Halogen-based flame retardants

Lately substitution of hazardous substances has become a hard task for fabric finishing companies. Substances such as easy-care products, fluorocarbons for water- and oil-repellent properties, various flame retardants (halogen or phosphor-based), plasticizers... 

Tried-and-tested flame retardant systems based on bromine compounds will continue to be used, with just a few exceptions, on account of their outstanding price-to-performance ratio. This is clear from printed circuits, where halogen-free reactive and additive FR systems are not able to displace the FR4 laminates based on tetrabromobisphenol-A (TBBA). Such is the case in Europe, at least. The situation could prove to be different in Asia, since the giants in the sector, such as Sony, have committed themselves to halogen-free printed circuits. In Europe and Asia, iimovations in flame retardant plastics continue to be in the field of halogen-fi ee systems. 

Some of this effort has been directed to replacement of tetrabromobisphenol A, a major reactive component of circuit boards (wiring boards). Extensive development of flame-retarded halogen-free circuit boards has taken place especially in Europe and the Far East to find alternatives to tetrabromobisphenol A-based epoxy circuit boards. The use of phosphate ester additives usually depresses the thermomechanical properties excessively, and so the use of reactive phosphorus compounds has been the preferred approach. 

For more than a decade potential environmental problems associated with organo-bromine flame retardant systems have motivated the search for non-halogen-based approaches to reduce polymer flammability. Initially, research focused on development of new phosphorus-based flame retardants, and numerous publications and patents have been issued in this area. Similarly motivated research has also produced nonhalogen flame retardant approaches based on other elements, such as boron and sihcon. At the same time, work on the use of additives, or flllers, with nanometer-scale primary particle sizes, produced polymer nanocomposites. These materials exhibit enhancement in a variety of physical properties at one-tenth the loading required when micrometer-size additives are used. ... 

Laurel Industries flame retardants are based on Thermoguard antimony-halogen and Pyronil brominated plasticizer technology and used to flame retard a variety of plastics and other materials. Synergistic action occurs when Thermoguard antimony is combined with a halogen which continues to be a very cost effective flame retardant technology for most polymers. 

In polymers such as polystyrene that do not readily undergo charring, phosphoms-based flame retardants tend to be less effective, and such polymers are often flame retarded by antimony—halogen combinations (see Styrene). However, even in such noncharring polymers, phosphoms additives exhibit some activity that suggests at least one other mode of action. Phosphoms compounds may produce a barrier layer of polyphosphoric acid on the burning polymer (4,5). Phosphoms-based flame retardants are more effective in styrenic polymers blended with a char-forming polymer such as polyphenylene oxide or polycarbonate. 

Usage of phosphoms-based flame retardants for 1994 in the United States has been projected to be 150 million (168). The largest volume use maybe in plasticized vinyl. Other use areas for phosphoms flame retardants are flexible urethane foams, polyester resins and other thermoset resins, adhesives, textiles, polycarbonate—ABS blends, and some other thermoplastics. Development efforts are well advanced to find appHcations for phosphoms flame retardants, especially ammonium polyphosphate combinations, in polyolefins, and red phosphoms in nylons. Interest is strong in finding phosphoms-based alternatives to those halogen-containing systems which have encountered environmental opposition, especially in Europe. 

A significant advance in flame retardancy was the introduction of binary systems based on the use of halogenated organics and metal salts (6,7). In particular, a 1942 patent (7) described a finish for utilizing chlorinated paraffins and antimony(III) oxide [1309-64-4]. This type of finish was invaluable in World War II, and saw considerable use on outdoor cotton fabrics in both uniforms and tents. 

Halogenated intermediates based on chlorendic anhydride and alkoxylated, brominated bisphenol are quite stable and are used extensively in flame-retarded high temperature compositions, but brominated aUcychcs, such as dibromotetrahydrophthahc resin, are rapidly dehydrohalogenated at lower temperatures. 

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). 

More recently, based on the results of an extensive series of small scale degradation studies, two additional mechanisms for the volatilization of antimony from antimony oxide/organohalogen flame retardant systems have been proposed (23,24). Of these two proposed mechanisms, [4] and [5], [4] does not involve HX formation at all and [5] suggests an important role for the direct interaction of the polymer substrate with the metal oxide prior to its reaction with the halogen compound. 

An overview is provided of ongoing risk assessments on halogenated phosphate ester flame retardants in Europe. On the basis of the so-called second and fourth Priority lists on Existing Chemicals (Council Regulation No793/93) three chlorinated phosphate ester flame retardants are selected. The selection is based on their hazard profile, volume and use pattern. The three substances involved are TCPP, TDCP and TCEP (Antiblaze V6 from Albemarle is also involved but, due to confidentiality, is not discussed. An outline is provided from a European point of view on topics such as methodology of risk analyses, data-gaps and worst case approach, industry involvement, downstream participation and possible impact of final report on industry. 2 refs. 

Examples of foaming agents are sodium bicarbonate, ammonium bicarbonate and nitroso compounds. Especially, when dinitroso-pentamethylenetetramine is used, foaming aids can be added that accelerate the decomposition and reduce the decomposition temperature. This is achieved, for example, with salicylic acid, or with urea. Various chlorine and bromine flame retardants can be used as halogen-based flame retardants, e.g., tetrabromobisphenol A de-rivates. 

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