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Surface modification of polyester synthetic leather with tetramethylsilane by atmospheric pressure plasma
Applied Surface Science 346 (2015) 270–277
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Surface modification of polyester synthetic leather with tetramethylsilane by atmospheric pressure plasma C.W. Kan a,∗ , C.H. Kwong a , S.P. Ng b a b
Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Hong Kong Community College, The Hong Kong Polytechnic University, Hong Kong
a r t i c l e
i n f o
Article history: Received 4 November 2014 Received in revised form 17 March 2015 Accepted 19 March 2015 Available online 31 March 2015 Keywords: Atmospheric plasma Hydrophobic Polyester Synthetic leather Contact angle
a b s t r a c t Much works have been done on synthetic materials but scarcely on synthetic leather owing to its surface structures in terms of porosity and roughness. This paper examines the use of atmospheric pressure plasma (APP) treatment for improving the surface performance of polyester synthetic leather by use of a precursor, tetramethylsilane (TMS). Plasma deposition is regarded as an effective, simple and singlestep method with low pollution. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) confirm the deposition of organosilanes on the sample’s surface. The results showed that under a particular combination of treatment parameters, a hydrophobic surface was achieved on the APP treated sample with sessile drop static contact angle of 138◦ . The hydrophobic surface is stable without hydrophilic recovery 30 days after plasma treatment. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polyester synthetic leather is a soft-napped material that resembles natural leather and its demand has grown substantially due to limited supplies and high cost of real animal leather. Owing to hydrophilic nature, it is easily stained. This problem can be alleviated by surface modification. Plasma treatment of polymer surface is a well-established technique because of its unique ability to fabricate a thin hydrophobic film on surfaces [1]. Atmospheric pressure plasma overcomes the disadvantages of low pressure plasma which is cumbersome and expensive for integration into in-line production process [2]. Atmospheric pressure plasma jet is an effective way to create a plasma zone with the movable jet [3]. Because of its stability and low toxicity, organosilane is one of the monomers used for fabricating hydrophobic surface on textile substrates. Nowling et al. [4] demonstrated the deposition of different organosilanes on plastic by means of plasma treatment. Organosilanes serve as a precursor, for example, tetramethylsilane (TMS) [5,6], hexamethyldisilane [7,8], tetramethylcyclotetrasiloxane (TMCTS) [4] and tetraethxysilane (TEOS) [4], forming a thin film on a surface, with a stable performance. Much works have been done on synthetic materials but scarcely on synthetic leather owing to its surface structures in terms of
∗ Corresponding author. Tel.: +852 2766 6531; fax: +852 2773 1432. E-mail address: [email protected] (C.W. Kan). http://dx.doi.org/10.1016/j.apsusc.2015.03.111 0169-4332/© 2015 Elsevier B.V. All rights reserved.
porosity and roughness. For instant, the capillary action involves in liquid absorbing on textile materials [9]. This paper presents results of a study of surface modification of synthetic leather with TMS by means of atmospheric pressure plasma treatment. Contact angle, FTIR and XPS analysis were applied to study surface chemical composition where SEM analysis was carried out in order to study the morphological changes. Multiple linear regression analysis was applied to study the correlation between the treatment parameters. 2. Experimental 2.1. Material 100% polyester synthetic leather was supplied by Fifield (Asia) Ltd. The synthetic leather has a hairy-like or suede-like surface structure and it was cut into size of width 1 cm × 2.5 cm for APP treatment. The sample was stored in a conditioning room at 65 ± 2% relatively humidity and 21 ± 1 ◦ C temperature for 24 h prior to experiment. The basic information of the synthetic leather is listed in Table 1. 2.2. Atmospheric pressure plasma (APP) treatment An atmospheric pressure plasma generator (AtomfloTM – 250, Surfx Technology, USA) was used for the APP treatment. Gas discharge was ignited by applying a radio frequency of 13.56 MHz. The
C.W. Kan et al. / Applied Surface Science 346 (2015) 270–277 Table 1 Basic information of polyester synthetic leather. Composition
Picture
100% microfiber polyester
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2.4. Scanning electron microscopy (SEM) JEOL Model JSM-6490 SEM was used and the samples were coated with gold before SEM analysis. Magnification of the image was 5000×. Accelerate voltage was 20 kV. 2.5. Fourier transform infrared spectroscopy (FTIR-ATR) Perkin Elmer spectrophotometer (Spectrum 100, Perkin Elmer Ltd.) equipped with an attenuated total internal reflectance (ATR) accessory was used to analyze chemical functionalities of samples. Zinc selenide crystal was used as ATR crystal. Each FTIR spectrum was obtained after an average of 64 scans with a resolution of 4 cm−1 . 2.6. X-ray photoelectron spectroscopy (XPS) XPS analysis was carried out by a SKL-12 spectrometer (Sengyeong, China) modified with VG CLAM 4 multi-channel hemispherical analysis equipped with Al/Mg twin anode. The spectrometer was operated with non-monochromatic Mg K␣ (1253.6 eV) radiation for the characterization of the plasma-modified substrate under vacuum condition of 8 × 10−8 Pa. XPS measurement was conducted with the sample perpendicular to the detector axis (normal takeoff angle is 90◦ ). To compensate for surface charging effects, all binding energies were referenced to C 1s peak at 285.0 eV. Spectra were analyzed with the aid of software XPSpeak. 2.7. Stain resistance
Fig. 1. Schematic diagram of APP treatment.
APP jet was placed vertically over the sample in the experiment. Fig. 1 schematically shows the experimental set-up for APP treatment. Helium was used as carrier gas and tetramethylsilane (TMS) (ACROS, 99%) was applied as the precursor. Fig. 2 shows the chemical formula of TMS. TMS was applied directly on the sample surface. Various combinations of treatment parameters, discharge power (80 W, 90 W, 100 W and 110 W), flow rate of helium (7.5 litres per minute (LPM), 10.0 LPM, 12.5 LPM, 15.0 LPM and 17.5 LPM), amount of TMS (0.1 ml, 0.15 ml, 0.2 ml and 0.25 ml), jet distance (10 mm, 15 mm, 20 mm and 25 mm) and treatment time (30 s) were used for making the hydrophobic film.
2.3. Contact angle and surface energy The surface hydrophobicity was quantified by measurement of sessile drop static contact angle with contact angle goniometer [10]. A drop of 5 l deionised water was probed on the sample surface. Droplets’ images were recorded by a high-resolution camera. Contact angle in the picture was precisely measured. Five readings were taken from each sample. Mean values of the readings were calculated. The measurement was done immediately after APP treatment. Greater the contact angle is, the more hydrophobic the surface will be.
Fig. 2. Chemical formula of TMS.
