As noted above, PP is a semi-crystalline polymer, which is what contributes to the haze appearance in its natural state. At the molecular level, semi-crystalline polymers exhibit organized and tightly packed molecular chains. The areas of crystallinity are called spherulites and can vary in shape and size with amorphous areas existing between the crystalline areas. The degree of crystallinity affects many characteristics of the polymer such as stiffness, as well as the chemical and thermal resistance of the material.
Polypropylene normally crystallizes slowly, forming collections of large and complex crystals or spherulites. The growth of these spherulites is initiated around microscopic “sites” naturally present in the material. This phenomenon is called nucleation. Without the addition of a clarifying additive, the size of the spherulites grow to be larger than the wavelength of visible light, resulting in light scattering and hazing the material.
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