4. Resistance to electrical tracing
Tracking, or leakage tracing, is the gradual formation of conductive pathways on the surface of a plastic insulating material under the combined effect of electrical stress and electrolytic impurities. For plastic insulating materials, a common electrical performance index is compared to the electric traceability index (Comparative Tracking Index, CTI), from the definition of the material is subjected to 50 drops of electrolyte during the maximum voltage value of the failure of the non-electrical tracing, the so-called failure of electrical tracing, i.e., the overcurrent, 0.5 A or greater current lasts for 2 s when the action; or continuous burning 2 s or more. To be more specific, the test voltage range of CTI is 100 ~ 600 V (50Hz), and the voltage increase or decrease is a multiple of 25 V. There are two types of electrolytes, Solution A is a 0.1 wt% ammonium chloride solution with a resistivity of about 3.95 Ohm-m; Solution B is 0.1 wt% ammonium chloride + 0.5 wt% sodium diisobutylnaphthalene sulfonate with a resistivity of about 1.98 Ohm-m; Solution B is more aggressive and is usually followed by a letter M after the CTI value. In addition, there is a concept of PTI (Proof Tracking Index), or Leakage Starting Index, which is the voltage resistance value of a material to withstand 50 drops of electrolyte without leakage starting.
CTI testing standards include IEC 60112, ASTM D3638 and GB/T 4207. For plastic insulating materials, the substrate, fillers and additives (flame retardants, plasticizers, etc.) all affect the CTI; from the point of view of formulation and processing, avoiding the precipitation of small molecules, the generation and accumulation of free carbon is the key to avoiding the precipitation of small molecules, and at the same time improving the appearance of the product's gloss and flatness. Take DuPont's Crastin® PBT as an example, the CTI is between 175 ~ 600 V. The addition of glass fiber and flame retardant will make the CTI lower to a certain extent. In addition, the CTI of materials such as
PPS and LCP is slightly lower, mainly because of the higher carbon content of the molecular structure. In short, for electrical and electronic equipment, plastic surface insulation, the overall consideration of the substrate, formulation and processing aspects.
5. Arc resistance
Plastic insulating materials arc resistance (Arc Resistance), refers to the material resistance caused by high-voltage arc deterioration of the ability to usually use the arc flame on the surface of the material caused by carbonization to the surface conductivity, material combustion, material melting (hole formation) time required to express (unit is s). The test generally uses high voltage, small current (12.5 kV voltage, 10 ~ 40 mA current), in the two electrodes generated between the arc, the role of the surface of the material, through the arc interval time is gradually shortened, the current is gradually increased, so that the material is subjected to progressively more severe combustion conditions until the destruction of the specimen, the record of the time elapsed from the generation of the arc until the destruction of the material. Compared with the “wet burning” of the trace resistance, the arc resistance belongs to the “dry burning”, which is to examine the insulating properties of the material surface by generating an electric arc again and again.
The main test standards for arc resistance are IEC 61621, ASTM D495 and GB/T 1411, and the arc resistance time of general plastic insulating materials ranges from tens of seconds to one or two hundred seconds; the longer the arc resistance time, the better the surface insulation performance. Similar to CTI, glass fibers, flame retardants and other fillers and additives in plastics, as well as the smoothness of the surface of the plastic, will affect the arc resistance of the material.
6. Corona resistance
High-voltage charged body, such as high-voltage power cables and their connectors, around the gas in the strong electric field will be localized free and discharge phenomenon, known as corona (Corona). Plastic
Insulation Materials in the corona discharge will be slowly destroyed, mainly due to the direct collision of charged particles, local high temperature, ozone and other oxidizing effects. Corona resistance (Corona Resistance) refers to the insulating material by corona discharge can resist the quality of the nature of the decline.
Corona resistance test standards are IEC 60343, ASTM D2275 and GB/T 22689. corona resistance is generally a test of the material's resistance to surface discharge breakdown ability, i.e. breakdown time. Corona-resistant plastic insulating materials, especially corona-resistant films, play an important role in high-frequency pulse power electronics. DuPont's Kapton® CRC
Polyimide film is marketed for its excellent corona resistance and is used in a variety of high-voltage environments where corona discharges are present, such as motors, generators, and transformers. kapton® 100CRC has a higher voltage withstand time in the presence of partial discharges (1,250 VAC/1050 Hz) than the common polyimide film Kapton® 100HN dozens of times. It is worth mentioning that the addition of inorganic nanoparticles is an important method of improving the corona resistance of plastic insulating materials.
7. Localized Discharge
Partial Discharge (PD) is an electrical discharge in which the insulation between conductors is only partially bridged by an electric field. Partial discharge generally occurs before the breakdown, the reason is mainly due to the existence of uneven composite media within the insulator, bubbles or air gaps, conductive impurities, resulting in a local electric field is too concentrated in a point and discharge. These bubbles or air gaps on the one hand, insulating materials in the manufacturing process is unavoidable, on the other hand, long-term operation due to temperature changes or electromagnetic forces caused by mechanical vibration and other factors. Partial discharge will accelerate the aging and breakdown of insulating materials, in the structural design, material selection and manufacturing should not be ignored. For plastic insulating materials, the structural design and manufacturing process should be considered together to avoid excessive manufacturing difficulties, such as thick-walled injection molding, air bubbles and other defects in the material, and exacerbate the partial discharge.
The main test standards for partial discharge are IEC 60270, ASTM D1868 and GB/T 7354. In the process of measurement, the amplitude of voltage, the frequency of voltage, the action time of voltage and the environmental conditions will affect the results of partial discharge. In addition, besides electrical measurement methods such as pulse current method, ultrasonic method and light wave method can also be used to detect partial discharges. The unit of partial discharge is Coulomb (C), 1 Coulomb is the amount of electricity that passes through the cross-sectional area of a wire in 1 second when there is a current of 1 Ampere in the wire (1C=1A-s); in general, the amount of partial discharge of the insulating product is required to be no more than 3
PC (3×10-12 C).
In summary, for the plastic insulating material itself, the electrical properties mainly include insulation resistance and resistivity, relative dielectric constant and dielectric loss, dielectric strength, resistance to electrical tracing, resistance to arcing, resistance to corona, leakage current and partial discharge. In fact, for different electrical, electronic and appliance products, there are different requirements and standards for the overall electrical properties of the product. Therefore, for the overall insulation performance of these products, the selection of plastic insulation materials and the design of the insulation structure should be considered. In short, for plastic insulation materials, the selection of materials to follow the physical principles (mechanical properties, thermal properties, electrical properties), manufacturing principles (manufacturing process), economic principles and safety principles to meet the insulation requirements of the final product.
