ISDCSTRIAL A N D ENGIATEBRINGCHEMISTRY
September, 1923
939
The More Complete Evaluation of the Pigment Reenforcement of Rubber’ By William B. Wiegand 64 QUEEN ST.,NORTH,KITCHENER, ONT.
T
HE proof resilience measured, for example, as foot pounds of energy stored in one cubic inch of the un-
zero concentration to the upper limits of pigmentation, gives the complete stress-strain picture of the series of compounds under consideration.
stressed sample, has attained wide acceptance as perhaps the most generally useful single criterion of the quality Definition of “ AA” Function of vulcanized rubber. It is most conveniently measured as It mill be seen that the “A” function includes not only the the area subtended on the strain axis by the stress-strain curve. As energy it comprises an intensity factor represented proof resilience due to pigmentation but also that of the base by load per square inch, and a capacity factor represented by mixing. In order to assess the reenforcing power of any pigment it -is therefore necesextension. It is thus’ f sary to separate these two TdE, the limits extending components. between zero elongation and “A” function is defined as J J T.dE.dV, where T = In Figure 3 is therefore the elongation a t rupture. load, E = elongation, V = volume per cent of pigment. shown the area lying beIts dimensions represented I t has the dimensions of a volume, and can conveniently tween the proof resilience (Mass) (Length)* be shown as a solid figure. are curve of the base mixing as by (Tirne)Z “AA” function is defined as excess proof resilience represented by the line A B clearly those of an area. developed by a pigment throughout its reinforcing and that part of the “A” I n the study of the physrange of concentrations. function curve which lies ical p r o p e r t i e s of com“A” function and “AA” function are discussed for pounded rubber the proof above it. We define this some of the more commonly used pigments and a reresilience as defined above is area in terms of foot pounds lationship suggested between them and pigment strucas ‘‘ AB” function. It repthus seen to exhibit the ture and piling. properties of a single memresents the entire reenforcber of a series of mixings ing properties of a pigment which may be regarded as a over the range of concentracontinualprogression from the uncompounded base to the up- tions in which the physical properties of the base mix are per limit of pigmentation as fixed either by choice or by the ab- enhanced. sorptive capacity of the base. Since, however, the proof reMathematical Relationships silience varies continuously with the concentration of the pigment, we define as the “A” function, the integral of the Proof resilience as f T. dE yields tensile as the first derivaproof resilience with respect to pigment concentration be- tive with respect to elongation, or tensile is represented by tween any assigned limits. It thus has the form the rate of change of proof resilience with respect t o elongation. ~
“A”
=
~~
~~
f“-. Ey. dV J
vo
It will be seen that the “A” function comprises an intensity factor represented by Ey, the proof resilience, and a capacity factor, dV. Since proof resilience is represented by
ss
TdE, substi-
tuting in the expression for “A” function, we have the following:
If the values for proof resilience are erected in the usual manner as ordinates, and pigment concentrations as abscissas, the numerical value of the “A” function is clearly represented by the area subtended on the abscissa by the proof resilience curve. It is represented by the shaded area in Figure 1. Reference to the expression for “A” function as a double integral of tensile with respect both to elongation and to (Mass)(Length)3 rolume loading, which has the dimensions (Time)* of a volume, suggests the graphic depiction of “A” function as in Figure 2. This figure, the volume of which represents the space filled by all the stress-strain curves, say, from 1 Presented before the Division of Rubber Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 5 t o IO,
3985.
V O L . LOADING Figure 1
“A” function, as f f T.dE.dV, likewise yields tensiie as the second derivative with respect both to volume of pigmentation and to elongation, or tensile comes out as the rate of change of “A” function with respect to both pigment loading and elongation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
940
E y d V yields proof resilience as its “A” function as first derivative with respect to volume, or proof resilience as represented by the rate of change of “A” function with respect to pigment loading.
