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The Migration of Styrene Butadiene Latex during the Drying of Coating Suspensions: When and How Does Migration of Colloidal Particles Occur? Yong-Hua Zang,*,† Juan Du,† Yanfen Du,† Zhenjuan Wu,† Shaoling Cheng,‡ and Yuping Liu§ †
Tianjin Key Lab of Pulp and Paper, and ‡College of Materials Science and Chemical Engineering, Tianjin Univeristy of Science & Technology, 29 at the 13th Street, TEDA, Tianjin, China, and § College of Chemistry, Nankai University Received May 31, 2010. Revised Manuscript Received October 15, 2010
Surface elemental compositions of model latex clay coatings on an impervious substrate consolidated under various conditions were measured using the XPS technique, in order to clarify when and how colloidal latex particles migrate to the surface during drying. Under similar drying conditions, surface carbon content decreased with the addition of a water-soluble polymer to the coating colors, while remaining virtually unchanged for coatings of different coat weights made with a given color, indicating that surface carbon content variation is mainly caused by migration of latex rather than of water-soluble polymer. The results also showed that for coatings made with a given suspension, surface carbon content decreased with increasing delay time between coating and heating. For coatings frozen during consolidation and dried by sublimation, surface carbon content increased with increasing drying time before freezing. These results suggest that for the model coatings studied, latex migration mainly occurs after coating application before capillary formation during the initial drying stage when coatings are in the liquid phase, contradicting both the conventional capillary transport and boundary wall migration mechanisms. An alternative mechanism which attributes latex migration to surface trapping effect and to higher Brownian mobility of the smaller latex particles compared with pigment appears to provide a systematically consistent explanation to those phenomena. The new particle migration mechanism implies that segregation of colloidal particles is a ubiquitous phenomenon that would occur not only during the drying of paper coatings but also during consolidation of colloidal films containing particles of different sizes. This is of great importance in the control of surface compositions of nanocomposite coatings.
1. Introduction Pigmented coatings have widely been used to improve surface properties in various industrial processes such as papers,1,2 paints,3,4 nanocomposite films, and thin layer catalysts.5 A coating color is a fairly well mixed colloidal suspension of mineral pigments, particulate latex or water-soluble polymer binders, and other minor functional additives such as dispersants and rheology modifiers.1,6-9 During coating application and consolidation, however, particulate latex may redistribute with respect to pigments due to shearing or along with movement of the aqueous phase toward the substrate and the coating surface,6-10 resulting in binder migration. Particle migration or redistribution during consolidation of colloidal dispersions is receiving increasing attention largely due to increased application of nanoparticles in areas such as cosmetics, ceramics, porous biomaterials, and “smart coatings” with self-cleaning or self-healing ability. The paper coating industry in particular has recognized decades ago the importance of latex *Corresponding authors. E-mail:
[email protected]; huazang06@ yahoo.ca. Tel.:86-22-60601996. (1) Lepoutre, P. Prog. Org. Coat. 1989, 17, 89. (2) Pan, S. X.; David, H. T.; Scriven, L. E. Tappi J. 1995, 78, 127. (3) Croll, S. G. J. Coat. Technol. 1986, 58, 41. (4) Diebold, M. P.; Bettler, C. R.; Mukoda, D. M. J. Coat. Technol. 2003, 75, 29. (5) Sun, J. K.; Velamakanni, B. K.; Francis, L. F. J. Colloid Interface Sci. 2004, 280, 387. (6) Vanderhoff, J. W.; Bradford, E. B. 1972 Tappi Coating Conference Proceedings; Tappi Press: Atlanta, GA, 1972, 173. (7) Hagen, K. G. Tappi J. 1986, 69, 93. (8) Hagen, K. G. Tappi J. 1989, 72, 77. (9) Lee, D. I.; Whalen-Shaw, M. Binder Migration in Paper and Paperboard Coating; Tappi Press: Atlanta, GA, 1993; Chapter 2. (10) Yamazaki, K.; Nishioka, T. Tappi J. 1993, 76, 79.
