Effect of electron beam irradiation on polypropylene films—dielectric and FT-IR studies
Introduction
The modifying effect of the ionizing radiation on the physical properties of polymeric materials is a subject of continuous extensive study [1], [8]. The electronic transport is for instance seriously affected by ionizing radiation and significant changes in both electrical and optical properties are induced (e.g. [2], [3]). Deep insight of, dielectric and mechanical properties for irradiated polymers associated with their structure were investigated [4], [5]. However, great attention was paid especially to polypropylene (PP) which belongs to the family of polyolefin’s. It is a vinyl polymer having hydrogen atom constituents (–H2C–CH(CH3)–)n. It has excellent mechanical and dielectric properties. It is the lightest known industrial polymer which finds its application mainly in electrical appliances.
Formation of free radicals, recombination and scission of bonds, cross-linking and different oxidation reactions [6], [7] are taken place during irradiation process on polypropylene and are closely related to different forms of radiation. Therefore, the results of these interaction processes are highly dependent on the energy and dose of the irradiation source. Since the radiation cross-linking and degradation processes occur simultaneously in most cases. However, several experiments with different incident energy and different doses as well as various forms of radiation are underway in order to study further the above problem.
In this context, the effect of gamma radiation were early investigated [8], [9] on polypropylene. A study was performed particularly in the dose range 7.3–198.6 kGy [8]. It was found that polypropylene degradation take place mainly due to oxidative degradation which is of concern for the integrity of electrical insulation, where new functional groups and structures are formed. Hydroperoxide and ketone functional groups are formed due to oxidation of radicals and their presence can change the polypropylene properties. Moreover, when 200 kGy gamma ray irradiation was applied, it induced cracks in the 40 μm thick polypropylene film. This degradation is due to both immediate and long term effects, i.e. direct radiation induced main chain scission and subsequent autoxidation of the active radical sites was formed during and following the irradiation process [9].
Effect of irradiation of polymers by heavy ions is more pronounced than by electrons or gamma rays [10]. For instance, irradiation of PP by 20 keV protons with an irradiation dose of 486 kGy resulted in strong cross-linking of macro-molecules which was observed for the first time, by Wang [11]. Further investigation was performed on PP samples by 100 keV protons with 200 kGy dose [12]. This experiment showed improvements in the mechanical properties meanwhile the IR spectra indicated conformational changes that took place with the formation of the double bonds. In another experiment, the mechanical and the thermal properties of polypropylene (PP) films were studied whereas they were bombarded with 62 MeV energetic proton beams [13]. It was found that by increasing the proton dose, an increase in crystallinity, and both decomposition temperature and melting point was induced, which indicates existence of strong cross-linking effect.
Electrons as low linear energy transfer (LET) radiation, compared to ions, are more often used and they mostly affect the physical and chemical properties of PP. The effect of 2 MeV electron irradiation on polypropylene at 23 kGy dose [14] was studied with thermo-gravimetric analysis, differential scanning calorimeter and X-ray diffraction analysis. The thermal decomposition temperature as well as the melting temperature was found to increase due to electron bombardment. In the same time, an increase in crystallinity of the polymer has also been observed after irradiation due to probable scission of tie chains (relief of stress in the crystal lamellae at the entry of tie molecules) and the growth of order as a result of reduction of molecular weight. In another work, 8 MeV electron beam irradiation rapidly deteriorated the mechanical properties of polypropylene when the irradiation dose reached 400 kGy [11]. However, degradation and cross-linking of polymers are competitive processes and are considered as a function of depth and dose when the polymer is irradiated by electron beam [7]. More studies are needed for further investigation and clarification any changes in the physical properties of PP when it is bombarded with high doses electron radiation.
The purpose of the present work is to investigate the effect of 1.5 MeV high dose electron beam on the dielectric properties and ac conductivity of polypropylene films. These properties are associated with the structure of PP films, therefore, this work is devoted to study the conformational structure induced in the PP films by electron beam.
Section snippets
Experimental work
Polypropylene films with an area of 2 × 2 cm2 and a thickness of 20 μm (El-Sheriff Company for plastics, Cairo, Egypt) were electron beam irradiated. The electron irradiation was performed in air and at room temperature using a 1.5 MeV electron beam from the ICT-type electron accelerator (NCRRT, AEA, Cairo, Egypt). The conveyer was attached to a cooling system in order to avoid temperature heating of the samples. The samples were subjected to various integral irradiation dose levels up to 1080 kGy,
FT-IR spectroscopic analysis
Radiation can induce structural changes in polymers and infrared absorption (FT-IR) spectroscopy is one of the techniques useful for the study of such changes occurred in these systems. The FT-IR spectra of the un-irradiated and the irradiated PP are shown in Fig. 1. All the observed dominant bands are the characteristic bands of the control PP(un-irradiated) sample indicating the isotactic nature of the polymer. The presence of the same main bands in the irradiated PP indicates that the
Conclusions
It is evident from FT-IR spectral analysis, that most types of the CH3 and CH2 group frequencies can be observed in both the un-irradiated as well as the 1.5 MeV electron irradiated polypropylene. FT-IR spectral analysis showed that the isotactic nature of the polymer was not destroyed by electron irradiation, even at the highest dose. Dielectric changes however occurred in the electron beam irradiated PP foils and they showed an increasing magnitude of the relaxation peak with electron dose.
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