Mechanical properties and electrical conductivity of carbon-nanotube filled polyamide-6 and its blends with acrylonitrile/butadiene/styrene

Abstract

Composites of polyamide-6 and carbon nanotubes (NT) have been prepared on a corotating twinscrew extruder. It is shown by transmission electron microscopy (TEM) that the nanotubes are dispersed homogeneously in the polyamide-matrix. The electrical conductivity of these composites was analyzed and compared to carbon black filled polyamide-6. It is found that the NT-filled polyamide-6 shows an onset of the electrical conductivity at low filler loadings (4–6 wt%). In agreement with rheological measurements this onset in the conductivity is attributed to a percolation of nanotubes in the insulating matrix polymer. Tensile tests of the NT-composites show a significant increase of 27% in the Young's modulus, however the elongation at break of these materials dramatically decreases due to an embrittlement of the polyamide-6. Blends of these composites and Acrylonitrile/butadiene/styrene (ABS) have been prepared by extrusion. It is shown by TEM measurements that the nanotubes are selectively located in the polyamide-6. These selectively filled polyamide-6/ABS-blends show a highly irregular, cocontinuous morphology. Due to the confinement of the conductive filler to one blend component these materials show an onset in the electrical conductivity at very low filler loadings (2–3 wt%). These findings are explained by a double percolation effect. The NT-filled blends show superior mechanical properties in the tensile tests and in IZOD notched impact tests.

Introduction

In recent years, there has been intense interest in multiphase polymer blends due to the potential opportunities of combining the attractive features of each blend component while at the same time reducing their deficient characteristics [1], [2]. However, binary blends of immiscible polymers generally exhibit poor mechanical properties due to a coarse and often unstable morphology, such as in polypropylene/polyamide blends [3]. Therefore it is essential to control and stabilize a desired type of morphology in a polymer blend in order to generate polymeric materials with favorable properties.

In the past it was shown that a cocontinuous morphology often accounts for a synergistic improvement of the blend properties [4], [5], [6], [7]. The thermodynamically unstable cocontinuous morphology is often observed for blends of a composition near phase inversion. Such morphologies can be stabilized by the use of a compatibilizer, which resides at the polymer/polymer interface and reduces the interfacial tension between the blend components [8], [9], [10] and prevents coalescence via steric stabilization [11], [12], [13], [14]. The compatibilizer can either be premade or formed in situ during melt processing (reactive compatibilization).

In the development of blends based on polyamides the concept of reactive compatibilization has been described extensively [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. Especially blends of polyamide-6 and acrylonitrile/butadiene/styrene (ABS) are of significant commercial interest, due to their improved toughness particularly at low temperatures. ABS consists of butadiene rubber dispersed in a matrix of poly(styrene-co-acrylonitrile) (SAN). The preferred route to compatibilze these blends has been to add a polymer, which is capable of reacting with the amine-functionality of the polyamide and which is also miscible with the SAN phase of ABS. The use of terpolymers of styrene, acrylonitrile and maleic anhydride as effective compatibilizers for polyamide-6/ABS-blends is described extensively in patents [25], [26], [27].

Steinmann et al. have shown that a cocontinuous morphology can also be stabilized by selectively filling one phase of a binary blend [28]. PS/PMMA-blends in which the PMMA phase was selectively filled with nanoscale glass beads were characterized with respect to the stability and the broadness of the phase inversion region. It was found that the filler not only stabilizes the cocontinuous morphology observed at the phase inversion concentration but also broadens the phase inversion region, the concentration range in which cocontinuity can be observed.

A common characteristic of most polymeric materials is their high electrical resistivity, which accounts for the insulating character of these materials. While for many applications this feature is highly desirable, there are also numerous applications for which an electrically conducting behavior would be advantageous. The conventional method to enhance electrical conductivity of polymers is to compound the polymer with conductive carbonaceous fillers. In this case with increasing filler content the electrical conductivity increases by orders of magnitude at the percolation threshold, which depends on the dispersion state and geometry of the filler. In order to obtain materials with high conductivity, high loading of conductive filler is needed. This, however, not only increases the final cost of the material due to the high cost of the filler, but also often impairs the mechanical properties of the material.

Recently, increasing attention has been given to the incorporation of conductive fillers into immiscible polymer blends in order to improve the conductivity at much lower filler contents due to a double percolation phenomenon. Double percolation refers to the percolation of a filler within one phase of a polymer blend (first percolation), which itself percolates in the blend (second percolation). Sumita et al. have shown that for HDPE/iPP blends electrical conductivity can be observed at carbon black loadings of less than 1 phr [29]. In this case, the filler is located preferentially in the HDPE domains of the blend. The electrical conductivity is explained by a double percolation phenomenon. Within the HDPE domains the filler percolates which causes a conductivity of the HDPE domains. These domains percolate in the blend to form a continuous phase. In a second publication Sumita et al. have examined the conductivity of short carbon fiber filled HDPE/iPP blends [30]. In this case the percolation threshold of the filler should be considerably lower than in the case of spherical fillers due to the high aspect ratio of fibers. It was shown that the percolation threshold could be reduced considerably by selectively filling only the HDPE domains. However, in both publications Sumita et al. have not examined the effect of the filler on the mechanical properties of the blend.

Soares et al. have shown that the electrical conductivity of polystyrene–rubber blends loaded with carbon black which is localized at the interface is dramatically decreased [31]. Obviously, only a match of interaction energies of all blend components yields such a morphology. Their results allow to conclude that the type of carbon black and the locus of its action is decisive for improved conductivity.

Recently carbon nanotubes have been described as effective conducting fillers for polymers. Nanotubes can be described as long and slender fullerenes, in which the walls of the tubes are hexagonal carbon (graphite structure). These tubes can either be single walled (SWNT) or multi-walled (MWNT). In a recent review, Thorstenson et al. have described the use of nanotubes to produce composite materials with superior electric properties [32]. Moreover, carbon nanotubes are described to significantly improve the stiffness of polymers due to the high elastic modulus of nanotubes.

Only in a very recent paper by Pötschke et al. the influence of processing conditions on the state of dispersion of MWNT in polymeric matrices has been described [33]. It was shown that the nanotubes have been dispersed uniformly through the matrix by compounding a polycarbonate master batch with the corresponding neat polymer in a twin-screw compounder. No signs of segregation or depletion of MWNT at the surface of extruded samples was found.

From the findings stated above, one can presume a number of effects caused by the addition of carbon nanotubes to polyamide-6/ABS-blends. Firstly, the stability of the cocontinuous morphology should be improved by the addition of a reactive compatibilizer and the filler, if one manages to confine the filler to one phase of the blend. Secondly, in this case electrical conductivity should be observed at rather small loadings of nanotubes in the blend due to a double percolation phenomenon [34]. Thirdly, due to the high stiffness of nanotubes the mechanical properties should be influenced substantially.

It is the aim of this paper to investigate systematically the filler dispersion and morphology of carbon nanotube filled polyamide-6 and its blends with ABS with respect to the electrical and mechanical properties of these materials. The percolation of carbon nanotubes is described by measurements of the electrical conductivity and is compared to results derived from rheological experiments.