Plastic is widely used as an electrical material because of its excellent insulation and moldability.
Usage is broadly divided into strong current and weak current. When used in electronic parts as a material for low current, it must have excellent high-frequency insulation. For that reason, the dielectric constant (epsilon) and dielectric loss tangent (tan delta) should be small. On the other hand, when used for low-frequency insulation as a high-voltage material, a material with excellent insulation resistance, dielectric breakdown strength, arc resistance, and tracking resistance is desired even if the dielectric characteristics are somewhat poor.
I will explain the following four items.
A Electrical insulation
B Dielectric properties
C Deterioration of electrical properties
D Chargeability
A-1 Electrical insulation
Materials that do not conduct electricity are generally called insulators. When an insulator is sandwiched between two electrodes and a DC voltage is applied,
You can see that very little current is flowing. This is called leakage current. The resistance against this leakage current is called "insulation resistance". The resistance between the opposing faces of a 1 cm square cube, defined as the "volume resistivity". The higher the value, the better the insulator. For the current flowing only on the surface
The resistance is called "surface resistance". Fig. 1 shows the measurement principle of both resistances.
A-2 Dielectric breakdown, dielectric strength
If you put an insulator between two electrodes and gradually increase the voltage, only a small amount of current flows at first, but when the voltage becomes very high, the current increases rapidly and a part of the insulator disappears. It melts and opens holes or carbonizes and breaks down, and loses its insulating properties. This phenomenon is called "dielectric breakdown". The maximum voltage that an insulating material can withstand per unit thickness of a sample is called "dielectric breakdown strength".
B Dielectric properties
When a voltage is applied to an insulator, a corresponding positive and negative charge is generated in the insulator. This phenomenon is called a dielectric phenomenon. Insulators are also called dielectrics because they cause this phenomenon. This is a very important property when insulating materials are used in the high frequency range. It is represented by a dielectric constant and a dielectric loss tangent tanσ. A material with a large value generates heat during use in a high-frequency circuit, resulting in large power loss. It can also cause material deterioration and insulation failure accidents.
C-1 Arc resistance
Even plastics with high withstand voltage will deteriorate due to partial discharge if they are used at high voltage for a long time. Arc resistance is determined by stepwise application of AC voltage between electrodes to generate discharge, and measuring how many seconds it takes to burn and lose insulation. When arc discharge occurs on the surface of the insulating plate, it is a measure of how much it can withstand, and there is a large difference depending on the type of plastic.
C-2 2 Tracking resistance
The surface is thermally degraded by arc discharge, and a carbonized electrical path is formed along the surface of the insulator. This phenomenon is called tracking, and the difficulty of occurrence is called tracking resistance.
D Chargeability
Plastic is easily electrified by friction and contact. It can suck in dust in the air and contaminate the product, or cause spark discharge to trigger an explosion. Electricity leakage occurs quickly in humid air, so the amount of charge decreases. In general, the higher the resistance value, the stronger the degree of dust collection contamination. As a rule of thumb, 10 to the 15th power ohms and above are extremely susceptible to contamination, and 10 to the 10th power and less do not cause much contamination. It is one of the standards for selecting antistatic grades.
▶Chemical properties of plastics
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