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Carbon and glass hierarchical fibers: Influence of carbon nanotubes on tensile, flexural and impact properties of short fiber reinforced composites
Materials and Design 43 (2013) 10–16
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Carbon and glass hierarchical fibers: Influence of carbon nanotubes on tensile, flexural and impact properties of short fiber reinforced composites S. Rahmanian a, K.S. Thean b, A.R. Suraya a,b,⇑, M.A. Shazed b, M.A. Mohd Salleh a,b, H.M. Yusoff b a b
Advanced Materials and Nanotechnology Lab, Institute of Advanced Technology, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia Department of Chemical and Environmental Engineering, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 21 April 2012 Accepted 15 June 2012 Available online 4 July 2012 Keywords: A. Nanocomposites A. Carbon nanotubes C. Vapor deposition E. Impact
a b s t r a c t Dense carbon nanotubes (CNTs) were grown uniformly on the surface of carbon fibers and glass fibers to create hierarchical fibers by use of floating catalyst chemical vapor deposition. Morphologies of the CNTs were investigated using scanning electronic microscope (SEM) and transmission electron microscope (TEM). Larger diameter dimension and distinct growing mechanism of nanotubes on glass fiber were revealed. Short carbon and glass fiber reinforced polypropylene composites were fabricated using the hierarchical fibers and compared with composites made using neat fibers. Tensile, flexural and impact properties of the composites were measured, which showed evident enhancement in all mechanical properties compared to neat short fiber composites. SEM micrographs of composite fracture surface demonstrated improved adhesion between CNT-coated fiber and the matrix. The enhanced mechanical properties of short fiber composites was attributed to the synergistic effects of CNTs in improving fiber–matrix interfacial properties as well as the CNTs acting as supplemental reinforcement in short fiber-composites. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) exhibit the combination of intriguing mechanical [1], thermal [2] and electrical [3] properties. Therefore, they possess great potential to reinforce high performance composites with wide structural applications [4,5]. Significant research attempts have sought to incorporate CNTs into fiber reinforced composites in order to improve the matrix [6,7] or fiber–matrix interfacial characteristics [8,9]. There are two main methods to fabricate these composites, either through the dispersion of CNTs in polymer (to perform as modified matrices or sizing agent) [10] or directly growing CNTs onto the fiber surfaces [11]. However, using the former method, good dispersion of disentangled CNTs in polymers is a primary challenge and cause filler aggregation phenomena. As a result, agglomeration sites act as stress concentration points and restrict the mechanical and physical performance of composites [12]. Such mentioned problems and also limitations in manufacturing process due to increasing viscosity of CNT-polymer matrices, have encouraged researchers to use the latter approach of directly growing or coating CNTs on fiber surfaces. These types of fibers are sometimes known as hierarchical fibers [13]. Other advantages of CNT-coated fibers are to improve the
⇑ Corresponding author at: Advanced Materials and Nanotechnology Lab, Institute of Advanced Technology, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia. Tel.: +60 3 89466285; fax: +60 3 86567120. E-mail address: [email protected] (A.R. Suraya). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.06.025
interfacial bonding between the fiber and matrix [14] as well as the additional support on the load transfer effects afforded by the CNTs [15]. Among various techniques to produce CNTs, chemical vapor deposition (CVD) is the most promising to fabricate CNT-coated fibers. CVD is an adaptable process since it enables the use of different kinds of hydrocarbons, substrates and catalysts. Hydrocarbon sources such as ethylene, acetylene, toluene, benzene are often employed along with Co, Fe or Ni as active catalysts to grow CNTs on substrates through CVD process. The structure and architecture of CNTs can be controlled through regulating the parameters of the CVD process like temperature, flow rate, and length of reaction time [16,17]. Some previous works demonstrated improvement in mechanical properties of these hierarchical composites while using distinctive conditions for the CVD process. Enhancement of interfacial shear strength (IFSS) of silica fibers by applying a two-step technique [18], significant increase in storage modulus of unidirectional carbon composites after low temperature CVD process [19] and improvement of interlaminar fracture toughness of aligned CNTs grown silicon carbide fabric composites [20] have been reported. Despite the extensive application of short fiber composites, previous researchers have mostly paid attention to the effects of CNTgrown fiber in composites reinforced by continuous carbon fibers or fabrics. Due to the wide application of short glass fiber reinforced composites, there is a need to understand more about the synthesis of CNTs on glass fiber as well as carbon fiber and their role in improving the mechanical properties of short fiber-composites.
