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1
Impression materials
Pavel Bradna
Institute of Dental Research
To prepare accurate and true replicas of oralstructures (teeth, mucosa)
Purpose
Step 1. Making a negative (impression)
Step 2. Prepare a model or a cast -accurate positive
The replica is prepared in 2 steps:
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Requirements1. Capable of plastic to rigid body transformation2. Acceptable for a patient
- non-toxic, non-irritant, tasteless, reasonablesetting time up to 5-7 min
3. Good handling properties – easy to prepare/mixplastic before set, but viscous enough not to flowout of a tray (thixotropic), adequate working andsetting times
After being set:4. Accuracy and detail reproduction (25-50µm),5. Dimensionally stable6. Resistant to mechanical stress - elastic and rigid7. Compatible with model materials8. Resistant to disinfectant solutions9. Cost effective
Important terms
• Elastic/plastic deformations• Strength• Pseudoplastic/thixotropic• Hydrophilic/hydrophobic• Working time• Setting time
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Elasticity/plasticityelastic/plastic deformation
stre
ss (f
orce
/m2 )
Describes behaviour of material under load
Region of elasticdeformations,
Hook‘s law applies
Region of plasticdeformations
Slope – Young‘s modulus,modulus of elasticity
Deformation
Maximum load atfracture – ultimate
strength
Proportional limit
5 N
When unloaded!fully recovers!
No load
Mechanical model of elastic behaviourA spring – ideally elastic behaviour (Hook‘s law)
Def
orm
a tio
n
t1loaded
Def
orm
atio
n
t2unloaded
Loaded
Unloaded
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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A dashpot – ideally plastic behaviour
Permanentdeformation
Def
orm
atio
n
t1loaded
t2unloaded
Mechanical model of plastic behaviour
When unloaded!No recovery!
Plastic =permanent/irreversibledeformation
defo
rmat
ion
Timet1 t2
Plastic/permanentdeformation
<
Recovery from deformation
Loaded for ashort time
Loaded fora long time
combination of a spring and a dashpot – a model ofviscoelasticity
Pouring aftermaterial
relaxation
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
5
% Recoveryfrom
deformation
Def
orm
atio
n
Time
The effect of recovery from deformationon the accuracy of a model
99% 95%
Plasticdeformation
Pseudoplasticity/thixotropy of fluids
Rate of deformation γ
Stre
ss τ
Gradually unloadedPseudoplastic
Gradually loaded
Gradually unloadedThixotropic (structure is
„broken“ under shear
Slope-viscosity
τ = ηγ.
.
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Hydrophilic/hydrophobic
Hydrophobic material(α>90o)
unable to wet humidsurfaces
Hydrophilic materialable to wet humid surfaces
Contactangle
Does not copy the wet surface
Examples of clinical significance of pseudoplasticproperties, hydrophilicity and high mechanicalresistance (tear strength) of impression materials
Compromised flow in smalldetails - too high viscosity
and hydrophobicity
Preparedtooth
Impressionmaterial
Fracture and lost detail dueto low strength
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Working time – a period from the start of mixing tothe final time at which the impression can be seatedin the mouth without its distorsion
Setting time – a period from the start of mixingtill the impression becomes elastic enough toresist deformation during its withdrawal from themouth
Impression com-pounds
ZnO-eugenol (ZOE)Impression plaster
Inelastic/rigid
Agar hydrocolloidAlginateElastomeric: Polysulfide Polyether Silicone
Elastic
ReversibleIrreversible
Classification of impression materials
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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1. Zinc-oxide eugenol impressionpastes (ZOE)
Inelastic/rigid impressionmaterials
Main indications: impression of edentulous ridges,surgical dressing
A. Irreversible
Setting reaction:
chelate structure
H2O, acetic acid, Zn acetate
A two-paste system composed of:
Paste A – ZnO/mineral or vegetable oil as a plasticizer
Paste B – oil of cloves with app. 85 % of eugenol orpure eugenol, resins and fillers, accelerators
-OH and methoxy groups
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Silicone impression materials
Low viscosity of ZOEimpression pastes – enablesmucostatic impressions
Comparison of viscosity changes during ZOE setting
