Process Development and Characterization of Spraying Carbon Nanofibers Over Fabrics for Reinforcing Polymer Composites
Ahmed Khattab,1 Pengfei Zhang,2 Wan Shou,3 Mohammad J. Khattak4 1 Department of Industrial Technology, College of Engineering, University of Louisiana at Lafayette, Lafayette, Louisiana 70504-2972 2
Department of Mechanical Engineering, College of Engineering, Louisiana State University, Baton Rouge, Louisiana 3
Department of Mechanical Engineering, College of Engineering, University of Louisiana at Lafayette, Lafayette, Louisiana 70504 4
Department of Civil Engineering, University of Louisiana at Lafayette, Lafayette, Louisiana, 70504
Process development and characterization of spraying carbon nanofibers (CNF) over carbon fiber fabrics for reinforcing polymer composites are presented in this study. The molded composite structure consists of a high-temperature polymer reinforced with carbon fiber fabrics sprayed with different dosages of carbon nanofibers. The materials were molded using vacuum assisted resin transfer molding process. Tensile testing and scanning electron microscopy (SEM) were used to characterize the molded materials. The results show that the tensile strength and modulus were both improved over the molded materials without CNF. Spraying CNF with a dosage of an 8 mg/mm2 of the used fabrics helped to increase the tensile strength by 12%. The tensile modulus increased by 28% with a CNF dosage of 16 mg/mm2. Uniform distribution of CNF was observed under SEM in the molded compoC 2013 Society of sites. POLYM. COMPOS., 00:000–000, 2013. V Plastics Engineers
INTRODUCTION Nanoreinforced polymer composites are currently receiving significant popularity in the field of materials development [1–6]. Carbon nanofiber (CNF) has been widely studied and demonstrated a high potential to be a
Correspondence to: Dr. Ahmed Khattab; e-mail:
[email protected] Contract grant sponsor: Louisiana Board of Regents Support Fund; contract grant number: LEQSF(2010-15)-LaSPACE; contract grant sponsor: NASA; contract grant number: NNX10AI40H. DOI 10.1002/pc.22816 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2013 Society of Plastics Engineers V
POLYMER COMPOSITES—2013
good candidate to improve the properties of polymer composites [1–6]. These properties include mechanical properties [2–4], thermal [4, 5] and electrical [5, 6]. Even dispersion of the nanoparticles is one of the major challenges, the smaller the nanoparticles, with a high aspect ratio, the stronger the tendency to agglomerate [7]. Improvements in dispersion techniques yield improvements in the resultant material properties. Conversely, an improper dispersion may lead to nanoparticles damage which deteriorates the material properties. Among the several manufacturing processes of fiber reinforced polymer composites, vacuum assisted resin transfer molding (VARTM) is a very cost-effective process in fabricating large structures with a high fiber volume fraction [8–10]. It is a closed process offering environmentally benign manufacturing through the reduction of volatile organic compound (VOC) emission. VARTM process can produce high quality composite materials with good mechanical properties [10]. VARTM can be used for manufacturing nanoparticles reinforced polymer composites [10, 11]. The most common technique in introducing nanoparticles into polymer composites involves mixing the nanoparticles with the matrix directly by sonication and/or mechanical shear. In liquid molding processes, such as VARTM, nanoparticles filtration through the distribution medium, during the mold filling stage, results in a poorly controlled and inconsistent distribution of the nanoparticles throughout the part with some uncertainty about the actual amount of the nanoparticles in the molded material. To directly deposit the nanoparticles onto the fabrics to fabricate advanced nanocomposites, several techniques
have been reported, such as electrophoretic deposition [12], solution filtering [13], spraying [14], and thermal chemical vapor deposition (CVD) [15]. In a preliminary investigation by Khattab et al. [16, 17], two different indirect dispersion techniques were evaluated to overcome the aforementioned problems of direct dispersion of nanoparticles, such as CNF, in polymers. The first technique used distilled water as a low viscous carrier for the CNF. The CNF-water mixture was injected into the mold first. Then, the mold was heated to evaporate the water before injecting the polymer. In the second technique, CNF were sprayed over the woven fabrics. The study showed the viability of the spraying technique as well as the need for more research to determine the optimum percentage of sprayed CNF to enhance the properties of the molded materials. In this article, the effects of spraying different CNF dosages over fabrics on the properties of fiber reinforced composites were investigated. Mixtures with different CNF concentration in acetone were prepared by sonication and high shear mixing. Then, the mixture was sprayed by a high air pressure gun, obtaining different dosages of CNF over fabrics (8, 12, 16, 20 mg/mm2). The sprayed fabrics were used as the reinforcement for manufacturing composites panels using VARTM. The molded materials were mechanically tested under tensile loading according to ASTM standard D3039. A scanning electron microscope (SEM) was used to analyze and characterize the dispersion of CNF and the fracture morphology of the molded materials.
