High-performance nanocomposites based on arcylonitrile-butadiene rubber with fillers of different particle size: Mechanical and morphological studies


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Acrylonitrile-butadiene rubber (NBR) nanocomposites with layered silicate (LS), calcium phosphate (CP), and titanium dioxide (TO) of different particle size were prepared in an open two-roll mixing mill at different filler loading in presence of
  High-Performance Nanocomposites Based on Arcylonitrile-Butadiene Rubber With Fillers of DifferentParticle Size: Mechanical and Morphological Studies Thomas P.C., 1 Tomlal Jose E, 1 Selvin Thomas P, 2 Sabu Thomas, 3 Kuruvilla Joseph 41 Research and Postgraduate Department of Chemistry, St. Berchmans College,Changanacherry, Kerala 686 101, India 2 Corporate R&D Division, HLL Lifecare Limited, Karamana, Trivandrum, Kerala 695 012, India 3 School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala 686 560, India 4 Department of Chemistry, Indian Institute of Space Science and Technology (IIST), ISRO PO,Thiruvananthapuram, Kerala 695 022, India  Acrylonitrile-butadiene rubber (NBR) nanocompositeswith layered silicate (LS), calcium phosphate (CP), andtitanium dioxide (TO) of different particle size were pre-pared in an open two-roll mixing mill at different fillerloading in presence of sulphur as vulcanizing agent. Thelayered silicate (LS) filled system showed outstandingenhancement in mechanical properties in comparisonwith nanocalcium phosphate (CP) and titanium dioxide(TO). The variations in properties can be attributed tothe extent of intercalation/exfoliation, which was highly influenced by the filler size. The layered silicate filledsystem at 20 phr showed nearly 349% increase in ten-sile strength compared to pure NBR whereas anincrease of 110% and 84% were shown by CP and TOfilled systems respectively. The modulus enhancementswere in the order of 200%, 63% and 22%, respectively compared to the unfilled system. The increase in tearresistance was in the order of 230%, 115%, and 41%respectively for the filled systems in comparison withunfilled NBR. The significant enhancements in mechani-cal properties were supported by the morphologicalanalysis.  POLYM. COMPOS., 31:1515–1524, 2010.  ª  2009Society of Plastics Engineers INTRODUCTION Hybrid inorganic-organic materials are promising sys-tems for a variety of applications due to their outstandingimprovements in material properties compared with virginpolymer or conventional micro composites and macrocomposites. Nylon 6-silicate is the first example of such ahybrid composite [1]. The improvements can include highmechanical [2] electrical [3], optical [4], thermal [5], anddecreased gas permeability [6]. These types of propertyenhancements are imparted by the physical presence of the nanoparticles and their interaction with the polymer matrix and the state of dispersion [7]. As nanoscale mate-rials have a large surface area for a given volume, manyimportant chemical and physical interactions are governedby surface properties; a nanostructure material can havesubstantially different properties from a larger-dimen-sional material of the same composition. Two particular characteristics of silicates are generally considered for layered silicate nanocomposites. These are the ability of the silicate particles to disperse into individual layers andsecond factor is the possibility to fine-tune their surfacechemistry through ion exchange reactions with organicand inorganic cations. The methods for the preparation of polymer nanocomposites include in situ polymerization,solution mixing, latex blending, melt mixing and an opentwo-roll mixing method. Recently, the two roll mill mix-ing technique has become the promising method for fabri-cating polymer clay nanocomposites because of the manyadvantages including the use of existing processing equip-ment and its eco-friendly acceptable nature [8]. The rub-ber-layered clay nanocomposites are highly relevant torubber industries and considered as an alternative to theconventionally and highly filled rubber compounds [9].Nitrile butadiene rubber (NBR), a well-known copoly-mer in the solid state or in the latex state, is one of themost widely used and commercialized elastomers that is Correspondence to : Kuruvilla Joseph; e-mail: kjoseph.iist@gmail.comDOI 10.1002/pc.20938View this article online at wileyonlinelibrary.com. V V C 2009 Society of Plastics Engineers POLYMER COMPOSITES—-2010  manufactured mostly by the emulsion polymerizationmethod. Because of