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carbon fiber composites
Composites Part B 160 (2019) 436–445
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
The effects of plasma surface treatment on the mechanical properties of polycarbonate/carbon nanotube/carbon fiber composites
T
Beom-Gon Choa, Sang-Ha Hwangb, Miseon Parkc, Jong Kyoo Parkc, Young-Bin Parka,∗, Han Gi Chaeb,∗∗ a Department of Mechanical and Aerospace Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea b School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea c The 4th R&D Institute, Agency for Defense Development, Yuseong P.O. Box 35, Daejeon, 34186, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon fibres Carbon nanotubes Polymer-matrix composites (PMCs) Surface treatments Mechanical properties
The effects of plasma surface treatment on the mechanical properties of multiscale hybrid composites consisting of polycarbonate (PC), carbon nanotube (CNT) and carbon fiber (CF) were investigated. Dynamic mechanical properties and impact energy absorption of multiscale hybrid composites fabricated under various processing conditions were measured and correlated with surface roughness and surface functionality. The highest roomtemperature storage modulus (E’) of 39 GPa and absorbed impact energy of 5.3 J were obtained from the plasmatreated PC/CNT/CF composite, which are increases by 387% and 194%, respectively, as compared to the neat PC/CF composite. Although the plasma treatment was more efficient for the PC/CF composites rather than the PC/CNT/CF composites, hybridization of CNT showed synergistic effects on the enhancement of mechanical properties due to the combination of increased surface roughness and functionality as well as the bridged modulus gap between PC and CF.
1. Introduction Carbon fiber reinforced plastics (CFRP) have drawn significant attention in the field of structural applications such as aerospace, automotive, marine, and sporting goods due to their excellent performance/ weight ratio [1–3]. Decades of research and development have been made to improve the properties of composite materials [4]. It has been generally accepted that the interfacial characteristics are one of the most critical parameters that determines the bulk composite properties. Many researches have been conducted to address this issue including chemical oxidation [5–8], high-energy beam irradiation [8–12], surface treatment by rare earth [8,13]. Most recently, introduction of carbon nanomaterials such as CNTs and or graphene modified resins opens the new possibility for the tailoring of interfacial characteristics [14–17]. One of the routes to hybridize CNT in CFRP has been explored by Thostenson et al. [18] and Kepple et al. [19]. Their approaches are to grow CNTs on the surface of carbon fibers, then process these “fuzzy fibers” with epoxy resin, exhibiting dramatically improved interfacial bond strength. The enhanced Mode II fracture behavior and interfacial shear strength (IFSS) of the composite laminates were also studied by
∗
Karapappas et al. [20] and Gojny et al. [21] respectively. One of the most interesting result reported by Frankland et al. [22] is multiscale hybrid micro-nanocomposites based on CNT/carbon fiber/epoxy, suggesting that the presence of CNTs near the surface of carbon fibers can improve the interfacial properties of the composites. Nonetheless, the multiscale hybrid composites still pose several challenges such as CNT dispersion and high viscosity of matrix especially in the case of using thermoplastics as matrix because of their high melt viscosity thereby limited dispersion and processing methods despite of the advantages over thermosets including shorter processing time and recyclability [23–27]. In addition, limited interfacial interaction is considered to limit the potential performance of multiscale hybrid composites [28–30]. Among the several studies including chemical and physical modification of carbon fiber, plasma treatment facilitates mass production capability through continuous process and is known to sustain the fiber intrinsic properties due to relatively low energy exposure [8]. In this study, to address the issues of manufacturing multiscale hybrid composites, such as poor embedding efficiency by high melt viscosity and fiber-matrix interfacial bonding, we adopt solution-based processing of polycarbonate (PC)/CNT/CF multiscale hybrid
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y.-B. Park), [email protected] (H.G. Chae).
∗∗
https://doi.org/10.1016/j.compositesb.2018.12.062 Received 29 August 2018; Received in revised form 13 December 2018; Accepted 14 December 2018 Available online 21 December 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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frequency of 39 Hz. The schematic description of plasma treatment apparatus is shown in Fig. 1 (a). A round nozzle with a diameter of 30 mm traveled perpendicular to the specimen conveying direction at the various speeds of 1, 2 and 6 m/min while the conveyor moved stepwise with a step of 30 mm. Woven carbon fiber without plasma treatment was also used to prepare as a control sample. Multiscale hybrid composites (PC/CNT/CF) were prepared in two steps: (1) ultrasonic-assisted CNT dispersion in chloroform followed by PC dissolution, and (2) impregnation of CNT/PC/chloroform solution into woven CF followed by evaporation of chloroform. The detailed processing steps are as follows. The CNT dispersion was optimized by preliminary study using UV–vis spectroscopy (UV-1800, Shimadzu, Japan) of CNT/PC solution (Fig. S1). To begin with, 10 mg of CNTs were pre-dispersed in 1 L of CHCl3 using a bath sonicator (Branson 5800, Branson Ultrasonic Corp.) for 2 h. Pre-dispersed solution was divided into 50 mL vials each containing 0.5 mg of CNT. The divided solutions were sonicated from 4 h to 40 h to analyze when the colloid stability appears and verify the tendency of absorbance values depending on sonication time varied with a specific wavelength, such as 315, 320, 325, and 330 nm. The absorption spectra of 1 wt. % CNTs in CHCl3 with different sonication time from 4 to 40 h using bath sonicator is shown in Fig. S1. When the sonication time increased from 4 to 40 h, the absorbance values have increasing tendency, suggesting the dispersion of CNT is improved upon prolonged sonication time. In addition, as shown in Fig. S1 (b), absorption intensity was investigated as a function of sonication time at various wavelengths to show at which sonication time, absorption intensity become stable. At various wavelengths, absorption intensity of CNT dispersion was dramatically increased after 8 h of sonication and become stable. This indicates that 8 h of sonication is good enough to obtain highly dispersed CNT solution.
