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Elastomeric Impression Materials (Desirable Properties (Adequate shelf…
Elastomeric Impression Materials
Desirable Properties
Adequate shelf life
Acceptable odour and taste
Biologically acceptable
Easily manipulated
Suitable working time
Suitable setting time
Readily wets oral tissues
Not affected by oral tissues
Does not release gas during setting
Introduction
Rubber Base (Elastomers)
Polysulfide
Silicone
Condensation
Addition
Polyether
Composition
Base
Polysulphide polymer (80%), titanium oxide or zinc(16%), sulphate, copper carbonate or silica
Catalyst
Lead oxide, dibutyl, sulphur (YUX) and magnesium stearate
Progress of Reaction
EXOTHERMIC REACTION
Lengthening of chains causes increase in viscocity
WATER OR HYDROGEN IS RELEASED AS A BY-RODUCT
Tray Adhesive
Enhances adhesion of impression material to tray
Prevents thermal shrinkage of impression material
Material does not adhere to acrylic trays; either have perforations or use adhesives
Properties
Setting Time
LONG SETTING TIME
Increased temperature decreases setting time
Use of retarder like oleic acid increases setting time
Water decreases setting time
Elastic Properties
Increases with time
HIGH PERMANENT DEFORMATION (2-3%)
Elastic recovery is sufficiently rapid to allow cast to be made soon after impression is made
Dimensional Accuracy and Stability
Good in the first few hours but decreases with time
Very small setting contraction
NOTE
prep needs to be dried beforehand so that the water produced by the reaction does not attract more water. When water is produced as by product and evaporates, it leaves voids in prep and decreases mechanical properties