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IMPRESSION MATERIALS Presented by- Dr Sakshi II Yr PG Dptmnt of Prosthodontics 1

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1

IMPRESSION MATERIALS

Presented by- Dr Sakshi II Yr PG

Dptmnt of Prosthodontics

2

WHAT IS AN IMPRESSION???

• negative replica of the hard and soft oral tissues

• Register or reproduce the form and relationship of the teeth and oral tissues

3

Purpose of Impression Materials

• Impression –negative reproduction of tissues• Positive cast made by filling the impression

with dental stone or other model material

4

Desirable qualities of Impression Materials

A pleasant odour, taste and acceptable colorAbsence of toxic or irritant constituentsAdequate shelf life for requirements of storage and distribution

5

Desirable qualities contd…

Easy to use with minimum of equipmentSetting characteristics which meet clinical requirementsLow enough viscosity to adapt to the oral tissues,yet be viscous enough to be contained in the impression tray

6

Desirable qualities contd…

Should have adequate wettability of the oral tissues Set impression should show adequate elastic recovery with no permanent deformation upon removal from mouth

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Desirable qualities contd….

Dimensionally stable after setting over temperature and humidity ranges normally found in clinical and lab procedures until the pouring of the castCompatible with cast and die materials

8

Desirable qualities contd….

Biocompatible ,non-toxic Must be of color and opacity that allows the dentist to evaluate the impressionReadily disinfected without significant loss of accuracy or loss of mechanical properties

9

Desirable qualities contd….

Materials ,associated processing time and equipment –cost effectiveNo release of gas or other by-products during setting of the impression materials

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CLASSIFICATON:

Classified on the basis of setting and elasticity.

Mode of setting rigid Elastic

Set by chemical reaction(irreversible)

Impression plaster, zinc oxide eugenol.

Alginate, polysulfide, polyether, silicone,

Set by temperature change(reversible)

Compound, waxes Agar hydrocolloid.

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13

Impr

essio

n M

ater

ials

Non-elastic

Elastic

Aqueous Hydrocolloids

Non-aqueous Elastomers

Polysulfide

Silicones

Polyether

Condensation

Addition

Agar (reversible)

Alginate (irreversible)

Plaster

Compound

ZnO - Eugenol

Waxes

O’Brien, Dental Materials & their Selection 1997

14

NON-ELASTIC IMPRESSION MATERIALS

NON-ELASTIC IMPRESSION MATERIALS

Impression plaster

Zinc-oxide eugenol

impression paste

Impression compound

15

Impression Plaster• used as mucostatic impression material for

making final impressions for edentulous patients

• Doesn’t compress and displace tissues during seating of tray due to its fluidity

16

Impression plaster contd…

• Applicable to patients with displaceable soft tissues that should be recorded in a passive state

17

Composition

• ß-calcium sulphate hemihydrate • Reacts with water to form calcium sulphate

dihydrate• W/P ratio– 0.5-0.6• Expansion and setting times controlled by

incorporating compounds designed to mediate handling properties

18

• Potassium sulphate added as an anti-setting expansion agent

• Borax(retarder)- added to the powder to balance the setting acceleration caused by Pot. Sulphate and to bring the setting time under control

19

• Alzarin red-to make clear distinction between the impression and model

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• Custom tray constructed using 1-1.5mm spacer with acrylic resin or shellac

• Impression plaster can be used as wash material

• Techinque- “Puddling” the impression into place

21

• With remaining plaster in tray, the tray is seated in single movement

• Then tray is gently moved from side-to side and antero-posteriorly to take advantage of fluidity of material

22

• Hemihydrate particles absorb moisture from the surface of the oral tissues allowing intimate contact between impression material and the tissues

23

• Plaster impression material –very brittle and fractures easily

• When undercut is involved, fracture the impression to facilitate removal from mouth

• Fragments reconstructed to form completed impression

24

• Beading of the impression done• Coated with separating medium and cast in

fresh plaster• Disinfection- achieved with a 10 min soak in

sod hypochlorite solution

25

• Not used regularly due to mechanical limitations

• Used frequently as occlusal registration material

26

Zinc-oxide Eugenol Impression Paste

• Composition-2 separate pastes dispensed in tubes

• One tube contains zinc oxide and vegetable or mineral oil

• Other tube contains eugenol and rosin

27

Composition

Components Percentage

Tube no 1 (base)

Zinc-oxide 87

Fixed vegetable or mineral oil 13

Tube no 2 (accelerator)

Oil of cloves or eugenol 12

Gum or polymerised rosin 50

Filler(silica type) 20

Lanolin 3

Resinous balsam 10

Accelerator solution(CaCl2) and color 5

28

Setting reaction of ZOE

• Ionic in nature• Requires ionic medium in which it can proceed

at any desired rate• 1st reaction-hydrolysis of zinc oxide to its

hydroxide form

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Zinc Oxide Eugenol

• When the 2 pastes are mixed,the phenol –OH of the eugenol acts as a weak acid and undergoes an acid-base reaction with zinc hydroxide

