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Improvement of mechanical properties and anticorrosion performance of epoxy coatings by the introduction of polyaniline/graphene composite ⁎
Yen-Ting Lina, Trong-Ming Donb, , Chong-Jun Wongb, Fan-Chun Mengc, Yi-Jun Linc, Shaio-Yen Leec, Chia-Fen Leed, Wen-Yen Chiua a
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan c Angstron Materials Asia Limited, Taipei 11491, Taiwan d Department of Cosmetic Science and Institute of Cosmetic Science, Chia Nan University of Pharmacy & Science, Tainan 71710, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Epoxy Polyaniline Graphene Anticorrosion Coatings
We reported the preparation of epoxy coatings incorporated with a composite filler of poly(styrenesulfonate)polyaniline/reduced graphene oxide (PSS-PANI/rGO) and their performance on the mechanical and anticorrosion properties. The rGO platelets were first dispersed in the PSS solution, followed by in situ oxidative polymerization of aniline. The resulting PSS-PANI/rGO composite was then blended with a bisphenol-A type epoxy at different loadings by tri-roller mill, and was subsequently cured with a curing agent. The ultimate tensile strength and tensile toughness of the epoxy composite at a loading of only 0.5 wt% PSS-PANI/rGO were improved by 39% and 127%, respectively, when compared to the respective values of the pristine epoxy. This was ascribed to their strong interfacial bonding upon curing by the reaction between the PANI and epoxy. Furthermore, the potentiodynamic polarization of the carbon steels coated with the epoxy/PSS-PANI/rGO composite revealed that the anticorrosion performance was greatly improved when compared to those with the neat epoxy and epoxy/rGO coatings. The superior anticorrosion effect was attributed to its larger tortuosity of diffusion pathways, improved interfacial strength between the epoxy and filler, and the passivation layer formed by the presence of polyaniline which was confirmed by X-ray photoelectron spectroscopy. The improved anticorrosion and toughness would allow the coatings to withstand externally mechanical impact and prevent corrosion effectively.
1. Introduction Since 2004 [1], graphene and graphene-derived materials have drawn a lot of attention both in academia and industry for their great potential in many applications. The exceptional properties such as great electrical [2] and thermal conductivity [3], high rigidity [4], high specific surface area, and optical transparency [5] of graphene have made it one of the most promising materials. One popular application of graphene and its derivatives is in the polymer composites as the reinforcing fillers [6, 7]. Among all the polymeric materials suitable for the fabrication of composites, epoxy resin is one of the most versatile and competitive materials [8–10], owing to their excellent properties such as high adhesion strength, easy processing, high modulus, and thermal stability. Recently, the addition of graphene platelets to further improve mechanical properties of epoxies has been studied. By the
inclusion of only 0.1 wt% of functionalized graphene platelets in an epoxy resin, Naebe et al. [11] found that the flexural strength and storage modulus were improved by 22% and 18%, respectively. In fact, epoxy resins have superior rigidity and mechanical strength, but they are intrinsically brittle. The toughening of epoxies has thus been a major issue in many years. The addition of rubber particles has become an effective way to improve the toughness, yet accompanied with decreases in thermal stability and mechanical strength. It has been therefore tried to increase the toughness of epoxies while maintaining the mechanical strength by using rigid nanoparticles such as silica, clay, carbon nanotubes (CNTs), and recently graphene platelets. Bortz et al. [12] reported enhancements of 63% and 111% in mode I fracture toughness and fracture energy, respectively, through the addition of 1 wt% of GO to an epoxy system. Moreover, the uniaxial tensile fatigue life was increased greatly up to 1580%. Though CNTs, as another
Abbreviation:poly(styrenesulfonate)-polyaniline/reduced graphene oxide, PSS-PANI/rGO ⁎ Corresponding author. E-mail addresses: [email protected] (Y.-T. Lin), [email protected] (T.-M. Don), [email protected] (F.-C. Meng), [email protected] (Y.-J. Lin), [email protected] (S.-Y. Lee), [email protected] (C.-F. Lee), [email protected] (W.-Y. Chiu). https://doi.org/10.1016/j.surfcoat.2018.01.050 Received 12 November 2017; Received in revised form 9 January 2018; Accepted 15 January 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Lin, Y.-T., Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2018.01.050
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performances of the coatings, neglecting the fact that an effective anticorrosion coating must also possess flexibility and toughness to maintain the intact with the surface as well as withstand the mechanical impact [36]. In this work, we tried to prepare anticorrosion coatings composed of both PANI and rGO which would have improved toughness and enhanced anticorrosion properties. First, PANI was synthesized by in situ polymerization of aniline on the surface of rGO in poly (styrenesulfonate) (PSS) solution to produce PSS-PANI/rGO as the reinforcing composite filler. The PSS-PANI/rGO was then blended with an epoxy resin and subjected to thermal cure. Thermal and mechanical properties of the epoxy composites were investigated by several instruments. Moreover, the epoxy composite resins were coated on carbon steel sheets and then subjected to cure. Potentiodynamic polarization technique was used in polarization of specimens for corrosion testing.
carbon-based material, have been a competitive candidate for the reinforcing filler, graphene is still a better material for improving the material properties. This is because the specific surface area of graphene is higher than those of CNTs and also the inner surface of the CNTs is inaccessible to polymer matrix. Rafiee et al. [13] compared the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled CNTs, and multi-walled CNTs. Their results showed that graphene platelets significantly out-performed CNT additives in improving mechanical properties of the epoxies, including fracture toughness, Young's modulus, tensile strength and fatigue resistance. They proposed that the superiority of graphene platelets over CNTs in terms of improving mechanical properties was related to their high specific surface area, enhanced interfacial adhesion and the planar geometry of graphene platelets. In a review paper published recently in 2015 [14], the authors summarized the effects of the addition of different nanoparticles such as single-walled CNT, double-walled CNT, multi-walled CNT, graphene, nanoclay and nanosilica on fracture toughness, mechanical strength and stiffness of the epoxy. They concluded that graphene and its related materials are ideal fillers for enhancing the epoxy performance with regard to some specific applications. They also pointed out that only limited work has been carried out to investigate how the matrix is bonded to the surface of graphene and the relationship between the interfacial bonding and the final performance of graphene/epoxy nanocomposite needs to be elucidated. In addition to mechanical reinforcement, the combination of epoxy and graphene also has potential in the field of anticorrosion coatings. The impenetrable two-dimensional structure [15] and high specific surface area of graphene can effectively prolong the tortuous pathways for gas and ions to penetrate through the protective coatings. Ramezanzadeh et al. [16] applied epoxy/amino-functionalized graphene oxide (FGO) on the mild steel substrates and examined the barrier and anticorrosion performance by electrochemical impedance spectroscopy. The charge transfer resistance of substrates coated with the epoxy/FGO was 88.7% higher than that with the pristine epoxy. Schriver et al. [17] studied the short-term and long-term performance of graphene coatings on Cu and Si substrates. Their results showed that although graphene indeed provided effective short-term oxidation protection, over long time scales it could promote more extensive wet corrosion than that for the bare Cu surface. They believed that in the long term, the conducting graphene coatings could maintain a conductive pathway on the copper surface, thus facilitating electrochemical reactions, while typical native oxides would passivate the surface and therefore terminate the electron transfer required for continued corrosion. Zhou et al. [18] also confirmed that the graphene coating could accelerate long-term oxidation and corrosion of an underlying copper substrate. Sun et al. [19] thus proposed a method for eliminating the corrosion-promotion effect of graphene. Instead of directly using graphene, they synthesized graphene/pernigraniline composite (GPC) which was then embedded into poly(vinyl butyral) (PVB) to prepare the coating for the protection of copper surface. The nonconductive pernigraniline, also known as the fully-oxidized-state polyaniline (PANI), served as an electrical insulator on the surface of reduced graphene oxide (rGO), thus minimizing graphene–graphene/metal connections while increasing the electrical resistance of coating. They showed that the GPC-PVB coating exhibited much better anticorrosion than the individual pernigraniline- or rGOmodified PVB from the results of both electrochemical measurement and scratch test. It is well-known that PANI, a conjugated polymer, is also useful in anticorrosion industrial. Many studies [20–33] have reported the passivation effect of PANI-derived coatings on metal surfaces, which could improve the anticorrosion effect even at low loadings of 1–3 wt% [30, 31]. Although many literatures have reported about the anticorrosion behavior of either PANI-containing or graphene-containing coatings, only a few researchers attempted to examine anticorrosion performance of surface coatings composed of both graphene and PANI [19, 34, 35]. Furthermore, most studies focused on the electrochemical
2. Material and methods 2.1. Materials Poly(sodium 4-styrenesulfonate) (PSS, average molecular weight = 1 × 106, 25 wt% in H2O) and ammonium persulfate ((NH4)2S2O8, 98%) were purchased from Sigma-Aldrich (USA). The reduced graphene oxide nanoplatelets (rGO, N002-PDE from Angstron Materials, USA) were comprised of few-layer graphene nanoplatelets with a lateral dimension of approximately 7 μm, according to the manufacture. Their oxygen contents were about 10–30 wt%. Acetone (C3H6O, 99.5%), aniline (C6H7N, 99.5%) and hydrochloric acid (HCl, 37% solution) were purchased from Acros Organics (USA). Diglycidyl ether of bisphenol-A epoxy resin (DGEBA, NPEL-128, epoxide equivalent weight = 184–190 g/eq) was purchased from Nan-Ya Plastics and curing agent (V009, containing 4,4′-methylene dianiline, 4-tert-butylphenol, and benzyl alcohol) was purchased from San-Chun Compounding Co. in Taiwan. A high carbon steel SK-5 purchased from China Steel Corporation (Taiwan) with 0.75–0.85% carbon and 0.60–0.90% manganese was used for the corrosion test. 2.2. Preparation of PSS-PANI/rGO composite fillers The rGO nanoplatelets were relatively hydrophobic because their oxygen contents were lower than typical values of graphene oxide [37, 38]. Therefore, the dispersion of rGO nanoplatelets in water was poor. According to previous literature [39, 40], the π-π interaction between the poly(styrenesulfonate) (PSS) and rGO would help the dispersion of rGO in water phase. In this study, 4.68 g of PSS solution (25 wt%) was first diluted by addition of 180 ml deionized water. Then, 0.17 g of rGO was slowly added to the PSS solution and sonicated for 10 min. Subsequently, aniline (0.50 g) was added slowly into the PSS/rGO solution, and the solution was also sonicated for 10 min. This solution was labeled as solution A. In order to initiate polymerization reaction, solution B was prepared by dissolving 2.77 g of ammonium persulfate as initiator in 20 ml of deionized water with the addition of 1 ml of 37% hydrochloric acid. Solution B was then slowly poured into solution A to initiate the polymerization of aniline. The reaction was carried out at 25 °C for 1 h with a mechanical stirring at 300 rpm. After reaction, the resulting PSS-PANI/rGO composite was collected by vacuum filtration, rinsed by 50 ml of 0.6 M KOH solution to neutralize the reaction product, and rinsed again by deionized water for several times before it was dried at 50 °C for 12 h. 2.3. Preparation of the epoxy/rGO and epoxy/PSS-PANI/rGO composite films A bisphenol-A type epoxy resin was first blended with various amounts of rGO or PSS-PANI/rGO by using a tri-roller mill. Subsequently, curing agent (V009) at a weight ratio of 11:20 to the 2
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Fig. 1. FT-IR spectra of the (a) rGO, (b) PANI, (c) PSS and (d) PSSPANI/rGO.
Q800 from TA, USA) at a frequency of 1 Hz. Samples with a thickness of 0.30 mm were heated from 30 °C to 180 °C at a heating rate of 3 °C/min to obtain the storage modulus (E′) and loss tan(δ) curves. Tensile mechanical properties of the neat epoxy and epoxy composites after cure were measured by a universal testing machine (AGS-F from SHIMADZU, Japan) according to the ASTM D-638 (type V specimens) method at a crosshead speed of 0.5 mm/min. The dumbbell-shaped specimens had an overall length of 63.5 mm and the gauge length was 7.62 mm. In addition to the initial modulus (E), ultimate tensile strength (UTS) and elongation at break (EB), the tensile toughness (TT) was also calculated by integrating the area under stress-strain curve as an indication of energy absorption capability. Five specimens were tested for each sample.
epoxy resin was added into the mixture by mechanical agitation. The well-mixed mixtures were coated on slides by a blade coater with a thickness of about 0.30 mm. They were cured at 25 °C for 7 days and then subjected to post-cure by further heating at 100 °C for 1 h and another 1 h at 130 °C. All samples were cut into rectangular-shape or dumbbell-shape specimens for the measurements of thermal and mechanical properties. 2.4. Structure and morphology of the PSS-PANI/rGO and epoxy/PSSPANI/rGO Structures of the PSS-PANI/rGO composite filler and its individual components were analyzed by Fourier transform infrared spectroscopy (FTIR, Spectrum 100 from Perkin Elmer, USA) to acquire the information of their functional groups and interactions among various components. Samples were ground with KBr powder and pressed into disks. FTIR spectra were then obtained by transmittance mode from 4000 to 400 cm−1 with a resolution of 4 cm−1. The arrangements of crystalline planes of the rGO and PSS-PANI/rGO were examined by an X-ray diffractometer (Ultima IV from Rigaku, USA) to obtain their diffraction patterns. Moreover, the morphology of the rGO and PSSPANI/rGO platelets as well as the cryo-fractured surface of the epoxy composite films were all observed by a scanning electron microscope (SEM, Nova NanoSEM 230, FEI, Japan).
