2222
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
strength of rubbers and latex films. The present practice of expressing the results on an ultimate tensile basis is totally misleading and tends to give the impression that compounding and vulcanization of Type I11 latices, at least, leads to reinforcement, when no reinforcement may actually take place. Since the actual tensile of compounded and uncompounded latex films was found to be the same in this study, i t must follow also, if there is no difference in chain packing in the two types of films, that the same links, and the same number of links per unit mea, are broken in both compounded and uncompounded films. If the breaking of carbon-to-carbon bonds is involved in film rupture, these facts must mean that the same bonds are broken In both films, and that sulfur cross linking on vulcanization does not contribute to polymer strength but only to its stiffening as reflected in extensibility. Another conclusion apparent from the data is t h a t Type 111 latices may differ widely in the strength of the polymer films. If the very low result for sample FS-13 is omitted, the spreads between maximum and minimum values amount to a factor of 3.6 in Tu, of 2.8 in Tc, and of 2.9 in S. The highest value of X observed, 20,700 pounds per square inch, compares not unfavorably Kith the actual tensiles of natural rubber vulcanizates, which range from about 25,000 to 35,000 pounds per square inch. tf the influences responsible for film tensile variation in Type TTT latex could be ascertained, and if these could be controlled
Vol. 40, No. 11
so as to yield consistently the high tensiles occasionally observed with films of this latex, we would have a polymer approaching fairly closely natural rubber latex films in strength. ACKNOWLEDGMENT
This study was sponsored by the Office of Rubber Reserve, Reconstruction Finance Corporation, as part of tho Government Synthetic Rubber Program. LITERATURE CITED
Blake, J . T., IND. ENQ.CHEM.,20, 1084 (1928). (2) Eaton, B. J., and Grantham, J., J. SOC. Chem. I n d . , 34, 989 (1915). ( 3 ) Greidor, H. W., J. IND. EM. CHEM.,14, 385 (1922); 15, 604 (1)
(1923).
(4)Maron, S.H., and Madow, B., IND. ENQ.CHEW,ANAL.ED.,20 545 (1948).
( 5 ) Schippel, H. F., J. IND. ENQ.CHE;~.,12, 33 (1920). (6) Shepard. N. A., in Alexander’s Colloid Chemistry,” Vol. I V , pp. 312-16, New York, Reinhold Pub. Gorp., 1932. (7) Shepard, N. A , , Street, J. N., and Park, C. R., in Davis and Blake’s “Chemistry and Technology of Rubber.” ChaD. XI. Kew York, Reinhold Pub. Gorp., 1937. (8) Stevens, H. P., J . SOC.Chem. I n d . , 35, 874 (1916) (9) Wiegand, W. B., Can. C h e m J.,4,160 (1920). RECEIVED October 3,1947. PreRented before the Division of Rubber Chem. istry a t the 112th Meeting of the AMERICANCHEMICAL SOCIETY, New York, N. Y.
FRITZ S. ROSTLER AND ICATHLEEN S. ROSTLER Golden Bear Oil Company, Oildale, Calif.
