Meaning
The phenomenon where the thermal properties of a polymer depend on its past cooling and heating cycles is an important consideration in polymer processing. This thermal behavior, known as thermal history hysteresis, is caused by the slow rate of molecular relaxation and crystallization in polymer matrices. It is observed when measuring the melting and crystallization temperatures during differential scanning calorimetry.
The temperature at which crystallization occurs during cooling is always lower than the subsequent melting temperature during heating.
Processing Impact
The speed of cooling during extrusion or injection moulding directly determines the crystalline structure of the polymer. Fast cooling rates suppress crystallization, resulting in a more amorphous structure than predicted by slow-cooling lab tests. This difference in crystallinity alters the mechanical properties of the finished part.
Moulders must manage this effect by controlling the mould temperature to achieve consistent part dimensions.
Analytical Characterization
To study this behavior, laboratories use controlled heating and cooling cycles in calorimetry. A first heating run is used to erase the thermal history of the sample. This allows the analyst to measure the intrinsic properties of the resin during a second run.
Comparing the first and second heating runs reveals the processing conditions the polymer experienced during manufacture.
Cooling Effect
Crystallization is a time-dependent molecular rearrangement. Rapid cooling locks the chains in a disordered state. This reduces the measured crystallinity.
Annealing can restore the stable crystalline state.
