Role of Graphene nano Platelets in Enhancing Properties for CNT-CB Filled Poly (1,4-cis-isoprene) based Rubber Nanocomposites.

Traditional fillers such as carbon black are well-known reinforcing filler since a century and used in various rubber products such as tires [1-3]. Recently, "Nanofillers" has emerged as new class materials and are extensively explored to improve dynamic mechanical properties of rubber nanocomposite. Apart from nanographitic fillers and carbon nanotubes, an increasing interest was evidenced for graphene, a two dimensional (2D) sheet made up of sp2 hybridized carbon atoms in an extended honey comb like structure. Such attention towards graphene is due to its wonderful thermal, electric and mechanical properties. [4-7] Comprehensive studies on graphitic nanofillers in elastomeric matrix are available and improved dynamic-mechanical properties are reported [5,7]. In our recent work, use of graphene, few layer graphene or graphitic nanofillers, nano-silica and carbon nanotubes demonstrates an improved rheological, mechanical and electrical property. However, challenges such as promoting uniform dispersion, stable filler networking, and enhanced polymer-filler interaction were found from investigations as reported [8-15]. The structure and dynamics of in-rubber interactions, stereochemistry of polymer chains and filler dispersion (such as orientation of filler particles, their aggregates) play an important role in understanding the hysteretic losses and improved dynamic-mechanical properties of filled rubber nanocomposites. However, nonlinear dependence of filled rubbers, also called Payne effect, could not be successfully implemented to determine such behavior with complete accuracy (due to features such as "quantum effects" exhibited by such nanoparticle) in case of nanofiller reinforced rubber matrices. Another important feature of rubber reinforcement is filler networking, which after attaining percolation limit that is long range filler's cluster aggregation in the rubber matrix may led to improved properties significantly [8-15, 16, 18, 19]. Now-a-days, hybrid filler systems are employed in which the traditional filler are mixed with so-called nanofiller particles and significant improved properties were reported. Such improved properties are described due to synergistic or mutually enhanced effect or due to interactive interaction of the filler nanoparticles with themselves or other hybrid filler species or with polymer chains of filled rubber matrix [19-25]. Recently, L. Bokobza et al demonstrated improved dispersion, tensile strength and electric properties via use of CB, CNT and CB+CNT hybrid fillers in SBR rubber matrix as reported [20]. H. Lorenz et al found technically promising synergetic effects with use of silica-CNT hybrid filler in elastomeric matrix and demonstrates improved dynamic-mechanical, thermal and fracture properties for the nanocomposites [21]. M. Galimberti et al demonstrates synergistic effect for CNT-CB and interactive effects in CB+CNT, nano-G+CB hybrid fillers based rubber nanocomposites [22,23]. A. Malas et al shows that mechanical, thermal and dynamic mechanical properties are higher for the modified expanded graphite (MEG) MEG/CB loaded natural rubber (NR) composites as compared to expanded graphite (EG) and EG/CB loaded NR composites [24]. L. L. Wang et al demonstrates that cofficient of fraction and wear rate for NBR/CB/EG composites show a decreasing trend with a rise in applied load and sliding velocity [25]. In this manuscript, rubber nanocomposites were prepared by melt mixing method based on carbon allotropes and poly (1,4-cis-isoprene) rubber (IR). Morphological and structural characteristics of nanofillers (xg C750, CNT, CB) were carried out through scanning electron microscopy (SEM). Firstly, the master batch was prepared by mixing IR with CB and CNT. Subsequently, fresh nanocomposites were obtained by increasing xg C750 (from 2 to 15 phr) concentration in the master batch using small Haake RheomixTM 600. Dynamic-mechanical tests were carried-out in torsion mode on raw nanocomposites, with strain sweep from 0.28% to 400%. Tensile strength and stability of filler network of vulcanizates were assessed by means of stress-strain and multi-hysteresis tests. Electrical properties were investigated through dielectric AC conductivity. The high and low temperature properties of vulcanizates performed through dynamic-mechanical thermal analysis (DMTA). The effects of increasing xg C750 loading into IR rubber matrix on dynamic-mechanical, thermal and electrical properties are demonstrated.

