Impression Materials Flashcards
All Elastic impression materials, which include alginates, agar and all elastomers are what in their unset state?
Polymers
All impression materials except agar do what on setting?
What does agar do instead?
- Chemically cross-link on setting
- Agar sets by a physical, reversible process
What are elastomers used for? (2)
- To produce an accurate replica of the teeth and supporting tissues
- For the construction of full and partial dentures, crowns, bridges and inlays
Compare the cost of elastomers to alginates and agar
Elastomers cost significantly more than alginates/agar
How are elastomers dispensed?
describe unset/set states
2 paste systems that are fluid in the unset state, which are cross-linked (elastic) in the set state
When does chemical cross linking occur in a two paste system?
As soon as the two components come into contact with each other, the chemical cross-linking reaction (setting) begins
When does chemical cross linking occur in a one paste system?
As soon as the LED light unit is directed on to the paste
How can two paste systems be supplied? (3)
- Two separate tubes
- Two tubs of putty
- Putty and tube
What are the advantages of supplying two paste systems in double barrel cartridges or pouches? (2)
- Avoid incomplete mixing
- Prevent introduction of air bubbles
In elastomers how are the polymer cross-links carried out?
Either condensation or addition reaction
How does the differing cross-link reaction in elastomers affect the dimensional stability of the set material?
- Addition reaction results in no by-products therefore material does not shrink on setting
- Condensation reaction results in the elimination of a small molecule thus resulting in shrinkage of the set material
What type of bonds are cross links?
Cross-links are covalent bonds joining one polymer chain to another
What are the different viscosities of elastomeric impression materials? (5)
- Putty
- Heavy
- Medium
- Light
- Wash
Describe the difference in amounts of filler used in Putty and Wash elastomeric impression materials?
- Putty has maximum amount of filler
- Wash has least amount of filler
When pairing different viscous pastes, what elastomeric impression material pastes would you usually combine together?
- Putty with wash material
- Heavy/Medium with light-bodied material
Why can’t the light-bodied/wash materials be used by itself?
Due to it containing little filler, it does not have the strength (and body), and so it will tear easily when removing the impression from the mouth
Explain the role of each material when combining a light-bodied and heavy-bodied material
- The light-bodied material records the fine detail
- The heavy-bodied material acts as a base to support the light material
Explain the ‘one stage’ putty and wash technique
- The wash material is syringed around the teeth or prepared tooth
- The putty is loaded into the tray and inserted immediately in the mouth after syringing the wash material in the mouth
Explain the ‘two stage no spacer’ putty and wash technique
- The putty is used first to take an impression of the
mouth before starting the preparation - After preparing the teeth the wash material is syringed on the prepared teeth and, the tray with putty impression is reinserted over the wash
Explain the ‘two stage with spacer’ putty and wash technique
- A polythene sheet is placed over the teeth
- The putty is then used to take an impression of the mouth by placing over the polythene sheet
- After set, the putty impression is removed from the mouth and so is the polythene sheet removed
- The wash material is placed in the space provided in the putty impression and the tray is seated back in the mouth
Explain the dual viscosity technique
- Heavy and light-bodied pastes are used in combination
- Heavy or medium-bodied material is extruded into the tray
- Light-bodied material is syringed around the teeth (area of interest) and can also be placed on the heavy/medium material in the tray
Which materials take longer to set, light or heavy bodied?
Light bodied material will take longer to set
What is the main advantage of using impression techniques?
Fine detail recorded
Give a disadvantage of the ‘one stage’ putty and wash technique
Both materials have to be mixed simultaneously
Give a disadvantage of the ‘two stage’ putty and wash technique
You have to take 2 impressions which can be long
What are condensation silicones commonly known as?
Polydimethyl siloxanes (PDMS)
Is PDMS/PVDMS hydrophobic or hydrophilic?
What does this mean when taking an impression?
- Hydrophobic
- Mouth has to be dry otherwise defects in impression material
What are the two pastes called in condensation silicones?
- Base paste
- Catalyst paste
What is the base paste of a condensation silicone made up of? (2)
- Silicone polymer with terminal OH-groups
- Inert filler
What is the catalyst paste of a condensation silicone made up of? (3)
(list the functions of each)
- Tetraethoxy orthosilicate (cross-linking agent)
- Dibutyl tin dilaurate (catalyst)
- Inert filler
In a condensation silicone reaction what product is lost in the reaction?
Where are parts of of this product from?
- C2H5OH (ethanol)
- C2H5 from Tetraethoxy Orthosilicate
- OH from base polymer
What are some advantages of condensations silicone? (4)
- Stronger and have better dimensional stability than alginates
- They are more elastic than polysulphides and polyethers, therefore they recover from deformation rapidly
- Tear strength is adequate
- Elongation at break is adequate
What are some disadvantages of condensations silicone? (5)
- Shrink in air
- Hydrophobic
- Expand in disinfectant
- Erratic setting behaviour when a liquid catalyst is used in place of a catalyst paste
- Limited shelf life, mainly with liquid catalyst
What are addition silicones commonly known as?
Polyvinyldimethyl siloxanes
PVDMS
What is the base paste of an addition silicone made up of? (2)
- Base paste
- Catalyst paste
What is the base paste of an addition silicone made up of? (2)
- Silicone polymer with terminal vinyl groups
- Inert Filler
What is the catalyst paste of an addition silicone made up of? (3)
- Silicone oligomer with Si-H groups (cross-linking agent)
- Platinum salt catalyst
- Inert filler
Draw/Give a vinyl group
CH2=CH
What are some advantages of addition silicone? (2)
- Best dimensional stability compared to all other impression materials
- Elastic recovery is very good
What are some disadvantages of addition silicone? (3)
- When pouring, free water in plaster/stone reacts with the unreacted Si-H groups in the set material, thus releasing hydrogen and giving a porous model (so wait 30 mins before pouring)
- Setting can be seriously impaired when handled with natural rubber gloves
- Hydrophobic material
