effect of clay dispersion on the cell structure of ldpe/clay nanocomposite foams

Effect of Clay Dispersion on the Cell Structure ofLDPE/Clay Nanocomposite Foams

S.M. Seraji, M.K. Razavi Aghjeh, M. Davari, M. Salami Hosseini, Sh. KhelgatiInstitute of Polymeric Materials, Polymer Engineering Department, Sahand University of Technology,Sahand New Town, Tabriz, Iran

In this study, the effect of clay and its dispersion stateon the cell morphology and foaming behavior ofchemically crosslinked polyethylene (PE) foams wereexamined. In addition, the effect of foaming process onthe clay morphology was also considered. It wasshown that the morphology of the clay before thefoaming process and its compatibility with PE matrixplay a major role in determining the final foam proper-ties. A PE-g-MA compatibilizer was used to increasethe melt intercalation of PE onto the clay galleries andto improve clay dispersion in the PE matrix. The uni-form dispersion of clay provided greater and well-dispersed nucleation sites. This led to smaller cell size,narrower cell size distribution, and higher cell density,and lower foam density. During the foaming process,intercalated clays were delaminated due to the rapidpolymer melt expansion that inhibited gas release andincreased foam expansion ratio. POLYM. COMPOS.,32:1095–1105, 2011. ª 2011 Society of Plastics Engineers

INTRODUCTION

Crosslinked closed-cell polyethylene (PE) foams are

found in many applications such as packaging, transporta-

tion, sports, and agriculture [1, 2]. The mechanical prop-

erties of these foams are strongly dependent on the foam

density, which is a complicated function of different pa-

rameters, such as crosslinking degree, foaming agent con-

tent, and the process conditions [3–7]. It was shown that

the cell structure including cell size, cell size distribution,

and cell density play a major role in determining the me-

chanical properties of the PE foams. Therefore, many

studies on PE foams have focused on the control of the

cell structure [4–10]. The additives, generally, micron-

sized inorganic particles, were employed in several

studies as nucleating agents to induce heterogeneous

nucleation for producing a large number of nucleation

sites [11–13].

In recent years, it was shown that the nanofillers, par-

ticularly nanoclays, could improve the cellular morphol-

ogy by enhancing the nucleation rate and retarding bubble

growth in the early stage of foaming process [14–16].

Furthermore, the nanometer dimension of nanoclays was

found to be beneficial for reinforcing foamed materials,

considering the thickness of foam cell walls in the micro-

meter range [14, 17].

Over the last decade, polymer/clay nanocomposites

have gained the interest of both industries and academic

institutions, because they often exhibit remarkable

improvements in material properties when compared with

virgin polymer or conventional micro- and macro-compo-

sites [18–20]. Many studies on polymer nanocomposite

foams have used CO2 as a blowing agent [21–29]. How-

ever, there is lack of information about the polymer nano-

composite foams prepared by chemical blowing agents.

Recently, Dong-Woo Kim et al. [30] prepared lower

density EVA/EtBC/Clay (Ethylene Vinyl Acetate/Ethylene

Butene Copolymer/Clay) nanocomposite foams without

sacrificing the foams’ mechanical properties. In addition,

Velasco et al. [31] focused on foaming behavior and used

a two-step compression-molding process to produce cross-

linked LDPE/Hectorite nanocomposite foams. They stud-

ied the effect of process conditions on the density, cell

structure, and crystalline characteristics of foams.

Clay dispersion plays an important role in determining

the final properties of the PE nanocomposites. On the

other hand, the interface of PE matrix and clay platelets

is more susceptible to cell nucleation. Therefore, it can be

predicted that the dispersion state of the clay is an impor-

tant parameter in determining the properties of PE/Clay

nanocomposite foams. Because of the lack of polar groups

in the PE backbone, homogeneous dispersion of the clay

layers in PE was presumed to be impossible. To enhance

the compatibility of PE and clay, PE was grafted with a

polar monomer, such as maleic anhydride (MA), which is

commonly used [32].

