The range included in the thermal properties of plastics is wide and includes the following items.
(1) Physical constants: thermal conductivity, specific heat, coefficient of thermal expansion
(2) Physical changes during heating: Thermal deformation (glass transition point, softening temperature, flow temperature, crystallization related)
(3)Deterioration of physical properties・・・Decrease in mechanical and electrical properties
④Heat resistance
Compared to metal materials, plastic materials are characterized by a much lower and narrower temperature range in which the above-mentioned state changes and physical property deterioration occur.
1. Thermal transition point
There are two points in which the properties of plastics change discontinuously with changes in temperature.
This is called glass transition point and melting point. The glass transition point is the temperature at which a soft rubber-like polymer is cooled and transitions to a glass state. The glass transition point is written as Tg and is also called the secondary transition point. Since it shows rubber-like elasticity when it exceeds Tg, it is an important property that is emphasized in practical use.
2.Heat resistance
There are the following two criteria for judgment.
Judged by thermal deformation
Based on appearance judgment
Thermoplastic plastics are usually judged by the degree of thermal deformation, and the following two are common.
Thermal deformation temperature (ASTM D648) ... defined by the degree of deformation of a rod-shaped test piece
Vicat softening point (ASTM D1525)・・・Defined by the penetration of the needle into the test piece
On the other hand, thermosetting plastics are evaluated for their heat resistance based on appearance changes according to JIS K6911.
3. Flame resistance
It does not imply fire resistance in the same sense as metal or concrete, but rather how well a plastic material resists burning when exposed to a flame.
The flame resistance of plastic materials is generally classified as follows.
(1) Noncombustibility・・・Those that do not burn continuously
(2) Self-extinguishing property: extinguishes immediately when the flame is removed under specified conditions
(3) Slow-burning property・・・Those with no self-extinguishing property but slow burning rate
Since Japanese electrical products have a large export ratio to the United States, the test method based on the UL standard is common for flame resistance.
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