2.6. Anticorrosion performance of the epoxy composite coatings Potentiodynamic polarization technique was used in polarization of specimens for the corrosion test. Carbon steel sheets (85 × 10 × 0.1 mm3) were first polished by SiC-600 paper, washed with acetone, blow-dried and affixed on slides (Supplementary Fig. S1). Specimens for polarization test were prepared by well-mixing the epoxy resin, filler and the curing agent by mechanical agitation. They were then coated on the carbon steel sheets with a thickness of ca. 50 μm using a coating rod. The coatings were cured at 25 °C for 7 days and post-cured at 100 °C for 1 h followed by another 1 h at 130 °C. The coated carbon steel sheets were pasted with insulating tapes and then immersed in the 5 wt% NaCl solution for 24 h. Their anticorrosion performance was measured by an electrochemical workstation (CHI440 from CH Instruments, USA) using a linear sweep voltammetry program (Autolab NOVA 1.11, Metrohm) at a scan rate of 0.05 V/s from −2.0 to 1.0 V. Platinum sheet and Ag|Ag+ electrode were used as the respective counter and reference electrodes. Furthermore, surface structures of the carbon steels being coated with various epoxy composites were analyzed. The coated carbon steels were first immersed in the 5 wt% NaCl solution for 7 days, and the coatings were then removed to expose their surfaces. The carbon steel sheets were cut into small pieces and analyzed by X-ray photoelectron spectroscopy (XPS Theta Probe, ThermoFisher Scientific, USA) to analyze their surface elements and verify the anticorrosion effect.
2.5. Thermal and mechanical properties of the epoxy composites The effects of the rGO and PSS-PANI/rGO fillers on the curing extent of the epoxy resin were investigated by differential scanning calorimetry (DSC, Q20 from TA, USA). Samples were prepared by mixing the epoxy resin and curing agent with or without the filler, and they were immediately transferred to the DSC for thermal scan from 0 °C to 180 °C at a heating rate of 10 °C/min. The reaction heat was then determined from the integration area of exothermic peak. Furthermore, glass transition temperature (Tg) of the pristine epoxy and epoxy composites after cure were also determined by DSC. Samples were first heated from 40 °C to 180 °C at a heating rate of 10 °C/min. They were cooled down to 40 °C and then re-heated again to 180 °C at the same heating rate. The Tg was determined from the second heating curve. Dynamic mechanical properties of the neat epoxy and epoxy composite films were measured by a dynamic mechanical analyzer (DMA, 3
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Fig. 2. XRD patterns of the (a) rGO, and (b) PSS-PANI/rGO platelets.
3. Results and discussion
covered with PSS-PANI.
3.1. Structure and morphology of the PSS-PANI/rGO
3.2. Curing of the epoxy and epoxy composites
The PANI was synthesized by oxidation polymerization in the PSS/ rGO aqueous dispersion. As shown in Fig. 1, successful polymerization of aniline was confirmed by FTIR analysis. For the original rGO platelets, the peaks at 1571 and 1174 cm−1 were attributed to the skeletal vibration of C]C and stretching vibration of CeO(H), respectively; on the other hand, the produced PSS-PANI/rGO platelets exhibited another two peaks at 1497 and 1581 cm−1 which could be ascribed to the skeletal vibrations of benzene and quinone rings in the PANI component. Moreover, the peaks appeared at 1010 and 1040 cm−1 for the PSS-PANI/rGO platelets were assigned to the in-plane skeleton vibration of the benzene ring and the symmetric vibration of SO3– group in the PSS component. The characteristic peaks of the produced PSSPANI/rGO confirmed the successful synthesis of the PANI. Moreover, XRD patterns of the rGO and PSS-PANI/rGO were obtained for analyzing their structures. As shown in Fig. 2(a), a broad diffraction peak centered at 2θ = 24.95° was observed for the rGO platelets which was equivalent to the interlayer spacing of 0.36 nm according to the Bragg's law. Zhang et al. [41] synthesized the rGO by modified Hummers method followed by hydrazine reduction and also found the diffraction peak around 2θ = 24.6°. As for the PSS-PANI/rGO, the diffraction peak of the rGO component remained at the same position, indicating that the interspacing of the platelets did not change. Moreover, there were diffraction peaks at 2θ = 21.32°, 29.96° and 31.00° corresponding to the (100), (211), and (020) planes of the PANI [42, 43]. These diffraction peaks suggested that highly ordered structure of PANI was present in the PSS-PANI/rGO composite. The morphologies of the as-prepared PSS-PANI/rGO after directly drying in the oven along with the original rGO powders are shown in Fig. 3. The average lateral size of the rGO platelets was about 8.5 ± 1.7 μm, while that of PSS-PANI/rGO platelets was about 11.5 ± 2.1 μm. The larger size of the PSS-PANI/rGO indicated that the dispersed rGO platelets were wrapped by the PSS-PANI and some of them might agglomerate during the drying process. Furthermore, the thickness of the rGO platelets was about 50 nm and increased to 100 nm for the PSS-PANI/rGO. The larger thickness and rougher surface of the PSS-PANI/rGO platelets further confirmed that the rGO platelets were
The rGO and PSS-PANI/rGO were then added into the epoxy resin separately as the potential reinforcing filler as well as the anticorrosion agent. However, their presence would have effects on the curing behavior of the epoxy resin. DSC was used to evaluate the curing behavior of the epoxy composites by simply measuring their curing heats which were determined from the integration areas under the exothermic peaks recorded from 0 to 180 °C at a heating rate of 10 °C/min. For comparison, we calculated the curing extents of all epoxy composites by dividing their curing heats with that of pristine epoxy. As shown in Fig. 4, all the epoxy composites had curing extents less than 100%, indicating their curing heats were lower than that of the pristine epoxy (280.2 J/ g). For the epoxy composite filled with rGO, the curing extent was 95.1% at the loading of 0.1 wt%, and it gradually decreased to 91.3% as the rGO loading increased to 1.0 wt%. The presence of rGO platelets thus hindered the curing of epoxy by increasing the viscosity of matrix and reducing the contact probability of reactants. Yet, the epoxy/PSSPANI/rGO composites had higher curing extents than the respective epoxy/rGO composites at the same loading content. For example, at the loading of 0.1 wt%, the curing extent of the epoxy/PSS-PANI/rGO was 97.6%, ca. 2.5% higher than that of the epoxy/rGO. It has been suggested that the reaction between the PANI and epoxy could occur at high temperatures [44–46]. By using the DSC technique, Palaniappan et al. [45] studied the curing kinetics of the epoxy with polyaniline. They found that the curing reactions of the epoxy and polyaniline could occur which followed an autocatalytic kinetics in the kinetic-control stage. Jang et al. [46] synthesized PANI nanorods and incorporated them into a liquid crystalline epoxy as curing agent. They found that the heat of cure was proportional to the content of PANI nanorods. To confirm the reaction between the PANI and epoxy resin, the PSS-PANI/ rGO was well-dispersed in the epoxy resin at 15 wt% without the addition of curing agent. The mixture was then maintained at 130 °C for 1 h. As shown in Fig. 5, the peaks of the epoxy/PSS-PANI/rGO mixture at 3497 and 915 cm−1, which were attributed to the respective stretching vibration of N-H in the PANI and the C-O deformation of epoxide group in the epoxy resin, were both reduced after heating at high temperature. This suggests that the reaction occurred between the 4
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Fig. 3. SEM images of the (a) rGO, and (b) PSSPANI/rGO platelets.
could be prepared with the composite filler up to 1.0 wt%.
epoxy resin and PANI. Cryo-fractured surfaces of the epoxy and epoxy composites after cure were examined by SEM as shown in Fig. 6. The smooth surface of the pristine epoxy resin shown in Fig. 6(a) suggested low capacity of energy absorption during crack propagation. In other words, this epoxy was intrinsically brittle. On the contrary, the epoxy/rGO composite presented a rougher fracture surface as revealed in Fig. 6(b) and 6(c). The impenetrable rGO fillers retarded the crack propagation by inducing the deflection and pinning of crack fronts. In addition, the rGO platelets in the epoxy matrix had almost the same size as the original rGO, indicating that the rGO did not agglomerate at this loading of only 0.5 wt%. Yet, microvoids were found near the rGO platelets, indicating weak interfacial bonding between the rGO and epoxy matrix. When the loading of rGO reached 1.0 wt%, the interfacial bonding became so weak that cracks started to appear on the composite film right after the curing process. Therefore, we could only prepare the epoxy composites with the rGO loading up to 0.5 wt% in this work. On the other hand, no microvoids were found on the fracture surface of the epoxy/PSS-PANI/ rGO composites as shown in Fig. 6(d)–(f). Stronger interfacial bonding was thus expected to be present between the PSS-PANI/rGO platelets and epoxy matrix. This is because the rGO platelets were covered with the PSS-PANI where the PANI could react with the epoxy as suggested from the previous results. The epoxy/PSS-PANI/rGO composites thus
3.3. Thermal and mechanical properties of the epoxy and epoxy composites Dynamic mechanical thermal properties including storage modulus (E′) and loss tan(δ) of the epoxy and epoxy composites after cure were measured at a frequency of 1 Hz as shown in Fig. 7. The peak temperature of the loss tan(δ) curve was defined as the glass transition temperature (Tg) of material. Moreover, The Tg values of the epoxy composites were also determined from the second heating curves in DSC. It is known that the Tg value represents the flexibility or rigidity of polymer chains. The higher the Tg, the higher the chain rigidity is. In an epoxy composite system, the Tg depends on the crosslinking density or curing extent, the type and content of filler and the interfacial bonding between the filler and matrix. The Tg values of the epoxy and epoxy composites are summarized in Table 1. Although all of the Tg values determined from DMA were higher than the respective Tg values of the epoxy composites at the same loading content, their changes with the filler loading were similar. After post cure at high temperatures, the neat epoxy had a Tg,DMA of 114 °C, and the addition of rGO or PSSPANI/rGO could raise the Tg of the material. The segmental motion of epoxy chains was thus restricted by the presence of more rigid fillers. However, the Tg increased with increasing the filler content only up to a Fig. 4. Curing extents of the epoxy/rGO and epoxy/PSS-PANI/ rGO composites. The weight ratio of the curing agent solution to the epoxy resin was 11/20 for all the samples.
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Fig. 5. FT-IR spectra of the epoxy/PSS-PANI/rGO without the addition of curing agent: (a) after heating for 1 h at 130 °C and (b) before heating. The PSS-PANI/rGO was added at 15 wt%.
Fig. 6. SEM images of the freeze-fractured surfaces of (a) neat epoxy; and epoxy composites filled with rGO at (b) 0.1 wt%, and (c) 0.5 wt%, and PSS-PANI/rGO at (d) 0.1 wt %, (e) 0.5 wt%, and (f) 1.0 wt%.
due to the weak interfacial bonding and the presence of microvoids around the rGO fillers. However, the decline in the E′ was not found for the epoxy/PSS-PANI/rGO composite when the PSS-PANI/rGO content was increased from 0.1 wt% to 0.5 wt%. In other words, improved modulus was still observed for the epoxy composite filled with 0.5 wt% PSS-PANI/rGO, though the improvement was not pronounced. As we compare the epoxy/PSS-PANI/rGO and epoxy/rGO systems at the same loading content of 0.5 wt%, there is a large difference between their E′ values. Table 2 shows that the E′ of the epoxy/PSS-PANI/rGO at 0.5 wt % loading is 2.34 GPa, while the value of the epoxy/rGO at the same loading is 1.88 GPa, even lower than the value of the neat epoxy. This is because further addition of PSS-PANI could increase the interfacial strength between the filler and the epoxy matrix as proved in the previous section. Yet, increasing the PSS-PANI/rGO content up to 1.0 wt%, a decrement in storage modulus was still found. This is probably caused by the decreased crosslinking density and the agglomeration of PSSPANI/rGO fillers in the epoxy matrix. Tensile mechanical properties of the epoxy and epoxy composites
certain point, after that it started to decrease. This is because the Tg not only depends on the presence of rigid filler but also on the crosslinking density or the curing extent of the epoxy. The previous results showed that the curing extent decreased with increasing the filler extent. In other words, the higher the filler content, the lower the crosslinking density. Therefore, as the filler content increased to a certain point, the effect of decreased crosslinking density would surpass that of the restricted segmental motion by the filler. Dynamic mechanical analysis also provided the storage modulus data (E′) of epoxy composites. The E′ at 30 °C of the neat epoxy and epoxy composites are also summarized in Table 1. By the addition of only 0.1 wt% rGO, the E′ was increased by 13.3% when compared to that of the neat epoxy. Yet, a further increase of the rGO content to 0.5 wt%, a decline in the E′ for the epoxy/rGO composite was observed. It is well known that mechanical properties of composite materials depend on the type, shape and the content of filler, and also the interfacial bonding between the filler and matrix. Therefore, the decrease in the modulus for the epoxy composite filled with 0.5 wt% rGO might 6
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Fig. 7. Dynamic mechanical properties including storage modulus (E′) and loss tan(δ) of the (a) epoxy/rGO, and (b) epoxy/PSS-PANI/ rGO composites at different loadings of fillers. Samples were cured at 25 °C for 7 days and followed by post-cure at 100 °C for 1 h and 130 °C for another hour.