• Forms a salt- zinc eugenolate

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ZOE contd…

• Two further coordinate bonds are formed by donation of pairs of electrons from methoxy oxygen to zinc

31

ZOE contd…

• Disadvantage-• Stinging or burning sensation caused by

eugenol • Orthoethoxybenzoic acid -substitute

32

Manipulation

• Mixed on oil impervious paper or glass mixing slab

• Proper proportion of two pastes obtained by squeezing 2 strips of paste of the same length,one from each tube ,onto the mixing slab

33

Manipulation of ZOE contd…

• Flexible stainless steel spatula used for mixing• 2 strips of contrasting colors combined with

the first stroke of the spatula ,mixing is continued for approx 1 min, until a uniform color achieved

34

Types of ZOE

• Classified as Hard paste(type I) soft paste(type II)• Final set for type I paste-10 min type II paste-15min Actual time shorter when setting occurs in

mouth

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• Shorten the setting time – by adding small amount of Zinc acetate or additional accelerator or a drop of water in the paste before mixing or by extending mixing time

• Prolonging the setting time- cool spatula and mixing slab

36

• Paste of thick consistency –compresses the tissues

• Thin,fluid material results in little or no compression

Advantage of heavier consistency – increased strength

37

Dimensional stability

• Negligible shrinkage(less than 0.1%) may occur during hardening

• No significant dimensional change• Impression can be preserved indefinitely

without change in shape provided the tray material is dimensionally stable

38

Disinfection

• 2 % alkaline glutaraldehyde solution• Immersed in solution for required time,rinsed

and poured immediately

39

Applications of ZOE

• Final impression of edentulous ridges• As a wash impression with other impression• As an interocclusal registration material• As a temporary liner material for dentures• As a surgical dressing

40

Impression compound• Also called “modelling plastic”• Thermoplastic material• Supplied in the form of cakes(red) and sticks

(green, gray or red)

41

Composition

• Mixture of –waxes(principal ingredient) thermoplastic resins filler(increase viscosity and

rigidity) coloring agent• Shellac, stearic acid and gutta-percha added to

improve plasticity and workability

42

Types of Impression compound

Type-I(Lower fusing)

Type- II(Higher fusing)

43

Type I (Lower fusing material)

• Cakes- as an impression material for completely edentulous patients, the material is softened by heat, inserted into the tray and placed against the tissues before it cools to a rigid mass

44

• Sticks- as a border molding material for the custom tray ,the material is used before making the final impression

45

Type II( Higher fusing material)

• Used as an adaptation material which requires more viscous properties

• Used for making primary impression of the soft tissues and then used a tray to support a thin layer of a second impression material such as ZnOE paste, hydrocolloids or nonaqueous elastomers

46

Manipulation

• Setting mechanism– reversible physical process

• Softening by heat – prerequisite • Preheated and used warm (~450C)• Then cooled to the intraoral temperature(370C)

at which it is fairly rigid

47

• Once the impression tray is seated , it should be held gently(passively) in position until the impression cools below the fusion temperature

48

• Softened by heat over flame(green stick) or in a temperature controlled water bath

• When direct flame is used, the material should be moved over the flame in such a manner that it will not be allowed to boil or ignite so that the constituents are volatilized

49

• Prolonged immersion or overheating in water bath makes the compound brittle or grainy due to leaching of low molecular weight ingredients

50

• Dimensional stability- allow thorough cooling of the impression before removal from the mouth and to construct the cast or die as soon as possible after the impression has been obtained(at least within the hour)

• Disinfection – 2% alkaline glutaraldehyde solution

51

Thermal Properties

a) Thermal conductivity-low thermal conductivitySignificance• During softening of the material, the outside will

soften first and inside last, so to ensure uniform softening, the material should be kept immersed for a long time in water bath

• The layer adjacent to tissues will remain soft . Thus it is important to cool the compound thoroughly before removing the impression

52

b)Coefficient of Thermal Expansion- high COTE (0.3% acceptable)

c)Glass Transition Temperature• The temperature at which the material loses

its hardness or brittleness on heating or forms a rigid mass upon cooling

• Approx 39°C

53

d)Fusion temperature– corresponds to a definite reduction in plasticity of the material during cooling

• Above this temperature, the material remains plastic while the impression is being made

• Approx 43.5°C

54

Significance of Fusion temp and Glass transition temp

• Above Fusion temp,the fatty acids are liquid and lubricate the softened material to form a smooth plastic mass while the impression is being obtained.