MaterialsSynthetic poly(1,4-cis-isoprene) (IR) with trade name SKI3, purchased from Nizhnekamskneftechim Export, mooney viscosity (ML1+4 at 100°C) as 70 M.U. was used as rubber matrix. Nanofillers used were commercially available exfoliated graphene nanoplatelets (xGnPs) of type xg C750 (purchased from XG Sciences, USA), Carbon Black of type Printex xe2 (obtained from Orion Engineered Carbons, Deutschland) and Carbon Nano Tubes (CNTs) with trade name NANOCYL NC 7000TM (obtained from Nanocyl S.A, Belgium). Sulphur based vulcanizing system was adopted. Other ingredients used were zinc oxide, stearic acid and cyclohexyl benzothiazol-2-sulfenamide (CBS). All ingredients were used as received without any further purification. The BET surface area of the nanofillers are described in table 1.

Dry melt mixing and Curing Preparation of Master BatchLarge Thermofisher-HaakeTM 3000 laboratory was employed at a rotor speed of 50 rpm and initial temperature of 50 oC for preparing master batch. As Step-1st, IR (100 phr) was feed and masticated into mixing chamber for 1-2 minutes. In 2nd step as filler's dispersing phase, the CB-Printex xe2 (20 phr) was added step by step into mixing chamber and mixing lasted for 4-5 minutes. Finally, CNTs were added and mixed for 3-4 minutes until a stable torque was achieved. The total mixing process for preparing master batches did not exceed 10 minutes. Master batch (containing 20 phr of CB-Printex xe2 and 2 phr of CNT) was lastly discharged and homogenized for 4-5 times on open mill.Preparation of nanocomposites Small HaakeTM 600 laboratory mixer was used for preparing rubber nanocomposites from IR/CB/CNT-master batch. At 50°C and rotor speed of 50 rpm, the IR -master batch was fed into the mixing chamber in 1st step and mixed for 1-2 minutes. In step-2 as part of filler dispersion, xg C750 was added into mixing chamber and mixing lasted for 3-4 minutes until a stable torque was achieved. The other ingredients were mixed in third (ZnO and stearic acid for 2 minutes) and in 4th step (sulphur and CBS) respectively for 2-3 minutes until torque reaches equilibrium before discharging nanocomposites from mixing chamber. The total duration for mixing was maintained below 10 minutes.

Nanocomposites were finally homogenized by passing on open mill for 4/5 times at room temperature and nip distance of 1 cm was maintained as reported [8-10].

Structural, dynamic-mechanical and thermal measurements The morphology of nanofiller's were investigated with scanning electron microscope (SEM) using Zeiss EVO MA 10, equipped with tungsten filament and carried out at a controlled voltage of 8 kV. Rheometric curves were obtained at 160°C, frequency of 1.667 Hz and a strain of 0.50%. Rubber Process Analyzer (RPA 2000) was used for performing strain sweep tests on raw nanocomposites at 1 Hz frequency, strain sweep from 0.28% to 400% and temperature of 80°C.

Dynamic Mechanical Thermal Analysis (DMTA) were carried out on 2 mm thick vulcanizates using the equipment "ARES, Rheometric Scientific" with a temperature sweep from -70°C to 80°C at 0.1% strain and 10 Hz frequency. A continuous supply of N2 was maintained to obtain cryogenic conditions for DMTA measurements. Tensile tests were performed on vulcanizates according to DIN 53 504 standards. An universal testing machine-Zwick /Roll Z010 equipped with a preload of 0.5 N, was used to perform tensile -stress-measurements. Strain rate was maintained at 200 mm/minute for tensile tests and 40 mm/minute for multi-hysteresis measurements.

