Gypsum products and its prosthodontic implications .pptx

gypsum products in dentistry

1. GYPSUM & GYPSUM PRODUCTS Presented By: Dr. Shweta Gaikwad Guided By :Dr. Amit Jagtap

2. Overview Introduction Classification of Gypsum products Production of gypsum products Setting of gypsum Factors affecting setting reaction Setting expansion Strength of set gypsum products Specialized gypsum products Care of gypsum Infection control References

3. Gypsum products used in dentistry are based on calcium sulphate hemi-hydrate (CaSO4)2·H2O. Their main uses are for casts or models, dies •The primary applications of gypsum products in dentistry include the production of study models for oral and maxillofacial structures and their use as auxiliary materials for dental laboratory operations involved in the production of dental prostheses. •They are also used to form models and casts on which wax dental prostheses and restorations are constructed. Introduction

4. CLASS 1 IMPRESSION PLASTER CLASS2 DENTAL PLASTER CLASS 3 DENTAL STONE (Hydrocal, Alpha hemihydrate) CLASS 4 DENTAL STONE (HIGH STRENGTH, LOW EXPANSION) (Die stone, Densite, Modified alpha hemihydrate) CLASS 5 DENTAL STONE (HIGH STRENGTH, HIGH EXPANSION) CLASSIFICATION ADA Specification number 25

5. IMPRESSION PLASTER For making impressions in complete denture and maxillofacial prosthetics (not used currently for this purpose). Bite registration material

6. TYPE 2 OR DENTAL PLASTER DENTAL PLASTER GYPSUM + HEAT (OPEN KETTLE @ 120ºC) Β- HEMIHYDRATE CALCIUM SULPHATE

7. DENTAL PLASTER For making study casts and models. To make molds for curing dentures. For mounting casts on articulator.

8. TYPE 3 OR DENTAL STONE (TYPE 3 GYPSUM) GYPSUM + STEAM & PRESSURE AT 125ºC ΑLPHA- HEMIHYDRATE CALCIUM SULPHATE

9. TYPE 3 OR DENTAL STONE 1.full or partial denture model 2.orthodontic model 3. for casts used to process dentures because the stone has enough strength for this purpose and the denture is easier to remove after processing

10. TYPE 4 OR DIE STONE (TYPE 4 DIE STONE) GYPSUM + 30% calcium chloride at 100ºC ΑLPHA- HEMIHYDRATE CALCIUM SULPHATE

11. TYPE 4 OR DENTAL STONE, DIE, HIGH STRENGTH, LOW EXPANSION SYNONYMS: Class II stone, die stone, densite, improved stone. USES Making dies used for inlay, crown and bridge wax patterns.

12. TYPE 5 OR DENTAL STONE, DIE, HIGH STRENGTH, HIGH EXPANSION USES: fabrication of die to aid in compensation for shrinkage of base metal alloy during solidification

13. Setting of gypsum

14. In sol state hemihydrate particles are converted to dihydrate and as the measured amount of water is consumed the mass converts to a solid gel. Colloidal theory hemihydrate + water --> colloidal state through sol -gel mechanism.

15. Hydration theory It suggests that rehydrated plaster particles join together through hydrogen bonding to the sulfate groups to form the set material.

16. Dissolution-precipitation theory

17. Solubility of the hemihydrate Rate of crystal growth Number of nuclei of crystallization Factors affecting setting reaction W/P RATIO Effect of Temperature & pH 1 2 3

18. W/P RATIO The ratio of the water to the hemihydrate powder is usually expressed as the W/P ratio Higher W/P ratio means less number of nuclei per unit volume W/P ratio setting time strength setting expansion

19. Gypsum Effect of temperature Solubility difference Hemihydrate Temperature Setting reaction Plaster-water mixture temperature 100 °C exceeds 50 °C Retardation NO REACTION 0- 50 degree ---------- little change

20. setting reaction Effect of pH pH 1 ∝ Liquids with low pH, such as saliva, retard the setting reaction

21. NaCl upto about2% NaSO4 upto 3.4% K2SO4 > 2 Set calcium sulfate dihydrate Borax < 0.08g/L Accelerators Accelerators & Retarders A cetates B orates B orax C itrates C olloids such as gelatine, glue, agar, coagulated blood Retarders The chemical that increases the rate of hemihydrate dissolution or precipitation of dihydrate accelerates the setting reaction.

