ORIGINAL PAPER

Mechanical, thermal and transport properties of nitrilerubber (NBR)nanoclay composites

Meera Balachandran & S. S. Bhagawan

Received: 26 May 2011 /Accepted: 28 November 2011 /Published online: 22 February 2012# Springer Science+Business Media B.V. 2012

Abstract The article describes the properties of nitrile rub-ber (NBR)nanoclay composites prepared by a two-stepmethod. viz. preparation of a 3:1 [by weight] masterbatch ofNBR and nanoclay followed by compounding on a tworoll mill and molding at 150 C and 20 MPa pressure.The tensile strength, elongation at break, modulus, stor-age modulus (E) and loss modulus (E) increased withthe nanofiller content, reached the maximum value at 5 phrand decreased thereafter. The solvent uptake, diffusion,sorption and permeation constants decreased with nanoclaycontent with the minimum value at 5 phr nanoclay. Themechanism of solvent diffusion through the nanocompo-sites was found to be Fickian. Thermodynamic constantssuch as enthalpy and activation energy were also evaluated.The dependence of various properties on nanoclay contentwas correlated to the morphology of the nanocomposites.supported by morphological analysis.

Keywords Nanocomposite . Organoclay . Rubber .

Mechanical properties . Dynamic mechanical properties .

Diffusion . Swelling

Introduction

Over the last two decades several studies have revealed thatthe properties of polymeric materials can be improved

significantly by addition of nanoparticles in the polymermatrix [16]. The most interesting feature of these polymernanocomposites is that the changes in propertiesmechan-ical, thermal, barrier and others are achieved at very lowconcentrations of the nanofiller compared to micro or macrosized fillers [79]. As the particle size of a materialdecreases, the specific surface area increases and the inter-action between the matrix and the filler also consequentlyincreases. This increase in fillermatrix interaction enhan-ces the properties of the nanocomposites [7]. The variouskinds of nanofillers studied are nanoclay, nanosilica, nano-alumina, nanocalcium carbonate, carbon nanotubes etc. Ma-jority of work on polymer nanocomposites is reported forthermoplastics [1, 2 and references therein]. However, rub-ber nanocomposites also offer ample scope for improvementin properties [3, 10]. Nanoclays are layered silicates whosesurface can be modified using organic modifiers through ionexchange reactions to increase their compatibility with poly-mer matrix. They also have the ability to disperse intoindividual layers in the matrix. A number of papers havebeen published on nanoclay composites based on elastomersincluding natural rubber [11, 12], styrene butadiene rubber[13], chloroprene rubber [14] ethylene propylene diene rub-bers [15, 16] and nitrile rubber [1730].

Nitrile rubber (NBR) is a special purpose rubber used inseveral applications that require oil resistance. There areseveral research papers that have reported enhanced me-chanical properties for NBR nanocomposites. Kim et.al.and Nah et.al. reported that mechanical properties of NBRcould be improved using long chain surface modifiedmontmorillonite [1820]. They prepared NBR nanocompo-sites by melt intercalation process. Ahmadi et.al. preparedNBRnanoclay hybrids in melt blender and subsequentlyused two-roll mill to prepare NBR nanocomposites[21].Hwang et.al. and Wu et.al. prepared NBR nanocomposites

M. Balachandran : S. S. Bhagawan (*)Department of Chemical Engineering and Materials Science,Amrita Vishwa Vidyapeetham,Coimbatore 641112, Indiae-mail: ss_bhagawan@cb.amrita.edu

M. Balachandrane-mail: b_meera@cb.amrita.edu

J Polym Res (2012) 19:9809DOI 10.1007/s10965-011-9809-x

with enhanced barrier properties by blending NBR latex andnanoclay [22, 23]. There are several studies on preparationand properties of NBRnanoclay composites prepared bysolution mixing [13, 24, 25]. Zhao et.al. reported that NBRnanocomposites prepared using two-roll mill mixing resultsin aggregation of nanoclay [ 26]. Gas barrier properties ofNBR composites have been found to show tremendousimprovement on incorporation of organomodified nanoclay[24, 27]. We had earlier reported that NBR nanoclay com-posites prepared using NBR-nanoclay masterbatchexhibited improved mechanical properties and increasedthe oil and heat ageing resistance of NBR [2830]. It maybe noted that reported literature on solvent transport prop-erties through nanocomposites are few.

