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Visualizing Flocculation and Adsorption Processes in Papermaking Using Fluorescence Microscopy Wesley L. Whipple* and C. Maltesh Polymer Science Department, Nalco Chemical Company, One Nalco Center, Naperville, Illinois 60563-1198 Received August 9, 1999. In Final Form: November 11, 1999 Polymer adsorption characteristics in complex papermaking systems have been elucidated using tagged reagents and the well-established technique of fluorescence microscopy. Interactions between polymers and components of papermaking slurries have been previously well researched, but the theories put forth are usually based on indirect inferences. Moreover, the use of simple model systems often does not permit correlation with real systems. The present study clearly shows that, under shear conditions and time scales prevalent on a paper machine, polymer partitions to inorganic fillers and fiber fines. In the absence of fillers, the polymer adheres to high surface area regions of the fiber, viz., fibrils that result from mechanical fiber processing operations. The roles of surface area, electrostatic interactions, and other papermaking operations are discussed in detail. We believe this study to be the first extension of fluorescence microscopy for visualizing polymer partitioning in complex systems such as papermaking slurries. On the basis of the data provided here, it should be facile to extend this application for studying polymer behavior in other systems such as sludge dewatering and mineral processing.
Introduction Papermaking involves the removal of water from a cellulose fiber slurry to form a fiber mat. This slurry is a mixture of cellulosic fibers, inorganic fillers (clay, titania, calcium carbonate), rosin, alum, starch, and water, brought together in predetermined proportions.1,2 Polymers of varying molecular weights are used in papermaking for operations such as retention and drainage, charge neutralization, strength improvement, coating, and prevention of deposits on the paper machine. In the design of improved reagents, and optimization of existing programs for papermaking, it is imperative to understand the complex interactions between the different substrates in the paper matrix. This knowledge could assist in the development of novel reagents with a specific purpose or target in a composite paper slurry. For example, polymers with an affinity only for titanium dioxide particles will increase retention of this expensive commodity and reduce waste. The nature of polymers used in papermaking is as diverse as the spectrum of macromolecules available to the papermaker. Polymers can differ with respect to charge, molecular weight, structure, and type. Even within a given class of polymers, differences due to the method of synthesis or source of natural polymer are observed. In paper matrixes, polymer-particle interactions depend on the polymer, as well as on the slurry composition. Different aspects of polymer adsorption and flocculation have been extensively studied using clean, fiber-only systems without cellulosic fines or inorganic fillers.3-6 The rapid adsorption of polymers to fiber and the probability for transfer and * To whom correspondence should be addressed: e-mail,
[email protected] (1) Roberts, J. C. Paper Chemistry; Chapman and Hall: New York, 1991. (2) Smook, G. A. Handbook For Pulp & Paper Technologists, 2nd ed.; TAPPI Press: Atlanta, GA, 1992. (3) Bown, R. Paper Chemistry; Roberts, J. C., Ed.; Chapman and Hall: New York, 1991; pp 162-197. (4) Swerin, A.; O ¨ dberg, L. Fundamentals of Papermaking Materials11th Fundamental Research Symposium; Pira International: Cambridge, U.K., 1997; Vol. 1, pp 265-350.
