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Tensile and flexural properties of graphene oxide coated-short glass fiber reinforced polyethersulfone composites
Composites Part B 99 (2016) 407e415
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Tensile and flexural properties of graphene oxide coated-short glass fiber reinforced polyethersulfone composites Sen-Sen Du a, c, Fei Li a, Hong-Mei Xiao a, Yuan-Qing Li b, Ning Hu b, Shao-Yun Fu b, * a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China College of Aerospace Engineering, Chongqing University, Chongqing 400044, China c University of Chinese Academy of Sciences, Beijing 100190, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 20 April 2016 Accepted 3 June 2016 Available online 6 June 2016
It is the first time to report the effects of graphene oxide (GO) coating on the mechanical properties of short glass fiber (SGF) reinforced polymer composites. GO-coated SGF reinforced polyethersulphone (PES) composites are manufactured using extrusion compounding and injection molding techniques. The micro-structures and morphologies of GO and GO-coated SGFs are investigated using scanning electron microscopy, small-angle X-ray scattering, atomic force microscope and Fourier transform infrared techniques. Then, the tensile and flexural properties of the GO-coated SGF/PES composites are systematically studied taking into account the effect of GO coating content. It is observed that both the tensile and flexural strengths are effectively enhanced by the GO coating on the SGF surfaces. This observation is mainly attributed to the enhanced interfacial adhesion between SGFs and PES due to the GO coating. Moreover, the tensile and flexural moduli are also improved by the addition of GO due to the fact that GO has a much higher modulus than the PES matrix. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Glass fibers Polymer-matrix composites (PMCs) Mechanical properties
1. Introduction In the past decades, short glass fiber (SGF) and short carbon fiber (SCF) reinforced polymer composites have been intensively investigated because of their ease of fabrication and superior mechanical properties. Particularly, low density thermoplastic composites based on SGFs and SCFs exhibit great potential in automotive or aviation industries as light-weight replacement materials. It is well known that the mechanical properties of short fiber reinforced polymer composites are critically related with fiber content, aspect ratio, orientation and fiber-matrix interfacial adhesion [1,2]. In order to achieve high mechanical performance for short fiber reinforced composites, a strong interfacial adhesion between short fibers and polymer matrix is necessary for transferring load from the matrix to the fibers [3,4]. To improve the fiber-polymer interfacial adhesion, the conventional methods include either enhancing the chemical bond between the fibers and the polymer matrix with coupling agent or introducing newly activated components or surface roughness onto
* Corresponding author. E-mail addresses: [email protected] (Y.-Q. Li), [email protected], [email protected]. cn (S.-Y. Fu). http://dx.doi.org/10.1016/j.compositesb.2016.06.023 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
the fiber surfaces with plasma treatment etc. [5,6]. Recently, graphene oxide (GO), which consists of a two-dimensional sheet of covalently bonded carbon atoms bearing various oxygen functional groups (e.g. hydroxyl, epoxy, and carbonyl groups) on their basal planes and edges, has shown a great potential as multifunctional sizing agent to enhance the interfacial bonding between the microsized fibers and the polymer [7e12]. The surface configuration of micro-sized fibers changed by GO sheets could enhance the strength and toughness of the fiber-matrix interfacial region [13]. Polyethersulphone (PES), an amorphous thermoplastic polymer, has attracted much attention due to its fascinating properties such as high glass transition temperature (Tg around 225 C), excellent thermal stability (operation temperature up to 180 C), great flame retardancy, low creep, high dielectric strength, extraordinary dimensional stability and chemical resistance etc. [14]. With these outstanding properties, PES is highly desirable as replacement for metals or ceramics in automotive, aerospace and microelectronics industries [14e16]. However, its relatively low mechanical properties still impede its broader applications. Recently, the fabrication of GO-coated SCF/PES composited was reported by our group [4]. The results reveal that GO coating on SCF surfaces leads to an obvious SCF/PES interfacial adhesion enhancement. A better stress transfer due to the enhanced
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interfacial bonding leads to the enhanced mechanical properties by the incorporation of GO-coated SCFs into the PES matrix compared to the un-treated SCF case [4]. However, comparing with SCF reinforced polymer composites, SGF reinforced composites are more attractive for industry applications because they have the most favorable cost-mechanical property relationships [17]. But, the knowledge obtained for SCF reinforced polymer composites cannot be simply transferred to the SGF reinforced polymer composites since SGFs and SCFs are two different types of micro-sized fibers. In order to explore the application of PES, it is of significance to study the enhancement effectiveness of SGFs by GO coating on PES. At the same time, based on our knowledge, there has been no report related with the effects of GO sheets on the interfacial characteristics between SGFs and PES and hence on the mechanical properties of SGF/PES composites. In the present work, short glass fiber reinforced PES composites are prepared using the melting blending and injection molding techniques. Meanwhile, to improve the mechanical properties of SGF/PES composites, GO as sizing agent is employed to improve the interfacial adhesion between SGFs and PES. First, the microstructures of GO and GO-coated SGFs are investigated using atomic force microscope (AFM), scanning electron microscopy (SEM), X-ray scattering (XRD), and Fourier transform infrared (FTIR). Then, the mechanical properties of the GO-coated SGF/PES composites are systematically investigated. The results reveal that the surfaces of SGFs are covered with wrinkled and roughened GO sheets after coating treatment. Importantly, the tensile and flexural properties of PES are effectively enhanced by the GO coating on SGF surfaces.
2. Experimental section 2.1. Materials Polyethersulfone granules (PES, E3010) was bought from BASF, Germany. Graphite powders were obtained from Qingdao AoKe ShiMo Co. Ltd, China. Short glass fibers (SGFs, 6 mm length, 552B) were provided by Zhejiang JUSHI, China. Ethanol, concentrated
sulfuric acid, potassium permanganate, hydrochloric acid and N,Ndimethyl-Formamide (DMF), were all obtained from Beijing Chemical Works and used as-received. Sodium nitrate was purchased from Tianjin JinKe Fine Chemical Industry Research Institute, China. All of the raw materials were used without any further purification and treatment.
