Journal of Nuclear Energy Science & Power Generation TechnologyISSN: 2325-9809

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Research Article, J Nucl Ene Sci Power Generat Technol Vol: 4 Issue: 1

Surface Treatment of Polypropylene Film using Cold Cathode Ion Source

Atta A1, Abdel Reheem AM2 and Abdel Rahman MM2*
1Radiation Physics Department, National Center for Radiation Research and Technology (NCRRT), Atomic Energy Authority (AEA), Cairo, Egypt
2Accelerators & Ion Sources Department, Nuclear Research Center, Atomic Energy Authority, Egypt
Corresponding author : Associate Prof. Moustafa Mohamed Abdelrahman
Accelerators and Ion Sources Department, Nuclear Research Center, Egypitation Atomic Energy Authority, Egypt, PO Box- 13759
Tel: +201122898811
E-mail: [email protected]
Received: August 28, 2014 Accepted: December 30, 2014 Published: January 27, 2015
Citation: Atta A, Abdel Reheem AM, Abdel Rahman MM (2015) Surface Treatment of Polypropylene Film using Cold Cathode Ion Source. J Nucl Ene Sci Power Generat Technol 4:1. doi:10.4172/2325-9809.1000129

Abstract

Surface Treatment of Polypropylene Film using Cold Cathode Ion Source

The main goal of this work was to examine the structural and optical changes in the Polypropylene (PP) films that treated by argon ion irradiation. This Polypropylene film was treated and modified by using a cold cathode ion source. The irradiation conditions (i.e., exposure time, beam current, and discharge current) were optimized to control the extent of surface modification. Argon ions of 1.5 keV are produced from the cold cathode ion source with an operating gas pressure of 2x10-4 mbar. The functional groups on the surface of samples were examined using FTIR spectrometer. The optical band gap and activation energy of polypropylene (PP) induced by an Ar plasma are determined. It was found that, the optical band gap, calculated from the absorbance spectra, decreased from 5.9 to 4.2 eV.

Keywords: Cold cathode ion source; Ion beam applications; Surface treatment; Polypropylene

Keywords

Cold cathode ion source; Ion beam applications; Surface treatment; Polypropylene

Introduction

Polypropylene (PP), known as polypropene, is a thermoplastic polymer used in a wide variety of applications including packaging and labeling, textiles (e.g., ropes, thermal underwear and carpets), stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and polymer banknotes. Ion beams are the basis of many industrial applications, mainly thin film growth, plasma etching and surface modification. Polypropylene was used for a wide variety of applications in many technological fields because it has some excellent bulk properties, such as high strength-to-weight ratio, good resistance to corrosion and relatively inexpensive to produce [1,2]. Polypropylene fibres have such excellent properties as low specific weight (0.91 g/cm3 only), high strength (42–53 cN/Tex), and good resistance to acids and alkalis, and they also possess good thermal resistance and anti-bacterial properties. PP fibres have been widely used in sportswear and industrial textiles, such as for filtration, composites, biomaterials and electronics. In these applications, the surface properties of the pp are particularly important. As a type of environmentally friendly physical surface modification technology, ion irradiation is now common in industry for it is a very effective way to give the hydrophilicity to polymeric fibers [1-5]. The use of ion beam surface modification technique can provide a differential partition or a layered structure in terms of tribological, chemical, physical and mechanical properties in a sample without changing the bulk properties [6]. Especially optical properties of PP have been studied to explore the decrease in band gap or activation energy for the application on band gap engineering and electronics [7,8].
For many applications in the plastics industry, the surface of polypropylene needs to be modified in order to improve its wettability and adhesion properties. One of the techniques used to modify the surface is the exposure of the polymer to either plasma or ion beams. In this work, the PP film was treated and modified by using a cold cathode ion source with varying ion irradiation conditions (i.e., exposure time, beam current, and discharge current), so as to examine the structural and optical changes in the PP films caused by argon ion irradiation.

Materials and Methods

Cold cathode ion source
The construction of the ion source is shown in Figure 1. It consists essentially of anode cylinder from stainless steel with 30 mm length and 22 mm diameter connected with positive voltage power supply up to +10 kV, two cathode discs with diameter equal to 22 mm and 2 mm thickness. The extractor electrode connected to negative voltage power supply, Faraday cup was placed at 30 mm distance from exit aperture of the ion source. Ion source characteristics were determined as shown in Figure 2. The parameters working pressure, Pr = 2 × 10-4 mmHg, discharge voltage, Vd = 900 volts, extraction voltage, Vext = 600 V, magnetic field intensity, g = 180 gauss. The obtained value of the output ion beam current is 210 μA at discharge current, Id = 6.5 mA.
Figure 1: Cold cathode ion source.
Figure 2: Variation of output ion beam and discharge current at pressure 2 √ó 10-4 mmHg.
Figure 2 shows the variation of output ion beam current with discharge current at Vd = 900, Vext = −600 V, p = 2 × 10-4 mmHg, g = 180 gauss. It was seen from this figure that, an increase of output ion beam current was accompanied by an increase of the discharge current where the characteristics of such discharge is characterized by abnormal glow [9].
Sample preparation
The sample considered in our investigations was PP (Polypropylene). The polymer samples, 50 μm thicknesses, were cut into 10 × 10 mm pieces, ultrasonically cleaned in alcohol to remove organic material and dried with hot air before the treatment.
Description of Characterization Techniques
FTIR spectra of the pristine and the irradiated samples were investigated using (FTIR-Beckman-4250) spectrophotometer in the range 400 cm-1 to 4000 cm-1 (NCRRT, AEA, Cairo, Egypt). X-Ray Diffraction XRD (NCRRT, AEA, Cairo, Egypt), scanning was carried out by a fully computerized X-ray diffractometer, (Shimadzu type XDDI). UV/Vis spectra in the wavelength range from 200 to 900 nm, using the UV–Visible spectrophotometer (model, CECIL 3041, UK).

