Preparation of graphene-glass fiber-resin composites

Composite Interfaces, 2018 https://doi.org/10.1080/09276440.2018.1439641

Preparation of graphene-glass fiber-resin composites and its electromagnetic shielding performance Xiangrong Wan, Hao Lu, Junfeng Kang, Sheng Li and Yunlong Yue School of Materials Science and Engineering, University of Jinan, Jinan, China

ABSTRACT

The surface of the glass fiber (GF) was modified by silane coupling agent (KH550) and bovine serum albumin (BSA), and then the graphene oxide (GO) was coated onto the modified surface of the glass fiber. Followed by a reduction reaction, the reduced graphene oxide (RGO) coated on glass fiber was obtained. Finally, the reduced graphene oxide-glass fibers (RGO-GF) were combined with unsaturated resins. The interfacial morphology of reduced graphene oxide-glass fibers was investigated by scanning electron microscopy (SEM). The structure of the materials was analyzed by Fourier transform infrared spectroscopy (FT-IR). The crystal phases of the material were identified by X - ray diffraction (XRD). The mechanical properties and electromagnetic shielding effectiveness of the sample were tested. The results showed that the interface between glass fibers and graphene binds more closely after the glass fibers was treated by KH550. The tensile strength of the RGO-GF composites reached 85.05 MPa. Compared with the GF composites, it increased by 51.4% when the glass fibers content was 30%. The shielding effectiveness of the composites reached 21.3 dB at the frequency range of 8.2– 12.4 GHz (x-band). Therefore, by coating the surface with reduced graphene oxide, the glass fibers can make a great shielding effect on the electromagnetic wave.

CONTACT Yunlong Yue

[email protected]

© 2018 Informa UK Limited, trading as Taylor & Francis Group

ARTICLE HISTORY

Received 20 October 2017 Accepted 1 February 2018 KEYWORDS

Graphene; glass fiber; composite materials; electromagnetic shielding

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1. Introduction Because of its high strength, high modulus, good corrosion resistance, good insulation and many other excellent properties, GF has been widely used in our daily life and industrial production [1]. For example, it can be filled into the resin as a reinforcement to make high strength composites, which were used in the aerospace, automation, construction and other fields. Besides this, GF with good dielectric properties was the main raw material of the printed circuit board in the electrical field. There are many ways to modify the glass fibers surface, which include surface coating treatment, grafting treatment, electrophoretic deposition treatment and other physical and chemical methods [2]. Graphene is the thinnest but hardest nanomaterial in the world. It has a single atomic layer structure and the carbon atoms form a six-membered ring structure in SP2 hybrid [3–6]. The outermost π-electron orbital of each carbon atom is perpendicular to the two-dimensional plane of graphene, and numbers of π-electron orbits form a delocalized electron network [7,8]. In this particular electronic arrangement, the internal electrons of graphene have moved in the direction of the external electric field so that the graphene obtains electrical conductivity [9,10]. The graphene’s electron mobility is more than 15,000 cm2/V • S, which is higher than the carbon nanotubes and silicon crystal at room temperature. However, its resistivity is only 10−6 Ω • cm which is much lower than copper and silver. It’s the smallest resistivity material in the world [11]. At present, Graphene used in the field of shield electromagnetic radiation is the most popular topic for its excellent conductivity.

COMPOSITE INTERFACES

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With the development of science and technology, the degree of electronization is getting higher and higher. Electronic equipment has almost occupied all the aspects of our production and life. Since the electromagnetic waves emitted by the electronic equipment are not only harmful to the human body, but also it makes the instruments interact each other. Therefore, electromagnetic shielding is a problem that all countries in the world is trying to solve. The research of glass fibers coated with graphene is still in the initial stage. Zhao et al. grafted carbon nanotubes layer by layer to carbon fiber surface to improve the interfacial properties of epoxy composites [12]. Yang et al. completed the preparation of graphene/ glass fiber core shell composites conductive materials by using chemical vapor deposition at low temperature [13]. Lamellar size of the deposited graphene is between 30 nm and 120 um. The minimum resistance of the film material reached 180 Ω∕sq by the four probe method. Hong et al. coated the hydroxylated modified carbon nanotubes onto the surface of the glass fiber by using physical adsorption and chemical grafting. The results showed that the conductivity of carbon nanotubes/glass fiber composites reached 20 S/m [14]. According to previous studies, the modified graphene was coated on the surface of the glass fiber by electrostatic adsorption in this work [15–17]. The GO on the surface of the glass fiber was reduced to RGO by sodium borohydride, and RGO-GF/resin composites with electromagnetic shielding effectiveness was prepared by combining with unsaturated resin. The process of coating glass fibers with graphene was studied by changing the amount of glass fiber in the resin composites. At the same time, the mechanical properties and electromagnetic shielding effectiveness of the composites were tested respectively.

