T OF MOISTURE ON GR-S Rate of Cure and Physical Properties IAN C. RUSH Canadian Synthetic Rubber Limited, Sarnia, Ontario, Canada
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OISTURE has been least three batches had Moisture definitely accelerates the rate of cure of GR-S; discussed as a factor been tested for each moisthe increase is a function of the moisture retained at the which might give rise to ture content and each completion of mixing and not of the total water added to variable rates of cure of method of addition. The the batch. The normal variation in moisture content of GR-S type polymer. This averages of the indiGR-S and carbon black as received are not sufficient to moisture might be present vidual results (stressaffect the rate of cure of GR-S, since these amounts are in the GR-S polymer itself strain data and per cent lost by evaporation during mixing. The action of the or in the compounding moisture retained) on moisture appears to be confined to the curing stage; it i n g r e d i e n t s used. Acbatches to which the may possibly act as an activator for thiazole-type accelercordingly, a program same amount of moisators with, perhaps, chemical reaction involved. Moiswas initiated in the ture was added, were ture does not affect physical properties of mill-mixed s p r i n g of 1944 t o then considered free from GR-S stocks, but may result in improved processing establish the influence day-to-day variations. characteristics and physical properties in factory of m o i s t u r e n o t o n l y operation. MIXING, CURING, on the rate of cure of GR-S TESTING but also on its ohvsical All batches in any one series were mixed from the same sample properties. Since that time two papers have been published on this subject by other investigators (1,6). The results reported of raw polymer. It should be noted here that the polymer used here verify some of the conclusions drawn by these investigators for different series was not the same. Before use the various samples were checked periodically for moisture content and were but seem t o be at variance with others. not used until they had reached equilibrium values of less than I n this study various amounts of water were added in the 0.1% (here considered 0% moisture). The chemical analyses following ways: by premixing with carbon black, by adding of these samples are shown in Table I. To ensure uniformity, directly on the mill rolls a t the completion of normal milling, sufficient compounding ingredients for all batches in this work and by soaking GR-S crumb in water. Curing curves were obtained for each batch and were used to evaluate the rate of were set aside. They were then desiccated as required for 24 hours before use. cure. To eliminate day-to-day variations in physical properties One operator carried out all mixing and curing operations, due to error in testing methods, three batches of different moisMixing was done on a 10 X 20 inch mill (gear ratio, 1 to 1.4). ture contents were mixed and tested on the same day. This same The official recipe of the War Production Board was used: GR-S. group was then remixed and tested on successive days until a t _
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0*45
t
/X I / - - - - -
l-*-
Z-o+-o WATCR ADDllD TO MILL ROLLS
2c-c-O-
SOAKLD IN WATLR
3d-A-&POLYMER
>A P
0.101 0
I
I
I
2
,
1
1
1
4 5 6 %WATER ADDED
7
...
1
I
7
8
9
WATER ADDEO TO CAR5ON WATLR ADDED ON MfLL R O L L S n POLYMER SOAKED IN WATER
10
0.0 (BASED ON POLYMER)
aFigure 1. Wect .of Water Added on Modulus Ratio
0.5
I.o
1.7
2.0
2.)
7.0
M
%WATER RETAINED
Figure 2. Modulus Ratio as a Function of Water Retained
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1946
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pared: time to maximum tensile (T,,,),time to constant modulus (Mmo = 1000 pounds per square inch), tensile ratio (TR), modulus ratio ( M R ) . Time to maximum tensile is inherently inaccurate because of the nature of the curve for tensile vs. time of cure and because of the poor reproducibility of the points of break of GR-S test strips. Time to constant modulus appears to give a good index of rate of cure if day-to-day variations in testing can be eliminated and if polymers of like gel contents are compared.
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TABLE I. CHIDUICAL ANALYSISOF SAMPLES(IN PERCENT) --Method To carbon blaok
Ethanol-toluene-meotrope extract Free fatty acid Free 608 Total asg NaCl
-I
-
WATER
ADDED TO
CARBON
2 -o--O- WATER ADDED ON MILL ROLLS
3-
I
OP
Figure 3.
