1. Introduction Graphene is a single layer of carbon atoms packed in a honeycomb structure [1]. Upon the discovery of free standing graphene in 2004, a revolutionary amount of research has been conducted to fully ex- ploit the exceptional properties of graphene [2–9]. However, it has been considered extremely challeng- ing to commercialize advanced products fabricated with graphene due to inability in mass production of high-quality graphene at low cost. Recently, graphene nano-platelets (GnPs) have been recognized as an inexpensive alternative to graphene due to its possi- bility of mass production at low cost [10, 11]. GnPs are layered graphene nano-crystals in the structure of platelets stacked by van der Waal’s forces [12]. The incorporation of GnPs into conventional poly- olefins has promising potential in a vast array of ap- plications due to its excellent thermal, mechanical, and electrical properties. Such composites can be employed in the fields of electronics, automobile, aerospace, sensors, and many more [13, 14]. Among many polymers, polypropylene (PP) is one of the 885 A study on the effects of graphene nano-platelets (GnPs) sheet sizes from a few to hundred microns on the thermal, mechanical, and electrical properties of polypropylene (PP)/GnPs composites Y-S. Jun, J. G. Um, G. Jiang, A. Yu * Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, N2L 3G1 Waterloo, Ontario, Canada Received 5 March 2018; accepted in revised form 16 May 2018 Abstract. Polypropylene (PP) is incorporated with four different grades (H100, M25, M5, and C300) of graphene nano- platelets (GnPs) via twin screw extrusion followed by injection moulding. The composites’ thermal stability, crystallization behaviour, tensile strength, and electrical property are carefully examined. The thermal stability is significantly enhanced with the incorporation of small-sized GnPs as shown by the 11.2% improvement in T 5% (the temperature at which 5 wt% of the mass loss occurs) and 5.1% improvement in T max (the temperature at which the maximum loss rate occurs). The thermal stabilizing effect of fillers can be significantly enhanced when they are well distributed with less aggregation as is the case for small-sized GnPs. The GnPs show a considerable nucleating effect on PP by increasing the crystallization temperature (T c ). The greatest improvement in tensile property is achieved with the use of small-sized GnPs. A 33.0% enhancement in tensile strength and 59.1% improvement of tensile modulus are obtained with the use of C300 and M5, respectively. The significantly increased thermal stability and mechanical property with small-sized GnPs are due to the fact that these small- sized fillers achieve a high degree of dispersion with less agglomeration as shown in the scanning electron microscope (SEM) images. However, the fillers with a large sheet size are still beneficial for purposes concerning electrical conductivity since the lowest percolation is obtained with H100. The greater the size of the GnPs, the smaller the percolation threshold of composites is exhibited. Keywords: polymer composites, polypropylene, graphene nano-platelets, electrical conductivity, tensile property eXPRESS Polymer Letters Vol.12, No.10 (2018) 885–897 Available online at www.expresspolymlett.com https://doi.org/10.3144/expresspolymlett.2018.76 * Corresponding author, e-mail: aipingyu@uwaterloo.ca © BME-PT