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Increase of Open-Cell Content By Plasticizing Soft Regions With Secondary Blowing Agent

Posted on: Sunday, 23 October 2005, 03:01 CDT

By Lee, Patrick C; Naguib, Hani E; Park, Chul B; Wang, Jin

This article describes the effects of n-butane mixed with primary CO2 as a secondary blowing agent on the cellpopulation density, the volume expansion ratio, and the open-cell content of low-density polyethylene (LDPE) and LDPE/polystyrene (PS) blends in extrusion. With the plasticizing effect of n-butane, a high open-cell content (up to 100%) over a wide range of processing temperatures was successfully achieved. POLYM. ENG. SCI., 45: 1445-1451, 2005. 2005 Society of Plastics Engineers

INTRODUCTION

Until recently, open-cell foams have been almost exclusively manufactured with polyurethane thermoset materials [I]. Because the manufacturing technologies for open-cell thermoplastic foams have not been developed extensively, only a few thermoplastic foams with very high open-cell contents are available [2-11]. For instance, open-cell thermoplastic loams have been produced by leaching a soluble filler from a polymer matrix [2]. They have also been produced by using: interpolymer blending [3], grafting of polymer resin [4, 5], blending with soft polymers [6, 7], or by punching foams with fine needles [8]. In addition, the stretching of mineral- filled polymers [9], adjustments to the amount of injected CO2, and changes to the temperatures of the extruding head and nozzle [10, 11] have been used.'

Our previous study successfully demonstrated an opencell foaming extrusion technology by achieving a high open-cell content (up to 99%) with LDPE/PS blends while using CO2 as a blowing agent and a small amount of crosslinking agent [12]. If thermoplastic foams with an open-cell content of up to 100% can be achieved over a wide melt temperature range, then numerous industrial applications such as filters, separation membranes, diapers, battery electrode supports, battery separators, and tissue attachments and growth could be made available al a lower manufacturing cost. In addition, these open- cell foams would demonstrate improved functionality (i.e., permeability of gas or vapor, selective osmosis, and absorbing and dampening of sound). Microcellular open-cell foams, in particular, will exhibit better properties for some of these applications.

Background

In our previous cell-opening study, three major strategies were applied to produce LDPE-based, open-cell foam structures [12]. First, a structural nonhomogeneity, consisting of hard and soft regions, in the polymer matrix was created by crosslinking. The basic concept of forming a hard/soft non-homogeneity in the matrix to promote cell opening in polyurethane foaming was adopted for producing LDPE-based open-cell thermoplastic material. Upon crosslinking 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., noncrosslinked sections) were opened in the thinning cell walls during cell V: growth to create interconnections between cells, the hard regions (i.e., crosslinked sections) held 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 noncrosslinked 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. Since cell opening is more susceptible with thinner cell walls, a large expansion ratio is desirable. At the same time, the noncrosslinked sections of the cell walls should remain soft enough for the entrapped gas to rupture them. In order to satisfy both requirements, and thereby maximize the open-cell content, the temperature had to be closely controlled. The noncrosslinked 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 noncrosslinked (i.e., soft) sections in the polymer matrix. Nonetheless, a thinner eell thiekness was preferred to inerease the degree of cell opening in the eell walls. Since the eell wall thickness of PE foams generally decreases at lower temperatures because of higher expansion ratios 113], an optimum temperature should be maintained to maximize the open-cell content.

Third, increasing cell density further decreased the cell wall thickness. Cell density was increased by blending a small 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 eell nuclei density through heterogeneous nucleation [14J, which in turn decreased the cell wall thickness, and thus favored cell wall opening.

All of these strategics were empirically proven to be effective means of cell opening. Due to the proper combinations of these strategies, a very high open-cell content (up to 99%) was successfully achieved at optimum temperatures.

