Types of Rubber
One rubber is obtained from natural sources in commercial quantities. In addition, a number of synthetic compounds are classified as rubbers. Smoked sheet and pale crepe represent the forms in which the major portion of natural rubber is commercially available.

Natural Rubber (NR)
Although natural rubber may be obtained from hundreds of different plant species, the most important source is the rubber tree. Natural rubber is cis-1,4-polyisoprene, containing average polymer chain.

The economic competition from synthetic rubbers has stimulated research and development in natural rubber by increasing productivity in the field, improving uniformity and quality of the product and packaging, and developing modified natural rubbers with specific properties.

Increased productivity has been achieved by increasing the yield of the trees by cross-pollination of high-rubber-yielding clones, use of chemical stimulants, and better tapping and collection methods. These methods have resulted in large increases in yields on Malaysian estates, with much greater increases anticipated. With improved productivity and better processing methods and controls, much better quality and uniformity have been achieved. This has made possible the development of standard Malaysian rubbers (SMR) to meet specifications on a number of properties, including dirt and ash content, viscosity, and copper and manganese content.
In addition, experimental work has been done on improved collection of latex by microtapping and collecting in polyethylene bags, and trees having three parts: a high-yielding trunk system grafted onto a strong root system and an improved, more prolific leaf system grafted onto the trunk.

Despite competition from synthetic rubbers and a great reduction in percentage use worldwide of natural rubber, the tonnage of natural rubber continues a steady growth in parallel with the growth of the industry.

Styrene–Butadiene Rubbers (SBR)
The extensive development of the synthetic rubber industry originated with the World War II emergency, but continued expansion has been the result of the superiority of the various synthetic rubbers in certain properties and applications. The most important synthetic rubbers and the most widely used rubbers in the entire world are the styrene–butadiene rubbers (SBR). Formerly designated GR-S, SBRs are obtained by the emulsion polymerization of butadiene and styrene in varying ratios.

When SBR is used in light-duty tires such as passenger car tires, the cold-rubber compounds have proved equal or superior to natural rubber treads. However, they are inferior to natural rubber for truck tires because of the greater heat buildup during flexing. The cold, oilextended type, prepared by the replacement of a portion of the polymer by a heavy-traction oil, accounts for more than 50% of the coldrubber production. Its advantage is primarily economic. Master batches, containing both oil and carbon black, have the advantage of process simplification.

In general, the compounding and processing methods for all the SBR types are similar to those of natural rubber. Although natural rubber is superior with respect to lower heat buildup, resilience, and hot tear strength, the SBR types are more resistant to abrasion and weathering. However, carbon black or some other reinforcing fillers must be added to the SBR to develop the best physical properties. Unlike natural rubber, SBR does not crystallize on stretching and thus has low tensile strength unless reinforced. The major use for SBR is in tires and tire products. Other uses include belting, hose, wire and cable coatings, flooring, shoe products, sponge, insulation, and molded goods.

cis-1,4-Polybutadiene (BR)
Work on the duplication of natural rubber stimulated interest in stereoregulated polymerization of butadiene, particularly of the high cis-l,4 structure.

Black-loaded vulcanizates of cis-1,4- polybutadiene rubbers exhibit good physical properties, such as lower heat generation, higher resilience, improved low-temperature properties, and greatly improved abrasion resistance. Processing properties are rather poor, but can be greatly improved by employing blends with natural rubber or SBR. Tests on retreaded passenger tires gave outstanding abrasion resistance and increased resistance to cracking as compared with natural rubber. In passenger tires, cis-1,4-polybutadiene improves treadwear by about 1% for each percent of polybutadiene in the tread compound. In truck tires, a blend with natural rubber gives 14% more wear than natural rubber alone.

Butyl Rubber
Isobutylene and isoprene or butadiene obtained from cracked refinery gases are the primary raw materials required for the manufacture of butyl rubber.

Neither neoprene nor butyl rubber requires carbon black to increase its tensile strength, but the reinforcement of butyl rubber by carbon black or other fillers does improve the modulus and increases the resistance to tear and abrasion. The excellent resistance of butyl rubbers to oxygen, ozone, and weathering can be attributed to the smaller amount of unsaturation present in the polymer molecule. In addition, these rubbers exhibit good electrical properties and high impermeability to gases. The high impermeability to gases results in use of butyl as an inner liner in tubeless tires. Other widespread uses are for wire and cable products, injection-molded and extruded products, hose, gaskets, and sealants, and where good damping characteristics are needed.

