Aliphatic bromine

Aromatic and aliphatic bromine compounds play an important role as industrial products, e.g. special products are widely used as flame retardants for polymeric materials (ref. 1). Because there is an increasing interest and concern about the behaviour and fate of anthropogenic compounds in the environment (ref. 2), we have studied the physical behaviour and chemical reactivity of these products which are relevant to the environment. The main object is the study of their thermal behaviour during incineration, as well as photolytic reactions. Of prime concern is... 

Polypropylene. 3 series of experiments, in which the first stage was performed with Rr2 solutions of 3 different concentrations, at 28 C and various time intervals are summarized in Figure 1. The amounts of NH Br found in the polymer Increase with B concentration to a maximum concentration of 5.7% of NH Br. The 0.1. values of the samples (Fig. 1) appear to correlate closely with the NH Br. Table I shows the average effectivity, e.g. the slope of the plot of O.I. vs % Br for the PP fabric to be 1.24. This value is seen in Table II to be considerably higher than the value of 0.6 reported by van Krevelen [12] for aliphatic and aromatic bromine compounds and of 0.5 found bv Green [13] for aliphatic Br compounds. According to Green [13], 10% of aliphatic bromine are needed to produce a PP with an O.I. of 23.5, as compared to 4,6% Br from NH Br, which produced an 0.1. of 24.2 in the present experiments. 

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. 

Kaspersma, J. Doumen, C. Munro, S. Prins, A. M. Fire retardant mechanism of aliphatic bromine compounds in polystyrene and polypropylene, Polymer Degradation and Stability, 2002, 77(2), 325-331. 

Aliphatic chlorine compounds find some utility as flame retardants for styrenic polymers, but aromatic chlorine compounds are probably too stable to be effective [21]. Aliphatic bromine compounds are too thermally unstable for com-... 

Because expanded polystyrene foam is processed at a lower temperature, aliphatic bromine compounds such as hexabromocyclododerane (HBCD) can be used for this application. The flame retardant levels in these systems are family low, typically less than 3wt%. These levels are sufficient to pass the Steiner Tunnel test, and synergists such as antimony trioxide are not necessary. 

Bromine compounds are also used as fire retardants. These compounds are about twice as effective as chlorine compounds on a weight basis, so that significantly lower concentrations are needed. However, bromine compounds are higher in cost than chlorinated compounds and are generally less stable under exposure to heat and light (29). Those compounds containing aromatic bromine are significantly more stable to heat and hydrolysis than the aliphatic type. Examples are decabromodiphenyl oxide (DBDPO), tetrabromobisphenol and tetrabromobisphenol A. A pentabromodiphenyl oxide blend is available for urethane foams and polyesters (34). Aliphatic bromine-type additives are used as flame retardants in plastic foams (polyurethane and polystyrene (33). 

The bromine linked to a double bond or linked to an aromatic nucleus are much more stable structures (not easily decomposed to HBr as dibromo neopentylglycol, a saturated aliphatic bromine compound). Thus, a very successful bromine containing diol, produced industrially [4, 24], is based on tetrabromophthalic anhydride. Tetrabromophthalic anhydride is reacted first with diethylene glycol and the resulting half ester is reacted with propylene oxide (PO) (reaction 18.5) [3]. 

Bromine-containing compounds on a weights basis are at least twice as effective as chlorine-containing ones. Because of the smaller quantities of bromine compounds needed, their used hardly influences the mechanical properties of the base resins and reduces markedly the hydrogen halide content of the combustion gases. Aliphatic, cycloaliphtic, as well as aromatic and aromatic-aliphatic bromine compounds are used as flame retardants. 

Aromatic-aliphatic bromine compounds, like the bis(dibromopropyl)-ether of tetrabromobisphenol A, or bromoethylated and aromatic ring-brominated compounds, such as l,4-bis-(bromoethyl)-tetrabromobenzene, combine the high heat stability of aromatic-bound bromine with the outstanding flame retardancy of aliphatic-bound bromine. They are used mainly as flame retardants for polyolefins, including polyolefin fibers. 

