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.
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.
SOURCES OF NATURAL RUBBER
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.
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
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.
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
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.
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.
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.
PROPERTIES OF 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.
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
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.
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.
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.
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
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.
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
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
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.
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
2. PERCENTAGE COMPOSITION OF RUBBER LATEX
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
Ribbed Smoked Sheets.
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
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.Packing.
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.
RUBBER MANUFACTURE AND PREPARATION
Growth of the Industry.
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
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
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
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.
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.
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.
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
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
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.
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,
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
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.
substances toughen the rubber, giving it added strength and resistance
to wear. These are known as
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.
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
agents, fillers, and other compounding ingredients are often termed
pigments, although true
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.
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
Three operations--frictioning, skim coating, and sheeting--are performed on the calender.
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
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
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.
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
Vulcanization or Curing.
the raw rubber, as received at the factory, is masticated, compounded, and prepared for various
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.
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
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.
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
Manufacture of Articles Directly from Latex.
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
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
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.
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
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
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.
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
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)
Open Steam Process.
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
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
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 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 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.
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.
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
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
intensified their researches on synthetic rubber just prior to World
War I and developed a
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).
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
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
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.
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
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
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
from the reactors is treated with a short-stopping agent to stop the
reaction, and with an
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.
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
Buna N (NBR).
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.
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.).
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
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.
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.
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.
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 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.
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.