Article: Silicone

Not to be confused with Silicon.

Silicone caulking can be used as a basic sealant against water and air penetration. Certain types are also used as a topseal in firestop applications, which are subject to stringent bounding. Oxime based, or "neutral cure" silicones are the best and most expensive, whereas acetic acid (vinegar) silicones occupy the lower price range. Silicone caulking cures by absorbing atmospheric moisture and the escape of certain ingredients, which activate crosslinking. First a "skin" is formed and slowly but surely, the paste turns to solid rubber. Upon combusting, a silica and carbon char is left behind. In firestopping, the thin rubber skin made of any elastomers including silicone minimises smoke transfer and serves to hold back the hose-stream test, which is mandatory in the US and well-liked in Canada - irrelevant most anywhere else.

Silicones, 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. The second largest group of silicone materials is based on silicone resins, which are formed by branched and cage-like oligosiloxanes.

Properties

Silicone oils, polymers and cured sealants are odorless and colourless, water resistant, chemical resistant, oxidation resistant, stable at high temperature, and have weak forces of attraction, low surface tension, low freezing points and do not conduct electricity. Silicone cauling is odourless only once cured. Silicones have many uses, such as lubricants, adhesives, sealants, gaskets, breast implants, pressure compensating diaphragms for drip irrigation emitters, dishware, Silly Putty, and many other products. Due to their thermal stability and relatively high melting and boiling points, silicones are often used where organic polymers are not applicable. Their unreactivity generally makes them non-toxic (see below). Simethicone, a silicone-based anti-foaming agent, has remained available as an over-the-counter drug and food additive.

Chemical terminology

A hypothetical silicone group. Such species do not exist, see text.

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

Safety

A controversy developed during the 1990s around allegations that silicone in breast implants was responsible for several diseases and disorders (e.g. scleroderma, arthritis) and chronic fatigue syndrome. Leakage and rupture of silicone from implants does occur, but further study needs to be done of possible risks. For more information, see breast implant.

Firestops

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. This was a winter installation with outdoor temperatures near -40°C. Now and then, the exterior door was left open, which affected ambient indoor temperatures and thus the "snapping" process of the foam, whose density varied widely within the seal. Silicone foam, once mixed, evolves hydrogen gas, which bubbles, thus creating a foam, as the liquids turn to solid rubber.

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. 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, which has led to many reportable events for the US Nuclear Regulatory Commission, to ensure proper installations in facilities under their jurisdiction.

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 can, at times, 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. In plumbing, the grease is typically applied to O-rings in faucets and valves - for example, all official Moen replacement cartridges are pre-lubricated with it. 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 to foul brake pads.

See also

• Firestop
• Passive fire protection
• Bounding
• Negev Nuclear Research Center
• Nuclear Reactor
• Nuclear Regulatory Commission