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Impression materials 1

Impression Materials

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Impression materials

Index1. Introduction 2.Types if impressions 3.Impression trays 4.Ideal requirements 5. Classification 6.Alginate 7. Agar 8.Polysulfide 9.Condensation silicone 10.Addition silicone 11.Polyether12. 13. 14.2

12.Viscoelastic property 13.Manipulation 14.Impression technique 15.Removal of impression 16.Preparation of stone cast and die 17.Dimensional stability 18.Elasticity 19.Tear strength 20.Biocompatibility 21.Common causes of failures 22.Comparison of properties 23.


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.

Preliminary impression


1. 2. 3. 4. 5.

diagnostic models custom trays provisional coverage orthodontic appliances pretreatment and posttreatm ent records

Types of dental impressions

Final impression most accurate reproduction of the teeth and surrounding tissues.


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

Bite registration reproduction of the occlusal relationship between the maxillary and mandibular teeth.


Provide an accurate registration of the patients centric relationship between the maxillary and mandibular arches.

Impression trays7


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


Types of impression trays8

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.


Quadrant tray

Section tray

Full arch tray


Criteria for tray selection11

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.


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.

Tray adhesives13

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


For condensation silicone impression material


Impression materials


Ideal requirements16

1. A pleasant odor, taste, and esthetic color 2. Absence of toxic or irritant constituents 3. Adequate shelf life for requirements of storage and distribution 4. Economically commensurate with the results obtained 5. Easy to use with the minimum of equipment 6. Setting characteristics that meet clinical requirements 7. Satisfactory consistency and texture 8. Readily wets oral tissues 9. Elastic properties with freedom from permanent deformation after strain

Ideal requirements17

10. Adequate strength so it will not break or tear on removal from the mouth 11. 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 die 12. Compatibility with cast and die materials 13. Accuracy in clinical use 14. Readily disinfected without loss of accuracy 15. No release of gas during the setting of the impression or cast and die materials

Thermal Non-elastic Chemical18

Wax Thermoplastic compounds Plaster Zinc oxide eugenol paste Reversible hydrocolloids Irreversible hydrocolloids

Aqueous Elastic Non-aqueous

Condensation polymerized Addition polymerized

Polysulfide Condensation polymerized silicone Addition polymerized silicone Polyether


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.


Potassium titanium fluoride

Gelation process21

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.

Role of retarder22

Setting time23

Fast setting alginate 1.5 to 3 minutes Normal setting alginate 3 to 4.5 minutes This is regulated by the manufacturer by the amount

of retarder added The 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.


Temperature (C)

Packaging and dispensing of alginate25


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 rubbermixing 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



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


Making the impression29

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

adhesive Thickness at least 3 mm Impression 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.

Permanent deformation30

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 ofthe 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.


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%.


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/cm The tear strength is a measure of the force/thickness

ratio needed to initiate and continue tearing.

Causes of distortion and dimensional changes in alginate impression33



Disinfection of the impression34

Iodophor Bleach Gluteraldehyde

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)

Compatibility with gypsum35

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. retarder In alginate impression, the amount is high enough to act as retarder.


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.

Shelf life37

2 major factors affecting shelf life

1. Storage temperature 2.Moisture contamination Available 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 environment.

Common causes of failures38

Moisture or debris on tissues


Agar reversible hydrocolloids40

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 43C (109F).



Thymol Glycerine Coloring and flavoring agents

Bactericidal agent Plasticizer


Tray material Packaged in plastic tubes. Used with water-cooled tray. Syringe material Packaged in plastic or glass cartridges that fit a syringe.


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 65C 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.