power of 240 w (20 Hz), pulse rate of 90% and a total time of 24 min. The sonication process included three cycles, 8 min for each individual cycle. There was an interval for cooling the mixture down with a 25-min period between two individual cycles. After the sonication process, another 25-min period was provided for mixture cooling. A mechanical shear mixing was then applied with a shear rate of 3,500 rpm for 3 min. Then the CNFacetone mixture was sprayed over one side of the reinforcement fiber fabrics. The fabrics were transferred to an air circulating oven under 100oC for 12 h before being used. The amount of CNF sprayed per unit area of the fabric was determined by the weight difference of the fabric before and after spraying.
EXPERIMENTAL
MATERIALS CHARECTERIZATION
Materials System
Mechanical Testing
Vapor-grown CNF (Polygraf III) produced by Applied Science was used in this study. This functionalized CNF has a diameter of 60–150 nm with a tensile modulus of 600 GPa and a tensile strength of 7 GPa. Five harness carbon fiber woven fabrics, produced by Hexcel, were used to provide the needed reinforcing component for composite structures. This material has a tensile strength of 4,278 MPa, density of 1,790 kg/m3 and fiber areal weight of 0.376 kg/m2. The resin used is Cycom 5250-4RTM produced by Cytec Engineered Materials. It is a one-part homogenous Bismaleimide (BMI) resin with some additives to improve toughness. This polymer was developed for use in high performance structural composites requiring high temperature, up to 204 C, use and increased toughness.
Tensile properties of the molded materials were determined according to ASTM standard D3039. Five specimens were tested for each case. The machine which was used in the test is Series 793 Materials Test System from MTS Systems Corporation. Hydraulic grips were used with a gripping pressure of 10 MPa. The tension test was performed using a constant head speed of 1.27 mm/min. An extensometer was used to record strain. The Specimens used in the test were 203.2 mm long, 25.4 mm wide. To prevent slippage of the specimen inside the
Composite Fabrication The composite panels used in this study were manufactured using a VARTM process with a dimension of 380 mm 3 175 mm. Two layers of carbon fiber fabrics were used. Five different cases were investigated. In Case 1 the composite panels were molded without CNF while in the other cases they were reinforced with CNF sprayed over fabrics with different dosages. Table 1 shows the designed and actual dosages of sprayed CNF over fabrics. The resin was pre-heated to 120˚C and then injected into the mold and heated from 120˚C to 188˚C at the rate of 1.67˚C per minute, after which it was cured at this temperature (188˚C) for 4 h. After curing, the mold was left to cool down to room temperature for demolding.
TABLE 1. The designed and actual dosages of sprayed CNF over fabrics.
Case No.
CNF Dispersion and Spraying Mixtures of CNF/Acetone with 2.5 mg/ml of CNF concentration in acetone were prepared by sonication and high shear mixing. The sonication was applied with a 2 POLYMER COMPOSITES—2013
1 2 3 4 5
Designed CNF Dosage, mg/mm2
Actual CNF Dosage, mg/mm2
Experimental Error, %
– 8 12 16 20
– 8.22 11.86 16.79 19.40
– 2.75 1.17 4.94 3.00
DOI 10.1002/pc
respectively. The molded material is a fiber dominated material. So, the test values have been normalized using the specimen thickness and reinforcement fiber areal weight [18]. The normalized values were calculated as follows: Normalized value5Test value 3 ðFV normalizing 3Tspecimen 3qf =FAW specimen Þ (1) where: FVnormalizing: chosen common fiber content (volume fraction) Tspecimen: specimen thickness qf: fiber density FAW: reinforcement fiber areal weight. The average value, standard deviation, and percentage of coefficient of variation were calculated as follows:
FIG. 1. Tensile stress–strain curve for the molded materials with and without CNF sprayed fabrics. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.].
hydraulic grips of the MTS machine, strips of emery cloth were glued to the ends of the specimens.
x5
Morphological Characterization
n X
! xi =n
(2)
i51
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! u n u X 2 t 2 xi 2n x =ðn21Þ Sn21 5
Microstructures of the composite specimens were characterized under a field emission scanning electron microscopy JEOL-6300FV for the molded materials with and without CNF. The specimens were cut through the crosssection of the tensile test failure region. Neat CNF as received from the vendor as well as the CNF sprayed over the fabrics were characterized under the SEM.
