Svnthetic Rubber
e R. L. BEBB, E. L. CARR, A N D L . B. WAKEFIELD The Firestone Tire & Rubber Co.,Akron, Ohio
T
HE rise of the synthetic
T h e preparation of synthetic rubber for general-purpose whose relative importance for rubber industry began initiation i n c r e a s e s on1 y use is discussed, particular reference being made to its with the GR-S program, alslightly as soap disappears origin abroad and subsequent development into an item of large scale manufacture i n this country. While emulsion by adsorption on the polymerthough prior t o 1941 a small monomer particle, and ( b ) industry was making butapolymerization is described most fully, mention is also made of the mass-polymerized Buna varieties. A discusthe monomer droplet, which diene-acrylonitrile c o p o 1ymers, neoprene (polychlorois a very minor locus. sion is given of the currently accepted theories of initiation Polymerization is initiated and propagation, both for polymerization at 50" C. and prene), a n d t h e T h i o k o l when free radicals generated for the currently popular 5 " C. polymerization employing p o l y s u l f i d e r u b b e r s . AB iron-complexed redox catalysis. Brief mention is made of by the "initiator" are capshown in Table I, the productured by the small amount t i o n of GR-S i n c r e a s e d specialty polymers of the oil-resistant type containing of monomer solubilized in the acrylonitrile as well as a recent research development, the rapidly between 1942 and soap micelles, the polymer1943, reaching a maximum in Alfin polymers. m o n o m e r p a r t i c l e being 1945. After t h a t year and formed. The rate of initiathe end of World War 11, tion is determined by a number of factors ('7, 8, 20, 67) among the return of natural rubber decreased the demand for the which are (a)rate of formation of free radicals; ( b ) concentration butadiene-styrene copolymer, but there was little change in the and kind of soap; and (c) rate of diffusion of mercaptan. The demand for the butadiene-acrylonitrile, neoprene, or butyl types. polymer-monomer particle grows too large to be held in the micelle; it then is found in the aqueous phase where it constitutes the principal locus of polymer growth-namely, the polymer-monomer Table I. U. S. Production of Synthetic Rubbera (Long Tons) particle. Period GR-S Butyl Neoprene N-Type Total The growing polymer-monomer particle adsorbs soap a t the 2,464 227 5,692 8,383 1941 ... expense of the micelles; the soap concentration in the aqueous 22,434 23 9,734 8,956 3,721 1942 phase soon falls below the critical micelle concentration, a t which 1,373 3 3 , 603 231,722 182,209 14,487 1943 16,812 762,630 18 890 670,268 56,660 1944 point the surface tension of the aqueous phase rises sharply. 4 7 :426 719,404 45,672 7,871 820,373 1945 47,766 740,026 73,114 613,408 5 738 1946 I n the GR-S recipe this effect occurs a t about 13% conversion. 68,824 31,495 408,858 515,795 6:618 ' 1947 After the disappearance of micellar soap the number of polymer 489,529 7,908 34,848 52,603 394,170 1948 11,072 35,215 52,237 392,690 295,166 1949 particles is fixed. Conversion increases by particle groivth 50,067 12,037 65,832 476,184 368,248 1950 through polymerization in the polymer-monomer particles. .I1 U. 6 . Dept. Commerce Industry Reports-Rubber (1947-50). 50 to 60% conversion all monomer droplets have been absorbed by polymer particles. At still higher conversions polymerization draws on monomer present in the polymer par Licle. This paper is directed toward a review of some important polymerization processes treating primafily the preparation of GR-S PROCESS butadiene-styrene copolymers in emulsion. The GR-S process by which most of the synthetic rubber n a? LOCUS OF POLYMERIZATION
The heterogeneous system comprising an emulsion polymerization recipe consists initially of an aqueous emulsifier phase and a hydrocarbon phase, with low concentrations of initiating and modifying compounds distributed between the two phases. The mechanism of the solubilization of hydrocarbons and of the initiation and growth of vinyl and diene polymers in soap emulsion has been elucidated in recent years. The theory of Harkins of the mechanism of emulsion polymerization, which was evolved on the basis of the work of numerous investigators (.@), has gained general acceptance. It successfully explains most of the phenomena. ilccording t o this theory there are two principal loci of polymerization: (a)principal Iocus of initiation-namely, the soap micelle in the core of which is dissolved a small amount of monomer; ( b ) principal locus of polymer growth-namely, the polymer-monomer particle itself. Minor loci of initiation are (a)the water phase, 724
produced throughout the war years of 1942 to 1945 was based on the following simple recipe ( 1 4 ) : Water Soap flakes Potassium persulfate Dodecyl mercaptan Styrene Butadiene Hydroquinone (stopping agent) Phenyl- @-naphthylamine (antioxidant)
Parts 180 5.0 0.3 0.5 25 75 0.1 1.25
A detailed description of the layout and operations involved in a typical GR-S plant of a capacity of 30,000 long tons of polymer per year has been given by Soday (69). The polymerization reaction was effected principally as a batch process in autoclaves of 3750-gallon capacity, at the batch temperature of 50" C. A reaction time of 12 t o 15 hours was required t o reach a conversion of about 75% where the polymerization process was stopped by addition of hydroquinone. Unreacted monomers were removed from the latex thus obtained; the butadiene by flash stripping,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic
-
the styrene by steam stripping under reduced pressure. The antioxidant, phenyl-@-naphthylamine, was added as an emulsion to the stripped latex. Isolation of the polymer with the antioxidant and soap acids intimately admixed was effected by fist creaming with brine and then coagulating with aqueous sulfuric acid or aluminum sulfate ( 4 ) . The polymer was finally obtained as dry crumbs compressed into a bale. The GR-S process was essentially an American development, particularly from the standpoint of high rate of polymerization and large scale production of high quality elastomer. The base retipe was known before World War I1 (34),but the rate of polymerization was low (as was later learned) because of inhibitors present in the monomers (19) and the soap (14). Moreover, the optimum polymer of which the recipe was capable had not been obtained. Private research in the thirties (66) and the Rubber Reserve Co. cooperative research program in the forties { I C ) so improved the process from the standpoint of rate and polymer quality t h a t the volume of synthetic rubber production was adequate t o supply the requirements of the United States throughout the war period. Soap flakes as a mixture of sodium salts of fatty acids were used as the emulsifier. It was soon discovered that linoleic acid and especially linolenic acid and the conjugated triene eleostearic acid in the soap