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Composites Part B 172 (2019) 447–457
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Radiation resistance of carbon fiber-reinforced epoxy composites optimized synergistically by carbon nanotubes in interface area/matrix Minjie Yan, Liangsen Liu, Li Chen, Nan Li, Yaming Jiang, Zhiwei Xu *, Miaolei Jing, Yanli Hu, Liyan Liu, Xingxiang Zhang Key Laboratory of Advanced Braided Composites, Ministry of Education, School of Textiles, Tianjin Polytechnic University, Tianjin, 300387, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Gamma irradiation Radiation resistance Secondary curing Enhance Carbon nanotubes
In order to investigate the effect of the addition of carbon nanotubes (CNTs) in carbon fibers reinforced epoxy matrix composites (CF/EP) on the ability of composites to resist γ radiation. We used γ irradiation and inves tigated composites, γ-CF/EP without CNTs; γ-CF/EP with CNTs in matrix (γ-CF/EP-CNTs); γ-CF/EP with CNTs in interface (γ-CF-CNTs/EP) and γ-CF/EP with CNTs both in the interface area and matrix (γ-CF-CNTs/EP-CNTs). The results show that the bending strength and bending modulus of γ-CF-CNTs/EP-CNTs are 28.59% and 41.44% higher than that of γ-CF/EP before fatigue testing, and 32.19% and 73.51% higher than that of gamma-CF/EP after fatigue testing. Ultrasonic C-scan microscopic examination after fatigue test showed that the internal structure damage of γ-CF-CNTs/EP-CNTs was the smallest, while γ-CF/EP was the largest. The storage modulus of the three modified composites are significantly improved compared with the unmodified composite, especially the storage modulus of γ-CF-CNTs/EP-CNTs is sharply improved from 2.53 GPa (γ-CF/EP) to 5.66 GPa (γ-CFCNTs/EP-CNTs). X-ray spectroscopy shows changes in the surface and internal valence content of the composites, further revealing the crosslinking reaction and the secondary curing reaction. Combined with scanning electron microscopy and crack propagation models, the microstructure and enhancement mechanisms of CNTs-reinforced composites for anti-gamma radiation are discussed. The CNTs synergistic enhance the composite’s anti-gamma radiation performance with excellent effects when CNTs are added together into matrix and interface area.
1. Introduction Its development is very rapid about the aerospace industry and nu clear reactors in recent decades, but the space or nuclear reaction ra diation environment is harsh toward materials [1,2]. Various irradiating rays exist in the atmosphere, such as gamma rays, protons, electrons, ultraviolet rays, etc [3]. γ ray is one of the most energetic energy and has a great destructive effect on materials [4]. The working period of ma terials will be reduced because γ-irradiation may cause degradation in material properties like thermal stability, mechanical, optical, magnetic properties, fatigue etc [5,6]. Traditional materials like lead, concrete and ceramic are heavy, poor in mechanical property, resistance to corrosion, resistance to radiation [7–9], etc, so some researchers focus on carbon fiber/epoxy composites, which can are used on the aerospace industry and nuclear reactors to provide power for future nuclear power plant and spacecraft [10,11]. Due to their resistant to radiation, high specific strengths, low densities, lightweight, rigidity, good fatigue,
resistance to creep properties and capability of molding into multiple shapes, etc, the fiber reinforced polymer composites are being increas ingly employed in the aerospace industry and nuclear reactors [12–17]. Carbon fibers (CFs) are selected as structural and functional materials for many applications due to its excellent mechanical properties, ther mal stability and radiation resistance [18–20]. CFs exhibit a crystalline graphite base with a non-polar surface and chemical inertness due to the high temperature carbonization/graphitization step in the manufacturing process. Some investigations have found that epoxy resin has excellent mechanical strength and heat stability as a new class of matrix material for advanced carbon fiber reinforced epoxy resin matrix composites (CF/EP) [21,22]. Epoxy resins exhibit good thermal stability and strong radiation resistance, but when exposed to high energy gamma rays, they degrade rapidly and mechanical properties are dras tically reduced. In addition, CF/EP exhibit reduced physical-chemical properties when exposed to high-energy gamma rays, especially resin degradation and interface area debonding [23,24], which are caused by
* Corresponding author. E-mail address: [email protected] (Z. Xu). https://doi.org/10.1016/j.compositesb.2019.04.041 Received 16 October 2018; Received in revised form 4 April 2019; Accepted 29 April 2019 Available online 30 April 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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the disadvantages of fibers and resin [25]. The interphase between the reinforcing CFs and the matrix that likes “heart” plays an important role in combining and transferring the internal pressure from matrix to the CFs [26,27]. Some experts found that carbon nanotubes (CNTs) can exhibit superior mechanical [28–32], electrical and thermal properties such as low density [33], high stiffness [34], radiation resistance etc [35, 36], which have been applied in many fields. The CNTs exhibited larger specific surface area and pore volume after gamma irradiation and gamma-radiation is better for rearrangement and/or reconstruction of functional groups on surface [37]. CNTs greatly enhance the mechanical properties of composites, which can be attributed to the chain entan glement and conformation in elastomers [38]. CNTs can also act as free radical scavenger because the amount of free radicals is reduced by the gamma irradiation process as the CNTs concentration increases [39–41]. In order to protect and improve the resin’s resistance to degradation and interfacial peeling of CF/EP, we chose CNTs as a modifier and free radical scavenger [42]. According to the characteristics of CFs surface and EP resin flow, the introduction of CNTs into CF/EP has been achieved. There are a variety of physical and chemical methods [14,16,43–45], like (1) chemical re actions between fibers and functionalized CNTs; (2) direct growth of CNTs on CFs surface by chemical vapor deposition; (3) infiltrating CFs in the solution of CNTs; (4) coating of CFs with CNTs- including sizing; (5) electrophoretic deposition of CNTs on CFs surface. Due to electropho retic deposition technology is an effective way to comprise CNTs in composites which has the advantages of uniform deposition and simplification and could be employed to continuous treating of CFs tows, so we take this approach. There are three different ways to add CNTs to optimize CF/EP: firstly, mixing CNTs completely throughout the matrix (matrix modification); secondly, attaching CNTs straightly onto CFs surface (interface area modification); and last but most important, the combination of one and two (matrix modification and interface area modification cooperation). There is a wide range and complex of chemical processes including chain scission and cross-linking that can be induced by radiochemical or thermal oxidation [46,47]. The purpose of our study was to investigate the mechanical and thermal properties of CNTs as reinforcements added to CF/EP composites after gamma ray irradiation at 7 MGy [48,49]. In this paper, the 7 MGy gamma irradiation dose was chosen to simulate the harsh radiation environment around the nuclear power plant and in space [50]. To investigate its ability to resist gamma- irradiation, the four composite samples were compared with that of the different adding techniques. CNTs are added to the base epoxy resin to enhance the corrosion resistance and degradation resistance of the composites [51, 52]. The introduction of CNTs at the interface area between the fiber and the resin is to prevent delamination and exfoliation from being affected by high energy gamma rays. In order to synergistically enhance the anti-decomposition and anti-debonding properties of the composite, CNTs are simultaneously introduced into interface area and matrix resin. Four different samples were placed together in the same irradia tion environment. In order to compare the radiation resistance of four different sam ples, we analyzed the macroscopic and microscopic damage of the samples after different tests. SEM (Scanning electron microscope) was used to observe the morphology of CNTs on the surface of CFs and section morphology after mechanical testing. Using three-point short beam bending method to explore the bending performance and fatigue resistance of four composite materials by using a universal testing ma chine [53–55]. Residual strength, which is often considered the fatigue life forecast of CF/EP, was acquired to clarify damage capacity of CF/E We introduced ultrasonic C-scan analysis of CF/EP after residual flexure strength testing. All tests were performed at normal temperature and the final values were measured as averages of five samples for each CF/E The thermal stability and crosslink density were characterized by using thermogravimetric analysis (TGA) [56–58]. Dynamic mechanical ther mal analysis (DMA) could express the viscoelasticity characteristics of
