Recent developments in the rubber industry

RECENT DEVELOPMENTS IN THE RUBBER INDUSTRY* GEORGE OENSLAGER, Tm B. F.GOOORICA CO.,AKRON,OHIO

The world's supply of rubber i s centralized in the Far Eastern plantations. B y W i n g it i s possible to treble the yield of rubber per tree per annum. A superior quality of tire cord i s now made from short staple American cotton. Titanium white and colloidal precipitated chalk are wed increasingly in the industry. Crude lauric acid i s replacing steark acid. Selenium, tetramethyl thiuram disuljide, and polynitro compounds hawe a limited use as wulcanizing agents. Organic accelerators of wulcanization and so-called age resistors are i n common use. A classification of these materials is given. Rubber latex i s being used increasingly i n the manufacture of articles of irregular shape. Manufacturing processes h a ~ ebeen refined. Plasticizers and internal mixers are replacing the two-roll mixing mill. Indiwidual molds for vulcanizing tires and tubes arelinding increased w e . . . . . . . Introduction Up to the year 1840 crude rubber had been used to a limited extent in the United States and Europe for the manufacture of elastic thread and waterproof clothing. The methods used involved either the spreading upon the fabric of a solution of crude or masticated rubber in turpentine or coaltar naphtha, or the application of rubber as formed in a thin sheet on a calender. The pioneer in the development of these mechanical methods, mastication and calendering, was the Englishman, Thomas Hancock. Unfortunately, goods prepared from crude rubber, although they had excellent waterproofing qualities, softened during w6rm weather and became tacky, causing them to pick up dirt, and in freezing weather they hardened to such an extent that they were unfit for wear. These objectionable features Charles Goodyear found could be removed by incorporating sulfur into the rubber and subsequently heating the finished product between 240' and 360°F. To these two pioneers of the rubber industry the world is indebted for the processes which are today in common use in the mannfacture of rubber goods. From the time of their discoveries until the beginning of the present century the rubber industry grew slowly, with scarcely a new departure or fundamental contribution on the part of their successors. The lack of progress was in part due to the secrecy which prevailed throughout the industry; this secrecy, however, succumbed as a result of the vigorous demands made upon the rubber industry in the early part of this century by the progressive and rapidly growing automobile industry which it served. This led to a change in viewpoint necessitating investigations not only into manufacturing operations but also into quality,

* This article is the basis of a lecture delivered on July 2, 1931, at the University of Michigan, Ann Arbor, Mich.,to members of the Summer School for EngineeringTeachers (sponsored by the Society for the Promotion of Engineering Education). 975

976

JOURNAL O F CHEMICAL EDUCATION

JUNE,

1932

cost, and available supply of all raw materials consumed in the manufacture of rubber goods. In the following article are sketched many of the significant steps in this story of progress. Crude Rubber For many years rubber has been obtained from a great variety of vines and trees growing in the tropical and subtropical portions of America and Africa. In the latter continent it was largely obtained from vines by tapping. The white milky liquid called latex which exudes when the tree or vine is wounded was frequently evaporated on the human body, the film of rubber obtained being rolled into balls, or it was allowed t o flow on the ground, TAPPING A RUBBER TREE coagulation taking place as a result of water absorption by the soil. The dirty coagulum after partially drying in the sun appeared on the market in the form of small balls. In America the greater part of the rubber was obtained in the valley of the Amazon River from the Hevea tree, which grows profusely scattered in dense forests. The rubber was obtained by rather crude methods; the hark was cut with an axe or a knife and the latex which exuded was collected in cups; the liquid latex was evaporated by pouring it on a paddle which was turned slowly over a low, smoky fire. On evaporation of the water in the latex, afilm of rubber was deposited; by repeating this operation a "biscuit" of rubber was built up. The first systematic attempt to produce rubber by the cultivation of rubber trees dates hack some fifty years when the English planted some

VOL.9. NO. 6 RECENT DEVELOPMENTS IN RUBBER INDUSTRY

977

Hevea trees in the botanical garden in Singapore. These trees prospered and gave promise of a new industry; plantations were started on a commercial scale and by 1904 eleven thousand acres were producing rubber on the island of Ceylon. Later, extensive plantations were started in the Federated Malay States, Java, and Sumatra. The grow in^.of rubber under carefully BTSCUITOF FINE PARAWITH A regulated conditions on the plantations SECTIONREMOVED TO SHOWTHE turned out to be very profitable and s~~~~~~~ large areas have been planted continuously until recent times. This has taken place concurrently with the tremendous growth in the automobile industry. The scientists in the employ of the English and the Dutch governments at the agricultural experiment stations in Kuala Lumpur, Federated Malay States, and Buitzenzorg, Java, have studied intensively the problems involved in the commercial production of rubber; as a result of their efforts it is now possible to obtain cultivated rubber in but few grades, uniformly dry, and having practically uniform qualities. They have also standardized the procedure followed in tapping the trees and have gone into the subject of plant breeding. Latex, which is a suspension of fine particlesof rubber in water, occurs in a network of interconnected tubes located in the barky tissues of the tree and in the leaves and roots. I t is obtained from the Hevea tree by tapping; the procedure commonly followed on the plantations involves cutting out a portion of the bark with a gouge similar to that used by carpenters; a groove about one-auarter of an inch dcep is made in the bark at an angh. of 4S0, the groo\.r only partially encircling the trcc. l'he latex as it r x u d t s follows 111,. channel and is col' lwtcd in small rups , .^. ... ,.& .,%-, a l m ~ tthe sizc of a .. teacup. Usually the trees are tapped every SMOKE HOUSEPOK RUBBER