composites. Furthermore, the effect of plasma treatment of CF surface on the mechanical properties of composites is also explored by changing treatment condition. The key mechanisms for interfacial adhesion by plasma treatment are: (1) removal of various contaminants from the surface of CFs, (2) increasing fiber surface roughness, which would provide more surface area for interaction, and (3) improving the chemical interaction between CF and PC by introducing functional groups [8,31]. The multiscale hybrid composite processed by optimal condition (highest CNT dispersion and highest surface functionality of CF without deterioration its property) exhibits nearly 200% improvement in absorbed impact energy as compared to the control composite. 2. Experimental 2.1. Materials Multi-walled carbon nanotubes (MWCNTs) with diameters ranging from 15 to 30 nm and purity higher than 95% were purchased from LG Chem (Korea). Polycarbonate (PC) was purchased from Honam Petrochemical (Korea) and woven carbon fiber (CF) with plain weave texture was purchased from Toray (Japan). Methanol and Chloroform were purchased from SK Chemicals (Korea) and used without further purification. 2.2. Plasma treatment and preparation of multi-scale hybrid composites Prior to preparation of composites, woven carbon fibers were treated using a plasma apparatus (3D i REV, Applied Plasma Inc., Korea) with an exposure power of 800 W. Compressed air was supplied into the plasma apparatus at a pressure of 150 MPa at an operation
Fig. 1. Schematic description of processing multiscale hybrid composites, (a) plasma treatment apparatus for CF surface treatment, (b) vacuum-assisted distillation set-up for CNT/PC/chloroform solution, and (c) fabrication sequence of composite laminate by solution based impregnation followed by hot-pressing. 437
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Fig. 2. The 3D images of CF surface and mean roughness (Ra) with different plasma scan rate: (a) neat CF, (b) 6 m/min, (c) 2 m/min and (d) 1 m/min. Topographical images and Ras shown in the figure only represents morphology within a single fiber not between fibers.
Fig. 3. The SEM images of CFs surface with various plasma scan rate: (a) neat CF, (b) 6 m/min, (c) 2 m/min and (d) 1 m/min.
of solvent was removed by vacuum distillation as illustrated in Fig. 1 (b). 2 layers of woven CFs (100 × 100 mm) were fully wetted by the PC/chloroform or CNT/PC/chloroform solution and dried in the oven at 45 °C for a day. Then, the dried composites were hot pressed at 215 °C for 15 min at a pressure of 10 MPa as shown in Fig. 1 (c).
Although it can be observed that the absorption intensity gradually increased with sonication time, this may be due to the fact that the prolonged sonication can break CNTs and reduce the length of CNTs. Therefore, the sonication time was set to 8 h. The prepared CNT dispersion under optimal condition (8 h sonication) was then added to the separately prepared PC solution (9.9 g/100 mL) and the excess amount
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Fig. 4. The SEM images of PC/CF composites with CNTs incorporation and plasma surface treatment (2 m/min speed): (a) PC/CF; (b) treated PC/CF; (c) PC/CNT/CF and (d) treated PC/CNT/CF.
2.3. Characterization
3. Results and discussion
2.3.1. Surface analysis of woven CF The surface morphology of woven CF was observed using a field emission scanning electron microscope (FE-SEM) (Nanonova 230, FEI, USA). The surface roughness of CF was also measured using a 3D measurement system (NV-3000, Nanosystems, Korea). The scanning area for surface roughness measurement was 120 μm (width) × 90 μm (length). To obtain the statistically meaningful result, 10 different points of a single filament (diameter of CF was 7 μm) and 5 different filament samples were analyzed. The chemical composition of CF surface was also characterized by X-ray Photoelectron Spectroscopy (XPS) (Thermo Fisher Scientific, Inc., USA) using a monochromatic Al Kα Xray source (1486.6 eV). The surface functional groups of CFs were investigated by using ATR-FTIR spectroscopy (Nicolet iS50, Thermo Fisher Scientific, Inc., USA) using Germanium (Ge) as an internal reflection element. The analysis was carried out in transmittance mode over the scanning range of 700−4000 cm−1.
The three-dimensional topographic profile and arithmetic mean roughness of CFs with or without plasma treatments are shown in Fig. 2. The arithmetic mean roughness (Ra) increased from 135 to 169 nm by the plasma treatment with a scan speed of 6 m/min, which may be mainly due to the removal of epoxy sizing layer, since it was observed that the Ra decreased by increasing plasma treatment time (by reducing the scan speed to 2 m/min). However, with the plasma treatment speed of 1 m/min, the Ra again increased by 360 and 343%, respectively, from those of control CF and sample with a plasma treatment speed of 2 m/min, respectively, suggesting that CF surface may have been etched away. SEM images shown in Fig. 3 confirms the damage on the CF surface. SEM images of the multiscale hybrid composites with or without CNT incorporation and plasma surface treatment are also shown in Fig. 4. It can be noted that the control composite sample exhibits relatively poor wetting behavior between matrix and CFs as compared to those of the plasma treated composites. The XPS C1s spectra of four types of composite samples are shown in Fig. 5 and summarized in Table 1. C1s peaks can be deconvoluted into CeC (284.5 eV), CeOH (285.7 eV), C]O (286.4 eV), COOH (288.4 eV) and CO32 −/π→ π ∗ (289.3 eV) [29]. By comparing the control CF and CF plasma-treated with a scan speed of 6 m/min, one can note that plasma treatment results in the removal of ketone and epoxy functional groups (peak 3). The normalized peak area of peak 3 summarized in Table 1 also indicating drastic decrease of peak area from 88.1% to 13% with only a small degree of plasma treatment, suggesting that the decomposition of C]O and CeOeC functional groups from epoxy-type sizing material on the CF surface [32]. The normalized area of peak 2 and peak 4 indicating COH and COOH groups drastically increased with plasma treatment from 0.9% to 8.9% and from 35.6% to 60.3%, respectively. This can be explained by the chain scission of