cleavage of polymer chains have been aptly described using these model systems.4,7-13 Electrostatic interactions and hydrogen bonding are proposed to be dominant in polymer-fiber interactions. Several insights are provided for polymer interactions in the presence of salts and other interfering dissolved and colloidal substances.10,14-16 Dissolved and colloidal materials originate from the wood source, recycled paper, additives, and the process water used for papermaking. Despite the fundamental understanding gained in polymer-fiber interactions, the results have limited applications to real papermaking systems. An important aspect not addressed in these adsorption studies is the effect of fibrillation and fines generated from mechanical fiber refining. Refining, or the process of separating individual fibers, develops desired properties such as strength, porosity, and opacity.3 During refining, the fibers break, thereby generating fines, and also expose complex surfaces which influence polymer adsorption characteristics. The cellulosic fines and their extracts can randomly coat, and alter, the surface characteristics of (5) Lindstro¨m, T. In Fundamentals of Papermaking. Transactions of the Ninth Fundamental Research Symposium Held at Cambridge: September 1989; Baker, C. F., Punton, V. W., Eds.; Mechanical Engineering Publications Limited: London, 1989; Vol. 1, pp 311-412. (6) Tanaka, H.; O ¨ dberg, L.; Wågberg, L.; Lindstro¨m, T. J. Colloid Interface Sci. 1990, 134 (1), 219-228. (7) O ¨ dberg, L.; Tanaka, H.; Glad-Nordmark, G.; Swerin, A. Colloids Surf., A 1994, 86, 201-207. (8) Tanaka, H.; O ¨ dberg, L.; Wågberg, L.; Lindstro¨m, T. J. Colloid Interface Sci. 1990, 134 (1), 229-234. (9) Tanaka, H.; Swerin, A.; O ¨ dberg, L. J. Colloid Interface Sci. 1992, 153 (1), 13-22. (10) Tanaka, H.; Swerin, A.; O ¨ dberg, L. Nord. Pulp Pap. Res. J. 1995, 10 (4), 261-268. (11) Tanaka, H.; Swerin, A.; O ¨ dberg, L. Langmuir 1994, 10, 34663469. (12) Tanaka, H.; Swerin, A.; O ¨ dberg, L. Tappi J. 1993, 76 (5), 157163. (13) Lundqvist, A.; O ¨ dberg, L.; Glad-Nordmark, G. Ber. Bunsen-Ges. Phys. Chem. 1996, 100 (6), 977-983. (14) Falk, M.; O ¨ dberg, L.; Wågberg, L.; Risinger, G. Colloids Surf. 1989, 40 (1-2), 115-124. (15) Wågberg, L.; O ¨ dberg, L. Nord. Pulp Pap. Res. J. 1989, 4 (2), 135-140. (16) O ¨ dberg, L.; Wågberg, L. Papier (Darmstadt) 1989, 43 (10A), V37-V38.
10.1021/la991076f CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000
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components in a papermaking slurry.17 Fillers, also considered fines, are typically added to a paper slurry to achieve desired final sheet optical properties and reduce the use of more expensive fibers. There is speculation regarding the location of polymers in these complex systems and how they partition between different substrates under varying shear conditions. There is a need for new techniques to determine where polymers adsorb and their relationship among other additives. Modern scanning probe microscopic tools are limited to model systems and are not effective for studying polymer adsorption in complex paper matrixes. This information can be obtained using epifluorescence microscopic techniques in tandem with fluorophore-labeled additives. Since the inception of fluorescence microscopy in the early 1900s, its growth has been fueled by the development of fluorescent stains, immunofluorescence techniques, and advances in microscopes and digital imaging technology.18 New applied microscopic techniques and instrumentation are periodically reviewed by Cooke.19 Goring and coworkers pioneered the use of ultraviolet microscopy in paper science and measured the distribution of lignin in the cell walls of different woods.20-22 This technique has also been used to define polymer location in polymerwood composites23,24 and in coated wood and paper25,26 and to measure the distribution of orthoquinones in wood.27 More recently, fluorescence confocal laser scanning microscopy (CLSM) has been used to characterize fibers,28,29 including fiber angle,30,31 fibrillation, fines location, and handsheet structure.32-34 Despite this prior work, there is relatively little precedence for the use of fluorescence microscopy as it pertains to polymer adsorption and flocculation studies. Epifluorescence microscopy is ideally suited for determining the role of polymers and additives in papermaking, particularly given the palette of fluorochromes and digital imaging techniques available.18,35 This technique is noninvasive and allows for determining the location of different substrates in a complex system, particularly when labeled with complementary fluorochromes. The success of this experiment is largely dependent on selectively coupling fluorochromes to additives or structures to be studied. In the present study, polymer adsorption to acid and alkaline papermaking slurries was studied through the (17) Jaycock, M. J.; Pearson, J. L.; Counter, R.; Husband, F. W. J. Appl. Chem. Biotechnol. 1976, 26 (7), 370-374. (18) Rost, F. W. D. Fluorescence Microscopy; Cambridge University Press: Cambridge, 1995; Vol. 2. (19) Cooke, P. M. Anal. Chem. 1998, 70 (12), 385R-423R. (20) Procter, A. R.; Yean, W. Q.; Goring, D. A. I. Pulp Pap. Mag. Can. 1967, 68 (9), T445-T460. (21) Fergus, B. J.; Goring, D. A. I. Holzforschung 1970, 24 (4), 118124. (22) Goring, D. A. I. Appita 1985, 38 (1), 31-40. (23) Furuno, T.; Goto, T. Mokuzai Gakkaishi 1973, 19 (6), 271-274. (24) Furuno, T.; Nagadomi, W.; Goto, T. Mokuzai Gakkaishi 1975, 21 (3), 144-150. (25) Vogelsang, J. Farbe Lack 1995, 101 (3), 281-284. (26) Quackenbush, D. W. Tappi J. 1984, 67 (5), 72-75. (27) Mislankar, A.; Darabie, A.; Reeve, D. W. J. Pulp Pap. Sci. 1997, 23 (2), J73-J76. (28) Gonzalez-Rio, F.; Martinez Nistal, A.; Alonso Guervos, M.; Astorga, R. Invest. Tec. Pap. 1997, 34 (131), 112-129. (29) Jang, H. F.; Seth, R. S. Tappi J. 1998, 81 (5), 167-174. (30) Batchelor, W. J.; Conn, A. B.; Parker, I. H. Appita J. 1997, 50 (5), 377-380. (31) Jang, H. F. J. Pulp Pap. Sci. 1998, 24 (7), 224-230. (32) Moss, P. A.; Retulainen, E.; Paulapuro, H.; Aaltonen, P. Pap. Puu 1993, 75 (1-2), 74-79. (33) Moss, P. A.; Retulainen, E. J. Pulp Pap. Sci. 1997, 23 (8), J382J388. (34) Ting, T. H. D.; Johnston, R. E.; Chiu, W. K. Appita J. 1998, 51 (4), 281-286.
