- Email: [email protected]
Electrically conductive green composites based on epoxidized linseed oil and polyaniline: An insight into electrical, thermal and mechanical properties
Composites Part B 136 (2018) 149–157
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Electrically conductive green composites based on epoxidized linseed oil and polyaniline: An insight into electrical, thermal and mechanical properties
MARK
Vinay Khandelwala, Sushanta K. Sahooa, Ashok Kumarb, Gaurav Manika,∗ a b
Department of Polymer and Process Engineering, IIT Roorkee Saharanpur Campus, Saharanpur 247001, India CSIR - National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Bio-epoxy resin Polymer-matrix composites (PMCs) Thermosetting resin Electrical properties
Renewable resource based electrically conductive composites were prepared using polyaniline (PANI) as a conductive filler and epoxidized linseed oil (ELO) as the matrix. Linseed oil (LO) was epoxidized to form ELO and characterized through 1H NMR and IR spectra. Bio-based ELO/PANI conducting composites were prepared by varying the PANI concentration with an aim of attaining the electrical conductivity in the antistatic range (10−8 to 10−3 S/cm) to replace its petro-based counterpart. Conductivity increased with PANI upto the order of 10−6 S/cm with percolation threshold at around 7% of PANI. The shear stress and viscosity of the uncured ELO resin and ELO/PANI resin mixture were studied as a function of shear rate. Differential scanning calorimetry (DSC) studies showed that addition of PANI had a minimal effect on ELO curing at all concentrations. Dynamic mechanical analysis indicated that PANI as a filler provided mechanical fortification in the rubbery region and increased glass transition temperature (Tg) significantly. Thermal stability of ELO remained almost unaffected with the PANI incorporation. Microscopic observation revealed good distribution of PANI in ELO matrix even at higher loading. Interestingly, tensile strength and Young's modulus increased by ∼8 and ∼27 folds, respectively, at 15% PANI content.
1. Introduction Electrically conductive polymer composites are prospective materials for several engineering applications such as electrically conductive adhesives, electromagnetic interference shielding materials, antistatic formulations, sensors, fuel cell, rechargeable batteries and electronic devices, etc. [1–5]. These composites are made up of host matrix that provides the mechanical strength and conductive fillers particles that offer electrical conduction [6]. Intrinsically conducting polymers (ICP) are an important class of conductive fillers that are environmentally stable and cost effective alternate to the traditional expensive metallic fillers which are prone to corrosion with the passage of time [4]. Among ICPs, Polyaniline (PANI) is one of the most studied polymers both by academicians and industries by virtue of its versatile nature like the ease of synthesis, good polymerization yield, low cost, adjustable conductivity and good environmental stability [5,7]. Despite possessing such advantages, researchers have not been able to exploit its properties because of its infusibility and insolubility [8]. A better way is the compounding of PANI with inherently insulating polymer matrices to develop conducting composites for various engineering applications. ∗
Epoxy is one of the commonly used thermosetting matrices for such composites because of its outstanding thermal and mechanical properties, minimal curing shrinkage, and ability to withstand harsh chemical environments. This has increased its suitability for numerous applications like structural materials and adhesives, coatings, aeronautical materials, electronic packaging and composites, etc. [9–18]. PANI doped with organic acids like camphorsulfonic acid (CSA) [19,20], dodecylbezenesulfonic acid (DBSA) [21,22] and p-toluenesulfonic acid (PTSA) [23,24] has been used earlier as preferable filler by various researchers. Peltola et al. [19] prepared PANI-CSA based epoxy adhesives having the conductivity in the range of 10−8-10−3 S/cm, (applicable for antistatic application) at less than 2% of PANI loading. Later, Tsotra et al. [21] formulated epoxy/PANI-DBSA composites, and obtained the conductivity of 2 × 10−7 S/cm at 10% PANI content which qualifies for electrostatic discharge (ESD) application. Soares et al. [25] reported the conductivity of epoxy/PANI-DBSA (7%) and epoxy/PANI-DBSA (18%) adhesives of the order of 10−8 and 10−5 S/ cm, respectively, through physical mixing method. Mir et al. [26] developed electrically conductive adhesives with inorganic acid (HCl) doped PANI and established the conductivity of the order of 10−6 S/cm
Corresponding author. E-mail address: [email protected] (G. Manik).
http://dx.doi.org/10.1016/j.compositesb.2017.10.030 Received 25 July 2017; Received in revised form 23 October 2017; Accepted 24 October 2017 1359-8368/ © 2017 Elsevier Ltd. All rights reserved.
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Merck, India. p-toluenesulfonic acid (PTSA) was obtained from Vetec Sigma, India. LO was procured from Himedia, India. Seralite (SRC-120), an acidic ion exchange resin, and magnesium sulfate (MgSO4) were purchased from SRL Pvt. Ltd, India. Glacial Acetic acid (CH3COOH), Hydrogen peroxide (H2O2) (30 wt%), and sodium carbonate (Na2CO3) were procured from Rankem, India. HHPA for curing was obtained from Sigma-Aldrich, USA. BF3-ethylamine complex was supplied by TCI, India.
at 10% PANI content. The selection of curing agent plays a crucial role in attaining the desired conductivity level. Basic amines, commonly used as cross-linking agents, tend to deprotonate PANI because of their basic nature, and thereby, reduce its conductivity [25]. Acid anhydrides such as hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, etc. are widely used as cross-linkers for epoxy resin along with an accelerator to reduce the curing temperatures. Since BF3-complex is a Lewis acid and does not affect the conductivity of PANI, it is mostly used as curing agent as well as an accelerator in cross-linking of epoxy resin [27]. The rapidly depleting fossil resources and increasing environmental sustainability concerns have motivated the researchers to explore renewable and/or bio-based feedstock as an alternative to petro-based materials [28,29]. In this context, vegetable oils are receiving overwhelming attention worldwide due to their ability to form pre-polymers, a plethora of availability, non-hazardous nature, environmental friendliness and biodegradability [30]. Vegetable oils are chemically composed of mainly triglycerides with long aliphatic carbon chain and multiple unsaturation contents which serve as active sites for direct polymerization or after modification [31]. Various modifications that may be carried out at the unsaturation of the fatty acid chain are acrylation, maleination, halogenation, ozonolysis, dimerization, metathesis, epoxidation and hydroxylation, etc. [32]. Among several modifications possible for plant oils, epoxidation is the most common, straight forward and inexpensive method. In situ method of epoxidation is widely used by researchers as it is solvent free and epoxy content of the epoxidized oil can be appropriately controlled by varying the reaction time [33–35]. Epoxidized oils are considered as renewable raw materials for the preparation of bio-based thermoset products for numerous engineering applications because of their relatively lower viscosity and better processability [35]. They are widely used as reactive diluents for epoxy thermosets for their strong viscosity reducing ability, and also as preferable pre-polymers for the synthesis of polyols used in the manufacturing of polyurethanes with superior thermal and mechanical performance [36,37]. Among all oils, Linseed oil (LO) is the most suitable oil due to its high unsaturation content (53% linolenic acid and 6.6 number of double bonds/molecule) that can be easily epoxidized and can yield higher epoxy values closer to that of commercial epoxy. Being a drying oil, it is frequently used as paint binder and wood finish. Additionally, it has also found applications in enamels, linoleum, oilcloth, printer's ink, and as waterproofing for raincoats [31,38]. Epoxidized linseed oil (ELO) is commonly used as plasticizers for PVC and epoxy resins to provide flexibility, and also for coatings and adhesive applications [39,40]. Many researchers have developed ELO based composites with interesting properties using various reinforcements [41,42]. Another instance of its use previously as a greener substitute involve development of polyhedral oligomeric silsesquioxane based bionanocomposite to reduce dependence on petro-based epoxy resin [43]. In the present study, we report the preparation of bio-based conducting composite taking ELO as bio-renewable matrix and PANI as conducting filler. LO was epoxidized via in situ method, and subsequently, the effect of PANI on the electrical, thermal and thermomechanical properties of ELO network cured with hexahydrophthalic anhydride (HHPA) and BF3-complex was investigated. To the best of our knowledge, no such system of bio-based epoxy resin/PANI has been studied previously. The work was undertaken with the aim of developing ELO/PANI composites with the target values of conductivity in the range (10−8 to 10−6 S/cm) applicable for antistatic coating/ESD application.
2.2. Synthesis of PTSA doped PANI PANI doped with PTSA was prepared via the conventional one phase polymerization technique in aqueous phase as reported in our earlier publication [44]. In a typical procedure, aniline (5 gm) and PTSA (10.22 gm) were mixed in distilled water (250 ml) and stirred for 1 h to make homogenous solution keeping the temperature between 0 and 5 °C. Subsequently, APS (12.25 gm) was added slowly to this mixture maintaining the molar ratio of PANI: PTSA: APS at 1:1:1. Stirring was continued for 5 h and followed by the addition of acetone (25 ml) to seize the polymerization. Solution was filtered, washed with distilled water and acetone and dried in oven at 65 °C for 24 h. 2.3. Epoxidation of LO Epoxidation of LO was performed in the presence of glacial CH3COOH and H2O2 in a three-necked round bottom flask equipped with magnetic stirrer, a dropping funnel and a thermometer as reported by Kim et al. [35]. Linseed oil (79 gm) and CH3COOH (30 gm) were added in the molar ratio of 1:1 (1 mol unsaturation in oil) to the round bottom flask along with 25% seralite SRC-120 (19.75 gm) (acidic ion exchange resin) and stirred for 30 min. To this solution, H2O2 (113 gm) was added dropwise in the ratio of 2:1 of H2O2 and oil unsaturation and the mixture was stirred for 5 h with the temperature maintained at 60 °C throughout the reaction (Fig. 1). After the completion of reaction, mixture was filtered with cheese cloth to remove seralite catalyst. Subsequently, ELO layer was separated and washed with 2% Na2CO3 solution and filtered using MgSO4 to remove moisture and dried overnight in vacuum oven at 60 °C. 2.4. Preparation of ELO/PANI composites The calculated amount of PANI and ELO were dispersed in acetone using ultrasonic bath (Labmann, LMUC-4) for 1 h. Mixture was stirred at 55–60 °C for 5 h to improve the dispersion quality and to remove the acetone completely from mixture. Curing agent along with the accelerator was added in the ratio of 44:5 per 100 parts of ELO as reported earlier [27]. The mixture obtained was poured in steel molds applied with silicon spray. The samples were then cured at 120 °C in a thermostatic oven for 5 h. Composites with 0, 3, 5, 7, 10, 15% PANI concentration are abbreviated as ELO, ELO/PANI-3, ELO/PANI-5, ELO/PANI-7, ELO/PANI10, ELO/PANI-15, respectively. 2.5. Characterization 2.5.1. LO/ELO characterization Epoxy equivalent weight (EEW) was determined by titration with 0.1 N HBr in glacial acetic acid as per the ASTM D 1652. Iodine value was obtained according to ASTM D 5768-02 using Wijs solution [45]. FT-IR spectra of LO and ELO were recorded with FT-IR spectrophotometer (Perkin Elmer FT-IR C91158, UK) within the range from 4000 cm−1 to 400 cm−1 with a 4 cm−1 resolution. 1H NMR spectra was obtained from JEOL resonance spectrometer (JNM-ECX-400II) using deuterated chloroform (CDCl3) as a solvent. Degree of epoxidation (DOE) was calculated using equation (1) as reported by Jebrane et al. [34].