2. Experimental 2.1. Materials E-Glass fiber was obtained from Taishan Glass Fiber Co., Ltd. Graphite powder (8000 meshes, purity of 99.95%) was purchased from Hengxing Chemical Manufacturing Co., LTD (Tianjin, China). 191#Universal Unsaturated Polyester Resin (Phthalic resin; viscosity: 0.35 Pa.S. at 25 °C; OH content: 24 mg KOH/g; Gel time: 8 min; Solid content: 65%) was provided by Rixin composite materials Co., Ltd. (Shandong, China). 98wt% concentrated sulfuric acid (H2SO4), 30wt% hydrogen peroxide (H2O2), the ethylene glycol ((CH2OH)2), ammonia (NH₃·H₂0), sodium borohydride (NaBH4), potassium permanganate (KMnO4), sodium nitrate (NaNO3), bovine serum albumin (BSA) and sodium hydroxide (NaOH) were purchased from the Sinopharm group Shanghai Chemical Reagent Company. The deionized water used in the experiment was made in the laboratory.

2.2. Experimental methods 2.2.1. Modification of glass fibers The glass fibers used in the experiment was placed in a muffle furnace and calcined at 500 °C for 120 min. The aim is to allow the surface of the glass fiber to be better bound to the coupling agent by removing the oily infiltrants onto the surface [18]. The organic matter on the surface of the glass fiber was removed by immersing the glass fiber in an acetone solution to 60 min. The –OH of the surface of the glass fiber was exposed by hydroxyl treatment of glass fiber to 60 min. The treated glass fiber was placed in KH550 silane coupling

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Figure 1. The diagram of graphene oxide modified glass fiber.

agent soaked for 30 min and dried 60 min at 50 °C. After that, immersing the glass fibers in standard BSA solutions to 60 min. At the end, Wash it with deionized water and dry. 2.2.2. Reduction of graphene oxide coated on the surface of glass fibers The graphene oxide used in the experiment was made by improved Hummers method [19,20]. The modified glass fiber was placed in graphene oxide dispersion agent (pH = 3; 1.5 mg/ml) for 30 min. Finally, the glass fiber was washed with deionized water and dried. The GO-GF were dispersed in the water, and adjust the pH of the solution to 10 by using 1 mol/L sodium hydroxide solution. The RGO-GF composites was obtained by dropping 15wt% sodium borohydride solution slowly and stirring it at 80 °C. Figure 1 is a graph of graphene oxide modified glass fibers. 2.2.3. Preparation of graphene-glass fiber/resin composites The prepared RGO-GF were subjected to chopping treatment (length 3–5 mm) and added them into the solution that includes the 191\ unsaturated polyester (UP) resin and accelerator. After that, the solution was stirred until the fibers were evenly dispersed in the solution. The solution was placed in a vacuum pump and evacuated for 30 min after the addition of the curing agent to the solution, and the purpose of the vacuum is to remove the bubbles [21]. Finally, the graphene-glass fibers/resin composites were obtained by casting in the mold.


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Figure 2. XRD diffraction analysis diagram of graphite and GO.

2.3 Testing and characterization The microstructure of graphene oxide was analyzed by Nicolet 380 scanning electron microscope manufactured by FEI Corporation of the United States. The diffraction peaks of graphene oxide were characterized by the D8ADVANCE XRD analyzer manufactured by Brooks, Germany，and using Ni filtered Cu K𝛼1 radiation with scanning speed of 2° (2𝜃 ). The group type of GO-GF was analyzed by Nicolet 380 type Fourier transform infrared spectrometer (standard 0.5 cm−1), and the sample was mixed with potassium bromide. The mechanical properties of the composites were tested by wdw-30 universal testing machine, and the sample size is 25 × 10 × 10 mm, and 10 specimens were prepared for

Figure 3. SEM figure of GO.

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each kind of composites. The electromagnetic shielding performance of the test materials was based on the Agilent N5234A model of the vector network analyzer, and the sample size is 22.4 × 10 × 10 mm, and 10 specimens were prepared for each kind of composites.

