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Polymer Testing 28 (2009) 548–559

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Polymer Testing

journal homepage: www.elsevier .com/locate/polytest

Material Properties

Influence of maleic anhydride grafted ethylene propylene dienemonomer (MAH-g-EPDM) on the properties of EPDMnanocomposites reinforced by halloysite nanotubes

Pooria Pasbakhsh, H. Ismail*, M.N. Ahmad Fauzi, A. Abu BakarSchool of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

a r t i c l e i n f o

Article history:Received 25 February 2009Accepted 14 April 2009

Keywords:Halloysite nanotubesEthylene propylene diene monomerNanocompositesMaleic anhydrideTransmission electron microscopy

* Corresponding author. Tel.: þ60 4 593 7788x6113E-mail address: hanafi@eng.usm.my (H. Ismail).

0142-9418/$ – see front matter � 2009 Elsevier Ltddoi:10.1016/j.polymertesting.2009.04.004

a b s t r a c t

Ethylene propylene diene monomer grafted with maleic ahydride (MAH-g-EPDM) wasprepared by peroxide-initiated melt grafting of MAH onto EPDM using a HAAKE internalmixer at 180 �C and 60 rpm for 5 min. The effect of MAH-g-EPDM compatibilizer on theinteractions, and tensile and morphological properties of halloysite nanotubes (HNTs)filled EPDM nanocomposites was investigated. The tensile properties of the nano-composites were influenced by two major factors. The hydrogen bonding between MAH-g-EPDM and HNTs, which was confirmed by attenuated total reflection Fourier transforminfrared spectroscopy (ATR-FTIR), as well as the formation of EPDM-rich and HNT-richareas, are the dominant effects on the tensile strength of the nanocomposites at low andhigh HNT loading, respectively. It was found that the cure time (t90), maximum torque(MH) and minimum torque (ML) of the compatibilized nanocomposites were increasedafter adding MAH-g-EPDM. The reinforcement mechanism of the compatibilized and un-compatibilized EPDM/HNT nanocomposites was also investigated based on morphologicalobservations of the nanocomposites.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Polymer matrices (e.g., thermosets, thermoplastics,elastomers) reinforced by nanofillers have attractedconsiderable attention in recent times due to their highermechanical, thermal and physical properties [1–6]. Rubber/Clay nanocomposites are one of the most promisingnanocomposite systems which are prepared by the incor-poration of layered silicates such as organo modifiedmontmorillonite (OMMT) into the rubbers [2,3,7–13].However, the preparation of EPDM/Clay nanocompositeshas been widely studied by researchers [3,8,9,11,13]. It hasbeen reported in our previous works [14,15] that theincorporation of halloysite nanotubes (HNTs) into EPDMcan increase the tensile, thermal, swelling and dynamic

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mechanical properties from 0 to 100 phr of HNT loading.Due to the fact that EPDM does not include any polargroups in its backbone, EPDM and HNTs are incompatible. Ithas also been reported by many researchers [3,16,17] thatthe properties (mechanical, thermal, rheological, barrier,etc.) of the rubber/clay nanocomposites are extremelyaffected by two important factors: the degree of dispersionof the nano-filler in the matrix and the compatibilitybetween the nano-filler and the rubber.

Melt grafting of unsaturated polar groups onto thepolymer backbone by using organic peroxides to func-tionalize polyolefins has been studied by variousresearchers [17,18]. To improve the compatibility betweena non-polar rubber such as EPDM and the nano-filler, meltgrafting of maleic anhydride (MAH) onto the rubber back-bone has also been done [16–18].

When a non-conjugated diene (a third monomer), isadded to the copolymerization of ethylene and propylene,the resulting rubber becomes a terpolymer, ethylene

Table 1Compositions of the HNT filled EPDM nanocomposites (phr).

Sample code EPDM MAH-g-EPDM

HNT ZnO Stearicacid

MBT TMTD Sulphur

EPDM/H0 100 0 0 5 1.5 0.8 1.5 1.5EPDM/H5 100 0 5 5 1.5 0.8 1.5 1.5EPDM/H10 100 0 10 5 1.5 0.8 1.5 1.5EPDM/H30 100 0 30 5 1.5 0.8 1.5 1.5EPDM/H100 100 0 100 5 1.5 0.8 1.5 1.5EPDM/H0/

MAH-g-EPDM80 20 0 5 1.5 0.8 1.5 1.5

EPDM/H5/MAH-g-EPDM

80 20 5 5 1.5 0.8 1.5 1.5

EPDM/H10/MAH-g-EPDM

80 20 10 5 1.5 0.8 1.5 1.5

EPDM/H30/MAH-g-EPDM

80 20 30 5 1.5 0.8 1.5 1.5

EPDM/H100/MAH-g-EPDM

80 20 100 5 1.5 0.8 1.5 1.5

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559 549

propylene diene terpolymer (EPDM) which can then bevulcanized by sulphur. The high mechanical, dynamic andelectrical properties as well as resistance to heat, aging andoxidation make EPDM the most suitable rubber for auto-motive sealing systems, electrical applications, buildingprofiles, roof sheeting and under the hood applications[19,20].

