tor. When litharge is used with methyl Tuads instead of zinc oxide as an activating agent, the lead dimethyldithiocarbamate is found to be the reaction product in the cured sample.
F i g u r e 7 . E m c t r o n P h o t o m i c r o g r a p h s o f K a l v a n ( I n d i c a t i n g P r i m , P a r t i c l e S i z e o f a b o u t 300 A . ) a n d o f W h i t i n g (right)
February, 1943 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 203
Fi g u r e 8 . Mi c r o p h o t o m e t e r Tr a c i n g s o f Sa m e Ca l c i u m Ca r b o n a t e Di f f r a c t i o n Li n e i n Ka l v a n a n d Gi l d e r’s
Wh i t i n g f o r Us e i n Pa r t i c l e Si z e De t e r m i n a t i o n s
O r ie n ta tio n o f V a r io u s M a te r ia ls
Zinc dimethyldithiocarbamate is strongly oriented in the rubber matrix on stretching. This shows in the x-ray diffrac
tion pattern by nonuniform intensity of the diffraction rings.
The degree of orientation may be judged by the sharpness of the arcs into which the rings are broken. Rubber crystallites are an example of a completely oriented material. The arcs are so sharp that they appear as spots. Figure 5 shows the orientation of some of the inorganic fillers in rubber. A dis
cussion of the orientation of these fillers is necessary to an understanding of the importance of the orientation of accel
erator salts in the rubber matrix.
A dry film from latex, stretched 550 per cent (Figure 5a), is shown as the pattern for rubber free from all fillers. This pattern was taken at a sample film distance of 3 cm.; therefore the rubber d i f f r a c t i o n
spots appear closer to the central portion of the film than in Figures 1, 2, 3, and 4.
The sample shown in Figure 56 is stretched 550 per cent. Here all the rings, in addition to the rubber interferences, are due to zinc oxide. The amount of accelerator used is so small that it does not show up. These rings are not uniformly intense, as they are for the zinc oxide in a fingerprint pattern, but appear with localized intensity maxima. This means that during stretch
ing of the rubber, the em
bedded zinc oxide grains are rotated and lined up
in a preferred direction with respect to the4 direction of stretching. It is possible to calculate from the positions of the maxima on the rings the actual arrangement, with the normal to the (0 0 1) planes parallel to the fiber or stretching axis. This means, in turn, that such a powerful bond exists between the zinc oxide grains and the rubber that the rubber does not tear away from the oxide surfaces during stretching but instead tends to pull all the grains into parallel alignment.
This is a distinct property of the zinc oxide and, to some ex
tent, of the compounding; for in the course of years of investi
gation many specimens containing 5 parts of zinc oxide showed no such fibering1 of these crystal grains at 550 per cent elongation.
An electron microscope picture of the zinc oxide com
pounded into this stock is shown in Figure 6. The individual particles are much longer in one direction than in the other two, and it may easily be understood how they would be oriented in the rubber matrix on stretching.
The stock shown in Figure 5c contains 25 parts of Kalvan, a finely divided calcium carbonate. Stretched only 200 per cent, the central rubber halo shows no evidence at this elon
gation of crystalline spots. The intense ring is characteristic of calcite, and all others may be identified by comparison with fingerprint patterns as belonging either to calcite or zinc oxide.
The rings are continuously uniform, indicating random ar
rangements of all crystal grains.
Figure 5d represents the same stock stretched 550 per cent.
Now the crystal fiber pattern for rubber has appeared; in addition the calcite lines, especially the faint line just outside the rubber interferences and the very intense ring, show defi
nite evidence of fibering. Again this means a remarkably powerful bond between Kalvan and rubber, and a sufficiently small size so that the grains rotate and align themselves with respect to the direction of stretch. The reverse is true as the rubber itself crystallizes; for on release of tension, the speci
men again reverts to an entirely random arrangement of rubber molecules and filler crystals. Fibering of the filler appears just when fiber spots appear for the rubber.
In Figure 5e the stock containing 25 parts of gilder’s whiting, a coarse calcium carbonate, is stretched 550 per cent.
Under these identical conditions the contrast with the pre
ceding pattern is remarkable. The rubber crystallizes at this elongation, but the whiting crystal grains are entirely unaf
fected by the stretching operation and remain in the original random arrangement. The grains are too large and the bond
1 K a t z , J . R ., a n d B i n g , K . , Z. angew. Chem., 3 8 , 4 3 9 ( 1 9 2 5 ) .
Fi g u r e 9 El e c t r o n Ph o t o m i c r o g r a p h s o f Cr y s t a l s o f Le d a t e
204 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 35, No. 2 is too weak to permit a response to stretching deformation;
the rubber must tear loose and, in a sense, flow around and past these particles like water in a stream around a fixed boulder; an exaggerated case is the same effect seen when rub
ber contains grains of sand. Stretching to break at about 700 per cent has no greater effect on the whiting grains.
