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epoxy composites and single carbon fiber at different temperatures
Composites Part B 91 (2016) 111e118
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Inherent and interfacial evaluations of carbon nanotubes/epoxy composites and single carbon fiber at different temperatures Zuo-Jia Wang a, Dong-Jun Kwon a, Jin-Yeong Choi a, Pyeong-Soo Shin a, Jin-Woo Yi b, Joon-Hyung Byun b, Hyung-Ik Lee c, Jong-Kyoo Park c, K. Lawrence DeVries d, Joung-Man Park a, d, * a
Department of Materials Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 660-701, Republic of Korea Korea Institute of Materials Science, Changwon 641-010, Republic of Korea c Agency for Defense Development, 4-R&D Center, Daejeon 305-600, Republic of Korea d Department of Mechanical Engineering, The University of Utah, Salt Lake City, UT 84112, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2014 Received in revised form 28 August 2015 Accepted 26 December 2015 Available online 3 February 2016
In order to develop an improved processability for machines and equipment operating at high temperatures, thermal degradation resistance of a high temperature epoxy network became a hot issue. Nanocomposites were not only used for improving mechanical and electrical properties. In here, the thermal stability of carbon nanotube (CNT)/epoxy nanocomposite was investigated by mechanical and interfacial evaluation. The dispersion state was monitored and observed by electrical resistance measurements as well as by using a scanning electron microscope (SEM). At low concentration of 0.3 vol%, the CNT reinforcement significantly improved the mechanical and interfacial properties of nanocomposites. At 150 C, the CNT/epoxy nanocomposites exhibited much better thermal stability than neat epoxy resin as a result of the thermal resistance of CNTs. Microstructural studies revealed that the uniformity of the CNT dispersion played an important role in this thermal resistance. Addition of low concentration CNT could improve both mechanical properties and thermal stability of epoxy resin for high temperature applications. © 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Nano-structures B. Thermomechanical D. Electron microscopy D. Thermal analysis
1. Introduction High temperature polymers able to operate continuously at high temperatures are an important class of polymers for use in electronic, automotive, and aerospace applications [1,2]. The majority of these high temperature polymers fall into one of six material types: epoxy, phenolic, bismalimide, cyanate ester, bismalimidetriazine (BT), and polyimide, with each material type having advantages and disadvantages compared to the others. Among these materials, epoxy tends to exhibit the best processability but poorer thermal stability [3]. It was reported that the maximum temperature of use for epoxy resins is only approximately 60 C in continuous exposure and about 100e120 C for short term exposures [4]. The behavior in fire of the epoxy resin is very important when used as composite
* Corresponding author. Department of Materials Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 660-701, Republic of Korea. Tel.: þ82 55 772 1656; fax: þ82 55 772 1659. E-mail address: [email protected] (J.-M. Park). http://dx.doi.org/10.1016/j.compositesb.2015.12.036 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
matrix. The poor fire resistance in high temperatures represents the main obstacles for the development of epoxy resin. The one of important objective of this study consists in solving these drawbacks. Some recent formulations have extended the continuous upper operating temperature. Generally prolonged exposure to high temperatures results in degradation of the epoxy resin, possibly causing a loss of mechanical strength and interface adhesion. The mass and adhesion loss are mutually dependent on chemical network degradation. Network degradation will affect loss of inherent properties by breaking bonds which will reduce the mechanical strength of the epoxy and lower the adhesion strength [5]. Typically these high temperature epoxy resins have greater crosslink densities and glass transition temperatures, providing a considerable increase in their thermal, dynamic, mechanical and adhesive properties. The thermal degradation of special designed network known to have good thermal stability is studied by Tg reduction measurements [6]. However, the demand of epoxy resins for high temperature applications cannot be satisfied by these virgin special designed epoxy resins as the developing of industry science.
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The polymer needs to be more stable to resist network degradation, leading to a degradation at longer time for a similar temperature. The appearance of high performance reinforcing materials provides a new entry to improve the thermal stability of epoxy resins. Polymer-based composites which are reinforced with nanoparticles are of great interest from an industrial and scientific perspective because they exhibit much better mechanical and thermal behavior and characteristics than more conventional composites [7e10]. Carbon nanotubes (CNT) have attracted a good deal of interest due to their unique one-dimensional structure and novel properties [11e13]. The outstanding intrinsic mechanical properties of CNT have resulted in considerable attention for use as reinforcement for composite materials [14,15]. These benefits associated with composite design could thus be implemented on the nanoscale. It has been reported on the effect of the addition of CNT to polymer composites on thermo-mechanical and ablation performance [16]. Studies have shown that nanocomposites containing CNT can achieve improved mechanical and interfacial properties by taking advantage of enhanced stress transfer and interfacial friction between individual nanotubes and the matrix [17,18]. Recent research demonstrates that by adopting the multiscale reinforcements, significant improvements are achieved in glass fiber and carbon fiber composites, especially for the fiberematrix interphase and matrix dominated performances [19,20]. CNT and carbon fibers were incorporated into an epoxy matrix to fabricate a high performance multiscale composite and to improve the stress transfer between epoxy and carbon fibers [21,22]. Comparing to composite laminate material without carbon nanotube reinforcements, there are modest improvements in the mechanical properties of strength and stiffness. However, a potentially significant increase was demonstrated for the long-term fatigue life of these carbon nanotube reinforced composite materials [23]. As a consequence, significant improvements in the mechanical properties of tensile strength, stiffness and resistance to failure resulted for this carbon fiber reinforced epoxy composite [24,25]. The contact angle reaches the minimum when the multi-walled CNT contents equal to 0.5 wt%. The composite transverse tensile strength shows a similar tendency as the contact angle with increasing the CNT content. The introduction of CNT at a proper content into the epoxy resin can significantly enhance the cryogenic CF/epoxy interfacial and normal adhesion property [26]. The interlaminar shear strength (ILSS) of the CFRP composites with 0.5 wt% CNTs increased by 12% compared to the neat CFRP composites [27]. It is clear that the introduction of CNT at a proper content into the epoxy resin can significantly enhance the CF/epoxy interfacial normal adhesion property [28,29]. In this work, CNT-epoxy nanocomposites with relatively uniform CNT dispersion were fabricated for relative high temperature applications. Thermal stability of nanocomposites was investigated using mechanical testing and interfacial evaluation methods. At 150 C, CNT/epoxy nanocomposites exhibited better mechanical and interfacial properties than did neat epoxy resin. Based on the morphology observation, electrical resistivity and thermal analysis, the uniform dispersion of the CNT in epoxy matrix contributed to the thermal resistance of the nanocomposites. The nanostructures of CNT could work as reinforcing and stress transferring phase to provide better mechanical and interfacial properties of CNT/epoxy nanocomposites.
