impresssion mat-rigid final

8/7/2019 impresssion mat-rigid final

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ImpressionImpression

materialsmaterials

Dr.SumitDr.Sumit GoelGoel


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An impression, in general terms, is a mark producedon a surface by pressure.

The word impression is derived from the Latin

word impression.


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DEFINITION

Impression MaterialImpression Material

Any substance or combination of substances used forAny substance or combination of substances used for

making an impression.making an impression.

(GPT(GPT--2001).2001).

Dental ImpressionDental Impression-- (negative reproduction)(negative reproduction)

A negative likeness or replica (in reverse) of oral tissuesA negative likeness or replica (in reverse) of oral tissues

and/ or teeth & adjacent structures for use inand/ or teeth & adjacent structures for use in

dentistry.(GPTdentistry.(GPT--2001).2001).


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IndicationsIndications

Diagnostic castsDiagnostic casts

preliminarypreliminary

opposingopposing IndirectIndirect

restorationrestoration

fixedfixed

removableremovable Bite registrationBite registration


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HISTORYHISTORY

Before middle of the 18Before middle of the 18thth centurycentury-- Blocks of ivory & bone as IM.Blocks of ivory & bone as IM.

During 18During 18thth centurycentury Bees Wax, used in making impression.Bees Wax, used in making impression.

1848 Colburn & Blake1848 Colburn & Blake GuttaGutta perchapercha as IM.as IM.

18441844 WescottWescott,, DwinelleDwinelle & Dunning& Dunning Used Plaster of Paris asUsed Plaster of Paris as

IM.IM.

J.W. Greene, P.T. GreeneJ.W. Greene, P.T. Greene 19001900 Discussed about manipulationDiscussed about manipulation

ofof ModellingModelling plasticplastic

19251925 AlphousAlphous PollerPoller 11stst suggested use of agar for dentalsuggested use of agar for dental

impressionsimpressions


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Late 1920sLate 1920s 11stst True functional impression waxesTrue functional impression waxes

developed.developed.

1930 Applegate1930 Applegate Series of true physiologic waxesSeries of true physiologic waxes

developed.developed.

19391939 TrapozzanoTrapozzano Early techniques using ZOE paste.Early techniques using ZOE paste.

S.S. WildingS.S. Wilding Patent to use alginate for impression.Patent to use alginate for impression.

Middle 1950sMiddle 1950s Elastomeric impression materialsElastomeric impression materials

introduced.introduced.


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CLASSIFICATION

Impression MaterialsImpression Materials

ElasticElastic InelasticInelastic

Hydrocolloid RubbersHydrocolloid Rubbers --Impression compoundImpression compound

Agar AlginateAgar Alginate --Impression waxesImpression waxes

-- Impression plastersImpression plasters

Polysulfides Polyether SiliconesPolysulfides Polyether Silicones --ZOEZOE

Condensation AdditionCondensation Addition


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1.1. NonNon--elasticelastic

2.2. ElasticElastic

AqueousAqueous (hydrocolloids)(hydrocolloids) AgarAgar

AlginateAlginate

NonNon--aqueousaqueous (elastomers)(elastomers)

PolysulfidePolysulfide

SiliconesSilicones CondensationCondensation

AdditionAddition

PolyetherPolyether


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1.1. By their generic chemical name:By their generic chemical name:--

silicone materials or ZOE materialssilicone materials or ZOE materials..

2.2. According to the manner in which they harden:According to the manner in which they harden:--

Chemical reactionChemical reaction TemperatureTemperature changechange

(irreversible)(irreversible) ((reversiblereversible)) ThermoplasticThermoplastic

-- ImpressionImpression PlasterPlaster

--Zinc oxide Eugenol Zinc oxide Eugenol -- Impression Impression compoundcompound

-- Alginate Alginate -- AgarAgar

-- Non aqueous elastomers Non aqueous elastomers -- ImpressionImpression WaxesWaxes

Other ways to classify :Other ways to classify :--


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3. According to the uses of materials in dentistry:-

a) Materials used for obtaining impressions in dentulousmouth-

-AlginateAlginate

-- AgarAgar can also be used for edentulous

-Non aqueous elastomersNon aqueous elastomers impressions

b) Materials used for obtaining impressions of edentulousmouth-

-- Impression compoundImpression compound

-- Impression plasterImpression plaster

-- Zinc oxide EugenolZinc oxide Eugenol

-- WaxWax

-- Non aqueous elastomersNon aqueous elastomers

-- AgarAgar

-- AlginateAlginate


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4. According to the viscosity or the tissue displacement:-

MucostaticMucostatic MucocompressiveMucocompressive

Impression plaster Impression compoundImpression plaster Impression compound

ZoEZoE imp. Pasteimp. Paste AlginateAlginate

Low Viscosity ElastomersLow Viscosity Elastomers AgarAgar

High Viscosity ElastomersHigh Viscosity Elastomers

5. According to the ability of the set material to bewithdrawn from undercuts:-

--ElasticElastic

-- InelasticInelastic


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Impression

Impression Tray

IMPRESSION MATERIALSIMPRESSION MATERIALSKey PropertiesKey Properties

AccuracyAccuracy

a.a. AccuracyAccuracy = ability to replicate the intraoral surface details= ability to replicate the intraoral surface details.

