Epoxy composites reinforced with multi-walled carbon nanotube/poly(ethylene glycol)methylether-coated aramid fiber

1 Introduction

The properties of aramid fibers make them suitable for use in numerous industries. Aramid fibers are employed in cables, optical fibers, ballistics, conveyer and transmission belts, linear tension members, tires, and cut and heat protection [1]. Owing to their stiffness, low density, and noble resistance to crack, these fibers are widely utilized in transportation and marine industries for constructing stronger, stiffer, lighter, and more durable materials [2]. In recent times, carbon nanotubes (CNTs) have been the focus of considerable research on innovative, next-generation materials with high stiffness and high strength [3]. CNTs have presented potential requisitions in composites based on their outstanding thermal, electrical, and mechanical properties, and on their low density and high aspect ratio [4]. For these reasons, there have been numerous efforts to use CNTs in the consolidation of composite resources to overcome the extent of performance of traditional materials. Nevertheless, research in this field has dealt with polymer/CNT composites that offer remarkable strengthening impact for the subsequent materials [5]. Moreover, the literature has shown that the fracture toughness, shear, thermal, flexural, and tensile properties of polymer-based composite materials improve considerably with a 0.1–5 wt.% loading of CNTs [6, 7]. Because of the interfacial interactions, mainly in the interphase of superficially functionalized polymer/CNT composite, recently few investigations have focused on the preparation and design of hybrid fiber/CNT composites by developing CNTs over fibers [8, 9]. The CNT coated fibers have potential to increase the thickness features of polymer-based composites and to harden the matrix/fiber interface [10, 11]. The CNTs in multi-scale hybrid composites not only can be grown directly over the fabric surface or fiber, but also can be disseminated in the matrix. The CNTs provide a potential to mend the thermal, electrical, and mechanical properties of the base materials [12]. It has been verified that the addition of a mere 1 wt.% of CNTs into the material matrix can increase the stiffness of the subsequent composites by as much as 36–42% and their tensile strength by 25% [13]. Furthermore, the existence of CNTs in the host matrix may alleviate the various downsides of traditional fiber composites, particularly the interlaminar properties and longitudinal compression [14, 15].

Thermosetting polymer matrices have contributed in fiber-integrated composites. In the thermosets, epoxy resin is the imperative resin. Generally, thermoplastic resins are amalgamated with epoxy resin to enhance the toughness [16, 17]. Typically, it is problematic to achieve homogeneous blends as intermolecular contacts are intricate in thermosetting blends relative to linear polymer blends. In recent times, several miscible epoxy blends have been investigated such as the blend of epoxy and polyethylene oxide (PEO). The epoxy/PEO blend was found to be miscible on treatment with phthalic anhydride (PA) and 4,4′-diaminodiphenylmethane (DDM). In aromatic amine curative systems, the miscibility was attributed to the formation of intermolecular hydrogen-bonding interactions [18]. The methods of melt blending and slurry (solution) approaches have been used for obtaining epoxy composites. Each of these methods yields a different set of mechanical properties. A disadvantage of the slurry method is the requirement of organic solvent for polymer solubilization, whereas the melt method often suffers from poor filler dispersion. An alternative technique is the use of infiltration to obtain composites with fine matrix-reinforcement interaction. Thermoset epoxy EPON Resin 862 (bisphenol F) and a curing agent (3:1 by weight) were introduced into the residue of a 1 wt.% nanocomposite after nitrogen gasification at room temperature under vacuum. A two-stage curing process is used to form epoxy/single-walled CNT (SWCNT) composites at 80°C for 2 h up to 150°C for 2 h [19]. In another attempt, epoxy-infiltrated hydroxyapatite composites were prepared [20]. A monoglycidyl ether-based epoxy resin and an Epodur 231 curing agent with a 100:27.5 ratio (wt/wt) were used. The epoxy and the hardener were mixed and heated to 60°C. Afterwards, the mixture was poured into a mold, permitting the immersion of hydroxyapatite pieces. Composites reinforced with epoxy aramid fibers have been reported to cure in an autoclave, under a pressure of 0.69 MPa, following a heating cycle of up to 181°C [21]. Epoxy hybrids reinforced with CNTs and aramid fibers have been produced. The thermoplastic nanocomposites were packed by CNTs, and the films of the nanocomposites were casted by CNT dispersion. The epoxy resin was thermally treated up to the melting point of the thermoplastic resin. The resin was permeated into preformed fibers, and the nanocomposite film was liquefied by means of the epoxy resin. The CNT particles were distributed in the sections between the aramid fibers. Finally, the epoxy resin was treated and the hybrid CNT/aramid fiber composite was manufactured. In this study, multi-walled carbon nanotubes were first functionalized with carboxylic acid (MWCNT-COOH) and then with poly(ethylene glycol) methyl ether (MWCNT-PEGME). The latter was adopted as the thermoplastic polymer because CNTs are known to disperse well in PEGME and to become miscible with the epoxy. Furthermore, the PEGME may reduce the processing temperature and time, which has become the current focus in the aerospace industry [22]. The aramid fibers were then coated with the homogeneous dispersion of MWCNT-PEGME using electrophoresis. Finally, the epoxy/aramid-MWCNT-PEGME composites were prepared. The epoxy/aramid-MWCNT-PEGME hybrid composites consisted of four components, namely, aramid fibers, carbon nanotubes, PEGME, and epoxy resin. Thermal and morphological characterization of the composites were performed to confirm the dispersion state of MWCNTs via differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM).

