Preparation of Improved Polyester
Scope of Process
Commercial materials were used as received. T h e 10gallon vessel was of stainless steel, rated for 150-p.s.i., working pressure, with a jacket for heating with steam and cooling with water, and having a blade stirrer driven a t 140 r.p.m. The vessel was charged with 10.83 pounds (0.1104 mole) of maleic anhydride, 32.90 pounds (0.2223 mole) of phthalic anhydride, 3.26 pounds (0.0429 mole) of propylene glycol, and 16.25 grams of lithium chloride. After closing, air was removed by evacuating and flushing with nitrogen. T h e contents were heated to 125' C., with stirring. One-half hour after reaching this temperature the propylene oxide, 18.00 pounds (0.310 mole)! was admitted slowly over a 2-hour period by controlled pressure displacement from a connected charge tank. An additional 50 minutes of reaction time yielded polyester Ivith an acid number of 46.8. The polyester was transferred portionwise to a 5-liter glass flask equipped with a stirrer, a gas inlet line, a thermometer, and an electrically heated mantle rated for over 200' C. Phosphoric acid, 85%, 0.5 mole per mole of lithium chloride present, \vas added. The flask was flushed with nitroqen and the contents were heated to 200' C . for 4.5 holm. The resin was then cooled to 150' C., O.OIO~oby weight of hydroquinone was added, and the resin was diluted finally with sufficient styrene to yield a 627, solution.
'To the extent that introduction of molecular weightregulating initiator permits, pol>-esters can be prepared using other glycols such as diethylene glycol and diprop) lene glycol, or even a dibasic acid such as adipic acid. Succinic anhydride and chloroendic anhydride are suitable. 1,2-Butylene oxide and epichlorohydrin are readily available epoxides that have been used to obtain additional modifications.
\Veight losses during the preparation and isomerization steps were small. Typical properties establishing the suitability of the polyester and its blend with styrene are shown in Table 111. Table IV lists physical data obtained for a typical sample of cured resin.
Literature Cited (1) Allied Chemical Corp., Belg. Patent 625,783 (April 11, 1963). (2) Batzer, H., Mohr, B., hfakromol. Chem. 8, 217 (1952). ( 3 ) Devoe and Raynolds Co., Brit. Patent 839,773 (June 29,1960).
(4) Feuer. S. S., Bockstahler, T. E., Brown, C. A,, Rosenthal, I., Znd. Eng. Chem. 46, 1643 (1954). (5) Fischer, R. F., J.Appl. Polymer Sci.7, 1451 (1963). (6) Fischer, R. F., J . PolymerSci. 44, 155 (1960). (7) Fischer, R. F. (to Shell Oil Co.), U. S. Patent 2,966,479 (Dec. 27, 1960). (8) Hayes, R. A. (to Firestone Tire and Rubber Co.), Zbid., 2,779,783 (Jan. 29, 1957). (9) Zbid.. 2,822,350 (Feb. 4, 1958). (10) Malkemus, J. D. (to Jefferson Chemical Co.), Zbid., 2,910,490 (Oct. 27, 1959). (11) Meyer, I,. S.,Reinforced Plastics Division, Society of Plastics Industry 5th Annual Conference. 1950, Sect. 7. (12) Park, R. E., Johnson, R. M., Jesensky, A. D., Cather, R. D., SPE J. 17, 1088 (1961). (13) Schwenk, E., Gulbins. K., Roth, M., Benzing, G., Maysenholder, R., Hamman, K., Makromol. Chem. 51, 53 (1962). (14) Turunen, L., IND. ENC. CHEM.PROD. RES. DEVELOP. 1, 40 (1962). RECEIVED for review September 3, 1963 ACCEPTED November 18. 1963
HEAT STABILITY OF BROMINATED EPOXY RESINS BA R
T J
.
B R E M M E R, Benzene Research Laboratory, Ths Dozv Chemical Co., Midland, Mich.
