IMPRESSION MATERIALS USED IN ORTHODONTICS

REACTOR PASTE

Tetraethyl orthosilicate Cross linking agent
Stannous octate Catalyst

 

CHEMISTRY AND SETTING REACTION

It is a condensation reaction

Polymerization occurs as a result of cross linkage between orthoethyl silicate and terminal hydroxyl group of dimethyl siloxane to form a three dimensional network

Stannous octate acts as a catalyst

Ethyl alcohol is by product. Its evaporation probably accounts for much of the condensation taking place in set silicone impression

     PROPERTIES

Pleasant colour and odour

Setting time is 6-8 minutes, mixing time is 45 seconds

Excellent reproduction of surface details

Dimensional stability is low due to high curing shrinkage (0.4-0.6%) and shrinkage due to evaporation of ethyl alcohol. To avoid this cast should be poured immediately.

Tear strength is 3000gm/cm2

It is hydrophobic.  So impression field should be dry

Can be electroplated with silver or copper

Evaporation of alcohol can lead to dimensional instability

Production of hydrogen can lead to pitting of dental stone surfaces.

ADDITION SILICONES

Superior to condensation silicones

Also called polyvinyl silioxane impression materials.

COMPOSITION

            Base:

Polymethyl hydrogen siloxane

Other siloxane prepolymers

Fillers

            Accelerator

Divinylpolydimethyl  siloxane

Other siloxane prepolymers

Fillers

Platinum salt-catalyst

CHEMISTRY AND SETTING REACTION

It is an addition reaction

In this case polymer is terminated with vinyl groups and it is cross linked with silane (hydride group). The reaction is activated by platinum salt

No reaction byproduct develops as long as correct proportions of vinyl siloxane and silane siloxane are maintained

-Sulfur compounds retard the setting of addition silicone.

PROPERTIES

Pleasant colour and odour

Good reproduction of surface details

Setting time is 4-7 minutes and mixing time is 45 seconds

It has the best dimensional stability among all elastomers. It has low curing shrinkage (0.17%) and low permanent deformation (0.05-0.3%)

Good tear strength (3000gm/cm2)

It is hydrophobic with contact angle 80deg. – 100deg. so impression field should be dry

Can be electroplated with copper/silver

Shelf life ranges from 1-2 years

POLYETHER

Mode of Supply:

Available as base and accelerate paste in tubes