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Microwave radiation effects on carbon fibres interfacial performance
Composites Part B 99 (2016) 398e406
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Microwave radiation effects on carbon fibres interfacial performance Ben Wang, Yugang Duan*, Jingjing Zhang, Xinming Zhao State Key Lab for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province, 710049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2015 Received in revised form 6 September 2015 Accepted 3 June 2016 Available online 8 June 2016
In this paper, microwave radiation, a fast and cost-effective industrial surface pretreatment method without chemicals, was employed to pretreat T300 carbon fibres and improve the carbon fibres/matrix interfacial properties. The microwave pretreatment mechanism on carbon fibres was studied in an experiment divided microwave treated carbon fibres into a microwave radiation section and a pure current section. The polarization current in the carbon fibre induced by microwave radiation helped microwave effects interact on the morphology, compositions, and structure of carbon fibres. Carbon fibres pretreated with short time microwave radiation had an increased thermodynamic work of adhesion and the interfacial shear strength, indicating microwave radiation pretreatment of carbon fibre promises to be an effective method of fiber treatment. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Carbon fibre Interface/interphase Surface analysis Surface treatments
1. Introduction Carbon fibres reinforced polymer composites are widely used in aerospace, energy, marine and automobile industries due to their outstanding properties such as high specific strength and stiffness, and lower density [1,2]. Composite properties are not only determined by the fibre and the resin, but also influenced by the interface between the two constituents. An effective adhesion of the carbon fibres/matrix interface has been considered as highperformance composite preconditional factor to obtain better stress transfer and crack resistance [3,4]. The carbon fibres/matrix interface formation in the composite preparation process could be considered in two stages. In the first impregnating stage, the resin spreads and impregnates on the fibre surface more easily with favorable wettability and high surface energy; In the second interface formation stage, the interface would be formed by the interaction between the resin and fibre surface, which is strongly influenced by the amount of functional groups and active surface area [5,6]. Commercially available carbon fibres are normally coated by a surface sizing layer to connect with the matrix and protect fibres from the damage. However, the tailored sizing is usually limited to match a singular kind of polymer besides the complicated sizing procedure [7,8]. To overcome the disadvantage of sizing, some carbon fibres surface pretreatments/modifications have been applied to provide an alternative interphase to improve
* Corresponding author. E-mail address: [email protected] (Y. Duan). http://dx.doi.org/10.1016/j.compositesb.2016.06.032 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
the carbon fibres/matrix interface cohesive force in the industrial processes while remaining cost-effective. They adopted gas [9], liquid [10], plasma [11], ultrasonic [12] and electrochemical approaches [3] as the fibre modification. But all these pretreatment methods are time-consuming, and some chemicals are needed, which increases the cost of the production. For example, aqueous ammonia was employed to pretreat carbon fibres to increase the surface roughness, but the pretreatment time of 48 h was needed for the optimized results [13]. Also, plasma with O2 required plasma generator and high O2 flux to increase the surface oxygen content of carbon fibres in a short time [14]. Microwave, a high-frequency electromagnet wave ranging from 100 MHz to 300 GHz, has been used in some applications according to its microwave effects including thermal and non-thermal effects. For thermal effects, dielectric materials can be heated fast due to dielectric loss, which could activate molecule to relax or break chemical bonds, and form new chemical bonds. Non-thermal microwave effects have been postulated to result from a direct stabilizing interaction of the electric field with specific (polar) molecules in the reaction medium that is not related to a macroscopic temperature effect [15]. For example, the focusing of electric fields at particle interfaces caused by microwaves can form plasma and generate polarity groups [16,17]. Other than microwave effects, conductive carbon fibres could be regarded as an electric dipole according to the skin effect (Electromagnetic radiation at high frequencies penetrates the near surface region of an electrical conductor), because the diameter of monofilament is only 7 mm far smaller than microwave wavelength (122 mm). Consequently, it
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could activate polarization current on carbon fibres surface during the microwave radiation [18]. Since these characters of carbon fibres, polarization current effect and microwave effects induced by microwave pretreatment are supposed to perform on carbon fibres simultaneously. However, the mechanism of these effects for the carbon fibres pretreatment on the fibre surface performance is not so clear. Therefore, the microwave radiation was employed in present work to pretreat T300 carbon fibres within a short time, and microwave effects were analyzed by dividing microwave treated carbon fibres into a microwave radiation section and a pure current section. The carbon fibres surface after being microwave treated was investigated through morphology, compositions and surface structure. The interface performance was characterized through the carbon fibres wettability against solvents and resin and the interfacial shear strength (IFSS). 2. Material and methods Fig. 1. Polarization current test illustration during the microwave radiation.
