Inorganic Pigments in Natural and Synthetic Rubber d LEONARD H. CQHAN' AND RUSSELL SPIEL3ldW
Witco Chemical Company, 295 Madison Avenue, Netu Y o r k , N . Y . T h e behavior i n elastomers of calcium carbonate pigments, which have fairly symmetrical shape, the same crystal structure, and similar surface nature depends on their average particle diameter-that is, on their surface area-in a manner similar to that previously observed for carbon blacks. When different inorganic pigments are compared, factors other than surface area-particle shape, crystal structure, and surface nature-must be considered. For example, the unsymmetrical shape of clay leads to higher stiffening (modulus) than would be expected from its surface area alone. Fine particle size inorganic pigments such as Witcarb R (precipitated calcium carbonate), Silene (calcium silicate), and Witco No. 1 clay reinforce elastomers in a manner analogous to carbon blacks. In
Lhe case of Witcarb R, studies of rubber properties as a function of pigment loading showed marked increase in tear, modulus, and hardness for natural rubber and the principal synthetics. Tensile was considerably increased in GR-S and GR-A type synthetics and was increased slightly in natural rubber and GR-11. An attempt was made to interpret the data by considering the separate effects of varying pigment surface area and varying volume of dispersed phase, both of which aceompanj loading changes of a finely divided reinforcing pigment. Differences in behavior of natural rubber and GR-M on the one hand and GR-S and GR-A on the other ma? be explained in part by the tendency of the former to crystallize.
REVIOUS publications (1, 2. 4-8,18) from the \ T h o Chemical and Cont,inental Carbon laboratories have indicated that the behavior of carbon black in elastomers dcpends principally on the particle size (average diameter and diameter distribution of the spherical units observable in a n electron micrograph), surface nature, and crystal or internal structure of the carbon particles. Other investigators (13, 15) have pointed out that properties imparted by carbon blacks to rubber mixes can be accounted for on the basis of three fundamental properties, surface area, pH, and structure. Surface area and pH correspond roughly t o particle size and surface nature, respectively, while structure appears t o be related t o t,he shape of the particles. The surface area of smoot,h, spherical, nonporous particles is determined by particle size-that is, by average diamet'er and diameter distribution of the unit particles. (In the rare instances in which porous particles are dealt with, the effect of internal surface must be considered separately in connection with pore Recent work has diameter and size of elastomer molecules.) indicated that, even where t,he surface area and other properties of two blacks appear to be the same, the properties may differ, possibly because of variations in the shape of the particle size distribution curves ( l a ) or variations in the shape of the particles or chains. pH is determined by surface nature and often is a convenient. indicator of this property. In P O I cases, ~ however, pH is not sufficient to give the whole story of surface nature. Specific surface properties sometimes vary, as indicatcd by differences in adsorption per unit area as well as by rariations in rubber properties, without any corresponding change in pH occurring-as in a series of furnace blaclcs of varying degrees of graphitization. Structure is defined as the tendency of carbons to form chain net,works (15). Previously this behavior was thought to involve attractive forces between separatc particles and to be caused primarily by the crystal or internal structure and possibly by the surface nature as well (1). Alt,hough thcsc forces appear to be partially responsible for chain and agglomerate formation (14), work n-ith the stereoscopic electron microscope (17) and work on the effect of mechanical attrition on carbon particles (9) point t,oward structure as primarily a particle shape phenomenon.
JT-hat was thought to be iiidividual particles clumpcd togctlier by agglomerating forces now seems to the authors to be more logically interpreted in many cases as particlcs fused together into a single irregular secondary particle, in the form of either a chain or a cluster. These groups appear much like snowballs TT hich, having been subjected t o sufficient pressure to cause slight melting, freeze together on release of the pressure. They probably originate during the formation of the carbon particles in the gas flame. The extent of grom th of the chains and clumps dctermines the asymmeti y of the secondary carbon pal ticles; this, in turn, affects the properties of the product, especially modulus and plasticity which depend to a n impoitant, degree on particle shape. Thus, the properties that determine behavior of carbon pigments may be stated as:
1
Present address, Witco Technical Service Laboratory, 719 First ArTe.,
New York 17, N. Y . 1
Present address, Witoo Research Laboratory, 6200 West 6lst ll, Ill.
