1304
INDUSTRIAL AND ENGINEERING CHEMISTRY
Suggestions made by Warren Stubblebine and C. C. Vogt of the Office of the Quartermaster General and the assistance of other individuals of the laboratories of The Connecticut Hard Rubber Co. are much appreciated. The inspiration provided by C. M. Doede of the latter company is especially appreciated. LITERATURE CITED
(1) Doede, C. M., Duke, N. A., and Glime, A. C., paper presented before Division of Rubber Chemistry, AM. CHEM.Soc., Wash-
inaton. D. C.. March 1951. (2) Fairbank, H. A:, Walker, C. A,, and Doede, C. M., Phus. Rev., 83, 205 (1951).
(3) Flory, P. J., and Rehner, J., J . Chem. Phys., 11,512 (1943).
Vol. 45, No. 6
(4) General Electric Co., Technical report on 81176 silicone polymer, April 3, 1951. (5) Jones. H. F., U. S. Patent 2,448,530 (Sept. 7, 1948). ( 6 ) Moakes, R. C. W., and Pyne, J. R., J . Rubber Research, 19, 77 (1950). ( 7 ) Rochow, E. G., “Chemistry of the Silicones,” 2nd ed., Kew York, John Wiley & Sons, 1951. (8) Warrick, E. L., U. S. Patent2,460,795 (Feb. 1, 1949). (9) Ibid., 2,541,137 (Feb. 13,1951). RECEIVEDfor review November 4. 1952. ACCEPTEDFebruary 20, 1953. Presented before t,he Division of Rubber Chemistry, ANERICANCHEMICAL SOCIETY,Buffalo, N. Y., 1952. Work done under G. S. Government Contrao t D-4-44- 109-QM-64, 195 1.
Polymerization of G eratures J
REVIEW OF RECENT DEVELOPMENTS L. H. HOWLAND, V. C. NEKLUTIN, R. L. PROVOST, AND F. A. MAUGER hTaugatuckChemical Division, United States Rubber Co., Naugatuck, Conn.
T
HE period immediately subsequent t o widespread acceptance of “cold” rubber as a superior tire tread polymer was marked by extremely rapid development of formulations for the polymerization of the new synthetic rubber. Efforts in this direction were sparked by the necessity for the GR-S producers to arrive at recipes which would permit maximum flexibility of operation, uniformity, economy, and ease in adjustment of reaction rates, and a t the same time eliminate certain objections to the early formulations such as tendencies toward latex instability. Neklutin et at. (9) gave some indication of the wide variety of formulations applicable to polymerization of GR-S at 41’ F. and mentioned the commercial status of several recipes. The present paper reviews more recent developments by the authors in polymerization techniques and mentions several developments now in the pilot plant stage which show promise of emergence to commercial status in the near future. POLYMERIZATION RECIPES
41° F. (STANDARD COLDRUBBER). The original sugar-free, ferrous pyrophosphate recipe described by Neklutin et al. (9) has been changed very little (Recipe I, Table I). T h e substitution of diisopropylbenaene monohydroperoxide (or p-menthane hydroperoxide) for the original cumene hydroperoxide has permitted a reduction in the ferrous sulfate-potassium pyrophosphate levels required to attain 60% conversion of monomers in the same length of time (approximately 14 hours). This recipe has been used commercially t o make GR-S 101 in the government-operated synthetic rubber plants. The sensitivity of the recipe to activator make-up, a t first considered disadvantageous, has since been overcome by improvements in preparation techniques, Just as the sugar-free, ferrous pyrophosphate recipe has been preferred in polymerizations where a t least part of the emulsifier is rosin soap, polymerizations emulsified with all-fatty-acid soap a t the present time employ a so-called polyamine activation system in place of ferrous pyrophosphate. The polyamine activators, first described by Whitby e t al. (17) and later studied extensively by other investigators (5,9),have been found to be very versatile, and the lack of necessity for complex activator
preparations has resulted in excellent reproducibility of reaction rates. I n polymerizations employing all-fatty-acid soap emulsification a t 41” F. (Recipe 11, Table I), diethylenetriamine (DETA) has been used in preference t o the amines of higher molecular weight such as the more active triethylenetetramine or tetraethylenepentamine, because of its economy, uniformity, and availability. Diethylenetriamine was the activator employed in the commercial production of GR-S X-565 mentioned in a previous paper (9) and i t has since been used in almost all cold GR-S where the polymerization emulsifier has been 100% fatty acid soap. Diethylenetriamine activation has also been employed in many cold, high-solids foam sponge latices, some of which are emulsified with mixtures of fatty acid and rosin acid soaps. One recent development which has improved the versatility of the diethylenetriamine-activated recipe is the use of small amounts of ferrous sulfate in conjunction with the polyamine. The effect of these small amounts of ferrous sulfate on reaction rate is shown in Figure 1. The iron salt is dissolved in the diethylenetriamine solution, so that charging is not complicated in any way. Sometimes it is desirable to use a small amount of sequestering agent along with the added iron, A recipe for polymerization of a rosin soap (Dresinate 731)emulsified system with tetraethylenepentamine has been re ported (9) and sequestering agents such as ethylenediaminetetraacetic acid have been found to have a beneficial effect in the recipe ( 1 1 ) . Several polymers have since been produced on a commercial scale using tetraethylenepentamine a8 the activator, but the nonuniformity and shortage of this amine have prevented extensive use of this system up to the present time. Development of 41” F. recipes employing rosin soap emulsification with the lower polyamines was retarded by the fact that they were considerably less active than the pentamine and reaction times were excessively long even a t high activator levels. The discovery that strong inorganic reducing agents have a beneficial effect on reaction rate when the lower polyamines are employed as activators (4)was thoroughly investigated. It was found that the most important variable in such a formulation was the amount of reducing agent added, the limiting conversion
June
.