2 Materials and methods

2.3 Preparation of modified MWCNT–coated aramid fibers by electrophoresis

Because of the ability of MWCNTs to respond to an electric field, electrophoresis was used as a simple technique for the deposition of nanotubes on the surface of aramid fibers [24]. PEGME-MWCNTs were deposited uniformly on the surface of aramid fibers from aqueous nanotube dispersions when a DC potential was applied between the aramid fibers and the counter electrode. Because of the negative charge, the MWCNTs migrated towards the positive aramid fiber electrode and were subsequently deposited on the fiber surface [25, 26]. Forty weight percent of aqueous nanotube dispersion was deposited uniformly on the surface of the aramid fibers. The weight percent of the CNT solution was taken to be constant each time for electrophoresis to ensure the uniform coating of nanotubes on the fibers. Figure 1 shows the deposition of the nanotubes on the surface of the aramid fibers via electrophoresis [27].

Figure 1:

Deposition of carbon nanotubes on the fiber surface by electrophoresis.

2.4 Preparation of the epoxy and aramid-MWCNT-PEGME fiber composites

To prepare the epoxy and aramid-MWCNT-PEGME hybrid composites, the aramid fibers were placed in a Petri dish. The mixture of the epoxy resin and the curing agent was poured on the modified fibers (3:1 by weight). The mixture was heated up to 100°C and smoothly shaken to allow the aramid-MWCNT-PEGME fibers to interact with the epoxy. Finally, the Petri dish was kept in a vacuum oven to cure the resin at 80°C for 4 h. The schematic formation of MWCNT-coated aramid fibers and epoxy composites is shown in Figure 2. Epoxy/aramid fiber composites using unmodified fibers were also prepared using the same technique. Sample designations used are given in Table 1.

Figure 2:

Formation of MWCNT-coated aramid fibers and epoxy composites.

Table 1

Sample designations used.

Sample code	Sample	Fiber content (wt.%)
MWCNT-PEGME	Poly(ethylene glycol) methyl ether-functionalized multi-walled carbon nanotube	–
Aramid-MWCNT-PEGME	Poly(ethylene glycol) methyl ether-functionalized multi-walled carbon nanotube-coated aramid fibers	40 wt.% CNTs coated on 10 g of fibers
Epoxy/aramid-MWCNT-PEGME 10	Epoxy and poly(ethylene glycol) methyl ether-functionalized multi-walled carbon nanotube-coated aramid fibers	10
Epoxy/aramid-MWCNT-PEGME 20	Epoxy and poly(ethylene glycol) methyl ether-functionalized multi-walled carbon nanotube-coated aramid fibers	20
Epoxy/aramid-MWCNT-PEGME 30	Epoxy and poly(ethylene glycol) methyl ether-functionalized multi-walled carbon nanotube-coated aramid fibers	30
Epoxy/aramid-MWCNT-PEGME 40	Epoxy and poly(ethylene glycol) methyl ether-functionalized multi-walled carbon nanotube-coated aramid fibers	40
Epoxy/aramid 10	Epoxy and unmodified aramid fibers	10
Epoxy/aramid 20	Epoxy and unmodified aramid fibers	20
Epoxy/aramid 30	Epoxy and unmodified aramid fibers	30
Epoxy/aramid 40	Epoxy and unmodified aramid fibers	40

