Flame retardants intumescent systems

Intumescent fire retardant additives undergo a thermal degradation process on heating, which produces a thermally stable, foamed, multicellular residue called intumescent char. When these substances are added to a polymeric material which is later involved in a fire, they produce an intumescent char which accumulates on the surface, while the polymer is consumed, providing insulation to the underling materials and partially protecting it from the action of the flame. 

The intumescent char acts essentially as a physical barrier to heat and mass transfer between the flame and the burning material. Thus, the process of pyrolysis of the polymer that produces combustible volatile products to feed the flame is reduced by a decrease in temperature, caused in turn by a lower heat supply from the flame. The diffusion of the volatile products towards the flame is hindered with further reduction of the flame feed. Furthermore, whatever may be the role of oxygen in the combustion process, its diffusion towards the polymer burning surface is also hindered, This series of events can lead to an interruption in the self-sustained combustion process because the flame is starved. 

the condensed phase mechanism of fire retardant intumescent systems aims at reducing the rate of pyrolysis of the polymer below the threshold for self-sustained combustion. This limits the production of volatile moieties and hence reduces undesirable secondary effects of volatiles combustion such as visual obscuration, corrosion and toxicity, which are typical of the widely used halogen containing fire retardants. 

Moreover, intumescent char adhesion to the surface of the burning polymer prevents molten inflamed polymeric particles from dripping, thus avoiding a source of fire propagation which is typical of some materials. 

The intumescent approach has been used for about 50 years in coatings for the protection of metal and wood structures [1,2]. The introduction of intumescent systems in the bulk of polymeric materials is relatively recent [3-5]. The early developments in intumescent additives for polymers were based on experience acquired in coating applications. Indeed, the empirical approach had led to a recognition of the need for compounds capable of supplying the charred residue (a carbonific ) and of blowing it to a foamed cellular structure ( spumific ) as components of formulations showing intumescent behaviour in coatings. 

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Pol5mrethane-phosphate combinations ate known to form flame retardant intumescent systems. However, the intumescent formulation caimot be permanent beeause of the water solubility of the phosphate. This problem could be solved by the teehnique of mieroeneapsulation. Microcapsules of diammoniiun hydrogen phosphate (DAHP) with a PU shell are synthesised. Cotton fabrics can be coated with PU formulations that enclose the microeapsules and show a signifieant FR effect. 

Flame retardants inorganic oxide and hydroxide systems Flame retardants intumescent systems ... 

Zhang et al. [31] studied fire-retardant properties of PP/IFR/LDH nanocomposites, which were comprised by a typical intumescent flame retardant (IFR) system and LDHs with different bivalent metal cations. The results of pk-HRR, pk-MLR, pk-EHC, and ignition time (IT) obtained from the cone calorimeter tests of various samples were listed in Table 8.1 [31]. It could been seen from Table 8.1 that the IT values of PP/IFR/LDH samples with different divalent metal cations of Zn, Mg, Cu, and Ca were 55, 55, 54, and 52 s, respectively, which are longer than 48 s of the PP/IFR sample without LDH. These data showed that the PP/IFR samples with LDHs are obviously harder to ignite than the sample only with the IFR. 

A novel intumescent flame-retardant (IFR) system composed of a phosphorous-nitrogen (PSPTR)-containing spiro triazine structure and phenol formaldehyde resin (PF) was investigated by Hu et al. (Hu et al. 2012). 

Y. Li, B. Li, J. Dai, H. Jia, and S. Gao, Synergistic effects of lanthanum oxide on a novel intumescent flame retardant polypropylene system. Polymer Degradation and Stability, 93(1) 9-16, January 2008. 

Protective Coatings. Some flame retardants function by forming a protective Hquid or char barrier. These minimize transpiration of polymer degradation products to the flame front and/or act as an insulating layer to reduce the heat transfer from the flame to the polymer. Phosphoms compounds that decompose to give phosphoric acid and intumescent systems are examples of this category (see Flame retardants, phosphorus flame retardants). 

