Polymerization of 2,S-Difluoro-
butadiene and 2-Chloro=3=fluorobutadiene L. B. WAKEFIELD Chemical and Physical Research Laboratories, Firestone Tire & Rubber Co., Akron, Ohio
'
T h e work was undertaken to determine the effect of introducing fluorine into butadiene upon low temperature resistance and general physical properties of polymers. The monomers studied were 2,3-difluorobutadiene and 2-chloro-3-fluorobutadiene,prepared by standard halogenation and dehydrohalogenation reactions. Both monomers were polymerized in emulsion; 2,3-difluorobutadiene also was copolymerized with styrene and with butadiene. The homopolymer of 2,3-difluorobutadiene and its copolymer with styrene were hard and Ieathery rather than rubberlike. The copolymer with butadiene had a greater ozone resistance than GR-S; it was comparable to polybutadienc in other physical properties, with the exception of cold resistance, in which it was decidedly inferior. Polymers of 2-chloro-3-fluorobutadiene resembled polychloroprene closely in ozone resistance, solvent resistance, and general tensile properties. However, they stiffened at a higher temperature than polychloroprene. The net effect of the introduction of fluorine into butadiene, therefore, was to decrease the cold resistance by a considerable amount, leaving the tensile properties and solvent resistance relatively unchanged. It was concluded that the use of fluorine-substituted dienes would not lead to superiority in either cold resistance or general physical properties.
a definite geometrical configuration in the monomer which would lead to a high degree of crystallinity in the polymer, but it was recognized that the weak polarity and small bulk of the fluorine atoms might give rise to relatively small interchain forces and low second-order transition points. The replacement of one fluorine atom by chlorine was expected t o give a polymer very much like polychloroprene in tensile strength and cold resistance but with added solvent and ozone resistance. The experiments described in the following sections on the polymerization of difluoro- and chlorofluorobutadiene represent work with a limited amount of monomer of unknown polymerization characteristics. It was uncertain whether the monomers would more nearly resemble butadiene or chlorobutadiene and whether the polymers would require a sulfur- or magnesia-type cure. Therefore it was impossible to adjust the polymerization details and curing recipes to an optimum level. The monomers were prepared by W. S. Barnhart and E. S. Hanson of the Plastics and Organic Chemicals groups of the Firestone Tire & Rubber Co. The chlorofluorobutadiene was prepared by the action of aqueous alkali on a liquid mixture of mono-, di;, and trifluoro compounds resulting from the reaction of hydrogen fluoride and 1,2,3,4-tetrachlorobutane;difluorobutadiene was obtained from 1,2,2,3,3,4-hexachlorobutaneby reaction with hydrogen fluoride and mercuric oxide at 50' C., separation of the difluoro derivat,ive, and dehydrochlorination with zinc. The physical properties of these materials are given in Table I.
T
HE polymerization of butadiene in the past has led to the preparation of a number of highly interesting materials. The polymers have excelled in cold resistance, and in recent times have shown good tread wear ( 8 ) ; in general, they left much to be desired with respect to processability, solvent resistance, and tensile strength. Chlorine-containing butadienes have shown considerable improvement in these properties, but at the expense of cold resistance. To develop polymers with improved cold resistance and a minimum loss in other properties, interest was turned toward fluorine-containing monomers in view of the lower melting point of fluorine compounds in comparison with their chlorine analogs. The literature contains several references to fluorine-containing elastomers (IO). An intensive study of the polymerization of 2fluorobutadiene has been made in this country. The polymers had a better balance of oil resistance and freedom from low temperature stiffening than did polychlorobutadiene controls. The polymerization of 2-chloro-3-fluorobutadiene was claimed by Bock in a German patent ( I ) but only general polymer characteristics were mentioned; cold resistance and tensile properties were not discussed. These data confirmed the interest in fluorinecontaining dienes in this study. The monomers 2,3-difluorobutadiene and 2-chloro-3-fluorobutadiene were of particular interest. The symmetrically placed fluorine atoms of 2,3-difluorobutadiene were expected to favor
PROPERTIES OF MONOMERS TABLE I. PHYSICAL Monomer 2-Chloro-3-fluorobutadiene "2,3-Difluorobutadiene
