- Asked 2/16
- 2070 Bhadra · 3 marks
- 2068 Magh · 2.5 marks
Short note: Silicones
Short answer
Silicones are polymers with a –Si–O– backbone and methyl side groups, made by hydrolyzing (CH3)2SiCl2 and condensing the silanediol. They are heat-stable (–50 to 250 °C), water-repellent, and electrically insulating. Uses include lubricants, sealants, flexible rubber, and heat-resistant coatings.
Write a short note on silicones.
Silicones are inorganic–organic polymers with a backbone of alternating silicon and oxygen atoms (–Si–O–Si–) and organic groups (usually methyl) attached to silicon, general formula [R₂SiO]ₙ.
Preparation
- Dimethyldichlorosilane is made by passing CH₃Cl over silicon at 300 °C with copper catalyst: Si + 2CH₃Cl → (CH₃)₂SiCl₂
- Hydrolysis: (CH₃)₂SiCl₂ + 2H₂O → (CH₃)₂Si(OH)₂ + 2HCl
- Condensation (loss of water) of the silanediol gives the linear polymer: n (CH₃)₂Si(OH)₂ → [–Si(CH₃)₂–O–]ₙ + n H₂O
Adding a little (CH₃)₃SiCl stops the chain (chain length control); adding CH₃SiCl₃ gives cross-links.
Types
| Type | Structure | Uses |
|---|---|---|
| Silicone oils/fluids | Short linear chains | Lubricants, hydraulic fluids, polishes, anti-foaming agents, cosmetics |
| Silicone greases | Oil + filler (silica) | High and low temperature lubricant, vacuum grease |
| Silicone rubbers | Long chains lightly cross-linked | Gaskets, seals, tubing, medical implants, cable insulation |
| Silicone resins | Highly cross-linked network | Heat-resistant paints, varnishes, electrical insulation, water repellents |
Properties and uses
Silicones are stable to heat (–50 to 250 °C), water repellent, chemically inert, good electrical insulators, and physiologically harmless.
- Lubricants and oils stable over a wide temperature range.
- Water-repellent coating for cloth, glass and building surfaces.
- Electrical insulation (high-temperature) and sealants for electronic devices.
- Rubber for gaskets, tubes, medical implants, keypads and bakeware.
- Cosmetics, polishes, anti-foam and mould-release agents.
✓ Answer checked by Claude Opus 5.5 (high effort)