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Influence of ethylene glycol pretreatment on effectiveness of atmospheric pressure plasma treatment of polyethylene fibers
Applied Surface Science 256 (2010) 3253–3258
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of ethylene glycol pretreatment on effectiveness of atmospheric pressure plasma treatment of polyethylene fibers Ying Wen a, Ranxing Li a, Fang Cai b, Kun Fu a, Shujing Peng a, Qiuran Jiang a, Lan Yao a, Yiping Qiu c,* a
Key Laboratory of Textile Science and Technology (Donghua University), Ministry of Education, China Key Laboratory of Eco-Textiles (Donghua University), Ministry of Education, China c Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, Shanghai 201620, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 May 2009 Received in revised form 3 December 2009 Accepted 6 December 2009 Available online 28 December 2009
For atmospheric pressure plasma treatments, the results of plasma treatments may be influenced by liquids adsorbed into the substrate. This paper studies the influence of ethylene glycol (EG) pretreatment on the effectiveness of atmospheric plasma jet (APPJ) treatment of ultrahigh molecular weight polyethylene (UHMWPE) fibers with 0.31% and 0.42% weight gain after soaked in EG/water solution with concentration of 0.15 and 0.3 mol/l for 24 h, respectively. Scanning electron microscopy (SEM) shows that the surface of fibers pretreated with EG/water solution does not have observable difference from that of the control group. The X-ray photoelectron spectroscopy (XPS) results show that the oxygen concentration on the surface of EG-pretreated fibers is increased less than the plasma directly treated fibers. The interfacial shear strength (IFSS) of plasma directly treated fibers to epoxy is increased almost 3 times compared with the control group while that of EG-pretreated fibers to epoxy does not change except for the fibers pretreated with lower EG concentration and longer plasma treatment time. EG pretreatment reduces the water contact angle of UHMWPE fibers. In conclusion, EG pretreatment can hamper the effect of plasma treatment of UHMWPE fibers and therefore longer plasma treatment duration is required for fibers pretreated with EG. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Atmospheric pressure plasma treatment Ethylene glycol UHMWPE fiber XPS Interfacial shear strength
1. Introduction Plasma surface treatment is a vital technology in many fields such as electronics, aerospace, automotive and biomedical industries [1] due to its ability to induce various surface modifications, such as etching [2], deposition [3] and plasma polymerization [4]. However, most of the current plasma treatments are carried out in low pressure or vacuum, which limits the application of this technology [5,6]. In contrast, atmospheric pressure plasma treatment does not require a vacuum system and therefore can greatly expand the scope of plasma treatments in materials processing [7]. Furthermore, different from low pressure plasmas, atmospheric pressure plasmas may treat materials containing adsorbed liquid. Our previous studies have shown that absorbed water in the substrate increases wettability and surface etching of the plasma treated aramid and polyamide fibers [8,9]. Alcohols are widely used in industry and research as reagents, solvents and fuels [10] which could be left on surface of polymers before plasma treatments and therefore interact with plasmas in subsequent plasma surface treatment processes. Indeed, in our previous studies, ethanol has been found to have
* Corresponding author. Tel.: +86 21 67792744; fax: +86 21 67792627. E-mail address: [email protected] (Y. Qiu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.12.015
detrimental influence on the effectiveness of plasma treatment on ultrahigh molecular weight polyethylene (UHMWPE) fibers [11]. However, little has been reported in the literature about how plasma treatment of polymeric materials is influenced by other commonly used alcohols, among which ethylene glycol (EG) is one of the most widely used chemical in wet treatment of fibrous materials. EG is a water soluble two-carbon diol with a molecular weight of 62.07 g/ mol. It is a viscous clear, colorless and odorless liquid used for a number of commercial purposes such as automobile antifreeze and aircraft deicing agent [12,13]. The adverse health effects of acute large ingestions of EG are well described in both animals and humans [14]. In plasma treatments EG is used as a cross-linking agent [15] and a solvent helped to calculate the surface free energy of indium-tin-oxide [16]. Owing to the wide application of EG in industry, the influence of EG absorbed into the substrate material on the effectiveness of atmospheric pressure plasma treatment could be of great importance to the industry. UHMWPE fibers possess high specific tensile modulus and strength. However, UHMWPE fibers have low surface energy and chemically inert surfaces, resulting in poor adhesion to many matrices. To enhance the surface wettability and adhesion properties of UHMWPE fibers, both chemical treatments and plasma treatments have been used [17]. Compared with the chemical treatments, plasma treatments make greater improvement of
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Fig. 1. The SEM micrographs for the surface of the UHMWPE fibers: (a) control, (b) plasma directly treatment for 1 lap, (c) plasma directly treatment for 2 laps, (d) EG0.15 mol/l pretreated followed by plasma treatment for 1 lap, (e) EG-0.15 mol/l pretreated followed by plasma treatment for 2 laps, (f) EG-0.3 mol/l pretreated followed by plasma treatment for 1 lap, (g) EG-0.3 mol/l pretreated followed by plasma treatment for 2 laps.