Stain resistance of polyester synthetic leather was evaluated by staining it with coffee and milk tea. Instant coffee (Nescafe, Premium white coffee, prepared by dissolving 35 g instant coffee powder in 180 ml distilled water (25 ◦ C) at room temperature) and instant milk tea (Lipton, Gold Milk Tea, prepared by dissolving 16.5 g instant milk tea powder in 150 ml distilled water (25 ◦ C) at room temperature) were used for simulating the actual use condition. A drop of coffee and milk tea was placed on the sample surface and the contact angle was measured. The larger the contact angle is, the better is the stain resistance. 2.8. Multiple linear regression model SPSS 14.0 was used for regression model development, to find the correlation between dependent and independent parameters. 3. Results and discussion 3.1. Effects of discharge power and flow rate of helium Fig. 3 shows results of measurements of the contact angles (CA) of polyester synthetic leather after TMS plasma treatment. Plasma treatment with 80 W resulted in a relatively smaller enhancement in CA. Thus, 80 W cannot provide sufficient power to maintain a stable discharge of TMS. On the contrary, 90 W provides sufficient power for discharging of TMS. Considerable amount of silicon compounds were deposited on the sample surface. 90 W power resulted in the greatest improvement in CA. Discharge power beyond 90 W (100 W and 110 W) did not show further improvement in CA. Low power is preferable on the grounds of energy savings aspect also. It is assumed that the higher the discharge power in APP treatment, larger is the quantity of plasma species generated and their reactivity on the material surface is increased. Discharge power beyond a threshold results in high temperature and the plasma jet becomes too hot, leading to degradation of synthetic leather surface [11,12]. As a result, the contact angle values are affected. Moreover, if more
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C.W. Kan et al. / Applied Surface Science 346 (2015) 270–277 Table 2 CA values of different samples (treatment parameters: discharge power = 90 W; flow rate of helium = 7.5 LPM; jet distance = 10 mm and treatment time = 30 s).
Fig. 3. CA of polyester synthetic leather after TMS plasma treatment (treatment parameters: amount of TMS = 0.2 ml, jet distance = 10 mm and treatment time = 30 s).
active plasma species are generated in the plasma zone, they may have higher chance of colliding with each other which may reduce the activity for the plasma treatment [11–13]. Consequently, the surface reaction induced by plasma treatment gets reduced and the increased discharge power does not impart significant changes in surface hydrophobicity. Therefore, satisfactory hydrophobic improvement can be achieved with discharge power 90 W. CA decreases as the flow rate of helium increases. As helium gas continuously comes out from the plasma jet, part of the TMS may be driven away instead of depositing on specimen surface. This becomes more prominent at higher flow rates (of helium). At high flow rate, the amount of helium species in the plasma increases and hence the concentration of TMS active species in the plasma gets diluted. The resident time of TMS in the plasma zone also decreased. And less activated monomers in the plasma as part of the energy was consumed by the ionization helium particles. Consequently, less TMS is deposited on the sample surface and less of hydrophobicity is the result. Flow rate of helium of 7.5 LPM is the lowest operating flow rate of the plasma jet. Therefore, the most effective power is 90 W, with flow rate of 7.5 LPM, that can be used for achieving a hydrophobic surface. The hydrophobic surface is robust and stable in time, without hydrophilic recovery 30 days after plasma treatment. When placing a DI water drop on the untreated sample surface, the water drop will be absorbed spontaneously and immediately. In fact, water drop was formed and then absorbed into the sample by capillary penetration [14] because of the porous and hairy structure the sample surface. Surface structure of TMS plasma modified polyester synthetic leather is revealed by means of SEM micrographs. Fig. 4(a)–(c) illustrates the surface morphology of polyester synthetic leather after TMS plasma treatment with different discharge powers. The untreated sample (Fig. 4(a)) shows a smooth surface. It is clear that granular patches were developed on the samples with discharge power of 90 W and 110 W, shown in Fig. 4(b) and (c), respectively. The roughness of fibre surface is the same for both discharge powers. This implies that beyond the optimum power (90 W), further enhancement of power does not increase the deposition [15]. The granular patches are believed to be silicon compounds deposited through plasma treatment. The chemical properties of the patches are revealed by FTIR and XPS analysis. 3.2. Effect of amount of TMS Table 2 shows the CA values of polyester synthetic leather after TMS plasma treatment with different amounts of TMS. Fig. 5(a)–(d)
Amount of TMS (ml)
Contact angle (◦ )
0.1 0.15 0.2 0.25
Absorbed immediately 110 ± 2 138 ± 2 130 ± 2
illustrates the surface morphology of polyester synthetic leather after TMS plasma treatment with different amounts of TMS. Fig. 5(a) shows quite a small number of granular patches deposited on the fibre surface. This indicates that 0.1 ml TMS does not provide sufficient organosilicons to enhance surface hydrophobicity. On the other hand, it is clear that more granular patches are developed on the samples with discharge power 0.15 ml, 0.2 ml and 0.25 ml, as shown in Fig. 5(b)–(d), respectively. More patches are developed on the sample with 0.15 ml and, therefore, the contact angle is greatly increased to around 110◦ . The roughness of samples with 0.2 ml and 0.25 ml is about the same. This implies that once the optimum deposition (with 0.2 ml TMS) is achieved, no further enhancement takes place even when more of the precursor is applied. It is assumed that with the increase in the amount of TMS in plasma treatment, the surface hydrophobicity should be enhanced. However, the results in this study show that there is no further increase in CA with increased use of TMS. The reason may be that the collision between helium plasma species and TMS species increases and therefore, fewer active TMS species can react with the synthetic leather surface leading to no further increase in CA value [8,9]. In the TMS plasma treated sample, the granular patches are believed to be silicon compounds deposited through plasma treatment. The chemical properties of the patches are revealed by FTIR and XPS analysis. 3.3. Effect of jet distance Jet distance is the distance between the plasma jet and the substrate surface. Table 3 shows CA values of specimens treated at different jet distances. The largest CA is 138◦ which is achieved at a jet distance of 10 mm. Starting from 10 mm onwards, the greater the separation, the smaller the CA is and it reaches a steady value at 25 mm. This is because fewer active species reach the specimen surface because of the longer separation. On the other hand, when the jet distance increases, velocity and activity of the active species in the plasma jet greatly decreases by the time they reach the synthetic leather surface and thus the surface reaction is inadequate [11,12]. In addition, the specimen suffers thermal degradation when the jet distance is less than 10 mm because the sample gets melted under excessive heat of the plasma jet [11,12]. 3.4. Optimum treatment parameters and stain resistance There is a pronounced improvement in CA value after TMS plasma treatment. By definition, a surface with a CA value greater than 90◦ is a hydrophobic surface [16]. In this study, when the treatment parameters for hydrophobization of polyester Table 3 CA values of different samples (treatment parameters: discharge power = 90 W; flow rate of helium = 7.5 LPM; amount of TMS = 2 ml and treatment time = 30 s). Jet distance (mm)
Contact angle (◦ )
10 15 20 25
138 110 102 102
± ± ± ±
2 2 2 2
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Fig. 4. SEM micrographs of polyester synthetic leather after TMS plasma treatment with discharge power (a) untreated, (b) 90 W, (c) 110 W (treatment parameters: amount of TMS = 0.2 ml, flow rate of helium = 7.5 LPM, jet distance = 10 mm and dwell time = 30 s).