Additional Strategy for Further Increase of Cell Opening

As a continuation of this previous work, efforts have been made to further increase the open-cell content over a broad spectrum of melt temperatures. If it were possible to produce thermoplastic foams with more than 95% open-cell content over a wide temperature range, manufacturability would be made easier and manufacturing costs would be lowered. In this study, a new strategy was applied to further increase cell opening by softening uncrosslinked sections in the cell walls with η-butane as a secondary blowing agent. Since η-butane diffuses out of the foams over time while leaving no residue behind, it is a preferred plasticizer in comparison to other conventional plasticizers. The expected effects of η-butane on the cell opening phenomena can be summarized as follows. First, since η-butane has a superior plasticizing effect in an LDPE melt, uncrosslinked sections, which experience substantial structural softening, can open up more easily when it is added. second, butane may affect the cell density and thereby affect the open-cell content. Since a high amount of blowing agent is known to increase the cell density and thereby decrease the cell wall thickness [15], the open-cell content will increase [12]. Third, added butane may also increase the expansion ratio during foaming and thereby increase the overall open-cell content. Since cell opening starts to occur during the initial expansion stage [16-18], the expansion ratio may increase first and then decrease back due to fast gas loss with a high open-cell content. Thus, the final expansion ratio may not be elevated once the foam has a high open- cell content [19] even with additional butane. In other words, adding butane will inerease the initial (and interim) expansion ratio and thereby the open-cell content. But when the open-cell content increases, the final expansion ratio may diminish because of gas loss. If gas escape is blocked in the case of thick samples by freezing the skin, the high expansion ratio will be sustained longer (or permanently) and, therefore, higher open-cell contents and large openings in the eell walls will be obtained [16]. However, for thin samples it is difficult to control the skin temperature separately; this strategy is not pursued in our study.

EXPERIMENTAL

Experimental Materials

The plastic material used in this study was commercially available low-density polyethylene (LDPE) resin (Novapol Polyethylene LC-0522-A) supplied by Nova Chemicals (Calgary, Alberta). The melt index (ASTM D 1238, 19O0C/ 2.16 kg) is 4.5 g/10 min and the density (ASTM D 792) is 0.922 g/cm3. This polyethylene resin is biologically and chemically inert, which is suitable for biomedical applications such as tissue attachment and growth. The other polymer resin used in the study was polystyrene (PS) from Nova Chemicals (PS 101). The melt index is 2.2 g/10 min and the density of this polystyrene is 1.040 g/cm^sup 3^.

The crosslinking agent was an organic dicumyl peroxide (Varox DCP- 40C) supplied by R.T. Vanderbilt Co. (Norwalk, CT). The melting or sublimation point of this chemical was 38C (100.4F) and the specific gravity was 1.6. This crosslinking agent is unstable and reactive with strong acids and strong oxidizers. The thermal behavior of this exothermic organic peroxide was examined using a DSC 2910 of TA Instruments (New Castle, DE), with a heating rate of 10C/min. The onset temperature was around 150C. A fixed amount of 0.4 wt% was used throughout the analyses.

The nucleating agent employed in this study was talc (particle size range 2-3 jam; Ingnia Polymers, Toronto, Canada). A fixed amount of 2 wt% was used throughout the experiments. The CO2 and n- butane, supplied by Matheson Gas Product (Joliet, IL), were utilized as blowing agents. The CO2 and n-butane were commercial grades with minimum 99.5% and 99% purity, respectively.

Experimental Setup

A single-screw foaming extrusion system was used in this study. The single extrusion system consists of a 5 hp extruder drive with a speed-control gearbox, a '' single-screw extruder (Brabender 05-25- 000, Germany), a mixing screw (Brabender 05-00-144), two positive displacement equipme\nts (ISCO syringe pump 26OD), a gear pump (Zenith, PEP-II 1.2 cc/rev), a dissolution enhancing device containing static mixers (Omega FMX8441S), two heat exchangers, a filament die with a length and diameter (L/D) ratio of 7.62/0.457 mm (0.3''/0.018'') or 2.54/0.457 mm (0.1''/0.018''), and a cooling sleeve. The extruder was used for plastieating the polymer resin and the gas-injection equipment, which were attached to the extruder at two separate points, were used for injecting the soluble amount of gases into the polymer melt. The gear pump controls the polymer melt flow rate, which was independent of temperature and pressure changes, and the dissolution-enhancing device ensured the homogeneity of the polymer/blowing agent solution. The heat exchangers provided uniform cooling for the polymer melt. Shaping and cell nucleation were accomplished in the filament die, and the cooling sleeve was used around the die to ensure its precise temperature control. Figure 1 shows a schematic of the overall setup.

FIG. 1. Single extruder system with two separate injection ports.

