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Foaming of Low-Density Polyethylene in a Dynamic Decompression and Cooling Process.


Effects of the properties of polymers and blowing liquids on the macrostructure and microstructure of foamed products in a dynamic decompression and cooling (DDC) process have been investigated. When the homogeneous solutions, prepared by the heating and mixing of the mixtures of low-density polyethylenes (LDPE) and chlorinated hydrocarbons under nitrogen ([N.sub.2]), go through a rapid pressure quench above the boiling point of the liquids, bubbles nucleate out through liquid/gas phase separation and grow through diffusion and expansion of the gaseous phases. Foam cell stabilization is improved by polymers exhibiting higher extensional hardening and blowing liquids possessing higher latent heat of evaporation. Resultant LDPE foams have mixed cell structures: in the skin parts, more cells are closed, but more cells are open in the core parts. In the polymer matrix, micromorphologies of granules, fibers, and fiber networks, with oriented lamellae, are observed. The formation of these complex structures is des cribed in terms of phase and deformation behaviors of the solutions.


Polymer foams can be manufactured by a number of different technologies [1]. Among them, the expansion process is the most widely used technique in which gas bubbles nucleate and expand by either decompression or heating of the system that contains polymers and foaming agents. Its foaming mechanism generally involves the pressure difference between the inside of the cell and the surrounding medium. In a 1932 patent [2], Munters and Tandberg first proposed a polymer foaming process that utilizes the expansion technique. The inventors produced melt solutions of polystyrene (PS) and methyl chloride ([CH.sub.3]Cl) by the heating and mixing of the solid/gas mixtures. The solutions were then extruded into the atmosphere wherein the volatile gas evaporates, cooling and expanding the melt, and therefore producing foamed articles. Subsequently in 1950, modifying this batch technique, mcIntire of the Dow Co. [3] introduced a continuous process, where an extruder produced continuously pressurized solutions of PS/[CH.sub.3]C1 and forwarded them to a storage tower. The solutions were then discharged into the atmosphere. In 1962 Rubens et al. [4], also from the Dow Co., introduced a new foaming agent of 1,2-dichlorotetrafluoethane in order to fabricate closed celled foams of polyethylene (PE) without crosslinking the polymer matrix. Simultaneously, in the 1960s, Blades and White of the DuPont Co. proposed a process that employs liquids as a blowing agent [5, 6]. Here, homogeneous mixtures of polymer/poor solvents are decompressed through a shaping die into a closed-cellular body. A number of thermoplastic foams are presented as examples in this patent, but they seem not to have been commercialized, as a 1976 paper of the DuPont Co. implied [7].

Recently, Apfel [8] proposed a technology called "Dynamic Decompression and Cooling (DDC) Process" that is originally directed at producing open-cell foams of glassy metals, and which utilizes a sudden pressure quench of the bulk system that contains immiscible pairs of melts and blowing liquids [9,10]. In the process, a melt seeded with droplets of a volatile liquid is rapidly decompressed, leading to a dynamic, superheated and supersaturated mixture; then the droplets explosively vaporize, taking their heat of vaporization from the melt and, therefore, homogeneously cooling and expanding it. Foaming of crystalline materials in DDC can yield glassy or crystalline products depending on the cooling rate of the melt. For polymer foaming, we employed poor solvents as a blowing liquid, following the previous method of White and Blades (5, 6), and then produced the open cell foams of poly(bulylene terephthalate, PBT) that exhibited variations in cell size and shape [11]. The open cell walls were also found to pos sess various micromorphologies of granules, fibers, sheets, and networklike structures, with oriented lamellae. It was thus conjectured that phase separation and flow-induced deformation of the solution were the mechanisms of DDC foaming to develop cellular and crystalline structures. However, the complex mechanisms of heat/ mass transfer in the system, explosively occurring during the initial foaming process, are not fully understood, owing partly to the limited amount of experimental data and partly to the highly transient nature of the process. In the present work, we extend our previous work by processing solutions of low-density polyethylenes (LDPE) and chlorinated hydrocarbons, which have different molecular structures and weights, and investigate the influences of the material properties on resultant foam structures. Our experiments therefore aim eventually at developing foams of uniform cell structures, either open or closed, by using the DDC process, to better understand the underlying mechanisms of structure formation in transient polymer foaming processes.


