Materials and Design 50 (2013) 322–331

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Materials and Design

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Improving performance of low-temperature hydrogenated acrylonitrilebutadiene rubber nanocomposites by using nano-clays

0261-3069/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.matdes.2013.02.072

⇑ Corresponding author. Address: No. 1, South Da Hong Men Road, FengtaiDistrict, Beijing, China. Tel.: +86 010 6838 3313; fax: +86 010 6838 2974.

E-mail address: zjhicca@iccas.ac.cn (J. Zhang).

Jihua Zhang ⇑, Lifeng Wang, Yunfeng ZhaoAerospace Research Institute of Material and Processing Technology, Beijing 100076, PR China

a r t i c l e i n f o

Article history:Received 2 December 2012Accepted 25 February 2013Available online 14 March 2013

Keywords:A. NanomaterialE. MechanicalH. Selection for material properties

a b s t r a c t

The modifications of low-temperature rubbers have received little attention for a long time despite oftheir importance in the field of gas or fluid sealing. In this article, the compounding of low-tempera-ture-grade hydrogenated acrylonitrile butadiene rubber (LTG-HNBR) with organically modified montmo-rillonite (OMMT) clays was fabricated for the purpose of improving bulk properties. Heat mixing is foundthe first step to achieve high performance of the composites due to its advantages in increasing disper-sivity of OMMTs. The results of SAXS and TEM show that the OMMTs are partially exfoliated and inter-calated in the matrix. The addition of OMMTs barely affects glassy temperature of matrix, but improvesits low-temperature elasticity. The amount of 10 phr organoclays is believed the optimum one to exploitthe clay networks where strong interactions with rubber cause dramatic reinforcement. Tensile strengthof its nanocomposites is even comparable to that with 35 phr carbon black. Moreover, the incorporationof organoclays reduces the oil-swelling of the bulk and improves its thermal stability. Therefore, weexpect that LTG-HNBR nanocomposites may be used to sever the oil-sealing applications at a lowtemperature.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Recently, hydrogenated acrylonitrile butadiene rubber (HNBR)has attracted the scientists’ great interests due to its practicalapplications, such as some key rubber components of automobilesand seals and gaskets for oil drilling devices. As a substitute forNBR, HNBR saturates its residual double bonds through the hydro-genation process and represents the specialty rubber with goodthermal, oil, UV, and ozone resistance [1,2]. However, low-temper-ature-grade of HNBR (LTG-HNBR) has received less attention thanothers in spite of its importance in the field of oil-sealing. Gener-ally, lowering acrylonitrile (ACN) content is a main technologicalroute to achieve LTG-HNBR products. But its flexible molecularchains caused by low ACN content of approximately 19 wt% areeasily piled up and then crystallized. The rubber will become stiffand lose the elasticity at a low temperature although it is not brit-tle even at the temperature of �60 �C [3]. This makes a fatal prob-lem with application. In order to solve it, a monomer with largeside groups is chemically introduced to reduce the pile-up of eth-ylene groups during the polymerization of LTG-HNBR, which thusremains rubber elasticity at the glassy temperature of below�30 �C [4]. But the reduction in crystallinity deteriorates otherbasic properties of rubber such as mechanical property and the

ability of oil or heat resistance. To make up for the property lossof the bulk, commercial product of in situ LTG-HNBR/zinc dimeth-acrylic salt (ZDMA) compounding has been fabricated to achievecombination properties of high strength, good low-temperature-elasticity and oil-resistance by adding the zinc oxide and metha-cylic acid (at a certain ratio) [5]. Yet, the hardness of this productis too high (Shore A hardness is even more than 90) and its vulca-nizates are easy to stick molds [6]. Such a rubber compoundingis always used by mixing itself with other HNBR products in thepractice, which conversely brings some new problems about itslow-temperature property [7]. Therefore, we believe that it be-comes an urgent need to prepare the LTG-HNBR rubbers with highperformance by other physical or chemical modifications.

Carbon black is the most widely used filler in rubber technologywhose reinforcing effects are caused by the fractal nature of bothcarbon black aggregates, larger-scale filler networks in the rubbermatrix and the surface activity [8]. Inorganic fillers (silica, lime-stone, kaolin, etc.) tend to form large aggregates in rubbers, leadingto drawbacks in processing and poor reinforcement [9]. Contrary tothem, the layered montmorillonite (MMT) has received specialattention due to its distinct characteristics arising from the nano-scopic dimensions and high aspect ratio, such as a large interfacialarea, and potential confinement effects on the polymer properties[10,11]. The academic theory illustrates that the glassy layeraround the fillers (or aggregates) is responsible for the reinforce-ment of rubber matrix because it acts as ‘‘glue’’ between the fillers,rendering the effect particularly strong when combined with a

