Key Words to Know Better about Adhesive (6) -- Thermal

11. HDT

Heat deflection temperature (HDT), also known as heat distortion temperature, is a measure of the ability of a material to resist deformation under load at elevated temperatures. It is the temperature at which a standard test specimen deflects a specified amount under a specified load.

The HDT test is commonly performed on plastics and other polymeric materials to determine their suitability for applications where they will be exposed to elevated temperatures. It is an important property to consider for materials used in manufacturing processes, electrical components, automotive parts, and other applications where the material will be subjected to high temperatures.

During the HDT test, a test specimen is subjected to a gradually increasing temperature while a three-point bend load is applied. The deflection of the specimen is monitored, and the HDT is determined as the temperature at which the deflection reaches a specified value, typically 0.25 or 0.5 millimeters.

The HDT value indicates the temperature at which a material begins to soften and lose its dimensional stability under load. Materials with a high HDT have better resistance to deformation and are better suited for applications requiring thermal stability.

It is important to note that the HDT is influenced by factors such as the type of polymer, molecular structure, crystallinity, filler content, and processing conditions during manufacturing. Different materials will have different HDT values, and it is crucial to choose a material with an HDT appropriate for the specific application and operating conditions.

In summary, heat deflection temperature (HDT) is a measure of a material's ability to resist deformation under load at elevated temperatures. It is determined through a standardized test and helps assess whether a material is suitable for use in applications involving high temperatures. Materials with higher HDT values have better dimensional stability at elevated temperatures.

The measurement of HDT is typically done using a three-point bending test method. A standardized test specimen in the form of a rectangular bar or disk is supported on two points while a load is applied at the center. The temperature is gradually increased at a specified rate, and the deflection of the specimen is measured.

There are two commonly used deflection criteria for determining the HDT: 0.25 mm and 0.5 mm. The temperature at which the specimen deflects by the specified amount is recorded as the HDT. The test is usually conducted according to specific standards such as ASTM D648 or ISO 75.

The HDT value is influenced by various factors such as the type of material, molecular structure, crystallinity, and processing conditions. Different materials will have different HDT values, and it is important to choose a material with an HDT suitable for the intended application.

The HDT is an indication of a material's ability to maintain dimensional stability and resist deformation under load at elevated temperatures. It is commonly used to assess the suitability of materials for applications such as manufacturing processes, electrical components, automotive parts, and other environments where exposure to high temperatures is expected.

12. Tg

Glass transition temperature (Tg) is the temperature at which an amorphous material undergoes a transition from a rigid, glassy state to a more flexible, rubbery state. It is a critical property that characterizes the behavior of many polymeric materials. Unlike crystalline materials that have a clear melting point, amorphous materials such as glass, plastics, and polymers soften gradually over a range of temperatures when heated.

The measurement of Tg is typically determined using differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA). In DSC, the heat capacity and energy changes of a material are measured as it is subjected to a controlled temperature ramp. Tg is detected as a sharp change in the heat capacity curve, indicating the transition from the glassy to the rubbery state.

Tg is influenced by factors such as molecular weight, molecular structure, chemical composition, and processing conditions. Materials with a higher Tg tend to have greater rigidity and improved dimensional stability at high temperatures. Conversely, materials with a lower Tg are more flexible and exhibit better impact resistance and elongation.

Understanding the Tg of a material is crucial for designing and selecting appropriate materials for specific applications. It plays a significant role in determining the processing conditions, maximum service temperature, and overall performance of polymers in various industries, including automotive, aerospace, electronics, packaging, and more.