Reinforcing effect of imidazole modified nanosilica on thermal and mechanical properties of anhydride based epoxy system

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ScienceDirect Materials Today: Proceedings 21 (2020) 1038–1043

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ICRACM-2019

Reinforcing effect of imidazole modified nanosilica on thermal and mechanical properties of anhydride based epoxy system Tankeshwar Prasada*, Sudipta Haldera, S. S. Dharb a

Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar-788010, Assam, India b Department of Chemistry, National Institute of Technology Silchar, Silchar, Assam, 788010, India

Abstract In this study, imidazole modified nanosilica (IS) was prepared from TEOS precursor by sol-gel via co-condensation method. The surface modification was confirmed by FTIR and SEM characterization. Further, epoxy nanocomposites were prepared using different content (0-3 wt %) of IS. TGA analysis demonstrated the significant effect of IS towards enhancement of thermal stability of epoxy composites at 1 wt% IS particle contents. The maximum enhancement in tensile strength of composites (1 wt% IS) was achieved by 45% with respect to neat epoxy. From these results, it confirms that the organic compatibility and chemical bonding at the interface around IS favours effective stress transfer to the matrix system. Thus, this work exposed the potential of imidazole modified nanosilica as a good choice of reinforcement in the field of composite industries. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of SIXTH INTERNATIONAL CONFERENCE ON RECENT ADVANCES IN COMPOSITE MATERIALS, ICRACM-2019. Keywords:Nnanoparticles; imidazole; sol-gel method; nanocomposites; tensile properties;

1. Introduction The recent advance in composite materials, the effective reinforcement and the resulting negative impact of high performance polymer have stimulated the use of renewable feedstock in academic research and the composite industry. The latest trends in the field of composite materials with the rapid progress in nanotechnology, nano-sized inorganic particles are also of great interest for improving material properties [1].

* Corresponding author. Tel.: +91-9126693169; fax: +91-384222479. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of SIXTH INTERNATIONAL CONFERENCE ON RECENT ADVANCES IN COMPOSITE MATERIALS, ICRACM-2019.

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Among these, Silica nanoparticles emerge as an attractive candidate for such applications because of their high elastic modulus, surface areas, thermal stability, low density and low material cost [2]. In most of the academic research work, authors are reported the impact of nano-sized fillers on mechanical and thermal property of epoxy system [3–5]. However, uniform dispersion of SiO2 into epoxy resin remained a technological challenge, because of their high surface polarity with silanol groups prone to aggregation tendency resulting structural defects. Hence weak adhesion [6] at the interface can be overcome by introducing some new aspect of surface modification of silica surface is essential. The enhancement in surface wettability [7] of SiO2 using different coupling agents have been proven to be effective reinforcements for epoxy system as the functional groups play the role in the curing process and thus integrate the epoxy networks through strong covalent bonding [8]. The cure kinetics of epoxyanhydride systems can be tailored via imidazole modified nanofiller to suit for healing applications. Recently, Qing Lyu [9] reported, the Imidazolium ionic liquid modified GO/epoxy system, The results yielded 12%, 26% and 52% increment in flexural strength, flexural modulus, and impact strength, respectively compared to control. Thus a series of literature has been reported related to influence of various silane and ionic liquid modified nanofillers on properties of epoxy system but limited reports are available based on imidazole modified nanosilica. In view of the above, reinforcing effect of IS towards mechanical and thermal properties of epoxy system are reporting first time. 2. Experimental Details 2.1. Materials Tetraethylorthosilicate (TEOS, purity 98.0%) and (3-Glycidyloxypropyl) trimethoxysilane (GPTMS, ≥98%) was purchased from Alfa Aesar India Pvt. Ltd. Chennai (India), Absolute Ethanol from Changshu Yangyuan chemical (china) and ammonia solution (25 % NH3 in water) were obtained from Merck Specialties Private Limited, Mumbai (India). Diglycidylether of bisphenol-A based epoxy resin (Araldite LY556, Epoxy content: 5.5 eq/kg, Viscosity: 10,000-12,000 mPa-s at 25°C), anhydride based hardener, (Aradur HY 906, Viscosity: 175-350 mPa-s at 25°C) and 1-methyl imidazole as an accelerator (DY 070, viscosity: 50 mPa.s at 25 ºC) were supplied by Huntsman India Pvt. Ltd., Gujarat, India. 2.2. Synthesis of IS nanoparticles Pure silica nanoparticles (PS) was synthesized using sol-gel method [10]. However, synthesis of imidazole modified silica nanoparticles (IS) was performed using one pot sol-gel method [11]. In a typical procedure, 2 ml (3 M) of deionized water and 4 ml (0.50 M) of TEOS were added to 30 mL of absolute ethanol and stirred vigorously for 30 min using magnetic stirrer. subsequently, 4 ml (1 M) of 28% ammonium solution was added via pipette and the reaction solution is stirred for 4 h. Consecutively, modifying agent GPTMS (10 wt % of TEOS) and 2 g (25 mmol) 1-methyl imidazole was added to the above reaction mixture, then the resultant solution was purged with dry nitrogen and stirred for another 8 h. The modified nanoparticles were purified and separated repeatedly via centrifuge using ethanol and distilled water and the powders were dried at 90ºC for 24 h in a vacuum oven. 2.3. Preparation of epoxy nanocomposites Epoxy composites was prepared by varying content of IS (0-3 wt. %) embedded into hardener and thoroughly stirred using high shear impellor for 10 min followed by probe type sonication for 30 min. Then, stoichiometric amount of epoxy resin and initiator D070 (weight ratio of epoxy: hardener: initiator is 100:95:2) was added to the above mixture and continued mixing for another 5 minutes to obtain a homogeneous mixture followed by degassed under vacuum for 30 min. Then, the mixture was poured into a silicone rubber mold and kept in vacuum oven for pre-curing at120ºC (2 hours) and post cured at 160ºC (8 hours). For comparison, composites with PS was also prepared and designated as PSC, ISC and neat epoxy (NE).

