Langmuir 1992,8, 2421-2436 2421 Concentrated Emulsions. 3. Studies on the Influence of Continuous-PhaseViscosity, Volume Fraction, Droplet Size, and Temperature on Emulsion Viscosity Ashok K. Das, Doble Mukesh, V. Swayambunathan, Dilip D. Kotkar, and Pushpito K. Ghosh'ft Alchemie Research Centre, P.O. Box 155, Thane-Belapur Road, Thane 400 601, Maharashtra, India Received September 30,1991. In Final Form: July 10, 1992 Explosive-typeemulsions with internal-phase fractions from 0.74 to 0.92 were studied to quantify the effects of average droplet size, continuous-phaseviscosity, volume fraction, and temperature on emulsion viscosity measured at low rates of shear. Plots of emulsion viscosity vs volume fraction of the dispersed phase ($1 were obtained at 80 "C for two different emulsions varying in continuous-phase composition, but having constant droplet size profiles. Emulsion viscosity increased steeply as the intemal phase increased, but varied inversely with average dropletradius. The effect of dropletsize became more prominent at higher values of 4. The viscosity of the emulsion (when corrected for the viscosity of the continuous phase) increased approximately linearly with increased internal phase when drop size distribution was held constant. A mathematical analysis is given that ascribes the increase to droplet distortion. At low shear, the studied emulsions exhibited Boltzmann temperature dependence to flow (E, = 2-6 kcal/mol) with breaks at 22 and 72 OC. The continuous phase also exhibited a break at 22 OC. Introduction The present investigation concerns the rheological iehavior of emulsion explosives for which a satisfactory quation of viscosity is not available. Such explosives are rater-in-oil (w/o) type emulsions. The aqueous oxidizer ihases contain dissolved inorganicnitrate salts maintained t a high level of supersaturation through emulsification rhich suppresses nucleation. The fuel phases are com- losed of oil, wax, and low HLB surfactants.' Glass nicroballoons and/or chemically generated voids are ncorporatedinto the emulsion matrix to create "hot spots" .wing adiabatic compression and thereby enhance det- nation sensitivity. The multiple requirements of oxygen dance, hydrophobicity, and high-energydensity require uch emulsions be of the w/o type, with 4 values ranging rom 0.88 to 0.92. Emulsion explosives and a variety of ther concentrated emulsions,dispersions, and foamshave radually gained increased attention over the last decade rith important developments in the study and applications f these non-Newtonian systemsS2 Perhaps the first experimental correlations of emulsion iscosity with volume fraction under different shear onditions of measurement were reported by Matsumoto + Present address: Research Centre, IC1 Specialties, Blackley, llancheeter M9 3DA, England. (1) (a) Bluhm, H. F. U.S. Patent 3,447,978, June 1969. (b) Bampfield, 1. A.; Cooper, J. In Encyclopedia of Emulsion Technology; Becher,P., Id.; Marcel Dekker: New York, 1988; Vol. 3, p 281. (c) Takauchi, F.; amamoto, K.; Sahai, H. J. Ind. Erplos. SOC., Jpn. 1982, 43, 285. (d) [attori,K.; Fukatau, Y.; Sahai, H. J. Ind. Ezplos. SOC., Jpn. 1982, 43, 95. (e) Villamagna,F. Determination of the Degree of Crystallization 1 Emulsion Explosivesby NMR. IC1ExplosivesGroupTechnicalCenter iternal Report; 1986; Parts I and 11. (2) (a) Princen, H. M. J. Colloid. Interface Sci. 1983, 91, 160. (b) rincen, H. M. J. Colloid. Interface Sci. 1985,105,150. (c) Princen, H. I.; Kiss, A. D. J. Colloid Interface Sci. 1986,112,427. (d) Reinett, D. ..; Kraynik, A. M. J. Colloid.Interface Sci. 1989,132,491. (e)Yoshimura, ..; Prud'homme,R. K.; Princen, H. M.; Kiss, A. D. J. Rheol. 1987, 31, 99. (0 Yoshimura, A.; Prud'homme, R. K. J. Rheol. 1988,32,53. (g) [offmann, H.; Ebert, E. Angew. Chem., Int. Ed. Engl. 1988,27,902. (h) lilliams, J. M.; Wrobleski,D. A. Langmuir 1988,4,44. (i) Williams, J. I. Langmuir 1991, 7, 1370. (j) Ishida, H.; Iwama, A. Combust. Sci. 'echnol. 1984,36,51. (k) Ruckenstein, E.; Park, J. S. J. Appl. Polym. ci. 1990, 40, 213. (1) Park, J. S.; Ruckenstein, E. J. Appl. Polym. Sci. #89,38, 453. and Sherman for microemulsions,3 and by Lissant for macro emulsion^.^ Work with concentrated macroemul- sions has been rigorously pursued only more recently. Pal and Rhodes have identified a normalized dispersed-phase volume fraction as the sole parameter responsible for the flow properties of the system inve~tigated.~ However, thew studies were confined to emulsionswith 4- N 0.80. While few detailed investigations have been reported for highly concentrated emulsions,it is qualitatively known through experiments that their rheological behavior is dependent not only on 4 but also on the viscosities of internal and external phases, the size and distribution of internal-phase droplets, interfacial tension, temperature, and the shear rate employed. Princen and Kiss have recently shown the dependence of yield stress and shear viscosityof highly concentrated o/w emulsions on some of the parameters listed abovee6 Much of the work reported by previous researchers pertains to measurements at intermediate to high rates of shear. We have studied the low or "zero" shear viscosities of emulsion explosives, because they most closely reflect the ambient viscosities of undisturbed samples. Indeed, "zero" shear viscosity is likely to influence the appearance and storage stability of emulsion explosives, the tautness of packed cartridges, and the ability of the matrix to prevent coalescence and escapeof the chemicallygenerated voids essential for detonation.' Further, because of the non-Newtonian nature of these emulsions, the study of viscosities of emulsions as a function of 4 and droplet size (r) would be meaningful only if such measurements are made within a range of shear rates for which all emulsions exhibit similar characteristics. This study delineates the effects of volume fraction, droplet size, continuous-phase viscosity, and temperature on emulsion viscosity. (3) Mataumoto, S.; Sherman, P. J . Colloid. Interface Soc. 1969, 30, (4) Lissant, K. J. Emulsions and Emulsion Technology; Marcel (5) Pal, R.; Rhodes, E. J. Colloid Interface Sci. 1986, 107, 301. (6) Princen, H. M.;Kiss, A. D. J. Colloid. Interface Sci. 1989,128,176. 525. Dekker: New York, 1974; Vol. 6, Part I. 0143-7463/92/2408-2421$03.00/0 0 1992 American Chemical Society