T
HE advantages of using premixed preparations of fillers and plasticizers in compounding rubber have been discussed (8). These advantages are, primarily, a saving in milling time, even power consumption during milling, elimination of dust in handling dry fillers, and elimination of the inconveniences involved in handling highly viscous plasticizers. In this previous study it Ras found that a fixed ratio of filler to plasticizer exists for each filler-plasticizer pair limiting the amount of plasticizer vhich can be premixed with a filler without impairing dispersion of the filler in a rubber mix. When a premixed preparation was used having plasticizer proportion below this limit, the resulting rubber compound had the filler dispersed at least as well as when the ingredients were incorporated separately, as evidenced by the physical properties and by microscopic observation of the vulcanized compound. If, on the other hand, this plasticizer limit was exceeded, dispersion mas poor and the physical properties of the vulcanized compound were lower than those obtained by separate incorporation of the two ingredients, and microscopic observation of the torn surface of the compound showed filler present in lumps instead of evenly dispersed. An attempt was made to relate the limit of plasticizer content of the premixed preparation t o the relative effect of the filler and the plasticizer on the plasticity of the masticated rubber ( 2 ) . tt was found t h a t in a number of cases the limiting amount of plasticizer coincided with the mixture in which the softening effect of the plasticizer was equivalent to the stiffening effect of the filler. I n other words, the concentration of plasticizer in the premixed preparation had to be low enough so that the plasticity of the rubber mix either remained constant or decreased when the premixed preparation was incorporated into rubber. A plas-
ticizer concentration high enough so that the premixed preparation had a softening effect on the rubber gave poor dispersion. The proportion of plasticizer t o filler in which softening effect of plasticizer is equivalent to stiffening effect of filler was called the “constant plasticity ratio,” for when incorporated into masticated rubber in any amount, the plasticity of t h e rubber matrix remains constant. With carbon blacks this general rule t h a t the constant plasticity mixture was the limit of plasticizer for good dispersion was found t o apply approximately t o the thermal blacks and to the furnace blacks; hon-ever, there were two d+ cided exceptions to the rule. With the structure blacks, lampblack, and acetylene black, good dispersion of the filler could be achieved by incorporating i t in the form of premixed preparations containing proportions of plasticizer high enough t o soften the batch considerably. With the channel blacks, the limit t o the amount of plasticizer which could be successfully premixed with the black was much lower than would be expected, so that for successful filler dispersion a premixed preparation had to be used with considerable stiffening effect ( 2 ) . SCOPE OF INVESTIGATION
Although the previous study had shown that the effect of 8 premixed plasticizer-filler preparation on the plasticity of the rubber mix was of importance, it was obvious t h a t other factors must enter into the picture. The purpose of the present study was t o find some rule applicable to all carbon blacks correlating other specific properties with the proportions of black and plasticizer which could be premixed for incorporation into rubber. I n order to arrive at the correlation sought in the form of a quantitative expression, the previously measured data were
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1948
T h i s paper presents the results of a continuation of a previously reported study dealing with premixing plasticizer and filler before incorporation into rubber. It was €ound in this earlier investigation that premixing of powdered fillers and liquid plasticizers before incorporation into rubber can successfully be accomplished in a number of cases only if the ratio of filler to plasticizer is such that the premixed preparation does not increase the plasticity of the rubber mix over the plasticity of the masticated rubber itself, This rule holds true for all fillers except those of very fine particle size (channel blacks) and blacks known as structure blacks. The reasons for these deviations from the rule were the object of the present investigation. The influence of particle size and affinity of black for plasticizer was investigated as factors influencing dispersibility, in addition to the relative stiffening effect of the fillers. Re(ating experimentally measurable data expressive of these properties to dispersibility of premixed preparations led to a mathematical formula which is applicable to all fillers, including channel blacks and structure blacks. Considerations and experiments on which this quantitative formula I s based are reported and experimental proof is presented for the validity of the expression.
critically reviewed and the possibilities investigated of using experimentally measurable data expressive of the specific properties of the blacks logically bearing on black dispersion. LMPORTANCE OF AFFINITY OF BLACK FOR PLASTICIZER
It is certainly not without significance t h a t all the mixtures of black and plasticizer which could be successfully incorporated were dry powders. In no case was i t possible t o disperse successfully a mixture which contained enough plasticizer t o be a paste. I t can be assumed from this fact t h a t it is necessary i n order to effect dispersion t o have some black present which is not coated with plasticizer as i t is in a paste and functions as a stiffening agent of the rubber matrix, thereby increasing the grinding power. This black which is not coated with plasticizer is here called the “free black” in t h e mixture. The amount of free black in a filler-plasticizer mixture can conveniently be determined by the indirect method of measuring the &mount of the plasticizer required t o give a stiff paste with the particular black. The free black in a premixed filler-plasticizer preparation will then be the amount of black over this proportion. Mathematically expressed, the proportion of the total black present as free black in any mixture of plasticizer and filler is 1
- xa
necessary t o effect dispersion of a black-plasticizer mixture will vary from one black t o another. Two factors should come into play in this connection: first, the difference between blacks in ease of dispersion, and, secondly, t h e effectiveness of the rubber matrix as a grinding agent. Blacks differ in the ease with which they can be dispersed in a rubber mix in t h a t the blacks with larger surface area (small particle size), other things being equal, are more difficult to dieperse than those of smaller area (large particle size). Assuming that dispersion depends on the free black in t h e filler-plasticizer mixture, blacks of larger surface area should need more frrr black t o effect dispersion. The effectiveness of the rubber matrix as a grinding agent for dispersing t h e black-plasticizer mixture depends on its stiffness The stiffness of the mix is in turn dependent on both t h e stiffening effect of the particular black in the black-plasticizer mixture and the softening effect of the particular plasticizer. The plasticity of the rubber mix can be used as a n experimentally measurable figure for the stiffness or grinding power of the matrix. The comparative stiffening effect of the blacks can be expressed as the amount of black per unit amount of rubber required t o lower the plasticity of a masticated rubber a given amount; the smaller this figure, the greater the stiffening effect The comparative softening effect of plasticizers @anbe expressed in the same way as the amount of plasticizer per unit amount of rubber required t o increase the plasticity the same amount. T o summarize, the minimum amount of free black necessarj to effect dispersion of a black-plasticizer mixture should vary directly with the surface area of the black, inversely with the stiffening effect of the black (or directly with the amount of the black necessary to reach a given stiffness), and directly with the softening effect of the plasticizer (or inversely with the amount of plasticizer t o reach a given softness). MATHEMATICAL EXPRESSION FOR PLASTICIZER LIMIT
The foregoing theoretical considerations can be expressed mathematically as follows:
or a
A
K S Cj
C,
a = weight of plasticizer per unit weight of black in the mixture A = weight of plasticizer per unit weight of black required t o form a stiff paste T o illustrate the calculation of t h e free black by a hypothet>ical example, if a mixture contains 0.20 gram of plasticizer per gram black, and 0.40 gram of the plasticizer is re uired t o form a stiff paste with the black, the proportion of free%lack in the mixture will be 1, 0.40 :0 20 or 1 0.50
-
meaning t h a t one half of the black in the mixture can be classed
as free black, and can be pictured as not being coated with plasticizer. RELATION O F F R E E BLACK TO SURFACE AREA AND STIFFENING EFFECT
It can be expected t h a t the degree of stiffness of the rubber matrix and, consequently, the minimum amount of free black
2223
a = ~ ( l -kSC/ - T )
= weight of plasticizer per unit weight of black in a premixed
preparation containing the highest amount of plasticizer which will disperse successfully = weight of plasticizer per unit weight of black required to form a stiff paste = a constant = surface area of the black per unit weight = parts by weight of black per unit weight of masticated rubber t o lower the plasticit,y a given amount = parts by weight of plasticizer per unit weight of masticated rubber t o raise the plasticity a given amount
If the assumptions on which this expression is based are correct, this quantitative expression should fit the experimental data measured on all blacks. The numerical value of the constant in this equation was determined by substituting the variable quantities for one blackplasticizer combination on which the limit of plasticizer allowable for successful dispersion had been determined very closely by numerous compounding tests. The limiting amount of plasticizer was calculated from the equation for mixtures of nine other blacks with this plasticizer, and for mixtures of two other plasticizers with this black. I n order to determine the validity of the equation, these calculated values were compared with compounding tests t o determine whether the calculated values fell between the composition of a premixed preparation which would disperse successfully and one which would not; the criterion for successful dispersion being
INDUSTRIAL AND ENGINEERING CHEMISTRY
2224
TABLE I. CALCULATION O F PLASTICIZER LIXIT I N
PREMIXED PREPARATIONS O F VARIOUS
EXTENDER
BLACKS WITH UNSaTURATED HYDROC.4RBON Plasticizer Required t o Raise Plastisity 0.100 Inch, G. Plasticizer per G . GR-S, C p 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22
Black Required
Type of Black MT
Brand Name of Black Thermax P-33 Pelletex Ststex B Statex K Standard Micronex
ET
SRF FF RF MPC
LB
Oil Absorption, G. Plasticizer per G. Black, A 0.31 0.46 0.83 0.97 1.14 1.31
HMF RF(H .
CF
t o Lower Plasticity
Surfme Area
0.100 Inch, G. Black per G. GR-S, Cj 2.10 1.00 0.60 0.425 0.37 0.30 0.45 0.35 0.36 0.28
of Black
b., S
Sq. M. per
9.8 20.5 32 .O 62.3 80.0 105.0 23.0 42.5 78.6 65.0
Vol. 40, No. 11
Calcd. Proportion Plasticizer Permissible in Mixture, G. Plasticizer per G. Black, a 0.15 0.22 0.425 0.32 0.28 0.26 1.05 0.93 0.51 1.55
on the porcelain plat,e at room teniperature designated in the A.S.T.X. method. The plast,icity measurements used in construct,ing Figures 1 and 2, from which values for C; and C , were read, were made on a Scot>t parallel plate plastometer wit,h both plat,ens flat. A temperature of 100" C. was used in the platens, and a weight, of 15 pounds on the spindle. The test specimen had original dimensions of approximately 17/~6inches in diamet'er (3.65 em.) and 0.6 inch high (1.5 mi.) and was compressed initially to 0.600 inch before st'art'ing the test. Measurements are the compression in 0.001 inch during a 2.5-minute compression period. The value of C J was arbitrarily selected t,o be the parts by weight' of black per unit weight of GR-S necessary to give a compression value of 0.100 inch lower than that of the masticated GR-S itself; that of C,, to be the parts by weight of plasticizer per unit weight of GR-S giving a compression value 0.100 inch higher than that oT t,he masticated GR-S.
200
41 /
DETERMINATION OF VALUE OF CONSTANT
WEIGHT PER UNIT WEIGHT OF GR-S
Figure 1.
Effect of Plasticizer on Plasticity
that a compound made with the premixed preparation of black and plasticizer gave a t least a5 good tensile and elongation a t break as a parallel mixture made by incorporating the two ingredients separately. Values for A were determined experimentally, and expressed as grams of plasticizer per gram of black. Values for C, and C, were read from the curves in Figures 1 and 2. These curves were drawn from experimental data. Values for surface area S in square meters per gram on all the blacks cxcept Philblack 0 were furnished by W. C. Goodrich of the Coluinbian Carbon Research Laboratories, and had been determined in thosp laboratories by the rub-out color test in mineral oil. The surface area value for Philblack 0 was furnished by the Rubber Chemicals Division of Phillips Petroleum Company, and had been measured by nitrogen adsorption.
The numerical value of the constant, L, was determined from experimental data on mixtures of an S R F black (Pelletex) with a inediuin viscous unsaturated hydrocarbon extender. (Naftolm MY, a product of Wilmington Chemical Corporation, was used as thc unsaturated hydrocarbon extender in the earlier part of the investigation, and Califlux G.P., a product of Golden Rear Oil Company, in the later part since i t was more readily available in the authors' laboratory. The two products are identical in roniposition and physical properties and are interchangeable.) According to repeated compounding tests a mixture of these two componeiits containing 0.37 gram of plasticizer per gram of black (274;. plasticizer, 73cI, black) gave satisfactory dispersion,
EXPERIMENTAL METHODS
The values for A , the number of grams of plasticizer required to form a stiff paste with 1 gram of the black, were determined by a modification of the American Societp for Trsting Materials' method for the determination of stiff paste oil absorption of pignients (I). Since the plasticizers used in the test are for the most part heavy viscous materials at room temperature, rather than liquid-like linseed oil for which the method is designed, the mixing was carried out on the polished stainless steel top of a hot platc heated with steam of no pressure, instead of
I 0.50
I 1.00
WEIGHT PER UNIT WEIGHT
Figure 2.
I 1.50
OF GR-S
Effect of Carbon Blacks on Plasticity
2.00
November 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
while a mixture containing 0.48 gram of plasticizer per gram of black (32.5y0plasticizer, 67.5% black) dispersed poorly and gave a compound having physical properties lower than those which could be achieved by incorporating the black and the plasticizer separately. I n order t o calculate the value of the constant, a value of 0.425 gram of plasticizer per gram of black, intermediate between the two, was used as limit of plasticizer content, a. It was found by test t h a t to form a stiff paste, 0.83 gram of the plasticizer is required per gram of the black, A . Reading from Figure 1 i t can be seen t h a t to lower the plasticity of GR-S the specified amount of 0.100 inch, 0.60 part by weight of S R F from Figure 2 i t black per unit weight of GR-S is required, C;, can be seen t h a t to raise the plasticity of GR-S the specified amount of 0.100 inch, 0.22 part by weight of medium viscous unsaturated hydrocarbon extender per unit weight of GR-S is required, C,. The surface area of this S R F black '4 32.0 square meters per gram. Substituting these values in the equation we have:
I---
:::
or
-k
2225
same amounts of black and plasticizer were incorporated scparately. Comparison of the tensile properties of the two compounds was used as a n indication of whether or not dispersion had been achieved. With lampblack and acetylene black the limit of plasticizer is very high, so t h a t any GR-S compound made with plasticizer t o filler proportion higher than the calculated plasticizer limit would have physical properties in such a low range t h a t any comparison between the test compound and the control would be meaningless. With acetylene black and lampblack, therefore, only the fact that a premixed preparation of lower plasticizer content than the calculated limit gives a good mixture was established. Results of the compounding tests are shown in Table 11. The test formula used for these tests, in parts by weight, was '
GR-S Plasticizer Filler Stearic acid Zinc oxide Benzothiazyl disulfide Diphenylguanidine Sulfur
X 32.0 0.60
100 .oo -4s indicated 48 indicated 0.50 5.00 1.60 0.2.5 Varied according to plasticizer content to give 1.80 on GR-S plus 3 . O O on plasticizer
~
0.22
k = 0.0056
This value for the constant is, of course, dependent on the units in which the variables are expressed. If any of the variables were expressed in units other than those used above, the value of the constant would have to be changed accordingly. TESTING VALIDITY OF THE FORMULA
Using the value of 0.0056 calculated for k , the values for a (highest proportion of plasticizer permissible in a premixed prcparation t o give good dispersion) were calculated for mixtures of the plasticizer-extender with nine other blacks. The blacks were chosen to include representatives of blacks of widely different particle size and structural characteristics. Compounding tests were made with each black t o verify the calculated value. Premixed preparations were made with each black having plasticizer proportions below and above the calculated limit. These preparations were then tested in a GR-S test compound together with a parallel mixture in which the
This is the same test formula used in the earlier study ( 2 ) . The results of the compounding tests in Table I1 show the validity of t h e formula. Where a premixed preparation was tested having lower plasticizer content than the calculated limit, tensile strength of the compound is substantially the same as t h a t of the compound in which black and plasticizer were incorporated separately. Where the calculated plasticizer limit was exceeded in the premixing of filler and plasticizer, the tensile strength of the resulting compound was noticeably lower than t h a t of the control. T h e only exception is in the case of Statex K, where the calculated limit was 0.28 gram of plast'icizer per gram of black, while a mixture having a plasticizer content of 0.33 gram of plasticizer per gram of black apparently dispersed satisfactorily. Considering the degree of error possible even in carefully controlled measurements of the variables in the calculation, this amount of error in the calculated limit can certainly be expected. T h e validity of the formula was checked further as t o its applicability to other plasticizers. For this purpose, two other plasticizers were used, pine t,ar, which has a greater plasticizing
WITH UNSATURATED HYDROCARBON EXTENDER TABLE 11. COMPOUNDING TESTS ON PREMIXED PREPARATIONS OF VARIOUS BLACKS
Type of Black
MT FT
SRF
FF RF
MPC
Brand Name Thermax
P-33 Pelletex
Statex B
Statex K
Standard Micronex
Calculated Limit, G. Plasticizer per G. Black
0.15 0.22 0.425 0.32
0.28 0.26
G. Plasticizer per G. Black in Test Compound
Parts on 100 OR-S Black Plastioiser
0.09
92
8
0.18
85
15
0.18
85
15
0.43
70
30
0.37
73
27
0.48
67.5
32.5
0.26
45
11.25
0.43
45
19.3
0.33
75
25
0.49
67
33
0.18
85
15
0.33
75
25
LB
Lampblack
1.05
0.67
48
32
HMF
Philblack A
0.93
0.82
55
45
1.00
50
50
0.49
67
33
1.00
50
50
1.50
40
60
RF(HAF)
CF
Philblack 0
Shawinigan acetylene black
0.51
1.55
Filler and Plasticizer Incorporation Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Premixed Separate Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate Premixed Separate
Properties a t Optimum Cure Tensile a t Elongation break, Ib./sq. in. a t break, %
,
INDUSTRIAL AND ENGINEERING CHEMISTRY
2226 T4ELE
111. CALCULATIOSO F PLASTICIZER LIMITIN PREMIXED PRBPARdTIONS O F SRF BLACK (PELLETEX) WITH VARIOUS PLASTICIZERS
0.60 0.60
Plasticizer Required to Raise Plasticlty 0.100 Inch, G . Plasticizer per G. GR-9, C, 0.27 0.22
0.60
0.105
*
TYPF,of Plasticizer White oil Unsaturated hydrocarbon extender Pine tar
TABLE Iv. Type of Plastioizer White oil
Brand h-ame of Plasticizer Squibb mineral oil Naftolen M V
Oil Absorption, G. Plasticiaer per G. Black, A 0.68 0.83
Surface Area, of Black, Sq. M. per G., 8 32.0 32.0
0.95
32.0
Pine t a r from Zophar Mills, Ino. COMPOUXDIXG
TESTSO N
Brand Name Squibb mineral oil
PREXIXED PREPARATIONS O F Calculated Limit, G. Plasticizer per G. Black 0.41
G. Plastioirer per, G. Black in Test Compound 0.32 0.56
Unsaturated hydrooarbon extender
Pins tar
Vol. 40, No. 11
Kaftolen M V
Pine tar from Zophar IUills, Inc.
0.425
0
Black Required to Lower Plasticity 0.100 Inch, G. Black per G. GR-S, e,
SRF BLACK (PELLETEX) P a r t s on 100 GR-S SRF black Plasticizer 38 12 52
18
0.37
73
27
0.48
67.5
32.5
0.11
90
10
effect, and a white oil, which has a lesser plasticizing effect than the unsaturated hydrocarbon extender used in calculating the constant. The limit of plasticizer content in a premixed preparation with SRF black was calculated for each of these plasticizers. These calculations, together with the figures for the unsaturated hydrocarbon extender, are tabulated in Table 111. The d a t a in Table IV, showing t h a t compounding tests verified the calculated limits, suggest t h a t the formula can be extended to predict the limit of plasticizer content for mixtures of any black with any plasticizer (the test compound used here was the same as used for the tests in Table 11). Further tests with other plasticizers of completely different chemical nature would be necessary before it could be stated with certainty t h a t such &n extension of the use of the formula is warranted, or whether the numerical value of the constant would have to be altered. In this formula all of the variables except C, and C, are independent of the polymer. Since i t was shown in the previous study (3) t h a t the ratio between softening effect of the plasticizer and stiffening effect of the filler &-as t h e same for all rubbers tested, the expression, C,/C,, is also independent of the polymer. This holds especially true for amounts of plasticizers which can conceivably be encountered in practical compounding. T h e maximum grinding power available for dispersion is given by the stiffness of the polymer itself; therefore, the dispersion attainable with premixed preparations containing plasticizers below the stipulated limit will be equal to, but not higher than, the dispersion attainable by direct incorporation of the filler into the masticated rubber. Therefore, although the tests reported in this study were made with GR-S, i t is reasonable t o assume t h a t the results are applicable to other polymers.
