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Curing epoxy with electrochemically synthesized GdxFe3-xO4 magnetic nanoparticles
Progress in Organic Coatings 136 (2019) 105245
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Curing epoxy with electrochemically synthesized GdxFe3-xO4 magnetic nanoparticles
T
⁎
Maryam Jouyandeha, Payam Zarrintajb, Mohammad Reza Ganjalia,c, , Jagar A. Alid, ⁎ Isa Karimzadehe, Mustafa Aghazadeha, Mehdi Ghaffarif, Mohammad Reza Saebg, a
Center of Excellence in Electrochemistry, School of Chemistry, College of Science, University of Tehran, Tehran, Iran Polymer Engineering Department, Faculty of Engineering, Urmia University, P.O. Box 57561511818-165, Urmia, Iran c Biosensor Research Center, Endocrinology & Metabolism Molecular-Cellular Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran d Department of Petroleum Engineering, Faculty of Engineering, Soran University, Kurdistan Region, Iraq e Department of Physics, Bonab Branch, Islamic Azad University, Bonab, Iran f Polymer Group, Faculty of Technical and Engineering, Golestan University, P.O. Box 155, Gorgan, Golestan, Iran g Department of Resin and Additives, Institute for Color Science and Technology, P.O. Box: 16765-654, Tehran, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cure Index Epoxy Fe3O4 nanoparticle Gadolinium Oxidation-resistant coatings
In this work, supermagnetic lanthanide-doped Fe3O4 nanoparticles were synthesized through electrodeposition method by partial doping in gadolinium (Gd) to be intended in the near future in developing oxidation-resistant epoxy-based coatings. Partial substitution of Fe3+ ions in Fe3O4 by Gd3+ lanthanide cations was confirmed by XRay diffraction (XRD) patterns and Fourier-transform infrared spectroscopy (FTIR) spectra. Field-emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometer (VSM) were employed to evaluate particle size distribution and magnetic nature of the nanoparticles, respectively. Low-filled nanocomposites containing 0.1 wt.% of Fe3O4 and Gd3+-doped Fe3O4 were prepared and their curing ability was assessed in terms of qualitative measurements on network formation by the use of Cure Index. Having a wide range of curing behavior led to development of Poor, Good, and Excellent cured coatings depending on nanoparticle bulk composition and heating rate applied in the nonisothermal differential scanning calorimetry (DSC). Overall, incorporation of Fe3O4 into epoxy decreased exothermic heat release of epoxy, while Gd3+-doped nanoparticles facilitated curing reaction between epoxy and aliphatic amine curing agent. Moreover, glass transition temperature of the prepared nanocomposites decreased compared to the blank epoxy. The developed epoxy/lanthanide-doped Fe3O4 nanocomposites are potential candidates for developing oxidation-resistant coatings thanks to the presence of Gd3+ cations.
1. Introduction Properties of organic coatings depend closely on the possibility and facility of network formation through crosslinking reaction between the thermoset resin and curing agent [1]. The effects of pigmentation [2–6], nanoparticle incorporation [7–11], and non-covalent and covalent surface functionalization of nanoparticles [12] on network formation in organic coatings have been already discussed. On the other hand, tuning properties of organic coatings by the proper modification of bulk composition of nanoparticles was rarely reported. Fe3O4 nanoparticles have been in the core of attention in the last decades thanks to their extraordinary potential [13,14] for applications such as adsorbent [15,16], photocatalyst [17] and therapeutic treatments [18,19]. They were also considered for development of anti-corrosion films and ⁎
coatings [20–22]. Control over energy harvesting devices is of prime importance in designing systems [23,24]. In this sense, controlled oxidation of Fe3O4 nanoparticles for applications in electromagnetic devices was practised [25]. Lanthanide-doped Fe3O4 nanoparticles were examined in design and manufacture of magnetic–luminescent nanocomposites [25,26]. Controlling the atomic composition of dopants in magnetic nanoparticles by partial substitution of Fe2+ and Fe3+ cations by transition metals or lanthanides give rise to control over the crystallinity of such hybrid nanoparticles for optimized synthesis of supermagnetic nanomaterials [27,28]. Synthesis of nanohybrids of rare-earth elements with Fe3O4 nanoparticles having variable particle size was practised in some previous works [29,30]. However, the potential of rare-earth element cations doped in Fe3O4 nanoparticles for participating in curing
Corresponding authors. E-mail addresses: [email protected] (M.R. Ganjali), [email protected] (M.R. Saeb).
https://doi.org/10.1016/j.porgcoat.2019.105245 Received 11 July 2019; Received in revised form 18 July 2019; Accepted 19 July 2019 Available online 05 August 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 136 (2019) 105245
M. Jouyandeh, et al.
2.4. Characterization
reaction of epoxy with amine curing agents was not reported. In this work, supermagnetic gadolinium (Gd)-doped Fe3O4 nanoparticles were synthesized by electrodeposition method to be intended in near future in developing oxidation-resistant epoxy-based nanocomposite coatings. Partial substitution of Fe3+ ions in Fe3O4 structure by Gd3+ rare-earth cations was carried out electrochemically. X-Ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were employed to study change in bulk composition upon insertion of Gd3+ cations into Fe3O4 atomic structure. Field-emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometer (VSM) were furthermore used for the analysis of size/size distribution and magnetic properties of the nanoparticles, respectively. Nanocomposites containing 0.1 wt.% of Fe3O4 and Gd3+-doped Fe3O4 were subjected to curing at different heating rates through nonisothermal differential scanning calorimetry (DSC). Network formation in the resulting nanocomposites was quantified by the use of Cure Index to label them as Poor, Good, and Excellent cured systems. Calculation of glass transition temperature of epoxy and its nanocomposites containing superparamagnetic iron oxide nanoparticles (SPIONs) allowed for assessing the effect of bulk composition alteration on the physical properties of epoxy.
