A Staffilndustrw CoUUaborative Report, WILL H. SHEARON, JR.
.
J. P. MCKENZIE AND MARTIN E. SAMUEL3
In collaboration with
Copolymer Corporation, Baton Rouge, La.
Associate Editor
T
HE history of the wartime development and improvement of processes for chemical rubber manufacture in this country has been told many times; i t stands out as one of the most remarkable tales in chemical engineering annals. Cosler and McCune (5) have assembled an excellent bibliography of 122 references on chemical and natural rubber in discussing advances in rubber during 1947. Breuer (3) has published four articles reviewing the extensive German patent literature, which led t o the development of the butadiene-copolymer type rubbers, identified as the Buna types i n Germany. The story of low temperature rubber may not present as vivid a picture as the wartime production of synthetic rubber, but i t is none the less interesting and certainly one of the most important developments since the American chemical rubber industry began. The phrase "low temperature rubber'' as used in this paper is applied t o copolymers of butadiene and styrene made at temperatures considerably lower than the standard temperature of 122' F., and specifically at temperatures of 41 ', 14 ', and 0 F., and even lower. It does not include Butyl rubber, a copolymer of; isobutylene and isoprene, normally made i n the region of - 150" F., other special-purpose products made at relatively low temperatures, among which are Ameripol, Butaprene, Ghemigum, and Perbunan, or chemical rubber possessing superior properties in applications involving exposure to low temperatures such as are encountered in Arctic regions. The birth of the Office of Rubber Reserve in 1940 and its deduction of GR-S polymer *(after n'surveyed) resulted in the operation of 15 GR-S plants in this country with a combined annual rated capacity of 705,000 long tons. The standard method .of production was emulsion polymerization at 122' F. It was known that polymerization at lower temperatures would produce rubber of better quality. At the inception of the government rubber program B. F. Goodrich recommended (IS)a polymerizat i o n temperature of 35" C. as producing a superior rubber. Recommendations (19) were made by Howland of U. S. Rubber e o . and Swaney of Standard Oil Development Co. as early as September 1943 for studies of low temperature polymerization. I n fact, as early as 1943 full scale production tests had been carried out, using a polymerization temperature of 104" F., at -the Torrance, Calif., plant operated by the Goodyear Tire and Rubber Company, by order of the Office of the Rubber Director. However, because of the critical need for an assured source of emergency rubber, i t was considered the better part of wisdom rto stick t o a process yhich utilized known recipes and methods
and ended with definite quantities of rubber with known qualities. All attempts to lower the temperature of polymerization from 122" F., while still using the regular GR-S recipe, had resulted in greatly increased times of reaction t o reach comparable conversions-approximately 30 hours at 104 O F. and 70 hours at 86" F., as compared t o 14 hours a t standard temperature. The entire United States chemical rubber program has been carried out, under the sponsorship of the Office of Rubber Reserve, as a n industry-wide cooperative effort. The Government has spent, t o date, approximately $20,000,000 on chemical rubber research and development. Results of this work, and additional information obtained in privately financed investigations, have been available to all participants in the program. From this coordinated effort has come the low temperature process that is of so much current interest.
O
Six Low Temperature Reactors at Copolymer Corporation
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The' fundamental basis for the successful development of low temperature rubber after the end of the war lay largely in the use of systems employing what are currently termed "speed-up" chemicals to increase the rate of polymerization at low temperatures. Stewart and Fryling (40) reported early in 1943 the use of certain redox systems in speeding up the polymerization of GR-S. The German experience with low temperature recipes has been reported by Livingston (88). In 1945 patents were issued to Fryling ( 1 4 ) and Stewart (39, 4 1 ) of The B. F. Goodrich Co., covering such systems. Goodyear reported in early 1946 to Rubber Reserve (18) that similar studies had been carried out in its laboratories before the war. These systems involved the use of both an oxidizing and a reducing agent,. Ot,her patent,s covering systems of oxidizingreducing agents have been issued in this country and in France and Gernmny ( 4 , 10, 44, 46). The teams of the U. S. Technical Missions to Europe in 1945-46 brought back to the United Stat,es reports (24, 29, 47) of German work on chemical rubber; these included work on a redox polymeri5ation syst,em, briefly reported by Weidlein ( 4 5 ) . Act,ually the use of a redox system in chemical rubber manufacture was developed independently by scientists in England, Germany, and the United States, but t'he work in England was fundamenhl research and that, in Germany had not, progressed beyond the laboratory or pilot' plant stage. The reports on the German work revived interest in the work done in t,his country on low temperature recipes which had lain more or less dormant during the war because of other productmion problems. The German methods commonly employed benzoyl peroxide as the oxidizing agent and sugars, salts of heavy metals, etc., as reducing agents. Work done in various research laboratories with benzoyl peroxide and sugars, using German emulsifiers and soaps, indicated t,hat the German recipes produced polymers of good quality but control was difficult. Fatty acid soap emulsifiers and potassium persulfate catalyst are commonly employed in the product'ion of regular GR-S. [The word catalyst as used by the rubber industry docs not have quite the meaning ordinarily associated with the term, but refers to the oxidizing agent which helps to promot,epolymerization and is normally consumed in productioni38).] Much work was done in the investigation of oxidizing-reducing syst,ems, and the following sequence of contributions appears to cover the significant development,s that influence the shift in emphasis to the low temperature polymerization process: In 1944, Reynolds reported (33) the use of diazo ethers, particularly p-methoxyphenyl, diazo-2-naphthyl et,her (hIDN), as catalyst-modifiers in polynierizat,ion of copolymer systems at normal reaction temperatures. Diazo aminobenzene had been proposed as an initiator in Russia as early as 1937 ( 2 ) . In 1945, Kolthoff, Dale, and Schot,t,(26) applied diazo-thio ethers in conjunction with potassium ferricyanide to obtain polymerization systems operating with satisfactory reaction at low temperatures. These experiments were conducted on a laboratory scale in bottles of 4-ounce to 1-pint. size. The theory of free radicals for keeping polymerization going is almost universally accepted, and with the MDX-ferricyanide activator, it is thought t'hat activation is brought, about by decomposition of the thio ether, with production of free radicals. The Government Laboratories, operated by Akron University for the Office of Rqbber Reserve, reported in early 1946 (32) on polymerizations which they conducted with the JIDN-ferricyanide formula of Kolthoff in &gallon reactors, at temperatures of 30", 20", lo", and 2" C. Enough polymer was obtained for complete laboratory evaluation of physical properties, which showed marked improvement in such charact,eristics as tensile strength, elongation, hysteresis-crack-growth balance, and processibility as the polymerization temperature was decreased. A month or so later, Goodyear reported ( 1 8 ) results of 5-gallon tests, stimulat,ed by the German work, using the redox type recipe with benzoyl peroxide as catalyst. These test's yielded un-