2.1. Materials Polyacrylonitrile-based T300 carbon fibres with sizing agent were produced by Toho Tenax in Japan, which was coded as CFs. A mild desizing procedure extracted the CFs by acetone for 48 h at 70 C was adopted to avoid deteriorating the morphology and properties of original fibres [13], and the obtain fibre was coded as CF. The mixture of diglycidyl ether of bisphenol A epoxy (DER331, Dow, USA) and amine-based curing agent (Isophorone, 2855-13-2, Evonik Degussa, German) with a 3:1 wt ratio was used as the resin matrix. 2.2. Surface pretreatment by microwave radiation A tow of carbon fibres including 3000 filaments was irradiated using a microwave generator (BMR-01, Kairui, China) with a emitting frequency of 2.45 GHz at 300 W. Due to the anisotropic dielectric behavior of carbon fibres, the fibre direction was perpendicular to the electric field direction for the maximum microwave absorbance [19]. The height between the generator and carbon fibres was 180 mm. To prevent carbon fibres from being ablated, the radiation time was arranged in cycles: 90 s for radiation followed by 60 s without radiation. In addition, the specimens treated by different radiation time were coded as CF-T (T stands for radiation time). 2.3. Methods 2.3.1. Polarization current test In order to evaluate the polarization current effect on carbon fibres during the microwave radiation, the polarization current was transmitted from the irradiated monofilament, and recorded as shown in Fig. 1. Both ends of the monofilament (200 mm) were pasted on a glass plate to maintain a straight line, and then connected with copper wires (1.5 mm2) by conductive adhesive (4 104 U cm, Jiuqi, YC-01, China) to form a conductive path. A 1MU resistor was used to magnify the voltage signal obtained by oscilloscope (maximum Sampling rate 4 109 Sa/s, RIGOL, DS4012, China). 2.3.2. Atomic force microscopy The carbon fibres morphologies were observed by Atomic Force Microscopy (AFM, NanoScope MultiMode III, USA) with a scanning speed of 1 mm/s and a scanning scope of 3 mm 3 mm. At least 3 samples was provided for measuring the roughness of carbon fibres.
2.3.3. X-ray photoelectron spectroscopy The carbon fibres surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS, Kratos AXIS ultra DLD, UK) with a monochromate Al Ka source (hn ¼ 1486.6 eV) at a power of 150 W (15 kV 10 mA). To compensate for the surface charging effect, all binding energies were referenced to Cls neutral carbon peak at 284.6 eV. 2.3.4. Raman spectroscopy The carbon fibres microscopic structure was analyzed by Raman Spectroscopy (Lab-RAM Aramis, JOBIN YVON HORIBA, France) using a HeeNe 20 mW laser with an excitation source of 514.5 nm. 2.3.5. Single fibre tensile testing The single fibre tensile testing was according to ASTM D337975. The specimen ends were attached to a cardboard tab using a quick-drying adhesive (Instant Krazy Glue, KG82048SN, USA) while under slight tension to ensure a consistent gauge length. The tab ends were gripped in the jaws of the testing machine, an INSTRON 1195 Tester with a 100 g load cell. The tab was then cut so that only the fibre was loaded during the test. Specimens with approximately 20 duplicate samples per set were tested under quasi-static tensile loading at a deformation rate of 0.1 mm/min. 2.3.6. Wettability The carbon fibres surface wettability was evaluated by contact angle and surface energy using a Dynamic Contact Angle Meter (DCAT 21, Germany). Deionized water (g ¼ 72.8 mN/m, gd ¼ 21.8 mN/m, gp ¼ 51.0 mN/m) and ethylene glycol (g ¼ 48.3 mN/m, gd ¼ 29.3 mN/m, gp ¼ 19.0 mN/m) were chosen as the testing liquid. The dispersive and polar components of carbon fibres are described in Eq (1):
qffiffiffiffiffiffiffiffiffiffiffi
g1 ð1 þ cos qÞ ¼ 2 gp1 gpf gf ¼ gdf þ gpf
(1)
where q is the contact angle between carbon fibres and the testing liquid; g is the surface tension; gd is the dispersive component; gp is the polar component; subscript l stands for the testing liquid, and subscript f stands for fibres. The wettability of carbon fibres/epoxy was directly measured by the contact angle, and the work of adhesion (WA) was calculated by the revised Young-Dupre [20] Eq (2).
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WA ¼ gf ð1 þ cos fÞ
(2)
Where f is the contact angle between the carbon fibres and cured epoxy. 2.3.7. Interfacial shear strength The interfacial evaluation equipment (MODEL HM410, Japan) was employed to determine the IFSS by microbond test. Monofilament specimens were adhered on a holder with a 70e90 mm embedded length using a fine-point applicator, and the monofilaments adhered by a resin microdroplet were cured in an oven under normal pressure at 90 C for 10 h. The force to pull the filament out of the cured epoxy resin was measured for each specimen, and IFSS tIFSS was calculated by the following equation:
.
tIFSS ¼ Fmax p$df $le
(3)
where Fmax is the maximum value of pull out force, df is the diameter of single fibre, and le is the microdroplet embedded length. 3. Results and discussion 3.1. Polarization current In the polarization current test, Fig. 2 shows that the voltage of two applied monofilament ends during microwave irradiation was recorded, which proves the f polarization current existence when microwave irradiates on the carbon fibre. In order to evaluate this influence of polarization current, a designed experiment was performed to separate the polarization current effect from the effects of microwave radiation on carbon fibre. In Fig. 1, the monofilament specimen was divided equally into a microwave radiation section and a pure current section. In the microwave radiation section, carbon fibre was directly subjected to the microwave radiation, and both the microwave and polarization current are supposed to have effects on carbon fibre surface. Specimens of this section are coded as CF-M. In the pure current section, the activated polarization current was transmitted from the microwave radiation section, and carbon fibre was affected by polarization current. Specimens of this section are coded as CF-E. The relative polarization current density was calculated by equation (4).