2363
a
Boil- Melting ing Pointa, Point, C. O C. n? 52-63 - 110 1.428
5.56.5 All boiling points uncorrected.
- 92
~
d': 1.OS5
c1 F, Calck- Ca1cd.Found, Found,
%
33.332.7 1,373 1.001 0.00.0
%
17.817.5
42.242.0
POLYMERIZATION DETAILS. All the polymerizations were effected in glass beverage bottles sealed with oil-resistant rubberlined crown caps. Because of the particularly small amounts of difluorobutadiene available, and its volatile nature, this monomer was stored in a capped bottle under nitrogen and transferred by hypodermic syrin e ( 7 , 8) to the polymerization bottle after the other ingredients Bad been added, the air swept out with nitrogen, and the bottle capped. Chlorofluorobutadiene, .because of its higher boiling point, could be added before purgmg with nitrogen and capping. Freezing of the ingredients in the bottle was undesirable because it was time-consuming and subjected the bottles to severe thermal strains which resulted in frequent losses through breakage, which often occurs in the polymerization bath. The progress of polymerization was followed by. determining the total solids of the latex ( 7 , 8). The convemon a t whwh optimum polymer properties would be obtained was not known. on the basis of GR-S experience, it was assumed to be 70 to 75%
2364
INDUSTRIAL AND ENGINEERING CHEMISTRY
in the case of difluorobutadiene, and 85 to 95% with chlorofluorobutadiene, from chlorobutadiene patent literature ( 3 , i i ) . Two er cent of phenyl-2-naphthylamine (per 100 polymer) was adfed as an antioxidant to the latex before coagulation. There was some question as to whether this was suitable for use with halogen-containing polymers, but as the German patent on chlorofluorobutadiene polymerization described its use in an example, it was considered proper to add it to both chlorofluoroand difluorobutadiene latices. Those polymerization recipes employing anionic emulsifiers were coagulated with salt and acetic acid, whereas the cationic recipe required salt and alcohol. The coagulum was washed on a mill and dried a t 70" C. in a mechanical-convection oven. Polychlorofluorobutadiene was compounded in a neopreneiype gum stock recipe as shown in Table 11; the difluorobutadiene polymers and copolymers were mixed in both gum and carbon black stock recipes (Table 11),of a type commonly used in evaluating GR-S-type polymers.
TABLE 11. COMPOUSDING RECIPES Type Stock Polymer Zinc oxide Stearic fcid Pip-pip Circo light oil E.L.C. Magnesia Sulfur Santocure E P C black Bardol Pine tar Phenyl-2-naphthylamine a
I. Neoprene Gum 100.0 5.0
11. GR-S Gum 100.0 5.0 1 .o
0.5 0.3
... ...
1.2
..I I.. I _
111.8 Piperidine pentarnethylene dithiocarbamate.
100.0 2.4 1.5
.. ..
...
2.0
...
... ..... . ... 109.2
1.7 1.2 45.0 4 .O 2.6 0.6 159.0
-
Stress-strain properties were determined by standard ASTILI procedures; hysteresis values were obtained by ueing a resonance vibrator of the type described by Dillon, Prettyman, and Hall (6). .Low temperature properties were determined by the method described by Liska (9). POLYMERIZATION OF DIFLUOROBUTADIENE
The initial polymerizations of 2,3-difluorobutadiene were carried out in an anionic (GR-S-type) recipe ( 4 ) . The polymerization rate for 2,3-difluorobutadiene alone was practically identical to that for a 75 to 25 butadiene-2,3-difluorobutadiene mixture, and both were noticeably faster than a 75 to 25 2,3difluorobutadiene-styrene mixture. The homopolymer was definitely rubbery, although shorter, harder, and tougher than GR-S; the copolymer with styrene was highly thermoplastic, being boardy and stiff a t room temperature but pliable when warm. Of all of the samples, the copolymer with butadiene was the softest and most rubbery but it was similar to polybutadiene, being weak and short. The polymerization data are given in Table I11 with a butadiene polymer for comparison.