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modified heat-softened products with greatly improved tensile strength and hot and cold resilience. Almost simultaneously, Firestone (22) reported bottle-scale results down t o 10" C. on a duplication of the German redox recipe Lvith t'he use of Americanmade emulsifiers in place of the German Fischer-Tropsch product, Mersolat. I n the meant,ime, work by Kolt'hoff (86)and Fryling (16) resuhed in a somewhat preferable formula for plant operation in that the use of MDN was eliminated and the more readily available cumene hydroperoxide was used as a catalyst. \.andenberg and Hulse (49) of the Hercules Experiment' Station discuss in detail the use of cumene hydroperoxide in a redox emulsion polymerization system; they point, out t,he advant#age of redox systems for obtaining fast, rates of polymerization with peroxides of high thermal stability-that, is, those which can be successfully used at low temperatures-and show the great superiority of cumene hydroperoxide over the conventional potassium persulfate initiator as well as the improvement over similar redox systems based on the common peroxides and per salt. With MDS-ferricyanide activation the reaction is faster a t lower temperatures than with cumene hydroperoxide and a suear-iron activator. However, difficulty is encountered in making MDN, because it is unstable at room temperature, and it. costs more than cumene hydroperoxide. The balance of favor has tipped toward cumene hydroperoxide at present, and MDN is not now being made commercially. These developments appear to have made possible a feasible low temperature polymerization process. The use of disproportionated rosin soap in GR-S polymcrization is already well known in the manufacture of GR-S-10 ( 1 , 6 ) . Dunbrook (9) has stated that the industry became interested in rosin soaps because of availabiltty, low cost, and the possibility of giving an improved rubber. General problems which presented themselves in connection with the use of ordinary rosin soaps were resolved with the introduction of the disproportionated soaps, consisting mainly of hydroabietic and dehydroabiet,ic acids. A discussion of chemical changes involved in the disproportionation reaction (27) states that cooperative studies by the Hercules PoTTder Company and a number of rubber companies have established that suitably refined disproportionated rosin gives uniformly acceptable rates of polymerization and polymers with outstanding physical properties. A bibliography of tests establishing this statement is included. The function of the soap emulsifier in the polymerization reaction is to solubilize the monomers and modifier and to act as the locus of reaction during the early stages of the polymerization ( 7 ) . I t is established that the rate of polymerization increases with increasing soap concentration, and although rosin soaps are not quite so efficient in this respect as fatty acid soaps, they produce a rubber with more tack. hlso, polymers made with rosin soap and cumene hydroperoxide show better qualities than those made with benzoyl peroxide. Those who favor the use of rosin acid soap do so because it materially reduces the tendency of the reacting mixture to "set up" as the temperature is decreased, and thus becomes particularly attractive at lowered temperatures, which are not below the freezing point of JTater. HoTvever, others in the industry feel that agents such as potassium oleate or potassium laurate can be used with equal success, and may be even superior for recipes suitable at temperatures below freezing. The first pilot plant production of t,he low t,emperature polymers \vas made by Phillips in the late spring of 1946; products were obtained both at, 41 ' and 14" F. Phillips emphasized from the start the desirability of using a modifier in the low- temperature system. At first a JIDN-potassium ferricyanide modifier was used t,o keep t,he polymeiization going and control its rate. A4ctually,pure potassium ferricyanide was not used, but a mixture of sodium and potassium ferricyanides marketed under the name Redsol. The Phillips pilot plant has a 260-gallon reactor and a refrigerating capacity of 20 tons at 0" F. I t can be opcratcd at tcmperatures from 0" to 150" F. A propane-methanol
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
refrigeration and coolant system is used. (A 20-gallon reactor and two 5-gallon vessels are also available for such service.) Shortly after the middle of 1946, Goodyear's interest in the low temperature product had developed t o the point where a continuous polymerization unit consisting of twelve 5-gallon reactors in series was operated a t 33" to 45" F. with fatty acid soap emulsifiers, but without a modifier, to produce 2000 pounds of polymer for laboratory and tire evaluation. By late 1946 and early 1947 substantial quantities of 41' F. polymer (5" C.) using a modifier were being made by Phillips. Serious interest in the possibilities of these polymers was evinced following road tests of tires made by Lake Shore Tire and Rubber Company from Phillips' 41" F. polymer. None of the government-financed pilot plants were equipped with refrigeration facilities; so in Marsh 1947 the Copolymer Corporation, a private corporation of Baton Rouge, La., asked the Office of Rubber Reserve for permission t o equip two 20-gallon reactors and the 500-gallon reactors in its pilot plant for low temperature rubber work, Permission was granted and the pilot installation was completed in June 1947. Results were promising and in October Copolymer was granted permission to convert half of its annually rated capacity of 30,000 long tons of GR-S to the production of 41" F. rubber. T h e plant went into operation on February 20, 1948. Although the U. S. Rubber Company made the first plant scale run of low temperature rubber on a n experimental basis in the summer of 1947, the Copolymer Corporation manufactured the first low temperature rubber in extended operations on a large scale after extensive evaluation of the product. Copolymer also was the first to use rosin soap as a n emulsifier. A run (42) was made at the Institute, W. Va., plant operated by U. S. Rubber Company, just prior to its shutdown. It was made a t 35" F., both unmodified and modified, in a n attempt to compare certain properties of unmodified, heat-softened rubber with those of modified rubber. As the results obtained were generally satisfactory, and the plant was being closed, further runs were not made. The run consisted of six batches of approximately 6000 pounds each; five of the batches contained ethylene glycol as an antifreeze whereas one batch did not. The formula employed resulted from work carried on earlier in pilot plants operated by U. S. Rubber a t Naugatuck, Conn., and Institute. Copolymer leases and operates a typical government-owned mutual plant, the first of the mutual plants to go into production (April 1, 1943). Companies participating in operation are the Armstrong Tire & Rubber Company, Natchez, Miss., Armstrong Tire and Rubber Company, West Raven, Conn., Dayton Rubber Xanufacturing Company, Gates Rubber Company, Lake Shore Tire and Rubber Company, Lee Rubber and Tire Corporation, Inland Tire and Rubber Company, Mansfield Tire and Rubber Company. President of the board is A. L. Freedlander of Dayton Rubber Company, and C. M. Hulings is operating vice president in charge of the plant. The Copolymer technical staff had, prior to the interest in 41 'F. rubber, done considerable work on polymerization a t 104" F. Tests a t Copolymer (35) of products made during the Torrance, Calif., run at 104" F. showed that there was no great difference betwren polymers of the same Mooney value made at 104" and 122' F., except in processing characteristics and tensile strength. However, certain types of high Mooney polymers made a t 104" F. gave superior tread wear with no unusual amount of cracking difficulties. In order to make high Mooney rubbrrs processible, it is necessary either to add softeners or to give the rubber mechanical treatment. One industry group thought it better to make high Mooney polymers with difficult processibility, add softeners at the processing plant, and still get excellent properties, whereas the other group stuck to the idea of using low temperature normal Mooney rubber having good