used as reinforcing materials in the nanocomposites. The CNT had a range of diameters (10e25 nm), lengths (5e10 mm) and density (1.995 g/cm3). Epoxy Resin (MY-720, Huntsman Co., USA) was used as the composite matrix. Fig. 1 shows the chemical structures of this epoxy resin and curing agent. The ratio of the epoxy resin to the curing agent was 7:3. Carbon Fiber (T700S, Toray Inc., Japan) was used as the reinforcing fiber. Acetone (Dae Jung Chemical, Co., Korea) was used as the CNT dispersing solvent.
2.2. Methodologies 2.2.1. Fabrication process of CNT/epoxy nanocomposites Fig. 2 shows details of the fabrication process for the CNT/epoxy nanocomposites used in this study [30]. First the epoxy resin was diluted using acetone with mechanical mixing. The CNT solvent mixture was then dispersed in the epoxy solution under sonication for 6 hours in a sealed beaker. The solvent was next evaporated under sonication for 12 hours. The residual solvent was further eliminated using a vacuum oven at 50 C for 24 hours. While several concentrations of CNT in the CNT/epoxy nanocomposites were investigated in previous studies, an optimal concentration of a uniform dispersion of CNT was found to be 0.3 vol% [31]. This was the concentration used in the composites in this study. After the dispersion and mixing process of CNT in the epoxy matrix, the curing process involved 4temperature steps: 1 hour at 80 C, followed by 1 hour at 100 C, 4 hours at 150 C, and then finally 2 hours at 177 C. The state of dispersion of CNT in the CNT/epoxy nanocomposites was observed using a SEM (Philips XL30 S FEG, Netherlands).
2.2.2. Electrical resistivity measurements of CNT/epoxy nanocomposites The electrical resistance of the CNT/epoxy nanocomposites was measured by the four-point method using a multimeter (HP34401A), as shown schematically in Fig. 3(a). The dimension of test specimens was 1 3 40 mm. Electrical contact points which located at regularly spaced intervals (5 mm), along the specimen using embedded copper wire without silver paste (copper wires were fixed in silicone mold before specimen preparation), were used to determine the electrical volumetric resistivity. The volumetric resistivity is the resistance per unit volume of the bulk material, and was used to provide a measure of the state of dispersion of the CNT filler in the nanocomposites. This electrical volumetric resistivity is determined by:
2. Experimental 2.1. Materials Multi-wall carbon nanotubes (CM-95, Hanwha Nanotech Co., Korea), produced by a chemical vapor deposition process, were
Fig. 1. Chemical structure of epoxy MY-720: (a) epoxy resin and (b) curing agent.
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Fig. 2. Schematic diagram of the fabrication process of CNT/epoxy nanocomposites.
rv ¼
Av Lec
Rv ðU$cmÞ
(1)
where rv is the resistivity, Rv is the measured electrical resistance, Av is the cross-sectional area, and Lec is the spacing between the electrical contact points. The relatively uniform dispersion of the CNT filler was thought to very favorably contribute to the thermal resistance of the nanocomposites. To prove that the CNT preferred orientation is negligible, the electrical resistivity of CNT/epoxy nanocomposites was measured in three orthogonal directions. The nanocomposites specimen was
prepared as an equilateral cubic (10 10 10 mm) as shown in Fig. 3(b), and the electrical resistivity in three orthogonal directions were measured. To progress the electrical resistivity measurement, copper wires were fixed on specimen surface with silver paste. The outer probes were fixed on surface in three orthogonal directions, and inner probes were spaced 1 mm apart. 2.2.3. Thermal analysis of CNT/epoxy nanocomposites The thermal stability of the produced materials was investigated using a TGA instrument (Q50, TA Instruments, U.S.A.). Measurements conducted in both air and nitrogen at dynamic scans of 10 C/min from 30 C to 900 C were used to compare the TGA of CNT/epoxy nanocomposites with that of neat epoxy resin.
Fig. 3. Schematic of the volumetric resistivity test specimens: (a) rectangular and (b) cubic.
