Prosthodontics.pptx
- 1. GYPSUM PRODUCTS – PART III
- 2. CONTROL OF THE SETTING TIME SOLUBILITY OF THE HEMIHYDRATE NUMBER OF NUCLEI OF CRYSTALLIZATION RATE OF CRYSTAL GROWTH
- 3. SOLUBILITY OF THE HEMIHYDRATE— • If the solubility of the hemihydrate is increased, supersaturation of the dehydrate is achieved faster, which accelerates the rate of dehydrate crystal deposition.
- 4. NUMBER OF NUCLEI OF CRYSTALLIZATION • Nucleation is the first step at which Ca2+ and SO4 2− in solution start to assemble into clusters on the nanometer scale, becoming stable under the operating conditions. • These stable clusters constitute the nuclei. The greater the number of nuclei of crystallization, the faster the dihydrate crystals will form and the sooner the mass will harden. • Any preexisting fine dihydrate particles will also serve as nuclei
- 5. RATE OF CRYSTAL GROWTH • Increasing or decreasing the rate of crystal growth will accelerate or retard the setting time
- 6. WATER POWDER RATIO • The weight (or volume) of the water divided by the weight of the hemihydrate powder is known as the water/powder ratio, which is abbreviated as W/P. • For example, if 100 g of dental stone is mixed with 28 mL of water, the W/P ratio is 0.28. • Important factor in determining the physical and mechanical properties of the final gypsum product.
- 7. • The use of a higher W/P ratio decreases the number of nuclei per unit volume. Consequently, the setting time is prolonged • An increase in the W/P ratio also causes a decrease in the strength and a decrease in the setting expansion of the set gypsum products
- 8. ACCELERATORS AND RETARDERS • Chemical modifiers have been used extensively to increase or decrease the setting time of gypsum products; they are called retarders and accelerators, respectively • The most commonly used accelerator is potassium sulfate, which is particularly effective in concentrations greater than 2%. • Slurry water flowing out from a model trimmer contains numerous fine gypsum particles that act as nuclei of crystallization and that can serve as an effective accelerator.
- 9. • Increasing the time or the speed of mixing - create more nuclei by crushing the developing dihydrate crystals into smaller particles, thereby accelerating the setting of the mix • 2% of the hemihydrate, sodium chloride is an accelerator. • Sodium sulfate has its maximum acceleration effect at approximately 3.4%. • Borax, a known retarder for gypsum setting, has been shown also to promote the growth of dehydrate crystals, but only at a concentration lower than 0.2 Mm (about 0.08 g/L).
- 10. MECHANISM OF SETTING EXPANSION • The crystallization of dihydrates can be pictured as an outgrowth of crystals from nuclei of crystallization. • Crystals growing from the nuclei can intermesh with and obstruct the growth of adjacent crystals.
- 11. • When the process repeats itself with thousands of the crystals during growth, an outward stress or thrust develops, producing an expansion of the entire mass. • The mixture exhibits an initial contraction followed by an expansion and smaller contraction
- 12. HYGROSCOPIC SETTING EXPANSION • Five stages • Under normal setting conditions, the crystals of the dihydrate begin to form on the nuclei. • Stage II -The water around the particles is reduced by hydration and these particles are drawn more closely together because surface tension of the water keeps the water surface area at a minimum
- 13. • STAGE III As the crystals of dihydrate grow, they contact each other and the water around the particles again decreases • STAGE IV -Further dihydrate growth consumes more water and should draw the crystals together as before, but the outward thrust of the growing crystals opposes this contraction • STAGE V Eventually, the crystals become intermeshed and entangled
- 15. NORMAL VS HYGROSCOPIC EXPANSION • Setting expansion without water immersion • The variables that can cause an increase or decrease in the normal setting expansion will have the same effect on the hygroscopic setting expansion. • The expansion that occurs under water • A physical phenomenon • For example, a reduction in the W/P ratio and increased spatulation will result in a higher hygroscopic expansion.
- 16. EFFECT OF WATER CONTENT • The wet strength (also known as green strength) and the dry strength. • The wet strength is the strength that is determined when water in excess of that required for hydration of the hemihydrate remains in the test specimen.
- 17. • When such excess water is removed by drying, the strength obtained is the dry strength • The dry strength may be two or more times as high as the wet strength.
- 18. TYPES OF GYPSUM PRODUCTS
- 19. IMPRESSION PLASTER • Composed of plaster of Paris (β-hemihydrate)to which modifiers have been added to regulate the setting time and setting expansion. • Impression plaster is rarely used any longer for making dental impressions because it has been replaced by less rigid materials such as the hydrocolloids and elastomers
- 20. • A water/powder ratio of approximately 0.5 to 0.6. • Its fluidity makes it suitable for making impressions of soft tissues in the uncompressed state
- 21. Uses • preliminary impressions • splinting transfer coping utilized to produce long-span implant-supported prostheses.