Although there are a number of studies on PE/Clay

nanocomposite foams, there has been no in-depth study

on the effect of clay dispersion on the different aspects of

these foams. Therefore, the main objective of the present

Correspondence to: Dr. M.K. Razavi Aghjeh; e-mail: [email protected]

DOI 10.1002/pc.21127

Published online in Wiley Online Library (wileyonlinelibrary.com).

VVC 2011 Society of Plastics Engineers

POLYMER COMPOSITES—-2011

work was to examine the effect of clay and its dispersion

on the properties of the PE/Clay nanocomposite foams.

Furthermore, the effect of the foaming process on clay

dispersion was also studied, along with the effect of clay

on the crosslinking degree of PE foams. In the subsequent

study, the effect of clay dispersion on the mechanical

properties of these PE/Clay nanocomposite foams will be

examined.

EXPERIMENTAL

Materials

In this study, LDPE-0020 (MFI ¼ 2.0 g/10 min; 2.16

kg, 1908C) from Bandar Emam Petrochemical Company,

Iran, were used. Furthermore, Dicumyl peroxide (DCP)

as a crosslink agent from AkzoNobel (Amersfoot, the

Netherlands), maleic anhydride grafted polyethylene (PE-

g-MA); E-142 (MFI ¼ 2.0 g/10 min; 2.16 kg, 1908C;degree of grafting ¼ 1.0 wt%) from Plusspolymers Com-

pany, India, as a compatibilizer; and Azodicarbonamide

(ADCA) as a foaming agent from Foco Company, South

Korea, were used. The montmorillonite used in this study

was dimethyl dioctadecyl ammonium modified montmo-

rillonite (CloisiteTM 15A) obtained from Southern Clay

Products, TX.

Sample Preparation

The compositions and corresponding codes of the dif-

ferent compounds prepared in this study are listed in

Table 1.

Preparation of Foamable Nanocomposites. MMT was

dried in a vacuum oven at 808C for 24 h to remove the

absorbed moisture prior to mixing. The nanocomposites

and reference samples were prepared using an internal

mixer (Brabender W50EHT) with a rotor speed of 60 rpm

at a temperature of 1208C. They were prepared using a

one-step method, unless otherwise specified. In this

method, different samples were prepared using different

feeding orders. The obtained compounds were marked as

PN-C-AD, PN-MA-C-AD, and PN-MA-AD-C (Table 1).

For example, in preparing PN-MA-AD-C, the PE and PE-

g-MA were first filled simultaneously and ADCA and

DCP were added to the melt mixture after 3 and 5 min,

respectively. After another 5 min, the clay was then incor-

porated to the mixture and compounding was continued

for up to 20 min.

To study the effect of process temperature on clay dis-

persion, and hence, its effect on the foam properties, a

foamable nanocomposite sample was prepared using a

two-step method as follows: Nonfoamable PE/PE-g-MA/

Clay nanocomposite (master batch) was first prepared at

1708C in the same internal mixer and discharged from the

chamber, cooled in air, and then ground. The grinded

master batch was filled into the mixer chamber at 1208C,

and then ADCA and DCP were incorporated into the

melted master batch and compounding was continued

until 20 min. This sample was marked as PN-MA-C-AD

(MB). To determine the effect of clay, two different

foamable composites without clay were similarly prepared

with and without PE-g-MA, and marked P-MA-AD and

P-AD, respectively. To prepare the foamable samples, the

chamber temperature was set at 1208C to inhibit ADCA

and DCP decomposition prior to compression molding.

For all the samples, the clay content and PE-g-MA/Clay

weight ratio were kept constant at 3 and 5 wt%, respec-

tively. The ADCA and DCP contents were also kept

constant at 10 and 1 phr, respectively, based on PEþPE-

g-MA.