Table 1 Glass transition temperature (Tg) and storage modulus (E′ at 30 °C) of the epoxy composites filled with different loadings of rGO and PSS-PANI/rGO. Sample
Filler (wt%)
Tg,DSC (°C)
Tg,DMA (°C)
E′ @30 °C (GPa)
Epoxy Epoxy/rGO
0 0.1 0.5 0.1 0.5 1.0
98 115 114 110 108 108
114 124 121 121 116 118
2.10 2.38 1.88 2.31 2.34 1.56
Epoxy/PSS-PANI/ rGO
Table 2 Tensile mechanical properties including initial modulus (E), ultimate tensile strength (UTS), elongation at break (EB) and tensile toughness (TT) of the epoxy composites filled with rGO and PSS-PANI/rGO at different loadings. Sample
Filler (wt%)
E (GPa)
Epoxy Epoxy/rGO
0 0.1 0.5 0.1 0.5 1.0
1.55 1.83 1.72 1.69 1.89 1.69
Epoxy/PSSPANI/ rGO
Samples were cured at 25 °C for 7 days, followed by post-cure at 100 °C for 1 h and 130 °C for another hour.
± ± ± ± ± ±
UTS (MPa) 0.05 0.26 0.29 0.11 0.04 0.15
43.1 49.1 44.5 59.2 64.9 51.4
± ± ± ± ± ±
EB (%) 6.7 9.9 10.2 7.1 6.0 12.3
3.6 4.0 3.5 5.2 5.0 3.5
TT (J/m3) ± ± ± ± ± ±
0.2 0.5 1.5 1.0 1.2 1.1
562 ± 93 774 ± 124 677 ± 330 1229 ± 228 1273 ± 344 678 ± 362
Samples were cured at 25 °C for 7 days and followed by post-cure at 100 °C for 1 h and 130 °C for another hour.
after cure were also measured in this study. Their stress–strain curves are presented in Fig. 8 where initial modulus (E), ultimate tensile strength (UTS), elongation at break (EB) and tensile toughness (TT) were all calculated. The tensile toughness was the absorbed energy per unit volume of the sample being extended to fracture by tensile test. The results are summarized in Table 2. It can be seen that similar trends were observed for the tensile mechanical properties as those found in the storage modulus data. The tensile mechanical properties of the epoxy composite added with the rGO at 0.1 wt% were higher than those of the neat epoxy, but became lower when the addition amount of rGO was increased to 0.5 wt%. This also can be ascribed to the weak
interfacial bonding and the microvoids around the rGO fillers. Owing to the stronger interfacial bonding in the epoxy/PSS-PANI/rGO composites, improved tensile mechanical properties were still observed for the epoxy composites when the PSS-PANI/rGO content was increased from 0.1 to 0.5 wt%. For example, the initial modulus was increased from 1.55 GPa for the neat epoxy to 1.69 GPa for the epoxy composite filled with 0.1 wt% PSS-PANI/rGO, and further increased to 1.89 GPa for the composite with 0.5 wt%. The ultimate tensile strength was also increased from the value of 43.1 MPa for the neat epoxy to 64.9 MPa for
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Fig. 8. Tensile stress-strain curves of the (a) epoxy/ rGO, and (b) epoxy/PSS-PANI/rGO composites after cure.
the epoxy composite filled with 0.5 wt% PSS-PANI/rGO. The increases in the initial modulus and ultimate tensile strength for the epoxy composite filled with 0.5 wt% PSS-PANI/rGO were thus about 22% and 51%, respectively, when compared to the values of the neat epoxy. Moreover, its elongation at break and tensile toughness were 39 and 127% higher than the respective values of the pristine epoxy. This is because the polyaniline on the surface of PSS-PANI/rGO platelets could react with the epoxy matrix during the cure. Interfacial bonding was thus enhanced greatly and enabled the transfer of loading stress to the platelets. Wang et al. [47] tried to prepare the epoxy composites filled with the GO and DGEBA-functionalized GO. They also found that without surface functionalization, the interfacial strength was weak between the GO filler and epoxy matrix; on the other hand, the compatibility and dispersion of GO sheets in epoxy matrix were effectively improved when the GO was surface-functionalized with the DGEBA. Xu and Gao [7] synthesized graphene-reinforced nylon-6 and reported that during polymerization, not only the GO was thermally reduced to graphene, but some nylon-6 chains were grafted on the graphene sheets. As a result, the graphene sheets could be homogeneously dispersed in the nylon-6 matrix. Naebe et al. [11] ascribed the enhancement of mechanical properties mainly to the uniform dispersion of functionalized graphene and strong interfacial bonding between the modified graphene and epoxy matrix. Yet, when the PSS-PANI/rGO loading was increased further to 1.0 wt%, the tensile mechanical properties of the epoxy composite became lower as shown in Table 2, which was similar to the previous observation in the dynamic mechanical properties. This is also ascribed to the decreased crosslinking density and agglomeration of PSS-PANI/rGO platelets.
CR =
Potentiodynamic polarization curves of the carbon steel electrodes coated with the epoxy and epoxy composites in the 5 wt% NaCl aqueous solution are presented in Fig. 9. The anticorrosion properties of the epoxy and epoxy composites could be evaluated from the corrosion potential (Ecorr), corrosion current density (Icorr), polarization resistance (Rp), and corrosion rate (CR). The Ecorr (V) and Icorr (μA/cm2) were determined by the intersection of the linear portions of the anodic and cathodic curves in the Tafel plots. According to Stern-Geary equation [48], the Rp could be calculated by the following equation:
βa βc 1 Icorr 2.303(βa + βc )
(2)
where k is a constant (3268.5 mol/A), M is the molecular weight of iron (56 g/mol), ρm is the density of iron (7.85 g/cm3), Icorr,b is the corrosion current density of bare carbon steel, and Icorr,c is the corrosion current density of the coated steel electrodes. The results are summarized in Table 3. Generally, higher values of Ecorr, Rp and lower values of Icorr and CR suggest better anticorrosion performance of the coatings. The bare carbon steel had high values of corrosion current density (Icorr) and corrosion rate (CR) and thus was easily subjected to corrosion. When it was coated with the neat epoxy, the Icorr and CR were decreased greatly to about 37.1 μA/cm2 and 0.865 mm/year, respectively, indicating a good physical insulation of epoxy coating. By introducing the rGO into the epoxy coating, the anticorrosion properties were further improved. In addition, the higher the rGO content, the higher the anticorrosion efficiency. At a loading of 0.5 wt%, the Icorr and CR of the epoxy/rGO coating decreased to only 0.093 μA/cm2 and 2.2 μm/year, respectively. According to literatures [16, 35], the presence of graphene platelets can effectively improve the protection efficiency by prolonging the tortuosity of diffusion pathways for corrosive reactants, such as O2 and Cl–. Furthermore, the epoxy/ PSS-PANI/rGO coatings had higher values of Ecorr than the neat epoxy and epoxy/rGO coatings. The positive shift of Ecorr was attributed not only to the stronger interfacial bonding between the epoxy and PANIcontaining fillers, but also to the passivation effect of PANI. Although the content of PANI in the epoxy/PSS-PANI/rGO coating might not be high, the good dispersion and high specific surface area of the PSSPANI/rGO platelets still made it possible that some PANI could directly attach to the carbon steel surface. It has been reported [21, 49] that once the PANI could contact with the iron surface, it would act as a redox catalyst, catalyzing the oxidation reaction on the iron surface, and forming a dense passivation layer that was composed of Fe2O3 and Fe3O4. The dense Fe3O4 passivation layer would cause a positive shift of the corrosion potential in the polarization curve and also protect the metal beneath the layer from further corrosion. Recent researches also show that the coatings containing only 1–3 wt% of PANI can make notable improvement in corrosion protection [30, 31]. In this work, the epoxy/PSS-PANI/rGO coating at a loading of 0.5 wt% exhibited the best anticorrosion performance among all the coatings. The anticorrosion efficiency became lower when the PSS-PANI/rGO loading in the epoxy composite was increased to 1.0 wt%, which was ascribed to the poor dispersion of filler. In literatures, Chang et al. [9] introduced graphene into epoxy at 1 wt% and reduced the corrosion current of epoxy-coated cold-rolled steel by 71%. By adding 7 wt% of PANI into epoxy, Radhakrishnan et al. [50] also reduced the corrosion current of epoxycoated stainless steel by one order. In our work, the corrosion current of the carbon steel coated with the epoxy/PSS-PANI/rGO (0.5 wt% in epoxy) was decreased by more than 3 orders of the value of the carbon
3.4. Anticorrosion evaluation
Rp =
kMIcorr ρm
(1)
where βa and βc are the reciprocals of the anodic and cathodic slopes: ΔE/Δ(log I), determined at the points + 70 and −70 mV from the lowest turning point, respectively. Furthermore, the CR of the specimens could be estimated as: 8
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Fig. 9. Tafel plots for the bare carbon steel and carbon steel electrodes coated with the neat epoxy and various epoxy composites.
Table 3 Electrochemical corrosion measurements of the bare carbon steel and epoxy, epoxy/rGO and epoxy/PSS-PANI/rGO coated carbon steels. Coating
Filler (wt%)
Ecorr (V)
Icorr (μA/cm2)
Rp (Ω)
CR (mm/year)
Bare carbon steel Epoxy Epoxy/rGO
– – 0.1 0.5 0.1 0.5 1.0
−1.37 −1.38 −1.43 −1.29 −1.21 −1.04 −1.17
321 37.1 0.831 9.32 × 10−2 1.28 5.19 × 10−3 6.52 × 10−2
7.38 × 10−5 5.74 × 10−3 0.323 2.56 0.172 55.0 4.78
74.8 0.865 1.94 × 10−2 2.17 × 10−3 2.98 × 10−2 1.21 × 10−4 1.52 × 10−3
Epoxy/PSS-PANI/ rGO
steel coated with the neat epoxy, indicating great improvement of corrosion protection. In order to understand the anticorrosion mechanism, XPS was used to analyze the surface structures of the carbon steels with various epoxy coatings. The coated carbon steels were soaked in the 5 wt% NaCl aqueous solution for 7 days, and the coatings were then removed to expose the surface structures of carbon steels for analysis. Fig. 10 shows their XPS spectra. The peaks at 710.4 eV and 724.0 eV were observed for all the bare carbon steel and carbon steels being coating with the epoxy and epoxy/rGO, which represented the Fe 2p3/2 and 2p1/2 peaks of Fe3+ ion [51], while an additional peak at 708.15 eV was found for the carbon steel coated with the epoxy/PSS-PANI/rGO. It was ascribed to the Fe 2p3/2 peaks of Fe2+ ion [51–53]. The results of XPS spectra thus confirmed the presence of Fe3O4 passivation layer only in the epoxy/PSS-PANI/rGO-coated carbon steel electrode, leading to the positive shift of the Ecorr. In summary, the anticorrosion mechanisms proposed for the epoxy/PSS-PANI/rGO coating were composed of the increased tortuosity, strong interfacial bonding between the filler and matrix, and passivation layer caused by the presence of PSS-PANI/rGO platelets as illustrated in Fig. 11.
