SOLUTION VISCOSITY OF GR-S Variation with Conversion W. A. SCHULZE AND W. W. CROUCH Hydrocarbon Chemical Company, Bartlesuille, Okla. Studies have been made of variations in the solution vismsity of GR-S polymers during polymerization with the object of providing a means of studying the behavior of mercaptan modifiers. The viscosity us. conversion curves depend on the amount and type of varioun mercaptans charged and correlate with the rates of depletion of the mercaptans. The polymer viscosities increase during the early stages of the polymerization reaction, usually reaching a maximum at monomer conversions of around 7570. At higher conversions insoluble gel usually begins to form, accompanied by a sharp decrease in the viscosity of the soluble portion. These effects are believed to be related to a change in the structure of the polymers at high conversions.
For the purpose of characterizing modifiers it is not required that absolute values of molecular weights be derived from the viscosity data. Instead, the data are expressed in the term Ln 7. , where qr is the viscosity of a dilute solution of the polymer relative to that of the solvent, and Cis the concentration in grams of solute per 100 milliliters of solution. Viscosity data were obtained with solutions of concentration* in the range of 0.15 t o 0.28 gram per hundred milliliters and employing viscometers of such dimensions (11) that no kinetic energy correction was required. Thus, according t o the terminology proposed by Cragg (2), the above term represents the inherent viscosity of the polymers as differentiated from the intrinsic viscosity, ( q ) , obtainable by extrapolation of the same term t o zero concentration (9).
T
HE properties of emulsion-polymerized butadiene-styrene MATERIALS USED
copolymers are known t o be affected profoundly by the presence in the polymerization charge of small quantities of certain modifiers, outstanding among which are the aliphatic mercaptans of more than six carbon atoms per molecule (14, 16). At normal conversions in the range of 70 t o 80% products varying in properties from a tough, unworkable material through all stages of plasticity up t o that of a soft, pasty mass can be produced simply by adding t o the charge the proper mercaptan in increasing quantities up t o a few tenths of a per cent of the weight of monomers. Thus, a means is provided by which the synthetic rubber producer might effectively control the plasticity of his product. The mercaptans are involved in the polymerization reaction ttnd are depleted from the charge during the polymerization process (6). Depending on whether monomers or mercaptans react at a relatively faster rate, the concentration of the latter relative t o that of the unreacted monomers in the reaction mixture may increase or decrease as the polymerization proceeds. In the latter case progressively tougher po!ymers are made during the polymerization process, the final product being an extremely heterogeneous mixture .of widely varying molecular weights, the average value of which is determined by the amount of mercaptan charged. A valuable help in understanding the behavior of a modifier would be afforded by a determination of the molecular weight of polymers present at all stages of the reaction. Of the available methods of estimating molecular weights of high polymers, the most rapid and convenient involves the determination of the viscosity of dilute solutions of the materials, from which by the proper equations weight average (10) molecular weights may be estimated (4, 16). This report describes an investigation t o determine the variation of solution viscosity with conversion of G R S polymers prepared in laboratory polymerization tubes, with the view of developing a rapid and useful procedure for studying the behavior of modifiers. Varying the amount of ditTerent mercaptan modifiers was also investigated and found t o be reflected in the viscosity us. conversion curves. Furthermore, it was observed that an unexpected maximum in the viscosity curve is usually reached, after which the viscosity of the soluble fraction decreases a t higher conversions. This is believed t o result from a change in the configuratioh of the polymer partirles at higher conversion.
Except for the mercaptans, the polymerization charge rocipe was identical with that employed by Craig @)-that is, butadiene 75 parts, styrene 25 parts, water 180 parts, fatty acid soap 5 parts, potassium persulfate 0.30 part, and mercaptan variable. The butadiene was Phillips Petroleum Company special purity grade product of 99.6% purity. It was taken gas phase from the storage cylinder; thus it was removed from the stabilizer and butadiene polymers that might be present. Dow Chemical Company high purity styrene was used. It wag fractionally distilled in a 2-foot Vigreux column; only the middle fractions were taken and stored in a refrigerator a t 0" C. until used. Distilled water was employed. Procter and Gamble Company Ivory flakes were employed a8 emulsifier in the earlier work. Later experiments made use of specially purified S.F. flakes purchased from the same company The potassium persulfate was Merck & Company, Inc., C.P. grade. Three mercaptans were employed. The tertiary dodecyl mercaptan used w a r a Phillips Petroleum Company commercial product made by catalytic addition of hydrogen sulfide t o a mixture of isomeric branched-chain olefins boiling in the C1p range. The tertiary hexadecyl mercaptan was a similar product obtained in an experimental trial run from a correspondingly higher boiling fraction of olefins. Properties of these two products are tabulated as follows:
Distillation range a t 5 mm., O C. First drop 507 over SO& over Molecular weight Mercaptan sulfur content, wt. 7 Mercaptan purity (calcd.), %
Tertiary Dodecyl Mercaptan
Tertiary Hexadeoyl Mercaptan
79.0 86.1 93.3 193.3 15.9 96.8
119.4 128.9 158.7 259
6.1
49.50
0 The impurities in the experimental batch of tertiary hexadecyl mercaptan were predominantly olefinic hydrocarborn tLat apparently do not affect the polymerization reaction.