CF/EP and was adopted to define storage modulus (E’) and loss factor (tanδ) of CF/EP before and after irradiation [59,60]. Other than that, the destructive form is taken by recording the trace of free radicals con centrations and surveying formation and ingredient change of surfaces and interiors of different samples before and after irradiation by X-ray photoelectron spectroscopy (XPS) [61,62]. In order to explain the damage mechanism in more detail, different schematics are introduced to simulate the crack propagation path. Due to the large amount of data and the length of the article, all data of the four unirradiated composites are reflected in the Supplementary Materials(SM). 2. Experimental 2.1. Materials We selected multi-wall CNTs with length of 10–20 μm, the diameter of 30–50 nm and purity over 95% which were purchased from Chengdu Organic Chemicals Co. Ltd, Chinese Academy of Sciences. The Commercially-available T700S CFs as reinforcement were purchased from Toray, with an average diameter of 7 μm were employed. Chang shu Jiafa provides chemical JC-02A modified matrix and JH-0511 modified 2-ethyl-4-methylimidazole as accelerator. We chose tetrahy drophthalic anhydride as curing agent, which was purchased from Wenzhou Qingming Chemical Co., Ltd. 2.2. Introducing CNTs into the surface of CFs and matrix To remove the commercial sizing, the original CFs was refluxed with acetone in the soxhlet apparatus for 48 h at 70 � C, because CNTs are difficult to stay on the virgin CFs surface. Subsequently, CFs were seasoned 8 h under 60 � C in the oven. In order to introduce groups into the CNT surface, CNTs were placed in a mixture of concentrated nitric acid (HNO3)/concentrated sulfuric acid (H2SO4) (1:3 v/v) at 80 � C for 8 h. The follow is the electrophoretic deposition parameters: concen tration of CNTs in distilled water 0.3 mg/ml, voltage 24V, time 5 min, electrode materials metal panel and electrode spacing 50 mm. Func tionalized CNTs were distributed in distilled water by an ultrasonic bath for 2 h to get 0.3 g/L uniform solution. After that, the unsized CFs tow was infiltrated into the uniform solution and then the process of elec trophoretic deposition. To achieve an average solution, functionalized CNTs were mixed in pure acetone for 2 h by ultrasonic bath. The solution was brewed with the mixture which was added in by accelerant, harder and epoxy resin well in a weight ratio of 1:70:100 in 60 � C vacuum oven. Then we used a vacuum pump to continuously pump the air at least three times until all acetone volatilized. The CNTs are mixed and dispersed in the matrix at a concentration of 0.35 wt%, which is almost the electrophoretic deposition process when the layered composite is completed. 2.3. Preparation of samples We use resin transfer molding technique to fabricate carbon fiber reinforced epoxy composites samples. Firstly, the fiber bundle is held horizontally in the groove of the mold. Secondly, the mixture was poured in the mold by resin transfer molding. Finally, the mold was put in a heating oven at 90 � C for 3 h, 120 � C for 3 h and 150 � C for 5 h. After curing, composite samples were named as CF/EP (CFs reinforced EP composites), CF-CNTs/EP (composites based on CNTs-modified CFs surface), CF/EP-CNTs (composites based on CNTs-reinforced EP) and CF-CNTs/EP-CNTs (composites based on CNTs-modified CFs and CNTsreinforced epoxy synergistic), respectively. 2.4. γ- irradiation exposure 60
Co γ ray radiation equipment (Institute of Technical Physics, Hei longjiang Academy of Sciences) was employed as radiation resource. 448
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The composite samples were exposed to 7 MGy γ-ray radiation in air at dose rate of 25 kGy/h and temperature of 13 � C. After irradiation, the composite samples were named as γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/ EP and γ-CF-CNTs/EP-CNTs, respectively.
the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs induced. At the same time we found a special phenomenon that four kinds of samples have undergone secondary curing. As we can see from Table 1, It can be obviously observed that when the irradiation dose at 7 MGy, the maximum storage modulus of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs slightly increase from 2.53 GPa to 2.76 GPa (γ-CF/EP), 2.87 GPa–3.75 GPa (γ-CF/EP-CNTs), 4.95 GPa–4.99 GPa (γ-CF-CNTs/EP) and 5.66 GPa–5.72 GPa (γ-CFCNTs/EP-CNTs).The tanδ value which equals the ratio of the loss modulus to the storage modulus, is used to characterize the internal friction of materials. It can be seen that the tanδ is relatively small in the glass transition zone and then passes through a maximum in the glass transition region with increasing temperature. Fig. S1(b) shows that CF/ EP, CF/EP-CNTs, CF-CNTs/EP and CF-CNTs/EP-CNTs have small dif ferences in thermal stability. Fig. 2(b) shows the tanδ of γ-CF/EP, γ-CF/ EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs slightly increase from 0.285 to 0.325, 0.446, 0.507 and the Tg is named as the temperature corresponding to the maximum value of tanδ peak, which is decreasing from 110.62 � C to 91.42 � C. By analyzing the DMA data before and after irradiation of compos ites, we believe that the increase in E’ indicates that the presence of CNTs significantly increases the stiffness of the composite over the entire temperature range. The deposition of CNTs on the CFs surface made a better interface area bonding between CFs and matrix (γ-CF-CNTs/EP), CNTs enhanced the matrix when CNTs were mixed with EP (γ-CF/EPCNTs) and CNTs play a dual role (γ-CF-CNTs/EP-CNTs), which can in crease the volume fraction of phases in the composite and reduce the effective polymer chain mobility of the interphase regions. The reason why the E’ of composites is increasing is that CNTs can effectively in crease the contact area between resin and resin, fiber and fiber, and resin and fiber in composite materials. In addition, when the composites is subjected to γ-irradiation, the high-energy stimulation segment and the molecules move violently, and the intermolecular contact is more suf ficient, so that the intermolecular connection is more tight. As we all know, γ-ray irradiation not only can conduct chemical bonds cleavage and chain broken, but also but it can promote bonding and cross-linking of bonds. We boldly speculated that the secondary cure of composites may be due to the cross-linking of bonds more than the chemical bonds breakage and chain scission. Although Tg of the modified composite is slightly reduced, its E’ is significantly larger than that of the original composites. We conclude that the reason for secondary solidification is
3. Results and discussion 3.1. SEM for surface characteristics of CFs Characterization of the CF surfaces before and after electrophoretic deposition of CNTs was performed by SEM imaging. Commercial T700 CFs had rough surface which could be seen at Fig. 1 (a).As could be seen, T700 CFs had glossy surface after refluxing in acetone for 48 h and the CNTs attached on the CF surface disappeared in Fig. 1(b) and in Fig. 1 (c), respectively. The randomly oriented CNTs were deposited on the fiber surface by the electrophoretic deposition method for 5 min after 30 s of sonication in Fig. 1(c). CNTs are successfully introduced on the CFs and distributed evenly, which may increase the chemical bonding of between CFs and matrix. 3.2. Changes in dynamic mechanical properties of composites Dynamic mechanical thermal analysis (DMA) can perform the microscopic relaxation movement of the composites before and after γ-ray irradiation and be used to define the loss factor (tanδ) and the storage modulus (E’) of the composites. The storage modulus that characterizes elastic behavior is the most important characteristic for evaluating the load carrying capacity of composites. Fig. 2 display the dynamic storage modulus (E0 )-temperature and damping factor (tanδ)temperature spectra of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs. Fig. S1(a) of SM provides DMA data for CF/EP, CF/ EP-CNTs, CF-CNTs/EP and CF-CNTs/EP-CNTs. It can be observed that as the temperature increases, the E’ value of the composites before and after irradiation is substantially unchanged in the glass state, and then a sharp drop occurs in the glass transition region. More detailed data for unirradiated composites is in Table S1 of SM. We can see the E’ of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased badly from 2.53 GPa (γ-CF/EP) to 5.66 GPa (γ-CF-CNTs/EPCNTs) at room temperature from the Fig. 2. (a) and Table 1. The increase in the E’ and Tg could be attributed to CNTs strengthen the connection between fiber and epoxy number and the link of the chemical bonds in
Fig. 1. Morphology of T700 CFs: (a) before desizing, (b) after desizing, (c) depositing CNTs. 449
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Fig. 2. DMA of composites: (a) storage modulus (E’) and (b) loss factor (tanδ).
temperatures of composites are closely related to their crosslink density. The results show that CNTs have little effect on the thermal properties of composites resistant to γ radiation. Although CNTs can act as a free radical scavenger to scavenge free radicals and prevent oxidation, the effect on crosslink density is not significant. In addition, composites resin decomposition temperature and Tg are consistent in DMA. This also explains the reason for the small difference in thermal stability.
Table 1 E’, E’ Max, tanδ and Tg of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CFCNTs/EP-CNTs. Sample code
E’ (GPa, 25 � C)
E’ Max(GPa)
Tan δ
T g (� C)
γ-CF/EP γ-CF/EP-CNTs γ-CF-CNTs/EP γ-CF-CNTs/EP-CNTs
25.3 28.7 49.5 56.6
27.6 37.5 49.9 57.2
0. 285 0. 325 0. 446 0. 507
110. 62 91. 42 109. 77 102. 61
3.4. Chemical bond changes on the surface and inside of composites
that gamma rays stimulate the vibration of free radicals and small molecules, so the segment movement and contact are more sufficient. For the reason that the E’ of the modified composite material is obvi ously increased, (1) it may be that the CNTs eliminate the radical to reduce the molecular motion; (2) the CNTs chemically react with other substances such as the matrix resin, resulting in an increase in the in ternal motion friction. We conclude that the Tg of the modified com posite decreases, which may be due to the high energy γ ray breaking the resin structure and molecular chain breakage, resulting a decrease in crosslink density.
