---
title: "Silicone rubbers: preparation and applications"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:silicone-rubbers"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/silicone-rubbers
site: BCT Engineering
---

# Silicone rubbers: preparation and applications

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2068 Bhadra, 2.5 marks
- Video: [SILICONES ELASTOMERS (RUBBERS) PROPERTIES SYNTHESIS AND USES](https://www.youtube.com/watch?v=hyKMJ9Ut4u8) — UMA BIO-Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/silicone-rubbers
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

Give the preparation and applications of silicone rubbers.

## Short answer

Silicone rubbers are prepared by hydrolysis of dimethyldichlorosilane to polydimethylsiloxane, then vulcanised with peroxide. They maintain elasticity from −50 to 250 °C, used for gaskets, seals, medical tubing, and electrical insulation.

## Answer

#### Preparation
1. Dimethyldichlorosilane is prepared: Si + 2CH₃Cl → (CH₃)₂SiCl₂ (Cu, 300 °C).
2. Hydrolysis and condensation give long linear polydimethylsiloxane chains of very high molecular mass:
   (CH₃)₂SiCl₂ + 2H₂O → (CH₃)₂Si(OH)₂ + 2HCl
   n (CH₃)₂Si(OH)₂ → [–Si(CH₃)₂–O–]ₙ + n H₂O
3. The polymer is mixed with a filler (silica) and vulcanised (cross-linked) by heating with a peroxide such as benzoyl peroxide, giving a flexible rubbery network.

#### Applications
Silicone rubbers keep their elasticity from about –50 °C to 250 °C and resist oxidation, ozone, water and chemicals.
- Gaskets, seals, O-rings and hoses for aircraft and engines.
- Insulation of electrical wires and cables; keypads of electronic devices.
- Medical tubing, catheters, implants; baby bottle nipples.
- Sealants for building and electronics, bakeware and kitchen moulds.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is dimethyldichlorosilane prepared?
   A. SiO2 + NaOH at 25 °C
   B. PCl5 + NH4Cl
   C. Si + 2CH3Cl with Cu catalyst at about 300 °C
   D. Si + H2O at 100 °C
   Answer: C. Direct process: silicon reacts with methyl chloride over copper. (Key point: Starting material)

2. What happens when (CH3)2SiCl2 is hydrolysed?
   A. It gives SiO2 and CH4 only
   B. It forms branched cross-linked silica
   C. It gives phenol and HCl
   D. It gives (CH3)2Si(OH)2, which condenses to linear [–Si(CH3)2–O–]n
   Answer: D. Hydrolysis then water loss condensation gives long polydimethylsiloxane chains. (Key point: Hydrolysis)

3. How is silicone rubber vulcanised?
   A. By heating with a peroxide such as benzoyl peroxide, after adding silica filler
   B. By heating with sulphur at 150 °C
   C. By cooling in water
   D. By adding NaOH
   Answer: A. Peroxide cross-links the chains into a rubbery network. (Key point: Vulcanisation)

4. Silicone rubbers keep their elasticity between about
   A. –200 °C and 20 °C
   B. –50 °C and 250 °C
   C. 0 °C and 50 °C
   D. 100 °C and 600 °C
   Answer: B. They have a wide working range and resist oxidation, ozone, water and chemicals. (Key point: Temperature range)

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