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In the present investigation, we have reported the synthesis of CNTs on carbon fiber and glass fiber via feeding a vaporized solution of benzene (carbon precursor) and ferrocene (catalyst precursor) into a custom-built CVD reactor in which fiber bundles were placed. The morphology of the CNT-coated on carbon as well as glass fibers were studied and compared using scanning electronic microscope (SEM) and transmission electron microscope (TEM). Short carbon fiber reinforced polypropylene (SCF–PP) composites and short glass fiber reinforced polypropylene (SGF–PP) composites were fabricated using the hierarchical fibers. To evaluate the effects of CNTs coating on various mechanical characteristics of short fiber composites, tensile, flexural and impact tests were carried out. Fracture surfaces of composites after impact test were examined through SEM micrographs to analyze the role of CNTs in the interfacial region of fiber–matrices.
blended in an internal mixer (Thermo Haake PolyDrive R600/ 610) equipped with a pair of roller-type blades. Melt blending process was set to a temperature of 170 °C [22] with a rotor speed of 55 rpm. After 5 min melting of PP, cut fibers were added to the melted PP. A batch of the blended composite was compressed into a 15 cm 15 cm mold with 1 mm and 3 mm thickness and compression molded at 170 °C, under a pressure of 150 kg/cm2 and cooled at 60 °C by use of HSINCHU Hot Press Machine. 2.4. Tensile, flexural and impact test
The fibers used in this investigation were unsized polyacrylonitrile (PAN)-based carbon fibers (Toho Tenax Co. Ltd) and commercially available E-glass fibers (Fibreglass Enterprise, China). Polypropylene (PP) (graded TitanPro SM950 from Titan Petchem(M) Sdn Bhd) was chosen as polymer matrices. PP is a general purpose thermoplastic with wide engineering and technological applications. Some physical and mechanical properties of these fibers are listed in Table 1.
Tensile and flexural tests were conducted using an Instron Universal Testing Machine at ambient temperature to determine the modulus of elasticity and strength of PP, SCF–PP, SGF–PP, CNT– SCF–PP, CNT–SGF–PP. Tensile tests followed ASTM D638 standard [23] using dog-bone bars, under a load of 1 kN at a crosshead speed of 5 mm/min [24]. Flexural tests were performed following ASTM D790 standard [25] by use of specimens with 124 mm height, 13 mm width and 3 mm thickness with the same crosshead speed. Several measurements were carried out for each test and the average values were presented. The 3 mm thick composite sheet was cut into rectangular bar with width of 13 mm and length of 64 mm for notched Izod impact test. A single-edge V-shape notch of 2 mm depth and a notch root radius of 0.25 mm were cut in the middle on the longer edge of the rectangular bar. The impact tests were carried out using Izod impact tester (MPm Pendulum 8000) with a pendulum speed of 3.46 m/s and according to ASTM D256 [26].
2.2. CVD synthesis for the growth of CNTs
2.5. Morphology characterization
The CVD set up consists of a single tubular quartz placed in a horizontal split furnace. Fiber tows are placed in the isothermal section of the quartz tube. A desizing process for the glass fibers was performed prior to growth of CNTs. To remove sizing of the glass fibers, a heating process at 600 °C for 30 min was conducted. The reactor was heated at a rate of 50 °C/min while argon gas was fed into the tube with 100 sccm flow rate until the desired temperature of 700 °C was achieved. Then hydrogen which acts as carrier gas was introduced into a container containing a solution of 0.2 g of ferrocene dissolved into 50 ml of benzene at a flow rate of 100 sccm. The chemicals were entrained into the reactor via the hydrogen flow while argon flow was discontinued. In the reactor, iron (Fe) nanoparticles were formed by thermal decomposition of ferrocene in the hydrogen atmosphere and were deposited onto the fiber surfaces. Carbon originating from decomposed benzene diffuses into the Fe catalyst and was crystallized to form CNTs. After 30 min, dense growth of CNTs on the surface of the fibers was achieved. Note that the process parameters used in this study were the optimized conditions found from previous work which has been reported elsewhere [21].
The morphology of CNTs coating was investigated using SEM (Hitachi S-3400 N). SEM was also used to inspect fracture surfaces from impact tests. In order to analyze the morphology of individual CNTs, TEM (Phillips HMG 400) was used.
2. Experimental details 2.1. Materials
2.3. Short fiber-composite fabrication The fabrication process of composite samples was the same for carbon and glass fibers. The CNT-coated fiber tows were chopped to a size of 2 mm using a universal cutting mill machine (Pulverisette 19). To prepare the composite, fibers (5 wt.%) and PP were
3. Results and discussion 3.1. Morphology of carbon nanotubes The morphology of CNTs are dependent on CVD process parameters like temperature, gas flow rate, catalyst concentration and also substrate. Very uniform and dense distribution of randomly orientated CNTs was achieved on both carbon and glass fibers, as shown in Fig. 1. SEM images of CNTs grown on carbon and glass fibers are shown at different magnifications in Fig. 1a–d respectively. The apparent thickness of the CNTs coating observed from SEM micrographs was about 7 lm on carbon fiber and 12 lm on glass fiber under similar process conditions. Representative TEM micrographs of individual CNTs, which appear to be of multi walled type, grown on respective fibers, are shown in Fig. 2. Encapsulated particles which are likely to be the Fe nanoparticle catalysts can be seen at the end of the tubes, indicating a tip-growth mechanism. Meanwhile, Fig. 3 shows the TEM images of individual CNTs that have been dislodged from the coatings and obtained through the dispersion in acetone followed by ultrasonication. Shown inset are the typical diameter distribution for the CNTs. As observed from Fig. 3a, the CNTs on carbon
Table 1 Material properties. Materials
Tensile strength (MPa)
Tensile modulus (GPa)
Density (g/cm3)
Diameter (lm)
Carbon fiber Glass fiber
3800 1770
231 72
1.8 2.5
6.5 14.7
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Fig. 1. SEM images of fibers (a) uniform CNTs grown on carbon fibers, (b) the length of CNTs on carbon fiber, (c) uniform CNTs grown on glass fibers, (d) CNTs grown vertically on glass fiber at the beginning and then entangled.
