Silicone | Definition, Composition, Properties, Uses, & Facts

silicone, 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. 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).

Composition, structure, and properties

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. 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 (CH2), methyl (CH3), or phenyl (C6H5), are attached to each silicon atom. A general formula for silicones is (R2SiO)x, where R can be any one of a variety of organic groups.

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major industrial polymers: Polysiloxanes (silicones)

The most common silicone compound, poly-dimethylsiloxane, can illustrate the central characteristics of the class. The starting material is metallic silicon, which is obtained from silica sand. Silicon is reacted with methyl chloride (CH3Cl) over a copper catalyst, forming dimethyldichlorosilane ([CH3]2Si[Cl]2). By reacting this compound with water, the chlorine atoms are replaced by hydroxyl (OH) groups. The resultant unstable compound, silanol ([CH3]2Si[OH]2), polymerizes in a condensation reaction, the single-unit molecules linking together to form poly-dimethylsiloxane with concomitant loss of water. The dimethylsiloxane repeating unit of the polymer has the following structure:

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. In addition, the Si-O bond is highly heat-resistant and is not readily attacked by oxygen or ozone. 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. 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.

Applications

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. Nonvulcanized, low-molecular-weight polysiloxane fluids are exceptionally stable to decomposition by heat, water, or oxidizing agents and are good electrical insulators. They make excellent lubricants and hydraulic fluids, as well as emulsions for imparting water repellency to textiles, paper, and other materials. Silicone resins are used in protective coatings and electrically insulating varnishes and for laminating glass cloth.

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 rubbers are usually strengthened by fillers such as silica; other fillers are mixed in to add bulk and colour. Valued for their electrical-insulating properties, chemical stability, and the wide temperature range over which they retain resiliency, silicone rubbers are used mainly in O-rings, heat-resistant seals, caulks, gaskets, electrical insulators, flexible molds, and (owing to their chemical inertness) surgical implants.

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History

Siloxanes were first characterized as polymers by the English chemist Frederic Stanley Kipping in 1927. Because Kipping thought that the structure of the repeating unit was essentially that of a ketone (the polymer chains formed by silicon atoms, with oxygen atoms attached by double bonds), he incorrectly called them silicones, a name that has persisted. In 1940 American chemist Eugene George Rochow at the General Electric Company laboratories in Schenectady, N.Y., U.S., prepared methyl siloxanes by the process that remains the basis of modern polymerization methods. Meanwhile, researchers at Corning Glass were exploring the production of silicones, and in 1943 Corning and the Dow Chemical Company formed the Dow Corning Corporation to produce silicone products.

The Editors of Encyclopaedia Britannica

This article was most recently revised and updated by Adam Augustyn

Silicones are a diverse family of synthetic polymers made up of Siloxane repeating units. Siloxane is a monomer that consists of silicon, oxygen, carbon, and hydrogen atoms. Silicones are colorless and rubber-like substances. They have a wide range of applications due to their unique combination of properties, including flexibility, heat resistance, low toxicity, and water repellency.

In this article, we will learn what are silicones, their sources, structure, formula, preparation, and applications along with their environmental implications.

What Are Silicones?

Silicones are Polymers with siloxane as a monomer. It is a chain of hydrogen, carbon, and silicon atoms that alternately combine with oxygen and silicon atoms. Silicone is also known as polysiloxane which is a polymer made up of siloxane. Silicones are used in countless applications that promote safety and well-being in daily life. They are widely utilized in a variety of industrial and consumer applications.

Sources of Silicone

The main sources of silicone are

• Natural – Origin Silica Sone (SiO

  2

  )

• Water

• Natural Gas – Drived Methanol

From these materials, through complex chemical reactions, silicone is synthesized.

Silicones Structure

Silicones are a family of synthetic polymers that are composed of repeating units of silicon, oxygen, carbon, and hydrogen atoms. The basic structure of silicones consists of a chain of alternating silicon and oxygen atoms, with organic groups attached to the silicon atoms. The most common organic groups used in silicones are methyl (CH₃) and phenyl (C₆H₅).

Silicone Formula

General formula of silicone is (R2SiO)x, where R belongs to any one of the organic groups.

Silicones are polymeric organosilicon compounds having Si – O – Si linkages. These compounds have the general formula (R2SiO)x where R is alkyl or aryl group. They may be linear, cyclic or cross-linked polymers.

Properties of Silicones

The properties of silicones are mentioned below:

• Silicones are chemically reactive and have poor thermal conductivity.

• It possesses strong ozone, UV, and oxygen resistance.

• It possesses both conductive and electrically insulative qualities.

• Silicones are known for their flexibility over a wide range of temperatures.

• Silicones are hydrophobic, meaning they repel water.

