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Bio-sourced electrically conductive epoxidized linseed oil based composites filled with polyaniline and carbon nanotubes
Composites Part B 172 (2019) 76–82
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Bio-sourced electrically conductive epoxidized linseed oil based composites filled with polyaniline and carbon nanotubes Vinay Khandelwal a, Sushanta K. Sahoo b, Ashok Kumar c, Sushanta K. Sethi a, Gaurav Manik a, * a
Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee Saharanpur Campus, Saharanpur, 247001, India Materials Science and Technology Division, CSIR – National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram, 695019, India c CSIR - National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi, 110012, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Polyaniline Electrical conductivity Bio-epoxy resin
In the present work, electrically conductive hybrid green composites were developed using epoxidized linseed oil (ELO) as bio-sourced matrix and, p-toluenesulfonic acid (pTSA) doped polyaniline (PANI) and carbon nanotubes (CNT) as two conductive fillers of varied nature. Based on percolation threshold, two different concentrations of CNT (0.1% and 0.4%) were added to ELO/PANI composites. Conductivity increased with the incorporation of CNT and maximum conductivity observed with 0.1% and 0.4% CNT was 1.0 � 10 5 and 4.5 � 10 5 S/cm respectively confirming the synergism among the fillers. The good distribution of both CNT and PANI in ELO matrix was confirmed by morphological studies. CNT addition increased the viscosity and induced the shear thinning behavior in ELO/PANI (5%) resin system. Energy storing ability as well as glass transition temperature (Tg) of the ELO/PANI (5%) composite were improved with CNT incorporation as revealed from viscoelastic studies. Tensile strength and elastic modulus showed increased values upon CNT insertion within composite. Impact strength improved with 0.1% CNT addition to ELO/PANI5 confirming an enhancement in toughening of composite. This investigation opens up the path to make electrically conductive composites with tunable con ductivity from renewable resource based material such as linseed oil.
1. Introduction In current years, application of nanotechnology to bio-based poly meric systems has created novel opportunities for improving the prop erties, retaining cost-effectiveness and eco-friendly nature [1]. Addition of nanofillers to plant oil-based polymers is found to improve the me chanical, thermal and electrical properties reasonably to be used as an alternative for depleting petro-sourced materials [2,3]. Among nano particles, carbon nanotubes (CNT) has attracted scientific and techno logical interest worldwide and CNT reinforced polymer composites have been shown to exhibit enhanced mechanical, thermal, and electrical properties compared to base polymers [4]. In the last decade, epoxidized vegetable oils like epoxidized soybean oil (ESO), epoxidized linseed oil (ELO), epoxidized castor oil (ECO) etc., have been shown as most versatile prepolymers in preparing multi functional composites [5,6]. Thielemans et al. [7] worked on stable dispersion of impure multiwalled carbon nanotubes (MWCNT) in acry lated ESO/styrene resin by stirring process to achieve improved prop erties. The modulus of the nanocomposite is increased by 30% due to the
addition of 0.28 wt% dispersed MWCNT. Miyagawa et al. [8] reported the synthesis of ELO based epoxy nanocomposite reinforced with fluo rinated single-wall carbon nanotubes (SWCNT) and compared the properties with neat matrix. Incorporation of 0.24 wt% fluorinated SWCNT into the bio-based epoxy resin increased the storage modulus by 14% and fracture toughness by 43% in comparison to neat matrix. Particularly, in order to improve the electrical properties of the composites, mostly CNT are used as reinforcing agents to form a well connected conductive network. Intrinsically conducting polymers (ICPs) are another predominant category of electrically conductive fillers. Polyaniline (PANI) is one of the most encouraging ICP for the prepara tion of such electrically conductive composites owing to its exceptional properties including lightweight, easy synthesis, high yield, adjustable conductivity, high environmental stability, etc. [9–12]. There are many researchers who have studied epoxy/PANI composites for several en gineering applications such as anti-corrosion coatings, antistatic for mulations, electrically conductive adhesive, electromagnetic interference (EMI) shielding, etc. [10,13–16]. However, there are very few articles reported on PANI functionalized plant oil composites. In one
* Corresponding author. E-mail addresses: [email protected], [email protected] (G. Manik). https://doi.org/10.1016/j.compositesb.2019.05.050 Received 7 February 2019; Received in revised form 29 March 2019; Accepted 5 May 2019 Available online 7 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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such instance, castor oil-based waterborne polyurethane dispersions with PANI were prepared and characterized for potential use in anti static and corrosion inhibition processes [17]. However, CNT or PANI individually have limitations in obtaining desired conductivity of the composites for specific applications as higher content of CNT is difficult to incorporate with better dispersibility, and surplus loading of PANI hinders the processability due to raised viscosity and forms unwanted aggregates. Use of a combination of two fillers is another approach in preparing such conductive composites where properties of both fillers complement each other in enhancing the overall properties of composite. Several articles are available on employing CNT with other carbon nanomaterials like carbon black [18,19], graphene nanoplatelets [20,21] as fillers in preparing epoxy composites. A few reports are also available on CNT and PANI filled epoxy composites showing better properties than either of the filler individually in epoxy resin [22,23] whereas the study of preparing such hybrid composites using plant oil as matrix is still not matured enough. However, the properties of hybrid polymer composite greatly depend on the chemistry of polymer matrix, properties of nanofillers, synergism among the fillers, degree of compatibility be tween fillers and matrix, processing techniques and polymerization methods [24–27]. In our previous work, we explored the effect of PANI on ELO based green composite and obtained highest conductivity of 6.18 � 10 6 S/cm at 15% PANI loading along with a 27 fold increase in Young’s modulus [28]. However, higher loading of PANI leads to difficulty in processing, so in the current investigation, we report a facile process to prepare bio-based green nanocomposite with two predetermined concentrations of CNT to ELO/PANI systems aiming to achieve a significant enhance ment in conductivity. Other properties such as mechanical, thermal, morphological and viscoelastic behavior of the hybrid composites have also been studied to reveal the suitability for its use as potential bio-based antistatic materials.