70
TEMP.-4loF.
70
TIME-HOURS
TIME
Figure 1. Effect of Added Ferrous Sulfate on Reaction Rate of Diethylenetriamine-Activated GR-S Recipe
- HOURS
Figure 2. Effect of Potassium Sulfite on Reaction of Rosin Soap-Emulsified TriethylenetetramineActivated GR-S Recipe
plant samples have indicated t h a t the polymer produced is fully equivalent to the standard cold rubber. T h e physical test data are given in Table 11. Attempts to develop a 41" F. rosin soap formula activated with diethylenetriamine met with little success until it was found t h a t sodium dithionite (sodium hydrosulfite) was a specific reductant for this system ( 1 ) . Bottle polymerkations indicated a level of 0.20 part of sodium dithionite per 100 parts of monomers as optimum; however, in 5-gallon reactors i t was shown that 0.20 part was too high a concentration and that reduction in the amount charged increased the limiting conversion. This TABLE I. POLYMERIZATION RECIPES behavior was directly opposed t o that exRecipe No. I I1 I11 IV V VI VI1 VI11 hibited by potassium sulfite in triethylTemperature, F. 41 41 41 41 0 0 41 41 enetetramine recipe mentioned above. 200 200 180 180 200 Water 200 200 240 The data are presented graphically in Fig45 ... ... ... ... ... ... 60 Methanol ... ... 15 ... ... ... . . . ... Ethylene glycol ure 3. As shown in this chart, a re75 75 75 71 71 71 75 75 Butadiene 25 25 25 25 29 29 29 duction in the sodium dithionite charge 25 Styrene *.. 4.7 1.05 ... 4.7 ... 4.7 Potash salt of fatty acidsa ... to 0.05 part permitted reactions to attain Potash salt of dispropor... 5.95 tionated rosin acids 4.0 ... ... 4.7 ... ... 60% conversion (Recipe IV, Table I). Sodium salt of dispropor.0.. 0. 5 ... 4.7 ... ... Although concentrations below 0.05 part tionated rosin acids ... 4.7 ... ... 0.05 ... 0.02 0.03 0.22 Potassium hydroxide were not investigated, i t has been shown 0.05 ... ... ... ... ... 0.05 ... Sodium hydroxide Sodium salt of naphthat h a t elimination of the sodium dithionite lenesulfonic acid confrom the charge resulted in excessively densed with formalde0.15 0.05 0.05 0.05 0.05 0.10 0.10 ... hyde long reaction times. The main drawback ... 0.40 ... 0.40 ... 0.50 0.30 0.30 Potassium chloride Trisodium Dhosuhatet o this recipe is the rapid deterioration * lZHz0 0.30 ... *.. .*. ... ... ... ... Sodium dithionite ... ... ... 0.05 ... 0.10 005 ... of sodium dithionite in solution. It is Potassium sulfite ... ... 0.50 ... 0.50 ... ... ... thus necessary to prepare fresh solution Cumene hydroperoxide (or) 0.12 0.15 ... ... ... ... ... for each charge, a procedure not readily Diisopropylbenzene monohydroperoxide 0.08 0.15 0.25 0.16 0.3 0.12 0.15 adaptable to large scale operations. Ferrous sulfate7Hz0 0.20 0:002 ... ... 0.002 0.006 0 . 1 8 0 . 2 5 SUBFREEZINQ RECIPES.On the basis max. Tetrapotassium pyrothat significant improvements have been phosphate 0 22 ... ... ... ... 0.18 ... made in the quality of GR-S as a remax. Tetrasodiiim pyrophossult of lowering the polymerization temphate.lOH?O ... ... ... ... ... ... 0.40 Diethylenetriamine ... o.'i25 ... 0.15 ... ... ... ... perature from 122" to 41" F., i t would be Triethylenetetramine ... ... 0.12 ... 0.28 0.28 ... ... Ethylenediaminetetraaoereasonable to expect further improve tic acid ... ... 0.025 ... ... .0.. .2 0 00.025 ... ment when polymerization is conducted tert-Ciz mercaptan 0.21 0.18 0.20 .20 0.20 0.20 0.20 Approx. time to 60% a t still lower temperatures; consequently conversion, hours 14 14 14 12 16 25 5 1.67 considerable work has been done in deFatty acids approved by Office of Synthetic Rubber, RFC. veloping suitable recipes for subfreezing
being controlled almost exclusively by this factor. This effect is depicted in Figure 2 for a recipe based on disproportionated rosin emulsification and triethylenetetramine (TETA) activation with potassium sulfite as the reductant (Recipe 111, Table I). Variation in the concentrations of amine, peroxide, sodium hydroxide, or sequestering agent at each of the levels of potassium sulfite shown had but minor effects on either the specific reaction rate or the ultimate conversion attained. Although this formula has not yet been used on a commercial scale, tests on large pilot
.
I
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~~
9 . .