Figure 5/ shows a specimen containing 25 parts Kalvan and 5 parts red oxide, stretched 675 per cent. It again demon
strates the marked fibering of the Kalvan grains. Exactly the same phenomenon is shown for any specimen containing Kalvan, from 5 to 75 parts. With 75 parts of Kalvan as a common commercial proportion, the fibering is prominent;
but on account of the preponderance of the mineral crystals which absorb much of the x-ray energy, it is difficult to repro
duce clearly the rubber interferences. Especially noteworthy is the enormous increase in tear resistance in increasing the
The differences between Kalvan and gilder’s whiting have already been noted in rubber. The differences in particle size and shape of these materials accounts for the difference in behavior. Figure 7 shows electron photomicrographs of these two materials; Figure 8 is a microphotometer tracing of the principal interference of calcite taken from x-ray patterns of these two materials for the purpose of computing particle
size. Gilder’s whiting is out of the range of the x-ray method.
Kalvan consists of plates about 300 A. in diameter, as shown by either the x-ray method or by direct measurement from the electron photomicrograph.
Figure 4 shows that Ledate (lead dimethyldithiocarbamate) is extremely highly oriented in the rubber matrix on stretch
ing, and the layer lines of Ledate even appear to be in the same position as the layer lines on the pattern for the inter
ferences from the rubber crystallites themselves. This indi
cates that an extraordinarily strong bond exists between the rubber crystallites and the lead salt. A picture of this lead salt was taken under the electron microscope (Figure 9);
the general shape of the material is that of long needles. Such a geometric shape would easily be completely lined up in the rubber matrix on stretching. Furthermore, these needles apparently grow or are made up from very small particles.
They are clearly seen at higher magnification. Perhaps these extremely small particles account for the extraordinary bond between rubber and Ledate.
Tuads and its derivatives give extremely highly oriented salts in the rubber matrix on stretching. Ordinary (or methyl) Tuads is frequently used in higher loadings of 3 to 5 per cent, especially in the compounding of reclaim rubber. The pres
ence in these stocks of considerable highly oriented salt may account for some of the desirable physical properties observed with these higher percentages. The Captax family of accelera
tors is not so highly oriented in the rubber matrix as the portion of the pressure-temperature curves for mixtures of 3 to 4 mole per cent hydrogen contained minima;
likewise, the paths of the bubble-point curves of mixtures containing greater than 4 mole per cent hydrogen suggested that minima would have been obtained for these mixtures if the measurements had been extended to higher temperatures.
These results are reported because they exemplify an interest
ing type of phase behavior which may be a characteristic property of a mixture consisting of a liquid and a slightly soluble gas, and because they will be useful in engineering calculations.
E x p e r im e n ta l A p p a ra tu s a n d P ro ced u re The apparatus used for obtaining the solubility data was a stainless steel bomb with a capacity of 400 cc. The sampling line was a 3/s inch o. d. X Vie inch i. d. steel tube which ex
tended to a point Vi inch from the bottom of the bomb. A thermowell of similar length was provided. Pressures were read from a calibrated Bourdon gage, and temperatures were measured by an iron-constantan thermocouple. The bomb was incased in an electrically heated jacket and was mounted in a motor-driven cradle so that it could be oscillated through a 60° angle. With each revolution of the driving motor the
E. E. N E L S O N AND W . S . B O N N E L L Gulf Research and Development Company, Pittsburgh, Penna.
longitudinal axis of the bomb passed through the horizontal, and thus it was possible to sample either the liquid or the vapor phase from the same sampling line by stopping the bomb motion at either the bottom or the top of the arc.
The procedure followed in making a series of solubility de
terminations was to charge the bomb with hydrocarbon to give a liquid volume of approximately 50 per cent of the total bomb capacity. After the bomb had been placed in the in
sulated jacket and heated to the desired temperature, hydro
gen was charged from a high-pressure cylinder. Samples were not taken until the bomb temperature had remained within 0.5° C. of the desired value for at least 1 0 minutes;
then the bomb motion was stopped in either the high or low position, depending upon which phase was to be sampled, and the sample line was purged by venting to ensure representa- lve sampling. The pressure and temperature were recorded e ore and after the sample was taken. Gas samples of ap
proximately 500 cc. were taken by displacement of brine and
^or hydroSen by being passed over copper oxide a .. ^ ^ is temperature the hydrogen was completely oxi ized, and the n-butane was not appreciably affected,
is was confirmed by the preparation and analysis of syn- e ic mixtures of pure hydrogen in pure n-butane. The
February, 1943 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 205
M O L % HYDROGEN IN t h e VAPOR PHASE
Fi g u r e 1 . So l u b i l i t t o f Hy d r o g e n* i n ti- Bu t a n e
water vapor formed in the oxidation of the hydrogen was con
densed in a small water layer present in the analytical ap
paratus; hence the humidity of the sample was not changed by any appreciable amount during analysis, and the decrease in volume represented the volume per cent hydrogen. Por
tions from each sample were analyzed until the results of successive analyses agreed within 0.2 per cent. The volume percentages of hydrogen determined by the oxidation analyses were corrected to mole percentages by compressibility factors for hydrogen and n-butane, and a correction was applied for the amount of water vapor present during analysis.
