Impression materials
Pavel Bradna
Institute of Dental Research
Purpose
To prepare accurate and true replicas of oral
structures (teeth, mucosa)
The replica is prepared in 2 steps:
Step 1. Making a negative (impression)
Step 2. Prepare a model or a cast accurate positive
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Requirements
1. Capable of plastic to rigid body transformation
2. Acceptable for a patient
- non-toxic, non-irritant, tasteless, reasonable
setting time up to 5-7 min
3. Good handling properties – easy to prepare/mix
plastic before set, but viscous enough not to flow
out of a tray (thixotropic), adequate working and
setting times
After being set:
4. Accuracy and detail reproduction (25-50µm),
5. Dimensionally stable
6. Resistant to mechanical stress - elastic and rigid
7. Compatible with model materials
8. Resistant to disinfectant solutions
9. Cost effective
Important terms
• Elastic/plastic deformations
• Strength
• Pseudoplastic/thixotropic
• Hydrophilic/hydrophobic
• Working time
• Setting time
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Elasticity/plasticity
elastic/plastic deformation
Describes behaviour of material under load
stress (force/m2)
Region of elastic
deformations,
Hook‘s law applies
Region of plastic
deformations
Maximum load at
fracture – ultimate
strength
Slope – Young‘s modulus,
modulus of elasticity
Deformation
Proportional limit
Mechanical model of elastic behaviour
Deformation
A spring – ideally elastic behaviour (Hook‘s law)
No load
5N
Loaded
Deformation
Unloaded
When unloaded
!fully recovers!
t1
t2
loaded unloaded
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Mechanical model of plastic behaviour
A dashpot – ideally plastic behaviour
permanent/irreversible
deformation
When unloaded
!No recovery!
Deformation
Plastic =
Permanent
deformation
t1
t2
loaded
unloaded
Recovery from deformation
combination of a spring and a dashpot – a model of
viscoelasticity
Loaded for
a long time
deformation
Loaded for a
short time
Plastic/permanent
deformation
Pouring after
material
relaxation
t1
<
t2
Time
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Deformation
The effect of recovery from deformation
on the accuracy of a model
99%
% Recovery
from
deformation
95%
Time
Plastic
deformation
Pseudoplasticity/thixotropy of fluids
Slopeviscosity
Stress τ
Gradually loaded
Gradually unloaded
Thixotropic (structure is
„broken“ under shear
Gradually unloaded
Pseudoplastic
.
τ = ηγ
.
Rate of deformation γ
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Hydrophilic/hydrophobic
Contact
angle
Hydrophilic material
Hydrophobic material
(α>90o)
unable to wet humid
surfaces
able to wet humid surfaces
Does not copy the wet surface
Examples of clinical significance of pseudoplastic
properties, hydrophilicity and high mechanical
resistance (tear strength) of impression materials
Impression
material
Prepared
tooth
Compromised flow in small
details - too high viscosity
and hydrophobicity
Fracture and lost detail due
to low strength
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Working time – a period from the start of mixing to
the final time at which the impression can be seated
in the mouth without its distorsion
Setting time – a period from the start of mixing
till the impression becomes elastic enough to
resist deformation during its withdrawal from the
mouth
Classification of impression materials
Irreversible
Reversible
Inelastic/rigid
ZnO-eugenol (ZOE) Impression comImpression plaster pounds
Elastic
Alginate
Elastomeric:
Polysulfide
Polyether
Silicone
Agar hydrocolloid
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Inelastic/rigid impression
materials
A. Irreversible
1. Zinc-oxide eugenol impression
pastes (ZOE)
Main indications: impression of edentulous ridges,
surgical dressing
Setting reaction:
-OH and methoxy groups
chelate structure
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 or
pure eugenol, resins and fillers,
accelerators
H2O, acetic acid, Zn acetate
(C) Institute of Medical Biochemistry, 1st Fac. Med., Charles University, and Institute of Dental Research in Prague