were retarders of polymerization (14). Partial selective hydrogenation to eliminate multiple unsaturation minimized thik type of retardation. Soaps of 12 t o 18 carbon atoms serve well in the GR-5 recipe whereas those of less than eight carbon atoms are nonemulsifying and those above 18 carbon atoms &renot sufficiently soluble in water (14). Low cost emulsifiers have been developed from rosin acids, by hydrogenation and disproportionation, t o remove powerful inhibitors of the type of abietic acid and also phenolic inhibitors ($1) The rosin acid liberated from the coagulation improved the tack of the polymer (10). This type emulsifier was used for Bome .time in the 50" C. GR-S recipe, and was later extensively used in cold recipes for obtaining fluid emulsifier solutions at low temperature (74). Diene monomers of a high degree of purity are available today. Butadiene dimer ( 1-vinyl-Aa-cyclohexene), spontaneously produced by thermal polymerization for which no retarder is known, has a slight inhibiting effect in the GR-S system (69). This conetitutes no serious problem since the rate of dimer formation is slow a t ordinary storage temperatures. I n the purification of 'butadiene and of isoprene the powerful inhibitors, 1,4-pentadiene and cyclopentadiene, are removed. Styrene of a high degree of purity is used in the production of GR-S. The monomer is stabilized with a low concentration of di-terl-butyl catechol which is not removed before polymerization .since the retarding effect on the rate is negligible (69). INITIATION AND MODIFICATION
The initiation couple, potassium persulfate-dodecyl mercaptan, i s eminently satisfactory at 50' C. for obtaining a practical rate and high quality elastomer. At higher temperatures-for example, 100" C.-the reaction can be made t o go t o completion in a few minutes (69). At lower temperatures polymerization is much slower but can be accelerated by adding potassium ferricyanide or acrylonitrile or by making a major change in the recipe t o include the redox activation systems discussed later, The mercaptan performs two functions (40'41): it initiates the polymerization chain by supplying free radicals through interaction with persulfate, and i t regulates the polymer chain length by acting as a chain transfer agent in terminating one chain and initiating another (68). The reactions involved are
KzSeOs f RSH
RS. April 1952
KHSO,
+ RS.
+- M -+ RSM. initiation
(11 (2)
Rubber-
+ nM +RS(Mn+l).propagation RS(hL+,). + RSH RS(M,+l)H + RS. termination and chain transfer RSM.
(3)
--f
(4)
The practical significance of modification is t h a t it produces polymers of sufficient plasticity t o permit handling in conventional rubber processing equipment, Below the modifying concentration of dodecyl mercaptan the polymer is relatively hard and considerable power is required t o work it. Moreover, such a polymer contains gel which adversely affects some physical properties, notably crack growth. Controlled gelation, however, using the special cross-linking agent, divinylbenzene, has been reported as valuable for improving the'processability of polymer in some applications (78). I n the modifying range of dodecyl mercaptan concentrations the plasticity of GR-S can be varied widely from stiff polymers of a Mooney plasticity value of 150 t o very soft polymers of about 15 Mooney. Polymers of about 50 Mooney plasticity are most satisfactory for factory processing. The effect of various concentrations of dodecyl mercaptan on GR-S properties is shown in Table I1 (14).
Table 11. Dodecyl Mercaptan,
%
Effect of Dodecyl Mercaptan o n G R - S Properties Hours at
Conversion,
n
12
% 0
12 ._
70
01 0 3 0.5
12 12 12
78
0 05
50" C.
78 78
Mooney Viscosity
( M L 4)
...
Hard __
Gel,
%
Vnr;'hinh
13
153 95
77
50
0
Xntrinsic Viscosity
...
... 1.2 1.2 2.1
A large number of mercaptans can function as initiators and modifiers (do), their effectiveness being a function of chain length and branching (20) and other structural features. Primary mercaptans exhibit a sharp maximum effectiveness for initiation at chain length of 10 to 12 carbon atoms. Tertiary mercaptans rapidly increase in initiation effectiveness until a chain length of 10 carbon atoms is reached, after which the effectiveness increases slowly, becoming constant at 14 to 16 carbon atoms. Quantitative studies have been made of the rate of consumption of mercaptans ( @ ) , and relationships have been established for calculating mo!ecular weight and intrinsic viscosity and for expressing modifier efficiency as a function of mercaptan consumption (29). Other methods of initiation which have been found effective in some degree are: varioue cobaltic complexes acting in the presence of mercaptans (11), aroyl disulfides plus persulfate (18), hydrogen peroxide with iron activation (bo), and diazothioethers (39). None of these methods has proved as satisfactory as the mercaptan-persulfate couple. Adequate stopping of the polymerization is necessary in order t o be able to strip the latex and still obtain a polymer of good quality. Hydroquinone at a concentration of 0.05 to 0.1% on the monomers was the most satisfactory stopping agent out of hundreds of organic compounds of various degrees of effectiveness tested in the 50" C. GR-S recipe. The stopping reaction destroys the initiation step by destroying the persulfate. I n addition t o the stopping agent a n antioxidant is necessary in the polymer to prevent oxidative deterioration. Phenyl-8-naphthylamine is the most commonly used compound for this purpose. Since considerable discoloration of the polymer results from the presence of phenyl-@-naphthylamine, less effective nondiscoloring antioxidants are used for obtaining lightcolored stocks for special applications (58). The monomer composition of the copolymer is determined by the reactivity ratios of the monomers and the initial charging ratio (61, '76). No other variables in the emulsion system influence the monomer composition of the polymer. For an initial charge ratio of 25 styrene t o 75 butadiene the initial polymer formed contains only 17.2% styrene (61). The styrene content
INDUSTRIAL AND ENGINEERING CHEMISTRY
725
2 L A S T B M E R S - S y n t h e t i c Rubberof the polymer increases with conversion until a t 100% conversion the styrene content becomes 25% of the copolymer. The whole polymer, therefore, is quite heterogeneous with respect to monomer composition (7‘6). The butadiene is present in the polymer chain predominantly as 1,4-units with about 22% present as 1,2- units ($6, 30, 39, 61). About 20y0 of the 1,4- units are in the cis configuration. At lower temperatures of polymerization the cis content is lower
This was proved by other oxidizing agents which alone could not initiate polymerization but when combined with the proper reducing agent caused polymer formation. The reaction between oxidant and reductant in the usual systems has generally been considered to take place indirectly, with an iron salt as intermediate.
