December 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
(6) Cohen, M., Compt. rend., 226,1366-8 (1948). (7) Cretcher, L. H., and Pittenger, W. H., J . Am. Chem. SOC.,46, 1503-4 (1924). (8) Davis, H. S.,and Carpenter, E. L. (to American Cyaqamid Co.), U. S. Patent 2,452,554 (Nov. 2,1948). (9) Davison, J. B., and O h , J. F. (to Sharples Chemicals, Inc:), Ibid., 2,494,610 (Jan. 17, 1950). (10) Erlenmeyer, E., and Darmatiidter, L., 2. Chem., 4, 342-3 (18fiS). (11) Fe&&’.P:, Berbe, F . , and Flamme, L. R., Bull. SOC. chim. Belges, 56, 349-68 (1947). (12) Fletcher, C. J. M., J . Am. Chem. SOC.,58,534--5 (1936). (13) Fletcher, C. J. M., and Rollefson, G. K., Ibid., 58, 2135-40 11938). (14) Gabel, G., Bull. SOC. chim. France, 41, 936-40 (1927). (15) Goepp, R. M., Jr., Fuller, D. L., Vaughan, W. E., a’nd Brandner, J. D., “Manufacture of Ethylene Oxide via Chlorohydrination of Ethylene,“ M A T Final Rept. 874,7 (1947). (16) Ibid., p. 12. (17) Ibid., p, 19. (18) Gomberg, M., J. Am. Chem. Soc., 41, 1414-31 (1919). (19) Green, A. D., and Waterman, W. W. (toStandard dil Development Co.), U. s. Patent 2,232,910 (Feb. 25, 1941). (20) Grignard, V., Bull. SOC. chim. France (3), 29,944-8 (1903). (21) Hibbert, H., U. S. Patent 1,270,759 (June 25, 1918). (22) Hopff, H. (to I. G. Farbenindustrie A.-G.), Ibid., 2,029,618 (Feb. 4, 1936). (23) I. G. Farbenindustrie A.-G., Brit. Patent 271,169 (Feb. 22, 1926). (24) Ibid., 292,066 (June 11,1927). (25) I. G. Farbenindustrie A.-G., French Addition Patent 39,773 (Feb. 17, 1931). (26) Irwin, C. F., and Hennion, G. F., J . Am. Chem. Soc., 63,858-60 (1941).
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(27) Kautter, C. T. (to Shell Development Co.), U. S. Patent 2,051,486 (Aug. 18,1936). (28) Kihner, F.. French Patent 763,107 (April 29, 1932). (29) Larson, A. T. (to E. I. du Pont de Nemoura & Co.), U. 5. Patent 2,153,064 (April 4,1939). (30) Lefort, T. E. (to SociBt6 franpaise de catalyse gBn6rali&e), I W . ,1,998,878 (April 23,1935). (31) Lenher, S.,J . A m . Chem. Soc., 53,2420-1 (1931). (32) Loder, D. J. (to E. I. du Pont de Nemours & Co.), U. 5. Patent 2.285.448 (June 9.1942). (33) MoBee, E. T., Ha& H.‘B., and Wiseman, P. A., I N ~ .E N ~ . CHEM., 37,432-8 (1945). (34) Matianon. C.. Moureu, H.. and Dode, M., Bull. 8oc. d i m . France (5), 1,1308-17 (1934). (35) Nenitzescu, C. D., and Scarlatescu, N., Ber., 68B, 587-91 (1935). (36) Perkins, G. A. (to Union Carbide and Carbon Corp.), U.5. Patent 2,042,862 (June 2,1936). (37) Reid, E. W., and Lewis, D. C. (to Carbide & Carbon Chemicals Corp.), Ibid., 1,904,013 (April 18,1933). (38) Rohm & Haas A.-G., French Patent 675,327 (May 17,’1929). (39) Twigg, G. H., Proc. Roy. Soc. (London), A188, 92-104, 105-22. 123-41 (1946). (40) Vaunhan. W. E.. .and Goem. R. M.. Jr.. “ProDosed Ethylene Oxide ’Manufacture via-Oxidation of Ethylene at Zwmkel near Gladbeck” FIAT Final Repl. 875 (1947). (41) Wurtz, A., Ann., 113,255-6 (1860). (42) Wurtz, A., Compt. rend., 48,101-4 (1859). (43) Ziese, W., Ber., 66B, 1965-72 (1933). 1 - - - 1 - - -
.
.-
I
R E C E I V ~April D 21, 1950.
Presented before the Divisions of Petroleum Chemistry and Gas and Fuel Chemistry, Symposium on Chemicals from, Petroleum, at the 117th Meeting of the A M ~ R I C ACNH E M I C A S~o c m w , Houston,Tex.
