Introduction to Elastomers

BY.DR HINA ADNANDNT 476

These are used where a high degree of accuracy is needed, especially in crown and bridge work. They have two main advantages over the Hydrocolloids - good tear resistance and dimensional stability. They are mainly hydrophobic rubber based materials. All of these materials come in different viscosity's ranging from low to high viscosity. The light bodied material maybe used as a wash impression over a medium or heavy-bodied material. There are two ways this can be carried out as described below.

ONE STAGE IMPRESSIONLight bodied impression material is placed in a syringe, and placed over the areas where high detail is required (e.g. over a crown preparation). Some is then squirted over the heavy-bodied impression material which has been loaded into an impression tray. The impression is then taken as normal. This technique saves time

TWO STAGE IMPRESSIONAn impression is taken with the heavy-bodied material. This is then removed from the mouth and inspected. The light bodied material is then prepared and again placed in a syringe. This is then squirted over heavy-bodied material and then impression relocated in its original impression.

Polysulphides

Used for crown and bridge work mainly, but also used for partial dentures, over dentures and implants. Two equal lengths are mixed together with a spatula for about a minute. The tray needs to be treated with an adhesive (rubber solution in acetone) to provide retention for the polysulphide. Taking the impression is delayed by 5 minutes before the impression is placed in the patients mouth - the final setting time is usually about 10 minutes from the start of mixing - this delay therefore decreases the amount of time the impression tray is in the patients mouth. A one or two stage impression technique may be used. Although dimensionally stable, the impression should be cast within 24 hours.

Polysulphides

Supplied as two pastes mixed in a 1:1 ratio.

BASE PASTE•Polysulphide (forms rubber on polymerization)•Filler (to give body)•Plasticizers (control viscosity)ACTIVATOR PASTE•Inert oil (forms a paste)•Sulphur (facilitates the reaction)•Lead oxide (causes polymerization and cross-linking)

PROPERTIES

Dimensional stabilityExcellent surface detail (is only used in special trays)Viscosity depends on the brand usedVery small setting contraction (0.3-0.4% over the first 24 hrs)Contraction on cooling from mouth to room temperatureVery good tear resistanceGood shelf lifeViscoelastic

ADVANTAGES

Dimensional stabilityAccuracyComes in a number of different viscosity'sLong working time (although this may be a disadvantage in some clinical situations)Long shelf life

DISADVANTAGES

Lead oxide in base paste may have toxic effectsStaining of clothes due to the Lead oxideMessy to work with - unpleasant rubbery smellCan only be used in a special tray

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 color.

Condensation Silicones

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

Polyethers

Used for crown and bridge work, partial dentures, implants and over dentures. Mixed in a 1:1 ratio until homogeneous colour, the amount of catalyst used can be used to control the setting time. Used in special or stock trays with an adhesive. A one or two stage technique can be used. Although dimensionally stable the impression should be cast within 24 hours.

THE END