DimensionalDimensionalStabilityStability

b.b. Dimensional StabilityDimensional Stability = ability to retain its absolute= ability to retain its absolutedimensional size over time.dimensional size over time.

TearTearResistanceResistance

c.c. Tear ResistanceTear Resistance = ability to resist tearing in thin sections= ability to resist tearing in thin sections(such as through the feather(such as through the feather--edged material within theedged material within thegingivalgingival sulcussulcus..


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REQUIREMENTS OF IMPRESSIONREQUIREMENTS OF IMPRESSION

MATERIALSMATERIALS

The requirements of impression materials can beThe requirements of impression materials can be

discussed as follows:discussed as follows:

1. Factors which affect the1. Factors which affect the accuracyaccuracy of the impression.of the impression.

2. Factors which affect the2. Factors which affect the dimensional stabilitydimensional stability of theof the

impression.impression.

3. Manipulative variables such as3. Manipulative variables such as ease of handling, settingease of handling, setting

characteristicscharacteristics etc.etc.

4. Additional factors.4. Additional factors.


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1.1. Accurate reproduction of surface detail:Accurate reproduction of surface detail:--a)a) Rheological properties:Rheological properties:--

-- A low viscosity or a degree of pseudo plasticity isA low viscosity or a degree of pseudo plasticity is

desirable.desirable.

b)b) Ability of impression material to adapt closely to bothAbility of impression material to adapt closely to both

the soft and hard tissues:the soft and hard tissues:--

-- The way in which the materials interacts with salivaThe way in which the materials interacts with saliva

affects the fine detail reproduction.affects the fine detail reproduction.


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2.2.D

imensional accuracy & stability:D

imensional accuracy & stability:--a)a) The type of impression tray:The type of impression tray:--

ii)) The rigidity of the tray:The rigidity of the tray:--

-- It may influence the accuracy of impression whenIt may influence the accuracy of impression when

Flexible trays are usedFlexible trays are used

ii)ii) Tray bond with impression material:Tray bond with impression material:--

-- A good bond between the tray & the impressionA good bond between the tray & the impression

material is important.material is important.


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b)b) Shrinkage of the impression material:Shrinkage of the impression material:--Whether impression material sets by a chemical reaction or someWhether impression material sets by a chemical reaction or somechange in physical state, both usually result in some shrinkage ofchange in physical state, both usually result in some shrinkage of

impression material.impression material.

i)i) Tray contracts & reduces the impression space.Tray contracts & reduces the impression space.

ii)ii) The impression material contracts towards the tray &The impression material contracts towards the tray &

decreases the impression space.decreases the impression space.


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c)c) Permanent set:Permanent set:--

The impression material must be sufficiently flexible to allowThe impression material must be sufficiently flexible to allow

removal from the undercut regions without causing distortion.removal from the undercut regions without causing distortion.

Elastic materials must be used, as most are actually viscoelasticElastic materials must be used, as most are actually viscoelastic

materials, there is a possibility of some permanent deformations.materials, there is a possibility of some permanent deformations.

Degree of distortion depends upon:Degree of distortion depends upon:--

ii) Severity of undercut) Severity of undercut

ii) Thickness of impression materialii) Thickness of impression materialiii) Time impression is kept in compressediii) Time impression is kept in compressed

statestate


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d)d) Good resistance to tearing:Good resistance to tearing:--

On removing elastic impression materials from undercutOn removing elastic impression materials from undercut

areas, they are often put under a considerable tensile stress.areas, they are often put under a considerable tensile stress.

Impression materials should have sufficient mechanicalImpression materials should have sufficient mechanical

strength not to tear or undergo permanent deformationstrength not to tear or undergo permanent deformationduring removal.during removal.

The thickest part of impression are compressed against theThe thickest part of impression are compressed against the

tray when they pass the widest part of the tooth crown.tray when they pass the widest part of the tooth crown.


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e)e) Storage stability:Storage stability:--

-- There is usually a significant delay between the making of anThere is usually a significant delay between the making of an

impression & when the model is pouredimpression & when the model is poured

-- Therefore, it is important that the impression material neitherTherefore, it is important that the impression material neither

shrinks, nor expands, nor distorts at all normal degrees ofshrinks, nor expands, nor distorts at all normal degrees oftemperature & humidity during this time period.temperature & humidity during this time period.

f)f) Compatibility with model & die materials:Compatibility with model & die materials:--

-- It is undesirable that the choice of the material for makingIt is undesirable that the choice of the material for makingcasts should be governed by the impression material.casts should be governed by the impression material.