3 Results and discussion

3.1 Tensile strength and modulus

The addition of modified nanotube-coated aramid fibers to the matrix may significantly improve the composite mechanical properties. The tensile properties are an important indicator of the composite mechanical properties. The improvement in tensile properties may be attributed to the formation of a better interface structure that allows the stress transfer from the matrix to the reinforcement [28]. Figure 3 shows the improvement in the mechanical properties of modified CNT-coated aramid fiber-reinforced epoxy composites. The figure shows a comparison of the properties of epoxy/aramid-MWCNT-PEGME 10, epoxy/aramid-MWCNT-PEGME 20, epoxy/aramid-MWCNT-PEGME 30, and epoxy/aramid-MWCNT-PEGME 40 composites. The tensile strength increased from 658 to 1198 MPa. In the composites, different reinforcements caused dissimilar tensile properties. As can be seen in the figure, the lowest tensile strength was found for neat epoxy at 658 MPa. The epoxy/aramid-MWCNT-PEGME 10 showed a tensile strength of 697 MPa; the epoxy/aramid-MWCNT-PEGME 20, around 999 MPa; and the epoxy/aramid-MWCNT-PEGME 30, 1030 MPa. The tensile strength was found to be higher for the epoxy/aramid-MWCNT-PEGME 40, which was around 1198 MPa. Literature studies have shown that even very low loadings of nanotubes may result in beneficial effects on the tensile strength of fiber-reinforced epoxy composites [29, 30]. Figure 4 shows a comparison of the tensile modulus of epoxy/aramid-MWCNT-PEGME composites. This study shows that the tensile modulus increased from 62 GPa (neat resin) to 96 GPa in the epoxy/aramid-MWCNT-PEGME 10 composite and to 180 GPa in the epoxy/aramid-MWCNT-PEGME 40 composite. The tensile modulus was found to be comparable to that found in the literature for fabricated epoxy/glass-fiber composites [31]. To assess the results, epoxy/aramid fiber composites using unmodified fibers were also studied. In this series, the tensile strength increased from 422 to 888 MPa. Moreover, the modulus increase was in the range of 34–89 GPa. The increase in the tensile strength and modulus was less pronounced as compared to that of the modified fiber series. The results indicated that the coating of the modified MWCNTs may significantly improve the tensile properties of the composites.

Figure 3:

Comparison of the tensile strength of epoxy/aramid-MWCNT-PEGME composites.

Figure 4:

Comparison of the tensile modulus of epoxy/aramid-MWCNT-PEGME composites.

3.2 Flexural properties

The flexural strength of the composites is shown in Figure 5. The flexural strength of the epoxy/aramid-MWCNT-PEGME composites was found to be a function of filler content. The epoxy/aramid-MWCNT-PEGME 40 composite showed the highest flexural strength, whereas the epoxy/aramid-MWCNT-PEGME 10–30 composites exhibited a lower strength. The maximum flexural strength for the epoxy/aramid-MWCNT-PEGME 40 composite was 1593 MPa, whereas that for the epoxy/aramid-MWCNT-PEGME 30 was 1525 MPa. The improved performance may be attributed to the enhanced interfacial adhesion between the matrix and the fibers. The epoxy/aramid-MWCNT-PEGME 20 and epoxy/aramid-MWCNT-PEGME 10 composites showed a relatively lower flexural strength of 1470 and 1338 MPa, respectively. In contrast, neat resin showed the lowest flexural strength of 1158 MPa. Here, the fiber-matrix adhesion and stress transfer at the interface are important parameters influencing the flexural properties of fiber-reinforced composites [32, 33]. In the series of epoxy and unmodified aramid fiber, the flexural strength of all the composites was lower relative to that of the modified fiber series. Epoxy/aramid 10, epoxy/aramid 20, epoxy/aramid 30, and epoxy/aramid 40 had a flexural strength of 1298, 1320, 1395, and 1412 MPa, respectively. Here, the epoxy/aramid 40 composite again had the highest flexural strength among the unmodified fiber series; however, the value was lower than that of the functional series.

