March, 1928
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
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New Process for Curing Cord Tires and Molded Tubes' Henry R. Minor GENERAL CARBONIC
CO ,
NEW
YORK
hT. Y .
and the amount will I-ary from bag to bag causing variation in the tire cure. The.spitting of dope when removing hot bags from tires causes further variation, loss, painful burns, and slippery floors. The air used to supply and maintain pressure frequently carries water and oil with it, so that examination of an air bag at the end of its life will frequently reveal a heterogeneous mixture of dope, oil, and water, and the wonder is that the bag lasted as long as it did. The last tires cured obviously received a very different cure than those when the air bag, was new. HOT-WATERPROCESS-This process is not new hut mi-. til recently it had not be,en generally adopted. Manuf a c t ur ers were hesitant about installing a process Processes Now Used A new process for the curing of cord tires and molded which from the standpoint tubes, with temperature-pressure control by means of of air-bag life and air-bag There are two well-known inert gas and steam, is presented for the consideration cost offered no apparent adprocesses-the air-glycerol of the rubber industry. This process is the logical vantages over the air-glycprocess and the hot-water sequential development of a number of years' labor erol process, while on the process. Not so well known devoted to this particular problem, during which other hand there was an is the carbon dioxide procperiod sufficient experimental and commercial applicae x p e n s i v e installation to ess, which has made intertion has been made to warrant the optimistic outlook consider-expensive to inesting progress i n r e c e n t held for it. stall and expensive to mainy e a r s b e c a u s e of sound It has the advantages of each of the older processes tain-the argument that by theory and improvements and the disadvantages of neither; it is distinctly differc u r i n g from two sides a in the process itself. quicker cure was obtained AIR-GLYCEROLPROCESS ent and, beyond the observation of certain simple precautions, apparently offers few difficulties in its applicabeing offset by the fact that -In the air-glycerol proction. in actual time of cure users ess it is customary to add of the air-glycerol process small amounts of glycerol were competing favorably or other preservative to the air bag from time to time as a preventive of oxidation, with the best in hot water. and in-curing to supply the pressire with air. This procWith the advent of thicker tires made necessary by the ess has the advantages that it is relatively simple, the phenomenal growth of busses, much greater interest has workmen are used to it, the reports of air-bag life appear to been evidenced in the hot-water process. With eight-, ten-, equal or exceed the best reports with other processes, and and twelve-ply tires in daily production it is obvious that the time of cure appears to be just as short as in any other. where heat is applied from the outside of the tire only the The air-glycerol process has its disadvantages, however. inner plies are going to be undercured. Further, if this is Leakage of air into the heater from the manifold and air- the best process for the larger sizes, why isn't it the best bag connections during the cure is an objectionable feature; process for all sizes? from two to four times as much air passes into the heater In this process it is customary to supply all the pressure to mix with steam as is actually required for pressure purposes. with hot water, air being removed from the bag by an initial Air settling to the bottom of the heater causes undercuring application of steam, which also serves to raise the initial of tires; wherever air pockets a badly cured tire results and temperature of the bag. The addition of heat units to the air mixed with steam interferes seriously with the heat trans- inside of a bag so that curing progresses from two sides tofer from steam to the mold. This condition has long been wards a center is manifestly an advantage, particularly in recognized by the rubber industry. Some companies en- view of the bus-tire development. When hot water is used deavor to improve conditions by agitation of the steam-air as the pressure medium there will be no leakage of air from mixture; others blow off large quantities of steam from the the manifold and bag connections to mix with the steam in bottom of the heater to remove air at a cost of from $25 to the heater; consequently the conditions of heat transfer $100 per day; still others both agitate and blow. It is doubt- from the steam to the mold are superior to what are obful, however, if conditions are ever equal to those where tained in the air-glycerol process. no air is present. The disadvantages of this process, aside from its high cost The dope used to protect the inner surface of the air bag of installation and maintenance, are the relatively short from the effects of oxidation softens the inner surface and in life of the air bags due to the weakening effect of water and a measure this must be compensated for by added thickness. the harsh treatment of the bags themselves, wet floors and I n adding dope to an air bag there is a gradual accumulation tables, the necessity of evacuating the bags with a costly 1 Received January 6, 1928. vacuum system, the cost of raising water to a high temperature
ODERX requirements in the curing of cord tires call for process improvements which will increase the uniformity of the product and permit equal quality in the larger sizes. Of the many factors bearing on this problem, curing conditions are probably of first importance. The manner in which a tire is cured not only affects the uniformity of the product but is a substantial item of the tire cost. Most tires are cured today in pot heaters holding from twenty to thirty tires piled one on top of the other, the green tires being placed in iron or steel molds and an expansible air bag inserted within each tire, the airbag valves being connected to a central manifold and the reiuired pressure being furnished by air, hot water, or other medium.