.. ---.-

978

JOURNAL OF CHEMICAL EDUCATION

JUNE,

1932

other day, a new strip of bark being removed. The flow of latex is extremely slow; immediately after tapping, it flows a t the rate of about two drops per second, diminishing in an hour to about a drop per minute. The average volume per tapping is approximately one fluid ounce. The rubber in the latex is coagulated with a 1% solution of glacial acetic or formic acid in water. In a short time a viscous coagulum is formed from which water exudes on standing or on pressing. This coagulum either is squeezed between smooth rolls to a thickness of about a quarter of an inch, the slabs then being dried in a smokehouse heated by a smoldering wood fire, or is washed and sheeted out in the form of crepe which is dried in a loft at atmospheric temperature. Latex fresh from the tree is about one-third rubber by weight; the rubber exists as small particles, either pear-shaped or sph&ical, having a diameter ranging between 0.5 and XOp, these particles having the usual Brownian movement. In solution there are also present various cyclic sugars, such as quebrachitol (methyl lev0 inositol) to the extent of about 3% by weight. Crude rubber, regard7 less of its source, is not a 3 pure material; plantation 5 rubber, for example, con%4 tains between 0.3 and 3 0.5% of ash, between 2.25 s .- 2 and 3.25% of proteins, z1 and between 3.0 and 1900 02 04 06 08 10 12 14 16 18 20 22 24 26 28 30 3.5% of so-called "resin" Year. soluble in acetone. The FIGURE1.-AREA 01.RWBER PLANTATIONS IN THE truerubber contentis ap. YEARS1900 TO 1930 proximately 93%. The presence of these impurities is a matter of considerable importance; the proteins modify slightly the physical properties of the rubber after mastication, and to a lesser degree after vulcanization; the resins, consisting largely of stearic, oleic, and linoleic acid, have a profound effect on the rate of vulcanization, as will be discussed later. It has long been known that there is a great difference in the amount of rubber produced by mature, individual trees; a few produce as much as twenty pounds per annum, while the average tree produces but three pounds; hence, it has become the custom when developing new plantations to use seeds selected from trees giving a high yield. About twelve years ago a further advance was made. It was believed that buds taken from trees giving a high yield of rubber would retain these characteristics when grafted on trees grown from selected seeds. Preliminary studies having proved satisfactory, several thousand acres were planted according to the following procedure: seeds are planted one foot apart in nursery beds;

9

g

-

V ~ L9,. No. 6 RECENT DEVELOPMENTS IN RUBBER INDUSTRY

979

.

when the plant has reached a height of about four feet, which requires about six months, a bud from a tree giving a high yield of rubber is grafted on the trunk a short distance above the ground; after about twenty days the union of the bud with the tree is complete; in order to force the development of the bud the trunk of the tree is cut off a foot above the bud; in about ten days sprouts begin to appear on the stump a t various places; all of these are removed except the one from the newly placed bud; when this bud has developed into a shoot about an inch long the young tree is transplanted to its final location in the plantation. In the course of about seven years the tree matures sufficiently to produce rubber in paying quantities. Up to date the results have been most gratifying; the yield of rub880,000

166

8oo,ooo

2 720.000

150 U 135.5

640,000

120%

560,000

105a

$ &.

a

2 C

0

-

2 480,WO e

a u L

400,000

753

3 320.000

60 .j

%

P.

.-p: 240,000

45.2

0

F.

8 160,000 H

30 g

t?

15 g

80,000 1900 02 04 06

'

9022

08 10 12 14 16 18 20 22 24 26 28 30

Year. FIGURE 2.--wORLD'S PRODUCTION, I N TONS. OF DURING THE

WILD AND

YEARS1900 TO 1930

PLANTATION RUBBER

ber from budded trees is claimed t o be between 800 and 1000 pounds per acre per annum, which is a large increase over the average production of about 300 pounds from trees grown from seed in the usual manner. It is very likely that areas to be planted in the future will have budded stock. The following data apply to a first-class rubber plantation with selected but not budded trees: Trees per acre Rubber produced per acre per year Rubber produced per tree per year Rubber produced per tree per day When tapped on alternate days (150 tappings per year) the production per tapping is

80 to 125 400 pounds (181 kg.) 4.0 pounds (1.81 kg.) 0.17 ounce (5.0 g.) 0.42 ounce (12.1 g.)

980


Average rubber content of the latex Yield of latex per tree, tapped on alternate days

JUNE,

1932

33% 1 25 ounces (36 cc.)

The number of employees on the plantations in the Far East is around eight hundred thousand, of whom only a few thousand are "whites." Prior to 1930 the cost of bringing a plantation into bearing was approximately 400 U. S. dollars per acre.

Cotton Fabric The rubber industry is a very large consumer of cotton fabrics; for example, a pneumatic tire contains approximately, by weight, one-third rubber, one-third cotton, and one-third pigments and other materials; garden hose, boots and shoes, and belts contain, respectively, one-third, one-third, and three-fourths their weight of cotton fiber. It is estimated that the rubber industry throughout the world consumes a t least three hundred million pounds per year of cotton spun and woven into various types of fabrics, which corresponds to approximately six per cent. of the total American crop. Of this cotton approximately two-thirds is used in the construction of pneumatic tires. Until within the last ten years it was thought throughout the industry that the tensile strength and elongation, which are the more important criteria of the service which a fabric will give, were largely determined by the length of the cotton fiber and it was customary to use in the better grades of goods long-staple fibers, which are much more expensive to produce than those having a short staple. f n cords intended for use in pneumatic tires Egyptian and Sea Island cottons were thought to be indispensable. The average length of the cotton fiber from different localities is about as follows: Sea Island Pima Imperial Valley Egyptian Texas Upland

13/a0 lS/sV 11/4"

l'/s"

lV

'%" During the past ten years some striking changes have taken place in the cotton and the automobile industries. Because of the difficulties and the expense involved in the growing of long-staple cotton and the unwillingness of the consumers to pay the price, the American farmer in the South has virtually standardized on a short-staple fiber having an average length of 1'/16", and to a limited extent on so-called Peeler cotton having a length between 13/16Vand 11/4". During this same period the automobile manufacturer has developed automobiles heavier in weight, with higher powered engines, making it possible to develop higher speeds, which necessitates more powerful brakes, all of which means that the rubber manufacturer has been forced to develop a more sturdy tire, the backbone of which is cotton

VOL.9, No. 6 RECENT DEVELOPMENTS IN RUBBER INDUSTRY

.

