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Ceramics International 40 (2014) 7439–7448www.elsevier.com/locate/ceramint

Development of butyl rubber–rutile composites for flexible microwavesubstrate applications

Janardhanan Chameswary, Mailadil T. Sebastiann

Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology (CSIR), Trivandrum 695019, India

Received 12 October 2013; received in revised form 6 December 2013; accepted 21 December 2013Available online 31 December 2013

Abstract

Butyl rubber–micron rutile (BR/MRT) and butyl rubber–nano rutile (BR/NRT) composites were prepared by sigma mixing. The effect ofmicron rutile and nano rutile content on dielectric properties at 1 MHz and 5 GHz were investigated. For 0.30 volume fraction (vf) of micron rutileloading, the composites have attained a relative permittivity (εr) of 8.59 and loss tangent (tanδ) of 0.0024 at 5 GHz, and for the same volumefraction of nano rutile content, the composite showed εr of 7.62 and tanδ of 0.0031 at 5 GHz. The thermal properties such as thermal conductivity(TC) and coefficient of thermal expansion (CTE) of both composites were studied as a function of filler loading. It is found that the nano rutileadded butyl rubber composites have better thermal properties than those micron composites. The stress–strain curves of both composites show themechanical flexibility of the composites. The experimental relative permittivity and thermal conductivity of both composites were compared withtheoretical models.& 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Composite; C. Thermal properties; C. Dielectric properties; Elastomer

1. Introduction

Stretchable electronics or elastic electronics is one of themost recently developed class of large area electronics [1].Stretchable electronic materials enable classes of applicationssuch as electronic eye cameras, artificial skins, flexible sensorsand actuators that cannot be achieved using conventional,wafer-based technologies [2]. Stretchable electronics can betwisted, folded and conformally wrapped onto arbitrarilycurved surfaces without any significant change in operatingcharacteristics. [3]. On plastic substrates, this deformation ispermanent. Elastomeric substrates allow circuits to go a shapebeyond: to reversible deformation and near-arbitrary dimen-sions [1]. Flexible and stretchable substrates are developed inorder to satisfy the present electronic world0s requirements.Beyond traditional electronics, potential stretchable applica-tions include biomedical, wearable, portable and sensorydevices, such as cyber skin for robotic devices and implantableelectronics. Stretchability can be introduced into the electronic

e front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2013.12.091

g author. Tel.: þ91 471 2515294; fax: þ91 471 249 1712.ss: mailadils@yahoo.com (M.T. Sebastian).

system by the use of suitable stretchable substrate materials[4]. The researcher Zhigang Wu from Uppsala University hasdevised a stretchable wireless sensor which can measureintensive body movements and the design enables wirelessmeasurement of repeated bending across movable parts [5].Materials to be used for stretchable and flexible substrate

applications should satisfy certain criteria which are mechanicalflexibility, low relative permittivity and low loss tangent, highthermal conductivity, low coefficient of thermal expansion etc.Elastomer ceramic composites can be used for these applications.The composite combines the mechanical flexibility and lowprocessing temperature of elastomer with excellent dielectric andthermal properties of ceramics. Butyl rubber is chosen as theelastomer matrix because of its excellent dielectric properties andgood weathering properties due to its paraffinic non polarcharacter [6]. Rutile is selected as the ceramic filler for thepresent study due to its excellent dielectric and thermal proper-ties. The microwave dielectric properties of rutile was firstreported by Cohen and has εr¼100, Q¼10,000 at 3.45 GHzand tauf¼þ400 ppm/1C [7].Several polymer ceramic composites were reported for

microwave substrate applications [8–11]. Recently elastomer

ghts reserved.

