Silicones

Silicones are a versatile group of synthetic polymers composed of repeating siloxane units, which include silicon, oxygen, carbon, and hydrogen atoms. These colourless, rubber-like substances are known for their flexibility, heat resistance, low toxicity, and water repellency. Due to these unique properties, silicones find a broad range of applications in various industries.

1.0What are Silicones?

Silicones are organosilicon polymers characterised by repeating siloxane units. Their production starts with alkyl or aryl-substituted silanes. The presence of nonpolar alkyl groups makes silicones water-repellent. Silicones are essential in numerous applications. They comprise silicon and oxygen atoms in a chain, often with added carbon and hydrogen. They are renowned for their high thermal stability and resistance to oxidation and chemicals. These properties make silicones invaluable as electrical insulators and in various medical procedures.

2.0General Properties of Silicones

Silicones exhibit several unique properties, including:

• Low thermal conductivity and chemical reactivity.
• Minimal toxicity.
• Ability to repel water and create watertight seals.
• High resistance to oxygen, ozone, and ultraviolet (UV) light.
• Both electrically insulative and conductive properties.
• High gas permeability and thermal stability.
• Excellent solvency for organic compounds.
• Silicones are hydrophobic, meaning they repel water.

3.0Structure of Silicones

Silicones differ from many industrial polymers in that their molecular backbone is not composed of carbon, which is a defining feature of organic compounds. This absence of carbon imparts unique properties to silicones.

 However, most silicones incorporate organic groups, such as methyl (CH₃) and vinyl (CH₂), attached to each silicon atom. Known as polymerised siloxanes or polysiloxanes, silicones feature an inorganic silicon-oxygen backbone chain (⋯−Si−O−Si−O−Si−O−⋯) with two organic groups attached to each silicon centre.

The general formula for silicone is (R₂SiO)_X, where R represents any one of the organic groups.

4.0Preparation of Silicon 

The most common method for producing silicones involves reducing silicon dioxide (silica) in the form of sand with carbon at high temperatures:

                        SiO₂(s) + 2C(s) → Si (s) + 2CO (g)

Metallic silicon is derived from silica sand.

• Reaction with Methyl Chloride: Silicon is reacted with methyl chloride (CH₃Cl) in the presence of a copper catalyst to produce dimethyldichlorosilane ([CH₃]₂Si[Cl]₂). This reaction involves:2RCl+Si→R₂SiCl₂

Here, R represents a methyl group (CH₃). This step involves passing methyl chloride vapour over heated silicon at about 550 K with a copper catalyst. The silicon reacts with the methyl chloride to form dimethyldichlorosilane and other chlorosilanes. The process can be represented as:                   

                     Si+2CH₃Cl →  [CH₃]₂ SiCl₂  + 2HCl

• Formation of Silanol: Dimethyldichlorosilane is then hydrolysed by reacting it with water. During this reaction, the chlorine atoms are replaced by hydroxyl groups (OH). This conversion results in the formation of silanol ([CH₃]₂Si[OH]₂):

                      [CH₃]₂ SiCl₂ + 2H₂O → [CH₃]₂Si(OH)₂  + 2HCl

The hydrolysis of dimethyldichlorosilane yields silanol, where each silicon atom is bonded to two hydroxyl groups.

Condensation Reaction: The silanol molecules undergo a condensation reaction, where the hydroxyl groups on adjacent silanol molecules react, releasing water and forming siloxane bonds (Si-O-Si). This process creates a network of linked silanol molecules. The condensation reaction can be expressed as:                      

                     2[CH₃]₂Si(OH)₂  →  [CH₃]₂Si-O-Si[CH₃]₂+2H₂O

This reaction continues, linking more silanol molecules to form a polymer chain.

Formation of Polydimethylsiloxane: As the condensation reaction proceeds, the linked silanol molecules form polydimethylsiloxane. This polymer consists of repeating units of dimethylsiloxane, with the release of water as a byproduct. The general structure of polydimethylsiloxane can be represented as:                       [-Si(CH₃)₂O-]_n      

In the polymerization process, long-chain polydimethylsiloxane is formed, where n denotes the number of repeating units in the polymer chain. This versatile polymer can be utilised in various applications, including silicone oils, rubbers, and resins.

Adjusting the reaction conditions and reactant ratios allows the length and properties of polymer chains to be controlled, creating various silicones tailored for specific applications.

5.0Types of Silicones

Linear Silicones are produced by hydrolysis and subsequent dialkyl or diaryl silicon chloride condensation. 

• Silicone rubbers are interconnected by methylene or similar groups.
• Silicone Resins: These are formed by blending silicones with organic resins, such as acrylic esters.

Cyclic Silicones: These are created by the hydrolysis of R₂SiCl₂.

Cross-Linked Silicones: These are formed through the hydrolysis of RSiCl_3.

6.0Compounds of Silicones

Silicones can take on various forms, with their chemical structure tailored to specific applications. Here are some common silicone compounds and their types:

• Siloxanes: The core building blocks of silicones are siloxanes, which feature a repeating pattern of silicon and oxygen atoms, with organic groups such as methyl, ethyl, or phenyl attached to the silicon. The general formula for a siloxane is R₂SiO, where R represents an organic group.
• Polydimethylsiloxane (PDMS): PDMS is a prevalent silicone compound comprising a linear or branched polymer chain made from repeating dimethylsiloxane units, denoted as (CH₃)₂SiO.
• Silicone Resins: These are highly cross-linked, three-dimensional networks of siloxane units. Extensive cross-linking imparts unique properties such as thermal stability and rigidity to these resins.
• Silicone Gels: Formed by cross-linking silicone fluids, silicone gels create a three-dimensional network that results in a gel-like texture, making them useful in applications requiring cushioning or thickening.
• High Consistency Silicone Rubber (HCR): HCR is a silicone rubber variant characterized by its high molecular weight, resulting in a thicker, more solid consistency suitable for applications requiring durable and robust materials.

7.0Applications of Silicones

• Consumer Products: Silicones enhance texture and performance and are commonly found in personal care items, including shampoos, conditioners, and cosmetics.
• Industrial Lubricants: Silicones are used as lubricants in machinery because they reduce friction and wear.
• Medical Applications: Medical-grade silicones are employed in healthcare products such as implants, catheters, and medical tubing, benefiting from their biocompatibility and flexibility.
• Electronics: In the electronics industry, silicones are valued for their insulating and protective qualities, helping to safeguard components and improve performance.
• Cookware: Silicone is used in cookware, such as heat-resistant potholders and baking mats, because it can withstand high temperatures and has non-stick properties.