TABLE 111. POLYMERIZATION OF DIFLUOROBUTADIENE IN ANIONICSYSTEM I
.
Difluorobutadiene Styrene Butadiene Soap flakes'" Watera
DDM
Potassium uersulfate Hours a t 50' C. Conversion, % ' Nature of polymer a
t..
1oo:o 5.0 180.0 0.5 0.3 16 70
Soft, rubbery Rubber Reserve Standard grade
I1 100.0
... ...
I11 75.0 25.0
...
I\-. TEXSILE STRENGTH OF DIFLUOROBUTADIENE POLYMERS
Stock type Monomers Ratio
Cure, Blinutee
(Cures a t 280' F.) Gum Black Gum Black D F B D a BDb$DFBD B D : D F B D BD 100 73:23 75: 2; 100
Modulus at 300%
40 80 120
200
Tensile strength
40 80 120 40
460
Elongation
Pn -"
120
Swelling in SR-6, % a t 24 hours 40 a Difluorobutadiene. 5 Butadiene,
,..
...
... ...
700
... ...
360
1125 1800 1975
150
550
,.. ,..
900 925
2225 2100 2250 460 330 320
150 150 100 300 240 180
1550 1900 1550 530 460 400
...
...
...
111. GR-S Black
... ... ...
2.0 4.0
T.4BLE
Vol. 43, No. 10
5.0 5.0 180.0 180.0 0.5 0.6 0.3 0.3 9.5 22 75 a4 Leath- Thermoplasery, tic tough
IV 25.0
...
75.0 5.0
180.0 0.5 0.3 9.5 74 Rubbery weai
described in Table I1 were fairly processable. All cures of the styrene copolymer and the longer cures of the homopolymer exhibited excessive porosity. Tensile data in Table Is' include one value for the homopolymer, mixed in the GR-S gum recipe (11, Table 11)the values for the butadiene-difluorobutadiene copolymer mixed in GR-S gum and black recipes (I1 and 111, Table 11),and those for a polybutadiene compounding control, mixed in a GR-S black recipe (111,Table 11). The tensile strength of the homopolymer gum stock is definitely higher than the butadiene copolymer, xhich is that normally obtained for GR-S-type copolymers, including polybutadiene. The second-order transition point of the honiopolymer was about 1O C. as obtained by dilatometer measurements. The carbon black stock of the butadiene copolymer showed some reinforcement had occurred, but the tensile strengths were only slightly above that of the polybutadiene control. The solvent swelling of the homopolymer gum stock (40-minute cure) in SR-6 (ASTM standard fuel No. 2) was also determined. The value in Table IV is somewhat greater than that quoted for Neoprene Type GN ( 6 ) , indicating that the polar influence of fluorine is much less than chlorine. On the basis of these data it was concluded that neither the homopolymer nor the copolymer with styrene possessed any unusual qualities and it was decided not to study them further. It was felt that a study should be made of copolymers with butadiene as these had given the best results. Because the products of the initial polymerization tests in a carbon black recipe had been distinguished by their poor processability, it was decided to use a cationic system, which gives more easily processable polymers, for further work. Aleo, to emphasize the contribution of the difluorobutadiene, its proportion in the monomer charge was increased to 50 parts. Csing the cationic recipe the polymer described in Table V wae obtained, It was mived in a tread stock, cured, and tested, giving the data shown in Table VI. The ozone resistance values (obtained for expoaure to a concentration of 25 parts of ozone per hundred million of air) indicate that the copolymer shows a great
TABLEv.
