SR - past, present and future.

SR - past, present and future

Unlike the other synthetic polymers, synthetic rubber was developed to duplicate an already known material, i.e., natural rubber. The latter was actually discovered in America as a result of Columbus' voyages, but did not enter into industrial technology until the early part of the 19th century, i.e., after the development of vulcanization. Since then there were continuous attempts to synthesize this unique elastic material by chemical methods. Incidentally, the name "rubber" is also unique to the English language, most other languages using some form of the original name ("caa-ochoe") used by the American Indians, and is said to have originated in 1770 with Joseph Priestley, who found this material useful in erasing pencil marks.

Early history

Long before the rise of synthetic polymers, which followed Staudinger's discovery of macromolecules in the 1920s, there were many attempts to synthesize rubber. These started shortly after Williams' classical analysis (ref. 1) of natural rubber in 1860, which showed it to consist of isoprene and its homologs. Both Bouchardat (ref.2) and Tilden (ref.3) then tried to "convert" isoprene into a rubberlike substance, using heat and strong acids, with little success.

Other investigators then tried using other dienes, e.g. butadiene, 2,3-dimethylbutadiene and piperylene, but were no more successful.

The first synthetic rubber to merit industrial production was the famous (or rather infamous!) "methyl rubber" produced in limited tonnage in Germany during the emergency of World War I. This was a polymer of 2,3-dimethylbutadiene produced by thermal polymerization at 30-70 [degrees] C in a process lasting from 2-6 months! This rubber was so bad that it was dropped immediately after the war.

First real synthetic rubber

During the 1920s, Staudinger's classic work on polymers and polymerization led to a more intelligent approach to the problem of synthetic rubber.

Thus, both in Germany and the USSR, the polymerization of butadiene by sodium metal produced the first commercial synthetic rubber, called "Buna" rubber, for obvious reasons. The Russians stayed with sodium polybutadiene for the next two decades, until the end of World War II, while the Germans changed over to the emulsion copolymerization of butadiene and styrene during the 1930s. This was still called "Buna" rubber, although they indicated the comonomer styrene by the name "Buna S". (They also developed nitrile rubber at that time, by the copolymerization of butadiene and acrylonitrile, naming it "Buna N".)

It was Buna S that was the basis for the rise of the giant synthetic rubber industry in the United States during the rubber emergency of World War II, starting with the shutdown of the supply of natural rubber from the Far East after Pearl Harbor.

Prior to that, there was no real interest in general-purpose synthetic rubber in the U.S., the only developments being in special oil-resistant rubbers, such as Thiokol in 1930 and Neoprene in 1932.

The phenomenal rise in production of butadiene-styrene copolymer in the USA between 1942 and 1945 represented a remarkable feat of technology, second only to the development of the atomic bomb, and led to a complete change in the world picture.

At that time, the U.S. was consuming about 1 million tons of natural rubber per annum, or roughly one-half of the world supply. We had a stockpile of about a one year's supply. The Government Synthetic Rubber Program then spent $700 million to build 51 plants to produce the necessary synthetic rubber to meet that demand, starting virtually from scratch. How successful that effort was is shown in table 1 (refs. 4 and 5). Today, this useful tire rubber is the dominant synthetic rubber and is the main component of automobile tires.

At the same time as the above developments in the rise of butadiene-styrene rubber, the Standard Oil Company (ref. 6) also used its contacts with the German company, IG Farbenindustrie, to obtain information leading to the development of butyl rubber. This was the first of the low-unsaturation synthetics so necessary for rubber of superior weathering properties.

Recent developments

The last 25 years witnessed two giant advances in synthetic rubber and rubber technology:

* stereospecific polymerization, and

* block copolymers (thermoplastic elastomers).

Stereospecific polymerization, i.e. the synthesis of such "stereo" rubbers as cis-1,4-polyisoprene (natural rubber) and cis-1,4-polybutadiene, as well as of the ethylene-propylene copolymers (EPDM), was the result of a revolutionary discovery of stereospecific polymerization catalysts by Ziegler in Germany and Natta in Italy. Block copolymers, especially the styrene-butadiene-styrene triblock copolymers which lead to thermoplastic elastomers, arose from the application of "living" anionic polymerization systems. This development opened a whole new area of rubber products, where the rubber is permanently thermoplastic, because it consists of a two-phase material in which a plastic component (e.g., polystyrene) is finely dispersed within the rubber matrix, to which it is chemically bonded, and thus acts as a thermoplastic crosslinking agent. This, then, made injection molding a viable technology in the rubber industry.

