Correlating Structural and Electrical Properties in Radiation-Cured Polymers B. Vissouvanadin1, G. Ranoux2, X. Coqueret2, S. Le Roy1, C. Laurent1 and G. Teyssedre1 1
Université de Toulouse and CNRS, LAPLACE (Laboratoire Plasma et Conversion d’Energie) 118 route de Narbonne, F-31062 Toulouse cedex 9, France 2 Université de Reims Champagne-Ardenne, CNRS UMR 7312 (Institut de Chimie Moléculaire de Reims) BP 1039, 51687 Reims Cedex 2, France *
Corresponding author:
[email protected]
Abstract- Radiation-cured epoxy and acrylate-based materials are used in a number of applications where curing under ionizing radiations is preferred to the classical thermal curing process. Because mechanical properties are often the key features in these applications much less attention has been paid to their electrical properties. We report on the dielectric properties of two classes of radiation-cured materials being typical of two classes of polymerization: free radical mechanism (acrylate-based formulation) and cationic mechanism (epoxy-based formulation). Questions arise as regards the electrical properties due to the complex formulation aiming at controlling the network properties (mechanical properties, degree of cure, glass transition temperature) and to the need of initiators in cationic polymerization with the inclusion of pair of ions in the network. Samples are 1mm-thick plaques, being cured under electronbeam. Different formulations are tested with the aim to investigate the influence of the network structure on electrical properties with specific emphasis on the nature of initiator (iodonium or sulfonium salts with various counter ions) and its concentration in epoxy-based formulation and reactive diluents in acrylate-based formulation. Structural and thermo-mechanical characterization is correlated with the electrical properties being current-voltage characteristics, space charge distribution and dielectric spectroscopy. Mechanisms driving the observed properties are presented and discussed.
properties of the material. Number of processing parameters such as the nature of monomer, radiation dose, the initiator content (for epoxies), and reactive diluent concentration may have a strong effect on the structural properties. In addition with the network, the presence of electrical charges, coming from ionization mechanism or from special components of the curable formulations, may have an impact on the dielectric strength of the material. The objective of this work is to gain a better understanding of the relation between network structure and electrical properties of radiation-cured materials. Dielectric characterization is performed on this family of materials and their properties are compared to these of thermal epoxy, considered as benchmark for the present study. II. EXPERIMENTAL A. Materials Three kinds of material have been investigated in this work: thermally-cured epoxy, radiation-cured epoxy and acrylate. Epoxy materials have been elaborated using the base model resin Diglycidyl Ether of Bisphenol A (DGEBA)
I. INTRODUCTION Radiation-induced crosslinking polymerization offers multiple advantages compared to conventional thermoset-based technology [1] . As radiation curing is achieved at ambient temperature, no heating is required reducing the process energy cost. No by-products, which mostly affect properties of the material, are generated unlike for thermal curing process. In addition, curing under ionizing radiation enables a fast manufacturing [2] . Two main chemistries have been widely used to manufacture materials, each of them potentially offering advantages from its specificities. Acrylate-based materials are obtained by free radical mechanism which occurs when electron beam interacts with monomers. Reactive diluents are often used to enhance mechanical and thermal withstand of the material by improving the reaction efficiency. For cationic process, initiator is required to trigger the polymerization of epoxy-based material. The use of some transfer agents and reactive diluents enable tailoring both mechanical and thermal
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Fig. 1: Structure of the different monomers and initiator used for the study
Thermal epoxy has been obtained by crosslinking reaction at elevated temperature between the DGEBA resin and a strong acid anhydride in the presence of a catalyst. The onium salt initiator for electron beam (EB) initiation were cumyltolyliodonium tetrakis(pentafluorophenyl) borate named DAIS for diaryl iodonium salt. Ethoxy (ETAC) and epoxy (EPAC) diacrylates monomers have been used for the fabrication of acrylate materials. Different nature of reactive diluents are used to