Preparation of Nanocomposite Foams. All the foam-

able samples were first compression-molded at a tempera-

ture of 1308C, and then the temperature was raised to

1658C and held for 5 min for precrosslinking of the

samples. Subsequently, the temperature was increased to

2058C and held for another 10 min. Then, the mold was

opened at the same temperature and the foam was

obtained.

Characterization

The degree of intercalation/exfoliation of foamable

nanocomposites and nanocomposite foams was evaluated

using X-ray diffractometry (XRD). A disk (1 mm in

thickness) of foamable nanocomposites was prepared by

compression molding at 1308C and 100 bar pressure for

2 min. In the case of foamed nanocomposites, the XRD

sample was cut from the prepared foams. The XRD pat-

tern of all the samples was obtained using TW3710 Phi-

lips X’Pert diffractometer with Cu Ka radiation. It was

scanned from 2h of 1.5–108 with a scanning speed

of 0.028 min21. The basal spacing of the silicate layer

(d-spacing) was calculated using Bragg’s equation.

TABLE 1. Compositions and the corresponding codes of different

compounds.

Code PE PE-g-MA ADCA DCP Clay

P-D 100 0 10 1 0

PN-C 100 0 0 0 3

PN-MA-C 100 15 0 0 3

PN-C-D 100 0 0 1 3

PN-C-AD 100 0 10 1 3

P-MA-D 100 15 0 1 0

P-MA-AD 100 15 10 1 0

PN-MA-C 100 15 0 0 3

PN-MA-C-D 100 15 0 1 3

PN-MA-AD-C 100 15 10 1 3

PN-MA-C-AD 100 15 10 1 3

PN-MA-C-AD(MB)a 100 15 10 1 3

a This sample was prepared using a two-step method.

1096 POLYMER COMPOSITES—-2011 DOI 10.1002/pc

The state of crosslinking was evaluated using three dif-

ferent methods: gel content measurement, differential

scanning calorimetry (DSC) studies, and rheometry. The

gel content of the samples was measured according to

ASTM D2765-90 as follows: 300 6 5 mg of the material

(initial weight) was extracted in 400 ml of boiling xylene

for 24 h. The remaining material (gel) was dried for 3 h

at 1408C in a vacuum oven and weighted. The gel content

was calculated using Eq. 1:

Gel Contentð%Þ ¼ Gel weight

Initial weight3100 ð1Þ

Following the curing process for foam production, the

thermal behavior of the foamable samples was studied

using a differential scanning calorimeter (DSC-200, F3

Maia, NETZSCH) with the same thermal program of the

compression-molding process.

Rheological studies were performed using a stress con-

trolled rheometer (MCR 301: Anton Paar) equipped with

parallel-plate geometry (diameter ¼ 25 mm, gap ¼ 1.5

mm). Frequency sweep tests were applied on the cross-

linked samples (without ADCA) at 2058C.The density of the foam samples was determined

according to ASTM D3575. The morphology of the foam

samples, i.e., the cell size and cell size distribution, were

examined using SEM analysis (Hitachi S-2400 SEM with

an electron potential of 25 kV). All the surfaces were gold

sputtered to ensure good conductivity of the electron beam,

and microphotographs were taken within a magnification

of 1003. SEM images were analyzed using Image Process-

ing software (Image J) to measure the cell size, cell size

distribution, and cell density using the following equations:

f ðxÞ ¼ 1

rffiffiffiffiffiffi2p

p exp

� ðx� lÞ2

2r2

!ð2Þ

N0 ¼ NM2

A

� �3=2qqf

� �ð3Þ

where l and r2 are the average cell size and variance,

respectively. N0, A, N, M, q, and qf are the cell density in

unit volume, SEM micrograph area, the number of cells

in area A, magnification of the SEM micrograph, polymer

density, and foam density, respectively [33]. Further study

of cell morphology was conducted using Image J software

and quantitative stereology method proposed by Rhodes

and Khaykin [34]. Stereological parameters considered in

this study are listed in Table 2.

