Clay-containing Polymer Nanocomposites || Foam Processing

11Foam Processing

11.1 IntroductionFor several decades, polymeric foams have been widely used as packing materials,

because they are lightweight, have a high strength/weight ratio, have superior insulating

properties, and exhibit high energy-absorption. Polymeric foams can be obtained using

several methods, and thermoplastic-based foams, in particular, are commercially

produced using one of the following three techniques [1]:

1. Direct extrusion, where a foam is directly obtained at the exit of an extrusion die,

normally with the use of a physical blowing agent, such as CO2 or N2.

2. A batch process, where a previously compounded polymeric material is foamed inside

an autoclave reactor using a high-pressure gas dissolution process.

3. During molding, where a previously extrusion-compounded thermoplastic-based

material is foamed inside a mold, such as compression or injection molding, and

temperature and pressure are simultaneously applied to gradually cross link, when

necessary, and foam the material.

With the last method, thermoplastic foam is usually obtained through the use of chemical

blowing agents. Whatever the foaming technique, it is important that the obtained foams

exhibit a closed-cell-type structure with thin polymer cell walls covering each cell. To

obtain a closed-cell cellular structure, the cell growthmust be perfectly controlled through

regulation of both the decomposition temperature of the chemical foaming agent and the

melt viscosity of the polymer. If the temperature is excessively high, the rate of decompo-

sition of the foaming agent will be rapid, and the melt strength of the polymer will be low,

resulting in coalescence and cell rupture; if the temperature is excessively low, not only

will the rate of decomposition of the foaming agent decrease and require longer foaming

times, the melt viscosity and strength of the base polymer will also be significantly higher,

restraining cell growth and resulting in only partially foamed products.

In the case of PNCs, the dispersed clay platelets not only alter the foaming character-

istics and expansion behavior of closed-cell foams but may also result in globally smaller

and more isotropic cells, by enhancing the thermomechanical properties of the foamed

material with respect to the neat polymer foam [2]. Moreover, dispersed clay platelets

may increase the melt strength of the base polymer, mainly during cell wall formation

and stretching, and stabilize the whole cell structure and limit cell coalescence. They

may also act as nucleating agents for bubble generation in foams in either a batch process

or direct extrusion [2, 3] where CO2 is used as a physical foaming agent. In both cases,

the authors concluded that small amounts of clay nanoparticles greatly reduced the cell

size of the foams and increased the cell density.

Clay-Containing Polymer Nanocomposites: From Fundamentals to Real Applications

© 2013 Elsevier B.V. All rights reserved.351

352 CLAY-CONTAINING POLYMER NANOCOMPOSITES

11.2 Preparation, Characterization, and Properties ofNanocomposite FoamsLinear polyolefins, such as neat PP, have some limitations in foam processing because

such polymers do not demonstrate a high strain-induced hardening, which is the primary

requirement to withstand the stretching force experienced during the latter stages of bub-

ble growth. The branching of polymer chains, grafting with another copolymer, or the

blending of branched and linear polymers are the common methods used to improve

the extensional viscosity of a polyolefin to make it suitable for foam formation. PPNCs

(clay-containing nanocomposites of PP) have already been shown to exhibit a high mod-

ulus and, under uniaxial elongation, a tendency toward strong strain-induced hardening

[4, 5]. On the basis of these results, Okamoto et al. [6, 7] first conducted foam processing of

PPNCs with the expectation that PPNCs would provide advanced foams with desirable

properties. They used a physical foaming method, that is, a batch process, to conduct

foam processing. This process consists of four stages: (a) saturation of CO2 in the sample

at the desired temperature, (b) cell nucleation when the release of CO2 pressure begins

(forming supersaturated CO2), (c) cell growth to an equilibrium size during the release

of CO2, and (4) stabilization of the cell via a cooling process of the foamed sample.

Figure 11.1 illustrates the autoclave setup used in their study. Figure 11.2 shows SEM

images of the fracture surface of PP-g-MA and various PPNC foams at various

Pressure gauge

Autoclave

Band heater

CO2 gas cylinderCooling water jacket

Sample

FIGURE 11.1 Autoclave setup. Source: Reproduced from

Nam, Maiti, Okamoto, Kotaka, Nakayama, Takada,

Ohshima, Usuki, Hasegawa, and Okamoto [6] by

permission of the Society of Plastics Engineers.

