Elastomers. Chapter HISTORY AND ECONOMICS

Transcription

1 Chapter 18 Elastomers 1. HISTORY AND ECONOMICS Although naturally occurring rubber from the tropical tree has been known for ages, the Spanish navigator and historian Gonzalo Valdez ( ) was the first to describe the rubber balls used by Indians. Natural rubber was brought back to Europe from the Amazon in 1735 by Charles Condamine, a French mathematical geographer, but it remained only a curiosity. Michael Faraday made a rubber hose from it in But it was not until Charles Goodyear discovered vulcanization in 1839 that natural rubber got its first wave of interest. As the story goes, Charles became so involved with his job that he set up a laboratory at home to study the chemistry of rubber. Because his wife hated the odor of his experiments, he could only continue his work at home when she was not around. While studying the effect of sulfur and other additives on the properties of rubber he was interrupted unexpectedly by his wife one day when she returned home early from shopping. He quickly shoved his latest mixture into the oven to hide it. As fate had it, the oven was lit, the rubber was vulcanized, and the modern era of elastomer research was born. His first patent covering this process was issued in Today both natural rubber, an agricultural crop, and synthetic elastomers are multi-billion dollar businesses. Looking back at Fig. 16.1, we see that Synthetic Rubber (NAICS ) totals $5.7 billion. It is a large area of polymer use and is 1% of Chemical Manufacturing. But in the related industry covering final end products called Plastics and Rubber Products

2 Table 18.1 U.S. Production of Synthetic Elastomers, Consumption of Natural Rubber Natural rubber 29% SBR 25 Polybutadiene 17 Ethylene-propylene 10 Nitrile 3 Polychloroprene 2 Miscellaneous 14 Source: Chemical and Engineering News, and Chemical Economics Handbook Manufacturing (NAICS 326), Rubber Products (NAICS 3262) totals $35.3 billion, of which Tires (NAICS 32621) makes up $15.4 billion, showing the dominance of the automobile tire market in this sector of the chemical industry. The top polymer production summary in Table 1.16 gives a numerical list of important synthetic elastomers. Styrene-butadiene rubber (SBR) dominates the list at 1.93 billion Ib for U.S. production. All other synthetic elastomers are much smaller. While elastomers had a slight increase in production from , only 0.5% annually, SBR was down 2.3% per year. From it was up 1.0% per year. The fastest growing elastomer is ethylene-propylene, up 5.2% annually for Table 18.1 gives a breakdown in percent production of synthetic elastomers and consumption of natural rubber in the U.S. 2. NATURAL RUBBER Natural rubber can be found as a colloidal emulsion in a white, milky fluid called latex and is widely distributed in the plant kingdom. The Indians called it "wood tears." It was not until 1770 that Joseph Priestly suggested the word rubber for the substance, since by rubbing on paper it could be used to erase pencil marks, instead of the previously used bread crumbs. At one time 98% of the world's natural rubber came from a tree, Hevea brasiliensis, native to the Amazon Basin of Brazil which grows to the height of 120 ft. Today most natural rubber is produced on plantations in Malaysia, Indonesia, Singapore, Thailand, and Sri Lanka. Other rubber-bearing plants

3 can be cultivated, especially from a guayule shrub, which is now more important than the tree. 3. VULCANIZATION The process that makes the chemistry, properties, and applications of elastomers so different from other polymers is cross-linking with sulfur, commonly called vulcanization. The modern method of cross-linking elastomers involves using a mixture of sulfur and some vulcanization accelerator. Those derived from benzothiazole account for a large part of the market today. Temperatures of O 0 C are typical. SH 2-mercaptobenzothiazole Zinc oxide and certain fatty acids (R COOH) are also added. Although this mechanism is by no means completely understood, it is proposed that the benzothiazole and zinc oxide give a zinc mercaptide, and this forms a soluble complex with the fatty acid. Reaction of this with Sg molecules gives a persulfidic complex (X = benzothiazole).

4 Interchange with the original complex leads to the formation of a mixture of polysulfidic complexes, which are considered to be the active sulfurating species. These complexes then react with the allyl carbons of rubber, the most reactive sites in the polymer. The cross-linking occurs by reactions of the following type, where Usually a mono- or disulfide cross-link occurs but larger numbers of sulfur atoms are possible. If the total percentage of sulfur in the material is <5%, it is usually very elastic. If >5% of sulfur is added, it produces a very hard, dark, nonelastic material called ebonite, sometimes used for things like combs and buttons.

5 4. ACCELERATORS In 1906 Oenslager and Marks at Diamond Rubber Co. (later B. F. Goodrich Co.) began working on accelerators for cross-linking with sulfur. These substances not only increase the rate of vulcanization but create a final product that is more stable and less susceptible to aging. Benzothiazoles now own 22% of the accelerator market, which is about 100 million Ib/yr. Other types of accelerators are sulfenamides (50%), dithiocarbamates (5%), thiurams (4%), and others (19%). The reason for the differences is that some cause very fast vulcanization rates like sulfenamides, and some slower like benzothiazoles. Sulfenamides such as N-cyclohexyl-2-benzothiazolsulfenamide can be made from benzothiazoles by reaction of an amine and an oxidizing agent such as NaOCl, HNO 2, or H 2 O REINFORCING AGENTS Even with vulcanization, however, many elastomers lack the balance of properties required for good wear. Reinforcing agents have been studied to strengthen the rubber mechanically. In 1912 the Diamond Rubber Co. found that addition of carbon to rubber tires caused them to last ten times longer than without this reinforcing agent. Rubber became the substance of choice for automobile tires and conveyor belts. Glass, nylon, polyester, and steel now aid carbon in reinforcement for many applications. Up to 20% of these reinforcing agents can increase the tensile strength of the rubber by 40%. 6. ANTIDEGRADANTS Most polymers are attacked by oxygen, ozone, and ultraviolet light. Rubber is one such polymer that is rapidly degraded in molecular weight and mechanical strength. Over 100 chemicals, collectively called age resistors or age antidegradants, are added to elastomers to keep them from becoming brittle, turning sticky, developing cracks, etc. Most oxidation inhibitors today are either amines, phenols, or phosphites. Phenols were suggested as early as 1870 to combat aging. Amines are now used more than phenols in elastomers. Combinations are often used for heat,

6 oxygen, ozone, UV, and moisture resistance. Two examples of amine age resistors are given here. The market is about 150 million Ib/yr, of which amines are about 60%. The market breakdown for antidegradants is phenylenediamines (50%), phenolics (13%), phosphites (13%), quinolines (10%), diphenylamines (6%), and others (8%). Other kinds of rubber chemicals are blowing agents, peptizers, and retarders. The total market for all chemical additives for rubber is over 250 million Ib/yr. 7. DEVELOPMENT OF SYNTHETIC RUBBER Until the 1930s natural rubber from Hevea brasiliensis was the only available elastomer. The United States had to, and still does, import every pound. Although research on synthetic substitutes began before 1940 in this country, World War II influenced speedy development of substitutes when our supply of natural rubber from the Far East was cut off. Gasoline had to be rationed not because of its shortage, but because of the automobile tire shortage. In 1910 scientists concluded that natural rubber was c/s-l,4-polyisoprene. In 1931 Du Pont introduced the first synthetic elastomer, polychloroprene (Neoprene, Duprene ), and Thiokol Corporation introduced a polysulfide rubber called Thiokol. Polychloroprene, although very expensive compared to polyisoprene, has superior age resistance and chemical inertness. It is also nonflammable. The Government Rubber Reserve Company in the 1940s pioneered the development of styrene-butadiene copolymers, by far the largest volume of synthetic rubber used today. Now usually known as SBR, it has also been called Buna-S, 5wtadiene with a sodium (Nd) catalyst and copolymerized