.
r ¼ IRMS p$d2
(4)
Where r represents the polarization current density, IRMS is the root mean square of the polarization current, and d is the monofilament diameter. The density of this acquired current can be as high as 9.26 A/cm2, which is close to the electrochemical treatment current [21]. It implies that the surface of carbon fibres might suffer a electrochemical erosion that result in surface modification when this high-energy and high-frequency polarization current impacts on carbon fibres. The further verifications and specific discussions are in the following microstructure characterizations. 3.2. Surface roughness The microwave radiation section and pure current section surface roughness was characterized to analyze the influence of the microwave effects and pure polarization current effect on carbon fibre. Fig. 3 shows 2D and 3D images of the surface topographies of carbon fibre for a better presentation of the shape and depth of the patterns. The AFM image of the desized carbon fiber shows a decreased roughening than the sized carbon fiber, thus indicating that the sizing agent was removed to reveal the carbon fiber smooth surface. There was a remarkable difference after the radiation in the pure current section and microwave radiation section. The surface of carbon fibre before radiation (Fig. 3b) was relatively smooth, but there were some parallel grooves distributed along the longitudinal direction of all treated fibres (Fig. 3cef), indicating both microwave effects and polarization current effect act on the surface. The arithmetic mean surface roughness (Ra) was measured and expressed in the top-right corner of the AFM image. The roughness increased with the radiation time, which indicates that microwave radiation played an etching effect on the carbon fibre surface. Also, these grooves caused by microwave radiation became wider and deeper with radiation time, obtaining a larger specific surface of carbon fiber. Fig. 4 further observes the groove surface. The groove depth in y direction of CF-M-180 decreased more dramatically than that of CF-M-90, suggesting that the microcrystallites structure on the surface was peeled with the increased radiation time. Meanwhile, some small peaks in the x direction appeared corresponding to the humps in the AFM image, which is similar to phenomena induced by other oxidation method [22], implying an oxidation occurred on the carbon fiber during the microwave radiation. Moreover, CF-M regarded as the cooperation of polarization current effect and microwave effects exhibited a larger roughness than the CF-E within the same radiation time. It proves that, besides the polarization current effect, microwave effects affected the carbon fibre surface topographies as well. 3.3. Surface composition
Fig. 2. The recording voltage during the microwave radiation for 90 s.
The chemical compositions of CFs and microwave treated CF were characterized by XPS spectrum and presented in Table 1. The data indicates that the oxygen/carbon atomic ratio increased in both sections, which suggests that the oxidation occurred during the microwave radiation. The polarization current explains how oxidation influenced the carbon fibre surface in the pure current section. Firstly, the high-density polarization current caused discharge phenomena at interface between the conductive carbon fibre and non-conductive air. This discharge phenomena caused some particular chemical phenomena like molecular break, and promoted the surface oxidation [16,23]. Secondly, high-frequency polarization current (2.45 GHz) increased the molecules effective
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Fig. 3. 2D and 3D AFM surfaces images of (a) CFs, (b) CF, (c) CF-E-90, (d) CF-M-90, (e) CF-E-180 and (f) CF-M-180.
Fig. 4. The groove depth along the longitudinal direction (x) and the transverse direction (y) by AFM: (a) CF-M-90 and (b) CF-M-180.
Table 1 The oxygen/carbon atomic ratio and chemical functional groups of CFs and microwave treated CF. Specimen
O/C
eCeCe
eCeOe
eC]Oe
eCOOe
CFs CF CF-E-90 CF-M-90 CF-E-180 CF-M-180
0.2501 0.1389 0.1518 0.1977 0.1811 0.2287
75.43% 91.7% 91.6% 78.9% 87.8% 65.2%
18.94% 4.63% 3.29% 11.34% 6.57% 20.8%
5.63% 1.8% 3.03% 7.64% 4.24% 10.6%
0.00% 1.8% 1.9% 2.1% 1.36% 3.17%
collision rate and further accelerated the surface the oxidation [24]. There was a relatively higher oxygen content increase (increased by 30.2% and 26.2% for 90 and 180 s, respectively) in the microwave radiation section than in the pure current section, inferring that microwave effects also oxidized the carbon surface besides polarization current effect. From discussions on microwave effects, the arc discharging phenomena when the microwave irradiates on the carbon fibre proves the forming of plasma and generate polarity groups [18,25]. Additionally, the high temperature caused by
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microwave promoted oxidation on the carbon fibre [16]. To observed the change of chemical functional groups on the surface, the narrow scan spectra in the C1s region (Fig. 5) was deconvoluted into four carbon-based functional groups including graphitic carbon (eCeCe, 284.6 eV), alcohol hydroxyl (eCeOe, 285.9e286.8 eV), carbonyl carbon (eC]Oe, 287.2e287.5 eV) and carboxyl functions or ester groups (eCOOe, 288.5e288.9 eV). Table 1 summarizes the relative contents of these functional groups. Most of the oxygen-containing functional groups of CF-E and CF-M were increased with the radiation time. However, the increased level of polarity groups in the pure current section was less than that in the microwave radiation section. Especially for
alcohol hydroxyl, only 6.57% in the pure current section compared with 20.8% in the microwave radiation section after 180 s radiation. The above results illustrate that polarity groups induced by oxidation primarily originated from microwave effects. Additionally, the content of polarity groups for CF-M-180 stayed a relatively high level, even higher than that of CFs, which suggests a strong chemical interaction was imposed on the interface [26]. 3.4. Surface structure The CF surface structure after pretreatment was characterized by Raman spectrum and deconvoluted in Fig. 6. According to
Fig. 5. The narrow scan spectra of the C1s peak region of (a) CFs, (b) CF, (c) CF-E-90, (d) CF-M-90, (e) CF-E-180 and (f) CF-M-180.
