Extruded Open-Cell Foams Using Two Semicrystalline Polymers with

Until recently, commercial open-cell foams have been almost ..... U.S.. Patent 6,414,047 B1, 2002. (8) Browers, S. D.; Wiegand, D. E. Oil Sorbent Mate...
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Ind. Eng. Chem. Res. 2006, 45, 175-181

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MATERIALS AND INTERFACES Extruded Open-Cell Foams Using Two Semicrystalline Polymers with Different Crystallization Temperatures Patrick C. Lee, Jin Wang, and Chul B. Park* Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, UniVersity of Toronto, Toronto, Ontario, Canada M5S 3G8

This paper presents an extrusion-based open-cell foaming process with polypropylene (PP)/polyethylene (PE) blends. The basic strategy for achieving a high open-cell content is to induce a hard/soft melt structure using two semicrystalline polymers with distinctively different crystallization temperatures (Tc) and to foam this inhomogeneous melt structure with supercritical CO2. The effects of polymer blending, die geometry, and temperature on cell opening were investigated in this study. Two blends of high-melt-strength (HMS) PP/metallocene PE (mPE) and linear PP/low-density PE (LDPE) were used as case examples. Introduction

Background on Previous Cell-Opening Strategies

Until recently, commercial open-cell foams have been almost exclusively manufactured with polyurethane thermoset materials.1 Because the manufacturing technologies for open-cell thermoplastic foams have not been developed extensively, very few thermoplastic foams with exceptionally high open-cell contents are available.2-10 For instance, some open-cell thermoplastic foams have been created by leaching a soluble filler from a polymer matrix.2 Such foams have also been produced by interpolymer blending,3 polymer resin grafting,4,5 soft polymer blending,6,7 and punching foams with fine needles.8 In addition, scientists have tried stretching mineral-filled polymers,9 adjusting the amount of injected CO2, and changing the temperatures of the extruding head and nozzle10,11 to yield better open-cell foams. Our previous study successfully demonstrated an open-cell foaming extrusion technology that achieved a high open-cell content (up to 99%) with low-density polyethylene (LDPE)/ polystyrene (PS) blends; CO2 was used as the blowing agent, and a small amount of cross-linking agent was applied as well.12 In a subsequent study, an open-cell content of up to 100% with LDPE/PS blends could be achieved over a wide melt temperature range using both CO2 and n-butane as blowing agents.13 Although these open-cell foams yielded high open-cell contents, it was observed by scanning electron microscopy (SEM) that most cell walls contained small “pinhole” openings or pores. If pore size can be enlarged, then open-cell foams would demonstrate improved functionality (i.e., better permeability of gas or vapor, selective osmosis, and absorption and dampening of sound), and thereby, numerous industrial applications such as filters, separation membranes, sound insulators, battery electrode supports, battery separators, and tissue attachments and growth supports could be made more effectively and efficiently. In this study, an attempt has been made to produce open-cell foams with high open-cell contents and larger pore sizes.

In our previous cell-opening studies, four major strategies were applied to produce LDPE-based, open-cell foam structures.12,13 First, a structural inhomogeneity in the polymer matrix, consisting of hard and soft regions, was created by cross-linking. The basic concept of forming a hard/soft inhomogeneity in the matrix to promote cell opening in polyurethane foaming was adopted for producing LDPE-based open-cell thermoplastic materials. Upon cross-linking at a high processing temperature, the polymer melts displayed nonuniform arrays of hard and soft regions throughout the polymer matrix. When the soft regions (i.e., non-cross-linked sections) were opened in the thinning cell walls during cell growth to create interconnections between cells, the hard regions (i.e., cross-linked sections) retained the overall cellular structure instead of allowing the cells to completely coalesce with each other. Second, the cell wall thickness was decreased by increasing the expansion ratio of the foam while maintaining the soft noncross-linked sections of the cell walls. As the expansion ratio of the foam was steadily increased, the processing temperature was closely controlled in an effort to open up the soft regions. Because cell opening is more likely with thinner cell walls, a large expansion ratio is desirable. At the same time, the noncross-linked sections of the cell walls should remain soft enough for the entrapped gas to rupture them. To satisfy both requirements and thereby maximize the open-cell content, the temperature had to be closely controlled. The non-cross-linked soft sections became hard at lower temperatures and therefore difficult to open. Thus, a high temperature was preferred at the time of foaming in order to obtain a greater contrast of crosslinked (i.e., hard) and non-cross-linked (i.e., soft) sections in the polymer matrix. Nonetheless, a thinner cell thickness was preferred to increase the degree of cell opening in the cell walls. Because the cell wall thickness of PE foams generally decreases at lower temperatures because of higher expansion ratios,13 an optimum temperature should be maintained to maximize the open-cell content. Third, increasing the cell density further decreased the cell wall thickness. Cell density was increased by blending a small

