Elsevier

European Polymer Journal

Volume 41, Issue 8, August 2005, Pages 1838-1845
European Polymer Journal

Injection-moulded α- and β-polypropylenes: I. Structure vs. processing parameters

https://doi.org/10.1016/j.eurpolymj.2005.02.020Get rights and content

Abstract

The paper studies the effects of mould temperature and holding pressure on the structure of neat and β-nucleated isotactic polypropylenes. Commercially available isotactic polypropylene was modified with a β-specific nucleator based on N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide in the concentration of 0.03 wt.%. From both the original material (α-iPP) and the β-nucleated material (β-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 °C in 10 °C steps, while in the P-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 α-iPP specimens; the skin thinned out and the spherulite size increased with mould temperature rise. On the other hand, in the case of β-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 α-iPP and β-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 β-form content, particularly in the skin of specimens. On the contrary, higher holding pressure depressed the crystallinity proportionally within the bulk of specimens.

Introduction

Isotactic polypropylene (iPP) represents one of the most commonly used polymeric materials. It has acquired this position among other polymers because of its versatility, relatively good mechanical properties, easy processability and recyclability and, generally, favourable price-to-performance ratio [1]. Supermolecular structure and, consequently, also end-use properties of isotactic polypropylene are essentially influenced by crystallization conditions, in particular due to iPP polymorphism. Generally, three basic crystalline forms of iPP (α, β, γ) can be identified [2]. However, only the monoclinic α- and the trigonal β-form possess application relevance. Commercial grades of iPP crystallize essentially into the most stable α-form with a sporadic occurrence of the β-form [3]. Nevertheless, when special crystallization procedures are used or, in particular, specific nucleators are added, the β-form can become a predominant crystalline form in common iPP articles [4], [5]. Recent studies have proved close interrelations between the β-form content and significant increase of toughness and cold drawability [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. As a consequence, the β-nucleator-doped iPP (β-iPP) is classified among regular polymeric materials as a suitable counterpart of the neat isotactic polypropylene (α-iPP).

Almost a third of polypropylene production is processed by means of injection moulding because the process produces a complex finished part in a single rapid and automatic operation [1]. Injection-moulded articles are distinguished by a complex morphology. Three or more regions can be identified throughout the wall thickness ranging from a non-spherulitic highly oriented skin to a spherulitic central core [15]. In the case of β-iPP, the scenario is even more complicated since β/α-form proportion is strongly influenced by processing conditions. Generally, the skin of β-iPP injection-moulded parts is composed of pure or nearly pure α-form while the core consists of a high amount of β-form [16]. Indeed, particular structure characteristics such as skin thickness, spherulite size or overall polymorphic distribution are highly sensitive to processing parameters; the interrelations between processing parameters, structure and properties of injection-moulded β-iPP have been investigated by several research teams. The effect of melt temperature was studied by Fujiyama [16], whereas Varga et al. [17] focused on the influence of injection speed on the skin thickness, the β-form content and the mechanical properties of injection-moulded β-iPP. However, only a few papers dealing with the effect of mould temperature [18] and no work focusing on the influence of holding pressure on the structure and the mechanical properties of injection-moulded β-nucleated isotactic polypropylene have been found. Indeed, an intimate knowledge of the sensitivity of this material to the processing set-up is a key premise for its successful processing and industrial use.

Thus, this work represents a comprehensive study of the interrelations between important processing parameters (mould temperature and holding pressure), the supermolecular structure and the mechanical properties of injection-moulded α-iPP and β-iPP. Due to a large scope, the results will be discussed in two parts; this paper (Part I) focuses on the effects of selected processing parameters on the morphology, while the following paper (Part II) will deal with the influence of mould temperature and holding pressure on the tensile properties of injection-moulded α-iPP and β-iPP.

Section snippets

Materials

Commercially available isotactic polypropylene Mosten 58.412 (α-iPP) supplied by Chemopetrol Litvínov a.s., Czech Republic, was used as a basic material throughout the study. The material is characterized by a melt flow index of 3 g/10 min (2.16 kg, 230 °C, ISO 1133), a weight-average molecular weight approx. 320 000 (GPC) and an isotacticity index of 98% (ISO 9113). In order to prepare β-iPP, specific β-nucleating agent NJ Star NU-100 (N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide; Rika Int.,

Morphology

Cross sections of the α-iPP specimens injection-moulded at various mould temperatures are shown in Fig. 1. The morphology consists of a spherulitic core (right-hand side) and non-spherulitic skin (left-hand side). From the comparison of individual cuts, two basic effects of mould temperature (MT) on the morphology are evident. Firstly, the spherulite size increases with rising MT. This effect is caused, as generally accepted, by reducing the intensity of nucleation with the increase of

Conclusions

Specific sensitivity of the structure of α- and β-iPP injection-moulded specimens to the mould temperature and holding pressure has been examined. Polarized-light microscopy revealed strong effect of the mould temperature on the morphology of α-iPP. The spherulite size of α-iPP specimens increases with mould temperature rise while the skin is thinned. High nucleation efficiency of a β-specific nucleator is demonstrated by virtually constant spherulite size in the core of β-iPP specimens, while

Acknowledgments

This work was supported by the Czech Science Foundation (project numbers 106/02/P144 and 106/05/0550). The authors also thank Mr. Jan Hrbáček and Mr. Tomáš Marek for their kind help in measurements.

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