Fig. 2. TEM micrographs of CNTs on (a) carbon fiber, (b) glass fiber. Some have encapsulated iron nanoparticles on the end whilst some are open (as indicated by the arrows).
fibers consisted of tubes which were about 20–50 nm in diameter. However, CNTs grown on glass fiber consisted of tubes with diameters of around 30–70 nm as shown in Fig. 3b. Noticeably, nanotubes grown on glass fibers were greater in mean diameter compared to nanotubes on carbon fibers. Such difference can probably be related to the size of the deposited catalyst nanoparticles. Iron mobility on substrate fibers is the key factor controlling diameter diversification [27] since other process parameters were the same for two kind of fibers. A higher iron mobility causes more agglomeration of nanoparticles, therefore leading to bigger nanoparticles and larger CNTs diameter. So it may be inferred that iron nanoparticle mobility on glass fiber would be higher than on carbon fiber. 3.2. Tensile properties All the mechanical properties that will be shown, are comparison between bulk mechanical properties of neat-fiber composites and hierarchical fiber composites to demonstrate the superiority
of the latter. Tensile stress–strain curves and tensile strength and Young’s modulus of investigated composites are presented in Fig. 4a and b respectively. Improvement of strength and stiffness of composites after CNT-coating are evident from Fig. 4a However, more comparison can be done regarding the quantities of these parameters using Fig. 4b. The tensile strength of pure PP is about 14 MPa which generally increases after the addition of fibers. This enhancement is higher for SCF–PP compared to SGF–PP most probably due to the higher stiffness of carbon fiber. Comparing the stiffness of composites fabricated from coated fibers with neat fibercomposites demonstrates a significant improvement in tensile modulus. It was found that growing CNTs on fibers increases tensile modulus around 57% and 40% for carbon and glass fiber composites respectively. The tensile strength of unreinforced matrices would increase by incorporating SCF. However, the strength remains unchanged for sized-SGF composite and shows a slight reduction in unsizedSGF-composite test. Such decrease of strength at low weight SGF fraction has been reported elsewhere [28,29] and can be explained
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Fig. 3. TEM images and distribution curve of CNTs tube diameter on (a) carbon fiber, (b) glass fiber.
Fig. 4. Tensile strength and modulus of short fiber–PP composites with 5 wt.% carbon fiber or glass fiber, before and after CNTs coating (a) stress–strain curves, (b) tensile strength and modulus.
due to defective flow of matrix around fiber causing voids at the fiber–matrix interface region which leads to easy pull out of fiber by low force. In this case the role of fiber as flaws dominates the reinforcement role of fiber because of low weight of fibers compared to the matrix [28,29]. Similar to stiffness, the strength of short fiber-composites with CNTs are higher than composites without CNTs for both carbon and glass fiber composites. Such improvements can be associated with improved stress transfer between the fiber and matrix [15]
due to the CNTs. In contrast to the strength of the composites, tensile modulus is not strongly influenced by interfacial adhesion since stiffness is measured at very low strain [29,30]. Such phenomena can be observed from the tests of SGF composite. In spite of increase in modulus, the strength of sized-SGF composite remains the same as PP and gets weaker for unsized-SGF composite. Considering the unique role of CNTs in modification of interfacial region properties, around 40% improvement of strength was achieved by use of CNTs coated SGF in composite.
