Effective epoxy composite coating mechanical/fracture toughness properties improvement by incorporation of graphene oxide nano-platforms reduced by a green/biocompataible reductant

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Abstract

In this study, graphene oxide nano-platforms were functionalized/reduced (rGO-His) by a green reductant based on histamine. Additionally, the rGO-His was modified by zinc metal cations in two ways (I and II) and characterized by FT-IR, XRD, XPS, HR-TEM and FE-SEM. Then, by dynamic mechanical thermal analysis (DMTA), tensile test and thermogravimetric analysis (TGA) the mechanical/thermal properties of the epoxy composite coatings filled with rGO-His and rGO-His-Zn (I and II) were evaluated. Results of tensile and DMTA analyses proved significant increment in tensile strength, elongation at break, cross-linking density and thermal stability of the coating in the presence of rGO-His.

Introduction

Discovery of graphene has opened a new point of view toward several structural applications like super capacitors [1], solar cells [2], sensors [3], actuators [4], polymer nanocomposites [5] and so on [6], [7]. Typically, graphene nanosheets are among the most widespread investigated types of carbon allotropes including fullerene (spherical shape), carbon nanotube (tubular form), graphite (hexagonal shape) and diamond [8]. It is common knowledge that graphene is a two-dimensional crystalline network composed of densely honeycomb carbon atoms with sp2 hybridization [9]. Actually, graphene has been famed because of its incredible elasticity, high tensile strength, lightness, high mechanical strength and huge surface area. Likewise, graphene accounts for high thermal conductivity even better than copper and other optical, chemical and electronic features that can easily surpass other inorganic nanomaterial [10], [11], [12]. It is interesting to note that polymers are generally cheap, lightweight, highly flexible and easy to fabricate. Epoxy resin gained tremendous attention thanks to virtuous chemical and corrosion resistance, low shrinkage upon curing process, low molecular weight and striking adhesion to substrate [13], [14], [15]. By the contrast, application of the thermosetting epoxy resin has been limited because of the brittleness and high sensitivity to temperature. The reaction of epoxy resin with hardener led to fabricate a highly cross-linked three-dimensional microstructure network that facilitated creep resistance and temperature tolerance. However, to obtain the desirable application, epoxy resin should be toughened and the temperature tolerance should be enhanced. Therefore, filling the epoxy resin with nanoparticles or toughening the epoxy resin via rubber would be effective [14], [16], [17], [18]. In the light of these facts, amalgamation of small portion of graphene nanosheets in the polymer matrix would result promising enhancement of the thermal and mechanical properties of polymer nanocomposites (PNCs) [19]. Generally, application of PNC is almost linked to the interfacial bonding between the polymer matrix and reinforcing agent and also the quality of dispersion. The strong Van der Waals forces between the graphene nanosheets derived from the ππ interactions and lack of any functional groups are responsible for the severe agglomeration of nanosheets in the polymer matrix. To tackle this problem, an oxidizing form of graphene family namely graphene oxide has been proposed [20], [21]. Researchers concluded that the inclusion of small portion of GO into epoxy matrix led to providential enhancement of tensile strength and Young’s modulus [22]. On the basal plane of GO nanosheets the epoxy and hydroxyl groups are existed while on the edges, carbonyl and carboxylic acid groups are available. Thus, GO easily disperses in water and the dispersion of GO in some organic solvent is still ambiguous [23]. A favorable procedure for increasing the quality of dispersion and reinforcing the interfacial bonding is GO nanosheets modification via organic compounds through covalent and/or noncovalent bonding that was accompanied with partial reduction of GO or functionalization [24], [25]. Inorganic nanoparticles like TiO2, ZnO, Al2O3, SiO2, Magnetic, Ag and Au have been used as modifiers of graphene oxide nanosheets [26]. In a research study polybenzoimidazole was covalently grafted on the graphene nanosheets to enhance the interface of filler and epoxy matrix and also the mechanical properties of the epoxy composite [27]. The presence of 0.25 wt% Poly (vinyl imidazole) (PVI) grafted graphene oxide nanosheets in the epoxy composite enhanced the Young’s modulus and tensile strength of the epoxy nanocomposite about 45.5% and 59.6%, respectively [17]. Ramezanzadeh et al. [28] prepared graphene oxide nanosheets with three different lateral sizes and covalently modified them with p-phenylenediamine. Results revealed that the small and medium lateral sizes of modified graphene oxide properly increased the glass transition temperature (Tg) and tensile strength of the epoxy composite. They expressed that the small lateral size nanoplatelete showed better dispersion in resin and resulted in greater mechanical strength. Lei et al. [29] has introduced the imidazole ring on GO surface through the addition of isophorone diisocyanate to utilize the co-curing effect of imidazole. The maximum tensile strength, flexural strength and impact strength of 40%, 10.8% and 42%, respectively, were obtained in the presence of 0.8% filler according to one pot method. Reduction of graphene oxide usually takes place through thermal and chemical reduction procedures [30], [31]. In the case of chemical reduction of GO, exerting hydrazine [32], hydroquinone [33] and dimethylhydrazine [34] as the most conventional reducing agents can ensure the high degree of reduction while they have been identified as environmental contaminates and high toxic materials. Additionally, the high degree of reduction decreases the interlayer distance and the number of layers in each stack increases. Therefore, the nanosheets tend to aggregation in the epoxy matrix. For sake of environment, green reducing agent like vitamin C [35], mashroom [36], zinc powder [37], aluminum powder [38], green tea [39] and so on have been utilized to obtain reduced GO with small defects in compared with strong toxic reducing agents.