(3)
i51
x CV51003Sn21 =
(4)
where: x: sample mean (average) Sn21: sample standard deviation CV: sample coefficient of variation, in percent n: number of specimens xi: measured property. Figure 1 shows the stress-strain curves for the different cases under investigation. It is obvious that the composite specimens failed immediately reaching to their ultimate tensile stress. A comparison between the tensile strength and tensile modulus for specimens molded with no CNF added, CNF dispersed in acetone and sprayed over the fabrics is shown in Table 2. The tensile strength was improved by adding CNF, indicating that adding CNF has a positive effect on the tensile strength. The maximum improvement in the tensile strength was shown with a CNF dosage of 8 mg/mm2, then it decreased slightly by
RESULTS AND DISCUSSIONS Tensile Properties Case number 5 with a 20 mg/mm2 CNF dosage failed to be molded due to an incomplete mold filling with the resin. This indicates that a CNF with this dosage reduced the preform permeability sharply. The tensile strength and modulus were determined for four Cases 1 to 4 with 0, 8, 12: 16 mg/mm2 CNF sprayed over the reinforcement fabrics. Five specimens of each case were tested. The average thicknesses for the specimens of Cases 1, 2, 3, and 4 were found to be 0.871, 0.982, 1.065, and 1.066 mm,
TABLE 2. Summary of normalized mechanical test results. Ultimate tensile strength MPa CNF dosage mg/mm [2] 0 8 12 16
DOI 10.1002/pc
Tensile modulus
Average (MPa)
Standard deviation (MPa)
Coefficient of variation (%)
Average (GPa)
Standard deviation (GPa)
Coefficient of variation (%)
729 814 805 808
17.2 11.8 13.7 28.3
2.4 1.4 1.7 3.5
55 57 67 70
2.1 1.1 7.1 0.8
3.8 1.9 10.6 1.1
POLYMER COMPOSITES—2013 3
FIG. 2. SEM images of (A) neat CNF and (b) sprayed CNF over the fabric.
adding CNF dosages higher than 8 mg/mm2. The composite material with no CNF has a tensile strength of 729 MPa with a 2.4% coefficient of variation. While the molded material with a CNF dosage of 8 mg/mm2 added has a tensile strength of 814 MPa with a 1.4% coefficient of variation. So, the composite material with CNF showed an increase of about 12% in the tensile strength than the composite panel molded without CNF. As shown in Table 2, the tensile modulus increased as a response to the increased CNF dosage. Table 2 shows that the tensile modulus of the composite that molded without CNF has a modulus of 55 GPa with a 3.8% coefficient of variation. Adding a CNF dosage of 16 mg/mm2 helped to increase the tensile modulus, by 28%, to 70 GPa with a coefficient of variation of 1.1%. SEM Characterization Characterization of the sprayed fabric under SEM showed that the sprayed CNF created a layer with a 3D nanostructure of CNF. Figure 2A depicts that obvious bundles and aggregates can be observed for the neat CNF as received from the vendor, the entanglement between
individual fibers is remarkable. Figure 2B shows a relative uniform distribution of the CNF sprayed over the fabric with a looser structure resulting from the sonication and spray technique. The fracture surfaces of the molded materials were observed under SEM as shown in Figures 3–7. The fracture surfaces were obtained by breaking the specimens in liquid nitrogen. Figure 3 shows the polymer composite made with CNF sprayed over fabrics with a dosage of 8 mg/mm2. Individual CNF were detected with a good distribution in the polymer matrix. SEM morphologies of selected specimens are shown in Figures 4–7A and B at two magnification factors 2,000 and 10,000, respectively. Figure 4 shows the polymer composite specimen molded without CNF. The observation, under the SEM, of the fiber surfaces shows smooth surfaces for the fibers without CNF sprayed, which may result from weak interfacial bonding between fiber and polymer. Figures 5–7 show the polymer composite specimens molded with CNF sprayed over fabrics with CNF dosages of 8, 12, 16 mg/ mm2, respectively. The surface of the fiber in the cases with spayed CNF was a rougher surface, than the case without CNF, with a thin layer of CNF attached to the
FIG. 3. SEM image of the molded composite materials with CNF sprayed over fabrics. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.].
