Flammability properties of polymer nanocomposites with single-walled carbon nanotubes: effects of nanotube dispersion and concentration
Introduction
There is a high level of interest in using filler particles having at least one nano-dimensional scale (nanofiller) for making polymeric nanocomposite materials with exceptional properties [1], [2]. One of the promising applications involves the improvement in flammability properties of polymers with nanofillers because one weak aspect of polymers is that they are combustible under certain conditions. These filled systems are attractive as possible alternatives to conventional flame retardants and furthermore they could simultaneously improve both physical and flammability properties of polymers. At present, the most common approach using nano scale particles is the use of layered silicates having large aspect ratios. The flame retardant (FR) effectiveness of clay-polymer nanocomposites with various resins has been demonstrated and several flame retardant mechanisms have been proposed [2], [3], [4], [5], [6], [7], [8], [9], [10]. It appears that the flammability properties of clay-polymer nanocomposites are not significantly affected by whether they are intercalated or exfoliated as long as they are nanocomposites rather than microcomposites. Significant reduction in heat release rate has been achieved with a clay content of about 5% by mass.
Carbon nanotubes are another candidate as a FR additive because of their highly elongated shape (high aspect ratio). This was demonstrated by using multi-walled carbon nanotubes (MWNT) in polypropylene (PP) [11], [12] and also in poly(ethylene vinyl acetate) [13]. The in situ formation of a continuous, network structured protective layer from the tubes is critical for significant reduction in heat release rate, because the layer thus acts as a thermal shield from energy feedback from the flame [12]. Single-walled nanotubes also have potential as flame retardants by the same mechanism. Despite reports of the exceptional physical properties of the nanocomposites with SWNT [14], [15], [16], [17], there are no published studies on the flammability of SWNT polymer nanocomposites. The dispersion of SWNT in polymers remains a challenge, so it is important to determine the effects of the nanotube dispersion on flammability properties. Thus, we investigate the effects of small quantities of single-walled carbon nanotubes and their dispersion in PMMA on the flammability properties of these nanocomposites.
Section snippets
Materials
The matrix polymer used in this paper is poly(methyl methacrylate) (PMMA) (Polysciences,1 Mw: 100,000 g/mol). SWNTs for the nanocomposites were synthesized by the high-pressure carbon monoxide method (HiPCo)[18]. The metal residue in the SWNTs is less than 13% by mass. The
Sample morphology
The distribution of the nanotubes in PMMA/SWNT(0.5%) was examined by optical microscopy to globally observe the dispersion of the nanotubes, as shown in Fig. 1. Fig. 1(a) indicates that the nanotubes are relatively uniformly distributed within the polymer matrix on a micrometer scale. By using a higher concentration of SWNT in the DMF suspension, the sample in Fig. 1(b) shows regions of nanotube aggregation. In this study, the former sample is designated as having ‘good dispersion’ and the
Discussion
The formation of a protective network layer covering the entire surface without any cracks or openings is critical for reducing the heat release rate and mass loss rate of the nanocomposites. Therefore, it is important to understand how the black discrete islands are formed instead of the formation of the continuous layer and how to avoid them. In the early stage of the gasification test, the upper part of the sample is heated and starts melting. When the temperature of the sample becomes high
Conclusion
PMMA/SWNT nanotube nanocomposites were prepared by the coagulation method and the effects of nanotube dispersion and concentration (up to 1% by mass) on the flammability properties of these nanocomposite were determined. A nanotube-containing network structured layer without any major cracks or openings was formed during the burning tests and covered the entire sample surface of the nanocomposite having good nanotube dispersion. However, the nanocomposite having poor nanotube dispersion or a
Acknowledgements
We thank Carbon Nanotechnologies Incorporated and Foster Miller Company for providing SWNTs, Mr Richard Harris for preparing the sample disks and Ms Caitlin Baum for making Fig. 12. F. Du and K. I. Winey acknowledge funding from the Office of Naval Research.
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