Preparation of high melt strength polypropylene by addition of an ionically modified polypropylene

Polypropylene-graft-maleic anhydride (PP-g-MAH) ionically modified with zinc(II) (PP-g-MAHZn) in varying amounts of ionic associations were prepared via melt reaction. Absorption bands in FTIR-ATR and shifted binding energy in X-ray photoelectron spectroscopy showed the formation of zinc carboxylates in PP-g-MAHZn. Decreased crystallinity, gradually increased viscosities, and storage modulus with the increment of zinc carboxylates in the PP-g-MAHZn were investigated by thermal and rheological analysis, which were attributed to the restriction of chain mobility via ionic interactions between polymer chains and aggregations of ionic domains. Furthermore, PP-g-MAHZn was introduced to polypropylene as an additive to promote foamability, and the PP/PP-g-MAHZn blends displayed higher viscosities and storage modulus compared with pristine PP in rheological measurements. In addition, the PP/PP-g-MAHZn blends were foamed by supercritical CO₂ and cryogenically fractured surfaces were observed by scanning electron microscope. PP/PP-g-MAHZn foams presented high closed-cell content, uniformity of cells, and increasing cell density owing to the combined effects of rheological properties, while maintaining the cell structures and good dispersion of the PP-g-MAHZn as nucleating sites. These results reveal that PP-g-MAHZn is a suitable additive to produce high melt strength PP for foam production.