Edentulous ridges - lowviscosity is favourable to
avoid displacement oftissues
rigid
ity
ZOE
Silicone
Advantages:1. Low viscosity – no compression of soft tissues
2. Dimensional stability (shrinkage less than 0.1 %) 3. Good surface detail reproduction
4. Low price
Disadvantages:
1. Cannot be used in deep undercuts
2. Eugenol allergy in some patients (o-ethoxy benzoic acid [EBA] to replace eugenol)
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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CaSO4.0.5H2O + 1.5H2O CaSO4.2H2O + heat +expansion
2. Impression plaster
CaSO4 . 0.5H2O β-hemihydratePotassium sulfate – to reduce expansion but accelerates
settingBorax – a retarder to prolonge setting timeDiatomaceous earth, quartz, lime - to make the plaster
more brittle
Composition:
Setting reaction:
app. 0.1 lin %
Main indication: impression of edentulous ridges
Advantages:
1. Cheap and long shelf life, easy to prepare 2. Very good surface detail reproduction
3. Excellent dimensional stability
Disadvantages: 1. Very rigid – often need to be fractured when
removed from the mouth 2. Fractures if undercuts are present 3. Non-toxic but may dry soft tissues - unpleasant to
patients
Old fashioned – not frequently used
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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3. Impression compounds(Kerr‘s, Stent‘s impression compounds)
Thermoplastic material (softens when heated to 50oCand hardens on cooling) for making impressions of
edentulous ridge, tooth impressions in a copper band
Composition:1. Resins (wax, shellac, guttapercha)2. Filler (talc)3. Lubricants (stearic acid, stearin)
B. Reversible
Advantages:1. Can be reused, easy to use
2. Non-irritant and non-toxic
Disadvantages
1. Poor dimensional stability2. Easy to distort when withdrawn from the mouth
Old fashioned – not frequently used
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Elastic impression materialsA. Hydrocolloid impression materials
B. Elastomeric impression materials
Silicone - condensation
- addition
Polyethers
Elastomeric (non-aqueous)(irreversible)
HydrocolloidIrreversible
ReversiblePolysulfide
A. Hydrocolloid impression materialsHydrocolloid – a colloidal system (particle size up to
app. 0.5 µm) with water as a dispersion medium so called HYDROCOLLOID SOL that can be
tranformed to a semisolid GEL
Cooling
Heating
Coagulation
reversible/agar sol gel
irreversible/alginate sol gel
Setting reaction
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Alginate impression materials(Irreversible hydrocolloid impression material,
preliminary, orthodontic impressions etc.)
Alginate chains M = 30 – 150 000
Form viscous sols and gels afteraddition of Ca+2 ions
Constituent units
B. Irreversible hydrocolloid
Based on natural polysaccharide – Na+, K+, triethanol aminealginate salts (isolated from brown seaweeds)
Setting/gelation reaction:
NanAlg + CaSO4 nNa2SO4 + CanAlg2H2O
powder gel
Egg-box structureCross-linked structure of alginate gels
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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1. Na/K alginate2. Calcium sulfate (CaSO4.2H2O, CaSO4.1/2 H2O)3. Diatomaceous earth (amorphous SiO2)4. Retarder – phosphates prolonge working time
2Na3PO4 + 3CaSO4 3Na2SO4 + Ca3(PO4)25. Setting accelerator - K2TiF6 (makes also stone surface
hard)6. Additives – glycol, paraffin oils – to agglomerate particles
and make material „dustless“
Composition:
Mixing ratio powder/water app. 10 g/20 mL
Properties:1. Set after mixing with water2. Shrink due to loss of water by Syneresis –
expression of water from the surface ofimpression (if contains Na2SO4 – decreasedquality of stone surface); by Evaporation ofwater from the surface
3. Imbibition – sorption of water causing adimensional change
4. Chromatic phase indicator may be incorporated tosignal the impression setting
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Dimensional changes of a cast prepared fromalginate impression stored in various environments
Phillips‘ Science of Dental materials
Store in a closedcontainer with atowel saturated
with water
0.00
0.20
0.40
0.60
0.80
1.00
0 5 10 15 20 25
Ypeen PremiumYpeenPhaseHydrogumElastic
Time /hours
Dim
ensi
onal
cha
nges
lin
%
Dimensional changes of a cast prepared fromvarious alginate materials with storage time
RH=100 %/23oC
Standardtypes
Prolongedpouring
time