added to the round bottom flask in 1:1 M ratio (1 mol oil unsaturation). AIER (25% Seralite SRC-120) was added to this solution and stirred continuously using magnetic stirrer. Hydrogen peroxide was added dropwise to this solution in 2:1 M ratio of hydrogen peroxide and unsaturation of oil, and the solution was further stirred for 5 h at 60 � C. Afterward, mixture was filtered using cheese cloth and ELO was extracted, washed using 2% sodium carbonate solution and subse quently filtered through magnesium sulfate in order to eliminate trap ped water and finally dried using oven at 60 � C for 12 h. 2.4. Synthesis of ELO/CNT composites CNT was first dispersed in acetone in the presence of ELO, aided by sonication. This was followed by stirring the mixture for 5 h at 55 � C in order to have better distribution and to eliminate the acetone. HHPA and BF3-complex in the proportion of 44:5 per 100 parts of ELO were added to this mixture and finally cured at 120 � C for 5 h [28]. In the prepared ELO/CNT composites, weight percentage of CNT was kept at 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5%. ELO/PANI composites were prepared as per the method adopted in our previous work [28]. 2.5. Synthesis of hybrid ELO/PANI/CNT composites CNT and ELO were added in acetone and dispersed with the help of ultrasonic bath. To this dispersion calculated amount of PANI was added and the mixture was sonicated again. The remaining procedure is same as that for preparing ELO/CNT composites. Binary composites are named as ELO/PANIx (where x ¼ 1, 3, 5, 7, 10, 15% PANI) and ELO/ CNTy (where y ¼ 0.05, 0.1, 0.2, 0.3.0.4, 0.5% CNT) and hybrid com posites are coded as ELO/PANIx/CNT0.1 and ELO/PANIx/CNT0.4. 2.6. Characterization The conductivity of the specimens was measured by Keithley 236 source measurement unit by two-probe method. For this, samples of circular geometry having diameter of 11.5 mm were polished up to a thickness level of nearly 0.7–0.8 mm and then coated with conductive silver paint on each side. Morphology of the fractured specimen was studied by using FE-SEM (field emission scanning electron microscopy), TESCAN MIRA 3 LM. The flow behavior of ELO based systems without the curing agent incorporation was analyzed with the help of parallel plate rheometer (Anton Paar, MCR-102) by changing the shear rate from 0.05 to 300 s 1 at room temperature. The thermomechanical behavior was studied using dynamic me chanical analyzer by NETZCH Instruments, (DMA 242). The specimens of dimension 40 � 12 � 3 mm3 were used for this study in a 3-point bending setup. Samples were tested with the heating rate of 5 � C/min, having frequency of 1 Hz and the temperature was varied from 60 to 150 � C. The thermal stability of the composites was examined using the thermal analyzer (EXSTAR SII 6300) by heating the samples from 30 to 700 � C at a heating rate of 10 � C/min under nitrogen environment. The tensile properties of the composite samples were determined as per ASTM D 3039 by using Universal Testing Machine (Instron 3382). The specimen having geometry 110 � 12 � 3 mm3 having a span length of 50 mm were tested at a cross head speed of 2 mm/min. Five specimens were tested and averaged for each sample to evaluate the properties.. The impact strength of the composite samples was obtained by using Izod impact tester (Tinouusolesan, UK) under the ASTM D 256. For this study, samples of dimension 60 � 12 � 3 mm3 were used and the results were averaged out of five specimens for each sample.
2. Experimental 2.1. Materials Linseed oil (LO) was purchased from Himedia, India. Hexahy drophthalic anhydride (HHPA) (95%) and BF3-ethylamine (97%) for curing of ELO were supplied by Sigma-Aldrich and TCI chemicals, respectively. Magnesium sulfate (99%) and acidic ion exchange resin (AIER), Seralite (SRC-120) were procured from SRL chemicals, India. Hydrogen peroxide (30 wt%), Acetic acid (99.5%), and sodium car bonate (99.9%) were obtained from Rankem chemicals, India. MWCNT (95%) having diameter and length of 30–50 nm and 10–30 μm, respec tively was supplied by SRL, India. Ammonium persulfate (APS) and aniline were supplied by Merck, India while p-toluenesulfonic acid (pTSA) was obtained from Vetec-Sigma, India. 2.2. PANI-pTSA synthesis PANI-pTSA was prepared as per the method reported in our previous work [22]. A predetermined amount of pTSA and Aniline (1:1 M ratio) were mixed in distilled water and mechanically stirred within temper ature range of 0–5 � C. Afterward, APS solution in water was transferred slowly to this mixture and stirred continuously for 5 h. Reaction was quenched using acetone and subsequently the reaction mixture was filtered through Buchner funnel. Green colored precipitate was thor oughly washed using distilled water followed by acetone several times and oven dried for 24 h at 65 � C. 2.3. Synthesis of ELO Epoxidation of LO was carried out as per the method described in our previous article [28]. In a typical procedure, LO and acetic acid were 77
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Fig. 1. Conductivity of (a) ELO/CNT composite and (b) ELO/PANI composite, ELO/PANI/CNT0.1 and ELO/PANI/CNT0.4 hybrid composites (Conductivity of the neat ELO, ELO/CNT0.05, ELO/PANI1 and ELO/PANI3 composites was below the measurement capacity of equipment, and hence was considered to be 10 14 S/cm).
Fig. 2. SEM micrographs of (a) ELO/PANI5 composite, (b) ELO/CNT0.1 composite, (c, d) ELO/PANI5/CNT0.1 hybrid composite and (e) ELO/PANI5/CNT0.4 hybrid composite.
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3-D conductive path in ELO and contribution of 0.1% CNT to the con ductivity is quite small. However, in ELO/CNT0.4 composite system, percolation threshold has already been established and therefore in ELO/PANI/CNT0.4 hybrid composites, the major contribution towards conductivity comes from CNT (0.4%). Increasing PANI content has a minimal role in enhancing the conductivity and it remained constant beyond 3% PANI, as the curve seems to be parallel to x-axis. Maximum conductivity obtained with ELO/PANI15/CNT0.4 is 4.53 � 10 5 S/cm which is practically equivalent to the conductivity observed in our earlier work (7.01 � 10 5 S/cm) wherein petro-based epoxy (DGEBF) was used as matrix containing same fillers content [22]. Since the percolation threshold in ELO/PANI composite is attained between 5 and 7% PANI content, hence a 5% PANI content was chosen to develop hybrid composites for further studies. 3.2. Morphological analysis Fig. 2 shows the SEM images of fractured surfaces of ELO/PANI5, ELO/CNT0.1 and ELO/PANI5/CNT composites. Although, dispersion of PANI is quite uniform in the ELO matrix still few agglomerates can be seen that are sparsely distributed (Fig. 2a). CNT also appears to be uniformly dispersed but this distribution is localized in nature forming CNT rich region in the matrix (Fig. 2b). Interestingly, in ELO/PANI5/ CNT0.1 composite (Fig. 2c and d) it can be seen that, PANI particles and CNT coexist together and no segregation is visible. Further, on a closer look at higher magnification (Fig. 2c), it can be observed that CNT seems to be intercalated in PANI aggregates (shown by white arrow) that also support the synergistic effect among fillers resulting in higher conduc tivity of hybrid composites. However, in case of ELO/PANI5/CNT0.4 (Fig. 2e), CNT agglomeration is observed near the PANI aggregates which is because of the strong van der Waals interactions between CNTs.