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INDUSTRIAL AND ENGINEERING CHEiMISTRY
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and pumping it a t a high pressure, the hazard attendant on its use, the effect of only a slight leakage when the medium pumped is of an incompressible nature, and that while the water cure transfers heat rapidly a t the start it slowly cools off and it is not possible to maintain uniform temperature conditions within the air bag. Curing by steam has been attempted many times, but the fact that for any particular temperature there is a corresponding pressure has hitherto limited its use to relatively low pressures. For example, in the curing of tubes on mandrils this operation is now conducted in open steam a t approximately 45 pounds steam pressure,. the temperature limitations preventing the use of higher pressures which would give admittedly superior products.
U POT H€.T€RS
1
Then 215
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Vol. 20, No. 3
35 = 180 pounds, which is the partial pressure
of the carbon dioxide under the desired conditions. * Assume original temperature of carbon dioxide as 100' F. =, 560" absolute, and final temperature of carbon dioxide as 260 F. = 720" absolute. 560 Then 180 X = 139.86 or absolute pressure of carbon
120 dioxide at 100" F., which = 125 pounds gage. That is, to obtain 260' F. within air bag, carbon dioxide should be supplied at 125 pounds gage and backed by the 200 pounds gage of steam. Pressure a n d Temperature Equilibria
As the temperature and pressure within the closed volume decrease because of heat transfer, additional steam supply enters, thus maintaining pressure equilibrium. Temperature
co2RETL'rQN TU GASOMETER
-
I
SUPPLr
P l a n S h o w i n g Method of Curing Tires by Means of COS G a s a n d S t e a m w i t h Recovery of Gas for R e - u s e
CARBONDIOXIDE PROCESS-This process has interested many because the bags are dry a t all times, a more uniform cure is obtained than with air-glycerol, and the air-bag life seems to be enhanced. On the other hand, the objectionable features of gas leakage into the steam of the vulcanizer still remain, and this process offers no means for adding heat to the inside of the tires. The slight saving over the air-glycerol process is too small to warrant any extensive installation in an industry where a process may see a complete change in the short space of one year. Principle of New Process
An effort has been made to provide a new process having the advantages of both the air-glycerol and the hot-water processes and the disadvantages of neither, as applied to the curing of cord tires and molded tubes. The process consists in first bringing a part of the pressure up to a predetermined point with an inert gas, such as carbon dioxide, then shutting off the gas and turning on steam a t the curing pressure and leaving it on throughout the period of the cure. The first effect of the steam is to heat the inert gas, causing it to expand and thus cutting down the partial pressure of the steam that can enter the bag, thereby insuring any desired inner temperature which may be easily computed for any particular case. The inert gas acts as a dry preservative for the inner surface of the air bag, preventing oxidation; and it also acts as an automatic valve controlling the amount of heat units that may be added to the bag. Phrased differently, a means is proposed which permits obtaining any temperature at any desired pressure always under inert gas conditions; a means which is applicable not only to curing of tires, molded tubes, and other processes in the rubber industry, but also to many processes in other industries. As a typical example, assume that tires are being cured a t 260" F. or 20 pounds gage, and that the pressure maintained on the air bag and thence to the tire in the mold is 200 pounds; required, the proper gage pressure of the carbon dioxide which when backed by 200 pounds of steam will maintain the same curing temperature within the air bag as exists without the mold. 200 pounds gage = 215 pounds absolute 260' F. = 20 pounds gage or 35 pounds absolute
equilibrium is dependent on partial pressure of steam and is best determined experimentally for each general case. The extremes of this process may be considered as ranging from inert gas at atmospheric pressure and steam at any desired pressure, in which case the temperature within the container would be practically the temperature of the maintaining steam, to the condition where the inert gas is supplied at the same pressure as the steam, when of course there would be no transfer of heat units. I n between there is any desired degree of variation. I n this method the gas is used in such fashion as to make of itself an automatic valve controlling, not only its own temperature, but also the number of heat units which may be added to it and thence to a neighboring medium such as an article to be cured or a chemical reaction to be effected. I n other words, by proper variation heat units may be added with but a small amount of condensation from the steam or there may be a sufficient flow of steam over an extended period of time such that a considerable condensation of moisture may take place.