Preparation of Silicones

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The most common method of making silicones is to reduce silicon dioxide (silica) in the form of sand with carbon at high temperatures:

SiO2(s) + 2C(s) → Si(s) + 2CO(g)

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Production Process of Silicones

The production of silicones involves several steps, from the extraction of raw materials to the synthesis of silicone polymers. Here’s a general overview of the silicone production process:

Step 1: Separate Silicon from Silica

• Natural compounds like silica are found all over the world. Almost every rock, clay, and sand on earth contains silica, which makes up over a quarter of the planet’s crust.

• In order to produce silicone, silicon must first be isolated from silica. In order to accomplish this, a large volume of quartz sand is heated up to 1800 degrees Celsius. As a result, pure, isolated silicon is obtained. In order to make fine powder, manufacturers grind it after it cools.

Step 2: Combine Powder With Methyl Chloride

• Mixing silicon powder with methyl chloride is the next step. Fluidized bed reactors are commonly used for this purpose.

• Methyl chlorosilane is formed, which contains atoms of chlorine as well as silicon.

• Dimethyldichlorosilane is the primary building block of silicone, and methyl chlorosilane consists of several compounds.

Step 3: Distil the Mixture

• Distillation process used to separate methyl chlorosilane’s components.

• The compounds contained in methyl chlorosilane can be used to make a variety of silicones.

• As each of these has a different boiling point, they can be separated by distillation, applying a different precise temperature to extract which one.

Step 4: Addition of Water

• Adding dimethyldichlorosilane to water causes hydrochloric acid and disilanol to separate after distillation.

• When the hydrochloric acid is applied, the disilanol is catalysed into polydimethylsiloxane.

• This also allows any air bubbles trapped in the solution to be released.

Step 5: Polymerization of the Silicone

• The polydimethylsiloxane molecule contains siloxane bonds.

• Depending on the properties of the final product, silicone can be polymerized in different ways before being poured into a silicone mold.

Compounds of Silicones

Silicones can exist in various forms, and their chemical structure depends on the specific type and intended application. Here are some common compounds and types of silicones:

1. Siloxanes:

   The fundamental components of silicones are siloxanes. They are composed of silicon and oxygen atoms arranged in an alternating pattern, with organic groups (like methyl, ethyl, or phenyl) joined to the silicon atoms. A siloxane’s general formula is R

   2

   SiO, where R stands for an organic group.

2. Polydimethylsiloxane (PDMS):

   PDMS is one of the most widely used silicone compounds. It is a linear or branched polymer made up of repeating units of dimethylsiloxane (CH

   3

   )

   2

   SiO.

3. Silicone Resins:

   Silicone resins are three-dimensional networks of siloxane units, forming highly cross-linked structures.

4. Silicone Gels:

   Silicone gels are formed by cross-linking silicone fluids, creating a three-dimensional network with a gel-like consistency.

5. High Consistency Silicone Rubber (HCR):

   HCR is a type of silicone rubber with a high molecular weight, providing a thicker and more solid consistency.

Applications of Silicones

1. Consumer Products:

   Silicones are widely used in personal care products such as shampoos, conditioners, and cosmetics.

2. Industrial Lubricants:

   Due to their lubricating properties, silicones are used as lubricants in machinery.

3. Medical Applications:

   Medical-grade silicones are used in various healthcare products, including implants, catheters, and medical tubing.

4. Electronics:

   Silicones are used in electronics for their insulating and protective properties.

5. Cookware:

   Silicone serves as an insulator in products like heat-resistant potholders.

Environmental Effects of Silicones

While silicones are generally considered safe and inert, their environmental effects have been a topic of discussion. Here are some key points regarding the environmental effects of silicones:

1. Persistence:

   Silicones are known for their stability and resistance to biodegradation. This persistence in the environment can lead to concerns about their long-term impact.

2. Bioaccumulation:

   Silicones are generally considered to have low potential for bioaccumulation in living organisms.

3. Aquatic Environment:

   The release of silicone-based products into water bodies, such as rivers or oceans, could potentially have environmental consequences.

4. Waste Management:

   Disposal of silicone-containing products at the end of their life cycle can contribute to landfill waste.

5. Recyclability:

   Some silicones can be recycled, while others may be more challenging to process in recycling facilities.

Also, Check

Silicones – FAQs

1. What are Silicones?

Silicones are synthetic polymers made from silicon, oxygen, carbon, and hydrogen atoms.

2. What are the Main Types of Silicones?

The main types of silicones include polydimethylsiloxane (PDMS), which is a methyl silicone, and phenyl silicones, which contain phenyl groups in addition to methyl groups.

3. What makes Silicones different from other Polymers?

The property of silicones such as flexibility, heat resistance, low toxicity, and water repellency make it different from other polymers.

4. What are the Common Applications of Silicones?

Common uses of silicones include personal care products (cosmetics, shampoos), industrial lubricants, medical devices and electronics.

5. Do Silicones have Environmental Implications?

Silicones are generally inert and stable but they do have certain environmental implications.

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