Some Properties of Butyl RubberCarbon.Black Systems F. P. FORD
AND A.
M. GESSLER
Esso Laboratories, Standard Oil Development Co., Linden, N. J.
T
HE effect of varying milling c o n d i t i o n s o n
several specific properties of Butyl rubber-carbon black systems has been discussed in a previous paper (7). I n that work, i t was shown that changes in mill roll clearances had a pronounced effect on the dispersion of S R F furnace black in Butyl rubber. The differences in dispersion were observed by means of the electron microscope and by photometric and plastometric methods. The relative size of S R F black agglomerates in different composhions was compared by examination of dilute suspensions in a photometer. Electron micrographs (Figure 1) were prepared from similar suspensions. The bulk viscosity was determined on carbon black masterbatches with a parallel .plate plastometer (6). These data are summarized in Table I. A continuation of the study of these systems has brought forth new information on the relationship of carbon pigments to Butyl polymers. Such data have been obtained by more detailed photometric and plastometric measurements. These techniques revealed changes taking place in Butyl-SRF systems upon exposure at different temperatures before vulcanization. The influence of vulcanizing conditions on the pigment habit of these systems was also investigated. In this work extensive use of electrical resistivity measurements was made. Viscosity tests on unvulcanized systems as well as conventional stress-strain measurements of the vulcanizates were also employed.
Table I. Carbon Black Dispersions 50 parts SRF in GR-1-16 Average Light Agglomerate Transmittance‘. Size, % Micron 20 0.17 25 0.26 57 0.35 53 0.39
Mill Setting, Inch 0.010 0.020 0.035 0.050 0.070 60 0.. 4.6 . . GR-1-15 n o t milled GR-1-15; milled 7 minutes, 0.010-inch setting GR-1-70, not milled 0.0068% solution.
Viscosity, X 10- Poises 39.6 47.9 54.6 51.3 6.. 9.2 20.9 19.2 54.5 ~
Most of the present study was devoted t o two systems--fine dispersions prepared on an 0.010-inch mill and gross dispersions prepared on an 0.070-inch mill. The practical effect of dispersion differences obtained from such variations in mixing conditions is illustrated in Figura 2. Analysis of changes taking place in such systems under a variety of conditions has led t o the conclusion that the existence of some form of association between polymer and pigments is quite probable. PHOTOMETRIC STUDIES
The further study of SRF-Butyl rubber systems in the unvulcanized state waa undertaken t o investigate a certain apparent lack of reproducibility in the photometric teat. It was discovered April 1952
for example, t h a t the light transmittance values of suspensions of a given set of compounds, prepared a t different times, were variable. At first it was thought that these differences were attributable t o changes in solvent or in the technique used in the preparation of the suspensions. Chemical analysis of the solvents eliminated the first possibility. While it was determined t h a t the time and vigor of agitation employed in making the suspensions had some . effect on their opacity, the observed differences could not be entirely accounted for in this way. A change in particle configuration or bound rubber content was suspected. A number of experiments were performed t o investigate such possibilities. I n each case a fine dispersion (0.010-inch mill setting) and a gross dispersion (0.070-inch mill setting) were made.
Previous studies have shown that differences in milling conditions cause significant variations i n the dispersion of SRF black i n Butyl rubber. Recent work has placed such differences on a more quantitative basis. This was achieved by the interrelation of photometric, rheological, and electrical measurements. These observations revealed new phenomena involving changes in the pigment-polymer habit of such systems. Analysis of these phenomena has led to the conclusion that they may be interpreted as evidences of pigment-polymer association.
