Synthetic Rubber and Plastics ERNST A. HAUSER Massachusetts Institute of Technology, Cambridge, Massachusetts
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HAT the recent developments in the field of plasbcs, pamcularly those connected with the production of synthetic rubber, will be forever considered shining examples of chemistry in operation is already an established fact. In the years just prior to our entry into this war we consumed over 700,000 long tons--or about 15,500,000 pounds--of rubber! With the enemy's occupation of Malaya and the Dutch East Indies we lost our main source of supply, since the amount of rubber imported from other areas has always been insignificant. Today we are assured that our production of synthetic rubber will shortly reach and even surpass the above figures. This is a miraculous achievement; a lasting monument to the ingenuity and resourcefulness of our chemists and engineers. It must be realized that it is not the purpose of this paper to give a detailed account of this development because such an undertaking would cover a number of volumes and would have to assume a considerable knowledge of organic chemistry on the part of the reader. In the following discussion it is the intention of the author to give the reader, even if he is not a trained chemist, as simple a picture as possible of the various reactions leading to the production of synthetic rubber and other plastics, and to offer as simple an explanation as possible for the differences in properties of the new synthetic products. This procedure, i t is hoped, will give the reader not only a fundamental idea of what synthetic rubber and other plastics represent, but also make him redly appreciate this tremendous development. Analysis. The first step in synthesizing a substance is to determine its composition, or, as the chemist calls this procedure, to analyze it. As far back as 1860 we knew that rubber could be broken down into a liquid which would boil a t only two-tenths of a degree above the normal temperature of our body. We also have known since then that the molecules of this liquid are composed of 5 carbon and 8 hydrogen atoms. It was named "isoprene." In the chemist's shorthand the formula for isoprene is written C6Hs. Isoprene. If one wanted to show not only the gross composition of the molecule, but also the way the elements are bonded together, one would put i t down as shown in Figure 1. We see from this formula that
every carbon atom (C) has four connections or bonds, whereas the hydrogen atom (H) has only one. The chemist speaks of carbon as being four-valent, and hydrogen, univalent. We also notice that the carbon atoms a t the ends of the molecule are tied to the center carbons by double bonds. Such a double bond is more reactive than a single one. Let us now compare this molecule with one person entering a room filled with people, all of whom have their arms close to their bodies. As long as the room is not overcrowded each will be able to move as freely as he pleases. This picture can well represent the movement of simple molecules, of which a free flowing liquid is composed. Polymerieation. Under certain conditions, as for example the influence of ultraviolet light, hydrochloric acid, and a number of other chemicals called catalysts, the isoprene molecule undergoes a change in the distribution of its bonds without altering its chemical composition (Figure 2). What does this change signify? The end carbon atoms are no longer saturated within the molecule and are looking around to join others which are in a similar condition. Let us assume that some of the people moving around in the aforementioned room have stretched out their arms and four or five people join hands, thus forming short chains. If they try to move through the crowd now they will 6nd it far more difficult than before. Frequent collisions with others will complicate their movement. This picture represents the change which the originally free-flowing isoprene undergoes when some of its molecules start to join upthe liquid becomes more viscous. The chemist using his shorthand way of expressing such a change would simply write (C6HII,),. Using the formulas showing the bond distribution, we would put it as shown in Figure 3. Such reaction is known as polymerization.
If the human chains in the room grow more numerous and longer, the moment will come when the crowd will be so entangled that free movement is impossible. This corresponds, for all intents and purposes, to the state of a solid. Transposing this picture to the isoprene molecule, we 6nally wind up with a snarl of the above-described isoprene chains. We have produced by synthesis a rubber-like solid (Figure 4). Butadiene. Unfortunately isoprene is not available
in sufficiently large quantities on which to base a synthetic rubber industry with a production capacity as large as is now needed. The Russians, who must he considered pioneers in this type of chemical research, as well as the Germans, realized this long ago. During the first World War Germany produced a synthetic rubber starting with butadiene, which is obtainable in large quantities from either petroleum or alcohol or, by more complex chemical reactions, from acetylene gas. Butadiene, which the chemist writes simply as ChHs is a gas a t any temperature above -5°C. It is very similar in its bond structure to isoprene (Figure 5). Comparing this configuration with that of isoprene we need no longer stretch our imaginations to see how such molecules, if brought into an active state, will also join to form thread- or chainlike aggregates. "Polybutadiene" as produced is today best known by the term "Buna rubber." Most of the Russian synthetic rubber production is based thereon. Nwprae. Another chemical compound, development of which originated entirely in this country, is known as "chloroprene" (C4H6Cl)(Figure 6).