Supercooling and Bubble Growth in the Polymer Melt

As solutions of polymer melts in blowing liquids are decompressed above the boiling point of the solvent, the volatile liquid phase vaporizes, taking latent heat from the melt and therefore cooling it. For the two species that are non-reacting and form a uniform phase, an energy balance equation is given [10]:

[delta]T ([[rho].sub.m][C.sub.p,m][V.sub.m] = [delta][H.sub.v,s][[rho].sub.s] [V.sub.s] -([delta][H.sub.f,m] + [delta][H.sub.c,m])[[rho].sub.m][V.sub.m] (1)

where, [delta]T is the reduction in temperature; the subscripts m and s denote respectively the polymer melt and the solvent; [rho] is the density; [C.sub.p] is the specific heat; V is the volume; [delta][H.sub.u] is the latent heat of vaporization; [delta][H.sub.f] is the heat of fusion; and [delta][H.sub.c] is the heat of crystallization.

In the DDC process, like other foaming technologies, the cooling rate of the polymer melts (dT/dt [approximate] ([delta]T/[delta]t) is of underlying importance in designing both macrostructures and microstructures of resultant foams. Bubble growth is related to vaporization of the solvent and thus to the rate of melt cooling, in which material properties such as density ([rho]), diffusivity (D), viscosity ([eta]), and surface tension ([gamma]), etc. are functions of transient temperature and pressure.

A growth model of a large number of spherical bubbles in polymer melts was proposed, which employs a cell model and assumes an ideal gas and a Newtonian liquid [12]. The bubbles are close in proximity and have a uniform radius R. This model has been modified to incorporate both non-Newtonian behavior of polymers and distributions of bubble sizes [13-15]. However, the bubble growth models are similar in principle and neglect the latent heat of the volatile phase, which does not behave a perfect gas law. Continuity and momentum balance yield [12]:

dR/dt = V + [R.sup.3]/4[eta]V = [([P.sub.g] - [P.sub.f]R - 2[gamma]] - R/3 dln[rho]/dt (2)

[partial]c/[partial]t = 9D[[rho].sup.2] [partial]/[partial]x[[(x/[rho] + [R.sup.3]).sup.4/3][partial]c/[partial]x] (3)

where, a convected coordinate, x = ([r.sup.3] - [R.sup.3])[rho]; V is the volume of a cell; [P.sub.g] and [P.sup.f] are, respectively, the pressure inside the gas bubble and the pressure in bulk liquid surrounding a cell; [gamma] is the surface tension; and c is the concentration of dissolved gas in the polymer solution. The density term in Eq 2 will dominate in bubble growth as the melt begins to solidify i.e., [eta] [right arrow] [infinity], or as the system approaches mechanical equilibrium. In the limit, the final bubble size ([R.sub.f]) is related to the initial bubble size ([R.sub.i]);

[R.sub.f] = [R.sub.i][[[rho]f/[rho]i].sup.-1/3] (4)

Phase Behavior of Polymer Solutions

For a binary polymer/solvent system, the thermodynamic equilibrium between the solvent-rich and polymer-rich phases is specified (16);

[[micro].sub.i] = [[micro]'.sub.i] (5)

where [[micro].sub.i] is the chemical potential of component i (1: solvent; 2: polymer segment). In general, binary solutions of polymers and poor solvents undergo phase separation with lowering temperature. The phase separation starts to occur at a critical temperature, [T.sub.c], where the following relationship is satisfied:

[partial][[micro].sub.i]/[partial][v.sub.j] = [[partial].sup.2][[micro].sub.i]/[partial][[v.sup.2].sub.j] = 0 (6)