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migration on the structure and printability of coated papers.1,9,11-17 In fact, excessive binder migration into base paper would cause dusting and picking during printing, because of lack of binder in the coating layer. Uneven binder migration toward the coating surface may cause print mottles, because nonuniformities in binder distribution at the coating surface may cause uneven ink transfer and nonuniform water and ink absorbency.10-14 As a result, large research efforts have been expanded to control binder redistribution and a lot of interesting results were generated, using various techniques such as visual analysis of scanning electron microscopic images15,17-19or modern surface chemistry analysis methods, such as ESCA,10,11,20 UV absorption,21,22 FTIR-ATR spectroscopy,23 and Raman spectroscopy.24 Different mechanisms6-9,17,25-29 have been advanced to describe binder migration processes. Among them are the Hagen7,8 modified (11) Zang, Y. H.; Aspler, J. S. J. Pulp Paper Sci. 1998, 24, 141. (12) Engstrom, G.; Strom, G.; Norrdahl, P. Tappi J. 1987, 70, 45. (13) Engstrom, G.; Rigdahl, M. Tappi J. 1991, 78, 147. (14) Zang, Y. H.; Aspler, J. S. Tappi J. 1995, 78, 147. (15) Heisei, E. J.; Cullen, D. W. Tappi J. 1965, 48, 80. (16) Aschan, P. J. Tappi, J. 1973, 56, 78. (17) Watanabe, J.; Lepoutre, P. J. Appl. Polym. Sci. 1982, 27, 4207. (18) Ma, Y.; Davis, H. T.; Scriven, L. E. Prog. Org. Coat. 2005, 52, 46. (19) Kentta, E.; Pohler, T.; Juvonen, K. Nord. Pulp Pap. Res. J. 2006, 21, 665. (20) Al-Turaif, H. A.; Bousfield, D. W. Nord. Pulp Pap. Res. J. 2005, 20, 335. (21) Malik, J. S.; Kline, J. E. Proceedings of the 1992 Tappi Coating Conference; Tappi Press: Atlanta, GA, 1992; Vol. 105. (22) Kline, J. E. Proceedings of the 1993 Tappi Coating Conference; Tappi Press: Atlanta, GA, 1993; Vol. 93. (23) Halttunen, M.; Loija, M. Proceedings of the 2001 Tappi Coating and Graphic Art Conference; Tappi Press: Atlanta, GA, 2001; p 203. (24) Titla, S.; Tripp, C. P.; Bousfield, D. W. J. Pulp Paper Sci. 2003, 29, 382. (25) Ranger, A. E. Pap. Technol. 1994, 35, 40.
Published on Web 11/02/2010
DOI: 10.1021/la103675f
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capillary transport6 and Ranger25 boundary wall effect mechanisms, which contradict each other but remain the most widely accepted concepts.9,12,20,24,29 Hagen7,8 proposed that latex particles were dragged to the coating surface by capillary flow due to water evaporation, implying that latex migration mainly occurred after a period of drying when pigment particles in the coating layer become close enough to form capillaries. In contrast, Ranger25 proposed that latex enrichment at the surface was mainly due to a boundary wall effect of the coating applicator, which caused a “near-wall depletion” of larger pigment particles (ranging from 0.2 to 2 μm) relative to latex (around 0.1-0.2 μm) due to volume exclusion, as observed in the field of rheology of suspensions.30 Ranger’s mechanism implies that small latex particles are enriched at the surface during coating application, and that binder migration is a process by which latex particles move away from the surface into the bulk by concentration diffusion or by capillary transport. More recently, Groves and co-workers 26,29 have critically reviewed the literature and argued against all published latex migration mechanisms. They26,29 found that while evaporation caused water-soluble starch to migrate to the coating surface, there was no enrichment of particulate latexes at the surface. They proposed that the excess of organic materials at the coating surface (used as proofs of latex migration in the literature) was caused by migration of water-soluble surfactants and dispersants contained in the suspensions, rather than by migration of latex binders. There are many reasons why binder migration has proved to be a controversial subject. One of the reasons is that binder migration during the manufacture of coated papers is strongly affected by many variables including substrate absorbency, coating color compositions, coating application process and conditions, coat weight, and drying strategy and conditions, making it difficult to isolate the effects of each factor. Another reason is that the migration of a water-soluble binder (such as starch and carboxymethyl cellulose/CMC) and a particulate latex binder may be controlled by different mechanisms.8,26 It is generally agreed that a watersoluble binder is an integral part of the aqueous phase and can migrate with the movement of water. However, latex particles are dispersed in water and can only be carried by viscous drag.8,9 This work presents a systematic