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
1306
TABLE 11. PHYSICAL TESTDATA O N POLYMER FROM POLYAMINE-~LCTIVATED, ROSINSOAP-EMVLSIFIED SYSTEN
A
Polymer Polymerization recipe ComDound Poiymer HAF black Zinc oxide Stearic acid Sulfur Mercaptobenzothiazyl disulfide Tests 300% modulus lb./sq. inch Tensile Ib./s&. inch Elongation, % 205' F. tensile, Ib./sq. inch Aged tensilea, Ib./sq, inch Heat buildups,
Polyamine (Recipe 111, Table I)
B Iron pyrophosphate (Recipe I, Table I)
100 55 3.0 2.0 2.0 1.75
55 3.0 2.0 2.0 1.75
Vol. 45, No. 6
mate stress-strain properties or abrasion resistance and that conversion has little effect, except that tensile strength and abrasion resistance suffer to some extent when conversion is allowed to proceed beyond about SOYo, although a t least part of the inferiority of the 90% conversion polymer in this work may be due to its slightly lower Mooney viscosity.
100
Cured at 292O F., Min. 25 50 100 25 50 100 25 50 100 50 100 50 100 50
O W
Aged flex crack 7.1 9.2 growthe, 0;OOl inch/kc. Abrasion ratingd 100 100 Geer oven 96 hours a t 212O F. b Goodrioh hexometer. c De Mattia average 3 cures. aged in Geer oven 96 hours a t 212O F. d Modified cambourn abrade;, std. cold rubber 2 100. Q
polymerization. Smith et al. (IS)show a straight-line relationship between latex film tensile strength and polymerization temperature down to 0" F., and Hart and Meyer ( 5 )show continued improvement in polymer structure as polymerization temperature is reduced from 122" to 0" F. However, tread compounds of rubbers made at 0' F. have been erratic in showing improvement over their 41 ' F. counterparts either in laboratory evaluations or in actual road-wear tests, although significant improvement in flexing was recognized a t an early date (6). There have been many hypotheses advanced in an attempt to explain the erratic behavior of the subfreezing polymers in showing the expected quality improvement.
TABLE111. PROPERTIES OF 0' F. FATTY ACID-EXULSIFIED POLYMERS AT VARYIXG CONVERSION AND MOONEY VISCOSITY Polymer No. Conversion, % hlooney viscosity, ML-4 Compound Polymer E P C black Zinc oxide Sulfur Mercaptobenzothiaz yl disulfide
C 60
D 60
E 60
F 70
G 80
H 90
50
62
82
65
59.5
42
100 40 5.0 2.0
100 40 5.0 2.0
100 40 5.0 2.0
100 40 5.0 2.0
100 40 5.0 2.0
40 5.0 2.0
1.75
1.75
1.75
1.75
1.75
1.75
800 590 510 780 1400 1060 1530 940 1930 1550 2030 1410 4050 3570 4060 3400 Tensile, lb./sq. inch 4060 3880 3740 4060 3730 3560 3530 3860 Elongation, % 740 800 750 900 570 540 690 700 460 540 580 430 Abrasion rating0 101 100 101 100 a Modified Lambourn abrader, polymer A (Table 11) = 100.
490 1060 1450 3290 4050 4040 800 690 590
470 880 1240 2460 3540 3260
100 25 50 100 25 50 100
99
100
800
700 550 90
Evaluation of polymer C by several consumers in a variety of applications did not reveal consistently significant superiority compared with 41' F. polymers. This was believed to be due, a t least in part, to the presence of fatty acid, so that further recipe development at 0" F. was confined to rosin soap emulsification.
The high iron levels required for suitable activation of 0" F. polymers cause deterioration in properties during compounding. Optimum Mooney viscosity and conversion have not been selected. Freezing point depressants necessarily used in the polymerization recipes alter molecular configuration by acting as chain transfer agents or by influencing solubility of modifier and other ingredients. Polymerization in an all-rosin soap system a t 0" F. has not been feasible. Shortstopping is inadequate. Besides eliminating the possibility of quality deterioration due t o the use of high iron levels in the iron pyrophosphate-activated recipes, the same advantages favor polyamine activation systems at 0" F. as at 41' F. For these reasons an attempt was made to develop a polyamine-activated recipe for use a t 0" F. This was accomplished fairly easily by adapting Recipe I11 (Table I ) t o a subfreezing fatty acid-emulsified system. The water phase (25y0methanol) was increased to 240 parts, the rosin soap replaced with fatty acid soap, the activation level increased, and a small amount of iron added t o improve reaction rate (Recipe V, Table I). Using this formulation, a series of polymers was prepared a t varying conversions and Mooney viscosities in order t o determine whether optimum properties were attained a t levels other than the normal 60% conversion and 50 t o 60 Mooney viscosity. The test results on this series of polymers are listed in Table 111. It is indicated that Mooney viscosity, within the range investigated, does not affect the ulti-
TIME -HOURS
Figure 3. Effect of Sodium Dithionite on Reaction of Rosin-Emulsified DiethylenetriamineActivated GR-S Recipe The effect of freezing point depressant on properties was investigated by preparing a polymer a t 41" F. according to Recipe I11 (Table I), except that 25% of the water was replaced by methanol, the most commonly used antifreeze. If the methanol exhibits any chain transfer activity a t 0" F. itself or influences it through effect on the mercaptan modifier, it was believed that this effect should be enhanced a t 41' F. and result in appreciable dif-
INDUSTRIAL AND ENGINEERING CHEMISTRY
June 1953
130T
in Table IV indicate that the use of methanol in the polymerizaOF METHANOL ON PROPERTIES OF.POLYMERtion recipe may result in some deterioration in quality. TABLEIV. EFFECT PREPARED AT 41" F. Although commercial 0" F. polymers had been produced with Polymer No. J (Av. 2 KandL fatty acid soap emulsification (X-464) as far back as 1948 (15)and Sets Data) (Av. Data) No Yes Methanol in recipe 55 53 Raw viscosit ML-4 a t 212O F. Cornpounded)$iscosity, ML-4 a t 212' F. 85 71 Cur$d a t 292 F., Tests Min. 300% modulus, lb./sq. inch 25 1570 1640 50 2020 2210 100 2300 2550 Tensile, Ib./sq. inch 25 3300 2890a 50 3690 3220" 3600 3270" 100 Elongation, % 25 540 480" 50 500 440" 100 420 3700 205' F. tensile, lb./sq. inch 50 2050 1440" 100 1930 1610" Aged tensile, lb./sq. incha 50 2950 2910 100 2540 2900 50-min. resilience R.T. 46.6 39.7" A t 212' F. 56.0 50.8= Heat build-upc, F. 50 79 89" Torsional hysteresis 50 0.124 0,1205 (285' F.) 100 0,110 0.115 Flex crack growthd, green, 0.001 inch/kc. 1.0 1.5" Flex crack growthd, agedb, 0.001 9.7 >12.3" inc h/ko , Abrasion rating6 100 93 Experimental polymer consistently inferior to control in these tests. b Geer oven, 96 hours a t 212O F. c Goodrich flexometer. d De Mattia average 3 cures. e Modified darnbourn abrader.
