Silicone Elastomer Developments 1967–1977

Abstract

Abstract Silicone rubber for purposes of this review will cover materials based on the polymer polydimethylsiloxane (PDMS). Of necessity, groups other than methyl as substituents on silicon will be included, and changes in the backbone structure from that of pure siloxane will be discussed. Rubbers in which polydimethylsiloxane is only a minor constituent will not be a part of this paper. The definition of what constitutes a rubber becomes blurred as one considers elastoplastic materials and rubbery resins. In this review, a rubber is a material whose properties and function when cured depend principally upon an elastic response to stress. Curing means that a network of crosslinked polymer is established by any one of a number of different vulcanization reactions. Silicone rubber is unique in the large number of choices available for forming crosslinks. A crosslinked network of silicone rubber polymers is relatively weak; hence reinforcement by small-particle active fillers, such as silicas, is essential. The volume of literature, even in this specialized elastomer field, is large. This decade, centered on a date roughly 30 years after work began in this field, shows a high level of research and development activity. Computer searches of literature are common and, depending on the specific profile, the references emerging number around 2500. Clearly, even to annotate such a bibliography is beyond the scope of this paper. This review is aimed at a discussion of the significant developments of this past decade. Of necessity, this requires a judgment of the significance of each publication, a decision which could be debated. Many references will be omitted from consideration, but those wishing a more complete bibliography can obtain it from any of a number of commercial computer searches. Reviews of silicone rubber began to appear only 8 years after work began. Many of these were aimed at acquainting engineers with the properties of these newer rubbers and were not comprehensive reviews. Two reviews did cover the field effectively. Lewis gave a summary of the first 19 years of work and showed the state of the art at that time. Hunter gave a complete statement of the history of steps taken to improve the strength of silicone rubber. Other reviews have appeared in the decade of this review. Many of these, particularly in eastern European countries, are aimed at acquainting users with the properties and technology of silicone rubbers. Lewis updated his earlier review as a segment of a book. A number of reviews of specialized areas have appeared—e.g., encapsulating and potting, adhesives, radiation chemistry, biomedical applications, and elastoplastics. Another segment of a book by Bobear covered some of the same areas as Lewis but included applications of the elastomers. Polmanteer reviewed the chemistry and phenomenological behavior of silicone rubber. A specialized review of only patents in the field of silicones contains a segment on silicone rubber. The coverage is from 1970 forward to publication in 1977, and as a result fewer patents are mentioned than are covered in the present review. This review will attempt to cover the significant developments during the past decade in all of the above areas.