Silicone

Silicones, also known as polysiloxanes, are a major class of organosilicon polymers characterised by repeating siloxane units, typically represented as –Si–O–Si–, with various organic substituents (R groups) attached to the silicon atoms. They occur as colourless oils, gels, elastomers, or rigid resins and are widely used in industrial, commercial, medical, and domestic applications due to their distinctive chemical and physical properties. Their versatility stems from the ability to vary chain lengths, branching, side-group chemistry, and degrees of crosslinking, giving rise to a broad range of materials such as silicone oils, silicone rubbers, silicone greases, resins, and caulking compounds.

Historical Development

The term silicone was introduced in 1901 by Frederic Kipping, who used it to describe polydiphenylsiloxane by analogy with the structure of ketones. Although he recognised that his compounds were polymeric rather than monomeric ketones, the nomenclature persisted. Subsequent structural elucidation revealed that these substances did not possess true ketone functionalities, making siloxane the more accurate modern term. Despite this, silicone remains firmly embedded in industrial and popular usage.
Significant advances in organosilicon chemistry occurred in the early twentieth century, culminating in the establishment of the silicone industry in the 1930s. James Franklin Hyde, often referred to as the “father of silicones”, developed processes for producing silicone polymers from silicon compounds and pioneered the manufacture of fused quartz, which later proved integral to telecommunications and electronics. His work led to the creation of Dow Corning, a partnership dedicated to the commercial production of silicone-based products.
Early investigations into silicon–oxygen polymerisation included the work of Alfred Stock and Carl Somiesky, who examined the hydrolysis of dichlorosilane and proposed mechanisms leading to polymeric products. Their studies provided the foundation for understanding how organosilicon monomers could react to form higher polysiloxanes over time.

Chemical Structure and Types

The fundamental structural motif of silicones is the siloxane backbone – a repeating chain of alternating silicon and oxygen atoms. Each silicon atom commonly bears two organic groups, such as methyl or phenyl substituents. By varying these groups and the degree of polymerisation, a vast array of silicone materials can be produced.
The two principal categories include:

• Linear polysiloxanes, exemplified by polydimethylsiloxane (PDMS), which is widely used as a silicone oil due to its viscosity stability and low surface tension.
• Branched or cage-like silicone resins, produced from multifunctional siloxane precursors and valued for their hardness, thermal resistance, and film-forming capacity.

Silicones may appear as liquids, gels, elastomers, or rigid plastics depending on molecular weight and crosslink density. Side-group chemistry and network structure also govern properties such as flexibility, thermal response, and chemical reactivity.

Synthesis and Processing

Most commercial silicones originate from dimethyldichlorosilane, synthesised through the direct reaction of silicon with methyl chloride. Hydrolysis of the dichloride yields silanol-terminated oligomers, which subsequently condense to produce linear or cyclic polysiloxanes:

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

Cyclic polysiloxanes form under certain conditions and may be ring-opened to yield linear polymers of controlled molecular weight. Crosslinking agents such as methyltrichlorosilane or methyltrimethoxysilane introduce branching, enabling the formation of silicone resins or elastomeric networks. Conversely, precursors bearing three organic groups serve as chain-terminating species.
For consumer products such as sealants and adhesives, silyl acetates replace silyl chlorides to avoid corrosive by-products. Their hydrolysis produces acetic acid, allowing for slower but safer curing, particularly in domestic caulking applications.

Combustion and Ceramic Conversion

When silicones burn in air, they yield solid silica (silicon dioxide), forming a fine white powder sometimes referred to as silica fume. Under inert conditions, the pyrolysis of polysiloxanes can produce silicon oxycarbide ceramics, a class of high-temperature materials used in advanced engineering applications. By incorporating functional groups such as vinyl or acrylate units, polysiloxanes can be cross-linked into preceramic polymers suitable for additive manufacturing, including stereolithography processes.

Physical and Chemical Properties

Silicones possess a set of highly desirable properties that account for their broad industrial use. Key characteristics include:

• Low thermal conductivity, enhancing their role in insulation.
• High chemical stability and resistance to oxidation, ozone, and ultraviolet radiation.
• Low toxicity, combined with physiological inertness, making them suitable for medical and food-contact applications.
• Wide thermal operating range, with consistent performance across extreme temperatures.
• Hydrophobicity and water-repellent behaviour, allowing the formation of watertight seals.
• Selective adhesion, with strong bonding to glass but limited adhesion to many other substrates.
• Resistance to microbial colonisation, aiding hygienic performance.
• Mechanical flexibility, including crease resistance and resilience.
• Variable electrical properties, enabling both insulation and controlled conductivity depending on formulation.
• High gas permeability, notably oxygen permeability, which is several hundred times that of butyl rubber—useful in specialised medical applications but unsuitable for high-pressure or vacuum sealing.

Specialty rubber sheeting can be manufactured to meet regulatory standards such as FDA compliance, extending silicone use to highly regulated industries requiring strict hygiene.

Applications Across Industries

Silicones have become integral to a wide range of technological and domestic applications. Major sectors include:

• Electrical and electronic engineering, where silicones serve as insulating materials, encapsulants, and protective coatings.
• Household products, including sealants, utensils, and lubricants.
• Automotive systems, such as gaskets, spark-plug wire insulation, brake lubricants, and flexible tubing. While silicones offer excellent thermal and chemical stability, care is taken in automotive paint shops to prevent contamination that could cause surface defects.
• Aerospace engineering, which relies on silicone gaskets, seals, and dampening components. Aerospace-grade silicones withstand large and rapid temperature fluctuations—from ground-level heat to near-freezing temperatures at altitude—while providing airtight sealing and resilience under vibration.
• Medical and dental fields, where silicones are used for implants, tubing, impression materials, and flexible components requiring biocompatibility.
• Textile and paper industries, where silicones provide coatings that enhance durability, water resistance, and finish quality.
• Office equipment and machinery, including keypads and flexible components requiring long-term mechanical stability.