Heat release rate char formation

With greatly reduced burning rate, heat release, and char formation, it is possible to combine nanofiller with traditional flame retardants to achieve satisfactory regulatory approvals such as various UL ratings. In fact, several studies have revealed synergistic effects of combinations of nanofillers with traditional microsized flame retardants. 

Cone calorimetry measurement has demonstrated that such a modification drastically reduces the peak heat release rate and facilitates the char formation. This serves as a physical barrier for the heat flux through the polymer surface [75]. 

Studies on various fillers in EVA compounds for comparison with ATH and magnesium hydroxide have found that hydrotalcite has promising FR effects. 50 wt% of hydrotalcite in EVA has a very low heat release rate and a low evolved gas temperature. The layered structure of hydrotalcite may provide an explanation. The formation of a char layer was observed, which retains an intumescent, fairly compact, surface during combustion conditions. 

The major approach now for EVA compounds, especially those designed for cable jackets, is the introduction of nano-clays to the mix. As little as a 3% loading of a nanofiller produces a marked effect on heat release. 5% of nanofiller reduces the peak heat release rate by almost 50% as well as shifting that peak towards longer time-lapse. The flame retardant properties are due to the formation of a char layer that acts as a Arm insulating layer and dramatically reduces the emission of volatiles towards the flame front. The silicate layers not only play the active role in char... 

Flammability resistance. An increased flammability resistance has been noticed as an important property enhancement involving nano-platelets incorporated into polymers involving exfoliated clay this involves the formation of a stable carbon/nano-platelet or nanofibre surface. This surface exhibits analogous characteristics to intumescent coatings, whereby the resultant char provides protection to the interior of the specimen by preventing continual surface regeneration of available fuel to continue the combustion process. The primary advantage noted with nano-filler incorporation is the reduction in the maximum heat release rate. 

Enhance char formation Lower heat release rate I New alloy system... 

The primary effect of the nano-clays seems to be related to char formation. The workers at NIST have found that a reduction in mass loss and heat release rate only starts once the surface of the polymer is at least partly covered by char. Beyer reported that, while no char was produced by burning unfilled EVA, the filled composite formed a strong char early in the process [54]. Once the amount of clay is taken into account, final char levels are often similar to unfilled polymer, indicating that while a stronger, more insulating, char may form and retard combustion, it is eventually consumed in this test. 

Figure 7.9 (A) Char formation of nano-LDPE after cone testing heat flux 35 kW/m and (B) Peak heat release rates of LDPE and nano-LDPE heat flux 35 kW/m. ...

Curves of heat release rate (HRR) versus time for intumescent EVA-based formulations (Figure 6.11) exhibit two peaks assigned to the development of intumescence. The first corresponds to formation of a protective layer, and the second corresponds to its destruction or failure. It clearly appears that when a nanocomposite is included in the formulation (in the matrix, in the carbonization agent, or in both), the first peak heat release rate (PHRR) is reduced (from about 340 kW/m to 200 kW/m ). However, the second peak decreases only when EVAnano is used, suggesting the formation of a stronger char. Work is in progress to explain these phenomena. 

POSS on the flame retardant performance was described for the formation of an oxidatively stable, uniformly covered nonpermeable surface char layer.Some previous studies demonstrated reasonably effective flame retardant performance of POSS, but a recent study with trisilanol phenyl POSS in PMMA did not show any flame retardant performance as measured in a cone calorimeter. It was suggested that POSS has the potential to reduce the heat release rate, but one must be careful in selecting the POSS material to be evaluated. 

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