Experimental Procedure

LDPE or LDPE/PS pellets, mixed with 2.0 wt% talc and 0.4 wt% crosslinking agents, were first fed into the barrel through the hopper and were completely melted by the screw motion. The designated amounts of CO2 (i.e., 8, 12, 15 wt%) and n-butane (i.e., 2, 4, 6 wt%) were then injected into two separate points on the extrusion barrel by two positive displacement pumps. They were subsequently mixed with the polymer melt stream in the barrel and eventually dissolved in the melt. The single-phase polymer/ gas solution went through the gear pump and was fed into the heat exchanger, where it was cooled to a preselected temperature. The cooled polymer/gas solution entered the die and foaming occurred at the die exit. All the other materials and processing parameters such as screw speed, gear pump speed (i.e., 12 RPM), barrel temperatures (i.e., 125-135C), and blowing agent content were fixed in order to obtain a constant 12 g/min gas/polymer mixture flow rate. Meanwhile, the synchronized melt and die temperatures were lowered step by step and samples were randomly collected at each set temperature once the system reached the equilibrium state.

Depending on the cell sizes, the foam samples were characterized using an optical microscope (Wild Heerbrugg, Switzerland) or a scanning electron microscope (SEM, Hitachi 510, Japan). The foam samples were dipped in liquid nitrogen and then fractured to expose the cellular morphology before the characterization of the foam structure. The open-cell content of the foam samples were measured using a gas pycnometer (Quantachrome, Boynton Beach, FL).

CHARACTERIZATION OF OPEN-CELLED FOAMS

Cell-Population Density and Volume-Expansion Ratio

FIG. 2. Cell density vs. butane content of LDPE (8 wt% CO2, 0% PS, 0.4 wt% crosslinking agent, L/O = 2.54/0.457 mm).

The detailed open-cell content calculation procedure is described in ASTM D6226-98.

RESULTS AND DISCUSSION

It is well known that the cell opening behavior strongly depends on cell wall thickness [12]. In other words, the cell density and expansion ratio, which determine the cell wall thickness, significantly affect cell opening. Therefore, cell density and volume expansion ratio are discussed first before introducing open- cell contents.

Cell-Population Density

The cell-population densities of LDPE with a fixed amount of CO2 (i.e., 8 wt%) and a varying amount of n-butanc (i.e., O, 2, 4, and 6 wt%) are graphed in Fig. 2. LDPE melt was extruded through a filamentary die with a dimension of L/D = 2.54/0.457 mm (or 0.1''/ 0.018''). The average value of cell densities over the processing melt temperatures was calculated and depicted for each combination of CO2 and n-butane contents. These average cell densities were in the range of 10^sup 6^ to 10^sup 7^ cell/cm^sup 3^, showing insensitivity to the n-butane contents. Hence, it is safe to assume that a small amount of n-butane (i.e., up to 6 wt%) has a negligible impact on cell densities. Similarly, n-butane showed almost no effect on the cell densities for the experiments with a fixed amount of CO2 (i.e., 12 wt%) and a varying amount of n-butane (i.e., O, 2, 4, and 6 wt%) (see Fig. 3).

All the cell densities were in the range of 10^sup 8^ to 10^sup 9^ cell/cm^sup 3^, and more than 10^sup 9^ cells/cm^sup 3^ were observed for the experiment with 15 wt% CO2 and 6 wt% η-butane blowing agent blending. For the 15 wt% CO2 experiment, only the blowing agent blending case (i.e., 15 wt% CO2 and 6 wt% n-butane) was studied. These experiments (i.e., 12 and 15 wt% CO2 injection cases) were performed with a filamentary die with a dimension of L/ D = 7.62/0.457 mm (or 0.3'/0.018'). Since according to Xu et al. [20] cell densities should not be affected by pressure increase as a result of die length increase (i.e., from 2.54-7.62 mm) if die diameter (i.e., 0.457 mm) remains the same, the cell-population densities were mostly governed by CO2 content and the heterogeneous nucleation mechanism. The results were due to PS addition, and thus not by the addition of n-butane.

FIG. 3. Cell density vs. butane content of LDPE/PS blends (10% PS, 0.4 wt% crosslinking agent, L/D = 7.62/0.457 mm).

FIG. 4. Volume expansion ratio vs. Butane content of LDPE (8 wt% CO2, 0% PS, 0.4 wt% crosslinking agent, L/D = 2.54/0.457 mm).