771
processibility without the addition of softeners. Present interest is in the 41' F. rubber rather than the 104" F. rubber. PILOT PLANT
The important difference between the regular GR-S process and the new low temperature process is to provide a cooling medium and a means of reducing the temperature in the system by approximately 80' F. The pilot plant refrigerating equipment a t Copolymer uses Freon and the main plant installation uses ammonia; the cooling medium in both cases is isopropanol solution. The refrigerating system in the pilot plant consists of two Mills Freon-12 units, each operated by a 7.5 h.p. explosionproof motor; each motor has a capacity of 51,200 B.t.u. per hour at a suction temperature of 5 " F. Each unit has a condenser cooled by plant refrigerating water and separate evaporators installed inside an 1800-gallon tank which also acts as a storage tank for the refrigerated coolant. Evaporator coils, consisting of 600 feet of 0.875-inch copper tubing, are installed in a steel box inside the tank and bafflesin the box ensure circulation of the coolant liquid over the evaporator coils. Pressure safety cutouts are installed on each compressor; these cut out at zero suction pressure or 150 pounds discharge pressure. The oooling medium is a 4oyO by volume solution of isopropanol which was chosen because of desirable viscosity characteristics at low temperatures. The coolant is circulated through the surge tank and also to the reactor jackets by means of a 1.5 h.p. J. B. Gould centrifugal pump with a 9.25-inch impeller operating at 1700 r.p.m. When cooling liquid is not required by the reactors it circulates through the surge tank. Temperature control bulbs for the coolant are of the liquid expansion type with a -20' to +55" F. range. These are placed in the bottom of the surge tank at the pump suction. The only change necessary in the reactor control instruments to change the pilot plant from regular runs to low temperature runs, was the installation of new liquid expansion bulbs with a range of -20" to +150." F. The coolant going to the jackets is divided into two streams: one flows directly from the surge tank and the other goes through a steam-jacketed pipe so t h a t the liquid can be warmed up by opening a manually operated valve. The flow of each of these streams t o the reactor jacket is instrument-controlled. All lines, the surge tank, and the reactors are cork-insulated. MAIN PLANT INSTALLATION
The areas of operation in the low temperature plant installation are similar to, or a part of, those in the regular GR-S plant and comprise storage, pigment preparation, reaction, recovery, and coagulation and finishing. Soday (37) in discussing the preparation and properties of GR-S, described the Copolymer regular GR-S operation in as much detail as was allowed by wartime restrictions. Pahl (31) discussed the engineering equipment in a typical GR-S plant and the improvements made over the war years. The purpose of this paper is to discuss the information not previously reported on the operation of the regular GR-S process and the points of difference between the regular process and the low temperature processes. TANKF A n M AND MONOMERHANDLING. Butadiene is received sometimes by tank car, sometimes by pipe line from the adjoining Esso Standard Oil Company butadiene plant. The butadiene received in tank cars usually contains about 200 p.p.m. of p-tertbutylcatechol as a polymerization inhibitor; this must be removed prior to use of the butadiene. [Roby (34) discusses in detail the factors affecting stability of butadiene during storage and processing.] Because the inhibitor readily forms the sodium salt, the raw monomer is given a dilute caustic wash prior to use. Butadiene received by pipe line usually contains about 5 t o 10 p.p.m. inhibitor. No attempt is made to remove the inhibitor from styrene prior t o use. The styrene and butadiene are received at about 98% purity or better and the concentration for the charge is 95.5%. This figure is used in order to maintain a n economical balance in the plant. Some plants get all their butadiene by pipe line from adjacent plants and charge it on a once-through basis. The recycle butadiene is returned to the source plant for purification;
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ment is affected by the peripheral speed of the agitator is probably due t o the (All quantities expressed in parts per hundred of charged monomer, by weight) fact that, with slower speeds, larger Original GR-S Present GR-S GR-S-10 Copolymer 41' F, particles are formed in the organic phase Butadiene 75 71 71 71 with correspondingly less surface and, Styrene 25 2Q 29 29 Oxidizing agent KzSz08 0 . 3 0 KzSzOa 0.23 KiSzOs 0 . 2 3 Cumene hydrotherefore, a reduced modifier requireperoxide 0 . 1 5 ment. With the tertiary CU mercaptan Activator ... ..., . . . . . .. ....... Dextrose 2.5 FeSOa. 12Hz0 0 . 1 used in the low temperature process, the NanPzO7 0.6 consumption of modifier is independent Emulsifiera (soap) F a t t y acid, F a t t y acid, Dresinate 731, Dresinate 731, of agitation (8). Kolthoff (2'7) as well 90% soln. 4.6 90% s o h . 4.3 anhydrous 4 . 3 anhydrous 4.7 Modifier Dodecyl merDodecyl merDodecyl merTertiary Ci2 meras Reynolds and eo-workers (53) attribcaptan 0 . 6 0 captan 0.45 captan 0 . 7 captan 0.2 ute the difference in behavior to a faster Total HzO 180.0 180.0 180.0 180.0 rate of solubilization into the focus of Reaction temp., F. 122-130 122 125 41 77 72 72 60 Conversion, % the reaction with tertiary mercaptans 13-15 14-16 20-22 Reaction time, hr. 15-17 Final ratio butadiene than with primary mercaptans of the t o styrene 79.21 76.5:23.5 76.5:23.5 76.5: 23.5 same molecular weight. The reactors a Quantities in p.p.m., based on dry weight. have been insulated with 4 inches of cork outside the inner jacket through which the coolant flows, and the inner 15 to 20y0 of the recycle monomer is added to fresh monomer. jackets have been tightened considerably to prevent loss of isoThe proportion of recycle material to fresh material will affect propyl alcohol. the reaction rate, either as diluents or impurities. Some impuriA typical reactor charge is as follows: ties, especially butadiene dimer, are polymerization inhibitors. For these reasons dead polymers are removed periodically by 317 1110 withdrawing a tank car of recycle monomer from the recovery 130 system and shipping it to a source plant for purification. 663
TABLE I. COMPARISON O F GR-S RECIPES
1030
