Material Properties

Morphological, thermal and tensile properties of halloysite nanotubesfilled ethylene propylene diene monomer (EPDM) nanocomposites

H. Ismail*, Pooria Pasbakhsh, M.N. Ahmad Fauzi, A. Abu BakarSchool of Materials and Mineral Resources Engineering, Department of Polymer Engineering, Universiti Sains Malaysia, Engineering Campus,14,300 Nibong Tebal, Penang, Malaysia

a r t i c l e i n f o

Article history:Received 1 May 2008Accepted 13 June 2008

Keywords:Halloysite nanotubesTransmission electron microscopyEthylene propylene diene monomerTensile propertiesThermal stability

a b s t r a c t

A novel ethylene propylene diene monomer/halloysite nanotubes (EPDM/HNT) nanocom-posite was prepared by mixing 0–100 parts per hundred rubber (phr) of HNTs with EPDMon a two-roll mill. The results obtained show that the tensile strength, elongation at break,tensile modulus at 100% elongation (M100) and crosslink density were tremendouslyincreased with increase of HNT loading. The thermal and flammability properties of nano-composites were enhanced, especially at HNT loading higher than 15 phr. Morphology ofthe fractured surfaces of EPDM/HNT nanocomposites were studied by SEM and TEM.The morphological study revealed that homogenously dispersed HNTs inside the EPDM,the interfacial and inter-tubular interactions between HNTs and EPDM as well as the for-mation of HNTs’ zig-zag structures, especially at high HNT loading, were the main reasonsfor the significant improvement in mechanical and thermal properties of EPDM/HNTnanocomposites.

! 2008 Published by Elsevier Ltd.

1. Introduction

The main reason for reinforcing rubbers is to improvethe mechanical and thermal properties as well as reducingcost, and sometimes weight, of the compounds. Carbonblack is a typical reinforcing material in rubbers. However,due to pollution issues and dark colour of the carbon black,several researchers have focused on development of otherreinforcements in rubber composites during the last fewyears [1–6]. Since the Toyota group first demonstratedthat the modulus, tensile strength and heat distortion tem-perature of polymer nanocomposites were significantlyincreased by adding 4.2 wt% of montmorillonite, silicatelayered clays have been widely used as fillers in the rubberand plastic industries [7,8]. More recently, some studieshave reported that by adding a small amount of carbonnanotubes (CNTs), the thermal stability, flammability and

mechanical properties of polymers were strongly improved[9–11].

Against the improvement in weight, strength andthermal properties obtained in polymer layered silicatenanocomposites, the preparation process and organictreatment of nanoclays are complex and costly [12]. Lay-ered silicates need to be exfoliated to separate the layersand obtain the dispersion needed for uniform propertiesin the nanocomposite materials [6,13].

Recently, halloysite nanotubes (HNTs) have been used asa new type of filler for polymers, such as epoxy, polypropyl-ene and polyvinyl alcohol, to improve the mechanicaland thermal properties of the composites [14–18].HNTs are a kind of naturally occurring aluminosilicate(Al2Si2O5(OH)4$H2O 1:1) with hollow nanotubular struc-ture. HNTs do not require exfoliation and they can easilybe dispersed in a polymer matrix, even at high loading(>30 parts per hundred rubber (phr)). This is due totheir unique crystal structure, low density of hydroxylfunctional groups and their tubular shape [18–20].

* Corresponding author. Tel.: !60 4 5996113; fax: !60 4 5941011.E-mail address: hanafi@eng.usm.my (H. Ismail).

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Polymer Testing

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0142-9418/$ – see front matter ! 2008 Published by Elsevier Ltd.doi:10.1016/j.polymertesting.2008.06.007

Polymer Testing 27 (2008) 841–850

Ethylene propylene dienemonomer (EPDM) is an unsat-urated polyolefin rubber which has attracted much atten-tion for outdoor applications such as automotive sealingsystems, building profiles, electrical power cables, whiteside walls of tires, roofing sheets, belting and sportinggoods as well as ablative and insulator compounds usedin solid propellant rocket motors. This is due to its remark-able ability to accept high loading of fillers and its strongresistance to oxygen, ozone, UV and heat [21–25].

