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ATR-FTIR
Applied Surface Science 253 (2007) 4367–4373 www.elsevier.com/locate/apsusc
RF-CF4 plasma surface modification of paper: Chemical evaluation of two sidedness with XPS/ATR-FTIR Halil Turgut Sahin * Suleyman Demirel University, Faculty of Forestry, Department of Forest Products Engineering, 32260 Isparta, Turkey Received 13 July 2006; received in revised form 29 August 2006; accepted 22 September 2006 Available online 2 November 2006
Abstract The study was performed to examine the correlation between the initial roughness and surface fluorination of paper under RF-CF4 plasma environment. Based on the experimental observations, a correlation was observed between surface fluorination and plasma parameters, e.g. RF-power, treatment time and gas pressure. The level of fluorination with RF-CF4 plasma treatment was found to be extensive in both side of paper. Even very short treatment time, as low as 1 min at 300 W power, provides effective implantation of fluorine (38.7%) on surfaces. It was observed that, CF4 plasma treatment had a significant effect on the molecular fragmentation on both side of paper. However, the felt side have a much stronger effect on plasma-induced dissociation and fluorination than in the wire side of paper. # 2006 Elsevier B.V. All rights reserved. PACS : 81.15.Fg; 52.38.Ph; 52.77.Bn; 52.40 Hx; 82.33.Xj Keywords: Plasma; Film deposition; Etching; Plasma surface interaction; Paper; Two sidedness
1. Introduction Cellulose fibers, the primary structural element of the sheet network, are the most important component influencing properties and end uses of paper. However, due to the production on conventioanl paper machines (Fourdriener), where the water is removed mainly through one side (bottom) of the sheets, this formation process has the potential of producing a sheet that differs in composition and microstructure on its two surfaces. This difference in the same sheet is usually called as the ‘two-sidedness’ of paper. The side that is in contact with the paper machine wire during process is called the wire side (bottom side) whereas the other side that contact with felted pres nip is called as felt side (top side) [1]. Because of the asymmetric water drainage on the wire section of paper machines, fibrous fines and fillers are usually lower on bottom compare to top side, and correspondingly coarse fibrous materials and fillers are enriched on the felt side of sheets [2]. Another factor that contributes to two-sidedness is
* Tel.: +90 246 211 3171; fax: +90 246 237 1810. E-mail address: [email protected] 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.09.052
the mechanical dewatering on the wire by high angle foils [3]. In addition, the wet-pressing during sheet formation have a distinctly asymmetric effect on the two sides of the sheet. For instance, the weave patterns of some felts can be transferred to paper surface, especially if the nip pressure is aggressive. The microstructural difference between the opposite sides of the same sheets have significant drawbacks in many paper grades. It leads to non-uniform fines distrubution in the z-direction, with difference on color, smoothness, density and gloss on the same paper products [2,3]. These structural differences can also show up as to be differnet liquid absorption property (e.g., water, printing ink). In addition, poorly bonded coarse fibrous and non-fibrous particles more easily detach from the felt side and cause important problems during off paper machine processing. Nissan was reported that the use of short hardwood pulps, secondary fibers, and high amounts of fillers such as; clay, talc, titanium dioxide, calcium carbonate have potantial to increase possible structural difference between paper surfaces [1]. However, the properties of the paper surface including twosidedness can be corrected some extent by using several approcahes. These, applying correct operational techniques ‘on machine’ and ‘off-machine’ processes such as care on
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calendering techniques, using appropriate dewatering equipment (twin-wire machines), surface sizing, and coatings [2,3]. Cold plasma treatments have began to advantageously replace conventional chemical processes on surface modification of synthetic and natural polymers [4]. In fact, these treatments offers a unique way of adding thin films to the surface of almost any substrate and are environmently friendly. Moreover, surface functionalization and modifications can be done in one step with a desired gas. Furthermore, plasma ˚, modifies the polymer surface layer at a depth up to 500 A depending on treatment external parameters (e.g., power, time, pressure), and leaves the bulk characteristics unaffected [4–7]. Since surface energy is a important property for polymers, regulated by a layer of molecular dimensions, plasma technology can effectively achieve modification of this nearsurface region without affecting the desirable bulk properties of the material. Consequently, these treatments have been widely applied in treatments of polymer materials with numerous environmental advantages [4–6]. Plasma induced surface reactions could be influenced by the surface properties of starting paper material. However, certain properties such as smoothness, texture and structure difference between wire and felt side of paper are customary to measure these properties on both sides. Plasma surface modification of cellulose based products have been studied in a number of researchers [6–11]. But up to date, there has been no any study on plasma exposure of specified paper surface at the same treatment conditions and their respond to surface chemical and physical modications. Many surface analyzing methods, including chemical functional group analysis, can be used to detect the chemical changes in a paper surface before and after modification reactions. X-ray photoelectron spectroscopy (XPS) is very powerful technique and gives information on the binding carbon to other atoms [12,13]. Infrared (IR) techniques give information of changes in functional group distribution such as aromatic, carbonyl, carboxyl, and double-bond content [14]. They are most probably important in surface modification research. In X-ray photoelectron spectroscopy (XPS), knowledge of elemental depth distributions within the first 10 nm is often essential. However, none of the conventional XPS methods can reliably discern, for example, a thin over layer on rough surfaces like paper. Therefore, reliable experimental data on the actual distributions of fluorine and other chemical groups on the paper surfaces are needed, although there are numerous XPS studies on cellulose fibers and paper. For that reason, the aim of this study was to find correlations between the felt and wire side of sheets under RF-CF4 RF plasma treatment conditions. It was also aimed to find alternative ways to improve certain characteristics of paper by plasma treatments that will be discuss another part of this study.
pilot Fordrinier machine with constant jet/wire speed. There were no additives contained rather than cellulose fibers in the sheet, which could influence the results. A capacitively coupled stainless steel parallel plate static RF-cold plasma reactor was used in all experiments. In this reactor installation, parallel electrodes inside the reactor enables the creation of uniform electric fields determined by the size of electrodes. The one grounded (cold) electrodes were placed at a distance of 3 cm on upper side of the powered (hot) electrode, which was placed in the center of the reactor. Typically, the procedure for an CF4 plasma treatment was as follows; the 30 cm 30 cm paper samples were placed in the center between the hot and the cold electrodes. After pumping the reactor to a base pressure <70 mTorr, it was flushed with an argon flow (100–300 mTorr) and discharged 300 W for 5 min, in order to remove contaminants from previous experiments. The samples were then treated with continuous plasma system (40 kHz) for a determined time. At the end of treatment, the argon flow was maintained for 2 min, after which the reactor was brought to atmospheric pressure with air. All treated samples were stored at room temperature. The plasma reactor installation is shown in Fig. 1. The external parameters used in this study were RF power, treatment time and pressure, choosen to be 10–500 W, 0.5–20 min, 100–500 mTorr, respectively. Carbontetrafluoride (CF4), which is chemically inert, thermally stable and is widely utilized in plasma research for various purposes was choosen to evaluate under RF-cold plasma environment with paper surfaces. The CF4 gas was purchased from AGA Gas Co. with a purity of at least 99% unless otherwise noted. The surface roughness measurement was carried out according to Tappi Test Method T 538 (Sheffielt method). It is a measurement of air flow between the specimen (backed by flat glass on the bottom side) and two pressurized, concentric annular lands that are impressed in to the sample from top.