DIFLUOROBUTADIENE IN CATIONIC SYSTEM
POLYAIERIZATIOS O F
50 50 200
During compounding these polymers exhibited abnormal behavior. The styrene copolymer was very tough and showed no tendency to break down on milling; on incorporation of the carbon black, it became hard and brittle. The homopolymer was fairly processable until the black was added, when it stiffened and became leathery. Only the copolymer with butadiene gave a satisfactory black stock. The gum stocks of all three polymers
8.6 2.0
Pinene mercaptan Potassium persulfate Hours a t 50' C. Conversion, % Xature of polymer a
Du Pont Aquarex NS.
1.35
0.3 21
78 Soft, rubbery
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INDUSTRIAL AND ENGINEERING CHEMISTRY
October 1951
it is almost completely dead; it reaches a bending modulus of TABLEVI. TEST DATAON BUTADIENE-DIFLUOROBUTADIENE 10,000 pounds per square inch some 20 degrees higher than the COPOLYMER
(Cures a t 280' F.) Polymer
Modulus a t 300%
Tensile strength
Elongation
C. at which bending modulus equals 10,000 Ib./sq. inch Rebound a t 25' C. Rebound at 100' C. Dynamic modulus (80 minutes), lb./sq. inch a t C.
Internal friction, kilopoises a t c.
Ozone resistance, hours a t equal rating
BD:DFBD 50:50
Cure, Minutes 40 80 120 160 40 80 120 160 40 80 120 160 80
... 1900 1850 1850 1525 280 250 250 210 -36
35 53
- 20 - 10
0
* $0 100 - 20 - 10
0 50 100
...
...
1475 1150 438 280
...
108 43 11 7.3 50
GR-S Control
550 925 1000 1025 3100 3250 3500 3650 790 630 620 630 -42 36 48 1293 1375 589 289
chlorobutadiene control. The ozone resistance of the polymers was determined on black stocks, which had been prepared by the addition of 45 parts of EPC black to the standard neoprene gum stock recipe. After a total of 184 hours of exposure (about equally divided between exposure to 25 parts of ozone per hundred million of air plus ultraviolet light, and to ozone alone) the samples had barely detectable signs of checking. However, no difference between them could be seen.
TABLE VIII. TESTDATA-POLYCHLOROFLUOROBUTADIENE Polymer
Modulus a t 300%, Ib./sq. inch
(Cures a t 280" F.) Chlorofluorobutadiene Cure, Minutes 20 125 40 175 80 275
Chlofobutadiene
175 325 350
...
Tensile strength, lb./sq. inch
108 57 24 8.6
20 40 80
1600 2300 1675
2125 2150 1950
Elongation, %
20 40 80
1100 1040
BOO
840 730 720
7-10
Rebound At 25' C., % At 100' C., %
60
6 62
45 72
Bending modulus, C. a t 10,000 lb./sq. inch
60
- 18
- 37
...
improvement in this direction; the values approach those obtained with chlorobutadiene polymers. The change in monomer ratio to 50 parts of butadiene to 50 of 2,3-difluorobutadiene caused a slight drop in tensile strength, down to that expected for polybutadiene. The low temperature bending modalus values indicate that the polymer stiffens a t a much higher temperature than would be expected; the fluorine atoms caused a much greater polymer chain interaction than was foreseen. The rebound data suggest that the copolymer is more efficient than GRS a t 100" C., but this picture is reversed by the resonance vibrator hysteresis data, for the internal friction of the copolymer is much greater than that of the GR-S control.
An estimation of the solvent resistance of the gum stock was obtained by a weight increase method. The data presented in Table IX show that in comparison to the polychlorobutadiene control (laboratory polymerized) the chlorofluorobutadiene polymer has a slightly greater swelling action. Neoprene Type GN has a comparable swelling behavior (6). ]
TABLEIX. SWELLINQ OF CHLOROFLUORO POLYMER IN SR-6
POLYMERIZATION OF CHLOROFLUOROBUTADIENE
Because of the close chemical similarity of chlorofluorobutadiene to chlorobutadiene, the polymerization recipe used (Table VII) was one mentioned in the patent literature on chlorobutadiene (11). When the chlorofluorobutadiene polymer was mixed in the neoprene-type gum stock, cured, and tested, the data recorded in Table VI11 were obtained. A laboratory prepared polychlorobutadiene control is included for comparison. The data indicate that in tensile properties the two polymers are similar, the chlorofluorobutadiene polymer having a slightly higher elongation a t break and a lower 300% modulus. In the rebound and low temperature bending modulus values, however, a noticeable difference appears. Even a t 100" C. the chlorofluorobutadiene polymer is distinctly lower in rebound, and at 25" C.