Several factors influence the high temperature performance of bromine-containing epoxy resins. Heat distortion temperatures and weight changes at 300°, 400°, and 450' F. are useful standards for comparison. A beneficial effect i s realized from a decrease in the amount of hydrolyzable chlorine in some commercial brominated epoxy resins. Addition of nonhalogen-containing epoxy resins, lowering the bromine content, and, in many cases, the presence of small amounts of antimony trioxide further improve the properties. The type o f curing agent i s of great importance, the best results being obtained with anhydride hardeners. Amine-type converters are inferior to a boron trifluoride-monoethylamine complex. Epoxy resins based on tetrabromobisphenol A have better high temperature properties than resins based on the asymmetric 2,4,6-tribromo-3-hydroxybenzoic acid. ERTAIN
bromine-containing epoxy resins provide excellent
C flame-retardant characteristics with little loss of other
desirable properties. One such resin is D.E.R. 542 epoxy resin (Dow Chemical Co.) which is essentially 2,2-bis [3,5dibromo-4-(2,3-epoxypropoxy)phen~d]propane,normally referred to as the diglycidyl ether of tetrabromobisphenol A. I t r a n replace a significant portion of epoxy in any formulation, imparting ample fire retardance with little or no degradation of the mechanical and electrical properties (5, 6 ) . In an effort to find fire-resistant epoxy resin systems with even better high temperature properties, the influence of several variables in brominated resins was investigated. Some years ago Belanger and Schulte (7) pointed out that the chlorine content in conventional epoxy resins has a direct influence on the heat distortion temperature (HD'T), as well as the electrical properties when the temperature is above the
heat distortion temperature. Furthermore, the choice of the converter was of importance. Wynstra (8) reported that the diglycid) 1 ether of tetrachlorobisphenol A, hardened with amine curing agents, deteriorated rapidly a t elevated temperatures. Cass and Reaville ( 3 ) noted that the same resin, cured with anhydride curing agents, gave excellent high temperature stability. Partansky (7) disclosed that the addition of small amounts of antimony trioxide affected the high temperature stability of bromine-containing epoxy resins. Finally, one would expect the basic structure of the brominated resin to influence its high temperature properties. Experimental
Tetrabromobisphenol A [4,4'-isopropylidenebis-(2,6-dibromophenol) 1 was allowed to react with epichlorohydrin, in a 1 to 10 mole ratio. A slightly overstoichiometric amount of 50% aqueous sodium hydroxide was added slowly from a VOL. 3
NO. 1
MARCH 1964
55
dropping funnel during the course of the reaction. The reaction temperature was maintained between 100' and 110' C. by continuously distilling off water-epichlorohydrin mixture, separating the two components, and returning the epichlorohydrin to the reaction vessel. After all the sodium hydroxide was added, the excess epichlorohydrin was separated by distillation and replaced with an equal volume of toluene. T h e toluene solution was filtered to remove the sodium chloride formed during the reaction and the toluene was evaporated to obtain the diglycidyl ether of tetrabromobisphenol A, TBBPA DGE (I). The semisolid crude resin was purified by several crystallizations from a mixture of equal weights of toluene and methyl ethyl ketone. A white crystalline having a point Of '16-17' c' and an active or hydrolyzable chlorine content of 0.1 1% was obtained. The bromine content was 47.9% and the epoxy equivalent weight 329; theoretical, 48.7% and 328, respectively.
0
Br
/\
CH2
Tests. Each of the nine resins or resin systems was cured with the above curing agents. BFZMEA was used a t 3 parts per hundred; the curing schedule was 4 hours a t 100' C. plus 15 hours at 160' C. MDA was employed in stoichiometric amounts. Curing was for 16 hours a t 55' C. and 2 hours a t 125" C., followed by 2 hours a t 175' C. MNA, 0.85 equivalent of anhydride per equivalent of epoxy with 1.5 parts per hundred of benzyldimethylamine, was the third converter. T h e cure schedule was 2 hours a t 90' C. and 20 hours a t 171' C. The choice of curing schedule and amount of converter used in each case was based on recommendations found in the literature or given by the suppliers of the curing agent and had been shown to give optimum properties in certain nonhalogenated systems. An is the cure cyc1e for the MNA-cured systems, where slightly lower than the recom-
0
Br
/\
\ Br
Br
CH3
I Previous work on D.E.R. 542 had indicated that the hydrolyzable chlorine content is practically the same as the total chlorine content. The hydrolyzable chlorine was determined by mild hydrolysis with sodium hydroxide, followed by chloride determination employing a potentiometric titration method, using standard alcoholic silver nitrate. Total chlorine was obtained by x-ray absorption. Because the amounts of total and hydrolyzable chlorine are normally the same, all chlorine found in crude TBBPA DGE is believed to be present in the form of the chlorohydrin, 11.
