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