Challenge to Extrusion of Low-Density Microcellular Polycarbonate

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Ind. Eng. Chem. Res. 2005, 44, 92-99

Challenge to Extrusion of Low-Density Microcellular Polycarbonate Foams Using Supercritical Carbon Dioxide John W. S. Lee, Kihyun Wang, and Chul B. Park* Department of Mechanical and Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, Ontario, Canada M5S 3G8

This research investigated the expansion behavior of extruded polycarbonate (PC) foams blown with supercritical (sc) CO2 to achieve low-density microcellular foams. The expansion behavior of PC foam was interpreted by the amount of gas retained in the cell structure in consideration of cell opening, cell-to-cell diffusion, and melt stiffening. The expansion ratio curve plotted against the die temperature showed a typical mountain shape, confirming our previous results. In addition, because the expansion behaviors and foam properties are strongly dependent on the cell density, the cell nucleation behaviors were also thoroughly investigated. Three filamentary dies were designed to investigate the effects of the die geometry. The pressure-drop rate had some effect on the cell density of PC foams as expected, i.e., a higher pressure-drop rate resulted in a higher cell density of PC foams. A high pressure-drop rate was also favorable for a high expansion ratio. By controlling all of these parameters, an expansion ratio of over 14 with a cell density of over 1010 cells/cm3 could be achieved. Introduction Microcellular foams are characterized by a cell size less than 10 µm and a cell density greater than 109 cells/ cm3. Because of this unique structure, microcellular plastics offer superior mechanical properties, such as impact strength, toughness, and fatigue life when compared with unfoamed polymers.1-7 Microcellular foams also show a good light-reflecting ability.8 Since microcellular foams were first developed in the early 1980s,9 microcellular foams have been produced by various processes such as batch processing,9,10 extrusion,11-15 injection molding,16 and so on. The extrusion process especially caught people’s attention because of its high productivity compared to batch processes. Many factors affect the cell nucleation and expansion behaviors of microcellular foams. First, different types of blowing agents behave differently in the polymer matrix because of their different solubilities and diffusivities. Inert gases such as CO2 have high diffusivities and low solubilities, whereas long-chain molecules such as butane have low diffusivities and high solubilities.11 High solubility and low diffusivity are favorable for expansion by increasing the amount of gas dissolved in the polymer matrix and retarding gas loss that occurs through diffusion out of the foam.12 The die pressure is another important factor that affects microcellular foams. The die pressure must be higher than the solubility pressure, i.e., the pressure required for the injected blowing agent to be dissolved completely in the polymer matrix.13 An insufficient pressure cannot form a one-phase polymer-gas solution, and this results in undesirable large voids in the foams due to undissolved gas pockets. The pressure-drop rate across the nucleation device also plays an important role in cell nucleation. Previously, Park et al.14 demonstrated that higher pressuredrop rates result in higher cell densities. Also, Xu et al. found that high pressure-drop rates are favorable not * To whom correspondence should be addressed. Tel.: 416978-3053. Fax: 416-978-0947. E-mail: [email protected].

only for high cell densities15 but also for a high expansion ratios.17 In contrast to the pressure-drop rate, the magnitude of the pressure drop has insignificant effects on the cell density as long as the pressure is higher than the solubility pressure.15 Recently, Naguib et al. investigated the mechanisms governing volume expansion of extruded polypropylene foams.18 They noted that the expansion behavior of foams was governed by different factors depending on the die temperature: loss of blowing agents at high temperatures and crystallization/stiffening of the polymer matrix at low temperatures. The expansion ratio plotted against the die temperature showed a typical “mountain” shape, and the expansion ratio increased as the content of blowing agents was increased. Another factor that affects the expansion ratio is the open-cell content inside the foam. Park et al. investigated open-cell formation in extruded LDPE/PS blends.19 Their research showed that there was an optimal temperature for maximizing the open-cell content. At low temperatures, cell walls became too stiff for cell opening, and therefore, the open-cell content increased as the temperature was increased in the low-temperature range. On the other hand, at high temperatures, the thickness of cell walls governed cell opening, and therefore, the open-cell content decreased as the temperature increased in the high-temperature range. However, the effect of the open-cell content on the expansion behavior was not discussed in the description of the fundamental volume expansion mechanisms. In 1999, Huang et al. reported open-cell foams of polycarbonate (PC) blown with CO2 in a co-rotating twin-screw extruder.20 Their studies showed that closedor open-cell structures might be formed, depending on the CO2 concentration, the melt temperature, and the die pressure. They could obtain only a 2-fold expansion because of the open cells formed in the foam. Recently, Gendron et al. published an excellent paper demonstrating the continuous extrusion of microcellular PC foam.11 They used CO2 and n-pentane as blowing agents and extensively investigated the effects of the blowing agent, processing pressure, and temperature on