more recently ( I O ) with a mixture of hydrogenated rosin and f a t t y acid soap (X-602), a substantial proportion of fatty acid was known to be necessary if reaction rates were not to be excessively long. Figure 4 illustrates the effect on reaction time of increasing the ratio of rosin to fatty acid in the soap. It is apparent that replacing up to 50% of fatty acid soap with rosin soap has only a slight retarding effect, but that replacing more than 50% of t h e fatty acid results in serious retardation, the reaction time to 60% conversion in the case of a n emulsifier composed of SOY0 rosin and 20% fatty acid being four times that required for 100% f a t t y acid emulaification. A great deal of experimental work was conducted with little success on formulations based on all-rosin soap or soaps containing a minimum of 85% rosin, as far as attainment of reasonably rapid reactions was concerned. I n this work it was noted that variations in the catalyst-activator levels, pH, etc., had
ferences in physical properties. As the inclusion of methanol in the recipe had a tendency t o decrease reaction rates, several minor modifications were made to adjust for this condition. Potassium ions replaced sodium ions. Triethylenetetramine waB replaced by tetraethylenepentamine. Peroxide and activator levels were raised 25%. Ferrous sulfate heptahydrate (0.003 part) was included in the recipe. The polymer was compared with its counterpart prepared without methanol (polymer A, Table 11). The test data listed
5
10
15
20
25
30
PERCENT METHANOL
Figure 5. Critical Micelle Concentration us. Per Cent Methanol in Water
PERCENT ROSIN
100
80
60
$@P 40
IN EMULSIFIER
20
0
PERCENT FATTY ACIDSOAP IM EMULSlFlER
Figure 4.
Effect of Rosin Soap Emulsifier on Reaction Time Temperature, 0' F.
practically no effect on reaction rate. This insensitivity of the recipe to the normal variables suggested the soap as being the primary rate-controlling ingredient. Accordingly, the potassium salt of disproportionated rosin was selected as a typical rosin soap and the potassium salt of fatty acids approved b y the Office of Synthetic Rubber, Reconstruction Finance Corp., was selected as the most commonly used fatty acid soap, and critical micelle concentrations of these two soaps under varying conditions were studied, using a modification of the method of Corrin, Klevens, and Harkins ( 8 ) . Figures 5, 6, and 7 show the effect of methanol concentration, temperature, and electrolyte concentration on the soap concentration at which micellization takes place. The effects of increasing methanol concentration in the water phase and of reducing temperature are much more pronounced in the case of the rosin soap, both serving t o raise the critical concentrations markedly. Electrolyte concentration has little effect on the critical concentrations at 0" F. in the presence of 25% methanol.
'
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
1308
Although it is clearly indicated that, under similar conditions, the critical concentration of the disproportionated rosin soap is many times higher than that of the fatty acid soap, the value of 0,016 mole per liter obtained for the rosin soap a t 0" F. in 25% methanol and in the presence of electrolyte is still considerably below the concentration actually present in a normal polymerization system containing approximately 5 parts of soap in 240 parts of miter phase (about 0.06 mole per liter). However, it
.018
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I
I
I
I
100
I20
Vol. 45. No. 6
were checked a t 0" F., soap concentrations of 5 . 3 and '7.0 parts giving approximately equal reaction times of about 100 hours in a recipe emulsified with 85% disproportionated rosin soap and 15% fatty acid soap and activated Tvith triethylenetetramine and ferrous sulfate.
TABLEV.
EFFECTO F S O A P CONCEXTRATION O S RE-4CTION
RATE
( 4 1 O F. rosin soap recipe, 25% methanol) Time t o 60% Conversion, Soap, Parts/100 Monomers Hours 3.3 30 4.0 17 4 7 12 6 0 12 10.0 22
,016
.014
.ole .OIC
B f .oat \ II +cn 0
3 -00
.m C
I
20
40 60 80 TEMPERATURE- O F .