Morphological images of the deposited SPIONs were observed on a FE-SEM microscope, Model; Mira 3-XMU and accelerating voltage of 100 kV. FT-IR spectra were recorded through Bruker Vector 22 IR spectrometer in the wavelength range of 4000–400 cm−1. XRD patterns of the synthesized SPIONs were recorded by PW-1800 X-ray diffraction with a Co Kα radiation. Magnetic behaviors of the fabricated SPIONs particles were also specified in the range of −20,000 Oe to 20,000 Oe at RT condition using a VSM instrument, Model: lakeshore 7400. The prepared epoxy nanocomposites were analyzed for curing potential by DSC (Perkin Elmer DSC 4000) under dynamic mode to study the effect of SPIONs and Gd3+-doped SPIONs on the state of cure of epoxy/amine system. DSC analysis was performed under nonisothermal mode at heating rates (β) of 5, 10, 15, and 20 °C.min−1 from 15 to 250 °C under nitrogen atmosphere with the flow rate of 20 mL.min−1. Moreover, glass transition temperature (Tg) of epoxy and its nanocomposites was obtained at β of 10 °C/min through heating-cooling-heating cycle. For this purpose, samples were first heated up from 15 to 250 °C, then cooled down to ambient temperature, and again reheated up to 250 °C at 10 °C.min−1.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Structure and morphology
Ferric nitrate nonahydrate (Sigma-Aldrich, 99.9%), ferrous chloride tetrahydrate (Sigma-Aldrich, 99.5%) and gadolinium nitrate hexahydrate (Sigma-Aldrich, 99.8%) were purchased and used without further purification. Epoxy matrix (diglycidyl ether of bis-phenol A, Epon-828) with epoxide equivalent weight of 185–192 g/eq and curing agent (triethylenetetramine, TETA) with hydrogen equivalent weight of 25 g/ eq were supplied by Hexion, Beijing (China) and used in stoichiometric amount (resin:hardener = 100:13) in nanocomposite preparation.
The XRD patterns of the deposited SPIONs are displayed in Fig. 2. In both patterns, the diffraction planes of (111), (220), (311), (400), (422), (511), (440), and (533), are observed. These XRD patterns are very similar to that reported for magnetite phase of iron oxide (JCPDS No: 88-0315) [33,34]. The crystallite sizes (D) of undoped-SPIONs and Gd3+-doped SPIONs were calculated using Debye–Sherrer equation (D = Kλ/βcosθ), and the values of 7.1 nm, and 9.7 nm were respectively obtained. Morphological characteristics of the synthesized SPIONs samples were characterized through FE-SEM observations and shown in Fig. 3. For both two deposited SPIONs powders, spherical particles with an average diameter of 15 nm are observed in the FE-SEM images, as reported in Refs. [33,34]. The FT-IR spectra for both SPIONs powders are given in Fig. 4. In both spectra, the IR bands located at 425-430 cm−1 and 550-560 cm−1 can be assigned to the stretching vibration modes of Fe2+–O–Fe2+ and Fe3+–O–Gd3+ bonds. Fig. 5 shows the measured magnetization data for both synthesized SPIONs. The superparamagnetic behaviors of both synthesized powders are observable from the S-shape and the absence of any loop in these VSM curves. The saturation magnetization (Ms), remanence (Mr) and coercivity (Ce) of samples were also obtained, which are: For undopedSPIONs; Ms = 72.96 emu/g, Mr = 0.95 emu/g and Ce=2.9 Oe [33], and for Gd3+-doped SPIONs; Ms = 40.67 emu/g, Mr = 0.33 emu/g and Ce=1.35 Oe [34]. These magnetic data proved the superparamagnetic behavior for both synthesized SPIONs. Notably, Gd3+-doped SPIONs exhibited better superparamagnetic behavior due to the reduced both Mr and Ce values.
2.2. Synthesis of SPIONs The cathodic electrochemical synthesis was used to synthesize undoped and Gd3+-doped SPIONs. The electrochemical set-up and conditions were those used in recent works [31,32]. A two-electrode electrochemical system including stainless steel cathode and graphite anode was constructed for deposition experiments. Two different baths were prepared for the synthesis of undoped and Gd3+-doped SPIONs, which were dissolved in ferric nitrate (2 g) + ferrous chloride (1 g) in one litter of H2O (in the case of undoped SPIONs), and 2 g ferric nitrate +1 g ferrous chloride +0.3 g gadolinium nitrate dissolved in 1 litter distilled water (for the synthesis of Gd3+-doped SPIONs). In the electrodeposition of both SPIONs, the applied parameters were Tbath = 25 °C, tsynthesis = 30 min and idepostion = 10 mA.cm−2. After each deposition run for 30 min, the synthesis process was stopped and the thin film deposited onto the surface of cathode was meticulously scrapped from the surface. The obtained wet powders were dried at 70 °C for 1 h. The prepared SPIONs were labeled undoped-SPIONs and Gd3+-doped SPIONs according to the chemical composition of the deposition bath. The structure of Gd3+-doped SPIONs is schematically shown in Fig. 1.