Halloysite nanotubes (Al2Si2O5(OH)4$nH2O) are a kindof naturally occurring aluminosilicate with nanotubularstructure and have been used as a new type of nano-reinforcement for polymers such as epoxy [5,21], poly-propylene [22], polyamide [23], polyvinilalcohol [24] andstyrene rubber [25,26] in recent years. The outer surface ofhalloysite is similar to SiO2, while the properties of theinner side and edges of the tubes could be considered asAl2O3 [27,28]. Due to the unique crystal structure of theHNTs, they do not need exfoliation, and due to the smallbasal spacing of the crystal planes, the intercalation of theHNTs by polymers and additives is hard to achieve [5,21].Furthermore, the polarity on the surfaces of the tubulesindicates that the HNTs would be well dispersed in thepolymer matrices.

The aim of present study was firstly to incorporatemaleic anhydride via peroxide-initiated melt grafting ontoEPDM and use it as a compatibilizer in EPDM/HNT nano-composites, which would result to higher tensile proper-ties. A further aim of this study was to compare the tensileproperties and morphological characteristics of un-compatibilized and compatibilized EPDM/HNT nano-composites and propose a reinforcement mechanism inorder to explain the possible interactions inside the EPDM/MAH-g-EPDM/HNT nanocomposites.

2. Experiments

2.1. Materials

EPDM Keltan, 778Z with ethylene content of 67%, ENB of4.3% and ML (1þ4) at 125 �C of 63 MM was used as thematrix. The HNTs, (ultrafine grade) were contributed byImerys Tableware Asia Limited, New Zealand, with bright-ness of 98.9% as measured by a Minolta CR300 using D65light source [29]. The DCP peroxide, maleic anhydride(MAH) and the other compounding ingredients such as zincoxide, stearic acid, sulphur, tetramethyl thiuram disulfide(TMTD) and 2-mercapto benzothiazole (MBT) were allsupplied by Bayer (M) Ltd.

2.2. Melt grafting

MAH-g-EPDM was prepared by peroxide-initiated meltgrafting of MAH onto the EPDM using a HAAKE internalmixer at 180 �C and 60 rpm for 5 min; all the conditions forpreparation of the MAH-g-EPDM are based on Grigoryevaand Karger-Kocsis [18] who have reported them asoptimum conditions to obtain the maximum graftingcontent of MAH-g-EPDM. The composition of the reactionrecipe was typically as follows: 39 g of EPDM, 2.5 wt% ofMAH, and 0.25 wt% of DCP. The components were mixed ina HAAKE mixer with optimum mixing volume of 44.1 cm3.All the reactants (EPDM, MAH and DCP) were dry mixed

together before their fast (<1 min) introduction into thepreheated mixing chamber.

2.3. MAH grafting efficiency

FTIR spectra were recorded on a FTIR spectrometermodel Perkin Elmer System 2000 in a range between550 and 4000 cm�1 with a 0.4 cm�1 resolution. Films of200–300 mm thickness were prepared by compression-molding using a hot press at 150 �C and 5 MPa pressure.The films were vacuum dried at 75 �C for 14 h to evaporatethe unreacted MAH.

2.4. Preparation of the EPDM/MAH-g-EPDM/HNTnanocomposites

The mixing of MAH-g-EPDM with the EPDM, HNTs andother compounding ingredients such as zinc oxide, strearicacid, MBT, TMTD and sulphur, as shown in Table 1, was doneusing a laboratory-sized two-roll mill (160 mm� 320 mm),model XK-160 at room temperature for 20 min. The vulca-nization behaviour of composites such as cure time (t90),scorch time (tS2), maximum torque (MH), minimum torque(ML) and cure rate index (CRI) were determined at 150 �Cusing a Monsanto Moving Die Rheometer (MDR 2000). Thecompounds were subsequently compression moulded at150 �C, based on respective t90 values.

