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 DOI: 10.1177/0095244310368125

2010 2010 42: 331 originally published online 31 MarchJournal of Elastomers and Plastics

Mohd and Che Husna AzhariNoraiham Mohamad, Andanastuti Muchtar, Mariyam Jameelah Ghazali, Dahlan Hj.

CompositesCharacteristics of Epoxidized Natural Rubber-Alumina Nanoparticles

Correlation of Filler Loading and Silane Coupling Agent on the Physical  

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Correlation of Filler Loading and SilaneCoupling Agent on the PhysicalCharacteristics of Epoxidized NaturalRubber-Alumina NanoparticlesComposites

NORAIHAM MOHAMAD,1,2,* ANDANASTUTI MUCHTAR,1

MARIYAM JAMEELAH GHAZALI,1 DAHLAN HJ. MOHD3

AND CHE HUSNA AZHARI1

1Faculty of Engineering, Universiti Kebangsaaan Malaysia43600 Bangi, Selangor, Malaysia2Universiti Teknikal Malaysia Melaka, 76109 Durian TunggalMelaka, Malaysia3Malaysian Nuclear Agency, 43000 Bangi, Selangor, Malaysia

ABSTRACT: Studies were conducted on the effects of filler loading and3-aminopropyltriethoxysilane (SCA) on the physical characteristics of epoxidizednatural rubber (ENR)-alumina nanoparticles composites (ENRAN). The fillerloading varied from 10, 20, 30, 40, 50, and 60 phr in the absence and presenceof 2 phr SCA in every formulation. The alumina nanoparticles have apositive interaction with the ENR matrix since they increase the glasstransition temperature of the composites [Mohamad, N., Muchtar, A., Ghazali,M.J., Dahlan, H.M. and Azhari, C.H. (2008). The Effect of Filler onEpoxidized Natural Rubber-Alumina Nanoparticles Composites, Eur. J. Sci.Res., 24: 538–547]. The addition of SCA decreased the maximum torque (MH)and the torque difference (MH�ML) of ENRANs compound as well as causingthe curing process to occur at a very fast rate. The spherical alumina particles

*Author to whom correspondence should be addressed. E-mail: noraiham@utem.edu.myFigure 2 appears in color online: http://jep.sagepub.com

JOURNAL OF ELASTOMERS AND PLASTICS Vol. 42–July 2010 331

0095-2443/10/04 0331–16 $10.00/0 DOI: 10.1177/0095244310368125� The Author(s), 2010. Reprints and permissions:http://www.sagepub.co.uk/journalsPermissions.nav

were dispersed in ENR matrix and a uniform particle distribution was observedunder scanning electron microscope enabling the increase of the torque values inthe composites.

KEY WORDS: epoxidized natural rubber, alumina nanoparticles, silanecoupling agent, cure characteristics, morphology.

INTRODUCTION

MUCH RESEARCH HAS been carried out in the development of nano-filled composites through the incorporation of nano-scale materials

such as ceramics and carbon in polymer matrix. For instanceorganoclays [1], carbon nanotubes [2], alumina nanoparticles [3], andsilica nanoparticles [4] have been added to polymers. Polymernanocomposites exhibit unique properties even by the addition of alow weight percent nanofillers (55 wt%) that cannot be obtained fromconventional or micro-scale fillers [5]. The incorporation of nanofillersenhances mechanical, electrical, optical, and other properties of polymercomposites without sacrificing too much of the good properties such astoughness being traded for stiffness as that found in rubber-filled carbonfibers [6]. Several polymers have been used for preparing polymernanocomposites such as elastomers (natural rubber (NR) [7,8],epoxidized natural rubber (ENR) [1,9], styrene-butadiene rubber(SBR) [10,11], chloroprene rubber (CR) [12], ethylene propylenediene monomer rubber (EPDM) [13], etc.), thermoplastics (nylon 6[14], polypropylene (PP) [15,16], polyethylene terephthalate (PET) [17],polymetylmetacrylate (PMMA) [18], polycarbonate [19,20], etc.), andpolymer blends [21,22].

