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Rapid Gel-Like Latex Film Formation through Controlled Ionic Coacervation of Latex Polymer Particles Containing Strong Cationic and Protonated Weak Acid Functionalities Gene D. Rose,* J. Keith Harris, Gordon D. McCann, Jamie M. Weishuhn, and Donald L. Schmidt 1712 Building, The Dow Chemical Company, Midland, Michigan 48674 Received July 7, 2004. In Final Form: October 11, 2004 We introduce a controlled ionic coacervation (CIC) process that rapidly forms uniform, gel-like latex films with significant mechanical integrity without loss of water from the film. This process uses latex particles that contain both strong cationic charges and weak protonated acid groups. An increase in pH ionizes the weak acid and triggers the rapid setting of the latex films. The necessary increase in pH can be achieved by coating the latex onto an alkaline surface (such as concrete) or by controlled release of a fugitive acid (such as carbon dioxide). We explore the effect of latex composition and concentration on this process. We show that the CIC process does not require a water-soluble polymer to obtain the rapid-set film properties. Our proposed mechanism for CIC process is consistent with models for rapid, irreversible, particle-particle aggregation.
Introduction Latex polymers are widely used in commercial coating and adhesive applications because these water-based systems have reduced volatile organic compound (VOC) emissions compared to solvent-borne formulations. In these applications, the latex-based formulations begin as dispersions of polymer particles stabilized by electrostatic or entropic repulsion. The physical properties of the coating or adhesive develop after dehydration forces the particles into contact. Particle coalescence and film formation then occur if the drying temperature is above the polymer glass transition temperature (Tg) or if a small amount of coalescing solvent is present. Interparticle diffusion of the polymer molecules slowly builds the mechanical strength of the film. The properties of the resulting film depend strongly on polymer composition and the method of latex synthesis, as well as its end-use formulation and method of application. As a result, literature on polymer latex film formation is extensive. Recent reviews summarize this literature.1,2 Latex-based formulations that rapidly develop mechanical integrity before appreciable dehydration has occurred are often desirable to prevent flow after application of the formulations. One way to reduce the flow upon application is to reduce the amount of water in the latex formulation; however, this also increases the viscosity of the formulation, which quickly leads to undesirable rheological properties for the application. Therefore, latex compositions that are stable in storage and rapidly “set” at the appropriate time without the addition of other materials (“single-pot” systems) are desirable to simplify the use of these products. Several methods of decreasing the set time using pH changes have been demonstrated. In one approach to obtain a single-pot, rapid-set formulation, Brown uses the * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Steward, P.; Hearn, J.; Wilkinson, M. C. Adv. Colloid Interface Sci. 2000, 86, 195-267. (2) Winnick, M. A. In Emulsion Polymerization and Emulsion Polymers; Lovell, P. A., El-Aasser, M. S., Eds.; John Wiley and Sons: New York, 1997; Chapter 14.
