Flammability properties of polymer nanocomposites with single-walled carbon nanotubes: effects of nanotube dispersion and concentration

doi:10.1016/j.polymer.2004.10.087

Polymer

Volume 46, Issue 2, 12 January 2005, Pages 471-481

https://doi.org/10.1016/j.polymer.2004.10.087 Get rights and content

Abstract

The effects of the dispersion and concentration of single walled carbon nanotube (SWNT) on the flammability of polymer/SWNT nanocomposites were investigated. The polymer matrix was poly (methyl methacrylate) (PMMA) and the SWNT were dispersed using a phase separation (‘coagulation’) method. Dispersion of SWNTs in these nanocomposites was characterized by optical microscopy on a micrometer scale. Flammability properties were measured with a cone calorimeter in air and a gasification device in a nitrogen atmosphere. In the case where the nanotubes were relatively well-dispersed, a nanotube containing network structured layer was formed without any major cracks or openings during the burning tests and covered the entire sample surface of the nanocomposite. However, nanocomposites having a poor nanotube dispersion or a low concentration of the nanotubes (0.2% by mass or less) formed numerous black discrete islands with vigorous bubbling occurring between these islands. Importantly, the peak heat release rate of the nanocomposite that formed the network layer is about a half of those, which formed the discrete islands. It is proposed that the formation of the discrete islands is due to localized accumulation of the nanotubes as a result of fluid convection accompanying bubble formation and rise of the bubbles to the surface through the molten sample layer and bursting of the bubbles at the surface. The network layer acts as a heat shield to slow the thermal degradation of PMMA.

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 M_w: 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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Flammability properties of polymer nanocomposites with single-walled carbon nanotubes: effects of nanotube dispersion and concentration

Abstract

Introduction

Section snippets

Materials

Sample morphology

Discussion

Conclusion

Acknowledgements

Polym Degrad Stab

Polym Degrad Stab

Polymer

Polymer

Chem Phys Lett

Proc Combust Inst

Polym Degrad Stabil

Adv Mater

SAMPE J

Chem Mater

Chem Mater

Chem Mater