Here, v is the volume fraction. The binodal curve that separates stable region from the metastable region is defined as the loci where the two phases are in equilibrium (Fig. 1). As the temperature decreases further, the solutions will pass through the spinodal, the limits of the metastable region, at which the second derivative of the Gibbs free energy ([delta]G) of mixing with respect to v vanishes;

[[partial].sup.2][delta]G/[partial][[v.sup.2].sub.j] = 0 (7)

It follows that in the metastable region, [[partial].sup.2][delta]G/[partial][[v.sup.2].sub.j] [greater than] 0,

while in the unstable region, [[partial].sup.2][delta]G/[partial][[v.sup.2].sub.j] [less than] 0.

chase separation will proceed either by nucleation and growth or spinodal decomposition, depending on the thermodynamic stability of the system. In the metastable region, the mixture is generally stable to small changes in composition, but unstable to large amplitude fluctuations by which phase transition occurs. As a result, molecules form clusters by impingement, which will grow continuously in size, initially by downhill diffusion of the molecules from the neighboring mother phase and in the later stage by a coalescence or ripening process. In the unstable regime, on the other hand, spinodal decomposition that is characterized by uphill diffusion of like molecules is a dominant mechanism for phase separation. A spinodally decomposed system exhibits highly interconnected phase morphologies, a morphology that is widely used to fabricate open, interconnected cellular structures of polymer foams [17-19].



Low-density polyethylene (LDPE) polymers having different melt indices (MI, g/10 min) were used in this experiment. The two LDPEs obtained from the Dow Chemical Co. had MI's of 0.22 and 25, and the other LDPE with MI of 2.3 was supplied by the Mobil Chemical Co. HPLC-grades of chloroform (CH[Cl.sub.3]) and methylene chloride (C[H.sub.2][Cl.sub.2]) with purity above 99.9% purchased from the Fisher Scientific Co. were used as blowing liquids. The typical properties of chlorinated hydrocarbons are given in Table 1.

Foam Formation

An apparatus that consisted of a pressure cell, a stirrer, and temperature/pressure controlling units was built by modifying a PARR[R] pressure vessel. A 50 ml glass crucible containing 27 wt% polymers in blowing liquids was fitted with a ceramic heater. This heating unit was placed inside the vessel and then pressurized with nitrogen ([N.sub.2]) in the range of 2.07 to 4.14 MPa. Subsequently, the system was heated up to temperatures of 120 to 200[degrees]C, above the crystalline melting point ([T.sub.m]) of LDPE, while stirring at 6 rpm with a turbine-type impeller that was located near the bottom of the vessel. A homogeneous one-phase solution thus formed was left for 2 min at a prescribed temperature to ensure the complete melting of any crystal remnants in the solution. Then the heater and stirrer were turned off, letting the system cool down naturally until the moment of decompression, when the release valve was quickly opened to the atmosphere. This operation led to the formation of foamed polymers ins ide the vessel. The temperature of the foam was recorded immediately after the system reached equilibrium at the atmospheric pressure. The whole cycle of the foaming process from heating to decompression took about 15 to 20 min.


Foam Density. The mean bulk densities of cylindrical foams were determined by measuring the weight and volume of the specimens. The samples with a length of 20 mm were prepared by freeze cutting using liquid nitrogen, and their diameters were measured by a video camera. The volumes of selected samples were also estimated by a water displacement method.

Differential Scanning Calorimeter. Thermal analyses were performed over the temperature range from 0 to 150[degrees]C with a Perkin-Elma Pyris-1 differential scanning calorimeter (DSC). The instrument was calibrated using pure indium metal. The sample with a weight of 10.0 [plus or minus] 0.2 mg was scanned at a heating rate of 20[degrees]C/min and its crystallinity was obtained by:

[X.sub.c](%) = [delta][H.sub.exp]/[delta][H.sup.o] x100 (8)

where [delta][H.sub.exp] = [delta][H.sub.melting] - [delta][H.sub.cold crystallization], [delta][H.sup.o] = 293 J/g given by Wunderlich and Cormier [20] as the heat of fusion of 100% crystalline polyethylene.