2. Experimental 2.1. Materials Aniline and Ammonium peroxydisulfate (APS) were purchased from 150
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 1. Epoxidation procedure for linseed oil.
DOE =
number of epoxide groups × 100 % number of starting double bonds
process, conversion of all unsaturated groups to epoxides is difficult to accomplish due to steric factors, electronic effects and side reactions [35].
(1)
2.5.2. Composite characterization Morphological characterization of the fractured specimen was carried out using field emission scanning electron microscopy (FE-SEM, TESCAN MIRA 3 LM). Samples were sputtered with gold-palladium alloy prior to analysis. The DC conductivity of the ELO/PANI samples was determined at room temperature by a two-probe method with Keithley 236 source measurement unit. Circular samples with 11.5 mm diameter were polished using sandpaper and maintained at a thickness between 0.7 and 0.8 mm. Both the sides of the pallet were painted with silver paste to have conductive contacts. The viscosity of uncured ELO resin and ELO/PANI blends was examined through parallel plate rheometer (Anton Paar, MCR-102) in the steady state mode. The samples were tested at 25 °C and shear rate varied between 0.05 and 500 s−1. Dynamic curing profile of specimen was examined through DSC via NETZCH Instruments, (DSC 200 F3). Freshly prepared uncured sample weighing 5–7 mg was heated at the ramping rate of 10 °C/min in N2 environment from ambient temperature to 220 °C. DSC curing parameters (Tonset, Tp and ΔH) were analyzed with Proteus software supplied by NETZCH. Dynamic mechanical study of the composite samples having geometry 40 × 12 × 3 mm was carried out using DMA by NETZCH Instruments, (DMA 242) in 3-point bending mode. Samples were scanned from −60 to 180 °C at the heating rate of 5 °C/min with frequency of 1 Hz. Thermal degradation behavior was analyzed by the thermal analyzer (EXSTAR SII 6300). About 10 mg of sample was heated from 30 to 700 °C at the rate of 10 °C/min under N2 atmosphere. The thermal parameters T5, T10 and T50 refer to the temperatures at 5 wt%, 10 wt% and 50 wt% weight losses, respectively. The tensile testing of specimens of ELO and ELO/PANI composites was carried out using Universal Testing Machine (Instron 3382) as per ASTM D 3039, with a span length of 50 mm and cross head speed of 2 mm/min. The dimension of the specimen was 110 × 12 × 3 mm.
3.1.2. FT-IR analysis FT-IR spectra of LO and ELO are depicted in Fig. 2. In LO, the band at 3012 cm−1 arises due to C-H stretching vibrations of = C-H and a weak band at 1652 cm−1 corresponds to stretching of CH=CH. The bands at 2929 and 2856 cm−1 are attributed to the asymmetric and symmetric stretching of methylene groups, while their asymmetric and symmetric bending vibrations appear at 1464 and 1378 cm−1, respectively. Signals at 1747 and 1165 cm−1 are ascribed to the stretching of C=O and C–O bonds of fatty acids, respectively. In the spectrum of ELO, the absence of the bands at 3012 and 1652 cm−1 signifies the removal of unsaturation during the epoxidation reaction. The most significant change after the epoxidation of linseed oil is the presence of oxirane group at 822 cm−1 in ELO [32,34,35,46]. 3.1.3. NMR analysis The 1H NMR spectra of LO and ELO are depicted in Fig. 3a and Fig. 3b, respectively. In LO, the multiplet at 4.1–4.35 ppm (2) is attributed to methylene hydrogens from the triglyceride moiety. Resonance between 5.21 and 5.45 ppm (1,3) indicates the presence of vinyl hydrogens and methine hydrogen from the glyceride group in LO. After epoxidation, the intensity of vinyl hydrogen at 5.27–5.45 ppm almost disappeared as shown in Fig. 3b. It indicates the loss of
3. Results and discussion 3.1. ELO characterization 3.1.1. Functional group analysis The EEW of ELO was established through HBr-COOH titration method and found to be 195. Iodine value of LO was determined to be 160 which was reduced to 27 after epoxidation. In the epoxidation
Fig. 2. FT-IR spectra of LO and ELO.
151
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 3. NMR spectra of (a) LO and (b) ELO.
unsaturation and change in hybridization of carbon from sp2 to sp3 type because of the formation of oxirane ring. In addition to this, the allyl protons in LO at 2.1 ppm (4) are shifted to 1.45 ppm (4) after epoxidation in ELO. The most representative signal of epoxide groups (–CH– protons) at 2.90–3.21 ppm (1) confirms the conversion to ELO [32,34,35]. The signal of methylene hydrogens from the triglyceride moiety (2) has been chosen as an internal standard for quantification, which does not interfere with other signals and remains unchanged after epoxidation. DOE was calculated to be 84.3% using equation 1 [34], in which the number of starting double bonds is taken as area measured under the signal at 5.27–5.45 ppm (1) in LO and number of epoxy groups taken as area measured under the signal at 2.90–3.21 ppm (1) in ELO.
in preparing electrically conducting composite without sacrificing the conductivity. 3.2.3. Rheological study It can be observed from shear stress vs. shear rate graph shown in Fig. 6a that uncured ELO resin and ELO/PANI-5 resin system exhibit linear relationship. The curve reveals that no shear thinning behavior takes place for ELO and ELO/PANI-5 resin system wherein pure Newtonian flow behavior is noticed. While, in the case of ELO/PANI-10 and ELO/PANI-15 resin system, initially slightly non-linearity is observed at low shear rate and transition takes place at high shear rate to linearity. At 10% and 15% of PANI content, the flow transition from Newtonian to non-Newtonian or pseudoplastic behavior is attributed to the textural changes in the samples [47]. This pseudoplasticity occurs at lower shear rate due to significant increase in rigidity of the system through formation of PANI agglomerates [48]. Initially, the weak dipole or van der Waal's interaction between PANI agglomerates hinders ELO chains alignment. As the shear rate increases, these weak interactions or agglomerates are broken and ELO chains align rapidly in the direction of increasing shear rate resulting in shear-thinning behavior [47]. This fact is better evident and confirmed from viscosity vs. shear rate graph as shown Fig. 6b. With increase in PANI loading, the viscosity of ELO resin gradually increased at all shear rates because of increased rigidity imparted by PANI fillers [48]. In the case of uncured ELO resin and ELO/PANI-5 resin system, the viscosity is independent of shear rate, i.e. remains constant confirming the flow of Newtonian nature. On the contrary, at 10% and 15% PANI loading, the viscosity decreased upto a shear rate of 100 s−1 and 250 s−1, respectively, but got saturated at higher shear values. This is because of higher heterogeneity incorporated in the system at increasingly higher PANI contents which resists the flow and alignment of polymer chains.
3.2. Composite characterization 3.2.1. Morphological study Fig. 4 depicts the micrograph of PANI particles and fractured surfaces of ELO and ELO/PANI composites. At lower concentration of PANI in ELO/PANI-5 (Fig. 4c), very few agglomerates are visible which signifies that PANI has a good dispersion in ELO matrix without any apparent phase separation. As the PANI content is increased beyond 5% to 10 and 15% respectively, it starts to increasingly agglomerate within the matrix, and the size and number of PANI rich regions becomes larger as evident from dotted white circles in Fig. 4d and e. However, the agglomerates are noticeably well dispersed in matrix at 10% and 15% PANI content. 3.2.2. Electrical conductivity The variation in DC conductivity of the ELO/PANI composites as a function of increasing PANI content is shown in the Fig. 5. It can be observed that conductivity increases with the increase in PANI concentration. Percolation threshold, i.e. the minimum concentration of conducting filler to form a continuous path within the matrix for electrical conduction is achieved near 7% of PANI having a conductivity of 1.27 × 10−7 S/cm which is about four orders higher than that of 5% PANI. After the percolation, the increase in conductivity is gradual and becomes almost constant after 10% of PANI. The maximum conductivity value of 6.18 × 10−6 S/cm was achieved at 15% PANI content. It is quite interesting to note that at nearly similar PANI content, the conductivity value was of the same order as obtained in our previous work (9.49 × 10−6 S/cm) where DEGBF-epoxy was taken as matrix [44]. Similar conductivity was also reported by Tsotra et al. (2.0 × 10−7 S/cm) for epoxy/PANI (10%) system as shown in Table 1. Thus, it indicates that ELO can successfully replace petro-based epoxy
3.2.4. Differential scanning calorimetry To investigate the influence of PANI on curing profile of ELO, nonisothermal curing curves of composite systems are shown in Fig. 7. In all composite systems, the single exothermic peak is observed within the range of 50–220 °C confirming cross-linking of ELO with HHPA in the presence of BF3-complex. A broader shoulder peak is noticed in all the systems revealing residual cure at higher temperature. It can be observed that, the curing onset temperature (Tonset) for ELO/PANI gradually increases with the increase in PANI concentration. However, the shift in Tonset is marginal even at 15% PANI content which reveals that PANI inhibits the curing initially to small extent which is insignificant. Similarly, on the addition of PANI, the peak temperature of curing (Tp) of ELO also shifts to slightly higher values along with a 152
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 4. SEM images of (a) PANI, (b) neat ELO, (c) ELO/PANI-5, (d) ELO/PANI-10 and (e) ELO/ PANI-15 composites.
temperature (Tg). Fig. 8a shows the E′ of cured ELO and ELO/PANI composites. It can be observed that in the glassy region (below 0 °C), the polymer chains are well restricted to move, and hence, the modulus is high for all the samples. The storage modulus of each sample remained constant in the lower temperature range. However, with an increase in temperature the polymer segmental motion takes place, and the samples undergo a transition in the range of 0–100 °C, and modulus sharply drops to lower values. Beyond the glassy region, a significant difference is observed in cured ELO and its composites with PANI reinforcement wherein E′ gradually increases with increase in the PANI loading [50]. The presence of benzene ring in PANI structure provides the mechanical fortification or stiffness to the samples. At elevated temperature (in rubbery state), ELO/PANI-15 shows greater storage modulus because of relatively higher PANI reinforcement (Table 3). While the flexible ELO starts to display viscous flow behavior, PANI hinders the segmental mobility polymer chain due to strong interfacial bonding and imparts rigidity to sample. The tan δ curves can be analyzed by the width of the curve and
decrease in enthalpy of curing (ΔH) as depicted in Table 2. This is attributed to the minor retardation effect of PANI on the cross-linking of ELO resin, and also reduction in the curable component (ELO) in the composites equivalent to added PANI content resulting in lower enthalpy of curing [21]. Additionally, incorporation of PANI leads to an increase in viscosity of resin which hinders the molecular movement, and hence, the rate of diffusion of reactants which affects the crosslinking process [49]. Interestingly, the difference in Tp and ΔH for ELO/ PANI-5 and neat ELO is minimal indicating that addition of a small amount of PANI has a negligible effect on ELO curing. This may be attributed to better dispersibility of lower PANI content (5%) within ELO matrix; a fact also supported by morphological studies analysis. 3.2.5. Dynamic mechanical properties Dynamic mechanical analysis (DMA) reveals the information about storage modulus (E′) that represents the elastic property or the energy storage in the material and loss tangent (tan δ), which measures the damping ability and is often used to determine glass transition 153
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 5. The electrical conductivity of ELO/PANI composites (conductivity for the neat ELO and ELO/PANI-3 composite was out of the range of the instrument, and therefore, was taken to be 10−14 S/cm).