Fig. 10. XPS spectra of the carbon steel surfaces which had been coated with the neat epoxy and epoxy composites and subsequently soaked in the 5 wt% NaCl aqueous solution for 7 days.
resin at different loadings to prepare epoxy composites. As measured by DSC, DMA, and tensile machine, the epoxy/PSS-PANI/rGO composite had better thermal and mechanical properties than the neat epoxy and epoxy/rGO, indicating that the PSS-PANI/rGO platelets were more compatible with the epoxy matrix than the rGO, upon which the PANI on the surface of PSS-PANI/rGO platelets could participate in the curing reaction of epoxy resin. The enhanced interfacial strength and the stiffness provided by the PSS-PANI/rGO platelets improved the tensile toughness by 127% for the epoxy/PSS-PANI/rGO at a loading of 0.5 wt %, when compared to that of the pristine epoxy. Moreover, in the potentiodynamic polarization analysis, the presence of rGO filler in the epoxy/rGO coatings could reduce the corrosion current density by increasing the tortuosity of diffusion pathways for gas and ions; whereas the carbon steel coated with epoxy/PSS-PANI/rGO coatings exhibited even higher corrosion potential and lower corrosion rate. The XPS spectra of the carbon steel surface confirmed the existence of Fe3O4 layer, which was ascribed to the passivation of carbon steel surface by
4. Conclusions In this study, the PSS-PANI/rGO composite was fabricated via insitu redox polymerization of aniline on the surface of PSS-dispersed rGO platelets. The structural analyses from FTIR spectra, XRD patterns and SEM pictures confirmed that the surface of rGO platelets was covered uniformly by the PSS-PANI. The rGO and PSS-PANI/rGO platelets were then used as the fillers to be blended with the bisphenol-A type epoxy 9
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Fig. 11. Schematic diagram of the proposed anticorrosion mechanism of epoxy composite coatings
PANI. The superior anticorrosion performance of the epoxy/PSS-PANI/ rGO coatings was due to the more compact filler-matrix interface and the presence of passivation layer. In summary, the incorporation of PSSPANI/rGO in epoxy composite enhanced the tensile toughness of the material and provided promising potential in anticorrosion applications. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2018.01.050.
sheets, Nano Lett. 8 (2008) 2458–2462. [16] B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian, M.H.M. Moghadam, Enhancement of barrier and corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide, Corros. Sci. 103 (2016) 283–304. [17] M. Schriver, W. Regan, W.J. Gannett, A.M. Zaniewski, M.F. Crommie, A. Zettl, Graphene as a long-term metal oxidation barrier: worse than nothing, ACS Nano 7 (2013) 5763–5768. [18] F. Zhou, Z. Li, G.J. Shenoy, L. Li, H. Liu, Enhanced room-temperature corrosion of copper in the presence of graphene, ACS Nano 7 (2013) 6939–6947. [19] W. Sun, L.D. Wang, T.T. Wu, Y.Q. Pan, G.C. Liu, Synthesis of low-electrical-conductivity graphene/pernigraniline composites and their application in corrosion protection, Carbon 79 (2014) 605–614. [20] B. Wessling, J. Posdorfer, Corrosion prevention with an organic metal (polyaniline): corrosion test results, Electrochim. Acta 44 (1999) 2139–2147. [21] B. Wessling, Corrosion prevention with an organic metal (polyaniline): surface ennobling, passivation, corrosion test results, Mater. Corros. 47 (1996) 439–445. [22] W.-K. Lu, R.L. Elsenbaumer, B. Wessling, Corrosion protection of mild steel by coatings containing polyaniline, Synth. Met. 71 (1995) 2163–2166. [23] P.J. Kinlen, Y. Ding, D.C. Silverman, Corrosion protection of mild steel using sulfonic and phosphonic acid-doped polyanilines, Corrosion 58 (2002) 490–497. [24] R.M. Torresi, S. de Souza, J.E.P. da Silvaa, S.I.C. de Torresi, Galvanic coupling between metal substrate and polyaniline acrylic blends: corrosion protection mechanism, Electrochim. Acta 50 (2005) 2213–2218. [25] P.J. Kinlen, A mechanistic investigation of polyaniline corrosion protection using the scanning reference electrode technique, J. Electrochem. Soc. 146 (1999) 3690. [26] L. Zhong, H. Zhu, J. Hu, S.H. Xiao, F.X. Gan, A passivation mechanism of doped polyaniline on 410 stainless steel in deaerated H2SO4 solution, Electrochim. Acta 51 (2006) 5494–5501. [27] B. Wessling, Passivation of metals by coating with polyaniline: corrosion potential shift and morphological changes, Adv. Mater. 6 (1994) 226–228. [28] D.W. DeBerry, Modification of the electrochemical and corrosion behavior of stainless steels with an electroactive coating, J. Electrochem. Soc. 132 (1985) 1022. [29] G. Mengoli, M.T. Munari, P. Bianco, M.M. Musiani, Anodic synthesis of polyaniline coatings onto Fe sheets, J. Appl. Polym. Sci. 26 (1981) 4247–4257. [30] A. Sakhri, F.X. Perrin, E. Aragon, S. Lamouric, A. Benaboura, Chlorinated rubber paints for corrosion prevention of mild steel: a comparison between zinc phosphate and polyaniline pigments, Corros. Sci. 52 (2010) 901–909. [31] H. Zhang, J. Wang, X. Liu, Z. Wang, S. Wang, High performance self-healing epoxy/ polyamide protective coating containing epoxy microcapsules and polyaniline nanofibers for mild carbon steel, Ind. Eng. Chem. Res. 52 (2013) 10172–10180. [32] M.F. Montemor, Functional and smart coatings for corrosion protection: a review of recent advances, Surf. Coat. Technol. 258 (2014) 17–37. [33] L.H.E. Santos, J.S.C. Branco, I.S. Guimarães, A.J. Motheo, Synthesis in phytic acid medium and application as anticorrosive coatings of polyaniline-based materials, Surf. Coat. Technol. 275 (2015) 26–31. [34] Y. Jafari, S.M. Ghoreishi, M. Shabani-Nooshabadi, Polyaniline/graphene nanocomposite coatings on copper: electropolymerization, characterization, and evaluation of corrosion protection performance, Synth. Met. 217 (2016) 220–230. [35] C.H. Chang, T.C. Huang, C.W. Peng, T.C. Yeh, H.I. Lu, W.I. Hung, C.J. Weng, T.I. Yang, J.M. Yeh, Novel anticorrosion coatings prepared from polyaniline/graphene composites, Carbon 50 (2012) 5044–5051. [36] P.A. Sørensen, S. Kiil, K. Dam-Johansen, C.E. Weinell, Anticorrosive coatings: a review, J. Coat. Technol. Res. 6 (2009) 135–176. [37] N. Morimoto, T. Kubo, Y. Nishina, Tailoring the oxygen content of graphite and reduced graphene oxide for specific applications, Sci. Rep. 6 (2016) 21715.
References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] H.B. Heersche, P. Jarillo-Herrero, J.B. Oostinga, L.M.K. Vandersypen, A.F. Morpurgo, Bipolar supercurrent in graphene, Nature 446 (2007) 56–59. [3] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902–907. [4] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [5] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [6] J. Liang, Y. Huang, L. Zhang, Y. Wang, Y. Ma, T. Guo, Y. Chen, Molecular-level dispersion of graphene into poly(vinyl alcohol) and effective reinforcement of their nanocomposites, Adv. Funct. Mater. 19 (2009) 2297–2302. [7] Z. Xu, C. Gao, In situ polymerization approach to graphene-reinforced Nylon-6 composites, Macromolecules 43 (2010) 6716–6723. [8] U. Vietri, L. Guadagno, M. Raimondo, L. Vertuccio, K. Lafdi, Nanofilled epoxy adhesive for structural aeronautic materials, Compos. Part B 61 (2014) 73–83. [9] K.-C. Chang, M.-H. Hsu, H.-I. Lu, M.-C. Lai, P.-J. Liu, C.-H. Hsu, W.-F. Ji, T.L. Chuang, Y. Wei, J.-M. Yeh, W.-R. Liu, Room-temperature cured hydrophobic epoxy/graphene composites as corrosion inhibitor for cold-rolled steel, Carbon 66 (2014) 144–153. [10] S. Komori, Y. Sakamoto, Development trend of epoxy molding compound for encapsulating semiconductor chips, in: D. Lu, C.P. Wong (Eds.), Materials for Advanced Packaging, Springer US, Boston, MA, 2009, pp. 339–363. [11] M. Naebe, J. Wang, A. Amini, H. Khayyam, N. Hameed, L.H. Li, Y. Chen, B. Fox, Mechanical property and structure of covalent functionalised graphene/epoxy nanocomposites, Sci. Rep. 4 (2014) 4375. [12] D.R. Bortz, E.G. Heras, I. Martin-Gullon, Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites, Macromolecules 45 (2012) 238–245. [13] M.A. Rafiee, J. Rafiee, Z. Wang, H. Song, Z.-Z. Yu, N. Koratkar, Enhanced mechanical properties of nanocomposites at low graphene content, ACS Nano 3 (2009) 3884–3890. [14] N. Domun, H. Hadavinia, T. Zhang, T. Sainsbury, G.H. Liaghat, S. Vahid, Improving the fracture toughness and the strength of epoxy using nanomaterials - a review of the current status, Nano 7 (2015) 10294–10329. [15] J.S. Bunch, S.S. Verbridge, J.S. Alden, A.M. van der Zande, J.M. Parpia, H.G. Craighead, P.L. McEuen, Impermeable atomic membranes from graphene
10
Surface & Coatings Technology xxx (xxxx) xxx–xxx
Y.-T. Lin et al.
[46] J. Jang, J. Bae, K. Lee, Synthesis and characterization of polyaniline nanorods as curing agent and nanofiller for epoxy matrix composite, Polymer 46 (2005) 3677–3684. [47] F. Wang, L.T. Drzal, Y. Qin, Z. Huang, Mechanical properties and thermal conductivity of graphene nanoplatelet/epoxy composites, J. Mater. Sci. 50 (2015) 1082–1093. [48] A.J. Bard, L.R. Faulkner, J. Leddy, C.G. Zoski, Electrochemical Methods: Fundamentals and Applications, Vol. 2 Wiley, New York, 1980. [49] D.E. Tallman, G. Spinks, A. Dominis, G.G. Wallace, Electroactive conducting polymers for corrosion control, J. Solid State Electrochem. 6 (2002) 73–84. [50] S. Radhakrishnan, N. Sonawane, C.R. Siju, Epoxy powder coatings containing polyaniline for enhanced corrosion protection, Prog. Org. Coat. 64 (2009) 383–386. [51] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. [52] S.G. Wang, G. Han, G.H. Yu, Y. Jiang, C. Wang, A. Kohn, R.C.C. Ward, Evidence for FeO formation at the Fe/MgO interface in epitaxial TMR structure by X-ray photoelectron spectroscopy, J. Magn. Magn. Mater. 310 (2007) 1935–1936. [53] B. Wessling, Scientific and commercial breakthrough for organic metals, Synth. Met. 85 (1997) 1313–1318.
[38] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228–240. [39] G. Wang, X. Shen, B. Wang, J. Yao, J. Park, Synthesis and characterisation of hydrophilic and organophilic graphene nanosheets, Carbon 47 (2009) 1359–1364. [40] S. Cho, J.S. Lee, J. Jun, S.G. Kim, J. Jang, Fabrication of water-dispersible and highly conductive PSS-doped PANI/graphene nanocomposites using a high-molecular weight PSS dopant and their application in H2S detection, Nano 6 (2014) 15181–15195. [41] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Graphene/polyaniline nanofiber composites as supercapacitor electrodes, Chem. Mater. 22 (2010) 1392–1401. [42] V.H. Nguyen, J.J. Shim, Green synthesis and characterization of carbon nanotubes/ polyaniline nanocomposites, J. Spectrosc. 2015 (2015) 1–9. [43] J. Pouget, M. Jozefowicz, A.e.a. Epstein, X. Tang, A. MacDiarmid, X-ray structure of polyaniline, Macromolecules 24 (1991) 779–789. [44] B. Sreedhar, S. Palaniappan, S. Narayanan, The effect of zeolite on the curing of epoxy resin by polyaniline: a kinetic study, Polym. Adv. Technol. 13 (2002) 459–465. [45] S. Palaniappan, B. Sreedhar, S.M. Nair, Polyaniline as a curing agent for epoxy resin: cure kinetics by differential scanning calorimetry, Macromol. Chem. Phys. 202 (2001) 1227–1231.