Halogenated flame-retardants may be either additive or reactive. The additive ones are aliphatic/aromatic or otherwise cyclic halogenated hydrocarbons. Generally their efficiency is inversely related to their thermal stability. The flame-retardant effect of the halogen atom (X) is increased by hydrogens on a vicinal carbon atom (as in aliphatic halogenated compounds) due to the ready scission of HX from the carbon chain. It is obvious, however, that these compounds are poor in thermal stability. The same is true for brominated flame-retardants they are more effective but less stable than chlorinated ones. For this reason, chlorinated paraffins and brominated aromatics are more commonly used as flame-retardants rather than aliphatic bromine compounds. 

A cold suspension of 3 eqs. AICI3 in dichloromethane at 0° treated with 6 eqs. borane-tcrt-butylamine, stirred for 10 min, a soln of 1 eq. 4 -methoxy-2-bromoaceto-phenone in the same solvent added, and stirred at 0° for 2 h before acidic (0.1 N HCl) work-up /7-methoxyphenethyl bromide. Y 86%. The reagent is mild, easy to handle, and selective, leaving ester groups, aromatic halogen (Cl, Br), and aliphatic bromine unaffected. F.e.s. C.K. Lau et al., J. Org. Chem. 54, 491-4 (1989). 

Organic Bromine. This is an efficient flame retardant that can be greatly syner-gized by addition of antimony trioxide. Since aliphatic bromine is too unstable for plastic processing, preferred compounds are polybrominated diphenyl ethers for thermoplastics, and tetrabromo bisphenol A and tetrabromophthalic anhydride for epoxies and polyesters. In a fire, it does produce smoke and toxic corrosive gases, so this must be considered in specific applications. In Europe, environmental concerns may limit the use of bromine. 

Tribroinoneopentyl alcohol (TBNPA) is a reactive FR containing more than 70% aliphatic bromine. It is exceptionally stable and is particularly suitable where thermal, hydrolytic, and light stability are required. It is highly soluble in polyether polyols, making it particularly suitable for use in polyurethane polymers. 

The type and the quality of the suspension agent system and the reaction temperature profile are critical to ensure good suspension stability, bead size distribution and a high conversion rate. These parameters also influence the properties of the final product. An aliphatic brominated compound is specifically added for the production of flame-retardant grades. 

As previously discussed, the key to HALS-flame retardant incompatibility is acid generation by the flame retardant, which in turn deactivates the HALS. The mechanism for bromine radical generation by flame retardants is quite structure dependent. Aliphatic brominated flame retardants are primarily decomposed thermally, which may occur during the extrusion process. Aromatic brominated flame retardants are relatively stable through the processing step but may generate bromine radicals during UV exposure. 

Figure 1.1 compares the flame retardant efficiency of aliphatic brominated flame retardant and aromatic brominated flame retardant. Because the thermal decomposition of the aliphatic flame retardant starts at temperatures below the thermal decomposition of pol5T)ropylene, it shows very good performance in polypropylene. In contrast, because the aromatic brominated fire retardant is significantly more stable, optimum debromination is not achieved at the temperature of decomposition of polypropylene, and this flame retardant shows inferior performance. 

FIGURE 1.1 Dependence of total flaming time of polypropylene measured in a UL-94 test on bromine content for an aliphatic brominated flame retardant and an aromatic brominated flame retardant. (From Ref. 23, cop)Tight 2001, Routledge/Taylor Francis Group, with permission.)... 

There is another condensed-phase mode of action that is specific for aliphatic bromine, and it is the opposite of char formation. Bromine radicals generated thermally at low temperature in the polymer melt can cause chain scission at tertiary C atoms.Examples of polymers where this mechanism is operational are polystyrene (foams) and polypropylene (preferably thin parts, films, or fibers). The decreased molecular weight causes fast dripping of the hot polymer, which cools the flame and eventually extinguishes it ... 

Finberg, I. Bar Yaakov, Y. Georlette, P. Squires, G. Geran, T. Fire retardant efficiency and properties of aliphatic bromine compounds in styrenic copolymers, in Proceedings of the 12th Conference on Recent Advances in Flame Retardancy of Polymeric Materials, Stamford, CT, 2001. 

The lower melting point and processing temperatures of polyethylenes permit the use of chlorinated paraffins, whereas PP requires stabler additives [19, 26]. Since PP is less stable than polyethylene, it usually requires higher concentrations of flame retardants. The decomposition temperature of the polymer must be matched to the decomposition temperature of the flame retardant in order to obtain maximum effectiveness thus polyethylene is better with stabler aromatic bromine, whereas PP is better with less stable aliphatic bromine [18, 19, 26]. 

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