INDUSTRIAL A N D ENGINEERING CHEMISTRY
age of raw rubber, and that we should produce synthetic rubber because it is not possible for the plantations in the Far East to supply a sufficient amount of crude rubber for our uses. There is no logic in these arguments, and an analysis of the situation leads us to believe precisely the reverse. Only seven thousand square miles of territory have been planted to rubber trees, and yet when one considers the possibilities the world over, he can conceive of something like seven millions of square miles of territory in which rubber trees can be grown. This may sound trite, but it is, nevertheless, significant. CosT-In this practical discussion we must again look a t the cost sheet. The cost of growing and producing crude rubber is not so low as it can be brought. Preswt indications are that it can be produced more economically on plantations of some kind than in the chemical factory. It mould seem that the planters can reduce their costs radically by an intensive labor-saving campaign. This, likewise, has been discussed a t length. Such elements of cost as the clearing of the jungle can un-
doubtedly be reduced by taking advantage of proper machinery and methods. Tapping schemes, while greatly improved over what they were ten years ago, are still too costly. The handling of latex from the tree t o the go-down requires more labor than it probably should, and the coagulation, washing, and drying, which are today intermittent processes, may be made continuous, provided the rubber trade will be willing, and no doubt it will be, to accept, instead of special types of crude rubber, one grade of a scientifically manufactured, dried, and pressed type in the form either of blocks or sheets. Conclusion
So the world has little to expect, and the planters nothing to fear, from synthetic rubber; and in the inevitable cost competition which would arise, were synthetic rubber to be produced, the planter would be able to deliver crude rubber on board ship at a profit to himself, and a t a price t o which synthetic rubber could not be brought.
Alternative Materials for Rubber Principles of Tread Wear and Resistance to Abrasion By Ellwood B. Spear RESEARCH A N D DEYELOPXENT LABORATORY, THERMATOMIC CARBOSCo., PITTSBVRGH, PA.
HE term “alternative,” in this connection, is meant
to include any material now known, or yet to be discovered, that may be used in the industry instead of rubber. At present, the technologist is familiar with many such substances, among which are iron, wood, cotton, wool, silk, leather, balata, gutta-percha, Bakelite, gums, resins, bitch, asphalt, porcelain, glass, shellac, and many others. It is obvious that no one of these substancw can be used universally in the place of rubber. For instance, shellac, porcelain, and glass are very good electric insulators, but cannot be employed where elastic properties are required. Balata is used successfully in belting and gutta-percha is employed in large quantities for insulation in submarine cables. The properties of rubber are so unique and in many cases so extraordinary that it is a fair question to mk whether or not it is wise to attempt to make a synthetic product that will hare properties identical with those of caoutchouc. Would it not be a more rational procedure to aim at the production of a synthetic product having properties suitable for a specific purpose? For instance, in the manufacture of insulated wire me do not require the high tensile, the estraordinary stretch, or the very great resistance to abrasion that are exhibited by rubber compounds. On the other hand, a low dielectric constant, a low power loss, and high electric insulation are essential. Many materials are used today for this purpose instead of rubber. Shellac, paper, asphalt, gutta-percha, and several other substances may he employed for certain types of insulated wire and cable. Why not synthesize a substance for use in insulated wire where especial attention is given to the dielectric constant, the power loss, the insulating properties, and chemical stability? One of the features of rubber with which the technologist has to contend is the poor aging properties in the presence of sunlight. Properly cured rubber will last for many years if protected from strong sunlight. The writer has used a rubber stopper in a wash bottle for over ten years and when circumstances forced him to part company with the wash