3.2. Cure labeling The effect of introducing of 0.1 wt.% of n SPIONs and Gd3+-doped SPIONs on curing reaction of epoxy/amine system was analyzed by dynamic DSC at β of 5, 10, 15 and 20 °C/min and shown in Fig. 6. As can be observed, unfilled and filled epoxy with SPIONs and Gd3+doped SPIONs show a single exothermic peak at all heating rates. It is inferred that the addition of SPIONs and Gd3+-doped SPIONs does not change the mechanism of epoxy curing reaction which supposed it proceeds mainly with the epoxy-amine ring opening addition reaction [35]. In addition, it can be speculated that SPIONs and Gd3+-doped
2.3. Preparation of epoxy nanocomposites Epoxy nanocomposite containing 0.1 wt.% of SPIONs (EP/SPIONs) and Gd3+-doped SPIONs (EP/ Gd3+-doped SPIONs) were prepared by sonicating nanoparticles in epoxy resin for 5 min and mixing by a mechanical mixer for 20 min with speed of 2500 rpm. The prepared epoxy nanocomposite dispersions were then mixed with stoichiometric amount of TETA at room temperature and froze at -10 °C to prevent them from pre-curing before DSC analyses. 2
Progress in Organic Coatings 136 (2019) 105245
M. Jouyandeh, et al.
Fig. 1. Schematic of the structure of the synthesized Gd3+-doped SPIONs.
Fig. 2. XRD patterns of the synthesized SPIONs samples. Fig. 4. FT-IR spectra of the synthesized (a) undoped-SPIONs and (b) Gd3+doped SPIONs.
Fig. 3. FE-SEM images of undoped-SPIONs and (b) Gd3+-doped SPIONs. 3
Progress in Organic Coatings 136 (2019) 105245
M. Jouyandeh, et al.
Fig. 5. VSM curves of the fabricated (a) undoped-SPIONs and (b) Gd3+-doped SPIONs.
[37,38]:
SPIONs might be able to hinder or facilitate cure reaction of epoxy which resulted in change of the shape of the DSC thermograms [36]. Additional information about the curing reaction of epoxy/amine systems were obtained from DSC thermograms (Fig. 6): the onset cure temperature (Tonset), the peak temperature (Tp), the endset temperature of cure reaction (Tendset), temperature range during which curing was completed (ΔT = Tendset - Tonset) and the total heat of cure (ΔH∞). Based on the ΔT and ΔH∞ values of the neat epoxy and its nanocomposites, the effect of addition of SPIONs and Gd3+-doped SPIONs was studied qualitatively in terms of Cure Index (CI) which defined as follows
ΔH ∗ =
ΔHC ΔHRe f
(1)
ΔT ∗ =
ΔTC ΔTRe f
(2)
CI = ΔH ∗ × ΔT ∗
(3)
where subscript C and Ref refer to the composite and reference epoxy, respectively. The values of curing reaction information at β of 5, 10, 15
Fig. 6. Dynamic DSC thermograms of neat epoxy and its nanocomposites at (a) 5 °C/min, (b) 10 °C/min, (c) 15 °C/min, (d) 20 °C/min. 4
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Table 1 Cure characteristics of the prepared epoxy nanocomposites as a function of heating rate. Designation
β(°C/min)
Tonset(°C)
Tendset(°C)
Tp(°C)
ΔT(°C)
ΔH∞(J/g)
ΔT*
ΔH*
CI
EP
5 10 15 20 5 10 15 20 5 10 15 20
31.8 35.4 36.9 47.8 30.5 39.4 49.4 53.9 21.2 39.5 38.9 43.9
82.9 94.7 102.1 109.5 86.6 97.3 104.8 110.6 88.8 101.6 107.3 112.2
160.9 156.7 185.7 197.5 147.5 162.3 177.9 190.2 162.2 162.8 183.9 192.6
129.2 121.3 148.8 149.7 117.0 122.8 128.5 136.3 140.9 123.3 145.0 148.7
353.5 402.1 405.8 370.0 303.2 334.9 345.5 369.8 373.5 380.4 350.4 441.9
n.a. n.a. n.a. n.a. 0.91 1.01 0.86 0.91 1.09 1.02 0.97 0.99
n.a. n.a. n.a. n.a. 0.86 0.83 0.85 0.99 1.05 0.94 0.86 1.19
n.a. n.a. n.a. n.a. 0.78 0.84 0.73 0.90 1.14 0.96 0.83 1.18
EP/SPIONs
EP/Gd3+-doped SPIONs
n.a. – not applicable (reference measurements). The CI value as Bold character (1.14) is representative of Poor cure, while Italic value (1.18) shows Excellent cure state.
3.3. Glass transition temperature
and 20 °C/min are summarized in Table 1. As expected, Tonset and Tp increased by increasing heating rate for the neat epoxy and its nanocomposites containing 0.1 wt.% of SPIONs and Gd3+-doped SPIONs. This suggests higher kinetic energy per molecule in the system, which reduces the time required for epoxy/amine curing reaction [39]. It is evident that DSC thermograms are correspondingly shifted towards higher temperatures to compensate for the restricted time [40,41]. As can be inferred, the values of ΔH for EP/SPIONs nanocomposite significantly dropped at all heating rates which indicate that epoxy/ amine curing reaction was hindered by SPIONs. Free orbitals of Fe cations are more likely to react with the free electron pair of the amine curing agent than oxirane ring of epoxy resin which leads to the reduction in the reactivity of amine groups of curing agent. Therefore, Adding SPIONs reduces the stoichiometric amount of curing agent and hinder the curing reaction. The properties of lanthanide doped magnetic nanoparticles depend on their selectively in substituting Fe3+ ions in the octahedral and tetrahedral sublattices. There is a large difference in ionic radii of Gd3+ dopant and Fe3+ cation in Fe3O4 lattice. The difficult accommodation of larger Gd3+ cation in the SPIONs lattice implies that some part of gadolinium form Gd(OH)3 [42]. Therefore, large Gd3+ ion prefers to occupy the octahedral sites of SPIONs lattice rather than the smaller tetrahedral sites. It is inferred that Gd3+ dopants tend to locate in the surface of SPIONs lattice. Moreover, the resistance to oxidation of SPIONs can be improved by substituting Fe3+ with rare earth elements dopant such as Gd3+ [43]. Accordingly, addition of Gd3+-doped SPIONs in the epoxy/amine system can improve the curing reaction which observed in rise of ΔH values compared to neat epoxy and EP/ SPIONs. For easier understanding of the effect of SPIONs and Gd3+-doped SPIONs on curing reaction of epoxy/amine system cure state in terms of CI was shown in the plot of ΔH* versus ΔT* (Fig. 7). Fig. 7 separated in to three regions with green, blue and red colors which show Excellent cure ( ΔT ∗ < CI < ΔH ∗), Good curing (CI > ΔH ∗) and Poor cure (CI < ΔT ∗) , respectively. As can be observed, CI of EP/SPIONs nanocomposite located in red regions regardless of heating rate which confirmed that SPIONs hindered curing reaction of epoxy/amine system by utilizing amine curing agent and reducing TETA stoichiometric amount. On the other hand, Gd3+-doped SPIONs labeled cure state of epoxy/amine Good and Excellent at low and high heating rates, respectively. β = 5 °C/min offered longer time for the reaction between cuing moieties, therefore higher ΔH (ΔH* > 1) obtained at wider temperature window (ΔT* > 1) for EP/ Gd3+-doped SPIONs which led to Good cure state. In addition, at β = 20 °C/min high kinetic energy per molecules increased the mobility of curing moieties and led to Excellent cure state.