2.2. Preparation of graphene oxide (GO) and GO-coated glass fibers GO was prepared using the modified Hummers method [18,19]. The obtained GO was subsequently dispersed in deionized water to form a suspension of 0.325 mg/ml. The non-exfoliated graphite oxide sheets were removed by centrifuge, then the GO was obtained by ultrasonic technique (1000 W) for 1.5 h and the homogeneous GO aqueous solution with a specific solubility of GO was formed for later use. A given amount of GO aqueous solution in terms of the prescribed formulation was used for treating SGFs. An appropriate amount of SGFs in the prescribed ratio was added to the GO aqueous solution under mild magnetic stirring for 24 h. After that, the resultant mixture was dried under vacuum at 100 C to remove water and then cooled naturally to room temperature. The GO concentration could be easily calculated in terms of the solubility of GO, the amount of GO aqueous solution and the amount of SGFs. The GO-coated SGFs with various GO concentrations were finally prepared. Moreover, the intensive stirring of GO-coated SGFs was conducted for examining the coating efficiency of the GO on to SGFs. The as-obtained GO-coated SGFs were mixed with PES to prepare GO-coated SGF/PES composite samples with different GO concentrations using a TSE-20/600-4-48 co-rotating twin-screw extruder (NanJing Ruiya Extrusion Systems LTD., China) at a screw speed of 30 rpm and a feed rate of 6 rpm. The temperature profile of the barrel was set at 360-365-370-375-380-380-375 C from the hopper to the die. The materials were crushed by a breaker and dried in an oven at 130 C for 6 h before injection molding. Standard tensile and flexural test bars were then obtained according to the recommendation of ASTM D 638-96 and ASTM D
Fig. 1. Evaluation of the coating efficiency of GO onto the surfaces of SGFs.
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Fig. 2. SEM images of (a) graphite and (b) GO; TEM image of (c) GO.
Fig. 3. AFM image (left) of graphene oxide sheet dispersion in water on freshly cleaved mica surface through drop-casting and height profile (right) indicating a sheet thickness of ~1 nm.
790-03, respectively by injection mold at the mold temperature of 180 C using HTF80X/1 plastic-injection molding machine (HaiTian International Holding LTD., China).
2.3. Characterization The morphology of GO sheets was characterized using an atomic force microscope (AFM) system (Veeco NsIV, USA). The as-obtained GO sheets were dispersed in water and dip-coated onto freshly cleaved mica surfaces before testing. The morphology of raw materials and PES composites were imaged by a Hitachi S-4800 Scanning electron microscope (SEM). The samples used for the SEM observation were prepared by directly spraying a thin layer of gold on the sample surfaces. X-ray diffraction (XRD) analysis of graphite, GO and GO-coated glass fibers were carried out by a X-ray diffractometer with Cu Ka radiation (k ¼ 1.5418 Å). Raman spectroscopy characterization was carried out on a RenishawinviaReflex (England) with a 532 nmAr laser. Fourier transform infrared (FTIR) measurements were conducted on an Excalibur 3100 FTIR (Varian, USA) between 500 and 4000 cm1. GO and glass fibers were mixed with KBr powders and pressed into tablets for FTIR characterization. The tensile and flexural properties were measured on an INSTRON 5882 Mechanical Tester under a 100 kN load cell with a crosshead speed of 2 mm/min. The mechanical properties were tested according to the recommendation of GB/T 2571-1995. At least six specimens were measured for each composition. The fracture surfaces and the fiber orientation distribution (FOD) of the specimens after impact testing were examined by SEM. After removing the PES matrix using DMF extraction, the optical images of glass fibers were taken using an optical microscope (OLYMPUS STM6, Japan), and the fiber lengths were measured by the computer software SemAfore 4.0 based on the glass fiber images obtained.
3. Results and discussion 3.1. Characterization of GO and GO-coated short glass fibers In order to evaluate the stability of GO on SGFs, the dried 0.5 wt% GO-coated SGFs were re-dispersed and intensively stirred in deionized water and then the coated-SGFs after intensive stirring were collected for SEM observation. As shown in Fig. 1, the GO was still firmly adsorbed on the surfaces of SGFs after intensive stirring according to the SEM image of the collected SGFs. This indicates that the GO has been successfully and firmly coated on the surfaces of SGFs. The SEM images for the raw graphite and the as-prepared GO as
Fig. 4. XRD patterns of graphite and GO.
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The X-ray diffraction (XRD) patterns of graphite powders and GO are displayed in Fig. 4. A sharp peak at 2q ¼ 26.5 can be distinguished for graphite, which is assigned to the diffraction of (002) plane of the well-ordered graphene with an interlayer spacing of 0.335 nm [23]. However, the sharp diffraction peak observed in the XRD pattern of pristine graphite completely disappears upon oxidation, and a new diffraction peak appears at 2q with a value of approximately 10.6e11.0 , corresponding to an interlayer spacing of 0.80e0.83 nm. The variation in the interlayer spacing of GO is a result of variation in the degree of oxidation on the graphite. This means that the graphite oxide has been successfully exfoliated to form layered GO nanosheets after oxidation and exfoliation. The d-spacing of GO can be calculated according to the Bragg’s law (1): nl ¼ 2d sinq
Fig. 5. FTIR spectra of the as-prepared GO and the SGFs.
well as the TEM image for the GO are presented in Fig. 2. Thick and flat layers appear for graphite as shown in Fig. 2a while a wrinkled morphology is observed for GO nanosheets as shown in Fig. 2b. Moreover, Fig. 2c shows that the GO nanosheets have a very thin crumpled silk veil wavy morphology [20]. An important property of GO, brought about from the hydrophilic nature of the oxygenated graphene layers, is its easy exfoliation in aqueous media [21]. To investigate the exfoliation degree of the graphite oxide in water, the AFM image of graphene oxide (GO) sheet dispersion in water is conducted. Fig. 3 exhibits a typical AFM image of GO sheets deposited onto a mica substrate from an aqueous dispersion. Analysis of the AFM image reveals that most GO sheets have heights of about 1 nm and lengths ranging from hundreds of nanometers to several micrometers, indicating fully exfoliated GO sheets were indeed achieved in water [22].
