Alginate-Irreversible Hydrocolloid


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Alginate-Irreversible Hydrocolloid

  2. 2. IMPRESSION MATERIAL DEFINATION A negative likeness or copy in reverse of the surface or object. An imprint of the teeth and adjacent structures for use in dentistry
  3. 3. Plaster Non-elastic Compound Waxes ZnO - Eugenol Aqueous Hydrocolloids Elastic Agar (reversible) Alginate (irreversible) Polysulfide Non-aqueous Elastomers Silicones Polyether Condensati on Addition O‟Brien, Dental Materials & their Selection 1997
  4. 4. ALGINATE Alginate is classified as irreversible hydrocolloid.  Hydrocolloid because it consists of particles of a gelatinous (colloidal) state in water (hydro) and irreversible because once it has jelled it cannot be returned to a liquid solution. 
  5. 5. The Word Alginate comes from the term “ALGIN”.  It is a peculiar mucous extract yielded by certain brown seaweed ( algae ) .  The term „ALGIN‟ was coined by chemist from scotland @ the end of 19th century. 
  6. 6. TYPES Type I – Fast setting. Type II – Normal setting.
  7. 7. MODES OF SUPPLY It is supplied as a powder that is packed: a) In bulk or in tins or in sackets (or) b) In preweighed individual containers. A plastic scoop is supplied for dispensing the bulk powder, & a plastic cylinder is supplied for measuring the water . 
  8. 8. APPLICATIONS 1) Used for impression making : a) when there are undercuts b) In mouth with excessive flow of saliva c ) For partial dentures with clasps. 2) For making preliminary impressions for complete denture. 3) For orthodontic and study models. 4) For duplicating models.
  9. 9. COMPOSITION :  Potassium alginate(15%)  Comes from seaweed; is used as a thickening agent.  Calcium sulfate(16%)  Reacts with the potassium alginate to form the gel.  Trisodium phosphate (2%)  Added to slow down the reaction time for mixing.
  10. 10.  Diatomaceous earth( 60 %) A  filler that adds bulk to the material. Zinc oxide (4%)  Adds  bulk to the material. Potassium titanium fluoride(3%)  Added so as not to interfere with the setting and surface strength.
  11. 11. PHYSICAL PHASES OF ALGINATE   The first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semiliquid form. The second phase is a gel. In the gel phase, the material is semisolid, similar to a gelatin dessert.
  12. 12. SETTING REACTION Reaction occurs by a chemical reaction.  Soluble alginate reacts with calcium sulphate to produce insoluble calcium alginate as a gel.  The production of calcium alginate is delayed by the addition of a third soluble salt to the solution,with which the calcium sulphate will react in preference to the soluble alginate to form an insoluble calcium salt. 
  13. 13. In a nutshell , when powder is mixed with water to obtain a paste ,two main reaction occur during setting : 1) 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3 Na2SO4 2) sodium alginate + CaSO4 + H2O -------- > Ca alginate + Na2SO4 (Powder ) (Gel) 
  14. 14.  First , sodium phosphate reacts with the calcium sulphate to provide adequate working time. Second ,after the sodium phosphate has reacted, the remaining calcium sulphate reacts with sodium alginate to form an insoluble calcium alginate which forms a gel with water.
  15. 15. WATER-TO-POWDER RATIO An adult mandibular impression generally requires two scoops of powder and two measures of water.  An adult maxillary impression generally requires three scoops of powder and three measures of water. 
  16. 16. MANIPULATION : Fluff or shake the powder to distribute the powder particles evenly  Use the specific measuring devices (water & powder) provided by the manufacturer for mixing  Follow the manufacturer‟s direction regarding the ratio of water to powder.  Use exact measurements 
  17. 17.  After the water and powder have been measured, place the water in a clean dry bowl. Shift the powder into the water. Adding the powder to the water ensures the powder particles are wet evenly. If mixed in reverse (the water is added to the powder) the chemical reaction will start early with some particles setting faster than others.
  18. 18.  Mix the alginate for the specific amount of time and using a stiff spatula “swipe” the alginate mass against the sides of the bowl to avoid entrapment of air in the mix.