2.5. X-Ray diffraction analysis (XRD)

The XRD patterns of HNTs, EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites were recorded by usinga Bruker Axs model D8 diffractometer. The basal spacing ofthe halloysite nanotubes before and after blending withEPDM and MAH-g-EPDM was calculated by using Bragg’slaw. The Cu Ka (l¼ 1.54060 Å) was operated at 40 kV and40 mA in combination with a Ni filter. The samples werescanned from 2q¼ 5 to 35�.

2.6. FTIR spectroscopy

Fourier transform infrared spectroscopy (FTIR) usinga Perkin Elmer System 2000 equipped with attenuated

Fig. 1. FTIR analysis of (a) EPDM and (b) MAH-g-EPDM.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559550

total reflectance (ATR) technique was employed to char-acterize the possible interactions between HNTs, EPDM andMAH-g-EPDM. FTIR spectra were conducted in a rangebetween 550 and 4000 cm�1 with a 0.4 cm�1 resolution.HNTs were ground thoroughly with KBr at approximately1–3% by weight and pressed into a pellet with a thickness ofabout 1 mm.

2.7. Tensile strength

After 24 h of storage, dumbbell shaped specimens werepunched from the moulded sheets by a tensile specimencutter. Modulus, tensile strength and elongation at break(Eb) were measured following ISO 37 using a universaltensile testing machine Instron 3366 at room temperature(25� 2 �C) at a crosshead speed of 500 mm/min.

2.8. Swelling properties

Swelling tests were done in toluene in accordance withISO 1817. Cured test pieces of the compounds of dimensions30� 5� 2 mm were weighed using an electronic balance.The test pieces were then immersed in toluene for 72 h and

5 10 20

Fig. 2. XRD pattern of: (a) HNT, (b) EPDM/H5, (c) EPDM/H5/MAH-g-EPDM, (d) EPDg-EPDM.

the pieces were weighed again. Calculation of the change inmass is as follows:

Swelling Percentage ¼ ½ðM2�M1Þ=M1� � 100 (1)

where M1 is the initial mass of specimen (g) and M2 is themass of specimen (g) after immersion in toluene.

2.9. Scanning electron microscopy (SEM) observations

The fracture surfaces of tensile samples of EPDM/HNTand EPDM/MAH-g-EPDM/HNT nanocomposites wereinvestigated by using a Supra-35VP scanning electronmicroscope (SEM). The main purpose of this evaluation wasto observe the degree of dispersion of halloysite nanotubesin the EPDM and to evaluate the bonding between the HNTsand EPDM. To prevent electrostatic charging duringobservation, a thin layer of Pd–Au was coated onto thesamples.

2.10. Transmission electron microscopy

A transmission electron microscope, Philips CM12(100 KV acceleration voltage) was used to study the

30

(a)

(b)

(c) (d)

(e) (f)

(g)

M/H10, (e) EPDM/H10/MAH-g-EPDM, (f) EPDM/H100, (g) EPDM/H100/MAH-

Table 2Diffraction pattern characteristics of HNTs and nanocomposites.

Sample 2q (�) d (nm)

HNT 12.19 0.725

EPDM/H5 11.37 0.77712.28 0.720

EPDM/H5/MAH-g 7.05 1.5212.1 0.73

EPDM/H10 11.4 0.77512.32 0.717

EPDM/H10/MAH-g 7.3 1.21012.35 0.77

EPDM/H100 12.18 0.715EPDM/H100/MAH-g 12.31 0.717

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559 551

dispersion of HNTs inside the EPDM matrix. To observe theEPDM/HNT naocomposites, ultra thin specimens wereprepared using a cryogenic Ultra microtome Leica-Reichertsupernova.

3. Results and discussion

3.1. FTIR analysis of MAH-g-EPDM

Fig. 1 shows a comparison between the FTIR spectra ofpure EPDM and MAH-g-EPDM in the 4000–550 cm�1

region. The absence of an absorption band at 700 cm�1

related to the carbon–carbon double bond of the MAH [18]confirmed the elimination of unreacted MAH by vacuumdrying of the MAH-g-EPDM.

FTIR spectra of the pure EPDM and MAH-g-EPDM inFig. 1 shows two absorption bands in the range of 1710–1719 cm�1 and 1770–1792 cm�1 which are attributed to theC]O symmetric stretching bonds. The presence ofabsorption bands at 1713 cm�1 and 1780 cm�1 indicate thegrafting of MAH onto EPDM.