J. Zhang et al. / Materials and Design 50 (2013) 322–331 323

high percolation of the filler network. In contrast with conven-tional (micro-) rubbers, rubber composite with MMT clay displaysa larger relative interphase volume whose percolation occurs atmuch lower volume fraction, with correspondingly large effectson the bulk properties, especially mechanical properties. Moreover,the compounding of NBR with organoclays has been suggested asimple method to enhance oil-resistance property of the bulk be-cause the dispersion of inorganic nano-fillers, in principle, greatlyincreases the tortuosity of the path that diffusing molecules mustfollow during the permeation through the polymeric membrane[12]. Hence this modification arouses us to construct an easy andpromising road to improve properties of the LTG-HNBR as theoil-sealing material.

In this article, the rubber composites were prepared by addingorganically modified MMT (OMMT) into LTG-HNBR matrix afterthey were mechanically blended on a two-roll mill and then vulca-nized. By combining the micro-morphology with X-ray scatteringexperiments, we investigated the nature of the clay-rubber inter-face, taking as a reference of the rubber bulk behavior. After that,some physical properties of LTG-HNBR composites, such as oil-resistance, tensile strength and low-temperature elasticity werestudied in detail. In addition, to give some evidences about the ori-gin of exceptional improvements on their properties, mechanismanalysis associated with clay dispersions or affinities was also dis-cussed in comparison with the conventional modifications.

2. Experimental details

2.1. Materials

LTG-HNBR (Zetpol 4310) with the acrylonitrile mass fraction ofabout 18.6 wt% and 5% residual double bonds was provided byZeon Co. (Tokyo, Japan). OMMT (I.30P, using octadecyl ammoniumchloride as the intercalant) was supplied by Nanocor Co., USA. Itsmorphology is recorded in Fig. S1. Other chemicals and ingredients,such as cure agent of dicumyl peroxide (DCP) and triallyl isocyan-urate (TAIC) were purchased from China.

2.2. Preparation

Various loadings of OMMTs were firstly added into LTG-HNBRwith the clay/rubber mass ratio of 0/100, 5/100, 10/100, 15/100and 20/100 by a mixing roller for 3 min at room temperature. Theywere then kneaded to prepare LTG-HNBR blends on a two-roll millwith 90 �C for 10 min. On the other hand, a comparison of process-ing method was made that some LTG-HNBR blends with the massratio of 10/100 were continued to mix for 10 min at room temper-ature. Other compounding and crosslinking additives (including3.0 phr ZnO, 2.0 phr stearic acid, 3.0 phr MgO, 3.0 phr antioxidant,6.0 phr DCP and 1.5 phr TAIC) were added. They were continued toknead at room temperature for 10 min. After that, the blends werehot-pressed under the pressure of 15 MPa and vulcanized at 160 �Cfor 20 min, then naturally cooled down to room temperature. Fi-nally, the vulcanized specimens were further put into an oven atthe temperature of 150 �C for 4 h in order to vulcanize completelyand avoid the aging effect on their short-term properties (second-step vulcanization) [13]. By this way, the vulcanized compoundingof LTG-HNBR with organoclays was obtained.

3. Characterization

3.1. Scanning electron microscopy (SEM)

Images of scanning electron microscope (SEM) attached withenergy dispersive spectrometer (EDS) unit under accelerating

voltage of 30 kV (S440, Leica Cambridge Ltd., England) were takenfrom the representative fractured surfaces of rubber compositesafter they were gold coated. The SEM specimens were preparedby fracturing them in liquid nitrogen as described in [8].

3.2. Transmission electron microscopy (TEM)

A Hitachi H-800-1 transmission electron microscope (TEM),which was purchased from Hitachi Ltd. (Tokyo, Japan), was usedto examine the morphology of the OMMTs in the composites,and the TEM specimens were prepared at �100 �C with a Reic-hert-Jung Ultra-cut microtome manufactured in Leica Camera AG(Leitz, Germany) and mounted onto 200-mesh copper grids byreferring to [6].

3.3. Small-angle X-ray scattering experiment (SAXS)

Small-angle X-ray scattering (SAXS) was used to study the nat-ure and extent of the dispersions of the clays in the filled speci-mens by referring to [14]. The SAXS patterns were obtainedusing a Bruker AXS (Cu Ka, the wavelength of k is 1.5406 Å) at40 kV and 40 mA in a transmission mode. The range of scatteringvectors q from the (nh nk nl) peak was 0–40 nm�1 which can be ex-pressed as.

q ¼ 4pnk

sin h; ð1Þ

where n is an integer, k is the wavelength of incident light, h is theangle between the incident light and the scattering planes. On thebasis of the Bragg’s law, the basal interplanar spacing (d) of the clayis estimated by a mathematical transform:

d ¼ 2pq: ð2Þ

3.4. Dynamic mechanical analysis (DMA)

According to the standard [15], dynamic viscoelastic measure-ments (DMA) were made with a dynamic mechanical analyzer(VA4000, METRAVIB Co., French) by using cylinder specimens (itsdiameter and height is 10 mm and 10 mm, respectively). The tem-perature dependence of compressing moduli was measured in arange of �60 to 60 �C at the constant frequency of 125 Hz andthe heating rate of 3 �C/min.

