WHAT IS VULCANIZATION? E. B. BhSCOCK, Tbr Firestone Tirc & Rubber Company, Akmn, Ohio
z,>lr, Blank, what is rubber?’’ After a pause this trim rcplied witli a sinile, ”12ubber-to turn and gaze intently--is a vert). T I lm it precrdes a noun such as ‘neck,’it is an adjeetim:.” The history of rubber is limited csseiitially to the activities of five nations-Spain, Portugal, Francc, Englmid: and tlie I-uiteii States. Christopher Columhus is thought to hare Iiren the first nhite man to become acquainted with rubber. It is rdateil t.liat on the occmion of his second voyage to America, 1493-96, l i e witnessed a game played by the inlrabitauts uf lIait,i using halls prepared from the gum of a tree. ’This story is open to question since there is no rubber-producinp: tree or shrub iudigcnous to Haiti. It is certain, however, tirat Spanish explorers iound rubber in tropical Mexico ahout w it hcing used by tlie natives in various ways in t,lieact,ivities of theiv primitive lives. The I’ortuguese missionaries were the first Europeans to csplorr the Amazon Valley, in 1731. They iound the natives making errtile articles of r:lothing--shocs, head gear, torches, aiid iitlier useful articles. The species of tree fouritl in the Amazon Valley is known as Hevea braziliensis and is the one which furnishes pract.ica1ly a l l of tlie ruhber of commerce today. In 1762 tlie first rubberp~iducingtree (Fuutumia e l a s t i c a ) discovered i n Africa was found in French Guinea and in 1765 the same species was riiscovered in Madagascar. L a t e r a n iinportant, rubber-producing vine known as Landoblpkia i n s found in Africa. In 1798 the Brit,isli discovered a r u b b e r tree knomii as Ficus elastica in Penang, an island of tlie Malay Peninsula. It is interesting to note that trees and vines with no direct hotanical relat i o n a n d growing i u widely separated localities in the tropics produce rubber. The secretions called “latex” which flow from these trees and vines w h the bark is wounded are found t o contain, aft.er varying proportions of resins and other inipurities are removed, a hydrocarbon of the same composition, reWorld’s Largest Jaoket Mold, Capable of Vulcarrirgardless of the botanical ing a Tire Up to 4 Feet i n Cross Section and 10 Feet origin of the secretion, i n Height for Earth-Moving Equipment
SE hundred years ago this country was just beginning
to emerge from afinancial disaster such as had not. been known in the Gnited States up to that time. Tlie financial storm which had been gathering t,lirough the preceiling years had burst with terrible fury in 1837 and had resulted in a general suspension of specie payments, dosing of banks. and financial ruin of i~tnrrnicrahlecorporations and individuals. These financial difficulties caused a change in political feeling and weakened the confidence oi t.lie people in the wisdom of the Democratic policy. Cont,rol of the IIoiise of Representatives was retained with difficulty by the Ifeniocratie party in the election of 1838. The policies of tbe party leaders, together aitlr the prevalent depression in business during t.he next two years, so weakened tho party that. i n the campaign of 1840 the candidacy of the Whig nominee, Ilarrison of Ohio, was supportmi with an enthusiasm such as ina. political campaigns have witnessed. Van Uuren was nomiuated bv the Democrats for another term, but General Harrison w& chosen president by an immense majority. The rubber industry at this time, prior to the discovery of vulcanization in 1839, was in a most. precarious condition. Xot only mere rubber maiiufacturers facing financial disaster because of the chaotic business conditions of the period, h t the quality of the industry’s products was so nusatisfactory that opportunities f o r expansion were limited. The years 1834-37 witnessed t h e failure of inany pioueer robber companies of tlie United Statea, including the Roxbury India Rubber Factory, which was the first chartered rubber company in this eouutry. WHAT is vulcanization? The term is easy to define but difficult to understand. What is rubber? Perhaps a review of the history and nature of this important comrnodit,y is essential as an introduction to my theme. PearSOIL ($) described an occasion wlieil during a discussion he turned to a wellknown wit and inquired,
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OCTOREH, 1Y3Y
The rleveloji~~reirt US ronr~esof sipply OS rii1ibr.r is a romantic story. The supply ohtaiiied ill the early periods rame natrirally as a result of trading witli natives. Shortly after the discovsy of vulcanization, nianuPact.iirers became a,ware of the necessity of developing mire stalde and more rxtensive swrces, but fur yesrs practically dl oi the world's silpply of good ruliher grew wild in t,lre/tingles uf Iirazil. The exploit, with mart far-reaching restilt.; was tint OF Sir Henry Wickham. After failing to estahlisli a rubber plant,alion ill Brazil, lie managed t o mrnggle out al,oirt seventy t,liousaiid Hez,eo braziiimsia seeds whiclr were rubsequrntly planted in Kew Uarderrs iu I.ondou. In 1876 he transported successfully about twroiy-eight Ii~mrlrerlyoorrg trim to tlie Middle East where they were planted on the islands of Ceylvn and Singapore. This was the beginning of t,he rubber plantations industry. 13s 1910 it had surpassed tlie production in Brazil, which st,eadily decreased in importance as R source of supply.