Ammonium polyphosphates, on the other hand, are relatively water insoluble, nonmelting solids with very high phosphorus contents (up to about 30%). There are several crystalline forms and the commercial products differ in molecular weights, particle sizes, solubilities, and so on. They are also widely used as components of intumescent paints and mastics where they function as the acid catalyst (i.e., by producing phosphoric acid upon decomposition). They are used in paints with pentaerythritol (or with a derivative of pentaerythritol) as the carbonific component and melamine as the spumific compound.22 In addition, the intumescent formulations typically contain resinous binders, pigments, and other fillers. These systems are highly efficient in flame-retarding hydroxy-lated polymers. 

Caze, C., Devaux, E., Testard, G., and Reix, T., New intumescent systems An answer to the flame retardant challenges in the textile industry, in Fire Retardancy of Polymers The Use of Intumescence, Le Bras, M., Camino, G., Bourbigot, S., and Delobel, R. (Eds.), 1998, Royal Society of Chemistry, London, pp. 363-375. 

In this chapter, we have discussed recent developments of intumescent flame-retarded materials in terms of reaction and resistance to fire. Research work in intumescence is very active. New molecules (commercial molecules and new concepts) have appeared. Nanocomposites are a relatively new technology in the held of flame retardancy. This technology gives the best results combined with conventional FRs and leads to synergistic effects with intumescent systems. Very promising developments in the synergy aspects are then expected and efforts should be continued in this way. 

Montaudo, G., Scamporrino, E., and Vitalini, D. 1983. Intumescent flame retardants for polymers. II. The polypropylene-ammonium polyphosphate-polyurea system. J. Polym. Sci., Polym. Chem. Ed. 21 3361-3371. 

Wang, D.L., Liu, Y., Wang, D.Y., Zhao, C.X., Mou, Y.R., and Wang, Y.Z. 2007. A novel intumescent flame-retardant system containing metal chelates for polyvinyl alcohol. Polym. Deg. Stab. 92 1555-1564. 

Recently, some reports have explored the potential of synergistic effect between silica and other flame retardants.53-55 For example, silica showed synergistic effect with alumina in polypropylene (PP)/ammonium polyphosphate (APP)—pentaerythritol (PER) intumescent-based system. The data indicate that the HRR values improved by incorporating silica into the intumescent-based formulation and the improvement was much more pronounced by combining both silica and alumina in the formulation. 

In an attempt to provide compatibility between the FR and polymer matrix, Wang et al. reported recently on a novel microencapsulated intumescent system containing 4A zeolite as a potential flame retardant for natural rubber (NR).61 The flame-retardant properties of NR composites loaded with different amounts of intumescent flame retardant (IFR), IFR-4A zeolite, and microencapsulated intumescent flame retardant (MIFR)-4A zeolite agents were studied and compared. The LOI data demonstrate that the NR composite filled with 50phr of MIFR-4A zeolite agent and 50phr of IFR-4A zeolite shows better FR properties as compared to NR and 50phr of IFR-filled systems. 

Hermansson, A., Hjertberg, T., and Sultan, B.A., Linking the flame-retardant mechanisms of ethylene-acrylate copolymer, chalk and silicone elastomer system with its intumescent behavior, Fire Mater. 2005, 29, 407. 

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... 

Fire-retarded materials functioning in the condensed phase, such as intumescent systems, form, on heating, foamed cellular charred layers on the surface, which protects the underlying material from the action of the heat flux or the flame. It is recognized that the formation of the effective char occurs via a... 

X-ray diffraction (XRD) has been poorly used to characterize the carbon phase of intumescent structure. Indeed, as shown previously, the carbon structure resulting from the development of the intumescent system is mainly disordered whereas XRD characterizes ordered structure. However, this technique may be of interest to study the carbonization process in the case of flame-retardant systems containing layered additives, such as expandable graphite,28,42 or even more in the case of lamellar nanocomposites, such as MMT-based nanocomposites. 

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