B. Wang et al. / Composites Part B 99 (2016) 398e406
previous reports [27,28], the curve D (1360 cm1) represents the mismatch at the edge of carbon atoms, implying defective graphene, and the curve G (1575 cm1) represents single crystal graphite. The area ratio of D (ID) to G (ID) is the description of graphitization. ID/IG is very sensitive to the microwave radiation time (Table 2), and this ratio in both sections increased with the radiation time, inferring that the microwave radiation on the carbon fibre surface disordered the graphitization. For graphite materials, ID/IG is supposed to have an inversely proportional relationship with the crystallite size (La). According to Baldan.M [28], the crystallite size on the surface of carbon fibre is given by the following formula:
403
La ¼ 4:4IG =ID
(5)
From equation (5), it illustrates that La is refined after the Table 2 Relative areas comparison of Raman peaks of untreated and microwave treated CF. Specimen
ID
IG
ID/IG
La (nm)
CF CF-E-90 CF-M-90 CF-E-180 CF-M-180
12580 15000 18345 18680 13430
9554 8781 8645 9745 6033
1.31 1.71 2.12 1.91 2.26
3.36 2.57 2.08 2.3 1.95
Fig. 6. Raman spectra fitted curves for (a) CF, (b) CF-E-90, (c) CF-M-90, (d) CF-E-180 and (e) CF-M-180.
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Fig. 7. The tensile strength of CFs and microwave treated CF.
Table 3 The contact angles and surface energies of CFs and microwave treated CF after pretreatment. The error bars are the standard deviation of the contact angle from three specimens. Specimen
Contact angle ( ) Deionized water
CFs CF CF-E-90 CF-M-90 CF-E-180 CF-M-180
77.1 77.3 76.8 73.7 75.3 72.4
± ± ± ± ± ±
0.4 1.9 0.7 0.8 2.3 1.2
Surface energy (mN/m) Ethylene glycol 65.6 67.0 66.4 62.9 64.3 61.4
± ± ± ± ± ±
0.8 0.7 1.3 0.9 0.9 1.7
E-51 epoxy 38.5 41.5 38.2 34.2 35.1 32.2
± ± ± ± ± ±
2.4 2.4 3.5 2.2 1.9 3.7
WA
gpf
gdf
gf
7.35 6.45 6.58 6.97 6.97 7.04
19.92 20.77 21.00 23.03 21.82 24.12
27.27 27.22 27.57 29.99 28.41 31.16
Fig. 8. IFSS of CFs and microwave treated CF/epoxy microdroplet composites. The error bars are the standard deviation of ten IFSS tests.
48.61 47.62 49.26 54.81 51.67 57.52
B. Wang et al. / Composites Part B 99 (2016) 398e406
radiation, thus indicating the graphite microstructure of carbon fibre decomposed and the micro-crystallites on the surface were peeled, which deteriorated the strength of carbon fibre [29]. Fig. 7 shows that the monofilament tensile strength decreased dramatically with increased radiation time. Therefore, the oxidation resulting from microwave radiation is the main reason for the deterioration. When microwave radiated on the carbon fibre surface, the graphitization disordered, and the graphite on the surface was etched to form grooves, as a result, the strength of monofilament became weaker [9]. The decreased level of tensile strength becomes sharp until a radiation time of 270 s (a decrease of 25% compared with CFs). Therefore, the radiation time should be limited within 180 s to ensure the tensile strength remains high enough for application.
405
to cooperate with microwave effects to affect the fibre surface performance. The roughness and polarity groups of the carbon fibre surface were increased after microwave radiation, which was attributed to the surface oxidation induced by the cooperation of microwave effects and polarization current effect. The decreased contact angle after the radiation indicated that the active chemical groups and the roughness of carbon fibre surface increased and resulted in better compatibility between carbon fibre and the matrix. The IFSS values after 180 s microwave radiation reached as high as 68.54 MPa, which increased by 98% compared with the CFs values. All these results indicated that the microwave radiation enhanced interface performance in a rather short time without chemicals. Consequently, the microwave radiation can be used as a promising low cost and rapid fibre surface pretreatment method of carbon fibre.
3.5. Surface wettability Acknowledgment Table 3 shows the wettability of CFs and microwave treated CF. The contact angles of carbon fibre against different solvents decreased after the radiation, which indicates improved wettability. Also, both the dispersive and polar components of surfacefree energy gradually increased after the radiation, and the changing tendency of carbon fibre surface energy was in accordance with the degree of surface roughness and polarity groups. The increased functional groups on the carbon fibre surface characterized in the XPS spectra contributed to the improved polar component [30]. Meanwhile, the depth of grooves etched by the microwave radiation explains the increased dispersive components [31]. The compatibility of irradiated carbon fibre with resin was evaluated by the contact angle between the carbon fibre and cured epoxy. Table 3 lists the thermodynamic work of adhesion of carbon fibre and resin. The thermodynamic work of adhesion of CF after 180 s irradiation increased as high as 121% compared with CFs. This result reveals that fibre was well impregnated by epoxy and the interactions between fibre, and epoxy would be more intense with microwave radiation [32]. 3.6. CF/epoxy interfacial shear strength Fig. 8 shows the average IFSS values of CFs and microwave treated CF. The figure shows that a 21% IFSS improvement was obtained after desizing. This result is similar with Ref. [33], which accounted it for the increment of the thermodynamic work of adhesion. Also, the IFSS values increased as high as 98% after 180 s radiation. Consequently, the improved interfacial adhesion between carbon fibre and epoxy is attributed to the additional physical and chemical effect due to the microwave radiation. For physical effect, more specific surface resulting from the increased surface roughness would be interlocked with resin, which is beneficial to stress transfer; for chemical interaction, polarity groups induced on the surface of carbon fibre are considered as the cooperation of polarization current effect and microwave effects, which promotes the chemical adhesion between carbon fibre and epoxy. This IFSS improvement (68.54 MPa for 180 s radiation on carbon fibre) is close or even higher than the data obtained by other treatments such as aqueous ammonia (38.9 MPa) [13], ultrasonic (66 MPa) [12], and electrochemical oxidation (61 MPa) [34], but less treated time is required. 4. Conclusions The microwave radiation adequacy to improve the interfacial adhesion has been demonstrated in the present work. High-energy polarization current in the filament was observed during the microwave radiation, which was regarded as another important factor
