Changes in the Rubber Industry during the Past Fifty Years’ By George Oenslager Tliie B P Gooonicn C o , A I R O N , Oniu
LJ
P TO the year 1840 unvulcauizcd rubber had been used
to a limited extent in the United States and Europe for the manufacture of elastic thread and waterproof clothing, and in Brazil for the manufacture of overshoes for export. The former were prepared either hy spreading a solution of rubber in turpentine or coal-tar naphtha upon the fabric or hy softening the rubher by mastication between revolving iron rolls, forming it into a thin sheet and applying it to tlie surface of the fabric. The shoes were prepared by evaporating the milky juice (later) of the rubber t.ree on forms imported from England and the United States. The most serious objection to goods so prepared was that, although the rubber had excellent waterproofing propert.ies, it softened during warm weather and became tacky, causing it to pick up dirt, and in freezing weather it hardened to such an e x t e n t t h a t goods c o n t a i n i n g i t were unfit for wear. Realizing that the future of t.hc industry dcpendcd upon the removal of these objectionable characteristics, Charles Goodyear set out to modsy the physical properties of crude rubber. After much e x p e r i m e n t a t i o n he found that a mixture of rubber and sulfur ARubber heated at temperatures between 220” and 360”F. was converted into a highly resilient product which was no longer soluble in such liquids as turpentine and coal-tar naphtha, which would not harden when expoRed to the usual changes in atmospheric temperature, and which after being stretched would return immedi~ttely,upom release, to practically its original lengt~h. This discovery, to which the term “vulcanization” was applied, became the basis of one of our greatest industries. The process as discovered by Goodycar lias remained practically unchanged; it has, hornever, been improved and greatly refined. To indicate tlic most important clianges in this industry, especially those in which scientific methods have played a part, is the purpose of this article. Rubber Production Until twenty-five years ago all the ruhber used in commerce was the now so-called “wild ruhhers” obtained from vines and trees growing in the tropics. When an incision is rnade in the bark of such plants there exudes a milky fluid coinmonly called lat.ex, holding in solution small quantities of sugars and proteids, and in suspension between 20 and 30 per cent of rubber by weight. The rubber is obtained from the latex either by evaporation or coagulation. As carried out in the Amazon River Valley the first process consists in 1
Received June 1. 1926.
repeatedly dippiug a wooden paddle into a tank of tlie latex and exposing the paddle to the smoke of a slow wood fire; the water evaporates and a thin film of rubber is left behind. By many repeated dippings and dryings a large hall of rubber is secured, which still contains about 15 per cent of water. In tropical Africa the coagulation is carried out either by allowing the latex to flow from the tree or vine to the ground, fermentation and absorption of water causing coagulation of the rnbher, or the latex is collected in a calabash, diluted with water, and allowed to coagulate by fermentation. With the exception of ruhher prepared in Brazil, all the wild rubbers are very dirty, containing as impurities bark, sand, and water. It is therefore necessary that they be washed and dried, involving a shrinkage of between 15 and 30 per cent. As tliis drying period in the early days c x t e n d e d o v e r one month, the capital investinent in crude rubber was much greater than today, when the rubber is dried either in vacuo between 2 and 4 hours or in cliambers lieated by highly humidified air for about 18 hours. As the crude rubber appearing in the world’s market fifty years ago Plmfafltm raried greatly in composition, d e p e n d i n g upon tlie vine or tree from which it was obtained, and upon the method of coagulation, and as these differences influenced the rate of vulcanization, the great skill in the compounding of ruhher goods consisted largely in blending the various grades so 21s to sccurc a uniform product. Today these difficulties havc disappeared to a large extent, because about 90 per cent of the world’s supply of rubber 110%’ comes from one type of tree, the Heuea braziliensis. At the present time most of the world’s rubber supply comes from the plantations in the Far East, the extent of which is indicated below, according to estimates made in 1825: Acres
British Maiays Java and Sumatra Ceylon India and Burma Other coiinfries mai
2,200,000
1.3~0,noo 480,000
imwo z15,oon
4.amoon es 8,800 iquelemi~e~
~n 1925 ttlese plantations contained about six hundred million trees ami produced 482,000 long tons of rubher. During the same year the rubber produced in the rest of the world was only 35,000 tons, a b u t two-thirds of which came from tlie Amazon River Valley. From these facts it is evident that 90 per cent of the world’s rubber is produced on plantations located in Dutch and British possessions.
