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2010 46: 173 originally published online 1 February 2010Journal of Cellular PlasticsMohammadi

Mohammad Mehdi Khorasani, Seyed Reza Ghaffarian, Amir Babaie and NaserNanocomposites Prepared by a High Temperature Process

Foaming Behavior and Cellular Structure of Microcellular HDPE  

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Foaming Behavior and CellularStructure of Microcellular HDPENanocomposites Prepared by a

High Temperature Process

MOHAMMAD MEHDI KHORASANI,1,2 SEYED REZA GHAFFARIAN,1,*AMIR BABAIE

1AND NASER MOHAMMADI

1

1Polymer Engineering Department, Amirkabir University of Technology

P.O. Box 15875/4413, Tehran, Iran2Petrochemical Research and Technology Company

P.O. Box 14965/115, Tehran, Iran

ABSTRACT: This article discusses the development of microcellular HDPEand HDPE–clay nanocomposites via a batch high temperature process usingsupercritical N2. The study incorporates the effects of clay content andnanocomposite microstructure on the foaming process performance and cellularmorphology under investigation. It was possible to produce much nucleated andmore expanded microcellular foams with the nanocomposites than with the pureHDPE, as over 14 vol% void fraction could be reached at 6 wt% clay containingnanocomposite as a good result for foaming by N2 gas via a batch foamingprocess. We found that the state of nanoparticle dispersion affects themicrocellular morphology, so a better dispersed nanocomposite results in amore nucleated system in the microcellular foaming process. Moreover, therelationship between crystalline morphology and cell structure was investigated.Crystallinity and melting point were the important parameters for controllingthe cell growth mechanism in this foaming method.

KEY WORDS: microcellular nanocomposites, batch foaming, cellular morphol-ogy, HDPE.

*Author to whom correspondence should be addressed. E-mail: sr_ghaffarian@aut.ac.irFigures 1–7, 9 and 10 appear in color online: http://cel.sagepub.com

JOURNAL OF CELLULAR PLASTICS Volume 46 — March 2010 173

0021-955X/10/02 0173–18 $10.00/0 DOI: 10.1177/0021955X09358237� The Author(s), 2010. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

INTRODUCTION

Microcellular foamed semi-crystalline polymers are moreintricate to achieve compared to the amorphous polymers [1].

Colton [2] described three major problems in microcellular foaming ofsemi-crystalline polymers: low gas solubility in the crystalline region,the requirement to foam near the melting temperature, and the changein physical size and structure of the crystallites.

The physical foaming process can be carried out batchwise orcontinuously [1–6]. Two alternatives have been proposed for batchprocessing of microcellular foams; the first consists of saturating thepolymer with gas (e.g., CO2 or N2) at room temperature and subsequentlyreleasing the gas and foaming the polymer by heating it to a temperatureabove its softening point in a high temperature bath [3,4]. However,during the transfer of gas-saturated materials to the high temperatureenvironment, diffusion would inevitably occur [6], leading to a lower celldensity and expansion ratio. Plus, the saturation time is very long (fromhours to days depending on diffusivity). This greatly limits theproductivity, especially for semicrystalline polymers [6].

In the second method, the polymer is saturated with gas at a relativelyhigh temperature and pressure (in supercritical conditions) followed byrapid depressurization to atmospheric pressure. This method takesadvantage of the depression of polymer softening temperature inducedby the presence of gas [7]. In addition, increasing degree of super-saturation by this method has also been reported by Sumarno et al. [8].