Ethylene-Propylene Polymers(EPM, EPDM)
Stereospecific catalysts are employed to make synthetic rubbers by the copolymerization of ethylene and propylene. Either monomer alone polymerizes to a hard, crystallizable plastic, but copolymers containing 35 to 65% of either monomer are amorphous, rubbery solids. Special catalysts must be employed because ethylene polymerizes many times faster than propylene. Best results seem to be obtained with complex catalysts derived from an aluminum alkyl and a vanadium chloride or oxychloride. Processing techniques and factory equipment used with other rubbers can also be applied to these copolymers. The mechanical properties of their vulcanizates are generally approximately equivalent to those of SBR. Terpolymers containing ethylene, propylene, and a third monomer, such as dicyclopentadiene, have become more popular because they contain unsaturation and thus may be sulfur- cured by using more or less conventional curing systems.

Neoprene (CR)
One of the first synthetic rubbers used commercially to the rubber industry, neoprene is a polymer of chloroprene, 2-chlorobutadiene- 1,3. In the manufacturing process, acetylene, the basic raw material, is dimerized to vinylacetylene and then hydrochlorinated to the chloroprene monomer.

Sulfur is used to vulcanize some types of neoprene, but most of the neoprenes are vulcanized by the addition of basic oxides such as magnesium oxide and zinc oxide. The cure proceeds through reaction of the metal oxide with the tertiary allylic chlorine that arises from the small amount of 1,2-polymerization that occurs. Other compounding and processing techniques follow similar procedures and use the same equipment as for natural rubber. One of the outstanding characteristics of neoprene is the good tensile strength without the addition of carbon black filler. However, carbon black and other fillers can be used when reinforcement is required for specific end-use applications that require increased tear and abrasion resistance.

The neoprenes have exceptional resistance to weather, sun, ozone, and abrasion. They are good in resilience, gas impermeability, and resistance to heat, oil, and flame. They are fairly good in low temperature and electrical properties. This versatility makes them useful in many applications requiring oil, weather, abrasion, or electrical resistance or combinations of these properties, such as wire and cable, hose, belts, molded and extruded goods, soles and heels, and adhesives.

Nitrile Rubber (NBR)
Much of the basic pioneering research on emulsion polymerization systems was with nitriletype rubbers. These rubbers, first commercialized as the German Buna N types in 1930, are copolymers of acrylonitrile and a diene, usually butadiene.

Both sulfur and nonsulfur vulcanizing agents may be used to cure these rubbers. Carbon black or other reinforcing agents are necessary to obtain the optimum properties. If proper processing methods are followed, the nitrile rubbers can be blended with natural rubber, polysulfide rubbers, and various resins to provide characteristics such as increased tensile strength, better solvent resistance, and improved weathering resistance. The nitrile rubbers have outstanding oil, grease, and solvent resistance. Consequently, the commercial usage of these rubbers is largely for items in which these properties are essential. Another major usage is the utilization of the latex form for adhesives and for the finishing of leather, impregnation of paper, and the manufacture of nonwoven fabrics.

cis-1,4-Polyisoprene (IR)
In 1954, synthetic cis-1,4-polyisoprene was made from isoprene with two different classes of catalysts. The first class includes lithium and the lithium alkyls. The second class uses a mixture of an aluminum alkyl and titanium tetrachloride, the system first used for the low-pressure polymerization of ethylene. Both catalyst system polymerizations are carried out in hydrocarbon solution and require highly puri- fied monomer and solvent. Traces of air, moisture, and most polar compounds adversely affect reaction rates, polymer properties, and structure.

The cis-1,4 polymer structure obtained with these catalysts is also characteristic of natural Hevea rubber. The presence of high cis content appears necessary for the desirable physical properties with Hevea in contrast to the inferior properties of emulsion-type polyisoprene, which contains mixed cis-, trans-, 1,2-, and 3,4- isoprene units.

This polymer and the corresponding butadiene polymer discussed above are called stereorubbers because of their preparation with stereospecific catalysts. The emulsion polymers are formed by a free-radical mechanism that does not permit control of the molecular structure. Stereorubbers are formed by anionic mechanisms that permit nearly complete control of the structure of the growing polymer chain in stereoregular fashion.