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Intertwined forest of CNTs coated on fiber surface can act as additional reinforcement and contribute in load carrying. As fibers were scattered randomly in the matrix, large quantities of CNTs would be positioned approximately in each direction of loading. As a result of high tensile modulus and strength of CNTs, significant enhancement in mechanical property of composite would be expected. Although most attention has been paid to the role of CNTs in interface or interlaminate of fiber reinforced composites, they possess potential capability to act as individual reinforcement in short fiber-composites. Therefore it can be deduced that there are two main advantages of CNT-coated fibers when used in short fiber-composites: firstly, improved fiber–matrix stress transfer and secondly synergic effects of fibers as well as CNTs as reinforcement. Meaningful raise in tensile modulus of CNTs grown fiber-composites can be related to these synergistic effects. 3.3. Flexural properties Similar to tensile properties, the same trend is observed in flexural tests as shown in Fig. 5. Utilizing CNT-coated fibers enhanced flexural modulus by 51% and 36% for SCF and SGF composites respectively. The CNT–SCF–PP composite with 35% rise and CNT– SGF–PP composite with 43% rise in flexural strength demonstrated the consequential role of CNTs in short fiber-composites. Such noticeable improvement can be attributed to efficient stress transfer between CNT-coated fibers and the PP matrix as well as the hybridization of composite through the addition of the CNTs. More discussion will be presented in Section 3.4 when discussing SEM images of fracture surfaces. Sized-SGF composite exhibited approximately the same flexural stiffness and slightly greater strength than the desized-SGF composite which are not significant compared to SGF composites containing CNT-coated fibers. 3.4. Impact property The impact strengths of the various composites are compared in Fig. 6. The results from Izod impact tests, show higher impact energy for neat glass fiber compared to neat carbon fiber composites. Such higher impact energy can be interpreted by the ability of the two kinds of composites to store energy in the fibers. Such energy matches the area under the tensile test curve of the fibers which is affected by elastic modulus and strain at failure point. The higher failure strain of glass fiber compared to carbon fiber [31] means that glass fibers can absorb more energy and thus increase the impact resistance of composites. Coating CNTs on fibers enhanced the Izod impact energy by approximately 34% for SCF-composite and
24% for SGF-composite (compared to sized-SGF composite). However, CNT–SGF–PP shows superior impact energy than CNT–SCF– PP with the same fiber length and weight fraction. Impact properties of composite are influenced by the mechanism of energy absorption and dissipation in materials. Indeed, such mechanism is also a key factor in the fracture toughness of materials. Regarding to the literature, several mechanisms of energy dissipation in short fiber-composites have been suggested: plastic deformation and fracture of matrices, plastic deformation and fracture of fibers, debonding of fiber–matrices interface and fiber pull out [32,33]. Surface modification through growing CNTs on fiber surfaces critically alters the fiber–matrix interfacial region, in addition to the CNTs themselves operating as nano-reinforcement in polymer. Consequently, achieved improvement in impact strength of CNT-coated fiber-composites was attributed to the improvement in the interface region of fiber and matrices. Despite the fact that upgrading the fiber–matrix interface would decrease impact strength in some researches [34], it needs to be optimized in order to improve both tensile and impact strength. Furthermore, the interaction of CNTs with the polymer matrix would bring about some extra energy dissipation occurrences such as nanotube pull out, fracture or bridging. The high aspect ratio and strength of CNTs can strongly influence energy consumption mechanism of polymer composites under fracture as reported elsewhere in impact tests of CNT–PP composites [35]. To further study the fracture behavior of CNT-coated fiber composites, the fractured surface of composite samples after impact test was viewed under SEM. The SEM micrograph in Fig. 7a and c shows a typical fractured surface of composite with neat carbon fibers and glass fibers, respectively. It can be seen that the fibers are smooth with minimal signs of any interfacial interaction between the fiber surface and the matrix, whilst empty holes in the matrix indicate the regions where fiber pull-out had occurred. In the case of composites with CNT-coated fibers, some fiber–matrix interaction is apparent as indicated by the relatively rough surface of the fractured fibers shown in Fig. 7b and d, unlike the smooth neat fibers shown previously. The presence of PP residue on the CNTcoated fiber surfaces, points to the enhanced adhesion between the fiber and the matrix. Such adhesion can be interpreted as effective grafting of CNTs on fiber and in addition micromechanical coupling between the CNTs and PP matrix. Such coupling is probably attributed to effective PP transfusion into the coated CNTs and consequent interlocking between the two materials. In general various mechanisms can be proposed regarding the promotion of energy dissipation in CNT-fiber composites:
Fig. 5. Flexural strength and modulus of short fiber–PP composites with 5 wt.% carbon fiber or glass fiber, before and after CNTs coating.
S. Rahmanian et al. / Materials and Design 43 (2013) 10–16
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Fig. 6. Impact strength of short fiber–PP composites with 5 wt.% carbon fiber or glass fiber, before and after CNTs coating.
Fig. 7. SEM images of fracture surface of composites after impact test (a) smooth surface of neat carbon fibers without CNTs, (b) PP sticks to the surface of CNTs grown carbon fibers, (c) smooth surface of neat glass fibers without CNTs, (d) PP sticks to the surface of CNTs grown glass fibers.
1. Strong CNT-fiber and CNT-matrix interactions increases fiber– matrix debonding energy. 2. Strong CNT-fiber and CNT-matrix interactions possess more strength so crack growth requires more energy. 3. Higher interfacial shear stress transfer between fiber and matrix increases the energy for fiber pull-out. 4. Conclusions The surfaces of carbon and glass fibers were modified successfully through growing dense and uniform CNTs coatings by the use of a floating catalyst chemical vapor deposition process. SEM micrographs demonstrated similar coating morphology on carbon and glass fibers, but TEM micrographs of individual CNTs demonstrated a larger diameter of CNTs on glass fibers compared to carbon fibers. Short fiber–polypropylene composites fabricated from CNT-coated fibers were found to exhibit superior tensile, flexural
and impact properties compared to neat short fiber-composites. Randomly orientated CNTs grafted onto the fiber surfaces, provide effective micromechanical coupling between the fiber and matrix, which was evident from SEM images of fractured surfaces, and improves the efficiency of stress transfer between the two. It was demonstrated that in addition to causing enhanced fiber–matrix interfacial bonding, CNTs also act as reinforcement thus lending additional enhancement in mechanical properties of short fibercomposites due to these synergistic effects.
Acknowledgements The authors would like to acknowledge the financial support given by Universiti Putra Malaysia under the Research University Grant Scheme and Ministry of Science Innovation and Innovation (MOSTI) Malaysia under the Brain Gain R&D program.
