ct of Atmospheric d to karious relati.ie humidities 11x3 shown good correlatiolr between moisture content of the mixed stock and the curing rate. Little effect on the other physical propertie? has been obsened. The rate of moisture absorption of three samples of raw polymer and their compounds has been determiiicd. These data show that the presence of only a trace of soap has a marked inAuence on the rake of inoiiture absorption.
The \miability of the curing rate of GK-S compositions has presented a serious problem, both to the factory compounder and to the laboratory charged with establishing and maintaining specifications. 3Ioisture content of the raw polymer has been found to have a definite effect on the curing rate, Using the GR-S specificition recipe, a study of the effect upon the physical characteristics of vulcanizates due to exposure of the mixed stocks
RECENT publication by Braendle aQd Kiegand ( 1 ) demonstrated that the moisture content of GR-S stocks has a marked effect on the rate of cure and, to a lesser degree, on the physical propertic. of the vulcanizates. Other unpublished d a b of The R. F. Goodrich Company and The Goodyear Tire ant1 Rubber Company show clearly that moisture hm a definite effect on the plasticity of raw GR-8 as vr.ell rn O i i the rate of cure of its compounds. Hall and Hall ( 8 ) iir England found, independently, the same effect, of moiiture on the rate of cure and processing. Natural rubber compositions show a similar effect due to moisture (3). It has been observed that, in testing a large master batch of GR-5 a t intervals, variations in ratc of CUPO and modulus values beyond experimental error occurred without any apparent relation to time of stnnding. The humidity of the storage room was one of the iincontrollcd variables. Therefore, at the request of the Committee on Specifications for Synthetic Rubber, an investigation of the effect of atmospheric humidity during storage of uncured st,ock on the stress-strain relations and the rate of cure of GR-S was undertaken.
28
2.4 R.S STOCKS.
kP W
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3
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4
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RAW 3lITERIALS AND MOISTURE DETERMINATIOh
14
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Three samples of GR-S were chosen because of their variation in soap content. They were supplied by tkic. Office of the Rubber Director and designated by tiic? following code: GR-S ORD High soap (HS\ Zero soap ( Z S ~
Soap Content 0.18% 0.70 0.0
The other ingredients used and their source were ac follow: carbon black, E. P. black; BRT S o . 7, zinc oxidc, sulfur, and mercaptobenzot,hiazole, Offce of Rubber Director. CRCDF.GR-S. Aloisture x a s determined by the hot mill method. A sample of a t least 200 grams wa: xyeighed t,o 0.1 gram, The volatile matter (moisturc
F. E. RUPERT .Wonsunto Chemical Company
Akron, Ohio
F. W . GAGE 1
4
6 8 !O DAYS EXPOSURE
12
14
378
Pittsburgh Plate Glass Cornpun2 Barberton, Ohio
INDUSTRIAL AND ENGINEERING CHEMISTRY
Apxil, 1945
and/or styrene) was driven off by passing this sample repeatedly through a mill with the rolL at a temperature of 210' to 230' E'. At the start a mill opening of approximately 0.1 inch was used, and this was reduced as rapidly as possible without converting the GR-S to crumb. When the opening had been reduced to about 0.01 inch, the rubber was weighed at intervals until constant veight was obtained. This usually required about 20 minutes. The following data were obtained: ORD GR-S, 0.14% volatile matter; XS GR-S, 0.18%; HS GR-S, 0.63%. CARBON BLACK. The moisture content in the black was determined, by heating at 105' C. for 24 hours, to be 1.30%. These determinations were made on the material as received without conditioning. MIXEDSTOCK. The hot mill method as described above was used on a sample of about 25 grams. In this case constant weight was attained in about 12 minutes. The following results were obtained: ORD GR-S, 0.09% volatile matter; ZS GR-S, 0.10%; HS GR-S, 0.13%.
379
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FIG. 3 EFFECT OF HUMIDITY ON 300%MODULUS OF 25-MINUTE CURE
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EFFECT O F HUMIDITY O N RATE OF CURE
d i - l - t ~
MILLING PRACTICE
WEIGHINGOF INGREDIENTS. All ingredients were weighed to within 1.0% of the weights specified. The final weight of the mixed batch was in each case less than the sum of the ingredients. The data obtained follow: Sum of weights of all ingredients in each case was 2616 grams except that the GR-S was increased by 10 grams in the case of the high soap GR-S. This allowance was made because of the relatively high moisture content of the original crude GR-S: GR-S ORD
zs
HS
Original Wt. 2616 g. 2616 2626
Final Wt. 2605 g. 2604 2607
Loss in Mixing 0.42% 0.46 0.72
MILLINGEQUIPMENT. The mill used for the mixing of the three stocks had rolls 8 inches in diameter and 16 inches in length with a working distance between the guides of 15.25 inches. The speed of the slow roll was 23 r.p.m., and the speed of the fast (back) roll was 30 r.p.m. Two batches were mixed from each of the GR-S samples and were thoroughly blended to give a theoretical batch weight of 2616 grams. The actual weight of each batch mixed was 1308 grams according to the following formula: GR-5 E. P. black BRT Np. 7
Zinc oxlde Sulfur .Meroaptobenzothiszole
100.0 50.0 5.0 5.0 2.0 1.6
__.
163.5
800 400 40 40 16
12 1308
During the mixing, the temperature of the rolls was determined a t various times by a Cambridge surface pyrometer. The temperature varied between 123" and 130' F. MIXINGAND MILLINGPROCEDURE. The following schedule was observed during the mixing of each batch. Whenever a cut is mentioned, it was made by cutting three quarters of the way across the roll and holding until the bank just disappeared.
When cuts each way are mentioned, they were made from alternate directions. Various steps in the mixing operation follow: GR-S was passed through the rolls twice, without banding at
a mill setting of 0.008 (i0.002) inch. Total time elapsed, 1 minute. GR-S mas banded on the front roll with mill set at 0.055 (10.005) inch and 8/4 cuts were made from alternate sides every 30 seconds. 'Iota1 time elapsed, 11minutes. Carbon black was added slowly and evenly across the mill; the mill was opened at intervals to maintain a constant bank. A */4 cut was made from each side when half of the black was added, and another from each side when all the black had been added except that which had fallen through the mill rolls. Then the black in the pan was added. Total time elapsed, 23 minutes. Melted softener was added. Softener had been kept a t about 210' F. for 30 minutes. Zinc oxide, sulfur, and accelerator were added carefully. Total time elapsed, 29 minutes. Three cuts were made from each side. Total time elapsed, 31 minutes. Batch was cut from mill and with mill setting of approximately 0.030 inch, the rolled stock was passed endwise through the mill six times. Total time elapsed, 33 minutes. The batch weight was checked a t this point. After a second batch was prepared by this procedure, the two were blended. The stock was passed through the rolls twice without banding at a mill setting of 0.008 inch. The stock was banded, and three 8 / d cuts were made from each direction and sheeted to approximately 0.085 inch. The batch was cooled to room temperature on a
380
INDUSTRIAL A N D ENGINEERING CHEMISTRY
I
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2500
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CNSILE
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In
Vol. 37, No. 4
zinc-covered surface and cut into 5 3 / , X 53/, inch slabs. When the batch had reached room temperature, the moisture in the mixed batch was determined by the hot mill method. COKDITIONING, CURING, AKD TESTING
Cnvulcanized slabs of each batch and of each type of GR-9 were placed in desiccators to give constant relat'ive humidities as follows:
,2000 CI
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Relative Humidity
w
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0% 60% 100%
G 1000 500
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While conditioning in the desiccators, the compounded slabs were placed on galvanized screens ('/r-inch mesh) which were separated by slabs of wood, '/,-inch thick. By this method practically the entire surface of each slab was exposed to the desired relative humidity. Slabs of each crude GR-S and of each of the compounded stocks were weighed accurately before being placed in t,he desiccators, and then veighed every 24 hours to determine the moisture pickup or loss. It should be noted that slabs exposed for 14 days a t 1 0 0 ~ orelative humidity showed signs of bloom, the nature of which was not determined. This w m slightly in evidence with those slabs exposed for 14 days to 60% relative humidity. Conditioning was done a t 25" ( =t2.0°)C. CURING. After exposure periods of 24 hours, 6 days, and 14 days, three sheets of each stock from each humidity condition were cured, one each a t 25, 50, and 90 minutes at 292' F. A mold similar to that shown in Figure 1 of A.S.T.M. (specification D15-41) was used, and all curing was done according to procedure described in the specification. TESTISG-. Only three strips were pulled in each case unless good agreement was not obtained. I n such instances more dumbbells were tested until satisfactory agreement was reached. The weight pickup of slabs during the 24 hours of elapsed time between curing and testing was determined for the 25-minute cures since these should show the greatest pickup. It was found t o be negligible and so was made only for the 1- and 6-day exposures. Constant humidity control during this period was not available, so relative humidity during these resting periods is recorded (Table I). RESULTS AND DISCUSSION
1000
500
0
i
Content of Desiccator Anhydrous CaCll 38.7% HiSO4 Water
60%R.H EXPO!