In order to further understand chemical interactions and more accurately detect changes in the content of individual elements induced by γ radiation, XPS spectra were conducted. Fig. S3, and Fig. S4 of SM, Fig. 4 and Fig. 5 show the XPS C1s spectra of the surface and interior of the composites, in which the spectra can be characterized by four special peaks. The four peaks were considered to be derived from C-C and C-O groups, both having a binding energy of 284.6 eV, having a C-N group of 285.7 eV, having a C¼O group of 288.62 eV and C-O, C-O-C group having 288.2 eV. The characteristics associated with these peaks are given in Table S2 of SM and Table 2, including the binding energy and area ratio. These peaks were optimized by changing their area while maintaining their positions. The XPS data for the non-irradiated composites are presented in Table S2. After the modified composite is irradiated with the same dose of gamma, the surface oxygen content is reduced, so the O/C ratio is increased. Regarding the change in the elements inside composite, the ratio of O/C shows the same pattern of change. This may be due to gamma irradiation causing chain breaks and small molecules to vola tilize on the surface of the modified composites and CNTs as a radical scavenger within it to reduce some of the functional groups. As we can
3.3. Thermal stability of composites The thermal stability of the composite was expressed by measuring the thermal decomposition temperature of the EP by TGA. The degra dation curve shows that the composites exhibit a two-stage decompo sition process in Fig. S2(a) of SM and Fig. 3(a). At 700 � C, the char yield of the composite before and after irradiation is shown in Fig. S2(b) of SM and Fig. 3(b). By comparing the thermal stability of the irradiated and non-irradiated composites, it was found that there was substantially no difference. Previous studies have shown that the thermal decomposition
Fig. 3. (a) The weight percentage versus temperature of composites and (b) TGA of the char yield of composites. 450
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Fig. 4. XPS C1s spectra of the composite surface after γ irradiation: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs.
modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs in crease percentage than γ-CF/EP are 6.27%, 14.23%, 28.59% and 5.56%, 18.92%, 41.44%, respectively. The addition of CNTs greatly enhances the mechanical γ radiation resistance of composites, especially for γ-CFCNTs/EP-CNTs. In order to further explore the improvement of γ radiation resistance of modified composites, the fatigue resistance test was performed in the next step and the result could be seen in Fig. 6 [13,63,64]. Residual strength of composites under fatigue loading is one of the important performance parameters, which is usually used to predict the fatigue life of composites. During the 40,000 bending fatigue cycle, a series of loosening phenomena occur in both the resin and the interface area region, such as interface area debonding and partial valence bond rupture. As the fatigue damage accumulates, the macroscopic mechan ical properties of composites change, such as reducing the bending strength and bending modulus of the composite. The bending strength and bending modulus of composites all decreased after the three-point bending fatigue test, but still maintained a high strength. After bending fatigue test, Fig. 6 show that the bending strengths of γ-CF/EP to γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased from 776.3 MPa to 825.5 MPa, 906.3 MPa, 1026.2 MPa and their bending modulus increased from 49.1 GPa to 57.2 GPa, 65.3 GPa, 85.2 GPa. Table 3 show specific bending strength and bending modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increase per centage than γ-CF/EP are 6.34%, 16.75%, 32.19% and 16.49%, 32.99%, 73.51% in detail, respectively. As revealed in Fig. 6 and Table 3, the residual bending strength and bending modulus retention of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs were higher than that of original sample γ-CF/EP, which indicating CNTs enhanced ability of composites to resist fatigue and γ radiation. Matrix cracking, demixing, joint cracking and fiber break occur when the composite sample is subjected to a force. The damage first appears on the surface of composites and then propagates along the
see, the area ratio in C1s spectra for C-C peak increases both externally and internally, while the area ratios for C-O, C-O-C peaks and total proportion of oxygen-containing functional groups (sum of C¼O, C-O-Cand C/O area ratio) decrease. The content of C-N bond increases significantly in the composite material. This is because the composite material is stimulated by high-energy γ-rays, and the N element carried by the accelerator reacts with CNTs and epoxy resin to form more chemical bonds, thus causing secondary curing reaction. The internal content of C-C bond in composites decreases less than others, when CNTs appear in both the interface area region and the resin and the C¼O bond content of γ-CF/EP is internal more than on the surface, while the other three modified composites are more on the surface than inside. This may be the presence of oxygen and moisture in the air, and a chemical re action occurs on the surface of the composite. This may be the presence of oxygen and moisture in the air, and a chemical reaction occurs on the surface of the composite. In addition, CNTs are present in the composite as a free radical scavenger and reduce the group containing C¼O bonds. Those explain why the composite has a secondary cure and the internal crosslink density are reduced as well as E’ increase and Tg decrease. 3.5. Mechanical properties of composites The bending properties of composites are one of the most important parameters of mechanical properties. The flexural strength and flexural modulus of the unirradiated composites are presented in Fig. S5 and Table S3 of SM. As we can see, the bending strength and bending modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs composites are larger than original sample γ-CF/EP and gradually in crease in Fig. 6. The bending strengths of γ-CF/EP to γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased from 875.6 MPa to 930.5 MPa, 1000.2 MPa, 1125.9 MPa, respectively and their bending modulus increased from 66.6 GPa to 70.3 GPa, 79.2 GPa, 94.2 GPa, respectively. Table 3 show specific bending strength and bending 451
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Fig. 5. XPS C1s spectra of the composite inside after γ irradiation: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs. Table 2 Characteristics of composite surface and internal gamma-irradiated C1s peaks. Area (%). Binding energy(eV)
γ-CF/EP γ-CF/EP-CNTs γ-CF-CNTs/EP γ-CF-CNTs/EP-CNTs
C-C(1) 284.6
C-N(2) 285.7
C-O(3) 286.6
C¼O(4) 288. 2
O/C
Surface
Inside
Surface
Inside
Surface
Inside
Surface
Inside
Surface
Inside
45. 49. 48. 49.
40. 64 47. 10 43. 14 47. 70
17. 83 15. 89 16. 61 16. 23
20. 17. 21. 19.
18. 17. 17. 17.
19. 19. 19. 17.
18. 16. 16. 17.
19. 15. 16. 16.
0. 0. 0. 0.
0. 29 0. 24 0. 21 0. 26
19 44 80 52
85 69 10 06
crack through matrix to fibers. Through analysis of bending strength and bending modulus before and after fatigue testing of composites, it was found that the fatigue resistance and anti-gamma irradiation of the modified composites also has greatly improved relative to the original sample. CNTs not only enhance fibers and resins, but also enhance chemical and mechanical riveting of resins and resins as well as fibers and fibers. CNTs could cause transverse crack propagation difficulty in resin and the local toughening and enhanced EP matrix in the interface region, which will release stress concentration, stress transfer and pre vent cracks from spreading to CFs surface. In composites, when the cracks encounter CNTs, it will change the original propagation path and it will produce more tiny cracks. The large number of tiny cracks can not only absorb and consume the crack energy, but also delay the occur rence of damage. CNTs are added to the resin or deposited onto the CFs surface, which plays an important role in the energy transfer or stress concentration of the resin or interface area. Therefore, the simultaneous addition of CNTs to the resin and interface area regions can effectively reduce stress concentration and energy consumption, which is to improve the mechanical bending properties and anti-gamma radiation
72 90 62 82
51 32 41 23
86 77 97 44
30 89 35 01
32 26 25 31
properties of the composite. 3.6. Microscopic exploration of damage In order to better understand the mechanism of CNTs increasing the fatigue resistance of composites, we used ultrasonic C-scan to analyze the composites after three-point bending fatigue test. Fig. 7 shows the ultrasonic C-scan of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs composite samples after three-point bending fa tigue test. Ultrasound C-scan in the study, red in the color bar indicates low attenuation or maximum amplitude of ultrasonic energy through the composite. Blue color represented the healthy material and red color represented the defects and debonding. The fatigue internal damage of the four unirradiated composites is reflected in Fig. S6 of SM. As we can see from the Fig. 7, the red area becomes less and less from γ-CF/EP to γ-CF-CNTs/EP-CNTs. The extent of damage for the γ-CF/EP-CNTs, γ-CFCNTs/EP and γ-CF-CNTs/EP-CNTs composites were significantly reduced than that of the γ-CF/EP composites, which may be due to the fact that transition interface zone and the interior of the EP give the 452
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Fig. 6. γ-CF/EP; γ-CF/EP-CNTs; γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs before and after fatigue test: (a) Bending strength and (b) Bending modulus.