* To whom correspondence should be addressed. E-mail: park@ mie.utoronto.ca. Tel.: 416-978-3053. Fax: 416-978-0947.

10.1021/ie050498j CCC: $33.50 © 2006 American Chemical Society Published on Web 11/11/2005

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amount of second-phase material (i.e., PS phase and talc) into the LDPE matrix. Blending of PS and talc into LDPE most likely increased the cell nuclei density through heterogeneous nucleation,14 which, in turn, decreased the cell wall thickness and, thus, favored cell wall opening. Finally, cell opening was promoted by softening the cell walls with n-butane as a secondary blowing agent. It is well-known that n-butane diffused into the LDPE melt can act as an effective plasticizer. Also, n-butane is a cleaner, friendlier plasticizer than other conventional plasticizers because it diffuses out of the foams over time while leaving no residue behind. All of these strategies were empirically shown to be effective means of cell opening. Through the proper combinations of these strategies, a very high open-cell content (up to 100%) was successfully achieved at optimum temperatures. Modified Cell-Opening Strategy To produce foams with high open-cell contents, the strategy of creating a structural inhomogeneity (i.e., hard/soft regions) in the melt was improved. Instead of using cross-linking, two semicrystalline polymers with different crystallization temperatures (Tc) were melt-blended in order to maximize the stiffness contrast between hard and soft regions in the polymer matrix. Because the cell-opening process occurs after a foam extrudate exits a die, the foam structure naturally experiences cooling with surrounding air. During this cooling process, the temperatures of semicrystalline polymers such as polypropylene (PP), LDPE, and metallocene PE (mPE) cross their crystallization temperatures, resulting in structural hardening or crystallization. If the melt structure has two blended polymers with different Tc values, a high-Tc polymer would crystallize first before a low-Tc polymer would. This means that, between two Tc values, the soft sections (i.e., a low-Tc polymer) would be almost liquidlike, and the hard sections (i.e., a high-Tc polymer) would be almost solidlike, creating a great stiffness contrast. If the cell-opening process occurs while the temperature of the foam structure is between two Tc values, then the chances for cell opening will be greatly improved because of the significant stiffness differences. Cell-Opening Mechanisms Two possible cell-opening mechanisms are proposed depending on the polymer blending ratios. First, when soft sections by a low-Tc polymer (e.g., mPE or LDPE) form minor and dispersed phases and hard sections by a high-Tc polymer (e.g., PP) form a major melt matrix, cell opening can be initiated and propagated through well-dispersed low-Tc polymer domains that are entrapped between growing adjacent cells, as shown in Figure 1a. Even though these soft domains can become elongated as cells grow (i.e., cell walls become thinner), cell opening is most likely to be initiated at the weakest cell wall sections because of the embedded soft polymer phases. Another cell-opening mechanism can occur when hard sections by a high-Tc polymer (e.g., PP) form minor and dispersed phases and soft sections by a low-Tc polymer (e.g., mPE or LDPE) form a major melt matrix. In this case, as shown in Figure 1b, the debonding between the soft polymer matrix and the undeformable hard spherical domains initiates cell opening during cell growth. For both cell-opening mechanisms, the processing temperature should be carefully controlled to maximize the cell opening. Experimental Section Experimental Materials. The plastic materials used in this study were commercially available LDPE (Novapol LC-0522-

