcohol) on properties of papers made from two different pulps. Appita J. 2009, 62 (4), 303. ¨ dberg, L.; Wagberg, L.; Lindstro¨m, T. Adsorption (11) Tanaka, H.; O of cationic polyacrylamides onto monodisperse polystyrene latices and cellulose fiber: Effect of molecular weight and charge density of cationic polyacrylamides. J. Colloid Interface Sci. 1990, 134 (1), 219. ¨ dberg, L.; Berg, J. C. Adsorption and (12) Einarson, M.; Aksberg, R.; O reconformation of a series of cationic polyacrylamides on charged surfaces. Colloids Surf. 1991, 53, 183. (13) Petlicki, J.; van de Ven, T. G. M. Adsorption of polyethylenimine onto cellulose fibers. Colloids Surf., A: Physicochem. Eng. Aspects 1994, 83, 9. (14) Wagberg, L.; Ha¨gglund, R. Kinetic of polyelectrolyte adsorption on cellulose fibers. Langmuir 2001, 17, 1096. (15) Li, H.; Du, Y.; Wu, X.; Zhan, H. Effect of molecular weight and degree of substitution of quaternary chitosan on its adsorption and flocculation properties for potential retention-aids in alkaline papermaking. Colloids Surf., A: Physicochem. Eng. Aspects 2004, 242, 1. (16) Lofton, C.; Moore, S. M.; Hubbe, M. A.; Lee, S. Y. Deposition of polyelectrolyte complexes as a mechanism for developing paper dry strength. Tappi J. 2005, 4 (9), 3. (17) Reginald Thio, B. J.; Meredith, J. C. Measurement of polyamide and polystyrene adhesion with coated-tip atomic force microscopy. J. Colloid Interface Sci. 2007, 314, 52.

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ReceiVed for reView June 19, 2009 ReVised manuscript receiVed September 16, 2009 Accepted September 18, 2009 IE900999N