Y. Wen et al. / Applied Surface Science 256 (2010) 3253–3258
adhesion between UHMWPE and resin, due to better penetration of the resin and enhanced mechanical keying [18]. In this study, UHMWPE fiber is selected as a model system to analyze the influence of EG on the outcome of atmospheric pressure plasma treatments. The fiber surface morphology, fiber surface chemical composition, adhesion strength of the fiber to epoxy, and wettability with and without EG pretreatment were investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), micro-bond pull-out test [19], and water contact angle measurement [20], respectively.
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Table 1 XPS elemental analysis of control and APPJ treated UHMWPE fibers. Sample
Control Plasma directly treated 1 lap EG-0.15 mol/l plasma treated 2 laps
Chemical composition (%)
Atomic ratio (%)
C
O
O/C
93.39 92.18 93.10
6.61 7.82 6.90
7.07 8.48 7.42
and -unpretreated UHMWPE fibers following the plasma treatment. The fibers were gold coated prior to being loaded into the SEM chamber.
2. Experiment 2.1. Materials
2.4. XPS measurement The UHMWPE fibers were in the form of a yarn composed of 240 monofilaments with a tensile modulus of 130 GPa and a single fiber diameter around 28 mm provided by Ningbo Dacheng Chemical Fiber Group. The matrix was epoxy resin prepared with Araldite LY 3600 CL and hardener Aradur 3600-1 CL (Ciba-Geigy, Basel, Switzerland) at a ratio of 7:3 (wt/wt). The EG was 99.99% pure provided by Sinopharm Chemical Reagent Co. Ltd. 2.2. Sample preparation and plasma treatment All the UHMWPE fibers including the control fibers were soaked and washed in acetone for 3 times and 5 min each time, and then washed in deionized water for 5 min, followed by drying at 80 8C for 1 h in a vacuum oven. EG was diluted with deionized water into solutions of two pretreatment concentrations of 0.15 and 0.3 mol/l, respectively. The fibers were then divided into three groups: control or untreated, plasma directly treated for 1 lap and 2 laps, and EG solution pretreated and then plasma treated for 1 lap and 2 laps. In the EG-pretreatment process, the samples were immersed in EG solutions for 24 h, and then squeezed between the two rollers on a padding machine under the pressure of 0.3 GPa for 3 turns and then balanced in a sealed container at 13 8C and 17% relative humidity for 24 h. The mass change (add-on) of the fibers was determined by weighing the fibers before and after the EG pretreatment using an analytical balance. The plasma treatments were carried out using an atmospheric pressure plasma jet (APPJ) system (Model AtomfloTM-R, Surfx Company, USA) with the discharge power of 30 W and a frequency of 13.56 MHz. Helium (99.99% pure) as the carrier gas was introduced into the nozzle at a flow rate of 20 L/min. The distance between the nozzle and the specimens was 2 mm. The specimens were fixed on a wood frame placed on a conveying belt moving at a rate of 2.17 mm/s. They were plasma treated using a round nozzle (diameter of 20 mm) for 1 or 2 laps. In each lap, fibers were moving under the nozzle for 2 min equivalent to 18.4 s stationary time. The treatment was performed in an environment of 14 8C and 35% relative humidity.