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–74487440

based composites were also widely used for the stretchablesubstrate applications. The microwave dielectric properties ofbutyl rubber based ceramic composites were reported bySebastian and co workers [12–16]. Crippa et al. synthesizedpolystyrene–rutile nano composites which exhibit high relativepermittivity and low dissipation factor in the frequency range10–106 Hz. They coated rutile nano particles with polystyreneand then dispersed into polystyrene matrix [17]. Ratheesh et al.prepared rutile filled PTFE composites and dielectric, thermaland mechanical properties were studied as a function of fillerloading. At optimum rutile loading of 67 wt%, the compositeattained a relative permittivity of 10.2 and loss tangent of0.0022 at X-band frequency region [18]. Ratheesh et al. alsoinvestigated the effect of nano rutile ceramic on the microwavedielectric properties of PTFE composites and compared theproperties with that of micron rutile–PTFE composites. It wasfound that PTFE–nano rutile composites have high losstangent and higher moisture content than that of microncomposites [19]. Nayak et al. prepared flexible polyur-ethane–titania composites by two different sequences ofmixing method and reported that preparation methods changethe morphology of the composites and thus results in thechange of dielectric properties [20]. Kashani et al. arrangedtitanium dioxide filler particles into a chain structure in asilicone rubber matrix by dielectrophoretic effect using analternative electric field and the composite achieved anincreased relative permittivity and reduced dielectric loss inthe orientation direction of filler particles [21]. Saritha et al.investigated the mechanical, thermophysical and diffusionproperties of TiO2 filled chlorobutyl composites [22].Eventhough rutile filled polymer and elastomer compositesare investigated, the microwave dielectric properties of butylrubber–rutile composites are not yet studied. The present paperfocuses on the microwave dielectric, thermal and mechanicalproperties of butyl rubber–rutile composites. The polymernano composites have found several industrial applicationsdue to its enhanced electrical, mechanical and thermal proper-ties when compared with that of the micron size filler polymercomposites. As the nano size filler particles have high specificsurface area, the improved properties can be achieved at lowfiller loading itself. Hence the present investigation alsofocuses on the effect of filler size on microwave dielectric,thermal and mechanical properties of butyl rubber–rutilecomposites.

Table 1Formulation of the composites used.

Ingredients1;# BR/MRT-0 BR/MRT-1 BR/MR

Butyl rubber 100 100 100Zinc oxide 5 5 5Stearic acid 3 3 3Tetra methyl thiuram disulphide 1 1 1Sulfur 0.5 0.5 0.5Micron rutile ()2;$ 0 (0) 10 (0.021) 25 (0.0

The composition of BR/NRT composite is same as BR/MRT composite. But the n#Parts per hundred.$The corresponding filler volume fraction is given in parenthesis.

2. Experimental

The micron rutile powder (average size of 2 μm) wasprepared from anatase by heating at 1200 1C for 4 h. Thepowder was then ground well and sieved through a 25 μmsieve. Nano rutile (average size 100 nm) was procured fromSigma Aldrich and was dried at 100 1C for 24 h before use.Butyl rubber–micron rutile (BR/MRT) and butyl rubber–nanorutile (BR/NRT) composites were prepared by sigma mixing asdescribed in our earlier paper [23]. The compositions of bothcomposites are given in Table 1. The BR/MRT compositeswere prepared with a volume fraction of micron rutile loadingfrom 0 to 0.40 vf. The nano rutile has higher surface area.Hence the possibility of agglomeration of nano particlesincreases with filler loading. This makes the processing ofbutyl rubber nano rutile composites difficult at higher fillerloading [24]. Hence a maximum loading of 0.30 vf of nanorutile is possible in the case of BR/NRT composites.The microstructures of the composites were examined using

a scanning electron microscope (SEM) (Jeol Model, JSM5600LV). The dielectric properties at 1 MHz were measuredfollowing the parallel plate capacitor method using a LCRmeter (Hioki Model, 3532-50). The microwave dielectricproperties were measured using a Split Post Dielectric Reso-nator (SPDR) with the help of a vector network analyzer(Agilent Technologies, E5071C, ENA Series) [25]. Themicrowave dielectric properties of both composites were alsoinvestigated after bending the samples manually by an angle of1801 and the bending cycle was repeated for 125 times. Thethermal conductivity of the composites was measured by laserflash technique using a thermal conductivity analyzer (FlashLineTM 2000, Anter Corporation, USA) by using the relation

TC¼ λ� Cp� ρ ð1Þwhere λ is the thermal diffusivity, Cp is the specific heatcapacity at room temperature and ρ is the density of thesample.The linear coefficient of thermal expansion of each compo-

site was measured using a dilatometer (Netzsch Model, DIL402 PC) in the temperature range from 30 1C to 100 1C. Themoisture absorption characteristics of the composites weremeasured following ASTM D 570-98 procedure [15] using thesamples with dimensions 50 mm� 50 mm� 2 mm. The sam-ples were weighed accurately and immersed in distilled water

T-2 BR/MRT-3 BR/MRT-4 BR/MRT-5 BR/MRT-6

100 100 100 1005 5 5 53 3 3 31 1 1 10.5 0.5 0.5 0.5

52) 50 (0.099) 100 (0.18) 200 (0.30) 300 (0.40)

ano rutile loading is limited to 0.3vf.

