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Plasma penetration depth and mechanical properties of atmospheric plasma-treated 3D aramid woven composites
Applied Surface Science 255 (2008) 2864–2868
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Plasma penetration depth and mechanical properties of atmospheric plasma-treated 3D aramid woven composites X. Chen a,b, L. Yao a,b, J. Xue a,b, D. Zhao a,b, Y. Lan a,b, X. Qian a,b, C.X. Wang a,b,c, Y. Qiu a,b,* a
Key Laboratory of Textile Science and Technology, Donghua University, Ministry of Education, China Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, Shanghai 201620, China c College of Textiles and Clothing, Yancheng Institute of Technology, Jiangsu 224003, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 July 2008 Received in revised form 12 August 2008 Accepted 13 August 2008 Available online 22 August 2008
Three-dimensional aramid woven fabrics were treated with atmospheric pressure plasmas, on one side or both sides to determine the plasma penetration depth in the 3D fabrics and the influences on final composite mechanical properties. The properties of the fibers from different layers of the single side treated fabrics, including surface morphology, chemical composition, wettability and adhesion properties were investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurement and microbond tests. Meanwhile, flexural properties of the composites reinforced with the fabrics untreated and treated on both sides were compared using threepoint bending tests. The results showed that the fibers from the outer most surface layer of the fabric had a significant improvement in their surface roughness, chemical bonding, wettability and adhesion properties after plasma treatment; the treatment effect gradually diminished for the fibers in the inner layers. In the third layer, the fiber properties remained approximately the same to those of the control. In addition, three-point bending tests indicated that the 3D aramid composite had an increase of 11% in flexural strength and 12% in flexural modulus after the plasma treatment. These results indicate that composite mechanical properties can be improved by the direct fabric treatment instead of fiber treatment with plasmas if the fabric is less than four layers thick. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Plasma treatment 3D composites Aramid fibers Interfacial adhesion Penetration depth Mechanical properties
1. Introduction Plasma treatment, as an important material modification technique, can be used to modify the chemical and physical properties of material surfaces without altering the bulk properties [1]. Adhesion of fibers to matrices is critical to mechanical and physical performance of composites [2–7]. However, most of the fibers used for composites do not have good adhesion with matrices. Therefore, plasma treatments have been applied to almost all kinds of reinforcing fibers [8]. In general, the mechanical properties of the fiber reinforced composites in which the plasma-treated fibers were used as reinforcements can be improved due to improved adhesion between the fiber and the matrix [9–15]. Treating composite preforms instead of fibers or yarns can be advantageous because
* Corresponding author at: Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, 2999 North Renmin Road, Songjiang District, Shanghai 201620, China. Tel.: +86 21 67792744; fax: +86 21 67792627. E-mail address: [email protected] (Y. Qiu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.028
yarns may be stored for long time before being woven into fabrics and then impregnated in a composite. Due to aging of the plasma treatment effect, by the time the yarns are consolidated in a composite, the plasma treatment effect could be significantly weakened [16,17]. Meanwhile, treating fabrics, especially net shaped preforms could minimize the application of the plasmas, saving energy and reducing cost [18,19]. However, how deep the plasma can penetrate into a preform determines the effectiveness of the plasma treatment. The penetration of plasmas into porous materials has been studied by many researchers [20–26]. Krentsel et al. [20,21] have demonstrated that the CF4 and C2F4 plasma–surface interaction is not just limited to the outer surfaces of a filter paper directly in contact with a low temperature plasma torch, but extends into the inner layers of the porous substrate. Mukhopadhyay et al. [22] have investigated the surface modification of porous filter papers using radio frequency plasma and found that hydrophobic coatings can be deposited on a stack of five porous filter papers and the extent of permeation can be controlled by altering the applied power of the treatment. Geyter et al. [23] have reported that process pressure has an important effect on the penetration of
X. Chen et al. / Applied Surface Science 255 (2008) 2864–2868
plasma through layers of textiles at medium pressure. Poll et al. [24] have found that the penetration of plasmas into textiles is inversely proportional to the pressure of the plasma treatments. Wang et al. have reported that helium/oxygen atmospheric pressure plasma jet is able to penetrate six layers of polyester fabrics with pore sizes of 200 mm and the penetration depth increases with the average pore sizes of fabrics [25]. It was also found that different treatment conditions would influence the penetration of plasmas through wool fabrics [26]. However, little has been reported on the effect of plasma treatment on the fabrics with more complicated structures such as 3D woven fabrics for 3D composites which have been more and more frequently used because of their damage tolerance and antidelamination properties [27]. The purpose of this study is to investigate the plasma penetration depth in 3D aramid woven preforms and the influences on the mechanical properties of the resultant composites. The plasma source in this research was an atmospheric pressure plasma jet as it is relatively intensive and versatile for treatments of preforms with any shapes. The surface morphology, chemical compositions, wettability and interfacial adhesion of the aramid fibers from different layers of the 3D preforms were studied respectively to determine the treatment effect and the penetration depth of the plasma through the fabric. Three-point bending test was performed to determine the influence of the plasma treatment on the composite properties.
