GYPSUM PRODUCTS in dental materials in prosthodontics

gypsum products in dental materials

1. PRESENTED BY : SAGAR MITRA 1 ST YEAR PGT GYPSUM PRODUCTS 1

2. Introduction Production of Gypsum Products Setting of Gypsum Products Setting Expansion Strength of Set Gypsum Products Types of Gypsum Products Manipulation of Gypsum Products Care of the Cast Disinfection of the cast Anusavice K.J.-“Phillips’ Science of Dental materials” CONTENTS 2

3. Gypsum (CaSO4•2H2O; calcium sulfate dihydrate ) is a mineral mined in various parts of the world, but it is also produced as a by- product of flue gas desulfurization in some coal-fired electric power plants. Products derived from gypsum is widely used in the field of dentistry. Anusavice K.J.-“Phillips’ Science of Dental materials” INTRODUCTION 3

4. Anusavice K.J.-“Phillips’ Science of Dental materials” TYPES OF GYPSUM: 4

5. Building construction Soil conditioning Food additives Pharmaceuticals Medical devices APPLICATION IN DENTISTRY For cast preparation. Models and dies. Impression Material As Investment Material Mounting of Casts As a mold material for processing of complete dentures Anusavice K.J.-“Phillips’ Science of Dental materials” USES: 5

6. Anusavice K.J.-“Phillips’ Science of Dental materials” USES 6

7. Anusavice K.J.-“Phillips’ Science of Dental materials” PRODUCTION OF GYPSUM PRODUCTS 7

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10. α- hemihydrate β- hemihydrate Formed when dihydrate is heated under steam pressure. Formed when dihydrate is heated in an open kettle or kiln. α- hemihydrate crystals are denser and have a prismatic shape. β -hemihydrate crystals are characterized by their “sponginess” and irregular shape. When hemihydrate particles are mixed with water the α- hemihydrate produces a much stronger and harder dihydrate structure. β - hemihydrate produces a less stronger and harder dihydrate structure. It requires less water as compared to β-hemihydrate. β- hemihydrate crystals require more water to wet the powder particles. Anusavice K.J.-“Phillips’ Science of Dental materials” 10

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12. Anusavice K.J.-“Phillips’ Science of Dental materials” SETTING OF GYPSUM PRODUCTS 12

13. There are three theories of gypsum setting: The colloidal theory proposes that, when mixed with water, hemihydrate enters into the colloidal state through a sol- gel mechanism. In the sol state , hemihydrate particles are hydrated to form dihydrate , thereby entering into an active state. As themeasured amount of water is consumed, the mass converts to a solid gel. The hydration theory suggests that rehydrated plaster particles unite through hydrogen bonding with sulfate groups to form the set material . Anusavice K.J.-“Phillips’ Science of Dental materials” THEORIES OF GYPSUM SETTING: 13

14. The most widely accepted mechanism is the dissolution- precipitation theory , which is based on dissolution of the hemihydrate particles in water followed by instant recrystallization to the dihydrate . This reaction has become possible because the solubility of hemihydrate in water is four times greater than that of the dihydrate near room temperature. Thus, the setting reactions occur as follows: When the hemihydrate is mixed with water, a suspension is formed that is fluid and workable. The hemihydrate dissolves until it forms a saturated solution of Ca2+ and (SO4) 2−. This saturated hemihydrate solution is supersaturated with respect to the solubility of the dihydrate ; precipitation of dihydrate occurs. Anusavice K.J.-“Phillips’ Science of Dental materials” 14

15. As the dihydrate precipitates, the hemihydrate continues to dissolve. The process proceeds as either new crystals form or further growth occurs on the crystals already present until no further dihydrate can precipitate out of solution. The setting reaction takes time to complete and changes in the mixture begin as soon as the hemihydrate and water are mixed together. There is less than 50% dihydrate present in Type IV and V stones, about 60% in Type II die materials, and over 90% in Type I plasters. Anusavice K.J.-“Phillips’ Science of Dental materials” 15

