PROCESS DEVELOPMENT OF TET RABRO M0 BIS PH EN0 L A HENRY E. HENNIS
Benzene Research Laboratory, The Dow Chemical Co., Midland, Mich. Tetrabromobisphenol A was prepared by the reaction of bisphenol A and bromine in an ethanol-water solvent. The composition of the solvent was designed to permit precipitation of the product as it was formed. Underbrominated bisphenol A and colored impurities were occluded in the precipitate, giving a finished material of low assay and high color. The product properties were considerably improved by heating the reaction mixture after bromine addition. This “digestion” permitted the release of underbrominated bisphenol A for complete bromination to tetrabromobisphenol A, leached a considerable amount of colored impurities, and increased the yield. This technique produced tetrabromobisphenol A without extensive purification in reasonably high purity and very good yield.
resins are used in many applications in ever-increasTheir application is limited, however, by their flammability. Recent patents (4,6) have indicated that epoxy resins incorporating 4,4’-isopropylidenebis-(2,6-dibromophenol), more commonly referred to as tetrabromobisphenol A: are fire-retardant. Tetrabromobisphenol A (11) is prepared by the reaction of bisphenol A (I) and bromine. T h e compound was first prepared by Zincke and Gruter (7) in 1905. They utilized glaciai POXY
E ing volume.
reaction of methanol and hydrogen bromide, presented a major toxicity problem. Dietzler’s patent (2) taught that a n
CH,OH
+ HBr
-P
CH3Br
+ H20
(2)
ethanol-water solvent system was workable, but inferior to methanol-water. The ethanol-water solvent would be highly desirable if an economical technique could be introduced to upgrade the product and decrease production variation. Research was initiated to achieve this goal. Experimental
CH3
I
Br
H C ) D T G r : H Br
+ 4HBr
(1)
CH3
I1 acetic acid as the reaction solvent and reported a melting point of 162-63’ C. for the chemical. Later investigators reported melting points of 155-60’ C . (3)and 160-61’ C. ( 7 ) for tetrabromobisphenol A, which is in agreement with Zincke and Griirer (7). In 1959 Sclinell (5) reported the melting point of 178-80’ C. and Dietzler (2) later stated that a highly purified sample melted a t 181-82’ C. Dietzler (2) was the first to conduct a considerable amount of development work toward a practical process for making tetrabromobisphenol A. His elegant procedure of employing an alkanol-water solvent permitted convenient isolation and purification of the product, because the tetrabromobisphenol A separated from solution in high yield as it was formed. T h e methanol-water solvent system was extensively studied and optimized and appeared to be the most practical. Large scale day-to-day production of tetrabromobisphenol A was anticipated in one of our associated plants and careful scrutiny of Dietzler’s method (2) by production personnel revealed that the procedure should be made safer with less production variation. The use of methanol presented a minor fire and toxicity problem. T h e presence of the colorless, odorless. and tasteless gas, methyl bromide, formed by the 140
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
Equipment. Plant batch bromination facilities were simulated by a 2-liter resin pot equipped with a dropping funnel, motor-driven stirrer, reflux condenser fitted with a Dean-Stark trap for distillate removal, and thermometers to measure reaction medium and reflux vapor temperatures. T h e bromine was weighed in the tared dropping funnel. Evaporation during the transfer of bromine from a weighing vessel to the dropping funnel caused considerable underbromination. Melting points were taken in capillary tubes in a n air bath and are uncorrected. The product softened considerably prior to melting, making it difficult to judge the lower limit of the melting point. The freezing point did not have this disadvantage and was adopted. The freezing points were determined by melting the product in a noninsulated standard 6-inch test tube, allowing it to cool while being stirred with a calibrated thermometer, and recording the freezing point plateau. Thus the freezing points in this work are lower than by the ASTM method. This simple method was adopted because it was felt that the relative freezing points were as significant as the absolute values. T h e color of the product was determined by the APHA method, employing a 10-gram sample dissolved in 50 ml. of methanol. The color faded for about 40 minutes and then was constant. Thus two APHA color values were recorded, one immediately after dissolving of the sample and the second an hour later. All other standard methods of color determination were even less satisfactory. Process. Bisphenol A (344 grams, 1.51 moles, epoxy resin grade, f.p. 154.0” C.), 483 grams of alkanol, and 228 grams of water were loaded in the resin pot. Complete solution was attained after stirring a few minutes. T h e reactor was immersed in an ice bath and 968 grams (6.05 moles) of bromine was added dropwise at a rate to maintain the desired reaction temperature. Then the reaction mixture was stirred and heated. T h e alkyl bromide was removed by the Dean-Stark trap when higher temperatures were needed than the presence
Table 1.