10.1021/ie0400402 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/03/2004

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 93

Figure 1. Schematic of tandem extrusion system.

the cell density of the microcellular PC foam. However, only a low expansion ratio was obtained from the microcellular PC foams, and the volume expansion behavior was not studied extensively. The cell-opening behavior was not studied, either. In this context, the present paper shows the expansion behavior and addresses strategies for extruding low-density microcellular PC foams with supercritical (sc) CO2. Because PC foams are highly susceptible to cell opening20 and because the expansion behavior of PC foams is significantly influenced by cell opening via gas loss, the open-cell content at each processing condition was measured and analyzed in this study. The obtained open-cell contents were used to explain the expansion behavior of PC foams. In addition, because the cell density also affects the expansion ratio, the cell nucleation behavior of PC foams was also investigated for varying controllable parameters, such as the blowing agent content, die geometry, die temperature, and so on. Experimental Section Materials. The polymer material used in this study was polycarbonate supplied by GE Polymerland. The grade was Lexan 101-112, a general-purpose grade with an average melt flow index of 7 dg/min. It had a density of 1.2 g/cm3, and its glass transition temperature was 149.7 °C. Prior to extrusion, the PC resin was dried for 12 h at 110 °C. The blowing agent used in this study was CO2 (Matheson, 99.5% purity). Experimental Setup. Figure 1 shows a schematic of the tandem extrusion system used in this study. It was a Brabender extruder with a static mixer and gear pump attached and retrofitted for gas injection. The first extruder was used for the plasticization of the polymer resin, while the second extruder was responsible for mixing and initial cooling of the polymer melt. The gear pump controlled the polymer melt flow rate, and the heat exchanger further homogenized and cooled the melt. scCO2 was supplied by a positive displacement pump, and a filamentary die was used in this tandem extruder. Design of Dies. Figure 2 shows the schematic of a typical filamentary die. Filamentary dies are characterized by the length and radius of the nozzle. Using a power law,21,22 the pressure drop and pressure-drop rate

Figure 2. Schematic of a typical filamentary die.

in the die were derived as follows15

L 1Qn ∆p ) -2m 3n+1 3 + nπ R n Q n+1 ∆p 1 ) -2m 3 + ∆t n πR3

[(

)]

( )( )

(1) (2)

where ∆p is the pressure drop across the nozzle of die, Q is the volume flow rate of the polymer-gas solution, m and n are the power-law constants of the material, L is the length of the nozzle, and R is the radius of the nozzle. The characteristic constants of PC, m ) 9794.89 N‚s0.5693/m2 and n ) 0.5693, determined from the experimental rheological data within the shear rate ranging between 101 and 104 s-1 at 270 °C,11 were used in the calculation. As shown in Figure 3, three dies were designed so that they have the same pressure drop but different pressuredrop rates to observe the effect of the pressure-drop rate on the microcellular foams.14,15 The detailed design procedure is described in refs 14 and 15, and the procedure for determining the geometries of the three dies is as follows: First, the volumetric flow rate was set to 7.83 × 10-8 m3/s, which is a typical value for the extrusion system used in this study. Second, three levels of pressuredrop rates of 5.000, 0.451, and 0.051 GPa/s were selected. The designed dies were calibrated to ensure the same pressure drop across each die during extrusion. The pressure drops measured for each die were recorded and compared. Table 1 shows the calibrated die geometries, calibrabrated pressure drops, and theoretical pressuredrop rates.

