Heat release rate polymer-carbon 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). 

Nanocomposites refer to the combination of nanosized fillers (10 m diameter) with polymers, rather than the combination of polymer matrix (filled with nanoparticles) and fiber reinforcement The most popular fillers used as fire retardants are layered silicates. Loading of 10% or less (by weight) of such fillers significantly reduces peak heat release rates and facilitates greater char production [7]. The char layer provides a shielding effect for the composites below and the creation of char also reduces the toxicity of the combustion products, as less carbon is available to form the CO and CO2. 

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... 

It is has been demonstrated that the addition of small quantities of carbon nanotubes (CNTs) can dramatically improve the thermal and mechanical properties of polymers. " In many cases, however, this enhancement of properties is limited by the degree to which the CNTs can be dispersed uniformly within the polymer matrix. This appears to be particularly true for flame-retarding applications. For example, Kashiwagi and co-workers demonstrated that the heat release rates from burning well-dispersed nanocomposites consisting of single-walled CNTs (SWCNTs) in poly (methyl methacrylate) were significantly lower than... 

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 mechanism of flammability reduction for polymer nanotube and nanofiber nanocomposites (Chapter 10) is similar to that for clays a nanofiUer-rich surface or barrier forms, which slows the rate of mass loss and therefore the rate of heat release. There is little reduction in the total heat release, indicating that the carbon nanoflbers and nanotubes only reduce the flammability of the... 

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