7.5C

lay

con

ten

t (

wt

%)

200 µm

130.6� C 134.7� C 139.2� C 143.4� C

4

2

0

Temperature (8C)

FIGURE 11.2 Scanning electron microscopy images for PP-g-MA and various PPCNs foamed at different

temperatures. Source: Reproduced from Nam, Maiti, Okamoto, Kotaka, Nakayama, Takada, Ohshima, Usuki,

Hasegawa, and Okamoto [6] by permission of the Society of Plastics Engineers.

Chapter 11 • Foam Processing 353

temperatures. The SEM images indicate closed-cell structures with cells that exhibit 12- or

14-hedron shapes, with the exception of PPNC4 and PPNC7.5 foams prepared at 130.6�C.The faces of the formed cells are mostly pentagons or hexagons, which express the most

energetically stable state of the polygon cells. The distribution function of cell sizes cal-

culated from the SEM images is shown in Figure 11.3. Based on the distribution curve,

the PPNC7.5 sample exhibited a bimodal distribution of cell sizes, whereas the other sam-

ples exhibited a Gaussian distribution. Another observation from Figure 11.3 is that the

width of the distribution peaks, which represent the polydispersity of the cell size,

became narrower with the addition of clay into the matrix (PPNC2 and PPNC4). This

behavior may be due to the heterogeneous clay acting as sites for cell nucleation, and

their uniform dispersion in the matrix, which, if present, leads to homogeneity in cell

size. However, the cell size of the prepared foams gradually decreases with increasing

clay content in the PPNCs. This behavior is due to the intrinsically high viscosity of

the materials with increased clay loadings. In contrast, the cell density of the foams

0 20 40 60 80 100 120 140 160 180 200

PP-MA134.7� C

Cell size / µm

40

30

20

10

0

18

12

6

0

12

8

4

0

12

8

4

0

PPCN2134.7� CF

ract

ion

/ %

PPCN4134.7� C

PPCN7.5134.7� C

FIGURE 11.3 Typical example for cell size distribution of foamed PP-MA and various PPCNs in experiment at

134.7°C. Averagevaluesd inmmandvariancessd2 inmm2 intheGaussianfit throughthedataare122.1and12.1 forPPMA

foam,95.1and9.8forPPCN2foam,64.4and3.1forPPCN4foam.Source:ReproducedfromNam,Maiti,Okamoto,Kotaka,

Nakayama, Takada, Ohshima, Usuki, Hasegawa, and Okamoto [6] by permission of the Society of Plastic Engineering.


behave in the opposite way. The characteristic parameters of the pre- and postfoamed

samples are summarized in Table 11.1.

Figure 11.4 shows the TEM images of (a) the structure of the nanocell wall and (b) the

junction of the three cell walls in the PPNC4 foam processed at 134.7�C. Interestingly, inFigure 11.4(a), the clay particles in the cell wall align along the interface between the solid

and gas phases; that is, the clay particles arrange along the boundaries of the cells. The

figure also shows the perpendicular alignment of the clay particles to the stretching or

elongating direction, which is the main cause of the strain-induced hardening behavior

in the uniaxial elongation viscosity [4]. During foam processing, a similar structure forma-

tion occurs with a different mechanism. Because of the biaxial flow of the material during

foam processing, the clay particles either turn their faces or assume a fixed face orienta-

tion and align along the flow direction, that is, along the cell boundary. This type of align-

ment behavior in clay particles may help cells withstand the stretching force, whichmight

Table 11.1 Characteristic Parameters of Pre- and Postfoamed PPMA and Various PPCNs

