Elastomeric Impression Materials in Dentistry

Elastomeric Impression Materials

Classify elastomeric impression materials. Describe the composition and setting reaction of addition and condensation silicone material. Enumerate differences between them.
Or
Classify elastomeric impression materials. Describe the composition and setting reaction of addition and condensation silicone material.
Answer:

Classification of Elastomeric Impression Material:

1. On the basis of their chemistry

• Polysulfide
• Condensation polymerizing silicones
• Addition polymerizing silicones
• Polyether.

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2. On the basis of their viscosity (ISO 4823: 2015)

• Type 0 — Putt consistency (very heavy)
• Type 1 — Heavy bodied consistency (tray consistency)
• Type 2 — Medium bodied consistency (regular-bodied)
• Type 3 — Light bodied (syringe consistency).

3. On the basis of wettability or contact angle

• Hydrophilic, if their contact angle is from 80°–105°
• Hydrophobic, if their contact angle is from 40°–70°.

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Differences between Condensation and Addition Silicone:

Composition of Addition Silicone:

• Base
  • Poly (methyl hydrogen siloxane)
  • Other siloxane prepolymers
  • Fillers (amorphous silica or fluorocarbon)
  • Palladium-hydrogen absorber
  • Retarders.
• Accelerator
  • Divinyl polysiloxane
  • Other siloxane prepolymers
  • Platinum salt—catalyst
  • Fillers.

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Setting Reaction of Addition Silicone:

It is an additional reaction. In this, the base polymer is terminated with vinyl groups and is cross-linked with silane. The reaction is activated by platinum salt. There are no by-products as long as there is a balance between the vinyl siloxane and silane siloxane. If unbalanced, hydrogen gas is produced causing air bubbles in stone models. To avoid this palladium is added to absorb hydrogen

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Composition of Condensation Silicone:

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Setting Reaction of Condensation Silicone:

It is a condensation reaction. Polymerization occurs as a result of cross-linkage between the ortho-ethyl silicate and the terminal hydroxyl group of the dimethylsiloxane to form a 3D network. Stannous octoate acts as the catalyst. The reaction is exothermic.

The ethyl alcohol formed as a byproduct evaporates gradually from the set rubber leading to shrinkage.