Peak heat release rate composites

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. 

Figure 11.30 presents the heat release rate curves of polypropylene (PP) and its composites with 1 wt% CNTs or Ceo-rZ-CNTs. The incorporation of CNTs considerably reduced the peak heat release rate (PHRR) of PP (reduction around 66). At the same loading level. 

Beyer clearly observed a delay in thermal degradation using TGA (in air) with the addition of a small amount of organoclay to EVA-ATH composite [20]. The char of the EVA-ATH-organoclay compound formed in a cone calorimeter was rigid, with only a few small cracks, whereas the char of the EVA-ATH composite was much less rigid (reduced mechanical strength) and had many big cracks. These observations allowed the author to explain the reduction of the peak heat release rate to 100 kW/m for the nanocomposite, compared to 200 kW/m for the EVA-ATH composite [21]. 

Similar results are also observed in PS nanocomposites [56], which were prepared by free radical polymerization of styrene monomers in the presence of ZnAl and MgAl LDHs intercalated with 4,4 -azobis(4-cyanopentanoate) anions (LDH-ACPA). An intercalated-exfoliated morphology is observed for the composites of ZnAl-ACPA, whereas MgAl-ACPA shows microcomposite formation. The cone calorimetry results show good correlation between the reduction in PHRR and dispersion, in which the reduction in the peak heat release rate for 10% ZnAl-ACPA is 35% relative to the pristine polymer, whereas a 24% reduction is recorded for MgAl-ACPA at a similar loading. 

Inan and co-workers study of the flammability of PA6-clay nanocomposites provides an elegant illustration of the dominant heat transfer roll that the char plays in controlling nanocomposite flammability.In these experiments PA6 nanocomposite samples were placed atop pure PA6 samples, these compression-molded composite samples were burned in a cone calorimeter, and the reduction in peak heat release rate for the composite sample was found to be 11% of that expected if the entire sample had been nanocomposite. Since only half of the composite contained clay, this magnitude of effect is surprising. Furthermore,... 

Ristolainen et al." used modified montmorillonite as a partial substitute for ATH in PP-ATH composites and observed enhanced flame retardancy with composites containing both fillers. Wilkie and Zhang" studied the fire behavior of PE combined with ATH and a modified montmorillonite. The combination of PE with 2.5% modified montmorillonite and 20% ATH gave a 73% reduction in the peak heat release rate, which was the same as that obtained when 40% ATH alone was used. A further increase in the montmorillonite loading did not improve the fire properties. Mechanical properties such as elongation at break could be improved in comparing compounds with or without montmorillonite at the same reduction in peak heat release rate. 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

When used in a flexible PU foam, smoke and toxic fume emissions are greatly educed compared with untreated foam or ones with halogenated FRs present. When utilised in a polypropylene composition, the heat release rate is much lower lhan for a halogen- or a phosphate-based FR, with a much-reduced peak. 

Cone Calorimeter data of a nylon-6,6 composition with PVA oxidized by KMn04 (Mn - chelate complexes) show improvement of peak rate of heat release from 476.7 kW/m (composition of nylon-6,6 with PVA) to 399.5 kW/m (composition of nylon-6,6 with PVA, oxidized by KMn04) [212]. On the other hand, the exothermal process of smoldering for the composition of nylon-6,6 with PVA, oxidized by KMn04 has been noted [213]. This reaction is evidently provided by chelated Mn-structures which increase the total heat release of nylon-6,6 with PVA, oxidized by KMn04 in comparison with nylon-6,6 with PVA. 

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