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Fig. 1. Schematic of the 3D woven preform.
Table 2 Densities and fiber volume fractions of untreated (control) and plasma-treated composites Composites
Untreated Plasma treated
Fiber volume fraction (%) Weft direction
Warp direction
Z direction
Overall
Specific density (g/cm3)
13.9 14.0
33.3 33.2
7.9 7.6
55.2 54.9
1.23 1.21
2. Experimental 2.3. Sample preparation 2.1. Materials Aramid yarns used in this study were Kevlar1129. The resin was an unsaturated polyester resin, AROPOLTM G 194-70RT from Ashland Chemical Company (Changzhou, Jiangsu, China), with styrene content of 42%. The physical properties of the fibers and the resin are shown in Table 1. The fabric used in the experiment was a 3D orthogonal woven fabric with four warp and five weft layers (Fig. 1). The fabric counts in warp and weft directions were 200 ends/10 cm and 230 picks/ 10 cm, respectively, while that of the Z yarn was 50 ends/10 cm. The thickness of the fabric was 6 mm.
After the plasma treatment, for the first group, the aramid yarns were pulled out of each weft layer of the treated fabrics. The single aramid fibers were selected randomly from the yarns. Layers counted from the treated side to the untreated one were respectively named the first to the fifth (see Fig. 1). For the second group, impregnation of the fabrics was achieved by vacuum assisted resin infusion molding method and curing at 25 8C over night. The obtained composites were then used for the bending test. Before the test, the composite specific densities and the fiber volume fractions were determined as showed in Table 2. 2.4. Surface morphology analysis
2.2. Plasma treatment The atmospheric pressure plasma treatment was carried out using a plasma jet, Corotec Plasma-Jet1 (PJ-1), at a sample moving speed of 4.3 mm/s. The excitation frequency was 50 Hz and the output power was 720 W. Before the treatments, all the fabrics were immerged into acetone for 2 h to remove potential surface contaminants and then dried in a vacuum oven at 100 8C for 1 h. Three groups of fabrics were prepared. In the first group, the fabrics for the fiber tests were treated on one side for three laps. In the second group, the fabrics for composite tests were treated on both sides and each side was also treated for three laps. The third group was the control or the untreated group. Table 1 Physical properties of the fibers and the resin
The surfaces of the aramid fibers from different layers of plasma-treated and -untreated fabrics were examined by a JEOL JSM-5600LV scanning electron microscope at a magnification of 10,000. The samples were coated with gold prior to the scanning electron microscopy (SEM) analysis. 2.5. Chemical composition analysis The chemical composition analysis of the fiber surfaces was performed using ESCALAB 250 photoelectron spectrometer (Thermal Electron VG Scientific, USA). The X-ray source was Mg Ka (hn = 1253.6 eV) and the take-off angle was 458. The pressure in the X-ray photoelectron spectroscopy (XPS) vacuum chamber was kept between 107 and 108 Pa. 2.6. Wettabilily
Properties
Kevlar1129
Unsaturated polyester resin
Linear density (dtex) Tensile strength (MPa) Tensile modulus (GPa) Failure strain (%)
3140 3450 97.0 3.3
– 55 3.8 –
The wettability for the aramid fibers from different layers of the plasma-treated and the -untreated fabrics was determined by measuring the water dynamic contact angle of the fibers using the Wilhelmy balance method performed on a DCA322 (Thermo Cahn Company, USA) automatic tensiometer interfaced with a computer.
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X. Chen et al. / Applied Surface Science 255 (2008) 2864–2868
This measurement was carried out at 20 8C and 65% relative humidity [28]. 2.7. Fiber/resin interfacial adhesion The interfacial adhesion between the aramid fibers and the resin was determined by measuring the fiber/resin interfacial shear strength (IFSS) using the micro-bond test. Immediately after the plasma treatment, micro-bond samples were prepared as described in [29] with unsaturated polyester resin and 4 phr (parts per 100 parts) of the harder (methylethyl ketone peroxide). After placing the resin beads on the fibers, the samples were cured for
2 h at 80 8C. The diameters of the fibers and the lengths of the beads were measured using an Olympus CH-2 microscope equipped with a Panasonic WV-GP410/A digital photomicrography system. The micro-bond test was performed at a displacement rate of 1 mm/ min on a XQ-1 fiber tensile testing machine with a load cell of 1 N capacity. 2.8. Composite bending test The flexural properties of the plasma-treated and the untreated 3D aramid fabric reinforced composites were measured using three-point bending test. The composites were first cut into
Fig. 2. SEM micrographs of the surfaces of aramid fibers from different layers of the treated fabric and the control: (a) first layer; (b) second layer; (c) third layer; (d) fourth layer; (e) fifth layer; (f) control.
