Fire Retardant Additives

The materials of attention in promoting fire safety are generally organic polymers, both natural, such as wood (qv) and wool (qv), and synthetic, nylon (see Polyamides), vinyl, and mbber (qv). Less fire-prone products generally have either inherently more stable polymeric stmctures or fire-retardant additives. 

Useful materials incorporating fire-retardant additives are not always straightforward to produce. Loadings of 10% are common, and far higher levels of flame retardants are used in some formulations. These concentrations can have a negative effect on the properties and functions for which the materials were originally intended. Product-specific trade-offs are generally necessary between functionaUty, processibiUty, fire resistance, and cost. 

Zinc Borates. A series of hydrated 2inc borates have been developed for use as fire-retardant additives in coatings and polymers (59,153). Worldwide consumption of these 2inc salts is several thousand metric tons per year. A substantial portion of this total is used in vinyl plastics where 2inc borates ate added alone or in combination with other fire retardants such as antimony oxide or alurnina trihydrate. 

A key property associated with chlorinated paraffins, particularly the high chlorine grades, is nonflammability, which has led to their use as fire-retardant additives and plasticizers in a wide range of polymeric materials. The fire-retardant properties are considerably enhanced by the inclusion of antimony trioxide. 

The self-extinguishing characteristics of the chlorine-containing resins are improved by incorporation of antimony oxide but this approach is not possible where translucent sheet is required. As an alternative to chlorine-based systems a number of bromine-containing resins have been prepared and, whilst claimed to be more effective, are not currently widely used. It is probably true to say that fire-retarding additives are used more commonly than polymers containing halogen groupings. 

Polyurethane foams do, however, suffer from one serious disadvantage. Unless modified they bum with copious evolution of smoke and toxic by-products, which has led to a number of fatal fires, particularly in domestic accommodation. To some extent the problem may be reduced by suitable upholstery covering, but as mentioned on p. 775 a number of countries have now made mandatory the use of fire retardent additives. At the time of writing there is considerable activity in the development of new safer systems, particularly in the use of amino materials such as melamine as additives. Further developments may also be expected in the near future. 

The use of copolymers is essentially a new concept free from low-MW additives. However, a random copolymer, which includes additive functions in the chain, usually results in a relatively costly solution yet industrial examples have been reported (Borealis, Union Carbide). Locking a flame-retardant function into the polymer backbone prevents migration. Organophosphorous functionalities have been incorporated in polyamide backbones to modify thermal behaviour [56]. The materials have potential for use as fire-retardant materials and as high-MW fire-retardant additives for commercially available polymers. The current drive for incorporation of FR functionality within a given polymer, either by blending or copolymerisation, reduces the risk of evolution of toxic species within the smoke of burning materials [57]. Also, a UVA moiety has been introduced in the polymer backbone as one of the co-monomers (e.g. 2,4-dihydroxybenzophenone-formaldehyde resin, DHBF). 

As expected, the EDS data set indicates that the polymeric matrix material (the PE-PP blend) is composed only of carbon (hydrogen is not detectable by this method). The particle, however, appears to be composed mainly of aluminum and oxygen along with small amounts of copper. The ratio of aluminum to oxygen is consistent with the chemical formula for aluminum oxide (A1203). The SEM-EDS results are consistent with aluminum oxide and traces of copper as the primary constituents of the particulate contamination. (Al2O3.3H20 is a commonly used fire-retardant additive in polymeric products.)... 

FIRE RETARDANT FILLERS. The next major fire retardant development resulted from the need for an acceptable fire retardant system for such new thermoplastics as polyethylene, polypropylene and nylon. The plasticizer approach of CP or the use of a reactive monomer were not applicable to these polymers because the crystallinity upon which their desirable properties were dependent were reduced or destroyed in the process of adding the fire retardant. Additionally, most halogen additives, such as CP, were thermally unstable at the high molding temperatures required. The introduction of inert fire retardant fillers in 1965 defined two novel approaches to fire retardant polymers. 

The surface area and degree of dispersion in the polymer matrix of the fire-retardant additive has a pronounced effect on its efficiency. Colloidal tin(IV) oxide is significantly more effective, in terms of its flame-retardant ability, than powdered tin(IV) oxide or B-stannic acid. 

Melamine and its salts are widely used in formulations of fire retardant additives, particularly of the intumescent type (4-71. The role played by melamine structures in these additives is however not yet understood. The thermal behaviour is of paramount importance in studies of the fire retardance mechanism. It is known that melamine undergoes progressive condensation on heating with elimination of ammonia and formation of polymeric products named "melam", "melem", "melon" (8.91. The following schematic reaction is reported in the literature (10-121 ... 

As a result of human health concerns, production of mirex ceased in 1976, at which time industrial releases of this chemical to surface waters were also curtailed. However, releases from waste disposal sites continue to add mirex to the environment. Virtually all industrial releases of mirex were to surface waters, principally Lake Ontario via contamination of the Niagara and Oswego Rivers. About 75% of the mirex produced was used as a fire retardant additive, while 25% was used as a pesticide. As a pesticide, mirex was widely dispersed throughout the southern United States where it was used in the fire ant eradication program for over 10 years. 