investigation to elucidate mechanisms that govern the migration of colloidal latex particles (0.1-0.2 μm) relative to micrometer-sized pigments (0.2-2 μm) during the drying of coating suspensions. By measuring surface elemental compositions of model coatings on an impervious substrate using the X-ray photoelectron spectroscopy (XPS) technique, it was shown that the latex particles did migrate to the surface when applied on nonabsorbent films. We have further analyzed when the latex migration takes place and found that the surface latex enrichment was induced during neither coating application nor after capillary formation, contradicting both the conventional capillary transport and the wall-effect migration mechanisms. A recently proposed particle migration mechanism appears to provide a systematically consistent explanation to the experimental results. (26) Groves, R.; Matthews, G. P.; Ridgway, C. J. In The Science of Papermaking; Bakers, C. F., Ed.; Pulp Paper Fundamental Res. Soc.: Oxford, UK, 2001; Vol. 2, p 1149. (27) Gagnon, R. E.; Parish, T. D.; Bousfield, D. W. Tappi J. 2001, 84, 66. (28) Sand, A.; Toivakka, M.; Hjelt, T. Nord. Pulp Pap. Res. J. 2008, 23, 52. (29) Groves, R. Proceedings of the 2003 PITA Coating Conference; Zeebra Publishing: Manchester, U.K., 2003; p 69. (30) Hartman-Kok, P. J. A.; Kazarian, S. G.; Lawrence, C. J. J. Colloid Interface Sci. 2004, 280, 511–517.
18332 DOI: 10.1021/la103675f
Zang et al.
2. Experimental Section 2.1. Materials and Methods. The pigment used was a commercial coating clay (with 90-92% by weight of particles finer than 2 μm and an average size of about 1.2 μm) from Maomin Technology, China. A film-forming, carboxylated styrene butadiene (SB) latex (with a glass transition temperature Tg of about 0 C and an average particle size of 0.158 μm as determined with a laser particle size analyzer) obtained from a coated paper mill was used as binder. The clay was predispersed at 68% solid (by weight) in distilled water to which 0.3 parts of a dispersing agent (sodium polyacrylate) per hundred parts of pigment (on a dry weight basis) was added to minimize the viscosity. Coating colors, with solid content (including pigment and latex) varying from 45% to 62% by weight, were then prepared by adding different quantities of latex and distilled water to the predispersed clay. The pH of the suspensions was adjusted to 7.8 using sodium hydroxide. Model clay coatings were prepared by applying the coating colors to nonporous polyester (Mylar) films with a draw down bar, as described in previous works.11,14 The use of an impervious substrate ensures that all the water lost is by evaporation from the coating surface, so that the point during drying when the latex migrates to the surface can be identified without interference of binder and water penetration into base paper. Gloss of the coatings during consolidation at room temperature (about 23 C) was monitored using a glossmeter to determine the first critical concentration (FCC), at which the water-air interface reaches surface defined by the constituent solid (skeletal) material of the porous medium, as described by Watanabe and Lepoutre.17 Solid content of the coating film during drying was calculated by weighing the coatings with an electronic balance, which was video recorded. Four series of coatings were prepared to investigate respectively the effects of drying conditions (e.g., delayed drying and freezedrying) and coating formulation on latex migration. The coatings in the first series were virtually identical in compositions and in coat weight, with drying condition as the only variable. These coatings were prepared using the same coating color with a binder content of 10 parts of SB latex per hundred parts (pph) of clay, an initial solid content of 50% by dry weight, and a coat weight of about 22 ( 2 g/m2, corresponding to a wet coating thickness of about 32 μm. This solid content is much lower than commonly used latex-clay coatings (about 60%) and is chosen to promote latex migration, if it does occur. Those coatings were first dried at a room temperature of 23 C for a predesigned delay time and then dried at a higher temperature of 80 ( 2 C within an electric hot-air heating oven. The second series was coatings with a coat weight of about 20 ( 2 g/m2, containing 15 pph latex, prepared with a suspension of 50% solid content. These coatings were either immediately frozen after coating by plunging into liquid nitrogen to retain structure of the initial wet coatings, or frozen in liquid nitrogen after drying in an oven at about 80 C for various drying times, ranging from 8 to 100 s. The frozen coatings were then dried by vacuum sublimation in a freeze-dryer (temperature