Table 2 summarized Tg values of the fully cured neat epoxy, EP/ SPIONs and EP/Gd3+-doped SPIONs nanocomposites cured at heating rate of 10 °C/min. As apparent, EP/SPIONs and EP/Gd3+-doped SPIONs nanocomposites show a Tg depression. The reason for Tg depression is the weak interaction between epoxy resin, SPIONs and Gd3+-doped SPIONs which is attributed to the lack of reactive groups on the surface of nanoparticles. Unstable interface between resin and nanoparticles creates a free volume in the vicinity of the nanoparticles for segmental mobility of polymer chains [44]. The decrease in the value of Tg is more noticeable in the case of EP/ SPIONs nanocomposite due to partially cured system because of hindrance effect of SPIONs. Considering Table 1 together with Table 2 provides a better description of the effect of SPIONs and Gd3+-doped SPIONs on cure state of nanocomposites and consequently Tg. As can be observed, ΔH value of EP/ Gd3+-doped SPIONs system is higher than EP/ SPIONs nanocomposite which resulted in denser cross-linked network. Therefore, the Tg value of the epoxy nanocomposite containing 0.1 wt.% of Gd3+-doped SPIONs is higher than SPIONs.
4. Conclusion Undoped and Gd-doped SPIONs are synthesized through one-step electrodeposition method. The synthesized SPIONs are analyzed through XRD, FE-SEM, FT-IR and VSM techniques. From XRD results the average crystallite size of undoped-SPIONs and Gd3+-doped SPIONs were calculated using Debye–Sherrer equation and the values of 7.1 nm, and 9.7 nm were respectively obtained. CI obtained from dynamic DSC measurements at β of 5, 10, 15 and 20 °C/min was considered to evaluate the cure state of epoxy/amine system in the presence of SPIONs and Gd3+-doped SPIONs. It was recognized that Gd3+doped SPIONs effectively enhance cure reaction of epoxy system at low and high heating rates which leads to Good and Excellent cure state in terms of CI. The Tg of fully cured nanocomposites was found to be lower than the neat epoxy, 0.1 wt.% of SPIONs and Gd3+-doped SPIONs decrease the Tg values 20 °C and 11 °C, respectively. This decrement indicates weak interface between nanoparticles and epoxy resin due to the lack of functional groups on the surface of nanoparticles, but still much more higher that the SPIONs incorporated sample.
Acknowledgments The financial support of this work by Iran National Science Foundation (INSF; Grant Number: 950019) and University of Tehran is gratefully acknowledged. 5
Progress in Organic Coatings 136 (2019) 105245
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Fig. 7. Cure state of EP/SPIONs and Gd3+-doped SPIONs in terms of CI at heating rates of 5, 10, 15 and 20 °C/min. Table 2 Glass transition temperature of the fully cured epoxy nanocomposites at β of 10 °C min−1. Designation
Tg (°C)
EP EP/SPIONs EP/Gd3+-doped SPIONs
120 100 109
[16]
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[18] [19]
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[40] M. Jouyandeh, O.M. Jazani, A.H. Navarchian, M. Shabanian, H. Vahabi, M.R. Saeb, Bushy-surface hybrid nanoparticles for developing epoxy superadhesives, Appl. Surf. Sci. 479 (2019) 1148–1160. [41] M. Jouyandeh, O.M. Jazani, A.H. Navarchian, M. Shabanian, H. Vahabi, M.R. Saeb, Surface engineering of nanoparticles with macromolecules for epoxy curing: development of super-reactive nitrogen-rich nanosilica through surface chemistry manipulation, Appl. Surf. Sci. 447 (2018) 152–164. [42] A. Budnyk, T. Lastovina, A. Bugaev, V. Polyakov, K. Vetlitsyna-Novikova, M. Sirota, K. Abdulvakhidov, A. Fedorenko, E. Podlesnaya, A. Soldatov, Gd3+-Doped magnetic nanoparticles for biomedical applications, J. Spectrosc. 2018 (2018). [43] C.R. De Silva, S. Smith, I. Shim, J. Pyun, T. Gutu, J. Jiao, Z. Zheng, Lanthanide (III)doped magnetite nanoparticles, J. Am. Chem. Soc. 131 (2009) 6336–6337. [44] F. Tikhani, M. Jouyandeh, S.H. Jafari, S. Chabokrow, M. Ghahari, K. Gharanjig, F. Klein, N. Hampp, M.R. Ganjali, K. Formela, M.R. Saeb, Cure index demonstrates curing of epoxy composites containing silica nanoparticles of variable morphology and porosity, Prog. Org. Coat. 135 (2019) 176–184.