981

cord. The cotton manufacturer has met the situation by more careful attention to all the details of manufacture, and by developing new types of construction. With short-staple fiber, which was regarded as unsatisfactory ten years ago, it is now possible to secure cord of a superior quality.* That this is true is evidenced by the fact that the life of a pneumatic tire is about double what it was eight years ago. The coustruction of cords now in common use by the t i e manufacturers in this country is as follows: about two-thirds of the cord used is the so-called hawser cord 23's/5/3, which means that five plies of yam No. 23 (yarn number equals the number of hanks of 840 yards each required to weigh one pound) having a right-hand twist are twisted into a ply of cord in which the twist is to the right, and of these cords three are assembled into one cord by twisting toward the left. The remaining third of the cord is divided between 13's/3/3, 15's/3/3, 17's/3/3, and 23's/4/3, most of which require the coarser yams. Some of the physical properties of cords are as follows: Carlruclion

P i b n Lcnglh

Tenrda Slrcnglh ol RuPIurc

Uongaliol of Ruplrrr

15's/3/3

l'/as" to ll/la" 13 to 16 lb. 18 to 22% The above cord is used in the manufacture of the cheaper grades of tires. 23's/5/3 ll/m# 16 to 18 lb. 21 to 22Y0 (American short-staple cotton) Largely used in the manufa~tureof tires intended for passenger cars. W's/5/3 l S / I to ~U 11/41 19 t# 21 lb. 20 to 24Y0 Long-staple Peeler or Egyptian cotton, largely used in the manufacture of bus and truck tires.

Pigments, Softeners, Fatty Acids, and Solvents Until ten years ago either the manufacturers of rubber goods were unable to set up specifications for the quality of the raw materials or the suppliers were unable to furnish exactly what was required of them. With the rapid growth of the rubber industry there has developed a clearer conception of the properties required in the various raw materials. It is now possible to secure better and cheaper products than ever before; for example, carbon black of uniform physical properties can now be purchased from a large number of manufacturers. As the result of research work on the part of the dye manufacturers a great variety of dyes which will withstand the process of vulcanization is available. Among the new materials which have recently been adopted are the following: titanium dioxide which, because of its intense whiteness and fineness of subdivision and permanence

* During the past ten years the ratio of the ultimate tensile strength of the fibers assembled in cord form to the tensile strength of the individual fibershas increased from fifty per cent. to seventy per cent.

982


JUNE,

1932

of color, is gradually replacing lithopone; a very finely divided whiting (precipitated chalk) prepared by the action of carbon dioxide on calcium hydroxide suspended in water containing certain colloidal materials in solution; a so-called "soft" carbon black different from ordinary carbon black in that i t imparts a more velvety feel and a lower modulus to the cured rubber; crude lauric acid (prepared from cocoanut oil) which is gradually replacing stearic acid. Of late there has been a tendency to replace benzol as a solvent for rnbber in cement form with commercial bexane as obtained from gasoline by fractional distillation. Hexane has two advantages over benzol: it boils a t a slightly lower temperature and within as narrow a range; moreover, its vapor is free from poisonous effects. Vulcanization and Vulcanizing Agents In the early part of the nineteenth century there was a considerable demand for fabrics made waterproof by the application of rubber. It was customary in those days to soften the rubber by working it between rolls and applying it to the fabric either in the form of a thin sheet on a calender, or of a cement, that is, a solution of rubber either in coal-tar naphtha or turpentine. As the surface of such rubber was very tacky, it was customary to reduce the tack by incorporating in the rnbber dough a material such as lime, magnesia, or whiting, which had a drying effect. Even then, the quality of these goods was poor; in the summer time the rubber surface became soft and tacky and in the wi5ter the film of rubber became hard and stiff; moreover, the rubber had slight resistance to abrasion, and if stretched it failed to return to its original shape. In 1839 Charles Goodyear discovered that by heating rubber into which sulfur had been incorporated the physical properties were profoundly changed; it lost its tackiness, returned to its original length with slight permanent set even after being highly stretched; it had high resistance to abrasion and no longer stiffened when exposed to a low atmospheric temperature nor softened a t a high temperature; in fact, it always returned substantially to its original shape and size after distortion. This discovery forms the basis of the rubber industry. A few years later Nelson Goodyear, brother of Charles Goodyear, discovered that by prolonged heating of rnbber with half its weight of sulfur a black, ebony-like product was obtained, quite flexible, having very high tensile strength and low elongation. This product became known as hard rnbber or ebonite. The effect of heating a mixture of 100 parts of rubb& and 66.7 parts of sulfur can be studied by the following procedure. Sheets of this mixture are heated (cured) between platens at a temperature of 33S°F., the time of heating being increased by 10-minute increments; determinations are made of the tensile strength and elongation in the usual manner. the results

VOL.9,NO. 6 RECENT DEVELOPMENTS IN RUBBER INDUSTRY

983

being plotted on coordinate paper (see Figure 3). It will be noted that after heating for a short time a product is obtained having a tensile strength of about 3800 pounds per square inch with an elongation of about 800%;

discovered the process for vulcanizing rubber.

after further heating during a short period the tensile strength and elongation rapidly decrease, the former falling to about 500 pounds per square

984


JUNE, 1932

inch. On further heating the tensile strength rapidly increases, the maximum finally reaching about 10,000 pounds per square inch, and the elongation decreases to about 3%. These two types of material, one with moderate tensile strength and great elongation, the other with high tensile strength and low elongation, represent the two extremes of products now manufactured of rubber, the one extreme being known as soft rubber and the other, hard rubber.* While the above physical changes are taking place as a result of vulcanization, there is a chemical combination of the rubber with sulfur. In the "soft" rubber stage the vulcanized rubber contains between 1.5 and 2.5% of combined sulfur; in the hard rubber stage it has a limiting value of about 32%, which corresiponds with a compound having the formula (C5H8S),. Fully cured h a d rubber, therefore, may be regarded chemically as u completely s a t u r a t e d compound, which fact explains its permanence ip the air; soft rubber, however, being only partially saturated with sulfur or Oo its equivalent, is still in a 100 condition to absorb o ~ y gen and is therefore s ~ s ceptible to atmospheric 10 20 30 40 50 60 70 80 oxidation. ** Minutes Cure at 170PC. FIGUKE 3.-Pno~nsssrv~ C ~ A N G E S IN TENSILE Sulfur still remains suSTRENGTHAND ELONGATION DUE TO VULCANIZA- preme m a wlcanizing TlON agent for rubber, but during recent years attempts have been made to secure unique or improved qualities by other agents ormethods. Peachey, for example, found it possible to vulcanize thin sheets of rubber by passing them alternately through hydrogen sulfide and sulfur dioxide gases a t ordinary temperature; the nuscent sulfur formed by the reaction of the two gases vulcanized the rubber. Ostromislenskii has found it possible to effect vulcanization by heating

'

I t is t o be noted that in the manufacture of soft ruhber goods i t is custonlary to employ not over 6 t o 8 % of sulfur on the weight of the rubber. ** According t o the above statement, vulcanization is theresult of thecombination of sulfur with rubber. The mechanism of the change in physical properties is in dispute; according t o one school, not only is there a direct combination of rubber and sulfur, but there is also a transformation of the rubber into another material, either by isom&zation or polymerization.