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2.2.4. Mechanical property measurements Tensile and compressive tests were used in the determination of mechanical properties of neat epoxy resin and the CNT/epoxy nanocomposites at different temperatures. Tensile dog-bone and Mini-Broutman compressive specimens were cast in silicone molds, according to standard specifications. The dimension of tensile specimens was 1 3 20 mm, and the practical compression area of Mini-Broutman specimens were 3 6 mm2. Mechanical tests were then performed using a Universal Testing Machine (LR 10 K, Lloyd Co., UK) at a test speed of 0.5 mm/min, and the stress direction in tensile and compressive testing was same as the horizontal direction during curing. A temperature chamber connected to a heating system providing accurate controlled thermal conditions during testing. The Mini-Broutman specimens have a necked-shape resulting in maximum axial stress in the center of the specimen. Other advantages of the shape of these specimens are they resist buckling and kicking and the specimen symmetry results in zero shear stress at the sample's center [32].
of CNT in epoxy matrix, the electrical properties of equilateral cubic type nanocomposites was confirmed using a cubic specimen in Fig. 3(b). The electrical resistivity results in multiple directions (x, y, z) were 12.9 (±1.20), 12.6 (±2.0), and 13.4 (±1.9) kU cm, respectively. The degree of CNT dispersion in the CNT/epoxy nanocomposites could be “indirectly” confirmed by electrical resistivity measurement. Better dispersion generally resulted in lower as well and more uniform electrical resistivity and standard deviation, which is attributed to a better distribution of the CNT in the matrix as well as more and better electrical contact. Fig. 6 shows the measured electrical resistivity of CNT/epoxy nanocomposites using four-point electrical resistance measurement method. It exhibited a lower and more uniform electrical resistivity, even compared with previous research of CNT/polymer composites [31]. The relatively constant value of the resistivity observed, across the five regions is seen as an indication that the CNT/epoxy nanocomposite had a high degree of uniformity of dispersion at 0.3 vol%.
2.2.5. Measurement of interfacial shear strength Interfacial shear strength (IFSS) of the carbon fiber and CNT/ epoxy nanocomposites was measured by a microdroplet pull-out test. The schematic of test systems was shown in Fig. 4. Single carbon fibers were fixed in a steel frame (60 100 mm) at regular separated distances (5 mm). Microdroplets of CNT/epoxy nanocomposites were formed on each fiber using a tip-pin and fiber. The microdroplet specimen was fixed for testing by a microvice using a specially-designed micrometer. The IFSS was calculated from the measured pullout force F using the equation:
3.2. Comparison of thermal stability
t¼
F pDf L
(2)
where Df and L are the fiber diameter and embedded length, respectively. 3. Results and discussion 3.1. Dispersion state of CNT fillers Fig. 5 shows SEM photographs of fracture surfaces for CNT/ epoxy nanocomposites with 0.3 vol% concentrations at 5000 and 10,000 magnifications. The white segments in SEM photographs represent the ends of CNT in the fractured surfaces and the fracture surface morphology illustrates that the CNT is reasonably uniformly dispersed in the matrix. To prove the uniform dispersion
Fig. 7 shows TGA test results for neat epoxy resin and the CNT/ epoxy nanocomposites. Fig. 7(a) shows the TGA patterns for the two materials studied in air. In this case oxidative branching and cross linking appeared to be the prevalent degradation mechanisms. The epoxy matrix exhibited good thermal resistance and was relatively stable below 100 C. Volatilization of epoxy resin started at about 100 C, and the fragmentation of epoxy network was nearly completed at 300 C. Gaseous components such as water, carbon dioxide and methane were released during this stage. With further heating, carboxylic acids were formed before finally decomposing into cresols, carbonic oxide and carbon dioxide [33]. The neat epoxy resin was completely degraded at approximately 650 C as the weight loss approached 100%. However, the CNT/ epoxy nanocomposites had slightly more residual mass due to the incomplete oxidation of the charred material at the higher temperatures. To remove the influence of oxidation reaction, thermal degradation was observed for the neat epoxy resin and CNT/epoxy nanocomposites tested in nitrogen, as shown in Fig. 7(b). Structural damage of the epoxy network could be observed at temperatures higher than 100 C. In the CNT/epoxy nanocomposites, the structural damage was attenuated because of the reinforcing effect of the CNT filler. The presence of CNT significantly affected the thermal degradation over the full range of temperatures studied. For temperatures above 450 C, dehydration occurred and a carbonlike structure was gradually formed. Beyond this temperature, the curve was approximately flat. The addition of CNT filler to the epoxy resin resulted in increased thermal stability, particularly at the higher temperatures. 3.3. Mechanical properties of CNT/epoxy nanocomposites
Fig. 4. Schematic of test systems for microdroplet pull-out test.
Fig. 8 shows tensile and compressive test results for neat epoxy resin at room temperature and 150 C. In Fig. 8(a), the tensile modulus decreased at 150 C, and the tensile strength also decreased from 90 ± 7 to 79 ± 10 MPa. Compressive test results were showed in Fig. 8(b). The compressive modulus and compressive strength exhibited lower results at 150 C. The compressive strength of neat epoxy resin decreased from roughly 105 ± 12 to 67 ± 5 MPa at 150 C. On the other hand, as clearly shown in Fig. 9, both the tensile strength and the compressive strength of CNT/epoxy nanocomposites exhibited substantial increases with the addition of small amounts of CNT filler compared to values for the neat epoxy resin. Apparently, the applied stresses
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Fig. 5. FE-SEM photographs of CNT/epoxy nanocomposites: (a) 5000 and (b) 10000 magnification.
are more effectively transferred between the matrix and the accumulated CNT networks, resulting in significant improvements in mechanical properties. The tensile strength of CNT/epoxy nanocomposites decreased from 120 ± 9 to 114 ± 5 MPa, and compressive strength decreased from 156 ± 21 to 136 ± 26 MPa when the temperature was increased to 150 C. Especially at 150 C, tensile strength and the compressive strength of CNT/epoxy nanocomposites exhibited less decrease than if did for neat epoxy resin. It appears that the addition of 0.3 vol% such randomlyoriented CNT nanostructures in CNT/epoxy nanocomposites contribute to maintaining the mechanical properties at 150 C. The schematics, in Fig. 8, illustrate a possible mechanism for the reinforcing effect of CNT on mechanical properties of nanocomposites at 150 C. As the temperature increased, the free volume in neat epoxy increased and a molecular tie chain might be broken as a result of thermal expansion. Therefore, both tensile and compressive strength of neat epoxy decreases significantly at 150 C. CNT/ epoxy nanocomposites, exhibit more limited thermal expansion and structure damage due to the CNT reinforcing network. At the same time, a part of the thermal energy is absorbed by CNT. This might explain (at least in part) the smaller lose in tensile and compressive strength of CNT/epoxy nanocomposites than in the neat resin.
