Deposition of polymer films in the diffuse coplanar surface discharge

Deposition of polymer films in the diffuse coplanar surface discharge M. Šimor 2, P. Sťahel 1, A. Brablec 1, Z.Navrátil 1, D. Kováčik 2, A. Zahoranová 2 V.Buršíková 1 and M. Černák 2 1 Department of Physical Electronics, Masaryk University, Brno, Czech Republic 2 Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, Slovakia Abstract The coplanar diffuse surface discharge is used for deposition of thin hydrophobic films. The films were deposited on a filter paper substrate from mixture of nitrogen and HMDSZ. The discharge was investigated by emission spectroscopy while the composition of deposited films was studied by IR spectrometry and mechanical properties were studied by depth sensing indentation technique. Surface energy and wettability of thin polymer - like layers were also estimated. Then, a significant decrease of free surface energy was achieved (from 53 mj/m 2 for the uncoated paper to about 11-23 mj/m 2 ). 1. Introduction Over the past two decades the plasma technique for surface activation of polymeric materials has been extensively reported, including the surface treatment of paper [1-3]. The majority of applications were made at reduced pressures (see, for example [1, 2]), where the spatially homogeneous low-temperature plasma can easily be generated and brought into direct contact with the treated surfaces. However, the use of expensive vacuum systems that force batch processing has discouraged this application of low-pressure plasmas in larger industrial scale for treatment of low-cost materials. Additional disadvantages of the lowpressure plasma technique are high power consumption, long processing times, and difficulty to scale-up of an experimental set- up to a large production reactor. Thus it is apparent that such low-pressure plasma treaters cannot be used in line with standard paper production lines. In a contrast, atmospheric-pressure plasma processes offer different advantages for the finishing of low-cost materials: costly vacuum equipment is unnecessary, processing times are reduced, and the plasma finishing is simpler in an in-line process. The most common systems tested are modifications of barrier ("silent" or "industrial-corona") discharge treaters widely used for the treatment of polymeric film [3]. In these cases the discharge is generated by applying a high frequency, high voltage signal to an electrode separated from a grounded plane by a discharge gap and a dielectric barrier. The treated material as, for example paper, is localised on the dielectric barrier surface. The main drawback of the barrier discharge devices is that the useful plasma conditions are achieved only in small volume plasma channels termed "streamers" developing perpendicularly to the paper surface. As a consequence, the plasma is in a very limited contact with the surface, which results in low processing speeds, typically in the order of 1 m/min. Moreover, because the plasma channels and arcing is an intrinsic phenomenon associated with this discharge type, localised arcing results in the formation of pinholes in the material being treated. To remedy the mentioned shortcomings of the standard barrier discharge treaters, a novel surface discharge type (the coplanar diffuse surface discharge CDSD) has been developed, whose major advantage is the generation of a thin layer of uniform atmospheric-pressure plasma with the power density as high as 100 W/cm 3 [4]. This system is protected by patent pending, and the laboratory tests have demonstrated the feasibility of the CDSD technique for surface activation of polypropylene and polyester nonwoven fabrics. In this paper preliminary results on surface using this novel discharge type are presented. 2. Experimental The experimental arrangement including the discharge electrode system is illustrated by Figure 1. The electrode system consisted of two systems of parallel striplike electrodes (1- mm wide, 50 µm thick, 150 mm long, 0.5 mm strip-to-strip; molybdenum) were embedded in 96 % alumna using a green tape technique. The thickness of the ceramic layer between the plasma and electrodes was 0.4 mm. A sinusoidal high- frequency high voltage (6 khz, up to 10 kv peak) was applied between both electrode systems. Such a discharge electrode arrangement and energisation were found to generate visually almost uniform plasmas of some