addition of a polyamine plus a volatile base, such as ammonia, to anionic latexes.3,4 At high pH, the formulation is stable. Upon application, the base is lost, the pH decreases, and the amine groups of the polyamine become protonated. These cationic chains quickly bind to the anionic charges on the particle surface. This not only decreases the Coulombic repulsion but also forms bridges between the particles causing the coating to set rapidly, even though most of the water has not evaporated. Thus, the set time depends on the loss of the volatile base rather than the rate of water evaporation. Kangas and Neuendorf also used the interaction of an amphoteric polyelectrolyte (e.g., a copolymer of 2-aminoethyl methacrylate [2-AEM] plus methacrylic acid [MAA]) plus latexes with “anionizable” groups (e.g., a copolymer containing acrylic acid [AA]) to achieve a rapid-set formulation.5 They achieved “coacervation” by adjusting the pH of the dispersed system plus amphoteric polyelectrolyte to a value such that the amphoteric polyelectrolyte was cationic (below its isoelectric point) and the groups on the dispersed particles were anionic. They defined coacervation broadly as any process that causes the particles of a dispersed system to agglomerate in large numbers, which they specifically stated “includes precipitation, gelation, flocculation, and coagulation”. They defined a variety of ways of initiating this coacervation, one of which was the use of a volatile base, e.g., ammonia, to lower the pH to the desired value. For example, with a latex copolymer containing AA and a poly(2-AEM/MAA) amphoteric polyelectrolyte, they used ammonium hydroxide to ionize the AA. Heating and mechanical foaming reduced the pH below the isoelectric point of the poly(2AEM/MAA) and resulted in “coacervation”. Shalbayeva et al.6 used ammonia as a “screening solvent” to overcome the difficulty of forming polyelec(3) Brown, W. ProceedingssInternational Conference in Organic Coatings: Waterborne, High Solids, Powder Coatings, 23rd, Athens, July 7-11, 1997; Institute of Materials Science: New Paltz, New York, 1997; pp 63-72. (4) Brown, W. Prog. Org. Coatings 1998, 34, 26. (5) Kangas, D. A.; Neuendorf, W. R. Coacervation of Anion-Containing Aqueous Disperse Systems with Amphoteric Polyelectrolytes. U.S. Patent 3,947,396, March 30, 1976.
10.1021/la048309+ CCC: $30.25 © 2005 American Chemical Society Published on Web 01/15/2005
Rapid Gel-Like Latex Film Formation
Langmuir, Vol. 21, No. 4, 2005 1193 Table 1. Typical Latex Polymerization Recipe ingredient
A
deionized water seed latex (18.2 wt% solids)
20.90 9.51
B
butyl methacrylate methyl methacrylate methacrylic acid
19.71 14.39 0.615
49.3 46.1 2.29
C
deionized water Ageflex FM1Q75MC (75 wt% active)
10.85 1.93
2.23
D
deionized water tert-butyl hydroperoxide (70 wt% active)
10.85 0.295
E
deionized water sodium formaldehyde sulfoxylate total
10.85 0.109
Figure 1. Schematic of particle-particle controlled ionic coacervation.
trolyte complexes of polyamines and poly(acrylic acid) with anionic latex particles by direct mixing. At high pH (>12), they obtained stable colloidal systems. Loss of ammonia initiated ionic interactions between the complementary charged polyelectrolytes and latex particles. Film properties varied between rubbery and glassy depending on the polyelectrolyte complex-to-latex ratio. Recently, Schmidt et al.7 introduced the concept of a “controlled ionic coacervation” (CIC) process for waterborne soluble polymers containing both acidic and cationic functionality that provides rapid-set coatings. They defined CIC as “a controlled aggregation of soluble polymer molecules without precipitation to yield clear, rapid-set films”. We extend the CIC concept to latex polymers containing both acidic and cationic functionality, in which the CIC is a “controlled aggregation of latex particles without coagulation or phase separation”. Ionization of the weak acid groups initiates the CIC process for these latexes. Thus, any latex whose particles have fixed cationic charges and weak acid groups is potentially a “coacervate latex”. Figure 1 illustrates this process schematically. After ionization of the acid groups, the coacervating latex rapidly develops mechanical integrity even before significant water loss. Furthermore, during this CIC process and throughout the dehydration process, the latex maintains a homogeneous, opaque, gel-like appearance that, upon drying, yields a clear, uniform film. In contrast, during a coagulation or flocculation process, the latex separates into a polymer and a serum phase. We show that, unlike the rapid-set latexes described above, our CIC process does not