Epoxy Resin

Epoxy resin is a type of thermosetting polymer, commercially available in different forms, has a wide range of application in aerospace, coating, electric insulation and marine application due to its outstanding performances, easy handling and low cost [1-5]. Epoxy high inner stress, brittle texture, inferior impact resistance, weak creep resistance and poor moisture resistance, limited its application in other industrial areas [6]. So, the epoxy was mixed with polymers[7], clays, metal [8], carbon nanotube [9], graphene[10], and graphene oxide[11] to overcome its limitations. Graphene oxide (GO) is a graphite derivative that produced through oxidation of graphite using various methods. Graphene oxide is a 2D sheet with oxygen functional at the surface (hydroxyl, carbonyl, and epoxide) and (carboxyl) at its edge [12,13]. Graphene oxide is a promising material as polymer reinforcement, due to its high mechanical properties[14], excellent thermal stability[15], and good insulation properties [16]. Moreover, graphene oxide high surface area, high aspect ratio, excellent sheet flexibility, reactivity sites, and good dispersion in most common solvent favored its use a composites material for polymeric material. The Graphene oxide tends to improve the mechanical properties, thermal stability, insulator, and dielectric properties of the various commercial epoxy resin. However, Graphene oxide sheets tend to aggregate either by Van der Waals force or ?-? stacking interaction, which would decrease the improvement effects of graphene sheets in epoxy matrix [17]. Therefore, graphene oxide was modified either through covalent bonds or by ?-? interaction to reduce these aggregation phenomena and enhanced epoxy properties. For instance, Wan et al improved epoxy tensile strength and modulus by 75% and 15% through mixing epoxy with graphene oxide modified with DGEBA (diglycidyl ether of bisphenol-A) [18].Also, Liu et al enhanced epoxy glass temperature and tensile strength by 26oC and 31% using grafted graphene oxide [5]. Sharmila et al prepared Graphene oxide modified with iron oxide nanoparticles, they epoxy composites shown improvements in the tensile, impact, and fracture strength [19]. Fluorinated polymer is polymer contain carbon-fluorine bonds and they has been applied in many domestic and industrial application due there excellent thermal stability, anti-chemical corrosion, weathering resistance, high dielectric conductivity, and low surface energy [20-22]. However, the high market price of the fluoropolymers and poor miscibility in epoxy resin restricted their applications [23]. Thermal stability, roughness, and corrosion resistance were improved for epoxy resin modified with a fluorinated polymer [24-27]. The fluorine atoms reduced the crosslinking density of the epoxy resin and fluoropolymer, reducing the mechanical properties of the epoxy [28]. The grafting of the fluorinated polymer into carbon base material such as carbon nanotube, fluorinated graphene, and fluorinated graphite may combine the excellent properties of carbon base material and fluorine group's properties. The miscibility was improved with enhancement of the mechanical properties, thermal stability, surface properties, and electric properties. Yang and coworkers grafted fluorinated polymer on reduced graphene oxide coated with dopamine. The modified graphene showed excellent dispersion in the ferroelectric polymer matrix with good high dielectric constant and low dielectric loss [29].Que et al modified epoxy resin (DGEBA) with trifluoromethyl containing polyimide structures. Compared with neat epoxy, they found that the modified epoxy shown better thermal stability with 18 to 55 % char yield at 800 0C, water contact angle (94.9–105.0 °), higher toughness, and lower water absorption (0.47–0.95 %). The tensile shear of modified epoxy was lower than the neat epoxy, the fluorine groups decreased epoxy surface energy which further reduced the adhesion strength compared with unmodified epoxy [7]. In this work, graphene oxide was modified with Hexadecafluoro (1, 10) decanediol through one-step reaction. Modified graphene oxide was mixed with epoxy resin in various loading. Then, the mechanical properties, thermal stability, water contact angle water uptake, and alkaline corrosion resistance were examined for epoxy composites. Results and Discussion:   The surface functionalization of GO was confirmed by FTIR, XRD, XPS, Raman spectroscopy, and TGA. Fig. (1) shows the FTIR spectrum of GO and GOFO. For GO typical peaks were observed at 3390 cm-1 (stretching vibration of C-OH), 1041 cm-1 (stretching vibration of C-O), 1374 