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Impression Materials

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Page 1: Impression Materials


Impression materials

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1. Introduction2. Types if impressions3. Impression trays4. Ideal requirements5. Classification 6. Alginate7. Agar8. Polysulfide9. Condensation silicone10. Addition silicone11. Polyether

12. Viscoelastic property13. Manipulation14. Impression technique15. Removal of impression16. Preparation of stone

cast and die17. Dimensional stability18. Elasticity19. Tear strength20. Biocompatibility21. Common causes of

failures22. Comparison of


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Impression materials are used to make an accurate replica of the hard and soft oral tissues.

The area involved may vary from a single tooth to the whole dentition, or an impression may be made of an edentulous mouth.

The impression stage is the first of many stages involved in the production of dentures, crowns, bridges, orthodontic appliances etc.

The impression gives a negative reproduction of the tissues, and by filling the impression with dental stone or other model material, a positive cast is made.

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Types of dental impressions

Click icon to add picturePreliminary impression

1. diagnostic models

2. custom trays

3. provisional coverage

4. orthodontic appliances

5. pretreatment and post‑treatment records

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Final impression –

• most accurate reproduction of the teeth and surrounding tissues.

• Used to make indirect restorations, partial or full dentures, and implants.

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6Bite registration –

• reproduction of the occlusal relationship between the maxillary and mandibular teeth.

• Provide an accurate registration of the patient’s centric relationship between the maxillary and mandibular arches.

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Impression trays

Characteristics – Must be sufficiently rigid to:

1. Carry the impression material into the oral cavity.2. Hold the material in close proximity to the teeth.3. Avoid breaking during removal.4. Prevent warping of the completed impression.

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Types of impression trays

Quadrant tray Covers one half of the arch.

Section tray Covers the anterior portion of the arch.

Full arch tray Covers the entire arch.

Perforated tray Holes in the tray create a mechanical lock to hold

the material in place.Smooth tray

Interior of the tray is painted or sprayed with an adhesive to hold the impression material.

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Quadrant tray Section


Full arch tray

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Criteria for tray selection

Feel comfortable to the patient. Extends slightly beyond the facial surfaces of

the teeth. Extends approximately 2 to 3 mm beyond the

third molar, retromolar, or tuberosity area of the arch.

Sufficiently deep to allow 2 to 3 mm of material between the tray and incisal or occlusal edges of the teeth.

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The use of custom trays for polyether and addition silicone impressions is not critical since these materials are stiffer and have less polymerization shrinkage than the polysulfide material.

When severe undercuts are present, the use of a custom tray should be avoided. Disposable stock trays are also used to support the putty when the putty-wash technique is used for making impressions.

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Tray adhesives

VPS adhesives (blue) For polyvinyl siloxane and polyether impression

materials. butyl rubber or styrene acrylonitrile for polysulfide impression material

poly dimethyl siloxane, other silicone, Ethyl silicate

For condensation silicone impression material

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Impression materials

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Ideal requirements

1. A pleasant odor, taste, and esthetic color2. Absence of toxic or irritant constituents3. Adequate shelf life for requirements of storage and distribution4. Economically commensurate with the results obtained5. Easy to use with the minimum of equipment6. Setting characteristics that meet clinical requirements7. Satisfactory consistency and texture8. Readily wets oral tissues9. Elastic properties with freedom from permanent deformation after strain

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Ideal requirements

10. Adequate strength so it will not break or tear on removal from the mouth11. Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for a period long enough to permit the production of a cast or die12. Compatibility with cast and die materials13. Accuracy in clinical use14. Readily disinfected without loss of accuracy15. No release of gas during the setting of the impression or cast and die materials

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Reversible hydrocolloids

Irreversible hydrocolloids

Condensation polymerized

Addition polymerized


Condensation polymerized silicone

Addition polymerized silicone






WaxThermoplastic compounds

PlasterZinc oxide eugenol paste

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The present alginate hydrocolloid, or irreversible impression material, was developed as a substitute for the agar impression material when its supply became scarce during World War II.

This material is based on a natural substance extracted from certain brown seaweed. The substance is called anhydro-P-d-mannuronic acid or alginic acid.

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Potassium titanium fluoride

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Gelation process

A reaction of soluble alginate with calcium sulfate and the formation of an insoluble calcium alginate gel.

Structurally, calcium ions replace the sodium or potassium ions of two adjacent molecules to produce a cross-linked complex or polymer network.

