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Rheology of Concentrated Dispersions of Copper Phthalocyanine Pigments in Organic Solvents D. F. Kevin Hughes and Ian D. Robb* Centre for Water Soluble Polymers, North East Wales Institute, Mold Road, Wrexham LL11 2AW, U.K. Received January 21, 1999. In Final Form: August 6, 1999 This paper deals with the rheology of concentrated (12-14% w/v) dispersions of copper phthalocyanine pigments in organic solvents. Dilute dispersions (0.3% w/v) of one of these pigments (Z-CuPc) had previously been shown to be stabilized by charge but flocculated by added polymer. The rheology of the concentrated dispersions is determined principally by the state of flocculation of the particles. For concentrated systems of Z-CuPc (14% w/v) containing polymer and organic solvent, G′ and G′′ increased with polymer concentration, probably due to the increased attraction between the particles arising from depletion flocculation. The apparent yield stress of a dispersion of an alternative pigment (B-CuPc), which did not flocculate even in the presence of 20-50% polymer, was much lower than that of Z-CuPc. Slip at the walls of the parallel plate geometry occurred after preshearing dispersions containing higher polymer concentrations. The preshearing appeared to allow the structure of the floc to shrink, leaving a solvent-rich layer adjacent to the plate surface. Addition of organic acids or electrolytes had little effect on the rheological properties of the concentrated dispersions.
Introduction The rheology of concentrated dispersions is of considerable scientific and industrial interest and has been reviewed in a number1-3 of papers. As might be expected, the rheology depends on the interaction between the particles and, in particular, whether they are stable or flocculated. For charge-stabilized particles, the viscosity and shear moduli have been shown to be determined by the interparticle repulsive potential4,5 and hence depend on the particle size, concentration, and ionic strength.6,7 The smaller the particles, the smaller the interparticle separation and hence the greater the interparticle repulsion and shear moduli. For sterically stabilized dispersions the rheology has been shown to depend on the interparticle interactions,8-10 which in turn depend on the extent of overlap of the extending stabilizing layers11-13 and the presence of the particles themselves.14,15 Below a critical volume fraction, the rheological behavior is Newtonian, but above this * To whom correspondence should be addressed. (1) Goodwin, J. W.; Hughes, R. W. Adv. Colloid Interface Sci. 1992, 42, 303. (2) Tadros, Th. F. Adv. Colloid Interface Sci. 1996, 68, 97. (3) Mewis, J.; D’Haene, P. Makromol. Chem., Macromol. Symp. 1993, 68, 213. (4) Forsyth, P. A.; Marcelja, S.; Mitchell, D. J.; Ninham, B. W. Adv. Colloid Interface Sci. 1978, 9, 37. (5) Buscall, R.; Goodwin, J. W.; Hawkins, M. W.; Ottewill, R. H. J. Chem Soc., Faraday Trans. 1 1982, 78, 2873, 2889. (6) Tadros, Th. F. Langmuir 1990, 6, 28. (7) Tadros, Th. F.; Hopkinson, A. J. Chem. Soc., Faraday Discuss. 1990, 41, 90. (8) de L. Costello, B. A.; Luckham, P. F.; Tadros, Th. F. Colloids Surf. 1988, 34, 301. (9) de L. Costello, B. A.; Luckham, P. F.; Tadros, Th. F. J. Colloid Interface Sci. 1992, 152, 237. (10) Tadros, Th. F.; Liang, W.; Costello, B.; Luckham, P. F. Colloids Surf. 1993, 79, 105. (11) Liang, W.; Tadros, Th. F.; Luckham, P. F. J. Colloid Interface Sci. 1992, 153, 131. (12) Andrews, D.; Jones, R.; Leary, B.; Boger, D. J. Colloid Interface Sci. 1992, 150, 84. (13) Kim, I. T.; Luckham, P. F. J. Colloid Interface Sci. 1991, 144, 174. (14) Gast, A. P.; Russel, W. B. J. Chem. Phys. 1986, 84, 1815. (15) Russel, W. B. J. Rheol. 1980, 24, 287.