The type of catalyst employed influences the structure of the polymer. The aluminum– titanium–catalyzed rubbers exhibit, on stretching, a gradual crystallization somewhat slower than natural, and they are readily processed because the molecular weight range tends to be less than that of natural rubber. The lithium-catalyzed rubbers contain about 93% cis-1,4 structure; they exhibit very little tendency to crystallize and cannot be processed satisfactorily until they have undergone substantial mastication to reduce their very high molecular weight values to a level more nearly resembling that of well masticated natural rubber. The differences in structure influence the properties of these rubbers. The aluminum–titanium- catalyzed rubber, because it crystallizes more readily, exhibits better hot tensile strength. The lithium-catalyzed polymer, as it is of higher molecular weight, exhibits higher resilience and less heat generation. Both types, when compounded and cured, produce physical properties that closely approach, or are equivalent to, natural rubber.

Tire tests with heavy-duty tires for trucks, buses, and airplanes have shown that with respect to wear and heat buildup the isoprene rubbers are comparable to natural rubber during tire operation. Polyisoprene rubber has passed qualification tests in high-speed jet aircraft tires to withstand landing speeds as high as 250 mi/h (112 m/s).

Other Rubbers
There are other specialty rubbers available that are important because of specific properties, but in the aggregate they make up only approximately 2% of the world production of synthetic rubbers.

Silicone Rubbers
Silicone rubber is a linear condensation polymer based on dimethyl siloxane. In the preparation, dimethyl dichlorosilane is hydrolyzed to form dimethyl silanol, which is then condensed to dimethyl siloxane, and this, upon further condensation, yields dimethyl polysiloxane, the standard silicone rubber.

Various types of silicone rubbers are produced by substituting some of the methyl groups in the polymer with other groups such as phenyl or vinyl. Advantages of this type of substitution are evidenced by improvements in specific properties. For example, the presence of phenyl groups in the polymer chain gives further improvement in low-temperature properties. Fluorine-containing side groups improve chemical resistance. Many types are commercially available, ranging from fluid liquids to tough solids.

Because sulfur is not effective for the vulcanization of most silicone rubbers, a strong oxidizing agent, such as benzoyl peroxide, is used; the cross-linking produced is random. Although the standard silicone rubbers are not reinforced by carbon black, the physical properties can be improved by the incorporation of various inorganic fillers such as titania, zinc oxide, iron oxide, and silica, which act as reinforcing and modifying agents. The physical, chemical, and electrical properties can be altered by varying the type and amount of these fillers. Carbon black can be used as a filler with vinyl-containing polymers.

In general, the silicone rubbers have relatively poor physical properties and are difficult to process. However, they are the most stable of rubbers and are capable of remaining flexible over a temperature range of –90 to 316°C. They are unaffected by ozone, are resistant to hot oils, and have excellent electrical properties. Their most extensive uses are for wire and cable insulation, tubing, packings, and gaskets in aerospace and aircraft applications. In the form of dispersions and pastes, they are used for dipcoating, spraying, brushing, and spreading. Silicone rubbers are important in medical and surgical devices because of their property, unique among elastomers, of being compatible with body tissues. Fast, automatic, economical injection molding of liquid silicones has been developed.

Hypalon
Hypalon is the Du Pont trade name for a family of chlorosulfonated polyethylenes prepared by treating polyethylene with a mixture of chlorine and sulfur dioxide, whereby a few scattered chlorine and sulfonyl chloride groups are introduced into the polyethylene chain. By this treatment, polyethylene is converted to a rubberlike material in which the undesirable degree of crystallinity is destroyed but other desirable properties of polyethylene are retained. The outstanding chemical stability of Hypalon results from the complete absence of unsaturation in the polymer chain.

Vulcanization is accomplished by means of metallic oxides, such as litharge, magnesia, or red lead, in the presence of an accelerator as a stock in itself. Hypalon can be blended with other types of rubbers to provide a wide range of properties.

Hypalon has extreme resistance to ozone. Its chemical resistance to strong chemicals, such as nitric acid, hydrogen peroxide, and strong bleaching agents, is superior to any of the commonly used rubbers. These vulcanizates also have good heat resistance, mechanical properties, and unlimited colorability. Typical applications include white sidewall tires and a variety of sealing, waterproofing, insulating, and molded items.

Epichlorohydrin Elastomers
These elastomers are polymers of epichlorohydrin. The two main types of epichlorohydrin elastomers are the homopolymer (CO) and the copolymer (ECO) of epichlorohydrin and ethylene oxide. The copolymer has a lower brittle point, better resilience, and a lower specific gravity than the homopolymer, but is not as good in high-temperature properties. Since the backbone of the molecule is saturated, these elastomers cannot be vulcanized with sulfur. Cross-linking is achieved by reaction of the chloromethyl side group with diamines or thioureas. A metal oxide is also required.