Results

FTIR
Figure 3 showed the peaks at 2950 and 2870–2880 cm−1 that assigned to the CH3 asymmetric and symmetric stretching modes, respectively, and the peaks at 2920 and 2836 cm−1 were assigned to the CH2 asymmetric stretching vibration modes. Clearly, from Figure 3, the Ar ion beam irradiation decreased the amount of CH3 groups in the PP and transformed them into CH2 groups inducing cross-linking in the PP. The treatment of polymer surface using Ar ion would break C–C bonds, leading to the production of carbon radicals. The Ar ions induced the radicals on PP surface and these radicals reacted with each other to form cross-linked layers [10].
Figure 3: FTIR spectra for the pristine and irradiated PP films with 1.5 kev argon ions.
Powder X-Ray Diffraction Study (XRD)
To analyze the changes in the crystalline properties of the PP polymers, X-ray diffraction studies have been performed as shown in Figure 4. The spectra show that all films are partially crystalline polymer. The main diffraction peak for PP occurs at 2Ѳ = 17.80. It indicates that, there is partially decrease in crystalline of PP by argon ion beam irradiation. The loss of crystallinity after ion beam irradiation at high fluencies could be due to scission processes at main chains.
Figure 4: XRD spectra for the pristine and irradiated PP films with 1.5 kev argon ions.
Optical properties
Figure 5 shows the relation between the absorbance and wavelength in nm of pristine and irradiated pp film with different ion flounces. An increase of ion flounces was accompanied by an increase of the absorbance at wavelength in range 200-500 nm.
Figure 5: UV-Vis spectra for the pristine and irradiated PP films with 1.5 kev argon ions.
Using the absorbance data, the optical band gap energy Eg of PP was obtained from Tauc expression [11]:
α =A(hν −Eg)r                 (1)
Where, A is a constant and hν is the incident photon energy. The optical band gap was calculated from the (hνα)2 versus hν plot (Figure 6) to the hν axis at α = 0.
Figure 6: Spectra for the pristine and irradiated PP films with 1.5 kev argon ions.
Figure 7 shows that the optical band gap of the PP decreased from 5.885 eV for Pristine to 4.2 eV at 1.5 × 1018 ion/cm2 Ar ion fluence. The Ar ion beam increased the formation of carbon–carbon double bonds as confirmed by FTIR analysis, which promoted the delocalization of charge carriers. As a result, the band gap and the activation energy decreased. Therefore, conjugation and carbonization seemed to be main process responsible for the decrease in the band gap of PP irradiated by Ar ion [12]. This decrease in the optical band gap may also be attributed to the creation of point defects during the ion implantation caused by the interaction of ions or secondary electrons with the lattice atoms and by inelastic multiple collisions [13,14].
Figure 7: The optical band gap for the pristine and irradiated PP films with 1.5 keV argon ions.
The urbach energy is one of the standard measurements of the inhomogeneous disorder in semiconducting materials [15]. The activation energy was calculated from the band tail using the Urbach rule:
               (2)
Where, B is a constant and Ea is the activation energy. Figure 8 indicates that as the Ar ion energy increased from 0 to 1.5 × 1018 ion/cm2. The activation energy gradually decreased from 3.6 to 1.22 eV. The decrease in the Ea of PP might be associated with formation of clusters of amorphous carbon within the Ar ion treated PP. The clusters of amorphous carbon and the point defects bring a higher carrier concentration on band structure [16]. Presence of these defects might lead to the formation of lower-energy states and decrease the gap between the conduction and valence bands in irradiated polymer. Consequently, the activation energy of the PP decreased with increasing Ar ion energy.
Figure 8: The band tail width for the pristine and irradiated PP films with 1.5 kev argon ions.
Carbonaceous clusters are supposed to be rich with charge carriers that enhance the electrical conductivity in ion irradiated polymers and consequently they also influence the optical properties of such materials. The number of carbon atoms (N) in a cluster is correlated with the optical energy band gap Eg using the following equation [16]:
               (3)
Figure 9 shows the change of number of carbon atoms per conjugation length (N) in carbon clusters as a function of ion fluency. One should note that the number of the carbon atoms in clusters increases with increasing the ion fluence (Table 1).
Figure 9: The number of carbon atoms (N) in a cluster as a function of ion-fluence for pp irradiated films with1.5 keV argon ions.
Table 1: Optical band gap energy (Eg), optical activation energy (Ea) and the no. of carbons atoms in cluster (N) for the pristine and irradiated pp fims with 1,5 kev argon ions.

Discussion

From the obtained results and above discussions, it can be concluded that: The PP film was treated and modified by using a cold cathode ion source with varying irradiation conditions (i.e., exposure time, beam current, and discharge current), so as to examine the structural and optical changes in the PP films caused by argon ion irradiation. The treatment of polymer surface using Ar ion would break C–C bonds, leading to the production of carbon radicals. The Ar ions induced the radicals on PP surface and these radicals reacted with each other to form cross-linked layers. The main diffraction peak for PP was obtained at 2Ѳ = 17.80. It was found, there was partially decrease in crystalline of PP by argon ion beam irradiation. An increase of ion flounces was accompanied by an increase of the absorbance at wavelength in range 200-500 nm. The Ar ion beam increased the formation of carbon–carbon double bonds as confirmed by FTIR analysis, which promoted the delocalization of charge carriers. As a result, the band gap and the activation energy decreased. Finally, the number of the carbon atoms in clusters increases with increasing the ion fluence.

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