($4).
C6Hj -O-O-C-C6H5
8
0
I1
II
Low Temperature Process. An important development in synthetic rubber polymerization that has taken place since V70rld War I1 has been the introduction of low temperature polymerization. While it had been known for some time that butadiene and styrene copolymers of the GR-S type showed somewhat improved laboratory test properties when made a t 30” C., tire tests on polymers made a t these temperatures showed them to be no better than GR-S, and the reaction rate was so slam that commercial production was completely impractical. I n the early part of the synthetic rubber program, no practical method was known for satisfactory polymerizations a t temperatures as low as 5“ C. At the end of the war, information was obtained ( 4 7 ) regarding the German redox formulas, and simultaneously initiation by diazo compounds was developed in this country for use a t 5’ C. Since that time, development has been rapid, and emulsion polymerizations have been effected a t temperatures as low as -40” C. (36). Three classes of low temperature activators have now been developed: the diazothioethers, the hydroperoxide-coniplexed iron type, and polyamine activators. DIAZOTHIOETHER SYSTEMS.The polymerization-initiating power of diazothioethers (prepared by coupling a diazonium salt with a thiol) was discovered in 1944 (66) in an attempt to combine the known initiating power of diazo compounds (such as diazoaminobenzene) (3, 13) with the modifying power of mercaptans. Later investigations by other v,-orkers (39) showed that the addition of an oxidizing agent, potassium ferricyanide, and a mercaptan so “activated” the diazothioether that it gave rapid polymerization rates a t 5” C. and could be used a t temperatures as low as -18“ C. ( 8 2 ) . A polymerization recipe for the most commonly used diazothioether follows:
NarPOa.12HzO Hours (5’ C . )
Conversion, yo Hercules, Dresinate 731. b 3: 1 : 1 mlxture of tertiary C I S Cin, , and Cis mercaptens. C 2-(4-Methoxybeniene diazomercapto) naphthalene.
70/30 250 5.0 0.4 0.3 0.3 0.5 3.5 60
Only limited use has been made of this system because of the superiority of the hydroperoxide-complexed iron systems. HYDROPEROXIDE-IRON COMPLEX SYSTEMS. These systems, which have been responsible for the production of nearly all the cold rubber made in this country, had their beginning in systems developed by German workers during World War I1 (37, 48). They arose from observations on the inhibiting effect of dissolved (“molecular”) oxygen on the polymerization of chloroprene. Exclusion of air led to increased polymerization rates, but induction periods were often met and the rates were erratic. The addition of noninhibiting reducing agents, such as sulfite or hydrosulfite, to remove the last traces of oxygen, greatly improved the rate and uniformity. Logemann, a t Leverkusen, and Monheim and Sohnke, at Hochst, discovered simultaneously that removal of oxygen was not the only action of the reducing agent; the reaction between the reducing agent and the peroxy compound (the “redox reaction”) was the polymerization-initiating step.
726
++
0
REDOX POLYMERIZATION
Butadiene/styrene Water Disproportionated rosin soap” Mixed tertiary mercaptansb Diazothioether KaFe (CN)6
+ Fe
-+ 0
/I
c~W~c-0.+ CBH~C-O-
+ Fe+++
Thus benzoyl peroxide reacts with ferrous iron to give a perbenzoyl radical, a benzoate ion, and ferric iron. Then the ferric iron is reduced to the ferrous state, so that the same step can again occur and the perbenzoyl or phenyl radicals initiate polymerieation. However, there are indications that hydroperoxides and reducing sugars may react directly with formation of free radicals which initiate polymerization ( 4 2 ) . The redox systems which mere first reported in this country were undependable (46). An active Rubber Reserve Co. research program soon effected many improvements, chief of which was the int,roduction of cumene hydroperoxide as oxidant. Others have reported the results of investigations which established optimum proportions of iron, complexing agent, hydroperoxide, and reducing agent (B,23,35). With sugar-cohtaining recipes and using cumene hydroperoxide, the hydroperoxide to iron to complexing agent mole ratio was found to be 1: 1: 1. The amount of sugar used wag not critical, and from 0.75 to 3.0 parts per hundred of monomers were commonly used. The iron complexing agent was pyrophosphate ion; initially the sodium salt was used, but later invest,igation ( 6 4 ) proved that use of potassium pyrophosphate gave much more uniform polymerization rates that also were somewhat faster. Since alkaline ferrous solutions are sensitive to air, it has been customary to prepare the mixture of the ferrous salt, potassium pyrophosphate, and sugar with exclusion of air. This mixture, normally a grayish-white suspension, is called the activator. The necessity for specifying the exact details of its preparat’ion for optimum results, including t.he proportions of ingredients, order of mixing, temperature, and time of “aging,” all suggest that very complex and little-understood phenomena are involved. Other iron sequestering agents, such as hydrosulfide and silicate ions, and the fatty acid soaps of iron have been used. Typical recipes for both a low-sugar and a sugar-free system ( 5 2 , 5 5 )are: X478 71/29 Butadiene/styrene 200 Water 0.10 Cumene hydroperoxide 0.18 Sulfole Baa ... Mixed tertiary mercapi;ans 4.5 Dresinate 214 0.16 Triton R-100 0.10 KC1 0.50 NasPOa.12HzO 0.115 FeSOa.7IlzO K*P?O, 0.155 ~.__ Dextrose 0.9 Time to 60% oonversion a t 5 O C., hours 13.8 Tertiary dodeoyl mercaptan. 3 : 1: 1 mixture of tertiary C I ~Cln, , and C16 mercaptans. ~
a b
X 526 71/29 215
0.12
...
