Char promoter

The use of phosphorus compounds as flame retardants has been reviewed by Lyons and others (1, 2, 3, 4 5). The mechanism of the action of this element is generally accepted to involve decomposition to produce acids which function as char promoters. Phosphorus compounds are particularly effective flame retardants for polyesters where they function to increase the char yields. 

The complete absence of cross-linking reactions prevents potential char-forming reactions being favored in the presence of conventional condensed-phase flame retardants, and hence, the most effective flame retardants for polyolefins are usually bromine-based so that flame inhibition in the vapor phase is effected or intumescent-based, where char-promotion arises from the flame retardant itself. 

While there is controversy as to whether or not this Cellulose species exists, experimental evidence for the Cellulose species was obtained by Price et al.,60 who suggested that it could be a free radical in nature. At lower temperatures, oxygen plays a dominant role in cellulose degradation, and pyrolysis is faster in an oxidative atmosphere than in an inert one.61 Oxygen catalyzes the formation of both volatiles and char-promoting reactions.62 At higher temperatures, the degradation products are little affected.61... 

Increased char promotion Reduced flammability Wider range of endotherm and water release Enhanced oxide thermal barrier) )... 

Reduced flammability and smoke emission Increased char promotion Reduced overall filler levels Suppression of phosphine formation by metal hydroxide... 

Colored formulations Low co-additive additions Enhanced flammability resistance/reduced smoke Improved processibility and physical properties Handling issues Char promotion Reduced filler levels Can be pigmented Reduced overall filler levels Color limitations Possible adverse toxicity effects... 

The addition of very small amounts of fine carbon fibers73 or polyacrylonitrile fibers74 can reduce the level of inorganic hydroxide required to achieve UL94 V-0 flammability ratings in polyolefin compounds. These secondary additives are thought to function as char promoters. 

Melamine diborate (MB), known in the fire-retardant trade as melamine borate, is a white powder, which can be prepared readily from melamine and boric acid. It is partly soluble in water and acts as an afterglow suppressant and a char promoter in cellulosic materials. Budenheim Iberica79 claims that, in a 1 1 combination with APP, MB (10%-15%) can be used for phenolic bound nonwoven cotton fibers. In general, melamine borate can be used as a char promoter in intumescent systems for various polymers including polyolefins or elastomers. However, its low dehydration temperature (about 130°C) limits its application in thermoplastics that are processed at above 130°C. Melamine borate is also reported to suppress afterglow combustion in flame-proofing textiles with APP or monoammonium phosphate to meet the German DIN 53,459 and Nordtest NT-Fire 002.80... 

Owing to the multiple combinations involving interfacial compatibilizers and char promoter agents, including various polymers, this category of FR systems appears promising and yet able to meet industrial demand based on the association of excellent FR and mechanical properties. Moreover, the emergence of synthetic anionic clays, like LDHs, offers new possible combinations. 

Kandola, B. K., Akonda, M. H., and Horrocks, A. R. Use of high-performance fibres and intumescents as char promoters in glass-reinforced polyester composites, Polym. Degrad. Stabil. 2005, 88, 123-129. 

Chem. Descrip. Zinc borate CAS 1332-07-6 EINECS/ELINCS 215-566-6 Uses Flame retardant, smoke suppressant for plastisols, coatings for cellulosics, textiles, and adhesives synergist in PVC and halogenated polyester formulations, and in elastomers, thermoplastic elastomers, polyamides, and polyolefins strong char promoter Properties Wh. free-flowing powd., nonhygroscopic 6 p avg. particle size 99.9% < 30 p sol. 0.5 g/100 ml water sp.gr. 2.50 bulk dens. 18.1 Ib/ff oil absorp. 39 ref. index 1.48 Zb -237 [Great Lakes]... 

Chem. Descrip. Zinc borate CAS 1332-07-6 EINECS/ELINCS 215-566-6 Uses Flame retardant, smoke suppressant for plastisols, coatings for textiles, adhesives synergist in PVC and halogenated polyester formulations, and in elastomers, thermoplastic elastomers, polyamides, and polyolefins strong char promoter... 

