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The History of Organic Accelerators in the Rubber Industry’ By W. C. Geer and C. W. Bedford THE B. F. GOODRICH Co., AKRON,OHIO
IPTORT is a fascinating subject, for it involves the known, and gives one the stimulus to speculation and research into the unknown. Indeed, written history in its various ramifications, whether of the individual, the state, or an industry, may be likened somewhat to a tree, yearly adding proved material and dropping away the unessential leaves; but composed of the solid matter of essential facts. Occasionally, the historical investigator goes into our forest of human knowledge, and from some particular phase of mankind’s activities tears away the forest covering and discloses a chapter, new only in its presentation, but his activities serve to give to the world a further knowledge of the facts upon which the present growth of his subject rests. This thought is quite true in the story of the organic accelerator in the rubber industry. And the story is worthy of the telling, since this group of substances has come to be so important in the rubber industry. Like many another human activity, many men in their studies used substances, but did not observe the properties of them, which later came to be prominent and important in the art. Sulfur
Even so fundamental a substance as sulfur traces its connection with rubber beyond the use made of it by Charles Goodyear. It takes away from his discovery no whit of priority to cite the names of men and the observations which they made prior to his day. I n the early days of the rubber industry, mixing mills driven by powerful motors or engines were but little used on account of the slight volume of rubber turned over in the various factories. Hence, much of the adaptation of rubber was performed through the medium of solutions. As early as 1831 Samuel Guthrie, of New York City, is said to have dissolved sulfur in turpentine, and to have used the mixture as a rubber solvent. Just what he did with it and what results were obtained are not known. I n 1832 Friedrich Ludersdorff published an observation to the effect that when rubber was dissolved in turpentine, and 3 per cent of sulfur dissolved with it and evaporated, the resulting film of rubber was less tacky on the surface than that obtained without the sulfur. But his obsewations went no further. I n 1839 Xathaniel Hayward patented in the United States a mixture of raw rubber with sulfur as an ingredient. The history of those days, coordinating as they did so closely with Charles Goodyear’s work, has shown us that Hayward did not know about the fundamental change wrought upon a mixture of rubber and sulfur by the application of heat. I n 1841, subsequent to the time when Goodyear discovered the effect of sulfur, J. A. Fanqhawe obtained a patent in England upon the use of sulfur in connection with rubber, again without mention of the use of heat. All these earlier students of the art missed the essential fact which was discovered by Goodyear in 1839. Much has been written upon the work of this founder of the rubber industry, and it will not be repeated here. It may be said, in passing, that Goodyear’s contribution had to do not alone with sulfur; Goodyear mixed rubber and sulfur and heated * Received January 24, 1925.
the mixture for a defined length of time, combining therefore the effects of sulfur, time, and temperature. He was the first to observe that this combination produced profound changes in the physical properties of rubber, not only drying the surface or “metallizing” it, as they called it in those days, but he observed that this combination caused the product to be stable in the presence of heat and cold, and that this heated mixture was more resistant to the deteriorating effect of the atmosphere and more serviceable because of its greater resistance t o wear. He conquered the chief obstacle to t h e growth of the rubber industry. Inorganic Accelerators
Substances in rubber which we now call inorganic accelerators were known relatively early. Andrew Ure in 1840 described the effect of the addition of lime for the purpose of “drying” rubber. Goodyear used lime and magnesia to avoid surface tackiness in his unvulcanized rubber mixtures. As soon as he discovered the effect of heat upon a mixture of rubber and sulfur, which, however, contained a lead compound, it is not surprising that so shrewd an observer should have commented upon the effect of inorganic oxides during vulcanization; and he therefore became the discoverer, not only of the effect of sulfur, temperature, and time, but also of the effect of inorganic accelerators. Organic Accelerators
