. .
CHARLES COODYEAR MEMORIAL LECTURE
Presented before the Division of Rubber Chemistry AMERICAN CHEMICAL SOCIETY
-1
THE SECOND MILE L. B. SEBRELL G o o d y e a r Tire a n d Rubber Company, Akron, Ohio
Synthetic rubber of good physical properties can be madej i n many respects it i s fully equal t o natural rubber; in others, synthetic rubber shows serious deficiencies, par. ticularly in i t s ability t o give satisfactory results in pure gum stocks or good tensile strengths at elevated temperatures. While there i s considerable work to b e done in improving synthetic rubber, the dangers arising from a lack of rubber may be considered largely removed. A n accurate plan of attack t o improve synthetic rubber must consist of the removal of cross linkages t o as great an ex-
tent as possible. Some radical change in the method of forming the rubberlike polymer will have t o be developed before the qualities of the synthetic rubber will greatly improve beyond those outlined i n this paper. The comparison between butadiene-styrene or -nitrile types and the Butyl type of rubber i s interesting. While Butyl i s inferior i n several respects to the other synthetics i t excels them i n other respects. Since it i s generally assumed to b e more nearly a straight-chain polymer, further development along this line would seem t o be i n order.
HE title “The Second Mile” was inspired by an address T given by W. E. Wickenden, of the Case School of Applied Science, upon an entirely different subject. Since this title
still more or less secret. It is also proposed to go into the probable structure of these polymers and to indicate some trends of research which might profitably be followed in order to improve them.
seemed to suit the synthetic rubber situation as it now exists, Wickenden’s permission was secured to use the title in connection with this lecture. In its broader aspects, the paper purports to cover the present status of the synthetic rubber situation. Natural rubber will be compared with the copolymers made from butadiene and styrene, butadiene and acrylonitrile, and the copolymers generally known as Butyl rubber. These three materials were chosen for the comparative study because they are the materials which the Government proposes to use in the largest quantities to alleviate the present shortage of rubber. It is hoped that the data will give a fairly accurate picture of the comparative or relative values of these synthetic rubbers and natural rubber. There will be no discussion of the methods of manufacture which are
Polymerization
The polymerization of organic substances has been known for considerably more than a hundred years. Berzelius (2) was the first to use the term “polymerization” and to define it as indicating those compounds which possess the same properties but have a different total number of atoms. Williams ($8) is generally credited with the k s t preparation of isoprene when he separated this material from the products of destructive distillation of rubber. However, Himly (IO),while investigating the fractional distillation of rubber, isolated a distillate which he called “Faradayin”. He also coined the term “Kautchin” (now known as dipentene) for one of the higher boiling fractions. However, these investigators did not again polymerize these rubberlike products to any compounds of a higher molecular weight. Bouchardat (3) is usually given the credit for first synthesizing a rubberlike material from a liquid of low molecular weight. In 1875 he advanced the idea that isoprene is a primary unit of natural rubber and succeeded in producing a rubbery polymer from the distillation products by the process of polymerization. He brought about this conversion by heating isoprene with fuming hydrochloric acid and obtained a product which he described as being elastic and possessing the characteristics of rubber. Tilden (19,2O) then showed that isoprene could be obtained by the pyrolysis of turpentine, and that by treatment with hydrochloric acid it would also undergo spontaneous polymerization. He also polymerized isoprene by the use of nitrosyl chloride.
* (Left] Rolling Mill for Crude Dry Chemigum, Synthetic Rubber M a d e by Goodyear Tire and Rubber Company
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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(Right)
Chemigum Emerging from (I Dryer Which Removes Water Collectted during the Process of Coagulation
* The work of Tilden was carried on by Hofmann (11) in Germany. Inconnection with Coutelle (I), Hofmann obtained the earliest German patent on the production of synthetic rubber by the heat polymerization of isoprene with or without polymerizing agents. In 1910 Matthews and Stran e (16) in England and Harries in Germany reported almost simultaneously that metallic sodium catalyzes the polymerization of isoprene to syntheticrubber. The claims of these early patents specify subjecting isorene to metals of the alkali or alkah e earth groups, their mixtures, alloys, or amalgamsin such a manner that the metals are wholly or largely in contact with the vapor of the hydrocarbon. The story of synthetic rubber manufacture in Germany during the First World War is well known. A derivative of butadiene (2,3-dimethylbutadiene)was polymerized by means of sodium to give methyl rubber. During 1914-18 some 2400 tons were said to have been manufactured. One or two different grades of this type of synthetic rubber were prepared at that time; almost all of the dimethylbutadienewas made from acetone. After the First World War several attempts were made to cafry on the development of synthetic rubber in various laboratories, both in the United States and abroad. It can safely be said that not until it was generally known in the United States that the Germans were again devoting serious attention t o the synthetic rubber problem was concerted effort put forth here to produce a good synthetic rubber. In Italy the polymerization of butadiene and related hydrocarbons by the sodium method has been fairly well developed, and Sam les received in this country have been of a relatively high quaEty.