Fig. 7. DSC curing profile of neat ELO and ELO/PANI composites.
Table 3. This indicates that the addition of PANI decreased the free volume content and restricted the movement of polymeric chains.
Table 1 Comparison of conductivity values obtained for ELO/PANI composites vs. literature value. S.No. 1 2 3 4 5
System Epoxy/PANI-DBSA (x = 18%) Epoxy/PANI-HCL (x = 15%) Epoxy/PANI-DBSA (x = 10%) Epoxy/PANI-PTSA (x = 15%) ELO/PANI-PTSA (x = 15%)
Conductivity (S/cm) −5
7.0 × 10 ∼10−5 2.0 × 10−7 9.49 × 10−6 6.18 × 10−6
Reference
3.2.6. Thermogravimetric analysis TGA analysis was performed to study the thermal stability and degradation behavior of cured ELO and ELO/PANI composites as shown in Fig. 9. Thermal stability parameters such as T5, T10, T50 and char yield at 600 °C are depicted in Table 4. It can be observed that ELO shows two stage degradation and addition of PANI does not change the basic degradation contour. First stage degradation starts at 175 °C that is correlated with the release of low molecular weight moieties and unreacted part of the system [52] while the second stage degradation begins at 345 °C which is due to the scission of main polymeric backbone chain. Increasing PANI content in ELO practically does not alter the T5, T10 and T50 values which indicate that ELO degradation behavior remained unaffected. However, the char residue increases with PANI concentration in the composites (Table 4).
[25] [4] [21] [44] Current study
x denotes PANI content.
intensity of the peak (Fig. 8b). In the current system, with an increase in PANI loading, the tan δ curve peak is lowered and more broadened as compared to the base matrix. Reduced intensity of loss tangent peak reveals the effectual stress transfer between the PANI and ELO matrix and good energy dissipation across the stronger interface [49]. Broadening of loss tangent curve occurs due to inhibition of the relaxation process within the composites due to the incorporation of PANI. Also the presence of different polymer chain lengths or structures yields a broader temperature range, to initiate considerable viscous chain movements [51]. With the addition of PANI, the peak position shift to higher temperature confirming higher Tg of the composites as shown in
3.2.7. Tensile testing Tensile testing was carried out to investigate the reinforcement effect of PANI on ELO composites. It can be seen from Table 5 that both the tensile strength and modulus appreciably increase with the PANI
Fig. 6. (a) Shear stress vs shear rate (b) viscosity vs shear rate for ELO (matrix) and blends.
154
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Table 2 DSC curing parameters of neat ELO and ELO/PANI composites.
Table 3 DMA parameters of cured neat ELO and ELO/PANI composites.
Samples
Tonset1(°C)
Tonset2 (°C)
Tp (°C)
ΔH (J/g)
Samples
E′ at 25 °C (MPa)
Tg (°C)
Intensity tan δ
Neat ELO ELO/PANI5 ELO/PANI10 ELO/PANI15
40.5 42.5 43.7 47.6
76.1 78.1 77.1 81.4
101.8 102.0 103.0 106.8
260.0 253.3 227.3 210.8
Neat ELO ELO/PANI-5 ELO/PANI-10 ELO/PANI-15
1595 2226 2649 2732
48 55 58 63
0.68 0.59 0.55 0.49
Tonset1 – temperature at which heat flow deviates from a linear response calculated manually. Tonset2 – onset temperature obtained using proteus software.
content in ELO. The increase in stiffness in the composites is provided by benzene rings structure of PANI while the strong interface between PANI and ELO matrix contributes to higher strength. Although, ELO showed inferior modulus and strength because of its flexible aliphatic chain structure, however, a significant increase of ∼8 folds in tensile strength and ∼27 folds in modulus was achieved with 15% PANI. Neat ELO matrix exhibits a much higher elongation at break (∼35%) revealing its outsized flexible nature whereas the addition of PANI reduces the elongation to some extent by incorporating brittleness. It can be concluded that both strength and modulus increase significantly with the addition of PANI without sacrificing much on elongation at break. This fact can be correlated with the increase in Tg and storage moduli as mentioned in dynamic mechanical study. It is noticeable that all samples are having relatively lower modulus and higher tensile strain confirming the elastomeric nature of the materials. Fig. 9. TGA curves of cured neat ELO and ELO/PANI composites.
4. Conclusions Table 4 TGA parameters of cured neat ELO and ELO/PANI composites.
ELO/PANI based electrically conductive composites were successfully prepared with percolation threshold obtained near 7% PANI content. A maximum conductivity of 6.18 × 10−6 S/cm was achieved with 15% PANI, which is in proximity to values obtained with petrobased epoxy counterpart. Dynamic curing thermograms showed that PANI impedes the curing of ELO to small extent at 15% PANI whereas, at a lower concentration of PANI (5%), it had negligible effect on ELO curing because of better dispersibility as noticed in the morphological study. Storage modulus increased with increment in PANI concentration especially in rubbery region and maximum Tg was observed at 63 °C for 15% PANI loading. TGA studies showed that addition of PANI does not alter the thermal decomposition profile of ELO. SEM images demonstrated that PANI has uniform dispersion in ELO matrix with no noticeable phase separation. Both the tensile strength and modulus
Neat ELO ELO/PANI-5 ELO/PANI -10 ELO/PANI -15
T5 (°C)
T10 (°C)
T50 (°C)
Char residue (wt %) at 600 °C
203 201 206 207
234 231 234 236
382 370 372 372
5.2 10.1 11.1 16.9
increased with PANI concentration with a modest drop in elongation at break. Since the obtained conductivity values are similar to that of petro-based epoxy/PANI systems, hence, we propose that bio-epoxy/ PANI composite can be a potential greener material for anti-static coatings/ESD applications.
Fig. 8. The dynamic mechanical behavior of cured neat ELO and ELO/PANI composites: (a) storage modulus and (b) tan δ.
155
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Table 5 Tensile properties of cured neat ELO and ELO/PANI composites.
[21]
Samples
Tensile Strength (MPa)
Young's Modulus (MPa)
Elongation at break (%)
ELO ELO/PANI5 ELO/PANO10 ELO/PANI15
2.04 ± 0.28 4.48 ± 0.16
9.15 ± 0.76 34.54 ± 4.08
34.87 ± 5.65 28.68 ± 5.95
6.47 ± 0.47
60.31 ± 1.71
25.31 ± 2.66
15.91 ± 0.95
249.69 ± 40.26
21.59 ± 4.68
[22]
[23]
[24]
Acknowledgements
[25]
Authors are thankful to the Ministry of Human Resource Development (MHRD), Government of India for financial support.
[26]
References
[27]
[1] Park O-K, Kim NH, Yoo G-H, Rhee KY, Lee JH. Effects of the surface treatment on the properties of polyaniline coated carbon nanotubes/epoxy composites. Compos Part B Eng 2010;41:2–7. http://dx.doi.org/10.1016/j.compositesb.2009.10.002. [2] Jakubinek MB, Ashrafi B, Zhang Y, Martinez-Rubi Y, Kingston CT, Johnston A, et al. Single-walled carbon nanotube–epoxy composites for structural and conductive aerospace adhesives. Compos Part B Eng 2015;69:87–93. http://dx.doi.org/10. 1016/j.compositesb.2014.09.022. [3] Kotsilkova R, Ivanov E, Bychanok D, Paddubskaya A, Demidenko M, Macutkevic J, et al. Effects of sonochemical modification of carbon nanotubes on electrical and electromagnetic shielding properties of epoxy composites. Compos Sci Technol 2015;106:85–92. http://dx.doi.org/10.1016/j.compscitech.2014.11.004. [4] Jia QM, Li JB, Wang LF, Zhu JW, Zheng M. Electrically conductive epoxy resin composites containing polyaniline with different morphologies. Mater Sci Eng A 2007;448:356–60. http://dx.doi.org/10.1016/j.msea.2006.09.065. [5] Bhadra S, Khastgir D, Singha NK, Lee JH. Progress in preparation, processing and applications of polyaniline. Prog Polym Sci 2009;34:783–810. http://dx.doi.org/10. 