Self-extinguishing behavior

In general, roughly LOI values of up to 27% signify flammability 33 higher LOI values signify self-extinguishing behavior in the UL 94 test (Table 15.2). However, there is no clear correlation between LOI and UL 94, and some extremes down to 22% and up to 47% (Figure 15.2, Table 15.2)... 

The LOI value for self-extinguishing behavior is often taken as 27, not 21 (which is the volume % concentration of oxygen in air), to correct for a lack of convective heating in the LOI test. 

While neither the MWNT elongated cylinders nor the exfoliated clay sheets resulted in self-extinguishing behavior, they did slow the rate of fire spread by reducing the rate of heat evolution. Both of these nano materials form a kind of synthetic char, more effectively than larger particulate fillers, and act as heat shields. It may be that combinations of several types of materials will prove most effective in reducing fire spread. 

By means of compounding, tailor-made engineering plastics parts are manufactured to satisfy every conceivable application. The warping problem, which existed in the past for the production of large area parts, e.g. for car body parts and bumpers, can be overcome by selected reinforcement/filler systems. Simultaneously the heat distortion temperature could be raised from about 150 °C for pure PBT homopolymer to approximately 210 °C for reinforced polymer. Self-extinguishing behavior was imparted by incorporation of flame retardant synergistic systems. Further, in the last thirty years, many commercial products have been developed by blending PBT with other polymers in the melt. Moreover, a variety of additives, fillers or reinforcements, which have been mentioned previously in this Chapter, may be added to the PBT blends. 

For epoxy resins containing no other flame retardants, extinguishing behavior requires about 16 to 20 percent chlorine [71], For polyesters containing no other flame retardants, self-extinguishing behavior requires about 21 percent alicyclic chlorine or 28 to 31 percent aromatic chlorine (in forms such as hexahalobenzenes and chlorinated biphenyls) [72]. 

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. 

A typical composition that was stated to be suitable for application on PP fibers and films consists of 2% Sb203 and 4% highly effective bromine-containing alicyclic compound. This composition would increase the LOI of PP from 17.4 to at least 25.2 which is adequate for self extinguishing behavior [43]. 

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