synthetic leather are set at discharge power = 90 W, flow rate of helium = 7.5 LPM, amount of TMS = 2 ml, jet distance = 10 mm and treatment time = 30 seconds, an optimum CA value of 138◦ was obtained, implying that the polyester synthetic fibre surface
had become hydrophobic when compared with CA of untreated specimen. Experimental results revealed that there is a noticeable improvement in stain resistance to both coffee and milk tea after
Fig. 5. SEM micrographs of polyester synthetic leather after TMS plasma treatment with different amounts of TMS (a) 0.1 ml, (b) 0.15 ml, (c) 0.2 ml, (d) 0.25 ml (treatment parameters: discharge power = 90 W; flow rate of helium = 7.5 LPM; jet distance = 10 mm and treatment time = 30 s).
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Fig. 6. FTIR spectra of polyester synthetic leather after TMS plasma treatment with flow rate of helium (a) 7.5 LPM, (b) 10 LPM, (c) 12.5 LPM, (d) 15 LPM, (e) 17.5 LPM, (f) untreated (treatment parameters: amount of TMS = 0.2 ml; discharge power = 90 W; jet distance = 10 mm and treatment time = 30 s).
plasma treatment with TMS (Table 4). Before plasma treatment, coffee and milk tea was absorbed immediately in the polyester leather surface, i.e. no stain resistant behaviour was noted. However, the plasma treatment increases the surface hydrophobicity in relation to the increase in CA values. 3.5. FTIR Table 5 shows the range of wave numbers in which functional groups and classes of compounds can be observed from the FTIR spectrum [16]. FTIR reveals the chemical compositions of the samples. Fig. 6 shows the FTIR spectra of polyester synthetic leather specimens treated with TMS plasma at different flow rates of helium. TMS plasma modified polyester synthetic leather exhibits the following features: increase in the peak of 2920 cm−1 , 2853 cm−1 (CH antisymmetric and symmetric stretching), 1462 cm−1 (CH2 bending (scissors) vibration), 1260 cm−1 (CH3 symmetric deformation), 1089 cm−1 , 1016 cm−1 (Si O Si), 847 cm−1 (Si CH3 rocking) and 721 cm−1 (CH2 rocking in methylene chains; intensity depends on chin length). As the flow rate of helium decreases (from 17.5 to 7.5 LPM), the silicon-related absorbance increases. The greatest intensity of the silicon related peaks occur at low flow rates of helium, 7.5 LPM. This indicates that low flow rate of helium facilitates the deposition of silicon compounds.
3.6. XPS The surface chemistry of polyester synthetic leather is investigated by XPS. The contents of carbon (C), oxygen (O) and silicon (S) were examined and the surface stoichiometry was revealed. Fig. 7 shows the wide scan of the XPS results of untreated and plasma modified polyester synthetic leather. The intensity of silicon compounds (Si 2p) is increased after TMS plasma treatment. The results reveal that silicon is deposited on sample surface after TMS plasma treatment. The spectra features detected, along with their binding energies and full width at half maximum (FWHM) of the treated sample are reported in Table 6. The carbon, oxygen signals
Table 5 Wave numbers (ranges) in FTIR. Range (cm−1 ) and intensity
Group and class
Assignment and remarks
2990–2850 (m-s)
CH3 and CH2 in aliphatic compounds CH2 in aliphatic compounds CH3 in aliphatic compounds CH3 in aliphatic compounds Si CH3 in silanes
CH antisymmetric and symmetric stretching CH2 bending (scissors) vibration Asymmetric CH3 deformation CH3 symmetrical deformation CH3 symmetric deformation Si O Si Si C stretch
1475–1450 (vs) 1645–1440 (m) 1380–1370 (s) 1280–1250 (vs)
Table 4 CA of different liquids (treatment parameter: discharge power = 90 W, amount of TMS = 2 ml, jet distance = 10 mm, treatment time = 30 s). Liquid Deionised water Coffee Milk tea
Untreated Absorbed immediately Absorbed immediately Absorbed immediately
APP treated ◦
138 ± 2 125 ± 2◦ 118 ± 2◦
1100–1000 (vs) 860–720 (vs) 850–810 (vs) 740–720 (w-m)
Si O Si in siloxanes Si C in organosilicon compounds Si CH3 in silanes (CH2 )n – in hydrocarbons
Si CH3 rocking CH2 rocking in methylene chains; intensity depends on chain length
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Fig. 7. Surface elementary composition of (a) untreated and (b) TSM plasma modified polyester synthetic leather under optimum condition. Table 6 XPS features of plasma modified polyester synthetic leather. Peak
Position (eV)
C 1s O 1s Si 2p
284.1–292.5 231.5–538.5 102.0–107.5
FWHM 1.00–1.73 1.18–2.02 1.94
Table 7 XPS position of C 1s features of plasma modified polyester synthetic leather. Peak
Feature
Peak position (eV)
C1 C2 C3
C H, C C C O C O, O C O
285.0 286.8 289.2
are composed of various peaks indicating the presence of different chemical surroundings for carbon and oxygen atoms. Table 7 shows the peak positions of C 1s features of plasma treated sample. Deconvolution study reveals that C 1s peak is assigned to be C H, C O and C O bonds are as shown in Fig. 8. It is clear that the C O and C O bonds intensities decreased after TMS plasma treatment. The atomic content of O-containing groups dropped after TMS plasma treatment. Chemical deposition is the basic mechanism of TMS plasma treatment. The results show a decrease in carbon (C) and oxygen (O) content and an increase in silicon (Si) content. Generally speaking, surface hydrophobicity can be determined by O-containing groups which have a strong affinity towards polar wetting agents. The O content decreased after plasma treatment (Table 8). The O/C ratio Table 8 Atomic percentage and atomic ratio of TMS plasma treated polyester synthetic leather.