Phosphorus is known as an effective char promoter. Charring limits the release of fuels to the flame and therefore reduces the heat released. Moreover, the char accumulates on the surface of the material and can act as a protective layer which limits the heat transfer from the flame to the condensed phase and the gas transfer from the pyrolysis zone to the flame. This phenomenon is called the barrier effect. Nevertheless, it should be noted that the presence of char is not a sufficient condition to observe an effective barrier effect. Its structure (cohesion, porosity, thickness) is also veiy important but rarely studied. Thermal stability is another important parameter to assess the reaction to fire of a material. Indeed, the higher is the degradation temperature of a polymer, the greater is the heat required to start its pyrolysis. Table 12.2 lists the effects of phosphorus-containing groups on the thermal stability and charring for a variety of polymers. 

Phosphorus is believed to act mainly as a char promoter. Therefore, a fraction of the fuel is trapped in the condensed phase and the total heat release (THR) is lowered. ° The decrease of the peak of the heat release rate (pHRR) in the PCFC is quite often accompanied by other minor peaks and/or by a decrease of the peak temperature. Therefore the PCFC results do not bring a definite conclusion about flame retardancy improvement. 

There are literature claims that phosphorus acts not only as a char promoter but also in the vapor phase, leading to a decrease in the effective heat of combustion (EHC). This activity could be either flame inhibition [i.e. incomplete combustion due to scavenging of radicals like H- or HO by phosphorus radicals in the vapor phase) or a change in the pyrolytic gases. If the decrease in the EHC is generally well observed, very few articles provide evidence of flame inhibition. 

According to the results of Ebdon et al. obtained from two series of MMA and styrene copolymers, char promotion could not be easily anticipated only from the phosphorus content. This means that a range of parameters modifies the efficiency of phosphorus as a flame retardant not only the phosphorus content, but also its chemical environment, its position in the macromolecule, and the host polymer. In this short chapter it is not possible to list in detail the influence of all these parameters. Therefore, only the main conclusions will be discussed in the following, from a narrow set of selected articles. 

Phosphorus could act as char promoter, but also in the vapor phase. The partition of phosphorus between condensed and vapor phases and its modes-of-action (as char promoter, flame inhibitor, etc.) depend strongly on various parameters. Phosphine oxide seems to be a relatively poor char promoter. Generally, a high oxidation state should allow more efficient... 

Jash and Wilkie [86] reported that even when the fraction of clay was as low as 0.1 wt% the PBQiR in a cone calorimeter was lowered by 40 %. Lee et al. [87] demonstrated that incorporation of 6, 8 and 10 wt% of MMT into epoxy resin increased linearly the char yield firom 9.1 to 15.4 % reducing the thermal degradation of the epoxy matrix. Nazare et al. [88] studied the flammability properties of unsaturated polyester resin with nanoclays using cone calorimetry. The authors verified that the incorporation of 5 wt% of nanoclays reduces the PHRR by 23-27 % and THR values by 4-11 %. While incorporation of condensed-phase flame retardants (such as ammonium polyphosphate, melamine phosphate and alumina trihydrate) reduce the PHRR and THR values of polyester resin, the inclusion of small amounts of nanoclay (5 % w/w) in combination with these char promoting flame retardants causes total reductions of the PHRR of polyester resin in the range 60-70 %. Ammonium polyphosphate, in particular and in combination with polyester-nanoclay hybrids show the best results compared to other flame retardants. 

A char residue from neat PP is left at 450°C (5%), due to charring promoted by oxidative dehydrogenation. Then it slowly decomposes on heating up to 600°C in air (Fig.4). On the other hand, thermal degradation of PP-MAPP-Cloisite 20A in air results to much more stable char form which doesn t oxidize even at 600°C (Fig. 4 and 5). 

The recent interest in the reported char-promoting functionalized dispersed nanoclays to yield nanocomposite structures having enhanced fire and mechanical properties, when the clays are present only at levels of 2 10%, prompts their investigation as potential fire retardants. Because of its wholly aliphatic hydrocarbon structure, neat polypropylene by itself bums very rapidly with a relatively smoke-free flame and without leaving a char residue. It... 

This occurs in the condensed phase and interferes with heat feedback from the burning gases in the flame to the decomposing polymer beneath. It also promotes the formation of a layer of char which further protects and insulates unbumed material. The smoke suppression effect may be viewed as a consequence of char promotion (that is carbon-rich particulates that would have otherwise become smoke, are locked up in the condensed phase as char). It is also likely that the very high surface area transition aluminium oxides formed during decomposition of ATH will adsorb many volatile species and fragments that could otherwise become smoke. 

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