2 Zinc Oxide EugenolThis 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.
5 1) Zinc Oxide Eugenol impression material 2) An impression taken in Zinc oxide eugenol
6 Composition BASE PASTE REACTOR PASTE Constituents of a typical zinc oxide eugenol paste are:BASE PASTEZinc oxideInert oils (plasticiser)Gum rosin (increases setting time and improves cohesion)REACTOR PASTEEugenolZinc 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.
7 zinc eugenolate (chelate) (salt) Setting reactionAcid – base reaction.Chelation reactionZinc oxide + water zinc hydroxideZinc hydroxide + 2 eugenol Base Acidzinc eugenolate (chelate) (salt)
8 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”.
9 Setting timeThe 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 timeTypes: S.TType I: hard minutesType II: soft minutesAccelerators: water, magnesium chloride, zinc acetate or alcohol.Retarders: inert oils such as olive oils, mineral oil.
10 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 reaction5. Cooling the glass slab and mixing spatula will increase STSetting time is normally 4-5 minutes.
14 properties Non toxic Adherence to tissues Good surface detail in thin sectionGood dimensional stability (little or no dimensional change on setting, 0.1% dimensional change during setting)Can be added to with fresh zinc oxide eugenolStable on storage and good shelf life
15 Advantages & Disadvantages 1. Dimensional stability2. Good surface detail3. Can be added to1. Cannot be used in very deep undercuts2. Only sets quickly in thin section 3. Eugenol allergy in some patients
16 Plaster of Paris (Non Elastic) Uses:Secondary impression or final wash in a special tray.Composition of powder:Calcium sulphate hemihydrate.Potassium sulphate or nitrate:AcceleratorFlavouring agent.Colouring agents.
17 Properties. Manipulation: Water / powder ratio. 55 to 60ml water to 100gm of powder.Separating medium alcoholic solution of varnish. Or soap suspensions.Properties.Storage problems: kept in sealed containers.
18 Reaction:Chemical reaction: crystalizationCaSO4 ½ H2O + H2O CaSO4.2H2O + heatDimensional changes:Setting expansion can be reduced to % by presence of K2SO4.Soluble impression plaster:Contains starch to facilitate the separation of impression from model by softening when immersed in hot H2O.