In this study, NBRnanoclay composites were preparedby a two stage compounding process. The morphology ofthe nanocomposites was analyzed using X-ray diffractionand transmission electron microscopy. The effect of nano-filler content on mechanical, dynamic mechanical and ther-mal properties of the NBR nanocomposites were studied.The effect of nanoclay content on the transport behavior wasalso investigated. The transport coefficients and the thermo-dynamic parameters for sorption of solvent trough the nano-composites along with gas permeation rate were alsoevaluated.

Experimental

Materials

Nitrile rubber (NBR) with 33% acrylonitrile content andhaving Mooney viscositiy of ML (1+4) at 100 C047.0was procured from Apar Industries Ltd., Mumbai, India.The nanoclay used was Cloisite 20A, a natural montmoril-lonite modified with quaternary ammonium salt (organicmodifierdimethyl dehydrogenated tallow, quaternary am-monium, cation exchange capacity 95 meq/100 g clay, 90%dry particle size less than 13 m and d00102.42 nm) wasprocured from Southern Clay Products, USA. Other com-pounding ingredients were obtained from standard suppliersin India. The formulation used for the study contained NBR(100 phr), sulfur (1.5 phr), zinc oxide (5 phr), stearic acid(1 phr), nanoclay (varied), dioctyl phthalate (DOP, variedmaintaining nanoclay : DOP ratio at 4:1), dibenzothiazoledisulfide (MBTS, 1.25 phr) and tetramethyl thiuram disul-fide (TMTD, 0.25 phr).

Preparation of NBR nanocomposites

A two-step method was employed to prepare NBR nanoclaycomposites. In the first step a masterbatch containing 3 partsNBR and 1 part nanoclay by weight was prepared using

Fissions Haake Rheocord 90. For masterbatch preparation,NBR was masticated at 60 rpm till the torque stabilized,followed by addition of nanoclay and mixing for 10 min. Inthe second step, the NBRnanoclay masterbatch was com-pounded with neat NBR along with other compoundingingredients in a two roll mill with friction ratio of 1.25.Nanocomposites with varying contents of nanoclay wereprepared. The rubber formulations were evaluated for curecharacteristics on an oscillating disc rheometer (TechProRheotech ODR- ASTM D-2084). The NBRNanoclaycomposites were compression molded at 150 C and20 MPa for the optimum cure time in a hydraulic press tomake approximately 2 mm thick rubber sheets. The sampleswere designated NBRNCLX, where X is the amount ofnanoclay (0, 2, 5, 7.5 and 10 phr). Thus NBRNCL5 indi-cates NBR nanoclay composite having a filler loading of5 phr.

Characterization

The intergallery distance of the nanocomposites was mea-sured using wide angle X-Ray diffraction studies on BrukerD8 ADVANCE X-ray diffractometer with Cu X-ray beam ofwavelength 0.154 nm. The diffraction curves were obtainedwithin the range of scattering angles (2) of 210 at a scanrate of 3/min. The d-spacing of the nanoclay was deter-mined from the 2 position of the diffraction peak based onBraggs law.

The morphology of the nanocomposites was examinedby transmission electron microscopy (TEM) images takenwith JEOL 2010 electron microscope with accelerator volt-age of 200 kV. The nanocomposite samples for TEM anal-ysis were prepared by ultra cryomicrotomy at -80C usingLeica Ultracut UCT. Freshly sharpened glass knives withcutting edge of 45 were used to get cryosections of 100 nmthickness.

To evaluate the tensile properties (strength, modulus andelongation at break) dumbbell specimens were punched outfrom the molded sheets and tested as per ASTM D412method on a Universal Testing Machine at a crossheadspeed 500 mm/min. For all mechanical properties, the aver-age of five experimental values is reported.

Dynamic mechanical analysis was done using DMAQ800. Rectangular specimens having dimensions 35122 mm were used. The dynamic moduli and mechanicaldamping (tan ) were evaluated. The dynamic analysiswas done in dual cantilever mode with temperature scanfrom -70 C to +70 C using a heating rate of 2 C/min atfrequency range 1 Hz10 Hz.