Fig. 6. Electrical resistivity measurement of CNT/epoxy nanocomposites using fourpoint method.
3.4. Interfacial properties of CNT/epoxy nanocomposites Fig. 10 shows typical curves of pull-out force versus extension for neat epoxy microdroplet tests, at the two temperatures. These plots show that the microdroplet's pull-out or slippage force was larger at 25 C than at 150 C. At 150 C, first slippage was
Fig. 7. TGA analysis of CNT/epoxy nanocomposites in: (a) air and (b) nitrogen.
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Fig. 8. Mechanical tests on neat epoxy: (a) tensile and (b) compressive.
Fig. 9. Mechanical tests on CNT/epoxy nanocomposites: (a) tensile and (b) compressive.
accompanied by a sudden drop in load to nearly zero, followed by subsequent increases and drops in load analogous to ‘slip-stick’ behavior often observed in adhesive peel tests. On the other hand for tests conducted at 25 C, at the first occurrence of any slippage the load dropped immediately to zero, somewhat analogous to brittle fracture. Fig. 11 shows the microdroplet failure load (the F in Equation (1) used to calculate the IFSS) between carbon fiber and epoxy resin at the two temperatures. It is clear that there are two distinct patterns or regions of behavior. At first, microdroplet slippage occurred when the applied force exceeded the value of the interfacial adhesion force (solid symbols). And then in the second region, fiber fracture occurred when the value of the interfacial adhesion force exceeded the carbon fiber's tensile strength (hollow symbols). The IFSS between carbon fiber and neat epoxy resin was calculated using the critical point in the test results. It decreased from 70 ± 9 MPa to 50 ± 6 MPa when the test temperature was increased from room temperature to 150 C. Fig. 12 shows typical force versus extension curves of CNT/epoxy nanocomposites microdroplet tests at the two temperatures. This figure also shows that the microdroplet's pull-out force was larger
at 25 C than at 150 C. Fig. 13 shows results similar to those in Fig. 11 but for CNT/epoxy nanocomposites microdroplet tests. The IFSS between carbon fiber and CNT/epoxy nanocomposites decreased from 86 ± 7 MPa to 73 ± 6 MPa when the temperature was increased from 25 C to 150 C. The experimental results on the mechanical and interfacial properties of neat epoxy and CNT/epoxy nanocomposites at the different temperatures are summarized in Table 1. At 150 C, CNT/epoxy nanocomposites exhibited lower interfacial adhesion with a carbon fiber than at room temperature. Illustration in Figs. 10 and 12 shows the comparison of pull-out failure patterns for neat epoxy and CNT/epoxy nanocomposites on carbon fiber before and after microdroplet pull-out tests. The pullout patterns illustrated in these photographs are consistent with the test curves. In the microdroplet test on the neat epoxy, a 90 mm microdroplet was fixed on carbon fiber, and epoxy microdroplet debonded and slid along the carbon fiber during pull-out test. Since the tensile strength of carbon fiber was higher than interfacial adhesion between carbon fiber and neat epoxy, epoxy microdroplet would not stay on carbon fiber under increased tensile stress. Although the sizes of two microdroplets were nearly
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Fig. 10. Force versus extension curves for a microdroplet pull-out test for neat epoxy at different temperatures.
Fig. 12. Force versus extension curves for a microdroplet pull-out test for CNT/epoxy nanocomposite at different temperatures.
Fig. 11. Microdroplet pull-out results for neat epoxy at different temperatures.
Fig. 13. Microdroplet pull-out results for CNT/epoxy nanocomposites at different temperatures.
same, the slippage force was higher for CNT/epoxy nanocomposites on carbon fiber than the neat epoxy case due to the better stress transferring effect by the nanostructures of CNT. After slippage, edge part of microdroplet remained at original position, because the interfacial adhesion was even higher than fracture force of microdroplet. In the case of CNT/epoxy composites, it exhibited a larger residual part of microdroplet due to the better interfacial adhesion compare with neat epoxy. 4. Conclusions Epoxy MY-720 is shown to have good thermal stability and reasonable mechanical properties at 150 C. However, CNT/epoxy nanocomposites with concentrations of CNT of only 0.3 vol% exhibited significantly better mechanical and interfacial properties than did the neat epoxy resin. Electrical resistivity analysis indicated that the CNT was uniformly dispersed in the composites
matrix which contributed to it effectively reinforcing the epoxy matrix. Major findings from this study were as follows: 1) Even at this relatively low concentration, the CNT reinforcement significantly improved the mechanical properties of these nanocomposites. 2) The IFSS between a carbon fiber and CNT/epoxy nanocomposites was also greatly improved over that between a carbon fiber and neat epoxy which is attributed to improved thermal stability due to the thermal resistance of nano-structures at the interface. 3) The mechanical and interfacial properties at 150 C were significantly worse for neat epoxy resin than those at room temperature. In contrast, CNT/epoxy nanocomposites had only slightly lower tensile and compressive strength at 150 C than those at room temperature.
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Table 1 Comparison of mechanical and interfacial properties of CNT/epoxy nanocomposites. [15] Material
Neat epoxy CNT/epoxy nanocomposites a
Temperature ( C) 25 150 25 150
Tensile strength (MPa) 90 79 120 114
a
(7) (10) (9) (15)
Compressive strength (MPa)
IFSS (MPa)
105 67 156 136
70 50 86 73
(12) (5) (21) (26)
[16] (9) (6) (7) (6)
Standard deviation.