The primary dodecyl mercaptan a sample of a commersial product purchased from Hooker Electrochemical Gompany. I t was known t o be a mixture of normal primary mercaptans, principally dodecyl, but also vontaining tetradecyl and hexa151
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 40, No. 1
(I), and determining the relative viscosity of the solution a t 25" * 0.1" C. in an A.S.T.M. No. 50 modified Ostwald viscometer. Flow time of benzene in these viscometers is around 200 seconds. The concentrations were determined fro ; the quantities of benzene and polymer employed or, in case gel was present, by evaporating an aliquot of the solution to dryness and weighing the residue. I n calculating the concentrations, no correction was made for aluminum soap and antioxidant contents of the polymers. These materials were present in all the polymers in increasing quantities at lower conversions and, if corrected for, would result in a small shift upward of all the inherent viscosity curves. Mercaptan depletion was determined employing a technique developed by Kolthoff and co-workers (7, 8). The entire content of a 10-gram tybe was coagulated in alcohol, and the unreacted mercaptan determined by an amperometric titration with silver nitrate solution. The coagulated polymer was recovered to determine the conversion.
DODECYL MERCAPTAN
X - 4 3 5 PART TERTIARY
DODECYL MERCAPTAN
VARIATIONS O F INHERENT VISCOSITIES AND MERCAPTAN DEPLETIONS
I n the first series of determinations tertiary dodecyl mercaptan was employed as modifier in two charge levels of 0.30 and 0.35 parts. Tubes were removed during a period of 11 to 20 hours polymerization time to give a range of conversions of 67 t o 89.5%. The data are shown in Figure 1. There was a rapid rise in the viscosity of the polymers, which reached a maximum at a conversion of around SO%, depending on the amount of mercaptan charged, and subsequently dropped rapidly. Along with or shortly after the initial drop in the solution viscosity of the polymer, gel formation was noted, which increased as the reaction proceeded. As the polymerization continued and gel content increased, the viscosity of the soluble fraction continued to drop. It is not to be inferred that the polymers were becoming more 1
70
75
,
eo
as
90
Ss
CONVERSION, %
2.4
Figure 1'. Inherent Viscosity and Benzene Solubility of GR-S Made at Various Conversions with 0.30 and 0.35 Parts Tertiary Dodecyl Mercaptan Modifier
decyl mercaptan to the extent of approximately 21 and 7010, respectively, as well as a small.amount of decyl mercaptan. POLYMERIZATION AND EVALUATION PROCEDURES
The polymerization procedure was a modification of that described by Fryling ( 5 ) . The polymerization charge of 10 grams of monomer and other ingredients in the proportions shown was introduced into a glass tube, sealed, and rocked in a water bath at 50" * 0.1' C. for various periods of time to obtain a range of polymer conversions. The tube was then opened; the latex was recovered quantitatively, mixed with 2y0 of the estimated polymer weight of phenyl-p-naphthylamine stabilizer, then coagulated with a few milliliters of 10% aluminum sulfate solution. After washing with water, the polymer was dried in an oven a t 75' C. and weighed to determine the yield. I n some of the later work, including the data presented in Figure 4 of this report, crown cap beverage bottles were used instead of sealed glass tubes as the polymerization reactors. (These experiments were started early in 1944 and n ere reported to Rubber Reserve during the period April 19 to December 27 of that year. A number of the new and improved laboratory techniques for studying emulsion polymerizations have been developed since that time.) The viscosities were determined by introducing a weighed sample of each polymer into benzene, allowing it to stand a t room temperature for 48 hours, filtering through a sintered glass plate
0.30 PART MODIFIER
0 0.45 0 0.55 V 0.70 1.00
0.60
40
, 65
io
I
1
CONVERSION, 45 81% 0
I
PART PART PART PART
d,
'MODIFIER MODIFIER MODIFIER MODIFIER
d5
1
Figure 2. Inherent Viscosity vs. Conversion of GR-S RIodifieh by Various Amounts of Tertiary Dodecyl Mercaptan
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1948
0
428
MERCAPTAN PART TERTIARY MERCAPTAN
3 IU
CONVERSION, %
Figure 3. Inherent Viscosity vs. Conversion of GR-S RIodified by Commercial Primary and Tertiary Dodecyl Mercaptans