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Dependence of pH and rigidity on time
4.5
5.5
6.5
7.5
8.5
0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00time [min]
0
50
100
150
rigid
ity
BlueBlue - - violetviolet
PinkPinkWhiteWhite
SettingSettingpH
Acid-base colour indicators (phenolftalein/thymolphtalein)
5 sec5 sec 35 sec35 sec
1 min 30 sec1 min 30 sec 2 min2 min
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Advantages:1. Very good biological tolerance2. Easy to use and mix
3. Fast setting 4. Low price
Disadvantages: 1. Poor dimensional stability
2. Setting dependent on water temperature and water hardness
3. Although 100 h pouring time is also recommended theyshould be poured as soon as possible
4. Sometimes problems with a model stone compatibility
Most frequently used impression material
Agar impression materials(Reversible hydrocolloid impression material)
Based on thermoreversible gelation of natural polysaccharide– agar (isolated from red algae/seaweeds)
Agarose, is a stronglygelling, non-ionicpolysaccharide
1,3- linked β-D-galactopyranose and
1,4-linked 3,6-anhydro-α-L-galactopyranose units
Agaropectin, is morecomplex polysaccharidehaving sulfate groups
B. Reversible hydrocolloid
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Agar sols form gels upon cooling of a hot solution to (30 – 40)°C
Agar gels melt to sols upon heating to (90 – 95)°C
Solrandom coils
Gelation IDouble helices
Gelation IIaggregation of
helices
Gelation:
1. Agar2. Borax to increase the gel strength3. Potassium sulfate as a gypsum hardener4. Water – dispersion medium
Composition:
Supplied in two forms – in tubes and cartridges
Tray material Filled tray
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Material in syringesapplied to teeth
Compartments for liquefying 100oC,storage 65oC and tempering the
material in a tray at 45oCAdvantages:
1. Very good biological properties2. Excellent surface detail reproduction
Disadvantages:1. Need special equipment (water bath) and special technique2. Dimensionally instable - evaporation or imbibition3. Low strength and poor tear resistance4. Slow setting time
100oC 65oC 45oC
Other applications of hydrocolloidmaterials
Reversible hydrocolloid impressionmaterials are used as duplicating materialsto prepare dental cast models (gypsum or
from investment materials)
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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C. Elastomeric (non-aqueous)impression materials
Polysulfide
Silicone - condensation
- addition
Polyethers
Elastomeric (non-aqueous)(irreversible)
Main indications· impressions for partial prostheses (removable)· impressions for crowns and bridges (fixed)· impressions for implantology
Synthetic polymers with rubber properties after setting
Polymerization shrinkage of elastomers isusually compensated for using a combinationof:
1. Highly filled-viscous material „Putty“for preliminary impression (low polymercontent - of low shrinkage)
2. Low filled-low viscosity „wash“ or „light“ impression material (high polymer content - higher elasticity buthigher shrinkage)
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Based on reaction between polymer with freemercaptan (-SH) groups and oxidizing agent PbO2which lenghtens and cross-links chains via reaction
of terminal and pendant -SH groups)
(Thiokol rubbers, mercaptan rubbers)The very first elastomeric impression material
Polysulfide impression materials
HSSH
SH
HS HS
HS
SH
HS
SH HS
SH
HS
PbO2 PbO + O-H2O
O
-S – S-
Linear polysulfide polymer
Catalyst
Cross-linkedpolymer
Setting reaction:
Water releasedincreases
contraction
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Composition:
Base paste:
polysulfide polymer, filler, plasticizer
Catalyst paste:
lead dioxide, sulfur, inert oil
Volume mixing ratio 1:1
Supplied in a two-paste system
1. Low price 2. Long working time
Advantages:
Disadvantages:
1. Should be poured within 0.5 – 1 hour2. Lead oxide may cause toxic effects3. Unpleasant mercaptane smell4. Long setting time app. to 10 min5. Poor elastic recovery – prone to plastic deformation
Old fashioned – not frequently used
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Polyether impression materials
Based on cross-linking of polyether chains viacationic polymerization of aziridine rings using a
aromatic sulfonate ester as an initiator
Initiation:
SO3R SO3-
+ R+
Propagation:
CH3-CH-CO-O-[CHR-(CH2)n –O]m-O-CO-CH-CH3
-
N
CH2 CH2
N
CH2 CH2
R++
N-RCH2
+CH2N
CH2 CH2
+
Ring opening
Hydrophilic part(ethylene oxide units)Linear polymer
Cross-linked structure
aziridinerings
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Composition:
Base paste:• Polyether• Filler, plasticizer
Catalyst paste:
• Sulphonic acid ester• Inert oil• Filler
Supplied in a two-paste system
1. Rather stiff when set material (difficult to remove from mouth)
2. Very expensive3. May cause allergic reaction due to the sulphonic acid
ester
Advantages: 1. Naturally hydrophillic2. Accurate and high dimensional stability3. Good elastic recovery4. Low setting contraction5. Excellent surface detail reproduction
Disadvantages:
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Silicone impression materials
Based on cross-linking polycondensation reaction ofhydroxy terminated polysiloxane prepolymer with
tetra alkoxy silanes catalyzed by dibutyl-tindilaurate (DBTD)
C-silicone impression materials(condensation silicones)
+ Si(OC2H5)4
DBTD
Alcohol releases from the reaction mixture andcontributes to contraction/shrinkage of the impression
Setting reaction - polycondensation
OH
OH
… … …
SiO
Si
O
O O
SiO
SiO
…..O
…..O
O
Si
O
O
O
…..O
…..O
… …
Si
O
…..O
O
Si
…..O
O
…
…..O
SiO
Si
HO
OHO
…..O
O
…
4C2H5OH
…
Linear polymer Cross-linked polymer
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Composition:
Base paste:• Hydroxyterminated polysiloxane polymer• Filler (cristobalite, talc, starch)
Catalyst:Liquid catalyst:• Cross-linking agent (e.g tetraethoxy silane) +
activator (dibutyl-tin dilaurate)Paste catalyst:• Cross-linking agent, activator, inert oil• Filler
Supplied as a two-component system
1. Hydrophobic 2. Shrinking of impression over time, pouring time till 4 h 3. Catalyst may cause allergic reaction 4. Difficult to dispense proper volumes of both components 5. Usually hand-mix version only
Advantages: 1. Accurate impressions when poured soon2. Good elastic recovery3. Lower price
Disadvantages:
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Based on cross-linking polyaddition reaction of vinylterminated polysiloxane polymer with
methylhydrogen silicone cross-linking agent in thepresence of platinium catalyst
A-silicone impression materials(addition silicones, vinyl siloxane, poly(vinyl siloxane))
Impression material, duplicating material,relining material, bite registration material,
root canal sealer
Pt may release H2 from water or –OHgroups from the reaction mixture causing
bubbles on a gypsum model
Setting reaction - polyaddition
Vinylpolymer
Cross-linked polymer
CH3
CH3
+CH=CH2Si
CH3
Si
…..O
CH3
CH3
CH3
…..O +CH=CH2
CH3SiO
Si
H
O
O
CH3
SiH CH3
H + SiCH2=CH
PtCH2-CH2
Si
CH3
Si
…..O
CH3
CH3
CH3
…..O CH2-CH2
CH3SiO
SiO
O
CH3
Si CH3
SiCH2-CH2
CH3
CH3
Cross-linking agent
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Composition:
Base paste:1. Vinyl terminated polysiloxane polymer2. Pt catalyst3. Filler (cristobalite, talc)4. Surfactant (hydrophilic agent)
Catalyst:1. Vinyl terminated polysiloxane polymer2. Cross-linking agent3. Filler, silicone oil (adjust viscosity of
duplicating materials)
Supplied as a two-component 1:1 system
1. Hydrophobic – necessary to add a surfactant 2. Setting inhibited by latex gloves or some
adstringents (sulfur, heavy metals) 3. Hydrogen release – surface bubbles – pouring time
1 h after removal from the mouth 4. High price
Advantages: 1. Accurate impressions, very low shrinkage2. Very good surface detail3. Highly elastic4. Perfect elastic recovery5. Dimensionally stable
6. Non-toxic and non-irritant
Disadvantages:
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
29
Verylow
Verylow
N
50
96
0.5
3-4
1.5
1powder
Algi-nate
YNNNNHydrogen release
70*-809850-6082Verylow
Contact angle [o]
highlowerVery highlowhighRelative cost
2525252525Detailreproduction [µm]
99.0-99.997.2-99.698.3-99.094.5-96.998.8Recovery fromdeformation [%]
0.01-0.20.2-1.00.2-0.30.4-0.50.01Contraction [lin %]after 24 h
4-75-85-67-103-5Setting time[min]
2-42-42-34-7-Working time[min]
2 pastes2 pastes2 pastes2 pastes1No of components
A-siliconeC-siliconePolyetherPolysulfideAgarProperty
Partly from Phillips‘ Science of dental materials
Typical properties of elastic impression materials
*Hydrophilic types
Examples of typical packages of impressionmaterials
PuttyLight
Light
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague