Gypsum products

COVERS Production of Gypsum Products
Setting of Gypsum Products
Setting Expansion
Strength of Set Gypsum Products
Types of Gypsum Products
Manipulation of Gypsum Products

1. GYPSUM PRODUCTS Dr. Kriti Trehan MDS 1 ST Year 16/1/18

2. CONTENTS Introduction Production of Gypsum Products Setting of Gypsum Products Setting Expansion Strength of Set Gypsum Products Types of Gypsum Products Manipulation of Gypsum Products

3. INTRODUCTION 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. TYPES Albaster - pure white ,fine grained and translucent .

4. Satin spar - fibrous needle like with silky lustre . Selenite - colourless , crystalline and transparent.

5. USES 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.

6. PRODUCTION OF GYPSUM PRODUCTS These materials are produced by calcining calcium sulfate dihydrate (gypsum). Commercially, the gypsum is ground and subjected to temperatures of 110 °C to 130 °C (230 °F to 266 °F) in open containers to drive off part of the water of crystallization.

7. The resulting particle is a fibrous aggregate of fine crystals with capillary pores known as plaster of Paris or dental plaster in dentistry. As the temperature is further raised, it becomes an anhydrite. This process is known as calcination . When gypsum is heated in a kettle, vat, or rotary kiln that maintains a wet environment; a crystalline hemihydrate called dental stone is produced in the form of rods or prisms.

8. Because of differences in crystal size, surface area, and degree of lattice perfection, the resulting powders are often referred to as α- hemihydrate for dental stone and β- hemihydrate for plaster of Paris. If the calcination process occurs under pressure in a 30% calcium chloride solution or in the presence of more than 1% of sodium succinate , the resulting hemihydrate crystals will be shorter and thicker than those produced in a closed container. Residual calcium chloride or sodium succinate is removed by washing the powder with hot water. This type of gypsum-producing product is called modified α- hemihydrate or die stone. These crystals require even less water for mixing.

9. 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.

10. SETTING OF GYPSUM PRODUCTS     The reaction between gypsum products and water produces solid gypsum, and the heat evolved in the exothermic reaction is equivalent to the heat used originally for calcination . Set gypsum products probably never attain 100% conversion unless they are exposed to high humidity for a long time. Therefore, there are unreacted hemihydrates remaining in the set materials .

11. SETTING REACTIONS 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 the measured 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 .

12. 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.

13. 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.

14. QUANTIFYING SETTING REACTIONS The time from addition of powder to the water until mixing is completed is called the mixing time . Mechanical mixing is usually completed in 20 to 30 seconds. Hand spatulation generally requires at least a minute to obtain a smooth mixture. The time from the start of mixing to the point where the consistency is no longer acceptable for the product’s intended purpose is the working time . Generally, a 3-minute working time should allow sufficient time for mixing, pouring an impression and a spare impression, and cleaning the equipment before the gypsum becomes unworkable.

15.   The elapsed time for each stage varies from material to material. The manufacturers usually provide this information with the product. The time that elapses from the beginning of mixing until the material hardens is known as the setting time.

16. TESTS FOR SETTING OF GYPSUM PRODUCTS 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 on the surface is taken up in forming the dihydrate , so that the mix loses its surface gloss and gains strength.

17. 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.

18. Vicat’s test for setting time : After initial setting the further reaction is determined by an instrument called Vicat penetrometer . The needle with a weighted plunger rod is supported and held just in contact with the mix . The time elapsed until the needle no longer reach the bottom of the mix is known as setting time. The elapsed time for each stage varies from material to material. The manufacturers usually provide this information with the product.

19. CONTROL OF THE SETTING TIME 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.

20. Rate of crystal growth —Increasing or decreasing the rate of crystal growth will accelerate or retard the setting time. In practice, these mechanisms have been incorporated in the formulation of the material by the manufacturer and by manipulation techniques performed by the operator. Impurities : Fine gypsum particle residues from incomplete calcination or addition by the manufacturer will shorten the setting time because of the increase in the number of nuclei. Fineness : Grinding of hemihydrate particles during manufacturing increases not only the rate of dissolution of the hemihydrate solution but also the number of nuclei. This increase in nuclei density results in a more rapid rate of crystallization.

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.

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

23. Material W/P Ratio Spatulation 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 Effect of Spatulation on Setting Time

24. MODIFIERS FOR CONTROLLING SETTING TIME 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.

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).

26. SETTING EXPANSION 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.

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.

28. Expansion under normal setting conditions 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 .

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.

30. Hygroscopic setting expansion 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 .

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).

32. CONTROL OF SETTING EXPANSION 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.

33. STRENGTH OF SET GYPSUM PRODUCTS   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.

34. 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.

35. EFFECT OF W/P RATIO   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 diametral tensile strength as high as 25% of the corresponding compressive strength. When materials are mixed at low W/P ratios, the diametral tensile strength is less than 10% of the corresponding compressive strength.

36. 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.

37. 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.

38. EFFECT OF MANIPULATION AND ADDITIVES 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.

39. SURFACE HARDNESS & ABRASION RESISTANCE 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%).

40. TYPES OF GYPSUM PRODUCTS ADA Specification No. 25 classifies five types of gypsum products, as shown in Table 9-5, with the property requirement for each type.

41. IMPRESSION PLASTER (TYPE I): 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.

42. 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.

43. 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.

44. 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.

45.   MODEL PLASTER (TYPE II) 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.

46. DENTAL STONE (TYPE III) 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).

47. 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

48. 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 .

49. 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.

50. DENTAL STONE, HIGH STRENGTH, HIGH EXPANSION (TYPE V) 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.

51. SPECIAL GYPSUM PRODUCTS 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

52. MANIPULATION OF GYPSUM PRODUCTS 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.

53. 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.

54. 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. 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.

55. MIXING

56. MIXING 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.

57. 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.

58. CARE OF THE CAST 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

59. INFECTION CONTROL 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 .

60. 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).

61. REFERENCES 1. Anusavice K.J.-“Phillips’ Science of Dental materials” 11 th edition , 2002. 2. Anusavice K.J.-“Phillips’ Science of Dental materials” 12 th edition, 2003 3. Craig’s R.G., Powers J.M. – “Restorative Dental Materials ” 13 th edition