Elastomeric impression materials Flashcards
What are polysulphides also known as?
- First dental elastomers
- Vulcanizing impression materials
- Mercaptan impression materials
- Thiokol
What colour is polysulphide?
Brown
What gives polysulphide its colour?
Lead dioxide polymer
What is the nature of the polymerization reaction of polysulphides?
Exothermic
What is the setting time of polysulphides?
8 - 12 minutes
Are polysulphides comfortable for the patient?
No. Unpleasant odour and colour. It’s messy and stains clothes.
How can you accelerate polysulphide setting time?
Heat and moisture.
Are polysulphides dimensionally stable?
No. Max permanent deformation.
How soon must cast be poured after polysulphide impression is made?
Within 30 minutes to preclude chances of deformation.
What causes dimensional change in polysulphides?
High amount of polymerisation shrinkage
What causes polymerisation shrinkage in polysulphides?
Loss of water, which is the byproduct of the polymerisation reaction.
How can polymerisation shrinkage and permanent deformation be avoided with polysulphides?
- Taking minimum quantity of impression material
2. Using a custom acrylic tray
One redeeming quality of polysulphide impression materials
Shows highest tear strength
What is the relation of polysulphide impression materials with water?
Hydrophobic. Impression area needs to be dried before making an impression with polysulphide.
What, along with high tear strength, makes polysulphides suitable for recording deep undercuts?
High flexibility - the material can be released from the undercut areas with minimum stress.
Biocompatibility of polysulphides.
Good – causes lowest cell death count.
What sets polysulphides apart from all the other impression materials?
They’re the only material that can be electroplated.
What are Room Temperature (RTV) silicones?
Condensation silicone
Which elastomeric impression material has the highest curing shrinkage?
Condensation silicone, because of evaporation of ethyl alcohol byproduct.
How soon must cast be poured after a c-silicone impression is made? Why?
30 minutes - to avoid curing shrinkage.
How is the deformation of condensation silicone compared to polysulphides?
C-silicones show less permanent deformation compared to polysulphides.
Relationship of c-silicones to water.
Hydrophobic.
Flexibility of c-silicones.
HIgher than polysulphides - can be removed from undercuts without distortion.
Shelf life of c-silicones
Limited, because of oxidation of the tin component.
Handling of c-silicones
Latex gloves contraindicated.
PVS Impression material
Addition silicones
Addition silicones, aka
- Pseudoplastic impression materials
- Polyvinyl siloxane
- Vinyl Poly siloxane
What is a big advantage a-silicones have over c-silicones and polysulphides?
No volatile byproduct.
How soon must cast be poured after an a-silicone impression is made?
Casts can be poured after several hours -
- Lowest curing shrinkage
- Lowest permanent deformation
- Best dimensional stability
Relation of PVS to water
Extremely hydrophobic
Setting time of pseudoplastic impression material
Least setting time of all elastomeric materials.
Handling of a-silicone
Latex gloves contraindicated - sulphur present in latex gloves inhibits setting of a-silicones.
Shelf life of PVS
Better than condensation silicones
Disinfection of a-silicones
Immersion in
- 10% hypochlorite
- 2% glutaraldehyde
Issues with a-silicones
Sometimes hydrogen may be evolved and may cause pinpoint voids in stone casts.
Which was the first elastomer to be developed specifically as an impression material?
Polyether
Stiffness of polyether
Stiffest of all elastomers - extremely difficult to remove from undercut areas because of high modulus of elasticity.
Tear strength of polyether
High - second only to polysulphides.
Dimensional stability of polyether
High - cast can be poured after several days and several times without altering dimensional stability.
Biocompatibility of polyether
Not good - highest cell toxicity.
Relationship of polyether with water
Only hydrophilic elastomeric impression material.