As reported by Grigoryeva and Karger-Kocsis [18], theabsorption bands in the region of 1770–1792 cm�1 can berelated to grafted anhydride. The absorption band at1713 cm�1 is attributed to the presence of dimericcarboxylic acid in MAH-g-EPDM. On the other hand, the

1060.0 1020 1000 980 9cm-1

1032.091010.97

1031.99 1015.82

1027.841016.77

1027.68 1016.54

1040

Fig. 3. FTIR analysis of (a) HNTs, (b) EPDM/H5, (c) EPDM

existence of OH groups in MAH-g-EPDM is confirmed bythe absorption band at 922 cm�1.

3.2. XRD analysis

Fig. 2 and Table 2 give the X-Ray diffraction pattern ofthe HNTs, un-compatibilized and MAH-g-EPDM compati-bilized EPDM/HNT nanocomposites with 5, 10 and 100 phrof HNT loading. The HNTs has a peak at 2q¼ 12.19�, whichcorresponds to a basal spacing of 0.725 nm. As given inTable 2, the EPDM/H5/MAH-g-EPDM nanocompositeshows two lower 2q peaks around 12.1� and 7.05� whichcorrespond to basal spacing of 1.52 nm and 0.73 nm,respectively. On the other hand, as presented in Table 2, thebasal spacing of the EPDM/H5 nanocomposite (0.77 nm,2q¼ 11.37�) was not increased as much as the compatibi-lized one (EPDM/H5/MAH-g). The reduction of 2q andincreasing of the basal spacing of the HNTs may be attrib-uted to the intercalation of the HNTs by the other materialssuch as EPDM, zinc oxide and stearic acid, and can clearlyconfirm the formation of nanocomposites. According toTable 2 and Fig. 2, a similar finding was also observed forthe basal spacing of HNTs in EPDM/H10 and EPDM/H10/MAH-g nanocomposites, which was increased to 0.775 nmand 1.21 nm, respectively. It is very clear that the interlayerspacing of the HNTs in compatibilized EPDM/HNT nano-composite at low HNT loading was much higher than un-compatibilized nanocomposites. It has been reported in ourprevious work [14] that HNTs may be intercalated by EPDM,ZnO and stearic acid in un-compatibilized samples. Thegreater intercalation of the HNTs inside the compatibilizednanocomposites may be attributed to the intercalation ofthe maleic anhydride into the HNTs interlayer space. On thecontrary, the HNTs in EPDM/H100 and EPDM/H100/MAH-g-EPDM nanocomposites were not intercalated. As shownin Fig. 2, the degree of intercalation has decreased withincreasing the HNT loading. There was no intercalation ofthe HNTs at 100 phr loading even in compatibilizednanocomposites.

As depicted in Fig. 2, between 15� and 30� those nano-composites with low HNT loading formed a semi-crystallinestructure. The formation of this semi-crystalline structure isdue to the breaking of some HNT planes ((020), (110), (002))

60 940 920 900 880.0

912.00

931.61 913.01

931.88

931.81 911.31

(b)

(c)

(d)

(a)

/H5/MAH-g-EPDM, (d) EPDM/H10/MAH-g-EPDM.

CH

CH3

*CH

CH3

CH CH2 2

C C

O OO

*CH

CH3

CH CH

C CO O

OOH

Si

O

Al

OH

ENB-EPDM

CH

C

OO O

C

CH

MAH

+

MAH-g-EPDM EPDM/MAH-g-EPDM/HNT

Halloysite nanotubes

Hydrogen bonding

Scheme 1. Possible interactions between MAH-g-EPDM and halloysite nanotubes.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559552

because of the penetration of the EPDM and other materialsinto the lumen structure of the HNTs. The disappearance ofthe semi-crystalline region in nanocomposites with 100 phrHNT loading (Fig. 2 (f) and (g)) may be related to thereduction of the breakage of these planes due to limitedpenetration of the materials into the HNTs.

3.3. FTIR analysis of EPDM/HNT nanocomposites

Attenuated total reflection Fourier transform infraredspectroscopy (ATR-FTIR) was used to estimate the possibleinteractions between HNTs, EPDM and other ingredientsinside the compatibilized and un-compatibilized EPDM/HNT nanocomposites. Fig. 3 (a–d) show a comparisonbetween ATR-FTIR spectra of HNTs, EPDM/H5, EPDM/H5/MAH-g-EPDM and EPDM/H10/MAH-g-EPDM nano-composites. According to the ATR-FTIR spectra of HNTs inFig. 3 (a), the absorption peaks around 912 cm�1 and1032 cm�1 are related to the Al–OH librations and Si–Ostretching bands, respectively. As depicted in Fig. 3 (b), no