The development of polymer-ceramic nanocomposites has created anumber of technologies and opportunities that can be applied to ENR. Ina previous research by Teh et al. [1], ENR is used as a compatibilizer inproducing natural rubber-organoclay nanocomposites. Organoclay canbe easily dispersed in polar polymers when compared to nonpolarpolymers such as NR [1]. ENR is miscible with more polar polymers [23]therefore offering unique properties such as good oil resistance, low gaspermeability, a higher wet grip, rolling resistance, and a higher tensilestrength. The oil resistance of ENR 50 vulcanizate is reported to almostmeet with the characteristics of medium-acrylonitrile-content nitrilerubber and also surpasses that of CR [23]. The resistance to air

332 N. MOHAMAD ET AL.

permeability of ENR 50 is also claimed to be comparable to butyl rubberand medium-acrylonitrile-content nitrile rubber [23]. Alumina has beenrecognized as a structural material with an extremely high melting point(20508C), a high degree of hardness, and is capable in taking on diverseshapes and functions [24].

The potential of polymer-filled alumina nanoparticle composites inwear and tribology, optical, and electrical have been studied by someresearchers [17,19,25]. The incorporation of nano-scaled alumina in PPhas improved the mechanical properties of the polymer composites [26]and increased the wear resistance of PET-filled alumina nanoparticlesby nearly two times over the unfilled polymer [17]. There have also beenseveral reports of improved ductility and toughness in brittle thermosetpolymers due to the addition of alumina nanoparticles [27,28]. In ourprevious study [29], the addition of alumina nanoparticles in ENR hasproved to accelerate the curing process to 40% and has also increased thetensile modulus at 100% and 300% elongation up to 100% compared tounfilled ENR.

The ENR-alumina nanoparticles composites can be prepared usingsimiliar procedures as those used for NR. Rubber nanocomposites withan exfoliated morphology have been successfully prepared by severalmethods such as in situ polymerization, solution blending, and directmelt compounding [22]. The solution method is widely used to preparenanocomposites, which can be applied to ENR but this method is veryinconvenient due to the use of organic solvents [8]. Hence, the meltcompounding method using the internal mixer has been used tosynthesize ENR-alumina nanoparticles composites.

Silane coupling agents are widely used in the rubber industry today.It is generally believed that they tend to promote adhesion through theformation of strong, primary interfacial bonds with both the polymersand inorganic substrates [30]. According to Zhao et al. [31] thehypothesis is that the amino groups would react with the epoxy andthe silane groups with the alumina surface, thus creating a relativelystrong nanoparticle/epoxy interface. However, it is required thatthe inorganic filler surface must contain enough active hydroxyls [30].The silane coupling agents are also believed to increase the reinforcingability of inorganic fillers to rubber matrix [30]. According toDharmendra et al. [32], the functionalization or surface treatment ofreinforcements with coupling agents is one of the methods to counteractthe negative effect of fillers on tensile strength. Tensile strength can beretained or even improved by appropriate functionalization of thereinforcements.

Physical Characteristics of ENRAN 333

EXPERIMENTAL

Materials

Table 1 shows the formulation used in this study. The MalaysianRubber Board supplied the ENR under the trade name, ENR 50 with a53% epoxidization. The average Mooney viscosities measured at ML(1þ 4) 1008C is 85.5 and the average specific gravity at �258C is0.9366. The Nanostructured & Amorphous Materials Inc., USA,supplied the Alumina nanoparticles with an average diameter of30–80 nm. Other compounding ingredients such as sulphur, zinc oxide,and stearic acid were purchased from Systerm/Classic Chemicals SdnBhd.; tetramethylthiuram disulfide (TMTD) from Aldrich Chemistry; N-cyclohexylbenthiazyl sulphenamide (CBS) and Flexsys America, USAsupplied the N-(1,3-Dimethylbutyl)-N0-phenyl-p-phenylenediamine(6PPD) while the 3-aminopropyltriethoxysilane (SCA) was supplied bySigma-Aldrich (USA). All materials are used in the original suppliedform and complete recipes are given in Table 1.