shear fracture failures like shear debonding failure and fiber breakage often occur at between fibers and resin interface area areas in Fig. 8. The pictures of the section can help us understand the enhancement mech anism of CNTs and how to increase fatigue resistance and finally improve the anti-radiation performance of modified composite mate rials. The Fig. 8 presents fracture morphology of the carbon fiber rein forced epoxy resin composite, which showing γ-CF/EP is obviously different from with other three γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CFCNTs/EP-CNTs composites. For the section of the primary CF composite, the CFs surface is quite clean and the epoxy remains less, indicating that the interface area is very loose. The surface of the CFs begins to become rougher than the original sample and the residual epoxy around it in Fig. 8 (c). In Fig. 8(e) and Fig. 8 (g), there are more resins attached to the CFs surface and we can notice that there is a certain amount of CNTs in the interface area, which indicated a strong interface phase. The same situation occurs after the three-point bending fatigue test in Fig. 8. (b), Fig. 8 (d), Fig. 8 (f) and Fig. 8 (h). After the fatigue test, the difference in the fracture surface of the sample is more epoxy resin residue and also more loose. After the fatigue test, the resin becomes brittle and the connection between the fibers and the resin becomes weak but still maintains certain strength. It can be seen that the fiber surface is smooth
Table 3 The percentage of increase in bending strength and bending modulus of com posites compared with γ-CF/EP before and after fatigue testing.
γ-CF/EP γ-CF/EP-CNTs γ-CF-CNTs/EP γ-CF-CNTs/EP-CNTs
Flexure strength
Flexure modulus
Before
After
Before
After
0 6.27% 14.23% 28.59%
0 6.34% 16.75% 32.19%
0 5.56% 18.92% 41.44%
0 16.49% 32.99% 73.51%
composites with great capacity for “damage accumulation” and pro motes cross-linking and bonding of CFs and resins due to the incorpo ration of CNTs in epoxy resins. 3.7. Interface area microstructure and enhancement mechanism antiγ-irradiation After three-point bending test, the cross-sectional structure was ob tained and its morphology was observed by SEM. It can be seen that
Fig. 7. Ultrasonic C-scans after three-point bending fatigue: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs. 453
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Fig. 8. Sectional morphology of composites: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs before fatigue test and (b) γ-CF/EP, (d) γ-CF/EP-CNTs, (f) γ-CF-CNTs/EP, (h) γ-CF-CNTs/EP-CNTs after fatigue test.
to rough and then breaks, which indicates that the interface area of γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs is get ting better and better. In the original sample, the interface area between the fiber and the resin was achieved by vanderWaals forces and partial chemical combination. The CNTs are uniformly distributed in the γ-CF/ EP-CNTs and form a bonding bridge at the interface area, which pro motes the connection between the fibers and the resin. For γ-CF-CNTs/ EP composites, some CNTs fall from the surface of the deposited CFs and are dispersed into the epoxy during the composite molding process, which may form a gradient interface region between the filler and the resin. The CNTs stand upright on the surface of the fiber by electro phoretic deposition technique, which increases the fiber-resin connec tion by mechanical interlocking and different chemical bonding. For γ-CF-CNTs/EP-CNTs composite, not only the formation of a gradient layer promotes cross-linking of the resin and the fiber, but also
mechanical interlocking, which together promoting the bond between the fiber and the resin. From the SEM at the cross section, it can be concluded that adding and not adding CNTs has a great effect on the bending properties of the composites. The key factor for anti-gamma irradiation of CFs reinforced epoxy resin composites are the stability of the epoxy resin and crosslink density of the interface area. To further illustrate the influence of CNTs on anti-gamma irradiation of the composite, we introduce schematic diagram. During the threepoint bending test, a possible form of composites breakage is to begin to produce larger cracks and then propagate to the internal fibers until they break which is illustrated in Fig. 9. Let us explore the radiation resistance of CNTs when the composite is under stress. As shown in Fig. 9 (a), the original sample exhibits a large crack gap and a small number of cracks and the crack propagation rarely changes direction directly to the fiber and causes delamination. This further explains that γ-CF/EP has 454
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The γ radiation resistance of γ-CF-CNTs/EP-CNTs is significantly improved, not only because of the high-quality radiation resistance of CNTs, but also because of the excellent fusion and combination of CNTs and fibers and resins, which synergistic enhance the γ radiation resis tance of composites.
weaker anti-gamma radiation capability. In Fig. 9(b), when the CNT is added to the matrix resin, the number of cracks increases and large cracks decrease, but there are still many debonding and delamination phenomena in the interface region. The direction of crack propagation is constantly changing and a large number of new cracks are produced to prevent the crack from directly reaching the CFs. The generation of tiny cracks reduces stress concentration and effectively consumes energy, which significantly increases the anti-gamma radiation of γ-CF/EP-CNTs In Fig. 9 (c), a transition layer occurs when CNTs are added to the interface area of the composite. When CNTs are added to the composite interface area, the number of cracks on the surface is very large, but during the transfer, as the transition layer approaching, the number of cracks increases sharply and the direction of transfer becomes chaotic. This proves that the transition layer hinders crack propagation, con sumes energy and reduces the rate of composite damage. There is no delamination or peeling at the interface area of the composite, indi cating that the CNTs enhance the fiber-resin connection and mechanical engagement at the interface area. CNTs are added to the interface region of the composite to form a transition layer, which can effectively improve the damage resistance and γ radiation resistance of γ-CF-CNTs/ E It can be seen from Fig. 9 (d) that the number of major cracks and microcracks in γ-CF-CNTs/EP-CNTs is much smaller and tens of times higher than that in γ-CF/EP and the crack propagation is in all di rections. During the propagation of cracks from the surface of B to the inside, the number constantly increasing, especially the number of cracks at the transition layer near the interface area is expanding dramatically. The main reason for adding CNTs to the matrix and interface area to synergistic enhance the gamma radiation resistance of the composite is that it promotes the attachment of the resin to the resin and the chemical bonding of the resin to the fibers. CNTs can be considered as an energy dissipating agent in the matrix and at the interface area. Because cracks act as a carrier of energy, when they are propagated in the matrix resin, they will change direction or split and energy will be weakened, dispersed and disappeared when they are in contact with CNTs. The CNTs act as a binder to form a transition layer at the interface area. When a crack propagates to the interface layer, the transition layer blocks crack propagation or splitting cracks, thereby reducing stress concentration and promoting energy dissipation.
4. Conclusions Through macroscopic analysis of the mechanical bending properties of irradiated samples, it was found that bending strength and bending modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased percentage than γ-CF/EP are 6.27%, 14.23%, 28.59% and 5.56%, 18.92%, 41.44% before fatigue testing and 6.34%, 16.75%, 32.19% and 16.49%, 32.99%, 73.51% after fatigue testing, especially γ-CF-CNTs/EP-CNTs grew most significantly. Ultrasonic C-scan images of microscopic detection further demonstrates that the modified com posites have better fatigue resistance and γ radiation resistance, espe cially γ-CF-CNTs/EP-CNTs. Based on the cross-section and internal cracks and introducing the crack propagation model, the anti-radiation and anti-fatigue enhancement mechanisms are discussed. Through the microscopic exploration of DMA and TGA, the storage modulus of γ-CF/ EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs is much higher than that of γ-CF/EP, but they have lower glass transition temperatures and thermal decomposition temperatures than γ-CF/E Using XPS to explore various valence bond contents and explain why CNTs act as free radical scavengers to eliminate free radicals and react with other substances as reactants, increasing crosslink density and increasing the difficulty of segment movement. These results explain that the chemical reaction of CNTs with EP and CFs enhance the chemical bonding of the matrix resin and the interface area region to absorb γ ray energy, hinder crack growth, reduce stress concentration and consume energy, which is the reason for the above-mentioned enhanced mechanical properties. Therefore, the simultaneous addition of CNTs into resin and interface area region not only effectively improves the mechanical bending properties and fatigue resistance of the composite, but also significantly improves its anti-γ radiation.