Utilization o f Butyl Rubber i n A u t o m o t i v e Inner T u b e s D. J. BUCKLEY, E. T. MARSHALL, AND H. H. VICKERS Esso hboratories, Standard Oil Development Company, Elizabeth, N . J . T h e wartime shortage of natural rubber created a phenomenal demand for the new polymer, Butyl rubber, primarily for the manufacture of automotive inner tubes. Extremely difficult problems were encountered in fostering the use of Butyl. A major crisis occurred when inner tube fabricators attempted to make Butyl passenger tubes on equipment designed for natural rubber at far above prewar rates. As a result, an extensive use-development program was carried out. A study of the automatic butt splicing of inner tubes led to mechanical improvements which aided materially in offsetting the characteristically differ-
ent nature of Butyl. A large scale factory test program was run to correlate polymer plant variables with inner tube plant experience. The results demonstrated particularly the mnsitivity of Butyl tube fabrication operations to small variations in factory conditions. The severe 1%6-47 winter spotlighted the tendency of Butyl passenger tubes to “buckle” when in subzero seryice in the northern United States and Canada. Extensive laboratory studies and field trials eventually showed that this buckling tendency could be overcome by proper compounding and curing of the Butyl tubes.
Y T H E middle of 1949, the rubber industry of the United Sthtes had consumed over, 650,000,000pounds of Butyl
its use by the fabricator of the end products. In the case of Butyl, such problems were greatly magnified because the development period was compressed abnormally under the influence of the tremendous war and postwar demand.. Three specific problems were encountered in the manufacture of Butyl inner tubes: difficulties in splicing the raw tubes, unexplainable variations in factory processing experience, and service failures of finished tubes in operation at subzero temperatures. A brief relation of the work t h a t was done in aiding in the solution of these three problems should be of value. Such experiences are felt t o be typical of the many predictable and unforeseen problems that may be encountered in the development of any new raw material. The achievement of Buccess in the use of Butyl t o fabricate inner tubes of high quality can be credited primarily to the rub-
rubber and had fabricated enough Butyl rubber inner tubes to equip completely over 50,000,000automotive vehicles. Yet Butyl was not invented until 1937 and was not commercially produced until March 1943. This phenomenal growth in the use of a new product is illustrated in Figure 1. The war, and the consequent shortage of natural rubber, provided the chief impetus for this great demand for synthetic polymers. Neoprene and Buna N, in commercial production in 1933 and 1941, respectively, reached high levels of use early in the war, but were then exceeded by Butyl as Butyl found i t P place in inner tube manufacture. No new material can be brought to a high level of use without encountering many problems, not only in its manufacture but in
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ber industry itself. Nothing could have been achieved without the constant effort of the rubber manufacturers to solve the problems they encountered with Butyl. The successful solution of these problems was of considerable concern to the Standard Oil Development Company, inventor and licensor of the Butyl process. A major independent effort was therefore made to aid the rubber industry in overcoming some of its problems.
Figure 1.
U. S. Consumption of Specialty Rubbers
Early expectations for Butyl, a copolymer of isobutylene with a small amount of diolefin (8), were high. As an almost completely saturated, yet vulcanizable polymer, it had the attractive possibility of falling midway between natural rubber and plastics, with the resistance to chemicals, sunlight, and aging of the plastics and the relatively permanent elasticity of the vulcanizable polymers. As early as 1939, large scale use of Butyl in the manufacture of cable insulation, tire curing bags, inner tubes, coated fabrics, and all types of mechanical goods was envisioned The deficiencies in product properties then apparent were the slow rate of vulcanization and the tendency to blister during vulcanization. These problems were largely overcome by improvements in the manufacturing process, by adjustment of the diolefin content of the polymer, and by extensive research in compounding (2, 6 ) . Butyl polymers with increased diolefin contents (designated as GR-1-15 and GR-1-25) were made available shortly after the commercial development of the standard polymer (GR-I). These polymers exhibit vulcanization rat>rs that are higher than the standard product. I n March 1941, the construction of a privately owned commercial scale Butyl plant a t Baton Rouge was decided upon by the Esso Standard Oil Company. In February 1942, however, before completion of this original plant, the Rubber Reserve Company (Reconstruction Finance Corporation) took over the project as part of the Government's wartime synthetic rubber program, and it was expanded to include additional units at Baton Rouge and a large plant a t Baytown, Tex. The Government furnished the capital for these plants, and construction and operation a t the two locations were carried on by the Esso Standard Oil Company and the Humble Oil an$ Refining Company, respectively, under contract with Rubber Reserve. The Canadian Government, acting as the Polymer Corporation, Ltd., also authorized the construction of a small Butyl plant a t Sarnia, Ontario. Early technical efforts of the plant operators and of the Standard Oil Development Company were weighted heavily in the direction of perfecting the low temperature polymerization technique used and adapting it to large scale continuous operation. While this road was an arduous one, it was grittifying that