Figure 6. Critical Micelle Concentration Temperature i n 25qo Methanol
US.
was noted that, as methanol concentration increased and particularly as temperature decreased, the definition of the critical concentration became less sharp-Le., the transition between dissolved soap and micellar soap was gradual. This indicates the possibility that even a t relatively high soap concentrations, the number of micelles formed is relatively small and that reaction rate is controlled primarily by the number of micelles available for initiation rather than by the activity of the catalyst system. If this is true, one might expect a marked divergence from normal in molecular weight distribution of the polymer formed, which could be manifest in its physical properties. Fractionation of rubbers polymerized a t 0" F. may provide data which d l serve to determine the validity of this hypothesis. One of the logical approaches to improvement in reaction rates of 0" F. polymerization with rosin soaps, in the light of this work, was to use larger than normal amounts of soap. This was first tried in a methanol-containing 41" F. recipe similar to Recipe I11 (Table I). It was found, according to the data given in Table T'. that 4.7 to approximately 6.0 parts of soap per 100 parts of monomers was optimum; reaction rate decreased a t either lower or higher soap levels. These results were unexpected, as in most polymerization recipes rate is directly proportional to soap concentration. The data serve to emphasize the unpredictable effect of employing methanol as a freezing point depressant but do not necessarily controvert the hypothesis that rate is controlled by micelle concentration. T h e indications obtained in Table V
PERCENT KCI ON SOAP
Figure 7. Critical Micelle Concentration v s . Electrolyte i n 25Yo Methanol I t was also found that operation a t methanol concentrations of less than 25% in the water phase, while successful in small reactors, was not possible in large vessels because the necessarily lower coolant temperature caused freezing of the latex on the walls of the reactor. The attempt to improve reaction rates was then continued by replacing part of the methanol antifreeze with ethylene glycol. I n the mixed soap (85 rosin-15 fatty acid), polyamine-iron activated formula, replacing '7 parts of methanol with glycol reduced reaction time a t 0' F. from about 100 hours to about 40 hours when 7.0 parts of soap were used per 100 parts of monomers. Increasing the soap concentration t o 10.0 and 15.0 parts increased reaction time to 50 and 56 hours, respectively. Replacing 15 parts of methanol with ethylene glycol resulted in a reaction time of 25 hours a t a soap level of 7.0 parts. Soap levels of 5.3 and 15.0 parts increased reaction time to 43 and 55 hours, respectively. The best formula is given in Table I as Recipe TI. Thus, although it has not yet been possible t o polymerize with all-rosin soap a t 0" F. in a reasonable time, the fatty acid content of the emulsifier has been reduced to the point where it is difficult to imagine that further reduction could have an appreciable
,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
June 1953
OF 0' F. POLYMERS PREPARED WITH ROSINTABLE VI. TESTING FATTY ACID SOAPRATIOOF 85/15
Polymer No. Polymerization temperature, Antifreeze Raw viscosity, ML-4
M
N Pa 0 0 41 Methanol None 100 75 55
' F. Cured a t 292' F., Min.
I.
Tread stock (HAF black, 55 parts) Green properties 300% modulus, lb./sq. inch Max. tensile lb./sq. inch Elongation 205O F., t e k l e , lb./sq. inch ' 50Lm&. resilience, %
k
25 50 100 50 50
n.1.
-
212O F. Heat build-uua. O F. Flex crack &owthe (0.001 inch/kc.), av. 3 cures Abrasion ratingd Aged properties, Geer oven, 96 hours a t 212O F. 100% modulus, lb./sq. inch Max. tensile Ib./sq. inch Flex crack gkowthc, 0.001 inch/kc., av. 3 cures 11. Carcass stock ( H M F black, 33 parts) Green properties 300% modulus, Ib./sq. inch
Max tensile lb./sq. inch Eloigation % 205' F. ten'sile, lb./sq. inch 60-min. resilience, % R.T. 212' F. Torsional hystereis (285' F.) Heat build-up6 O F. Flex crack gro&thc, 0.001 inch/kc., av. all cures Aged properties, Geer oven, 96 hours a t 212' F. 100% modulus, Ib./sq. inch
Max. tensile, lb./sq. inch a Control polymer. b Goodrich Flexometer. C De Mattia. d Modified Lambourn abrader.
,-
1
50
41.5 52.7 98
0.6 ... 25 50 100
1300 1130 1070 2960
3 6 . 5 45.0 46.5 50.2 106 108 0.6 118
1.1 100
1370 1440 1400 1550 1200 1430 2890 2780
7.5
11.1
12.5
60 60
630 1100 1350 1600 2950 570 1260
700 1250 1400 1530 2800 500 1140
1140 1560 1700 1700 2000 320 600
60 60
50 57 0.097 75
46 57 0.076 63
49 63.5
0.3
0.3
1.3
640 860 840 1000 2170
660 750 750 840 2330
720 750 840 600 1860
30 45 60 90
30 45 60 90
..