0
Br
CHI
mended temperatures were employed. Higher curing temperatures sometimes result in higher heat distortion temperatures. But the more severe conditions were not used here for fear of early decomposition, which does occur, as shown below. The differences in high temperature properties were tested by determining the heat distortion temperature following ASTM D 648-56. Comparable heat distortion data for two standard nonbromine-containing epoxy resins were also
OH
Br
I
2 - 0 - c ~ ,-CH-CH Br
/
2-~i
\
I
CHI
Br
I1 T o obtain various levels of hydrolyzable chlorine, measured amounts of concentrated hydrochloric acid were added to the recrystallized TBBPA DGE kept in a toluene solution. Resins having three different chlorine levels were thus obtained (A, B, and C, Table I). The three resins were also mixed with 27, antimony trioxide. Some of resin A was mixed with an equal amount of D.E.N. 438 resin [poly(glycidyl ether) of Novolac having an average functionality of 3.51. Another mixture was made by blending equal parts of resin A and D.E.R. 331 resin [essentially the diglycidyl ether of 2,2-bis(p-hydroxyphenyl)propane]. One epoxy resin was prepared from 2,4,6-tribromo-3hydroxybenzoic acid. I t had an epoxy equivalent weight of 262 and a hydrolyzable chlorine content of l . l O ~ , . This material, mainly 2,4,6-tribromo-3-(2,3-epoxypropoxy)-2,3epoxypropyl ester, is referred to as resin D. Br 0
obtained. Weight losses of '/2 X '/2 X '/z inch cubes a t 300', 400', and 450' F. were measured. Thermogravimetric curves were also obtained from ground resin samples. The apparatus consisted of a spring deflection type thermogravimetric balance similar to the American Instrument Co. Thermograph. All analyses were run on about 50-mg. samples, in an air atmosphere, with the temperature increase programmed a t about 3.7' C. per minute. The flammability of several brominated and nonhalogencontaining systems was determined following the test procedure as described in ASTM D 635-56. Unfilled castings were used having a '/*-inch thickness. Six specimens were tested; each specimen was ignited twice if nonburning or self-extinguishing. The self-extinguishing time in seconds was determined. This
Table II.
Heat Distortion Temperatures (' C.) Curing Agent
Resin System
Br
1
Br O-CH2-CH-CH2
\0/ The curing agents used were boron trifluoride monoethylamine (BF,MEA), methylenedianiline (MDA), and methyl nadic anhydride (MNA). Table 1. Code
Epoxy equivalent Chlorine, yo
56
Three Chlorine levels in TBBPA A B
329 0.11
375 1.11
DGE
Resin A Plus 270 Sbz03 Resin B Plus 2% Sb203 Resin C Plus 2'30 Sbz03 Resin A and D.E.N. 438 resin, 50/50 Resin A and D.E.R. 331 resin, 50/50 Resin D D.E.N. 438 resin
187 186 172 173 156 154 212 183 153 230
D.E.R. 331 resin
125
c
421 2.06
l&EC P R O D U C T RESEARCH A N D DEVELOPMENT
BFJLEA M D A
a
After additional cure of 4 hours at 200aC.
5 hours at 200' C.
b
182 181 161 162 145 141 187 161 128 177 207a i57
MNA
159 161 150 151 145 144 175 151 140 185 188b 157 150b
After additional cure of
Figure 1.
Weight loss at 400' F. of BFBMEA-cured systems
is not required in the ASTM test but gives good comparative data. Results and Discussion
Heat distortion temperatures are tabulated in Table 11. Decrease in H D T with increasing chlorine content is less pronounced with MNA. The HDT's obtained with MNA are lower than with BFSMEA and MDA in systems containing TBBPA DGE, which is somewhat unusual, since the struc-
Table 111. Heat Distortion Temperatures (' C.) of Resin B Cured with M N A at Different levels and after Additional Postcure Equivalent of Additional Anhydride per Standard Cure of 5 Hours Epoxy Equivalent Cure at 200" C.
0 . 8 5 ( 4 0 . 5 phr)= 0 . 9 2 5 ( 4 4 . 3 phr) 1 . 0 ( 4 8 . 0 phr) a
150 154 148
146 144 146
Parts per hundred parts resin.