its processability, oil, fuel and chemi-cal resistance as well as its good mechanical propertiesafter vulcanization, NBR has found a variety of applica-tions in different fields. However; NBR exhibits a poor tensile strength and low resistance without vulcanizationand reinforcing fillers. Hence fillers such as carbon black,metals and ceramics were used to improve the mechanicalproperties of NBR [10]. For example carbon blacks areexcellent fillers due to their strong interaction with NBR,but the resulting materials have limitations associatedwith the decreased processability at higher filler loading.Recently, silica has been used in NBR because of itsgood interaction and eco-friendly issues caused by carbonblack [11, 12]. However it is difficult to get uniform dis-persion of silica particle in NBR matrix due to particleaggregation perhaps caused by hydrophilic and polar hydroxyl groups on the particle surfaces [13]. This is whymuch attention has recently been given to the use of lay-ered silicates as reinforcing filler in NBR [14, 15].In the present work, three different types of nanocom-posites having NBR matrix with different fillers namelyTiO 2 , Ca 3 (PO 4 ) 2 , and layered silicate (montmorillonite1.44P) as dispersed phases were studied by an open two-roll mixing method in presence of sulphur as vulcanizingagent. The fillers TiO 2  and Ca 3 (PO 4 ) 2  are having sphericalshape while the silicate filler is layered in shape. The sizefactor of the fillers in the nanometer units are in the order LS  \  Ca 3 (PO 4 ) 2  \  TiO 2 . The aim of this work is tostudy the mechanical and morphological changes causedby the extent of intercalation/exfoliation of the particlesbased on the particle geometry in the polymer matrixunder the influence of conventional vulcanizing system. EXPERIMENTAL  Materials The nitrile butadiene rubber (NBR) under the tradename (NBR-553) having Mooney viscosity (ML 1 þ 4  100 8 C)40.00 and bound acrylonitrile content 33.90% was suppliedby Apar Industries, Mumbai, India. The layered silicate(I.44P) was obtained from Nanocor China. Nanomer I.44Pis onium ion modified MMT clay containing 60% clay(CAS No. 1318-93-0) and 40% dimethyl dialkyl (C14-18)ammonium organic modifier. Calcium phosphate in thenanometer (particle size 40 nm) range was prepared in our laboratory by Thomas et al. [16] and titanium dioxideunder the trade name KEMOX-RC 800PG, having particlesize 190 nm was supplied by Kerala Metals and MineralsLtd (KMML),Kollam, Kerala, India. The compoundingingredients, such as vulcanizing agents and acceleratorswere procured from M/s Bayer India Ltd., Mumbai, India. Sample Preparation Different mixes were prepared according to the speci-fied formulations given in Table 1. The mixing processwas carried out using a laboratory two-roll mixing mill(150  3  300 nm 2 ) at a friction ratio 1:1.4. The machinewas water cooled during the mixing operation. The com-pounding was performed at room temperature keeping thenip gap; mill roll speed ratio and the time taken for mix-ing process (16 min overall time) kept same in all themixes. The mixes were designated as NBR/TO, NBR/CPand NBR/LS representing different fillers such as TiO 2 ,Ca 3 (PO 4 ) 2 , and layered silicate respectively in sulphur vulcanizing system. The fillers were heated in an oven at100 8 C for 8 hrs to remove the moisture, kept in a desicca-tor and used for the composite preparation. NBR wasmasticated for 2 min before the ingredients were added inthe same order as mentioned above. The samples werethen cured at 160 8 C in an electrically heated hydraulicpress to their respective cure time ( t  90 ) at a pressure of 150 kg/cm 2 . Characterization The curing properties were studied using Elastograph‘‘Vario’’ 67.98 (Gottfert; Germany) rheometer by placing7–10 g of compound in the heating chamber of the rhe-ometer operated at 160 8 C. Torque maximum, torque mini-mum, scorch time and cure time were obtained from therheometer observations. Separate rheographs were takenfor each composition to get the respective cure time.Tensile and tear tests were performed on dumbbell andcrescent shaped specimens according to ASTM standardsD 412 and D 624 on Instron 4411(England) UTM at across head speed of 500 and 100 mm/min, respectively.Five samples were tested for tensile and tear and the aver-age of the values were taken in each case. Hardness wastested according to the ASTM D 2240 standard. Thecross-link density was calculated by equilibrium swellingmethod in toluene at 298 K for 72 hrs in order to achievethe equilibrium swelling condition. Circular samples with2 cm diameter were used for the analysis. The cross-linkdensity  u  can be calculated from the swelling methodusing the equation [17]The cross link density  #  ¼  12  M  c ð 1 Þ TABLE 1. Compositions of the mixes.Ingredient Quantity (phr)NBR 100Zinc oxide 5Stearic acid 1TDQ 1CBS 0.75Sulfur 2Fillers (LS, CP, and TO) VariableCBS,  N  -cyclohexyl benzothiazole-2-sulphenamide; TDQ, 2,2,4 tri-methyl-1,2 dihydroquinoline. 