Untreated Plasma treated
C
O
Si
N
O/C
Si/O
75.67 75.17
21.38 21.01
1.03 4.47
1.14 N/A
0.28 0.26
0.05 0.22
Fig. 8. XPS deconvoluted C 1s peak (a) untreated sample and (b) plasma modified polyester synthetic leather under optimum condition.
is an indicator of surface polarity of a substrate [17]. It describes the relative quantity of polar O-containing functionalities on a sample surface. The lower the ratio, the more hydrophobic a sample surface is. The O/C ratio drops after TMS plasma treatment (Table 8). At the same time, the absence of polar nitrogen (N)-containing group is also an indicator of the increase of surface hydrophobicity. The small amount of N-containing groups in the untreated sample is absent after plasma treatment. On the other hand, the silicon (Si) – containing groups are increased, as reported in Table 8. The Si-containing groups are increased 3 times more than the original. Si-containing groups are successfully deposited on the sample surface after TMS plasma treatment. The Si/O ratio is increased, as shown in Table 8. It is greatly boosted (4.4 times) after treatment. The Si/O is important to predict surface hydrophobicity, the higher the ratio, the more the hydrophobic surface is. It is because the ratio indicates that the hydrophilic oxygen content is reduced or silicon related groups are successfully deposited on the surface, or both happed at the same time. The XPS result confirmed the FTIR result that Si-containing groups are increased after plasma treatment. FTIR and TMS results show that the granular patches observed on the fibres of plasma treated samples (Fig. 6) are silicon compounds deposited through plasma treatment. TMS plasma treatment successfully enhances surface hydrophobicity of polyester synthetic leather. The improvement of surface hydrophobicity Table 9 Significance of the model parameters. Sig. (1-tailed) Flow rate of helium Amount of TMS Discharge power
0.101 0.000 0.000
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Table 10 Coefficients of the regression model.
Flow rate of helium Amount of TMS Discharge power
Unstandardized coefficients
Standardized coefficients
Collinearity statistics tolerance
−1.330 373.241 0.100
−0.143 0.536 0.554
1.000 1.000 1.000
Table 11 Comparison between predicted and experimental CA values. Flow rate of helium (LPM)
Amount of TMS (ml)
Discharge power (W)
Predicted CA (◦ )
Experimental CA (◦ )
Difference between predicted and experimental value
7.5 15.0 20.5 15.0
0.3 0.3 0.2 0.1
120 120 50 80
133.6 123.7 2.7 5.0
134 117 0 0
0.4 6.7 2.7 5.0
is attributed to the deposition of organosilicon by plasma treatment [8,18–20]. 3.7. Multiple linear regression analysis The multiple linear regression is applied to analyze the effects of independent variables, discharge power, amount of TMS and flow rate of helium on a dependent variable, contact angle (CA) by means of software SPSS. Table 9 indicates all the parameters are highly related to CA (within 95% confident interval) except flow rate of helium. In fact, as discussed before, the presence of helium serves as a medium and stabilizer for plasma generation as well as plasma jet operation. A low flow rate is preferable for plasma deposition. According to SPSS analysis, the adjusted R2 is 0.615, which means 61.5% of the variance in CA can be predicted from the independent variables. Sweet and Grace-Martin suggested that the fitness of the regression model is acceptable if the R-square is greater than 0.6 [21]. R-square statistics indicate the model fitness by representing the percentage of dependent variations fits into the linear regression model. From Table 9, the significance of F-test for this regression model is 0.000, and the model is highly significant to predict CA with the independent variables together within 95% confidence level. According to Table 10, contributions of independent variables are compared by standardized coefficients which convert the variables into the same scale. The regression coefficient of flow rate of helium is −0.143. The negative result indicates that the CA decreases as the flow rate of helium increases. This agrees with results in previous findings that low flow rate facilitates plasma deposition. On the other hand, the regression coefficients are 0.536 and 0.554 for amount of TMS and discharge power, respectively. The positive sign indicates that the CA decreases as the discharge power or amount of TMS increases. Having the highest coefficient (0.554) among all independent variables, discharge power contributes strongly to CA. All relationships are highly statistically significant (significance: 0.000–0.047). The lowest significance (0.047) from flow rate of helium indicates that flow rate of helium provides a relatively low contribution to plasma deposition. In fact, flow rate of helium provides a medium for discharging instead of being a critical factor for the deposition. The collinearity statistics tolerance demonstrates the proportion of variation of an independent variable not explained by the linear relationship with other independent variables. A value close to 1 indicates no multicollinearity problem whereas a value close to 0 indicates multicollinearity problem. Table 10 shows that the tolerance values of all the independent variables are equal to one. Hence, all the independent variables are highly correlated. Table 10 shows that the unstandardized coefficients which show the net
Table 12 Minimum values of operating parameters. Flowrate of helium (LPM)
Amount of TMS (ml)
Discharge power (W)
7.5
0.1
80
effect of the parameters. The constant is the Y-intercept, while the rest are the regression coefficients of the slope of the line which describes the relationship between the independent and dependent variables. Putting up all together, there is a linear relationship, as shown in the following regression equation: Predicted CA = −100.33 − 1.33flow rate of helium + 373.241amount of TMS + 1.1discharge power This equation predicts CA values with respect to the independent variables. The agreement between the predicted and experimental values is good, as shown in Table 11. It indicates that analysis is reliable. The derived equation is based on the results under different machine parameters. It is valid for the parameters within the range of machine settings. For instant, the minimum operating flow rate of helium is 7.5 LPM. The equation predicts the resulting CA after plasma treatment within the operating parameters. Table 12 shows the minimum values of operating parameters. 4. Conclusion Enhancement of surface hydrophobicity of polyester synthetic leather was achieved by means of atmospheric plasma treatment under controlled conditions. The parameters were the key factors for achieving the result. TMS was used as the chemical for plasma hydrophobization due to its advantages from the safety perspective and hydrophobicity improvement. A low flow rate of helium 7.5 LPM is suggested as it stabilizes plasma ionization without hindering chemical deposition. Sufficient discharge power (90 W) is required to stabilize ionization without degrading the sample surface. With the optimum treatment parameters, a highly hydrophobic surface of polyester synthetic leather was achieved with deionised water contact angle of 138◦ . The hydrophobic surface is robust and stable in time, without hydrophilic recovery 30 days after plasma treatment. SEM, FTIR and XPS results confirmed that silicon compounds were successfully deposited on the sample surface and the reduction of the atomic content of polar Ocontaining groups. Multiple linear regression analysis confirmed the correlations between plasma process parameters. Amount of TMS and discharge power showed significant correlations to the surface hydrophobic enhancement. In addition, it was revealed that
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after TMS plasma treatment, good stain resistance to coffee and milk tea can be achieved. Acknowledgement Authors would like to thank the Hong Kong Polytechnic University for the financial support of this work. References [1] K. Tsougeni, N. Vourdas, A. Tserepi, E. Gogolides, Langmuir 25 (2009) 11748–11759. [2] V. Raballand, J. Benedikt, A. von Keudell, Appl. Phys. Lett. 92 (2008) 091502. [3] C. Cheng, Z. Liye, R.J. Zhan, Surf. Coat. Technol. 200 (2005) 6659–6665. [4] G.R. Nowling, M. Yajima, S.E. Babayan, M. Moravej, X. Yang, W. Hoffman, R.F. Hicks, Plasma Sources Sci. Technol. 14 (2005) 477–484. [5] J.L.C. Fonseca, D.C. Apperley, J.P.S. Badyal, Chem. Mater. 5 (1993) 1676–1682. [6] P. Favia, R. Lamendola, R. d’Agostino, Plasma Sources Sci. Technol. 1 (1992) 59–66.
