Effect of graphene oxide addition on the interlaminar shear property of carbon fiber-reinforced epoxy composites

NEW CARBON MATERIALS Volume 32, Issue 1, Feb 2017 Online English edition of the Chinese language journal Cite this article as: New Carbon Materials, 2017, 32(1): 48-55

RESEARCH PAPER

Effect of graphene oxide addition on the interlaminar shear property of carbon fiber-reinforced epoxy composites Xiao Han, Yan Zhao*, Jian-ming Sun, Ye Li, Jin-dong Zhang, Yue Hao School of Materials Science and Engineering, Beihang University, Beijing 100191, China

Abstract:

Carbon fiber-reinforced composites were manufactured by hot pressing stacked carbon fiber prepregs using graphene oxide

(GO)-modified epoxy resin as the matrix. Tetrahydrofuran was used as the solvent to disperse GO in the epoxy resin. Results showed that a homogeneous GO-modified epoxy system could be obtained, which was stable for approximately 3 h, long enough to produce the prepreg. The incorporation of 0.10 wt% GO into the epoxy resin achieved the largest interlaminar shear strength (ILSS) of 96.14 MPa for laminates, 8.05% higher than that without GO. Also, the glass transition temperature of the composite was increased by approximately 5 °C. The improvement of ILSS could be attributed to the toughening of the epoxy resin and an improvement in the interfacial adhesion between carbon fibers and epoxy matrix. Key Words: Graphene oxide; Composite; Carbon fiber; Epoxy resin; ILSS

1 Introduction

multiscale graphene/carbon polymer composites.

Carbon fiber reinforced polymer composites (CFRPs) have been increasingly used in high-performance applications such as aerospace, military, automotive and sport industries, owing to their improved in-plane mechanical properties such as high tensile strength, high elastic modulus and excellent stiffness offered by reinforcing fibers [1]. While the fiber-dominated in-plane properties outperform those of many structural materials made from monolithic metals and alloys, the Z-axis through-the-thickness properties, such as delamination resistance, are relatively inferior due to an apparent low performance of matrix-dominated interface. Therefore, the improvement of such properties may expand the applications of composite to wide fields [2-4].

Herein we propose a process for preparing CFRP composites using epoxy resin with well dispersed GO nanosheets as matrix. For the first time, tetrahydrofuran (THF), a specific organic solvent for composite processing, was selected in this work to modify epoxy resin, owing to its superior properties for achieving homogeneous GO dispersion over other traditional organic solvents. The THF was easily removed by a rotary evaporator under vacuum, leading to a well-dispersed GO in epoxy resin. Interlaminar shear strength (ILSS) of the composites was measured using short beam shear (SBS) test to examine the effect of GO addition on the through-the-thickness property of CFRPs.

With the development in nanocomposite technology, nanofillers are considered as ideal reinforcement candidates to improve the interlaminar mechanical properties of CFRP laminates [5-8], which include various carbon nanomaterials, such as carbon blacks, carbon nanotubes, fullerene and carbon nanofibers [9-16]. Graphene, as a new candidate of carbon nanomaterials with a two-dimensional structure, attracted broad interests of science researchers, owing to its high specific surface area, excellent mechanical properties and thermal performance [17-21]. However, to the best of our knowledge most research work is merely focused on graphene-modified polymer and their mechanical properties [22-27] and there is little investigation in literature about

2 2.1

fiber

co-reinforcement

for

Experimental Synthesis of GO

Modified Hummers’ method was utilized to produce graphite oxide from natural graphite powder which was oxidized by potassium nitrate and potassium permanganate in the presence of sulfuric acid [28]. Our previous work indicated that a complete oxidation was achieved by slightly raising reaction temperature and extending reaction time [29]. Afterwards, ultra-sonication was applied to exfoliate graphite oxide to GO and to break the GO agglomeration.

Received date: 10 Oct 2016; Revised date: 30 Dec 2016 *Corresponding author. E-mail: [email protected] Copyright©2017, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-5805(17)60107-0

Xiao Han et al. / New Carbon Materials, 2017, 32(1): 48-55

Fig.1 Fabrication process of GO/carbon fiber reinforced epoxy composites.

2.2

Preparation of GO dispersion in epoxy resin

GO was dispersed into the epoxy resin (EP) with the aid of THF. Because of the hydrophilic nature of the oxygenated graphene layers, three different solvents, namely de-ionized water (DI water), acetone and THF, were attempted to disperse GO to form a GO suspension. After ultra-sonication for 2 h, EP, supplied by Beijing Institute of Aeronautical Materials, was added into the suspension prior to the addition of a curing agent (Diamino Diphenyl Sulfone, Aldrich). The mixture was subsequently stirred using a rotary evaporator under vacuum to expel the solvent. The GO-modified EP was dissolved in acetone with the curing agent, and sonicated for another 2 h. The GO-modified EPs with four different GO contents of 0.05, 0.10, 0.20 and 0.40 wt% were prepared. 2.3

Fabrication of composite laminates

The GO-modified EP was brushed onto plies of carbon fiber fabrics (CCF300, supplied by WeiHaiTuoZhan Fiber Co. Ltd. in China) with a unit weight of 154 g/m2 to prepare GO/CF/EP prepregs. After cut into definite sizes, the prepregs were put into a vacuum drying oven to remove the remaining acetone for 3 h. Then the laminates were stacked on a steel mold and hot-pressed with a hot-pressing machine. They were cured at 150 °C for 1 h and post-cured at 180 °C for another 2 h. A low pressure of 0.6 MPa was applied throughout the whole curing process to maintain a laminate thickness of 2.0 ±

0.1 mm. The composite laminates had a fiber volume fraction in the range of (60 ±3)%. 2.4

Characterization

Fourier transform infrared (FT-IR) spectra were recorded on a Nexus 670 spectrometer, in the range of 500-4000 cm−1. X-ray photoelectron spectroscopy (XPS) measurements were performed using an AXIS ULTRADLD spectrometer with a monochromated Al-Kα radiation (1486.6 eV). Atomic force microscopy (AFM) images were obtained by a Multimode Nano4 in tapping mode. Transmission electron microscopy (TEM) was performed with a FEI Tecnai G2 F20 microscope using sliced thin sections of GO/CF/EP composites with a thickness of approximately 40 nm. The three-point SBS test was carried out to determine the effect of the GO addition on ILSS of the unidirectional carbon fiber composites according to the JC/T773-2010 standards. The dimension of specimens for the three-point SBS test was 20.0×10.0×2.0 mm3. The tests were performed in an Instron mechanical testing machine (Instron5565-5KN, UK) using a cross-head speed of 1 mm/min. ILSS was calculated with the following formula: ILSS 

0.75P bh

where P is the failure load and b and h are the specimen width and thickness, respectively. Five replicate specimens were prepared for SBS tests.

Xiao Han et al. / New Carbon Materials, 2017, 32(1): 48-55

The Tg of GO/CF/EP composites with a size of 35×8×2 mm3 were tested by a DMA982 produced from P Oxygen Kin-Elm Oxygen CO. Ltd., operating in the three-point bending mode at an oscillation frequency of 1 Hz. Data were collected from room temperature to 250 °C at a heating rate of 5 °C/min. The SEM images of the laminate fracture surfaces were obtained on a FEI Quanta200F field-emission SEM system (operated at 4 kV). The samples were coated with Au by sputtering prior to the observation.

3 3.1

used in this work may induce ring-opening reaction of epoxy groups, creating new C-N bonds [33,34]. On the other hand, as the surface of carbon fibers contains hydroxyl and carboxylic acid groups, their exposure to the GO nanosheets randomly distributed nearby may form strong hydrogen bonds, leading to more intense interfacial interactions. Taken together, the GO nanosheets dispersed in the inner matrix region may chemically connect themselves to epoxy component through C-N bonds. And the GO nanosheets at the interfacial region may act as the bridge to connect carbon fibers and epoxy resin through hydrogen bonding and mechanical interlocking.

Results and discussion 3.2

Characterization of GO

As shown in Fig. 2a, the following functional groups were identified by FT-IR spectroscopy, O-H stretching vibrations (3 417 cm-1), C=O stretching vibrations (1 727 cm-1), C=C from unoxidized sp2 C=C bonds (1 611 cm-1), and epoxy C-O stretching vibrations (1 219 cm-1) and alkoxy C-O stretching vibrations (1 047 cm-1) [30]. Besides, we utilized XPS to analyze GO sample. The C1s spectrum of GO showed that percentage of sp3C was almost equal to that of sp2C resulting from the oxidation, and peaks corresponding to -C-O group (286.9 eV), -C=O group (287.4 eV) and -COO- group (288.9 eV) were clearly exhibited [31]. These observations of FT-IR and XPS data are consistent, both indicating that GO contains a wide range of oxygen functional groups including hydroxyl, epoxide, carboxyl and carbonyl groups [32], which may play an important role in the mechanical reinforcement of GO/CF/EP composites. On the one hand, GO with pendant oxygen-containing groups across the surfaces may form chemical bonds with resin matrix during the curing process because the amine hardener, DDS,