2. Materials and methods All plasma treatments were conducted on standard full bleached, white spruce Kraft papers (75 g/m2) that made on a
Fig. 1. Capacitively coupled stainless steel parallel plate static RF-cold plasma reactor.
H.T. Sahin / Applied Surface Science 253 (2007) 4367–4373
Surface chemical characteristics and atomic composition of plasma treated and untreated paper samples were evaluated by the use a Perkin-Elmer PHI 5400 XPS spectrometer (MgX-ray source; 15 kV and 300 W). Samples were mounted on doublesided silicone free polymeric adhesive tape. Typical pressures during analysis were between 10 9 and 10 8 Torr. The survey spectra were collected in the range of 0–1000 eV binding energies with a resolution of 1.0 eV. After the survey spectra, the high resolution XPS multiplex spectra were collected for all elements identified from the survey scans (e.g., C1s, O1s, F1s, etc.,). The energy resolution for a multiplex scan has been set at 0.05 eV with pass energy at 35.75 eV. The surface atomic concentration was calculated with the software based on the peak intensity corrected for atomic sensitivity factors. Attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) was used to evaluate the chemical groups on the plasma functionalized paper surfaces. An ATI-Mattson research Series IR was used provided with a GRASEBY-Special Benchmark Series ATR in-compartment P/N 11160 unit. All ATR-FTIR measurements were performed under liquid nitrogen blanket. Data were collected in the 100–4000 cm 1 wavenumber region with three scans for each sample.
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chemically different, are sufficiently similar to allow a consistent surface modification under RF-CF4 plasma treatment. Atalla reported that structural change in cellulose during papermaking could be possible but basic chemical structure of cellulose was occur in sheet structure [15]. Fig. 2 shows the XPS deconvoluted high resolution (HR) carbon spectra (C1s) of untreated paper. The deconvolution used Gaussian peak shapes and integrated background subtraction. For the fits, the peak positions were fixed according to tabulated chemical shifts [12,16] and guidelines established for paper [7,13,17]. According to Hua’s group, the pure cellulose (a-cellulose) should have a two-component HR C1s XPS spectra that are C–OH group at 286.7 eV and O–C–O group at 288 eV with the ratio of 0.83:0.17, respectively. However, the different atomic composition of paper surfaces are also reflected in the nature of the relative HR C1s peaks. In addition to the theoritacal C–O (C2) and O–C–O peaks (C3), the existence of a significantly intense C–C peak (C1) at 285 eV could be observed in both surface of untreated paper samples (Fig. 2a and b). Cellulose and its derivatives have intensively been studied by XPS in a number of researchers [7,12,13,16]. Consistently a tri-modal HR C1s XPS diagrams have been exhibited regardless
3. Results and discussions Sheffield surface roughness method was used to determine initial raughness properrites of untreated paper. As expected, specially prepared additive free sheet’s wire side have approximately 10% higher initial roughness value compare to felt side. Because paper is composed of a randomly felted layer of fiber, it follows that the structure of opposite surfaces has a varying degree of surface topographical property as measured in this study. According to the XPS survey scans, all sample surfaces consisted mainly of carbon (C1s) and oxygen (O1s) that chracteristics for cellulose. However, the relative surface atomic composition exhibits different consantration of carbon and oxygen elements. The atomic compositions of untreated paper substrate with theorotical values of cellulose are listed in Table 1. Since hydrogen atoms are not included, untreated paper (cellulose) was expected to show relative oxygen to carbon ratio of 0.833 (C: 54.5% and O: 45.5%). But, accordance our XPS measurements, indicating the wire side have slightly higher carbon (57.9 versus 56.7%) and lower oxygen (42.1 versus 43.3%) in comparison to felt side. This is somewhat surprising considering the differences in starting materials and processing for the paper substrate. However, this variance indicates that the types of surface present (cellulose), although they might be Table 1 The surface atomic compositional changes of untreted paper substrate with theorotical values of cellulose
Felt side Wire Side Theoretical value
C
O
O/C
C–C
C–O
O–C–O
56.7 57.9 54.5
43.3 42.1 45.5
0.76 0.73 0.83
24.7 19.5 –
50.5 62.2 83.33
24.81 18.3 16.66
Fig. 2. Comparative HR C1s XPS spectrum of untreated paper: (A) wire side; (B): felt side.
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Table 2 RF-power effects on atomic concentration (%) of paper samples after CF4 plasma treatment determined by XPS (T = 10 min and p = 300 mTorr) Power (W)
0 100 300 500
O
C
F
O/C
F/C
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
42.1 25.1 21.2 13.0
43.3 17.8 12.9. 16.1
57.9 44.0 42.1 41.1
56.7 50.7 41.4 46.7
0 30.9 36.7 46.0
0 31.5 45.7 37.2
0.73 0.57 0.5 0.32
0.76 0.35 0.31 0.34
0 0.70 0.87 1.1
0 0.62 1.1 0.80
of the source of the cellulosic substrate. It has been suggested that the 285.0 eV peak (C1) is probably due to X-ray induced cellulose decomposition and/or the presence of impurities like lignin and fatty acids [16,18,19]. From these information, it can be concluded that, oxidized hydrocarbon type contaminants are present on the additive free paper substrate, with exception of the basic organochemical structure of cellulose. The deposition of fluorine atoms on the paper surface by CF4 RF-plasma treatment is evidenced by the presence of F1s peak at 689 eV binding energy on XPS spectra (not shown). The survey XPS data indicate the presence of carbon, oxygen and very high concentrations of fluorine atoms on the plasma treated papers. Based on survey XPS studies, a comparative summary of the surface atomic compositional changes with plasma treatment (as RF-power) are shown in Table 2. It can clearly seen that initial microstructurel differences of a paper sheet affect the subsequent fluorination and surface modification characteristics. In all cases, the level of fluorination was found to be extensive on both felt and wire side of paper. The highest amount of fluorine incorporation (46%) was observed in wire side after 10 min of treatment at 500 W RF-power. In general, higher power and prolonged treatment gives higher fluorination, which also favors removal of oxygen from the surfaces. However, the relative surface atomic composition of the two surfaces are quite different. The surface fluorination of paper appear to be well correlated on plasma RF power-treatment time (Fig. 3), RF power-pressure (Fig. 4) and treatment time-pressure (Fig. 5). These comparison between the surfaces and the measured results reveals that the fluorination response of a sheet with initial roughness can be
Fig. 3. RF-plasma power and treatment time effcets on surface fluorination of paper.