The thermal stability of the samples from 35 to 500 Cwas investigated using NETZSCH STA 409 CD TGA-DTAsimultaneous analyzer at a heating rate of 10 C/min innitrogen atmosphere.

Page 2 of 10 M. Balachandran, S.S. Bhagawan

The gas permeation rate through the nanocomposites wastested using Lyssy L 1005000 manometric gas permeabil-ity tester with oxygen gas at 10 mL/h and 23 C.

Transport properties were studied using rectangular sam-ples of size 20202 mm that were cut from the NBRnanoclay composites. The edges of the samples were slight-ly curved to obtain uniform absorption. The thickness andinitial weight of samples were measured. The samples werecompletely immersed in toluene in glass diffusion bottleskept at uniform temperature. The samples were removedfrom the solvent at specific time intervals, excess solventat the surface removed using filter paper and weighed. Thesamples were returned to the solvent in the diffusion bottleimmediately. The process was continued until equilibriumswelling is reached. At time t of immersion, the mole per-cent uptake Qt for solvent was determined using the formula

Qt Mt Mo MW=Mo

100 1

where Mt is the mass of sample after time t of immersion,Mo the initial mass of the sample and MW is the molecularweight of the solvent. The sorption isotherms were plottedwith the mole percentage uptake of solvents for the nano-composites versus square root of time. The diffusion andpermeability coefficients were then calculated. The experi-ments were conducted at 30, 50 and 70 C. The valuesreported here is the average of three experiments.

Results and discussion

Morphology

XRD patterns indicate the state of dispersion of nanoclay inthe polymer matrix. Nanoclay exhibits a peak correspondingto the d-spacing between the layers of the nanoclay.When nanoclay is delaminated and exfoliated in thematrix, the XRD peaks disappear [36]. When the nano-clay forms intercalated structures in the polymer matrix,a shift in position of peak occur corresponding to alarger d value [31]. Figure 1 shows XRD patterns forthe NBR nanocomposites.

For Cloisite 20A, the peak in the XRD pattern occurred at203.775, corresponding to d spacing of 2.3387 nm. ForNBR nanoclay composites containing 2 and 5 phr nanoclay,no peaks were observed indicating exfoliation. For nano-composite with 10 phr nanoclay, the XRD peak shifted to5.12 (corresponding to d01.7247 nm). The occurrence ofpeak suggested that the nanoclay was not exfoliated in theNBR matrix and formed aggregates. The decrease in inter-layer distance is due to reaggregation of silicates, which canbe attributed to participation of the alkyl groups in Cloisite20A in the curing reaction during vulcanisation [11, 32, 33].

The reduction in intensity of peak (compared to nanoclay) at10 phr nanoclay content showed that some amount of dis-persion of the clay had taken place. The intercalation ofnanoclay in NBRnanoclay systems is due to the interac-tion between the butadiene segments of NBR and organicsurface of the modified nanoclay [13]. It can be inferredthat, NBRnanoclay composites form a mixture of inter-calated and exfoliated structures.

Transmission electron microscopy was used to study thenanostructure of NBRlayered silicate composites. TheTEM micrographs of NBR nanocomposites containing 2, 5and 10 phr are shown in Fig. 2(a), (b) and (c), respectively.At low concentration of nanoclay (2phr), the nanoclay wasdispersed as single platelets as well as small aggregatesconsisting of few stacks of clay platelets. At 5 phr, thenanoclay was relatively more dispersed in the NBR matrixas exfoliated platelets with lesser stacks of clay platelets. Athigher nanoclay content, i.e., 10 phr, the nanoclay formedaggregates. These stacks gave rise to the peak observed inthe XRD pattern.

Mechanical properties

The stressstrain characteristics of the NBR nanocompositesare shown in Fig. 3. The stress increased continuously withstrain, as seen in typical synthetic elastomers. As nanoclaycontent in the composites increased, the stress in the nano-composites increased for the same strain level up to 5 phrand then decreased. The mechanical properties of theNBRnanoclay composites are given in Table 1. Thetensile strength increased rapidly with increasing claycontent in the range 25 phr; at 7.5 phr the value oftensile strength showed a gradual decrease. The tensilemoduli of nanocomposites increased with increasing claycontent up to 7.5 phr and with further addition of nano-clay the moduli decreased. The ratio of modulus of filledcompounds (M100f) to that of unfilled NBR (M100u) is ameasure of the reinforcing effect of the nanoclay filler.This ratio increased with nanoclay content, reached peakvalue at 57.5 phr and then decreased.