'
Figure 5
The GR-S samples were selected to demonstrate a wide range of moisture absorption rates. While it was realized that there are several constit,uents of GR-S which might influence the rate of moisture absorption, it was assumed that the soap content would be a Satisfactory index of this property and selection was made on this basis. The rates of moisture absorption of the three GR-8
April, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
381
1
1 1 I I I I samples are illustrated in Figure 1. It is EFFECT OF MOISTURE CONTENT IN CARBON BLACK O N observed that the rate of absorption of mois3DULU5 ture by raw GR-S is roughly proportional to the humidity of the atmosphere in which it is conditioned. However, the presence of only a trace of soap has a marked influence on moisture absorption. The ORD sample contains 25% of the soap content of the high soap sample; yet under conditions of 100% relative humidity its moisture pickup, over and above that of the sample with 0% soap content, is 85% of that of the high soap sample. On the other hand, the absorption of moisture by the compounded stocks shows a more direct relation with the soap content. This is illustrated in Figure 2. The object of this investigation was to determine the effect of this absorbed moisture on the rate of cure as indicated by the stressstrain characteristics of the vulcanizates. To 3Ak tk0 illustrate this effect, the 300% modulus values of the 25-minute cured stocks were selected Figure 6 since these stocks are definitely undercured and naturally show greater variations than those stocks nearer the optimum cure. This I I 1 I J5 AND TENSILE O F HEVEA R U E # E 4STOCK use of modulus seems justified since all sheets of each base polymer were from one blend of laboratory-mixed stocks. Figure 3 presents graphically the 300% modulus figures of the 25-minute cured stocks of the three compounds after various conditions of exposure. This chart demonstrates that the rate of cure increases with increased humidity and increased length of exposure It has been shown that moisture absorption of the compounded stocks is roughly proportional to the soap content of the GR-S, but a study of the effect of this absorbed moisture on the rate of cure fails to reveal that the changes in rate of cure can be traced entirely to the variation in soap content. The changes in rate of cure appear to be more directly a function of relative humidity and time of exposure. By soap content is meant, not the effect of the soap as such, but the effect it exerts through its influence on moisture pickup. SURE- 35'C. Figure 4 presents the same data in modified form by plotting the time required to attain Figure 1 a 300% modulus of 1000 pounds per square inch. Here again the marked effect of both humidity and time of exposure on rate of cure are illustrated, gressively higher tensiles of the undercured stocks as the huwithout much correlation between soap content and rate of cure. midity and time of exposure increase. Figure 5 gives both 300% modulus and tensile figures for the The results of this investigation are substantiated by the data obtained by the Carbon Black Nomenclature Committee in a three stocks for all cures, humidities, and exposures. I t appears that the optimum physical properties vary but little as a result study of the influence of moisture in carbon black upon rate of of exposure to high humidities for relatively long periods of time, cure. Figure 6 illustrates changes in modulus and tensile figures due to increasing moisture content of the carbon black. However, the increase in rate of cure is illustrated by the proLittle change in the optimum physical properties is observed, but there is a decided increase in the modulus and tensile values of the undercured vulcanizates with increasing moisture content. TABLE I. WEIGHT PICKUP O F CURED SLABS" DURING %-HOUR To compare the results of this investigation with similar rePERIOD BETWEEN CURING AND TESTINQ sults obtained on natural rubber, data were taken from a report Conditions of Storage of the A.C.S. Physical Testing Committee (3). Using the % Rel. TEmp., % Wt. Daya of Exposure hum. C. GR-9 Change before Cure following formula, data were obtained for stocks exposed to 10, 22 24 ORD -0.06% 1 40, 70, and 100% relative humidities at 23" and 35" C. for 2 days ZS