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Figure 1. Fluorescent rhodamine monomer a. Table 1. Summary of Tagged Cationic Polymers Studied polymer
molar fraction of a
reduced viscosity (dL/g)a
A B
0.0006 0.0011
16.8 10.4
a
Determined in 1 M NaNO3, at 30°C, 450 mg/L polymer
use of tagged flocculants and other reagents, coupled with fluorescence microscopy. Polymer adsorption experiments were conducted using simulated furnishes in a Britt dynamic drainage jar. This jar has been used for studying chemical effects in papermaking systems under variable shear without interference from filtration and mechanical entrapment.36,37 Materials and Methods FluorescentRhodamine-BMonomer.Thevinylicrhodamine-B monomer, a (Figure 1, ex/em 561/583 nm), was synthesized by coupling hydroxyethyl methacrylate with rhodamine-B according to the procedure of Ward.38 Crude fluorescent monomer a was purified by preparative TLC using Whatman 60 Å silica gel plates prior to use (6:4 methanol/ethyl acetate, Rf 0.65). Fluorescent Cationic Poly(acrylamide). The fluorescent polymer A, a terpolymer with 90 mol % acrylamide and 10 mol % dimethylaminoethyl acrylate, methyl chloride quaternary salt (DMAEA‚MCQ), and the fluorescent rhodamine-B monomer,38 a (Figure 1), was synthesized using conventional water-in-oil emulsion polymerization in the following manner.39 An aqueous monomer solution was made-up by stirring together 0.047 g of the vinylic rhodamine-B monomer a, 18.2 g of a 49.6% aqueous solution of acrylamide, 0.45 g of adipic acid, 1.35 g of NaCl, 3.41 g of a 80.3% aqueous solution of DMAEA‚ MCQ, 8.90 g of water, and 0.18 g of a 5% aqueous solution of EDTA‚4Na+. The components were stirred until in solution. The oil phase was prepared by heating a mixture of 11.7 g of Escaid-110 (Exxon Chemical Co.), 0.23 g of Tween-61, and 0.68 g of Span-80 until the surfactants dissolved (≈55 °C). The oil phase was charged into a 125 mL baffled reaction flask. With vigorous stirring, the monomer solution was added over 1 min. The resulting mixture was stirred for 1 h at 45 °C. A mixture of 0.0100 g of AIBN (2,2′-azobis(isobutyronitrile)) and 0.0014 g of AIVN (2,2′-azobis(2,4-dimethylvaleronitrile)) was added to the water-in-oil emulsion. The polymerization was carried out under a N2 atmosphere for 4 h at 45 °C, then the temperature was raised to 70 °C for 1 h. A dry polymer, with >98% fluorescent tag a incorporation, was obtained by precipitating the emulsion polymer in a 1:1 MeOH/acetone mixture. Tag incorporation was measured by gel phase chromatography (GPC) using refractive index and fluorescent detectors in series. Polymer B was synthesized in a similar manner (Table 1). (35) Rost, F. W. D. Fluorescence Microscopy; Cambridge University Press: Cambridge, 1996; Vol. 1. (36) Britt, K. W.; Unbehend, J. E. Tappi 1976, 59 (2), 67-70. (37) Unbehend, J. E.; Britt, K. W. Pulp Paper: Chemistry and Chemical Technology, 3rd ed.; Wiley: New York, 1981; Vol. 3, pp 15931607. (38) Ward, W. J.; Cramm, J. R.; Reed, P. E.; Johnson, B. S. US Patent 5772894, June 30, 1998. (39) Baade, W.; Hunkeler, D.; Hamielec, A. E. J. Appl. Polym. Sci. 1989, 38 (1), 185-201.