Acknowledgments This work was supported by Agency for Defense Development under the contract UE135026GD, 2013e2015.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Inherent and interfacial evaluations of carbon nanotubes/epoxy composites and single carbon fiber at different temperatures Zuo-Jia Wang a, Dong-Jun Kwon a, Jin-Yeong Choi a, Pyeong-Soo Shin a, Jin-Woo Yi b, Joon-Hyung Byun b, Hyung-Ik Lee c, Jong-Kyoo Park c, K. Lawrence DeVries d, Joung-Man Park a, d, * a
Department of Materials Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 660-701, Republic of Korea Korea Institute of Materials Science, Changwon 641-010, Republic of Korea c Agency for Defense Development, 4-R&D Center, Daejeon 305-600, Republic of Korea d Department of Mechanical Engineering, The University of Utah, Salt Lake City, UT 84112, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 October 2014 Received in revised form 28 August 2015 Accepted 26 December 2015 Available online 3 February 2016
In order to develop an improved processability for machines and equipment operating at high temperatures, thermal degradation resistance of a high temperature epoxy network became a hot issue. Nanocomposites were not only used for improving mechanical and electrical properties. In here, the thermal stability of carbon nanotube (CNT)/epoxy nanocomposite was investigated by mechanical and interfacial evaluation. The dispersion state was monitored and observed by electrical resistance measurements as well as by using a scanning electron microscope (SEM). At low concentration of 0.3 vol%, the CNT reinforcement significantly improved the mechanical and interfacial properties of nanocomposites. At 150 C, the CNT/epoxy nanocomposites exhibited much better thermal stability than neat epoxy resin as a result of the thermal resistance of CNTs. Microstructural studies revealed that the uniformity of the CNT dispersion played an important role in this thermal resistance. Addition of low concentration CNT could improve both mechanical properties and thermal stability of epoxy resin for high temperature applications. © 2016 Elsevier Ltd. All rights reserved.
Keywords: A. Nano-structures B. Thermomechanical D. Electron microscopy D. Thermal analysis
1. Introduction High temperature polymers able to operate continuously at high temperatures are an important class of polymers for use in electronic, automotive, and aerospace applications [1,2]. The majority of these high temperature polymers fall into one of six material types: epoxy, phenolic, bismalimide, cyanate ester, bismalimidetriazine (BT), and polyimide, with each material type having advantages and disadvantages compared to the others. Among these materials, epoxy tends to exhibit the best processability but poorer thermal stability [3]. It was reported that the maximum temperature of use for epoxy resins is only approximately 60 C in continuous exposure and about 100e120 C for short term exposures [4]. The behavior in fire of the epoxy resin is very important when used as composite
* Corresponding author. Department of Materials Engineering and Convergence Technology, Engineering Research Institute, Gyeongsang National University, Jinju 660-701, Republic of Korea. Tel.: þ82 55 772 1656; fax: þ82 55 772 1659. E-mail address: [email protected] (J.-M. Park). http://dx.doi.org/10.1016/j.compositesb.2015.12.036 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
matrix. The poor fire resistance in high temperatures represents the main obstacles for the development of epoxy resin. The one of important objective of this study consists in solving these drawbacks. Some recent formulations have extended the continuous upper operating temperature. Generally prolonged exposure to high temperatures results in degradation of the epoxy resin, possibly causing a loss of mechanical strength and interface adhesion. The mass and adhesion loss are mutually dependent on chemical network degradation. Network degradation will affect loss of inherent properties by breaking bonds which will reduce the mechanical strength of the epoxy and lower the adhesion strength [5]. Typically these high temperature epoxy resins have greater crosslink densities and glass transition temperatures, providing a considerable increase in their thermal, dynamic, mechanical and adhesive properties. The thermal degradation of special designed network known to have good thermal stability is studied by Tg reduction measurements [6]. However, the demand of epoxy resins for high temperature applications cannot be satisfied by these virgin special designed epoxy resins as the developing of industry science.
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The polymer needs to be more stable to resist network degradation, leading to a degradation at longer time for a similar temperature. The appearance of high performance reinforcing materials provides a new entry to improve the thermal stability of epoxy resins. Polymer-based composites which are reinforced with nanoparticles are of great interest from an industrial and scientific perspective because they exhibit much better mechanical and thermal behavior and characteristics than more conventional composites [7e10]. Carbon nanotubes (CNT) have attracted a good deal of interest due to their unique one-dimensional structure and novel properties [11e13]. The outstanding intrinsic mechanical properties of CNT have resulted in considerable attention for use as reinforcement for composite materials [14,15]. These benefits associated with composite design could thus be implemented on the nanoscale. It has been reported on the effect of the addition of CNT to polymer composites on thermo-mechanical and ablation performance [16]. Studies have shown that nanocomposites containing CNT can achieve improved mechanical and interfacial properties by taking advantage of enhanced stress transfer and interfacial friction between individual nanotubes and the matrix [17,18]. Recent research demonstrates that by adopting the multiscale reinforcements, significant improvements are achieved in glass fiber and carbon fiber composites, especially for the fiberematrix interphase and matrix dominated performances [19,20]. CNT and carbon fibers were incorporated into an epoxy matrix to fabricate a high performance multiscale composite and to improve the stress transfer between epoxy and carbon fibers [21,22]. Comparing to composite laminate material without carbon nanotube reinforcements, there are modest improvements in the mechanical properties of strength and stiffness. However, a potentially significant increase was demonstrated for the long-term fatigue life of these carbon nanotube reinforced composite materials [23]. As a consequence, significant improvements in the mechanical properties of tensile strength, stiffness and resistance to failure resulted for this carbon fiber reinforced epoxy composite [24,25]. The contact angle reaches the minimum when the multi-walled CNT contents equal to 0.5 wt%. The composite transverse tensile strength shows a similar tendency as the contact angle with increasing the CNT content. The introduction of CNT at a proper content into the epoxy resin can significantly enhance the cryogenic CF/epoxy interfacial and normal adhesion property [26]. The interlaminar shear strength (ILSS) of the CFRP composites with 0.5 wt% CNTs increased by 12% compared to the neat CFRP composites [27]. It is clear that the introduction of CNT at a proper content into the epoxy resin can significantly enhance the CF/epoxy interfacial normal adhesion property [28,29]. In this work, CNT-epoxy nanocomposites with relatively uniform CNT dispersion were fabricated for relative high temperature applications. Thermal stability of nanocomposites was investigated using mechanical testing and interfacial evaluation methods. At 150 C, CNT/epoxy nanocomposites exhibited better mechanical and interfacial properties than did neat epoxy resin. Based on the morphology observation, electrical resistivity and thermal analysis, the uniform dispersion of the CNT in epoxy matrix contributed to the thermal resistance of the nanocomposites. The nanostructures of CNT could work as reinforcing and stress transferring phase to provide better mechanical and interfacial properties of CNT/epoxy nanocomposites.