Solvent, Reaction Temperature, and Heating Temperature Data
Reaction Heating6 Prodyt Productt Yield, Run Solventa Temp., O C. Temp., C. F.P., C. Color, APHA % 25 164 155,110 94.6 25 6 EtOH 172 58 64,50 94.2 25 7 EtOH 175 83 132,95 96.0 25 8 EtOH 95.0 174.5 83 150,110 EtOH 15 9 95.0 174.5 160,118 83 EtOH 35 10 93.3 171.5 185,130 83 EtOH 45 11 93.7 175.5 90,30 83 n-PrOH 25 12 91 . O 171.5 130,52 83 IsoPrOH 25 13 88.5 178.5 80,45 83 n-BuOH 25 14 94.9 167 170,132 83 MeOH 15 25 First value determined immediately after dissolving b All experiments utilized 30-minute heating period. a Mixture of alcohol indicated and water. sample. Second value was color 1 hour after dissolving when fading had ceased
of a low-boiling alkyl bromide would permit. T h e reaction mixture was then cooled to 2 5 " C. and the product collected on a Biichner funnel. T h e product was first washed by pulling a 130-gram and two 440-gram portions of water through the cake and then slurry-washed until the washes indicated a p H of 5. About 10 slurry washes were required. T h e product was dried in a n oven a t 60 O C. for 36 hours. Results and Discussion
I t was first necessary to conduct some exploratory experiments introducing techniques that would upgrade the product to a desirable level with the ethanol-water solvent system. Dietzler (2) had described a run utilizing a n ethanol-water solvent and had obtained tetrabromobisphenol A as a light tan powder (m.p. 170-74.5' C.) in 93.5% yield. We repeated this experiment and obtained product of equivalent quality. Infrared spectroscopic analysis indicated that the major impurity was tribromobisphenol ,4(111). This had to be a ten-
When bisphenol A was brominated in ethanol and water added, the product set to a concrete-like mass on the filter when separation of product and mother liquor was attempted. Further study in this approach was discontinued. The alternative procedure gave promising results. A heating period of approximately 30 minutes a t 75' to 80' C. pot temperature yielded a very light tan tetrabromobisphenol A (m.p. 174.579' C.) in 96.0% yield. This also improved the crystalline nature of the product, so that it was much easier to filter and wash. Visual observation indicates that a considerable amount of free bromine is present in the reaction mixture and the brown bromine color gradually decreased during heating until it was absent. Figure 1 illustrates the manner in which this heating period was conducted. T h e plotted temperature was recorded in the bromination vessel. Time 0 represented bromine addition completion. Heat was applied to the vessel and the pot temperature subsequently increased. Boiling started a t 74' C. ; the distillate was primarily ethyl bromide formed by the reaction of ethanol and hydrogen bromide. Failure to remove this distillate would have brought the system to a state of reflux and
+
CHICH~OH HBr I11 tative conclusion, because pure tribromobisphenol A was not available for a n analytical standard. Experiments were then designed in an effort to reduce the amount of this impurity. This presence of detectable amounts of tribromobisphenol A indicated underbromination. The futility of using an excess of bromine to encourage complete reaction had been revealed by Dietzler (2). Excess bromine did not change the melting point. Rather, it had a n adverse effect, in that the product was deep pink in color. The product could be upgraded considerably (white. m.p. 179-80' C ) by heating it in a n ethanolwater mixture to leach out impurities. The yield (78.5%) was expectedly low on this first pass. The mother liquor from this digestion procedure had to be recycled into the next bromination reaction to make this a practical process. Product dissolved in this recycle mother liquor plus the product prepared in the bromination reaction made the succeeding reaction mixture so thick that it could not be stirred. The lack of response in upgrading the product by a n excess of bromine (2) and the improvement of the product by a digestion procedure strongly indicated that the problem was a n occlusion of underbrominated bisphenol A in the precipitate. Two solutions to this problem Jvere apparent. T h e precipitation of the product could be delayed until bromination was complete or the product could be dissolved to allow the release of underbrominated bisphenol A for complete reaction.
4
CH&H2Br
+ HgO
(3)
the pot temperature would have decreased as more ethyl bromide was formed. The ethyl bromide was therefore removed to drive the temperature even higher to dissolve more product and thereby release more occluded impurities. Distillate removal was discontinued a t 83' c. and the pot temperature decreased as shown. 90 r
/ DISTILLATE REMOVAL DISCONTINUED
70 -L
/
I
-.
