IMPRESSION MATERIALS USED IN ORTHODONTICS

Heavy bodied

COMPOSITION

 

     BASE PASTE     FUNCTION
Polysulfide polymer – 80-85% This is further polymerized and cross linked to form rubber
Filler (Titanium dioxide zinc sulfate) – 16-18% To provide required strength
Plasticizer (Dibutyl Phthalate) To provide appropriate viscosity
Small quantity of sulphur – 0.5% To accelerate the reaction

 

Accelerator paste

Lead dioxide – 60-68% To react with thiol group causing setting and give brown colour
Filler and plasticizer To provide strength and to give adequate viscosity
Oleic / stearic acid Act as retarder to control rate of setting reaction

 

CHEMISTRY AND SETTING REACTION

The main component of polysulfide is a multifunctional mercaptan (-SH) or polysulfide polymer. These linear polymer contains pendent –SH groups

Reaction yields water as by product

PROPERTIES

Unpleasant odour and colour

It has a long Setting time of 8 to 12minutes

Mixing time is 45 seconds

Excellent reproduction of surface detail

Dimensional stability:

The curing shrinkage is high (0.45%)

Has the highest permanent deformation (3.5%)

This improves with time.  So pouring of model should be delayed by half an hour

Tear strength is 4000 gms/cm2

It has good flexibility.  A 2mm spacing in the tray is sufficient to make the impression.

It is hydrophobic.  So impression area should be dry

Shelf life is good i.e. 2 years

SILICONE RUBBER IMPRESSION MATERIALS

            Types:

            Two types of silicone impression materials are available based on the type of polymerization reaction occurring during setting

Condensation silicone

Addition silicone

MODE OF SUPPLY

Available as base and catalyst in paste form. The condensation silicone catalyst can also be supplied as a liquid. The putty is supplied in jars.

Condensation silicone is available in three viscosities.

Light bodied

Medium bodied

Putty

CONDENSATION SILICONES

BASE PASTE

Hydroxyl terminated polydimethyl siloxane Undergoes cross linking to form rubber
Colloidal silica Filler