quite well predicted. As mention above, the level of fluorination was found to be extensive on both side of paper. Even very short treatment time, as low as 1 min at 300 W provides effective implantation of a high amount (38.7%) fluorine on paper surfaces. It is notable that after the fluorine concentration reached a maximum, it decreased with increasing further treatments at the both surface of sheet. Those indicates, the initial roughness on opposite surfaces of the same paper was responded different mechanism in similar conditions. For verifying this assumptions, the surface oxygen to carbon relative ratio (O/C) was plotted against plasma treatment RF power (Watts). Sapieha et al., and some researchers was shown that XPS could be used in evaluating the surface chemical property of paper surface for which has been based on determination of oxygen-to-carbon (O/ C) atomic ratios [16,18].
Fig. 4. RF-plasma power and pressure effcets on surface fluorination of paper.
Fig. 5. RF-plasma treatment time and pressure effcets on surface fluorination of paper.
H.T. Sahin / Applied Surface Science 253 (2007) 4367–4373
Fig. 6. RF-plasma treatment power effect on surface relative atomic composition change (O/C).
The dependence of oxygen to carbon ratio as a function of RF power, both for felt and wire side using pressure of 300 mTorr, is shown in Fig. 6. It was shown that O/C ratio directly proportional dependence on plasma RF-power for wire side of paper. It was also observed that, wire side usually have higher O/C realtive ratio compare to felt side under similar RF CF4 plasma environment. This suggests oxygen removal from the paper is strictly related to the surface topography, hence the differenet mechanism should control both effects the increased fluorine results in a decreased oxygen or O/C atomic ratio. This is probably due to either thin film formation and/or ablative etching effects of the CF4 gas under low pressure plasma environments. The quantitative evaluation of the XPS data was carried out by deconvolution of HR C1s spectrum using curve fitting program. The approach used here was to fit peaks to groups of known species having well characterized binding energy shifts. Using the same methodology for each spectrum relative comparisons can be made between treatments. The HR C1s XPS spectra of paper (wire side) treated with RF-CF4 plasma environment is presented in Fig. 7. The spectrum clearly reveals diversity of fluorine based functionalities on paper surfaces. All XPS diagrams from CF4–plasma treated samples shown an octa-modal pattern. Besides chracteristics tri-modal (C–C; C1 at 285 eV, C–O; C2 at 286.7 eV, and O–C–O; C3 at 288 eV) peak pattern of cellulose, the presence of CF3–*CO–O and CF2–CF*–C; C4 at 289.2 eV, CF3–CF2*–O; C5 at 291.2 eV, CF2*–CF and CF2–*CF2–C; C6 at 292.6 eV, CF2–*CF2–O; C7
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Fig. 7. The HR C1s XPS spectra of paper (wire side) treated with RF-CF4 plasma (P = 300 W, T = 10 min, p = 300 mTorr).
at 293 eV, and CF3–CF; C8 at 295 eV new groups. Furthermore, Fluorine based functionalities (CF*–CF2), which have the marginally similar binding energy as O–C–O at 288 eV, apparently also resulted in increased CFx-related peak surface areas. It should be noted that the assignments of the resolved HR C1s XPS peaks into atoms with non-equivalent positions should be considered as being only suggestive. The curve fitting procedure is an over simplicition owing to the multitude of peaks considered. Based on HR C1s XPS studies, the percent surface area of nonequivalent carbon peaks with treatment time and RF power are listed in Tables 3 and 4. In general, plasma exposure causes diminution of the C–C and C–O functionalities but fairly intense C–C (285 eV); C–O (286.7 eV) and O–C–O (288 eV) non-equivalent C 1s binding energy peaks can be identified in all plasma-treated samples. Even 1 min treatment time, the lowest amount of C–C group was observed in both felt and wire side of paper (12.5 versus 11.7%) (Table 3). Further treatment at various plasma conditions could influence surface functionalities some degree. It was significant to observe that the formation of new functionalities (C4–C8) are due to modification of C–O and O–C–O linkages in cellulose microstructure. The decrease of C–O and C–O–C peak areas at longer treatment times such as; 5–15 min can be attributed to plasma-induced cleavage of pyrasonic ring in cellulose, followed by recombination and grafting of the new CFx functionalities. The less distinctive C–C (285 eV) area resulted through deconvolution and the high fluorinated functional
Table 3 Relative HR C1s surface composition of untreated and CF4-plasma treated paper (P = 185 W, p = 300 mTorr) Time (min)
0 1 5 10 15
C–C
C–O
O–C–O and CF*–CF2
CF3–*CO–O and CF2–CF*–C
CF3–CF2*–O
CF2*–CF and CF2–*CF2–C
CF2–*CF2–O
CF3–CF
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
– 11.4 13.1 12.1 11.7
– 6.7 7.8 4.7 5.8
– 5.5 5.1 11.4 12.0
– 7.0 4.0 1.9 3.5
– 3.0 1.6 9.3 8.6
– 3.1 1.4 1.6 2.3
– 2.5 0.3 4.1 5.1
– 4.0 1.0 1.3 1.6
Wire
Felt
Wire
Felt
Wire
Felt
Wire
19.5 12.5 14.7 12.6 17.8
24.7 11.7 18.3 15.9 18.6
62.2 33.7 36.4 23.8 22.7
50.5 35.2 39.6 53.4 37.0
18.3 14.4 14.6 14.7 11.2
24.8 18.6 17.8 13.9 19.2
– 17.0 14.2 12.0 10.8
Felt – 14.0 10.0 7.4 12.0
