Impression materials Flashcards
Definition of Impressions [1]
Impressions are negative reproductions of dental structures
Requirements of an impression material [10]
- Capable for plastic to rigid transformation
- Acceptable for patient (Non-toxic,-irritant, setting time up to 5-7 minutes)
- Good handling properties (Easy to mix, thixotropic, adequate working and setting time)
- Accuracy and details production (Ability to replicate the intraoral surface details)
- Dimensionally stable (Ability to retain its absolute dimensional size over time)
- Resistance to mechanical stress (Elastic and rigid)
- Tear resistance ( ability to resist tearing in thin sections such as through the feather-edged material within the gingival sulcus)
- Compatible with model materials
- Resistance to disinfectants
- Cost effective
Classification of impression material [2]
- Rigid/inelastic (Old, not important)
2. Flexible/elastic (Elastomers & Hydrocolloids)
When are rigid/inelastic materials used? [1]
What is their problem? [1]
Used when there are no undercuts and for edentulous patients
Problem is any significant deformation produces a permanent deformation
What are the four rigid/inelastic materials?
- Impression compound
- Impression plaster
- Zinc oxide eugenol
- Impression waxes
What are impression compound made of? [3]
- resins (wax),
- filler (soapstone)
- lubricants (stearic acid)
Properties of impression compound [9]
- Poor surface detail
- High coefficient of thermal expansion (contracts up to 0.3% when removed from mouth to room temp.)
- Distorts when removed over undercut areas
- Mucodisplacive
- Poor dimensional stability
- Can be modified by re-heating
- Non-toxic and non-irritant
8 Good shelf life
Advantages of impression compound? [5]
Disadvantages of impression compound? [3]
Ad: o Non-toxic/ irritate o Reusable o Can be reheat and re-adapted o Support other materials for wash impressions o Mucocompressive
Disad:
o Poor surface details
o Poor dimensional stability
o Will distort if remove from undercuts
What is impression plaster’s composition? [1]
What does it mix with to set? [1]
What does it become when set? [1]
Calcium sulphate hemihydrate
Mix with water (Often adds anti-expansion solution)
Becomes dihydrate
Properties of impression plaster [8]
- Excellent at recording fine detail (because very fluid when inserted in the mouth)
- Dimensionally stable if anti-expansion solution is used
- Fractures if undercuts are present
- Mucostatic
- Needs to be treated with a separating medium (e.g. soap/varnish) before being cast in stone or plaster
- Exothermic setting reaction
- On storage dimensionally stable but a small amount of shrinkage may occur
- Non-toxic but may be unpleasant due to dryness and heat evolved during setting (exothermic)
Advantages of impression plaster? [4]
Disadvantages of impression plaster? [3]
Ad: o Good dimensional stability o Good surface detail o Rate of setting reaction can be controlled by clinician o Mucostatic
Disad: o Good dimensional stability o Good surface detail o Rate of setting reaction can be controlled by clinician o Mucostatic
What is ZOE composed of? [3+3]
Base paste:
- Zinc oxide
- Inert oils (plasticiser)
- Hydrogenated resins(increases setting time and improves cohesion)
Reactor paste
- Eugenol
- Zinc acetate (accelerator)
- Fillers (talc or kaolin)
Properties of ZOE [5]
- Recording edentulous ridges in a close fitting special tray or the patients existing dentures
- Mixed in a 1:1 paste ratio
- Used in thin sections only (2-3mm) as a wash impression
- Vaseline is used as a separating agent on those areas requiring protection (soft tissues, teeth, etc)
- Firm pressure is used during impression taking
Problem of ZOE? [1]
The set material contains both some unreacted zinc oxide and eugenol. Therefore any movement of the tray as the paste is hardening will lead to a deformed, inaccurate impression
What does ZOE setting time depend on? [3]
What is the normal setting time for ZOE? [1]
- Accelerator additives (Zinc acetate/acetic acid)
- Exposure to moisture on mixing or the addition of water will accelerate the reaction
- Increasing temperature causes a fasting setting time
Setting time is normally 4-5 minutes
Advantages of ZOE? [3]
Disadvantages of ZOE? [3]
Ad:
o Good dimensional stability
o Good surface detail
o Mucostatic/ Mucodisplacive
Disad:
o Only sets quickly in thin section
o Eugenol allergy
o Cannot be used in deep undercuts
What is the composition of impression waxes? [2]
What is the ratio? [1]
- Low melting paraffin wax and beeswax
- 3:1 ratio
When are impression waxes used? [1]
What precaution should one take after taking the impression? [1]
Used to correct small imperfections (Airblows) in other impressions such as ZOE impressions
Cast should be poured immediately (avoid distortion)
What are flexible elastic materials? [1]
When are they used? [2]
These materials can be stretched and bent to a fairly large degree without suffering any deformation
- They are used for recording the patient’s mouth where undercuts are present.
- For RPD, overdentures, implants, crowns and bridge work.