To evaluate the dispersion state of the clay via rheo-

logical measurements, frequency sweep tests were per-

formed on the foamable nanocomposites and reference

samples in the range of 0.1–500 s21 at a temperature of

1308C and with amplitude of 1%, to maintain the

response of the materials in the linear viscoelastic regime,

using the same rheometer.

RESULTS AND DISCUSSION

XRD Results

The XRD patterns of clay reflect the ordered arrange-

ment of silicate layers. The penetration of polymer into

clay interlayer (intercalation) results in an increase in the

d-spacing and a shift in XRD peaks toward lower angles.

A further shift to lower angles and the broadening or

disappearance of characteristic XRD peaks indicate par-

tial or complete exfoliation of the ordered clay structure

[35].

Figure 1 shows the XRD spectra of neat clay along

with two different foamable nanocomposites with and

without PE-g-MA (PN-C-AD and PN-MA-C-AD),

and two different nonfoamable nanocomposites with and

without PE-g-MA (PN-C and PN-MA-C). The XRD trace

of the Cloisite 15A clay exhibited bimodal basal reflection

TABLE 2. Stereological parameters considered in this study [34].

PT Total no. of test points PT ¼ m2

PH Sum of hit points

PI Sum of the intersection points in the test area

VV Volume density, volume of the features per

unit test volume Vv ¼ PH

m2

SV Surface density, surface area of the features

per unit test volume Sv ¼ 2PI

m2dS/V Specific surface, surface area of features per

unit test volume of the featuresS

V¼ 2PI

PHd

L3 Mean intercept length of the features, MILL3 ¼ 2PHd

PI

k Mean free distance between features MFDk ¼ L3ð1� VVÞ

Vv

D Mean diameterD ¼ 3PHd

PI

m is the number of grid lines in horizontal and vertical directions.

d is the distance between two test points.

FIG. 1. XRD patterns of different foamable nanocomposites and neat

clay. [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com.]

DOI 10.1002/pc POLYMER COMPOSITES—-2011 1097

at 2h of 2.918 and 7.308, representative of 3.1 and 1.2 nm

d-spacing, respectively. Compounding of the clay with

LDPE (PN-C) did not shift the clay (001) peak to lower

angles, showing that the diffusion of the PE molecules

onto the clay galleries did not occur. The clay diffraction

peak for PN-MA-C appeared at lower angles (2h ¼ 2.478)than that of the neat clay and PN-C sample. This is indic-

ative of intercalation of PE-g-MA and/or PE molecules

onto the clay basal spacing [32]. A comparison of the

XRD patterns of the samples with and without ADCA

and DCP (PN-C with PN-C-AD and PN-MA-C with PN-

MA-C-AD) demonstrated that the presence of ADCA and

DCP does not clearly change the d-spacing of the clay in

both cases, with and without compatibilizer. However, a

slight reduction in 2h and peak intensity may be the result

of the orientation of clay tactoids around the ADCA solid

particles due to the higher affinity of clay to ADCA than

PE.

Figure 2 shows the XRD patterns of three different

foamable samples with the same compositions, but with

different feeding orders. The results show that the feed-

ing order has no distinct effect on the intercalation of the

PE molecules onto the d-spacing of clay. However, using

the two-step method and/or feeding of the clay before

the addition of ADCA resulted in better clay dispersion.

The lower peak intensity of PN-MA-AD-C sample may

also be due to the clay orientation around the ADCA

particles.

Figure 3 illustrates the comparison of the XRD spec-

tra of different nanocomposite foams (after foaming

process). A comparison of the results of this figure with

those presented in Figs. 1 and 2 (before the foaming

process) suggests that the foaming process has an influ-

ential effect on the increase in layer spacing as well as

dispersion of the clay platelets. This effect is found to

be more obvious for the PN-MA-C-AD (MB) sample.