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Table 4 Relative HR C1s surface composition of untreated and CF4-plasma treated paper (T = 10 min, p = 300 mTorr) Time (min)
0 100 300 500
C–C
C–O
O–C–O and CF*–CF2
CF3–*CO–O and CF2–CF*–C
CF3–CF2*–O
CF2*–CF and CF2–*CF2–C
CF2–*CF2–O
CF3–CF
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
– 14.2 9.7 6.7
– 7.8 6.5 6.7
– 8.5 4.8 1.5
– 4.3 2.3 4.6
– 3.0 2.9 1.0
– 3.0 1.9 2.7
– 1.6 0.9 0.9
– 4.2 2.2 3.0
Wire
Felt
Wire
Felt
Wire
Felt
Wire
19.5 13.1 16.1 18.2
24.7 13.6 17.3 14.4
62.2 28.4 36.6 43.6
50.5 30.0 48.8 44.5
18.3 15.5 16.6 16.7
24.8 20.4 14.7 15.3
– 15.7 12.4 11.3
content of the paper allows us to suggest that a large number of C–CFx bonds are present in addition to the C–O linkages in the structure of the plasma-treated surface layers. As seen in Table 3, peak area ratios corresponding to CF3–CF2*–O are higher in the case of the wire side substrates (11.4, 13.1, 12.1 and 11.7, respectively) in comparison to those of the felt side of samples (6.7, 7.8, 4.7 and 5.8, respectively). This resulted a more intense free radical mediated surface reactions under plasma, in situ conditions. Possible reaction pathways for a CF4 gas under RF-plasma glow discharge were reported by researchers [20–23]. The presence of an alternating RF-electromagnetic field across a plasma causes electron acceleration, which in turn leads to bond cleavage and ionization of CF4 molecules. Since HF has high bond strength, the H and F atoms generated in the initial reactions could abstract other F and H atoms, leading to generation of other radicals in the polymer chain. It is probable that UV emission from CF4-plasma (glow discharge) is the primary cause of the free radical formation for implanting CFx-groups on the paper surface. Hua’s group and Sahin reported that free radical formation in the cellulose is possible in several ways: hydroxyl abstraction from cellulose chains, and C–C, C–O–C bond scission by electron or ion bombardment [16,24]. It is reasonable to conclude that the formation of fluorine containing functionalities effects intense macromolecular ablation and chain scission on cellulose macromolecule. The decreased C–OH and C–O–C surface
Felt – 17.0 6.3 8.9
areas in high-resolution XPS spectra verify this assumption. It was clear that interaction between CF4 plasma gas and surface topography are important. This is an interesting observation, which suggests heterogeneity of paper surfaces. Finally, ATR-FTIR method for detecting intermediate structures as well as fluorinated compounds in CF4 plasma treated samples are likely to be useful for the clarification of the deoxidation and intensive fluorination phenomena. The comparative diagrams of untretaed and RF-CF4 plasma treated papers of both sides are presented in Fig. 8. One cannot identify significant differences in the absorption patterns of the untreated felt (Fig. 8a) and wire surface (Fig. 8b) diagrams, which is indicative that the two structures are made of identical linkages; C1 group vibration 893 cm 1; C–C strecthing 1000 cm 1; C–C–O stretching 1060 cm 1; C–O– C symetric stretching 1120 cm 1; CH2 bending 1370 cm 1 and C–OH (in-plane OH bending) 1400 cm 1. RF-CF4 plasma treated surfaces (Fig. 8c and d) exhibit the chracteristic multimodal broad absorption in the 1100–1500 cm 1 region and thus chracteristics for multiple fluorine containing macromolecules including fluorinated polymers. According to Bellammy, multiple fluorine-atoms containing larger molecules usually have a broad absorption in the 1000–1400 cm 1 range, with complex series of bands associated with C–F vibrations [25]. The more intense in-plane OH vibrations (1390–1420 cm 1) and O–C–O and C–OH peak areas (1400 cm 1) in IR spectra indicates the generation of new surface chemistry containing functionalities which are related to the plasma-induced surface cleavage and followed by recombination and grafting of the new Fluorine functionalities in cellulose structure. 4. Conclusion
Fig. 8. Comparative FTIR diagrams of CF4 plasma treated paper: (A) wire side; (B) felt side; (C) plasma treated wire side; (D) plasma treated felt side of paper.
It has been shown that the RF-CF4 plasma exposure is caused intensive fluorination in both side of paper. The highest fluorine concentration (46%) was found at the 500 W and 10 min treatment conditions. This also corresponded to the lowest O/C ratio observed in this study. The felt side that have denser and less fines usually have higher atomic florine content compare to wire side under similar CF4 plasma conditions. High-resolution XPS data collected from plasma-exposed, additive-free paper surfaces, in the C1s binding energy zones clearly indicate that there are significant differences in the relative concentrations of the surface functionalities implanted under the RF-CF4 plasma conditions.
H.T. Sahin / Applied Surface Science 253 (2007) 4367–4373
It was also found that the molecular structure of the cellulose is degraded upon prolonged exposure time and treatment conditions. It can be concluded that the structural difference between the opposite sides of the same paper sheet is a significant respond in CF4 plasma environments. This creates novel surface characteristics of paper, which can not be realized by conventional treatments.