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Table 2. Description of Paper Slurries Studied (slurries made up at 0.5 wt % total solids) % pulp slurry type
hardwood
softwood
alkaline acid
0.24 0.28
0.16 0.18
Table 3. Dosing Sequence for Polymer Adsorption in Britt Jar t)0s t ) 10 s t ) 20 s t ) 40 s
commence shear via mixingsadd furnish add starch (and alum) as required add flocculant sample for microscopic analyses
Rhodamine-B labeled polymers used in this study are summarized in Table 1. For adsorption studies, 1500 mg/L aqueous polymer solutions were prepared either from the dry polymer or by inverting the water-in-oil polymer emulsion. Flocculant activity was not influenced by the presence of low levels of fluorescent tag (less than 1 per 900 repeat units), as has also been reported earlier.6 Cationic Poly(acrylamide). A copolymer of 90 mol % acrylamide and 10 mol % DMAEA‚MCQ commercially available from Nalco Chemical Co. (5-10 × 106 g/mol) was used as received in studies requiring unlabeled flocculant. Fluorescent Microspheres. FluoSpheres carboxylate-modified microspheres (F-8787) were obtained from Molecular Probes. The spheres (2% in water) had a charge of 0.43 mequiv/g, a particle diameter of 20 nm ((15.8%), and excitation/emission wavelengths of 505/515 nm. Synthetic Alkaline and Acid Furnish. Kraft-bleached softwood and hardwood pulps were obtained as dry sheets (lap) from Champion International. The sheets were then pulped in deionized water and mechanically refined (Canadian Standard Freeness40 350-360 mL) using a Valley Beater to fibrillate the fibers and also generate cellulosic fines similar to those encountered in paper mills. These fibers were mixed in appropriate ratios along with inorganic fillers to generate a synthetic papermaking slurry (furnish). The pH was adjusted to simulate acidic or alkaline paper systems. The composition of the two furnishes is described in Table 2. The ionic strength of the water used was ∼0.1 mol/L and composed of common ionic species encountered in paper mills (Na+, Ca2+, Mg2+, Cl-, HCO3-, SO42-). Starch. Cationic substituted potato starch of ≈300 000 g/mol from Avebe BA of The Netherlands was used as received. This product contained 5-10 glycidyl trimethylammonium chloride (GTMAC) groups for every 100 glucose units. Alum. A 50% aqueous solution of aluminum sulfate, Al2(SO4)3‚ 16H2O was prepared prior to use. Inorganic Fillers. Ground calcium carbonate (GCC) from Omya Chemical Corp. was used as received. The particle size was specified to be 0.7 µm with a surface area of 11.6 m2/g. It must be noted that GCC contains a polyacrylate dispersant to facilitate dispersion of the material in water. However, the low level of dispersant present did not influence or bias polymer adsorption and filler flocculation. Calcined kaolinite clay (particle size ≈ 2 µm) from Thiele Kaolin, Sandersville, GA, was used as received. Adsorption Conditions in Britt Jar. A standard baffled Britt jar was used, and the stirring speed was set depending upon the program being evaluated. The screen in the Britt jar was 200 mesh (76 µm), and 500 mL of slurry was used for each experiment. The dosing sequences used are listed in Tables 3 and 4. Sample Preparation. At the time indicated in Tables 3 and 4, furnish was withdrawn from the Britt jar using a transfer pipet with a 7 mm opening. A drop of furnish was placed on a 25 × 75 mm glass microscope slide and covered with a 22 × 22 mm cover glass. (40) Measured according to TAPPI Standard Test Method: T 227 om-94. Freeness is the ability of a fiber slurry to drain water and depends on the nature of fiber and its fibrillation as well as the fines content in the system. Canadian Standard Freeness (CSF) measures the drainage of 1 L of 0.3 weight % pulp slurry through a calibrated screen. Fines are loosely defined as all material finer than 76 µm (-200 mesh).