In the previous studies, the batch processes of microcellular foamgeneration on HDPE system were usually conducted at low saturationtemperatures near room temperature by using CO2 gas as a blowingagent. Doroudiani et al. [9] presented the effect of crystallinemorphology on microcellular foaming of HDPE. They reported thatthe morphology of semicrystalline polymers has a great influence on thesolubility and diffusivity of the blowing agent and the cellular structureof the resulting foam in microcellular processing. They also used blendsof HDPE and PP to produce materials with a variety of crystalline andphase morphologies to enhance the subsequent microcellular foaming[10]. Rachtanapun et al. [11–13] carried out a series of works on themicrocellular foaming of HDPE/PP blends and showed that blendingfacilitates the formation of microcellular structures in polyolefinsbecause of the poorly bonded interfaces of immiscible HDPE/PPblends, which favor cell nucleation.

In the last decade, considerable efforts have been devoted to polymernanocomposite foams [14–36]. The combination of nanoparticles and

174 M. M. KHORASANI ET AL.

foaming technologies such as supercritical fluid foaming, ease theformation of these new materials [22]. Compared with the conventionalmicron-sized filler particles employed in various foaming processes,nano-sized particles may in fact offer unique properties. The extremelyfine dimensions and large surface area of nanoparticles provide muchmore intimate contact between the particles, polymer matrix, and gasphases [17–20]. The interfaces between the nano-fillers and polymer arepotential nucleation sites, where heterogeneous nucleation can beencouraged by a change in surface tension or the wetting factor betweenthe filler and polymer interface [23,24]. The importance of nanoparticlesin tuning the process for microcellular manufacturing is well attainablefrom the work of Lee et al. [19]. They have studied the effects of a smallamount of clay content and its dispersion on the cell morphology ofnanocomposite foams during extrusion foaming process. It is reportedthat for PE/clay nanocomposite foams blown with supercritical CO2,50.1 wt% of clay addition can produce the microcellular foam structurewith a cell density of4109 cells/cm3 and a cell size of �5 mm, which is aconsiderable achievement in a continuous process.

The heterogeneous nucleation induced by nano-scaled nucleatingagents decreases considerably the energy barrier for cell nucleation,increases the cell nucleation rate, decreases the cell diameter, and isexpected to narrow the cell size distribution as well [25,26]. As morebubbles start to nucleate concurrently, there is a less amount of gasavailable for bubble growth, leading to a reduction of cell size [6].Nanoparticles can also reinforce the micro-scaled cell walls, thusachieving macroscopic enhancement of mechanical properties [27].Therefore, what distinguishes clay nanoparticles from traditionalmicro-fillers is that the role of nanoparticles in polymer foaming is notmerely limited to the nucleation of cells, they also promote thefoamability of polymers and foam morphologies [25].

Recently, some researchers have investigated the effect of clayparticles on the structure and properties of HDPE microcellular foamsprepared by a batch process. Lee et al. [15] employed HDPE–claynanocomposites to make novel nanocomposite foams using supercriticalCO2 as a blowing agent. It was shown that compared to pure HDPEfoams, the addition of clay increased the cell density and reduced the cellsize of nanocomposites. Jo and Naguib [29–31] also investigated theeffects of nanoclay and foaming conditions on the foam morphology andmechanical properties of microcellular HDPE–clay nanocompositefoams prepared by batch foaming process using CO2. Tensile propertiesof HDPE–clay foams are improved when compared to that of pureHDPE foams.

Foaming Behavior and Cellular Structure of Microcellular HDPE 175

So far we have not come across any reported research on the prepara-tion of HDPE and its nanocomposite microcellular foams by N2 in highsaturation temperature. The objective of this study is to investigate thecapabilities of the high temperature process by using N2 for the batchmicrocellular foaming of a highly crystalline polymer, HDPE, and tostudy the effect of nanoparticles with different compatibilizing agents onthe performance of this method. Furthermore, a correlation betweennanocomposite structure and cellular morphology has been established.

