Chapter 6
The Effect of Hydrophilic Nonionogenic Comonomers on Flow Properties of Carboxylated Latexes 1
Jaromir
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2
Šňupárek , Otakar Quadrat , J i ř íHorský , and Martin Kaška
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1
Department of Polymeric Materials, University of Pardubice, 532 10 Pardubice, Czech Republic Institute of Macromolecular Chemistry, Academy of Science of the Czech Republic, 162 06 Prague, Czech Republic
2
Different copolymer latexes containing hydrophilic nonionic functional groups were thickened with a model alkali¬ -swellable latex based on ethyl acrylate / methacrylic acid / Ν,Ν'-methylenebisaerylamide copolymer and the thickening efficiency was investigated. The thickening process was closely related mainly to the volume fractions of both the nonionic and the alkali-swellable latex particles.
Copolymer latexes containing hydroxyl or amide groups are frequently utilized in water-borne coatings. The reactive functional groups are loci for polymer chain crosslinking with melamine resins or with polyisocyanates. Highly carboxylated latex copolymers that are alkali-soluble or alkali-swellable increase the viscosity of water-borne vehicles. Copolymers comprising both nonionogenic comonomers as they are, e.g., hydroxyethyl methacrylate or
© 2002 American Chemical Society
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
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72 methacrylamide, and ionogenic acrylic or methacrylic acid are frequently used in shell layers in core-shell particles. Steep increases in viscosity during neutralization of such copolymer latexes is important for the vehicle rheology; however, it also may cause difficulties in their practical application. Thus, a good knowledge of the flow behavior of such vehicles is very important. At present, the mechanism of such thickening processes is not fully understood. It is assumed (1-3) that, after neutralization of the originally acidic material, electrostatic repulsions between ionized carboxylic groups attached to polymer chains of the latex and thickener leads to particle swelling, which increases steric and electrostatic particle interactions. Also bridging flocculation or volume restricting flocculation may appear (1,2). These structural changes result in high resistance to deformation in the flow field and, consequently, in an increase in non-Newtonian viscosity, yield stress, and dynamic moduli of the thickened system. Flow properties of three model latexes of copolymers containing a small amount of acrylic acid, as well as a reactive hydrophilic monomer, either methacrylamide (MAAm) or 2-hydroxyethyl methacrylate ( H E M A ) in two of the latexes, and thickened with a model alkali-swellable thickener (AST) have been investigated using capillary viscometry and dynamic and steady shear measurements.
Experimental Latex Preparation The basic copolymer latex comprising butyl acrylate, styrene, and acrylic acid (L-0) and the other two also with M A A m (L-M) or H E M A (L-H) comonomers have been prepared by a non-seeded semi-continuous emulsion copolymerization with a monomer emulsion feed. The solids content of the prepared latexes was 50 wt%. The A S T has been prepared similarly by emulsion copolymerization of ethyl acrylate, methacrylic acid and N , N methylenebisacrylamide. In the case of the carboxyl-containing dispersion, the polymerization was carried out only to a low (7 wt%) solids content to reduce unpredictable crosslinking of particles by chain transfer. The pH of the resulting latexes was 2-3. The emulsion polymerization procedure that was used guaranteed a homogeneous statistical copolymer composition of latex particles (4-6). The hydrodynamic diameter of the latex particles as measured by dynamic light scattering (Auto-Sizer LoC, Malvern Instruments, U K ) ranged between 150 and 170 nm. The copolymer compositions are given in Table I.
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
73 Table L Composition of Latexes
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Monomer Butyl acrylate Styrene Methacrylamide 2-hydroxyethyl methacrylate Acrylic acid Ethyl acrylate Methacrylic acid Ν,Ν'-methylenebisacrylamide
(wt%)
L-0
L-M
L-H
56.5 41.5
51.5 36.5 10
60 28
2
2
AST
10 2 82.5 15 2.5
Materials Technical grade monomers, ethyl acrylate, η-butyl acrylate, and acrylic acid (Chemical Works, Sokolov, Czech Republic), styrene (Kauëuk Kralupy, Czech Republic), 2-hydroxyethyl methacrylate, methacrylic acid and NJVmethylenebisacrylamide (Rohm, Darmstadt, Germany) were used. Polymerizations were carried out in a 2.5 L stirred glass reactor under a nitrogen atmosphere using a redox ammonium peroxodisulfate-sodium bisulfite initiating system (for hydroxy containing samples) or ammonium peroxodisulfate alone (for carboxylated low-solids dispersion of the thickener). Disponil A E S 60 (sodium poly(ethylene glycol) alkylaryl ether sulfate, Henkel, Germany) in an amount of 2 wt% of the active component (relative to the monomers) was used as an emulsifier.
Sample Preparation and Viscometry In this study we aimed at the thickening of medium-concentrated latexes (25 wt%). Thickened samples were prepared by addition of dilute aqueous ammonia to mixtures of originally acidic latexes with the alkali-swellable dispersion to reach the required pH, 9.2-9.3. pH measurements were carried out with a digital pH meter (Radiometer, Copenhagen) with a combined electrode, G K 2321 C. Viscosity measurements were performed at 25 °C with samples stabilized for one day after adjustment of the pH. The relative viscosity η = η/η where η is the viscosity of latices and η is that of the dispersion medium, was measured using an Ostwald capillary viscometer. Rheological experiments were performed using a Weissenberg τ
01
Daniels et al.; Polymer Colloids ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
0
74 rheogoniometer (model R-16, U K ) with a cone-and-plate system with a cone diameter of 5 cm and a complementary angle of 2°.
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Results and Discussion Capillary viscometry indicated that the dependencies of viscosity of different neutralized latexes on their particle concentration considerably differ as is shown in Figure 1. It could be assumed that during neutralization of the originally acid latexes the effective hydrodynamic volume of latex particles (intrinsic viscosity [rj]) increased due to electrostatic interactions of ionized carboxylic groups, which was manifested in an increase of the latex viscosity. The [η] values were obtained as an intercept of the φ / 1 η η vs. Φι, plot (Figure 2) according to the Mooney equation (7) Γ
T?r = exp{[7]]
