Ind. Eng. Chem. Res. 2007, 46, 9103-9106
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Effect of Recycling on the Properties of Paper Surfaces Adam Brancato, Frances L. Walsh, Ronald Sabo, and Sujit Banerjee* Institute of Paper Science & Technology, School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0620
AFM surface adhesion measurements made on virgin and recycled bleached kraft pulp show that recycling increases the apparent hydrophilicity of the fiber surface. Yet, the water retention values and strength decreases as expected, which is consistent with internal cross-linking of the bonding sites and a reduction in hydrophilicity. Recycling does not affect the amount of monolayer water bound to the fiber surface indicating that the pore water is reduced but not the water bound to fiber surfaces. It is proposed that the contact area between the AFM tip and the fiber is greater for recycled material than for virgin. This could be caused by fiber shrinkage, changes in the angle of contact, lamination of fibrils, or other processes. Hence, in this instance, the surface adhesion values are more a measure of the topography of the surface than of its chemistry. An application to newsprint is illustrated. Introduction Recycling paper fibers irreversibly changes their structure.1,2 Both the water retention value (WRV, the water remaining in the pulp after centrifugation) and the tensile strength of fibers decrease upon recycling. Most of these changes occur over the first recycle after which the effect progressively declines. Fibrils and other bonding sites are not fully rehydrated when the dried fiber is repulped, which reduces their ability to hydrogen bond with one another. A reduction in hydrogen-bonding potential implies a reduction in the hydrophilicity of the fiber, which is consistent with the decreased ability of the fiber to hold water. This is further supported by the finding that recycling increases the contact angle of water on fiber.3-5 We have studied the effects of recycling on the properties of the fiber surface using atomic force microscopy (AFM) to probe the hydrophobicity of the surface and have found that recycling sharply increased the surface adhesion of a silicon tip to fiber. This suggests that the surface becomes nominally more hydrophilic upon recycling, which conflicts with the observations described above. In this paper we reconcile the difference and propose a mechanism for the AFM results. Experimental Section Never-dried thermomechanical pulp (TMP) and bleached softwood kraft pulp were obtained from Augusta Newsprint and Stora Enso’s Wisconsin Rapids mill, respectively. They were refined in a Valley Beater using TAPPI procedure T 200 om89.6 The pulps (20 g dry weight) were recycled in batches. They were processed for 15 000 revolutions in 2 L of water in a British disintegrator. Handsheets (5 g dry weight) were then prepared in a TAPPI circular handsheet mold and dried in a controlled environment under TAPPI standard conditions6 for 48 h. The handsheets were repulped, new handsheets were prepared as above, and the cycle was repeated three times. WRV was determined with TAPPI procedure UM 256.6 AFM measurements were made with an MFP-3D instrument from Asylum Research (Santa Barbara, CA). The 10-nm radius tips (AC240TS) also obtained from Asylum, had nominal spring constants of 2 N/m. The actual spring constant was calculated * To whom correspondence should be addressed. E-mail: sb@ gatech.edu.