X. Chen et al. / Applied Surface Science 255 (2008) 2864–2868
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120 mm 15 mm specimens. Three-point bending tests were conducted on an Instron Model 3365 universal testing machine at a constant cross head speed of 2 mm/min with a 5 kN load cell. The span of the beam was set to 80 mm. All the composites were tested along their warp directions. 3. Results and discussion 3.1. Surface morphology The surfaces of the aramid fibers from the treated and the control fabrics are shown in Fig. 2. It appears that the plasma treatment etched the surfaces of the fibers. Compared with the control fiber, the fibers from all layers of the treated fabric showed a certain degree of increased surface roughness. The roughness is the most obvious for the first layer. This is different from the results reported in our previous publications when aramid fibers were treated with atmospheric pressure plasma jet using helium as the carrier gas [30]. This is because the current plasma device is a corona discharge device with a much larger power compared with the one in our previous studies (720 W vs. 10 W). 3.2. Chemical composition analysis XPS analysis detected significant differences in the chemical composition between the treated groups and the control group as showed in Table 3. For the fibers from the first layer of the fabric, the atomic percentage of oxygen decreased 14.3% while the percentage of nitrogen increased 220%. The atomic ratio of (O + N)/ C increased by 15.8% which could mean more functional groups were introduced onto the surface of the fibers. Since the original atomic ratio of nitrogen and oxygen are both close to 15%. The increase of nitrogen content and the decrease of oxygen made the surface composition closer to the original surface content of the fiber. Therefore the change of the surface chemical composition could result from the etching of the oxidized fiber surface as reported in the literature [31]. Table 4 represents the results of deconvolution analysis of the C1s peaks. For the fibers from the first layer, the number of functional groups such as C–O, C–N, C O and –COOH increased to Table 3 Surface chemical compositions of untreated (control) and plasma-treated aramid fibers Fibers
Untreated First layer Second layer Third layer Fourth layer Fifth layer
Atomic concentration (%)
Atomic ratio
C
O
N
N/C
O/C
(O+N)/C
78 75 78 77 77 78
20 17 15 15 16 19
2 8 7 8 7 3
0.03 0.11 0.09 0.10 0.09 0.04
0.25 0.22 0.20 0.20 0.21 0.25
0.28 0.33 0.29 0.30 0.30 0.29
Table 4 Deconvolution analysis of C1s peaks for untreated (control) and plasma-treated aramid fibers Fibers
Untreated First layer Second layer Third layer Fourth layer Fifth layer
Carbon bonds (%)
Fig. 3. The dynamic contact angles of fibers from different layers of the treated fabric and the control.
50% from 28% for the untreated fiber. This could not only improve the wettability of the fibers but also facilitate the formation of strong secondary bonds between the fibers and the resin. Compared to the control group, the fibers from the second layer to the fifth layer showed a declining trend in the surface chemical changes, which indicates that the effect of the plasma treatment was less profound for the fibers located in the inner layers. 3.3. Wettabilily The dynamic contact angles of the aramid fibers are showed in Fig. 3 in which ua is the advancing contact angle, and ur is the receding contact angle. The advancing and receding contact angles of the first two layers were both considerably reduced after the treatment while the bottom three layers were at almost the same level as the control sample. The advancing and receding contact angles of control group were 72.58 and 69.78, respectively, while those of the first layer decreased about 68 (about 8%) and 5.48 (about 7.8%); those of the second layer decreased about 3.28 (about 4.4%) and 2.68 (about 3.7%). This corresponds well with the SEM and XPS analysis results discussed in the previous sections. Owing to the tight structure, the properties of the fibers from different layers were modified to different degrees because the top layers blocked the pathway of the plasma. 3.4. Fiber/matrix interfacial adhesion The IFSSs, ti, was calculated using the following equation [32] derived from the well-known shear-lag model
ti ¼
C–O/C–N
C
72 50 52 55 65 70
20 38 35 32 25 21
3 9 9 9 7 6
O
COOH 5 3 4 3 3 4
(1)
where pmax is the peak load, A is the cross-sectional area of the fiber, L is the imbedded length, r is the equivalent fiber radius calculated from the fiber cross-sectional area and n is defined as n¼
C–C
n pmax cothðnL=rÞ 2A
1=2 Em Ef ð1 þ nm ÞlnðR=rÞ
(2)
where Em = 3.8 GPa is the Young’s modulus of the matrix, nm = 0.45 is the Poisson’s ratio of the matrix measured in our laboratory, Ef = 97 GPa is the fiber tensile modulus, R is the radius of the epoxy beads, and r is the apparent radius of the fiber calculated from the image of the fiber from the microscope. The IFSSs of the aramid fibers from the treated fabric and the control are shown in Table 5.
X. Chen et al. / Applied Surface Science 255 (2008) 2864–2868
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Table 5 Interfacial shear strengths (IFSS) of untreated (control) and plasma-treated fibers with unsaturated polyester resin Sample Untreated First layer Second layer Third layer Fourth layer Fifth layer *
Number of specimens
IFSS (MPa)
S.D. (MPa)
c*
40 14 14 15 19 20
43.7 70.2a 56.9b 46.7c 43.6c 43.7c
12.8 9.9 6.3 6.2 7.8 4.2
Values with different superscript letters are statistically different at P < 0.05.
