In situ formation of zinc phthalate as a highly dispersed β-nucleating agent for mechanically strengthened isotactic polypropylene

Abstract

Introducing β-nucleating agents (β-NAs) is a convenient and reliable method for obtaining isotactic polypropylene (iPP) having a high β-crystal content. Nevertheless, the poor dispersion and agglomeration of β-NAs limit their nucleation efficiency. In this paper, a new strategy-involving in situ generation of β-NAs-is described to improve the dispersion of NAs in the polymer matrix. Here, zinc phthalate (ZnPht) is reported as a new β-NA for iPP; the self-dispersed ZnPht(IS) was generated in situ by adding its reaction precursors-phthalic anhydride (Pht) and zinc oxide (ZnO)-during the extrusion of iPP. Compared with the situation involving the pre-addition of ZnPht, the use of ZnPht(IS) resulted in a higher nucleation efficiency and greater selectivity toward the β-crystal form; indeed, the content of β-crystal forms (kβ) improved from 0.74 for ZnPht/iPP to 0.97 for ZnPht(IS)/iPP. Moreover, the impact strength of the iPP prepared using 0.2 wt% ZnPht(IS) was nearly 2.9 times greater than that of neat iPP; for the iPP nucleated with ZnPht, it was approximately 2.0 times greater at the same loading content. To explain these phenomena, we propose a new mechanism for the self-dispersion of β-NA during its in situ generation; evidence from Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy was consistent with our proposed mechanism. This strategy and method of preparing β-NA in situ has several attractive features that should encourage the production and application of β-iPP. Furthermore, this strategy might potentially be extended to solve the issues of poor dispersion and serious agglomeration for other additives in other polymers.