Ultrasound-Enhanced Spray Drying of Polymer Micro-emulsions for Nanoparticles Synthesis
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Ultrasound-Enhanced Spray Drying of Polymer Micro-emulsions for Nanoparticles Synthesis* Shirley C. Tsai Department of Chemical Engineering, California State University, Long Beach, CA C. Y. Chen, Y. L. Song and C. S. Tsai Institute for Applied Science and Engineering Research, Academia Sinica, Taipei, Taiwan H. S. Tsai and W. Y. Chiu Department of chemical Engineering, National Taiwan University, Taipei, Taiwan
Abstract - In this paper we report the experimental results that polymer micro-emulsions (up to 10 wt%) behave like Newtonian liquids in ultrasoundmodulated two-fluid (UMTF) atomization. Specifically, the emulsion drops generated are much smaller and more narrowly sized than those generated in conventional ultrasonic atomization (without air). After spray drying, the PMMA emulsion drops yield uniform nanoparticles that were originally dispersed in the micro-emulsion. Contrary to the conventional view that viscoelastic liquids are more difficult to atomize than Newtonian liquids, we found that ultrasound enhanced atomization of viscoelastic liquids such as gel-forming xanthan gum solutions. Under similar aerodynamic conditions using UMTF atomization at 54 kHz, 40-µm-diameter uniform drops were obtained for Xan solutions, but not for Newtonian aqueous glycerol of same viscosity. I. Introduction Ultrasound-modulated two-fluid (UMTF) atomization [1-3] was employed in the present study to explore potential applications in atomization of polymer micro-emulsions and gel-forming viscoelastic polymer solutions. This new atomization (spray) technique uses ultrasound to control the drop size and size distribution of two-fluid atomization (i.e. liquid atomization by high-velocity air). Compared to conventional ultrasonic atomization, UMTF atomization of Newtonian liquids such as water and aqueous glycerol can yield much smaller and more uniform drops with peak diameter (the drop diameter where the maximum occurs in a drop size distribution) determined by the third harmonic of the ultrasound
[2,3]. Whether or not polymer micro-emulsions will behave like Newtonian liquids is yet to be determined. Since ultrasound is often used to measure the elastic properties of materials, one of the objectives of the present study is to determine whether or not ultrasound will enhance atomization of aqueous solutions of gel-forming polymers such as viscoelastic xanthan gum (Xan). This study is particularly relevant because viscoelastic liquids are in general more difficult to atomize than Newtonian liquids, and elastic polymer beads are very useful for strength reinforcement in composite materials. II. Experimental As shown in Fig. 1, the UMTF atomizer consists of an annulus for airflow and a Sono-Tek ultrasonic nozzle. The cross sectional area of the annulus is adjustable in order to vary the air velocity while maintaining a constant air-to-liquid mass ratio (mA/mL). The Sono-Tek ultrasonic nozzle consists of a central tube for liquid flow, around which a pair of washer-shaped ceramic (PZT) piezoelectric transducers is sandwiched in
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