epoxy composites using a simple process for depositing commercially functionalized carbon nanotubes on the fibers

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Interfacial improvement of carbon fiber/epoxy composites using a simple process for depositing commercially functionalized carbon nanotubes on the fibers Min Li, Yizhuo Gu *, Yanan Liu, Yanxia Li, Zuoguang Zhang Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, 37 Xue Yuan Road, Haidian District, Beijing 100191, PR China

A R T I C L E I N F O

A B S T R A C T

Article history:

An aqueous suspension deposition method was used to coat the sized carbon fibers T700SC

Received 15 March 2012

and T300B with commercially carboxylic acid-functionalized and hydroxyl-functionalized

Accepted 9 September 2012

carbon nanotubes (CNTs). The CNTs on the fiber surfaces were expected to improve the

Available online 17 September 2012

interfacial strength between the fibers and the epoxy. The factors affecting the deposition, especially the fiber sizing, were studied. According to single fiber-composite fragmentation tests, the deposition process results in improved fiber/matrix interfacial adhesion. Using carboxylic acid-functionalized CNTs, the interfacial shear strength was increased 43% for the T700SC composite and 12% for the T300B composite. The relationship between surface functional groups of the CNTs and the interfacial improvement was discussed. The interfacial reinforcing mechanism was explored by analyzing the surface morphology of the carbon fibers, the wettability between the carbon fibers and the epoxy resin, the chemical bonding between the fiber sizing and the CNTs, and fractographic observation of crosssections of the composites. Results indicate that interfacial friction, chemical bonding and resin toughening are responsible for the interfacial improvement of nanostructured carbon fiber/epoxy composites. The mechanical properties of the CNT-deposited composite laminate were further measured to confirm the effectiveness of this strategy.  2012 Elsevier Ltd. All rights reserved.

1.

Introduction

In recent years, the introduction of carbon nanotubes (CNTs) into conventional continuous fiber-reinforced polymer composites to create hierarchical reinforcement structures is a topic of significant current interest [1,2]. Due to their unique structures, outstanding strength and modulus, excellent electrical and thermal properties, CNTs have been exploited to improve mechanical and functional properties of highperformance fiber composites. Recent research demonstrates that by adopting the multiscale reinforcements, significant improvements are achieved in glass fiber and carbon fiber composites, especially for the fiber–matrix interphase and

matrix-dominated out-of-plane performances, such as interfacial shear strength (IFSS) [3–7], interlaminar fracture toughness [8,9], fatigue resistance [10,11], glass transition temperature [12] and electrical conductivity [13]. Moreover, CNTs also act as damage [14] or strain sensors [15] to monitor local strain, initiation of microcracks and accumulation of damage in fiber composites for realizing the capability in health monitoring. Dispersion of CNTs into the composite matrix and direct attachment of CNTs onto the fiber surface are two kinds of strategies for forming CNT-based hierarchical composites [1,2]. Due to the conveniences of fabricating composites using traditional processing methods and controlling the content,

* Corresponding author: Fax: +86 010 82338699. E-mail address: [email protected] (Y. Gu). 0008-6223/$ - see front matter  2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.09.011

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orientation and dispersion state of CNTs at fiber/matrix interphase region, many researchers have paid attentions to direct attachment of CNTs onto fiber tows and fiber fabrics. There are four different techniques to directly place CNTs on fiber surface given in the literature so far: (1) direct growth of CNTs on fibers by means of chemical vapor deposition [4–7]; (2) electrophoretic deposition of CNTs on fiber surface [13], (3) chemical reactions between functionalized CNTs and fibers [16,17], (4) depositing CNT including coating of fibers with CNT-containing sizing (a kind of thin polymer coating) [3,12,14,15,18] and spraying CNT-containing solution on fiber fabrics [8–11]. Among these methods, the fourth approach is the most uncomplicated, and introduces no damage to the fibers (even healing surface defects and improve fiber tensile property [18–21]). Furthermore, the introduction of functional groups on the surface of CNT is generally believed to be an effective way for exploiting the advantages of the CNT. The CNT-depositing technology can use commercially functionalized CNTs, so it does not need complex chemical treatments for functionalizing CNTs, which are often necessary for other processes of introducing CNTs on fibers [1,2]. Consequently, the CNT-depositing technology is easy to adopt at large-scale for practical applications. For the purpose of improvement in interfacial property of composites, the CNT-sizings have been used in glass fiber composites, while the spraying technology of the depositing CNTs is adopted for carbon fiber composites. For example, Godara et al. [3] reported an over 90% improvement in IFSS of a CNT-sizing glass fiber/epoxy composite based on the results of single-fiber push-out microindentation. Besides, Kwon and co-workers [8,9] realized significant increase in the interlaminar fracture toughness of carbon fiber/vinylester composite laminate by means of spraying CNTs, and the maximum increase for Mode II values (GIIC) was about 30%. These results reveal that the deposited-CNT technology is a promising method for strengthening interfacial bonding between matrix and fiber, helping improve composite mechanical properties. In this paper, we delivered CNTs to the surface of carbon fiber by immersing the fiber bundles in CNT-containing aqueous suspension, which takes advantages of the aforementioned sizing and spraying methods. In order to improve the dispersibility of CNTs and the bonding between CNTs and the bulk materials, the commercially functionalized CNTs were adopted. This work aims to improve the interfacial strength between carbon fiber and epoxy matrix, and understand the reinforcing mechanism using the deposited-CNTs. Many studies have demonstrated that the micromechanical tests, such as single-fiber composite fragmentation test (SFFT) [4–6] and single fiber–microdroplet tensile test [7,22], are effective means for evaluating the interfacial properties of the fiber/ polymer composites modified with CNTs. In the current study, single-fiber fragmentation tests were performed to investigate the IFSS of the CNT-coated carbon fiber/epoxy composites. Two kinds of sized carbon fibers with distinct characteristics of surface morphology and two types of commercially functionalized multi-walled CNTs (MWCNTs) were used for evaluating the deposition states of the CNTs at different conditions. Focusing on the interfacial reinforcing mechanism, the effects of the deposited-CNTs on the fiber surface morphology, the