The characteristic peak of clay disappeared after the

sample underwent the foaming process. The foaming

process and its effect on clay dispersion can be

described as follows:

The gas produced via decomposition of ADCA is dis-

solved in a polymer melt under high pressure. A portion

of the gas can also diffuse to the clay interlayer before

the foaming process, particularly in samples with a higher

degree of intercalation. When the pressure is removed

rapidly (mold is opened), the gas phase nucleates and

starts to grow because of thermodynamic instability. This

growth allows the polymer melt to expand. Rapid expan-

sion of polymer melt induces an effective elongation flow

field on the clay platelets, leading to an increase in layer

spacing. Delamination of clay layers induces a vacuum

between the clay interlayer, which forces the polymer

melt to diffuse into the clay interlayer. Greater interaction

between the matrix and clay platelets results in increased

layer spacing. It should be noted that after pressure is

removed, the expansion of part of gas located in the clay

interlayer can induce a positive pressure force that intensi-

fies the increase in d-spacing of the clay, leading to its

exfoliation. Exfoliated nanoclays can provide larger sur-

face area for cell nucleation.

Rheology of the Foamable Nanocomposites andReference Samples

Figure 4 shows the complex viscosity (g*) and storage

modulus (G0) versus angular frequency (x) for P-AD, P-

MA-AD, PN-C-AD, and PN-MA-C-AD foamable com-

pounds. The results show that PE-g-MA could increase

the viscosity and elasticity of the foamable compound

containing clay (PN-C-AD vs. PN-MA-C-AD). In the

compound without clay, a little increase in its viscoelastic

properties was observed with addition of PE-g-MA (P-AD

vs. P-MA-AD). Therefore, it can be concluded that the

increased viscoelastic properties using PE-g-MA for

FIG. 2. XRD patterns of different foamable nanocomposites prepared

with different feeding orders. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

FIG. 3. XRD patterns of different nanocomposite foams (after foam-

ing). [Color figure can be viewed in the online issue, which is available



samples containing clay, is mainly in the result of higher

intercalation and better clay dispersion. These results are

consistent with those of XRD studies (see Fig. 2).

Degree of Crosslinking and Foam Density

Gel content. The results of the gel content of the differ-

ent foamed samples are listed in Table 3. The presence of

clay is found to increase the gel content, both with and

without PE-g-MA (P-AD vs. PN-C-AD and PN-MA-AD

vs. PN-MA-C-AD). This may be due to the bridge effect

of clay layers between the PE molecules or gels, leading

to the creation of physical network and lower solubility

of PE in xylene. The use of PE-g-MA compatibilizer

leads to a slight decrease in the gel content (P-AD vs. P-

MA-AD and PN-C-AD vs. PN-MA-C-AD), which may be

due to the different molecular structures of PE and PE-g-

MA, and different affinity of DCP to PE-g-MA than that

of PE.

Thermal Behavior. To provide further evidence on the

observed trend in gel content and the results of rheologi-

cal studies, a DSC study was undertaken on the same

samples but without ADCA, to prevent overlapping of the

endo/exo term effects of DCP and ADCA. It should be

noted that the heating program was the same as that

applied in foam compression molding. Table 3 indicates

the results of curing heats collected from the DSC curves

for different samples. The heat of curing follows the same

trend as gel content and rheological behaviors. In other

words, the curing heats of the samples P-C-AD and

P-MA-AD have the maximum and minimum values of

curing heat, respectively.

Rheology of the Nonfoamable Nanocomposites and

Reference Samples. As the viscosity of the melt mix-

ture has a great effect on the cell structure of the foams

[36, 37], the rheological behavior of the different samples

was examined. As the density of a foam sample affects

its melt viscosity, the rheological behavior of the cross-

linked samples without blowing agent was compared

instead of crosslinked foam samples, under the assump-

tion that decomposed ADCA has the same effect on the

melt viscosity of different samples. Figure 5 shows the

complex viscosity (g*) and storage modulus (G0) versus

angular frequency (x) for P-D, P-MA-D, PN-C-D, and

PN-MA-C-D crosslinked nonfoamable compounds. The

results strongly support the findings of gel content mea-

surements and show that the presence of clay and

PE-g-MA increases and decreases the melt viscosity,

respectively.