The surface chemical composition of the control and the treated UHMWPE fibers were evaluated by the Thermo ESCALAB 250 X-ray photoelectron spectrometer. The spectra were collected using Mg K (h = 1253.6 eV) with pass energy of 20 eV. The X-ray source power was 300 W and the take-off angle was 458. The pressure within the XPS chamber was between 107 and 108 Pa. The C 1s and O 1s core level spectra were recorded to determine the surface composition of the fibers. In deconvolution analysis of the XPS peaks, the spectra were deconvoluted using Gaussian peak profiles and a linear background. 2.5. Wettability measurement The wettability of the fibers is represented by the static water contact angle of the fiber. Before test the fibers were washed by deionized water for 5 min and dried at 80 8C for 1 h in a vacuum oven. The sessile drop technique was applied using a JC2000A Stable Contact Angle Analyzer to observe the digital images of distilled water droplets on the fibers [20]. 2.6. Micro-bond pull-out test All the single fiber specimens for micro-bond test were prepared the same way as reported previously [8]. Small epoxy beads were put onto fibers and then cured at 80 8C for 2 h. The embedded length and the diameter of each bead as well as the diameter of the fiber were measured using an optical microscope. The micro-bond pull-out test was conducted on a single fiber tensile testing machine (XQ-1, Shanghai Lipu Research Institute, China) with a micro-vise. The upper clamp displacement rate was 1 mm/min. The interfacial shear strength (IFSS), ti, was calculated using the following equation derived from the well-known shear-lag model [21]:
ti ¼
n pmax cothðnL=rÞ 2A
(1)
2.3. Scanning electron microscopy A JSM-5600LV Scanning Electron Microscopy (SEM) System was employed to observe the surface morphology of the EG-pretreated
where pmax is the peak load, A is the cross-sectional area of the fiber, L is the embedded length, r is the equivalent fiber radius calculated from the fiber cross-sectional area, and n is
Table 2 XPS elements analysis for control and APPJ plasma treated UHMWPE fibers. Sample
Control Plasma directly treated 1 lap EG-0.15 mol/l plasma treated 2 laps
Relative area corresponding to different bonds (%) C–C (285 eV)
C–O (286.4 eV)
C5 5O (287.8 eV)
O–C5 5O (289.3 eV)
92.5 86.5 90.9
7.5 11.6 7.6
0 0.8 0.3
0 1.1 1.2
Y. Wen et al. / Applied Surface Science 256 (2010) 3253–3258
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defined as [21] n¼
1=2
Em E f ð1 þ nm Þ lnðR=rÞ
(2)
where Em = 2.69 GPa [22] is the Young’s modulus of the matrix, nm = 0.4 [22] is the Poisson’s ratio of the matrix. Ef = 130 GPa for the UHMWPE fiber is the fiber tensile modulus, R is the radius of the epoxy beads, and r is the same as defined previously.
Fig. 2. Potential chemical reactions during plasma treatment of polyethylene fibers with EG pretreatment.
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2.7. Statistical analysis The IFSS datum and contact angle data were analyzed using one-way analysis of variance (ANOVA) and Tukey’s pair-wise multiple comparison [23]. A p-value smaller than 0.05 was considered statistically significant. 3. Results and discussion 3.1. Surface morphology Surface morphology of control and treated UHMWPE fibers are shown in Fig. 1. All samples have longitudinal straits which inherently exist as a result of gel spinning process for manufacturing UHMWPE fibers. No obvious difference in surface roughness can be observed at the 5000 magnification among fibers from all groups. Residual EG pretreatment was observed on some of the EGpretreated fibers.
Fig. 3. Water contract angle of UHMWPE fibers vs. pretreatment for APPJ treated for 1 lap (&), and APPJ treated for 2 laps (!).
3.2. XPS analysis 3.3. Wettability of fibers The surface chemical compositions for the surfaces of the UHMWPE fibers in the control, the plasma treated only and plasma treated with EG-pretreatment groups were revealed through XPS analysis. Table 1 represents the elemental compositions and the atomic ratios of O/C derived from the XPS spectra. The APPJ plasma treatment oxidized the UHMWPE fiber surface showing a higher oxygen concentration than that of the control fibers [9,19] while the oxygen concentration of EG-pretreated group is between those of the control and the APPJ treated only group. The oxidation seemed to be hampered by the existence of EG. In order to identify what chemical functional groups were introduced to the surface of the fibers after the APPJ treatment, deconvolution analysis was carried out to obtain the concentration of each chemical component included in the C 1s peak using XPSPEAK software as shown in Table 2. The degree of fitting for each curve was higher than 99%. Four types of bonds represented by four subpeaks were fitted into the C 1s spectra: the aliphatic carbon atoms (285 eV), C–O bonds (286.4 eV), C5 5O bonds (288.1 eV), and O5 5C–O bonds (289.4 eV). After the direct APPJ treatment, C–C bonds reduced obviously due to the surface oxidation in-plasma or post-plasma treatments, and more C–O bonds were generated. However, for the fibers with EG pretreatment, the concentration of C–C decreased less compared with the plasma directly treated fibers. This may be attributed to a thin layer of EG on fiber surface preventing fibers from being oxidized by plasmas. There were also small percentages of C5 5O and O5 5C–O groups on the surface of the plasma treated fibers as observed in our previous studies [11]. The chemical reactions between activated species in the plasma and the fiber as well as EG potentially are presented in Fig. 2. During the plasma treatment, UHMPE fiber is excited by plasma particles, and generated active sites, such as R*, R–O*, R– C*5 5O. At the same time, ethylene glycol (EG) might lose some H atoms and form M–CH2–O*, M–C*H–OH. Reacted with ozone or atomic oxygen, they can be transformed into M–CH2–O*, M– C5 5O–H, M–C5 5O–O*, M–C5 5O–OH, M–C*5 5O, and react with the active sites on the excited fiber surface. Therefore, ester groups might be formed through the reaction. Also produced are ether and ketone groups, but the numbers are limited. It is also possible that the formed M–CH2–O–CH2–O–M, M–C5 5O–H, M–C5 5O–OH can react with EG molecules and give birth to M–C5 5O–CH2–M and M–C5 5O–O–CH2–M oligomers. It is believed that these chemical reactions led to the surface composition change of the fibers pretreated with EG.
