Self-diagnostic carbon nanocomposites manufactured from industrial epoxy masterbatches
Introduction
Carbon nanotubes and graphene are two carbonaceous materials which have been under extreme focus in recent years as additives for polymer matrix composites (PMC’s) [1], [2], exhibiting high thermal and electrical conductivity, chemical and thermal stability, and relatively high mechanical properties [3], [4], [5], [6]. A wide range of applications has been identified, such as gas detectors [7], [8], semi-conducting materials [9], [10], aerospace materials and structures [11], [12], [13] and different types of sensors [14], [15].
An area which has attracted particular attention is the field of nanocomposite self-diagnostic sensor materials [16]. Through the addition of the carbon nanoparticles to electrically non-conductive thermoplastic and thermoset matrices, electrically conductive nanocomposites can be produced, granted that the electrical percolation limit is reached or surpassed. Such composites display piezo-resistivity caused by three main factors: (1) the destruction of the percolation network through reduction of the number of connections between individual particles, (2) deformation of the particles present within the network causing an increase in their intrinsic resistivity due to dimensional change, and (3) change of current flow due to the tunneling mechanism [17], [18], [19], [20]. Piezoresistivity based self-diagnostic materials allow measurement of stresses where traditional sensors cannot be used and may help in streamlining designs for a variety of applications listed previously.
Masterbatches can be particularly useful for the large volume production of such nanocomposites. Masterbatches are concentrates of carbon nanoparticles embedded or pre-mixed in a selected matrix. From an industrial point of view, such masterbatches allow economical integration of advanced materials into existing thermoset/thermoplastic products production lines with minimum changes required as to manufacturing route. Apart from the manufacturing advantages, the embedded particles have a much lower chance of becoming airborne, hence presenting no health hazards, which have to be counteracted if nano-additions are used in powder form and can be economically unviable [21], [22], [23], [24], [25], [26]. However, very little to no literature exists pertaining to piezoresistivity of such nanocomposites manufactured using masterbatch technology.
The relationship between the weight percentage of nanoparticles and the resistivity of nanocomposites has been studied both for carbon nanotubes [27], [28] and graphene and its derivatives [29], with nanoparticles in powder form for thin film and bulk samples [30], [31], [32], [33], [34], [35], [36]. Masterbatches have been also used is such studies, both for thermoplastic and thermosetting matrices [30], [31], [32]. The general trend is that the higher the amount of addition of such nanoparticles, the lower the achieved resistivity. A clear relationship between attained resistivity and piezoresistive response has not been made, owing to differences in processing and materials.
Where piezoresistivity is concerned, lower concentrations of CNT’s, nearing the percolation threshold, result in a higher piezoresistive response. This has been seen for both thin films [37], [38], [39], [40], [41], [42], [43] and bulk materials [24], [44], [45], [46], [47], [48], [49]. Literature concerning the use of graphene and its derivatives is sparser compared to that for nanotubes and is also more recent. The conclusions regarding sensitivity and weight percentage are consistent with CNT studies, where a lower weight percentage yields higher sensitivity [50], [51], [52]. Some studies have investigated the piezoresistive response of multiple types of graphene based nanofillers at multiple weight percentages but have focused on film type materials utilizing nanoparticles in powder form [53], [54]. A majority of the studies however, focus on the piezoresistive response of one type of graphene/derivative nanoparticle in thin film form or as separate attachable sensors [51], [55], [56], [57], [58].
Strain measurement on piezoresistive samples simultaneous with electrical measurement becomes difficult when extensometers are used, prompting the use of non-contact extension measurement systems [49], such as optical Digital Image Correlation, used in the present study.
In the cited works, masterbatch technology is rarely used; comparative studies between CNT and graphene (or graphene derivative) materials made from masterbatches are scarce. With different dispersion and processing methods, the results vary greatly for the same weight percentage. To the best of the present authors’ knowledge, no publications have utilized a common, economically scalable manufacturing route for CNT and graphene/graphene derivative masterbatches or identified a clear relationship between electrical resistivity and piezoresistivity.
This work aims at bridging gaps in knowledge identified in the review presented above. It compares the electrical resistivity and piezoresistive response of epoxy matrix nanocomposites produced using an inexpensive, scalable production route from masterbatch precursors of SWCNT, MWCNT, graphene, reduced graphene oxide and nitrogen doped graphene as well as identifying a clear performance relationship. This information can be used as a starting point in self-diagnostic material development for particular applications as well as large volume production. The work also contributes to development of best practices for simultaneous measurement of electrical conductivity (placement of the electrodes) and extension (non-contact optical full-field methods).
Section snippets
Materials
Six masterbatches containing different carbon nanoparticles were used for the production of nanocomposite samples: single wall carbon nanotubes (SWCNT’s), two types of multiwall carbon nanotubes (MWCNT’s), graphene (G), reduced graphene oxide (RGO), and nitrogen doped graphene (NDG), with their properties provided by the manufacturer listed in Table 1.The range of aspect ratio values for CNTs was estimated based on given data.
Masterbatches were diluted in EPOLAM 2031 (DGEBA based) to the
Graphene and graphene derivatives
Fig. 2 shows SEM images taken of the fracture surface of G, NDG, and RGO samples (respectively) at 3000× and 12,000× magnifications.
The microstructure reveals significant differences in the fracture surfaces of G, NDG, and RGO samples. In G samples, agglomerates are difficult to identify and are comparatively smaller as compared to NDG and RGO samples. More importantly, the distances between the agglomerates seen in G samples are seen to be larger than those in NDG and RGO samples, with little
Conclusions
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Masterbatch-based self-diagnostic nanocomposites: Piezoresistive self-diagnostic nanocomposites have been displayed to be successfully produced using CNT masterbatch technology. They display the ability to sense tensile strain and provide a possibility to replace traditional sensors for composite structural health monitoring. Industrially scalable technology and techniques can be used for manufacturing, making them functional, economical, and health hazard-free. Graphene/derivative based
CRediT authorship contribution statement
Hassaan A. Butt: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization. Stepan V. Lomov: Conceptualization, Methodology, Formal analysis, Writing - review & editing, Visualization, Supervision. Iskander S. Akhatov: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision. Sergey G. Abaimov: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Project
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors of this publication would like to acknowledge Ivan Sergeichev, Stepan Konev and Boris Voloskov from the Mechanical Testing Laboratory at the Center of Design, Manufacturing and Materials at Skoltech for approving, arranging and conducting the required mechanical testing for this study. The authors would also like to acknowledge Yaroslava Shakhova from the Advanced Imaging Core Facility at Skoltech for authorizing and conducting the necessary microstructural analysis. Special mention
Data availability statement
The data is currently part of an ongoing study and can be provided upon reasonable request.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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