1341–1347. [35] S. Ghiyasi, M.G. Sari, M. Shabanian, M. Hajibeygi, P. Zarrintaj, M. Rallini, L. Torre, D. Puglia, H. Vahabi, M. Jouyandeh, Hyperbranched poly (ethyleneimine) physically attached to silica nanoparticles to facilitate curing of epoxy nanocomposite coatings, Prog. Org. Coat. 120 (2018) 100–109. [36] M. Jouyandeh, Z. Karami, O.M. Jazani, K. Formela, S.M.R. Paran, A. Jannesari, M.R. Saeb, Curing epoxy resin with anhydride in the presence of halloysite nanotubes: the contradictory effects of filler concentration, Prog. Org. Coat. 126 (2019) 129–135. [37] M. Jouyandeh, S.M.R. Paran, A. Jannesari, M.R. Saeb, ‘Cure Index’ for thermoset composites, Prog. Org. Coat. 127 (2019) 429–434. [38] M. Jouyandeh, S.M.R. Paran, A. Jannesari, D. Puglia, M.R. Saeb, Protocol for nonisothermal cure analysis of thermoset composites, Prog. Org. Coat. 131 (2019) 333–339. [39] V. Akbari, F. Najafi, H. Vahabi, M. Jouyandeh, M. Badawi, S. Morisset, M.R. Ganjali, M.R. Saeb, Surface chemistry of halloysite nanotubes controls the curability of low filled epoxy nanocomposites, Prog. Org. Coat. 135 (2019) 555–564.
7
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Curing epoxy with electrochemically synthesized GdxFe3-xO4 magnetic nanoparticles
T
⁎
Maryam Jouyandeha, Payam Zarrintajb, Mohammad Reza Ganjalia,c, , Jagar A. Alid, ⁎ Isa Karimzadehe, Mustafa Aghazadeha, Mehdi Ghaffarif, Mohammad Reza Saebg, a
Center of Excellence in Electrochemistry, School of Chemistry, College of Science, University of Tehran, Tehran, Iran Polymer Engineering Department, Faculty of Engineering, Urmia University, P.O. Box 57561511818-165, Urmia, Iran c Biosensor Research Center, Endocrinology & Metabolism Molecular-Cellular Sciences Institute, Tehran University of Medical Sciences, Tehran, Iran d Department of Petroleum Engineering, Faculty of Engineering, Soran University, Kurdistan Region, Iraq e Department of Physics, Bonab Branch, Islamic Azad University, Bonab, Iran f Polymer Group, Faculty of Technical and Engineering, Golestan University, P.O. Box 155, Gorgan, Golestan, Iran g Department of Resin and Additives, Institute for Color Science and Technology, P.O. Box: 16765-654, Tehran, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Cure Index Epoxy Fe3O4 nanoparticle Gadolinium Oxidation-resistant coatings
In this work, supermagnetic lanthanide-doped Fe3O4 nanoparticles were synthesized through electrodeposition method by partial doping in gadolinium (Gd) to be intended in the near future in developing oxidation-resistant epoxy-based coatings. Partial substitution of Fe3+ ions in Fe3O4 by Gd3+ lanthanide cations was confirmed by XRay diffraction (XRD) patterns and Fourier-transform infrared spectroscopy (FTIR) spectra. Field-emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometer (VSM) were employed to evaluate particle size distribution and magnetic nature of the nanoparticles, respectively. Low-filled nanocomposites containing 0.1 wt.% of Fe3O4 and Gd3+-doped Fe3O4 were prepared and their curing ability was assessed in terms of qualitative measurements on network formation by the use of Cure Index. Having a wide range of curing behavior led to development of Poor, Good, and Excellent cured coatings depending on nanoparticle bulk composition and heating rate applied in the nonisothermal differential scanning calorimetry (DSC). Overall, incorporation of Fe3O4 into epoxy decreased exothermic heat release of epoxy, while Gd3+-doped nanoparticles facilitated curing reaction between epoxy and aliphatic amine curing agent. Moreover, glass transition temperature of the prepared nanocomposites decreased compared to the blank epoxy. The developed epoxy/lanthanide-doped Fe3O4 nanocomposites are potential candidates for developing oxidation-resistant coatings thanks to the presence of Gd3+ cations.
1. Introduction Properties of organic coatings depend closely on the possibility and facility of network formation through crosslinking reaction between the thermoset resin and curing agent [1]. The effects of pigmentation [2–6], nanoparticle incorporation [7–11], and non-covalent and covalent surface functionalization of nanoparticles [12] on network formation in organic coatings have been already discussed. On the other hand, tuning properties of organic coatings by the proper modification of bulk composition of nanoparticles was rarely reported. Fe3O4 nanoparticles have been in the core of attention in the last decades thanks to their extraordinary potential [13,14] for applications such as adsorbent [15,16], photocatalyst [17] and therapeutic treatments [18,19]. They were also considered for development of anti-corrosion films and ⁎
coatings [20–22]. Control over energy harvesting devices is of prime importance in designing systems [23,24]. In this sense, controlled oxidation of Fe3O4 nanoparticles for applications in electromagnetic devices was practised [25]. Lanthanide-doped Fe3O4 nanoparticles were examined in design and manufacture of magnetic–luminescent nanocomposites [25,26]. Controlling the atomic composition of dopants in magnetic nanoparticles by partial substitution of Fe2+ and Fe3+ cations by transition metals or lanthanides give rise to control over the crystallinity of such hybrid nanoparticles for optimized synthesis of supermagnetic nanomaterials [27,28]. Synthesis of nanohybrids of rare-earth elements with Fe3O4 nanoparticles having variable particle size was practised in some previous works [29,30]. However, the potential of rare-earth element cations doped in Fe3O4 nanoparticles for participating in curing
Corresponding authors. E-mail addresses: [email protected] (M.R. Ganjali), [email protected] (M.R. Saeb).