Vol. 42, No. 12
the first commercial product, as is frequently the case, was superior in many of its properties to the pilot plant product. The unique properties of Butyl rubber, particularly its air retention (8 to 10 times better than natural rubber) and its resistance to aging and to tearing, led to its early consideration :is an inner tube material ( 7 ) . Butyl tubes were found to perform so satisfactorily that, by early 1944, Butyl had become the only substitute for natural rubber in inner tubes acceptable to the military authorities. Butyl was channeled by government directive into inner tube use, as quickly as it became available, by the simple process of prohibiting the use of natural rubber in such tubes. Satisfactory inner tubes could not be produced from GR-S. Extensive laboratory programs were carried on throughout the prewar and war periods on improving and modifying the product, improving the manufacturing process, and furthering product utilization. Additional valuable information was obtained from factory testing and fleet testing programs carried on in conjunction with technical sales service activities (by the Enjay Company, Inc.,. formerly Stanco Distributors, Inc.). It was not until January 1, 1946, however, that the supply of Butyl was sufficient to divert its use from military and essential truck and bus inner tubes to passenger car inner tubes. The competition for the postwar civilian market WBS then on in earnest and thp difficulties experienced with Butyl came rapidly to the forefront. One of the serious handicaps to the rapid conversion to the use of Butyl rubber was that passenger inner tube fabricating equipment, designed and modified over a period of many years for efficient use with natural rubber, was suddenly called upon to handle a totally new polymer. An unlimited civilian market waa suddenly open, and inner tube production' rates were set at the maximum attainable, well in excess of normal prewar rates
Figure 2. Commercial Butt-Splicing Machine Used in Studies of Butyl Inner Tube Splicing
December 1950
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INDUSTRIAL AND ENGINEERING CHEMISTRY
on natural rubber. Thib: combination of circumstances gave rise to a number of problems ill the adaptation of Butyl to inner tube manufacture, centered around ( 1 ) nonuniformity of the polymer, which caused unpredictable variations in factory processing; (2) insufficient tackiness of Butyl compounds for satisfactory butt splicing of inner tubes; and ( 3 ) excessive cold flow of the uncured stocks during the forming operations. Canadian fabricators, who had started production of Butyl passenger tubes more than 8 year earlier than in the United States, were not as concerned by these processing difficulties, presumably because of their lower production line speeds. They had, however, in the winter of 1944-45, experienced a number of customer complaints about Butyl inner tubes which, while in cold weather service, had formed longitudinal folds or “buckles” which caused failures by chafing through a t the cremes. Adjustments for tube failures were necessary on about 1% of the Canadian tubes produced. THE SPLICING PROBLEM
Automobile inner tubes are made by extruding a continuous tube of compounded uncured rubber, cutting this into proper lengths, applying a prefabricated valve, and then splicing the tube ends together to form a ring. The tube is then inflated to stretch it into a doughnut shape, after which it is vulcanized in a curing mold. Adequate adherence of the valve to the Butyl tube was an early problem worked on in the laboratory, but this difficulty was eventually solved satisfactorily. For many years inner tubes were spliced by cutting the tube ends to a feather edge, coating the ends with a cement, and fitting the two ends together by hand to form a lap splice. Later, automatic butt-splicing machines were developed, such as the Goodyear machine illustrated in Figure 2. These involved clamping the tube ends, cutting these ends in a vertical plane, and pressing the frwhly cut ends together in perfect alignment to form a butt splice. These automatic machines could be operated a t high speeds with less than 1% of defective splices. The sudden advent of Butyl changed the splicing picture considerably. The uncured Butyl compound was found to be less elastic in nature and was easily damaged, yet it would resist flattening in the splicing machine clamps, resulting in weak splices. Any weak splice or damaged portion would cause complete failure when stressed in the subsequent inflating and forming operation because Butyl, unlike natural rubber, would concentrate any strain a t the weakest point. To help overcome these difficulties, tube manufacturers developed a technique of chilling the splice before forming. This technique, and caretul placement of the tube in the die, permitted them to use the butt-splicing machines with partially atisfactory results. Tubes rejected because of defective splices ranged from 2 to lo%, although many of these rejects could be repaired and sold as seconds. As Butyl came into general use, getting uniformly satisfactory splices was so important to tube manufacturers that an experimental program appeared desirable, to determine if any fundamental changes could be made in the Butyl that would reduce the high number of rejects and make the splicing of Butyl comparable to that of natural rubber. With this object in mind, a number of inner tube manufacturing plants were visited in the summer of 1945 to observt: closely the difficulties involved. Jn almost all thee plants, the same types of splicing defects were found. These defects were classified and samples were taken for detailed study. In order to ascertain the causes of these defects and to investigate the effects of different compounds and polymers on splicing, an experimental tube manufacturing operation was set up in the laboratory, complete in every detail from Banbury mixer and mills to extruder, splicing machine, chilling table, forming ring, and curing mold. The splicing machine and attendant equipment in the tube laboratory were similar to
