![]() Vulcanized silicone rubber is prepared in two principal forms: (1) as room-temperature-vulcanizing (RTV) elastomers, which are low-molecular-weight liquids that are cast or molded into desired shapes and then interlinked at room temperature, and (2) high-temperature-vulcanizing (HTV) elastomers, which are higher-molecular-weight gums that are mixed and processed like other elastomers. Silicone resins are used in protective coatings and electrically insulating varnishes and for laminating glass cloth. They make excellent lubricants and hydraulic fluids, as well as emulsions for imparting water repellency to textiles, paper, and other materials. Nonvulcanized, low-molecular-weight polysiloxane fluids are exceptionally stable to decomposition by heat, water, or oxidizing agents and are good electrical insulators. Polysiloxanes are manufactured as fluids, resins, or elastomers, depending on the molecular weight of the polymers and the degree to which the polymer chains are interlinked. On the other hand, the Si-O bond is susceptible to hydrolysis and attack by acids and bases, so silicone plastics and rubbers are relatively weak and readily swollen by hydrocarbon oils. As a result, silicones are remarkably stable, and they have the lowest glass-transition temperature (the temperature below which the molecules are locked in a rigid, glassy state) and the highest permeability to gases of any polymer. In addition, the Si-O bond is highly heat-resistant and is not readily attacked by oxygen or ozone. Siloxane molecules rotate freely around the Si-O bond, so, even with vinyl, methyl, or phenyl groups attached to the silicon atoms, the molecule is highly flexible. The dimethylsiloxane repeating unit of the polymer has the following structure: The resultant unstable compound, silanol ( 2Si 2), polymerizes in a condensation reaction, the single-unit molecules linking together to form poly-dimethylsiloxane with concomitant loss of water. By reacting this compound with water, the chlorine atoms are replaced by hydroxyl (OH) groups. Silicon is reacted with methyl chloride (CH 3Cl) over a copper catalyst, forming dimethyldichlorosilane ( 2Si 2). The starting material is metallic silicon, which is obtained from silica sand. The most common silicone compound, poly-dimethylsiloxane, can illustrate the central characteristics of the class. Major industrial polymers: Polysiloxanes (silicones) A general formula for silicones is (R 2SiO) x, where R can be any one of a variety of organic groups. This lack of carbon in the polymer backbones makes polysiloxanes into unusual “inorganic” polymers-though in most members of the class two organic groups, usually vinyl (CH 2), methyl (CH 3), or phenyl (C 6 H 5), are attached to each silicon atom. The silicones differ from most industrial polymers in that the chains of linked atoms that make up the backbones of their molecules do not contain carbon, the characteristic element of organic compounds. Their chemical inertness, resistance to water and oxidation, and stability at both high and low temperatures have led to a wide range of commercial applications, from lubricating greases to electrical-wire insulation and biomedical implants (such as breast implants). Silicone, also called polysiloxane, any of a diverse class of fluids, resins, or elastomers based on polymerized siloxanes, substances whose molecules consist of chains made of alternating silicon and oxygen atoms. ![]() SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions. ![]()
0 Comments
Leave a Reply. |