The state of dispersion of nanoclay in the NBR matrixand the interfacial interaction between the nanoclay andNBR determines the mechanical properties of the nanocom-posites [24, 34, 35]. The increase in tensile strength of NBR-nanoclay composites, even at low concentration of nanoclaywas due to the exfoliation and uniform distribution of nano-clay in the rubber matrix as evident from the morphologystudies, rigidity of the nanoclay and the interaction betweenNBR and organo-modified nanoclay [29]. At 10 phr, thedispersion of nanoclay in NBR matrix is poor because of theformation of aggregates. This decreased the interaction be-tween the nanoclay and NBR and consequently decreasedthe tensile strength. The elongation at break also showed

Properties of NBRnanoclay composites Page 3 of 10

increase with increasing nanoclay content. The increase inelongation is partly due to the plasticizing effect of thealkylammonium ion in the organo-modifier of the nanoclaywhich is located at the rubber-nanoclay interface [19]. Athigher nanoclay content, the formation of non-exfoliatedaggregates made the composites stiffer. This resulted inreduction in elongation at break.

Dynamic mechanical behaviour

The response of material to oscillatory deformation wasmeasured using dynamic mechanical analysis (DMA).DMA was performed to yield storage modulus (E), lossmodulus (E) and tan (E/E) as a function of temperature.Figure 4 shows the dynamic elastic (storage) modulus E forneat NBR and NBR nanoclay composites as a function oftemperature. The values of tan, E and Tg (glass transitiontemperature) obtained from DMA of NBRnanoclay com-posites are shown in Table 2. The dynamic mechanicalproperties are influenced by nanoclay in the NBR nanocom-posites. NBRnanoclay composites showed enhancement instorage modulus above the glass transition temperature Tg atall concentrations of nanoclay. The high aspect ratio of

nanoclay and formation of intercalated and exfoliated structurein the rubber matrix that resulted in enhanced interfacialinteraction between the nanoclay and NBR is the reason forthe improvement in E of the nanocomposites [36]. As in thecase of static mechanical properties, the storage modulus andloss modulus values showed peak values at 5 phr nanoclaycontent. At higher nanoclay contents, the formation of aggre-gates leads to a decrease in the dynamic moduli. Below Tg,there was no significant change in E values with varyingnanoclay content, owing to the suppression of the mobilityof the polymer segments near the interface in the rubberyplateau region [5].

The coefficient C, calculated using Eq. (2) is a measure ofthe effectiveness of the filler on the moduli of the composites[37].

C E0G=E0R compositeE0G=E0R re sin 2

where EG and ER are the storage modulus values in theglassy and rubbery region, respectively. The higher the valueof the coefficient C, lower the effectiveness of the filler. Themeasured values of E at -50 C and +50 C were used as EGand ER, respectively. From the values of C, given in Table 2,

Fig. 1 XRD pattern ofNBR-nanoclay composites

Fig. 2 TEM micrographs of NBRnanoclay composites for different nanoclay contents a 2 phr b 5 phr and c 10 phr

Page 4 of 10 M. Balachandran, S.S. Bhagawan

it can be noted that the effectiveness of the filler increased upto 5 phr and thereafter decreased at 10 phr of nanoclay. Sinceexfoliation and uniformity in dispersion were highest at 5 phr,the stress transfer between the matrix and the filler henceeffectiveness of the filler was maximum at this concentrationof nanoclay.

The effect of nanoclay content on loss factor (tan ) as afunction of temperature at frequency of 1 Hz is shown inFig. 5. Incorporation of nanoclay lowered the peak value oftan and thereby reduced the damping properties of thesystem. The lowest value of tan was at 5 phr nanoclayloading. At 10 phr, the peak value was lower than that ofunfilled rubber but higher than that at 5 phr nanoclay.However, there was no change in Tg values due to theaddition of nanoclay. The area under the peak in tan vs.temperature curve is a measure of energy dissipated [20]. Asseen from the curve, there was marginal narrowing of peaksand reduction of damping properties was marginal.

