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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


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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


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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


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.

.


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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


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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


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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


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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)


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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


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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



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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


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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


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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


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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


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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


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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


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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


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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)


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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)


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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


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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



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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


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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:


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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


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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:


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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


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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:


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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


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