POLYMER SOAKED
r:o
Ik
io
IN WATER
2.5
RETAiNED ( m E P ON POLYMER)
J
Relation between Water Added and Water Retained by a Batch
of Moisture Addition-
On mill
Polymer soaked in water
6.9s 4.83 0.06 0.89 0.75
7.36
6.85 5.10 0.2.9 1.26 0.85
rolls
5.00
0.12 0.87 0.65
Tensile ratio (9)appears useful; i t is the tensile of a marked undercure divided by the maximum tensile. This eliminates the first objection to "time to maximum tensile" (i.e., trying to pick the point at which the tensile reaches a maximum from a very flat curve), but i t does not obviate the disadvantage of poor reproducibility of tensile tests. Modulus ratio appears to be as good as or better than any of the previous methods. Because of the nature of the curve for modulus os. time of cure, i t is unnecessary to use the modulus of the optimum cure. Therefore, a marked undercure and a marked overcure can be used-in this case, the ratio of the 15- to the 90-minute moduli, where both moduli are taken at 300% elongation. An increase in the rate of cure is thus indicated by an increase in the modulus ratio. RATE OF CURE
100 parts; carbon, 50; B R T No. 7, 5 ; zinc oxide, 5 ; sulfur, 2; a n d Captax, 1.5. I n the first series moisture was added to predried carbon black immediately before mixing and dispersed as thoroughly as possible by hand. Where wet polymer was used, the weight of CR-S per batch was varied so that exactly 100 parts of dry pplymer were used. Milling times were held constant at the time specified in the official Rubber Reserve Company's procedure for testing GR-S. The additional time required to add t h e water directly to the mill rolls at the completion of normal mixing was roughly proportional to the amount of water added. This additional milling time probably had little effect on modulus since the batches were first removed from the mill for weighing and thus cooled. Further, additional milling at that stage would tend to lower and not raise the modulus and would not, therefore, show up a n increase in rate of cure. Curing was at 292' F. In the first series where the moisture was added to the carbon black, the moisture retained was taken as the difference between the actual batch weight and an assumed normal dry batch weight. This assumed weight was based on some forty dry batches for which the maximum variation in weight was less than 0.08%. I n the second series the moisture retained was determined by weighing the mixed batches immediately before and immediately after the addition of water. I n the third series the water added by soaking the GR-S crumb was determined by the standard R R C hot mill method. The water retained was then determined on an assumed normal dry batch weight as in the first series. Stress-strain data were determined on a Scott L-6 tensile tester which was thoroughly overhauled and cross-checked with the National Bureau of Standards, machines immediately before this program started and rechecked periodically during the course of the work. Considerable difficulty arose in this work in finding an accurate index to rate of cure. Four indices were investigated and com-
Table I1 shows the increase in rate of cure indices with increasing moisture content for the various series. These data ar9 plotted in Figure 1 against the total moisture added to the batches. If the acceleration of the rate of cure were a function of the total water added to a batch, one would expect that these curves would at least be parallel. I n an attempt to explain these differences, the results were replotted in Figure 2, the
TABLE 11. EFFECT OF METHOD OF ADDINGWATERON MODULUS RATIO
1 6 8 9 2 3 4 5 13
1 4 5 6
7 8 9 10 11 12 13 ' 1 6
a
Moisture Added t o Carbon Blaok 238 1413 0.0 1377 220 0.0 272 1486 0.0 280 1428 0.67 347 1470 0.83 6.0 1470 440 1.5 7.0 1450 483 1.9 8.0 1497 2.25 600 9.0 1510 2.42 673 10.0 Moisture Added to Mill Rolls 1410 165 0.0 0.0 1525 205 0.375 1.0 1540 245 0.59 2.0 1586 357 1.20 3.0 1595 475 1.50 4.0 1600 545 1.54 5.0 1655 615 2.0 6.0 1640 605 2.0 7.0 1660 675 2.8 8.0 1645 675 3.0 9.0 1580 680 3.12 10.0 Raw Polvmer ___ . ~ "Crumb Soaked in Water 1380 160 0.0 0.0 1400 180 0.0 1.6 1370 185 0.0 2.7 1400 180 0.0 3.5 1335 160 0.25 4.5 1425 190 0.83 5.6
0.0 1.0 3.0 4.0
3 2 4 6 Based on polymer.
