Influence of a nucleating agent on the crystallization behaviour of isotactic polypropylene and elastomer blends

Abstract

The influence of a nucleating agent on the crystallization behaviour of isotactic polypropylene (iPP), in their blends with poly(styrene-b-ethylene butylene-b-styrene) (SEBS), and a metallocenic ethylene–octene copolymer (EO) was investigated by DSC, optical microscopy and real-time small and wide angle X-ray scattering (SAXS and WAXS) experiments using synchrotron radiation. In non-nucleated iPP/SEBS blends, the crystallization of the iPP matrix occurred in the presence of the styrenic domains which induced a nucleating effect on the process, as observed in the synchrotron experiments. The metallocenic elastomer did not affect the crystallization behaviour of iPP in the iPP/EO blends in non-isothermal experiments, however, the development of crystallinity in the elastomer was restricted. In the nucleated isotactic polypropylene/elastomer blends a significant increase in the crystallinity and the crystallization rate of the iPP matrix was observed due to the presence of the nucleating agent. However, the nucleating efficiency of the additive was strongly affected by the nature and content of the elastomeric component. The nucleating agent efficiency was higher in the presence of the ethylene–octene component than the styrenic elastomer.

Introduction

Isotactic polypropylene, iPP, is one of the most important commercial thermoplastics with excellent properties. However, a great disadvantage which limits its application range is its low impact strength, especially at low temperature. A large number of studies have been carried out over the years to improve its toughness, and blending with an elastomer is one of the main methods used [1], [2], [3], [4]. The elastomers which have been employed in this type of blends are mainly ethylene–propylene rubber (EPR) and ethylene–propylene–diene terpolymer (EPDM) [5], [6], [7], [8], [9], [10], [11], [12], although styrene–ethylene butylene–styrene triblock copolymer (SEBS) [13], [14], [15], [16], [17], [18] and recently metallocenic ethylene–octene copolymers (EO) [19], [20], [21], [22] have also been tested.

It is well established in polymer blends that the incorporation of one component in order to improve a certain property may result in the detriment of another. It is often necessary to add several components in order to achieve an optimum balance of all the application properties. In the case of these polypropylene blends, the enhancement in toughness by blending with elastomers is accompanied by a decrease of stiffness [2]. However, the improvement of both the impact resistance and the stiffness of isotactic polypropylene has a practical importance in order to extend its range of applications. Several strategies have been used to achieve a balance between both properties and adding an inorganic filler or another semicrystalline polymer are amongst those most studied [23], [24], [25], [26].

On the other hand, the crystallization of iPP is controlled by the nucleation stage, and the addition of specific additives or nucleating agents to shorten the induction time of crystallization and accelerate the formation of crystalline nuclei is a technique commonly used in the polymer industry to shorten injection-moulding cycle times, thus reducing production costs. Furthermore, such agents generate smaller spherulites and increase crystallinity, thus improving the optical and mechanical properties. In previous studies [27], [28], [29], [30], [31], we have analyzed the influence of a series of nucleating agents on the structure, morphology, crystallization behaviour and mechanical properties of isotactic polypropylene. An organic phosphate derivative has been found to have a very high nucleating efficiency achieved even at very low concentrations which provoked an important increase in the flexural modulus [27], [31]. In this regard, the use of a nucleating agent as a third component in the isotactic polypropylene/elastomer blends appears to be a promising alternative to achieve a good balance of properties, although only very few studies have appeared [32], [33].

From the numerous published works on isotactic polypropylene/elastomer blends, there are few details available on the crystallization behaviour of the matrix, and in most cases the main interest is in the mechanical performance of the blends. However, in the presence of a nucleating agent which can significantly modify the crystallization behaviour of iPP, it seems very important to consider the effect of both the additive and the elastomer on the capacity of the matrix to crystallize.

The aim of this work is to investigate the influence of an organic phosphate derivative as a nucleating agent of the monoclinic phase of iPP in the crystallization process of iPP/SEBS and iPP/EO blends. The crystallization behaviour of the non-nucleated blends was determined under different crystallization conditions and as a function of the elastomer concentration, for comparison with the nucleated iPP/elastomer blends. This study is part of a broad project related to the development of high impact resistance and high modulus materials based on iPP and the influence of different parameters on these properties.