• Thus all impressions with compound should be made above this temperature

55

• Once the impression tray is seated,it should be held firmly in position until first fusion temperature and later the glass transition temperature is reached

• Thus, impression is made above the fusion temperature and removed after it cools down to its glass transition temperature

56

Important considerations for proper use of impression compound

• Low thermal conductivity- adequate time needed to attain thorough heating and cooling

• Incorporation of water(wet kneading)- excessive flow of the material at mouth temp. producing distortion as the impression is removed from mouth

57

• Tray used for impression must be strong and rigid enough to support the material and to avoid distortion of the impression

• Relatively high viscosity limits its ability to record fine details

• Cast should be poured as soon as possible to minimise distortion due to relaxation of the compound

58

• Safely disinfected by immersion in sod hypochlorite, iodophors or phenolic glutaraldehydes

59

Elastomeric Impression Materials

ELASTOMERIC IMPRESSION MATERIALS

Aqueous hydrocolloids

Agar-agar Alginate

Non-aqueous elastomers

Polysulfides

Silicones Polyethers

60

AQUEOUS HYDROCOLLOIDS

61

Agar • Reversible hydrocollloid• Physical change of agar from sol to gel induced

by lowering temperature• Gel liquefies to sol when heated to a

temperature known as liquefaction temperature(700C-1000C)

62

• When sol is cooled, it becomes gel at a point known as the gelation temperature( btw 370C and 500 C)

• Thus called reversible hydrocolloid

63

• Gelation temp- critical for impression making• If too high,heat from the sol may injure the

oral tissues• If too low,below oral temperature,impossible

to make impression because the sol will not convert to a gel

64

• Polysaccharide- extracted from certain types of seaweed

• Water major constituent• Supplied as gel• Available in tray and syringe consistencies• Tubes used to fill water cooled trays and

cartridges used with syringes

65

Composition Agar Gelling agent

Borax Improves strength

Potassium sulphate Gypsum hardener

Alkyl benzoates Preservatives

Water Reaction medium(>80%)

Coloring agents

Flavouring agents

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• Fillers such as diatomaceous earth,wax, clay,silica,rubber and similar inert powders– used to control strength , viscosity and rigidity

• Thymol and glycerine added as bactericidal agent and plasticiser

67

Making the Agar impression

• Process requires a 3 compartment conditioning unit for the agar tray material

• Allows liquefaction,storage and tempering• Syringe material used only in liquefaction and

storage compartments

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Liquefy the hydrocolloid gel in the tube in boiling water at 1000C for minimum 10 min

tube then placed in a storage bath at 65°C to retain the sol condition until needed

impression tray filled with hydrocolloid sol from the tube taken from storage bath , gauze pad placed over the top of the tray material

Tray placed in water filled tempering compartment(at abt 45°C)

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Just before tempering completed,syringe material taken directly from storage compartment and applied to the prepared teeth

• Note--- tempering time-3-10 min• if >10 min,partial gelation occurs• syringe material doesn’t require

tempering bcoz maintained in fluid state to enhance adaptation to tissues

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• Syringe material first applied to the base of the preparation,then remainder of the prepared tooth is covered

• Tip of the syringe is held close to the tooth and it remains embedded below the surface of the syringe material to prevent entrapment of air bubbles

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• Water soaked outer layer of hydrocollloid loaded tray and the gauze covering the tray impression material are removed to ensure firm bonding to the syringe hydrocolloid

• Tray immediately brought into position,seated with light pressure and held with a very light force

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• Gelation accelerated by circulating cool (18-21°C)through tray for 3-5 min

• During gelation process, tray must be held in mouth until gelation has proceeded to a point at which gel strength is sufficient to resist deformation or fracture

• Tray removed with a snap

74

Distortion during gelation

• Some contraction occurs due to physical change (sol gel)

• If held rigidly in the tray,shrink towards the center of its mass,thus creating larger dies

• Rapid cooling may cause stress concentration near the tray

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Dimensional stability

• Storage medium- 2% potassium sulfate solution or 100% relative humidity

77

Compatibility with Gypsum

• Contains borax- retarder for setting of gypsum products

• Deficiency of gypsum setting can be overcome by--Immersing agar impression in a solution containing a gypsum accelerator(2% pot sulfate solution) prior to pouring of the impression

• By incorporating gypsum surface hardener in the material such as sulfate

78

• Disinfection House hold bleach or iodophors

• Accuracy Most accurate of impression materials

79

Alginate • Irreversible hydrocolloid• Most widely used material in dentistry• Developed as a substitute for agar

80

Advantages

• Ease of manipulation• No need of expensive

equipments• Relatively low cost• Comfort to patients

• High viscosity• Ability to displace

tissues

Disadvantages

81

Potassium or sodium alginate dissolves in water and reacts with calcium ions

Calcium sulphate dihydrate A reactor ,reacts with potassium alginate to form a dihydrate insoluble alginate gel

Zinc oxide Filler particles, affects properties and setting time

Potassium titanium fluoride Accelerator ,counteracts the inhibiting effect of the hydrocolloid on the setting of stone,ensures good quality surface of the cast

Diatomaceous earth Filler particles, controls the consistency of the mix and the flexibility of the set alginate

Trisodium phosphate Retarder,controls the settting time to produce either regular or fast set alginates