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Microwave radiation effects on carbon fibres interfacial performance Ben Wang, Yugang Duan*, Jingjing Zhang, Xinming Zhao State Key Lab for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi Province, 710049, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2015 Received in revised form 6 September 2015 Accepted 3 June 2016 Available online 8 June 2016
In this paper, microwave radiation, a fast and cost-effective industrial surface pretreatment method without chemicals, was employed to pretreat T300 carbon fibres and improve the carbon fibres/matrix interfacial properties. The microwave pretreatment mechanism on carbon fibres was studied in an experiment divided microwave treated carbon fibres into a microwave radiation section and a pure current section. The polarization current in the carbon fibre induced by microwave radiation helped microwave effects interact on the morphology, compositions, and structure of carbon fibres. Carbon fibres pretreated with short time microwave radiation had an increased thermodynamic work of adhesion and the interfacial shear strength, indicating microwave radiation pretreatment of carbon fibre promises to be an effective method of fiber treatment. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Carbon fibre Interface/interphase Surface analysis Surface treatments
1. Introduction Carbon fibres reinforced polymer composites are widely used in aerospace, energy, marine and automobile industries due to their outstanding properties such as high specific strength and stiffness, and lower density [1,2]. Composite properties are not only determined by the fibre and the resin, but also influenced by the interface between the two constituents. An effective adhesion of the carbon fibres/matrix interface has been considered as highperformance composite preconditional factor to obtain better stress transfer and crack resistance [3,4]. The carbon fibres/matrix interface formation in the composite preparation process could be considered in two stages. In the first impregnating stage, the resin spreads and impregnates on the fibre surface more easily with favorable wettability and high surface energy; In the second interface formation stage, the interface would be formed by the interaction between the resin and fibre surface, which is strongly influenced by the amount of functional groups and active surface area [5,6]. Commercially available carbon fibres are normally coated by a surface sizing layer to connect with the matrix and protect fibres from the damage. However, the tailored sizing is usually limited to match a singular kind of polymer besides the complicated sizing procedure [7,8]. To overcome the disadvantage of sizing, some carbon fibres surface pretreatments/modifications have been applied to provide an alternative interphase to improve
* Corresponding author. E-mail address: [email protected] (Y. Duan). http://dx.doi.org/10.1016/j.compositesb.2016.06.032 1359-8368/© 2016 Elsevier Ltd. All rights reserved.
the carbon fibres/matrix interface cohesive force in the industrial processes while remaining cost-effective. They adopted gas [9], liquid [10], plasma [11], ultrasonic [12] and electrochemical approaches [3] as the fibre modification. But all these pretreatment methods are time-consuming, and some chemicals are needed, which increases the cost of the production. For example, aqueous ammonia was employed to pretreat carbon fibres to increase the surface roughness, but the pretreatment time of 48 h was needed for the optimized results [13]. Also, plasma with O2 required plasma generator and high O2 flux to increase the surface oxygen content of carbon fibres in a short time [14]. Microwave, a high-frequency electromagnet wave ranging from 100 MHz to 300 GHz, has been used in some applications according to its microwave effects including thermal and non-thermal effects. For thermal effects, dielectric materials can be heated fast due to dielectric loss, which could activate molecule to relax or break chemical bonds, and form new chemical bonds. Non-thermal microwave effects have been postulated to result from a direct stabilizing interaction of the electric field with specific (polar) molecules in the reaction medium that is not related to a macroscopic temperature effect [15]. For example, the focusing of electric fields at particle interfaces caused by microwaves can form plasma and generate polarity groups [16,17]. Other than microwave effects, conductive carbon fibres could be regarded as an electric dipole according to the skin effect (Electromagnetic radiation at high frequencies penetrates the near surface region of an electrical conductor), because the diameter of monofilament is only 7 mm far smaller than microwave wavelength (122 mm). Consequently, it
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could activate polarization current on carbon fibres surface during the microwave radiation [18]. Since these characters of carbon fibres, polarization current effect and microwave effects induced by microwave pretreatment are supposed to perform on carbon fibres simultaneously. However, the mechanism of these effects for the carbon fibres pretreatment on the fibre surface performance is not so clear. Therefore, the microwave radiation was employed in present work to pretreat T300 carbon fibres within a short time, and microwave effects were analyzed by dividing microwave treated carbon fibres into a microwave radiation section and a pure current section. The carbon fibres surface after being microwave treated was investigated through morphology, compositions and surface structure. The interface performance was characterized through the carbon fibres wettability against solvents and resin and the interfacial shear strength (IFSS). 2. Material and methods Fig. 1. Polarization current test illustration during the microwave radiation.