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Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Carbon and glass hierarchical fibers: Influence of carbon nanotubes on tensile, flexural and impact properties of short fiber reinforced composites S. Rahmanian a, K.S. Thean b, A.R. Suraya a,b,⇑, M.A. Shazed b, M.A. Mohd Salleh a,b, H.M. Yusoff b a b
Advanced Materials and Nanotechnology Lab, Institute of Advanced Technology, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia Department of Chemical and Environmental Engineering, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 21 April 2012 Accepted 15 June 2012 Available online 4 July 2012 Keywords: A. Nanocomposites A. Carbon nanotubes C. Vapor deposition E. Impact
a b s t r a c t Dense carbon nanotubes (CNTs) were grown uniformly on the surface of carbon fibers and glass fibers to create hierarchical fibers by use of floating catalyst chemical vapor deposition. Morphologies of the CNTs were investigated using scanning electronic microscope (SEM) and transmission electron microscope (TEM). Larger diameter dimension and distinct growing mechanism of nanotubes on glass fiber were revealed. Short carbon and glass fiber reinforced polypropylene composites were fabricated using the hierarchical fibers and compared with composites made using neat fibers. Tensile, flexural and impact properties of the composites were measured, which showed evident enhancement in all mechanical properties compared to neat short fiber composites. SEM micrographs of composite fracture surface demonstrated improved adhesion between CNT-coated fiber and the matrix. The enhanced mechanical properties of short fiber composites was attributed to the synergistic effects of CNTs in improving fiber–matrix interfacial properties as well as the CNTs acting as supplemental reinforcement in short fiber-composites. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Carbon nanotubes (CNTs) exhibit the combination of intriguing mechanical [1], thermal [2] and electrical [3] properties. Therefore, they possess great potential to reinforce high performance composites with wide structural applications [4,5]. Significant research attempts have sought to incorporate CNTs into fiber reinforced composites in order to improve the matrix [6,7] or fiber–matrix interfacial characteristics [8,9]. There are two main methods to fabricate these composites, either through the dispersion of CNTs in polymer (to perform as modified matrices or sizing agent) [10] or directly growing CNTs onto the fiber surfaces [11]. However, using the former method, good dispersion of disentangled CNTs in polymers is a primary challenge and cause filler aggregation phenomena. As a result, agglomeration sites act as stress concentration points and restrict the mechanical and physical performance of composites [12]. Such mentioned problems and also limitations in manufacturing process due to increasing viscosity of CNT-polymer matrices, have encouraged researchers to use the latter approach of directly growing or coating CNTs on fiber surfaces. These types of fibers are sometimes known as hierarchical fibers [13]. Other advantages of CNT-coated fibers are to improve the
⇑ Corresponding author at: Advanced Materials and Nanotechnology Lab, Institute of Advanced Technology, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia. Tel.: +60 3 89466285; fax: +60 3 86567120. E-mail address: [email protected] (A.R. Suraya). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.06.025
interfacial bonding between the fiber and matrix [14] as well as the additional support on the load transfer effects afforded by the CNTs [15]. Among various techniques to produce CNTs, chemical vapor deposition (CVD) is the most promising to fabricate CNT-coated fibers. CVD is an adaptable process since it enables the use of different kinds of hydrocarbons, substrates and catalysts. Hydrocarbon sources such as ethylene, acetylene, toluene, benzene are often employed along with Co, Fe or Ni as active catalysts to grow CNTs on substrates through CVD process. The structure and architecture of CNTs can be controlled through regulating the parameters of the CVD process like temperature, flow rate, and length of reaction time [16,17]. Some previous works demonstrated improvement in mechanical properties of these hierarchical composites while using distinctive conditions for the CVD process. Enhancement of interfacial shear strength (IFSS) of silica fibers by applying a two-step technique [18], significant increase in storage modulus of unidirectional carbon composites after low temperature CVD process [19] and improvement of interlaminar fracture toughness of aligned CNTs grown silicon carbide fabric composites [20] have been reported. Despite the extensive application of short fiber composites, previous researchers have mostly paid attention to the effects of CNTgrown fiber in composites reinforced by continuous carbon fibers or fabrics. Due to the wide application of short glass fiber reinforced composites, there is a need to understand more about the synthesis of CNTs on glass fiber as well as carbon fiber and their role in improving the mechanical properties of short fiber-composites.
