Fire-Reta rda nt Unsaturated Polyesters Comparisons of Analogous Brominated and Chlorinated Reactive Intermediates John A. Schneider,'R. Garth Pews, and Jefferson D. Herring Research and Development, The DOLLChemical Co., Midland, Mich. 48640
The effectiveness of bromine vs. chlorine has been determined for the fire-retardant unsaturated polyester intermediates: dihalomaleic anhydrides, dihaloneopentyl glycols, tetrahalophthalic anhydrides, a n d tetrahalobisphenol A-ethylene oxide derivatives. N e w polyesters were prepared from 1,4,5,6,7,7-hexabromo-[2,2,1]-5-heptene-2,3-dicarboxylic anhydride, and from 1,4,5,6-tetrabromo-7-oxo-5-norbornene-2,3-dicarboxylic dimethyl acetal.
S e v e r a l articles and reports have discussed the use of halogenated intermediates to prepare fire-retardant polyesters. This report introduces some new intermediates and compares the effectiveness of chlorine and bromine analogs. Of all the techniques used to produce fire-retardant polyesters, the use of reactive fire-retardant intermediates has the least detrimental effects on the other desirable properties of a polyester: physical. thermal, chemical. and electrical. At present. four such raw materials are marketed for polyesters: chlorendic anhydride, tetrabromophthalic anhydride. tetrachlorophthalic anhydride, and 2,2-bis(bromomethyl)-1.3-propanediol. Although it has often been indicated t h a t bromine compounds are better fire-retardant intermediates than chlorine compounds, few references compare the effectiveness of bromine and chlorine analogs. P a p e et al. (1968) compared the relative efficiencies of tetrabromophthalic and tetrachlorophthalic anhydrides. They found bromine superior to chlorine: 1 3 5 bromine was as effective as 22'; chlorine. This paper compares the effectiveness of bromine L S chlorine in some fire-retardant polyester intermediates. Experimental
Ultraviolet spectra were determined with a Cary recording spectrophotometer. Model 15. N M R spectra were obtained with a Varian A-60, using tetramethylsilane as an internal standard. Infrared spectra were obtained with a Beckman IR-9 spectrometer. The mass spectrum was obtained on a CEC' 21-llOB (direct-probe) instrument. I n every case. the spectrum was consistent with the proposed structure. and gave the correct molecular weight and isotope ratios for the number of halogens present in the molecule. All melting points were taken on a Thomas-Hoover melting point apparatus.
1,4,5,6,7,7-Hexabromo-5-norbornene-2,3-dicarboxy~ic Anhydride (IV). Hexabromocyclopentadiene (107.5grams, 0.20
' T o whom correspondence should be sent
anhydride
mole), maleic anhydride (30.0 grams, 0.30 mole), and chlorobenzene (250 ml.) were heated under reflux for 24 hours. After cooling, the solvent was evaporated in vacuo and the product recrystallized from ethyl acetate-hexane to give material of melting point > 250" C (dec); infrared (split mull) 1561 (Br-C= C-Br) and 1781,1789 cm-' (O=C0-C=O); N M R (acetone - d e ) 6 4.58 ppm ( s ) ; mass spectrum m / e 632,553,525,481, and 455. Anal. Calcd. for C9HrBrs03:C , 16.93; H. 0.32; Br, 75.23. Found: C. 17.0; H , 0.16; Br, 75.4. 1,2,3,4-Tetrabromo-5,5-dimethoxycyclopentadiene. Hexabromocyclopentadiene (54 grams. 0.1 mole) was dissolved in dimethoxyethane (250 ml), and the solution cooled to -78' C in a three-necked, round-bottomed flask (500 ml) equipped with magnetic stirrer, dropping funnel, condenser, thermometer, and nitrogen inlet. After the temperature had decreased to -78" C, a sodium methoxide solution, prepared from 12.8 grams of 85% sodium methoxide and anhydrous methanol (75 ml), was added dropwise. When the addition was complete, the cooling bath was removed and the homogeneous solution allowed t o warm to room temperature. At approximately 0"C , the sodium bromide began to precipitate from the reaction. After 2 hours a t room temperature, the reaction mixture was poured onto an acidified ice-water mixture. The off-white precipitate was collected on a filter and dried to give 42.6 grams (96%) of crude ketal (mp 95-100" C ) : recrystallization from methanol, mp 104-05" C, infrared (split mull) 2999, 2952, 1450, and 1450 cm-' (C-H), 1573 cm-l ( C B r = C B r ) , 10801200 cm-' (C-0); N M R (CC1,) 6 3.29 (s, OCH,); UV,,, (hexane) 318 mp ( t 1700).