wettability of epoxy on the fiber, chemical bonding, and fracture pattern at the interface region were further analyzed, and the effect of surface functional groups of the CNTs was discussed. The tensile property and the interlaminar shear strength of the CNT-coated carbon fiber composite laminate were measured and the results were analyzed with respect to the effectiveness of the depositing CNTs.

2.

Experimental

2.1.

Materials

Commercially-available, sized carbon fibers, T700SC yarns of 12000 filament count and T300B yarns of 3000 filament count (Toray), were used for this study. The diameters of the two carbon fibers are 7 lm. The cross section of T700SC is circular, and that of T300B is irregular circular or elliptical, resulted from different spinning methods of the polyacrylonitrile fibers. The sizing contents of the two carbon fibers are both 1 wt.%, but the chemical constitutions of the two kinds of sizings are different [23]. Two types of functionalized MWCNTs with carboxylic acid (referred as COOH–CNTs) and hydroxyl groups (referred as OH–CNTs), respectively, were produced and supplied by Shenzhen Nanotech Port Co., Ltd. These nanotubes have the lengths between 0.2 and 2 lm with the diameters about 20– 40 nm, and the carbon purity is larger than 97%. An epoxy resin YD-128 based on diglycidylether of bisphenol A (KUKDO Chemical Co., Ltd.) with hardener EC301 (a polyether amine based curing agent, produced by BASF Chemical Co.) was used as matrix. The epoxy resin has a low viscosity, good mechanical adhesion and is very suitable for fabricating composite parts using resin transfer molding (RTM).

2.2.

Preparation of CNT-coated fibers

Before preparation of the CNT-coated fibers, the nanotube materials were dispersed in de-ionized water by ultrasonication (bath sonicator, sonic power 100 W, sonic frequency 80 kHz) for 3 h to obtain the stable suspension at a concentration of 0.05 wt.%. Then, the carbon fiber bundles were immersed in a bath containing the suspension for 20 min, and were dried at 70 C for 2 h, followed by drying at 120 C for 2 h. The procedure of production of the CNT-coated fibers is illustrated in Fig. 1. To examine the structures of the CNTs after the dispersing process, transmission electron microscope (TEM) images (JEOL, JEM 2100F, 200 kV) of the CNTs in the suspension were taken. In order to increase the content of CNTs in the aqueous suspension, another CNT-suspension was prepared, which contained a kind of non-ionic surfactant polyoxyethylene octyl phenyl ether (Triton X-100, supplied by Shantou Xilong Chemical Factory) with the weight ratio of 5:1 to CNTs. This surfactant has been used to disperse CNTs in epoxy resin [24,25], and the maximum content of CNTs in de-ionized water containing Triton X-100 is 0.3 wt.% in our laboratory. The process of production of the CNT-coated fibers using Triton X-100 is the same as that without Triton X-100.

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Immersion

Sized fibers

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Aqueous suspension with CNTs

CNT-deposited fibers

Fig. 1 – Schematic of the CNT-depositing process on carbon fibers.

Like the sizing and the spraying methods mentioned in Section 1, our route of depositing functionalized CNTs is also undamaged for fiber and the process is not complex. Moreover, this method does not need to remove industrial sizing and the deposition process can be used for various types of carbon fiber reinforcements, such as yarn, fabric and preform.

2.3.

Characterization of CNT-coated fibers

2.3.1.

Characterization of functionalized CNTs

TEM was used to observe the dimensions of the functionalized CNTs. Fourier transform infrared spectrometer (FTIR) was carried out using Thermo Nicolet Nexus 470 Spectrometer for confirming the functional groups on the surfaces of CNTs. The FTIR spectra were collected in the wave number range of 4000–400 cm1 at a resolution of 4 cm1. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Ka radiation. The base pressure was about 3 · 109 mbar. The kinetic energy of photoelectrons was determined with a hemispheric analyzer set to pass energy of 100 eV for wide-scan spectra and 30 eV for high-resolution spectra, respectively. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. Quantitative elemental compositions were determined from peak areas. The high-resolved spectra were dissected by means of the spectra deconvolution software.