The inorganic accelerators were not all valuable, for inany reasons. They were expensive and not effective in increasing the quality of the mixtures containing them. It was but a few years ago that the industry consumed a relatively large volume of soft wild rubber, such as caucho ball, the African grades, and others. But the strength and other properties were so different from the properties obtained from t h e hard Fine Para grades that rubber men ever dreamed of something to bring those qualities closer together. They used to say, “Can we not make the cheap grades equal t o the good ones in time of vulcanization as well as in resulting physical properties?” It was this desire that brought about, as much as any one thing, the research work into the effect of organic substances in rubber mixtures. The use of non-nitrogenous organic acid accelerators apparently dates from the early use of wild rubber, although, as we now know, they could not have become effective until after Goodyear’s knowledge of the effect of metallic oxides, sulfur, and heat were known. So it was not until 1904 that C. 0. Weber discovered the value of these acids. He added organic acids, such as stearic and oleic, to what were known as “lowgrade” rubbers, and thus supplied a natural deficiency in the acid content of certain grades of rubber and caused them to vulcanize much better. Dr. Weber, therefore, may be called, not only the father of rubber chemistry, but the discoverer of the non-nitrogenous organic acid accelerator. Nitrogenous Organic Accelerators
Antedating the work of Dr. Keber men used inorganic compounds containing nitrogen. Thomas Rowley, in 1881, was granted a patent on the use of ammonia gas in the vulcanizing chamber. He uced ammonium salts and even
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necessary. He chose aniline and phenylhydrazine, each of which he felt would fulfil the requirements. On June 1, 1906, he prepared a mixture which consisted of Mexican rubber2 250, sulfur 20, zinc oxide 15.5, and aniline 15. This mixture was cured under 40 pounds of steam for 1, 1.5, 2 , and 3 hours. The results in his notebook for the I-hour cure were marked “excellent.” Three-hour cures were usual in those days. Between June 2 and 17, 1906, he prepared many mixtures in which aniline was used both with and without zinc oxide; and in addition he tested the effect of dimethylaniline, diphenylamine, naphthylamine, nitrobenzene, and acetanilide. It is interesting to note at this stage that he also tested tetraethyl lead. The results of his laboratory work on the use of aniline were so pronounced that immediate tests were authorized for factory use. h formula was prepared using aniline in an inner tube and in a friction compound, the records of which have been preserved. This was prepared prior to June 22, 1906. On June 22 aniline was used in a solid tire recipe. From that time with increasing frequency aniline appears in the factory formulas of the Diamond Rubber Company. The practice of the use of aniline continued from then on until, by improved research work, other organic accelerators came into use, and it was not abandoned as a factory accelerator until about July, 1914. hlr. Oenslager was very early dissatisfied with the use of aniline oil, because it was a liquid and because it had poisonous effects and therefore required great care in its use. He remembered then a solid derivative of aniline which he had prepared during his college work, so between July 17 and 20 he made several tests of the effect of sulfocarbanilide. During July he also tested urea, acetamide, cinchonine, toluidine, triphenylguanidine, anthraquinone, CGH$CN, thiourea, and hydrobenzamide. During August, 1906, he increased the use to include carbanilide, p-nitrosodimethylaniline, hexamethylenetetramine, aniline sulfate, oxamide, uric acid, amidoazobenzene, and hydrazobenzene. During this period also he ran various tests upon sulfocarbanilide, and made the further discovery that zinc oxide was essential to the use of many, if not all, of these organic accelerators. One of hlr. Oenslager’s mixes, consisting of caucho 300, sulfur 18, and sulfocarbanilide 10, was found to require 3 hours at 40 pounds of steam to give a proper cure, whereas the addition of zinc oxide gave cures in only ten minutes. In order to arrive a t the proper amount of accelerator to use, he prepared mixes comprising Mexican rubber 300, zinc oxide 187, and sulfur 21, to which he added sulfocarbanilide, 20, 10, 5 , and 0 parts. These mixes were cured for 10, 15, 30, 60, 120, and 180 minutes a t 40 pounds of steam, and the notation was made that with 20 parts of the accelerator the mix cured in 10 minutes, with 10 parts it cured in 45 minutes, with 5 parts in 60 minutes, and without accelerator the mix was uncured in 3 hours. Mr. Oenslager, therefore, began the use of zinc oxide in connection with organic accelerators, a practice, either in pure gum or compounded mixes, which has come down to us and is still used. Sulfocarbanilide was used in the factory on September 19, 1906, in a pneumatic tire tread formula, and this substance became one of the factory accelerators that has continued in practical use to the present day. On September 20 this same substance was used in the plant in inner tube and gum friction compositions, in combination with zinc oxide. During the later months of 1906 many formulas were written in which sulfocarbanilide was used. On January 4, 1907, the rapid accelerating action of sulfocarbanilide and the low temperature at which it operated 2
This was a factory name for extracted Pontianac.