1,4,5,6-Tetrabromo-7-oxo-5-nonbornene-2,3-dicarboxylic Anhydride Dimethyl Acetal (VII). 1,2,3,4-Tetrabromo-5,5dimethoxycyclopentadiene (50 grams, 0.113 mole) and maleic anhydride (12.0 grams, 0.120 mole) were refluxed in toluene (125 ml) for 3 hours. After cooling, the product was collected by filtration, washed with carbon tetrachloride, and dried to give 52.7 grams (86%) of a white solid, mp 24445. C; infrared 1785 c m - ' ( C = O j , 1565 cm-' ( C = C ) ; N M R (CDCl,),;,3.92 (CH-C=0),63.68and3.64 ( s , O C H I j . Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 4, 1970
559
Anal. Calcd. for C I I H $ B r I O IC. : 24.44: H. 1.49. Br. 59.25. Found: C, 24.60; H, 1.45: Br. 59.10. Polyester Preparation. This polyester preparation is used as an example. All the preparations used the same temperature program. A mixture of 50.4 grams (0.34 mole) of phthalic anhydride. 38.2 grams (0.06 mole) of IV, 39.2 grams (0.40 mole) of maleic anhydride. 64.0 grams (0.84 mole) of' propylene glycol, and 25 ml of xylene was stirred and heated for about 12 hours. The reaction temperature was controlled by a programmer; the maximum temperature reached was 190"C. The water given off was collected in a Dean-Stark water trap. T h e acid number was then determined, using 0.1N alcoholic KOH. When t h e proper acid number was obtained (about 301, t h e mixture was cooled to 120" C , and 0.08 gram of hydroquinone added. After stirring for a few minutes, 80 ml of styrene were added. When a homogeneous solution was obtained. the hot mixture was filtered into a bottle and the Gardner color number determined. Casting Preparation. T h e product mixture obtained from the polyester preparation was treated with 3.4 grams of benzoyl peroxide, put into a n oven. and cured for 1I hours. The curing temperature was 30 minutes a t 50' to 60" C , 10 minutes at 60" to 80'C,35 minutes a t 80'C,and 15 minutes from 80' to 110°C. T h e resin was then removed from the mold and submitted to the fire-retardant tests. Flammability Determinations. The castings were evaluated by the HLT-15 intermittent flame test. the ASTM Dr-i s i (Globar) flame test. the ASTM D 635-56T test. or the oxygen index flammability test.
Table I. Tetrabromophthalic vs. Tetrachlorophthalic Anhydride"
% CI
Br
Intermediate
560
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
ASTM D-757,
rating
mm/min
Phthalic anhydride
0
0
0
Tetrabromophthalic anhydride (I)
18.5 22
23
... ... ...
...
23
60 80 100 0
Tetrachloropht halic anhydride (11)
Burns freely 7.5 5
20.0
' Pape et a/ (1968).