1016/j.progpolymsci.2009.04.003. [6] Yang Q, Wei S, Cheng G. Preparation of conductive polyaniline/epoxy composite. Polym Compos 2006;27:201–4. http://dx.doi.org/10.1002/pc.20176. [7] Oyharçabal M, Olinga T, Foulc M-P, Lacomme S, Gontier E, Vigneras V. Influence of the morphology of polyaniline on the microwave absorption properties of epoxy polyaniline composites. Compos Sci Technol 2013;74:107–12. http://dx.doi.org/ 10.1016/j.compscitech.2012.10.016. [8] Hatamzadeh M, Jaymand M. Synthesis of conductive polyaniline-modified polymers via a combination of nitroxide-mediated polymerization and click chemistry. RSC Adv 2014;4:28653–63. http://dx.doi.org/10.1039/c4ra00864b. [9] Petrie E. Epoxy adhesive formulations. McGraw-Hill Education; 2005. [10] Tan SG, Chow WS. Thermal properties, fracture toughness and water absorption of epoxy-palm oil blends. Polym Plast Technol Eng 2010;49:900–7. http://dx.doi.org/ 10.1080/03602551003682042. [11] Guadagno L, Sarno M, Vietri U, Raimondo M, Cirillo C, Ciambelli P. Graphenebased structural adhesive to enhance adhesion performance. RSC Adv 2015;5:27874–86. http://dx.doi.org/10.1039/C5RA00819K. [12] Vietri U, Guadagno L, Raimondo M, Vertuccio L, Lafdi K. Nanofilled epoxy adhesive for structural aeronautic materials. Compos Part B Eng 2014;61:73–83. http://dx. doi.org/10.1016/j.compositesb.2014.01.032. [13] Guadagno L, Vietri U, Raimondo M, Vertuccio L, Barra G, De Vivo B, et al. Correlation between electrical conductivity and manufacturing processes of nanofilled carbon fiber reinforced composites. Compos Part B Eng 2015;80:7–14. http:// dx.doi.org/10.1016/j.compositesb.2015.05.025. [14] Guadagno L, Raimondo M, Vertuccio L, Mauro M, Guerra G, Lafdi K, et al. Optimization of graphene-based materials outperforming host epoxy matrices. RSC Adv 2015;5:36969–78. http://dx.doi.org/10.1039/C5RA04558D. [15] Guadagno L, Naddeo C, Raimondo M, Barra G, Vertuccio L, Sorrentino A, et al. Development of self-healing multifunctional materials. Compos Part B Eng 2017;128:30–8. http://dx.doi.org/10.1016/j.compositesb.2017.07.003. [16] Guadagno L, Naddeo C, Raimondo M, Barra G, Vertuccio L, Russo S, et al. Influence of carbon nanoparticles/epoxy matrix interaction on mechanical, electrical and transport properties of structural advanced materials. Nanotechnology 2017;28:094001. http://dx.doi.org/10.1088/1361-6528/aa583d. [17] Liu S, Chevali VS, Xu Z, Hui D, Wang H. A review of extending performance of epoxy resins using carbon nanomaterials. Compos Part B Eng 2017. http://dx.doi. org/10.1016/j.compositesb.2017.08.020. [18] Ren X, Peng S, Zhang W, Yi C, Fang Y, Hui D. Preparation of adipic acid-polyoxypropylene diamine copolymer and its application for toughening epoxy resins. Compos Part B Eng 2017;119:32–40. http://dx.doi.org/10.1016/j.compositesb. 2017.03.019. [19] Peltola J, Cao Y, Smith P. Epoxy adhesives made with inherently conducting polymers. Adhes Age 1995;38:18–20. [20] Lu J, Moon K-S, Kim B-K, Wong CP. High dielectric constant polyaniline/epoxy
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39]
[40]
[41]
[42]
[43]
[44]
156
composites via in situ polymerization for embedded capacitor applications. Polym Guildf 2007;48:1510–6. http://dx.doi.org/10.1016/j.polymer.2007.01.057. Tsotra P, Friedrich K. Thermal, mechanical, and electrical properties of epoxy resin/ polyaniline-dodecylbenzenesulfonic acid blends. Synth Met 2004;143:237–42. http://dx.doi.org/10.1016/j.synthmet.2003.12.016. Jia W, Tchoudakov R, Segal E, Narkis M, Siegmann A. Electrically conductive composites based on epoxy resin containing polyaniline-DBSA- and polyanilineDBSA-coated glass fibers. J Appl Polym Sci 2004;91:1329–34. http://dx.doi.org/10. 1002/app.13301. Belaabed B, Wojkiewicz JL, Lamouri S, El Kamchi N, Redon N. Thermomechanical behaviors and dielectric properties of polyaniline-doped para-toluene sulfonic acid/ epoxy resin composites. Polym Adv Technol 2012;23:1194–201. http://dx.doi.org/ 10.1002/pat.2029. Jeevananda T, Palaniappan S. Siddaramaiah. Spectral and thermal studies on polyaniline-epoxy novolac resin composite materials. J Appl Polym Sci 1999;74:3507–12. http://dx.doi.org/10.1002/(SICI)1097-4628(19991227)74. 14 < 3507::AID-APP26 > 3.0.CO;2-H. Soares BG, Celestino ML, Magioli M, Moreira VX, Khastgir D. Synthesis of conductive adhesives based on epoxy resin and polyaniline.DBSA using the in situ polymerization and physical mixing procedures. Synth Met 2010;160:1981–6. http://dx.doi.org/10.1016/j.synthmet.2010.07.021. Mir IA, Kumar D. Development of polyaniline/epoxy composite as a prospective solder replacement material. Int J Polym Mater Polym Biomater 2010;59:994–1007. http://dx.doi.org/10.1080/00914037.2010.504150. Oyharçabal M, Olinga T, Foulc MP, Vigneras V. Polyaniline/clay as nanostructured conductive filler for electrically conductive epoxy composites. Influence of filler morphology, chemical nature of reagents, and curing conditions on composite conductivity. Synth Met 2012;162:555–62. http://dx.doi.org/10.1016/j.synthmet. 2012.02.011. Llevot A. Sustainable synthetic approaches for the preparation of plant oil-based thermosets. J Am Oil Chem Soc 2017;94:169–86. http://dx.doi.org/10.1007/ s11746-016-2932-4. Baroncini EA, Kumar Yadav S, Palmese GR, Stanzione JF. Recent advances in biobased epoxy resins and bio-based epoxy curing agents. J Appl Polym Sci 2016;133:44103. http://dx.doi.org/10.1002/app.44103. Bobade SK, Paluvai NR, Mohanty S, Nayak SK. Bio-based thermosetting resins for future generation: a review. Polym Plast Technol Eng 2016;55:1863–96. http://dx. doi.org/10.1080/03602559.2016.1185624. Meier MAR, Metzger JO, Schubert US. Plant oil renewable resources as green alternatives in polymer science. Chem Soc Rev 2007;36:1788–802. http://dx.doi.org/ 10.1039/b703294c. Mahendran AR, Aust N, Wuzella G, Kandelbauer A. Synthesis and characterization of a bio-based resin from linseed oil. Macromol Symp 2012;311:18–27. http://dx. doi.org/10.1002/masy.201000134. Campanella A, Baltanas MA, Capel-Sanchez MC, Campos-Martin JM, Fierro JLG. Soybean oil epoxidation with hydrogen peroxide using an amorphous Ti/SiO2 catalyst. Green Chem 2004;6:330–4. http://dx.doi.org/10.1039/b404975f. Jebrane M, Cai S, Sandström C, Terziev N. The reactivity of linseed and soybean oil with different epoxidation degree towards vinyl acetate and impact of the resulting copolymer on the wood durability. Express Polym Lett 2017;11:383–95. http://dx. doi.org/10.3144/expresspolymlett.2017.37. Kim JR, Sharma S. The development and comparison of bio-thermoset plastics from epoxidized plant oils. Ind Crops Prod 2012;36:485–99. http://dx.doi.org/10.1016/ j.indcrop.2011.10.036. Zhang C, Xia Y, Chen R, Huh S, Johnston PA, Kessler MR. Soy-castor oil based polyols prepared using a solvent-free and catalyst-free method and polyurethanes therefrom. Green Chem 2013;15:1477–84. http://dx.doi.org/10.1039/c3gc40531a. Sahoo SK, Mohanty S, Nayak SK. Toughened bio-based epoxy blend network modified with transesterified epoxidized soybean oil: synthesis and characterization. RSC Adv 2015;5:13674–91. http://dx.doi.org/10.1039/C4RA11965G. Sharma V, Banait JS, Kundu PP. Swelling kinetics of linseed oil-based polymers. J Appl Polym Sci 2009;111:1816–27. http://dx.doi.org/10.1002/app.29006. Yim Y-J, Rhee KY, Park SJ. Fracture toughness and ductile characteristics of diglycidyl ether of bisphenol-A resins modified with biodegradable epoxidized linseed oil. Compos Part B Eng 2017;2017(131):144–52. http://dx.doi.org/10.1016/J. COMPOSITESB. 07.047. Es-Haghi H, Mirabedini SM, Imani M, Farnood RR. Mechanical and self-healing properties of a water-based acrylic latex containing linseed oil filled microcapsules: effect of pre-silanization of microcapsules' shell compound. Compos Part B Eng 2016;85:305–14. http://dx.doi.org/10.1016/j.compositesb.2015.09.015. Samper MD, Petrucci R, Sánchez-Nacher L, Balart R, Kenny JM. New environmentally friendly composite laminates with epoxidized linseed oil (ELO) and slate fiber fabrics. Compos Part B Eng 2015;71:203–9. http://dx.doi.org/10.1016/j. compositesb.2014.11.034. Deng Y, Paraskevas D, Tian Y, Van Acker K, Dewulf W, Duflou JR. Life cycle assessment of flax-fibre reinforced epoxidized linseed oil composite with a flame retardant for electronic applications. J Clean Prod 2016;133:427–38. http://dx.doi. org/10.1016/j.jclepro.2016.05.172. Lligadas G, Ronda JC, Galià M, Cádiz V. Bionanocomposites from renewable Resources: epoxidized linseed Oil−Polyhedral oligomeric silsesquioxanes hybrid materials. Biomacromolecules 2006;7:3521–6. http://dx.doi.org/10.1021/ BM060703U. Khandelwal V, Sahoo SK, Kumar A, Manik G. Study on the effect of carbon nanotube on the properties of electrically conductive epoxy/polyaniline adhesives. J Mater Sci Mater Electron 2017;28:14240–51. http://dx.doi.org/10.1007/s10854-0177282-y.
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
behavior of functionalized soybean oil modified toughened epoxy/organo clay nanocomposite. Prog Org Coatings 2015;88:263–71. http://dx.doi.org/10.1016/j. porgcoat.2015.07.012. [50] Belaabed B, Lamouri S, Wojkiewicz JL. Curing kinetics, thermomechanical and microwave behaviors of PANI-doped BSA/epoxy resin composites. Polym J 2011;43:683–91. http://dx.doi.org/10.1038/pj.2011.46. [51] Sahoo SK, Mohanty S, Nayak SK. Synthesis and characterization of bio-based epoxy blends from renewable resource based epoxidized soybean oil as reactive diluent. Chin J Polym Sci 2015;33:137–52. http://dx.doi.org/10.1007/s10118-015-1568-4. [52] Kumar S, Samal SK, Mohanty S, Nayak SK. Epoxidized soybean oil-based epoxy blend cured with anhydride-based cross-linker: thermal and mechanical characterization. Ind Eng Chem Res 2017;56:687–98. http://dx.doi.org/10.1021/acs. iecr.6b03879.