2.3.2.

Surface morphology measurements

After the CNT-depositing treatment, the carbon fiber bundles were examined with a CamScan-Apollo300 scanning electron microscope (SEM) operating at 20 kV to evaluate the adhering state of CNTs to the fiber surfaces. The values of surface roughness of the carbon fibers with and without the coating of the CNTs were determined by means of atomic force microscopy (AFM) scan (Veeco, Dimension icon). The scan area was 3 · 3 lm and the arithmetic average roughness, Ra, was calculated using the installed software (NanoScop Analysis) with 5 positions.

2.3.3.

Contact angle measurements

The dynamic contact angles at ambient temperature were tested by the modified Wilhelmy plate method using a surface tension apparatus (Dataphysics, DCAT21) to determine the wettability between the CNT-coated carbon fiber and the epoxy resin YD-128. Five single carbon fibers were first connected to a circular clamp in equidistance, and the assembly was hung on the arm of electronic balance. Then, the fibers

were inserted into the epoxy resin at a stage motor speed of 0.01 mm/s with surface detection threshold of 0.15 mg and immersion depth of 3 mm. The dynamic contact angle was calculated from Wilhelmy equation: F ¼ Pcl cos h

ð1Þ

where F is the balance force, P is the perimeter of fiber, cl is the surface tension of epoxy resin and h is the dynamic contact angle. The average dynamic contact angle was obtained from 5 tests for each kind of fiber.

2.3.4.

Infrared spectrometer analysis

In order to confirm whether the functionalized CNTs can react with the industrial sizing of the carbon fiber to form chemical bonding, FTIR was carried out using Thermo Nicolet Nexus 470 Spectrometer. The fiber sizing was obtained from the raw carbon fibers by acetone extraction at 85 C for 24 h. Then, the extracted sizing was dispersed in acetone, and was mixed with CNTs (the weight ratio between the sizing and the CNTs is 100:7.5) by ultrasonication for 10 min. The mixture was dried at 70 C in vacuum oven for 4 h to remove the acetone. This sample was further heated at 120 C for 2 h to accelerate the possible reaction between the CNTs and the sizing. Finally, the FTIR spectra of the extracted sizing and the mixtures of the CNTs and the extracted sizing were collected in the wave number range of 4000–400 cm1 at a resolution of 4 cm1.

2.4. Preparation of single fiber-composite sample and fragmentation tests To assess the influence of the CNTs on the interfacial bonding, the IFSS of the CNT-coated carbon fiber in the epoxy is tested by means of single fiber-composite fragmentation. The procedures of preparation of the samples and measurement are as follows. The single carbon fibers were carefully separated from the deposited-CNT roving. Then, the single fiber was positioned in the center of a mould with a dogbone shaped cavity. The two ends of the fiber extended over the mould and were stuck to a piece of plastic with 1.2 g, in order to hold the fiber straight in the sample. Subsequently, the epoxy resin was mixed with the curing agent at 100:30 by weight, and was cast into the mould, covering the fiber completely. Curing temperature and procedure were 3 h at 120 C in vacuum oven, avoiding any obvious voids in the sample. After the specimen had cooled, they were removed from the mold. The surfaces of the cured specimen were polished for facilitating the observation of the single fiber. The specimen was fixed to a micro-tension apparatus equipped with a 1 kN capacity load cell. The measurement

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under a tensile load was taken at a crosshead speed of 0.76 lm/min, and the tensile stress–strain curve was recorded. During the testing, entire single fiber fragmentation was monitored by polarized light microscope and the number of fiber fragment was counted within the gauge length of 20 mm. After the saturation point was reached, i.e. the number of fiber fragment stopped increasing, the tension test was continued until the sample was broken at the middle region of the sample. Then, the fracture cross-section of the broken sample was observed under SEM, and the state of interfacial bonding was evaluated. By assuming a constant shear stress, IFSS, sIFSS, can be estimated from Kelly–Tyson model [4–6,26]: sIFSS ¼ rf df =ð2lc Þ

ð2Þ

lc ¼ 4l=3

ð3Þ

where df is the fiber diameter, rf is the fiber strength at the critical fragment length, lc, which can be obtained from the mean fiber fragment length, l, at crack saturation. rf can be calculated based on the following Eq. [4]: rf ¼ r0 ðl0 =lc Þ1=b

ð4Þ

where l0 is initial length of the single carbon fiber (20 mm in this study), r0 is the fiber tensile stress at the point of first fiber fragment, and b is Weibull shape parameter determined from the curve of fiber tensile stress vs. fragment length. Therefore, when comparing two fibers of equal strength, strength distribution and diameter, the fiber with the shortest critical length will have the highest IFSS. In this work, at least five specimens were tested for each case to obtain average IFSS and critical fragment length. In addition, the elastoplasticity of the matrix during fragmentation test can be used to characterize the fiber/matrix adhesion and stress transfer [27–29]. Thus, the effects of deposited-CNTs on the interfacial

bonding were also evaluated according to the results of birefringence effects.