= Inner hydroxyl group

= Al atom

= Si atom = O atom

= hydroxyl group inside theHNTs between the sheets

×

Scheme 2. Crystalline structure

shift was detected in the absorption peaks at 912 cm�1 and1032 cm�1 which is attributed to the non-polar character-istics of EPDM. On the other hand, as shown in Fig. 3 (c), forthe FTIR spectra of compatibilized EPDM/HNT nano-composite with 5 phr of HNT loading, the absorption bandof Si–O stretching at 1032 cm�1 and Al–OH librations at912 cm�1 have shifted to 1027 cm�1 and 932 cm�1,respectively. The 5 cm�1 blueshift in Si–O stretching andthe 20 cm�1 redshift in Al–OH group is related to theformation of hydrogen bonding between outer and innersurfaces of the HNTs with MAH-g-EPDM, according toScheme 1. Du et al. [25] have also used ATR-FTIR to char-acterize the formation of hydrogen bonding betweencarboxylated butadiene-styrene and HNTs. By increasingthe HNT loading from 5 to 10 phr loading (Fig. 3 (d)), theAl–OH group spectra at 912 cm�1 appeared again. Thesimultaneous presence of these two peaks at 912 cm�1 and932 cm�1 may be attributed to the incomplete entrapmentof the materials inside the lumen of the HNTs due to theincrease of the HNT loading.

Octahedralsheet

Tetrahedralsheet

of halloysite nanotubes.

Fig. 4. Tensile strength of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559 553

A crystalline structure of HNTs is shown in Scheme 2.Consequently, the Al–OH group is located inside the tubeswhile the outer surface of HNTs is covered by the Si–O.HNTs have a dioctahedral 1:1 layered alominosilicate,consisting of two different interlayer surfaces. As shown inScheme 2, aluminium atoms makes an octahedral structurewith oxygen atoms and OH group is situated on one side ofthe lamella, while silicon–oxygen atoms are located on theother side of the lamella. As reported by Guimares et al.[30], there are two kinds of OH groups in the HNT structure,outer and inner OH groups which are located in the tetra-hedral and octahedral sheets of HNTs, respectively.

3.4. Tensile properties

As shown in Fig. 4, the tensile strength of EPDM/MAH-g-EPDM/HNT nanocomposites from 0 to 30 phr loading isincreased by compatibilization of the nanocomposites withMAH-g-EPDM except at 100 phr HNT loading. The incre-ment of tensile strength of compatibilized EPDM/HNTnanocomposites from 0 to 30 phr HNT loading in compar-ison to the un-compatibilized EPDM/HNT nanocompositesis due to the formation of hydrogen bonding andimprovement in the interfacial interactions between HNTsand EPDM in the presence of compatibilizer, as confirmedby FTIR results. This enhancement in the tensile strength bymixing the MAH-g-EPDM with EPDM clay nanocomposites

Fig. 5. Eb of EPDM/HNT and EPDM/MAH

is in good agreement with the results which have beenreported by Chow et al. [16]. The decrease of the tensilestrength of EPDM/H100/MAH-g-EPDM nanocomposites incomparison to EPDM/H100 nanocomposites can be attrib-uted to the bad dispersion of the HNTs inside the EPDM inthe presence of MAH-g-EPDM at high HNT loading, whichwill be discussed later in the morphological observations.

Fig. 5 demonstrates the elongation at break (Eb) of theEPDM/HNT nanocomposites with and without the MAH-g-EPDM. As shown in Fig. 5, by adding MAH-g-EPDM, the Eb

of the compatibilized nanocomposites were decreased atHNT loading higher than 10 phr. However, the Eb of the un-compatibilized EPDM compound (EPDM/H0) in theabsence of HNT is increased 43% by adding 20 phr of theMAH-g-EPDM.

The tensile modulus at 100% elongation (M100) of theEPDM/HNT nanocomposites with and without MAH-g-EPDM is illustrated in Fig. 6. The presence of maleicanhydride groups makes a stronger interaction betweenhalloysite nanotubes and EPDM in comparison to thenanocomposites without MAH-g-EPDM. This kind ofimprovement in the tensile modulus has also been repor-ted by Mohammadpour and Katbab [3]. They havementioned that the degree of reinforcement is much moresignificant in the presence of the MAH-g-EPDM as a com-patibilizer in organo modified montmorillonite (O-MMT)filled EPDM nanocomposites.

-g-EPDM/HNT nanocomposites.