PIGMENT AREA. The term "pigment area" is a bit misleading inasmuch as there is no actual pigment added to the raw materials. I t is in this area that the emulsifier, activator, and shortstop dispersion are prepared, as is the antioxidant emulsion which is added to the latex before finishing. The old south 3000-gallon wooden soap tank (37) has been given over to rosin soap preparation and the north tank retained for stearate soaps. New 4000-gallon steel storage tanks have been added for soap, one for each of the two types. From an equipment standpoint it is interesting t o note that these new tanks, together with a new activator storage tank, and the 10,000-gallon surge tank, are old tank cars (or halves)-a provision to cope with the postwar steel and equipment shortage. The rosin soap used is a Hercules Powder Company product, Dresinate 731; this is put into a 6% water solution in the preparation tank along with small amounts of caustic and sodium phosphate. The solution does not gel on standing. Therefore, it is not necessary to use heat to accomplish solution as with stearate soap. A 25-gallon wooden tank, originally intended for preparation of buffer solutions, is now used for activator solution make-up for the new rubber; a 2500-gallon steel tank has been added for activator storage. The activator solution (Table I) is made up as follows: The sodium pyrophosphate is added slowly to water heated to between 120' and 130 O F. Bfter all the pyrophosphate is in solution the glucose is added and dissolved. The temperature of the resulting solution is raised to boiling and maintained a t 212' F. for 30 minutes. It is then cooled to 80" to 90" F. and the ferrous sulfate is dissolved as the final step. The catalyst, cumene hydroperoxide, more properly referred t o as an oxidant or initiator, comes in galvanized drums as 67 to 70% solution in cumene; the modifier is a tertiary Cl9 mercaptan (thiol) derived from petroleum sources; this also comes in drums. The mercaptan serves as a chain modifying agent; it cuts down cross linkages and increases plasticity. Mooney value and processibility of the final rubber are largely dependent on the type and concentration of the modifier. Both the catalyst and the modifier are added directly t o the reaction mixture without further preparation. REACTORS.Twelve of the 25 glass-lined jacketed reactors (3750-gallon capacity) used in the regular GR-S plant have been refitted as low temperature reactors. Pahl (31) points out improvements in the Brumagim-type agitators. The significant departures from the earlier data given by Soday (37) are reduction in speed and change in diameter and angle of pitch of the agitator blades. Copolymer has reduced the speed of the agitators for both regular and low temperature GR-S from 300 r.p.m., given by Soday, to 115 to 120 r.p.m. This has resulted in a considerable decrease in the amount of modifier used in the preparation of GR-S and GR-8-10. The fact that the modifier require-
18-18
12-15
The charge amounts to approximately 8000 pounds of raw monomer. At 6070 conversion and 10% nonhydrocarbons left in the finished rubber, approximately 5333 pounds of rubber will be produced per reactor, per charge. All raw materials charged to the reactors, with the exception of catalyst and modifier, are charged through the feedstock cooler. The monomers, emulsifier, 580 gallons of the water, the activator, and the modifier are charged a t substantially the same time; the cumene hydroperoxide is added last with the rest (450 gallons) of the charge water t o ensure that the reaction does not begin a t a temperature above 41 F. Experience has shown that cumene hydroperoxide is extremely sensitive to acid, and as the mercaptan modifier is subject to oxidation by the cumene hydroperoxide with resultant formation of acids which neutralize the trace of caustic used to stabilize the cumene hydroperoxide, the two are kept separate until they reach the reactor. Butadiene and styrene blends are piped from the tank farm through meters; the soap and activator solutions, also metered, come from storage tanks and a sufficient quantity in the line to the charge pumps is maintained a t all times. The charge water is the regular plant circulating water which is well water and does not require treatment. It is pumped diremly from the pond through a meter to the cooler and reactors. The cumene hydroperoxide and the mercaptan are charged from drums to separate weigh scales and then directly to the reactors. The cooling system used i n the process has two purposes, the precooling of the raw materials so that batches will be charged t o the reactors a t the correct temperature and maintenance of the correct batch temperature in the reactors during the polymerization period. The refrigerant in the main plant system is ammonia, instead of Freon as used in the pilot plant. The system is a n ammonia vapor system with two 300-h.p. gas-driven Clark Super 2-cycle right angle engine compressors; each has a rated refrigeration capacity of 190 tons. The ammonia leaves the compressors at a final pressure of 200 pounds per square inch. The coolant is joy0isopropanol-water solution, stored in the 10,000-gallon surge tank, and circulated through the cooling system by a 1300-gallonper-minute centrifugal pump. This is a n electrically powered pump and a n additional steam-powered pump is kept as a standby in case*of power failure. The coolant is pumped to the ammonia evaporator at approximately 30" F. and leaves a t about 15" F. In the event it is
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773
Condensed Flow Chart for 4 1 O F. Polymer
+
cooled below 15’ F., there is a line by-passing the evaporator with a n instrument-controlled valve which opens t o allow the flpw of sufficient 30” F. liquid to adjust the temperature of the coolant. From the evaporator the isopropanol is forced through a header from which lines run to the feedstock cooler, reactor jackets, and surge tank. The amount of coolant needed to maintain the reactors at 41 O 5’. is controlled separately in each reactor by a Taylor temperature recorder-controller system. From the reactors the coolant returns t o the surge tank. It flows through the feedstock cooler only during the charging period. The feedstock cooler is also controlled by a temperature recorder-controller system and a coolant line By-passes the cooler for use in the event of a leak in the coolant inlet line. Hydrostatic pop valves to take care of expansion of liquid in case of line blocks are on the coolant lines entering the eva orator, leaving the cooler, and by-passing the cooler. I n norma? operation the entire quantity of coolant contained in the surge tank, reactor jackets, and piping, is more than 10,000 gallons. Therefore, a n auxiliary surge tank has been provided in the event that it becomes necessary, t o drain the lines and equipment. The polymerization time for 41 O F. rubber has been 20 to 22 hours, as compared with about 14 hours for a 122” F. rubber. (Polymerization times in the pilot plant have been only 17 to 18 hours, and an effort is being made to reduce the plant time.) It is generally thought t h a t polymerization is initiated by free radical resultink from oxidation of ferrous pyrophosphate by the hydroperoxide. The iron is reduced by the glucose subsequently, thus functioning in a true catalytic sense. The more expensive and scarce fructose is preferable to glucose, but glucose can be used. Vandenberg and Hulse (4.3) point out t h a t the
