PP normally crystallizes slowly and forms relatively large complex crystal aggregates known as spherulites. The growth of these spherulites is generally initiated around microscopic "defects" naturally present in the material. This phenomenon is called nucleation.

Without Clarifier, the size of these spherulites is generally larger than the wavelength of visible light resulting in light scattering and hazing of the material.

Nucleation and Clarification
Adding a clarifier to PP is equivalent to introducing "artificial defects" in which spherulites can initiate their growth. When PP is clarified with Millad® 3988, the rate of crystal initiation is increased by many orders of magnitude throughout the polymer. Because many more crystals are growing in the same amount of space, they are all much smaller in size. The result is crystals smaller than the wavelength of visible light that allow light to pass through, imparting enhanced clarity. The figure below illustrates this process.





To be efficient the molecules introduced in the melt have to modify the kinetics of crystallization of the polymer. This is the reason why a clarifying agent has to fulfill the following requirements:

• Insoluble in the polymer
• Small particle size (between 1 and 10 microns, or smaller)
• Homogeneous dispersion

The mechanism of nucleation is very complex and the effect of a clarifying agent depends on numerous parameters like the nature of the polypropylene (homopolymer, random copolymer, block copolymer), the melt index, the polydispersity index, processing conditions and even the polymerization process.