Sample Tf/°C r/g mL�1 d/mm Nc/cells.mL�1.107 d/mm

PP-g-MA 0.854

PP-g-MA foam 130.6 0.219 74.4 1.8 11.88

PP-g-MA foam 134.7 0.114 122.1 0.48 9.07

PP-g-MA foam 139.2 0.058 155.3 0.25 5.56

PP-MA foam 143.4 0.058 137.3 0.35 6.46

PPCN2 0.881

PPCN2 foam 130.6 0.213 72.5 1.99 10.76

PPCN2 foam 134.7 0.113 95.1 1.01 6.76

PPCN2 foam 139.2 0.058 133.3 0.39 4.62

PPCN2 foam 143.4 0.113 150.3 0.26 10.68

PPCN4 0.900

PPCN4 foam 130.6 0.423

PPCN4 foam 134.7 0.196 64.4 2.92 8.41

PPCN4 foam 139.2 0.193 93.4 0.96 11.98

PPCN4 foam 143.4 0.341 56.1 3.52 15.08

PPCN7.5 0.921

PPCN7.5 foam 130.6 0.473

PPCN7.5 foam 134.7 0.190 35.1 18.35 4.30

PPCN7.5 foam 139.2 0.131 33.9 22.00 2.70

PPCN7.5 foam 143.4 0.266 27.5 34.2 5.11

Source: Reproduced fromNam,Maiti, Okamoto, Kotaka, Nakayama, Takada, Ohshima, Usuki, Hasegawa, and Okamoto [6] by permission

of the Society of Plastics Engineers.

A 500 nm B 500 nm

FIGURE 11.4 Bright-field transmission electron micrographs for PPCN4 foamed at 134.7°C: (a) monocell wall;

(b) junction of three contacting cells. Source; Reproduced from Nam, Maiti, Okamoto, Kotaka, Nakayama, Takada,

Ohshima, Usuki, Hasegawa, and Okamoto [6] by permission of Society of Plastic Engineering.



otherwise break the thin cell wall, and thereby help improve the strength of the foam

toward stretching. The authors suggest that the clay particles seem to act as a secondary

layer to protect the cells from being destroyed by external forces. The clay particles can

also be randomly dispersed in the central part of the junction, see Figure 11.4(b), marked

with an arrow. Such behavior of clay particles presumably reflects the effect of the stag-

nation flow region of the material under the growth of three contacting cells.

Antunes et al. [1] used the compression-molded foaming process for the preparation of

PP and its clay-containing nanocomposite foam. The main objectives of their study were

to determine the influence of foaming on the dispersion of the MMT particles within the

PP matrix and to elucidate how their delamination affects the final foam characteristics.

The authors also used a chemical foaming agent in addition to MMT.

The cellular structure andmorphology was characterized using SEM and TEM, and the

results showed that the dispersed MMT particles in the nanocomposite thermally stabi-

lized the polymer during its expansion and enlarged the foaming-temperature-processing

window. The authors found that these effects are more pronounced in the presence of a

chemical foaming agent, which indicates that its sudden thermal decomposition locally

increased the thermal gas pressure. Moreover, similar to the results in the previous report,

dispersed MMT particles globally reduced the cell size for similar foaming times and

expansion ratios and narrowed the cell size distribution, which resulted in the nucleation

of cell bubbles during the early stages of foaming and an increase in themelt strength dur-

ing cell growth. All these conclusions were supported by TEM studies.

Similar to the case of PP–clay nanocomposites, various authors have reported the prep-

aration, characterization, and properties of PE–clay nanocomposite foams [8–12]. For

example, Velasco et al. [8] reported the preparation of cross-linked LDPE–hectorite nano-

composite closed-cell foams using a two-step compression-molding process. The LDPE–

hectorite nanocomposites were prepared using the hectorite (chemically modified with

dimethyl dehydrogenated tallow ammonium chloride) master batch, part of the previ-

ously compounded LDPE (PE0) and 3 wt % (PE3) and 7 wt % (PE7) hectorite composites.

Details of the sample preparation can be found elsewhere [8].

The foam processing method used by Velasco et al. consists of a first step, where low

temperatures and constant pressure were used to gradually cross link and, at the same

time, initiate the nucleation of the gas bubbles (favored at lower temperatures) as well

as bubble growth. In a second step, higher temperatures were used to complete the expan-

sion of the already prefoamed sample. The results showed that dispersed hectorite plate-

lets not only affected the foaming process but also changed the cell size, distribution, and

shape of the foams.

