In the case of Extra High Voltage (EHV) power cables, the conductor shield (Semiconductive), Insulation (XLPE), Insulation shield (Semiconductive), Metallic sheath and Oversheath are sequentially designed from the conductor of the cable. In EHV power cables, the Oversheath layer functions to protect the Power cable from the external environment. However, in the process of cable underground layering the cable, the Oversheath layer constituting the outermost layer may be damaged by various external stress, and if the degree of damage is critical. The damage can create cracks or holes in the cable. After installation, the voltage applied to the Metallic sheath and electricity is detectide from the Oversheath layer to diagnose the occurrence of damage such as cracks or holes in the Oversheath layer. For the diagnosis, an excellent electrical conductivity of the Oversheath layer is required. However, due to the large quantity of eco-friendly metal oxide-based flame retardant applied to the layer to impart flame retardant properties, conductive carbon black is limited in forming a network, which led to the application of a larger volume of conductive carbon black. [1] As a large amount of flame retardants filler and conductive filler are applied, the weight of the material increases and mechanical properties, such as flexibility and impact resistance, becomes weak. The decrease in flexibility impairs workability in the winding and installation steps after the cable manufacturing. In addition, carbon black has a limit in securing the electrical properties required for EHV power cables. In this study, by applying CNT (carbon nanotube), a conductive nanomaterial, together with the existing conductive carbon black, excellent electrical properties are secured even with a small amount of conductive filler, and at the same time, nano fusion composite material technology with excellent impact resistance was studied.

Compounding

The structure of the semiconductive flame retardant compound is shown in (Table 1). The matrix was constructed by using MDPE (medium density polyethylene) and POE (polyolefin elastomer) as the base polymer. For flame retardant properties, a metal oxide based flame retardant was applied. Magnesium hydroxide (MDH) in the form of fine particles with a particle size of 1.4 μm was used to mechanical properties through excellent dispersion properties. [2][3] In addition to the commonly used furnace black grade carbon black for electrical conductivity, high-purity CNTs with a purity of 97% or more were mixed with carbon black and used.[4] Other additives such as antioxidants and lubricants were used together. For MDPE and POE used as base polymers, materials with similar melting temperature (melt temperature_Tm) and flowability (melting index_MI) were selected for extrudability and mechanical properties as well as compatibility between the two base polymers.

Table 1: Composition of specimens.

The base polymer, flame retardant, conductive filler (C/B + CNT), and other additives were compounded in a pilot scale process as shown in Figure 1. After processing at 170°C for 20 minutes using a kneader mixer (5?), using a single screw extruder (50∅), a pellet-type semi-conductive flame-retardant compound sample was manufactured by strand-cutting.

Figure 1: Kneader compounding process schematic.

The sample prepared in the form of pellets was dried for 24 hrs in a heating oven preheated to 60?, and using a hot-press, a flat type press sheets (200mm (W)× 200mm(L)) with 1.0mm(T) and 3.0mm(T) were applied at 180?. It was molded for 5 minutes under a pressure condition of 200 kgf.

Test Method

Volume Resistivity: Volume resistivity properties were evaluated according to ASTM D 991 standard. The volume resistivity specimen was manufactured with a width of 30.0 mm and a length of 115.0 mm on a 1.0 mm thickness press sheet. It was evaluated using a JIG with a distance between electrodes of 50.0mm. The test environment was evaluated at a room temperature (23°C) and a high temperature (90°C), respectively, and the sample was loaded in preheated oven to high temperature (90°C) and heated for 60 minutes before evaluation.

Figure 2:  Izod impact tester schematic.

Mechanical Properties: The Izod impact test indicates the energy required to break a notched specimen under standard conditions. In the impact strength test, the initial potential energy is converted to kinetic energy by pendulum motion according to “the law of conservation of energy”, and then converted back to potential energy in addition to the energy consumed for fracture of the specimen, through which the impact strength is evaluated.

Figure 3: Dumbbell specimen for Tensile strength test.