Mixing and Curing Characteristics Measurement

The melt compounding process is performed according to ASTMD-3192 [33] and carried out using a Haake internal mixer working at atemperature of 908C with a rotor speed of 60 rpm for 6 min [34]. TheENR is masticated for 1 min before all the other ingredients are added(excluding sulphur) and mixed for another 4 min. Finally, the sulphur is

Table 1. Typical formulations.

Ingredients Loadings (phr)a

ENR 50 100Zinc oxide 2.0Sulphur 1.6Stearic acid 1.5CBSb 1.9TMTDc 0.96PPDd 2.0SCAe 2.0Alumina 10, 20, 30, 40, 50, 60

aParts per hundred rubber.bN-cyclohexylbenthiazyl sulphonamide.cTetramethylthiuram disulfide.dN-(1,3-Dimethylbutyl)-N0-phenyl-p-phenylenediamine.e3-aminopropyltriethoxysilane.

334 N. MOHAMAD ET AL.

added and mixed for another 1 min before the mixture is dumped andleft to cool down to room temperature.

From this stock, five unvulcanized samples (approximately 8 g) are cutto allow testing of the curing characteristics in accordance to the TestMethod ASTM D-2084 [33] using a Monsanto ODR 2000 Moving DieRheometer (Alpha Technologies, Swindon, UK) at 1508C. The meanvalue for minimum torque, maximum torque, scorch time, and cure timeis then calculated for every formulation.

Swell Measurements

Cured test pieces with dimensions of 30� 20� 3 mm3 are left to soakand absorb (swell) in toluene, a solvent for 24 h at room temperature.Q (the weight of toluene absorbed per gram of rubber hydrocarbon) isdetermined according to the following Equation (1) [23]:

Q ¼Ws �Wds

Wd � 100=Wtð1Þ

where Ws is the swollen weight, Wds is the deswollen weight, Wd is thedry weight, and Wt is the formula weight. This test is repeated foranother four trials per sample to increase the confidence level.

Measuring of Glass Transition Temperature Using theDifferential Scanning Calorimetry

A differential scanning calorimeter (DSC) (Perkin Elmer Jade) is usedto obtain the Tg of the vulcanized samples with a temperature rampfrom �608C to 1208C at 108C/min. The measurement is performed threetimes for every sample. The maximum and minimum values are thendiscarded, and the mean and standard deviation of the remaining valuesare taken to ensure the validity of the data.

Morphological Examination Using Scanning ElectronMicroscope

An examination of the composite surfaces is carried out using ascanning electron microscope (SEM) ZEISS (EVO 50) at a magnificationof 2000�. For every sample, when taking micrographs a minimum ofthree frames at each magnification is taken to ensure a high confidencelevel in the analysis. The objective is to get information on qualitativefiller dispersion and particle shape. The micrographs are captured undervariable pressure using the backscattered electron.

Physical Characteristics of ENRAN 335

RESULTS AND DISCUSSION

Effect on Curing Characteristics

Figure 1 shows the maximum torque, MH and the minimum torque,ML of untreated and treated ENRAN. The MH increases when fillerloading is increased. The addition of SCA in ENRAN during mixingdecreases the MH when compared to the untreated ENRAN. MH isregarded as the measure of the composite modulus as well as thereinforcing effect factor of the filler on the matrix [1,35]. Even thoughthe presence of SCA is expected to strengthen the bonding between thefiller and matrix, however, in this case it shows a contrasting effect. Theaddition of SCA decreases the MH value of ENRAN and the disparitybecomes larger when filler loading is increased. The reduction ofcompatibility between the alumina nanoparticles and the ENR matrixmay be attributed to the less active -OH being available on the surface ofalumina nanoparticles [30]. In addition, the effect may be less prominentwhen SCA is directly just mixed into the mixture during thecompounding process when compared to other research conducted[30–32] where pre-treatment is carried out on the filler surfaces.