Per Cent Weight Loss after 2000 Hours at 300' F. Curing Agent Resin System BFaMEA M D A MAVA Resin A 1 .IO 0.41 0.16 0.34 P l U S 2% Sb203 0.77 0.21 Resin B 1.80 1.I2 0.84 1.59 Plus 2% Sb203 1.08 0.26 Resin C 0.96 36.0 0.28 Plus 270 Sb208 1.75 24.0 0.14 0.42 0.06 Resin A and D.E.N. 438 resin, 50/50 0 . 2 4 Resin A and D.E.R. 331 resin, 50/50 0 . 5 6 1.73 0.15 Resin D 2.25 20.0 Table IV.
Decomposed completely.
turally similar D.E.R. 331 resin shows a different tendency. A failure to reach optimum cure conditions for the MKAcured bromine-containing systems was thought to be the cause of this anomaly. However, as Table I11 clearly shows, additional curing of resin B for 5 hours a t 200' C. does not improve the H D T . Only slightly better results n e r e obtained when the amount of MNA per equivalent of epoxy was increased from 0.85 to 0.925 mole. Further increase to 1 mole gave a subsequent drop in the H D T . At optimum conditions, the MN.4-cured bromine-containing systems have about the same H D T as the related nonhalogenated D.E.R. 331 resin. The H D T is the same or less, respectively, when D.E.R. 331 resin is cured with MDA or BFSMEA, lvhile an improvement is noticed when the brominated resin is cured with either of the latter curing agents. The halogenated resin follovJs more closely a trend also found in the less related D.E.N. 438 resin. This somewhat surprising behavior of the brominated resin has not been satisfactorily explained. The decrease in H D T with increasing chlorine content is no doubt mainly due to a corresponding lowering of the degree of cross-linking in the cured resin. The high values obtained with the mixture of resin A and D.E.N. 438 resin, a highly cross-linked system, support this statement. Since the chlorine content in TBBPA DGE goes hand in hand Lvith the degree of cross-linking in the cured resin, it can be said that the hydrolyzable chlorine content has a n effect on H D T values. The addition of small amounts of antimony trioxide does not influence the H D T to any extent. The asymmetric resin has a poorer H D T than resin B, in spite of the fact that its chlorine content is the same, The dissimilarity in structure must be the cause of the difference in this physical property. Per cent Tveight loss data (Table IV) indicate that most samples show only minor weight losses after 2000 hours a t 300' F. (149' C.). Notable exceptions are resin C having a high chlorine content and resin D cured with MDA. Some trends are becoming noticeable, although not too definite as VOL. 3
NO. 1
MARCH 1964
57
T I M E , HOURS
Figure 2, Weight loss at cured systems
400' F. of
methylenedianiline-
yet. I t appears that lo\ver chlorine content gives lobver weight loss, except for resin C cured with MNA4. Addition of 2% antimony trioxide is advantageous in most cases. Resin A mixed \vith D . E . S . 438 resin performs much better than resin A blended with D.E.R. 331 resin. Continuous exposure a t 400' F. (204' C.) brings out the differences more dramatically. Figure 1 shows the heat-loss curves of BF&lEA-cured systems. Resin A performs better than resin B, lvhich in turn is superior to the more chlorinecontaining resin C. Resin D is inferior to the resins based on tetrabroinobisphenol A. Addition of conventional rpoxy resins to resin A, resulting in a decrease of bromine content, gives a higher heat stability. Best results are obtained when epoxy novolac is blended in. Similar results are obtained with MDA, as shown in Figure 2. Resins B and C are again poorer than A. In Figure 3, the excellent performance of resin A cured with MNA can be noted. Resin C j which has the highest chlorine content, shows LIP slightly better than resin B at 300" and 450" F. A similar observation was made by Belanger and Schulte
0
IO
20
30
Figure 3. 58
40
50
60
Weight loss at
70