Fig. 6. Tensile modulus at 100% of elongation (M100) of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559554

3.5. Scanning electron microscopy

The fractured surfaces of EPDM/HNT and EPDM/MAH-g-EPDM/HNT nanocomposites are presented in Fig. 7. Fig. 7

Fig. 7. Tensile fractured surfaces of EPDM/HNT nanocomposites: (a) EPDM/H0,composites: (d) EPDM/H0/MAH-g-EPDM, (e) EPDM/H30/MAH-g-EPDM and (f) EPD

(a) and (d) compares the fractured surfaces of EPDM/H0and EPDM/H0/MAH-g-EPDM nanocomposites. It can beseen from these two images that the EPDM/H0/MAH-g-EPDM has a rougher surface in comparison to EPDM/H0,

(b) EPDM/H30 and (c) EPDM/H100; and EPDM/MAH-g-EPDM/HNT nano-M/H100/MAH-g-EPDM.

Fig. 8. Tensile fractured surfaces of EPDM/HNT nanocomposites: (a) EPDM/H10, (b) EPDM/H10/MAH-g-EPDM.

Fig. 9. Tensile fractured surfaces of EPDM/HNT nanocomposites EPDM/H100/MAH-g-EPDM. The presence of EPDM and HNT-rich areas.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559 555

and this is in good agreement with the tensile propertiesresults. The increasing of the tortuous path and roughnessof the compatibilized MAH-g-EPDM nanocomposites incomparison to the un-compatibilized nanocomposites isclearly observed in Fig. 7 (b and e) and Fig. 7 (c and f). Thepresence of maleic anhydride which was grafted onto theEPDM increased the interfacial bonding between EPDMand HNTs. The increase of the interfacial bonding between

Fig. 10. Tensile fractured surfaces of EPDM/HNT nanocomposit

HNTs and matrix increased the roughness of the fracturedsurfaces. The roughness of the fractured surfaces isincreased by increasing of the HNT loading from 0 to100 phr loading.

Fig. 8 (a) and (b) compare the fractured surfaces of theun-compatibilized and compatibilized EPDM/HNT nano-composites with 10 phr HNT loading. It is shown that bothsamples, EPDM/H10 in Fig. 8 (a) and EPDM/H10/MAH-g-EPDM in Fig. 8 (b), have quite good dispersion of HNTsinside the matrix, indicating the ability of HNTs to dispersehomogenously. It is also observed that the interfacebetween HNTs and matrix in EPDM/H10/MAH-g-EPDMnanocomposite is blurred compared to EPDM/H10 due tothe HNTs being wrapped in the matrix, and no debondedtubes and cavities can be seen in compatibilized EPDM/H10/MAH-g-EPDM nanocomposite. This all indicates that,after compatibilization of EPDM/HNT nanocomposites withMAH-g-EPDM, the compatibility between HNTs and EPDMinside the nanocomposites is increased by formation ofinterfacial interactions (hydrogen bonding) between HNTsand EPDM at low HNT loading.

It is noteworthy to mention that the fractured surfacesof EPDM/MAH-g-EPDM/HNT nanocomposites at high HNTloading have two different domains, as depicted in Fig. 9:HNT-rich area and EPDM-rich area. The EPDM-rich area

es: (a) EPDM/H100, and, (b) EPDM/H100/MAH-g-EPDM.

Fig. 11. TEM images of EPDM/HNT nanocomposites: (a) EPDM/H10, (b) EPDM/H100; and EPDM/MAH-g-EPDM/HNT nanocomposites, (c) EPDM/H10/MAH-g-EPDM, (d) EPDM/H100/MAH-g-EPDM.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559556

was circled in Fig. 9 while the other regions belong to theHNT-rich area. The formation of these two phases at highHNT loading is attributed to the polarity differencebetween polar HNTs, MAH-g-EPDM and non-polar EPDMdue to the high concentration of the HNTs and low quantityof the MAH-g-EPDM. The MAH-g-EPDM is more polar andthe HNTs prefer to interact with OH groups of maleicanhydride by hydrogen bonding. Ye et al. [5] have reportedthat two phase structures, HNT-rich and epoxy-rich phases,were formed in epoxy/HNT nanocomposites. Wang et al.[17] have also demonstrated the preference of silicatelayers to be dispersed and exfoliated by the EPDM-g-MAHrather than non-polar poly (trimethylene terephthalate)

Fig. 12. TEM images of EPDM/MAH-g-EPDM/HNT nanocomposites (a) EPDM/H10/MHNT loading, (b) EPDM/H100/MAH-g-EPDM showing the formation of two phases

PTT matrix inside PTT/EPDM-g-MAH/organoclay ternarynanocomposites, which led to creation of two differentphases.