2 types of flexible elastic materials? [2]
- Elastomers
2. Hydrocolloids
What is sol state? [1]
Relatively fluidic state when the solute particles present are dispersed thoughout the medium (usually liquid)
What is gel state? [1]
Viscous and jelly-like state when the particles become attached to each other, forming a loose network with the medium which restricts movement of the solute molcules.
What are the phases between sol, gel and sol-gel states? [3]
Before gel point (sol) –>fluid
At gel point –> loss of fluidity
After gel point –> gel
When are elastomers used? [3]
What are the 2 main properties over hydrocolloid? [2]
- Used when high degree of accuracy is needed
- Bridge
- Crown
Ad:
- Good tear resistance
- Dimensional stability
Main composition of elastomers [2]
One of them contains a few other things
- Flexible matrix
- Multifunctional prepolymer, oligomer or polymer (not from monomer)
- Crosslinking agent
- Curing agent (‘catalyst or initiator)
- Modifiers (accelerators, retarders, plasticizers, flavoring agents, colorants) - Filler/extender
What are the four main groups of elastomers? [4]
- Polysulphide
- Polyether
- Polysiloxane (silicone rubber: Condensation and addition silicone)
- ‘Hybrid’
Composition of polysuphide? [6]
Base paste:
- Polysulphide prepolymer
- Filler (give body) (TiO2)
- Plasticizers (control viscosity)
Activator paste:
- Lead oxide (cause polymerization)
- Sulphur (facilitates reaction)
- Inert oil (forms a paste)
What type of reaction is the setting reaction for polysulphide? [1]
Condensation polymerization
What is the normal setting time for polysulphide? [1]
10 minutes
How to use polysulphide? [6]
- 2 equal lengths (1:1) are mixed together with a spatula for about 1 min
- The tray needs to be treated with an adhesive (rubber solution in acetone) to provide retention for the polysulphide
- Wait for 5 mins, then place in patients mouth
- Final setting time is usually about 10 mins from the start of mixing
- A 1 or 2 stage impression technique may be used
- Although dimensionally stable, the impression should be cast within 24 hours
Advantages of polysulphide? [5]
Disadvantages of polysulphide? [5]
Ad: o Dimensional stable o Accuracy o Long working time o Long shelf life o Comes with number of different viscosities
Disad: o Dimensional stable o Accuracy o Long working time o Long shelf life o Comes with number of different viscosities
Properties of polysulphide [7]
- Dimensionally stable
- Excellent surface detail (only used in special trays)
- Viscosity depends on the brand used
- Very small setting contraction (0.3-0.4% over the firs 24 hrs)
- Contract on cooling from mouth to room temperature
- Very good tear resistance
- Good shelf life
- Viscoelastic
What is the type of setting reaction for polyether? [1]
Ionically initiated additional polymerization
Composition of polyether? [8+4]
Base: 1. Polyether prepolymer 2. Colloidal silica 3. Plasticizer (high and low viscosity) 4. Pigments 6, Flavorings 7. Triglucerides 8. Accelerators
Activator 1. Alkyl benzene sulphonate ester 2. Fillers 3 Plasticizer 4. Pigments
Properties of polyether [8]
- Hydrophilic (absorbs water)
- Good shelf life of up to 2 years
- Good elastic recovery
- Non toxic
- Low setting contraction
- Low tear strength
- Excellent surface detail
- Good dimensional stability
Advantages of polyether? [6]
Disadvantages of polyether? [4]
Ad: o Good dimensional stability o Accuracy o Ease of use o Good on undercuts o Excellent surface detail o Low setting contraction
Disad: o Allergic reaction (sulphonate ester) o Poor tear strength o Short working time o Stiff set material (hard to remove from mouth)
Instruction for use of polyether? [4]
- Mix the polymer and sulphonate ester together, they react to form a stiff polyether rubber
- Setting time occurs in about 6 minutes
- Usually only comes in 1 viscosity - regular bodied, but can also come as light + heavy bodied (diluent)
4, Heat and moisture speed up the setting reactions, Plasticizer thins the mixture
How are polysiloxane (silicone rubber) classified? [2]
By the type of chemical reaction by which they set:
- Condensation silicone
- Addition silicone
In condensation silicone setting reaction, what is the catalyst precursor? [1]
What also plays an important role in that reaction? [1]
Precursor is Tin octoate
Atmospheric H2O play an important role
Composition of silicone [9+3]
Base paste:
- Polydimethysiloxane (PDMS)
- Micronised Talc
- Calcium carbonate
- Pyrogenic silica
- Paraffin Wax
- Micronised quartz
- Corn Starch
- Water
- Colourant
Activator paste:
- Ethyl orthosilicate (TEOS)
- Tin Octoate
- Tri-isooctylamine
Instruction for use of condensation silicone [2]
- Mix 2 pastes to react, cross-linking occurs and setting takes about 7 minutes
- Ethanol gas is evolved on setting which leads to surface pitting and roughened surface to the resulting model