EXPERIMENTAL

Materials

HDPE (Rigidex HD5218EA, Tabriz Petrochemical Co., Iran) with adensity of 0.952 g/cm3 (ISO 1872/1) and a melt index of 18 g/10 min (ISO1133) was used. The maleic anhydride-grafted HDPE (HD-g-MA,Fusabond E MB100D, DuPont, Canada) with MFI¼ 2.0 g/10 min andthe maleic anhydride-grafted LLDPE (LL-g-MA, Orevac 18302,ARKEMA) with MFI¼ 1.2 g/10 min were used as coupling agents.Organically modified clay with dimethyl dehydrogenated tallow alkylammonium (OMMT, Cloisite20A, Southern Clay Products) wasemployed as a layered silicate. A commercial grade N2 (purity: 99.8%)was utilized as the blowing agent without any further purification.

Preparation and Characterization of HDPE–ClayNanocomposites

The organoclay was dried in a vacuum oven for 24 h at 1008C prior tomixing. HDPE–clay nanocomposites were prepared by melt mixingHDPE, OMMT, and the compatibilizer in the mixing chamber of aBrabender laboratory internal mixer. The mixing temperature was setat 1808C, the rotor speed was 70 rpm, and the mixing time was 10 min.Based on the prior studies [37,38], for all nanocomposites the ratio ofcompatibilizer to Cloisite20A OMMT was kept at 3. The compositions ofnanocomposites studied in this work are summarized in Table 1.Nanocomposites comprising of HD-g-MA as the compatibilizer werecalled the HD group and the other set called the LL group ofnanocomposites. Pure HDPE and the prepared nanocomposites werethen molded into sheets of 1.3 mm in thickness by hot pressing at 1808Cand 20 MPa, followed by cooling to room temperature by the waterquenching method. The sheets were used for nanocomposite structurecharacterization and microcellular foaming experiments.

176 M. M. KHORASANI ET AL.

The characterization of nanocomposites structure was investigated byXRD method. The XRD patterns were recorded with an X-raydiffractometer (Phillips, X’Pert) equipped with Cu-Ka radiation(0.1542 nm). Measurement was performed in 2� ranges from 1.58 to 108.

Rheological measurements were conducted in oscillatory shear in aRheometric-Mechanical Spectrometry (RMS, Anton Paar Physica) witha parallel plate over a frequency range of 0.01–1000 rad/s at 1908C.Dynamic strain sweeps were initially performed to define the limits ofthe linear viscoelastic regime.

The melting point, crystallization temperature, and crystallinity of thebase sheets were investigated with a differential scanning calorimeter(DSC, SW8.01 METTLER) at a scanning rate of 58C/min under anitrogen environment.

Microcellular Foaming Procedure

A new batch foaming method based on the high temperature processwas utilized in this study. A laboratory autoclave thermoregulated byelectrical heaters was designed and used for saturating and thenfoaming of the samples. The saturation vessel was slowly charged withN2 gas up to a desired saturation pressure, 90 bar at a saturationtemperature of 1268C. The saturation temperature is closely near themelting points of the samples by an approximately 4–58C difference.Contact time was set as 1 h to fully saturating of the samples. Once therequired saturation time was reached, the vessel was immediatelydecompressed to atmospheric pressure. Super-saturation conditions canbe achieved either by decompression and/or heating processes.Therefore, the samples were kept in the vessel for a desired foamingtime to be heated by the heaters. Flowing of cold nitrogen gas by anintensive stream into the vessel terminated the foaming process andfroze the internal foamed structure. Foaming time was kept 3 s in all

Table 1. The prepared HDPE nanocomposites composition.

Sample nameHD-g-MA

(wt%)LL-g-MA

(wt%)Organoclay

(wt%)

N2-HD 6 – 2N4-HD 12 – 4N6-HD 18 – 6N2-LL – 6 2N4-LL – 12 4N6-LL – 18 6

Foaming Behavior and Cellular Structure of Microcellular HDPE 177

experiments to prevent from blow out and destruction of samples. Thelength of foaming time was accounted based on the start moment ofdecompression step to the start moment of cooling step.