https://doi.org/10.1016/j.porgcoat.2019.105245 Received 11 July 2019; Received in revised form 18 July 2019; Accepted 19 July 2019 Available online 05 August 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 136 (2019) 105245
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2.4. Characterization
reaction of epoxy with amine curing agents was not reported. In this work, supermagnetic gadolinium (Gd)-doped Fe3O4 nanoparticles were synthesized by electrodeposition method to be intended in near future in developing oxidation-resistant epoxy-based nanocomposite coatings. Partial substitution of Fe3+ ions in Fe3O4 structure by Gd3+ rare-earth cations was carried out electrochemically. X-Ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were employed to study change in bulk composition upon insertion of Gd3+ cations into Fe3O4 atomic structure. Field-emission scanning electron microscopy (FE-SEM) and vibrating sample magnetometer (VSM) were furthermore used for the analysis of size/size distribution and magnetic properties of the nanoparticles, respectively. Nanocomposites containing 0.1 wt.% of Fe3O4 and Gd3+-doped Fe3O4 were subjected to curing at different heating rates through nonisothermal differential scanning calorimetry (DSC). Network formation in the resulting nanocomposites was quantified by the use of Cure Index to label them as Poor, Good, and Excellent cured systems. Calculation of glass transition temperature of epoxy and its nanocomposites containing superparamagnetic iron oxide nanoparticles (SPIONs) allowed for assessing the effect of bulk composition alteration on the physical properties of epoxy.
Morphological images of the deposited SPIONs were observed on a FE-SEM microscope, Model; Mira 3-XMU and accelerating voltage of 100 kV. FT-IR spectra were recorded through Bruker Vector 22 IR spectrometer in the wavelength range of 4000–400 cm−1. XRD patterns of the synthesized SPIONs were recorded by PW-1800 X-ray diffraction with a Co Kα radiation. Magnetic behaviors of the fabricated SPIONs particles were also specified in the range of −20,000 Oe to 20,000 Oe at RT condition using a VSM instrument, Model: lakeshore 7400. The prepared epoxy nanocomposites were analyzed for curing potential by DSC (Perkin Elmer DSC 4000) under dynamic mode to study the effect of SPIONs and Gd3+-doped SPIONs on the state of cure of epoxy/amine system. DSC analysis was performed under nonisothermal mode at heating rates (β) of 5, 10, 15, and 20 °C.min−1 from 15 to 250 °C under nitrogen atmosphere with the flow rate of 20 mL.min−1. Moreover, glass transition temperature (Tg) of epoxy and its nanocomposites was obtained at β of 10 °C/min through heating-cooling-heating cycle. For this purpose, samples were first heated up from 15 to 250 °C, then cooled down to ambient temperature, and again reheated up to 250 °C at 10 °C.min−1.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Structure and morphology
Ferric nitrate nonahydrate (Sigma-Aldrich, 99.9%), ferrous chloride tetrahydrate (Sigma-Aldrich, 99.5%) and gadolinium nitrate hexahydrate (Sigma-Aldrich, 99.8%) were purchased and used without further purification. Epoxy matrix (diglycidyl ether of bis-phenol A, Epon-828) with epoxide equivalent weight of 185–192 g/eq and curing agent (triethylenetetramine, TETA) with hydrogen equivalent weight of 25 g/ eq were supplied by Hexion, Beijing (China) and used in stoichiometric amount (resin:hardener = 100:13) in nanocomposite preparation.
The XRD patterns of the deposited SPIONs are displayed in Fig. 2. In both patterns, the diffraction planes of (111), (220), (311), (400), (422), (511), (440), and (533), are observed. These XRD patterns are very similar to that reported for magnetite phase of iron oxide (JCPDS No: 88-0315) [33,34]. The crystallite sizes (D) of undoped-SPIONs and Gd3+-doped SPIONs were calculated using Debye–Sherrer equation (D = Kλ/βcosθ), and the values of 7.1 nm, and 9.7 nm were respectively obtained. Morphological characteristics of the synthesized SPIONs samples were characterized through FE-SEM observations and shown in Fig. 3. For both two deposited SPIONs powders, spherical particles with an average diameter of 15 nm are observed in the FE-SEM images, as reported in Refs. [33,34]. The FT-IR spectra for both SPIONs powders are given in Fig. 4. In both spectra, the IR bands located at 425-430 cm−1 and 550-560 cm−1 can be assigned to the stretching vibration modes of Fe2+–O–Fe2+ and Fe3+–O–Gd3+ bonds. Fig. 5 shows the measured magnetization data for both synthesized SPIONs. The superparamagnetic behaviors of both synthesized powders are observable from the S-shape and the absence of any loop in these VSM curves. The saturation magnetization (Ms), remanence (Mr) and coercivity (Ce) of samples were also obtained, which are: For undopedSPIONs; Ms = 72.96 emu/g, Mr = 0.95 emu/g and Ce=2.9 Oe [33], and for Gd3+-doped SPIONs; Ms = 40.67 emu/g, Mr = 0.33 emu/g and Ce=1.35 Oe [34]. These magnetic data proved the superparamagnetic behavior for both synthesized SPIONs. Notably, Gd3+-doped SPIONs exhibited better superparamagnetic behavior due to the reduced both Mr and Ce values.