Structural and Morphological characterizations of nanofillers The structural and morphological characteristics of the nanofillers were investigated though scanning electron microscope (SEM) technique. The representative micrographs of nanofillers (xg C750, CNT- NC 7000 and CB-Printex) are shown in figures 1.

The xg C750 shows ruptured or damaged "platelet-like" morphology showing exfoliated randomly arranged graphene layers (figure 1 (a, b)). Such damaged graphene layers in the stack could be due to various vigorous thermo-acidic treatments [8] made in order to achieve maximum exfoliation and high surface area. Such exfoliated staked graphene layers shows transparent appearance with evidenced corrugated morphology over stacked graphene layers and nearer to aggregated filler particles. SEMs micrographs of CNT-NC 7000 are reported in figure 1 (c, d). Fewer aggregate rather smaller agglomerates were evidenced which were however not tightly bundled or entangled. At higher magnification, the CNTs show a broad length distribution with random orientation. Average diameters of such "tube-like" particles largely are in nanometer ranges while others are in submicron region and very few in micron range were evidenced. Micrographs of CB-Printex are reported in figure 1 (e, f). A uniform distribution of the filler particles were evidenced while few aggregates and very few agglomerates were noticed. Prominent "voids" in aggregated particles were clearly evidenced at higher resolution (figure 1f). Additionally, randomly arranged smaller "platelets-like" structures were noticed.

Characterizations of IR nanocompositesRheometry Optimized rheometric curves for hybrid filler system with increasing filler concentration of xg C750 (from 0 to 15 phr) are presented in figure 2a. It was observed that an increasing concentration of filler results in increased torque and decreased scorch and curing time as reported [10, 22, 23]. Such effects would to be due to establishment of filler networking or interactive effects of individual carbon nano-fillers among themselves/hybrid species or improved dispersive forces (such as van d. waal interactions) because of filler's influence in rubber matrix as reported recently [8-10, 22, 23]. Therefore, it was seen that increasing filler concentration in rubber matrix has positive influence in rubber vulcanization process. Additionally, other features such as enhanced torque at scorch regime were not evidenced, thereby confirming non-occurrence of filler flocculation behavior. A little reversion that is relative decrease of increased modulus at vulcanization maturation (after t'90) was observed.

A comparative study of scorch time (t'10) with increasing filler loading for xg C750 in IR based hybrid nanocomposites, are presented in figure 2b. The decreasing nature of scorch time (t'10) with increasing filler loading for xg C750 was evidenced. It was found that xg C750 influence scorch by accelerating it, which became more pronounced with increasing filler loading. Such scorch acceleration for increasing xg C750 in IR nanocomposites could be due to high surface area of xg C750 that provide high interfacial area of filler to interact with polymer chains. The high surface energy heterogeneity of xg C750 that provide high dispersive forces of filler to interact with polymer also plays a significant role in enhancing heat transfer thereby accelerating scorch time and favoring early network formation as reported recently [8-10, 22, 23]. A sharp decrease of scorch time in nanocomposites could also be proposed due to formation several intermediate products or by-products such as amino groups during dry mixing that favor vulcanization reaction in shorter time. Additionally, high thermal conductivity of the filler also plays a significant role in decreasing scorch time and it's proposed to be another reason for sharp fall of scorch time for xg C750 filled IR nanocomposites.

Strain sweep tests were employed to investigate the visco-elastic behavior of the nanocomposites containing hybrid filler system. The filler networking was investigated by determining the dependence of G' modulus at minimum deformation G'(min) on the filler content of xg C750 in hybrid filler system into IR matrix as presented in figure 3a. It was observed that the characteristic plateau of storage modulus (G') increases at lower strain amplification with increasing filler concentration and decreases with increasing strain as reported [10, 22, 23].