plastic as the polymerizations continued a t higher conversion; the opposite is the case ( 1 4 ) . If the polymer samples are arranged in order according t o conversion, it is readily observed that products of reduced plasticity are obtained as the polymerization proceeds, in spite of the decrease of solutiop viscosity of the sol fraction. One obvious explanation of decrease in viscosity of the soluble portion a t high conversion is that the higher-molecular-weight fractions of the polymer are removed selectively in forming the gel. Although this may no doubt account for a part of the effect, the maximum is reached and some decrease in solution viscosity obtained before any appreciable amount of filterable gel had formed (Figure 1). Results from many additiofial experiments have shown that solution viscosities frequently undergo considerable decrease a t higher conversions without any apparent formation of gel. Figure 2 presents a second series of curves employing a wider range of mercaptan charges. With the increase in the mercaptan charge the maximum in the viscosity curve occurs a t a higher conversion and a t a lower viscosity value. Figure 3 compares solution viscosity variation of polymers modified by tertiary and commercial primary dodecyl mercaptans. The mercaptan charges were adjusted to obtain the maxima in the two curves a t approximately the same conversion. Figure 4 illustrates a series of curves obtained with various
153
amounts of tertiary hexadecyl mercaptan modifier. Although there is some rise in solution viscosity of the polymers with conversion, it is much ,more gradual and shows less tendency t o approach a maximum than do those polymers made with the lower mercaptans. Figure 5 illustrates the depletion of the three types of mercaptans during the polymerization. Under the conditions of these experiments the tertiary dodecyl mercaptan is depleted most rapidly of the three, except a t very low conversions, and has very nearly disappeared from the charge by 7001,conversion. This correlates with the fact illustrated in Figure 3 that solution viscosity of polymers made with this mercaptan decreases most rapidly a t high conversions. The rate of reaction of tertiary hexadecyl mercaptan is a p proximately linear with polymer conversion; thus, the ratio of the amount of mercaptan to that of unreacted monomers present in the charge remains approximately constant throughout the polymerization, with the result that relatively constant modification is obtained with this modifier throughout the polymerization process. The data presented here were obtained in polymerization reactions under conditions of mild agitation and are not duplicated when different type reactors, such as stirred autoclaves, are employed. Subsequent experiments have shown that the mercaptan depletion and inherent viscosity curves are affected considerably by an increase in agitation, particularly with primary mercaptans and tertiary hexadecyl mercaptan, both of which deplete more rapidly with increased agitation. It has been shown (la, 13) that the rates of diffusion of these mercaptans from the oil phase to the reaction loci are the controlling factors in their modifying action; therefore, it is understandable that their rates of reaction are affected by a change in. agitation. SIGNIFICANCE OF VISCOSITY CURVES
The rise in solution viscosity with conversion of GR-S modified by primary or tertiary dodecyl mercaptans is to, be expected in view of the property of these mercaptans of depleting rapidly from the reaction mixture during polymerization. It was, however, unexpected that the viscosity should reach a maximum value and drop a t high conversions, particularly in view of the fact that the plasticity of the product continues t o decrease
I
Q
V 0
1.50 PARTS MODIFIER 1.60 PARTS MODIFIER i,M PARTS MODIFIER
40
50
O
eo
70
I
80
PRIMARY OODECYL MERCAPTAN
1
90
CONVERSION,
Figure 4. Inherent Viscosity of GR-S Modified by Various Amounts of Commercial (49.5Yo) Tertiary Hexadecyl Mercaptan
CONVERSlON, %
Figure 5.
Depletion of Various Mercaptans from GR-S during Polymerization
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INDUSTRIAL AND ENGINEERING CHEMISTRY
throughout this period. Part of the decrease in solution viscosity can no doubt be attributed to a real decrease in the average molecular weight of the soluble portion of tbe polymer as a result of removal of the higher molecular weight polymers selectively in forming the gel. However, it is believed that a further contributing factor is a change in the structure of the polymer particles as the polymerization proceeds. The rapid formation of gel at high conversions suggests that cross linking between the polymer chains is occurring, which no doubt affects the molecular structure of even that fraction of the polymer that remains in solution. It is believed that this effect may act to reduce the solution viscosity to a greater extent than it is increased by the accompanying rise in molecular weight. Solution viscosity values are little indication of the properties of these polymers made at conversions above that of the maximum point in the viscosity os. conversion curve, particularly if gel is present. A polymer of a given inherent viscosity made at a higher conversion may be very different from one of the same viscosity recovered at a conversion below that point. ACKNOWLEDGMENT
The authors wish to acknowledge the invaluable assistance of the various workers participating in the Rubber Reserve Research and Development Program whose numerous unpublished reports have contributed materially in this work.