The foam density (�f) was measured by a buoyancy method using adensity determination kit with the resolution of 10�4 g. The volumeexpansion ratio measured as the ratio of the unformed sample density tothe foam density.

The cellular structure of foams was characterized by scanningelectron microscopy (SEM, Phillips XL30) after breaking them in theliquid nitrogen. The average cell size and cell density were analyzed byutilizing the ‘ImageJ’ software. The cell density was calculated as thenumber of cells per unit volume with respect to the unfoamed polymer.The cell density was calculated by using the following equation:

Nc ¼n

A

� �3=2

��p

�f

where n is the number of cells in the defined area A, and �p and �f are thedensity of the unfoamed and foamed polymer, respectively.

Sorption Experiments

Saturation of the samples with N2 was used to study the effect of claycontent on the solubility of N2 in the samples. Experiments wereperformed at 808C and 90 bar for 1 h to prevent foaming of the samples,which could introduce some errors on solubility measurements. N2

uptake (solubility) was measured by weight gain immediately afterpressure release.

RESULTS AND DISCUSSION

Structure of Nanocomposites

X-Ray spectra of nanocomposites containing different clay content,with two distinct compatibilizers are shown in Figure 1. At XRDspectrum of Cloisite20A OMMT, two peaks are observed: first at 2�¼ 3.7(001) related to interlayer space of clays that are modified withalkilammonium and other one is in 2�¼ 7.12 correlated to unmodifiedlayers. No reflection peak is discernible in the X-ray diffraction patternof N2-HD sample indicating that exfoliated structure has been achieved.For N4-HD there is substantial reduction in the intensity of diffractionpeak comparing Cloisite20A OMMT, suggesting an intercalated structure.

178 M. M. KHORASANI ET AL.

XRD pattern of N6-HD shows that d-spacing are enlarged from 24 to27.4 A as well as a decrease of intensity of peak. It is well known thatclay concentration affects dispersion performance and when the clayamount increases the degree of exfoliation decreases [38].

For the nanocomposites containing LL-g-MA as the compatibilizer, asimilar trend has also been observed by increasing the clay content.Appearance of a peak in high 2� range for all samples compatibilizedwith the LL-g-MA shows that this compatibilizer cannot enlarge theinterlayer distance of unmodified clay layers.

Cloisite 20A

N2-HD

N2-LL

Arb

itray

inte

nsity

N4-HD

N6-HD

N6-LL

1 2 3 4 5 6

Degrees (2q)

7 8 9 10

N4-LL

Figure 1. XRD spectra of Cloisite20A OMMT and HDPE–clay nanocomposites.

Foaming Behavior and Cellular Structure of Microcellular HDPE 179

Totally it can be said that HD-g-MA has a great effect on the dispersionof layers throughout the HDPE matrix, but using of LL-g-MA can resultin a lesser modification of the clay structure in the same matrix.

Rheological Behavior

The measurement of rheological property of polymer nanocompositesunder molten state is crucial to gain fundamental understanding ofstructure of these materials. Generally, the rheological behavior ofpolymer nanocomposite melts strongly depends on their nanostructure,state of dispersion–distribution, and interfacial properties [39].

The effect of OMMT content and compatibilizer type on the rheologicalproperties of HDPE was also investigated, as is shown in Figures 2 and 3.Addition of OMMT leads to a great increase of viscosity and storagemodulus of HDPE matrices. Complex viscosity for all cases shows aunique trend of frequency dependency. A viscosity upturn in very lowfrequencies followed by a Newtonian decrease and ultimately a plateau-like terminal zone can be observed obviously. Changes in storage moduliare also in an adaptive and confirming manner. Increase in storagemoduli and viscosity upturn, especially at low frequencies which does notexist for corresponding matrices, could be attributed to existence of anetwork microstructure, and whatever the amount of increase is greaterthe performed network are stronger and would be said that the clays aredispersed better. The smaller viscosity and storage moduli of N2-LLsample compared to those of N2-HD are remarkable. This effect is due tobetter miscibility and compatibility of HD-g-MA with HDPE matrix.