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3. Manipulative variables:-

a) Setting time:-

- Impression materials should have a setting time under the controlof the operator.

- For convienence & comfort, ideal impression material has a long

working time & short setting time.

b) Setting temperature:-

- The impression material should set or harden at or near mouth

temperature.

c) Ease of manipulation:-

- The setting characteristics of the material have an important effect

on ease of handling.


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4. Additional factors:-

1. A pleasant odour, taste & esthetic color.

2. Absence of toxic or irritant constituents.3. Adequate shelf life for requirement of storage & 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 & texture.

8. Readily wets oral tissues.

9. Elastic properties with freedom from permanent deformation

after strain.


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10. Adequate strength so it will not break or tear on removal from the

mouth.

11. Dimensional stability over temperature & humidity ranges

normally found in clinical & laboratory procedures for a period

long enough to permit the production of a cast & die.

12. Compatibility with cast & die materials.

13. Accuracy in clinical use.

14. Readily disinfected without loss of accuracy.

15. No release of gas during the setting of impression or cast &die

materials.


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IMPRESSION COMPOUND

- One of the oldest dental impression materials.

- It is rigid, thermoplastic material with a glass transition

temperature of about 39C.


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USES:-

1. As a preliminary impression for the construction

of individual trays.

2. To modify the fit of stock trays.

3. As a base in wash impression techniques.

4. To obtain peripheral seal.

5. For compression impressions.6. In operative dentistry.


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COMPOSITION:-

Ingredient Parts

1) Rosin 30

2) Copal resin 30

3) Carnauba wax 10

4) Stearic acid 5

5) Talc 756) Coloring agent Appropriate amount

- Composition varies from one product to another.


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1. WAXES:

- Bees wax was used as an impression material & it may still be

one of the ingredients in some modern products.

- They are tasteless, odourless, colourless & greasy to touch.

- Waxes used in impression compound include beeswax &

colophany.


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2. RESINS:

- They are amorphous organic substances, insoluble in water.

- Naturally occuring resins used in impression compound are

shellac, dammar, rosin or sandarac.

- This component is primarily responsible for the transition from a

solid to a liquid state.

3. PLASTICISERS:

- Waxes & resins, if used on their own, would tend to produce a

brittle material with a tendency towards tackiness.

- Brittleness is overcome by the addition of plasticisers, such as

shellac, glycerin, gutta percha & stearic acid.


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4. PIGMENTS:- A variety of pigments of coloring agents are added for the

purpose of identifying different brands or those of differentsoftening temperatures.

- Rouge which produces a characteristic red color is the most

common pigment.

5. FILLERS:

- Inert materials added to strengthen or change the physical

properties of impression compound.

- They are chemically distinct from the principal ingredients.


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- Waxes or resins are the principal ingredients & compose the matrix.

- Matrix is mostly non-crystalline in order to know the desirable

thermoplastic properties of the compound.

- This results in a relatively high flow & a low strength even at room

temperature.

- Consequently, a filler is added that can reduce the plasticity of the

matrix material by frictional action.

- Hence, both, the viscosity at temperatures above that of the mouth &

the rigidity of the compound at lower temperatures will be enhanced.

- Fillers also reduce the adhesiveness of the softened material to the

oral tissues & minimises shrinkage due to thermal contraction.

- Fillers commonl used are chalk limestone talk iron oxide etc.


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TYPES:-

Based on the variation of the temperature at which softening takes place:-

1. High- fusing or Tray compound/ Type II compound2. Low- fusing or Impression compound/ Type I compound

- Tray compound is used in the construction of dentures to form a tray

that can be employed with other types of impression materials for

reproducing the mouth tissues.

- Type I compound is referred to as a true impression compound & is used for

impressions of edentulous mouths.

- Tray compound is more viscous when it is softened & more rigid when it is

hardened as compared to impression compound.


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PROPERTIES:-1) Fusion Temperature:

- The plateau or horizontal straight- line portion of the curve,

characteristic of a pure crystalline material is ill- defined.

- The fusion temperature of approx. 43.5C is not a solidificationtemperature since the glass transition temperature for this

particular compound is approx. 39C.


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- The practical significance of fusion temperature is that it

indicates a definite reduction in plasticity during cooling.

- The fusion temperature of impression compound shouldbe such that at the time of insertion it should flow

enough to register the details of the oral mucosa & not

cause discomfort or tissue irritation.

2) Thermal Conductivity:- Thermal conductivity is low.

- During softening of the material, the outside always softens first

& the inside last.


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3) Thermal Contraction:

- The average linear contraction of impression compound frommouth temperature to room temperature of 25C may vary

between 0.3 & 0.4%.

- The volume expansion over the same temperature range may be

as great as 1.38 to 2.29%.