Conditioning unit45

65 C


Permanent deformation and Flexibility47


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%


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

Causes of failure of agar impression49


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

Laminate technique51

Agar-Alginate combination technique Syringe material agar Tray material alginate


ing equipment the bond between the agar and the alginate is not always sound ter-cooled impression trays the higher viscosity of the alginate material displaces the agar hydro n of the procedure the dimensional inaccuracy of the alginate hydrocolloid limits its us more compatible with gypsum model materials than alginates


Non-aqueous elastomeric impression materials52

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.Condensation polymerization Addition polymerization

1. Polysulfide 2.Condensation silicone 3.Addition silicone 4.polyether


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.

CompositionBase paste:


Reactor paste:

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

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

Polysulfide Reaction55 Chain lengthening

--SH O

HS---------------------SH O S H O = Pb = O H S Pb O








Condensa tion reaction

S + 3PbO + HH2 2O O S


Cross linking

mercaptan + lead dioxide


polysulfide rubber + lead oxide + waterMoisture and temperature Alter the reaction

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


Available in light, medium, heavy viscosities. Same matched tubes to be used Pseudoplasticity Setting time 5-8 min

Temperature humidity Loss of Water Altering base :accelerator ratio


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


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.

Advantages and disadvantages



Lower cost compared to silicones and polyethers Long working time High tear strength High flexibility Good detail reproduction Less hydrophobic Long shelf life.

Disadvantages Poor dimensional stability

water by-product pour within one hour single pour, second pour less accurate.

Custom trays Messy bad odor (thiol group in base paste) may stain clothing (lead dioxide in catalyst paste) Long setting time.

Condensation silicone60

Condensation silicone rubber impression

materials are used mainly for fixed partial denture impressions. They are ideal for single-unit inlays.


Base paste

Reactor paste or liquid.

poly(dimethylsiloxane) with Organo tin compound-

terminal hydroxy groups (dibutyl tin dilaurateor OH), stannous octoate orthoalkylsilicate Oily diluent Filler- 37%to 75%colloidal Organic dyes or pigments silica or microsized metal(added to form paste)or copper carbonate.


CH3 C2H5O Si OC2H5

Condensation Silicone Reaction

HO Si O Si - O - H CH3 62CH3 CH3 CH3 n

HO Si O Si - O - H CH3 CH3



n - Hydroxyl terminated polydimethyl siloxane CH3 CH3

Tetraethyl ortho silicate

stannous octoateOC2H5 n Si

exother mic

HO Si O Si - O CH3 CH3 CH3 CH3



HO Si O Si - O CH3 CH3 n


one drop per inch of extruded

base paste (for base paste/liquidcatalyst system)

Also available in putty consistency setting time (6 to 8 minutes)

Temperature Altering base catalyst ratio

Advantages and disadvantagesAdvantages Better elastic properties Clean, pleasant Stock tray


Disadvantages Hydrophobic Poor dimensional stability

high shrinkagepolymerization evaporation of ethanol


Good working and setting

time Less distortion on removal.

pour immediately

within 30 minutes

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

Addition silicone65

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.

CompositionBase paste Polymethyl hydrogen siloxane Other siloxane prepolymers Fillers Hybrid silicone


Catalyst paste Divinyl polydimethyl siloxane Platinum salt(catalyst) Retarders Fillers. Platinum or palladium to absorb the hydrogen.


Platinum salt CH3 CH3 O

---O Si CH2 - CH2 - Si CH3 O CH3 CH3

CH3 - Si - CH2 - CH2 Si O --O


Secondary reaction between moisture and residual

hydrides of the base polymer can lead to the development of hydrogen gas.

tinum or palladium added to the formulation acting as H2 scavengers Waiting for an hour or more before pouring a cast

Prevention the Can cause pin-point voids on cast


Available in light, medium, heavy and putty consistency. 2 paste system Monophase system Putty in 2 jars Putty-wash

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

Major disadvantagesHYDROPHOBIC Causes distortion or loss of details at the margin due to undetected moisture. Also prevents gypsum products from accurate reproduction of details. Prevention adding nonionic surfactant to the paste renders the surface hydrophillic.