Water contact angles of the control and the plasma treated UHMWPE fibers are shown in Fig. 3. The water contact angles decreased for the plasma treated directly group, that agreed with our previous study [24]. The reduced water contact angle could be mainly due to the increased O/C ratio and more C–O bonds for the plasma treated fibers as shown in the XPS results. In addition, the EG pretreatment also reduced the water contact angle of the UHMWPE fiber due to the fact that EG is a hydrophilic substance. The higher the concentration of EG, the lower the water contact angle. However, the thin layer of EG did not improve IFSS between the fiber and epoxy because these small molecules did not bond to the polyethylene molecules on the fiber surface. The EG molecules would even become a lubricant on the fiber/epoxy interface, reducing the interfacial bonding even further as the IFSS values for the three of the four EG-pretreated groups are smaller than that of the control. In addition, a longer treatment time will generally lower the water contact angle for all groups except for the EG-0.3 mol/l group which had the same water contact angle for the different plasma treatment durations probably due to the existence of the EG layer that could not be etched away by the plasma even after 2 laps of plasma treatment. 3.4. Interfacial shear strength As shown in Table 3, without EG pretreatment, the IFSS values for the fiber to epoxy were significantly increased by APPJ treatment, consistent with our previous studies in which APPJ plasma treatment significantly enhanced the adhesion of aramid fibers and UHMWPE fibers to epoxy [9]. In addition, as the plasma treatment time increased, the IFSS values for the fibers decreased a little. It is possible that prolonged Table 3 Interfacial shear strength of control and APPJ treated UHMWPE fibers determined by micro-bond pull-out test. Treatment index
Sample size
Control Plasma directly treated 1 lap Plasma directly treated 2 laps EG-0.15 mol/l plasma treated 1 lap EG-0.15 mol/l plasma treated 2 laps EG-0.3 mol/l plasma treated 1 lap EG-0.3 mol/l plasma treated 2 laps
45 35 30 33 35 26 35
IFSS (MPa) Mean
S.D.
6.13a 17.16b 12.38c 3.98a 12.96c 5.89a 3.24a
1.73 2.82 8.77 3.31 5.57 2.67 3.69
Means with different letters are statistically different at p < 0.05.