16. Anusavice K.J.-“Phillips’ Science of Dental materials” QUANTIFYING SETTING REACTIONS 16

17. The tests done for setting of gypsum products are listed as below Loss of gloss test. Gillmore’s test. Vicat test for setting time. Loss of gloss test : As the reaction proceeds, the excess water onthe surface is taken up in forming the dihydrate , so that the mix loses its surface gloss and gains strength. Anusavice K.J.-“Phillips’ Science of Dental materials” TESTS FOR SETTING OF GYPSUM PRODUCTS 17

18. GILLMORE’S TEST : When the mix no longer leaves an impression when penetrated by Gillmore needle , which has a tip 2.12 mm in diameter and weighs 113.4 g, the time elapsed is called the initial setting time . At this point, the mass still has no measurable compressive strength and the cast cannot be safely removed from the impression. The elapsed time at which a heavier Gillmore needle, weighing 453.6 g and with a tip 1.06 mm in diameter, leaves only a barely perceptible mark on the surface is called the final setting time. Anusavice K.J.-“Phillips’ Science of Dental materials” 18

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20. Theoretically, there are at least three mechanisms that can achieve such control. These include: Solubility of the hemihydrate - If the solubility of the hemihydrate is increased, supersaturation of dihydrate is achieved faster, which accelerates rate of dihydrate crystal deposition. Number of nuclei of crystallization - Nucleation is the first step at which Ca 2+ and SO 4 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. Anusavice K.J.-“Phillips’ Science of Dental materials” CONTROL OF THE SETTING TIME 20

21. 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. The use of a higher W/P ratio decreases the number of nuclei per unit volume. Consequently, the setting time is prolonged. Anusavice K.J.-“Phillips’ Science of Dental materials” 21

22. Mixing : Within practical limits, the longer and the more rapidly the gypsum product is mixed, the shorter is the setting time as the crystals are broken up by the spatulation process, which results in more nuclei of crystallization. Temperature : the difference in solubility between hemihydrate and gypsum becomes smaller with increasing temperature, and this condition lowers the driving force for forming the dihydrate ; it also results in a slower setting reaction Anusavice K.J.-“Phillips’ Science of Dental materials” 22

23. Material W/P Ratio Spatulatio n Turns Setting Time Model Plaster 0.50 20 14 min 0.50 100 11 min 0.50 200 8 min Dental Stone 0.30 20 10 min 0.30 100 8 min Anusavice K.J.-“Phillips’ Science of Dental materials” Effect of Spatulation on Setting Time 23

24. Chemical modifiers have been used extensively to increase or decrease the setting time of gypsum products; they are called retarders and accelerators, respectively. The chemical that increases the rate of hemihydrate dissolution or precipitation of dihydrate accelerates the setting reaction. 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 Anusavice K.J.-“Phillips’ Science of Dental materials” MODIFIERS FOR CONTROLLING SETTING TIME 24

25. At a concentration of 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 dihydrate crystals, but only at a concentration lower than 0.2 mM (about 0.08 g/L). Anusavice K.J.-“Phillips’ Science of Dental materials” 25

26. Regardless of the type of gypsum product selected, an expansion of the mass can be detected during the change from the hemihydrate to the dihydrate. Depending on the composition of the gypsum product, this observed linear expansion may be as low as 0.06% or as high as 0.5%. 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. 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. Anusavice K.J.-“Phillips’ Science of Dental materials” SETTING EXPANSION 26

27. Therefore, the structure immediately after setting is composed of interlocking crystals between which are micropores and pores containing the excess water required for mixing. The onset of initial setting occurs at approximately the minimal point of the curve, the point at which expansion begins. Anusavice K.J.-“Phillips’ Science of Dental materials” 27

28. STAGE I: Imagine that the initial mix is represented in the top row of by the three round particles of hemihydrate surrounded by water. 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 . Anusavice K.J.-“Phillips’ Science of Dental materials” Expansion under normal setting conditions 28

29. 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. Anusavice K.J.-“Phillips’ Science of Dental materials” 29

30. The hygroscopic setting expansion is a physical phenomenon and is not caused by a chemical reaction any more than is the normal setting expansion. STAGE I: 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. STAGE II: 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 . Anusavice K.J.-“Phillips’ Science of Dental materials” Hygroscopic setting expansion 30