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4) Flow:- The flow of impression compound can be beneficial or it can be a

source of error.- The compound should soften at a point just above mouth

temperature & in this state exhibit adequate flow to adapt closely

to the tissues & register surface detail.

- Once the compound has solidified, any deformation should becompletely elastic, so that the impression can be withdrawn

without distortion of flow.


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- The flow properties at mouth temperature (37C) & slightly above

45 C are very important.

- Minimum flow is required at mouth temperature, but sufficientflow is needed at a temperature

only 8C above mouth

temperature to record the detail

in the impression.- Compounds have widely differing

flow values between 1& 8% at

37C & between 81&93%

at4

5C.


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- The higher amount of flow required of the impression compound at 45C is based onits use as a final impression material,where as the tray compound is used only for the

initial impression(tray), & a second impression material such as zincoxide eugenol

paste will be used in the tray to record the final impression.

- Impression compound is the most viscous of the impression materials. Viscosity is

about 70 times greater than that for impression plaster & more than 100 times

greater than values for some of the light- bodied elastomers.

- The very high viscosity of impression compound is significant in 2 ways:-

1) It limits the degree of fine details which can be recorded in an impression.

2) It characterizes compound as a muco-compressive material.


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5) Dimensional Stability:

- Softening the compound by a method that will not affect its physical properties

adversely by overheating or prolonged heating is important.

- In the mouth, adequate cooling of the compound is essential to avoid distortion

when the impression is removed.

- If the surface of the compound is hard, but the inside is soft, a relaxation will

occur immediately after the impression is withdrawn.

- Storage in a warm environment or for extended periods of time promotesdimensional changes.

- So, a cast or die should be constructed as soon as possible after the impression

has been obtained at least within the first hour.


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- 3 factors that combine to produce significant internal stresses within

the compound impression are:-

1) The high value of coefficient of thermal expansion.

2) The poor thermal conductivity.

3) The relative large temperature drop from softening temperature to room

temperature.

6) Reproduction of Details:

- Impression compound has a high viscosity, so reproduction of

surface detail is not very good.

7) Rigidity:

- Impression compound is fairly rigid after setting & has poor elastic properties.


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ClinicalPresentation of the Compound:-

- For recording prosthetic impressions, such as preliminary

impressions of edentulous arches the material is supplied in

rectangular/circular sheets about 5-7.5 cm and 0.65 mm thick.- Peripheral seal materials, supplied as stick forms of 10 cm long and

1cm in diameter are used for border extensions on impression trays.

- For copper brand impressions of inlays and crowns, the material is

supplied in stick form. Working temperature of green stick isl22-129 F black stick is 133-135 F.


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Manipulation:-

Cooling:-

Disinfection of the Impression:-- The impression should first be rinsed to remove blood, debris &

saliva.

- Disinfection is done by immersion for a minimum of 10min. &

maximum of 30min.in a glutaraldehyde(eg. Cidex) or an iodophor(eg. Biocide).

- A 20min. immersion in 2% ID210 solution has no adverse effectson the dimensional stability or surface detail reproduction.

- ID210 is a virucidal, synergistic combination of aldehydes,

uaternar ammonium com ound & a non-toxic surfactant.


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- Zinc-oxide eugenol impression pastes produce a rigidimpression with a high degree of accuracy & good

reproduction of surface detail.

ZINC-OXIDE EUGENOL

IMPRESSION PASTE


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USES:-1) Cementing medium

2) Surgical dressing

3) Temporary filing material4) Root canal filling material5) Temporary relining material for dentures

6) For stabilization of base pastes in bite registration

7) Bite registration material8) In full denture prosthodontics to register final impressions under

saddle areas at try-in of the cast framework.

9) In partial denture prosthodontics as corrective washes.

10) In cases exhibiting pronounced nausea.


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TYPES:-

- Impression pastes may be classified on the basis of eugenol content

as:

1) Eugenol containing impression pastes

2) Non-eugenol pastes

- ZOE impression pastes are available in 2 types as hard & soft set.

- Soft set material is tougher& not as brittle.- Hard set material generally has a more fluid consistency when

mixed, a shorter final setting time & a higher resistance to

penetration when set.


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COMPOSITION:-- The major constituents are zinc-oxide & eugenol with additional

components.

- This impression material is dispensed as two separate pastes.

Tube No. 1

1) Zinc oxide 80-87%

2) Fixed vegetable or mineral oil 13-15%

Tube No. 2

1) Oil of Cloves/eugenol 12-15%2) Gum or Polymerised rosin 50%

3) Filler 16-20%

4) Lanolin 3%

5) Resinous balsam 10%

6) Accelerator, colour & moisture 4-5%


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Zinc oxide:

- It is a reactive ingredient which takes part in the setting reaction.

- The type of zinc oxide used may be critical i.e. it should be finelydivided & should contain a very slight amount of water.

Fixed vegetable or mineral oil:

- The zinc oxide base component is mixed with inert oils to form a

paste.- The inert oils may be mineral oil or vegetable oil like olive oil or

linseed oil.

- It acts as a plasticiser & also aids in masking the action of eugenol

as an irritant.


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Eugenol:- Oil of cloves contain 70-85% eugenol.- It is sometimes used in preference to eugenol because it reduces the burning

sensation in the soft tissues.Gum Rosin:

- It is an important constituent & can be incorporated in either the zinc oxide or the

eugenol component or may be mixed in both.

- The rosin gives body & coherence to the mixed material & imparts the

thermoplastic properties to the set impression, so that it can be softened in hotwater for easy removal from the cast.

- Carnauba wax, Kauri gum or Coumarone resins can be substituted for gum rosin.

- Rosin facilitates the speed of the reaction & a smoother, more homogenous

product results.

- It is an inert material used with eugenol to form a paste of suitable consistency.


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Resinous Balsam:

- Canada balsam & Peru balsam are used to increase flow & improve mixing

properties.Accelerator:

- It can be incorporated in either one or both pastes.

- Magnesium chloride/ calcium chloride hasten the normally slow- setting reaction

of a simple zinc oxide & eugenol mixture to a point convenient for clinical

impression procedures.

- Certain soluble salts & zinc salts such as zinc acetate are efficient accelerators.

- Other chemicals commonly used are metallic salts of hydrochloric & nitric acids,

primary alcohols & glacial acetic acid.

- Addition of water decreases the setting time of ZOE product.


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CHEMISTRY:-

- The reaction between zinc oxide & eugenol is very complex & has

never been completely defined.

OHOCH3 Structural formula for EUGENOL.

CH2-CH=CH2

ZnO + H2O Zn(OH)2 This reaction takes place either inZn(OH)2 + 2HE ZnE2 + 2H2O solution or at the surface of the

Base Acid Salt zinc oxide particles.

(Eugenol) (Zinc Eugenolate)


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OH H2C=HC-H2C

OCH3+ ZnO H2O O CH3

CH2-CH=CH2 (excess) O Zn O

(eugenol) H3C O

(zinc eugenolate) CH2-CH=CH2


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e

PROPERTIES:-1) Se tting Time :

- Se tting time is important, since the re must be sufficie nt time for mixing, fillingthe tray & se ating the impre ssion.

- Once the mate rial, in a plastic condition, has b e e n carrie d to the mouth, only aminimal time shoulde lapse be fore the impre ssion harde ns.

- 2 se tting time s, initial & final, are re cognise d by the Ame rican De ntalAssociation Spe cification No. 16 for de ntal impre ssion paste s.

- The initial se tting time is the pe riod from the be ginning of mixing until themate rial ce ase s to pull away or string out whe n its surface is touche d with ame tal rod of spe cifie d dime nsions. The impre ssion should be se ate d in the mouth be fore the initial se t. Initial se t may vary be tw e e n 3&6min.

- The final se t occurs whe n the rod fails to pe ne trate the surface of the spe cime nmore than 0.2mm unde r a load of 50gm. Final se t should occur within 10min.forType I paste s & 15min.for Type II paste s.


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Control of Setting Time:

- The setting time of ZOE is not as easily controlled by the operator.

- Methods by which the operator may control the setting time are:-

a) By adding small amount of zinc acetate or other accelerators.

Also, a small drop of water or alcohol may be mixed into the

paste containing eugenol before blending the 2 pastes.

b) When the setting time is too short, the cause is usually high

humidity & or temperature. Cooling the spatula & mixing slab

may help increase setting time.

c) The setting time can also be prolonged by the addition of certain

inert oils &waxes during the mixing, such as olive oil, mineral oil

& petrolatum.


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2) Consistency & Flow:

- The consistency & flow of the freshly mixed paste when it is impressed against the

tissues are of considerable clinical importance.

- A paste of thick consistency or high viscosity can compress the tissues, whereas athin, fluid material results in an impression that copies the tissues in a relaxed

condition with little or no compression.

- The thickness of the paste used is normally around 1mm.

3) Rigidity:- The major restriction on the use of these materials is their lack of elasticity.

- The set material may distort or fracture when removed over undercuts.

- The materials are sometimes used to record small undercuts in soft tissues but the

tendency of some pastes to flow under relatively small pressures should be

remembered.


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4) Strength:- It is not a critical requirement for this impression material since it is

supported by a tray & is not used in areas where it would be

required to withstand extensive deformation & stress during

removal from the mouth.

5) Hardness:

- The hardness of the set material varies with different products.- It is measured by noting the extent to which a loaded needle will

penetrate a small specimen of the set material.

- It is expressed in terms of millimeters penetrated in a given time.

- Hardness values for ZOE pastes may vary from 0.2-2.5mm.


