Synthetic rubber mechanism

When the physical modification method is used, PS is modified by mechanical stirring with various synthetic rubbers such as polybutadiene, polybutadiene styrene, polyisopropene, polychloropropene, polybutadiene styrene-acrylonitrile copolymers. In the chemical modification, PS is modified with polyfunctional modificators in the presence of cationic catalysis. 

Synthetic rubbers (elastomers) are long-chain polymers with special chemical and physical as well as mechanical properties. These materials have chemical stability, high abrasion resistance, strength, and good dimensional stability. Many of these properties are imparted to the... 

Pressing is carried out within a cemented carbide die between two steel or cemented carbide punches. In order to impart enough mechanical strength to the blank to permit further manipulation without risk, removable organic binders (camphor, natural or synthetic waxes, latex or synthetic rubber, methyl polymethacrylate, polyvinyl alcohol, carboxymethylcellulose, ammonium alginate) are mixed into the powder, dissolved in a convenient volatile solvent. Some of these also act as lubricants thus minimizing the wear on the die. 

When many molecules combine the macromolecule is termed a polymer. Polymerization can be initiated by ionic or free-radical mechanisms to produce molecules of very high molecular weight. Examples are the formation of PVC (polyvinyl chloride) from vinyl chloride (the monomer), polyethylene from ethylene, or SBR synthetic rubber from styrene and butadiene. 

The Polystyrene Family. After 1945 there was a drastic drop in the demand for styrene monomer because the need for synthetic rubber suddenly declined. On the other hand, a big demand existed for goods in the civilian sector and this provided a strong impetus to the expansion of polystyrene. In many cases, however, the mechanical strength of polystyrene was inadequate and this initiated numerous research efforts, especially in the immediate postwar period. 

Conjugated dienes such as 1,3-butadiene very readily polymerize free radically. The important thing to remember here is that there are double bonds still present in the polymer. This is especially important in the case of elastomers (synthetic rubbers) because some cross-linking with disulfide bridges (vulcanization) can occur in the finished polymer at the allylic sites still present to provide elastic properties to the overall polymers. Vulcanization will be discussed in detail in Chapter 18, Section 3. The mechanism shown in Fig. 14.3 demonstrates only the 1,4-addition of butadiene for simplicity. 1,2-Addition also occurs, and the double bonds may be cis or trans in their stereochemistry. Only with the metal complex... 

Several flexible polymers, such as natural rubber (NR) synthetic rubber (SR) polyalkyl acrylates copolymers of acrylonitrile, butadiene, and styrene, (ABS) and polyvinyl alkyl ethers, have been used to improve the impact resistance of PS and PVC. PS and copolymers of ethylene and propylene have been used to increase the ductility of polyphenylene oxide (PPO) and nylon 66, respectively. The mechanical properties of several other engineering plastics have been improved by blending them with thermoplastics. 

Phillips Petroleum in the United States [23] developed a propellant composed of ammonium nitrate as oxidant and rubber as a combustible and binding agent. The rubber consists of synthetic rubber and such typical rubber ingredients as carbon black (to improve the mechanical properties of rubber), an accelerator and an inhibitor (to prevent oxidation). To endow the rubber with sufficient plasticity... 

The most important contribution in the field of simultaneous dehydrogenation, condensation, and dehydration made by Russian chemists is the synthesis of butadiene from ethanol over a double oxide catalyst by the method of Lebedev. Much has been published on this process. Lebedev s interest in rubber synthesis began with his researches on conversions of dienes in 1908 and his method of synthesis of butadiene was reported in 1927. An experimental synthetic rubber plant was founded for research in the field and the studies on the mechanism of formation of butadiene and of polymerization were continued after Lebedev s death by his students (103,104,105,188,190,378). A survey of the properties and methods of preparation of butadiene was published by Petrov (289). 

The major general purpose rubbers are natural rubber, styrene-butadiene rubber, butadiene rubber, isoprene rubber, and ethylene-propylene rubber. These rubbers are used in tires, mechanical goods, and similar applications. Specialty elastomers provide unique properties such as oil resistance or extreme heat stability. Although this differentiation is rather arbitrary, it tends also to classify the polymers according to volumes used. Styrene-butadiene rubber, butadiene rubber, and ethylene-propylene rubber account for 78 percent of all synthetic rubber consumed. 

Wallace Carothers will be the subject of one of our Polymer Milestones when we discuss nylon in Chapter 3. Among his many accomplishments in the late 1920s and early 1930s, Carothers and his coworkers made a major contribution to the discovery and eventual production of the synthetic rubber, polychloroprene. It was synthesized from the diene monomer, chloroprene, CH2=CCI-CH=CHr Chloroprene, which is a very reactive monomer—it spontaneously polymerizes in the absence of inhibitors— was a product of some classic studies on acetylene chemistry performed by Carothers and coworkers at that time. In common with butadiene and iso-prene, in free radical polymerization chloroprene is incorporated into the growing chain as a number of different structural isomers. Elastomeric materials having very different physical and mechanical properties can be made by simply varying the polym-... 

Wallace H. Carothers, a brilliant organic chemist who was principally responsible for the development of nylon and the first synthetic rubber (Neoprene), was born in 1896 in Burlington, Iowa. As a youth, Carothers was fascinated by tools and mechanical devices and spent many hours experimenting. In 1915 he entered Tarkio College in Missouri. Carothers so excelled in chemistry that even before his graduation, he was made a chemistry instructor. 

Emulsion polymerization first gained industrial importance during World War II when a crash research program in the United States resulted in the production of styrene-co-butadiene [SBR] synthetic rubber. The Harkins-Smith-Ewart model [5-6] summarized the results of early research, which focussed on this and similar systems. Current thinking is not entirely in accord with this mechanism. It is still worthwhile to review it very brielly here, however, because it is still widely referenced in the technical literature and because some aspects of the model provide valuable insights into operating procedures. 

Copolymers may be produced by step reaction or by chain reaction polymerization in similar mechanisms to those of homopolymerization. The most widely used synthetic rubber (SBR) is a copolymer of styrene (S) and butadiene (B). Also, ABS, a widely used plastic, is a copolymer or blend of polymers of acrylonitrile, butadiene, and styrene. A special... 

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cycliza-tion to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create in-termolecular cross-links that render the exposed areas insoluble. 

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