Fig. 3. Viscosity vs. shear rate for ELO and its blends.
3. Results and discussion 3.1. Electrical conductivity Variation of electrical conductivity with the addition of CNT to ELO matrix is depicted in Fig. 1a. It can be observed that conductivity starts to increase beyond 0.05% of CNT and increases by 8 orders i.e. 10 6 S/ cm from 10 14 S/cm with 0.4% CNT addition to ELO with percolation threshold established between 0.2 and 0.3% CNT. However, in ELO/ PANI composite conductivity increases sharply beyond 3% PANI and the percolation is realized between 5 and 7% PANI as shown in Fig. 1b. Maximum conductivity in ELO/PANI composites is found to be 6.18 � 10 6 S/cm at 15% PANI loading, which is at par with that of ELO/CNT0.4 composite. The incorporation of higher PANI content (15%) in ELO is needed to achieve a conductivity equivalent to ELO/ CNT0.4 composite which is due to PANI’s lower inherent conductivity. To study the significance of synergistic effect between PANI and CNT inside the ELO matrix, two concentrations of CNT i.e. 0.1% (below percolation) and 0.4% (above percolation) were added to ELO/PANI composites. It can be observed from Fig. 1b that with the addition of just 0.1% CNT to ELO/PANI3 system conductivity increased as high as 6 orders which confirms that both the fillers support the conductive network of each other within ELO matrix. Further, this harmony among fillers is quite evident till 7% PANI and starts to diminish afterward and conductivity of ELO/PANI15 and ELO/PANI15/CNT0.1 are practically same. This is because, in ELO/PANI7 system, PANI has already formed a
3.3. Rheological study Viscosity is an essential parameter in ascertaining the processing conditions for preparing composites for various applications, especially for paint and coating purpose. Fig. 3 depicts the viscosity behavior as a function of shear rate for neat ELO, ELO/PANI5 and ELO/PANI5/CNT systems. It can be observed that uncured ELO and ELO/PANI5 resin systems display a linear relationship between viscosity and shear rate, thereby revealing pure Newtonian flow behavior. Further, slightly enhanced near zero shear rate viscosity of 1.3 Pa s is observed for ELO/PANI5/CNT0.1 system due to the addition of 0.1% CNT as compared to 0.9 Pa s for ELO/PANI5. A minimal shear thinning is also perceived at initial shear rates that gets saturated at higher shear values. However, with the incorporation of higher content (0.4%) of CNT, viscosity attained a further significantly high value of 3.3 Pa s at
Fig. 4. Illustration of (a) Storage modulus, E’ and (b) tan δ versus temperature for cured ELO and ELO/PANI5 ELO/PANI5/CNT composites. 79
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Table 1 DMA parameters of ELO, ELO/PANI5 and ELO/PANI5/CNT composites. Samples
E ’ (MPa) at 25 � C
Tg (� C)
tan δ (Intensity)
ELO ELO/PANI5 ELO/PANI5/CNT0.1 ELO/PANI5/CNT0.4
1595 2226 2589 2850
48.2 54.6 58.1 63.2
0.68 0.59 0.56 0.54
near zero shear rate. Higher filler loading leads to reduced inter-particle distance that results in higher entanglement among CNT [29]. The strong van der Waals interactions among CNT offer the resistance to flow of ELO chains and hence increase the viscosity. This effect is quite evident at low shear rates and as the shear rate increases, these in teractions between CNT starts weakening and shear thinning is observed in accordance with theoretical expectations and previous experimental observations in composites [30]. 3.4. Dynamic mechanical properties
Fig. 5. TGA curves of cured neat ELO, ELO/PANI5 and ELO/PANI5/ CNT composites.
DMA depicts the information about the stiffness of the polymers along with the temperature dependent molecular chain relaxations occurring within polymers. In order to study the effect of CNT on ther momechanical properties of ELO/PANI composites, the variation in storage modulus (E0 ) and tan δ (ratio of loss modulus and storage modulus during applied alternating strain cycle) were evaluated as a function of temperature and are shown in Fig. 4. The overall viscoelastic response of composites resembles to that of thermoset amorphous polymers, displaying a vigorous drop in modulus in the proximity of the glass transition on account of the enhanced chain mobility. The E0 values at different temperatures, peak intensity of tan δ curve and glass transition temperature Tg are collectively presented in Table 1. It can be noticed from Fig. 4a that at room temperature (25 � C), among all samples, neat ELO behaves similar to a rubbery-like material with lower E0 value of 1595 MPa, which rise to 2226 MPa, a ~40% raise due to addition of 5% PANI owing to mechanical fortification or stiffness offered by PANI [28]. Incorporation of CNTs improved the E0 value of ELO/PANI5 composite significantly both in viscoelastic and rubbery region, having a value of 2850 MPa at 25 � C, an increment of 28% observed with 0.4% CNT loading compared to its base matrix system (ELO/PANI5). This confirms the reinforcement effect of the uniformly dispersed CNTs, improvement in modulus and load bearing capacity of the composite and good matrix-filler interaction. Analogous increments have been reported for epoxy/carbon nanofiber composite [31] and for halloysite nanotubes (HNT) reinforced epoxidized Hemp Seed Oil [32]. In loss factor (tan δ) curve (Fig. 4b), a single relaxation peak is noticed for all samples clearly revealing homogeneity in crosslinked network. PANI addition lowers the peak intensity and shifts the peak towards right [28]. The peak of tan δ gradually shifted towards higher temperatures with increase in CNT content on account of the restricted mobility as well as relaxation of the crosslinked matrix chains at the ELO CNT interface. Tg has been reported as the temperature corre sponding to tan δ curve peak and it is noteworthy to see that Tg increased from 54.6 to 63.2 � C at 0.4% loading of CNT within the ELO/PANI5 matrix. This confirmed effective molecular packing, reduced free vol ume content, and increased crosslink density by incorporation of CNT. The intensity of the loss factor peak signifies the chain segmental mobility of a polymer at relaxation temperature. Fig. 4b and Table 1 show broadening of the tan δ curve and substantial reduction in the peak intensity from neat ELO matrix to the ELO/PANI5 composite due to heterogeneity of the matrix and restricted mobility of crosslinked ELO polymer chains by stiff PANI polymer chains. But, no significant change in peak intensity is observed on adding CNT filler into ELO/PANI sys tem. It is because the uniformity of the polymer matrix structure de creases to some extent after addition of CNT. Some part of the polymer chains retain the conformation or morphology of unfilled ELO/PANI
Table 2 Tensile properties of ELO, ELO/PANI5 and ELOPANI5/CNT composites. Specimen
Ultimate Tensile Strength (MPa)
Elastic Modulus (GPa)
Elongation at break (%)
Neat ELO ELO/PANI5 ELO/PANI5/ CNT0.1 ELO/PANI5/ CNT0.4
2.04 � 0.28 4.48 � 0.16 5.26 � 0.71
9.15 � 0.76 34.54 � 4.08 55.67 � 7.47
34.87 � 5.65 28.68 � 5.95 22.07 � 1.35
42.51 � 0.63
95.81 � 6.65
18.38 � 5.83
matrix, whereas the molecular mobility of another polymer chains (which are in contact with CNT) reduces. Thus, the overlapping of peaks is observed with decrease in peak intensity, while the integral intensity of the tan δ peak remains almost unchanged for nanocomposite system. It reveals that the CNTs improve the matrix modulus without sacrificing its energy dissipating ability. The tan δ curve width indicates the structural homogeneity of a polymer and it shows that the glass transition of composites takes place over a wide range of temperature. Upon addition of PANI to ELO, broadening of the curve is seen. Additional broadening is noticed with the addition of CNT, higher for ELO/PANI5/CNT0.4, which may be attributed to a higher number of branching modes, a broader distribu tion of chain structures and larger interfacial volume. A similar finding was reported for polyamide-6/HNT [33] and LO/POSS nanocomposites [34]. On increasing the concentration of CNT, the density of nanoparticle-matrix interactions appears to be comparatively higher and interfacial area also increased. This leads to the molecular motions at the interfacial area and pays efficiently to the damping. 3.5. Thermogravimetric analysis The thermal degradation study of cured ELO, ELO/PANI5 and ELO/ PANI5/CNT composites has been carried out in order to examine their thermal stability and corresponding TGA curves are shown in Fig. 5. Thermogram of ELO majorly demonstrates two stage decomposition out of which initial degradation begins around 175 � C which is linked to the discharge of low molecular weight fragments such as unreacted HHPA, BF3-complex, etc. [35]. While second step degradation starts at around 345 � C that signifies the scission of crosslinked structure and polymeric backbone and continues till 600 � C. It is noteworthy to see that presence of CNT and PANI does not change the basic decomposition contour of ELO. However, incorporation of PANI and CNT enhanced the stability of 80
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0.1% CNT addition validating the synergistic effect taking place among two different class of fillers. However, the effect of 0.4% CNT addition was less evident since at this CNT content, the percolation was already achieved and the contribution of increasing PANI content towards conductivity was minimal. Highest conductivity obtained for ELO/ PANI15/CNT0.4 hybrid composites was 4.5 � 10 5 S cm 1. Apart from the improvement in the electrical conductivity, FE-SEM revealed that PANI (5%) has fairly good dispersion in ELO matrix and CNT addition has no observable effect on PANI distribution. Viscosity vs. shear rate measurements showed that neat ELO and ELO/PANI5 showed Newto nian behavior while addition of CNT to ELO/PANI5 mixture enhanced the viscosity and displayed shear thinning behavior. E0 and Tg increased with the insertion of PANI and CNT into ELO matrix. Maximum incre ment in Tg of about ~9 � C, was observed for ELO/PANI5/CNT0.4 compared to ELO/PANI5. Thermal degradation of ELO with the incor poration of PANI and CNT remained practically unaffected. Tensile strength and Young’s modulus were found to increase with PANI and CNT concentrations accompanied with a significant reduction in elon gation at break and impact strength. The obtained properties revealed the suitability of the formulated composites for specific applications e.g. antistatic formulation. Acknowledgements
Fig. 6. Impact strength of neat ELO, ELO/PANI5 and ELO/PANI5/ CNT composites.
The author, Vinay Khandelwal is thankful for the financial support to Ministry of Human Resource Development, Govt. of India. Authors are also grateful to technical staff of polymer and process engineering department of IIT Roorkee.
these composites at higher temperatures (405–629 � C). 3.6. Mechanical properties
References
Tensile properties of ELO, ELO/PANI5 and ELO/PANI5/CNT com posites are listed in Table 2. It can be observed that the addition of PANI (5%) doubles the tensile strength of ELO while modulus is increased more than ~3 folds with ~18% reduction in strain to failure. The in crease in modulus may be attributed to stiffness offered by rigid PANI chains while higher strength is the result of good interfacial bonding between ELO and PANI [28]. Addition of CNT (0.1%) to ELO/PANI5 system has marginally increased the tensile properties. However, higher CNT (0.4%) content has significant contribution in enhancing the strength and modulus by ~9.5 folds and ~3 folds, respectively, accompanied with a dip of 36% in elongation at break. This increment is attributed to the reinforcing effect of CNT and the strong interface adhesion between ELO resin and CNT, resulting in an improved load transfer. In order to investigate the influence of addition of PANI and CNT on toughening ability of ELO matrix, Impact test has been carried out and the results presented in Fig. 6. It can be observed that impact strength decreases on addition of PANI (5%) by 22% due the inherent rigidity or brittleness of PANI that leads to reduction in ductility of ELO. Addition of 0.1% CNT enhances the impact strength of ELO/PANI5 by 10% because of the homogeneous CNT dispersion that curtails the stress concentration nuclei, and the better matrix CNT interfacial adhesion that can offer an effectual hindrance for crack propagation [32]. On the contrary, higher loading of CNT (0.4%) leads to a considerable decay in impact strength because of formation of aggregates. The presence of CNT clusters may nucleate small cracks which lead to premature failure.