0.24 4.0 0.15
...
0.30 0.18 0.20
13.8
Recent work has developed other hydroperoxides of greater polymerization-initiating power (17 , $1, 7 9 ) . Rubber Reserve specifications now allow the interchangeable use of cumene hydroperoxide, diisopropylbenzene hydroperoxide, or p-menthane hydroperoxide in low temperature polymers. This has been possible as changes in the oxidant affect only the reaction rate, and economic or local operating considerations may often dictate changes from the original oxidants. Since p-menthane hydroperoxide is not derived from benzene and hence is more attractive on a supply basis, it is expected eventually to replace other oxidants to a large extent.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Synthetic HYDROPEROXIDE-POLYAMINE SmrEMs. Because the previous class of activators introduces iron which might catalyze spbsequent autoxidation and cause polymer degradation, a search was made for other activating systems free from heavy metals. Such a requirement has been met in the use of mixtures of hydroperoxides and polyamines (77). These recipes have been of great interest because the absence of any added iron salt made possible the production of nonstaining, nondiscoloring low temperature polymers. They also have the further advantage of being much simpler in operation, since there is no complex activator solution to be made up, A typical polymerization recipe (39) follows: Butadiene/styrene Water Mixed tertiary mercaptans Rubber Reserve potassium soap Triton R-100 KCI
72/28 180
0.2 4.5
0.1
0.5
KQH
0.1
Time to 60% conversion at 5' C.. hours
8
Cumene hydroperoxide Tetraethylenepentamine
0.2 0.2
This system is not strictly iron-free. Purification of all materials to an iron content below l p.p.m. reduced the polymerization rate considerably. Consequently, the reaction mechanism may be fundamentally the same as that of the previously discussed complexed-iron systems. The structure of the amine has a marked effecton its activating power. Monoamines (primary, secondary, or tertiary) and simple diprimary amines as trimethylenediamine have no significant activating effect; ethylenediamine and propylenediamine have small activating power. The polyethylene and polypropylene polyamines [H2NCH(CHs)CH*NHCH&H( CH8)NHa 1, however, give a strong activation, the effect in the ethylene series increasing from diethylenetriamine through tetraethylenepentamine and thereafter falling slightly. All the active amines contain two primary amino groups, one or more secondary amino groups, and not more than two carbon atoms separating the nitrogen atoms OONTINUOUS OPERATION
'
A
The batchwise production of GR-S was accompanied by a pilot plant examination of the continuous technique. At first, three %gallon autoclaves were connected in series; later, two 5-gallon reactors were added (58). The basic GR-S charge was divided into three feed streams: distilled water (180 parts), soap (5 parts), and mercaptan (0.5 part); butadiene (75 parts) and stgrene (25 parts); and distilled water (20 parts) and potassium persulfate (0.3 part). Preliminary pilot plant runs indicated that the mercaptan should be added with the soap; if mixed with the monomers, its efficiency was reduced. A more desirable modifier usage resulted when the mercaptan was divided and added to the first three of the five reactors The addition of 0.1 part with the charge, 0.1 part into the second reactor, and 0.3 part into the third reactor gave a product with a plasticity more nearly in line with the control. The properties of a 50% conversion polymer most nearly duplicated those of a batchwise control; polymers made t o higher conversions were tougher and lower in tensile strength and elongation. The temperature schedule of 118" F. in the first reactor and 122 " F. in the remaining four produced a much shorter holdup time with little loss in properties. Variations in the method of catalyst addition caused no significant change in the over-all rate of the process, although an initial charge of 0.2 part in the first stage and 0.1 part in the third stage of the five-stage process was claimed t o soften the product. The conversion of the government synthetic rubber plants to the cold process (5' C.) after pilot plant testing (44, 46) led to plant installations designed for continuous operation in the 3750gallon autoclaves. The following formula was selected:
April 1952
Rubber-
Butadiene Styrene Cumene hydroperoxide Sulfole (modifier) Dresinpte 214 Trisodlum phosphate Activator Ferrous sulfate Potassium pyrophosphate Glucose Water
Parts 71.00 29.00 0.10 0.16
4.70 0.50
0.10 0.20 0.80 180.00
The primary advantage claimed for continuous operation in
GR-S manufacture is the ability t o operate with the autoclaves full rather than at the 90% loading practiced in batch operation. This permits a theoretical capacity increase of 15 to 20%, based on comparable reaction times. In early practice, the time required for batchwise preparation of cold rubber was shorter than in the continuous process, and a 4% increase in capacity was realized. Greater experience in producing cold rubber has considerably improved this aspect. Additional advantages included the more uniform product obtained from a continuous plant with uniform process control and reduced surges on utilities. A reduced operating labor cost was claimed with reduced process and refrigeration equipment requirements. It was also found possible to reduce utility requirements by using waste heat exchange to cool the feedstock streams entering the first reactor. GERMAN EMULSION POLYMERS
