Nanocomposite polymers, fire cone calorimeter

This chapter is organized in the following way. First, we present some common techniques for characterizing the dispersion of nanoclays in polymer blends. The dispersion level has been shown to have a fundamental effect on the fire performance of polymer-clay nanocomposites (PCNs), as an exfoliated or intercalated polymer-clay system seems to enjoy reduced flammability. Second, the effects of nanoclays on the viscosity of polymer blends are discussed. With increased temperature in the condensed phase during combustion, most polymers (and hence polymer blends) have sufficiently low viscosity to flow under their own weight. This is highly undesirable, especially when the final products will be used in vertical orientation, because the melt can drip, having the potential to form a pool fire, which can increase fire spread. The results on thermal stability are presented next, followed by those for the cone calorimeter. The quantitative effects of nanoclays on the... 

Specific aspects of barrier formation were discussed above. A silicate or sihcate-char surface layer acting as a barrier for heat and mass transport is probably the main general fire retardancy mechanism of all layered-silicate nanocomposites. Most sources claim that this mechanism is responsible for the strongly improved performance in a cone calorimeter test. In particular, the strong reduction in PHRR is used to propose that layered silicates are the most promising approach for fire retardancy of polymers. However, the barrier effects and their influences on cone calorimeter results are not described in detail, so that the specific characteristics of these mechanisms are unclear. 

The final need is to understand the degree to which a nanocomposite reduces flammability and its relevance to actual fire risk scenarios. This subject is covered extensively in Chapter 5, but it is worth revisiting here. To some extent it can be argued successfully that cone calorimeter results for a polymer nanocomposite are not relevant to a final fire risk scenario at all, yet cone calorimeter does yield fundamental flammability performance data for a material. It is necessary to put... 

The observation that polymer-clay nanocomposites have significantly lower peak heat release rates (PHRRs) when compared to the pure polymer [47] stimulated a dramatic effort focused on the evaluation of the flame-retardant potential of clay dispersed in polymer. The decrease in PHRR can be related directly to the decrease in the spread of fire from one combustible material to another. This affect is directly applicable to definition (1) above. Figures 8.1 and 8.2 contain a comparison of cone calorimeter results for a pure polymer and a nanocomposite of that polymer with montmorillonite. 

Polymer-clay nanocomposites that have the best fire-retardant performance evaluated by the cone calorimeter have clay particles oriented parallel to the surface of the sample. Vertical fire retardant tests (UL-94) of these samples do not demonstrate improved performance because the edges of the particles are exposed to the fire. The edges of clay are very thin (approximately 1 nm). Hence, the mechanisms predicated on barriers provided by the clay are not applicable. Mechanisms associated with increased melt viscosity are apparent with vertical fire-retardant testing. Dripping during the burning of the vertical samples is greatly reduced [3]. 

---