Post Crystallization (Cold Crystallization)

The post crystallization of semi-crystalline plastics occurs primarily at elevated temperatures and increased molecular mobility above the Température de Transition VitreuseThe glass transition is one of the most important properties of amorphous and semi-crystalline materials, e.g., inorganic glasses, amorphous metals, polymers, pharmaceuticals and food ingredients, etc., and describes the temperature region where the mechanical properties of the materials change from hard and brittle to more soft, deformable or 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 Temperatures and EnthalpiesThe enthalpy of fusion of a substance, also known as latent heat, is a measure of the energy input, typically heat, which is necessary to convert a substance from solid to liquid state. The melting point of a substance is the temperature at which it changes state from solid (crystalline) to liquid (isotropic melt).melting temperature (see fig.1).

Denser packing of the molecules can cause shrinkage or distortion with crack formation in a plastic or rubber part.

Fig.1: Post crystallization (from Gottfried Wilhelm Ehrenstein, Sonja Pongratz: Beständigkeit von Kunststoffen, p.30)

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 ContrainteLa Contrainte est définie par un niveau de force appliquée sur un échantillon d’une section bien définie. (Contrainte = force/surface). Les échantillons qui possèdent une section rectangulaire ou circulaire peuvent être comprimés ou étirés. Les matériaux élastiques comme les élastomères peuvent être étirés jusqu’à 5 à 10 fois leur longueur initiale.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.

Application Examples

Example of Post CristallisationCrystallization is the physical process of hardening during the formation and growth of crystals. During this process, heat of crystallization is released.Crystallization

This heating curve shows the glass transition, post crystallization and melting of PET (polyethylene terephthalate).

Instrument: DSC 214 Polyma

Test conditions:

  • 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
Fig. 2: Exothermal post crystallization of PET at 131°C (peak temperature)

Example of post crystallization and melting of volcanic rock

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.

Test conditions:

  • Temperature range: RT to 1250°C under nitrogen
  • Heating rate: 10 K/min
Fig. 3: DSC 404 F1 measurement on volcanic rock, temperature range: RT to 1250°C, 10 K/min

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