cm-1 (deformation vibration of C-OH), 1261 cm-1 and 877 cm-1 (stretching vibration of C-O-C), and 1734cm-1 (stretching vibration of C=O). The peak at 1619 cm-1 is attributed to the skeletal vibration of the graphitic of GO [30]. After reaction with FO new peaks appeared. The peaks at 1178 cm-1 (stretching vibration of C-F) [31], at 2966 cm-1 (bending stretching of C-H), and at 1441 cm-1 (stretching vibration of C-H) from the perfluoro compound. The peak at 1080 cm-1 was related to stretching vibration of ester bond C-O-C. These results indicated that FO was successfully grafted onto GO surface.The XRD patterns of Graphite, GO, and GOFO was shown in Fig. (2) and the interlayer distance was calculated using Bragg equation. Pristine graphite is shown a typical peak at 26.410 with an interlayer spacing of 0.337 nm [32]. This peak shifted to 11.020 (corresponding to an interlayer space of 0.802 nm) after graphite was oxidized to GO, that was due to the formation of oxygen-containing functional groups on a graphite oxide surface and water molecules trapped between the layers [33]. The functionalization of GO with FO shifted the peak from 11.020 to 10.080 (corresponding to an interlayer space of 0.676nm), suggested that GO was modified with FO. Another broad diffraction peak was observed at 22.760, which suggests the formation of few layers of reduced graphite oxide. To further study the structural differences between graphite, GO, and GOFO, the graphitic crystal size was calculated using Sheerer equation and the number of layers was calculated based on crystal size. Graphite had a large crystal size of 21.28nm with 63-64 graphene layers. After oxidation to GO, the crystal size decreased to 10.92nm and the number of layers decreased to 10-11 graphene layers. The crystal size of GOFO was 3.78nm with 5 -6 layers of the modified graphene sheet. In order to study the exfoliation of GOFO into the epoxy matrix, the XRD patterns of EGOFO composites and neat epoxy were investigated as shown in Fig. (3). For neat epoxy, two broads peaks centered at ?70 and ?180 were observed due to the amorphous nature of neat epoxy. All Epoxy composites show the same characteristic diffraction peak as neat epoxy and no diffraction peaks of GOFO at 10.080 and 22.760 were observed [34]. That's confirmed that GOFO was highly exfoliated into the epoxy matrix. Although, the highly exfoliated level of GOFO sheets in the epoxy matrix cannot represent a good dispersion was obtained in the epoxy composites [35]. The thermographic analysis in Fig. (4) represents the thermal stability of GO and GOFO under a nitrogen atmosphere. Graphene oxide shows 8.404% weight loss between 50 0C -100 0C, due to evaporation of residual water trapped between the layers. A significant weight loss of 29.53% was observed between 100 0C – 250 0C, that's due to decomposition of labile oxygen functional groups to H2O, CO, and CO2 [22,10].Compared to GO, GOFO was more stable with 3.48% weight loss was observed between 50 0C -100 0C can be assigned to evaporate of residual water trapped between the layers, which was lower than GO. Between 100 0C – 250 0C, 8.18% weight loss was found due to decomposition of the reaming oxygen functional groups. Then, 12.44% weight loss was observed between 250 0C – 550 0C, that can be assigned to the decomposition of the bond between GO and perfluoro compound. In addition, compared with 48.59% weight loss at 700 0C for GO, GOFO undergoes 20.83% weight loss. The above data can be due to GO, bosses high amount of oxygenated functional groups, while GOFO possesses low oxygenated functional groups due to functionalization by FO. The above results confirmed that GOFO was more stable than GO.Raman spectroscopy was applied to explore the structural variations between GO and GOFO. Generally, the graphitic material shows two characteristic peaks, the G peak (~1575 cm?1) arises from first-order scattering of the E2g photon of sp2 C atoms and the D peak (~1350 cm?1) from a breathing mode of ?