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Comparison of viscosity changes during ZOE setting
Silicone impression materials
Low viscosity of ZOE
impression pastes – enables
mucostatic impressions
Edentulous ridges - low
viscosity is favourable to
avoid displacement of
tissues
rigidity
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)
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2. Impression plaster
Main indication: impression of edentulous ridges
Setting reaction:
CaSO4.0.5H2O + 1.5H2O
CaSO4.2H2O + heat +expansion
Composition:
app. 0.1 lin %
CaSO4 . 0.5H2O β-hemihydrate
Potassium sulfate – to reduce expansion but accelerates
setting
Borax – a retarder to prolonge setting time
Diatomaceous earth, quartz, lime - to make the plaster
more brittle
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
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B. Reversible
3. Impression compounds
(Kerr‘s, Stent‘s impression compounds)
Thermoplastic material (softens when heated to 50oC
and 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)
Advantages:
1. Can be reused, easy to use
2. Non-irritant and non-toxic
Disadvantages
1. Poor dimensional stability
2. Easy to distort when withdrawn from the
mouth
Old fashioned – not frequently used
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Elastic impression materials
A. Hydrocolloid impression materials
B. Elastomeric impression materials
Irreversible
Hydrocolloid
Reversible
Polysulfide
Elastomeric (non-aqueous)
(irreversible)
Silicone - condensation
- addition
Polyethers
A. Hydrocolloid impression materials
Hydrocolloid – 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
Setting reaction
reversible/agar
sol
irreversible/alginate sol
Cooling
Heating
Coagulation
gel
gel
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B. Irreversible hydrocolloid
Alginate impression materials
(Irreversible hydrocolloid impression material,
preliminary, orthodontic impressions etc.)
Based on natural polysaccharide – Na+, K+, triethanol amine
alginate salts (isolated from brown seaweeds)
Constituent units
Alginate chains M = 30 – 150 000
Form viscous sols and gels after
addition of Ca+2 ions
Setting/gelation reaction:
NanAlg + CaSO4
powder
H2O
nNa2SO4 + CanAlg2
gel
Egg-box structure
Cross-linked structure of alginate gels
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Composition:
1. Na/K alginate
2. Calcium sulfate (CaSO4.2H2O, CaSO4.1/2 H2O)
3. Diatomaceous earth (amorphous SiO2)
4. Retarder – phosphates prolonge working time
2Na3PO4 + 3CaSO4
3Na2SO4 + Ca3(PO4)2
5. Setting accelerator - K2TiF6 (makes also stone surface
hard)
6. Additives – glycol, paraffin oils – to agglomerate particles
and make material „dustless“
Mixing ratio powder/water app. 10 g/20 mL
Properties:
1. Set after mixing with water
2. Shrink due to loss of water by Syneresis –
expression of water from the surface of
impression (if contains Na2SO4 – decreased
quality of stone surface); by Evaporation of
water from the surface
3. Imbibition – sorption of water causing a
dimensional change
4. Chromatic phase indicator may be incorporated to
signal the impression setting
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Store in a closed
container with a
towel saturated
with water
Dimensional changes of a cast prepared from
alginate impression stored in various environments
Phillips‘ Science of Dental materials
Dimensional changes of a cast prepared from
various alginate materials with storage time
RH=100 %/23oC
Dimensional changes lin %
1.00 Ypeen Premium
Ypeen
Phase
0.80 Hydrogum
Elastic
Standard
types
0.60
0.40
Prolonged
pouring
time
0.20
0.00
0
5
10
15
20
25
Time /hours
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Dependence of pH and rigidity on time
8.5
150
Blue - violet
White
6.5
100
r ig id ity
Pink
7.5
pH
Setting
50
5.5
4.5
0
0:00
2:00
4:00
6:00
8:00
time [min]
10:00
12:00
14:00
Acid-base colour indicators (phenolftalein/thymolphtalein)
5 sec
35 sec
1 min 30 sec
2 min
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Most frequently used impression material
Advantages:
1. Very good biological tolerance