Synthetic rubber today

The role of synthetic rubber today is well illustrated by looking at rubber consumption in the USA. We produce a total of about 2 million metric tons of synthetic rubber per annum (of which about 2/3 consists of the two principal tire rubbers, SBR and polybutadiene). In contrast, the U.S. consumption of natural rubber represents only about 1/4 of total rubber use. On a world scale, the ratio of natural to synthetic is somewhat more favorable (about 1/3).

Synthetic rubbers today fall into two broad categories, which govern their technology. These are:

* Rubber vulcanizates, where the original long-chain macromolecules are crosslinked into a permanent network by reaction with a chemical crosslinking agent (vulcanization). These are the descendants of Charles Goodyear's original process of sulfur vulcanization of natural rubber.

* Thermoplastic elastomers, where the rubber chains are bound into a network held together by thermoplastic domains, and which can therefore be molded and remolded by heat.

The rubber vulcanizates

The most recent edition of Rubber Technology (ref. 7) provides a good idea of the large variety of synthetic rubbers available today. These are listed, by type, together with their chemical structure and properties in table 2. No attempt was made to show the various grades available for each elastomer, since these generally comprise large numbers, and are available in the industrial literature.

An examination of table 2 reveals two remarkable features: a) the wide range in properties (and uses) of the various synthetic rubbers, and b) the variety of crosslinking agents required to vulcanize the elastomers.

It is obvious that synthetic rubbers today offer a wide variety of physical and chemical properties not obtainable from natural rubber. Thus the legendary impermeability of butyl rubber to air has revolutionized tire construction. The impressive oil and solvent resistance of nitrile has enabled the use of rubber in many necessary applications, including the ubiquitous gasoline hose at service stations. The silicones, with their unique molecular structure of alternating silicon and oxygen atoms, have enabled the use of rubber over a broad range of temperatures, from sub-zero (refrigerator door gaskets) to oven heat (200 [degrees] C). Then we have the unbelievable fluorocarbon rubbers, with their ability to resist a variety of solvents as well as temperatures as high as 300 [degrees] C (48 hrs.) and 200 [degrees] C for continuous use.

The unique "castable" polyurethanes exhibit amazingly high resistance to abrasion, a highly desirable property for footwear and toys, subjected as these products are to the known destructive propensities of feet and kids, respectively. The highly saturated hydrocarbon rubbers, butyl and EPDM, have brought a new dimension into the long-term aging and weathering of rubber insulation. In fact, EPDM rubber is enjoying a rapid growth as single-ply roll-roofing material, replacing the venerable asphalt-impregnated roofing.

Some of the more recent developments in synthetic rubber have been equally exciting. Thus the hydrogenated nitriles have added the extra aging property of saturated hydrocarbons to the well-known solvent resistance of the nitriles. The propylene oxide rubbers have unusually high resistance over a range of temperatures, even better than natural rubber, as well as good ozone resistance. The polyalkylene sulfides, one of the first synthetic rubbers produced in this country, have outstanding solvent, aging and ozone resistance.

As for the vulcanization process, it is obvious from the list in table 2 that the crosslinking of synthetic rubber chains into networks has gone far beyond the original sulfur-based process pioneered by Charles Goodyear in 1839. This is natural rubber, and can be crosslinked by other agents. Thus the chlorine-containing Neoprene is best crosslinked by the reaction of the allylic chlorine atoms (from 1, 2-enchainment) with metal oxides, such as magnesium or zinc oxide. Diamines can also react with this polymer, as a crosslinker, and the same reaction can be carried out with the chlorine or bromine atoms of the halobutyls. As a matter of fact, one of the methods for making otherwise incompatible blends of butyl with natural rubber or SBR is to use chlorobutyl or bromobutyl in a zinc oxide/sulfur vulcanization system. Diamines are also the crosslinkers for the fluorocarbon rubbers, by reaction with the active fluorine atoms on the polymer chains.

When the polymer chain has no satisfactory reactive points, such as active side-groups or double bonds, the organic peroxides can be relied on to crosslink the chains. Hence peroxide vulcanization is used for silicone rubber and for some of the low-unsaturation hydrocarbon rubbers. This type of crosslinking depends on the ability of the peroxide to abstract hydrogen atoms from hydrocarbons, creating radicals along the polymer chain, and these subsequently couple to form crosslinks. Hence this method is useful when no other reactive groups are available as substituents on the rubber molecule.