Table 6 Flexural properties of untreated (control) and plasma-treated composites Composites
Untreated Plasma treated
Number of samples
9 9
roughened and the polar groups were introduced after the plasma treatment, while the treatment effect weakened for the fibers from the inner layers. The wettability of the aramid fibers increased after the plasma treatment, while the inner layer fibers were influenced to a lesser degree. The IFSS of the aramid fibers with unsaturated resin increased after the plasma treatments; however the effect decreased for the inner layers. Air atmospheric pressure plasma produced by corona discharge was able to penetrate into the first two layers in 3D aramid woven fabric. The flexural strength and the modulus of the 3D aramid woven composite increased after the reinforcing fabric was treated on both sides with the plasma jet. Acknowledgements
Flexural strength (MPa)
Flexural modulus (GPa)
Mean
STDV
Mean
STDV
201 223
7 19
7.58 8.47
0.31 0.91
It can be seen that the IFSS value of the plasma-treated fibers from the first and the second layer increased 61% and 31%, respectively from that of the control, indicating improved fiber– matrix adhesion. No observable difference in IFSS was found between the control group and the bottom three layers. The reason that the IFSS were improved for the fibers from the first two layers could be a combination of roughened fiber surfaces, increased polar groups and improved surface wettability for these fibers. 3.5. Composite flexural performances The flexural properties of the two samples are showed in Table 6. For plasma-treated composites, the flexural strength and the modulus increased 11% and 12%, respectively, from those of the control (223 MPa vs. 201 MPa, P = 0.011 and 8.45 GPa vs. 7.58 GPa, P = 0.024). This outcome suggests that plasma treatment for 3D aramid fabrics can improve the mechanical properties of the composites even when the penetration depth of the treatment is limited. In summary, plasma jet treatment of 3D preforms can penetrate at least two layers of the fabric. The treatment made the surface of the fibers rougher with more polar groups and improved wettability. Consequently the IFSS of these fibers to the resin increased and the composite with the plasma-treated preform had an improved flexural performance. Therefore, using atmospheric pressure plasma to treat both sides of a 3D preform, the plasma treatment effect can reach all the layers when the preform has less than five layers. 4. Conclusions In this research, 3D aramid fabrics were treated by atmospheric pressure plasma jet with corona discharge. The properties of the aramid fibers from each layer of the fabric, including the surface morphology, chemical composition, wettability and interfacial bond strength with resin were investigated. The bending properties of 3D aramid fabric reinforced composites were also studied. The results showed that the surfaces of the aramid fibers were
This project was partially sponsored by the National High Technology Research and Development Program of China (No. 2007AA03Z101), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0526), National College Student Innovative Research Program, and Donghua University Undergraduate Education Reform Project. We want to thank Prof. Jin Zhang for her help in treating the fabrics. References [1] R. Li, L. Ye, Y.-W. Mai, Composites 28 (1996) 73–86. [2] M.O.H. Cioffi, H.J.C. Voorwald, L.R.O. Hein, L. Ambrosio, Composites 36 (2005) 615–623. [3] D.A. Biro, G. Pleizier, Y. Deslandes, J. Appl. Polym. Sci. 47 (1993) 883–894. [4] D.M. Choi, C.K. Park, K. Cho, C.E. Park, Polymer 38 (1997) 6243–6249. [5] S.-G. Lee, T.-J. Kang, T.-H. Yoon, J. Adhesion Sci. Technol. 12 (1998) 731–748. [6] J.F. Friedrich, P. Rohrerand, W. Saur, T.H. Gross, A. Lippitz, W. Unger, Surface Coat. Technol. 59 (1993) 371–378. [7] N.V. Bhat, D.J. Upadhyay, J. Appl. Polym. Sci. 86 (2002) 925–936. [8] S.J. Luo, W.V. Ooij, J. Adhesion Sci. Technol. 16 (2002) 1715–1735. [9] T. Peijs, H.A. Rijsdijk, J.M.M. Kok, P.J. Lemstra, Compos. Sci. Technol. 52 (1994) 449–466. [10] G.J. Filatovs, J. Mater. Sci. L.&T. 12 (1993) 664–665. [11] D.W. Woods, I.M. Ward, J. Mater. Sci. 29 (1994) 2572–2578. [12] D.W. Woods, P.J. Hine, I.M. Ward, Compos. Sci. Technol. 52 (1994) 397–405. [13] D.N. Hild, P. Schwartz, J. Mater. Sci. Mater. Med. 4 (1993) 481–493. [14] W. Weisweiler, K. Schlitter, Thin Solid Films. 207 (1992) 158–165. [15] E. Ebert, W. Weisweiler, The Interfacial Interactions in Polymeric Composites, Proc. NATO Adv. Study Inst. Antalya/Kemer,Turkey June 15–26,1992, pp. 287– 230. [16] C.S. Zhang, P. Chen, B. Sun, W. Li, B.C. Wang, J. Wang, Appl. Surface Sci. 254 (2008) 5776–5780. [17] Y. Ren, C.X. Wang, Y.P. Qiu, Surface Coat. Technol. 202 (2008) 2670–2676. [18] F.H. Su, Z.Z. Zhang, F. Guo, H.J. Song, W.M. Liu, Wear 261 (2006) 293–300. [19] S. Kaplan, Surface Coat. Technol. 186 (2004) 214–217. [20] E. Krentsel, S. Fusselman, H. Yasuda, T. Yasuda, M. Miyama, J. Polym. Sic. Part A: Polym. Chem. 32 (1994) 1839–1845. [21] E. Krentsel, H. Yasuda, M. Miyama, T. Yasuda, J. Polym. Sci. Part A: Polym. Chem. 33 (1995) 2887–2892. [22] S.M. Mukhopadhyay, P. Joshi, S. Datta, J. macdaniel, Appl. Surf. Sci. 201 (2002) 219–226. [23] N.De. Geyter, R. Morent, C. Leys, Plasma Sources Sci. Technol. 