Fig. 9. Possible fracture mechanism of composite materials (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs. 455
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Acknowledgments
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Radiation resistance of carbon fiber-reinforced epoxy composites optimized synergistically by carbon nanotubes in interface area/matrix Minjie Yan, Liangsen Liu, Li Chen, Nan Li, Yaming Jiang, Zhiwei Xu *, Miaolei Jing, Yanli Hu, Liyan Liu, Xingxiang Zhang Key Laboratory of Advanced Braided Composites, Ministry of Education, School of Textiles, Tianjin Polytechnic University, Tianjin, 300387, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Gamma irradiation Radiation resistance Secondary curing Enhance Carbon nanotubes
In order to investigate the effect of the addition of carbon nanotubes (CNTs) in carbon fibers reinforced epoxy matrix composites (CF/EP) on the ability of composites to resist γ radiation. We used γ irradiation and inves tigated composites, γ-CF/EP without CNTs; γ-CF/EP with CNTs in matrix (γ-CF/EP-CNTs); γ-CF/EP with CNTs in interface (γ-CF-CNTs/EP) and γ-CF/EP with CNTs both in the interface area and matrix (γ-CF-CNTs/EP-CNTs). The results show that the bending strength and bending modulus of γ-CF-CNTs/EP-CNTs are 28.59% and 41.44% higher than that of γ-CF/EP before fatigue testing, and 32.19% and 73.51% higher than that of gamma-CF/EP after fatigue testing. Ultrasonic C-scan microscopic examination after fatigue test showed that the internal structure damage of γ-CF-CNTs/EP-CNTs was the smallest, while γ-CF/EP was the largest. The storage modulus of the three modified composites are significantly improved compared with the unmodified composite, especially the storage modulus of γ-CF-CNTs/EP-CNTs is sharply improved from 2.53 GPa (γ-CF/EP) to 5.66 GPa (γ-CFCNTs/EP-CNTs). X-ray spectroscopy shows changes in the surface and internal valence content of the composites, further revealing the crosslinking reaction and the secondary curing reaction. Combined with scanning electron microscopy and crack propagation models, the microstructure and enhancement mechanisms of CNTs-reinforced composites for anti-gamma radiation are discussed. The CNTs synergistic enhance the composite’s anti-gamma radiation performance with excellent effects when CNTs are added together into matrix and interface area.
1. Introduction Its development is very rapid about the aerospace industry and nu clear reactors in recent decades, but the space or nuclear reaction ra diation environment is harsh toward materials [1,2]. Various irradiating rays exist in the atmosphere, such as gamma rays, protons, electrons, ultraviolet rays, etc [3]. γ ray is one of the most energetic energy and has a great destructive effect on materials [4]. The working period of ma terials will be reduced because γ-irradiation may cause degradation in material properties like thermal stability, mechanical, optical, magnetic properties, fatigue etc [5,6]. Traditional materials like lead, concrete and ceramic are heavy, poor in mechanical property, resistance to corrosion, resistance to radiation [7–9], etc, so some researchers focus on carbon fiber/epoxy composites, which can are used on the aerospace industry and nuclear reactors to provide power for future nuclear power plant and spacecraft [10,11]. Due to their resistant to radiation, high specific strengths, low densities, lightweight, rigidity, good fatigue,
resistance to creep properties and capability of molding into multiple shapes, etc, the fiber reinforced polymer composites are being increas ingly employed in the aerospace industry and nuclear reactors [12–17]. Carbon fibers (CFs) are selected as structural and functional materials for many applications due to its excellent mechanical properties, ther mal stability and radiation resistance [18–20]. CFs exhibit a crystalline graphite base with a non-polar surface and chemical inertness due to the high temperature carbonization/graphitization step in the manufacturing process. Some investigations have found that epoxy resin has excellent mechanical strength and heat stability as a new class of matrix material for advanced carbon fiber reinforced epoxy resin matrix composites (CF/EP) [21,22]. Epoxy resins exhibit good thermal stability and strong radiation resistance, but when exposed to high energy gamma rays, they degrade rapidly and mechanical properties are dras tically reduced. In addition, CF/EP exhibit reduced physical-chemical properties when exposed to high-energy gamma rays, especially resin degradation and interface area debonding [23,24], which are caused by
* Corresponding author. E-mail address: [email protected] (Z. Xu). https://doi.org/10.1016/j.compositesb.2019.04.041 Received 16 October 2018; Received in revised form 4 April 2019; Accepted 29 April 2019 Available online 30 April 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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the disadvantages of fibers and resin [25]. The interphase between the reinforcing CFs and the matrix that likes “heart” plays an important role in combining and transferring the internal pressure from matrix to the CFs [26,27]. Some experts found that carbon nanotubes (CNTs) can exhibit superior mechanical [28–32], electrical and thermal properties such as low density [33], high stiffness [34], radiation resistance etc [35, 36], which have been applied in many fields. The CNTs exhibited larger specific surface area and pore volume after gamma irradiation and gamma-radiation is better for rearrangement and/or reconstruction of functional groups on surface [37]. CNTs greatly enhance the mechanical properties of composites, which can be attributed to the chain entan glement and conformation in elastomers [38]. CNTs can also act as free radical scavenger because the amount of free radicals is reduced by the gamma irradiation process as the CNTs concentration increases [39–41]. In order to protect and improve the resin’s resistance to degradation and interfacial peeling of CF/EP, we chose CNTs as a modifier and free radical scavenger [42]. According to the characteristics of CFs surface and EP resin flow, the introduction of CNTs into CF/EP has been achieved. There are a variety of physical and chemical methods [14,16,43–45], like (1) chemical re actions between fibers and functionalized CNTs; (2) direct growth of CNTs on CFs surface by chemical vapor deposition; (3) infiltrating CFs in the solution of CNTs; (4) coating of CFs with CNTs- including sizing; (5) electrophoretic deposition of CNTs on CFs surface. Due to electropho retic deposition technology is an effective way to comprise CNTs in composites which has the advantages of uniform deposition and simplification and could be employed to continuous treating of CFs tows, so we take this approach. There are three different ways to add CNTs to optimize CF/EP: firstly, mixing CNTs completely throughout the matrix (matrix modification); secondly, attaching CNTs straightly onto CFs surface (interface area modification); and last but most important, the combination of one and two (matrix modification and interface area modification cooperation). There is a wide range and complex of chemical processes including chain scission and cross-linking that can be induced by radiochemical or thermal oxidation [46,47]. The purpose of our study was to investigate the mechanical and thermal properties of CNTs as reinforcements added to CF/EP composites after gamma ray irradiation at 7 MGy [48,49]. In this paper, the 7 MGy gamma irradiation dose was chosen to simulate the harsh radiation environment around the nuclear power plant and in space [50]. To investigate its ability to resist gamma- irradiation, the four composite samples were compared with that of the different adding techniques. CNTs are added to the base epoxy resin to enhance the corrosion resistance and degradation resistance of the composites [51, 52]. The introduction of CNTs at the interface area between the fiber and the resin is to prevent delamination and exfoliation from being affected by high energy gamma rays. In order to synergistically enhance the anti-decomposition and anti-debonding properties of the composite, CNTs are simultaneously introduced into interface area and matrix resin. Four different samples were placed together in the same irradia tion environment. In order to compare the radiation resistance of four different sam ples, we analyzed the macroscopic and microscopic damage of the samples after different tests. SEM (Scanning electron microscope) was used to observe the morphology of CNTs on the surface of CFs and section morphology after mechanical testing. Using three-point short beam bending method to explore the bending performance and fatigue resistance of four composite materials by using a universal testing ma chine [53–55]. Residual strength, which is often considered the fatigue life forecast of CF/EP, was acquired to clarify damage capacity of CF/E We introduced ultrasonic C-scan analysis of CF/EP after residual flexure strength testing. All tests were performed at normal temperature and the final values were measured as averages of five samples for each CF/E The thermal stability and crosslink density were characterized by using thermogravimetric analysis (TGA) [56–58]. Dynamic mechanical ther mal analysis (DMA) could express the viscoelasticity characteristics of