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–7448 7441

for 24 h. The samples were then taken out and again weighedafter removing the excess water from the surface. The volume% of water absorption was then calculated using the relation,

Volume % water absorption ¼ ðWf �WiÞ=ρwðWf �WiÞ=ρwþWi=ρc

� 100 ð2Þ

where wi and wf are the initial and final weights of the sampleand ρw and ρc are the densities of distilled water and sample,respectively.

Tensile tests of both composites were carried out in aUniversal Testing Machine (Hounsfield Model, H5K-S UTM)with a rate of grip separation of 500 mm/min.

3. Results and discussion

3.1. Microstructure

The morphology of filler particles and composites are shownin Fig. 1. The micron size rutile ceramic consists of flake likeparticles with an average size of 2 μm as shown in Fig. 1(a).The nano size rutile consists of agglomerated and irregularlyshaped particles with an average size of 1 μm and composedof nano particles of average size 100 nm and is depicted inFig. 1(b). Fig. 1(c) and (d) shows the fractured SEM images ofBR/MRT-5 and BR/NRT-5, respectively. A homogenousdispersion of filler particles in the matrix can be seen fromboth the figures eventhough some pores are present due to theagglomeration of filler particles at higher filler loading.

3.2. Dielectric properties

Fig. 2(a) and (b) shows the variation of relative permittivityof butyl rubber–micron rutile and butyl rubber–nano rutile

Fig. 1. SEM image of (a) micron rutile powder (b) nano rutile po

composites as a function of filler loading at 1 MHz and 5 GHz,respectively. The relative permittivity of both the compositesincreases with filler content at both the frequencies which isexpected since the relative permittivity of rutile is higher thanthat of butyl rubber matrix. As the ceramic content increases,the connectivity among the filler particles increases andthereby dipole–dipole interaction increases [26]. Consequentlyrelative permittivity of the composites increases with fillerloading. It is worth to be note that the relative permittivity ofmicron rutile filled composite shows higher relative permittiv-ity than that of nano composites. Generally the relativepermittivity of nano composite is higher than that of microncomposite. This unusual behavior may be due to the differencein morphology of micron rutile and nano rutile. Furtherdetailed studies are needed to understand this unusual beha-viour of butyl rubber–rutile composites.The loss tangent is an important factor affecting the

frequency selectivity of materials. Fig. 3(a) and (b) showsthe variation of loss tangent of butyl rubber with micron rutileand nano rutile content at 1 MHz and 5 GHz, respectively.From the figures it is clear that the loss tangent of both thecomposites increases with filler loading at both the frequen-cies. The accumulation of charges at the interfaces causespolarization in heterogeneous systems and the relaxation ofthis polarization causes loss at low frequencies. As the fillercontent increases the interfacial area increases and conse-quently the loss tangent of composites increases. It is alsonoted that the loss tangent of BR/NRT composite is higherthan that of BR/MRT composites. The increased lattice straindue to high surface area of nano rutile particles and highermoisture content [19,27] in the butyl rubber–nano rutilecomposites contribute to higher loss tangent of BR/NRTcomposites.

wder (c) fractured surface of BR/MRT-5 and (d) BR/NRT-5.

Fig. 2. Variation of εr of BR/MRT and BR/NRT composites with filler content(a) at 1 MHz and (b) 5 GHz.

Fig. 3. Variation of tanδ of BR/MRT and BR/NRT composites with fillercontent (a) at 1 MHz and (b) 5 GHz.