In the present work, thermal, physical and tensile prop-erties of EPDM nanocomposites filled by halloysite nano-tubes were studied. The morphological characteristics ofEPDM/HNT nanocomposites and their reinforcing mecha-nism were also examined.

2. Experimental

2.1. Materials

The halloysite nanotubes, (ultrafine) were supplied byImerys Tableware Asia Limited, New Zealand. The elemen-tal composition of HNTs is as follows (wt%): SiO2, 49; Al2O3,34.8; Fe2O3, 0.35; TiO2, 0.12; Na2O, 0.25; MgO, 0.15 [26].After 24 h drying at 80 "C in an oven, the density of HNTswas found to be 2.136 g cm#3, measured by aMiromeritices,Accupyc 1330 (gas pycnometer). The EPDM Keltan, 778Zwith ethylene content of 67% and ENB of 4.3% and ML(1!4) 125 "C of 63 MMwas used as thematrix. The densityof EPDM, measured by a Precisa (XT220A), is 0.823 g cm#3.Zinc oxide, stearic acid, sulphur, tetramethyl thiuram disul-fide (TMTD) and 2-mercapto benzothiazole (MBT) were allobtained from Bayer (M) Ltd and used as received.

2.2. Specimen preparation

The compounding of EPDM, HNT and other ingredientssuch as zinc oxide, strearic acid, MBT, TMTD and sulphurwas done on a laboratory-sized (160 mm$ 320 mm) two-roll mill, model XK-160. Table 1 shows the compositionsof HNT filled EPDM nanocomposites. The curing times(t90) of the compounds were obtained by using a MonsantoMoving Die Rheometer (MDR 2000) at 150 "C. The com-pounds were subsequently compression moulded at150 "C, based on the respective t90 values.

2.3. Measurements

2.3.1. X-ray diffraction analysis (XRD)The XRD patterns of HNTs and EPDM/HNT nanocompo-

sites with addition of 5 phr and 50 phr of HNT were

recorded by a Bruker Axs model D8 diffractometer. Thebasal spacing of the halloysite nanotubes before and afterblending with EPDM was calculated by using Bragg’s law.The Cu Ka (l% 1.54060 Å) was operated at 40 kV and40 mA in combination with a Ni filter. The samples werescanned from 2q% 5–30".

2.3.2. Scanning electron microscopy (SEM)The morphologies of HNTs and tensile fracture sur-

faces of EPDM/HNT nanocomposites were observed undera Supra-35VP field emission scanning electron micro-scope. Before being examined, the HNTs were stirredwith methanol to prepare a nonaggregated sample andthe nanocomposites were coated with a thin layer ofPd–Au, to prevent electrostatic charging during evaluation.

2.3.3. Transmission electron microscopy (TEM)A transmission electron microscope, Philips CM12

(100 KV acceleration voltage) was used to study the nano-tubular shapes of the HNTs and their dispersion inside theEPDMmatrix. The halloysite was suspended in ethanol andafter 1 min it was pipetted from the suspension and a drop-let was placed on a carbon thin film coated 400 mesh cop-per grid. To observe the EPDM/HNT naocomposites, ultrathin specimens were prepared using a cryogenic ultramicrotome Leica–Reichert Supernova.

2.3.4. Measurement of tensile propertiesCompression molded sheets having a thickness of about

2 mm were punched to prepare the dumb-bell shapedspecimens for tensile testing. Tensile testingwas conductedfollowing ISO 37 using a universal tensile testing machineInstron 3366 at room temperature (25& 2 "C) and ata crosshead speed of 500 mm/min. Tensile modulus at100% elongation (M100), tensile strength and elongationat break (Eb) were recorded.

2.3.5. Crosslink densityThe crosslink density of specimenswasmeasured on the

basis of the rapid solvent-swelling measurements (tolueneuptake for 72 h at 25 "C) by applying the Flory–Rehnerequation [27]:

Mc %h# rPVsV1=3

r

i.hln'1# Vr( ! Vr ! cV2

r

i(1)

Vr % 1='1! Qm( (2)

where Mc is the molecular weight, r is the density of theEPDM (0.823 g cm#3), Vs is the molar volume of the solvent(107.0 ml/mol for toluene), Vr is the volume fraction of theswollen rubber,Qm is the swelling weight of the EPDM/HNTcompounds in toluene and c is the interaction coefficientbetween the rubber network and solvent (0.49) which iscalculated by Eq. (3). The degree of crosslink density (V)is given by Eq. (4):

c % 'ds # dr(V0=RT (3)