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A. Thermal Epoxy Current-voltage characteristics at various temperatures for thermal epoxy are shown in Fig. 2. The threshold field is at about 10 kV/mm and does not depend on temperature. However, below the threshold field, the slope of I-E characteristic is less than 1 and tends to decrease with temperature. This tendency is often encountered when conduction is injection limited. For higher fields, the I-E characteristics follow a non-linear behavior probably due to the effect of space charge. Furthermore, space charge measurements reveal the presence of a low negative charges density within the bulk of material at 25°C while positive charges dominate at 50°C for field above 10 kV/mm. In the investigated temperature range, thermal epoxy exhibits a rather low conductivity (order of magnitude 3.10-16S/m at 25°C and activation energy # 0.6 eV for field below the threshold. -9
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Results on permittivity and dielectric losses as a function of temperature for various frequencies (from 10 to 106 Hz) are presented in Fig. 3. We observe very small variation of permittivity and loss for temperatures less than 80°C for any frequency. For permittivity, a step is observed at around the Tg of the material (120°C) whose position moves toward higher frequencies as the temperature increases. Losses increase with temperature and decrease with frequency. Such phenomenon corresponds to the effect of electrical conduction. 10
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B. Thermal, Mechanical and Dielectric characterization Both Differential Scanning Calorimetry in the temperature modulated mode (MDSC) and Dynamic Mechanical Analysis (DMA) have been used to assess thermal and mechanical properties of the materials. For DSC, the Tg is obtained by differentiating the reversing heat capacity as a function of temperature while for DMA the glass transition temperature is marked by a peak on the loss modulus vs. temperature curve. In addition to the Tg, DMA and MDSC give information regarding the microstructure of the formed network. Three types of techniques have been used to analyze the dielectric properties of the manufactured materials. Current measurements have been performed on gold-metallized plaque samples (50mm electrode diameter), at 3 different temperatures for fields between 2 and 25kV/mm. To avoid possible memory effects due to sample pre-stressing, a new sample is used for each test at a given temperature. Conductivity and its dependence with field and temperature can be estimated from the value of the quasi-stationary charging current (taken after 1 h of polarization). Threshold field (characterizing the transition between two conduction regimes, mostly from ohmic to space charge limited) can also be assessed from the plot of conduction current v.s. applied field. At low frequency (typically less than 0.1 Hz), total dielectric loss (including polarization and dc conduction) can be computed using timefrequency transformation of charging current while discharging current gives only the polarization component. Relevant information regarding dielectric relaxation phenomena can be obtained and correlated to DMA analysis. Space charge has been measured on gold coated samples by mean of the Pulsed Electro-acoustic Method (PEA) [3]. Measurements were performed at 2 different temperatures (25 and 50°C) and for fields ranging from 5 to 20 kV/mm. Each polarization step lasts 1 h and is followed by a depolarization procedure of the same duration. Dielectric spectroscopy has been performed on various formulations. Frequency-domain dielectric measurements were performed in the range between 10-1 and 107 Hz during a heating run from -80 to 200°C using a Novocontrol Alpha-a high resolution dielectric analyzer.
III. RESULTS AND DISCUSSION
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improve/modulate the thermo-mechanical properties (toughness, Tg) of the material by modifying chain length. Various types of reactive diluents have been studied: results on hexane diol diacrylate (HDDA) and Isobornyl acrylate (IBOA) are briefly reported in this work. They are introduced before EB curing in a proportion of 10% wt. to a 6/3 ratio of EPAC/ETAC mixture. Plaque samples, 1mm-thick, have been poured into aluminum molds and cured using EB radiation produced by pulsed accelerator (10 Mev, 20 kW). The dose rate was fixed at 15 kGy/s and the deposited dose is function of the number of passes applied to the sample (50 kGy per pass for doses up to 100 kGy). For cationic process, post curing at 180°C for 90 min is achieved to complete the reticulation process.
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B. Radiation-cured Epoxies: effect of DAIS content DMA and DSC analyses reveal that the glass transition temperature of EB-cured DAIS-containing epoxies increases till about 170°C with the conversion rate. However, higher conversion rate, obtained for higher DAIS content, result in a decrease of the glass transition temperature [4], Fig. 4.
