- Email: [email protected]
Carbon Fiber Reinforced Polymers modified with thermoplastic nonwovens containing multi-walled carbon nanotubes
Composites Science and Technology 173 (2019) 110–117
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Carbon Fiber Reinforced Polymers modified with thermoplastic nonwovens containing multi-walled carbon nanotubes
T
Kamil Dydeka,∗, Paulina Latko-Durałeka, Anna Boczkowskaa, Michał Sałacińskib, Rafał Kozeraa a b
Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Wołoska 141, 02-507, Warsaw, Poland Air Force Institute of Technology, ul. Księcia Bolesława 6, 01-494, Warsaw, Poland
ARTICLE INFO
ABSTRACT
Keywords: A. Carbon nanotubes A. Polymer-matrix composites (PMCs) B. Electrical properties B. Mechanical properties Thermoplastic nonwovens
The main purpose of this work was to improve the electrical conductivity of Carbon Fiber Reinforced Polymers (CFRP) by implementing novel thermoplastic nonwovens doped with carbon nanotubes. For this, two types of nonwovens containing carbon nanotubes were produced by the extrusion and thermal pressing of fibers. Nonwovens were placed between each layer of prepregs and CFRPs were fabricated using an out-of-autoclave method. It was found that implementation of nonwovens with 7 wt% of multi-walled carbon nanotubes resulted in improved surface and volume electrical conductivity in all directions. Microstructure analysis revealed the good quality of the produced laminates and the random distribution of the nonwovens in the composite panels. Examination of loss and storage moduli by dynamic mechanical analysis showed the higher flexibility of the laminates and the appearance of an additional glass transition peak due to the presence of copolyamide in the nonwovens used.
1. Introduction Nowadays, changes in the design and type of materials used in aircraft structures occur very quickly. There is a noticeable tendency to use very stiff materials that are as light as possible in order to reduce the whole weight of aircraft and thus increase the weight of the transported load, and to reduce emissions of carbon dioxide and other harmful substances [1]. For these reasons, the use of polymer matrix composites reinforced with carbon fibers with high specific stiffness and strength is still growing. The best example is the Boeing 787 Dreamliner, in which the mass fraction of composites applied in the wings, fuselage and tail is about 50%, while the volume fraction is almost 80% [2]. However, CFRPs are much less electrically conductive than metals, therefore the lighting strike protection of these aircraft against catastrophic structural damage, hazardous electrical shocks to passengers, and loss of flight control is not ensured because the current from a lightning strike cannot find conductive paths. In this case the protection of composite structures carrying fuel becomes even more crucial, as fuel vapors could ignite due to electrical sparking from lightning strikes [3,4]. Moreover, composite materials with low electrical conductivity cannot shield electronic parts from electromagnetic interference because they do not work as a Faraday Cage, in which electromagnetic waves are prevented from entering any closed surface bounded by conductive materials [5]. ∗
In order to increase resistance and protection from lightning strikes, electrically conductive meshes or tapes are implemented in the outer composite layer of aircraft. They are made of copper, aluminum or nickel, which not only add more weight, but their production cost is also too high [6]. Metallic meshes can be replaced by electrically conductive fibers or woven/nonwoven fabrics that consist of polymer coated with copper, silver or nickel particles [7]. However, they still have high weight, low flexibility, and contain a chemical binder in the structure which can be released during the CFRP manufacturing process. Hence, the new approaches include improved electrical conductivity of CFRP without increasing its weight, whilst still maintaining high mechanical properties. One idea is to incorporate carbon nanotubes (CNTs) due to their low density and outstanding mechanical and electrical properties [8]. There are different ways of introducing CNTs into CFRPs: the first involves mixing epoxy resin with CNTs for infusion [9]; however, the most promising approach is the application of semi-finished products as this eliminates the usage of powder CNTs, which are harmful. One example is buckypaper, which is a thin film produced by filtering a highly concentrated suspension of dispersed CNTs or by gas-phase catalytic growth [2]. The electrical conductivity of buckypaper is 103–105 S/m and the density is 0.25–0.83 g/cm3. However, the main disadvantage is their brittleness, which causes technical problems and poor infiltration during the CFRP fabrication process [10–14]. Another
Corresponding author. E-mail address: [email protected] (K. Dydek).
https://doi.org/10.1016/j.compscitech.2019.02.007 Received 20 November 2018; Received in revised form 28 January 2019; Accepted 6 February 2019 Available online 08 February 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
solution described in the literature is the usage of prepregs with CNTs deposited on their surface [15]. A new idea developed by our team is the application of nonwovens made of thermoplastic polymers doped with CNTs [16,17]. Commercial thermoplastic nonwovens made of different polymers do not contain any conductive particles and are used as interlayers or surface finishing in CFRPs to increase their fracture toughness and prevent delamination or cracks [18,19]. Hence, implementation of semi-finished products containing thermoplastic polymer and conductive nanoparticles can increase not only the mechanical properties of CFRPs, but also their electrical conductivity. Nonwovens containing CNTs can be produced with various methods: the melt-blown method [20], by pressing thermoplastic fibers containing CNTs [16], by spraying CNTs on dry carbon fibers [21], by growing CNTs on carbon fibers [22], by coating polymeric fabrics with thin layer of CNTs [23], or by electrospinning [24]. In this paper, thermoplastic nonwovens containing multi-walled carbon nanotubes (MWCNTs) were fabricated from extruded fibers by thermal pressing. They were interleaved between each layer of the commercial prepregs with the out-of-autoclave method of CFRP fabrication. The main goal of this work is to show the effect on the electrical and mechanical properties of CFRPs of two different types of thermoplastic nonwovens based on copolyamides. The selected copolyamides differed in melt viscosity and melting point, which can affect the creation of conductive paths by carbon nanotubes between carbon fabrics and consequently affect the electrical conductivity of CFRPs.
agglomerates of MWCNTs, compared with coPA1566, which has lower viscosity [27]. Finally, the presence of larger agglomerates made the extrusion of fibers more difficult and therefore thicker fibers for coPA1566+7 wt% MWCNTs were obtained. In the next step, the extruded fibers were cut into 70 mm-long pieces that were then randomly deposited on a special carrier and pressed on both sides using a thermomechanical press (LEMUR) according to patent PL 221848 B1 [28]. The areal weight of each nonwoven was 15 g/ m2. Images of both types of produced nonwovens are shown in Fig. 1c and d. After pressing, the overall thickness of the nonwovens was lower than for fibers: 76 ± 9.0 μm for nonwoven based on coPA1330 and 109 ± 13.0 μm for coPA1566. After pressing, the fibers had elliptical cross sections. 2.2. Laminate manufacturing Two types of carbon-epoxy unidirectional prepregs were used: TC E722-02 (TenCate Advanced Composites, UK) and HexPly® NCHR 913 (Hexcel, USA). Both of them contained 12 k of carbon fibers and 35% weight fraction of epoxy resin. The areal weight of both prepregs was 130 g/m2. Six types of laminates were manufactured using the out-ofautoclave method (Table 1). Each panel was fabricated from 14 prepreg layers. Nonwovens with 7 wt%MWCNTs were placed between each layer of prepreg. Reference panels (1T and 4H) were fabricated without nonwovens. After lay-up, the laminates were degassed, vacuum bagged on a glass plate, consolidated at 75 °C for 30 min, and then consolidated again at 170 °C for 120 min under vacuum. The heating speed and cooling rate were 3 °C/min in each case. After curing, the composite panels were removed from the vacuum bag, then trimmed and cut into appropriate test specimen dimensions. The thickness of the laminates and the weight fraction of carbon fibers are included in Table 1.