Coloring agents

Flavoring agents

Composition

82

Modified alginates

• Dustless alginates• Include polyethylene glycol or polypropylene

glycol on the alginate powder to agglomerate the particles

• Color indicators added to reveal the stage of setting reaction

83

Modified alginates

• Two paste alginate materials• One paste contains sol of

alginate,fillers,retarders and other ingredients like glycols and dextrose

• Other paste contains gypsum dihydrate, fillers, retarder , glycerol or glycol, gypsum surface modifier and some silicone oil

84

Gelation process

• Typical sol-gel reaction• Soluble alginate reacts with calcium ions from

calcium sulphate and forms insoluble calcium alginate

• Production of calcium alginate- rapid,doesn’t allow sufficient working time

• Retarder trisodium phosphate added to extend working time

85

Setting reaction of alginate

(a) K2n-Alginate+ n CaSO4 nK2SO4 + Can-alginate

(b)2Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4

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Controlling Setting Time

• Ideal W/P ratio- 20 ml water/8gms of powder 40 ml water/16gms of powder• Powder should be weighed not measured• Approx 2.5:1• Slight modification in W/P ratio affects 2

important properties--- tear strength elasticity

88

• Thus setting time best regulated by amount of retarder added during manufacturing

• Fast-set alginate- 1.5-3min• Regular set alginate- 3-4.5 min• Can also be influenced by altering the

temperature of water

89

• Cool water in hot weather• Precool mixing bowl and spatula• Tap water-contains certain levels of metallic

ions(Ca,Mg)• Tap water with a high hardness may accelerate

setting time

90

Preparation of Alginate Impression Materials

• Measured powder added slowly to premeasured water already poured into clean rubber bowl

• Powder incorporated into water by carefully mixing with a metallic spatula flexible enough to adapt well to the wall of the mixing bowl

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• Avoid incorporating excessive air into the mix• Vigorous figure of 8 stropping motion • Mixing time- 45sec to 1 min• Result should be a smooth creamy mixture

that doesn’t drip off the spatula when raised from bowl

92

• Mechanical mixing devices• Include rotating mixing bowl , mechanical

mixer with time-control unit, a vaccum mixer for water/powder mixing

• Advantages- convenience, speed and reduction of human error

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Making the alginate impression

• Perforated metal tray preferred• Thickness of alginate impression between the

tray and the tissues should be at least 3mm• Compressive strength doubles during first 4 min

after gelation,but doesn’t increase appreciably thereafter

• Improve elasticity over time which minimizes distortion of the material during impression removal(undercut areas)

94

Compressive strength of an Alginate gel as a function of Gelation time

Time from Gelation(min) Compressive strength(KPa)

0 330

4 770

8 810

12 710

16 740

95

• Alginate impression should not be removed from the mouth for at least 3 min after gelation has occurred

• Tear strength increased when the impression is removed along a vertical path with a snap

• Speed of removal- between rapid movement and a slower rate

96

STRENGTH• Manufacturer’s directions should be followed• Any deviation from instructions can have

adverse effects on the gel strength

SHELF LIFE• 2 factors affecting shelf life- storage

temperature and moisture contamination

97

Dimensional stability

• Syneresis- loss of water when exposed to air at room temperature associated with shrinkage

• Imbibition- swelling of the impression if immersed in water

98

How can distortion be minimised??

• Poured immediately after making impression• If pouring delayed, then rinsed in tap

water,disinfected wrapped in a surgical paper towel saturated with water and placed in a sealed plastic bag or humidor

99

Compatibility with Gypsum

• Poatssium titanium fluoride-surface hardener or gypsum hardener

• Solubility-1.3gm per 100 ml of water at 20°C• Fluoride on surface of alginate will form fine

calcium fluoride precipitates with the calcium from the gypsum

• Fine calcium fluoride particles become nuclei that accelerate the setting of gypsum

100

• Rough stone surface will result if excess rinsing water collected on the surface of the impression at the time of pouring the stone mixture

• A dried gel results in its adherence to the surface of the cast which results in tearing upon removal

101

• Surface of impression should be shiny but with no visible water film or droplets at the time of pouring

• Stone cast or die should be kept in contact with the impression for a minimum of 30 min, preferably for 60 min before the impression is separated from the cast

102

Disinfection

• Household bleach(1-10 dilution) iodophors or synthetic phenols

• After rinsing,disinfectants sprayed on exposed surface

• Impression immediately wrapped in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min

103

• Wrapped impression removed from the bag, unwrapped, rinsed and shaken to remove excess water

• Then poured with stone of choice AccuracyNot capable of reproducing the finer details

compared with other impression materials

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Laminate technique(Alginate -Agar method)

• Modification of agar procedure• Agar in tray replaced with a mix of chilled

alginate that bonds to the agar expressed from a syringe

• Alginate gels by chemical reaction while agar gels by means of contact with the cool alginate rather than water circulating through the tray