Thermal behavior

Thermo gravimetric analysis was performed in nitrogen atmo-sphere to study the thermal stability of NBRnanoclay com-posites. The thermal stability factors, viz. initial decomposing

temperature (IDT), temperature at the maximum rate of heatloss (Tmax) and the char content at 500 C were calculatedfrom the TGA thermograms and are listed in Table 3. Thethermal stability of the composites was enhanced on additionof nanoclay. In neat rubber, the initial decomposition temper-ature (IDT), the temperature at which the degradation starts isaround 401C. On addition of nanoclay, there was no changein IDT. The thermograms for the NBR nanocomposites aregiven in Fig. 6. However, the temperature at which maximumrate of decomposition occurs increased with increased nano-clay content. The enhanced thermal stability of NBR nano-composites is due to the restricted thermal motion of thepolymer chains in the silicate layers of the nanoclay [24].The char content of the nanocomposites at 500 C increasedwith nanoclay content.

Gas permeation rate

The oxygen permeation rate through the NBR nanocompo-sites is given in Table 4. It is observed that at lower nanoclaycontents (2 and 5 phr) the permeation rate decreased appre-ciably. Exfoliation of clay improves the barrier characteristicsof polymers by providing tortuous path for the diffusion of gas

Fig. 3 Stressstrain Characteristics of NBRnanoclay composites

Table 1 Mechanical Properties of NBRNanoclay Composites

Name NanoclayContent phr

TensileStrength MPa

Elongation atbreak %

M100 MPa M200 MPa M300 MPa M100f/M100u

NBRNCL0 0 2.19 558 0.54 0.97 1.18 1.00

NBRNCL2 2 3.08 593 0.72 1.10 1.58 1.33

NBRNCL5 5 6.98 721 1.08 1.82 2.41 1.98

NBRNCL7.5 7.5 4.51 575 1.22 1.84 2.26 1.98

NBRNCL10 10 4.01 595 0.79 1.38 1.87 1.45

Fig. 4 Storage Modulus (E) vs. Temperature plots of NBRnano-clay composites at 1 Hz

Properties of NBRnanoclay composites Page 5 of 10

molecules through the polymer [27]. The dispersion and ex-foliation of the nanoclay platelets increased the path lengthrequired to transport the permeating molecule through therubber matrix thereby decreasing the rate of transport [38].At higher nanoclay contents, the lengths of the tortuous pathdecreased due to formation of aggregates and lesser extend ofexfoliation resulting in increased gas permeation rate throughthe composites.

The suitability of Nielsons model to predict the barrierproperty of NBR nanocomposites was studied. The model isgiven by Eq. (3) [27].

kc=km 1 8 = 1 a8=2 3where kc and km are the gas permeability of the compositeand matrix respectively, is the volume fraction of the fillerin the matrix and is the aspect ratio of the filler (taken as100 [39]). Figure 7 shows the relative permeability of (kc /km)of NBR nanocomposites, both experimental and those pre-dicted byNielsons theory as a function of nanoclay content. Itwas found that Nielsons theory was satisfactory at lowernanoclay concentrations, whereas at higher nanoclay contents,the experimental values were well above the theoretical pre-diction. Nielsons model assumes uniform arrangement ofclay platelets in the polymer matrix. This assumption is not

valid in the case of nanocomposites with higher nanoclaycontent as the nanoclay formed agglomerates and the disper-sion of nanofiller is not uniform.

Transport characteristics

The effects of nanoclay content on the diffusion, sorptionand permeation of toluene through NBR-nanoclay compo-sites were studied. The transport behavior through compo-sites depends on the type of filler, matrix, temperature,reaction between solvent and the matrix, etc. Hence thestudy of the transport process through composites can beused as an effective tool to understand the interfacial interac-tion and morphology of the system. The swelling behavior ofNBRnanoclay composites was assessed by calculatingswelling coefficient, using the equation [40]

b M1Mo Mo

x1s 4

where Mo and M are the mass of the sample before swellingand after swelling respectively and s is the density of thesolvent. Table 4 shows that the swelling coefficient decreaseswith increasing nanoclay content. The sorption curves (Qt(moles of solvent sorbed per 100 g of rubber) vs. t) are