Sample Polychlorofluorobutadiene Polychlorobutadiene
2 hours 115 95
Wt. Increase, %, After Immersion 5 24 hours hours 134 138 98 103
Stress relaxation crystallinity tests on both the chlorofluoroand difluorobutadiene polymers indicate them to be more crystalline than Neoprene Type GN. SUMMARY
Both 2,3-difluorobutadiene and 2-chloro-3-fluorobutadiene polymerize in emulsion. 2,3-Difluorobutadiene copolymerizes with styrene or butadiene. Both the homopolymer and the styrene copolymer suffer either from difficulty in compounding and curing, or from poor physical properties when considered as an elastomer. The butadiene coTABLE VII. POLYMERIZATION OF CHLOROFLUOROBUTADIENE polymer had a much greater ozone resistance than GR-S. It was comparable to polybutadiene in other physical properties Chlorofluorobutadiene 100.0 Water 190.0 with the exception of low temperature resistance, in which it was N wood rosin 4.0 Sulfur 0.6 decidedly poorer than GR-S. Sodium hydroxide 0.8 Polymers of 2-chloro-3-fluorobutadiene, when prepared in Ammonium persulfate 1.0 Sodium dinaphthylmethane neoprene-type polymerization recipes, have a very high ozone sulfonate 0.8 resistance, similar to that of polychlorobutadi ene, and have equal Hours a t 1 5 O C. 9 Conversion, % 95 tensile properties and solvent resistance. T he polymer stiffens Xature of polymer Tough, a t a higher temperature than does polychlor obutadiene and has rubbery a much lower rebound at room temperature.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2366
ACKNOWLEDGMENT
(5)
The author wishes to express his appreciation to the Firestone Tire & Rubber Co. for permission to publish this work and to F. W. Stavely for his continued interest in the problem.
Vol. 43, No. 10
Dillon, J. H., Prettyman, I. B., and Hall, G. L., J . A p p l i e d Phys., 15,309-23 (1944).
(6) Du Pont de Nemours and Co., Inc., E. I., Rubber Chemicals Div., Bull. BL223 (April 15, 1948). (7) Harrison, S. A., and Meincke, E. R.. Anal. Cheni., 20, 47-8 (1948).
LITERATURE CITED
(1)
(8)
Rock, W. (to I. G. Farbenindustrie), Ger. Patent
737,276 (July
9, 1943). (2) Chern. Inds., 67, 29 (July 1950). A. M, (to E. 1. du Pant de Nemours and Co,, Inc,j, 8, (3) collins, Patent 2,264,173 (Nov. 25, 1942). (4)Craig, D. (to B. F. Goodrich co.1, Ihid., 2,362,052 (Nor-. 7 , 1944).
u.
J
Houston, R. J.,Ibid., 20,49-51
(1948).
Liska3J' w ' 2 IND' 36140-6 (1y44)' (10) Mochel, W.E., Salisbury, L. F., Barney, A. L., Coffman, I).D., and Mighton, C. J., Ibid., 40,2285-9 (1948). (11) Wilder, F. N. (to E. I. du Pont de Nemours and C o . , Iuc.), Brit. Patent 587,804 (May 6, 1947). RECEIVED March 2, 1931. Presented before the Division of Rubber Chemist r y of the AMENICAN CHESIICAL SOCIETY.Washington, D. C., 1951.
rolvsis of J
0
ERNEST G. LONG' AND KENNETH A. KOBE University of Texas, Austin, Tex.