g a ~ eto the substatice the name rnbber, udticli is i i s e d t,oday rr.liewrcr the English language is spoken. The rubber industry really dates only from 1819, when tire manufacture of rubber ~ o o t l swas hegriri in Englaiid. l3y 1832 the indiist1.y Iiad spread to America. One OS the greatest names conncrted with ~nai~uf;tcturiiig of rubber articles prior to 1839 was t,hat of Cl~arlesMacintosh, mho estahlisl~eda factory in Manchester, England. It was he wlio f o i i i x l that it, was posvilile to riissolre crude rubtier in coal-tar ~ i ~ ~ h t lami i i i sorreisfillly waterproof Salirics for iisc as clothing. The raincoat irl,ir.li he developt~ilarai ~~lrielr is still linoi\-rr as a ninrlrintosli was not ser\-iceat~le,I~owc\w, because of tbe poor resistance of r~irvnleanized robl)er to chimging wcatber con,-as a contemporary oS C1,nrles Macintosh; altliougli the history of the industTy is riot clear on this subject, i t appears that the first successfol mbber t,ire was
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INDU81‘111Al. AND ENGINEERING CHEMISTRY
invented by Hancock. The companies of Haucock and Macintosh were consolidated in 1830, and i t is certain that. they wereproducinga solid rubber tire as early as 1846. The first recorded use of rubber for vehicle tires was in 1835. These tires consisted of a rubber cushion applied to the ordinary iron tire and covered with an iron ring or tire so as to get elasticity without subjecting the rubber surface to the road. The first pneumat.ic tire was patented by W. 13. Thomson, an Englishman, in 1845.
VOL. 31, NO. 10
will produce vulcanization, sulfur is still used universally. Sulfur presumably reacts with the rnhber at the double bonds, progressively forming thioether linkages or sulfur bridges between tho already highly polymerized molecules. If this progresses to the extent of satnrat,ion of all double bonds, then hard rubber results with almost complete loss of the original elasticity and improvement in chemical inertness. When a soft rubber compound is vulcanized, only a small percentage (0.1 or even less) of the amount of EARLY in 1839 Charles sulfur required for saturation is allowed to react. Goodyear, experimenting That this reaction is still with rubber compounds in incompletely understood is t.he kitchen of his humble illustrated by the fact that home in Woburn, Mass., the physical characteristics discovered vulcanization. of tlre vulcanized product The exact circumstances are not known. Perhaps arc not. necessarily dependent upon the amount. of the heating of the sample into which sulfur had been sulfur reacted. The accelmixed was accidental. erator used has a profound Probably it was the result effect upon tlre rate of thc reaction arid u p o n t h e of the still modern cut-andt,ry method. There is no quantity of sulfur desirable evidence that lie proceeded to give the best physical step by step with a keen charaetcristics which, in tho sense of direction toward a ultimate analysis, are tlie clearly visualized result, measure of degree of vulwhich is t h e o r t h o d o x canii,at’ion. Technical progress of the legend of the modern aprubber industry has been proach to scientific problems. Keen observation, o u t s t a n d i n g in modern times, but the scientific trail good rcasoning, and insatiahle curiosity have been blazers of the last three ganI’ooring A i r t w ( L a t e x Sponge) Compound into Mold for Vulfanisation erations of mhbor cheniists important contributors to h a v e faced m a n y disthe scientific progress of the couraeements similar to rubber iudustrv. Ccrtainlv in the experinktal ewir;nment of Charles Goodyear there those to which Charles Goodyear was subjected. The princiwas no atmosphere of mysterious apparatiis, assorted glasspal raw material, rubber, is a product of nature whicli varies ware, white uniforms, and sterilized surroundine;i that are with the type and age of tho tree from which it is draurn and synonymous with the work of elremifitsas depicted in tcniay’s with methods of preparation, packing, storage, shipment, and advertising and publicity. preliminary handling in factories. This necessitates positive vulcanization as ordinarily conceived can be defined as the and unusually careful control. The sulfur which Charles process of combining sulfur with rubber urrdcr the influence Goodyear found to be essential is today the universally used of heat. Violent controversies raged for years between chemvulcanizing agent,, but the quantity in any specific compound ists and physicists concerniug what actually happens to bring themhherused, may rangefrom I to50percentoftheu~eiglit,of depending upon t:he properties desired in the finished product. about the remarkable changes that occur as the result of this process. Is it a chemical change or purely a physical pheCharles Goodyear found many ingredients that improved properties for specific purposes, but thcse have now become nomenon? I shall remain on safe ground by stating simply that sulfur goes into chemical combination with rubber; so numerous that infinit,e possibilities exist in varying the most rubber technologists agree that the results are due to properties of rubber mixtures. The tcmperat,ure of vulcombined chemical and physical forces. canization may range from that of superheated steam down Most of the recently published definitions are in terms of to the temperature nf the room in which the work is done. the changes in properties that are brought about by the The time of vulcanization may be measured in minutes or process. Williams (f) states it: “In the broadest sense, hours. vulcanization to produce soft rubber is any treatment which decreases the plasticity of the rubbcr at the same time mainOTHER factors have stimulated the manufactiirers of rubber goods to achieve unusual technical advances. The taining the elasticity.” Various other physical and chemical changes accompany commodity rubber furnishes an excellent example. In 1910 a vulcanization. There is a decrease in tlie solubility or swellcorner of the Brazilian rubber market carried the price of ing tendency of the rubber in solvents, the tensile strength is rubber to $3.06 per pound. The industry was in financial increascd, the ability to maintain flexibility at lower temperacliaos in 1922 when the price of rubber tonolied 13 cents per tures is improved, and its resistance to heat is improved pound. The British Rubber Restriction Act of that year drove the price of rubber to ii, high of $1.23 in 1925 and in decidedly. June, 1932, rubber sold for less than 3 cents per pound. Although the addition of a number of materials such as selenium, tellurium, trinitrobenzene, and benzoyl peroxide During a majority of the past thirty-five years it has been
OCTOBER, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
possible for a rubber manufacturer to see the market price of his principal commodity fall to less than half or rise to more than double the cost of his inventory. Potentialities of future research and development in the rubber industry appear almost limitless in the light of the advancement that has been made in recent years. Thousands of new llSeS have been discovered, and the rate of their development can reflect only great credit On the courage and vision of its scientists and management. Its record of service
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and contributions to civilization may be offered today as a fitting tribute to the memory of the man whose sacrifices and courage in discovering the basic process of vulcanization made them possible. Literature Cited (1) Davis and Blake, ‘‘Chemistry and Technology of Rubber,” p. 237 (1937). (2) Pearson, H. C., “Rubber Tires,” p. 9 , India Rubber Publishing Co., 1906.