retardant properties. The higher the chlorine content, the lower the ratio of S b O required for equivalent fire retardancy (Robitschek and Bean. 1954). However. below 2Orc) these workers certain levels of chlorine content found that even excessive amounts of S b O will not compensate for the chlorine deficiency. Normally, a polyester based on chlorendic anhydride (111) will contain no more than 27 to 3 0 5 aliphatic chlorine. (1
X
bo
X
X
II
Discussion
The effectiveness of aromatic bromine os. aromatic chlorine is illustrated in the work of Pape et al. (1968). on the use of tetrahalophthalic anhydrides in fire-retardant polyesters. The results of their study (Table I ) demonstrated t h a t polyesters prepared from tetrabromophthalic anhydride (I) were of better over-all quality than those prepared from tetrachlorophthalic anhydride (11). I compared to I1 was found to give '.(1) more formulation versatility: ( 2 ) faster esterification: (3) lower viscosity resins; (4) higher flexural strength: ( 5 ) lower heat distortion temperature, and (6) comparable water absorption." The results in Table I support Kametz's report (1967) that a t least 3OLCaromatically bound chlorine is needed to get self-extinguishing properties in a polyester. T o do this, another chlorinated compound must be used with 11. since it contains only 49.5'1 chlorine. When a polyester is prepared from propylene glycol and a 1 to 1 ratio of I1 and maleic anhydride, only 19.2', chlorine is present in the resin when diluted with 30'; styrene. Even with I , 23'; bromine is necessary for a n HLT-15 rating of 100 (Pape et ai.,1968). Fenimore and Martin (1966) reported that only 21.5"; bromine is required to obtain a 100 value using I . Polyesters from I have good physical properties. but normally darken upon weathering. Usually, aromatic bromines are stable to heat and normal conditions. but sensitive to ultraviolet radiation. I n the case of I . the bromines are probably made even more labile, because of the strong electronwithdrawing properties of the two carboxyl groups, the known ability of aromatic carbonyl materials to function as ultraviolet absorbers, or both. The most widely used fire-retardant intermediate for polyesters is chlorendic anhydride. T h e chlorine content is 58'; and. therefore. usually a synergist-e.&?., S b O or a phosphorus compound-is required to obtain good fire-
HLT-15
0
Ill
IP
X=CI X:Br
The apparent advantages of bromine compared to chlorine prompted the authors to prepare the bromine analog of chlorendic anhydride, and t o determine its utility in fireretardant polyesters. The synthesis of 1,4,5,6,7,7-hexabromo-5-heptene-2,3-dicarboxylicanhydride (IV) was achieved by refluxing hexabromocyclopentadiene a n d maleic anhydride in chlorobenzene for 24 hours. When incorporated into a standard polyester resin (see Experimental), a clear b u t yellowed casting (Gardner color of 18) was obtained (Table 11). A similar yellowing is associated with fire-retardant resins derived from hexachlorocyclopentadiene adducts-eg., chlorendic anhydride a n d Chloran (V) (Gardner values of 4)-but is enhanced when bromine is substituted for chlorine. Roberts and coworkers (Roberts et ai., 1964) have postulated that this color problem was caused by t h e presence of activated hydrogens in the 2.3-position of chlorendic anhydride. Attack of these hydrogen atoms would readily lead to elimination of hydrogen chloride and splitting out o f the endomethylene bridge. I n the presence of oxygen, the bridge would yield phosgene and eventually hydrogen chloride. These workers further postulated that if the n-hydrogens could be removed or replaced by some nonreactive species. greater thermal and oxidative stability should result, These postulations led to the synthesis of Chloran ( V ) , which exhibited improved weathering properties but no significant improvement in the initial Gardner color of the resin. The high Gardner colors of hexachlorocyclopentadiene-based polyesters can perhaps be explained from the results of polyesters prepared from the ketals (YI
Table II. 5-Norbornene Derivatives ASTM D-757, Intermediate
Oo /
Br
O h
... ... ...
III
CI
16.5 28.0 29.0
vI
...
267
Gordner color
...
...
4
4.6 17 18
20
90
... ...
VI1
mm/min
96
10.2 14.2
IV
Rating
...
20.0 16.3 12.2
10.0 19.3
... ...
0
I
.
1.5
.
0 0
...
...
...
20 100
... ...
2.0 1.5
Table 111. Dihalomaleic Anhydrides Intermediate
0
Br-C-C
I1
I1
Br
O h
O h
CI
Gardner color
20.2 15.2 10.0
... , .. , . .
9
...
17
9
Remarks
S o t enough maleic
anhydride to permit crosslinking
0
Br-C-C
I1 0 0
c1-c-c II c1-c-C
I1
.
I
.