CF/EP and was adopted to define storage modulus (E’) and loss factor (tanδ) of CF/EP before and after irradiation [59,60]. Other than that, the destructive form is taken by recording the trace of free radicals con centrations and surveying formation and ingredient change of surfaces and interiors of different samples before and after irradiation by X-ray photoelectron spectroscopy (XPS) [61,62]. In order to explain the damage mechanism in more detail, different schematics are introduced to simulate the crack propagation path. Due to the large amount of data and the length of the article, all data of the four unirradiated composites are reflected in the Supplementary Materials(SM). 2. Experimental 2.1. Materials We selected multi-wall CNTs with length of 10–20 μm, the diameter of 30–50 nm and purity over 95% which were purchased from Chengdu Organic Chemicals Co. Ltd, Chinese Academy of Sciences. The Commercially-available T700S CFs as reinforcement were purchased from Toray, with an average diameter of 7 μm were employed. Chang shu Jiafa provides chemical JC-02A modified matrix and JH-0511 modified 2-ethyl-4-methylimidazole as accelerator. We chose tetrahy drophthalic anhydride as curing agent, which was purchased from Wenzhou Qingming Chemical Co., Ltd. 2.2. Introducing CNTs into the surface of CFs and matrix To remove the commercial sizing, the original CFs was refluxed with acetone in the soxhlet apparatus for 48 h at 70 � C, because CNTs are difficult to stay on the virgin CFs surface. Subsequently, CFs were seasoned 8 h under 60 � C in the oven. In order to introduce groups into the CNT surface, CNTs were placed in a mixture of concentrated nitric acid (HNO3)/concentrated sulfuric acid (H2SO4) (1:3 v/v) at 80 � C for 8 h. The follow is the electrophoretic deposition parameters: concen tration of CNTs in distilled water 0.3 mg/ml, voltage 24V, time 5 min, electrode materials metal panel and electrode spacing 50 mm. Func tionalized CNTs were distributed in distilled water by an ultrasonic bath for 2 h to get 0.3 g/L uniform solution. After that, the unsized CFs tow was infiltrated into the uniform solution and then the process of elec trophoretic deposition. To achieve an average solution, functionalized CNTs were mixed in pure acetone for 2 h by ultrasonic bath. The solution was brewed with the mixture which was added in by accelerant, harder and epoxy resin well in a weight ratio of 1:70:100 in 60 � C vacuum oven. Then we used a vacuum pump to continuously pump the air at least three times until all acetone volatilized. The CNTs are mixed and dispersed in the matrix at a concentration of 0.35 wt%, which is almost the electrophoretic deposition process when the layered composite is completed. 2.3. Preparation of samples We use resin transfer molding technique to fabricate carbon fiber reinforced epoxy composites samples. Firstly, the fiber bundle is held horizontally in the groove of the mold. Secondly, the mixture was poured in the mold by resin transfer molding. Finally, the mold was put in a heating oven at 90 � C for 3 h, 120 � C for 3 h and 150 � C for 5 h. After curing, composite samples were named as CF/EP (CFs reinforced EP composites), CF-CNTs/EP (composites based on CNTs-modified CFs surface), CF/EP-CNTs (composites based on CNTs-reinforced EP) and CF-CNTs/EP-CNTs (composites based on CNTs-modified CFs and CNTsreinforced epoxy synergistic), respectively. 2.4. γ- irradiation exposure 60
Co γ ray radiation equipment (Institute of Technical Physics, Hei longjiang Academy of Sciences) was employed as radiation resource. 448
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The composite samples were exposed to 7 MGy γ-ray radiation in air at dose rate of 25 kGy/h and temperature of 13 � C. After irradiation, the composite samples were named as γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/ EP and γ-CF-CNTs/EP-CNTs, respectively.
the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs induced. At the same time we found a special phenomenon that four kinds of samples have undergone secondary curing. As we can see from Table 1, It can be obviously observed that when the irradiation dose at 7 MGy, the maximum storage modulus of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs slightly increase from 2.53 GPa to 2.76 GPa (γ-CF/EP), 2.87 GPa–3.75 GPa (γ-CF/EP-CNTs), 4.95 GPa–4.99 GPa (γ-CF-CNTs/EP) and 5.66 GPa–5.72 GPa (γ-CFCNTs/EP-CNTs).The tanδ value which equals the ratio of the loss modulus to the storage modulus, is used to characterize the internal friction of materials. It can be seen that the tanδ is relatively small in the glass transition zone and then passes through a maximum in the glass transition region with increasing temperature. Fig. S1(b) shows that CF/ EP, CF/EP-CNTs, CF-CNTs/EP and CF-CNTs/EP-CNTs have small dif ferences in thermal stability. Fig. 2(b) shows the tanδ of γ-CF/EP, γ-CF/ EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs slightly increase from 0.285 to 0.325, 0.446, 0.507 and the Tg is named as the temperature corresponding to the maximum value of tanδ peak, which is decreasing from 110.62 � C to 91.42 � C. By analyzing the DMA data before and after irradiation of compos ites, we believe that the increase in E’ indicates that the presence of CNTs significantly increases the stiffness of the composite over the entire temperature range. The deposition of CNTs on the CFs surface made a better interface area bonding between CFs and matrix (γ-CF-CNTs/EP), CNTs enhanced the matrix when CNTs were mixed with EP (γ-CF/EPCNTs) and CNTs play a dual role (γ-CF-CNTs/EP-CNTs), which can in crease the volume fraction of phases in the composite and reduce the effective polymer chain mobility of the interphase regions. The reason why the E’ of composites is increasing is that CNTs can effectively in crease the contact area between resin and resin, fiber and fiber, and resin and fiber in composite materials. In addition, when the composites is subjected to γ-irradiation, the high-energy stimulation segment and the molecules move violently, and the intermolecular contact is more suf ficient, so that the intermolecular connection is more tight. As we all know, γ-ray irradiation not only can conduct chemical bonds cleavage and chain broken, but also but it can promote bonding and cross-linking of bonds. We boldly speculated that the secondary cure of composites may be due to the cross-linking of bonds more than the chemical bonds breakage and chain scission. Although Tg of the modified composite is slightly reduced, its E’ is significantly larger than that of the original composites. We conclude that the reason for secondary solidification is
3. Results and discussion 3.1. SEM for surface characteristics of CFs Characterization of the CF surfaces before and after electrophoretic deposition of CNTs was performed by SEM imaging. Commercial T700 CFs had rough surface which could be seen at Fig. 1 (a).As could be seen, T700 CFs had glossy surface after refluxing in acetone for 48 h and the CNTs attached on the CF surface disappeared in Fig. 1(b) and in Fig. 1 (c), respectively. The randomly oriented CNTs were deposited on the fiber surface by the electrophoretic deposition method for 5 min after 30 s of sonication in Fig. 1(c). CNTs are successfully introduced on the CFs and distributed evenly, which may increase the chemical bonding of between CFs and matrix. 3.2. Changes in dynamic mechanical properties of composites Dynamic mechanical thermal analysis (DMA) can perform the microscopic relaxation movement of the composites before and after γ-ray irradiation and be used to define the loss factor (tanδ) and the storage modulus (E’) of the composites. The storage modulus that characterizes elastic behavior is the most important characteristic for evaluating the load carrying capacity of composites. Fig. 2 display the dynamic storage modulus (E0 )-temperature and damping factor (tanδ)temperature spectra of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs. Fig. S1(a) of SM provides DMA data for CF/EP, CF/ EP-CNTs, CF-CNTs/EP and CF-CNTs/EP-CNTs. It can be observed that as the temperature increases, the E’ value of the composites before and after irradiation is substantially unchanged in the glass state, and then a sharp drop occurs in the glass transition region. More detailed data for unirradiated composites is in Table S1 of SM. We can see the E’ of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased badly from 2.53 GPa (γ-CF/EP) to 5.66 GPa (γ-CF-CNTs/EPCNTs) at room temperature from the Fig. 2. (a) and Table 1. The increase in the E’ and Tg could be attributed to CNTs strengthen the connection between fiber and epoxy number and the link of the chemical bonds in
Fig. 1. Morphology of T700 CFs: (a) before desizing, (b) after desizing, (c) depositing CNTs. 449
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Fig. 2. DMA of composites: (a) storage modulus (E’) and (b) loss factor (tanδ).
temperatures of composites are closely related to their crosslink density. The results show that CNTs have little effect on the thermal properties of composites resistant to γ radiation. Although CNTs can act as a free radical scavenger to scavenge free radicals and prevent oxidation, the effect on crosslink density is not significant. In addition, composites resin decomposition temperature and Tg are consistent in DMA. This also explains the reason for the small difference in thermal stability.
Table 1 E’, E’ Max, tanδ and Tg of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CFCNTs/EP-CNTs. Sample code
E’ (GPa, 25 � C)
E’ Max(GPa)
Tan δ
T g (� C)
γ-CF/EP γ-CF/EP-CNTs γ-CF-CNTs/EP γ-CF-CNTs/EP-CNTs
25.3 28.7 49.5 56.6
27.6 37.5 49.9 57.2
0. 285 0. 325 0. 446 0. 507
110. 62 91. 42 109. 77 102. 61
3.4. Chemical bond changes on the surface and inside of composites
that gamma rays stimulate the vibration of free radicals and small molecules, so the segment movement and contact are more sufficient. For the reason that the E’ of the modified composite material is obvi ously increased, (1) it may be that the CNTs eliminate the radical to reduce the molecular motion; (2) the CNTs chemically react with other substances such as the matrix resin, resulting in an increase in the in ternal motion friction. We conclude that the Tg of the modified com posite decreases, which may be due to the high energy γ ray breaking the resin structure and molecular chain breakage, resulting a decrease in crosslink density.