43
1309
factor in future studies on polymerization, particularly a t very low temperatures. SHORTSTOPPING AGENTS
The early cold rubber was shortstopped with di-tert-butyl hydroquinone. This material, in addition to being costly, was rather difficult to handle, because it is insoluble in water and had to be added as a ball-milled water dispersion. Although it was a very excellent shortstop under ideal conditions of usage, there was a tendency for the solid shortstop to be incompatible with the latex and erratic shortstopping resulted. Dinitrochlorobenzene in styrene solution was next employed extensively. This material was relatively inexpensive and was compatible with the latex. However, the desirability of finding a watersoluble shortstopper continued to spur efforts in this direction, particularly in view of the fact that dinitrochlorobenzene was not completely satisfactory from the point of view of the consumer. At about this time it was found that hydroquinone could be used as a shortstop in sugar-free cold recipes ( I d ) . This material was the standard shortstop for 122" F. GR-S but had not been applicable to cold recipes up to this time. The data presented in Table VI1 indicate that hydroquinone is effective in the ironpyrophosphate recipe (Recipe I, Table I), the polyamine recipe (Recipe 11, Table I), and the ferrous silicate recipe discussed in a previous paper (9),provided the latex is either heated immediately after shortstopping or a small amount of hydrogen peroxide is used in conjunction with the hydroquinone. I n sugar-containing recipes the oxidation of the hydroquinone to quinone, which is the actual shortstop ( 8 ) , is probably suppressed by the sugar. However, even in the sugar-free system, the hydroquinone is apparently not oxidized rapidly enough by the residual peroxide, so that either t h e temperature must be raised quickly or an oxidizing agent must be employed in conjunction with the hydroquinone. The oxidation is slow a t room temperature in the acid environment afforded by the hydroquinone, so that the two materials can be premixed and stored. Oxidation in the alkaline medium afforded by the latex is very rapid. This is shown by an immediate darkening of the solution and precipitation of quinhydrone upon addition of a small amount of caustic to the mixed solution. I n actual plant practice. it was not found necessarv to use hydrogen perixide,-as the polymerized and shortstopped latex is normally immediately heated after discharging from the reactor. In special cases where it would be necessary to hold the latex in the reactor, the peroxide could be added separateiy. Smith et al. (14) mentioned the dithiocarbamates as shortstops in connection with high-solids-type cold latices and listed
effect on polymer properties. Nevertheless, some work is still being done in the direction of obtaining an all-rosin soap recipe for subfreezing polymerization. Tests conducted on the 0" F. polymers prepared with the 85/15 ratio of rosin to fatty acid soaps still do not indicate significant improvement over 41' F. cold rubber in stress-strain properties but improved resistance to abrasion and cracking is indicated, although the former improvement may in TABLEVII. HYDROQUINONE SHORTSTOPPING OF SUGAR-FREE RECIPES part be due to the higher raw Mooney viscosity. Shortstop, Parts/100 Mooney These data are given in Table VI. However, in the % Solids Viscosity Parts Monomers __carcass compound both room temperature and hot HydroUnAged UnAged Recipe R u n N o . quinone Hz02 aged 24 hr. aged 24 hr. tensiles are considerably improved, these properties promoting interest in tire carcass applications. I. Iron pyrophosphate Latex temp. increased It is now felt that, except for the possible deleslowly to R.T. 1 0.15 .. 21.2 23.7 .. ., .. 2 0.15 .. 20.722.1 .. terious effect of methanol, many of the justifiable Latex heated quickly objections to previously tested 0" F. polymers, with to R.T. 1 0.10 .. .. . . 56 53 2 0.10 66 65 regard to their status as truly representative prodAged a t 41° F. 1 0.15 0.'015 20:2 2012 61 60.5 2 0.15 0.015 2 2 . 8 22.3 ., . . ucts of polymerization a t that temperature, have Latex heated slowly 0.15 0.015 21.4 21.4 61 57 been removed. It still remains to prepare a polyto R.T. 1 2 0.15 0.015 20.9 21.0 68 52 mer a t 0' F. in the presence of 100% dispropor3 0.10 21.1 30.8 61 132 4 o : i 5 0.01 20.3 20.1 45 45 tionated rosin soap. The question of effectiveness 5 0.10 0.01 21.8 23.4 51 55 11. Pol amine of shortstopping a t the very low temperature also zatex heated quickly is as yet unanswered, although the conventional to R.T. 1 0.10 .. 20.5 20.2 39 46 tests indicate that the dithiocarbamates are fully III. Ferrous silicate 2 0.15 .. 21.0 21.2 59 55 effective. It is believed that shortstopping should Latex heated quickly t o R.T. 1 0.15 .. 21.4 21.5 76 71 be considered an integral part of the polymerization 2 0.10 .. 20.9 25.6 69 95 and that closer attention should be paid to this
INDUSTRIAL AND ENGINEERING CHEMISTRY
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several advantages for the dithiocarbamates over such materials as dinitrochlorobenzene and di-tert-butyl hydroquinone. The dithiocarbamates are preferred over hydroquinone because of cost, effectiveness, and freedom from staining and discoloration. The first dry rubber produced commercially (February 1950) using dithiocarbamate shortstop was GR-S X-565. The sodium dimethyldithiocarbamate originally used with this polymer was found t o lead to the formation of precoagulum unless the shortstop was added in combination with a soap or added in very dilute solution. For high-solids latices, the dimethylammonium salt was recommended ( 1 4 ) . It has since been found that the potassium salt may be used universally without danger of precoagulum formation. This permits use of solutions of higher concentration, which is important in eliminating dilution of high solids types, and also eliminates the added expense and handling involved in employing additional soap. The data in Table VI11 show the complete freedom from precoagulum formation afforded by potassium dimethyldithiocarbamate. The fact that one of the government synthetic rubber plants has been using potassium dimethyldithiocarbamate exclusively for over one year in both low solids and high solids cold production, is evidence of the commercial applicability of this discovery. The interest in dithiocarbamatetype shortstops by the synthetic rubber industry is indicated by the fact that a majority of GR-S polymers are now being shortstopped with these materials either alone or in combination with auxiliary shortstops.
