Addition Silicone Impressions In Fixed Prosthodontics

Addition Of Silicone Impressions In Fixed Prosthodontics

Addition Silicone

In addition, silicone is also known as polyvinyl siloxane.

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

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.

Properties of Addition Silicone

• Addition Silicone odor and color is pleasant.
• Addition Silicone can lead to allergic reactions.
• Reproduction of surface details is excellent.
• The setting time is 5 to 9 minutes. The mixing time is 45 seconds.
• Working time is extended by chilling the tubes.
• Dimensional stability is good. Curing shrinkage is low (0.17%) and permanent deformation is very low, i.e. 0.05 to 0.3%.
• Tear strength is good, i.e. 3,000 g/cm.
• Material is hydrophobic so care should be taken while pouring impression.
• Addition Silicone can be electroplated from silver or copper
• Flexibility is low. Extra spacing should be provided in the impression tray. Care should be taken at the time of removing stone cast from the impression to avoid breakage.
• Addition Silicone shelf-life ranges from 1 to 2 years.

Advantages of Addition Silicone

• Most dimensionally stable impression material.
• These impressions can be kept for 7 days after taking an impression.
• They are available in various consistencies so can be used in various clinical situations.

Disadvantages of Addition Silicone

• They are inherent and have a hydrophobic nature.
• A rounded margin with loss of detail in impression is caused by moisture contamination of the prepared tooth.
• Sulfur contamination from latex gloves inhibit setting reaction of additional silicones.
• As residual hydrides in set material react with moisture present in the atmosphere and evolve hydrogen gas, due to this cast cannot be poured readily.
• It should be poured after 30 minutes for gypsum casts and only after 24 hours for epoxy dies.

Manipulation of Addition Silicone

An equal length of base and accelerator is extruded onto the mixing pad side by side without touching. The accelerator paste is then incorporated into the base paste. Mixing is done using a tapered stiff bladed spatula. A streak-free mix is obtained in 45 seconds.