In order to further understand chemical interactions and more accurately detect changes in the content of individual elements induced by γ radiation, XPS spectra were conducted. Fig. S3, and Fig. S4 of SM, Fig. 4 and Fig. 5 show the XPS C1s spectra of the surface and interior of the composites, in which the spectra can be characterized by four special peaks. The four peaks were considered to be derived from C-C and C-O groups, both having a binding energy of 284.6 eV, having a C-N group of 285.7 eV, having a C¼O group of 288.62 eV and C-O, C-O-C group having 288.2 eV. The characteristics associated with these peaks are given in Table S2 of SM and Table 2, including the binding energy and area ratio. These peaks were optimized by changing their area while maintaining their positions. The XPS data for the non-irradiated composites are presented in Table S2. After the modified composite is irradiated with the same dose of gamma, the surface oxygen content is reduced, so the O/C ratio is increased. Regarding the change in the elements inside composite, the ratio of O/C shows the same pattern of change. This may be due to gamma irradiation causing chain breaks and small molecules to vola tilize on the surface of the modified composites and CNTs as a radical scavenger within it to reduce some of the functional groups. As we can
3.3. Thermal stability of composites The thermal stability of the composite was expressed by measuring the thermal decomposition temperature of the EP by TGA. The degra dation curve shows that the composites exhibit a two-stage decompo sition process in Fig. S2(a) of SM and Fig. 3(a). At 700 � C, the char yield of the composite before and after irradiation is shown in Fig. S2(b) of SM and Fig. 3(b). By comparing the thermal stability of the irradiated and non-irradiated composites, it was found that there was substantially no difference. Previous studies have shown that the thermal decomposition
Fig. 3. (a) The weight percentage versus temperature of composites and (b) TGA of the char yield of composites. 450
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Fig. 4. XPS C1s spectra of the composite surface after γ irradiation: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs.
modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs in crease percentage than γ-CF/EP are 6.27%, 14.23%, 28.59% and 5.56%, 18.92%, 41.44%, respectively. The addition of CNTs greatly enhances the mechanical γ radiation resistance of composites, especially for γ-CFCNTs/EP-CNTs. In order to further explore the improvement of γ radiation resistance of modified composites, the fatigue resistance test was performed in the next step and the result could be seen in Fig. 6 [13,63,64]. Residual strength of composites under fatigue loading is one of the important performance parameters, which is usually used to predict the fatigue life of composites. During the 40,000 bending fatigue cycle, a series of loosening phenomena occur in both the resin and the interface area region, such as interface area debonding and partial valence bond rupture. As the fatigue damage accumulates, the macroscopic mechan ical properties of composites change, such as reducing the bending strength and bending modulus of the composite. The bending strength and bending modulus of composites all decreased after the three-point bending fatigue test, but still maintained a high strength. After bending fatigue test, Fig. 6 show that the bending strengths of γ-CF/EP to γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased from 776.3 MPa to 825.5 MPa, 906.3 MPa, 1026.2 MPa and their bending modulus increased from 49.1 GPa to 57.2 GPa, 65.3 GPa, 85.2 GPa. Table 3 show specific bending strength and bending modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increase per centage than γ-CF/EP are 6.34%, 16.75%, 32.19% and 16.49%, 32.99%, 73.51% in detail, respectively. As revealed in Fig. 6 and Table 3, the residual bending strength and bending modulus retention of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs were higher than that of original sample γ-CF/EP, which indicating CNTs enhanced ability of composites to resist fatigue and γ radiation. Matrix cracking, demixing, joint cracking and fiber break occur when the composite sample is subjected to a force. The damage first appears on the surface of composites and then propagates along the
see, the area ratio in C1s spectra for C-C peak increases both externally and internally, while the area ratios for C-O, C-O-C peaks and total proportion of oxygen-containing functional groups (sum of C¼O, C-O-Cand C/O area ratio) decrease. The content of C-N bond increases significantly in the composite material. This is because the composite material is stimulated by high-energy γ-rays, and the N element carried by the accelerator reacts with CNTs and epoxy resin to form more chemical bonds, thus causing secondary curing reaction. The internal content of C-C bond in composites decreases less than others, when CNTs appear in both the interface area region and the resin and the C¼O bond content of γ-CF/EP is internal more than on the surface, while the other three modified composites are more on the surface than inside. This may be the presence of oxygen and moisture in the air, and a chemical re action occurs on the surface of the composite. This may be the presence of oxygen and moisture in the air, and a chemical reaction occurs on the surface of the composite. In addition, CNTs are present in the composite as a free radical scavenger and reduce the group containing C¼O bonds. Those explain why the composite has a secondary cure and the internal crosslink density are reduced as well as E’ increase and Tg decrease. 3.5. Mechanical properties of composites The bending properties of composites are one of the most important parameters of mechanical properties. The flexural strength and flexural modulus of the unirradiated composites are presented in Fig. S5 and Table S3 of SM. As we can see, the bending strength and bending modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs composites are larger than original sample γ-CF/EP and gradually in crease in Fig. 6. The bending strengths of γ-CF/EP to γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased from 875.6 MPa to 930.5 MPa, 1000.2 MPa, 1125.9 MPa, respectively and their bending modulus increased from 66.6 GPa to 70.3 GPa, 79.2 GPa, 94.2 GPa, respectively. Table 3 show specific bending strength and bending 451
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Fig. 5. XPS C1s spectra of the composite inside after γ irradiation: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs. Table 2 Characteristics of composite surface and internal gamma-irradiated C1s peaks. Area (%). Binding energy(eV)
γ-CF/EP γ-CF/EP-CNTs γ-CF-CNTs/EP γ-CF-CNTs/EP-CNTs
C-C(1) 284.6
C-N(2) 285.7
C-O(3) 286.6
C¼O(4) 288. 2
O/C
Surface
Inside
Surface
Inside
Surface
Inside
Surface
Inside
Surface
Inside
45. 49. 48. 49.
40. 64 47. 10 43. 14 47. 70
17. 83 15. 89 16. 61 16. 23
20. 17. 21. 19.
18. 17. 17. 17.
19. 19. 19. 17.
18. 16. 16. 17.
19. 15. 16. 16.
0. 0. 0. 0.
0. 29 0. 24 0. 21 0. 26
19 44 80 52
85 69 10 06
crack through matrix to fibers. Through analysis of bending strength and bending modulus before and after fatigue testing of composites, it was found that the fatigue resistance and anti-gamma irradiation of the modified composites also has greatly improved relative to the original sample. CNTs not only enhance fibers and resins, but also enhance chemical and mechanical riveting of resins and resins as well as fibers and fibers. CNTs could cause transverse crack propagation difficulty in resin and the local toughening and enhanced EP matrix in the interface region, which will release stress concentration, stress transfer and pre vent cracks from spreading to CFs surface. In composites, when the cracks encounter CNTs, it will change the original propagation path and it will produce more tiny cracks. The large number of tiny cracks can not only absorb and consume the crack energy, but also delay the occur rence of damage. CNTs are added to the resin or deposited onto the CFs surface, which plays an important role in the energy transfer or stress concentration of the resin or interface area. Therefore, the simultaneous addition of CNTs to the resin and interface area regions can effectively reduce stress concentration and energy consumption, which is to improve the mechanical bending properties and anti-gamma radiation
72 90 62 82
51 32 41 23
86 77 97 44
30 89 35 01
32 26 25 31
properties of the composite. 3.6. Microscopic exploration of damage In order to better understand the mechanism of CNTs increasing the fatigue resistance of composites, we used ultrasonic C-scan to analyze the composites after three-point bending fatigue test. Fig. 7 shows the ultrasonic C-scan of the γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs composite samples after three-point bending fa tigue test. Ultrasound C-scan in the study, red in the color bar indicates low attenuation or maximum amplitude of ultrasonic energy through the composite. Blue color represented the healthy material and red color represented the defects and debonding. The fatigue internal damage of the four unirradiated composites is reflected in Fig. S6 of SM. As we can see from the Fig. 7, the red area becomes less and less from γ-CF/EP to γ-CF-CNTs/EP-CNTs. The extent of damage for the γ-CF/EP-CNTs, γ-CFCNTs/EP and γ-CF-CNTs/EP-CNTs composites were significantly reduced than that of the γ-CF/EP composites, which may be due to the fact that transition interface zone and the interior of the EP give the 452
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Fig. 6. γ-CF/EP; γ-CF/EP-CNTs; γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs before and after fatigue test: (a) Bending strength and (b) Bending modulus.