TABLEVIII. COACULUX-FORMING TENDENCIES OF DITHIOCARBAMATE SHORTSTOPS
Recipe
GR-S 1500 (low solids) X-683 (high solids)
% Coagulum Formed on Addition of 2.5 Parts of 20% Solution h-a salt K salt 0.9s Nil 0.04 Nil
Because of the fact that dithiocarbamates are used as vulcanization accelerators, there was considerable hesitancy on the part of many compounders in accepting these materials as components of the polymer. However, careful investigation showed that as long as large excesses of dithiocarbamate were avoided, the rate of cure of the solid polymers was not affected significantly. It is reasonable t o assume that whatever excess shortstop is present is either washed out or decomposed during the coagulation. FAST 41' F. RECIPES
One of the most convenient methods of increasing synthetic rubber production is t o increase reactor productivity by shortening the reaction cycle. The limit imposed here is the heat transfer efficiency of the jacketed polymerization vessel, which is largely a matter of surface to volume ratio. Lowering the coolant temperature in the reactor jacket to provide a larger temperature differential does not necessarily improve the over-all heat transfer, because the colder wall temperature increases the viscosity of the latex adjacent thereto and results in a higher latex film coefficient. There is also danger of freezing the latex on the reactor wall, in addition to the consideration of higher refrigeration costs if the coolant is to be provided a t temperature differentials of 25' F. or greater. EXTERNAL HEATEXCHAPCCE. I n accordance with the limitations described, it was considered desirable t o investigate the feasibility of installing additional heat exchange surface and, for preliminary work, increased heat exchange surface external to the reactor was chosen. This was done in the government-owned pilot plant a t Naugatuck, Conn. (16), first in a reactor of 80gallon capacity and finally in a reactor of 500-gallon capacity.
Vol. 45, No. 6
I n the 80-gallon reactor (about 22 square feet of jacket area), the charge was continuously circulated through an ordinary shell and tube heat exchanger (154 square feet) by means of a DeLavitE IMO pump similar to that described by Smith et al. ( 1 4 ) . Reaction times as short as 3 hours could be maintained with this system a t a circulation rate of about 30 gallons per minute of latex without the batch losing temperature control. The recipe used was basically Recipe I of Table I, except that 0.15 part of diisopropyl benzene monohydroperoxide (DIBHP) was used with 0.30 part of ferrous sulfate heptahydrate and 0.48 part of sodium pyrophosphate decahydrate, potash fatty acid soap was employed, and water was increased to 240 parts. On the basis of successful experience in the 80-gallon reactor system, a 600-gallon reactor was connected through the DeLaval IMO pump to a Walker-Wallace Model HERE plate-type heat exchanger. This heat exchanger was described in detail by Smith et al. (14),who conducted the first evaluation of this equipment for concentration of high solids latices. It was used in the polymerization process described here for the same reason it was selected for latex concentration-Le., ease of cleaning. The flow sheet for polymerization heat exchange is identical with that shown by Smith et al. ( 1 4 ) for latex concentration, if a reactor is used in place of a stripping vessel and coolant is substituted for hot water . With the arrangement described above, reaction times of approximately 5 hours could be obtained a t a latex circulation rate of about 30 gallons per minute. The heat transfer coefficient for the heat exchanger was approximately 80 B.t.u. per hour per sq. foot-' F. Recipe VII, Table I, was used for the 500-gallon batches. On a commercial scale, more recently, an increase in the heat transfer capacity of polymerization vessels has been obtained by installation of a tube bundle internal to the reactor and circulating coolant through the bundle as well as through the reactor jacket. These internal coils have roughly one third of the jacket area, but their heat transfer coefficients are about double that of the jacket, so that reaction rates can be about doubled with this system of operation in contrast t o the external system where a still greater increase can be obtained. The internal tube bundles for cold rubber polymerization were first installed a t the plant operated by the Phillips Chemical Co. a t Borger, Tex. ( 7 ) , and have since been used a t other locations. FASTCONTINUOUS POLYMERIZATION. Some preliminary pilot plant work has been done on the application of the plate heat exchanger as a continuous polymerizer (16). Ease of dismantling and cleaning of equipment of this type during the development stage made it preferable over straight tubular polymerizers. Also, the increased turbulence obtained in the plate-type machine for the same flow rate is thought to be an advantage which would minimize fouling as well as phase separation. In initial break-in runs, Recipe VIII, Table I, was employed, the ingredients being charged continuously through rotameters in two streams from preesured vessels. This recipe had given a reaction time of 1.67 hours in smaller reactors. A dilute soap solution containing the peroxide was metered in one stream and all other ingredients, including the balance of the soap solution, were metered in the other stream. T h e flow ratio was about 1 t o 2 for the respective streams. At a total flow rate of approximately 1.0 gallon per minute (cycle time of 20 minutes) a reasonable conversion was attained. Positive flow control was attained for only short periods because of mechanical difficulties and the experience was not representative of the recipe, since, although the pressured charge vessels were maintained a t 41" F., heat gains between these vessels and the exchanger resulted in an inlet temperature somewhat about 41' F. Exit temperature of t h e latex was maintained rather well a t 41 a F. Although considerable work on the continuous system is required, the early runs amply demonstrated the feasibility of this type of operation. Further pilot plant work has been delayed
INDUSTRIAL AND ENGINEERING CHEMISTRY
lune 1953
pending installation of additional equipment. scheduled to be resumed in the near future.