2.2. Synthesis of SPIONs The cathodic electrochemical synthesis was used to synthesize undoped and Gd3+-doped SPIONs. The electrochemical set-up and conditions were those used in recent works [31,32]. A two-electrode electrochemical system including stainless steel cathode and graphite anode was constructed for deposition experiments. Two different baths were prepared for the synthesis of undoped and Gd3+-doped SPIONs, which were dissolved in ferric nitrate (2 g) + ferrous chloride (1 g) in one litter of H2O (in the case of undoped SPIONs), and 2 g ferric nitrate +1 g ferrous chloride +0.3 g gadolinium nitrate dissolved in 1 litter distilled water (for the synthesis of Gd3+-doped SPIONs). In the electrodeposition of both SPIONs, the applied parameters were Tbath = 25 °C, tsynthesis = 30 min and idepostion = 10 mA.cm−2. After each deposition run for 30 min, the synthesis process was stopped and the thin film deposited onto the surface of cathode was meticulously scrapped from the surface. The obtained wet powders were dried at 70 °C for 1 h. The prepared SPIONs were labeled undoped-SPIONs and Gd3+-doped SPIONs according to the chemical composition of the deposition bath. The structure of Gd3+-doped SPIONs is schematically shown in Fig. 1.
3.2. Cure labeling The effect of introducing of 0.1 wt.% of n SPIONs and Gd3+-doped SPIONs on curing reaction of epoxy/amine system was analyzed by dynamic DSC at β of 5, 10, 15 and 20 °C/min and shown in Fig. 6. As can be observed, unfilled and filled epoxy with SPIONs and Gd3+doped SPIONs show a single exothermic peak at all heating rates. It is inferred that the addition of SPIONs and Gd3+-doped SPIONs does not change the mechanism of epoxy curing reaction which supposed it proceeds mainly with the epoxy-amine ring opening addition reaction [35]. In addition, it can be speculated that SPIONs and Gd3+-doped
2.3. Preparation of epoxy nanocomposites Epoxy nanocomposite containing 0.1 wt.% of SPIONs (EP/SPIONs) and Gd3+-doped SPIONs (EP/ Gd3+-doped SPIONs) were prepared by sonicating nanoparticles in epoxy resin for 5 min and mixing by a mechanical mixer for 20 min with speed of 2500 rpm. The prepared epoxy nanocomposite dispersions were then mixed with stoichiometric amount of TETA at room temperature and froze at -10 °C to prevent them from pre-curing before DSC analyses. 2
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Fig. 1. Schematic of the structure of the synthesized Gd3+-doped SPIONs.
Fig. 2. XRD patterns of the synthesized SPIONs samples. Fig. 4. FT-IR spectra of the synthesized (a) undoped-SPIONs and (b) Gd3+doped SPIONs.
Fig. 3. FE-SEM images of undoped-SPIONs and (b) Gd3+-doped SPIONs. 3
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Fig. 5. VSM curves of the fabricated (a) undoped-SPIONs and (b) Gd3+-doped SPIONs.
[37,38]:
SPIONs might be able to hinder or facilitate cure reaction of epoxy which resulted in change of the shape of the DSC thermograms [36]. Additional information about the curing reaction of epoxy/amine systems were obtained from DSC thermograms (Fig. 6): the onset cure temperature (Tonset), the peak temperature (Tp), the endset temperature of cure reaction (Tendset), temperature range during which curing was completed (ΔT = Tendset - Tonset) and the total heat of cure (ΔH∞). Based on the ΔT and ΔH∞ values of the neat epoxy and its nanocomposites, the effect of addition of SPIONs and Gd3+-doped SPIONs was studied qualitatively in terms of Cure Index (CI) which defined as follows
ΔH ∗ =
ΔHC ΔHRe f
(1)
ΔT ∗ =
ΔTC ΔTRe f
(2)
CI = ΔH ∗ × ΔT ∗
(3)
where subscript C and Ref refer to the composite and reference epoxy, respectively. The values of curing reaction information at β of 5, 10, 15
Fig. 6. Dynamic DSC thermograms of neat epoxy and its nanocomposites at (a) 5 °C/min, (b) 10 °C/min, (c) 15 °C/min, (d) 20 °C/min. 4
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Table 1 Cure characteristics of the prepared epoxy nanocomposites as a function of heating rate. Designation
β(°C/min)
Tonset(°C)
Tendset(°C)
Tp(°C)
ΔT(°C)
ΔH∞(J/g)
ΔT*
ΔH*
CI
EP
5 10 15 20 5 10 15 20 5 10 15 20
31.8 35.4 36.9 47.8 30.5 39.4 49.4 53.9 21.2 39.5 38.9 43.9
82.9 94.7 102.1 109.5 86.6 97.3 104.8 110.6 88.8 101.6 107.3 112.2
160.9 156.7 185.7 197.5 147.5 162.3 177.9 190.2 162.2 162.8 183.9 192.6
129.2 121.3 148.8 149.7 117.0 122.8 128.5 136.3 140.9 123.3 145.0 148.7
353.5 402.1 405.8 370.0 303.2 334.9 345.5 369.8 373.5 380.4 350.4 441.9
n.a. n.a. n.a. n.a. 0.91 1.01 0.86 0.91 1.09 1.02 0.97 0.99
n.a. n.a. n.a. n.a. 0.86 0.83 0.85 0.99 1.05 0.94 0.86 1.19
n.a. n.a. n.a. n.a. 0.78 0.84 0.73 0.90 1.14 0.96 0.83 1.18
EP/SPIONs
EP/Gd3+-doped SPIONs
n.a. – not applicable (reference measurements). The CI value as Bold character (1.14) is representative of Poor cure, while Italic value (1.18) shows Excellent cure state.