Similarly, Seraji et al. [12] conducted foam processing of clay-containing nanocom-

posites of PE and tried to understand the effect of the dispersion of the clay on the

morphological and mechanical properties of the nanocomposite foams. The foamable

nanocomposite sample was prepared using a two-step process. First, a master batch of

PE–PP-g-MA–clay was prepared using an internalmixer (BrabenderW50EHT) with a rotor

speed of 60 rpm at 170�C. The prepared master batch was collected in the air and

Table 11.2 Compositions and Corresponding Codes of Various PEs and TheirNanocomposite Samples

Sample 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

aThis sample was prepared using a two-step method.

Source: Reproduced from Seraji, RazaviAghjeh, Davari, Salami Hosseini, and Khelgati [12] by permission of the Society of Plastic

Engineering.


grounded. The grounded master batch was then mixed with ADCA (azodicarbonamide,

chemical foaming agent) and DCP (dicumyl peroxide) using the same internal mixer at

120�C for 20 min. The compositions and abbreviations of various samples are summa-

rized in Table 11.2. For foam processing, samples were first compressionmolded at a tem-

perature of 130�C; the temperature was subsequently raised to165�C and maintained at

this temperature for 5 min for precrosslinking. The temperature of the compression-mold

was then increased to 205�C and held at this temperature for another 10 min. Finally, the

mold was opened at the same temperature, and the foam was obtained.

Figure 11.5 shows SEM images of various foam samples, and their cell size distributions

are presented in Figure 11.6. The average cell size, cell density, and variance of the various

foams are summarized in Table 11.3. As evident from the figures and table, the dispersed

silicate layers have a strong effect on the cell-size distribution and cell density. Moreover,

the compatibilizer PE-g-MA has also been shown to profoundly influence the cell size and

density. In the presence of PE-g-MA, the increase in the cell density and the decrease in the

average cell size may be due to the nucleation. The increased cell density and decreased

average cell size may also be due to the different viscosities of the PE and PE-g-MAmatri-

ces, which also lead to different bubble growth rates. This difference, in turn, increases the

cell-size distribution. Huang, Wang, and Sun [13] report similar behavior in the case of

PP–HDPE–clay systems.

The SEM images and cell-size-distribution figures also indicate that the incorporation

of clay, in the absence of the PE-g-MA compatibilizer, reduces the average cell size and

cell-size distribution and increases cell nucleation, which leads to an increase in cell den-

sity. This observation again supports the strong nucleation role of dispersed silicate layers

for cell growth, which is responsible for the higher cell density. Furthermore, dispersed

SEM MAG: 100 xSEM HV: 15.00 kVDate(m/d/y): 03/12/09

Det: SE DetectorWD: 26.5740 mmVac: HiVac

VEGA\\ TESCAN

RAZI200 mm



VEGA\\ TESCAN

RAZI200 mm



VEGA\\ TESCAN

RAZI200 mm

A B

C D



VEGA\\ TESCAN

RAZI200 mm

FIGURE 11.5 Scanning electron microscopic images of different nanocomposite foams: A, P-AD; B, P-MA-AD; C,

PN-C-AD; and D, PN-MA-C-AD(MB) (for compositions please see Table 11.2). Source; Reproduced from Seraji, Razavi

Aghjeh, Davari, Salami Hosseini,and Khelgati [12] by permission of the Society of Plastic Engineering.


clay particles increase the viscosity of the polymer melt, inhibit further cell growth, and,

therefore, decrease the mean cell size and increase the foam density. Finally, in the case of

nanocomposite foams, the uniform distribution of the cell size may be due to the lower

activation energy of nucleation near the clay surface.

Based on the previously discussed results, we conclude that the presence of a compa-

tibilizer (PE-g-MA) and clay could increase and decrease, respectively, the broadness of

the cell-size distribution. The use of both the compatibilizer and the clay significantly

decreased the foam density and increased the cell density. We believe such results are

related to the role of the compatibilizer to facilitate the high level of dispersion of the sil-

icate layers in the PEmatrix. The high level of dispersion of clay platelets provides a greater

polymer–clay interface area and therefore induces greater cell nucleation. Furthermore,

higher delamination of clay particles leads to a higher melt viscosity, which inhibits gas

diffusion through the cells and thus leads to a greater number of cells with smaller sizes.

On the basis of these conclusions, the morphology and location of the clay particles in

different foamsamples after the foaming process is schematically presented in Figure 11.7.