Suspensions were prepared by weighing a ’/rgram sample on an analytic balance. A quantity of solvent, equivalent to 50 ml. of solution per l / ~gram of compound, was measured out in a buret. Either heptane or cyclohexane was used. Although cyclohexane is a more powerful solvent for Butyl, in dilute suspensions identical photometric readings were obtained with either solvent. The samples of compound and the solvent were placed in 2-ounce, square transparent bottles with tight fitting screw-type plastic caps. The bottles were then placed on a slowly revolving wheel for approximately 16 hours. The wheel had a maximum diameter of 13 inches and revolved approximately once every 1.5 minutes. Under these conditions a freshly prepared Butyl-SRF disperaion gave uniform homogeneous sus ensions. These 1% ’ suspensions were carefully diluted t o O.Olg/, concentration, and light transmittance values were obtained on the Diller colorimeter using calibrated test tubes as the sample holder. In repeated tests, the fine dispersion gave a value of from 12 t o 14% light transmitted, while the gross dispersion had readings of from 29 to 32’% light transmitted. Several different batches of such dispersions were allowed t o age both at room temperature and at 65’ C. before being placed in suspension in cyclohexane. Readings were obtained on such samples a t various exposure times. The results of several experiments are shown in Figure 3. It is quite apparent that the large clumps of black in the coarse dispersion had undergone some change in their configuration upon standing a t room temperature. After several weeks i t apparently has undergone an improvement in dispersion. Actually this is not the case, as will be shown, but certainly some change has taken place. A more marked reaction evidently occurs a t 65’ C., m might be expected. The fine dispersion shows a relatively rapid increase in light transmittance value, while initially the gross dispersion undergoes a decrease in value. An explanation for thjs phenomenon is that bound rubber formation is taking place. In a fme dispersion this can occur immediately as soon tt8 proper conditions are provided, specifically increased temperature. A coarse dispersion, however, must first undergo some change in its configuration, so that subsequent polymer “wetting” or insolubilization may take place. Hence, an initial increase in opacity of the dilute suspensions-decreased light transmittanceis explainable on the basis of a reduction in particle size. This decrease in size, then, permits bound rubber to form more readily. The evidence for bound rubber formation is Bound enough, but
INDUSTRIAL AND ENGINEERING CHEMISTRY
819
LLASTOMERS-Compounding as yet it has not been possible to place it on a quantitative basis for Butyl-SRF systems. As a Butyl-SRF compound ages it gradually develops signs of insolubility, whereas a fresh mixture will yield a homogeneous suspension in which discrete particles can be observed only microscopically. An aged composition produces coarse suspensions in which large specks of composition are quite visible to the naked eye. These clumps are essentially insoluble and, in time, comprise the major portion of the suspensions. Under such conditions, photometric analysis becomes quite inaccurate and of little value in the study of surh phenomena. A
Table 11. Effect of Aging on Viscosity -Aging Conditions
Viscosity X 10-6 Poises 0.010-inch mill 0.070-inch%lll 43,1 72.0 42.3 76.3 73.4 I
.
.
50.7
5 wks. at room temp. 8 wks. a t room temp.
52.6
79.8 86.4 85.5
48.4 47.1
85.2 80.4 80.4
sa.7 -73.2 54.0
...
C
What apparently does take place is a gradual sorption of poly~lic~r and pigment. Presumably, such association takes place whet11f.r the carbon particles are discrete or in agglomeratefi. Admittedly this analysis does not take into account possil~l(* thixotropic effects. A thorough investigation of such fa.ctor,w should be made. 'C'nfortunately, this is beyond the scope of tlie present paper. However, viscosity determinations on pure gum butyl before and after milling (Table I) and on pigmented systsnw over varying periods of time ,do not support the thought t h t thixotropic effects are a doniinant factor in these systems. ELECTRICAL R E ~ I S T I V I T Y MEASUREMENTS
Black Dispersions A. B. C. D. E.
0.010-inch mill setting 0.021-inch mill setting 0.034-inch mill setting 0.059-inch mill setting 0.068-inch mill setting
Determination of the bound rubber cont,ent of Butyl-SRF systems might be made by other methods. Diffusion methods employing dialysis sacs made of pure gum Butyl vulcanizates have been suggested by an associate. On the other hand, channel black forms very tight gel structures with Butyl, and the determination of the bound rubber content of such syst,enis is quite simple and reproducible. A technique similar to that suggested by Sperberg and his associates (16) has been found to be a convenient method of measuring t'he bound rubber content of unvulcanized But.yl-channel black systems. Unfortunately the carbon gels produced by furnace blacks in Butyl are not stable enough for determination by this method. Possibly a modified procedure, based on polar solvents or los.er temperatures, would be feasible. It might be concluded from the photometric work described above that aging of Butyl-SRF mixtures brings about a definite improvement in dispersion. Such a view would be based on the fact that the polymer is gradually made insoluble, or adsorbed, by the pigment. However, in previous work ( 7 )it was shown that differences in dispersion, i.e., differences in degree of agglomeration, could be detected by plastometric measurements. Therefore, parallel plate viscosities were obtained on the compositions which had been aged a t 65' C. Both the fine and coarse dispersions showed a slight, gradual increase in viscosity upon aging a t this temperature (Table 11). Had a significant improvement in dispersion (agglomerate disaggregation) occurred, a lower viscosity would have been expected. On the other hand, the increase in viscosity was not sufficient to indicat'e any considerable increase in agglomerate size. The present authors offer the following explanation for these phenomena. A change in dispersion occurs principally through severe mechanical working of a mix. Under static conditions there is no reason t o suppose a movement of carbon particles either in building up or reduction of agglomerate structures (18). 820
Since it has already been shown that the degree of dispersion of carbon particles may vary considerably, it is reasonal~lrto expect that such differences may have an influence on electrical resistivity. A number of investigators (I, 2, 6, 9, le! 1 4 ) have utilized this effect in studying carbon black dispersion. Usually: however, such work has been confined t o vulcanized systems. 111 the present investigations particular emphasis has been plxrerl on resistivity measurements of unvulcanized compoxitioiir. These determinations were made in a conventional manner using a guarded ring electrode and n weighted electrode. The specimens were approximately 6 X 6 X 0.075 inches in size, and wcw covered on one side with 0.001-inch aluminum foil before testing. A maximum charge of 540 volts, supplied by dry batteriw was used in this work.
Figure 2. Calendered Butyl Compositions A. B.
Fine dispersion, 0.010-inch mill setting Coarse dispersion, 0.070-inch mill setting
Saniples were prepared for resistivity measurements eithei, h y calendering or by molding in a 6 X 6 X 0.075 inch tensile pat1 mold. The temperatures and pressures a t which these oper:itions were performed had an important bearing on resistivity anti will be discussed. I n view of photometric and plastometric data it is of interest to consider the resistivity of fine and coarse dispersions and the effect of aging on this property in t,he urlvulcanized state. It was readily demonstrated t,hat fine dispersions had a relatively high resistivity, 10" ohm-cm., while coarse dispersions were in the range of 108 t o l o 9 ohm-cm. specific resistivty. These effects were quite reproducible. The effects of aging on such
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
Vol. 44, No. 4
L
LASTOMERS-Compounding-
systems are shown in Figure 4. Table 111. Effect -of Electrical Field on Viscosity The most conspicuous change Dispersion noted was the increase in resis7 X 10-6 Poises tivity of the gross dispersion. This fine, 0.010 inch 44.0-45.7 71.3-73.6 Coarse 0.070 inch approaches the value obtained on Coarse: molded, control 57.7-63.7 for resistivity drift 54.1-56.4 Coarse, molded, tested a well-dispersed system. A second tI AGED AT 65.C. phenomenon which seems to be a 0 *. A reproducible one is that the resis50 tivity of a good mixture shows a used were calendered t o a gage of 0.075 t o 0.080 inch a t a temslight initial drop which is followed perature of 300" F. Processing a t high temperatures apparently by a recovery t o a higher level, 20 disturbs the particle configuration t o a lesser extent than the near the limit of instrumental acshearing forces involved when such systems are forced to flow curacy. The resistivity values are slowly at low temperatures and high pressures. The resistivity a specimen slightly higher for DAYS changes before this experiment are indicated in Table IV. which has been subjected to test on Figure 3. Drift in several occasions than for another Light Transmittance of SRF-Butyl Comspecimen of the same composipounds tion which rested for 17 days with Table IV. Resistivity Changes during Aging at Room Temperature no t a t s . In all cases the resistibity it; in the order of 1013to 1014 ohm-cm. When aged at 65 O C. Resistivity Changes, Ohm-Cm. Aging Time 0.010-inch mill setting 0.070-inch mill setting the resistivity values converge more rapidly and come t o equiiibOriginal, no aging 3 5 x 1014 1 1 x 109 niim a t about 1O1O ohm-cm. All these changes are quite con2 day 3 o x 1014 7 9 x 10'0 siqtent with the light transmittance effects described earlier. 1 week 4 3 x 1013 1 . 6 X 101' 1 month 6 5 X 10'4 1 z x 1013 Two obvious differences exist between resistivity of r a w rompounds and VU]citnizates of identical comThe high resistivity obtained in the gross dispersion, upon position. Raw compounds standing at room temperature, is quite transient at elevated temrome t o equilibrium a t 1Olo > le AGED AT ROOM TEMPI perature. While both samples showed a lower initial resistivity t 0 1 1014 ohm-cm., depending II at 65" C. then at room temperature, the coarse dispersion showed upon temperature, w hi1 e a much greater drop than the fine dispersion. As a matter of OIO"MILL W cured specimens have a rr9 fact, both systems approach their original unaged resistivity specific resistivity of lo' or $ value. -8 S 10 14 108 ohm-cm., a t the highest. This change, however, is reversible. In a final experiment on t h e same specimens after they had rested approximately 4 months, both fine and coarse dispersions were found t o approach a value The show that unvulcanized other a pronounced distinction samples drift in is around 3.0 X 101*ohm-cm. specific resistivity. electrical resistivity, during ll exposure t o a n electrical 10 9 potential, while vulcaniaates are quite stable i n this re8 50 100 110 apect. The effect of elecHOURS trical field on viscosity is shown in Table III. The Figure 4. Drift in Electrical Resistivity of SRFm a g n i t u d e of electrical Butyl Systems drift in uncured compounds is quite striking, as seen in Figure 5 . This plot indicates the drift observed in a compound subjected t o a constant electrical potential for a period of about 27 hours. A change in resistivity from >10l4 a t the start to < l o 8 ohm-cm. was recorded after 24 hours under the w constant imposition of an electrical potential. This suggests s 7some form of particle alignment induced by the current. HowL ever, the effect on the visco-elastic flow properties of the compound is negligible, indicating that any new alignment which may have been set up was comprised of weaker forces than those obtained in the case of gross dispersions produced by mechanical techniques. The evidence for this lies in parallel plate viscosity data, which indicate only a slight difference in viscosity between portions of the samples exposed t o current flow and parts not Two distinct types of resistivity drift have now been described so treated. I n fact, the exposed portion had a slightly lower qualitatively. An unvulcaniaed Butyl rubber-furnace black viscosity than the other portion. Previous data had demonsystem when allowed to rest, without mechanical, thermal, or strated that larger agglomerates resulted in increased viscosity electrical disturbances, will tend t o approach an equilibrium in contrast to thk situation under discussion. resistivity value of -1014 ohm-cm. This is true even if the speciAdditional drift determinations were performed on another men has initially a low resistivity, -108, by virtue of gross dispair of similar samples at 65" C. The results of this experiment persion. On the other hand, when such systems of high resisare shown in Figure 6. At 65" C. and constant application of tivity are exposed t o a continuous flow of direct current, they electrical potential, the drift from 1014t o 108 ohm-cm. is much tend to drift in the opposite direction. This drift is temperature more rapid, taking place in about 140 minutes. The specimens AGED AT ROOM TEMP.
.
W
~k+-%%-$
.
5
:ri E l
k p i -*I
April 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
82 1
ELASTOMERS-Compounding dependent, but in any case the system approaches a minimum value of -los ohm-em. Both of these effects are somewhat transient. I n the case of a gross dispersion which attains high resistivity upon aging under static conditions, it has been found that this high resistivity can be dissipated simply by raising the temperature. On the other hand, the low resistivity obtained in such systems by electrical forces is reversed upon standing under static conditions. Although the reproducibility of the resistivity data is a measure
DISPERSION [ 01D"MILL) A C O A R S E DISPERSION ( 070"MILL)
.FINE
significance. Samples prepared a t room temperature had, a8 expected, high resistivity. In the case of the gross dispersion the value is higher than usual, probably because of the effect of molding under high pressure as suggested previously. It is especially noteworthy that these nonvulcaniaing systems, even when exposed t o curing conditions 10 minutes a t 320" F., do not undergo the large decrease in resistivity recorded in vulcanizates. ' ~ regardless of dispersion. The value obtained is ~ 1 0 ohm-em., Furthermore, this value is essentially the same as t h a t obtained upon prolonged exposure to the milder temperature of 65" C. (149" F.). The 1O1O value obtained is apparently quite stable as evidenced by tests made on heat treated specimens after standing a t rest for four months. These data seem to indicate that vulcanizing agents alone do not increase the conductivity of polymer-SRF systems. Neither does thermal treatment In the absence of curatives have the effect of true vulcanization on resistivity. Apparently, the maximum resistivity changes are brought about only by actual vulcanization. This is a remarkable conclusion which suggests some form of polymer-sulfur-carbon black interaction. Before attempting any theoretical explanation of these phenomena, however, it will be useful to consider some additional experiments. PLASTOMETRIC STUDIES
25
50
75
IO0
I25
MINUTES
Figure 6. Drift in Resistivity of Unvulcanized B u t y l - S R F S y s t e m s Under c o n s t a n t electrical potential at 65' C.
of the precision of these measurements, this exploratory work was intended t o be primarily qualitative rather than quantitative in nature. It is visualized t h a t in future work quantitative determinations would be useful. I n this connection, knowledge of experimental variables effecting such measurements acquired in the present work will be particularly valuable. VULCANIZATION STUDIES
These observations led t o a study of resistivity changes occurring during, or as a result of, vulcanization The effects obtained were studied in two systems-mixtures containing vulcanizing ingredients and those with no vulcanizing agents. Fine and coarse furnace black dispersions were prepared in each group and all compounds were molded both a t room temperature and at vulcanizing temperature. The basic formulation was polymer, 100 parts; S R F black, 50 parts; zinc oxide, 5 parts; tetramethyltkiuram disulfide, 1 part; and sulfur, 2 parts. The last three ingredients were added only in the case of vulcanizing combinations. I n such cases a large masterbatch containing these curatives in Butyl was made. The carbon black was then added to this masterbatch under carefully controlled conditions. In this way complete formulations were prepared mithout disturbing the particular pigment dispersions which xere being evaluated. The data obtained in this study were shown in Table V. The outstanding fact regarding vulcanizing compositions is that the mere presence of sulfur, zinc oxide, and accelerator is not sufficientt o alter the resistivity of the systems. Samples prepared a t room temperatures, and hence not vulcanized, had the same resistivity as both fine and gross dispersions containing no curatives. However, when molded for 10 minutes at 320" F., the resistivity dropped t o a normal value of -lo7 t o lo8 ohm-cm. It is worth noting t h a t in this experiment the gross dispersion exhibited less change than the fine one. It is further evident that there was a slight but definite difference in resistivity caused by varying the molding pressure. The latter observation is consistent with data reported by Bulgin ( 2 ) . I n the C & B of ~ nonvulcanizing systems the data areof comparable
822
The effect on carbon black dispersion of variations in the amount of physical work done on such systems has been described. The measurement of rheological and elccG.ical properties has been discussed and some suggestion made of changes occurring in pig ment-particle configuration during vulcanization. It is therefore of interest' t o measure the changes in viscosity or flow properties of Butyl-furnace black systems under such Conditions. Investigation of this problem was carried out on the same five samples which were discusfied briefly (Figure 1 and Table I; and in more detail in an earlier paper ( 7 ) . Inasmuch as these compounds contained no vulcanizing agents i t wa.8 possible to measure the viscosity changes incurred by exposure t o high temperature and pressure. These differences are shown ir* Table VI. The viscosity test specimens were exposed to high temperatures (10 minutes a t 320" F.) in a laboratory press in small rings, 1inch inside diameter by 0.150 inch usually employed in preforming the test specimens for the parallel plate plastometer. The data indicate t h a t the large differences in viscosity existing in the original compounds are minimized by exposure to vulcanizing conditions in this manner. I n most instancet; the magnitude of viscosity change is directly related to the de-
Table V.
Resistivity Changes d u r i n g Vulcanization of Butyl-SKF S y s t e m s Resistivity Change -
Molding Conditions
0.010-inch compd.
0.070-inch compd-
COMPOUNDS CONTAINING VGLC.4SlZINQ AQEXTS 2.9 x 108 2 . 6 X 10' 10 min. a t 320° F., 200 lb./sq. inch 3 . 8 X 10' 10 min. a t 320' F., 2000 lb./eq. inch 84 . 92 X x 10' 10s 0 > 1014 2 hr. a t room temp., a t low pressure 2 hr. at room temp., a t low pressure, 1 .o x 100 1 j x 1013 aged 1 day CoupouNDs
VITHOUT
10 min. a t 320' F., original 10 min. a t 320' F aged 2 days 10 inin. a t 320° F:: aged 4 weeks 2 hr. a t room temp., 20,000 lb./sq.
inch 2 hr. a t room temp., 20,000 lb./sq. inch, aged 1 day
VULCANIZING AQEJTS 2 . 6 X 10'0 5 . 0 X 10" 3 . 1 x 10'0 7 2.2 .6 X x 10s lo'@ 3 . 4 x 10'0 5.5
x 1013 > 10"
S H E E T S CALENDERED AT 300' I". AGEDAT 1.5 x Original 8.9 x 16 hr. st 65' C. 3.1 X 110 hr. at 65' C. Same specimen after resting 4 months a t room temp. 1.6 X a
1 . 2 x 10" 2 . 6 X 101p
65' I?., NO PRESSURE' 4 . 5 x 100 10'4 7.6 x 109 10" 1 . 2 x 10'0 1O'O
100
5.7
x
100
No vulcanizing agents present.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 4
-ELASTOMERS-Compounding gree of agglomeration: the coarser dispersions showed the greatest loss in viscosity. The one exception is in the case of the'mix prepared on the 0.050-inch gage mill. It is not easy t o explain this discrepancy. However, in the earlier paper (7) i t was pointed out that this compound differed from the others in that the mill bank constituted a substantially greater proportion of this composition than any of the others. It is possible that this results in a more thorough blending of the compound and possibly the agglomerates were more tightly held in the polymer mass than was the case with the other compounds. If this were true, i t is a t least conceivable that these agglomerates were less readily fractured during the hot molding operation.
'"7 *FINE
DISPERSION (0IO"MILL)
ACOARSE DISPERSION
In
(.070"MI LL)
% STRAIN
Figure 7. Effect of Dispersion on Tensile Properties
during the vulcanization process. The higher modulus occurring in such products may be attributed t o the viscous effects of the large agglomerates. The superior reinforcing properties of fine dispersions are, however, reflected in higher ultimate tensile strength and better abrasion resistance. Also characteristic of true reinforcement is the lag in modulus a t low extensions followed by higher values as orientation effects are induced at increased strains. SUMMARY AND DISCUSSION
Several facts have been established as a result of this work. The degree of carbon black dispersion or, to be more specific, the degree of disaggregation obtained in SRF black distributed in Butyl rubber, is influenced to a significant extent by the amount of mechanical work done on such mixtures. These differences have been observed repeatedly by means of the electron microscope and photometric and rheological tests. The electrical properties of SRF-Butyl systems are quite sensitive t o the conditions t o which they are exposed. Changes in electrical resistivity are brought about by variations in temperature or pressure as well as by electrical forces. Most of such changes, however, are reversible, except in the case of vulcanizates. The vulcanization reaction has an important influence on the electrical properties of such systems. Although the process of vulcanization tends t o minimize the differences in S R F pigment dispersion, there is considerable evidence that such differences may persist in vulcanizates and have an important effect on their physical properties.
Nevertheless, i t may be concluded that the effect of heat and pressure on such systems is to minimize existing differences in carbon black dispersions. These changes, unlike electrical drifts, are more permanent and essentially irreversible. This explains why dispersion differences which may give serious processing difficulties in production cannot be readily detected by means of laboratory tests based on the vulcanizate properties of such systems. WLCANIZATE PROPERTIES
It cannot be stated positively, however, that there are not some subtle differences in the physical properties of these vulcanizates. These properties have been measured in several different ways. In all cases a polymer masterbatch containing all vulcanizing ingredients was used in preparing compounds with varying degrees of S R F black dispersion. Test specimens were prepared by vulcanizing the samples simultaneously in the same multicavity molds. Stress-strain curves (Figure 7 ) were obtained on compounds vulcanized for 10 minutes a t 320" F. At low elongations the coarse dhpersion has a distinctly higher modulus than the h e one, while the latter has a greater ultimate tensile strength of 260 pounds per square inch, an improvement of almost 16%. Tests with the Taber abrader indicate that a definitely higher rate of abrasion loss may be expected with coarse dispersions than with fine ones. After 1000 cycles, the better dispersion had an 0.142% abrasion loss as compared with 0.17070 for the coarse dispersion. These data support the belief that large agglomerates of furnace black occurring in gross dispersions will persist t o some extent
Table VI. Effect of Vulcanizing Conditions o n Viscosity of Carbon Black Dispersions 50 parts SRF in GR-1-18 Mill setting inch 0.010 0.020 0.035 0.050 0.070 Origipal vis'oosity, x IO* poises 39.6 47.9 54.6 51.3 69.2 Viscosity after 10 min. at 320' F. 36.1 34.4 36.9 45.1 43.9 Difference, ave. of 4 determinations 3.5 13.5 17.7 6.2 25.3
April 1952
In view of all these facts one may speculate upon possible explanations for the observed phenomena. Several authors (8,6,14) have, indeed, pointed out that increased shearing action on polymer-pigment systems, obtained by more severe milling conditions, does bring about gross differences in ''dispersion." There has always been, however, a need for more precise teehniques of measuring and evaluating these differences. It is hoped that the several procedures discussed in this paper will prove to be useful in such applications. I n the present work the polymer-pigment habit in Butyl rubber-SRF systems has been described by these methods. However, the theoretical explanation for some of the phenomena, such as photometric or electrical drifts, is not immediately obvious. Bulgin (a) and Parkinson and Blanchard ( l a ) hsve conddered the possibility of changes in carbon structure under various conditions, such as Brownian movement. I n the present work i t has been shown that photometric and electrical resistivity changes are not accompanied by a comparable alteration in rheological properties. How then may these concomitant but not complementary effectsbe explained? By what mechanism may such changes occur? A hypothetical case is illustrated in
INDUSTRIAL A N D ENGINEERING CHEMISTRY
823
-ELASTOMERS-CompoundingFigure 8, in which some liberties have been taken with an electron micrograph of SRF black. Here the formation of continuous chains for passage of a current in the presence of an electrical field is proposed. Such a mechanism is suggested by work of Cohan and Watson (J), Voet and Suriani (IQ), and Pohl ( I S ) . Under the influence of an electrical field dipoles may be induced on suspended particles. The particles can then coalesce into conductive chains throughout the rubber matrix. They do t.his more readily when the temperature is raised and the viscosity of thc rubber matrix reduced. This electrical drift in unvulcanized m s is reversible, for the compounds in Figure 6 returned l o their original values upon standing a few weeks. Furthermore there appears t o be no good reason why formation of conductive c h i n s by a mechanism involving induced dipoles could not take place without fracture of existing agglomerates. This view is indtxed consistent with the present data. It has already been suggested that the changes in light traminit t;ince values are related t o the formation of bound Butyl. [rt,r,e, too, i t seems reasonable to expt:ct that some portions of ciirhon aggregates can form an association with the polymer without mtually breaking up tho agglonierate structure. Thus, iwund rubber formation could proceed without altering the rheo1ogic:;d properties appreciably. The initial drop in light transmittance value in the case of very coarse dispersions might be euplainrd by the hypothesis that the smelling of the polymer in solution does actually cause fracture of very large agglomerates. There is, indeed, some theoretical evidence to support the conw p t of c;trbon black-polymer int ction of a t,ype that could produce bound rubber. Nauntoii and Waring ( I O ) , Gehman and Field ( 8 ) , and Thornhill and Smith ( 1 8 ) have suggested t h a t swing bonds exist between polymer and pigment and that these bonds may be related to unsaturation. In reviewing work in this field, Parkinson (11) proposed the existence of prima.ry vnlonce linkages between carbon pigment and polymers. This roncept has been advanced more recently by the work of Stearns : ~ n d,Johnson (16). It should perhaps be pointed out t,hat these concept.s must be approached with some caution. The distinctions between primary valence linkages, van der Waals forces, and physical associations are difficult if not impossible t o detect i n such systems as the ones under discussion. It is reasonable t o assume, however, t h a t some such forces are involved in bound ruhber as well as reinforcing phenomena. It, was suggested earlier that an interact,ion between carbon 4)lack and the cross-linking polymer occurs during vulcanization with sulfur. This thought was based on several observations n u d e on the electrical properties of vulcanized and unvulcanized pyst,eins. It was found that the presence of sulfur, and other curatives, in mere physical admixture in a polymer-black system has no effect on the electrical resistivity. Likewise, the application of vulcanizing temperatures, in the absence of vulcanizing agents, does not bring about the degree of change promoted by actmualvulcanization. In speculating on the reasons for this behavior, it is convenient to accept certain concepts which have been reasonably well established. Vulcanization is believed t o be a series