2. Materials and methods 2.1. Nonwovens fabrication Two types of thermoplastic copolyamides (trade names: Griltex®1330A (coPA1330) and Griltex®1566 (coPA1566) supplied by EMS Griltech (Switzerland) were used as a polymer matrix to produce nonwovens. The selected polymers belong to the group of hot melt adhesives, which possess sticky properties when melted and have good compatibility with epoxy resin. They consist of polyamide 6 and 66 segments, whose melting points are much lower than for typical polyamides and occur at 125–135 °C for coPA1330 and at 115–125 °C for coPA1566. They have different melt viscosities of 1200 Pa s and 800 Pa s for coPA 1330 and coPA 1566, respectively. Both copolyamides were mixed with 7 wt% MWCNTs (trade name NC7000, Nanocyl) using an industrial twin-screw extruder machine by Nanocyl, Belgium. The calculated electrical percolation threshold for coPA1330 was found to be 2.0–3.0 wt%MWCNTs and for coPA1566 was almost 1.0 wt% MWCNT [25]. In the presented studies a much higher CNT content was used (7 wt% MWCNTs) since it resulted in greater improvement of electrical conductivity as shown in our previous results [26]. Subsequently, pellets of coPA1330+7 wt% MWCNTs and coPA1566+7 wt% MWCNTs were processed directly into fibers using a laboratory twinscrew extruder machine (HAAKE MiniLab, ThermoScientific, Germany) equipped with a circle nozzle. For both materials, the extrusion temperature was adjusted to 180 °C and the screw speed was set at 40 rpm. For coPA1330+7 wt% MWCNTs, the average diameter of the extruded fibers (measured with SEM images) was 100.6 ± 12.04 μm; for coPA1566+7 wt%MWCNTs fibers the diameter was 294.7 ± 29.13 μm. The differences in the obtained thicknesses of the nanocomposite fibers are related to the size of MWCNT agglomerates in the pellets and the effect of the viscosity of the copolyamides. As presented in Fig. 1a and b, a masterbatch of coPA1330+7 wt%MWCNTs contains far fewer and smaller MWCNT agglomerates, which are dispersed homogenously (images taken from a light optical microscope). In turn, for coPA1566+7 wt%MWCNTs, larger agglomerates that were not uniformly distributed were found. Such variances in the dispersion and distribution of MWCNTs are caused by the viscosity of the coPAs used. Mixing the more viscous coPA1330 with MWCNTs introduces higher shear forces that result in more effective breaking up of primary
2.3. Methods Electrical volume conductivity of the fabricated CFRP was measured in three directions: along the carbon fibers (X-direction), perpendicular to the carbon fibers (Y-direction) and through the laminate thickness (Z-direction). Samples for the test with dimensions 60 mm × 10 mm (for measurements in X and Y directions) and 15 mm × 15 mm (for measurements in Z direction) were cut from different sections of the composite panels. The volume electrical conductivity was measured using the Keithley 6221/2182A device equipped with a measuring stand with copper electrodes. Good contact between electrodes and sample was maintained by the application of silver paste. The electrical surface conductivity was measured for samples with dimensions of 75 mm × 75 mm, which were also cut from different sections of the laminates. The test was carried out according to ASTM D-257 using the 6517B Electrometer/High Resistance Meter equipped with an 8009 Test Fixture. Each sample was placed between concentric stainless-steel ring electrodes with a conductive rubber pad which maintained perfect contact between the sample and electrodes. The voltage applied was 1 V, time of the test was 15 s and the number of readings was set to 5. The diameters of the extruded fibers were determined using a Scanning Electron Microscope (SEM, TM 3000 Hitachi, Japan) with applied voltage 5 kV. The observations of the microstructure and quality of the produced CFRP laminates were performed using SEM (TM 3000 Hitachi, Japan). In this case the applied voltage was 15 kV. SEM was also used to analyze the propagation of cracks in the laminate after short-beam shear tests. The dispersion and distribution of MWCNTs in the final composite panels were examined with a high-resolution SEM SU70 Hitachi, Japan. The influence of thermoplastic nonwovens on the glass transition temperature of epoxy was analyzed by Dynamic Mechanical Analysis (DMA). The test was performed using a DMA Q800 (TA Instruments, USA) in dual cantilever mode according to ASTM D7028-07 from 0 °C to 111
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 1. Materials based on coPA1330+7 wt%MWCNTs (a, c) and coPA1566+7 wt%MWCNTs (b, d): light optical images of the initial pellets (a, b), structure of thermoplastic nonwovens (c, d).
220 °C with a heating rate of 2 °C/min at a frequency of 1 Hz and with an amplitude of 30 μm. Samples for the test were 60 mm long and 10 mm wide. The mechanical properties of the CFRP composites were measured in a short-beam shear test (according to ASTM D2344) using an MTS Q/ Test 10 universal testing machine. Six specimens (25 mm length, 6.4 mm width) were cut from different sections of each composite panel along the carbon fibers. The test was performed at room temperature at a speed of 1 mm/min.
the Z-direction (Fig. 2a), the improvement of the electrical conductivity of laminates fabricated from TenCate prepreg was about 92% and 84% for interleaves made of coPA1330+7 wt%MWCNTs and coPA1566+7 wt%MWCNTs nonwovens, respectively. In the case of laminates based on Hexcel prepreg, this increase was a little higher (about 106%) for panels interleaved with coPA1330+7 wt%MWCNTs nonwovens (5H), but was significantly improved (about 227%) (3.14 S/ m) when the nonwovens were made of coPA1566+7 wt%MWCNTs (6H). In both cases the increase of the conductivity through the thickness of the laminates was caused by the presence of conductive paths formed by carbon nanotubes introduced to the composite structure with the thermoplastic nonwovens [29]. Because the electrical conductivity along the carbon fibers (Fig. 2b) was much higher than through the laminate thickness, the observed changes of electrical conductivity in the X-direction are less than in the Z-direction. Application of thermoplastic nonwovens between the TenCate layers increased the electrical conductivity in the X-direction by about 85% for coPA1330+7 wt %MWCNTs and 67% for coPA1566+7 wt%MWCNT nonwovens. In turn for Hexcel-based composites, the improvement was lower than 10% for both types of thermoplastic nonwovens. Finally, electrical volume conductivity measured perpendicular to the carbon fibers (Y-
3. Results 3.1. Electrical conductivity The results of the electrical volume conductivity of the manufactured CFRP panels in Z, X and Y directions are shown in Fig. 2a–c, respectively. It is clearly seen that implementation of both types of nonwovens containing 7 wt% MWCNTs between prepreg layers resulted in an increase of electrical conductivity in all directions. However, dependent on the type of nonwovens used (based on coPA1330 or coPA1566), the increase of electrical conductivity was not the same. In Table 1 Characteristics of the manufactured laminates. Material
Laminate designation
Thickness [mm]
Carbon fibers' weight fraction [%]
TenCate reference TenCate + coPA1330+7 wt%MWCNT TenCate + coPA1566+7 wt%MWCNT Hexcel reference Hexcel + coPA1330+7 wt%MWCNT Hexcel + coPA1566+7 wt%MWCNT
1T 2T 3T 4H 5H 6H
1.93 2.03 2.11 1.89 2.00 2.07
66.6 63.2 62.8 66.9 64.4 63.3
112
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 2. Electrical conductivity for CFRP panels: a) dz direction, b) dx direction, c) dy direction (throughout the thickness), d) surface.
direction, Fig. 2c) was improved for all types of composite panels and was the highest for laminate 2T (90%), then for 3T (54%), 6H (43%) and 5H (25%). Although this improvement was better for TenCate based panels, CFRP laminates made of Hexcel prepreg showed higher electrical conductivity in all directions than those made of TenCate, as shown in Fig. 2. Similarly to the volume electrical conductivity, the implementation of thermoplastic nonwovens containing 7 wt%MWCNTs also caused an increase in the electrical surface conductivity (Fig. 2d) because the electrical surface conductivity was affected by the current flowing partially in the sample volume [30]. For laminates based on TenCate, the increase in the electrical surface conductivity was 237% for nonwovens made of coPA1330+7 wt%MWCNTs and 306% for panels in which nonwovens were made of coPA1566+7 wt%MWCNTs. A slightly lower improvement of electrical surface conductivity was achieved for CFRP panels based on Hexcel prepreg. For laminate 5H interleaved with coPA1330+7 wt%MWCNTs nonwovens, the increase was about 115%; in turn, for panel 6H containing coPA1566+7 wt%MWCNTs nonwovens, it was only about 90%. Again, composite panels manufactured from Hexcel prepregs show higher overall electrical surface conductivity than those based on TenCate.
fiber layers are also visible; they are randomly distributed because the fibers from the nonwovens are also distributed arbitrarily. The thermoplastic bridges are rich in carbon nanotubes, which makes the creation of conductive paths possible. However, it should be noted that the CNT-doped thermoplastic bridges are not uniformly distributed, which may lead to discrepancies in the electrical conductivity measured on specimens taken from different sections of the same CFRP panel. In order to explain the increase of electrical conductivity as the result of implementation of thermoplastic nonwovens containing MWCNTs between the prepreg layers, the microstructure observations were also carried out using high-resolution SEM. For the analysis, the images were collected for CFRP panels based on Hexcel prepreg because their electrical conductivity was the highest. Fig. 4a and b shows the interface boundary between the thermoplastic nonwoven doped with CNT and carbon fibers in which nonwovens seem to adhere very well to the carbon fibers without any visible delamination or debonding. This is due to the sticky properties of the copolyamides used. Fig. 4a and b also confirm that the MWCNTs are well dispersed and randomly distributed in the thermoplastic nonwoven. The combination of good dispersion of MWCNTs and good adhesion between carbon fibers and nonwovens is responsible for the improvement of electrical conductivity, especially through the laminate thickness [31].
