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34 Cards in this Set
- Front
- Back
Alginate
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Irreversible Hydrocolloid
Alginic acid is reacted with calcium sulfate dihydrate and water to form a calcium alginic gel and potassium sulfate |
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Agar
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Reversible Hydrocolloid
Composed of linear carb chains, similar to alginic acid without carboxylic acid groups Liquefies at 70-100 celcius Gels at 30-50 celcius Reversible because you can heat and cool it |
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Polysulfides
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Use for removal of partial dentures
Reactive terminal mercaptan (-SH) groups and reactive sulfur groups extended from within the polymer allow cross linking during polymerization |
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Advantages of Polysulfides
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Low cost (compared to silicones and polyethers)
Long working time High tear strength High flexibility Good detail reproduction |
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Disadvantages of Polysulfides
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Porr dimensional stability (water by-product, pour within 1 hr, Single pour)
Custom Trays Messy (paste-paste mix, bad odor, may stain clothing) Long setting time |
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Polysiloxane (Condensation silicones)
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Curing involves tri- or tetra-functional alkyl silicate crosslinking hydroxyl terminated silicone polymers by condensation rxns.
A catalyst (metal organic ester) is needed. Loss of rxn by-products (ethanol) resulted in significant dimension changes over time |
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Polyvinyl Siloxanes (addition silicones)
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Base contains filler particles, vinyl dimethly polydimethyl siloxanes, polymethyl hydrogen siloxane (-SH)
Accelerator contains filler vinyl dimethyl polydimethyl siloxanes and chloroplatinic acid (catalyst) Moisture of impurities (silanol) can react w hydride and yield hydrogen gas, whihc can be eliminated through absorption by adding palladium or platinum salts |
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Advantages of Polyvinylsiloxanes (addition silicones)
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Highly accurate
High dimensional stability Stock or custom trays Multiple casts Easy to mix Pleasant odor |
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Disadvantages of Polyvinylsiloxanes (addition silicones)
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Expensive
Sulfur inhibits set (latex gloves, Fe and Al sulfate retraction solution) Short working time Lower tear strength |
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Polyethers
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Base is a low molecular weight polyether with a reactive imine ring
Polymerization occurs via cationic polymerization opening of the active imine ring Resulting polymer is a tetrahydrofuran/ethylene oxide copolymer It can absorb water when immersed in water |
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Polyethers
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Another class is based on alkoxysilane terminated polyether
Polymerization occurs via condensation of alkoxysilane group System with alkoxysilane claims to be free of water absorption |
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Advantages of Polyethers
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Highly accurate
Good dimensional stability Stock or custom trays Good surfae detail Pour within one week (keep dry) Multiple casts Good wettability |
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Disadvantages of Polyethers
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Expensive
Short working time Rigid (difficult to remove from undercuts) Bitter taste Low tear resistance Absorbs water (changes dimension) |
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Surface Wettability
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Impression materials can be classified as readily wettable by gypsum (hydrophilic) or resistant to wetting (hydrophobic)
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Which materials are hydrophilic and easiest to pour?
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Irreversible hydrocolloid (Alginate)
Reversible hydrocolloid Polyether |
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Wettability by water
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hydrocolloids > polyether > hydrophilic addition silicone > polysulfide > hydrophobic addition silicone (condensation silicone)
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Higher contact angle
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= greater probability of air entrapment (bubbles) during pouring
= More readily repelled by hemorrhage or other sources of moisture |
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Working/Setting time of Polysulfides
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Working = 3-6 minutes
Setting = 6-10 minutes |
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Working/Setting time of Addition Silicones
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Working = 2 min
setting = 4-6 min |
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Working/Setting time of Polyethers
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working = 2-3 min
setting = 3-5 min |
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Best working/setting?
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Use Polysulfides or Addition Silicones for multiple preps.
Polyethers can be utilized when rapid sets are required |
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Tear Strength
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maximum load divided by the thickness of the specimen
&uarr as viscosity &uarr |
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What has greatest tear strength?
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Light-body Polysulfide
Polysulfides use when multiple subgingival areas are present |
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What is the Stiffest material?
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Polyethers - require care in patients with long teeth or tissue undercuts - consider low viscosity materials
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Impression Tray
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be rigid
dimensionally stable allow uniform layer of impression material retain impression material correct extenstion distinct seating |
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Thixotropic
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a material that becomes more fluid when the shear rate is increased by deforming or disturbing it
This property allows making impression with Monophase (single viscosity) |
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Thixotropic Agent
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Used to increase viscosity of material to reduce slumping during processing
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Viscoelasticity
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material has viscous and elastic characteristics when undergoing deformation
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Fast Removal
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elastic property
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Slower Removal
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facilitates plastic deformation
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Polyvinylsilicone catalyst
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platinum salt
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Polyether catalyst
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aromatic sulfonate
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How much shrinkage occurs in first hour?
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half
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What material is most stable?
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Addition Silicone > Polyethers > Polysulfides
Use Addition Silicones when undercuts are present |