2.3.2. Mechanical properties Dynamic mechanical properties of the multiscale hybrid composites were measured by dynamic mechanical analyzer (DMA) (Q800, TA Instruments, Inc., USA). The experiments were carried out with tensile mode, temperature ramp in the range from RT to 200 °C (2 °C/min) and frequency sweep with multiple strain frequencies (0.1, 1, 10 Hz). The amplitude of deformation and pre-load were 5 μm and 0.01 N, respectively. The absorbed impact energy of composites was measured by drop weight impact test. 7 J of kinetic energy was applied by impact testing system (INSTRON 9350, USA). Four types of samples were selected based on the DMA results and tests were repeated at least 5 times for each sample type. 439
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Fig. 5. C1s spectra of CFs with various plasma scan rate: (a) neat CF, (b) 6 m/min, (c) 2 m/min and (d) 1 m/min. Table 1 Surface elements of CFs and normalized peak area of specific functional groups with respect to CeC bond depending on plasma treatment speeds. Treatment speed (m/min)
0 6 2 1
Surface elements (atomic %)
Normalized peak area
C
O
N
O/C
CeOH
C=O
COOH
CO32 −/ π → π ∗
73 73 78.9 80.1
21.6 19 16.3 14
4 4.4 3.3 3
30 26 20.7 17.5
0.9 8.9 9.4 7.5
88.1 19.4 19.7 13
35.6 60.3 63.2 52.9
5.3 25.1 17.3 16.7
respectively due to excessive plasma etching caused decomposition of oxygen functional groups from the damaged CF surface [33–38]. This result suggests that prolonged plasma treatment does not necessarily result in a high content of oxygen functional group, and one should try to find an optimal processing condition for the sized CF. ATR-FTIR spectroscopy was used to investigate the change of functional groups on the CF surface by plasma treatment. Fig. S2 shows the FT-IR results of plasma treated CF. The intensity of C]O group assigned to 1800 cm−1 decreased as they are exposed more time under the plasma because the epoxy sizing layer was removed. Meanwhile,
epoxy, leading to the formation of COH or COOH functional groups in the early stage of plasma treatment. Therefore, overall decrease in oxygen content (C1s/O1s) predominantly took place with the removal of epoxy sizing followed by gradual decrease after complete removal of epoxy sizing with higher degree of plasma treatment (2 m/min and 1 m/min scan rate), which is well agreed with surface roughness results. However, COH and COOH groups decreased abruptly with further decrease of plasma scan rate. When the scan rate of plasma treatment decreased from 2 m/min to 1 m/min, the normalized peak area of COH and COOH decreased from 9.4% to 7.5% and from 63.2% to 52.9%, 440
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Fig. 6. Dynamic mechanical behavior of composites with various plasma scan rate: (a), (b) without CNT hybridization, (c), (d) with CNT hybridization.
Fig. 7. Storage modulus of two types of composites (with or without CNTs) depending on different strain frequencies at 80 °C: (a), (d) 0.1 Hz, (b), (e) 1 Hz and (c), (f) 10 Hz.
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Fig. 8. The storage modulus of two types composites depending on hybridization of CNTs with various plasma scan rate and different frequencies at 170 °C: (a), (d) 0.1 Hz, (b), (e) 1 Hz and (c), (f) 10 Hz.
Fig. 10. Arrhenius plots of α-transitions for PC/CF and PC/CNT/CF composites.
Fig. 9. The comparison of % increases for storage moduli of multiscale hybrid composites from control sample: (a) PC/CNT 1 wt.%/CF composite prepared by 2 m/min plasma treatment and solution casting shows 387% increases from PC/ CF composite, (b) polyacrylonitrile (PAN)/CNT 1 wt.%/CF composite prepared by functionalization of CNT and wet spinning shows 37% increases from PAN/ CF composite [14], (c) polyamide 6 (PA 6)/CNT 0.1 wt.%/glass fiber (GF) composite prepared by melt compounding shows 30% increases from PA 6/CF composite [45], (d) epoxy/double walled CNT(DWCNT) 0.05 wt.%/CF composite prepared by resin infusion shows 25% increases from epoxy/CF composite [46], (e) poly(ether ether ketone) (PEEK)/single walled CNT (SWCNT) 1 wt.%+poly(1-4-phenylene ether-ether sulfone) (PEES)/GF composite prepared by laser treatment and melt compounding shows 22% increases from PEEK/GF composite [47], and (f) PC/CNT 2 wt.%/CF composite prepared by melt mixing shows 9.7% increases from PC/CF composite [40].
COOH group assigned to 1680 cm−1 increased with high plasma treatment (2 m/min scan rate shown by green line) because of the surface oxidation. With decreasing scan rate further to 1 m/min, the band intensity for both C]O and COOH groups were abruptly diminished because of peeling CF's surface as a resulted of excessive plasma treatment, which is in good agreement with XPS results. The plasma treatment has enough energy to break the chemical bonds of the various functional groups on the CF surface leading formation of radicals. As a result, the oxygen functional groups such as COH and COOH are removed or rearranged by reaction between radicals and oxygen become unpredictable [33]. The effects of plasma treatment and hybridization of CNTs on 442
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Fig. 11. The impact test results from drop weight testing: photographs of (a) PC/CF composite and (b) PC/CNT/CF composite treated by plasma with speed of 2 m/ min after drop weight impact; (c) load versus time plot and (d) impact energy of PC/CF composites with or without plasma treatment and CNTs hybridization.