2.5.

Composite laminate fabrication and test procedure

Composite laminates were fabricated using vacuum assisted resin infusion molding. Several layers of the raw and the COOH–CNT coated T700SC unidirectional fabric cloths (200 g/m2 of areal density) were, respectively, laid up according to the required thickness of mechanical testing standard. The carbon fiber preform was sealed in a vacuum bag, and the epoxy resin is then infused into the carbon fibers under vacuum (>98 kPa) at ambient temperature. After the infiltration of epoxy, the composites were cured for 3 h at 120 C. The weight contents of carbon fiber for different composite laminates were 56%. Test specimens were cut from the cured composite laminates. The tensile modulus and strength of composites were measured on a universal testing machine (Instron 3382) using rectangular shaped specimens made in accordance with American Society for Testing and Materials (ASTMs) standard D3039. In addition, the interlaminar shear strengths (ILSSs) of laminates were tested according to ASTM standard D2344. A minimum of five specimens were tested for each set of conditions.

3.

Results and discussion

3.1.

Surface characteristics of CNT-deposited carbon fibers

We first examined the morphology of the CNTs on the surface of the carbon fiber after deposition process. The surface morphologies of the raw carbon fibers obtained from SEM are shown in Fig. 2a and b, indicating that T700SC has smooth

Fig. 2 – SEM images of: (a) as-received T300B carbon fiber, (b) as-received T700SC carbon fiber, (c) CNT-deposited carbon fibers without surfactant, and (d) CNT-deposited carbon fibers with surfactant.

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surface, while the surface of T300B is obviously rough with lots of grooves along fiber axial. The typical SEM image of the CNT-deposited carbon fiber is shown in Fig. 2c, which identifies the presence of the CNTs on the fiber surface with random orientation and uniform dispersion. Fig. 2d shows the surface morphology of the fibers after depositing CNTs with the surfactant Triton X-100. It can be seen that CNT-containing Triton X-100 forms coating not only on the fiber surface, but also among the individual filaments. The similar phenomenon can be found on the coated glass fibers with phenoxy-based sizing containing MWCNTs [4], which resulted in bundling of the fibers (fiber adhering to one another). This adhesion among fibers might influence the impregnation of the fibers with the resin matrix and production of the composite. Moreover, our recent work shows that Triton X-100 has a degradation effect on the interphase bonding of the studied composite. Liao et al. also found similar results, which were attributed to bundling of the fibers and

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degrading the interfacial adhesion between matrix and CNTs [30]. Adopting the CNT-deposition process without Triton X100, T300B and T700SC with COOH–CNTs and OH–CNTs were obtained, and their SEM micrographs are shown in Fig. 3a–d. These micrographs reveal that there are more CNTs adhering to T300B than those adhering to T700SC, especially for the case of the depositing OH–CNTs. These results can be explained from the surface morphology of the carbon fiber and the dimensions of the CNTs. It has been mentioned above that T700SC and T300B have smooth surface and rough surfaces, respectively. Obviously, the grooves on the surface of the T300B are favorable for ‘‘catching’’ nanotubes. The COOH–CNTs have larger length and aspect ratio than the OH–CNTs, as shown in Fig. 4. Fig. 4 shows that the COOH– CNTs have the lengths between 1 and 2 lm with the diameters about 20–40 nm, and the OH–CNTs have the lengths between 0.2 and 1 lm with the diameters about 20–40 nm. In

Fig. 3 – Typical SEM micrographs of the CNT-deposited fiber surface: (a) T700SC with COOH–CNTs, (b) T300B with COOH–CNTs, (c) T700SC with OH–CNTs, (d) T300B with OH–CNTs, (e) unsized T700SC with COOH–CNTs, and (f) T700SC with COOH–CNTs after water flushing.

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Fig. 4 – TEM images of (a) carboxylic acid-functionalized CNTs and (b) hydroxyl-functionalized CNTs after dispersing treatment.

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Fig. 5 – FTIR spectrum of (a) COOH–CNTs and (b) OH–CNTs.

addition, from the results of FTIR, hydroxyl groups (around 3550 cm1) and carbonyl groups (around 1760 cm1) can be simultaneously found on the COOH–CNTs (Fig. 5a), and hy-

Table 1 – The composition of COOH–CNTs and OH–CNTs. CNTs

C 1s (%)

O 1s (%)

N 1s (%)

COOH–CNTs OH–CNTs

93.5 92.6

5.8 5.9

0.7 1.5

droxyl groups (around 3550 cm1) exist on the OH–CNTs (Fig. 5b), confirming the functional groups on the CNTs. Table 1 shows the chemical composition of the carbon nanotubes obtained from XPS measurements. For the COOH–CNTs, there are C, O and N elements, while similar contents of C and O elements exist on the OH–CNTs. The deconvolutions of the C 1s peaks for COOH–CNTs and OH–CNTs are shown in Fig. 6. Four peaks are found for C1s COOH–CNTs, attributed to C–C bonds, COOH groups, C–O groups and p–p* transitions [31]. For OH–CNTs, four peaks are shown, which are assigned to C–C bonds, N–C@O bonds, C–OH groups and p–p* transitions [31]. The corresponding

Fig. 6 – XPS deconvolution spectra of C 1s peak for: (a) COOH–CNTs and (b) OH–CNTs.