31. STAGE III: As the dihydrate crystals continue to grow and contact each other, no reduction in the distance between crystals is expected. STAGE IV: The crystals grows much more freely during the early stages, before the intermeshing finally prevents further expansion (stage V . Anusavice K.J.-“Phillips’ Science of Dental materials” 31

32. A lower W/P ratio and a longer mixing time will increase the setting expansion. Potassium sulphate (accelerator) — 4% solution decreases setting expansion from 0.5% to 0.06% Sodium chloride 2%(accelerator) and ground gypsum increases setting expansion. Anusavice K.J.-“Phillips’ Science of Dental materials” CONTROL OF SETTING EXPANSION 32

33. EFFECT OF WATER CONTENT The strength of plaster or stone increases rapidly as the material hardens after the initial setting time. However, the free water content of the set product definitely affects its strength. For this reason, two strength properties of gypsum are reported: 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. When such excess water is removed by drying, the strength obtained is the dry strength. Anusavice K.J.-“Phillips’ Science of Dental materials” STRENGTH OF SET GYPSUM PRODUCTS 33

34. Model Plaster: 12.5 Mpa Dental stone:31 Mpa Die stone : 45 Mpa 34 Anusavice K.J.-“Phillips’ Science of Dental materials”

35. o The dry strength may be two or more times as high as the wet strength. Microwave irradiation has been used to speed up the drying of gypsum casts. Anusavice K.J.-“Phillips’ Science of Dental materials” 35

36. As previously noted, 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. Material that is mixed at a high W/P ratio has a tensile strength as high as 25% of the corresponding compressive strength. When materials are mixed at low W/P ratios, the tensile strength is less than 10% of the corresponding compressive strength. Anusavice K.J.-“Phillips’ Science of Dental materials” EFFECT OF W/P RATIO 36

37. The compressive strength is inversely proportional to the W/P ratio. Model plaster has the greatest quantity of excess water, whereas high- strength dental stone contains the least excess water. High- strength dental stone is the densest and thus shows the highest compressive strength. Model plaster is the most porous and thus shows the lowest compressive strength. Anusavice K.J.-“Phillips’ Science of Dental materials” 37

38. A plot of the strength as a function of the W/P ratio for the five different types of gypsum products used in dentistry. The strength ranges represent the wet strength at 1 hour. The strength increases as the specimens dry and it can double in a week. Anusavice K.J.-“Phillips’ Science of Dental materials” 38

39. The spatulation time also affects the strength of the plaster, an increase in mixing time increases the strength to a limit that is approximately equivalent to that of hand mixing for 1 minute. If the mixture is overmixed , the gypsum crystals will be broken up and the final product will hold less crystalline interlocking structure. The addition of an accelerator or retarder lowers both the wet strength and the dry strength of the gypsum product. Such a decrease in strength can be partially attributed to the salt added as an adulterant and to the reduction in inter crystalline cohesion. Anusavice K.J.-“Phillips’ Science of Dental materials” EFFECT OF MANIPULATION AND ADDITIVES 39

40. Surface hardness of gypsum materials is related to their compressive strength. Surface hardness increases at a faster rate than the compressive strength. Abrasive Resistance of gypsum products (for high- strength dental stone) increases by 15- 41% when impregnated with epoxy resins. Surface hardness of set gypsum is improved by mixing stone with a hardening solution containing colloidal silica( about 30%). Anusavice K.J.-“Phillips’ Science of Dental materials” SURFACE HARDNESS & ABRASION RESISTANCE 40

41. ADA Specification No. 25 classifies five types of gypsum products, as shown in Table , with the property requirement for each type. Anusavice K.J.-“Phillips’ Science of Dental materials” TYPES OF GYPSUM PRODUCTS 41

42. Impression plaster is a β- calcium sulfate hemihydrate used at 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, a characteristic of mucostatic impression material. Because of its rigidity, the use of impression plaster has been suggested for making preliminary impressions or splinting transfer coping utilized to produce long- span implant- supported prostheses. Anusavice K.J.-“Phillips’ Science of Dental materials” IMPRESSION PLASTER (TYPE I): 42

43. Potassium sulfate is added as an anti-setting expansion agent to reduce the setting expansion and 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. As an alternative, an antiexpansion solution containing potassium sulfate, borax, and pigment may be used with a standard white plaster. Anusavice K.J.-“Phillips’ Science of Dental materials” 43