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[12] G.M. Dorris, D.G. Gray, The surface analysis of paper and wood fibers by ESCA (electron spectroscopy for chemical analysis): I application to cellulose and lignin, Cell. Chem. Technol. 12 (1978) 9. [13] W.K. Istone, X-ray photoelectron spectroscopy, in: T.E. Conners, S. Banerjee (Eds.), In: Surface Analysis of Paper, CRC Press, 1995, pp. 235–300. [14] M.A. Friese, S. Banerjee, FT-IR spectroscopy, in: T.E. Conners, S. Banerjee (Eds.), In: Surface Analysis of Paper, CRC Press, 1995, pp. 119–234. [15] R.H. Atalla, Structural change in cellulose during papermaking and recycling. Material interaction relevant to recycling of wood-based material, in: Rowell, et al. (Eds.), Proceeding of Materials Research Society Symposium; 1992 April 27–29, San Francisco, CA, 1992. [16] Z.Q. Hua, R. Sitaru, F. Denes, R.A. Young, Mechanisms of oxygen and argon-RF-plasma induced surface chemistry of cellulose, Plasmas Polym. 2 (3) (1997) 199–223. [17] D.G. Gray, The surface analysis of paper and wood fibers by ESCA. III. Interpretation of carbon (1s) peak shape, Cell. Chem. Technol. 12 (1978) 735. [18] S. Sapieha, M. Verreault, E.J. Klemberg-Sapieha, E. Sacher, R.M. Wertheimer, X-ray photoelectron study of the plasma fluorination of lignocellulose, Appl. Surf. Sci. 44 (1990) 165–169. [19] L.-S. Johansson, J.M. Campbell, K. Koljonen, P. Stenius, Evaluation of surface lignin on cellulose fibers with XPS, Appl. Surf. Sci. 144–145 (1999) 92–95. [20] F. Hochart, J. Levalois-Mitjaville, R. Jaeger, L. Gengembre, J. Grimblot, Plasma surface treatment of poly acrylonitrile films by fluorocarbon compounds, Appl. Surf. Sci. 142 (1999) 574–578. [21] I.H. Loh, M. Klausner, R.F. Baddour, R.E. Cohen. Surface modifications of polymers with fluorine-containing plasmas: deposition versus replacement reactions, 1987, 27 (11): 861–867. [22] K. Sugiyamau, K. Kiyokawa, H. Matsuoka, A. Itou, K. Hasegawa, K. Tsutsumi, Generation of non-equilibrium plasma at atmospheric pressure and application for chemical process, Thin Solid Films 316 (1998) 117–122. [23] T. Yasuda, T. Okuno, K. Tsuji, H. Yasuda, Surface-configuration change of CF4 plasma treated cellulose and cellulose acetate by interaction of water with surfaces, Langmuir 12 (1996) 1391–1394. [24] H.T. Sahin, Odun ve selulozda meydana gelen renk degismeleri uzerine arastırmalar, SDU Orman Fakultesi Dergisi, Sayı 2 (2002) 57–70. [25] L.J. Bellamy, The IR Spectra of Complex Molecules, Methuen & Co. Ltd./Wiley & Sons, Inc., London, 1966.
RF-CF4 plasma surface modification of paper: Chemical evaluation of two sidedness with XPS/ATR-FTIR Halil Turgut Sahin * Suleyman Demirel University, Faculty of Forestry, Department of Forest Products Engineering, 32260 Isparta, Turkey Received 13 July 2006; received in revised form 29 August 2006; accepted 22 September 2006 Available online 2 November 2006
Abstract The study was performed to examine the correlation between the initial roughness and surface fluorination of paper under RF-CF4 plasma environment. Based on the experimental observations, a correlation was observed between surface fluorination and plasma parameters, e.g. RF-power, treatment time and gas pressure. The level of fluorination with RF-CF4 plasma treatment was found to be extensive in both side of paper. Even very short treatment time, as low as 1 min at 300 W power, provides effective implantation of fluorine (38.7%) on surfaces. It was observed that, CF4 plasma treatment had a significant effect on the molecular fragmentation on both side of paper. However, the felt side have a much stronger effect on plasma-induced dissociation and fluorination than in the wire side of paper. # 2006 Elsevier B.V. All rights reserved. PACS : 81.15.Fg; 52.38.Ph; 52.77.Bn; 52.40 Hx; 82.33.Xj Keywords: Plasma; Film deposition; Etching; Plasma surface interaction; Paper; Two sidedness
1. Introduction Cellulose fibers, the primary structural element of the sheet network, are the most important component influencing properties and end uses of paper. However, due to the production on conventioanl paper machines (Fourdriener), where the water is removed mainly through one side (bottom) of the sheets, this formation process has the potential of producing a sheet that differs in composition and microstructure on its two surfaces. This difference in the same sheet is usually called as the ‘two-sidedness’ of paper. The side that is in contact with the paper machine wire during process is called the wire side (bottom side) whereas the other side that contact with felted pres nip is called as felt side (top side) [1]. Because of the asymmetric water drainage on the wire section of paper machines, fibrous fines and fillers are usually lower on bottom compare to top side, and correspondingly coarse fibrous materials and fillers are enriched on the felt side of sheets [2]. Another factor that contributes to two-sidedness is
* Tel.: +90 246 211 3171; fax: +90 246 237 1810. E-mail address: [email protected] 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.09.052
the mechanical dewatering on the wire by high angle foils [3]. In addition, the wet-pressing during sheet formation have a distinctly asymmetric effect on the two sides of the sheet. For instance, the weave patterns of some felts can be transferred to paper surface, especially if the nip pressure is aggressive. The microstructural difference between the opposite sides of the same sheets have significant drawbacks in many paper grades. It leads to non-uniform fines distrubution in the z-direction, with difference on color, smoothness, density and gloss on the same paper products [2,3]. These structural differences can also show up as to be differnet liquid absorption property (e.g., water, printing ink). In addition, poorly bonded coarse fibrous and non-fibrous particles more easily detach from the felt side and cause important problems during off paper machine processing. Nissan was reported that the use of short hardwood pulps, secondary fibers, and high amounts of fillers such as; clay, talc, titanium dioxide, calcium carbonate have potantial to increase possible structural difference between paper surfaces [1]. However, the properties of the paper surface including twosidedness can be corrected some extent by using several approcahes. These, applying correct operational techniques ‘on machine’ and ‘off-machine’ processes such as care on
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calendering techniques, using appropriate dewatering equipment (twin-wire machines), surface sizing, and coatings [2,3]. Cold plasma treatments have began to advantageously replace conventional chemical processes on surface modification of synthetic and natural polymers [4]. In fact, these treatments offers a unique way of adding thin films to the surface of almost any substrate and are environmently friendly. Moreover, surface functionalization and modifications can be done in one step with a desired gas. Furthermore, plasma ˚, modifies the polymer surface layer at a depth up to 500 A depending on treatment external parameters (e.g., power, time, pressure), and leaves the bulk characteristics unaffected [4–7]. Since surface energy is a important property for polymers, regulated by a layer of molecular dimensions, plasma technology can effectively achieve modification of this nearsurface region without affecting the desirable bulk properties of the material. Consequently, these treatments have been widely applied in treatments of polymer materials with numerous environmental advantages [4–6]. Plasma induced surface reactions could be influenced by the surface properties of starting paper material. However, certain properties such as smoothness, texture and structure difference between wire and felt side of paper are customary to measure these properties on both sides. Plasma surface modification of cellulose based products have been studied in a number of researchers [6–11]. But up to date, there has been no any study on plasma exposure of specified paper surface at the same treatment conditions and their respond to surface chemical and physical modications. Many surface analyzing methods, including chemical functional group analysis, can be used to detect the chemical changes in a paper surface before and after modification reactions. X-ray photoelectron spectroscopy (XPS) is very powerful technique and gives information on the binding carbon to other atoms [12,13]. Infrared (IR) techniques give information of changes in functional group distribution such as aromatic, carbonyl, carboxyl, and double-bond content [14]. They are most probably important in surface modification research. In X-ray photoelectron spectroscopy (XPS), knowledge of elemental depth distributions within the first 10 nm is often essential. However, none of the conventional XPS methods can reliably discern, for example, a thin over layer on rough surfaces like paper. Therefore, reliable experimental data on the actual distributions of fluorine and other chemical groups on the paper surfaces are needed, although there are numerous XPS studies on cellulose fibers and paper. For that reason, the aim of this study was to find correlations between the felt and wire side of sheets under RF-CF4 RF plasma treatment conditions. It was also aimed to find alternative ways to improve certain characteristics of paper by plasma treatments that will be discuss another part of this study.