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were used to practical advantage in quick-vulcanizing repair compounds for pneumatic tires. His records upon the effect of aging are clear. Having had the experience with the poor aging from the use of mercuric iodide, his records were carefully kept, and observations made and entered in his notes, which go to show that the new organic accelerators were not deteriorating the rubber mixtures. Since February 8, 1907, nearly every formula in the records of the Diamond Rubber Company contains a nitrogenous organic accelerator. Mr. Oenslager’s work went on with increasing interest. d list of substances tested shows the wide range of his thinking along the line of organic compounds as vulcanization aids. I n the month of September, 1906, he tested nitronaphthalene, ammonium sulfocyanate, quinoline, xylidene, diamido diphenyl, diphenyl urea, phenylenediamine and tetraethyl lead. During October his list of retests included a further study of aniline, sulfocarbanilide, phenylthiourea, urea, tetraethyl lead, p-nitrosodimethylaniline, and dinaphthylthiourea. He early saw the necessity of uniformity of dispersion, and worked out the principle of using a master batch of rubber containing a higher quantity of accelerator, from which the proper amount of this master batch could be weighed out and mixed into the rubber mixture, thus giving a better distribution. This history of the use of organic accelerators in the rubber industry is brief, somewhat too brief, possibly. It has not been carried beyond the early discoveries, and does not attempt to be complete. The purpose has been to write a synopsis of the first research work upon organic accelerators available to us; and in particular, which is the point to be emphasized, a description of that research work which was followed by prompt application in rubber goods manufactured in a factory, and sold to the trade. So far as our records go, hlr. Oenslager was the first one to discover definitely the accelerating effect of these nitrogenous organic compounds in rubber mixtures, and to direct the use of them into actual factory practice. Many chemists have contributed largely and with ability to the development and knowledge of these accelerators. To them likewise belongs large and enduring credit. Practical Results
What were the practical results of these discoveries? To the manufacturer it permitted tires and other rubber goods to be vulcanized in from one-third to one-fifth the time formerly required. Factory vulcanizers are now “turned over” rapidly and efficiently. To the consumer have come higher service values. Rubber products are better and the world recognizes the improvement. The discovery of the value of organic compositions as accelerators of vulcanization started something! Thousands of them are now known, and the rubber cherpist finds it necessary to exercise judgment in using them. Rubber compounds are many; their construction is dictated by highly specialized service requirements. It has necessitated the concentrated research of many aggressive and splendidly trained chemists to develop the knowledge of accelerator values of these substances. Very few of the accelerators originally worked out are now in use. It is, however, a tribute to hIr. Oenslager that a t least one of them, discovered by him, has maintained its position and is still used in considerable quantity. Thiocarbanalide is a striking illustration of one of the first organic accelerators used in practice, and which is still used. because although considered today a slow and \Teak accelerator, its cost is sufficiently low to warrant the use of larger volumes in order to obtain desired results. During the last twenty-five years rubber-manufacturing methods hare rapidly changed and the industry has grown
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by leaps and bounds. To show the precise effect in dollars and cents of the use of accelerators is a difficult task. Conclusions from figures can be only rough estimates. I n 1906 the total amount of crude rubber used in the United States was 27,344 long tons; in 1924, 301,778 long tons. I n 1906 there were 107,000 automobiles, which a t the rate of 6 tires per car per year consumed 642,000 tires. I n those days, 3500 miles per tire was considered reasonable service. Nowadays, fabric tires run a yearly average around 7000 miles, and cord tires 15,000 to 20,000 miles. The averages show for 1924 (with its 17,897,000 automobiles) for replacement purposes about 2.4 tires per car per year. These improvements came about, not alone because of accelerators, but by virtue of a number of other factors working together; the making of tires upon machines, improvements in processes, improvement in the fabric used, better rubber mixtures, and more careful factory control-all contributed to the service improvements. We know of no way accurately to estimate money saved due t o the differences in quality. The inorganic accelerator is valuable, but we have stated that the organic accelerator has given to the consumer additional service qualities. It is not too much to say that the resistance of modern compounds to aging has been improved by at least one year. We are inclined t o believe that, were there t o be no organic accelerators today, it is probable that in tires alone there would be not less than 3 tires per car per year which would require additional cost t o the user. Since the total bill for tires and tubes during the year 1924 amounted to $196,930,000 it is probable that this chemical research work has saved the motorists an amount equal to 25 per cent of the present expense, or in round figures, say, $50,000,000 yearly.
