Influence of different carbon nanotubes on the mechanical properties of epoxy matrix composites – A comparative study

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Abstract

Carbon nanotubes (CNTs) in general are considered to be highly potential fillers to improve the material properties of polymers. However, questions concerning the appropriate type of CNTs, e.g., single-wall CNTs (SWCNT), double-wall CNTs (DWCNT) or multi-wall CNTs (MWCNT), and the relevance of a surface functionalisation are still to be answered. This first part of the study focuses on the evaluation of the different types of nanofillers applied, their influence on the mechanical properties of epoxy-based nanocomposites and the relevance of surface functionalisation. The nanocomposites produced exhibited an enhanced strength and stiffness and even more important, a significant increase in fracture toughness (43% at 0.5 wt% amino-functionalised DWCNT). The influence of filler content, the varying dispersibility, the aspect ratio, the specific surface area and an amino-functionalisation on the composite properties are discussed and correlated to the identified micro-mechanical mechanisms.

Introduction

In recent years, the development of nanocomposites has become an attractive new subject in materials science. Nanoparticles in general are regarded as high potential fillers to improve the mechanical properties of polymers. Furthermore, dependent on the applied type of filler, nanoparticles can influence the electrical and thermal conductivity of the final nanocomposite.

Interesting candidates with potentially unique properties are carbon nanotubes (CNTs) [1], [2], [3], [4]. CNTs exhibit an exceptionally high stiffness and strength [5], [6], [7], [8], a diameter dependent specific surface area (SSA) of up to 1300 m2/g [9], as well as an aspect ratio in the range of several thousands. According to their graphitic structure, CNTs possess a high thermal conductivity and an electrical conductivity, which can be either semi-conducting or metal-like.

The combination of the previously mentioned material properties makes CNTs highly desirable candidates to improve the properties of polymers. Besides, the development of CNT/polymer nanocomposites opens new perspectives for multi-functional materials, e.g., conductive polymers with improved mechanical performance and with a perspective of damage sensing and “life”-monitoring.

In order to efficiently exploit the potential of CNTs to improve the mechanical performance of polymers, one has to be aware of aggravated challenges when comparing with conventional micro-scaled filler particles. The extraordinary large SSA of nanoparticles, being several decades larger compared to conventional reinforcement fibres (e.g., short carbon fibres: SSA  1 m2/g), leads to special challenges, which can be summarised in (i) an appropriate dispersion of the reinforcements in the matrix, (ii) a sufficient interfacial bonding and (iii) receiving representative information about nano-structural influences (CNT-structure-property relationship and nano- (micro-) mechanical mechanisms).

The surface area of nanotubes can act as desirable interface for stress transfer, but undesirably induces strong attractive forces between the CNTs themselves, leading to excessive agglomeration behaviour. The SSA of CNTs is dependent on the diameter and the number of sidewalls [9], where a maximum is provided by SWCNTs. Additionally, SWCNTs have the largest aspect ratio compared to CNTs consisting of multiple layers. In order to minimise the SSA, SWCNTs form aggregates of bonded and aligned CNT bundles, called nano-ropes. These ropes, consisting of ten to hundreds of individual tubes, are difficult to separate and infiltrate with matrix.

The other extreme are MWCNTs. Having a much larger diameter and consisting of several concentric walls, these nanotubes provide a SSA of only 200 m2/g or less. Therefore, MWCNTs exhibit a much better dispersibility, but they provide smaller interface for stress transfer and a lower aspect ratio. Furthermore, the stress transfer between the concentric layers has to occur via interlayer shearing to be transferred by van der Waals forces, which are relatively weak. As a conclusion, MWCNTs can considered to be less efficient concerning a mechanical reinforcement. We also have chosen to use DWCNTs for this study, possessing a SSA of 600–800 m2/g, giving a compromise between dispersibility and reinforcing potential.

Various methods to disperse nanotubes in polymer resins, such as stirring and sonication, have been reported in the literature [10], [11]. Most of these methods are either limited in capacity or not powerful enough to separate the agglomerates into individual nanotubes. One common technique to distribute CNTs in epoxies is the sonication technique. A pulsed ultrasound separates the CNTs within agglomerates and disperses them in the matrix effectively. However, this method is only manageable for small batches due to the extreme reduction of the vibrational energy with increasing distance from the sonotrode. Another detrimental effect of this method is the reported rupture of the CNTs, caused by the local energy input [12], [13], [14], resulting in a reduction of the effective tube length.

