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- Extremely fast polymer crystallization
When semi-crystalline polymers crystallize from the melt (typically during the cooling phase of a process), the lamellae organize from a primary nucleus to form complex macro-structures called spherulites. These spherulites continue to grow until they impinge on an adjacent spherulite at which point the growth ceases.
Properties of the polymers, including optical and physical characteristics depends on the end size of the spherulite structures.
Furthermore, for crystal growth to start a primary process called nucleation has to occur: this is basically the formation of a focal center around which the lamellae can organize themselves. The secondary process of crystal growth follows nucleation and is characteristic for polyolefins.
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Three phases of primary nucleation are:
- Spontaneous nucleation which refers to homogeneous nucleation under the influence of super-cooling.
- Orientation-induced nucleation, which is a result of alignment of crystals (often due to process flow dynamics).
- Heterogeneous nucleation.
The third part of primary nucleation involves the addition of a foreign phase presenting a new surface on which crystal growth can occur. Typically, this foreign phase takes the form of a nucleating agent which has been designed to have a good epitaxial match with the growing polymer crystal.
Figure 1 shows an illustration of the heterogeneous nucleation process versus a non-nucleated resin for comparison:
Figure 1: Illustration of the crystallization process in non-nucleated or nucleated matrix
In nucleated Polyolefins, crystallization occurs earlier in the cooling process and happens at a faster rate. This allows decreased cooling time of the polymer. Also, nucleation density is much higher and crystal spherulite size is much smaller.
Polypropylene is recognized as a relatively easy material to nucleate. This is because the rate of crystallization is low enough to allow the nucleating agent to have a direct impact on the nucleation density. Conversely, polyethylene has an extremely fast rate of crystallization, rendering most nucleating agents as ineffective. This is probably the reason there has been such minimal commercial activity in the area of nucleating agents for polyethylene.