The post crystallization of semi-crystalline plastics occurs primarily at elevated temperatures and increased molecular mobility above the glass transition. It can, however, also take place at room temperature, e.g., for rubber or TPU.
Post crystallization is a change in the physical structure which leads to an increase in the degree of crystallization and the lamella thickness as well as to the perfection of the crystal structure. During post crystallization (cold crystallization), at the transition zone between the existing crystalline structures and the amorphous regions, new ordered structures (crystallites) grow. These newly formed crystals can be differentiated from the pre-existing ones by means of their lower melting temperature (see fig.1).
Denser packing of the molecules can cause shrinkage or distortion with crack formation in a plastic or rubber part.
Post crystallization constitutes a physical aging process in the sense in which it is referred to in DIN 50035.
Physical aging processes are always the result of thermodynamically unstable states (residual stress, orientations, imperfect crystal structure) caused by the cooling conditions during processing.
Such conditions cause plastics to solidify in a supercooled melt, so the resulting structure has no equilibrium.
Physical aging processes are accelerated by the impact of temperature.
This heating curve shows the glass transition, post crystallization and melting of PET (polyethylene terephthalate).
Instrument: DSC 214 Polyma
- Temperature range of 0°C to 290°C under nitrogen: 40 ml, 60 ml (P2, protective)
- Heating/cooling rates: 10 K/min
- Sample mass: 12 mg in pierced Concavus® crucibles
Natural materials such as rocks are difficult to analyze in terms of their chemical composition. Such materials are generally a mixture of different oxides, sulfates or carbonates. Volcanic rock is usually solidified from molten magma and primarily contains a variety of oxides.
This example, presented in Fig. 3, shows a measurement on such a material using the DSC 404 F1. It can clearly be seen that it has solidified almost completely amorphously.
The glass transition was between 623°C and 655°C.
Post crystallization and melting were detected at 884°C and 1111°C (peak temperature).
The heat released during crystallization was comparable to the heat of fusion, indicating the highly amorphous nature of this mixture.
- Temperature range: RT to 1250°C under nitrogen
- Heating rate: 10 K/min