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4. Conclusions Electrically conductive bio-sourced ELO based composites were prepared using PANI and CNT as fillers and their electrical, morpho logical, thermal and mechanical properties were evaluated. Percolation threshold of ELO/PANI and ELO/CNT composites were observed to be between 5 and 7% PANI and 0.2–0.3% CNT, respectively. CNT was added to ELO/PANI system to prepare the hybrid composites and the resultant conductivity was found to be several orders higher with just 81
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Bio-sourced electrically conductive epoxidized linseed oil based composites filled with polyaniline and carbon nanotubes Vinay Khandelwal a, Sushanta K. Sahoo b, Ashok Kumar c, Sushanta K. Sethi a, Gaurav Manik a, * a
Department of Polymer and Process Engineering, Indian Institute of Technology Roorkee Saharanpur Campus, Saharanpur, 247001, India Materials Science and Technology Division, CSIR – National Institute for Interdisciplinary Science and Technology (NIIST), Thiruvananthapuram, 695019, India c CSIR - National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi, 110012, India b
A R T I C L E I N F O
A B S T R A C T
Keywords: Polymer-matrix composites (PMCs) Polyaniline Electrical conductivity Bio-epoxy resin
In the present work, electrically conductive hybrid green composites were developed using epoxidized linseed oil (ELO) as bio-sourced matrix and, p-toluenesulfonic acid (pTSA) doped polyaniline (PANI) and carbon nanotubes (CNT) as two conductive fillers of varied nature. Based on percolation threshold, two different concentrations of CNT (0.1% and 0.4%) were added to ELO/PANI composites. Conductivity increased with the incorporation of CNT and maximum conductivity observed with 0.1% and 0.4% CNT was 1.0 � 10 5 and 4.5 � 10 5 S/cm respectively confirming the synergism among the fillers. The good distribution of both CNT and PANI in ELO matrix was confirmed by morphological studies. CNT addition increased the viscosity and induced the shear thinning behavior in ELO/PANI (5%) resin system. Energy storing ability as well as glass transition temperature (Tg) of the ELO/PANI (5%) composite were improved with CNT incorporation as revealed from viscoelastic studies. Tensile strength and elastic modulus showed increased values upon CNT insertion within composite. Impact strength improved with 0.1% CNT addition to ELO/PANI5 confirming an enhancement in toughening of composite. This investigation opens up the path to make electrically conductive composites with tunable con ductivity from renewable resource based material such as linseed oil.
1. Introduction In current years, application of nanotechnology to bio-based poly meric systems has created novel opportunities for improving the prop erties, retaining cost-effectiveness and eco-friendly nature [1]. Addition of nanofillers to plant oil-based polymers is found to improve the me chanical, thermal and electrical properties reasonably to be used as an alternative for depleting petro-sourced materials [2,3]. Among nano particles, carbon nanotubes (CNT) has attracted scientific and techno logical interest worldwide and CNT reinforced polymer composites have been shown to exhibit enhanced mechanical, thermal, and electrical properties compared to base polymers [4]. In the last decade, epoxidized vegetable oils like epoxidized soybean oil (ESO), epoxidized linseed oil (ELO), epoxidized castor oil (ECO) etc., have been shown as most versatile prepolymers in preparing multi functional composites [5,6]. Thielemans et al. [7] worked on stable dispersion of impure multiwalled carbon nanotubes (MWCNT) in acry lated ESO/styrene resin by stirring process to achieve improved prop erties. The modulus of the nanocomposite is increased by 30% due to the
addition of 0.28 wt% dispersed MWCNT. Miyagawa et al. [8] reported the synthesis of ELO based epoxy nanocomposite reinforced with fluo rinated single-wall carbon nanotubes (SWCNT) and compared the properties with neat matrix. Incorporation of 0.24 wt% fluorinated SWCNT into the bio-based epoxy resin increased the storage modulus by 14% and fracture toughness by 43% in comparison to neat matrix. Particularly, in order to improve the electrical properties of the composites, mostly CNT are used as reinforcing agents to form a well connected conductive network. Intrinsically conducting polymers (ICPs) are another predominant category of electrically conductive fillers. Polyaniline (PANI) is one of the most encouraging ICP for the prepara tion of such electrically conductive composites owing to its exceptional properties including lightweight, easy synthesis, high yield, adjustable conductivity, high environmental stability, etc. [9–12]. There are many researchers who have studied epoxy/PANI composites for several en gineering applications such as anti-corrosion coatings, antistatic for mulations, electrically conductive adhesive, electromagnetic interference (EMI) shielding, etc. [10,13–16]. However, there are very few articles reported on PANI functionalized plant oil composites. In one
* Corresponding author. E-mail addresses: [email protected], [email protected] (G. Manik). https://doi.org/10.1016/j.compositesb.2019.05.050 Received 7 February 2019; Received in revised form 29 March 2019; Accepted 5 May 2019 Available online 7 May 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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such instance, castor oil-based waterborne polyurethane dispersions with PANI were prepared and characterized for potential use in anti static and corrosion inhibition processes [17]. However, CNT or PANI individually have limitations in obtaining desired conductivity of the composites for specific applications as higher content of CNT is difficult to incorporate with better dispersibility, and surplus loading of PANI hinders the processability due to raised viscosity and forms unwanted aggregates. Use of a combination of two fillers is another approach in preparing such conductive composites where properties of both fillers complement each other in enhancing the overall properties of composite. Several articles are available on employing CNT with other carbon nanomaterials like carbon black [18,19], graphene nanoplatelets [20,21] as fillers in preparing epoxy composites. A few reports are also available on CNT and PANI filled epoxy composites showing better properties than either of the filler individually in epoxy resin [22,23] whereas the study of preparing such hybrid composites using plant oil as matrix is still not matured enough. However, the properties of hybrid polymer composite greatly depend on the chemistry of polymer matrix, properties of nanofillers, synergism among the fillers, degree of compatibility be tween fillers and matrix, processing techniques and polymerization methods [24–27]. In our previous work, we explored the effect of PANI on ELO based green composite and obtained highest conductivity of 6.18 � 10 6 S/cm at 15% PANI loading along with a 27 fold increase in Young’s modulus [28]. However, higher loading of PANI leads to difficulty in processing, so in the current investigation, we report a facile process to prepare bio-based green nanocomposite with two predetermined concentrations of CNT to ELO/PANI systems aiming to achieve a significant enhance ment in conductivity. Other properties such as mechanical, thermal, morphological and viscoelastic behavior of the hybrid composites have also been studied to reveal the suitability for its use as potential bio-based antistatic materials.