V % 1='2Mc( (4)

Table 1Compositions of the HNT filled EPDM nanocomposites

Composite (phr) C.1 C.2 C.3 C.4 C.5 C.6 C.7 C.8

EPDM 100 100 100 100 100 100 100 100HNT 0 5 10 15 30 50 70 100ZnO 5 5 5 5 5 5 5 5Stearic acid 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5MBT 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8TMDT 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Sulphur 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

H. Ismail et al. / Polymer Testing 27 (2008) 841–850842

where ds and dr are the solubility parameters of the solventand rubber network, respectively. R is the universal gasconstant (8.314 J/mol K) and T is the absolute temperature.

2.3.6. Thermal gravimetric analysis (TGA) and flammabilityTGA analysis was performed using a Perkin–Elmer Pyris

6 TGA analyser from 25 "C to 600 "C at a heating rate of20 "C/min under nitrogen atmosphere.

The UL-94 vertical burning test was carried out usinga Plastics HVUL horizontal vertical flame chamber (AtlasFire Science Products, Chicago, USA). The compressionmolded specimens were punched to prepare127$12.7$ 3 mm3 sheets. The methane tank pressureregulator was set to 20 psi. All test bars underwent twotrials; each trial consisting of ignition for 10 s. After 10 sthe flamewas removed and the ignition timewas recorded.The burning behaviour was also observed. The burningbehaviour was classified according to ISO 1210.

3. Results and discussion

3.1. X-ray diffraction analysis

XRD patterns of the HNTs, EPDM/HNT (5 phr) andEPDM/HNT (50 phr) are displayed in Fig. 1. The values of2q" and their relative basal spacing are also presented inTable 2. The HNTs show a diffraction peak at 2q% 12.19"

which is related to the 0.725 nm basal spacing for the 001peak. As shown in Table 2, (composites C.2 and C.6 with 5and 50 phr HNT loading), there were two reflection (001)peaks for HNTs which were dispersed in EPDM nanocom-posites. The first peak for C.2 and C.6 is attributed to thedisplacement of the (001) peak to lower angles at(2q% 11.37") and (2q% 11.78"), respectively. These peakswhich were located at lower angles can confirm the pres-ence of structures with limited intercalation and can beattributed to the formation of nanocomposites. Due to thenon-polar characteristics of EPDM and having little differ-ences in the basal spacing of EPDM/HNT nanocomposites(0.053 nm and 0.026 nm related to C.2 and C.6), theobserved layer expansion may be attributed to the adsorp-tion of curatives such as ZnO and stearic acid in the HNTgalleries. Apart from the first peak, another peak observedat a higher angle (2q% 12.29") and (2q% 13.76") for C.2 andC.6, respectively, might be related to some de-intercalationof the HNTs. As shown in Table 2 and confirmed by Jousseinet al. [19], halloysite-7 Å gives another two reflection peaksat 02, 11 bands and 002 plane which are related to 2qw 20"

and 2qw 24.9", respectively. As also presented in Fig. 1 andTable 2, the basal spacing of these bands and plane ofEPDM/HNT compounds at 5 phr and 50 phr of HNT loadingwere slightly increased.

3.2. Microstructure

3.2.1. Microstructure of HNTsFig. 2 illustrates the SEM and TEM micrographs of HNTs.

Apparently,most of theHNTs are tubular in shapewith typicaldimensions of 150 nm–2 mm long, 20–100 nmouter diameter

6 10 20 30

12.19°20.06°

19.24°

11.37°

11.78° 19.61°

c

b

a

Fig. 1. XRD pattern of EPDM/HNT nanocomposite: (a) HNT, (b) EPDM/HNT-5 phr and (c) EPDM/HNT-50 phr.

Table 2Diffaction pattern characteristics of HNT and EPDM/HNT nanocomposites

Specific plane (001) (020), (110) (002)

Diffaction pattern 2q" d (nm) 2q" d (nm) 2q" d (nm)

HNT 12.19 0.725 20.06 0.4423 24.92 0.357

EPDM/HNT 5 phr 11.37 0.778 19.24 0.461 24.47 0.363(Composite C.2) 12.29 0.720

EPDM/HNT 50 phr 11.78 0.751 17.90 0.495 24.37 0.365(Composites C.5) 13.76 0.643 19.61 0.453

Fig. 2. (a) SEM micrograph of HNTs and (b) TEM micrograph of HNTs.