Foam Density. The results of density measurements are

also presented in Table 3. The results show that the pres-

ence of clay may increase or decrease the foam density

depending on the dispersion state of the clay before the

foaming process. It was shown that at constant blowing

agent content, the density of conventional crosslinked PE

foams is proportional to the gel content, which is a func-

tion of crosslink density [38, 39]. This trend was not

observed in our experiments. While poor dispersion of

clay could lead to both higher gel content and higher den-

sity (P-AD vs. PN-C-AD), better dispersion of clay could

also lead to higher gel content but remarkably lower

TABLE 3. Density, gel content, and DCS results for different samples.

Sample code

Gel

content

(%)

Heat of

curing

(mw mg21)

Density

(kg m23)

P-AD 56.05 – 52 6 2

PN-C-AD 62.86 – 72 6 2

P-MA-AD 54.62 – 50 6 2

PN-MA-C-AD 61 – 44.5 6 2

P-D – 0.076 –

PN-C-D – 0.096 –

P-MA-D – 0.068 –

PN-MA-C-D – 0.082 –

PN-MA-AD-C 54.8 – 46 6 2

PN-MA-C-AD(MB) 62.62 – 42 6 2FIG. 5. Complex viscosity (g*) and storage modulus (G0) vs. angular

frequency (x) for different nonfoamable compounds (temp ¼ 2058C).

FIG. 4. Complex viscosity (g*) and storage modulus (G0) versus angu-

lar frequency (x) for different foamable compounds (temp ¼ 1308C).


density (P-MA-AD vs. PN-MA-C-AD). In this sample,

the presences of clay increased the gel content and melt

viscosity. However, better dispersion of clay in this sam-

ple, as revealed by XRD and rheological studies provided

much more interface of polymer/clay. As the polymer/

clay interface was found to be more susceptible for nucle-

ation, in the foaming stage, a large number of nucleates

were generated [14]. On the other hand, during the foam-

ing process, delaminated clay layers can act as barriers to

prevent escape of the gas, which in turn leads to greater

expansion of polymer melts and, therefore, lower foam

density. Undelaminated clay tactoids provide less nuclea-

tion sites and lower barrier effect when compared with

delaminated clay layers [27]. Thus, it can be concluded

that the increase in the density of the former sample is

related to the increased gel content and melt viscosity.

SEM Results

Effect of Compatibilizer on the Cell Structure of Dif-

ferent Nanocomposite Foams. The cell structure of PE

nanocomposite foams is strongly affected by the clay con-

tent, dispersion, and compatibility of clay with the poly-

mer matrix. To enhance the compatibility and adhesion of

FIG. 6. SEM images of different nanocomposite foams. A: P-AD, B: P-MA-AD, C: PN-C-AD and D: PN-MA-C-AD(MB).


PE and clay, a compatibilizer such as PE-g-MA is used

[32]. The presence of such a compatibilizer may affect

the cell structure, depending on its polarity, molecular

weight, and miscibility with the main matrix. Therefore,

to identify the independent effect of clay on the cell struc-

ture, the morphology of four different foam samples was

analyzed. Figure 6 shows the SEM micrographs of P-AD,

P-MA-AD, PN-C-AD, and PN-MA-C-AD foam samples.

The cell size distributions of these samples are shown

in Fig. 7, and the data on the results are presented in Ta-

ble 4. The presence of PE-g-MA decreased the average

cell size and increased the cell size distribution and cell

density. The increase in the cell density and decrease in

the average cell size may be related to increased nuclea-

tion in the presence of compatibilizer [40, 41]. The inter-

face between the compatibilizer and PE matrix was found

to have much lower activation energy for bubble nuclea-

tion, and therefore can provide more cells in unit volume,

leading to a decrease in the mean cell size [42]. On the

other hand, the different viscosities of the components

(PE and PE-g-MA) led to different bubble growth rates,

which in turn increased the cell size distribution [43]. The

results also showed that the addition of clay, without

compatibilizer, reduced the average cell size and cell size

distribution, and increased the cell density. The presence

of clay tactoids facilitated cell nucleation, leading to an

increase in cell density [10]. On the other hand, the

higher viscosity of the polymer melt in the presence of

clay inhibited cell growth, and therefore, decreased the

mean cell size and increased the foam density (Table 3).