The thermoplastic elastomers

This class of synthetic rubbers had its start approximately 25 years ago as a development of anionic "living" polymerization (refs. 8 and 9). The latter refers to the polymerization of monomers, such as butadiene, isoprene, styrene, etc., by organometallic initiators, preferably of the lithium type. These systems are referred to as "living" since each initiator molecule is capable of starting a polymer chain, which then continues to grow as long as any monomer is available. In other words, with suitable precautions, the growing chains never terminate (or "die"). Hence, these are ideal systems for the synthesis of block copolymers, by sequential addition of different monomers.

The type of block copolymers which originally gave rise to the thermoplastic elastomers were the triblock copolymers, polystyrene-polybutadiene-polystyrene (ref. 10), developed by the Shell Chemical Co. under the tradename of "Kraton." A schematic of the molecular structure and morphology of these materials is shown in figure 1. As can be seen, the polystyrene blocks, being incompatible with the polybutadiene, separate out as domains dispersed in and chemically bound to the polybutadiene matrix, thus forming a rubber network held together by thermoplastic polystyrene "crosslinks." It is not surprising, therefore, that such a material exhibits the properties of a vulcanized rubber at moderate temperatures (resistance to flow, good retraction, etc.) but flows freely at elevated temperatures.

The tensile properties of such thermoplastic rubbers are well illustrated in figure 2 and table 3 for a series of styrene-isoprene-styrene triblock copolymers of varying composition and molecular weight (refs. 8 and 9). It can be seen that the modulus of these rubbers is governed mainly by the polystyrene ("filler") content, more or less independently of molecular weight, while the tensile strength is largely independent of both factors, except at very low molecular weight of the polystyrene. What is also notable is the very high strengths obtainable in these materials. This is undoubtedly due to the high regularity of block size obtained in anionic polymerization, leading to a very uniform morphology, which, in turn, results in a uniform network structure. The sharp drop in tensile strength at low polystyrene levels is obviously due to the much greater compatibility of the short polystyrene chains with the polyisoprene, resulting in a homogeneous rather than a two-phase morphology, to the point where a network no longer exists.

The development of these triblock copolymers thus gave rise to a whole new concept of "thermoplastic rubber," where a network of "soft," rubbery chains is held together by a dispersion of a "hard" thermoplastic phase. Since then, this concept has been applied to a variety of block and graft copolymers having different properties tailored to specific uses. For example, although the styrene-diene triblock copolymers can be ideally synthesized by anionic polymerization, they suffer from the poor temperature and solvent resistance of the polystyrene and the poor aging of the polydiene. (This has been somewhat improved by hydrogenation of the polydiene blocks.) Unfortunately, the anionic mechanism of polymerization is limited to very few practical monomers. Hence other approaches have been used.

One highly desirable thermoplastic elastomer (TPE) is, of course, a block (or graft) copolymer of EPDM and polypropylene, which would have the good aging of the EPDM and the good temperature and solvent resistance of the polypropylene "crosslinks." However, it has not been found possible to create such a block copolymer by a "living" polymerization method. Instead, a mixed block and graft system can be made by subjecting a mixture of the two polymers to high shear at elevated temperature, at which point the linear polymer chains are continuously ruptured and recombine into various block and graft copolymers. The morphology of such materials is much more irregular than that of the anionic triblocks so that tensile strength drops, as expected, but the desirable improvement in resistance to temperature and solvents of the "olefinic TPE's" is notable.

Using various polymerization and interpolymer reactions, a constantly increasing array of thermoplastic elastomers has become available during the past decade or so. A list of commercially available types is shown in table 4, but this may already be outdated by the time of publication of this article, since this is a rapidly expanding field! As can be seen, these TPEs have been classified according to the nature of the "soft" and "hard" segments making up their structure, since their thermal and solvent resistance are mostly dependent on the nature of the "hard" blocks, which form the network junctions, and these can be either crystalline or amorphous. No attempt has been made to list all of the products by trade names, but the latter may be found in the appropriate technical publication (refs. 11 and 12).

The future

The art of forecasting may appear to be quite difficult but is actually relatively simple. Since "the future" usually encompasses a period of anywhere from 20 years to centuries, the time of reckoning is quite far away (and often past the lifetime of the prophet)! Hence, if the prophecy is proven completely wrong, no one would remember it anyway. But should it actually be vindicated, there could always be some scholar who may discover it in the literature and loudly marvel at the remarkable powers of the long-gone soothsayer.

In the case of synthetic rubber, it might be best to consider possible future developments separately for the vulcanizates and for the thermoplastic elastomers.

The rubber vulcanizates

On reviewing the array of vulcanizates listed in table 2, one has the feeling that there is now available a variety of polymer chain structures offering a wide range of chemical and physical properties.