44. Manipulation Because freshly mixed plaster is too fluid to be retained in a stock tray, a custom tray can be constructed using a 1- to 1.5- mm spacer 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. With the remaining plaster in the tray, the tray is seated in a single movement. Then the tray is gently moved from side to side and anteroposteriorly to take advantage of the fluidity of the material. In view of the fluidity of the material, the resulting impression may be difficult to remove. The plaster impression material is very brittle and fractures easily. Anusavice K.J.-“Phillips’ Science of Dental materials” 44

45. 45 Anusavice K.J.-“Phillips’ Science of Dental materials” IMPRESSION PLASTER

46. Impression plaster containing potato starch is termed as soluble plaster. Impression made from this is poured and after it hardens it is immersed in hot water which further leads to swelling of the starch and easy retrievability of the cast. Contains dental plaster, k2so4, borax and colouring agents. 46 Anusavice K.J.-“Phillips’ Science of Dental materials” SOLUBLE PLASTER

47. When the impression involves an undercut area, it is necessary to fracture the impression to facilitate removal from the mouth. The fragments are then reconstructed to form the completed impression. Long, narrow strips of wax can be adapted around the periphery of the impression. This is called beading. The impression is then coated with a thin layer of separating medium and cast in fresh plaster; otherwise, separation would be impossible. Disinfection of a plaster impression can be achieved with a 10- min soak in sodium hypochlorite solution, as described previously. Anusavice K.J.-“Phillips’ Science of Dental materials” 47

48. 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 according to the limits cited in the ADA specification or ISO standard. It is usually marketed in the natural white color, thus, contrasting with stones, which are generally colored. Anusavice K.J.-“Phillips’ Science of Dental materials” MODEL PLASTER (TYPE II) 48

49. With the advent of hydrocolloid impression material , the improved hardness of α- hemihydrate made stone dies workable and the indirect wax pattern possible. 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). Anusavice K.J.-“Phillips’ Science of Dental materials” DENTAL STONE (TYPE III) 49

50. It is intended for the construction of casts in the fabrication of full dentures to fit soft tissues. For this application, a slight setting expansion can be tolerated in casts that reproduce soft tissues, but not when teeth are involved. 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 Anusavice K.J.-“Phillips’ Science of Dental materials” 50

51. 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 . Anusavice K.J.-“Phillips’ Science of Dental materials” DENTAL STONE, HIGH STRENGTH (TYPE IV) 51

52. A hard surface is necessary for a die stone because the tooth preparation is covered with wax and carved flush with the margins of the die. A sharp instrument is used for this purpose; therefore, the stone must be resistant to abrasion. It is fortunate that the surface hardness increases more rapidly than the compressive strength because the surface dries more rapidly. This is a real advantage in that the surface resists abrasion, whereas the core of the die is tough and less subject to accidental breakage. Anusavice K.J.-“Phillips’ Science of Dental materials” 52

53. This gypsum product exhibits an even higher compressive strength than the Type IV dental stone by lowering the W/P ratio even further than that used for Type IV stone. In addition, the setting expansion has been increased from a maximum of 0.10% to 0.30%. Thus, higher expansion is required in the stone die to aid in compensating for the alloy solidification shrinkage. The use of a Type V stone may also be indicated when the expansion achieved during the fabrication of cast crowns is inadequate. Anusavice K.J.-“Phillips’ Science of Dental materials” DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION (TYPE V) 53

54. The orthodontist prefers a white stone or plaster for study models and may even treat the surface with soap to increase their sheen. These products generally have a longer working time, which reduces void formation and facilitates trimming. The use of an articulator makes it necessary to mount the casts using a gypsum- producing product. These materials are referred to as “mounting” stones or plasters. They are fast setting and have low setting expansion. The mounting plaster has a sufficiently low strength to permit easy trimming and facilitate separating the cast from the articulator mounting plates Anusavice K.J.-“Phillips’ Science of Dental materials” SPECIAL GYPSUM PRODUCTS 54

55. Die stone mixed with investment materials. Mixed with colloidal sillica liquid. The whole complex is then invested and mixed as divestment and water. These help to compensate for the shrinkage of the wax pattern in different cases. 55 Anusavice K.J.-“Phillips’ Science of Dental materials” Divestment:

56. In practice, clinicians and technicians must not only produce a cast using a gypsum-producing material, but they must also store the powder properly and maintain the cast in its best condition for subsequent procedures. CARE OF GYPSUM PRODUCTS The hemihydrate of gypsum absorbs water from the air readily. For example, if the relative humidity of the surroundings exceeds 70%, the plaster absorbs sufficient moisture from the air to start a setting reaction. The first hydration probably produces a few crystals of gypsum on the surface of the exposed hemihydrate crystals. These gypsum crystals can act as nuclei of crystallization and accelerate the setting reaction when they are mixed with water. Anusavice K.J.-“Phillips’ Science of Dental materials” MANIPULATION OF GYPSUM PRODUCTS 56

57. If the hydration is allowed to continue, this process results in the hemihydrate crystals being completely covered with dihydrate crystals. Under these conditions, the water penetrates the dihydrate coating with difficulty and the setting time is prolonged. Therefore, 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. When gypsum products are stored in closed containers, the setting time is generally retarded only slightly, approximately 1 or 2 min per year. This may be counteracted by a slight increase in the mixing time if necessary. Anusavice K.J.-“Phillips’ Science of Dental materials” 57

58. 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. This characteristic may vary from product to product, and it will pack more densely if the container remains undisturbed. When the container is shaken, the packed particles will be loosened and the volume will increase as a result of air entrapment. Anusavice K.J.-“Phillips’ Science of Dental materials” PROPORTIONING 58

59. If mixing is performed by hand, the bowl should be parabolic in shape, smooth, and resistant to abrasion. The spatula should have a stiff blade and a handle that is convenient to hold. Method of mixing:- Add measured water Gradual addition of the preweighed powder The mixture is then vigorously stirred, with periodic wiping of the inside of the bowl with the spatula. Anusavice K.J.-“Phillips’ Science of Dental materials” MIXING 59

60. The mixing should continue until a smooth mix is obtained, usually within a minute. Trapping of air should be avoided while mixing to avoid porosity – weak spots & surface inaccuracies. Longer spatulation = increases working time. After mixing, the use of a vibrator of high frequency and low amplitude is helpful in reducing air entrapment. Anusavice K.J.-“Phillips’ Science of Dental materials” 60

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62. Once the setting reactions in the cast have been completed, its dimensions will be relatively constant under ordinary conditions of room temperature and humidity. However, it is sometimes necessary to soak the gypsum cast in water in preparation for other procedures. When a dry cast is immersed in water, negligible expansion may occur if the water is saturated with calcium sulfate. If the water is not saturated, dissolution of gypsum will occur. For example, a stone cast immersed in a container under running water will lose approximately 0.1% of its linear dimension for every 20 min of immersion. Anusavice K.J.-“Phillips’ Science of Dental materials” CARE OF THE CAST 62

63. If an impression has not been disinfected, it is necessary to disinfect the stone cast. Disinfection solutions that do not adversely affect the quality of the gypsum product can be used. Dental stone containing a disinfectant may also be used. Useful disinfectants for stone casts include spray disinfectants, hypochlorites , & iodophores Anusavice K.J.-“Phillips’ Science of Dental materials” INFECTION CONTROL 63

64. 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). Anusavice K.J.-“Phillips’ Science of Dental materials” 64

65. Anusavice K.J.- “Phillips’ Science of Dentalmaterials ” 11 th edition , 2002. Anusavice K.J.- “Phillips’ Science of Dentalmaterials ” 12 th edition, 2003 Craig’s R.G., Powers J.M. – “Restorative Dental Materials” 13 th edition Nejatian , T., Firouzmanesh , P., & Ajaz , A. (2019). Dental gypsum and investments. Advanced Dental Biomaterials, 37–54. doi:10.1016/b978-0-08-102476-8.00003- 7 BASIC DENTAL MATERIALS 3rd edition by John J Manappallil Materials used in dentistry by S . Mahalakshmi Anusavice K.J.-“Phillips’ Science of Dental materials” REFERENCES 65

66. Thank You Anusavice K.J.-“Phillips’ Science of Dental materials” 66