-point photons of A1g symmetry. Also, the intensity ratio ID/IG can be used to evaluate the graphitization degree of the graphitic material, the lower value represented a high degree of graphitization [30]. Fig. (5) shows Raman spectrum results of GO and GOFO. The G and D bands of GO were located at 1599.9 and 1357 cm-1 receptivity, as reported in previous reports [30,36,37]. After functionalization with FO, the G band was shifted to a lower wavelength at 1589 cm-1, no obvious changes were observed for the D band. In addition, the ID/IG intensity ratio of GO increases from 0.96 to 1.062 for GOFO. The changes in the intensity ratio imply that GOFO of graphene structure was restored, which can be due to the removal of oxygen functional groups. The GO and GOFO were characterized using X-ray photoelectron spectroscopy (XPS), the results are shown in Fig. (5) and the elemental analysis is tabulated in Table (1). The GO broad spectrum shows two peaks at binding energies around 286.1 eV and 532.4 eV, which were assigned to C1s and O1s, respectively. The atomic composition of graphene oxide was obtained from XPS broad spectrum and revealed the presence of carbon (71.88%) and oxygen (28.12%). In contrast, the GOFO broad spectrum displayed three peaks at binding energies around 286.1 eV, 532.4 eV, and 686 eV, they can be assigned to C1s, O1s, and F1s respectively. The atomic composition of GOFO obtained from the broad spectrum shown the existence of carbon (71.66%), oxygen (27.34%), and fluorine (1%). The carbon-to-oxygen ratio increased from 1.6 for GO to 2.6 for GOFO, due to functionalization with FO. The C1s XPS spectrum of GO and GOFO are shown in Fig. (6). The C1s spectra can be decomposed into four characteristic peaks: C=C/C-C (non-oxygenated carbon, 284.5 eV), C-OH/C-O-C (hydroxyl, epoxy, and 286eV), C=O (carbonyl, 286.8 eV), and COOH (carboxyl, 288.9 eV) [38]. In comparison, the decomposed C1s spectrum of GOFO shows additional peaks at 287.02eV, 290 eV, and 292 eV, assigned to -C-O- (ether bond), C-F (fluorine), and C-F2 (fluorine) respectively[39]. As shown in Fig. (5), compared with XPS C1s spectrum of GO (Fig. (6)), the peak intestines of C-O and C-O-C were significantly decreased, new peaks appeared. The above results indicated the attachment of FO to graphene oxide surfaces via the reaction with hydroxyl groups of FO with C-O groups on the GO surfaces [40]. As shown in the SEM images of EGOFO composites, the GOFO sheets were well dispersed in the epoxy matrix. GOFO nano-sheets were surrounded tightly by epoxy resin, which indicates a good interfacial interaction between GOFO and the epoxy matrix. Such good interfacial bonding improves the stress transfer from the epoxy matrix to the GOFO nano-sheet and thus enhanced the mechanical properties of epoxy composites. However, due to the presence of incompatible fluorine groups on the GOFO, micro-voids can be observed and such voids could enhance crack initiation which leads to decrease the mechanical properties of epoxy composites[41]. SEM analysis of epoxy composites: The SEM images of epoxy and epoxy composites at fracture site after the tensile test are shown in Fig. (7). The images were used to examine the dispersion and the compatibility of GOFO in the epoxy matrix. Fig. (7a,b) shows neat epoxy exhibited relatively smooth surface with a river-like pattern in the fracture direction, confirmed the brittle nature of epoxy [42]. In contrast epoxy composites displayed rougher, fluctuant surfaces with GOFO nano-sheet pulled out of the epoxy matrix Fig. (7 c,e). Also, epoxy composites SEM images shown that GOFO nano-sheets were well dispersed in the epoxy matrix, a well interfacial interaction between the nano-sheets and epoxy composites Fig. (7 d,f). The good interfacial bonds can effectively transfer the stresses from the epoxy matrix to the GOFO nano-sheets and thus enhanced the mechanical properties of epoxy composites [41]. However, several micro-voids can be observed in all epoxy composites that can be due to the presence of fluorine groups. The low compatibility between the epoxy resin and fluorine groups caused the appearances of micro-voids. Cracks can initiate easily in the micro-voids, then spread through epoxy composites reducing epoxy composites strength. Mechanical properties of epoxy composites: Tensile test:The tensile test was conducted to investigate the effects of GOFO on the epoxy resin performance. Fig. (8) shows the stress-strain curves of neat and epoxy composites, and the results were summarized in table (2). The tensile modulus of pure epoxy was 471 MPa and its changes to 486 MPa (increased by 3.2 % with 0.1 wt. %), 507 MPa (increased by 7.6 % with 0.3wt. %), and 530 MPa (increased by 12.52 % with 0.5 wt. %). The highest tensile modulus was 12.52% compared to neat epoxy corresponding to 0.5% GOFO loading. The increase in the tensile modulus was due to the good dispersion and interfacial interaction between the GOFO and epoxy matrix. That's led to the good transfer of stress from the matrix to GOFO sheets. However, the tensile strength decreased by 18.35%, 22.15%, and 20.25% with 0.1 wt. %, 0.3 wt. %, and 0.5 wt. %. GOFO loading. The formation of micro-voids between the GOFO sheets in the epoxy matrix due to incompatibility between GOFO sheet and epoxy