A Microcellular Foaming Simulation System with a High Pressure-Drop Rate Qingping Guo, Jin Wang, and Chul B. Park* Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, UniVersity of Toronto, Toronto, M5S 3G8, Ontario, Canada Masahiro Ohshima Department of Chemical Engineering, Kyoto UniVersity, Kyoto 606-8501, Japan In this paper, we undertook an experimental and theoretical analysis of the pressure-drop behaviors of a batch foaming system with a visualization window that was designed for microcellular foaming simulation. A polystyrene (PS)-CO 2 system was used in the experiment and analysis. The maximum pressure-drop rate achievable was 2.5 GPa/s from the designed system. Some experimental simulation results at high pressure- drop rates and at low pressure-drop rates are also discussed. We observed that the application of a higher pressure-drop rate results in a higher cell density (and, thereby, a smaller cell size) for plastic foams. This confirms that the pressure-drop rate is one of the most important parameters to control the cell density of plastic foams. In addition, the results show that the content of the blowing agent (CO 2 ) dissolved into a given polymer has a significant effect on bubble nucleation and growth. Introduction In the past decade, efforts have been made to reduce cell size and enhance cell size uniformity in microcellular plastic foams, because plastic foams with a finer cell size and a more uniform distribution exhibit better mechanical and thermal properties. 1-6 Park and co-workers 7,8 have shown that the pressure-drop rate has a strong role in determining the cell density of foams in a continuous extrusion processes. In reality, the pressure drop is not instantaneous; it occurs over a finite time period. During the course of the pressure drop, the gas in the solution will either diffuse into the nucleated cells or nucleate new cells to reduce the free energy of the system. The higher the pressure drop, the more gas that is used for cell nucleation instead of for cell growth. Consequently, the cell density (i.e., the number of cells per unit volume of unfoamed plastic) is expected to be large. Therefore, a high pressure-drop rate is a critical factor in microcellular plastic foam processing. In addition, Park et al. 9 have demonstrated that the content of the blowing agent (gas) dissolved into a given polymer has a significant effect on bubble nucleation and growth. Generally, the more gas that is dissolved into a polymer, the smaller the bubble size and the larger the bubble number. The amount of gas in a polymer is decided by the solubility of the gas in that polymer and the given saturation conditions (i.e., the initial temperature and pressure). Thus, the saturation conditions are yet other critical factors to be considered in the microcellular plastic foaming process. A fundamental understanding of how the properties of materials and the processing parameters affect the foaming processes is required to manufacture high-quality plastic foams. Experimental and computer simulation studies based on certain fundamental properties (i.e., the thermophysical and rheological properties of polymer/gas mixtures) are important to elucidate the cell nucleation and growth behaviors in foam processing. Hitherto, much research has been conducted on foaming processes, from both experimental and theoretical viewpoints. Most experimental studies have focused on the use of the actual processing equipment while varying the processing conditions, material compositions, and system configurations. Efforts have been made to investigate the effects of these parameters on cell nucleation and growth behaviors. However, because of the numerous parameters that have a role in the actual foaming procedure, very limited success has been achieved in identifying the fundamental mechanisms of cell nucleation and expansion using the processing equipment. 7,10-16 To better understand the nature of the various foaming mechanisms, some researchers have mounted a visual window in the mold and/or die. 12-16 For example, Villamizar et al. 12 performed visual observation experiments on the injection foam molding of a mixture of polyethylene and a chemical blowing agent, using a rectangular mold cavity with glass windows on both sides. Han and co- workers 13,14 performed a visual observation of bubble nucleation in shear flow of a mixture of polystyrene (PS) and trichloro- fluoromethane, using a light scattering technique; they reported several types of nucleation mechanisms. Xanthos et al. 15 recently developed an in-line optical method that is used to generate the solubility data of inert gases in PS and poly(ethylene terephthalate) (PET) in single- or twin-screw extruders with gas injection capabilities. Shimoda et al. 16 conducted visual observa- tion experiments of polypropylene mixed with isobutene in a shear flow. All of these studies focused on visualization during the actual processing, and valuable information about bubble formation was obtained. However, because of the dynamic nature of the flow during actual processing, achieving control of the parameters in the visualization window was not easy and the information derived from the visualization window was limited. However, some researchers have focused on experimental simulations under static conditions; these situations allow for better control of the processing parameters when studying cell nucleation and growth behaviors. With the help of a high-speed camera, a microscope, and image processing, Ohyabu et al. 17 designed a high-pressure visualization device to observe the early stages of bubble nucleation and growth behaviors of molten polypropylene when it is dissolved with supercritical carbon dioxide. Oshima and co-workers 18-20 also designed and * To whom correspondence should be addressed. Tel.: +01-416- 978-3053. Fax: +01-416-978-3053. E-mail address: park@ mie.utoronto.ca. 6153 Ind. Eng. Chem. Res. 2006, 45, 6153-6161 10.1021/ie060105w CCC: $33.50 © 2006 American Chemical Society Published on Web 08/08/2006