  19. 19. SETTING TIME OF ALGINATE Since alginate reacts or gels chemically, temperature is a major factor in the setting time. The colder the temperature of the water the longer it takes to set; conversely, the higher the water temperature the faster it sets. Ideal water temperature is 68 degrees or room temperature.
  20. 20. LOADING OF TRAY A perforated tray is used so that the material is forced out slightly through the holes in the tray during loading,thereby loacking itself mechanically into the tray.  The surface of the alginate in the tray may be smoothened out by moistening the finger with water and running it over the surface of the alginate. 
  21. 21. SEATING THE TRAY Before making the upper impression, saliva should be wiped off from the palatal region.  In order to prevent the material from flowing along the throat and causing nausea to the patient,the posterior portion may be seated first and then the anterior portion seated properly.  Since the material sets from tissues towards periphery,any movement during gelation may result in distortion.  So once the tray is seated ,it must be held in place firmly without any movemnets. 
  22. 22. REMOVAL OF THE IMPRESSION An alginate impression when set develops a very effective peripheral seal.  This seal should be freed by running the finger around the periphery.  The impression must be removed suddenly ,with a jerk.  After removal from the mouth, - washed with water to remove saliva. -cast should be poured as soon as possible. 
  23. 23. ALGINATE SETTINGS     Normal set alginate  Working time of 2 minutes and a setting time of up to 41/2 minutes after mixing. Fast set alginate  Working time of 11/4 minutes and a setting time of 1 to 2 minutes. Working time  The time allowed for mixing the alginate, loading the tray, and positioning the tray in the patient's mouth. Setting time  The time required for the chemical action to be completed.
  24. 24. TAKING AN ALGINATE IMPRESSION  Explain the procedure to the patient:  The material will feel cold, there is no unpleasant taste, and the material will set quickly.  Breathe deeply through your nose to help you relax and be more comfortable.  Use hand signals to communicate any discomfort.
  25. 25. AN ACCEPTABLE ALGINATE IMPRESSION       There is a complete "peripheral roll," which includes all of the vestibular areas. The tray is not "overseated," which would result in exposure of areas of the impression tray. The impression is free from tears or voids. There is sharp anatomic detail of all teeth and soft tissues. The retromolar area, lingual frenum, tongue space, and mylohyoid ridge are reproduced in the mandibular impression. The hard palate and tuberosities are recorded in the maxillary impression.
  26. 26. CAUSES FOR DISTORTION AND DIMENSIONAL CHANGE OF ALGINATE If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition.  If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis. 
  27. 27. RECENT ADVANCES IN ALGINATES Dust free alginatesDe-dusting agent (glycerin or glycol) Siliconized alginatessilicon polymers are added to strengthen the material Alginates containing disinfectantseg quaternary ammonium salts or chlorohexamine are added to alginate powder. Hard and soft set alginatesby adjusting the amount of fillers
  28. 28. PROPERTIES 1)FLEXIBILITY It is about 14% at a stress of 1000g/cm2. Lower w/p ratio results in lower flexibility. 2)ELASTICITY AND ELASTIC RECOVERY alginate are highly elastic but less when compared to the agar. 97.3% elastic recovery occurs. Permanent deformation is less if the impression is removed from the mouth quickly.
  29. 29. 3) STRENGTH The compressive strengths of alginate gels range from 5000 to 8000 g/cm2. The tear strengths vary from 350 to 700 g/cm2. Both of these properties are time dependent with higher values being obtained if the time of removal is delayed.
  30. 30. 4) DIMENSIONAL STABILITY : Alginate impression loses water by evaporation and shrinks on standing in air. If it is placed in water it absorbs water and swells. Therefore ,cast should be poured immediately after making the impression.
  31. 31. 5)SHELF LIFE Alginate impression material have a short shelf life. They deteriorate rapidly when stored at elevated temperatures in a humid environment. Therefore ,it is better not to stock more than 1 year supply .
  32. 32. ADVANTAGES OF ALGINATE It makes an accurate impression  It allows for undercuts  The process is not time consuming  It is easy to work with, has good viscosity & is low in adhesive qualities  It is comfortable to the patient  Inexpensive to the dentist  Requires little armamentarium 
  33. 33. DISADVANTAGES 1) 2) 3) Can‟t be corrected. Cannot be used alone for compressing the tissues. It is not accurate enough for crown and bridge impressions.