The ML of every batch is also shown in Figure 1. The ML increaseswith the increasing filler loading and the addition of SCA.

45

40

35

30

25

20

15

10

5

00 10 20 30

MH (untreated ENRAN)

ML (untreated ENRAN)

MH (treated ENRAN)

ML (treated ENRAN)

Alumina nanoparticles (phr)

Tor

que

(dN

m)

40 50 60 70

FIGURE 1. Minimum torque, ML and maximum torque, MH of untreated and treatedENRAN.

336 N. MOHAMAD ET AL.

The incorporation of alumina nanoparticles has proportionally increasedthe ML in every sample but a progressive increase is observed with thepresence of SCA in the samples. The ML is related to the viscosity ofnanocomposites [1,29]. So, the increase in the ML is an indication of anincrease in the viscosity. For untreated ENRAN, the increase of ML canbe attributed to higher filler–matrix interaction. The alumina nanopar-ticles most likely interact with ENR particles hence chemical bondsoccur between the silanol groups on the alumina surfaces and epoxidegroups of ENR. This is proven by the glass transition temperatureshown in Figure 2. The increase in Tg is an indication of the goodadhesion between the filler and the matrix [6,29]. According to Ajayanet al. [6] if a polymer is absorbed on the filler surface, the glass transitiontemperature is stabilized or increased and could be as high as 308C. But,the Tg of untreated ENRANs decreases as the filler loading is increased.This proves that the presence of SCA in the compound weakens theadhesion between ENR and alumina nanoparticles.

Besides that, the addition of fillers of a smaller size tends to imposeextra resistance to flow due to a higher restriction of molecular motionof the macromolecules which results in a higher viscosity [29]. However,a further increase of ML in treated samples may be due to theinteraction of different charges in the compound caused by thefunctional groups of the SCA. According to Zhao et al. [31] the SCA

–13

–14

–15

–16

–17

–18

–19

–20

0 10 20 30

Untreated ENRAN Treated ENRAN

Alumina nanoparticles (phr)

Gla

ss tr

ansi

tion

tem

pera

ture

, Tg

(°C

)

40 50 60

FIGURE 2. Glass transition temperature of untreated and treated ENRAN.

Physical Characteristics of ENRAN 337

contains two components, which are the amino group and the silanegroup and these groups will interact with different constituents in thecompounds. This phenomenon interferes with the movement of the fillerand matrix particles and increases the viscosity of the compound.

The torque difference, MH�ML for untreated and treated ENRANshows a similar trend with the maximum torque (Figure 3). TheMH�ML is a measure of the dynamic shear modulus, which indirectlyrelates to the crosslink density of the nanocomposites [23,29]. Theuntreated ENRAN shows significant increase in the MH�ML whenfiller loading is further loaded from 10 to 60 phr. The increase ofMH�ML values may suggest that alumina nanoparticles also act ascross-linking agents in the matrix [29] and form additional crosslinkagesthrough chemical reactions as depicted in Figure 4. However, theaddition of SCA has negative effects on the MH�ML values and the

40

35

30

25

20

15

10

5

0

10 20 30

Untreated ENRAN Treated ENRAN

Alumina nanoparticles (phr)

Max

. tor

que-

min

. tor

que,

MH

–M

L(d

Nm

)

40 50 60

FIGURE 3. Torque difference, MH�ML.

OH Al2O3

Al2O3

CH3

CH3

CH3

Molding

CH2

CH2

CH2

H2C+

H2C

H2CC C

C

C C

C

O

OH

OH

O

OH

H

H

Alumina ENR

FIGURE 4. Proposed reaction between alumina nanoparticles and ENR.

338 N. MOHAMAD ET AL.

increase of alumina nanoparticles loading in the samples is no longercapable of improving the crosslink density in the compounds. Thepresence of SCA has disturbed the ability of the alumina nanoparticlesas crosslinking agents as well as reducing the compatibility betweenENR and alumina. These results are proven by the swell measurementthrough an expression in Equation (1).