3.2. Microstructure
3.3. Mechanical properties
The microstructure images of the manufactured CFRP panels are presented in Fig. 3, with visible layers of carbon fibers and implemented nonwovens. From the cross-section of the reference laminates based on Hexcel prepreg (Fig. 3b), fewer voids than in the TenCate reference panel can be observed (Fig. 3a). Hence, the reference panel based on Hexcel prepreg showed higher electrical conductivity. The addition of both types of thermoplastic nonwovens (Fig. 3c–f) between the prepreg layers contributed to reducing the number of voids compared to the reference laminates. The far fewer voids in the panels manufactured with Hexcel prepreg led to increased electrical conductivity. In these images, thermoplastic bridges between the carbon
In order to determine the interlaminar shear strength (ILSS) of the fabricated laminates, a short-beam strength test was performed. It was expected that thermoplastic nonwovens would increase the CFRPs' resistance against delamination [32,33]. In the case of the laminates based on TenCate prepreg, nonwovens caused a decrease in the ILSS from 67.0 ± 1.14 MPa (panel 1T) to 58.6 ± 3.78 MPa (panel 2T) for nonwovens made of coPA1330+7 wt%MWCNT and to 66.8 ± 3.55 MPa (3T) for laminates interleaved with coPA1566+7 wt %MWCNT nonwovens. The opposite situation was observed for the laminates manufactured with Hexcel prepreg. Here, implementation of 113
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 3. Microstructure observations of laminates: a) TenCate reference (1T), b) Hexcel reference (4H) c) TenCate + coPA1330+7 wt%MWCNTs (2T), d) Hexcel + coPA1330+7 wt%MWCNTs (5H), e) TenCate + coPA1566+7 wt%MWCNTs (3T), f) Hexcel + coPA1566+7 wt%MWCNTs (6H).
thermoplastic nonwovens increased the ILSS of the laminates, but only at a negligible level. The interlaminar shear strength of the reference laminate based on Hexcel prepreg (4H) reached 86.4 ± 2.04 MPa; this is an increase of about 2.4 MPa and 1 MPa for laminate 5H and 6H, respectively. In this case the use of a hot melt adhesives doped with MWCNTs caused crack bridging [34,35]. Moreover, the presence of thermoplastic polymer in the laminates led to a different type of cracking of the samples. In the case of the reference laminates (1T and 4H), after reaching the maximum force the samples broke in a brittle way, whereas after reaching the maximum force the laminates with the addition of thermoplastic nonwovens deformed plastically. Finally, one distinct crack was observed for the reference laminate (Fig. 5a), and several smaller cracks were observed for laminates modified with thermoplastic nonwovens (Fig. 5b). In addition, observations were made from the top of the sample surface. In the case of the reference laminate (4H), carbon fibers that were pulled out of the polymer matrix were observed (Fig. 5c), but this phenomenon was not observed for laminates modified with thermoplastic nonwovens (Fig. 5d). In addition, the cracks were smaller in the modified laminate than in the reference laminate. This explains the higher strength of laminates containing thermoplastic nonwovens with CNT. The reason that the laminates produced from two different prepregs displayed different mechanical properties could be also associated with the surface preparation of the carbon fibers used (i.e. sizing was applied). The lack of
improved ILSS in laminates produced using TenCate prepreg could be associated with the worse adhesion between the thermoplastic nonwoven and the carbon fibers. In Fig. 6a, debonding between CNT-doped copolyamide and carbon fiber is visible, while in the case of Hexcel prepreg the adhesion seems to be much better, as shown in Fig. 6b. 3.4. Dynamic mechanical analysis DMA results are presented in Fig. 7. It can be seen from the loss modulus results (Fig. 7b) that there is one peak at the curves of the reference panels. For laminate 1T (TenCate prepreg) it occurs at 126.9 °C and for laminate 4H (Hexcel prepreg) it occurs at 177.7 °C, which is related to the glass transition temperature of the epoxy resin. In the case of laminates containing thermoplastic nonwovens, an additional peak appeared at about 39–42 °C; this comes from segments of polyamide 6 which are present in the copolyamide macromolecules used for fabrication of the nonwovens [36]. However, incorporation of thermoplastic nonwovens based on copolyamides with 7 wt% MWCNTs did not affect the glass transition temperature of the epoxy resin [37]. CFRP panels interleaved with thermoplastic nonwovens also show changes in the storage modulus values, as presented in Fig. 7a. For all laminates, the storage modulus decreases in comparison to the reference panels due to the presence of thermoplastic polymer, which has lower stiffness than epoxy.
Fig. 4. Microstructure observation of CFRP: a) laminate 5H, magnification 50 k; b) laminate 6H, magnification 50 k. 114
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 5. Microstructure observations after ILSS tests: a) panel 4H; b) panel 5H; c) image from the top of sample 4H; d) image from the top of sample 5H.
Fig. 6. Adhesion between the melted nonwoven and the prepreg layer: a) laminate 3T; b) laminate 6H.
4. Conclusions
has a higher melting point and viscosity, leads to greater improvement of electrical conductivity due to better dispersion of CNTs caused by higher shear forces during the extrusion of thermoplastic fibers. The good adhesion between nonwovens and carbon fibers in the case of Hexcel prepreg was found to be the consequence of the usage of copolyamides from the group of hot melt adhesives. Implementation of thermoplastic nonwovens into CFRP increases the flexibility of the CFRP, as is confirmed by the lower storage modulus shown by the DMA test. On the other hand, the presence of thermoplastic copolyamide in the laminates does not affect the glass transition temperature of the epoxy resin, but the additional peak in the loss modulus curve corresponds to the glass transition temperature of polyamide 6. The analysis of the mechanical properties in terms of the interlaminar shear strength showed that – dependent on the type of prepreg used– the implementation of nonwovens decreased (for TenCate based panels) or slightly increased their ILSS (for Hexcel based panels). This may be caused by their different adhesion to carbon fibers due to the different sizing methods applied to the carbon fibers. In the case of TenCate prepreg, debonding between CNT-doped copolyamide and carbon fibers was observed, which caused poor adhesion. The opposite observations were made for Hexcel-based panels. Moreover, laminates interleaved with nonwovens containing MWCNTs exhibit smaller cracks than the
In this work the effect of thermoplastic nonwovens containing 7 wt % MWCNTs on the electrical, thermal and mechanical properties of CFRPs was investigated. Two types of nonwovens based on two commercial copolyamides were fabricated by pressing the extruded CNTdoped fibers in laboratory conditions. In the next step, nonwovens were implemented between the layers of the two commercial prepregs using an out-of-autoclave technique. The application of CNT-doped nonwovens did not affect the CFRP fabrication process. Moreover, they are easy to handle and safe for workers. It was found that electrical volume conductivity as well as electrical surface conductivity were improved when compared to the reference panels. In the case of electrical volume conductivity, the highest improvement was about 227% (3.14 S/m) through the laminate thickness for panels based on Hexcel prepreg and coPA1566+7 wt%MWCNTs nonwovens. In the case of electrical conductivity measured along and perpendicular to the carbon fibers, the improvement was less than 50%. Laminates based on TenCate prepreg show lower electrical conductivity in comparison to laminates based on Hexcel prepreg. The obtained results confirmed that thermoplastic nonwovens doped with carbon nanotubes create conductive bridges between carbon fiber layers. The nonwoven based on coPA1330, which 115
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 7. Dynamic mechanical analysis of laminates: a) storage modulus and b) loss modulus.
reference panel. Also, the top view of the specimens after ILSS tests confirmed a different cracking mechanism when the reinforcement fibers were bonded by thermoplastic polymer. The GSM of thermoplastic nonwovens containing carbon nanotubes should be further adjusted, taking into account the improvement of both the electrical conductivity and mechanical properties of CFRP.
[5] M. Gagné, D. Therriault, Lightning strike protection of composites, Prog. Aero. Sci. 64 (2014) 1–16 https://doi.org/10.1016/j.paerosci.2013.07.002. [6] X. Ma, F. Scarpa, H.X. Peng, G. Allegri, J. Yuan, R. Ciobanu, Design of a hybrid carbon fibre/carbon nanotube composite for enhanced lightning strike resistance, Aerosp. Sci. Technol. 47 (2015) 367–377 https://doi.org/10.1016/j.ast.2015.10. 002. [7] F.L. Zhou, R.H. Gong, I. Porat, Nano-coated hybrid yarns using electrospinning, Surf. Coat. Technol. 204 (2010) 3459–3463 https://doi.org/10.1016/j.surfcoat. 2010.04.021. [8] O. Gohardani, M.C. Elola, C. Elizetxea, Potential and prospective implementation of carbon nanotubes on next generation aircraft and space vehicles: a review of current and expected applications in aerospace sciences, Prog. Aero. Sci. 70 (2014) 42–68 https://doi.org/10.1016/j.paerosci.2014.05.002. [9] D. Quan, Enhancing mode-I and mode-II fracture toughness of epoxy and carbon fibre reinforced epoxy composites using multi-walled carbon nanotubes, Mater. Des. 143 (2018) 81–92 https://doi:10.1016/j.matdes.2018.01.051. [10] I.W.P. Chen, R. Liang, H.B. Zhao, B. Wang, C. Zhang, Highly conductive carbon nanotube buckypapers with improved doping stability via conjugational crosslinking, Nanotechnology 22 (2011) 7 https://doi.org/10.1088/0957-4484/22/48/ 485708. [11] W. Ma, L. Song, R. Yang, T. Zhang, Y. Zhao, L. Sun, et al., Directly synthesized strong, highly conducting, transparent single walled carbon nanotube films, Nano Lett. 7 (2007) 2307–2311 https://doi.org/10.1021/nl070915c. [12] D. Wang, P.C. Song, C.H. Liu, W. Wu, S.S. Fan, Highly oriented carbon nanotube papers made of aligned carbon nanotubes, Nanotechnology 19 (2008) 6 https://doi. org/10.1088/0957-4484/19/7/075609. [13] J. Zhang, D. Jiang, H.X. Peng, F. Qin, Enhanced mechanical and electrical properties of carbon nanotube buckypaper by in situ cross-linking, Carbon 63 (2013) 125–132 https://doi.org/10.1016/j.carbon.2013.06.047.