ferrous sulfate is of major importance in eliminating initial periods of very slow polymerization. The use of disproportionated or dehydrogenated rosin soap has generally meant higher modifier consumption, but, as shown in Table I, there is a considerable reduction in the amount of modifier used in t h e low temperature process from t h a t used in We other recipes primarily because temperature acts as a regulator for chain growth-that is, the lower the temperature, the less branching occurs. Although some modifiers are consumed at a greater rate with rosin soap, the tertiary mercaptans are not so affected. Fryling points out ( l a ) that the slower polymerization rates, usually observed with tertiary mercaptan modifiers, are, in part, a consequence of the lower concentration resulting from more efficient modification as compared with primary mercaptans of the same molecular weight. His results show that the use of a blend of tertiary mercaptans appears t o be a practical and effective solution to the problem of modifying dehydrogenated rosin soap recipes. I n the Phillips pilot plant a mixture of tertiary Clz, Cia, and Clemercaptans known as MTM, or mixed tertiary mercaptans, is used; Copolymer a t the present timeis using the tertiary CISmodifier. I n regular GR-S production and in low temperature reactions using MDY activator, the modifier is required to initiate the reaction; with cumene hydroperoxide this is not the case. At the end of the reaction, as determined by the control laboratory, an “instruction to drop” is issued to the reactor
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operator. The charge is dropped into a cold blondomn tank and the shortstopping agent is added simultaneously. The function of the shortstop is to stop polymerization; it muvt alsci pi event further polymerization a t the temperatures encountered in the recovery system. An attempt was made to use hvdroquinone, the shortstop used in the iegulai GR-S process, but this was unsuccessful even R ith quantities thiee times the noimal amounts. Furfural was used as the shortstop in the pilot plant and F a s quite satisfactory, it is particularly desirable from the standpoint of cost, hut in practice it has so far proved impossible t o separate it from the styrene in the recovery system. Di-trrtbutyl hydroquinone, vihich is soluble in the oil phase, has proved satisfactory, particularly in preventiqg a continuance of the reaction a t the 120" F. temperature of the recovery system. I t is added as a dispersion in a solution of Dresinate 731; the dispersion contains 10% of the hydroquinone derivative and 13.5% of dry Dresinate 731; it is stabilized by grinding in a Charlotte colloid mill. The control laboratory in the reactor area, xhich determines the end of the reaction, performs three specific functions. Here the solutions being charged to the reactors are analyzed, the per cent conversion and LIooney level of the latex in the reactors are determined as well as the raw viscosity of the finished product. The second is the most important of the functions. Each hatch is tested one or more times, as required, to predict the time a t which the desired conversion and Xooney level will be attained. Conversion is generally a straight-line function of time, but the incremental Mooney value varies rather ridely. Desired reactor Mooney is governed by the property of the finished bale and the latex during the various stages of processing. The control laboratory analyzes latex for solids and pH during stripping operations and in the latex blend tanks. Facilities are maintained in the control laboratory for coagulating, washing, and drying the rubber crumb as in the plant process. The four steel, glass-lined blowdown tanks have a 7500-gallon capacity each; three were taken from the regular GR-S unit and one was the old off-grade blowdowm. The primary blowdowns are not jacketed, but the secondary blowdowns are jacketed so that warm water can be circulated to warm the latex and generate sufficient pressure (20 pounds per square inch) to transfer it to the recovery system. These jackets are not welded to the tanks as was the case with the reactors, but are supported by legs because it was not considered practicable to try
Vol. 40, No. 5
to weld jackets t o tanks already glass-lined. The primary blowdown tanks are essentially latex surge tanks. The heat is supplied to the jackets of the secondary blowdowns by circulating stripped styrene water from the recovery system stripping column with the addition of sufficient st,eam to maintain a 1500" F. temperature in t,he jackets. If t,he t,emperature of the water from the column is too high or the supply fails, permitting live steam to enter t,he jackets, the control will open a valve and allow make-up water to enter the system. Latex level in the secondary bloivdoims is controlled by an automatic liquid level f l o ~ controller. The recovery system has been \?-ell described by Pahl and Soday (31, 37). The unreacted butadiene is removed in two successive horizontal flash tanks and the styrene steam-stripped under vacuum, after which the &ripped latex is pumped t,o t,he finishing area. &xrrIoxIDxm. Plieriyl-~-riaphtliylar~iirie(PBX) is the antioxidant generally used in the preparation of regular GR-S; it is introduced as a dispersion in water to add 1.25y0 PBK t o the GR-S contained in the latex. RLE, a high temperature reaction product of acetone and diphenylamine in liquid form, may also be used. I t is dispersed with tall oil and caustic in agitated tanks; the 1.5% BLE may be added t o the latex storage tank in the finishing area or continuously to the latex stream as it .enters the coagulating unit. Either antioxidant may he used in the low temperat,ure process; its main function is to prevent oxidation of the polymer during drying operations; Copolymer, at present, uses BLE. CREAMING, COAGULATING, AND FINISHING. The stripped lstex from the latex sthrage tank is creamed with brine (a saturated solution of rock salt with calcium and magnesium salts removed) and coagulated with sulfuric acid. Low temperature latex appears to have less mechanical stability t,han either GR-S or GRS-10 latex and the p H maintenance is very crit,ical since it has been observed that p H plays an important part, in controlling the particle size of the coagulum. In the creaming and coagulating step, the brine, latex, and acidified serum are fed through a pump a t a normal rate much lower than the capacity (120 gallons per minute) of the pump. Approximately 35 gallons per minut'e of latex, 4 gallons of 25.50;: brine, and 10 gallons of serum go into the pump; in the pump and exit line the latex is creamed and partially coagulated; the slurry goes t o the SUOgallon coagulating tank. There is an automatic overflow from the coagulating tank to the 1900-gallon holding t,ankfrom which the coagulum goes to a vihrat,ing screen for dewatering. The serum from the screen runs by gravity to a storage t'ank having a p H controller in the top; from this sulfuric acid is added to bring the p H to a narrow range of 1.8 to 2.0. Serum flows from this tank a t the rat'e of 60 gallons per minute into the coagulat'ing tank. The crumb rubber is reslurried n-ith fresh water, filtered on an Oliver filter, shredded and dried in a Coe 3-pass dryer, and weighed and baled int,o 75-pound bales of rubber with a 4minute Mooney.of 60 * 5 (large rotor). DISPOSITION OF PRODUCT
811 of Copolymer's initial production, with the exception of small quantities supplied to other companies for evaluation, is being supplied to Copolymer's subscribing companies. This is the allocation procedure usually followed by Rubber Reserve when ney products are involved. POLY MER PROPERTIES