inorganic filler type % CaCO3 clay
0.10 0.04
fines content (% of solids)
slurry pH
31 20
8.0 4.8
Table 4. Dosing Sequence for Polymer and FluoSphere Adsorption in Britt Jar time (s)
agitator speed (rpm)
action
0 10 20 50 60 80
750 750 2000 750 750 750
commence shear via mixingsadd furnish add cationic starch (usually 25 mg/L) add flocculant reduce the shear via mixing speed add the FluoSphere sample for microscopic analyses
Table 5. Filter Blocks Used on the Fluorescence Microscope filter block
excitation filter (nm) (band pass)
dichromatic mirror (nm) (ref short pass)
suppression filter (nm) (long pass)
I3 N2
450-490 530-560
510 580
520 580
Microscopy. Fluorescence microscopy was performed on a Leitz Diaplan microscope. Incident illumination was with a 50 W Hg lamp. Images were obtained at magnifications of 100× (N.A. 0.25, 200 µm scale), 200× (N.A. 0.45, 100 µm scale), and 400× (N.A. 0.70, 60 µm scale). The Leitz filter blocks used are summarized in Table 5. Fluorochromes selected had excitation/ emission wavelengths such that potential interference from the paper matrix was avoided. Electronic images were obtained using a Dage-MTI DC300 three-color CCD camera and processed using Image ProPlus (3.0) software. Pictures were transferred to Microsoft PowerPoint, annotated, and printed on a Tektronix Phaser 450 printer for publication.
Results and Discussion Polymer Adsorption: Fiber Alone. The adsorption of fluorescent polymer A (10 mg/L) on a refined kraft softwood pulp slurry (0.05 wt % solids) without filler was studied microscopically. The fiber and fibrils are shown in Figure 2a. By use of epifluorescence illumination (Figure 2b, filter block N2), adsorption of polymer was observed on both fiber and fibril. The concentration of the polymer appears greater on the fibrils, correlating to the greater surface area available for adsorption. The surfaces exposed upon fibrillation are believed to be more receptive to chemical additives,3 and Figure 2 clearly shows this. In the absence of inorganic fillers, polymer partitions to high surface area regions of the fiber that may correspond to areas of higher charge. Carboxylate groups on the surface of cellulose fibers impart an inherent anionic nature that can be altered with changes in pH. At an alkaline pH, the fibers uncoil to a greater extent than at acidic pH values causing the fibrils to extend outward to greater distances. This results in fibers with more exposed surface area and promotes interactions with the cationic polymer.41 Having demonstrated the utility of fluorescence microscopy in paper-related systems, further studies were conducted with simulated furnishes in the Britt jar. Polymer Adsorption: Synthetic Alkaline Furnish. Results of studies performed to determine where fluorescent polymer B partitions in a synthetic alkaline furnish containing fiber and CaCO3 filler are shown in Figure 3. Parts a and b of Figure 3 illustrate the location of polymer (41) Aloi, F. G.; Trsksak, R. M. In Retention of Fines & Fillers During Papermaking; Gess, J. M., Ed.; TAPPI Press: Atlanta, GA, 1998; pp 61-108.
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Figure 2. Adsorption characteristics of polymer A on kraft softwood fiber: (a) brightfield micrograph (400×) of fiber (1) and fibril (2); (b) epifluorescence micrograph (400× filter block N2), same location and focal plane as (a). Greatest concentration of fluorescent polymer (3) is located at fibrils. Annotation key: 1, fiber; 2, fibril; 3, rhodamine B labeled polymer (red); 4, 505/515 FluoSphere (green); 5, CaCO3 filler floc; 6, fiber fine; 7, TiO2/clay filler floc; 8, no fluorescent polymer on fiber; *, out of focus.
Figure 3. Synthetic alkaline furnish treated with 5 mg/L fluorescent polymer B. Location of polymer in the floc: (a) brightfield micrograph (100×) of fiber/filler floc, CaCO3 filler flocs (5) appear dark; (b) epifluorescence micrograph (100×, filter block N2) corresponding to (a), concentration of labeled polymer greatest at the filler floc, some out-of-plane fluorescence observed; (c) same sample as (a), location 2, brightfield micrograph (200×) of single fiber, CaCO3 flocs adsorbed to fibrils; (d) epifluorescence micrograph (200×, filter block N2) of (c), same focal plane. Concentration of polymer greatest with filler floc. Annotation key: 1, fiber; 2, fibril; 3, rhodamine B labeled polymer (red); 4, 505/515 FluoSphere (green); 5, CaCO3 filler floc; 6, fiber fine; 7, TiO2/clay filler floc; 8, no fluorescent polymer on fiber; *, out of focus.