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S. Rahmanian et al. / Materials and Design 43 (2013) 10–16
In the present investigation, we have reported the synthesis of CNTs on carbon fiber and glass fiber via feeding a vaporized solution of benzene (carbon precursor) and ferrocene (catalyst precursor) into a custom-built CVD reactor in which fiber bundles were placed. The morphology of the CNT-coated on carbon as well as glass fibers were studied and compared using scanning electronic microscope (SEM) and transmission electron microscope (TEM). Short carbon fiber reinforced polypropylene (SCF–PP) composites and short glass fiber reinforced polypropylene (SGF–PP) composites were fabricated using the hierarchical fibers. To evaluate the effects of CNTs coating on various mechanical characteristics of short fiber composites, tensile, flexural and impact tests were carried out. Fracture surfaces of composites after impact test were examined through SEM micrographs to analyze the role of CNTs in the interfacial region of fiber–matrices.
blended in an internal mixer (Thermo Haake PolyDrive R600/ 610) equipped with a pair of roller-type blades. Melt blending process was set to a temperature of 170 °C [22] with a rotor speed of 55 rpm. After 5 min melting of PP, cut fibers were added to the melted PP. A batch of the blended composite was compressed into a 15 cm 15 cm mold with 1 mm and 3 mm thickness and compression molded at 170 °C, under a pressure of 150 kg/cm2 and cooled at 60 °C by use of HSINCHU Hot Press Machine. 2.4. Tensile, flexural and impact test
The fibers used in this investigation were unsized polyacrylonitrile (PAN)-based carbon fibers (Toho Tenax Co. Ltd) and commercially available E-glass fibers (Fibreglass Enterprise, China). Polypropylene (PP) (graded TitanPro SM950 from Titan Petchem(M) Sdn Bhd) was chosen as polymer matrices. PP is a general purpose thermoplastic with wide engineering and technological applications. Some physical and mechanical properties of these fibers are listed in Table 1.
Tensile and flexural tests were conducted using an Instron Universal Testing Machine at ambient temperature to determine the modulus of elasticity and strength of PP, SCF–PP, SGF–PP, CNT– SCF–PP, CNT–SGF–PP. Tensile tests followed ASTM D638 standard [23] using dog-bone bars, under a load of 1 kN at a crosshead speed of 5 mm/min [24]. Flexural tests were performed following ASTM D790 standard [25] by use of specimens with 124 mm height, 13 mm width and 3 mm thickness with the same crosshead speed. Several measurements were carried out for each test and the average values were presented. The 3 mm thick composite sheet was cut into rectangular bar with width of 13 mm and length of 64 mm for notched Izod impact test. A single-edge V-shape notch of 2 mm depth and a notch root radius of 0.25 mm were cut in the middle on the longer edge of the rectangular bar. The impact tests were carried out using Izod impact tester (MPm Pendulum 8000) with a pendulum speed of 3.46 m/s and according to ASTM D256 [26].
2.2. CVD synthesis for the growth of CNTs
2.5. Morphology characterization
The CVD set up consists of a single tubular quartz placed in a horizontal split furnace. Fiber tows are placed in the isothermal section of the quartz tube. A desizing process for the glass fibers was performed prior to growth of CNTs. To remove sizing of the glass fibers, a heating process at 600 °C for 30 min was conducted. The reactor was heated at a rate of 50 °C/min while argon gas was fed into the tube with 100 sccm flow rate until the desired temperature of 700 °C was achieved. Then hydrogen which acts as carrier gas was introduced into a container containing a solution of 0.2 g of ferrocene dissolved into 50 ml of benzene at a flow rate of 100 sccm. The chemicals were entrained into the reactor via the hydrogen flow while argon flow was discontinued. In the reactor, iron (Fe) nanoparticles were formed by thermal decomposition of ferrocene in the hydrogen atmosphere and were deposited onto the fiber surfaces. Carbon originating from decomposed benzene diffuses into the Fe catalyst and was crystallized to form CNTs. After 30 min, dense growth of CNTs on the surface of the fibers was achieved. Note that the process parameters used in this study were the optimized conditions found from previous work which has been reported elsewhere [21].
The morphology of CNTs coating was investigated using SEM (Hitachi S-3400 N). SEM was also used to inspect fracture surfaces from impact tests. In order to analyze the morphology of individual CNTs, TEM (Phillips HMG 400) was used.
2. Experimental details 2.1. Materials
2.3. Short fiber-composite fabrication The fabrication process of composite samples was the same for carbon and glass fibers. The CNT-coated fiber tows were chopped to a size of 2 mm using a universal cutting mill machine (Pulverisette 19). To prepare the composite, fibers (5 wt.%) and PP were
3. Results and discussion 3.1. Morphology of carbon nanotubes The morphology of CNTs are dependent on CVD process parameters like temperature, gas flow rate, catalyst concentration and also substrate. Very uniform and dense distribution of randomly orientated CNTs was achieved on both carbon and glass fibers, as shown in Fig. 1. SEM images of CNTs grown on carbon and glass fibers are shown at different magnifications in Fig. 1a–d respectively. The apparent thickness of the CNTs coating observed from SEM micrographs was about 7 lm on carbon fiber and 12 lm on glass fiber under similar process conditions. Representative TEM micrographs of individual CNTs, which appear to be of multi walled type, grown on respective fibers, are shown in Fig. 2. Encapsulated particles which are likely to be the Fe nanoparticle catalysts can be seen at the end of the tubes, indicating a tip-growth mechanism. Meanwhile, Fig. 3 shows the TEM images of individual CNTs that have been dislodged from the coatings and obtained through the dispersion in acetone followed by ultrasonication. Shown inset are the typical diameter distribution for the CNTs. As observed from Fig. 3a, the CNTs on carbon
Table 1 Material properties. Materials
Tensile strength (MPa)
Tensile modulus (GPa)
Density (g/cm3)
Diameter (lm)
Carbon fiber Glass fiber
3800 1770
231 72
1.8 2.5
6.5 14.7
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Fig. 1. SEM images of fibers (a) uniform CNTs grown on carbon fibers, (b) the length of CNTs on carbon fiber, (c) uniform CNTs grown on glass fibers, (d) CNTs grown vertically on glass fiber at the beginning and then entangled.