cago
St., Chi-
Particle size (diameter distribution of the spheres which may occur either as separate particles or as units of a secondary chain or cluster). Shape of the secondary chains or clusters (particle shape or sti ucture). Suiface nature (chemical constitution and arrangement of surface layers). Crystal or internal structure of the particles. For inorganic pigments, the same factors must bc considered ith the exception that shape differences are usually caused by crystal habit-that is, diffcrenccs in shape of the units themselves (needles, platelets, etc.) and not by fused sccondary particles Also, whercas differences in crystal or internal structure of commei cia1 carbon blacks are limited somen-hat in scope, the n-ide variation in chemical constitution of inoi ganic pigrncnts mag' make this factor moie important. TT
PARTICLE SIZE AND SHAPE
I n the electron micrographs in Figure 1, the paiticlr size and shape of m o carbon pigments may be compared with thosc of the inorganic pigments discussed in this paper, Avci age particle dimensions and external surface area calculated from these photographs are givm in Table I. As the densities of the pigments are different, surface area per unit volume as n.cll as surface area per gram is included. The latter area is important when the pigments are compared in rubber on a weight loading basis, the former when comparison is made on a volume basis.
2204
November 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
Witcarb R X 50,000
Witoo No. 1 Clay X 20,000
Ultrafine Precipitated Calcium Carhonate
Natural Hydrated Aluminum Silicate
Continental A (Witco No. 1) X 50,000
Continex S R F X 50,000
Channel Black M e d i u m Processing)
Furnace Black (Semireinforoing)
Silene X 50,000
W-itcarb R-12 X 20,000
Precipitated Hydrated Calcium Silicate
Precipitated Calcium Carbonate
2205
Figure 1
Particle diameter distribution curves obtained from the electron micrographs appear in Figure 2 where the frequency is plotted against the average diameter or, in the case of unsymmetrical particles, the average value of the largest linear dimension.
In Figure 3, the external surface area distribution curves are plotted for the same pigments. The area distribution curves and the total area are important in determining those properties which depend on surface, such as accelerator adsorption, resilience, and, possibly, tensile strength. Provided the specific surface is
2206
INDUSTRIAL AND ENGINEERING CHEMISTRY
Principal chemical constituent
Witco AA Whiting Natural CaCOs
Micronized Whiting Natural CaCOz
Witcarb R-12 Pptd. CaCOa
Crystal typeb
Calcite
Calcite
Calcite
Kaolinite
Csloite
Av. diam., mp
3900
1500
145c
Thjck 50; wide 170:
Kobo C
Surface aresc Sq. m.jg. Sq. m. jcc. Sludge p H 1% adsorption mg./g. Oil absorptiohd, % Color Specific gravity Apparent densitye, lb./cu. it.
0.55 1.5 9.1 5 10.6 White 2.73
1.4 4 8.6 10 14.9 White 2.73
13 35 9.1 10 32 White 2.68
17 49 4.7 9 33 Light cream 2.6
32 85 11.3 17 37 White 2.65
53
37
21
Vol. 40, No. 11
PROPERTIES OF PIGKEXTS TABLE I. PHYSICAL Witco No. 1 Clay Hydrated aluminum silicate
35
Witcarb R Ultra-fine wtd. CaCOI
MT Black Carbon
Silene" Pptd. hydrated calcium silicate
16
10
53 116 9.4 16 87 White 2.1
7.3 18 30 Gray 1.8
28
30
Quasigraphitic 74 c
Quasigraphitic 27 c
SRF
Quasigraphitic 2700
36 C
Carbon
Witco No.1 (Continental A) Carbon
Continex
18
YO 54 9.3 43 57 Black 1.8
96 173 4.1 278 81 Black 1.8
20-25/
20-251
This material, althoughin commercial use a t the time this work was begun, has been superseded b y a finer product, Silene EF. Determined from x-ray diffraction patterns. Determined from electron microscope photographs. d Spatula method. 6 Pour method. f Pellet form.
a
b
kno-m the particle size distribution curves (Figure 2) are useful as an indicator of particle asymmetry. For two pigments having the same largest dimension distribution curves, the one R ith the largest area must have the most asymmetrical particles and, other things being equal, would give higher modulus in rubber (10). For example, clay has a distribution curve in Figure 2 similar t o that for R-12 but an area per unit volume over 307' greater; this indicates greater asymmetry and, hence, a higher modulus The shape of the carbon black particle units is approximately spherical, the Witcarbs appear to be composed of tiny crystals which also are fairly symmetrical. Witco KO. 1 clay, as noted, is markedly unsymmetrical; the particle consists of thin platelets only about one fifth the thickness of the larger dimension of the flat side. The thickness of the clay platelets was estimated from a photograph of a mixture of clay and Silene; the density of the clay particles appeared slightly greater than the Silene particles Fhich had-an average diameter of 36 milli-
AIERAGE
9
Figure 2
.
E
2 +
PAR-ICLE
2
I
I
I
L-
i
Figure 4
/L
DnIMFTiR,
3
N A T U R e L RULISER
I
cR's
0
I
'i
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
November 1948 INORGANIC P I G M E N T S SURFACE A R E A
CARBON
PlGMENTS
INORGANIC
SO M K R CC
PIGMENTS
C A R B O N
SURFACE A R E A ,
2207
PIGMLNI-I
regular agglomerates may be more important.
So M K R C
4000
2900
SURFACE N A T U R E AND C R Y S T A L
STRUCTURE
0
MOOULUS
d
"
$00
-
I
709
s? 600
ULTIMATE ELONGATION
2400
-
1800
I
r-
Figure 5 INORGANIC P I G M E N T S SURFACE AREA,
C4RION
'p
PIGMENTS
Sa M PER CC
Some attempt has been made to indicate the surface nature by A A T 3004 means of adsorption and pH data in Table GR-M X HYCAR I. The crystal or inOR-I A ternal structure is roughly indicated also I by the chemical conMODULUS AT I00 % stitution and crystal type. Much work remains to be done in determining the effect of these factors. At present i t is known only that the surface and the crystal structure of Figure 7 a calcite particle are different from these of PIGMENT s ' c A R ON P I G MCNT s a hydrated calcium SUR,FACE A + E Y So M,PER G7 silicate and that two pigments of these materials cannot be"-expected to behave: in exactly the same manner in elastomers even when their particle size and shape are identical. Surface nature determines both the binding pr adsorptive forces between the pigment particles and the elastomer molecules and the cohesive forces which hold together pigment agglomerates. The pigmentelastomer binding determines the amount of rubber immobilized on the surface of the pigment-hence, the Figure 8 * ratio of volume of pigment to freerubber. Since the volume ratio is one of the important factors determining modulus, plaaticity, hardness, etc., these properties are dependent on the surface as well as on the particle shape. On the other hand, the adhesive forces holding agglomerates together will determine the degree of dispersion obtained under given mixing conditions. The effective particle size of the pigment, therefore, will depend also on surface nature as a result of the influence of the latter on diepersion. Thus, properties, such as tensile strength and resilience, which depend on particle size, in turn, are dependent to some extent on the surface. Variability of surface nature and ease of dispcrsibility in a given elastomer formulation may be responsible for results, such as those obtained with Silene, where the tensile strength (Figure 7) is much lower than would be expected from its fine particle size.
,
INORGANIC
I
N
Figure 6
microns. These dimensions for clay are supported by x-ray data (11). The effect of particle asymmetry shows up clearly in the properties imparted to the various rubbers by clay; the modulus of the vulcanizates (Figures 5 and 7)is abnormally high except in the curve for GR-I where 300% elongation is too low for accurate modulus measurement. At greater elongations, the modulus of clay stocks is also above normal for GR-I. In Figure 1, Silene appears to be ellipsoidal, with a ratio of major to minor axis of about 1.5. It is possible that this slight asymmetry may be due to the adjustment of the microscope. In any casc, Silene i s much more symmetrical than clay. The apparent slight asymmetry of Silene probably contributes to its high modulus in some elastomers but other factors such as wrface nature, crystal structure, and tendency t o form stable ir-
, ,9
2208
INDUSTRIAL AND ENGINEERING CHEMISTRY
u I
z
6
Figure 10
WITCARB LOADINO,
Figure 11
P A R T S BY WEIGHT
Figure 12
Vol. 40, No. I1 velop maximuin propertks.) A similar trend in tensile and other properties carries over cvcn to groups including both furnace and channrl blacks provided the shape of the secondary aggregates, surface naturc, and crystal structure do not vary too widely. Calcium carbonates have a n advantage over carbon blacks in the study of the effect of particle size 011 rubber properties, inasmuch as other differences in t,hc samples can be eliminated more readily. X-ray data show that all of the calcium carbonates (Figure 4) arc reasonably pure calcitesthat is, they all have the same crystal structure. Electron microscope picturcs indicate that, the particlcs are singlc, fairly symmetrical crystals, so that differences in particle shape are unimportant. Surface nature probably varies slightly beca.use of variations in impurities, such as free limc, but as the particles are 98 to 99% calcite, these differences probably are secondary. Thus, the important difference in the calcium carbonates plotted in Figure 4 is in average particle size and the results show clearly that tensilr and tear increase arid abrasicii loss decrease Jvith decreasing size. The fact that, t h t w t,rcnds arP the samf: as for differcnl cnhanncl blacks (6, 8 ) indi-
pi,rviably in surface nature or crystal struct>ure. The shape of the secondary particles should also be similar. Otherwise, irregular variation in modulus might be expected when this property is plottcd ayainst partirk: size.
EFFECT OF PARTICLE SIZE OF CALCIUhI CARBONATE PIGMENTS ON ELASTOMER PROPERTIES
COMPARISOS OF CALCIUM CARBOIiATE A N D OTHER PIGMENTS
-4considerable amount of work (1-8, 16, f7) has shown that, for carbon blacks of similar surface nature, crystal structure, and secondary particle shape, properties in various rubbers follow a regular t,rend when plotted against particle size. For example, the references cited show that, in a series of channel blacks which are similar except for differences in particle size, tensile strength and resistance to tern both increase whereas abrasion loss decreases as particle size decreases t o about, 20 to 25 mp (for still smaller size blacks dispersion and cure retardation both become problems so that special formulation usually is required to dc-
The physical properties of several inorganic and carbon pigments appear in Table I. I n Figures 5 through 8, the behavior of these pigments is shown as a function of their specific surface area in natural and synthetic rubbers. The forniulations in which the pigments wcre tcstcd are given in Table 11. The same code for identifying the various elastomers is used in all the graphs. Ignoring the values for clay and Silene which differ in surface nature, pmticle shape, and crystal structure, as well as in surface area, the trends for the calcium carbonate pigments in most cases are similar to those shonn by the carbon blacks.
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1948
2209
EFFECT OF lNCREASING LOADINGS OF WITCARB R
.
The conclusions concerning the effect of surface area and other fundamental properties of pigments on their behavior in elastomers were based on a comparison of different pigments a t the same volume or weight loading in various elastomers. It is of interest to consider also changes in propertiesof elastomers resultingfrom addition of increasing amounts of pigment. When the pigment loading is increased, two factors are changed aimultaneously. First, the pigment surface area per unit volume of elastomer increases; and secondly, the volume of inelastic dispersed phase also increases. The effect on the properties of the elastomer depends on the relative importance of these two factors. Figures 9 through 14 illustrate the dependence of the physical properties of natural and several synthetic rubbers on the loading of Witcarb R , a fincly divided precipitated calcium carbonate.
Figure 14
crystallizing polymers-GR-S and Hycar. The crystallites formed on stretching natural rubber and G R - N reinforce the gumstock but, in the case of loaded stocks, they aggravate WITC&RB LOADING, PARTS BY WEICU'I the volume dilution factor by increasing the ratio of inelastic t o elastic phase a t a given pigFigure 13 ment loading. This may be one reason why the maximum in the tensile against loading curve TENSILE, TEAR,AND ELONGATION.Figure 4 shows that, for occurs a t lower loadings for the crystallizing polymers. stocks containing a fixed loading of calcium carbonate pigment, tensile strength increases as surface area increases. It might These considerations regarding tensile apply equally well to have been expected that the influence of volume dilution alone the tear results shown in Figure 10 except that here crystallizawould be to reduce tensile strength as the amount of the elastic tion probably is less important (because of smaller elongations phase which binds the final compound together is decreased. produced in the tear test) and the degree of improvement obResults obtained with varying loadings of relatively coarse fillers such as ground whitings (where surface area is small, even for high tained in all the elastomers is more nearly the same. Despite loadings) appear t o confirm this. As a result of these opposing the absence of marked crystallization during this test the maxima tendencies, it would appear that on addition of a high specific in the tear curves for natural and GR-iLI still occur a t lower area reinforcing pigment, such as Witcarb R, tensile would inloadings than for GR-S and Hycar which apparently have not crease at volume loadings which would be moderate compared to the elastomer volume; reach a maximum; and then fall off as reached their maxima at the highest loadings studied. This bethe volume dilution effect became predominant. This effect is havior indicates that other factors besides crystallization must shown in Figure 9 except for GR-I; even in GR-I it is possible be considered before a complete explanation can be given of the that some increase in tensile over the gum stock might be found if relative tear resistance of the different polymers. loadings belon- 25 part? by weight had been studied. The absence of a maximum or its displacement to very low loadings may Corresponding to the large increase in tensile of Hycar and be associated with the low unsaturation of GR-I; possibly the GR-S there also is an increase in ultimate elonyation (Figure 11). double bonds form the points at which the polymer chains are GR-M and rubber maintain good elongation, a t least a t low anchored t o the pigment particles. Natural rubber and loadings, whereas the elongation of GR-I decreases sharply GR-M do not show as marked a n increase in tensile as the nonfrom the beginning.