for each tip with the Sader method.7 The AFM was initially used in the tapping mode. If good tip-to-surface contact was made, then the instrument was placed in contact mode and force measurements were taken. Each point was measured twice, and the values were averaged if the results were within 10% of each other. Measurements were made from at least ten separate locations. At least three test strips (one from each handsheet from each preparation) were analyzed using at least two separate areas on each strip, for a minimum of 60 force measurements for each refining and recycling level. Surface adhesion measurements made by AFM are influenced by humidity.8-10 In order to evaluate the significance of this variation, unrefined bleached kraft handsheets, both virgin and thrice-recycled, were tested at 20, 50, and 80% RH. These conditions were obtained by mixing water-saturated and dry air streams. The results, presented in Table 1, show a small increase in surface adhesion with increasing humidity for the virgin handsheets and a larger increase for the thrice-recycled samples. However, these changes are small compared to those resulting from refining and recycling. Our measurements were made at 21 °C and at a relative humidity of 50%, and small variations in humidity would not significantly affect the results. AFM measurements on single fibers were made by draining a dilute solution of never-dried fiber onto filter paper. A clean glass slide was pressed on the paper so that the fibers transferred to the slide. Most of the fibers were cleanly separated. The slide was dried at 70 °C under TAPPI standard conditions.6 AFM measurements were taken (in air) on a single fiber. The tip was withdrawn, water was added to the slide to cover the fiber, the system was allowed to equilibrate for an hour, and the force measurement was retaken. The slide was then dried for 24 h, and the process was repeated. An OpTest Laboratory Fiber Quality Analyzer (FQA) was used to determine the average fiber length for each level of refining and recycling of the bleached kraft pulp. Tritium pulp: water partition coefficients (Kp/w) were measured as described by Walsh and Banerjee.11 Four determinations were made for each sample. Results and Discussion Most properties of paper deteriorate upon recycling; strength, swelling capacity, fiber thickness, and its ability to hold water are all reduced. Drying irreversibly closes some of the void
10.1021/ie070826a CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2007
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Table 1. Effect of Humidity on Surface Adhesion
Table 3. Effect of Recycling on the Properties of Refined Bleached Kraft Pulp
surface adhesiona (nN)
a
RH (%)
virgin
recycled
20 50 80
26 (4) 30 (5) 32 (6)
80 (8) 97 (7) 108 (15)
The standard deviation is in parentheses.
Table 2. Effect of Recycling on WRV and Surface Adhesiona no. of recycles 0
1
2
WRV (g/g) surface adhesion (nN)
1.4 (0.1) 21 (4)
TMP 1.3 (0.2) 31 (5)
1.2 (0.2) 30 (10)
WRV (g/g) surface adhesion (nN) tensile strength (kN/m)
1.79 (0.02) 28 (4)
Kraft 1.73 (0.03) 68 (8)
1.65 (0.02) 85 (9)
1.65 (0.02) 94 (7)
2.13 (0.06)
1.91 (0.05)
1.79 (0.06)
1.73 (0.04)
a
3
recycling levela
WRVb (g/g)
Kp/wc
surface adhesionc (nN)
U0 U1 U2 U3 M0 M1 M2 M3 H0 H1 H2 H3
1.8 1.7 1.7 1.7 2.1 1.5 1.4 1.4 2.4 1.7 1.6 1.7
0.11 (0.08) 0.21 (0.05) 0.18 (0.05) 0.12 (0.02) 1.0 (0.4) 0.4 (0.4) 0.5 (0.03) 0.7 (0.4) 0.7 (0.3) 0.8 (0.3) 0.7 (0.4) 0.8 (0.4)
28 (5) 68 (8) 85 (9) 94 (7) 35 (4) 69 (10) 93 (10) 106 (9) 29 (6) 82 (9) 93 (9) 98 (8)
a U, M, and H represent unrefined, medium refined, and highly refined pulps; the numbers represent the level of recycling. b The standard deviation is 0.1. c The standard deviation is in parentheses.
The standard deviation is in parentheses.
spaces in the fiber network through cross-linking,12 interfibril cross-linking,13 and other means. Lactone formation14 and cocrystallization of cellulose15 have also been invoked. Regardless of the mechanism, recycling reduces the number of hydrogen bonds available to bond one fiber to another. TMP is less affected than kraft pulp because the lignin sterically hinders interfiber bonding. The effect of recycling on TMP and kraft pulps is listed in Table 2. The WRV decreases but only minimally. As shown later, a much larger decrease occurs for the refined pulps. Tensile strength measured for the kraft fibers shows the expected decrease. The strength of the TMP sheets also decreased, but it was too low for accurate measurements to be taken. The FQA results (for kraft) showed no decrease in fiber length indicating that recycling did not shorten the fibers in our experiments. AFM has been previously used to image the surface of fibers after refining and other treatments.16 In this study we use the technique to measure nanoscale tack rather than as an imaging device for reasons that will be discussed later. Recycling increased the AFM surface adhesion values sharply for kraft and to a lesser extent for TMP as shown in Table 2. The effect was especially pronounced for the first kraft recycle. Additional measurements were made with bleached kraft pulp refined to two levels. The unrefined pulp had a freeness (a measure of drainage) of 673 mL of CSF. The freeness of the refined pulps (designated as medium and highly refined) was 423 and 240 mL of CSF, respectively. The unrefined and refined materials were each recycled three times, and the measurements listed in Table 3 were made at each stage. Refining creates more surface by fibrillating the fibers, and the WRV increases as a result. Recycling progressively reduced the WRV, especially for the refined fibers. These results are completely in line with previous findings. Refining the pulp did not significantly alter the AFMmeasured surface adhesion. For example, the first values in each set of data in Table 3 are similar, indicating that refining does not appear to change the nature of the surface on the AFM scale. The surface adhesion values in Table 3 rise with progressive recycling, and the increase is roughly similar across all the levels of refining as shown in Figure 1. The sheets were recycled between measurements, and the AFM tip would not contact the same fiber after each recycle. Results from measurements made on a single fiber in both wet and dry states are illustrated in
Figure 1. Effect of recycling on the surface adhesion of unrefined (circles), medium refined (triangles), and highly refined (squares) fibers.