2.1. Materials Polyacrylonitrile-based T300 carbon fibres with sizing agent were produced by Toho Tenax in Japan, which was coded as CFs. A mild desizing procedure extracted the CFs by acetone for 48 h at 70 C was adopted to avoid deteriorating the morphology and properties of original fibres [13], and the obtain fibre was coded as CF. The mixture of diglycidyl ether of bisphenol A epoxy (DER331, Dow, USA) and amine-based curing agent (Isophorone, 2855-13-2, Evonik Degussa, German) with a 3:1 wt ratio was used as the resin matrix. 2.2. Surface pretreatment by microwave radiation A tow of carbon fibres including 3000 filaments was irradiated using a microwave generator (BMR-01, Kairui, China) with a emitting frequency of 2.45 GHz at 300 W. Due to the anisotropic dielectric behavior of carbon fibres, the fibre direction was perpendicular to the electric field direction for the maximum microwave absorbance [19]. The height between the generator and carbon fibres was 180 mm. To prevent carbon fibres from being ablated, the radiation time was arranged in cycles: 90 s for radiation followed by 60 s without radiation. In addition, the specimens treated by different radiation time were coded as CF-T (T stands for radiation time). 2.3. Methods 2.3.1. Polarization current test In order to evaluate the polarization current effect on carbon fibres during the microwave radiation, the polarization current was transmitted from the irradiated monofilament, and recorded as shown in Fig. 1. Both ends of the monofilament (200 mm) were pasted on a glass plate to maintain a straight line, and then connected with copper wires (1.5 mm2) by conductive adhesive (4 104 U cm, Jiuqi, YC-01, China) to form a conductive path. A 1MU resistor was used to magnify the voltage signal obtained by oscilloscope (maximum Sampling rate 4 109 Sa/s, RIGOL, DS4012, China). 2.3.2. Atomic force microscopy The carbon fibres morphologies were observed by Atomic Force Microscopy (AFM, NanoScope MultiMode III, USA) with a scanning speed of 1 mm/s and a scanning scope of 3 mm 3 mm. At least 3 samples was provided for measuring the roughness of carbon fibres.
2.3.3. X-ray photoelectron spectroscopy The carbon fibres surface chemical composition was determined by X-ray photoelectron spectroscopy (XPS, Kratos AXIS ultra DLD, UK) with a monochromate Al Ka source (hn ¼ 1486.6 eV) at a power of 150 W (15 kV 10 mA). To compensate for the surface charging effect, all binding energies were referenced to Cls neutral carbon peak at 284.6 eV. 2.3.4. Raman spectroscopy The carbon fibres microscopic structure was analyzed by Raman Spectroscopy (Lab-RAM Aramis, JOBIN YVON HORIBA, France) using a HeeNe 20 mW laser with an excitation source of 514.5 nm. 2.3.5. Single fibre tensile testing The single fibre tensile testing was according to ASTM D337975. The specimen ends were attached to a cardboard tab using a quick-drying adhesive (Instant Krazy Glue, KG82048SN, USA) while under slight tension to ensure a consistent gauge length. The tab ends were gripped in the jaws of the testing machine, an INSTRON 1195 Tester with a 100 g load cell. The tab was then cut so that only the fibre was loaded during the test. Specimens with approximately 20 duplicate samples per set were tested under quasi-static tensile loading at a deformation rate of 0.1 mm/min. 2.3.6. Wettability The carbon fibres surface wettability was evaluated by contact angle and surface energy using a Dynamic Contact Angle Meter (DCAT 21, Germany). Deionized water (g ¼ 72.8 mN/m, gd ¼ 21.8 mN/m, gp ¼ 51.0 mN/m) and ethylene glycol (g ¼ 48.3 mN/m, gd ¼ 29.3 mN/m, gp ¼ 19.0 mN/m) were chosen as the testing liquid. The dispersive and polar components of carbon fibres are described in Eq (1):
qffiffiffiffiffiffiffiffiffiffiffi
g1 ð1 þ cos qÞ ¼ 2 gp1 gpf gf ¼ gdf þ gpf
(1)
where q is the contact angle between carbon fibres and the testing liquid; g is the surface tension; gd is the dispersive component; gp is the polar component; subscript l stands for the testing liquid, and subscript f stands for fibres. The wettability of carbon fibres/epoxy was directly measured by the contact angle, and the work of adhesion (WA) was calculated by the revised Young-Dupre [20] Eq (2).
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WA ¼ gf ð1 þ cos fÞ
(2)
Where f is the contact angle between the carbon fibres and cured epoxy. 2.3.7. Interfacial shear strength The interfacial evaluation equipment (MODEL HM410, Japan) was employed to determine the IFSS by microbond test. Monofilament specimens were adhered on a holder with a 70e90 mm embedded length using a fine-point applicator, and the monofilaments adhered by a resin microdroplet were cured in an oven under normal pressure at 90 C for 10 h. The force to pull the filament out of the cured epoxy resin was measured for each specimen, and IFSS tIFSS was calculated by the following equation:
.
tIFSS ¼ Fmax p$df $le
(3)
where Fmax is the maximum value of pull out force, df is the diameter of single fibre, and le is the microdroplet embedded length. 3. Results and discussion 3.1. Polarization current In the polarization current test, Fig. 2 shows that the voltage of two applied monofilament ends during microwave irradiation was recorded, which proves the f polarization current existence when microwave irradiates on the carbon fibre. In order to evaluate this influence of polarization current, a designed experiment was performed to separate the polarization current effect from the effects of microwave radiation on carbon fibre. In Fig. 1, the monofilament specimen was divided equally into a microwave radiation section and a pure current section. In the microwave radiation section, carbon fibre was directly subjected to the microwave radiation, and both the microwave and polarization current are supposed to have effects on carbon fibre surface. Specimens of this section are coded as CF-M. In the pure current section, the activated polarization current was transmitted from the microwave radiation section, and carbon fibre was affected by polarization current. Specimens of this section are coded as CF-E. The relative polarization current density was calculated by equation (4).