0.168 0.160 0.183 0.196 0.236 0.299 0.333 0.401 0.446 0.125 0.135 0.155 0.225 0.298 0.340 0.372 0.370 0.406 0.411 0.430 0.116 0.128 0.135 0.128 0.120 0.133
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findings are in substantiaf agreement with others previously reported (4). 7000 The calculation of \\ater retained in the series where i t was added to the TENSILE carbon black is admittedly inaccurate 2600 since an error of 0.1% in weighing n-ould give an absolute error of 0.163% in the calculated moisture retained. If the experimental values of per cent 2200 moisture retained for the serier in which the moisture was added to the vi carbon black are corrected t o curve 1, 1800 'f: Figure 3, the curve for modulus ratio w us. water retained parallels moie closdy 2 v) the curve for the moisture addition on 2 the mill rolls and gives further cvidence 1400 of a break above 2% moisturc retained. Figure 4 shows typical curing Curves for a normal dry batch and the same io00 stock plus 2.4% moisture retained. The difference in the rate of cure is apparent. Braendlo and Wiegand's paper ( 1 ) 600 on this subject shows a graph of per cent moisture in polymer against time TIME OFCURE (MlNj to best cure, in which an increase 70 40 50 10 70 80 200 10 in nioirture content of GR-8 poly. . mer from 0.1 to 0.5y0 is shonm tjo Figure 4 . ( 'omparison of Curing ( : I J ~ Ie s decrease the time to best cure from 100 to 70 minutes at 280" F. Further, two thirds of t>his difference, or 20abscissa being changed to per cent moisture retained when the minute curing time, is shcwn to occur between moisture conbatch is finally removed from the mill. tents of 0.13 and 0.19%. I t seems unlikely that a difference The curves of Figure 2 check more closely than those of of 0.06% water could cause such a large variation in thc rat&of Figure 1. Figure 2 seems to show that there is little acceleration cure. Moreover, the cntirc moisture range of their graph is far of the rate of cure until the moisture retained exceeds 0,25$Zc. below the minimum raw polymer moisturc content which the Between 0.25 and about 2% there is a considerable increase in present data shorn is necessary for any acceleration of the rate of the rate of cure. If the data are plotted as the logarithm of cure. A study of the dat'a used by these investigators reveals modulus ratio against moisture retained, this region appears as that they used polymers from three different sources. While a straight line, probably indicating chemical reaction during GI%-Sfrom any one producing plant should have a uniform rate cure. Above 2% moisture retained, the curves flatten out and of cure, GR-S from different plants may not have. For example, thus show little further acceleration of the rate of cure. This GR-S coagulated wit'h salt and acid contains up to 0.75% sodium region, however, is probably of little practical value since there chloride, a known accelerator of cure. It is to be expected, would be considerable danger of the stock blistering during cure. therefore, that this material will be faster curing than alumCurve 3, Figure 1, is a straight line and indicates a practically coagulated GR-S which contains no sodium chloride. constant rate of cure. In Figure 2, however, this curve is so compressed that the experimental points appear to cover only the initial stage of slight acceleration; that is, most of the O F hIOiSTURE ON P I I Y s I C i l L PROPERTIES TABLE 111. EFFECT moisture added to the polymer in this manner is lost during OF GR-S milling. Since i t was impossible to handle polymer with higher Code % 1Ioisture 70Mobture T w s o a = 1200, EYsoo = 1200, initial moisture contents than those shown, this curve could not NO. Added" Retained" Lb./Sq. In. 75 be continued. It appears likely that, if this curve could be Moisture Incorporated through Carbon Black extended, i t would show the same region of rapid acceleration 0.0 575 1 0 0 as the other two curves. 590 1.0 0 0 6 595 3.0 0 0 8 The curves of Figure 3 show that the earlier the moisture is 600 9 4.0 0 67 2 6.0 590 0 53 added in the mixing procedure, the lower the amount of water 585 7.0 1 5 3 retained. The relatively small difference between curves 1 and 590 8.0 1.9 4 ;95 0 . 0 2 . 2 5 5 3 and the large difference between curves 1 and 2 are explained 080 2.42 10.0 13 by the rapid increase in the batch temperature when the carbon Moisture Added on Mill Rolls black is added. The ordinates a t 0% water retained for curves 0.0 0.0 1 1 and 3 are of particular interest. No moisture is retained, and 0.375 1.0 4 2.0 0.59 5 thus there is no acceleration in the rate of cure until the carbon 1.20 3.0 6 black contains 3y0 moisture (on the polymer) or the polymer 1.50 4.0 7 1.54 5.0 8 itself contains 3.5$Zn moisture. Since the maximum moisture 2.0 6.0 9 2.0 7.0 10 content of carbon black as received is 2% (1) of the polymer 2.8 8.0 11 weight and since the maximum moisture content of specification 3.0 9.0 12 8.12 10.0 13 GR-S is 0.75%, i t appears that no moisture will be retained from a Based on polymer. either of these sources; therefore, neither source should contribute to variations in the rate of cure as far as moisture is concerned.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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I n the same paper the following statement appears: normally occurring amounts of moisture in carbon black are held so tightly that they do not effect cure." According t o the data presented here, this water does not affect cure, not because i t is held tightly by the carbon black, but because it is lcst to the batch by evaporation. , EFFECT OF MOISTURE ON PHYSICAL PROPERTIES
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ingredients are in the batch. The probable explanation of this improvement is that water chills the Banbury batch and thereby reduces the heat deterioration of GR-S. Further, at the reduced temperature the stock would be less plastic, more work would be done on it, and physical properties would thus be enhanced. This effect is probably secondary to the reduction in heat deterioration. Since laboratory mill-mixed batches are much cooler than Banbury batches mixed in the factory, i t is not surprising that no improvement is noted in laboratory evaluations.