Acknowledgments The studies were carried out within grant no. DOB-1-3/1/PS/2014, financed by the National Center for Research and Development. References [1] P. Feraboli, M. Miller, Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike, Compos. Part A-Appl S 40 (2009) 954–967 https://doi.org/10.1016/j.compositesa.2009.04.025. [2] E.J. Siochi, J.S. Harrison, Structural nanocomposites for aerospace applications, Mater. Res. Soc. 40 (2015) 829–835 https://doi.org/10.1557/mrs.2015.228. [3] Y. Hirano, S. Katsumata, Y. Iwahori, A. Todoroki, Artificial lightning testing on graphite/epoxy composite laminate, Compos. Part A-Appl S 41 (2010) 1461–1470 https://doi.org/10.1016/j.compositesa.2010.06.008. [4] J. Gou, Y. Tang, F. Liang, Z. Zhao, D. Firsich, J. Fielding, Carbon nanofibre paper for lightning strike protection of composite materials, Compos. Part B Eng. 41 (2009) 192–198 https://doi.org/10.1016/j.compositesb.2009.06.009.
116
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al. [14] H. Chu, Z. Zhang, Y. Liu, J. Leng, Self-heating fibre reinforced polymer composite using meso/macropore carbon nanotube paper and its application in deicing, Carbon 66 (2014) 154–163 https://doi.org/10.1016/j.carbon.2013.08.053. [15] Y. Akcin, S. Karakaya, O. Soykasap, Electrical, thermal and mechanical properties of CNT treated prepreg CFRP composites, Mater. Sci. Appl. 7 (2016) 465–483 https:// doi.org/10.4236/msa.2016.79041. [16] P. Latko, W. Rumiński, A. Boczkowska, Carbon-nanotubes doped veils, Compos. Struct. 134 (2015) 52–59 https://doi.org/10.1016/j.compstruct.2015.07.115. [17] P. Latko, R. Kozera, A. Salinier, A. Boczkowska, Non-woven veils manufactured from polyamides doped with carbon nanotubes, Fibres Text. East. Eur. 6 (2013) 45–49. [18] G.W. Beckermann, K.L. Pickering, Mode I and Mode II interlaminar fracture toughness of composite laminates interleaved with electrospun nanofibre veils, Compos. Part A-Appl S 72 (2015) 11–21 https://doi.org/10.1016/j.compositesa. 2015.01.028. [19] M. Kuwata, P.J. Hogg, Interlaminar toughness of interleaved CFRP using nonwoven veils: Part 2. Mode-II testing, Compos. Part A-Appl S 42 (2011) 1560–1570 https://doi.org/10.1016/j.compositesa.2011.07.017. [20] Z. Lewandowski, A. Ziabicki, L. Jarecki, The nonwovens formation in the meltblown process, Fibres Text. East. Eur. 15 (2007) 77–81. [21] M. Fogel, P. Parlevliet, P. Olivier, E. Dantras, Manufacturing of conductive structural composites through spraying of CNTs/epoxy dispersions on dry carbon fiber plies, Compos. Part A-Appl S 100 (2017) 40–47 https://doi.org/10.1016/j. compositesa.2017.04.020. [22] T.R.P. Pozegic, I. Hamerton, J.V. Anguita, W. Tang, P. Ballocchi, P. Jenkins, et al., Low temperature growth of carbon nanotubes on carbon fibre to create a highly networked fuzzy fibre reinforced composite with superior electrical conductivity, Carbon 74 (2014) 319–328 https://doi.org/10.1016/j.carbon.2014.03.038. [23] R.R. Bonaldi, E. Siores, T. Shah, Characterization of electromagnetic shielding fabrics obtained from carbon nanotube composite coatings, Synth. Met. 187 (2014) 1–8 https://doi.org/10.1016/j.synthmet.2013.10.003. [24] S. Mazinani, A. Ajji, C. Dubois, Morphology, structure and properties of conductive PS/CNT nanocomposite electrospun mat, Polymer 50 (2009) 3329–3342 https:// doi.org/10.1016/j.polymer.2009.04.070. [25] P. Latko-Durałek, J. Macutkievic, Ch. Kay, A. Boczkowska, T. McNally, Hot-melt adhesives based on co-polyamide and multi-walled carbon nanotubes, J. Appl. Polym. Sci. 135 (2017) 1–15 https://doi.org/10.1002/app.45999. [26] P. Latko, A. Boczkowska, Fabrication of carbon nanotubes-doped veils, in:
[27] [28] [29]
[30] [31]
[32] [33]
[34] [35]
[36] [37]
117
P.Ch. Wölcken, M. Papadopoulos (Eds.), Smart Intelligent Aircraft Structures (SARISTU), Springer, 2015, pp. 815–824 https://doi.org/10.1007/978-3-31922413-8_43. G.R. Kasaliwal, A. Göldel, P. Pötschke, G. Heinrich, Influences of polymer matrix melt viscosity and molecular weight on MWCNT agglomerate dispersion, Polymer 52 (2011) 1027–1036 https://doi.org/10.1016/j.polymer.2011.01.007. A. Boczkowska, P. Latko, Sposób wytwarzania włókien i włókniny z nanorurkami węglowymi. Patent nr PL 221848 B1, 2016. M. Yourdkhani, W. Liu, S. Baril-Gosselin, F. Robitaille, P. Hubert, Carbon nanotubereinforced carbon fibre-epoxy composites manufactured by resin film infusion, Compos. Sci. Technol. 166 (2018) 169–175 https://doi.org/10.1016/j.compscitech. 2018.01.006. M. Lisowski, Pomiary Rezystywności I Przenikalności Elektrycznej Dielektryków Stałych, Oficyna Wydawnicza Poltechniki Wrocławskiej, 2004. N. Yamamoto, R. Guzman de Villoria, B.L. Wardle, Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes, Compos. Sci. Technol. 72 (2012) 2009–2015 https://doi.org/10.1016/j.compscitech.2012.09.006. B. Beylergil, M. Tano, E. Akta, Effect of polyamide-6,6 (PA 66) nonwoven veils on the mechanical performance of carbon fiber/epoxy composites, Compos. Struct. 194 (2018) 21–35 https://doi.org/10.1016/j.compstruct.2018.03.097. H. Xu, X. Tong, Y. Zhang, Q. Li, W. Lu, Mechanical and electrical properties of laminated composites containing continuous carbon nanotube film interleaves, Compos. Sci. Technol. 127 (2016) 113–118 https://doi.org/10.1016/j.compscitech. 2016.02.032. S.U. Khan, J.K. Kim, Interlaminar shear properties of CFRP composites with CNFbucky paper interleaves, ICCM International Conferences on Composite Materials, 2011. M. Tehrani, A.Y. Boroujeni, T.B. Hartman, T.P. Haugh, S.W. Case, M.S. Al-Haik, Mechanical characterization and impact damage assessment of a woven carbon fiber reinforced carbon nanotube–epoxy composite, Compos. Sci. Technol. 75 (2013) 42–48 https://doi.org/10.1016/j.compscitech.2012.12.005. R. Greco, L. Nicolais, Glass transition temperature in nylons, Polymer 17 (1976) 1049–1053 https://doi.org/10.1016/0032-3861(76)90005-7. N. Zheng, Y. Huang, H.Y. Liu, J. Gao, Y.W. Mai, Improvement of interlaminar fracture toughness in carbon fiber/epoxy composites with carbon nanotubes/ polysulfone interleaves, Compos. Sci. Technol. 140 (2017) 8–15 https://doi.org/10. 1016/j.compscitech.2016.12.017.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Carbon Fiber Reinforced Polymers modified with thermoplastic nonwovens containing multi-walled carbon nanotubes
T
Kamil Dydeka,∗, Paulina Latko-Durałeka, Anna Boczkowskaa, Michał Sałacińskib, Rafał Kozeraa a b
Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Wołoska 141, 02-507, Warsaw, Poland Air Force Institute of Technology, ul. Księcia Bolesława 6, 01-494, Warsaw, Poland
ARTICLE INFO
ABSTRACT
Keywords: A. Carbon nanotubes A. Polymer-matrix composites (PMCs) B. Electrical properties B. Mechanical properties Thermoplastic nonwovens
The main purpose of this work was to improve the electrical conductivity of Carbon Fiber Reinforced Polymers (CFRP) by implementing novel thermoplastic nonwovens doped with carbon nanotubes. For this, two types of nonwovens containing carbon nanotubes were produced by the extrusion and thermal pressing of fibers. Nonwovens were placed between each layer of prepregs and CFRPs were fabricated using an out-of-autoclave method. It was found that implementation of nonwovens with 7 wt% of multi-walled carbon nanotubes resulted in improved surface and volume electrical conductivity in all directions. Microstructure analysis revealed the good quality of the produced laminates and the random distribution of the nonwovens in the composite panels. Examination of loss and storage moduli by dynamic mechanical analysis showed the higher flexibility of the laminates and the appearance of an additional glass transition peak due to the presence of copolyamide in the nonwovens used.