Sepierllber, 1926:
903
'The plautation indust.ry ha3 been subjected to a very careful study by scientists on the plantations and a t the government experimental stations in the Far East,. Such problems as proper spacing of t.rees, time of tapping, rate of bark removal, budding, plant diseases, and mci,hods of eoaylation lis\-c heen carefully sbudied and all available information has heen passcd on to the plant,ers in order that the plantations may bc opratcd at the highest degree of efficiency and proiIn(:c a product uniformly clean and having a uniform rate of vulcanization. Suine of the more important facts about the indust,ry are as follo\r-s: On a well-managed plantation the uumber of trees 1JCr acre is hetween one hundred and one hundred and twenty-five. Tapping takes placc on alternate days; the yicld of latex per brce per tapping is about 30 cc., rontaining about 10 grains oE rubber, representing R prodnction of about one pouud of rubber per acre pcr day. After liaving been bulked in a large stoneware or tile tauk holding from 100 to 1000 gallons, tlre latex is coagulated by means of dilute acetic acid in an amount equivalent to about 1.3 parts of glacial acid to 100 parts of rubber. (About 1500 ions of glacial acetic acid per year are used on the plantations of t,hc Far East.) The eoagnlum is either shected out and rvdsircd with \rater between revolving corrugated iron rolls, tlre resulting sheet being hung up to dry for about one month, forming a p r d u d known as pale crepe; or the coagulum is rqucezcd between sinootlr iron rolls in tbc prcsence of water, thc sheets k i n g hung up to dry in the ~ a r i i i smoky , atmosphere of a don-burning wood fire; such rubber is known as smoked shcets. Beeause of the great care t.akcn in its preparation, plantation rubber, with the exccption of tlie outside layers in contact with the wood container, is elem-so clean, in fart,, that about three-fourt,hs of it is now used without xashing, the remaining onefourth being washed preliminary to use in the manufacture of inner tubes and rubber tissues. Thc placing of the growing of rubber on a pernranent scientific and business foundation on a tremendous scale is a wonderful achievement. It is the most important contribut.ion to the rubber industry since tho discovery of vulcanization by Goodyear. For their foresight, courage, and persistenc,e the pioneer plantem deserve great credit. Dispersed Rubbe
In order to use of solvents for rubber and a t the same time secure hettcr res u l t s , some manufacturers are now impregn a t i n g f a b r i c s with rubber by drawing them through a bath of latex which, preserved with a m m o n i a , h a s hecn shippd from the Far East plantations. Fabrics are now being successfully rubberized Tappin&! a Rubber Tree by passing them through a bath of latex in which the ininutc particles of rubber have been vulcanized by digesting the latex with a water-soluble polysnlfide or with sulfur in the presencc of an organic accelerator. An artificial latex is now being manufactured from rubber, either by emulsifying a benzene-rubber cement in water in the presence of a protective colloid, the solvent being removed by boiling off, or by mixing the rubber on a mill with about one-fifth its weight of glue swelled in wat,er. Under
proper conditions this mixture can be made to disintegrate in watcr, forming 5 finely divided emulsion. I3y a recently developed process rubber is being electrodeposited on molds of a suitable construction. This process depends upon the fact that when a direct current is passed through rnblm latex, to which there may have been added finely divided pigments and sulfur, the rubber and other suspended materials are deposited uniformly as a film on the tinode or on a porous mold surrounding the anode. This process lcnds itself to the manufacture of a great variety of objects, in particular sue11 articles as bathing caps, surgeons' gloves, tohaceo pouches, ctc, Reclaimed Rubber Although mrious schemcs had bccn proposed for rendering worn-out rubber goods fit for re-use in the rubber business, completely successful results were not obtained until tlit: eighties, when the Mitcliell process WES dcveloped. This. theso-callediicid nroc-