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Factories have increased in size. \Ye can, however, roughly estimate the additional investment that might be required in the curing rooms for molds and vulcanizers, were the organic accelerator to be removed from our undertakings. In the old days, tires were vulcanized with but few molds in a heater. These molds were bolted together, put on a car, and run into a heater where they were cured a t from 3 to 4 hours. It was a slow process. Today we have automatic conveyers that carry molds to heaters, and upon discharge bring them back to the workmen. Machinery has minimized effort. These heaters are now large vertical presses which hold from 20 t o 30 molds in them a t a time. Hydraulic pressure is used to close them. The time of vulcanization varies from 1 to 1.5 hours. For purposes of rough estimate, it is probable that without the organic accelerator it would be necessary at least to double the investment required in molds and in the heaters and other apparatus incident to the vulcanization of the product. The present-day investment in molds and vulcanizers for the purpose of turning out a matter of 50 million tires per year is probably, in round figures, $25,000,000; in buildings necessary to house these, say, $25,000,000 more. It is likely that all other molds, presses, and buildings would require $30,000,000 in addition-a total of $8O,OOO,OOO. These are rough estimates only. If we assume double the time of vulcanization required (if no organic accelerators were used) our estimate would show that the organic accelerator has probably saved the rubber industry an investment of $40,000,000. Yes, the industry has benefited, and so has the consumer of rubber goods, because organic accelerators of vulcanization were discovered and have been used! ~
Earning Power of Research HE Sun-Maid Raisin Growers' Association, comprising over 85 per cent of the raisin growers of California, was confronted with a problem which is common t o many industries. The raisin growers have years of over-production, the quality of their raw product is not uniform, and even under the best of circumstances there are wastes produced during the preparation of raisins for human food consumption which should be conserved. Unless a use could be developed for raisins in the form of a product that would not compete with the raisins themselves, they would be eternally subject t o violent market fluctuations and uncertain costs of production that are t o be avoided for any enterprise. An announcement has just been made that research applied to this problem has found the solution. To maintain a uniform high-quality product, this association of growers must segregate the lower grades of raisins, and this tonnage, together with any surplus crop beyond the market requirements, as well as the waste material, is to be converted into a neutral fruit sugar sirup, for which there is already a great demand. This sirup of dextrose and levulose not only has great sweetening power but is nondrying and therefore in great demand by the baking trade for use in breads, cakes, and icings, which must be kept a t the right degree of moisture. The business management of this raisin growers' association followed the method that is so often recommended but too seldom heeded by manufacturers. They secured a chemical engineer t o organize a research staff to investigate the engineering and chemical problems and t o determine the economics of the situation. F. M. de Beers was chosen for this task, and he engaged as his associates J. K. Dale, H. W. Denny, F. E. Twining, and the Miner Laboratories. Cooperating with this group were members of the staff of the University of California, both at Berkeley and at Davis, where stock-feeding tests
are being made. Following the necessary research, a process was developed based upon the facts determined. It could not have been developed by any other method. Next came the semicommercial plant, which has been operated 24 hours a day for nearly 9 months, converting thousands of tons of raisins into sirup and producing many carloads of stock feed. The sirup and feed have been sold, thereby paying for a large part of the expense of development, and enough data were secured to guide the group in the designing of the first unit of the commercial plant. Contracts for this unit have been placed. Research through all these agencies of scientific study will be continued in order to improve the process, reduce costs, and improve the final product. The four main buildings of this sirup unit are placed on an eight-acre tract well served by railroads with adequate provisions for expected increases in capacity. This work on the part of an association of eighteen thousand growers is due largely t o the foresight of Ralph P. Merritt, its president and managing director. This organization distributed over 600 tons of raisins per day t o the consuming public during 1924, and in a business so large it is obvious that great quantities of stems, seeds, cap-stems, and low-grade raisins must be handled. Estimates as to available tonnage, yield of sirup, costs, etc., have been made, but it is too early to discuss these details even if the policy of the association would permit such authorized publicity. Besides sirup and stock feed, cream of tartar will be prepared as a by-product, and a press cake containing lime, phosphates, and nitrogenous matter will be offered as a fertilizer. So once again we have an illustration on a scale adequate to demonstrate the earning power of research and a model to which we can refer with pride in urging others t o consider what chemistry might do for them in stabilizing markets, eliminating wastes, and adding t o revenue, if but given the opportunity.