They exhibit outstanding ease of processing; extreme impermeability to gases; moderate tensile strength and elongation; high modulus; good abrasion resistance; good heat aging; excellent resistance to hot oil, water, perchloroethylene, acids, bases, and ozone; good lowtemperature properties; and electrical properties ranging between those of a poor insulator and a good conductor, depending on compounding. These properties make the epichlorohydrin elastomers useful in gaskets for oil field specialties, diaphragms, and pump and valve parts; hose for low-temperature flexibility, oil and fuel resistance, or gas impermeability applications, and mechanical goods such as belting, wire, and cable.

Thermoplastic Elastomers
Proper choice of catalyst and order of procedure in polymerization have led to development of thermoplastic elastomers. The leading commercial types are styrene block copolymers with a structure that, unlike the random distribution of monomer units in conventional polymers, consists of polystyrene segments or blocks connected by rubbery polymers such as polybutadiene, polyisoprene, or ethylene-butylene polymer. These types can be designated SBS, SIS, and SEBS, respectively. Other types of thermoplastic elastomers, such as the polyester type, have also been developed. SBS, SIS, and SEBS polymers, when heated to a temperature above the softening point of the styrene blocks, can be processed and shaped like thermoplastics but, when cooled, act like vulcanized rubber, with the styrene blocks serving as cross-links. Thus, it is unnecessary to vulcanize. Also scrap or excess material can be reused.

Thermoplastic elastomers are very useful in providing a fast and economical method of producing a variety of products, including molded goods, toys, sporting goods, and footwear. One of the disadvantages for many applications is the low softening point of the thermoplastic elastomers. Attempts to modify the composition and structure to raise the softening temperature have generally resulted in inferior rubbery properties.

Fluoroelastomers
These are basically copolymers of vinylidene fluoride and hexafluoropropylene. Because of their fluorine content, they are the most chemically resistant of the elastomers and also have good properties under extremes of temperature conditions. They are useful in the aircraft, automotive, and industrial areas. Polyurethane Elastomers

Polyurethane elastomers are of interest because of their versatility and variety of properties and uses. They can be used as liquids or solids in a number of manufacturing methods. The largest use has been for making foam for upholstery and bedding, but difficulties have been encountered in public transportation due to flammability.

Polysulfide Rubbers
These rubbers have a large amount of sulfur in the main polymer chain and are therefore very chemically resistant, particularly to oils and solvents. They are used in such applications as putties, caulks, and hose for paint spray, gasoline, and fuel.

Polyacrylate Rubbers
Polyacrylate rubbers are useful because of their resistance to oils at high temperatures, including sulfur-bearing extreme-pressure lubricants.

Products
A wide variety of products have a rubber, or elastomer, as an essential component. Most rubber products contain a significant amount of nonrubber materials, used to impart processing, performance, or cost advantages. The automotive industry is the biggest consumer of rubber products. About 60% of all rubber used goes into the production of passenger, bus, truck, and off-the-road tires. In addition, a typical automobile contains about 68 kg of rubber products such as belts, hose, and cushions. Outside the automotive area, a wide variety of rubber products are produced, including such familiar items as rubber bands, gloves, and shoe soles.

The formulation and manufacture of rubber products is as diverse as the wide variety of applications for which they are intended This discussion is limited to the more common methods of manufacture and those that apply to the widest variety of products. In general, the manufacture of rubber products involves compounding, mixing, processing, building or assembly, and vulcanization.

Compounding
 The technical process of determining what the formulation components of a rubber product should be is called compounding. This term is also applied to the actual process of weighing out the individual components in preparing for mixing.

Rubber is the backbone of any rubber product. Natural rubber, obtained from the latex of rubber trees, accounts for about 23% of all rubber consumed in the United States. The balance is synthetic rubber. Of the synthetic rubbers, SBR (styrene–butadiene rubber) and polybutadene rubber are most important, accounting for 71% of synthetic rubber production. A variety of specialty synthetic rubbers, such as butyl, EPDM, polychloroprene, nitrile, and silicone, account for the balance of synthetic rubber production.

Choice of the rubber to be used depends on cost and performance requirements. The specialty rubbers often give superior performance properties but do so at higher product cost. Many rubber products contain less than 50% by weight of rubber. The balance is a selection of fillers, extenders, and processing or protective coatings.

In products other than tires, clays may be added as extenders, silicas as reinforcing agents, and plasticizers for flex or fire-retardant properties; colors or brightening agents may also be used.