(1)
where l is the X-ray wavelength, n is the diffraction series and d is the interlayer spacing. The calculated value of l for graphite and graphite oxide is 3.35 Å and 8.66 Å, respectively, implying that the sample is expanded when graphite is oxidized. However, the peak of GO is weaker than graphite oxide, implying that the GO layers have been disordered [24]. The FTIR spectrum of GO is shown in Fig. 5. It can be seen that the predominant characteristic absorption signals include a broad and intense peak at 3420 cm1 attributed to OeH stretching of carboxyl groups, an intense peak at 1730 cm1 assigned to C]O stretching of carbonyl groups, a weak peak at 1620 cm1 corresponding to C]C skeletal vibration of un-oxidized graphitic domain and a strong peak at 1366 cm1 attributed to an alkoxy CeOH stretching vibration. In addition, an absorption peak at 1053 cm1 assigned to the stretching vibration of CeO [25]. The peak at 1210 cm1 is attributed to the symmetrical stretching vibration of epoxy ring (CeOeC) while the peaks observed at 907 cm1 and 800 cm1 are assigned to the asymmetric stretching and in-plane bending vibrations of CeOeC [26]. The FTIR spectrum of SGFs is also shown in Fig. 5. It exhibits that SGFs contain OH
Fig. 6. SEM images for (a, b) the raw short glass fibers and (c, d) the coated short glass fibers with the 0.5 wt% GO.
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Fig. 7. Raman spectra of graphite, GO, SGFs and GO-coated SGFs.
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groups. As well known, that reactive groups (namely epoxy groups) can react with hydroxyl to form ether bond, indicating that GO tend to react with SGFs and form strong bonding [4]. The SEM images for the morphologies of the raw SGFs and GOcoated SGFs are shown in Fig. 6. As shown in Fig. 6a and b, the surfaces of the raw SGFs are quite smooth. After the GO treatment, the SGF surfaces become rough and the typical wrinkled morphology of GO-wrapped SGFs can be clearly observed as shown in Fig. 6c and d. The treated short glass fibers with the 0.5 wt% GO are fully covered by GO as shown in Fig. 6d. This indicates that GO has been successfully coated on SGF surfaces. Raman spectroscopy is another useful and non-destructive technique used to reveal the structural information of carbonbased materials such as carbon nanotubes, graphite and GO. Typically, the Raman spectrum of GO consists of two predominant D and G bands as shown in Fig. 7. The D and G bands are represented by the peaks at approximately 1350 cm1 and 1580 cm1. The G band corresponds to the optical E2g phonons at the Brillouin zone center due to bond stretching of the sp2 carbon pairs. The D band is associated with the second-order of zone-boundary phonons where it is activated by defects [27]. For GO-coated SGFs, the two
Fig. 8. Effects of GO content on (a) tensile strength and (b) Young’s modulus of the GO-coated SGF/PES composites.
Fig. 9. Effect of the GO content on the (a) flexural strength and (b) flexural modulus of the GO-coated SGF/PES composites.
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Fig. 10. SEM images of the pulled-out fibers of (a) raw SGF/PES composite, (b) 0.05 wt% GO-coated SGF/PES composite, (c) 0.1 wt% GO-coated SGF/PES composite, (d) 0.2 wt% GOcoated SGF/PES composite, (e) 0.5 wt% GO-coated SGF/PES composite and (f) 1 wt% GO-coated SGF/PES composite.
peaks can also be clearly observed although the content of GO is only 0.5 wt%. However, for raw SGFs, no peak is observed. So Raman spectroscopy further demonstrates the successful coating of GO on SGF surfaces. 3.2. Mechanical properties of PES composites Tensile tests were performed on the dumbbell shaped samples prepared using an injection molding machine. The results for the tensile strength of PES composites are given in Fig. 8a as a function of the GO weight percentage from zero to 1.0 wt%. It is observed that the tensile strength of the PES composites increases from 108 to 119 MPa and then slightly decreases with further increasing the GO content. The maximum composite strength at the 0.5 wt% GO content corresponds to an increase of 10.2% compared to the untreated SGF case. This result indicates that a small amount of GO can effectively enhance the tensile strength of the composites. It is because that the interfacial bonding between SGFs and PES has been enhanced by the GO coating on SGF surfaces to be shown later and thus there will be a better load transfer from the PES to the SGFs, allowing the composites to bear a higher applied load [28].
The results for the Young’s modulus of the PES composites are shown in Fig. 8b as a function of the GO content. The Young’s modulus of composites increases with increasing the GO content and reaches the plateau level after the 0.5 wt% GO content, corresponding to an increase of 25.4% compared to the un-treated case. It is well known that GO has a much higher Young’s modulus than the PES matrix, thus the increase of the composite Young’s modulus with increasing the GO content is understandable. The flexural strength of the PES composites with various amounts of GO is depicted in Fig. 9a. It can be observed that the flexural strength of the composites increases from 160 MPa to the maximum value of 175 MPa at the 0.5 wt% GO content and then slightly decreases with further increasing the GO content. The maximum strength corresponds to an increase of 9.4% compared to the un-treated SGF case. This observation is due to the fact that the interfacial bonding between SGFs and PES is improved by the GO coating to be shown later. At the same time, the flexural modulus of the composites shown in Fig. 9b exhibits a consistent increasing tendency with increasing the GO coating content. The introduction of GO coating leads to a maximum increase of 15.9% in the flexural modulus at the 1.0 wt% GO content compared to the un-treated
Fig. 11. Photomicrographs of the collected short glass fiber samples after removing the PES matrix from the composites: (a) un-treated, (b) 0.05 wt% GO, (c) 0.1 wt% GO, (d) 0.2 wt% GO, (e) 0.5 wt% GO and (f) 1.0 wt% GO.
