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Rubber, a substance obtained from a wide variety of plants growing principally in the tropics and secreting a milky liquid in the roots, stem, branches, leaves, or fruit, or in tubes under the bark. This liquid is not the sap of the plant, and its function in the life of the plant is not well understood. The milky liquid, called latex, contains tiny globules that can be isolated by proper treatment in the form of a coherent, elastic solid known as crude, or raw, rubber.


The English word "rubber" was coined by Joseph Priestley, the discoverer of oxygen, from its use in rubbing out pencil marks. The term "India rubber" was given to it because it was first brought to Europe from the West Indies, the land that Columbus and his contemporaries thought was India. The word "elastomer" is now generally used to describe materials that have rubberlike properties.From excavations of ruins in Central and South America, it seems likely that rubber was known to the Maya Indians as early as the 11th century. It is reported that the Spaniards under Columbus saw the natives of Haiti using rubber in various ways. Long afterward, in 1731, Portuguese missionaries exploring the Amazon Valley found the Indians there using rubber to make useful articles.

The first record of scientific importance was written by Charles Marie de la Condamine, who was commissioned in 1735 by the Academy of Science of Paris to measure a degree of the meridian in South America. In his eight years of travel he made many observations and sent home, among other things, specimens of rubber. He wrote, "The natives prepare watertight shoes of one piece from this material.
They also spread it over an earthen flasklike form, and when the fluid has solidified, they break up the earthen form, remove the fragments through the neck of the bottle, and thus obtain a light, unbreakable vessel suitable for holding liquids."


Crude or natural rubber is of two types: wild rubber, collected from naturally occurring trees, shrubs, and vines; and plantation rubber, from cultivated trees and plants. Throughout the 19th century the bulk of all crude rubber of commerce was wild rubber collected from the several million Hevea brasiliensis trees in the tropical forests of Latin America, and from trees and vines in Africa. The production of wild rubber reached a peak of 70, 000 tons in 1912.


The country around the mouth and the valley of the Amazon River in South America continues to be the most important source of wild rubber. The species yielding the best quality are Hevea brasiliensis and, less important commercially, Hevea benthamiana. The natives obtained the wild rubber from the latex by immersing wooden paddles in it and then rotating them over a fire of burning nuts.Under the influence of the heat and smoke, a ball of rubber known as a biscuit is formed.


Although the Hevea brasiliensis tree is the principal commercial source of rubber, a number of other plants furnish minor amounts. Guayule is a woody shrub grown wild in northern Mexico. It has been cultivated in the same region and in the southwestern United States. The shrub grows several feet high and belongs to the Compositae family and to the genus Parthenium. The plant thrives best at altitudes of between 3, 000 and 7, 000 feet (910-2, 130 meters) with an average yearly rainfall of 10 to 15 inches
(250-375 mm). Guayule rubber is found in all parts of the shrub except the leaves and, in the ultivated shrub, amounts to about 20 percent of the plant's dry weight.After being dug or pulled out of the ground, the shrub is cured in the field for several days, then baled and stored until needed. In the mill it is chopped up and the chips are dried down to about 15 percent moisture content. It is
then crushed, mixed with water, and fed into a series of revolving tubes lined with rough silica bricks and about one-third filled with flint pebbles. The pebbles thoroughly macerate the mixture, freeing the rubber from the fiber. The mixture is then discharged into a vat, where the water-logged wood sinks and the rubber floats and is skimmed off. After further cleaning, the rubber is dried and pressed into 100-pound (45-kg) cakes for shipment. Although this process recovers a satisfactory proportion of
the rubber from the shrub, it is cumbersome and expensive, and results in a mixture of rubber with foreign substances.

Guayule rubber extracted from the wild plant contains between 20 and 25 percent of resin, whereas the cultivated guayule contains about 16 percent. The rubber hydrocarbon content of the commercial resinous guayule rubber runs about 70 percent. Guayule rubber is usually blended with higher grade rubbers in the manufacture of rubber products, but if deresinated it would be satisfactory for most articles. The resins in guayule have definite value as an aid to processing when mixed with high-grade plantation


Gutta-percha and balata are obtained from the latex cells of different trees belonging to the family Sapotaceae. Most of the gutta-percha comes from Malaysia, Borneo, Sumatra, and Java.

Treatment of the gutta-percha latex varies somewhat with the species from which it is obtained. In the early days the custom was to fell the trees of certain species (Pallaquium for instance) and collect the latex by making cuts around the trunk at intervals of 12 to 18 inches (30-45 cm). In this method the latex coagulates in the cuts, and coagula are scraped off with a knife. After the mass of crude material is freed from impurities by boiling in water, it is rolled into sheets. These sheets become
hard upon cooling, and are then ready for shipment. Extraction methods in which the gutta-percha is obtained by extracting the leaves and tapping are practiced to some extent. Latex from trees of the Payena species, which can be tapped more satisfactorily than Pallaquium, should be collected not later than an hour after sunrise, since otherwise an objectionable discoloration develops. This latex is coagulated by boiling. The resulting crude gutta-percha is cut into pieces, softened in hot water,
washed in a washing mill, forced through a strainer, washed again, worked in a kneading machine, and finally sheeted out in slabs 5 feet (150 cm) in length and varying from 1/8 to 1/4 inch (3-6 mm) in thickness. Commercial gutta-percha contains from 10 to 60 percent of resin, consisting of albane (a crystalline resin) and flauvile (a yellow amorphous resin) in the ratio of about 2 to 1.


Balata is obtained from certain species of Mimusops, found in Trinidad, the Guianas, Venezuela, and adjacent regions. To facilitate collection of the latex, the trees were formerly felled. In Guyana and Suriname the trees are always tapped. The latex is allowed to ferment for two or three days, after which time the upper layer coagulates.
The coagulated layer is removed and dried after it has attained a thickness of about 1/4 inch (6 mm). Attempts to make use of extraction methods such as those used with gutta-percha have not been very successful. Commercial balata is softer than gutta-percha, since it contains more of the softer resin, flauvile.


Jelutong, derived from the various species of Dyera, is used extensively in the manufacture of chewing gum. The latex from the tree is coagulated with acetic acid and purified by boiling with water.

Other Varieties.

Attempts have been made to cultivate numerous other rubber-bearing trees belonging to different botanical genera, such as Castilla, Manihot, Ficus, Kickxia, Funtumia, and Landolphia. During World War II, when rubber was very scarce, a large number of rubber-bearing plants were investigated. Among these were goldenrod, which Edison had studied years before; Cryptostegia, a vine of the milkweed family; and Koksagya, a kind of dandelion brought from the Soviet Union. Some produced rubber in amounts too small to be of importance. Others would have needed special processing methods to isolate the rubber from large amounts of resin. Moreover, none of the rubber-bearing plants was available in sufficient quantities to meet immediate requirements, and the rubber was generally inferior to natural plantation rubber. The plan of utilizing such rubber-bearing plants was therefore abandoned in favor of synthetic rubber.


Commercial crude, or raw, rubber is a tough, noncrystalline elastic solid having a specific gravity of 0.911 and a refractive index of 1.5910. Its composition varies with different latexes and according to the method of preparation on the plantation. A typical analysis is shown in Table 1.

Component Smoked Sheet Pale Crepe
Moisture 0.6% 0.4%
Acetone-soluble material 2.9 2.9
Proteins 2.8 2.8
Ash 0.4 0.3
Water-soluble material 1.0 1.0
Esters insoluble in acetone 1.0 1.0
Rubber hydrocarbon 91.3 91.6
100.0% 100.0%

Rubber does not dissolve in water, alcohol, or acetone, but it swells and disperses in benzene, toluene, gasoline, carbon bisulfide, turpentine, chloroform, carbon tetrachloride, and other halogen-containing solvents, producing viscous cements that find application as adhesives.