15 (2006) 78–84. [24] H.U. Poll, U. Schladitz, S. Schreiter, Surface Coat. Technol. 142–144 (2001) 489– 493. [25] C.X. Wang, Y. Ren, Y.P. Qiu, Surface Coat. Technol. 202 (2007) 77–83. [26] C.X. Wang, Y.P. Qiu, Surface Coat. Technol. 201 (2007) 6273–6277. [27] D. Brown, M. Morgan, R. McIlhagger, Composites 34 (2003) 511–512. [28] C.M. Weikart, M. Miyama, H.K. Yasuda1, J. Colloid Interface Sci. 211 (1999) 28–38. [29] Y. Qiu, S. Deflon, P. Schwartz, J. Adhesion Sci. Technol. 7 (1993) 1041–1049. [30] L. Liu, Q. Jiang, T. Zhu, X. Guo, Y. Sun, Y. Guan, Y. Qiu, J. Appl. Polym. Sci. 102 (2006) 242–247. [31] S. Rebouillat, T.M. Escoubes, Z.R. Gauthier, Z.A. Vigier, J. Appl. Polym. Sci. 58 (1995) 1305–1315. [32] D. Hull, T.W. Clyne, An Introduction to Composite Materials, Cambridge, 1996, pp. 133–155.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Plasma penetration depth and mechanical properties of atmospheric plasma-treated 3D aramid woven composites X. Chen a,b, L. Yao a,b, J. Xue a,b, D. Zhao a,b, Y. Lan a,b, X. Qian a,b, C.X. Wang a,b,c, Y. Qiu a,b,* a
Key Laboratory of Textile Science and Technology, Donghua University, Ministry of Education, China Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, Shanghai 201620, China c College of Textiles and Clothing, Yancheng Institute of Technology, Jiangsu 224003, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 July 2008 Received in revised form 12 August 2008 Accepted 13 August 2008 Available online 22 August 2008
Three-dimensional aramid woven fabrics were treated with atmospheric pressure plasmas, on one side or both sides to determine the plasma penetration depth in the 3D fabrics and the influences on final composite mechanical properties. The properties of the fibers from different layers of the single side treated fabrics, including surface morphology, chemical composition, wettability and adhesion properties were investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle measurement and microbond tests. Meanwhile, flexural properties of the composites reinforced with the fabrics untreated and treated on both sides were compared using threepoint bending tests. The results showed that the fibers from the outer most surface layer of the fabric had a significant improvement in their surface roughness, chemical bonding, wettability and adhesion properties after plasma treatment; the treatment effect gradually diminished for the fibers in the inner layers. In the third layer, the fiber properties remained approximately the same to those of the control. In addition, three-point bending tests indicated that the 3D aramid composite had an increase of 11% in flexural strength and 12% in flexural modulus after the plasma treatment. These results indicate that composite mechanical properties can be improved by the direct fabric treatment instead of fiber treatment with plasmas if the fabric is less than four layers thick. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Plasma treatment 3D composites Aramid fibers Interfacial adhesion Penetration depth Mechanical properties
1. Introduction Plasma treatment, as an important material modification technique, can be used to modify the chemical and physical properties of material surfaces without altering the bulk properties [1]. Adhesion of fibers to matrices is critical to mechanical and physical performance of composites [2–7]. However, most of the fibers used for composites do not have good adhesion with matrices. Therefore, plasma treatments have been applied to almost all kinds of reinforcing fibers [8]. In general, the mechanical properties of the fiber reinforced composites in which the plasma-treated fibers were used as reinforcements can be improved due to improved adhesion between the fiber and the matrix [9–15]. Treating composite preforms instead of fibers or yarns can be advantageous because
* Corresponding author at: Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, 2999 North Renmin Road, Songjiang District, Shanghai 201620, China. Tel.: +86 21 67792744; fax: +86 21 67792627. E-mail address: [email protected] (Y. Qiu). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.028
yarns may be stored for long time before being woven into fabrics and then impregnated in a composite. Due to aging of the plasma treatment effect, by the time the yarns are consolidated in a composite, the plasma treatment effect could be significantly weakened [16,17]. Meanwhile, treating fabrics, especially net shaped preforms could minimize the application of the plasmas, saving energy and reducing cost [18,19]. However, how deep the plasma can penetrate into a preform determines the effectiveness of the plasma treatment. The penetration of plasmas into porous materials has been studied by many researchers [20–26]. Krentsel et al. [20,21] have demonstrated that the CF4 and C2F4 plasma–surface interaction is not just limited to the outer surfaces of a filter paper directly in contact with a low temperature plasma torch, but extends into the inner layers of the porous substrate. Mukhopadhyay et al. [22] have investigated the surface modification of porous filter papers using radio frequency plasma and found that hydrophobic coatings can be deposited on a stack of five porous filter papers and the extent of permeation can be controlled by altering the applied power of the treatment. Geyter et al. [23] have reported that process pressure has an important effect on the penetration of