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• Agar contacts the prepared teeth ,maximum detail reproduced

• Equipment cost low, less preparation time needed

• Main disadvantages- bond btw agar and alginate not always

sound higher viscosity of alginate displaces agar

during seating dimensional inaccuracy of alginate limits its

use to single units

107

NON-AQUEOUS ELASTOMERS

108

Elastomeric impression materials

• Comprise a group of synthetic polymer-based impression materials that are chemically cross-linked when set and can be stretched

• Rapidly recover to their original dimensions

109

Based on the backbone of polymer chains

Elastomers

Polysulfide Silicones Polyethers

110

Elastomers

• Supplied in 2 components: base paste catalyst paste• Formulated in several consistencies in increasing

content of filler Extra light body Light body Medium or regular body Heavy body Putty(extra heavy)

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• Extra low and putty available only for condensation and addition silicones

• Polysulfide provided only in light body and heavy body

• No heavy body for condensation silicone

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• Different colored pastes dispensed either through a spiral mixing tip or in equal lengths on a mixing pad

• Setting occurs through a combination of chain-lengthening polymerization and chemical cross-linking by either a condensation or addition reaction

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• 3 types of systems available to mix the catalyst and base : hand mixing, static automixing, dynamic mechanical mixing

• To record soft tissues for edentulous patients under minimum compression: polysulfide or addition silicones

(free flowing, minimum viscosity)

114

• If moderate compression required: medium-viscosity polysulfides, addition silicones, polyethers

• Undergo shrinkage upon polymerisation• Condensation type silicones undergo

additional contraction

115

• Polysulfides and condensation silicones : highest dimensional changes during setting

• Addition silicones and polyethers: lesser dimensional changes

116

Polysulfides • First synthetic elastomeric impression

material introduced in 1950• 2 paste system• Available in low, medium and high

consistencies• Made up of a base and accelerator/reactor• Brands- COE-FLEX,PERMALASTIC,NEOPLEX etc

117

Composition The BasePolysulfide polymer(-SH,mercaptan gp) Principal ingredient

Titanium oxide and Zinc Fillers

Sulphate ,copper carbonate or silica Strengthener

Dibutyl phthalate Plasticizer (confers viscosity to base)

The AcceleratorLead dioxide, hydrated copper oxide or organic peroxide

Reactor

Sulfur Promoter, accelerates the reaction

Oleic acid or Stearic acid Retarder, controls setting reaction

118

• Working time: 5-7 min( longest among elastomers)

• Setting time: 8-12 min• Pouring the cast: impression must be poured

within 30 min to 1 hr

119

• Each paste supplied in a dispensing tube with approx sized bore diameters at the tip

• Equal lengths of paste extruded from each tube to provide the correct ratio of polymer to cross-linking agent

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• Reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete

• Resilient network started to form• During final set, a material of adequate

elasticity and strength is formed that can be removed past undercuts

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• Polymerisation results in chain lengthening and cross –linking with an increase in molecular weight

• Setting indicated by change of the color of the paste to dark- brown or gray- brown

• Color- presence of lead oxide

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Polymerisation of ploysulfide impression material SH groups interact with oxygen released from lead dioxideCompletion of the condensation reaction results in water as a by-productPendant –SH for cross-linking and terminal for chain lengthening

123

• Hot and humid conditions accelerate the setting of polysulfide impression material

• Reaction slightly exothermic and yields water as a by-product

124

• Good flexibility• High tear strength• Hydrophobic• Messy, stains clothes and has an offensive

odor• Uses—impression for crown and bridge edentulous impressions

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Condensation silicone• Followed in 1955• Supplied as two-paste system or base-paste and a

low viscosity liquid catalyst or a two-putty system• Putty used as tray material in conjunction with a

low-viscosity silicone • Referred to as the Putty-wash technique• Brands- Speedex

126

Composition The Base paste

- -hydroxyl-terminated polydimethyl siloxane

High molecular weight polymer

Silica or calcium carbonate Fillers

The Liquid Accelerator

Tin octoate Metal organic ester

Orthoalkyl silicate

Oil-based diluents

Thickening agents Increase viscosity

127

Condensation silicone

• Working time: 3 min• Setting time: 6-8 min• Impression must be poured as soon as

possible within first 30 min

128

• Curing involves a reaction of tri- and tetra-functional alkyl silicates in the presence of stannous octoate as a catalyst

• Sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a 3-D network

129

Condensation polymerisation of alpha-omega hydroxy –terminated poly(dimethyl siloxane) with tetraethyl orthosilicate in the presence of stannous octoate(catalyst)This reaction results in the release of ethanol molecules

130

• By-product: Ethyl alcohol• Subsequent evaporation accounts for much of

the contraction that takes place in the setting impression

• Extra-heavy or putty consistency developed to counteract the large polymerisation shrinkage