in a fiber-filler floc, showing polymer preferentially adsorbed to the CaCO3 filler. More detailed information
is obtained by observing a single fiber away from the main fiber mass (Figure 3c,d). The isolated fiber, with associated
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Figure 4. Synthetic alkaline furnish treated with 5 mg/L unlabeled 10% cationic poly(acrylamide) flocculantssimilar to polymer A, blended with 0.0011 mol fraction of fluorescent tag a: (a) brightfield micrograph (100×) of fiber/filler floc. CaCO3 filler flocs (5) appear dark; (b) epifluorescence micrograph (100×, filter block N2) corresponding to (a). Unbound tag predominantly associated with fiber fines (6) and, in an isolated instance, with fiber (1); (c) magnification of (a) (400×) with fiber fine (6) in focus; (d) epifluorescence micrograph (400×, filter block N2) of (c), same focal plane. Unbound fluorescent tag a preferentially adsorbed to fiber fine. Annotation key: 1, fiber; 2, fibril; 3, rhodamine B labeled polymer (red); 4, 505/515 FluoSphere (green); 5, CaCO3 filler floc; 6, fiber fine; 7, TiO2/clay filler floc; 8, no fluorescent polymer on fiber; *, out of focus.
fibrils and CaCO3 filler flocs, is shown in Figure 3c. The fluorescent polymer is located with the filler flocs (Figure 3d), which are adsorbed to the fibrils (Figure 3c). The weak, diffuse fluorescence located on the lower surface of the fiber in Figure 3d is from an out-of-focus filler floc. The physical location of polymer in a matrix containing fiber and filler is a topic of much debate. Using kinetic and shear-induced-collision arguments, some mechanistic models suggest that high molecular weight polymers are predominantly adsorbed on fibers.3,4 In contrast, it was proposed elsewhere that filler particles will adsorb a large amount of polymer due to a high particle number concentration and high specific surface area.42 By preadsorbing polymer to clean fibers, then introducing model and natural fillers, it was established that polymer can transfer from fiber to filler.4 Results from paper machines support the importance of polymer-filler interactions. Preflocculated filler is retained to greater extents than untreated filler.43 Hence it can be inferred that in a composite paper slurry, predominant polymer adsorption to the fibers will lead to low filler retention. (42) Swerin, A.; Risinger, G.; O ¨ dberg, L. Nord. Pulp Pap. Res. J. 1996, 11 (1), 30-35. (43) Lambert, B. P.; Lowes, J. US Patent 3873336, March 25, 1975.
A combination of refined fibers and fillers similar to those encountered on paper machines was used in our study. This allows for competitive polymer adsorption to be studied. Polymer adsorption and transfer between different substrates will occur concurrently. The contact time between polymer and furnish was roughly 20 s prior to sampling. Microscopic observations were conducted within 5 min of sampling. It is unlikely that high molecular weight polymer will transfer between substrates in a low shear environment during this time period.12 For the synthetic alkaline furnish, polymer is associated with filler flocs that are located on fibrils (Figure 3). The ground calcium carbonate filler used in this system had a particle size of ≈0.7 µm. The network of filler flocs seen in Figure 3a is significantly larger, an obvious result of polymer adsorption (Figure 3b). Furthermore, in stark contrast to Figure 2, there is no evidence of polymer adsorption to the fiber surface. The results presented here suggest preferential polymer adsorption to calcium carbonate filler. The filler flocs adhere to the fibrils on the fiber surface. The preferential adsorption of cationic flocculant to the ground calcium carbonate (GCC) filler may seem an apparent contradiction to prior published work on polymer adsorption to fibers. However, upon closer scrutiny, the
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Figure 5. Synthetic alkaline furnish treated with 5 mg/L unlabeled 10% cationic poly(acrylamide) flocculant and 5 mg/L fluorescent microparticles (0.43 mequiv/g): (a) brightfield micrograph (100×) of fiber/filler floc; (b) phase contrast epifluorescence micrograph (100×, filter block I3) corresponding to (a). Fiber morphology along with location of fluorescent microparticle observed. Concentration of the fluorescent microparticle is greatest at the filler floc. Annotation key: 1, fiber; 2, fibril; 3, rhodamine B labeled polymer (red); 4, 505/515 FluoSphere (green); 5, CaCO3 filler floc; 6, fiber fine; 7, TiO2/clay filler floc; 8, no fluorescent polymer on fiber; *, out of focus.