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Role of retarder

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Setting time

Fast setting alginate – 1.5 to 3 minutesNormal setting alginate – 3 to 4.5 minutesThis is regulated by the manufacturer by the

amount of retarder addedThe clinician can alter the setting time by the

temperature of water added. NOT BY THE AMOUNT OF ADDED WATER.

A 1 min reduction in setting time occurs for each 10 °C of temperature increase.

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Temperature (°C)

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Packaging and dispensing of alginate

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The measured powder is sifted into premeasured water that has already been poured into a clean rubber bowl.

The powder is incorporated into the water by carefully mixing with a metal or plastic spatula that is sufficiently flexible to adapt well to the wall of the mixing bowl.

A vigorous figure-8 motion is best, with the mix being swiped or stropped against the sides of the rubber-mixing bowl with intermittent rotations (180°) of the spatula to press out air bubbles. All of the powder must be dissolved.

Mixing time - 45 sec to 1 min

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The result should be a smooth, creamy mixture that does not readily drip off the spatula when it is raised from the bowl.

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Making the impression

Perforated tray is generally used.Plastic tray or metal rim lock tray – alginate

tray adhesiveThickness – at least 3 mmImpression should not be removed from the

mouth at least for 3 minutes after gelation. (compressive strength doubles in the first 4 mins after gelation)

Not left in the mouth for longer than 6 – 7 mins.

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Permanent deformation

The ANSI/ADA Specification requires that the recovery from deformation be more than 95% when the material is compressed 20% for 5 seconds at the time it would normally be removed from the mouth.

Lower permanent deformation (higher accuracy) occurs

(1) when the percent compression is lower (adequate bulk of the material)

(2) when the impression is under compression a shorter time (snap removal)

(3) when the recovery time is longer, up to about 8 minutes after the release of the load.

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The ANSI/ADA Specification permits a range of 5% to 20% at a stress of 1000 g/cm2, and most alginates have a typical value of 14%.

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Compressive strengths range from 5000 to 9000 g/cm2. (Acc. to ADA should be at least 3750 g/cm2)

Tear strengths vary from 380 to 700 g/cmThe tear strength is a measure of the

force/thickness ratio needed to initiate and continue tearing.

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Causes of distortion and dimensional changes in alginate impression



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Disinfection of the impression


After the impression is rinsed thoroughly, the disinfectant can be sprayed liberally on the exposed surface. The impression is then wrapped immediately in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min.

Alternate method is immersion in the disinfectant solution (not more than 10 mins)

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Compatibility with gypsum

Water content of hydrocolloid inhibits setting of gypsum at the surface.

The gelation process of alginate impression produces not only insoluble calcium alginate but also sodium sulfate.

Sodium sulfate – low conc. – accelerator high conc. – retarderIn alginate impression, the amount is high

enough to act as retarder.

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The pouring of a stone mixture to fill the impression should start from one end of the arch.

somewhat superior stone surfaces may be obtained if the impression is placed in a humidor while the stone hardens.

The filled impression should never be immersed in water while the stone sets.

The stone cast or die should be kept in contact with the impression for a minimum of 30 min, preferably for 60 min.

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Shelf life

2 major factors affecting shelf life – 1. Storage temperature2. Moisture contaminationAvailable in individually sealed pouches or in

bulk form in cans.It is best not to stock more than 1 year's

supply in the dental office. The material should be stored in a cool, dry


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Common causes of failures

• Improper mixing • Prolonged mixing• Excessive gelation• W/P ratio too low

Grainy material

• Inadequate bulk• Premature removal from the mouth• Moisture contamination• Prolonged mixing


• Undue gelation preventing flow• Air incorporated during mixing

External bubbles

Irregularly shaped voids

• Moisture or debris on tissues

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•Inadequate cleaning of impression•Excess water left in impression•Premature removal of the impression•Model left in impression too long•Improper manipulation of stone

Rough chalky stone


•impression not poured immediately•Movement of tray during gelation•Premature removal from mouth•Improper removal from mouth


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Agar reversible hydrocolloids

The setting of a reversible hydrocolloid, often called gelation, is a solidification process that involves phase changes from sol to gel states.

This changes is brought about by temperature change.

The gel-to-sol and sol-to-gel transformations are dependent on time and temperature. The liquefaction and gelation temperatures are different (the latter being lower), and the effect is called hysteresis. A typical value of the gelation temperature is 43°C (109°F).

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Thymol Bactericidal agentGlycerine PlasticizerColoring and flavoring agents

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Tray material Packaged in plastic tubes. Used with water-cooled

tray.Syringe material

Packaged in plastic or glass cartridges that fit a syringe.