7 with styrene, or GR-S, Government Rubber tyrene. Although it took many years to develop, it is now the rubber of choice for most applications today, especially automobile tires. Polyisobutylene, commonly called butyl, was first developed in 1937 by Esso Research and Engineering Co. Its main repeating unit is isobutylene but it contains some isoprene for cross-linking. Originally butyl was used for automobile tire inner tubes, where it was replaced in 1955 by tubeless tires. However, most inner tubes still employed today are butyl rubber. In addition to being used for engine mounts and suspension bumpers, it has found large volume uses as liners in reservoirs and in irrigation projects. Hypalon" chlorosulfonated polyethylene was introduced by Du Pont in Although not a high volume rubber it has found use in coatings and hoses. 8. CATALYSTS AND MECHANISMS The mid-1950s saw the first commercial production by Goodrich, Firestone, and Goodyear of polymers with stereochemistry which is consistent or regular. In the early 1950s Karl Ziegler in Germany and Giulio Natta in Italy found catalysts that polymerized olefins with regular configurations. The Ziegler-Natta catalysts were primarily a combination of a transition metal salt (TiCIs or TiCU) and an organometallic compound (EtsAl). By proper manipulation of the ratio of these two substances either cw-1,4- or trans-l,4-po\yisoprene from isoprene can now be prepared. The mechanism of Ziegler-Natta polymerization was given for polypropylene in Chapter 14, Section 2.4. Review this and work through the mechanism with an elastomer monomer such as butadiene. Many of the synthetic elastomers now made are still polymerized by a free radical mechanism. Polychloroprene, polybutadiene, polyisoprene, and styrene-butadiene copolymer are made this way. Initiation by peroxides is common. Many propagation steps create high molecular weight products. Review the mechanism of free radical polymerization of dienes given in Chapter 14, Section 2.2. Butyl rubber, polyisobutylene, is an example of cationic polymerization with an acid. Review Chapter 14, Section 2.3. A small amount of isoprene is added to enable cross-linking during vulcanization through the allylic sites.

8 The more complex structure of this polymer must therefore be 9. SBR VS. NATURAL RUBBER By far the largest selling elastomers are SBR and natural rubber. SBR at 1.93 billion Ib/yr accounts for about 35% of the U.S. synthetic rubber market and 25% of the total rubber market. The U.S. imports about 2.2 billion Ib of natural rubber per year. A distant third is polybutadiene at 1.33 billion Ib. In 1940 natural rubber had 99.6% of the U.S. market. Today it has only 29%. In 1950 synthetic elastomer consumption passed natural rubber use in the U.S. Since then it has been a battle between the leading synthetic, SBR, and the natural product. It is apparent that these two polymers are very important. Table 18.2 summarizes and compares them by their properties. Table 18.2 SBR and Natural Rubber Property Natural SBR Tensile, strength, psi 4,500 3,800 Percent elongation % modulus, psi 2,500 2,500 Temperature range for use, 0 C -60 to to 100 Degree of elasticity Excellent Good Tear resistance Good Moderate Abrasion resistance Moderate Good Age resistance Poor Moderate Solvent resistance Poor Poor Gas impermeability Good Moderate Uniformity Variable Constant Versatility Lower Higher Processibility Easier Harder Tolerance for oil and carbon additives Lower Higher Price stability Bad Good Percent of U.S. market 29 25

9 Natural Rubber Consumption Billions of Pounds SBR Production Year Figure 18.1 U.S. consumption of natural rubber vs. SBR production. (Source: Chemical and Engineering News and Chemical Economic Handbook) The balance between natural rubber and SBR is a delicate one. Natural rubber has made a comeback and reversed its downward trend. Developments of rubber farming have raised the yield from 500 Ib/acre/yr to 2,000-3,000. Petrochemical shortages and price increases have hurt SBR. Finally, the trend toward radial-ply tires, which contain a higher proportion of natural rubber, favors this comeback. Fig 18.1 shows the U.S. natural rubber consumption trends vs. U.S. SBR production, where this "bounceback" of the natural rubber market is very evident from 1980 to the present. The competitive price structure for these two elastomers through the years has been very evident, and their prices are never too far apart. 10. TIRES No discussion of elastomers is complete without a mention of tire technology. About 70% of all synthetic elastomers in the U.S. are used in tires. About 264 million tires are produced in the U.S. annually, 217 million for cars and the rest for trucks and busses. A typical tire is made up of four parts: (1) the tread, which grips the road; (2) the sidewall, which protects the

10 Tread Carcass Sidewall Liner Figure 18.2 Parts of a typical tire. (Source: Wittcoff and Reuben, Industrial Organic Chemicals in Perspective. Part Two: Technology, Formulation, and Use, John Wiley & Sons, Reprinted by permission of John Wiley & Sons, Inc.) sides of the tire; (3) the liner, which prevents air loss; and (4) the carcass, which holds the layers together (Fig. 18.2). The tire is about 50% rubber by weight. Carbon black (as a reinforcing agent), extender oil, and the tire cord in the carcass make up the rest. The cord was rayon for many years. Glass fiber has also been popular. But now nylon, polyester, and steel are the major cord components. Steel became most popular in radial tires of the 1980s and is growing in importance as the primary reinforcing agent. About 75% of car radial tires and 92% of truck radials are steel belted. The tread must have the best possible "grip" to the road. Grip is inversely related to elasticity, and natural rubber has good elasticity but poor grip, so no natural rubber is used in automobile tire treads. Treads are blended of SBR and polybutadiene in an approximate ratio of 3:1. Truck tire treads do have natural rubber, between %, to avoid heat buildup and because grip is not so necessary in heavy trucks. Aircraft tires consist of 100% natural rubber. The carcass requires better flexing properties than the tread and is a blend of natural rubber and SBR, but at least 60% of natural rubber. The sidewalls have a lower percentage of natural rubber, from 0-50%. The liner is made of butyl rubber because of its extreme impermeability to air. The most important single trend in the U.S. tire market is the switch from cross-ply and belted bias-ply to radial-ply tires. Radials held only 8% of the

11 Cross-ply Belted bias-ply Radial Figure 18.3 Types of plies in tires. (Source: Writeoff and Reuben, Industrial Organic Chemicals in Perspective. Part Two: Technology, Formulation, and Use, John Wiley & Sons, Reprinted by permission of John Wiley & Sons, Inc.) U.S. car tire market in 1972, but by 1977 it had grown to 50% and it is now 89%. The difference in the three is shown in Fig A tire carcass contains plies of rubberized fabric. In the cross-ply the cords cross the tire at an angle. In the belted bias-ply the cords cross at an angle and an additional belt of fabric is placed between the plies and the tread. In the radial-ply the cords run straight across the tire and an extra belt of fabric is included. Radial tires have better tread wear average (66,000 miles radial, 40,000 miles bias-ply) and better road-holding ability. However, they are more easily damaged on the sidewall and they give a less comfortable ride. They also require a higher proportion (80% vs. 50%) of the more expensive natural rubber. It seems likely that the popularity of radial-ply tires will continue, and natural rubber consumption may continue its comeback. Today the elastomer can be reclaimed from discarded tires. Over 2 billion are available for recycling. Most reclaiming of the elastomer is done by an alkali process with 5-8% caustic soda and heating. Reclaiming is not profitable unless it costs no more than half as much as pure elastomer, since reclaimed material contains only 50% elastomer hydrocarbon. Approximately 0.66 billion Ib of elastomer is reclaimed each year in the U.S., only about 10% of the total elastomers used. Efforts are also being made to burn discarded tires for fuel to generate electricity, since each tire contains energy equivalent to 2.5 gallons of oil as fuel, enough to heat an average house for a day.