Storage modulus (G', in kPa) of hybrid/IR filled rubber nanocomposites as a function of filler loading are comparatively presented in figure 3b. An influence of increasing loading of xg C750 in improving storage modulus for IR based hybrid nanocomposites could be due to many reasons. Firstly, xg C750 which is characterized with high surface area provide high interfacial area for filler to interact with polymer chains in rubber matrix. Therefore, it could results in restricting mobility of polymer chains around or absorbed on the surface of nanofillers, leading to formation of a stiffened interphase and hence increasing modulus. Secondly, higher loading (>3 phr of xg C750) may lead to the formation of long range filler-filler interacting networks inside the rubber matrix thereby resulting in an improved modulus of IR based hybrid filler's nanocomposites. Thirdly, the interactive effect of nanofiller's among themselves or with the rubber matrix results an enhanced storage modulus after 3 phr loading of xg C750 in IR hybrid nanocomposites. Recently, it was reported in literature that in composites based on hybrid filler systems such as CB/CNT [22,23], CB/nanoG [10,22], (OC, NC, nanoG, CNT) were found to develop a synergism with CB. The pronounced enhancement of G' (in particular after 3 phr of xg C750) and the findings reported in figure 3c shows that xg C750 is able to develop an enhancing effect within CB+CNT filled rubber nanocomposites. It would be due to factors affected by filler's size, morphology, orientation, dispersion and loading in IR hybrid nanocomposites. Higher dispersion that is dispersion of filler's particle as primary or individual particle, and higher exfoliation is helpful in formation of improved interfacial interaction between filler and rubber matrix affecting the storage modulus as reported [10, 20-22].

Tensile strength Stress-strain curves for hybrid/IR system shows marked improvement in tensile strength properties with increasing xg C750 concentration in IR rubber matrix (figure 4a). Improved tensile properties could be due to better filler networking, higher filler-polymer interaction that would lead to improved dispersive forces and favorable nanofiller's orientation or interactive features of filler's individually or with hybrid species. It was found that nanocomposites containing xg C750 are able to attain remarkable stress and improved elongations at break that became more pronounced after 10 phr loading. It could be due to enhancing or synergistic effect of xg C750 interaction with hybrid species as reported. [10, 21, 22, 26]. For a given polymer and cure system, the impact of the filler network, both in its strength and architecture affects the dynamic modulus and hysteresis under dynamic strain as reported [27].

The multi-hysteresis stress-strain was carried out for hybrid/IR system. In figure 4b, we have presented multi-hysteresis for hybrid system containing 0 and 2 phr of xg C750 filler grade in IR matrix master batch (detailed in experimental section). During multi-hysteresis cyclic strain it was found, that a stable filler network can reduce the hysteresis of the filled rubber, the breakdown and reformation of the filler network could cause an additional energy dissipation (as can be seen especially for 1st cycle) that resulting in higher hysteresis losses. It could be due to strain amplification by stiffer filler clusters and cyclic breakdown and re-aggregation (healing) of softer, already damaged filler clusters. In simpler way, one can hypothesize that all soft clusters are broken at the turning points of the cycle and the mechanical energy stored in these strained clusters is completely dissipated; that is only irreversible stress contributions result. Theoretically, the cluster mechanics of the material is complicated to be understood fully due to the fact that not all soft clusters are broken at the turning points of a cycle [27]. A comprehensive for 5 phr, 10 phr and 15 phr of xg C750 filled IR master batch are shown in figure 4c. It was also reported that the filler network can substantially increase the effective volume of the filler due to rubber trapped in the agglomerates, leading to high elastic modulus at low strains as reported [27].

Dynamic mechanical temperature analysis (DMTA analysis) The mechanical performance can be further evaluated through DMTA test to demonstrate the reinforcing efficiency of the nanofillers and the extent of polymer-filler interaction at extreme temperature as reported [28]. Figure 5a and 5b presents the behavior of modulus and loss tangent (tan d). The glass transition temperature (Tg) can be obtained from the maximum peak in the tan d curve and complex modulus (G*), and it can be observed that the Tg of composites containing 15 phr of xg C750 (-65.4°C) was ~3°C higher than that of vulcanizates containing no xg C750 (-63.6°C).