pilot Fordrinier machine with constant jet/wire speed. There were no additives contained rather than cellulose fibers in the sheet, which could influence the results. A capacitively coupled stainless steel parallel plate static RF-cold plasma reactor was used in all experiments. In this reactor installation, parallel electrodes inside the reactor enables the creation of uniform electric fields determined by the size of electrodes. The one grounded (cold) electrodes were placed at a distance of 3 cm on upper side of the powered (hot) electrode, which was placed in the center of the reactor. Typically, the procedure for an CF4 plasma treatment was as follows; the 30 cm 30 cm paper samples were placed in the center between the hot and the cold electrodes. After pumping the reactor to a base pressure <70 mTorr, it was flushed with an argon flow (100–300 mTorr) and discharged 300 W for 5 min, in order to remove contaminants from previous experiments. The samples were then treated with continuous plasma system (40 kHz) for a determined time. At the end of treatment, the argon flow was maintained for 2 min, after which the reactor was brought to atmospheric pressure with air. All treated samples were stored at room temperature. The plasma reactor installation is shown in Fig. 1. The external parameters used in this study were RF power, treatment time and pressure, choosen to be 10–500 W, 0.5–20 min, 100–500 mTorr, respectively. Carbontetrafluoride (CF4), which is chemically inert, thermally stable and is widely utilized in plasma research for various purposes was choosen to evaluate under RF-cold plasma environment with paper surfaces. The CF4 gas was purchased from AGA Gas Co. with a purity of at least 99% unless otherwise noted. The surface roughness measurement was carried out according to Tappi Test Method T 538 (Sheffielt method). It is a measurement of air flow between the specimen (backed by flat glass on the bottom side) and two pressurized, concentric annular lands that are impressed in to the sample from top.
2. Materials and methods All plasma treatments were conducted on standard full bleached, white spruce Kraft papers (75 g/m2) that made on a
Fig. 1. Capacitively coupled stainless steel parallel plate static RF-cold plasma reactor.
H.T. Sahin / Applied Surface Science 253 (2007) 4367–4373
Surface chemical characteristics and atomic composition of plasma treated and untreated paper samples were evaluated by the use a Perkin-Elmer PHI 5400 XPS spectrometer (MgX-ray source; 15 kV and 300 W). Samples were mounted on doublesided silicone free polymeric adhesive tape. Typical pressures during analysis were between 10 9 and 10 8 Torr. The survey spectra were collected in the range of 0–1000 eV binding energies with a resolution of 1.0 eV. After the survey spectra, the high resolution XPS multiplex spectra were collected for all elements identified from the survey scans (e.g., C1s, O1s, F1s, etc.,). The energy resolution for a multiplex scan has been set at 0.05 eV with pass energy at 35.75 eV. The surface atomic concentration was calculated with the software based on the peak intensity corrected for atomic sensitivity factors. Attenuated total reflectance-fourier transform infrared spectroscopy (ATR-FTIR) was used to evaluate the chemical groups on the plasma functionalized paper surfaces. An ATI-Mattson research Series IR was used provided with a GRASEBY-Special Benchmark Series ATR in-compartment P/N 11160 unit. All ATR-FTIR measurements were performed under liquid nitrogen blanket. Data were collected in the 100–4000 cm 1 wavenumber region with three scans for each sample.
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chemically different, are sufficiently similar to allow a consistent surface modification under RF-CF4 plasma treatment. Atalla reported that structural change in cellulose during papermaking could be possible but basic chemical structure of cellulose was occur in sheet structure [15]. Fig. 2 shows the XPS deconvoluted high resolution (HR) carbon spectra (C1s) of untreated paper. The deconvolution used Gaussian peak shapes and integrated background subtraction. For the fits, the peak positions were fixed according to tabulated chemical shifts [12,16] and guidelines established for paper [7,13,17]. According to Hua’s group, the pure cellulose (a-cellulose) should have a two-component HR C1s XPS spectra that are C–OH group at 286.7 eV and O–C–O group at 288 eV with the ratio of 0.83:0.17, respectively. However, the different atomic composition of paper surfaces are also reflected in the nature of the relative HR C1s peaks. In addition to the theoritacal C–O (C2) and O–C–O peaks (C3), the existence of a significantly intense C–C peak (C1) at 285 eV could be observed in both surface of untreated paper samples (Fig. 2a and b). Cellulose and its derivatives have intensively been studied by XPS in a number of researchers [7,12,13,16]. Consistently a tri-modal HR C1s XPS diagrams have been exhibited regardless
3. Results and discussions Sheffield surface roughness method was used to determine initial raughness properrites of untreated paper. As expected, specially prepared additive free sheet’s wire side have approximately 10% higher initial roughness value compare to felt side. Because paper is composed of a randomly felted layer of fiber, it follows that the structure of opposite surfaces has a varying degree of surface topographical property as measured in this study. According to the XPS survey scans, all sample surfaces consisted mainly of carbon (C1s) and oxygen (O1s) that chracteristics for cellulose. However, the relative surface atomic composition exhibits different consantration of carbon and oxygen elements. The atomic compositions of untreated paper substrate with theorotical values of cellulose are listed in Table 1. Since hydrogen atoms are not included, untreated paper (cellulose) was expected to show relative oxygen to carbon ratio of 0.833 (C: 54.5% and O: 45.5%). But, accordance our XPS measurements, indicating the wire side have slightly higher carbon (57.9 versus 56.7%) and lower oxygen (42.1 versus 43.3%) in comparison to felt side. This is somewhat surprising considering the differences in starting materials and processing for the paper substrate. However, this variance indicates that the types of surface present (cellulose), although they might be Table 1 The surface atomic compositional changes of untreted paper substrate with theorotical values of cellulose
Felt side Wire Side Theoretical value
C
O
O/C
C–C
C–O
O–C–O
56.7 57.9 54.5
43.3 42.1 45.5
0.76 0.73 0.83
24.7 19.5 –
50.5 62.2 83.33
24.81 18.3 16.66
Fig. 2. Comparative HR C1s XPS spectrum of untreated paper: (A) wire side; (B): felt side.