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the plasma treatment could etch away the fiber surface more but might make the initially rough fiber surface somewhat smoother and thus lead to a lowered interfacial bonding as reported in literature [25]. However, for the relatively high EG concentration (EG-0.3 mol/ l) pretreated groups, the IFSS was hardly improved compared with the control. For the relatively low EG concentration (EG-0.15 mol/l) pretreated group, only when the fibers were plasma treated for a longer time (2 laps), was the IFSS improved significantly. Therefore, the effect of plasma treatment was neutralized by the pretreatment of EG. 4. Conclusion The atmospheric pressure plasma treatment was used to modify the surface morphology and increase the oxidation of fiber surface so as to enhance the adhesion of UHMWPE fibers to epoxy resin. However, when the UHMWPE fibers pretreated with EG/ water solution, the IFSS between fibers and epoxy resin was even lower than that of the control before the EG layer on the fiber surface was etched away by the plasma. This is mainly due to the EG layer on the fiber surface acted as a barrier to the plasma treatment or as a lubricant on the fiber/epoxy interface. Therefore, the EG layer on the fiber surface has to be removed before the plasma treatment can be effective. Acknowledgements This work was jointly supported by the National High Technology Research and Development Program of China (no. 2007AA03Z101), Natural Science Foundation for the Youth (no. 50803010), and the Program of Introducing Talents of Discipline to Universities (no. B07024), and the National College Student Innovation Research Program.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of ethylene glycol pretreatment on effectiveness of atmospheric pressure plasma treatment of polyethylene fibers Ying Wen a, Ranxing Li a, Fang Cai b, Kun Fu a, Shujing Peng a, Qiuran Jiang a, Lan Yao a, Yiping Qiu c,* a
Key Laboratory of Textile Science and Technology (Donghua University), Ministry of Education, China Key Laboratory of Eco-Textiles (Donghua University), Ministry of Education, China c Department of Textile Materials Science and Product Design, College of Textiles, Donghua University, Shanghai 201620, China b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 May 2009 Received in revised form 3 December 2009 Accepted 6 December 2009 Available online 28 December 2009
For atmospheric pressure plasma treatments, the results of plasma treatments may be influenced by liquids adsorbed into the substrate. This paper studies the influence of ethylene glycol (EG) pretreatment on the effectiveness of atmospheric plasma jet (APPJ) treatment of ultrahigh molecular weight polyethylene (UHMWPE) fibers with 0.31% and 0.42% weight gain after soaked in EG/water solution with concentration of 0.15 and 0.3 mol/l for 24 h, respectively. Scanning electron microscopy (SEM) shows that the surface of fibers pretreated with EG/water solution does not have observable difference from that of the control group. The X-ray photoelectron spectroscopy (XPS) results show that the oxygen concentration on the surface of EG-pretreated fibers is increased less than the plasma directly treated fibers. The interfacial shear strength (IFSS) of plasma directly treated fibers to epoxy is increased almost 3 times compared with the control group while that of EG-pretreated fibers to epoxy does not change except for the fibers pretreated with lower EG concentration and longer plasma treatment time. EG pretreatment reduces the water contact angle of UHMWPE fibers. In conclusion, EG pretreatment can hamper the effect of plasma treatment of UHMWPE fibers and therefore longer plasma treatment duration is required for fibers pretreated with EG. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Atmospheric pressure plasma treatment Ethylene glycol UHMWPE fiber XPS Interfacial shear strength
1. Introduction Plasma surface treatment is a vital technology in many fields such as electronics, aerospace, automotive and biomedical industries [1] due to its ability to induce various surface modifications, such as etching [2], deposition [3] and plasma polymerization [4]. However, most of the current plasma treatments are carried out in low pressure or vacuum, which limits the application of this technology [5,6]. In contrast, atmospheric pressure plasma treatment does not require a vacuum system and therefore can greatly expand the scope of plasma treatments in materials processing [7]. Furthermore, different from low pressure plasmas, atmospheric pressure plasmas may treat materials containing adsorbed liquid. Our previous studies have shown that absorbed water in the substrate increases wettability and surface etching of the plasma treated aramid and polyamide fibers [8,9]. Alcohols are widely used in industry and research as reagents, solvents and fuels [10] which could be left on surface of polymers before plasma treatments and therefore interact with plasmas in subsequent plasma surface treatment processes. Indeed, in our previous studies, ethanol has been found to have
* Corresponding author. Tel.: +86 21 67792744; fax: +86 21 67792627. E-mail address: [email protected] (Y. Qiu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.12.015
detrimental influence on the effectiveness of plasma treatment on ultrahigh molecular weight polyethylene (UHMWPE) fibers [11]. However, little has been reported in the literature about how plasma treatment of polymeric materials is influenced by other commonly used alcohols, among which ethylene glycol (EG) is one of the most widely used chemical in wet treatment of fibrous materials. EG is a water soluble two-carbon diol with a molecular weight of 62.07 g/ mol. It is a viscous clear, colorless and odorless liquid used for a number of commercial purposes such as automobile antifreeze and aircraft deicing agent [12,13]. The adverse health effects of acute large ingestions of EG are well described in both animals and humans [14]. In plasma treatments EG is used as a cross-linking agent [15] and a solvent helped to calculate the surface free energy of indium-tin-oxide [16]. Owing to the wide application of EG in industry, the influence of EG absorbed into the substrate material on the effectiveness of atmospheric pressure plasma treatment could be of great importance to the industry. UHMWPE fibers possess high specific tensile modulus and strength. However, UHMWPE fibers have low surface energy and chemically inert surfaces, resulting in poor adhesion to many matrices. To enhance the surface wettability and adhesion properties of UHMWPE fibers, both chemical treatments and plasma treatments have been used [17]. Compared with the chemical treatments, plasma treatments make greater improvement of
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Fig. 1. The SEM micrographs for the surface of the UHMWPE fibers: (a) control, (b) plasma directly treatment for 1 lap, (c) plasma directly treatment for 2 laps, (d) EG0.15 mol/l pretreated followed by plasma treatment for 1 lap, (e) EG-0.15 mol/l pretreated followed by plasma treatment for 2 laps, (f) EG-0.3 mol/l pretreated followed by plasma treatment for 1 lap, (g) EG-0.3 mol/l pretreated followed by plasma treatment for 2 laps.