10
Xot enough maleic anhydride to permit crosslinking
0
I'
0
CI,
,CI
CH,O,
x-
,0CH3
v
II
P
and L'II). As is seen from Table 11. ketals VI and VI1 give polyester resins of markedly improved Gardner colors-1 and 2 1 , s . -1 for chlorendic anhydride. These results suggest that the high Gardner color obtained with hexahalocylopentadiene adducts are due to the presence of halogen in the endomethylene bridge. and not to the lability o f t h e methine hydrogens in t h e 2.3-position o f t h e 5-norbornene system.
A further comparison of the greater effectiveness of bromine compared to chlorine in the 5-norbornyl systems is illustrated in the work of Kovacs and Marvel (1967). who studied the fire-retardant characteristics of the vinyl esters I-j-heptene-2-carboxylic acid of 1,4,5.6.7.5-hexahaloi2.2.1 (VIII and IX) when copolymerized with acrylonitrile. The flammability of the copolymer was determined on pressed films. The results showed that copolymers containing u p to 18.35' chlorine or 6.35' bromine burned; however. copolymers containing more than 25(, chlorine or 9.8'; bromine did not support combustion. I n summary. bromine was about three times as effective as chlorine in this system. The dihalomaleic anhydrides provide another system for comparing bromine and chlorine in fire-retardant polyesters. Dibromomaleic anhydride was prepared as described in a British patent (Monsanto. 1966). The initial polyesters prepared from this material were dark. Gardner color of 9. This was surprising. since the vinylic halogens were expected to have good stability. However. further work showed that t h e material contained impurities such as dihromosuccinic anhydride, which could not be removed completely by distillation or recrystallization. Similar color problems were found with dichloromaleic anhydride. The maximum ratio of bromine that can be contained in a maleic anhydride-phthalic anhydride-propylene glycol polyester resin (30$ styrene). using dibromomaleic anhydride in place ofmaleic anhydride. is 20.1' (Table 111). The maximum ratio of chlorine using dichloromaleic anhydride is 17'r (30'~ styrene), These intermediates, like chlorendic anhydride, contain a n inactive double bond and do not enter into the crosslinking mechanism. Thus. maleic anhydride must be present. T o get enough bromine or chlorine into the polyester to improve fire-retardant properties. t h e maleic anhydride is reduced t o a point where such poor castings are obtained that fire-retardant tests cannot be carried out on them. Another reactive intermediate that contains vinyl halo(Kostelitz and gens is 2,3-dibromo-2-butene-1.4-diol Pompon, 1 9 6 7 . Difficulties in preparing sufficient quantities of the analogous dichloro compound prevented a comparison study of these diols. The polyesters we prepared from 2.3-dibromo-2-butene-1.4-diol were dark (Gardner value o f 9 ) and gave a HLT-15 value of 100 a t lt5c,bromine.
Table IV. Dihaloneopentyl Glycols ASTM635
HIT-15 lntermediote
CH2Br
I
HOCH?-C-CH?OH
O h
Br
% CI
15 16
... ... ... ...
15
(With
5 10
rating
rating, mm
0
20 80
31.7 SE" 15.9 NB"
100
CH2Br 0.55
P)
I HOCH,-C-CHIOH
I
(With
0.5['>P)
17.7
Char S B
freely 16 NB
CHjC1 " SE. Self-extinguishing, ' KB. Sonburning.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
561
At present, 2,2-bis(bromomethyl)-1,3-propanediol (X, is being widely evaluated as a fire-retardant agent for polyesters. I t s utility is due t o the unique structure of the neopentyl system, stable aliphatic bromines, and lack of hydrogens 9 to the bromine atoms. This intermediate has t h e fire-retardant effectiveness of a n aliphatic bromine compound, but greatly improved light stability (Larsen, 1969). Using the ASTM-635 test: a polyester containing 10'; bromine using this compound is rated nonburning; a t 6? bromine, it is rated self-extinguishing. Only 1 6 5 bromine is necessary to rate 100 on the HLT-15 test (Table I V ) . 2,2-Bis(chloromethyl)-1,3-propanediolhas been investigated in polyesters. Its low chlorine content (41.55) makes it impossible t o obtain fire-retardant polyesters using it as the only source of fire-retardant properties (Celanese Corp., 1960). A polyester prepared from it can contain no more than 1 7 . 7 5 chlorine. A polyester containing 12.55 chlorine [from 2,2-bis(chloromethyl)-1,3-propanediol] has been reported t o have better weatherability than a similar nonhalogenated polyester (Celanese Corp., 1960). Polyesters prepared from these materials also have improved chemical resistance. Bisphenol A-alkylene oxide adducts are used in polyesters because the resins have good chemical resistance. Polyesters prepared from the tetrabromobisphenol A-ethylene oxide adduct have shown low flammability, high mechanical Table V. Tetrahalobisphenol A-Ethylene Oxide Adducts ASTM D-757,
Yo
Intermediate
Br
Yo CI
mm/min
7.2 2.3
17.8
Table VI. Results of Limiting Oxygen Index Flammability Test on Various FR Polyesters FR
Yo Br
Yo CI
101 value
I
9.1 13.4
...