4 POLYMER COMPOSITES—2013
DOI 10.1002/pc
FIG. 4. SEM micrographs for the molded composites without nanofibers: (A) 2,000 magnification and (B) 10,000 magnification.
fibers, which can be an indication of a better interfacial bonding. The CNF dispersed in the matrix with the crack bridging effect in a micro/nanoscale, as shown in Fig. 5B, can hinder the crack propagation. It is believed that a small amount of CNF coating the fibers would positively affect the fiber/ resin interfacial bonding. However, a large amount of CNF would negatively affect materials performance because a thicker layer of CNF would form over the fabrics and blocks the resin to penetrate through and wet the fibers. Figures 5 and 6 show the composite material specimens with CNF dosages of 8 and 12 mg/mm2, respectively. As shown in the figures, the fibers surfaces are rough and coated with a thin layer of CNF/matrix. However, a relatively thick layer of CNF/matrix can be seen completely covering the woven fabrics is shown in Figure 7 for a composite material specimen with a CNF dosage of 16 mg/mm2. CONCLUSIONS In this paper, the effects of spraying CNF over carbon fiber fabrics were investigated and evaluated in molding CNF reinforced high-temperature polymer composite
materials via VARTM. Tensile testing and scanning electron microscopy were used to characterize the molded materials. The following conclusions can be drawn from the generated results: 1. Spraying CNF with a dosage of an 8 mg/mm2 of the used fabrics helped to increase the tensile strength by 12%. 2. Spraying CNF with a dosage of a 16 mg/mm2 helped to increase the tensile modulus by 28%. 3. Fabrics with a 20 mg/mm2 CNF dosage failed to be molded due to an incomplete mold filling with the resin. This indicates that a CNF with this dosage reduced the preform permeability sharply. 4. SEM images showed uniform distribution of the sprayed CNF in both sprayed fabrics and fractured surfaces.
To achieve accurate control of nanoparticles spray, a novel design and development of a fully automated spray system is needed. The study shows the need for more research to determine the optimum percentage of sprayed CNF to better enhance the properties of the molded materials. It is believed that a relation between the CNF sprayed dosage and the resin viscosity should be established to optimize the molding process. In general, the
FIG. 5. SEM micrographs for the molded composites with nanofibers coated over woven fabrics with a CNF dosage of 8 mg/mm2: (A) 2,000 magnification and (B) 10,000 magnification.
DOI 10.1002/pc
POLYMER COMPOSITES—2013 5
FIG. 6. SEM micrographs for the molded composites with nanofibers coated over woven fabrics with a CNF dosage of 12 mg/mm2: (A) 2,000 magnification and (B) 10,000 magnification.
FIG. 7. SEM micrographs for the molded composites with nanofibers-coated over woven fabrics with a CNF dosage of 16 mg/mm2: (A) 2,000 magnification and (B) 10,000 magnification.
spraying technique has a high potential to control the exact amount of CNF to be added to the composites with the capability to selectively reinforce localized areas of the fabrics to accommodate a stress distribution caused by an applied load for a given application. In essence, not only it can eliminate the shortcomings of the direct dispersion of nanoparticles in polymers, hence lead to welldispersed reinforcements, but it can potentially allow for an unprecedented 3D multi-scale control of reinforcement distribution. This technique will allow an engineer to control the reinforcement type, volume fraction, aspect ratio, and size of reinforcing particles at specific locations in the composite part. This can enable precision 3D microstructural control for targeted properties and applications. The authors are in the process of designing and developing an automated nanoparticles spraying system for selective reinforcement. ACKNOWLEDGMENTS This research was supported by the Louisiana Board of Regents Support Fund, [contact number LEQSF(2010-15)LaSPACE] and the support of NASA [grant number 6 POLYMER COMPOSITES—2013
NNX10AI40H] through a subcontract with LSU. The authors wish to acknowledge the support of the Airtech Advanced Materials Group, Hexcel Corporation, and Cytec Engineered Materials. REFERENCES 1. E. Thostenson, C. Li, and T. Chou, Compos. Sci. Technol., 65, 491 (2005). 2. D. Bortz, C. Merino, and I. Martin, Compos. Sci. Technol., 72, 446 (2012). 3. C.S. Lim, M.E. Guzman, and B. Minaie, Carbon, 54, 489 (2013). 4. M. Hossain, M. Dewan, M. Hosur, and S. Jeelani, Compos. Part B, 44, 313 (2013). 5. G. Sui, S. Jana, W. Zhong, M. Fuqua, and C. Ulven, Acta Materialia, 56, 2318 (2008). 6. G.D. Liang and S.C. Tjong, IEEE Transact Dielectrics Electrical Insulation, 15, 214 (2008). 7. H. Khare and D. Burris, Polymer, 51, 719 (2010). 8. S. Ghose, K.A. Watson, D.C. Working, E.J. Siochi, J.W. Connell, and J.M. Criss, High Performance Polym., 18, 527 (2006).
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