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Effects of Shear Stress and Pressure Drop Rate on Microcellular Foaming Process |
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Lee Chen, Himanshu Sheth, and Xiang Wang Abstract The effect of shear stress and pressure drop rate on cell nucleation density during the foaming process has been studied using a Foaming Process Simulator. The major part of this simulator is a test chamber that can be temperature controlled and pressurized with gas. Shear stress can be applied to the polymer melt through a rotor. With this simulator, the effects of shear stress and pressure drop rate can be investigated separately. Three materials have been tested: PS, unfilled HDPE, and HDPE filled with 5% talc. It was found that both shear stress and pressure drop rate have significant effect on cell nucleation density, although shear stress is generally dominant. The effect of shear stress or pressure drop rate becomes more critical when the saturation pressure is low. The transition pressure is about the 1,500 psi for PS or unfilled HDPE. The transition pressure reduces to about 500 psi with the filled HDPE, since the filler acts as a nucleation agent and reduces the requirement on saturation pressure for sufficient nucleation. Introduction Cell structure of microcellular foams is strongly affected by how fast the pressure is released as well as the shear stress in the die flow. Figure 1 shows the cell structure of rigid PVC foams made at different extrusion throughputs.   It is clear that the cell density is much higher for the sample made with a higher throughput. It is also seen that the cell density is much higher at the surface area where shear stress is high. It is important to clarify the major factor behind the difference in cell density. The effects of shear stress and pressure drop rate are the main hypothesis. However, the effects of these on the microcellular foaming process have not been well studied. Although much research work has been contributed to the microcellular foaming process, most of the work was done with a solid-state batch process (1) with which it is difficult to investigate any effects of shear stress or pressure drop rate. More recent research work did use continuous processes and the effects of shear stress and pressure drop rate have been noted. Park, Baldwin and Suh (2,3) studied the effect of pressure drop rate on cell nucleation in a continuous process of microcellular foam. With different nozzle sizes, a significant difference in cell density was observed in a foam extrusion process. Biesenberger and Lee (4) studied foam-enhanced devolatilization (DV) and indicated that when solvent level is low it is necessary to introduce deformation to achieve appreciable foam DV. A cavity model (5), which was developed previously for nucleation in blood vessels during diving, was introduced to explain their observation. A recent study (6) by M. Favelukis et al reported similar results for bubble nucleation and growth in shear flow. Lee (7) investigated the effects of shear stress and pressure on nucleation in LDPE foam extrusion with HCFC as a blowing agent. He found that cell nucleation increases with shear rates, which was calculated from throughput and die-end opening. Since pressure can also be a factor for cell nucleation, he later made more detailed investigations with Kim (8) on both shear stress and pressure on the nucleation with the same material and blowing agent. However, as indicated in some of their papers, pressure gradient is hardly separable from shear in an actual die flow. In addition, the thermal effects and possible two-phase flow are coupled in the results from extrusion experiment. Investigations that can separate these factors are highly desirable. In this work, the effects of shear stress and pressure drop rate on nucleation in polymer foams have been studied with the foaming process simulator. The simulator has been presented previously (9). The major part of the simulator is a test chamber that holds the polymer samples. The chamber can be pressurized with gas up to 5,000 psi and heated up to 450°F. The gas pressure is monitored by a pressure transducer and recorded by a data acquisition. The gas pressure is released through an air-controlled valve or regular valve. The shear stress can be applied to the polymer melt through a rotor. Compared to the studies with a traditional batch process, the simulator operates under similar conditions in continuous processes like extrusion or injection molding. Both gas absorption and foaming occur in the molten state. The temperature and pressure drop during foaming can be controlled in the similar range as continuous processes. Proper shear rate can be applied to the polymer melt. Compared to the studies with a commercial extruder or an injection molding machine, the effects of shear stress and pressure drop rate can be investigated separately. Three materials were investigated: polystyrene (PS), unfilled high density polyethylene (HDPE) and HDPE filled with 5% of talc. It was found that both shear stress and pressure drop rate have significant effects on cell nucleation density, although shear stress is generally dominant. The effect of shear stress or pressure drop rate becomes more critical when the saturation pressure is low. In other words, the effect of pressure drop rate or shear stress becomes more critical when the driving force is insufficient for nucleation. The transition pressure is about 1,500 psi for PS or unfilled HDPE. The transition pressure reduces to about 500 psi with the filled HDPE, since the filler acts as a nucleation agent and reduces the requirement of saturation pressure for sufficient nucleation. Experiment Foaming Process Simulator
A sample holder was used to hold samples when several were to be foamed at the same time. It is made of brass sheets and contains several layers. Materials Three polymers were investigated in this work, PS (Fina 585), unfilled HDPE (Aquistar LP5403) and the same HDPE filled with 5% (by weight) Talc LG445, with a nominal size of about 5 microns. The polymer samples were extruded with a slit die (1.75"wide and 0.030" gap), obtaining polymer sheets about 0.050" thick. The filled HDPE samples were made by adding filler concentrate. CO2 was used as the blowing agent. Test Without Shearing The solid cover was used. Samples were cut into pieces and put into a sample holder. Each layer held one sample. The test chamber was heated to the test temperature, normally 265°F. A relatively low melt temperature was used to avoid cell coalescence during the time the test chamber was opened to get the foam samples. Once thermal equilibrium was established, CO2 was injected to pressurize the test chamber to the designed saturation pressures. A data logger was used to record the chamber pressure. The pressure curve became flat when the polymer samples were saturated. The gas pressure was then released at the desired pressure drop rate. The chamber was opened quickly to get the foam samples. The pressure release was controlled to get different drop rates. The high pressure drop rate was achieved using an air-controlled Hipco 3/8" valve. With a 1/4" valve the medium pressure drop rate was obtained. The low pressure drop rate was controlled manually with a regular valve. The initial pressure drop rates at a pressure of 21.1 MPa (3,000 psi) are about 0.15, 0.02, and 0.0015 GPa/s, respectively. Actual pressure drop curves are shown in Figure 3, marked as high dp/dt, medium dp/dt and low dp/dt respectively, noticing that the initial pressure drop is always higher and eventually reduced.  