3.5. Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared spectra (the wavenumber range of400–4000 cm�1) were obtained from an accumulation of 100 scansat a resolution of 2 cm�1 (FTS3000, Bio-Rad Co., USA). The spectrawere taken at ambient temperature and the specimens wereground in KBr, as described elsewhere [16].

3.6. Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC) measurements werecarried out according to [17] with PerkinElmer Pyris1 calorime-ter, USA. All specimens were heated from �150 to 50 �C at arate of 10 �C/min to investigate their glass-transitiontemperature.

3.7. Elasticity testing at the low temperature

The elasticity at low-temperature was determined by compres-sion coefficient at low temperature (K) according to the standard[18]:

324 J. Zhang et al. / Materials and Design 50 (2013) 322–331

K ¼ h2 � h1

h0 � h1; ð3Þ

where h0 is the initial height of the specimen, h1 is its compressedheight at the low-temperature preset by the measurement (in ourexperiments, the initial compressed ratio was 30%, or h1 = 0.7h0),h2 is its recoverable height when the compressed force is releasedat the low-temperature.

3.8. Mechanical properties

Tensile and tear tests were carried out at room temperature byusing a Testometric Universal Tester M350-20 kN at a crossheadspeed of 500 mm/min in accordance with the Chinese standards[19,20]. In tear tests, the trouser-shaped samples were chosen touse. At least five specimens were tested for each rubber compositeand their mean value was recorded.

3.9. Swelling properties

On the basis of the standard [21], the ability of oil-resistancewas characterized by the equilibrium volumetric swelling ratio inthe hydraulic pressure oil (No. 10, meeting the technology stan-dard of [22]): DV = (V1 � V0)/V0, where V1 is the swelling volumeof the specimen in the medium and V0 is its initial volume driedin the air.

3.10. Thermogravimetric analysis (TGA)

By referring to [23], thermogravimetric analyses (TGA) of allspecimens were performed by using a TA instrument (Mettler-to-ledo STARe system TGA/DSC1, Switzerland) under a nitrogen atmo-sphere at a purge rate of 20 ml/min. The specimens were heatedfrom 30 �C to 700 �C at a rate of 10 �C/min.

4. Results and Discussions

4.1. Optimal mixing step and its SEM analysis

To achieve good performances of the rubber composites, opti-mal preparation is firstly discussed. Two mixing steps are used tocompare: one is at the high temperature of 90 �C; the other is atthe room temperature. Table 1 lists various properties of theLTG-HNBR composites mixed by different steps, including mechan-ical, low-temperature elasticity and oil-swelling behaviors. Betterbasic properties of LTG-HNBR composites are immediately discov-ered in the case of being mixed at 90 �C owing to its outstandingmechanical property. The tensile strength of the specimen pre-pared by heat mixing is even more three times than the one atthe room temperature. To explore the origin of property differ-ences from the mixing steps, their micro-morphologies on the

Table 1Various physical properties of LTG-HNBR nanocomposites including 10 phr OMMTafter different mixing steps.

Heatmixing

Room-temperaturemixing

Hardness/Shore A 59 55Tensile strength (MPa) 9.61 2.28Elongation at break (%) 268 110Permanent set (%) 10.2 2Tearing strength (KN/m) 19 13.8Compression coefficient K �30 �C 0.69 0.67�35 �C 0.22 0.20Swelling ratio (at 100 �C for 72 h, in No.

10 oil)1.394 1.396

fracture surfaces are examined (see Fig. 1). Simultaneously, localspectrograms of EDS attached to the SEM are recorded to distin-guish the dispersed clays from other rubber compounding, suchas ZnO or MgO. Note that the scanned elements of dispersed fillersinclude C, O, Si, Mg and Al, which are the characteristic onesbelonging to organoclays (see Fig. 1b). OMMT-clays are brokendown into some small layers and uniformly dispersed in the matrixafter they were mixed at the temperature of 90 �C and then vulca-nized. The lengths of nano-clay are roughly estimated between 50and 400 nm. It is said that the platelet-like OMMTs has been wellblended in the rubber matrix by the heat mixture step. On theother side, Fig. 1c shows the SEM image of the rubber compositewhich was mixed at the room temperature. There are some obvi-ous agglomerations of fillers (their sizes are beyond 10 lm) besideswell-dispersed layered ones. The element analysis displays thatthese agglomerations come from OMMT clays. One magnifiedSEM image for such agglomeration is exhibited in the black frameinset of Fig. 1d. It is very close to the morphology of original clayswhich are stacked by lots of layered clays (see Fig. S1). The effectsof mixing temperature on the dispersion of clays are similar to thatof OMMT/silicone rubber system, which heavily affected the basicproperties of rubber composites [24]. Therefore, heat-kneading isbelieved a key mixing step to obtain optimal properties of LTG-HNBR composites, which has been used in the follow-upexperiments.