Retaining a high aspect ratio of the CNTs, enabling an efficient load transfer to the nanotube, is of special interest towards an improvement of the mechanical performance of a CNT-reinforced polymer. The calandering method, introduced previously [15], has turned out to be effective with regard to the achieved dispersion and exhibits the opportunity of up-scaling the capacity to reach technical demands. Therefore, we believe this method to be a very promising approach to solve the issue of dispersion of nanoparticles for resin systems in the future.

The second issue in the development of CNT/polymer composites is the interfacial adhesion between the CNTs and the matrix polymer. A sufficient stress transfer from the matrix to the tubes is required to efficiently exploit the potential of CNTs as structural reinforcement. The interfacial bonding between CNT and matrix can be improved by functionalising the CNT-surface. The introduction of tailored chemical groups (e.g., amino-, carboxyl- or glycidyl-groups for epoxies) enables covalent bonding between CNTs and epoxy, improving the interfacial stress transfer and positively affecting the dispersibility of the nanofiller. The described effect of a functionalisation on the mechanical properties has been predicted by simulations [16] and also experimentally proved by previous work [15], [17], [19].

The third main issue is a comprehensive knowledge about the integrity of the CNTs resulting in improved effectiveness on the mechanical and physical properties. According to the production techniques, as chemical vapour deposition (CVD), electric arc-discharge method, laser ablation and other catalytic processes, the nanotubes possess deviations of mechanical and physical properties. Influencing parameters are the defect-density and distribution (degree of graphitisation), the curvature, the aspect ratio, the length- and diameter distributions, the density and the purity. Furthermore, one will find numerous variations in CNT length and diameter and a distribution of different chiralities in one batch as well. As a conclusion, similarities can be drawn to polymers, where it is a common and established procedure to describe properties and structural features by the use of distributions (chain length, molecular weight, tacticity, etc.).

The function of CNTs as electrically conductive additive is generally understood and has already been commercially exploited. The reinforcing potential of CNTs on the other hand is still widely undeveloped and needs further basic research. Progress reported in the literature [15], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], can be summarised in that way, that there are still difficulties in achieving a reinforcing effect of CNTs in epoxy matrices, to prove their potential as structural modifier.

Allaoui et al. [18] investigated the influence of MWCNTs in a rubbery epoxy matrix. The addition of up to 4 wt% MWCNTs led to a significant increase in strength and Young’s modulus. However, the suggested preparation method, using solvents (methanol), stirring and evaporation, did not lead to an adequate distribution. Agglomerates in the dimension of several tens of micrometers could still be detected by light microscopy. In this context, the authors would like to mention, that light microscopy may be a preliminary method in order to get an idea of the overall distribution of agglomerates, but it is definitely not suitable to substantiate the discussion about the dispersion of CNTs, due to the limited resolution, insufficient for nanofillers.

The same group [19] reported an influence of length and aggregate size of MWCNTs on the improvement in mechanical and electrical properties of a standard DGEBA-based nanocomposite. Composites containing 0.5–4 wt% CVD-grown MWCNTs exhibited an increased Young’s modulus, but a reduction in fracture strain. The reduction in fracture strain was explained in terms of the existence of agglomerates, leading to local defects enhancing early failure. Furthermore, a certain dependence of the mechanical behaviour on the aggregate size could be found.

Liao et al. [20] investigated the influence of SWCNTs on the thermo-mechanical properties of epoxy-based nanocomposites. The composites were produced by the use of sonication and the application of solvents and surfactants to disperse the nanotubes. A dependence of the achieved dispersion on the relative improvement of the mechanical properties was observed. The preparation route was considered to be an effective method in order to disperse CNTs. However, in their case, the incorporation of SWCNTs led to a reduction of the glass transition temperature, being explained by a reduction of the interfacial adhesion due to the use of a surfactant and the mentioned solvent.

Miygawa and Drzal [21] reported an increase in the storage modulus of epoxy-based nanocomposites containing fluorinated SWCNTs. A linearly decreasing glass transition temperature with increasing filler content was also observed. This result could indicate that a fluorination of CNTs may not be a suitable functionalisation to significantly improve the interfacial adhesion. A correlation of fluorinated SWCNTs to PTFE- or other matrix incompatible particles could be drawn, which would induce a similar behaviour.