0.3 mm thickness in nitrogen and ambient air at atmospheric pressure. The power density of 50 W/cm 3 was kept in all experiments of this study. Fig.1 Arrangement of coplanar barrier discharge reactor. The films were deposited on a filter paper substrate from the mixtures of hexamethyldisilazane - C 6 H 19 Si 2 N (HMDSZ) with nitrogen. Pure nitrogen was added to the mixture of HMDSZ and nitrogen. The HMDSZ vapours were obtained bubbling of the nitrogen through glass bottle containing the monomer. Filter paper strips were used as the substrates. The paper was pulled 0.4 mm above the insulating plate and 0.1 mm above the discharge with constant rate in the zone of after-glow. For characterisation of the mechanical properties polycarbonate and glass substrates were used. The total surface energy of the deposited film was investigated by means of the sessile drop technique using the Surface Energy Evaluation System (SEE System) [5, 6]. The contact angles were measured directly from the image of the solid-liquid meniscus of a liquid drop set taken with CCD camera. For the determination of total free surface energy from the contact angle measurement the so-called acidbase theory we used. This theory [7, 8] enables to determine polar and apolar part of free surface energy and the electron-acceptor and electron-donor parameters of the surface tension. Wetting properties were studied by means of industrial permeability tests. The permeability was measured as a time necessary for penetration of 5 ml of testing liquid through the sample to special wettable paper ERT FF3. This test is consistent with tests ISO 9073-8-1995 The composition of the film was studied by means of Infrared Spectroscopy (IR). The spectra were taken from the filter papers samples by a Bruker Spectra 22 Spectrometer, using a ZnSe crystal, with a 45 incidence angle. 20 scans were co-added for all samples to reduce noise-to-signal ratio. The resolution was 2 cm 1. From each sample were taken 3 spectra, which were furthermore manipulated by means of spectrometer software: spectra were averaged, normalised and after then were devise differential spectra with standard sample. The mechanical properties were studied by means of the depth sensing indentation technique using a Fischerscope H100 tester. The Jobin YVON TRIAX 550 monochromator equipped with the CCD detector, recorded the spectra emitted by the discharge. 3. Results and discussion In the first part of the presented study, the discharge properties are investigated by optical emission spectroscopy. The free surface energy and wettability of the coated papers are studied in second part. Chemical composition of deposited films is investigated in the next subsection while mechanical properties of the systems are presented in last part. 3.1 Optical emission spectroscopy Emission spectra of the discharge in nitrogen were recorded in the range 300 800 nm. Typical spectra of the discharge created in the mixture of nitrogen and HMDSZ vapours are shown in the Fig.2. The spectrum is plotted in the range 300-500 nm, because above 500 nm only the second spectral order was registered.

Fig.2: The emission spectrum of the surface discharge burning in nitrogen with admixture of HMDSZ. The spectra consist of the molecular bands of second positive system of nitrogen (C 3 Π u B 3 Π g ). When the monomer was added into the nitrogen, intensive bands of CN violet system ( 2 Π 2 Σ) at 388 nm and 422 nm were observed. Intensity of N 2 and CN system depended on the flow rate of organosilicon. Therefore integrated intensity of the CN band at 388 nm and integrated intensity of N 2 system was calculated. The integrated intensity CN/N 2 ratio as a function of the flow rate of HMDSZ admixed to the 6 l/min of pure nitrogen is shown in the Fig. 3. Fig.3: Ratio of integrated intensities of CN and N 2 bands for different flow rates HMDSZ. The full line serves as a guide of eyes

The CN/N 2 ratio in the beginning increases with increasing flow rate of organosilicon, than it decreases. The maximal decomposition of monomer is at 0.03 flow rate ratio. The vibrational temperature was calculated from the bands of second positive system of nitrogen N 2 0-2, N 2 1-3 and N 2 2-4. The value of the vibrational temperature varied only slightly with flow rate of organosilicon admixed to pure nitrogen, and its value was at about 1800 K in all cases. 3.2 Surface energy and wettability Thin polymer-like layers were deposited on the filter paper substrate in order to reduce the water penetration and decrease the free surface energy. In Fig. 4 the total surface free energy and its polar and apolar parts are shown as a function of flow rate ratio HMDSZ/N 2. 24 Surface energy [mj/m 2 ] 18 12 6 γ tot γ LW 0 0.00 0.05 0.10 0.15 0.20 γ AB Q HMDSZ /Q nitrogen Fig.4 Total surface energy γ tot and its polar γ AB and a polar γ LW part of samples deposited from different gas mixtures. The dash lines serve as a guide of eyes. There was observed a significant decrease in surface free energy for the whole range HMDSZ to nitrogen flow rate ratio (0.017 < Q HMDSZ /Q O2 < 0.20) comparing to the paper surface energy (53 mj/m 2 ). At first, the surface energy decreases (in the range from Q HMDSZ /Q O2 = 0.017 up to 0.034) but then the increase is observed for higher concentration of HMDSZ in nitrogen. Similar dependencies were observed for the polar and the apolar part of the surface energy. The total surface free energy and its components increased when CN/N 2 ratio (see Fig. 3) decreased and the dependencies show the similar evolution. The acid and basic component of the polar part of free surface energy is not plotted. The Fig. 5 shows the images of the water drop set on the coated and uncoated substrate. In case of uncoated paper the contact angle is at about 60 o, however after the deposition the water contact angle increased to 120 o. Fig.5: Image of the uncoated paper substrate (A.) and coated filter paper (B.)