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Fig. 12. Glass fiber-length distribution histograms of the un-treated and GO-coated SGF/PES composites: (a) un-treated, (b) 0.05 wt% GO, (c) 0.1 wt% GO, (d) 0.2 wt% GO, (e) 0.5 wt% GO and (f) 1.0 wt% GO.
case. The enhancement in the composite flexural modulus is attributed to the high modulus of GO. 3.3. Micrographs of the fracture surfaces of the PES composites The tensile fracture surfaces of the composites are observed using SEM. And the micrographs for the fracture surfaces are shown
in Fig. 10. Fig. 10a shows that the surfaces of the raw SGFs are quite smooth, indicating a relatively weak interfacial adhesion between SGFs and PES matrix. Fig. 10bef exhibits that the rough surfaces of the GO-coated SGFs are observed after GO-coating. There is a big amount of PES matrix attached on the SGF surfaces for the coated SGF cases. The average diameter from over 20 measurements of the SGFs are 9.32, 10.85, 12.06, 12.89, 13.06, 12.86 mm respectively for
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Fig. 13. SEM images of the fracture surfaces of the (a) un-treated SGF/PES composite, (b) 0.05 wt% GO-coated SGF/PES composite, (c) 0.1 wt% GO-coated SGF/PES composite, (d) 0.2 wt% GO-coated SGF/PES composite, (e) 0.5 wt% GO-coated SGF/PES composite and (f) 1 wt% GO-coated SGF/PES composite.
the cases of 0 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt% and 1.0 wt% GO. Namely, the average diameter of the pulled-out SGFs increases with increasing the SGF content up to 0.5 wt% and afterwards decreases with further increasing the GO content. It is well known that a relatively big amount of polymeric matrix attached on pulled-out short fibers corresponds to a relatively strong interfacial bonding [29]. Thus, it is clear that the SGF/PES interfacial adhesion is the optimal for the 0.5 wt% GO-coated SGF case while a further increase in the GO content to 1.0 wt% will lead to a reduced interfacial adhesion strength but it is still much better than for the untreated case. For the high (1.0 wt%) GO content case, the reduced interfacial adhesion strength should be attributed to the GO agglomeration [3] occurring on the SGF surfaces. The composite tensile strength is closely related to the SGF/PES interfacial adhesion strength and a strong interfacial adhesion strength corresponds to a high composite strength [2]. Therefore, it is understandable that the composite strength increases initially with increasing the GO content up to 0.5 wt% and then decreases with further increasing the GO content. The pulled-out GO-treated SGFs (Fig. 10) with a larger average diameter than the un-treated SGFs (Fig. 6a and b) indicates that the GO coated on the SGFs is not split off from the fiber surfaces after extrusion compounding and injection molding processing, confirming that the GO coating has a good stability on the SGF surfaces.
important roles in determining the final fiber length of the composites. Fiber orientation affects the mechanical properties of composites [30], thus an attempt was tried to get SEM and optical images from the cut and polished cross sections of the samples in order to measure fiber orientation. However, the SEM and optical images obtained from the polished cross sections are quite indistinct especially for the GO-coated SGF cases with an improved interfacial adhesion. Even an etching solvent such as DMF was used, the PES matrix could not be removed from the SGF/PES interfaces. Therefore, a qualitative characterization of fiber orientation is accepted here from the fracture surfaces of the samples. From the SEM micrographs for the fracture surfaces of the SGF/PES composites as shown in Fig. 13, it can be seen that the fibers are preferentially aligned along the injection molding direction (transverse to the sample cross-section). This is understandable since the SGF content and the processing conditions are fixed for the investigated cases. In summary, the above results show that the effects of GO coating on the fiber length and orientation are negligible. Therefore, the enhancements in the composite tensile and flexural strength via the GO coating on the SGF surfaces are mainly attributed to the enhanced SGF-PES interfacial adhesion. The improvements in the tensile and flexural modulus are due to the fact that the modulus of GO is much higher than the PES matrix.
3.4. Glass fiber-length and orientation
4. Conclusions
It is well known that fiber length is a major influencing factor on the mechanical properties of short fiber reinforced composites [28]. For injection molded composites, fiber breakage takes place during processing [17]. The results for the effect of the GO content on average glass fiber length are presented in Fig. 11 and Fig. 12. The photomicrographs of the collected SGFs after removing PES matrix from these composites using DMF are shown in Fig. 11. The fiber length is then measured for over 800 fibers for each composition using the software SemAfore 4.0 and the results for the SGF length distribution histograms are presented in Fig. 12. As shown in Fig. 12a-f, the mean fiber length is not significantly affected by the GO coating on the SGFs. Namely, the average glass fiber length is in the narrow range of 112e122 mm for all the investigated cases. Thus, it can be considered that the GO coating has a negligible effect on the final fiber length. This is understandable since the SGF content is fixed and the processing conditions are the same for all the investigated cases, which play
In conclusion, the SGF surfaces were covered with the wrinkled and roughened GO sheets after coating treatment. It was shown that the GO coating effectively improved the interfacial adhesion of SGF/PES composites. The GO-coated SGF reinforced PES composites were fabricated using the extrusion compounding and injection molding techniques and the reinforcing effect of GO as a coating on SGFs has been examined for enhancing the mechanical properties of the SGF/PES composites. The results showed that the GO coating at a proper content could simultaneously enhance the tensile strength, Young’s modulus, flexural strength and flexural modulus. The PES composite with the 0.5 wt% GO coating showed the maximum improvement of 10.2% and 25.4% for the tensile strength and Young’s modulus, and the maximum improvement of 9.4% and 15.9% for the flexural strength and flexural modulus, respectively. Therefore, the GO-coated SGFs are believed to be a promising reinforcement for fabrication of high performance short glass fiber reinforced composites.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Tensile and flexural properties of graphene oxide coated-short glass fiber reinforced polyethersulfone composites Sen-Sen Du a, c, Fei Li a, Hong-Mei Xiao a, Yuan-Qing Li b, Ning Hu b, Shao-Yun Fu b, * a