A-Closed vessel D-Release to atmosphere B S o u r c e of inert gas supply as COa E-To recovery apparatus C-High pressure steam supply F-Gage Temperature Pressure Control by Means of Inert Gas a n d Steam
It is impracticable to use air in place of inert gas in this process, as the oxidation of the inside of the air bag would be greatly promoted by the increased temperature obtained. Temperature equilibrium is obtained immediately and continues substantially constant throughout the cure. The reason for this is obvious. At the start of operations there is direct contact between the inert gas and steam a t an initial temperature higher than that represented by the final partial
March, 1928
INDUSTRIAL AND ENGINEERING CHEMISTRY
pressure which results from the expansion of the inert gas, and this intimate mixing takes place within an insulated space, thereby insuring B rapid rise of temperature. The partial pressure of the steam must, however, be high enough to compensate for such heat transfer as may be taking place, and it should be borne in mind that such small condensation as takes place serves to decrease the total volume, increasing slightly the partial pressure of the inert gas and decreasing the partial pressure of the maintaining steam. Thermocouple readings taken within an air bag indicate that the temperature of the pressure gas is very sensitive to changes in steam pressure. I n the curing of molded tubes it is desirable to have the initial pressure of the gas sufficiently high to form the tube because of the rapidity of temperature rise within the tube. Carbon Dioxide as Inert Gas
Carbon dioxide as used in this process should increase air-bag life. This gas has been used in the past on any and all kinds of air bags on the theory that with an inert gas, compounding played no part. On the contrary, it has been proved in practice that an air bag heavily loaded with gas black does not react favorably with carbon dioxide, the bag becoming porous and the gas black a t curing temperatures acting to break down the carbon dioxide. Iron oxide acts in a somewhat similar manner. Moisture, is essential to the securing of the best results from carbon dioxide. Practically all tests in the past have been made with gas liquefied a t high pressure in cylinders and the users have not stopped to consider that this product was an excellent dehydrating agent. So long as the air bag is not used beyond seventyfive heats it is not so important, but beyond this point the cumulative effect of a dehydrating agent becomes increasingly evident both in the superficial appearance of the interior of the bag and in its physical characteristics. Carbon dioxide used only in the initial cure of an air bag has been found by laboratory test to increase the modulus of elasticity about 20 per cent as compared with the usual air cure and the superficial appearance after being used with
293
small and the cost negligible. I n general, only one-third of the pressure is supplied with gas and if desired two-thirds of this may be easily recovered for re-use. The inert gas used in this process is ample in amount for complete protection of the inside of the air bag. There is no accumulation in the air bag; consequently the cure does not vary from bag to bag as in the air-glycerol process. Carbon dioxide has long been known by the rubber industry as not only an economical inert gas easily applied, but as having a definite preservative effect on the rubber itself. Apparatus has recently been developed which greatly facilitates its use. Substitution of Steam for Air Pressure
By substituting steam for the air ordinarily used in maintaining pressure, certain economies are effected; air costs from 2/8 to 1 cent per tire, whereas a similar amount of steam costs but 0.002 cent. Moreover, the leakage of air into heaters is eliminated; the importance of this factor has already been emphasized. Leakage of steam from the manifold connections would benefit rather than harm curing conditions. The amount is very small compared with the volume of steam used within the heater. By eliminating air in the heater the tires a t the bottom of the heater receive exactly the same cure as those at the top. Hitherto this has been one of the main points in favor of the hot-water process. Table I-Tests on Molded Tubes Cured on Carbon Dioxide [IO-minute cure at 45 pounds (3.1 kg. per sq. cm.)pressure]
LOADAT
PRESSURE 751bs. (5.3 kg.) air 75 lbs. COz 3 7 . 5 lbs. (2.7 kg.)
