Ethylene propylene diene monomer (EPDM) is a representative terpolymer being copolymerized by ethylene, propylene, and non-conjugated diolefine. These belong to the polydiolefine family with the molecular backbone being completely saturated and only the branch containing unsaturated double bonds. The molecular structure characteristics of EPDM lead to high resistances to oxidation, chemical corrosion, and optical radiation, as well as the excellent vulcanization properties [1
]. Among all the rubber, EPDM possesses the lowest specific gravity and can persist preferable electrical properties, even after absorbing a large number of fillers and oil. In particular, EPDM is essentially non-polar and not compatible with polar molecules, resulting in a considerably low rate of water absorption and the extraordinary insulating performances [2
]. Although EPDM has been preferably used as a qualified insulation material in high voltage direct current (HVDC) cable accessories, there are still unaddressed technological issues for the interface between cross-linked polyethylene (XLPE) and reinforced insulating EPDM, primarily due to the discrepant electrical conductivity between XLPE and EPDM [3
]. Accordingly, space charges are liable to accumulate at the interface between cables and insulation accessories under DC electric field, giving rise to severe local distortion of electric field in cable accessories, and eventually causing electrical breakdown [4
]. Furthermore, converter transformer and other non-synchronous apparatuses will produce transient overloading in the operating HVDC cables, which can also exacerbate space charge accumulations in cable accessories. The flaws in electrical conductivity of dielectric materials, that could not be pertinently utilized for cable accessories, are now substantially restricting the development of HVDC transmission [6
Since the nano-dielectrics (polymer dielectric nanocomposites) were proposed in the 1990s, the underlying modification mechanism and realizing their applications in industrial production of insulated cables are always the most important issues. It is found that the dielectric properties of polymers can be significantly amended by filling nanoparticles. In order to investigate novel insulation materials to homogenize the electric field in cable accessories, the nonlinear nanocomposites have been focused as prospective candidates by which the electrical conductivity or dielectric permittivity will suddenly increase with the increment of applied electric field strength, due to back-to-back Schottky barrier formed by the accumulated charges at nano-interfaces [7
]. The insulating materials with nonlinear dielectric properties can homogenize electric field distribution and thus reduce space charge accumulation caused by electric field distortion. Non-linear dielectric materials can overcome the technical problems that cannot be solved in traditional fabrication processes and structure design of cable manufactures. The nonlinear conductivity of nanocomposites depends on the effective contact area between nanoparticles and polymer matrix, which requires the nanofiller to be controlled with respect to the minimized size and highly dispersed distribution [11
]. Meanwhile, in order to obtain nonlinear composites, the high content of non-linear nanofillers to achieve nonlinear conductivity will cause the deterioration of mechanical properties and breakdown strength. Therefore, these drawbacks and difficulties in developing cable accessories by nanodielectrics technology make it almost impossible to be fulfilled in practical industrial productions. Novel modification strategies are urgently needed to circumvent the inevitable limitations of nanodielectrics. At present, the other effective methods of modifying polymer insulation materials include ultra-clean process, blending, and chemical modification [12
]. Based on the first-principles calculations, the energetic features of intrinsic traps, introduced by physical and chemical defects, have been reported to reasonably suggest that the polar group can present deep traps in polymer materials [13
]. Recent reports indicated that the excellent dielectric properties of modified polymers via chemical grafting are attributed to the trapping mechanism of space charge suppression and breakdown strength improvement [15
]. However, due to low gasification temperature, the grafted molecules are liable to vaporize in chemical grafting reactions to form gas bubbles in the pipeline of polymer material production, which will intensively deteriorate the insulation performance. It is inevitable for chemical grafting reactions to produce by-product impurity, which need to be carried out at high temperature and pressure for a long time, leading to severe mechanical degradation of polymer materials. Therefore, the traditional chemical method of grafting micro-molecules cannot be applied for industrial production of cable accessories.