Figure 3. Action of Compacting Spring in ButtSplicing Machine To obtain better splicing of Butyl rubber inner
tubes
those used in most manufacturing plants, and the same defects seen in plant operations were easily reproduced in the laboratory. These defects were then studied minutely from all angles and segregated into types. J t was found that, regardless of the stock condition, adjustment of the machine, or care of the operator, inherent weaknesses in the splice a t times resulted in complete failure, particularly if the splices were not chilled before forming. This indicated that the trouble, as the manufacturers had suggested, might be in the fundamental properties of the Butyl. A simple test, however, made by cutting the uncured tube into thin strips all along the splice and stretching each strip, showed that one place adjacent t o the fold was always weak, while other points could be many times stronger than required. This indicated that the source of trouble might be in the machine, which had not been developed originally to splice Butyl. This was a new approach to the problem. Close observation of the cutting of the tube ends in the splicing machine showed that, while the knife moved in a true vertical plapc, the tube stock was distorted under the clamping and cutting pressures, so that the resultant cut was concave. The laminated clamp, which was used to hold the tube firm during the cutting and splicing operation, did not always completely flatten the tube, but left a hollow core at the folds. This hollow core not only permitted greater deflection under the knife, resulting in an excessively concave cut, but also prevented proper confinement of the tube stock when the two freshly cut ends were pressed together, resulting in a generally weak condition a t the folds. A machine employing a solid clamp was not subject to this difficulty, but it could not cope with variations in tube width and gage, and so was not considered an answer to this particular defect. To help strengthen the splice at the fold, a compacting spring
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INDUSTRIAL AND ENGINEERING CHEMISTRY
- 591 100
Vol. 42, No. '12
-
Figure 4.
CONSECUTIVE GROUP NUMBERS ( 840 LBS. BUTYL PER GROUP) Comparison of Two Duplicate 36,000-Pound Lots of Butyl i n
Inner Tube Factory Test
fitting under the knife block was developed, as shown in Figure 3. .This spring not only prevented the concave cut by counteracting the clamping pressure, but also pushed horizontally, packing extra stock into the hollow core of the fold and holding it there while the cut was made (see reference lines drawn on tube in Figure 3). This compacting of stock into the hollow fold placed extra material where needed, compensating for the lack of confinement, and resulted in a uniformly strong splice. In order to overcome pinching and misalignment defects, the flexible laminated clamp arm was given a concave shape, so that on closing it made first contact with the folds of the tube. This pushed the folds into the corners of the die, permitting the center part of the tube to rise or buckle but eventually to flatten out uniformly across the die. Among other benefits, this specially shaped clamp permitted the matching together of unequal width ends of tubes, because it could press-fit a wide tube end into one die and a narrow tube end into the other die. Several other minor modifications added to the stability of the operation and a few special adjustments and techniques were developed, resulting in a simple splicing operation that produced excellent splices with Butyl tubes .without chilling and with less than 1% defects. The developments when tested in plant operations gave highly &isfactory results. It was noticed, however, that proper maintenance of the splicing machine was extremely important. Apparently, Butyl could be spliced successfully with a modified machine on a production basis, but it w a s still somewhat more difficult to splice than natural rubber, and the machine required careful adjustment and maintenance. After the problem had been solved to this measure of satisfaction, a motion picture was prepared showing the equipment used, with slow-motion shots illustrating how the various defects were caused on the original machine and how the modified machine eliminated these defects. This motion picture was shown to the inner tube industry early in 1947 and accomplished its objective. Practically all the plants that used this type of splicing machine either modified their own machines or bought
machines that had been redesigned by the manufacturers, incorporating the modifications at least in principle. Many fabricators using different types of machines were aided by the results shown, and modified their own machines to achieve improved results. Although this splicing study cost approximately $100,000, it was considered an important factor in aiding inner tube fabricators to lower costs of processing Butyl rubber. FACTORY TEST OF UNIFORMITY
