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Silicon, silica, silicone--what's the difference? Silicon is a chemical element, one of the 109 known substances into which all matter can be resolved. Second only to carbon in its presence on earth, one-quarter of the earth's crust is silicon. Carbon is also the only element capable of establishing more compounds than silicon; thus, silicon possesses immense potential for commercial application. One of the premier semi-conducting elements, silicon is used in many electronic devices, such as transistors and computers.
However, one does not find silicon alone in nature. It always exists as silica or silicates. Silica is silicon dioxide (SiO2), commonly found in sand and quartz. A silicate is a compound made of silicon, oxygen, and at least one metal, sometimes with hydrogen, sometimes without it. Talc, all gemstones but diamond, and clays are natural silicates. The most widely recognized synthetic form is sodium silicate, or water glass, a combination of silica with sodium and hydrogen. Capable of ". . . combining chemically with most metallic oxides,"[7] silica is important in both the chemical and industrial realms. It appears in a wide range of products, from glass to cosmetics to the amorphous silica gel we find with our new shoes. (Materials lacking the molecular lattice structure of a solid state are amorphous, for example, all liquids. Thus, an amorphous form of a material possesses the same atomic makeup as the crystalline version, but without a "highly ordered geometry" [8].)
Silicone is a synthetic polymer, or macro-molecule, whose backbone is a repeating chain of Si-O molecules, with various organic groups attached to the silicon. The most common silicone is PDMS, poly-dimethylsiloxane [(CH3)2Si-O), the foundation of all silicones. Silicones have been introduced into many products, from cosmetics to building materials to computers. Commercial silicones are separated into six groups according to the number of repeating units and the number of ties linking these units: ". . . fluids, emulsions, compounds, lubricants, resins, and elastomers."[9] A fluid is characterized by its uniform change in shape or direction when an outside force is applied, so that it includes all liquids. Typically, silicone fluids are straight chains of PDMS ending in trimethylsyl groups, ranging in viscosity from that of water to fluids that cannot be poured. An emulsion is a stable combination of at least two immiscible liquids, one present in the other as droplets, for example, oil in water. A compound in this context is a formulated or compounded mixture. Lubricants are "materials of low viscosity that separate moving solids, used to minimize friction and wear."[10] A silicone resin is a substance which is ". . . non-combustible, electrically nonconductive, hard and glassy when . . . cold, and soft and sticky . . . [above] . . . the glass transition point."[11] Finally , an elastomer describes a synthetic polymer which behaves much like natural rubber, i.e. can be stretched to several times its size and still snap back to its original length.
Silicone gels lie somewhere between a fluid and an elastomer. They are "lightly crosslinked" PDMS molecules, forming a three dimensional array "which . . . [are] . . . swollen with PDMS fluids to give a sticky, cohesive mass without form. . . "[12] and , thus, are more complex than a fluid; however, the gel's composition is less dense than that of the elastomer, which has only minute portions of its fluid not crosslinked. An elastomer surrounds the gel in silicone breast implants.