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6) Dimensional Stability:

- The dimensional stability of most of the impression pastes is quite

satisfactory.

- ZOE impression materials shrink in the order of 0.1% or less at theend of 30min. after mixing.

- No significant further change occurs at the end of 24 hours & thesematerials can be considered to be reasonably stable in dimension.

7) Reproduction of Details:

- The relatively low initial viscosity of the mixed paste, coupled with

its pseudoplastic nature, allows fine detail to be recorded in the

impression.


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-Manipulation

- Pouring of the cast

- Disinfection of the impression.


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Non eugenol pastes

- Occasionally, eugenol may promote an allergic response in some

patients . There may be a stinging or burning sensation when eugenolcontacts soft tissues.

- Alternative non-eugenol materials are based upon the reaction

between zinc oxide and a carboxylic acid such as orthoethoxybenzoic

acid, to form an insoluble soap (saponification reaction). Thecarboxylic acid may be present as a liquid or as a powder dispersed in

a medium such as ethyl alcohol.

ZnO + 2RCOOH -> (RCOO) 2 Zn + H20

- The reaction is not greatly affected by temperature or humidity.


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IMPRESSION PLASTER

- It is a rigid, mucostatic impression material that sets by

chemical reaction.


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COMPOSITION:-- It consists of essentially the - hemihydrate of calcium sulphate

which is calcined.

- Impression plaster consists of:-

1) Accelerator:

- They are the chemicals that are added to reduce the setting time

& control the setting the setting expansion.

- Potassium sulfate, potassium chloride & potassium nitrate

accelerate the setting time & reduce setting expansion.

- Potassium sulphate acts by increasing the solubility of the

hemihydrate.


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2) Retarder:

- They are the chemicals that are added to increase the setting time.

- Sodium carbonate, sodium bicarbonate, potassium carbonate &

borax can be used as retarders.

- A balanced blend of accelerator & retarder to the raw hemihydrate

base plaster can reduce the setting expansion & at the same timecontrol setting expansion.

- Typical combinations are potassium sulfate- borax & potassium

sodium tartarate- sodium citrate.


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3) Colouring agents:

- Alizarin red is a commonly used pigment.

4) Starch:

- The addition of starch gives a soluble impression plaster.

- When these soluble plasters are used, the impression can be

separated from the cast by immersion of both in hot water.

- Starch also produces a very smooth mix, with the added advantage

that it is slightly sticky & so will adhere to the tray.


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5) Gums:

- Natural gums like tragacanth are added to change the handling

characteristics of the plaster mix.

- They give cohesion to the mass & facilitate border molding of the

impression in the mouth before the plaster sets.

6) Flavoring agents:

- These may be used to offset the bland taste of the plaster.

- Additives may be supplied as an aqueous solution to be mixed withplaster.

Potassium sulphate 4%

Borax 0.4-1%

Alizarin red 0.04%

This solution is called AE or anti-expansion solution.


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PROPERTIES:-1) Setting Time & Setting Expansion:

- The setting time of impression plaster is largely determined by the

controlling agents added by the manufacturer.

- Variation of the W:P ratio gives some control but at the same time

influence the consistency of the mass & the strength.

- The salts of many metals will reduce the setting time of plaster &

many of them also reduce the setting expansion.

- The addition of4% potassium sulphate will make the settingexpansion clinically negligible but, at the same time will reduce

the setting time to about 1min.

- Addition of 0.4% borax to the potassium sulphate solution will

give a setting time of about 3min., which is a convenient time.


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- Setting time & expansion can be controlled by the operator by:-

1) W:P ratio ed ST & ed SE

2) Spatulation time ed ST & ed SE

3) in Temperature upto 30C es ST

2) Accuracy:-

- The mixed impression material is initially very fluid & is capable of

recording soft tissues in the uncompressed state.

- Impression plaster allows very intimate contact between the

impression material, combined with the ability to remove moisture

from tissue & a minimal dimensional change on setting & results in

a very accurate impression.


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3) Storage:-

- If impression plaster is exposed to air under conditions of high relative humidity

during storage, it will absorb water from the atmosphere.

- This causes crystals of calcium sulfate dihydrate to form on the surface of the

hemihydrate crystals.

4) Rigidity:-- After setting, the plaster impression material is very brittle.

- It can undergo virtually no compressive or tensile strain without fracturing.

5) Impression plasters are non-toxic, but the water absorbing nature of these

materials often causes patients to complain about a very dry sensation after

having impressions recorded.

6) This material is most dimensionally stable & exhibits little dimensional change.


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MANIPULATION:-

1) Proportioning & Mixing:-

-Impression plaster should be mixed in a rubber bowl using a stiff,

wide bladed spatula.

- The material is used at a higher W:P ratio than is normally used for

modelling plasters.

- The fluid mix is required to enable fine detail to be recorded in the

impression & to give the material mucostatic properties.