COz 1,. _" Wl I h n (5.5 k g.) steam
ELONOA-
LOADAT
TENSILE AT ELONGATION ELONGATION BREAK 400%
200%
TION AT
BREAK
400 450
181 204
1525 1600
692 726
3025 3100
213.7 218.3
730 730
500
227
1700
732
3375 2 3 7 . 7
730
.L".
AGED 6 DAYS I N GEER OVEN
7.5 lbs. air 7 5 lbs. COz 3 7 . 5 Ibs. COz lbs. steam
+ 80
576 576
261 261
2050 1975
931 897
3400 3575
239.4 251.7
700 730
600
272
2125
965
3900
274 7
730
/NDIV/DUAL uiRr,cp Plan S h o w i n g Method of Curing Molded Tubes w i t h C O PGas a n d S t e a m
air-glycerol subsequent thereto would indicate a continuation of this marked improvement. Condensation is of importance, particularly in molded tubes. Only so much condensation can take place as is necessary to bring the weight of rubber up to the curing temperature. I n the case of a 6-inch molded tube, assuming an equal amount of heat from both inside and outside, the condensation would amount to approximately 5 ounces of water, all of which would quickly evaporate on release of pressure. Table I gives the results of some preliminary tests on molded tubes. The amount of inert gas used in this process is relatively
The pressure means, being gaseous, are completely evacuated by the blow-down at the end of cure. It is not necessary to collapse the air bags by a costly vacuum system as in the hot-water process with its loss of valuable time and its weakening effect on the bags. Number of Heat Units and Rate of Heat Transfer
This new process permits the addition of as many or more heat units than may be added by the hot-water process. Assume an air bag of '/z cubic foot volume. If hot water is added a t a temperature of 350" F. and during the cure drops to 250" F., the total number of B. t. u.'s available is
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62'5 X 100 -
= 3125. If in this new process only 4 pounds 2 of steam are condensed, 4000 B. t. u.'s are released for heating and there is a vast difference between removing 30 odd pounds of water and a matter of 4 pints, most, if not all, of which may itself evaporate on release of pressure. As a matter of fact, fewer heat units need be added to the bag because of reduced thickness required as compared with general hotwater practice. It was recently stated2that if heat were transmitted equally to two sides of a rubber slab '/z inch thick it would take 15 minutes for the center of the slab to reach the impressed temperature (in this case 280" F.), but if the slab were 1 inch thick an hour would be required. This emphasizes the advantage of the relatively thin bag that may be used in the new process and its bearing on the rate of heat transfer. A further important fact is that the conductivity of a mixture of one-third gas and two-thirds steam is much greater than that of static water. The uniformity of heat transfer compares favorably with the hot-water process. Pressure being supplied by steam, * Blaker and Shade, India Rubber World, 76, 313 (1927)
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considerable leakage may exist without affecting the cure, whereas with a liquid medium a small leak will cause pronounced change in pressure conditions. I n the hot-water process owing to change in water density during progress of cure, there is a marked difference in the temperature of the water a t different points in the bag, while a t the top there is a variable pocket of air and steam. In the new process temperature conditions are practically constant, owing to rapid diffusion of gases mixed under conditions of curing temperature and pressure, and if the particular conditions applied call for a large addition of heat units it is simple to secure a continuous movement of the steam-gas mixture circumferentially. Advantage i n Manufacture of Molded Tubes
Molded tubes are now being cured by this process with a 25 per cent reduction in curing time and without any moisture remaining in the tube. The quality of molded tubes cured with carbon dioxide and steam is outstanding as compared with tubes cured by any other process. Under the severe treatment accorded molded tubes by the busses, this superior quality is a factor of first importance.
