Polypropylene is a semi-crystalline polymer as many other polymers are, i.e., it comprises amorphous and crystalline regions. When polypropylene crystallizes from the melt, the onset of the phase transformation is the formation of small nuclei.
Once this primary nucleation has occured, the crystal can grow and form spherical macrostructures called spherulites. The use of nucleating agents leads to higher nucleus density allowing for the formation of a larger number of spherulites during the cooling of the melt, whereas in non-nucleated Polyproyplene the spherulites are typically less numerous and are smaller.
Pigments also very often interfere with the crystallisation process during molding, leading to dimensional stability problems like distortion also called warping.
EFFECT OF NUCLEATING AGENTS
The change of the number and size of spherulites by nucleating agents affects the properties and the processing of polypropylene and expands its application window significantly.
- Improved physical and mechanical properties
Nucleating agents increase several physical and mechanical properties of polypropylene such as stiffness, impact properties, hardness, heat distortion temperature, etc. The improvement of mechanical properties generally enables downgauging, thinwalling and weight reduction of the finished parts.
- Reduced cycle-time and increased output
The overall rate of crystallization of polypropylene increases when using nucleators. A higher crystallization speed ensures a faster solidification of the molten polymer upon cooling. This leads to a reduction of cycle-time in molding processes and generally to increased output as well.
- Improved optical properties
The smaller size of spherulites improves the transparency or haze as well as the clarity of polypropylene articles. The strongest clarifying effect is however obtained by clarifying agents (see next section).
EFFECT OF PIGMENTS
The degree of crystallinity and the speed of crystallisation is determining the final properties of an plastic article. Organic pigments are known to influence these parameters during the cooling phase of plastic processing, particularly in HDPE injection moulding.
This can result in : - reduced mechanical stability
- impact on dimensional stability
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Typical applications where this effect becomes important are containers, crates and caps & closures.
Ciba Specialty Chemicals has developed based on innovative technology, a series of organic products which have no or only very little influence on the dimensional and mechanical stability during HDPE processing.
Non and Low warping pigments for HDPE
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