FIGURE 11.6 Cell-size distribution of various PE-based nanocomposite foams (for compositions please see

Table 11.2). Source; Reproduced from Seraji, Razavi Aghjeh, Davari, Salami Hosseini, and Khelgati [12] by permission

of the Society of Plastic Engineering.

Table 11.3 Compositions and Corresponding Codes of Various PEs and TheirNanocomposite Samples

Sample Code Variance/mm2 Cell Density/cm�3 Average Cell Size/mm

P-AD 1.378�10�3 5.42�106 114

PN-C-AD 0.589�10�3 3.05�107 60.1

P-MA-AD 1.431�10�3 2.89�107 63.8

PN-MA-C-AD 0.793�10�3 3.40�107 82.8

PN-MA-AD-C 1.303�10�3 1.66�107 84.1

PN-MA-C-AD(MB) 0.574�10�3 5.93�107 56.7

Source: Reproduced from Seraji, Razavi Aghjeh, Davari, Salami Hosseini, and Khelgati [12] by permission of the Society of Plastic

Engineering.


The intercalation of clay before the foaming process led to its higher exfoliation of silicate

layers during the foaming process, refer to Table 11.2. Such a process leads to a greater

barrier effect. In contrast, unintercalated clays were found to be oriented around the cells

during the foaming process and exhibited a reduced barrier effect (PN-C-AD, Table 11.2).

Like Seraji et al.[12], Zeng et al. [2] arrived at the same conclusion in the case of PS/clay

and PMMA/clay nanocomposite foams: the improvement in the properties of polymer

nanocomposite foam is directly related to the degree of dispersion of the silicate layers

in the nanocomposite. In their study, Zeng et al. [2] synthesized a series of PS and PMMA

Unintercalated clay

Foaming process

Foaming process

Intercalated clay

FIGURE 11.7 The mutual effect of clay dispersion and foaming process. Source; Reproduced from Seraji, Razavi

Aghjeh, Davari, Salami Hosseini,and Khelgati [12] by permission of the Society of Plastic Engineering.


nanocomposites using an in situ polymerization method. Two types of organically

modified MMTs, such as C20A and MHABS (2-methacryloyloxyethylhexadecyldimethy-

lammonium bromide-modified MMT), were used for the preparation of nanocomposites

with PS and PMMA. They also used a two-stage method for the preparation of PS and

PMMA nanocomposites. For example, a PS–clay (20 wt %) master batch was prepared

using in situ polymerization of styrene in the presence of an organoclay. The master

batch was then diluted in PS to prepare nanocomposites with the desired clay content

using a microcompounder at 200�C and 250 rpm. In the case of PMMA–MHABS com-

posites, the free PMMA was first extracted from the PMMA–MHABS composite using

soxhlet extraction (dichloromethane as solvent). The dried unextracted portion was then

melt-blended with PS to prepare a PS– (MHABS–PMMA) composite. The (PS–MHABS) –

PMMA composite was also prepared by melt-blending PS–MHABS and PMMA. These

two materials have the same weight composition (PS–PMMA–MHABS¼86:9:5). Based

on XRD and TEM observations, refer to Figure 11.8, the authors claimed that exfoliated

PS and PMMA nanocomposites were prepared using a two-stage processing method.

However, we do not agree with the interpretation of Zeng et al. [2]. TEM images clearly

indicate the formation of highly intercalated nanocomposites; however, an improvement

in the silicate layers’ delamination was clearly observed when a two-stage processing

technique was used.

Zeng et al. [2] conducted foam processing of various PS and PMMA nanocomposites

via a batch process using CO2 as a foaming agent, similar to the method of Okamoto

et al. [6]. The foam cell morphologies of neat polymers and those of their nanocomposite

samples are presented in Figure 11.9. The morphological analysis shows that dispersed

clay particles undoubtedly serve as a heterogeneous nucleation agent and strongly affect

the cell size and density. In the case of highly intercalated nanocomposites, such as

100 nm

100 nm

100 nm

100 nm

100 nm

A B

C

E

D

FIGURE 11.8 Bright-field transmission electron microscopy images of various nanocomposites. (a) PS–C20A

(5 wt%), (b) PS–MHABS (20 wt %), (c) PS–MHABS (5 wt %, after extrusion and injection molding), (d) PMMA–C20A

(5 wt %), (e) PMMA–MHABS (5 wt %). Source; Reproduced from Zeng, Han, Lee, Koelling, and Tomasko [2] by

permission of Wiley-VCH, Germany.