1.0E+06

1.0E+05

1.0E+04

Com

plex

vis

cosi

ty (

Pa

s)

1.0E+03

1.0E+020.01 0.1 1 10

N2-HDN2-LLN4-HDN4-LLN6-HDN6-LLHDPE

Frequency (rad/s)

100 1000

Figure 2. Complex viscosity of HDPE and nanocomposites measured by a parallel plate

rheometer at 1908C.

180 M. M. KHORASANI ET AL.

Based on XRD results, exfoliated structure of clay in N2-HD comparedwith exfoliated-intercalated structure of N2-LL forms a stronger net-work, resulting in such a rheological property difference. By increasingclay concentration to 4 wt% and 6 wt% we could not distinguish betweenrheological properties of samples derived from two compatibilizers, butsamples comprising of HD-g-MA have a slightly larger quantity incomplex viscosity and storage moduli. These different effects are relatedto the degree of network-like structure formed by the nano-fllers. As aresult of structure similarity observed from XRD results, dispersedparticles in these samples are not fully modified and so, this lowmodification level leads to an intercalated morphology. Hence, they showvery low sensitivity to miscibility of compatibilizer.

By the above interpretation, we could find very good conformitybetween the structure and rheological properties. It can be a powerfulguidance to explanation of clay role in microcellular foaming process.

Crystallinity and Thermal Properties

Crystalline structure of HDPE has been affected by interference of clayand compatibilizer in nanocomposites studied. Figure 4 shows thatmelting point (Tm) increased and crystallization peak temperature (Tc)decreased for all of nanocomposites compared to those of HDPE. Meltingpoint enhancement arises from a change of lamellar ordering in thecrystallite structure; also decrease of crystallization peak temperature is

1.0E+05

1.0E+06

1.0E+04

1.0E+03

Sto

rage

mod

ulus

(P

a)

1.0E+02

1.0E+010.01 0.1 1 10

N2-HDN2-LLN4-HDN4-LLN6-HDN6-LLHDPE

Frequency (rad/s)

100 1000

Figure 3. Storage moduli of HDPE and nanocomposites measured by a parallel plate

rheometer at 1908C.

Foaming Behavior and Cellular Structure of Microcellular HDPE 181

related to the promoting effect of heterogeneous nucleation on the crysta-llization kinetics, induced by clay platelets. As seen in Figure 5, crystallinecontent of HD group nanocomposites is larger than that of HDPE and theother group has a lower content. But in both types of nanocomposites this

135

TmTc

130

125

120

115

110

105

Tem

pera

ture

(°C

)

HDPE

N2-HD

N4-HD

N6-HD

N2-LL

N4-LL

N6-LL

Figure 4. Melting point (Tm) and crystallization peak temperature (Tc) of HDPE and

nanocomposites used to microcellular processing; Data measured by DSC from sheet

samples.

70

60

50

40

30

20

10

0

Cry

stal

linity

(%

)

HDPE

N2-HD

N4-HD

N6-HD

N2-LL

N4-LL

N6-LL

Figure 5. Crystallinity of HDPE and nanocomposites used to microcellular processing;

data measured by DSC from sheet samples.

182 M. M. KHORASANI ET AL.

parameter decreases with increasing clay content. The outbreak ofheterogeneous nucleation enhances the crystalline fraction of the firstgroup of nanocomposites, although in higher clay loading, the limitingeffect of clay particles on the movement of chains helps in lowering thefraction of crystalline regions. Use of LL-g-MA has a destructive effect onthe crystallization of matrix chains due to its LLDPE backbone and so thedifference in crystallization behavior. The effect of clay increasingremains unchanged in the second group of nanocomposites too.