results presented here support and extend the findings in the literature. Lundqvist et al. have clearly demonstrated that preadsorbed polymer transfers between polymer and filler surfaces.13 For instance, polymer first preadsorbed to fiber and then contacted with GCC filler will transfer to the filler, and this redistribution varies inversely with polymer-fiber preadsorption times. Drawing a parallel between their work and the conditions studied here, the preadsorption time of polymer to fiber in our system is effectively zero. Under these conditions, Lundqvist et al. predict that the polymer will partition (or transfer) almost entirely to the filler particles. In practice, polymer is almost always added to a complete papermaking slurry and rarely only to fiber. The visual evidence for the preferential partitioning of polymer to filler is potentially an important finding for papermaking. It is reported that the surface area for kraft softwood fibers refined to a freeness of 350 CSF is about 5 m2/g.44 It can be assumed that kraft hardwood fibers refined to similar freeness levels possess a surface area of the same order. As mentioned earlier, the surface area of the GCC filler was 11.6 m2/g but filler constitutes only 20% of the solids in this paper slurry. The fiber and fiber fines have a higher exposed surface area for reagent interaction. The results shown here illustrate the preferential adsorption of polymer to filler particles. The total surface area available for adsorption will depend on the nature of the fiber, level of fines, pH, and the presence of interfering species. It is therefore possible that the mechanism of polymer adsorption will be system specific. In the composite cationic polymer-filler-fiber system studied here, the location of filler flocs on fibrils implies the prevalence of electrostatic interactions. Shear-induced collision and surface area models may be inappropriate to explain polymer adsorption in chaotic systems such as papermaking. Role of Unbound Tag. Controls were performed using synthetic alkaline furnish to determine any bias that unbound tag might introduce. For these experiments, a “dyed” polymer solution of untagged flocculant, with similar chemistry to polymer B, was prepared by blending (44) Biermann, C. J. Handbook of Pulping and Papermaking; Academic Press: New York, 1996; p 139.
in appropriate levels of rhodamine tag a to a 2500 mg/L solution of commercial Nalco flocculant. Photomicrographs from these experiments are shown in Figure 4. Consistent with the behavior of polymer B in Figure 3a, filler flocs are seen in a paper matrix (Figure 4a). In contrast, fluorescence was observed predominantly with fiber fines (Figure 4b,d). However, in an isolated instance, a single fiber surface, unique from the other fibers, was saturated with tag (Figure 4b). This control experiment clearly illustrates that adsorption characteristics of the tag differ (Figure 4b) from that of the tagged polymer (Figure 3b). Polymer Adsorption: Microparticle Programs. In recent years, microparticle programs have become popular in papermaking. These programs are defined by the use of inorganic or organic charged colloidal particles (bentonite clay, colloidal silica, or poly(acrylic acid)) and by their addition points.45-47 High molecular weight flocculant is usually added upstream to the microparticle prior to a high shear environment to maximize contact time and mixing. The colloidal particles are added downstream to the polymer after the last point of high shear. The polymer flocs formed are broken down but re-form upon contact with the charged microparticles. The reflocculated slurry contains smaller, denser flocs resulting in better filler retention and faster water removal from the papermaking matrix, yielding paper with superior properties. The mechanism of these microparticles is often unclear; however, electrostatic interactions are believed to be dominant. The microparticles are mainly anionic and are used with cationic, anionic, and nonionic flocculants. The partitioning of microparticles to different components in a papermaking matrix is uncertain and may vary between various systems. To gain a better understanding of the interactions occurring in microparticle programs, the adsorption characteristics of fluorescent microparticles and cationic flocculant were studied with synthetic alkaline furnish. Carboxylate-modified FluoSpheres (20 nm, 0.43 mequiv/ g, ex/em 505/515) available from Molecular Probes were used as fluorescent microparticles in these experiments. (45) Begala, A. J. US Patent 5098520, March 24, 1992. (46) Langley, J.; Holroyd, D. US Patent 4753710, June 6, 1988. (47) Honig, D. S.; Harris, E. US Patent 5167766, December 1, 1992.
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Figure 6. Synthetic alkaline furnish treated with 5 mg/L fluorescent polymer B and 5 mg/L fluorescent microparticles (0.43 mequiv/g): (a) brightfield micrograph (400×, focal plane 1) of fiber, fibrils, and filler floc, focal plane 1; (b) epifluorescence micrograph (400×, filter block N2, focal plane 1), flocculant (3) preferentially adsorbed to CaCO3 filler, the filler flocs are adsorbed to the fibrils; (c) epifluorescence micrograph (400×, filter block I3, focal plane 1), concentration of fluorescent microparticles greatest in filler floc, microparticles also adsorbed on fibrils; (d) phase contrast epifluorescence micrograph (400×, filter block N2, focal plane 2), fiber and filler floc morphology, along with location of fluorescent microparticle observed; (e) phase contrast epifluorescence micrograph (400×, filter block I3, focal plane 2), fiber and filler floc morphology, along with location of fluorescent microparticle observed. Annotation key: 1, fiber; 2, fibril; 3, rhodamine B labeled polymer (red); 4, 505/515 FluoSphere (green); 5, CaCO3 filler floc; 6, fiber fine; 7, TiO2/clay filler floc; 8, no fluorescent polymer on fiber; *, out of focus.