Chemical Unsaturation of Rubbers Vulcanized with Polynitro Compounds and Benzoyl Peroxide, and Its Possible Bearing on Vulcanization' Harry L. Fisher* and A. E. Gray* THEB. F. GOODRICH COMPANY, AKRON,OHIO
TUDIES on the chemical unsaturation of ordinary vulcanized rubber show that vulcanization has caused no change in the unsaturation of the rubber hydrocarbon beyond that which can satisfactorily be accounted for by the chemical combination of sulfur on the basis of one atomic equivalent of sulfur to a C5Hs group.4 If such is the case with sulfur vulcanization, it becomes very desirable to know whether there is any change in the unsaturation when rubber is vulcanized with substances other than sulfur-namely, polynitro compounds and benzoyl peroxide. Do these substances or their decomposition products also add to the olefin bonds and thus similarly reduce the unsaturation of the rubber hydrocarbon, or do they simply act in a catalytic fashion and change the rubber hydrocarbon without changing its unsaturation? I n an effort to throw some light on this interesting and intricate problem, samples of rubber were vulcanized with dinitrobenzene, trinitrotoluene, and benzoyl peroxide, and the unsaturation was determined by the Kemp-Wijs method with iodine ~ h l o r i d e . ~These samples dissolved with consid.erable difficulty, but a modified procedure, in which each sample cut into very thin pieces was allowed to swell in the solvent for many hours, worked very well. The results so far obtained are not so complete as
S
1 Presented before the Rubber Division at the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927. Received October 25, 1927. 2 Present address, U. S. Rubber Co., 561 West 58th S t . , N e w York, N. Y. 8 Present address, Lehigh Unibersity, Eeth!ehem, Pa. 4 Spence and Scott, Kolloid-Z.. 8, 308 (1911); also private communication from Kemp, author of the Kemp-Wijs method for the determination of the rubber hydrocarbon, Ind. Eng. Chem., 19, 531 (1927); and unpublished work of the writers. 6 Ostromislenski, J. Russ. Phys. Chcm. Soc., 47, 1462, 1885 (1915); C . A . , 10, 1943, 3177 (1916).
the writers would like to have them, because each one changed his business connection before the full plans could be realized. However, the results appeared so satisfactory that it has seemed worth while to publish the work as far as it has gone. Experimental
(A) In a preliminary set of experiments with the two polynitro compounds the pale crepe used was not previously analyzed for its unsaturation. The value for the rubber hydrocarbon (CsH8)used was an average value, 91.7 per cent. The solution method was used in all these analyses, after acetone extraction and drying i n vacuo over concentrated sulfuric acid. Experiment ( 1 ) RATIO 100 9.5 10.5 3.0
Pale crepe Litharge Gas black m-Dinitrobenzene
BATCH Grams 400 38 42 12
492
Vulcanized 60 minutes at 141" C., 15 by 20 by 0.2 cm. (6 by 8 by 2!/3 inch) sheet; tensile, 200.50 kg. per sq. cm. (2852 Ibs. per sq. in.) ; elongation, 613 per cent.
Experiment ( 2 ) Pale crepe Litharge Trinitrotoluene
Grams 100 8 4s
112
Mixed on a small laboratory mill; vulcanized as gravity disks, 20 minutes at 135" C. 8
Stevens, J. Soc. Chem. Ind., 36, 107 (1917).