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2.4. Characterization Methods FTIR spectra of silica particles were recorded with PerkinElmer spectrum 100 series equipment to identify the presence of functional groups. The samples were prepared by mixing fine grounded powder of silica particles with spectroscopic grade of KBr and converted to thin film of pellet form. The morphology and the particle size were examine under SEM (Zeiss, supra 55) at an accelerating voltage of 5 kV. Degradation behavior of epoxy composites were investigated by simultaneous thermal analyzer (NETZSCH STA 449 F3 Jupiter, Germany). The test was carried out by heating of solid sample (10 mg) in a temperature range 30 to 800ºC at a heating rate of 10ºC/min under nitrogen gas purging (30 ml/min) using sample of Al2O3 pan as a reference material. The tensile test of nanocomposites was performed by computerized universal tensile machine (INSTRON, Model 5969, USA) at a cross-head speed of 1 mm/min. The dog-bone shaped tensile samples with dimension of 90*10*5 mm with gauge length of 50 mm were prepared according to the ASTM D638 (Type-V) standard followed by grinding and polishing. 3. Results and discussion 3.1. Characterization of IS nanoparticles FTIR spectra of PS and IS nanoparticles are shown in Fig. 1. The IR peak of PS exhibits stretching and bending vibration modes of OH groups of physically adsorbed water molecules and silanol groups at 3428 and 1671 cm-1. The characteristic peaks near 1085 and 800 cm-1 were related to the symmetric and asymmetric stretching vibration of Si-O-Si bond and no band at 2875 cm–1 and 2945 cm–1 for -CH2 stretching peaks was observed. Hence no unreacted TEOS was present during synthesis of PS nanoparticles. However, the absorption peaks of IS at 1085 cm-1 is shifted to little lower wave number compare to PS, indicating the faster condensation rate of GPTMS in presence of 1-methyl imidazole. The appearance of new sharp absorption peak at around 1386, 2875, and 2945 cm-1 indicates the presence of C-N asymmetric stretching, -CH2 asymmetric and symmetric stretching mode of vibration [12]. The presence of these bands confirmed that the imidazole moiety is covalently attached to the surface of silica particles.

Fig. 1. FTIR spectra of pure silica nanoparticles (PS) and imidazole modified silica nanoparticles (IS).

A Typical SEM image illustrating the morphology and particle size of PS and IS nanoparticles as shown in Fig. 2 (a) and (b). The morphology of both of the particles is spherical in shape and the average particle size of PS and IS nanoparticles was observed as 170 nm and 8 nm respectively. The average particle size reduction of IS compares to PS nanoparticles is an indication of covalent bonding of imidazole groups to the surface of SiO2. The particle size

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reduction of IS can be tuned by controlling the degree of condensation rate between GPTMS and imidazole. As imidazole acts as a base catalyst due to its nucleophilic nature of pyridine type of nitrogen resulting strong interparticle interactions between imidazole and epoxy group of GPTMS linked within the siloxane network [13].

Fig. 2. SEM image of (a) pure silica nanoparticles (PS) and (b) imidazole modified silica nanoparticles (IS).