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Surface modification of polyester synthetic leather with tetramethylsilane by atmospheric pressure plasma C.W. Kan a,∗ , C.H. Kwong a , S.P. Ng b a b
Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Hong Kong Community College, The Hong Kong Polytechnic University, Hong Kong
a r t i c l e
i n f o
Article history: Received 4 November 2014 Received in revised form 17 March 2015 Accepted 19 March 2015 Available online 31 March 2015 Keywords: Atmospheric plasma Hydrophobic Polyester Synthetic leather Contact angle
a b s t r a c t Much works have been done on synthetic materials but scarcely on synthetic leather owing to its surface structures in terms of porosity and roughness. This paper examines the use of atmospheric pressure plasma (APP) treatment for improving the surface performance of polyester synthetic leather by use of a precursor, tetramethylsilane (TMS). Plasma deposition is regarded as an effective, simple and singlestep method with low pollution. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) confirm the deposition of organosilanes on the sample’s surface. The results showed that under a particular combination of treatment parameters, a hydrophobic surface was achieved on the APP treated sample with sessile drop static contact angle of 138◦ . The hydrophobic surface is stable without hydrophilic recovery 30 days after plasma treatment. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Polyester synthetic leather is a soft-napped material that resembles natural leather and its demand has grown substantially due to limited supplies and high cost of real animal leather. Owing to hydrophilic nature, it is easily stained. This problem can be alleviated by surface modification. Plasma treatment of polymer surface is a well-established technique because of its unique ability to fabricate a thin hydrophobic film on surfaces [1]. Atmospheric pressure plasma overcomes the disadvantages of low pressure plasma which is cumbersome and expensive for integration into in-line production process [2]. Atmospheric pressure plasma jet is an effective way to create a plasma zone with the movable jet [3]. Because of its stability and low toxicity, organosilane is one of the monomers used for fabricating hydrophobic surface on textile substrates. Nowling et al. [4] demonstrated the deposition of different organosilanes on plastic by means of plasma treatment. Organosilanes serve as a precursor, for example, tetramethylsilane (TMS) [5,6], hexamethyldisilane [7,8], tetramethylcyclotetrasiloxane (TMCTS) [4] and tetraethxysilane (TEOS) [4], forming a thin film on a surface, with a stable performance. Much works have been done on synthetic materials but scarcely on synthetic leather owing to its surface structures in terms of
∗ Corresponding author. Tel.: +852 2766 6531; fax: +852 2773 1432. E-mail address: [email protected] (C.W. Kan). http://dx.doi.org/10.1016/j.apsusc.2015.03.111 0169-4332/© 2015 Elsevier B.V. All rights reserved.
porosity and roughness. For instant, the capillary action involves in liquid absorbing on textile materials [9]. This paper presents results of a study of surface modification of synthetic leather with TMS by means of atmospheric pressure plasma treatment. Contact angle, FTIR and XPS analysis were applied to study surface chemical composition where SEM analysis was carried out in order to study the morphological changes. Multiple linear regression analysis was applied to study the correlation between the treatment parameters. 2. Experimental 2.1. Material 100% polyester synthetic leather was supplied by Fifield (Asia) Ltd. The synthetic leather has a hairy-like or suede-like surface structure and it was cut into size of width 1 cm × 2.5 cm for APP treatment. The sample was stored in a conditioning room at 65 ± 2% relatively humidity and 21 ± 1 ◦ C temperature for 24 h prior to experiment. The basic information of the synthetic leather is listed in Table 1. 2.2. Atmospheric pressure plasma (APP) treatment An atmospheric pressure plasma generator (AtomfloTM – 250, Surfx Technology, USA) was used for the APP treatment. Gas discharge was ignited by applying a radio frequency of 13.56 MHz. The
C.W. Kan et al. / Applied Surface Science 346 (2015) 270–277 Table 1 Basic information of polyester synthetic leather. Composition
Picture
100% microfiber polyester
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2.4. Scanning electron microscopy (SEM) JEOL Model JSM-6490 SEM was used and the samples were coated with gold before SEM analysis. Magnification of the image was 5000×. Accelerate voltage was 20 kV. 2.5. Fourier transform infrared spectroscopy (FTIR-ATR) Perkin Elmer spectrophotometer (Spectrum 100, Perkin Elmer Ltd.) equipped with an attenuated total internal reflectance (ATR) accessory was used to analyze chemical functionalities of samples. Zinc selenide crystal was used as ATR crystal. Each FTIR spectrum was obtained after an average of 64 scans with a resolution of 4 cm−1 . 2.6. X-ray photoelectron spectroscopy (XPS) XPS analysis was carried out by a SKL-12 spectrometer (Sengyeong, China) modified with VG CLAM 4 multi-channel hemispherical analysis equipped with Al/Mg twin anode. The spectrometer was operated with non-monochromatic Mg K␣ (1253.6 eV) radiation for the characterization of the plasma-modified substrate under vacuum condition of 8 × 10−8 Pa. XPS measurement was conducted with the sample perpendicular to the detector axis (normal takeoff angle is 90◦ ). To compensate for surface charging effects, all binding energies were referenced to C 1s peak at 285.0 eV. Spectra were analyzed with the aid of software XPSpeak. 2.7. Stain resistance
Fig. 1. Schematic diagram of APP treatment.
APP jet was placed vertically over the sample in the experiment. Fig. 1 schematically shows the experimental set-up for APP treatment. Helium was used as carrier gas and tetramethylsilane (TMS) (ACROS, 99%) was applied as the precursor. Fig. 2 shows the chemical formula of TMS. TMS was applied directly on the sample surface. Various combinations of treatment parameters, discharge power (80 W, 90 W, 100 W and 110 W), flow rate of helium (7.5 litres per minute (LPM), 10.0 LPM, 12.5 LPM, 15.0 LPM and 17.5 LPM), amount of TMS (0.1 ml, 0.15 ml, 0.2 ml and 0.25 ml), jet distance (10 mm, 15 mm, 20 mm and 25 mm) and treatment time (30 s) were used for making the hydrophobic film.
2.3. Contact angle and surface energy The surface hydrophobicity was quantified by measurement of sessile drop static contact angle with contact angle goniometer [10]. A drop of 5 l deionised water was probed on the sample surface. Droplets’ images were recorded by a high-resolution camera. Contact angle in the picture was precisely measured. Five readings were taken from each sample. Mean values of the readings were calculated. The measurement was done immediately after APP treatment. Greater the contact angle is, the more hydrophobic the surface will be.
Fig. 2. Chemical formula of TMS.