I1 I11
...
17.0 8.4 16.4 20.6 26.4
21.9 23.1 20.6 21.1 21.6 24.7 29.1 25.8 19.9 20.6 20.7 24.5 18.1 20.5 23.1 25.7 28.3
intermediate
... ...
... ...
IV VI
14.0
VI1
14.0
... ...
...
x
562
0
6.3 10.3 14.3 20.6
...
...
12.0 16.2 20.0
... ... ...
... ...
...
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4,1970
0.30 -
," 0.280
E
0.26 W
> X
0
0.24 -
f
E _I
0.22 -
DI 8R OM 0N EOP E N T Y L GLYCOL V.1"
0
4
I 8
I
I
12
16
1 20
I
I
24
28
32
'10 HALOGEN
Figure 1. Effect of halogen content on LO1 values
stability. exceptional heat stability, and outstanding water resistance. This intermediate also offers formulation variation (Farbenwerke Hoechst. 1962). Work in the Dow laboratories with the ethylene oxide adducts of tetrabromobisphenol A and tetrachlorobisphenol A have indicated that bromine is roughly twice as effective as chlorine on a weight basis (Table VI. One might conclude from the fire-retardant data in Tables I and V that the tetrahalobisphenol A-ethylene oxide adducts are more effective fire-retardant agents than the tetrahalophthalic anhydrides. This is so, even though both contain aromatic halogens. One must be cautious in assuming this, however, since the results of the ASTM D-757 test can vary, depending on the technique. and this work was done by two different investigators. Also. the inherently different structures of the two polyesters might contribute to differences in their flammability. By comparing the effect of I and X on the fire-retardant properties of bromine polyesters, it might be assumed that aliphatic bromine compounds are approximately 11 times more effective than aromatic bromine compounds. This is only one case. and the inherently different structures of the polyesters may also contribute to the difference in flammability o f t h e resins. In general, all the aliphatic bromine intermediates exhibited comparable results. The brominated cyclopentadiene adducts appeared t o be slightly more effective than 2,2bis(bromomethyl)-1,3-propanediol (X). T h e cyclopentadiene adducts normally required 15%-bromine t o get a 100 rating on the HLT-15 test, while X normally requires 16'; bromine. This slight difference could be due t o the varied stability ofthe bromines in a norbornene svstem. Since the completion of this work, we have noticed that more interest is being placed on the GE oxygen index flammability test as a means of measuring fire retardancy (Davis et al., 1970).This test measuyes the minimum percentage of oxygen needed to support combustion. Table VI contains some limiting oxygen index ( L O I ) values for the polyesters prepared during this study. The effect of halogen content on the LO1 values was rather thoroughly studied for polyesters prepared from 111 and X (Figure 1). The halogen content of the various polyesters
was determined analytically b y atomic activation: therefore. the data points are exact numbers and not approximations. The chlorendic anhydride resin climbs to an LO1 value in the area of0.21 at 8'/chlorine and remains relatively steady to about the 19' chlorine level. At this point there is a rapid increase in the LO1 value. The dihomoneopentyl glycol resin has a steady increase in LO1 values up to about the 8' bromine level. Then, the LO1 values increase more rapidly with increased halogen content up to lj' bromine. where the curve decreases in slope. Ac knowledgment
\Ve acknowledge the advice and aid of C . \Y.Roberts. E. R. Larsen. and W . C. Weaver ofthe Dow Chemical Co. literature Cited Celanese Corp., Brit. Patent 836,437 (June 1,1960). Davis. J . H.. Litton. R. K.. Calendine. R. H.. Sect. 4-A. p. 1. 25th Annual Technical Conference. Reinforced Plastics Composites Division. Society of the Plastics Industry. 1970.