[45] Pan X, Sengupta P, Webster DC. Novel biobased epoxy compounds: epoxidized sucrose esters of fatty acids. Green Chem 2011;13:965–75. http://dx.doi.org/10. 1039/c0gc00882f. [46] Chen J, Soucek MD, Simonsick WJ, Celikay RW. Epoxidation of partially norbornylized linseed oil. Macromol Chem Phys 2002;203:2042–57. http://dx.doi.org/10. 1002/1521-3935(200210)203. 14 < 2042::AID-MACP2042 > 3.0.CO;2–0. [47] Sirqueira ADS, Teodoro Júnior D, Coutinho MDS, Neto ASDS, Silva ADA, Soares BG. Rheological behavior of acrylic paint blends based on polyaniline. Polímeros 2016;26:215–20. http://dx.doi.org/10.1590/0104-1428.2178. [48] Kurbatov VG, Indeikin EA. Influence of the polyaniline structure on the properties of epoxy compounds and materials. Russ J Appl Chem 2015;88:138–42. http://dx. doi.org/10.1134/S1070427215010206. [49] Sahoo SK, Mohanty S, Nayak SK. Study of thermal stability and thermo-mechanical
157
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Electrically conductive green composites based on epoxidized linseed oil and polyaniline: An insight into electrical, thermal and mechanical properties
MARK
Vinay Khandelwala, Sushanta K. Sahooa, Ashok Kumarb, Gaurav Manika,∗ a b
Department of Polymer and Process Engineering, IIT Roorkee Saharanpur Campus, Saharanpur 247001, India CSIR - National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Bio-epoxy resin Polymer-matrix composites (PMCs) Thermosetting resin Electrical properties
Renewable resource based electrically conductive composites were prepared using polyaniline (PANI) as a conductive filler and epoxidized linseed oil (ELO) as the matrix. Linseed oil (LO) was epoxidized to form ELO and characterized through 1H NMR and IR spectra. Bio-based ELO/PANI conducting composites were prepared by varying the PANI concentration with an aim of attaining the electrical conductivity in the antistatic range (10−8 to 10−3 S/cm) to replace its petro-based counterpart. Conductivity increased with PANI upto the order of 10−6 S/cm with percolation threshold at around 7% of PANI. The shear stress and viscosity of the uncured ELO resin and ELO/PANI resin mixture were studied as a function of shear rate. Differential scanning calorimetry (DSC) studies showed that addition of PANI had a minimal effect on ELO curing at all concentrations. Dynamic mechanical analysis indicated that PANI as a filler provided mechanical fortification in the rubbery region and increased glass transition temperature (Tg) significantly. Thermal stability of ELO remained almost unaffected with the PANI incorporation. Microscopic observation revealed good distribution of PANI in ELO matrix even at higher loading. Interestingly, tensile strength and Young's modulus increased by ∼8 and ∼27 folds, respectively, at 15% PANI content.
1. Introduction Electrically conductive polymer composites are prospective materials for several engineering applications such as electrically conductive adhesives, electromagnetic interference shielding materials, antistatic formulations, sensors, fuel cell, rechargeable batteries and electronic devices, etc. [1–5]. These composites are made up of host matrix that provides the mechanical strength and conductive fillers particles that offer electrical conduction [6]. Intrinsically conducting polymers (ICP) are an important class of conductive fillers that are environmentally stable and cost effective alternate to the traditional expensive metallic fillers which are prone to corrosion with the passage of time [4]. Among ICPs, Polyaniline (PANI) is one of the most studied polymers both by academicians and industries by virtue of its versatile nature like the ease of synthesis, good polymerization yield, low cost, adjustable conductivity and good environmental stability [5,7]. Despite possessing such advantages, researchers have not been able to exploit its properties because of its infusibility and insolubility [8]. A better way is the compounding of PANI with inherently insulating polymer matrices to develop conducting composites for various engineering applications. ∗
Epoxy is one of the commonly used thermosetting matrices for such composites because of its outstanding thermal and mechanical properties, minimal curing shrinkage, and ability to withstand harsh chemical environments. This has increased its suitability for numerous applications like structural materials and adhesives, coatings, aeronautical materials, electronic packaging and composites, etc. [9–18]. PANI doped with organic acids like camphorsulfonic acid (CSA) [19,20], dodecylbezenesulfonic acid (DBSA) [21,22] and p-toluenesulfonic acid (PTSA) [23,24] has been used earlier as preferable filler by various researchers. Peltola et al. [19] prepared PANI-CSA based epoxy adhesives having the conductivity in the range of 10−8-10−3 S/cm, (applicable for antistatic application) at less than 2% of PANI loading. Later, Tsotra et al. [21] formulated epoxy/PANI-DBSA composites, and obtained the conductivity of 2 × 10−7 S/cm at 10% PANI content which qualifies for electrostatic discharge (ESD) application. Soares et al. [25] reported the conductivity of epoxy/PANI-DBSA (7%) and epoxy/PANI-DBSA (18%) adhesives of the order of 10−8 and 10−5 S/ cm, respectively, through physical mixing method. Mir et al. [26] developed electrically conductive adhesives with inorganic acid (HCl) doped PANI and established the conductivity of the order of 10−6 S/cm
Corresponding author. E-mail address: [email protected] (G. Manik).
http://dx.doi.org/10.1016/j.compositesb.2017.10.030 Received 25 July 2017; Received in revised form 23 October 2017; Accepted 24 October 2017 1359-8368/ © 2017 Elsevier Ltd. All rights reserved.
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Merck, India. p-toluenesulfonic acid (PTSA) was obtained from Vetec Sigma, India. LO was procured from Himedia, India. Seralite (SRC-120), an acidic ion exchange resin, and magnesium sulfate (MgSO4) were purchased from SRL Pvt. Ltd, India. Glacial Acetic acid (CH3COOH), Hydrogen peroxide (H2O2) (30 wt%), and sodium carbonate (Na2CO3) were procured from Rankem, India. HHPA for curing was obtained from Sigma-Aldrich, USA. BF3-ethylamine complex was supplied by TCI, India.
at 10% PANI content. The selection of curing agent plays a crucial role in attaining the desired conductivity level. Basic amines, commonly used as cross-linking agents, tend to deprotonate PANI because of their basic nature, and thereby, reduce its conductivity [25]. Acid anhydrides such as hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, etc. are widely used as cross-linkers for epoxy resin along with an accelerator to reduce the curing temperatures. Since BF3-complex is a Lewis acid and does not affect the conductivity of PANI, it is mostly used as curing agent as well as an accelerator in cross-linking of epoxy resin [27]. The rapidly depleting fossil resources and increasing environmental sustainability concerns have motivated the researchers to explore renewable and/or bio-based feedstock as an alternative to petro-based materials [28,29]. In this context, vegetable oils are receiving overwhelming attention worldwide due to their ability to form pre-polymers, a plethora of availability, non-hazardous nature, environmental friendliness and biodegradability [30]. Vegetable oils are chemically composed of mainly triglycerides with long aliphatic carbon chain and multiple unsaturation contents which serve as active sites for direct polymerization or after modification [31]. Various modifications that may be carried out at the unsaturation of the fatty acid chain are acrylation, maleination, halogenation, ozonolysis, dimerization, metathesis, epoxidation and hydroxylation, etc. [32]. Among several modifications possible for plant oils, epoxidation is the most common, straight forward and inexpensive method. In situ method of epoxidation is widely used by researchers as it is solvent free and epoxy content of the epoxidized oil can be appropriately controlled by varying the reaction time [33–35]. Epoxidized oils are considered as renewable raw materials for the preparation of bio-based thermoset products for numerous engineering applications because of their relatively lower viscosity and better processability [35]. They are widely used as reactive diluents for epoxy thermosets for their strong viscosity reducing ability, and also as preferable pre-polymers for the synthesis of polyols used in the manufacturing of polyurethanes with superior thermal and mechanical performance [36,37]. Among all oils, Linseed oil (LO) is the most suitable oil due to its high unsaturation content (53% linolenic acid and 6.6 number of double bonds/molecule) that can be easily epoxidized and can yield higher epoxy values closer to that of commercial epoxy. Being a drying oil, it is frequently used as paint binder and wood finish. Additionally, it has also found applications in enamels, linoleum, oilcloth, printer's ink, and as waterproofing for raincoats [31,38]. Epoxidized linseed oil (ELO) is commonly used as plasticizers for PVC and epoxy resins to provide flexibility, and also for coatings and adhesive applications [39,40]. Many researchers have developed ELO based composites with interesting properties using various reinforcements [41,42]. Another instance of its use previously as a greener substitute involve development of polyhedral oligomeric silsesquioxane based bionanocomposite to reduce dependence on petro-based epoxy resin [43]. In the present study, we report the preparation of bio-based conducting composite taking ELO as bio-renewable matrix and PANI as conducting filler. LO was epoxidized via in situ method, and subsequently, the effect of PANI on the electrical, thermal and thermomechanical properties of ELO network cured with hexahydrophthalic anhydride (HHPA) and BF3-complex was investigated. To the best of our knowledge, no such system of bio-based epoxy resin/PANI has been studied previously. The work was undertaken with the aim of developing ELO/PANI composites with the target values of conductivity in the range (10−8 to 10−6 S/cm) applicable for antistatic coating/ESD application.
2.2. Synthesis of PTSA doped PANI PANI doped with PTSA was prepared via the conventional one phase polymerization technique in aqueous phase as reported in our earlier publication [44]. In a typical procedure, aniline (5 gm) and PTSA (10.22 gm) were mixed in distilled water (250 ml) and stirred for 1 h to make homogenous solution keeping the temperature between 0 and 5 °C. Subsequently, APS (12.25 gm) was added slowly to this mixture maintaining the molar ratio of PANI: PTSA: APS at 1:1:1. Stirring was continued for 5 h and followed by the addition of acetone (25 ml) to seize the polymerization. Solution was filtered, washed with distilled water and acetone and dried in oven at 65 °C for 24 h. 2.3. Epoxidation of LO Epoxidation of LO was performed in the presence of glacial CH3COOH and H2O2 in a three-necked round bottom flask equipped with magnetic stirrer, a dropping funnel and a thermometer as reported by Kim et al. [35]. Linseed oil (79 gm) and CH3COOH (30 gm) were added in the molar ratio of 1:1 (1 mol unsaturation in oil) to the round bottom flask along with 25% seralite SRC-120 (19.75 gm) (acidic ion exchange resin) and stirred for 30 min. To this solution, H2O2 (113 gm) was added dropwise in the ratio of 2:1 of H2O2 and oil unsaturation and the mixture was stirred for 5 h with the temperature maintained at 60 °C throughout the reaction (Fig. 1). After the completion of reaction, mixture was filtered with cheese cloth to remove seralite catalyst. Subsequently, ELO layer was separated and washed with 2% Na2CO3 solution and filtered using MgSO4 to remove moisture and dried overnight in vacuum oven at 60 °C. 2.4. Preparation of ELO/PANI composites The calculated amount of PANI and ELO were dispersed in acetone using ultrasonic bath (Labmann, LMUC-4) for 1 h. Mixture was stirred at 55–60 °C for 5 h to improve the dispersion quality and to remove the acetone completely from mixture. Curing agent along with the accelerator was added in the ratio of 44:5 per 100 parts of ELO as reported earlier [27]. The mixture obtained was poured in steel molds applied with silicon spray. The samples were then cured at 120 °C in a thermostatic oven for 5 h. Composites with 0, 3, 5, 7, 10, 15% PANI concentration are abbreviated as ELO, ELO/PANI-3, ELO/PANI-5, ELO/PANI-7, ELO/PANI10, ELO/PANI-15, respectively. 2.5. Characterization 2.5.1. LO/ELO characterization Epoxy equivalent weight (EEW) was determined by titration with 0.1 N HBr in glacial acetic acid as per the ASTM D 1652. Iodine value was obtained according to ASTM D 5768-02 using Wijs solution [45]. FT-IR spectra of LO and ELO were recorded with FT-IR spectrophotometer (Perkin Elmer FT-IR C91158, UK) within the range from 4000 cm−1 to 400 cm−1 with a 4 cm−1 resolution. 1H NMR spectra was obtained from JEOL resonance spectrometer (JNM-ECX-400II) using deuterated chloroform (CDCl3) as a solvent. Degree of epoxidation (DOE) was calculated using equation (1) as reported by Jebrane et al. [34].
