X-Ray Structure of RubberCarbon Black Mixtures J
S. D. GEHMAN AND J. E. FIELD The Goodyear Tire and Rubber Company, Akron, Ohio
T
for rubber are available in a recent comprehensive review ( 7 ) . HE position of carbon black as an indispensable comSome effects of fillers on the x-ray structure of rubber have pounding ingredient for present-day rubber technology been noted in isolated instances, but investigations were not has led to many experimental and theoretical inquiries pursued. regarding the manner in which it functions to improve the properties of rubber into which it is incorporated. The genCompounds Used eral term “reinforcement” is used to express improvements Table I lists the vulcanized compounds used. They were in physical properties brought about by the mixing of powders mixed on a laboratory mill. The milling was carried out in into rubber. Shepard, Street, and Park (16) reviewed the each case until a good dispersion was ensured. They mere concepts which have been employed in efforts to define reinvulcanized in the form of 2-mm. gage test sheets from which forcement. Reinforcing effects are made evident in measnarrow strips were dyed for the x-ray exposures. urements of plasticity, viscosity of solutions, stress-strain I n addition to the vulcanized compounds, diffraction patrelations, tear, and abrasion resistance. By all criteria which terns were obtained for several unvulcanized mixtures of have been proposed, channel black heads the list of reinforcing carbon black and rubber, sopowders although there is called master batches. They some indication that even are listed in Table 11. more pronounced reinforcing effects can be obtained with The effects on the x-ray structure of stretched rubfiner carbon blacks. Procedure ber caused by the incorporation of carbon hlacks of Available explanations of different average particle sizes were investigated. reinforcement constitute an The x-ray diffraction patterns The minimum elongation necessary for the fiber were obtained with a General involved maze of contradicdiagram to appear was determined by photometric Electric x-ray diffraction unit tory evidence, speculations, and C u I L r a d i a t i o n . T h e measurements of the patterns for a range of and controversies which is in CUKBradiation was filtered out elongations. For the same black, the required by a nickel foil (gage, 0.8 mil), strong contrast to the pracelongation decreased as the loading increased. For Operating voltage was 35,000 tical importance of having a the different blacks, the elongations necessary for volts, current 25 milliamperes. clear understanding of what All exposures were made at the appearance of the fiber diagram were in the occurs. The literature on the room temperature from 2 to 6 same order as the average particle sizes of the hours. subject has been ably reblacks, the loading being the same. A Zeiss nonrecording microviewed by Shepard, Street, The results were interpreted as making evident photometer was used for measand Park (26). Characterisu r e m e n t s of p h o t o g r a p h i c the existence of a region of enhanced stress in the tics of a powder which have density. For each stock a series vicinity of the particles. This field of stress was of patterns were taken at differbeen held responsible for reinalso apparent from the increased length of the difent elongations, and the density forcement are of the AI diffraction spot was fraction spots, a n effect found to depend upon the measured. These values, particle size of the black. The fact t h a t the finer plotted against the elongation, blacks required lower elongations for the fiber dia1. Particle size gave curves which intercepted 2. Particle shape gram was attributed to the much greater number of the elongation axis and thus 3. Chemical composition determined the lowest elongaparticles present for the same volume loading. 4. Surfare activity tion at which an appreciable The bearing of the x-ray results on various a. Flocculated systems amount of diffraction occurred. theories advanced in regard to reinforcement by b. Dispersed systems For each such curve, the films carbon black is discussed, as is the possible relation c. Plastic solid films were taken on the same pinhole, d . Heat effects with the same exposure times of the observed x-ray structure to the general physiand the same operating condical effects of reinforcement. I t is concluded that tions, and were developed simulthe x-ray data can be explained on the basis of the t aneously. The tube ran In the work here reported, known stresses in the vicinity of large inclusions i n steadily b u t an additional experimental evidence from caution was taken to :er:f stretched rubber. The field of stress shown by the the possibility of error from this x-ray diffraction studies on x-ray patterns to exist about the particles implies cause as well as from variarubber into which a series t h a t the rubber molecules are firmly attached to tion in film sensitivity. At of carbon blacks had been the surface and t h a t the rubber works a t a higher the termination of each exi n c o r p o r a t e d is presented posure, the film was marked point on its stress-strain curve. The reduction in near the edge with a control and discussed in its bearing plasticity which might reasonably be expected due density obtained by exposure on some of the above explanato the immobilized molecules appears sufficient to to the incident beam, weakened tions. account for the rest of the results, in a general way. b y passage t h r o u g h a n Descriptions of the general aluminum strip. This exposure time (a minute or so) was x-ray diffraction phenomena 1401
INDUSTRIAI, AND ENGINEERING CHEMISTRY
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TABLEI. VULCANIZED Comouxos
TABLE11. M.m-m~BATCHES ldontifyinz NO. 17
1s 19 20
21 22 23 24
25
Smoked Sheet 100 100 1W 100
Acetone-Extd. Crpp?
48 ehsnncl black
., . .. ..
87 channel blsrk
, ,
...
100
... ...
100
...
...
Carbon Blhok
...
48 Superapaetra 117 Superspectrn
100 100
IOU
always t h e same for the Same curve. Any corrections indicated its necessary bv the control mot were alw& smkll (rarely more than 10 per cent), and the effect on the intercept of the curve was usually neelieible. All of the curve^ were no; f i n on the same pinhole, however, and in this case the expmnre times to secure suitable pattern int e n s i t i e s varied f r o m c u r v e t o curve. For the different pinholes, the exposure time for the control spat was alrvays made proportional to the expaosure time for the patterns so that the curves could be put on the same scnle. This cross comparison of films from different pinholes has no bearing on the d e t e r m i n a t i o n of t h e minimum elongations since each individual curve wm run on the same pinhole. It merely served BS a coiivenient control for seouring comDarable intensities on d l the films and in no sense served as a means for securing the absolute intensities, which were not required.
.......
4R channel bin& 48 ohnnnel blnrk 48 G ~ s t e x
%8Gvatar
iitPSi.i,> Acid
... .... .. ... 1 I'
a 1
I'
A.
VOL. 32.
\io. i n
not noticeably broader so that the Same size for the crystallites and t.he same degee of lattice perfection may be inferred. The particle size of chaiinel black is of the same order of inagnitiide as the size deduced for the rubber crystallites which are responsible for the x-ray d i f f r a c t i o n pattern of stretched rubber. The relatively sharp spots obtained for stretched rubber at higher elongations show that the rubber crystallites embeiliied i n amo-rphous rubber a t lower elongations do not cause poor alignmerrt~ of crystallites formed by further stretching, in contrast to the effects with foreign particles such as carbon black. If the rubber for the higher loadings of carbon hlack existeii as the dispemerl phase, as suggested by Roiry (3) and by Wiegand (19), it might, 1x2 expected that there would he some marked differences in the x-ray structure a t the higher loadings. On the contrary, there is a continuous, monotonic change i n the patterns LIS the loading is increased. There is
KO bbrk
8 . 11 channel black
Vulcanized RubberCarbon Black Mixtures Figure 1 shows the progressive effect of increased loadings of channel black on the x-ray diagram at 400 per cent elongation. At this elongation the fiber diagram is well d e v e l o p e d . The black does not interfere with the formation of the rubber erystallites but causes a less periect alignment so that the spots o i the normal rubber diagram are spread out into arcs. The arcs are
D. 87 channel black FIGURE 1. EFFECT OF INCREA~ED LOADINGS OP C'HANNEL HI.ACKON X-RAY DIAGRAM C . 48 channel black AT
400 PERCENTELONGATION
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no evidence here that individual rnhher macromolecules are surrounded by carbon black particles, in which case crystallization should be hindered or entirely prevented, or at least the lattice should he distorted. The general appearanre of the patterns is readily interpreted in a straightforward way in terms of results published concerning the stress about an inclusion in a sheet of stretched ruhber ( 1 , 18). If a plane sheet of rubber containing a h a r d inclusion is stretched, the lines of stress are distorted in the neighborhood of the inclusion. Consequently the F:longation, 300 per cent; no hlaok I