- 22. COMPOSITION • Potassium sulfate is added as an anti-setting expansion agent to reduce the setting expansion • A retarder like borax is added to the powder to balance the setting acceleration caused by the potassium sulfate and to bring the setting time under control. • A pigment, such as alizarin red, may be added to make a clear distinction between the impression and the model after casting of the model
- 23. MANIPULATION • A custom tray can be constructed -with acrylic resin or shellac. • Preliminary impressions can be made with dental compound, and impression plaster can be used as the wash material • The technique for inserting the impression into the mouth involves “puddling” the impression into place.
- 24. DISADVANTAGES • Difficult to remove. • The plaster impression material is very brittle and fractures easily. • When the impression involves an undercut area, it is necessary to fracture the impression to facilitate removal from the mouth.
- 25. MODEL PLASTER • This model plaster or laboratory Type II plaster is now used principally to fill a flask used in denture construction when setting expansion is not critical and the strength is adequate • It is usually marketed in the natural white color, thus, contrasting with stones, which are generally colored
- 26. DENTAL STONE (TYPE III) • Type III stone has a minimal 1-hour compressive strength of 20.7 MPa (3000 psi), but it does not exceed 34.5 Mpa (5000 psi). USES : • Construction of casts in the fabrication of full dentures to fit soft tissues. • Type III stones are preferred for casts used to process dentures because the stone has enough strength for this purpose and the denture is easier to remove after processing.
- 27. • The minimal time allowed for setting varies from 45 to 60 minutes, depending on the rate of setting of the stone or plaster and the type of impression material used.
- 28. DENTAL STONE, HIGH STRENGTH (TYPE IV) • The principal requisites for a die material are strength, hardness, and minimal setting expansion. • To obtain these properties, modified α-hemihydrate is used. • The cube-shaped particles and the reduced surface area produce such properties without undue thickening of the mix. This material is also called die stone
- 29. • USED in for cast after tooth preparation – sharp instrument used – resistant to abrasion • The surface resists abrasion, whereas the core of the die is tough and less subject to accidental breakage
- 30. DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION (TYPE V) • This gypsum product exhibits an even higher compressive strength than the Type IV dental stone. • In addition, the setting expansion has been increased from a maximum of 0.10% to 0.30% • The rationale for increasing setting expansion is that certain newer alloys, such as base metal, have a greater casting shrinkage than do the traditional noble metal alloys
- 31. • Indication • Expansion achieved is cast crowns is inadequate. • Contraindication • One should avoid the use of Type V stones for producing dies for inlays and onlays since the higher expansion may lead to an unacceptably tight fit
- 32. CARE OF GYPSUM PRODUCTS • It is important that all gypsum products be stored in a dry atmosphere. • The best means of storage is to seal the product in a moisture-proof metal container • WHY - the setting time is generally retarded only slightly, approximately 1 or 2 min per year
- 33. PROPORTIONING • The recommended W/P ratio should be used. • The water and powder should be measured by using an accurate graduated cylinder for the water volume and a weighing balance for the weight of powder. • The powder should not be measured by volume (as by using a scoop) as it does not pack uniformly. • Preweighed envelopes are very popular
- 34. MIXING AND POURING • Hand mixing - the bowl should be parabolic in shape, smooth, and resistant to abrasion. • The spatula - a stiff blade and a handle that is convenient to hold. • A measured amount of water is placed in the bowl and the weighed powder is sifted into the water as initial hand mixing is performed. • The mixture is then vigorously stirred, with periodic wiping of the inside of the bowl with the spatula to ensure wetting of all of the powder and breaking up of any agglomerates or lumps. • The mixing should continue until a smooth mix is obtained, usually within a minute. • A longer spatulation time drastically reduces the working time
- 35. • Entrapment of air in the mix must be avoided • since porosity can lead to weak spots and surface inaccuracies. • • After mixing, the use of a vibrator of high frequency and low amplitude is helpful in reducing air entrapment. • The preferred method of mixing is to use a mechanical mixer under vacuum
- 36. CARE OF CAST • The safest method for soaking the cast is to place it in a water bath with gypsum debris remaining on the bottom of the container to provide a saturated solution of calcium sulfate. • If the storage temperature is raised to between 90 °C and 110 °C (194 °F to 230 °F), shrinkage occurs, along with loss of strength as the water of crystallization is removed and the dihydrate reverts to the hemihydrate form. • As a rule of thumb, it is not safe to store or heat a stone cast in air at a temperature higher than 55 °C (130 °F).
- 37. DISINFECION OF GYPSUM CASTS • Disinfection solutions can be used that do not adversely affect the quality of the gypsum cast. • Alternatively, disinfectants can be incorporated either in the powder or dissolved in the mixing water
- 38. THANK YOU