Histamine is a biogenic amine retained the role of a chemical mediator of hypersensitivity and several physiological processes. Histamine has been found in all human and animal tissues particularly in mast cells, basophiles, lymphocytes, gastric mucosa, lungs and liver and also effectively in epidermis and neurotransmitters of central nervous system. Accordingly, histamine mediates inflammation, neural modulation and gastric acid secretion [40], [41], [42]. Intriguing point is the presence of histamine in plants like stinging Nettle and also in various foods. In fact, histamine is a low molecular weight hormone that is composed of an amine–ethyl part and an imidazole ring in the two tautomer face which can be produced through the decarboxylase of l-histidine [43]. Additionally, the affinity between the metal cations like zinc and GO nanosheets is not strong enough. Therefore, introducing a mediator like histamine would be effective in creation of strong bond through organo–metal complex formation.

In this investigation, we have modified graphene oxide nanosheets with green reducing agent namely histamine and in the next step the zinc cations were adsorbed on rGO-His nanosheets. The samples were characterized by FT-IR, XRD, Raman spectroscopy, XPS, FE-SEM, HR-TEM and zeta potential analyses. The rGO-His and rGO-His-Zn were augmented with epoxy resin to fabricate polymer nanocomposite. The thermal/mechanical properties of the samples were investigated by DMTA, tensile and TGA tests.

Section snippets

Materials

Expandable graphite, purchased from Kropfmuehl Graphite Co. Germany, was used for GO synthesis. Histamine dihydrochloride was supplied from Sigma-Aldrich Co. and used as green biogenic reductant. Analytical grade of potassium permanganate (KMnO4, Merck CO., Germany), sodium nitrate (NaNO3, Merck CO., Germany), hydrogen peroxide (H2O2, Merck CO., Germany), zinc nitrate tetrahydrate (Zn(NO3)2·4 H2O, Merck), hydrochloric acid (HCl, 37%, Fluka) and sulfuric acid (H2SO4, 98%, Fluka) were prepared.

FT-IR analysis

FT-IR analysis was performed to characterize the interactions between the histamine and GO nanosheets. According to Fig. 2, the absorption peaks located at 3735 cm−1, 2950 cm−1, 1734 cm−1 and 1630 cm−1 are associated with Osingle bondH stretching, Csingle bondH stretching vibration, Cdouble bondO stretching vibration and vibration of Cdouble bondC, respectively. Additionally, the peaks located at 1380 cm−1 and 1056 cm−1 wavenumbers are attributed to the stretching vibration of Csingle bondOH, and the stretching vibration of epoxide groups (Csingle bondOsingle bondC),

Conclusions

This article has highlighted the importance of functionalization of GO nanosheets with histamine as a green reducing agent and zinc cations on the mechanical and thermal properties of epoxy-based nanocomposites. An encouraging finding is about the role of histamine on GO nanosheets that is functionalization and reduction of platelets simultaneously. Furthermore, findings corroborate the idea that there is a complex formation between the zinc cation and the histamine molecules grafted on GO

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      Therefore, nanoparticles in the epoxy coating based on their physical and chemical natures may reduce the chemical crosslinking density. On the other hand, the calculated crosslinking density in the presence of nanoparticles is apparent and depends on both the chemical crosslinking density and elastic modulus of the incorporated nanoparticles [57]. The GO nanosheets have a high elastic (about 207.6 ± 23.4 GPa) modulus [14].

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