T h e hydrolysis of mercuric acetate makes impossible a simple solution of this salt in watei. The minimum concentration of acetic acid necessary to prevent precipitation of mercuric oxide has never been determined. The extent of hydrolysis of mercuric acetate has been determined for concentrations up to saturation over the temperature range -1.5" to 100" C. Pure mercuric acetate can be prepared from solutions containing more acetic acid than the amount formed dn hydrolysis. In mercuration processes i t is necessary to exceed this concentration.
acetic acid, and water may be in equilibrium with (a) aohd mercuric acetate, as along a simple solubility curve, or ( b ) solid mercuric oxide, formed under conditions of hydrolysis, or (c) with mercuric acetate and mercuric oxide, as a t the intersection of solubility and hydrolysis curves. Thus the hydrolysis-solubility curve of mercuric acetate represents one branch of the ternary solubility isotherm for which the solid phase is mercuric oxide. The aceti6 acid concentration in the equilibrium hydrolysis solution represents the minimum acetic acid concentration possible in the ternary system for any specified temperature and concentration of mercuric acetate. EXPERIMEIYTAL
0
RGANIC mercurials are of considerable interest because of their properties as bactericides and fungicides as well as their properties as chemical intermediates. An extensive literature exists concerning their preparation, properties, and reactions (1). However, there are few data relative to the medium which is almost invariably used in mercuration processes. Data on this mercurating medium (the system, mercuric acetateacetic acid-water) are needed if the study of the reactions is to be put on a fundamental basis. In addition, data on this system are needed in the general study of salt solubility for a system in which considerable hydrolysis of the salt occurs. For this particular salt, these data are required for proper control of the crystallization process by which a salt of high purity can be produced. The usual mercurating agent for organic compounds is mercuric acetate which is normally introduced as an aqueous acetic acid solution. Although there are several references to the water solubility of mercuric acetate without mention of hydrolysis ( 2 , S ) , mercuric acetate does not dissolve in water to form a simple binary solution. When pure mercuric acetate is dissolved in water, hydrolysis occurs, producing acetic acid and precipitating mercuric oxide according to Equation 1. Hg(0Ac)z
+ HzO
2HOAc
+ HgO(s)
(1)
The reaction is reversible, and the equilibrium is affected by temperature and concentration. At temperatures above the freezing point of acetic acid, the solution of mercuric acetate, 1
Present address, Johns-bhnville Research Center, RIanville, N. J
Determination of Mercury. Mercuric ion wm determined by precipitating mercuric sulfide Kith hydrogen sulfide in a dilutc aliquot. Standard laboratory procedure was used. Check analyses shoxed accuracy of 1 part in 2000. Determination of Acetic Acid. The hydrolysis of the mercuric acetate prevented a direct deterinination of the acetic acid by titration. Instead, it was necessary to determine the total acetate present and, by subtracting the acetate equivalent to the mercuric ion as mercuric acetate, t o calculate the acetic acid acetate by difference. A simple direct titration with standard sodium hydroxide (0.08-0.20 A') using phenolphthalein as an indicator gave accurate results, and this method was used at all times. Addition of excess base and back-titration with hydrochloric acid gave the same results as a direct titration with base. At the end point the solution is one of sodium acetate in water. The solubility of the mercuric oxide, 0.000237 gram-mole per liter (4),is negligible. Determination of Water. Water was calculated by subtracting the percentage of mercuric acet,ate and acetic acid from 100%.
TABLE I. EFFECTO F MERCURIC OXIDE ON ACETATETITRATION Bample x-1 2-2
2-3 x-4 2-5 37-6 2-7
Mercuric Oxide
A-&OH, RI1.
None None Small amount all dissolved Twice x-3 all 'dissolved 3 times 5-5, all dissolved 4 times 2-3,some excess HgO 6 times 2-3,some excess HgO
14.73 14.74 14.78 14.74 14.74 14.74 14 74
