Intumescent chars

Intumescent layers of such thicknesses are not difficult to achieve, but let us consider some limitations to this approach as well as some clues to improving intumescent char or char-like barriers. 

FIGURE 6.12 (See color insert following page 530.) HRR as a function of time of pure TPU and TPU/ FQ-POSS composite (external heat flux = 35kW/m2) (a) and intumescent char residue at the end of the cone experiment (b). 

Myers et al. reported that partially dehydrated APB is an effective intumescent flame retardant in thermoplastic polyurethane.77 APB at 5-10 phr loading in TPU can provide 7- to 10-fold improvement in burn-through test. It is believed that in the temperature range of 230°C-450°C, the dehydrated APB and its released boric oxide/boric acid may react with the diol and/or isocyanate, the decomposed fragments from TPU, to produce a highly cross-linked borate ester and possibly boron-nitrogen polymer that can reduce the rate of formation of flammable volatiles and result in intumescent char. 

The formation of borophosphate partially explains the good performance when APP and boric acid are mixed together in the epoxy resin. Indeed, in that case good mechanical resistance of the intumescent char is observed as borophosphate is a hard material, which also shows a good thermal stability. As a conclusion, the boron containing compounds provide the good structural properties of the char, whereas the phosphorus ensures the adhesion of the char to the steel. 

Finally, the mechanical destruction of the intumescent char is also an important task for investigation. If a char has good structural, morphological, and heat insulative properties but is easily destroyed under mechanical action, its efficiency is totally lost in the turbulent regime of combustion. 

Qu, B. and Xie, R. 2003. Intumescent char structures and flame-retardant mechanism of expandable graphite-based halogen-free flame-retardant linear low density polyethylene blends. Polymer International 52(9) 1415-1422. 

At Bolton, we also have attempted to introduce volatile and possible vapor phase-active, phosphorus-based FR components in back-coating formulations.60 62 The selected FRs included tributyl phosphate (TBP), a monomeric cyclic phosphate Antiblaze CU (Rhodia Specialties) and the oligomeric phosphate-phosphonate Fyrol 51 (Akzo). When combined with an intumescent char-forming pentaerythritol (PER) derivative (NH1197, Chemtura) and applied as a back-coating on to cotton and polypropylene substrates, significant improvements in overall flame retardancy were observed. 

Also a hindering effect toward oxygen diffusion, owing to the intumescent char layer formed, may be responsible for CO increase when oxygen concentration is low, incomplete combustion is favored over a complete one. The char morphology in presence of EG is particular the char has the so-called worm like structure (Figure 25.4) due to the expansion of EG. 

Three different types of halogen-free intumescent charring agents have been identified [17] (Table 7.1). All those compounds lead to the formation of a superficial char layer that prevents further decomposition but they act in three different ways ... 

The presence of phosphorus in the polymer backbone has a veiy practical consequence, quite apart from the structural issues. Phosphorus is one of the most important elements that prevent the combustion of organic materials. The presence of both phosphorus and nitrogen is synergistic. Thus, the phosphorus-nitrogen backbone in polyphosphazenes ensures that many poly(organophosphazenes) are not only nonflammable but also quench combustion of other compounds with which they are in contact. The mechanism of this fire suppression is believed to be both an interruption of the free radical processes that occur in a flame and the formation of an intumescent char that shields the material from the ingress of oxygen. 

Work based in Hungary and France has been examining the claims for a siloxane compoimd to act as a synergist for the protection of polyolefins. Organoboroxo-siloxane (OBSi) is added with APP and pentaerythritol to polypropylene. The material appears to increase the viscosity of the polymer composition and provide some plasticity to the resultant intumescent char. The plasticity allows for better prevention of char cracking and so provides improved flame retardancy. The increased melt viscosity is created by the product of BSi-pentaerythritol, formed during the compound preparation, and the improved char plasticity is the result of products formed at high temperatures from BSi and APP. 

The materials based on the combinations of APP with a sufficient amount of ATH or melamine may be considered of interest for cable insulation based on EVA. These formulations bum with the formation of a protective intumescent char and were superior to non-intumescent ATH-EVA ones. The CO production for the APP-melamine containing EVA was also reduced to a marked degree by comparison with the ATH-EVA system. However, in the case of the APP-ATH-EVA formulation, the total CO yield increased to some extent by comparison with the ATH-EVA mixture. 

Work in China has shown the synergistie effect of silicotungistic acid (SiW ) on pol q)ropylene flame retarded by an intumescent FR (NP28 phosphorus-nitrogen compound). The tungsten compound increased the thermal stability of the PP formulation at temperatures above 500 °C. The SiWi2 could efficiently promote the formation of compact intumescent charred layers with phosphocarbonaceous structures. 

A new approach to render fibre-reinforced rigid composite materials flame retardant is undertaken by the utilisation of complex fibrous-intumescent chars. The use of flame retarded cellulosics fabric, surface coated with an interactive intumescent as an additional reinforcement in an otherwise conventional structure has been studied. Thermal analysis has shown that when heated, all components decompose by chemically interactive mechanisms leading to a char-bonded stmcture and the residual mass of char formed is higher than expected above 450 °C, even in the case where polyester resins are present. Not only are greater fiactions of char formed above 450 °C but the chars formed are more resistant to oxidation than the respective components (resin, traditional fabric and coated cellulose). Thus composites comprising these various components will have significantly improved fire performance. 

Polymer + Intumescent Char Fo (APP or other cosynergists) rming System... 

Char-forming thermoplastics often swell and intumesce during their degradation (combustion), and the flame-retardant approach is to promote the formation of such intumescent char. 

As thermal analysis has showed APP destabilizes nylon 6, since the thermal decomposition is observed at a temperature 70°C lower than that of the pure nylon 6 [141]. However, the Intumescent layer effectively protects the underlying polymer from the heat flux and therefore in the configuration of the "linear pyrolysis" experiments, the formulation nylon 6/APP (40%) decomposes more slowly than pure polymer [141]. These experiments prove the fire retardant action of the intumescent char. 

The combustion behavior of melamine pyrophosphate and dimelamine phosphate are different from those of melamine and the other melamine salts (Table 3.4.1). The former are ineffective at low concentrations (> 15%) and become effective at a loading of 20-30% because the intumescent char is formed on the surface of burning specimens. The mechanism of fire retardant action of both melamine pyrophosphate and dimelamine phosphate is similar to that of APP since, by analogy with ammonia melamine volatilizes, whereas the remaining phosphoric acids produce esters with nylon-6, which are precursors of the char [146]. Some part of the freed melamine condenses probably forming melem and melon [147]. 

In regard to flammability, LCPs have an oxygen index ranging from 35 to 50 percent. When exposed to open flame they form an intumescent char that prevents dripping and... 

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