H. Ismail et al. / Polymer Testing 27 (2008) 841–850 843

and 5–30 nm inner diameter. Beside the tubular shape, othermorphologies such as short tubular, pseudo spherical, platyand semi rolled HNTs can be seen from the TEM images.Clearly, HNTs have the same geometry as multi wall carbonnanotubes (MWCNTs), but with the advantage in comparisonto CNTs that they do not stick in the matrix, which makesthem easier to disperse in a viscous polymer [15].

3.2.2. TEM of nanocompositesThe microstructures of EPDM/HNT nanocomposites

with 10 and 100 phr of HNT loading were also observedby TEM. The observation can be reported as follows:

(1) Good dispersion of HNTs inside the EPDM: Fig. 3(a) and(b) shows the dispersion of 10 phr and 100 phr of HNTsinside the EPDM matrix, respectively. Evidently, HNTswere well dispersed in the matrix, even with 100 phrloading. The dark circular dots are believed to be theends of the HNT tubes. As reported by Levis and Deasy[20], the silica is positioned on the outer surfaces ofHNTs, whereas the alumina locates on the inner wallsand edges of the tubules. Because of the special charac-teristics of HNTs, such as straight and tubular morphol-ogy, low hydroxyl density on the surfaces, unusualcharge distribution and unique crystal structure, theHNTs can be homogenously and easily dispersed insidethe EPDMmatrix. The distinctive dispersion of HNTs inother polymer matrices has been reported by variousresearchers [15,17,20].

(2) Inter-tubular interaction: as can be seen in Fig. 4(a) and(b), the lumen structure of the HNTs can be filled by theEPDM and other ingredients. The image contrastsinside the lumen structure of HNTs are clearly seen inFig. 4(a) and (b) and this may be attributed to fulland partial penetration of materials inside the HNTs’lumen structures. The penetration of EPDM and othermaterials inside the tubes would form inter-tubularinteractions between matrix and HNTs.

(3) Interaction between HNTs: Fig. 5(a)–(c) illustrates theinteraction between the HNTs in the EPDM/HNT nano-composites with 10 phr and 100 phr HNT loading,respectively. Beside the well-dispersed HNTs inside theEPDM in Fig. 5(a), a zig-zag structure (shown by a circle)was formed by edge-to-edge and face-to-edge interac-tions between the HNTs. At high HNT loading (100 phr),as shown in Fig. 5(b) and (c), in those regions with highconcentration of HNTs, not only do they avoid agglomer-ation, but also they tend to create zig-zag structures. Con-sequently, with increasing HNT content, the amount ofzig-zag structures was increased. The creation of edge-to-edge and face-to-edge structures is directly related tothe chemical composition of HNTs.

Halloysite is chemically similar to kaolinite witha dioctahedral 1:1 layered aluminosilicate and consistsof two different interlayer surfaces [15]. On the otherhand, the chemical composition of halloysite indicatesthat HNTs contain 0.35 wt% Fe2O3 which is positionedon the inner walls of tubules. As reported by Liu et al.

Fig. 3. TEM images of EPDM/HNT nanocomposites: (a) EPDM/HNT (10 phr), (b) EPDM/HNT (100 phr) (magnification: 21,000$).

Intertubularinteractions

Intertubularinteractions

a b

Fig. 4. Inter-tubular interactions: (a) EPDM/HNT (10 phr), (b) EPDM/HNT (100 phr) (magnification: 70,000$).

H. Ismail et al. / Polymer Testing 27 (2008) 841–850844

[17] and Frost et al. [28], the inner and outer surfaces oftubules are covered by hydroxyl groups and oxygenatoms, respectively. The charge distribution, unusualcrystal shape and non-polar characteristics of EPDMare the main reasons for edge-to-edge and face-to-edge interactions between HNTs which make thezig-zag structures in the EPDM/HNT nanocomposites,particularly at high HNT loading. As reported by Levisand Deasy [20], because the inner and outer faces ofthe tubule walls normally carry net negative charges

and their edges are amphoteric with positive chargeat low pH, face-to-edge attachments occurred in aque-ous suspension below pH 6 and facilitate binding ofcations to the unreacted faces.

