Phosphorus-based flame

The mode of action of phosphorus-based flame retardants is believed to take place in either the condensed or the vapor phase (refs. 1,2) depending on the type of phosphorus compound and the chemical composition of the polymer. Phosphorus has been reported to be 3 to 8 times more effective than bromine depending on the polymer type (ref. 3). 

In 2000, NEC developed an epoxy resin with what it describes as a fire-retardant structure that avoids the need for either TBBA or phosphorus-based flame retardants in circuit boards. The new resin contains a metal hydroxide retardant. The company claims the new board is almost totally free of pollutants, and is easy to process and thermally recycle. By also integrating flame retardant properties within the board, use of the metal hydroxide is minimised, while offering good electrical properties, higher heat resistance and improved processing characteristics. ... 

Figure 75. Flammability of the electrolytes containing various phosphorus-based flame retardants (FR). All electrolytes are composed of these flame retardants in 1.0 m LiPFe/EC/EMC. (Reproduced with permission from ref 529 (Eigure 1). Copyright 2003 The Electrochemical Society.)...

Condensed-Phase Mechanisms. The mode of action of phosphorus-based flame retardants in cellulnsic sy stems is probably best understood. Cellulose decomposes by a noncalalyzed route lo tarry depolymerization products, notably levoglucosan, which then decomposes to volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. However, when catalyzed by acids, the decomposition of cellulose proceeds primarily as an endothermic dehydration of the carbohydrate to water vapor and char. Phosphoric acid is particularly efficaceous in this catalytic role because of its low volatility (see Phosphoric Acids and Phosphales). Also, when strongly heated, phosphoric acid yields polyphosphoric acid which is even more effective in catalyzing the cellulose dehydration reaction. The flame-retardanl action is believed to proceed by way of initial phosphory lation of the cellulose. 

The largest volume use of phosphorus-based flame retardants may be in plasticized vinyl. Other use areas for phosphorus flame retardants are flexible urethane foants. polyester resins and other thermoset resins, adhesives. textiles. polycarbonate-ABS blends, and some Other thermoplastics. Development efforts are well advanced lo find applications for phosphorus flame retardants, especially ammonium polyphosphate combinations, in polyolefins, and red phosphorus in nylons, Interest is strong in finding phosphorus-bused alternatives to those halogen-containing systems which have encountered environmental opposition, especially in Europe. 

Two examples have been selected to demonstrate this process. The first involves a selective catalysts development program at Akzo Nobel under collaboration with Mark E. Davis of the California Institute of Technology (Caltech). Catalysts with greater selectivity were needed to improve the performance of a product line of phosphorus-based flame retardants and functional fluids. The Akzo Nobel... 

The major developments in reactive phosphorus-based flame retardants for epoxy resins to 2005 have been well reviewed.52 It will suffice here, therefore, to outline just the major developments and to highlight the most recent work. 

The use of phosphorus-based flame retardants in combination with other, better established, flame retardants is most effective in situations in which the combination proves synergistic. However, as yet our understanding of such synergistic effects is far from complete and more fundamental work is required in this area Work in which the gaseous and solid products of combustion, with and without the presence of flame retardants, are carefully analyzed. Such analyses can now be undertaken more readily than in the past, owing to the relatively recent development of techniques such as gas-phase FT-infrared spectroscopy and laser-pyrolysis time-of-flight mass spectrometry for the identification of volatiles, and solid-state NMR spectroscopy and x-ray photoelectron spectroscopy for the analysis of chars. 

Levchik, S. V. and Weil, E. D., A review of recent progress in phosphorus-based flame retardants, J. Fire... 

Levchik, S.V., Camino, G., Costa, L., and Levchik, G.F. 1995. Mechanism of action of phosphorus-based flame retardants in nylon 6.1. Ammonium polyphosphate. Fire Mater. 19 1-10. 

Weil, E.D. Levchik, S.V. Ravey, M. Zhu, W.M. A survey of recent progress in phosphorus-based flame retardants and some mode of action studies. Phosphorus, Sulfur Silicon Relat. Elem. 1999, 144, 17-20. 

The next major class of flame retardant additives that are nonhalogenated is the phosphorus-based flame retardants, but even these materials have some regulatory environmental concerns.Other nonhalogenated flame retardants that are not phosphorus-based exist, including mineral fillers (i.e., Al(OH)3, Mg(OH)2), expandable graphite, mela-mine, and polymer nanocomposites combined with other flame retardants.Each of these materials has its own advantages and disadvantages, and effectiveness in one polymer system often does not translate into another system. 

Weil, E. D. Phosphorus Based Flame Retardants, in Flame Retard. Polym. Mater. 1978, 2, 103-133 Levin, M. Atlas,... 

Phosphorus-based flame retardants include tris(l,3-dichloroisopropyl) phosphate, used in polyurethane foams and polyester resins. Once again, there is debate concerning toxic side-effects of such products although these flame retardants may save lives, they produce noxious fumes during a fire. 

Figure 11.19 Structures of phosphorus-based flame retardants.

Inorganic phosphorus compounds are also used as flame-retardants. Elemental red phosphorus itself is applied, for example, in polyurethane foams and more recently in polyamides. Some marketed types of red phosphorus-based flame-retardants are collected in Table 5.6. 

Toldy, A., Toth, N., Anna, P., and Marosi, G. Synthesis of phosphorus-based flame retardant systems and their use in an epoxy resin. Polymer Degradation and Stability, 91, 585-592 (2006). 

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