As Butyl rubber manufacturing operations improved, the uniformity of the product likewise improved. A point was reached, however, where Butyl of apparently good uniformity was being produced, but inner tube factories were still experiencing large and unexplainable process variations. This problem was attacked vigorously from the standpoint of developing improved test methods to detect possibly indiscernible variations in product quality, and smoothing out process variables in manufacturing. Investigations of splicing and of other variables, such as tack (S), compounding, and processing behavior (9, IO), were already in progress in the laboratories. I n order to obtain a clearer insight into the fabricating difficulties being encountered, the Butyl Rubber Operators' Committee of the Office of Rubber Reserve recommended in August 1946 an actual inner tube factory test program. Accordingly, arrangements were made to carry out a comprehensive test program a t the KO.2 plant of the Robbins Tire and Rubber Company a t Tuscumbia, Ala. This was a large modern factory, producing only inner tubes, which seemed very suitable for the test program visualized. The layout of the plant facilitated tracing test polymers through various stages of the inner tube operations. The Robbins management and personnel were very cooperative. A disadvantage of the plant was that only one oversized tuber was used for extruding all sizes of tubes, but this did not seem important. The testing a t Tuscumbia was started on October 1, 1946, and continued for 8 weeks. The only changes made in the normal manufacturing procedure followed by the plant were of a
Dpcernber 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
minor nature designed to improve segregation and identification of the stocks. Extremely careful observation of all stages of the fabricating operation was deemed essential, however. For this purpose, a force of approximately 50 men was assembled from the various technical groups concerned with the manufacture, development, and sales of Butyl. The entire inner tube plant production train, from the unloading of the raw materials to the final boxing of the inner tubes, was divided into nine operations. A n observer was stationed a t each operation during all operating periods t o record pertinent data and to note any discrepancies. Considerable testing equipment was added to the plant laboratory to permit much more extensive examination of materials in process and finished products. For the test program, a number of large lots of both regular and s ecial types of Butyl were prepared in the governmentownecfButy1 plants a t Baytown, Tex., and Baton Rouge, La. The boxes containing the test polymers were numbered conthey were filled in the Butyl lants and were run secutively through the inner tube plant in that same orxer. This permitted correlation of polymer plant variables with tube plant ex erience. The large lots of regular production polymer provicfed long enough runs to establish base conditions at the Robbins Quadruplicate 36,000-pound lots of .standard Butyl were prepared at Baytown b segregatmg consecutive boxes in groups of four, each box in eacz group receiving the same number. By re-sorting, four reference lots of polymer were obtained in which the polymer from any one box was essentially identical to that from any of the other three boxes of the same number. While the box-to-box variations were the same as for regular GR-I, the variations were duplicated in each of the 36,000ound batches. Special pol mer lots included several of much getter uniformity produced %y the use of special expedients in the polymer plant, some of varying degrees of moisture content, some polymer that had previously given trouble in the Robbins plant, aqd some that had been plasticized from higher molecular weights to the specification range (GR-I-SOP). All in all, ap roximately 1,200,000 pounds of polymer were made available k r testing, almost all of which was used in the program.
&ktj
The attempts to correlate variations in the inner tube plant operations with changes in polymer quality were concentrated primarily on the data regarding rejected tubes, both cured and uncured. Although variations in polymer might have a bearing on operations prior to the tubing procedure, the effects would be of importance principally through their contributions to the number of inner tubes rejected. I t was quickly realized that reject values were affected inarkedly by slight changes in mechanical operation and personnel factors as well as by polymer ,variations: It was found impossible to segregate rejects due to polymer alone from rejects due to other causes, because all influencing factors were too intimately related. Because the major difficulties with Butyl seemed to concern its deformation properties and its tack quality, i t was decided that rejects that had a direct bearing on deformation and tack would be most pertinent in correlating properties of the Butyl with plant experience. A performance index was therefore set up to cover only defects believed most sensitive to polymer quality. One of the most interesting correlating charts, presented in Figure 4; shows the results obtained when two of the duplicate 36,000-pound batches were run th+ough the plant 16 days apart. The Mooney viscosity values of the tubed stock substantiate the essential identity of the lots, yet the reject records as measured by the performance index (based upon the five classes of rejects considered to be most affected by polymer quality), and by the total cured rejects, are almost mirror images of each other. These and other data suggested strongly that variables other than the polymer uniformity contributed heavily to tube plant variations. It was also apparent that the tubing operations were sensitive to small variations in any of the factors contributing to rejects. There was no consistent evidence that a major improvement could be achieved by narrowing the Mooney viscosity specifications for Butyl or by improving the polymer uniformity. It