bottle the stopper was still in good condition. Inner tubes will last for years in pneumatic tires for passenger cars barring accidents, provided that the climate is moderate and the speed reasonable so that the tires never become very hot. Severtheless, if a portion of one of these tubes is exposed to sunlight for a few months, the material will be absolutely destroyed so that it may be easily torn with the fingers. Is it possible to make a synthetic product that will be suitable for inner tubes and have aging properties superior to those of rubber? Another weakness of rubber comes to light when we consider the short life in steam hose. Here it is essential that the substance be impermeable to hot water or steam and have a high resistance to the chemical effects of the two last mentioned. I n steam hose we do not require high tensile, high stretch, or very great flexibility. Heat aging coupled with impermeability and moderate flexibility are the prerequisite qualities. For use in rubber shoes, rubber has some strong and some objectionable properties. I t is waterproof, flexible, resists wear very well, but the impermeability is not wholly an advantageous factor. Rubber does not permit ventilation for the feet and therefore rubbers or rubber boots become uncomfortable in a short time so that many people refuse to wear them or even rubber-soled shoes. It is possible to make a substance reasonably waterproof and a t the same time more permeable to perspiration than rubber is. The writer made determinations some years ago showing that the permeability to perspiration could be very greatly increased without unduly impairing the water-proof qualities. If one is to stand in water all day, then rubber meets the requirements for footwear quite well. On the contrary, for winter wear or intermittent wet and dry use the waterproof quality is exaggerated and the permeability not great enough to make the shoes comfortable. Can a synthetic substance be made which will be reasonably waterproof when used in footwear and still be more comfortable than rubber? As a material for solid and pneumatic tires, we are obliged
INDUSTRIAL A N D ENGINEERING CHEMISTRY
to confess that rubber is not only unsurpassed but that it has no competitor. It must be realized, of course, that rubber requires to be strengthened by another substance such aa cotton in the carcass and steel wire in the bead. This means that rubber itself is not sufficiently strong at lower elongations to withstand the stress in pneumatic tires. It is extremely improbable that a substance can be made which will have sufficient strength to hold its shape in pneumatic tires and a t the same time have the properties which are necessary for inner tubes, frictions, and tread stocks. If the organic chemist desires to make a synthetic product that will replace rubber in any of the various uses to which rubber is put, he should know fairly accurately what properties his product must have. The man who should be in the best position to give this information is the rubber technologist. The subject is very broad and it would require a great deal of time to present the facts even if the necessary data were a t hand. Because of space limitations, the remainder of this article is confined to a consideration of the principles involved in tread wear and resistance to other types of abrasion. Principles of Tread Wear a n d Resistance to Abrasion CASE A, STEEL TIRE-Let us assume that a rigid steel-tired wheel having a mass M is rolling in the direction of the upper arrow, Figure 1, with a uniform velocity V and strikes the solid wall D. If M and V are not too great the wheel will strike the wall and rebound without any great damage t o either the wheel or the wall except a t the points of contact E and F, the surface of the tire and wall, respectively.
The kinetic energy of the rolling mass is represented by the formula 1/pMV2. The work done is that which is necessary t o bring the wheel to a state of rest, or to reduce V t o V O ,preliminary t o reversing the direction of the wheel. Disregarding the rolling friction, the total work done in stopping the wheel is dissipated in damage t o the surface, in heat generated, and in , t o the wheel in the kinetic energy equal to l / ~ M V 1 zimparted direction of the lower arrow. For a wheel of comparatively rigid construction l/2MV12 is quite small. The heat generated is due t o the distortion of the materials of the wheel and the wall and also t o the crushing or deformation of the surfaces E and F. CASE B, SOLID RUBBERTmE-Let us now assume the wheel t o be constructed as before except that the tire is made of solid rubber. The total mass and velocity are t o remain as in case A . As soon as the surfaces E and F come into contact, the wheel begins t o slow down, the rubber squeezes out somewhat in all directions, and therefore slides along the surface F, resulting in more or less abrasion of the rubber. Because the rubber of the tire will store up energy, much of which is recoverable, the wheel will reverse its direction, rebound much farther than in case A , and acquire a kinetic energy equal to ‘ / Z M V Z ~It . is t o be noted t h a t 1/2MV22 > l/~MV12. This is because the rubber of the tire will store up more energy than the steel tire will. The total work done is the same as in case A , but the distribution is entirely different. I n case A the time taken to change the velocity from V t o V Ois very much shorter than in case B. In other words, the negative acceleration a1 is much greater in case A than is a2 in case B . Force is defined as mass times the acceleration. Therefore, in case A , F1 = Mal.
