Elastomers Flashcards
What is the fundamental chemistry behind elastomers, and how does cross-linking affect their properties?
Elastomers are formed by polymerization with cross-linking of polymer chains. Cross-linking:
- Generates elastic properties
- Causes fluid-to-solid transition
- May produce byproducts (H₂O, H₂, alcohol) affecting dimensional stability
What are the key material properties evaluated in BDS2 vs. BDS3?
BDS2:
Surface detail reproduction
Flow/viscosity
Contact angle/wettability
Elastic recovery
Stiffness
Tear strength
Mixing/working time
BDS3 adds:
Shore A hardness
Shark fin test
Setting shrinkage
Dimensional stability
Thermal expansion coefficient
Biocompatibility
What is the shore a hardness and shark fin test?
Shore A Hardness:
Measures the final hardness of set material
Affects ease of removal and handling
Higher values indicate harder materials
Shark Fin Test:
Specialized test measuring flow under pressure
Simulates clinical conditions better than simple flow tests
Longer “fin” indicates better flow under pressure
How do you assess the quality of surface interaction between material and tooth/soft tissue surfaces?
Three key measurements:
Viscosity - must flow readily
Surface wetting - must make intimate contact
Contact angle - determines detail reproduction quality
Each factor must be optimized for successful impression
What determines accuracy?
- surface reproduction (ISO)
- visco-elasticity / elastic recovery
In terms of viscosity, what is its relationship to impression material performance?
Viscosity:
Measures material’s ability to flow
Determines potential for close contact with tissues
Affects quality of surface detail recording
Must be appropriate for specific clinical application
What makes an ideal viscosity for impression materials, and how does it vary by application?
Must be low enough to flow into surface details
High enough to maintain position
Varies by use:
- Light body: Very low for detail
- Medium body: Moderate for general use
- Heavy body: Higher for tray stability
How does contact angle (θ) relate to material wettability, and what makes a smaller contact angle advantageous?
Smaller contact angle indicates better wettability
Better wettability means more intimate contact with tooth/soft tissue surfaces
Determines how well material envelops hard/soft tissue surfaces
Critical for recording fine detail
Explain what happens with a large contact angle at the tooth surface?
- Material forms distinct globules
- Spaces form between globules
- Results in incomplete surface replication
- Parts of tooth surface remain unrecorded
- Creates potential inaccuracies in final impression
Explain what happens with a small contact angle at the tooth surface?
- Material flows continuously across surface
- No spaces between material contact points
- Complete surface coverage
- All surface details captured
- Results in accurate impression
How does contact angle affect impression quality, and what makes a silicone “hydrophilic”?
Large contact angle: Creates spaces between material globules, incomplete surface replication
Small contact angle: Complete surface contact and replication
Hydrophilic silicones incorporate non-ionic surfactant to:
Wet tooth surface better
Improve compatibility with water-containing die materials
What are the key features of hydrophilic silicones and their advantages?
Incorporate non-ionic surfactant
Advantages:
Better wetting of tooth surface
More easily wetted by water-containing die materials
Better performance in moist oral environment
Compare and contrast the behavior of hydrophobic vs. hydrophilic impression materials in the oral environment.
Hydrophobic:
Large contact angles
Poor performance in moisture
May require aggressive drying
More likely to trap air bubbles
Hydrophilic:
Small contact angles
Better performance in moisture
More forgiving technique
Better detail reproduction
What are the ISO 4823:2000 standards for surface detail reproduction?
Three standard notch dimensions:
A = 20μm
B = 50μm
C = 75μm
These determine material’s ability to reproduce fine detail
How does viscoelastic behavior affect impression removal, and what is the optimal removal technique?
Materials exhibit permanent deformation after stretching/compression
- Faster removal (sharp pull) results in less permanent strain
- Lower deformation occurs with shorter load time
- 100% elastic recovery is ideal but rarely achieved
