What Are Siloxanes

What Are Siloxanes

Siloxanes are a class of silicone compounds with silicon-oxygen-silicon (Si-O-Si) bonds as their core structure, in which silicon atoms in the molecule are connected by oxygen atoms and bonded with organic groups (e.g., methyl, phenyl, etc.).

Chemical Structure

Core structure: The basic unit of a siloxane is the Si-O-Si bond, and the silicon atom is usually connected to two organic groups (R) to form a linear, cyclic or cross-linked network structure.

Physical and chemical properties

• Thermal stability: stable from -50°C to 250°C, some high-temperature silicone resins are resistant to 500°C or more.

• Hydrophobicity: low surface tension (approx. 20 mN/m), excellent water repellency, used in building sealants and fabric treatments.

• Electrically insulating: low dielectric constant (2.5-3.0), resistant to arc and corona discharge, used for electronic component encapsulation.

• Chemically inert: resistant to acid and alkali, ozone and UV, high physiological compatibility, suitable for medical devices and implants.

Industrial Preparation

• Raw material: Dimethyl dichlorosilane (CH₃)₂SiCl₂ is used as the main material, and mixed cyclosiloxanes (DMC) are generated through hydrolysis, polycondensation and cracking reactions.

• Production process:
  Hydrolysis and condensation: chlorosilanes react with water to form silanols, which are further condensed to form siloxane chains or rings.
  Catalytic polymerization: Use of acidic or basic catalysts (e.g. KOH, H₂SO₄) to regulate molecular weight and cross-linking degree.

• Distillation and purification: Separation of siloxane monomers with different ring numbers (e.g. D3, D4, D5) for specific applications.

Structural Classification

Linear Siloxanes

• Polydimethylsiloxanes (PDMS): Chemical formula [(CH₃)₂SiO]ₙ, linear polymers with a wide range of molecular weights (from low to silicone oils and high to silicone rubber). Used in lubricants, defoamers and cosmetics (e.g., hair conditioners).

• Hydride Siloxanes: contain Si-H bonds and are classified as cross-linkers (e.g. CHT's HYDROSIL® series) and chain extenders used to modulate elastomer hardness and ductility6.

Cyclic Siloxanes

• D3 (Hexamethylcyclotrisiloxane): C₆H₁₈O₃Si₃, CAS 541-05-9, used as structuring control agent for silicone rubber.

• D4 (Octamethylcyclotetrasiloxane): C₈H₂₄O₄Si₄, CAS 556-67-2, restricted by the EU REACH regulation for use in wash-off cosmetics.

• D5 (Decamethylcyclopentasiloxane): C₁₀H₃₀O₅Si₅, CAS 541-02-6, high volatility, widely used in skin care and hair care products.

• D6 (Dodecamethylcyclohexasiloxane): C₁₂H₃₆O₆Si₆, CAS 540-97-6, low bioaccumulation, environmentally friendly option to replace D4/D5.

Branched/Crosslinked

• Silicone Resins: Three-dimensional network structure, high temperature resistant (>500°C), used in electronic encapsulation and fireproof coatings.

• Carborane-Siloxanes: Carborane-containing groups, resistant to radiation and extreme temperatures, used in spacecraft seals.

Function Modification Classification

Organomodified Siloxanes

• Polyether-Modified Siloxanes (Polyether-Modified): such as TEGO® Glide series, containing EO/PO chain segments, used as coating leveling agents and wetting agents.

• Amino Siloxanes: Containing -NH₂ groups, they enhance fiber softness and antistatic properties.

Specialty Siloxanes

• Hydrosiloxanes: Used for cross-linking medical grade silicone products such as catheters and implants.

• Fluorosiloxanes: Containing fluorine groups, they are chemically resistant and are used for corrosion-resistant coatings in semiconductor manufacturing.

Fields of application

Industrial Materials

• Sealants & Lubricants: PDMS for weathering sealants and hydrocosiloxanes for high-temperature lubrication.

• Electronic Packaging: Silicone resins protect circuit boards from thermal stress and moisture.

Consumer Goods

• Cosmetics: D5/D6 is used as a volatile carrier to give skin care products a silky texture.

• Kitchenware: Silicone products contain D3-D6 and need to pass migration tests to ensure safety.

Medical & Environmental

• Biocompatible materials: Cross-linked silicone is used in artificial organs and wound dressings.

• Biomethane purification: Cyclic silicone (D4-D6) needs to be removed from biogas to prevent engine corrosion.

Environmental and Safety Controversies

• D4/D5: classified as PBT (Persistent, Bioaccumulative, Toxic) substances by the EU, restricting their use in personal care products.

• Replacement trend: industry shifting to D6 and modified silicones (e.g. phenylsiloxanes) to reduce ecological risks.