Thermal properties of engineering plastics, 7 parameters that designers must know!
Time：2023-03-23 10:27:53 / Popularity： / Source：
In selection of engineering plastics, material engineers must consider not only mechanical properties, aging resistance, etc. of material, but also its thermal properties.
There are many thermal performance tests of engineering plastics, what do we need to know?
1. Continuous working temperatureIndicates degree of heat resistance of plastic molded products. Generally divided into physical heat resistance and chemical heat resistance, the former refers to performance of plastic molding style of specified shape to maintain shape of product under specified heating temperature; the latter refers to thermal stability of molecular bonds in plastic molding material.
It mainly represents heat resistance of plastic molding styles under no load conditions. Long-term working temperature of common engineering plastics such as PEEK is 260 ℃.
2. Glass transition temperatureAt temperatures above Tg, amorphous polymers will become soft and rubbery. It is important to ensure that use temperature of amorphous polymer is below Tg in order to obtain desired mechanical properties.
3. Heat distortion temperatureHeat deflection temperature is a colloquial term for deflection temperature under load. It is a method of determining rigidity of plastics at high temperature: under a certain load, heating is continued at a certain speed until pattern shows temperature at which deformation amount is indicated.
In amorphous plastics, deflection temperature under load represents the lower limit temperature close to glass transition temperature, so it can be used as a practical reference index to some extent. However, in crystalline plastics, deflection temperature under load is temperature between glass transition point and crystal melting point, so it is a meaningless temperature both theoretically and practically, and deviation of measurement results is also obvious. This parameter is used to measure ability of different materials to withstand temperature under short-term heating and under load conditions accordingly.
4. Linear expansion coefficientCoefficient of linear expansion refers to expansion ratio of a plastic molded product when temperature rises by 1℃ under a certain pressure, expressed as coefficient of linear expansion relative to unit length. This coefficient is one of important indicators to understand degree of dimensional change of product as temperature of plastic molded product increases.
5. Thermal conductivityThermal conductivity represents amount of heat that passes through a plastic product with a temperature difference of 1℃ and a thickness of 1cm in 1s. It is a reference index for studying thermal insulation of target product when designing product.
6. Specific heat capacitySpecific heat capacity refers to heat capacity per unit mass (1 g) of plastic molded product. Generally, it represents amount of heat required to raise temperature of a plastic molded product by 1℃. Specific heat capacity is same as thermal conductivity, which is reference data for studying temperature rise of target product during product design.
7. Embrittlement temperatureA measure of low temperature mechanical behavior of plastics. When sample is impacted by a hammer with a certain energy, temperature at which cracking probability of sample reaches 50% is called embrittlement temperature.
Determination of embrittlement temperature: Clamp one end of specified shape and size with experimental clamps, fix it in experimental tank, immerse it in specified low-temperature heat transfer medium at all levels, place it for 2.9-3.1min, hit it once with a hammer, find embrittlement temperature from result of damage to pattern.
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