Figure 5:

Comparison of the flexural strength of epoxy/aramid-MWCNT-PEGME composites.

3.3 Fracture surface analysis

The tensile fracture surface of the tested specimens was examined using SEM (Figure 6). In order to examine the failure mechanism of the composites, the micrographs of epoxy/aramid-MWCNT-PEGME 10 (Figure 6A), epoxy/aramid-MWCNT-PEGME 30 (Figure 6B), and epoxy/aramid-MWCNT-PEGME 40 (Figure 6C and D) are presented. The micrographs show that the fiber filaments were fully covered with the epoxy resin. As can be seen clearly in the figure, there was no separation of the fibers from the matrix owing to the fine interaction between the components. In Figure 6A–C, there were very few fibers where the pull-out phenomenon was observed. Interface debonding, matrix cracking, or interlaminar delaminations were not observed either. For comparison purposes, the micrograph for neat epoxy is presented in Figure 6F. Pure epoxy showed a gyroid-type morphology with fewer pores in the structure. To study the interfacial interaction between the epoxy and the fibers, a higher resolution image of epoxy/aramid-MWCNT-PEGME 40 was made at 20 μm, which provided some interesting information about the matrix/fiber bonding. A strong adhesion was observed at the interface that encapsulated the fibers in the matrix, which caused a strong bonding. At higher resolution (Figure 6E), micro-cavities were observed due to fiber pull out. The occurrence of smaller arches means that the interfacial bonding between the fiber and the matrix was strong, which could provide an efficient stress transfer from the matrix. This caused an improvement in the mechanical properties of the composites.

Figure 6:

FESEM images of (A) epoxy/aramid-MWCNT-PEGME 10, (B) epoxy/aramid-MWCNT-PEGME 30, (C) epoxy/aramid-MWCNT-PEGME 40, (D) epoxy/aramid-MWCNT-PEGME 40, and (E) epoxy/aramid-MWCNT-PEGME 40 at 20 μm, and (F) neat epoxy at 2 μm.

3.4 DSC study

To investigate the effects of loading of aramid fibers coated with modified MWCNTs on the thermal properties of epoxy resin, thermal analysis was performed using DSC. Figure 7 shows that the T_g value of neat epoxy was 162°C, whereas for varying weight percentages of reinforced (10, 20, 30, and 40 wt.%) composites the T_g value was 164°C, 168°C, 170°C, and 173°C, respectively. The higher values of T_g may be attributed to the introduction of MWCNT-coated aramid fibers, which interacted with the matrix to restrict the mobility of the epoxy network. Hence, the matrix-filler interaction between MWCNT-coated fibers and epoxy caused a chain confinement. The T_g value of the unmodified epoxy/aramid composites was found to be relatively lower, in the range of 163–165°C. Moreover, electrophoresis was found to be an efficient technique for forming MWCNT-coated aramid fibers for use as a reinforcement [34].

Figure 7:

DSC thermograms of epoxy/aramid-MWCNT-PEGME at 10°C/min in N₂.

4 Conclusions

The purpose of the current research was to determine whether electrophoresis is an efficient method for coating aramid fibers homogeneously with modified nanotubes. Reinforcement with modified aramid-MWCNT-PEGME fibers made it possible to tailor the mechanical properties of novel epoxy composites by means of selective content. In order to completely inject the epoxy resin into the aramid fibers, PEGME-MWCNTs were coated over the fiber surface via electrophoresis. In this way, it was possible to distribute the nanotubes in the desired region between the fibers. FESEM observation indicated that the composites have a fine matrix-filler interaction, with least fiber pull-out phenomenon. The epoxy/aramid-MWCNT-PEGME 40 composite showed superior mechanical properties compared with the other composites. The difference in the property was significant compared with the neat epoxy as well as with the composites with lower loading level. Epoxy/aramid fiber composites were also prepared using unmodified fibers, and the tensile strength, modulus, flexural strength, and T_g were studied. The properties were found to be lower than those for the epoxy/aramid-MWCNT-PEGME composites. The manufacture of novel high-performance multi-scale hybrid composites is particularly important to achieve advanced engineering applications.