Rubber in the Automotive Industry From the Viewpoint of the Rubber Technologist S. M. CADWELL, R. A. MERRILL, SLOMAN, AND F. L. YOST
A picture is presented of the close coopera-
c. M.
tion between the rubber and automotive industries which has made the automobile by far the largest consumer of rubber and has built the rubber industry to its present proportions. Some of the major advances in the rubber industry are listed as well as the role chemistry has played in these advances. Some of the more recent improvements which are just going into the automobile are described. Finally, data are presented for the first time on the static fatigue properties of rubber and on the interrelation of static and dynamic fatigue problems in the design of rubber parts.
United States
amounted to approximately 40 billion dollars. This is the difference between the amounts actually paid for tires and tubes and the amounts which would have been paid if tire quality and price had remained a t the 1920 levels. It ignores the fact that vehicle mileage would probably have been considerably smaller if tire costs per mile had not come down. It is occasionally worth while to review the more important physical properties which make rubber unique, to remind us what an unusual substance we have as a basic material, They are as follows: Flexing endurance, stands great distortion without injury. Tensile strength is as high as about 30,000 pounds per square inch, measured on cross section a t break. Stretch can be adjusted to almost any desired value over a wide range. Great range in properties, from soft to hard rubber. Hysteresis provides self-dampening. High capacity to absorb energy. Resistance to chemicals is outstandingly good. Highly resistant to abrasion. High dielectric strength makes rubber a splendid insulator. High coefficient of friction against nearly all surfaces. Waterproofness. Low permeability to most gases and liquids. Readily molded, a thermosetting plastic. Available in a range of color. Available in liquid form as dispersion or cement. Low in material cost.
A
.
Rubber Company, Detroit, Mich.
BOUT 80 per cent of the rubber consumed in the United States is used in the automotiveindustry. Rubber was necessary for the development of the modern automobile, and the popularity of the automobile has, in turn, tremendously enlarged the rubber business. The parallel growth of the rubber and automotive industries is apparent in Figure 1, where the consumption of rubber is compared with the registration of motor vehicles in the United States for the past thirty years. The modern automobile contains about two hundred different parts made wholly or in considerable part of rubber. Including the tires and tubes, the weight of these materials is about 145 pounds per car. Rubber is used in the automobile in practically all of its available forms either as latex, cements, soft rubber, hard rubber, or sponge rubber. Every important characteristic of rubber is utilized. Rubber has a wider range of properties than any other single material, and automotive engineers are t o be congratulated on their progressiveness in using these unique and valuable properties to improve their product. The rubber industry also has risen to the occasion and made remarkable advances which have contributed greatly t o the wider use of rubber in the automobile and truck. The rubber technologist and the rubber industry in general have enormously reduced the cost and improved the quality of the rubber parts offered to the automotive industry. Tires and tubes are the most important of these products, and Figures 2 to 5 show the tremendous progress which has been made in reducing the cost and improving their quality. Knowing the vehicle registration for each year and estimating the average mileage per car per year as 10,000 we can calculate the savings to the motoring public resulting from the reduced cost of tire mileage (Figure 5 ) . During the last twenty years the accumulated savings calculated in this way have
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
March, 1941
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If we examine the two hundred different uses of rubber in the average automobile, we find that at least one, and usually several, of the unique properties of rubber have caused it to be chosen over other materials. I n the tire assembly, for example, rubber is used in the tube because flexibility and low permeability to air are required. Rubber is employed in the tire carcass because it will withstand repeated flexing, and will insulate the cotton cords and prevent them from chafing against one another. We use rubber in the tire treads because no other material will provide the necessary resistance to abrasion, absorption of shock, waterproofing of the structure, and friction against road surfaces.
3. The development of antioxidants by chemists to improve performance and t o lengthen enormously the useful life and reduce the cost of rubber products. 4. The use of carbon black in treads by rubber technologists to improve wear. 5. The development of adequate means of adhesion of rubber t o metal by rubber technologists which permits the fabrication of parts comprising these two classes of materials. Such parts are used largely to dampen vibration. 6 . The development of improved methods of fabricating rubber parts. 7. The development of improved fabrics to be used with rubber.
