Flame retardant mechanism of polyamide 6–clay nanocomposites

Abstract

The thermal and flammability properties of polyamide 6/clay (2 and 5% by mass fraction) nanocomposites were measured to determine their flame retardant (FR) performance. The gasification process of the nanocomposite samples at an external radiant flux of 50 kW/m² in a nitrogen atmosphere was observed, and the residues collected at various sample mass losses were analyzed by thermogravimetric analysis, transmission electron microscopy, and X-ray diffraction to determine the content of the residue and to understand the FR mechanism of the nanocomposites. The analysis of the floccules of blackened residues shows that up to 80% by mass of the residues consists of clay particles and the remainder is thermally stable organic components with possible graphitic structure. Furthermore, clay particles are stacked in the carbonaceous floccule residues and the d-spacing of the clay platelets is in the range of 1.3–1.4 nm as compared to the well exfoliated original sample. The accumulation of the initially well-dispersed clay particles in the sample on the burning/gasifying sample surface are due to two possible mechanisms. One is recession of the polymer resin from the surface by pyrolysis with the de-wetted clay particles left behind. Another mechanism is the transportation of clay particles pushed by numerous rising bubbles of degradation products and the associated convection flow in the melt from the interior of the sample toward the sample surface. Numerous rising bubbles may have another effect on the transport of clay particles. Bursting of the bubbles at the sample surface pushes the accumulated clay particles outward from the bursting area and forms the island-like floccules instead of forming a continuous net-like structure of a clay filled protective layer. Therefore, both PA6/clay nanocomposite samples did not produce sufficient amounts of protective floccules to cover the entire sample surface and vigorous bubbling was observed over the sample surface which was not covered by the protective floccules.

Introduction

Polymer–clay nanocomposites have attracted a great deal of interest due to their improved mechanical, thermal and biodegradability properties [1], [2], [3], [4], [5], [6]. Furthermore, an improvement in the flammability properties of polymers has been achieved with polymer–clay nanocomposites, which could provide an alternative to conventional flame retardants (FR) [7], [8], [9], [10], [11], [12], [13], [15]. Several mechanisms have been proposed to describe the observed improvement in flammability properties of polymers by the formation of polymer–clay nanocomposites. One of them is the reduction in heat release rate due to the formation of a protective surface barrier/insulation layer consisting of accumulated clay platelets with a small amount of carbonaceous char [8], [11]. Another mechanism proposed by Wilkie et al. is radical trapping by paramagnetic iron within the clay [12]. They showed that even when the clay was as low as 0.1% by mass fraction, the peak heat release rate of the clay/polystyrene nanocomposite was lowered by 40%, a value not much different from that observed with higher amounts of clay.

In our recent study of a silica based polymer nanocomposite, it was observed that the accumulated amount of silica particles on the sample surface, and their coverage over the exposed sample surface during burning, have a significant effect on the reduction of heat release rate of poly(methyl methacrylate) [16]. Both the accumulation and the surface coverage are affected by melt flow and by the bubbling of evolved degradation products near the sample surface. Therefore, it is important to understand the fate of clay particles in the molten layer near the sample surface to see if this sheds light on the FR mechanism. The key question is where are the initially well-dispersed clay particles during burning? The accumulation of clay particles near the sample surface and their area coverage over the degrading sample surface could be critical factors in determining their FR effectiveness.

In this study, the accumulation of clay particles and their coverage over the sample surface were measured by video images. Thermal gravimetric analyses and X-ray diffraction (XRD) measurement were conducted on residues collected at various times with samples having lost different fractions of the initial sample mass during the gasification experiment. The FR mechanism of the polymer–clay nanocomposite is explained on the basis of the above described results and other measurements of melt viscosity and of clay characteristics in the collected residues.