Fig. 2. TEM micrographs of CNTs on (a) carbon fiber, (b) glass fiber. Some have encapsulated iron nanoparticles on the end whilst some are open (as indicated by the arrows).
fibers consisted of tubes which were about 20–50 nm in diameter. However, CNTs grown on glass fiber consisted of tubes with diameters of around 30–70 nm as shown in Fig. 3b. Noticeably, nanotubes grown on glass fibers were greater in mean diameter compared to nanotubes on carbon fibers. Such difference can probably be related to the size of the deposited catalyst nanoparticles. Iron mobility on substrate fibers is the key factor controlling diameter diversification [27] since other process parameters were the same for two kind of fibers. A higher iron mobility causes more agglomeration of nanoparticles, therefore leading to bigger nanoparticles and larger CNTs diameter. So it may be inferred that iron nanoparticle mobility on glass fiber would be higher than on carbon fiber. 3.2. Tensile properties All the mechanical properties that will be shown, are comparison between bulk mechanical properties of neat-fiber composites and hierarchical fiber composites to demonstrate the superiority
of the latter. Tensile stress–strain curves and tensile strength and Young’s modulus of investigated composites are presented in Fig. 4a and b respectively. Improvement of strength and stiffness of composites after CNT-coating are evident from Fig. 4a However, more comparison can be done regarding the quantities of these parameters using Fig. 4b. The tensile strength of pure PP is about 14 MPa which generally increases after the addition of fibers. This enhancement is higher for SCF–PP compared to SGF–PP most probably due to the higher stiffness of carbon fiber. Comparing the stiffness of composites fabricated from coated fibers with neat fibercomposites demonstrates a significant improvement in tensile modulus. It was found that growing CNTs on fibers increases tensile modulus around 57% and 40% for carbon and glass fiber composites respectively. The tensile strength of unreinforced matrices would increase by incorporating SCF. However, the strength remains unchanged for sized-SGF composite and shows a slight reduction in unsizedSGF-composite test. Such decrease of strength at low weight SGF fraction has been reported elsewhere [28,29] and can be explained
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Fig. 3. TEM images and distribution curve of CNTs tube diameter on (a) carbon fiber, (b) glass fiber.
Fig. 4. Tensile strength and modulus of short fiber–PP composites with 5 wt.% carbon fiber or glass fiber, before and after CNTs coating (a) stress–strain curves, (b) tensile strength and modulus.
due to defective flow of matrix around fiber causing voids at the fiber–matrix interface region which leads to easy pull out of fiber by low force. In this case the role of fiber as flaws dominates the reinforcement role of fiber because of low weight of fibers compared to the matrix [28,29]. Similar to stiffness, the strength of short fiber-composites with CNTs are higher than composites without CNTs for both carbon and glass fiber composites. Such improvements can be associated with improved stress transfer between the fiber and matrix [15]
due to the CNTs. In contrast to the strength of the composites, tensile modulus is not strongly influenced by interfacial adhesion since stiffness is measured at very low strain [29,30]. Such phenomena can be observed from the tests of SGF composite. In spite of increase in modulus, the strength of sized-SGF composite remains the same as PP and gets weaker for unsized-SGF composite. Considering the unique role of CNTs in modification of interfacial region properties, around 40% improvement of strength was achieved by use of CNTs coated SGF in composite.