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Table 2 RF-power effects on atomic concentration (%) of paper samples after CF4 plasma treatment determined by XPS (T = 10 min and p = 300 mTorr) Power (W)
0 100 300 500
O
C
F
O/C
F/C
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
42.1 25.1 21.2 13.0
43.3 17.8 12.9. 16.1
57.9 44.0 42.1 41.1
56.7 50.7 41.4 46.7
0 30.9 36.7 46.0
0 31.5 45.7 37.2
0.73 0.57 0.5 0.32
0.76 0.35 0.31 0.34
0 0.70 0.87 1.1
0 0.62 1.1 0.80
of the source of the cellulosic substrate. It has been suggested that the 285.0 eV peak (C1) is probably due to X-ray induced cellulose decomposition and/or the presence of impurities like lignin and fatty acids [16,18,19]. From these information, it can be concluded that, oxidized hydrocarbon type contaminants are present on the additive free paper substrate, with exception of the basic organochemical structure of cellulose. The deposition of fluorine atoms on the paper surface by CF4 RF-plasma treatment is evidenced by the presence of F1s peak at 689 eV binding energy on XPS spectra (not shown). The survey XPS data indicate the presence of carbon, oxygen and very high concentrations of fluorine atoms on the plasma treated papers. Based on survey XPS studies, a comparative summary of the surface atomic compositional changes with plasma treatment (as RF-power) are shown in Table 2. It can clearly seen that initial microstructurel differences of a paper sheet affect the subsequent fluorination and surface modification characteristics. In all cases, the level of fluorination was found to be extensive on both felt and wire side of paper. The highest amount of fluorine incorporation (46%) was observed in wire side after 10 min of treatment at 500 W RF-power. In general, higher power and prolonged treatment gives higher fluorination, which also favors removal of oxygen from the surfaces. However, the relative surface atomic composition of the two surfaces are quite different. The surface fluorination of paper appear to be well correlated on plasma RF power-treatment time (Fig. 3), RF power-pressure (Fig. 4) and treatment time-pressure (Fig. 5). These comparison between the surfaces and the measured results reveals that the fluorination response of a sheet with initial roughness can be
Fig. 3. RF-plasma power and treatment time effcets on surface fluorination of paper.
quite well predicted. As mention above, the level of fluorination was found to be extensive on both side of paper. Even very short treatment time, as low as 1 min at 300 W provides effective implantation of a high amount (38.7%) fluorine on paper surfaces. It is notable that after the fluorine concentration reached a maximum, it decreased with increasing further treatments at the both surface of sheet. Those indicates, the initial roughness on opposite surfaces of the same paper was responded different mechanism in similar conditions. For verifying this assumptions, the surface oxygen to carbon relative ratio (O/C) was plotted against plasma treatment RF power (Watts). Sapieha et al., and some researchers was shown that XPS could be used in evaluating the surface chemical property of paper surface for which has been based on determination of oxygen-to-carbon (O/ C) atomic ratios [16,18].
Fig. 4. RF-plasma power and pressure effcets on surface fluorination of paper.
Fig. 5. RF-plasma treatment time and pressure effcets on surface fluorination of paper.
H.T. Sahin / Applied Surface Science 253 (2007) 4367–4373
Fig. 6. RF-plasma treatment power effect on surface relative atomic composition change (O/C).
The dependence of oxygen to carbon ratio as a function of RF power, both for felt and wire side using pressure of 300 mTorr, is shown in Fig. 6. It was shown that O/C ratio directly proportional dependence on plasma RF-power for wire side of paper. It was also observed that, wire side usually have higher O/C realtive ratio compare to felt side under similar RF CF4 plasma environment. This suggests oxygen removal from the paper is strictly related to the surface topography, hence the differenet mechanism should control both effects the increased fluorine results in a decreased oxygen or O/C atomic ratio. This is probably due to either thin film formation and/or ablative etching effects of the CF4 gas under low pressure plasma environments. The quantitative evaluation of the XPS data was carried out by deconvolution of HR C1s spectrum using curve fitting program. The approach used here was to fit peaks to groups of known species having well characterized binding energy shifts. Using the same methodology for each spectrum relative comparisons can be made between treatments. The HR C1s XPS spectra of paper (wire side) treated with RF-CF4 plasma environment is presented in Fig. 7. The spectrum clearly reveals diversity of fluorine based functionalities on paper surfaces. All XPS diagrams from CF4–plasma treated samples shown an octa-modal pattern. Besides chracteristics tri-modal (C–C; C1 at 285 eV, C–O; C2 at 286.7 eV, and O–C–O; C3 at 288 eV) peak pattern of cellulose, the presence of CF3–*CO–O and CF2–CF*–C; C4 at 289.2 eV, CF3–CF2*–O; C5 at 291.2 eV, CF2*–CF and CF2–*CF2–C; C6 at 292.6 eV, CF2–*CF2–O; C7
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Fig. 7. The HR C1s XPS spectra of paper (wire side) treated with RF-CF4 plasma (P = 300 W, T = 10 min, p = 300 mTorr).