Figure 2. Effect of recycling on the surface adhesion of single fibers.
Figure 2. The profiles in Figures 1 and 2 track each other, confirming that the changes observed originate at the fiber level. The gap between the air and water profiles in each panel of Figure 2 is probably an outcome of capillary action. The difference is smaller for TMP than for kraft because TMP is less porous than kraft with proportionately fewer capillaries. The magnitude of the change in surface adhesion is surprisingly high. It is difficult to come up with any scenario where the first recycle would double the hydrophilicity of the surface, especially in light of the water retention values, which suggest the opposite trend. A possible explanation is that the apparent increase in hydrophilicity does not derive from a chemical change at all but is an outcome of greater contact between the AFM tip and the fiber surface. This could derive from fiber shrinkage and other changes induced by recycling.17 A possible configuration is shown in the schematic in Figure 3 where the bonding sites on the fiber surface are drawn closer together upon
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Figure 3. Schematic of the proposed interaction of an AFM tip with the surface of virgin (left) and recycled fiber (right).
recycling and the AFM tip contacts a larger number of these sites. This reasoning is consistent with the decrease in sheet strength caused by recycling. Surface shrinkage would promote internal hydrogen bonding and would reduce bonding across fibers. Such a reduction has been measured by confocal laserscanning microscopy.18 It is known that fibrils extending out from the fiber surface collapse during recycling and laminate onto the surface.20 This would increase the local density of the surface and add to the number of sites available for contact with the AFM tip. Also, the angle of contact between the fiber and the surface could change upon recycling, which would alter the area of contact.21 Let us now consider the effect of recycling on the surface of a fiber. Kp/w is the (dimensionless) partition coefficient of tritiated water between fiber and water and is a measure of the tightly bound water content of pulp.11 It is obtained by adding tritiated water to a pulp/water suspension whereupon the tritium partitions between the bulk water and the pulp. The free water associated with the pulp is removed with a dry, water-miscible, aprotic solvent such as acetone, which physically displaces the free water without exchanging with the tritium. At this stage the only tritium attached to the pulp should be associated with the hydroxyl and carboxylic acid groups in the pulp matrix and with bound water. The tritium tied up with the bound water far exceeds that associated with the exchangeable groups in pulp. This pulp-associated tritium is removed by washing the pulp with water, which exchanges the tritium back into the aqueous phase. Hence, Kp/w is essentially related to the monolayer water bound to pulp, and, hence, to the wet surface area of the fibers. Kp/w values for the unrefined pulp recycled to different levels are included in Table 3. They range between 0.1 and 0.2. The measurement uncertainty increases rapidly for the refined pulps because internal delamination exposes new surface, and the sampling errors are more pronounced.22 Kp/w for the unrefined pulp ranges between 0.1 and 0.2, whereas values for the refined pulps are much higher. Clearly, the effect of recycling is much less than that of refining. Thus, recycling does not change the surface area of the fibers much, certainly in comparison to even moderate refining. This does not contradict the AFM findings where the surface becomes compacted, because different aspects of the fiber surface are involved. Kp/w measures the area of the fiber surface capable of holding monolayer water, which discounts any void spaces. Shrinkage of the surface will reduce void spaces but should not materially affect the surface area of the fibers themselves. Similarly, collapse of the fibrils should reduce the fiber surface area only slightly but should have a much stronger effect on surface density. Hence, it is possible for recycling to lead to a more compact mat surface without significantly altering the surface area of the fibers. The contact angle of water on fiber increases upon recycling,3-5 which suggests that the fiber becomes nominally less hydrophilic. However, both the chemistry and the roughness of the surface affect the contact angle. While the reduced hydrogen bonding ability of the surface would decrease hydrophilicity, the surface is known to “wrinkle”, i.e. become rougher upon recycling.23 Roughness can dramatically increase the contact
Figure 4. Surface adhesion measurements of reel samples taken from a newsprint mill. Each value was averaged over 60 points from each of the 11 locations.