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Intertwined forest of CNTs coated on fiber surface can act as additional reinforcement and contribute in load carrying. As fibers were scattered randomly in the matrix, large quantities of CNTs would be positioned approximately in each direction of loading. As a result of high tensile modulus and strength of CNTs, significant enhancement in mechanical property of composite would be expected. Although most attention has been paid to the role of CNTs in interface or interlaminate of fiber reinforced composites, they possess potential capability to act as individual reinforcement in short fiber-composites. Therefore it can be deduced that there are two main advantages of CNT-coated fibers when used in short fiber-composites: firstly, improved fiber–matrix stress transfer and secondly synergic effects of fibers as well as CNTs as reinforcement. Meaningful raise in tensile modulus of CNTs grown fiber-composites can be related to these synergistic effects. 3.3. Flexural properties Similar to tensile properties, the same trend is observed in flexural tests as shown in Fig. 5. Utilizing CNT-coated fibers enhanced flexural modulus by 51% and 36% for SCF and SGF composites respectively. The CNT–SCF–PP composite with 35% rise and CNT– SGF–PP composite with 43% rise in flexural strength demonstrated the consequential role of CNTs in short fiber-composites. Such noticeable improvement can be attributed to efficient stress transfer between CNT-coated fibers and the PP matrix as well as the hybridization of composite through the addition of the CNTs. More discussion will be presented in Section 3.4 when discussing SEM images of fracture surfaces. Sized-SGF composite exhibited approximately the same flexural stiffness and slightly greater strength than the desized-SGF composite which are not significant compared to SGF composites containing CNT-coated fibers. 3.4. Impact property The impact strengths of the various composites are compared in Fig. 6. The results from Izod impact tests, show higher impact energy for neat glass fiber compared to neat carbon fiber composites. Such higher impact energy can be interpreted by the ability of the two kinds of composites to store energy in the fibers. Such energy matches the area under the tensile test curve of the fibers which is affected by elastic modulus and strain at failure point. The higher failure strain of glass fiber compared to carbon fiber [31] means that glass fibers can absorb more energy and thus increase the impact resistance of composites. Coating CNTs on fibers enhanced the Izod impact energy by approximately 34% for SCF-composite and
24% for SGF-composite (compared to sized-SGF composite). However, CNT–SGF–PP shows superior impact energy than CNT–SCF– PP with the same fiber length and weight fraction. Impact properties of composite are influenced by the mechanism of energy absorption and dissipation in materials. Indeed, such mechanism is also a key factor in the fracture toughness of materials. Regarding to the literature, several mechanisms of energy dissipation in short fiber-composites have been suggested: plastic deformation and fracture of matrices, plastic deformation and fracture of fibers, debonding of fiber–matrices interface and fiber pull out [32,33]. Surface modification through growing CNTs on fiber surfaces critically alters the fiber–matrix interfacial region, in addition to the CNTs themselves operating as nano-reinforcement in polymer. Consequently, achieved improvement in impact strength of CNT-coated fiber-composites was attributed to the improvement in the interface region of fiber and matrices. Despite the fact that upgrading the fiber–matrix interface would decrease impact strength in some researches [34], it needs to be optimized in order to improve both tensile and impact strength. Furthermore, the interaction of CNTs with the polymer matrix would bring about some extra energy dissipation occurrences such as nanotube pull out, fracture or bridging. The high aspect ratio and strength of CNTs can strongly influence energy consumption mechanism of polymer composites under fracture as reported elsewhere in impact tests of CNT–PP composites [35]. To further study the fracture behavior of CNT-coated fiber composites, the fractured surface of composite samples after impact test was viewed under SEM. The SEM micrograph in Fig. 7a and c shows a typical fractured surface of composite with neat carbon fibers and glass fibers, respectively. It can be seen that the fibers are smooth with minimal signs of any interfacial interaction between the fiber surface and the matrix, whilst empty holes in the matrix indicate the regions where fiber pull-out had occurred. In the case of composites with CNT-coated fibers, some fiber–matrix interaction is apparent as indicated by the relatively rough surface of the fractured fibers shown in Fig. 7b and d, unlike the smooth neat fibers shown previously. The presence of PP residue on the CNTcoated fiber surfaces, points to the enhanced adhesion between the fiber and the matrix. Such adhesion can be interpreted as effective grafting of CNTs on fiber and in addition micromechanical coupling between the CNTs and PP matrix. Such coupling is probably attributed to effective PP transfusion into the coated CNTs and consequent interlocking between the two materials. In general various mechanisms can be proposed regarding the promotion of energy dissipation in CNT-fiber composites:
Fig. 5. Flexural strength and modulus of short fiber–PP composites with 5 wt.% carbon fiber or glass fiber, before and after CNTs coating.
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Fig. 6. Impact strength of short fiber–PP composites with 5 wt.% carbon fiber or glass fiber, before and after CNTs coating.
Fig. 7. SEM images of fracture surface of composites after impact test (a) smooth surface of neat carbon fibers without CNTs, (b) PP sticks to the surface of CNTs grown carbon fibers, (c) smooth surface of neat glass fibers without CNTs, (d) PP sticks to the surface of CNTs grown glass fibers.
1. Strong CNT-fiber and CNT-matrix interactions increases fiber– matrix debonding energy. 2. Strong CNT-fiber and CNT-matrix interactions possess more strength so crack growth requires more energy. 3. Higher interfacial shear stress transfer between fiber and matrix increases the energy for fiber pull-out. 4. Conclusions The surfaces of carbon and glass fibers were modified successfully through growing dense and uniform CNTs coatings by the use of a floating catalyst chemical vapor deposition process. SEM micrographs demonstrated similar coating morphology on carbon and glass fibers, but TEM micrographs of individual CNTs demonstrated a larger diameter of CNTs on glass fibers compared to carbon fibers. Short fiber–polypropylene composites fabricated from CNT-coated fibers were found to exhibit superior tensile, flexural
and impact properties compared to neat short fiber-composites. Randomly orientated CNTs grafted onto the fiber surfaces, provide effective micromechanical coupling between the fiber and matrix, which was evident from SEM images of fractured surfaces, and improves the efficiency of stress transfer between the two. It was demonstrated that in addition to causing enhanced fiber–matrix interfacial bonding, CNTs also act as reinforcement thus lending additional enhancement in mechanical properties of short fibercomposites due to these synergistic effects.
Acknowledgements The authors would like to acknowledge the financial support given by Universiti Putra Malaysia under the Research University Grant Scheme and Ministry of Science Innovation and Innovation (MOSTI) Malaysia under the Brain Gain R&D program.
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