$9)
sibly the particle size of the synthetic latex might be directly connected with the physical properties to be obtained from synthetic rubbers. It is common knowledge that the present types of synthetic rubber do not give as good results when vulcanized in pure gum stocks as they do when substantial quantities of carbon black are added. This brings up the question of whether or not carbon black, being of submicroscopic size, acts as a grinding medium; and whether, upon being milled with the synthetic rubber, it has a pronounced surface effect upon the particles of synthetic latex or acts as a shearing medium to break them down and reveal less highly polymerized rubber in the interior of these small particles. This assumption is based upon the fact that, in milling, the original latex particles are disintegrated; it has long been known that the outer shells of these particles possess a somewhat higher degree of polymerization than the interior of the natural latex particle. Further studies with the ultramicroscope may disclose additional data of value in determining the difference in particle size between natural and synthetic latex and supply the reason for the differences and the possible significance of such variations. X - R a y Structure o f Synthetic Rubber
A considerable amount of attention has been given to the
Letex
There is an outstanding difference in the particle size of natural latex and the synthetic latex of any of the copolymers made by the emulsion processes above referred to. Figure 1 represents natural and synthetic latex, respectively. The synthetic latex, when examined with the ordinary microscope, is of so small a particle size as to be almost invisible. The electron microscope shows the presence of particles, but these particles may be merely aggregations of larger clumps of molecules. In discussing the difference in particle size with A. R. Kemp, of the Bell Telephone Laboratories, he pointed out that pos-
x-ray diagrams of various synthetic rubbers, but it has not been possible to apply the results of such studies directly to the process of improving the copolymer types of synthetic rubber now under consideration. The general character of the x-ray diffraction results with synthetic rubbers was discussed in B previous paper (16). The copolymers of butadiene and styrene and of butadiene and acrylonitrile are amorphous under all conditions. Polyisobutylene and polychloroprene, on the other hand, develop a crystalline structure upon stretching. Stretched Butyl B rubber shows the same crystalline structure as polyisobutylene. Thiokol has a somewhat crystalline structure, both stretched and unstretched. In the cases where crystalline structures are obtained, it is evident that the x-ray patterns can be used to secure information on the chain form. They can be used to evaluate in various indirect ways the strength of the intermolecular forces
. I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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Figure 1.