Just how the tree synthesizes the rubber hydrocarbon is not known. Unvulcanized rubber becomes soft and sticky in warm weather and brittle in cold weather. When heated to about 350°F. (177°C.) in the absence of air, rubber decomposes and yields small amounts of isoprene as one of the products.

Furthermore, rubber belongs to the class of organic compounds referred to as unsaturated compounds, which show considerable reactivity with other chemical reagents. Thus rubber reacts with hydrochloric acid to form rubber hydrochloride; and with chlorine, both by addition and substitution, to form chlorinated rubber. Atmospheric oxygen attacks rubber slowly, causing it to become hard and brittle, but ozone attacks it more rapidly. Oxidizing agents such as nitric acid, potassium permanganate, and
hydrogen peroxide oxidize the rubber. It is unaffected by alkalis or moderately strong acids. It also reacts with hydrogen, sulfur, sulfuric acid, sulfonic acids, oxides of nitrogen, and a great many other reagents, forming so-called derivatives, some of which have found commercial application.

The rubber hydrocarbon is a chemical compound composed of two elements, carbon and hydrogen, and having the general formula (C5H8)n.

The rubber hydrocarbon is present in latex as a suspension of minute particles varying in size from 4 to 20 millionths of an inch (0.1-0.5 micrometer). The largest particles can be viewed under the ultramicroscope and are seen to be in constant motion, exhibiting the phenomenon known as Brownian movement.

Each rubber particle carries a negative charge. If a current is passed through latex, the rubber migrates toward the anode, or positive electrode, and is deposited upon it.
This property is made use of commercially to coat metal objects. The surfaces of the rubber particles contain adsorbed proteins present in the serum, which act to prevent the latex particles from coming together and coagulating the rubber. By changing the material adsorbed on the surface of the hydrocarbon particle in rubber latexes, the charge can be reversed and the rubber particles are then deposited on the cathode.

Rubber has two important properties that make it useful as an article of commerce. In the vulcanized state it is elastic, and after stretching returns to its original shape; and in the unvulcanized state it is plastic; that is, it flows under the effect of heat or pressure. One property of rubberlike materials is unique: when stretched they become warm, and when contracted they become cool. Conversely, rubber contracts when heated and elongates when cooled, exhibiting what is known as the Joule effect. When stretched several hundred percent, rubber orients to such an extent that it exhibits a crystalline X-ray fiber diagram. Hevea rubber has the cis configuration; balata and gutta percha have the trans form. As a poor conductor of electricity it is valuable as an electric insulator.


Nearly all plantation rubber comes from cultivated groves of Hevea brasiliensis, which grows to a height of 40 to 80 feet (12-24 meters) at maturity, or to about 100 feet (30 meters) under ideal conditions. Adult trees generally range from 3 to 4 feet (90-120 cm) in diameter. They thrive on
many types of soil but require a warm, moist climate with an annual rainfall of more than 80 inches (2, 000 mm).

Plantation rubber dates back to 1876, when Sir Henry A. Wickham brought 70, 000 seeds of Hevea brasiliensis to England, where nearly 3, 000 were germinated successfully. Young trees were sent to Ceylon (Sri Lanka), Malaya (West Malaysia), and other parts of the East. From 1900 on, exports of plantation rubber steadily increased until in 1914 they exceeded those of wild rubber.

In 1982, almost 4 million metric tons of plantation rubber were produced in the world. About 40 percent was from estates comprising plantations of 100 acres (40 hectares) or more. About 90 percent of all plantation rubber comes from southeast Asia. The remainder is produced in Africa, India, South America, and the Philippines.


On modern rubber estates, nursery bud-grafted seedlings are planted in regular rows on cleared jungle land. From 100 to over 200 are planted per acre (40-80 per hectare), but by the time they reach bearing age (5 to 7 years after planting), about 100 to 120 trees per acre (40-48 per hectare) remain. An ordinary Hevea tree will yield about 4 to 5 pounds (1.8-2.2 kg) of rubber per year, depending on
the frequency of tapping; but with modern methods of seed selection and bud grafting, yields as high as 12 to 16 pounds (5.4-7.2 kg) have been obtained. Bud grafting consists of grafting a dormant bud from a proved high-yielding tree to a seedling one to two years old. After several months the bud forms a healthy bud shoot termed a scion, which grows to form the new tree. The seedling is then cut off just above the bud patch.


Rubber in the form of latex is obtained from the Hevea tree by tapping, a delicate and important operation. The vessels or tubes containing the latex are located under the outer bark of the tree in the cortex. The latex tubes in the cortex are separated from the inner woody tissue by a thin paperlike layer known as the cambium. In order to cause the latex to flow, it is necessary to make an incision through the bark of the tree and through the cortex. In the process of tapping, a U-shaped tool is used in cutting away a narrow strip of the bark, care being exercised not to penetrate the cambium and thus injure the tree. Tapping is started about 4 to 5 feet (120-150 cm) above the ground by making a slanting cut into the bark extending from a third of the way to halfway around the tree. A spout is then driven into the bottom of the cut, and a small glass or china cup is attached to collect the latex. Successive tappings are made by cutting a very thin shaving approximately inch (1 mm) along the lower side of the channel. Tapping is continued to within 6 inches (15 cm) of the ground, and then the other half of the tree is tapped in a similar manner. Tapping is started early in the morning, and the latex flows for about two hours. The trees are tapped every other day throughout the year or every day
in alternate two-week periods, each tree being tapped about 165 times a year. Much greater amounts of latex can be obtained by alternate tapping, which gives the tree proper rest periods.

The amount of rubber in the latex varies mainly with the age of the tree, the highest rubber content recorded being 69.5 percent in latex from 30-year-old trees that had long rest periods from tapping. The composition of rubber latex from 4-year-old and 10-year-old trees is shown in Table 2.

The treatment of the latex in the field and factory varies somewhat on different plantations. Sometimes a preservative such as ammonia or sodium sulfite is added to the latex cup at the time of tapping. Either ammonia alone or ammonia with pentachlorphenol is used if the latex is to be exported as such; sodium sulfite is used if the latex is to be manufactured into pale crepe. The latex is then carried
to the collecting station. One worker can tap 200 to 350 trees daily and collect about a teacupful of latex per tree.

Age of Trees
Content 4 Years Percent 10 Years Percent
Acetone-soluble material (resin, fatty acid, etc.) 1.22 1.65
Protein 1.47 2.03
Ash 0.24 0.70
Rubber hydrocarbon (caoutchouc) 27.07 35.62
Water 70.00 60.00

Pale Crepe.

At the station, the latex is weighed and strained to remove dirt, pieces of bark, and lumps of partially coagulated latex. If the latex is to be marketed as such, additional ammonia is added as a preservative before concentrating it. The latex from the tree contains 30 to 40 percent rubber and is diluted to 15 to 20 percent before the rubber is coagulated. In case the latex is to be used for the
production of pale crepe, sodium bisulfite is added as a bleaching agent and preservative prior to coagulation. The coagulation is carried out in aluminum-lined or stainless-steel vats or tanks by adding a weak solution of formic or acetic acid. The latex being coagulated is allowed to stand for several hours, or overnight, after which the rubber has risen to the top in the form of a doughlike curd. The large pieces of coagulum are broken into smaller ones by passing through a machine known as a
cracker. The rubber is then washed by passing through corrugated rolls, which roughen and crinkle the sheets to resemble crepe paper. The rolls of the wash mill revolve at different speeds and a stream of water is sprayed over the rubber. Finally the rubber is sheeted on a mill with smooth rolls. The sheets of rubber are then about 4 feet (1.2 meters) long, 18 inches (46 cm) wide, and about 1/8 inch (3 mm) thick. This rubber, known as pale crepe, is ready for shipment after it has been dried in hot air. Being clean and practically colorless, it is preferred where delicate color, whiteness, and transparency of rubber goods are desired. Its chief uses are for the manufacture of crepe shoe soles, certain surgical and medical goods, light-colored cements, and some types of electrical wire insulation.