(4) Orientation: the circular dark dots shown in Fig. 6 rep-resent the cross sections of HNTs. The presence of thesecross sections reveals that the HNTs had a three dimen-sional orientation inside the matrix. A close look atFig. 6 also indicated that the cross sections of HNTsare not exactly circular but more oval shape. This

a b

c

Fig. 5. TEM images of zig-zag structures which is shown by a circle, formed by face-to-edge (black arrows) and edge-to-edge (white arrows) interactions betweenHNTs in: (a) EPDM/HNT (10 phr) (magnification: 22,000$), (b) EPDM/HNT (100 phr) (magnification: 35,000$) and (c) EPDM/HNT (100 phr) (magnification:22,000$).

Fig. 6. TEM image showing an oval dot which reveals the cross section of HNT.

H. Ismail et al. / Polymer Testing 27 (2008) 841–850 845

would imply that the HNTs had random dispersion inthe EPDM matrix; because the microtome knife cutthe HNTs with a tilt angle to their axis [29].

3.3. Tensile properties

Fig. 7 clearly shows that the tensile strength increasedas the loading of HNT was increased from 0 to 100 phr.

The tensile strength of EPDM nanocomposites filled with30 phr and 100 phr of HNT was 217.4% and 873.4% higherthan unfilled EPDM, respectively. In fact, at higher loadingof HNTs, the tensile strength increases significantly. Theincrease in tensile strength, especially at high HNT loading,is attributed to several factors, i.e. good dispersion of HNTsinside the EPDM, inter-tubular interactions between HNTsand EPDM, the edge-to-edge and face-to-edge interactionsbetween HNTs which make zig-zag structures, and thethree dimensional orientations of HNTs inside the nano-composites. Strong interactions allow more efficient loadtransfer and, hence, better mechanical performance, asreported by Yang et al. [30]. Beside the superior ability ofHNTs to disperse inside the polymer matrixes at high load-ing, in comparison to the other polymers, EPDM has theunusual ability to accept high loadings of fillers [21].

The fracture surfaces of tensile specimens at lowmagni-fication (50$) are shown in Fig. 8. Comparing Fig. 8(a) and(f) reveals how the roughness and tortuous path of the frac-ture surfaces increased with increase of the HNT loading.This could be explained by the good interactions betweenthe HNTs as well as between the HNTs and matrix.

1.32 1.94 2.21 2.854.192

5.42

0

2

4

6

8

10

12

14

0 5 10 15 30 50 70 100

Ten

sile s

tren

gth

(M

Pa)

HNT loading (Phr)

8.15

12.86

Fig. 7. Tensile strength of EPDM/HNT nanocomposites.

Fig. 8. Tensile fracture surfaces of EPDM/HNT nanocomposites with (a) 0 phr, (b) 5 phr, (c) 10 phr, (d) 30 phr, (e) 50 phr and (f) 70 phr (magnification: 50$).

H. Ismail et al. / Polymer Testing 27 (2008) 841–850846

Fig. 9(a)–(d) also presents the tensile fracture surfaces ofEPDM/HNT nanocomposites with various HNT loading at5000$ magnification. Fig. 9 shows that there is a goodadhesion between well-dispersed HNTs and EPDM at dif-ferent HNT loadings (shown by black arrows). On the otherhand, as shown in Fig. 9(d) and discussed earlier in TEMresults, the edge-to-edge and face-to-edge interactionsbetween HNTs were increased at high HNT loading.

Elongation at break (Eb) of the EPDM/HNT nanocompo-sites at different HNT loading (0–100 phr) is shown inFig. 10. It is obvious that Eb of the nanocomposites increaseswith increasing HNT loading from 0 to 100 phr. The Eb ofthe EPDM/HNT nanocomposites with 30 phr and 100 phr

of HNT was 140% and 305% higher than the unfilledEPDM, respectively. This enhancement, particularly athigh filler loading (>30 phr), is a unique behaviour of thesystem and, hence, improves the stiffness as well as ductil-ity in the nanocomposites. The interfacial and inter-tubularinteractions between HNTs and EPDM as well as thehomogenous dispersion of HNTs inside the EPDM areresponsible for increasing the stiffness and ductility ofnanocomposites.

