Electron Beam Irradiation Effects on Dielectric Parameters of SiR–EPDM Blends

The survival of an electrical system is mostly governed by the endurance limit of the dielectric material employed in it. The five different compositions of SiR–EPDM blends were prepared. Electron beam radiation has been widely used in the cable manufactur‐ ing industries in order to increase the life of the cable. Hence, the five blends were irradiated to 5, 15 and 25 Mrad dose levels by electron beam accelerator. The dielectric parameters such as breakdown voltage (BDV), dielectric strength (DS), dielectric constant (DC), and dissipation factor (DF) were measured as per ASTM/IEC standards. This chapter evaluates the effect of electron beam irradiation on dielectric parameters of SiR–EPDM blends.


Introduction
Electron beam irradiation has been effectively utilized in power cable industry and identified as one of the most advanced processing techniques. The products processed with electron beam radiation, experience shorter exposure time, which could result in less oxidative effects on certain materials. It is essential to investigate the effect of electron beam irradiation upon the dielectric parameters of the five different compositions of SiR-EPDM blends [1][2][3][4]9]. Hence, the samples of SiR-EPDM were irradiated to 5, 15 and 25 Mrad dose levels by electron beam accelerator. The new functional groups formed during blending and after the electron beam irradiation were investigated through physicochemical investigation techniques like Fourier transform infrared spectroscopy (FTIR). To observe the morphological changes and also to identify the elemental composition, scanning electron microscope (SEM) analysis and energy dispersive X-ray analysis (EDXA) were performed on SiR-EPDM blends. The effects of electron beam irradiation on the dielectric parameters of various compositions of SiR-EPDM blends were reported.

Electron beam irradiation
The five different compositions of SiR-EPDM blends were irradiated up to 5, 15 and 25 Mrad doses using an electron beam accelerator of 1.5 MeV rating at M/S Siechem Industries, Pondicherry, India.

Dielectric characterization
In order to analyze the dielectric behavior of SiR-EPDM blends in harmful environments, blends have been tested as per ASTM/IEC standards.

Breakdown voltage and dielectric strength
As per standard ASTM D 149 (IEC 60243), BDV and DS were measured. The sample dimensions of 5 × 5 × 0.3 cm were placed between two electrodes, and the voltage was increased at a fixed rate of 500 V/s. The voltage at which dielectric breakdown occurs was measured as BDV. DS was calculated.

Dielectric constant and dissipation factor
DC and DF were measured as per ASTM D 150 (IEC 60250) at 1 MHz. The sample dimensions were 5 × 5 × 0.3 cm.

Physicochemical investigations
Various physicochemical techniques such as FTIR, EDXA and SEM were used to identify the nature of changes in the electron beam irradiated samples of SiR-EPDM blends.

Fourier transform infrared spectroscopy (FTIR) analysis
FTIR spectra of electron beam irradiated samples were taken using Perkin Elmer spectrophotometer, in the wave number ranging from 500 cm −1 to 4000 cm −1 . The number of scans for each IR spectrum was 4.

Energy dispersive X-ray (EDXA) analysis
EDXA analysis has been performed using EDXA analysis setup (Make HITACHI), in order to determine the elemental composition of the materials at the surface of the electron beam irradiated samples of SiR-EPDM blends.

Scanning electron microscopy (SEM) analysis
SEM analysis was performed using a scanning electron microscope (Make HITACHI) with a magnification of 5-300,000, in order to study the morphology of the surface of electron beam irradiated samples of SiR-EPDM blends.

Dielectric characterization of virgin SiR-EPDM blends
The virgin SiR rich blends (A and B) have higher breakdown voltage (BDV) and dielectric strength (DS), when compared to remaining blends. This may be due to the occurrence of maximum self cross-linking during the blending process itself. During the blending process, the cross-linking reaction has taken place between the side chains of SiR and EPDM. The blend C and EPDM rich blends (D and E) were found to have lesser values of BDV, DS, and higher values of DC and DF in comparison with SiR rich blends (A and B).   The BDV and DS of SiR-rich blends (A and B) and EPDM-rich blends (D and E) reduced for all doses of electron beam. The BDV and DS of the blend C improved for all doses of electron beam. The DC of the blend D and E has been improved at 5 and 5/25 Mrad respectively.