added to the round bottom flask in 1:1 M ratio (1 mol oil unsaturation). AIER (25% Seralite SRC-120) was added to this solution and stirred continuously using magnetic stirrer. Hydrogen peroxide was added dropwise to this solution in 2:1 M ratio of hydrogen peroxide and unsaturation of oil, and the solution was further stirred for 5 h at 60 � C. Afterward, mixture was filtered using cheese cloth and ELO was extracted, washed using 2% sodium carbonate solution and subse quently filtered through magnesium sulfate in order to eliminate trap ped water and finally dried using oven at 60 � C for 12 h. 2.4. Synthesis of ELO/CNT composites CNT was first dispersed in acetone in the presence of ELO, aided by sonication. This was followed by stirring the mixture for 5 h at 55 � C in order to have better distribution and to eliminate the acetone. HHPA and BF3-complex in the proportion of 44:5 per 100 parts of ELO were added to this mixture and finally cured at 120 � C for 5 h [28]. In the prepared ELO/CNT composites, weight percentage of CNT was kept at 0.05, 0.1, 0.2, 0.3, 0.4 and 0.5%. ELO/PANI composites were prepared as per the method adopted in our previous work [28]. 2.5. Synthesis of hybrid ELO/PANI/CNT composites CNT and ELO were added in acetone and dispersed with the help of ultrasonic bath. To this dispersion calculated amount of PANI was added and the mixture was sonicated again. The remaining procedure is same as that for preparing ELO/CNT composites. Binary composites are named as ELO/PANIx (where x ¼ 1, 3, 5, 7, 10, 15% PANI) and ELO/ CNTy (where y ¼ 0.05, 0.1, 0.2, 0.3.0.4, 0.5% CNT) and hybrid com posites are coded as ELO/PANIx/CNT0.1 and ELO/PANIx/CNT0.4. 2.6. Characterization The conductivity of the specimens was measured by Keithley 236 source measurement unit by two-probe method. For this, samples of circular geometry having diameter of 11.5 mm were polished up to a thickness level of nearly 0.7–0.8 mm and then coated with conductive silver paint on each side. Morphology of the fractured specimen was studied by using FE-SEM (field emission scanning electron microscopy), TESCAN MIRA 3 LM. The flow behavior of ELO based systems without the curing agent incorporation was analyzed with the help of parallel plate rheometer (Anton Paar, MCR-102) by changing the shear rate from 0.05 to 300 s 1 at room temperature. The thermomechanical behavior was studied using dynamic me chanical analyzer by NETZCH Instruments, (DMA 242). The specimens of dimension 40 � 12 � 3 mm3 were used for this study in a 3-point bending setup. Samples were tested with the heating rate of 5 � C/min, having frequency of 1 Hz and the temperature was varied from 60 to 150 � C. The thermal stability of the composites was examined using the thermal analyzer (EXSTAR SII 6300) by heating the samples from 30 to 700 � C at a heating rate of 10 � C/min under nitrogen environment. The tensile properties of the composite samples were determined as per ASTM D 3039 by using Universal Testing Machine (Instron 3382). The specimen having geometry 110 � 12 � 3 mm3 having a span length of 50 mm were tested at a cross head speed of 2 mm/min. Five specimens were tested and averaged for each sample to evaluate the properties.. The impact strength of the composite samples was obtained by using Izod impact tester (Tinouusolesan, UK) under the ASTM D 256. For this study, samples of dimension 60 � 12 � 3 mm3 were used and the results were averaged out of five specimens for each sample.
2. Experimental 2.1. Materials Linseed oil (LO) was purchased from Himedia, India. Hexahy drophthalic anhydride (HHPA) (95%) and BF3-ethylamine (97%) for curing of ELO were supplied by Sigma-Aldrich and TCI chemicals, respectively. Magnesium sulfate (99%) and acidic ion exchange resin (AIER), Seralite (SRC-120) were procured from SRL chemicals, India. Hydrogen peroxide (30 wt%), Acetic acid (99.5%), and sodium car bonate (99.9%) were obtained from Rankem chemicals, India. MWCNT (95%) having diameter and length of 30–50 nm and 10–30 μm, respec tively was supplied by SRL, India. Ammonium persulfate (APS) and aniline were supplied by Merck, India while p-toluenesulfonic acid (pTSA) was obtained from Vetec-Sigma, India. 2.2. PANI-pTSA synthesis PANI-pTSA was prepared as per the method reported in our previous work [22]. A predetermined amount of pTSA and Aniline (1:1 M ratio) were mixed in distilled water and mechanically stirred within temper ature range of 0–5 � C. Afterward, APS solution in water was transferred slowly to this mixture and stirred continuously for 5 h. Reaction was quenched using acetone and subsequently the reaction mixture was filtered through Buchner funnel. Green colored precipitate was thor oughly washed using distilled water followed by acetone several times and oven dried for 24 h at 65 � C. 2.3. Synthesis of ELO Epoxidation of LO was carried out as per the method described in our previous article [28]. In a typical procedure, LO and acetic acid were 77
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Fig. 1. Conductivity of (a) ELO/CNT composite and (b) ELO/PANI composite, ELO/PANI/CNT0.1 and ELO/PANI/CNT0.4 hybrid composites (Conductivity of the neat ELO, ELO/CNT0.05, ELO/PANI1 and ELO/PANI3 composites was below the measurement capacity of equipment, and hence was considered to be 10 14 S/cm).
Fig. 2. SEM micrographs of (a) ELO/PANI5 composite, (b) ELO/CNT0.1 composite, (c, d) ELO/PANI5/CNT0.1 hybrid composite and (e) ELO/PANI5/CNT0.4 hybrid composite.
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3-D conductive path in ELO and contribution of 0.1% CNT to the con ductivity is quite small. However, in ELO/CNT0.4 composite system, percolation threshold has already been established and therefore in ELO/PANI/CNT0.4 hybrid composites, the major contribution towards conductivity comes from CNT (0.4%). Increasing PANI content has a minimal role in enhancing the conductivity and it remained constant beyond 3% PANI, as the curve seems to be parallel to x-axis. Maximum conductivity obtained with ELO/PANI15/CNT0.4 is 4.53 � 10 5 S/cm which is practically equivalent to the conductivity observed in our earlier work (7.01 � 10 5 S/cm) wherein petro-based epoxy (DGEBF) was used as matrix containing same fillers content [22]. Since the percolation threshold in ELO/PANI composite is attained between 5 and 7% PANI content, hence a 5% PANI content was chosen to develop hybrid composites for further studies. 3.2. Morphological analysis Fig. 2 shows the SEM images of fractured surfaces of ELO/PANI5, ELO/CNT0.1 and ELO/PANI5/CNT composites. Although, dispersion of PANI is quite uniform in the ELO matrix still few agglomerates can be seen that are sparsely distributed (Fig. 2a). CNT also appears to be uniformly dispersed but this distribution is localized in nature forming CNT rich region in the matrix (Fig. 2b). Interestingly, in ELO/PANI5/ CNT0.1 composite (Fig. 2c and d) it can be seen that, PANI particles and CNT coexist together and no segregation is visible. Further, on a closer look at higher magnification (Fig. 2c), it can be observed that CNT seems to be intercalated in PANI aggregates (shown by white arrow) that also support the synergistic effect among fillers resulting in higher conduc tivity of hybrid composites. However, in case of ELO/PANI5/CNT0.4 (Fig. 2e), CNT agglomeration is observed near the PANI aggregates which is because of the strong van der Waals interactions between CNTs.