Micrographs of flocs obtained from the treatment of synthetic alkaline furnish with 5 mg/L of cationic flocculant (10 mol % cationic, 5-10 × 106 g/mol), followed by 5 mg/L of FluoSpheres are shown in Figures 5 and 6. As in Figure 3a, the location of the filler flocs in the fiber matrix is shown in the brightfield micrograph 5a. Micrograph 5b was produced using a combination of transmitted light-phase contrast and epifluorescence microscopy (filter block I3). This allowed for the simultaneous observation of fluorescing and nonfluorescing portions. By use of this technique, weak, out-of-focus fluorescence was eliminated in the micrograph. Although the greatest concentration of FluoSpheres was located
predominantly with the CaCO3 filler flocs (Figure 5b), a significant number of FluoSpheres adsorb on the fiber fibrils (Figure 6c). When different substrates are labeled with complementary fluorochromes, their location and adsorption characteristics can be observed using different filter blocks. An adsorption experiment was conducted where synthetic alkaline furnish was treated with rhodamine-B labeled polymer B, followed by FluoSpheres. Micrographs in Figure 6 show flocculated CaCO3 adsorbed to fibrils on the side and bottom of the fiber. Micrographs at two different focal planes were obtained to show the threedimensional nature of the filler floc-fiber complex. In
Flocculation and Adsorption Processes in Papermaking
Langmuir, Vol. 16, No. 7, 2000 3131
Figure 7. Synthetic acid furnish treated with 5 mg/L fluorescent polymer B. Adsorption characteristics of polymers and filler particles on the fiber: (a) brightfield micrograph (400×) of fiber, fibrils, and filler, no starch or alum used in flocculation experiment; (b) epifluorescence micrograph (400×, filter block N2) corresponding to (a), polymer and filler adsorbed to fibrils (3, 7), no fluorescence observed on the main fiber surface; (c) brightfield micrograph (400×) of fiber, fibrils, and filler, starch (25 mg/L) and alum (50 mg/L) included in the flocculation experiment; (d) epifluorescence micrograph (400×, filter block N2) corresponding to (c), a significant amount of polymer on the fiber surface, particularly adsorbed to the fibrils (in focus). Annotation key: 1, fiber; 2, fibril; 3, rhodamine B labeled polymer (red); 4, 505/515 FluoSphere (green); 5, CaCO3 filler floc; 6, fiber fine; 7, TiO2/clay filler floc; 8, no fluorescent polymer on fiber; *, out of focus.
panels a-c of Figure 6, the upper surface of the fiber is in focus, and in panels d-e in Figure 6, the lower surface is in focus. CaCO3 filler flocs are observed, which adsorb to the fibrils on the fiber. Polymer is adsorbed to the CaCO3 filler but is not detected on the fiber surface (Figure 6b). The fluorescent microparticles were observed both with the CaCO3 filler floc and on the fibrils. Epifluorescence micrographs in Figure 6d,e were obtained in the presence of low-level polarized transmitted light. This technique was employed to show the fiber morphology in addition to the location of the fluorescent substrates. The microparticle is primarily associated with fibrils and filler floc, i.e., chiefly where the polymer partitions. This suggests that polymer-microparticle interactions are important to the success of the program. Since the polymer used here was cationic and the microparticle was anionic, electrostatic interactions can be expected to dominate in this system. Additional work using anionic flocculants or tagged starch is suggested. Polymer Adsorption: Synthetic Acid Furnish. Acid papermaking is another industrially important system that utilizes fillers other than calcium carbonate. Furthermore, the customary presence of alum in this system offers an alternative system to investigate using fluores-
cence microscopy. Compared to the synthetic alkaline furnish, the synthetic acid furnish studied here has a lower concentration of fines and therefore a lower surface area. Other differences are described in the Experimental Section. The effect that cationic starch and alum have on the adsorption of polymer B for a synthetic acid furnish is illustrated in Figure 7. Without starch and alum, the polymer and small filler flocs (