Vot 18, No. 11
In case B, Fa = Mas Fi >. R. AS ai > 4
From this consideration alone the damage done to the surface of the steel tire should be greater than that on the rubber tire. There are, however, other important aspects t o be discussed. When the rubber tire touches the wall, the rubber deforms so t h a t the bearing surface is much greater than if the tire were made of steel. Moreover, the amount of bearing surface increases continuously until the direction of the movement of the wheel is reversed. Therefore, the pressure per square inch of bearing surface is much less in case B than in case A . This factor should tend t o decrease the abrasion. On the other hand, as already pointed out, the rubber slips along the surface F during the deformation and this movement should increase the abrasion. These two considerations are cardinal to the theory of tread wear. CASEC, PNEUMATIC TIRE-Once more let us assume that the wheel has the mass M and velocity V as before, but this time a pneumatic tire is substituted for the steel or solid rubber tire. The time taken to reduce the V t o Ve will be much greater than in the case of A , or even B; for not only does the rubber squeeze out under the forces applied, but the deflection of the tire is much greater as the carcass can be relatively easily deformed under a nearly constant air pressure inside the tire. If the product l/,MVZ is large enough the pneumatic tire will be deflected as far as the rim will permit. The greater deflection of the pneumatic tire results in a still smaller negative acceleration I n this instance FS = Mas. By a process of reasoning simi03. lar to that indicated under case B , a1 > CYZ > as K > FZ >Fa. Furthermore, the bearing surface is greater than in either of the preceding cases. If these were the only considerations involved in tread wear, the damage done t o the tread surface should be least in the case of the pneumatic tire. It must not be forgotten, however, that the movement of the tread surface at the points of contact with the wall increases with the deflection of the tire. This, of course, results in the greatest abrasive action in case C. We have, therefore, the greater the deflection of the tire the smaller the damage to the surface as a result of the impact and, on the other hand, the greater the deflection the greater the abrasive action due to the slipping of the rubber on the surface of the obstacle, road, etc.
Another principle involved in tread wear should be recognized a t this stage. Let us suppose a steel, a solid, and a pneumatic-tired wheel, respectively, having the same mass in each case, strike an obstacle such as a stone or brick lying on the road in the direct path of the wheel. Assuming that the obstacle is not crushed in any of the three cases and that its total vertical height is X, the weight of the steel-tired wheel must be raised X distance against the gravitational forces. This is not true of the solid rubber nor the pneumatictired wheels. In fact, under certain circumstances the obstacle may sink into the pneumatic tire so far that the wheel is raised only a small fraction of X . Thus the total work done in raising the wheel against gravitational forces is least in the case of the pneumatic tire. The wearing effect of slippage of the tread along the road due to the deflection of the tire can easily be demonstrated and is occasionally encountered by the autoist. If the front wheels of an automobile are run parallel to each other and therefore parallel to the direction of travel, the side buttons of a nonskid design wear down much faster than the center buttons. I n practice, however, the front wheels are given some “gather” in order to improve the ease of steering and to prevent shimmying. This “gather” causes an abrasive action on the tread which is greatest