control PC/CF composite showed higher plasma treatment dependency (increased by 287% at 2 m/min) than that of multiscale hybrid composites (increased by 142% at 2 m/min), suggesting that the plasma treatment efficiency for the control PC/CF composite is better than that of the multiscale hybrid composite. It is also noted that the E′ drastically decreased by the extensive plasma treatment (1 m/min) for both the control and the multiscale hybrid composites, suggesting that the prolonged plasma treatment can induce the damage on CF surface as discussed earlier, leading to the overall decrease in fiber property as well as the composite property. On the other hand, the normalized E′ of the various composites at 170 °C are also analyzed (Fig. 8), showing that the extent of reinforcement efficiency above Tg exhibits for the PC/ CF composite is much lower than that of the PC/CNT/CF composite. For comparison, the % increase of E’ of the various multi-scale hybrid composites are shown in Fig. 9 [40–42]. The PC/CNT/CF composite prepared in the current study (plasma treatment speed of 2 m/min) exhibited improvement by 387% as compared to the control PC/CF composite, while other studies showed less than 50% improvement. This suggests that the surface treatment significantly enhanced the interfacial strength between CF and PC/CNT matrix [43]. The frequency dependence experiments by DMA can be analyzed by the Arrhenius equation and the activation energy (Ea) was calculated as shown in Fig. 10. The Ea of the PC/CF composites was highest (1768 kJ/ mol) at the plasma treatment speed of 2 m/min, while that of the control sample showed only 622 kJ/mol, suggesting that the surface treatment of CF dramatically increased the interfacial interaction with PC matrix. On the other hand, the PC/CNT/CF composites exhibited much less dependence on the CF surface treatment. The Ea of the best PC/CNT/CF composite (plasma treatment speed at 2 m/min) was 686 kJ/mol while that of the control composite was 636 kJ/mol. This
dynamic mechanical properties was assessed under both isothermal (frequency sweep mode) and non-isothermal (temperature sweep mode) conditions. As shown in Fig. 6 (a) and (c), the highest storage moduli (E′) are observed when the plasma treatment speed was 2 m/ min both in the control PC/CF and PC/CNT/CF composites, confirming the optimal plasma treatment condition is 2 m/min for a given system. It can be also noted that CNT containing multiscale hybrid composite exhibits excellent E′ as compared to those of the control specimen under all the plasma treatment conditions up until glass transition temperature of PC (∼150 °C). The initial E′ of the control PC/CF composite with a plasma treatment speed of 2 m/min was about 32 GPa while that of the PC/CNT/CF composites prepared at the same condition was 39 GPa. The loss factor, tan δ, represents the damping behavior of molecular motion and the peak temperature is generally accepted as a glass transition temperature (Tg) of polymer matrix. It is quite interesting to note that the control PC/CF composites showed decrease in Tg by plasma treatment (Fig. 6 (b)), while those of the PC/CNT/CF composites increased upon plasma treatment (Fig. 6 (d)). This suggests that the CNT can substantially constrain the molecular motion of PC [39]. Therefore, although the plasma treatment is negatively affecting on the Tg, the CNT incorporation compensates such effect and even leads to the overall increase in Tg of the multiscale hybrid composites. The DMA results under different dynamic frequencies (0.1, 1, and 10 Hz) at 80 °C are also shown in Fig. 7. As noted earlier, the plasma treatment speed of 2 m/min appears to be the best condition for the CF surface treatment. In addition, the CNT incorporation led to the 300% increase in E′ at 1 Hz even without plasma surface treatment (control samples shown in Fig. 7(b)). For fair comparison to assess the effect of plasma treatment, the normalized E′ of the treated specimens by that of the control composites are also shown Fig. 7 (e). At a strain frequency of 1 Hz, the 443
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References
confirms the normalized E′ shown in Fig. 7 (d, e, and f) where the PC/ CF composites showed better % increase as compared to those of the PC/CNT/CF composites by optimizing surface treatment condition. Nonetheless, the absolute dynamic mechanical property (E’) of the multi-scale hybrid composite is superior to that of the control PC/CF composite because of the reinforcement effect of the CNTs. Along with dynamic mechanical testing, impact response and absorbed energy measurements of the various composites were conducted by drop weight test set-up. Fig. 11 shows the photographs of the PC/ CNT/CF composite before and after drop weight impact test. Based on the digital image analysis, impact spot penetration and permanent indentation occurred mainly due to the fiber fracture and hysteresis, not because of the different damage modes such as splitting, edge delamination or bottom layer delamination. Absorbed energy in an impact event can be calculated from load–deflection curves. Characteristic of load–deflection curves also includes some useful tips in assessing damage process of composite structures. Therefore, several load–deflection curves of samples, in this study, for varied impact energies are given in Fig. 11 (c) and (d), respectively. From the initial stage of loaddeflection curve, linear slope provides information about bending stiffness while subsequent step curve provides non-linear permanent and vibrational energy absorbing capability of composite [44]. Among the composite specimens, PC/CF composite at plasma treatment speed of 2 m/min showed the highest energy absorbing capability by elastic deformation (4.3 J) with peak load of 1.4 kN. The PC/CNT/CF composite prepared at the plasma treatment speed of 2 m/min showed second highest energy absorbability by elastic deformation (3.3 J) with peak load of 1.2 kN. However, the highest total absorbed energy was achieved with 2 m/min PC/CNT/CF composite (5.3 J) instead of 2 m/ min PC/CF composite (5 J), indicating that hybrid multiscale composite is capable of absorbing more energy from vibrational deformation followed by initial impact event 4. In addition, it is noteworthy that the total energy absorbed by PC/CNT/CF composite (5.3 J) was increased by 194% than that of the neat PC/CF composite (1.8 J).
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4. Conclusions This study showed that the plasma surface treatment of CF fabric and CNT incorporation can significantly improve the dynamic mechanical property (storage modulus as high as 39 GPa) and energy absorbing capability (as high as 5.3 J) of the multiscale hybrid composite although the plasma surface treatment is very efficient in the case of PC/CF composites under optimal processing condition. In this context, it should also be noted that the current performance of conventional composites has been achieved after more than half a century of processing optimization. The multiscale hybrid composites are still in the early stage of development and very limited studies have been carried out to date. Therefore, the experimental results reported in this paper should be considered as the baseline, and further study will increase the performance of multiscale hybrid composites beyond those reported in this study.
Acknowledgments This work was supported by the Korean Government (Ministry of Trade, Industry and Energy, MOTIE, and Defense Acquisition Program Administration, DAPA) through Institute of Civil-Military Technology Cooperation.
Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.compositesb.2018.12.062. 444
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
The effects of plasma surface treatment on the mechanical properties of polycarbonate/carbon nanotube/carbon fiber composites
T
Beom-Gon Choa, Sang-Ha Hwangb, Miseon Parkc, Jong Kyoo Parkc, Young-Bin Parka,∗, Han Gi Chaeb,∗∗ a Department of Mechanical and Aerospace Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea b School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulju-gun, Ulsan, 44919, Republic of Korea c The 4th R&D Institute, Agency for Defense Development, Yuseong P.O. Box 35, Daejeon, 34186, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon fibres Carbon nanotubes Polymer-matrix composites (PMCs) Surface treatments Mechanical properties
The effects of plasma surface treatment on the mechanical properties of multiscale hybrid composites consisting of polycarbonate (PC), carbon nanotube (CNT) and carbon fiber (CF) were investigated. Dynamic mechanical properties and impact energy absorption of multiscale hybrid composites fabricated under various processing conditions were measured and correlated with surface roughness and surface functionality. The highest roomtemperature storage modulus (E’) of 39 GPa and absorbed impact energy of 5.3 J were obtained from the plasmatreated PC/CNT/CF composite, which are increases by 387% and 194%, respectively, as compared to the neat PC/CF composite. Although the plasma treatment was more efficient for the PC/CF composites rather than the PC/CNT/CF composites, hybridization of CNT showed synergistic effects on the enhancement of mechanical properties due to the combination of increased surface roughness and functionality as well as the bridged modulus gap between PC and CF.
1. Introduction Carbon fiber reinforced plastics (CFRP) have drawn significant attention in the field of structural applications such as aerospace, automotive, marine, and sporting goods due to their excellent performance/ weight ratio [1–3]. Decades of research and development have been made to improve the properties of composite materials [4]. It has been generally accepted that the interfacial characteristics are one of the most critical parameters that determines the bulk composite properties. Many researches have been conducted to address this issue including chemical oxidation [5–8], high-energy beam irradiation [8–12], surface treatment by rare earth [8,13]. Most recently, introduction of carbon nanomaterials such as CNTs and or graphene modified resins opens the new possibility for the tailoring of interfacial characteristics [14–17]. One of the routes to hybridize CNT in CFRP has been explored by Thostenson et al. [18] and Kepple et al. [19]. Their approaches are to grow CNTs on the surface of carbon fibers, then process these “fuzzy fibers” with epoxy resin, exhibiting dramatically improved interfacial bond strength. The enhanced Mode II fracture behavior and interfacial shear strength (IFSS) of the composite laminates were also studied by
∗
Karapappas et al. [20] and Gojny et al. [21] respectively. One of the most interesting result reported by Frankland et al. [22] is multiscale hybrid micro-nanocomposites based on CNT/carbon fiber/epoxy, suggesting that the presence of CNTs near the surface of carbon fibers can improve the interfacial properties of the composites. Nonetheless, the multiscale hybrid composites still pose several challenges such as CNT dispersion and high viscosity of matrix especially in the case of using thermoplastics as matrix because of their high melt viscosity thereby limited dispersion and processing methods despite of the advantages over thermosets including shorter processing time and recyclability [23–27]. In addition, limited interfacial interaction is considered to limit the potential performance of multiscale hybrid composites [28–30]. Among the several studies including chemical and physical modification of carbon fiber, plasma treatment facilitates mass production capability through continuous process and is known to sustain the fiber intrinsic properties due to relatively low energy exposure [8]. In this study, to address the issues of manufacturing multiscale hybrid composites, such as poor embedding efficiency by high melt viscosity and fiber-matrix interfacial bonding, we adopt solution-based processing of polycarbonate (PC)/CNT/CF multiscale hybrid
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Y.-B. Park), [email protected] (H.G. Chae).
∗∗
https://doi.org/10.1016/j.compositesb.2018.12.062 Received 29 August 2018; Received in revised form 13 December 2018; Accepted 14 December 2018 Available online 21 December 2018 1359-8368/ © 2018 Elsevier Ltd. All rights reserved.
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frequency of 39 Hz. The schematic description of plasma treatment apparatus is shown in Fig. 1 (a). A round nozzle with a diameter of 30 mm traveled perpendicular to the specimen conveying direction at the various speeds of 1, 2 and 6 m/min while the conveyor moved stepwise with a step of 30 mm. Woven carbon fiber without plasma treatment was also used to prepare as a control sample. Multiscale hybrid composites (PC/CNT/CF) were prepared in two steps: (1) ultrasonic-assisted CNT dispersion in chloroform followed by PC dissolution, and (2) impregnation of CNT/PC/chloroform solution into woven CF followed by evaporation of chloroform. The detailed processing steps are as follows. The CNT dispersion was optimized by preliminary study using UV–vis spectroscopy (UV-1800, Shimadzu, Japan) of CNT/PC solution (Fig. S1). To begin with, 10 mg of CNTs were pre-dispersed in 1 L of CHCl3 using a bath sonicator (Branson 5800, Branson Ultrasonic Corp.) for 2 h. Pre-dispersed solution was divided into 50 mL vials each containing 0.5 mg of CNT. The divided solutions were sonicated from 4 h to 40 h to analyze when the colloid stability appears and verify the tendency of absorbance values depending on sonication time varied with a specific wavelength, such as 315, 320, 325, and 330 nm. The absorption spectra of 1 wt. % CNTs in CHCl3 with different sonication time from 4 to 40 h using bath sonicator is shown in Fig. S1. When the sonication time increased from 4 to 40 h, the absorbance values have increasing tendency, suggesting the dispersion of CNT is improved upon prolonged sonication time. In addition, as shown in Fig. S1 (b), absorption intensity was investigated as a function of sonication time at various wavelengths to show at which sonication time, absorption intensity become stable. At various wavelengths, absorption intensity of CNT dispersion was dramatically increased after 8 h of sonication and become stable. This indicates that 8 h of sonication is good enough to obtain highly dispersed CNT solution.