4.2. TEM observations

In order to display the dispersion of nano-clays in rubber ma-trix, their morphologies are checked by TEM. Fig. 2 shows repre-sentative TEM images of the LTG-HNBR nanocompositesincluding 5 phr and 10 phr clays, respectively. There are obviouslytwo phase domains in the composites, that is, matrix and dispersedOMMT phase. It can be seen that OMMT clays are partially exfoli-ated and intercalated in the rubber matrix, which is agreed withother HNBR (not low-temperature grade) compounds with clays[13]. This is always found in most nanocomposites composed byplastic polymer or rubber due to their low extent (<10%) of exfoli-ation for clays [11,25,26]. Careful comparison is made that thereare more intercalated structures (thick layer-bundles) of clays inthe case with 10 phr OMMTs than that with 5 phr OMMTs. Thetrends are developed to agglomerate in the matrix when theiramounts are above 10 phr. Moreover, such clay-agglomerationscan be also observed from the local SEM images, such as the nano-composites with 20 phr clays (see Fig. S2). It is noticeable that theaggregations or agglomerations of plate-like OMMTs would reducetheir phase interfaces and thus suppress the interactions of claynetworks with the matrix.

To accurately describe the dispersivity of clay particles on theproperties of rubber matrix, a digital analysis for the clay particlesis further conducted. Main attentions are paid on the nanocompos-ites with 5 phr and 10 phr OMMTs because other measurements,such as SEM observation, are difficult to make a nice distinctionfor fine clay-structures at such two contents. Relying on the pro-gress of Paul and his colleagues’ works [27], the TEM images ofpolymer compounding with clays can be simply digitized. Thusthe length, thickness and aspect ratio of the traced particles issimultaneously analyzed by software Image-Pro Plus. Fig. 3 showsa series of representative histograms of particle length and aspectratio for HNBR nanocomposites containing various amounts ofOMMTs. All of the features, whether particle length or aspect ratio,show broad distributions. On the other hand, special defined sizesof dispersed clays are also calculated on base of imaging analysis(i.e., the number and weight averages of the length, thicknessand aspect ratio, see [28]). The statistical results of particle analysison nanocomposites containing 5 phr and 10 phr OMMTs are listed

Fig. 1. The fractured-surface SEM images of the LTG-HNBR nanocomposites with 10 phr organoclays by using different mixture steps: (a) mixing at the temperature of 90 �Cand (b) its local EDS spectrogram for nanofillers; (c) mixing at room temperature and (d) its magnified inset.

Fig. 2. Representative TEM images of the rubber nanocomposites including different OMMT amounts: (a) 5 phr and (b) 10 phr.

J. Zhang et al. / Materials and Design 50 (2013) 322–331 325

in Table 2. Note that the average particle lengths, ln or lw, for thenanocomposite with 10 phr OMMTs are a bit larger than that with5 phr one. The situation parallels to the average particle thick-nesses. However, a contrary trend is seen in their average aspectratios, either for the number average ones or the weight averageones. This suggests that a small amount of OMMT is easy to ratherexfoliate than intercalate because two-roll mill can provide stron-ger shear stress of matrix to peel less clays when everything else isequal. Moreover, the higher the amounts of OMMTs in the matrixare, the closer to the original clays their contour features are (i.e.,a large length and thickness), implying the development of beingbundled or agglomerated of clay-platelets with the increase ofOMMT amounts. This is consistent with the SEM observations.On the other hand, we can also address that all data of aspect ratiois roughly close between the nanocomposites with 5 phr and10 phr OMMTs. This means that there would be tremendousimprovements on basic property of the composite with 10 phrOMMTs owing to the increment of clay amounts. In addition, ourdata of average aspect ratio seems smaller than that of Paul’s re-ports about ethylene–vinyl acetate (EVA) copolymers [28], whichresults primarily from the high shear ability from their twin screwextruder and viscosity of matrix at the mixing temperature.