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Figure 3. Pressure drops and pressure-drop rates of three dies. Table 1. Summary of Die Characteristics calibrated die geometry (in.)

calibrated pressure drop (psi)

die

L

D

270 °C

240 °C

theoretical pressure-drop rate (GPa/s)

1 2 3

0.025 0.100 0.220

0.024 0.040 0.064

1020 980 890

1710 1680 1640

5.000 0.451 0.051

Experimental Procedure. The PC resins were processed using the tandem extruder with injection of scCO2. Experiments were carried out using the three dies at three different CO2 contents while varying the die temperature. The die temperature was carefully decreased from 260 °C to the lowest possible temperature with a decrement of 5 °C. The pressure changes with the decrease of temperature were recorded at the steady state of each processing condition. The foam samples were randomly selected at each processing condition and were characterized three times to minimize the fluctuations of the measured data. Characterization of Foams. The foam samples were characterized by the expansion ratio, the cell population density, and the open-cell content. The foam density was determined by the water displacement method (ASTM D792-00). The expansion ratio was calculated from the ratio of the bulk density of pure PC (F0 ) 1.2 g/cm3) and the measured density of the foam sample (Ff). The cell density was calculated on the basis of micrographs obtained by scanning electron microscopy (SEM). The samples were first dipped in liquid nitrogen and then fractured to expose the cellular morphology. The fracture surfaces were then gold-coated using a sputter coater for enhancing conductivity, and the microstructures were examined using a Hitachi 510 SEM instrument. Then, the cell densities were calculated according to the equation23,24

N0 )

( ) nM2 A

3/2

Φ

(3)

where n is the number of bubbles in the micrograph, A is the area of the micrograph, M is the magnification factor of the micrograph, and Φ is the expansion ratio.

Figure 4. Effect of die temperature on the expansion ratio of PC microcellular foams using various die geometries: (a) 3, (b) 5, and (c) 7 wt % CO2.

The open-cell content was measured using a stereotype gas pycnometer from Quantachrome Corp. This gas pycnometer was utilized to determine the true volume and density of the solid, based on method ASTM D622698.19 The open-cell content was calculated using the pycnometer readings and sample geometric information. The principle of open-cell measurement can be found in ref 19. Results and Discussion Effects of Processing Conditions on the Expansion Ratio. Figure 4a-c shows the expansion ratios plotted against the die temperature for different die geometries and CO2 contents. At temperatures higher than 180 °C, the expansion ratios showed almost the


Figure 6. Comparison between expansion ratio and open-cell content.

Figure 5. Effect of die temperature on the open cell content of PC microcellular foams using various die geometries: (a) 3, (b) 5, and (c) 7 wt % CO2.

same low values below 2 regardless of the die geometry and CO2 content. However, as the temperature was lowered, the expansion ratios for 5 and 7 wt % CO2 started to increase. It was observed that the expansion ratio was a strong function of the CO2 content. In the case of 3 wt % CO2, even though the temperature was further decreased, the expansion ratio remained almost the same at 3. It seemed that the 3 wt % amount of CO2 was not enough for high expansion. Also, because of the low CO2 content, the viscosity of the polymer melt (and therefore the die pressure) was very high. Consequently, it was hard to decrease the die temperature below 165 °C because of the pressure surge. In the cases of 5 and 7 wt % CO2, the expansion ratio graphs clearly showed a mountain shape,18 confirming

that the expansion ratio was significantly affected by gas loss and melt stiffening.18 However, there was no significant difference between the 5 and 7 wt % CO2 samples. The maximum expansion ratios for both 5 and 7 wt % CO2 were obtained at 160 °C. On the other hand, because of the lower viscosity through the plasticizing effect of CO2, it was possible to decrease the die temperature further to near 150 °C unlike in the 3 wt % CO2 case. The pressure-drop rate also had an effect on the expansion ratio. The effect was negligible for the PC foams blown with 3 wt % CO2, but it became more significant when the CO2 content was increased to 5 and 7 wt %. It turned out that a high pressure-drop rate is favorable for high expansion, as shown in our previous study.17 Effects of Open-Cell Content on the Expansion Ratio. Figure 5a-c shows the open-cell contents plotted against the die temperature with various die geometries and CO2 contents. The open-cell content curves showed a similar pattern in most cases. The open-cell graph could be divided into three zones: a high-temperature zone (zone 1) in the range of 180-200 °C, an intermediate temperature zone (zone 2) in the range of 165-180 °C, and a low-temperature zone (zone 3) in the range of 145-165 °C. In the high-temperature zone (zone 1), the open-cell content was low. It seems that the rapid loss of gas through fast diffusion of CO2 at high temperature resulted in a low expansion ratio.18 As a result, a large cell-wall thickness was produced with some cell opening.12 However, the open-cell content would not be that high, about 40% as shown in Figure 5, because of the large cell-wall thickness.19 As the temperature was lowered (in zone 2), the diffusivity decreased, and thus, the escaping ratio of gas decreasd.12,18 As a consequence, more gas was retained in the foam, and the expansion ratio tended to increase.12 However, this temporary increase in the expansion ratio (in the initial hump) decreased the cellwall thickness, and therefore, the cell walls were more easily opened.19 As a result, the initially expanded foam collapsed back to have a low expansion ratio. The opencell content was very high over 80% in this temperature range of 165-180 °C, as shown in Figure 5.