.
r ¼ IRMS p$d2
(4)
Where r represents the polarization current density, IRMS is the root mean square of the polarization current, and d is the monofilament diameter. The density of this acquired current can be as high as 9.26 A/cm2, which is close to the electrochemical treatment current [21]. It implies that the surface of carbon fibres might suffer a electrochemical erosion that result in surface modification when this high-energy and high-frequency polarization current impacts on carbon fibres. The further verifications and specific discussions are in the following microstructure characterizations. 3.2. Surface roughness The microwave radiation section and pure current section surface roughness was characterized to analyze the influence of the microwave effects and pure polarization current effect on carbon fibre. Fig. 3 shows 2D and 3D images of the surface topographies of carbon fibre for a better presentation of the shape and depth of the patterns. The AFM image of the desized carbon fiber shows a decreased roughening than the sized carbon fiber, thus indicating that the sizing agent was removed to reveal the carbon fiber smooth surface. There was a remarkable difference after the radiation in the pure current section and microwave radiation section. The surface of carbon fibre before radiation (Fig. 3b) was relatively smooth, but there were some parallel grooves distributed along the longitudinal direction of all treated fibres (Fig. 3cef), indicating both microwave effects and polarization current effect act on the surface. The arithmetic mean surface roughness (Ra) was measured and expressed in the top-right corner of the AFM image. The roughness increased with the radiation time, which indicates that microwave radiation played an etching effect on the carbon fibre surface. Also, these grooves caused by microwave radiation became wider and deeper with radiation time, obtaining a larger specific surface of carbon fiber. Fig. 4 further observes the groove surface. The groove depth in y direction of CF-M-180 decreased more dramatically than that of CF-M-90, suggesting that the microcrystallites structure on the surface was peeled with the increased radiation time. Meanwhile, some small peaks in the x direction appeared corresponding to the humps in the AFM image, which is similar to phenomena induced by other oxidation method [22], implying an oxidation occurred on the carbon fiber during the microwave radiation. Moreover, CF-M regarded as the cooperation of polarization current effect and microwave effects exhibited a larger roughness than the CF-E within the same radiation time. It proves that, besides the polarization current effect, microwave effects affected the carbon fibre surface topographies as well. 3.3. Surface composition
Fig. 2. The recording voltage during the microwave radiation for 90 s.
The chemical compositions of CFs and microwave treated CF were characterized by XPS spectrum and presented in Table 1. The data indicates that the oxygen/carbon atomic ratio increased in both sections, which suggests that the oxidation occurred during the microwave radiation. The polarization current explains how oxidation influenced the carbon fibre surface in the pure current section. Firstly, the high-density polarization current caused discharge phenomena at interface between the conductive carbon fibre and non-conductive air. This discharge phenomena caused some particular chemical phenomena like molecular break, and promoted the surface oxidation [16,23]. Secondly, high-frequency polarization current (2.45 GHz) increased the molecules effective
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Fig. 3. 2D and 3D AFM surfaces images of (a) CFs, (b) CF, (c) CF-E-90, (d) CF-M-90, (e) CF-E-180 and (f) CF-M-180.
Fig. 4. The groove depth along the longitudinal direction (x) and the transverse direction (y) by AFM: (a) CF-M-90 and (b) CF-M-180.
Table 1 The oxygen/carbon atomic ratio and chemical functional groups of CFs and microwave treated CF. Specimen
O/C
eCeCe
eCeOe
eC]Oe
eCOOe
CFs CF CF-E-90 CF-M-90 CF-E-180 CF-M-180
0.2501 0.1389 0.1518 0.1977 0.1811 0.2287
75.43% 91.7% 91.6% 78.9% 87.8% 65.2%
18.94% 4.63% 3.29% 11.34% 6.57% 20.8%
5.63% 1.8% 3.03% 7.64% 4.24% 10.6%
0.00% 1.8% 1.9% 2.1% 1.36% 3.17%
collision rate and further accelerated the surface the oxidation [24]. There was a relatively higher oxygen content increase (increased by 30.2% and 26.2% for 90 and 180 s, respectively) in the microwave radiation section than in the pure current section, inferring that microwave effects also oxidized the carbon surface besides polarization current effect. From discussions on microwave effects, the arc discharging phenomena when the microwave irradiates on the carbon fibre proves the forming of plasma and generate polarity groups [18,25]. Additionally, the high temperature caused by
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microwave promoted oxidation on the carbon fibre [16]. To observed the change of chemical functional groups on the surface, the narrow scan spectra in the C1s region (Fig. 5) was deconvoluted into four carbon-based functional groups including graphitic carbon (eCeCe, 284.6 eV), alcohol hydroxyl (eCeOe, 285.9e286.8 eV), carbonyl carbon (eC]Oe, 287.2e287.5 eV) and carboxyl functions or ester groups (eCOOe, 288.5e288.9 eV). Table 1 summarizes the relative contents of these functional groups. Most of the oxygen-containing functional groups of CF-E and CF-M were increased with the radiation time. However, the increased level of polarity groups in the pure current section was less than that in the microwave radiation section. Especially for
alcohol hydroxyl, only 6.57% in the pure current section compared with 20.8% in the microwave radiation section after 180 s radiation. The above results illustrate that polarity groups induced by oxidation primarily originated from microwave effects. Additionally, the content of polarity groups for CF-M-180 stayed a relatively high level, even higher than that of CFs, which suggests a strong chemical interaction was imposed on the interface [26]. 3.4. Surface structure The CF surface structure after pretreatment was characterized by Raman spectrum and deconvoluted in Fig. 6. According to
Fig. 5. The narrow scan spectra of the C1s peak region of (a) CFs, (b) CF, (c) CF-E-90, (d) CF-M-90, (e) CF-E-180 and (f) CF-M-180.