Ribbed Smoked Sheets.

About 85 percent of the rubber is given an alternative treatment to produce ribbed smoked sheets. In this operation, the rubber is coagulated in troughs usually 10 feet (3 meters) long, 4 feet (1.2 meters) wide, and 16 inches (41 cm) deep. A 1 percent solution of the coagulant, either acetic acid or formic acid, is added, and partitions 11/2 inches (3.8 cm) apart are inserted into slots in the trough. The coagulant will cause the latex to thicken slowly and then set to a fairly firm coagulum after a few
hours or overnight. After the coagulum has become firm enough to be sheeted out, the slabs are passed through successive pairs of smooth rollers and finally through a pair of rollers which have a characteristic ribbed marking engraved on them. The sheets are washed by spraying clean water on the coagulum during the rolling process and the finished sheet is soaked in running water for 15 to 30 minutes. The sheets are then placed in a smokehouse to dry by the heat from open wood fires. The temperature is maintained at 115°F. to 120°F. (46°C. to 49°C.), and the length of time required for the smoking and drying process is between 5 and 12 days. The smoking process hinders the formation of molds and bacteria and aids in the preservation of the rubber against oxidation. Smoked sheets have a brown color, whereas crepe rubber is light yellow.Concentrated Latex.

Increasing amounts of rubber are exported directly as latex. Before shipment, however, the latex is generally concentrated to 60 percent rubber content by creaming, evaporation, or centrifuging. Concentrating of the rubber globules in latex by one of these methods is analogous to the process of separating cream from milk.


Crude rubber is generally packed in wooden cases or burlap bags. A case of ribbed smoked sheet contains 224 pounds (101 kg) of rubber, whereas a bale of rubber packed in burlap contains 250 pounds (113 kg) of rubber. Sheeted rubber has been used as a covering for baled rubber. The cover of sheet rubber is cemented to the bale and reinforced with steel bands.


Growth of the Industry.

The first English rubber factory in England was established in 1820, by Thomas Hancock. Three years later, Charles Macintosh of Scotland patented a process for waterproofing fabrics, in which a benzene solution of rubber was applied to the fabric, this rubber coating then being covered with another layer of fabric. The name "mackintosh" is still widely used for a certain type of waterproof garment. The
first factory established in the United States for the manufacture of rubber goods such as clothing and shoes was the Roxbury India Rubber firm at Roxbury, Mass., in 1833. It produced waterproof clothing, shoes, life preservers, and wagon covers, which were, however, affected by temperature changes. Edwin M. Chaffee, of the Roxbury factory, thought that these difficulties were brought about by the use of solvents. In 1836 he invented and built a large machine (first known as the "monster, " later as the
calender) for applying rubber directly to the fabric without the use of a solvent. However, it took vulcanizing to make the commercial use of rubber practicable.

Charles Goodyear tried for nearly ten years to find a means of rendering rubber firm and yet flexible regardless of temperature. In January 1839, at Woburn, Mass., when he accidentally dropped part of a mixture of rubber, white lead, and sulfur upon a hot stove, he reached the solution to his problem. The charred lump could be bent when cooled and easily stretched without breaking; despite repeated
stretching, it would snap back to its original shape. Heat no longer made it sticky, nor did cold make it stiff. At the time of Goodyear's discovery, Hancock in England was also confronted with the problem of overcoming the adhesiveness and tackiness of rubber, and he too had tried mixtures of rubber and sulfur to overcome these defects. After being shown some samples of Goodyear's product, he rediscovered the process of heating rubber with sulfur, called vulcanization, a term proposed by a friend of Hancock's.

Following the discovery of vulcanization, factories engaged in the production of rubber boots, and shoes and waterproof fabrics began to appear throughout New England. The movement of the rubber-manufacturing industry to the Middle West began in 1870, when Dr. Benjamin Franklin Goodrich
moved a rubber factory from Hastings-on-Hudson, N.Y., to Akron, Ohio. This factory, since then grown to immense size, manufactured a wide range of rubber articles, using the slogan "Everything in Rubber."

In view of the subsequent invention of the automobile, Dr. Goodrich's move was a fortunate one. He may have been influenced by the proximity of the carriage-making industry, to which rubber manufacturers supplied tires. When the Middle Western carriage manufacturers changed over to automobiles, they found a source of tires already developed in Ohio. Therefore it was natural that the Ohio rubber companies should develop into huge corporations when the automobile was put into mass production.


In past years it was necessary to wash a large part of the raw rubber at the factory to remove sand, bark, and other impurities prior to fabrication into rubber goods. Washing was particularly necessary for wild rubbers, which accounted for a large percentage of commercial crude rubber. Washing is done mechanically by passing the rubber between two corrugated rolls of a wash mill and spraying it with a continuous stream of water. The rolls tear the rubber apart and the stream of water washes away the dirt and bark. The rubber is washed on several mills, each successive mill having more finely corrugated rolls. Finally the rubber is sheeted out on a mill with the rolls set close together. The sheeted rubber is then hung on a drier to remove moisture. In modern practice, with high-grade plantation rubber furnishing most of the rubber of commerce, the washing operation can be eliminated.
However, small amounts of wild rubber and even of plantation rubber are washed for certain special uses such as drug, medical, and surgical goods.


One of the most important properties of rubber, its plasticity, is utilized in the production of rubber goods. In order to mix the rubber with compounding ingredients, it first must be softened or plasticized by mechanical working. The rubber is said to be broken down in this process which is known as mastication. Hancock's discovery in 1820 that rubber could be masticated was of the greatest technical importance to the rubber industry. His masticator consisted of a spiked rotor revolving in a spiked hollow cylinder, and the machine was operated by manpower. In modern practice, the rubber industry uses three types of machines prior to incorporating compounding ingredients into the rubber. These are the rubber mill, the Banbury mixer, and the Gordon plasticator.

Rubber Mill.

A rubber mill consists of two horizontal steel rolls revolving in opposite directions and at different speeds. Crude rubber is passed through the nip between the two revolving rolls. In this operation, the rubber is subjected to compression and shearing and is kneaded until it is properly broken down, in which condition it is soft and pliable. A great deal of heat is evolved during the milling process, and the steel rolls are therefore hollow to permit the circulation of cooling water. Breakdown of the rubber is more rapid if the rolls are kept cool. Oxygen, necessary to break down rubber on the mill, is furnished by the surrounding air. The larger mills have rolls 84 inches (213 cm) long and 26 inches (66 cm) in diameter. A 60- by 22-inch (152- by 56-cm) roll is also used extensively. An 84-inch mill can masticate and compound 170 pounds (77 kg) of rubber. The finished batch of stock weighs about 275
pounds (125 kg), varying with the compounding ingredients. Prior to 1925, most rubber was masticated on mills, but thereafter the mills were largely superseded by the Banbury mixer and the Gordon plasticator, which cost less to operate.

Banbury Mixer.