composites. Furthermore, the effect of plasma treatment of CF surface on the mechanical properties of composites is also explored by changing treatment condition. The key mechanisms for interfacial adhesion by plasma treatment are: (1) removal of various contaminants from the surface of CFs, (2) increasing fiber surface roughness, which would provide more surface area for interaction, and (3) improving the chemical interaction between CF and PC by introducing functional groups [8,31]. The multiscale hybrid composite processed by optimal condition (highest CNT dispersion and highest surface functionality of CF without deterioration its property) exhibits nearly 200% improvement in absorbed impact energy as compared to the control composite. 2. Experimental 2.1. Materials Multi-walled carbon nanotubes (MWCNTs) with diameters ranging from 15 to 30 nm and purity higher than 95% were purchased from LG Chem (Korea). Polycarbonate (PC) was purchased from Honam Petrochemical (Korea) and woven carbon fiber (CF) with plain weave texture was purchased from Toray (Japan). Methanol and Chloroform were purchased from SK Chemicals (Korea) and used without further purification. 2.2. Plasma treatment and preparation of multi-scale hybrid composites Prior to preparation of composites, woven carbon fibers were treated using a plasma apparatus (3D i REV, Applied Plasma Inc., Korea) with an exposure power of 800 W. Compressed air was supplied into the plasma apparatus at a pressure of 150 MPa at an operation
Fig. 1. Schematic description of processing multiscale hybrid composites, (a) plasma treatment apparatus for CF surface treatment, (b) vacuum-assisted distillation set-up for CNT/PC/chloroform solution, and (c) fabrication sequence of composite laminate by solution based impregnation followed by hot-pressing. 437
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Fig. 2. The 3D images of CF surface and mean roughness (Ra) with different plasma scan rate: (a) neat CF, (b) 6 m/min, (c) 2 m/min and (d) 1 m/min. Topographical images and Ras shown in the figure only represents morphology within a single fiber not between fibers.
Fig. 3. The SEM images of CFs surface with various plasma scan rate: (a) neat CF, (b) 6 m/min, (c) 2 m/min and (d) 1 m/min.
of solvent was removed by vacuum distillation as illustrated in Fig. 1 (b). 2 layers of woven CFs (100 × 100 mm) were fully wetted by the PC/chloroform or CNT/PC/chloroform solution and dried in the oven at 45 °C for a day. Then, the dried composites were hot pressed at 215 °C for 15 min at a pressure of 10 MPa as shown in Fig. 1 (c).
Although it can be observed that the absorption intensity gradually increased with sonication time, this may be due to the fact that the prolonged sonication can break CNTs and reduce the length of CNTs. Therefore, the sonication time was set to 8 h. The prepared CNT dispersion under optimal condition (8 h sonication) was then added to the separately prepared PC solution (9.9 g/100 mL) and the excess amount
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Fig. 4. The SEM images of PC/CF composites with CNTs incorporation and plasma surface treatment (2 m/min speed): (a) PC/CF; (b) treated PC/CF; (c) PC/CNT/CF and (d) treated PC/CNT/CF.
2.3. Characterization
3. Results and discussion
2.3.1. Surface analysis of woven CF The surface morphology of woven CF was observed using a field emission scanning electron microscope (FE-SEM) (Nanonova 230, FEI, USA). The surface roughness of CF was also measured using a 3D measurement system (NV-3000, Nanosystems, Korea). The scanning area for surface roughness measurement was 120 μm (width) × 90 μm (length). To obtain the statistically meaningful result, 10 different points of a single filament (diameter of CF was 7 μm) and 5 different filament samples were analyzed. The chemical composition of CF surface was also characterized by X-ray Photoelectron Spectroscopy (XPS) (Thermo Fisher Scientific, Inc., USA) using a monochromatic Al Kα Xray source (1486.6 eV). The surface functional groups of CFs were investigated by using ATR-FTIR spectroscopy (Nicolet iS50, Thermo Fisher Scientific, Inc., USA) using Germanium (Ge) as an internal reflection element. The analysis was carried out in transmittance mode over the scanning range of 700−4000 cm−1.
The three-dimensional topographic profile and arithmetic mean roughness of CFs with or without plasma treatments are shown in Fig. 2. The arithmetic mean roughness (Ra) increased from 135 to 169 nm by the plasma treatment with a scan speed of 6 m/min, which may be mainly due to the removal of epoxy sizing layer, since it was observed that the Ra decreased by increasing plasma treatment time (by reducing the scan speed to 2 m/min). However, with the plasma treatment speed of 1 m/min, the Ra again increased by 360 and 343%, respectively, from those of control CF and sample with a plasma treatment speed of 2 m/min, respectively, suggesting that CF surface may have been etched away. SEM images shown in Fig. 3 confirms the damage on the CF surface. SEM images of the multiscale hybrid composites with or without CNT incorporation and plasma surface treatment are also shown in Fig. 4. It can be noted that the control composite sample exhibits relatively poor wetting behavior between matrix and CFs as compared to those of the plasma treated composites. The XPS C1s spectra of four types of composite samples are shown in Fig. 5 and summarized in Table 1. C1s peaks can be deconvoluted into CeC (284.5 eV), CeOH (285.7 eV), C]O (286.4 eV), COOH (288.4 eV) and CO32 −/π→ π ∗ (289.3 eV) [29]. By comparing the control CF and CF plasma-treated with a scan speed of 6 m/min, one can note that plasma treatment results in the removal of ketone and epoxy functional groups (peak 3). The normalized peak area of peak 3 summarized in Table 1 also indicating drastic decrease of peak area from 88.1% to 13% with only a small degree of plasma treatment, suggesting that the decomposition of C]O and CeOeC functional groups from epoxy-type sizing material on the CF surface [32]. The normalized area of peak 2 and peak 4 indicating COH and COOH groups drastically increased with plasma treatment from 0.9% to 8.9% and from 35.6% to 60.3%, respectively. This can be explained by the chain scission of