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Table 2 – C 1s peak fitting results for CNTs with peak positions in parentheses. CNTs

C–C% (eV)

O–C@O% (eV)

C–O% (eV)

N–C@O% (eV)

p–p*% (eV)

COOH–CNTs OH–CNTs

72.7 (284.9) 76.3 (284.8)

12.0 (287.1) —

11.7 (286.0) 18.7 (286.2)

— 4.3 (289.0)

3.6 (290.3) 0.7 (291.1)

binding energies and concentrations of these peaks are listed in Table 2. It can be seen that the content of active groups including COOH and C–O groups on the COOH–CNTs is larger than that on the OH–CNTs. Different functional groups can influence the dispersibility of CNTs. In fact, we found that the COOH–CNTs were more stable in de-ionized water than the OH–CNTs, which is attributed to more functional groups on the surface of COOH–CNTs (Table 2). These differences in the dimensions of CNTs and the dispersibility of CNTs are believed to be the main reasons that give more uniform and thicker CNT coatings on the fiber surfaces using the COOH–CNTs. We also used unfunctionalized MWCNTs to prepare the CNT-coated carbon fibers. It was found that obvious aggregation happened in the case of

pristine MWCNTs after mixing MWCNT/water suspension. For functionalized MWCNTs, good dispersion can be achieved in water via ultrasonication. However, for unfunctionalized MWCNTs, effective coating of MWCNTs to carbon fiber surface cannot be obtained due to poor stability of suspension. The similar result was reported in recent published paper [32]. Notice that the stable dispersion of CNTs in a suspension is the key of our proposed CNT-depositing technology, and improving the stability of MWCNT suspension is important for the distribution of CNTs on carbon fibers and the fabrication of CNT containing carbon fiber composites. From this point of view, CNTs with more functional groups are favorable for the suspension of dipping process.

Table 3 – Dynamic contact angles between the CNT-deposited carbon fiber and epoxy resin and the surface roughness of the corresponding carbon fiber. Fiber type

Raw T700SC

T700SC with COOH–CNTs

T700SC with OH–CNTs

Raw T300B

T300B with COOH–CNTs

T300B with OH–CNTs

Contact angle/ Surface roughness Ra/nm

85.9 ± 1.0 4.6 ± 1.5

77.4 ± 2.1 39.6 ± 6.3

64.8 ± 0.7 13.3 ± 3.9

75.1 ± 2.4 35.1 ± 4.7

70.7 ± 1.9 31.1 ± 6.4

47.6 ± 1.6 49.6 ± 6.7

Fig. 7 – AFM topographical images of different fibers: (a) raw T700SC, (b) T700SC with deposited COOH–CNTs, (c) T700SC with deposited OH–CNTs, (d) raw T300B, (e) T300B with deposited COOH–CNTs, and (f) T300B with deposited OH–CNTs.

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In order to further evaluate the effects of the CNTs on the carbon fibers, the wettability of fiber surface by epoxy resin was analyzed using dynamic contact angle. The contact angles between various carbon fibers and the epoxy resin, which can be seen in Table 3, clearly demonstrate a decrease in the contact angle after CNT-depositing process, indicating the improvement in surface wettability. Some research suggested that a good wettability of carbon fiber by polymer could achieve after adhering CNTs to the fiber, due to the increases in surface roughness [5] and active chemical group on the fiber [22]. The AFM topographic images

(a)

of different carbon fibers are presented in Fig. 7, and the corresponding roughness parameters, Ra, derived from the AFM scans are shown in Table 3. Obvious increases in surface roughness are observed for the T700SC with deposited COOH–CNTs (up to 760%) and the T300B with deposited OH– CNTs (about 41%). Comparing the changes of the contact angle with the changes of Ra, it seems that surface roughness plays an important role in the wettability of the CNT-deposited carbon fiber with rough surface, and chemical group on CNTs is significant for the wettability of the CNT-deposited carbon fiber with smooth surface.

(b) 100

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Transmittance / %

Transmittance / %

80 60 40 epoxy group

60 40 epoxy group 20

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o

Before 120 C treatment o After 120 C treatment

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Transmittance / %

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3000 -1

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Fig. 8 – FTIR spectrum of (a) extracted sizing of T700SC, (b) the mixture of the T700SC sizing and COOH–CNTs, (c) the mixture of the T700SC sizing and OH–CNTs, (d) extracted sizing of T300B, (e) mixture of the T300B sizing and COOH–CNTs and (f) mixture of the T300B sizing and OH–CNTs.

3.2.