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China College of Aerospace Engineering, Chongqing University, Chongqing 400044, China c University of Chinese Academy of Sciences, Beijing 100190, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 20 April 2016 Accepted 3 June 2016 Available online 6 June 2016
It is the first time to report the effects of graphene oxide (GO) coating on the mechanical properties of short glass fiber (SGF) reinforced polymer composites. GO-coated SGF reinforced polyethersulphone (PES) composites are manufactured using extrusion compounding and injection molding techniques. The micro-structures and morphologies of GO and GO-coated SGFs are investigated using scanning electron microscopy, small-angle X-ray scattering, atomic force microscope and Fourier transform infrared techniques. Then, the tensile and flexural properties of the GO-coated SGF/PES composites are systematically studied taking into account the effect of GO coating content. It is observed that both the tensile and flexural strengths are effectively enhanced by the GO coating on the SGF surfaces. This observation is mainly attributed to the enhanced interfacial adhesion between SGFs and PES due to the GO coating. Moreover, the tensile and flexural moduli are also improved by the addition of GO due to the fact that GO has a much higher modulus than the PES matrix. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Glass fibers Polymer-matrix composites (PMCs) Mechanical properties
1. Introduction In the past decades, short glass fiber (SGF) and short carbon fiber (SCF) reinforced polymer composites have been intensively investigated because of their ease of fabrication and superior mechanical properties. Particularly, low density thermoplastic composites based on SGFs and SCFs exhibit great potential in automotive or aviation industries as light-weight replacement materials. It is well known that the mechanical properties of short fiber reinforced polymer composites are critically related with fiber content, aspect ratio, orientation and fiber-matrix interfacial adhesion [1,2]. In order to achieve high mechanical performance for short fiber reinforced composites, a strong interfacial adhesion between short fibers and polymer matrix is necessary for transferring load from the matrix to the fibers [3,4]. To improve the fiber-polymer interfacial adhesion, the conventional methods include either enhancing the chemical bond between the fibers and the polymer matrix with coupling agent or introducing newly activated components or surface roughness onto
* Corresponding author. E-mail addresses: [email protected] (Y.-Q. Li), [email protected], [email protected]. cn (S.-Y. Fu). http://dx.doi.org/10.1016/j.compositesb.2016.06.023 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
the fiber surfaces with plasma treatment etc. [5,6]. Recently, graphene oxide (GO), which consists of a two-dimensional sheet of covalently bonded carbon atoms bearing various oxygen functional groups (e.g. hydroxyl, epoxy, and carbonyl groups) on their basal planes and edges, has shown a great potential as multifunctional sizing agent to enhance the interfacial bonding between the microsized fibers and the polymer [7e12]. The surface configuration of micro-sized fibers changed by GO sheets could enhance the strength and toughness of the fiber-matrix interfacial region [13]. Polyethersulphone (PES), an amorphous thermoplastic polymer, has attracted much attention due to its fascinating properties such as high glass transition temperature (Tg around 225 C), excellent thermal stability (operation temperature up to 180 C), great flame retardancy, low creep, high dielectric strength, extraordinary dimensional stability and chemical resistance etc. [14]. With these outstanding properties, PES is highly desirable as replacement for metals or ceramics in automotive, aerospace and microelectronics industries [14e16]. However, its relatively low mechanical properties still impede its broader applications. Recently, the fabrication of GO-coated SCF/PES composited was reported by our group [4]. The results reveal that GO coating on SCF surfaces leads to an obvious SCF/PES interfacial adhesion enhancement. A better stress transfer due to the enhanced
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interfacial bonding leads to the enhanced mechanical properties by the incorporation of GO-coated SCFs into the PES matrix compared to the un-treated SCF case [4]. However, comparing with SCF reinforced polymer composites, SGF reinforced composites are more attractive for industry applications because they have the most favorable cost-mechanical property relationships [17]. But, the knowledge obtained for SCF reinforced polymer composites cannot be simply transferred to the SGF reinforced polymer composites since SGFs and SCFs are two different types of micro-sized fibers. In order to explore the application of PES, it is of significance to study the enhancement effectiveness of SGFs by GO coating on PES. At the same time, based on our knowledge, there has been no report related with the effects of GO sheets on the interfacial characteristics between SGFs and PES and hence on the mechanical properties of SGF/PES composites. In the present work, short glass fiber reinforced PES composites are prepared using the melting blending and injection molding techniques. Meanwhile, to improve the mechanical properties of SGF/PES composites, GO as sizing agent is employed to improve the interfacial adhesion between SGFs and PES. First, the microstructures of GO and GO-coated SGFs are investigated using atomic force microscope (AFM), scanning electron microscopy (SEM), X-ray scattering (XRD), and Fourier transform infrared (FTIR). Then, the mechanical properties of the GO-coated SGF/PES composites are systematically investigated. The results reveal that the surfaces of SGFs are covered with wrinkled and roughened GO sheets after coating treatment. Importantly, the tensile and flexural properties of PES are effectively enhanced by the GO coating on SGF surfaces.
2. Experimental section 2.1. Materials Polyethersulfone granules (PES, E3010) was bought from BASF, Germany. Graphite powders were obtained from Qingdao AoKe ShiMo Co. Ltd, China. Short glass fibers (SGFs, 6 mm length, 552B) were provided by Zhejiang JUSHI, China. Ethanol, concentrated
sulfuric acid, potassium permanganate, hydrochloric acid and N,Ndimethyl-Formamide (DMF), were all obtained from Beijing Chemical Works and used as-received. Sodium nitrate was purchased from Tianjin JinKe Fine Chemical Industry Research Institute, China. All of the raw materials were used without any further purification and treatment.