used as reinforcing materials in the nanocomposites. The CNT had a range of diameters (10e25 nm), lengths (5e10 mm) and density (1.995 g/cm3). Epoxy Resin (MY-720, Huntsman Co., USA) was used as the composite matrix. Fig. 1 shows the chemical structures of this epoxy resin and curing agent. The ratio of the epoxy resin to the curing agent was 7:3. Carbon Fiber (T700S, Toray Inc., Japan) was used as the reinforcing fiber. Acetone (Dae Jung Chemical, Co., Korea) was used as the CNT dispersing solvent.
2.2. Methodologies 2.2.1. Fabrication process of CNT/epoxy nanocomposites Fig. 2 shows details of the fabrication process for the CNT/epoxy nanocomposites used in this study [30]. First the epoxy resin was diluted using acetone with mechanical mixing. The CNT solvent mixture was then dispersed in the epoxy solution under sonication for 6 hours in a sealed beaker. The solvent was next evaporated under sonication for 12 hours. The residual solvent was further eliminated using a vacuum oven at 50 C for 24 hours. While several concentrations of CNT in the CNT/epoxy nanocomposites were investigated in previous studies, an optimal concentration of a uniform dispersion of CNT was found to be 0.3 vol% [31]. This was the concentration used in the composites in this study. After the dispersion and mixing process of CNT in the epoxy matrix, the curing process involved 4temperature steps: 1 hour at 80 C, followed by 1 hour at 100 C, 4 hours at 150 C, and then finally 2 hours at 177 C. The state of dispersion of CNT in the CNT/epoxy nanocomposites was observed using a SEM (Philips XL30 S FEG, Netherlands).
2.2.2. Electrical resistivity measurements of CNT/epoxy nanocomposites The electrical resistance of the CNT/epoxy nanocomposites was measured by the four-point method using a multimeter (HP34401A), as shown schematically in Fig. 3(a). The dimension of test specimens was 1 3 40 mm. Electrical contact points which located at regularly spaced intervals (5 mm), along the specimen using embedded copper wire without silver paste (copper wires were fixed in silicone mold before specimen preparation), were used to determine the electrical volumetric resistivity. The volumetric resistivity is the resistance per unit volume of the bulk material, and was used to provide a measure of the state of dispersion of the CNT filler in the nanocomposites. This electrical volumetric resistivity is determined by:
2. Experimental 2.1. Materials Multi-wall carbon nanotubes (CM-95, Hanwha Nanotech Co., Korea), produced by a chemical vapor deposition process, were
Fig. 1. Chemical structure of epoxy MY-720: (a) epoxy resin and (b) curing agent.
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Fig. 2. Schematic diagram of the fabrication process of CNT/epoxy nanocomposites.
rv ¼
Av Lec
Rv ðU$cmÞ
(1)
where rv is the resistivity, Rv is the measured electrical resistance, Av is the cross-sectional area, and Lec is the spacing between the electrical contact points. The relatively uniform dispersion of the CNT filler was thought to very favorably contribute to the thermal resistance of the nanocomposites. To prove that the CNT preferred orientation is negligible, the electrical resistivity of CNT/epoxy nanocomposites was measured in three orthogonal directions. The nanocomposites specimen was
prepared as an equilateral cubic (10 10 10 mm) as shown in Fig. 3(b), and the electrical resistivity in three orthogonal directions were measured. To progress the electrical resistivity measurement, copper wires were fixed on specimen surface with silver paste. The outer probes were fixed on surface in three orthogonal directions, and inner probes were spaced 1 mm apart. 2.2.3. Thermal analysis of CNT/epoxy nanocomposites The thermal stability of the produced materials was investigated using a TGA instrument (Q50, TA Instruments, U.S.A.). Measurements conducted in both air and nitrogen at dynamic scans of 10 C/min from 30 C to 900 C were used to compare the TGA of CNT/epoxy nanocomposites with that of neat epoxy resin.
Fig. 3. Schematic of the volumetric resistivity test specimens: (a) rectangular and (b) cubic.