where the pressure per square inch is greatest-namely, along the center rib or center buttons. The effect of the rapid abrasion of the side buttons is often partially provided for in practice by adopting the SOcalled “flat” tread. The wear on the center buttons of the rear tires is due in large part to the torque caused by the engine and to an effect in the opposite direction as a result of the action of the brakes. These facts are well known, of course, to - tire engineers. There is another effect of. the slippage along the road surface that is very pronounced in the case of certain fread designs. Frequently one notices a tread worn into waves
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Stock 1284 1285 1286 1287 1288
Carbon code G G P-18 P-18 C
on 100 vol. rubber 14 20 14 20 14
Cure Min. at 40 Ibs. steam 45 45 35 35 35
Elong. at break 698 673 753 742 695
p T 8 N S I L B AT-
1445 44 5 606 572
500% elona. 2796 3520 1625 2265 1859
Break 5170 5166 4983 5187 4282
Resilient enerey _. 1090 1191 925 995 775
CO 72.7 82.7 59.0 62.0 61.1
TO ABRASION N. J. Zinc Co. Goodrich
36‘30‘ 38’40’ 33’26’ 350 0’ 31O35’ 24Ey T P X 100
machine 100 122 62 85 35
machine 100 103 102 104 74
C is the concavity factor [Wiegand, THISJOURNAL, 11, 625 (1925)j calculated according to the formula C = where Ey = energy of resilience in foot pounds per cubic inch, and T P = tensile in pounds per square inch times per cent elongation at break divided by 10,000. b 0 is the angle which the straight line joining the origin with the point of rupture on the curve makes with the extension coardinate. The values given for 0 are relative to one another only when all curves are plotted on the same scale. The values for any set of rurves depend upon the scale chosen for the ordinates and abscissas, respectively.
where the wave length corresponds to the pattern of the nonskid design. This is due to the fact that the button is acting under forces one component of which is in the direction of travel. These forces cause the button to be distorted forward every time it comes in contact with the road. This is often referred to as the rolling compression wave. As the wheel turns on its axis, the back end of the button is released first and exerts a pulling action on the remainder of the button. In other words, a portion of the button is being dragged over the surface of the road before the weight has been entirely removed. This is frequently spoken of as the “kickback” and results in the button being worn faster on one end than on the other. It may be of interest to the automobile owner that a tread on a pneumatic tire would wear away in a few thousand miles under normal load if the surface of the road were always dry and free from extremely finely divided material which acts as a lubricant. This phenomenon of unusually rapid wear has often been demonstrated while testing tires on an indoor track. It will be evident from what has been said that a soft stock will be distorted underneath a load to a greater extent than a stiffer stock and, therefore, the slipping of the rubber on the road surface will be greater in the case of the softer stock. Assuming approximately the same proof resilience in both cases, the soft tread stock should wear faster than a stiffer stock when used for solid or pneumatic tires. This is in entire agreement with the known facts. The whole question is reversed, however, if these stocks are used for lining a discharge chute for sand or gravel where the load is comparatively small. Here the stiffer stock, because of its very stvness, resists the movement of the sharp particles and therefore suffers greater abrasion; whereas the softer stock retreats a t the points of contact, the particle rolls past, and the rubber returns to its original position little the worse for the encounter.