Fig. 10 shows the tensile fractured surfaces of EPDM/HNT nanocomposites which confirm the lower tensilestrength and Eb of the compatibilized EPDM/HNT nano-composites at 100 phr HNT loading (Fig. 10 (b)) incomparison to un-compatibilized EPDM/H100 nano-composites (Fig. 10 (a)). Although the creation of fibrilstructures (which are shown by arrows) due to the com-patibilization effect of maleic anhydride is clearly seen inEPDM/H100/MAH-g-EPDM, there are still some unfilledcavities in EPDM/H100/MAH-g-EPDM nanocomposites in

AH-g-EPDM showing a very good dispersion of HNTs inside the EPDM at lowof HNT-rich and EPDM-rich regions.

HNTs

= Al-hydrogen bonding

= Sulphur crosslinking

EPDM rich

= EPDM chains

HNT rich

= Si-hydrogen bonding

a b

c d

Scheme 3. Reinforcement mechanism of un-compatibilized EPDM/HNT nanocomposites: (a) low HNT loading, (b) High HNT loading; and compatibilized EPDM/HNT nanocomposites: (c) low HNT loading, (d) high HNT loading.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559 557

comparison to the EPDM/H100 nanocomposites, whichmay be related to an insufficient amount of EPDM-g-MAH(20 phr) for compatibilizaton of EPDM/HNT nano-composites at 100 phr loading of HNTs.

3.6. Transmission electron microscopy

Fig. 11 demonstrates the comparison between thedispersion of the HNTs inside the compatibilized and un-compatibilized nanocomposites with 10 and 100 phr HNTloading. As shown in Fig. 11 (a) and (c), the dispersion of theHNTs inside the EPDM/H10/MAH-g-EPDM is increased incomparison to EPDM/H10 nanocomposites. On the otherhand, it is very clear from Fig. 11 (b) and (d) that the HNTsare more dispersed in EPDM/H100 in comparison to EPDM/H100/MAH-g-EPDM. A better depiction of good and baddispersion of HNTs at 10 and 100 phr HNT loading incompatibilized EPDM/HNT nanocomposites is shown inFig. 12 (a) and (b), respectively. As described before, theHNTs prefer to interact with the polar MAH-g-EPDMinstead of non-polar EPDM due to the formation ofhydrogen bonding between Si–O and Al–OH groups ofHNTs and OH groups of MAH-g-EPDM. By increasing theHNT loading from 0 to 100 phr, the fraction of MAH-g-EPDM to HNT loading is decreased, and the amount of

MAH-g-EPDM is not sufficient for compatibility of HNTs athigh loading. This mismatch created two phases inside thenanocomposites, particularly at high HNT loading(100 phr).

3.7. Reinforcement mechanism

The proposed reinforcement mechanism of compatibi-lized and un-compatibilized EPDM/HNT nanocomposites atlow and high HNT loading is illustrated in Scheme 3. Thismechanism is concluded from the FTIR, XRD, SEM andTEM results which have been discussed earlier. For un-compatibilized EPDM/HNT nanocomposites, as illustratedin Scheme 3 (a) and (b) and reported in our previous work[14], because of the straight and tubular morphology andunique crystal structure of the HNTs, they can be homo-genously and easily dispersed inside the EPDM matrix.According to Scheme 3(b) and depicted in Fig. 11(b), inthose regions with high concentration of HNTs, they tend toform zig-zag structures due to the edge-to-edge and face-to-edge interactions between them.

Regarding Scheme 3 (c) and (d) and earlier discussion,there are two effects which compete with each other: Thehydrogen bonding between MAH-g-EPDM and HNTs, andthe formation of EPDM and HNT-rich areas which is

Table 3Curing properties of composites.

Sample code T90 (min) Ts2 (min) MH (dN m) ML (dN m) CRI¼ 100/(t90� tS2)

EPDM/H0 20.75 7.40 13.18 0.47 7.49EPDM/H0/

MAH-g22.07 6.98 13.34 0.92 6.62

EPDM/H5 15.53 3.97 14.31 0.46 8.65

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559558

influenced by the ratio of MAH-g-EPDM to HNT loading. Asshown in Scheme 3 (c), at low HNT loading the effect ofhydrogen bonding plays the leading role which results inincreased tensile strength, but at high HNT loading theformation of EPDM/HNT-rich areas has the dominant effectresulting in reduction of the tensile strength and very largedecrease in Eb.