angle, and wrinkling would magnify the effect of recycling on contact angle.24 The various parameters used to track the effect of recycling nominally seem to provide conflicting results. The drop in WRV suggests a reduction in hydrophilicity, which is reinforced by the contact angle and tensile strength measurements. Yet, the AFM work points to an apparent increase in hydrophilicity. The apparent conflict arises because the different techniques measure different attributes of the fiber. The lower WRV results from a collapse of the pores, which reduces the water-carrying capacity of the fiber. This same collapse compacts the surface, which increases the area available for contact with the AFM probe tip and leads to higher surface adhesion values. Hence, the AFM responds more to changes in the packing density of the surface than to changes in surface chemistry. It would be difficult to image these changes with our 10-nm AFM tip because the changes involved occur at a much smaller scale. For the changes to be imaged they would have to occur at a 100 nm scale, in which case the AFM tip would not see much change at the 10 nm level. Finally, we describe an application of AFM suggested by the results in Table 2. Reel samples were collected from a recycle newsprint mill in Georgia during June/July 2007, and surface adhesion values measurements taken from several locations on the sheet are illustrated in Figure 4. Except for the June 29 sample, the profiles are similar with the maxima centered at the 15-30 nN region. This falls within the general range expected for virgin fiber as per Table 2. The June 29 profile is noteworthy for two reasons. First, the force constants are much lower than those seen for the other days, which suggests that the surface is more hydrophobic than that of virgin fiber. A hydrophobic contaminant would seem to be responsible. Regardless of the reason, the difference between the profiles between June 28 and 29 illustrates the variability that can occur over a day. This is most likely caused by changes in furnish. In conclusion, we have shown that recycling paper substantially increases the apparent hydrophilicity of the surface as measured by AFM force constants, which runs counter to commonly accepted viewpoints. It is proposed that the apparent increase in hydrophilicity does not result from any intrinsic change in the chemistry of the fiber but from the fact that recycling changes the area and angle of contact between the AFM tip and the fiber surface and compacts the surface through fiber shrinkage, lamination of fibrils, etc. This is not to discount
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chemical changes that are known to occur but to emphasize that the impact of topographical changes is more significant in the context of our measurements. In a practical application, AFM is clearly able to highlight day-to-day changes in the surface characteristics of newsprint. Acknowledgment A.B. acknowledges scholarship support from IPST. This study was funded by an industrial consortium comprising Stora Enso, Georgia-Pacific, Bowater, Abitibi, and Eka Chemicals. Literature Cited (1) Jayme, G. Mikro-Quellungsmessungen an Zellstoffen. Wochenbl Papierfabr. 1944, 6, 187. (2) Howard, R. C. The Effects of Recycling on Paper Quality. J. Pulp Pap. Sci. 1990, 16, J143. (3) Tze, W. T.; Gardner, D. J. Contact Angle and IGC Measurements for Probing Surface-Chemical Changes in the Recycling of Wood Pulp Fibers. J. Adhes. Sci. Technol. 2001, 15 (2), 223. (4) Okayama, T. The Effects of Recycling on Pulp and Paper Properties. Kami Pa Gikyoshi 2002, 56 (7), 62. (5) Klungness, J. H.; Caulfield, D. F. Mechanisms Affecting Fiber Bonding During Drying and Aging of Pulps. Tappi J. 1982, 65 (12), 94. (6) TAPPI. TAPPI Test Methods; TAPPI Press: Atlanta, GA, 1994. (7) Sader, J. E.; Chon, J. W. M.; Mulvaney, P. Calibration of Rectangular Atomic Force Microscope Cantilevers. ReV. Sci. Instrum. 1999, 70, 3967. (8) Sedin, D. L.; Rowlen, K. L. Adhesion Forces Measured by Atomic Force Microscopy in Humid Air. Anal. Chem. 2000, 72, 2183. (9) Sugawara, Y.; Ohta, M.; Konishi, T.; Morita, S.; Suzuki, M.; Enomoto, Y. Effects of Humidity and Tip Radius on the Adhesive Force Measured with Atomic-Force Microscopy. Wear 1993, 168, 13. (10) Jones, R.; Pollock, H. M.; Cleaver, J. A. S.; Hodges, C. S. Adhesion Forces Between Glass and Silicon Surfaces in Air Studied by AFM: Effects of Relative Humidity, Particle Size, Roughness, and Surface Treatment. Langmuir 2002, 18, 8045. (11) Walsh, F. L.; Banerjee, S. Characterization of Thin Water Layers in Pulp by Tritium Exchange. Part 1: Methods Development. Holzforschung 2007, 16, 115. (12) Lindstro¨m, T. In Paper, Structure and Properties; Bristow, J. A., Kolseth, P., Eds.; Marcel Dekker; New York, 1986; pp 99-109.