in resins based on bisphenol A. The anomalous results may be attributed to the amount of MXA used, since no attempts were made to find the most favorable concentration of this curing agent. The weight losses in resin D as well as in resin A mixed with D.E.R. 331 resin and D.E.X. 438 resin are as expected. Methylenedianiline is not a good curing agent for brominated epoxy resins when high temperature properties are desired. This is expressed in Figure 4, where MDA is compared with BFShlE.4 and MNA in a resin system containing epoxy novolac. The best results are obtained Irith the anhydride curing agent. This parallels results obtained by Delmonte (4, who found that diglycidyl ether of bisphenol A cured with methyl nadic anhydride loses less weight a t elevated temperature than the same resin cured with BF3ME.4. However, the per cent bromine in the formulation containing the anhydride is lower than in the corresponding system cured with B F M E A , 16.4 and 23.3, respectively. The lower bromine content could a t least in part account for the superior performance of the MYA-cured system. Delmonte further reported that the standard liquid epoxy resin, cured with methylenedianiline, is more stable a t elevated temperatures than the same resin cured with BFSMEA. Bromine-containing systems do not follow this pattern. The beneficial effect of 2% antimony trioxide is shown dramatically in Figure 5 . S o t in all cases is this improvement as evident as it is here. For instance, when an amine curing agent is used, addition of 2% Sb203 can hardly be regarded as advantageous (see Table IV). However, the effect of 2% of this additive with the other two curing agents is mostly favorable, although in some isolated cases antimony trioxide seemed to be slightly detrimental. All tests were run with one level of the trioxide, although there is probably a n optimum amount of Sb203 for each system. These levels have not yet been established. Figures 1 through 5 indicate that there was an initial low rate of weight loss followed by a rapid increase and then tapering off. During the rapid weight loss, the samples puffed UP or disintegrated. Gases: presumably bromine-containing decomposition products, evolved. The shapes of the curves suggest an autocatalytic mechanism of decomposition. A similar behavior was found by Buchoff and Sherwin (2) in an epoxy resin system cured with hexachloro-endo-methylene tetrahydrophthalic anhydride (HET anhydride).
80 90 100 T I M E , HOURS
400" F. of
I&EC P R O D U C T RESEARCH A N D DEVELOPMENT
110
120
130 140
150
160
methyl nadic anhydride-cured systems
170
180
30 25 v)
g 20 -I
i-
I15
E W B
'0
P
2 W
5
0
a 0 W 0 a
IO
20
30
40
50
Figure 4.
60
70
80 90 100 T I M E , HOURS
Effect of curing agent
110
120
I30 140 150 160 170 180
on weight loss at 400" F.
Resin A and D.E.N. 438 resin, 5 0 / 5 0
At 450' F. (232' C.) the weight loss of bromine-containing epoxy resins is rather rapid. Table V gives the weight losses after 4 hours. The same pattern found a t 300' and 400' F. is apparent. The best system, resin A mixed with D.E.N. 436 resin cured with MNA, lost only 1.9% by weight after 14 hours a t 450' F. Thermogravimetric Analyses. The weight loss, with increasing temperatures. of a ground cured resin sample provides another way to measure the thermal stability of a system. Thermogravimetric curves in Figure 6 indicate again that resin B is more stable than resin C, while the addition of D.E.R. 331 resin to resin A results in better high temperature stability. The beneficial effect of antimony trioxide can be read from Figure 7. That MDA is a poor curing agent for brominated resins is shown once more in Figure 6. The superiority of the MNA converter over BFaME4 catalyst is not too evident. since the thermogravimetric curves cross each other. Flammability. The main advantage of bromine-containing epoxy resins is their flame-retardant characteristics. T h e
effect of several levels of bromine on the self-extinguishing time is noticeable: as shown in Table VI. High bromine content gives short self-extinguishing times. If no bromine is present, the flame is not extinguished in the case of D . E . R . 331. MNA-cured D.E.N. 436 is surprisingly classified as selfextinguishing by this test. However, the self-extinguishing time is long compared to bromine-containing systems.
Table V.
Per Cent Weight Loss after 4 Hours at 450' F.
Resin System
Resin A Plus 27, Sb2Oa Resin B Plus 2Y0 S b 2 0 3 Resin C Plus 2'70 Sb?Oa Resin A and D.E.N. 438 Resin, 50/50 Resin A and D.E.R. 331 resin, 50/50
Curing Agent BFsMEA JMNA 2.38 0.24 2.00 0.44 2.58 9.32 2.45 0.73 5.59 1.24 2.45 0.56 0.50 0.24 0.96 0.58
60
55 50 v,
45
vr
0 -I
40
I-
I
2 35 w
'
30
I-
z
$
25
a w
a
20 15 10
5 0 0
IO
20
30
40
50
60
70
80
90
100
110
120
130 140 150 160 170
180
TIME,HOURS
Figure 5.