The German philosophy of the preparation of a general purpose synthetic rubber differed from the concept of GR-S. The Bunas as produced were stiffer than GR-S and required softening just before being processed. A single product could thereby serve many purposes by being softened t o different degrees. Buna S-1 was made from a butadiene/styrene (70/30) charge polymerized to 60% conversion in a Nekal B X (di-sec-butylnaphthalene sodium sulfonate)-sodium linoleate solution ( 2 7 ) . The technique of heat softening of synthetic rubber wft~developed for the processing of Buna S, the softening process being dependent on the presence of oxygen. It could be effected in an oven or in an autoclave a t 1to 3 atmospheres and temperatures from 130" to 150" C. ( 2 5 ) . A change from Buna S to Buna S-3 (60,7 2 ) was necessitated by a linoleic acid shortage which developed during World War 11. Linoleic acid was used as a soap, imparting both modifying and emulsifying, action t o the system. The new charge included diisopropylxanthogen disulfide (Diproxid) as the modifying agent (0.06 to 0.1 part), butadiene (68 t o 70parts), styrene (30 t o 32 parts), Nekal BX (2.85 to 3.1 parts), paraffin acids (0.5 pars), sodium hydroxide (0.32 to 0.5 part), potassium persulfate (0.4 to 0.45 part), Diproxid (0.06 t o 0.1 part), and treated water (61). The charge was polymerized a t 45" to 50" C. to a conversion of 60%. Eight lines of six 20-cubic meter reactors (5280 gallons) were operated continuously a t Hlils, five reactors being used a t one time with a holdup time of 30 hours. The Diproxid was added in equal amounts to three of the reactors corresponding to approximately 14, 25, and 43% conversion. Latex from the last reactor was filtered and stabilized with 3 parts of phenyl-& naphthylamine; unreacted monomer was recovered by a vacuum steam distillation. The latex was coagulated in a 2'/~-inch glass pipe containing two special tees for the introduction of latex and coagulant. The latex entered the first tee a t 30" to 40' C. receiving a spiral motion by the shape of the nozzle. Brine was introduced into the side of the same tee directly opposite t o the latex stream. The creamed latex moved about 24 inches to the second tee where bisulfite solution was added to agglomerate the creamed particles. In addition to the general purpose Buna 5-3, special purpose Buna copolymers were available. These included high styrene
INDUSTRIAL AND ENGINEERING CHEMISTRY
727
ELASTOMERS-Synthetic Table 111.
Rubber
Production of Perbunan Types (Tons) i n Germany
Perbunan Perbunan special (dry basis) Perbunan Extra Igetex
1939 960
1940 1941 1942 1943 1944 (to Oct. 26) 1692 2433 2835 3341 2802
97 114 72 49 93 93 69 92 136 230 222 234 Made irregularly a t rate of '/z ton per month.
of trade-mark names in volumes 8s shown in Table I. Production of the Perbunan types in Germany is listed in Table 111. The outstanding characteristic of these polymers is their oilresistance which depends on the amount of bound acrylonitrile. Since the low temperature flexibility is also dependent on the nitrile, two types of polymer have been marketed-one offering high oil-resistance (Buna NN and Perbunan Extra) and one which has moderate oil-resistance and favorable low temperature characteristics (Buna N and Perbunan) ( 5 6 , 5 7 ) . The polymerization charges resembled those for the styrene analogs:
types, low odor and iron-free preparations, oil resistant polymers, and nonstaining varieties. SYNTHETIC LATEX
Toward the end of 1943, attention was given t o special preparations for latex applications ( 1 ) . The first latex made available was Type I, the latex from which solid GR-S was obtained. It, therefore, contained 75: 25 butadiene t o styrene ratio, was stabilized by the addition of 1.5 to 2.0% phenyl-@-naphthylamineand was offered at a total solids of 28 t o 30%. Type I1 latex differed only in containing no antioxidant. Type I11 was the first latex prepared by a special recipe. It had a 50:50 monomer ratio made in a potassium soap of crude rosin a t a total solids of 35 t o 40% (35') Later in the GR-S program, other special latices were developed including Type V which was polymerized t o 60% solids t o be competitive with high solids natural latex and t o avoid concentrating a lower solids latex. The technique of producing Type V latex ( 5 )involved producing a latex of materially larger particle size. This was done by reducing the soap in the original charge t o less than 2 parts with a reduction in the water charge. T h e production required special agitation to decrease the period during which the latex was unstable; this permitted better heat transfer and smoother operation. In addition, increments of soap were introduced a t carefully selected points t o produce a mechanically stable, fluid latex. The h a 1 Type V charge contained a low water t o monomer ratio. and a soap charge t o 2 t o 3 parts, of which part was added initially and part incrementwise. This technique increased the latex particle size to 3000 A. as compared with 800 t o 900 A. for normal GR-S charge. Other workers (9) have pointed out similar difficulties in producing high solids latices. First, the charges showed a viscosity peak a t about 20% conversion which could be overcome by suitable agitation and the proper selection and use of emulsifiers, stabilizers, and viscosity controlling agents. Secondly, the higher monomer charge produced more heat during the polymerization; this could be handled by the use of reflux condensers or less active recipes. Thirdly, since high solids latices died out more readily than did their low solids counterparts, increment additions of catalyst were necessary. Several Igetex latices were manufactured in Germany at 45% solids or less (6, 64). Some were prepared a t the concentration at which they were marketed; others were concentrated, either by creaming or by the Stockpunkt method. The latter technique involved adjusting the electrolyte level of the latex and cooling with agitation until gelation occurred. The gel could then be separated from the serum as a fine precipitate which, on warming t o room temperature, reverted to a higher solids latex (6). OIL-RESISTANT POLYMERS
The oil-resistant butadiene-acrylonitrile copolymers were developed in Germany ( 6 , 7 0 , 7 S )and shipped to the United States in volumes reaching 150 tons in 1939. At the beginning of World War 11, several companies in the United States undertook to prepare the acrylonitrile rubbers, marketing them under a series 128
Butadiene Acrylonitrile Water Potassium persulfate
Parts Perbunan (Standard Oil N. J.) 74 26
180 0.3
...
h-ekal BX (an alkylaryl sulfonate) Oleic acid Sodium oleate
1 4
Sodium psrophpsphate Sodium hydroxide
0.3 0.05-0.1
Buna K (I. G. Farben) 74 26 150 0.2
..3.. ...6 ...
...
Diproxid Mercaptan Temp.,
C.