1516  POLYMER COMPOSITES—-2010  DOI 10.1002/pc  where  M  c  is the molecular weight of the polymer betweenthe cross-links and the same can be calculated with thefollowing equation [18]  M  c  ¼  q p V  s  V  1 = 3rf  ln ½ 1    V  rf   þ  V  rf   þ  v V  2rf  ð 2 Þ V  s  is the molar volume of the solvent,  q p  is the density of the polymer,  v  is the interaction parameter (0.435) for thetoluene-NBR system [9].  V  rf   is the volume fraction of rubber in the solvent-swollen filled sample.  V  rf   is givenby the equation of Ellis and Welding [19]. V  rf   ¼ð d     fw Þ = q p ð d     fw Þ = q p  þ  A s = q s ð 3 Þ where  d   is the deswollen weight,  f   is the volume fractionof filler in the rubber vulcanizate and  w  is the initialweight of the sample,  q p  is the density of the polymer,  q s is the density of the solvent and  A s  is the amount of thesolvent absorbed.Thermodynamical parameters of rubber elasticity arecrucial to obtain a deeper understanding of mixing inrubber/nanofillers. The expansion of rubber in presenceof a solvent will significantly modify the conformationalentropy  D S  and the elastic Gibbs free energy  D G.  Thechange in elastic Gibbs free energy can be determinedfrom the Flory-Huggins equation [20, 21] as shownbelow D G  ¼  RT  ½ ln ð 1    V  rf  Þ þ  V  rf   þ  v V  2rf   ð 4 Þ From the statistical theory of rubber elasticity, the con-formational entropy  D S  can be calculated using the equa-tion [9] D S  ¼  D GT  5The morphology of the cryofractured composites wereanalyzed by using scanning electron microscopy (SEM),transmission electron microscopy (TEM), atomic forcemicroscopy (AFM) and small angle X-ray scattering(SAXS). Scanning electron microscopy images of thesamples were taken using JEOL 6400 WINSEM model(Jeol, Tokyo, Japan) at 5 KeV. Transmission electronmicrographs of the samples were taken in a LEO 912Omega transmission electron microscope with an acceler-ation voltage of 120 kV. The specimens were preparedusing an Ultra cut E cryomicrotome. Thin sample speci-mens of about 100 nm were obtained with a diamondknife at  2 100 8 C. Atomic force microscopy had donewith Park systems (XE-100 AFM) Park SYSTEMS,South Korea. X-ray diffraction patterns of the nitrile rub-ber nanocomposites were taken using Ni- filtered Cu K a radiation at a generator voltage of 60 kV, and a generator current of 40 mA and wavelength of 0.154 nm at roomtemperature (Bruker-D 8). RESULTS AND DISCUSSION Curing Behavior  The vulcanization characteristics are expressed in termsof torque (min), torque (maxi), scorch time  t  s1 , optimumcure time  t  90 , cure rate ( t  90  2  t  s1 ) and torque increase.The rheometric properties of the gum and nanocompositesat 10 phr filler loading are given in Table 2. The increasein maximum and minimum torques as well as their differ-ence is higher with LS and CP filled system compared toTO filled composites. This suggests some reinforcementfor both LS and CP with the polymer matrix. This hap-pens when fillers are intercalated and/or exfoliated. Thepattern of dispersion is clear from the TEM pictures andthe tensile properties of the composites. The maximumvalues torques associated with LS filled system can beattributed to the intercalation/exfoliation of the layeredsilicate by nitrile rubber matrix [22]. The cure time  t  90 values indicate that both LS and CP accelerate the vulcan-ization. In the case of LS this effect is observed for other rubbers, perhaps linked to a transitional metal complexingin which sulphur and amine groups of the intercalantsparticipate [8, 23]. The scorch time analysis showed thatthere is marginal decrease in scorch time as a result of the addition of TO filler and this indicates that the TO fil-ler, as such, has little influence on the cure reactions of nitrile rubber due to the inert chemical nature. However organically modified layered silicate and CP fillersshowed comparatively more reduction in the scorch timeof the nitrile rubber. These fillers behave like a vulcaniz-ing accelerator for NBR decreasing the scorch time. Suchaccelerating effect has already been reported by Wu et al.in NBR/organosilicate system [9]. This may be due to thecomplex formation with amines and sulphur containingcompounds which facilitates the formation of elementalsulphur [8] and this may be reason for the reductionin scorch time and cure time of the filled nitrile rubber systems.  