acid, is subjected to Bieeuit of Fine Para with a SecHon the action of water Removed to Show the SPueture or of steam i n a jacketed kettle at a temperature of about 350" 1". for about 14 hours. By this treatment the acidised scrap becomes plastic. After washing on a mill to remove dirt and metallic particles, i t is dried, worked between revolving rolls, and strained through a screen for the removal of metallic particles. This process is not commercially successful for reclaiming pneumatic tires and other materials containing much free sulfur. Such articles are reclaimed by the Marks, the socalled alkali, process, which consists in digesting the finely divided scrap in a caustic soda solubion of 5 to 6 per cent concentration at a temperature of 363" or 387" F. during a period of 18 or 12 hours, respectively. B y this treatment the free sulfur and the fabric go into solution. After washing, the now plasticized rubber is treated as described under the acid process. Such reclaimed rubber, frequently called "shoddy," is n,a.nufactured in enormous quantities (approximately 140,000 tons in the United States in 1925) for use in rubber mixings, either by itself or admixed with new rubber. mihile in many respects not equal to new rubber, it is good enough for manufacturing a great variety of articles where low price is indis pcnsablc and the bighe& quality is not required. Vulcanizing Agents
The material stiil almost exclusively used for imparting to rubber the physical properties associated with vulcanization is sulfur. Although selenium, certain dinitro compounds such as m-dinitrobenzene, and certain organic polysuKdes produce a similar vulcanieiug effect on rubber, their nsc is very small. Sulfur chloride, in the vapor state or in solution, is still commonly used for vulcanizing articles of light. \might. Fillers, and Reenforcing and Coloring Materials Fifty years ago the principal materials used for fillers were whiting, barytes, and substitutes formed by the action of
sulfur or sulfur chloride on vegctable oils; for coloring purposes, red oxide of iron, Indian &, golden antimony, and lamp black; for purpose of improving the insulating properties and facilitat,ing the tubing and calendering processes, ozokcritc, paraffin, m d palm oil; for imparting toughness and resistance to abrasion, zinc oxide, light carbonate of magnesia, and white lead. All these materials are in common w e today, with the addition of a great variety of anilinc dyes and lakes, blanc fixe (precipitated barium sulfate), sublimed white lead (basic lead sulfate), anti many finely around minerals such as calcite, slate, asbestine, and clay. The most. ininortant a.ddition to this list, however, is gas or mrhon black, which is produced by the incomplete combunt i o n o i natural gas. (.ki)out 37,000 toils or carbon black, equivalent to about 45 per cent of that prodnced, were used in the rnbher industry in the Unit.ec1 Statcs in 1925.) This material, having a low specific gravity and being the most finely divided pigment comnicrcially a v a i 1a b l e , imparts to vulcanized rubber a greater resistance to abrasion tban that imparted by any other material known. It is now very comm o n l y used i n t i r e treads to the extent of Smoke Hovse for Drying and Smoking l5 to 30 per cent by Rubber weight. BY . reolncing in part the zinc oxide iimierly used in light-colored treads, the service of the treads tins been practically doubled.
