DM9 Impression Materials Flashcards
What are the 3 general classifications of impression materials
Hydrocolloids
Elastomers
Non-elastic materials
What types of Hydrocolloids are there
Reversible (Agar)
Irreversible (Alginates)*
What is the difference in setting reaction between agar and alginates
Agar - physical process
Alginates - cross linking reaction
What kinds of elastomer impression materials are there
Silicones
Polysulfides
Polyethers
What kinds of silicone elastomers are there
Condensation
Addition
In what state are all elastic impression materials before setting
They are all in the form of polymers in the unset state
How do the polymers of all elastic impression materials set
They chemically cross-link on setting, either when the powder and liquid (alginates), or two pastes, are mixed together
What is the only exception to the normal method of setting for elastic impression materials
Agar
Name some of the desirable qualities of an ideal impression material
- Pleasant odour, taste and aesthetic colour
- Adequate shelf life for requirements of storage and distribution
- No toxic irritants
- Economically commensurate with results obtained
- Easy to use
- Good consistency + texture
- Readily wets oral tissues
- elastic properties with no permanent deformation after strain
- dimensional stability with temp and humidity
What are the uses of elastomers
- Produce an accurate replica of the teeth and supporting tissues
- For the construction of full and partial dentures, crowns, bridges and inlays
Why are elastomers used for their uses and describe the cost
Due to their strength and dimensional stability
COST - significantly more than alginates/agar
When does the cross linking reaction in 2 paste systems begin
As soon as the 2 components come into contact with each other
When does the cross linking reaction in 1 paste systems begin
As soon as the LED light unit is directed on to the paste, the chemical setting reaction begins
Name a form of a one paste system
Composites
How can the 2 pastes of impression materials be found
- 2 separate tubes
- 2 tubs of putty, or putty and tube
- Double barrel cartridges
- Pouches for pentamidine
Why are double barrel cartridges and pouches for pentamix used
- Avoid incomplete mixing and prevents the introduction of air bubbles
What kind of cross-linking reactions are there
Condensation
Addition - affects the dimensional stability of the set material
What is the difference between addition and condensation cross-linking reactions
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 different viscosities do elastomeric impression material pastes come in
- Putty
- Heavy
- Medium
- Light
- Wash
What determines the viscosity of elastomeric impression material pastes
The amount of filler present and the molecular weight of the polymer
What different viscosity pastes are usually used together in 2 paste systems
Putty + Wash
Heavy/Medium + Light
Why can’t light-bodied/wash materials be used by themselves
As they contain little filler they don’t have the strength, so will tear easily when removing the tray/impression from the mouth
What are the different roles of the lighter bodied material and the heavier bodied material
Lighter records the fine detail and the heavier acts as the base to support the light material
Describe the twin mix/one stage Putty and Wash technique
Wash material is syringed around the teeth and prepared tooth
Describe the twin mix/one stage Putty and Wash technique
Wash material is syringed around the teeth and prepared tooth
Putty is loaded into the tray and inserted immediately into the mouth after syringing the wash material in the mouth
Describe the double impression/two stage Putty and Wash technique with no spacer
- 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 putty on the tray and reinsert the tray over the wash
Describe the Double impression/two stage Putty and wash technique with a spacer
- A polythene sheet (spacer) 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
- The wash material is placed in the space provided in the putty impression and the tray is seated back in the mouth
Describe 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 and can also be placed on the heavy/medium material in the tray
- Tray is seated in mouth and materials are allowed to set
When are dual viscosity and putty and wash techniques used
when recording a lot of detail like when making a crown
AY BAWS CAN I HABE DE NOTE PLZ
Light bodied material will take longer to set
What are the advantages of using impression techniques (putty and wash/dual viscosity)
Fine detail recorded e.g. for a crown
What are the disadvantages of the one stage impression techniques
Both materials (each as a two paste system) have to be mixed simultaneously
What are the disadvantages of the two stage impression techniques
Time taken to take two impressions
Name some kinds of elastomers
Condensation silicones
Addition silicones
Impregum penta
Polysulfides
What are condensation silicones commonly known as
Poly-dimethyl siloxanes (PDMS)
NB - hydrophobic
Why does the mouth need to be very dry when taking an impression with a condensation silicone
Causes defects in impression material due to presence of saliva and blood etc.