sion without taking some other factors into consideration. The values for the proof resilience are fixed by the tensile, the elongation, and the stiffness. I n order that the resilient energy of a soft stock may be comparatively high, the tensile and elongation at break must be unusually great. This is brought out quite clearly by Curves 1285 and 1287, Figure 2. The proof resilience of stock 1285 is represented by areas A and B, whereas that of stock 1287 is the sum of the areas ’4and C. The area B is considerably greater than C, although the tensiles of the two stocks are almost identical and the per cent elongation a t break of stock 1287 is much greater than that of 1285. (Table I ) Practical experience has shown that stocks such as 1285 will wear better in treads for pneumatic tires than stocks similar to 1287. On the other hand, 1287 will withstand sand abrasion better than 1285. This reversal is predicted from the abrasion results given in the two last columns in Table I. By means of simple arithmetical calculations it can be shown that there is some degree of parallelism between the results given by the New Jersey Zinc Company’s machine and the values for resilient energy, the concavity factor or -5000
The Loose Abrasive Machine
This explains why the loose abrasive machine of the Goodrich type always shows a smaller difference between a stiff and a softer stock than do machines similar to the type made by the New Jersey Zinc Company. I n fact, the Goodrich machine has shown repeatedly that the softer stocks will withs>and sand abrasion much better than the very stiff stocks if the proof resilience of the two are nearly equal. I n other words, the loose abrasive machine does not test the ability of the stock to carry the load without undue distortion. The relationships between the results given by the abrasion machines, tread wear, and resistance to abrasion, the shape of the stress-strain curves and resilient energy will be discussed under the next heading. Relation of Stress-Strain Curves and Proof Resilience to Tread Wear and Resistance to Abrasion
Proof-resilience determinations, while undoubtedly of great value t o the rubber technologist, cannot be accepted as safe predictions of tread wear and resistance to sand abra-
300 500 700 PER CENT ELONGATfON Figure 2
the angle 8 given in Table I. On the other hand, these values must be interpreted differently to bring them into harmony with the results obtained by the loose abrasive machine. It is apparent, however, that the latter can be used to indicate the superiority of stocks having the higher resilient energy provided that this is due to high tensile and elongation and not to greater rigidity. Compare stocks 1286 and 1288. It is evident that 8 must be smaller for stock to be used for a sand chute than for those employed for treads. For use in solid tires heat conductivity and hysteresis are also important factors; otherwise, the solid tire may “blow out” during hot weather if the load is heavy and the speed relatively high. It will be evident from a consideration of Figure 2 that the angle 8 becomes greater as the tensile increases and the elonga-
INDUSTRIAL d N D ENGINEERING CHEMISTRY
tion assumes a lower value. I n the case of steel 0 is comparatively large, probably in the neighborhood of 80 degrees. As pointed out in the preceding paragraphs, if 0 has a low value the tread stock does not carry the load without undue distortion and hence will not give satisfactory wear on a pneumatic tire. It has also been shown that steel, where e is comparatively great, does not wear well because the forces a t the instant of contact are high enough to assume the dominating role. It is interesting to speculate as to t,he optimum values 0 may have in an ideal tread stock. I n other words, for given values of resilient energy and the concavity factor, how large may become before the impact forces offset the advantages gained by producing a stock t o resist distortion? Likewise the question may be propounded -for use in linings to resist abrasion, how small may 0 become if the resilient energy can be maintained a t a comparatively high value? The analysis of tread wear discussed in the preceding paragraphs should aid in correcting an impression frequently voiced by rubber technologists that the extraordinarily
Vol. 18, No. 11
high elongation of rubber compounds before rupture is unnecessary. If the arguments presented above are sound, high stretch is essential. We may say with confidence to the organic chemist that, for use in tread stocks a t least, compounds containing substitutes for rubber must show high resilient energy and a comparatively high rigidity. Moreover, if his stocks are to be employed as a wear-resisting material in lined chutes for sand, gravel, and other abrasives, the resilient energy should be as high as is consistent with rather low rigidity. Resistance to tear is doubtless another important factor in tread wear and in withstanding the action of abrasives. I t seems probable that this factor is intimately connected with high tensile and elongation. There is, however, some lack of unanimity among rubber technologists on this point. Acknowledgment
The author gratefully acknowledges helpful suggestions by R. E. Day and valuable aid by R. L. Moore in the preparation of this article.