2. Experimental 2.1. Materials Aniline and Ammonium peroxydisulfate (APS) were purchased from 150
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 1. Epoxidation procedure for linseed oil.
DOE =
number of epoxide groups × 100 % number of starting double bonds
process, conversion of all unsaturated groups to epoxides is difficult to accomplish due to steric factors, electronic effects and side reactions [35].
(1)
2.5.2. Composite characterization Morphological characterization of the fractured specimen was carried out using field emission scanning electron microscopy (FE-SEM, TESCAN MIRA 3 LM). Samples were sputtered with gold-palladium alloy prior to analysis. The DC conductivity of the ELO/PANI samples was determined at room temperature by a two-probe method with Keithley 236 source measurement unit. Circular samples with 11.5 mm diameter were polished using sandpaper and maintained at a thickness between 0.7 and 0.8 mm. Both the sides of the pallet were painted with silver paste to have conductive contacts. The viscosity of uncured ELO resin and ELO/PANI blends was examined through parallel plate rheometer (Anton Paar, MCR-102) in the steady state mode. The samples were tested at 25 °C and shear rate varied between 0.05 and 500 s−1. Dynamic curing profile of specimen was examined through DSC via NETZCH Instruments, (DSC 200 F3). Freshly prepared uncured sample weighing 5–7 mg was heated at the ramping rate of 10 °C/min in N2 environment from ambient temperature to 220 °C. DSC curing parameters (Tonset, Tp and ΔH) were analyzed with Proteus software supplied by NETZCH. Dynamic mechanical study of the composite samples having geometry 40 × 12 × 3 mm was carried out using DMA by NETZCH Instruments, (DMA 242) in 3-point bending mode. Samples were scanned from −60 to 180 °C at the heating rate of 5 °C/min with frequency of 1 Hz. Thermal degradation behavior was analyzed by the thermal analyzer (EXSTAR SII 6300). About 10 mg of sample was heated from 30 to 700 °C at the rate of 10 °C/min under N2 atmosphere. The thermal parameters T5, T10 and T50 refer to the temperatures at 5 wt%, 10 wt% and 50 wt% weight losses, respectively. The tensile testing of specimens of ELO and ELO/PANI composites was carried out using Universal Testing Machine (Instron 3382) as per ASTM D 3039, with a span length of 50 mm and cross head speed of 2 mm/min. The dimension of the specimen was 110 × 12 × 3 mm.
3.1.2. FT-IR analysis FT-IR spectra of LO and ELO are depicted in Fig. 2. In LO, the band at 3012 cm−1 arises due to C-H stretching vibrations of = C-H and a weak band at 1652 cm−1 corresponds to stretching of CH=CH. The bands at 2929 and 2856 cm−1 are attributed to the asymmetric and symmetric stretching of methylene groups, while their asymmetric and symmetric bending vibrations appear at 1464 and 1378 cm−1, respectively. Signals at 1747 and 1165 cm−1 are ascribed to the stretching of C=O and C–O bonds of fatty acids, respectively. In the spectrum of ELO, the absence of the bands at 3012 and 1652 cm−1 signifies the removal of unsaturation during the epoxidation reaction. The most significant change after the epoxidation of linseed oil is the presence of oxirane group at 822 cm−1 in ELO [32,34,35,46]. 3.1.3. NMR analysis The 1H NMR spectra of LO and ELO are depicted in Fig. 3a and Fig. 3b, respectively. In LO, the multiplet at 4.1–4.35 ppm (2) is attributed to methylene hydrogens from the triglyceride moiety. Resonance between 5.21 and 5.45 ppm (1,3) indicates the presence of vinyl hydrogens and methine hydrogen from the glyceride group in LO. After epoxidation, the intensity of vinyl hydrogen at 5.27–5.45 ppm almost disappeared as shown in Fig. 3b. It indicates the loss of
3. Results and discussion 3.1. ELO characterization 3.1.1. Functional group analysis The EEW of ELO was established through HBr-COOH titration method and found to be 195. Iodine value of LO was determined to be 160 which was reduced to 27 after epoxidation. In the epoxidation
Fig. 2. FT-IR spectra of LO and ELO.
151
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 3. NMR spectra of (a) LO and (b) ELO.
unsaturation and change in hybridization of carbon from sp2 to sp3 type because of the formation of oxirane ring. In addition to this, the allyl protons in LO at 2.1 ppm (4) are shifted to 1.45 ppm (4) after epoxidation in ELO. The most representative signal of epoxide groups (–CH– protons) at 2.90–3.21 ppm (1) confirms the conversion to ELO [32,34,35]. The signal of methylene hydrogens from the triglyceride moiety (2) has been chosen as an internal standard for quantification, which does not interfere with other signals and remains unchanged after epoxidation. DOE was calculated to be 84.3% using equation 1 [34], in which the number of starting double bonds is taken as area measured under the signal at 5.27–5.45 ppm (1) in LO and number of epoxy groups taken as area measured under the signal at 2.90–3.21 ppm (1) in ELO.
in preparing electrically conducting composite without sacrificing the conductivity. 3.2.3. Rheological study It can be observed from shear stress vs. shear rate graph shown in Fig. 6a that uncured ELO resin and ELO/PANI-5 resin system exhibit linear relationship. The curve reveals that no shear thinning behavior takes place for ELO and ELO/PANI-5 resin system wherein pure Newtonian flow behavior is noticed. While, in the case of ELO/PANI-10 and ELO/PANI-15 resin system, initially slightly non-linearity is observed at low shear rate and transition takes place at high shear rate to linearity. At 10% and 15% of PANI content, the flow transition from Newtonian to non-Newtonian or pseudoplastic behavior is attributed to the textural changes in the samples [47]. This pseudoplasticity occurs at lower shear rate due to significant increase in rigidity of the system through formation of PANI agglomerates [48]. Initially, the weak dipole or van der Waal's interaction between PANI agglomerates hinders ELO chains alignment. As the shear rate increases, these weak interactions or agglomerates are broken and ELO chains align rapidly in the direction of increasing shear rate resulting in shear-thinning behavior [47]. This fact is better evident and confirmed from viscosity vs. shear rate graph as shown Fig. 6b. With increase in PANI loading, the viscosity of ELO resin gradually increased at all shear rates because of increased rigidity imparted by PANI fillers [48]. In the case of uncured ELO resin and ELO/PANI-5 resin system, the viscosity is independent of shear rate, i.e. remains constant confirming the flow of Newtonian nature. On the contrary, at 10% and 15% PANI loading, the viscosity decreased upto a shear rate of 100 s−1 and 250 s−1, respectively, but got saturated at higher shear values. This is because of higher heterogeneity incorporated in the system at increasingly higher PANI contents which resists the flow and alignment of polymer chains.
3.2. Composite characterization 3.2.1. Morphological study Fig. 4 depicts the micrograph of PANI particles and fractured surfaces of ELO and ELO/PANI composites. At lower concentration of PANI in ELO/PANI-5 (Fig. 4c), very few agglomerates are visible which signifies that PANI has a good dispersion in ELO matrix without any apparent phase separation. As the PANI content is increased beyond 5% to 10 and 15% respectively, it starts to increasingly agglomerate within the matrix, and the size and number of PANI rich regions becomes larger as evident from dotted white circles in Fig. 4d and e. However, the agglomerates are noticeably well dispersed in matrix at 10% and 15% PANI content. 3.2.2. Electrical conductivity The variation in DC conductivity of the ELO/PANI composites as a function of increasing PANI content is shown in the Fig. 5. It can be observed that conductivity increases with the increase in PANI concentration. Percolation threshold, i.e. the minimum concentration of conducting filler to form a continuous path within the matrix for electrical conduction is achieved near 7% of PANI having a conductivity of 1.27 × 10−7 S/cm which is about four orders higher than that of 5% PANI. After the percolation, the increase in conductivity is gradual and becomes almost constant after 10% of PANI. The maximum conductivity value of 6.18 × 10−6 S/cm was achieved at 15% PANI content. It is quite interesting to note that at nearly similar PANI content, the conductivity value was of the same order as obtained in our previous work (9.49 × 10−6 S/cm) where DEGBF-epoxy was taken as matrix [44]. Similar conductivity was also reported by Tsotra et al. (2.0 × 10−7 S/cm) for epoxy/PANI (10%) system as shown in Table 1. Thus, it indicates that ELO can successfully replace petro-based epoxy
3.2.4. Differential scanning calorimetry To investigate the influence of PANI on curing profile of ELO, nonisothermal curing curves of composite systems are shown in Fig. 7. In all composite systems, the single exothermic peak is observed within the range of 50–220 °C confirming cross-linking of ELO with HHPA in the presence of BF3-complex. A broader shoulder peak is noticed in all the systems revealing residual cure at higher temperature. It can be observed that, the curing onset temperature (Tonset) for ELO/PANI gradually increases with the increase in PANI concentration. However, the shift in Tonset is marginal even at 15% PANI content which reveals that PANI inhibits the curing initially to small extent which is insignificant. Similarly, on the addition of PANI, the peak temperature of curing (Tp) of ELO also shifts to slightly higher values along with a 152
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 4. SEM images of (a) PANI, (b) neat ELO, (c) ELO/PANI-5, (d) ELO/PANI-10 and (e) ELO/ PANI-15 composites.
temperature (Tg). Fig. 8a shows the E′ of cured ELO and ELO/PANI composites. It can be observed that in the glassy region (below 0 °C), the polymer chains are well restricted to move, and hence, the modulus is high for all the samples. The storage modulus of each sample remained constant in the lower temperature range. However, with an increase in temperature the polymer segmental motion takes place, and the samples undergo a transition in the range of 0–100 °C, and modulus sharply drops to lower values. Beyond the glassy region, a significant difference is observed in cured ELO and its composites with PANI reinforcement wherein E′ gradually increases with increase in the PANI loading [50]. The presence of benzene ring in PANI structure provides the mechanical fortification or stiffness to the samples. At elevated temperature (in rubbery state), ELO/PANI-15 shows greater storage modulus because of relatively higher PANI reinforcement (Table 3). While the flexible ELO starts to display viscous flow behavior, PANI hinders the segmental mobility polymer chain due to strong interfacial bonding and imparts rigidity to sample. The tan δ curves can be analyzed by the width of the curve and
decrease in enthalpy of curing (ΔH) as depicted in Table 2. This is attributed to the minor retardation effect of PANI on the cross-linking of ELO resin, and also reduction in the curable component (ELO) in the composites equivalent to added PANI content resulting in lower enthalpy of curing [21]. Additionally, incorporation of PANI leads to an increase in viscosity of resin which hinders the molecular movement, and hence, the rate of diffusion of reactants which affects the crosslinking process [49]. Interestingly, the difference in Tp and ΔH for ELO/ PANI-5 and neat ELO is minimal indicating that addition of a small amount of PANI has a negligible effect on ELO curing. This may be attributed to better dispersibility of lower PANI content (5%) within ELO matrix; a fact also supported by morphological studies analysis. 3.2.5. Dynamic mechanical properties Dynamic mechanical analysis (DMA) reveals the information about storage modulus (E′) that represents the elastic property or the energy storage in the material and loss tangent (tan δ), which measures the damping ability and is often used to determine glass transition 153
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Fig. 5. The electrical conductivity of ELO/PANI composites (conductivity for the neat ELO and ELO/PANI-3 composite was out of the range of the instrument, and therefore, was taken to be 10−14 S/cm).