22. Spatulation Setting time Effect of spatulation MIXING TIME: Time from addition of powder to water until mixing is completed. Mechanical mixing:20-30 secs Hand spatulation:1 min. WORKING TIME: : The time available to use a workable mix, one that maintains an even consistency that may be manipulated to perform one or more tasks. General 3 min working time is adequate SETTING TIME :When the powder is mixed with water, the time that elapses from the beginning of mixing until the materials hardens is known as setting time. Break up dihydrate crystals during the early stages of setting, thus producing more nuclei on which crystallization can be initiated

23. 1. Loss of gloss test for initial set. Occurs when excess water in the mix is taken up in forming the dihydrate, so the mix loses its gloss. This occurs approx. at 9 mins. and the mass has no measurable compressive strength. TEST TO DETERMINE SETTING OF GYPSUM

24. Initial Gilmore test for initial set Gilmore test for final setting time:- Gilmore needle test Gilmore needle(1lb, 1/24”) barely perceptible mark 45-60min Gilmore needle: ¼ lb , 1/12” no longer leaves an impression is called the initial set. This time is marked by a definite increase in strength. 13 mins

25. Vicat penetrometer test The setting material is indented by a needle of 1mm diameter Under a load of 300gm.setting time is achieved when needle can no longer penetrate to a depth of 2 mm into the material.

26. SETTING EXPANSION? Crystals growing from the nuclei can intermesh with and obstruct the growth of adjacent crystals. The crystallization of dihydrates can be pictured as an outgrowth of crystals from nuclei of crystallization. Therefore, the structure immediately after setting is composed of interlocking crystals between which are micropores and pores containing the excess water required for mixing. On drying, the excess water is lost, resulting in a porous structure.

27. CONTROL OF SETTING EXPANSION At higher W/P ratios, fewer nuclei of crystallization per unit volume are present compared with the greater density of nuclei in thicker mixes. Because it can be assumed that the space between the nuclei is greater in such a case, it follows that there is less growth interaction of the dihydrate crystals and less outward thrust. However, the most effective method by which to control setting expansion is through the addition of chemicals by the manufacturer

28. NORMAL SETTING EXPANSION Stage I: 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. 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

29. HYGROSCOPIC SETTING EXPANSION Stage I of hygroscopic setting condition shows an identical mixture of hemihydrate (the area delineated by the dashed circle) under water (the area outside of the dashed circle). The hydration of hemihydrate particles here would proceed as usual. Since they are under water, the water consumed by hydration will be immediately replenished by the immersion water and the distance between the particles would remain the same ( stage II ). As the dihydrate crystals continue to grow and contact each other, no reduction in the distance between crystals is expected ( stage III ). This means that the crystals will grow much more freely during the early stages ( stage IV ), before the intermeshing finally prevents further expansion ( stage V ). Consequently, the observed setting expansion that occurs when the gypsum product sets under water is greater than that which occurs during setting in air.

30. EFFECT OF WATER CONTENT 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. Dry strength may be two or more times as high as the wet strength. this phenomenon STATES that as the last trace of water is lost, fine crystals of gypsum precipitate serve as anchors between the larger crystals The set plaster or stone is porous, and the greater the W/P ratio, the greater the porosity. The greater is the W/P ratio, the less is the dry strength of the set material, because fewer dihydrate crystals are available per unit volume EFFECT OF W/P RATIO STRENGTH OF SET GYPSUM PRODUCTS

31. In general, an increase in mixing time increases the strength to a limit that is approximately equivalent to that of hand mixing for 1 minute. The addition of an accelerator or retarder lowers both the wet strength and the dry strength of the gypsum product which can be partially attributed to the salt added as an adulterant and to the reduction in intercrystalline cohesion EFFECT OF MANIPULATION AND ADDITIVES