Electron Micrograms of Natural (left) and Synthetic
and the relations between molecular length, molecular length distribution, and molecular mobility, which in turn finds its reflection in the plastic properties. The x-ray diffraction results with Butyl I3 rubber are especially interesting when correlated with stress-strain curves. Figure 2 shows the amorphous pattern for unstretched Butyl rubber and the crystalline pattern which develops upon stretching. Unlike Hevea and GR-S, Butyl rubber does not show very marked improvement in tensile strength due to incorporation of gas black, although typical reinforcement occurs for such properties as abrasion, tear, and modulus. Since the tensile strength is closely connected with the crystallinity a t high elongations, x-ray diffraction results might be expected to throw some light on this apparently contradictory result. It was found that the crystallization phenomena in loaded and unloaded Butyl rubber stocks were analogous to thosr with Hevea (7). Evidence of crystallization began to appear a t about 500 per cent elongation for the gum stock and at about 200 per cent for a compound loaded with 60 parts of gas black. Spreading of the diffraction spots into longer arcs for the loaded stock indicated that the alignment of the crystallites in the direction of stretching was less perfect than for the gum stock. Thus, although higher elongations are required for crystallization in Butyl rubber as compared to Hevea, the effect of carbon black on the patterns is similar. The stress-strain curves in Figure 3 indicate that there may be a simple explanation of the apparent lack of tensile reinforcement. Between 800 and 900 per cent elongation, the stress-strain curve for the Butyl rubber gum stock rises abruptly, reflecting the onset of a highly crystalline structure which is also indicated by the sharp, intense diff raction pattern. The fact that crystallization sets in a t higher elongations when the molecules are already well aligned probably contributes to this perfection, as does the simple and regular chain form of the Butyl rubber molecules. Thus, in the gum stock, the ultimate tensile strength of the material is nearly realized due to its own crystal reinforcement. The black can contribute little further in this respect, and its interference in the alignment of the crystallites may actually work against a higher tensile strength. For Hevea, on the other hand, the onset of crystallization is more gradual and the resulting structure less regular. The black can furnish effective an-
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chorage points to prevent failure from starting between the crystallites, and a large improvement in tensile strength results. For copolymers, which invariably show amorphous structures, the use of x-ray diffraction as a means for investigating the molecular structure is severely limited. It is true that, even in the case of liquids, the x-rays reveal a fundamental type of molecular structure which consists of a statistical space d i s t r i b u t i o n of n e i g h b o r i n g molecules. The only information to be (right) Latices (X 1300) obtained directly from the analysis of such patterns is the average distance of atom neighbors. A comparative study of these amorphous patterns for copolymers has been found to yield some interesting results, even without carrying through the involved mathematical analysis required for their exact interpretation. The patterns shown in Figures 4, 5, and 6 were taken by J. E. Field, using strictly monochromatic CuKa radiation securedby reflection of the x-ray beam from a rock salt crystal. The polybutadiene pattern (Figure 4) is a broad halo, indicating an amorphous structure. I n contrast, the pattern for polyacrylonitrile shows a sharp diffraction ring, proving the existence of small crystalline regions in random orientation. The pattern for polystyrene indicates that, in addition to the usual halo for liquids, there is an inner ring corresponding to a larger molecular spacing. Thus, these three polymers give patterns which are readily distinguishable. We wished t o know to what extent these structures occurred in copolymers.
Unrtrotshed
Figure 2.
Strotcned
X-Ray Patterns of Butyl Rubber
Figure 5 shows the pattern for an emulsion copolymer with a 75-25 starting ratio of butadiene to acrylonitrile. This appears to be the pattern of a homogeneous structure. When the starting ratio was 50-50, the crystalline ring of polyacrylonitrile became plainly evident. Two conclusions can be drawn. Either some or all of the acrylonitrile units of the chain molecules were sufficiently numerous and flexible to come within their normal range of action and assume the same
ture the inner ring is related is not definitely
mixtures of polybutadiene and polystyrene latices. T h i r t y per cent of polystyrene could be readily detected by the presence of the inner ring. Although this method of x-ray analysis is not so sensitive as might be desired, it sliows that for copolymerization of either acrylonitrile or styrene with butadiene, the structure of polyacrylonitrile 2nd oi polystyrene, respectively, tends to make its appearance when the number of molecules approximates the number of butadiene molecules. I n an experiment with a three-component system, with starting ratios of 50, 25, und 25 of butadiene, acrylonitrile, and styrene, respectively, the pattern of Figure 0 was obtained. T h c p r e s e n c e of p o l y s t y r e n e s t r u c t u r c is plainly shown by the inner halo, indicating a different character of reaction for this system as compared to the two-component system.
Polybutadiene
Polyacrylonitrile
Figure 4.
75 butsdiene-P5 acrylonitrile
Figure 5.