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2) Impression Making:-

- Freshly mixed plaster is too fluid to be used in a stock impression

tray & is normally used in a special tray, constructed using a

1-1.5mm spacer. The tray may be made from acrylic resin or shellac

- Another technique is to record the plaster impression as a wash in a

preliminary compound impression.

- The compound is deliberately moved during setting to create space

for the plaster wash.- One technique for recording impressions of undercut areas, commonly used before

the advent of elastic materials, was to allow the impression plaster to set & then to

fracture it in order to facilitate removal from mouth. Material is weak & easily

fractures due to high W:P ratio.


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3) Separating Media:-

- Alcoholic solutions of varnishes & waxes, soap solutions, water

glass( 60% solution of sodium silicate in water), liquid paraffin & avariety of commercial preparations may be used.

4) Disinfection:-

- Impression plaster may be immersed for 20min in 2%ID210 solution.


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IMPRESSION WAX

- Waxes were first used in dentistry in the early 18th

century for the purpose of recording impressions of

edentulous mouths.


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USES:-

1) For corrective impressions of free- end saddle areas of partial

dentures.2) In edentulous mouths.

3) Used in small quantities to fill in areas of impressions in which

insufficient material has been used or in which an air blow has

caused a defect.

4) As a bite- registration material.

5) A mixture of border corrective & denture body corrective waxes is

used in patients with heavy masticatory muscuature.


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TYPES:-

- Dental waxes are classified as:

1) Pattern wax

2) Processing wax

3) Impression wax

- Impression wax is of 2 types:1) Corrective wax

2) Bite registration wax


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COMPOSITION:-

- Waxes used in dentistry may be composed of natural & synthetic

waxes, gums, fats, fatty acids, oils, natural & synthetic resins &

pigments of various types.

- Particular working characteristics of each of the dental waxes are

achieved by the blending of appropriate natural & synthetic waxes& resins & other additives.


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Natural waxes Synthetic waxes Additives

Mineral Acrawax C Stearic acid

- Paraffin Aerosol OT Glyceryl tristearate

- Microcrystalline Castorwax Oils

- Barnsdahl Flexowax C - Turpentine

- Ozokerite Epolene N-10 Color

- Ceresin Albacer Natural resins

- Montan Aldo 33 Rosin

Plant Durawax 1032 - Copal

- Carnauba - Dammar

- Ouricury - Sandrac- Candelilla - Mastic

- Japan wax - Shellac

- Cocoa butter - Kauri

Insect- Beeswax Synthetic resins

Animal- Spermaceti - Elvax

- Polyethylene


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WAXES:-

- Waxes are inorganic polymers consisting of hydrocarbons & their

derivatives(esters & alcohols).- The average molecular weight of a wax blend is about400 to 4000

which is low compared with structural acrylic polymers.

- The chemical components of both natural & synthetic waxes

impart characteristic physical properties to the wax, which are ofprimary interest because the specific physical properties of a wax

or wax blend, determine its usefulness for intended applications.


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a) Paraffin Waxes:-

- These are obtained principally from the high boiling point fractions of petroleum

and are mixtures of chiefly straight- chain saturated hydrocarbons containing

26-30 carbon atoms.

- The melting range is low (50-70C).

- Presence of oils lower the melting temperature.

- Paraffin waxes used in dentistry are refined waxes & have less than 0.5% oil.

- It is used in inlay & modelling waxes.

b) Ceresin:-

- It is derived from wax- bearing distillates from natural mineral petroleum

refining or lignite refining.

- They may be used to increase the melting range of paraffin waxes.


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c) Carnauba & Ouricury Waxes:-

- Carnauba contains 85% alkyl esters of various kinds.

- These waxes are characterized by high hardness, brittleness & high melting

temperatures.

- They have melting temperatures from 84 to 91C, whereas ouricury melts at 79

to 84C.

- They possess the outstanding quality of increasing the melting range & hardness

of paraffin waxes.- eg. The addition of 10% of carnauba wax to paraffin wax with a melting range of

20C will increase the melting range to 46C.

- The addition of ouricury waxes produces a similar effect, but they are less

effective than carnauba wax.


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d) Beeswax:-

- It is an insect wax and is a complex mixture of esters consisting

mainly of myricyl palmitate , saturated and unsaturatedhydrocarbons and high molecular weight organic acids.

- The melting temperature is 60C to 70C.

- It is brittle at room temperature but becomes plastic at body

temperature.

- It is used to modify the properties of paraffin waxes.


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SYNTETIC WAXES:-

- These are complex organic compounds of varied chemical compositions.

- Although differing chemically from natural waxes, they possess certain physical

properties( melting temperature & hardness) akin to those of the natural waxes.- They may differ from natural waxes in certain characteristics because of the high

degree of refinement they possess, in contrast to the contamination that is

frequently present in waxes from natural sources.