at 293 eV, and CF3–CF; C8 at 295 eV new groups. Furthermore, Fluorine based functionalities (CF*–CF2), which have the marginally similar binding energy as O–C–O at 288 eV, apparently also resulted in increased CFx-related peak surface areas. It should be noted that the assignments of the resolved HR C1s XPS peaks into atoms with non-equivalent positions should be considered as being only suggestive. The curve fitting procedure is an over simplicition owing to the multitude of peaks considered. Based on HR C1s XPS studies, the percent surface area of nonequivalent carbon peaks with treatment time and RF power are listed in Tables 3 and 4. In general, plasma exposure causes diminution of the C–C and C–O functionalities but fairly intense C–C (285 eV); C–O (286.7 eV) and O–C–O (288 eV) non-equivalent C 1s binding energy peaks can be identified in all plasma-treated samples. Even 1 min treatment time, the lowest amount of C–C group was observed in both felt and wire side of paper (12.5 versus 11.7%) (Table 3). Further treatment at various plasma conditions could influence surface functionalities some degree. It was significant to observe that the formation of new functionalities (C4–C8) are due to modification of C–O and O–C–O linkages in cellulose microstructure. The decrease of C–O and C–O–C peak areas at longer treatment times such as; 5–15 min can be attributed to plasma-induced cleavage of pyrasonic ring in cellulose, followed by recombination and grafting of the new CFx functionalities. The less distinctive C–C (285 eV) area resulted through deconvolution and the high fluorinated functional
Table 3 Relative HR C1s surface composition of untreated and CF4-plasma treated paper (P = 185 W, p = 300 mTorr) Time (min)
0 1 5 10 15
C–C
C–O
O–C–O and CF*–CF2
CF3–*CO–O and CF2–CF*–C
CF3–CF2*–O
CF2*–CF and CF2–*CF2–C
CF2–*CF2–O
CF3–CF
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
– 11.4 13.1 12.1 11.7
– 6.7 7.8 4.7 5.8
– 5.5 5.1 11.4 12.0
– 7.0 4.0 1.9 3.5
– 3.0 1.6 9.3 8.6
– 3.1 1.4 1.6 2.3
– 2.5 0.3 4.1 5.1
– 4.0 1.0 1.3 1.6
Wire
Felt
Wire
Felt
Wire
Felt
Wire
19.5 12.5 14.7 12.6 17.8
24.7 11.7 18.3 15.9 18.6
62.2 33.7 36.4 23.8 22.7
50.5 35.2 39.6 53.4 37.0
18.3 14.4 14.6 14.7 11.2
24.8 18.6 17.8 13.9 19.2
– 17.0 14.2 12.0 10.8
Felt – 14.0 10.0 7.4 12.0
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Table 4 Relative HR C1s surface composition of untreated and CF4-plasma treated paper (T = 10 min, p = 300 mTorr) Time (min)
0 100 300 500
C–C
C–O
O–C–O and CF*–CF2
CF3–*CO–O and CF2–CF*–C
CF3–CF2*–O
CF2*–CF and CF2–*CF2–C
CF2–*CF2–O
CF3–CF
Wire
Felt
Wire
Felt
Wire
Felt
Wire
Felt
– 14.2 9.7 6.7
– 7.8 6.5 6.7
– 8.5 4.8 1.5
– 4.3 2.3 4.6
– 3.0 2.9 1.0
– 3.0 1.9 2.7
– 1.6 0.9 0.9
– 4.2 2.2 3.0
Wire
Felt
Wire
Felt
Wire
Felt
Wire
19.5 13.1 16.1 18.2
24.7 13.6 17.3 14.4
62.2 28.4 36.6 43.6
50.5 30.0 48.8 44.5
18.3 15.5 16.6 16.7
24.8 20.4 14.7 15.3
– 15.7 12.4 11.3
content of the paper allows us to suggest that a large number of C–CFx bonds are present in addition to the C–O linkages in the structure of the plasma-treated surface layers. As seen in Table 3, peak area ratios corresponding to CF3–CF2*–O are higher in the case of the wire side substrates (11.4, 13.1, 12.1 and 11.7, respectively) in comparison to those of the felt side of samples (6.7, 7.8, 4.7 and 5.8, respectively). This resulted a more intense free radical mediated surface reactions under plasma, in situ conditions. Possible reaction pathways for a CF4 gas under RF-plasma glow discharge were reported by researchers [20–23]. The presence of an alternating RF-electromagnetic field across a plasma causes electron acceleration, which in turn leads to bond cleavage and ionization of CF4 molecules. Since HF has high bond strength, the H and F atoms generated in the initial reactions could abstract other F and H atoms, leading to generation of other radicals in the polymer chain. It is probable that UV emission from CF4-plasma (glow discharge) is the primary cause of the free radical formation for implanting CFx-groups on the paper surface. Hua’s group and Sahin reported that free radical formation in the cellulose is possible in several ways: hydroxyl abstraction from cellulose chains, and C–C, C–O–C bond scission by electron or ion bombardment [16,24]. It is reasonable to conclude that the formation of fluorine containing functionalities effects intense macromolecular ablation and chain scission on cellulose macromolecule. The decreased C–OH and C–O–C surface
Felt – 17.0 6.3 8.9
areas in high-resolution XPS spectra verify this assumption. It was clear that interaction between CF4 plasma gas and surface topography are important. This is an interesting observation, which suggests heterogeneity of paper surfaces. Finally, ATR-FTIR method for detecting intermediate structures as well as fluorinated compounds in CF4 plasma treated samples are likely to be useful for the clarification of the deoxidation and intensive fluorination phenomena. The comparative diagrams of untretaed and RF-CF4 plasma treated papers of both sides are presented in Fig. 8. One cannot identify significant differences in the absorption patterns of the untreated felt (Fig. 8a) and wire surface (Fig. 8b) diagrams, which is indicative that the two structures are made of identical linkages; C1 group vibration 893 cm 1; C–C strecthing 1000 cm 1; C–C–O stretching 1060 cm 1; C–O– C symetric stretching 1120 cm 1; CH2 bending 1370 cm 1 and C–OH (in-plane OH bending) 1400 cm 1. RF-CF4 plasma treated surfaces (Fig. 8c and d) exhibit the chracteristic multimodal broad absorption in the 1100–1500 cm 1 region and thus chracteristics for multiple fluorine containing macromolecules including fluorinated polymers. According to Bellammy, multiple fluorine-atoms containing larger molecules usually have a broad absorption in the 1000–1400 cm 1 range, with complex series of bands associated with C–F vibrations [25]. The more intense in-plane OH vibrations (1390–1420 cm 1) and O–C–O and C–OH peak areas (1400 cm 1) in IR spectra indicates the generation of new surface chemistry containing functionalities which are related to the plasma-induced surface cleavage and followed by recombination and grafting of the new Fluorine functionalities in cellulose structure. 4. Conclusion
Fig. 8. Comparative FTIR diagrams of CF4 plasma treated paper: (A) wire side; (B) felt side; (C) plasma treated wire side; (D) plasma treated felt side of paper.
It has been shown that the RF-CF4 plasma exposure is caused intensive fluorination in both side of paper. The highest fluorine concentration (46%) was found at the 500 W and 10 min treatment conditions. This also corresponded to the lowest O/C ratio observed in this study. The felt side that have denser and less fines usually have higher atomic florine content compare to wire side under similar CF4 plasma conditions. High-resolution XPS data collected from plasma-exposed, additive-free paper surfaces, in the C1s binding energy zones clearly indicate that there are significant differences in the relative concentrations of the surface functionalities implanted under the RF-CF4 plasma conditions.
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It was also found that the molecular structure of the cellulose is degraded upon prolonged exposure time and treatment conditions. It can be concluded that the structural difference between the opposite sides of the same paper sheet is a significant respond in CF4 plasma environments. This creates novel surface characteristics of paper, which can not be realized by conventional treatments.
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