VOL.9, NO. 6 RECENT DEVELOPMENTS I N RUBBER INDUSTRY

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rubber with 3% iits weight of symmetrical trinitrobenzene plus considerable zinc oxide, or with 3% its weight of m-dinitrobenzene plus considerable litharge. Most of the physical properties of rubber vulcanized in this way are inferior to those secured by vulcanizing with sulfur. Such vulcanized rubber, however, has some unique properties which adapt it to uses in a small way in certain lines of rubber manufacture. Lately a commercial product known as Tuads (which chemically is tetramethyl thiuram disulfide) incorporated into rubber to the extent of 4%. with or without the addition of about 1% of sulfur, has been used in certain types of rubber goods which are exposed to the deleterious effects of prolonged heating in service; for example, inner tubes for the larger sizes of automobile tires carrying heavy loads. The vulcanization is effected by a loosely combined atom of sulfur in the disulfide. Selenium and tellurium, which chemically resemble sulfur, have been successfully used during the past few years; selenium, in particular, is being employed to a very limited extent in the manufacture of rubber belts, insulated wire, and oil-resistant compounds. It is customary to use selenium to the extent of about 1% on the weight of the rubber, along with an equal amount of sulfur and the necessary amount of organic accelerator. Among the advantages claimed for the product are increased abrasive resistance, rigidity, stiffness, and resistance to oil. Compounds containing selenium are said to have remarkably long life in service. Accelerators of ~ulc&zation C

In 1840, shortly after discovering the principles underlying the vulcanization of rubber, Charles Goodyear found that by the addition of lime, magnesia, or litharge to a compound containing F i e Para rubber and sulfur the time required to secure the optimum physical properties could be shortened, with considerable improvement in quality; for example, a mixture containing 100 parts of Fine Para and 10 parts of sulfur has an optimum cure of 210 minutes a t 140°C.; if the sulfur is reduced to 6 parts, and 10 parts of litharge are added, the tensile strength is increased from 2800 pounds per square inch to 3250 pounds, with a reduction in til?e of cure from 210 to 25 minutes a t a temperature of 140°C. This increase in the rate of vulcanization with a corresponding improvement in the physical properties by the use of materials such as litharge, lime, or magnesia does not, however, hold for many grades of rubber such as "Africans" and "Central Americans," which were in common use up to twenty years ago. Such rubbers were regarded as being inferior in quality and brought a low price in the world's markets. It became apparent some twenty-five years ago to a few of the scientists connected with the rubber industry that the difference in the rate of vulcanization and in the physical properties of the vulcanized rubber obtained from the high-grade and the second-grade crude rub-

986


JUNE,

1932

bers was in some way associated with the impurities present, which could be removed by extraction with a solvent, such as acetone. After considerable research it was found impossible to separate and identify the impurities present in a high-grade rubber such as Fine Para. Accordingly, a study was made of the effect of different types of organic compounds on the rate of vulcanization. It soon became apparent that many organic compounds which either were basic in nature or contained an amino or nitroso group had a profound effect upon the rate of vulcanization and upon the quality of the vulcanized product. .The results of this investigation were promptly put into effect on a large scale in one American factory with great secrecy and with great profit, the materials then used being thiocarbanilide and i mnn p-aminodimethylaniline; 8 materials of this type .+ .M were t e r m e d "organic 1500 6 : accelerators." D u r i n g the next ten years knowl" h: edge of this work spread loo0 throughout the industry. An enormous amount of study has been de5' voted to this subject and 500 a .hundreds of organic compounds having accelerat"7 ing powers have been 150 300 450 GOO 750 900 1050 1200 1350 discovered and experiPer Cent. Elongation. mented with on a factory Temp. of Vulcanization-148'C. scale. T h e y may be Mixture: Para Rubber 92.5% Sulfur 7.5% classified as follows: lW% 1. Amino derivatives of aldehydes; e. g. hexamethvlenetetramine, aldehyde ammonia, ethylideneaniline, butylideneaniline. 2. Aliphatic amines and piperidines, also their addition products; e. g., the dimethylamine salt of dithiocarbamic acid, the piperidine salt of pentamethylenedithiocarbamic acid. 3. Aromatic amines and compounds formed by their reaction with carbon disulfide; e. g., o-ditolylthiourea. 4. Guanidines; e. g., mono-, di-, and tri-phenylguanidine. 5. Nitroso bodies; e. g., nitrosodimethylaniline. 6. Thiuram disulfides, formed by the oxidation of dithiocarbamates. 7. Xanthates; e . g., zinc isopropylxanthate. 8. Salts of dithio acids; e. g., zinc salt of dithiofuroic acid. 9. Thiazoles; e. g., mercaptobenzothiazole.

-

W"""

$2

$8

*

.

VOL. 9, No. 6 RECENT DEVELOPMENTS I N RUBBER INDUSTRY

. 987

In spite of the use of organic accelerators, manufacturers found that there is frequently a variation in the rate of cure of different types of plantation rubber and between differentlots of the same type of rubber. It was established in 1923 that the cause thereof was a variation in the amount of stearic, oleic, and linoleic acids which occur naturally in rubber to the extent of about 1.4%. This difficulty was overcome by adding to the rubber mix, stearic acid to the extent of between 0.5 and 3.0% of the rubber used. During the past three years stearic acid has been partly replaced by crude lauric acid, manufactured from cocoanut oil; not only is it equally effective,but it serves the additional purpose of softening the rubber in the uncured state, thereby facilitating the process of mixing. Among the accelerators in common use in the rubber industry throughout the world are the following: Mercaptobenzothiaz0 30 60 90 5 2 0 150 180 210 240 270 ole (trade name, CapMinutes Cure. tax) ; the accelerator 1 2 3 4 5 most generally used Para Rubber 100.0 100.0 100.0 1f10.0 100.0 in the rubber industry, s,,lrur 10.0 10.0 6.0 6.U 6.0 - - ~ ~ 0.75 especially in the manu- Diphenylguanidine Tetra Methyl facture of tires; it may Dlsulfide 0.125 - 10.0 10.0 10.0 be used in compounds Zinc Oxide 100 Litharge curing a t medium tem110.0 120.0 116.0 116.75 116.125 p e r a t u r e s such as 2400F.; it produces a FIGURE ~.-~LTIMATETENSILE O F FINE PARARUBBER VULCANIZED AT 14I0C. DURrNO VARIOUSPEBIODS OP TIME fast rate of cure a t high temperatures such as 290°F.; it has the very desirable property of not being adsorbed by carbon black. Aldehyde amines, next in importance to Captax; several accelerators of this type are adsorbed somewhat by carbon black and hence are not much used in tire tread mixings; excellent for compounds containing rubber, sulfur, and a small amount of zinc oxide; polybutyraldehyde aniline is the most widely used accelerator of this class; others of less importance are aldehyde ammonia, hexamethylenetetramine. heptaldehyde aniline, and ~~~