of complex cheniical reactions in which strong intermolecular forces are formed between rubber molecules ( 1 7 ) . These changes are thought t o involve reactions with the alpha-methylene carbon atoms or double bonds, with a possible reduction of polymer unsaturation. These chemical bonds between rubber molecules account for t h e conversion of the soft, plastic unvulcanized rubber into a tough, elastic, infusable, and insoluble vulcanizate. The magnitude of the permanent change in resistivity upon actual vulcanization suggests some form of interaction between polymer, pigment, and curatives. Some observers have proposed that this is simply a matter of flocculation, but this idea does not seem t o fit all the experimental observations. Another possibility is t h a t this is a viscosity effect. Viscosity being temperature dependent, a lower resistivity is obtained a t vulcanizing tempera824
ture!, the particular pigment configuration responsible for low rebistivity becoming permanent upon vulcanization. It i,q true t h a t resistivity does decrease rapidly during vulcanization. That this change is permanent and irreversible can undoubtedly be attributed t o the fact that a rigid polymer network is formed which does not permit any easy change in pigment p configuration. However, Etill anot,her hypothesis is suggested by the present work. There appears to be a possibility t,hat,the strong polymer cross-linking forces created by vulcanization disrupt weakt,r polymer-pigment bonds. In this way tht: pigment particles are forced into more conductive chains. This concept presents an interesting possibility--if some method of strengthening polymer-pigment bonds can be devised, a true enhancement in polymer reinforcement may be realized. It is acknowledged that some of the points made and conclusions suggested in t.his paper are of a controversial nature. This i~ unavoidahle and even defiirable in a field wherc further investigations are needed. I t is hoped t,hat these circumstances will stimulate additional investigations---],articularl?- where further elucidation would contribute to a hetter rinctei~standingof rcinforcing phenomena. Studies of other types of pigments as well as all carbon black types would sccm especially worthwhile at, this time. The practical value of sonic of t h c p e findings should be obvious. Their application t o spc?c*ificproblems in the f h i c a tion of inner tubes, c,oated fahrics, insulated wire, ;ind fiimilar products is mggested. ACKNOWLEDGMENT
The authors are indebted to A. 1‘. h\lottlau for the electi.o[i microgiaphs and t o D. 1,. Crow for the illustrations and graphs. Most of the laboratory experiments were performed bj. F. J . Blash. The writers received valuable assistance and suggestions from many other associates during t>hecourse of the work and in preparation of the manuscript. LITERATURE CITED Blanchard, A. F., a n d Parkinson, D., Proc. dnd Iiuhber Tech. Conj., L o n d o n , 1948, 414 [ R i d h i (:hem. and Technol., 23, A15 (1950)l. Bulgin, D., Trans. Inst. Rubber I d . , 21, 188 (1045) [Rubbwr Chern. and Technol., 19, 667 (1946)l. Cohan, I,. IT., Proc. 2nd Rubber Tech. Conj., L o i i d o ? ~1948, , 365. Cohaii, 1,. H., a n d W a t s o n , J. 1%.L.. Rubber A g e ( N . Y . ) ,68, 687 (1951). D a n n e n b r r g , E. M., J o r d a n , M. IC., and Stokes, C . A,, I n d i a Rubber W o r l d , 122, 663 (1950). Dienes, G . J., a n d Klenim, €1. F., J . AppZirtE Phi/s., 17, 488
(1946). Ford, F. P., a n d h f o t t l a u , A. Y., Rubber A g e (Ar. Y.), 70, No. 4, 157 ( J a n u a r y 1952). Gehnian, S. D., a n d Field, S. E., IND. ENG.CHEM.,32, 2401 (1940). Morris, R. E., a n d Hollister, J. W., Ibid., 40, 2326 (1948). N a u n t o n , TV. J. S.,a n d Waring, ,I. R. S., Proc. Rubber Tech. Conf., L o n d o n , 1938; Tians. Inst. Ruhber I d . , 14,340 (1939). Parkinson, D., “Advances in Colloid Science,” Vol. 2, g . 423, New York, Interscience Publishers, 1946. Parkinson, D., a n d Blanchard, A. F.,T r a n s . Inst. Rubber Iud., 23, 259 (1948) [Rubber Chem. a n d Technol., 22, 118 (1949)l. Pohl, 13. A,, J . A p p l i e d P h y s . , 22, 869 (1951). Rperberg, L. R., P o p p , G. E., mid Biard, C. C., Ezabbw -Age (17~. Y.), 67, 561 (1950). Speiberg, L. R., Svetlik, J. F., a n d Bliss, L. .L, IND.E h w . CHBM.,41, 1641 (1949). Stearns, R. S., a n d Johnson, B. L., Ibid., 4 3 , 146 (1951); Rubher Chem. and Technol.. 24. 597 (1951). (17) Stiehler, R. D., a n d Wakelin, J. H . ,
[email protected].,39, 1647 (1947) [Rubber Chem. a n d Technol., 21, 325 (1948)l. (18) Thornhill, F. S.,a n d Smith, W. R., IND. Esa. CHEM.,3 4 , 218 (1942). (19) Voet, A., a n d Suriani, L. R., J . Colloid Sci., 6 , 155-61 (1951).
RECEIVED for review September 15, 1951.
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
ACCEPTEDFebruary 1, 1952.
Vol. 44, No. 4