Morphology Figure 1 shows the X-ray diffraction of layered silicateand their NBR hybrids. No peak was observed for pureNBR, indicating its amorphous nature. Layered silicateexhibits a single peak at 2 h  of 3.77 8 . When small amount TABLE 2. Cure characteristics at 10 phr of filler loading.Characteristics NBR NBR/TO NBR/CP NBR/LSTorque  M  L max (dNm) 0.26 0.29 0.33 0.48Torque  M  H max (dNm) 8.20 8.86 9.04 9.44Scorch time  t  S 1 (min) 2.31 2.18 1.97 1.81Cure time  t  90  (min) 10.58 9.60 8.43 8.11Cure rate  t  90 2  t  S 1 (min) 8.27 3.42 6.46 6.30Torque increase (  M  H 2  M  L ) 7.94 8.57 8.71 8.96 DOI 10.1002/pc  POLYMER COMPOSITES—-2010  1517  of LS was incorporated in NBR, peak shifted to 2.12 8 which is an indication of the fact that the LS is partiallyexfoliated or intercalated at this particular filler loading.Again at 10 phr layered silicate loading a single promi-nent peak appeared at an angle of 2 h  of 2.58 8  which isslightly higher than that observed at 5phr filler loading.However, at higher filler loading (20 phr) peaks wereobserved at 2.89 8 , 4.71 8 , and 9.37 8 . The first peak at2.89 8  testifies the intercalation of the polymer matrix in tothe layered silicate structure whereas the two peaksobserved at 4.71 8  and 9.37 8  are attributed to the de-inter-calation or particle agglomeration at higher filler loading.Similar results have been reported by Lee and co-workersin Nanocomposites prepared from nitrile rubber usingorganophilic layered clay [24]. The calcium phosphateand titanium dioxide filled systems show no reflections 2 h [ 15 as being amorphous.The morphological analysis of the nanocomposites wasdone by using equipments such as TEM, SEM and AFM.The transmission electron microscopy (TEM), which pro-vides clear evidence for the delamination of layered sili-cate in NBR matrix. As shown in Fig. 2, TEM micro-graph for LS/NBR nanocomposites at 10 phr reveal thatthere is no considerable cohesion of the silicate in theNBR matrix, and the silicate particles are appeared to bedispersed more uniformly in comparison with other fillersnamely CP and TO. Particle agglomeration is appeared inthe NBR/CP and NBR/TO composites which can beattributed to the less effective interaction between thepolymer matrix and fillers. Scanning electron micrographs(SEM) of the nitrile rubber composites at10 phr filler loading displayed in Fig. 3. These pictures reveal theagglomeration tendency of the CP and TO fillers in com-parison with layered silicate. AFM images of the surfacesof the composites were analyzed to understand filler dis-persion behavior in the matrix. The phase images of thevirgin polymer and the composites with different fillers at10 phr loading were given in Fig. 4a–d. Figure 4a showsthe phase image of the neat matrix. It can be seen thatthe neat matrix shows a smooth surface in comparisonwith other filler loaded samples. The comparison of surfa-ces (Fig. 4b–d) revealed that more uniform particle dis-persion is associated with layered silicate. Hence it can beconcluded that effective filler-matrix interaction followsthe order NBR/LS [ NBR/CP [ NBR/TO  Mechanical Properties Nanomaterials provide reinforcing efficiency becauseof their high aspect ratios [25]. The properties of theresulting nanocomposite are greatly influenced by the sizescale of its component phases and the degree of mixingbetween the two phases. Depending on the nature of thecomponents used and the method of preparation, signifi-cant differences in composite properties may be obtained.Figure 5 represents the stress- strain graph for different FIG. 1. X-ray diffraction pattern of nitrile rubber nanocomposites.FIG. 2. TEM pictures of nitrile rubber composites at 10 phr loading of (a) NBR/LS, (b) NBR/CP, and (c) NBR/TO. 1518  POLYMER COMPOSITES—-2010  DOI 10.1002/pc  FIG. 3. SEM pictures of nitrile rubber composites at 10 phr loading of (a) NBR/LS, (b) NBR/CP, and (c) NBR/TO.FIG. 4. AFM images of virgin polymer and composites at 10 phr loading. (a) Virgin polymer, (b) NBR/LS,(c) NBR/CP, and (d) NBR/TO. [Color figure can be viewed in the online issue, which is available atwileyonlinelibrary.com.] DOI 10.1002/pc  POLYMER COMPOSITES—-2010  1519
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