32. When strength, hardness and accuracy are required dental stones are normally used in preference to dental plaster. The stone materials are less likely to be damaged during the laying down and carving of a wax pattern and give optimal dimensional accuracy. Thus, these materials are used when any work is to be carried out on the model or die as would be the case when constructing a denture on a model or a cast alloy crown on a die. When mechanical properties and accuracy are not of primary importance the cheaper dental plaster is used. Thus, plaster is often used for mounting stone models onto articulators and sometimes for preparing study models

33. SPECIALIZED GYPSUM PRODUCTS DENTAL CASTING INVESTMENTS: Adding a refractory material like silica or quartz or crystobalite to dental plaster or stone permits it to withstand high temperatures. These are called dental casting investments. Uses: To prepare refractory molds for casting dental alloys. DIVESTMENT: It is a combination of die stone and gypsum-bonded investment mixed with colloidal silica. The advantage of divestment is that the wax pattern does not have to be removed from the die, thus distortion of the pattern can be avoided. Uses: To make refractory dies.

34. CARE OF GYPSUM If the gypsum cast has to be soaked in water it must be placed in a water bath in which plaster debris is allowed to remain constantly on the bottom of the container to provide a saturated solution of calcium sulfate at all times. This is known as ‘slurry water’. If the cast is washed in ordinary water, surface layer may dissolve, hence slurry water is used to preserve surface details. Such a procedure also causes a negligible expansion. All gypsum casts must be handled carefully as any departure from the expected accuracy may result in a poorly fitting appliance.

35. STORAGE OF GYPSUM kept in air tight containers. kept clean with no dirt or other foreign bodies.

36. INFECTION CONTROL Gypsum products may be disinfected by: 1. Immersing cast in a disinfection solution. 2. Addition of disinfectant into the stone. 3. Overnight gas sterilization while treating patients known to have an infection (impractical for routine use).

37. STRENGTH OF SET GYPSUM PRODUCTS WATER CONTENT W/P RATIO MANIPULATION& ADDITIVES

38. RESIN MODIFIED STONES: It is a blend of synthetic resin and alpha gypsum. These stones are less brittle, have improved surface smoothness and increased resistance to abrasion. Their compressive strength can be as high as 79 MPa. Example: ResinRock, Whipmix corporation SYNTHETIC GYPSUM: It is possible to make alpha and beta hemihydrate from the by-products during the manufacture of phosphoric acid. More expensive than that made from natural gypsum. However, manufacture is difficult and a few have succeeded (e.g., Japan and Germany).

39. MOUNTING PLASTER: Plaster used for attaching the cast to the articulator is known as mounting plaster. Important properties for these products include a low setting expansion (0.09) which is important for the accuracy of the mounting, low strength (12 MPa) which allows easy separation of the cast and fast setting time (3 minutes). FAST SETTING STONE: These are exceptionally fast setting stones (2 minutes) with an early high compressive strength (1 hour - 41 MPa) which allows separation of the cast. Example: Snap stone (Whipmix).

40. REFERENCES Anusavice, Phillips’ Science of Dental Materials , 12th edition, India, Elsevier publication Mahalaxmi S. Materials used in dentistry, 2nd edition, Wolters Kluwer Pvt Ltd, New Delhi Craig’s Restorative Dental Materials, 12th edition?Jaypee Brothers Medical Publishers.

41. QUESTION & ANSWERS

42. Thus it , Accelerates setting time; reduces setting expansion, effectively reducing vertical dimension change and preventing the laboratory errors. Anti-expansion solution COMPOSITION: 6% potassium sulphate 0.6% borax Potassium sulphate can reduce the setting expansion by reduction of the axial ratio of the growing needle like arrays of the crystals. But its drawback is that potassium sulphate can not only reduce the setting expansion but also accelerate the setting reaction. This acceleration of the reaction rate is due to increased rate of hemi-hydrate dissolution Thus, potassium sulphate alone will cause inadequate working time and a retarder is needed to use it Borax which is a retarder is used in combination to apply it practically. Nyan M. Effect of anti-expansion solution on setting expansion and setting time of dental plaster. Myanmar Dental Journal. 2020;27(1):60-4.