Plastics, most of which are insulating materials (generally speaking, the resistivity of insulating materials is above 107 Ohm-m, i.e., the conductivity is below 10-7 S/m). Plastic insulating materials are important materials for the electrical and electronic industry (Electric and Electronic, E&E), and their rational selection and application is the key to ensure the quality and reliability of electrical and electronic equipment. Plastic insulating materials in the application of the basic properties include electrical properties, mechanical properties, chemical properties, environmental properties, economic and other properties, here mainly introduces the electrical properties, specifically including volume and surface resistance, dielectric constant, dielectric loss, dielectric strength, resistance to electrostatic tracings, resistance to electric arc, corona resistance, partial discharge and so on.
The performance of plastic insulating materials in the electric field, we usually use dielectric properties to describe, specifically including dielectric conductivity, dielectric polarization, dielectric loss and dielectric strength of the four basic properties, and its corresponding characteristic parameters are resistivity (ρv, ρs), relative dielectric constant (εr), dielectric loss angular tangent (tan δ) and dielectric strength (Ej). Simply put, plastic insulating materials undergo conductivity, polarization, loss, and breakdown in an electric field. Generally speaking, the insulation of a plastic part includes surface insulation and internal insulation. Surface insulation mainly includes properties such as surface resistance, resistance to electrical tracing, arc resistance, corona resistance, etc., while internal insulation includes properties such as volume resistance, dielectric constant, dielectric loss, dielectric strength, partial discharge, etc.
1. Insulation resistance and resistivity
Insulation resistance is one of the basic parameters to characterize the properties of insulators, the insulation resistance of an insulator consists of two parts, the volume resistance (Volume Resistance, Rv) and the surface resistance (Surface Resistance, Rs), the corresponding resistivity are the volume resistivity (ρv) and surface resistivity (Surface Resistivity, Rs), respectively. The corresponding resistivities are Volume Resistivity (ρv) and Surface Resistivity (ρs), respectively. From the definition, the volume resistance is placed in the specimen "two" on the opposite surface of the two electrodes between the added DC voltage and flow through the two electrodes between the steady-state current quotient, volume resistivity that is, the volume resistance per unit volume; Surface Resistivity is placed in the specimen "a" surface on the two electrodes, the surface resistivity is placed in the specimen "a" surface on the two electrodes. surface resistance is in the specimen "a" surface of the two electrodes of the voltage added between the two electrodes and the current flowing through the two electrodes of the quotient of the surface resistivity that is the unit area of the surface resistance. In fact, plastic insulating materials in the electric field, there will also be a very small current through, this current through the phenomenon, known as leakage, through the current is called leakage current (Leakage Current).
The main test standards for volume and surface resistance of plastic insulating materials are IEC 60093, ASTM D257 and GB/T 1410. It is worth noting that the test conditions and environmental conditions such as temperature, humidity, electric field strength and irradiation will have an effect on the insulation resistance of plastics. The volume resistivity of common plastic insulating materials is between 107 ~ 1016 Ω-m and the surface resistivity is between 1010 ~ 1017 Ω. Generally speaking, the resistivity of non-polar polymers is slightly larger than that of polar polymers, but due to the great differences in material composition, manufacturing process, and testing conditions, even the performance of the same material varies greatly.
2. Dielectric constant and dielectric loss
Relative permittivity (also called Relative Permittivity, εr) is the quotient of the capacitance between the electrodes of a capacitor and the vacuum capacitance of the same electrode configuration when the space around the electrodes is completely filled with insulating material. Dielectric constant is the product of relative dielectric constant and vacuum dielectric constant. Dielectric Loss Angle (δ), is the residual angle of the phase difference between the voltage applied to a capacitor with insulating material as the dielectric and the resulting current. The tangent of the dielectric loss angle (also known as the dielectric loss factor, Dissipation Factor, tanδ) is the ratio of active power to reactive power consumed by the insulating material when a voltage is applied, i.e., the tangent of the loss angle δ. In layman's terms, the source of dielectric constant is the plastic insulating material polarized in the electric field, forming an inverse electric field, which reduces the electric field strength of the capacitor; the source of dielectric loss is the plastic insulating material polarized in the electric field, absorbing electrical energy and dissipating it in the form of heat.
Plastic insulation material relative dielectric constant and dielectric loss factor of the test standards are mainly IEC 60250, ASTM D150 and GB/T 1409. here to mention the frequency of the two effects (50Hz ~ 1GHz), the general plastic
Insulation Materials, with the increase in the frequency of the electric field, the dielectric constant decreases, the dielectric loss increases. Common non-polar or slightly polar plastics, such as polyethylene, polystyrene, polytetrafluoroethylene and other pure hydrocarbon plastics, the relative permittivity is very small (about 2 ~ 3), the dielectric loss factor is also very small (10-8 ~ 10-4); Polar plastics, such as PVC, phenolic resins, nylon, etc., their relative permittivity is larger (4 ~ 7), the dielectric loss factor is larger (0.01 ~ 0.2). Like resistivity, the dielectric constant and dielectric loss of plastic insulating materials are also affected by material composition, manufacturing process, and testing conditions.
3. Dielectric Strength
Dielectric Strength (Dielectric Strength) test is divided into two types, i.e. breakdown test and voltage withstand test. Breakdown test is in the continuous voltage test, the specimen occurs when the breakdown voltage, that is, the breakdown voltage (Breakdown Voltage or Puncture Voltage), the unit thickness of the breakdown voltage that the dielectric strength (kV/mm). Withstand voltage test is in the step-by-step voltage, the specimen withstand the highest voltage, that is, withstand voltage (Withstand Voltage or Voltage Resistance); in the voltage level, the entire test specimen does not occur within the breakdown. It is worth noting that, during the test, there is a possibility of flashover, that is, the specimen and the electrodes around the loss of insulation properties of the gas or liquid medium, causing the test circuit.
The main standards for testing the dielectric strength of plastic insulation materials are IEC 60243, ASTM D149, GB/T 1408 and GB/T 1695, of which GB/T 1695 is a test method for vulcanized rubber. It is worth mentioning that the test of material dielectric strength is affected by voltage waveform and frequency (DC, industrial frequency; lightning shock), voltage action time, thickness and inhomogeneity of the specimen and environmental conditions. The dielectric strength of common general-purpose and engineering plastic plates and sheets is around 10 ~ 60 kV/mm, and the dielectric strength of films such as polypropylene, polyester and polyimide is around 100 ~ 300 kV/mm.