Pol~riyrens
X-Ray Patterns of Polymers
50 butadlane-50 acrylonitrile
X-Ray Patterns of Copolymers
5 0 butadiene-50 styrene
INDUSTRIAL AND ENGINEERING CHEMISTRY
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tion of the rubber will be insoluble in benzene regardless of concentration or time of exposure to the solvent. The insoluble portion will be swollen to a degree dependent, in turn, upon the degree of branching and cross linkage of the insoluble fraction. The determination of Figure 6. X-Ray Pattern of benzene solubility and 50 Butadiene-25 Acrylonitrileswelling index involves 25 Styrene the extraction of finely divided polymer a t 35" C. for 16 hours and separation of the sol and gel phases with a fine-mesh Monel screen. The solubility is determined by evaporation of an aliquot portion of the filtrate and is expressed as per cent of the original rubber soluble in benzene. The swelling index is defined as cc. of solution retained by swollen gel grams undissolved rubber and is calculated by subtracting from the original volume of the solvent the volume of the filtrate and dividing by the weight of undissolved polymer. The swelling indices were determined for a large number of butadiene-styrene rubbers of varying benzene solubilities. A good correlation was found for these values as Figure 8 shows. As would be expected, the insoluble portions of highly insoluble rubbers are less swollen than the insoluble fractions of rubbers which are almost completely benzene soluble. The correlation of these values of swelling indices and benzene solubilities has been close enough so that the swelling index is no longer measured. VISCOSITYMEASUREMENTS AND CALCULATED AVERAGE MOLECULAR WEIGHTS. Viscosity measurements were made of the benzene-soluble portions of butadiene-styrene polymers
Figure 7.
Microphotometer Curves for Polystyrene
in an attempt to determine average molecular weights of the rubbers. The determinations were made in dilute benzene solution (0.1 to 0.3 gram per 100 cc.) in an Ostwald capillary viscometer at 25" C. The exact concentration of the solution was determined after the viscosity measurements. The preliminary tests showed considerable deviation from linearity
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of ? f s p plotted against concentration. It was found, however, that an excellent linearity is obtained by plotting log,, qR against concentration. This correlation was pointed out for a large number of polymers by Kemp and Peters (IS). This relation permits the calculation of average molecular weights, if a satisfactory constant is determined by comparison with osmotic pressure measurements or freezing point determinations. Since osmotic pressure determinations have not yet been made for our samples, we prefer to report our results in terms of intrinsic viscosity, [TI, defined as the natural logarithm of the relative viscosity divided by concentration. In cases where we have used calculated average molecular weights based on viscosity measurements, we have arbitrarily adopted the constant used by Kemp and Peters, who calculated weight average molecular weights from viscosity data for rubber, neoprene, and Buna 85 as follows: mol. wt. =
log;?^ x 0.75 x
104
where C is expressed in unit moles per liter. Their constant was based on cryoscopic measurements for various fractions of natural rubber. Although its use for the synthetics is not rigidly justified, the values used in our work serve the purpose of expressing relative molecular weights. Calculated on this basis, the average molecular weights for completely benzenesoluble butadiene-styrene rubber of good quality are in the range 40 to 50,000. 80 1
I
Swellins Index of Inrol. Portion
Figure 8. Relation of Swelling Indices to Benzene Solubilities for 7 5 Butadiene25 Styrene Rubber
A sample of completely benzene-soluble butadiene-styrene (75-25) made in our pilot plant was checked by viscosity measurements and then submitted to the laboratories of E. I. du Pont de Nemours & Company, Inc., for ultracentrifuge tests. Through the courtesy of Cole Coolidge, G. D. Patterson, E. D. Bailey, and J. B. Nichols, molecular weight distribution and average molecular weights were obtained. The average molecular weight found for this sample by the ultracentrifuge was 92,500. This value is approximately twice that calculated from relative viscosity determinations using the formula and constant referred to above. This value was 44,000. Until a check against osmotic pressure measurements is available, no absolute value is attached to the molecular weights calculated from viscosity values. Since the Staudinger relation applies only to linear molecules, its application to a polymer which may be considerably branched would be expected to give low calculated molecular weights. MOLECULAR WEIGHTDISTRIBUTION.The molecular weight distribution obtained at the du Pont laboratories by ultracentrifuging a chloroform solution of butadiene-styrene rubber is shown in Figures 9 and 10. This particular sample is somewhat richer in extremely low-molecular-weight material than other butadiene-styrene rubbers tested. Before the ultracentrifuge data had been obtained, our laboratory had investigated available methods of obtaining approximate molecular weight distributions. Our object