2. 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 they
should be poured as soon as possible
4. Sometimes problems with a model stone compatibility
B. Reversible hydrocolloid
Agar impression materials
(Reversible hydrocolloid impression material)
Based on thermoreversible gelation of natural polysaccharide
– agar (isolated from red algae/seaweeds)
Agarose, is a strongly
gelling, non-ionic
polysaccharide
Agaropectin, is more
complex polysaccharide
having sulfate groups
1,3- linked β-D-galactopyranose and
1,4-linked 3,6-anhydro-α-L-galactopyranose units
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Gelation:
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
Sol
random coils
Gelation II
aggregation of
helices
Gelation I
Double helices
Composition:
1. Agar
2. Borax to increase the gel strength
3. Potassium sulfate as a gypsum hardener
4. Water – dispersion medium
Supplied in two forms – in tubes and cartridges
Tray material
Filled tray
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100oC
Material in syringes
applied to teeth
Advantages:
65oC
45oC
Compartments for liquefying 100oC,
storage 65oC and tempering the
material in a tray at 45oC
1. Very good biological properties
2. Excellent surface detail reproduction
Disadvantages:
1. Need special equipment (water bath) and
special technique
2. Dimensionally instable - evaporation or imbibition
3. Low strength and poor tear resistance
4. Slow setting time
Other applications of hydrocolloid
materials
Reversible hydrocolloid impression
materials are used as duplicating materials
to prepare dental cast models (gypsum or
from investment materials)
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C. Elastomeric (non-aqueous)
impression materials
Synthetic polymers with rubber properties after setting
Polysulfide
Elastomeric (non-aqueous)
(irreversible)
Silicone - condensation
- addition
Polyethers
Main indications
· impressions for partial prostheses (removable)
· impressions for crowns and bridges (fixed)
· impressions for implantology
Polymerization shrinkage of elastomers is
usually compensated for using a combination
of:
1. Highly filled-viscous material „Putty“
for preliminary impression (low polymer
content - of low shrinkage)
2. Low filled-low viscosity „wash“ or
„light“ impression material (high
polymer content - higher elasticity but
higher shrinkage)
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Polysulfide impression materials
(Thiokol rubbers, mercaptan rubbers)
The very first elastomeric impression material
Based on reaction between polymer with free
mercaptan (-SH) groups and oxidizing agent PbO2
which lenghtens and cross-links chains via reaction
of terminal and pendant -SH groups)
Setting reaction:
Linear polysulfide polymer
HS
SH
HS
HS
O
SH
HS
SH
HS
SH
HS
SH
HS
Catalyst
PbO2
-H2O
PbO + O
Water released
increases
contraction
-S – S-
Cross-linked
polymer
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Composition:
Supplied in a two-paste system
Base paste:
polysulfide polymer, filler, plasticizer
Catalyst paste:
lead dioxide, sulfur, inert oil
Volume mixing ratio 1:1
Advantages:
1. Low price
2. Long working time
Disadvantages:
1.
2.
3.
4.
5.
Should be poured within 0.5 – 1 hour
Lead oxide may cause toxic effects
Unpleasant mercaptane smell
Long setting time app. to 10 min
Poor elastic recovery – prone to plastic deformation
Old fashioned – not frequently used
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Polyether impression materials
Based on cross-linking of polyether chains via
cationic polymerization of aziridine rings using a
aromatic sulfonate ester as an initiator
Initiation:
+
SO3R
R+
SO3-
Propagation:
Linear polymer
Hydrophilic part
(ethylene oxide units)
CH3-CH-CO-O-[CHR-(CH2)n –O]m-O-CO-CH-CH3
Ring opening
N
CH2
CH2
N
CH2
aziridine
rings
-
N +
CH2
+CH
2
+
R+
CH2
N-R
CH2
CH2
Cross-linked structure
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Composition:
Supplied in a two-paste system
Base paste:
•
•
Polyether
Filler, plasticizer
Catalyst paste:
•
Sulphonic acid ester
•
Inert oil
•
Filler
Advantages:
1.
