Structure and properties of polypropylene/graphene nanoplatelets microcomposites: effect of graphene size.

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The aim of this work is to assess the potential of graphene nanoplatelets (GNP) according to their production method to improve properties of composites based on polypropylene and elaborated by melt mixing. The effects of GNP size on structural, thermal, rheological and mechanical properties of composites were investigated. Three grades of GNPs, obtained by a thermomechanical process and commercialized by Knano under KNG-180, KNG-150 and KNG-G5 references, were considered in this work. KNG-180 and KNG-150 GNPs were obtained from graphite through a three step process: i) graphite was intercalated by sulfuric acid under stirring and then washed and dried, ii) then, it underwent a thermal treatment and finally iii) the obtained expanded graphite was ultrasonicated in hydroalcoholic solution. From this step, particles were considered as graphene nanoplatelets [1]. An additional ball milling process in good solvent, followed by centrifugation, enabled to obtain thinner KNG-G5 from KNG-180 [2]. Composites were elaborated by melt mixing in an internal mixer using different conditions and thus mixing energies. The dispersion state of the composites was evaluated at different scales using SEM, TEM and X-ray diffraction. Thermal, rheological and mechanical properties of the composites were investigated. Although mechanical delamination followed by centrifugation enables to improve dispersion state of GNP in thermoplastics, PP/GNP mixtures have to be considered as non-exfoliated microcomposites. Even if, different mixing conditions were investigated, they did not affect the dispersion state. However, thermal, rheological and mechanical properties of PP/KNG-G5 composites were outstandingly close to those of thermoplastic/montmorillonite or reduced graphene nanocomposites and show positive results regarding the use of this graphene manufacturing process without chemical treatment. Ackowledgements GNPs were kindly provided by the CNRS research group project n°3661 "GDR PolyNano". References 1 G. Chen, C. Wu, W. Weng, D. Wu, W. Yan, Preparation of polystyrene/graphite nanosheet composite, Polymer. 44, 1781 (2003) 2 W. Zhao, M. Fang, F. Wu, H. Wu, L. Wang, G. Chen, Preparation of graphene by exfoliation of graphite using wet ball milling, J. Mater. Chem. 20, 5817 (2010)

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