In the present study, in order to fulfill the prospective molecular modification of polymer insulation materials, a new tactical scheme of initiating crosslinking reactions of the EPDM and the auxiliary crosslinking agents with polar-group by ultraviolet (UV) irradiation is employed. This results in the crosslinked EPDM, with preferable dielectric properties, to be specifically utilized in industrial productions of cable terminations. Accordingly, we adopt N.N-m-phenylene dimaleimide (HVA2), triallyl isocyanurate (TAIC) and trimethylolpropane trimethacrylate (TMPTMA) as auxiliary crosslinking agents, and benzophenone (BP) as photoinitiator to carry out crosslinking reactions under UV irradiation. Theoretically, the first-principles electronic structure calculations indicate that electronic bound states with the energy level located in the band-gap of EPDM arise after EPDM molecules are connected by auxiliary crosslinkers, which account for the underlying mechanism of increasing conductivity and breakdown strength. The electric field distributions in cable terminations are constructed with modified crosslinked EPDM, and are finally investigated by finite element numerical simulations.
2. Experiments, Theoretical Calculations, and Finite Element simulations
2.1. Material Synthesis
The melting blend and hot-pressing approaches are employed through the synthesis of modified EPDM with the basic raw materials being presented as follows: EPDM (4725P, American DuPont Co., Ltd., Chicago, Illinois, USA) as parent material, benzophenone (BP, Jinleiyuan Chemical Co., Ltd., Lianyungang, China) for initiating grafting reaction under photon irradiation, auxiliary crosslinking agents (HVA2, TAIC and TMPTMA, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), all of which are in the purity higher than 95%. In the melting blend process of preparing the initial mixtures, the pristine EPDM is melted uniformly in Torque Rheometer (RM200C, Hapro Company Ltd., Harbin, China) at 140 °C for 11 min with a stirring speed of 60 rpm, and then 2 wt % BP photon-initiator and 1 wt % auxiliary crosslinking agent are added, then blended for 4min and cooled down to room temperature so as to obtain the uniform mixture materials, which will eventually be pressed into film specimens under a pressure of 15 MPa for 30 min at 140 °C. For photon-initiated crosslinking reactions, the prepared hot-pressed blend is firstly treated in a flat vulcanizer at 120 °C with the pressure being increased by 5 MPa per 5 min from 0 to 15 MPa so as to make the material melt, and then the melt material is irradiated by a light source array of UV LED units (NVSU233A-U365, Riya Electronics Chemistry Co., Ltd., Shanghai, China) for 2 s on a pre-irradiation platform at the normal pressure and room temperature in air atmosphere. The electrically modified EPDM materials are finally achieved after short-circuit degassing at 80 °C for 48 h in a vacuum oven so as to eliminate the residual impurities of small molecules. In photon-initiated crosslinking process under UV irradiation, the power and wavelength of light-emitting are controlled on 1.0 W, and 365 nm, respectively, and light incident direction is 60° angle with the plane of thin film sample. Besides, in order to ensure homogeneous crosslinking reactions across the film plane of EPDM materials, the samples are mounted on the conveyor belt at a constant speed of 1.5 mm/s with a distance of 15 mm between the film plane and UV light source.
2.2. Characterization and Measurement
In order to verify whether the auxiliary crosslinkers are grafted onto EPDM molecular chains through UV-initiated crosslinking reactions, the molecular structures of prepared homologous composites and crosslinked EPDM are characterized by Fourier Transform Infrared (FT-IR) Spectroscopy (FT/IR-6100, Jiasco Trading Co., Ltd., Shenyang, China) in the spectral range of 500–4000 cm−1 with the scanning resolution of 2 cm−1. In accordance to the standards of GB/T 2951.21-2008, and ASTM D 2765-2011, respectively, the crosslinking degrees of EPDM are verified through thermal elongation and gel extraction experiments in which the prepared materials are pressed into dumbbell-shaped samples under 0.2 MPa and then degassed in a vacuum oven at 200 °C for 30 min.
Electrical conductivity is tested by a three-electrode system at various temperatures from 30 to 70 °C for the circular film samples of 50 mm diameter and 300 µm thickness with the evaporated aluminum electrodes on both sides. The protective electrode in annular shape (inner and outer diameters of 54, and 76 mm, respectively) encircles the disc of measuring electrode (50 mm in diameter) on one side of the film samples, and the circular electrode with a larger diameter of 78 mm on the other side is used for applying high voltage. The three-electrode system is composed of a high-voltage DC power supply (continuously adjustable output voltage from 0 to 15 kV), an Ammeter with a testing range of 10−4–10−15 A) and an oven with a maximum operating temperature of 200 °C. After the tested samples are preheated in the oven for 1 h with the protection, high voltage and measuring electrodes being connected to ground, DC power supply, and Ammeter, respectively, DC power voltage is increased gradually to each level of testing voltage (electric field covering the range of 3–40 kV/mm) keeping for 1 h, on which the stable conductance current and voltage values are recorded. In order to reduce random error in testing experiments, three identical samples are prepared for each testing point of electrical conductance under the same condition, by which the averaged values are obtained as the final results to be plotted into conductivity-electric field curves. The DC dielectric breakdown strength of the circular film samples with a diameter of 80 mm and a thickness of 0.1 mm are tested with asymmetric columnar electrodes (25 and 75 mm in diameter for high voltage, and ground electrodes, respectively) by recording the maximum voltage before the sample breakdown when the applied electric field is raised continuously at a constant speed of 4 kV/s.