Stain resistance of polyester synthetic leather was evaluated by staining it with coffee and milk tea. Instant coffee (Nescafe, Premium white coffee, prepared by dissolving 35 g instant coffee powder in 180 ml distilled water (25 ◦ C) at room temperature) and instant milk tea (Lipton, Gold Milk Tea, prepared by dissolving 16.5 g instant milk tea powder in 150 ml distilled water (25 ◦ C) at room temperature) were used for simulating the actual use condition. A drop of coffee and milk tea was placed on the sample surface and the contact angle was measured. The larger the contact angle is, the better is the stain resistance. 2.8. Multiple linear regression model SPSS 14.0 was used for regression model development, to find the correlation between dependent and independent parameters. 3. Results and discussion 3.1. Effects of discharge power and flow rate of helium Fig. 3 shows results of measurements of the contact angles (CA) of polyester synthetic leather after TMS plasma treatment. Plasma treatment with 80 W resulted in a relatively smaller enhancement in CA. Thus, 80 W cannot provide sufficient power to maintain a stable discharge of TMS. On the contrary, 90 W provides sufficient power for discharging of TMS. Considerable amount of silicon compounds were deposited on the sample surface. 90 W power resulted in the greatest improvement in CA. Discharge power beyond 90 W (100 W and 110 W) did not show further improvement in CA. Low power is preferable on the grounds of energy savings aspect also. It is assumed that the higher the discharge power in APP treatment, larger is the quantity of plasma species generated and their reactivity on the material surface is increased. Discharge power beyond a threshold results in high temperature and the plasma jet becomes too hot, leading to degradation of synthetic leather surface [11,12]. As a result, the contact angle values are affected. Moreover, if more
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C.W. Kan et al. / Applied Surface Science 346 (2015) 270–277 Table 2 CA values of different samples (treatment parameters: discharge power = 90 W; flow rate of helium = 7.5 LPM; jet distance = 10 mm and treatment time = 30 s).
Fig. 3. CA of polyester synthetic leather after TMS plasma treatment (treatment parameters: amount of TMS = 0.2 ml, jet distance = 10 mm and treatment time = 30 s).
active plasma species are generated in the plasma zone, they may have higher chance of colliding with each other which may reduce the activity for the plasma treatment [11–13]. Consequently, the surface reaction induced by plasma treatment gets reduced and the increased discharge power does not impart significant changes in surface hydrophobicity. Therefore, satisfactory hydrophobic improvement can be achieved with discharge power 90 W. CA decreases as the flow rate of helium increases. As helium gas continuously comes out from the plasma jet, part of the TMS may be driven away instead of depositing on specimen surface. This becomes more prominent at higher flow rates (of helium). At high flow rate, the amount of helium species in the plasma increases and hence the concentration of TMS active species in the plasma gets diluted. The resident time of TMS in the plasma zone also decreased. And less activated monomers in the plasma as part of the energy was consumed by the ionization helium particles. Consequently, less TMS is deposited on the sample surface and less of hydrophobicity is the result. Flow rate of helium of 7.5 LPM is the lowest operating flow rate of the plasma jet. Therefore, the most effective power is 90 W, with flow rate of 7.5 LPM, that can be used for achieving a hydrophobic surface. The hydrophobic surface is robust and stable in time, without hydrophilic recovery 30 days after plasma treatment. When placing a DI water drop on the untreated sample surface, the water drop will be absorbed spontaneously and immediately. In fact, water drop was formed and then absorbed into the sample by capillary penetration [14] because of the porous and hairy structure the sample surface. Surface structure of TMS plasma modified polyester synthetic leather is revealed by means of SEM micrographs. Fig. 4(a)–(c) illustrates the surface morphology of polyester synthetic leather after TMS plasma treatment with different discharge powers. The untreated sample (Fig. 4(a)) shows a smooth surface. It is clear that granular patches were developed on the samples with discharge power of 90 W and 110 W, shown in Fig. 4(b) and (c), respectively. The roughness of fibre surface is the same for both discharge powers. This implies that beyond the optimum power (90 W), further enhancement of power does not increase the deposition [15]. The granular patches are believed to be silicon compounds deposited through plasma treatment. The chemical properties of the patches are revealed by FTIR and XPS analysis. 3.2. Effect of amount of TMS Table 2 shows the CA values of polyester synthetic leather after TMS plasma treatment with different amounts of TMS. Fig. 5(a)–(d)
Amount of TMS (ml)
Contact angle (◦ )
0.1 0.15 0.2 0.25
Absorbed immediately 110 ± 2 138 ± 2 130 ± 2
illustrates the surface morphology of polyester synthetic leather after TMS plasma treatment with different amounts of TMS. Fig. 5(a) shows quite a small number of granular patches deposited on the fibre surface. This indicates that 0.1 ml TMS does not provide sufficient organosilicons to enhance surface hydrophobicity. On the other hand, it is clear that more granular patches are developed on the samples with discharge power 0.15 ml, 0.2 ml and 0.25 ml, as shown in Fig. 5(b)–(d), respectively. More patches are developed on the sample with 0.15 ml and, therefore, the contact angle is greatly increased to around 110◦ . The roughness of samples with 0.2 ml and 0.25 ml is about the same. This implies that once the optimum deposition (with 0.2 ml TMS) is achieved, no further enhancement takes place even when more of the precursor is applied. It is assumed that with the increase in the amount of TMS in plasma treatment, the surface hydrophobicity should be enhanced. However, the results in this study show that there is no further increase in CA with increased use of TMS. The reason may be that the collision between helium plasma species and TMS species increases and therefore, fewer active TMS species can react with the synthetic leather surface leading to no further increase in CA value [8,9]. In the TMS plasma treated sample, the granular patches are believed to be silicon compounds deposited through plasma treatment. The chemical properties of the patches are revealed by FTIR and XPS analysis. 3.3. Effect of jet distance Jet distance is the distance between the plasma jet and the substrate surface. Table 3 shows CA values of specimens treated at different jet distances. The largest CA is 138◦ which is achieved at a jet distance of 10 mm. Starting from 10 mm onwards, the greater the separation, the smaller the CA is and it reaches a steady value at 25 mm. This is because fewer active species reach the specimen surface because of the longer separation. On the other hand, when the jet distance increases, velocity and activity of the active species in the plasma jet greatly decreases by the time they reach the synthetic leather surface and thus the surface reaction is inadequate [11,12]. In addition, the specimen suffers thermal degradation when the jet distance is less than 10 mm because the sample gets melted under excessive heat of the plasma jet [11,12]. 3.4. Optimum treatment parameters and stain resistance There is a pronounced improvement in CA value after TMS plasma treatment. By definition, a surface with a CA value greater than 90◦ is a hydrophobic surface [16]. In this study, when the treatment parameters for hydrophobization of polyester Table 3 CA values of different samples (treatment parameters: discharge power = 90 W; flow rate of helium = 7.5 LPM; amount of TMS = 2 ml and treatment time = 30 s). Jet distance (mm)
Contact angle (◦ )
10 15 20 25
138 110 102 102
± ± ± ±
2 2 2 2
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Fig. 4. SEM micrographs of polyester synthetic leather after TMS plasma treatment with discharge power (a) untreated, (b) 90 W, (c) 110 W (treatment parameters: amount of TMS = 0.2 ml, flow rate of helium = 7.5 LPM, jet distance = 10 mm and dwell time = 30 s).
synthetic leather are set at discharge power = 90 W, flow rate of helium = 7.5 LPM, amount of TMS = 2 ml, jet distance = 10 mm and treatment time = 30 seconds, an optimum CA value of 138◦ was obtained, implying that the polyester synthetic fibre surface
had become hydrophobic when compared with CA of untreated specimen. Experimental results revealed that there is a noticeable improvement in stain resistance to both coffee and milk tea after
Fig. 5. SEM micrographs of polyester synthetic leather after TMS plasma treatment with different amounts of TMS (a) 0.1 ml, (b) 0.15 ml, (c) 0.2 ml, (d) 0.25 ml (treatment parameters: discharge power = 90 W; flow rate of helium = 7.5 LPM; jet distance = 10 mm and treatment time = 30 s).