The Banbury is an enclosed internal mixer similar to a dough mixer, with two rotors revolving in opposite directions and at different speeds in a water-cooled chamber. The rubber is forced between the rotors and also between the rotors and the walls of the chamber. A lid operated by a hydraulic ram forces the stock onto the rotors. The rubber and the compounding ingredients are fed
through a hopper at the top. The standard-size Banbury can handle about 400 pounds (180 kg) of stock per batch, and the large size can take up to 1, 000 pounds (450 kg). The order of mixing the compounding ingredients with the rubber is much the same as in milling. After a prescribed mixing period, the stock is discharged through the bottom of the chamber in the form of large lumps, and it is necessary to
sheet the rubber on a mill afterward. The development of the Banbury greatly reduced the time needed to soften the rubber and to mix in the compounding ingredients. A large mixer will process up to 4, 000 pounds (1, 800 kg) of stock per hour. Banburys do most of the mixing in modern plants.

Gordon Plasticator.

In a modern factory processing large quantities of rubber, the crude, tough rubber as received from the plantation in bales is cut into pieces by powerful mechanically operated knives. The rubber is then put through a Gordon plasticator, a large single-barrel, single-screw machine similar in principle to a sausage grinder. The rubber is churned about by the screw, and this violent mechanical action causes the temperature of the plasticized rubber to rise to from 315°F. to 360°F. (157°C. to 182°C.) in the few minutes required for it to pass through the plasticator. Breakdown of the rubber is largely due to heat, only a small part being due to mechanical action. The plasticator has a far greater capacity for softening rubber than either a mill or a Banbury mixer. A single-screw machine will turn out 7, 000 to 10, 000 pounds (3, 000-4, 500 kg) per hour.

The rubber plasticized in a Gordon plasticator must be cooled as it leaves the machine; otherwise, part of it will melt as a result of rapid oxidation at the higher temperature. The use of a water spray lowers the surface temperature of the rubber to the point where there will be no appreciable oxidation. Frequently, rubber that has been softened by a single pass through the plasticator is still further softened by a second and even a third pass through the machine.

The use of pelletizers, machines which cut the rubber into small pellets or slabs of uniform size and shape, has facilitated weighing and handling operations in rubber processing. Rubber is fed into the pelletizer after it comes from the plasticator. The resulting pellets are mixed with carbon black and oils in a Banbury mixer, forming the masterbatch which is also pelletized. Mixing with the curing agents, sulfur, and accelerators follows. After blending in a Banbury, this stock is ready to be made into various products.


A simple compound of rubber and sulfur would have only limited practical use. In order to improve the physical properties of rubber and make it more serviceable for various applications, it is necessary to modify the properties by the addition of other substances. All the materials mixed with the rubber before vulcanization, including sulfur, are known as compounding ingredients. They produce both chemical and physical changes in the rubber. Their function is to modify the hardness, strength,
and toughness, and to increase resistance to abrasion, oil, oxygen, chemical solvents, heat, and cracking. Different formulas are employed for different applications.


Certain chemical substances, termed accelerators, when used with sulfur shorten the time of vulcanization and improve the physical properties of rubber. Examples of inorganic accelerators are white lead, litharge (lead monoxide), lime, and magnesia. Organic accelerators, discovered in
1906 by Oenslager, are much more active and are an important part of almost every rubber compound. The
proportion required is relatively small, usually 0.5 to 1.0 part per 100 of rubber being sufficient. Most accelerators require the presence of zinc oxide to exert their full effectiveness, and some require an organic acid, such as stearic acid. Modern compounding formulas, therefore, usually contain zinc oxide and stearic acid.

Softeners and Plasticizers.

Softeners and plasticizers are generally necessary to reduce the time of milling and the temperature during processing. They also aid in the dispersion of the compounding ingredients by exerting a swelling or solvent action on the rubber. Vegetable oils, paraffin oil, waxes, oleic and stearic acids, pine tar, coal tar, and rosin are typical softeners.

Reinforcing Agents.

Some substances toughen the rubber, giving it added strength and resistance to wear. These are known as
reinforcing agents. Carbon black in very finely divided form is the most common reinforcing ingredient; it is relatively cheap and is one of the most effective known.The tread stock of an automobile tire contains approximately 45 parts of carbon black to 100 parts of

Other widely used reinforcing agents include zinc oxide, magnesium carbonate, silica, calcium carbonate, and certain clays, but all of these have less reinforcing values than carbon black.


In the early days of the rubber industry and prior to the advent of the automobile, a number of substances were added to rubber to cheapen it. Reinforcing action was not too essential, and such substances were used principally to add bulk and weight. They are known as fillers or inert compounding ingredients. Barites, whiting, some clays, and diatomaceous earth are common fillers.


The use of antioxidants to preserve the desirable properties of rubber during aging and in service dates from the period after World War I. Like accelerators, the antioxidants are complex organic compounds which when added in a concentration of 1 to 2 parts per 100 of rubber keep it from becoming hard and brittle. Exposure to air, ozone, heat, and light are the principal causes for aging of
rubber. Certain antioxidants also protect the rubber from the effects of flexing and the action of heat.


Reinforcing agents, fillers, and other compounding ingredients are often termed pigments, although true
pigments that impart color to rubber articles are also used. Zinc oxide, titanium oxide, zinc sulfide, and lithopone are used as white pigments. Chrome yellow, iron oxide, antimony sulfide, ultramarine, and lampblack are used for different color effects.


After crude rubber has been plasticized and mixed with the compounding ingredients, it is further processed before vulcanization in the form of the finished article. The type of treatment depends on the use for which the rubber is intended. Calendering and tubing are employed extensively in the preparation of the rubber for final use.

Calenders are large machines designed for sheeting rubber or applying it to fabric. A standard calender usually consists of three horizontal rolls arranged vertically one above the other, though four-roll and five-roll calenders are used for certain operations. The hollow rolls vary in size up to 100 inches (254 cm) in length by 30 inches (76 cm) in diameter, and are provided with steam and cold water
to control the temperature vital for uniformity of gauge (thickness) and smoothness of surface. Adjacent rolls revolve in opposite directions, and the speeds of the individual rolls and the spacing between adjacent rolls can be accurately controlled.

Three operations--frictioning, skim coating, and sheeting--are performed on the calender.


Fabric such as cotton or rayon is frictioned by passing it between the lower and middle rolls. The rubber is fed from a bank between the upper and middle rolls, and a thin sheet passes over the middle roll and is forced onto the fabric, which becomes completely coated with the rubber. In the
frictioning operation, the lower roll runs at a slower speed than the middle roll, thus wiping as well as squeezing the rubber into the interstices between the cords of the fabric.

Skim Coating.

Coating or skimming of fabric with a thin sheet of rubber is accomplished in a manner similar to frictioning. The rubber is fed between the two upper rolls and the fabric passes between the lower two. For coating, however, the two lower rolls are run at the same speed. A thin sheet of rubber is pressed against the cloth but is not forced into the interstices of the fabric as in the case of frictioning. With four-roll calenders, it is possible to coat both sides of the fabric. Frequently the same fabric
is first frictioned with rubber and then coated, the operations being applied either to one side or both sides.


Rubber is sheeted on calenders by feeding rubber, previously plasticized on a mill, between the upper and middle rolls. A thin sheet of rubber passes halfway around the center roll and is removed between the center and lower roll and wound in cloth liners to prevent sticking. The calenders are run at different speeds depending upon the type of stock, the thickness, and the use for which the product is intended. Calenders are used extensively for sheeting footwear rubber.


The tubing machine is used for the shaping of tubing, hose, pneumatic-tire treads, solid-tire treads, inner tubes, channel rubber for automobiles, and other articles. It consists of a steel cylindrical barrel surrounded by a jacket to permit heating or cooling. A tight-fitting screw forces the unvulcanized rubber, previously warmed on a mill, through the cylindrical barrel to a head, into which
is inserted a removable die that determines the shape of the extruded strip. The extruded stock is usually cooled with a spray of water. The tread section of tires is extruded from a tubing machine in this manner. Inner tubes are also extruded in a continuous section and cut to length after leaving the machine. Many articles such as channel strips and small tubing are extruded to size and shape and are then vulcanized. Other articles, such as tread sections, are assembled on a tire body and are later molded to modified shapes.

Vulcanization or Curing.

After the raw rubber, as received at the factory, is masticated, compounded, and prepared for various
manufacturing uses by calendering, tubing, or shaping by other means, it is necessary to vulcanize the finished article to make it serviceable. Vulcanization is carried out in a number of ways. Many articles are given their final form and are vulcanized simultaneously by the application of heat and pressure in metal molds. Automobile tires after assembly on a drum are shaped to size and then
vulcanized in engraved steel molds. The molds are stacked one on top of the other in vertical pot heaters, and steam is admitted to the closed heater. An air bag resembling an inner tube is inserted in the unvulcanized casing. Air, steam, hot water, or combinations of these are introduced into the air bag by flexible connections of copper tubing, expanding the tire carcass and causing the rubber to flow
into the design of the mold. In modern practice, an increasing number of tires are cured in individual
vulcanizers called watchcase molds. The castings are hollow inside to permit circulation of the steam, hot water, and air used to supply the heat. The molds are opened automatically at the proper time.

Tire presses have been developed which, in one operation, insert the curing bag, cure the tire, and remove the bag from the cured tire. In these presses, the curing bag is installed as part of the curing press. Preforming of the tire around the bag and removal of the tire are done automatically. Inner tubes are cured in similar molds having a smooth facing. The average curing time for an inner tube is about 7 minutes at 310°F. (154°C.). A 6:00 x 16 automobile tire is cured for approximately 30 minutes at 300°F. (150°C.). At lower temperatures, longer times are required.

Many articles of smaller shape, especially mechanical goods, are cured in metal molds which are placed between parallel plates of a hydraulic press. The platens of the press are hollow to permit heating with steam. The article receives its heat only by transfer of heat from the hollow platens and through the metal mold. Steam does not come in contact with the article.

A great many articles are vulcanized by heating in air or carbon dioxide. Rubberized cloth, clothing, raincoats, and boots and all other kinds of rubber footwear are vulcanized in this manner. The operation is usually carried out in large horizontal steam-jacketed vulcanizers. Hot air is circulated through the inside of the vulcanizers. Rubber stocks cured in dry heat usually contain smaller percentages of sulfur to avoid blooming of part of the sulfur on the surface. Accelerators are used to shorten the time of cure, which in general is longer than the time required for open steam cures or press cures.

Some rubber goods are cured by immersion in hot water under pressure. Sheet rubber for thread and tape is wound between layers of muslin on a drum and vulcanized in hot water under pressure.

Rubber hose, insulated wire, and atomizer bulbs are vulcanized in open steam. The vulcanizers are usually horizontal cylinders with tight-fitting covers. Fire hose is vulcanized with steam on the inside and thus forms its own vulcanizer. The rubber core is drawn into the tube of braided cotton, the couplings are attached, and steam under pressure is kept inside for the required time.

Vulcanization without heat can be accomplished with sulfur chloride, either by immersion in a solution or by exposure to the vapors. Only thin-walled sheets or articles such as aprons, bathing caps, finger sheaths, and surgical gloves are cured in this way, since the reaction is rapid and the solution does not penetrate far. An aftertreatment with ammonia is necessary to remove the acid formed during

Uses of Vulcanized Rubber.

Vulcanized rubber has many valuable properties that make possible its use in the home and in industry and contribute much to man's comfort, health, and pleasure. In addition to good tensile strength, the desirable properties are its flexibility, resilience, electrical resistance, elasticity, resistance to abrasion, low permeability to gases and liquids, chemical resistance, and low heat conductivity.

In a great variety of articles such as tubes, hose of all kinds, boots, waterproof clothing, and bathing apparel, vulcanized rubber is used because of its impermeability to liquids and gases. Rubber is applied primarily because of its low water-absorbing capacity and remarkable resistance to electrical currents in such uses as insulation for wire and cables, instrument panels, and electrician's gloves. Its elasticity is utilized in the manufacture of rubber bands, surgical goods, mechanical rubber goods, and wearing apparel. Its resilience and abrasion resistance make it ideal for tires, heels and soles, conveyor belts, and as vibration dampers and shock absorbers for heavy machinery and in automotive parts. Imperviousness to many chemicalsmakes its application in the laboratory and in the chemical industry indispensable. The largest amount is consumed in the manufacture of tires and tubes for automobiles, airplanes, and bicycles. The second largest use is in the manufacture of mechanical goods, a large part of which is utilized in automobiles.

Building of a Tire.

In the early days of the automobile-tire industry, the body of the tire was built upon a circular core and partially vulcanized. The tread, also partially vulcanized, was then applied and vulcanization of the whole tire was completed. Later the body and tread were assembled on the core and vulcanized simultaneously. After 1925, the core was gradually replaced by a flat revolving collapsible drum. By
this method, called the flat-band method, the tire is constructed by first building up the body with flat strips or plies of rubberized cord fabric cut on the bias. Each layer is applied to the drum with the cords at right angles to those in the previous layer. After the first two layers are in position, the beads consisting of hoops of steel wire covered with rubber are put in position on each side
of the fabric, and the fabric is rolled over. The beads serve to keep the finished tire on the rim. Additional plies are then added to build a four-, six-, or eight-ply tire. Chafing strips are put around the bead for protection against rim wear. The cushion, made of a rubber compound softer than that in the tread, is applied in the center just under the tread. The tread and sidewall, extruded in
one piece, are put in place and rolled down to produce the finished tire. The drum is then collapsed and the tire is removed. The tire is then placed on a conveyor and taken to the shaping machine, also referred to as the expanding machine, which shapes the tire to a form that looks like a huge doughnut. At the same time, an air bag is inserted into the tire, to provide pressure from within the casing
during the vulcanizing operation and to provide heat, since the air bag is filled with steam or hot water during cure. The air bags are removed after vulcanization and are reused again and again for subsequent cures.

Manufacture of Articles Directly from Latex.

The direct use of latex in rubber-goods manufacture has increased markedly since the 1930's. One of the principal uses is in the manufacture of latex foam mattresses for homes, hospitals, and hotels, and for seat and chair cushions in automobiles, trucks and buses, theaters, and homes. The latex is concentrated by centrifuging on the plantations to a rubber content of 60 percent and is shipped in this form. The compounding ingredients such as sulfur, zinc oxide, accelerator, and other chemicals are
added to the latex as fine dispersions in an aqueous emulsion, since the dry powders do not mix well with latex. The compounded latex is then run into the bowl of a frothing machine or beater, where mechanical whipping with air transforms it into foamed latex, a light, creamy froth or foam containing millions of air bubbles. The texture and density of the cured latex foam can be controlled by the
amount of air whipped into the latex. The more air it contains, the lighter will be the finished product.

In large-scale production, the compounded latex is continuously mixed with air in an internal mixer of the type used in making marshmallows and cake. The latex and air are metered into this machine in a controlled ratio depending on the density of the latex foam desired. Subsequently, either in the foaming machine or in an auxiliary mixer, zinc oxide and a gelling agent are metered and mixed into the foam. The latex foam is then poured into molds for shaping. These molds are usually designed to give
additional holes or openings in the finished product to increase its lightness and cushioning qualities. The molds are conveyed into a tunnel-type oven where curing with steam at atmospheric pressure is accomplished. After curing, the article is removed from the mold, washed in running water, and passed through squeeze rolls to remove excess water. Drying is completed in circulating air ovens
which are sometimes augmented by high-frequency heating to shorten the drying time. The vulcanized latex is known as latex foam rubber.

Considerable latex is used for impregnating tire-cord fabric to be used in the manufacture of the plies of automobile tires. The latex is first compounded and the fabric is passed from a roll through the latex, then through a dryer and rolled up on another drum.

Some articles such as gloves and toys are produced by dipping plaster of paris or porcelain forms of the desired shape into compounded concentrated latex. A coating of latex adheres to the form and is stripped from it after vulcanization. Rubber thread for clothing, formerly made by cutting from a vulcanized sheet, is formed by extruding compounded latex through an orifice into a coagulating
bath. The vulcanized thread can then be wound with cotton, rayon, or silk. Latex is also used to hold together the individual fibers of curled hair used as a stuffing for furniture and automobile cushions.When used to impregnate paper, latex gives rigidity to it and holds the fibers together. The impregnated paper can then be coated with varnish or lacquer and finds application as table and wall covering. Latex is also used to seal tin cans to make them airtight.

Rubber Cements.

Rubber cements for adhesive purposes are made by milling rubber to a point where it is partially broken down and is more readily soluble in solvents such as petroleum naphtha or benzene. The degree of breakdown on the mill determines the viscosity of the cement: the greater the milling, the lower the viscosity. The masticated rubber is put into churns fitted with a paddle stirrer, and the solvent is
added. The rubber and solvent are agitated until the rubber is completely dissolved. Rubber cements are used for adhering paper to paper, rubber to rubber, or rubber to metal, and for impregnating cloth with rubber. Thin-walled goods such as surgical gloves are also made by dipping a porcelain form into a rubber cement. The operation is repeated until the desired thickness is obtained. (See also Adhesives.)


Hard-rubber goods differ chiefly from soft-rubber goods in the amount of sulfur used in vulcanization. When the amount of sulfur used in the compounding formula is more than 5 percent, the rubber goods are spoken of as hard-rubber goods. As much as 47 parts of sulfur can be combined with 100 parts of rubber, resulting in a compound containing 32 percent sulfur. Such a product is hard and tough, and is known as ebonite because of its resemblances to ebony wood. It is also called vulcanite or hard rubber. The term hard rubber is generally applied to vulcanites of rubber and sulfur containing more than 20 percent of combined sulfur. Hard-rubber compositions have good electrical properties and are used in the electrical industry for insulating purposes and in switchboard panels, plugs, sockets, telephone receivers, and storage-battery cases. Hard-rubber pumps, pipes, valves, and fittings are used in the chemical industry in applications where resistance to corrosion is required. Children's toys are another source of hard-rubber consumption.


Much of the rubber in worn-out tires, tubes, and other articles can be reclaimed by suitable processes, and the product may be fabricated into a variety of useful articles. In general, reclaimed rubber is used to lower the cost of such goods as floor mats, garden hose, footwear, wire insulation, rubber utensils, hard-rubber products, and tubing. For some purposes, reclaimed rubber has desirable properties that cause it to be used in preference to natural rubber.

No process is known for recovering rubber in its original form once it has been vulcanized. To do this, it would be necessary to remove the sulfur from the rubber without affecting its structure in any way. Reclaimed rubber is essentially plasticized and softened vulcanized rubber containing nearly all the sulfur used in vulcanization and most of the compounding ingredients employed in the manufacturing of the specific article.

Scrap Rubber.

Scrap rubber is classified according to product and original use, for example, as inner tubes, pneumatic tires, boots and shoes, solid tires, and mechanical goods. It is stored in huge piles in reclaiming plants. Pneumatic-tire scrap is the most important type. The tires are sorted according to age, condition, and size. First the beads are removed by a cutting machine and are not salvaged further. The tires are then disintegrated by passing through a cracker, which crushes them preparatory to grinding. The cracked scrap is next passed over a magnetic separator, which removes loose pieces of metal. The rubber is then passed through the shredder, or hog, where it is ground to pieces varying from 1/4 to 1 inch (6-25 mm) in size. Much of the fabric is also separated prior to the actual

Mark Process.

There are a number of methods for reclaiming rubber. The one in general commercial use is essentially the Mark process, patented in 1899. This method consists in digesting the finely divided scrap-rubber containing fabric with caustic-soda solution and a coal-tar or petroleum naphtha in large pressure cookers or digesters at a temperature of about 375°F. (190°C.) for a period of 12 to 20 hours. The digesters are steam-jacketed and are capable of reclaiming from 5, 000 to 6, 000 pounds (2, 265-2, 720 kg)
of rubber in one batch. The dilute caustic solution destroys most of the fabric during the digestion process, and the naphtha and other oils that are added serve to swell and soften the rubber. After this digestion treatment, the rubber is washed thoroughly and is then dried, screened to remove traces of metal, massed, and refined. The refining operation consists in passing the rubber through tightly set rolls of a number of small mills known as refiners. The rolls of the refiner are set closer
together in each succeeding operation, so that the sheet that emerges from the last refiner has a thickness of only several thousandths of an inch. The sheeted rubber passes from the last refiner to a windup drum from which the reclaimed rubber is cut in the form of laminated slabs.

Open Steam Process.

The open steam or pan process is sometimes used for reclaiming scrap which contains little or no fabric, such as inner tubes, tire treads, and some mechanical goods. The scrap is ground finer than for the alkali digestion method. Oils and concentrated caustic soda solution are thoroughly mixed with the ground scrap, and the mixture is placed in pans to a depth of several inches. The pans are then heated
with live steam in a horizontal cylindrical vessel. After being cooked, the scrap is dried and removed from the pans. The refining operations are the same as for the alkali digestion process.

Wartime Uses.

During World War II reclaimed rubber became of major importance to the U.S. war effort because of the shortage of natural rubber after the Japanese conquest of Malaya. Under government sponsorship, 759, 274 metric tons of scrap rubber were collected and became available for reclaiming. Tires made of reclaimed rubber served to tide over a critical period in 1942 and 1943 while the manufacture of synthetic rubber was getting under way.


A number of products are made from natural rubber by chemical reaction and classed as rubber derivatives. Many of these have found useful application and are manufactured commercially.

Chlorinated Rubber.

Chlorinated rubber is prepared by passing chlorine into a solution of rubber in carbon tetrachloride. The chlorinated rubber containing about 65 percent chlorine is isolated as a white powdery material. Its principal application is as a component of paints or lacquers where resistance to corrosion by acids and alkalis is required.

Rubber Hydrochloride.

Rubber hydrochloride is prepared by passing hydrogen chloride gas into a solution of rubber in benzene. The resulting rubber hydrochloride, a white powder containing between 29 and 30 percent of chlorine, can be cast as a clear, flexible film. Considerable quantities are marketed in the form of a transparent packaging film known as Pliofilm.

Cyclized Rubber.

When rubber is heated with sulfuric acid, sulfonic acids, or certain metal salts such as stannic chloride, it undergoes a transformation into a tough, gutta-percha-like product. The rubber is said to be cyclized or isomerized and has the same chemical composition as the original rubber. Cyclized rubber has good adhesive properties and can be bonded to metal as a protection against corrosion.It can also be compounded with various pigments to give molding powders known commercially as Plioforms.