Electron Scanning Microscope (SEM). Morphologies of the foams produced were observed with a Philips XL30 scanning electron microscope (SEM). The instrument was operated at 20 to 30 KV with a working distance between the detector and sample of [sim] 10 cm. Fresh surfaces of the samples normal and parallel to the flow direction (FD) were prepared by freeze fracturing using liquid nitrogen. The fresh surfaces were then coated with a thin layer of carbon material to prevent charging by the electron beam.

Wide Angle X-ray Diffractometer. Wide angle X-ray diffraction (WAXS) patterns were taken using a Brucker GADD X-ray diffractometer equipped with the Cr target generator. The X-ray beam was monochromatized to obtain CrK[alpha] radiation. The middle part of foams was carefully cut out along the flow direction (FD) and then divided into two specimens: the skin and core parts. The specimens were mounted on the instrument so that the X-ray beam was normal to the surface of the foam. WAXS patterns were then recorded on flat films. 2[theta] scans were conducted over the Bragg angles from 5[degrees] to 75[degrees] and the [110] pole densities were collected along the azimuthal angle.


Supercooling of the DDC Process

The temperature reduction ([delta]T) induced by DDC for the LDPE with MI of 25/chlorinated hydrocarbon solution system was computed using Eq 1. The [delta][H.sub.f,m] of 35.7 cal/g for highly crystalline DDC foams and the [C.sub.p,m] of 0.6122 cal/K * g at l10[degrees]C were used [21]. It was assumed that during the foaming thermal equilibrium occurs so rapidly that the melt volume is at uniform temperature. It is noteworthy that the latent heats ([delta][H.sub.v]) of the blowing liquids vary with temperature: at low temperatures below 100[degrees]C, the values of [delta][H.sub.v] increase with decreasing molecular weight, but they decrease rapidly to zero as the temperature approaches a critical condition [22]. At 110[degrees]C, methylene chloride (C[H.sub.2][Cl.sub.2]) has higher [delta][H.sub.v] than chloroform (CH[Cl.sub.3]).

As shown in Fig. 2, both computed and experimental results exhibit the same tendency that the temperature reduction ([delta]T) increases with decreases in concentration of the polymer and with increases in latent heat of the blowing liquids. However, the calculated values are much larger than the experimental results, primarily due to the thermal inertia resulting from the slow cooling of the pressure vessel. The process may need a different configuration. In particular, the LDPE resin with MI of 0.22 revealed small [delta]T, frequently leaving the unsolidified melt at the bottom of the glass container.

Foam Density as Functions of Process Conditions

The mean densities of LDPE foams produced by the DDC process showed considerable fluctuations depending on the sampling position. The skin and bottom parts of the foams were much denser than the core and upper parts because of the restriction of melt expansion in a glass container. At high expansion rates above 3.5 MPa/s, i.e., the decompression duration below 1 sec, the mean densities of the foams decreased with increases in the decompression pressure ([P.sub.d]) and temperature ([T.sub.d]), but the variations are not substantial (Fig. 3). At constant [P.sub.d] and [T.sub.d], however, the mean foam densities increased appreciably, 90 to 160 kg/[m.sup.3], with increases in the decompression duration ([t.sub.d]). A minimum density of [sim]90 Kg/[m.sup.3] was achieved, which is relatively high as compared to that of the low-density extruded foams ([[rho].sub.f] [less than or equal to] 10 Kg/[m.sup.3]). At elevated temperatures, the foaming mixtures must have lower viscosity and higher diffusivity associated with molecular motion, the conditions that favor a high expansion of the polymer melt. However, the retarded decompression at low expansion rates suppresses the explosive evaporation of the volatile phase; therefore, the gaseous molecules may readily escape from the melt at relatively high temperatures without causing a large degree of expansion. At high decompression temperatures above 200[degrees]C, on the other hand, the LDPE melt cooled little by decompression, producing foamed sheets with high densities, [[rho].sub.f] [greater than or equal to] 500 Kg/[m.sup.3].