It is difficult to see what new types of chain structures could be developed that would offer substantial improvements in properties and still be economically feasible. Short-range improvements in base polymer, compounding and vulcanization can be expected in the immediate future, but revolutionary changes in the distant years are difficult to anticipate.

The most challenging problem in rubber technology is the creation of a rubber which could withstand ultra-high temperatures, i.e., ca. 1000 [degrees] C. This has been a goal of aerospace scientists and engineers for many years. since such a material would be of special importance for use in rocket engines, etc.

It is obvious that organic polymers could not fill this need; the question is whether an "inorganic" polymer could be developed having the necessary rubbery properties. To date a number of inorganic materials have been used for such applications, but these have been rigid ceramics, not rubbers. Hence the solution of this problem lies in the future.

The thermoplastic elastomers

Since this development is so recent, it is not difficult to predict that it has a very promising future. This is especially true because of the unique potential of these materials.

In the first place, they have enabled the use of rapid molding methods, e.g., injection molding, for the manufacture of rubbery products. Secondly, they can be used to prepare a range of materials, from soft to hard rubbers all the way to high-impact plastics.

The combinations of the various polymers shown in table 4 represent only the beginning of a new technology. As a matter of fact, these materials are sometimes referred to as "elastoplastics" instead of thermoplastic rubbers.

The significance of this is that it is leading to the disappearance of the boundary between rubber and plastics, and this is of special relevance to the automotive industry, which is one of the major consumers of these materials.

Both in the short term and from a long-range view, therefore, these materials appear to represent the "wave of the future."

Table : Table 1 - world SR production 1933-45 (thousands of long tons)

Year         USSR             Germany        USA         Canada
           (SK rubber)(a)     (Buna S)     (GR-S)(b)     (GR-S)
1933            2.2              -           -             -
1934            11.1             -           -             -
1935            25.6             -           -             -
1936            44.2             -           -             -
1937            25.0             2.1         -             -
1938            53.0             4.0         -             -
1939            78.5            20.6         -             -
1940             -              37.1         -             -
1941             -              65.9        0.23           -
1942             -              94.2         3.7           -
1943             -              110.6       182.3          2.5
1944             -              97.5        670.3         32.1
1945             c               -          719.4         36.6

(a) Sodium polybutadiene (b) Wartime name for butadiene-styrene copolymer (now SBR) (c) At the conclusion of the war, the USSR was producing about 100,000 tons of SK rubber. [Tabular Data Omitted]

Table : Table 3 - tensile strength of SIS block copolymers

Wt. % styrene     Molecular wt. [(x10.sup.-3])     Tensile strength
                    S        I             S              MPa
    40             21.1     63.4          21.1           31.0
    40             13.7     41.1          13.7           30.6
    30             13.7     63.4          13.7           32.1
    20             13.7    109.4          13.7           27.0
    20              8.4     63.4           8.4           16.0
    19              7.0     60.0           7.0            2.2
    11              5.0     80.0           5.0            0

[Tabular Data Omitted]

Literature cited

[1.] G. Williams, Proc. R. Soc. (London), 10, 516 (1860); J. Chem. Soc., 15, 110 (1862). [2.] G. Bouchardat, Compt. Rend., 89. 1117 (1879). [3.] W.A. Tilden, J. Chem. Soc., 45, 411 (1884). [4.] M. Morton, J. Macromol. Sci., Chem., A15(7), 1289 (1981). [5.] R.F. Dunbrook, in Synthetic Rubber, G.S. Whitby, Ed., Wiley, New York, 1954, Chap. 2. [6.] F.A. Howard, Buna Rubber. The Birth of an Industry, Van Nostrand, New York, 1947. [7.] Rubber Technology. Maurice Morton, Ed., Van Nostrand Reinhold, New York, 1947. [8.] M. Morton, Encyclopedia of Polymer Science and Technology. Vol. 15, John Wiley & Sons, New York, 1971, p. 508. [9.] M. Morton, Anionic Polymerization: Principles and Practice, Academic Press, New York, 1983, Chap. 9 and p. 201. [10.] J.G. Holden and R. Milkovich, Belgian Patent 627,652 (1963); U.S. Patent 3,265,765 (1965). [11.] B.M. Walker, Handbook of Thermoplastic Elastomers. Van Nostrand Reinhold, New York, 1979. [12.] G. Holden, "Thermoplastic Elastomers," Chap. 16 in Rubber Technology. 3rd Edition, M. Morton, Ed., Van Nostrand Reinhold, New York, 1987.