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Fig. 6. FTIR spectra of polyester synthetic leather after TMS plasma treatment with flow rate of helium (a) 7.5 LPM, (b) 10 LPM, (c) 12.5 LPM, (d) 15 LPM, (e) 17.5 LPM, (f) untreated (treatment parameters: amount of TMS = 0.2 ml; discharge power = 90 W; jet distance = 10 mm and treatment time = 30 s).
plasma treatment with TMS (Table 4). Before plasma treatment, coffee and milk tea was absorbed immediately in the polyester leather surface, i.e. no stain resistant behaviour was noted. However, the plasma treatment increases the surface hydrophobicity in relation to the increase in CA values. 3.5. FTIR Table 5 shows the range of wave numbers in which functional groups and classes of compounds can be observed from the FTIR spectrum [16]. FTIR reveals the chemical compositions of the samples. Fig. 6 shows the FTIR spectra of polyester synthetic leather specimens treated with TMS plasma at different flow rates of helium. TMS plasma modified polyester synthetic leather exhibits the following features: increase in the peak of 2920 cm−1 , 2853 cm−1 (CH antisymmetric and symmetric stretching), 1462 cm−1 (CH2 bending (scissors) vibration), 1260 cm−1 (CH3 symmetric deformation), 1089 cm−1 , 1016 cm−1 (Si O Si), 847 cm−1 (Si CH3 rocking) and 721 cm−1 (CH2 rocking in methylene chains; intensity depends on chin length). As the flow rate of helium decreases (from 17.5 to 7.5 LPM), the silicon-related absorbance increases. The greatest intensity of the silicon related peaks occur at low flow rates of helium, 7.5 LPM. This indicates that low flow rate of helium facilitates the deposition of silicon compounds.
3.6. XPS The surface chemistry of polyester synthetic leather is investigated by XPS. The contents of carbon (C), oxygen (O) and silicon (S) were examined and the surface stoichiometry was revealed. Fig. 7 shows the wide scan of the XPS results of untreated and plasma modified polyester synthetic leather. The intensity of silicon compounds (Si 2p) is increased after TMS plasma treatment. The results reveal that silicon is deposited on sample surface after TMS plasma treatment. The spectra features detected, along with their binding energies and full width at half maximum (FWHM) of the treated sample are reported in Table 6. The carbon, oxygen signals
Table 5 Wave numbers (ranges) in FTIR. Range (cm−1 ) and intensity
Group and class
Assignment and remarks
2990–2850 (m-s)
CH3 and CH2 in aliphatic compounds CH2 in aliphatic compounds CH3 in aliphatic compounds CH3 in aliphatic compounds Si CH3 in silanes
CH antisymmetric and symmetric stretching CH2 bending (scissors) vibration Asymmetric CH3 deformation CH3 symmetrical deformation CH3 symmetric deformation Si O Si Si C stretch
1475–1450 (vs) 1645–1440 (m) 1380–1370 (s) 1280–1250 (vs)
Table 4 CA of different liquids (treatment parameter: discharge power = 90 W, amount of TMS = 2 ml, jet distance = 10 mm, treatment time = 30 s). Liquid Deionised water Coffee Milk tea
Untreated Absorbed immediately Absorbed immediately Absorbed immediately
APP treated ◦
138 ± 2 125 ± 2◦ 118 ± 2◦
1100–1000 (vs) 860–720 (vs) 850–810 (vs) 740–720 (w-m)
Si O Si in siloxanes Si C in organosilicon compounds Si CH3 in silanes (CH2 )n – in hydrocarbons
Si CH3 rocking CH2 rocking in methylene chains; intensity depends on chain length
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Fig. 7. Surface elementary composition of (a) untreated and (b) TSM plasma modified polyester synthetic leather under optimum condition. Table 6 XPS features of plasma modified polyester synthetic leather. Peak
Position (eV)
C 1s O 1s Si 2p
284.1–292.5 231.5–538.5 102.0–107.5
FWHM 1.00–1.73 1.18–2.02 1.94
Table 7 XPS position of C 1s features of plasma modified polyester synthetic leather. Peak
Feature
Peak position (eV)
C1 C2 C3
C H, C C C O C O, O C O
285.0 286.8 289.2
are composed of various peaks indicating the presence of different chemical surroundings for carbon and oxygen atoms. Table 7 shows the peak positions of C 1s features of plasma treated sample. Deconvolution study reveals that C 1s peak is assigned to be C H, C O and C O bonds are as shown in Fig. 8. It is clear that the C O and C O bonds intensities decreased after TMS plasma treatment. The atomic content of O-containing groups dropped after TMS plasma treatment. Chemical deposition is the basic mechanism of TMS plasma treatment. The results show a decrease in carbon (C) and oxygen (O) content and an increase in silicon (Si) content. Generally speaking, surface hydrophobicity can be determined by O-containing groups which have a strong affinity towards polar wetting agents. The O content decreased after plasma treatment (Table 8). The O/C ratio Table 8 Atomic percentage and atomic ratio of TMS plasma treated polyester synthetic leather.