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That is, the sensitivity of the fractionation is less than in the low-molecular-weight range. Since this method of fra;tionation gives cuts which are still quite heterogeneous, the data are to be used only for comparative purposes. The separation is not sufficiently precise to warrant drawing 1 .o 5 complete distribution curves. X Since the successive extraction of the polymer n samples by several solvent blends is time con3 suming (about a week for each complete determin& ation), a rapid method has been tested by which 3 0.5 a fractionation is completed in less than 24 hours. It had been observed that the viscosities for the ,+ S O L RUDOFR (RUREAU OF STAIIDARDS) fractions of butadiene-styrene rubbers were additive, and that good agreement was found between the intrinsic viscosity (or calculated molecular weight) calculated from the amount and viscosity 100 PO0 300 400 500 600 700 of the separate fractions, and the value obMolecular Weisht X 1 0 - 8 tained by a viscosity determination of the soluFigure 9. Molecular Weight Distributions of Natural and Synthetic Rubbers tion of the entire unfractionated rubber. This relation suggested fractional extraction by the following method : Several duplicate samples of:the rubber are extracted for 16 hours with a series of pewas not to obtain precise distribution curves, but to develop troleum ether-benzene blends of the compositions used in suca rapid convenient procedure which would permit a study of cessive extractions. The amount and viscosity of the rubber the changes in relative amounts of low-, intermediate-, and high-viscosity fractions with change of polymerization conditions and of mechanical treatment of the rubber. The following method was adopted: A sample is extracted (by the same method described for benzene solubilities) with a poor solvent, and the amount and relative viscosity of the extracted polymer are determined. This extraction is then repeated on the undissolved polymer, using progressively richer blends of the poor solvent with a good solvent. The solvents for the butadiene-styrene polymers are petroleum ether (30-60’ C.) and benzene. As Table I shows, it is sometimes necessary to use alcohol-petroleum ether blends for the first fractions if the sample is especially rich in low-molecularweight polymer. The data for polymer A in Table I and Figure 11indicate the results of our fractionation of a portion of the batch from which the sample submitted to du Pont was taken. Similar fractional extraction data are shown for other butadiene-styrene rubber samples to indicate the different types of distribution obtained. Figure 10. Molecular W e i ht Distribution of I n the cases where high-viscosity fractions were found, no Butadiene-St yrene%ol ymer fraction of intermediate average viscosity was found. These data do not indicate that the intermediate material was absent, but that the solvent blends, sufficiently rich in benzene to extracted by each blend is determined, and the amounts and dissolve the intermediate portion, also extract,the very highest. viscosities of the individual fractions calculated from those of the combined fractions are actually measured. For our purposes the agreement between this “simultaneous” extraction TABLEI. SUCCESSIVE FRACTIONAL EXTRACTIONS OF and the more valid “successive” extraction was highly satisBUTADIENE-STYRENE POLYMERS factory. The agreement is shown in Table I1 in which the of otal various fractions are combined as shown. Rubber IntrinCslcd. Hydro- sic Via- Viscosity EFFECT OF MILLING. ON SOLUBILITY AND VISCOSITY MOLECUPolymer Solvent Blenda, Vol. yo carbon cosity Mol. Wt. LAR WEIGHT.The tests of solubility and viscosity must be A (9% non80/20 P. E:./EtOH 5.1 0.33 6.500 made upon a sample of known history in regard to the amount rubber) 100 P. E . 15 2 0.46 9,000 95/5 P. E./benaene 26.3 0.92 18,000 of mechanical working the sample has received. Table I11 93/7 P. E./benzene 13.0 1.53 30,000 P. E./beneene 9l/9 24.4 3.92 77,000 shows the effect of milling upon the benzene solubility and 90/10 P. E./benzene 14.6 4.85 95,000 viscosity of butadiene-styrene rubbers of varying solubilities 100 P.E . 7.8 0.42 8,000 in the unmilled condition. !,“,’9eT95/5 P. E./benzene 10.8 0.56 11,000 50’7 in91/9 P. E./benzene 16.1 1.23 24,000 All samples tested could be milled to complete solubility, solu%le 89/11 P. E./bencene 9.9 1.75 34 000 but the viscosity of the rubber after milling to complete solu88/14 P.E./benmne 5.0 3.74 73:500 bility is lowest for polymers of low original solubility. The C ( 9 7 non100 P. E. 7.7 0.49 9,500 rubper) 95/5 P. E./benzene 9.6 0.52 10,000 viscosity of rubbers originally 100 per cent soluble is likewise 9l/9 P. E./benzene 29.3 1.28 25,000 shown to be lowered by milling. Since any application of the 89/11 P. E./benzene 11.1 2.47 48,000 86.5/13.5 P. E./benzene 29.5 6.05 118,500 rubber must involve milling of the crumb or massed sheet 12.0 6.24 28/62 P. E./benaene 122.000 polymer, our test now measures solubility before and after 5 P. E. Petroleum ether. standardized milling and the viscosity after milling. ?