require an additional water-soluble or water-dispersible polyelectrolyte or polyamine to obtain this rapid-set feature. We also show that any condition that raises the pH of the latex enough to cause ionization of the weak acid groups will readily initiate the CIC process. We describe processes for initiating the CIC, provide an analysis of how the latex composition and concentration affects the controlled ionic coacervation process, and propose an explanation for the development of the rapid increase in the mechanical properties of films formed by the controlled ionic coacervation. Experimental Section Latex Synthesis. We prepared the latexes evaluated in this study using a seeded, continuous monomer addition, free radical, emulsion polymerization. The monomers used in the latex synthesis were butyl acrylate (BA) [141-32-2], methyl methacrylate (MMA) [80-62-6], methacrylic acid (MAA) [79-41-4], 2-((methacryloyloxy)ethyl)trimethylammonium chloride (QMA) [5039-78-1] (Ageflex FM1Q75MC, Ciba Specialty Chemicals, Tarrytown, NY), and 2-aminoethyl methacrylate hydrochloride (2-AEM) [247094-2] (99%, Acros Organic, New Jersey). We used tert-butyl hydroperoxide (t-BHP) [141-32-2]/sodium formaldehyde sulfoxylate (SFS) [149-44-0] as the initiator. Table 1 describes a typical latex recipe. We used all chemicals as received. We prepared most of our polymers using a computer-controlled, pressurized, stirred, one-gallon stainless steel reactor. All (6) Shalbayeva, G. B.; Nikolayeva, T. V.; Mil’chendo, Ye. N.; Kalyuzhnaya, R. I.; Zezin A. B. Polym. Sci. U.S.S.R. 1984, 26, 1421-1427. (7) Schmidt, D. L.; Mussell, R. D.; Rose, G. D. J. Coat. Technol. 2003, 75, 59-64.
wt% mol% (as received) (as actives)
stream
100
Table 2. Seed Latex Polymerization Recipe stream
ingredient
wt% (as received)
A
deionized water styrene Arquad 18-50 (50 wt% active) aqueous FeSO4‚7H2O solution (0.25 wt%)
71.0 3.54 7.09 0.0442
B
styrene
14.2
C
deionized water aqueous hydrogen peroxide (30 wt% active) total
3.54 0.590 100
polymerizations were conducted under a nitrogen atmosphere, and all of the feed tanks and lines were also under a nitrogen atmosphere. All of stream A was loaded directly into the reactor. The reactor was heated to the desired temperature (typically 90 °C), and the remaining streams were added continuously at a uniform rate, generally over 4 h. The reactor temperature was maintained at the polymerization temperature for an additional 60 min to complete the conversion of the monomers. The latexes were filtered using a 200-mesh screen stainless steel sieve to remove any coagulated polymer that may have been generated during the polymerization. Seed latexes were prepared using a one-gallon, stainless steel reactor and a continuous addition polymerization process similar to that used to prepare the final latexes. Table 2 describes the recipe for the seed latex. Stream A was loaded into the reactor and it was preheated to 70 °C. Next, streams B and C were fed continuously over 3 h, and this was followed by a 30-min finishing step at the polymerization temperature of 70 °C. The volumeaveraged mean diameter of five different batches of seed latex recipe prepared in the one-gallon reactor was 42 nm (standard deviation ) 5.5 nm), as measured by hydrodynamic chromatography. The seed particle diameter and the desired particle size of the final latex determined the amount of seed used in the final polymerization, but it was always less than 5 wt% (based on the monomer weight in the final polymerization). No surfactant was added in the final latex polymerization; thus, the amount of surfactant in the final latex was always less than 1.0 wt%. The seed latexes and final latexes were also prepared in oneor two-liter, three-neck flasks using syringe pumps (Orion Research, Inc., Boston, MA) to control the monomer addition rates. A detailed example of this procedure is given elsewhere.8 No significant difference in the CIC behavior was observed between latexes prepared in the one-gallon versus the glass reactors. Latex Characterization. Standard latex analyses included pH, particle size by hydrodynamic chromatography9 (calibrated with polystyrene latex standards), solids concentration by a (8) Harris, J. K.; Schmidt, D. L.; Rose, G. D. Fast-Setting Latex Coatings and Formulations. U. S. Patent 5,997,952, December 7, 1999. (9) Small, H. J. J. Colloid Interface Sci. 1974, 48, 147-161.
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microwave oven (Labwave 9000, CEM Corporation, Matthews, NC), and residual MMA and BA by gas chromatography. Typical values for the final latexes were a pH of 2-3, particle diameters of 100-160 nm, solids of 36-48 wt% and