Vol. 17, No. 9
show little or no intensity of reenforcement, compensated to some degree by a well-maintained capacity factor. Table 11-“ AA” F u n c t i o n a -DATA EY-7 PIGMENT WIEGAND GREIDER Carbon black 4960 474;7 Magnesium carbonate .... 2687 Zinc oxide 1813 , 1432 China clay 47 1 1904 Whiting 90 .... Lithopone .... 0 Barytes 45 .... Colloidal barium sulfate .... 0 For the method of calculation of energies and of “ A A ” Function see paper “A Convenient Formula for the Calculation of Rubber Energy, by Sheppard, I n d i a Rubber World, October 1, 1921. This method of calculation introduces an error small relative to the experimental error in the determination of the stress-strain curves.
Significance in Pigment Research
/I
ELONGATION Figure 2
-
Application to Published Data
To illustrate the application of “A” and ‘‘ AA” functions to rubber compounding, certain of the published data upon the rubber stress-strain curve as influenced by pigmentation have been recalculated and tabulated to exhibit proof resilience as a function of volume concentration. Table I-Proof
Volume
loading
Nil
2
4 5 6 8 9 10 12 15 20 25 30 35 40 50 75 100
CARBON
BLACK
W
G
450
455
--
Resilience Developed by P i g m e n t a t i o n Wiegand’s data W C Greider’s data (Foot Dounds Der cubic inch) MAG~BARYTBSNBSIUY COLZINC CAR- CHINA WHIT. LITHO-WATER LOIOXIDE BONATB CLAY ING PONE FLOATEDVAL W G G W G W G W G 450 455 455 450 455 450 455 450 455 482 582 532 463 437 46 453 529 495 455 645 666 430 628 432 450 438 548 522 665 525 623 4 54 585 640 446 448 540 615 455 560 365 448 438 535 518 473 404 491 408 451 428 468 494 315 358 415 305 387 387 300 290 400 395 411 317 288 250 318 205 217 265 115 175 129 120 112
These two aspects of pigment reenforcement-namely, its intensity a t optimum concentration and the rate of advance and decline of reenforcement viewed as a function of concentration-are not without significance when considering the mechanism of pigment performance. I n the first place, it seems clear that the normal decline in proof resilience of a mixing due to the dilution of the rubber phase can be arrested or reversed only by pigments which show either extreme fineness of subdivision or a very high degree of adhesion. With barytes, whiting, and lithopone this rate of addition of surface tension energy is not sufficient to offset the decline in energy due to dilution. It will next be observed that in the case of the reenforcing pigments proper there are marked differences in the trend of the proof resilience curve. Carbon black and zinc oxide are similar in showing a fairly smooth progression to the optimum point, and have similarly gradual declines. Magnesium carbonate and china clay, on the other hand, exhibit a much narrower range of reenforcement.
~~~
572 620 576
649
629 642 633 603 554 437 168
681 664 606 507
The data shown in Table I are represented graphically in Figure 4. From the above data the ‘ I AA” function has been calculated for a few of the more commonly used compounding ingredients. It will be seen that this inclusive index to reenforcement traverses a very wide range of reenforcing values, running from approximately five thousand energy units to nil. Clearly the “ S A ” function, viewed as an index to reenforcement, reflects not only the performance of a pigment at, say, its optimum concentration, but over its entire useful range. Carbon black takes first place because it maintains a high degree of reenforcing power over a wide range of loading. Magnesium carbonate, although yielding similarly high reenforcing power a t optimum concentrations, suffers more rapid decay of reenforcement as the loading increases. Zinc oxide shows a low intensity of resnforcement over a fair range. China clay advances rapidly to a moderate peak, followed by rapid decay. Lithopone, whiting, and barytes
W
u
Z
w
-I t W LL
0 0
IL
a
VOL.
