J O U R N A L OF COLLOID SCIENCE 18, 95--97
A N O M A L O U S BEHAVIOR OF A E R O S O L P R O D U C E D BY ATOMIZATION OF M O N O D I S P E R S E P O L Y S T Y R E N E LATEX 1 G. Langer and J. M. Pierrard Armour Research Foundation, Illinois Institute of Technology, Chicago 16, Illinois Received June 18, 1962; revised September 14, 1962.
In a previous paper (1) the production of residue particles in the atomization of polystyrene latex was discussed. When water evaporated from the latex droplets, the stabilizer associated with the latex bound some of the water, thus producing a residue. Data on these residue particles were limited, and the interference from polystyrene agglomerates was not fully defined. Further data have now become available to clarify these points. An electrostatic classifier developed at Armour Research Foundation was used to analyze the aerosol produced from the atomization of a monodisperse polystyrene latex of 0.814 micron diameter particles. The photographs in Fig. 1 show the spectrum of particles found at a series of points in the classifier. At the first sampling point polystyrene agglomerates consisting of large particles were found. At the next point single polystyrene particles were found, followed by increasingly smaller residue particles at the last two points. The concentration of the residue particles exceeded that of the polystyrene particles by a factor of approximately 20. The particles were examined with an oiled 100X objective, but the resolution precluded size determination and, also, the droplets spread out. The use of an electron microscope was precluded by the heat effect of the beam on the stabilizer droplets. Therefore, the size of the droplets was inferred from calibration of the classifier with a salt aerosol. It is interesting to note the sharp cutoff between the polystyrene and the residue particles. This was also evident from visual observation. The polystyrene deposit was red in a glancing light beam, while the residue material was gray to blue. These results are in agreement with the particle counter data (1), which showed the presence of extraneous particles by light-scattering measurements of particles larger than 0.5 micron. We attempted to remove the residue material by heating the aerosol, but this also destroyed the polystyrene? The work was performed under Contract AT 11(11-1)-578 for the AEC's Fallout Studies Branch, Division of Biology and Medicine. 95
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FIG. 1. Classification of an aerosol produced by atomization of a 0.814-micron diameter polystyrene latex suspension. a. 1.0-micron polystyrene agglomerates 4.5 cm. from the inlet of the classifier. b. 0.8-micron single polystyrene particles 7.5 cm. from the inlet. c. 0.7-micron residue particles 9.5 cm. from the inlet. d. 0.6-micron residue particles 11.5 cm. from the inlet. Lasen (2) also observed these residue particles. He passed the aerosol through a bed of 1.5-mm. glass spheres to remove the smaller residue particles by diffusion. This was effective, as it reduced the total number of residue particles from 105 to 102/cc. without much loss of polystyrene. O'Konski (3) also dealt with the problem of possible interference from the stabilizer residue. He estimated that the stabilizer residue particles, assuming that all the water evaporated, should be no larger than 0.08 micron. When he counted with his light-scattering instrument, evidence of spurious particles was noted at 0.3 micron and below. This was taken as
ATOMIZATION OF MONODISPERSE POLYSTYRENE LATEX
evidence of stabilizer particles. Why larger stabilizer particles were not found as clearly shown in our photomicrographs is a matter of conjecture. Our suggestion is that this is due to bound water enlarging the pure stabilizer particles. Although our aerosol drying procedures were similar to those of O'Konski's, the design of the ARF particle counter is such that the air sheath surrounding the particles in the counting chamber may contain some moisture. If the stabilizer material is very hygroscopic, this would also explain the larger sizes found in Fig. 1, because the samples were handled in room air. Also the possibility exists that different stabilizers are used. Dow Chemical Company does not disclose their composition. I~EFERENCES 1. LANGER, G., J. Colloid Sci. 15, 357 (1960). 2. LASEN, L., Z. Physik 160, 516 (1960). 3. O'KONSKI, C. T., Anal. Chem. 27, 694 (1955).