X. Chen et al. / Applied Surface Science 255 (2008) 2864–2868
plasma through layers of textiles at medium pressure. Poll et al. [24] have found that the penetration of plasmas into textiles is inversely proportional to the pressure of the plasma treatments. Wang et al. have reported that helium/oxygen atmospheric pressure plasma jet is able to penetrate six layers of polyester fabrics with pore sizes of 200 mm and the penetration depth increases with the average pore sizes of fabrics [25]. It was also found that different treatment conditions would influence the penetration of plasmas through wool fabrics [26]. However, little has been reported on the effect of plasma treatment on the fabrics with more complicated structures such as 3D woven fabrics for 3D composites which have been more and more frequently used because of their damage tolerance and antidelamination properties [27]. The purpose of this study is to investigate the plasma penetration depth in 3D aramid woven preforms and the influences on the mechanical properties of the resultant composites. The plasma source in this research was an atmospheric pressure plasma jet as it is relatively intensive and versatile for treatments of preforms with any shapes. The surface morphology, chemical compositions, wettability and interfacial adhesion of the aramid fibers from different layers of the 3D preforms were studied respectively to determine the treatment effect and the penetration depth of the plasma through the fabric. Three-point bending test was performed to determine the influence of the plasma treatment on the composite properties.
2865
Fig. 1. Schematic of the 3D woven preform.
Table 2 Densities and fiber volume fractions of untreated (control) and plasma-treated composites Composites
Untreated Plasma treated
Fiber volume fraction (%) Weft direction
Warp direction
Z direction
Overall
Specific density (g/cm3)
13.9 14.0
33.3 33.2
7.9 7.6
55.2 54.9
1.23 1.21
2. Experimental 2.3. Sample preparation 2.1. Materials Aramid yarns used in this study were Kevlar1129. The resin was an unsaturated polyester resin, AROPOLTM G 194-70RT from Ashland Chemical Company (Changzhou, Jiangsu, China), with styrene content of 42%. The physical properties of the fibers and the resin are shown in Table 1. The fabric used in the experiment was a 3D orthogonal woven fabric with four warp and five weft layers (Fig. 1). The fabric counts in warp and weft directions were 200 ends/10 cm and 230 picks/ 10 cm, respectively, while that of the Z yarn was 50 ends/10 cm. The thickness of the fabric was 6 mm.
After the plasma treatment, for the first group, the aramid yarns were pulled out of each weft layer of the treated fabrics. The single aramid fibers were selected randomly from the yarns. Layers counted from the treated side to the untreated one were respectively named the first to the fifth (see Fig. 1). For the second group, impregnation of the fabrics was achieved by vacuum assisted resin infusion molding method and curing at 25 8C over night. The obtained composites were then used for the bending test. Before the test, the composite specific densities and the fiber volume fractions were determined as showed in Table 2. 2.4. Surface morphology analysis
2.2. Plasma treatment The atmospheric pressure plasma treatment was carried out using a plasma jet, Corotec Plasma-Jet1 (PJ-1), at a sample moving speed of 4.3 mm/s. The excitation frequency was 50 Hz and the output power was 720 W. Before the treatments, all the fabrics were immerged into acetone for 2 h to remove potential surface contaminants and then dried in a vacuum oven at 100 8C for 1 h. Three groups of fabrics were prepared. In the first group, the fabrics for the fiber tests were treated on one side for three laps. In the second group, the fabrics for composite tests were treated on both sides and each side was also treated for three laps. The third group was the control or the untreated group. Table 1 Physical properties of the fibers and the resin
The surfaces of the aramid fibers from different layers of plasma-treated and -untreated fabrics were examined by a JEOL JSM-5600LV scanning electron microscope at a magnification of 10,000. The samples were coated with gold prior to the scanning electron microscopy (SEM) analysis. 2.5. Chemical composition analysis The chemical composition analysis of the fiber surfaces was performed using ESCALAB 250 photoelectron spectrometer (Thermal Electron VG Scientific, USA). The X-ray source was Mg Ka (hn = 1253.6 eV) and the take-off angle was 458. The pressure in the X-ray photoelectron spectroscopy (XPS) vacuum chamber was kept between 107 and 108 Pa. 2.6. Wettabilily
Properties
Kevlar1129
Unsaturated polyester resin
Linear density (dtex) Tensile strength (MPa) Tensile modulus (GPa) Failure strain (%)
3140 3450 97.0 3.3
– 55 3.8 –
The wettability for the aramid fibers from different layers of the plasma-treated and the -untreated fabrics was determined by measuring the water dynamic contact angle of the fibers using the Wilhelmy balance method performed on a DCA322 (Thermo Cahn Company, USA) automatic tensiometer interfaced with a computer.