During extrusion process, the head pressure is generally higher than the saturation pressure for the gas contained in the polymer. A series of tests were performed to simulate this process. When polymer was completely saturated at a certain gas pressure, the chamber pressure was quickly increased to 4,000 psi for two seconds then immediately released at a medium pressure drop rate. The test chamber was then quickly opened to get the foam samples. Test With the Shear The cover with the rotor was used. Only one sample could be tested each time. A sample of 15~20 grams, depending on the density of the material, was put into the chamber. Gas was injected with the rotor turning at 60 rpm. When the polymer was saturated (the pressure curve became completely flat), the speed of the rotor was increased to about 240 rpm (giving a shear rate of about 200 1/s) for about 20 seconds, then stopped (the rotor stopped instantly because of the break system). The gas was released immediately after stopping the rotor, at the medium pressure drop rate (about 0.02 GPa/s). Due to elasticity, the shear stress did not release immediately after the rotor stopped. Therefore, the effect of stress on foaming could be investigated. The test chamber was opened quickly to get the foam samples. Results and Discussion The densities of the collected samples were measured using a density scale (Mettler Toledo, AG 105). The cell structure was analyzed with a scanning electronic microscope (SEM). The cell nucleation density (number of cells per cubic centimeter of solid material) was then calculated. Here we assumed no cell coalescence or collapse because the melt temperature was relatively low and there was no shear deformation after the foaming. Comparisons of cell nucleation density are shown in Figure 4 to 7, with different pressure drop rates as well as with shear stress. In these figures, the x-axis is the saturation pressure at which the polymer samples get saturated with gas. Higher saturation pressure gives higher gas level. For unfilled HDPE, about 5% CO2 is absorbed at a saturation pressure of 3,000 psi (9). Figure 4 shows the results with unfilled HDPE. At a low pressure drop rate, a cell density of 107/cm3 (assume this is the definition of microcellular foam and corresponding cell size is about 55 microns at 50% density reduction) is achieved only at a saturation pressure of 3,000 psi. The same cell density of 107/cm3 can be achieved at a saturation pressure of 2,000 psi with the medium or high pressure drop rate, and at a saturation pressure of 1,500 psi with the shear stress. Except the samples made with shear stress, no proper foaming was observed at a saturation pressure of 1,000 psi or lower. It is also noticed that the effect of pressure drop rate or shear stress becomes more significant when the saturation pressure is low. In other words, the shear stress or pressure drop rate becomes more critical when the driving force is insufficient for cell nucleation.   Very similar results were obtained with PS, as shown in Figure 5, except that the difference between low and medium pressure drop rates seems to be much more significant. It is also noticed that the cell density is generally much higher with PS at high saturation pressures, compared with unfilled HDPE. Comparisons of results are shown in Figure 6 for HDPE filled with 5% talc. The results seem to be quite different with filled polymer. The effects of pressure drop rate and shear stress are similar over a wide range of saturation pressure. However, almost identical results can be observed if we focus on the results at very low saturation pressures (500 psi or lower). Because the filler acts as a nucleation agent, the cell nucleation density remains relatively high until the saturation pressure goes down to 500 psi. Again, we can see that the shear stress has the highest impact on nucleation density. With the shear, uniform foam was obtained when the saturation pressure went to as low as 200 psi.   Figure 7 shows a comparison of the tests when the foam samples were suddenly pressurized to 4,000 psi immediately followed by pressure release at the medium pressure drop rate. The high pressure does help to gain cell nucleation density, similar to higher dp/dt. Conclusions/Recommendations The foaming process simulator has been successfully used for foaming process study. The effects of shear stress and pressure drop rate on cell nucleation density have been investigated with three materials: PS, unfilled HDPE, and HDPE filled with 5% talc. The following conclusions have been reached. Both shear stress and pressure drop rate have significant effects on cell nucleation density during the foaming process. The effects of shear stress or pressure drop rate become more critical when the saturation pressure, or the amount of gas in the polymer, is low. In other words, the shear stress or pressure drop rate becomes more critical when the driving force is insufficient for cell nucleation. The transition point when these effects become critical is about 1,500 psi for unfilled HDPE or PS. The transition pressure reduces to about 500 psi with filled HDPE, since the filler acts as a nucleation agent and reduces the requirement on saturation pressure for sufficient nucleation. The shear stress has generally a much higher impact on cell nucleation over pressure drop rate. The amount of gas required for sufficient nucleation is significantly reduced by introducing shear stress. References
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The foaming process simulator has been presented previously (9) and is briefly explained here. The major part of the simulator is the test chamber (Figure 2), which holds the polymer samples. There are two types of test chambers. One is used for the foaming test without shear (Figure 2b). The other has a rotor to apply shear stress on the polymer samples (Figure 2a). The rotor is driven by a variable motor. The motor has a break system so that it can be stopped instantly. The test chamber can be pressurized up to 5,000 psi and heated up to 450°F. The gas pressure in the test chamber is monitored with a pressure transducer and recorded by a data logger.