Preparation and characterization of graphite nano-platelet (GNP)/epoxy nano-composite: Mechanical, electrical and thermal properties

Highlights

• Mechanical, thermal and electrical properties of graphite nano-platelet/epoxy composite.

• Electrical percolation threshold was achieved at 0.3 wt%, which is higher than the percolation threshold of carbon nanotube/epoxy composite.

• Improvement in the fracture toughness of pure epoxy by 36% due to the geometry of the filler.

• Crack deflection and micro-cracking are the major toughening mechanisms in GNP/epoxy composite.

Abstract

Epoxy based polymer nano-composite was prepared by dispersing graphite nano-platelets (GNPs) using two different techniques: three-roll mill (3RM) and sonication combined with high speed shear mixing (Soni_hsm). The influence of addition of GNPs on the electrical and thermal conductivity, fracture toughness and storage modulus of the nano-composite was investigated. The GNP/epoxy prepared by 3RM technique showed a maximum electrical conductivity of 1.8 × 10⁻⁰³ S/m for 1.0 wt% which is 3 orders of magnitude higher than those prepared by Soni_hsm. The percentage of increase in thermal conductivity was only 11% for 1.0 wt% and 14% for 2.0 wt% filler loading. Dynamic mechanical analysis results showed 16% increase in storage modulus for 0.5 wt%, although the Tg did not show any significant increase. Single edge notch bending (SENB) fracture toughens (K_IC) measurements were carried out for different weight percentage of the filler content. The toughening effect of GNP was most significant at 1.0 wt% loading, where a 43% increase in K_IC was observed. Among the two different dispersion techniques, 3RM process gives the optimum dispersion where both electrical and mechanical properties are better.

Introduction

Graphite nano-platelets (GNPs) are a new class of filler which consist of small stacks of graphene and are usually 1–15 nm thick. Compared with clay, they have a similar layered structure but with better mechanical properties. Generally, these graphite nano-platelets are prepared by intercalating graphite either with metal ions or by acid treatment. This is further exfoliated by thermal treatment to yield GNPs [1], [2]. Similar to carbon nanotubes (CNTs), these two-dimensional layered structures also possess excellent electrical and thermal conductivities along with high modulus. However, the above mentioned properties strongly depend on the number of layers stacked in GNPs, the degree of crystallinity in the graphitic plane, their aspect ratio and the order of stacking [3], [4]. Perhaps the most interesting application for this material is to use them as filler in composite structures. They have already shown promising results in the field of polymer composites as sensor, thermal interface materials and to create electrically conducting polymers [5], [6]. Polymers are well known for their specific strength and flexibility but have poor fracture toughness. To overcome this shortcoming, often they are reinforced with micron-sized or nano-sized fillers like silica, nano-clay, carbon black (CB) and carbon nanotubes (CNTs) [7]. Similar to other polymer nano-composites, when used as filler in a polymer matrix, the glass transition temperature, modulus and fracture toughness of the polymer is improved upon addition of GNPs [8], [9], [10].

Though, CNTs are ideal for toughening the particles, one disadvantage is the increase in viscosity due to entanglement and high surface area of the tubes which affects the processing. Unlike CNTs, GNPs have a 2-D layered structure which has higher surface area enabling better stress transfer and also lower viscosity of the composite compared to CNTs during fabrication [11]. However, the already existing problem of dispersing the nanofillers in a polymeric matrix and improving the compatibility between the filler and matrix holds good for dispersing GNPs too. Owing to the large specific surface area, they are more prone to form agglomerates which in turn will affect the overall properties of the nano-composite [12], [13]. The effect of the dispersion method on the properties of graphene/graphite nano-platelet composite and functionalization of the filler are being investigated extensively [14], [15].

Investigations on the mechanical aspects of GNP/epoxy, pertaining to its fracture toughness and flexural modulus have shown significant improvement with an increase in the loading and are greatly dependent on the lateral flake size of the filler [16]. In another study on graphene epoxy composite at low filler content of 0.1 wt%, a noticeable increment of 31 wt% in modulus and 40% in fracture toughness was observed [17].

Thermal conductivities of GNP/silicone composite prepared by three-roll milling technique improved by 18% for 25 wt% loading of commercially available GNPs [18]. Comparing the thermal conductivities using different fillers like neat graphite, expanded graphite and graphene nano-platelets in an epoxy matrix, there was a marginal increase in thermal conductivity for GNPs and this value decreased as the filler content increased beyond 0.6 vol%. This is attributed to the poor dispersion of the filler [19], [20]. Another study based on non-covalent functionalisation of graphene sheets dispersed in epoxy reported 800% increase in thermal conductivity for 4 wt% of filler loading. This is attributed to strong interaction between the matrix and filler, which reduces the interfacial thermal resistance and also homogeneous dispersion due to functionalisation [21].

The object of this work is to analyse the influence of addition of GNPs to epoxy matrix processed by three – roll milling technique, in terms of mechanical, thermal and electrical properties. To analyse the effect of processing technique on the electrical properties, an additional processing method was employed. This study gives an overview of the overall performance of the GNP/epoxy nano-composite prepared by 3RM technique.