The formation of β-polypropylene crystals in a compatibilized blend of isotactic polypropylene and polyamide-6

Compatibilized polypropylene (PP)/polyamide (PA6) blends with and without β nucleating agent (β-NA) are prepared, and are designated as Blend-0.3 and Blend-0, respectively. The melting and crystallization characteristic of the blends crystallized under different cooling rates and different crystallization temperatures are studied. It is observed that high β-PP content can be developed in Blend-0.3 only at slow cooling rates (<5°C/min), whereas high α-PP content is formed at fast cooling rates. Isothermal crystallization analysis of Blend-0 indicates that PA6 is an effective NA for α-PP in the lower temperature range, whereas the α-nucleating effect disappears in the higher temperature range. Blend-0.3 can, therefore, be viewed as a system containing both α- and β-NAs, simultaneously. PA6 is competing with β-NA in inducing PP crystallization. Under the normal injection of Blend-0.3, the melt will be cooled through the higher temperature that favors the effectiveness of β-NA rapidly because of the faster cooling rate. However, the α-nucleation effect from PA6 predominate at the lower temperature. This explains the difficulty in obtaining high β-PP content in Blend-0.3 from injection molding.