The effect of HNT loading on the tensile modulus (i.e.stress at 100% elongation, M100) of EPDM/HNT nanocom-posites is shown in Fig. 11. The M100 also showed anincreasing trend from 0 to 100 phr HNT loading by a factor

Fig. 9. Tensile fracture surfaces of EPDM/HNT nanocomposites with (a) 5 phr, (b) 10 phr, (c) 30 phr and (d) 70 phr (magnification 5000$).

151

0

100

200

300

400

500

600

700

0 10 15 30 50 70 100

Elo

ng

atio

n a

t b

reak (

%)

HNT loading (Phr)

5

223 241299

361 400

506

613

Fig. 10. The effect of HNT loading on the elongation at break of EPDM/HNTnanocomposites.

1.07 1.16 1.21 1.241.52

1.852.22

2.99

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 30 50 70 100

M100 (

MP

a)

HNT loading (Phr)

Fig. 11. The effect of HNT loading on M100 of EPDM/HNT nanocomposites.

H. Ismail et al. / Polymer Testing 27 (2008) 841–850 847

of 3. TheM100 for samples with 30 phr of HNT is about 42%higher thanM100 for the unfilled EPDM. The improvementof theM100 of EPDM/HNT nanocomposites is related to thestrong interactions between HNTs, EPDM and other ingre-dients on the surface and inner walls of HNTs as well asthe homogenous and intercalated EPDM/HNT structure.The reinforcing mechanism of inter-tubular diffusion ofEPDM inside the HNTs is the same as occluded rubber,which had been described by many researchers in rub-ber-carbon black compounds [31,32]. Kohls et al. [33]have reported that the most important effect of filler–rubber interaction has to do with the occlusion of rubberwhich is trapped between or within aggregates of filler.Therefore, the penetrated rubber inside and between thetubules acts as part of the filler network and increasesthe stiffness of the EPDM/HNT nanocomposites.

The mechanical properties determined in this studyreveal that the tensile strength,M100 and Eb were simulta-neously increased. As stated by Yang et al. [30], such simul-taneous increases in strength, stiffness (modulus) andductility (Eb) of polymer composites is an advantage, whichhas been obtained by incorporation of HNTs in the EPDM.

3.4. Crosslink density

The influence of HNT loading on crosslink density andmolecular weight of the EPDM/HNT nanocomposites isshown in Fig. 12. The crosslink density of the EPDM/HNTnanocomposites were increased with addition of HNT,and this is probably due to the interfacial and inter-tubularinteractions between EPDM and HNTs. The increase in

crosslinking is accompanied by decrease in molecularweight. Due to the presence of EPDM and other ingredientsinside the tubules, and edge-to-edge as well as the face-to-edge interactions between HNTs, the free volume betweenEPDMmolecules is reduced leading to less solvent swelling.Increasing crosslink density due to EPDM/HNT interactionsand formation of rubber structure [31] would decreasemolecular mobility inside and around the tubules and,therefore, lead to higher stiffness as shown in the tensilemodulus.

3.5. Thermal properties

The thermogravimetric analysis (TGA) results for theEPDM/HNT nanocomposites are shown in Fig. 13 andTable 3. The weight loss was measured as a function oftemperature. Table 3 shows the temperature at 5%weight loss, the maximum weight loss (%) and the tem-perature at maximum weight loss rate. The temperatureat 5% weight loss decreased with increasing HNT contentat 5 and 10 phr HNT loading but increased from 15 to100 phr HNT loading. As listed in Table 3, the tempera-ture at 5% weight loss for EPDM/HNT (70 phr) andEPDM/HNT (100 phr) were 15 "C and 20 "C higher thanunfilled EPDM, respectively. TGA results in Fig. 13 andTable 3 also clearly show the decreasing trend in maxi-mum weight losses of EPDM/HNT nanocomposites withincrease of HNT loading from 0 to 100 phr. This revealsthat the char residue of EPDM/HNT nanocomposites sig-nificantly increased in comparison to the EPDM matrix.The thermal stability is enhanced by increasing thechar residue as the formation of char hinders the diffu-sion of the volatile decomposition products [13,34–36].Similar enhancement in the thermal stability wasobserved from the slight increase in the temperatureat maximum weight loss rate of nanocomposites from0 to 100 phr HNT loading.