2.3. Molecular Model and Theoretical Methodology
The molecular models of EPDM molecules are chemically connected by auxiliary crosslinker and initially constructed with random distributed torsion, by which the EPDM molecules of 20 polymerization degree are crosslinked by auxiliary crosslinker molecule near the middle position of EPDM backbone chain, based on rotational isomeric state (RIS) model [17
]. The constructed initial polymer configurations are geometrically optimized to structural relaxation by total energy functional minimization with conjugated gradient algorithm in first-principles calculations [18
], so that the energy change, atomic force and displacement are theoretically evaluated to be lower than 1.0 × 10−5
eV/atom, 0.03 eV/Å, and 0.001 Å, respectively. The electronic structures are calculated based on the molecular orbitals and electronic density of states to investigate the band-edge features and grafting-introduced trap states. The first-principles calculations are performed by employing the scheme of all-electron and numerical atom-orbitals as implemented in DMol3 program of Materials studio 8.0 software package (Accelrys Inc., Materials Stutio v126.96.36.1993, San Diego, CA, USA), as the detailed methodology adopted in calculations listed in Table 1
2.4. Finite Element Simulations
In order to investigate the electric field homogenization, caused by appropriately increasing the conductance in the reinforced insulation layer of modified EPDM, the HVDC cable termination under a voltage level of 200kV is simulated by finite element numerical schemes of electric-thermal field coupling [20
] for the structure schematically shown in Figure 1
. The cable termination is modeled by the geometry as follows: Diameter of core is 38mm, and the thicknesses of XLPE insulation, inner shield, outer shield and reinforced insulation are 16, 2, 1, and 68 mm, respectively. In our simulations, the core and ambient temperatures are set at 70 and 20 °C, with the electrical and thermal parameters of each constituting part, as listed in Table 2
, substantially approaching the actual cable termination [21
]. As implemented in the multi-physics field coupling module of COMSOL finite-element software package, the electric field distributions in cable termination are particularly evaluated, which are self-consistently coupling with thermal field. In the finite element simulations with COMSOL, the free triangular mesh generation is adopted to refine local mesh at the position where electric field changes greatly in cable termination. According to Delaunay triangulation algorithm, the model is divided into 211593 elements in total with the maximum and minimum elements are adjusted until the obtuse-angle triangulation disappears. The element growth rate is set to 1.5 and the relaxation degree of narrow region is set to 1 in mesh generation. The chosen mesh allows the finite-element solutions to be obtained independent of element size.
Employing auxiliary crosslinking agents, a specific crosslinked EPDM material with significantly increased conductivity and adequate insulation strength has been developed by UV-initiation crosslinking technique. The underlying mechanism of promoting electrical conductance are elucidated by quantum mechanics calculations in combination with conductivity-electric field variation curves, at various temperatures of operating insulated cable. The electrical compatibility of modified EPDM materials is proven by finite element numerical simulations of electric field distributions, in order to avoid electric field distortion in cable termination. It has been demonstrated that UV cross-linking technique can successfully initiate crosslinking reactions of EPDM molecules and auxiliary crosslinking agents to achieve modified crosslinked EPDM materials with sufficient crosslinking degree and excellent thermal elongation performance. Compared with pure EPDM, higher conductivity has been acquired by the modified EPDM materials with auxiliary crosslinkers, by which HVA2 represents prominent efficiency for ameliorating electrical performances. The first-principles calculations show that multiple electronic states have been engendered by the grafted auxiliary crosslinkers which will reduce band-gap and increase the density of state, in both conduction and valance bands, consequently leading to increments of carrier mobility and density. Finite element numerical simulations verify that EPDM-HAV2 can most effectively homogenize electric field distribution at the root of cable termination.