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2.2.4. Mechanical property measurements Tensile and compressive tests were used in the determination of mechanical properties of neat epoxy resin and the CNT/epoxy nanocomposites at different temperatures. Tensile dog-bone and Mini-Broutman compressive specimens were cast in silicone molds, according to standard specifications. The dimension of tensile specimens was 1 3 20 mm, and the practical compression area of Mini-Broutman specimens were 3 6 mm2. Mechanical tests were then performed using a Universal Testing Machine (LR 10 K, Lloyd Co., UK) at a test speed of 0.5 mm/min, and the stress direction in tensile and compressive testing was same as the horizontal direction during curing. A temperature chamber connected to a heating system providing accurate controlled thermal conditions during testing. The Mini-Broutman specimens have a necked-shape resulting in maximum axial stress in the center of the specimen. Other advantages of the shape of these specimens are they resist buckling and kicking and the specimen symmetry results in zero shear stress at the sample's center [32].
of CNT in epoxy matrix, the electrical properties of equilateral cubic type nanocomposites was confirmed using a cubic specimen in Fig. 3(b). The electrical resistivity results in multiple directions (x, y, z) were 12.9 (±1.20), 12.6 (±2.0), and 13.4 (±1.9) kU cm, respectively. The degree of CNT dispersion in the CNT/epoxy nanocomposites could be “indirectly” confirmed by electrical resistivity measurement. Better dispersion generally resulted in lower as well and more uniform electrical resistivity and standard deviation, which is attributed to a better distribution of the CNT in the matrix as well as more and better electrical contact. Fig. 6 shows the measured electrical resistivity of CNT/epoxy nanocomposites using four-point electrical resistance measurement method. It exhibited a lower and more uniform electrical resistivity, even compared with previous research of CNT/polymer composites [31]. The relatively constant value of the resistivity observed, across the five regions is seen as an indication that the CNT/epoxy nanocomposite had a high degree of uniformity of dispersion at 0.3 vol%.
2.2.5. Measurement of interfacial shear strength Interfacial shear strength (IFSS) of the carbon fiber and CNT/ epoxy nanocomposites was measured by a microdroplet pull-out test. The schematic of test systems was shown in Fig. 4. Single carbon fibers were fixed in a steel frame (60 100 mm) at regular separated distances (5 mm). Microdroplets of CNT/epoxy nanocomposites were formed on each fiber using a tip-pin and fiber. The microdroplet specimen was fixed for testing by a microvice using a specially-designed micrometer. The IFSS was calculated from the measured pullout force F using the equation:
3.2. Comparison of thermal stability
t¼
F pDf L
(2)
where Df and L are the fiber diameter and embedded length, respectively. 3. Results and discussion 3.1. Dispersion state of CNT fillers Fig. 5 shows SEM photographs of fracture surfaces for CNT/ epoxy nanocomposites with 0.3 vol% concentrations at 5000 and 10,000 magnifications. The white segments in SEM photographs represent the ends of CNT in the fractured surfaces and the fracture surface morphology illustrates that the CNT is reasonably uniformly dispersed in the matrix. To prove the uniform dispersion
Fig. 7 shows TGA test results for neat epoxy resin and the CNT/ epoxy nanocomposites. Fig. 7(a) shows the TGA patterns for the two materials studied in air. In this case oxidative branching and cross linking appeared to be the prevalent degradation mechanisms. The epoxy matrix exhibited good thermal resistance and was relatively stable below 100 C. Volatilization of epoxy resin started at about 100 C, and the fragmentation of epoxy network was nearly completed at 300 C. Gaseous components such as water, carbon dioxide and methane were released during this stage. With further heating, carboxylic acids were formed before finally decomposing into cresols, carbonic oxide and carbon dioxide [33]. The neat epoxy resin was completely degraded at approximately 650 C as the weight loss approached 100%. However, the CNT/ epoxy nanocomposites had slightly more residual mass due to the incomplete oxidation of the charred material at the higher temperatures. To remove the influence of oxidation reaction, thermal degradation was observed for the neat epoxy resin and CNT/epoxy nanocomposites tested in nitrogen, as shown in Fig. 7(b). Structural damage of the epoxy network could be observed at temperatures higher than 100 C. In the CNT/epoxy nanocomposites, the structural damage was attenuated because of the reinforcing effect of the CNT filler. The presence of CNT significantly affected the thermal degradation over the full range of temperatures studied. For temperatures above 450 C, dehydration occurred and a carbonlike structure was gradually formed. Beyond this temperature, the curve was approximately flat. The addition of CNT filler to the epoxy resin resulted in increased thermal stability, particularly at the higher temperatures. 3.3. Mechanical properties of CNT/epoxy nanocomposites
Fig. 4. Schematic of test systems for microdroplet pull-out test.
Fig. 8 shows tensile and compressive test results for neat epoxy resin at room temperature and 150 C. In Fig. 8(a), the tensile modulus decreased at 150 C, and the tensile strength also decreased from 90 ± 7 to 79 ± 10 MPa. Compressive test results were showed in Fig. 8(b). The compressive modulus and compressive strength exhibited lower results at 150 C. The compressive strength of neat epoxy resin decreased from roughly 105 ± 12 to 67 ± 5 MPa at 150 C. On the other hand, as clearly shown in Fig. 9, both the tensile strength and the compressive strength of CNT/epoxy nanocomposites exhibited substantial increases with the addition of small amounts of CNT filler compared to values for the neat epoxy resin. Apparently, the applied stresses
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Fig. 5. FE-SEM photographs of CNT/epoxy nanocomposites: (a) 5000 and (b) 10000 magnification.
are more effectively transferred between the matrix and the accumulated CNT networks, resulting in significant improvements in mechanical properties. The tensile strength of CNT/epoxy nanocomposites decreased from 120 ± 9 to 114 ± 5 MPa, and compressive strength decreased from 156 ± 21 to 136 ± 26 MPa when the temperature was increased to 150 C. Especially at 150 C, tensile strength and the compressive strength of CNT/epoxy nanocomposites exhibited less decrease than if did for neat epoxy resin. It appears that the addition of 0.3 vol% such randomlyoriented CNT nanostructures in CNT/epoxy nanocomposites contribute to maintaining the mechanical properties at 150 C. The schematics, in Fig. 8, illustrate a possible mechanism for the reinforcing effect of CNT on mechanical properties of nanocomposites at 150 C. As the temperature increased, the free volume in neat epoxy increased and a molecular tie chain might be broken as a result of thermal expansion. Therefore, both tensile and compressive strength of neat epoxy decreases significantly at 150 C. CNT/ epoxy nanocomposites, exhibit more limited thermal expansion and structure damage due to the CNT reinforcing network. At the same time, a part of the thermal energy is absorbed by CNT. This might explain (at least in part) the smaller lose in tensile and compressive strength of CNT/epoxy nanocomposites than in the neat resin.