Crystalline Structure

The DSC scans of as-received LDPE resins generally revealed double melting peaks (Fig. 4a). However, the LDPE with MI = 2.3 displayed more than two peaks. This resin may possess different fractions of short and long chain branches, which increase melt strength but disrupt chain packing. DDC foaming tended to diminish the melting peaks or move them to high temperature regions, suggesting structural changes present in the foams. This agrees well with variations in crystalline fractions (Fig. 4b). As-received LDPE resins had crystallinities ([X.sub.c]) around 45%, but the two LDPEs with low MI were similar in level of [X.sub.c]. The reason for this may be differences in branch point defects and topological constraints, which interrupt the crystallization process. The DDC foams were more crystalline than the resins, revealing [X.sub.c]'s around 50%. During the foaming process, the polymers went through thermomechanical and solvent-induced crystallization, increasing their crystallinities. The foam with MI = 2.3 had a minimum [X.sub.c] with a broad melting endotherm in its DCS thermogram.

Consistent with the results of DSC scans, the DDC foams exhibited distinct crystalline character in WAXS diffraction (Fig. 5). Both equatorial and meridional patterns comprised distinct (110) and (200) peaks, which were more intense in the equatorial diagrams. However, the crystalline reflections changed little with variations in MI and expansion ratio. These features agree with those of flat film patterns, which revealed the intense Debye rings of the (110) and (200) planes on the azimuth. It was also found that the (110) pole figure was largely isotropic: the pole density changed marginally along the equator ([phi]) but remained relatively constant along the azimuth ([chi]). The WAXS results suggest quite ordered but little oriented crystals present the skin parts of the foams.

Cellular Structure

Foaming of LDPEs in the DDC process generated foams of a mixed cell structure with the cell size ranging from 50 to 1000 [micro]m in diameter (Fig. 6). In the skin parts, more cells were open; however, more cells were closed in the core parts. At MI = 25, the foams revealed largely open cell structures, and decreasing MI tended to better stabilize foam cells. This improved cell stability was attributed to increased resistance to thinning of cell walls as well as to retardation of melt drainage from the membranes [23]. The foams of MI = 2.3 had smaller cell size than those of MI = 0.22, probably owing to a high degree of side-chain branches in the molecule.

During the foaming, nucleated gas bubbles grow by the pressure gradient and the diffusion of the gas phase, developing stress at the cell wall of the polymer. If the stress exceeds the modulus of the material, the cell membrane will fail. In general, LDPE, as compared to high density PE, exhibits high degrees of strain-induced hardening due to its molecular mechanism, which imparts to the polymer a better stability in film blowing and which should help stabilize bubbles in foaming as well. Therefore, the mixed cell foams would result from the nonuniform distributions of stresses over the melt during the foaming periods. Obviously, the core and upper parts of the foams underwent more expansion than the skin and bottom parts because of the geometry of a glass container. Parameters such as localized liquid/liquid phase separation and varying rate of nucleation and growth for individual bubbles could also be responsible. Bubbles nucleated in the later stage owing to different mechanisms of nucleation or bubbles grown slowly owing to fluctuation of the temperature may remain closed without cell failure.