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–74487442

3.3. Theoretical modeling of relative permittivity

The prediction of relative permittivity of a composite is veryimportant for the design of composite materials since thedielectric properties of the composites are affected not only bythe relative permittivities of the constituent phases but also byother factors such as the morphology, dispersion and theinteraction between the two phases. The following modelswere used to predict the relative permittivity of the presentcomposites [28].Lichtenecker equation:

ln εef f ¼ ð1�vf Þln εmþvf ln εf ð3ÞMaxwell–Garnet equation:

εef f ¼ εm2εmþεf þ2vf ðεf �εmÞ2εmþεf �vf ðεf �εmÞ

ð4Þ

Jayasundere–Smith equation:

εef f ¼εmð1�vf Þþεf vf ½ð3εm=εiþ2εmÞ�½1þðð3vf ðεf �εmÞ=εiþ2εmÞÞ�

1�vf þvf ½ð3εm=εf þ2εmÞ�½1þðð3vf ðεf �εmÞ=εf þ2εmÞÞ�ð5Þ

Effective medium theory (EMT):

εef f ¼ εm 1þ vf ðεf �εmÞεmþnð1�vf Þðεf �εmÞ

� �ð6Þ

where εeff, εf, εm are the relative permittivity of the composites,filler and matrix, respectively. vf is the volume fraction of thefiller and n is a shape factor determined empirically.Fig. 4 shows the comparison of experimental εr of compo-

sites with theoretical values of εr. The measured relativepermittivity of both butyl rubber–micron rutile andbutyl rubber–nano rutile composites shows deviation fromMaxwell–Garnet equation and Jayasundere–Smith equation.Maxwell Garnet equation is valid only at very low fillerloading since the model considers the composite as sphericalinclusions surrounded by sufficient concentric layers of hostmatrix. For higher filler loading the filler particles may not becompletely surrounded by the polymer matrix, it showsdeviation at higher ceramic content [29]. The JayasundereSmith equation also shows considerable deviation fromexperimental data. The filler particles are assumed to bespherical with an equal radius in Jayasundere–Smith model[30]. Since the micron rutile particles have flake like morphol-ogy (Fig. 1(a)), it shows deviation from experimental εr. Thenano particles show aggregating tendency and are not identicalspheres (Fig. 1(b)), the measured relative permittivity of BR/NRT composite also shows deviation. From Fig. 4 it is clearthat the experimental εr of nano composite is in goodagreement with Lichnetcker equation and effective mediumtheory (EMT). Ratheesh et al. used Lichnetcker equationand EMT model for PTFE–rutile nano composites [19].The Lichnetcker equation is a simple logarithamic law ofmixing and holds well for butyl rubber–nano rutile composites.This model is matching with BR/MRT composites upto amicron rutile loading of 0.30 vf and shows deviation at higherceramic loading. This may be due to the agglomeration of filler

Fig. 4. Comparison of theoretical and experimental relative permittivity of BR/MRT and BR/NRT composites at 5 GHz.

Fig. 5. Variation of relative permittivity of (a) BR/NRT and (b) BR/MRTcomposites with bending.

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–7448 7443

particles at higher loading. The EMT model reported by Raoet al. [31] is suitable for predicting the effective relativepermittivity of present composites since it involves a morphol-ogy factor ‘n’. The value of n for the BR/NRT composite is0.17 and is almost matching with the n value reported forPTFE–rutile nano composites [19]. The n value for BR/MRTcomposite is 0.14 since the morphology of micron rutilepowder is different from nano rutile powder. The morphologyfactor represents only the morphology of ceramic material.Hence the two different n values. This model also showsdeviation at higher micron rutile loading and this may be dueto the imperfect dispersion of filler particles at high loading.

3.4. Bending

Figs. 5 and 6 show the effect of repeated bending on themicrowave dielectric properties of butyl rubber–micron rutileand butyl rubber–nano rutile composites, respectively. FromFig. 5(a) and (b), it is clear that the relative permittivity of bothBR/MRT and BR/NRT composites is independent of bending.The loss tangent of both composites shows only smallvariation with repeated bending as shown in Fig. 6(a) and (b).

3.5. Temperature dependence of relative permittivity at 1 MHz

The relative permittivity of materials should be stable withinthe operational range of temperature for practical applications.Fig. 7(a) and (b) shows the temperature variation of relativepermittivity at 1 MHz of butyl rubber–nano rutile and butylrubber–micron rutile composites, respectively. It is worth to benoted that all the compositions of both composites were foundto be almost thermally stable within the measured temperaturerange. It is found that the relative permittivity of the compositesdecrease with temperature and this may be due to the decreasein polarizability of dipoles with temperature [32].

