The role of nanodispersion on the fire performance of organoclay–polyamide nanocomposites

Abstract

This work has investigated the importance of nanoscale dispersion upon the fire performance of Nylon-6 nanocomposites by characterising the role of the char layer and its formation in reducing peak heat release rate during combustion. To do this, a series of layered silicate nanocomposites systems using Nylon-6 were prepared using twin screw extrusion techniques at different levels of clay addition and different processing temperatures. This work has shown that the addition of layered silicates improves the peak heat release rate in a synergistic manner, by forming a tough char layer that prevents or hinders the transfer of combustible products into the gaseous phase. Differences in nanoscale dispersion of the clay as measured using transmission electron microscopy (TEM) correlated strongly with changes in fire performance according to cone calorimetry measurements. Corresponding changes in the thermal decomposition behaviour (thermogravimetric analysis) and microstructure and elemental composition of the char layer (SEM) were used to further understand the fire retardation mechanism in the condensed phase. Comparison of the rate of mass loss with the heat release rates and XRD of the char were further used to investigate the role of the char layer in improving fire performance.

Introduction

Since the discovery of layered silicate polyamide nanocomposites by the Toyota Research Central Laboratory in the 1990s there has been a renaissance of research in both academia and industry, aimed both at understanding the fundamental principles behind nanocomposite formation and also the development of commercial applications. Along with the well known effects upon mechanical properties, concurrent improvements in fire performance for low levels of nanoclay addition have also generated significant interest. Early research, using cone calorimetry showed that the peak heat release rate (PHRR) could be reduced by 63% during combustion, compared to the based polymer, at levels of only 5 wt% [1], [2] for Nylon-6 (amongst other polymers). Comparison of the mass loss rate with the PHRR suggested that the fire retardancy mechanism occurred primarily in the condensed phase through char formation and not to any significant level in the gaseous phase. As a result, attention has also focussed on understanding the role of the chemical structure and morphology of the char layer in controlling fire performance. Investigation of the microstructure of the post-combustion char layer by Qin et al. [3] found increased levels of Si on the surface and attributed this to the formation of a tough, ceramic char layer and subsequent improvements in PHRR. Kashiwagi et al. [4] also investigated the microstructure of the char layer of Nylon-6 nanocomposites and found significant improvement in the fire performance measured using cone calorimetry. They reported that protective “floccules” consisting of 80% of clay particles and 20% of a carbonaceous graphitic structure, formed on the sample surface shielded the polyamide from the external thermal radiation and heat feedback from the flame. The effect of silicate addition upon polymer degradation and subsequent char formation in determining fire performance has also been highlighted by Song et al. [5] and Le Bras and Bourbigot [6]. The former group of researchers showed that char yields as determined via thermogravimetric analysis (TGA) closely correlated with decreases in PHRR, while the latter reported a similar relationship when using a nanofiller in a ternary blend [7], [8]. In both cases, the nano-reinforced char layer was visually observed to be tougher than the unmodified polyamide counterpart, thereby improving barrier performance while also promoting intumescence.

The importance of the quality of the dispersion whether a microcomposite, an intercalated and/or an exfoliated nanocomposite is also an important factor in determining fire performance. Currently, there are mixed results in the literature in regard to this. Morgan et al. [9], for example, reported that there was little effect upon fire performance whether the nanocomposite was exfoliated or intercalated. They reported improvements of around 30% regardless of whether the polymer was intercalated or exfoliated. In contrast, Wang et al. [10] reported a 40% decrease in PHRR for a 5 wt% organo-modified montmorillonite nanocomposite but only achieved a 16% improvement for the corresponding microcomposite based on an unmodified clay.

This paper seeks to contribute to the understanding of nanoscale dispersion in a layered silicate nylon nanocomposite system on fire performance. The fire performance of clearly defined intercalated and exfoliated Nylon-6 nanocomposites structures will be compared and discussed. Characterisation of the nanocomposites is determined using transmission electron microscopy (TEM), X-ray diffraction (XRD), cone calorimetry and thermogravimetric analysis (TGA). Investigation of the microstructure of the char layer after combustion was investigated using scanning electron microscopy (SEM), electrodispersive spectroscopy (EDS) and XRD.