Effect of fiber sizing on CNT-deposition state

Since the CNTs were deposited on the sized fibers, the effect of the sizing agent on the deposition of the CNTs was studies by coating COOH–CNTs on the unsized T700SC, which was prepared by acetone extraction. As shown in Fig. 3e, fewer CNTs adhere to the surface of the unsized T700SC compared to the case for sized T700SC (Fig. 3a). Furthermore, there are many CNTs remaining on the sized fiber (Fig. 3f), after the CNT-deposited fiber bundles were flushed by water under vacuum pressure, simulating the process of vacuum assisted RTM. These results indicate the importance of the fiber sizing for the deposition of CNTs on carbon fiber. Since the sizings of the T300B and T700SC are both epoxy type [23] and functionalized CNTs might react with epoxy [11,33], it is supposed that COOH–CNTs and OH–CNTs might react with the active groups in the sizings. In order to confirm this hypothesis, FTIR of the extracted sizings and the mixture of the functionalized CNTs and the extracted sizings were determined, as presented in Fig. 8. For the sizings without CNTs (Fig. 8a and d), the epoxy groups (around 900 cm1) can be found and the curves of FTIR change little after 120 C treatment. For the T700SC sizing containing CNTs (Fig. 8b and c) and the T300B sizing containing CNTs (Fig. 8e and f), the epoxy groups decrease after 120 C treatment, indicating chemical bonding between the epoxy in the sizings

(a)

80

IFSS / MPa

70

As-received Depositing CNTs with carboxylic acid group Depositing CNTs with hydroxyl group

60 50 40 30

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and carboxylic acid or hydroxyl groups on the CNTs. The covalent bonding between the CNTs and the sizing can provide strong adhering force of the CNTs on fiber surface and can reduce the amount of CNTs dispersing in matrix, which drop from fiber during fabrication of the composite. Consequently, the compatibility of the deposited-CNTs with the properties of the fiber sizing should be regarded as a critical factor for preparing the multiscale reinforcement.

3.3. Influence of deposited CNTs on interfacial bonding of single-fiber composite The IFSS of the carbon fiber/epoxy composites with and without deposited CNTs were tested by SFFT, and the measured data are given in Fig. 9. As shown in Fig. 9a, the deposition of the COOH–CNTs gives rise to a significant increase of the IFSS (about 43%) for the T700SC composite. Moreover, the IFSS of the T300B composite increases by around 12% and 11% after COOH–CNTs and OH–CNTs depositions, respectively (Fig. 9a). However, there is 17% decrease in the IFSS of the T700SC composite after OH–CNTs depositing (Fig. 9a). The critical fragment lengths (Fig. 9b) show that the composite specimens with the COOH–CNT deposited T700SC fiber and the CNT deposited T300B fibers display shorter fragment lengths than those with the raw carbon fibers. These results are consistent with those of the measured IFSS, demonstrat-

(b) 1200 Critical fragment length / um

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As-received Depositing CNTs with carboxylic acid group Depositing CNTs with hydroxyl group

800

600

400

200

20

T700SC

T300B

T700SC

T300B

Fig. 9 – The effects of the deposited-CNTs on the interfacial properties of the single fiber composites, error bars indicate standard deviation: (a) interfacial shear strength and (b) critical fragment length.

Fig. 10 – Birefringence effects of fragmented specimens at 4.0% tensile strain: (a) raw T700SC, (b) T700SC with deposited COOH–CNTs, (c) T700SC with deposited OH–CNTs, (d) raw T300B, (e) T300B with deposited COOH–CNTs, and (f) T300B with deposited OH–CNTs.

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ing that the CNT-deposition process significantly improves fiber/matrix interfacial adhesion. Fig. 10 demonstrates the photoelastic effects of the single fiber fragments in composite. The results of birefringence at fragment regions are consistent with the IFSS and the critical fragment length. Photoelastic observations of the raw fibers near the fiber breaks show a thin, flat region of birefringence followed by a bulge of birefringence (Fig. 10a and d). This birefringent behavior is an indication of interfacial debonding, which is associated with low fiber–matrix adhesion. In Fig. 10b, e and f, the CNT-deposited fiber breakages create obvious matrix cracks perpendicular to the fiber and between adjacent fragment ends, which produce bright ellipsoidal regions because of high shear stresses at the fiber ends. This is a consequence of a strong interface. In addition, T700SC with deposited OH–CNTs exhibits similar fiber breakage mode as raw T700SC, and more serious interfacial debonding is depicted in Fig. 10c. Some investigations have indicated that the structure and surface chemical property of CNTs have strong influences on the interphase of fiber reinforced resin matrix composites [11,34]. Figs. 9 and 10 show that depositing COOH–CNTs gives higher improvement in interfacial bonding than depositing OH–CNTs. This is attributed to the differences in the surface chemical characteristics of the CNTs. The COOH–CNTs have more surface functional groups than the OH–CNTs (Table 2), which leads to a better dispersion of the COOH–CNTs in the aqueous suspension and offers more active groups to react with the epoxy groups in the fiber sizing and the matrix, favoring the interfacial bonding. In addition, as shown in Fig. 4, the COOH–CNTs have higher aspect ratio than the OH–CNTs, which benefits for improving the IFSS of the composite containing the COOH–CNTs. Figs. 9 and 10 demonstrate that depositing OH–CNTs on T700SC leads to a decrease in the interfacial bonding compared to the raw T700SC composite, indicating worse fiber/ matrix adhesion. The degradation of the T700SC composite with OH–CNTs in IFSS is attributed to a low amount of the OH–CNTs adhering to T700SC (Fig. 3c). Kwon and co-workers [8] found that a low amount of CNTs, spraying on carbon fiber fabric, gave a decrease in GIIC of carbon fiber/polymer composite. They believed that lower amount of CNTs at the interface did not provide proper mechanical interlocking while serving as a localized defect because of a lower bonding between CNT and polymer. Besides the data about interfacial bonding, fragmentation test can provide fiber tensile strength and its distribution. According to the fiber tensile stress at the point of fiber fragment, the fiber tensile strength at different length was obtained. Note that the fiber tensile stress was calculated from the tensile strain and the fiber tensile modulus (230 GPa for T700SC and T300B). As given in Eq. (4), r0 denotes the carbon