Fig. 7. DSC curing profile of neat ELO and ELO/PANI composites.
Table 3. This indicates that the addition of PANI decreased the free volume content and restricted the movement of polymeric chains.
Table 1 Comparison of conductivity values obtained for ELO/PANI composites vs. literature value. S.No. 1 2 3 4 5
System Epoxy/PANI-DBSA (x = 18%) Epoxy/PANI-HCL (x = 15%) Epoxy/PANI-DBSA (x = 10%) Epoxy/PANI-PTSA (x = 15%) ELO/PANI-PTSA (x = 15%)
Conductivity (S/cm) −5
7.0 × 10 ∼10−5 2.0 × 10−7 9.49 × 10−6 6.18 × 10−6
Reference
3.2.6. Thermogravimetric analysis TGA analysis was performed to study the thermal stability and degradation behavior of cured ELO and ELO/PANI composites as shown in Fig. 9. Thermal stability parameters such as T5, T10, T50 and char yield at 600 °C are depicted in Table 4. It can be observed that ELO shows two stage degradation and addition of PANI does not change the basic degradation contour. First stage degradation starts at 175 °C that is correlated with the release of low molecular weight moieties and unreacted part of the system [52] while the second stage degradation begins at 345 °C which is due to the scission of main polymeric backbone chain. Increasing PANI content in ELO practically does not alter the T5, T10 and T50 values which indicate that ELO degradation behavior remained unaffected. However, the char residue increases with PANI concentration in the composites (Table 4).
[25] [4] [21] [44] Current study
x denotes PANI content.
intensity of the peak (Fig. 8b). In the current system, with an increase in PANI loading, the tan δ curve peak is lowered and more broadened as compared to the base matrix. Reduced intensity of loss tangent peak reveals the effectual stress transfer between the PANI and ELO matrix and good energy dissipation across the stronger interface [49]. Broadening of loss tangent curve occurs due to inhibition of the relaxation process within the composites due to the incorporation of PANI. Also the presence of different polymer chain lengths or structures yields a broader temperature range, to initiate considerable viscous chain movements [51]. With the addition of PANI, the peak position shift to higher temperature confirming higher Tg of the composites as shown in
3.2.7. Tensile testing Tensile testing was carried out to investigate the reinforcement effect of PANI on ELO composites. It can be seen from Table 5 that both the tensile strength and modulus appreciably increase with the PANI
Fig. 6. (a) Shear stress vs shear rate (b) viscosity vs shear rate for ELO (matrix) and blends.
154
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Table 2 DSC curing parameters of neat ELO and ELO/PANI composites.
Table 3 DMA parameters of cured neat ELO and ELO/PANI composites.
Samples
Tonset1(°C)
Tonset2 (°C)
Tp (°C)
ΔH (J/g)
Samples
E′ at 25 °C (MPa)
Tg (°C)
Intensity tan δ
Neat ELO ELO/PANI5 ELO/PANI10 ELO/PANI15
40.5 42.5 43.7 47.6
76.1 78.1 77.1 81.4
101.8 102.0 103.0 106.8
260.0 253.3 227.3 210.8
Neat ELO ELO/PANI-5 ELO/PANI-10 ELO/PANI-15
1595 2226 2649 2732
48 55 58 63
0.68 0.59 0.55 0.49
Tonset1 – temperature at which heat flow deviates from a linear response calculated manually. Tonset2 – onset temperature obtained using proteus software.
content in ELO. The increase in stiffness in the composites is provided by benzene rings structure of PANI while the strong interface between PANI and ELO matrix contributes to higher strength. Although, ELO showed inferior modulus and strength because of its flexible aliphatic chain structure, however, a significant increase of ∼8 folds in tensile strength and ∼27 folds in modulus was achieved with 15% PANI. Neat ELO matrix exhibits a much higher elongation at break (∼35%) revealing its outsized flexible nature whereas the addition of PANI reduces the elongation to some extent by incorporating brittleness. It can be concluded that both strength and modulus increase significantly with the addition of PANI without sacrificing much on elongation at break. This fact can be correlated with the increase in Tg and storage moduli as mentioned in dynamic mechanical study. It is noticeable that all samples are having relatively lower modulus and higher tensile strain confirming the elastomeric nature of the materials. Fig. 9. TGA curves of cured neat ELO and ELO/PANI composites.
4. Conclusions Table 4 TGA parameters of cured neat ELO and ELO/PANI composites.
ELO/PANI based electrically conductive composites were successfully prepared with percolation threshold obtained near 7% PANI content. A maximum conductivity of 6.18 × 10−6 S/cm was achieved with 15% PANI, which is in proximity to values obtained with petrobased epoxy counterpart. Dynamic curing thermograms showed that PANI impedes the curing of ELO to small extent at 15% PANI whereas, at a lower concentration of PANI (5%), it had negligible effect on ELO curing because of better dispersibility as noticed in the morphological study. Storage modulus increased with increment in PANI concentration especially in rubbery region and maximum Tg was observed at 63 °C for 15% PANI loading. TGA studies showed that addition of PANI does not alter the thermal decomposition profile of ELO. SEM images demonstrated that PANI has uniform dispersion in ELO matrix with no noticeable phase separation. Both the tensile strength and modulus
Neat ELO ELO/PANI-5 ELO/PANI -10 ELO/PANI -15
T5 (°C)
T10 (°C)
T50 (°C)
Char residue (wt %) at 600 °C
203 201 206 207
234 231 234 236
382 370 372 372
5.2 10.1 11.1 16.9
increased with PANI concentration with a modest drop in elongation at break. Since the obtained conductivity values are similar to that of petro-based epoxy/PANI systems, hence, we propose that bio-epoxy/ PANI composite can be a potential greener material for anti-static coatings/ESD applications.
Fig. 8. The dynamic mechanical behavior of cured neat ELO and ELO/PANI composites: (a) storage modulus and (b) tan δ.
155
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
Table 5 Tensile properties of cured neat ELO and ELO/PANI composites.
[21]
Samples
Tensile Strength (MPa)
Young's Modulus (MPa)
Elongation at break (%)
ELO ELO/PANI5 ELO/PANO10 ELO/PANI15
2.04 ± 0.28 4.48 ± 0.16
9.15 ± 0.76 34.54 ± 4.08
34.87 ± 5.65 28.68 ± 5.95
6.47 ± 0.47
60.31 ± 1.71
25.31 ± 2.66
15.91 ± 0.95
249.69 ± 40.26
21.59 ± 4.68
[22]
[23]
[24]
Acknowledgements
[25]
Authors are thankful to the Ministry of Human Resource Development (MHRD), Government of India for financial support.
[26]
References
[27]
[1] Park O-K, Kim NH, Yoo G-H, Rhee KY, Lee JH. Effects of the surface treatment on the properties of polyaniline coated carbon nanotubes/epoxy composites. Compos Part B Eng 2010;41:2–7. http://dx.doi.org/10.1016/j.compositesb.2009.10.002. [2] Jakubinek MB, Ashrafi B, Zhang Y, Martinez-Rubi Y, Kingston CT, Johnston A, et al. Single-walled carbon nanotube–epoxy composites for structural and conductive aerospace adhesives. Compos Part B Eng 2015;69:87–93. http://dx.doi.org/10. 1016/j.compositesb.2014.09.022. [3] Kotsilkova R, Ivanov E, Bychanok D, Paddubskaya A, Demidenko M, Macutkevic J, et al. Effects of sonochemical modification of carbon nanotubes on electrical and electromagnetic shielding properties of epoxy composites. Compos Sci Technol 2015;106:85–92. http://dx.doi.org/10.1016/j.compscitech.2014.11.004. [4] Jia QM, Li JB, Wang LF, Zhu JW, Zheng M. Electrically conductive epoxy resin composites containing polyaniline with different morphologies. Mater Sci Eng A 2007;448:356–60. http://dx.doi.org/10.1016/j.msea.2006.09.065. [5] Bhadra S, Khastgir D, Singha NK, Lee JH. Progress in preparation, processing and applications of polyaniline. Prog Polym Sci 2009;34:783–810. http://dx.doi.org/10. 1016/j.progpolymsci.2009.04.003. [6] Yang Q, Wei S, Cheng G. Preparation of conductive polyaniline/epoxy composite. Polym Compos 2006;27:201–4. http://dx.doi.org/10.1002/pc.20176. [7] Oyharçabal M, Olinga T, Foulc M-P, Lacomme S, Gontier E, Vigneras V. Influence of the morphology of polyaniline on the microwave absorption properties of epoxy polyaniline composites. Compos Sci Technol 2013;74:107–12. http://dx.doi.org/ 10.1016/j.compscitech.2012.10.016. [8] Hatamzadeh M, Jaymand M. Synthesis of conductive polyaniline-modified polymers via a combination of nitroxide-mediated polymerization and click chemistry. RSC Adv 2014;4:28653–63. http://dx.doi.org/10.1039/c4ra00864b. [9] Petrie E. Epoxy adhesive formulations. McGraw-Hill Education; 2005. [10] Tan SG, Chow WS. Thermal properties, fracture toughness and water absorption of epoxy-palm oil blends. Polym Plast Technol Eng 2010;49:900–7. http://dx.doi.org/ 10.1080/03602551003682042. [11] Guadagno L, Sarno M, Vietri U, Raimondo M, Cirillo C, Ciambelli P. Graphenebased structural adhesive to enhance adhesion performance. RSC Adv 2015;5:27874–86. http://dx.doi.org/10.1039/C5RA00819K. [12] Vietri U, Guadagno L, Raimondo M, Vertuccio L, Lafdi K. Nanofilled epoxy adhesive for structural aeronautic materials. Compos Part B Eng 2014;61:73–83. http://dx. doi.org/10.1016/j.compositesb.2014.01.032. [13] Guadagno L, Vietri U, Raimondo M, Vertuccio L, Barra G, De Vivo B, et al. Correlation between electrical conductivity and manufacturing processes of nanofilled carbon fiber reinforced composites. Compos Part B Eng 2015;80:7–14. http:// dx.doi.org/10.1016/j.compositesb.2015.05.025. [14] Guadagno L, Raimondo M, Vertuccio L, Mauro M, Guerra G, Lafdi K, et al. Optimization of graphene-based materials outperforming host epoxy matrices. RSC Adv 2015;5:36969–78. http://dx.doi.org/10.1039/C5RA04558D. [15] Guadagno L, Naddeo C, Raimondo M, Barra G, Vertuccio L, Sorrentino A, et al. Development of self-healing multifunctional materials. Compos Part B Eng 2017;128:30–8. http://dx.doi.org/10.1016/j.compositesb.2017.07.003. [16] Guadagno L, Naddeo C, Raimondo M, Barra G, Vertuccio L, Russo S, et al. Influence of carbon nanoparticles/epoxy matrix interaction on mechanical, electrical and transport properties of structural advanced materials. Nanotechnology 2017;28:094001. http://dx.doi.org/10.1088/1361-6528/aa583d. [17] Liu S, Chevali VS, Xu Z, Hui D, Wang H. A review of extending performance of epoxy resins using carbon nanomaterials. Compos Part B Eng 2017. http://dx.doi. org/10.1016/j.compositesb.2017.08.020. [18] Ren X, Peng S, Zhang W, Yi C, Fang Y, Hui D. Preparation of adipic acid-polyoxypropylene diamine copolymer and its application for toughening epoxy resins. Compos Part B Eng 2017;119:32–40. http://dx.doi.org/10.1016/j.compositesb. 