Cellular Morphology of Microcellular Nanocomposites

Investigation of microcellular foaming of HDPE by the high tempera-ture process route, and studying the effect of clay on the performance ofthis method and structure of microcellular foamed nanocomposites werethe objectives of this work. In Figure 6 we can see the strong effect of clayon the performance of the foaming method. By loading 2 wt% of clay,expansion ratio increased obviously and this trend continued. Foamstructure reached to over 14 vol% void content at 6 wt% clay, which is agood result for microcellular foaming by N2 gas via a batch foamingprocess. It is known that the foamability of polymers is affected by thesorption of gas in the polymer [11]. The presence of nano-fillers wasshown to promote the accumulation of gas on the polymer–particleinterface [6]. Figure 7 shows that the amount of N2 gas dissolved in allnanocomposite samples is higher than the neat HDPE. Well dispersednanocomposite samples (N2-HD, N2-LL) shown high solubility as a resultof a great polymer–particle interface, but increasing the clay up to4–6 wt% could not enhance the solubility of nanocomposites due to thepoor dispersion of clay. Even an evident solubility decrement is seen forHD group samples resulting from their high crystallinity in addition tothe poor dispersion of clay particles present in amorphous regions. It hasto be noted that N2 could be dissolved in amorphous interlamellar regionsonly. But solubility measurements were achieved at 808C and it could bepredicted that this parameter could be increased at the processtemperature (1268C), especially for nanocomposites containing higherclay nanoparticles in their composition, because in such a temperaturesome imperfect crystals would be annihilated and a larger portion ofamorphous phase exposed to the supercritical N2.

On this basis, the higher foam expansion ratio at high clay contents wasresulted from some uptake in gas absorption and hence provided moreheterogeneous nucleating sites as the effects of higher clay loading.In equal nanoparticle content, the clay dispersion state, induced bycompatibilizer type, influences the mentioned mechanisms tries to

Foaming Behavior and Cellular Structure of Microcellular HDPE 183

increase the volume expansion ratio. N2-HD and N2-LL that havedifferent nanocomposite structures, fully exfoliated and intercalatedstructure respectively, show a slightly different expansion ratio, but othersamples having intercalated structures, in fixed clay content, showsimilar expansion ratios.

Structural differences observed in nanocomposites lead to differentmicrocellular morphologies for their foams. The cellular structure of theHDPE and HDPE–clay nanocomposite foams are shown in Figure 8, and

1.12

1.14

1.16

LL group

HD group

1.1

1.08

1.06

1.04

1.02

10 1 2 3

Clay content (%)

4 5 6 7

Exp

ansi

on r

atio

Figure 6. Expansion ratio as a function of clay content in two groups of nanocomposites.

0.16

0.18

0.2

LL group

HD group

0.14

0.12

0.1

0.08

0.060 1 2 3

Clay content (wt%)

4 5 6 7

Sol

ubili

ty o

f N2

(wt%

)

Figure 7. Measured solubility of N2 in the HDPE and HDPE/clay nanocomposites as afunction of clay content.

184 M. M. KHORASANI ET AL.

Figure 8. SEM micrographs of microcellular foam of: (a) pure HDPE, (b) N2-HD,

(c) N2-LL, (d) N4-HD, (e) N4-LL, (f) N6-HD, (g) N6-LL, showing cellular morphology

obtained in each system.

Foaming Behavior and Cellular Structure of Microcellular HDPE 185

the morphological characteristics of these foams (cell density andaverage cell size) are observed in Figures 9 and 10.

At first, abrupt increase of cell nucleation in both types ofnanocomposites from that of neat HDPE took place naturally. But wecould see two opposite trends for cell nucleation by increasing clayconcentration. The cell density was found to increase linearly versus clayconcentration in nanocomposite foams of HD group, but decreasedobviously by that in the LL group of nanocomposite foams.