fiber tensile strength at 20 mm length, and Weibull shape parameter b is used to estimate the length scale effect on fiber tensile property, where high b means narrow fiber strength distribution. These properties for various single fiber specimens are listed in Table 4. It clearly shows that the changes of both r0 and b after CNT-depositing process are negligible, which proves that the surface treatment using CNT coating introduces no damages on the carbon fibers.

3.4.

Interfacial reinforcing mechanism

As aforementioned, significant improvements in IFSS for CNT-deposited carbon fiber composites are observed compared to raw carbon fiber composites, except for the sample using the OH–CNT-deposited T700SC. There have been some mechanisms, which are proposed for explaining the interfacial improvement of fiber/polymer composite using CNTs [4– 7,11,17,22]. These interfacial reinforcing mechanisms can be summarized as five aspects: (1) Van der Waals binding due to increased surface area of fiber, (2) mechanical interlocking of CNTs with matrix, (3) good surface wettability of fiber by polymer, (4) chemical bonding between CNTs and bulk materials including fiber and matrix, and (5) local stiffening or strengthening of polymer matrix near fiber/matrix interface. According to the results of the morphology and roughness of the CNT-depositing fiber surface, it can be believed that Van der Waals binding and mechanical interlocking, increasing the interfacial friction and restricting the movement of the different phase of the materials at the composite interface, play important roles in the interfacial improvement. In addition, FTIR of the mixtures of the fiber sizing and the CNTs demonstrate the chemical reactions between the functionalized CNTs and the epoxy groups, and support the viewpoint of the chemical bonding at the interphase. However, the surface wettability might have negligible effect on the interfacial bonding of the composites. It is noted that the OH–CNTdeposited T700SC has much smaller contact angle with the epoxy resin than the as-received T700SC (Table 3), but it gives weaker interfacial bonding (Fig. 9). In general, epoxy resin is believed to have good wettability on raw sized carbon fibers, which could be one of the reasons for the negligible effect of the change of fiber wettability on the interfacial bonding of the composites. We next investigated fracture mechanism of the single-fiber composite by SEM characterizations. Fig. 11 presents typical SEM images of the fracture morphology before and after COOH–CNT coating. It is clear that obvious cracks appear at the joint of the matrix–fiber and indicate the fracture planes initiated at the interface between fiber and matrix, regardless of the CNT depositing. This observation suggests that the fracture modes of the studied carbon fiber/epoxy composites are unaffected by the CNT depositing. The original carbon fi-

Table 4 – Measured mechanical property of single fiber specimens based on Eq. (4). Fiber type

Raw T700SC

T700 with COOH–CNTs

T700 with OH–CNTs

Raw T300B

T300 with COOH–CNTs

T300 with OH–CNTs

Fiber tensile strength r0/MPa Weibull shape parameter b

4830 ± 391 6.03 ± 0.24

5037 ± 598 6.16 ± 0.50

4991 ± 184 6.74 ± 0.80

3968 ± 345 7.29 ± 1.07

4064 ± 138 7.41 ± 0.60

3981 ± 276 7.10 ± 0.80

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119

Fig. 11 – Fracture morphology of single fiber-composite fragmentation tests: (a) raw T700SC, (b) T700SC with COOH–CNTs, (c) raw T300B, and (d) T300B with COOH–CNTs. ber composites failure mostly clean fiber–matrix interface fractures (Fig. 11a and c), while the resin matrix obviously remains attached to fibers for the CNT-deposited samples (Fig. 11b and d). It therefore confirms that the depositing of the CNTs on carbon fiber help in creating local interlocking between the fibers and the matrix and increasing the frictional force at the interphase. Furthermore, it is worth noting that the fracture behavior at the matrix near the interphase changes after the CNT depositing. Fig. 11 show the composites containing CNTs exhibit significantly rougher fracture surfaces compared to the composites without CNTs, indicating a toughening effect of the CNTs on the matrix. During the composite fabrication, some CNTs drop from fiber surface and disperse into polymer, so that the toughening mechanism to the matrix might work. Our recent work exhibits that the toughness of the epoxy matrix significantly increases at very low content of the functionalized CNTs [35]. The toughening of polymer matrix near fiber/matrix interface is favorable for releasing stress concentration and hindering the propagation of crack, resulting in a stronger interphase. Overall, the interfacial reinforcing mechanism of the carbon fiber/epoxy composite due to the depositing CNTs is attributed to interfacial friction, chemical bonding and resin toughening, which are strongly depended on the properties of CNT and carbon fiber.