when mixed with sulfur in the ratio of 100:6’/4, vulcanized in about 2 hours at 287’ F., forming a prodnot of very high commercial value; by the addition of between 10 and 30 per cent of litharge the time of vulcanization could he reduced to between 15 and 30 minutes. Rubbers such aa the Congos, Kassai, Benguela, and Caueho, even after prolonged heating with sulfur aceording to the conditions given above, gave a vulcanized product of low tensile strength and poor resistance to tear. Moreover, while by the addition of such materials as litharge, lime, or red lead, the time of vulcanization could be greatly reduced, the physical properties, though greatly improved, were still much inferior te those associated with Fine Para. It was customary, therefore, to blend the inuch choaper slow-vulcanizing rubbers witil the more expensive ones, such as Para. By this procedure i t was possible to secure a product of excellent quality for many purposes. The canse underlying this variation in the vulcanization rate of rarious grades and types of rubber, the procedure to he followed in overcoming these variations, and most imp r t a n t , the controlling of the rate of vulcnnization at will, have received careful study during the past ten years. The following important facts have been discovered: I-The Fast-curing properties OF such rubbbers as Para, Manicoba, and most plantation rubbers are due to the presence of small quantiticr of nitrogenous substances and OF organic acids, such as oleic, stearic, and linoleic acid: by the addition OF basic materials such as lime, magnesia, and litharge, metallic salts soluble in rubber are Formed which greatly accelerate the rate
of vulcanization. 2-By the addition of about one per cent of oleic or stearic acid or the zinc or lead salts thereof, or of such complicated mixtures of acidic substances as occur in pine tar, the rate of vulcanization OF slow-curing rubbers is greatly increased, appioaching that of tbe Fast-curing rubbers. 3-By the addition of organic bases and their derivatives, thio compounds, and polysulfides the rate of vulcanization is greatly increased. 4-The physical properties of vulcanized soft rubber are OF the highest order when the vulcanization takes place during a short period of time.
Solvents and Solvent Recovery
In the early days of the industry the spreading and irnpegnation of fabrics with rubber cements containing gasoline, coal-tar naphtha, benzene, eta., wa8 carried out in the open air; no attempts were made to recover tho solvent. At present in many factories these processes are carried out in closed chambers making i t possible to recover the vapors of the evaporated solvent either by nhsorbing them in activated charcoal or by condensing them by meam of cooling mils or by compression. As a precaution against explosions the atniosphere used in some types of apparatus is cooled and washed flue gas from the boiler house. By any of these systems i t is possible to recover at least two-thirds of the solvent. Accelerators of Vulcanization Goodyear found that the rate a t which vulcanization t.akes place could be greatly increaaed by the use in the rubber mix of materials such as lime, magnesia, red lead, white lead, a.nd litharge. Thcse materials, the now so-called inorganic accelerators of mlcanization, were the only ones in common use until 1906 when, hy the introduction of organic accelerators in the United States, they became relatively less important. One of the great problems confronting the manufacturer twenty years ago was the variat,ion in the rate of vulcanization of the various types of rubber then available. For example, rnhhcrs such as Fine Para, Manicoba, and Cameroon,
Pale Crepe Rubber Drying in a Loft
With these facts in mind it is now cnstoma a rnbber mix of the bet.ter grades, to add an organic accelerator and to provide for the presence of a suitable organic acid and litharge or zinc oxidc Among the accelerators in common use are the aldehyde amines, di- and triphenylguanidine, di-a-tolylguanidine, ethylidene aniline, formaldehyde aniline, hexaniethylenetetramine, thiooarbanilide, p-aminodimethylaniline, zinc xanthate, and mercapto-benzothiazole; also the following which, because of their great activity and the low temperature at which they are effective, are known as ultra-acoelcrators: tetramethylthinram mono- and disulfide, the zinc and lead salts of dimetliyldithio-carbamic acid, and piperidinium pentamethylene-thiocarbamate. The manufacture
September, 1926
INDUS"RI.4 L A N D EA~G~.VEl$RIzVVG CHE.IlIS1'RY
903