Dispersion of GO

AFM is currently one of the foremost methods to characterize nanomaterials in height dimension [35-37]. Fig. 3 shows the AFM image of exfoliated GO in water, acetone and THF. As shown in Fig. 3a, GO dispersed in water are flat nanosheets with an average thickness of about 0.8 nm, indicating that graphite oxide was successfully exfoliated into single-layer GO sheet after sonication. Considering that GO would be incorporated into CFRP afterwards and the presence of water in composite could have detrimental impact on the porosity and mechanical property, the dispersion of GO in acetone, the most commonly used solvent in composite processing, was investigated, as shown in Fig. 3b. Thickness of GO sheet in acetone exceeds 100 nm, which is over a hundred times as thick as that in water. This might be caused by the agglomeration of GO in acetone whose polarity is smaller than water. In order to break GO stacking and obtain well-dispersed GO, THF, as an alternative organic solvent was studied as well. The AFM image of GO (Fig. 3c) illustrates that the thickness of each GO nanosheet is approximately 2-4 nm. And its lateral dimension is at micrometer scale, similar

Fig. 2 Characterization of GO (a) FT-IR spectrum and (b) XPS spectrum of C1s. Table 1

Composition of GO surface from XPS data. sp2C

sp3C

-C-O

-C=O

-COO-

Binding energy (eV)

284.8

285.3

286.9

287.4

288.9

Percentage (%)

25.00

21.88

27.20

23.59

2.34

Xiao Han et al. / New Carbon Materials, 2017, 32(1): 48-55

Fig. 3 AFM images of GO in different solvent:s (a) DI-water (b) acetone and (c) THF.

Fig. 4 TEM micrographs of the GO/epoxy nanocomposite plate containing (a) 0.10wt% GO and (b) 0.40wt% GO.

to that in water and acetone. Compared with acetone, THF matches better with GO nanosheets owning to the principle of the dissolution in the similar structure. In other words, there is strong interaction between graphene oxide and THF molecules, which leads to the improvement on disperse ability of GO nanosheets in THF.

was stable within 3 h. After held for 12 h, precipitation of agglomerated GO nanosheets on the bottom of the vials occurred for the dispersion containing 0.20 and 0.40 wt% of GO. In conclusion, the GO dispersion in EP containing curing agent is stable for a period of time, which was sufficient long enough to complete the composite processing in the next step.

The dispersion quality of GO in epoxy matrix was inspected by TEM, as given in Fig. 4. The photographs of nanocomposites containing 0.10 and 0.40 wt% of GO revealed a good disperse ability of GO nanosheets in the matrix with a low GO content and a severe aggregation of GO layers with a high GO content [38]. Fig. 5 illustrates the digital images of GO dispersion in epoxy with curing agent dissolved in acetone. Five vials contained different GO contents, namely 0, 0.05, 0.10, 0.20 and 0.40 wt% from the left to right. As shown in Fig. 5b, the color of the dispersion turned dark from light brown with increasing the GO content. After held at room temperature for 3 h, they still remained uniform, indicating that the dispersion

Fig. 5 Vials containing dispersions of GO with different contents in epoxy resin after (a) 0 h, (b) 3 h and (c) 12 h.

Xiao Han et al. / New Carbon Materials, 2017, 32(1): 48-55

Fig. 6 (a) Geometry and dimensions of SBS test specimen and (b) ILSS of CFRP composites versus GO contents.

Fig. 7 SEM images showing the representative morphologies of fracture surfaces of (a,c) the control sample, and (b,d) the sample containing 0.10 wt% of GO. Table 2

Thermal-mechanical properties of the control and GO-modified samples. Storage modulus G’ (Pa)

Control sample Sample containing 0.10wt% of GO

3.3

≈7×1010 ≈9×1010

Interlaminar shear property

Fig. 6 presents the average ILSS of the composites with various GO contents as determined by SBS tests. Compared with the control group, the average ILSS value increased from 88.98 to 91.41 and 96.14 MPa with the GO content of 0.05 and 0.10 wt%, respectively. However, the ILSS slightly decreased to 88.15 and 84.20 MPa with a further increase of the GO content to 0.20 and 0.40 wt%.