3
-
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POLYMER
-29.3
I
R
POLYMER C 11.1
INTERPRETATION OF TESTS. It has recently been pointed out by W. 0. Baker of the Bell Telephone Laboratories that the described method of determining polymer solubilities (which has been used in several laboratories) may include as “benzene soluble”, polymer which is colloidally and not molecularly dispersed. Regardless of the exact nature of the solutions obtained, the methods described have been useful in the evaluation of the effect of polymerization variables upon the quality of the rubber produced. They have been applied to the study of reaction temperature, catalyst, and modifier concentration, polymer yield, emulsifier type, and other variables of the reaction. Electrical Properties o f Synthetic Rubber
The electrical properties of synthetic rubbers can be fairly well anticipated from their chemical constitution and the known principles and theories of dielectrics (6). I n general, hydrocarbons show low dielectric constants and losses, with electrical properties insensitive t o temperature. In accordance with this, butadiene-styrene rubbers, polyisobutylene, and Butyl rubber have excellent properties as electrical insulators. Any electric power absorption observed for these uncompounded rubbers can almost certainly be attributed t o traces of impurities or moisture absorption due to impurities. To this extent there is a field of application here for synthetic rubber in which it is not fundamentally a t any disadvantage to natural rubber. For compounded stocks of these hydrocarbon rubbers, the electrical properties will be almost entirely determined by the ingredients which it is found necessary to add to secure desirable physical properties. Since these hydrocarbon rubbers are so inactive electrically, a t least for frequencies up t o a few megacycles, electrical measurements on them require the highest degree of precision, are influenced by traces of im-
purities, and in this range afford little insight into the structure. The electrical properties of butadiene-acrylonitrile rubbers are less interesting from a technical standpoint because they are so poor. They have a considerable degree of scientific interest because they can be used to study the molecular structure of the polymer in respect to the movement of segments of the chain molecules under the action of an alternating electric field. For the butadiene-acrylonitrile rubbers, there are polar groups attached to the long-chain molecules. These attempt to align themselves to an applied electric field, so that, in case of an alternating field, molecular rotations occur and power is absorbed due t o internal viscosity. In addition to this type of power absorption, due to dipole rotation, there are undoubtedly other losses due to mechanisms such as ionic conduction since the direct-current resistivities of the butadiene-acrylonitrile rubbers are relatively OW (24). Table IV gives typical results of electrical measurements on gum stocks of natural rubber, several hydrocarbon synthetic rubbers, and a nitrile rubber. The measurements were made in the Goodyear laboratories by R. B. Stambaugh, using General Radio audio- and radiofrequency bridges. Comparison of Synthetic Rubbers in Vibration
In a paper from our research laboratories, given before the Society of Automotive Engineers in Detroit in 1941, a method of determining the hysteresis and internal friction of rubber stocks was described. The application of this method to synthetic rubber has been extended to give the comparison between natural rubber and the synthetic rubbers which are under discussion in this paper. The measurement of synthetic rubbers when subjected to a vibratory driving force of known magnitude offers a convenient and accurate method of evaluating the stiffness and resilience for small deformations such as occur in many important applications. The technique has been described (8, 16, 17). Although general practice in testing the physical properties of synthetic rubber is t o w e a test formula containing gas black, it seemed worth while to determine whether vibration
TABLE11. COMPARISON OF SUCCESSIVE AND SIMULTANEOUS METHODS O F FRACTIONAL EXTRACTION OF BUTADIENE-STYRENE POLYMERS Calcd. Viscosity Mol. Wt.
< 12,000 12,000-30,000 30,00&60,000 60,000-S6,000 > 85.000
% ’ of Total Rubber Hydrocarbon Successive method Simultaneous method 17.2 20.3 26.8 13.0 24.4 14.6
30.8 15.9 24.6 10.9
10
20
30
!