Fig. 3. Viscosity vs. shear rate for ELO and its blends.
3. Results and discussion 3.1. Electrical conductivity Variation of electrical conductivity with the addition of CNT to ELO matrix is depicted in Fig. 1a. It can be observed that conductivity starts to increase beyond 0.05% of CNT and increases by 8 orders i.e. 10 6 S/ cm from 10 14 S/cm with 0.4% CNT addition to ELO with percolation threshold established between 0.2 and 0.3% CNT. However, in ELO/ PANI composite conductivity increases sharply beyond 3% PANI and the percolation is realized between 5 and 7% PANI as shown in Fig. 1b. Maximum conductivity in ELO/PANI composites is found to be 6.18 � 10 6 S/cm at 15% PANI loading, which is at par with that of ELO/CNT0.4 composite. The incorporation of higher PANI content (15%) in ELO is needed to achieve a conductivity equivalent to ELO/ CNT0.4 composite which is due to PANI’s lower inherent conductivity. To study the significance of synergistic effect between PANI and CNT inside the ELO matrix, two concentrations of CNT i.e. 0.1% (below percolation) and 0.4% (above percolation) were added to ELO/PANI composites. It can be observed from Fig. 1b that with the addition of just 0.1% CNT to ELO/PANI3 system conductivity increased as high as 6 orders which confirms that both the fillers support the conductive network of each other within ELO matrix. Further, this harmony among fillers is quite evident till 7% PANI and starts to diminish afterward and conductivity of ELO/PANI15 and ELO/PANI15/CNT0.1 are practically same. This is because, in ELO/PANI7 system, PANI has already formed a
3.3. Rheological study Viscosity is an essential parameter in ascertaining the processing conditions for preparing composites for various applications, especially for paint and coating purpose. Fig. 3 depicts the viscosity behavior as a function of shear rate for neat ELO, ELO/PANI5 and ELO/PANI5/CNT systems. It can be observed that uncured ELO and ELO/PANI5 resin systems display a linear relationship between viscosity and shear rate, thereby revealing pure Newtonian flow behavior. Further, slightly enhanced near zero shear rate viscosity of 1.3 Pa s is observed for ELO/PANI5/CNT0.1 system due to the addition of 0.1% CNT as compared to 0.9 Pa s for ELO/PANI5. A minimal shear thinning is also perceived at initial shear rates that gets saturated at higher shear values. However, with the incorporation of higher content (0.4%) of CNT, viscosity attained a further significantly high value of 3.3 Pa s at
Fig. 4. Illustration of (a) Storage modulus, E’ and (b) tan δ versus temperature for cured ELO and ELO/PANI5 ELO/PANI5/CNT composites. 79
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Table 1 DMA parameters of ELO, ELO/PANI5 and ELO/PANI5/CNT composites. Samples
E ’ (MPa) at 25 � C
Tg (� C)
tan δ (Intensity)
ELO ELO/PANI5 ELO/PANI5/CNT0.1 ELO/PANI5/CNT0.4
1595 2226 2589 2850
48.2 54.6 58.1 63.2
0.68 0.59 0.56 0.54
near zero shear rate. Higher filler loading leads to reduced inter-particle distance that results in higher entanglement among CNT [29]. The strong van der Waals interactions among CNT offer the resistance to flow of ELO chains and hence increase the viscosity. This effect is quite evident at low shear rates and as the shear rate increases, these in teractions between CNT starts weakening and shear thinning is observed in accordance with theoretical expectations and previous experimental observations in composites [30]. 3.4. Dynamic mechanical properties
Fig. 5. TGA curves of cured neat ELO, ELO/PANI5 and ELO/PANI5/ CNT composites.
DMA depicts the information about the stiffness of the polymers along with the temperature dependent molecular chain relaxations occurring within polymers. In order to study the effect of CNT on ther momechanical properties of ELO/PANI composites, the variation in storage modulus (E0 ) and tan δ (ratio of loss modulus and storage modulus during applied alternating strain cycle) were evaluated as a function of temperature and are shown in Fig. 4. The overall viscoelastic response of composites resembles to that of thermoset amorphous polymers, displaying a vigorous drop in modulus in the proximity of the glass transition on account of the enhanced chain mobility. The E0 values at different temperatures, peak intensity of tan δ curve and glass transition temperature Tg are collectively presented in Table 1. It can be noticed from Fig. 4a that at room temperature (25 � C), among all samples, neat ELO behaves similar to a rubbery-like material with lower E0 value of 1595 MPa, which rise to 2226 MPa, a ~40% raise due to addition of 5% PANI owing to mechanical fortification or stiffness offered by PANI [28]. Incorporation of CNTs improved the E0 value of ELO/PANI5 composite significantly both in viscoelastic and rubbery region, having a value of 2850 MPa at 25 � C, an increment of 28% observed with 0.4% CNT loading compared to its base matrix system (ELO/PANI5). This confirms the reinforcement effect of the uniformly dispersed CNTs, improvement in modulus and load bearing capacity of the composite and good matrix-filler interaction. Analogous increments have been reported for epoxy/carbon nanofiber composite [31] and for halloysite nanotubes (HNT) reinforced epoxidized Hemp Seed Oil [32]. In loss factor (tan δ) curve (Fig. 4b), a single relaxation peak is noticed for all samples clearly revealing homogeneity in crosslinked network. PANI addition lowers the peak intensity and shifts the peak towards right [28]. The peak of tan δ gradually shifted towards higher temperatures with increase in CNT content on account of the restricted mobility as well as relaxation of the crosslinked matrix chains at the ELO CNT interface. Tg has been reported as the temperature corre sponding to tan δ curve peak and it is noteworthy to see that Tg increased from 54.6 to 63.2 � C at 0.4% loading of CNT within the ELO/PANI5 matrix. This confirmed effective molecular packing, reduced free vol ume content, and increased crosslink density by incorporation of CNT. The intensity of the loss factor peak signifies the chain segmental mobility of a polymer at relaxation temperature. Fig. 4b and Table 1 show broadening of the tan δ curve and substantial reduction in the peak intensity from neat ELO matrix to the ELO/PANI5 composite due to heterogeneity of the matrix and restricted mobility of crosslinked ELO polymer chains by stiff PANI polymer chains. But, no significant change in peak intensity is observed on adding CNT filler into ELO/PANI sys tem. It is because the uniformity of the polymer matrix structure de creases to some extent after addition of CNT. Some part of the polymer chains retain the conformation or morphology of unfilled ELO/PANI