Fig. 6. Electrical resistivity measurement of CNT/epoxy nanocomposites using fourpoint method.
3.4. Interfacial properties of CNT/epoxy nanocomposites Fig. 10 shows typical curves of pull-out force versus extension for neat epoxy microdroplet tests, at the two temperatures. These plots show that the microdroplet's pull-out or slippage force was larger at 25 C than at 150 C. At 150 C, first slippage was
Fig. 7. TGA analysis of CNT/epoxy nanocomposites in: (a) air and (b) nitrogen.
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Fig. 8. Mechanical tests on neat epoxy: (a) tensile and (b) compressive.
Fig. 9. Mechanical tests on CNT/epoxy nanocomposites: (a) tensile and (b) compressive.
accompanied by a sudden drop in load to nearly zero, followed by subsequent increases and drops in load analogous to ‘slip-stick’ behavior often observed in adhesive peel tests. On the other hand for tests conducted at 25 C, at the first occurrence of any slippage the load dropped immediately to zero, somewhat analogous to brittle fracture. Fig. 11 shows the microdroplet failure load (the F in Equation (1) used to calculate the IFSS) between carbon fiber and epoxy resin at the two temperatures. It is clear that there are two distinct patterns or regions of behavior. At first, microdroplet slippage occurred when the applied force exceeded the value of the interfacial adhesion force (solid symbols). And then in the second region, fiber fracture occurred when the value of the interfacial adhesion force exceeded the carbon fiber's tensile strength (hollow symbols). The IFSS between carbon fiber and neat epoxy resin was calculated using the critical point in the test results. It decreased from 70 ± 9 MPa to 50 ± 6 MPa when the test temperature was increased from room temperature to 150 C. Fig. 12 shows typical force versus extension curves of CNT/epoxy nanocomposites microdroplet tests at the two temperatures. This figure also shows that the microdroplet's pull-out force was larger
at 25 C than at 150 C. Fig. 13 shows results similar to those in Fig. 11 but for CNT/epoxy nanocomposites microdroplet tests. The IFSS between carbon fiber and CNT/epoxy nanocomposites decreased from 86 ± 7 MPa to 73 ± 6 MPa when the temperature was increased from 25 C to 150 C. The experimental results on the mechanical and interfacial properties of neat epoxy and CNT/epoxy nanocomposites at the different temperatures are summarized in Table 1. At 150 C, CNT/epoxy nanocomposites exhibited lower interfacial adhesion with a carbon fiber than at room temperature. Illustration in Figs. 10 and 12 shows the comparison of pull-out failure patterns for neat epoxy and CNT/epoxy nanocomposites on carbon fiber before and after microdroplet pull-out tests. The pullout patterns illustrated in these photographs are consistent with the test curves. In the microdroplet test on the neat epoxy, a 90 mm microdroplet was fixed on carbon fiber, and epoxy microdroplet debonded and slid along the carbon fiber during pull-out test. Since the tensile strength of carbon fiber was higher than interfacial adhesion between carbon fiber and neat epoxy, epoxy microdroplet would not stay on carbon fiber under increased tensile stress. Although the sizes of two microdroplets were nearly
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Fig. 10. Force versus extension curves for a microdroplet pull-out test for neat epoxy at different temperatures.
Fig. 12. Force versus extension curves for a microdroplet pull-out test for CNT/epoxy nanocomposite at different temperatures.
Fig. 11. Microdroplet pull-out results for neat epoxy at different temperatures.
Fig. 13. Microdroplet pull-out results for CNT/epoxy nanocomposites at different temperatures.
same, the slippage force was higher for CNT/epoxy nanocomposites on carbon fiber than the neat epoxy case due to the better stress transferring effect by the nanostructures of CNT. After slippage, edge part of microdroplet remained at original position, because the interfacial adhesion was even higher than fracture force of microdroplet. In the case of CNT/epoxy composites, it exhibited a larger residual part of microdroplet due to the better interfacial adhesion compare with neat epoxy. 4. Conclusions Epoxy MY-720 is shown to have good thermal stability and reasonable mechanical properties at 150 C. However, CNT/epoxy nanocomposites with concentrations of CNT of only 0.3 vol% exhibited significantly better mechanical and interfacial properties than did the neat epoxy resin. Electrical resistivity analysis indicated that the CNT was uniformly dispersed in the composites
matrix which contributed to it effectively reinforcing the epoxy matrix. Major findings from this study were as follows: 1) Even at this relatively low concentration, the CNT reinforcement significantly improved the mechanical properties of these nanocomposites. 2) The IFSS between a carbon fiber and CNT/epoxy nanocomposites was also greatly improved over that between a carbon fiber and neat epoxy which is attributed to improved thermal stability due to the thermal resistance of nano-structures at the interface. 3) The mechanical and interfacial properties at 150 C were significantly worse for neat epoxy resin than those at room temperature. In contrast, CNT/epoxy nanocomposites had only slightly lower tensile and compressive strength at 150 C than those at room temperature.
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Table 1 Comparison of mechanical and interfacial properties of CNT/epoxy nanocomposites. [15] Material
Neat epoxy CNT/epoxy nanocomposites a
Temperature ( C) 25 150 25 150
Tensile strength (MPa) 90 79 120 114
a
(7) (10) (9) (15)
Compressive strength (MPa)
IFSS (MPa)
105 67 156 136
70 50 86 73
(12) (5) (21) (26)
[16] (9) (6) (7) (6)
Standard deviation.
Acknowledgments This work was supported by Agency for Defense Development under the contract UE135026GD, 2013e2015.
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[21]
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