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Liquefy the hydrocolloid gel in boiling water. The material must be held at this temperature for a minimum of 10 mins.

After it has been liquefied, it may be stored in the sol condition at 65°C until it is needed for injection into the cavity preparation or for filling a tray.

a gauze pad is placed over the top of the tray material, and the tray is placed in the water-filled tempering container (45° C) of the conditioning unit. The syringe material, on the other hand, is never tempered.

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Conditioning unit

65 °C

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Permanent deformation and Flexibility

The ANSI/ADA Specification requires that the recovery from deformation be greater than 96.5% after the material is compressed 20% for 1 second.(99%)

The ANSI/ADA Specification requirement for flexibility allows a range of 4% to 15%

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The compressive strength of a typical agar hydrocolloid impression material is 8000 g/cm2.

The tear strength of agar hydrocolloid impression materials is about 800 to 900 g/cm, which is higher than the ANSI/ADA Specification requirement of 765 g/cm

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Causes of failure of agar impression

• Inadequate boiling• Storage temperature too low• Storage time too long

Grainy material

• Water soaked tray material surface not removed

• Premature gelation of either material

Separation of tray and syringe material

• Inadequate bulk• Premature removal from mouth• Syringe material partially gelled

when tray was seatedTearing

• Gelation of syringe material; prevents flowExternal bubbles

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• Material too cold

Irregularly shaped voids

• Inadequate cleaning of impression• Excess water left in impression• Premature removal of the

impression• Model left in impression too long• Improper manipulation of stone

Rough chalky stone


Distortion• impression not poured

immediately• Movement of tray during

gelation• Premature removal from mouth• Improper removal from mouth• Use of ice water during initial

stages of gelation

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Laminate technique

Agar-Alginate combination techniqueSyringe material – agarTray material – alginate

Advantages • simplification of heating

equipment• the elimination of water-

cooled impression trays• Overall simplification of the

procedure• agar hydrocolloid is more

compatible with gypsum model materials than alginates

Disadvantages • the bond between the agar

and the alginate is not always sound

• the higher viscosity of the alginate material displaces the agar hydrocolloid during seating

• the dimensional inaccuracy of the alginate hydrocolloid limits its use to single units.

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Non-aqueous elastomeric impression materials

A Group of flexible chemical polymers, which are either chemically or physically cross-linked. Generally they can be easily stretched and rapidly recover their original dimensions when applied stress are released.

1. Polysulfide 2. Condensation silicone3. Addition silicone4. polyether

Condensation polymerization

Addition polymerization

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Polysulfide rubbers are widely used for fixed partial denture application, due to their high accuracy and relatively low cost.

These materials are useful for multiple impressions when extra time is needed.

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Base paste: mercaptan or polysulfide polymer (80 to 85%) lithopone and titanium dioxide (16 to 18%) dibutyl phthalate

Reactor paste: lead dioxide (reactor) (60-68%) lithophone or titanium dioxide (filler) Dibutyl phthlalate (plasticizer) (30-35%) Sulfur (3% ) (accelerator) oliec acid or stearic acid (retarder)

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--SH HS---------------------SH HS--











OO = Pb = O





+ 3PbO + H2O

mercaptan + lead dioxide polysulfide rubber + lead oxide + water

Polysulfide Reaction

Linear polymer ,with 1 mol% of branches to provide pendant mercaptan groups

Condensation reaction

Chain lengthening

Cross linking



Moisture and temperatureAlter the reaction

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Available in light, medium, heavy viscosities.

Same matched tubes to be usedPseudoplasticitySetting time 5-8 min

• Temperature• humidity• Loss of Water • Altering base :accelerator ratio

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Setting time is 6 – 10 mins; which is more than other elastomeric impression materials.

Very good tear resistance; can withstand about 700% tensile strain before tearing.

Least stiff material – most easily removed from undercut areas.

They are considered as viscoelastic and recover only slowly and not completely after being compressed or stretched.

Dimensional changes are mainly due to 2 reasons – Continued setting reaction By-product

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The use of lead compounds in polysulfide materials has been questioned because of the known toxic effects of lead.

There have been some limited reports of allergic responses to these materials, from people with an allergy to latex products.

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Advantages and disadvantages

AdvantagesLower cost

compared to silicones and polyethersLong working timeHigh tear strengthHigh flexibilityGood detail reproductionLess hydrophobicLong shelf life.

DisadvantagesPoor dimensional

stability water by-product pour within one hour single pour, second pour

less accurate.Custom traysMessy

bad odor (thiol group in base paste)

may stain clothing (lead dioxide in catalyst paste)

Long setting time.

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Condensation silicone

Condensation silicone rubber impression materials are used mainly for fixed partial denture impressions.

They are ideal for single-unit inlays.

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Base pastepoly(dimethylsiloxane)

with terminal hydroxy groups (-OH),

orthoalkylsilicateFiller- 37%to

75%colloidal silica or microsized metal(added to form paste)or copper carbonate.

Reactor paste or liquid.Organo tin compound-

dibutyl tin dilaurate or stannous octoate

Oily diluent Organic dyes or


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HO – Si – O – Si - O - H



CH3 n

HO – Si – O – Si - O - H



CH3 n




HO – Si – O – Si - O -



CH3 n

HO – Si – O – Si - O -



CH3 n





Condensation Silicone Reaction

stannous octoate



α-ω Hydroxyl terminated polydimethyl siloxane

Tetraethyl ortho silicate


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one drop per inch of extruded base paste (for base paste/liquid catalyst system)Also available in putty consistencysetting time (6 to 8 minutes)

TemperatureAltering base catalyst ratio

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Advantages and disadvantages

AdvantagesBetter elastic

propertiesClean, pleasantStock tray

putty-washGood working and

setting timeLess distortion on


Disadvantages HydrophobicPoor dimensional

stability high shrinkage

polymerization evaporation of ethanol

pour immediately within 30 minutes

poor wettability poor to adequate shelf life.Slightly more expensive.

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Addition silicone

These impression materials are also called vinyl polysiloxane or polyvinyl siloxane impression materials.

They represent an advance in accuracy over condensation silicone.

Due to their high accuracy, these materials are suitable for fixed and removable partial denture impressions.

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Base pastePolymethyl hydrogen siloxaneOther siloxane prepolymersFillersHybrid silicone

Catalyst pasteDivinyl polydimethyl siloxanePlatinum salt(catalyst)RetardersFillers.Platinum or palladium –to absorb the


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- Si – CH3


CH3 - Si -


---O – Si – CH2 - CH2



CH2 - CH2 – Si – O ---



Platinum salt

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Secondary reaction between moisture and residual hydrides of the base polymer can lead to the development of hydrogen gas.

Can cause pin-point voids on

the cast

Prevention – • Platinum or palladium added

to the formulation acting as H2 scavengers

• Waiting for an hour or more before pouring a cast

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Available in – light, medium, heavy and putty consistency.

2 paste systemMonophase systemPutty in 2 jarsPutty-wash

A silicone is most sensitive to temperature changes.Custom tray is required except for putty- wash


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Major disadvantages

HYDROPHOBICCauses distortion or

loss of details at the margin due to undetected moisture.

Also prevents gypsum products from accurate reproduction of details.

Prevention – adding non-ionic surfactant to the paste renders the surface hydrophillic.


Natural latex glovesSome vinyl gloves

containing sulfur stabilizers.

Prevention – recording the impression without touching the oral structures with gloves.

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Advantages Dimensionally stable pour up to one week

Stock or custom traysShorter setting timeLess distortion on removalAdequate tear strength.Multiple castsEasy to mixHighly accuratePleasant odor

DisadvantagesExpensiveSulfur inhibits set

latex gloves Ferric and Alsulfate

retraction solutionShort working timeLower tear strengthPossible hydrogen gas release

bubbles on die palladium added to absorb hydrophilic material also still needs to be carefully handled

for a very dry field.

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Polyether rubbers are used for accurate impressions of a few prepared teeth without severe undercuts.

Their high stiffness and short working time restricts their use to impressions of a few teeth.

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Base paste:Polyether polymerColloidal silica-fillerGlycolether or phthalate-plasticizer

Accelerator paste:Alkyl aromatic sulfonateFillerPlasticizer.Thinner-octyl phthalate

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MonophaseAutomatic mixing devicesThinner addedLess sensitive to temperature changes.