Cell density enhancement by increasing clay concentration as anucleating agent is a common result reported by Lee et al. [15] for

1.0E+13

1.0E+12

1.0E+11

Cel

l den

sity

(ce

lls/c

m3 )

1.0E+09

1.0E+10

1.0E+080 1 2 3

LL groupHD group

Clay content (wt%)

4 5 6 7

Figure 9. Cell density of microcellular nanocomposites.

Cel

l siz

e (µ

m)

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3

LL groupHD group

Clay content (wt%)

4 5 6 7

Figure 10. Cell size of microcellular nanocomposites.

186 M. M. KHORASANI ET AL.

HDPE–clay nanocomposites, but the second behavior needs to bediscussed. It said that using LL-g-MA as a compatibilizer in thenanocomposites leads to a weaker dispersion of clay platelets inthe HDPE matrix and so remains a series of effects that can be seenin the rheological properties and crystallinity. On the other hand, in2 wt% clay, N2-LL has a greater cell density than N2-HD although N2-HD has a higher effective particle concentration due to its claydispersion structure mentioned. We can relate this phenomenon tobetter nucleation performance of clay particles dispersed by compatibi-lizing effects of LL-g-MA. Non-wetting surface is one of the qualities forideal nucleants outlined by McClurg [3]. LL-g-MA has a low miscibilitywith the matrix and so clay particles surfaces remain nonwettedcompared to HD-g-MA. This dramatically decreases the energy barrierfor cell nucleation and increases the cell nucleation rate.

Changes in cell density reversed since here and HD group foams showa larger cell density from LL group foams. Two important proofs forcedsuch an observation. Preliminary experiments indicate that at high clayconcentration LL-g-MA is unable to disperse clay particles or enlarge thed-spacing of clay layers as well as HD-g-MA and hence, more clay stackscause lower effective particle concentration and smaller effectivenucleation sites. On the other hand, this phenomenon could be affectedby resonance effect of cell coalescence or cell wall destruction in the cellgrowth step of the foaming process. It was seen that, in the samples withhigh clay concentration, absorbed gas increased radically and so, aftersolid-state nucleation, the hydrostatic stress caused by the presence ofthe saturating gas within the polymer rises abruptly [40]. Then the cellgrowth step starts very quickly. In this step a great amount ofhydrostatic stress loads on the thinning cell walls frequently.Rheological properties of N4-HD, N4-LL, and N6-HD, N6-LL pairsshowed a little bit difference, which is not enough for the interpretationof such morphological difference. But we have to notice that this steptakes place in the solid state too, and crystallinity difference for eachpair, observed in Figure 5, can lead to a different strength for therubber-like material present in the growing cell walls. We can see thecell wall deterioration in SEM micrographs of nanocomposite foams N4-LL and N6-LL obviously. Also, according to Figure 4, these two sampleshave a shorter interval to their melting point compared to N4-HD andN6-HD and so they have a lower strength opposite to the cell growinghydrostatic tension. Finally, it can be said that, LL group samples have alooser system in the process conditions and cell coalescence happens inthis group severely. Average cell sizes (shown in Figure 10) are in aconsistent manner with the measured cell density results.

Foaming Behavior and Cellular Structure of Microcellular HDPE 187

CONCLUSIONS

In this study, for the first time microcellular foaming of high densitypolyethylene was conducted by the high temperature process method inwhich saturation process takes place in high temperature even near themelting point of polymer. This method did not show a good efficiency forpure HDPE in terms of expansion ratio and cell density, but theentrance of clay particles improved the volume expansion and micro-cellular structure by increasing the cell nucleation and N2 sorption.It was found that the state of nanoparticle dispersion affects themicrocellular morphology so that a better dispersed structure makesmore nucleation sites available. Also, nonwetted clay particle surfacesare better sites for nucleation. Crystalline morphology, obtained by eachnanocomposite system, could play an important role in cell growthmechanism of this process because solid-state nucleation was followedby a cell growth in a softened system strengthened by its degree ofcrystallinity and melting point.

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