Smoke opacity is another increasingly important characteristic measured by optical density. Fire-retardant additives can be halogenated or halogen-free, which reduces the corrosivity, toxicity and pollution risks. 

Brominated polystyrene is marketed as a fire-retardant additive. 

Due to the chlorine content, oxygen indices are higher than those of the polyethylenes, for example 23 to 25 without fire-retardant additives, but they will act as a combustible material in the event of fire. Combustion products include hydrochloric acid and carbon monoxide, both toxic gases. 

The principle uses of the zinc borate Zn[B304(0H)3] are as a polymer additive and preservative for wood composites, such as oriented strand board (OSB). As a polymer additive it functions as a fire retardant synergist and modifier of electrical and optical properties. Its function as a fire retardant additive is discussed further below. A substantial amount of Zn[B304(0H)3] is used to improve the tracking index, which is an important performance criterion for polymers, such as polyamides (nylon) and polybutyl teraphtha-lates (PBT), used in electrical applications. 

Electrical and office equipment enclosures, such as computer cases, copier cases and telecommunications equipment, are conventionally prepared from thermoplastic resins such as PC, ABS, and poly(propylene). These materials have the advantageous properties of toughness, flexibility and the ability to meet the UL specifications by including fire retardant additives. 

In the case of brominated additives the process is less studied, for decabromodiphenyl oxide, a widely used brominated fire-retardant additive, Sb8OnBr2 is the dominant oxybromide, whereas Sb405Br2 was not detected in measurable amounts. It was assumed that, if this last is formed by bromination of Sb8OnBr2, it eliminates SbBr3 relatively rapidly either by thermal disproportionation or by chemical reaction with decabromodiphenyl oxide.42... 

Decabromodiphenyl ether continues to be a popular fire-retardant additive for polyamide 6, its cost, high bromine content, and good thermal stability make it an attractive product. In Europe and the United States, most of its applications have been replaced by brominated polystyrene in order to address environmental issues. Brominated polystyrene is less expensive than the other polymeric fire retardants and has very good thermal stability. Moreover, it contributes to good electrical tracking index. On the other hand, it has lower efficacy as a fire retardant. 

Poly(pentabromobenzyl acrylate), another polymeric fire retardant, is particularly suitable for use with polyamides whether or not they contain fiber reinforcement. Its advantages over other fire-retardant additives result from a combination of its polymeric nature, high bromine content, and thermal stability. 

As already seen in Section 4.3, the primary action of halogen fire-retardant action for polypropylene is in the gaseous phase, thus the fire-retardant additives for polypropylene are often based on aliphatic bromine compounds in order to develop bromine at its low ignition temperature. 

Liquid chlorinated paraffins are the main halogen-containing fire-retardant additives used for poly(vinyl chloride) often in combination with a phosphate ester. In this case, the chlorinated paraffins have the secondary function of plasticizers. The thermal degradation mechanism of chlorinated paraffins is similar to that of poly(vinyl chloride), so in this case poly(vinyl chloride) stabilizers have also the secondary function to stabilize chlorinated paraffins. 

For more demanding applications, tetrabromophthalate ester is a thermally stable liquid fire-retardant additive with a bromine content of approximately 45wt%. Decabromodiphenyl ether is used for foamed soft PVC for thermal insulation even if diphenyl ether-free systems have been developed because of environmental concerns. 

Camino, G. Costa, L. Thermal degradation of a highly chlorinated paraffin used as a fire retardant additive for polymers, Polymer Degradation and Stability, 1980, 2(1), 23-33. 

Duquesne, S., Le Bras, M., Bourbigot, S., Delobel, R., Vezin, H., Camino, G., Eling, B., Lindsay C., and Roels, T. 2003. Expandable graphite A fire retardant additive for polyurethane coating. Fire Mater. 27 103-117. 

There are also a number of other hydroxides and hydroxycarbonates, which are increasingly being used as alternatives to halogen- and phosphorus-containing fire-retardant additives, since they are perceived to have less adverse impact on the environment. 

E. Kandare, D. Hall, D.D. Jiang, and J.M. Hossenlopp, Development of new fire retardant additives based on hybrid inorganic-organic nanodimensional compounds Thermal degradation of PMMA composites, in Fire and Polymers IV, Materials and Concepts for Hazard Prevention, C.A. Wilkie and G.L. Nelson (Eds.), ACS Symposium Series, American Chemical Society, 922, 2005. 

The efficiency of intumescent fire retardants could be enhanced by interlayers that deliver the active components to the surface (shown by two examples). The fire-retardant additives, delivered to the surface at early stage of combustion, accelerate the formation of protecting surface layer that hinders the degradation of the underlying material. This coating structure could be reinforced by an interlayer of ceramizing capability (e.g., polyborosiloxane). Phosphorus-free intumescent fire-retardant system could be formed by using such additive. 

Fire-Retardant Additives/Copolymers in Synthetic Fibers.744... 

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