volume fraction, it changes to elastic and solid-like with an apparent yield stress, and the elastic shear modulus increases with respect to the viscous modulus. At constant volume fraction, the viscosity and yield value increase with decreasing particle size. For systems of aggregated polystyrene particles in water, yield stresses are observed above concentrations where networks form and the shear modulus has been found16 to scale approximately with the fourth power of the particle phase volume. In contrast17 aggregated hydrophobed silica in hexadecane had a G′ power dependence on solid-phase volume of 3, though this system showed a gel/liquid transition with increasing temperature. Particles flocculated by the presence of excess unadsorbed polymer (i.e. by depletion flocculation) show18-20 an expected increase in storage and loss modulii with increasing polymer concentration. The rheology of organic pigments dispersed in organic solvents is of importance to paints and inks, though much less work has been reported for these systems than for aqueous solvents. Copper phthalocyanine (CuPc) pigments are the most ubiquitous and commercially valuable class of organic pigments.21,22 The a- and b- forms of CuPc are the only polymorphs widely used,22 though other forms, such as d-, e-, and p-, have been generally described as distorted a-forms.23,24 The rheology of dispersions of CuPc pigment in solvents containing polymers has been reported,25 showing that with increasing pigment concen(16) Buscall, R.; Mills, P. D. A.; Goodwin, J. W.; Lawson, D.W. J. Chem Soc., Faraday Trans. 1 1988, 84, 4249. (17) Chen, M.; Russel, W. B. J. Colloid Interface Sci. 1991, 141, 564. (18) Liang, W.; Tadros, Th. F.; Luckham, P. F. J. Colloid Interface Sci. 1993, 160, 183. (19) Patel, P. D.; Russel, W. B. J. Colloid Interface Sci. 1989, 131, 201. (20) Nashima, T.; Furusawa, F. Colloids Surf. 1991, 55, 149. (21) McKay, R. B. Pigment Dispersion in Apolar Media. In Interfacial Phenomena in Apolar Media; Dekker, H., Eicke, Parfitt, G. D., Eds.; 1987; New York, Chapter 9, p 361. (22) Fryer, J. R.; McKay, R. B.; Mather, R. R.; Sing, K. S. W. J. Chem. Technol. Biotechnol. 1981, 31, 371. (23) Honigmann, B. J. Paint Technol. 1966, 38, 77. (24) Herbst, W.; Merkle, K. Deutsche Farben-Z. 1970, 24, 365. (25) Mani, S.; Grover, S.; Bike, S. G. Rheol. Acta 1996, 35,329.
10.1021/la990055i CCC: $18.00 © 1999 American Chemical Society Published on Web 10/07/1999
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tration both storage and loss modulii increased, whereas tan δ decreased. In a previous paper,26 the colloid stability of dilute dispersions (0.3 w/v %) of two CuPc pigments (where the interparticle separation was several microns) in organic solvents was reported, showing electrostatic repulsion to be the main factor contributing to stability, even in low dielectric solvents. Addition of a hexahydropyrimidine polymer (molecular mass 500-35000 at concentrations of 2-50%) caused flocculation of one of the chargestabilized pigments. For such a dilute colloidal system the molecular mass of the added polymer is too small for it to flocculate by a bridging mechanism, indicating that flocculation (over such a wide polymer concentration) was by depletion. In more concentrated systems of these pigments, (14% w/v) where the average interparticle separation was only a fraction of a micron, the presence of polymer caused the pigment to form large flocs. In this paper, the effect of pigment phase volume and polymer concentration on the rheology of these concentrated dispersions in organic solvents containing various additives has been studied. In addition the occurrence of slip at the smooth surfaces of the parallel plates containing the flocculated dispersions was investigated.
Hughes and Robb Table 1. Composition of Dispersions Used in Rheological Measurements sample no.
polymer wt %
CuPc wt %
MPA wt %
additive wt %
Z 56 Z 46 Z 42 Z 39 Z* 50 B 51 Z 42/3 EA Z 42/6 BA Z 42/10 DA
56 46 42 39 50 51 42 42 42
14 14 14 14 12.5 12 14 14 14
30 40 44 47 37.5 37 40 37 33
0 0 0 0 0 0 3 (EA) 6 (BA) 10 (DA)
Experimental Section Two types of CuPc pigments were used: Zeneca Monastral Blue CSN (Z-CuPc) and BASF Heliogen Blue L7080 (B-CuPc). Z-CuPc was an R-stabilized crystal form27 with a BET surface area of 58 m2 g-1 and a density of 1.6 g cm-3. It was not known if any surface treatment had been added to the pigment. B-CuPc was a β-crystal form27 with a BET surface area of 63 m2 g-1 and a density of 1.6 g cm-3. No additives had been added to the pigment. They were heated at 120 °C to constant weight and stored in a desiccator. The solvent, 1-methoxy propyl-2-acetate (MPA), was manufactured by BASF and distilled before use. The polymer used, Laropal A81, was a polydisperse hexahydropyrimidine with a molecular mass distribution27 from 500 to 35 000 and was used as received. It was prepared by condensing 1 mol of urea with 3 mol of isobutyraldehyde to give an aldehyde of the following structure,
which was further condensed to give the polymer. Use of polymer in MPA solvent was necessary to raise the viscosity of MPA sufficiently to allow dispersion of the original pigment by milling. Three organic acids were used: ethanoic acid (EA) 99%; benzoic acid (BA) 99.5%; and 1-dodecanoic acid (DA) 99%. The acids were purchased from Sigma-Aldrich and used as received. Tetrabutylammonium bromide (TBAB), an electrolyte soluble in organic solvents, was obtained from Lancaster Synthesis, 98% pure. Dispersions of the pigments in varying concentrations of polymer and solvent were prepared by milling in a Netsch Zeta or Eiger 250 mill, and the particle size distribution was measured on a Malvern 3000 zetasizer. Acids or electrolyte were added to the dispersions by substituting some of the MPA solvent and keeping the pigment concentration constant. Rheological measurements were made on a TA Instruments Carri-Med CSL2 500 controlled stress rheometer usually using a parallel plate at 500 µm separation. Samples were loaded onto the parallel plate apparatus with a minimum of shearing and left for 10 min, and rheological measurements were taken over the following 10-20 min. No (26) Hughes, D. K.; Robb, I. D.; Dowding, P. Langmuir in press. (27) Data from manufacturer.