THE RUBBER INDUSTRY,
1839-1939
W . A. GIBBONS, United States Rubber Co., Passaic, N. J.
H E importance of Goodyear’s invention is strikingly revealed by a consideration of the changes which it brought about. Before the discovery of vulcanization there was a small and struggling rubber industry. Rubber had been used by the Indians as a waterproofing material, and some rubber manufacturing was started in Europe and the United States early in the nineteenth century. While natives worked principally with latex, European and American manufacturers endeavored to use the dry rubber prepared by the natives from latex. Some experimental work had been done on methods of handling rubber. For example, it was known that rubber could be softened by heat or mechanical action and that it could be dissolved in certain solvents and spread on fabrics. It was observed that rubber so treated had reduced strength, increased tackiness, and shorter life. On this account these processes were suitable only for the manufacture of those articles which did not require a rubber of high strength or elasticity. For products requiring a greater strength or stretch, other methods were used. The Brazilian rubber prepared by smoking latex on poles, and known as Para rubber, was relatively tough, hard, and elastic. Hancock used this rubber in the manufacture of rubber thread, bands, and rings. The process consisted of cutting the desired shape from the crude mass or biscuit. Only certain portions of the biscuits were of suitable shape for this cutting operation; the remainder was used for other purposes. To minimize this disadvantage, Hancock sent sticks of a certain size to Brazil and ordered biscuits of elongated shape, which could be cut more economically. The process of manufacture was started in Brazil and completed in Europe. Obviously a method which depended so much on obtaining the raw material in proper geometrical form left much to be desired. The buying of crude rubber for such applications must have attained the dignity of a fine art. Another product which required strength and lasting qualities was the rubber overshoe. I n order to secure an article of the proper shape, with sufficient toughness to give any service, it was necessary to make the shoe by applying latex to a rough last and then drying. This was done by the natives in South America, and shoes of this type were shipped in considerable quantities. To sum up the situation: Rubber was used principally because it was flexible and waterproof; it could be plasticized and shaped, but only by impairing other important properties; if articles requiring strength and stretch were made,
T
they were cut from selected pieces of the crude material; the products were in general of short life and low durability; they were of limited utility because they were extremely sensitive to heat and cold. The discovery of vulcanization made it possible to use the plastic properties of rubber to bring.it to the desired shape, and to convert it to a harder, tougher, and more permanent material. Rubber was the first thermosetting plastic. Rubber, like other raw materials such as steel, gtass, concrete, and paint, has dual properties important to its industrial use. In its initial stages it is soft and plastic, which permits it to be brought to the desired shape. It can then be converted to a permanent form. Gone forever were the days of selecting pieces of crude rubber of the desired shape from which to cut out parts for the fabrication of articles of inadequate elasticity, strength, and permanence. By this invention the manufacturer was liberated from a troublesome restriction as to raw material and process, and was enabled to make a product much better suited to the purpose in hand. To mankind there was made available a material so abounding in useful properties that after one hundred years of constant development, its new applications are still increasing. It is interesting to note that Goodyear’s objective, although definite, was modest in comparison with the event. He had been confronted with the deterioration in properties which resulted from plasticizing and shaping rubber. His avowed purpose was to restore the original properties of the rubber: “As early as the year 1800, wherever the properties of India rubber became known and appreciated, it became a subject of much inquiry and experiment to ascertain if there was any way by which it could be dissolved, and afterwards restored to its original state. This was the ultimatum sought after by great numbers who occupied themselves in experiments with it, especially those of the medical profession, as well as by the writer in all his early experiments. It was not thought of or expected (certainly not by the writer) materially to improve upon the original good qualities of the gum.” Let us consider briefly the technical aspects of Goodyear’s invention. What he did was to make a mixture of 25 parts of rubber, 5 parts of sulfur, and 7 parts of white lead, and subject this mixture to heat. He found that when a piece of this mixture was heated on a stove it was changed. The result was the first piece of vulcanized rubber. Geer ( 3 )has given a vivid description of this event. He pointed out that the only