Recent Advances The main advances in the rubber industry in recent years which have led to the present extensive and advantageous use of rubber in the modern automobile and truck are as follows: 1. The develo ment of rubber plantations t o yield a reasonably ches a n 8 reasonably uniform product. 2. The &velopment of accelerators of vulcanization by chemists to improve the uniformity and aging of rubber products and to reduce their cost.
Chemists and rubber technologists played a major role in five of the seven major advances. The cooperation between the automotive and rubber industries has been such that each of these advances has been rapidly incorporated into rubber parts for the automotive industry, and the automotive industry has taken full and immediate advantage of the advances made. With such a record of past cooperation it is obvious that the rubber industry welcomes the constructive criticism made by automotive engineers and fully realizes that the continued healthy growth of both industries demands that we meet each prob-
INDUSTRIAL AND ENGINEERING CHEMISTRY
372
Vol. 33, No. 3
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% CONSTANT SHEAR STRAIN FIGURE 6 lem in a cooperative spirit with the thought of finding a solution as soon as possible for our mutual benefit. Some recent advances in rubber are being utilized more and more widely by the automotive engineer. One of the most important is the foam-sponge rubber seat made from latex. These seats provide the most comfortable ride achieved to date because they conform to body contours and effectively damp high-frequency vibrations. A new type of rubber composition battery separator is replacing wooden battery separators to improve the performance of batteries considerably. Rayon heavy-service tires definitely give greater mileage and less trouble, in addition to saving about 4 per cent of the gasoline. Rayon passenger tires are definitely safer for highspeed driving, and the use of both classes of rayon tires is steadily increasing because they represent a major contribution to cheaper, safer transportation. I n retrospect we see a period of rapidly increasing use of rubber in the automotive industry, we see cars enormously improved by the use of rubber, and we see a rubber industry growing by leaps and bounds t o keep pace with the rapidly growing automotive industry. The airplane and tractor also utilize a large amount of rubber, and we find more and more being put into railroad and street cars. It is estimated that there are about 200 pounds of rubber in the modern Pullman car and about 500 pounds in the street car. Railroad cars use rubber for bumpers and street cars use it for springs under the whole car and under some of the auxiliary equipment.
FIGURE 7
phenomenon ordinarily defined as the progressive loss of strength due t o successive cycles of stress. A somewhat less familiar form of dynamic fatigue in some materials is the progressive loss of elongation properties with successive cycles of strain. Static fatigue is the progressive breakdown of a material under the influence of a static load; we believe it is due to the existence within the body of the material of randomly oriented regions of weakness, to stress concentrations, or to variations in elastic and plastic properties. Static fatigue of rubber is a newly recognized phenomenon and is normally one of the limiting factors in the specification of working stresses in the design of a rubber part. The load required t o break a rubber sample in a test machine is a function of the speed a t which the load is applied; it is greater, the greater the speed. If a rubber sample is not pulled to break in a test machine but is kept under a constant load slightly less than its breaking load in the ordinary test machine, the sample will break in a short time owing to the continued high static stress. If various lower loads are imposed on similar samples, the static fatigue life varies with the load and is greater, the lower the load. For some years the United States Rubber Company has been investigating the dynamic and static fatigue characteristics of rubber under a variety of test conditions. Until recently the main emphasis has been on dynamic fatigue, with the result that our present information on static fatigue is meager compared to that on dynamic fatigue. However, the importance of static fatigue from a design standpoint justifies calling attention to the phenomenon and making some general statements concerning it.
Fatigue Studies The rubber industry has been remiss in that it has not supplied automotive and other engineers with comprehensive engineering data on rubber. Realizing that the wider use of rubber in the future depends to a large extent upon the availability of such engineering data, the United States Rubber Company has tried to serve the rubber and other industries by developing and presenting such information. A paper was published last year on the dynamic fatigue properties of rubber'. During the intervening year we have been studying the static fatigue properties of rubber, and a brief summary of our work to date can be given. Rubber is subject to two types of fatigue-dynamic and static. The dynamic fatigue of a material is a well-known 1
Cadwell, Merrill, Sloman, and Yost, IND. ENQ.CHEM.,Anal. Ed., 12,
19-23 (1940).