3.5. Influence of deposited CNTs on mechanical property of composite laminate The T700SC composite laminates with and without COOH– CNT coating were prepared using vacuum assisted resin infu-

sion molding, and the thicknesses and fiber contents of the two kinds of laminates are almost same. Therefore, the mechanical properties were compared to demonstrate the effects of deposited CNTs on the composite performance. The tensile modulus and strength of the laminates are given in Table 5. The laminates with CNTs and without CNTs have the same tensile moduli, which are determined by the tensile modulus of fiber and the fiber content. Moreover, an approximately 11.2% increase in tensile strength is obtained for the samples with COOH–CNTs, compared to the neat samples. Fig. 12 shows the typical tensile stress–strain curves of the raw and COOH–CNT deposited T700SC carbon fiber composite laminates. It can be seen that obvious damages appear at smaller tensile strain for the raw sample, while the tensile stress of the sample with CNTs decreases significantly at larger strain. The deposited-CNTs improve the interfacial bonding of the composite, which favors improving the interfacial stress transfer from the matrix to the fibers and results in an increase on the tensile strength. In addition, Table 5 demonstrates the ILSS of the laminates with COOH–CNTs is increased by 12.9%, resulted from the effect of the CNTs on the toughening of matrix and the improvement of interfacial adhesion. These experimental results indicate that the improvements can be achieved for the nanostructured carbon fiber composite, including the fiber–matrix interphase and matrix-dominated performances. It should be noticed that the surface characteristics of CNTs must be considered for the improvement of the mechanical property of composite. Using CNTs with enough amounts of functional groups is the key of the CNT-depositing technology. More functional groups can provide good dispersibility of CNTs in the suspension, which

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Table 5 – Mechanical properties of the COOH–CNT deposited T700SC carbon fiber composite laminates fabricated using vacuum assisted resin infusion molding. Mechanical property

Tensile modulus (GPa)

Tensile strength (MPa)

Raw (without CNT) CNT-deposited

90.5 ± 4.6 90.6 ± 2.4

1459 ± 56 1622 ± 65

Tensile stress / MPa

Raw CNT-deposited

1200

31 ± 1.3 35 ± 2.1

reinforcing mechanism of deposited-CNTs is discussed. These results suggest that interfacial friction, chemical bonding and resin toughening near the interphase are responsible for the increased interfacial bonding between the CNThybridized carbon fibers and the epoxy matrix. The improvements in the tensile strength and the interlaminar shear strength of the CNT-deposited laminates are also obtained. The proposed method is a promising way to manufacture multiscale reinforcement, which can improve the mechanical properties of carbon fiber-reinforced polymer composites.

1600 1400

ILSS (MPa)

1000 800 600 400 200 0 0.0

0.5

1.0

1.5

2.0

2.5

Acknowledgments

Tensile strain / %

Fig. 12 – Tensile stress–strain curves of raw and COOH–CNT deposited T700SC carbon fiber composite laminates.

improves the distribution of CNTs on the surface of carbon fiber, resulting in strong interfacial friction in the composite. Moreover, more functional groups benefit the chemical bonding between the CNTs and the bulk materials and resin toughening near the interphase. These effects can improve the interfacial strength according to the interfacial reinforcing mechanism discussed in Section 3.4. Therefore, the mechanical properties of composite are expected to be further enhanced by means of adopting CNTs with suitable surface functional groups.

4.

Conclusions

Carboxylic acid- and hydroxyl-functionalized MWCNTs were directly deposited onto the carbon fiber surface using an aqueous suspension immersion method without the removal of the commercial sizing. The surface functional groups of CNTs are believed to be a key factor for the stability of MWCNT suspension and effective coating of MWCNTs to carbon fiber surface. The functionalized MWCNTs adhering to the carbon fiber are uniformly distributed with random orientation, and lead to an increase in surface roughness compared to the original fiber. The properties of the CNTs, the surface morphology of the raw fiber and the fiber sizing have significant effects on the deposition state of the CNTs. After CNT deposition, the interfacial shear strengths of the carbon fiber/epoxy composites are improved as shown by single-fiber composite fragmentation tests, especially for the T700SC with COOH–CNTs, which have more functional groups than OH– CNTs. Through the wettability of carbon fiber by epoxy resin, infrared spectrometry of the mixture of fiber sizing and CNTs, topography of the fiber surface and fracture morphology of cross-sections of the single-fiber composites, the interfacial

This work was supported by funding from the National Natural Science Fund Program of China (Grant No. 51103003). The authors would like to acknowledge Dr. Jingming Wang for helpful discussions.

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