Composition and Structure of Hevea Rubber By R. P. Dinsmore THEGOODYEAR TIRE& RUBBERC O , AKRON,OHIO
In 190j Willstatters showed cyclooctadiene was a ring of eight carbon atoms. Harriesg a t this time was able to form levulinic aldehyde and acid from ozonide, and this was taken as evidence that the structure of rubber was 1,j-dimethyl cyclooctadiene. Harries reasoned that if, as he believed, two isoprenes condensed to form the above compound, two butadienes should give Willstatter’s cyclooctadiene. This proved to be the case. A t this time Harries proposed the partial valence explanation of polymerization-two partial valences for each double bond. In 1910 Pickleslo raised objection to partial valence theory because: (1) polymerization does not reduce unsaturation and (2) polymers persist after destructive distillation, indicating strong bonds. Pickles proposed a long chain structure with free end bonds. In 1912 Barry and Weidert’l worked out a formula for vulcanized rubber, based on the amount of sulfur (2.5 per cent) required t o render rubber insoluble in benzene. The formula was (C10H16)2S2= 2500, from which x would equal 18+, In 1921 “alpha is0 rubber” (CloHl~)zwas prepared by Harries.12 Molecular weight showed z = 8. This was apparently not a true hydride, hut in 1922 rubber hydride was prepared by Pummerer and Burkard.’3 It behaved somewhat like rubber but oxidized readily. In 1922 a nonoxidizing hydro rubber was prepared by Staudinger and Fritchi.I4 A compound C~oHloowas isolated from its decomposition products, indicating high mo,lecular weight of original polymer and strong polymer bonds. This caused Staudinger to postulate primary valence linkage. C h e m i c a l Composition I n 1924 methyl and ethyl hydro rubber were made by THE RUBBERHYDROCARBON-In 1860 Greville \Villiamsl * Staudinger and Widmer.l5 obtained isoprene and couchine from rubber by distillation. In 1925-26 rubber heated to 250-270’ C. was shown by I n 1879 Bouchardotz condensed isoprene to dipentene Staudinger and Geiger16 to form a product they called “polyIn 1884-5 Wallach3 proved dipentene and couchine to be cyclo rubber,” losing all but one-fifth of its double bohds. “ I n identical. ternal cyclization” .was proposed as an explanation by StaudI n 1892 Tilden4 found t h a t dipentene decomposed to form inger. isoprene and t h a t isoprene could be polymerized to a rubberI n 1926 Fisher and Gray17 also made polycyclo rubber a t like material. temperatures 340-345” C. Harries5 in 1902 obtained two isomers of dipentene from rubber. In 1926 Staudinger and Widmer’s showed that the hydride of Gladstone and Hibbertc in 1888 analyzed rubber and showed polycyclo rubber can be formed. Staudinger and Geigerlg show the purified material corresponded very closely to ( C S H S ) ~ . that gutta-percha and balata yield .the same hydro and polycyclo They also obtained optical evidence t h a t there were three double rubber as Hevea. It may be pointed out here that all known bonds for ten carbon atoms. Their compounds were not abderivatives of the rubbers just mentioned are the same regardless solutely pure. Harries’ in 1904 obtained more accurate information regarding of the rubber. Recent work of Staudinger and Wehrli (private comrnunicaunsaturation by formin’g rubber ozonide, which he proved to be tion) shows t h a t traces of bromine or trichloroacetic acid cause C10H1606 thus indicating only two double bonds for ten carbon enormous reductions in the viscosity of a rubber cement and atoms. solutions of other large molecules. They cite this fact as evidence of a long hydrocarbon chain with end valences too weak t o * Numbers in t e x t refer to bibliography a t end of article.
HAT human minds are led into error alike by inadequate
hypotheses and by faulty ones is not a new observation. Yet the frequency with which its truth is demonstrated is sufficient excuse for calling it to mind. If the fact is well known, it is not treated with sufficient consideration. I n the study of rubber we find that progress has been hampered by hasty generalization and lack of a suitable hypothesis. Some investigators, obtaining a few facts which indicate the importance of chemical composition, jump to the conclusion that nothing else matters. Others make the same assumption regarding physical structure. Still others, intent on accumulating facts, misinterpret conditions and obtain a distorted image of the facts observed because they have no well-defined idea of the relation of their observations to the group of phenomena which constitute the whole. It is therefore believed that a better knowledge of the composition and structure of rubber mill be promoted by an orderly presentation of the facts nom known and, where the evidence is meager or contradictory, by calling attention to such discrepancies. By this means the errors in present hypotheses will be made evident and the foundation laid for a more suitable assumption.