It could be because of the fact that the xg C750 can enforce restriction to the polymer chain mobility due of the strong interfacial adhesion or higher polymer-filler interaction (since xg C750 has higher surface activity) between IR and xg C750 platelets. The area under tan d curve under different temperatures indicates the total amount of energy that can be absorbed by a material. It can be concluded that the introduction of xg C750 improves the overall stiffness of IR master batches; its elasticity is not affected significantly. Such characteristics are exciting, because most reinforcement will inevitably lead to a higher rigidity as reported [29].

Dielectric AC Conductivity PropertiesThe dielectric AC conductivity and properties were measured within frequency range from 10-2 to 106 Hz (figure 6a and 6b). The conductivity was studied for filler concentration of 0 phr, 2 phr, 5 phr, 10 phr and 15 phr of xg C750 in IR master batch. A good conductivity of the hybrid system was seen, which increases with increasing concentration of xg C750 into IR master batch. At lower loadings, the dielectric conductivity plateau remains almost similar at small frequency range and conductivity is not affected significantly. A higher conductivity of greater than 1.4*10-1 S/cm was observed at hybrid system containing 15 phr of xg C750 in IR matrix. To obtain enhanced electrical properties dielectric AC conductivity was taken at very low frequency (0.1 Hz) and plotted against filler volume fraction (figure 6c). A significant improvement of conductivity was observed after 3 phr which could be due to filler improved filler networking and establishment of long range interactions of filler-filler particles at such concentration. The dielectric conductivity of hybrid vulcanizates system with varying content of xg C750 exhibit enhanced properties at lower values due to high electron mobility of nanoparticles in IR matrix.

It has been demonstrated that the use of nanofillers such as xg C750 in hybrid filler systems brings significant improvement in dynamic-mechanical, thermal and di-electric properties of IR based rubber nanocomposites. The nanocomposites were successfully prepared by dry melt mixing method. From rheometric studies, it was found that the scorch time (t'10) was found to decrease with increasing filler concentration which was more pronounced with increasing concentration of xg C750 in IR rubber matrix.

From strain sweep measurements, it was found that the non-linear characteristic plateau of storage modulus at low strain amplitudes was obtained with increasing concentration of xg C750 in rubber matrix. Enhanced dynamic properties were observed at xg C750 loading of 3 phr in IR master batches. From stress-strain measurements, it was observed that stresses at all the elongations remarkably increase with the filler content in IR matrix. Multi-hysteresis stress-strain investigations show the first cycle exhibits higher energy dissipation than the third cycle and it was demonstrated that a stable filler networking can reduce hysteresis losses. The glass transition temperature (Tg) can be obtained from the maximum peak in the tan d curve and G*, and it can be observed that the Tg of composites containing 15 phr of xg C750 (-65.4 oC) was ~3°C higher than that of composites containing 0 phr of xg C750 (-63.6 oC). The xg C750 based rubber vulcanizates shows di-electric conductivity of ~1.4x10-1 S/cm at 15 phr filler loading of xg C750 in hybrid system which is more than sufficient for automobile applications such as tire industry.

Nanofillers * Graphene nanoplatelets * Rubber Nanocomposites * Hybrid FillerRubber nanocomposites based on commercially available carbon allotropes and poly (1,4-cis-isoprene) (IR) were investigated. The carbon allotropes used were exfoliated graphene nanoplatelets (xGnPs) of type xg C750, carbon nanotubes (CNT-NC7000) and carbon black (CB-Printex xe2). The master batch based on CB-CNT was first prepared by mixing IR with CB and CNT. Final nanocomposites were obtained by adding xg C750 (from 2 to 15 phr). Role of increasing xg C750 loading in rubber matrix on dynamic-mechanical, thermal and electrical properties is reported. A stability of filler networking was evidenced as demonstrated through multi-hysteresis tests.