'"1 10
10
Figure 1. heating
20
30
40 TIME, MIN.
50
60
70
Reaction mixture temperature profile
VOL. 2
NO. 2
JUNE 1 9 6 3
80
during
141
160
I76
140
ci n
I-
6
a
=-
172
s
u
120
s
z
5
W
168 U.
100
0 0 n U
I3 0
U
164
80
160
60 0
10
20
30
H E A T TIME,
Figure 2.
40
50
60
10
Figure 3.
Product freezing point increase during heating
Figure 2 illustrates the relationship of heating period time and product freezing point. After a maximum heating temperature of 83' C. was attained, the freezing point of the product increased no less than 14' C. Infrared spectroscopic analysis estimated that product assay increased from 88.5 to 98.5%. Heating beyond 30 minutes after maximum temperature did not increase the freezing point. This was expected, because a heating period of 30 minutes was required for bromine color disappearance in the exploratory experiments. Figure 3 shows the effect of the length of the heating period on the color of the product in APHA units. Two values were recorded because of the fading. The color of the tetrabromobisphenol A rapidly decreased during the first 20 minutes and then increased. I t is thought that color leaching is the primary process during the first part of the heating period, and that color impurity formation and deposition overcompensate color leaching during the latter portion. Bisphenols are unstable in a highly acidic environment and form colored products. .41so one cannot ignore the possibility of a marked solvent polarity change due to ethanol depletion and water formation from ethyl bromide formation. I t is likely that the colored impurities could be first leached and then redeposited primarily from a solvent change. The fact that the freezing point and yield are not lowered during the latter part of the heating period indicates insignificant product degradation. Figures 2 and 3 indicate that one cannot have the least color and the highest assay in a single run. A run using 20-minute heating would give tetrabromobisphenol A with lowest color, but with a freezing point about 1' C. lower than a run with 30-minute heating. Table I shows some of the more interesting effects in this work. Heating period temperatures were varied in runs 6 to 8 and the results support earlier conclusions. High temperature heating periods promote high assay and color. whereas an intermediate temperature (run 7) gives a product of lower color and purity. The variation of the temperature during bromine addition (runs 8 to 11) gave results which indicated that high quality tetrabromobisphenol A could not be made by exclusion of underbrominated bisphenol A by means of a high reaction temperature during bromine addition. The highly colored products obtained at the higher temperature runs were probably due to excessive aniounts of free bromine, contributing to oxidation ieactions. 142
0
MIN.
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
20
30 40 H E A T I N G TIME, MIN.
50
60
Product color variation during heating
Runs 12 to 15 show the results of the use of alcohols other than ethanol as solvent components: the solubility of the product in the solvent and the ease of the alcohol to form alkyl bromides and thus change the polarity of the solvent contribute to the purity, color, and yield of product. n-Propyl- and nbutyl alcohol-water combinations gave tetrabromobisphenol A with good properties, but in lower yields. Isopropyl alcoholand methanol-\vater systems were unsatisfactory, probably because of alcohol depletion by alkyl bromide formation. The methanol-water system which was so successful in Dietzler's method (2) was the least satisfactory when a heating period was utilized. Much of this work was rather dependent on the assumption that tribromobisphenol A (111) was the major impurity. This compound was recently synthesized in this laboratory and conclusively shown by infrared spectroscopy to be the major impurity. The method of synthesizing tribromobisphenol A and other intermediates encountered in the bromination of bisphenol A to tetrabromobisphenol A will be published soon. The use of an ethanol-water solvent for the bromination of bisphenol A to prepare tetrabromobisphenol A and a heating period to upgrade the product was successfully scaled u p in the manufacturing plant. Plant- and laboratory-prepared products were almost indistinguishable in freezing point, color, and physical appearance. Acknowledgment
The author is grateful to J . D. Wenger, J. P. Easterly, Jr., L. R . Thompson, and L. R . Collins for the experimental work. literature Cited
(1) Bralley, J. A., Pope, F. B., Brit. Patent 614,235 (Dec. 13, 1948). (2) Dietzler, A. J., U. S.Patent 3,029,291 (April 10, 1962). (3) Marsh, P. B., Butler, M. L., Clark, M. S.,Ind. Eng. Chem. 41.> 2176 ~- - (1949). (4)Nametz, R. C., U. S.Patent 3,058,946(Oct. 16, 1962). (5) Schnell, H., Ind. Eng. Chem. 51, 157 ( 1 9 5 9 ) . (6) Wismer, M., U. S. Patent 3,016,362(Jan. 9, 1962). (7) Zincke, T., Gruter, M., Ann. 343,86 (1905). \ - -
- / -
RECEIVED for review January 23, 1963 ACCEPTED March 15, 1963 Presented in part, Division of Industrial and Engineering Chemistry. 142nd Meeting, ACS, Atlantic City, N. J., September 1962.