Calcd. Proportion Plasticizer Permissible in Miuture, G. Plasticizer per G. Black, a 0.41 0.42i 0
WITH VARIOUS
Filler and Plaatiriapr Plasticizer Incorporation Premixed Separate Premixed Separate Premixed Separate Premixed Separate
PLASTICIZERS
Properties a t Optimum Cure Tensile nt in. aElongation t break, % a)./sq. t break, break Ib./sq. id. a t break, % 1220 400 1310 420 830 410 1180 460 1680 3 80 1700 430 1510 470 1690 460
Premixed Separate
1790 2070
340 320
magnified in the calculation. I n these cases little reliance could be placed on the calculated value. SUMMARY
It has been shown (3) t h a t i t is not always possible to premix successfully filler and plasticizer before incorporation into rubber, t h a t there is a n upper limit t o the amount of plasticizer which can be mixed with the filler without interfering with filler dispersion, and that this limit is sharp with reinforcing fillers, especially carbon blacks, depends on the particular filler and the particular plasticizer in the mixture, and appears to be independent of the polymer because premixed preparations which disperse successfully in GR-S also disperse successfully in other rubbers. A quantitative formula, developed from theoretical considerations for calculating the limit of a plasticizer which can be premixed with a filler, relates this limit to the surface area of the filler, the stiffening effect of the filler on the rubber, the softening effect of the plasticizer on the rubber, and the absorptive capacity of the filler for the plasticizer, This formula was found t o give values for a wide range of blacks which could be verified by compounding tests. (Plasticizer-filler mixtures satisfying the requirements of the calculated limits are being used successfully on a factory scale. Commercial premixed preparations are Naftex, produced by Wilmington Chemical Corporation, and Caloblack, produced by Golden Bear Oil Company. The process is protected by patents now pending.) Calculations and compounding tests carried out with two other plasticizers on one of the blacks indicate general applicability of the formula. T h e principal practical conclusion from this study is t h a t a lasticizer-filler mixture can be used successfully to facilitate andling and milling if the plasticizer content of the mixture is below the limits calculated from the formula presented. I n other words, fillers and plasticizers should not be premixed in arbitrary proportions, but the limits stipulated by the formula must be observed if the maximum grinding power inherent in the polymer is t o be utilized for dispersion.
f:
DEGREE OF ACCURACY OF CALCULATED LIMITS
Although all the instanccs tested in this study verified the calculated plasticizer limit closely, this was a carefully controlled study, with all test data collected under conditions as closely parallel as possible. T h e error involved would be greater under other circumstances-for example, if the data used in the calculation were taken at random from the literature. For practical application of the formula a wide safety margin is advisable. I n a few cases the calculated value, from purely mathematical considerations, will be at best only a n approximation. A study of the formula will reveal t h a t the mathematical expression, 1 kS (C,/C,), is of such a nature t h a t when lis (C,/C,) approaches unity, small errors in experimental data will be
ACKNOWLEDGMENT
The work reported in this paper was started by the authors a t the University of Delaware and concluded at the laboratories of Golden Bear Oil Company. The assistance of Rachel E. Morrison of the University of Delaware, who measured some of the values for oil absorption reported in this study, is gratefully acknomledged. LITERATURE ClTED
(1) Am. Soo. Testing Materials Standards, Part 11,p. 998, Am. SOC. Testing Materiale Designation D281-31 (1946). (2) Rostler, F. S., and du Pant, 13.I., IND.ENG.CHEM.,39, 1311 (1947). RECEIVED March 8, 1948.