3.6. Thermal conductivity

The substrate materials should have high thermal conduc-tivity in order to dissipate the heat generated during theoperation of electronic devices [33]. The thermal propertiesof the composites are influenced by dispersion and orientationof the filler particles, the filler aspect ratio, the relative ratio ofthermal conductivity of the filler and the matrix etc. Fig. 8shows the variation of thermal conductivity of butyl rubber–micron rutile and butyl rubber–nano rutile composites withvolume fraction of ceramic filler. The thermal conductivity ofboth composites increases with filler loading since rutile hashigh thermal conductivity than that of butyl rubber matrix.This may be due to the formation of continuous thermallyconductive chains as the filler-filler contact increases withceramic loading [22,34]. Literature survey reveals that the highaspect ratio filler added composite have high thermal con-ductivity than that of lower aspect ratio filler [35]. Hence onecan expect higher thermal conductivity (TC) for BR/MRTcomposites. But from the figure it is evident that the TC ofbutyl rubber–nano rutile composites is higher than that of butylrubber–micron rutile composites for the same filler content.Wang et al. reported that flake like filler could not easily formconductive chains [36]. Since the micron rutile particles haveflake like structure, the BR/MRT composite have lowerthermal conductivity than that of BR/NRT composites. The

Fig. 6. Variation of loss tangent of (a) BR/NRT and (b) BR/MRT compositeswith bending.

Fig. 7. Temperature dependence of εr at 1 MHz (a) BR/NRT and (b) BR/MRTcomposites.

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–74487444

particle size also plays a major role in thermal properties forcomposites. The nano particles can achieve higher packingdensity of filler in matrix and thereby thermal conductivityincreased for nano composites [37]. Meera et al. observed ahigher TC for 12–13 nm silica filled natural rubber compositesthan that of 190 nm TiO2 filled natural rubber composites [38].

Fig. 8. shows the variation of thermal conductivity of BR/MRT and BR/NRTcomposites with ceramic loading.

3.7. Theoretical modeling of thermal conductivity

Researchers used several theoretical approaches for predict-ing the thermal conductivity of polymer ceramic composites.The following theoretical models were used to predict thethermal conductivity of present composites [39,40]

Series mixing rule

1kc

¼ vfkf

þ vmkm

ð7Þ

Parallel mixing rule

kc ¼ vf kf þvmkm ð8ÞGeometric mean model:

kc ¼ kfvf km

1� vf ð9ÞMaxwell–Eucken model

kc ¼ kmkf þ2kmþ2vf ðkf �kmÞkf þ2km�vf ðkf �kmÞ

� �ð10Þ

Cheng Vachon model

1kc

¼ 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCðkm�kf Þ½kmþBðkf �kmÞ�

p

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–7448 7445

ln

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi½kmþBðkf �kmÞ�p þ B

2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCðkm�kf Þ

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi½kmþBðkf �kmÞ�

p � B2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiCðkm�kf Þ

p þ 1�B

kmð11Þ

where B ¼ffiffiffiffiffiffiffiffiffiffiffi3vf =2

pC ¼ �4

ffiffiffiffiffiffiffiffiffiffiffi2=3vf

pwhere kc is the effective

thermal conductivity of the composite, km and kf are thethermal conductivity of matrix and filler, and vm and vf are thevolume fractions of matrix and filler, respectively.

Fig. 9 shows the comparison of experimental and predictedthermal conductivity of BR/MRT and BR/NRT compositeswith varying filler content. The physical structures assumed inthe series and parallel models are of layers of the phasesaligned either perpendicular or parallel to the heat flow. Theseries and parallel model of TC gives only lower and upperlimits of thermal conductivity values of composites respec-tively [39]. It is clear from figure that the experimental valuesof both the composites lies within the range of these twomodels. The Geometric mean model is in good agreement withexperimental values of both the composites. Maxwell Euckenmodel assumes randomly distributed and non interactinghomogeneous spheres in a homogeneous medium. The Chengand Vachon assumed a parabolic distribution of the discontin-uous phase in the continuous phase based on Tsao0s model[41] which gives the thermal conductivity of two phase solidmixture. The experimental values are found to be higher thanthat predicted by Maxwell Eucken and Cheng Vachon model.It is very difficult to predict the thermal conductivity ofdifferent materials due to the wide variations in filler geometry,orientation and dispersion. Also the Kapitza resistance, [42]the interfacial boundary thermal resistance between the fillerparticles and the matrix is not considered while calculating thethermal conductivity of composites. It arises from the combi-nation of poor mechanical or chemical adherence at theinterface and a mismatch in CTE. No experimental methodseems to be available for the direct measurement of interfacialthermal resistance [43]. Hence the theoretical and experimentalTC values are not in agreement.