1. Introduction Nowadays, changes in the design and type of materials used in aircraft structures occur very quickly. There is a noticeable tendency to use very stiff materials that are as light as possible in order to reduce the whole weight of aircraft and thus increase the weight of the transported load, and to reduce emissions of carbon dioxide and other harmful substances [1]. For these reasons, the use of polymer matrix composites reinforced with carbon fibers with high specific stiffness and strength is still growing. The best example is the Boeing 787 Dreamliner, in which the mass fraction of composites applied in the wings, fuselage and tail is about 50%, while the volume fraction is almost 80% [2]. However, CFRPs are much less electrically conductive than metals, therefore the lighting strike protection of these aircraft against catastrophic structural damage, hazardous electrical shocks to passengers, and loss of flight control is not ensured because the current from a lightning strike cannot find conductive paths. In this case the protection of composite structures carrying fuel becomes even more crucial, as fuel vapors could ignite due to electrical sparking from lightning strikes [3,4]. Moreover, composite materials with low electrical conductivity cannot shield electronic parts from electromagnetic interference because they do not work as a Faraday Cage, in which electromagnetic waves are prevented from entering any closed surface bounded by conductive materials [5]. ∗
In order to increase resistance and protection from lightning strikes, electrically conductive meshes or tapes are implemented in the outer composite layer of aircraft. They are made of copper, aluminum or nickel, which not only add more weight, but their production cost is also too high [6]. Metallic meshes can be replaced by electrically conductive fibers or woven/nonwoven fabrics that consist of polymer coated with copper, silver or nickel particles [7]. However, they still have high weight, low flexibility, and contain a chemical binder in the structure which can be released during the CFRP manufacturing process. Hence, the new approaches include improved electrical conductivity of CFRP without increasing its weight, whilst still maintaining high mechanical properties. One idea is to incorporate carbon nanotubes (CNTs) due to their low density and outstanding mechanical and electrical properties [8]. There are different ways of introducing CNTs into CFRPs: the first involves mixing epoxy resin with CNTs for infusion [9]; however, the most promising approach is the application of semi-finished products as this eliminates the usage of powder CNTs, which are harmful. One example is buckypaper, which is a thin film produced by filtering a highly concentrated suspension of dispersed CNTs or by gas-phase catalytic growth [2]. The electrical conductivity of buckypaper is 103–105 S/m and the density is 0.25–0.83 g/cm3. However, the main disadvantage is their brittleness, which causes technical problems and poor infiltration during the CFRP fabrication process [10–14]. Another
Corresponding author. E-mail address: [email protected] (K. Dydek).
https://doi.org/10.1016/j.compscitech.2019.02.007 Received 20 November 2018; Received in revised form 28 January 2019; Accepted 6 February 2019 Available online 08 February 2019 0266-3538/ © 2019 Elsevier Ltd. All rights reserved.
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
solution described in the literature is the usage of prepregs with CNTs deposited on their surface [15]. A new idea developed by our team is the application of nonwovens made of thermoplastic polymers doped with CNTs [16,17]. Commercial thermoplastic nonwovens made of different polymers do not contain any conductive particles and are used as interlayers or surface finishing in CFRPs to increase their fracture toughness and prevent delamination or cracks [18,19]. Hence, implementation of semi-finished products containing thermoplastic polymer and conductive nanoparticles can increase not only the mechanical properties of CFRPs, but also their electrical conductivity. Nonwovens containing CNTs can be produced with various methods: the melt-blown method [20], by pressing thermoplastic fibers containing CNTs [16], by spraying CNTs on dry carbon fibers [21], by growing CNTs on carbon fibers [22], by coating polymeric fabrics with thin layer of CNTs [23], or by electrospinning [24]. In this paper, thermoplastic nonwovens containing multi-walled carbon nanotubes (MWCNTs) were fabricated from extruded fibers by thermal pressing. They were interleaved between each layer of the commercial prepregs with the out-of-autoclave method of CFRP fabrication. The main goal of this work is to show the effect on the electrical and mechanical properties of CFRPs of two different types of thermoplastic nonwovens based on copolyamides. The selected copolyamides differed in melt viscosity and melting point, which can affect the creation of conductive paths by carbon nanotubes between carbon fabrics and consequently affect the electrical conductivity of CFRPs.
agglomerates of MWCNTs, compared with coPA1566, which has lower viscosity [27]. Finally, the presence of larger agglomerates made the extrusion of fibers more difficult and therefore thicker fibers for coPA1566+7 wt% MWCNTs were obtained. In the next step, the extruded fibers were cut into 70 mm-long pieces that were then randomly deposited on a special carrier and pressed on both sides using a thermomechanical press (LEMUR) according to patent PL 221848 B1 [28]. The areal weight of each nonwoven was 15 g/ m2. Images of both types of produced nonwovens are shown in Fig. 1c and d. After pressing, the overall thickness of the nonwovens was lower than for fibers: 76 ± 9.0 μm for nonwoven based on coPA1330 and 109 ± 13.0 μm for coPA1566. After pressing, the fibers had elliptical cross sections. 2.2. Laminate manufacturing Two types of carbon-epoxy unidirectional prepregs were used: TC E722-02 (TenCate Advanced Composites, UK) and HexPly® NCHR 913 (Hexcel, USA). Both of them contained 12 k of carbon fibers and 35% weight fraction of epoxy resin. The areal weight of both prepregs was 130 g/m2. Six types of laminates were manufactured using the out-ofautoclave method (Table 1). Each panel was fabricated from 14 prepreg layers. Nonwovens with 7 wt%MWCNTs were placed between each layer of prepreg. Reference panels (1T and 4H) were fabricated without nonwovens. After lay-up, the laminates were degassed, vacuum bagged on a glass plate, consolidated at 75 °C for 30 min, and then consolidated again at 170 °C for 120 min under vacuum. The heating speed and cooling rate were 3 °C/min in each case. After curing, the composite panels were removed from the vacuum bag, then trimmed and cut into appropriate test specimen dimensions. The thickness of the laminates and the weight fraction of carbon fibers are included in Table 1.