40
30
Time, hours
10-14
25-30
Bound nitrile
26-28
26
The production of the oil-resistant rubbers has been batchwise in equipment similar to that used for the styrene rubbers, producing polymers of definite plasticities through control of the mercaptan type, concentration, and technique of addition ( 2 , 56). SODIUM POLYMERIZATION
Historically, the polymerization of a diene in the absence of water was of early significance having been used by the Germans during World War I to manufacture methyl rubber. The mass technique was developed further by the Germans for making the sodium-catalyzed Bunas 32, 85, and 115 and by the Russians for making their SKA and SKB. The three German polymers were numbered to designate a viscosity index and degree of polymerization (43). Buna 32 was a honeylike, viscous polymer with a molecular weight of 30,000 and was used in Germany primarily for softening less processable rubbers (16). Buna 85, with a molecular weight of about 80,000, was the principal German sodium rubber a t the time of allied occupations. It was used primarily as a polymer extender. Buna 115 was considered too difficult to process and therefore found only limited use. A batch process was used to make Buna 32 and Buna 115, whereas the Buna 85 process n-as a continuous one. Butadiene used in the sodium process was especially purified t o remove all traces of aldehydes and water, inasmuch as the former was claimed to widen the molecular weight distribution of the polymer and the latter t o poison the catalyst (16,47). Buna 32 was made in 8-cubic meter stainless steel autoclaves equipped with baffles and rotated in a water bath. Butadiene containing 9.1% vinyl chloride as a modifier and 0.5% dry sodium sand was charged to the autoclave and polymerized at 80' C. The pressure remained a t 7 to 10 atmospheres for the 8-hour reaction period and dropped to 1 to 2 atmospheres at 95% conversion of the monomer. The product was washed with water to remove the sodium and stabilized with phenyl-p-naphthylamine or aldol-or-naphthylamine. Buna 85 was made in continuous units consisting of horizontal, jacketed cylindrical steel vessels approximately 15 feet long, containing a hollow screw. The pitch of the screw as well as the
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distance from the reactor shell to the core of the screw increased gradually along the length of the reactor providing a n increasingly greater volume as the polymer formed. The temperature was controlled by passing cooling water through the jacket as well as through the hollow screw. The polymerization was catalyzed by potassium dispersed (10 to 15 microns) in Buna 32. Dioxane (0.5 t o 1.0%) was pumped simultaneously into the butadiene inlet line to serve as a diluent to prevent local overheating during the reaction. The 2% potassium paste was fed into the reactor at 0.570 potassium per 100 kg. of 99.8% butadiene. Polymerization was conducted a t 70" C. with a 1.5, to 2-hour holdup. The product was extruded from the polymerizer and mixed with 2% phenyl-@naphthylamine with 1% fatty acid to neutralize the residual catalyst and potassium hydroxide. The reactors a t Schkopau were said to produce 120 tons per month on a 24-hour cycle. The sodium polybutadiene gave a tensile strength of 1960 pounds per square inch, whereas the emulsion product showed 1120 pounds per square inch. The addition of styrene to the sodium-catalyzed butadiene charge was found to shorten the polymer chain as did 2-butene (71). Reference is also found to the use of styrene in the polymerization of the Buna 85 type, b u t no record was found for its commercial application (15). Copolymers of butadiene and styrene were prepared in the United States as part of the Rubber Reserve program (49). The laboratory results showed a 14-hour polymerization time for the preparation of polybutadiene and a 10-hour cycle for the 75: 25 butadiene to styrene charge. A large decrease in catalyst was necessary before any appreciable change in rate occurred, although a reduction in exposed surface of the catalyst increased the polymer viscosity and lowered the benzene solubility, producing a higher molecular weight. The total polymerization time at 30" C. was not markedly different from that of a 10" C. charge, but the polymer had a lower viscosity. A polymer made at 50" C. showed a shorter reaction time due primarily to a reduced induction period. I n this and subsequent work (63),it was observed that the sodium copolymers had a higher styrene content than did the original charge. This was contrary to the emulsion polymers where the initial polymer from a 75:25 butadiene to styrene charge contained 17.2% styrene. Large amounts of the copolymer, designated S-BS, were made in bombs consisting of 13-inch lengths of 3-inch pipe capped at both ends. The bomb was charged with powdered sodium as a 20% dispersion in xylene, and placed in a salt-ice mixture for the addition of the butadiene-butane mixture. The typical charge was: butadiene 75 parts, styrene 25 parts, isobutane 75 parts, and sodium catalyst, 0.3 part. The reaction took about 20 hours to reach 90 to 100%. Polymer made by this process to a Mooney plasticity of 69 (ML 4 at 212" F.) showed processing characteristics equal to natural rubber and superior to a GR-S sample. Flex life was much better than for the emulsion sample. Tensile strength, elongation, abrasion resistance, and aging characteristics were similar to the emulsion rubber. The outstanding fault of the sodium polymer was the tendency to become brittle at about -40" F. compared to -55" F. for natural rubber and -70" F. for GR-S. The superior processing quality and better balance of flex cracking and hysteresis, as well as the poorer brittle point were confirmed by other workers using polymers prepared without the addition of isobutane (36). ALFIN CATALYSIS
A new type solution catalyst was developed under the Rubber Reserve program with the observation that the complexes formed by sodium compounds of mixtures of alcohols and olefins were effective in catalyzing polymerization of dienes and olefins (68).