2411
appeared that major contributions to the processing of GR-I into inner tubes could be made by reducing the sensitivity of the operations, from the viewpoint of either the polymer or the mechanical or operational factors. In the studies of special lots of polymer, it was found that plasticized Butyl behaved as well as or better than any of the regular Butyl, that moisture in the raw polymer up to about 0.3% did not adversely affect inner tube plant operations, and that the tubing operation rather than moisture in the polymer was responsible for porosity in the tubed stock. Atmospheric conditions were studied carefully, but no correlation could be observed in the short period of the test. This factory test program, although costing over $lOO,OOO, furnished useful information for the inner tube industry and for the guidance of further laboratory studies. The results pointed particularly to the necessity for reducing the sensitivity of the tube manufacturing process to small variations in factory operations. Improvements in factory procedures and in polymer characteristics could contribute to this reduction in sensitivity. COLD WEATHER PERFORMANCE
When Butyl began to be generally used in the manufacture of passenger inner tubes, i t became apparent that such tubes were susceptible to an unusual type of service failure associated with cold weather-the formation of a longitudinal fold or buckle, usually in the crown area of the tube, when the tube wm operated * in subzero service. No such difficulty was encountered with truck and bus tubes, which were heavier in gage and operated at higher inflation pressures. While the passenger tube failures were limited primarily to Canada ( I ) and the north central tier of states in the United States, the problem was considered sufficiently serious to preclude the future use of Butyl in passenger car inner tubes, despite the fact that the percentage of failures waa only about 1.5% of the tubes produced in Canada and considerably less than that in the United States. Here was a problem that had to be overcome to the complete satisfaction of the rubber industry, if the use of Butyl in the largevolume passenger inner tube field was to be continued. The problem waa attacked from a number of different angles, but it was essential that the nature of the formation of the buckles be understood before other lines of research could be fully effective. Investigation showed that the mechanism of buckling in Butyl tubes a t low temperatures could be traced to the fundamental molecular nature of the polymer. The configuration of the Butyl polymer molecule is such that it results in hindered polymer chain movement (6, 9 ) . The possibilities of chain movement are further hindered when the temperature is lowered. The elasticity of Butyl is thereby reduced a t lower temperatures. When a tire runs over the road, a complex set of strains is set up in the tire. Considering only the obvious and major strains involved along internal surfaces, i t is found that there is a compressive strain in the shoulder region and a tensile strain in the crown area. These strains are indicated in Figure 5 on the tire crow section and in the strain diagram which results when strain is plotted as a function of the distance around the tire from shoulder to shoulder. Air prwsure forces the tube against the tire and the tube must therefore undergo the same dimensional changes m the tire. When the forces creating a tensile strain in the center of the crown area are operative in the tire, they are also operative in the tube, owing to interfacial friction. At reduced temperatures, the force required to extend a Butyl tube over the central tension area sometimes exceeds that which can be transmitted to the'tube by virtue of the interfacial friction between tube and tire. The need for the tube to extend and follow the contour of the inside of the tire is satisfied by m o v e ment in an area of zero stress, in the region between the cornpression and tension strains. On rapid repetition of this localized stretching process, an excesa of material accumulates in the
INDUSTRIAL AND ENGINEERING CHEMISTRY
2412
central area of tlie crown, thinning the tube sidewall. B ( ~ a u s c the tube under the influence of the air pressure must conform to the inner contour of the tire, the excess of material satisfies this condition by folding over or buckling in the manner shown in Figure 6. The inner edge of the buckle then chafes against the inner wall of the tube and eventually results in a failure
Vol. 42, No. 12
of 'the carbon black. Relittively coarse particlt~rurbon bl:tc:ks (HMF, SRF, and MT) are therefore preferred. The internal viscwsity of Butyl can be effectivcly rcducrcl t y the action of plasticizclrs sucth as petrolcum oil plastickc.rs arid estrrs of low viscosity, which inweasr t,hr sl);iti:tl :tr,r:tiigc.nients of the polynicr chttins and thus provitli, fu ' t w molecular movemrnt. The n c a t u,sult is t: x)l't(,r, mow r w I t type of inncr tutir.
Thv combination of irici,e:iscd (1l:isticnity and reduced interri:tl viscosity which were found to cwiitrihute t)rneficaially to low trinperature performance was rt?ducd t o pr:tc.tice i n the foim of espcrimental Butyl tubes. Butyl Tuhe S o . 273
I
268 ROAD SURFACE
271
274
COMPRESSION
Figure 5.
