Silicone

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Not to be confused with Silicon.

Silicones (more accurately called polymerized siloxanes or polysiloxanes) are inorganic-organic polymers with the chemical formula [R₂SiO]_n, where R = organic groups such as methyl, ethyl, and phenyl. These materials consist of an inorganic silicon-oxygen backbone (...-Si-O-Si-O-Si-O-...) with organic side groups attached to the silicon atoms, which are four-coordinate. In some cases organic side groups can be used to link two or more of these -Si-O- backbones together. By varying the -Si-O- chain lengths, side groups, and crosslinking, silicones can be synthesized with a wide variety of properties and compositions. They can vary in consistency from liquid to gel to rubber to hard plastic. The most common type is linear polydimethylsiloxane or PDMS (a silicone oil). The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.

Silicone caulking can be used as a basic sealant against water and air penetration.	Chemical structure of polydimethylsiloxane (PDMS).	A hypothetical silicone group. Such species do not exist, see text.	Self-leveling silicone firestop system used around copper pipe through-penetrations in a two-hour fire-resistance rated concrete floor assembly.
Self-levelling silicone firestop installation in mechanical service penetration in 2 hour rated concrete floor.	Faulty Sakno Silicone Foam Firestop Installation in Calgary Sewage Treatment Plant in the 1980's, to seal opening above a fire door in a cast concrete fire separation.	Silicone "foamfixer" pump used to apply silicone foam firestop materials.	Silicon dioxide ingredient used in the manufacture of all silicones. This is what remains when one burns silicone. If one burns silicone caulking or foam, see Gerald W. Brown, the white powder inside the charred remains is silica fume.

1 Chemical terminology
2 Uses of Silicone
3 See also
4 External links

[edit] Chemical terminology

Silicone is often mistakenly referred to as "silicon". Although silicones contain silicon atoms, they are not made up exclusively of silicon, and have completely different physical characteristics from elemental silicon.

The word "silicone" is derived from ketone. Dimethylsilicone and dimethyl ketone (a.k.a. acetone) have analogous formulas, thus it was surmised incorrectly that they have analogous structures. The same terminology is used for compounds such as silane (an analogue of methane). A true silicone group with a double bond between oxygen and silicon does not exist (see figure). Polysiloxanes are called "silicone" due to early mistaken assumptions about their structure.

[edit] Uses of Silicone

[edit] Sealants in building construction and maintenance

One-part silicone sealants are in common use to seal gaps, joints and crevices in buildings. Silicones are abundantly available for this purpose, in professional, as well as retail grades. One-part silicones cure by absorbing atmospheric moisture, which helps in the professional installation. To make a very smooth silicone seal, apart from masking the edges with tape, if practical, professional caulkers often wet wooden spoons and assorted, hand-crafted wooden tools, which they soak in water with diluted dishsoap. The silicone will not stick to wet, soapy wood, which makes this ideal for creating a perfectly smooth surface. Do-it-yourselfers typically use a moistened finger to trace neat beads into silicone caulking.

Similar methods work for urethane caulking, against which silicones compete quite heavily. White silicones frequently turn slightly yellow over time.

The strength and reliability of silicone rubber is widely acknowledged in the construction industry. Automotive body manufacturing plants and paint shops must avoid the presence of all silicones, as a mere hint of its presence in any form can cause severe failures in automotive paints. Vendors and contractors in such plants are often requested to verify in writing that they will not bring any silicones into the plant.

In the plumbing and automotive fields, silicone grease is often used as a lubricant. Automotive spark plug wires are often insulated by multiple layers of silicone. Such construction prevents spark from jumping to an adjacent wire, causing a misfire. It also minimizes RFI, which can interfere with an engine management computer. In plumbing, the grease is typically applied to O-rings in faucets and valves. In the automotive field, silicone grease is typically used as a lubricant for brake components since it is stable at high temperatures, is not water-soluble, and is far less likely than other lubricants to foul brake pads.

[edit] Cooking applications

Silicone is also impregnated into parchment paper and used as a non-stick material for applications such as baking and steaming. The silicone also makes the paper heat- and grease-resistant. This allows the paper to line cookie sheets and act as a replacement for greasing, thereby speeding mass production of baked goods. It is also commonly used in pouch cooking, where ingredients are sealed into a container made of parchment paper and allowed to steam.

Silicone rubber is also used to make utensils (notably spatulas) and bakeware.

[edit] Electronic components

Electronic components are sometimes protected from environmental influence by enclosing them in silicone. This increases the stability against mechanical shock, radiation, vibration and especially the electrical insulation. Silicone are selected over polyurethane or epoxy encapsulation when a wide operating temperature range is required (-150 to 600 F). Silicones also have the advantage of little exothermic heat rise during cure, low toxicity, good electrical properties and high purity. Therefore it is used when durability and high performance are demanded of components under hard conditions, as in space (satellite technology).Silicones, however are relatively expensive and can be attacked by solvents. (See Resin casting for how silicones can be dispensed in circuit board production).

[edit] Menstrual cups

A Menstrual cup is a type of cup or barrier worn inside the vagina during menstruation to collect menstrual fluid. Menstrual cups are often made of silicone for its durability and reusability.

[edit] Silicone breast implants

Controversy developed in the 1980s and 1990s around claims that the silicone gel in breast implants was responsible for a number of systemic health problems, including autoimmune diseases and cancer. Multiple lawsuits claiming injury from implants resulted in the 1998 bankruptcy of Dow Corning and a moratorium on the use of silicone implants for breast augmentation in the US and Canada pending study. However, multiple studies and expert review panels performed worldwide since then have consistantly concluded that women with silicone breast implants are no more likely to develop systemic illness than women without breast implants. In 2006 both Health Canada and the US FDA adopted positions similar to other countries in permitting the use of silicone implants for cosmetic breast augmentation in their respective countries.

[edit] Firestops

Silicone foams have been used in North American as well as the Israeli Dimona nuclear reactor buildings in an attempt to firestop openings within fire-resistance rated wall and floor assemblies to prevent the spread of flames and smoke from one room to another. (The Israelis wisely switched to the somewhat more expensive yet much safer "elastomer" version of this product, which avoids most safety concerns associated with the foamed version.) Silicone foam firestops have been the subject of serious controversy and press attention due to lack of proper bounding and smoke development due to the pyrolysis of combustible components within the foam, hydrogen gas escape, shrinkage and cracking. These problems have been exposed by whistleblower Gerald W. Brown and have led to a large number or reportable events among licensees (operators of nuclear power plants) of the Nuclear Regulatory Commission (NRC). When properly installed, silicone foam firestops can be fabricated for building code compliance. Advantages include flexibility and high dielectric strength. Disadvantages include poor bounding, combustibility (hard to extinguish) and significant smoke development.