PS–C20A or PMMA–C20A, most clay particles exist as tactoids. In the case of delaminated

nanocomposites, such as (PS-MHABS) –PMMA and PS– (MHABS–PMMA) nanocompo-

sites, clay particles exist as stacks of a few layers. The distance between the stack layers

is usually higher than the effective radius of gyration of a polymer chain. Therefore, many

more clay platelets are in contact with polymer chains and CO2, and this increased contact

FIGURE 11.9 Scanning electronmicroscope images of various foams. (a) neat PS, (b) PS–C20A (5wt%), (c) PS–MHABS

(20 wt %), (d) neat PMMA, (e) PMMA–C20A (5 wt %), (f) PMMA–MHABS (5 wt %), (g) (PS–MHABS) –PMMA,

(h) PS–(PMMA–MHABS). (a–c, g, h) Magnification 400�, scale bar 50 mm; (d–f) Magnification 1600�, scale bar 20 mm.

Source; Reproduced from Zeng, Han, Lee, Koelling, and Tomasko [2] by permission of Wiley-VCH, Germany.


provides a significantly greater interfacial area for CO2 absorption and cell nucleation.

Therefore, the effective particle concentration is substantially higher, which results in a

higher nucleation rate and, ultimately, a higher cell density. When more cells nucleate,

a similar amount of gas is available for bubble growth, which leads to a reduction of cell

size [2]. In another report, Wee, Seong, and Youn [14] conducted foam processing of PS–

clay nanocomposites and arrived at the same conclusions. In a recent report, Ngo et al.

[15] attempted to fine-tune the morphology and density of PS–clay nanocomposite foam

cells by improving the interfacial interaction between the PS matrix and an organically

modified clay surface.

A number of articles have been published on the structure and properties of various

types of PNC foams [16–27]. For example, in the case of rigid foam PVC–clay nanocom-

posites, Alian and Abu-Zahra [21] found that the specific compressive strength, flexural

modulus, and density of PVC were improved by nanocomposite formation with clay.

The tensile strength and modulus of elasticity of PVC had shown slight deterioration after


nanocomposite formation, whereas the impact strength and specific flexural strength

showed no significant changes in the presence of the nanoclay. Choongee and Naguib

[20] developed a constitutive model for tensile behavior of PMMA–clay nanocomposite

foams and elucidated the effects of intercalated and agglomerated nanoclays. As was

the case with PNCs, the authors also considered the polymer–filler interaction for the

development of this model, and the proposed properties were explained in terms of

the morphologies of the dispersed clay, including the aspect ratio and the distance

between dispersed clay particles. To validate their model, Choongee and Naguib [20] con-

ducted foam processing of various PMMA nanocomposites that contained 0, 0.5, 1, and 2

wt % clay via a batch process using CO2 as a foaming agent; they also performed tensile

tests on the foam samples, refer to Figure 11.10. The results showed that the tensile

00

10

20

30

40

50

1 2 3

2.0%

1.0%0%

0.5%

Strain (%)A

B

Str

ess

(MP

a)

4 5 6

00

10

20

30

40

50

1 2 3

2.0%1.0%0%

0.5%

Strain (%)

Str

ess

(MP

a)

4 5 6

FIGURE 11.10 Tensile stress–strain scans of PMMA–clay nanocomposite foams containing 0, 0.5, 1, and 2 wt % clay

loadings: (a) experimental curves and (b) theoretical curve obtained by determining the material parameter t

properly, using the proposed constitutive model. Detailed theory can be found in ref. [20]. Source; Reproduced from

Choonghee, and Naguib [20] by permission of the Society of Plastic Engineering.


modulus, the strength, and the elongation at break were improved at a clay loading of

0.5 wt %. Clay loadings greater than 0.5 wt % led to decrease tensile properties because

of the agglomeration of clay particles in nanocomposite foams at clay loadings greater

than 0.5 wt %. If the material parameter t is properly determined, the proposed cons-

titutive model can predict the tensile behavior of the intercalated and agglomerated

PMMA–clay nanocomposite foams. Detailed theory can be found elsewhere [20].