f
Mechanics of Tire a n d T u b e Deformation 272
It was denionstrated experinientally that the buckling occurs only within the first minute of operation of the tire in subzero service. After the first few revolutions of the wheel, sufficient heat is generated in the tire to raise the temperature of the tube above the buckling level. The above esplanation of the mechanism of buckling wits evolved only :ifter considerable study carried out in cooperation with the laboratories of the Polymer Corporation, Ltd., opcrator of the Canadiari Butyl plant. I t thus appears that buckling in Butyl inner t,ubes car1 be prevc~ntetlby any mechanism whicbh allows the tube to follow. the tire in the dimensional Yense during deformation on the: road. It has been shown repeatedlg that even a weak adhesive bond between tube and tire is sufficient to offset buckling tendencies; a light coating of soap solution on the surface of the tube will eliminate buckling. Lubrication of the surface of the tube with a dusting materid such R S talc contributes to buckling tendericies anti should be avoided. Although surh techniques could be employed effectively to reduce buckling, i t was the prinie objective of the work to produce improveyentty in t,he Butyl inner tube itself. The iniprovements developed during this program can be sumnnarized R J fallows : Effective State of Vulcanization. A high state of vulcanization of elastomers results in higher elasticity and therefore in less tendency t o accumulate material in the center of the crown i t i w of the tube during the repeated extensions in the cold rolling tire. The higher state of vulcanization could be achieved by the use of more unsaturated types of Butyl, by the use of higher than normal curing temperatures, and by compounding modificaGR-I-15tions, More highly unsaturat,ed polymers-i.e., were produced in the governmenbowiied Butyl plants m d were gradually adopted by the inner tube industryv. The use of higher curing temperatures than the usual 307' F. was studicd i n the laboratory, using a Dowtherm heRting system for a sfandard tube curing mold. This work established the fact that curing temperatures of 350" F. and higher were feasible anti tieneficial. Dissemination of this information resulted in the eventual adoption of higher curing tcmpcAratures by the inner tube fabricators. Reduction of Internal Viscosity. The hindered moleculm motion, or internal viscosity, of Butyl vulcanizates can be effectively modified by conipounding techniques (4). Carlion blacks are normally used in Butyl compounding t o impart added toughness and strength. However, they also reduce the elasticity of the vulcanizate by contributing t o the internal viscosity of the system. This effect is greater the smaller the particle size
Type Reference oi control tube produced from standard Butyl (GR-I) by conventional manufacturing practices and vulcanized a t 320° 1.". Experimental tube prepared with faster curing Butyl (GR-1-15] and coarse particle carbon black; vulcanized a t 338O P. Experimental tuhe p r e p a i d with high molecular weiaht Butyl (GR-1-17], Relected rarhon black types (HMI.' and SRF), and a relatively high petroleum oil plasticizer content (15 parts on 100 parts of polymer); vulcanized at 338O F. Experimental tube prepared with high molecular weight Butyl (GR-1-17] and selected carbon blacks, specially vulcanized (at X 3 X 0 F.)with the aid of Polyac. h higher concentration of petroleum oil plasticizer content w m employed (25 parts on 100 parts of polymer). Experimental tube prepared with high molecula? weight Butyl (GK-1-17] specially .i-ulcanized (at 338' F.) with the aid of Polyac. .4n ester plasticizer (25 parts of trioctyl phosphate on 1 0 0 parts of polymer) was used in the preparation of this tube.
To verify in practice the findings o f t,he laboratory program, a fleet of cars for road tests under cold road donditions was established. Wheel amemblies were chilled to low tempraturw in an isopropyl alcohol-dry ice bst,h (-25" and -40 I?,) and quickly mounted OII the test car, which was then driven over a prescribed test run. The cooling arid driving cycle was repeated as many times %Y the experiments dictittd. The progress of buckle formation was followtd by x-ray rsamination of the tutw while still mounted within tlie tire. The ocrurrence of :t t,uc:klv could Iw tliscerned by x-ray, b t ~ c a ~the . ~folded area g:tvi: :i dt.ris;c%rp i t tctrri on thcr fluc.)rr.sc.c~ritw r w i i I L W ~ for t>snmination.
Figure 6.
Buckled Butyl Inner T u b e Cross section
I N D U S T R I A L A N D ENGINEER’ING CHEMISTRY
December 19% n
R
TUBE
Figure 7.
FORMULATION
n
NUMBERS
Butyl Inner T u b e Field T e s t
Winter 1948-49, Minneapoli8, Minn., and Cut Bank, Mont.