Table 2 Tensile properties of ELO, ELO/PANI5 and ELOPANI5/CNT composites. Specimen
Ultimate Tensile Strength (MPa)
Elastic Modulus (GPa)
Elongation at break (%)
Neat ELO ELO/PANI5 ELO/PANI5/ CNT0.1 ELO/PANI5/ CNT0.4
2.04 � 0.28 4.48 � 0.16 5.26 � 0.71
9.15 � 0.76 34.54 � 4.08 55.67 � 7.47
34.87 � 5.65 28.68 � 5.95 22.07 � 1.35
42.51 � 0.63
95.81 � 6.65
18.38 � 5.83
matrix, whereas the molecular mobility of another polymer chains (which are in contact with CNT) reduces. Thus, the overlapping of peaks is observed with decrease in peak intensity, while the integral intensity of the tan δ peak remains almost unchanged for nanocomposite system. It reveals that the CNTs improve the matrix modulus without sacrificing its energy dissipating ability. The tan δ curve width indicates the structural homogeneity of a polymer and it shows that the glass transition of composites takes place over a wide range of temperature. Upon addition of PANI to ELO, broadening of the curve is seen. Additional broadening is noticed with the addition of CNT, higher for ELO/PANI5/CNT0.4, which may be attributed to a higher number of branching modes, a broader distribu tion of chain structures and larger interfacial volume. A similar finding was reported for polyamide-6/HNT [33] and LO/POSS nanocomposites [34]. On increasing the concentration of CNT, the density of nanoparticle-matrix interactions appears to be comparatively higher and interfacial area also increased. This leads to the molecular motions at the interfacial area and pays efficiently to the damping. 3.5. Thermogravimetric analysis The thermal degradation study of cured ELO, ELO/PANI5 and ELO/ PANI5/CNT composites has been carried out in order to examine their thermal stability and corresponding TGA curves are shown in Fig. 5. Thermogram of ELO majorly demonstrates two stage decomposition out of which initial degradation begins around 175 � C which is linked to the discharge of low molecular weight fragments such as unreacted HHPA, BF3-complex, etc. [35]. While second step degradation starts at around 345 � C that signifies the scission of crosslinked structure and polymeric backbone and continues till 600 � C. It is noteworthy to see that presence of CNT and PANI does not change the basic decomposition contour of ELO. However, incorporation of PANI and CNT enhanced the stability of 80
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0.1% CNT addition validating the synergistic effect taking place among two different class of fillers. However, the effect of 0.4% CNT addition was less evident since at this CNT content, the percolation was already achieved and the contribution of increasing PANI content towards conductivity was minimal. Highest conductivity obtained for ELO/ PANI15/CNT0.4 hybrid composites was 4.5 � 10 5 S cm 1. Apart from the improvement in the electrical conductivity, FE-SEM revealed that PANI (5%) has fairly good dispersion in ELO matrix and CNT addition has no observable effect on PANI distribution. Viscosity vs. shear rate measurements showed that neat ELO and ELO/PANI5 showed Newto nian behavior while addition of CNT to ELO/PANI5 mixture enhanced the viscosity and displayed shear thinning behavior. E0 and Tg increased with the insertion of PANI and CNT into ELO matrix. Maximum incre ment in Tg of about ~9 � C, was observed for ELO/PANI5/CNT0.4 compared to ELO/PANI5. Thermal degradation of ELO with the incor poration of PANI and CNT remained practically unaffected. Tensile strength and Young’s modulus were found to increase with PANI and CNT concentrations accompanied with a significant reduction in elon gation at break and impact strength. The obtained properties revealed the suitability of the formulated composites for specific applications e.g. antistatic formulation. Acknowledgements
Fig. 6. Impact strength of neat ELO, ELO/PANI5 and ELO/PANI5/ CNT composites.
The author, Vinay Khandelwal is thankful for the financial support to Ministry of Human Resource Development, Govt. of India. Authors are also grateful to technical staff of polymer and process engineering department of IIT Roorkee.
these composites at higher temperatures (405–629 � C). 3.6. Mechanical properties
References
Tensile properties of ELO, ELO/PANI5 and ELO/PANI5/CNT com posites are listed in Table 2. It can be observed that the addition of PANI (5%) doubles the tensile strength of ELO while modulus is increased more than ~3 folds with ~18% reduction in strain to failure. The in crease in modulus may be attributed to stiffness offered by rigid PANI chains while higher strength is the result of good interfacial bonding between ELO and PANI [28]. Addition of CNT (0.1%) to ELO/PANI5 system has marginally increased the tensile properties. However, higher CNT (0.4%) content has significant contribution in enhancing the strength and modulus by ~9.5 folds and ~3 folds, respectively, accompanied with a dip of 36% in elongation at break. This increment is attributed to the reinforcing effect of CNT and the strong interface adhesion between ELO resin and CNT, resulting in an improved load transfer. In order to investigate the influence of addition of PANI and CNT on toughening ability of ELO matrix, Impact test has been carried out and the results presented in Fig. 6. It can be observed that impact strength decreases on addition of PANI (5%) by 22% due the inherent rigidity or brittleness of PANI that leads to reduction in ductility of ELO. Addition of 0.1% CNT enhances the impact strength of ELO/PANI5 by 10% because of the homogeneous CNT dispersion that curtails the stress concentration nuclei, and the better matrix CNT interfacial adhesion that can offer an effectual hindrance for crack propagation [32]. On the contrary, higher loading of CNT (0.4%) leads to a considerable decay in impact strength because of formation of aggregates. The presence of CNT clusters may nucleate small cracks which lead to premature failure.
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4. Conclusions Electrically conductive bio-sourced ELO based composites were prepared using PANI and CNT as fillers and their electrical, morpho logical, thermal and mechanical properties were evaluated. Percolation threshold of ELO/PANI and ELO/CNT composites were observed to be between 5 and 7% PANI and 0.2–0.3% CNT, respectively. CNT was added to ELO/PANI system to prepare the hybrid composites and the resultant conductivity was found to be several orders higher with just 81
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