4.3. SAXS characterization

SAXS provides a supplement of microstructure informationabout the distribution of clays in the rubber matrix to TEMobservations. Then SAXS scattering patterns of neat rubber, origi-nal OMMT and corresponding rubber nanocomposites are re-corded. Fig. 4a shows 2D scattering striations of organoclayand its nanocomposites. Note that the scattering patterns be-come more distinct for nanocomposites than the original OMMT,which is attributed to the higher aspect ratio of peeled clays inthe rubber matrix according to [29]. In agreement with TEM re-sults, the multi-peak-structures in the intensity-scattering vector(q) plots (see Fig. 4b) show that OMMTs mainly present interca-lated ones in LTG-HNBR matrix, however no peak appears for theneat rubber due to its amorphous state. For nanocomposites, thed-spacing of the clays is estimated from the scattering patternsaccording to the Bragg’s law. It is seen that the d-spacing((001) primary peak) enlarges the interlayer space of OMMTsfrom 2.22 nm to 3.21 nm upon 5 phr one is mixed with rubbermatrix. Further increases of respectively 3.41 nm, 3.42 nm and3.45 nm are observed when more OMMTs (10 phr, 15 phr and20 phr, correspondingly) are added into the matrix. Moreover,

Fig. 3. Histograms of particle length and aspect ratio for rubber nanocomposites with different OMMT amounts: (a and b) 5 phr; (c and d) 10 phr.

326 J. Zhang et al. / Materials and Design 50 (2013) 322–331

the scattering intensity is also improved owing to the increase ofclay amounts (above 10 phr). Fig. 4c shows the OMMT contentdependence of d-spacing ((001) peak) and corresponding scat-tering angle 2h. The angle of 2h can be expressed by using thevector q

2h ¼ 2� arcsinkq4p

� �ð4Þ

It is easily found that the 2h value of nanocomposite with 10 phrclays almost reaches a minimum one although it still decreases abit with the increment of OMMT amounts.

4.4. The interaction analysis by dynamic mechanical properties andFTIR

The effects of OMMT loadings on the dynamic mechanical prop-erties of the vulcanized composites are studied by DMA over awide temperature range (�60–60 �C). The loss factor tand and stor-age modulus E’ as a function of temperature is shown in Fig. 5. It isfound that tand peaks of nanocomposites shift to higher tempera-tures when clay amounts increase. The similar research has beenreported that tand peaks of NBR composites with clays shifted tothe higher temperature whereas the ones with carbon black didnot really shift because of the stronger interaction between claysand the matrix [17]. So this implies the similar restrain of clayson the mobility of LTG-HNBR chains at such a frequency. The rela-tion is held between tand (tandc) of rubber composites with fillersand the loss factor (tandR) of neat rubber by Ziegel and Romanov[30]:

tan dc ¼tan dR

1þ 1:5UOMBð5Þ

where UOM is volume fraction of fillers and B is the parameter of theinteraction between fillers and neat rubber. Due to being frozen ofchain segments, the loss factor peaks (tandc,peak) of nanocompositesdisplay the intensity of rubber’s interactions with organoclays. Therelationship between the loss factor peak ratio of nanocomopsitesto bulk one (tandc, peak/tandR,peak) and UOM is drawn in Fig. 5b.We calculate the interaction parameter B of the clays for all nano-composites by referring to Eq. (5) (see Fig. S3). Note that the valueof B firstly increases and then decreases with the increments ofOMMT amounts. Its peak value belongs to the nanocomposite with10 phr clays, which indicates again the good filler–matrix affinity atsuch content. Fig. 5c shows the temperature dependence of storagemodulus E’ for rubber nanocomposites with various OMMTamounts. Note that the moduli E’ of LTG-HNBR nanocompositesare beyond that of the bulk one at the same temperature. The in-crease of the modulus imparted by active nanoclays may be re-garded as the product of two terms: one involving ahydrodynamic effect arising from the inclusion of rigid fillers. It isquantitatively taken into account by the Guth expression [31]:

E ¼ E0ð1þ 2:5UOM þ 14:1U2OMÞ ð6Þ

where E0 is the modulus of the unfilled matrix. Since the anisotropiccharacter is important for layered OMMTs, a shape factor has to betaken into account in predicting a clay-reinforced nanocompositemodulus. For nonspherical particles similar to rod-like fillers, ashape factor f (i.e., the aspect ratio of particles) is introduced [32]:

Table 2The statistical results of particle analysis for different nanocomposites.