What do condensation silicones consist of
Base paste and Catalyst paste
What does the base paste of condensation silicones contain
- Silicone polymer with terminal OH- group
- Inert filler
What does the catalyst paste of condensation silicones contain
- Tetraethoxy orthosixicate (cross-linking agent)
- Dibutyl tin dilaurate (catalyst)
- Inert filler
What part of the tetraethoxy orthosilicate takes part in the cross linking reaction
the 4 C2H5 ethyl groups
Describe the condensation reaction of condensation silicones
Base paste polymer will lose terminal OH and react with C2H5 on tetraethoxy orthosilicate
This forms C2H5OH as the molecule that is lost from the molecules
Whatre the advantages of condensation silicones
- Stronger and better dimensional stability than alginates
- More elastic than polysulfides and polyethers so they recover from deformation quickly
- Tear strength is adequate
- Elongation at break is adequate
What are the disadvantages of condensation silicones
- Shrink in air due to condensation cross linking reaction (much less than alginates)
- Hydrophobic - material will shy away from moisture so detergents are incorporated to add gettability
- These materials may expand in disinfecting solutions depending on the hydrophilic agents incorporated as material may take on too much water
- Mouth dry as possible
- 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
PolyVINYLdimethylsiloxanes (PVDMA)
NB - Hydrophobic
What are some examples of trade name addition silicones
Virtual, President, Peramgum, Aquasil, Extrude
What is the base paste of addition silicones made of
- Silicone polymer with terminal vinyl groups
- Inert filler
What are the catalyst pastes of addition silicones made up of
- Silicone oligomer with Si-H groups (cross-linking agent)
- Platinum salt catalyst
- Inert filler
What is the vinyl group on the Si polymer in the base paste of addition polymers
CH2=CH
Describe the structure of oligomer cross linkers in addition polymers (found in the catalyst paste)
Si-O groups with one methyl and one Hydrogen group on them, the H is the part that acts as the cross linker
Describe the addition reaction of addition silicones
The CH2=CH vinyl group gains the hydrogen from the oligomer catalyst and there is an Si-CH2-CH2-Si link between base and catalyst
What are the advantages of addition polymers
- Best dimensional stability compared to all other impression materials - no by-products
- Elastic recovery is very good, they are more elastic than polysulfides and polyethers, therefore they recover from deformation rapidly
What are the disadvantages of addition polymers
- 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. Recommended that pouring of the impression should be carried out half an hour after impression taking
- % elongation is generally less than that of condensation Si, as is the tear strength, but both are fine
- Setting can be seriously impaired when handled with natural rubber gloves (S poisons Pt catalyst)
- Hydrophobic = shy away from moisture, detergents added to add wettability
- Mouth still needs to be dry
- Hydrophilic addition silicones may expand in disinfecting solutions
What are polyethers commonly known as and are they hydrophobic of hydrophilic
Impregum - hydrophilic
What impregum products can cause tissue hypersensitivity
Alkyl benzene sulphate
What does the base paste of impregum/polyethers contain
- Polyether polymer (terminal ethylene-imine groups)
- Fillers
- Plasticisers (high and low viscosity)
- Pigments
- Flavourings
- Triglycerides
What does the catalyst paste of impregum/polyethers contain
- Initiator (cationic starter - sulphonium tetra borate salt)
- Fillers
- Plasticisers
- Pigments
What do inorganic fillers do
High rigidity of the impression and help maintain dimensional stability
What do plasticisers do
Adjust the viscosity of the paste
What do triglycerides do to polyethers
Increase the intrinsic viscosity*: i.e. resilience coupled with flowability under pressure
*viscosity diminishes under the influence of an increasing outside force or shearing speed, when influence goes away, viscosity increases again
How do polyethers set
By a cationic addition reaction via ring opening
What are the advantages of polyethers/impregum
- Good dimensional stability in AIR (make sure material is not stored in a humid environment)
- Quick setting compared to polysulfide and reliable
- Clean to handle and odourless
How do the properties of impregum penta soft differ from impregum penta
Composition is modified to give:
- Easier removal
- Improved taste
- Optimal convenience and handling
How are the different properties of impregum penta soft achieved
- Reducing the proportion of filler - decreases final hardness
- Proportion high and low viscosity plasticisers
What are the disadvantages of polyethers/impregums
- Original impregum - very stiff material and 50% elongation = tearing on withdrawal
- Permanent deformation - material recovers slowly and not completely after deformation
- Dimensional stability on prolonged contact with water or water vapour - the material swells (do not wrap in damp napkin) disinfection is a problem
What does the literature say on how to disinfect impregum penta and penta soft
Disinfect in a standard glutataldehyde solution for 10 mins and then rinse for 15 secs
What does the base paste of polysulfide rubber impression materials contain
- Polysulfide polymer
- Filler e.g. ZnS/TiO2
- Plasticiser (usually phthalate ester)
What does the catalyst paste of polysulfide rubber impression materials contain
- Lead dioxide (PbO2, cross-linking agent)
- Filler (as in base)
- Plasticiser (an in base)
- Sulfur (~1%)
- Stearic or oleic acid
How do polysulfide rubbers set
Thiokol reaction - condensation polymerisation
What is the by product of the thiokol condensation polymerisation reaction
Water
What are the advantages of using polysulfide rubbers
It is the strongest of all impression rubbers (elongation at break ~500%)
What are the disadvantages of using polysulfide rubbers
- Dimensional stability - shrinkage on setting (0.1-0.2%) due to condensation reaction
- Slow setting
- Dirty to handle and has an unpleasant smell
- Elastic recovery is not as good as the silicone and polyether materials (permanent set)
What impression materials should be avoided
- Photo-curing elastomers
- Siliconised alginates
What types of non-elastic impression materials are there and when are they used
- Impression plaster
- Impression compound
- Zinc oxide eugenol
All used in edentulous cases
What are the properties of impression plasters
- Mucostatic impression material - doesn’t displace soft tissue
- No trays required
- Material sets hard
- Only used in edentulous cases
What are the properties of impression compounds
- Poor thermal conductivity/flow properties
- It will not reproduce undercut areas
- Occasionally used as first impressions for edentulous cases
- Mucocompressive impression material - displaces buccal and lingual soft tissue
- Advantage of high viscosity - if full depth of sulcus is required to support subsequent denture - achieved with this material
What are the properties of Zinc Oxide/Eugenol impression pastes
- Brittle material when set - bits can fracture off
- Accurate in thin section
- Initial low viscosity and pseudoplasticity
- Mucostatic impression material
Describe the thermal expansion of zinc oxide/eugenol impression pastes
- high coefficient of thermal expansion
- Mouth temp to RT = 12C difference-ish
- Shrinkage of 0.26%
