Elsevier

Carbon

Volume 50, Issue 15, December 2012, Pages 5380-5386
Carbon

Size and synergy effects of nanofiller hybrids including graphene nanoplatelets and carbon nanotubes in mechanical properties of epoxy composites

https://doi.org/10.1016/j.carbon.2012.07.021Get rights and content

Abstract

We study mechanical reinforcement in a widely used epoxy matrix with the addition of graphene nanoplatelets (GnPs) and various mixture ratios of carbon nanotubes (CNTs) with GnPs. Two different dimensions of GnPs were used with flake sizes of 5 μm and 25 μm to investigate the influence of nanofiller size on composite properties. In GnP reinforced composites, bigger flakes showed greater reinforcement at all GnP concentrations as they actively control the failure mechanisms in the composite. In the mixture samples, highest CNT content (9:1) showed marked improvement in fracture toughness of 76%. The CNT:GnP ratio is an interesting factor significantly influencing the properties of the epoxy based nanocomposites. The combination of high aspect ratio of CNTs and larger surface area of GnPs contribute to the synergistic effect of the hybrid samples. Thermal conductivity consistently increases with incorporation of GnPs in the matrix. Transmission electron microscopy (TEM) images confirm the uniform nanofiller dispersion achieved in the composites. For the hybrid samples CNTs are seen to align themselves on the GnP flakes creating an inter-connected strong nanofiller network in the matrix. The homogeneous nanofiller dispersions have been achieved by high shear calendaring which is a method capable of being industrially scaled up.

Introduction

Ever since the discovery of carbon nano materials their remarkable properties have attracted large attention. Due to their unique structures CNT and graphene are two of the toughest materials ever tested and possess excellent electrical, thermal and electronic transport properties [1], [2], [3]. In case of mechanical properties, a single graphene layer exhibited Young’s modulus of ca: 1 TPa, and strength of ca: 130 GPa [2]. Such promising properties have made them ideal candidates for polymer reinforcement. Polymer composites reinforced with CNT [4], [5], [6], graphene [7], [8] and graphite nanoplatelets [9] have exhibited significant improvements observed in the mechanical and other properties. It has been reported recently that graphene/graphite nanoplatelets, which are commercially available and are cost effective are being successfully used as nanofillers for composites with enhanced functional properties [10], [11]. The larger surface area of GnPs increases the contact area with the polymer matrix maximizing the stress transfer from the polymer to the nanoplatelets. However, like CNTs the problem of dispersion is strongly present for GnPs as well due to strong van der Waals forces and π−π inter-planer stacking. Commercial GnPs are available in a variety of particle sizes and it is important to determine the influence of size if any, on the properties of the composites made with GnPs.

Hybrid composites have lately attracted the attention of researchers with different mixtures being tried out e.g. multi-walled CNT with carbon black [12] few layer graphene with single walled CNT and nano diamonds [13] and MWCNT with GnP [14]. It is an open question to completely understand the synergistic effect brought about by the combinations. Improvement in mechanical properties and thermal conductivity have been reported but the ratio of the nanofiller mixture is an important factor governing the reinforcing capabilities of the composites [13], [14]. Kumar et al. [15] suggested that by bringing together two nanofillers like CNT and GnP they form a co-supporting network of both fillers like a hybrid net structure in which the platelet geometry shields the tube fillers from fracture and damage during processing whilst still allowing full dispersion of both during high power sonication, thus causing improved properties. Shin et al. reported ultra-tough fibers containing CNT- reduced graphene oxide flake mixture. This is attributed to the formation of an interconnected network of partially aligned nanofillers during solution spinning [16].

In this paper we investigate the size effect of the GnPs on the mechanical and thermal properties of epoxy/GnP composites. We compare two commercially available varieties of GnPs which differ in their surface area (flakes of size 5 μm and 25 μm respectively) but have similar thicknesses. Varying amount of both kinds of GnPs were used to synthesize composites and their mechanical and thermal properties were thoroughly studied. We selected the GnP size that demonstrated higher improvement in composite properties to prepare the hybrid composites with MWCNT to study the synergistic effects of the two highly potent nanofillers in the matrix. A relatively low nanofiller concentration of 0.5 wt.% was selected for the mixture samples as from our previous research [17], [18] we have observed that at this concentration there is substantial improvement in properties and a good nanofiller distribution can be achieved by the employed dispersion technique. Different nanofiller ratios were tried out to understand the optimum loading for improved properties and maximum synergy achievable.

Section snippets

Materials

Commercially available graphene nanoplatelets (GnP) from XG Sciences Inc., (xGnP), Michigan, USA were used as fillers for the composites. The grade M GnPs are constituted of a few sheets of graphene stacked together and has thickness ∼6–15 nm. Two different GnP particle sizes of average diameter 5 μm and 25 μm as shown in Fig. 1a and b respectively were used to study the influence of size variation in properties. Multi-walled carbon nanotubes (MWCNT) from Nanocyl SA, Belgium (NC 3150), were used

Dispersion of nanofillers

Nanofiller dispersion is a well-known challenge since CNT and GnP have an inherent tendency to agglomerate and good dispersion is an important factor for mechanical reinforcement [17]. The main reasons behind agglomeration is the van der Waals attraction and the large surface areas of the nanofillers [21], for GnPs additional π–π bonding also accounts for the stacking of the individual sheets. The CNTs were acid oxidized to inhibit agglomeration and ensure better linkage with the polymer

Conclusions

The GnP particle size has a pronounced effect on the mechanical properties and thermal conductivity of the epoxy composites. The larger flakes succeed in better reinforcement of the composites as observed from both fracture toughness and three-point bending tests highlighting the importance of nanofiller size in this class of nanoparticles. The fracture toughness increased steadily up to 82% over neat epoxy with 2 wt.% GnP of 25 μm size and the flexural modulus exhibited the highest improvement

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