Comparative study of gamma-irradiated PP and PE polyolefins part 2: Properties of PP/PE blends obtained by reactive processing with radicals obtained by high shear or gamma-irradiation

Highlights

• Addition of a γ-irradiation step in a PP/PE reactive blending.

• Radicals are not the same in our PP/PE blend and in the genuine γ-PP and γ-PE.

• Secondary interactions and covalent bonds at the interface of the γ-PP and γ-PE.

• Improvement of mechanical properties with γ-irradiation.

Abstract

Compared with neat polypropylene (PP), the addition of low contents (typically <20wt %) of polyethylene (PE) can improve the impact strength of PP based blends. In order to improve tensile properties as well, reactive extrusion has been carried out with radicals obtained by high shear or γ-irradiation. The addition of a γ-irradiation step in the processing sequence, between extrusion and injection, can be considered as an efficient way for improving the properties of our PP/PE blends. The improvement of mechanical properties has been discussed in terms of creation of secondary interactions or covalent bonds at the interface of the γ-irradiated polymers, γ-PP and γ-PE. On one hand, the rheological measurements have highlighted an increase of the complex viscosity of the γ-irradiated blends at low frequencies. On the other hand, the EPR spectrum simulation shows that the radicals are not the same in the blend and in the genuine γ-PP and γ-PE. Besides, the increase of temperature in the sample affects the shape of the EPR spectra, revealing that some macroradicals react one to another, which creates new macroradical structures of rather alkoxyl type.

Introduction

Polypropylene and polyethylene are among the most common polyolefins in the plastic worldwide production. Individually, they are used in a lot of diversified fields with applications owing to their particular mechanical properties and chemical inertness [66], [48], etc. Mixing them together offers many intermediate desirable properties even if they are not miscible that can be a limitation to the development of their blends [1], [39], [58], [69]. Nevertheless, a lot of studies have been developed over the past two decades to improve their properties by several routes based mainly on i) controlling the cooling rate [9], [58], [61], [65], the amount and composition of the dispersed phase [1], [7], [29], [63], ii) adding copolymers or fillers in the formulations [ [8], [14], [15], [16], [25], [26], [27], [30], [31], [34], [35], [36], [38], [39], [51], [70]]. Most of these studies point out the presence of some miscible domains between polypropylene PP and polyethylene PE under specific conditions. Furthermore, the addition of little amounts of copolymers or fillers has an impact on the morphology of the blends as well as on their final mechanical properties. Authors put an emphasis on the direct impact of copolymers or fillers on the interfacial tension between PP and PE, by creating a “core–shell” structure between the polymer matrix and the dispersed phase. This structure can significantly enhance the mechanical properties.

Other studies investigate the reactive blending of polyolefins. In this context, γ-irradiation is gaining interest. Initially applied to sterilize implants in the medical industry [ [17], [37], [50]], this technology is now used to modify both molecular structure and chemistry of materials in the solid state (Part I [5], [19], [20]; 2006 [12], [23], [42], [49], [53], [56], [57]; Valenza 1993). It is based on the creation of macroradicals through covalent bond cleavage without necessity of dissolving and/or purifying the materials [11]. The main chemical transformations in the polymer are due to oxidations, cross-linking and further chain scission reactions.

The main purpose of γ-irradiation process is often to increase the compatibility between polymers or between polymers and fillers. For instance [3]; tried to improve the compatibility of polyamide 6 PA6 and PP by addition of a maleic anhydride grafted polypropylene obtained under γ-irradiation of polypropylene and maleic anhydride. They observed the formation of peroxides in a pre γ-irradiated PP for doses up to 100 kGy that favour direct chemical interactions with the amino groups of PA6, thus increasing the miscibility of both phases as revealed by smooth surface fractures [67]. have investigated the effect of γ-irradiation over the mechanical properties of PA/linear low density polyethylene LLDPE blends. Adding 10 wt % of γ-LLDPE increase the elongation at break from 8 to 21% and the impact strength from 39 to 65 J m⁻¹ (with Izod method) compared to the neat blend, without any other treatment or addition of compatibilizer. Another application of γ-irradiation consists in enhancing the coating properties of polyolefins [6]. have studied the impact of γ-irradiation on the mechanical properties of PP filled with sisal or wood flours. Improvement of elongation at break was marked out for the composites obtained with γ-irradiated PP matrices at 25 kGy (i.e. 25 kJ kg⁻¹). Moreover, doses higher than 25 kGy do not affect the impact strength of the composite while this latter decreases for neat γ-irradiated PP.

The objective of this work is to reinforce PP/PE blends with radical reactions induced by physical routes: i) high shear or ii) γ–irradiation. Whereas no residual macroradicals have been detected on our samples processed under high shear, their presence after γ–irradiation was confirmed using Electron Paramagnetic Resonance EPR spectroscopy according to part 1 [71] of this study. Then, parameters influencing the radical reactions – shear or γ–irradiation – were studied with particular attention on thermal, rheological, morphological as well as mechanical properties. The reinforcement expected during the blending process should enable to manufacture lighter parts from our materials. In addition, the present study proposes an alternative for polymer recycling without need of perfect sorting [39], [10] as it is a mere adjustment of the classical processing steps and formulation that adds value to polyolefin blends.