The synthesis of rubber as produced by the tree has never been accomplished in the laboratory. The so-called synthetic rubbers are elastic materials, termed elastomers, resembling the natural product in chemical and physical properties but differing from it in structure.

Early Types.

In 1826 Michael Faraday showed that the composition of rubber corresponded to the formula (C5H8)n; and in 1860 Greville Williams, another English chemist, decomposed rubber by heat and isolated a chemical substance called isoprene, having the formula C5H8. In 1879, the French chemist Bouchardat succeeded in changing isoprene into a rubberlike material, one of the first synthetic rubbers produced. However, Sir William A. Tilden in England was the first to prepare a rubberlike material from isoprene, which
he synthesized from turpentine. From 1908 to 1910, Hoffman and Harries in Germany and Mathews and Strange in England carried out intensive studies on the polymerization of butadiene, isoprene, and dimethylbuta diene. They discovered the method of preparing synthetic rubbers by polymerizing these hydrocarbons with sodium, a process which was later used commercially by Germany during World War I.

In 1910 the price of natural rubber reached $3 a pound. The increasing requirements for natural rubber and the consequent high price led to the first efforts in the United States to produce synthetic rubber. Kyrides and Earle, working in the laboratories of the Hood Rubber Company, synthesized dimethylbutadiene and polymerized it to synthetic rubber. At the same time, David Spence of the
Diamond Rubber Company synthesized isoprene from turpentine and prepared synthetic rubber on a pilot-plant scale.

The Germans intensified their researches on synthetic rubber just prior to World War I and developed a
commercially successful process. During World War I, Germany manufactured 2, 300 tons of methyl rubber, which was produced by polymerizing dimethylbutadiene with sodium metal.

In the United States, interest in synthetic rubber remained dormant until 1921, when Ostromislensky and Maximoff began work on synthetic rubber for the United States Rubber Company. In the following year, they produced a synthetic rubber from butadiene by the emulsion process.

Synthesis of Natural Rubber (Cis-1, 4-Polyisoprene and Cis-1, 4-Polybutadiene).

Natural rubber (from Hevea braziliensis) has a structure composed of 97.8 percent cis-1, 4-polyisoprene. The synthesis of cis-1, 4-polyisoprene was accomplished by several different routes with the use of stereoregulating catalysts, and this made possible the production of synthetic elastomers of definite stereochemical structure. Ziegler catalyst, so named from its discoverer, is composed of aluminum triethyl and titanium tetrachloride; it causes isoprene molecules to combine (polymerize) to form giant
molecules of cis-1, 4-polyisoprene (a polymer). Similarly, lithium metal or alkyl and alkylene lithium compounds, such as butyl lithium, serve to polymerize isoprene to cis-1, 4-polyisoprene. Polymerizations with these catalysts are conducted in solution using petroleum hydrocarbons as solvents. Synthetic cis-1, 4-polyisoprene has the properties of natural rubber and can be used as a direct replacement for natural rubber in the manufacture of rubber goods. (See also Plastics.)

Polybutadiene having 90 to 95 percent cis-1, 4 structure has also been synthesized by means of stereoregulating catalysts of the Ziegler type, for example, aluminum triethyl and titanium tetraiodide. Other stereoregulating catalysts such as cobalt chloride and aluminum alkyls also produce high cis-1, 4-polybutadiene (95 percent). Butyl lithium is also capable of polymerizing butadiene but
produces polybutadiene having a lower cis-1, 4 structure (about 35-40 percent). Cis-1, 4-polybutadiene has a remarkably high resilience and can be used as an extender for natural rubber.


In 1920, while attempting to prepare a new antifreeze from ethylene dichloride and sodium polysulfide, J. C. Patrick discovered instead a new rubberlike substance that he termed Thiokol.

It was not until 1930, however, that Thiokol was produced commercially. Thiokol has excellent resistance to gasoline and aromatic solvents. It ages well and has good tear resistance and low permeability to gases. Though not a true synthetic rubber, it nevertheless finds wide use as a
specialty rubber.


A rubberlike polymer, or elastomer, known as neoprene, based on the researches of J. A. Nieuwland of the University of Notre Dame, was announced by the DuPont Company in 1931. Neoprene is manufactured from acetylene, which, in turn, is made from coal, limestone and water. The acetylene is first polymerized to vinyl-acetylene, to which hydrochloric acid is added to produce chloroprene. Chloroprene is then polymerized to neoprene. In addition to being oil-resistant, neoprene has good heat and hemical
resistance and is used in the manufacture of hose, tubing, gloves, and mechanical parts such as gears, gaskets, and belting. During World War II, neoprene was manufactured under the government-sponsored synthetic-rubber program and was designated as GR-M.

Buna S (SBR).

In 1935, Germany announced the commercial product Buna synthetic rubber. The word "Buna" is derived from the first two letters of the words "butadiene" and "natrium." Butadiene is the main chemical raw material, and sodium (natrium) was used as the catalyst in the polymerization. Two types of Buna rubbers, Buna S and Buna N, were produced in Germany. Buna S, a copolymer of butadiene and styrene, was used as a general-purpose rubber for tires and tubes. Buna N is discussed under Buna N (NBR), below.

After the Japanese conquest of Malaya, 90 percent of the natural-rubber supply from the Far East was cut off from the United States, which had a stockpile sufficient for less than one year's normal peacetime requirements. Already, however, in May 1941, U.S. industry and the federal government had set up a cooperative synthetic-rubber program based on previous research and pilot-plant work of U.S. rubber, chemical, and oil companies, some of which had shared German patents under cartel agreements. This program was expanded to produce all synthetic rubber needed.

A synthetic rubber of the Buna-S type, designated as GR-S, was decided upon as the general-purpose rubber for tires and other articles necessary to prosecute the war. It was manufactured by the emulsion process of polymerization, in which soap is the emulsifying agent and the synthetic rubber is a copolymer of about 75 percent butadiene and 25 percent styrene. The butadiene used in the first plants to be completed was made from alcohol, but increasing amounts were made from petroleum. The styrene was manufactured from benzene, a coal-tar product, and from ethylene obtained from petroleum.

GR-S, now designated SBR, is manufactured in large jacketed reactors, also known as autoclaves, into which are charged the butadiene, styrene, soap, water, the catalyst (potassium persulfate), and a regulator (a mercaptan). The soap and water serve to emulsify the butadiene and styrene and bring these chemicals into intimate contact with the catalyst and the regulator. The contents of the reactor are
heated to approximately 122°F. (50°C.) and stirred for a period of 12 to 14 hours, during which time the rubber is formed by a polymerization process. The resulting latex, which contains the rubber, is in very small particles, and has a milky appearance, very much resembling the natural latex from the tree.

The latex from the reactors is treated with a short-stopping agent to stop the reaction, and with an
antioxidant to preserve the rubber. It is then stripped of excess butadiene and styrene. To separate (coagulate) the rubber from the latex, it is treated either with a solution of sodium chloride (table salt) and acid or with a solution of aluminum sulfate, which causes the rubber to separate in the form of a fine crumb. The crumb is then washed, dried in an oven, and pressed into bales.

SBR is the mostly widely used of all the elastomers. The largest single use is in automobile tires. SBR is similar in properties to natural rubber. Although it is not oil-resistant and is generally poor in chemical resistance, it shows excellent resistance to impact and abrasion.

Latices for Emulsion Paints.

Styrene-butadiene latices are extensively used in emulsion paints in which the latex is compounded with the usual paint pigments. In these applications the styrene content of the latex is found to be above 60 percent.

Cold Rubber and Oil-Extended Cold Rubber.