Effect of antimony trioxide on weight loss at
400' F.
VOL. 3
NO. 1
MARCH 1964
59
Table VI. Flammability Tests Curing Agent, 7*Bromine in Phr Cured Composition Classijication
Resin System
Resin B Resin B and D.E.R. 331 resin 50/50
MNA (48) BFsMEA (3) MDA (20) MNA (60) MNA (84) MNA (81)
D.E.N. 438 resin D.E.R. 331 resin a
Time elapsed before,flnme ceased after ignition source was removed.
31.8 22.4 19.6 14.7
Self-Extinguishing Time, Secondsa Max. Au. 0 0 4 1
Nonburning Self-extinguishing Self-extinguishing 5 Self-extinguishing 15 0 Self-extinguishing 105 0 Burning 208* Time elajsed to burn 4 inches; did not extinguish by itsclf.
3 8 97
205b
Conclusions
I
I
0
200
IO0
300
TEMPERATURE
Figure 7.
500
400
600
,'C.
Acknowledgment
Thermogravimetric curves
The author thanks J. G. Cobler for the thermogravimetric analyses. The helpful assistance and active interest of H. H. Pendred are also gratefully acknowledged.
Methyl nadic anhydride
80 ln
I O '
I
literature Cited (1) Belanger, \V. T., Schulte, S. A., Mod. Plastics 37, No. 3, 154
70
ln
9
(1959).
60
(2) Buchoff, L. S., Sherwin, W. R., Rubber Plastics Age 43, No. 9, 975 (1962). (3) Cass, R. A , , Reaville, E. T., Division of Paint, Plastics and
I-
=
2
50
W
3
40
z
30
V
20 n
10 0
0
200
IO0
300
400
500
600
TEMPERATURE , O C .
Figure 8.
Thermogravimetric curves
Effect of curing agent, resin B
60
Printing Ink Chemistry, 136th Meeting, .4CS, Atlantic City, N. J., 1959, preprint booklet pp. 77-84. 4) Delmonte, J., Chem. Eng. Progr. 5 8 , No. 10, 51 (1962). D ow Chemical Co., Red-bordered data sheet "Self-Extinguishing Epoxy Resins Containing Bromine." (6) Parr, F. T., National Conference on Electrical Insulation, February 1962, AIEE T-137-65. (7) Partansky, A. M., Dow Chemical Co., private communication. (8) LVynstra, John, Division of Paint, Plastics and Printing Ink Chemistry, 131st Meeting, ACS, Miami, Fla., 1957, preprint booklet pp. 19-27. RECEIVED for review August 12, 1963 ACCEPTEDDecember 9, 1963 Division of Organic Coatings and Plastics Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963.
h
I-
w
Factors influencing the high temperature performance of several bromine-containing epoxy resin systems were determined by comparing heat distortion temperatures and weight changes of the cured resins kept a t elevated temperatures for prolonged periods of time. The importance of these tests was stressed by Delmonte ( 4 ) ,who indicated that weight loss, particularly if the effect of extended exposure is introduced, is related to many factors, such as dimensional stability, strength retention, and electrical parameters. With the exception of some MNA-cured systems, increasing the amount of hydrolyzable chlorine in bromine-containing epoxy resin systems resulted in poorer high temperature performance. The choice of curing agent is very important; the amine-type converters were inferior to BF3MEA. Best results were obtained with a n anhydride curing agent. Improved high temperature resistance was obtained in many cases by the addition of small amounts of antimony trioxide. This additive had no influence when methylenedianiline was used as the curing agent. Low percentages of antimony trioxide did not change the heat distortion temperature in any case. The performance a t elevated temperatures was improved substantially when the amount of bromine was decreased to a more practical level by addition of nonhalogencontaining epoxy resins. Incorporation of D.E.N. 438 epoxy novolac resin gave more improvement than D.E.R. 331 resin. Epoxy resins made from tetrabromobisphenol A had excellent high temperature properties, when compared to resins based acid. This good peron 2,4,6-tribromo-3-hydroxybenzoic formance of the bisphenol A-derived fire-resistant formulations is believed to be due to the symmetrical structure and the less crowded aromatic rings of tetrabromobisphenol A.
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