Vol. 40, No. 1
LITERATURE CITED
(1) Baker, W.O.,Fuller, C. S., and Heiss, J. H., Jr., J . A m . Chem. Soc., 63,2142 (1941). (2) Cragg, L. H., J. Colloid Sci., 1, 261 (1946). (3) Craig, D.,U. S. Patent 2,362,052(Nov. 7, 1944). (4) Ewart, R. H., in Mark and Whitby’s “Advances in Colloid Science, Vol. 11, Scientific Progress in Field of Rubber and Synthetic Elastomers,” pp. 228-39, Interscience Publishers, Inc., 1946. (5) Fryling, C. F.,IND.ENG.CHEM.,ANAL.ED., 16, 1 (1944). (6) Hobson, R. W., Pierson, R. M., and Borders, A. M., Goodyear Tire & Rubber Co., private communication (iMar. 23,1943). (7) Xolthoff, I. M., and Harris, W. E., IND.ENG.CHEM.,ANAL. ED., 18, 161 (1946). (8) Xolthoff, I. M., and Harris, W. E., Univ. of Minn., private communication (June 15, 1943). (9) Kraemer, E. O.,IND.ENG.CHEM.,30, 1200 (1938). (10) Kraemer, E. O.,and Lansing, W. D., J . ‘Phys. Chem., 39, 153 (1935). (11) Raaschou, P.E.,IND..ENQ. CHEM.,ANAL.ED., 10,35 (1938). (12) Reynolds, W.B., Univ. of Cincinnati, private communication (Nov. 1944). (13) Smith, W.B., J. Am. Chem. SOC.,68,2064 (1946). (14) Starkweather, H.W., et al., IND.ENG.CHEM.,39,210 (1947). (15) Staudinger, H., “Die hochmolekularen organischen Verbindungen,” Berlin, Julius Springer, 1932. (16) Wollthan, H.,Drewer, M., and Becker, W., U. 6. Patent 2,281,613 (May 5, 1942). RECEIVED November 21, 1946.
Properties of Monocrystalline Ammonium Nitrate Fertilizer PHILIP MILLER’ AND W. C. SAEMAN Tennessee Valley Authority, Wilson Dam, Ala. Monocrystalline ammonium nitrate, produced on a pilot plant scale in a continuous vacuum crystallizer, was tested for resistance to shattering under impact and for behavior of the conditioned material during prolonged bag storage and when used in fertilizer distributors. These tests indicated that it was equal or superior to commercially available forms of ammonium nitrate for use as fertilizer. The previously reported adverse effect of low porosity and nonspherical shape on fertilizer properties for monocrystalline ammonium nitrate was not encountered with the present iinproved product. In addition to the favorable economics and product quality of cofitinuous vacuum crystallization, it is the least hazardous of the processes available for commercial use.
A
LARGE part of the United States and Canadian wartime capacity for producing synthetic ammonia arid nitric acid is now being utilized for the production of ammonium nitrate ,fertilizer. Such production has been limited by plant capacity for the production of solid ammonium nitrate in a form suitable for fertilizer use, and several producers have recently announced plans for construction of new plants for this purpose (9). Current production in the Cnited States is virtually all by the batch graining method ( 6 ) , which was designed originally to meet military requirements. A continuous spray granulation method (prilling} is used in Canadian plants. Although both processes give products that can be conditioned satisfactorily for 1
Present address, H . K. Ferguson Company, New York, N. Y .
fertilizer use by means of coating or dusting agents, the spraygranulated material is coarser and more free from fines, and requires less conditioning (IO). It is probable that new installations for ammonium nitrate fertilizer production will utilize some continuous process, such as spray granulation or crystallization, rather than batch graining. The Tennessee Valley Authority, which operates an ammonium nitrate graining plant, recently completed a pilot plant investigation in which the Oslo-Krystal crystallization process was successfully modified for the production of monocrystalline ammonium nitrate well suited for fertilizer use. This study of the crystallizing process was described in a recent paper ( 7 ) . Sufficient product was prepared in the pilot plant for a thorough evaluation of its storage and drillability properties, and the present paper gives the results of that study. Sprayed and grained ammonium nitrate and nitrate of soda, all obtained from commercial sources, were included in the tests to provide a basis of comparison. The properties of sprayed, grained, and monocrystalline ammonium nitrate intended for fertilizer use have been compared previously by Ross et al. (11, l a ) . The monocrystalline material tested in their work was produced in an earlier pilot plant investigation, made under the auspices of the Office of Production Research and Development ( I S ) , in which the crystallizing process differed significantly irom that used by TVA ( 7 ) . The OPRD material was quite different in appearance from the crystals tested in the present work and was, in general, markedly inferior to the latter in physical behavior, as indicated both by the