2.2. Preparation of graphene oxide (GO) and GO-coated glass fibers GO was prepared using the modified Hummers method [18,19]. The obtained GO was subsequently dispersed in deionized water to form a suspension of 0.325 mg/ml. The non-exfoliated graphite oxide sheets were removed by centrifuge, then the GO was obtained by ultrasonic technique (1000 W) for 1.5 h and the homogeneous GO aqueous solution with a specific solubility of GO was formed for later use. A given amount of GO aqueous solution in terms of the prescribed formulation was used for treating SGFs. An appropriate amount of SGFs in the prescribed ratio was added to the GO aqueous solution under mild magnetic stirring for 24 h. After that, the resultant mixture was dried under vacuum at 100 C to remove water and then cooled naturally to room temperature. The GO concentration could be easily calculated in terms of the solubility of GO, the amount of GO aqueous solution and the amount of SGFs. The GO-coated SGFs with various GO concentrations were finally prepared. Moreover, the intensive stirring of GO-coated SGFs was conducted for examining the coating efficiency of the GO on to SGFs. The as-obtained GO-coated SGFs were mixed with PES to prepare GO-coated SGF/PES composite samples with different GO concentrations using a TSE-20/600-4-48 co-rotating twin-screw extruder (NanJing Ruiya Extrusion Systems LTD., China) at a screw speed of 30 rpm and a feed rate of 6 rpm. The temperature profile of the barrel was set at 360-365-370-375-380-380-375 C from the hopper to the die. The materials were crushed by a breaker and dried in an oven at 130 C for 6 h before injection molding. Standard tensile and flexural test bars were then obtained according to the recommendation of ASTM D 638-96 and ASTM D
Fig. 1. Evaluation of the coating efficiency of GO onto the surfaces of SGFs.
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Fig. 2. SEM images of (a) graphite and (b) GO; TEM image of (c) GO.
Fig. 3. AFM image (left) of graphene oxide sheet dispersion in water on freshly cleaved mica surface through drop-casting and height profile (right) indicating a sheet thickness of ~1 nm.
790-03, respectively by injection mold at the mold temperature of 180 C using HTF80X/1 plastic-injection molding machine (HaiTian International Holding LTD., China).
2.3. Characterization The morphology of GO sheets was characterized using an atomic force microscope (AFM) system (Veeco NsIV, USA). The as-obtained GO sheets were dispersed in water and dip-coated onto freshly cleaved mica surfaces before testing. The morphology of raw materials and PES composites were imaged by a Hitachi S-4800 Scanning electron microscope (SEM). The samples used for the SEM observation were prepared by directly spraying a thin layer of gold on the sample surfaces. X-ray diffraction (XRD) analysis of graphite, GO and GO-coated glass fibers were carried out by a X-ray diffractometer with Cu Ka radiation (k ¼ 1.5418 Å). Raman spectroscopy characterization was carried out on a RenishawinviaReflex (England) with a 532 nmAr laser. Fourier transform infrared (FTIR) measurements were conducted on an Excalibur 3100 FTIR (Varian, USA) between 500 and 4000 cm1. GO and glass fibers were mixed with KBr powders and pressed into tablets for FTIR characterization. The tensile and flexural properties were measured on an INSTRON 5882 Mechanical Tester under a 100 kN load cell with a crosshead speed of 2 mm/min. The mechanical properties were tested according to the recommendation of GB/T 2571-1995. At least six specimens were measured for each composition. The fracture surfaces and the fiber orientation distribution (FOD) of the specimens after impact testing were examined by SEM. After removing the PES matrix using DMF extraction, the optical images of glass fibers were taken using an optical microscope (OLYMPUS STM6, Japan), and the fiber lengths were measured by the computer software SemAfore 4.0 based on the glass fiber images obtained.
3. Results and discussion 3.1. Characterization of GO and GO-coated short glass fibers In order to evaluate the stability of GO on SGFs, the dried 0.5 wt% GO-coated SGFs were re-dispersed and intensively stirred in deionized water and then the coated-SGFs after intensive stirring were collected for SEM observation. As shown in Fig. 1, the GO was still firmly adsorbed on the surfaces of SGFs after intensive stirring according to the SEM image of the collected SGFs. This indicates that the GO has been successfully and firmly coated on the surfaces of SGFs. The SEM images for the raw graphite and the as-prepared GO as
Fig. 4. XRD patterns of graphite and GO.
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The X-ray diffraction (XRD) patterns of graphite powders and GO are displayed in Fig. 4. A sharp peak at 2q ¼ 26.5 can be distinguished for graphite, which is assigned to the diffraction of (002) plane of the well-ordered graphene with an interlayer spacing of 0.335 nm [23]. However, the sharp diffraction peak observed in the XRD pattern of pristine graphite completely disappears upon oxidation, and a new diffraction peak appears at 2q with a value of approximately 10.6e11.0 , corresponding to an interlayer spacing of 0.80e0.83 nm. The variation in the interlayer spacing of GO is a result of variation in the degree of oxidation on the graphite. This means that the graphite oxide has been successfully exfoliated to form layered GO nanosheets after oxidation and exfoliation. The d-spacing of GO can be calculated according to the Bragg’s law (1): nl ¼ 2d sinq
Fig. 5. FTIR spectra of the as-prepared GO and the SGFs.
well as the TEM image for the GO are presented in Fig. 2. Thick and flat layers appear for graphite as shown in Fig. 2a while a wrinkled morphology is observed for GO nanosheets as shown in Fig. 2b. Moreover, Fig. 2c shows that the GO nanosheets have a very thin crumpled silk veil wavy morphology [20]. An important property of GO, brought about from the hydrophilic nature of the oxygenated graphene layers, is its easy exfoliation in aqueous media [21]. To investigate the exfoliation degree of the graphite oxide in water, the AFM image of graphene oxide (GO) sheet dispersion in water is conducted. Fig. 3 exhibits a typical AFM image of GO sheets deposited onto a mica substrate from an aqueous dispersion. Analysis of the AFM image reveals that most GO sheets have heights of about 1 nm and lengths ranging from hundreds of nanometers to several micrometers, indicating fully exfoliated GO sheets were indeed achieved in water [22].