2. Materials and methods 2.1. Nonwovens fabrication Two types of thermoplastic copolyamides (trade names: Griltex®1330A (coPA1330) and Griltex®1566 (coPA1566) supplied by EMS Griltech (Switzerland) were used as a polymer matrix to produce nonwovens. The selected polymers belong to the group of hot melt adhesives, which possess sticky properties when melted and have good compatibility with epoxy resin. They consist of polyamide 6 and 66 segments, whose melting points are much lower than for typical polyamides and occur at 125–135 °C for coPA1330 and at 115–125 °C for coPA1566. They have different melt viscosities of 1200 Pa s and 800 Pa s for coPA 1330 and coPA 1566, respectively. Both copolyamides were mixed with 7 wt% MWCNTs (trade name NC7000, Nanocyl) using an industrial twin-screw extruder machine by Nanocyl, Belgium. The calculated electrical percolation threshold for coPA1330 was found to be 2.0–3.0 wt%MWCNTs and for coPA1566 was almost 1.0 wt% MWCNT [25]. In the presented studies a much higher CNT content was used (7 wt% MWCNTs) since it resulted in greater improvement of electrical conductivity as shown in our previous results [26]. Subsequently, pellets of coPA1330+7 wt% MWCNTs and coPA1566+7 wt% MWCNTs were processed directly into fibers using a laboratory twinscrew extruder machine (HAAKE MiniLab, ThermoScientific, Germany) equipped with a circle nozzle. For both materials, the extrusion temperature was adjusted to 180 °C and the screw speed was set at 40 rpm. For coPA1330+7 wt% MWCNTs, the average diameter of the extruded fibers (measured with SEM images) was 100.6 ± 12.04 μm; for coPA1566+7 wt%MWCNTs fibers the diameter was 294.7 ± 29.13 μm. The differences in the obtained thicknesses of the nanocomposite fibers are related to the size of MWCNT agglomerates in the pellets and the effect of the viscosity of the copolyamides. As presented in Fig. 1a and b, a masterbatch of coPA1330+7 wt%MWCNTs contains far fewer and smaller MWCNT agglomerates, which are dispersed homogenously (images taken from a light optical microscope). In turn, for coPA1566+7 wt%MWCNTs, larger agglomerates that were not uniformly distributed were found. Such variances in the dispersion and distribution of MWCNTs are caused by the viscosity of the coPAs used. Mixing the more viscous coPA1330 with MWCNTs introduces higher shear forces that result in more effective breaking up of primary
2.3. Methods Electrical volume conductivity of the fabricated CFRP was measured in three directions: along the carbon fibers (X-direction), perpendicular to the carbon fibers (Y-direction) and through the laminate thickness (Z-direction). Samples for the test with dimensions 60 mm × 10 mm (for measurements in X and Y directions) and 15 mm × 15 mm (for measurements in Z direction) were cut from different sections of the composite panels. The volume electrical conductivity was measured using the Keithley 6221/2182A device equipped with a measuring stand with copper electrodes. Good contact between electrodes and sample was maintained by the application of silver paste. The electrical surface conductivity was measured for samples with dimensions of 75 mm × 75 mm, which were also cut from different sections of the laminates. The test was carried out according to ASTM D-257 using the 6517B Electrometer/High Resistance Meter equipped with an 8009 Test Fixture. Each sample was placed between concentric stainless-steel ring electrodes with a conductive rubber pad which maintained perfect contact between the sample and electrodes. The voltage applied was 1 V, time of the test was 15 s and the number of readings was set to 5. The diameters of the extruded fibers were determined using a Scanning Electron Microscope (SEM, TM 3000 Hitachi, Japan) with applied voltage 5 kV. The observations of the microstructure and quality of the produced CFRP laminates were performed using SEM (TM 3000 Hitachi, Japan). In this case the applied voltage was 15 kV. SEM was also used to analyze the propagation of cracks in the laminate after short-beam shear tests. The dispersion and distribution of MWCNTs in the final composite panels were examined with a high-resolution SEM SU70 Hitachi, Japan. The influence of thermoplastic nonwovens on the glass transition temperature of epoxy was analyzed by Dynamic Mechanical Analysis (DMA). The test was performed using a DMA Q800 (TA Instruments, USA) in dual cantilever mode according to ASTM D7028-07 from 0 °C to 111
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 1. Materials based on coPA1330+7 wt%MWCNTs (a, c) and coPA1566+7 wt%MWCNTs (b, d): light optical images of the initial pellets (a, b), structure of thermoplastic nonwovens (c, d).
220 °C with a heating rate of 2 °C/min at a frequency of 1 Hz and with an amplitude of 30 μm. Samples for the test were 60 mm long and 10 mm wide. The mechanical properties of the CFRP composites were measured in a short-beam shear test (according to ASTM D2344) using an MTS Q/ Test 10 universal testing machine. Six specimens (25 mm length, 6.4 mm width) were cut from different sections of each composite panel along the carbon fibers. The test was performed at room temperature at a speed of 1 mm/min.
the Z-direction (Fig. 2a), the improvement of the electrical conductivity of laminates fabricated from TenCate prepreg was about 92% and 84% for interleaves made of coPA1330+7 wt%MWCNTs and coPA1566+7 wt%MWCNTs nonwovens, respectively. In the case of laminates based on Hexcel prepreg, this increase was a little higher (about 106%) for panels interleaved with coPA1330+7 wt%MWCNTs nonwovens (5H), but was significantly improved (about 227%) (3.14 S/ m) when the nonwovens were made of coPA1566+7 wt%MWCNTs (6H). In both cases the increase of the conductivity through the thickness of the laminates was caused by the presence of conductive paths formed by carbon nanotubes introduced to the composite structure with the thermoplastic nonwovens [29]. Because the electrical conductivity along the carbon fibers (Fig. 2b) was much higher than through the laminate thickness, the observed changes of electrical conductivity in the X-direction are less than in the Z-direction. Application of thermoplastic nonwovens between the TenCate layers increased the electrical conductivity in the X-direction by about 85% for coPA1330+7 wt %MWCNTs and 67% for coPA1566+7 wt%MWCNT nonwovens. In turn for Hexcel-based composites, the improvement was lower than 10% for both types of thermoplastic nonwovens. Finally, electrical volume conductivity measured perpendicular to the carbon fibers (Y-
3. Results 3.1. Electrical conductivity The results of the electrical volume conductivity of the manufactured CFRP panels in Z, X and Y directions are shown in Fig. 2a–c, respectively. It is clearly seen that implementation of both types of nonwovens containing 7 wt% MWCNTs between prepreg layers resulted in an increase of electrical conductivity in all directions. However, dependent on the type of nonwovens used (based on coPA1330 or coPA1566), the increase of electrical conductivity was not the same. In Table 1 Characteristics of the manufactured laminates. Material
Laminate designation
Thickness [mm]
Carbon fibers' weight fraction [%]
TenCate reference TenCate + coPA1330+7 wt%MWCNT TenCate + coPA1566+7 wt%MWCNT Hexcel reference Hexcel + coPA1330+7 wt%MWCNT Hexcel + coPA1566+7 wt%MWCNT
1T 2T 3T 4H 5H 6H
1.93 2.03 2.11 1.89 2.00 2.07
66.6 63.2 62.8 66.9 64.4 63.3
112
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 2. Electrical conductivity for CFRP panels: a) dz direction, b) dx direction, c) dy direction (throughout the thickness), d) surface.
direction, Fig. 2c) was improved for all types of composite panels and was the highest for laminate 2T (90%), then for 3T (54%), 6H (43%) and 5H (25%). Although this improvement was better for TenCate based panels, CFRP laminates made of Hexcel prepreg showed higher electrical conductivity in all directions than those made of TenCate, as shown in Fig. 2. Similarly to the volume electrical conductivity, the implementation of thermoplastic nonwovens containing 7 wt%MWCNTs also caused an increase in the electrical surface conductivity (Fig. 2d) because the electrical surface conductivity was affected by the current flowing partially in the sample volume [30]. For laminates based on TenCate, the increase in the electrical surface conductivity was 237% for nonwovens made of coPA1330+7 wt%MWCNTs and 306% for panels in which nonwovens were made of coPA1566+7 wt%MWCNTs. A slightly lower improvement of electrical surface conductivity was achieved for CFRP panels based on Hexcel prepreg. For laminate 5H interleaved with coPA1330+7 wt%MWCNTs nonwovens, the increase was about 115%; in turn, for panel 6H containing coPA1566+7 wt%MWCNTs nonwovens, it was only about 90%. Again, composite panels manufactured from Hexcel prepregs show higher overall electrical surface conductivity than those based on TenCate.
fiber layers are also visible; they are randomly distributed because the fibers from the nonwovens are also distributed arbitrarily. The thermoplastic bridges are rich in carbon nanotubes, which makes the creation of conductive paths possible. However, it should be noted that the CNT-doped thermoplastic bridges are not uniformly distributed, which may lead to discrepancies in the electrical conductivity measured on specimens taken from different sections of the same CFRP panel. In order to explain the increase of electrical conductivity as the result of implementation of thermoplastic nonwovens containing MWCNTs between the prepreg layers, the microstructure observations were also carried out using high-resolution SEM. For the analysis, the images were collected for CFRP panels based on Hexcel prepreg because their electrical conductivity was the highest. Fig. 4a and b shows the interface boundary between the thermoplastic nonwoven doped with CNT and carbon fibers in which nonwovens seem to adhere very well to the carbon fibers without any visible delamination or debonding. This is due to the sticky properties of the copolyamides used. Fig. 4a and b also confirm that the MWCNTs are well dispersed and randomly distributed in the thermoplastic nonwoven. The combination of good dispersion of MWCNTs and good adhesion between carbon fibers and nonwovens is responsible for the improvement of electrical conductivity, especially through the laminate thickness [31].