131

Addition silicone• Commonly referred to as Polyvinyl siloxanes(PVS)• Supplied as low, medium, high and very high

consistencies• Based on silicone prepolymers that carry vinyl

and hydrogen side groups which can polymerize by addition polymerization

• Brand name- Aquasil

132

Composition The Base

Polymethylhydrosiloxane Low molecular weight polymer

Fillers

The Accelerator

Divinyl polymethyl siloxaneOther siloxane pre-polymers

Platinum salt Catalyst

Reatrder Controls working and setting times

133

• Working time: 2-4.5 min• Setting time: 3-7 min• Cast can be poured upto 1 week after making

the impression

134

• Reaction activated by a platinum salt catalyst (chloroplatinic acid) without the release of by-

products• In presence of impurites or moisture,

secondary reaction takes place between the residual hydrides and moisture leading to evolution of hydrogen gas

135

Hydrogen atoms along the backbone str of PVS chain move to the vinyl group during addition polymerisation(top)Final str after platinum salt has initiated the addition polymerisation reaction(bottom)The zigzag line at the other end of divinylpolysiloxane represents repeating units of dimethylsiloxane with a vinyl terminal

136

• This can cause minute gaseous voids in the gypsum casts and reduce the effectiveness of cross-linking polymer structure

• Automatic mixing systems simplified their manipulation, reduced voids in impressions, reduced the amount of material wasted and reduced the sensitivity of their mixing technique

137

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Advantages

• Most elastic of currently available materials• Virtually negligible distortion upon removal

from undercuts• Exceptional accuracy in reproducing anatomic

details • Dimensional stability allows pouring long after

impression making• Excellent occlusal record registration material

139

Disadvantages

• Inherent hydrophobic natureNon-ionic surfactant wetting agent added to

silicone paste rendering the surface of the impression more hydrophillic and called hydrophilized addition silicone

140

• Sulfur contamination from natural latex gloves inhibits the setting of addition silicone

• Touching the tooth with latex gloves before seating the impression can inhibit the setting of critical surface next to tooth

141

Polyether • Supplied as two-paste system in low, medium

and high consistencies• 2 types: 1. based on ring-opening polymerization

of aziridine rings which are at the end of branched polyether molecules

2. based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups

• Brands- IMPREGUM

142

1st type

• Main chain probably a copolymer of ethylene oxide and tetrahydrofuran

• Cross-linking and setting promoted by an initiator and an aromatic sulfonate ester

• R alkyl group• Produces cross-linking by cationic

polymerisation via the imine end groups • Supplied as 2 pastes: base and accelerator

143

Composition

The Base

Polyether polymer

Colloidal silica Filler

Glycol ether or pthalate Plasticizer

The Accelerator

Alkyl aromatic sulfonate Initiator

Filler

Plasticizer

144

• Working time: 2.5 min• Setting time: 4.5 min• Poured upto 1 week of storage

145

Initiator ,aromatic sulfonate ester dissociates and forms alkyl cations that bind the nitrogenatoms of the azridine ring terminals of the prepolymer(top,left)The arrows indicate binding between the cations (R) with nitrogen atomsThis action opens up the ring,and the reacted pre-polymer (center) now has 2 ethylene imine terminals(-NR-CH 2-C+H2) which can react with nitrogen atoms of adjacent unreacted prepolymers.(R2 Aziridine ringChain propagation polymerization reaction yields a larger molecule(right) which continues growing by binding with aziridine rings of additional unreacted prepolymersPolymerisation reaction terminates when the growing chain combines with a counter ion

146

2nd type

• Based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups

• Mechanism similar to condensation silicones• Material often called hybrid• Behave very much like the 1st type due to

ether linkages

147

• High degree of wettability• Inherent hydrophillic

nature• Relative stiffnessExcellent material for good

duplication of fine details and rigid support for pick-up copings

148

Making impressions with elastomeric materials

• Fabrication of gypsum models ,casts and dies involves 6 major steps:

1. Preparing a tray2. Managing tissue3. Preparing the material4. Making the impression5. Removing the impression6. Preparing stone casts and dies

149

1.Impression trays

• Custom tray recommended to reduce the quantity of material required

• In case of severe undercuts, custom tray avoided

• Prior to impression making, uniform thickness of tray adhesive applied

150

2.Tissue management

• Displace the gingival tissues, control gingival haemorrhage and control sulcular fluids to ensure access for the tooth preparation and making impression

• Gingival retraction cord- most commonly used

151

3.Manipulation of impression materials

• Supplied for 3 modes: hand mixing, static mixing and dynamic mechanical mixing

152

Hand mixing

• Dispense the same length of materials onto a mixing pad or glass slab

• Catalyst paste first collected on stainless steel spatula and then spread over base paste

• Mixture is then spread over the mixing pad• Mass is then scraped up with the spatula

blade and spread uniformly back and forth on the mixing pad

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• Process continued until the mixed paste is uniform in color with no streaks of the base or catalyst appearing in the mixture

