Presentation on theme: "Impression material. Zinc Oxide Eugenol This material is used for recording edentulous ridges in a close fitting special tray or the patients existing."— Presentation transcript:
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
Composition Constituents of a typical zinc oxide eugenol paste are: BASE PASTE Zinc oxide Inert oils (plasticiser) Gum rosin (increases setting time and improves cohesion) REACTOR PASTE Eugenol 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 crabs 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 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 time Setting 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 reaction 4. 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.
properties Non 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. oStorage problems: kept in sealed containers.
Reaction: Chemical reaction: crystalization CaSO 4 ½ H 2 O + H 2 O CaSO 4.2H 2 O + heat Dimensional changes: Setting expansion can be reduced to % by presence of K 2 SO 4. Soluble impression plaster: Contains starch to facilitate the separation of impression from model by softening when immersed in hot H 2 O.