988


JUNE,1932

trimene base (triethyltrimethylenetriarnine), which give excellent results around 270°F. or over. Guanidznes; di-o-tolylguanidine and diphenylguanidine are in common use; the latter accelerator, which was formerly used in large quantities in tire treads, because of its being badly adsorbed by carbon black is gradually giving way to mercaptobenzothiazole. Tetramethylthiuram monosulfide (trade names, Thionex and Monex), also Tetramethylthiuram disulfide (trade name, Tuads), in a class by themselves and commonly known as ultra-accelerators, because of their effectiveness a t temperatures ranging between 212' and 240°F., commonly used where rapid vulcanization is desired. Other accelerators in common use for special purposes are the piperidine salt of pentamethylene dithiocarbamic acid, the-zinc salt of dimethyldithiocarbamic acid, methyleneparatoluidine, and thiocarbanilide. Most organic accelerators are characterized by the fact that they are either crystalline or resinous materials having a melting point below 150°C., or oils easily incorporated into rubber and preferably soluble in rubber, non-toxic, and not staining the finished product. The amount used is between 0.25 and 2.0% of the weight of the rnbber; about four thousand tons are consumed annually throughout the world. In order to obtain a clear insight of the effect of organic acceleratov on the rate of vulcanization and on the improved physical properties which they impart, reference should be inade to Figures 4 and 5, comparing the results obtained with rubber and sulfuralone, and rubber and sulfur with the addition of several organic and one inorganic accelerators. To the manufacturer of rnbber goods the introduction of organic accelerators has been of tremendous importance; in addition to a slight reduction in material costs, i t has reduced the investment in molds, wlcanizers, other equipment, and factory buildings by about one-half; due to improved quality, the service given by an article such as an automobile tire has been more than doubled. Use of Age Resistors in Vulcanized Rubber Certain grades of crude rubber, notably Fine Para, when exposed during a few weeks to direct sunlight and air undergo no great change; other grades, however, either because of depolymerization or oxidation, or both, first turn tacky on the surface, then become brittle, the whole mass finally becoming resinous. If the resin and other impurities in Fine Para are removed by extraction with acetoneor alcohol, or if the proteins and resinous materials are removed by digestion with a dilute alkaline solution, the treated rubber deteriorates very rapidly, becoming tacky on the surface and finally changing into a brittle resin. From this it follows that certain

VOL.9, No. 6 RECENT DEVELOPMENTS I N RUBBER IWDUSTRY

989

grades of crude rubber contain materials which protect i t more or less from atmospheric oxidation. Such materials are known as anti-oxidants. Unfortunately, when even the best grades of rubber are wlcanized their resistance to atmospheric oxidation frequently is greatly diminished. In a short space of time, either a few months or a few years a t the most, they begin to harden and lose their resistance to abrasion and stretching. This characteristic weakness of vulcanized rubber is especially marked in rubber mixings containing large amounts of carbon black. In their endeavors to find materials which will protect vulcanized rubber from deterioration under ordinary atmospheric conditions chemists have studied exhaustively the non-rubber constituents in crude Fine Para but have been unable accurately to determine their chemical composition and structure. They have, therefore, during the past few years turned their attention to developing synthetic materials having certain spec5c properties as anti-oxidants. Up to the present time they have met with considerable success and as a result of their efforts a large number of these chemicals, termed "anti-agers," "age resistors," or "anti-oxidants," are now on the market and are in common use. These chemicals may be classified as follows: 1. Compounds containing an hydroxyl group hound to an aromatic nucleus; for

example, . d-phenylphenol. 2. Compounds containing a primary amino group hound to an aromatic nucleus; for example, diaminadiphenylmethsne. ' 3. Compounds containing s secondary amino group; for example, phenyl-8naphthylamine, diphenylethylenediamine. 4. Aldehyde amine reaction products; for example, aldol-a-naphthylarnine ~