Y. Wen et al. / Applied Surface Science 256 (2010) 3253–3258
adhesion between UHMWPE and resin, due to better penetration of the resin and enhanced mechanical keying [18]. In this study, UHMWPE fiber is selected as a model system to analyze the influence of EG on the outcome of atmospheric pressure plasma treatments. The fiber surface morphology, fiber surface chemical composition, adhesion strength of the fiber to epoxy, and wettability with and without EG pretreatment were investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), micro-bond pull-out test [19], and water contact angle measurement [20], respectively.
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Table 1 XPS elemental analysis of control and APPJ treated UHMWPE fibers. Sample
Control Plasma directly treated 1 lap EG-0.15 mol/l plasma treated 2 laps
Chemical composition (%)
Atomic ratio (%)
C
O
O/C
93.39 92.18 93.10
6.61 7.82 6.90
7.07 8.48 7.42
and -unpretreated UHMWPE fibers following the plasma treatment. The fibers were gold coated prior to being loaded into the SEM chamber.
2. Experiment 2.1. Materials
2.4. XPS measurement The UHMWPE fibers were in the form of a yarn composed of 240 monofilaments with a tensile modulus of 130 GPa and a single fiber diameter around 28 mm provided by Ningbo Dacheng Chemical Fiber Group. The matrix was epoxy resin prepared with Araldite LY 3600 CL and hardener Aradur 3600-1 CL (Ciba-Geigy, Basel, Switzerland) at a ratio of 7:3 (wt/wt). The EG was 99.99% pure provided by Sinopharm Chemical Reagent Co. Ltd. 2.2. Sample preparation and plasma treatment All the UHMWPE fibers including the control fibers were soaked and washed in acetone for 3 times and 5 min each time, and then washed in deionized water for 5 min, followed by drying at 80 8C for 1 h in a vacuum oven. EG was diluted with deionized water into solutions of two pretreatment concentrations of 0.15 and 0.3 mol/l, respectively. The fibers were then divided into three groups: control or untreated, plasma directly treated for 1 lap and 2 laps, and EG solution pretreated and then plasma treated for 1 lap and 2 laps. In the EG-pretreatment process, the samples were immersed in EG solutions for 24 h, and then squeezed between the two rollers on a padding machine under the pressure of 0.3 GPa for 3 turns and then balanced in a sealed container at 13 8C and 17% relative humidity for 24 h. The mass change (add-on) of the fibers was determined by weighing the fibers before and after the EG pretreatment using an analytical balance. The plasma treatments were carried out using an atmospheric pressure plasma jet (APPJ) system (Model AtomfloTM-R, Surfx Company, USA) with the discharge power of 30 W and a frequency of 13.56 MHz. Helium (99.99% pure) as the carrier gas was introduced into the nozzle at a flow rate of 20 L/min. The distance between the nozzle and the specimens was 2 mm. The specimens were fixed on a wood frame placed on a conveying belt moving at a rate of 2.17 mm/s. They were plasma treated using a round nozzle (diameter of 20 mm) for 1 or 2 laps. In each lap, fibers were moving under the nozzle for 2 min equivalent to 18.4 s stationary time. The treatment was performed in an environment of 14 8C and 35% relative humidity.