Age Resistors It is &tima& that during 1925 their pr&n&ion in thk U n i t d is generally recognia& that oxidation is olie of States amounted to three thousand tons. principal factors in the deterioration of rubber goods on Tlle advantages of a properly selected o w n i c accelerator standing. T~ overcoine this, inhibitors of oxidation, suci~ over the inorganic accelerators in the manufacture of most as certain aromatic dialnines and co,nyma&,ns of certain kinds of soft rubber goods are numerous. While lime, aldehydes and amines, are now beginning to find an extended litharge, and magnesia are excellcnt for use with fwt-vuleanwith good results, ieing rubbers, they are technically effective a t temperatures Hard Rubber not lower than about 240" F., the time factor increminr: very rapidly with R lowering of the temperature. For any The principal changes in this hrancli of the industry indesired temperature of vulcanization some suitable organic ~ d v ethe use of finely pulverized hard rubber scrap, reaccelerator is available. It is necessaey, however, for most claimed rubber, mineral fillers of a fibrous nature, tars, oils, manufacturing purposez to choose an accelerator which will asphalts, resins, ct,c.; in the manufacture of a cheap produrt not be appreciably effective below 180" F.,since this tem- suitable for use in the aotomohilc industry-hatterg jars, for perature is often attained during the mixing and calendering example. Organic accclerators are now being used in the operations. While lead conipoiinds color the rubber black, better grades. and lime and magnesia alter the color of many aniline dyes Probiems of the Future and lakes, a properly chosen organic accelerator is without As tile supply of natural gas is being rapidly depleted, the effect in this respect.. The chief advantage of organic over inorganic accelerators, however, lies in the fact that they roht of gas black %-illbe increased u n l w the present 3 per add greatly to the flexibility of the manufacturing processes cent efficiency of the process is greatly improved. I t may and make it possihle to secure a superior article at a lower ultiinately become necessary to develop a new pigment havcost. The economical advantage of their use is difficult to ing properties equivalent to those of gas black. A more important problem is the produetion synthetically estimate. Bedford and Geer are of the opinio of a rubber having qualities equivalent to those of natural the cost of pneumatic tires to the consumers rnhher a t a sufficiently low price. Of ahnost equal imporStates alone was fifty millioii dollars less that been had not organic accclerators been availsble, and that tnnce is the working out of a successful and cheap process for the true devulcaniaation of rubber whercby it is restored the plant investment was fifty million dollars le t.o its original physical condition. otherwise ~vouldhave been. of these accelerators has become a smcial industiv
Explosives 1876-1926' By Hugo Schlatter K.cX*R"s
& CoMPnriu, J.mxv"ao. CO"
IIEK we compare the explosivesindustry as it cxisted in 1876 with that of the present day, we are improssed not only by its enormous expansion but by the technical advances made in the past fifty years. It cannot be questioned that the material progress of our country, urhich we are daily witnessing, would not have been possible if this industry had lagged behind, as explosives enter into the production of almost everything we use. Our railroads, our vast steel buildings, our automobiles, the s p l e n d i d r o a d s over which we travel, t h e irrigation projects which have extended our arable domain, indeed, most of the conveniences and necessitios of our life could not have reached their present state of development without modern explosives which enable us to procure onr raw m a t e r i a l s i n great quantities and a t low cost. *Received Junes. 1926.
Explosives Heye Been of
Dynamite
When we speak oE the great advances made since 18i6 we must not forget that the foundations for them had been laid in the preceding half-century. Sobrero and Schoenbein, working in their laboratories, had discovered nitroglycerol and nitrocellulose thirty years earlier, but these discoveries had k i n dormant until Nobel introduced his dynamite, a mixture of three parts of nitroglycerol and one part of kieselguhr or diatomaceous earth, in themiddlesixties. In 1576 nitroglycerol was being made and used in limited quantities in the United States, and small dynamite factories existed in C a l i f o r n i a , New Jersey, and Massachusetts. But they had to contend with the prejudices of the consumers as well as with the active o p Great Ald In the C o n ~ t r ~ e f i oofn the Ed0 Csnal at position of the estabWaterford, N Y.
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