The water penetration time for deposited film is given in Table 1. It is necessary to say that the water permeability depends not only on the surface properties but also on the thickness of the deposited film and roughness of the substrate. In case of big substrate roughness the thickness of the film plays crucial role. The Table shows that water penetration time increases with increasing flow rate ratio. The film deposited from higher concentration of HMDSZ vapours is characterised by small water permeability. This effect can be explained with different structure and higher thickness of the deposited film compare to films deposited from the lower concentration of HMDSZ and N 2. Q HMDSZ /Q N2 Penetration time [s] 0 Filter paper 3 1 0.0017 111 2 0.033 432 3 0.067 159 4 0.13 2002 Table 1. Penetration times corresponding to films deposited from different gas mixtures 3.3. Chemical composition The attenuated total reflectance Fourier transform infrared spectroscopy (ATR - FTIR) was used in order to monitor and to specify the functional groups on the surface after the plasma treatment. The characteristic spectra of the film deposited from HMDSZ are shown on the figure 6. Most dominant are absorption peaks at 760-860 cm -1 corresponding to Si-C stretching bonds. The increase at 1000-1130 cm -1 can be correlated with Si-O-Si or Si-O-C stretching bonds. The CH 3 bonds represent the absorption peak at 1255-1280 cm -1. Absorbance [a.u.] 0.04 0.03 0.02 0.01 800 (Si-C) (Si-CH 3 ) 1260 (Si-O-Si) 1130 1087 1013 c b a 0.00 1400 1300 1200 1100 1000 900 800 700 d 842 757 Wavenumber [cm -1 ] Fig.6: FTIR spectra of the coated films deposited on the paper substrate for different flow rate ratio monomer/nitrogen: 1 0. 016, 2 0.032, 3 0.128, 4 0.2. 3.4 Structure, morphology and mechanical properties As in the case of films deposited in surface discharge [6], the plasma deposition of that hydrophobic films did not affect the appearance of the paper surface, i.e. we did not observe any yellowing of the substrate material as it was referred in [9]. The coatings exhibited good abrasion resistance and the microhardness of the films. The mechanical properties of films were measured on films deposited on polycarbonate plate, and

microhardness was determined to about 0.1-0.2 GPa. This value is comparable with the microhardness of plastics. 4. Conclusion A new deposition technique based on atmospheric pressure coplanar barrier discharge was developed in order to protect the paper surface against air pollutant, humidity and UV radiation. Highly hydrophobic plasma polymer coatings were deposited on the filter paper substrate from mixtures of HMDSZ with nitrogen. The free surface energy decreased from 53 mj/m 2 for the uncoated paper to about 11-23 mj/m 2. A substantial decrease of the water permeability was achieved in all cases. After the optimisation of deposition conditions this deposition technique may be used for example for the protection of the surfaces of wood, paper or other nature wettable materials. Acknowledgement This work has been supported by Grant Agency of Czech Republic under the contract numbers. 202/03/0708 and 202/02/D097. References [1] Z. Q. Hua, R. Sitaru, F. Denes, and R. A. Young: Plasmas and Polymers 2(3), 196-220 (1998). [2] F. Denes and R. A. Young, Surface Modification of Polysaccharides under Cold Plasma Conditions, in Structural Diversity and Functional Versatility of Polysaccharides, Ed. Severian Dumitriu, Marcel Dekker Inc., New York, Basel, Hong Kong (1998). [3] S. D. Lee, S. Manolache, M. Sarmadi and F. Denes, Deposition of High Fluorine Content Macromolecular Thin Layers under Continuous-Flow-System Corona Discharge Conditions, Polymer Bulletin 43, 409-416 (1999). [4] M Šimor, J. Ráhel J., P. Vojtek, M. Černák, A. Brablec, Appl. Physics Letters 81 (2002) 2716. [5] http://www.seesystems.wz.cz [6] P. Sťahel, V. Buršíková, Z. Navrátil, A. Záhoranová, J. Janča, this conference [7] R. J. Good: Contact Angle, Wettability and Adhesion ed. K. L. Mittal 3 (1993). [8] N. Inagaki, Plasma Surface Modification and Plasma Polymerisation, Technomic Publication Company, Lancaster (1996). [9] L. Ruys, A. Saey, M. Van Lancker, Y. Rogister, J. Knott, 3. Internationale Fachtagung N-D-Plasma- Technologie, Wuppertal (1995).