Type of elastomer Chemical composition Polymer as received Zinc oxide Light calcined magnesia Pine tar Hard paracumarone indene resin Stearite Stearite A Sulfur
Natural Rubber (Smoked Sheet) Natural polyisoprene Ab BO 100 100
5
... 2
5 ... 2
'3"
...
i:75
2.75
... ...
... ...
1.25
1.26
...
...
...
...
GR-S (Buna S) Butadienestyrene copolymer 100 5
... 30 ' 1
...
.
3 1 1.5
...
...
GR-M (Neoprene) Transpolychloroprene
Ab
BC
100
100 3
5 4
...
10 0.5
... ...
... . . I
. .
... I
.
.
4
...
10 4 ...
...
GR-I (Butyl) Isobutyleneisoprene copolymer
100 5
... ... ... ... 1 2
... ... ...
...
...
...
..,
. .
1 0.5
GR-Aa (Hycar OR-15) Butadieneacrylonitrile copolymer
100 5
30' 30 ,..
2 0.5 0.5
...
40 vol. 30 vol. 100 by wt. 100 by wt. 100 by wt. Other tests using Chemigum and Perbunan (18) in the same formula gave results similar to those shown for Hycar OR-15 in Figures 7 through 13. b A = formula used in increased loading tests (Figures 9 through 14). 0 B = formula used in pigment comparison tests (Figures 5 through 8 ) . a
MODULUS, HARDNESS, PLASREBOUNC. From the values for calcium carbonate pigments (AA whiting, micronized whiting, Witcarb R12, and Witcarb R ) in Figures 5 through 8, it is evident that modulus, hardness, and plasticity either increase slightly or are practically constant as surface area increases. Theoretical considerations and also data on the behavior of coarse fillers indicate t h a t these three properties should also increase with the volume of dispersed phase, a t least in the loading range covered in this paper. Here, both factors, surface area and volume of dispersed phase, are working in the same direction and steadily increasing values for all elastomers can be expected. The data in Figures i 2 , 1-3, and 14 show -that
TICITY, A N D
TABLE 11. FORMULAS FOR PIGMENT TESTS
2210
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
modulus, hardness, and plasticity actually do behave in this way. Rebound (Figure 14) behaves similarly except that there is a regular decrease with loading. Rebound also decreases with area (Figure 5 ) and with volume loading of coarse fillers. The foregoing results cover natural rubber and four types of synthetics (GR-S, GR-&I, GR-I, and Hycar). Tests on other GR-A type synthetic rubbers, such as Chemigum and Perbunan, give results similar to those shown for Hycar (19). ACKNOWLEDGMENT
The authors wish to express thcir thanks to the many members of the Witco Research and Technical Service Laboratories who
assisted in obtaining the data and preparing the figures; to their former associates C. Yoran and H. F. Schn-arz; t o P. L. Copeland, of the Illinois Institute, for taking the electron microscope photographs; and to L. R. Sperberg and W. R. Smith for their helpful suggestions and review of the manuscript. LITERATURE CITED
(1) Cohan, L. H., Chem. Eng.News, 23,2078-85 (1945). (2) Colian, L. H., and Mackey, J. F., IND. ENG.CHEaS., 35, 806-8
(1943). (3) Cohan, L. H., and Mackey, J. F., I n d i a Rubber W o r l d , 107, 469-71 (1943). (4) Cohan, L. H.. and Illackey, J. F., Rubber Age, 55, 583-5 (1944).