Figure 1. Particle size distribution of Z-CuPc after milling and additional sonication: (9) 9 passes through Zeta mill, unsonicated; (0) 9 passes through Zeta mill followed by sonication. time dependence of the rheological parameters was noticed under these conditions, indicating that slip at the plate walls was not a problem. In addition, measurements25 on similar systems using a parallel plate geometry with different gaps showed no evidence of slip. However, if, after loading, the samples were subject to steady preshear (at 100 Pa stress for 1 min), G′ and G′′ were found to be time dependent. This time dependence found with the parallel plate was compared with a splined cup and bob geometry. The compositions of the samples made are shown in Table 1. Samples having concentrations of polymer lower than 39% were not prepared satisfactorily, as the viscosity of the dispersing medium was too low to allow full dispersion of the pigment in the mills.
Results Light and electron microcopy showed the particle size distribution of the pigments to be in three broad ranges: ∼0.02 µm, which corresponded to the primary particles having a surface area of 50-60 m2/g; these particles were usually agglomerated into particles between 0.5 and 2.0 µm. The size distributions of the Z-CuPc pigment particles after grinding and sonication measured by PCS are shown in Figure 1. These micron-sized particles were capable of forming large flocs up to about 50 µm, depending on solution conditions. Oscillatory measurements were performed on the samples Z 46, Z 42, and Z 39 to establish the linear viscoelastic region and the data are shown in Figure 2. The measurements were made 10 min after loading the rheometer, and the samples were not subject to any preshearing before measuring. The data show that the linear region extends to about 0.02 m rads (1.5 Pa) for the Z 39 system but to about 0.05 m rads (3.75 Pa) for the Z 42 and Z 46 systems. Deviation from linearity occurred
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Figure 4. Shear stress as a function of shear rate for Z-CuPc dispersions (constant pigment phase volume) and coresponding polymer/solvent systems: (2) Z 39; (4) Z 39 (no CuPc); (!) Z 42; (0) Z 42 (no CuPc); (b) Z 46; (O) Z 46 (no CuPc).
Figure 2. G′ and G′′ versus oscillation displacement for Z 46, Z 42, and Z 39 dispersions: (2) G′ for Z 39; (4) G′′ for Z 39; (!) G′ for Z 42; (0) G′′ for Z 42; (b) G′ for Z 46; (O) G′′ for Z 46.
Figure 5. G′ and G′′ for Z 46 and Z 39 samples (measured with parallel plates) after preshearing: (2) G′ for Z 39; (4) G′′ for Z 39; (b) G′ for Z 46; (O) G′′ for Z 46.
Figure 3. Shear stress as a function of shear rate for Z-CuPc and B CuPC dispersions: (O) B 51; (2) Z 56; (!) Z 50.
at higher torque or displacement with increasing polymer concentration, indicating that the floc strength increased with increasing polymer concentration. For the systems studied here, measurements below 10 µNm were considered to be within the linear region. The apparent yield stresses (Figure 3) of the three different dispersions Z 56, Z* 50, and B 51 were obtained by applying an increasing shear stress (0-50 Pa over 2.5 min) and measuring the strain rate. No preshearing of the sample was applied. The apparent yield stresses, 13.6, 10, and 0.25 Pa, respectively, were taken as the stresses at the lowest reliable shear rate, in these experiments 0.015 s-1. The results clearly depend on the experimental conditions but do give valid relative values of the yield stress. The data show that the apparent yield stress increases with increasing pigment phase volume and nature of the pigment with B-CuPc having a much lower yield stress than Z-CuPc. The effect of polymer concentration on the rheology of the Z-CuPc dispersions is shown in Figure 4. The viscosity of the dispersion decreases with decreasing polymer concentration, especially at low shear rates, that is, below 0.1 s-1. Lower polymer concentrations would reduce the
viscosity of the media in which the pigments were dispersed, and the shear stress/shear rates of these media (without pigment particles) are also shown in Figure 4. While the decreasing polymer concentration does reduce the viscosity of the background media, at low shear rates (0.1 s-1) the polymers contribute