The influence of oxo-fluorination of MWCNTs on the fracture toughness of epoxy matrix nanocomposites has been investigated by Park et al. [22]. A certain increase in fracture toughness by SENB-testing (single edge notch bending) was shown. The results were related to the modified surface polarity of the CNTs, leading to an improved bonding to the matrix by polar interactions. In contrast to the neat fluorination, this functionalisation generates additional hydroxyl groups on the CNT surface, enabling hydrogen bonds to the epoxy matrix.

Promising results, concerning a mechanical reinforcement of epoxy resins by the use of CNTs were recently reported by Zhu et al. [23], [24]. They showed a significant progress in improving the CNT dispersion and matrix adhesion by adding 1 wt% alkylamino-functionalised SWCNTs to an epoxy matrix thus, resulting in an increased strength and Young’s modulus. The presented functionalisation assimilates the polarity and directly incorporates the SWCNTs into the epoxy network. Their results were in agreement with our results [15], [17], [25] using smaller quantities of amino-functionalised CNTs. In addition to the improvement in strength and stiffness, an increased glass transition temperature was observed. This substantiates previous results [25], [26], reporting a similar influence of the direct incorporation of CNTs into the matrix by covalent bonding. Tailored functional groups induce the formation of covalent bonds to the matrix material. We would like to point out that the surface chemistry of carbon nanotubes (introduction of functional groups), could be regarded as the most important aspect in efficiently exploiting the benefits of CNTs for a mechanical reinforcement.

This brief description shows the diversity of possible variables in developing CNT/epoxy nanocomposites. A comparison of results reported is often difficult, due to the usage of different epoxy matrices, with different processing techniques and parameters, as well as the choice of CNTs with different pre-treatments (e.g., functionalisation), from various sources with again different types and qualities. Presently, CVD is regarded as the most relevant manufacturing process for CNTs, due to the high production capacity fulfilling technical and in the near future industrial demands as well [27], [28].

In this part of the study, we describe the influence of different types of CVD-grown CNTs (single-, double- and multi-wall CNTs) on the mechanical properties of epoxy-based nanocomposites. We produced nanocomposites using identical processing conditions, by systematically varying CNT-type, filler content and surface functionality. All results are compared to reference materials, as the neat epoxy matrix and a nanocomposite containing carbon black. Keeping most variables constant, it was the aim of this study to compare the influence of these different types of CNTs on the mechanical properties and to appraise the real potential of CNTs as structural modifiers of epoxy-based composites. The modification of the electrical and thermal conductivity by CNTs will be presented separately [32].

Section snippets

Epoxy matrix system

The epoxy matrix used in this study consists of a modified DGEBA-based epoxy resin (L135i) with an amine hardener (H137i), supplied by Bakelite MGS Kunstharzprodukte GmbH, Stuttgart/Germany. This epoxy system is a standard resin for infusion processes (e.g., RTM – resin transfer moulding) and it is characterised by its low viscosity (ηRT = 250 MPa s).

Carbon nanotubes

All CNTs used in this comparative study were produced via the CVD-route:

(a) Single-wall carbon nanotubes

Single-wall carbon nanotubes were purchased

Influence of SSA and surface functionalisation of the CNTs on the dispersibility in epoxy resin

Dispersing nanoparticles in a chosen matrix system is a challenging process. As already mentioned in the introduction, nanoparticles exhibit a SSA, which is several orders of magnitude larger than that of conventional fibre-reinforcements (e.g., SWCNT: SSA  1300 m2/g). In order to homogeneously disperse nanoparticles in the epoxy and to properly impregnate the potential surface area of the nanofiller, a huge energy input has to be ensured to overcome the nanotube–nanotube adhesion by van der

Summary and conclusions

Nanocomposites consisting of a DGEBA-based epoxy matrix system and different types of carbon nanotubes were produced by calandering. The composites were investigated regarding the dispersibility of the CNTs in the matrix and the efficiency concerning a mechanical reinforcement. The obtained data were discussed in terms of influencing parameters as (i) specific surface area (SSA), (ii) chemical functionalisation of the surface, and (iii) particle shape and correlated to identified

Acknowledgements

The companies Bakelite MGS Kunstharzprodukte GmbH/Stuttgart and Exakt Vertriebs GmbH/Hamburg, Germany are acknowledged for the supply of the epoxy resin and the mini-calander, respectively. The European Commission (Scientific-Network: “Carbon Nanotubes for Future Industrial Composites: theoretical potential versus immediate application (CNT-Net)”; Contract No. G5RT-CT-2001-050206) is gratefully acknowledged for financial support.

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