The amounts ofOMMT (phr)

Total numberof particles

Number averageparticle length(ln, nm)

Number averageparticle thickness(tn, nm)

Weight averageparticle length(lw, nm)

Weight averageparticle thickness(tw, nm)

Number averageparticle aspectratio (l/t)n

ln/tn Weight averageparticle aspectratio (i/t)w

lw/tw

5 96 190.9 30.5 212.3 45.1 12.2 6.3 16.4 4.710 124 199.6 36.5 228.9 52 10.1 5.5 14.6 4.4

J. Zhang et al. / Materials and Design 50 (2013) 322–331 327

E ¼ E0ð1þ 0:67fUOM þ 1:62f 2U2OMÞ ð7Þ

By applying the data of Table 2 into Eq. (7), the modulus hasbeen foreseen to become larger with the fraction increase ofUOM. For example, the modulus of nanocomposite with 5 phrOMMTs (UOM = 0.022) is calculated as E = 1.30E0 (its (l/t)n of 12.2)while that one with 10 phr OMMTs (UOM = 0.044) is E = 1.61E0

(its (l/t)n of 10.1). The second term involves the effects of the phys-ical cross-linking density from polymer–filler interactions [11,33–35] which is therefore evaluated by FTIR. Fig. 6 shows the FTIRspectra at the wavenumber range of 750–1350 cm�1 for originalOMMT, neat rubber and corresponding nanocomposites. Thereare four components of Si–O vibration for the organoclays [16]:peak I (1101 cm�1), peak II (1035 cm�1), peak III (1001 cm�1) andpeak IV (978 cm�1), all of which belong to in-plane mode exceptpeak II (the out-of-plane mode). By comparison with [16,36], theseabsorptions are moved to lower wavenumbers which is caused bydifferent clay products. Among them, the position and width ofpeak II is most sensitive to the interlayer spacing of clays and thuschosen as a primary observed object. Note that the peak II, which isbarely seen for original OMMT, becomes very visible in LTG-HNBRnanocomposites. This is well explained that dispersion oforganoclays is intercalated ones to expand their gallery heights

Fig. 4. SAXS scattering patterns of neat rubber and corresponding nanocomposites: (a) 2content dependence of d-spacing (the black color square) and 2h at (001) peak (the bluereader is referred to the web version of this article.)

according to [36]. With the increase of organoclay amounts(15 phr), the absorption becomes relatively narrower and its inten-sity is higher than that with 10 phr OMMTs. Meanwhile, its peakposition is shifted to the higher wavenumber than that of originalorganoclays, which even reaches 1081 cm�1, implying the stronginteraction of pristine clays with rubber molecules. That meansthat the shield effects of long alkyl groups from the surfactantson the pristine MMT platelets have been partly replaced by theirinteractions with rubber chains: (1) the entanglements of surfac-tants’ long alkyl tails; (2) the affinity of pristine clay. Besides, thearea ratios of peak III against peak IV for the nanocomoposites isalso calculated by software Origin where the one from 15 phrOMMT is lower, suggesting a larger gallery height. This agrees withSAXS results. In accordance with [29], it is said in our cases that theeffects of organoclays on the modulus of the bulk are more com-plex than the hydrodynamic reinforcement mode predicted byEq. (7).

4.5. DSC analysis and low-temperature elasticity

Low-temperature property of rubber is determined by theglassy transition temperature (Tg) because its service temperature

D SAXS patterns; (b) the plots of intensity versus scattering vector q; (c) the OMMTcolor circle). (For interpretation of the references to color in this figure legend, the

Fig. 5. Dynamic mechanical properties for neat rubber and corresponding nano-composites: (a) temperature dependence of tand; (b) the plot of the ratio of tandc,

peak to tandR,peak versus fraction UOM; (c) temperature dependence of storagemodulus E’.

Fig. 6. FTIR spectra of neat rubber, organoclay and corresponding nanocompositesat the wavenumber range of 750–1350 cm�1.

Fig. 7. DSC curves of neat rubber and corresponding nanocomposites.

328 J. Zhang et al. / Materials and Design 50 (2013) 322–331

is always limited at the rubbery state. Fig. 7 shows DSC curves ofLTG-HNBR nanocomposites with various OMMT loadings (none,5 phr, 10 phr, 15 phr and 20 phr, respectively). Note that the valuesof Tg in DSC curves hardly shift for all nanocomposites when theOMMT loadings increases, which is different from the reportedOMMT/NBR (including 34 wt% ACN) compounds [17]. We ascribeit to the complex interactions of the clays’ surfactants with less po-lar rubber matrix [37,38]. There are two distinct interactions be-tween rubber matrix chains and the surfactants of clays whichare described as the ‘‘hooks’’ by us: one is ‘‘external hook’’ to con-nect outer surface of exfoliated OMMT platelets or their bundles,and the other is ‘‘inner’’ one to interact with the internal surfacesof the clay interlayers. Their effects are schematically demon-

strated in Fig. 8. By using the ‘‘external hook’’, the distance amongchemical crossliking point of the matrix is extended by the aid ofsurfactant’s long-chain tails (i.e. plasticization effect). This has

Fig. 8. Interaction sketch between the long alkyl tails of surfactant on the pristine MMT and rubber chains.

Table 3Compression coefficient (K) at different temperature for various nanocomposites.