2.
3.
4.
5.
Naturally hydrophillic
Accurate and high dimensional stability
Good elastic recovery
Low setting contraction
Excellent surface detail reproduction
Disadvantages:
1. Rather stiff when set material (difficult to remove
from mouth)
2. Very expensive
3. May cause allergic reaction due to the sulphonic acid
ester
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Silicone impression materials
C-silicone impression materials
(condensation silicones)
Based on cross-linking polycondensation reaction of
hydroxy terminated polysiloxane prepolymer with
tetra alkoxy silanes catalyzed by dibutyl-tin
dilaurate (DBTD)
Setting reaction - polycondensation
DBTD
O
…..O
Si O
O
O
O
O Si
Si
O
O Si
O
…
…
…
…
O
Linear polymer
Si
O
HO Si
…..O
OH
…..O
…..O
Si
…..O
+ Si(OC2H5)4
O
…..O
O
O
4C2H5OH
…..O
O
HO Si
OH
…
Si
…
…
…
O
…..O
Cross-linked polymer
Alcohol releases from the reaction mixture and
contributes to contraction/shrinkage of the impression
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Composition:
Supplied as a two-component system
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
Advantages:
1. Accurate impressions when poured soon
2. Good elastic recovery
3. Lower price
Disadvantages:
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
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A-silicone impression materials
(addition silicones, vinyl siloxane, poly(vinyl siloxane))
Impression material, duplicating material,
relining material, bite registration material,
root canal sealer
Based on cross-linking polyaddition reaction of vinyl
terminated polysiloxane polymer with
methylhydrogen silicone cross-linking agent in the
presence of platinium catalyst
Setting reaction - polyaddition
CH3
…..O
Si CH=CH2 +
CH3
H Si
…..O
CH3
CH3
O
CH3 Si
O
CH3
…..O
CH3
O
Si CH=CH2
+
H Si
H +
CH3
O
CH3
CH3 Si
O
CH2=CH Si
CH3
CH3
CH3
Vinyl
polymer
Pt
Crosslinking agent
…..O
O
Si CH2-CH2 Si CH3
CH3
CH2-CH2 Si
CH3
Si CH2-CH2 Si CH3
CH3
Cross-linked polymer
Pt may release H2 from water or –OH
groups from the reaction mixture causing
bubbles on a gypsum model
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Composition:
Supplied as a two-component 1:1 system
Base paste:
1.
2.
3.
4.
Vinyl terminated polysiloxane polymer
Pt catalyst
Filler (cristobalite, talc)
Surfactant (hydrophilic agent)
Catalyst:
1. Vinyl terminated polysiloxane polymer
2. Cross-linking agent
3. Filler, silicone oil (adjust viscosity of
duplicating materials)
Advantages:
1. Accurate impressions, very low shrinkage
2. Very good surface detail
3. Highly elastic
4. Perfect elastic recovery
5. Dimensionally stable
6. Non-toxic and non-irritant
Disadvantages:
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
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Typical properties of elastic impression materials
Property
No of components
Algi- Agar
nate
1
Polysulfide Polyether C-silicone
A-silicone
1
2 pastes
2 pastes
2 pastes
2 pastes
powder
Working time
[min]
1.5
-
4-7
2-3
2-4
2-4
Setting time
[min]
3-4
3-5
7-10
5-6
5-8
4-7
Contraction [lin %]
after 24 h
0.5
0.01
0.4-0.5
0.2-0.3
0.2-1.0
0.01-0.2
Recovery from
deformation [%]
96
98.8
94.5-96.9
98.3-99.0
97.2-99.6
99.0-99.9
Detail
reproduction [µm]
50
25
25
25
25
25
Hydrogen release
N
N
N
N
N
Y
Contact angle [o]
Very
low
Very
low
82
50-60
98
70*-80
Relative cost
Very
low
high
low
Very high
lower
high
*Hydrophilic types
Partly from Phillips‘ Science of dental materials
Examples of typical packages of impression
materials
Putty
Light
Light
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