Manufacture+of+PBDE's


 * 1.7 Manufacture of PBDEs**

Tetra, Penta, Octa, and Decabromodiphenyl Ethers

The following narrative will outline the historical development of polybrominated diphenyl ether flame-retardant molecules. Note that polybromodiphenyl ether or polybromodiphenyl oxide both refer to the same biphenyl structure. Use of Friedel-Crafts catalyst in a traditional approach to brominating aromatic compounds typically results in only a dibrominated derivative. Tetrabrominated diphenyl compounds were made and reported early in the last century (Cook, 1910). This process treated diphenyl oxide at room temperature with an excess of bromine with iodine as a catalyst. U.S. patent 2022634 attributed to Dow Chemical (1933) describes how to make a mixture of halogenated diphenyl oxides with more than four, but less than ten halogen atoms. The Dow method involved a higher reaction temperature so the mixture is kept in a fluid (melted) condition (no mention of solvents being added). In the case of bromination, the reaction temperature must be greater than 75ºC, or greater than 150ºC for chlorination, both using iron from the reaction vessel as a catalyst. No mention made of efforts to achieve maximum halogenation. Rudimentary purification was attained by washing with water, followed with dilute alkali or extraction with chlorobenzene to remove hydrogen bromide impurities, and then distilled under strong vacuum. The process made halogenated derivatives which could be separated as tetra through to heptaBDE.

The products varied from a nearly colorless, oily liquid to a semi-solid resin, or crystalline solid, with increasing halogen content. At that time it was feasible to isolate the individual compounds from the mixture, yet was not considered economic, nor was it necessary to do so. Dow discovered these mixtures of brominated or chlorinated diphenyl oxide remained stable at high temperature, and exhibited high dielectric constants as compared to petroleum oil. The mixtures were also non-inflammable and non-corrosive. Dow found use for similar chlorinated biphenyl mixtures as heat transfer fluids in transformers and capacitors (polychlorinated biphenyls, PCB's). These halogenated diphenyl oxides were useful as extreme-pressure additives in lubricants. The solid and semi-solid brominated diphenyl mixtures exhibited elasticity and transparency, and were considered useful in varnishes and plastics.

The pentabrominated diphenyl ether for use as a fire-retardant is outlined in G.B. (Great Britain) Patent 874006 (1960) currently unavailable for review. Pentabrominated diphenyl ether initially entered the market in about 1965 for use as an additive in rigid molded plastics. The release of bromine is responsible for the fire-retardant effect, so the most potent fire-retardant molecule should be maximally brominated. In order to add a halogen to the phenyl ring the aromaticity must be broken which requires high energy of activation, thus each subsequent addition of a halogen is increasingly difficult. In the case of highly-brominated aromatic compounds, the reaction is also difficult because the products are solid and noted to agglomerate during the reaction. The more complete halogenation reactions were enabled by a catalyst such as iron or aluminum halides using methylene bromide as a solvent. The solvent enables easier stirring. The octabromodiphenyl ether derivatives are outlined by U.S. Patents 3763248 and 3833674 (both Ethyl Corp) in reaction to the initially favorable reports of effectiveness and rapid market acceptance of pentabromodiphenyl ether used as a fire-retardant. The initial patent (1973) represented an attempt to increase the potency of the molecule by further bromination. The latter patent (1974) describes an improvement to remove color-bodies by isolating the product with a precipitant such as methanol.

The decabromodiphenyl ether is described in detail by several U.S. Patents: 3959387, 4521633, and 4849549, among others. The 3959387 patent describes a process for polybrominated diphenyl oxides using methylene bromide as a solvent, with reaction temperatures from room temperature to 200ºC.

The use of pure methylene chloride requires costly purification steps, both to recover the solvent, and to remove transhalogenation by-products (bromochloromethane, and dibromomethane) in both the product, and in the solvent. These by-products unfavorably influence the transparency and fire-retardant properties of the final decabromodiphenyl ether mixture. The U.S. Patent 4849547 by Bromine Compounds Limited (1987) describes how to prepare a higher-purity (>96% pure) decabromodiphenyl ether that will not discolor at upon heating to 280ºC for two hours. It was determined that a reaction temperature greater than 80ºC lead to reduced thermal stability. The purified fire-retardant is initially colorless, and if prepared at less than 80ºC, the fire-retardant will remain colorless even as it is heated. A mixture of at least two of the following three solvents: dichloromethane, bromochloromethane, dibromomethane was used to enable a reflux temperature approaching 80º C. The optimum catalysts used were aluminum, aluminum trichloride, or aluminum tribromide. The thermally-stable decabromodiphenyl ether allows the final polymer mixture to remain colorless, despite heat in a typical plastic-mold. This preparation scheme may represent an epitome of development of polybrominated diphenyl ethers. Decabromodiphenyl ether contains about 83.3% bromine, with a total molecular weight of 959.2. Purified decabromodiphenyl ether is completely brominated, yet remained colorless to produce cosmetically-appealing melt or blow-molded products.

Cook, A. (1910). Phenyl ether and some of its derivatives. Journal of the American Chemical Society. 32: 1285-1294. [| http://pubs.acs.org/toc/jacsat/32/10]

Patent files:

Next page