Compression K OMMT content/phr

0 5 10 15 20

Temperature �30 �C 0.54 0.65 0.69 0.66 0.66�35 �C 0.13 0.20 0.22 0.20 0.21

J. Zhang et al. / Materials and Design 50 (2013) 322–331 329

been verified by Miwa’s result that the surfactant on the OMMT re-duces the interaction between polymer and pristine clay and en-hances the mobility of polymer chains [39]. On the other hand,companying with the pristine clay platelets, the ‘‘inner hook’’ ofsurfactants among clay interlayers would firmly pin LTG-HNBRchains (i.e. confinement of layered clays). Together with them,the temperature of Tg maintains a constant for various LTG-HNBRnanocomposites. Such variation of Tg can be more seen in the non-polar or less polar rubber composites with clays, such as ethylene–propylene–diene rubber (EPDM) [40,41] and chlorobutyl rubber(CIIR) ones [42]. It is therefore said that the study for the low-tem-perature property of nanocomposites is needed to consider pristineMMT itself as well as the surfactants of organoclays. In contrastwith Fig. 5, it is emphasized that the temperature of Tg correspond-ing to tand peak (shown in DMA) is more sensitive than DSC ones,which displays the response of matrix and the interaction ofOMMT/rubber at a given frequency [23,42]. As a result, the DSCobservations confirm that LTG-HNBR nanocomposites would bestatic elastic ones at the near temperature of �30 �C. But the de-gree of its elasticity is directly dominated by the crosslinking net-work and the content of rubber polymer. The compressioncoefficients at low temperature (K) are measured and listed in Ta-ble 3. Note that there is only minute difference of elasticity at thetemperature of �30 �C for various composites; however, that of theneat rubber is the lowest one. Owing to good affinities of OMMTswith rubber chains, nanocomposites gain high elasticity. A close

Table 4The mechanical properties of neat rubber, its nanocomposites and a compared case with

OMMT content (phr)

0 5

Hadness/Shore A 46 53Tensile strength (MPa) 1.03 1.94Elongation at break (%) 89 100Permanent set (%) 3.0 2.2Tearing strength (KN/m) 8.25 11.3Swelling ratio(at 100 �C for 72 h, in No.10 oil) 1.42 1.40

comparison reveals that the low-temperature elasticity of thenanocomposite with 10 phr OMMTs is the best one in all speci-mens including the neat one, which is similarly seen at the temper-ature of �35 �C. Obviously, optimal filler–matrix interaction andappropriate amount of OMMTs allows reaching the mountain ofthe low temperature elasticity for rubber nanocomposites. Thesubsequent experiment at such clay content shows that high elas-ticity (the value of K is 0.42) can be achieved at the lower temper-ature of �40 �C if 10 phr plasticizer (bis [2-(2-butoxyethoxy)ethyl]adipate, Thiokol TP95, Rohm and Haas Company) was added.Therefore, LTG-HNBR nanocomposite is a promising elastomer forthe low-temperature applications.

4.6. Mechanical properties

Table 4 lists the mechanical properties of LTG-HNBR nanocom-posites with various amounts of OMMTs. It is found that tensilestrength of nanocomposites increases dramatically with the incre-ment of OMMT content. For example, it has grown more than 10times for the case with 20 phr OMMTs (11.1 MPa) in contrast withthe neat one. This tremendous advances (9.6 MPa) exhibit in thecase with 10 phr OMMTs which has been predicted by digitalTEM analysis. Afterwards, a tendency of the tensile strength to in-crease runs slowly with the increments of OMMT amounts, such as20 phr one. Improved hardness, the elongation at break and tearingstrength of rubber nanocomposites are in concordance with tensileresults. Their permanent set is also raised by the enhancements ofthe viscosity from networks of OMMT fillers. When comparing theorganoclay with carbon black as filler for LTG-HNBR, the reinforce-ment of the organoclay is more noticeable. An example is takenwhen high abrasion furnace black (N330) with the amount of10 phr was added into the matrix. As expected, its tensile strengthis only 2.4 MPa which is far less than that one with the same claycontent. Nah et al. brought forward the model to explain the rein-forcement of clays on polymer nanocomposites that the increase of

10 phr carbon black (N330).

N330 content (phr)

10 15 20 10

59 61 64 519.61 10.29 11.12 2.4268 257 261 15810.2 11.5 14.4 5.219 19.5 22.3 13.7

1.39 1.38 1.38 1.41

Fig. 9. The plot of the swelling ratio Qr of the rubber versus clay fraction UOM for therubber nanocomposites in hydraulic oil.

Fig. 10. The TG curves of rubber and its nanocomposites containing 10 phr OMMTs.