Mixing
This step accomplishes an intimate and homogeneous mix of the formulation components. Most large-volume stocks are mixed in internal mixers; these operate with two winged rotors with the compound ingredients forced into the rotors by an external ram. There are thousands of different recipes, each designed for different purposes and each requiring a mixing procedure of its own. In addition, various manufacturers differ in their ideas as to precisely how the mixing operation should be conducted, and the equipment in various plants differs widely. The trend is toward more automation in this process, both in weighing and changing the ingredients into the mixer and in handling the stock after discharge. Forming

Usually, forming operations involve either extrusion into the desired shape or calendering to sheet the material to some specified gauge or to apply a sheet of the material to a fabric.

Building operations, varying from simple to complex, are required for products such as tires, shoes, fuel cells, press rolls, conveyor belts, and life rafts. These products may be built by combining stocks of different compositions or by combining rubber stocks with other construction materials such as textile cords, woven fabric, or metal. For a few products no building operations are required. For example, many molded products are made by extruding the rubber, cutting it into lengths, and placing them in the mold cavities. Also, extruded products that are given their final shape by the die used in the extruder are ready for the final step of vulcanization, with no intermediate building operations.

The building operations for tires vary widely, depending on the kind of tire to be built and the equipment available in the particular plant. For example, the cord reinforcement may be of polyester, nylon, or steel wire; the number of plies required increases as the size and service requirements increase; the tread proper may be of a different composition than the tread base and sidewalls; one of the sidewalls may be made of a white stock; the construction may be of the tubeless variety, in which case an air-barrier ply of rubber is applied to the inside surface of the first ply; a puncture-sealing layer may be applied to the inside surface; or there may be several bead assemblies instead of only one.

Increasing popularity of radial tires has caused some basic changes in tire manufacturing operations. In a radial tire, the reinforcement cords in the body, or carcass, of the tire are placed in a radial direction from bead to bead. Radial tires require special building drums and result in a green tire of a different appearance from the barrel-shaped green biasply tire.

Vulcanization
 The final process, vulcanization, follows the building operation or, if no building operations are involved, the forming operation. Vulcanization is the process that converts the essentially plastic, raw rubber mixture to an elastic state. It is normally accomplished by applying heat for a specified time at the desired level. The most common methods for vulcanization are carried out in molds held closed by hydraulic presses and heated by contact with steam-heated platens, which are a part of the press in open steam in an autoclave; under water maintained at a pressure higher than that of saturated steam at the desired temperature; in air chambers in which hot air is circulated over the product; or by various combinations of these methods.

The vast majority of products are sulfurcured; that is, sulfur cross-links join the rubber chairs together. For special applications, vulcanization may take place without the use of sulfur, for example, in resin- or peroxide-initiated crosslinks or metal-oxide-cured polychloroprene.

The time and temperature required for vulcanization of a particular product may be varied over a wide range by proper selection of the vulcanizing system. The usual practice is to use as fast a system as can be tolerated by the processing steps through which the material will pass without “scorching,” that is, without premature vulcanization caused by heat during these processing steps. Rapid vulcanization affects economies by producing the largest volume of goods possible from the available equipment. This is particularly the case for products made in molds, because molds are costly, and their output is determined by the number of heats that can be made per day.

The rate of vulcanization increases exponentially with an increase in temperature; hence, the tendency is to vulcanize at the highest temperature possible. In practice, this is limited by many factors, and the practical curing temperature range is 127 to 171°C. There are numerous exceptions both below and above this range, but it probably covers 95% of the products made.

Finishing operations following vulcanization include removal of mold flash, sometimes cutting or punching to size, cleaning, inspection for defects, addition of fittings such as valves or couplings, painting or varnishing, and packing. Latex Technology

In addition to the technology of solid rubber, there is a completely different, relatively small but important part of the rubber industry involving the manufacture of products directly from natural rubber latex from the tree, synthetic latex from emulsion polymerization, or aqueous dispersions made from solid rubbers. In latex technology, materials to be added to the rubber are colloidally dispersed in water and mixed into the latex, a process involving the use of lighter equipment and less power than the mixing of solid rubber compounds. The latex compound can then be used in a variety of processes such as coating or impregnating of cords, fabrics, or paper; in paints or adhesives; molding (such as in toys); dipping (for thin articles like balloons, or household and surgeon’s gloves); rubber thread (for garments); and production of foam. Latex technology is particularly important in producing articles for medical and surgical uses.