Volume Expansion Ratio

Figures 4 and 5 show the final expansion ratios of the extruded foams. The volume expansion ratios did not increase with the addition of n-butane. Most of the volume expansion ratios of foam samples were within 2-4-fold, regardless of the n-butane content. Because of the fast cell growth rate in the LDPE and CO2 system [21], an initial hump [16] was always observed when CO2 was used as a major blowing agent. It was observed that the addition of butane did not change the profile of extruded foam even at a low temperature, and the expansion rate and extruded foam profile were governed by the major blowing agent of CO2 due to its high diffusivity. As discussed in our previous studies [16, 22], the formation of an initial hump always causes severe gas loss during foaming. Because of fast foam growth, the cell walls became thin much too quickly while the melt was still hot, and the cell-to-cell diffusion of n-butane occurred faster than in the normal case of n- butane only. Therefore, the escape of n-butane was facilitated by CO2. However, the loss of CO2 was also promoted by the plasticizing effect of n-butane. On the other hand, it is also believed that the openings in the cell walls promoted gas loss significantly. As the open-cell content increased with the addition of butane, gas escape was expedited and the final expansion ratio was small because of shrinkage.

FIG. 5. Volume expansion ratio vs. Butane content of LDPE/PS blends (10% PS, 0.4 wt% crosslinking agent, L/D = 7.62/0.457 mm).

FIG. 6. Open-cell content vs. butane content of LDPE (8 wl% CO2, 0% PS, 0.4 wt% crosslinking agent, L/D = 2.54/0.457 mm).

Open-Cell Contents

As the n-butane content increased from O to 6 wt% (in steps of 2 wt%), so too did the open-cell contents of LDPE with a fixed amount of CO2 (i.e., 8 wt%) increase dramatically from 60 to 85%, as shown in Fig. 6. Similarly, the open-cell contents of the LDPE/PS blend with a fixed 12 wt% CO2 increased, as shown in Fig. 7. For the experiment with a 15 wt% CO2 and 6 wt% n-butane blend, an average open-cell content of 96.7% was successfully achieved (Fig. 7).

Since cell density was not largely influenced by the presence of butane, butane's effects were considered negligible in this respect. However, the addition of butane must have affected the initial expansion ratio and thereby increased openings in the cell walls. But most important, cell opening must have been increased through the plasticizing effect of butane. In other words, the softness of the uncrosslinked sections in the cell walls substantially increased the chances for cell opening, even at low temperatures. According to our previous study [12], cell wall strength governed the open-cell content al low temperatures. Due to the residual n-butane in the polymer matrix, the stiffness of the cell wall decreased, yielding a higher chance of cell wall rupture. With the aid of n-butane, a range of optimum processing temperatures for the maximum open-cell conlenl was broadened, as shown in Fig. 8. One hundred percent of the open-cell contents were successfully achieved al the processing melt temperature range of 80-110C.

CONCLUSION

In this study it is claimed that open-cell (up to 100%), microcellular (more than 109 cells/cm^sup 3^) foams were successfully achieved using a blowing agent thai blends CO2 and n- butane. The addition of n-butane did not improve cell nucleation. However, due to a strong plasticization effect, the open-cell contenls were increased significantly over a wide range of processing temperatures.

NOMENCLATURE

FIG. 7. Open-cell content vs. butane content of LDPE/PS blends (10% PS, 0.4 wt% crosslinking agent, L/D = 7.62/0.457 nun).

FIG. 8. Open-cell content vs. melt temperature of LDPK/PS=90/10 blend (15 wt% CO2, 6 wt% butane, 0.4 wt% crosslinking agent, LfD = 7.62/ 0.457 mm).

REFERENCES

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21. P.C. Lee, "Manufacture of Low-Density, Microcellular, OpenCell Thermoplastic Foams with Large Intercellular Pores," Ph.D. Thesis, University of Toronto (2005).

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Patrick C. Lee, Hani E. Naguib, Chul B. Park, Jin Wang

Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Rd., Toronto, Ontario M5S 3G8, Canada

Correspondence to: Chul B. Park, e-mail: park@mie.utoronto.ca

Contract grant sponsors: Eastman Kodak; NSERC; Consortium for Cellular and Micro-Cellular Plastics (CCMCP).

DOI 10.1002/pen.20422

Published onlineo September2005 in Wiley lntcrScience(www.intcrscience. wiley.com).

2005 Society of Plastics Engineers

Copyright Society of Plastics Engineers Oct 2005

Source: Polymer Engineering and Science

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