• 2 putty systems(condensation and addition silicone) dispensed by volume using equal number of scoops of each material

• Knead the material with fingers until a uniform color is obtained

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Static mixing• Transforms 2 fluid(or paste-like)

materials into a homogenous mixture without mechanical mixing

• Device used- gun for compressing materials into a 2-cylinder cartridge, which contains the base and catalyst separately, as well as mixing tip

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• Mixing tip is made of helical mixer elements in a cylindrical housing

• Mixer elements are series of alternating right and left –turn 180°helixes positioned so that leading edge of one element is perpendicular to the trailing edge of the next

• Length of each material is the same as the inner diameter of the cylinderical housing

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Dynamic mechanical mixing

• Device uses motor to drive parallel plungers,forcing the materials into a mixing tip and out into an impression tray or syringe

• Motor driven impeller mixes the materials as they are extruded through the tip

• Materials supplied in collapsible plastic bags housed in cartridge

• Polyether and addition silicone

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4.Making an impression

• 3 techniques:• Multiple-mix technique• Monophase technique• Putty wash technique

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Multiple-mix technique

• Syringe material(light body)and tray material(heavy body)

• Lighter material injected within or around the tooth preparation

• Filled tray then inserted in the mouth and seated over the syringe material

• Tray material force the syringe material to adapt to the prepared tissues

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Monophase technique

• Medium body polyether and addition silicone • Only one mixture is made and a part of the

material is placed in the tray and another portion in syringe for injection in the prepared tissues

• Success depends on pseudoplastic (shear thinning) property of material

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Putty –wash technique

• Originally developed for condensation silicone to minimize the effect of associated dimensional changes

• Thick putty material placed in stock tray and a primary impression made

• Space for light-body “wash” material provided • Mixture of thin consistency wash material

placed into putty impression and preparation

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5.Removal of the impression

• Shouldn’t be removed until curing progressed sufficiently to provide adequate elasticity ,so distortion doesn’t occur

• Typically impression should be ready for removal within at least 10 min from time of mixing,allowing 6-8 min for impression to remain in mouth

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• Mechanics of removing impression– separation at the impression/tissue interface and stretching of the impression

• 1st step to break the physical adhesion between the tissue and the impression

• Polyether requires extra effort• 2nd step stretches the impression enough to pass

under the height of contour of hard tissue to remove impression

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6.Preparation of stone casts and dies

• Silicones– hydrophobic• Surfactant sprays- debubblizers improve

surface wettability of silicone impression material for stone slurry

• Dilute solution of soap also acts as surfactant• Polyether n polysulfide- don’t require

surfactant

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Properties of elastomeric impression materials

1. Working and setting time2. Reproduction of oral structure detail3. Rheological properties4. Elasticity and visco-elasticity5. Tear strength6. Dimensional stability7. Disinfection8. Wettability and hydrophillization9. Biocompatibility

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1. Working and setting timeMean working time(min) Mean setting time(min)

Impression material 23°C 37°C 23°C 37°C

POLYSULFIDE 6.0 4.3 16.0 12.5

CONDENSATION SILICONE 3.3 2.5 11.9 8.9

ADDITION SILICONE 3.1 1.8 8.9 5.9

POLYETHER 3.3 2.3 9.0 8.3

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2.Reproduction of detail

• Record detail to the finest degree• When stone poured on the surface, finest

details not always reproduced

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3.Rheological properties

• Viscosity and flow behaviour depends upon - ease of mixing -air entrapment during mixing -tendency of trapped air to escape before the

impression is made• All elastomers exhibit shear-thinning

characteristics before setting

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• 2 categories of shear thinning phenomena—• Pseudoplasticity • Thixotropy • Pseudoplastic material- displays decreasing

viscosity with increasing shear stress and recovers its viscosity immediately upon a decrease in shear stress

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• Thixotropic material- doesn’t flow until sufficient surface energy in the form of an impact force or vibration force is applied to overcome the yield stress of material

• Extreme shear thinning- material retains an immobile state at rest but flows freely under stress

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Significance of shear thinning

• Exhibited by addition silicone and polyether impression materials

• Enable the clinician to use a monophase impression making technique to capture details needed for fixed prostheses

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4. Elasticity and viscoselasticity

• Explained well by Maxwell- Voigt model• Relative amount of permanent deformation• Addition silicone<polyether<polysulfide• Recovery of elastic deformation following

strain is less rapid for the polysulfide material than for other three types of impression materials

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MAXWELL-VOIGT MODEL

A-Maxwell-Voigt model in a stress-free stateB-during loading,only S1 spring contracts in response to loadC-when loading continues,the pistons in dashpot D1 and D2 move proportionally to the duration of loading . S2 spring contracts alongwith dashpot D2. No change in S1D-the moment the load is released,S1 spring recovers instantly, whereas rest of the elements remain unchanged. S2 should also recover instantly but retarded by the sluggishness of dashpot D2E- as time passes, S2 spring recovers and extends dashpot D2 slowly near to its original position. Dashpot D1 remains unchanged