Rolle von Graphen-Nanoplatelets für die verbesserten Eigenschaften von CNT-Ruß gefüllten Poly-(1,4-cis-isopren) - basierenden Kautschuk-Nanokompositen Nanofüllstoffe * Graphene * nanoPlatelets * Kautschuk-Nanokomposite Hybrid-Füllstoffsysteme Es wurden Kautschuk Nanokomposite untersucht, die auf kommerziell verfügbaren Kohlenstoff-Allotropen und Poly(1,4-cis-Isopren) (IR) basieren. Die eingesetzten Kohlenstoff-Allotrope waren exfolierte Graphen-Nanoplatelets (xGnPs) der Type xg c750, Kohlenstoff-Nanoröhrchen (CNT)-NC 7000) und Ruß (Printex xe2). Der auf Ruß-CNT basierende "Masterbatch" wurde zunächst durch Mischen von IR mit Ruß und CNT hergestellt. Die fertigen Nanokomposite wurden durch die Zugabe von xg C750 (im Bereich von 2 bis 15 phr) erhalten. Es wurde der Einfluss der Erhöhung des Füllgrads an xg C750 auf die dynamisch-mechanischen, auf die thermischen und elektrischen Eigenschaften der Kautschukmatrix untersucht. Die Stabilität des Füllstoffnetzwerks wurde durch Multihysterese-Messungen gezeigt. Measured BET surface area of the carbon nanofillers used in present investigationsS. No.Carbon NanofillerBET Surface Area1.Carbon Black (CB)-Printex xe2~1120 m2/g2.Graphene Nanoplatelets xg C750~ 820 m2/g3.Carbon Nano Tubes NC7000~ 270 m2/g Measured BET surface area of the carbon nanofillers used in present investigationsS. No.Carbon NanofillerBET Surface Area1.Carbon Black (CB)-Printex xe2~1120 m2/g2.Graphene Nanoplatelets xg C750~ 820 m2/g3.Carbon Nano Tubes NC7000~ 270 m2/g Measured BET surface area of the carbon nanofillers used in present investigations S. No. Carbon Nanofiller BET Surface Area 1. Carbon Black (CB)-Printex xe2 ~1120 m2/g 2. Graphene Nanoplatelets xg C750 ~ 820 m2/g 3. Carbon Nano Tubes NC7000 ~ 270 m2/g 1

Figures and Tables: By a kind approval of the authors.

Fig. 1: SEM micrographs at different magnifications of xg C750 (a,b) and CNT-NC 7000 (c,d) CB-Printex (e,f).

Fig. 2: (a) Rheometric curves for Hybrid/IR system based nanocomposites containing xg C750 concentration from 0 to 15 phr; (b) The t'10 decreasing behaviour in Hybrid/IR nanocomposites containing different filler concentration of xg C750.

Fig. 3: (a) Storage modulus (G', kPa) as a function of different strains (increasing from 0.28% to 300%) for Hybrid/IR nanocomposites; (b) Comparative description of storage modulus with different filler loadings of xg C750 for hybrid/IR nanocomposites; (c) synergestic effect demonstrated from storage modulus as a function of filler loading for xg C750 in Hybrid/IR nanocomposites.

Fig. 4: (a) Stress-Strain curves obtained from Hybrid/IR nanocomposites; (b) Multi-hysteresis Stress-Strain curves comparatives for 0 phr and 2 phr of xg C750 in Hybrid/IR nanocomposites; (c) Multi-hysteresis Stress-Strain comparative for 5 phr, 10 phr and 15 phr of xg C750 in Hybrid/IR nanocomposites.

Fig. 5: (a) DMTA comparative of Hybrid/IR system based nanocomposites containing xg C750 concentration from 0 to 15 phr; (b) tan d comparative of Hybrid/IR system based nanocomposites containing xg C750 concentration from 0 to 15 phr.

Fig. 6: (a) Di-electric AC conductivity of Hybrid/IR composites; (b) Enhanced dielectric properties after 2 phr as demonstrated from di-electric AC conductivity as a function of filler loading for xg C750 in Hybrid/IR nanocomposites.

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