3. Results and discussion 3.1. XRD analysis of GO and graphite Figure 2 shows the XRD results of GO and graphite. It can be seen that natural graphite have a sharp and strong diffraction peak at 2θ = 26°. It’s the (002) crystal characteristic peak of graphite. GO has a weaker and wider diffraction peak at 2θ = 11° corresponding to the (001) plane [22–24]. According to the Prague equation: 2dsin𝜃 = n𝜆, the scale of the natural flake graphite is 0.334 nm; the interlayer spacing of the graphene oxide prepared by the laboratory is 0.802 nm. It can be concluded that the increase in the interlayer distance of graphene is due to the fact that the graphene oxide contains a large amount of oxygen-containing groups [22,25]. Eventually, the crystal structure of graphene was destroyed. The differences of characteristic peak diffraction angle and peak strength between GO and graphite showed that GO was prepared successfully. 3.2. SEM analysis of GO The SEM characterization of the sample is shown in Figure 3. It can be seen that the GO samples are thin and have transparent layer structure. This is because that the graphite becomes very easy to peel after the ultrasonic wave in the aqueous solution. The surface of GO has a lot of folds. It’s because that GO has large specific surface areas and can’t spread completely in the material surface [26].

Figure 4. FT-IR diagrams of different curing processes of BSA.


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Figure 5. SEM pictures of GO-GF (a) 0 mg/ml GO (b) 1.0 mg/ml GO (c) 1.5 mg/ml GO (d) 2.0 mg/ml GO.

3.3. FT-IR of BSA modified GF Figure 4 is FT-IR of samples after curing bovine serum albumin with different processes. Three different curing processes are: Curve (a) represents the glass fiber directly curing bovine serum albumin; Curve (b) represents the cure of bovine serum albumin after hydroxylation of the glass fiber; Curve (c) represents the cure of bovine serum albumin after the glass fiber has been hydroxylated and modified with a coupling agent. As can be seen from the graph, broad and strong absorption peaks exist near the 3400 cm−1 in these three processes and they are the absorption peaks of hydroxyl groups. Small and sharp absorption peaks appear near 2900 cm−1 in curve (c), which are caused by C-H stretching vibration [27,28]. With the improvement of the process, the peaks strengthen at 3000 cm−1 is enhanced. The change of the three curves explains that with the improvement of the immobilized process, the immobilized effect of BSA increases obviously. 3.4. SEM of GO-GF After the glass fibers were modified by different concentration GO sheets, the changes of surface topography for it were verified by the SEM. In Figure 5(a), the surface of GF without being modified by GO is smooth and has a little defects which were caused by alkali etching [29]. Some GO sheets appeared at GF surface as it showed in Figure 5(b), but only few GO sheets coated on the surface of GF. Most of the GF surface was exposed. In Figure 5(c), the surface of the glass fiber covered a large area of GO sheets when the graphene oxide

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Figure 6. FT-IR of GO and GO-GF.

concentration was 1.5 mg/ml. The surface of GF had obvious wrinkle effect. As shown in Figure 5(d), the GO layers gathered on the GF surface because of the GO layers spread hard in water when the GO concentration is 2.0 mg/ml. The wrinkles in Figure 5(c) and Figure 5(d) were caused by the inhomogeneity of the distribution of the graphene oxide sheets in the fiber sizing. On the other hand, the Vander Waals force among the nanosolids can also cause the graphene oxide to enrich together [30]. Thus, the graphene oxide in the surface of fiber can’t achieve uniform and stable effect when the graphene oxide concentration is too large. At the same time, the thicker graphene sheet has a poorer binding capacity to the glass fiber surface. The result shows that 1.5 mg/ml GO dispersion agent is the most suitable concentration for the experiment. 3.5. FI-IR of GO and GO-GF The FI-IR of GO and GO-GF as shown in Figure 6. In the graph, the absorption peaks corresponding to each vibration in the infrared spectrum changed obviously when the graphite oxide was introduced to the surface of glass fiber. The strongest absorption peaks appeared at 3405 cm−1, where the characteristic absorption peaks of the stretching vibration corresponded to hydroxyl groups. Furthermore, they are the characteristic absorption peaks of stretching vibration of C=O group at 1610 cm−1 and characteristic absorption peaks of in-plane bending vibration of carboxylic acid COH. At 1031 cm−1, glass fibers coated with graphene oxide exhibited an apparent absorption peak compared with individual graphene oxide, and it is the stretching vibration absorption peak of Si-O-Si bond formed when glass fibers was combined with silane coupling agent. Therefore, the graphene oxide still carries a large amount of oxygen-containing functional groups when the graphene oxide


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Figure 7. FT-IR of GO and RGO.

