Zinc Oxide Eugenol Impression material

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.

1) Zinc Oxide Eugenol impression material

2) An impression taken in Zinc oxide eugenol

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.

Setting reaction

Acid – base reaction.

Chelation reaction

Zinc oxide + water zinc hydroxide

Zinc hydroxide + 2 eugenol

Base

Acid

zinc eugenolate (chelate) (salt)

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”.

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.

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.

o Manipulation.

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

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

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.

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.

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.