Static Fatigue Curve Static fatigue occum not only in rubber stocks but also in rubber-to-metal bonds in the case of bonded parts. The main emphasis in our work has been on the study of static fatigue of the bond. Figure 6 shows the static fatigue of the bond for a stock with a shear modulus2 of about 80 pounds per square inch used in a shear sandwich of the type indicated. The rubber elements in the sandwich were approximately 0.75 X 0.75 X 0.35 inch. The samples were subjected to various constant shearing deflections a t 100" F. The average static fatigue lives of such samples in days are plotted on a log scale against the imposed shearing strain. The curve is not
* The shear modulus is the shearing stress divided by the shearing strain, a8 determined from the average slope of the shearing stress-strain curve out to about 30 per cent shearing strain.
March, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
developed below about 90 per cent shear, and the fatigue lives for these lower shearing strains cannot be obtained by extrapolating this curve since, for low shears, the curve actually rises more rapidly than is indicated for moderately high shears.
373
The static fatigue life of a rubber stock or of a rubber and metal part depends, in a complex manner, on the conditions under which it is used and on the design of the part. It is lower, the greater the operating stresses and the higher the temperature. Stocks with good static fatigue lives can be obtained through proper compounding. The static fatigue life of the bond can be increased to a considerable extent by careful design.
General Picture of Fatigue in Rubber
2 LINEAR STRAIN AT MINIMUM; +(IOO)% FIOURBI 8 The static fatigue curves for all types of deformations have this general form for all stocks, whether plotted as a function of constant strain or constant load. For any particular case the actual magnitudes of the static fatigue lives may be greater or less than those shown in Figure 6, depending on many factors: the shape of the unit, the stock, the cure, the temperature of the rubber, and the type and amount of deformation. That point must be emphasized. Figure 6 refers only t o a given stock, a particular type of deformation, a particular temperature, and a given type and shape of sample,. If any of these features is changed, the resulting static fatigue life curve will have the same general form but may be higher or lower than this particular curve or may even cross it. The static fatigue life is lower, the higher the temperature of the rubber, and is lower for constant loads than for constant strains corresponding initially to the constant loads.
General Features of Static Fatigue Static fatigue breaks often occur suddenly without previous indication of impending failure such as a pronounced increase in drift just before break. For a given load and temperature the lives of a number of supposedly identical samples are likely to vary widely. Under the same test conditions the static fatigue lives of different stocks and of their rubber-to-metal bonds vary radically. Figure 7 compares the static fatigue of the rubber-tometal bonds of two different stocks tested in the manner already discussed. Curve I is the same as previously shown in Figure 6. Curve I1 is for a softer stock with shear modulus about 65 pounds per square inch. The curves cross, principally because of the high extensibility of the softer stock. However, this variation in static fatigue life is not due solely to the difference in modulus of the two stocks, because there does not seem to be any particular dependence on the modulus. When the static fatigue resistances of rubber-to-metal bonds of two stocks of approximately the same low shear modulus (about 65 pounds per square inch) but different chemical composition were studied in simple shear tests, it was found that, under the same conditions of testing, the bonds for one stock had enormously higher static fatigue resistance than did those of the other stock although the strengths of the units for short time tests were about the same. Beyond a certain optimum cure, the static fatigue resistance of a stock or of its bond to metal is lower, the greater the cure.
Figure 8 shows how the dynamic fatigue of rubber for a particular oscillation stroke varies with the minimum strain in the oscillation cycle. The majority of our dynamic fatigue tests were run a t such speeds that the samples were broken before static fatigue became an important consideration. Therefore, except for very high minimum strains this is essentially a true dynamic fatigue life curve. If our dynamic fatigue tests had lasted for several years instead of several weeks or months, static fatigue (that is, progressive breakdown due to the average value of the strain) would become an important consideration and would reduce the life values for even moderate minimum elongations. This is illustrated by Figure 9 which shows both the static fatigue curve and three dynamic fatigue life curves obtained a t different frequencies. Up to this point the static fatigue life has been given in time units of days to complete rupture while the dynamic fatigue life has been given as number of cycles to complete rupture. I n Figure 9 both types of fatigue are expressed in time units. It is obvious that this can be done for dynamic fatigue values if the frequency of the test ma&hine is taken into account. These four curves refer to the same stock and to a given constant temperature of the rubber. The fatigue life axis is a log scale with arbitrary time units, and is the same for both the static and the dynamic fatigue curves. The abscissas refer, in arbitrary units, either to the percentage linear strain in the rubber a t the minimum in the oscillation cycle for the dynamic case or to the imposed strain in the static case. The oscillation stroke for all the dynamic curves is assumed to be the same, and hence corresponding points such as A , B, and C represent the same number of cycles of vibration to rupture. They represent different times to rupture because the samples for the different curves I, 11, 111, are assumed to be vibrated a t different frequencies.