Figure 8. Raman spectra of GO and RGO.

was introduced in the glass fibers, and can be combined with the resin better [31,32]. It also showed that the surface of the glass fibers had been covered with GO.

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3.6. The identification and characterization of RGO The glass fibers surface attachments were individually analyzed by Fourier transform infrared spectroscopy and Raman analyzer. Infrared spectrum was shown in Figure 7. The FT-IR spectra of GO showed a strong absorption band at 3405 cm−1 due to the stretching of −OH . The band at 1730 cm−1 corresponded to the C=O stretching vibrations, and the peak at 1624 cm−1 belonged to C=C bond, and the bands at 1425 and 1072 cm−1 originated from stretching C–O groups vibrations. Infrared spectrogram proved that the intensity of the characteristic peaks at 3400, 1730, 1624, 1425 and 1072 cm−1 significantly decreased after the reduction of GO. Furthermore, the content of oxygen groups in GO was greatly reduced, and the conjugate structure was restored [33]. These observations demonstrated that GO had been reduced into RGO. Raman spectroscopy is very effective in characterizing the structure and properties of carbon nanomaterials. In the Raman spectrum, the D band is related to the defects of the graphene edge and the amorphous structure, and the G band is connected with the SP2 hybridization of carbon atoms. The size of SP2-domains is usually measured by the intensity ratio of the D/G (ID/IG). The larger the ID/IG value is, the greater the degree of oxidation of graphene is. The smaller the ID/IG value is, the higher the degree of reduction of graphene is. In Raman spectra Figure 8, the D and G bands of GO and RGO are centered at 1350 and 1590 cm−1 respectively. By calculation, the results of ID/IG of GO is 0.94. According to the improvement of carbon atoms in graphene by SP2 hybridization in the process of reduction and the recover of the conjugated structure of graphene, it is predicted that ID/IG of RGO should be smaller than ID/IG of GO. However, the experimental results showed that RGO ID/ IG = 1.04 was greater than GO. The same experimental phenomena also existed in graphene nanotubes and graphene nanoparticles [34–37]. The explanations of this phenomenon are as follows: On the one hand, the surface defects of graphene were increased when GO was

Figure 9a. The compressive strength of the composites.


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Figure 9b. The tensile strength of the composites.

reduced; on the other hand, although C atoms formed SP2 hybrids, the C–C bonds on the graphene sheets were broken down after the graphene oxide was reduced, so that the relative size of SP2-domains decreased and led to the increase in ID/IG. On the basis of the above analysis, it can be seen that GO has been better reduced to RGO. 3.7. Mechanical properties testing of composites The compressive and tensile properties of RGO-GF/UP and GF/UP composites are shown in Figures 9a and 9b, respectively. It can be seen that the mechanical properties of RGO-GF composites have been enhanced. The mechanic performance of RGO-GF composites is generally higher than the GF composites when they have the same content. The improvement of the mechanical properties of the RGO-GF composites is mainly due to the improvement of the microstructure of the interface between the fibers and the matrix [38–40]. In the preparation of RGO-GF composites, the glass fibers were treated with silane coupling agent and formed a Si–O–Si covalent bond with the silane coupling agent, so they will be closely linked together. The chemical reactions will occur between GO and silane coupling agents of the surface of glass fibers when the GO dispersion agents are introduced in the surface of the glass fibers. The –CH3 bond on the silane coupling agents will react with the hydroxyl and carboxyl groups on the GO to form the C-O covalent bond. As a result, the GO adheres to the glass fibers surface. In summary, GO and glass fibers combine together firmly through the chemical bond, hydrogen bond. Thus, the interface of them is very strong. Besides, graphene has the characteristics of high aspect ratio and two-dimensional sheet geometry and so on. The crack tip is able to walk along the graphene surface when the crack in the materials is deflected [41–43]. The path of the crack is more distorted. Thereby, it can better release the pressure so that the toughness of the material is improved. In addition, the compressive strength of GF/UP composites has a down trending with the increase of fibers content. The

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maximum compressive strength of the composites is 96.24 MPa when the doping content of the glass fibers is 20wt%. Compared with pure resin materials (83.21 MPa), it increased by 15.6%. The tensile strength of the composites is the highest, which is 85.05 MPa, when the contents of glass fiber is 30wt%. 3.8. The EMSE test of composites The electromagnetic shielding effectiveness of composites is usually expressed as SE. The value of SE was obtained by calculating the ratio of the intensity of the electromagnetic wave before and after passing through the materials. SE was calculated by the following equation [44].