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previous reports [27,28], the curve D (1360 cm1) represents the mismatch at the edge of carbon atoms, implying defective graphene, and the curve G (1575 cm1) represents single crystal graphite. The area ratio of D (ID) to G (ID) is the description of graphitization. ID/IG is very sensitive to the microwave radiation time (Table 2), and this ratio in both sections increased with the radiation time, inferring that the microwave radiation on the carbon fibre surface disordered the graphitization. For graphite materials, ID/IG is supposed to have an inversely proportional relationship with the crystallite size (La). According to Baldan.M [28], the crystallite size on the surface of carbon fibre is given by the following formula:
403
La ¼ 4:4IG =ID
(5)
From equation (5), it illustrates that La is refined after the Table 2 Relative areas comparison of Raman peaks of untreated and microwave treated CF. Specimen
ID
IG
ID/IG
La (nm)
CF CF-E-90 CF-M-90 CF-E-180 CF-M-180
12580 15000 18345 18680 13430
9554 8781 8645 9745 6033
1.31 1.71 2.12 1.91 2.26
3.36 2.57 2.08 2.3 1.95
Fig. 6. Raman spectra fitted curves for (a) CF, (b) CF-E-90, (c) CF-M-90, (d) CF-E-180 and (e) CF-M-180.
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Fig. 7. The tensile strength of CFs and microwave treated CF.
Table 3 The contact angles and surface energies of CFs and microwave treated CF after pretreatment. The error bars are the standard deviation of the contact angle from three specimens. Specimen
Contact angle ( ) Deionized water
CFs CF CF-E-90 CF-M-90 CF-E-180 CF-M-180
77.1 77.3 76.8 73.7 75.3 72.4
± ± ± ± ± ±
0.4 1.9 0.7 0.8 2.3 1.2
Surface energy (mN/m) Ethylene glycol 65.6 67.0 66.4 62.9 64.3 61.4
± ± ± ± ± ±
0.8 0.7 1.3 0.9 0.9 1.7
E-51 epoxy 38.5 41.5 38.2 34.2 35.1 32.2
± ± ± ± ± ±
2.4 2.4 3.5 2.2 1.9 3.7
WA
gpf
gdf
gf
7.35 6.45 6.58 6.97 6.97 7.04
19.92 20.77 21.00 23.03 21.82 24.12
27.27 27.22 27.57 29.99 28.41 31.16
Fig. 8. IFSS of CFs and microwave treated CF/epoxy microdroplet composites. The error bars are the standard deviation of ten IFSS tests.
48.61 47.62 49.26 54.81 51.67 57.52
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radiation, thus indicating the graphite microstructure of carbon fibre decomposed and the micro-crystallites on the surface were peeled, which deteriorated the strength of carbon fibre [29]. Fig. 7 shows that the monofilament tensile strength decreased dramatically with increased radiation time. Therefore, the oxidation resulting from microwave radiation is the main reason for the deterioration. When microwave radiated on the carbon fibre surface, the graphitization disordered, and the graphite on the surface was etched to form grooves, as a result, the strength of monofilament became weaker [9]. The decreased level of tensile strength becomes sharp until a radiation time of 270 s (a decrease of 25% compared with CFs). Therefore, the radiation time should be limited within 180 s to ensure the tensile strength remains high enough for application.
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to cooperate with microwave effects to affect the fibre surface performance. The roughness and polarity groups of the carbon fibre surface were increased after microwave radiation, which was attributed to the surface oxidation induced by the cooperation of microwave effects and polarization current effect. The decreased contact angle after the radiation indicated that the active chemical groups and the roughness of carbon fibre surface increased and resulted in better compatibility between carbon fibre and the matrix. The IFSS values after 180 s microwave radiation reached as high as 68.54 MPa, which increased by 98% compared with the CFs values. All these results indicated that the microwave radiation enhanced interface performance in a rather short time without chemicals. Consequently, the microwave radiation can be used as a promising low cost and rapid fibre surface pretreatment method of carbon fibre.
3.5. Surface wettability Acknowledgment Table 3 shows the wettability of CFs and microwave treated CF. The contact angles of carbon fibre against different solvents decreased after the radiation, which indicates improved wettability. Also, both the dispersive and polar components of surfacefree energy gradually increased after the radiation, and the changing tendency of carbon fibre surface energy was in accordance with the degree of surface roughness and polarity groups. The increased functional groups on the carbon fibre surface characterized in the XPS spectra contributed to the improved polar component [30]. Meanwhile, the depth of grooves etched by the microwave radiation explains the increased dispersive components [31]. The compatibility of irradiated carbon fibre with resin was evaluated by the contact angle between the carbon fibre and cured epoxy. Table 3 lists the thermodynamic work of adhesion of carbon fibre and resin. The thermodynamic work of adhesion of CF after 180 s irradiation increased as high as 121% compared with CFs. This result reveals that fibre was well impregnated by epoxy and the interactions between fibre, and epoxy would be more intense with microwave radiation [32]. 3.6. CF/epoxy interfacial shear strength Fig. 8 shows the average IFSS values of CFs and microwave treated CF. The figure shows that a 21% IFSS improvement was obtained after desizing. This result is similar with Ref. [33], which accounted it for the increment of the thermodynamic work of adhesion. Also, the IFSS values increased as high as 98% after 180 s radiation. Consequently, the improved interfacial adhesion between carbon fibre and epoxy is attributed to the additional physical and chemical effect due to the microwave radiation. For physical effect, more specific surface resulting from the increased surface roughness would be interlocked with resin, which is beneficial to stress transfer; for chemical interaction, polarity groups induced on the surface of carbon fibre are considered as the cooperation of polarization current effect and microwave effects, which promotes the chemical adhesion between carbon fibre and epoxy. This IFSS improvement (68.54 MPa for 180 s radiation on carbon fibre) is close or even higher than the data obtained by other treatments such as aqueous ammonia (38.9 MPa) [13], ultrasonic (66 MPa) [12], and electrochemical oxidation (61 MPa) [34], but less treated time is required. 4. Conclusions The microwave radiation adequacy to improve the interfacial adhesion has been demonstrated in the present work. High-energy polarization current in the filament was observed during the microwave radiation, which was regarded as another important factor
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