shear fracture failures like shear debonding failure and fiber breakage often occur at between fibers and resin interface area areas in Fig. 8. The pictures of the section can help us understand the enhancement mech anism of CNTs and how to increase fatigue resistance and finally improve the anti-radiation performance of modified composite mate rials. The Fig. 8 presents fracture morphology of the carbon fiber rein forced epoxy resin composite, which showing γ-CF/EP is obviously different from with other three γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CFCNTs/EP-CNTs composites. For the section of the primary CF composite, the CFs surface is quite clean and the epoxy remains less, indicating that the interface area is very loose. The surface of the CFs begins to become rougher than the original sample and the residual epoxy around it in Fig. 8 (c). In Fig. 8(e) and Fig. 8 (g), there are more resins attached to the CFs surface and we can notice that there is a certain amount of CNTs in the interface area, which indicated a strong interface phase. The same situation occurs after the three-point bending fatigue test in Fig. 8. (b), Fig. 8 (d), Fig. 8 (f) and Fig. 8 (h). After the fatigue test, the difference in the fracture surface of the sample is more epoxy resin residue and also more loose. After the fatigue test, the resin becomes brittle and the connection between the fibers and the resin becomes weak but still maintains certain strength. It can be seen that the fiber surface is smooth
Table 3 The percentage of increase in bending strength and bending modulus of com posites compared with γ-CF/EP before and after fatigue testing.
γ-CF/EP γ-CF/EP-CNTs γ-CF-CNTs/EP γ-CF-CNTs/EP-CNTs
Flexure strength
Flexure modulus
Before
After
Before
After
0 6.27% 14.23% 28.59%
0 6.34% 16.75% 32.19%
0 5.56% 18.92% 41.44%
0 16.49% 32.99% 73.51%
composites with great capacity for “damage accumulation” and pro motes cross-linking and bonding of CFs and resins due to the incorpo ration of CNTs in epoxy resins. 3.7. Interface area microstructure and enhancement mechanism antiγ-irradiation After three-point bending test, the cross-sectional structure was ob tained and its morphology was observed by SEM. It can be seen that
Fig. 7. Ultrasonic C-scans after three-point bending fatigue: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs. 453
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Fig. 8. Sectional morphology of composites: (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs before fatigue test and (b) γ-CF/EP, (d) γ-CF/EP-CNTs, (f) γ-CF-CNTs/EP, (h) γ-CF-CNTs/EP-CNTs after fatigue test.
to rough and then breaks, which indicates that the interface area of γ-CF/EP, γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs is get ting better and better. In the original sample, the interface area between the fiber and the resin was achieved by vanderWaals forces and partial chemical combination. The CNTs are uniformly distributed in the γ-CF/ EP-CNTs and form a bonding bridge at the interface area, which pro motes the connection between the fibers and the resin. For γ-CF-CNTs/ EP composites, some CNTs fall from the surface of the deposited CFs and are dispersed into the epoxy during the composite molding process, which may form a gradient interface region between the filler and the resin. The CNTs stand upright on the surface of the fiber by electro phoretic deposition technique, which increases the fiber-resin connec tion by mechanical interlocking and different chemical bonding. For γ-CF-CNTs/EP-CNTs composite, not only the formation of a gradient layer promotes cross-linking of the resin and the fiber, but also
mechanical interlocking, which together promoting the bond between the fiber and the resin. From the SEM at the cross section, it can be concluded that adding and not adding CNTs has a great effect on the bending properties of the composites. The key factor for anti-gamma irradiation of CFs reinforced epoxy resin composites are the stability of the epoxy resin and crosslink density of the interface area. To further illustrate the influence of CNTs on anti-gamma irradiation of the composite, we introduce schematic diagram. During the threepoint bending test, a possible form of composites breakage is to begin to produce larger cracks and then propagate to the internal fibers until they break which is illustrated in Fig. 9. Let us explore the radiation resistance of CNTs when the composite is under stress. As shown in Fig. 9 (a), the original sample exhibits a large crack gap and a small number of cracks and the crack propagation rarely changes direction directly to the fiber and causes delamination. This further explains that γ-CF/EP has 454
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The γ radiation resistance of γ-CF-CNTs/EP-CNTs is significantly improved, not only because of the high-quality radiation resistance of CNTs, but also because of the excellent fusion and combination of CNTs and fibers and resins, which synergistic enhance the γ radiation resis tance of composites.
weaker anti-gamma radiation capability. In Fig. 9(b), when the CNT is added to the matrix resin, the number of cracks increases and large cracks decrease, but there are still many debonding and delamination phenomena in the interface region. The direction of crack propagation is constantly changing and a large number of new cracks are produced to prevent the crack from directly reaching the CFs. The generation of tiny cracks reduces stress concentration and effectively consumes energy, which significantly increases the anti-gamma radiation of γ-CF/EP-CNTs In Fig. 9 (c), a transition layer occurs when CNTs are added to the interface area of the composite. When CNTs are added to the composite interface area, the number of cracks on the surface is very large, but during the transfer, as the transition layer approaching, the number of cracks increases sharply and the direction of transfer becomes chaotic. This proves that the transition layer hinders crack propagation, con sumes energy and reduces the rate of composite damage. There is no delamination or peeling at the interface area of the composite, indi cating that the CNTs enhance the fiber-resin connection and mechanical engagement at the interface area. CNTs are added to the interface region of the composite to form a transition layer, which can effectively improve the damage resistance and γ radiation resistance of γ-CF-CNTs/ E It can be seen from Fig. 9 (d) that the number of major cracks and microcracks in γ-CF-CNTs/EP-CNTs is much smaller and tens of times higher than that in γ-CF/EP and the crack propagation is in all di rections. During the propagation of cracks from the surface of B to the inside, the number constantly increasing, especially the number of cracks at the transition layer near the interface area is expanding dramatically. The main reason for adding CNTs to the matrix and interface area to synergistic enhance the gamma radiation resistance of the composite is that it promotes the attachment of the resin to the resin and the chemical bonding of the resin to the fibers. CNTs can be considered as an energy dissipating agent in the matrix and at the interface area. Because cracks act as a carrier of energy, when they are propagated in the matrix resin, they will change direction or split and energy will be weakened, dispersed and disappeared when they are in contact with CNTs. The CNTs act as a binder to form a transition layer at the interface area. When a crack propagates to the interface layer, the transition layer blocks crack propagation or splitting cracks, thereby reducing stress concentration and promoting energy dissipation.
4. Conclusions Through macroscopic analysis of the mechanical bending properties of irradiated samples, it was found that bending strength and bending modulus of γ-CF/EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs increased percentage than γ-CF/EP are 6.27%, 14.23%, 28.59% and 5.56%, 18.92%, 41.44% before fatigue testing and 6.34%, 16.75%, 32.19% and 16.49%, 32.99%, 73.51% after fatigue testing, especially γ-CF-CNTs/EP-CNTs grew most significantly. Ultrasonic C-scan images of microscopic detection further demonstrates that the modified com posites have better fatigue resistance and γ radiation resistance, espe cially γ-CF-CNTs/EP-CNTs. Based on the cross-section and internal cracks and introducing the crack propagation model, the anti-radiation and anti-fatigue enhancement mechanisms are discussed. Through the microscopic exploration of DMA and TGA, the storage modulus of γ-CF/ EP-CNTs, γ-CF-CNTs/EP and γ-CF-CNTs/EP-CNTs is much higher than that of γ-CF/EP, but they have lower glass transition temperatures and thermal decomposition temperatures than γ-CF/E Using XPS to explore various valence bond contents and explain why CNTs act as free radical scavengers to eliminate free radicals and react with other substances as reactants, increasing crosslink density and increasing the difficulty of segment movement. These results explain that the chemical reaction of CNTs with EP and CFs enhance the chemical bonding of the matrix resin and the interface area region to absorb γ ray energy, hinder crack growth, reduce stress concentration and consume energy, which is the reason for the above-mentioned enhanced mechanical properties. Therefore, the simultaneous addition of CNTs into resin and interface area region not only effectively improves the mechanical bending properties and fatigue resistance of the composite, but also significantly improves its anti-γ radiation.
Fig. 9. Possible fracture mechanism of composite materials (a) γ-CF/EP; (b) γ-CF/EP-CNTs; (c) γ-CF-CNTs/EP and (d) γ-CF-CNTs/EP-CNTs. 455
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Acknowledgments
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