SULFUR CONTAMINATION Natural latex gloves Some vinyl gloves containing sulfur stabilizers. Prevention recording the impression without touching the oral structures with gloves.




Dimensionally stable pour up to one week

Stock or custom trays Shorter setting time Less distortion on removal Adequate tear strength. Multiple casts Easy to mix Highly accurate Pleasant odor

Expensive Sulfur inhibits set latex gloves Ferric and Alsulfate retraction solution Short working time Lower tear strength Possible hydrogen gas


bubbles on die palladium added to absorb hydrophilic material also still needs to be carefully handled for a very dry field.


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.

compositionBase paste: Polyether polymer Colloidal silica-filler Glycolether or phthalate73

Accelerator paste: Alkyl aromatic sulfonate Filler Plasticizer. Thinneroctyl phthalate



Monophase Automatic mixing devices Thinner added Less sensitive to temperature changes.

Advantages and disadvantagesAdvantages: Highly accurate Good dimensional stability Stock or dual-arch trays Good elasticity. Good surface detail Pour within one week


Disadvantages: Expensive Short working time Rigid

difficult to remove from undercuts

Bitter taste stiffness Absorbs water

kept dry

Multiple casts Good wettability


General properties

Viscoelastic properties77

Elastic solid SPRING

Viscous liquid OIL DASHPOT


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


base paste and catalyst system paste and liquid catalyst system base

2 putty system

Putty and liquid catalyst system


Automatic mixing devices81



Multiple mix technique

Single viscosity technique

Simultaneous putty-wash technique

2 stage putty-wash technique


Removal of the impression85

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.

Preparation of stone casts and dies86

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.

Dimensional stability87

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.


The relative amount of permanent deformation following

strain in compression increases in the following order : 1. addition silicone 2.condensation silicone 3.polyether 4.Polysulfide The stiffness increases in following order: 1. Polysulfide 2.Condensation silicone 3.addition silicone 4.polyether

Tear strength89

Tear strength increases in following order:

1. Hydrocolloids (agar and alginate) 2.silicones (addition and condensation) 3.Polyether 4.polysulfide. Tear strength is also influenced by the consistency and the manner of removal of the materials.


Cell toxicity polysulfide: lowest cell death count

polyether: highest cell death count Most likely biocompatibility problem segment of impression material lodged in a patients gingival sulcus


Common causes of failure of elastomeric impression material92

Rough or uneven surface on impression

1. 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



1. Too rapid polymerization, preventing flow 2.Air incorporated during mixing Irregularly shaped voids 1. Moisture or debris on the surface Rough chaulky stone cast 1. Inadequate cleaning of impression 2.Excess water left on surface of the impression 3.Excess wetting agent left on impression 4.Premature removal of cast 5. Improper manipulation of stone 6.Failure to delay pour of addition silic0ne at least 20 mins

Causes of distortion

1. Resin tray not aged sufficiently 2. 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 material 6. Insufficient relief for the reline material (if such technique is used) 7. Continued pressure against impression material that has developed elastic properties 8. Movement of tray during polymerization 9. Premature removal from mouth 10.Improper removal from mouth 11.Delayed pouring of the polysulfide 0r condensation silicone impression


Comparison of Properties95

Working time longest

to shortest polysulfide > silicones > polyether

Setting time shortest

to longest polyether < silicones < polysulfide

Comparison of Properties96

Stiffness most to least polyether > addition silicone > condensation silicone > polysulfides

Tear strength greatest to least polysulfide > addition silicone > polyether = condensation silicone.

Comparison of Properties97

Cost lowest to highest polysulfide polyether 0.2%> polysulfide 0.4%> condensation silicone0.6%Phillips 1996



K. J. : Phillips science of dental materials, 11th edition. Powers J. M., Sakaguchi R. L. : Craigs restorative dental materials, 12th edition OBrien W. J. : Dental materials and their selection, 4th edition McCabe J. F., Walls A.W.G. : Applied dental materials, 9th edition