FATIGUE LIFE IN ARBITRARY TIME UNITS
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STATIC FATIGUE CURVE CORRESPONDING
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Curve I represents what would be obtained in a high-speed fatigue machine. The speed of the machine is such that the complete dynamic fatigue life curve falls well within the static fatigue life curve. Hence it represents the true dynamic
314
INDUSTRIAL AND ENGINEERING CHEMISTRY
fatigue case. In general, our dynamic fatigue testing has been of this type. For a machine of somewhat lower speed, curve I1 would be obtained. Out to the point of intersection of curves I1 and IV the time intervals for curve I1 are constant multiples of those for curves I. Beyond the intersection of curve I1 with curve IV i t is impossible to increase the effective fatigue life of the rubber by increasing the minimum strain in the oscillation cycle. This is true because the intersection of curves I1 and IV represents the point where static fatigue becomes the determining feature in the life of the sample. Further increase in minimum strain will merely carry the fatigue life down along curve IV. Curve 111,for a low-speed machine, shows the same general behavior as curve 11. However, i t intersects IV a t a lower minimum strain than does 11. Increasing the minimum strain beyond that a t the point of intersection actually reduces the fatigue life instead of carrying i t to the higher dynamic fatigue values which would be found were static fatigue not the most important consideration. The conclusions to be drawn from these curves are: (1)
Vol. 33, No. 3
If rubber is worked in extension and if the conditions of vibration of the rubber are such that static fatigue can become an important consideration, there is a limit to the time life that can be obtained for the rubber by increasing the minimum strain in the oscillation. (2) This limit is the time corresponding t o the intersection of the static and dynamic fatigue curves. (3) The lower the vibration frequency of the unit, the lower is the minimum strain a t which this intersection takes place. These conclusions also apply to any other type of deformation. I n practical designing, both types of fatigue must be allowed for. If the amplitude and frequency of vibration of a part are large, the part may fail primarily because of dynamic fatigue; if the amplitude and frequency are small, the part may fail mainly as a result of static fatigue. For these reasons, any choice of working stresses must be a compromise so determined that both the static and the dynamic fatigue lives of the part are adequate. PRESENTED before the Division of Rubber Chemistry at the 100th Meeting of the American Chemical Society, Detroit, Mich.
THE ALCHEMIST B y Charles Meers Webb
WE
are again indebted to Chester G . Fisher for a photograph of the original painting which is the subject of No. 123 in the Berolzheimer series of Alchemical and Historical Reproductions. The original, signed by the artist, and dated 1858, is now in Mr. Fisher’s collection, having formerly been in the possession of Prince Demidoff of Russia. Webb was born near London, England, on July 16, 1830, and died a t Duesseldorf, Germany, on December 11, 1896. He studied at the academies at Amsterdam, Antwerp, and Duesseldorf, and became thoroughly imbued with the style of the Dutch school, although he was much influenced by Camphausen. He settled in Duesseldorf in 1848 and did many genre subjects. While his human figures remind one forcibly of Isabey’s work, there is considerable resemblance to that of Teniers, the Younger, particularly the furnace and still, so frequently used by the latter. In this case the alchemist was evidently a believer in “studying the literature”, for he possesses at least seven books, a larger number than are shown in any other member of the series. D. D. BEROLZHEIMER
East 41st Street New York, N. Y. 50
The lists of reproductions appear as foII0ws: 1 to 96.
January, 1939, issue! pa e 124; 97 U, 120,January, 1941, page 114. An addhona! reproduchon appears each month.