According to Manna et al. [36], in the presence of SCA, the aminegroups of the SCA react with the epoxy groups of the ENR to form C–Nbonds between ENR and the SCA, whereas the triethoxy silanefunctionality will react with the silanol groups on the alumina surface,as proposed in Figure 5. However, this is not the issue in this study. Thereaction between alumina nanoparticles and SCA may fail or occur in avery small percentage due to the inefficient mixing mechanism and alower availability of the active OH groups at the alumina surfaces. Butat the same time, amine groups in SCA may successfully form somebonding with the epoxide groups in the ENR. The failure of the silane

H OC C

CH3

Al2O3

Al2O3

NH2(CH2)3–Si–(OC2H5)3

Alumina

Molding

OH

OH

H

H

C C

CC

CH3

CH3

NH2(CH2)5–Si–(OC2H5)3

NH2(CH2)3–Si–(OC2H5)3

H2C

H2C

CH2

CH2

3-aminopropyltriethoxysilane

CH2 OHH2C

ENR

+ +

FIGURE 5. Proposed reaction between ENR and alumina nanoparticles with theaddition of SCA.

Physical Characteristics of ENRAN 339

groups in SCA to form any bonding with the alumina nanoparticles mayinterrupt the continuity of the crosslink in the ENRANs. Furthermore,previous chemical reaction that occurred directly between ENR andalumina nanoparticles (refer Figure 4) is less prominent since the bondenergy of C–O is greater than that of C–N, therefore, C–N is more activein the reaction than C–O [37]. As a result, the C–N bonds between ENRand SCA are more favored than the C–O bonds between ENR with thesilanol groups of alumina nanoparticles. As a consequence, the mixing orcompounding torque of the treated samples is lower.

Figure 6 shows the relationship between the weights of tolueneuptake per gram of rubber, Q against filler loading for both untreatedand treated ENRANs. It can be seen that the Q values decrease with theincreasing amount of alumina nanoparticles in the composites. But, Qvalues increase in treated ENRAN as the SCA is added. The Q value isinversely proportional to the crosslink density in composites. So, thedecrease in the Q values is a good indicator in the enhancement ofthe crosslink density. The increase in 3D networks (crosslink) in thecomposites has reduced the capability of the composites to absorbtoluene since there are less open chains in the composites for the tolueneto penetrate [29]. However, the ability of ENRANs to resist thepenetration of the toluene decreases with the presence of the SCA.These results prove that the SCA has reduced the compatibility betweenalumina nanoparticles and ENR in the composites, as discussed earlier.

Figure 7 shows the scorch, t2 and cure time, t90 for untreated andtreated ENRAN. The presence of alumina nanoparticles have resulted in

2.5

2

1.5

1

0.5

010 20 30

Untreated ENRAN Treated ENRAN

Alumina nanoparticles (phr)

Wei

ght o

f tol

uene

upt

ake

per

gram

of

rubb

er, Q

(g)

40 50 60

FIGURE 6. Weight of toluene uptake per gram of rubber, Q of untreated and treatedENRAN.

340 N. MOHAMAD ET AL.

a significant decrease in t2 and t90. This is due to the effect of the filler’ssurface area [29], where the smaller the particle size, the larger is thesurface area available for a reaction. Hence, there is a greaterinteraction between the fillers and the matrix and this will thereforeaccelerate the process and shorten the curing time. This claim is inagreement with Ishak and Bakar [38] who report that fillers play animportant role in accelerating the vulcanization process. Besides that,the presence of the silanol groups on the alumina surfaces can also helpto increase the curing rate of the composites [39]. The cure acceleratorused will be absorbed by the hydrogen bonds in the silanol groups andthis reduces the t90. However, the t2 and t90 values are further decreasedwhen the SCA is added into the formulation. The reduction of the t2 andt90 of the compound may be attributed to the production of thedecomposition product from the SCA that can act as a vulcanizationactivator or accelerator. The decomposition product accelerates thecuring process and shortens the t2 and the t90.