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Improvement of mechanical properties and anticorrosion performance of epoxy coatings by the introduction of polyaniline/graphene composite ⁎
Yen-Ting Lina, Trong-Ming Donb, , Chong-Jun Wongb, Fan-Chun Mengc, Yi-Jun Linc, Shaio-Yen Leec, Chia-Fen Leed, Wen-Yen Chiua a
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan c Angstron Materials Asia Limited, Taipei 11491, Taiwan d Department of Cosmetic Science and Institute of Cosmetic Science, Chia Nan University of Pharmacy & Science, Tainan 71710, Taiwan b
A R T I C L E I N F O
A B S T R A C T
Keywords: Epoxy Polyaniline Graphene Anticorrosion Coatings
We reported the preparation of epoxy coatings incorporated with a composite filler of poly(styrenesulfonate)polyaniline/reduced graphene oxide (PSS-PANI/rGO) and their performance on the mechanical and anticorrosion properties. The rGO platelets were first dispersed in the PSS solution, followed by in situ oxidative polymerization of aniline. The resulting PSS-PANI/rGO composite was then blended with a bisphenol-A type epoxy at different loadings by tri-roller mill, and was subsequently cured with a curing agent. The ultimate tensile strength and tensile toughness of the epoxy composite at a loading of only 0.5 wt% PSS-PANI/rGO were improved by 39% and 127%, respectively, when compared to the respective values of the pristine epoxy. This was ascribed to their strong interfacial bonding upon curing by the reaction between the PANI and epoxy. Furthermore, the potentiodynamic polarization of the carbon steels coated with the epoxy/PSS-PANI/rGO composite revealed that the anticorrosion performance was greatly improved when compared to those with the neat epoxy and epoxy/rGO coatings. The superior anticorrosion effect was attributed to its larger tortuosity of diffusion pathways, improved interfacial strength between the epoxy and filler, and the passivation layer formed by the presence of polyaniline which was confirmed by X-ray photoelectron spectroscopy. The improved anticorrosion and toughness would allow the coatings to withstand externally mechanical impact and prevent corrosion effectively.
1. Introduction Since 2004 [1], graphene and graphene-derived materials have drawn a lot of attention both in academia and industry for their great potential in many applications. The exceptional properties such as great electrical [2] and thermal conductivity [3], high rigidity [4], high specific surface area, and optical transparency [5] of graphene have made it one of the most promising materials. One popular application of graphene and its derivatives is in the polymer composites as the reinforcing fillers [6, 7]. Among all the polymeric materials suitable for the fabrication of composites, epoxy resin is one of the most versatile and competitive materials [8–10], owing to their excellent properties such as high adhesion strength, easy processing, high modulus, and thermal stability. Recently, the addition of graphene platelets to further improve mechanical properties of epoxies has been studied. By the
inclusion of only 0.1 wt% of functionalized graphene platelets in an epoxy resin, Naebe et al. [11] found that the flexural strength and storage modulus were improved by 22% and 18%, respectively. In fact, epoxy resins have superior rigidity and mechanical strength, but they are intrinsically brittle. The toughening of epoxies has thus been a major issue in many years. The addition of rubber particles has become an effective way to improve the toughness, yet accompanied with decreases in thermal stability and mechanical strength. It has been therefore tried to increase the toughness of epoxies while maintaining the mechanical strength by using rigid nanoparticles such as silica, clay, carbon nanotubes (CNTs), and recently graphene platelets. Bortz et al. [12] reported enhancements of 63% and 111% in mode I fracture toughness and fracture energy, respectively, through the addition of 1 wt% of GO to an epoxy system. Moreover, the uniaxial tensile fatigue life was increased greatly up to 1580%. Though CNTs, as another
Abbreviation:poly(styrenesulfonate)-polyaniline/reduced graphene oxide, PSS-PANI/rGO ⁎ Corresponding author. E-mail addresses: [email protected] (Y.-T. Lin), [email protected] (T.-M. Don), [email protected] (F.-C. Meng), [email protected] (Y.-J. Lin), [email protected] (S.-Y. Lee), [email protected] (C.-F. Lee), [email protected] (W.-Y. Chiu). https://doi.org/10.1016/j.surfcoat.2018.01.050 Received 12 November 2017; Received in revised form 9 January 2018; Accepted 15 January 2018 0257-8972/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Lin, Y.-T., Surface & Coatings Technology (2018), https://doi.org/10.1016/j.surfcoat.2018.01.050
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performances of the coatings, neglecting the fact that an effective anticorrosion coating must also possess flexibility and toughness to maintain the intact with the surface as well as withstand the mechanical impact [36]. In this work, we tried to prepare anticorrosion coatings composed of both PANI and rGO which would have improved toughness and enhanced anticorrosion properties. First, PANI was synthesized by in situ polymerization of aniline on the surface of rGO in poly (styrenesulfonate) (PSS) solution to produce PSS-PANI/rGO as the reinforcing composite filler. The PSS-PANI/rGO was then blended with an epoxy resin and subjected to thermal cure. Thermal and mechanical properties of the epoxy composites were investigated by several instruments. Moreover, the epoxy composite resins were coated on carbon steel sheets and then subjected to cure. Potentiodynamic polarization technique was used in polarization of specimens for corrosion testing.
carbon-based material, have been a competitive candidate for the reinforcing filler, graphene is still a better material for improving the material properties. This is because the specific surface area of graphene is higher than those of CNTs and also the inner surface of the CNTs is inaccessible to polymer matrix. Rafiee et al. [13] compared the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled CNTs, and multi-walled CNTs. Their results showed that graphene platelets significantly out-performed CNT additives in improving mechanical properties of the epoxies, including fracture toughness, Young's modulus, tensile strength and fatigue resistance. They proposed that the superiority of graphene platelets over CNTs in terms of improving mechanical properties was related to their high specific surface area, enhanced interfacial adhesion and the planar geometry of graphene platelets. In a review paper published recently in 2015 [14], the authors summarized the effects of the addition of different nanoparticles such as single-walled CNT, double-walled CNT, multi-walled CNT, graphene, nanoclay and nanosilica on fracture toughness, mechanical strength and stiffness of the epoxy. They concluded that graphene and its related materials are ideal fillers for enhancing the epoxy performance with regard to some specific applications. They also pointed out that only limited work has been carried out to investigate how the matrix is bonded to the surface of graphene and the relationship between the interfacial bonding and the final performance of graphene/epoxy nanocomposite needs to be elucidated. In addition to mechanical reinforcement, the combination of epoxy and graphene also has potential in the field of anticorrosion coatings. The impenetrable two-dimensional structure [15] and high specific surface area of graphene can effectively prolong the tortuous pathways for gas and ions to penetrate through the protective coatings. Ramezanzadeh et al. [16] applied epoxy/amino-functionalized graphene oxide (FGO) on the mild steel substrates and examined the barrier and anticorrosion performance by electrochemical impedance spectroscopy. The charge transfer resistance of substrates coated with the epoxy/FGO was 88.7% higher than that with the pristine epoxy. Schriver et al. [17] studied the short-term and long-term performance of graphene coatings on Cu and Si substrates. Their results showed that although graphene indeed provided effective short-term oxidation protection, over long time scales it could promote more extensive wet corrosion than that for the bare Cu surface. They believed that in the long term, the conducting graphene coatings could maintain a conductive pathway on the copper surface, thus facilitating electrochemical reactions, while typical native oxides would passivate the surface and therefore terminate the electron transfer required for continued corrosion. Zhou et al. [18] also confirmed that the graphene coating could accelerate long-term oxidation and corrosion of an underlying copper substrate. Sun et al. [19] thus proposed a method for eliminating the corrosion-promotion effect of graphene. Instead of directly using graphene, they synthesized graphene/pernigraniline composite (GPC) which was then embedded into poly(vinyl butyral) (PVB) to prepare the coating for the protection of copper surface. The nonconductive pernigraniline, also known as the fully-oxidized-state polyaniline (PANI), served as an electrical insulator on the surface of reduced graphene oxide (rGO), thus minimizing graphene–graphene/metal connections while increasing the electrical resistance of coating. They showed that the GPC-PVB coating exhibited much better anticorrosion than the individual pernigraniline- or rGOmodified PVB from the results of both electrochemical measurement and scratch test. It is well-known that PANI, a conjugated polymer, is also useful in anticorrosion industrial. Many studies [20–33] have reported the passivation effect of PANI-derived coatings on metal surfaces, which could improve the anticorrosion effect even at low loadings of 1–3 wt% [30, 31]. Although many literatures have reported about the anticorrosion behavior of either PANI-containing or graphene-containing coatings, only a few researchers attempted to examine anticorrosion performance of surface coatings composed of both graphene and PANI [19, 34, 35]. Furthermore, most studies focused on the electrochemical
2. Material and methods 2.1. Materials Poly(sodium 4-styrenesulfonate) (PSS, average molecular weight = 1 × 106, 25 wt% in H2O) and ammonium persulfate ((NH4)2S2O8, 98%) were purchased from Sigma-Aldrich (USA). The reduced graphene oxide nanoplatelets (rGO, N002-PDE from Angstron Materials, USA) were comprised of few-layer graphene nanoplatelets with a lateral dimension of approximately 7 μm, according to the manufacture. Their oxygen contents were about 10–30 wt%. Acetone (C3H6O, 99.5%), aniline (C6H7N, 99.5%) and hydrochloric acid (HCl, 37% solution) were purchased from Acros Organics (USA). Diglycidyl ether of bisphenol-A epoxy resin (DGEBA, NPEL-128, epoxide equivalent weight = 184–190 g/eq) was purchased from Nan-Ya Plastics and curing agent (V009, containing 4,4′-methylene dianiline, 4-tert-butylphenol, and benzyl alcohol) was purchased from San-Chun Compounding Co. in Taiwan. A high carbon steel SK-5 purchased from China Steel Corporation (Taiwan) with 0.75–0.85% carbon and 0.60–0.90% manganese was used for the corrosion test. 2.2. Preparation of PSS-PANI/rGO composite fillers The rGO nanoplatelets were relatively hydrophobic because their oxygen contents were lower than typical values of graphene oxide [37, 38]. Therefore, the dispersion of rGO nanoplatelets in water was poor. According to previous literature [39, 40], the π-π interaction between the poly(styrenesulfonate) (PSS) and rGO would help the dispersion of rGO in water phase. In this study, 4.68 g of PSS solution (25 wt%) was first diluted by addition of 180 ml deionized water. Then, 0.17 g of rGO was slowly added to the PSS solution and sonicated for 10 min. Subsequently, aniline (0.50 g) was added slowly into the PSS/rGO solution, and the solution was also sonicated for 10 min. This solution was labeled as solution A. In order to initiate polymerization reaction, solution B was prepared by dissolving 2.77 g of ammonium persulfate as initiator in 20 ml of deionized water with the addition of 1 ml of 37% hydrochloric acid. Solution B was then slowly poured into solution A to initiate the polymerization of aniline. The reaction was carried out at 25 °C for 1 h with a mechanical stirring at 300 rpm. After reaction, the resulting PSS-PANI/rGO composite was collected by vacuum filtration, rinsed by 50 ml of 0.6 M KOH solution to neutralize the reaction product, and rinsed again by deionized water for several times before it was dried at 50 °C for 12 h. 2.3. Preparation of the epoxy/rGO and epoxy/PSS-PANI/rGO composite films A bisphenol-A type epoxy resin was first blended with various amounts of rGO or PSS-PANI/rGO by using a tri-roller mill. Subsequently, curing agent (V009) at a weight ratio of 11:20 to the 2
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Fig. 1. FT-IR spectra of the (a) rGO, (b) PANI, (c) PSS and (d) PSSPANI/rGO.
Q800 from TA, USA) at a frequency of 1 Hz. Samples with a thickness of 0.30 mm were heated from 30 °C to 180 °C at a heating rate of 3 °C/min to obtain the storage modulus (E′) and loss tan(δ) curves. Tensile mechanical properties of the neat epoxy and epoxy composites after cure were measured by a universal testing machine (AGS-F from SHIMADZU, Japan) according to the ASTM D-638 (type V specimens) method at a crosshead speed of 0.5 mm/min. The dumbbell-shaped specimens had an overall length of 63.5 mm and the gauge length was 7.62 mm. In addition to the initial modulus (E), ultimate tensile strength (UTS) and elongation at break (EB), the tensile toughness (TT) was also calculated by integrating the area under stress-strain curve as an indication of energy absorption capability. Five specimens were tested for each sample.
epoxy resin was added into the mixture by mechanical agitation. The well-mixed mixtures were coated on slides by a blade coater with a thickness of about 0.30 mm. They were cured at 25 °C for 7 days and then subjected to post-cure by further heating at 100 °C for 1 h and another 1 h at 130 °C. All samples were cut into rectangular-shape or dumbbell-shape specimens for the measurements of thermal and mechanical properties. 2.4. Structure and morphology of the PSS-PANI/rGO and epoxy/PSSPANI/rGO Structures of the PSS-PANI/rGO composite filler and its individual components were analyzed by Fourier transform infrared spectroscopy (FTIR, Spectrum 100 from Perkin Elmer, USA) to acquire the information of their functional groups and interactions among various components. Samples were ground with KBr powder and pressed into disks. FTIR spectra were then obtained by transmittance mode from 4000 to 400 cm−1 with a resolution of 4 cm−1. The arrangements of crystalline planes of the rGO and PSS-PANI/rGO were examined by an X-ray diffractometer (Ultima IV from Rigaku, USA) to obtain their diffraction patterns. Moreover, the morphology of the rGO and PSSPANI/rGO platelets as well as the cryo-fractured surface of the epoxy composite films were all observed by a scanning electron microscope (SEM, Nova NanoSEM 230, FEI, Japan).