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during polymerization to form chainlike structures. This synthetic rubber is known as "Buna S'(Figure 8). Buna N . If we combine butadiene with acrylonitrile we produce a synthetic rubber known as "Perbunan" or "Buna N." This rubber is characterized by its high resistance to solvents such as gasoline (Figure 9). These represent the most important synthetic rubbers now under consideration, with the exception of bntyl rubber and thiokol, which will he discussed later. H
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Vulcanieatwn. Let us go back for a moment to the snarled-up isoprene chains. We grab the two ends on opposite sides of the snarl and pull. If we are clever we will eventually manage to elongate the snarl, hut we will have difficulties in pulling it apart. If we release the snarl i t will remain elongated. The same thing happens if you stretch a piece of natural rubber. However, most of us are familiar only with the ruhber goods we buy in stores, and we all know that if we stretch a ruhber hand and let go, i t snaps back. This rubber band is vulcanized rubber, that is, rubber which has been treated with sulfur. We believe that the sulfur ties the isoprene chains together by forming a link or bridge between them a t the previously discussed reactive spots, namely, the double bonds (Figure 10).
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STYRENE
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FIGURE 7
A comparison with the isoprene molecule shows that the only diierence between these two compounds is the replacement of the methyl (CH1) group with a chlorine atom. Again we can readily see the possibility of producing chain-aggregates. This synthetic rubber is known as "neoprene." Buna S. During recent years somewhat more complicated polymers have been produced for the main purpose of improving certain specific properties. For example, if we take butadieue and styrene (Figure 7), the latter being well known for the production of clear, transparent plastics, we can join these together H
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You see that we need only a few such bridges to make the structure fully coherent. Now the elongated chains will retract as soon as the strain is released. The rubber chemist terms this reaction vulcanization. To speak in terms of the plastic chemist, we have converted the originally thermoplastic or heat-softenable compound into a thermo-set one. AU the synthetic rubbers we have so far mentioned are therefore vulcanizable in accordance with their structural composition. Vislanex. Several years ago it was found that a certain fraction of crude oil, known as "isohuteue" (C4H8)can be polymerized to a rubber-like consistency. The product is known as "vistanex" (Figure 11).
gun blisters of our bombers, vinyl resins, cellulose nitrate, acetate, and casein resins.
Butyl Rubber. As we see, this chain of polyisobutene does not carry any double bond and therefore cannot be changed from its thermoplastic condition to a thermoset one. It is not vulcanizable. American ingenuity overcame this problem by joining isobutene with small amounts of butadiene or isoprene during polymerization. The result was "butyl rubber" (Figure 12). Now the "copolymer" is vulcanizable.
CH=CH~ PARADIVINYL BENZENE
POLYSTYRENE CHAINS LINKED WITH PARA-DIVINYLBENZENE
FIGURE 14
Condensation. The best-known thermosetting plastics are the phenol-formaldehyde resins, the ureaformaldehyde resins, and the melamine-formaldehyde resins. Although no conclusive answer is yet available as to how the reaction between phenol and formaldehyde proceeds until the solid resin generally known as "Bakelite" is formed, it is assumed that we first obtain an intermediate compound which aggregates in three Styrene. In connection with Buna S we have al- dimensions by the splitting out of water. ready mentioned the compound styrene. This, too, A chemical reaction in which two or more molecules is a liquid composed of molecules carrying a short uncombine with the separation of water or some other saturated chain connected with a benzene ring. By simple substances is generally termed condensation. polymerizing such molecules we again form a chain Thiokol. A great number of other thermosetting structure. Eventually we will have produced a solid, resins are the result of such condensation reactions. known as "polystyrene" (Figure 13). However, this is not unique to plastics, since a condensation reaction is also responsible for the formation of the only better known synthetic rubber-like material so far not mentioned. It is known under the trade name "thiokol." If one allows the chemical ethylene dichloride to react with sodium polysulfide, one obtains a chain polymer containing the hydrocarbon radical of the former, and the sulfur of the latter, while sodium chloride is eliminated (Figure 15).
The chains so formed are not interconnected, and therefore the shape of the product can be changed upon the application of heat and strain just like unvulcanized ETHYLENE SWlUM rubber. We have the typical example of a thermoplasTHIOXOL OlCHLORlOC TETR*SVWOE tic resin. However, if we introduce a compound known as paradivinyl benzene into the polymerization, we get an interlinking of chains comparable to the vulcanizaAlthough the reader must realize that the above distion of rubber. We have formed a thermo-set plastic (Figure 14). cussion is all but complete, it should offer a simple Thennobhstic Resins. This shows clearlv the dif- picture of what synthetic rubbers are, what they are ference between what we generally term a thermo-set composed of, why they exhibit properties similar to and a thermoplastic resin. But i t also shows that the natural product, and how and why their developnatural, as well as synthetic rubbers, have close rela- ment is closely related to that of the so-called synthetic tion to what we usually call "plastics." Other thermo- plastics. Also i t is hoped that it will show the complexplastic resins are the acrylic resins, familiar to us in the ity of the problem.