(1)
where l is the X-ray wavelength, n is the diffraction series and d is the interlayer spacing. The calculated value of l for graphite and graphite oxide is 3.35 Å and 8.66 Å, respectively, implying that the sample is expanded when graphite is oxidized. However, the peak of GO is weaker than graphite oxide, implying that the GO layers have been disordered [24]. The FTIR spectrum of GO is shown in Fig. 5. It can be seen that the predominant characteristic absorption signals include a broad and intense peak at 3420 cm1 attributed to OeH stretching of carboxyl groups, an intense peak at 1730 cm1 assigned to C]O stretching of carbonyl groups, a weak peak at 1620 cm1 corresponding to C]C skeletal vibration of un-oxidized graphitic domain and a strong peak at 1366 cm1 attributed to an alkoxy CeOH stretching vibration. In addition, an absorption peak at 1053 cm1 assigned to the stretching vibration of CeO [25]. The peak at 1210 cm1 is attributed to the symmetrical stretching vibration of epoxy ring (CeOeC) while the peaks observed at 907 cm1 and 800 cm1 are assigned to the asymmetric stretching and in-plane bending vibrations of CeOeC [26]. The FTIR spectrum of SGFs is also shown in Fig. 5. It exhibits that SGFs contain OH
Fig. 6. SEM images for (a, b) the raw short glass fibers and (c, d) the coated short glass fibers with the 0.5 wt% GO.
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Fig. 7. Raman spectra of graphite, GO, SGFs and GO-coated SGFs.
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groups. As well known, that reactive groups (namely epoxy groups) can react with hydroxyl to form ether bond, indicating that GO tend to react with SGFs and form strong bonding [4]. The SEM images for the morphologies of the raw SGFs and GOcoated SGFs are shown in Fig. 6. As shown in Fig. 6a and b, the surfaces of the raw SGFs are quite smooth. After the GO treatment, the SGF surfaces become rough and the typical wrinkled morphology of GO-wrapped SGFs can be clearly observed as shown in Fig. 6c and d. The treated short glass fibers with the 0.5 wt% GO are fully covered by GO as shown in Fig. 6d. This indicates that GO has been successfully coated on SGF surfaces. Raman spectroscopy is another useful and non-destructive technique used to reveal the structural information of carbonbased materials such as carbon nanotubes, graphite and GO. Typically, the Raman spectrum of GO consists of two predominant D and G bands as shown in Fig. 7. The D and G bands are represented by the peaks at approximately 1350 cm1 and 1580 cm1. The G band corresponds to the optical E2g phonons at the Brillouin zone center due to bond stretching of the sp2 carbon pairs. The D band is associated with the second-order of zone-boundary phonons where it is activated by defects [27]. For GO-coated SGFs, the two
Fig. 8. Effects of GO content on (a) tensile strength and (b) Young’s modulus of the GO-coated SGF/PES composites.
Fig. 9. Effect of the GO content on the (a) flexural strength and (b) flexural modulus of the GO-coated SGF/PES composites.
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Fig. 10. SEM images of the pulled-out fibers of (a) raw SGF/PES composite, (b) 0.05 wt% GO-coated SGF/PES composite, (c) 0.1 wt% GO-coated SGF/PES composite, (d) 0.2 wt% GOcoated SGF/PES composite, (e) 0.5 wt% GO-coated SGF/PES composite and (f) 1 wt% GO-coated SGF/PES composite.
peaks can also be clearly observed although the content of GO is only 0.5 wt%. However, for raw SGFs, no peak is observed. So Raman spectroscopy further demonstrates the successful coating of GO on SGF surfaces. 3.2. Mechanical properties of PES composites Tensile tests were performed on the dumbbell shaped samples prepared using an injection molding machine. The results for the tensile strength of PES composites are given in Fig. 8a as a function of the GO weight percentage from zero to 1.0 wt%. It is observed that the tensile strength of the PES composites increases from 108 to 119 MPa and then slightly decreases with further increasing the GO content. The maximum composite strength at the 0.5 wt% GO content corresponds to an increase of 10.2% compared to the untreated SGF case. This result indicates that a small amount of GO can effectively enhance the tensile strength of the composites. It is because that the interfacial bonding between SGFs and PES has been enhanced by the GO coating on SGF surfaces to be shown later and thus there will be a better load transfer from the PES to the SGFs, allowing the composites to bear a higher applied load [28].
The results for the Young’s modulus of the PES composites are shown in Fig. 8b as a function of the GO content. The Young’s modulus of composites increases with increasing the GO content and reaches the plateau level after the 0.5 wt% GO content, corresponding to an increase of 25.4% compared to the un-treated case. It is well known that GO has a much higher Young’s modulus than the PES matrix, thus the increase of the composite Young’s modulus with increasing the GO content is understandable. The flexural strength of the PES composites with various amounts of GO is depicted in Fig. 9a. It can be observed that the flexural strength of the composites increases from 160 MPa to the maximum value of 175 MPa at the 0.5 wt% GO content and then slightly decreases with further increasing the GO content. The maximum strength corresponds to an increase of 9.4% compared to the un-treated SGF case. This observation is due to the fact that the interfacial bonding between SGFs and PES is improved by the GO coating to be shown later. At the same time, the flexural modulus of the composites shown in Fig. 9b exhibits a consistent increasing tendency with increasing the GO coating content. The introduction of GO coating leads to a maximum increase of 15.9% in the flexural modulus at the 1.0 wt% GO content compared to the un-treated
Fig. 11. Photomicrographs of the collected short glass fiber samples after removing the PES matrix from the composites: (a) un-treated, (b) 0.05 wt% GO, (c) 0.1 wt% GO, (d) 0.2 wt% GO, (e) 0.5 wt% GO and (f) 1.0 wt% GO.