It is well accepted, as reported in previous studies[23,34,37,38], that the best thermal stability is obtainedfor polymer clay nanocomposites with filler contentbetween 6 and 8 phr, which was attributed to the exfolia-tion and well-dispersed nanoparticles at low clay content.

As the barrier properties of HNTs may be inferior tosilicate layered nanofillers [14] and the temperature at 5%weight loss didn’t change much at low HNT loading, itcould be concluded that the improvement of thermal

0 20 40 60 80 100

Mc

(g

/mo

le)

Cro

ss

lin

kin

g d

en

sit

y

(m

ole

/Cm

3)

HNT loading (phr)

1.50E+072.00E+072.50E+073.00E+073.50E+074.00E+074.50E+075.00E+075.50E+076.00E+07

5.00E-097.00E-099.00E-091.10E-081.30E-081.50E-081.70E-081.90E-082.10E-082.30E-08 Crosslinking density

Mc

Fig. 12. The crosslinking density and molecular weight of nanocomposites.

Fig. 13. TGA curve of EPDM, HNT and EPDM/HNT nanocomposites.

Table 3The thermal stability parameters of EPDM/HNT nanocomposites

Composites Loading(phr)

Temperatureat 5% weightloss

Maximumweight loss (%)

Temperature atmaximum weightloss rate ("C)

C.1 0 423 94.4 488C.2 5 417 91.2 492C.3 10 413 88.3 491C.4 15 425 84.9 492C.5 30 426 77.8 493C.6 50 428 69.0 494C.7 70 438 63.0 497C.8 100 443 56.7 503HNT – 491 19.1 524

H. Ismail et al. / Polymer Testing 27 (2008) 841–850848

stability of EPDM/HNT nanocomposites, especially at HNTloading higher than 15 phr, is attributed to many factors,including good dispersion of HNTs, entrapment of degrada-tion products of EPDM inside the lumen structure of thetubules, interfacial and inter-tubular interactions betweenEPDM and HNTs as well as the formation of zig-zag struc-tures. Recently, Du et al. [14] have reported that the degra-dation products of PP may be entrapped inside the lumensof HNTs, resulting in effective delay in mass transport andremarkably increased thermal stability of PP/HNTnanocomposites.

The flammability ranking based on UL-94 verticalburn test is presented in Table 4. The unfilled EPDM isunclassified because the total ignition of 5 specimenshad passed 250 s, as specified by the standard. Althoughadding 5–15 phr of HNT had strongly decreased thedripping and total ignition time, the EPDM/HNT was stillunclassified as the flame finally reached to the top of thespecimen. As recorded in Table 4, the addition of HNTsfrom 0 to 100 phr had reduced the flammability ofEPDM/HNT nanocomposites.

4. Conclusions

EPDM/HNT nanocomposites were prepared with differ-ent loading of HNTs from 0 to 100 phr. X-ray diffractionanalysis indicated limited intercalation of HNTs in EPDMmatrix. The morphological investigations by TEM revealthat the dispersion of HNTs inside the EPDM matrix washomogenous and three dimensional. There were also inter-facial and inter-tubular interactions between HNTs andEPDM, and edge-to-edge and face-to-edge interactionsbetween HNTs (zig-zag structure), especially at high HNTloading, which are the main reasons for the simultaneousincreases in tensile strength, stiffness and ductility ofEPDM/HNT nanocomposites. The good thermal stabilityand flame retardancy of nanocomposites resulted fromthe entrapment of degradation products of EPDM insidethe lumen structure of tubules.

Acknowledgements

The authors wish to acknowledge the financial supportprovided by USM short term grant (Ac No.: 6035261).

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Table 4Classification of flammability of EPDM/HNT nanocomposites according toUL-94 test

Composites Classification Drippingobserved

Specimenburns up toholdingclamp

Total flamingcombustionfor all 5specimens (s)

C.1 Unclassified Yes Yes 396C.2 Unclassified Yes Yes 342C.3 Unclassified No Yes 288C.4 Unclassified No Yes 231C.5 V-2 No No 191C.6 V-1 No No 154C.7 V-1 No No 99C.8 V-0 No No 46

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