2.6. Anticorrosion performance of the epoxy composite coatings Potentiodynamic polarization technique was used in polarization of specimens for the corrosion test. Carbon steel sheets (85 × 10 × 0.1 mm3) were first polished by SiC-600 paper, washed with acetone, blow-dried and affixed on slides (Supplementary Fig. S1). Specimens for polarization test were prepared by well-mixing the epoxy resin, filler and the curing agent by mechanical agitation. They were then coated on the carbon steel sheets with a thickness of ca. 50 μm using a coating rod. The coatings were cured at 25 °C for 7 days and post-cured at 100 °C for 1 h followed by another 1 h at 130 °C. The coated carbon steel sheets were pasted with insulating tapes and then immersed in the 5 wt% NaCl solution for 24 h. Their anticorrosion performance was measured by an electrochemical workstation (CHI440 from CH Instruments, USA) using a linear sweep voltammetry program (Autolab NOVA 1.11, Metrohm) at a scan rate of 0.05 V/s from −2.0 to 1.0 V. Platinum sheet and Ag|Ag+ electrode were used as the respective counter and reference electrodes. Furthermore, surface structures of the carbon steels being coated with various epoxy composites were analyzed. The coated carbon steels were first immersed in the 5 wt% NaCl solution for 7 days, and the coatings were then removed to expose their surfaces. The carbon steel sheets were cut into small pieces and analyzed by X-ray photoelectron spectroscopy (XPS Theta Probe, ThermoFisher Scientific, USA) to analyze their surface elements and verify the anticorrosion effect.
2.5. Thermal and mechanical properties of the epoxy composites The effects of the rGO and PSS-PANI/rGO fillers on the curing extent of the epoxy resin were investigated by differential scanning calorimetry (DSC, Q20 from TA, USA). Samples were prepared by mixing the epoxy resin and curing agent with or without the filler, and they were immediately transferred to the DSC for thermal scan from 0 °C to 180 °C at a heating rate of 10 °C/min. The reaction heat was then determined from the integration area of exothermic peak. Furthermore, glass transition temperature (Tg) of the pristine epoxy and epoxy composites after cure were also determined by DSC. Samples were first heated from 40 °C to 180 °C at a heating rate of 10 °C/min. They were cooled down to 40 °C and then re-heated again to 180 °C at the same heating rate. The Tg was determined from the second heating curve. Dynamic mechanical properties of the neat epoxy and epoxy composite films were measured by a dynamic mechanical analyzer (DMA, 3
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Fig. 2. XRD patterns of the (a) rGO, and (b) PSS-PANI/rGO platelets.
3. Results and discussion
covered with PSS-PANI.
3.1. Structure and morphology of the PSS-PANI/rGO
3.2. Curing of the epoxy and epoxy composites
The PANI was synthesized by oxidation polymerization in the PSS/ rGO aqueous dispersion. As shown in Fig. 1, successful polymerization of aniline was confirmed by FTIR analysis. For the original rGO platelets, the peaks at 1571 and 1174 cm−1 were attributed to the skeletal vibration of C]C and stretching vibration of CeO(H), respectively; on the other hand, the produced PSS-PANI/rGO platelets exhibited another two peaks at 1497 and 1581 cm−1 which could be ascribed to the skeletal vibrations of benzene and quinone rings in the PANI component. Moreover, the peaks appeared at 1010 and 1040 cm−1 for the PSS-PANI/rGO platelets were assigned to the in-plane skeleton vibration of the benzene ring and the symmetric vibration of SO3– group in the PSS component. The characteristic peaks of the produced PSSPANI/rGO confirmed the successful synthesis of the PANI. Moreover, XRD patterns of the rGO and PSS-PANI/rGO were obtained for analyzing their structures. As shown in Fig. 2(a), a broad diffraction peak centered at 2θ = 24.95° was observed for the rGO platelets which was equivalent to the interlayer spacing of 0.36 nm according to the Bragg's law. Zhang et al. [41] synthesized the rGO by modified Hummers method followed by hydrazine reduction and also found the diffraction peak around 2θ = 24.6°. As for the PSS-PANI/rGO, the diffraction peak of the rGO component remained at the same position, indicating that the interspacing of the platelets did not change. Moreover, there were diffraction peaks at 2θ = 21.32°, 29.96° and 31.00° corresponding to the (100), (211), and (020) planes of the PANI [42, 43]. These diffraction peaks suggested that highly ordered structure of PANI was present in the PSS-PANI/rGO composite. The morphologies of the as-prepared PSS-PANI/rGO after directly drying in the oven along with the original rGO powders are shown in Fig. 3. The average lateral size of the rGO platelets was about 8.5 ± 1.7 μm, while that of PSS-PANI/rGO platelets was about 11.5 ± 2.1 μm. The larger size of the PSS-PANI/rGO indicated that the dispersed rGO platelets were wrapped by the PSS-PANI and some of them might agglomerate during the drying process. Furthermore, the thickness of the rGO platelets was about 50 nm and increased to 100 nm for the PSS-PANI/rGO. The larger thickness and rougher surface of the PSS-PANI/rGO platelets further confirmed that the rGO platelets were
The rGO and PSS-PANI/rGO were then added into the epoxy resin separately as the potential reinforcing filler as well as the anticorrosion agent. However, their presence would have effects on the curing behavior of the epoxy resin. DSC was used to evaluate the curing behavior of the epoxy composites by simply measuring their curing heats which were determined from the integration areas under the exothermic peaks recorded from 0 to 180 °C at a heating rate of 10 °C/min. For comparison, we calculated the curing extents of all epoxy composites by dividing their curing heats with that of pristine epoxy. As shown in Fig. 4, all the epoxy composites had curing extents less than 100%, indicating their curing heats were lower than that of the pristine epoxy (280.2 J/ g). For the epoxy composite filled with rGO, the curing extent was 95.1% at the loading of 0.1 wt%, and it gradually decreased to 91.3% as the rGO loading increased to 1.0 wt%. The presence of rGO platelets thus hindered the curing of epoxy by increasing the viscosity of matrix and reducing the contact probability of reactants. Yet, the epoxy/PSSPANI/rGO composites had higher curing extents than the respective epoxy/rGO composites at the same loading content. For example, at the loading of 0.1 wt%, the curing extent of the epoxy/PSS-PANI/rGO was 97.6%, ca. 2.5% higher than that of the epoxy/rGO. It has been suggested that the reaction between the PANI and epoxy could occur at high temperatures [44–46]. By using the DSC technique, Palaniappan et al. [45] studied the curing kinetics of the epoxy with polyaniline. They found that the curing reactions of the epoxy and polyaniline could occur which followed an autocatalytic kinetics in the kinetic-control stage. Jang et al. [46] synthesized PANI nanorods and incorporated them into a liquid crystalline epoxy as curing agent. They found that the heat of cure was proportional to the content of PANI nanorods. To confirm the reaction between the PANI and epoxy resin, the PSS-PANI/ rGO was well-dispersed in the epoxy resin at 15 wt% without the addition of curing agent. The mixture was then maintained at 130 °C for 1 h. As shown in Fig. 5, the peaks of the epoxy/PSS-PANI/rGO mixture at 3497 and 915 cm−1, which were attributed to the respective stretching vibration of N-H in the PANI and the C-O deformation of epoxide group in the epoxy resin, were both reduced after heating at high temperature. This suggests that the reaction occurred between the 4
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Fig. 3. SEM images of the (a) rGO, and (b) PSSPANI/rGO platelets.
could be prepared with the composite filler up to 1.0 wt%.
epoxy resin and PANI. Cryo-fractured surfaces of the epoxy and epoxy composites after cure were examined by SEM as shown in Fig. 6. The smooth surface of the pristine epoxy resin shown in Fig. 6(a) suggested low capacity of energy absorption during crack propagation. In other words, this epoxy was intrinsically brittle. On the contrary, the epoxy/rGO composite presented a rougher fracture surface as revealed in Fig. 6(b) and 6(c). The impenetrable rGO fillers retarded the crack propagation by inducing the deflection and pinning of crack fronts. In addition, the rGO platelets in the epoxy matrix had almost the same size as the original rGO, indicating that the rGO did not agglomerate at this loading of only 0.5 wt%. Yet, microvoids were found near the rGO platelets, indicating weak interfacial bonding between the rGO and epoxy matrix. When the loading of rGO reached 1.0 wt%, the interfacial bonding became so weak that cracks started to appear on the composite film right after the curing process. Therefore, we could only prepare the epoxy composites with the rGO loading up to 0.5 wt% in this work. On the other hand, no microvoids were found on the fracture surface of the epoxy/PSS-PANI/ rGO composites as shown in Fig. 6(d)–(f). Stronger interfacial bonding was thus expected to be present between the PSS-PANI/rGO platelets and epoxy matrix. This is because the rGO platelets were covered with the PSS-PANI where the PANI could react with the epoxy as suggested from the previous results. The epoxy/PSS-PANI/rGO composites thus
3.3. Thermal and mechanical properties of the epoxy and epoxy composites Dynamic mechanical thermal properties including storage modulus (E′) and loss tan(δ) of the epoxy and epoxy composites after cure were measured at a frequency of 1 Hz as shown in Fig. 7. The peak temperature of the loss tan(δ) curve was defined as the glass transition temperature (Tg) of material. Moreover, The Tg values of the epoxy composites were also determined from the second heating curves in DSC. It is known that the Tg value represents the flexibility or rigidity of polymer chains. The higher the Tg, the higher the chain rigidity is. In an epoxy composite system, the Tg depends on the crosslinking density or curing extent, the type and content of filler and the interfacial bonding between the filler and matrix. The Tg values of the epoxy and epoxy composites are summarized in Table 1. Although all of the Tg values determined from DMA were higher than the respective Tg values of the epoxy composites at the same loading content, their changes with the filler loading were similar. After post cure at high temperatures, the neat epoxy had a Tg,DMA of 114 °C, and the addition of rGO or PSSPANI/rGO could raise the Tg of the material. The segmental motion of epoxy chains was thus restricted by the presence of more rigid fillers. However, the Tg increased with increasing the filler content only up to a Fig. 4. Curing extents of the epoxy/rGO and epoxy/PSS-PANI/ rGO composites. The weight ratio of the curing agent solution to the epoxy resin was 11/20 for all the samples.
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Fig. 5. FT-IR spectra of the epoxy/PSS-PANI/rGO without the addition of curing agent: (a) after heating for 1 h at 130 °C and (b) before heating. The PSS-PANI/rGO was added at 15 wt%.
Fig. 6. SEM images of the freeze-fractured surfaces of (a) neat epoxy; and epoxy composites filled with rGO at (b) 0.1 wt%, and (c) 0.5 wt%, and PSS-PANI/rGO at (d) 0.1 wt %, (e) 0.5 wt%, and (f) 1.0 wt%.
due to the weak interfacial bonding and the presence of microvoids around the rGO fillers. However, the decline in the E′ was not found for the epoxy/PSS-PANI/rGO composite when the PSS-PANI/rGO content was increased from 0.1 wt% to 0.5 wt%. In other words, improved modulus was still observed for the epoxy composite filled with 0.5 wt% PSS-PANI/rGO, though the improvement was not pronounced. As we compare the epoxy/PSS-PANI/rGO and epoxy/rGO systems at the same loading content of 0.5 wt%, there is a large difference between their E′ values. Table 2 shows that the E′ of the epoxy/PSS-PANI/rGO at 0.5 wt % loading is 2.34 GPa, while the value of the epoxy/rGO at the same loading is 1.88 GPa, even lower than the value of the neat epoxy. This is because further addition of PSS-PANI could increase the interfacial strength between the filler and the epoxy matrix as proved in the previous section. Yet, increasing the PSS-PANI/rGO content up to 1.0 wt%, a decrement in storage modulus was still found. This is probably caused by the decreased crosslinking density and the agglomeration of PSSPANI/rGO fillers in the epoxy matrix. Tensile mechanical properties of the epoxy and epoxy composites
certain point, after that it started to decrease. This is because the Tg not only depends on the presence of rigid filler but also on the crosslinking density or the curing extent of the epoxy. The previous results showed that the curing extent decreased with increasing the filler extent. In other words, the higher the filler content, the lower the crosslinking density. Therefore, as the filler content increased to a certain point, the effect of decreased crosslinking density would surpass that of the restricted segmental motion by the filler. Dynamic mechanical analysis also provided the storage modulus data (E′) of epoxy composites. The E′ at 30 °C of the neat epoxy and epoxy composites are also summarized in Table 1. By the addition of only 0.1 wt% rGO, the E′ was increased by 13.3% when compared to that of the neat epoxy. Yet, a further increase of the rGO content to 0.5 wt%, a decline in the E′ for the epoxy/rGO composite was observed. It is well known that mechanical properties of composite materials depend on the type, shape and the content of filler, and also the interfacial bonding between the filler and matrix. Therefore, the decrease in the modulus for the epoxy composite filled with 0.5 wt% rGO might 6
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Fig. 7. Dynamic mechanical properties including storage modulus (E′) and loss tan(δ) of the (a) epoxy/rGO, and (b) epoxy/PSS-PANI/ rGO composites at different loadings of fillers. Samples were cured at 25 °C for 7 days and followed by post-cure at 100 °C for 1 h and 130 °C for another hour.