Cold rubber is a special type of SBR rubber. It is produced at 41°F. (5°C.) and provides better tire wear than standard SBR which is made at 122°F. (50°C.). The wear rating of tires is still further improved if cold rubber is made very tough. This is done by adding to the base latex certain petroleum oils, known as oil extenders. The amount of oil added depends on the degree of toughness desired; for
tougher rubber, more oil is used. The addition of oil acts as a plasticizer for the tough rubber. The properties of oil-extended cold rubber are equal to those of regular cold rubber.

Buna N (NBR).

Together with Buna S, the Germans also developed an oil-resistant type of synthetic rubber known as Perbunan, or Buna N. This rubber became available in the United States just prior to World War II and was manufactured in large tonnage during the war, being designated as GR-A by the government (the present designations are NBR and nitrile). The principal constituent of NBR is also butadiene, and this is copolymerized with acrylonitrile by essentially the same process used to manufacture SBR.
Grades of NBR differ in the amount of acrylonitrile that they contain, the amount varying from 15 to 40 percent, depending on the intended use of the rubber. The nitriles are resistant to oil in proportion to the amount of acrylonitrile in the rubber. NBR was used in wartime applications where oil resistance was required, such as hose, self-sealing fuel cells, and automotive parts.

Butyl Rubber.

Butyl rubber, another synthetic rubber, was announced in 1940 and manufactured on a large scale under the government's synthetic-rubber program as GR-I. Butyl rubber is outstanding for its low permeability to gases; an inner tube of this material retains air ten times longer than one made of natural rubber. Butyl rubber is made by polymerizing isobutylene obtained from petroleum with a small amount of isoprene at a temperature of -150°F. (-100°C.).

The polymerization is not an emulsion process but is carried out in an organic solvent, such as methyl chloride. The properties of butyl rubber can be greatly improved by heat treating master-batches of butyl rubber and carbon black at temperatures of 300°F. to 450°F. (150°C. to 230°C.). Recently butyl rubber has found further application in tire treads because of its good riding qualities, freedom from squeal, and excellent traction. Facilities for manufacturing butyl rubber are being greatly
expanded to meet the increasing demand. Butyl rubber is incompatible with natural rubber and SBR and therefore cannot be blended with these rubbers. However, when butyl rubber is partially chlorinated to chlorobutyl rubber it becomes compatible with natural rubber and SBR. Chlorobutyl rubber retains low permeability to gases. Advantage can be taken of this property in blends of chlorobutyl rubber with
natural rubber or SBR which are used as inner liners of tubeless tires.

Ethylene-Propylene Rubber.

Copolymers of ethylene and propylene can be prepared over a wide range of composition and molecular weight. Elastomers containing 60 to 70 percent of ethylene can be cured with peroxides and have good vulcanizate properties. The ethylene-propylene rubber has excellent weathering and ozone resistance characteristics, good heat stability and oil resistance, good wear, but poor impermeability to air.
This rubber can be made from cheap raw materials and finds numerous applications in the industry.

The most widely used type of ethylene-propylene rubber is EPDM (ethylene-propylene-diene monomer). It is used mostly in wire and cable coverings, single-ply roofing, and oil additives. Its light weight and excellent resistance to ozone and weathering have spurred its use in both new and replacement roofing.


Vistanex, or polyisobutylene, is a polymer of isobutylene, also prepared at low temperatures. It has rubberlike properties but, unlike rubber, is a saturated hydrocarbon and therefore not subject to vulcanization. Polyisobutylene is resistant to the action of ozone.


Koroseal, a rubberlike material, is a plasticized polyvinyl chloride prepared from vinyl chloride, which, in turn, is made from acetylene and hydrochloric acid. Koroseal has outstanding resistance to oxidizing agents such as ozone, nitric acid, and chromic acid and is, therefore, used as a tank lining where corrosion is a problem. It is impermeable to water, oil, and gases, and, thus, finds application as a coating for cloth and paper. The calendered material is used for raincoats, shower curtains, and drapes. Low water absorption, high dielectric strength, nonflammability, and resistance to aging make plasticized polyvinylchloride resins desirable as wire and cable insulation.


A class of elastomers known as polyurethanes is finding application as foams, adhesives, coatings, and molded goods. The manufacture of polyurethanes involves several distinct steps. A polyester is first produced from the reaction of a dicarboxylic acid, such as adipic acid, and a polyhydric alcohol, such as ethylene glycol or diethylene glycol. The polyester is treated with a diisocyanate, such as tolylene-2, 4-diisocyanate or methylene diphenylene diisocyanate. The product of this reaction is treated with water and a suitable catalyst, such as N-ethyl-morpholine, to yield a resilient or flexible polyurethane foam. With additional diisocyanate, molded articles, such as tires, are produced. By varying the glycol and the dicarboxylic acid in the polyester preparation, it is possible to produce polyurethanes which can be used as adhesives or which can be fabricated into rigid foams, flexible foams, or molded articles. Polyurethane foams are flame resistant, have high tensile strength, and excellent tear and abrasion resistances. They have exceptionally high load-bearing capacities and age well. Vulcanized polyurethane rubbers have high tensile strength, excellent abrasion and tear resistances, and good oil resistance; they age well. A polyurethane rubber obtained from a polyether instead of a polyester has been developed. It has both good low-temperature and good aging properties.

Silicone Rubber.

The silicone rubbers are unsurpassed in their serviceability over a wide temperature range, from -100°F. (-73°C.) to 600°F. (315°C.). Tensile strengths of nearly 2, 000 pounds per square inch (140 kg/sq cm) have been obtained in vulcanized silicone stocks. Their aging and electrical properties are also very good.


This chlorosulfonated polyethylene elastomer is prepared by treating polyethylene with chlorine and sulfur dioxide. Vulcanized Hypalon is extremely resistant to ozone and weathering and has good heat and chemical resistance.

Fluorine-Containing Elastomers.

Kel-F elastomer is a copolymer of chlorotrifluoroethylene and vinylidene fluoride. This rubber has good heat and oil resistance. It is resistant to corrosive chemicals, nonflammable, and is serviceable from -15°F. (-26°C.) to 400°F. (200°C.). Viton A and Fluorel are copolymers of hexafluoropropylene and vinylidene fluoride. These elastomers have excellent resistance to heat, oxygen, ozone, weathering, and sunlight. They have moderately good low-temperature properties, being serviceable to -5°F. (-21°C.). These fluorine-containing elastomers are used in applications where resistance to heat and oils is required.

Specialty Elastomers.

Specialty elastomers are produced with a wide variety of physical properties. Most of these elastomers are quite expensive. The most important types include acrylics, chlorosulfonated polyethylenes, copolyester ethers, epichlorohydrins, fluorinated polymers, and thermoplastic block copolymers. They are used in seals, gaskets, hoses, wire and cable covers, roofing, and adhesives.

Soviet Types.

In the Soviet Union synthetic rubber was first produced in 1932. The earliest products were SKB and SKA, both manufactured by polymerizing butadiene with sodium metal. The butadiene for SKB was obtained from ethyl alcohol distilled from grain and potatoes; that for SKA was obtained from petroleum. During World War II the Soviets also manufactured neoprene, calling it Sovprene. More recent Soviet types of synthetic rubber include SKD (polybutadiene), SKS (styrene-butadiene), SKI (polyisoprene), SKEP and SKEPT (ethylene-propylene rubbers), BK (butyl rubber), SKT (silicone), SKF (fluorine-containing elastomers), and SKU (polyurethane). See also Chemistry, Organic; Fluorocarbons; Plastics; Silicones.


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