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 beta nucleating agent (beta-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 beta-PP content can be developed in Blend-0.3 only at slow cooling rates (<5 degrees C/min), whereas high alpha-PP content is formed at fast cooling rates. Isothermal crystallization analysis of Blend-0 indicates that PA6 is an effective NA for alpha-PP in the lower temperature range, whereas the alpha-nucleating effect disappears in the higher temperature range. Blend-0.3 can, therefore, be viewed as a system containing both alpha- and beta-NAs, simultaneously. PA6 is competing with beta-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 beta-NA rapidly because of the faster cooling rate. However, the alpha-nucleation effect from PA6 predominate at the lower temperature. This explains the difficulty in obtaining high beta-PP content in Blend-0.3 from injection molding. POLYM. ENG. SCI., 51:403-410, 2011. (C) 2010 Society of Plastics Engineers.