April 1952
Rubber
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After considerable work, the complex of sodium isopropoxide and allyl sodium was selected for extensive evaluation. Polymerization was effected in pentane solution,'with a 65 to 90% yield of polymer resulting in 30 minutes a t 30" C. Polybutadiene and butadiene-styrene copolymers ranging from 90: 10 to 70:30 were compared with emulsion analogs. I n general, they had higher gel contents, indicating a greater degree of cross linkage, and contained more external double bonds, indicating a larger amount of 1,2- polymerization. The Alfin polymers exhibited higher tensile strength and better abrasion resistance than did the emulsion counterparts but had higher stiffening and freezing points (12). LITERATURE CITED
(1)Anon., I n d i a Rubber World, 109, 577 (1944). (2) Arundale, E., U. S. Patent 2,434,536(1948). (3) Balandina, V.,Bull. acad. sci. U.R.S.S., 1936,423-33. (4)Bebb, R. L., U. S. Patent 2,459,740(1949). ENC.CBEM.,40, 1473 (5) Borders, A. M.,and Pierson, R. M., IND. (1948). (6) Braaier, S. A., et al., U. S. Dept. Commerce, OTS, PB 23858 (1945). (7) Carr, C. W., Kolthoff, I. M., Meehan, E. J., and Sternberg, R. J., J . Polymer Sci., 5, 191 (1950). (8) Cam, C. W., Kolthoff, I. M., Meehan, E. J., and Williams, D. E., Ibid., 5, 201 (1950). (9) Chittenden, F. D.,McCleary, C. D., and Smith, H. S.,India Rubber World, 113,814 (1946). (10) Cuthbertson, C. R.,Coe, W. S., and Brady, J. L., IND.ENG. CHEM.,38, 975 (1946). (11) Deanin, R., Lindsay, R. D., and Leventer, S. E., J . Polymer Sci., 3, 421-32 (1948). (12) D'Ianni, J. D., Naples, F. J., and Field, J. E , , IND.ENG.CHEM., 42, 95 (1950). (13) Dogadkin, B., and Vinogradova, M., Colloid J. (U.S.S.R.), 3, 129 (1937). (14) Dunbrook, R. F., I n d i a Rubber World, 117, 203, 355 (1947); 117,486,617,745 (1948). (15) Ebert, G., Heidebrock, R., and Orth, P., U. S.Patent 2,209,746 (1940). (16) Fennebresque, J. D., Monrad, C. C., and Troyan, J. E., U. S. Dept. Commerce, OTS, PB 512 (1945). (17) Fisher, G. S., Goldblatt, L. A,, Kniel, I., and Snyder, A. D., IND. ENG.CREM.,43, 671 (1951). (18) Frank, R.L.,Blegen, J. R., and Deutschman, A,, Jr., J. Polymer Sci., 3, 58 (1948). (19) Frank, R. L.,Blegen, J. R., Inskeep, G. E., and Smith, P. V., IND.ENO. CHEM.,39, 893 (1947). (20) Frank, R.L., Smith, P. V., Woodward, F. E., Reynolds, W. B., and Canterino, P. J., J . Polymer Sci., 3, 39 (1948). (21) Fryling, C. F.,and Follett, A. E., Ibid., 6,59 (1951). (22) Fryling, C. F., Landes, S. H., 8t. John, W. M., and Uraneck, C. A,, IND. ENG.CHEM.,41, 986 (1949). (23) Fryling, C. F., and St. John, W.M., Ibid., 42, 2164-70 (1950). (24) Fuller, C. S., Bell Telephone System Teoh. Pubs., Monograph B1045 (1946). (25)Hagen, H., Kautschuk, 14. 203-10 (1938); 15, 88-95 (1939). (26) Hampton, R. R.,Anal. Chem., 21, 923 (1949). (27) Handley, E. T., et al., U. 8. Dept. Commerce, OTS, PB 193 (1945). (28)Harkins, W. D., J. Am. Chem. SOC.,69, 1428 (1947);J . Polymer Sci., 5, 217 (1950). (29) . . Harris. W. E..and Kolthoff, I. M., J. Polymer Sci., 2, 72, 82 (1947). (30)Hart, E. J., and Meyer, A. W., J. Am. Chem. SOC.,71, 1980 11949). (31)H&S,-J: T., Drake, A. E., and Pratt, Y.T., IND. ENG.CHEM., 39, 1129 (1947). (32) Hobson, R. W., and D'Ianni, J. D., Ibid., 42, 1572 (1950). (33) Howland, L. H., Peaker, C. R., and Holmberg, A. W., I d a Rubber World, 109, 579 (1944). (34)I. G. Farbenindustrie, French Patent 843,903 (1939). (35)Johnson. P. H., Brown, R. R., and Bebb, R. L., IND.ENG. CHEM.,41, 1617-21 (1949). (36) Juve, A. E., Goff, M. M., Schroeder, C. H., Meyer, A. W., and Brooks, M.C., Ibid., 39, 1490 (1947). (37)Kern, W., Makromol. Chem., 1, 209 (1948). (38) Kitchen, L. J., Albert, H. E., and Smith, G. E. P., Jr., IND. ENG.CHEM.,42, 675 (1950). (39) Kolthoff, I. M.,and Dale, W. J., J . Polymer Scd., 3,400 (1948); 5, 301 (1950). (40) Kolthoff, I. M.,and Harris, W. E., Ibid., 2, 41,49 (1947). (41) Kolthoff, I. M.,and Lee, T. S., Ibid., 2, 206 (1947).
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TONBEWS-Synthetic Rubber(42) (43) (44) (45) (46) (47) (48) (49)
Kolthoff, I. M., and Nedalia, A. I., Ibid., 5 , 391 (1950). Konrad, E., A n g e w . Chenr., 62, 491 (1950). Larson. hl. W., Chem,. E n y . P m g r e s s , 47, 270 (1951). Laundrie, R. IT.,Rowland, E. E . , Snyder, A. D., Taft, W. K., and Tiger, G. *I., IND.ENO.CHEM.,42, 1439 (1950). Livingston, J. PI-.,Chem. Eng. A’ews, 27, 2444 (1949). Livingston, J. W.,[T. S. Dept. Commerce, OTS, PB 517 (1945). Marvel, C. S., Ibid., PB 11193. hfarvel, C. S.,Bailey, W.,T., and Inskeep, G. E., J . Polymer
Schateel, R. A , and White, W.L., U. 8.Dept. Commerce, OTS, PB 214 (1945). Schulze, SIT. A., and Crouch, W. W., J . Am. Chem. Sac., 70, 3891 (1948).