I n order to evaluate the effect of moisture variation on physical properties, it is necessary to pick equivalent rates of cure. As LITERATURE CITED a basis the point was chosen where the 300% modulus equals (1) Braendle, H. A., and Wiegand, W. G . , IND.ENQ.C H E ~ 36, ~ . ,724 1200 pounds per square inch ( M =~1200). ~ ~~ ~and elonga~ ~ i l ~ (1944). The results tion of various batches are compared at this point. (2) Cohrtn, L. H.. and Steinbery, M., IND. ENG.CEEM.,ANAL.E D , (Table 111) indicate no significant variation in physical properties 16, 15 (1944). (3) Macey, J. H., private communication, Nov. 27, 1944. with moisture content. (4)Poulos, T.,Vila, G. R., and Shepard, M. G., private communicaIt has been reported (3) that in factory operation the addition tion, Dee. 9, 1943. Qf water $0 Banbury batches results in improved Processing (5) Rupert, F. E., and Gage, F. W., IND.ENG.CHEM.,37,378 (1945). characteristics and physical properties. This improvement is pRnaENTsD before meeting of the Chemical Institute of Canada in Quebec, observed only when water is added after the compounding June 5, 1945.
Catalytic Conversion of Hydrocarbons Catalytic Promoting Effect of Antimony Tetroxide in the Aromatization of Hydrocarbons with a Chromia-Alumina Catalyst F. E. FISHER, H. C. WATTS, G. E. HARRIS, AND C. M. HOLLENBECK Skelly Oil Company, Puwhuska, Oklu. T h e activity of the catalyst in the catalytic aromatization of aliphatic hydrocarbons was improved to an appreciable extent by the addition of antimony tetroxide to the chromia-alumina catalyst. The aromatization reaction was carried out on three different types of charge stocks: a heptane fraction from a natural gasoline, a heavy straight-run gasoline, and a thermally cracked gasoline. Both the liquid and gaseous products were analyzed. The production of aromatics during any one reaction period increased through a maximum and then decreased. The yield of gaseous products paralleled the formation of aromatics until excessive cracking superseded the dehydrogenation-cyclization reaction.
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N THE catalytic conversion of aliphatic hydrocarbons to aromatics, considerable effort has been focused on the oxides of the metals of Group VI of the periodic table as catalysts. Chromium and molybdenum oxides have received special attention as catalysts for the dehydrogenation cyclization of hydrocarbons. Mittasch et al. ( 6 ) patented a process in 1933 for the production of aromatic * hydrocarbons from low-boiling nonaromatic hydrocarbons using the oxides of the metals of the sixth group alone or in admixture with other materials. Moldavskil et aE. (7)published data in 1936, showing the aromatization of saturated and olefinic hydrocarbons when passed over chromium sesquioxide. Grosse (3)was issued two patents in 1938 on a process for converting aliphatic hydrocarbons of 6 t o 12 carbon atoms to aromatic hydrocarbons, using a catalyst comprising the oxides of the metals of Group VI deposited on an activated alumina ocarrier.
Considerable effort has also been spent in improving the activities and efficiencies of these catalysts in the conversion process (1). Various other materials have been mixed with the catalyst, especially various metallic oxides from other groups of the periodic table. However, attention has also been paid to improving catalytic activity by changing the structural nature of the catalyst particles either by the method of manufacture or by special treatments of the compounded catalyst. This paper deals with the activity improvement or promoting effect of antimony tetroxide upon a chromium sesquioxide catalyst. Although this combination of metallic oxides is the same as that described by Burk and Hughes (a), an important difference is found in the method of preparation. The catalyst used in these experiments was made by depo.6 ting chromium trioxide and.antimony tetroxide on an activau2- cilumina carrier instead of coprecipitating the three oxides as a gel, as Burk and Hughes did. CATALYSTANDAPPARATUS
Chromium trioxide was dissolved in water, and the resulting solution was mixed thoroughly with 8-14 mesh, Alorco grade A, activated alumina. The chromium trioxide was added in such proportions that the resulting dried, ignited catalyst contained 8% by weight CrzOa. The wet catalyst was dried slowly with constant stirring at 110" C. in a hot air oven until it appeared to be as dry as the original alumina. The dried catalyst was then moistened with acetone, and the calculated quantity of finely powdered antimony pentoxide was added so that 8 or 10% by weight of the dry, ignited catalyst was antimony tetroxide. The