2.3.2. Mechanical properties Dynamic mechanical properties of the multiscale hybrid composites were measured by dynamic mechanical analyzer (DMA) (Q800, TA Instruments, Inc., USA). The experiments were carried out with tensile mode, temperature ramp in the range from RT to 200 °C (2 °C/min) and frequency sweep with multiple strain frequencies (0.1, 1, 10 Hz). The amplitude of deformation and pre-load were 5 μm and 0.01 N, respectively. The absorbed impact energy of composites was measured by drop weight impact test. 7 J of kinetic energy was applied by impact testing system (INSTRON 9350, USA). Four types of samples were selected based on the DMA results and tests were repeated at least 5 times for each sample type. 439
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Fig. 5. C1s spectra of CFs with various plasma scan rate: (a) neat CF, (b) 6 m/min, (c) 2 m/min and (d) 1 m/min. Table 1 Surface elements of CFs and normalized peak area of specific functional groups with respect to CeC bond depending on plasma treatment speeds. Treatment speed (m/min)
0 6 2 1
Surface elements (atomic %)
Normalized peak area
C
O
N
O/C
CeOH
C=O
COOH
CO32 −/ π → π ∗
73 73 78.9 80.1
21.6 19 16.3 14
4 4.4 3.3 3
30 26 20.7 17.5
0.9 8.9 9.4 7.5
88.1 19.4 19.7 13
35.6 60.3 63.2 52.9
5.3 25.1 17.3 16.7
respectively due to excessive plasma etching caused decomposition of oxygen functional groups from the damaged CF surface [33–38]. This result suggests that prolonged plasma treatment does not necessarily result in a high content of oxygen functional group, and one should try to find an optimal processing condition for the sized CF. ATR-FTIR spectroscopy was used to investigate the change of functional groups on the CF surface by plasma treatment. Fig. S2 shows the FT-IR results of plasma treated CF. The intensity of C]O group assigned to 1800 cm−1 decreased as they are exposed more time under the plasma because the epoxy sizing layer was removed. Meanwhile,
epoxy, leading to the formation of COH or COOH functional groups in the early stage of plasma treatment. Therefore, overall decrease in oxygen content (C1s/O1s) predominantly took place with the removal of epoxy sizing followed by gradual decrease after complete removal of epoxy sizing with higher degree of plasma treatment (2 m/min and 1 m/min scan rate), which is well agreed with surface roughness results. However, COH and COOH groups decreased abruptly with further decrease of plasma scan rate. When the scan rate of plasma treatment decreased from 2 m/min to 1 m/min, the normalized peak area of COH and COOH decreased from 9.4% to 7.5% and from 63.2% to 52.9%, 440
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Fig. 6. Dynamic mechanical behavior of composites with various plasma scan rate: (a), (b) without CNT hybridization, (c), (d) with CNT hybridization.
Fig. 7. Storage modulus of two types of composites (with or without CNTs) depending on different strain frequencies at 80 °C: (a), (d) 0.1 Hz, (b), (e) 1 Hz and (c), (f) 10 Hz.
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Fig. 8. The storage modulus of two types composites depending on hybridization of CNTs with various plasma scan rate and different frequencies at 170 °C: (a), (d) 0.1 Hz, (b), (e) 1 Hz and (c), (f) 10 Hz.
Fig. 10. Arrhenius plots of α-transitions for PC/CF and PC/CNT/CF composites.
Fig. 9. The comparison of % increases for storage moduli of multiscale hybrid composites from control sample: (a) PC/CNT 1 wt.%/CF composite prepared by 2 m/min plasma treatment and solution casting shows 387% increases from PC/ CF composite, (b) polyacrylonitrile (PAN)/CNT 1 wt.%/CF composite prepared by functionalization of CNT and wet spinning shows 37% increases from PAN/ CF composite [14], (c) polyamide 6 (PA 6)/CNT 0.1 wt.%/glass fiber (GF) composite prepared by melt compounding shows 30% increases from PA 6/CF composite [45], (d) epoxy/double walled CNT(DWCNT) 0.05 wt.%/CF composite prepared by resin infusion shows 25% increases from epoxy/CF composite [46], (e) poly(ether ether ketone) (PEEK)/single walled CNT (SWCNT) 1 wt.%+poly(1-4-phenylene ether-ether sulfone) (PEES)/GF composite prepared by laser treatment and melt compounding shows 22% increases from PEEK/GF composite [47], and (f) PC/CNT 2 wt.%/CF composite prepared by melt mixing shows 9.7% increases from PC/CF composite [40].
COOH group assigned to 1680 cm−1 increased with high plasma treatment (2 m/min scan rate shown by green line) because of the surface oxidation. With decreasing scan rate further to 1 m/min, the band intensity for both C]O and COOH groups were abruptly diminished because of peeling CF's surface as a resulted of excessive plasma treatment, which is in good agreement with XPS results. The plasma treatment has enough energy to break the chemical bonds of the various functional groups on the CF surface leading formation of radicals. As a result, the oxygen functional groups such as COH and COOH are removed or rearranged by reaction between radicals and oxygen become unpredictable [33]. The effects of plasma treatment and hybridization of CNTs on 442
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Fig. 11. The impact test results from drop weight testing: photographs of (a) PC/CF composite and (b) PC/CNT/CF composite treated by plasma with speed of 2 m/ min after drop weight impact; (c) load versus time plot and (d) impact energy of PC/CF composites with or without plasma treatment and CNTs hybridization.
