Impression materials agar and alginate

Impression materials agar and alginate

1. Impression materials

2. Negative likeness of oral structures Dental IMPRESSION

3. Opp. MODEL: POSITIVE likeness of oral structures:- Die: model of a single tooth Cast: model of more than one tooth

4. Elastic Hydrocolloids Synthetic rubbers Poly sulphide , polyether Silicone base Non elastic: Plaster of paris – impression plaster ZnO paste Impression waxes High and low fusing impression compounds Basic classification

5. An impression tray is a container which holds the impression material as it sets, and supports the set impression until model is poured in stone. Impression trays can be separated into two main categories- stock trays and special trays. Impression trays

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7. Stock trays: Metal – re- usable/ sterilizeable Plastic trays Custom trays: Made of visible light cure material – VLC Self cure/ cold cure/ auto cure acrylic resin Shellac resin Types of trays

8. Metal trays Perforated Non - perforated

9. Trays for edentulous pt

10. Hydro – water Colloid - mixture Mixture/ suspension in water Hydrocolloids

11. Reversible: Agar – agar Irreversible: Alginate Classification of Hydrocolloids

12. A revrsible elastic hydrocolloid impression material setting by physical changes, It gives good detail reproduction. CHEMISTRY Agar is a hydrophilic colloid extracted from certain types of seaweed. It is a sulphuric ester of linear polymer of glactose. SUPPLIED as Gel in collapsible tubes (tray material) Cylinders in a glass jar (syringe material) In bulk containers Agar Agar

13. Agar (8 -15 %) Basic constituent Borates (.2 - .5%) Improves strength and act as gypsum retarder. Potassium sulfate. (1 – 2%) Act as gypsum hardener. Fillers(.5 – 1%) hard waxes improves strength, Thixotropic material. ( .3 - .5 %) a plasticizer. E.g. glycerine and thymol. Alkyl benzoates. (.1%) preservative increases self life, Coloring and flavoring agent. Traces. Water (80%) dispersion medium. Composition

14. Liquefaction temperature Agar gels have fibrils held together by secondary molecular forces, which are weak. They break at increasing temperature to sol form. This increased temperature is called liquefaction temperature and is 71-100’C . Gelation temperature The sol can be converted to gel by decreasing the temperature below liquefaction. it is called gelation temperature. It varies from 37 o C-50 o c Hysteresis the temp lag between these two states. This helps in using agar as an impression material. This is the manipulation time, Gel/sol transition

15. The gelation temp.is critical If too high’ injury to oral tissues If too low, Inadequate flow properties Viscosity of sol. After liquefying, the material must be sufficiently viscous so that it does not flow out of the tray. the viscosity should be adequate that it readily records every detail of the teeth and soft tissues. Viscoelastic properties. It should recover completely when load is removed. Significance. The impression should be removed with a single snap along the long axis of the tooth. Minimize distortion due to this behaviour. Detail reproduction

16. Used as final imp. Of the dentate patients requiring rpd , fpd . Widely used as cast duplication. Duplication procedure: A duplicating flask is used. The master cast is placed in it. The agar is liquefied at 70 to 100c ; then tempered to 50c and then poured into the duplicating flask, master cast arrangement through an opening and filled. It is allowed to set and the master cast is removed with a jerk. The working cast is poured immediatley Uses

17. Detail reproduction is very good. Can record undercut areas correctly. Distortion on removal is prevented due to elastic recovery. Well tolerated by the patient. Can be used. Advantages

18. Cannot be electroplated Thin sections of impression tears easily. Multiple models cannot be poured. Special armamentarium required Gypsum hardener required. Sterilization of imp. Difficult. Disadvantages

19. Alginate The word alginate comes from term algin. The term was coined by scottish chemist. It is an elastic irreversible hydrocolloid. It is irreversible because it sets with chemical reaction. The reaction results in cross linking of polymer chain. Types Type 1. –Fast setting 1-2 min Type 2 . Slow setting - 2 - 4 min.

20. Composition Alginic acid is prepared from marine plant. And is a linear polymer of anhydro alpha-D mannuronic acid of high molecular weight. The solutions of potassium and sodium salts of alginic acid reacts calcium salt and produce an insoluble elastic gel. 1. Sodium or potassium alginate salts (18%) --- To dissolve in water. Calcium sulfate dihydrate (14%); To react with dissolved alginate to form insoluble calciun alginate.

21. Sodium phosphate(2%): to react with calcium sulphateand serve as a retarder. Diatomaceous earth or silicate powder (56%): to control consistency of the mix and flexibility of the impression. Potassium sulphate or potassium zinc chloride [(or silicates or borates(10%)]:to counteract the inhibiting effect of alginate on setting of gypsum.

22. Organic glycol (small): to coat the powder particles to minimize dust during dispensing (dustless alginate). Disinfectants: quaternary ammonium compounds or chlorhexidine (1-2%): to provide self disinfection. Wintergreen, peppermint, aniseed (traces): to provide pleasant taste. Pigments (traces): to colour.

23. Gelation process Sol- gel reaction/chemical reaction. Calcium sulfate reacts with sodium alginate or potassium alginate so rapidly to form insoluble calcium alginate that it does not allow sufficient working time. Trisodium phosphate, retarder, first reacts with calcium sulfate till no supply of retarder. 2Na 3 Po 4 + 3CaSo 4 =Ca 3 (Po 4 ) 2 +3Na 2 So 4

24. Remaining calcium sullfate reacts with sodium or potassium alginate and forms insoluble calcium alginate and so provide adequate working time. K 2 Alg + nCaSo 4 = nK 2 So 4 + Ca n Alg

25. Gel structure Insoluble salt formed by reaction of sodium alginate in solution with calcium salts. Calcium ions replace sodium ions in adjacent molecules and form cross linking between molecules Such polymer molecules constitutes brush heap structure of gel Final structure is brush heap of calcium alginate fibril network enclosing unreacted sodium alginate sol, excess water, filler particles and reaction byproducts like sodium sulfate and calcium phosphate.

26. Control of gelation time Time from start of mixing till material is no longer adhesive when touched by clean dry finger. Type 1 =1-2 min fast set Type 2 =2-4.5 slow set Control by manufacturing: By amount of retarder added during manufacturing

27. Control by clinicians: By altering w:p ratio Changing mixing time By altering temperature of water: Increase temp decrease gelation time Premature gelation cause distortion of impression and rendered useless Prolonged gelation time is tedious for both patient and dentist.