3.2. Microstructure
3.3. Mechanical properties
The microstructure images of the manufactured CFRP panels are presented in Fig. 3, with visible layers of carbon fibers and implemented nonwovens. From the cross-section of the reference laminates based on Hexcel prepreg (Fig. 3b), fewer voids than in the TenCate reference panel can be observed (Fig. 3a). Hence, the reference panel based on Hexcel prepreg showed higher electrical conductivity. The addition of both types of thermoplastic nonwovens (Fig. 3c–f) between the prepreg layers contributed to reducing the number of voids compared to the reference laminates. The far fewer voids in the panels manufactured with Hexcel prepreg led to increased electrical conductivity. In these images, thermoplastic bridges between the carbon
In order to determine the interlaminar shear strength (ILSS) of the fabricated laminates, a short-beam strength test was performed. It was expected that thermoplastic nonwovens would increase the CFRPs' resistance against delamination [32,33]. In the case of the laminates based on TenCate prepreg, nonwovens caused a decrease in the ILSS from 67.0 ± 1.14 MPa (panel 1T) to 58.6 ± 3.78 MPa (panel 2T) for nonwovens made of coPA1330+7 wt%MWCNT and to 66.8 ± 3.55 MPa (3T) for laminates interleaved with coPA1566+7 wt %MWCNT nonwovens. The opposite situation was observed for the laminates manufactured with Hexcel prepreg. Here, implementation of 113
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 3. Microstructure observations of laminates: a) TenCate reference (1T), b) Hexcel reference (4H) c) TenCate + coPA1330+7 wt%MWCNTs (2T), d) Hexcel + coPA1330+7 wt%MWCNTs (5H), e) TenCate + coPA1566+7 wt%MWCNTs (3T), f) Hexcel + coPA1566+7 wt%MWCNTs (6H).
thermoplastic nonwovens increased the ILSS of the laminates, but only at a negligible level. The interlaminar shear strength of the reference laminate based on Hexcel prepreg (4H) reached 86.4 ± 2.04 MPa; this is an increase of about 2.4 MPa and 1 MPa for laminate 5H and 6H, respectively. In this case the use of a hot melt adhesives doped with MWCNTs caused crack bridging [34,35]. Moreover, the presence of thermoplastic polymer in the laminates led to a different type of cracking of the samples. In the case of the reference laminates (1T and 4H), after reaching the maximum force the samples broke in a brittle way, whereas after reaching the maximum force the laminates with the addition of thermoplastic nonwovens deformed plastically. Finally, one distinct crack was observed for the reference laminate (Fig. 5a), and several smaller cracks were observed for laminates modified with thermoplastic nonwovens (Fig. 5b). In addition, observations were made from the top of the sample surface. In the case of the reference laminate (4H), carbon fibers that were pulled out of the polymer matrix were observed (Fig. 5c), but this phenomenon was not observed for laminates modified with thermoplastic nonwovens (Fig. 5d). In addition, the cracks were smaller in the modified laminate than in the reference laminate. This explains the higher strength of laminates containing thermoplastic nonwovens with CNT. The reason that the laminates produced from two different prepregs displayed different mechanical properties could be also associated with the surface preparation of the carbon fibers used (i.e. sizing was applied). The lack of
improved ILSS in laminates produced using TenCate prepreg could be associated with the worse adhesion between the thermoplastic nonwoven and the carbon fibers. In Fig. 6a, debonding between CNT-doped copolyamide and carbon fiber is visible, while in the case of Hexcel prepreg the adhesion seems to be much better, as shown in Fig. 6b. 3.4. Dynamic mechanical analysis DMA results are presented in Fig. 7. It can be seen from the loss modulus results (Fig. 7b) that there is one peak at the curves of the reference panels. For laminate 1T (TenCate prepreg) it occurs at 126.9 °C and for laminate 4H (Hexcel prepreg) it occurs at 177.7 °C, which is related to the glass transition temperature of the epoxy resin. In the case of laminates containing thermoplastic nonwovens, an additional peak appeared at about 39–42 °C; this comes from segments of polyamide 6 which are present in the copolyamide macromolecules used for fabrication of the nonwovens [36]. However, incorporation of thermoplastic nonwovens based on copolyamides with 7 wt% MWCNTs did not affect the glass transition temperature of the epoxy resin [37]. CFRP panels interleaved with thermoplastic nonwovens also show changes in the storage modulus values, as presented in Fig. 7a. For all laminates, the storage modulus decreases in comparison to the reference panels due to the presence of thermoplastic polymer, which has lower stiffness than epoxy.
Fig. 4. Microstructure observation of CFRP: a) laminate 5H, magnification 50 k; b) laminate 6H, magnification 50 k. 114
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 5. Microstructure observations after ILSS tests: a) panel 4H; b) panel 5H; c) image from the top of sample 4H; d) image from the top of sample 5H.
Fig. 6. Adhesion between the melted nonwoven and the prepreg layer: a) laminate 3T; b) laminate 6H.
4. Conclusions
has a higher melting point and viscosity, leads to greater improvement of electrical conductivity due to better dispersion of CNTs caused by higher shear forces during the extrusion of thermoplastic fibers. The good adhesion between nonwovens and carbon fibers in the case of Hexcel prepreg was found to be the consequence of the usage of copolyamides from the group of hot melt adhesives. Implementation of thermoplastic nonwovens into CFRP increases the flexibility of the CFRP, as is confirmed by the lower storage modulus shown by the DMA test. On the other hand, the presence of thermoplastic copolyamide in the laminates does not affect the glass transition temperature of the epoxy resin, but the additional peak in the loss modulus curve corresponds to the glass transition temperature of polyamide 6. The analysis of the mechanical properties in terms of the interlaminar shear strength showed that – dependent on the type of prepreg used– the implementation of nonwovens decreased (for TenCate based panels) or slightly increased their ILSS (for Hexcel based panels). This may be caused by their different adhesion to carbon fibers due to the different sizing methods applied to the carbon fibers. In the case of TenCate prepreg, debonding between CNT-doped copolyamide and carbon fibers was observed, which caused poor adhesion. The opposite observations were made for Hexcel-based panels. Moreover, laminates interleaved with nonwovens containing MWCNTs exhibit smaller cracks than the
In this work the effect of thermoplastic nonwovens containing 7 wt % MWCNTs on the electrical, thermal and mechanical properties of CFRPs was investigated. Two types of nonwovens based on two commercial copolyamides were fabricated by pressing the extruded CNTdoped fibers in laboratory conditions. In the next step, nonwovens were implemented between the layers of the two commercial prepregs using an out-of-autoclave technique. The application of CNT-doped nonwovens did not affect the CFRP fabrication process. Moreover, they are easy to handle and safe for workers. It was found that electrical volume conductivity as well as electrical surface conductivity were improved when compared to the reference panels. In the case of electrical volume conductivity, the highest improvement was about 227% (3.14 S/m) through the laminate thickness for panels based on Hexcel prepreg and coPA1566+7 wt%MWCNTs nonwovens. In the case of electrical conductivity measured along and perpendicular to the carbon fibers, the improvement was less than 50%. Laminates based on TenCate prepreg show lower electrical conductivity in comparison to laminates based on Hexcel prepreg. The obtained results confirmed that thermoplastic nonwovens doped with carbon nanotubes create conductive bridges between carbon fiber layers. The nonwoven based on coPA1330, which 115
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al.
Fig. 7. Dynamic mechanical analysis of laminates: a) storage modulus and b) loss modulus.
reference panel. Also, the top view of the specimens after ILSS tests confirmed a different cracking mechanism when the reinforcement fibers were bonded by thermoplastic polymer. The GSM of thermoplastic nonwovens containing carbon nanotubes should be further adjusted, taking into account the improvement of both the electrical conductivity and mechanical properties of CFRP.
[5] M. Gagné, D. Therriault, Lightning strike protection of composites, Prog. Aero. Sci. 64 (2014) 1–16 https://doi.org/10.1016/j.paerosci.2013.07.002. [6] X. Ma, F. Scarpa, H.X. Peng, G. Allegri, J. Yuan, R. Ciobanu, Design of a hybrid carbon fibre/carbon nanotube composite for enhanced lightning strike resistance, Aerosp. Sci. Technol. 47 (2015) 367–377 https://doi.org/10.1016/j.ast.2015.10. 002. [7] F.L. Zhou, R.H. Gong, I. Porat, Nano-coated hybrid yarns using electrospinning, Surf. Coat. Technol. 204 (2010) 3459–3463 https://doi.org/10.1016/j.surfcoat. 2010.04.021. [8] O. Gohardani, M.C. Elola, C. Elizetxea, Potential and prospective implementation of carbon nanotubes on next generation aircraft and space vehicles: a review of current and expected applications in aerospace sciences, Prog. Aero. Sci. 70 (2014) 42–68 https://doi.org/10.1016/j.paerosci.2014.05.002. [9] D. Quan, Enhancing mode-I and mode-II fracture toughness of epoxy and carbon fibre reinforced epoxy composites using multi-walled carbon nanotubes, Mater. Des. 143 (2018) 81–92 https://doi:10.1016/j.matdes.2018.01.051. [10] I.W.P. Chen, R. Liang, H.B. Zhao, B. Wang, C. Zhang, Highly conductive carbon nanotube buckypapers with improved doping stability via conjugational crosslinking, Nanotechnology 22 (2011) 7 https://doi.org/10.1088/0957-4484/22/48/ 485708. [11] W. Ma, L. Song, R. Yang, T. Zhang, Y. Zhao, L. Sun, et al., Directly synthesized strong, highly conducting, transparent single walled carbon nanotube films, Nano Lett. 7 (2007) 2307–2311 https://doi.org/10.1021/nl070915c. [12] D. Wang, P.C. Song, C.H. Liu, W. Wu, S.S. Fan, Highly oriented carbon nanotube papers made of aligned carbon nanotubes, Nanotechnology 19 (2008) 6 https://doi. org/10.1088/0957-4484/19/7/075609. [13] J. Zhang, D. Jiang, H.X. Peng, F. Qin, Enhanced mechanical and electrical properties of carbon nanotube buckypaper by in situ cross-linking, Carbon 63 (2013) 125–132 https://doi.org/10.1016/j.carbon.2013.06.047.