The study of the cold buckling of Butyl inner tubes began in 1945 and continued a t an accelerated pace for 4 years, as the
problem became more acute. Close cooperation was maintained with the Polymer Corporation in Canada and many rubber companies. Field tests in the winters of 1946-47 and 1947-48 provided additional information to aid compounding, vulcanization, and mounting studies. The culmination of this program came with the development of the experimental tubes described above and their field testing on a large scale during the winter of 194849. For this purpose, field tests were set up in Minneapolis, Minn., in the area around Cut Bank, Mont., and in the western plains of Canada. The activity in Minneapolis amounted essentially to the operation of a winter experimental station, where all aspects of the driving tests were under close control. Through the courtesy of the Durkee-Atwood Company, approximately 33 cars were fitted with experimental Butyl tubes and x-rayed weekly during the progress of the driving testa, which extended from December 1948 to March 1949, inclusive. During this test, 24 subzero days were orficially recorded, the lowest temperature reaching 18’ F. Experimental Butyl inner tubes were also mounted in approximately 50 cars in Montana in the spring of 1948 and allowed to run to April 1949, when they were removed and examined visually for buckles. During this test period in Montana, 42 subzero days were recorded with an official low of -34’ F. iri Cut Bank where most of the test cars were being operated. The results of these winter field trials &reshown in Figure 7 . Here Butyl inner tube types developed in the laboratory program are compared to a conventional type of Butyl tube manufactured in the past. The conventional.Buty1 tube (No. 273), used rn the control, showed a high incidence of failure due to buckle formation. Experimental tubes 268 and 271 showed marked improvemefit over the control. Experimental tubes 274 and 272 passed through the tests without formation of any buckles. As a means of demonstrating that Butyl inner tubes could be developed which would withstand very rigorous weather conditions, a special field test was run in the cities of Fort William, Winnipeg, Regina, Calgary, and Edmonton in western Canada, using a tube plasticized with trioctyl phosphate (No. 272). Approximately 380 cars were fitted with experimental tube 272 in December 1948, and allowed to run through January, February, and March 1949. During this time the number of subzero days in the various test localities ranged from 36 to 60. Minimum temperatures as low as -43” F. were officially recorded in a number of the Canadian cities. I n April a large proportion of the experimental tubes, selected a t random, were
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\
2413
examined by x-ray teohnique and all were found to be free from buckles. This was considered a sufficiently valid sample to warrant the conclusion that all the tubes had successfully withstood one of the coldest winters in the history of western Canada. No reports of abnormal failure of any of these Butyl tubes due to cold weather conditions were received from any of the five Canadian cities. During this same period it was observed generally that Butyl inner tubes n o t specifically prepared for low temperature service were giving the same types and percentages of failures recorded in previous winters. The buckling of Butyl passenger inner tubes in cold weather service was a difficulty not considered in the initial stages of the Butyl develop ment. The problem was solved only after considerable technical effort over more than 4 years at a cost of approximately $600,000. Not only was a successful solution achieved, b u t some of the most desirable compounds developed, those which used higher molecular weight Butyl polymers plasticized with petroleum oils, were actually cheaper and more easily processed than the conventional compounds previously used. The program thus led to the rapid a d o p tion by the industry of the high molecular weight, fast-curing types of Butyl (GRI-17 and GR-1-18) as the standard raw materials for Butyl passenger inner tubes.
ACKNOWLEDGMENT
The development of the utilization of Butyl discussed herein is the result of the efforts of many individuals and groups within the Standard Oil Development, Company, with the assistance of other affiliated organizations of the Standard Oil Company (N. J.), particularly that of the Enjay Company, Inc. Special acknowledgment should be given to the Polymer Corporation, Ltd., for information concerning its concurrent studies of the cold buckling of inner tubes. The cooperation of the rubber industry and of the Office of Rubber Reserve (Reconstruction Finance Corporation) also materially aided these efforts. T h e authors are indebted to F. P. Baldwin for the schematic analysis of the nature of buckle formation in inner tubes.
LITERATURE CITED
(1) Adams, R. J., Buckler, E. J., and Wanless, G. G., Rubber Technology Conference, London, 1948. (2) Baldwin, F. P., Turner, L. B., and Zapp, R. L., IND. ENG. CHEM.,36,791 (1944). (3) Beckwith, R. K., Welch, L. M., Nelson, J. F., Chaney, A. L., and MoCracken, E. A,, Ibid., 41,2247 (1949). (4) Buckley, D.J., and Eby, L. T., “Viscoelastic Study of Modulus,” Division of Rubber Chemistry, 113th Meeting, AM. CEIW. SOC., Chicago, 1948. ( 5 ) Bunn, C. W., Advances in Colloid Sci., 2, 125 (1946). (6) Haworth, J. P., and Baldwin, F. P., IND.ENG.CHEM.,34, 1301 (1942). (7) Lightbown, I. E., Verde, L. S., and Brown, J. R., Jr., Ibid., 39. 141 (1947). (8) Thomas, R..M., Lightbown, I. E., Sparks, W. J., Frolich, P. K., and Murphree, E. V.,Ibid., 32, 1283 (1940). (9) Zapp, R. L.,and Baldwin, F. P., Ibid., 38,948 (1946). (10)Zapp, R. L., and Gessler, A. M., Ibid., 36,656 (1944). RECEIVED April 26, 1950. Presented before the Divisions mf Petroteurn Chemistry and Gas and Fuel Chemistry, Symposium on Chemicals from Petroleum, st the 117th Meeting of the AMERICAN CHEMICAL SOCIETY, Houston, Tex.