3.3. Glass transition temperature
and 20 °C/min are summarized in Table 1. As expected, Tonset and Tp increased by increasing heating rate for the neat epoxy and its nanocomposites containing 0.1 wt.% of SPIONs and Gd3+-doped SPIONs. This suggests higher kinetic energy per molecule in the system, which reduces the time required for epoxy/amine curing reaction [39]. It is evident that DSC thermograms are correspondingly shifted towards higher temperatures to compensate for the restricted time [40,41]. As can be inferred, the values of ΔH for EP/SPIONs nanocomposite significantly dropped at all heating rates which indicate that epoxy/ amine curing reaction was hindered by SPIONs. Free orbitals of Fe cations are more likely to react with the free electron pair of the amine curing agent than oxirane ring of epoxy resin which leads to the reduction in the reactivity of amine groups of curing agent. Therefore, Adding SPIONs reduces the stoichiometric amount of curing agent and hinder the curing reaction. The properties of lanthanide doped magnetic nanoparticles depend on their selectively in substituting Fe3+ ions in the octahedral and tetrahedral sublattices. There is a large difference in ionic radii of Gd3+ dopant and Fe3+ cation in Fe3O4 lattice. The difficult accommodation of larger Gd3+ cation in the SPIONs lattice implies that some part of gadolinium form Gd(OH)3 [42]. Therefore, large Gd3+ ion prefers to occupy the octahedral sites of SPIONs lattice rather than the smaller tetrahedral sites. It is inferred that Gd3+ dopants tend to locate in the surface of SPIONs lattice. Moreover, the resistance to oxidation of SPIONs can be improved by substituting Fe3+ with rare earth elements dopant such as Gd3+ [43]. Accordingly, addition of Gd3+-doped SPIONs in the epoxy/amine system can improve the curing reaction which observed in rise of ΔH values compared to neat epoxy and EP/ SPIONs. For easier understanding of the effect of SPIONs and Gd3+-doped SPIONs on curing reaction of epoxy/amine system cure state in terms of CI was shown in the plot of ΔH* versus ΔT* (Fig. 7). Fig. 7 separated in to three regions with green, blue and red colors which show Excellent cure ( ΔT ∗ < CI < ΔH ∗), Good curing (CI > ΔH ∗) and Poor cure (CI < ΔT ∗) , respectively. As can be observed, CI of EP/SPIONs nanocomposite located in red regions regardless of heating rate which confirmed that SPIONs hindered curing reaction of epoxy/amine system by utilizing amine curing agent and reducing TETA stoichiometric amount. On the other hand, Gd3+-doped SPIONs labeled cure state of epoxy/amine Good and Excellent at low and high heating rates, respectively. β = 5 °C/min offered longer time for the reaction between cuing moieties, therefore higher ΔH (ΔH* > 1) obtained at wider temperature window (ΔT* > 1) for EP/ Gd3+-doped SPIONs which led to Good cure state. In addition, at β = 20 °C/min high kinetic energy per molecules increased the mobility of curing moieties and led to Excellent cure state.
Table 2 summarized Tg values of the fully cured neat epoxy, EP/ SPIONs and EP/Gd3+-doped SPIONs nanocomposites cured at heating rate of 10 °C/min. As apparent, EP/SPIONs and EP/Gd3+-doped SPIONs nanocomposites show a Tg depression. The reason for Tg depression is the weak interaction between epoxy resin, SPIONs and Gd3+-doped SPIONs which is attributed to the lack of reactive groups on the surface of nanoparticles. Unstable interface between resin and nanoparticles creates a free volume in the vicinity of the nanoparticles for segmental mobility of polymer chains [44]. The decrease in the value of Tg is more noticeable in the case of EP/ SPIONs nanocomposite due to partially cured system because of hindrance effect of SPIONs. Considering Table 1 together with Table 2 provides a better description of the effect of SPIONs and Gd3+-doped SPIONs on cure state of nanocomposites and consequently Tg. As can be observed, ΔH value of EP/ Gd3+-doped SPIONs system is higher than EP/ SPIONs nanocomposite which resulted in denser cross-linked network. Therefore, the Tg value of the epoxy nanocomposite containing 0.1 wt.% of Gd3+-doped SPIONs is higher than SPIONs.
4. Conclusion Undoped and Gd-doped SPIONs are synthesized through one-step electrodeposition method. The synthesized SPIONs are analyzed through XRD, FE-SEM, FT-IR and VSM techniques. From XRD results the average crystallite size of undoped-SPIONs and Gd3+-doped SPIONs were calculated using Debye–Sherrer equation and the values of 7.1 nm, and 9.7 nm were respectively obtained. CI obtained from dynamic DSC measurements at β of 5, 10, 15 and 20 °C/min was considered to evaluate the cure state of epoxy/amine system in the presence of SPIONs and Gd3+-doped SPIONs. It was recognized that Gd3+doped SPIONs effectively enhance cure reaction of epoxy system at low and high heating rates which leads to Good and Excellent cure state in terms of CI. The Tg of fully cured nanocomposites was found to be lower than the neat epoxy, 0.1 wt.% of SPIONs and Gd3+-doped SPIONs decrease the Tg values 20 °C and 11 °C, respectively. This decrement indicates weak interface between nanoparticles and epoxy resin due to the lack of functional groups on the surface of nanoparticles, but still much more higher that the SPIONs incorporated sample.
Acknowledgments The financial support of this work by Iran National Science Foundation (INSF; Grant Number: 950019) and University of Tehran is gratefully acknowledged. 5
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Fig. 7. Cure state of EP/SPIONs and Gd3+-doped SPIONs in terms of CI at heating rates of 5, 10, 15 and 20 °C/min. Table 2 Glass transition temperature of the fully cured epoxy nanocomposites at β of 10 °C min−1. Designation
Tg (°C)
EP EP/SPIONs EP/Gd3+-doped SPIONs
120 100 109
[16]
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