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Advantages and disadvantages

Advantages:Highly accurateGood dimensional stabilityStock or dual-arch traysGood elasticity.Good surface detailPour within one week

kept dryMultiple castsGood wettability

Disadvantages:ExpensiveShort working timeRigid

difficult to remove from undercutsBitter tastestiffnessAbsorbs water

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General properties

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Viscoelastic properties

Elastic solidSPRING

Viscous liquidOIL


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The amount of permanent deformation exhibited by an elastomeric impression material should be clinically negligible, provided that:

(1) the material has adequately gelled(2) negligible pressure is applied to the tray during polymerization(3) the impression has been removed rapidly along the path of tray insertion(4) the undercuts present in the cavity preparation are minimal

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base paste and catalyst system

base paste and liquid catalyst


2 putty systemPutty and

liquid catalyst system

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Automatic mixing devices

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Multiple mix


Monophase or single viscosity




2 stage putty wash

technique or reline


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Multiple mix techniqueSingle viscosity technique

Simultaneous putty-wash technique 2 stage putty-wash technique

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Removal of the impression

Should not be removed until it has adequately gelled

The borders of the tray should be pried loose parallel to the path of insertion until air leaks into the tray.

Then the tray can be removed rapidly with minimal rotation or twisting.

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Preparation of stone casts and dies

Polyether and addition silicone – multiple casts can be poured because of their excellent dimensional stability

Polyvinyl siloxane – hydrophobic nature – epoxy resin

To pour casts in gypsum a surfactant needs to be applied to the surface of the impression. Soap slurry can be used.

Hydrophilic addition silicone exhibit a contact angle of 30 - 35° with water.

Pouring stone cast in polyether is easier than silicone impressions.

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Dimensional stability

There are five major sources of dimensional change:

(1) polymerization shrinkage(2) loss of a by-product (water or alcohol) during the condensation reaction,(3) thermal contraction from oral temperature to room temperature(4) imbibition when exposed to water; disinfectant or a high humidity environment over a period of time(5) incomplete recovery of deformation because of viscoelastic behavior.

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The relative amount of permanent deformation following strain in compression increases in the following order :

1. addition silicone2. condensation silicone3. polyether4. PolysulfideThe stiffness increases in following order:1. Polysulfide2. Condensation silicone3. addition silicone4. polyether

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Tear strength

Tear strength increases in following order:1. Hydrocolloids (agar and alginate)2. silicones (addition and condensation)3. Polyether4. polysulfide.Tear strength is also influenced by the

consistency and the manner of removal of the materials.

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Cell toxicity – polysulfide: lowest cell death count

polyether: highest cell death countMost likely biocompatibility problem – segment of impression material lodged in a patient’s gingival sulcus

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Common causes of failure of elastomeric impression material

Rough or uneven surface on impression1. Incomplete polymerization caused by

premature removal from the mouth, Improper ratio or mixing of components, or presence of oil or other organic material on the teeth(for A silicone, agents that contaminate the material and inhibit polymerization)

2. Too rapid polymerization from high humidity or temperature

3. Excessively high accelerator/base ratio

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93Bubbles1. Too rapid polymerization, preventing flow2. Air incorporated during mixingIrregularly shaped voids1. Moisture or debris on the surfaceRough chaulky stone cast 1. Inadequate cleaning of impression2. Excess water left on surface of the impression3. Excess wetting agent left on impression4. Premature removal of cast5. Improper manipulation of stone6. Failure to delay pour of addition silic0ne at least

20 mins

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Causes of distortion

1. Resin tray not aged sufficiently2. Lack of adhesion of rubber to the tray caused by not

applying enough coats of adhesive, filling the tray with material too soon after applying adhesive or using wrong adhesive

3. lack of mechanical retention for those materials where adhesive is ineffective

4. Development of elastic properties in the material before tray is sealed

5. Excessive bulk of material6. Insufficient relief for the reline material (if such technique

is used)7. Continued pressure against impression material that has

developed elastic properties8. Movement of tray during polymerization9. Premature removal from mouth10. Improper removal from mouth11. Delayed pouring of the polysulfide 0r condensation silicone


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Comparison of Properties

Working time longest to shortest

polysulfide > silicones > polyether

Setting time shortest to longest

polyether < silicones < polysulfide

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Comparison of Properties

Stiffness most to least

polyether > addition silicone > condensation silicone > polysulfides

Tear strength greatest to least

polysulfide > addition silicone > polyether = condensation silicone.

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Comparison of Properties

Cost lowest to highest

polysulfide <condensation silicone < addition silicone < polyether

Dimensional stability best to worst

addition silicone 0.15%> polyether 0.2%> polysulfide 0.4%> condensation silicone0.6% Phillip’s 1996

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Anusavice K. J. : Phillips’ science of dental materials, 11th edition.

Powers J. M., Sakaguchi R. L. : Craig’s restorative dental materials, 12th edition

O’Brien W. J. : Dental materials and their selection, 4th edition

McCabe J. F., Walls A.W.G. : Applied dental materials, 9th edition

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