system due to the presence of fluorine groups. Fracture test:  Ã‚  The effects of GOFO loading on the fracture properties are shown in Table (2) and the stress-strain curves of fracture analysis were shown in Fig. (9). EGOFO composites shown a battened in the fracture modulus and strength to those of neat epoxy upon different loading. The fracture modulus was 1510MPa for neat epoxy. With 0.1, 0.3, and 0.5 wt. % loading of FO, the fracture modulus increased to 2340 ±17.61 MPa, 2324 ±73.5 MPa, and 2450 ±45 MPa respectively, these were 54.97, 53.84, and 62.85% respectively. However, for fracture strength, it's behaved differently with increasing GOFO concentration. The fracture strength decreased by 13.12% with the addition of 0.1 wt.% GOFO loading and then increased to 13.61% and 16.98% with 0.3 wt.% and 0.5 wt.% GOFO loading respectively. The increase in the fracture modulus and strength can be attributed to improvement in the interfacial interaction between the GOFO and epoxy matrix. That's allowed better load transfer from the epoxy matrix to the GOFO sheets, which increased both fracture modulus and strength. Thermal stability of epoxy composites:The Thermal stability of the GOFO nanocomposites was investigated by TGA as shown in Fig. (10) and Table (3).T10% and T50% are the temperature of sample weight loss reached 10% and 50 % respectively. Tdec is the sample decomposition temperature. All the specimens showed similar thermal degradation behavior with main weight loss between 3500C to 4500C. They exhibited different weight loss during the onset decomposition stages as shown in the enlarged window. The incorporation of GOFO significantly enhanced the thermal stability of epoxy composites in different trends. Epoxy composite 0.3 wt. % GOFO loading has a higher T5%, T50%, and Tdec compared with other epoxy composites. The T5% and T50% increased by ?13 and ?150C respectively based on pure epoxy. The Tdec increased by ?13 with the same loading compared to pure epoxy. According to the literate, there are two factors effects on the thermal stability of polymer composites: the formation of networks via covalent linkages between them and the crosslinking density [43,44]. The existence of the oxygen functional groups at the edge and the surface of the GOFO, can react with the epoxy resin and increased the crosslinking of the composites. The XRD data showed that GOFO doesn't have a uniform crystal shape compared to graphite, that makes the GOFO has a poor thermal conductivity. Therefore, with good dispersion of GOFO, it will act as a barrier and reduce the thermal conductivity of the epoxy composites [44]. Water uptake epoxy composites:The influences of water absorbed by the polymeric material can be significantly reduced by incorporating nano-size fillers into polymers [45,46].The water uptake of neat epoxy and its composites filled with GOFO were shown in Fig. (11). The addition of GOFO nano-fillers significantly decreased the amount of water absorbed by epoxy composites. The water uptake by neat epoxy was 0.432%, which decreased to 0.317% , 0.304% , and 0.308% for epoxy composites with 0.1wt.% , 0.3wt.% , and 0.5 wt.% GOFO loading respectively. this phenomenon implied that the addition of nano-size sheet with high aspect ratio provided tortuous pathways for water molecules to enter the epoxy composites and acted as a barrier water transportation through epoxy composites [47]. Another reason can be due to the hydrophobic nature of fluorine groups on GOFO, which can immobilized water molecules and prevented it from diffusion in epoxy matrix[47]. The hydrophobic behavior of GOFO composites:Fig. (12). Shown the air/water contact angle of pure epoxy and its composites. The measurement was performed at room temperature with three repetitions for each sample. From Fig. (12), the incorporation of GOFO increased the contact angle of all epoxy composites. The contact angle increased from 68.70 o  ± 1.73 for the pure epoxy to 83.53 o  ± 1.13 for 0.1 wt. % GOFO, 98.65 o  ± 2.49 for 0.3 wt. % GOFO, and 89.22 o ± 0.70 for 0.5 wt. % GOFO. The changes in the contact angle indicate that epoxy composites show more hydrophobic effects compared to pure epoxy [48]. The GOFO tends to immigrate to the surface of epoxy, due to the low surface energy of the fluorine atoms. They altered the surface energy and increased the hydrophobicity of the epoxy surface. But, with 0.5 wt. % GOFO loading, the contact angle decreased to 89.22 o ± 0.70, the aggregated GOFO nano-sheet at the surface can slightly reduce the polarity of the surface and increased the effects of the reaming oxygenated function groups of the graphene sheet.