ASSINGMENTNON.ELASTIC IMPRESSION MATERIAL....!1. Impression compound2. Impression Plaster3. Zinc Oxide Eugenol4. Impressio...
DISADVANTAGES  Cannot be used for mucocdisplacive impressions  Cannot be added to  Properties affected by operator handlin...
2. Reusable (but with re-use the constituents are leached out)3. Can be reheated and readapted4. Can support other materia...
ZINC OXIDE (excess)ZINC OXIDE (UNREACTED)+        ->+EUGENOLEUGENOLThe set material contains both some unreacted zinc oxid...
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  1. 1. ASSINGMENTNON.ELASTIC IMPRESSION MATERIAL....!1. Impression compound2. Impression Plaster3. Zinc Oxide Eugenol4. Impression WaxesImpression PlasterThe plaster should be mixed with water or an anti-expansion solution in the ratio of 100g to 50-60ml. The mix should be a smooth paste, free of air bubbles, which may appear on the surface ofthe impression leading to inaccuracy. The tray should be loaded and "puddled" into position, andheld in place until set. As the impression is removed from the mouth it is not uncommon forpieces of plaster around the periphery of the impression to fracture off. These pieces should beretrieved and glued back onto the impression before it is cast. Long narrow strips of wax are thenfit around the periphery of the impression just below where it ends. This is called beading. Theimpression is then coated with a thin layer of separating medium and cast in fresh plaster. Thebeading provides a clear indication of where the impression ends; the creation of this level areaprevents over-trimming and over-extension.CHEMISTRYImpression plaster is based on calcined (??????) calcium sulphate hemihydrate, which reactswith water to form a hard mass of calcium sulphate dihydrate.This setting reaction is associated with an expansion of 0.3-0.6%. When this is within the confinesof an impression tray it will lead to a significant reduction in accuracy.Mixing the plaster with anti-expansion solution (containing 4% potassium sulphate and 0.4%borax) will reduce this. The potassium sulphate reduces expansion to 0.05%, but this alsoaccelerates the setting reaction, and borax is added as a retarder, which gives more time to takethe impression.PROPERTIES Excellent at recording fine detail (because very fluid when inserted in mouth) Dimensionally stable if anti expansion solution used Fractures if undercuts present Mucostatic Needs to be treated with a separating medium (e.g. varnish or soap solution) before being castin stone or plaster Exothermic setting reaction On storage dimensionally stable but a small amount of shrinkage may occur Non toxic but may be unpleasant due to dryness and heat evolved during settingADVANTAGES Good surface detail Excellent dimensional stability Rate of the setting reaction can be controlled by the clinician
  2. 2. DISADVANTAGES Cannot be used for mucocdisplacive impressions Cannot be added to Properties affected by operator handling technique Taste and roughness may cause the patient to vomitImpression CompoundThe compound disc is completely immersed in a water bath at 55-60oC for about 4-5 minutes toensure complete softening. Gauze is placed at the bottom of the water bath to preventadherence. However if it is left in too long some of the constituents may be leached out into thewater bath, altering the properties of the material (it is often the plasticiser stearic acid that isleached out). If the compound is kneaded water will become incorporated and act as a plasticiser.The compound is loaded on to the tray and firm pressure is used to seat the tray home in themouth. After the impression has been taken it is carefully examined, a common mistake is notsoftening the compound enough. In this is the case the impression can be reheated in a separatebowl of water (cross infection control) and the impression is repeated.Properties of Impression CompoundCHEMISTRYMade up of 3 constituents1. Resins (e.g. wax)2. Filler (e.g. talc or soapstone)3. Lubricants (stearic acid or stearin)It is a thermoplastic compound i.e. it softens when heated and hardens when cooled. Thisprocess does not involve a chemical reaction.Impression compound is available in 2 forms:1. Stick form - used for modifying impression trays. Softening temperature is 700C2. Sheet form - used to take impressions of the mouth. Softening temp is 55-600C.PROPERTIES Poor surface detail High coefficient of thermal expansion (contraction of up to 0.3% when removed from mouth toroom temperature) Distorts when removed over undercut areas Mucocodisplacive Poor dimensional stability Can be modified by re-heating Non toxic and non irritant Good shelf lifeADVANTAGES1. Non irritant and non toxic
  3. 3. 2. Reusable (but with re-use the constituents are leached out)3. Can be reheated and readapted4. Can support other materials for wash impressions5. MucocompressiveDISADVANTAGES1. Poor dimensional stability2. Poor surface detail3. Expansion coefficient4. Will distort if removed from undercutsImpression WaxesWaxes are thermoplastic materials, which flow at mouth temperature and are soft a roomtemperature. They do not set by chemical reaction. Normally used to correct small imperfection(e.g. airblows) in other impressions, especial zinc oxide impressions. They consist of acombination of a low melting paraffin wax and beeswax in a ratio of approximately 3:1 to ensurethe wax flows at room temperature. A cast should be poured up immediately after taking theimpression to avoid distortion which readily occurs in wax. These materials are not commonlyused.Zinc Oxide-EugenolImpression MaterialThis material is used for recording edentulous ridges in a close fitting special tray or the patientsexisting 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 requiringprotection (soft tissues, teeth etc). Firm pressure is used during impression taking.Properties of Zinc-Oxide EugenolCHEMISTRYConstituents of a typical zinc oxide eugenol paste are:BASE PASTE Zinc oxide Inert oils (plasticiser) Hydrogenated resins (increases setting time and improves cohesion)REACTOR PASTE Eugenol Zinc acetate (accelerator) Fillers (talc or kaolin)Some pastes contain a substitute for eugenol e.g. a carboxylic acid.The 2 pastes come in contrasting colours and are dispensed in a 1:1 ratio. They are mixed to givea paste of even colour. The setting reaction is shown below:
  4. 4. ZINC OXIDE (excess)ZINC OXIDE (UNREACTED)+ ->+EUGENOLEUGENOLThe 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, inaccurateimpression.Setting time depends on:1. Accelerator additives (e.g. zinc acetate, acetic acid)2. Exposure to moisture on mixing or the addition of water will accelerate the reaction3. Increasing temperature causes a faster setting reactionSetting time is normally 4-5 minutes.PROPERTIES Non toxic Adherence to tissues Mucostatic or mucocodisplacive (depending on brand used) Good surface detail in thin section Good dimensional stability (little or no dimensional change on setting, 0.1% dimensionalchange during setting) Can be added to with fresh zinc oxide eugenol Stable on storage and good shelf lifeADVANTAGES1. Dimensional stability2. Good surface detail3. Can be added to4. Mucostatic or mucocodisplaciveDISADVANTAGES1. Cannot be used in very deep undercuts2. Only sets quickly in thin section3. Eugenol allergy in some patients

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