Peak heat release rate reduction

Blends made by extrusion have created difficulties and show a degradation of the molecular weight with development of colour. Processing leads to more exfoliation of platelets in the clay that causes two competing effects. A reduction of the peak heat release rate (HRR) because of the exfoliated platelets, versus degradation processes that increase the peak HRR. The dominant effect in low ash and/or low shear situations is reduction of peak HRR. The opposite is true with high ash or high shear since the peak increases. 

Flammabihty studies using cone calorimetry at 50 kW m heat flux showed that incorporation of 5 wt% organoclay reduced peak heat release rate (PHRR) by 23-27% and total heat release (THR) values by 4—11%, depending on the clay modification. However, no simple correlation was observed between the FR efficiency and the degree of day exfoliation. The synergistic effect was observed for the combination of ammonium polyphosphate and 5 wt% amount of nanoday, which resulted in the total reduction of the PHRR of polyester resin in the range 60-70%. 

Herbert has carried out a comprehensive study of the effect of ATH on the fire resistance of a peroxide crosslinked EVA (18% vinyl acetate), using the cone calorimeter [34]. This work showed that ATH was an effective fire retardant in this test, with significant reductions in most parameters, notably reduced peak heat release rate and smoke level. 

One of the major objectives in fire retardancy emphasizes a reduction in the peak heat release rate. This, in turn, reduces the fire propagation rate. 

The great improvements in flame retardancy caused by the organoclays also opened the possibility of decreasing the level of ATH within the EVA polymer matrix. The content of ATH needed to maintain 200 kW/m as a peak heat release rate could be decreased from 65 to 45 wt% by the presence of only 5 wt% organoclays within the EVA polymer matrix. Reduction in the total amount of these fillers resulted in improved mechanical and rheological properties of the EVA-based nanocomposite. 

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. 

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. 

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