Farbenwerke Hoechst. Brit. Patent 893,674 (April 11.1962). Fenimore, C. P.. Martin. F . H.. Mod. Plast. 141 (Sovember 1966). Kostelitz. O., Pompon. .J. B.. Fr. Patent 1,502,049 (Nov. 18. 1967 1. Kovacs, J . . Marvel. C. S..J . P o / . ~ mSei. . 5. 1229 (1967). Larsen, E. R.. \;ol, 29. No. 2. p. 375, Division of Organic Coatings and Plastics Chemistry. 158th Meeting. ACS, New York. September 1969. Monsanto Co.. Brit. Patent 1,026,412 (April 20.1966). Nametz. R. C.. I n d . Eng. Chem. 59, No. 5. 99 (19671. Pape. R . G.. S u l p h . R. J . . Nametz. R . C., Sect. 19-A. p. 1. 23rd Annual Technical Conference. Reinforced Plastics Composites Division. Society of the Plastics Industry. 1968. Roberts, C. W.. Haigh, D. H.. Rathsack. R. ,J., J . , A p p l . PoIym. Sci. 8, 363 (1964). Robitschek, P.. Bean, C. T.. I n d . Enq. Chcm. 46. 1628 (1954).
RECEIVED for review September 8. 1969 ACCEPTED August 31, 1970 Division of Organic Coatings and Plastics Chemistry. 133th Sleeting. ACS. New Y o l k . S . Y..Septeinher l%9.
Lead Dioxide-Gra phite Electrode Ramaswamy Thangappan', Subramanyam Nachippan', and Srinivasa Sampath3 Central Electrochemical Research Institute, Karaikudi-3, Tamil Nadu, India The lead dioxide electrode is used as a substitute for the conventional graphite and platinum electrodes in cells for the preparation of chlorates, perchlorates, chloralkali, hypochlorite, etc. Stationary or flowing electrolytes are used with or without addition agent, the anode being stationary or rotating for the electrodeposition from an aqueous bath of a suitable lead salt, of lead dioxide on graphite, carbon, or other metallic substrate. PbOe was deposited onto a substrote like graphite and carbon rods and plates of different sizes, from an electrolyte containing lead nitrate and copper nitrate in a fluidized bed of an inert material. The substrate material is the anode; the cathode is copper. The lead dioxide obtained is smooth, dense, hard, uniform, and free from pinholes and adheres to the surface of the substrate material.
T h e lead dioxide electrode is gaining importance as an anode in electrolyte cells for several processes. Although the electrodeposited lead dioxide is somewhat brittle, it has adequate strength for handling when plated on suitable substrates in the form of rods and plates. I t is inert
' To whom correspondence should be sent. ' Present address, Hindustan Photo Films Manufacturing
Co., Ltd., Ootacamund. India 'Present address, Western India Match Co., Ltd., Ambernath, India
to most of the oxidizing agents and strong acids. I t is a better electrical conductor than many metals and graphite. It is very hard, dense, and metallic in appearance. Two forms of lead dioxide have been reported, the orthorhombic a-PbOl and the tetragonal &PbOr. The PbOA electrodeposited from the lead nitrate bath is i3-PbO2, which has a higher oxygen overvoltage than the a-PbOl (Grigger, 1964). These properties favor it as a substitute for graphite and platinum anodes. And it has been suggested and used as an anode material for several electroInd. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 4, 1970
563