Acknowledgments The studies were carried out within grant no. DOB-1-3/1/PS/2014, financed by the National Center for Research and Development. References [1] P. Feraboli, M. Miller, Damage resistance and tolerance of carbon/epoxy composite coupons subjected to simulated lightning strike, Compos. Part A-Appl S 40 (2009) 954–967 https://doi.org/10.1016/j.compositesa.2009.04.025. [2] E.J. Siochi, J.S. Harrison, Structural nanocomposites for aerospace applications, Mater. Res. Soc. 40 (2015) 829–835 https://doi.org/10.1557/mrs.2015.228. [3] Y. Hirano, S. Katsumata, Y. Iwahori, A. Todoroki, Artificial lightning testing on graphite/epoxy composite laminate, Compos. Part A-Appl S 41 (2010) 1461–1470 https://doi.org/10.1016/j.compositesa.2010.06.008. [4] J. Gou, Y. Tang, F. Liang, Z. Zhao, D. Firsich, J. Fielding, Carbon nanofibre paper for lightning strike protection of composite materials, Compos. Part B Eng. 41 (2009) 192–198 https://doi.org/10.1016/j.compositesb.2009.06.009.
116
Composites Science and Technology 173 (2019) 110–117
K. Dydek, et al. [14] H. Chu, Z. Zhang, Y. Liu, J. Leng, Self-heating fibre reinforced polymer composite using meso/macropore carbon nanotube paper and its application in deicing, Carbon 66 (2014) 154–163 https://doi.org/10.1016/j.carbon.2013.08.053. [15] Y. Akcin, S. Karakaya, O. Soykasap, Electrical, thermal and mechanical properties of CNT treated prepreg CFRP composites, Mater. Sci. Appl. 7 (2016) 465–483 https:// doi.org/10.4236/msa.2016.79041. [16] P. Latko, W. Rumiński, A. Boczkowska, Carbon-nanotubes doped veils, Compos. Struct. 134 (2015) 52–59 https://doi.org/10.1016/j.compstruct.2015.07.115. [17] P. Latko, R. Kozera, A. Salinier, A. Boczkowska, Non-woven veils manufactured from polyamides doped with carbon nanotubes, Fibres Text. East. Eur. 6 (2013) 45–49. [18] G.W. Beckermann, K.L. Pickering, Mode I and Mode II interlaminar fracture toughness of composite laminates interleaved with electrospun nanofibre veils, Compos. Part A-Appl S 72 (2015) 11–21 https://doi.org/10.1016/j.compositesa. 2015.01.028. [19] M. Kuwata, P.J. Hogg, Interlaminar toughness of interleaved CFRP using nonwoven veils: Part 2. Mode-II testing, Compos. Part A-Appl S 42 (2011) 1560–1570 https://doi.org/10.1016/j.compositesa.2011.07.017. [20] Z. Lewandowski, A. Ziabicki, L. Jarecki, The nonwovens formation in the meltblown process, Fibres Text. East. Eur. 15 (2007) 77–81. [21] M. Fogel, P. Parlevliet, P. Olivier, E. Dantras, Manufacturing of conductive structural composites through spraying of CNTs/epoxy dispersions on dry carbon fiber plies, Compos. Part A-Appl S 100 (2017) 40–47 https://doi.org/10.1016/j. compositesa.2017.04.020. [22] T.R.P. Pozegic, I. Hamerton, J.V. Anguita, W. Tang, P. Ballocchi, P. Jenkins, et al., Low temperature growth of carbon nanotubes on carbon fibre to create a highly networked fuzzy fibre reinforced composite with superior electrical conductivity, Carbon 74 (2014) 319–328 https://doi.org/10.1016/j.carbon.2014.03.038. [23] R.R. Bonaldi, E. Siores, T. Shah, Characterization of electromagnetic shielding fabrics obtained from carbon nanotube composite coatings, Synth. Met. 187 (2014) 1–8 https://doi.org/10.1016/j.synthmet.2013.10.003. [24] S. Mazinani, A. Ajji, C. Dubois, Morphology, structure and properties of conductive PS/CNT nanocomposite electrospun mat, Polymer 50 (2009) 3329–3342 https:// doi.org/10.1016/j.polymer.2009.04.070. [25] P. Latko-Durałek, J. Macutkievic, Ch. Kay, A. Boczkowska, T. McNally, Hot-melt adhesives based on co-polyamide and multi-walled carbon nanotubes, J. Appl. Polym. Sci. 135 (2017) 1–15 https://doi.org/10.1002/app.45999. [26] P. Latko, A. Boczkowska, Fabrication of carbon nanotubes-doped veils, in:
[27] [28] [29]
[30] [31]
[32] [33]
[34] [35]
[36] [37]
117
P.Ch. Wölcken, M. Papadopoulos (Eds.), Smart Intelligent Aircraft Structures (SARISTU), Springer, 2015, pp. 815–824 https://doi.org/10.1007/978-3-31922413-8_43. G.R. Kasaliwal, A. Göldel, P. Pötschke, G. Heinrich, Influences of polymer matrix melt viscosity and molecular weight on MWCNT agglomerate dispersion, Polymer 52 (2011) 1027–1036 https://doi.org/10.1016/j.polymer.2011.01.007. A. Boczkowska, P. Latko, Sposób wytwarzania włókien i włókniny z nanorurkami węglowymi. Patent nr PL 221848 B1, 2016. M. Yourdkhani, W. Liu, S. Baril-Gosselin, F. Robitaille, P. Hubert, Carbon nanotubereinforced carbon fibre-epoxy composites manufactured by resin film infusion, Compos. Sci. Technol. 166 (2018) 169–175 https://doi.org/10.1016/j.compscitech. 2018.01.006. M. Lisowski, Pomiary Rezystywności I Przenikalności Elektrycznej Dielektryków Stałych, Oficyna Wydawnicza Poltechniki Wrocławskiej, 2004. N. Yamamoto, R. Guzman de Villoria, B.L. Wardle, Electrical and thermal property enhancement of fiber-reinforced polymer laminate composites through controlled implementation of multi-walled carbon nanotubes, Compos. Sci. Technol. 72 (2012) 2009–2015 https://doi.org/10.1016/j.compscitech.2012.09.006. B. Beylergil, M. Tano, E. Akta, Effect of polyamide-6,6 (PA 66) nonwoven veils on the mechanical performance of carbon fiber/epoxy composites, Compos. Struct. 194 (2018) 21–35 https://doi.org/10.1016/j.compstruct.2018.03.097. H. Xu, X. Tong, Y. Zhang, Q. Li, W. Lu, Mechanical and electrical properties of laminated composites containing continuous carbon nanotube film interleaves, Compos. Sci. Technol. 127 (2016) 113–118 https://doi.org/10.1016/j.compscitech. 2016.02.032. S.U. Khan, J.K. Kim, Interlaminar shear properties of CFRP composites with CNFbucky paper interleaves, ICCM International Conferences on Composite Materials, 2011. M. Tehrani, A.Y. Boroujeni, T.B. Hartman, T.P. Haugh, S.W. Case, M.S. Al-Haik, Mechanical characterization and impact damage assessment of a woven carbon fiber reinforced carbon nanotube–epoxy composite, Compos. Sci. Technol. 75 (2013) 42–48 https://doi.org/10.1016/j.compscitech.2012.12.005. R. Greco, L. Nicolais, Glass transition temperature in nylons, Polymer 17 (1976) 1049–1053 https://doi.org/10.1016/0032-3861(76)90005-7. N. Zheng, Y. Huang, H.Y. Liu, J. Gao, Y.W. Mai, Improvement of interlaminar fracture toughness in carbon fiber/epoxy composites with carbon nanotubes/ polysulfone interleaves, Compos. Sci. Technol. 140 (2017) 8–15 https://doi.org/10. 1016/j.compscitech.2016.12.017.