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
Graphical abstract
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.
Section snippets
Polymers
The two polyolefins used in this study are i) a polypropylene PPH 7060 (Total Petrochemicals, France), called PP; and ii) a high density polyethylene Lupolen 4261 AIM (LyondellBasell, France), called PE. The melt flow index (MFI) of these two polyolefins are 12 g/10min (503 K and 2.16 kg) and 15 g/10 min (463 K and 21.6 kg), respectively.
For the blends, 80 wt% of PP and 20 wt% of PE have been used. The corresponding notation is PP/PE.
Twin screw extruder
Polyolefin blends have been prepared using a ZSE 18 HPE
Impact of high shear and γ-irradiation on reactions affecting neat PP and PE
The absolute complex viscosity |η*| and the storage modulus G′ of PP and PE that underwent TSE and γ-irradiation are given in Fig. 3 a and b respectively. The rheological properties of neat PP and PE are given as references. As far as neat PP is concerned, both high shear TSE and γ-irradiation decrease both |η*| and G′ whereas the opposite behaviour is observed for PE. Hence, those results allow us to assume that, for PP, the main side reactions that occur are β-scissions of chains unlike for
Conclusions
In this study, it is shown that high shear extrusion do not create enough radicals through bond breakage nor enough cross recombination to improve significantly the mechanical properties of the PP/PE blend. Then, the radical concentrations were increased using γ-irradiation of PP/PE blends between extrusion and injection stages. The effect of such an irradiation was investigated looking at the morphological, thermal, rheological and mechanical characteristics. In addition, both experimental and
Acknowledgement
Authors wish to acknowledge Région Rhône Alpes for PhD grant for this project. They also thank Ionisos Company and as well as Dr Sophie Rouif for gamma-irradiation and valuable discussions. They wish to acknowledge P. Alcouffe and the ‘‘Centre de Microstructures et d'analyses, plateforme Lyon 1″ of the University Lyon1 for his assistance in the SEM characterization and valuable discussions.
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