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

Abstract

Polypropylene (PP) is incorporated with four different grades (H100, M25, M5, and C300) of graphene nanoplatelets (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 smallsized 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.

Melt-Mixed 3D Hierarchical Graphene/Polypropylene Nanocomposites with Low Electrical Percolation Threshold

“…However, it could be associated with the existence of a multi-step percolation behavior where a tunneling mechanism dominates the current flow between the conductive particles [46,55]. Most importantly, the percolation threshold reported in this study is significantly lower than those reported previously for polypropylene [40,41,56] using melt mixing methods [27] and other similar polymers [57]. An overview of the reported electrical percolation values for a number of different polypropylene/graphene systems prepared with various melt mixing protocols is presented in Table 1.…”

“…This behavior can be associated with nucleation effects and has been reported in the past for a number of fillers including clay, nanotubes and graphitic platelets [35,61,62]. Polypropylene has a very rich polymorphism form and it has been reported that filler inclusion has an effect of the overall morphology of the system [56]. For this reason, we have investigated the effect of the filler in the overall crystallinity and the resulting morphology.…”

“…Graphene has been reported of having superior barrier properties [2,3], therefore it is expected that it can improve the material thermal stability by creating a tortuous path to the various volatiles that are created by thermal decomposition in the bulk of the sample. In other words, we can expect two competing mechanisms to be present namely efficient heat transfer into the bulk of the sample and creation of barriers that can slow down thermal decomposition [56]. In our system, we observe that even at low filler loadings of the order of 0.2 or 0.5 wt.% the thermal stability of the material is substantially increased indicating that the material benefits from the existence of a tortuous path created by the filler.…”

“…Enrichment of charges in the bright area cause the electrical interaction with the AFM tip, in particular disturbing it therefore creating a phase contrast, thus indicating that different parts of the filler contribute differently to the conduction process. Polypropylene has a very rich polymorphism form and it has been reported that filler inclusion has an effect of the overall morphology of the system [56]. For this reason, we have investigated the effect of the filler in the overall crystallinity and the resulting morphology.…”

“…This improvement in thermal stability can be associated with the inherent property of graphene to act as a mass transport barrier that inhibits the transport of polymer degradation by-products to the surface [43]. Traditionally, the polymer decomposition at the initial stages has been reported to reduce the material thermal stability when carbonaceous fillers are used [43,56,69]. In this work such behavior is not observed as we can see the general trend of increasing thermal stability to be maintained even at low loadings (see Figure S1).…”