Effect on Morphology

The SEM micrographs in Figure 8 show the effect of filler loading andSCA on the morphology of the composites. The dark phase represents theENR matrix and the bright phase corresponds to the alumina particles.The micrographs show that the ENRANs have sequentially changed froma random to a regular spatial particle distribution (more uniformdistribution) in low to higher filler loading composites. However, the

0

0.5

1

1.5

2

2.5

3

3.5

10 20 30

t90 (untreated ENRAN)

t90 (treated ENRAN)

t2 (untreated ENRAN)

t2 (treated ENRAN)

Alumina nanoparticles (phr)

Tim

e (m

in)

40 50 60

FIGURE 7. Scorch time, t2 and cure time, t90 of untreated and treated ENRAN.

Physical Characteristics of ENRAN 341

alumina nanoparticles in every samples are poorly dispersed. There arefillers that form agglomerates presence in the composites. The size ofagglomerates is ranged from nanosize to approximately 20 mm. As thefiller loading increases, the density of the agglomerates in both treatedand untreated ENRAN also increases. The presence of a higher amount offillers that is distributed uniformly in the compound imposes an extraresistance to flow [29]. This will impart more resistance on the chainmobility during deformation and increases their viscosity and modulus,which relates to ML and MH values (refer to Figure 1). These results arein agreement with Mohamad et al. [29] and Ismail et al. [40] where a

FIGURE 8. SEM micrographs at magnification of 2000� for (a) untreated ENRAN-10 phr alumina nanoparticles, (b) treated ENRAN-10 phr alumina nanoparticles,(c) untreated ENRAN-30 phr alumina nanoparticles, (d) treated ENRAN-30 phr aluminananoparticles, (e) untreated ENRAN-60 phr alumina nanoparticles, and (f) treatedENRAN-60 phr alumina nanoparticles.

342 N. MOHAMAD ET AL.

higher amount of alumina fillers increases the stiffness of the composites(modulus) due to the restriction of the chain movement duringdeformation and a high filler–matrix interaction.

As observed in the micrographs, the presence of SCA affects the shapeand size of the dispersed alumina particles. The alumina particlesappear smaller and more sphere-like in the treated ENRAN if comparedto the untreated ENRANs. The manner of filler dispersion anddistribution in treated ENRAN should result in better cure properties.But, the presence of SCA in treated ENRAN decreases the MH values aswell as the stiffness of the compound since it may create a weak adhesionbetween filler and the ENR matrix as an effect of nonpre-treatment onfiller surface prior to the compounding process. The SCA seems topromote higher filler–filler interaction among the alumina particlesrather than improve the filler–matrix interaction.

CONCLUSIONS

The cure characteristics are affected by the crosslink density,adhesion between filler and matrix and filler dispersion. The increaseof alumina nanoparticles loading in untreated ENRAN has proved toincrease the torque difference as well as to accelerate the curing process.The alumina nanoparticles show a good compatibility with ENR matrixsince it increases the glass transition temperature. The addition of SCAhas negative effects on the cure characteristics as it decreases the MHand MH–ML values but increases the ML value. The SCA interrupts thecompatibility between alumina and ENR and decreases the Tg of thetreated ENRAN. The uniformly distributed alumina particles increasesthe stiffness of the compound as the filler loading increases but the effectreduces as SCA is added into the compound.

ACKNOWLEDGMENTS

The authors acknowledge the Ministry of Higher Education Malaysiaand Universiti Kebangsaan Malaysia (UKM) for granting theFundamental Research Grant Scheme (UKM-RS-02-FRGS0003-2007)to carry out this project. We wish to sincerely thank the MalaysiaNuclear Agency and Rubber Research Institute of Malaysia forproviding the equipment and technical assistance to carry out theexperiments. The first author is grateful to Universiti Teknikal MalaysiaMelaka (UTeM) for granting concession and study.

Physical Characteristics of ENRAN 343

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