The surface chemical composition of the control and the treated UHMWPE fibers were evaluated by the Thermo ESCALAB 250 X-ray photoelectron spectrometer. The spectra were collected using Mg K (h = 1253.6 eV) with pass energy of 20 eV. The X-ray source power was 300 W and the take-off angle was 458. The pressure within the XPS chamber was between 107 and 108 Pa. The C 1s and O 1s core level spectra were recorded to determine the surface composition of the fibers. In deconvolution analysis of the XPS peaks, the spectra were deconvoluted using Gaussian peak profiles and a linear background. 2.5. Wettability measurement The wettability of the fibers is represented by the static water contact angle of the fiber. Before test the fibers were washed by deionized water for 5 min and dried at 80 8C for 1 h in a vacuum oven. The sessile drop technique was applied using a JC2000A Stable Contact Angle Analyzer to observe the digital images of distilled water droplets on the fibers [20]. 2.6. Micro-bond pull-out test All the single fiber specimens for micro-bond test were prepared the same way as reported previously [8]. Small epoxy beads were put onto fibers and then cured at 80 8C for 2 h. The embedded length and the diameter of each bead as well as the diameter of the fiber were measured using an optical microscope. The micro-bond pull-out test was conducted on a single fiber tensile testing machine (XQ-1, Shanghai Lipu Research Institute, China) with a micro-vise. The upper clamp displacement rate was 1 mm/min. The interfacial shear strength (IFSS), ti, was calculated using the following equation derived from the well-known shear-lag model [21]:
ti ¼
n pmax cothðnL=rÞ 2A
(1)
2.3. Scanning electron microscopy A JSM-5600LV Scanning Electron Microscopy (SEM) System was employed to observe the surface morphology of the EG-pretreated
where pmax is the peak load, A is the cross-sectional area of the fiber, L is the embedded length, r is the equivalent fiber radius calculated from the fiber cross-sectional area, and n is
Table 2 XPS elements analysis for control and APPJ plasma treated UHMWPE fibers. Sample
Control Plasma directly treated 1 lap EG-0.15 mol/l plasma treated 2 laps
Relative area corresponding to different bonds (%) C–C (285 eV)
C–O (286.4 eV)
C5 5O (287.8 eV)
O–C5 5O (289.3 eV)
92.5 86.5 90.9
7.5 11.6 7.6
0 0.8 0.3
0 1.1 1.2
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defined as [21] n¼
1=2
Em E f ð1 þ nm Þ lnðR=rÞ
(2)
where Em = 2.69 GPa [22] is the Young’s modulus of the matrix, nm = 0.4 [22] is the Poisson’s ratio of the matrix. Ef = 130 GPa for the UHMWPE fiber is the fiber tensile modulus, R is the radius of the epoxy beads, and r is the same as defined previously.
Fig. 2. Potential chemical reactions during plasma treatment of polyethylene fibers with EG pretreatment.
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2.7. Statistical analysis The IFSS datum and contact angle data were analyzed using one-way analysis of variance (ANOVA) and Tukey’s pair-wise multiple comparison [23]. A p-value smaller than 0.05 was considered statistically significant. 3. Results and discussion 3.1. Surface morphology Surface morphology of control and treated UHMWPE fibers are shown in Fig. 1. All samples have longitudinal straits which inherently exist as a result of gel spinning process for manufacturing UHMWPE fibers. No obvious difference in surface roughness can be observed at the 5000 magnification among fibers from all groups. Residual EG pretreatment was observed on some of the EGpretreated fibers.
Fig. 3. Water contract angle of UHMWPE fibers vs. pretreatment for APPJ treated for 1 lap (&), and APPJ treated for 2 laps (!).
3.2. XPS analysis 3.3. Wettability of fibers The surface chemical compositions for the surfaces of the UHMWPE fibers in the control, the plasma treated only and plasma treated with EG-pretreatment groups were revealed through XPS analysis. Table 1 represents the elemental compositions and the atomic ratios of O/C derived from the XPS spectra. The APPJ plasma treatment oxidized the UHMWPE fiber surface showing a higher oxygen concentration than that of the control fibers [9,19] while the oxygen concentration of EG-pretreated group is between those of the control and the APPJ treated only group. The oxidation seemed to be hampered by the existence of EG. In order to identify what chemical functional groups were introduced to the surface of the fibers after the APPJ treatment, deconvolution analysis was carried out to obtain the concentration of each chemical component included in the C 1s peak using XPSPEAK software as shown in Table 2. The degree of fitting for each curve was higher than 99%. Four types of bonds represented by four subpeaks were fitted into the C 1s spectra: the aliphatic carbon atoms (285 eV), C–O bonds (286.4 eV), C5 5O bonds (288.1 eV), and O5 5C–O bonds (289.4 eV). After the direct APPJ treatment, C–C bonds reduced obviously due to the surface oxidation in-plasma or post-plasma treatments, and more C–O bonds were generated. However, for the fibers with EG pretreatment, the concentration of C–C decreased less compared with the plasma directly treated fibers. This may be attributed to a thin layer of EG on fiber surface preventing fibers from being oxidized by plasmas. There were also small percentages of C5 5O and O5 5C–O groups on the surface of the plasma treated fibers as observed in our previous studies [11]. The chemical reactions between activated species in the plasma and the fiber as well as EG potentially are presented in Fig. 2. During the plasma treatment, UHMPE fiber is excited by plasma particles, and generated active sites, such as R*, R–O*, R– C*5 5O. At the same time, ethylene glycol (EG) might lose some H atoms and form M–CH2–O*, M–C*H–OH. Reacted with ozone or atomic oxygen, they can be transformed into M–CH2–O*, M– C5 5O–H, M–C5 5O–O*, M–C5 5O–OH, M–C*5 5O, and react with the active sites on the excited fiber surface. Therefore, ester groups might be formed through the reaction. Also produced are ether and ketone groups, but the numbers are limited. It is also possible that the formed M–CH2–O–CH2–O–M, M–C5 5O–H, M–C5 5O–OH can react with EG molecules and give birth to M–C5 5O–CH2–M and M–C5 5O–O–CH2–M oligomers. It is believed that these chemical reactions led to the surface composition change of the fibers pretreated with EG.