SE = 20lg(Ei ∕Et )

(1)

In the formula (1), SE is the total electromagnetic shielding effect of the materials, Ei and Et represent electric field intensity of incident and transmitted waves, respectively. Electromagnetic shielding theory explains that electromagnetic waves will lose energy when it passes through shielding materials. In theory, there are three different mechanisms for electromagnetic shielding [45–47].

SE = SER + SEA + SEM

(2)

In the formula (2) [45], SER is the surface reflection efficiency of shielding materials. SEA represents the absorption efficiency of shielding materials. SEM represents multiple reflections losses inside the shielding materials.

) ( SER = 168 − 10 lg fur ∕𝜀r

Figure 10. The EMSE tests of UP, GF/UP and RGO-GF/UP composites.

(3)


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Figure 11. The EMSE test results of the composites with different content of RGO-GF.

SEA = 1.314d(fur ∕𝜀r )0.5

(4)

SEM = 20lg(1 − e𝜆∕4.3362 )

(5)

In the three formulas above, f is the frequency of electromagnetic waves; 𝜀r represents the electrical conductivity of shielding materials; ur represents the permeability of shielding materials; and d is the thickness of the materials. The value of SEM could be neglect when the frequency of the electromagnetic field is greater than 20 kHz. Figure 10 shows the x-band electromagnetic shielding effectiveness tests of UP, GF/UP and RGO-GF/UP composites. The mass fraction of fibers in GO/UP and RGO/UP composites was 40%. The pristine UP composites almost have no electromagnetic shielding ability. The shielding effectiveness of UP composites is 2.1 dB at 8.2 GHz. The electromagnetic shielding effectiveness of composites has a certain improvement after the GF reinforced UP composites. The shielding effectiveness of the resin has slightly increased after the introduction of fibers. The electromagnetic shielding effectiveness of RGO-GF composites is remarkably improved, as can be seen from the graph. The reason is that RGO can interact with the external electromagnetic field and increase the loss of the electromagnetic wave when the conductive network is formed in the composites [48–50]. The EMSE test results of the composites in x-band were shown in Figure 11. In composite materials, the mass fraction of RGO-GF is 0, 10, 20, 30 and 40%, respectively. Correspondingly, SE value of composites are respectively 10, 12.9, 16.2 and 21.3 dB at 12.4 GHz. SE values of the composites is the highest when the fibers content is 40%, which

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Figure12a. The absorption shielding efficiency of the composites.

Figure12b. The reflection shielding efficiency of the composites.

is because the bubbles in the materials are difficult to be eliminated when fiber content is very high. The EMSE values of composites increase with the number of glass fibers content in composites. The reason is RGO has nice electrical conductivity. An irregularly conducting network formed into RGO-GF and linked to the resin matrix when the glass fibers is uniformly dispersed in the composite material. A better conductive network gradually formed in the resin matrix with the increase of glass fibers mass fractions [51,52]. The ability of


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composites carriers to interact with electromagnetic fields is enhanced in a perfect conducting network, and the electromagnetic shielding performance of the composites is improved [48,53] . At the same time, dipolar polarization relaxation is caused by the orientation of different oxygen atoms connected by polar carbon atoms under the effect of electric field. The composites interface is the center of polarization relaxation. The relaxation can make the RGO absorb electromagnetic waves and increase the loss of electromagnetic energy [47,54–56]. So RGO-GF composites have good electrical conductivity. Figure 12a shows the absorption and reflection shielding efficiency of the composite with GF content increasing from 20 to 40%. According to the shielding principle formula: SE = SER + SEA, the EMSE value of the material depends on the its absorption and reflection losses. The SEA values of the composites correspond to 13, 14 and 18.2 dB when the mass fraction of RGO-GF in the composites is 20, 30 and 40%. The values of SEA increase with the number of the fibers content and the frequency of the electromagnetic wave. The values of SER are 1.5, 2.1 and 2.7 dB respectively when the content of RGO-GF in composites is 20, 30 and 40%. SER increase with the number of fibers content, and decreases with the increase of frequency. The absorption loss of composite materials is greater than its reflection loss from the numerical in Figure 12b, so absorption loss is the main mechanism of electromagnetic shielding of composite materials. Except for using unsaturated polyester resin as matrix, other polymers with excellent properties as matrix have also been studied. For example, using polyurethane, phenolic resin to make composites with excellent electromagnetic shielding properties. Zhu et al. use core-shell Fe–Silica nanoparticles to reinforce polyurethane composites. The Fe–SiO2/ polyurethane polymer nanocomposites absorber with 1.8 mm thickness shows a good electromagnetic wave absorption performance (RL