Heat release rate polymer-clay nanocomposites

The accumulation of clay at the surface acts thus as a barrier which limits heat transfers and reduces the release of combustible volatiles into the flame. A substantial decrease in the peak heat release rate of the nanocomposite (25 to 50%) can be achieved compared to the neat polymer (Bourbigot et al, 2006). However, this effect is very dependent on the quality of dispersion of the nanoparticles within the host matrix, and a high degree of exfoliation is usually targeted in order to maximize both the mechanical and fire properties (Hackman and Hollaway, 2006). Other types of nanoparticles, such as silica (Si02), titanium dioxide (Ti02), carbon nanotubes or silesquioxane, have also proven to have significant flame-retardant properties (Laoutid et al., 2009). 

The heat release rate curves shown in Fig. 4A are consistent with the characteristic burning patterns of intermediate thick, non-charring samples (II). The PHRR values for PE-ZCHS-5 and PE-ZCHS-10 nanocomposites are reduced by 27 and 25% relative to the pure PE respectively. For smectite clay/polymer nanocomposites, reduction in PHRR has been shown to be correlated with nanodispersion of the additive in the polymer matrix (72). With HDS and related layered metal hydroxide additives, we have also only found PHRR reduction in the case of PVE with CHDS, a system with some... 

Polymer clay nanocomposites exhibit very low flammability. For instance, the heat release rate during the combustion of polyamide 6-clay nanocomposite is reduced by 63% with a clay content of 5wt%. The nanocomposite structure also enhances the property of the char through reinforcement of the... 

The flammability behaviour of clay-polymer nanocomposites could be restricted by incorporating the nano-clay as reinforcement in limited volume fraction. The heat release rates also are found to be diminished substantially by nano-clay incorporation. The flammability resistance can be enhanced by the incorporation of nano-clay platelets without compromising other properties [114]. This improvement in flammability resulted in development of Wire Cable jacket material [115]. 

The comprehensive flame retardation of polymer-clay nanocomposite materials was reported by Dr. Jeff Gilman and others at NIST [7]. They disclosed that both delaminated and intercalated nanoclays improve the flammability properties of polymer-layered silicate (clay) nanocomposites. In the study of the flame retardant effect of the nanodispersed clays, XRD and TEM analysis identified a nanoreinforced protective silicate/carbon-like high-performance char from the combustion residue that provided a physical mechanism of flammability control. The report also disclosed that The nanocomposite structure of the char appears to enhance the performance of the char layer. This char may act as an insulation and mass transport barrier showing the escape of the volatile products generated as the polymer decomposes. Cone calorimetry was used to study the flame retardation. The HRRs (heat release rates) of thermoplastic and thermoset polymer-layered silicate nanocomposites are reduced by 40% to 60% in delaminated or intercalated nanocomposites containing a silicate mass fraction of only 2% to 6%. On the basis of their expertise and experience in plastic flammability, they concluded that polymer-clay nanocomposites are very promising new flame-retarding polymers. In addition, they predict that the addition... 

In summary, significant reduction in the peak heat release rate for the PA 6/clay nanocomposites was achieved by the formation of protective floccules on the polymer surface, which shielded the PA 6 from external thermal radiation and feedback from the flame. That is, the carbonaceous floccules acted as thermal insulation. 

Combustion of polymeric materials involves a complex process, where both condensed and vapor-phase reactions occur at exposed surfaces that are sources of flame and/or thermal radiation of the most common parameters measuring the flammability of polymeric materials are heat release rate (HRR) and mass loss rate (MLR) from cone calorimetry. Recently, nanocomposites containing nanoparticles have been of great interest in the composite industries. In particular, polymer blends containing clays have not been comprehensively studied for their flammability, in spite of the fact that most plastic products are made out of blends of more than two polymer. Furthermore, because the dispersion of nanoparticles is a key factor in determining the HRR and MLR of nanocomposites [23-26], we investigated correlations between flammability and dispersion in air and under nitrogen, especially for polymer blends. 

Thermosetting nanocomposites exhibit a reduced rate of heat release compared to neat polymer. However, the approach to nanocomposites itself is not sufficient to comply with the actual fire test standards. For this reason, traditional flame retardants are currently used in combination with nanofillers, and researchers are focusing on the individuation of synergistic systems. As an alternative to the most common cationic clays, anionic clays show improved performance in terms of flame retardancy. Epoxy nanocomposites based on anionic clay exhibit unique self-extinguishing behavior in a UL-94 horizontal burning test never observed before in a pure nanocomposite. The formation of a continnous intu-mescent ceramic layer on the surface of a polymer during combustion reduces the heat release rate to a higher extent than do montmorillonite nanocomposites. 

Polymer-clay nanocomposites reduce flammability by slowing the mass loss rate of fuel to the flame, thus keeping the heat release rate (HRR) low (Chapter 3). However, the material will eventually bum completely, leaving only a small amount of noncombusted carbon, with very little reduction in... 

Because the release rate of flammable fuel is reduced, the heat release rate is reduced as well. However, the char-clay barrier only slows the release of fuel—it does not fully prevent it—so a polymer nanocomposite will slowly bum until almost all the carbon mass has been pyrolyzed and combusted, which means that the total heat release for a polymer-clay nanocomposite is unchanged from that of the base polymer, but the peak heat release and average heat release rates are lowered. 

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. 

Cone calorimetric evaluations of polymer-clay nanocomposites indicate that PHRR and mass loss rate (MLR) can be significantly reduced when compared to the pure polymer. However, in many cases, the ignition temperature is lower, the total heat released (THR) has not changed, and the total mass loss (TML) has not changed for the polymer-clay nanocomposites when compared to the pure polymer. An examination of the flame-retardant behavior of polymer-clay nanocomposites indicates that the presence of the clay delays the decomposition of polymer in the cone calorimeter test and does not prevent the decomposition. These observations in relation to the definitions listed above for flame retardants excludes clay from being considered to be a flame retardant in the same category as commercially available flame retardants. Because of these inadequacies, considerable effort has been made to identify synergies that may exist between commercial flame retardants and clay in polymer. 

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