3.2. Thermal stability of nanocomposites The TGA curves of epoxy composites containing 1 wt % of PS and IS nanoparticles are shown in Fig. 3. The plots clearly reveal a single step degradation behavior for all types of nanocomposites. The initial decomposition temperature (TIDT, 5% wt. loss), maximum decomposition temperature (TMDT, 50% wt. loss) and char yield percentage (CYP) of the composites was measured from TGA plots. It was found that the decomposition temperature of ISC-1 is slightly improves in TIDT (353ºC) by 16°C and 7°C with respect to NE (337°C) and PSC-1 (344ºC) respectively. Moreover, the increase in TIDT may indicate the homogeneous dispersion of the particles which acts as obstacles to heat flow through the epoxy system [14]. The char yield percentage (CYP) of ISC was also found again higher than the ISC nanocomposites may be due to rigidity of the particles at 1 wt % with effective dispersion. Therefore, the overall thermal stability of ISC is much better than that of EP and PSC nanocomposites. Thus, it conclude the IS nanoparticles have the potential to restrict the thermal decomposition of epoxy system.

Fig. 3. TGA thermograms of epoxy composites containing 1 wt% of PS and IS nanoparticles.

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3.3. Tensile properties of nanocomposites Fig 4 (a) represents stress–strain curves for epoxy composites containing 1 wt% of PS and IS. The ISC composites at 1 wt% of IS exhibits highest tensile strength (84 MPa) and the failure strain (16 %). Effect of PS and IS on tensile strength and modulus of epoxy composites are shown in Fig. 4 (b). For epoxy composite with 1 wt% of IS, the tensile strength increased by 45% (84.0±3.6 MPa) with respect to NE. The increase in tensile modulus represent the improved stiffness, on the other hand the increase in elongation at break represent the improved ductility [15]. Further, increase of IS content, the tensile strength start decreases. From these results, it is clear that the reinforcing capability of IS was better than that of PS.

Fig. 4. (a) Representing stress–strain curves for epoxy composites containing 1 wt% of PS and IS; (b) Effect of PS and IS on tensile strength of epoxy composites.

4. Conclusion IS nanoparticles was prepared by sol-gel method. The attachment of imidazole group on the surface of silica was confirmed by FTIR and SEM characterization. From TGA results, a significant improvement in thermal stability of ISC nanocomposites was found at lower wt % of IS particle contents. The ISC nanocomposites also revealed a significant improvement in tensile strength of epoxy nanocomposites (1 wt %). This result confirms the effective stress transfer from the IS nanoparticles to the epoxy matrix may be due to enhancement in surface wettability of silica particles resulting strong covalent bonding at the interface. Such noticeable improvement in thermal and tensile properties demonstrates potential use of IS as a reinforcement in epoxy resin system. Acknowledgements The author(s) would like to acknowledge the SAIF, IIT Bombay (India) for providing facility of FTIR characterization. The author(s) also thanks to TEQIP-III, NIT Silchar, Assam (India) for provide financial support to attend the conference of ICRACM 2019 at IIT (BHU), Varanasi, UP (India) along with the publications. References [1] H. Gu, C. Ma, J. Gu, J. Guo, X. Yan, J. Huang, Q. Zhang, Z. Guo, Journal of Materials Chemistry C 4 (2016) 5890–5906. [2] F. Hussain, M. Hojjati, M. Okamoto, R.E. Gorga, Journal of Composite Materials 40 (2006) 1511–1575. [3] C. Velasco-santos, A.L. Martı, R. Ruoff, V.M. Castan, Chem. Mater. 15 (2003) 4470–4475. [4] S. Halder, T. Prasad, N.I. Khan, M.S. Goyat, S. Ram Chauhan, Materials Chemistry and Physics 192 (2017) 198–209. [5] T. Prasad, S. Halder, M.S. Goyat, S.S. Dhar, Polymers and Polymer Composites 39 (2018) 135–145. [6] Xu Q. Jie, S.B. Wang, F.F. Chen, T.C. Cai, X.H. Li, Z.J. Zhang, Nanomaterials and Nanotechnology 6 (2016) 1–7. [7] Y. Zhang, S. Huang, RSC Advances 6 (2016) 26210–26215. [8] E.S.A. Rashid, M.F.A. Rasyid, H.M. Akil, K. Ariffin, C.C. Kooi, Journal of Materials: Design and Applications 225 (2011) 160–170. [9] C. Zhang, X. Mi, J. Tian, J. Zhang, T. Xu, Polymers 9 (2017). [10] A. Paula, A. Carvalho, B.G. Soares, S. Livi, European Polymer Journal 83 (2016) 311–322.

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