(13) Laivins, G. V.; Scallan, A. M. In Products of Papermaking, Baker, C. F., Ed.; Trans. 10th Fundamental Research Symposium; Pira International: Oxford, 1993; pp 1235-1260. (14) Fernandes Diniz, J. M. B.; Gil, M. H.; Castro, J. A. A. M. Hornificationsits Origin and Interpretation in Wood Pulps. Wood Sci. Technol. 2004, 37, 489. (15) Newman, R. H. Carbon-13 NMR Evidence for Cocrystallization of Cellulose as a Mechanism for Hornification of Bleached Kraft Pulp. Cellulose 2004, 11, 45. (16) Hanley, S. J.; Gray, D. G. In Surface Analysis of Paper; Conners, T. E., Banerjee, S., Eds.; CRC Press: Boca Raton, FL, 1995. (17) Vieira, M. G. A.; Estrella, L.; Silva, M. A.; Rocha, S. C. S. Shrinkage of Recycled Paper Sheet During Drying. Drying Technol. 2006, 24 (4), 465. (18) Somwang, K.; Enomae, T.; Onabe, F. Effect of Fiber Hornification in Recycling on Bonding Potential at Interfiber Crossings: Confocal LaserScanning Microscopy. Kami Pa Giyoshi (Jpn. Tappi J.) 2002, 56 (2), 79. (19) Haggkvist, M.; Li, T.-Q.; Odberg, L. Effects of Drying and Pressing on the Pore Structure in the Cellulose Fibre Wall Studied by 1H and 2H NMR Relaxation. Cellulose 1998, 5, 33. (20) Wang, X.; Maloney, T. C.; Paulapuro, H. Internal Fibrillation in Never-Dried and Once-Dried Chemical Pulps. Appita J. 2003, 56 (6) 455. (21) Me´ndez-Vilas, A.; Gonza´lez-Martı´n, M. L.; Labajos-Broncano, L.; Nuevo, M. J. Experimental analysis of the influence of surface topography on the adhesion force as measured by an AFM. J. Adhes. Sci. Technol. 2002, 16 (13), 1737. (22) Walsh, F. L.; Banerjee, S. Characterization of Thin Water Layers in Pulp by Tritium Exchange. Part 2: Effect of Refining on Water Absorption. Holzforschung 2007, 16, 120. (23) Eriksson, I.; Lunabba, P.; Pettersson, A.; Carlsson, G. Recycling Potential of Printed Thermomechanical Fibers for Newsprint. Tappi J. 1997, 80 (7), 151. (24) Grundke, K.; Bogumil, T.; Gietzelt, T.; Jacobasch, H.-J.; Kwok, D. Y.; Neumann, A. W. Wetting Measurements on Smooth, Rough and Porous Solid Surfaces. Prog. Colloid Polym. Sci. 1996, 101, 58.
ReceiVed for reView June 15, 2007 ReVised manuscript receiVed September 22, 2007 Accepted October 1, 2007 IE070826A
