Injection-moulded alpha- and beta-polypropylenes: I. Structure vs. processing parameters

Abstract

The paper studies the effects of mould temperature and holding pressure on the structure of neat and beta-nucleated isotactic polypropylenes. Commercially available isotactic polypropylene was modified with a beta-specific nucleator based on N,N'-dicyclohexylnaphthalene-2,6-dicarboxamide in the concentration of 0.03 wt.%. From both the original material (alpha-iPP) and the beta-nucleated material (beta-iPP), dog bone-shaped test specimens were injection-moulded, using two sets of processing parameters. In the T-set the mould temperature was varied within the range of 40-120 degrees C in 10 degrees C steps, while in the beta-set the holding pressure was changed from 5 to 13 MPa in 1 MPa steps. Other processing parameters were kept on the same level. Polarized-light microscopy showed a strong effect of mould temperature on the morphology of alpha-iPP specimens; the skin thinned out and the spherulite size increased with mould temperature rise. On the other hand, in the case of alpha-iPP only the skin thickness was correspondingly affected, while the spherulite size remained virtually constant, independent of the mould temperature changes. At the same structure level, both alpha-iPP and beta-iPP specimens were insensitive to holding pressure variations. Polymorphic composition derived from wide-angle X-ray scattering displayed similar range of changes induced by variations of the processing parameters for both materials. The increase of mould temperature positively influenced the crystallinity and the P-form content, particularly in the skin of specimens. On the contrary, higher holding pressure depressed the crystallinity proportionally within the bulk of specimens. (c) 2005 Elsevier Ltd. All rights reserved.