Limited Oxygen Index (LOI): The oxygen index property of the sample was tested at room temperature (ASTM D 2863). Measure the minimum oxygen demand required for burning of the test specimen. Oxygen and nitrogen gas replace the inside of the glass column where burning takes place. At this time, the oxygen concentration is controlled by adjusting the flow rate of each gas. The flame retardant properties of the semi conductive flame retardant compound can be judged by evaluating the flammability according to the oxygen concentration.

Figure 4: LOI tester schematic.

Mooney Viscosity: Mooney viscosity was evaluated according to ASTM D 1646 standard. After preheating at 160 °C for 1 minute, the value was measured 4 minutes after starting the rotor, and the value was expressed as ML 1+4. Here, M means Mooney viscosity, L is rotor size (Rotor size: L-type-Ø38.10×5.5 mm, S-type-Ø30.48×5.5 mm), and “1” is for 1 minute It is the warm-up time, and “4” is the time the rotor rotates at 2rpm for 4 minutes.Test Results

Volume Resistivity

In general, the electrical conductivity properties of a semiconductive layer in a power cable can be explained by a volume resistivity value. In the case of a general semiconducting compound for power cables, it has the value of the volume resistance between 10⁰ ~ 10⁴ Ω?cm. In a high temperature environment, as the distance between the carbon black particles increases due to the thermal expansion of the semiconductive layer, the volume resistivity increases compared to room temperature.

Figure 5: SEM images of Carbon Black & CNT.

Carbon nanotubes, which are conductive fillers with a linear structure, suppress thermal expansion as they act as reinforcing materials in the compound [5]. In addition, since it has excellent electrical conductivity compared to carbon black, the conductive filler, which is 66% reduced compared to T-3, which is a single structure of carbon black, reduces the variation in high temperature volume resistance compared to room temperature, and confirms better electrical properties [6].

Figure 6: The results of Volume resistivity.

It is confirmed that the conductive filler having high rigidity compared to the base polymer inhibits hardness and elongation. Hardness and elongation are related to the flexibility of the material, and flexibility is required for the cable to be wound on the drum after the cable is manufactured. In particular, it was confirmed that the formulation of carbon nanotubes with a high rigidity characteristic of a linear structure greatly improved the impact strength [7-9].

Figure 7: The results of Tensile test.

Figure 8: The results of Impact strength and Hardness.

Limited Oxygen Index (LOI) Properties

In the case of phosphorus-based or halogen-based flame retardant, excellent flame retardant properties can be imparted to the compound even with a small amount. However, since toxic substances are emitted during burning, an inorganic metal oxide flame retardant was used in this study. [10-12].The flame retardant performance tends to be weak as the specimen is easily deformed by the heat of combustion during flame retardancy evaluation. In the case of T-4 and T-5 samples that were prescribed in a large amount compared to other samples in which carbon nanotubes were not prescribed, the distortion or deformation of the specimen during combustion was significantly less, and the evaluation of the oxygen index was also excellent.

Figure 9:  The results of LOI test.

Mooney Viscosity Properties

Semiconductive flame retardant compounds are used in the extrusion process of power cables, and their rheological properties directly affect processability. Therefore, the viscosity of the conductive filler (C/B + CNT) was quite high in the structure with a large amount of prescription, and in particular, the total conductive filler had a linear structure and high rigidity in the T-4 sample, which was prescribed in the same amount as the T-2 sample. Due to the tube, it was confirmed that the rheological properties were clearly weak in the rheological properties were clearly weak compared to flowability. [13-15]. In addition, as a small amount of carbon nanotubes was mixed with carbon black and prescribed, the proportion of the conductive filler in the compound could be reduced up to 65wt%. For this reason, superior rheological properties were confirmed compared to the T-2 structure using carbon black alone, and it works advantageously in terms of process ability.

Figure 10: The results of mooney viscosity.