control PC/CF composite showed higher plasma treatment dependency (increased by 287% at 2 m/min) than that of multiscale hybrid composites (increased by 142% at 2 m/min), suggesting that the plasma treatment efficiency for the control PC/CF composite is better than that of the multiscale hybrid composite. It is also noted that the E′ drastically decreased by the extensive plasma treatment (1 m/min) for both the control and the multiscale hybrid composites, suggesting that the prolonged plasma treatment can induce the damage on CF surface as discussed earlier, leading to the overall decrease in fiber property as well as the composite property. On the other hand, the normalized E′ of the various composites at 170 °C are also analyzed (Fig. 8), showing that the extent of reinforcement efficiency above Tg exhibits for the PC/ CF composite is much lower than that of the PC/CNT/CF composite. For comparison, the % increase of E’ of the various multi-scale hybrid composites are shown in Fig. 9 [40–42]. The PC/CNT/CF composite prepared in the current study (plasma treatment speed of 2 m/min) exhibited improvement by 387% as compared to the control PC/CF composite, while other studies showed less than 50% improvement. This suggests that the surface treatment significantly enhanced the interfacial strength between CF and PC/CNT matrix [43]. The frequency dependence experiments by DMA can be analyzed by the Arrhenius equation and the activation energy (Ea) was calculated as shown in Fig. 10. The Ea of the PC/CF composites was highest (1768 kJ/ mol) at the plasma treatment speed of 2 m/min, while that of the control sample showed only 622 kJ/mol, suggesting that the surface treatment of CF dramatically increased the interfacial interaction with PC matrix. On the other hand, the PC/CNT/CF composites exhibited much less dependence on the CF surface treatment. The Ea of the best PC/CNT/CF composite (plasma treatment speed at 2 m/min) was 686 kJ/mol while that of the control composite was 636 kJ/mol. This
dynamic mechanical properties was assessed under both isothermal (frequency sweep mode) and non-isothermal (temperature sweep mode) conditions. As shown in Fig. 6 (a) and (c), the highest storage moduli (E′) are observed when the plasma treatment speed was 2 m/ min both in the control PC/CF and PC/CNT/CF composites, confirming the optimal plasma treatment condition is 2 m/min for a given system. It can be also noted that CNT containing multiscale hybrid composite exhibits excellent E′ as compared to those of the control specimen under all the plasma treatment conditions up until glass transition temperature of PC (∼150 °C). The initial E′ of the control PC/CF composite with a plasma treatment speed of 2 m/min was about 32 GPa while that of the PC/CNT/CF composites prepared at the same condition was 39 GPa. The loss factor, tan δ, represents the damping behavior of molecular motion and the peak temperature is generally accepted as a glass transition temperature (Tg) of polymer matrix. It is quite interesting to note that the control PC/CF composites showed decrease in Tg by plasma treatment (Fig. 6 (b)), while those of the PC/CNT/CF composites increased upon plasma treatment (Fig. 6 (d)). This suggests that the CNT can substantially constrain the molecular motion of PC [39]. Therefore, although the plasma treatment is negatively affecting on the Tg, the CNT incorporation compensates such effect and even leads to the overall increase in Tg of the multiscale hybrid composites. The DMA results under different dynamic frequencies (0.1, 1, and 10 Hz) at 80 °C are also shown in Fig. 7. As noted earlier, the plasma treatment speed of 2 m/min appears to be the best condition for the CF surface treatment. In addition, the CNT incorporation led to the 300% increase in E′ at 1 Hz even without plasma surface treatment (control samples shown in Fig. 7(b)). For fair comparison to assess the effect of plasma treatment, the normalized E′ of the treated specimens by that of the control composites are also shown Fig. 7 (e). At a strain frequency of 1 Hz, the 443
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References
confirms the normalized E′ shown in Fig. 7 (d, e, and f) where the PC/ CF composites showed better % increase as compared to those of the PC/CNT/CF composites by optimizing surface treatment condition. Nonetheless, the absolute dynamic mechanical property (E’) of the multi-scale hybrid composite is superior to that of the control PC/CF composite because of the reinforcement effect of the CNTs. Along with dynamic mechanical testing, impact response and absorbed energy measurements of the various composites were conducted by drop weight test set-up. Fig. 11 shows the photographs of the PC/ CNT/CF composite before and after drop weight impact test. Based on the digital image analysis, impact spot penetration and permanent indentation occurred mainly due to the fiber fracture and hysteresis, not because of the different damage modes such as splitting, edge delamination or bottom layer delamination. Absorbed energy in an impact event can be calculated from load–deflection curves. Characteristic of load–deflection curves also includes some useful tips in assessing damage process of composite structures. Therefore, several load–deflection curves of samples, in this study, for varied impact energies are given in Fig. 11 (c) and (d), respectively. From the initial stage of loaddeflection curve, linear slope provides information about bending stiffness while subsequent step curve provides non-linear permanent and vibrational energy absorbing capability of composite [44]. Among the composite specimens, PC/CF composite at plasma treatment speed of 2 m/min showed the highest energy absorbing capability by elastic deformation (4.3 J) with peak load of 1.4 kN. The PC/CNT/CF composite prepared at the plasma treatment speed of 2 m/min showed second highest energy absorbability by elastic deformation (3.3 J) with peak load of 1.2 kN. However, the highest total absorbed energy was achieved with 2 m/min PC/CNT/CF composite (5.3 J) instead of 2 m/ min PC/CF composite (5 J), indicating that hybrid multiscale composite is capable of absorbing more energy from vibrational deformation followed by initial impact event 4. In addition, it is noteworthy that the total energy absorbed by PC/CNT/CF composite (5.3 J) was increased by 194% than that of the neat PC/CF composite (1.8 J).
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4. Conclusions This study showed that the plasma surface treatment of CF fabric and CNT incorporation can significantly improve the dynamic mechanical property (storage modulus as high as 39 GPa) and energy absorbing capability (as high as 5.3 J) of the multiscale hybrid composite although the plasma surface treatment is very efficient in the case of PC/CF composites under optimal processing condition. In this context, it should also be noted that the current performance of conventional composites has been achieved after more than half a century of processing optimization. The multiscale hybrid composites are still in the early stage of development and very limited studies have been carried out to date. Therefore, the experimental results reported in this paper should be considered as the baseline, and further study will increase the performance of multiscale hybrid composites beyond those reported in this study.
Acknowledgments This work was supported by the Korean Government (Ministry of Trade, Industry and Energy, MOTIE, and Defense Acquisition Program Administration, DAPA) through Institute of Civil-Military Technology Cooperation.
Appendix A. Supplementary data Supplementary data related to this article can be found at https:// doi.org/10.1016/j.compositesb.2018.12.062. 444
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