Schulre, W.A., Tucker, C. hI., and Crouch, W. W., IND.EXG. CHBM.,41, 1599 (1949).
Semon, W. L., Chem. Eng. News, 24, 2900 (1946). Shearon, W. H., Jr., McKenaie, J. P., and Samuels, M. E., IND.ENG.CHEM.,40, 769-777 (1948). Smith, W . V., J . Am. Chem. SOC.,68, 2059, 2064, 2069 (1946). Snyder, H. R . , Steward, J. M., Allen, R. E., and Dearborn, R. J., Ibid., 68, 1422 (1946). Soday, Frank J., Trans. Am. Innl. Chem. Engrs., 42, 647 (1946). StBcklin, P., editors, T. R. Dawson and J. R. Scott, “Proceedings Rubber Technology Conference,” p. 434, Cambridge, W. Heffer and Sons, Ltd., 1938. Talalay, A, and Magat, hI., “Synthetic Rubber from Alcohol,’’ Xew York, Interscience Publishers, Inc., 1945. Tech. Inds. Intelligence Comm., Rubber Subcomm., U. S.Dept. Commerce, OTS, PB 13358 (1948). U. S. Patent 1,973,000 (1934). Vandenberg, E. J., and Hulse, G. E., IND.ENG. CHEM.,40,
Sci.. 1. 275 119461.
Marvel, C. S., Deanin, R., Claus, C J , Wyld, W. B., and Seite, R. L., Ibad., 3, 350 (1948). (51) Meehan, E. J., Ibzd., 1, 318 (1946). (52) Mitchell, J. M . , Spolsky, R., and Williams, H. L., IND.ENG. CHEM.,41, 1592 (1949). (53) Morton, A. A , et al., J . Am. Chem. Soc., 68, 93 (1946); 69, (50)
160, 161, 167, 172, 950, 1675 (1947); 481, 487 (1949). (64)
70, 3132 (1948);
71,
Naunton, W. J. S.,et al., U. S. Dept. Commerce, OTS, PB 32161 (1945).
Neklutin, V. C., Westerhoff, C. B., and Howland, L. H., IND. ENG.CHEM.,43, 1246 (1951). (56) Nelson, J. F., and Vanderbilt, B., Rubber Technology Conf., London, Preprint KO. 15 (1948). (57) Newton, R. G., and Scott, J. R., J . Rubber Research, 13, 1-19 (55)
(1944). (68) Owen, J. J., Steele, C. T., Parker, P. T., and Carrier, E. W., IND.ENG.CHEM.,39, 110 (1947). (59) Rabjohn, Norman, Dearborn, R. J., Blackburn, W. E., Inskeep, G. E., Snyder, H. R., and Marvel, C. S.,J . Polymer Sci., 2, 488 (1947). (60) Rubber Reserve Co., U. S. Dept. Commerce, OTS, PB 13523 (1945). (61) Saffer, A,, and Johnson, B. L., IND. ENG. CHEX., 40, 538 (1948).
932 (1945).
Wall, F. T., J . Am. Chem. Soc., 67, 1929 (1945). Wall, F. T., Banes, F. W.,and Sands, G. D., J . Am. Chem. Soc., 68, 1429 (1946).
Whitby, 0.S.,Wellman, N., Floutr, V. W.,and Stephens, H. L., IND.ENG.CHEM.,42, 445 (1950). (78) White, L. M., Ebers, E. S.,Shriver. G. E., and Breck, S., Ibid., (77)
37, 770 (1945). (79)
Wicklate, J. E., Kennedy, T. J., and Reynolds, W. B., J . Polymer Sei., 6, 45 (1951).
RECEIVED for review September 17. 1951.
ACCEPTED January 25, 1952.
Relationship between Molecular Structure and Physical Properties S. D. GEHMAN The Goodyear Tire and Rubber Co.,Akron 16, Ohio
This paper brings together information on relationships between the most important features of molecular structure and the physical properties of synthetic rubbers. The molecular characteristics of greatest significance for the physical properties are considered to be the nature of t h e monomer units, molecular weight, cross linking, details of chain structure such as cis-trans isomerism and side vinyl groups, and chain branching. Observed variations in properties may be dominated by the detail of structure being investigated, but other uncontrolled deviations in structure occur, especially during processing and vulcanization. Thus quantitative correlations of structure and properties are usually possible only under ideal circumstances and very carefully controlled conditions. In general, each feature of structure exerts its most conspicuous influence o n a rather limited group of physical properties. The chemical nature of the monomer units determines the intermolecular forces and influences especially the temperature range in which rubber elasticity is exhibited, the swelling in organic liquids, and the permeability to gases. Molecular weight distribution is most significant for processability. Tensile strength and modulus are sensitive t o low molecular weights but become insensitive to higher molecular weights. Cross linking in the raw polymer gives rise to gel and affects the processability. The cross-linked network formed upon vulcanization is controlling for the tensile properties and for sta-
730
bility under stress. Regularity in the geometrical form of the chain molecules appears to be favorable for low hysteresis and conducive to crystallization under stress. This leads to improved tensile strength and flex life. Effects of chain branching are rather obscure but explain variations in properties not otherwise accounted for.
D
EFINITE features of molecular structure are required for a material that exhibits rubberlike elasticity. These involve
both geometrical characteristics of the molecules and favorable intermolecular forces. The molecules must be long-chain or threadlike molecules which, owing to thermal agitation, assume random configurations. Theory also requires chemical bonds or cross linkages at intervals along the chains, so that they are connected at these points and form a network structure. The intermolecular forces, or forces between neighboring molecules or groups of atoms, are variously designated as secondary valence forces, van der Waals forces, or cohesive forces. They must be sufficiently weak to permit configurational changes of the molecules and deformation of the network structure by relatively low stresses and yet strong enough to provide adequate tensile strength. The retractive force of rubber arises from the tendency of the chain molecules to resume their random configurations when these are altered by stretching. Judging from the wide range of rubberlike properties observed, these conditions are complied with to various degrees and in
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