Understanding these five temperature points is crucial for selecting and processing plastics:
Glass Transition Temperature (Tg): The temperature at which an amorphous polymer (or the amorphous regions of a semi-crystalline polymer) transitions from a hard, glassy state to a soft, rubbery (high-elastic) state. Tg is the lowest temperature at which polymer chain segments can move freely, and it typically represents the upper service temperature limit for amorphous products.
Melting Temperature (Tm): For semi-crystalline polymers, Tm is the temperature at which the ordered, crystalline regions transform into a disordered, viscous fluid. This is often called the melting point. Tm is the lower processing temperature limit for crystalline plastics.
Flow Temperature (Tf): For amorphous polymers, Tf is the temperature at which the material changes from a rubbery (high-elastic) state to a viscous fluid capable of flow. Tf is the lower processing temperature limit for amorphous plastics.
No-Flow Temperature (NFT): The highest temperature at which a plastic will not flow under a specific pressure (e.g., 50 MPa). It is determined experimentally by progressively heating the material under pressure until melt flow is observed; the NFT is then defined as 10°C below the temperature where flow first occurs.
Decomposition Temperature (Td): As the temperature of a polymer melt is increased beyond the processing range, molecular chain degradation accelerates. Td is the temperature at which significant chain degradation occurs, leading to loss of properties, gas evolution, and discoloration. Processing must be carried out well below Td.
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