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This measurement was carried out at 20 8C and 65% relative humidity [28]. 2.7. Fiber/resin interfacial adhesion The interfacial adhesion between the aramid fibers and the resin was determined by measuring the fiber/resin interfacial shear strength (IFSS) using the micro-bond test. Immediately after the plasma treatment, micro-bond samples were prepared as described in [29] with unsaturated polyester resin and 4 phr (parts per 100 parts) of the harder (methylethyl ketone peroxide). After placing the resin beads on the fibers, the samples were cured for
2 h at 80 8C. The diameters of the fibers and the lengths of the beads were measured using an Olympus CH-2 microscope equipped with a Panasonic WV-GP410/A digital photomicrography system. The micro-bond test was performed at a displacement rate of 1 mm/ min on a XQ-1 fiber tensile testing machine with a load cell of 1 N capacity. 2.8. Composite bending test The flexural properties of the plasma-treated and the untreated 3D aramid fabric reinforced composites were measured using three-point bending test. The composites were first cut into
Fig. 2. SEM micrographs of the surfaces of aramid fibers from different layers of the treated fabric and the control: (a) first layer; (b) second layer; (c) third layer; (d) fourth layer; (e) fifth layer; (f) control.
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120 mm 15 mm specimens. Three-point bending tests were conducted on an Instron Model 3365 universal testing machine at a constant cross head speed of 2 mm/min with a 5 kN load cell. The span of the beam was set to 80 mm. All the composites were tested along their warp directions. 3. Results and discussion 3.1. Surface morphology The surfaces of the aramid fibers from the treated and the control fabrics are shown in Fig. 2. It appears that the plasma treatment etched the surfaces of the fibers. Compared with the control fiber, the fibers from all layers of the treated fabric showed a certain degree of increased surface roughness. The roughness is the most obvious for the first layer. This is different from the results reported in our previous publications when aramid fibers were treated with atmospheric pressure plasma jet using helium as the carrier gas [30]. This is because the current plasma device is a corona discharge device with a much larger power compared with the one in our previous studies (720 W vs. 10 W). 3.2. Chemical composition analysis XPS analysis detected significant differences in the chemical composition between the treated groups and the control group as showed in Table 3. For the fibers from the first layer of the fabric, the atomic percentage of oxygen decreased 14.3% while the percentage of nitrogen increased 220%. The atomic ratio of (O + N)/ C increased by 15.8% which could mean more functional groups were introduced onto the surface of the fibers. Since the original atomic ratio of nitrogen and oxygen are both close to 15%. The increase of nitrogen content and the decrease of oxygen made the surface composition closer to the original surface content of the fiber. Therefore the change of the surface chemical composition could result from the etching of the oxidized fiber surface as reported in the literature [31]. Table 4 represents the results of deconvolution analysis of the C1s peaks. For the fibers from the first layer, the number of functional groups such as C–O, C–N, C O and –COOH increased to Table 3 Surface chemical compositions of untreated (control) and plasma-treated aramid fibers Fibers
Untreated First layer Second layer Third layer Fourth layer Fifth layer
Atomic concentration (%)
Atomic ratio
C
O
N
N/C
O/C
(O+N)/C
78 75 78 77 77 78
20 17 15 15 16 19
2 8 7 8 7 3
0.03 0.11 0.09 0.10 0.09 0.04
0.25 0.22 0.20 0.20 0.21 0.25
0.28 0.33 0.29 0.30 0.30 0.29
Table 4 Deconvolution analysis of C1s peaks for untreated (control) and plasma-treated aramid fibers Fibers
Untreated First layer Second layer Third layer Fourth layer Fifth layer
Carbon bonds (%)
Fig. 3. The dynamic contact angles of fibers from different layers of the treated fabric and the control.