Table 2 Dynamic mechanical analysis of NBRNanoclay compo-sites at 1 Hz

Sample tan Emax Tg (C) from C E0G=E0R comp

E0G=E0R re sin

max (MPa) tan E DSC

NBRNCL0 1.3 221 8. 14 22.5 1

NBRNCL2 1.3 247 8 14 22.4 0.9

NBRNCL5 1.1 281 10 16 24.2 0.5

NBRNCL7.5 1.3 272 9 14 24.6 0.6

NBRNCL10 1.2 249 8 16 24.8 0.6

Fig. 5 Effect of Nanoclay content on tan of NBR- nanoclay compo-sites at 1 Hz

Table 3 Thermal Stability factors of NBR-nanoclay compositesobtained from TGA

Name NanoCaCO3Content phr

IDT ( C) Tmax (C) Char %

NBRNCL0 0 401 450 9.42

NBRNCL2 2 392 538.4 10.11

NBRNCL5 5 393 610 10.72

NBRNCL10 10 397 607 18.41

Fig. 6 Thermograms for NBR- nanoclay composites

Page 6 of 10 M. Balachandran, S.S. Bhagawan

shown in Fig. 8 for varying nanoclay content. . Figure 8 showsthe effect of nanoclay content on the toluene uptake with time.The sorption of toluene was reduced for the nanofilled compo-sites compared to unfilled rubber. The dispersion of nanoclayin the rubber matrix created tortuous path for the transport ofthe solvent. The Qt vs t curve showed two distinct regionsan initial steep region with high sorption rate due to largeconcentration gradient and a second region exhibiting reducedsorption rate that ultimately reaches equilibrium sorption. Thesorption rate and equilibrium solvent uptake of NBR nano-composites reduced with increased nanoclay content. Beyond5 phr the solvent uptake increased slightly. This was due to theformation of agglomerates of nanoclay which is evident fromthe TEM micrographs.

The crosslink density () was calculated from the sorp-tion data using Eq. (5) [4042]

u 1=Mc 5where Mc is the molecular weight of the polymer betweenthe crosslinks. Mc is calculated using Eq. (6) [4042]

Mc PVs81=3

ln1 8 8 c82 6

where Vs is the molar volume of the solvent, P is thedensity of the polymer, is the interaction parameter and

8 is the volume fraction of rubber in the solvent-swollenfilled sample. 8 is given by Ellis and Welding equation as[4042]

8 D FMO P=D FMO P= AP S=

7

where D is deswollen weight, f is fraction of insolublecomponents, Mo is weight of sample taken, P and s aredensities of the polymer and solvent respectively and AS isthe weight of the absorbed solvent. The solvent interactionparameter is obtained from the equation [42]

c g VSRT

dS dP 2 8

where s is the solubility parameter of the solvent (18.2MPa1/2

for toluene) [43], P is the solubility parameter of the polymer(19.4 MPa1/2 for NBR) [43], + is the lattice constant(generally taken as 0.34 for elastomersolvent systems),Vs is the molar volume of the solvent (106.3 mL/gmol), Ris the universal gas constant and T is the temperature inKelvin. The estimated values of Mc for NBRnanoclaycomposites are tabulated in Table 4. Nanoclay filled sys-tems have lower Mc values , i.e. lower molar mass

Table 4 Oxygen Permeation Rate and Swelling Coefficient and Crosslink Densities of NBR-nanoclay composites

Sample Oxygen permeationrate ml/m2/day

Swelling Coefficient() cm3/g

Molar mass betweencrosslinks (Mc) g/mol

Crosslink density104

(gmol/cm3)

NBRNCL0 932.3 2.77 3445 1.45

NBRNCL2 600.4 2.31 2622 1.91

NBRNCL5 610.5 2.26 2643 1.89

NBRNCL7.5 778.6 2.29 2787 1.79

NBRNCL10 861.8 2.28 2872 1.74

Fig. 7 Plot of oxygen permeability ratio of NBRnanoclay compos-ite to matrix, kc/km as a function of nanoclay content Fig. 8 Sorption Isotherms for NBR- nanoclay composites at 30 C

Properties of NBRnanoclay composites Page 7 of 10

between crosslinks than unfilled NBR and Mc decreasedwith increasing nanoclay content. As the value of Mcdecreased, the available volume between adjacent cross-links decreased. The decrease in volume restricts the dif-fusion process. The slight increase in Mc at nanoclaycontent greater than 5 phr is due to the aggregation ofthe nanofiller as seen in the TEM micrographs. The valuesof crosslink density calculated supported this observation.The crosslink density values of NBRnanoclay compo-sites are tabulated in Table 4. As the nanofiller contentin the composites increased, the crosslink density alsoincreased, the peak value occurring at 5 phr.

