Halogen-based fire retardants

Toxicity and environmental concerns led to submission of a proposal to European Union to ban the use of PBDPE in 1989 (111-4301-89-EN Draft). The proposal was rejected on the basis of recommendations issued by a thorough debate between scientists, regulators, producers, and users of fire retardants, stating that banning would involve an unacceptable fire risk since alternatives were not available to replace halogen-based fire retardants with comparable effectiveness. Ever since, fire-retardant research has been mostly devoted to the development of nonhalogenated replacements for the halogen fire retardants. 

Though closed-cell rigid polyurethane foams are excellent thermal insulators, they suffer form the drawback of unsatisfactory fire resistance even in the presence of phosphorus- and halogen-based fire retardants. In this context, polyisocyanurates, which are also based on isocyanates, have shown considerable promise. Isocyanurate has greater flame resistance then urethane. Although rigid polyurethane is specified for the temperatures up to 200°F (93°C), rigid polyisocyanurate foams, often called trimer foams, withstand use temperatures to 300°F (149°C). Physical properties and insulation efficiency are similar for both types. 

First of all, halogen-based fire retardant additives (especially bromin-ated compounds associated with the antimony trioxide) are widely used. These systems release obscuring, corrosive and toxic smoke when they perform their fire retardant action. More, some of them release super toxic compoimds ( dioxins and polybrominated dibenzofurans) when exposed to heat during manufacturing or in fire. A continuous trend is the development of polymeric materials with reduced fire hazard, in order to meet the requirement of the international regulations (5th OECD Draft Status report (04/1993) and UN Environmental Program 1st Draft Report (01/1993)). 

Whilst rigid closed-cell polyurethanes are excellent thermal insulators they do suffer from a limited and often unsatisfactory level of fire resistance, even in the presence of phosphorus-containing and halogen-containing fire retardants. Considerable promise is now being shown by the polyisocyanurates, which are also based on isocyanate chemistry. 

Halogen-based flame retardants have served a great need for effective flame retardancy for several years. Due to relatively recent environmental concerns, there is a continuing trend toward the development of nonhalogenated materials to replace these systems. While this has been underway for quite some time, it does not appear that nonhalogenated materials will be available in the near future. Hence it appears that there is still a need for these materials to prevent fires. 

Polyolefins When used in conjunction with a halogen-based flame retardant, this zinc borate can partially replace antimony oxide (30%-40%) and still maintain the same fire test performance. In addition, it can improve aged elongation properties, increase char formation, and decrease smoke generation. The B203 moiety in zinc borate can also provide afterglow suppression (Table 9.6). 

M.S. Cross, The Development and Application of Halogen-Free Tin-Based Fire Retardant Additives in EVA, Eng. D. Thesis, Brunei University, Uxbridge, UK, 2004. 

PUR Implies occupational exposure to highly toxic isocyanides in the production, processing, manufacturing and in fires. PUR is commonly expanded with CFC. Halogen-based flame retardants are frequently used in the production of PUR. 

With the exception of alumina hydroxides, fire-retardant phosphorus compounds are now used in greater quantities than any of the other main types of retardants, that is, antimony oxide, borates or halogenated hydrocarbons. Advantages claimed for organophosphorus-based fire retardants are relatively low toxicity and minimum harmful volatiles under burning conditions [37]. 

Four halogen free, fire retardant resins, based on orthophthalic resin, filled with aluminium trihydrate. Approved for marine application by Det Norske Veritas. 

Under these conditions, regulations impose the use of materials possessing thermal stability as well as efficient fire retardant properties. In parallel, emissions of smoke must be low, not very opaque, not very toxic, and not very corrosive. This evolution toward greater safety seriously limits the use of many materials and involves the rejection of solutions largely used so far and, in particular, halogen-based flame retardants, on account of environmental concerns. Moreover, analysis of various statistics on plastic consumption (180 MT/yr, with a global annual growth rate of approximately 8%) shows the economic importance of this field and illustrates the world s industrial stake in it. 

Cables are available in a variety of constmctions and materials, in order to meet the requirements of industry specifications and the physical environment. For indoor usage, such as for Local Area Networks (LAN), the codes require that the cables should pass very strict fire and smoke release specifications. In these cases, highly dame retardant and low smoke materials are used, based on halogenated polymers such as duorinated ethylene—propylene polymers (like PTFE or FEP) or poly(vinyl chloride) (PVC). Eor outdoor usage, where fire retardancy is not an issue, polyethylene can be used at a lower cost. 

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. 

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

The traditional halogen fire retardants used in styrenic copolymers are decabromodiphenyl ether and octabromodiphenyl ether, tetrabromobisphenol A, bis(tribromophenoxy) ethane, ethylene bis-tetrabromophthalimide, and chlorinated paraffins. Actually the octabromodiphenyl ether has been banned on precautionary principles, as will be explained below. The fire-retardant capabilities of the more effective halogen-containing compounds are in line with the quantity of halogen in the final polymer blend, with consideration for the use of synergists. Thus, the practical utility of these flame-retardant compounds (once the issue of degradation temperature is resolved) is often based on their ability to be blended into the polymer and to not substantially affect the physical properties of the polymers. 

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. 

This chapter has provided a concise account of an important type of flame retardants based on silicon. This class of flame retardants may provide an opportunity to develop systems for fire retardancy that are environmentally friendly. It seems that there is a growing interest in this type of flame retardant, and this trend most likely will continue, given the increasing concern over the release of the halogenated species into the environment. 

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