Fig. 9. Comparison of theoretical and experimental thermal conductivity ofBR/MRT and BR/NRT composites with filler loading.

3.8. Thermal diffusivity and specific heat capacity

Fig. 10(a) and (b) shows the variation of thermal diffusivityand specific heat capacity of butyl rubber–nano rutile and butylrubber–micron rutile composites, respectively. The thermaldiffusivity of both composites increases with filler loading.Similar observation was reported by Meera et al. in TiO2 andsilica nanoparticle filled natural rubber composites [38]. Thespecific heat capacity of both BR/MRT and BR/NRT compo-sites decreases with filler loading. This is due to the lowspecific heat capacity of rutile than that of rubber matrix.

3.9. Coefficient of thermal expansion

The variation of coefficient of thermal expansion of bothBR/MRT and BR/NRT composites with filler content is givenin Fig. 11. The CTE of both composites were reduced withceramic addition as expected since the CTE of rutile (9.2 ppm/1C) is less than that of rubber matrix (191 ppm/1C). The freevolume of polymer decreases as the ceramic loading increasesand thereby thermal expansion of the composite is suppressed[44]. The coefficient of thermal expansion of BR/NRTcomposite is much lower than that of BR/MRT composites.

Fig. 10. Variation of thermal diffusivity and specific heat capacity of (a) BR/NRT and (b) BR/MRT composites with ceramic loading.

Fig. 11. Variation of coefficient of thermal expansion of BR/MRT and BR/NRT composites with ceramic loading.

Fig. 12. Variation of moisture absorption with ceramic loading of BR/MRTand BR/NRT composites.

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–74487446

This may be due to the more physical crosslinking points andincreased mechanical interaction between filler and matrix inthe nano rutile filled butyl rubber composite [37].

3.10. Moisture absorption

The presence of moisture content will affect the electricalproperties of composites since water is polar molecule. Fig. 12shows the variation of moisture absorption of BR/MRT andBR/NRT composites with ceramic content. The volume % ofmoisture content of both composites increases with fillerloading. It can be also seen that the nano composites havehigher moisture content than that of micron composites. Thenano rutile particles absorb more moisture content due to itslarge surface area. Hence BR/NRT composites have moremoisture content than that of BR/MRT composites.

Fig. 13. Stress–strain curves of BR/MRT and BR/NRT composites.

3.11. Mechanical properties

Fig. 13 shows the stress strain curves of BR-0, BR/MRT-5and BR/NRT-5. From the figure it is clear that the stressneeded for elongation increases with filler content. It is alsoevident that the stress needed for nano rutile filled butyl rubbercomposite is higher than that of micron rutile filled butylrubber composites. Fu et al. reported that the particle size,particle–matrix interface adhesion and particle loading are themain factors which affect the mechanical properties ofparticulate filled polymer composites [45]. The more uniformdispersion of nano particles in the rubber matrix is responsiblefor the high stiffness of BR/NRT-5 composite. Both thecomposites are not broken even upto an elongation of1000%. Hence the present composite is suitable for flexibleand stretchable applications.

4. Conclusions

The butyl rubber–micron rutile and butyl rubber–nano rutilecomposites were prepared by sigma mixing. The effect of fillerloading and filler size on dielectric, thermal and mechanicalproperties of the composites was explored. The relativepermittivity and loss tangent of BR/MRT composite for 0.30volume fraction of filler loading were 8.59 and 0.0024respectively and that of BR/NRT composite for the same fillercontent were 7.62 and 0.0031 at 5 GHz, respectively. Thethermal and mechanical properties of both the composites wereimproved with filler loading. The measured properties revealthat both butyl rubber–micron rutile and butyl rubber–nano

J. Chameswary, M.T. Sebastian / Ceramics International 40 (2014) 7439–7448 7447

rutile composites are suitable candidates for flexible andstretchable microwave substrate applications.

Acknowledgement

The authors are grateful to the Council of Scientific andIndustrial Research (CSIR), India for the award of SeniorResearch Fellowship. The authors are thankful to Dr. P.Prabhakar Rao and Mr. M. R. Chandran for SEM, and Mr.Brahmakumar for tensile measurements.

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