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Figure 8. Effect of die temperature on the cell density of PC microcellular foams using various die geometries: (a) 3, (b) 5, and (c) 7 wt % CO2.

Figure 7. Die pressure plotted against die temperature: (a) die 1, (b) die 2, and (c) die 3.

As the temperature was further decreased (in zone 3), the melt strength increased. Consequently, the open-cell content decreased (Figure 5), and the expansion ratio increased (Figure 4). The inverse relationship between the open-cell content and the expansion ratio is clearly demonstrated in Figures 4 and 5. However, Figure 5a for 3 wt % CO2 does not show zone 3, because it was impossible to decrease the die temperature below 165 °C because of the pressure surge mentioned earlier. As the open-cell content decreases, the sharp increase in the expansion ratio indicates that the role of the open-cell

content in determining the final expansion ratio of PC foam is significant. As the temperature was further decreased, the opencell content remained constant at low temperature, but the expansion ratio started to decrease because of the stiffening of the melt.16 These results conformed to our previous observations.12,17,18,25,26 Figure 6 summarizes the expansion behavior of PC foams as a function of temperature. In zone 1, significant gas loss due to the high diffusivity of gas governed the expansion ratio. In zone 2, significant gas loss due to the high open-cell content governed the expansion ratio. In zone 3, the decreased amount of lost gas due to the lowered open-cell content increased the expansion ratio.


Figure 9. Cell density change with respect to die temperature.

However, the high stiffness of the PC matrix governed the expansion ratio when the temperature was further decreased. Effects of Processing Conditions on the Cell Density. Park and Suh13 and Xu et al.15 demonstrated that the processing pressure must be higher than the solubility pressure to dissolve all of the injected gas into the polymer melt. Figure 7a-c shows the solubility pressures, which were estimated using the solubility data10 and Henry’s law,27 for all of the injected gas contents and for the actual observed die pressures for each die. Regardless of the die, the threshold temperature, the temperature corresponding to the point at which the die pressure curve crosses the solubility pressure curve, for 3, 5, and 7 wt % CO2 was observed to be 225, 210, and 200 °C, respectively. This indicates that, if the temperature is higher than these threshold temperatures for each gas concentration, the gas cannot dissolve into the polymer melt. As a consequence, the cell density will decrease dramatically. Most previously conducted microcellular experiments12,15,17,24 showed that the cell density was independent of the temperature, as long as the die pressure was higher than the solubility pressure. However, in the case of PC foams, a unique behavior of the cell density versus the temperature was observed. Figure 8a-c shows that the cell density tended to increase as the die temperature decreased. Even when the die pressure was higher than the solubility pressure (when the die temperature was decreased below the threshold temperature), because of the active cell coalescence, the

Figure 10. SEM images of cell morphologies when die 1 were used.

cell density was decreased significantly during the cell growth stage. However, as the melt strength increased with a decrease in the temperature, cell coalescence was decreased, and consequently, most nucleated cells survived in the cell growth stage. It should be noted that the cell density tended to converge to a certain value as the temperature decreased (Figure 8). It is also worth noting that the range of temperature where active deterioration of cell density had occurred shifted to the left with increasing CO2 content because of the higher plasticizing effect of CO2. This shift was observed more clearly when comparing 3 and 5 wt % CO2. Figure 9 presents a summary of the cell density change with respect to the die temperature. The high cell density obtained at low temperature seemed to be the nucleus density. The lowered cell density at higher temperatures up to the solubility temperature seemed to be due to the active coalescence of cells. Finally, when the temperature was higher than the solubility temperature, the cell density was further decreased because of incomplete dissolution of the injected gas as well as cell coalescence. The cell density slightly increased as the gas content increased, as observed in previous studies.15,23 The die geometry also exhibited some effects on the cell density, but compared to high-impact polystyrene (HIPS)14 and polystyrene (PS),15 the sensitivity of the cell density to the die geometry was not high. This indicates that the pressure-drop rate required for microcellular foaming of PC is not high and, therefore, the die design for PC foam would be easier compared to that for HIPS and PS. However, it appeared that a die with very low pressure-drop rate showed low expansion ratios because of premature cell growth. Therefore, the die design needs to be carefully conducted for the promotion of large expansion. Effects of Die Pressure and Die Temperature on Cell Morphology. Figures 10-12 show the cell morphologies of the foams obtained from dies 1-3, respectively, for varying CO2 contents. These SEM images clearly show how the cell morphologies change from a nonuniform large-celled structure to a uniform microcellular one. It is interesting to see the changes across the solubility temperatures for each CO2 content (i.e., 225, 210, and 195 °C for 3, 5, and 7 wt %, respectively, as shown in Figure 7). It is obvious that the cell

