1. Field of the Invention
This invention relates to the purification of brominated aromatic compounds such as decabromodiphenyl ether and pentabromophenol and more particularly to a process for purifying aromatic compounds containing residual free bromine and by-product hydrogen bromide.
2. Description of the Prior Art
High levels of purity are required for brominated aromatic compounds which have found utility as flame retardant agents in polymer compositions. In particular, it is important that such brominated products have extremely low levels of residual impurities such as free bromine, hydrogen bromide, retained catalysts, by-product bromine containing derivatives and the like since the presence of such impurities can have undesirable effects on the polymer compositions in which such agents are used. Purity is particularly important from the standpoint of color, and thermal stability under the processing conditions in commercial molding operations is likewise an essential parameter.
High purity is an especially important consideration in the case of decabromodiphenyl ether, a flame retardant agent that has found wide application as an additive for high impact polystyrene used in television cabinets and other consumer appliances.
The preparation of decabromodiphenyl ether by the direct bromination of diphenyl oxide using excess bromine itself as a reaction solvent, as disclosed in British patent specification No. 1,411,524, published Oct. 29, 1975, and in Stepniczka, U.S. Pat. No. 3,965,197, issued June 22, 1976, produces highly brominated derivatives containing undesirably large amounts of occluded free bromine and by-product hydrogen bromide.
The cited British specification describes the use of various chemical purification treatments (e.g., injection of sulfur dioxide to convert occluded bromine into hydrobromic acid and injection of ethylene to convert residual bromine into dibromoethane). The cited U.S. patent merely describes the recovery of crude brominated product and gross separation of bromine therefrom by injecting superheated steam and by washing with hot dilute hydrochloric acid and hot water. The patent does not describe purification of the crude reaction project.
Traditional purification methods such as recrystallization techniques are usable only with difficulty with materials such as decabromodiphenyl ether because its limited solubility in available solvents makes recrystallization both cumbersome and uneconomical.
Burk, U.S. Pat. No. 3,733,366, granted May 15, 1973, describes a procedure for decolorizing brominated biphenyl obtained by treating biphenyl with bromine or bromine chloride in methylene chloride solvent in the presence of aluminum chloride catalyst. The patentee describes heating the product to a temperature between about 100.degree. C. and about 160.degree. C. with a preferred temperature range being about 110.degree. C. to about 150.degree. C. (Col. 2 lines 7-13). The disclosed heating step may take place optionally in the presence of solvents such as ethylene dibromide, toluene and xylenes. The patent is concerned only with decolorizing brominated biphenyl and contains no disclosure that would aid one in purifying materials such as decabromodiphenyl ether using a heat treating step.
Britton, U.S. Pat. No. 2,022,634, issued Nov. 26, 1935 relates to the halogenation of diphenyl oxide and describes (page 2, col. 1, lines 3-7) heating the reaction mixture to above 75.degree. C., preferably between 250.degree. and 350.degree. C., to destroy bromine addition products. Britton's products are not completely brominated, nor does the patentee recognize that free bromine, rather than bromine addition products, may be removed by such a heating step.
Nagy et al, U.S. Pat. No. 3,752,856, issued Aug. 14, 1973, is directed to a process for producing brominated compounds in which an intimate mixture between bromine and the organic starting material is obtained by physical mixing with a sigma type blender. After bromination, the apparatus is flushed with dry air, preferably under lowered pressure, to remove the residual hydrobromic acid and, if applicable, excess bromine, and the whole mass is allowed to cool while continuing the grinding operation. The brominated aromatic product is said to be removed in the form of a powder as a rule. (Column 3, lines 5-11).
The patentee suggests that gaseous ammonia be passed into the apparatus to neutralize "the hydrobromic acid and possibly the bromine retained by the product" and goes on to describe purification of "[t]he crude product" by "washing with acidified water or better still, by wet grinding in the presence of a dilute inorganic acid, followed by washing with water and by drying." The reference also suggests that "[r]ecrystallization in an appropriate solvent offers another possibility of purification."
Nagy et al's Example III is a preparation of decabromodiphenyl in which the crude product was "heated at the temperature of 200.degree. C. under normal pressure and under a nitrogen flow . . . ," the resulting product containing over 2 weight percent ammonium and aluminum bromide impurities. In Example IX of the patent, crude decabromodiphenyl was first subjected to dry air at 70.degree. C. under vacuum (20 mm Hg) followed by heating at 150.degree.-200.degree. C. "under a flow of air." Recovery was completed by washing the "crude product" with aqueous NaOH. In Example IV, which is directed to decabromodiphenyl ether, purification is carried out by recrystallization in chlorobenezene.
Brackenridge, U.S. Pat. No. 3,833,674, issued Sept. 3, 1974, describes a process for polybromination of aromatic compounds including diphenyl ether by reacting an aromatic compound with bromine in the presence of methylene bromide solvent. The patent states (column 2, lines 11-15) that "a main improvement" of the invention is the isolation of the product by precipitation from the reaction mixture by adding methanol or the like as a precipitant.
Brackenridge describes four approaches to enhancement of product purity. (Column 4, line 46, et seq.) More particularly, the patent suggests the exclusion of light from the methylene bromide and methanol and from the reaction and recovery steps, minimization of temperature, and use of distilled solvent and precipitating agent. (Id., lines 60-68).
As a result, the prior art has failed totally to appreciate the applicability of the techniques of the present invention to the purification of brominated aromatic compounds such as decabromodiphenyl ether. In particular, the prior art has failed to recognize the importance of the crude decabromodiphenyl ether being thermally stable, much less that thermal stability is directly controlled by the substantial exclusion of aliphatic and alicyclic hydrocarbon impurities from the raw materials employed. The art further fails to disclose the importance of precise particle size control of the brominated aromatic product to be subjected to the heating step in achieving desired elimination of bromine, bromide, and other impurities.
Accordingly, it is a primary object of this invention to obtain a process for producing purified brominated aromatic compounds that is superior to the techniques that heretofore have been employed.
Another object is to provide a method of the character described that may be economically employed in purifying decabromodiphenyl ether.
A still further object is to provide a method of obtaining thermally stable decabromodiphenyl ether that may be purified in accordance with the present invention.