Silicone

Silicones are largely inert compounds with a wide variety of forms and uses. Typically heat-resistant, nonstick and rubberlike, they are frequently used in cookware, medical applications, sealants, lubricants and insulation.

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Properties

Some of the most useful properties of silicone include:

1. Thermal stability (constancy of properties over a wide operating range of −100 to 250 °C).
2. Though not lipophilic, the ability to repel water and form watertight seals.
3. Excellent resistance to oxygen, ozone and sunlight.
4. Flexibility.
5. Good electrical insulation.
6. Nonstick.
7. Low chemical reactivity.
8. Low toxicity.
9. High gas permeability: at room temperature (25 °C) the permeability of silicone rubber for gases like oxygen is approximately 400 times that of butyl rubber, making silicone useful for medical applications (though precluding it from applications where gas-tight seals are necessary).

Technical details

More precisely called polymerized siloxanes or polysiloxanes), silicones are mixed 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 siloxane is linear polydimethylsiloxane (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.

Synthesis

Silicones are synthesized from chlorosilanes, tetraethoxysilane, and related compounds. In the case of PDMS, the starting material is dimethylchlorosilane, which reacts with water as follows:

n [Si(CH₃)₂Cl₂] + n [H₂O] → [Si(CH₃)₂O]_n + 2n HCl

During polymerization, this reaction evolves potentially hazardous hydrogen chloride gas. For medical uses, a process was developed where the chlorine atoms in the silane precursor were replaced with acetate groups, so that the reaction product of the final curing process is nontoxic acetic acid (vinegar). As a side effect, the curing process is also much slower in this case. This is the chemistry used in many consumer applications, such as silicone caulk and adhesives.

Silane precursors with more acid-forming groups and fewer methyl groups, such as methyltrichlorosilane, can be used to introduce branches or cross-links in the polymer chain. Ideally, each molecule of such a compound becomes a branch point. This can be used to produce hard silicone resins. Similarly, precursors with three methyl groups can be used to limit molecular weight, since each such molecule has only one reactive site and so forms the end of a siloxane chain.

Modern silicone resins are made with tetraethoxysilane, which reacts in a more mild and controllable manner than chlorosilanes.

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 (see figure) does not exist in nature; chemists find that the silicon atom forms a single bond with each of two oxygen atoms, rather than a double bond to a single atom. Polysiloxanes are called "silicone" due to early mistaken assumptions about their structure.

Uses

Aquarium joints

Aquarium manufacturers have used silicone sealant exclusively from its inception in order to join glass plates, making aquariums of every size and shape.

Glass joints made with silicone sealant can withstand hundreds of metric tons of pressure, making obsolete the original aquarium construction method using angle-iron and putty.

Automotive

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.

Automotive spark plug wires are often insulated by multiple layers of silicone to prevent sparks from jumping to adjacent wires, causing misfires. The insulation also minimizes RFI, which can interfere with an engine management computer.

Automotive body manufacturing plants and paint shops must avoid the presence of all silicones, as they may cause "fish eyes," small, circular craters that appear in the finish.

Cookware

Silicone is becoming an important product in the cookware industry, particularly bakeware.

Dry cleaning

Liquid silicone can be used as a dry cleaning solvent. Touted as an "environmentally friendly" alternative to the traditional perchloroethylene (or perc) solvent, the decamethylpentacyclosiloxane (D5) process has been patented by the company GreenEarth Cleaning.

The solvent degrades into silica and trace amounts of water and CO2, and waste produced from the D5 drycleaning process is nontoxic and nonhazardous. This significantly reduces the environmental impact of a typically high-polluting industry.

Additionally, liquid silicone is chemically inert, meaning it does not react with fabrics or dyes during the cleaning process. This reduces the amount of fading and shrinking that most dry-cleaned garments experience.

Electronic

Electronic components are sometimes protected by enclosing them in silicone to increase stability against mechanical and electrical shock, radiation and vibration.

Silicones are used when durability and high performance are demanded of components under hard conditions, as in space (satellite technology). They 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.

The use of silicones in electronics is not without problems, however. Silicones are relatively expensive and can be attacked by solvents.^[1] Silicone easily migrates as either a liquid or vapor onto other components.

Silicone contamination of electrical switch contacts can lead to failures by causing an increase in contact resistance, often late in the life of the contact, well after any testing is completed.^[2] Use of silicone-based spray products in electronic devices during maintenance or repairs can cause later failures.

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 Israeli installation subsequently switched to an "elastomer" version of this product, which avoids most safety concerns associated with the foamed version.

Silicone foam firestops have been the subject of controversy and press attention due to smoke development from 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 combustibility (hard to extinguish) and significant smoke development.

Haircare products

Silicones are ingredients in some leave-in hair conditioner products. These lessen frizz by using silicone's water resistance to prevent humidity from entering a dry hair shaft.

Medicine

Silicone, particularly the gel form, is used in bandages and dressings, in breast implants and a variety of other medical uses.

Silicone is a common material for use in molds for behind-the-ear style hearing aids.

Menstrual cups

Often made of silicone for its durability and reusability.

Moldmaking

Two-part silicone systems are used to create rubber molds which can be used for production casting of resins, foams, rubber and low-temp alloys.

A mold made of silicone generally requires little or no mold release or surface preparation as most materials do not adhere to moldmaking silicone.

Plumbing and building construction

The strength and reliability of silicone rubber is widely acknowledged in the construction industry.

One-part silicone sealants and caulks are in common use to seal gaps, joints and crevices in buildings. One-part silicones cure by absorbing atmospheric moisture, which helps in the professional installation.

In plumbing, silicone grease is typically applied to O-rings in faucets and valves.

Versatile applications

Silicone Industry Trade Organizations

The leading global manufacturers of silicone base materials belong to three regional organizations: the European Silicone Center(CES) in Brussels, Belgium; the Silicone Environment Health and Safety Council(SEHSC) in Washington, USA; and the Silicone Industry Association of Japan (SIAJ) in Tokyo, Japan. A fourth organization, the Global Silicone Council (GSC) acts as an umbrella structure over the regional organizations. All four are nonprofit making and have no commercial role. Their primary mission is to promote the safety of silicones from a health, safety and environmental perspective. As the European chemical industry is getting prepared to implement the REACH legislation, CES is leading the formation of a consortium^[3] of silicones, silanes and siloxanes producers and importers to facilitate data and cost sharing.

See also

References

1. ^ See Resin casting for how silicones can be dispensed in circuit board production.
2. ^ See chapter 16.4.1 of Electrical Contacts: Principles and Applications By Paul G. Slade (image available online via GoogleBooks). Also, "A Comparison for the Effects of Various Forms of Silicon Contamination on Contact Performance" by Witter, G. (Published in "Components, Hybrids, and Manufacturing Technology, IEEE Transactions on" Mar 1979).
3. ^ REACH consortium