Water contact angles of the control and the plasma treated UHMWPE fibers are shown in Fig. 3. The water contact angles decreased for the plasma treated directly group, that agreed with our previous study [24]. The reduced water contact angle could be mainly due to the increased O/C ratio and more C–O bonds for the plasma treated fibers as shown in the XPS results. In addition, the EG pretreatment also reduced the water contact angle of the UHMWPE fiber due to the fact that EG is a hydrophilic substance. The higher the concentration of EG, the lower the water contact angle. However, the thin layer of EG did not improve IFSS between the fiber and epoxy because these small molecules did not bond to the polyethylene molecules on the fiber surface. The EG molecules would even become a lubricant on the fiber/epoxy interface, reducing the interfacial bonding even further as the IFSS values for the three of the four EG-pretreated groups are smaller than that of the control. In addition, a longer treatment time will generally lower the water contact angle for all groups except for the EG-0.3 mol/l group which had the same water contact angle for the different plasma treatment durations probably due to the existence of the EG layer that could not be etched away by the plasma even after 2 laps of plasma treatment. 3.4. Interfacial shear strength As shown in Table 3, without EG pretreatment, the IFSS values for the fiber to epoxy were significantly increased by APPJ treatment, consistent with our previous studies in which APPJ plasma treatment significantly enhanced the adhesion of aramid fibers and UHMWPE fibers to epoxy [9]. In addition, as the plasma treatment time increased, the IFSS values for the fibers decreased a little. It is possible that prolonged Table 3 Interfacial shear strength of control and APPJ treated UHMWPE fibers determined by micro-bond pull-out test. Treatment index
Sample size
Control Plasma directly treated 1 lap Plasma directly treated 2 laps EG-0.15 mol/l plasma treated 1 lap EG-0.15 mol/l plasma treated 2 laps EG-0.3 mol/l plasma treated 1 lap EG-0.3 mol/l plasma treated 2 laps
45 35 30 33 35 26 35
IFSS (MPa) Mean
S.D.
6.13a 17.16b 12.38c 3.98a 12.96c 5.89a 3.24a
1.73 2.82 8.77 3.31 5.57 2.67 3.69
Means with different letters are statistically different at p < 0.05.
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the plasma treatment could etch away the fiber surface more but might make the initially rough fiber surface somewhat smoother and thus lead to a lowered interfacial bonding as reported in literature [25]. However, for the relatively high EG concentration (EG-0.3 mol/ l) pretreated groups, the IFSS was hardly improved compared with the control. For the relatively low EG concentration (EG-0.15 mol/l) pretreated group, only when the fibers were plasma treated for a longer time (2 laps), was the IFSS improved significantly. Therefore, the effect of plasma treatment was neutralized by the pretreatment of EG. 4. Conclusion The atmospheric pressure plasma treatment was used to modify the surface morphology and increase the oxidation of fiber surface so as to enhance the adhesion of UHMWPE fibers to epoxy resin. However, when the UHMWPE fibers pretreated with EG/ water solution, the IFSS between fibers and epoxy resin was even lower than that of the control before the EG layer on the fiber surface was etched away by the plasma. This is mainly due to the EG layer on the fiber surface acted as a barrier to the plasma treatment or as a lubricant on the fiber/epoxy interface. Therefore, the EG layer on the fiber surface has to be removed before the plasma treatment can be effective. Acknowledgements This work was jointly supported by the National High Technology Research and Development Program of China (no. 2007AA03Z101), Natural Science Foundation for the Youth (no. 50803010), and the Program of Introducing Talents of Discipline to Universities (no. B07024), and the National College Student Innovation Research Program.
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