E x p e r im e n ta l D a ta
The experimental results obtained at 23.9°, 82.2°, and 115.6° C. are summarized in Table I, and the data are plotted
Fi g u r e 2 . Bu b b l e- Po i n t Cu r v e s f o r Mi x t u r e s o f Hy d r o g e n a n d 7 1 - Bu t a n e
T h e s o lu b ility o f h y d r o g e n in n - b u t a n e w as d e te r m in e d a t 2 3 .9 °, 82.2% a n d 115.6° C ., a n d a t p r e ssu r e s u p to 1 0 0 a tm o s p h e r e s . A t p r e ssu r e s b e lo w 30 a tm o s p h e r e s , s o lu b il
it y d ecr e a se s w it h in c r e a s e in te m p e r a tu r e , fr o m 30 t o 40 a tm o s p h e r e s te m p e r a tu r e h a s l i t t l e e ffec t o n s o lu b ilit y , a n d a b o v e 40 a tm o s p h e r e s s o lu b ilit y in c r e a s e s w it h i n c rea se in te m p e r a tu r e over t h e t e m p e r a tu r e r a n g e in v e s tig a t e d . A t c o n c e n tr a tio n s o f h y d r o g e n a b o v e 3 m o le p er c e n t , t h e s h a p e s o f t h e P -T b u b b le - p o in t cu rv es d eriv ed fr o m t h e s o lu b ilit y d a ta s u g g e s t t h e e x is t e n c e o f a re g io n o f is o b a r ic r e tr o g r a d e c o n d e n s a tio n a t t e m p e r a tu r e s fa r b e lo w t h e c r itic a l r e g io n o f t h e m ix tu r e s .
D a l a rep o r te d b y K ay (2) fo r t h e s y s t e m h y d r o g e n -n a p h th a s h o w a s im ila r p h a s e b e h a v io r , a n d i t is b e lie v e d t h a t t h e m e a s u r e m e n ts o n h y d r o g e n - n - b u t a n e c o n s t i
t u t e a d d it io n a l e v id e n c e in s u p p o r t o f K a y ’s s u g g e s tio n t h a t t h is m a y b e a c h a r a c te r is tic p ro p erty o f a g e n e r a l c la s s o f m ix tu r e s c o n s is t in g o f a liq u id a n d a s lig h t ly so lu b le g a s.
in convenient form in Figures 1 and 2. The relation between total pressure and solubility at constant temperature is given by the curves of Figure 1, which also includes some data on vapor composition. The vapor phase samples were taken p rim arily to determine whether a single- or two-phase con
dition existed within the bomb; although these vapor phase data are incomplete, they have been included since they will be useful in estimating phase equilibria. Figure 1 shows that the constant-temperature lines intersect in the range of 3 to 4 mole per cent hydrogen. Below about 3 mole per cent, solubility decreases with increase in temperature for a given pressure. For mixtures containing 3 to 4 mole per cent hydrogen, composition is substantially independent of tem
perature over the range investigated, and for mixtures con
taining more than 4 mole per cent, solubility increases with an increase in temperature.
T a b l e I. So l u b i l i t y Da t a f o r Hy d r o g e n i n n -- Bu t a n e T e m p e r a t u r e , P r e s s u r e , .--- M o l e % H r i n : --- •
° C . ( ° F . ) A t m . A b s . L i q u i d p h a s e V a p o r p h a s e
2 3 . 9 2 2 . 2 2 . 0 8 3 . 2
(7 5 ) 7 8 . 6 6 . 2
1 0 3 . 0 7 . 4
8 2 . 2 4 2 . 6 4 . 0 6 2 . 7
( 1 8 0 ) 6 0 . 8 6 . 3
6 9 . 0 7 . 0
9 3 . 6 9 . 9 8 3 . 3
1 0 5 . 8 1 1 . 1
1 1 5 . 6 3 8 . 6 • 3 . 2
( 2 4 0 ) 4 8 . 9 5 . 1 4 2 . 0
6 5 . 9 7 . 5
7 1 . 0 8 . 3
9 1 . 8 1 1 . 1 6 2 . 4
206 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 33, No. 2 When the liquid phase data of Figure 1 were replotted as
bubble-point pressure against temperature, the curves of Figure 2 were obtained. The accuracy of these curves is questionable since only three points were available for draw
ing each curve; however, it is clear that a minimum occurs in the bubble-point curve for mixtures containing 3 to 4 mole per cent hydrogen, and the curvature of the bubble-point curves of mixtures containing greater than 4 mole per cent is such that a minimum would probably have been obtained if the measurements had been extended to higher temperatures.
L I Q U ID
F i g u r e 3 . P -T B o r d e r C u r v e f o r a M i x t u r e H a v i n g a M i n i m u m P o i n t i n I t s B u b b l e - P o i n t C u r v e
The solubility data reported here are not to be considered as derived from precision measurements; however, the data are sufficiently accurate for engineering purposes. The method of analysis was checked by preparing synthetic mix
tures of pure hydrogen in pure n-butane. Analyses of these mixtures indicated that the degree of accuracy to be expected from the method of analysis was ±0 .2 mole per cent hydro
gen; thus the possible percentage error in solubility caused by inaccuracy in analysis would be relatively high at low concen
trations, but would be as low as 2 per cent for a mixture con