Fujimoto et al. [28] report, for the first time, the preparation and characterization of

neat PLA and its clay-containing nanocomposite foams with structures that ranged from

microcellular to nanocellular. The foams were prepared via a batch process. This report

was the first to elaborate the possibility of preparing biodegradable nanocellular poly-

meric foams via nanocomposite technology. The authors investigated the effect of the clay

dispersion in the PLA matrix on the cellular dimension and density of the foam. In a typ-

ical preparation process, PLA–MMTC18 and PLA–MMT 3C18 (whereMMTC18 andMMT3C18 represent MMTmodified with octadecylammonium and octadecyltrimethylammo-

nium cations, respectively) nanocomposite sheets, which had been previously prepared

through melt mixing and isothermally crystallized, are cut into 2 cm�2 cm pieces and

placed in an autoclave connected to a CO2 cylinder. The samples are then saturated with

CO2 at elevated temperatures (140–165�C) and pressures (�10 MPa). The samples are

thenmaintained under these conditions for 2 h. Subsequently, the CO2 is quickly released

from the autoclave (within 1 s). After the CO2-saturated neat PLA and nanocomposite

sheets are removed from the autoclave, the samples are immersed immediately in a sil-

icon–oil bath maintained at the desired temperature for a fixed time (30 s, known as

the foaming time). The foamed samples are next quenched in an ethanol–water (1:1) mix-

ture, washed in ethanol for at least 30 min, and dried under vacuum at 30�C for 48 h to

remove traces of ethanol and water. The two clay types used had different organo-

modifiers, their dispersion was consequently different: PLA–MMT C18 showed well-

ordered, intercalated, and flocculated nanocomposite structures; and MMT 3C18 showed

a disordered intercalated structure, which revealed a relatively better distribution of the

clays. Indeed, the effective distances between the stacks in PLA–MMT 3C18 were signifi-

cantly greater than those in PLA–MMTC18, which suggests that intercalated silicate layers

aremore homogeneously and finely dispersed in the case of PLA–MMT 3C18 than in PLA–

MMTC18. Homogeneous cells were formed in the case of nanocomposite foams, whereas

neat PLA foams showed a nonuniform cell structure with large cell size, refer to

Figure 11.11. The foam cell size became smaller after the nanoparticles were added.More-

over, better dispersions of the clays in the PLA matrix resulted in smaller foam cell sizes

and greater cell densities. According to the authors, this result was due to nucleation of the

foams by the silicates. The authors claim that the incorporation of nanoclay induced het-

erogeneous nucleation because the activation energy barrier was lower than that in

homogeneous nucleation. Similarly, Di et al. [29] showed that cell density increases with

an increase in clay loading. Ema, Ikeya, and Okamoto [30], however, demonstrated that

processing conditions strongly affect the morphology and properties of PLA–clay nano-

composite foams.

Neat PLA PLA/C18MMT5

PLA/qC18MMT5

A B

C

FIGURE 11.11 SEM images of freeze–fracture surface of (a) neat PLA, (b) PLA–C18 MMT5, and (c) PLA–qC18 MMT5

foams. Source; Reproduced from Fujimoto, Sinha Ray, Okamoto, Ogami, Yamada, and Ueda [28] by permission of

Wiley-VCH, Germany.


11.3 ConclusionsIn summary, the structure, morphology, and properties of PNC foams can be controlled

through adjustment of the foaming conditions, clay loading, and the interaction between

the polymer and the dispersed clay particles. Cell-nucleation efficiency, which is affected

by the size, shape, and distribution of the clay platelets, could be improved considerably if

an exfoliated-type nanocomposite structure is achieved, because finer particles reduce

the nucleation energy for the growth of the gaseous phase. At the same time, an

exfoliated-type structure and particle orientation during foaming due to preferred poly-

mer stretching would locally increase the melt strength of the base polymer and thereby

allow a the growth of larger cells without cell wall rupture or cell coalescence. In addition

to the fact that the particles act as a reinforcing agent, this approach could expand

the range of properties of these materials and create mechanically improved foams.

Another important issue has emerged from studies, indicating that supercritical CO2

not only serves as a processing–foaming agent but apparently plays a role as a clay disper-

sion agent.


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Clay-containing Polymer Nanocomposites || Foam Processing