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• Polyvinyl siloxane- exhibit most elastic recovery

• Distortion on removal from undercuts is virtually non-existent

• If material is in advanced stage of elasticity, and compressed excessively during seating of impression,distortion can occur when the material elastically rebounds

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• MOE in increasing order• Polysulfide<condensation silicone<addition

silicone< polyether

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5.Tear strength

• Low viscosity materials used in interproximal and subgingival areas

• Measures the resistance of an elastomeric material to fracture when subjected to a tensile force acting perpendicular to a surface flaw

• Tear strength in increasing order• Silicones<polyethers<polysulfides

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• Influenced by consistency and manner of removing the materials

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6. Dimensional stability• Dimensional accuracy v/s dimensional stability• 6 major sources of dimensional change1. Polymerisation shrinkage2. Loss of condensation reaction by-product3. Thermal contraction from oral temp to room temp4. Absorption of water or disinfectant over a period of

time5. Incomplete recovery of deformation because of plastic

deformation6. Incomplete recovery of deformation because of

viscoelastic behaviour

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• Absorption of water or fluids- negatively affects polyether impression

• Simultaneous leaching of water soluble plasticizer

• Stored in dry(relative humidity<50%),cool environment to maintain its accuracy

• Should never be left for protracted periods in disinfecting solutions

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7.Disinfection • Polysulfides and silicones--Glutaraldehydes,

chlorine compounds ,iodophors,phenolics• Polyether- chlorine compounds or iodophors

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8.Wettability and hydrophilization

• Silicones-most hydrophobic• Polyethers-hydrophillic• Spray surfactant on hydrophobic impressions

prior to pouring with gypsum• Non-ionic surfactant added during

manufacturing

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• Hydrophyllized PVS- ether group –hydrophillic and oriented towards the surface when the surfactant migrates by diffusion to the surface region

• Depends on the contact angle made by water droplets with the impression surface and surface energy

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• As soon as hydrophillized PVS encounters oral fluids during impression making, begins to release surfactant to its surroundings

• Adequate amount of surfactant trapped on the surface of set PVS impressions ,thus providing hydrophilicity for pouring of gypsum dies

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9.Biocompatibility

• Tests covered in ISO 10993-5,Biological Evaluation of Medical Devices:Tests for in-vitro cytotoxicity

• Polysulfide – lowest cell death count• Polyether –highest cell toxicity scores

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• Elastomer induced biocomaptibility problem– fragment of impression material trapped in patient’s gingival sulcus

• Causes severe gingival imflammation• Can also occur in 2nd stage implant surgery• Contact dermatitis from Polyether in dentists

or dental technicians

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10. Shelf life

• Don’t deteriorate appreciably in tube or container before the expiration date when stored in dry , cool environment

• Clear liquid expressed along material-plasticizer segregation

• Indicates manufacturing error or excessive temperature extremes during storage

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Effects of mishandling

• Failure to produce an accurate epoxy or gypsum die or cast

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Common failures occurring with use of elastomeric impression materials

Type of failure Causes

Rough or uneven impression surface Incomplete polymerization,improper ratio or mixing of components,presence of oil or plaque on teeth

Surface agents (latex for PVS) inhibit polymerization

Too rapid polymerization from high humidity or temp

Excessively high accelerator /base ratio(cond silicone)

Bubbles Air incorporated during mixing

Irregularly shaped voids Moisture debris on teeth surfaces

Rough or chalky stone casts Inadequate cleaning of impression

Excess water not blown off impression

Excess wetting agent left on impression

Premature removal of cast,improper w/p ratio of stoneFailure to delay pour(PVS) that doesn’t contain Palladium salt for 20 min

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Distortion Resin tray not aged sufficiently , still undergoing polymerisation shrinkage

Lack of adhesion of elastomer to tray

Excessive bulk of material

Lack of mechanical retention to the impression tray

Excessive bulk of material

Insufficient relief for the reline material

Development of elastic properties in material before tray is fully seated

Continued pressure against impression material that developed elastic properties

Movement of tray during polymerization

Premature/improper removal from mouth

Delayed pouring of polysulfide or condensation silicone impression

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Comparative properties of elastomeric impression materials

Property Polysulfide Condensation silicone

Addition silicone

Polyether

WT(min) 4-7 2.5-4 2-4 3ST(min) 7-10 6-8 4-6.5 6

Tear strength(N/m) 2500-7000 2300- 2600 1500-4300 1800-4800

Percent contraction(at 24h) 0.40-0.45 0.38-0.60 0.14-0.17 0.19-0.24

Contact angle(°) 82 98 98/53 49

Hydrogen gas evolution N N Y N

Automatic mixing N N Y Y

Custom tray Y N N N

Unpleasant odour Y N N N

Multiple casts N N Y Y

Stiffness 3 2 2 1

Distortion on removal 1 2 4 3

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