Impression material. Zinc Oxide Eugenol This material is used for recording edentulous ridges in a...
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Transcript of Impression material. Zinc Oxide Eugenol This material is used for recording edentulous ridges in a...
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Zinc Oxide Eugenol Impression material
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Zinc Oxide Eugenol
This material is used for recording edentulous ridges in a close fitting special tray or the patients existing dentures. The material is mixed in a 1:1 paste ratio and used in thin sections only (2-3mm) as a wash impression.
Vaseline is used as a separating agent on those areas requiring protection (soft tissues ,skin etc). Firm pressure is used during impression taking.
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1) Zinc Oxide Eugenol impression material
2) An impression taken in Zinc oxide eugenol
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CompositionConstituents of a typical zinc oxide eugenol paste are:
BASE PASTEZinc oxide Inert oils (plasticiser) Gum rosin (increases setting time and improves cohesion)
REACTOR PASTEEugenol Zinc acetate/Mg Cl (accelerator) Fillers (talc or kaolin)Some pastes contain a substitute for eugenol e.g. a
carboxylic acid./chlorothymol NON EUGENOL TYPE The 2 pastes come in contrasting colours and are dispensed in a 1:1 ratio. They are mixed to give a paste of even colour.
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Setting reaction
Acid – base reaction.
Chelation reaction
Zinc oxide + water zinc hydroxide
Zinc hydroxide + 2 eugenol
Base
Acid
zinc eugenolate (chelate) (salt)
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Chelation:Is the attachment of a molecule or ion to metal atom at more than one point.
The term is derived from the Greek word meaning “crab’s claw” indicating two pronged attachment.
• A central zinc atom is held by two eugenol “claws”.
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Setting time
The set material contains both some unreacted zinc oxide and eugenol.
Any movement of the tray as the paste is hardening will lead to a deformed, inaccurate impression.
Setting time
Types: S.T
Type I: hard 10 minutes
Type II: soft 15 minutes
1. Accelerators: water, magnesium chloride, zinc acetate or alcohol.
2. Retarders: inert oils such as olive oils, mineral oil.
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Factors affecting the setting timeSetting time depends on:1. Accelerator additives (e.g. zinc acetate, acetic
acid)2.Retarders (inert oils such as olive oils, mineral oil.
3. Exposure to moisture on mixing or the addition of water will accelerate the reaction4. Increasing temperature causes a faster setting reaction
5. Cooling the glass slab and mixing spatula will increase ST
Setting time is normally 4-5 minutes.
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o Manipulation.
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propertiesNon toxic Adherence to tissues Good surface detail in thin section Good dimensional stability (little or no
dimensional change on setting, 0.1% dimensional change during setting)
Can be added to with fresh zinc oxide eugenol
Stable on storage and good shelf life
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Advantages & Disadvantages ADVANTAGES DISADVANTAGES
1. Dimensional stability
2. Good surface detail
3. Can be added to
1. Cannot be used in very deep undercuts
2. Only sets quickly in thin section 3. Eugenol allergy in some patients
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Uses:
Secondary impression or final wash in a special tray.
Composition of powder:
1. Calcium sulphate hemihydrate.
2. Potassium sulphate or nitrate:
Accelerator
3. Flavouring agent.
4. Colouring agents.
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Manipulation:
Water / powder ratio.
55 to 60ml water to 100gm of powder.
Separating medium alcoholic solution of varnish. Or soap
suspensions.
Properties.o Storage problems: kept in sealed containers.
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Reaction:
Chemical reaction: crystalization
CaSO4 ½ H2O + H2O CaSO4.2H2O + heat
Dimensional changes:
Setting expansion can be reduced to + 0.06%
by presence of K2SO4.
Soluble impression plaster:
Contains starch to facilitate the separation of impression from model by softening when
immersed in hot H2O.