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Fig. 12. Glass fiber-length distribution histograms of the un-treated and GO-coated SGF/PES composites: (a) un-treated, (b) 0.05 wt% GO, (c) 0.1 wt% GO, (d) 0.2 wt% GO, (e) 0.5 wt% GO and (f) 1.0 wt% GO.
case. The enhancement in the composite flexural modulus is attributed to the high modulus of GO. 3.3. Micrographs of the fracture surfaces of the PES composites The tensile fracture surfaces of the composites are observed using SEM. And the micrographs for the fracture surfaces are shown
in Fig. 10. Fig. 10a shows that the surfaces of the raw SGFs are quite smooth, indicating a relatively weak interfacial adhesion between SGFs and PES matrix. Fig. 10bef exhibits that the rough surfaces of the GO-coated SGFs are observed after GO-coating. There is a big amount of PES matrix attached on the SGF surfaces for the coated SGF cases. The average diameter from over 20 measurements of the SGFs are 9.32, 10.85, 12.06, 12.89, 13.06, 12.86 mm respectively for
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Fig. 13. SEM images of the fracture surfaces of the (a) un-treated SGF/PES composite, (b) 0.05 wt% GO-coated SGF/PES composite, (c) 0.1 wt% GO-coated SGF/PES composite, (d) 0.2 wt% GO-coated SGF/PES composite, (e) 0.5 wt% GO-coated SGF/PES composite and (f) 1 wt% GO-coated SGF/PES composite.
the cases of 0 wt%, 0.05 wt%, 0.1 wt%, 0.2 wt%, 0.5 wt% and 1.0 wt% GO. Namely, the average diameter of the pulled-out SGFs increases with increasing the SGF content up to 0.5 wt% and afterwards decreases with further increasing the GO content. It is well known that a relatively big amount of polymeric matrix attached on pulled-out short fibers corresponds to a relatively strong interfacial bonding [29]. Thus, it is clear that the SGF/PES interfacial adhesion is the optimal for the 0.5 wt% GO-coated SGF case while a further increase in the GO content to 1.0 wt% will lead to a reduced interfacial adhesion strength but it is still much better than for the untreated case. For the high (1.0 wt%) GO content case, the reduced interfacial adhesion strength should be attributed to the GO agglomeration [3] occurring on the SGF surfaces. The composite tensile strength is closely related to the SGF/PES interfacial adhesion strength and a strong interfacial adhesion strength corresponds to a high composite strength [2]. Therefore, it is understandable that the composite strength increases initially with increasing the GO content up to 0.5 wt% and then decreases with further increasing the GO content. The pulled-out GO-treated SGFs (Fig. 10) with a larger average diameter than the un-treated SGFs (Fig. 6a and b) indicates that the GO coated on the SGFs is not split off from the fiber surfaces after extrusion compounding and injection molding processing, confirming that the GO coating has a good stability on the SGF surfaces.
important roles in determining the final fiber length of the composites. Fiber orientation affects the mechanical properties of composites [30], thus an attempt was tried to get SEM and optical images from the cut and polished cross sections of the samples in order to measure fiber orientation. However, the SEM and optical images obtained from the polished cross sections are quite indistinct especially for the GO-coated SGF cases with an improved interfacial adhesion. Even an etching solvent such as DMF was used, the PES matrix could not be removed from the SGF/PES interfaces. Therefore, a qualitative characterization of fiber orientation is accepted here from the fracture surfaces of the samples. From the SEM micrographs for the fracture surfaces of the SGF/PES composites as shown in Fig. 13, it can be seen that the fibers are preferentially aligned along the injection molding direction (transverse to the sample cross-section). This is understandable since the SGF content and the processing conditions are fixed for the investigated cases. In summary, the above results show that the effects of GO coating on the fiber length and orientation are negligible. Therefore, the enhancements in the composite tensile and flexural strength via the GO coating on the SGF surfaces are mainly attributed to the enhanced SGF-PES interfacial adhesion. The improvements in the tensile and flexural modulus are due to the fact that the modulus of GO is much higher than the PES matrix.
3.4. Glass fiber-length and orientation
4. Conclusions
It is well known that fiber length is a major influencing factor on the mechanical properties of short fiber reinforced composites [28]. For injection molded composites, fiber breakage takes place during processing [17]. The results for the effect of the GO content on average glass fiber length are presented in Fig. 11 and Fig. 12. The photomicrographs of the collected SGFs after removing PES matrix from these composites using DMF are shown in Fig. 11. The fiber length is then measured for over 800 fibers for each composition using the software SemAfore 4.0 and the results for the SGF length distribution histograms are presented in Fig. 12. As shown in Fig. 12a-f, the mean fiber length is not significantly affected by the GO coating on the SGFs. Namely, the average glass fiber length is in the narrow range of 112e122 mm for all the investigated cases. Thus, it can be considered that the GO coating has a negligible effect on the final fiber length. This is understandable since the SGF content is fixed and the processing conditions are the same for all the investigated cases, which play
In conclusion, the SGF surfaces were covered with the wrinkled and roughened GO sheets after coating treatment. It was shown that the GO coating effectively improved the interfacial adhesion of SGF/PES composites. The GO-coated SGF reinforced PES composites were fabricated using the extrusion compounding and injection molding techniques and the reinforcing effect of GO as a coating on SGFs has been examined for enhancing the mechanical properties of the SGF/PES composites. The results showed that the GO coating at a proper content could simultaneously enhance the tensile strength, Young’s modulus, flexural strength and flexural modulus. The PES composite with the 0.5 wt% GO coating showed the maximum improvement of 10.2% and 25.4% for the tensile strength and Young’s modulus, and the maximum improvement of 9.4% and 15.9% for the flexural strength and flexural modulus, respectively. Therefore, the GO-coated SGFs are believed to be a promising reinforcement for fabrication of high performance short glass fiber reinforced composites.
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