What are the 2 main classifications of impression materials?
Elastic
Non Elastic
What are the two states that hydrocolloids exist in and explain these
Sol = fluid with low viscosity, random arrangement of polysaccharide chains
Gel = high viscosity, organised arrangement of polysaccharide chains
The strength of a hydrocolloid depends on two things. What are they?
- Concentration of fillers
2. Concentration of fibrils
What is syneresis and imbibition and why does this happen with hydrocolloids?
Syneresis = gel molecules come closer and water is squeezed out
Imbibition = water is taken up in presence of moisture due to hydrophilic property
What makes agar reversible and alginate irreversibel?
Agar is formed with physical bonds only meaning that energy provided can break down the matrix and go back to sol state.
Alingate forms with chemical bonds which cannot be broken with heat.
Why do we want to place impression materials in the mouth in the sol state?
Allow flowability into the surface detail
What is found in the powder of aliginate?
- Powder of sodium alginate and gypsum (calcium sulphate)
How does alginate change from a sol to a gel state?
Sol state = sodium alginate
Gel state = calcium alginate (calcium cross links with alginate chains)
Why is sodium phosphate added to impression materials?
To control working time
Why do we not like bubbles in alginate?
- Weak point where impression can tear
- Rise to the impression surface and cause inaccuracy
- Lead to more water absorption
How do calcium ions become available to cross link with in alginate?
- Calcium salts start dissolving when water is added
Explain how trisodium phosphate reduces the reaction rate?
Tri sodium phosphate have 3 sodium ions per phosphate.
What are the main properties of alginate?
- Hydrophilic gives them good flow
- Easy to mix
- Fairly cheap
- Can control setting time
- Good shelf life
- Mucostatic
- Good working and setting time
- Poor dimensional stability
- Poor tear resistance
- Messy so can cause patient to gag
How do addition cured silicones set?
- Cross linking of hydrogen across double bond to cause setting
- Presence of platinum catalyst
What are the main properties of addition cured silicones?
- Good dimensional stability
- Good accuracy
- Available in different viscosities
- HYDROGEN EVOLUTION : platinum catalyst reacts with hydrogen leads to surface pitting and creates a rough surface
How does condensation silicones set and what is the catalyst used?
Two molecules react together (hydroxy terminated poly(dimethyl siloxane) in presence of alkyl silicate (cross-linking agent)
Tin catalyst
What molecule is released in condensation silicones?
2 ethyl alcohol molecules
What are the main properties of condensation cured silicones?
- Short working time
- Near ideal elastic recovery
- Hydrophobic (can cause blow holes with saliva present)
- Good tear strength
- High amount of shrinkage
- Accurate
- Easy to use
What are polyethers supplied as?
Two pastes
- Polyether with filler and plasticiser
- Catalyst paste (enhance polymerisation)
What type of setting reaction do polyethers undergo and how long does it take?
Addition polymerisation
6-7 mins
Are there any bi products in the poly ether setting reaction?
No - no shrinkage
What are the main properties of polyethers?
Hydrophilic so flows over surface tissues.
Good shelf life
Good elastic recovery
Low tear strength
Rapid setting time, short working time
Stiff so hard to remove it out mouth
Only can be used as 1 viscosity
During the setting of alginate impression materials, what happens?
A) Trisodium phosphate reacts with sodium alginate
B) Trisodium phosphate reacts with calcium sulphate
C) Colloidal changes to sol
D) Material in contact tight soft tissues sets last
B
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Biocompatability24
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Metals in the mouth0
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Polymers and Composites30
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Dental Composites39
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Failure of Materials 128
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Failure of Materials 228
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Amalgam Restorations29
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Principles of adhesion23
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Clinical aspects of adhesion23
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GIC and RMGIC30
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Impression materials - properties26
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Impression Materials - Types23
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Cements26