I
40
50
60
% Relation in Dynamic Resilience between Channel Black and G u m Stocks
Dynamic Resilience of Channel Bldck Stock,
Figure 12.
6 X
Starting Ratio for Polymer Butadiene Acrylonitrile 50 70
50 30
Butadiene
Styrene
75
25
0 Natural Rubber
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TABLE111. EFFECTOF MILLINQ ON BENZENE SOLUBILITY AND CALCULATED VISCOSITYMOLECULAR WEIGHTOF BUTADIENEPOLYMERS STYRENE No Treatment Calcd. viscosity mol. wt. of ? e 2 2 sol. portion 50,000 100 47,000 100 20,000 67 20,000 55 19,000 59 19.000 37 15,000 31 14,000 43 20
sample
....
a
Milled 5 Min on Ti htSet 8-111.Lib. Milkat
400
80' F.
sol in "bnxdne 100 100 100 100 100 100 100 100 100
Calcd. viscosity mol. wt. 37,000 39,000 31,000 39,000 38,000 32,000 27,000 29,000 25,000'
I9
-415 61
m
5 8 300
-
821
30 310
286
-
Required 15 minutes to solubilize. .in
MEASUREMENTS TABLEIV. RESULTSOF ELE~CTRICAL 1-Kilocycle Frequency DieleoPower tric factor, constant % z.59 0.271 P.38 0,302 2.70 0.342 12.85 5.73
Type of Rubber Natural Butyl Butadiene-styrene Butadiene-acrylonitrile
8
301 /-I
0
'E
2
15
Eel?
I
5 0 7 40t
g ; p:
1-Megacycle Frequency DieleoPower tric factor, constant % 2.52 0.695 2.20 . . .. 2.49 1.02 11.0 25.9
_ _ _ _ _ _ - - - - - ___._--.-. ______.__--- _ _ _ - - ---*-
PO
/.----
IO
----
/-/-
L 30 90 Temperature,
' c.
Figure 13. Effect of Temperature on Dynamic Modulus and Dynamic Resilience of Various Channel Black Stocks Buta S Natural rubber
-B u t a N ---
--------_._._ Butyl
measurements on gum stocks might not give a more fundamental comparison of the elastic properties of the polymer itself. By systematic variation of the conditions during polymerization, a series of butadiene-acrylonitrile rubbers was prepared which showed a wide range in dynamic resilience. These were tested when compounded as gum stocks and as tread stocks. The results are shown in Figure 12. There is an essentially linear correlation between the resilience of the tread stocks and the gum stocks. The straight line shown was drawn between the origin and the point for natural rubber. The points, especially those for the 70/30 nitrile polymers and
Figure 14. Effect of Milling on Plasticity and Recover of Various Rubbers ( I O Kg. Weight, 70" C., 15-Minute Preleat)
Figure 15. Effect of Milling on Extrusion Characteristics of Various Rubbers at PO0 Pounds per Square Inch and 92" C. Left-hand bar of each pair, original; right-hand bar, milled.
the two styrene polymers, fall so near the straight line that for most purposes it can be assumed that the relative values for the gum stocks persist after the addition of gas black. Furthermore, it can be inferred that carbon black reinforcement affects the resilience of both natural and synthetic rubbers in the same w-ay and by essentially the same mechanism. The general rule appears to be that 40 parts of gas black in 100 parts of polymer cause a 40 per cent decrease in the resilience from the value for the gum stock. It is advantageous in comparing the properties of various synthetic rubbers to include measurements over a range of temperature since, in many cases, the properties change much more rapidly with temperature in the operating range than do those of natural rubber. Figure 13 shows the dynamic modulus for tread stocks of natural, Butyl, butadiene-styrene, and butadiene-acrylonitrile rubbers against the temperature for a range from room temperature to 90" C. The effect of temperature in this range on the dynamic modulus of natural rubber is much less than for the other rubbers. At lower temperatures the dynamic modulus for natural rubber would also begin to show a large temperature dependence. The curves illustrate a fundamental difference between these synthetic rubbers and natural rubber
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1000 150
800 100 600
50
400
eo0
150
100
50 w Y
._ P I
c
6
g 130
v) K J
:
Ec 1000 Y
L
110
3
E
rn
b
a
90
800
70 600
50 400
30 eo0
10
1000
I70
150 800
100
600
50
400
--.- --.!