vs. temperature and frequency are shown in Fig.7. Between 50°C and 150°C, permittivity increases for frequencies below 1 Hz (Fig. 7 a) and could be due to the effect of space charge induced macroscopic polarization. -7
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Permittivity collapses from 160°C to 180°C which correspond to the Tg range of the material and increases again for higher temperatures. From 50°C and on, losses increase with temperature and higher values are obtained at lower frequencies (Fig. 7 b). Conduction processes may be responsible for such an increase. However, losses remain lower than for thermal epoxy especially at high temperature. For example at 10Hz and 160°C, losses are 10 times lower compared to thermal epoxy probably because of the difference in the glass transition temperature of about 50°C between the two materials. Changing the initiator content does not induce a strong effect regarding both permittivity and dielectric loss of the material in the investigated frequency range. 10
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Current-voltage characteristics at various temperatures for EB-cured epoxy with 1% DAIS are presented in Fig. 5. The threshold field is approximately 10kV/mm and remains relatively stable with temperature as for thermal epoxy. The slope of I-E characteristic at 55°C is less than unity meaning that conduction may be limited by charge injection. Conductivity at 25°C is about two orders of magnitude higher than for thermal epoxy. The presence of DAIS salt may increase charge mobility by providing shallow traps in the dielectric. DAIS also provides ionic carriers, but these do not appear to be mobile due to their size. Space charge measurements did not reveal evidence of ionic transport. The activation energy of the conductivity (below the threshold field) is estimated to 1.5 eV. Reducing the DAIS content to 0.25% does not change the conductivity at 25°C but tends to decrease its activation energy – i.e. its dependence with temperature. This can be explained by the difference in the Tg which is estimated to be 184°C and 173°C for 0.25% and 1% DAIS, respectively. The material with higher Tg seems less sensitive to temperature increase with regards conductivity. Space charge measurements (results not shown for sake of space saving) reveal charge injection at both electrodes at 25°C for fields from 5 to 20 kV/mm. Charge densities increase with electrical stress. DAIS salt is likely acting as charge acceptor (both for negative and positive charges). At 40°C, PEA measurement shows reduction in the amount of homocharges. Charge mobility is thought to increase with temperature; conduction may be electrode limited at elevated temperature. Dielectric spectroscopy shows that for temperature below 50°C, permittivity is about 4 and is stable with frequency. The evolution of losses between -80 and 50°C for various frequencies is shown in Fig. 6. The observed relaxation process corresponds to the β -relaxation of the polymer caused by local motion of chain segments, conformational rearrangements or rotation of the phenyl groups. Dielectric permittivity and losses
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C. Radiation-cured Acrylates Current versus applied field characteristics are represented in Fig. 8 for EB-cured acrylate in the presence of HDDA. The threshold field is estimated to be around 10kV/mm as for epoxy materials. In addition, at 60°C, the slope of I-E in log-log plot is lower than unity which may be attributed to an injection limited conduction phenomenon. It is also noteworthy that for higher field, the slope of current vs. field tends to decrease with an increasing temperature and is lower than 2 which may be related to a kind of hopping conduction rather than a pure SCLC mechanism. Conductivity of the specimen at 25°C is estimated around 3.1014 (S/m) for fields lower than 10kV/mm which is similar to the value obtained in the case of radiation-cured epoxy with DAIS initiator. The activation energy is about 0.8eV.
TABLE. 1: Parameters derived from H-N fit of losses curves T(°C) τ(s) α β 40°C 0.94 0.5 2.05*103 60°C 0.7 1 270
Space charge profiles during volt-off at different times are represented in Fig. 10. Little charge is detected in the bulk whereas influence charges are visible at both electrodes. These charges are attributed to slow polarization effect and might give a contribution on the dielectric loss. 0.8
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Fig. 8: Current-Voltage characteristics at different temperatures of EBcured acrylate in presence of HDDA reactive diluents.
Dielectric losses are computed from the depolarization currents (to avoid contribution of conductivity) using the Hamon approximation. It is seen (Fig. 9) that there is a loss peak which shifts toward higher frequency as the temperature increases showing a temperature-activated phenomenon. 2
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IV. CONCLUSION Dielectric tests have been performed on both radiationcured polymers and thermal epoxy. Materials processed by irradiation show higher conductivity than thermal epoxy. It has been found that for radiation-cured epoxy, the dependence of conductivity with temperature may be lowered by reducing initiator content. Furthermore, due probably to a higher value of glass transition temperature, these materials exhibit lower losses at high temperature (above 150°C) in comparison to thermal epoxy. Finally, radiation-cured materials (acrylate and epoxy) show similar dielectric performances.
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Spectra of Fig.9 were fitted using the Havriliak-Negami model (H-N) which enables to assess the characteristic relaxation time τ of the material, the broadness (α) and the asymmetry of loss peak (β) parameters (TABLE. 1). At 40°C, α is close to unity, meaning the presence of a single relaxation time but the value of β (0.7) describes an asymmetric behavior. The loss peak becomes quasi-symmetric at 60°C but α deviates
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X. Coqueret, “Obtaining high performance polymeric materials by irradiation,” Radiation Chemistry, EDP Sciences, 2008. R. L. Clough, “High Energy radiation and polymers: A review of commercial process and emerging applications”, Nucl. Instr. Meth. Phys. B, vol 185, pp. 8-33, 2001. T. Maeno, T. Futami, H. Kushibe, T. Takada and C.M. Cooke, “Measurement of Spatial Charge Distribution in Thick Dielectrics Using the Pulsed Electroacoustic Method”, IEEE Trans. Electr. Insul., vol. 23, pp. 433-439, 1988. G. Ranoux, M. Molinari, X. Coqueret, “Thermo-mechanical properties and structural features of diglycidyl ether of Bis phenol a cationically cured by e-bean radiation”, Radiat. Phys. Chem., pp. 1-6, 2011.