LOADING Figure 3
These differences are of interest both on theoretical and practical grounds. Viewed as an index to particle structure, it is suggested that there may be a relation between the shape of the “A” function curve and the conformation of the individual particle. I n a pigment the particles of which exhibit acicular, lamellar, or, in general, highly irregular surfaces, it can be readily understood that with increasing concentration there is a t first added to the system a rapidly increasing superficial area of contact between pigment and rubber phase, more rapid in fact than in the case of a pigment, the particles of which approximate spherical shape. (This follows from the obvious consideration that for a given mass the spherical
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1925 Greider
zoo
Wiegand
CAR. B L A C K
CAR BLACK
-
20 40 V O L . LOADING
650
200
200
600
ZINC O Y I D E 20
20
-
40
VOL. LOADING
I
2oo 40
Z I N C OXIDE
20
40
600
eo0
:z:
20
CLAY
40
FDO
wJ
- _
51 .“I400 L
-*-\,
0 L
EOO
COL BAR
SULPHATE
2oo
~ARYTES
20
-
40
VOL. L O A D I N G
600
40
Figure 4
shape offers minimum surface.) Since there is unquestionably a parallelism between the development of superficial area and the reenforcement of the compound, such irregularly
941
shaped pigments show a rapid advance to their optimum reenforcing power. On the other hand, with increasing loading, pigments of the approximately spherical class exhibit striking advantages. As with small pigment concentrations it may be assumed that the pigment particles are disposed neither in strictly cubical nor in strictly tetrahedral piling, but in a piling that may be regarded as continuously intermediate between these limits, it is necessary to assume that as the concentration approaches the saturation point there will be a tendency, at least on the part of pigments of the spherical class, toward the tetrahedral disposition. (The phenomena described by Obsorne Reynolds are of interest in this connection. The effect of similarity of electrical charge upon the particles may also be regarded as operating toward tetrahedral piling.) Such a tendency will clearly preserve the superficial contact area between pigment and rubber phases and, which is also of cardinal importance, make for uniform interparticle distances and therefore uniform thicknesses of rubber within the interstices between the particles. This will assume a more uniform disposition of stress in the rubber phase and thus notably improve the tensile and resilient properties of the sample. The decay in proof resilience with very high percentages of such pigments may be regarded as due to the dilution effect upon the rubber plus the loss of surface consequent upon ultimate agglomeration. In respect to pigments of the other class-namely, those the particles of which depart sensibly from sphericity-it seems clear that increased loadings impose much earlier agglomeration or contact of the individual particles, thus inhibiting the progress toward tetrahedral disposition, cutting down the rate of development of surface, and so yielding much earlier to the degrading effect of dilution. Just as in vulcanization there is a contest between the setting-up effect of sulfur and the breaking-down effectof heat, so in the compounding of rubber we may picture a contest between the rate of addition of surface energy by the pigment phase and the decline in the resilience of the rubber phase through dilution. From the practical side the trend of the “A” function curve is not without utility. There is now an increasingly important series of rubber compounds based upon the incorporation in very high percentages of, for example, carbon black, which exhibit high rigidity in combination with high extensibility and high tensile strength and resilience. These mixings have attained commercial importance in that they have extended the use of rubber into fields formerly preempted by the more rigid materials. High modulus of rigidity may be attained through the incorporation, in sufficiently high percentages, of practically any compounding ingredient. A very rigid compound may be produced, for example, by the addition of a very large percentage of whiting or barytes. Such products are practically useless, however, because the high rigidity has been attained a t the expense of tensile strength and extensibility-i. e., proof resilience. The prosecution of research in this interesting range of high modulus compounds is greatly facilitated by a close study of the trend of the “A” function curve. The most satisfactory results are clearly obtained by the choice of a pigment which unites as high as possible a value for the ‘‘ A A” function, with a minimum rate of decline of the curve in the higher concentrations. At the present time properly made carbon black exhibits the most favorable combination of these properties. Acknowledgment The writer gratefully acknowledges the assistance of H. A. Braendle in the preparation and calculation of the data of this paper.