Dielectric performance of virgin and electron beam irradiated SiR-EPDM blends
For SiR-EPDM blends, it has been observed that cross-linking and chain scission may modify the macromolecular chains of the material. The consequence is the change in the dielectric parameters of the material. The effect of dominant mechanism can be noted from the changes in dielectric parameters.

FTIR analysis
FTIR spectra of electron beam irradiated samples of SiR-EPDM blends were obtained to identify the mechanism for the change in dielectric parameters after the electron beam irradiation. FTIR spectra of the virgin and electron beam irradiated samples of three compositions of SiR-EPDM blends are depicted in Figures 5-7 respectively. The FTIR investigations on electron beam radiated samples revealed that the radiation has induced the chemical and morphological changes. The variation in dielectric parameters was validated through FTIR spectra. It depicts the occurrence of new functional groups along with the % absorbance and the corresponding wave number. Tables 1 and 2 list the correlation of variation in dielectric parameters of electron beam irradiated samples of SiR rich blends and EPDM rich blends and blend C using FTIR respectively.   The BDV, DS, and DF of the SiR rich blends (A and B) found to reduce for all doses of electron beam irradiation. This is due to the disappearance of acid (COOH) group in them. The BDV and DS of the blend C is improved for all doses of electron beam irradiation. This is due to the appearance of Si-O-Si group at 1019, 1018, and 1019 cm −1 with 29, 20, and 23% absorbance in it. The dielectric constant is improved at 5 Mrad. This may be due to the appearance of =C-H (Alkene, bending, strong). The DF has been reduced at 15 Mrad. This may be due to the disappearance of =C-H (Alkene, bending, strong). The maximum improvement in DC of blend D occurred at 5 Mrad. This may be due to the increase in Si-O-Si group at 1018 cm −1 with 34% absorbance and also due to the shifting of alcohol (OH)-free group to higher wave number [16][17][18]. The maximum improvement in DC of blend E has occurred at 25 Mrad. This may be due to the increase in alcohol (OH)-free group at 3795 cm −1 with 138% absorbance.      Table 3. Inferences from EDXA curves of electron beam irradiated samples of SiR rich blends (A and B).

Doses/elements Carbon wt (%) Silicon wt (%)
Oxygen wt (%)  Table 4. Inferences from EDXA curves of electron beam irradiated samples of EPDM rich blends (D and E) and blend C.

SEM analysis
It is observed from Figure 11(a1, a2) that the surface of SiR rich blend (A) has larger number of cracks. This may be due to the decrease in silicon content for 15 Mrad dose of electron beam irradiation (inferred from EDXA analysis), but the surface of blend B and EPDM rich blends (D and E) has smaller number of cracks. This is validated through the increase in oxygen and silicon concentrations (inferred from EDXA analysis). The surface of blend C has smaller cracks. This may be due to the reduction in oxygen and silicon concentrations. The availability of white particles on the surface of the blends B, D, and E may be due to the decrease in carbon content in them (inferred from EDXA curves).   It is observed from Figure 12(a1, b1) that the surface of SiR rich blend (A) has large number of cracks. This may be due to the decrease in silicon content for 25 Mrad dose of electron beam irradiation (inferred from EDXA analysis), but the surface of blend B and EPDM rich blends (D and E) has smaller number of cracks. This is validated through the increase in oxygen and silicon concentrations from EDXA analysis. The surface smoothness of blend C is moderate. This may be due to the reduction in oxygen and silicon concentrations.

Conclusion
The blend C is found to have the improved BDV and DS values for all doses of electron beam irradiation. Also a significant improvement in DC has been noticed at 5 Mrad in blends C, D, and at 5 and 25 Mrad in blend E respectively. A considerable improvement in DF has been observed at 15 and 25 Mrad in blend C and at 25 Mrad in blend D respectively. Hence, it is concluded that blend C and EPDM rich blends are found to have improved dielectric performance after the electron beam exposure.