Untreated Plasma treated
C
O
Si
N
O/C
Si/O
75.67 75.17
21.38 21.01
1.03 4.47
1.14 N/A
0.28 0.26
0.05 0.22
Fig. 8. XPS deconvoluted C 1s peak (a) untreated sample and (b) plasma modified polyester synthetic leather under optimum condition.
is an indicator of surface polarity of a substrate [17]. It describes the relative quantity of polar O-containing functionalities on a sample surface. The lower the ratio, the more hydrophobic a sample surface is. The O/C ratio drops after TMS plasma treatment (Table 8). At the same time, the absence of polar nitrogen (N)-containing group is also an indicator of the increase of surface hydrophobicity. The small amount of N-containing groups in the untreated sample is absent after plasma treatment. On the other hand, the silicon (Si) – containing groups are increased, as reported in Table 8. The Si-containing groups are increased 3 times more than the original. Si-containing groups are successfully deposited on the sample surface after TMS plasma treatment. The Si/O ratio is increased, as shown in Table 8. It is greatly boosted (4.4 times) after treatment. The Si/O is important to predict surface hydrophobicity, the higher the ratio, the more the hydrophobic surface is. It is because the ratio indicates that the hydrophilic oxygen content is reduced or silicon related groups are successfully deposited on the surface, or both happed at the same time. The XPS result confirmed the FTIR result that Si-containing groups are increased after plasma treatment. FTIR and TMS results show that the granular patches observed on the fibres of plasma treated samples (Fig. 6) are silicon compounds deposited through plasma treatment. TMS plasma treatment successfully enhances surface hydrophobicity of polyester synthetic leather. The improvement of surface hydrophobicity Table 9 Significance of the model parameters. Sig. (1-tailed) Flow rate of helium Amount of TMS Discharge power
0.101 0.000 0.000
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Table 10 Coefficients of the regression model.
Flow rate of helium Amount of TMS Discharge power
Unstandardized coefficients
Standardized coefficients
Collinearity statistics tolerance
−1.330 373.241 0.100
−0.143 0.536 0.554
1.000 1.000 1.000
Table 11 Comparison between predicted and experimental CA values. Flow rate of helium (LPM)
Amount of TMS (ml)
Discharge power (W)
Predicted CA (◦ )
Experimental CA (◦ )
Difference between predicted and experimental value
7.5 15.0 20.5 15.0
0.3 0.3 0.2 0.1
120 120 50 80
133.6 123.7 2.7 5.0
134 117 0 0
0.4 6.7 2.7 5.0
is attributed to the deposition of organosilicon by plasma treatment [8,18–20]. 3.7. Multiple linear regression analysis The multiple linear regression is applied to analyze the effects of independent variables, discharge power, amount of TMS and flow rate of helium on a dependent variable, contact angle (CA) by means of software SPSS. Table 9 indicates all the parameters are highly related to CA (within 95% confident interval) except flow rate of helium. In fact, as discussed before, the presence of helium serves as a medium and stabilizer for plasma generation as well as plasma jet operation. A low flow rate is preferable for plasma deposition. According to SPSS analysis, the adjusted R2 is 0.615, which means 61.5% of the variance in CA can be predicted from the independent variables. Sweet and Grace-Martin suggested that the fitness of the regression model is acceptable if the R-square is greater than 0.6 [21]. R-square statistics indicate the model fitness by representing the percentage of dependent variations fits into the linear regression model. From Table 9, the significance of F-test for this regression model is 0.000, and the model is highly significant to predict CA with the independent variables together within 95% confidence level. According to Table 10, contributions of independent variables are compared by standardized coefficients which convert the variables into the same scale. The regression coefficient of flow rate of helium is −0.143. The negative result indicates that the CA decreases as the flow rate of helium increases. This agrees with results in previous findings that low flow rate facilitates plasma deposition. On the other hand, the regression coefficients are 0.536 and 0.554 for amount of TMS and discharge power, respectively. The positive sign indicates that the CA decreases as the discharge power or amount of TMS increases. Having the highest coefficient (0.554) among all independent variables, discharge power contributes strongly to CA. All relationships are highly statistically significant (significance: 0.000–0.047). The lowest significance (0.047) from flow rate of helium indicates that flow rate of helium provides a relatively low contribution to plasma deposition. In fact, flow rate of helium provides a medium for discharging instead of being a critical factor for the deposition. The collinearity statistics tolerance demonstrates the proportion of variation of an independent variable not explained by the linear relationship with other independent variables. A value close to 1 indicates no multicollinearity problem whereas a value close to 0 indicates multicollinearity problem. Table 10 shows that the tolerance values of all the independent variables are equal to one. Hence, all the independent variables are highly correlated. Table 10 shows that the unstandardized coefficients which show the net
Table 12 Minimum values of operating parameters. Flowrate of helium (LPM)
Amount of TMS (ml)
Discharge power (W)
7.5
0.1
80
effect of the parameters. The constant is the Y-intercept, while the rest are the regression coefficients of the slope of the line which describes the relationship between the independent and dependent variables. Putting up all together, there is a linear relationship, as shown in the following regression equation: Predicted CA = −100.33 − 1.33flow rate of helium + 373.241amount of TMS + 1.1discharge power This equation predicts CA values with respect to the independent variables. The agreement between the predicted and experimental values is good, as shown in Table 11. It indicates that analysis is reliable. The derived equation is based on the results under different machine parameters. It is valid for the parameters within the range of machine settings. For instant, the minimum operating flow rate of helium is 7.5 LPM. The equation predicts the resulting CA after plasma treatment within the operating parameters. Table 12 shows the minimum values of operating parameters. 4. Conclusion Enhancement of surface hydrophobicity of polyester synthetic leather was achieved by means of atmospheric plasma treatment under controlled conditions. The parameters were the key factors for achieving the result. TMS was used as the chemical for plasma hydrophobization due to its advantages from the safety perspective and hydrophobicity improvement. A low flow rate of helium 7.5 LPM is suggested as it stabilizes plasma ionization without hindering chemical deposition. Sufficient discharge power (90 W) is required to stabilize ionization without degrading the sample surface. With the optimum treatment parameters, a highly hydrophobic surface of polyester synthetic leather was achieved with deionised water contact angle of 138◦ . The hydrophobic surface is robust and stable in time, without hydrophilic recovery 30 days after plasma treatment. SEM, FTIR and XPS results confirmed that silicon compounds were successfully deposited on the sample surface and the reduction of the atomic content of polar Ocontaining groups. Multiple linear regression analysis confirmed the correlations between plasma process parameters. Amount of TMS and discharge power showed significant correlations to the surface hydrophobic enhancement. In addition, it was revealed that
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after TMS plasma treatment, good stain resistance to coffee and milk tea can be achieved. Acknowledgement Authors would like to thank the Hong Kong Polytechnic University for the financial support of this work. References [1] K. Tsougeni, N. Vourdas, A. Tserepi, E. Gogolides, Langmuir 25 (2009) 11748–11759. [2] V. Raballand, J. Benedikt, A. von Keudell, Appl. Phys. Lett. 92 (2008) 091502. [3] C. Cheng, Z. Liye, R.J. Zhan, Surf. Coat. Technol. 200 (2005) 6659–6665. [4] G.R. Nowling, M. Yajima, S.E. Babayan, M. Moravej, X. Yang, W. Hoffman, R.F. Hicks, Plasma Sources Sci. Technol. 14 (2005) 477–484. [5] J.L.C. Fonseca, D.C. Apperley, J.P.S. Badyal, Chem. Mater. 5 (1993) 1676–1682. [6] P. Favia, R. Lamendola, R. d’Agostino, Plasma Sources Sci. Technol. 1 (1992) 59–66.
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