Table 1 Glass transition temperature (Tg) and storage modulus (E′ at 30 °C) of the epoxy composites filled with different loadings of rGO and PSS-PANI/rGO. Sample
Filler (wt%)
Tg,DSC (°C)
Tg,DMA (°C)
E′ @30 °C (GPa)
Epoxy Epoxy/rGO
0 0.1 0.5 0.1 0.5 1.0
98 115 114 110 108 108
114 124 121 121 116 118
2.10 2.38 1.88 2.31 2.34 1.56
Epoxy/PSS-PANI/ rGO
Table 2 Tensile mechanical properties including initial modulus (E), ultimate tensile strength (UTS), elongation at break (EB) and tensile toughness (TT) of the epoxy composites filled with rGO and PSS-PANI/rGO at different loadings. Sample
Filler (wt%)
E (GPa)
Epoxy Epoxy/rGO
0 0.1 0.5 0.1 0.5 1.0
1.55 1.83 1.72 1.69 1.89 1.69
Epoxy/PSSPANI/ rGO
Samples were cured at 25 °C for 7 days, followed by post-cure at 100 °C for 1 h and 130 °C for another hour.
± ± ± ± ± ±
UTS (MPa) 0.05 0.26 0.29 0.11 0.04 0.15
43.1 49.1 44.5 59.2 64.9 51.4
± ± ± ± ± ±
EB (%) 6.7 9.9 10.2 7.1 6.0 12.3
3.6 4.0 3.5 5.2 5.0 3.5
TT (J/m3) ± ± ± ± ± ±
0.2 0.5 1.5 1.0 1.2 1.1
562 ± 93 774 ± 124 677 ± 330 1229 ± 228 1273 ± 344 678 ± 362
Samples were cured at 25 °C for 7 days and followed by post-cure at 100 °C for 1 h and 130 °C for another hour.
after cure were also measured in this study. Their stress–strain curves are presented in Fig. 8 where initial modulus (E), ultimate tensile strength (UTS), elongation at break (EB) and tensile toughness (TT) were all calculated. The tensile toughness was the absorbed energy per unit volume of the sample being extended to fracture by tensile test. The results are summarized in Table 2. It can be seen that similar trends were observed for the tensile mechanical properties as those found in the storage modulus data. The tensile mechanical properties of the epoxy composite added with the rGO at 0.1 wt% were higher than those of the neat epoxy, but became lower when the addition amount of rGO was increased to 0.5 wt%. This also can be ascribed to the weak
interfacial bonding and the microvoids around the rGO fillers. Owing to the stronger interfacial bonding in the epoxy/PSS-PANI/rGO composites, improved tensile mechanical properties were still observed for the epoxy composites when the PSS-PANI/rGO content was increased from 0.1 to 0.5 wt%. For example, the initial modulus was increased from 1.55 GPa for the neat epoxy to 1.69 GPa for the epoxy composite filled with 0.1 wt% PSS-PANI/rGO, and further increased to 1.89 GPa for the composite with 0.5 wt%. The ultimate tensile strength was also increased from the value of 43.1 MPa for the neat epoxy to 64.9 MPa for
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Fig. 8. Tensile stress-strain curves of the (a) epoxy/ rGO, and (b) epoxy/PSS-PANI/rGO composites after cure.
the epoxy composite filled with 0.5 wt% PSS-PANI/rGO. The increases in the initial modulus and ultimate tensile strength for the epoxy composite filled with 0.5 wt% PSS-PANI/rGO were thus about 22% and 51%, respectively, when compared to the values of the neat epoxy. Moreover, its elongation at break and tensile toughness were 39 and 127% higher than the respective values of the pristine epoxy. This is because the polyaniline on the surface of PSS-PANI/rGO platelets could react with the epoxy matrix during the cure. Interfacial bonding was thus enhanced greatly and enabled the transfer of loading stress to the platelets. Wang et al. [47] tried to prepare the epoxy composites filled with the GO and DGEBA-functionalized GO. They also found that without surface functionalization, the interfacial strength was weak between the GO filler and epoxy matrix; on the other hand, the compatibility and dispersion of GO sheets in epoxy matrix were effectively improved when the GO was surface-functionalized with the DGEBA. Xu and Gao [7] synthesized graphene-reinforced nylon-6 and reported that during polymerization, not only the GO was thermally reduced to graphene, but some nylon-6 chains were grafted on the graphene sheets. As a result, the graphene sheets could be homogeneously dispersed in the nylon-6 matrix. Naebe et al. [11] ascribed the enhancement of mechanical properties mainly to the uniform dispersion of functionalized graphene and strong interfacial bonding between the modified graphene and epoxy matrix. Yet, when the PSS-PANI/rGO loading was increased further to 1.0 wt%, the tensile mechanical properties of the epoxy composite became lower as shown in Table 2, which was similar to the previous observation in the dynamic mechanical properties. This is also ascribed to the decreased crosslinking density and agglomeration of PSS-PANI/rGO platelets.
CR =
Potentiodynamic polarization curves of the carbon steel electrodes coated with the epoxy and epoxy composites in the 5 wt% NaCl aqueous solution are presented in Fig. 9. The anticorrosion properties of the epoxy and epoxy composites could be evaluated from the corrosion potential (Ecorr), corrosion current density (Icorr), polarization resistance (Rp), and corrosion rate (CR). The Ecorr (V) and Icorr (μA/cm2) were determined by the intersection of the linear portions of the anodic and cathodic curves in the Tafel plots. According to Stern-Geary equation [48], the Rp could be calculated by the following equation:
βa βc 1 Icorr 2.303(βa + βc )
(2)
where k is a constant (3268.5 mol/A), M is the molecular weight of iron (56 g/mol), ρm is the density of iron (7.85 g/cm3), Icorr,b is the corrosion current density of bare carbon steel, and Icorr,c is the corrosion current density of the coated steel electrodes. The results are summarized in Table 3. Generally, higher values of Ecorr, Rp and lower values of Icorr and CR suggest better anticorrosion performance of the coatings. The bare carbon steel had high values of corrosion current density (Icorr) and corrosion rate (CR) and thus was easily subjected to corrosion. When it was coated with the neat epoxy, the Icorr and CR were decreased greatly to about 37.1 μA/cm2 and 0.865 mm/year, respectively, indicating a good physical insulation of epoxy coating. By introducing the rGO into the epoxy coating, the anticorrosion properties were further improved. In addition, the higher the rGO content, the higher the anticorrosion efficiency. At a loading of 0.5 wt%, the Icorr and CR of the epoxy/rGO coating decreased to only 0.093 μA/cm2 and 2.2 μm/year, respectively. According to literatures [16, 35], the presence of graphene platelets can effectively improve the protection efficiency by prolonging the tortuosity of diffusion pathways for corrosive reactants, such as O2 and Cl–. Furthermore, the epoxy/ PSS-PANI/rGO coatings had higher values of Ecorr than the neat epoxy and epoxy/rGO coatings. The positive shift of Ecorr was attributed not only to the stronger interfacial bonding between the epoxy and PANIcontaining fillers, but also to the passivation effect of PANI. Although the content of PANI in the epoxy/PSS-PANI/rGO coating might not be high, the good dispersion and high specific surface area of the PSSPANI/rGO platelets still made it possible that some PANI could directly attach to the carbon steel surface. It has been reported [21, 49] that once the PANI could contact with the iron surface, it would act as a redox catalyst, catalyzing the oxidation reaction on the iron surface, and forming a dense passivation layer that was composed of Fe2O3 and Fe3O4. The dense Fe3O4 passivation layer would cause a positive shift of the corrosion potential in the polarization curve and also protect the metal beneath the layer from further corrosion. Recent researches also show that the coatings containing only 1–3 wt% of PANI can make notable improvement in corrosion protection [30, 31]. In this work, the epoxy/PSS-PANI/rGO coating at a loading of 0.5 wt% exhibited the best anticorrosion performance among all the coatings. The anticorrosion efficiency became lower when the PSS-PANI/rGO loading in the epoxy composite was increased to 1.0 wt%, which was ascribed to the poor dispersion of filler. In literatures, Chang et al. [9] introduced graphene into epoxy at 1 wt% and reduced the corrosion current of epoxy-coated cold-rolled steel by 71%. By adding 7 wt% of PANI into epoxy, Radhakrishnan et al. [50] also reduced the corrosion current of epoxycoated stainless steel by one order. In our work, the corrosion current of the carbon steel coated with the epoxy/PSS-PANI/rGO (0.5 wt% in epoxy) was decreased by more than 3 orders of the value of the carbon
3.4. Anticorrosion evaluation
Rp =
kMIcorr ρm
(1)
where βa and βc are the reciprocals of the anodic and cathodic slopes: ΔE/Δ(log I), determined at the points + 70 and −70 mV from the lowest turning point, respectively. Furthermore, the CR of the specimens could be estimated as: 8
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Fig. 9. Tafel plots for the bare carbon steel and carbon steel electrodes coated with the neat epoxy and various epoxy composites.
Table 3 Electrochemical corrosion measurements of the bare carbon steel and epoxy, epoxy/rGO and epoxy/PSS-PANI/rGO coated carbon steels. Coating
Filler (wt%)
Ecorr (V)
Icorr (μA/cm2)
Rp (Ω)
CR (mm/year)
Bare carbon steel Epoxy Epoxy/rGO
– – 0.1 0.5 0.1 0.5 1.0
−1.37 −1.38 −1.43 −1.29 −1.21 −1.04 −1.17
321 37.1 0.831 9.32 × 10−2 1.28 5.19 × 10−3 6.52 × 10−2
7.38 × 10−5 5.74 × 10−3 0.323 2.56 0.172 55.0 4.78
74.8 0.865 1.94 × 10−2 2.17 × 10−3 2.98 × 10−2 1.21 × 10−4 1.52 × 10−3
Epoxy/PSS-PANI/ rGO
steel coated with the neat epoxy, indicating great improvement of corrosion protection. In order to understand the anticorrosion mechanism, XPS was used to analyze the surface structures of the carbon steels with various epoxy coatings. The coated carbon steels were soaked in the 5 wt% NaCl aqueous solution for 7 days, and the coatings were then removed to expose the surface structures of carbon steels for analysis. Fig. 10 shows their XPS spectra. The peaks at 710.4 eV and 724.0 eV were observed for all the bare carbon steel and carbon steels being coating with the epoxy and epoxy/rGO, which represented the Fe 2p3/2 and 2p1/2 peaks of Fe3+ ion [51], while an additional peak at 708.15 eV was found for the carbon steel coated with the epoxy/PSS-PANI/rGO. It was ascribed to the Fe 2p3/2 peaks of Fe2+ ion [51–53]. The results of XPS spectra thus confirmed the presence of Fe3O4 passivation layer only in the epoxy/PSS-PANI/rGO-coated carbon steel electrode, leading to the positive shift of the Ecorr. In summary, the anticorrosion mechanisms proposed for the epoxy/PSS-PANI/rGO coating were composed of the increased tortuosity, strong interfacial bonding between the filler and matrix, and passivation layer caused by the presence of PSS-PANI/rGO platelets as illustrated in Fig. 11.
