Elastic Impression Materials, Introduction to Elastomers,Silicones

Silicones

The silicone impression materials are classified according to the type of chemical reaction by which they set.

Addition

Condensation

Addition silicones

Can be used as a one or two stage technique. May be used in special or stock trays. The very heavy bodied materials are measured in scoops and are mixed by hand until homogeneous in colour.

Example of an addition silicone 

      An impression taken in Xantopren Green                    Xantopren impression with beading

               Addition silicone impression material being used to take an impression of implants 

Properties of Addition Silicones

CHEMISTRY

These materials are often termed vinyl polysiloxanes. Supplied in 2 pastes or in a gun and cartridge form as light, medium, heavy and very heavy bodied. One paste contains a polydimethylsiloxane polymer in which some methyl groups are replaced by hydrogen. The other paste contains a pre-polymer in which some methyl groups are replaced by vinyl groups, this paste also contains a Chloroplatinic acid catalyst. On mixing, in equal proportions, crosslinking occurs to form a silicone rubber. Setting occurs in about 6-8 minutes.

PROPERTIES

• Good shelf life
• Dimensionally stable
• Moderate tear strength
• Excellent surface detail
• No gas evolution
• Non toxic and non irritant

ADVANTAGES

1. Accurate
2. Ease of use
3. Fast setting
4. Wide range of viscosity's

DISADVANTAGES

1. Hard to mix
2. Sometimes difficult to remove the impression from the mouth
3. Too accurate in some circumstances (cast produced is not sufficiently oversized)

Condensation Silicones

CLINICALLY

Used for crown and bridge work mainly, but also for partial dentures, implants and overdentures. Used in stock trays or special trays. One or two stage impression stage. Although dimensionally stable the impression should be cast within 24 hours.

CHEMISTRY

Supplied as a paste and liquid or two pastes, in light, medium, heavy or very heavy bodied (putty).

BASE PASTE

• Silicone polymer with terminal hydroxy groups
• Filler

CATALYST PASTE

• Crosslinking agent (organohydrogen siloxane)
• Activator (dibutyl-tin dilaurate)

On mixing the two pastes react, cross linking occurs and setting takes about 7 minutes.
The setting reaction is a condensation reaction.
Hydrogen gas is evolved on setting which leads to surface pitting, and a roughened surface to the resulting model.

PROPERTIES

• Hydrophobic
• Hydrogen gas evolution on setting
• Moderate shelf life
• Moderate tear strength
• Good surface detail
• Shrinking of impression over time
• Non toxic and non irritant
• Very elastic (near ideal)

ADVANTAGES

1. Accurate
2. Ease of use
3. Can be used on severe undercuts

DISADVANTAGES

1. Hydrogen evolution
2. Liquid component of paste/liquid system may cause irritation