The diffusivity (D) of the nanocomposites was calculatedusing the Eq. (9) given below [42, 4446]

D p h4Q1

29

where h is the thickness of the sample, is the slope of thesorption curves before attaining 50% equilibrium (the initiallinear portion of the curve) and Q is the equilibrium sol-vent uptake.

The sorption coefficient was calculated using Eq. (10)[42, 4446]

S M1MP

10

where, M is the mass of solvent taken up at equilibriumswelling and MP is the mass of the sample.

The net diffusion through polymer depends on the differ-ence in the amount of penetrant molecules between the two

surfaces. Hence, the permeability can be expressed as[42, 4446]

P D S 11where D is the diffusivity and S is the solubility. Sol-ubility is taken as mass of solvent sorbed per unit massof the sample.

The diffusion, sorption and permeability coefficients ofNBRnanoclay composites at 30 C, 50 C and 75 C aregiven in Table 5. The diffusion of solvent through a com-posite depends on the geometry of the filler (size, shape, sizedistribution, concentration, and orientation), properties ofthe filler, properties of the matrix, and interaction betweenthe matrix and the filler [46]. The transport coefficients forthe nanocomposites are considerably lower than those of theunfilled NBR. The diffusion of the penetrant solventdepends on the concentration of free space available in thematrix to accommodate the penetrant molecule [44]. Theaddition of nanoclay reduced the availability of these freespaces, restricted segmental mobility of the rubber matrixand create tortuous path for transport of solvent moleculesthrough the nanocomposites. However, at nanoclay contentgreater than 5 phr, there was an increase in diffusion andpermeation coefficients. In this case also the increase intransport coefficients can be attributed to the aggregationof the nanofiller. Similar trends were observed for sorptionstudies conducted at 50 C and 75 C also. As the temper-ature increased, the coefficient of diffusion also increasedfor all the samples. With increase in temperature, the ther-mal energy increased and consequently molecular vibration

Table 5 Transport Coefficients and Thermodynamic Parameters of NBR- nanoclay composites

Sample Diffusion Coefficient(D107) m2/s

Sorption Coefficient(S) g/g

Permeability Coefficient(P107) m2/s

ED, kJ/mol EP, kJ/mol Hs kJ/mol

30 C 50 C 75 C 30 C 50 C 75 C 30 C 50 C 75 C

NBRNCL0 6.58 8.11 8.46 2.39 2.32 2.16 15.7 18.8 18.3 4.86 2.88 1.98

NBRNCL2 6.02 7.77 8.18 2.00 1.94 1.88 12.0 15.1 15.4 5.93 4.73 1.20

NBRNCL5 5.20 6.64 7.51 1.96 1.81 1.73 10.2 12.0 13.0 7.15 4.77 2.38

NBRNCL7.5 5.36 6.89 7.77 1.98 1.87 1.76 10.6 12.9 13.6 7.20 4.83 2.36

NBRNCL10 5.37 7.10 7.81 1.98 1.87 1.76 10.6 13.3 13.8 7.24 4.99 2.25

Table 6 n and k values fordiffusion of toluenethrough NBRnanoclaycomposites

SAMPLE 30 C 50 C 75 C

n k (g/gmin2) n k (g/gmin2) n k (g/gmin2)

NBRNCL0 0.484 0.076 0.495 0.068 0.507 0.071

NBRNCL2 0.494 0.058 0.507 0.063 0.514 0.068

NBRNCL5 0.485 0.061 0.528 0.054 0.489 0.076

NBRNCL7.5 0.465 0.070 0.519 0.057 0.486 0.079

NBRNCL10 0.501 0.055 0.505 0.062 0.496 0.075

Page 8 of 10 M. Balachandran, S.S. Bhagawan

of solvent molecules, the free void volume in the polymermatrix and flexibility of the polymer chains increased [47].As a result, the diffusion coefficients of the NBR compositesincreased at higher temperatures. The transport coefficientsat 50 C and 75 C are also shown in Table 5.

