Injection-moulded α- and β-polypropylenes: I. Structure vs. processing parameters
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.
References (21)
- et al.
Polymer
(1996) - et al.
Polymer
(1996) - et al.
J Mater Process Technol
(1997) - et al.
Polypropylene—the definitive user’s guide and databook
(1998) - et al.
J Appl Phys
(1959) Crystallization, melting and supermolecular structure of isotactic polypropylene
J Macromol Sci Phys
(2002)Polym Eng Sci
(1996)- et al.
Polym Eng Sci
(1996) - et al.
J Appl Polym Sci
(1996)