EPDM/H5/MAH-g

18.98 4.20 14.73 0.99 6.76

EPDM/H10 13.73 2.72 14.95 0.49 9.08EPDM/H10/

MAH-g17.3 3.00 15.3 1.02 6.99

EPDM/H30 14.03 1.41 17.61 0.65 7.92EPDM/H30/

MAH-g15.95 1.48 18.75 1.47 6.91

EPDM/H100 18.12 1.01 25.12 1.88 5.84EPDM/H100/

MAH-g20.81 1.08 27.27 4.84 5.07

3.8. Curing properties

Table 3 gives the comparison between the curingproperties of EPDM/HNT and EPDM/MAH-g-EPDM/HNTnanocomposites. From Table 3, it can be concluded that thecuring time (t90), scorch time (ts2), MH and ML of thesamples are increased due to adding 20 phr of MAH-g-EPDM. The increase of the curing time of the compatibi-lized nanocomposites in comparison to un-compatibilizednanocomposites at similar HNT loading may be attributedto the cure retardency effect of maleic anhydride [31]. Thereactions between some maleic anhydride groups of theMAH-g-EPDM with accelerator species causes delay in theoptimum cure time (t90). On the other hand, these reactionsincreased interfacial bonding between HNTs and EPDM inthe presence of MAH-g-EPDM. The enhancement of the MH

and ML of the EPDM/MAH-g-EPDM/HNT nanocompositesin comparison to EPDM/HNT nanocomposites is due to theimprovement in the interfacial interactions. ML is an indi-cation of the processability of the compounds. As given inTable 3, ML is increased by adding MAH-g-EPDM whichshows the lower processability of the compatibilizednanocomposites due to higher shearing needed for mixing.

Table 3 also shows the effect of HNT loading on thecuring properties of EPDM/HNT nanocomposites. The curetime is decreased from 0 to 30 phr of HNT loading while itincreased again from 30 to 100 phr loading. The effect ofHNT loading and the entrapment of accelerators and EPDMinside the lumen of the HNTs are two factors which wouldcompete with each other. At low HNT loading, the presenceof the HNTs accelerates the vulcanization of the nano-composites but at high HNT loading, because the lumenstructure inside the tubes is a good place to attract theaccelerators, by adding more HNTs more accelerators

Fig. 13. The comparison between swelling percentage of the comp

would become entrapped inside the lumen of the HNTswhich would slow the vulcanization process.

3.9. Swelling properties

The swelling percentage is the measurement of thedegree of crosslinking, reduction in swelling indicatingincrease of crosslink density. Fig. 13 gives a comparisonbetween the swelling percentages of compatibilized andun-comaptibilized EPDM/HNT nanocomposites at differentHNT loadings. It is observed from the figure that theswelling percentage of the nanocomposites is decreaseddue to both compatibilization and HNT loading effects.Compatibilization of the nanocomposites by adding MAH-g-EPDM formed hydrogen bonding between HNTs andEPDM which increased the ability of EPDM chains to extenddue to toluene diffusion. It has been reported by variousresearchers [32,33] that increasing the interaction betweenpolymer and filler would lead to an increase in crosslinkdensity and reduction in solvent uptake. The increase of thecrosslinking density of the nanocomposites by addingMAH-g-EPDM is in good agreement with the tensilemodulus (M100) and maximum torque results discussedearlier.

atibilized and uncompatibilized EPDM/HNT nanocomposites.

P. Pasbakhsh et al. / Polymer Testing 28 (2009) 548–559 559

4. Conclusions

MAH-g-EPDM has been successfully prepared by meltcompounding of the EPDM, maleic anhydride and DCP. XRDpatterns indicated that the degree of intercalation of theHNTs inside the compatibilized EPDM/HNT nanocompositesis much higher than the un-compatibilized nano-composites. The enhancement of the tensile properties isdue to the creation of an interphase between EPDM and HNTby MAH-g-EPDM which helps to make stronger interfacialinteractions. On the other hand, the presence of this com-patibilizer reduced the curing time (t90) but increased themaximum and minimum torques as well as swelling resis-tance of the compatibilized EPDM/HNT nanocomposites incomparison to un-compatibilized EPDM/HNT nano-composites. Morphological observations revealed theformation of two different phases of EPDM-rich and HNT-rich areas which can be the main reason for reduction of thetensile strength and Eb at high HNT loading.

Acknowledgment

The authors wish to acknowledge the financial supportprovided by USM short term grant (Ac No.: 6035261).Pooria Pasbakhsh would like to thank Universiti SainsMalaysia for the financial support under USM fellowshipscheme for his PhD study.

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