The energy required for the diffusion or permeation ofsolvent molecule is computed using Arrhenius equation [44],

X X0eEX RT= 12where X is D or P, Xo is a constant representing either Do or Po(diffusion and permeation coefficients extrapolated to zeropermeant concentration), R is the gas constant, T is the tem-perature in Kelvin and Ex is the activation energy. The activa-tion energy for diffusion (ED) is obtained from the slope of lnD versus 1/T plot. The activation energy of diffusion oftoluene through NBR nanocomposites was found to be higherthan that of unfilled polymer. The nanofillers have higherspecific surface area which leads to enhanced rubber-fillerinteraction resulting in enhanced reinforcement. As the nano-filler content increases, the activation energy needed for dif-fusion also increases. The activation energy for permeation(EP), evaluated using the Arrhenius equation showed similartrends as ED. The enthalpy of sorption Hs was determinedby van Hoff equation [48].

EP Hs ED 13It is observed that the sorption is an exothermic process. Thevalue of Hs increases with increasing nanofiller content.

To evaluate the mechanism of sorption, the solvent uptakedata of the nanocomposites were fitted to the equation [45, 46]

logQt=Q1 logk nlog t 14

where Qt is the mol% increase in uptake at time t, Q is themol% increase in uptake at equilibrium, and k is a constantcharacteristic of the sample which indicates the interactionbetween the sample and solvent. The values of n and k weredetermined by linear regression analysis is tabulated inTable 6. Generally, the diffusion behavior of polymeric com-posites can be classified according to the relative mobility ofthe penetrant and of the polymer segments into (i) Case I orFickian diffusion, (ii) Case II diffusion and (iii) non Fickian oranomalous diffusion. For a Fickian mode of diffusion, thevalue of n is equal to 0.5 and the predominant driving forcefor diffusion is the concentration gradient. The rate of diffu-sion is much less than the rate of relaxation of polymer chains.In Case II diffusion, n is equal to 1 and the rate of diffusion ismuch higher than the relaxation process. When the value fallsbetween 0.5 and 1, the diffusion is anomalous and the rate ofdiffusion becomes comparable with the rate of relaxation ofpolymer chains [49]. In the case of NBR- nanoclay the value ofn at room temperature (30 C) is almost equal to 0.5 and thediffusion is fickian type, controlled by concentration dependent

diffusion coefficient. In rubbery polymers, well above theirglass transition temperatures, the polymer chains adjust quick-ly to the presence of penetrant molecules and hence they do notexhibit anomalous behavior. It is observed that as the temper-ature increases, the value of n increases for NBR- nanoclaycomposites, implying that the diffusion tend to be of anoma-lous type.

Conclusion

The tensile strength and modulus of NBRnanoclay com-posites increased with nanoclay content, upto 5 phr.. XRDand TEM investigations showed exfoliated and few interca-lated structures at low nanoclay content. At higher concen-trations, the nanoclay had tendency to form agglomerates.Addition of nanoclay enhanced the storage modulus, lossmodulus and thermal stability of nanocomposites. The trans-port behavior of solvent through the nanocomposites wasinvestigated using sorption isotherms. The diffusion, sorp-tion and permeation coefficients for diffusion of toluenethrough NBR- nanoclay composites were evaluated andfound to be decreasing with nanofiller content. The activa-tion energy for diffusion and permeation for the nanocom-posites were higher than that of neat NBR. The diffusion oftoluene through NBRnanoclay composites was largelyFickian.

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Page 10 of 10 M. Balachandran, S.S. Bhagawan

Mechanical, thermal and transport properties of nitrile rubber (NBR)nanoclay compositesAbstractIntroductionExperimentalMaterialsPreparation of NBR nanocompositesCharacterization

Results and discussionMorphologyMechanical propertiesDynamic mechanical behaviourThermal behaviorGas permeation rateTransport characteristics

ConclusionReferences