Normal progress in the improvement of materials generally improves some properties a t the expense of others. The significant thing about chemical rubber manufactured a t a low temperature is that the molecular structure is so altered as t o improve a large number of properties. This is indicated in the name which Copolymer has applied to its 41 'F. product-Ultipara-although there is no intention of claiming that the ultimate in chemical rubber has been attained. A contributing factor to the improvement in properties is that low Rfooney rubber made a t low temperature has half or less the proportion of low-molecular-weight
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1948
775
material, which degrades its physical TABLE 11. LABORATORY PHYSICAL DATA-COPOLYMERCORPORATION properties, that is present in GR-S. The Compound A Compound B low t e m p e r a t u r e polymer possesses 100.0 Polymer 100.0 Polymer superior processing properties, when E P C black 50.0 50.0 HAF black evaluated from both laboratory and 5.0 Coal tar softener ( B R T P7) 5.0 Coal tar softener (BRT #7) Zinc oxide 3 .O 5.0 Zinc oxide factory standpoints; laboratory tests 1 0 Stearic acid 1.5 Stearic acid Captax Santocurea Variable 1.5 have been substantiated by tire tests in DPGa 1.75 Variable Sulfur passenger and truck-size tires. A noteSulfur 2.0 worthy result was the exceptionally high Test conditions: Mooney viscosity, 4-minute reading large rotor, 212O F. Stress-strain tests, A.B.T.M. D412-41, a t 82O F. Temperature rise, Goodrich flexometer. 143 lb./sq. in. load: 0.175-inch wear resistance without tread cracking stroke; 1800 r.p.m ' 25-minute test a t looo F. Flex life, De Mattia hexameter; needle-pierced specimen. A.S.T.M. D'd30-40, method B ; data reported as thousands of flexing cycles to produce an 0.50found in the tires made from 41" F. inch'crack a t 317 cycles per minute and 82' F. Resilience, Lupke-type resiliometer with 100-cm. rubber. That this is extremely good pendulum; measurements a t 82' F. news is well established by Juve's disUitiTensile mate Modulus cussion ( 2 3 ) of the seriousness of crackStren th, Elonga- a t 300y0 Temp. growth tendencies in GR-S. Tests (3'6) Mooney ComLb,/&. tion, ElongaRise, Flex ResiliPolymer Viscosity pound In. % tion A T o F. Life ence, % have shown that the abrasion resistance Cumene hydroper65 A 4150 680 950 101 31.0 49.6 of the 41" F. polymer made on a pilot B 4025 490 1975 111 74.0 54 6 oxide, 41' F. plant scale by the Phillips Petroleum GR-6-10 52 A 2975 600 975 100 14 0 44 0 B 2825 500 1350 112 22.0 46.5 Company is 20 to 40% superior to that of GR-S-10. After aging for 24 hours at 212' F The following discussion of tire comCumene hydroper65 A 3350 500 1600 85 15.0 51.0 B 3775 360 3100 84 20.0 55 6 oxide, 41° F. poundihg and processing and of physi88 6.6 46.0 GR-S-10 52 A 2450 380 1775 cal properties presents, as a confirmaB 2950 350 2320 '94 14.0 48 0 tion of the outstanding properties of low a Accelerator adjustments were made t o compensate for differences in rate of cure. temperature rubber, a joint discussion of test results with the 41" F. rubber, made by Copolymer and Phillips (Philbility, extrudes faster, and furnishes a smoother and markedly prene A) and the 14" F. rubber made by Phillips (Philprene superior product. Evidence. of superiority also was found B). Phillips has evaluated these polymers at, a high temperawhen low temperature polymer was factory-processed under the ture (200' F.). same conditions as GR-S and GR-S-10 without difficulty] despite the fact that the former possessed 10 to 20 points higher, crude PROCESSIBILITY. The crude polymer can be stretched into and compounded, Mooney viscosity than the latter two. Procalmost endless filaments when elongated slowly. This is an indiessing temperatures are higher with the low temperature polymer cation of polymolecularity which is a property not possessed by because of incorporation of the furnace blacks, but it has shown either GR-S or GR-S-IO. This is evidence of unusual properties improved resistance to scorching. I n some instances stocks in the crude state and it is not surprising that factory processimade from low temperature polymer attained what are normally bility of the low temperature polymer is superior to that of GR-S considered excessively high processing temperatures, but no or GR-S-10. A salient feature of the polymer in this respect is evidence of scorching was found a t the Natchez, Miss., plant of easy wetting or incorporation of pigments and reinforcing the Armstrong Tire and Rubber Company on stocks extruded a t agents. Phillips' 41 ' rubber without softener, was found t o be 310' F. This tendency of the polymer to process hot may be far superior to either natural rubber or GR-S in rate of black overcome to a great extent by keeping the Mooney viscosity low incorporation. This means that Phillips' 14 O or 41 F. rubbers and utilizing higher softener levels. When the Mooney viscosity in certain factory operations will require shorter mixing time readings on low temperature polymer are observed throughout than now used with GR-S and natural rubber. The 41 F. polythe test, a n 8 to 10 point drop is found from the 1.5-minute readmer is especially receptive to the recently developed high-abrasion ing to the 4-minute reading. Such a negative Y value is indicafurnace blacks whereby vulcanizates are obtained with high tentive of the breakdown char:cteristics of the polymer. Tire tests sile strength ,resilience, and flex life, and with excellent aging indicate that Phillips' 41 F. rubber can be compounded a t characteristics. Further, the polymer possesses excellent milla-
TABLE 111. LABORATORY PHYSICAL DATA-41 CompoundA
.
F. POLYMER, PHILLIPS PETROLEUM COMPANY
Compound B 41' F. polymer (Phillips) or GR-9-10 100 HAF black 45 100 . .. Zinc oxide 3 50 50 Stearic acid 2 4 10 3 3 Agerite 1.2 2 2 Sulfur 1.75 1.65 1.15 Santocure 0.95 Ciroo oil 2.75 Paraflux 2.26 Test conditions: Stress-strain tests, A.S.T.M. D412-41 a t 80' and 200' F. Temperature rise, Goodrich flexometer; 143 Ib./sq. in. load; 0.175-inch stroke; 1800 r.p.m 25-minute test at looo F. Flex life, De Mattia flexometer; chisel-pierced specimen; data reported as thousands,of flexing cycles to pioduce a 1.0-inch crack ai'500 cycles per minute and ZOOo F. Resilience, Yerzley oscillograph. Resilience determined a t inertia load required to effect a deflection of from 16 to 20%. Measurements a t SOo F. Shore hardness, instantaneous reading with Shoxe Type A Durometer. Compression set, %, modified A.S.T.M. method B; 2 hours a t 212O F. a t 35% deflection plus 1-hour relaxation a t 212' F. GR-S 41° F. polymer (Phillips) E P C black Asphalt #6 Zinc oxide Sulfur Santocure
..,
100
.
Polymer
Phillips GR-S GR-S-10
Compound
A B A B
Tensile Strength Lb./Sq. In. 80° F. ZOOo F.
Ultimate Elongation, % SOo F. 200° F.
80' F.
SOo F.
2460 3970 2350 2830
Modulus a t 300% Elongation SOo F. 200° F.
Oven aged for 24 hours a t 212' F. 80' F. 395 2420 400 2700 320 2160 330 2550
Temp. Compression Rise, Set, % AT F.
Resilience
Flex Life
Shore Hardness
Vol. 40, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
776
TABLE Iv. LaBORATORY P H Y S I C A L DAT.4-14
F.P O L Y M E R ,
PHILLIPS PETROLEUM
C0.\WA4SY
(Test conditions same as shown in Table 111) Compound A
Compound B 100 60 3 10 1 1.20 1.75
Santocure Sulfur
Polymer
Compound
Phillips
A B A B
Tensile Strength Lb./Sq. In.
Ultimate Elongation,
4000 3600 2600 3000
480 450 360 370
SOo F.
200' F.
80'F.
"70
200' F.
Modulus at. 300% Elongation 80'
F.
Temp. i A T o F.
compression ~
200' F.
Set, yo
~
~ , Resilience
Flex Life
70 65 66 64
14
Shore Hardness
Oven aged for 24 hours a t 212O F
GR-S
GR-S-10
2100 2 100 2000
2400
8 7 8
7
61 73 62 69
7 4 4
65 64 66 65
e
substantially higher black loadings than can be tolerated in either GR-S or natukal rubber. I n a competitive industry this is a very important factor.