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morphologies were very nonuniform above these temperatures, most likely because of the large undissolved gas pockets as discussed above. However, nonuniformity with large pockets was still observed below the solubility temperatures (e.g., the cell morphologies observed at 190 and 200 °C for 5 wt % CO2 in Figures 10-12). This supports the previous discussion of active cell coalescence below the solubility temperature. However, as the temperature decreased further, the large cells were removed, and uniform cell structures were obtained with a dramatic increase in the cell density, as shown in Figure 8 (for example, see the data for 170 °C, 5 wt % in Figures 8 and 10). Even after a uniform structure was obtained, a high open-cell content was observed (Figure 5). This means that there is a temperature window in which cell opening in the cell walls is actively taking place but the cells maintain their cellular structure without being coalescenced completely. Therefore, this temperature range is desirable for the production of open-celled PC foams. As the temperature was decreased further, cell opening was prevented and closed cells were observed (Figures 5 and 10). Correspondingly, the expansion ratio was increased (Figure 4) while the cell density was maintained (Figure 8).

discussed. The expansion ratio plotted against the die temperature showed a typical mountain shape. Because PC foams are highly susceptible to cell opening, the expansion behaviors of PC foams were significantly affected by the amount of gas lost through openings in the cell walls. The expansion ratio increased as the content of CO2 increased. The die pressure had to be higher than the solubility pressure to dissolve completely all of the injected gas. In addition, it was reconfirmed that a high pressure-drop rate was favorable for high expansion ratios and high cell densities. Having too low of a pressure-drop rate decreased both the expansion ratio and the cell density. By controlling all of these parameters, an expansion ratio of over 14 with a cell density of over 1010 cells/cm3 could be achieved.

Conclusion

(1) Baldwin, D. F.; Suh, N. P. Microcellular Poly(ethylene terephthalate) and Crystallizable Poly(ethylene terephthalate): Characterization of Process Variables. SPE ANTEC Technol. Pap. 1992, 38, 1503.

In this paper, strategies for achieving low-density polycarbonate microcellular foams using scCO2 were

Acknowledgment This project was financially supported by NSERC, and the Consortium for Cellular and Microcellular Plastics (CCMCP). Their financial supports are greatly appreciated. Literature Cited

