6 августа, 2015 
Malyar Buna S (as it was first known) was developed in the late 1930s in Germany as a synthetic natural rubber. When the United States needed a synthetic polymer as a replacement for natural rubber as part of the war effort in the early 1940s, chemists at several rubber companies were familiar with that technology and tried to duplicate it. Attempts at developing an emulsion polymerization process failed until Waldo Semon of BF Goodrich consulted with Victor Mills of Procter & Gamble. Ivory soap (99 and 44/100ths pure!) was being used as an emulsifier for the process and it was determined that the small amount of perfume in the soap (part of the other 56/100ths) was a catalyst contaminant. Once the perfume was removed from the soap, the polymerization process was successfully developed, and styrene-butadiene rubber (SBR), which was then called GRS (General — purpose Rubber — Styrene), became the fastest growing of all synthetic rubbers because of its similarity to natural rubber in processing and performance after cure. More than 50 synthetic rubber plants were built by industry during the war period, under the auspices of the federal government, with the most volume allocated to SBR.
While quite similar to natural rubber, there were differences which prevented really high growth for this copolymer after the war. The molecular weight of SBR from those original processes was only about 100,000 compared to over 1 million for natural rubber. Further, differences in tensile strength, cohesive strength, elongation, hot tear strength, aging, and other properties existed which permitted natural rubber to regain most of its earlier market share. The low cost of SBR, however, made it an excellent blending elastomer. Modifications to the base polymers and continued work with compounding, processing and curing conditions improved the performance of these copolymers. New polymerization processes by the 1950s began to improve many of the original polymer properties in comparison to natural rubber.
Standard SBR materials are made from an emulsion polymerization process and are available in more than 100 grades, but only a few are used as a base for adhesives. The two basic processes for producing these many grades can be either a ‘‘hot’’ or ‘‘cold’’ process, depending on the polymerization temperature, with hot polymerization being the preferred process. Hot polymerized SBR typically yields a lower molecular weight polymer, but with a wider molecular weight distribution which provides for a more balanced polymer. The styrene content can also be varied to enhance certain properties. Emulsion process polymers are often called ‘‘random’’ SBR because there is no control of the attachment sites for the styrene monomer when polymerized. These polymers are often blended with other polymers to lower cost and increase compatibility with various resins, plasticizers, and fillers.
In the mid-1960s, a new form of SBR was introduced called a ‘‘block’’ copolymer. Produced by a solution polymerization process, this material exhibited an ordered molecular structure with the styrene monomer located at the ends of the butadiene monomer chain. In addition, other monomers such as isoprene, ethylene, butylene, and others, could be added to the polymer chain, which further modified basic properties. These materials possess a continuous rubber phase for resilience and toughness, and a discontinuous plastic phase for solubility and thermoplasticity. A variety of different grades are also available for this type of SBR, with differences in molecular weight, differences in the types of monomers used, differences in structural configuration, and differences in the ratio of endblock to midblock. Both emulsion and solution polymerized grades of SBR are available as solvent-based and water-based adhesives and sealants. Block copolymers are extensively used for hot melt formulations and both water-based and solvent-based pressure sensitive adhesive applications. Today, SBR elastomers are the most popular elastomers used for the manufacture of adhesives and sealants.
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