PHYSICAL PROPERTIES. The 41" F. polymer has been evaluated for vulcanizate physical properties in a number of formulations by Copolymer, and both the Phillips' 41 ' F. polymer and 14 O F. polymer have been evaluated by Phillips. Table I1 gives data for tread-type stocks made from Copolymer's 41" F. rubber, compounded with channel black and with furnace black. The use of GR-8-10 instead of GR-S as a control polymer in the physical test da a was prompted by the fact that GR-S-10 is made in the same emulsification SJ stem as the low temperature polymer; hence, the comparison between the t v o is more valid. Table I11 gives data for tread-type stocks compounded from Phillips' 41 F. rubber, GR-S, and GR-S-10, and Table IV compares Phillips' 14" F. rubber and GR-S; the compounding formulas are of a type similar to those used in Table 11. The absolute states of cure betn-een compounds il and B are a t slightly different levels. The compounds having equal compression set valves are a t equivalent states of cure. I n every case Phillips' 41 or 14" F. polymer is directly comparable to the GR-S or GR-S-10 with which it is compared. The properties at high temperature (200" F.) given in the Phillips tests should be noted. On the basis of these and other laboratory tests, as well as laboratory evaluations of factory-mixed stocks, it is evident that in general the 41 ' F. polymer is superior in quality to GR-S-10, in tensile strength, elongation, flex life, resilience, and resistance t o accelerated aging. Hysteresis properties are approximately equivalent at the same modulus, and the relation of the flex life of the low temperature polymer to that of GR-S-10, a t equivalent hysteresis, shows considerable superiority for the former. If Phillips' 41 O F. polymer at 50 pounds per hour of black loading is used as the basis for comparison, it is found that it is equal to or superior to natural rubber in stress-strain properties, abrasion resistance, and extrusion properties, but inferior in hysteresis. Increasing the Xooney level of the raw polymer does not improve the abrasion resistance if sufficient softener is added to the compounded elastomer and if the compounded stock is masticated sufficiently to assure its satisfactory extrusion properties. The low Moonev rubbers lvhich require no softener for plasticization appear most suitable for commercial exploitation. The results reported on the tire tread formulations are based on Tables 11, 111, and IV. Government road test data have not been accumulated in sufficient quantity to warrant conclusive evaluation of the product in actual use. Tests are under way and results should be available in the near future.
PROSPECTS
The properties of the 14" F. rubber show considerable general improvement over those of GR-S-IO, but have not shown consistent improvement in all properties over those of the 41" F. rubber. Phillips has made largc pilot plant batches of 14" F. rubber, but considerable more work, now under way, must be carried out before final conclusions can be drawn. U. S. Rubber has developed formulas and produced numerous pilot plant batches of 0 " F. polymer. The product has been evaluated in the laboratory and is now being tested in tires. The Government Laboratories have recentlv produced 2000 pounds of 14' F. polymer, using a recipe developed by Goodyear; this polymer will be tested in truck tires. Firestone has reported (81) bottle-scale polymerizations in the redox system t o -40" F. and tire tests are being made. The future for low temperature chemical rubber promises to be of great interest. Copolymer's satisfaction with the new product may be judged by the fact that it expects to ask the Office of Rubber Reserve for permission t o convert the other half of its plant to the l o ~ vtemperature process. Companies working on low temperature polymerization are concentrating on increasing the rate of polymerization at 41 O F. to bring it within the 12-hour range of GR-S and GR-S-10. To do this a number of changes in the emulsifier and modifier 1% ill be tried as well as sugarless recipes. The sugarless cumene hydroperoxide recipe, first proposed by Phillips (16) to give' rapid reactions a t 14' F., contains methanolas the antifreeze. I n the pilot plant they have successfully made a polymer using a sugarless recipe at 14" F. in 10 hours. At its Borger, Tex., plant the United States Rubber Company is equipping the minimum installation necessary for a semicommercial exploratory job on low temperature rubber. It plans t o do development work on plant scale starting with 41" F. rubber and then going t o 0 " F. Preliminary tests indicate that 0 " F. rubber is a t least the equal of natural rubber in tensile strength and elongation and it is expected to be superior to natural rubber in crack-resistance. The work a t the Borger plant involves the conversion of one txelfth of its annually rated capacity of 45,000 long tons to low temperature installations; the refrigerating system provides for the operation of six reactors a t 41 O F., or two or three a t 14" F., or one a t 0" F. ,4n ammonia refrigerating system will be used with methanol as both the coolant and the antifreeze. Recipes mill include oleate and laurate soaps. The kettles to operate a t 0 " F. Fill be sheathed in an 8-inch layer of cork. If results are satisfactory U. S. Rubber may convert the entire plant to low temperature installations later, as no basic changes are required and the cost difference will probably be small in comparison with the advantages obtained. U. S. Rubber expects that recipes
May 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
will be discovered which will enable the production of approximately as much rubber at 0" F. as at 122"F., and that continuous polymerization a t low temperatures will provide the answer t o a number of problems. Continuous olymerization a t 122" F. is in the operation at all of the G R - i plants except Firestone, Akron, U. S. Rubber, Naugatuck, Conn., Copolymer, and General Tire and Rubber Company, Baytown, Tex. Owen (50) describes such a process on a laboratory scale and Francis (11) discusses the plant scale process. Hucks (80) states that the large scale manufacture of polymers containing isoprene, either singly (polyisoprene), or in combination with styrene, or in trimonomer polymers of isoprene, butadiene, and styrene is of particular and growing interest. There are still many problems t o be solved in chemical rubber manufacture a t temperatures lower than the freezing point of water. Foremost are the design of refrigerating systems and problems introduced the necessity of adding a n antifreeze t o the charge. Experimental techniques at these lower temperatures are more difficult; activating systems that will go faster at lower temperatures will have t o be found; and the selection of a suitable emulsifier presents a problem. Even at 14' F., the use of rosin soaps, so successful at 41 O F., has not yet been proved possible. However, the increase in viscosity (as the temperature is lowered) with fatty acid soaps has been minimized by the use of soaps such as oleates and laurates which will not gel at the temperatures used. Another problem involved in the use of a n antifreeze is the additional stripping load put on the recovery system, necessitating greater condensing capacity. The capacity of the refrigerating equipment drops rapidly as the temperature is lowered; this is evidenced by the fact that the U. S. Rubber installation will have only one sixth the refrigerating capacity at 0 O F. as at 41 O F . Associated with this problem is the fact that the speed of reaction must be maintained a t a uniform rate; new activators must be found that will give suitable rates of polymerization a t the temperatures involved. If this were not the case, there would be an excess of refrigeration capacity available. Another interesting development is use of the sugarless recipe. The activator solution containing sugar, employed at 41' F., gives a slow reaction a t 14' F.; conversely, until recently it has been impossible t o use the 14"F. sugarless recipe a t 41 O F. Every effort is being made to develop a successful sugarless recipe that can be used at 41 F. on a plant scale. Because of the relatively high cost of the sugar portion of the recipe, this development will mean a considerable saving t o the industry. Great interest has been shown by the armed forces in a rubber with significant crack-growth resistance-such as shown by 41 O F. polymers in tires for large trucks and other equipment and also in a rubber which will withstand arctic temperatures. Perhaps a high butadiene (90%) rubber or a polybutadiene rubber will be the answer. The next development in low temperature manufacturing will probably be at -4' F. (-20' C.). ACKNOWLEDGMENT
This paper follows the usual pattern for staff-industry collaborative reports and is built around a specific plant installation, that of the Copolymer Corporation. The careful review of the background events which led to current developments in the industry and the discussion of the future of low temperature rubber emphasize the pioneering of the companies cooperating in the field of low temperature rubber-work that has made possible a plant installation such as Copolymer. The cooperation given the authors by C. F. Fryling, J. E. Troyan, and L. R. Sperberg of the Phillips Petroleum Company, L. H. Howland of the U. S. Rubber Company, J. D. D'Ianni, R."W. Hobson, and J. B. Mitchelson of the Goodyear Tire and Rubber Company, E. B. Babcock and R. F. Dunbrook of the Firestone Tire and Rubber Company, and R. A. Crawford and E. K. Bean of The B. F. Goodrich Company, and their advice during the preparation of the manuscript is particularly acknowledged.