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 99 (2) Collias, D. I.; Baird, D. G.; Borggreve, R. J. M. Impact Toughening of Polycarbonate by Microcellular Foaming. Polymer 1994, 25, 3978. (3) Collias, D. I.; Baird, D. G. Tensile Toughness of Microcellular Foams of Polystyrene, Styrene-Acrylonitrile Copolymer, and Polycarbonate, and the Effect of Dissolved Gas on the Tensile Toughness of the Same Polymer Matrices and Microcellular Foams. Polym. Eng. Sci. 1995, 35, 1167. (4) Seeler, K. A.; Kumar, V. Tension-Tension Fatigue of Microcellular Polycarbonate: Initial Results. J. Reinf. Plast. Compos. 1993, 12, 359. (5) Matuana, L. M.; Park, C. B.; Balantinecz, J. J. Structures and Mechanical Properties of Microcellular Foamed Polyvinyl Chloride. Cell. Polym. 1998, 17, 1. (6) Matuana, L. M.; Park, C. B.; Balantinecz, J. J. Processing and Cell Morphology Relationships for Microcellular Foamed PVC/ Wood-Fiber Composites. Polym. Eng. Sci. 1997, 37, 1137. (7) Shimbo, M.; Baldwin, D. F.; Suh, N. P. The Viscoelastic Behavior of Microcellular Plastics with Varying Cell Size. Polym. Eng. Sci. 1995, 35, 1387. (8) Kabumoto, A.; Yoshida, N.; Itoh, M.; Okada, M. Light Reflecting Plate. U.S. Patent 5,844,731, 1998. (9) Matini, J.; Waldman, F. A.; Suh, N. P. The production and analysis of microcellular thermoplastic foams. SPE ANTEC Technol. Pap. 1982, 28, 674. (10) Liang, M.; Wang, C. Production of Engineering Plastics Foams by Supercritical CO2. Ind. Eng. Chem. Res. 2000, 39, 4622. (11) Gendron, R.; Daigneault, L. E. Continuous Extrusion of Microcellular Polycarbonate. Polym. Eng. Sci. 2003, 43, 1361. (12) Park, C. B.; Behravesh, A. H.; Venter, R. D. Low-Density Microcellular Foam Processing in Extrusion Using CO2. Polym. Eng. Sci. 1998, 38, 1812. (13) Park, C. B.; Suh, N. P. Filamentary Extrusion of Microcellular Polymers Using a Rapid Decompressive Element. Polym. Eng. Sci. 1996, 36, 34. (14) Park, C. B.; Baldwin, D. F.; Suh, N. P. Effect of Pressure Drop Rate on Cell Nucleation in Continuous Processing of Microcellular Polymers. Polym. Eng. Sci. 1995, 35, 432. (15) Xu, X.; Park, C. B.; Xu, D.; Pop-Iliev, R. Effects of Die Geometry on Cell Nucleation of PS Foams Blown with CO2. Polym. Eng. Sci. 2003, 43, 1378.

(16) Xu, J. Method for manufacturing foam material including systems with pressure restriction element. U.S. Patent 6,322,347, 2001. (17) Xu, X.; Park, C. B. Effects of the Die Geometry on the Expansion Ratio of Polystyrene Foams Blown with Carbon Dioxide. SPE ANTEC Technol. Pap. 2003, 49, 1823. (18) Naguib, H. E.; Park, C. B.; Reichelt, N. Fundamental Foaming Mechanisms Governing Volume Expansion of Extruded PP Foams. J. Appl. Polym. Sci. 2004, 91, 2661. (19) Park, C. B.; Padareva, V.; Lee, P. C.; Naguib, H. E. Extruded Open-Celled LDPE and LDPE/PS Foams Using NonHomogeneous Melt Structure. J. Polym. Eng., accepted June 2004. (20) Huang, Q.; Seibig, B.; Paul, D. Polycarbonate Gollow Fiber Membranes by Melt Extrusion. J. Membr. Sci. 1999, 161, 287. (21) Ostwald, W. Ueber die Geschwindigkeitsfunktion der Viskositat disperser Systeme. I. Kolloid Z. 1925, 36, 99. (22) Bird, R. B.; Armastrong, R. C.; Hassager, O. Dynamics of Polymeric Liquids: Fluid Mechanics; Wiley: New York, 1977. (23) Park, C. B.; Cheung, L. K. A Study of Cell Nucleation in the Extrusion of Polypropylene Foams. Polym. Eng. Sci. 1997, 37, 1. (24) Park, C. B., Behravesh, A. H.; Venter, R. D. Chapter 8: A Strategy for Suppression of Cell Coalescence in the Extrusion of Microcellular HIPS Foams. In Polymeric Foams: Science and Technology; Khemani, K, Ed.; American Chemical Society: Washington, DC, 1996; p 115. (25) Naguib, H. E.; Park, C. B.; Lee, P. C. Effect of Talc Content on the Volume Expansion Ratio of Extruded PP Foams. J. Cell. Plast. 2003, 39, 499. (26) Naguib, H. E.; Park, C. B.; Panzer, U.; Reichelt, N. Strategies for Achieving Ultralow-Density Polypropylene Foams. Polym. Eng. Sci. 2002, 42, 1481. (27) Van Krevelen, D. W. Properties of Polymers; Elsevier Scientific Publishers Company: new York, 1990.

Received for review January 30, 2004 Revised manuscript received September 21, 2004 Accepted September 29, 2004 IE0400402

Challenge to Extrusion of Low-Density Microcellular Polycarbonate

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