Foam Morphology

DDC-produced LDPE foams revealed, in general, two different morphologies of granules and fibers in SEM micrographs (Fig. 7), which reflect the mechanisms of their formation: local fluctuations of the concentration and solvent- and flow-induced crystallization of the polymer. The morphologies of the inside closed cell walls changed considerably with bubble expansion. Some cells that had diameters below 400 [micro]m showed uniformly distributed granules on their wall surfaces, which consisted of smooth surfaces and aligned along the circumferential direction (CD). Phase separation and rapid cooling of the solutions, which tend to reduce or shrink the surface area of the cell walls, may be responsible for this morphology. Crystal lamellae that had a thickness of a few microns were also developed on the wall strut. Increasing expansion stretched and flattened the surfaces giving rise to fiber structures that were oriented along the CD. Around the fibers were dispersed granules with diameters of 1 to 10 [micro]m that possessed either smooth or rough surfaces, characteristically similar to those isolated globules generated from dilute PE solutions through liquid-liquid phase separation (17). A fiber network with thicknesses of [sim] 10 [micro]m was also seen.

The character of the mixed cell morphologies implies that the solutions may go through local fluctuations of the concentration, resulting in microdomains enriched in polymer, probably due to the nonuniformity of the initial state at the metastable regime. A subsequent depletion of the solvent phase may lead to the formation of granules on the wall surface. Fiber structures coexisting with the particles may therefore be generated. The smooth and rough surfaces of the particles, on the other hand, were attributed to crystallization of the polymer (18). In heterogeneous nucleation the outward growth of lamellae owing to foreign particles acting as heterogeneities was found to form a rough surface, while homogeneous nucleation resulted in a smooth, folded crystal surface. The lamellar thicknesses could also vary, possibly being much thinner in homogeneous nucleation.

Open cell structures exhibited diverse, quite well-developed lamellar habits with thicknesses of less than 1 [micro]m, the texture of which was characterized by orientation of the chains along the flow direction of the volatile phase. Oriented lamellae interconnected laterally were also found. These observations agree in general with the previous results of poly(butylene terephthalate, PBT) foams that comprised open cell structures (11). In contrast, an LDPE specimen that crystallized from the melt under free of stress conditions was found to have a round-shaped lamellar-like structure on the surface with a mean diameter as small as [sim] 20 A.


DDC foaming of solutions of LDPEs in hydrocarbon liquids has produced foams of a mixed cellular structure. In the skin and bottom parts, more cells are closed; but more cells are open in the core and upper parts, primarily owing to the restriction of expansion in a confined space. In the foaming process in which the melts are contained inside the vessel, the formation of cellular structure depends greatly on characteristics of heat/mass transfer in the solutions. Material parameters such as extensional hardening of the melts and latent heat of evaporation of the liquids also have influence on foam cells stabilization. However, the effects are limited by the configuration of the DDC apparatus. The polymer matrix, on the other hand, includes micromorphologies that consist of granules, fibers, and network-like fiber structures. In the closed cell walls, the polymer chains orient along the circumferential direction, while the crystallites that are aligned preferentially in the flow direction of the volatile phas e occur in the open cell walls. It is found that DDC foaming develops micromorphologies primarily through phase separation in the solutions and flow-induced deformation of the polymers; therefore, the control of both will eventually result in polymer foams with planned macrostructures and microstructures.


The authors gratefully acknowledge the NASA Office for Microgravity Materials Science for financial support (Grant No. NAG8-1461). They also thank Prof. Jordan and Dr. Gromek at The University of Connecticut for the support of X-ray measurements for this study. The LDPE polymers investigated were kindly offered by the Dow Co. and the Mobil Co.

(*.) Corresponding author.


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Table 1. Physical Properties of the Chlorinated Hydrocarbon Blowing
Solvent Molecular Melting Boiling
 Weight Point Point
C[H.sub.2][Cl.sub.2] 84.9 g/mol -96.7[degrees]C 40.5[degrees]C
CH[Cl.sub.3] 119.4 -63.5 61.2
Solvent Density Critical Conditions
C[H.sub.2][Cl.sub.2] 1.336 g/[cm.sup.3] 237.0[degrees]C
CH[Cl.sub.3] 1.489 263.4
C[H.sub.2][Cl.sub.2] 6.10 MPa
CH[Cl.sub.3] 5.47

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Publication:Polymer Engineering and Science
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Date:May 1, 2001
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