Vol. 40, No. 11
(5) Cohan, L. H., and Steinberg, hl., IXD. ENG.CHEM.,36, 715 (1944). (6) Cohan, L. H., and Steinberg, M., Rubber Age and Synthetics (London), 25,275-8 (1945). (7) Continental Carbon Go., “Conductivity of Natural and Synthetic Rubber Channel Black Stocks,” Rept. C 1, New York. June 1,1943. (8) Continental Carbon Co., “Physical Properties and Behavior in Rubber of Continental Carbon Blacks,” Rept. R 1, New Pork. 1943. (9) Dobbin; R. E., and Rossman, R. P., IXD.ENG.CHEai., 38 1145-58 (1946). (10) Guth, E. J., J . Applied Phys., 16, 20-5 (1945). (11) Hendrick, 9 . B., ”Structure and Base Exchange Properties of Clays,” presented before Catalysis Conference, Am. -4ssoc A d v . Sei., Gibson Island, Md.; June 1945. (12) Smit.h, W, R., “Carbon Black Behavior,” presented before the High Polyiner Conference Am. Assoc. Adv. Sci., Gibson Island, Md., June 1945. (13) Sweitzer, C. W., and Goodrich, W. C., Rubber Age, 55, 469-78 (1914). (14) Watson, J. H. L., J . Applied Phys., 17, 121-37 (1946). (15) Wiegand, 7%’. B., Can. Chem. and Process I n d . , 28, 151-62, 213. (1944). (16) Wiegand, W. B., India Rubber W o r l d , 105, 2%-2 (1911). (17) Wiegand, W. B., and Ladd, W. A., Rubber A g e , 50, 431-6 (1942). (18) Witco Chemical Co., Carbon Black Manual, New York, 1946. (19) T;Vitco Chemical C o . , New York, Witcarb R, B u l l . 45-2 (1945): 46-2 (1946). RECEIVED January 16,1947.
renew d
J . M . WILLIS, L. B. WAKEFIELD, R. H. POIRIER,
AI\’D E. The Firestone Tire 6% Rubber Company, Akron, Ohio
RI. GLYRIPH
A series of isoprene-styrene copolymers varying in monomer ratio from l O O / 0 to 60/40 has been prepared in the GR-S recipe. Physical tests in a tread recipe show superiority for these polymers over GR-S in efficiency. A number of activated recipes have been applied to the 75/25 isoprene-styrene ratio at 50 O C. The use of ferricyanide or acrylonitrile activation offer the most immediate possibilities for plant usage because of the small deviation from conditions for w-hich GR-S plants were designed. Little
difference in physical properties was noted for polyisoprenes prepared in a redox system at temperatures ranging from 40 O to 10 C. Isoprene-styrene (75/25) copolymers with varying gel characteristics were prepared in a GR-S system. As the gel varied from low to high temporary to high permanent, the efficiency increased at the expense of cut-growth resistance. Tests o n polymers prepared in the pilot plant have substantiated results obtained on polymers prepared in the laboratory.
0
and study a series of isoprene-styrene copolymers with a fairiv wide range of monomer ratios. Earlier work on the substitution of isoprene for butadiene in GR-S indicated that much slover polymerization rates are obtained with the grades of isoprene available. It has beeD found that, in some cases, even sulfone-purified isoprene givea slower rates than butadiene. Therefore, various methods of activation have been evaluated as means of obtaining standard timc cycles for plant use. In the plant preparation of isoprene analogs of GR-S, it was observed that the modificr charge could be reduced to give a polymer with a Mooney plasticity of 65 (containing gel, however) a-ithout affectini processibility. This lower modifier requirement has bee,i evident throughout the isoprene development. It was necessary, therefore, that attention be directed toward gel-containing isoprene rubbers with particular referexe to physical properties. This paper describes the preparation and properties of polymers made with varying monomer ratioe (ixluding polyisoprene), various activated charges, and a range of gel contents.
NE of the most serious shortcomings of GR-S when used in tires has been its low efficiency as evidenced by a much higher running temperature than natural rubber (4,6‘, 7 ) . This is a particular disadvantage in truck tires where thick sections prevent dissipation of the heat. Early workers in the field of synthetic rubber found t h a t isoprene-containing polymers did not have this inherent disadvantage (1). This is consistent with the properties of natural rubber wherein the units of the chain are of the isoprene type (3, 6, 8). One aspect of the work which was undertaken at Firestone involved testing isoprene as a butadiene replacement in an effort to arrive a t a definite conclusion on the relative merits of isoprene-styrene and butadiene-styrene copolymers. TWOstandard experimental types were taken as the basis of this work, X-141 being a 75/25 isoprene-styrene rubber prepared in a dehydrogenated-rosin soap system and X-46 being the same rubber prepared with fatty acid soap. Since it was possible t h a t a 75/25 monomer ratio would not give the best balance of properties, particularly if the end use were in a special purpose compound, it seemed of interest t o prepare
