On clarification of haze in polypropylene

The mechanism of reducing light scattering in isotactic polypropylene (i-PP), through the addition of so-called clarifying agents, is studied with small-angle light scattering (SALS) and scanning electron microscopy (SEM). The clarifying agents used in this study depict monotectic phase behavior with i-PP, crystallizing in a relatively narrow concentration range in a nanofibrillar network, providing an ultrahigh nucleation density in the i-PP melt. It is found that the clarifying effect, a dramatically increased transparency and reduced haze, that occurs within the aforementioned additive concentration range, coincides with a change in morphology from strongly scattering spherulites to shish-kebab-like crystalline structures, as evidenced by in situ SALS measurements and confirmed by SEM images. A simple scaling law, relating the diameter of the shish-kebab structures to the fibril diameter and volume fraction of the clarifying agent is proposed, suggesting that the performance of a (fibril-forming) clarifying agent will improve by reducing the fibril diameter and/or increasing the volume concentration of the clarifying agent.