- Following are some of the synthetic waxes available:

a) Polyethylene waxes: melt at 100 to 105C & properties are similar to paraffin wax

b) Polyoxyethylene glycol waxes: melts at 37 to 63C .

c) Halogenated hydrocarbon waxes: prepared by reaction of chlorine with natural wax

d) Hydrogenated waxes:prepared by reaction of hydrogen with natural waxes

e) Wax esters:produced from the reaction of fatty alcohols & acids.


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Gums:- These are complicated substances, many of which are mixtures containing largely

carbohydrates & when they are mixed with water, they either dissolve or form

sticky, viscous liquids.

- Many waxes obtained from plants & animals resemble gums in appearance.- Gum arabic & tragacanth are 2 natural gums that do not resemble waxes in either

their properties or composition.

Fats:

- Waxes are harder & have higher melting temperature than fats, but resemble fats

in some ways.- Both are tasteless, odourless & colorless in pure form & are greasy to touch.

- fat may be used to increase the melting range & hardness of compounded wax.

Resins:

- They are complex, amorphous mixtures of organic substances. Natural resins are

obtained from trees & plants, except shellac, which is produced by insects.


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PROPERTIES:-

1) Solid-Solid Transition Temperature:-

- A solid-solid transition occurs where the stable crystal lattice form

(orthorhombic in most dental waxes) commences to change to ahexagonal form which is present below the melting point of the wax.

2) Thermal Expansion & Contraction:-

- Waxes expand when subjected to a rise in temperature & contracts

as the temperature is decreased.- This fundamental property may be altered slightly when various

waxes are blended, but the response to thermal changes cannot be

reduced to negligible values.


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3) Mechanical Properties:-- Both the physical & mechanical properties that characterize a dental wax are

determined primarily by the amount of hydrocarbons & ester waxes present, by

the molecular weight distribution of each component & by the amount of

impurities present.4) Flow:-

- Waxes deform when subjected to a load for a period of time.

- The property of flow is visualised as resulting from the slippage of molecules over

each other.

- Impression wax exhibit high flow at mouth temperature & distort on withdrawal

from undercuts.

- Flow is low when the temperature of the wax is below the solid-solid transition

temperature.

- As temperature is raised above the transition temperature flow property increases.


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5) Internal Stresses:-

- Waxes have low thermal conductivities, so making it difficult to

achieve uniform heating.

- If a wax is moulded or adapted to shape without adequate heating

to above the solid-solid transition temperature, considerable

stresses will be set in the material.

- If the wax is subsequently warmed, relief of the stresses will occur,

resulting in distortion.


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Corrective Impression Wax:-

- Corrective impression wax is used as wax veneer over an original

impression to contact and register the details of soft tissues.- It is claimed that it records the mucous membrane and underlying

tissues in a functional state in which movable tissue is displaced to

such a degree that functional contact with the base of the denture is

obtained.

- Corrective waxes are formulated from hydrocarbon waxes such asparaffin, ceresin & beeswax & may contain metal particles.

- The flow of several corrective waxes measured by penetration at

37C is 100%.- These waxes are subjected to distortion during removal from mouth.


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Bite Registration Wax:-- Bite registration wax is used to articulate accurately certain

models of opposing quadrants.

- The wax bite registration of the copper formed die must provide

proximal & occlusal relations.

- Bite registrations frequently are made from 28 gauge casting wax

sheets or from hard baseplate wax, but waxes identified as bite waxes

appears to be formulated from beeswax or hydrocarbon waxes such

as paraffin or ceresin.- Certain bite waxes contain aluminium or copper particles.

- The flow of several bite waxes at37C ranges from 2.5% to 22%indicating that these waxes are susceptible to distortion on removal

from the mouth.


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MANIPULATION:-

- Wax can be used in a fashion similar to that for ZOE paste on the

surface of a primary impression or custom impression tray.

- The major use of waxes as an impression material may occur during

full denture impression techniques.

- In case of impression techniques for partial dentures, it must be used

in a rigid base plates that fits positively against the natural standing

teeth.

- A cast must be prepared immediately from such an impression to

avoid distortion.


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CONCLUSION


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REFERENCES

1) A historical review of complete denture impression materials-

JADA- 1975,91(11) : 1037-1040

2) Anusavice K.J. Phillips Science of Dental Materials

11th edition, 2003

3) Combe E.C. Notes on Dental Materials 6th edition , 1992

4) Craigs R.G., Powers J.M. Restorative Dental Materials

11thedition, 2002

5) Determination of properties of impression materials-Br. Dent. J 1988, 164(4) : 221-225.

6) Ferrecane J.L.- Materials in Dentistry- Principles&Applications

7) Mc Cabe J.F. Applied Dental Materials 7th edition , 1992

8) Philli R W Ski S i f D t l M t i l 9th diti 1992

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