Figure 1. Two possible cell-opening mechanisms. Table 1. List of Polymer Blends polymer HMS branched PP (WB 130): mPE (Affinity PL1840)

linear PP (HE351FB): LDPE (Novapol LC-0522-A)

weight fraction (%) 100:0 90:10 75:25 70:30 60:40 100:0 75:25 50:50 25:75 0:100

A, MI ) 4.5 g/10 min) supplied by NOVA Chemicals; mPE (Affinity PL1840, MI ) 1.0 g/10 min) obtained from Dow Plastics; and linear PP (HE351FB homopolymer, MI ) 12.0 g/10 min) and high-melt-strength (HMS) branched PP (WB 130, MI ) 2.4 g/10 min), both supplied by Borealis Gmbh. In this study, two types of polymer blends were used: linear PP was blended for L-PP/LDPE blends, and HMS branched PP was blended for HMS-PP/mPE blends. All of the combinations of blends investigated, including weight fractions, are summarized in Table 1. CO2, supplied by BOC Gas Co., was utilized as a blowing agent. The CO2 was a commercial grade with a minimum 99.5% purity, where the sum of N2, O2, and CH4 was less than 0.5%. Polymer Blend Preparation and Characterization. Two types of semicrystalline polymer blends (i.e., HMS-PP/mPE and L-PP/LDPE) were prepared for open-cell foaming experiments using a counter-rotating twin-screw extruder (C. W. Brabender D6/2 C) driven by a 3.7-kW (5-hp) dc motor. After these polymers had been melt-blended and pelletized, a small amount of each blend (i.e., 10 mg) was analyzed by differential scanning calorimetry (DSC, DSC 2910 from TA Instruments) at a cooling rate of 10 °C/min to measure crystallization temperatures. As shown in Figure 2, two separate crystallization peaks were clearly detected for each blending ratio of both types of polymer blends. HMS-PP/mPE blend morphologies after the melt-blending process with the counter-rotating twin-screw extruder were investigated with scanning electron microscopy (SEM). Figure 3 shows that the spherical mPE domains were uniformly dispersed in the PP matrix. As the mPE content increased from 10 to 30 wt %, both the size and the number of mPE domains

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Figure 2. DSC thermograms of polymer blends: (a) HMS-PP/mPE blends, (b) L-PP/LDPE blends.

increased accordingly; the mean size changed from 0.6 to 1.2 mm, and the number in an area of 10 mm2 increased from 15 to 28. At 40 wt % mPE content, a co-continuous structure of HMS-PP/mPE was observed. According to Paul and Barlow,15 the percolation point of blends, where the co-continuous phase is obtained, is typically obtained when the viscosity ratio becomes almost the same as the volume ratio, i.e.

φ1 η1 ) φ2 η2

(1)

where φ is the volume fraction of each phase. Because the viscosity ratio of mPE/HMS-PP was around 0.4 (shear rate ) 600 s-1, temperature ) 180 °C), the co-continuous structure would occur at 40/60 vol % of mPE/HMS-PP. In this case, the volume fraction and weight fraction of mPE/HMS-PP would be almost the same because of the similar bulk densities of the two polymers (i.e., mPE, 0.902 g/cm3; HMS-PP, 0.905 g/cm3). Extensional viscosities of HMS-PP/mPE blends were estimated using Cogswell’s method16

[

ηe ) η

]

(n + 1)∆Pe 1.89τw

(2)

where ηe is the extensional viscosity, η is the shear viscosity,

∆Pe is the excess entry pressure loss, τw is the capillary wall shear stress, and n is the power-law exponent of the shear viscosity. η, ∆Pe, τw, and n were measured in the shear rate range between 500 and 5000 s-1 at 180, 200, and 220 °C using a twin-bore Rosand Precision advanced extrusion capillary rheometer (RH-720). In addition, extensional viscosities of HMS-PP and mPE at three different strain rates (i.e., 1, 5, and 20 s-1) and temperatures (130, 150, and 170 °C for mPE and 170, 190, and 210 °C for HMS-PP) were measured using a Sentmanat Extensional Rheometer (SER) by Xpansion Instruments. Experimental Setup. Figure 4 shows a schematic of the tandem extrusion system used in this study. It consists of a 5-hp extruder driver with a speed control gearbox (Brabender, prep center), a first 0.75-in. extruder (Brabender, 05-25-000) with a mixing screw, a second 1.5-in. extruder with a built-in 15-hp variable-speed drive unit (Killion, KN-150), a positive displacement pump for injecting the blowing agent into the polymer melt, a gear pump (Zenith PEP-II 1.2 cm3/rev), a heat exchanger (for cooling the polymer melt) that contains homogenizing static mixers, a filament die with length/diameter (L/D) ratios of 7.62 mm (0.3 in.)/0.457 mm (0.018 in.) or 38.1 mm (1.5 in.)/1.02 mm (0.040 in.), and a cooling sleeve for precise control of the die temperature. The first extruder is used for plasticating the polymer resin. The second extruder provides mixing and initial cooling of the polymer melt. The gear pump controls the polymer melt flow rate, independent of temperature and pressure changes. The heat exchanger provides further cooling for the polymer melt to suppress cell coalescence. Shaping and foaming are done in the filament die. Experimental Procedure. The HMS-PP/mPE or L-PP/LDPE pellets were first fed into the barrel through the hopper and were completely melted by the screw motion. A specified amount of CO2 (i.e., 8 wt %) was then injected into the extrusion barrel by a positive displacement pump, mixed with the polymer melt stream in the barrel, and eventually dissolved in the melt. When the gas was injected into the extrusion barrel, the remaining section of the first screw and the second screw generated shear fields to completely dissolve the gas in the polymer melt via convective diffusion. The single-phase polymer/gas solution went through the gear pump and was fed into the heat exchanger, where it was cooled to the desired temperature. The cooled polymer/gas solution entered the die, and foaming occurred as the pressure decreased at the die exit. While all other materials and processing parameters, including the screw speeds (i.e., 40 rpm for the first screw and 4 rpm for the second screw), the gear pump speed (i.e., 12 rpm), the blowing agent content (i.e., 8 wt %), and the barrel temperatures (i.e., 160, 180, and 185 °C for the first screw and 180, 180, and 180 °C for the second screw) were held constant in order to obtain a constant 12 g/min flow rate of the gas/polymer mixture, the synchronized melt and die temperatures were lowered step by step, and samples were randomly collected at each set temperature only after the system had reached the equilibrium state. The foam samples were characterized using a scanning electron microscope (Hitachi 510). The foam samples were dipped in liquid nitrogen and then fractured to expose the cellular morphology before characterization of the foam structure. The open-cell contents of the foam samples were measured using a gas pycnometer (Quantachrome Co.). Results and Discussion Effect of Polymer Blending on Cell Morphologies. Because the expansion ratio and the cell density directly influence the

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Figure 3. HMS-PP/mPE blending morphologies: (a)HMS-PP/mPE ) 100/0, (b)HMS-PP/mPE ) 90/10, (c)HMS-PP/mPE ) 75/25, (d)HMS-PP/mPE ) 70/30, (e)HMS-PP/mPE ) 60/40.

Figure 4. Experimental setup of tandem extrusion system.

open-cell content by changing the cell wall thickness, the effect of polymer blending on the expansion ratio and the cell density are discussed first. Then, the effect on the open-cell content is discussed. (i) Volume Expansion Ratio and Cell-Population Density. For both HMS-PP/mPE and L-PP/LDPE blends, the effects of polymer blending on the volume expansion ratio and the cell density were similar: as the content of the soft polymer (i.e., low-Tc polymer) increased in the blends, the expansion ratio decreased noticeably because of the lower melt strength, while the cell densities increased slightly through heterogeneous nucleation14 (Figure 5). The expansion ratios of both HMS-PP/ mPE and L-PP/LDPE blends reached up to 30-fold without any soft polymer addition. However, after being blended with a soft polymer, the expansion ratios were reduced to 14 and 17 for HMS-PP/mPE ) 60/40 and L-PP/LDPE ) 50/50 blends, respectively. When the soft polymer became a major phase, almost no foam expansion was observed (i.e., for L-PP/LDPE ) 25/75 and 0/100). The cell densities of both types of blends steadily increased as the second-phase content increased. The

cell densities reached maximum values (up to 109 cells/cm3) at 60/40 and 50/50 ratios for the HMS-PP/mPE and L-PP/LDPE blends, respectively. Although the cell densities increased slightly from blending, thereby decreasing the cell wall thickness, the overall cell wall thickness increased because of the much more significant decrease in the expansion ratios. As noted in the previous section, cell opening is more challenging with thicker cell walls. Therefore, unless the strategy of using two polymers with different Tc values works over the strategy of thin cell wall thickness, the open-cell contents using polymer blending will decrease. (ii) Open-Cell Content. The open-cell contents for both HMS-PP/mPE and L-PP/LDPE blend foams were carefully examined to investigate the effect of blending two semicrystalline polymers on open-cell contents. The open-cell contents of blending experiments remained high even though the cell wall thickness was increased because of lower expansion ratios as the mPE or LDPE content increased in the matrix (Figure 6a,b). For both HMS-PP/mPE and L-PP/LDPE blends, the optimum temperature range for the maximum open-cell content

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Figure 5. Volume expansion ratios and cell densities of HMS-PP/mPE and L-PP/LDPE blends [8 wt % CO2, L/D ) 7.62 mm (0.3 in.)/0.457 mm (0.018 in.)].

Figure 7. Extensional viscosities of mPE and HMS-PP: (a) Extensional viscosities of HMS-PP/mPE blends at 180 °C, (b) maximum extensional viscosities of HMS-PP and mPE.

Figure 6. Open-cell contents of HMS-PP/mPE and L-PP/LDPE blends [8 wt % CO2, L/D ) 7.62 mm (0.3 in.)/0.457 mm (0.018 in.)].

gradually shifted to lower temperatures as the content of soft polymer increased. It is worth noting that high open-cell contents were successfully obtained over a broad temperature range for HMS-PP/mPE ) 60/40 and L-PP/LDPE ) 50/50 blend experiments as shown in Figure 6a and b, respectively. This supports the theory that the proposed strategy of embedding relatively soft polymer domains in the relatively hard polymer matrix indeed enhanced the chance of cell opening. To explain this phenomenon, extensional viscosities of HMS-PP and mPE were estimated using a Rosand capillary rheometer and the Cogswell method. Figure 7a shows that the extensional viscosity decreased as the mPE content increased, which indicates easier rupture of the cell walls containing mPE domains. As the foaming

temperature decreased, the extensional viscosity of HMS-PP increased quickly, whereas that of mPE was still low. This was because the crystallization temperature of HMS-PP was higher than that of mPE. This resulted in instability in the cell wall, which caused it to rupture. Figure 7b shows the maximum extensional viscosities of HMS-PP and mPE at three different temperatures and strain rates. The extensional viscosities of HMS-PP were 1 order of magnitude higher than those of mPE for all strain rates at 170 °C. Although the data are directly comparable at 170 °C only, it is evident that the difference in the extensional viscosities of two polymers would be much more significant at lower temperatures. This clearly supports the existence of instability in the cell wall arising from differences in the extensional viscosities of HMS-PP and mPE, which eventually led to cell wall rupture. When the extensional viscosity of the soft phase reached the maximum point during cell growth, the cell wall started to rupture. It is worth noting that the maximum extensional viscosity decreased significantly as the strain rate increased. This indicates that faster cell growth would be helpful in promoting easier cell opening in foaming, if all other parameters such as gas amount, temperature, cell wall thickness, etc., remain essentially constant. Larger pore sizes were also observed in SEM micrographs when PP was foremost in the blending matrix. This is most likely due to high expansion ratios; expansion ratios were maintained higher than 10-fold (Figure 5). Our previous studies using LDPE and PS employed expansion ratios of less than

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Figure 8. Volume expansion ratios and cell densities of HMS-PP/mPE blends (8 wt % CO2).

Figure 9. Open-cell contents of HMS-PP/mPE blends [8 wt % CO2, L/D ) 38.1 mm (1.5 in.)/1.02 mm (0.040 in.)].

5-fold.12,13 It seemed that higher extensional stress due to higher expansion ratios was applied to the cell walls, causing small pores to become larger ones. In L-PP/LDPE blend foaming experiments, the open-cell contents were observed to be high for low LDPE contents up to 50 wt % because of the high expansion ratios above 15-fold (Figure 5). However, as the LDPE content exceeded 50 wt % in the L-PP/LDPE blends, the open-cell contents decreased dramatically, as shown in Figure 6b because of the low expansion ratio (i.e., less than 3-fold, Figure 5). Effect of Die Geometry on Cell Morphologies. In this study, two filament dies with length/diameter (L/D) ratios of 7.62 mm (0.3 in.)/0.457 mm (0.018 in.) and 38.1 mm (1.5 in.)/1.02 mm (0.040 in.) were used to vary the pressure-drop rate (dP/dt). A pressure-drop rate (dP/dt) in a filamentary die can be approximated by the equation17

( )( )

1 dP ≈ -2m 3 + dt n

n

q πr3

n+1

(3)

where q is the volumetric flow rate, r is the radius, and m and n are material characteristic constants. The approximate values of dP/dt using a high-dP/dt die (i.e., L/D ) 7.62 mm/0.457 mm) and a low-dP/dt die (i.e., L/D ) 38.1 mm/1.02 mm) were 3.20 and 0.13 GPa/s, respectively (i.e., m ) 8874 N s0.34/m2, n ) 0.34, q ) 2 × 10-7 m3/s, and temperature ) 180 °C). Both expansion ratios and cell densities were higher in the experiments using a higher-dP/dt die (i.e., dP/dt ) 3.20 GPa/s) (Figure 8). Because of the higher expansion ratios and cell densities, it is evident that the cell wall thickness decreased and thereby increased the chance of cell opening. The increase in the open-cell contents using a higher-dP/dt die is clearly shown in Figures 6a and 9. Effect of Die Temperatures on Cell Morphologies. As discussed in the previous studies,12,13 it was observed that the open-cell content was a sensitive function of the melt temperature. Most open-cell content curves in Figures 6 and 9 show a mountain shape with respect to temperature, indicating that the open-cell content is governed by two mechanisms. There exists an optimum temperature for a maximum open-cell content. Below this temperature, the melt strength of the cell wall increases and becomes the dominant factor preventing cell opening. Above the optimum temperature, the cell opening is governed by the thickness of the cell walls. At high temperatures, achieving thin cell walls is challenging because of the

Figure 10. Cell densities and volume expansion ratios of HMS-PP/mPE blends [8 wt % CO2, L/D ) 7.62 mm (0.3 in.)/0.457 mm (0.018 in.)].

high rate of blowing agent loss (i.e., high gas diffusivity) through the hot skin. These results confirm those of the previous studies. As shown in Figure 10a, the cell densities of HMS-PP/mPE blends were insensitive to the die temperature. This means that the effect of cell density on the cell wall thickness due to the temperature is minimal and, thereby, has little effect on cell opening. However, the volume expansion ratios are noticeably sensitive to the temperature (Figure 10b). A large expansion ratio (or equivalently, a thin cell wall thickness) is favorable to cell opening. Thus, most trends in the open-cell contents similarly follow those of the expansion ratios. It is evident that the cell wall thickness decreases at low melt temperatures, which positively affect cell opening. However, the actual open-cell content, below a certain temperature (i.e., the optimum temperature), decreases because of the increased stiffness of the cell walls, despite the thinner cell walls (Figure 6a).