12 11. IMPORTANT ELASTOMERS We will finish this chapter with the following sections that give many of the details for elastomers including chemical structure, manufacturing process, some properties, and main uses. Some familiarity with these elastomers is essential Natural Rubber, NR, cjs-l,4-polyisoprene 1. Manufacture Biological polymerization in rubber tree See Fig for biosynthesis 98% cis configuration, MW = 350, , Properties, see table Uses 76% in tires, other miscellaneous uses 4. Economics Radial tires, favoring natural rubber, gave good growth since Will slow now that radial tires no longer increasing No production in U.S. U.S. consumption from imports at 2.2 billion Ib/yr 11.2 Styrene-Butadiene Rubber, SBR 5 Buna-S, GR-S 1. Manufacture Introduced in 1933 Emulsion and solution polymerization Free radical catalyst at low temperatures 75% Butadiene by weight, 85% butadiene molar

13 1,2- and 1,4-Butadiene units mixed 2. Properties, see Table Uses Tires and tire products, including tread rubber, 77%; mechanical goods, 15%; automotive, 5%; miscellaneous, 3% 4. Economics 2000 Production at 1.93 billion Ib SBR suffering since 1980 Change to radial tires, favoring natural rubber, now complete Replacement automotive parts a growing use Annual change of 1.0% 11.3 Polybutadiene, BR 1. Manufacture Introduced in 1955 Solution and emulsion polymerization Ziegler-Natta catalysis Mostly cis configuration 2. Properties Excellent abrasion resistance Low temperature flexibility Poor traction 3. Uses Tires and treads for automobiles, trucks, and buses, 72%; highimpact resin modification, 25%; industrial products (conveyor belts, hoses, seals, and gaskets) and other applications, 3% 4. Economics 2000 Production at 1.33 billion Ib Tire use expanding Impact modifier for styrene growing

14 11.4 Ethylene-Propylene, EPDM, EPM, EP 1. Manufacture Introduced in 1963 Ziegler-Natta catalysis EP is abbreviation, EPM means ethylene and propylene only, EPDM means ethylene, propylene, and dimer Most, about 85%, of EP is EPDM 55% Ethylene, 40% propylene, 5% dimer for cross-linking 2. Properties Low-temperature flexibility Good age, heat, and abrasion resistance 3. Uses Automotive, 44%; roofing membrane, 18%; oil additive, 10%; wire and cable, 8%; miscellaneous, 20% 4. Economics 2000 Production 0.76 billion Ib Increase 5.2%/yr Fastest growing elastomer Growing markets in automotive, building materials, and petroleum additives 11.5 Butyl Rubber, Polyisobutylene 1. Manufacture Introduced in 1937 Low-temperature solution polymerization

15 Cationic initiation % Isoprene added for cross-linking 2. Properties Low permeability to air and water Weather resistance Noise and vibration resistance 3. Uses Tires, tubes, and other pneumatic products, 83%; automotive mechanical goods, 6%; adhesives, caulks, and sealants, 6%; pharmaceutical uses, 4%; miscellaneous, 1% 11.6 Nitrile Rubber, Poly(butadiene-aciylonitrile), NBR 1. Manufacture Introduced in 1937 Emulsion polymerization Free radical catalyst 10-40% Acrylonitrile 2. Properties Solvent, fat, and oil resistance Wide temperature performance Low coefficient of friction 3. Uses Hose, belting, and cable, 28%; O-rings and seals, 20%; latex, 15%; molded and extruded products, 15%; adhesives and sealants, 10%; sponge, 5%; footwear, 2%; miscellaneous, 5% 11.7 Polychloroprene, CR 1. Manufacture Introduced in 1931 Emulsion polymerization Free radical catalysis Mostly trans configuration 2. Properties

16 Figure 18.4 Research size equipment for ply building in tires to test the usefulness of various fibers as the plies in tires. (Courtesy of Du Pont) Flame, solvent, age, and heat resistance 3. Uses Mechanical rubber goods, 30%; automotive, 30%; adhesives, 20%; construction, 5%; coated fabrics, 5%; miscellaneous, including wire and cable, 10% Suggested Readings Chemical Profiles in Chemical Marketing Reporter, , , , , , and Kent, Riegel's Handbook of Industrial Chemistry, pp Wittcoff and Reuben, Industrial Organic Chemicals in Perspective. Part Two: Technology, Formulation, and Use, pp