2017.03.019. [19] Peltola J, Cao Y, Smith P. Epoxy adhesives made with inherently conducting polymers. Adhes Age 1995;38:18–20. [20] Lu J, Moon K-S, Kim B-K, Wong CP. High dielectric constant polyaniline/epoxy
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38] [39]
[40]
[41]
[42]
[43]
[44]
156
composites via in situ polymerization for embedded capacitor applications. Polym Guildf 2007;48:1510–6. http://dx.doi.org/10.1016/j.polymer.2007.01.057. Tsotra P, Friedrich K. Thermal, mechanical, and electrical properties of epoxy resin/ polyaniline-dodecylbenzenesulfonic acid blends. Synth Met 2004;143:237–42. http://dx.doi.org/10.1016/j.synthmet.2003.12.016. Jia W, Tchoudakov R, Segal E, Narkis M, Siegmann A. Electrically conductive composites based on epoxy resin containing polyaniline-DBSA- and polyanilineDBSA-coated glass fibers. J Appl Polym Sci 2004;91:1329–34. http://dx.doi.org/10. 1002/app.13301. Belaabed B, Wojkiewicz JL, Lamouri S, El Kamchi N, Redon N. Thermomechanical behaviors and dielectric properties of polyaniline-doped para-toluene sulfonic acid/ epoxy resin composites. Polym Adv Technol 2012;23:1194–201. http://dx.doi.org/ 10.1002/pat.2029. Jeevananda T, Palaniappan S. Siddaramaiah. Spectral and thermal studies on polyaniline-epoxy novolac resin composite materials. J Appl Polym Sci 1999;74:3507–12. http://dx.doi.org/10.1002/(SICI)1097-4628(19991227)74. 14 < 3507::AID-APP26 > 3.0.CO;2-H. Soares BG, Celestino ML, Magioli M, Moreira VX, Khastgir D. Synthesis of conductive adhesives based on epoxy resin and polyaniline.DBSA using the in situ polymerization and physical mixing procedures. Synth Met 2010;160:1981–6. http://dx.doi.org/10.1016/j.synthmet.2010.07.021. Mir IA, Kumar D. Development of polyaniline/epoxy composite as a prospective solder replacement material. Int J Polym Mater Polym Biomater 2010;59:994–1007. http://dx.doi.org/10.1080/00914037.2010.504150. Oyharçabal M, Olinga T, Foulc MP, Vigneras V. Polyaniline/clay as nanostructured conductive filler for electrically conductive epoxy composites. Influence of filler morphology, chemical nature of reagents, and curing conditions on composite conductivity. Synth Met 2012;162:555–62. http://dx.doi.org/10.1016/j.synthmet. 2012.02.011. Llevot A. Sustainable synthetic approaches for the preparation of plant oil-based thermosets. J Am Oil Chem Soc 2017;94:169–86. http://dx.doi.org/10.1007/ s11746-016-2932-4. Baroncini EA, Kumar Yadav S, Palmese GR, Stanzione JF. Recent advances in biobased epoxy resins and bio-based epoxy curing agents. J Appl Polym Sci 2016;133:44103. http://dx.doi.org/10.1002/app.44103. Bobade SK, Paluvai NR, Mohanty S, Nayak SK. Bio-based thermosetting resins for future generation: a review. Polym Plast Technol Eng 2016;55:1863–96. http://dx. doi.org/10.1080/03602559.2016.1185624. Meier MAR, Metzger JO, Schubert US. Plant oil renewable resources as green alternatives in polymer science. Chem Soc Rev 2007;36:1788–802. http://dx.doi.org/ 10.1039/b703294c. Mahendran AR, Aust N, Wuzella G, Kandelbauer A. Synthesis and characterization of a bio-based resin from linseed oil. Macromol Symp 2012;311:18–27. http://dx. doi.org/10.1002/masy.201000134. Campanella A, Baltanas MA, Capel-Sanchez MC, Campos-Martin JM, Fierro JLG. Soybean oil epoxidation with hydrogen peroxide using an amorphous Ti/SiO2 catalyst. Green Chem 2004;6:330–4. http://dx.doi.org/10.1039/b404975f. Jebrane M, Cai S, Sandström C, Terziev N. The reactivity of linseed and soybean oil with different epoxidation degree towards vinyl acetate and impact of the resulting copolymer on the wood durability. Express Polym Lett 2017;11:383–95. http://dx. doi.org/10.3144/expresspolymlett.2017.37. Kim JR, Sharma S. The development and comparison of bio-thermoset plastics from epoxidized plant oils. Ind Crops Prod 2012;36:485–99. http://dx.doi.org/10.1016/ j.indcrop.2011.10.036. Zhang C, Xia Y, Chen R, Huh S, Johnston PA, Kessler MR. Soy-castor oil based polyols prepared using a solvent-free and catalyst-free method and polyurethanes therefrom. Green Chem 2013;15:1477–84. http://dx.doi.org/10.1039/c3gc40531a. Sahoo SK, Mohanty S, Nayak SK. Toughened bio-based epoxy blend network modified with transesterified epoxidized soybean oil: synthesis and characterization. RSC Adv 2015;5:13674–91. http://dx.doi.org/10.1039/C4RA11965G. Sharma V, Banait JS, Kundu PP. Swelling kinetics of linseed oil-based polymers. J Appl Polym Sci 2009;111:1816–27. http://dx.doi.org/10.1002/app.29006. Yim Y-J, Rhee KY, Park SJ. Fracture toughness and ductile characteristics of diglycidyl ether of bisphenol-A resins modified with biodegradable epoxidized linseed oil. Compos Part B Eng 2017;2017(131):144–52. http://dx.doi.org/10.1016/J. COMPOSITESB. 07.047. Es-Haghi H, Mirabedini SM, Imani M, Farnood RR. Mechanical and self-healing properties of a water-based acrylic latex containing linseed oil filled microcapsules: effect of pre-silanization of microcapsules' shell compound. Compos Part B Eng 2016;85:305–14. http://dx.doi.org/10.1016/j.compositesb.2015.09.015. Samper MD, Petrucci R, Sánchez-Nacher L, Balart R, Kenny JM. New environmentally friendly composite laminates with epoxidized linseed oil (ELO) and slate fiber fabrics. Compos Part B Eng 2015;71:203–9. http://dx.doi.org/10.1016/j. compositesb.2014.11.034. Deng Y, Paraskevas D, Tian Y, Van Acker K, Dewulf W, Duflou JR. Life cycle assessment of flax-fibre reinforced epoxidized linseed oil composite with a flame retardant for electronic applications. J Clean Prod 2016;133:427–38. http://dx.doi. org/10.1016/j.jclepro.2016.05.172. Lligadas G, Ronda JC, Galià M, Cádiz V. Bionanocomposites from renewable Resources: epoxidized linseed Oil−Polyhedral oligomeric silsesquioxanes hybrid materials. Biomacromolecules 2006;7:3521–6. http://dx.doi.org/10.1021/ BM060703U. Khandelwal V, Sahoo SK, Kumar A, Manik G. Study on the effect of carbon nanotube on the properties of electrically conductive epoxy/polyaniline adhesives. J Mater Sci Mater Electron 2017;28:14240–51. http://dx.doi.org/10.1007/s10854-0177282-y.
Composites Part B 136 (2018) 149–157
V. Khandelwal et al.
behavior of functionalized soybean oil modified toughened epoxy/organo clay nanocomposite. Prog Org Coatings 2015;88:263–71. http://dx.doi.org/10.1016/j. porgcoat.2015.07.012. [50] Belaabed B, Lamouri S, Wojkiewicz JL. Curing kinetics, thermomechanical and microwave behaviors of PANI-doped BSA/epoxy resin composites. Polym J 2011;43:683–91. http://dx.doi.org/10.1038/pj.2011.46. [51] Sahoo SK, Mohanty S, Nayak SK. Synthesis and characterization of bio-based epoxy blends from renewable resource based epoxidized soybean oil as reactive diluent. Chin J Polym Sci 2015;33:137–52. http://dx.doi.org/10.1007/s10118-015-1568-4. [52] Kumar S, Samal SK, Mohanty S, Nayak SK. Epoxidized soybean oil-based epoxy blend cured with anhydride-based cross-linker: thermal and mechanical characterization. Ind Eng Chem Res 2017;56:687–98. http://dx.doi.org/10.1021/acs. iecr.6b03879.
[45] Pan X, Sengupta P, Webster DC. Novel biobased epoxy compounds: epoxidized sucrose esters of fatty acids. Green Chem 2011;13:965–75. http://dx.doi.org/10. 1039/c0gc00882f. [46] Chen J, Soucek MD, Simonsick WJ, Celikay RW. Epoxidation of partially norbornylized linseed oil. Macromol Chem Phys 2002;203:2042–57. http://dx.doi.org/10. 1002/1521-3935(200210)203. 14 < 2042::AID-MACP2042 > 3.0.CO;2–0. [47] Sirqueira ADS, Teodoro Júnior D, Coutinho MDS, Neto ASDS, Silva ADA, Soares BG. Rheological behavior of acrylic paint blends based on polyaniline. Polímeros 2016;26:215–20. http://dx.doi.org/10.1590/0104-1428.2178. [48] Kurbatov VG, Indeikin EA. Influence of the polyaniline structure on the properties of epoxy compounds and materials. Russ J Appl Chem 2015;88:138–42. http://dx. doi.org/10.1134/S1070427215010206. [49] Sahoo SK, Mohanty S, Nayak SK. Study of thermal stability and thermo-mechanical
157