~

~

~~~

Among the anti-oxidants now commonly used are the following: Trnda Nnmr

AgeRite Resin AgeRite Powder and Neozone D AgeRite White AgeRite Gel Antox Neozone Neozone A Resistox Stabilite VGB BLE

Chcmirol Campmilion

Aldol-a-naphthylamine Phenyl-B-naphthylamine ~i-8-~-naphthyl-~-ihenylenediamine Ditolylamine petroleum wax 9-Aminophenol A mixture of phenyl-a-naphthylamine. st&ic acid and 2,4-diaminotoluene Phenyl-n-naphthylamine 9.P-Diaminodiphenylmethane Diphenylethylenediamine An acetaldehyde-aniline compound An acetone-amine compound

Most of the above materials occur in commerce either as oily liquids, powders, or resinous materials, all of them easily incorporated into the rubber during the mixing operation. The amount required to produce the

990


JUNE, 19'32

desired effect varies with the nature of the compounding ingredients; usually it is between 0.5 and 1% on the weight of the rubber. Inasmuch as the deterioration of vulcanized rubber usually progresses slowly it has become necessary to develop rapid methods for duplicating natural aging or deterioration. At the present time there are two widely used methods: one, devised by Dr. W. C. Geer, according to which test strips of rubber are suspended up to 21 days in an oven at 70°C., through which is passed a continuous supply of air; the other, devised by Messrs. Bierer and Davis, according.to which the test pieces are kept in a bomb under pressure of 300 pounds per square inch of oxygen a t either 60 or 70°C. during a period up to 96 hours. The effect of exposure of the rubber to hot air or hot, compressed oxygen under the above conditions is measured by changes in tensile strength, elongation, tear, abra&ve resistance, or other physical properties. In order to evaluate completely the aging qualities of rubber goods it is necessary to apply both of these methods of accelerated aging. Attempts have been made to correlate these two methods with natural aging, that is, exposure to air a t ordinary temperatures in diffused daylight. With some types of compounds it has been found, for example, that 2 days' exposure in the Geer oven or 5 hours' exposure in the BiererDavis bomb a t 70°C., or 10 hours at 60°C., is approximately equal to one year's natural aging. By properly interpreting the results of these two rapid methods of testing it is now possible for the manufacturer to design compounds which will have a proloflged life in service. It is not to be forQ

Aging Results: Tire Tread Compound "Smoked sheets" rubber Sulfur Zinc oxide Carbon black Mineral rubber Palm oil Hexamethylene tetrandne Phenyl-D-naphthylamine

55.20 lb. 2.25 " 15.00 " 20.00 " 5.00 " 2.00 " 0.55 " 0.00 " 100.00 " Cured 45 min. at 294°F. (146'C.)

.

55.20 1b. 2.25 " 15.00 " 20.00 " 5.00 " 2.00 " 0.55 " 0.50 " 100.50 "

OVENA E ~ N O

Tcnrilc

Tcnrila

Slrewlh

Elongolion,

%

Slrendh. Lb,/Sp.In.

Elongalion.

Original 7 days aging 14 " "

4000 2300 1250

680 500 310

3950 3000 2550

680 590 510

"

1350 754

460 320

2900 2700

590 590

Lb./So.I;.

%

BOMB AomO

48 hr. 72 "

VOL.9, NO. 6 RECENT DEVELOPMENTS I N RUBBER INDUSTRY

991

gotten, however, that not one commercial anti-oxidant yet discovered will prolong the life of rubber indefinitely. A great deal of study is being devoted to discovering better and cheaper anti-oxidants than those now in use. The following examples show the beneficial effects of anti-oxidants in typical tire tread and carcass stocks. Aging Results: Tire Friction Compound "Smoked sheets" rubber "Pale crepe" rubber Sulfur Zinc oxide Stearic acid Vulcone* Phenyl-8-naphthylainiile

45.00 lh.

45.00 lb

5.00 " 0.32 " 100 " 0.00 "

" " "

5.00 0.32 1.00 . 0.50 -100.00 " 100.50 Cured 30 min. at 294°F. (146°C.)

. Original 7 days aging " 14 " 21 " "

3150 2900 2500 1450

800 700 680 600

2930 2020

776 700

.

3100 3200 2750 2300

" "

..

'

730 710 660 660

BOMB ACING

72 hr. g(i ,a

"

3.

3250 2900

700 680

* Prepared by the condensation of aliphatic aldehydes with aniline, followed by polymerization. Industrial Uses for Rubber Latex One of the most interesting recent developments in the rubber industry has been the increased use of rubber latex. Although it has long been known that latex upon evaporation leaves a film of rubber, no practical commercial application of this property was made until within the last ten years. The importance of latex in the direct manufacture of rubber goods may be grasped from the fact that during the year 1930 there were imported into the United States several million gallons of latex; the dry rubber content of this latex was 2500 tons. Latex is imported in two forms; either in its original condition as obtained from the tree, with the addition of a small amount of a preservative such as ammonia, or in a concentrated form with the addition of stabilizing materials such as soaps, gums, et cetera. The latex in the unconcentrated form constitutes probably about three-quarters of the total imported into this country. It is used by one manufacturer of cord tires; the web of cords is impregnated and covered with a layer of rubber, not by the usual

992


.

JUNE,

1932

procedure of frictioning and coating on a calender, but by drawing i t through a bath of a compounded latex containing pigments, sulfur, accelerators, et cetera, the impregnated web then being dried by passing it over heated drums. The concentrated latex used in the manufacture of rubber goods either by the so-called anode process or by the dipping process is imported in several forms: a so-called Revertex, prepared by evaporating latex a t the plantation in a rotary drier until the total solids are between 75 and 78% (the evaporated latex contains a small amount of added soaps for the purpose of stabilization); Uterex, prepared a t the plantation by removing from the latex a portion of the water by means of a centrifuge of the milk separator type, the rubber content of the latex thereby being increased to about 60%; Lotol, prepared by the creaming of latex to which has been added between 0.1 and.0.2% of a solution of Irish Moss or gum tragacanth; and Vultex, which is vulcanized latex having a rubber content of about 50%. Two methods of manufacturing so-called dipped goods in the rubber industry from iatex are now coming into common use. Until recently such goods were prepared almost exclusively by dipping into rubber cement' a form having the desired shape, withdrawing it, allowing the solvent to evaporate, and redipping it until the desired thickness was obtained. In the manufacture of surgeons' gloves, for example, having a thickness of 0.008 to 0.010 of an inch, six to eight dips were required, whereas in the case of electriaans' gloves, having a thickness of 0.040 of an inch, twenty to twenty-four dips were necessary. After $he complete evaporation of the solvent the formed article was "cured either by exposure to vapors of sulfur chloride or by dipping into a dilute solution of sulfur chloride in benzol or carbon disulfide,or by heating in open steam, in which case i t was necessary to use sulfur in the dipping cement. This process involved the use of large quantities of inflammable solvents which were a serious item of expense and danger to the manufacturer. In the dipping process using latex as a source of rubber a form of a suitable material is coated with a film of a solution which a d s as a coagulant for rubber; for example, solutions of barium, strontium, calcium, and magnesium chlorides. The form is dipped into the latex compounded according to the quality of the goods desired and is allowed to remain therein for a few minutes. If a thicker deposit is required, the process is repeated. The coagulnm still on the form, after drying for complete removal of water, is vulcanized in open steam or in hot air. The anode process depends on the fact that when an electrical current is passed through latex the rubber particles, being negatively charged, move toward the anode and are discharged with the formation of a coagulum. In this process the pigments and the sulfur having a particle size of 0.5 to 2.0 mp are added to the latex. which is then brought to a pH value of 9.5