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Conclusion In this paper, the effect of inducing an inhomogeneous melt structure by blending two polymers; the effect of die geometry; and the effects of melt temperature on the open-cell content, cell density, and volume expansion ratio were investigated. The following conclusions can be drawn: 1. When low-Tc polymer domains are well-dispersed and embedded in a high-Tc polymer matrix, the chance of cell opening is increased during the cell growth process. 2. When high-Tc polymer domains are dispersed in a low-Tc polymer matrix, cell opening occurs by debonding between two phases during expansion. However, high open-cell contents were not observed because of expansion ratios that were too low. 3. Higher open-cell contents are achieved using a high-dP/dt die. This is due to the thinner cell walls from higher expansion ratios and cell densities. 4. There is an optimum processing temperature for achieving the maximum open-cell content. 5. A high open-cell content of up to 99% can be successfully achieved by optimizing the polymer blending ratios, processing temperature, and dP/dt with 8 wt % CO2. Acknowledgment The authors are grateful to Eastman Kodak and Consortium for Cellular and Microcellular Plastics (CCMCP) for the financial support of this project. Literature Cited (1) Klempner, D.; Frisch, K. C. Handbook of Polymeric Foams and Foam Technology; Hanser Gardner: New York, 1991. (2) Thomas, C. R. The Formation of Cellular Plastics. Br. Plast. 1965, 38, 552. (3) Chaudhary, B. I.; Barry, R. P. Extruded Noncrosslinked Foams Made From Ethylene-Stylene Interpolymer. Foams 1999, 99, 19.

(4) Kaji, K.; Hatada, M.; Yoshizawa, I.; Kohara, C. Preparation of Hydrophilic Polyethylene Foam of Open Cell Type by Radiation Grafting of Acrylic Acid. J. Appl. Polym. Sci. 1989, 37, 2153. (5) Kozma, M. L.; Bambara, J. D.; Hurley, R. F. Open Cell Foamed Articles Including Silane-Grafted Polyolefin Resins. U.S. Patent 5,859,076, 1999. (6) Chaudhary, B. I.; Malone B. A. Method of enhancing open cell formation in alkenyl aromatic polymer foams. U.S. Patent 5,962,545, 1999. (7) Abe, S. Polyolefin Foam and Polyolefin Resin Composition. U.S. Patent 6,414,047 B1, 2002. (8) Browers, S. D.; Wiegand, D. E. Oil Sorbent Material Made by Opening Cells of a Closed Cell Foam. U.S. Patent 4,183,984, 1980. (9) Hale, W. R.; Dohrer, K. K.; Tant, M. R.; Sand, I. D. A Diffusion Model For Water Vapor Transmission Through Microporous Polyethylene/ CaCO3 Films. Colloids Surf. A 2001, 187, 483. (10) Huang, Q.; Seibig, B.; Paul, D. Polycarbonate hollow fiber membrances by melt extrusion. J. Membr. Sci. 1999, 161, 287. (11) Huang, Q.; Seibig, B.; Paul, D. Melt Extruded Open-Cell Mircocellular Foams for Membrane Separation: Processing and Cell Morphology Relationship. J. Cell. Plast. 2000, 36, 112. (12) Park, C. B.; Padareva, V.; Lee, P. C.; Naguib, H. E. Extruded OpenCelled LDPE and LDPE/PS Foams Using Non-Homogeneous Melt Structure. J. Polym. Eng. 2005, 25, 239. (13) Lee, P. C.; Naguib, H. E.; Park, C. B.; Wang, J. Increase of OpenCell Content by Introducing a Secondary Blowing Agent. Polym. Eng. Sci. 2005, 45, 1445. (14) Park, C. B.; Cheung, L. K.; Song, S. W. The Effect of Talc on Cell Nucleation in Extrusion Foam Processing of Polypropylene with CO2 and Isopentane. Cell. Polym. 1998, 17, 221. (15) Paul, D. R.; Barlow, J. W. Polymer Blends (or Alloys). J. Macromol. Sci., ReV. Macromol. Chem. 1980, C18, 109. (16) Cogswell, F. N. Measuring the Extensional Rheology of Polymer Melts. Trans. Soc. Rheol. 1972, 16, 383. (17) Xu, X.; Park, C. B.; Xu, D.; Pop-Iliev, R. Effect of the Die Geometry on Cell Nucleation of PS Foams Blown with CO2. Polym. Eng. Sci. 2003, 43, 1378.

ReceiVed for reView April 26, 2005 ReVised manuscript receiVed October 7, 2005 Accepted October 18, 2005 IE050498J