777
LITERATURE CITED
Azorlosa, J. L., presented before the High Polymer Forum at the 112th Meeting of the A.C.S., New York, N. Y. Balandina, V., et al., Bull. acad. sci. U.R.S.S., 1936, 423-33. Breuer, F. W., I n d i a Rubber World, 109, 585-6, 590 (March 1944); 110, 56-7, 63 (April 1944): Rubber Age, 54, 229-34, (December 1943) : 336-9 (January 1944). Britton, E. C., and LeFevre, W. J., U. S. Patent 2,333,633 (Nov. 9, 1943).
Coder, V. A., and McCune, S. W., 111, India Rubber World, 117,481-5 (1948).
Cuthbertson, G. R., Coe, W. S., and Brady, J. L., IND. ENG CHEM.,38,975-6 (1946). Dunbrook, R. R., I n d i a Rubber World, 117, 203-7 (1947). Ibid., 355-9 (1947). Ibid., 486, 552 (1948). E. I. du Pont de Nemours and Go., Fr. Patent 812,267 (Aug. 4, 1937).
Francis, D. H., Chem. Eng. Progress, to be published. Fryling, C. F., IND. ENG.CHEM.,40,926 (1948). Fryling, C. F., to Office of Rubber Reserve, unpublished report, Dee. 22, 1941. Fryling, C. F., U. S. Patent 2,379,431 (July 3, 1945); 2,383,055 (Aug. 21, 1945). Fryling, C. F., and St. John, W. M., Hydrocarbon Chemical Co. to Office of Rubber Reserve, private communication, Oct. 14, 1946.
Fryling, C. F., St. John, W. M., and Uraneck, C. A., Hydrocarbbn Chemical Company to Office of Rubber Reserve, private conlmunication, Sept. 30, 1947. Hays, J. T., Drake, A. E., and Pratt, Y. T., IND. ENG.CHEM.,39, 1129-32 (1947).
Hobson, R. W., Clowney, J. Y., and and Lawrence, J. M., unpublished report to Office of Rubber Reserve, April 24, 1946. Howland, L. H., and Swaney, M. W., to Office of Rubber Reserve, unpublished report, Sept. 13, (943. Hucks, W. R., India Rubber World, 116,347-50 (1948). Johnson, P. H., Brown, R. R., and Bebb, R. L., presented before the High Polymer Forum a t the 113th Meeting of the A.C.S., Chicago, Ill. Johnson, P. H., Buskey, F. A., Bebb, R. L., to Office of Rubber Reserve, unpublished report, April 25, 1946. Juve, A. E., IND. ENG.CHEM.,39, 1494-98 (1947). Kolthoff, I. M., Univ. Minn. to Office of Rubber Reserve, private communication, Aug. 28, 1946. Ibid.,'July, August, and September 1946. Kolthoff, I. M., Dale, W. J., Schott, J. M., Univ. of Minn., unpublished communication, Oct. 15, 1945. Kolthoff, I. M., and Harris, W. E., J. Polgmer Sci., 2, 41-8 (1947).
Livingston, J., U. 5.Dept. of Commerce, OTS Rept., PB 13356. Marvel, C. S., Ibid., PB 32161, Nov. 28, 1945. Owen, J. J., Steele, C. T., and Parker, P. T., IND. ENG.CHEM., 39, 110-13 (1947).
Pahl, W. H., Chem. Eng. Progress, 43, 515-22 (1947). Piltz, W. A., Government Laboratories, Univ. of Akron, unpublished communication, Feb. 19, 1946. Reynolds, W. B., Univ. of Cincinnati, unpublished communication, Aug. 22, 1944. Roby, R. K., Wiese, H. K., and Morrell, C. E., IND.ENG. CHEM.,36, 3-7 (1944). Samuels, M. E., Copolymer Corp. to Office of Rubber Reserve, private communication, Jan. 19, 1945. Schulze, W. A., Reynolds, W. B., Fryling, C. F., Sperberg, L. R., and Troyan, J. E., India Rubber World, 117, 739-12 (1948). Soday, F. J., Trans. Am. Inst. Chem. Engrs., 42, 647-61 (1946). Starkweather, H. W., et al., IND. ENG.CHEM.,39, 210 (1947). Stewart, W. D., U. S. Patents 2,380,474-77 (July 31, 1945), in particular 2,380,476: 2,388,372-3 (Nov. 6, 1945). Stewart, W. D., and Fryling, C. F., t o Office of Rubber Reserve, unpublished communication, March 22, 1943. Stewart, W. D., and Zwicker, B. M. B., U. S. Patent 2,380,617 (July 31, 1945).
U. 9. Rubber Co., Rept. to Office of Rubber Reserve, Aug. 18, 1947.
Vandenberg, E. J., and Hulse, G. E., IND.ENG.CHEM.,40, 932 (1948).
Voss, Eisfeld, and Freudenberger (to I. G. Farbenindustrie, A. G.), Ger. Patent 664,337 (May 13, 1933). Weidlein, E. R., Jr., CHEM.ENG.NEWS,2 4 , 7 7 4 (1946). Youker, M. A., U. S. Patent 2,365,035 (Dec. 12, 1944). Youker, M. A., and Copeland, N. A., Rept. of Rubber Subcommittee Mission, Joint Intelligence Objectives Agency, t o Office of Rubber Reserve, March 14, 1946. R ~ C ~ I VMarch E D 31, 1948.