The uniform distribution of cell size may be due to the

lower activation energy of nucleation near the clay plate-

lets. From these results, it can be concluded that the

FIG. 7. Cell size distribution of different nanocomposite foams.

TABLE 4. Morphology characteristics of different nanocomposite

foams.

Sample code

Variance

(lm2)

Cell density

(cell cm23)

Average

cell size

(lm)

P-AD 1.378E-3 5.42Eþ6 114

PN-C-AD 0.589E-3 3.05Eþ7 60.1

P-MA-A D 1.431E-3 2.89Eþ7 63.8

PN-MA-C-AD 0.793E-3 3.40Eþ7 82.8

PN-MA-AD-C 1.303E-3 1.66Eþ7 84.1

PN-MA-C-AD(MB) 0.574E-3 5.93Eþ7 56.7

FIG. 8. Schematic presentation of the mutual effect of clay dispersion and foaming process.


presence of PE-g-MA and clay could increase and

decrease the cell size distribution broadness, respectively.

The use of both PE-g-MA and clay remarkably decreased

the foam density and increased the cell density. The latter

sample had narrower cell size distribution when compared

with the foam sample that contained PE-g-MA but

broader than neat PE foam. This was observed in the

results of the competitive effects of clay and PE-g-MA on

the cell size distribution. Thus, a compatibilizer (PE-g-MA) was found to strongly enhance clay dispersion. The

dispersion of clay was found to provide greater clay/

matrix interface that is more susceptible to cell nuclea-

tion. This was observed to increase the cell density as

well. On the other hand, higher dispersion of clay was

found to lead to higher melt viscosity, which inhibited

gas diffusion through the cells, and thus led to a greater

amount of cells with small sizes.

The morphology and location of the clay in different

foam samples after the foaming process is schematically

presented in Fig. 8. The intercalation of clay before foam-

ing process led to its exfoliation during the foaming

process [PN-MA-C-AD (MB)]. This type of dispersion

FIG. 9. SEM images of different nanocomposite foams prepared with different feeding orders. A: P-AD, B:

PN-MA-AD-C, C: PN-MA-C-AD and D: PN-MA-C-AD(MB).


provided more barrier effect. Unintercalated clays were

found to be oriented around the cells during the foaming

process and exhibited lower barrier effect (PN-C-AD).

This type of clay arrangement may be the main reason

for decreased peak intensity and cannot be an evidence

for exfoliated morphology [44, 45].

Effect of Compounding Sequence on the Cell Structure

of Different Nanocomposite Foams. Figure 9 shows

the SEM images of different nanocomposite foams pre-

pared with different feeding orders beside the PE foam.

The corresponding cell size distribution of the samples

and the collected data are presented in Fig. 10 and Table

4, respectively. The results clearly show that for all the

feeding orders, the cell size is smaller and cell size distri-

bution is narrower than that of neat PE foam. Moreover,

the results show that the feeding order has an influential

effect on the cell size, cell density, and cell size distribu-

tion. These results indicate that feeding of clay before the

addition of ADCA and DCP leads to lower cell size and

narrower cell size distribution. Pre-compounding of the

clay with PE/PE-g-MA mixture at higher temperature

(1708C) was found to have a significant effect in decreas-

ing the cell size and narrowing the cell size distribution,

as demonstrated by the XRD results (see Fig. 2).

Quantitative Stereology. Figure 11 shows the grid

structure used for calculating stereological parameters for

all the samples according to Rhodes and Khaykin [34].