1

VOL. 9, No. 6 RECENT DEVELOPMENTS I N RUBBER INDUSTRY

993

and a conductivity of 0.0045 to 0.0050 mho. This latex is placed in a tank in which is suspended a cathode, usually an iron plate, and an anode of zinc which has the dimensions and shape of the article to be manufactured. A current having a density between 0.05 and 1.0 ampere per square inch is applied; the density and the time of flow of the current depend upon whether thin articles such as surgeons' gloves or thick articles such as electricians' gloves are to be deposited. During the passage of the current zinc oxide is formed in small quantities a t the anode and the negative charge of the latex particles is neutralized, causing coagulation. After a sufficiently thick deposit has been formed the anode is removed from the latex bath and after complete drying in air is heated either in steam or hot water in order to vulcanize the deposited rubber. The quality of articles prepared from latex is much superior to that of goods made from rubber cement; the tensile strength varies between 4500 and 5000 pounds per square inch, with an ultimate elongation of about 900'%; their aging properties and their ability to withstand repeated hoiling in water followed by drying out, as, for example, in the sterilization of surgeons' gloves, are excellent; their electrical properties are exceptionally good; for example, electricians' gloves having a thickness of 0.040 of an inch have a uniform breakdown test of 26,000 volts. Perhaps the greatest advantage resulting from these processes is the possibility of building up a t a small expense rubber deposits on a great variety of articles having odd and irregulaf shapes. When made by hand such articles are costly. From properly compounded latex a deposit of rubber, either soft or hard, may be built up on irregularly shaped articles such as plating racks and dipping tanks used for electroplating, forming racks, washing trays, fan blades, including fans for handling corrosive fumes and suspensions of abrasive materials, spinnerette tubes for the rayon industry, perforated metal and screens for wet screening, and a great variety of other objects used in industry. A process has recently been worked out for the conversion of rubber into artificial latex. This may be accomplished by the prolonged mastication of rubber in an enclosed mixer with the slow addition of an aqueous solution of a protective colloid, such as glue. The glue solution is taken up by the rubber in the form of a very fine emulsion; when the concentration reaches a certain point an inversion of the structure results; the rubber breaks down into very fine particles having dimensions of between 1 and 3 mp, the glue solution then becoming the continuous outer phase. At the present time only a relatively small quantity of artificial latex is used in industry, mainly because of its excessive cost; however, when i t is desired to secure a thin, tacky film of rubber the artificial latex may be used to advantage. Since such latex improves adhesion between rubber and fabric plies. it finds a limited use in the spreading of fabrics.

994


JUNE.

1932

Latex is also being used on a small scale in the manufacture of a paper which is translucent, with a high tensile strength and great resistance to tear, also to a limited degree in the manufacture of imitation leather. During the past year one manufacturer has brought out a carpet in which the pile, instead of being locked in place by the warp in the burlap backing, is made to adhere to the backing by means of a rubber latex. It is claimed that by this method it is possible to manufacture carpets having long life and lending themselves to a great variety of artistic effects. Mechanical Equipment Following the example set by the automotive industry, the rubber industry has adopted mechanical handling and straight-line production as far as possible; also the use of controlling instruments and recording devices to make possible a greater accuracy and uniformity in plasticity and thickness of the component parts of articles before assembly in their final form. A great deal of progress has been made in obtaining a uniform product during the calendering operation. In order to maintain uniformity of gage and,smoothness of surface, the temperature of the calender rolls is kept uniform. This is accomplished by means of a thermocouple in constant contact with the surface of the roll; this thermocou~leby means of aDI ' X L T \-UI.CANIZEK I W K CIIRISCI'STUpropriate apparatus may regulate MATIC l ' 1 R l i ~ AND I N N E E TIJBW the supply of cooling water passing through the roll. The thickness and the weight of the calendered sheet of rubber, or of the frictioned and coated fabric, are indicated continuously at the calender by various instruments; one of these, known as the "Verigraph," operates by changes in electrostatic capacity of part of anelectrical system; another instrument, known as a magnetic gage, operates by changes in the reactance in a portion of an electrical circuit. These instruments record on a moving sheet of paper the actual thickness, with an accuracy of one-half of one-thousandth of an inch, as well as variations in weight from a predetermined standard in terms of ounces per square yard. In factories having a daily capacity of 20,000 tires or over it has been found economical to adopt the continuous calendering of the cord fabric used in building the tire carcass; the cords as they unwind from spools

.

VOL.9. NO. 6 RECENT DEVELOPMENTS IN RUBBER INDUSTRY

995

assembled on a creel are gathered together in a web and are usually coated sim~~ltaneously on both sides by means of a four-roll calender. By this arrangement a continuous web of cord is delivered to the calender without any break whatever, it only being necessary to replace the spools as fast as they become empty. I t is thus possible to operate a calender continuously for a week or longer, if desired. I t has long been the custom to plasticize the rubber by "working" it on large mills as a preliminary to the incorporation of the compounding ingredients; the mills used for this purpose consist of two horizontal, cored rolls of chilled steel cooled internally with water; these rolls operate a t differentspeeds and in opposite directions; the space between them is adjustable. As a result of the shearing effect during the working, commonly called mastication. the rubber is softened; a t the same time, duc to stresses set up in the rubber and to electrical charges on its surface, there is a slight chemical combination w i t h the oxygen of the air, the amount combined probably being under two-tenths of one per cent. The rubber after being "worked on the mill t e n minutes or longer becomes soft and ROLLIbflI.~ YSED FOR MASTICATING RUBBER A N D plastic and while in that I~conponnnxc COMPOUNDING INOREDIENTS condition it is ready for the incorporation of the various compounding ingredients. For many years the rolls in common use have had a length and diameter of 84 and 24 inches, respectively, the ratio of revolution of the two rolls being from l:ll/& to 1:l1j2. The charge of rubber for such a mill is about 220 pounds and the power required for operation is about 100 b.p. During the past few years a new type of machine has appeared in this country for the mastication of crude rubber, known as the Gordon Plasticator; this apparatus consists of two Archimedes screws about 20 inches in diameter and about 6 feet long, operating in water-jacketed cylinders placed vertically over one another, the upper screw discharging into the lower one in an appropriate manner. The crude rubber is fed continuously through a hopper attached to the top cylinder and during its passage