Operations are
SUMMARY
-
I n the proper environment, the lower polyethylene polyamines, diethylenetriamine and triethylenetetramine, can be successfully employed as activators for all-rosin soap-emulsified 41’ F. polymerizations. At 0” F., reasonable polymerization rates were obtained in a system emulsified with 85% disproportionated rosin soap and 15% fatty acid soap when the freezing point depressant consisted of a mixture of methanol and ethylene glycol in a ratio of 3 t o 1, 25% of the water having been replaced with the antifreeze. D a t a showing the effect on polymer quality, of methanol when employed as an antifreeze agent in polymeriza, tion, indicate that i t may cause a slight deterioration in tensile strength, flex crack resistance, and abrasion resistance of tread vulcanizates. A method for shortstopping of 41 O F. polymerizations with hydroquinone is described. A study of the dithiocarbamate shortstops in cold polymerizations indicated that the potassium salt of dimethyldithiocarbamic acid is superior t o the sodium salt from the point of view of precoagulum formation tendency. Successful pilot plant polymerizations were conducted in 3 to 5 hours’ reaction time, using reactors equipped with a latex circulating pump and an external heat exchanger. A successful continuous polymerization was also conducted using a platetype heat exchanger as the reactor and a residence time of less than 0.5 hour. ACKNOWLEDGMENT
The authors wish t o acknowledge the assistance of W. H. Leukhardt and C. V. Bawn who conducted the experimental work, and the work of R. W. Brown and J. A. Reynolds of the research staff who made valuable contributions to the development work reported herein.
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The various projects were carried out under the sponsorship of the Reconstruction Finance Gorp., Office of Synthetic Rubber, as part of the government synthetic rubber program. Thanks me due to that organization for permission t o publish this paper. LITERATURE CITED
Brown, R. W., U. S. Rubber Co., unpublished work. Corrin, M. L., Klevens, H. B., and Harkins, W. D., J. Chem. Phys., 14, 480-6 (August 1946). Embree, W. H., Spolsky, Roman, and Williams, H. L., IND. ENG.CHEM.,43, 2553-9 (1951). Goodrich Chemical Co., B. F., private communication to RFC, Office of Rubber Reserve, 1950. Hart, E. J., and Meyer, A. W., J. Am. Chem. SOC.,71, 1980-5 (1949). Howland, L. H., Messer, W. E., Neklutin, V. C., and Chambers, V. S., Rubber Age ( N . Y . ) ,64, 459-64 (1949). Kirkpatrick, S. D., Chem. Eng., 58,148-52 (November 1951). McCleary, C. D. (to U. S. Rubber Co.), U. S. Patent 2,457,701 (Dec. 28, 1948). Neklutin, V. C., Westerhoff, C. B., and Howland, L. H., IND. ENG.CHEM.,43, 1246-52 (1951). Phillips Chemical Co., private communication to RFC, Office of Rubber Reserve, 1951. Provost, R. L. (to U. S. Rubber Co.), U. S. Patent 2,560,741 (July 17, 1951),2,577,432 (Dec. 4, 1951). Reynolds, J. A., U.S. Rubber Co., unpublished work. Smith, H. S,, Werner, H. G., Madigan, J. C., and Howland, L. H., IND. ENG.CHEM.,41, 1584-7 (1949). Smith, H. S., Werner, H. G., Westerhoff, C. B., and Howland, L. H., Ibid., 43, 212-16 (1951). U. S. Rubber Co., private communication to RFC, Office of Rubber Reserve, 1948. U. S. Rubber Co., private communication to RFC, Office of Rubber Reserve, 1951. Whitby, G. S., Wellman, N., Floutz, V. W., and Stephens, H. L., IND. ENG.CHBM.,42,445-52 (1950). RECEIVED for review October 24, 1952. ACCEPTED February 2, 1953. Presented before the Division of Rubber Chemistry a t the 123rd Meeting of the AMERICAN CHEMICAL SOCIETY, Los Angeles, Calif.
Reaction Times for
Polvmerization of Cold GR-S J
B. C. PRYOR, E. W. HARRINGTON, AND DONALD DRUESEDOW B. F. Goodrich Chemical Co., Cleveland, Ohio
T
HE improved quality of finished rubber products made from cold GR-S, polymerized at 41” F. instead of the 122’ F.
A
temperature used for regular G R S , has resulted in constantly increasing customer demand. Plant modernization in 1952 increased cold rubber production capacity t o about 75% of the total. A typical comparison between “cold” GR-S and regular “hot” QR-S illustrates the improvement.
Wltimate tensile lb /s ultimate elongstioh
2 inch
300% modulus, lb./sq. inch Rebound % De Matt;& flex resistance (flexurest o failure) T r e a d wem index, passenger tires
Typical Tire Tread Compound Regular OR-8 Cold GR-S 3300 4250 630 710 1040 910
33 100,000 100
38 320,000 115-120
More detailed comparisons of hot and cold GR-S may be found i n the literature (3, 8). Production of cold GR-S started on a small commercial scale in 1948, and larger scale production was under way in 1949 in
several plants. This paper discusses the progress made in improving capacity for cold rubber production a t the RFC-owned OR-S plant a t Port Neches, Tex., operated by the B. F. Goodrich Chemical Co. Cold rubber production was made possible by the discovery of redox initiation, which permits practical polymerization rates at low temperatures. Redox polymerization can be described as the chemically controlled production of free radicals in the presence of the monomers by action between a reducing agent and a n oxidizing agent. Most present production processes utilize a n organic hydroperoxide, which is decomposed by a ferrous iron complex, producing free radicals and ferric iron. The free radicals initiate chain polymerization of the butadiene and styrene. Reducing sugars or certain other reducing agents may be used to regenerate the ferrous iron. Production of cold GR-S X-542 started a t the Port Neches plant in August 1949 using the polymerization formula known as the low-sugar recipe (9).