Fig. 10. XPS spectra of the carbon steel surfaces which had been coated with the neat epoxy and epoxy composites and subsequently soaked in the 5 wt% NaCl aqueous solution for 7 days.
resin at different loadings to prepare epoxy composites. As measured by DSC, DMA, and tensile machine, the epoxy/PSS-PANI/rGO composite had better thermal and mechanical properties than the neat epoxy and epoxy/rGO, indicating that the PSS-PANI/rGO platelets were more compatible with the epoxy matrix than the rGO, upon which the PANI on the surface of PSS-PANI/rGO platelets could participate in the curing reaction of epoxy resin. The enhanced interfacial strength and the stiffness provided by the PSS-PANI/rGO platelets improved the tensile toughness by 127% for the epoxy/PSS-PANI/rGO at a loading of 0.5 wt %, when compared to that of the pristine epoxy. Moreover, in the potentiodynamic polarization analysis, the presence of rGO filler in the epoxy/rGO coatings could reduce the corrosion current density by increasing the tortuosity of diffusion pathways for gas and ions; whereas the carbon steel coated with epoxy/PSS-PANI/rGO coatings exhibited even higher corrosion potential and lower corrosion rate. The XPS spectra of the carbon steel surface confirmed the existence of Fe3O4 layer, which was ascribed to the passivation of carbon steel surface by
4. Conclusions In this study, the PSS-PANI/rGO composite was fabricated via insitu redox polymerization of aniline on the surface of PSS-dispersed rGO platelets. The structural analyses from FTIR spectra, XRD patterns and SEM pictures confirmed that the surface of rGO platelets was covered uniformly by the PSS-PANI. The rGO and PSS-PANI/rGO platelets were then used as the fillers to be blended with the bisphenol-A type epoxy 9
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Fig. 11. Schematic diagram of the proposed anticorrosion mechanism of epoxy composite coatings
PANI. The superior anticorrosion performance of the epoxy/PSS-PANI/ rGO coatings was due to the more compact filler-matrix interface and the presence of passivation layer. In summary, the incorporation of PSSPANI/rGO in epoxy composite enhanced the tensile toughness of the material and provided promising potential in anticorrosion applications. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2018.01.050.
sheets, Nano Lett. 8 (2008) 2458–2462. [16] B. Ramezanzadeh, S. Niroumandrad, A. Ahmadi, M. Mahdavian, M.H.M. Moghadam, Enhancement of barrier and corrosion protection performance of an epoxy coating through wet transfer of amino functionalized graphene oxide, Corros. Sci. 103 (2016) 283–304. [17] M. Schriver, W. Regan, W.J. Gannett, A.M. Zaniewski, M.F. Crommie, A. Zettl, Graphene as a long-term metal oxidation barrier: worse than nothing, ACS Nano 7 (2013) 5763–5768. [18] F. Zhou, Z. Li, G.J. Shenoy, L. Li, H. Liu, Enhanced room-temperature corrosion of copper in the presence of graphene, ACS Nano 7 (2013) 6939–6947. [19] W. Sun, L.D. Wang, T.T. Wu, Y.Q. Pan, G.C. Liu, Synthesis of low-electrical-conductivity graphene/pernigraniline composites and their application in corrosion protection, Carbon 79 (2014) 605–614. [20] B. Wessling, J. Posdorfer, Corrosion prevention with an organic metal (polyaniline): corrosion test results, Electrochim. Acta 44 (1999) 2139–2147. [21] B. Wessling, Corrosion prevention with an organic metal (polyaniline): surface ennobling, passivation, corrosion test results, Mater. Corros. 47 (1996) 439–445. [22] W.-K. Lu, R.L. Elsenbaumer, B. Wessling, Corrosion protection of mild steel by coatings containing polyaniline, Synth. Met. 71 (1995) 2163–2166. [23] P.J. Kinlen, Y. Ding, D.C. Silverman, Corrosion protection of mild steel using sulfonic and phosphonic acid-doped polyanilines, Corrosion 58 (2002) 490–497. [24] R.M. Torresi, S. de Souza, J.E.P. da Silvaa, S.I.C. de Torresi, Galvanic coupling between metal substrate and polyaniline acrylic blends: corrosion protection mechanism, Electrochim. Acta 50 (2005) 2213–2218. [25] P.J. Kinlen, A mechanistic investigation of polyaniline corrosion protection using the scanning reference electrode technique, J. Electrochem. Soc. 146 (1999) 3690. [26] L. Zhong, H. Zhu, J. Hu, S.H. Xiao, F.X. Gan, A passivation mechanism of doped polyaniline on 410 stainless steel in deaerated H2SO4 solution, Electrochim. Acta 51 (2006) 5494–5501. [27] B. Wessling, Passivation of metals by coating with polyaniline: corrosion potential shift and morphological changes, Adv. Mater. 6 (1994) 226–228. [28] D.W. DeBerry, Modification of the electrochemical and corrosion behavior of stainless steels with an electroactive coating, J. Electrochem. Soc. 132 (1985) 1022. [29] G. Mengoli, M.T. Munari, P. Bianco, M.M. Musiani, Anodic synthesis of polyaniline coatings onto Fe sheets, J. Appl. Polym. Sci. 26 (1981) 4247–4257. [30] A. Sakhri, F.X. Perrin, E. Aragon, S. Lamouric, A. Benaboura, Chlorinated rubber paints for corrosion prevention of mild steel: a comparison between zinc phosphate and polyaniline pigments, Corros. Sci. 52 (2010) 901–909. [31] H. Zhang, J. Wang, X. Liu, Z. Wang, S. Wang, High performance self-healing epoxy/ polyamide protective coating containing epoxy microcapsules and polyaniline nanofibers for mild carbon steel, Ind. Eng. Chem. Res. 52 (2013) 10172–10180. [32] M.F. Montemor, Functional and smart coatings for corrosion protection: a review of recent advances, Surf. Coat. Technol. 258 (2014) 17–37. [33] L.H.E. Santos, J.S.C. Branco, I.S. Guimarães, A.J. Motheo, Synthesis in phytic acid medium and application as anticorrosive coatings of polyaniline-based materials, Surf. Coat. Technol. 275 (2015) 26–31. [34] Y. Jafari, S.M. Ghoreishi, M. Shabani-Nooshabadi, Polyaniline/graphene nanocomposite coatings on copper: electropolymerization, characterization, and evaluation of corrosion protection performance, Synth. Met. 217 (2016) 220–230. [35] C.H. Chang, T.C. Huang, C.W. Peng, T.C. Yeh, H.I. Lu, W.I. Hung, C.J. Weng, T.I. Yang, J.M. Yeh, Novel anticorrosion coatings prepared from polyaniline/graphene composites, Carbon 50 (2012) 5044–5051. [36] P.A. Sørensen, S. Kiil, K. Dam-Johansen, C.E. Weinell, Anticorrosive coatings: a review, J. Coat. Technol. Res. 6 (2009) 135–176. [37] N. Morimoto, T. Kubo, Y. Nishina, Tailoring the oxygen content of graphite and reduced graphene oxide for specific applications, Sci. Rep. 6 (2016) 21715.
References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] H.B. Heersche, P. Jarillo-Herrero, J.B. Oostinga, L.M.K. Vandersypen, A.F. Morpurgo, Bipolar supercurrent in graphene, Nature 446 (2007) 56–59. [3] A.A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C.N. Lau, Superior thermal conductivity of single-layer graphene, Nano Lett. 8 (2008) 902–907. [4] C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the elastic properties and intrinsic strength of monolayer graphene, Science 321 (2008) 385–388. [5] R.R. Nair, P. Blake, A.N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, N.M.R. Peres, A.K. Geim, Fine structure constant defines visual transparency of graphene, Science 320 (2008) 1308. [6] J. Liang, Y. Huang, L. Zhang, Y. Wang, Y. Ma, T. Guo, Y. Chen, Molecular-level dispersion of graphene into poly(vinyl alcohol) and effective reinforcement of their nanocomposites, Adv. Funct. Mater. 19 (2009) 2297–2302. [7] Z. Xu, C. Gao, In situ polymerization approach to graphene-reinforced Nylon-6 composites, Macromolecules 43 (2010) 6716–6723. [8] U. Vietri, L. Guadagno, M. Raimondo, L. Vertuccio, K. Lafdi, Nanofilled epoxy adhesive for structural aeronautic materials, Compos. Part B 61 (2014) 73–83. [9] K.-C. Chang, M.-H. Hsu, H.-I. Lu, M.-C. Lai, P.-J. Liu, C.-H. Hsu, W.-F. Ji, T.L. Chuang, Y. Wei, J.-M. Yeh, W.-R. Liu, Room-temperature cured hydrophobic epoxy/graphene composites as corrosion inhibitor for cold-rolled steel, Carbon 66 (2014) 144–153. [10] S. Komori, Y. Sakamoto, Development trend of epoxy molding compound for encapsulating semiconductor chips, in: D. Lu, C.P. Wong (Eds.), Materials for Advanced Packaging, Springer US, Boston, MA, 2009, pp. 339–363. [11] M. Naebe, J. Wang, A. Amini, H. Khayyam, N. Hameed, L.H. Li, Y. Chen, B. Fox, Mechanical property and structure of covalent functionalised graphene/epoxy nanocomposites, Sci. Rep. 4 (2014) 4375. [12] D.R. Bortz, E.G. Heras, I. Martin-Gullon, Impressive fatigue life and fracture toughness improvements in graphene oxide/epoxy composites, Macromolecules 45 (2012) 238–245. [13] M.A. Rafiee, J. Rafiee, Z. Wang, H. Song, Z.-Z. Yu, N. Koratkar, Enhanced mechanical properties of nanocomposites at low graphene content, ACS Nano 3 (2009) 3884–3890. [14] N. Domun, H. Hadavinia, T. Zhang, T. Sainsbury, G.H. Liaghat, S. Vahid, Improving the fracture toughness and the strength of epoxy using nanomaterials - a review of the current status, Nano 7 (2015) 10294–10329. [15] J.S. Bunch, S.S. Verbridge, J.S. Alden, A.M. van der Zande, J.M. Parpia, H.G. Craighead, P.L. McEuen, Impermeable atomic membranes from graphene
10
Surface & Coatings Technology xxx (xxxx) xxx–xxx
Y.-T. Lin et al.
[46] J. Jang, J. Bae, K. Lee, Synthesis and characterization of polyaniline nanorods as curing agent and nanofiller for epoxy matrix composite, Polymer 46 (2005) 3677–3684. [47] F. Wang, L.T. Drzal, Y. Qin, Z. Huang, Mechanical properties and thermal conductivity of graphene nanoplatelet/epoxy composites, J. Mater. Sci. 50 (2015) 1082–1093. [48] A.J. Bard, L.R. Faulkner, J. Leddy, C.G. Zoski, Electrochemical Methods: Fundamentals and Applications, Vol. 2 Wiley, New York, 1980. [49] D.E. Tallman, G. Spinks, A. Dominis, G.G. Wallace, Electroactive conducting polymers for corrosion control, J. Solid State Electrochem. 6 (2002) 73–84. [50] S. Radhakrishnan, N. Sonawane, C.R. Siju, Epoxy powder coatings containing polyaniline for enhanced corrosion protection, Prog. Org. Coat. 64 (2009) 383–386. [51] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials, Appl. Surf. Sci. 254 (2008) 2441–2449. [52] S.G. Wang, G. Han, G.H. Yu, Y. Jiang, C. Wang, A. Kohn, R.C.C. Ward, Evidence for FeO formation at the Fe/MgO interface in epitaxial TMR structure by X-ray photoelectron spectroscopy, J. Magn. Magn. Mater. 310 (2007) 1935–1936. [53] B. Wessling, Scientific and commercial breakthrough for organic metals, Synth. Met. 85 (1997) 1313–1318.
[38] D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruoff, The chemistry of graphene oxide, Chem. Soc. Rev. 39 (2010) 228–240. [39] G. Wang, X. Shen, B. Wang, J. Yao, J. Park, Synthesis and characterisation of hydrophilic and organophilic graphene nanosheets, Carbon 47 (2009) 1359–1364. [40] S. Cho, J.S. Lee, J. Jun, S.G. Kim, J. Jang, Fabrication of water-dispersible and highly conductive PSS-doped PANI/graphene nanocomposites using a high-molecular weight PSS dopant and their application in H2S detection, Nano 6 (2014) 15181–15195. [41] K. Zhang, L.L. Zhang, X.S. Zhao, J.S. Wu, Graphene/polyaniline nanofiber composites as supercapacitor electrodes, Chem. Mater. 22 (2010) 1392–1401. [42] V.H. Nguyen, J.J. Shim, Green synthesis and characterization of carbon nanotubes/ polyaniline nanocomposites, J. Spectrosc. 2015 (2015) 1–9. [43] J. Pouget, M. Jozefowicz, A.e.a. Epstein, X. Tang, A. MacDiarmid, X-ray structure of polyaniline, Macromolecules 24 (1991) 779–789. [44] B. Sreedhar, S. Palaniappan, S. Narayanan, The effect of zeolite on the curing of epoxy resin by polyaniline: a kinetic study, Polym. Adv. Technol. 13 (2002) 459–465. [45] S. Palaniappan, B. Sreedhar, S.M. Nair, Polyaniline as a curing agent for epoxy resin: cure kinetics by differential scanning calorimetry, Macromol. Chem. Phys. 202 (2001) 1227–1231.
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