What is condensation silicone used for? [3]
- Crown and bridge work mostly
- Partial dentures, implants and overdentures
- Stock trays and special trays
Within how many hours should condensation silicone be cast? [1]
Within 24 hours
Properties of condensation silicone [8]
- Hydrophobic
- Ethanol gas evolved on setting
- Moderate shelf life
- Moderate tear strength
- Good surface detail
- Shrinking of impression over time
- Non toxic and non irritant
- Very elastic (near ideal)
Advantages of condensation siliconer? [4]
Disadvantages of condensation silicone? [3]
Ad:
- Accurate
- Ease of use
- Very elastic (near ideal), used on severe undercuts
- Good surface detail
Disad:
- Ethanol evolution
- Liquid component of paste/ liquid system cause irritation
- Shrinking of impression over time
What type of react is the setting reaction for condensation silicone? [1]
Condensation reaction
Composition of addition silicone? [6+5]
Base
- Hydrogen siloxane
- Silicone oil
- Fillers
- Pigments
- Hydrophilizers
- Vinyle siloxane
Catalyst
- Vinyl siloxane
- Silicone oil
- Fillers
- Pigments
- Platinum complex
What type of react is the setting reaction for addition silicone? [1]
Addition polymerisation reaction
What is evolved when addition silicone is set? [1]
Hydrogen gas
Instruction for use of addition silicone [2]
- Mixing in equal portions.
- Crosslinking occurs to form a silicone rubber
- Setting time is about 6-8 minutes
Properties of addition silicone [6]
- Good shelf life
- Dimensionally stable
- Moderate tear strength
- Excellent surface detail
- Hydrogen gas evolved when setting
- Non toxic and non irritant
Advantages of addition siliconer? [4]
Disadvantages of addition silicone? [4]
Ad:
- Accurate
- Ease of use
- Fast setting
- Wide range of viscosities
Disad:
- Hard to mix
- Sometimes difficult to remove the impression from the mouth
- Too accurate in some circumstances (Cast produced is not sufficiently oversized)
- Hydrogen evolved when setting
Hybrid’s claimed advantages: [6]
And the facts [2]
Claimed ad:
- Optimised advantages from both addition silicone and polyether
- Hydrophilic
- Dimensional accuracy
- High tear strength
- Better elasticity
- High resistance to deformation
Facts:
- Unknown chemistry (Probably a kind of vinylsiloxane-polyether copolymer, with Pt catalyst)
- Lab studies showed comparable performance with addition silicone and polyether
Revise properties comparison on ppt
Go revise
What is hydrocolloid? [2]
A colloid is a state of matter in which individual particles of one substance, are uniformly distributed in a dispersion medium of another substance
When the dispersion medium is water, it is termed a hydrocolloid
Which are the 2 types of hydrofcollloids? [2]
Agar (reversible)
Alginate (Irreversible)
Hydrocolloids are placed in the mouth in the ___ state when it can record sufficient detail, then removed when it has reach the ___ state
sol
Gel
Hydrocolloids materials especially the alginates, may display _________ with some makes of dental stone. The resultant model may show reduced ______ ______ and possibly ________ _______ and _______
incompatibility
Surface hardness
surface irregularities
roughness
Composition of agar [4]
- Agar
- Borax, increases the gel strength
- Potassium sulfate -hardener
- Water - dispersion medium
At what temperature does agar sols forms gels upon cooling of a hot solution? [1]
Agar gels melt to sols upon heating to what temperature? [1]
30-40oC
90-95oC
Agar chemistry:
in sol agar forms ____
in Gelation I agar forms ____
in Gelation agar forms ____
Random coils
Double helices
Aggregation of helices
Instruction for use of agar [6]
- Supplied in sealed tube to prevent evaporation of water
- The tubes are heated in boiling water (in water bath) for 10-45mins
- Once the impression is taken, the tray can be cooled with water to aid gel formation
- A higher temperature is needed to convert the gel into a sol
- The first material to set is that which is in contact with the tray since it is cooler than tissues
- Thus it is the material in contact with the tissue with stays in the sol state for the longest time
Properties of agar [8]
- Good surface detail
- Can be used on undercuts, but liable to tear on deep undercuts
- Evaporation or imbibtion
- Non toxic and non irritant
- Slow setting time
- Poor tear resistance
- Adequate shelf life
- Can be sterilised by an aqueous solution of hypochlorite
Advantages of agar? [2]
Disadvantages of agar? [2]
Ad:
- Good surface detail
- Reusable and easily sterilised
Disad:
- Need special equipment (water bath) and special technique
- Dimensionally instability
Composition of alginate [8]
- Sodium/potassium alginate
- Calcium sulfate
- K2SO4, silicates, borates
- Sodium phosphate
- Diatomaceous earth
- Colourant
- Flavouring
- Disinfectant