Loss modulus G” (Pa)

tanδ

Tg (°C)

≈2.5×109

0.4396

192.9

≈1×109

0.3380

197.3

It is well known that the ILSS of CFRP composites is mainly determined by the shear properties of the matrix and the fiber–matrix interfacial bonding given the same fiber volume fraction [39]. To better understand the enhancement mechanism of the ILSS value in our study, the fracture surfaces of specimens were characterized by SEM. As shown in Fig. 7a, the fracture surface of the control sample, parallel to the carbon fiber direction, exhibited a smooth river line structure, which is the characteristic of brittle EP that is peeled out of carbon fibers, forming the crack. In contrast, a large

Xiao Han et al. / New Carbon Materials, 2017, 32(1): 48-55

3.4

Fig. 8 DMTA spectra of (a) the control sample and (b) the sample containing 0.10wt% of GO.

amount of resin adhesive uniformly arranged on the fracture surfaces can be observed in the composites containing 0.10wt% of GO as shown in Fig. 7b, forming much rough surfaces. Many researchers have reported that graphene platelets significantly enhance fracture toughness, fracture energy and resistance to fatigue crack growth of nanocomposite [23,40,41]. The GO nanosheets dispersed in matrix may be chemically connected to EP through forming C−N bonds, or mechanically interlock with EP due to the wrinkled surface texture of the GO platelets. These interactions play important roles in transferring loading more effectively within the matrix and preventing crack propagation by delaying micro-crack growth, coalescence and thus laminate failure. The addition of nano-sized fillers to an EP matrix could also increase thermal residual stresses on the surface of the fibers, and therefore the fiber-matrix interfacial bonding, leading to the improved ILSS [42]. Besides, GO, with oxygen functional groups on the monolayer graphitic nanosheet structure, can serve as a “transfer medium” from inorganic graphitic fibers to organic resin composed of epoxy macromolecules, bridging the gap between reinforcement and matrix within the interface areas, and therefore improving the interfacial strength to some extent. As a consequence, it can be observed in Fig. 7c and d that the fracture surface, perpendicular to carbon fiber direction, of laminate with 0.10 wt% of GO is less rugged and the fiber pulls out and hole hence remained are less obvious, indicating that fiber–matrix interfacial adhesion is improved. The enhancement mechanism discussed above is based on the situation that GO is well dispersed as shown in Fig. 4a. The introduction of GO with an appropriate content results in the improvement of resin toughness and the adhesion between carbon fibers and EP, thus increasing ILSS. But an excessive amount of GO in matrix indeed had the tendency to agglomerate, as shown in Fig. 4b. The micro-sized cracks might initiate at the GO aggregates and propagated throughout the samples, which leads to the reduction of ILSS at a high GO concentration [43].

Thermal-mechanical property

Dynamic mechanical thermal analysis (DMTA), performed at a set frequency to determine the storage modulus G’, loss modulus G”, and phase lag tanδ of the composite samples over a range of temperatures, is an efficient way to investigate the relationship among structures, molecular motions and intrinsic properties of the material [44,45]. As shown in Fig. 8, the Tg of composites is increased slightly after the incorporation of 0.10 wt% of GO, which might be caused by the effective physical cross-link points formed in existence of GO that hinder the movement of polymer chains, leading to a slightly high Tg [46]. Additionally, DMTA from another perspective reflects the interface behavior of polymer-matrix composite. During deformation, a composite with worse interfacial bonding will tend to consume more energy than the same composite with better interfacial bonding [47,48]. Table 2 compares the average G” and tanδ of laminates containing 0.10 wt% of GO to those of the control sample made from pristine epoxy matrix. As indicated in the table, the GO modification on epoxy matrix resulted in a decrease of loss modulus and tanδ. These results could be attributed to a substantial increase in fiber/matrix bonding strength by the incorporation of GO layers.

4

Conclusions

We demonstrated the improvement of interlaminar shear property of carbon fiber epoxy matrix composites by GO-modification. The following conclusions can be drawn from this investigation: The successfully exfoliated GO nanosheets with a thickness of 2-4 nm could form a homogeneous dispersion in THF to further prepare stable GO-modified epoxy matrix. The experimental results showed that ILSS of laminates varied with GO content and dispersion state. A maximum enhancement of ILSS by 8.05% was reached at the GO content of 0.10 wt% as compared with laminates without GO. Micrographic examinations showed that the interlaminar shear property enhancements should be attributed to the improvements of resin toughness and the adhesion between carbon fibers and epoxy resin, which were caused by the introduction of GO.

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