In this study, weakness in various physical properties could be effectively improved by combining the existing covering compound material technology for power cables and nano material technology. It was confirmed that excellent electrical properties could be secured even with a significantly smaller addition of conductive filler by hybridizing carbon black, which is a conductive filler and carbon nanotubes (CNT) having dozen times or more excellent electrical conductivity. In addition, effective reduction of the conductive filler improves impact resistance and flexibility, thereby preventing the anticorrosive layer located at the outermost part of the Extra-high voltage cable from being destroyed and damaged by external force. Due to a small amount of conductive filler, the compound has a relatively low specific gravity, which can help to reduce the weight of the cable. In addition, the effective weight loss of the filler works advantageously in terms of processability compared to a structure in which a large amount of filler is prescribed.  Lastly, in the case of a general semiconducting compound using carbon black as conductive filler, volume resistivity properties tend to be increased due to thermal expansion in a high temperature environment. However, through this study, it was confirmed that the problem of weakening electrical properties at high temperatures was resolved for a nano-fusion composite semiconducting flame retardant compound using carbon nanotubes as a nanomaterial technology.

1. Lee MH. Flame Retardancy and Physical Properties of Ethylene Vinyl Acetate/Aluminum Trihydroxide Composites. Polymer (Korea). 2015; 39: 433-440.
2. Chen H, Wen X, Guan Y, Min J, Wen Y, Yang H, et al. Effect of particle size on the flame retardancy of poly (butylene succinate)/ Mg (OH)₂ Fire and Materials. Wiley. 2016; 40: 1090-1096.
3. Cardenas MA, LOpeza G, Mitrea IJ, Merinoab JC, Pastorab JM, MartInezc JDD, et al. Mechanical and fire retardant properties of EVA/clay/ATH nanocomposites – Effect of particle size and surface treatment of ATH filler. Polymer Degradation and Stability. 2008; 93: 2032 - 2037.
4. Santos J Silva A, Bretas R. Using the carbon nanotube (CNT)/CNT interaction to obtain hybrid conductive nanostructures. AIP Conference Proceedings. 2015; 1664: 0700210 - 0700215.
5. Won HJ. A Study on the Effect of Fiber Orientation on Impact Strength and Thermal Expansion Behavior of Carbon Fiber Reinforced PA6/PPO Composites. Composites Research. 2014; 27: 52-58.
6. Kim KJ. The effect of CNTs Diameters on electrical resistivity, thermal conductivity and tensile strength of CNT-polyamide composites. J Korean Society of Mechanical Technology. 2016; 18: 250-255.
7. Abbasi SH, Juhani AA, Hamid AU, Hussein I. Effect of aspect ratio, surface modification and compatibilizer on the mechanical and thermal properties of ldpe-mwcnt nanocomposites. E-Polymers. Walter de Gruyter GmbH. 2011; 11.
8. Choi, YJ, Lee K, Kim K, Lee YS. mechanical properties of epoxy composites reinforced with carbon nanotube and oxyfluorinated powdered-carbon fiber. Polymer Korea. The Polymer Society of Korea. 2017; 41: 835-843.
9. Gao T, Cho JU. A study on impact property of sandwich composite of carbon-fiber-reinforced plastic. J the Korean Society of Mechanical Technology. 2014; 16: 1339-1344.
10. Tang H, Chen K, Li X, Ao M, Guo X, Xue D. Environment-friendly, flame retardant thermoplastic elastomer–magnesium hydroxide composites. Functional Materials Letters. World Scientific Pub Co Pte Lt. 2017; 10: 1750042.
11. Lee BJ. Mechanical properties and flame retardancy of rigid polyurethane foam using new phosphorus flame retardant. KSIEC. 2016; 27: 577-582.
12. Choi SS. A study on the flame retardant properties of EPDM rubber mixed with phosphorus and halogen compound. Elastomer. 2002; 37: 224-233.
13. Du D, Wang J, Smith JN, Timchalk C, Lin Y. Biomonitoring of Organophosphorus agent exposure by reactivation of cholinesterase enzyme based on carbon nanotube-enhanced flow-injection amperometric detection. Analytical Chemistry. American Chemical Society (ACS). 2009; 81: 9314-9320.
14. Shin KM. Study on the Melt Flow and Physical Properties of Nylon 66/Carbon Filler Composite with Processing Aid. Polymer (Korea). 2018; 42: 478-484.
15. Zare Yasser. The complex viscosity of polymer carbon nanotubes nanocomposites as a function of networks properties. Carbon Letters. 2019; 29: 535-545.