330 J. Zhang et al. / Materials and Design 50 (2013) 322–331

the crack path around these silicate layers (‘zig-zag’ route) is capa-ble of dissipating input energy to withstand greater stress thanthose with carbon black [43]. This is true for the polyurethane[44], carboxylated hydrogenated nitrile butadiene rubber (XHNBR)matrix composites [45] and equally valid in our nanocomposites.Only if more carbon black (35 phr) is added (tensile strength ofits composites is 10.1 MPa), can the rubber compounding obtainequal mechanical properties. However, high contents of carbonblack give rise to a marked loss in the low-temperature elasticityof the LTG-HNBR composite (its compression coefficient K at thetemperature of �30 �C is only 0.39).

4.7. Swelling study

The degree of swelling of LTG-HNBR composites at equilibriumis an important property to evaluate the number of filler–rubberattachments and their ability of medium-resistance, especiallyfor oil. To study the equilibrium of the swelling, we choose differ-ent soaking time (24 h, 48 h, 72 h and 120 h, respectively) at thetemperature of 100 �C. Without the consideration of possibleanisotropic swelling caused by the structures of OMMTs [46], anaverage swelling volume is measured dependent on Archimedes’principle. It is found that the swelling ratios of all specimens keepconstant (an equilibrium swelling) after they were soaked for 72 h.The equilibrium swelling ratios of various nanocomposites arelisted in Table 4. Note that the nanocomposites have better barrierproperties for hydraulic oil by comparison with the neat rubberone. It is well explained that the dispersion of nanofillers increases

the tortuosity of the path to diffuse oil molecules during the per-meation [12]. Under the assumption that the clays do not swellat all, one can calculate the equilibrium swelling ratio of the rubberphase Qr from the equilibrium swelling ratio Q of the compositeand from the filler volume fraction UOM [47]:

Qr ¼ ðQ �UOMÞ=ð1�UOMÞ ð8Þ

The relationship between the swelling ratio Qr of the rubber andclay fraction UOM is drawn in Fig. 9. Note that the swelling of rubberfor all nanocomposites is less than the neat one. The swelling ofrubber reaches a low point when the OMMT content is between10 phr and 15 phr. Bokobza et al. investigated the natural rubberreinforced by clays and proposed that the extent of restricted swell-ing for the rubber matrix reflects the interactions of organoclayswith them [47,48]. So this strengthens the fact that the interactionof OMMTs with the rubber matrix at such content is very strong. Itis also stressed that the dispersion and morphology state of fillersheavily affects the barrier property of the bulk for oil liquid.

4.8. Thermal stability

In order to display the thermal stability or thermal resistantproperties, the TG curves of neat rubber and corresponding com-posite containing 10 phr OMMTs are shown in Fig. 10. Note thatthe mass of neat HNBR is only 6.4% after the decomposition, its on-set decomposition temperature is 372 �C, and the ending decom-position temperature is 498 �C. In contrast, the residual mass ofnanocomposite is 20.1%, its onset and ending decomposition tem-perature is 392 �C and 508 �C, respectively. The addition of OMMTsobviously improves the thermal stability of rubber bulk just as de-scribed in OMMT/EPDM system [40]. Moreover, the decompositionof the surfactants on the organoclays is still not seen in the plotsowing to their strong interactions with matrix. The improved ther-mal stability of nanocomposite is attributed to the action of claylayers as superior insulator, mass transport barrier to the volatileproducts generated or the presence of inorganic phases like, SiO2,Al2O3, and MgO in clay particles during decomposition [49]. In-spired by the swelling properties, the slowing down of the escapeof the volatile products in nanocomposites is also from the laby-rinth paths of the silicate layers in the polymer matrix.

5. Conclusion

The LTG-HNBR nanocomposites with organoclays were success-fully prepared by thermal mixing. The OMMTs form partially inter-calated and exfoliated morphologies in the rubber matrix. Theiragglomerations are seen in the nanocomposites including largeamounts of OMMTs. It is noticeable that there are strong clay-net-works in the matrix where the addition of 10 phr is the optimumamount to exploit it. Thus the mechanical properties of LTG-HNBRare greatly improved by the good dispersion of clays. Due to theconfinement of layered structures and plasticization effect oflong-chain surfactants on organoclays, the nanocomposites barelychange glassy temperature (Tg) of the bulk, but improve its low-temperature elasticity. The experimental results show that theincorporation of OMMTs is helpful to improve the oil-resistanceand the thermal stability of the nanocomposites. Therefore,OMMT/LTG-HNBR nanocomposites are expected to have importantapplications as low temperature oil-sealing-materials with highperformance.

Acknowledgment

This work was supported by the National Natural Science Foun-dation of China (51103033).

J. Zhang et al. / Materials and Design 50 (2013) 322–331 331

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.matdes.2013.02.072.

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