Table 5 shows the calculated stereological parameters for

different foam samples.

The most obvious difference between the morphologi-

cal and stereological data is the mean diameter of the

cells. The calculated mean cell sizes using stereological

method are about half of those calculated from morpho-

logical method. However, it is interesting to note that the

trend of variation of cell size with different parameters is

the same for two different methods. The former is due to

the different methods of calculating of the mean cell size

in these methods. While in morphological method the

mean diameter of different cells is directly measured

using a two-dimensional image, in stereological method

the hydraulic diameter is calculated using a three-dimen-

sional analysis. So, these differences come from the

non-spherical nature of the cells. It seems that the results

obtained using stereological analysis is more reliable than

those obtained from morphological analysis.

FIG. 10. Cell size distribution of different nanocomposite foams pre-

pared with different feeding orders.

FIG. 11. The grid structure used for calculating stereological parame-

ters. [Color figure can be viewed in the online issue, which is available


TABLE 5. Stereological parameters calculated for different foams.

Sample code PH PI VV SV (lm21) S/V (lm21) MFD (lm) MIL (lm) D (lm)

P-AD 201.333 1256.667 0.8948 0.1117 0.1245 3.9335 32.2227 48.3355

P-MA-AD 194.333 1838.333 0.8637 0.1634 0.1890 3.3786 21.2118 31.8117

PN-C-AD 197.333 2150.333 0.8770 0.1911 0.2178 2.6063 18.3901 27.5851

P-MA-C-AD 202 1423.667 0.8977 0.1265 0.1409 3.2511 28.4690 42.7035

P-MA-AD-C 202.333 1397.667 0.8992 0.1242 0.1381 3.2888 29.164 43.7467

PN-MA-C-AD(MB) 190 1972.333 0.8444 0.1753 0.2076 3.5556 19.2711 28.9067


CONCLUSION

The effect of clay and its dispersion on the foaming

behavior and cell structure of PE nanocomposite foams

were studied. The use of a proper compatibilizer, PE-g-MA, increased intercalation of clay, which in turns led to

its better dispersion in the PE matrix. Uniform dispersion

of intercalated clay led to higher cell density and nar-

rower cell size distribution, and therefore, lower foam

density, which was thought to be due to higher nucleation

extent and barrier effect of delaminated clay layers during

the foaming process.

Without the use of any compatibilizer, clay had a poor

dispersion in PE matrix. In this case, although the clay

increased the cell density due to the lower activation

energy of nucleation near the clay tactoids, the extent of

nucleation was lower than that of the one that showed

better dispersion. Lower barrier effect of the uninterca-

lated clay favored the gas escape during the foaming pro-

cess, leading to a lower expansion ratio, and therefore,

higher foam density. On the other hand, interchelated

morphology of the clay changed to exfoliated morphology

after the sample underwent foaming process and inhibited

gas escape. Therefore, it can be concluded that the pres-

ence of clay may decrease or increase the foam density,

depending on the compatibility of the PE and clay, as

well as the dispersion state of the clay before the foaming

process.

Furthermore, it was shown that feeding order strongly

affected the clay dispersion and cell structure of the pro-

duced foam. Feeding of clay before the addition of ADCA

and DCP resulted in better dispersion, lower gel content,

and therefore, lower foam density. Pre-compounding of

the clay with PE and PE-g-MA in higher temperatures

before compounding with ADCA and DCP improved the

dispersion of clay and led to the lowest foam density. The

foaming process strongly affected the clay morphology.

Intercalated nanocomposites with better interaction of PE/

Clay were exfoliated during the foaming stage, which pro-

vided more barrier effect to gas escape, resulting in higher

expansion ratio and lower foam density.

The calculated mean cell size using 3D stereological

method were about half of those calculated from 2D mor-

phological method, but the results of stereological method

supported the morphological data in terms of the trend of

variation of cell size with different parameters.

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effect of clay dispersion on the cell structure of ldpe/clay nanocomposite foams

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