996


.

Jmrr, 1932

through the machine is subjected to vigorous "working." It is discharged from the lower cylinder in the form of a tube between 6 and 12 inches in diameter. In cases where it is desired to secwe greater plasticity the rubber is given a second pass through the plasticator. If the rubber is given but one pass, the capacity of the machine is 6500 pounds of rubber per hour, which is about five times the capacity of the conventional 84inch mill. The power required is about 500 h.p. By the use of this machine it is claimed there is an economy in power of about 7%, in floor space of about 50%, and in labor 66%. The incorporation of the pigments into the rubber is effected on rolls similar to those used for mastication as des-mibed above. Here again, the old style roll mill has been giving way to the so-called "internal mixers" which h a v e been adopted during recent years in factories devoted to the largescale production of automobile tires. In t h i s t y p e of mill, represented b y t h e Banbury mixer, totating blades, and not smooth rolls, are used. In some respects these machines resemble dough mixers or the Werner and Pfleiderer mixers in common use in factories handling plastics. Unfortunately, because of the impossibility of providing a large cooling surface in the jacket of this type of machine i t is difficult to keep certain types of batch, such as tread stock, below the temperature of curing; hence, it is frequently necessary to use the following procedure: all the pigments, oils, et cetera, with the exception of sulfur, are mixed with the rubber in the Banbnry machine, requiring about 10 minutes; the stock as i t issues from the machine a t a temperature of around 300°F. is transferred to an 84-inch roll mill and, after it is cooled sufficiently, the sulfur is incorporated in the usual manner. The Banbnry mixer has high capacity per unit of time; five mixers, for example, of size No. 11 are equivalent to nineteen

VOL.

9, NO. 6 RECENT DEVELOPMENTS IN RUBBER INDUSTRY

997

84-inch roll mills in volume of output. By their use the mixing cost is reduced by about one-third. A marked change is taking place in the type of heaters used in the vulcanization of tires and inner tubes. Until recently practically all tires were cured in molds externally heated by steam, these molds, 24 to 30 in number, being piled on a vertical hydraulic ram enclosed in a steam-jacketed kettle. After closing the kettle, heat is applied by means of steam a t a temperature between 260' and 300°F.; the time of cure for tires used on pleasure automobiles (4 to 6 plies) is between 50 and 100 minutes; for heavy duty tmck tires having between 10 and 16 plies, the curing time is between 2 and 6 hours. By means of individual vulcanizers, now being adopted extensively, each tire is cured in a steam-heated, cored mold which is cons t r u c t e d in halves; the lower half of the vulcanizer is stationary, while the upper half is either movable in a vertical direction or may be tilted back, depending on the design. This type of vulcanizer has been made practically automatic; after the tire containing a n "air bag" or "water bag" is placed in the mold, a button is pushed; the mold closes and locks automatically and air or superheated water is turned into the bag and the heating is continued during a predetermined time, which may be adjusted a t will by means of a clock arrangement operating various valves; a t the end of the cure the water or air is blown out of the bag, the mold opens and the tire is lifted away from the mold; these operations are performed automatically. This seems to be the last word in vulcanizing machinery. Unit vulcanizers are now commonly used for vulcanizing inner tubes. The uncured tube, formed either by extrusion from a tubing machine or by building up on a drum in an appropriate manner, is vulcanized in an individual vulcanizer resembling those described above; the temperatures commonly used are between 280' and 310°F.; the time of cure is between 5 and 20 minutes. Such tubes are known as molded tubes.

Summary of Past Developments During the past twenty-fwe years the noteworthy advances in the rubber industty have included the following developments: the replacement of the so-called wild rubbers lacking uniformity in quality with rubber produced in a systematic manner under regulated conditions on plantations; the production of fabrics by improved methods of manufacture from cotton grown exclusively in the United States which will meet the extreme requirements of the modem automobile tire; the improvement in tread wear of tires by the introduction of finely divided pigments, such as carbon black; the marked improvement in the physical properties of rubber goods made possible by the use of organic accelerators of vulcanization. An improvement in the resistance to perishing of rubber goods. by the use of antioxidants; fabrication from latex by methods new to the industry of articles with improved physical properties; general mechanization of production operations with improvement in quality and reduction in costs. Prospects I t is noteworthy that the rapid growth in the rubber industry during the past thirty years has been accelerated by demands made by large consumers, such as the automobile industry. These demands made it necessary for the rubber business to rely upon the guidance of technically trained men. On such men depends in a large measure the progress to be made in the future. Among the problems to be f a v d are the following: the production of a fine white pigment equivalent to carbon black in its physical properties; the discove~yof new anti-oxidants which will more effectively protect rubber goods against atmospheric oxidation, a t the same time making them more resistant to the action of sunlight and to cracking due to long-continued flexure; the increased use of the so-called cyclic rubbers and thermoplastics obtained from rubber by reacting upon it with condensing and isomerizing agents; the use of rubber as a raw material in the chemical industry for the synthesis of new products; the development of a process for the true reclaiming of scrap rubber goods, restoring the rubber to its original condition; the invention of methods for the synthesis of rubber and isomeric forms thereof from hydrocarbons, such as petroleum. A study of some of these problems will present serious difficulties, but no more serious than those confronted by the technical men of the last generation. It is to be hoped that the technical men of today realize that everything is not yet known about rubber and opportunities for making improvements and discoveries are just as great, if not greater, than ever before.

&&@a

VOL.9, NO. 6 RECENT DEVELOPMENTS I N RUBBER INDUSTRY .

999

Previous Adicles on Rubber Appearing in the Journal of Chemical Education BERGENBR, H. D., "The Relation of Chemistry t o the Rubber Industry" '(prize essay), J. CHEM.EDUC.,6, 1259--64 (July-Aug., 1929). BMUER,0 . L., "The Outlook for Synthetic Rubber," Qid., 6, 1286-92 (July-Aug., 1929). Cox. M., "Chemistry in Relation to the Rubber Industry" ( p h e essay), ibid., 3, 1044-50 (Sept., 1926). DALES,B., "The Electrodeposition of Rubber," ibid., 6, 2235-8 (Dec., 1929). FISHER.H. L., "Rubber: Newer Theoretical and Practical Developments," ibid., 8, 7-29 (Jan., 1931). KELLY AND BRUSON, "The Chemistry of Rubber," ibid., 3, 253-66 (Mar., 1926). LAWRENCE, J. C., "Pioneers in the Commercial Development of Rubber," ibid.. 7, 1788-801 (Aup.. 1930). M c P m n s o ~ A. , J., "Reclaimed Rubber" (Circular of the Burmu of Standards), ibid., 8, 2478 (Dec., 1931). TAnon, B. S., "The Chemist in the Rubber Plant," ibid., 8, 182938 (Sept., 1931). WATERS,C. E., "The Work of the Bureau of Standards on Rubber." ibid., 3,291-5 (Mar., 1926). -

Recent developments in the rubber industry

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