50% from 28% for the untreated fiber. This could not only improve the wettability of the fibers but also facilitate the formation of strong secondary bonds between the fibers and the resin. Compared to the control group, the fibers from the second layer to the fifth layer showed a declining trend in the surface chemical changes, which indicates that the effect of the plasma treatment was less profound for the fibers located in the inner layers. 3.3. Wettabilily The dynamic contact angles of the aramid fibers are showed in Fig. 3 in which ua is the advancing contact angle, and ur is the receding contact angle. The advancing and receding contact angles of the first two layers were both considerably reduced after the treatment while the bottom three layers were at almost the same level as the control sample. The advancing and receding contact angles of control group were 72.58 and 69.78, respectively, while those of the first layer decreased about 68 (about 8%) and 5.48 (about 7.8%); those of the second layer decreased about 3.28 (about 4.4%) and 2.68 (about 3.7%). This corresponds well with the SEM and XPS analysis results discussed in the previous sections. Owing to the tight structure, the properties of the fibers from different layers were modified to different degrees because the top layers blocked the pathway of the plasma. 3.4. Fiber/matrix interfacial adhesion The IFSSs, ti, was calculated using the following equation [32] derived from the well-known shear-lag model
ti ¼
C–O/C–N
C
72 50 52 55 65 70
20 38 35 32 25 21
3 9 9 9 7 6
O
COOH 5 3 4 3 3 4
(1)
where pmax is the peak load, A is the cross-sectional area of the fiber, L is the imbedded length, r is the equivalent fiber radius calculated from the fiber cross-sectional area and n is defined as n¼
C–C
n pmax cothðnL=rÞ 2A
1=2 Em Ef ð1 þ nm ÞlnðR=rÞ
(2)
where Em = 3.8 GPa is the Young’s modulus of the matrix, nm = 0.45 is the Poisson’s ratio of the matrix measured in our laboratory, Ef = 97 GPa is the fiber tensile modulus, R is the radius of the epoxy beads, and r is the apparent radius of the fiber calculated from the image of the fiber from the microscope. The IFSSs of the aramid fibers from the treated fabric and the control are shown in Table 5.
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Table 5 Interfacial shear strengths (IFSS) of untreated (control) and plasma-treated fibers with unsaturated polyester resin Sample Untreated First layer Second layer Third layer Fourth layer Fifth layer *
Number of specimens
IFSS (MPa)
S.D. (MPa)
c*
40 14 14 15 19 20
43.7 70.2a 56.9b 46.7c 43.6c 43.7c
12.8 9.9 6.3 6.2 7.8 4.2
Values with different superscript letters are statistically different at P < 0.05.
Table 6 Flexural properties of untreated (control) and plasma-treated composites Composites
Untreated Plasma treated
Number of samples
9 9
roughened and the polar groups were introduced after the plasma treatment, while the treatment effect weakened for the fibers from the inner layers. The wettability of the aramid fibers increased after the plasma treatment, while the inner layer fibers were influenced to a lesser degree. The IFSS of the aramid fibers with unsaturated resin increased after the plasma treatments; however the effect decreased for the inner layers. Air atmospheric pressure plasma produced by corona discharge was able to penetrate into the first two layers in 3D aramid woven fabric. The flexural strength and the modulus of the 3D aramid woven composite increased after the reinforcing fabric was treated on both sides with the plasma jet. Acknowledgements
Flexural strength (MPa)
Flexural modulus (GPa)
Mean
STDV
Mean
STDV
201 223
7 19
7.58 8.47
0.31 0.91
It can be seen that the IFSS value of the plasma-treated fibers from the first and the second layer increased 61% and 31%, respectively from that of the control, indicating improved fiber– matrix adhesion. No observable difference in IFSS was found between the control group and the bottom three layers. The reason that the IFSS were improved for the fibers from the first two layers could be a combination of roughened fiber surfaces, increased polar groups and improved surface wettability for these fibers. 3.5. Composite flexural performances The flexural properties of the two samples are showed in Table 6. For plasma-treated composites, the flexural strength and the modulus increased 11% and 12%, respectively, from those of the control (223 MPa vs. 201 MPa, P = 0.011 and 8.45 GPa vs. 7.58 GPa, P = 0.024). This outcome suggests that plasma treatment for 3D aramid fabrics can improve the mechanical properties of the composites even when the penetration depth of the treatment is limited. In summary, plasma jet treatment of 3D preforms can penetrate at least two layers of the fabric. The treatment made the surface of the fibers rougher with more polar groups and improved wettability. Consequently the IFSS of these fibers to the resin increased and the composite with the plasma-treated preform had an improved flexural performance. Therefore, using atmospheric pressure plasma to treat both sides of a 3D preform, the plasma treatment effect can reach all the layers when the preform has less than five layers. 4. Conclusions In this research, 3D aramid fabrics were treated by atmospheric pressure plasma jet with corona discharge. The properties of the aramid fibers from each layer of the fabric, including the surface morphology, chemical composition, wettability and interfacial bond strength with resin were investigated. The bending properties of 3D aramid fabric reinforced composites were also studied. The results showed that the surfaces of the aramid fibers were
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